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Abstract The Langmuir-Blodgett method for producing colloidal monolayers on substrates was learned. It was applied to produce highly ordered monolayers of colloidal polystyrene particles of 125nm and 400nm diameters. A Brewster angle imaging system to allow investigating the macroscopic surface dynamics of Langmuir films was built. It provided a tool for improvements in the general coating method. The produced monolayers have long continuity, high order and large area coverage as determined by observations of atomic force and scanning electron microscopy. A parameter to characterise the process, the peak surface pressure, was identified and its use is discussed. The skill developed here to produce coatings was applied to produce a colloidal etching barrier and a lift-off mask at the request of colleagues from other departments. The usefulness of this coatings is still being evaluated.

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Table of Contents ABSTRACT 2

TABLE OF FIGURES 5 LIST OF TABLES 8 NOMENCLATURE 8

1 INTRODUCTION 9 1.1 PROJECT OBJECTIVE 9 1.2 SELF-ASSEMBLY OF FILMS FROM COLLOIDS 9 1.3 FORMATION AND ALTERATION OF MONOLAYERS 11 1.4 THE SUB-PHASE 13 1.5 CHARACTERISTICS OF LANGMUIR FILMS 13 1.6 MONITORING THE PROCESS 14

1.6.1 Surface Pressure – The Wilhelmy Plate[20] 14 1.6.2 Visual Inspection – The Brewster Angle Microscope 16

1.7 MONOLAYER TRANSFER – THE LANGMUIR-BLODGETT METHOD 16 1.8 ADDITIVES 17

2 EXPERIMENTAL 18 2.1 THE LANGMUIR-BLODGETT TROUGH 18 2.2 THE WILHELMY PLATE METHOD FOR MEASURING SURFACE PRESSURE 20 2.3 EVALUATION OF CONTAMINANTS ON THE LIQUID SURFACE 20 2.4 THE BREWSTER ANGLE MICROSCOPE 22 2.5 FORMING A LANGMUIR FILM 23

2.5.1 Consumable Substances 23 2.5.2 Addition of Colloidal Material 24 2.5.3 Surface Relaxation 25 2.5.4 Characteristic Curves (Isotherms) 25

2.6 MONOLAYER TRANSFER BY LANGMUIR-BLODGETT METHOD 26 2.7 EVALUATION OF COATING QUALITY 26 2.8 SURFACE DYNAMICS 26

3 RESULTS 27 3.1 EXPERIMENT PREPARATION – PROBES 27 3.2 SUSPENSION PROCESS 28

3.2.1 Bulk vs Surface Suspension 30 3.3 ISOTHERMS 30 3.4 ISOTHERM FEATURES AND SP DEPENDENT VARIATIONS 31 3.5 COATING ANOMALIES 35 3.6 SURFACE DYNAMICS 36 3.7 GEOMETRY AND THE EFFECT ON THE TRANSFER PROCESS 37 3.8 PEAK SURFACE PRESSURE AND THE COLLOID SOLUTION VOLUME 37 3.9 QUANTIFYING SOLUTION VOLUME AND AREA COVERAGE 38 3.10 SURFACTANTS 39 3.11 COATINGS 41 3.12 USING THE COATINGS 44

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4 DISCUSSION 46 4.1 CHOICE OF PROBE AND SURFACE DYNAMICS 46 4.2 INTERFACES, ANOMALIES AND ORIENTATION 48 4.3 SURFACE RELAXATION 50 4.4 SUSPENSION METHODS AND PEAK SURFACE PRESSURE 51 4.5 COMPRESSION CURVES AND THE OPERATING POINT 53 4.6 SURFACE COVERAGE VS MINIMISING DOMAIN-BOUNDARY DEFECTS 54 4.7 ANALYSING COMPRESSION CURVES 54 4.8 SURFACTANTS AND THE LANGMUIR-BLODGETT PROCESS 55 4.9 MIXTURE TRANSFER 55 4.10 COATINGS 56 4.11 AN ATTEMPT AT QUANTIFYING THE PROCESS 57

5 EXTENDED WORK 58 5.1 PHOTOCATALYTIC SELF-CLEANING SURFACES 58

5.1.1 Background 58 5.1.2 Results and Discussion 59

5.2 WATER FILTRATION MEMBRANE 60 5.2.1 Background 60 5.2.2 Results and Discussion 62

6 CONCLUSIONS 63

7 APPENDIX 64 7.1 COLLOID SOLUTION VOLUME CALCULATION 64

8 REFERENCES 66

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Table of Figures Figure Caption Page

1.1 Illustration of hexagonal packing of spherical colloids showing possible lattice defects: domain boundary defects (dashed line), missing colloid defect (x), defects resulting from different size colloids (A) and a double stack defect (grey)

10

1.2 Single layer (SL) and double layer (DL) PS colloidal nanosphere masks (colloid diameter of 264nm) and the corresponding shadow and the final vapour deposited silver on mica structures imaged by AFM.

12

1.3 Scanning electron microscopy (SEM) images of PS spheres, 310nm in diameter, patterned by alignment and filling of laser cut grooves measuring 1.5μm wide and 0.36μm deep. Inset shows magnified image.

12

1.4 SEM images of PS (100nm in diameter) lattice before (A) and after (B) isotropic etching by oxygen plasma at room temperature for 2 minutes.

13

1.5 SEM images of PS (540nm in diameter) mask annealed by microwave pulses in a water/ethanol/acetone mixture. (A) to (F) signify the amount of annealing where (F) is the most annealed.

13

1.6 Ideal isotherm showing gas, liquid, solid phase regions. Purple line highlighting almost linear region within the liquid phase region.

14

1.7 Brewster angle microscopy principle. ΘB≈53° for n1=1 (air) and n2=1.33 (water) 16

1.8 Langmuir-Blodgett sample coating method depicting monolayer transfer and moving barriers.

17

2.1 KSV NIMA Langmuir-Blodgett trough with well including a force metre and dipper arm. 18

2.2

Dipper clamp holding two standard microscope slides parallel to the barrier interface and submerged in the sub-phase within the well. Next to the samples, the Wilhelmy plate (paper type) is suspended from a hook connected to a digital force meter above and is perpendicular to the compressing barriers. Arrows along dashed line indicate the movement direction of the barriers.

19

2.3 Section of a Langmuir trough and one barrier showing the concave and convex menisci formed on the barrier and trough edge, respectively. Trough overfilled to highlight the two menisci.

19

2.4 Cleanliness compression measurement, using a paper probe, showing a sharp increase in SP as the area is reduced indicating a high contamination

21

2.5

Custom built BAM system showing the light source and diffuser as well as the camera used mounted along the 53° vector from the normal to the liquid surface by means of the 3D printed camera mount and positioning grooves. Polariser (not shown) is attached directly in front of the camera lens.

22

2.6 Semi-submerged glass slide with one tip resting on edge of the trough. Dashed lines highlight the submerged outlines of the slide and a red line highlights the glass-water-air interface.

24

2.7 Method used for applying liquid mixtures to the body of a liquid 25mm below the surface without disturbing the SP readings.

25

3.1 SP value variation over time for the initial immersion of a Wilhelmy probe. 27

3.2 Complete relaxation plot of 125nm, 200μL, colloid mixture showing the material addition, PSP and relaxation regions.

28

3.3 Relaxation plots for 60, 90, 120 and 200μL colloid solutions (diluted 1:4 EtOH). Dashed lines represent a DI water substitute for each of the measurement sets. Numbers indicate mean PSP for each of the measurement sets.

29

3.4 Isotherms for 125nm PS spheres diluted 1:4 EtOH showing three plots for each of the solution mixtures; 60, 90 and 120μL.

30

3.5 SP fluctuations at the gas-liquid phase transition region of a 125nm diameter PS spheres 90μL colloid mixture isotherm.

31

3.6 SP drop in the liquid phase prior to reaching the target SP of 37mN.m-1. 32

3.7 Barrier oscillation in response to measured SP when maintaining a target SP of 37mN.m-1 for 5 minutes.

33

Table 1 – List of figures (page 1 of 3)

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Figure Caption Page

3.8 (A) AFM and (B) optical microscope images of domain boundaries for the case of 5 minutes of barrier oscillations at the target SP of 27mN.m-1

34

3.9

(A) AFM and (B) optical microscope images of domain boundaries for the case of 15 minutes of barrier oscillations at the target SP of 27mN.m-1. Dashed lines in (A) highlight the approximate domain boundaries. Dark lines in (B) are multilayer regions at the domain boundaries.

34

3.10 Optical microscope image of chains of PS colloids (125nm diameter) observed in uncoated regions.

35

3.11 (A) AFM and (B) optical microscope images of voids observed within large continuous monolayers of PS colloids (125nm diameter). The diameters of the elliptical void in (A) is 5 and 6μm.

35

3.12 Compressed solid layer depicting grating structures as a result of reaching a solid phase. Inset showing magnified section.

36

3.13 (A) Wilhelmy plate pushed from original rest position following a deposition and (B) returned to original resting point following aspiration of colloidal material off the liquid surface. Dashed line shows original resting position.

36

3.14 Possible sample orientations with respect to the barriers 37

3.15 Pure colloidal solution (diluted 1:4 EtOH) as a function of PSP. Mean values per solution volume are plotted as well as a corresponding exponential fit. The fitted equation and fit value are also shown.

37

3.16 Calculated, empirical and modified colloidal solution volume as a function of the gas-liquid transition region area coverage (for depositing monolayer films on a liquid surface).

38

3.17 Triton X-100 relaxation curves corresponding to measurements tabulated in Table 3.3. 39

3.18 Triton X-100 isotherm curves corresponding to measurements tabulated in Table 3.3. 40

3.19 Monolayer coating of 125nm PS spheres showing an (left)AFM of a highly ordered lattice and a corresponding (right)FFT of the height sensor data (colour scale is in nm).

41

3.20 Standard glass microscope slide coated with 125nm diameter PS spheres showing no macroscopically visible multilayers or large discontinuities. Yellow-white regions correspond to the uncoated substrate while the blue regions are monolayers.

41

3.21 Monolayer coating of 400nm PS spheres showing an (left)AFM of a highly ordered lattice and a corresponding (right)FFT of the height sensor data (colour scale is in nm).

42

3.22

Standard glass microscope slide coated with 400nm diameter PS spheres showing the optical properties originating from the periodicity of the monolayer and multilayer structures as different colours. Blue/green regions correspond to monolayers while pink and white-opaque regions correspond to multilayers. Curved regions at the beginning of the monolayer are also shown.

42

3.23

Monolayer coating of 400nm PS spheres showing an (left)AFM of a highly ordered lattice with defects and a corresponding (right)FFT of the height sensor data (colour scale is in nm).

(left) White square on AFM image indicate a magnified region shown in Figure 3.24 while

the inset shows the magnified particle indicated by the white arrow.

43

3.24 AFM image of a coating defect for the square region indicated on Figure 3.23 for a 400nm PS

sphere monolayer coating. Sphere diameters are 1: 399nm and 2: 296nm. Inverted colours for clarity only.

43

3.25

SEM images of 125nm diameter PS spheres on quartz as pillar structures are etched through the colloid mask. (A)Spheres on quartz, unprocessed. Etched for (B)1 minute (C)2 minutes and (D)15 minutes. Black and yellow arrows in (B) and (C) indicate the PS and quartz respectively. In (D), the PS spheres are removed at the end of the etching process using an oxygen plasma process.

44

3.26

SEM images of pillar structures etched in quartz through a 125nm diameter PS spheres colloidal mask. (A) Large area coverage for a 5 minute etch. (B) Dense packing for a 3 minute etch. (C) Enlarged section showing 470nm tall pillars with 125 and 160nm top and base diameters respectively.

45

Table 1 – List of figures (page 2 of 3)

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Figure Caption Page

4.1 Surface dynamics depicted by black arrows indicating flux of solid material on the liquid surface and transfer to a sample. Yellow arrow indicates plate movement direction.

46

4.2 Wilhelmy plate when static and pushed (blue arrow) in the gas and solid phases showing the observed surface fixed point of the plate in the solid phase only.

47

4.3 Illustration of multiple coating observations on a microscope slide showing a large gap in the coating as well as chains along coating direction and voids within a coated layer.

48

4.4

Picture of a microscope slide coated by 125nm diameter PS spheres showing monolayer domains, multilayers and chain structure regions. The back of the slide was painted black to improve the reflection contrast (prevent reflections from the back of the slide). White-yellow and blue regions correspond to uncoated and coated parts of the substrate respectively. Speckled white-yellow regions between the blue regions are breaks in the monolayer. Curved regions at the beginning of the coating are also shown.

48

4.5 Illustration of sample coating both good and bad simultaneously due to insufficient and local SP regions.

50

4.6

Proposed surfactant modification of colloids to produce open-packed hexagonal structures. (A)The modified hexagonal packing and (B)enlarged section showing the radius (r), amphiphilic tail spacing (Δ) and surfactant organisation. (C)The functionalised colloids organise above the sub-phase liquid.

55

5.1 Rutile-Anatase layered structure (anatase on the surface) coated with a colloid monolayer and etched using RIE to produce a large surface area pillar structure.

58

5.2

(Top left) AFM of a clean anatase TiO2 surface. (Bottom left) AFM of a monolayer coating of 125nm PS spheres on an anatase TiO2 surface (same sample as in top left but different region). To the right of the two AFM images a corresponding FFT of the height sensor data (colour scale is in nm) is shown.

59

5.3

Process flow of fabricating a porous chrome template filled with silica fibres. (A)Depositing hexagonally packed, 400nm diameter, PS spheres. (B)Shrinking the PS spheres by means of oxygen plasma. (C)Depositing a chrome layer. (D)Dissolving the remaining PS spheres and lifting-off the chrome on top of the spheres. (E)Selectively etching the silicon substrate. (F)Growing silica fibres in the template pores to achieve the complete membrane.

61

5.4 (left)AFM of a silicon substrate coated with 400nm diameter PS spheres and the corresponding (right)FFT, of the height sensor data (colour scale in nm). Smaller contamination spheres can be distinguished to be in the red range of the colour scale.

62

7.1 Perfect hexagonal packing of spheres with radius r and the smallest unit cell. 64

Table 1 – List of figures (page 3 of 3)

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List of Tables

Table Caption Page

2.1 Solvents used during experiments. 23

3.1 Relaxation experiment measurement sets. 28

3.2 PSP values for bulk and surface addition methods as well as a minimum and average PSP values for the case a of a flume method. Addition of only water is also shown.

30

3.3 Triton X-100 diluted mixtures (1:1000) volumes and corresponding measured PSP. 39

7.1 Parameter list 65

Table 2 – List of tables

Nomenclature

Abbreviations Term

AFM Atomic Force Microscopy

BAM Brewster Angle Microscope

EDTA Ethylenediaminetetraacetic Acid

EtOH Ethanol

FFT Fast Fourier Transform

IPA Isopropanol

NIST National Institute of Standards

NSL Nanosphere Lithography

OP Operating Point

POM Polyoxymethylene

PS Polystyrene

PSP Peak Surface Pressure

PTFE Polytetrafluoroethylene

Pt-rod Wilhelmy platinum rod probe

RIE Reactive Ion Etching

SDS Sodium dodecyl sulphate

SEM Scanning Electron Microscopy

SP Surface Pressure

TMD Traceable Mean Diameter

Table 3 – List of abbreviations used

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1 Introduction

1.1 Project Objective The objective of this project is to produce masks to form dense and highly ordered nanoscale structures. This is accomplished by harnessing the self-arranging properties of submicron particles, colloids. The process of attaining such masks is achieved by studying and improving upon the existing Langmuir-Blodgett method for monolayer formation and coating. The processes involved in engineering structures on the nanoscale is a major subject of interest in the last century and more so in the last 20 years. Many approaches can be taken to realise them and most require a photolithographic stage at some point that is limited by the wavelength of light. To overcome this, a new approach must be taken. The self-organisation properties of nanospheres to form ordered monolayers may provide a solution and is the subject of this project. The methods to create monolayers of colloidal polystyrene (PS) spheres were studied and characterised with a goal of producing highly ordered masks directly onto substrates. The masks can be used as deposition or etching barriers as well as lithographic and lift-off masks. Monolayer films were made for use as masks for a range of other applications. The apparatus used here is a Langmuir-Blodgett trough. Due to the lack of documentation and parameters in the field of colloidal lithography, including in peer-reviewed, published papers, it was necessary to pursue a basic and fundamental approach prior to moving on to making actual devices. For that purpose, the process of attaining a colloidal monolayer on a substrate was studied. This study included investigating the suspension of a colloidal solution, the formation of ordered monolayers, the deposition of the films and geometrical factors and their implications on the successful transfer that were not all expected.

1.2 Self-Assembly of Films from Colloids When small microscopic particles are mobile within a phase, they may impinge on one another and have a tendency to stick together. This nucleation promotes further aggregation and so macroscopic structures may naturally occur. This is the basis of colloidal techniques and through control of different parameters, the aggregation can be controlled to form ordered (crystalline) structures. As in all crystallization processes, the cohesive forces[1] between the particles drive the assembly process toward the organization of lowest energy. The quality and rate of this crystallization process are influenced by many variables such as concentration of particles, agitation, temperature and impurities. Particles, which are smaller than 1μm and larger than 1nm in diameter, dispersed in a containing material (liquid) and that are insoluble in that material are termed colloids. For experimentation and fabrication uses, these particles are typically spherical and symmetric. The most widely experimented material is PS. Only spherical colloids are considered here.

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Packing of spherical particles to high densities has been widely examined in the literature[2] and a single solution is found when considering spherical particles of uniform dimensions. The same packing pattern appears in natural structures from subatomic to macroscopic scales. This structure follows a hexagonal pattern (Figure 1.1) and when considered for uniform spherical particles will result in maximum two-dimensional packing density (D) given by equation (1.1) in units of spheres per unit area for spheres of radius r.

𝐷 =1

2√3𝑟2 (1.1)

Nucleation at different regions result in lattice mismatch at crystal boundaries which manifest as defects as shown in Figure 1.1. The quality of the crystal is evaluated by the area of a single ordered domain and the defect densities which correspond to any deviation from the continuous hexagonal lattice, including empty lattice points or a double stack that does not disrupt the full structure. A double stack is an additional colloid placed on a layer above the intended final crystal plane. The crystalline structures of interest are usually of a single monolayer or a few stacked layers in height and so their continuous crystalline order can be characterised for uses in numerous applications where highly ordered structures and a uniform periodicity on the nanoscale is desired.

Figure 1.1 – Illustration of hexagonal packing of spherical colloids showing possible lattice defects: domain boundary defects (dashed line), missing colloid defect (x), defects resulting from different

size colloids (A) and a double stack defect (grey) Monolayers are the main interest of colloidal research and have been investigated over many decades but have yet to play a significant role in modern industrial processing methods. This is due to a number of reasons of which the primary ones are process consistency and repeatability of their formation outside of laboratory conditions. Multi-layered crystals are typically formed by a repeated dispersion-evaporation process where a single layer is deposited at a time however spatially controlled methods of bulk growth[3] have been demonstrated in the literature. The optical properties of colloidal dispersions are rather unremarkable and are typically characterised as highly dispersive media due to the complete lack of structural order. The attractiveness of colloids is mainly in the self-arrangement and organization properties they possess to form structures that are on the nanoscale, beyond the capabilities of conventional photolithography processes. Highly ordered structures made of colloidal particles have optical properties that result solely from their ordered structure and periodicity. These can be fabricated in dimensions smaller than the wavelength of visible light producing optical effects such as photonic band gaps[4]. This, however, is most prominently observed for single (or stacked) monolayers that have long continuity and minimum defects. Such materials are attractive for many optical technologies and most notably revolve around their, wavelength selective, highly reflective or anti-

A

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reflective attributes which makes them useful as high performance Bragg reflectors or waveguides[4] to name just a few applications. This only accounts for using the self-arranged colloids as the main building block of the material and as such the material variability also provides other traits which are material specific. A further benefit arises from the possibility of coating non-planar surfaces. If colloidal particles with electrical properties are used, it may allow for device fabrication on scales surpassing current fabrication techniques in very high densities, although addressing a single colloidal particle by making an electrical contact to it, once a layer is complete, is a significant challenge. Restricting the movement of the colloidal particles to a narrow plane allows for thin (two-dimensional) ordered structures to appear, rather than bulk, three-dimensional structures. The restriction to a plane can be realised by dispersing the colloids on a surface. Colloids deposited on a liquid will ideally not penetrate the interface unless agitated by either mechanical or thermal sources and will therefore form a suspension at the interface. The suspension has sufficient lateral spatial movement allowing the colloids to arrange to their lowest energy state. PS is particularly useful for realisation of suspensions on the surface of water as it has a density that is very close to water allowing PS spheres to ‘float’ on the water surface without sedimenting. Upon shrinking of the surface of the liquid, the colloids interact and organize and the formation of a stable monolayer is possible. Such layers are typically termed Langmuir films, named after Irving Langmuir who pioneered the scientific treatment of surface floatation of adsorbents[5].

1.3 Formation and Alteration of Monolayers Forming and depositing monolayers can be done in many ways, none one of which provide as much control over the process as the Langmuir-Blodgett method. The Langmuir-Blodgett method takes the approach of growing a high quality crystal and then transferring it to a substrate. The principles of operation of the apparatus did not vary by much since it was invented by Langmuir and Blodgett in 1934[6] while the trough and film formation is attributed to Langmuir and the transfer method to Blodgett. The process of forming a monolayer on the liquid surface involves three major steps; Cleaning of the trough, barriers and surroundings, transfer of a colloidal suspension to the liquid surface and compression of the liquid surface until a criterion such as a given surface pressure (SP) or surface area is achieved. Due to the small dimensions of colloidal particles and their ability to form highly ordered structures, colloidal crystals are an attractive method for creating lithographic masks as an evaporation/etching barrier for shaping a substrate as first proposed by Deckman et al[7]. This makes the technique very attractive for semiconductor and optoelectronics fabrication as the masks can be made in dimensions smaller than what is currently possible with conventional photolithography. A typical source wavelength of 193nm allows reliable fabrication of features of about 40nm while newer systems using EUV (extreme ultraviolet) sources (13.5nm wavelength) allow fabrication of features that may reliably go down to 30nm or less[8]. The generated masks may be of single or multiple stacked monolayers and result in different structures as shown in Figure 1.2. The method of using spheres for lithography is typically called Colloidal Lithography, although Nanosphere Lithography (NSL) and Natural Lithography[7] also appear in the literature. Using colloids as masks presents a difficulty however, as the crystal must be formed on the substrate material. A method to reliably grow or transfer the colloidal crystal to the substrate surface is necessary and is a major challenge for the usage of colloids in general, not only for lithography. Among the many experimented methods[9], the Langmuir-Blodgett method[6] has the most control over the process while another popular and successful method is spin-coating[7] which allows for deposition of large areas at relatively low cost. The formed colloidal lithographic masks are beneficial when compared to conventional photolithography as there are no optically generated artefacts (blurred edges) as the

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colloids are in contact[10] with the substrate. Using colloidal lithography, naturally occurring and biologically inspired structures, such as the moth-eye structure[11], can be made.

Figure 1.2 – Single layer (SL) and double layer (DL) PS colloidal nanosphere masks (colloid diameter of 264nm) and the corresponding shadow and the final vapour deposited silver on mica structures

imaged by AFM. Adapted from [12] Colloids may form continuous monolayers on entire surfaces. If, however only a certain region is to be covered, introduction of topographical variations in the accepting substrate will allow the colloids to occupy and fill grooves and trenches. Such an example was demonstrated[13] by spin coating a dispersion on a laser-machined sample shown in Figure 1.3. Alignment and filling of grooves have yet to be demonstrated in a Langmuir-Blodgett process since the transferred monolayer will follow the topography and provide good step coverage when using this method.

Figure 1.3 – Scanning electron microscopy (SEM) images of PS spheres, 310nm in diameter,

patterned by alignment and filling of laser cut grooves measuring 1.5μm wide and 0.36μm deep. Inset shows magnified image. Adapted from [13]

Manipulation of a monolayer structure after transfer to a substrate is possible by mechanical, optical and chemical means. Mechanical implies using techniques such as AFM (contact-mode) to physically carve structures or extract single particles from the lattice. Optical techniques include laser etching of the lattice while chemical relates to subjecting the lattice to conditions that uniformly alter the entire lattice. An example of the latter case is demonstrated in Figure 1.4 and Figure 1.5. Figure 1.4, presents the results of subjecting the PS spheres to isotropic etching by oxygen plasma. This resulted in the uniform shrinking of the particles about their centres and uniformly across the sample. Figure 1.5 on the other hand presents the opposite case where the spheres were swollen by annealing in a water-ethanol-acetone mixture to reduce the periodic gaps between them.

DL

SL

10μm

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Figure 1.4 – SEM images of PS (100nm in diameter) lattice before (A) and after (B)

isotropic etching by oxygen plasma at room temperature for 2 minutes. Adapted from

[14]

Figure 1.5 – SEM images of PS (540nm in diameter) mask annealed by microwave

pulses in a water/ethanol/acetone mixture. (A) to (F) signify the amount of annealing where (F) is the most annealed. Adapted

from [15]

1.4 The Sub-phase Deionised (DI) water (H2O) is the most used liquid substance as the phase on which spheres are suspended in a Langmuir-Blodgett process. This phase is commonly referred to as the sub-phase. Water, at 20°C, the reference substance for metric density, has a density of ρ= 998.21 kg.m-3[16]. Most inorganic solid materials have densities higher making them likely to sediment if suspended in water. This makes forming suspensions that are localised to the liquid surface challenging as any colloid that strays below the surface is unlikely to reincorporate and will be lost (when considering the formation of a Langmuir film). The most experimented colloidal substance is PS, due to the ease at which colloids of such material can be fabricated at very high accuracy. Furthermore, PS has a density higher than, although close to, water (ρ=1050 kg.m-3[17]), which makes it less prone to sediment. Dilutions of colloidal solutions with a volatile solvent (that has a lower density than water such as ethanol, EtOH) significantly improves the segregation of the colloids to the surface rather than to the bulk of the sub-phase[18]. The exact dilution ratios are not widely reported and are often regarded as ‘trade secrets’.

1.5 Characteristics of Langmuir Films Quantitative evaluation of Langmuir films is typically approached by analysing isotherms. In the context of Langmuir films the isotherm is a plot of the change in SP as a function of the surface area at constant temperature. The curve can be very complex for typical organic molecules with active groups. In the case of films formed by unmodified PS colloids the situation is generally simplified and only includes three main regions of interest on the curve. An ideal isotherm curve for spherical colloids is given in Figure 1.6. The three regions on Langmuir film isotherms are generally termed gas, liquid and solid phases. These terms relate to the degree of spatial freedom that molecules, segregated to the surface, posses. The gas phase describes a high degree of freedom and the solid phase describes a very low, to negligible, degree of freedom in the 2D plane (defined by the liquid surface). The liquid phase is a narrow region where the molecules have short range and restricted freedom. The liquid phase is observed when a continuous, but not necessarily complete, monolayer is formed but the layer still possesses some flexibility to rearrange and, ideally, moves as a single crystal domain. In actual films, due to the way the colloids interact, small domains nucleate first within the gas phase. These small domains are forced together by the compression process and yield continuous layers that will have gaps (nano/microscopic to large millimetre long gaps).

B A

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Figure 1.6 – Ideal isotherm showing gas, liquid, solid phase regions. Purple line highlighting almost

linear region within the liquid phase region. In the solid phase region, high SP is relieved by the displacement of colloids to higher planes above the monolayer. For that reason, when forming monolayers, the solid phase region must be avoided so multiple layers will not be present. This is mentioned in particular, as multilayer generation using a Langmuir-Blodgett method is not a controlled process and will not produce distinct double or triple layers as may be desired but rather local, undefined, multi-layered regions. For fabricating highly ordered monolayers, the region of interest on an isotherm is the linear region that appears within the liquid phase and includes the majority of the region. The transition points between the gas-liquid phases and liquid-solid phases are not uniquely defined. For the purpose of this work, these are taken as the onset and conclusion of the linear region of the liquid phase. The ideal point for fabrication would be the highest SP in the liquid region (or even slightly within the solid phase[6]). It is recommended that the operating point (OP) is set to a lower SP value in order to reduce the risk of creating unwanted multi-layers, when approaching and maintaining the OP. It is reasonable to choose the middle of the liquid region as the position of the OP.

1.6 Monitoring the Process When investigating Langmuir films, monitoring systems that allow observation of large areas are required as the films typically extend over several square centimetres. When investigating any system, it is beneficial if a minimal disturbance is introduced, and is especially true when observing Langmuir films. Systems that may provide non-invasive inspection of such films include optical methods based on reflection and diffraction effects of the film. Brewster angle microscopy, BAM, is an example of such a system. A limiting factor for optical methods is that low intensity light must be used to prevent gradient forces from affecting the growth if a narrow beam is used[19]. However, these systems cannot directly provide a unique criterion, although it is possible, and may not be applicable for all materials. Therefore, an invasive method must be employed in the form of a Wilhelmy plate (or probe) and is the main approach used in monitoring the growth of Langmuir films (see section 1.6.1).

1.6.1 Surface Pressure – The Wilhelmy Plate[20] Monitoring the SP, or tension, of the surface layer during the formation of Langmuir films is an integral part of any experiment, as no other quantitative means to evaluate the state of the film exists, which is as universal. The Wilhelmy plate method to measure the SP of a liquid surface, named after its inventor Ludwig Wilhelmy, provides such quantitative means. The method requires partially immersing a plate of known dimensions in a liquid while measuring the force acting on the

Surf

ace

Pre

ssu

re

Surface Area

Gas Solid

Liquid

Ideal Isotherm

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plate due to a concave meniscus (the liquid wets the plate) forming at the plate-liquid interface. The critical dimensions are those defined by the circumference of contact defined by the interface at the plate-liquid intersection. The plate has to be partially submerged in the liquid such that the meniscus is not broken at any point and the plate is perpendicular to the liquid surface such that the circumference defining the liquid-plate cross-section is not altered through the experiment. The meniscus formed must be concave making the plate material dependent on the liquid investigated, hydrophilic plate for hydroxyl liquids such as water or lipophilic for oils. The force is translated into SP (N/m) by equation (1.2).

𝑆𝑃 = 𝐹

𝐶𝑖𝑐𝑜𝑠𝜃

(1.2) Where F is the measured force in Newtons, θ is the wetting angle of the meniscus that forms on the plate and Ci is the interfacial circumference defined by the liquid-plate cross-sectional interface (for typical rectangular plates; 𝐶𝑖 = 2𝑤 + 2𝑡 where w and t are the width and thickness of the plate respectively). The method further provides a methodology for measuring the wetting of a surface if the SP of a liquid is known. For most applications, equation (1.2) is usually simplified by assuming a fully wetted plate (θ=0). Therefore, the simplified equation, and the one used here, is shown in (1.3).

𝑆𝑃 = 𝐹

𝐶𝑖=

𝐹

2𝑤 + 2𝑡

(1.3) As mentioned earlier, the composition of the plates used in this technique can be made of any hydrophilic material when investigating the surface of water. A simple plate to use is made of paper. Typically, laboratory grade filter paper is used. The paper type is useful as it is porous and so it absorbs the liquid and promotes wetting of any type of liquid be it for hydrophilic or lipophilic sub-phases so that the concave meniscus is obtained (making the approximation of θ=0 acceptable in both cases). This type of plate is also cheap and disposable but still provides sufficiently accurate results when studying Langmuir films. A known shortcoming of the paper plate is apparent when submersion over long periods is considered. Then the paper may and will absorb some of the suspended particles (as in the chromatography effect[21]) and will gradually lower the SP. Using the Wilhelmy plate method, the SP of liquids can be continuously monitored as a function of many parameters of which time and the varying liquid surface area are some of the more useful ones.

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1.6.2 Visual Inspection – The Brewster Angle Microscope Brewster angle microscopy enables observation of surface structures in contrast to a uniform dielectric background. This is achieved by utilising the Brewster angle, Equation (1.4), at which p-polarised light (electric component of light parallel to plane-of-incidence) is completely transmitted from the lower dielectric material (ε1) to the higher dielectric (ε2) material while only s-polarised light (electric component of light wave perpendicular to plane-of-incidence) is reflected.

𝜃𝐵 = tan−1 √𝜀2

𝜀1= tan−1

𝑛2

𝑛1 (1.4)

Figure 1.7 – Brewster angle microscopy principle. ΘB≈53° for n1=1 (air) and n2=1.33 (water)

Incorporating a polariser in the correct orientation, s-polarised light is blocked from the reflection path. A spatially resolved light sensor, such as a CCD or a CMOS sensor positioned in the reflection path at the Brewster angle with respect to the normal to the water surface will not detect any light reflections off the water surface due to the removal of the s-polarised light from the source beam (Figure 1.7A). When a thin dielectric is introduced, in the form of a colloidal layer, the effective Brewster angle will change (the primary reflection is a function of n1 and n3 in this case) and p-polarised light will again be reflected and sensed by the detector, as shown in Figure 1.7B. The resulting image as detected by the sensor provides a highly contrasted picture of surface segregated material with respect to regions on the water surface that are clean.

1.7 Monolayer Transfer – The Langmuir-Blodgett Method The Langmuir-Blodgett method relies on transferring a monolayer to a semi-submerged, perpendicular substrate (sample) by the controlled extraction of the substrate from the sub-phase through the monolayer as shown in Figure 1.8. As the monolayer is transferred to the substrate, the SP drops due to the reduced surface coverage by the depleting monolayer. In response, the area of the trough is reduced by moving the barriers to maintain a constant SP.

n2

n1

θB

θB θ

B

Monolayer n

Source

P-Polariser Sensor

No Reflection

A B

n3

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Figure 1.8 – Langmuir-Blodgett sample coating method depicting monolayer transfer and moving

barriers. Adapted from [22] The starting position of the substrate depends on the material forming the film and the sub-phase liquid and as such is described here only for the case of PS colloids on a water sub-phase. The substrate begins submerged prior to depositing any colloids and is extracted during the coating process. The colloids transfer to the substrate due to cohesive forces, Van der Waals forces, between the suspended colloids and the adhesive forces between the colloids and the substrate. The method is sensitive to external variations such as temperature, mechanical and acoustic vibrations and contamination, among many others, but still permits fabrication of high quality films in ambient conditions and in conventional laboratories (opposed to cleanrooms) as reported here. Other methods, such as the Langmuir-Schaefer[23], that originated from the Langmuir-Blodgett method exist and typically only vary by sample-liquid surface orientations. Another widely used and successful method involves spin coating[7] and shown to produce highly ordered structures. Immersion of a substrate once a layer was deposited will result in detachment of colloids from the sample and the loss of the ordered lattice. Therefore, the transferred lattice must be treated as a water-soluble substance and may limit further processing. However, careful wetting of the colloidal lattice by applying a small droplet is theoretically feasible without damaging the ordered lattice.

1.8 Additives The use of additives in the experimental formation of Langmuir films is a common practice. The additives may come in the form of dilutions with volatile solvents, water or surfactants. Surfactants are reported[24] to improve or reduce the segregation of colloids to the sub-phase surface and reduce nucleation in the gas phase. Based on their concentration, surfactants promote[15], [25] the self-arrangement of colloids into continuous films. Surfactants have also shown to promote formation of controlled multilayer structures[26] and improve monolayer dispersion when spin coating is used (wetting of the coated surface)[12]. These additives appear in many forms in the literature and their successful experimental concentrations are not widely reported apart from the name of the substance. A common surfactant used is Triton-X100, which is an amphiphilic non-ionic surfactant molecule. The 100 in the name refers to the length of the carbon chain forming the hydrophobic tail. Surfactants are typically added to batches of aqueous colloidal solutions when they are packed and sent to consumers to prevent aggregation and extend their shelf life as in the case for the colloidal sources used here (see section 2.5.1). It is apparent the presence of surfactants influences the process but their significance is unknown and unquantified.

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2 Experimental

2.1 The Langmuir-Blodgett Trough The Langmuir-Blodgett trough and accompanying apparatus used here is shown in Figure 2.1. It is a KSV NIMA Langmuir-Blodgett medium deposition trough (model KN2002). The apparatus includes trough for containing a sub-phase liquid, typically water. The trough depth is 4mm and has a deeper region at its centre that is 20mm wide, 56mm long and 60mm deep and is referred to as the well. The width and length of the trough are 75.2mm and 360mm respectively. The dimensions relate to the top of the trough. Two computer-controlled barriers lay flush on top of the trough and can be moved along the length of the trough. The barriers do not extend below the top of the trough and so they do not penetrate the bulk of the sub-phase when the trough is filled. In the fully open position, the area enclosed by the barriers and the long edge of the trough is 232.4cm2 and corresponds to an enclosed trough length of 309mm. A digital force metre is placed above the trough to monitor the SP. The metre can be oriented and positioned at different distances from the trough surface. To the metre is attached a lightweight vertical hook from which a Wilhelmy, SP monitoring, probe is hung (only hook visible in Figure 2.1). The quoted SP sensitivity of the metre is 4μN.m-1. For the purposes of this work, the resolution of the force metre is beyond the observed values and is in most cases rounded to 10μN.m-1

Figure 2.1 – KSV NIMA Langmuir-Blodgett trough with well including a force metre and dipper arm.

Adapted from [22] Controlled extraction of samples is done by a computer-controlled, motorized, sample holder (dipper) that is positioned in the centre of the trough (between the barriers) and also centred between the short edge of the well and the SP probe. The dipper and Wilhelmy plate orientation along with the well in the trough are shown in Figure 2.2 while the metal rod probe is shown in Figure 2.5. The orientation of the sample and the Wilhelmy plate relative to the compression direction shown in Figure 2.2 is as recommended in the literature[6] and in the user manual[27] of the manufacturer of the trough. The orientation is such that the submerged samples are positioned parallel to the compressing interface (the barriers) while the plate is positioned perpendicular to the compression direction. Both sample and probe should be at maximum distance from each other and any other interface. When only measuring surface properties, no sample present, the probe is positioned at the middle of the trough (well).

Trough

Dipper

Force Metre

Well

Barriers

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For practical reasons the trough is made of a hydrophobic material such that water is well contained within and allows for over-filling of the trough without spillage, which is required for experiment preparation (surface cleaning by aspiration). An important requirement of the trough is that the liquid surface area enclosed between the barriers is never broken and exposed to other regions. This is accomplished through the hydrophilic properties of the material composing the barriers that form a concave meniscus on the barrier walls. The two menisci for a trough, overfilled with water, is seen in Figure 2.3. The trough is made from PTFE (polytetrafluoroethylene) while the barriers are made of POM (polyoxymethylene).

Figure 2.2 – Dipper clamp holding two standard microscope slides parallel to the barrier interface and submerged in the sub-phase within the well. Next to the samples, the Wilhelmy plate (paper

type) is suspended from a hook connected to a digital force meter above and is perpendicular to the compressing barriers. Arrows along dashed line indicate the movement direction of the barriers.

Figure 2.3 – Section of a Langmuir trough and one barrier showing the concave and convex menisci formed on the barrier and trough edge, respectively. Trough overfilled to highlight the two menisci.

Concave

Convex

Barrier

Trough

Wilhelmy Plate

Samples

Well

Dipper Clamp

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2.2 The Wilhelmy Plate Method for Measuring Surface Pressure The SP was monitored continuously during all processes with the force metre. The type of probe used varied during the experimentation as will be discussed in Section 4.1. The two Wilhelmy probe types that were used are a paper plate and a metal rod. All measurements were performed relative to the SP of pure water, 72.869mN.m-1 at 20°C[28], as no alteration to the sub-phase liquid were made throughout the experiments. The paper plate probe was supplied by BiolinScientific and was made of a rectangular piece of filter paper with dimensions of 24x10x0.1mm with a circular section removed at one end, 3mm in diameter, to allow it to be hung. The metal rod is made out of a circular rod of platinum and measures 983.4μm in diameter. The end of the rod is a hook to allow it to be hung and the length of the rod, not including the hook, is 52mm. The paper plate and platinum rod are abbreviated here as plate and Pt-rod respectively. The positioning of the probes relative to the trough and barriers as well as their submerged depth, was the same for both types and is shown in Figure 2.2 and Figure 2.5. The adjustment is performed by positioning of the force metre while a probe is hung from the attached hook. The lateral positioning of the probes is such that the probe is 20mm from the long edge of the trough and in the middle of the trough (between the barriers). The vertical positioning of the probes is such that they are approximately one-third submerged under the liquid surface. The probe is mounted at the beginning of any process and a resting period is required before reliable readings can be made. This period was fixed at 20 minutes for the plate and 30 minutes for the Pt-rod probes and was empirically deduced from the results in section 3.1.

2.3 Evaluation of Contaminants on the Liquid Surface Cleaning of the trough and barriers is an essential part of any film formation procedure although a monolayer can be successfully formed without it. Contaminants may severely alter the SP readings from the Wilhelmy plate sensor especially when small surface areas are inspected. As the subject of this research is formation of highly ordered nanostructures, minimising contaminants is a requirement as these may break continuous films by introducing large defects. The cleaning process involves washing of the trough and barriers under flowing DI water in combination with pure, non-denatured, EtOH. A detergent is used when necessary if adequate cleanliness of the trough cannot be achieved, typically once a week, even when the trough is not continuously used. An aspirator is used on a slightly over-filled trough to remove contaminants from the liquid surface and the refilling-aspiration sequence is repeated until a clean surface, as rigidly defined here, is measured. The process described here provides a way to investigate the cleanliness of a liquid surface. This is performed by reducing the trough area from its maximum to its minimum by full movement of the barriers while observing the SP of the liquid surface. When a contaminant is present on the surface it will cause the SP to rise as the area is reduced. As the contaminant cannot escape the bound region the rise is equivalent to increasing the concentration of a particulate in a solution except that in this case the solution is a 2D media. The result of such an experiment is shown in Figure 2.4 for a contaminated and clean surface. It is important to note that it can be initially misleading to think that the trough was clean if the barriers are not fully compressed. As the contaminations will only appear when the surface concentration is maximised (at approximately 10% of the original area) as seen by a sharp rise in SP at approximately 34cm2.

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Figure 2.4 – Cleanliness compression measurement, using a paper probe, showing a sharp increase

in SP as the area is reduced indicating a high contamination Acceptable values for a trough to be considered clean have been experimentally and rigidly defined as SP<0.2mN.m-1 and SP<0.7mN.m-1,when the surface area is fully compressed, for the paper plate and Pt-rod type probes respectively and are specific to the apparatus and the lab environment of this project. The values are in particularly sensitive to the submersion level of the probe. The values are applicable for all measurements performed here unless otherwise specified. The higher value for the Pt-rod probe is due to the increased sensitivity and fluctuations of the probe and does not represent a decrease in required cleanliness.

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2.4 The Brewster Angle Microscope It is useful to be able to visually investigate the dynamics on the liquid surface in real-time on a macroscopic scale during the Langmuir-Blodgett process. A simple BAM system provides a method to do so and was constructed for the purpose of this project. The BAM built for this project is a macro-inspection system rather than a microscope. The system incorporates a high-definition computer webcam positioned at approximately 53° to the trough (as water is used as the sub-phase at all times). A linear polariser is installed on the webcam rather than on the source due to impracticalities in blocking or polarising all other light sources (the trough is located below a large window). A light source, comprising of an LED array is employed and diffused to prevent saturating the light sensor on the webcam by strong narrow beams. The diffuser used was filter paper positioned approximately 4mm from the LED array. Positioning and mounting of the webcam is done with a custom made, 3D-printed, stand that allows positioning of the camera at different distances along the 53° vector from the liquid surface by means of positioning grooves. The distance adjustment is crucial due to the lack of lens system and provides a sufficient means to adjust the focal region. The system is shown in Figure 2.5.

Figure 2.5 – Custom built BAM system showing the light source and diffuser as well as the camera

used mounted along the 53° vector from the normal to the liquid surface by means of the 3D printed camera mount and positioning grooves. Polariser (not shown) is attached directly in front of the

camera lens.

LED Array

Diffuser

Camera Mount Positioning

Grooves

Camera

Polariser

Wilhelmy Rod Probe

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2.5 Forming a Langmuir Film

2.5.1 Consumable Substances The substances used include colloidal aqueous suspensions as well as solvents and DI water. The DI water used here for dilutions, cleaning and as the liquid sub-phase was purified in the laboratory to a resistivity of 15MΩ.cm using a PureLab Option ELGA DV 35. The solvents used included two categories namely, for cleaning and for suspension mixtures and are tabulated in Table 2.1.

Solvent Supplier Grade Purity Additives Used for EtOH Sigma-Aldrich Spectroscopic >99.8% - Mixtures

EtOH VWR Spectroscopic >99.9% - Mixtures

EtOH Sigma-Aldrich Anhydrous Reagent

90% 5% IPA 5% Methanol

Cleaning

IPA - Isopropanol

Table 2.1 – Solvents used during experiments. The reasoning for not using the spectroscopic grade EtOH for cleaning was due to practicality as the amounts used for cleaning were large (tens of millilitres compared to micro to a few millilitres for the mixtures). Both 125nm and 400nm diameter colloid PS spheres used were received suspended in DI water and sourced from ThermoFisher Scientific (Nanosphere™ product). The 125nm suspension contained 1.146% weight solids of which 1.033% PS spheres. The remaining solid material amounted to 0.113% weight and included a surfactant, variant of SDS (sodium dodecyl sulphate) and a preservative, EDTA (ethylenediaminetetraacetic acid). The sphere diameter was accurate to ±1nm according to a supplied NIST (National Institute of Standards) Traceable Mean Diameter certification (TMD). The 400nm suspension contained approximately 1% weight solids. The solid material was predominantly PS (as in the case of the 125nm spheres) and included an SDS variant and EDTA. The sphere diameter was accurate to ±9nm according to the supplied NIST TMD certification. The surfactant used is Triton® X-100 (supplied by AppliChem-Panreac) at a purity of >98.99% and came in a viscous liquid form. For the experiments performed, the pure solution was diluted 1:1000 with DI water.

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2.5.2 Addition of Colloidal Material Transfer of a colloid solution to the surface of a liquid is not a trivial matter and many experimental methods exist. Two methods for adding colloidal solutions to a liquid surface are described here and include a surface addition method as well as a bulk suspension method. In both cases, a Gastight®, 100μL, syringe is used with a 25-gauge needle to transport the colloidal mixtures.

2.5.2.1 Surface The surface method used to transfer the colloidal mixtures employed a flume that served as a method for gradually transporting the solution to the surface. The flume is formed from a conventional glass microscope slide that was thoroughly cleaned and placed in the sub-phase. The flume is placed so that only one of its four corners is outside of the liquid and rests on the trough side as shown in Figure 2.6. The flume presents a solid plane, almost parallel with the liquid surface, on to which solution drops can gently flow and subsequently gradually spread and interact with the liquid surface.

Figure 2.6 – Semi-submerged glass slide with one tip resting on edge of the trough. Dashed lines

highlight the submerged outlines of the slide and a red line highlights the glass-water-air interface.

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2.5.2.2 Bulk The bulk addition method is performed to investigate the effects of bulk suspended particles on the SP readings and the dynamics of the solvents and colloids in the bulk during the addition process. The addition is performed through a plastic cone held perpendicularly by the dipper and partially submerged in the liquid sub-phase. The submerged cone sectioned off a part of the liquid surface. The liquid mixture transported through a syringe needle placed through the cone such that the tip of the needle is approximately 25mm below the surface and facing away from the cone at an angle. The cone provided shielding from SP reading variations caused during the insertion and extraction of the needle. After the transfer is complete, the cone is withdrawn by the dipper at 1mm.min-1 and a compression of the surface is performed. The orientation of the cone needle with respect to the liquid surface is shown in Figure 2.7.

Figure 2.7 – Method used for applying liquid mixtures to the body of a liquid 25mm below the

surface without disturbing the SP readings.

2.5.3 Surface Relaxation To increase the experimental repeatability, the SP relaxation upon adding a solution to a surface was investigated. This was performed by depositing solutions (diluted 1:4 with EtOH) using the flume method (Section 2.5.2.1) in four measurement sets as listed in Table 3.1. Each set consisted of a repeated suspension of the same solution and a control suspension, which consisted of DI water as a substitute to the aqueous colloidal solution. The surface was allowed to relax for a period 60 minutes after the suspension transfer was complete. The SP was recorded from the beginning of the transfer process and until 60 minutes have elapsed.

2.5.4 Characteristic Curves (Isotherms) The characteristic curve (isotherm) in the context of this work refer to a plot of the SP as a function of the liquid surface area. The surface area is changed by moving the barriers without a mounted sample unless specified otherwise. The barrier compression is always performed in a symmetric manner such that the midpoint between the barriers is maintained at the centre of the trough at all times. Compressions are performed at a constant rate of 5mm.min-1 unless specified otherwise and the SP is recorded. A compression is performed after the surface is allowed to relax for a period of time (see section 2.5.3). The compression is stopped when the barriers activate an end-stop switch that defines the minimum surface area of 22.3cm2.

Air

Plastic cone

Liquid 25mm

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Consequentially, the linear region of the liquid phase is defined by the common range of SP values observed in a series of isotherm measurements (at least three). The OP may lay anywhere along the linear region, and locating it is the objective of measuring the isotherms.

2.6 Monolayer Transfer by Langmuir-Blodgett Method The Blodgett deposition/transfer method relies on an extraction/dipping process of a sample through a Langmuir film typically within the liquid phase range. The orientation of the substrate is typically perpendicular to the liquid surface. The substrate is thoroughly cleaned with Piranha solution[29] (Caro’s acid) cleaning and washed with DI water prior to mounting and submersion. Figure 2.2 depicts an immersed substrate prior to the addition of colloids. After the substrate is suspended and submerged, the colloidal solution is added to the sub-phase by the flume method (see section 2.5.2.1). The Langmuir film is then formed by symmetric compression of the surface layer by the barriers as in the case for attaining an isotherm (see section 2.5.4) except that the compression is controlled to maintain the SP at the desired value, the OP, until the entire process is complete and the sample is fully coated. A film is assumed to have formed when the OP is reached. After the OP is reached, the substrate is extracted from the liquid, through the formed film, at a constant, controlled rate of 1 or 0.5mm.min-1. A constant SP is maintained by the moving barriers, contracting or expanding the surface area according to the measured value returned from the continuous readings from the Wilhelmy probe. The barrier movement speed can be limited by a user-defined parameter to prevent the barrier oscillation amplitude from overshooting (due to spikes in the measured SP) and compressing beyond the liquid phase. An equation to calculate the minimum barrier movement rate based on the mounted sample dimensions and the extraction rate is shown in (2.1).

𝛥𝐵 = (2𝑤𝑠 + 2𝑡𝑠)𝑅𝑒

𝑤𝑏 (2.1)

In equation (2.1), Re is the sample extraction rate, wb is the barrier width and ws and ts are the mounted sample width and thickness respectively (assuming the sample is extracted perpendicular to the liquid surface). Here the barrier movement rate is set to 1mm.min-1, which is approximately 40% faster than the minimum value attained from the equation for a standard microscope slide extracted at 1mm.min-1.

2.7 Evaluation of Coating Quality The coating is qualitatively evaluated by three criteria: surface coverage, defects and order. The surface coverage is evaluated by visual inspection of a sample, either by the naked eye or by using an optical microscope. The evaluation is a qualitative way of grading samples for optimising the process. Defects are of two types; disruptive and non-disruptive faults caused by missing or extra particles or contaminants and were evaluated by AFM (Atomic force microscopy) imaging. The order is evaluated by AFM and the fast Fourier transform (FFT) of the captured image. When evaluating the order, the area of a continuous ordered domain and the order within it are considered. All AFM imaging was performed in air on a Bruker Icon AFM with OTESPA-R3 probes in soft-tapping mode to prevent altering the investigated layer.

2.8 Surface Dynamics The dynamics of solid material on the liquid surface were observed by visual inspection using the BAM system built here. The observations are done from the onset of, and during, the liquid phase on all measurements (isotherms and depositions). If the surface is uniform, no visual dynamics are observed, it is assumed that a perfect monolayer is present.

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3 Results For the purpose of familiarising and improving upon trials, a number of tests were performed to inspect the factors that may influence the quality of the final transferred film. These tests include inspection of the suspension process, the determination of the OP, the macroscopic observation of the surface dynamics of a Langmuir film and the implications of geometry and orientation of the sample substrate and the Wilhelmy plate or rod.

3.1 Experiment Preparation – Probes The SP as a function of time was measured from the time at which the probe was initially immersed. The result is plotted in Figure 3.1 for the plate and Pt-rod probes. It is noted that the SP determined using the paper plate probe decreases with time while the SP determined using the Pt-rod probe increases with time. Furthermore, the deviations observed for the Pt-rod are much more prominent than for the plate.

Figure 3.1 – SP value variation over time for the initial immersion of a Wilhelmy probe.

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3.2 Suspension Process After adding liquids, and mixtures, to the liquid surface, a variation of SP over time is observed. This variation is depicted in Figure 3.2 and has three regions of interest. A rising curve that corresponds to material being added to the surface. A peak surface pressure (PSP), that corresponds to the maximum SP observed and a following decreasing curve that is termed here as the ‘relaxation’. The relaxation curve has two regions, exponential followed by an almost linear curve.

Figure 3.2 – Complete relaxation plot of 125nm, 200μL, colloid mixture showing the material

addition, PSP and relaxation regions. To investigate the effects of the relaxation on the final SP, four sets of measurements of SP over time were made. Each set consists of three iterations of the same volume of diluted colloidal mixture and an additional reference mixture as described in Table 3.1.

Measurement set

Iterations Colloid Suspension

Volume (μL) Dilutant

Volume (μL)

1 1 DI Water 60

EtOH 240 3 PS 60

2 1 DI Water 90

EtOH 360 3 PS 90

3 1 DI Water 120

EtOH 480 3 PS 120

4 1 DI Water 200

EtOH 800 3 PS 200

Table 3.1 - Relaxation experiment measurement sets.

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All measurements were performed for a period of 60 minutes and the results are plotted below in Figure 3.3 for the first 30 minutes after the suspension process completed. During the last 30 minutes, no notable change is observed and each line decreases slowly by a seemingly linear manner. Graphs are offset in time to have their PSP aligned at time equals zero (material addition region omitted, see Figure 3.2). The large variations observed at approximately 2.5 minutes, on all plots to some degree, are due to tipping of the glass slide that forms the flume and vibrations caused when covering the apparatus. These disturbances do not alter the continuity of the plots.

Figure 3.3 – Relaxation plots for 60, 90, 120 and 200μL colloid solutions (diluted 1:4 EtOH). Dashed

lines represent a DI water substitute for each of the measurement sets. Numbers indicate mean PSP for each of the measurement sets.

From the relaxation data it is observed that the PSP varies between measurement sets as well as within each set. The average PSP within each set is shown in Figure 3.3 and was calculated to be 6.3, 8.6, 11 and 15mN.m-1 for the 60, 90, 120 and 200μL sets respectively. The reference mixtures are observed to be well below any of the colloidal mixtures with the highest PSP of 5mN.m-1 for the 200μL case.

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3.2.1 Bulk vs Surface Suspension To confirm that the PSP is sensitive only to the surface (rather than the bulk of the subphase), a measurement of a 90μL colloid solution (diluted 1:4 EtOH) added below the surface (bulk) was performed as described in section 2.5.2.2. A reference diluted DI water solution addition to the bulk was also performed. A conventional direct suspension of a colloidal mixture was made by applying the mixture drops directly onto the liquid surface from the syringe needle tip (without a mediator such as a flume). An addition of 450μL, undiluted water, was also performed (accounts for an equivalent volume of 90+4×90μL to the diluted measurements). The relaxation PSP for the four cases is shown in Table 3.2 and includes as a reference the lowest and average PSP recorded from all measurements during the entirety of the project for 90μL colloidal mixtures.

Method Substance PSP (mN.m-1)

Bulk 90μL Colloid mixture 2.97

Bulk 90μL Water mixture 0.98

Surface - Direct (no mediator) 90μL Colloid mixture 7.4

Surface - Flume 90μL Colloid mixture 8.4* (9.2+)

Surface - Flume 450μL Water (undiluted) 0.4

*Lowest and +average values observed during the entirety of project Table 3.2 – PSP values for bulk and surface addition methods as well as a minimum and average PSP

values for the case a of a flume method. Addition of only water is also shown.

3.3 Isotherms In the process of forming Langmuir films , the sought after phase is the liquid phase and in particular, the operating point within. By measurement of isotherms for different colloidal solution amounts, the characteristic transition points of the gas-liquid and liquid-solid phases can be obtained. This has been performed for the first three of the relaxation experiments listed in Table 3.1, excluding the three DI water experiments, and is plotted in Figure 3.4 (for comparison with the ideal curve see Figure 1.6).

Figure 3.4 – Isotherms for 125nm PS spheres diluted 1:4 EtOH showing three plots for each of the

solution mixtures; 60, 90 and 120μL.

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3.4 Isotherm Features and SP Dependent Variations During the process of performing measurements, features that vary from the conventional ideal isotherm (see Section 1.5) are observed and are shown below. The response of the barriers to fluctuations in SP is also observed. Figure 3.5 presents the gas-liquid transition region of a 90μL solution mixture isotherm where abrupt drops in surface pressure are observed, as the area is restricted. This is observed for more than 80% of the isotherms performed when the plate probe was used (subsequently identified as an artefact, see section 4.1) and never for the Pt-rod.

Figure 3.5 – SP fluctuations at the gas-liquid phase transition region of a 125nm diameter PS spheres

90μL colloid mixture isotherm.

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Fluctuations in SP are observed within the liquid phase during isotherm compressions and compressions to a target OP(when performing a Langmuir-Blodgett deposition process). In Figure 3.6, a 0.58mN.m-1 drop in SP is observed during a compression to target very close to the target SP (the OP). The variations around the target SP point correspond to 5 minutes in which the SP was maintained close to the target value by the moving barriers. It was observed that SP fluctuations within the liquid phase manifested in large regions (mm2 to cm2) of multilayers whereas fluctuations within the gas phase had no distinct impact but were associated with an artefact further examined in section 3.6.

Figure 3.6 – SP drop in the liquid phase prior to reaching the target SP of 37mN.m-1.

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Whilst performing a deposition using the Langmuir-Blodgett method, it was possible to designate a period between reaching the target SP and the onset of a coating (extraction of the sample). In that period, the SP is maintained at (about) the target SP by the barriers reacting to the measured SP as shown in Figure 3.6 where the target SP of 37mN.m-1 is maintained. The barrier position and SP over time for Figure 3.6 are shown in Figure 3.7. It is seen that the barriers are oscillating as a response to the SP readings from the Wilhelmy probe while maintaining the target SP within ±0.36mN.m-1. The approximate amplitude of the barrier oscillation is on average 50μm. The oscillation of the barriers appears to be superimposed on a saturating curve that depends on time. Prolonged testing periods showed that the graph continues to rise at a slow rate over several hours (more than 5 hours). The overall area reduction during the 5 minutes when the barriers maintain the target SP amounts to approximately 0.9cm2. The barrier position shown in Figure 3.7 corresponds to the position of one barrier and as the barriers move in a symmetric manner, the area reduction is calculated by introduction of a factor of two for the barrier position.

Figure 3.7 – Barrier oscillation in response to measured SP when maintaining a target SP

of 37mN.m-1 for 5 minutes.

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Two identical experiments were performed, in one of them the barriers maintained the SP and oscillated for 5 minutes and in the second case for 15 minutes before the monolayer was transferred to the substrate. Optical microscope as well as AFM images of the domain boundaries of the two samples are shown in Figure 3.8 and Figure 3.9 for the 5 and 15 minute cases respectively. The microscope images were taken using the AFM built-in microscope and are meant for qualitative evaluation purposes only and the scales presented are rough guidelines. The AFM images were performed on domain boundaries. The domain boundaries were uniform in their properties for the entire coating and the figures given represent the general case and not a unique feature of a particular region. In Figure 3.8A, large exposed regions between two large monolayer domains are observed. In contrast, in Figure 3.9A, three large monolayer domains are observed to be packed together (improved coverage) but their domain boundaries contain many defects especially above the monolayer plane.

Figure 3.8 – (A) AFM and (B) optical microscope images of domain boundaries for the case of 5

minutes of barrier oscillations at the target SP of 27mN.m-1

Figure 3.9 – (A) AFM and (B) optical microscope images of domain boundaries for the case of 15

minutes of barrier oscillations at the target SP of 27mN.m-1. Dashed lines in (A) highlight the approximate domain boundaries. Dark lines in (B) are multilayer regions at the domain boundaries.

100μm

B A

50μm

B A

Monolayer

Monolayer

Exposed Substrate

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3.5 Coating Anomalies When investigating coated samples, a few noticeable features appeared in the form of chains of colloids, several tens of spheres in width, as well as elliptical and circular voids within large ordered domains as shown below in AFM and microscope images (Figure 3.10 and Figure 3.11). The chains can be as long as several millimetres while the voids ranged from one to tens of micrometres in diameter. The voids are oval and, for most cases, symmetric and have an easily deduced centre point. For the case of the chain features, Figure 3.10, only an optical microscope image was obtained as capturing an acceptable quality image of a narrow chain was not possible within the allocated time.

Figure 3.10 – Optical microscope image of chains of PS colloids (125nm diameter) observed in

uncoated regions.

Figure 3.11 – (A) AFM and (B) optical microscope images of voids observed within large continuous

monolayers of PS colloids (125nm diameter). The diameters of the elliptical void in (A) is 5 and 6μm.

100μm

20μm

B A

Monolayer

Exposed Substrate

Exposed Substrate

Colloid Chain

Sample Extraction Direction

36 | P a g e

3.6 Surface Dynamics Visual inspection of the compression process yields clues towards formation of better layers and was performed for all processes through the BAM system. It was apparent that visual features are only noticeable when approaching the liquid phase (section 1.5) while the solid phase was very distinct and showed lines forming a grating-like structure parallel to the moving barriers as shown in the inset of Figure 3.12.

Figure 3.12 – Compressed solid layer depicting grating structures as a result of reaching a solid

phase. Inset showing magnified section. Upon further inspection of the formation of the liquid phase, it was apparent that regions that are observed to have localised grating structures appear on the surface at restricted regions. Such regions appeared between the Wilhelmy plate and the side of the trough during the extraction process of samples. This manifested in lateral movement of the plate as it was pushed away from its vertical position by a few millimetres, and rotated, as shown in Figure 3.13.

Figure 3.13 – (A) Wilhelmy plate pushed from original rest position following a deposition and (B) returned to original resting point following aspiration of colloidal material off the liquid surface.

Dashed line shows original resting position. The movement was commonly observed during the recording of isotherms as SP variations in the gas-liquid transition region when the Wilhelmy plate probe was used as shown in Figure 3.5. The grating structures discussed earlier are not clearly observed in Figure 3.13 as the movement of the plate occurred during the coating of the sample (removed prior to taking the picture) and was within the liquid phase. The material removed off the liquid surface, during a deposition, has a large area of approximately 33.84cm2, two standard microscope slides placed back to back, Figure 3.13A. Aspirating the solid material off the surface relieved the SP and allowed most of the liquid phase to relax to the lower energy gas phase, Figure 3.13B.

Wilhelmy Plate

Grating Lines

A B

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3.7 Geometry and the Effect on the Transfer Process Investigation of the influence of sample orientation was performed by mounting samples at 45° and 90° with respect to the generally recommended orientation (parallel to the barriers) and repeated twice as shown in Figure 3.14. Due to the width of the well in the trough, the samples consisted of standard glass slides cut in half along their lengths.

Figure 3.14 – Possible sample orientations with respect to the barriers.

The sample mounted at 45° showed the most continuous interface with the material around it when compared to the recommended and 90° samples. The assessment was evaluated by visual inspection through the BAM system only. The 90° sample promoted two distinct solid regions on the liquid surface at the two narrow edges (see grating structures in inset of Figure 3.12).

3.8 Peak Surface Pressure and the Colloid Solution Volume The relation between the PSP measured and the solution volume was calculated for all available data collected and plotted in Figure 3.15. This was done to evaluate if a trend exists between the PSPs and the colloid solution volumes.

Figure 3.15 – Pure colloidal solution (diluted 1:4 EtOH) as a function of PSP. Mean values per

solution volume are plotted as well as a corresponding exponential fit. The fitted equation and fit value are also shown.

y = 25.052e0.1401x

R² = 0.999

10

100

1000

5 7 9 11 13 15 17 19 21 23 25

Co

lloid

So

luti

on

Vo

lum

e (µ

L, lo

g sc

ale)

Peak Surface Pressure (mN.m-1)

125nm Diameter Spheres

400nm Diameter Spheres

Mean

Expon. (Mean)

Barrier

Recommended (Parallel) 90⁰ 45⁰

38 | P a g e

3.9 Quantifying Solution Volume and Area Coverage The solution volume for the 125nm spheres was experimentally determined by trial and error. This volume permits a full coating of a 60mm long sample such as a standard microscope slide (26mm wide and 1.1mm thick). The limit was decided by the physical limit of barrier movement that corresponds to maximum compression. When transitioning to the 400nm spheres, the necessary volume was calculated by modifying the exact calculation (described in section 7.1) by evaluation of the gas-liquid transition points recorded when experimenting with the 125nm spheres. The measured transition values are plotted as blue (125nm diameter PS spheres) and orange (400nm diameter PS spheres) circles in Figure 3.16.

Figure 3.16 – Calculated, empirical and modified colloidal solution volume as a function of the gas-

liquid transition region area coverage (for depositing monolayer films on a liquid surface). The solid lines in the figure correspond to the calculation, equations ( 7.10 ) and ( 7.11 ) for the 125nm and 400nm spheres respectively. The mean area, at the gas-liquid transition per solution volume, is plotted as black circles. A linear fit is made for the 125nm mean values, dashed blue line in figure. The fit represents the empirical solution volume to area relation and has a slope of 1.81μL.cm-2, a factor of 2.28 difference from the exact calculation. The 400nm solution line is scaled by the change in slope calculated between the two 125nm lines and is represented by a dashed orange line. The gas-liquid transition area, for a 200μL solution of 125nm spheres, is approximately 105cm2 and corresponds to 610.6μL of 400nm sphere solution. Subsequent gas-liquid transitions and the corresponding mean values are plotted for a 610μL solution, 400nm spheres. The measured values of gas-liquid transition areas showed a large deviation range varying from 13 to 55.4cm2 (200μL, 125nm spheres solution and 610μL, 400nm spheres solution respectively) making the calculation improper.

Calculated Measured

Mean Lin. Fit (Mean)

Calculated Adjusted Calc.

Measured

– 125nm diameter – 125nm diameter – Both – 125nm diameter – 400nm diameter – 400nm diameter – 400nm diameter

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3.10 Surfactants The result of addition of a surfactant (Triton X-100) on monolayer formation was examined. To gain an estimate for the quantities required, five relaxation measurements were made, as listed in Table 3.3, using Triton X-100 diluted with DI water (1:1000).

Measurement # Mixture Volume (μL) Increments (μL) PSP (mN.m-1)

1 13 1 10

2 30 10 15.6

3 45 5 20

4 100 10 25.5

5 1000 100* 40.2

*Poor volume control during transfer, number is approximate. Table 3.3 – Triton X-100 diluted mixtures (1:1000) volumes and corresponding measured PSP.

The relaxation plots for the measurements in Table 3.3 are shown below in Figure 3.17. It is noted that, when compared to relaxation plots of 125nm colloid mixtures (see Figure 3.2), the Triton X-100 has distinct addition steps and the SP per addition can be intuitively deduced. In contrast, the colloid relaxation plots, although addition steps can be observed, have more of a continuous increasing value (see Figure 3.2).

Figure 3.17 – Triton X-100 relaxation curves corresponding to measurements tabulated in Table 3.3.

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The average increase in SP for each aliquot of Triton X-100 was calculated for the first measurement as 0.77mN.m-1.μL-1. When observing the higher mixture volumes, the increase in SP per addition is observed to saturate for SPs above approximately 15mN.m-1 and the SP increase per microliter decreases. When performing isotherms for the five measurements, the liquid phase was never reached. The isotherms are presented in Figure 3.18.

Figure 3.18 – Triton X-100 isotherm curves corresponding to measurements tabulated in Table 3.3.

Coating attempts using 125nm PS colloid mixtures of 90μL added with 30μL of 1:1000 Triton X-100 produced no distinct change of the coated monolayers when inspected with AFM.

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3.11 Coatings From the results and insights gained during the project, coatings that were highly ordered and covered large areas were obtained. Shown here are representative results obtained for 125nm and 400nm spheres evaluated by AFM and by the FFT of the height sensor data.

A monolayer coating of 125nm PS spheres on a glass substrate is shown in Figure 3.19. The figure includes the corresponding FFT highlighting the highly ordered lattice. Very small defects are present in the form of missing lattice points and resulted in the slight deformation of the lattice around the defect.

Figure 3.19 – Monolayer coating of 125nm PS spheres showing an (left)AFM of a highly ordered

lattice and a corresponding (right)FFT of the height sensor data (colour scale is in nm). A macroscopic picture of the full microscope slide imaged in Figure 3.19 is shown below in Figure 3.20. The monolayer is clearly seen as blue regions whereas the exposed substrate is seen as bright, yellow-white, regions.

Figure 3.20 – Standard glass microscope slide coated with 125nm diameter PS spheres showing no macroscopically visible multilayers or large discontinuities. Yellow-white regions correspond to the

uncoated substrate while the blue regions are monolayers.

~60mm

~26

mm

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A monolayer coating of 400nm PS spheres on a glass substrate is shown in Figure 3.21. A corresponding FFT is also presented. Distinct linear domain boundaries can be seen as fractures along crystal domains and do not appear to significantly disturb the lattice. Missing lattice points (single spheres) are noted and closer examination is presented in Figure 3.23 and Figure 3.24. Single spheres appear above the monolayer surface but are few in number and do not disturb the surface.

Figure 3.21 – Monolayer coating of 400nm PS spheres showing an (left)AFM of a highly ordered

lattice and a corresponding (right)FFT of the height sensor data (colour scale is in nm). A macroscopic picture of the full microscope slide imaged in Figure 3.21 is shown below in Figure 3.22. The optical properties of the colloid layers are clearly seen as colourful patches. Monolayers are observed as green and blue whereas the multilayers are seen as pink-white and opaque regions. A curved region is observed at the position where the deposition began.

Figure 3.22 – Standard glass microscope slide coated with 400nm diameter PS spheres showing the

optical properties originating from the periodicity of the monolayer and multilayer structures as different colours. Blue/green regions correspond to monolayers while pink and white-opaque

regions correspond to multilayers. Curved regions at the beginning of the monolayer are also shown.

Blue/Green Monolayers

Pink/White-Opaque Multilayers

Curvature

43 | P a g e

A square, 25μm side, AFM image is shown in Figure 3.23 and a corresponding FFT. It is observed to be highly ordered, despite missing lattice points. These empty, 400nm, lattice points were observed to contain smaller particles as shown in Figure 3.24. It is noted that for all points examined, the smaller particles were symmetric hemispheres (cannot confirm the underside of the particle). The smaller particles were measured to be 296nm in diameter (line 2 in Figure 3.24) while the dimensions of the 400nm spheres were confirmed as 399nm (line 1 in Figure 3.24).

Figure 3.23 – Monolayer coating of 400nm PS spheres showing an (left)AFM of a highly ordered

lattice with defects and a corresponding (right)FFT of the height sensor data (colour scale is in nm). (left) White square on AFM image indicate a magnified region shown in Figure 3.24 while the inset

shows the magnified particle indicated by the white arrow.

Figure 3.24 – AFM image of a coating defect for the square region indicated on Figure 3.23 for a

400nm PS sphere monolayer coating. Sphere diameters are 1: 399nm and 2: 296nm. Inverted colours for clarity only.

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3.12 Using the Coatings A demonstration of one application of the fabricated colloid masks shown previously, section 3.11, is presented here using the example of etching pillars into a quartz sample coated with 125nm PS colloids. SEM imaging as well as reactive ion etching (RIE) and oxygen plasma processes curtesy of A. Taylor. The process of generating the pillars is shown in Figure 3.25 for different etching periods. The unmodified colloids on the quartz substrate can be seen in (A). In (B), the partially etched surface (1 minute etch) can be seen with unetched regions (yellow arrow) below the spheres (black arrow). The spheres are observed to react to the etching (as seen by the surface roughness) but not as much as the substrate and still maintain their spherical shape. In (C), the etching was prolonged for 2 minutes and approximately 100nm tall pillars are produced. Again, the round shape of the sphere was maintained, although is now hemispherical and the surface is even rougher then in (B). The final step involved the complete removal of the PS spheres by using oxygen plasma. (D) Shows 500nm tall quartz pillars, etched for 15minutes and the PS spheres were removed. The densely packed hexagonal patterns observed for the pillars in (D) represent the small ordered domains seen surrounding the void in Figure 3.11.

Figure 3.25 – SEM images of 125nm diameter PS spheres on quartz as pillar structures are etched through the colloid mask. (A)Spheres on quartz, unprocessed. Etched for (B)1 minute (C)2 minutes

and (D)15 minutes. Black and yellow arrows in (B) and (C) indicate the PS and quartz respectively. In (D), the PS spheres are removed at the end of the etching process using an oxygen plasma process.

C 1µm

D

400nm

A

B

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The large area coverage is demonstrated in Figure 3.26A for 5 minute etched pillars for a 2025μm2 area. In B, a dense packing pattern for a 3 minute etch is shown. In C, a close-up of some 470nm tall pillars is shown where a tapered shape is observed; narrower at the top and wider at the base of the pillar.

Figure 3.26 – SEM images of pillar structures etched in quartz through a 125nm diameter PS spheres

colloidal mask. (A) Large area coverage for a 5 minute etch. (B) Dense packing for a 3 minute etch. (C) Enlarged section showing 470nm tall pillars with 125 and 160nm top and base diameters

respectively.

400nm

125nm

160nm

A B

C

470nm

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4 Discussion

4.1 Choice of Probe and Surface Dynamics During the characterisation process of the Langmuir-Blodgett method, it was apparent that the initial choice of Wilhelmy probe, the paper plate, was problematic and was not viable when performing depositions as it gave inconsistent readings. An investigation into the issue resulted in the observation of physical drift of the Wilhelmy plate as seen in Figure 3.13. The drift appeared as the plate was pushed away from the centre of the trough and away from the trough edge (towards the mounted sample and to the left). An explanation may lay in assuming a macro surface dynamics picture as shown in Figure 4.1. In the figure, black arrows indicate the assumed (and observed) flux of solid material on the surface. As shown, the only material escape point is where material is deposited on the sample and can be treated as a SP relieving region (low SP) when a coating is performed. This results in solid material accumulating between the paper plate and the side of the trough. On the other side of the plate, material is depleted as it coats the sample. This was observed through the BAM as a local solid phase (grating structures, see inset in Figure 3.12) between the plate and the probe while a liquid, or gas, phase was present on the other side of the plate This could be thought of as generation of local SP regions and a resulting SP gradient along the -y direction. The outcome is an imbalance in the SP sensed by the probe, which macroscopically displaces (yellow arrow) the probe from the ideal position and skews the results.

Figure 4.1 – Surface dynamics depicted by black arrows indicating flux of solid material on the liquid

surface and transfer to a sample. Yellow arrow indicates plate movement direction.

x

y

x

z

Sample extraction

Sample

Wilhelmy plate Well

Barriers

Submerged portion of sample

47 | P a g e

The movement is most prominent when the liquid phase is approached (observed through the BAM when solid material is visible on the surface). It is therefore assumed that the plate is initially pushed out of its original position and is then locked in place. This was strengthened by a simple observation of the plate when in the solid phase. By applying a small amount of force at the plate hooking point, it was observed that the plate is fixed at the sub-phase surface and had a hinge-like effect as shown in Figure 4.2. The hook can be displaced by a few millimetres, in the solid phase, before the solid material on the surface gives way and the observed fixed point is freed, as in the gas phase.

Figure 4.2 – Wilhelmy plate when static and pushed (blue arrow) in the gas and solid phases showing

the observed surface fixed point of the plate in the solid phase only. To avoid the problems caused by the paper probe, a different, less common Wilhelmy probe was sought in the form of a platinum rod. This probe works in the same principle as the plate but has a smaller interfacial circumference making it less prone to generate isolated regions around it and is recommended[30] when small surface areas are to be examined although a quantified value for ‘small’ was not found. The probe also possess a significantly larger mass (compared to the wet paper plate) and eliminates the hinging effect. The shortcoming of using such a probe, and is the case for any probe of a smaller circumference, is that it becomes much more sensitive to SP variations and resulted in increasing the cleanliness threshold as described in section 2.3. The Pt-rod probe resolved all issues regarding probe drift (Figure 3.13) and SP variations at the gas-liquid transition region (Figure 3.5) are not observed with the Pt-rod and are therefore solely attributed to the plate drift and the generation of regions with local different SP due to the plate geometry. In the experiment that the SP value of the pure water addition was measured (section 3.2.1), no relaxation curve is observed and the SP simply rose and maintained the value of 0.4mN.m-1. It is therefore imperative that the submersion of the probe remains consistent throughout the experimentation, as an increase in the water level will superimpose a constant SP value for all subsequent measurements.

Static Solid Phase Gas Phase

Wilhelmy Plate

Force Meter Hook

Fixed Point

Observed Fixed Point

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4.2 Interfaces, Anomalies and Orientation The reliability of the Wilhelmy probes is heavily dependent on the quality of the meniscus formed. As the meniscus may extend along the liquid surface plane up to a few millimetres away from the plate-liquid intersection, any solid interfaces in close proximity can result in large errors in the readings. These surfaces may be the trough edge, barriers or even a mounted substrate. Following from the findings in section 4.1, an in-depth observation of the coating process was made to investigate anomalies in the coating uniformity. The Langmuir-Blodgett process did not consistently, and reproducibly, fully coat a sample and resulted in regions of different coating properties along the length of a microscope slide. The regions varied from completely uncoated to being coated by thin lines that, when investigated, were long chains of spheres along the coating direction with chain widths ranging from single to several tens of spheres. These features are illustrated in Figure 4.3 along the length of a coated microscope slide. A further anomaly showed oval uncoated regions, voids, in the midst of large coated regions and is illustrated as well.

Figure 4.3 – Illustration of multiple coating observations on a microscope slide showing a large gap in

the coating as well as chains along coating direction and voids within a coated layer. Macroscopic inspection of the reflections off a coated slide can be used to locate and recognize some features such as monolayers, multilayers and the chain structures as shown in Figure 4.4 by their reflective appearnce.

Figure 4.4 – Picture of a microscope slide coated by 125nm diameter PS spheres showing monolayer domains, multilayers and chain structure regions. The back of the slide was painted black to improve

the reflection contrast (prevent reflections from the back of the slide). White-yellow and blue regions correspond to uncoated and coated parts of the substrate respectively. Speckled white-

yellow regions between the blue regions are breaks in the monolayer. Curved regions at the beginning of the coating are also shown.

Intended coating Region

Coated

Sample Extraction Direction

Chains Voids

76mm

Samp

le Extraction

D

irection

Monolayer Domains

Exposed Substrate

Multilayers

Chains Curvature

49 | P a g e

It is assumed that the chains are due to SP variations during the coating process. This occurs as the barriers do not respond quickly enough to variations in SP or that local phase regions appeared in proximity to the probe. The existence of these local regions could not be verified to be the cause but were observed as discussed in section 4.1. The chain structure is assumed to be the by-product of the cohesive forces acting between the spheres, which appear to preferentially stick to each other rather than randomly, or evenly, disperse on the substrate surface. In fact, it was rarely observed that single spheres appear on a surface at long distances from other spheres. With the added observation that the spheres may form the chain structure, one can therefore deduce that the cohesive forces between the spheres are greater than the adhesive forces between the spheres and the substrate. The chains appeared to be an indication of insufficient compression due to either too rapid sample withdrawal rate or an insufficient barrier movement rate. On the other hand, a false reading from the probe may be the result of local variations in colloid concentration on the surface, which may be the culprit. The voids that appeared presented very symmetrical geometries and relatively clean borders. That is to say, barely any single spheres, or small domains, strayed within the area of the void (see section 3.5). The voids did not severely disturb the continuous monolayers. Several hypotheses were made based on the symmetry and appearance of the void structure. It is difficult to provide a conclusive reasoning to the origin of the voids. We can consider two types of impurities; surfactants on the substrate surface or air bubbles. Air bubbles within the sub-phase that are bound to the substrate may generate an additional/modified liquid-air interface and cause the anomalous shape. Otherwise, contamination is likely to be the cause and may be due to a surfactant contamination of the liquid or an adsorbent that was not properly removed from the substrate prior to mounting in the trough. No further investigation was made and the voids are not deemed too destructive, when compared to the chain structures. These structures can be beneficial and desired if such geometries can be readily reproduced. To manage the interfering features, chains and uncoated regions, a visual inspection of the process through the BAM was performed for the entirety of a coating process (100 minutes). It was observed that the meniscus formed on the substrate presented not only a transfer method for the Langmuir film but also a barrier. Figure 4.5 illustrates this and shows that the film remains at the bottom of the concave meniscus unless the SP (SP2 in Figure 4.5) behind it is sufficiently high that the colloids are pushed up the meniscus and towards the substrate, as in the case of the good coating and SP1. It was further observed that sharp edges, such as at the corners of samples, were more frequently devoid of solid material on the meniscus forming on them. The bare regions can be distinctly observed by the BAM and also on the coated samples by rounded-off, curved, edges at the top of a coating as shown in Figure 3.22 and Figure 4.4 and so do not appear to be dependent on the sphere diameter.

50 | P a g e

Figure 4.5 – Illustration of sample coating both good and bad simultaneously due to insufficient and

local SP regions. Inspection of the recorded SP values during the coating process showed that the barriers responded accordingly to the SP readings and so it is assumed that the main culprit in the creation of improper coatings is the local SP values. The findings reported in Section 3.6 explained the inconsistent SP measurements observed especially when coating a sample but only treated the probe in use, paper plate, as the culprit. Changing to the Pt-rod type seemed to resolve most of the issues for small samples (apart from local SP regions that were occasionally observed). Investigation of the flow of solid material around the sample (such as the solid material flux theory presented in section 4.1) resulted in testing the sample orientation to improve the material flow around the sample. The testing was limited in scope and only a general approach attained and showed to improve the uniformity of the SP across the trough when the sample is mounted at any angle θ for values between 90°>θ>0° from the recommended sample mounting (see Figure 3.14). The angle was limited by the well width and practically only allowed for a maximum angle of 38° (from the parallel to the barriers) when standard microscope slides were mounted. All successful samples that showed uniform coatings (not necessarily without breaks) were mounted at 38° (see Figure 3.14).

4.3 Surface Relaxation The multiple sequential steps involved in the Langmuir-Blodgett process require a better understanding of the periods involved for the separate processes and their effect on the fabricated (coated) sample. The relaxation plots, as termed here and presented in section 3.2, demonstrate a method to characterise the process of adding mixtures to a surface. In the literature, such descriptions rarely appear in the context of Langmuir-Blodgett processes and are more widely discussed when a liquid surface pressure/tension is examined such as in the case of evaluating the oil-air SP equilibrium period based on the measurement of the surface tension[28] and present a similar relaxation profile to the one shown in Figure 3.2. The relaxation plots observed always contained a rising edge, corresponding to material addition to the surface. The slope of which depended heavily on the user and addition method. An observation of the material addition region presented a useful way for self-evaluation when performing experiments and during fabrication. It allowed refinement and improved consistency of the transfer method as it was primarily skill dependent. Observations of clear distinct steps when a surfactant (Triton X-100) was applied to the surface hints at a property that the colloid solution lack, which is preferential relocation to interfaces. As the surfactant is an amphiphilic molecule, it readily segregates to the liquid-air interface, despite having a higher density than water, and as such, it is

Good Coating

Sample Extraction Direction

Bad Coating

No Colloids On The Concave Meniscus

Air

Liquid

SP1 SP2

51 | P a g e

assumed that the majority of the molecules appear on the surface shortly after addition. This generates clear, sharp, steps in SP compared to noisy and sharp steps as seen in Figure 3.2. The idea of mixing surfactants and colloids to modify their interaction is not new[24], [25], [31]. It is a common practice in the chemical and colloid production industry to prevent aggregation of the solids in aqueous solutions. Such was the case for the source of colloids used here (see section 2.5.1) that contained small amounts of an SDS variant surfactant as well as EDTA (a chelating agent). It is assumed that EDTA is added to prevent metals from precipitating[32] but its exact effect on this process is unknown. The relaxation time, the period of time from the moment the material addition is complete until the surface is at a near-stable state (for compression in particular), has been observed to be approximately 10 minutes. The relaxation has two regions; it begins with a sharp exponential-like decrease followed by an essentially asymptotically decreasing slope. Performing compressions within the exponentially decreasing region produced inconsistent isotherms and coatings. While 10 minutes appears sufficient to avoid the exponential region, in practice, 30 minutes is the period used when producing monolayers, as a precautionary measure, and no degradation was observed when extending this period up to an hour. A trial that extended over more than 12 hours showed that the linear-slope region did not vary until a drastic deformation of the liquid surface occurred, for example, when the sub-phase evaporated and did not completely fill the trough a sharp increase in SP is seen. The linearly decreasing region in the relaxation curves (see Figure 3.2) is attributed to evaporation. This assumption is based on the 12 hours test but no further investigation was performed.

4.4 Suspension Methods and Peak Surface Pressure The process of transferring colloidal mixtures to the liquid surface is paramount to the success of a coating process. It is the first point where the process may fail in its entirety for multiple reasons. The primary one being insufficient material segregation to the surface. The common method reported in the literature (if at all noted in the publication) for transferring colloidal mixtures to the liquid surface is by applying a colloidal solution directly to the liquid surface as a drop, withdrawn from a tip of a syringe needle. This method was not used here as it produces widely inconsistent results and is heavily dependent on skill. The flume method was found to produce the most consistent results when transferring the colloidal mixtures. Although, skill remains a major factor, it appears to be less dominant but no in-depth comparison was made. An investigation into methods to transfer colloids can form an entire project by its own right. When using the flume method, if the colloid mixture contains any volatile solvents, the water sub-phase covering most of the flume will partially retreat exposing a region of the slide that is slightly below the liquid surface. A successful transfer is observed by the naked eye as a strong disturbance (rippling) to the liquid surface for a short period of time. This, however, cannot trivially and quantitatively, define the success of the process and only presents a means of verifying that some surface suspension, vs bulk suspension, was achieved. In considering the small increase in PSP for the three DI water control measurements (Figure 3.3), it is apparent that the main contributor to the SP is the solid material (the colloids). Thus, verifying that the increased PSP is primarily due to the solid addition (colloids) to the surface and not due to the diluting solvent or any other parameter. The PSP appears to be a property of the particular mixture. As such, the PSP was successfully used throughout the project as a measure of evaluation of successful mixture transfer to a liquid surface. The PSP variation measured for all mixture group (containing a given volume of colloid added) throughout the project only varied by a maximum of ±1mN.m-1. It is therefore assumed that the higher PSP corresponds to more solid material

52 | P a g e

segregated. Thus, the PSP was reliably used as a unique measure of a successful mixture transfer process. The results attained in section 3.2.1 present some confirmation of the benefits of using the flume method and combined with experience presents a recommendation towards future work. As discussed above, the PSP is a reliable measure of the colloid material transfer to the surface. From the results (section 3.2.1), it is apparent that the PSP of 7.4mN.m-1 for the conventional direct transfer methodology is inferior to the flume method and presents a decrease of 1mN.m-1 from the lowest SP value recorded for the same mixture during the entire project. The process of making addition of mixtures to the bulk of a sub-phase resulted in surprising visual phenomena where the suspension of the colloidal bulk addition visibly rose to the surface (dispersive white ‘cloud’ of colloids rapidly rising to the surface) and generated a distinctive rippling that is associated with material segregating to the liquid-air interface. The recorded PSP value of 2.97mN.m-1 represents a significant segregation to the surface and an isotherm could be performed to verify this. On compression, the maximum SP reached was 23.4mN.m-1, indicating that the liquid phase was reached and colloids were definitely present on the surface. In contrast the reference bulk addition (water and EtOH mixture), showed a low PSP (0.98mN.m-1) and can be attributed primarily to the increased submersion of the probe. Prior to performing the measurement on addition of colloids to the bulk of the sub-phase, it was assumed that no significant amount of colloids would reach the surface. The PSP values recorded in section 3.2.1 demonstrated that material did reach the surface, and is detected by the PSP. The experiments support the notion of adding a low-density solvent to the mixture, as it appears to promote the colloids towards the sub-phase surface and may even reincorporate bulk suspended colloids back onto the surface.

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4.5 Compression Curves and the Operating Point When first approaching the project and the field of Langmuir-Blodgett coatings, isotherms appeared as a reliable characterisation method. However, treatment of isotherms as characteristic curves is problematic. As the self-organizing properties of the colloids is hampered when the liquid phase is approached (due to restriction of movement), each compression will be unique and depend on prior stages namely, the mixture transfer, the compression parameters and the colloid-colloid interactions in the gas phase. This is observed in Figure 3.4 where no isotherm curve can be observed to reproduce another curve beyond the gas phase. When comparing the 90μL mixture set (which included the first trials) to the two other sets that, although still vary, are much more similar. It was apparent that skill and practice in performing the transfer as well as the ambient conditions might have a significant effect on repeatability. Thus, the isotherms exact shape varies from the ideal case in Figure 1.6. It is noted however that the temperature conditions were not maintained as preferably required (by the definition of an isotherm, same/equal temperature). This was due to the laboratory ambient conditions that prevented any kind of temperature control and resulted in temperature ranges of approximately 23° to 27°C between and during measurements. As the colloids interact and spontaneously form monolayers in the gas phase, nucleation occurs due to cohesive forces. When compressing the gas phase into the liquid phase, the same surface states will never be the same for any two processes. This could in prospect be prevented by introducing a ‘lubricant’ or spacer of sort that will hamper the self-organizing properties until the particles are forced together and will be further discussed in section 4.8. All that was noted so far amounts to the realisation that exact analysis of transition points will not be a reliable measure for all cases in the scope of this work. Therefore, no distinct conclusion can and should be made regarding the transition points as thermodynamic equilibrium is not reached and although the terms gas, liquid and solid are used, the transitions between them are not uniquely defined. Although the transition are not specifically defined, by measuring isotherms the OP can be empirically determined and good monolayers produced (see section 3.11). It is found that the most reliable operating point lies as close to the onset of the linear region (from the gas phase) as possible while maintaining sufficient SP clearance from the onset corresponding to the maximum SP swing observed during the oscillating barriers period (see Figure 3.7, where an oscillation amplitude swing of ~0.4mN.m-1 is observed).

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4.6 Surface Coverage vs Minimising Domain-Boundary Defects During the experimentation and fabrication of many samples, it was apparent that a non-quantifiable relation exists between the coverage and the highly ordered domain boundaries. This was especially apparent when trying to improve coverage, as reported in section 3.4, by extending the period between reaching the OP and initializing the coating (sample extraction) process. During that period, the barriers oscillate as a response to the SP readings from the probe in order to maintain the target SP. The amplitude of the barrier oscillation can be damped by adjusting the maximum barrier movement parameters. The period of the oscillations can be indirectly adjusted by varying the probe-sampling frequency. These variations were not attempted for lack of time. The surface coverage observed for the 15 minute oscillation case (Figure 3.9) was significantly better than for the 5 minute oscillation case (Figure 3.8). It was evaluated qualitatively that nearly no exposed regions of the substrate were visible for the 15 minute case. However, although the domains for the 15 minute case were large and devoid of large gaps, the domain boundaries contained many defects in the form of multilayers along the domain boundaries (dashed white lines in Figure 3.9A). It is therefore recommended that the barrier oscillation period be adjusted based on the required result; small, highly ordered and boundary defect-free domains or large domains with many defects at the domain boundaries. Although, for the latter case, a highly ordered lattice still exists but the continuity due to the many fragmented domains will imply a non-continuous crystal plane orientation.

4.7 Analysing Compression Curves Observing the compression curves permits evaluation of expected defects such as multilayers on a coated sample. When observing, for instance, the SP drop seen in Figure 3.6, it can be expected that a multilayer exists on the Langmuir film and may be present when transferred to the sample. In fact, SP drops along any point that vary from the continuous plot will indicate that a multilayer has formed at a local region on the film and can be observed through the BAM as local grating structures on the film. In Figure 3.11(A), the lattice surrounding the void is formed from many small domains consisting of approximately 16 spheres each. The small domains appear to be very ordered within themselves, do not align with their surrounding domains, but still present good surface coverage. This was observed in a few other samples where the OP was too low and increasing it resulted in large continuous domains such as the one shown in Figure 3.19 and Figure 3.21. These features are not treated as failed coatings but rather as a different kind of colloid structure that may be useful for some applications.

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4.8 Surfactants and the Langmuir-Blodgett Process Surfactants are commonly used to modify and improve the wetting of substrate surfaces[12], [31], to enhance the self-organization properties[24], [25] and also as a means of modifying the crystal lattice by modifying the surface of the colloids with an amphiphilic surfactant[33]. A brief investigation was performed into the possibility of using a surfactant to improve the monolayer order and formation process (see section 3.10). The surfactant Triton X-100 was chosen based on the frequent mention in the literature where it is used to produce highly ordered films. When investigating these reports however, never once was the rationale for choosing Triton X-100 given. In most cases, the surfactant was not used in a trough but rather in a petri dish (lack of liquid-surface compression method such as the moving barriers) and the quantities used are not always specified. Another surfactant frequently reported to produce large monolayer was SDS[31] and was not tried, due to time constraints. Using SDS would be an interesting choice as the colloid suspensions used here (see section 2.5.1) contained small amounts of it to prevent aggregation. In hindsight, SDS would have been a better first trial substance rather than Triton X-100. The results (section 3.10) showed no benefit of using Triton X-100 as an additive. However, it did demonstrate the high surface selectivity (by observing the distinct step additions in the relaxation plots, Figure 3.17) of the surfactant compared to the colloid mixtures (Figure 3.2). Based on the results and literature findings[18], [34], it is suggested that modifying the surface of the PS colloids with amphiphilic molecules, such as Triton X-100, may improve the surface segregation. It may also produce open-packed hexagonal structures (Figure 4.6A and B) where the order is maintained but the spacing is dependent on the length of the surfactants hydrophobic tail (~Δ/2 in Figure 4.6B) rather than on the colloid diameter. This modification may also prevent nucleation at the gas phase, to some degree, and may act as a lubricant of sorts even when compressed.

Figure 4.6 – Proposed surfactant modification of colloids to produce open-packed hexagonal

structures. (A)The modified hexagonal packing and (B)enlarged section showing the radius (r), amphiphilic tail spacing (Δ) and surfactant organisation. (C)The functionalised colloids organise

above the sub-phase liquid.

4.9 Mixture Transfer The flume method (see section 2.5.2.1) used for transferring the colloids proved successful for forming films. It was not extensively compared to other methods but was nonetheless shown to have equal, or better, surface segregation than the commonly used method of adding drops directly to the surface. These findings are based on the PSP readings that were found to be a reliable parameter for detecting the colloid segregation to the surface. The PSP can therefore be used to monitor the addition progress when adding mixtures to a liquid surface. Monitoring the PSP while adding a colloidal mixture can be halted when a target PSP is reached, thus eliminating the need for accurate mixture volume measurements and may allow for a more reproducible methodology when

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fabricating Langmuir-Blodgett films. This may also support future large-scale integration of the Langmuir-Blodgett process in conventional nanofabrication facilities, perhaps one more, small, step towards reproducibility and consistency in this field. New addition methods can be envisioned to utilize the PSP parameter such as spray deposition. This method might have a small spraying nozzle through which the diluted colloid mixture can be deposited a few millimetres above the sub-phase surface and halted when the target PSP is reached. This kind of method cannot be reliably used without the PSP parameter as there will be many losses as not all of the sprayed material will necessarily reach the surface making volume measurements pre-deposition mostly irrelevant.

4.10 Coatings The purpose of the project was to form highly ordered films. This target was met and the very successful results are shown in section 3.11. The results show highly ordered colloidal lattices as seen by AFM and FFTs for both 125nm and 400nm diameter PS colloids. Furthermore, the macroscopic surface coverage (see Figure 3.20, Figure 3.22 and Figure 4.4) and the demonstrated patterns that result from them (see Figure 3.25 and Figure 3.26) are comparable to, and may even surpass, those found in the literature (especially to the highly successful spin coating method[7]). The etched structures (pillars) produced, and shown in Figure 3.25 and Figure 3.26, verify the application of such masks as a method to produce nanoscale engineered structures and is regarded as a confirmation of the success of the project. The mostly continuous, 125nm diameter PS sphere, coating shown in Figure 3.20 is a monolayer with minimal defects at all points investigated over an area of 100μm2, containing about 7000 spheres. This area was randomly picked after inspecting approximately 20 regions across the length and width of the slide. Investigation of the 400nm coatings showed that a contamination was present in the nanospheres supplied, as seen by smaller ~296nm (Figure 3.24) and ~130nm (Figure 3.23, inset) diameter spheres within the main 400nm lattice. Observation of the full lattice showed that the small spheres did not disrupt the order but rather only occupied empty points as seen in Figure 3.21 and Figure 3.23. The origin of the empty lattice points is assumed to be due to the small spheres, but may have occurred for a different reason. The origin of the contaminations of the smaller, 296nm diameter particles (Figure 3.24) is unknown as these appear to be very symmetric and similar to the rest of the PS spheres but such size spheres were not used or present in the laboratory. It is postulated that small amounts of a volatile solvent that is incompatible with PS, such as acetone, leaked into the pure source and shrunk[15] a small portion of the spheres. For the case of the small, 130nm particles (Figure 3.23, inset) it is assumed these were 125nm spheres from previous trials that were not completely removed (cleaned).

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4.11 An Attempt at Quantifying the Process The quantities of colloidal solution volume used here are 200μL and 610μL for the 125nm and 400nm diameter PS spheres respectively. The values of the colloid volumes and the dilution used here were empirically deduced although a calculated methodology was attempted, as described in section 3.9. The resulting spreads of the measured data are too large to qualify for any exact analysis, especially for the 400nm diameter sphere case (610μL solution) where the measured values spread across a range of 42.4cm2 (out of the full 210.1cm2 range possible by the barrier movement). Many other parameters recorded during the project were plotted, evaluated and deemed inconsistent except for the case of the PSP values as a function of the colloidal solution volume as shown in Figure 3.15 where an exponential fit was made for the mean of the solution volumes for both 125 and 400nm diameter PS spheres. The fit appears to be very good but it is acknowledged that the data set is rather small and that the PSP variations per solution volume are large (0.9 and 2.6mN.m-1 for smallest and largest spreads respectively). Nonetheless, the consistency at which these values can be reproduced within these ranges, when compared to all other parameters examined, may provide a plausible direction to continue investigating in a field where reproducible results are hardly found. My first improvement to the reproducibility would include a pump to automate the mixture addition with the flume method based on monitoring the SP (to a target PSP).

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5 Extended Work The successful coatings fabricated in previous sections allowed fabricating samples as part of two collaborations. The fabricated samples are for two different purposes, one will be used as an etching barrier for a RIE process while the second will be used as a lift-off mask for a metal deposition. The films appear to be useable but the final conclusion will be based on work done elsewhere.

5.1 Photocatalytic Self-Cleaning Surfaces 5.1.1 Background Self-cleaning surfaces are a major subject of study and most prominently for healthcare applications such as self-cleaning surfaces and equipment[35]–[37] for hospitals and laboratories where harmful organic pollutants may exist and pose harm to humans. Such surfaces may be catalysed by natural light and in that case, fall under the category of photocatalytic surfaces.

Investigation of the TiO2 rutile-anatase electron transport is an example of a photocatalytic interface that, if formed into a large surface area, may form a self-cleaning surface. The photocatalytic process involves electron transfer between the two crystal structures and results in an oxidising surface that may efficiently kill harmful organisms in its vicinity as previously demonstrated[38]. A method to produce the required large interfacial surface area is by etching through a layered rutile-anatase structure. Etching dense patterns to expose a large surface area of the active interface can be done by means of colloidal lithography. By coating a rutile-anatase layered structure with a PS colloid mask, a RIE etching process can be performed to create densely packed pillars as is illustrated in Figure 5.1.

Figure 5.1 – Rutile-Anatase layered structure (anatase on the surface) coated with a colloid

monolayer and etched using RIE to produce a large surface area pillar structure.

Quartz Quartz

PS

Anatase

Rutile

RIE

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5.1.2 Results and Discussion A colloidal lithography mask was fabricated (125nm diameter PS colloids) on a quartz slide coated with a rutile-anatse layered structure as shown in Figure 5.1. AFM images and FFTs of the coated and uncoated mask are shown in Figure 5.2. The anatase surface (Figure 5.2 top-left) is observed as rough, compared to the uncoated substrate region in Figure 3.8A, and lacks periodicity as seen by the corresponding FFT (Figure 5.2 top-right). In comparison, the monolayer of the coated sample (Figure 5.2 bottom-left) is shown to follow the surface topography while still maintaining a long-range periodic structure as observed by the corresponding FFT (Figure 5.2 bottom-right). The periodicity of the coated sample is blurred by the topography variation and not a result of disorder in the mask.

Figure 5.2 – (Top left) AFM of a clean anatase TiO2 surface. (Bottom left) AFM of a monolayer coating of 125nm PS spheres on an anatase TiO2 surface (same sample as in top left but different region). To

the right of the two AFM images a corresponding FFT of the height sensor data (colour scale is in nm) is shown.

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The coated samples were then etched using RIE and the PS spheres stripped using a Piranha cleaning process[39] to produce a dense, hexagonally packed, pillar structure similar to what is shown in Figure 3.25D. The structure is described as ‘similar’ due to the surface variation between the flat quartz and the rough anatase surfaces. The etched samples submitted for photocatalytic analysis. At the time of writing, no photocatalytic results have been returned on the success of the process although it is expected that further samples will need to be supplied to optimise the process, in particularly the etching depth.

5.2 Water Filtration Membrane 5.2.1 Background Liquid, in particularly water, filtration is a widely sought after and developed for numerous reasons. Be it for health purposes or filtration for industrial processes, a high quality water filter will usually be desired. The quality of liquid filters is in their ability to reliably separate particles from the main volume and typically appear as a membrane between two liquid compartments. Highly selective filters exist in nature in the form of cross-membrane channels in biological cells such as the Aquaporin membrane channel[40], [41] that exists in plants and even in humans[42] and selectively regulates water molecule transfer. Fabricating biomimetic membranes based on the aquaporin principle may prove desirable. Silica fibres grown inside a porous alumina membrane show promise in mimicking the aquaporin membrane filter and may be tuneable for a variety of particle sizes. However, incomplete filling of the asymmetric alumina pores results in an unacceptable leakage[43]. Formation of a porous metallic film with precisely defined pores will permit growing the silica fibres at higher accuracy. Thus, the sought after filter structure may be fabricated and is the subject of the collaboration described here. It is emphasised that the alumina pores are too large to perform the required filtering and only act as the filter body and growth template for the silica fibres.

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Colloidal lithography is utilized to form a lift-off mask and allow the formation of a porous metal film with pores accurately defined by the sphere diameter at high densities. This can be accomplished by depositing a PS spheres mask on a silicon substrate and the full process is shown in Figure 5.3. An oxygen plasma treatment (see section 1.3) will shrink the PS spheres about their centres. Chrome can then be deposited on the shrunk-colloid coated sample. The colloids are then stripped off along with any metal that was deposited on top of them (lift-off). The result is a patterned chrome film with accurately defined pours corresponding to the shrunk PS sphere diameters.

Figure 5.3 – Process flow of fabricating a porous chrome template filled with silica fibres.

(A)Depositing hexagonally packed, 400nm diameter, PS spheres. (B)Shrinking the PS spheres by means of oxygen plasma. (C)Depositing a chrome layer. (D)Dissolving the remaining PS spheres and lifting-off the chrome on top of the spheres. (E)Selectively etching the silicon substrate. (F)Growing

silica fibres in the template pores to achieve the complete membrane.

Silicon Silicon Silicon

Silicon

Deposit PS Spheres Oxygen Plasma Chrome Deposition

Silica Fibres Growth Selective Si Etch Lift-off

Top View

Side View

Top View

Side View

A B C

F E D

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5.2.2 Results and Discussion For the purpose of fabricating the mask described in Figure 5.3 above, it was opted to use a larger sphere diameter, 400nm, as the spheres had to be reduced in size and 125nm diameter may be too small when testing and optimising the process (in particularly the shrinking process). The fabricated masks were deposited using the Langmuir-Blodgett method on square (1cm2) silicon wafers and followed the optimisation process that was studied and experimented with the 125nm spheres. A sample of the result is shown in Figure 5.4.

Figure 5.4 – (left)AFM of a silicon substrate coated with 400nm diameter PS spheres and the

corresponding (right)FFT, of the height sensor data (colour scale in nm). Smaller contamination spheres can be distinguished to be in the red range of the colour scale.

Figure 5.4 represents the average layer quality where both contaminants (smaller diameter spheres) and slight topography variations can be seen as previously observed (section 3.11). The best layer images are shown in Figure 3.21 and Figure 3.23. Nonetheless, the presented monolayer order in Figure 5.4-right is more than enough for the purpose of optimising the process and producing a proof of principle. At the time of writing, no results have been returned on the success of the shrinking process and more samples are to be fabricated to allow optimising the sphere-shrinking process.

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6 Conclusions The process of producing Langmuir-Blodgett monolayer coatings of polystyrene nanospheres on glass substrates was practiced, improved and characterised. It is concluded that the current state of the field of colloidal lithography requires reproducible parameters that can be demonstrated in any laboratory and the SP of the liquid surface appears to be a promising candidate. A distinct lack of crucial parameters was noted when researching the literature and, in most cases, will not allow reproducing a reported experiment unless the authors are contacted and are willing to share the results. This appears to be against the spirit of research and hinders progress in this field and appears to have begun about 20 years ago as earlier papers describe the work in much more detail. It is concluded here that the PSP observed when monitoring the addition of diluted colloidal solutions to a liquid surface relates to the proportion of colloids that successfully segregate to the surface. Extending the measurement sets attained here to other colloid solutions of different volumes may allow characterising and quantifying the proportion of spheres that have been successfully transferred to the surface. This may allow for quantitative evaluation of other methodologies to transfer such solutions to a liquid surface. The PSP was not previously regarded or reported in the literature. It may allow improving the overall reproducibility of the Langmuir-Blodgett method and shows promise in possibly extending the method into industrial processes where yield and reproducibility are a necessity. Addition of volatile solvents to the colloidal solution was demonstrated to promote the segregation to the surface. Experimentation with the surfactant Triton X-100 yielded no improvement in the formed monolayers. Nonetheless, the use of surfactants in the process is postulated to have some promising aspects in particular if the surface of the colloids is modified with a surfactant. The Wilhelmy plate probe is found inappropriate for use when measuring SP in Langmuir troughs due to the mechanical effect of macroscopic surface dynamics. The platinum rod probe showed no degradation of results while avoiding the issues caused by the plate. The coating quality may be improved by further investigation into the surface dynamics of solid material suspended on a liquid surface. Methodologies for optimising films are mentioned. These involve investigating relaxation periods and the associated barrier response to the measured SP values. Following such methods allowed improving the monolayer surface coverage and continuity while minimising defects. Following these results, highly ordered samples were produced. The successful coatings allowed for a functional colloidal lithography work to be performed. This included forming a 125nm diameter PS sphere etching mask and a 400nm diameter PS sphere lift-off mask for a colleagues in another departments. At the time of writing, no results have been returned on the success of the masks.

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7 Appendix 7.1 Colloid Solution Volume Calculation The formula used to calculate the necessary volume of colloidal solution, V, to cover a desired surface area, A, was calculated by assuming a perfect hexagonal packing of spheres of radius r as shown in Figure 7.1.

Figure 7.1 – Perfect hexagonal packing of spheres with radius r and the smallest unit cell.

The smallest unit cell within hexagons is the equilateral triangle which is defined here to have a side

length of 2r and a subsequent height of ℎ = 𝑟√3. The unit cell has an area, Aunit, given by equation ( 7.1 ).

𝐴𝑢𝑛𝑖𝑡 = 𝑟2√3 ( 7.1 )

A sphere located in the middle of an air-liquid interface will have a cross-sectional area 𝐴𝑠 = 𝜋𝑟2. As each section of the three spheres occupying a unit cell has an area that corresponds to one sixth of a full sphere (equilateral triangle), the total full spheres within the unit cell is half (0.5 of a sphere). Thus, the calculated coverage ratio of a triangle unit cell, c, is given by equation ( 7.2 ).

𝑐 =0.5𝐴𝑠

𝐴𝑢𝑛𝑖𝑡=

𝜋

2√3 ( 7.2 )

The actual area, AC, that will be covered by spheres from the desired area is given by ( 7.3 ).

𝐴𝑐 = 𝐴 × 𝑐 =𝐴𝜋

2√3 ( 7.3 )

The number of spheres, NS, required to cover the actual area can then be calculated as given by equation ( 7.4 ).

𝑁𝑠 =𝐴𝑐

𝐴𝑠=

𝐴

𝑟22√3 ( 7.4 )

The total volume of spheres (or solids), VT, can now be calculated from the volume of a single

sphere, 𝑉𝑠 =4

3𝜋𝑟3, and is given by equation ( 7.5 ).

𝑉𝑇 = 𝑉𝑠𝑁𝑠 =2𝜋𝑟𝐴

3√3 ( 7.5 )

From equation ( 7.5 ) the mass of the spheres, mS, can be calculated and is done by accounting for their density, given by DS, and is given in equation ( 7.6 ).

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𝑚𝑠 = 𝑉𝑇𝐷𝑠 =2𝜋𝑟𝐴𝐷𝑠

3√3 ( 7.6 )

The colloids are available as aqueous solutions of pure DI water that are specified with a percent weight of solids, w%, and allows calculating the mass of water, mW, that contains the necessary amount of spheres. The calculation is given in equation ( 7.7 ). (w% is given as a percentage, e.g. 1 as opposed to 0.01)

𝑚𝑤 = 𝑚𝑠(100 − 𝑤%) =2𝜋𝑟𝐴𝐷𝑠

3√3(100 − 𝑤%) ( 7.7 )

The volume of water is then calculated from the mass by using the density of water, DW, which is then added to the total volume of spheres, VT, to calculate the final total solution volume in litres and is given in equation ( 7.8 ).

𝑉 = (𝑉𝑇 +𝑚𝑤

𝐷𝑤) × 103 =

2𝜋𝑟𝐴

3√3× 103 (1 +

𝐷𝑠

𝐷𝑤

(100 − 𝑤%)) ( 7.8 )

For the purposes of this project, DS, DW and w% are constants and given below in Table 7.1. The final solution equation contains only three variables (one of which, the radius, will be a constant of the spheres used) and is given in equation ( 7.9 ).

Parameter Value Value Description

DS 1050 (kg.m-3) Polystyrene

DW 998.21 (kg.m-3) Water at 4°C

w% 1 (%) Specified by manufacturer

Table 7.1 – Parameter list.

𝑉 = 105.14 ×2𝜋𝑟𝐴

3√3× 103 = 127.13 × 103𝑟𝐴 ( 7.9 )

The result, equation ( 7.9 ), is a straight line with a base positive slope of 127.13E+3 that is modified by the sphere radii. For the case of 125nm and 400nm diameter spheres, equation ( 7.9 ) becomes equations ( 7.10 ) and ( 7.11 ) respectively and provide the required volumes in litres when an area is given in m2.

𝑉125 = 7.95 × 10−3𝐴 ( 7.10 )

𝑉400 = 25.43 × 10−3𝐴 ( 7.11 )

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