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Development of Organosilane Nanovessels for Nanocrystallization

Daniel J. Sobczynski, Sunxi Wang, Guangzhao Mao*

Department of Chemical Engineering and Materials Science, Wayne State University, 5050

Anthony Wayne Drive, Detroit, Michigan 48202, USA

Abstract

The ability to design high-throughput devices at the nanolevel poses great possibilities for

various applications such as catalysis and detection. In this study, organosilane nanovessels are

used as “nano-beakers” for crystallization of alkanes. Chemical vapor deposition of

organosilanes using a polystyrene template has shown to be a reliable way to create robust

nanopatterns. Due to the highly reactive nature of organosilanes, self-polymerization reactions

often result in nanostructures with height profiles greater than monolayer expectations. The

resulting nanopattern using 900 [nm] polystyrene particles as a template was found to have an

average nanovessel volume of ~284,000 [nm3] and average contact angle of 82+6˚ (42 10 [µL]

drop sample size). These multilayered structures can act as vessels for nanoconfinement of

alkanes and possibly drugs such as aspirin. The volume of alkane deposited was ~5 x the volume

of the nanovessel. Contact angle analysis as well as AFM phase imaging indicates that alkane

material was concentrated near the nanovessel and dispersed in the interstitial area between the

rings, illustrating the alkane affinity for the nanovessel structure.

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Introduction

OTS monolayers have been widely studied1-4. Within the last few years, the OTS system has

been extended to create the patterned monolayer ring structures via particle lithography and

chemical vapor deposition5-7. The motivation to use chemical vapor deposition is two-fold. First,

OTS deposition from liquid solutions has been shown to leave large aggregates on the surface.

Vapor-phase deposition allows for a better controlled reaction, vastly reducing the presence of

bulk deposits. Second, the method developed by Garno, Li and co-workers requires the use of a

polystyrene nanosphere template. This template “mask” is not bonded to the surface and thus a

liquid solution may cause the template to flow in a liquid system. Another important aspect of

this reaction system is OTS’s strong reactivity to water. It has been shown that due to this

sensitivity, the monolayer OTS ring structures can easily develop into multilayers, due to self-

polymerization reactions of OTS. These multilayer structures can be described as “nano-beakers”

or “nanovessels” inside which nanocrystals can form. This paper will discuss the reactions

conditions used to create a reliable “nanovessel” pattern. This study then aims at creating alkane

and aspirin crystals inside the nano-vessels by simple spin/dip-coating and heating techniques.

Also, the vapor deposition reaction conditions are studied and their effect on the nanopattern.

Experimental Section

1. Materials

The following reagents were used without further purification: Octadecyltrichlorosilane

(OTS, Gelest), 3-aminopropyltrimethoxysilane (APTMS, Gelest), HAuCl4∙3H2O (99.999%,

Aldrich), 11-mercaptoundecanoic acid (HS(CH2)10COOH, MUA, 95%, Aldrich), CH3OH

(99.8%, Sigma), H2O2 (30% in water, Fisher), H2SO4 (pure, Fisher), NaCl, MgCl2, K2SO4 (solid,

>99%, Fisher), CH3CH2OH (200 proof, Fluka), NaBH4 (≥99%, Fluka), HCl (>37%, Fluka),

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NH3∙H2O (>29%, Fluka), HNO3 (69%, Fluka), and tetraethylammonium hydroxide (TEAOH,

~40% in water, Aldrich). One-side polished N type silicon (111) wafers (test grade, with

resistivity of 1 to 20 Ω∙cm and thickness of 525 ± 50 μm) were purchased from Wafer World Inc.

Grade 2, muscovite mica was purchased from Mica New York Corp and hand cleaved before

use. Deionized water (DI H2O) from Barnstead Nanopure water purification system (resistivity

18MΩ∙cm) was used to prepare all aqueous solutions. Before use, all the glassware was cleaned

with freshly prepared aqua regia (HNO3/HCl = 1:3, % v/v), rinsed abundantly with DI water and

blown dried with compressed N2.

2. Particle Lithography

Preparing substrates

Silicon dioxide substrates were cleaned with piranha (3:1 volume ratio of H2SO4 and H2O2

for 1 hour, highly corrosive and needs to be handled carefully) or RCA treatment (at 60 °C in

1:2:8 HCl/H2O2/DI water (by volume) for 30 min, and then in 1:2:7 NH4OH/H2O2/DI water (by

volume) for another 30 min). Then the samples were washed with copious amounts of DI water

and blow dried with compressed N2 before any further reaction steps.

Preparing the polystyrene nanosphere template

900 nm size standard polystyrene (PS) latex spheres (1% w/v in water) were purchased

from Thermo Scientific. To remove the contaminants such as the surfactants, the suspension was

centrifuged for 15 min at 13000 rpm. The transparent supernatant above the solid pellet was

decanted and replaced with DI water. 20 µL of 1 wt% PS solution was dropped on the substrates

and dried at ambient (relative humidity, RH~40%) for 30-45 min to allow the monodisperse

spherical particles to order into a two-dimensional crystal structure through immersion capillary

forces8. The samples were exposed to 30 minutes of vacuum and then purged with N2. Then, the

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sample was placed in a reaction vessel (100 [mL] or 3 [L]) that had also been vacuumed and

purged with N2 for 30 minutes.

Chemical Vapor Deposition

100 µL of OTS was then placed inside the bottom corner of the 100 [mL] jar containing

the sample. For reactions done in a 3 [L] desiccator, 100 µL of OTS was placed in a glass Petri

dish next to another Petri dish which contained the sample. For reactions done in the 3 [L]

desiccator, saturated salts were used to control the humidity and the HX71-MA humidity sensor

purchased from Omega was used. The saturated salts were placed in the desiccator after it had

been vacuumed for 30 minutes and purged with nitrogen. The salts were allowed to equilibrate

inside the desiccator for 30 minutes before the reaction was performed. CVD was carried out by

heating in an oven at 70˚C for 90 minutes. The gas deposition is adopted to allow slow rate of

deposition and surface arrangement of OTS molecules. 100 [mL] and 3 [L] reaction vessels were

used to study the reactor volume effect and different reaction durations from 10 [min] to 90

[min] were used to study the reaction time effect. After the reaction, the template was then

sonicated in a 1:1 volume ratio of ethanol: DI water to remove the nanosphere template and the

revealed organosilane nanopatterns were imaged with AFM under tapping mode in ambient air.

Alkane Deposition

A 40 [µL] drop of 3.5 [mM] docosane dissolved in chloroform was deposited on the

nanopattern substrate and quickly covered with a glass Petri dish. This was done to prevent the

chloroform from evaporating too quickly, causing a poor formation of a thick alkane film. Under

the optical microscope, crystalline domains should cover the sample at this point. The sample

was then heat for ~25 [min] at 90˚C and then cooled at -20˚C for another 25 [min]. Prior to

cooling, the visible layer is no longer present and it is believed that much of the docosane has

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dispersed into the rings and also formed big crystals, leaving other areas of the substrate empty.

After cooling, the sample is imaged in tapping mode AFM.

3. Characterization

Atomic force microscopy (AFM) images were obtained with either an E scanner

(maximum scan area = 14.2 × 14.2 μm2) or J scanner (maximum scan area = 125 × 125 μm2)

(Nanoscope IIIa, VEECO). Height, amplitude, and phase images were obtained in tapping mode

in ambient air. Uncoated silicon probes (TESP, VEECO) with a factory-specified spring constant

of 40 N/m, length of 125 μm, width of 40 μm, and nominal tip radius of curvature less than 10

nm were used. The scan rate used was in the range of 0.5–1 Hz depending on the scan size.

Integral and proportional gains were approximately 0.4 and 0.8, respectively. All AFM height

images are reported unless specified. Height images have been plane-fit in the fast scan direction

with no additional filtering operation. Images are analyzed using the Nanoscope software from

Digital Instruments (Version 5.12). The section analysis is used to measure the height and lateral

dimensions of the nanofeatures on the substrates. The root–mean square surface roughness Rq is

calculated from variations in AFM height images taken from the mean data plane according

to Rq = √∑zi

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n, where zi is the height value and n is the number of pixels in an image.

The surface hydrophobicity was measured with an NRL contact angle goniometer (Model

100, Rame-Hart) in the laboratory atmosphere. A 10 μL water droplet was placed on the

substrate and the static contact angles were measured on both sides of the droplet. Three droplets

were placed at various spots on the substrate and the average readings are reported. The typical

error is ±3 ̊.

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Results and Discussion

1. Treatment Methods: Piranha versus RCA

Past literature has reported the use of both piranha and RCA as possible treatment

methods for cleaning silicon surfaces3,5-7. In order to determine which method is better, X-ray

reflectivity and AFM imaging was done to compare piranha and RCA treated substrates. The

results are shown below in Figure I.

Figure I. (Left) AFM height image of piranha treated Si substrate with scan size of 4 × 4 μm2 and z-range of 5

nm; (Right) X-ray reflectivity profile of RCA treated Si substrate.

Table I. Comparison of XRR fitting results of piranha and RCA treated Si substrates.

The results from X-ray reflectivity (XRR) (Table I) show only minimal differences in the

roughness values and thus the conclusion is that either method is suitable to form a smooth

hydrophilic layer on silicon dioxide. Piranha treatment is the main method used for this study

due to ease of preparation.

2. Crystallinity of Nanosphere Template

As mentioned in the experimental section, once the substrates have been cleaned by

either piranha or RCA methods, a polystyrene nanosphere template is used to selectivity collect

water at the base of the particles, forming trapped water menisci which serve as reaction sites for

chemical vapor deposition of highly reactive species like OTS. Ideally, the nanosphere template

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should be packed in an ordered 2-D format (Figure II-a) everywhere but often times the results

shown not only areas with defects (Figure II-b) but absence of the template altogether (Figure II-

c).

a b c

Figure II. 900 [nm] particle template and 90 min CVD of OTS nanopattern. (a) AFM height image of well

ordered ring structures, a result of a well-ordered template; (b) Area with defects, indicated by disordered

domains of ring structures; (c) Areas that do not contain any rings, indicating a lack of polystyrene particle

menisci development. Data scale = 80 [nm], Image size = 10 x 10 µm2.

Thus, it can be difficult to quantify exactly how many rings are present on the surface for a given

density due to these problems. One possible solution may be to dry the polystyrene templates

under high humidity conditions and use a slightly higher concentration8.

3. Multilayered and Monolayer Organosilane Nanopattern Formation

Garno, Li, and co-workers reported monolayer ring structures using particle lithography

combined with vapor deposition performed in a 100 [mL] jar. It was found in our studies that

using the same basic procedure, with a few modifications such as drying in vacuum, multilayered

ring structures develop only. Monolayer ring structures were fabricated only when the reaction

vessel volume was 3 [L].

These experiments were done for a range of humidity values but, as seen from Figure III-

a, there is no clear dependence of ring structure height on humidity. It appears that for very low

humidity (~4%), almost no ring structure forms and eventually no impact is since in higher

ranges (40-50%). However, when experiments at low humidity values were performed in a 100

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[mL] jar for the same reaction time, higher ring structures developed consistently. Although not

shown, there has been evidence of higher ring structures for reactions in the 3 [L] desiccator, but

these results were not shown to be consistent as seen in the 100 [mL] case. Thus, there appears to

be no clear relationship between OTS ring height and relative humidity of the reaction vessel.

The contact angle of patterned substrates done in the 3 [L] desiccator were typically ~48˚ (Figure

III-b) indicating a surface that contains almost no OTS between the ring structures. Different

humidity values were achieved by using saturated salt solutions placed at the bottom of the 3 [L]

desiccator.

a b

Figure III. 900 [nm] particle template and 90 min CVD of OTS nanopattern in a 3 [L] reaction vessel. (a) Plot

of relativity humidity just before the CVD step for 3 [L] and 100 [mL] volume reactors. (b) Histogram of

contact angle for all different samples under various humidity values.

The reasoning behind the formation of multilayered ring structures lies in the high

susceptibility of OTS to self-polymerize7,9, forming a 3D network, resulting in “nano-beakers” as

opposed to monolayer ring structures. Typical AFM images of the nanopattern in 3 [L] and 100

[mL] reaction volumes are shown below in Figure IV.

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a b

Figure IV. AFM images (top) and corresponding height profile (bottom) for 90 min CVD of OTS

nanopattern using a 900 [nm] particle template. (a) Nanopatterns in 3 [L] reaction vessel; (b) 100 [mL].

When performing reactions in the 100 [mL] jar as compared to the 3 [L] reaction vessel, the 100

[mL] jar heats up faster, and the reasoning is that this leads to a higher pressure of OTS,

increasingly the likelihood of self-polymerization reactions, causing multilayered nanostructures

to develop as opposed to the monolayer seen in the 3 [L] case. The multilayered nanostructures

(Figure IV-b) resemble nanovessels. These patterns will be used for nanoconfinement studies.

4. Reaction Time Study

Using the 900 [nm] particle template and the 100 [mL] jar as the reaction vessel, 10 [min], 30

[min], 90 [min], and 721 [min] reactions were done to investigate the effect of reaction time on

the nanopattern. As shown in Figure V-a, the contact steadily increased as a function of reaction

time, however the ring height proved to remain basically constant for all reaction times. The

relative humidity (at ~24˚C) was 5-10% just before the reaction started for these experiments.

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a b

Figure V. Reaction time study of 900 [nm] particle template nanopattern. (a) Contact angle dependence on

CVD reaction time (b) High ring structures (15-20 [nm]) formed after only 10 min of reaction in CVD.

The nanowall structures thus react and form very quickly (Figure V-b), whereas the deposition of

OTS in the surface background steadily increases, evidenced by the contact angle dependence on

reaction time. The quick formation of the nanopattern is likely due to the localization of water at

the base of the spheres, leaving other areas dry. Based on this result, it is seen that after 90 [min],

the substrate contact angle no longer depends on reaction time to a significant extent. A 90 [min]

reaction time will be used for all the studies in this report since it provides consistent ring

structures and a relatively hydrophobic surface (≈82˚).

5. Characterization of Multilayered Nanopatterns

Since the multilayered nanopatterns were consistently produced under the reaction

conditions described in the Experimental Section, characterization of the nanovessel dimensions,

pattern density, and surface composition through contact angle analysis is now performed. The

end goal is to calculate the nanovessel inner volume. Although the nanovessel is better described

as “cone-shaped”, a reasonable approximation is to consider the volume to be a cylinder. If we

assume this is the case, the volume of the nanovessel can be calculated by the following

equation:

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𝑉𝑣𝑒𝑠𝑠𝑒𝑙 =𝜋 ∗ 𝑑𝑖𝑛𝑛𝑒𝑟

2 ∗ 𝐻

4 (𝐸𝑞. 1)

In this equation, 𝑑𝑖𝑛𝑛𝑒𝑟 refers to the inner diameter of the OTS wall structure, and 𝐻 the height

of the OTS wall structure. Calculations were performed for the outer and inner diameters, as well

as the height of the OTS nanowall structure. The nanovessel volume is simply the volume

encased by the OTS structure. Figure VI shows show the calculations were performed for height,

𝐻, and inner diameter, 𝑑𝑖𝑛𝑛𝑒𝑟 .

a b

Figure VI. (a) Nanowall height calculation; (b) Outer (red) and inner (green) diameter calculations.

Based on several samples, average 𝐻, 𝑑𝑖𝑛𝑛𝑒𝑟 and 𝑑𝑜𝑢𝑡𝑒𝑟 values were calculated to construct an

average 𝑉𝑣𝑒𝑠𝑠𝑒𝑙 to be ≈284,000 [nm3] (2.84 x 10-10 [nL]) from Eq.1. The average height value

was 14.2+3.9 [nm], the outer diameter, 379.2 + 45.8 [nm], and the inner diameter, 159.5 + 31.5

[nm]. Figure VII shows the histograms and average values for all of these parameters.

a N = 220 b N = 130 c N = 130

Figure VII. Histograms for calculations: (a) Height (b) Outer and (c) inner diameter. (N = sample size, fitted

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to normal distribution)

The next calculation is the nanostructure density for a 1 [cm] x 1 [cm] substrate. This was done

by counting the number of structures on several 10 [um] x 10 [um] images and scaling up to the

total substrate size. Based on this calculation, there are ideally (123.5 + 2.9) x 106

structures/substrate. However, this assumes that there are no defects or patches without rings. As

described earlier, it is know that this is not the case. The amount of defects are quite small and

amount to ~625,000 / substrate (assuming structures are located everywhere). This is ~0.5% of

the total number of rings. Thus, the main problem in developing a consistent template is limiting

the amount of area that is unoccupied, or the “empty” regions shown in Figure II-c.

The final characterization calculation was contact angle analysis which provides insight into the

chemical nature of the surface. Water is chosen as the liquid for contact angle measurements and

10 [µL] droplets are used for each measurement. For each measurement, the average contact

angle of both sides of the droplet is used (illustrated in Figure VIII-a). This is done to provide a

more consistent average and representation of the data. 3 droplets are taken for each sample in

approximately the center of the substrate. A total of 42 droplets are measured from various

samples and the average value was found to be 82 + 6˚ (Figure VIII-b). This range of contact

angle suggests a surface that is on the border of hydrophilic to hydrophobic. Typically, > 90˚ is

considered “hydrophobic” if water is the wetting liquid, however even in this range of ≈ (76-88)

˚, these samples still repel water from the surface.

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a 𝜃 =𝜃1+𝜃2

2

b N = 42

Figure VIII. Contact angle measurements. (a) Each of the 42 droplet values are based on the average contact

angle from both sides of the droplet. (b) Histogram of the 42 droplets (=N) measured on nanovessel pattern

substrates.

The contact angle can also be predicted from theory using the Cassie Law equation:

cos 𝜃𝑐 = 𝛾1 cos 𝜃1 + 𝛾2 cos 𝜃2 (𝐸𝑞. 2)

where 𝛾1 and 𝛾2 are the fractions of the surface composed of components 1 and 2, 𝜃1 and 𝜃2, the

contact angle for each component, and 𝜃𝑐 the contact angle for the heterogeneous surface. Based

on this equation, the experimentally measured contact angle (≈82˚) is the value for 𝜃𝑐 . In the

case of this sample, it is composed of silicon dioxide and OTS. We will call these components 1

and 2. The contact angle for water on silicon dioxide and OTS are known values,1,3 and will be

taken to be 0˚ and 110˚. Thus, the only unknown parameters are 𝛾1 and 𝛾2, the surface fractions,

which can be adjusted to yield the experimentally determined value for 𝜃𝑐 . The values for 𝛾1 and

𝛾2 turn out to be ~.65 and .35. From Eq.2, a surface that has contains SiO2 between the nanowall

structures will have a contact angle ~36˚. This figure assumes that rings are present everywhere

on the substrate. For a substrate with OTS between the ring structures and inside each structure,

the angle is ~110˚. For the sample in this study (𝜃 ≈ 82˚), Cassie’s Law suggests that the

background surface between the rings is ~40% SiO2 and ~60% OTS. In these studies, we have

assumed the nanowall to be filled with OTS inside since the actual structure of the wall is a cone

and only the very bottom of the structure would likely contain SiO2 (Figure IX).

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Figure IX. Consideration of the wall structure. Top (AFM height image) and bottom (height profile of top

image). Only the very bottom is likely to have not reacted with OTS (indicated by dark red in the center)

and therefore when using Eq.2, the surface area of OTS includes the inside of the ring.

6. Characterization of Alkane Filled Nanovessels

After performing the alkane deposition steps (outlined in the Experimental Section) on the

nanovessel substrates, calculation of the height, diameter, volume, and contact angle was

performed (Figure 8) and compared to the numbers obtained for an empty nanovessel. From

these calculations, the alkane was shown to not only fill the vessel, but also the outside of the

nanovessel. This is expected since the nanovessel is essentially made of alkane material and thus

there should be a strong affinity of the alkanes for the nanovessel material in general. As shown

in Figure X-a, the average height was 18.6 + 2.6 [nm], and the outer diameter 430.6 + 52.8 [nm].

Based on these values, the volume of alkane deposited per ring was ~2.2 x 106 [nm3] (~2.2 x 10-9

[nL]). This corresponds to ~5 x Vempty, or the volume of the empty nanovessel.

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a b c

Figure X. Alkane structure characterization. (a) Height calculation, (b) diameter calculation, and (c) contact

angle study. From top to bottom for a, b, and c: AFM height image, corresponding height profile, and

histogram for N=30 sample size with normal distribution fit.

The contact angle was measured at various points throughout the alkane deposition

procedure. After heating and cooling, the contact angle was ~91˚(Figure X-c, top), which is

slightly higher than prior to deposition. It is believed that although a thick layer of alkane

develops on the surface after the first step, the heating/cooling step causes the docosane to be

localized near the ring areas rather than uniformly dispersed. Also, heating/cooling is likely to

cause crystallization of large alkanes which also reduce the contact angle. As a result, the surface

coverage of OTS/alkane (they are treated as one) material is increased about 10% overall after

alkane deposition. In terms of the surface composition, the background area is predicted to be

composed of ~63% OTS/alkane and ~37% SiO2.

Since the goal of this research is to study the deposition of alkane inside the vessel, it is

important to show that the alkane is localized only near the nanovessel structures and nowhere

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else. One way to do this is by considering the AFM phase images shown in Figure XI.

Comparing the phase image obtained for the empty nanovessel pattern to the image after alkane

deposition, it is seen that the peaks are much more pronounced near the vessel after alkane

deposition. The degree of peak is indicative of hydrocarbon like material. Thus, the fact that

more pronounced peaks occur near the ring after alkane deposition indicate a stronger presence

of alkane material near the ring. Although the phase image cannot necessarily tell what materials

the surface is composed of, the difference in phase angle suggests a difference in composition.

Since the rings are empty prior to alkane deposition, the difference in phase must be due to the

presence of alkane localized at the vessel site. This shows the affinity of the alkane for the vessel

structure.

a b

Figure XI. AFM phase image comparison. (a) Phase image for a nanopattern filled with alkanes. (b) A phase

image for the empty nanoring pattern. The phase image in part (a) has much more pronounced peaks near

the ring, suggesting the composition of alkane material localized at the vessel as compared to the surface in

between the vessels.

Conclusion

Using particle lithography combined with chemical vapor deposition of organosilanes

often results in nanovessels rather than monolayer ring structures. When experiments were done

in a 3 [L] reactor volume at various humidity values, the overall ring height was approximately

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the monolayer (2.62 [nm]) but often showed slightly higher structures of ~5-6 [nm]. A similar

experiment was done in a 100 [mL] jar under a pure a nitrogen environment and always resulted

in nanopatterns with structures of ~14 [nm], for various different reaction times. Both

experiments were performed for the same amount of time but one difference is that the 3 [L]

takes a longer time to heat up than the 100 [mL] jar and also the vapor pressure of OTS is less in

the larger reactor volume. This is thought to affect the extent of deposition, and thus the height of

the rings in the 3 [L] case. At this point, it is still unclear exactly why the height is different in

each case.

Since the patterns that displayed higher structures (~14 [nm]) could be reliably produced,

the dimensions of the nanostructures were measured and found to be consistent and the volume

of each vessel (~2.84 x 10-10 [nL]) is an extremely small vessel, making it a unique structure to

house materials for crystallization. The deposition of alkanes resulted in vessels that were filled

inside and also on the outer wall of the nanostructure. The volume of alkane deposited per ring is

~5x the volume of the empty nanovessel, showing the expected affinity of the alkane for the

OTS composed vessel.

The contact angle after alkane deposition is slightly higher than for the empty pattern and

based on Cassie’s Law, the composition of the area between the vessels is ~63% OTS/alkane.

This shows that the vessels are filled while the interstitial space between structures remains

heterogeneous. AFM phase images show a localization of alkane material near the vessel as

compared to the bulk background composition.

Future studies will include X-ray diffraction to determine the crystallinity of the filled

vessels and experiments with aspirin in attempt to synthesize nanocrsytals of drug materials.

Acknowledgements

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This work has been made possible by the generous donations from the National Science

Foundation. Also, research consultation has been provided by the Garno group, in particularly,

Dr. Kathie Lusker, working in the Chemistry Department at Louisiana State University. Also,

Dr. Jinping Dong, now a Senior Research Associate in the IT Characterization at the University

of Minnesota has also provided correspondence throughout the study of this work.

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