Fabrication Processes for Nanomaterials

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Lecture 5

TOP DOWN

BOTTOM UP

Fabrication MethodsAtom by atom assembly

Like bricklaying, move atoms into place one at a time usingtools like the AFM and STM

Chisel away atomsLike a sculptor, chisel out material from a surface until thedesired structure emerges

Self assemblySet up an environment so atoms assemble automatically.Nature uses self assembly (e.g., cell membranes)

IBM logo assembled from individual xenon atoms

Polystyrenespheres self-assembling

Source: http://www.phys.uri.edu/~sps/STM/stm10.jpg; http://www.nanoptek.com/digitalptm.html

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Top Down

NANOPARTICLES

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Mechanical Attrition

Imposing an extremely highdeformation onto the material,structural refinement occurs byshearing and breaking down ofexisting structures and phases

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Mechanical Attrition – Ball Milling

Schematic diagram of high energy attritor mill

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Mechanical Attrition – Ball MillingRepeated welding, fracturing and rewelding of particles

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Mechanical Attrition – Ball MillingControl Factors:

Composition of particles/powders

Size of particles/powders

Milling speed (energy of impact)

Milling temperature

Milling atmosphere

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Laser AblationAdvantages•Possible high purity of the nanomaterial,•material variety, and the• in situ dispersion of the nanoparticles in a variety ofliquids• allowing safe and stable handling of the colloids.

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Synthesis of Nanoparticles by PLA in a liquid

To synthesize Ag (Cu2O @ Cu) NPs by PLA, ahigh purity silver (copper) slice is placed onthe bottom of glass vessel containing 20 mlof distilled water.It is irradiated with focused output of 1064nm of pulsed Nd:YAG laser (Spectra PhysicsInc. USA) operating at fixed energy for 30minutes.This results a yellow color in Ag (light greenin Copper) colloidal solution.

Ag Nanoparticles

UV visible absorption spectrum: Theabsorption band at ~398 nm is due toSPR of Ag nanoparticles, confirms theformation of pure Ag nanoparticles.

TEM image of Ag samplesynthesized at 40 mJ/pulse laserenergy confirms formation ofnanoparticles with average particlesize 2-3 nm.

The inset shows SAED pattern,indicates formation of crystallinenanoparticles

Nano-Particle Preparation by PLA (dry)

IEEE International Nanoelectronics Conference 3 8 January 2010 slide 7KMUTT

Lens

Argon gas inlet

Nd YAG Laser ( Miyachi)

(1064 nm, 1 Hz)

Minror

RotatingTarget

Motor

Fig 1. Schematic of pulsed laser ablation on a rotating target

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Bottom UP!

NANOPARTICLES

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Chemical SynthesisProcesses involving molecular assembly by interactive forcesbetween atoms and molecules, chemical bonds and Van der Waalsforces to form aggregates of atomic and/or molecular units

Major processes: sol gel process and chemical vapour depositionprocess

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The sol-gel process versatile solution process for making ceramics and glassmaterials. Applying the sol gel process,ultra fine or spherical shaped powders,thin film coatings,ceramic fibers,microporous inorganic membranes, monolithic ceramics andglasses, orextremely porous aerogel materials.

The sol gel process

https://www.llnl.gov/str/May05/Satcher.ht

Possibilities"sol" is cast into a mould,

a wet "gel" will form.

With further drying and heat treatment,the "gel" is converted into dense ceramic or glass articles.

If the liquid in a wet "gel" is removed under a supercriticalcondition,a highly porous and extremely low density material called"aerogel" is obtained.

Aerogel properties can be changed by adding different precursor molecules.

(a) an aluminum oxide foam prepared from aluminum nitrate has a cluster morphology thatresults in

(b) an opaque aerogel.

(c) Using aluminum chloride as the precursor produces an aerogel with fibrous morphology,resulting in

(d) a stronger foam that is also translucent.

https://www.llnl.gov/str/May05/Satcher.ht

Possibilities, cont.Adjust viscosity of a "sol"ceramic fibres can be drawn from the "sol".

Ultra fine and uniform ceramic powdersprecipitation,spray pyrolysis, oremulsion techniques

Limitations of sol gel synthesisPresursorsoften expensivesensitive to moisture

ProcessTime consuming

Dimensional changeshrinkage

Limits largescale

production

cracking

Advantages of sol gel synthesisPrecursorsVolatile

Densification T’s lowChemical conditions mildHighly porous and nanocrystalline materials

Easily purified– high purityproducts

Silica particles produced by sol gel

Colloids

SolutionsHave small particles (ions or molecules)Are transparentDo not separateCannot be filteredDo not scatter light.

A mixture of water H2O and ethanolCH3CH2OH is homogeneous

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ColloidsCannot be filtered

Do not “settle”

Have medium size particles

Separated with semipermeable membranes

Scatter light (Tyndall effect)Dispersion of matter in size from about 1 1000nm

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The TyndallEffect

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Sols – ink, paint

Colloid Examples

Natural aerosols – Fog, clouds

Made aerosol sprays, smogSolid aerosols, smoke, dust

Foam, whipped cream, shaving lather

Emulsions mayonnaise, milk

Gels – Jelly, butter

Pearl

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Examples of ColloidsFog/smoke

Whipped cream

Milk

Cheese

Blood plasma

Pearls

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Properties of colloids

Kinetic Properties: Motion of the particleswith respect to the dispersion mediumThermal motionBrownian movementDiffusionOsmosis

Gravity or centrifugal fieldSedimentation

Viscous flow

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Colloidal dispersions are more stable than suspensions and emulsions, due toSmaller particle size

Brownian movement

In 1889 G.L. Gouy found that the "Brownian" movement was more rapid for smallerparticles (we do not notice Brownian movement of cars, bricks, or people)

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Spherical colloidal particles of radius 30nm and density 1.35 g cm 3 are suspended in water (at298K, density 1.0 g cm 3, viscosity 1.0x10 3 Pa s).

Calculate the average distance from the origin travelled by a particle in 1 minute due tosedimentation and Brownian motion.

Diffusion distance: Sedimentation distance:

<x2>1/2 = (2Dt)1/2 = (2kTt/6 R)1/2 vt=2R2 gt/9

k = 1.3806488 × 10 23 m2 kg s 2 K 1

Colloids

LyophilicSolvent lovingHydrophilic

LyophobicSolvent hatingHydrophobic

Amphiphillic

Both!!!!

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Amphophiles

A colloid in which dispersed phase consists ofmicelles.Example: surfactant (surface active agent).

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Micelles

aggregates of amphophilicmoleculesCritical micelle concentrationConcentration at which micelle isformed

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Concentration at the critical micelle concentration.

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Colloids & nanocrystalsNumber of nucleation sites formed initially determines number of particlesproduced› Determined by amount of citrate addedIf initial AuCl4 is kept the same more nucleation sites means final particleswill be smallerFor monodisperse sol need to form nucleation sites quickly andsimultaneously› Hard to do !Gold particles are neutral cores surrounded by AuCl2 ions› Colloid stabilised by electric double layer

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How can we make nanoparticles?

Mixing hexadecyltrimethylammoniumbromide pentanol micelles of CdCl2with similar micelles containing Na2Sproduces nanoparticle CdS

since the aqueous solution serves as ananoreactor the particles cannotgrow bigger than the micelle

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Nano films and other structures

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Sol Gel Possibilities

Thin films on a substratespin coating or dipcoating.

Sol gel depostion of thin films

limitations

Spin coatingVariables??

Dip coatingVariables??

Fibre coatingVariables??

Sol gel coatings on particles

SnO2 coated with SiO2,diameter 27nm

Corrosion Protection

Anti-reflective coatings

ELECTROSPINNING

Type I collagen with randomly orientedfibers (left) and aligned fibers (right)

SEM of electrospunelastin at 250 mg/ml.

a) electrospun composite structure (Left),b) SEM of tubular electrospun composite (Middle),c) SEM of electrospun blend of collagen type I, collagen

type III, and elastin (Right)

a) Cotton gauze (left)b) Bovine electrospun fibrinogenmat (right) for potential use as atissue engineering scaffold orwound dressing.

SEM of randomly oriented PLA electrospun fromchloroform (Left) and HFP (Right)

Advanced Drug Delivery Reviews, 59(14), 1413 1433, 2007.

"Diagram by Joanna Gatford at The New Zealand Institute for Plant and Food Research Ltd"

Charge distribution changes

"Diagram by Joanna Gatford at The New Zealand Institute for Plant and Food Research Ltd"

What might control the size of the nano-fibres?

Parameters

1. Molecular weight, molecular weight distribution and architecture (branched,linear etc.) of the polymer

2. Solution properties (viscosity, conductivity and surface tension)

3. Electric potential, flow rate and concentration

4. Distance between the capillary and collection screen

5. Ambient parameters (temperature, humidity and air velocity in the chamber)

6. Motion and size of target screen (collector)

7. Needle gauge

Collectors

Plate

Disc

DrumMandrel

Images courtesy of Mecc Co. Ltd, Fukuoka, Japan

Spinnerets

Clip/tube Tubeless Co axial Heated

Images courtesy of Mecc Co. Ltd, Fukuoka, Japan

Plate - Good for making random mats

Typically, collection area: less than 370cm2 (corresponding to apiece of A5)

Images courtesy of Mecc Co. Ltd, Fukuoka,

DiscCreation of highly aligned fiber sheet

highly aligned fiber sheet can be obtained by the disc collectorcompared to

the drum collector.

Creation of fiber bundle

fiber bundle can be made by this collector.

Typically:Disc circumference: about 600mmRotation Speed : about 63 to 1,890 m/min. (100 to 3,000 rpm)

Images courtesy of Mecc Co. Ltd, Fukuoka, Japan

Drum-Creation of random mesh and aligned fiber sheetThe drum collector can make not only random mesh but alsoaligned fiber sheet.

Function of drum rotation speedRandom mesh – low rotation speedAligned fiber sheet high rotation speed

Typically,Collection area about 1,300cm2 (corresponding to 2 sheets of A4papers)Rotation speed: about 63 to 1,890 m/min. (100 to 3,000 rpm)

Images courtesy of Mecc Co. Ltd, Fukuoka, Japan

Rotating Drum

Mandrel - Creation of tubular nanofibrous assembly

Rod with different diameter can be used as a mandrel.

Possible to create tubular nanofibrous assembly with differentinner diameter.

Attachable Mandrel Diameter : 0.5 to 10 mm

Length of nanofibrous assembly: 150 mm

Images courtesy of Mecc Co. Ltd, Fukuoka,

Clip/Tube

Images courtesy of Mecc Co. Ltd, Fukuoka, Japan

TubelessMinimize waste of solution

The syringe can be directly attached to the spinneret.

No changes of hardware are required to used this spinneret.

Various types of solutions can be electrospun in a short time.

Images courtesy of Mecc Co. Ltd, Fukuoka,

Some Referenceshttp://en.wikipedia.org/wiki/Electrospinning

http://du2012 grp038 01.blogspot.com/2012/04/electrospinning process.html

http://www.people.vcu.edu/~glbowlin/research.html

http://www.people.vcu.edu/~glbowlin/electrospinning.htm

Vapour Deposition Techniques

Plasma and Vapour DepositionProcesses generally involvinga. creation of vapour phase,b. vapour transport from a source to substrate andc. deposition of the vapour phase on the substrate

Two main processes: physical vapour deposition (PVD) andchemical vapour deposition (CVD)

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Deposition ProcessSchematic diagramshowing theprocesses involvedin nucleation ofdeposited film

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Comparison of the coating processes

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Deposition ProcessLayer Mode: growth ofthin layer withcomplete coverage ofthe substrate surface

Island mode: distinctand separate clusters ofthe condensed phasefollowed bycoalescence of theclusters

Mixed mode: formationof a monolayerfollowed by the growthof cluster islands

TWO TYPES OF THIN FILM DEPOSITION: CVD AND PVD -very low gas pressure or high vacuum

CVDReactive gases interactwith substrateUsed to deposit Si anddielectricsGood film qualityGood step coverage

PVDUsed to deposit metalsHigh purityLine of sight

inert gas environment is usually maintained,but reactive gas may be injected into the

chamber for formation of specific compounds

PVDPHYSICAL VAPOUR DEPOSITION

PHYSICAL VAPOUR DEPOSITION (PVD)

Working ConceptPVD processes are carried out under vacuum conditions.The process involved four steps:1. Evaporation2. Transportation3. Reaction4. Deposition

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Physical Vapor Deposition: PVD

ADVANTAGES: LIMITATIONS:Line of sight

Shadowing

Thickness uniformity

Difficult to evaporate materials with lowvapor pressures

2 types: evaporation and sputtering

Versatile – deposits almostany material

Very few chemical reactions

Little wafer damage

sputtering

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Microstructure of coatings

SEM images of fractured surface of PVD coatings on nitrided steel: a– monolayer TiN, b– monolayer TiCN and c– multilayer TiAlN

Nucleation and grain growth are involved in coating deposition process on the substrate

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Nanocomposite coatings (1)

2 phases appear during the deposition process:nanocrystalline AlTiN (hard) and amorphous Si3N4 (tough)

Advantages:• very high nanohardness andtoughness• very high temperature resistance

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Nanocomposite coatings (2)

Physical Vapour DepositionControl Factors:

1. Energies of emitted atoms/molecules

2. Target to substrate separation and arrangement

3. Chamber pressure and partial pressure of reactive gas

4. Substrate bias voltage and temperature

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CVDCHEMICAL VAPOUR DEPOSITION

COLORLESS GEMCUT FROMDIAMOND GROWNBY CHEMICALVAPORDEPOS I T ION

Chemical Vapour DepositionMicrofabrication processes widely use CVD to depositmaterials in various forms, including: monocrystalline,polycrystalline, amorphous, and epitaxial.These materials include: silicon, carbon fibre, carbonnanofibres, filaments, carbon nanotubes, SiO2, silicongermanium, tungsten, silicon carbide, silicon nitride,silicon oxynitride and titanium nitride.CVD process is also used to produce synthetic diamonds.

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Thermal CVD system

http://www.iljinnanotech.co.kr/en/material/r-4-4.htm

Precurser GasFor growing Carbon Nanotubes

Carbon Nanotubes

http://www.iljinnanotech.co.kr/en/material/r-4-4.htm

Chemical Vapour DepositionPrecursor gases are used in the process

A reactive gas/gas mixture is often impinged on the substrate

Process is conducted at a high temperature such that molecularfragments and free atoms are formed and react with reactive gasesto form a desired coating on the substrate

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Working Concept

Chemical vapor deposition (CVD) results from the chemical reaction of gaseous precursor(s) at aheated substrate to yield a fully dense deposit.

Variables:temperature, pressure, and concentrations

Metal deposition

metal halide (g) metal(s) + byproduct (g)

Ceramic deposition

metal halide (g) + oxygen/carbon/nitrogen/boron source (g) ceramic(s) + byproduct (g)

UNIT IV LECTURE 3 93

Example

Advantages of CVD1. Can be used for a wide range of metals and ceramics2. Can be used for coatings or freestanding structures3. Fabricates net or near net complex shapes4. Is self cleaning—extremely high purity deposits (>99.995% purity)5. Conforms homogeneously to contours of substrate surface6. Has near theoretical as deposited density7. Has controllable thickness and morphology8. Forms alloys9. Infiltrates fiber preforms and foam structures10. Coats internal passages with high length to diameter ratios11. Can simultaneously coat multiple components12. Coats powders

UNIT IV LECTURE 3 96

Issues related to thin film deposition

Quality:CompositionDefect density (e.g. pinholes)ContaminationMechanical and electrical propertiesGood adhesionMinimum stress

TopographyUniform thickness on non planar surfacesStep coverageConformal coverage: uniformSpace filling in holes, channelsVoids

Various types of CVD:

Atmospheric pressure – APCVD

Low pressure – LPCVD

Plasma enhanced – PECVD

High density plasma HDPCVD

Major TechniquesMechanical attrition – top down processRapid condensation and solidificationChemical synthesisPlasma and vapour depositionLithography

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Laser AblationConventional nanoparticle generation routes•mechanical milling and grinding• drawbacks related to the purity ,

•chemical generation methods like the sol gelprocess•variety of accessible expensive materials.

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