Ch 2 Nanomaterials Synthesis

8/13/2019 Ch 2 Nanomaterials Synthesis

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Diah Susanti, Ph.D



Nanotechnology Definition

Nanotechnology is a technology of design, fabrication andapplications of nanostructures and nanomaterials.

Nanotechnology also includes fundamental understandingof physical properties and phenomena of nanomaterialsand nanostructures.

Study on fundamental relationships between physicalproperties and phenomena and material dimensions in thenanometer scale is also referred to as nanoscience.

In US, nanotechnology has been defined as being“concerned with materials and systems whose structuresand components exhibit novel and significantly improves

physical, chemical and biological properties, phenomenaand processes due to their nanoscale size.”



In order to explore novel physical properties andphenomena and realize potential applications ofnanostructures and nanomaterials, the ability tofabricate and process nanomaterials andnanostructures is the first corner stone innanotechnology.



1. Technologies in Nanomaterials SynthesisMany technologies have been explored to fabricate nanostructures

and nanomaterials. These technical approaches can be grouped

in several ways. One way is to group them according to thegrowth media:1. Vapor phase growth: laser reaction pyrolysis for nanoparticle

synthesis, atomic layer deposition (ALD) for thin filmdeposition, metal organic chemical vapor deposition(MOCVD)

2. Liquid phase growth: colloidal processing for the formation ofnanoparticles and self assembly of monolayer.

3. Solid phase formation: phase segregation to make metallicparticles in glass matrix and two-photon inducedpolymerization for the fabrication of thee-dimensional

photonic crystals, milling process to form powder materials.4. Hybrid growth, including vapor-liquid-solid (VLS) growth ofnanowires, reactive sputtering (PVD) (solid-gas), czocharlskimethod (solid-liquid)



•Pyrolisis is a thermochemical decomposition of organic material at elevatedtemperatures without the participation of oxygen. It involves the simultaneouschange of chemical composition and physical phase, and is irreversible. The word iscoined from the Greek-derived elements pyro "fire" and lysis "separating“ •

A laser is a device that emits light (electromagnetic radiation) through a process ofoptical amplification based on the stimulated emission of photons. The term "laser"originated as an acronym for Light Amplification by Stimulated Emission of Radiation.The emitted laser light is notable for its high degree of spatial and temporalcoherence.

Fig. 1 - Schematics of the set-up for lasersynthesis of nanoparticles from gas-phase

reactants Source: ENEA

Fig. 2 - Picture of the set-up forlaser synthesis of nanoparticles

from gas-phase reactants - Source:ENEA

LASER PYROLYSIS



Laser Pyrolysis using CO2 gas In the process of CO2 laser pyrolysis, the condensable products result from laser

induced chemical reactions at the crossing point of the laser beam with themolecular flow of gas or vapour–phase precursors (Fig. 1).

The pre-requisite for energy coupling into the system, leading to moleculardecomposition, is that at least one of the precursors absorbs through aresonant vibrational mode the infrared (IR) CO2 laser radiation tuned at about10mm .

Alternatively, an inert photo-sensitizer is added to the vapour phase mixture.The high power of the CO2 laser induces the sequential absorption of several IRphotons in the same molecule, followed by collision assisted energy pooling

leading to a rapid increase in the average temperature in the gas through V-T(vibration-translation) energy transfer processes, often accompanied by theappearance of a flame in the interaction volume.

If the molecules are excited above the dissociation threshold, moleculardecomposition, eventually followed by chemical reactions, occurs with theformation of condensable and/or volatile products.

Compared with other vapor-phase synthesis methods, it is fairly evident that

laser pyrolysis permits highly localized and fast heating (leading to rapidnucleation) in a volume that can be limited to a few hundred mm3, followed byfast quenching of the particle growth (in a few ms). As a result, nanoparticles with average size ranging from 5 to 30 nm and narrow size distribution areformed in the hot region. Unavoidable agglomeration, however, occurs whenthe nanoparticles leave the high temperature region since coalescence becomesmuch slower than coagulation.



The ENEA set-up for the production of nanopowders by CO2 laser pyrolysis isshown in Fig. 1 and Fig. 2. The CO2 laser beam (l=10.6 mm) is focused by acylindrical lens at the centre of the reaction chamber where it orthogonallyintersects the reactant gas flow.

The maximum laser power is 1.2 kW and the laser beam intensity in the focalregion can be varied in the range 2-4 kW/cm2. Reactant gases enter thechamber through the inner tube of a coaxial stainless steel nozzle. An inert gas(helium or argon) flows through the outer tube with the purpose of confiningand cooling the particles. The pressure in the reaction chamber is keptconstant by a throttle valve and measured by a capacitance gauge. Typical cellpressures are in the range 300-700 Torr.

In the laser pyrolysis process, the reactants are most often in the gas-phase,however in some cases, liquid precursors are either the only choice or the mostadvantageous from an economical point of view. The use of liquid precursors ismade possible by bubbling the inert gas, or one of the gas-phase reactants,through a heated jar containing the liquid precursor to carry out its vapor intothe chamber.

After leaving the hot reaction zone, the produced particles are driven by the gas

flow through a chimney into a removable bag, located in a tank between thereaction chamber and the vacuum pump.

http://www.enea.it/it/produzione-scientifica/energia-ambiente-e-innovazione-1/anno-2011/n.%204-5%202011%20Luglio-ottobre2011/Bacteria-endosimbionts-a-source-of-innovation-in-biotechnology-for-vector-borne-diseases-control



Laser Pyrolysis Application The technique of laser pyrolysis of gas-phase reactants

has been applied to the synthesis of a large variety ofoxide (TiO2, SiO2, Al2O3, Fe2O2) and non-oxide (Si,SiC, Si3N3, Si/C/N, Si/Ti/C, MoS2) nanopowders.



Ultrasonic-Spray Pyrolysis

Ultrasonic-spray pyrolysis to deposit ZnO on top of glass substrate as

Transparent Conductive Oxide (TCO) glass for solar cell application

Source: W. Widiyastuti, Adhi Setiawan, Sugeng Winardi, TantularNurtono, Suci Madhania, Diah Susanti, “The influence of Al dopant

precursors on the characteristics of ZnO fine particles prepared byultrasonic spray pyrolysis”, Procedia Engineering 50 ( 2012 ) 152 – 158



Atomic Layer Deposition (ALD) Atomic Layer Deposition (ALD) is a true

nanotechnology, allowing ultra-thinfilms of a few nanometres to bedeposited in a precisely controlled way.

There are two defining characteristics of

Atomic Layer Deposition ALD: Self-limiting atomic layer-by-layer

growth

Highly conformal coating

These characteristics offer many benefits insemiconductor engineering, MEMS(micro-electromechanical systems) andother nanotechnology applications.



The benefits of ALD

As the ALD process deposits precisely one atomic layer in

each cycle, complete control over the deposition process isobtained at the nanometre scale

Conformal coating can be achieved even in high aspectratio and complex structures

Pin-hole and particle free deposition is achieved

A very wide variety of materials is possible with AtomicLayer Deposition ALD:

Oxides, including Al2O3 HfO2, HfSiO, La2O3, SiO2, STO,

Ta2O5, TiO2, ZnO Nitrides, including AlN, HfN, SiNx, TaN, TiN

Metals, including Cu, Pt, Ru, W



ALD cycle for Al2O 3 deposited using TMA (trimethylaluminium) and O2 plasma.

Only step 3 varies between H2O for the thermal process or O2 plasma:

A. TMA chemisorbtion B. TMA purge C. O 2 plasma D. Short post plasma purge



Colloidal Process: Sol-Gel Sols and gels are two forms of colloids. A sol is a colloidal

suspension of solid particles in a liquid, whereas a gel is acolloidal suspension of liquid droplets in a solid. A sol-gel process involves a transition from an ‘aqueous’ sol form

to a ‘solid’ gel form. In the sol-gel process, the precursors(starting compounds) for the preparation of a colloid consist of ametal or metalloid element surrounded by various ligands. For

example, common precursors for tungsten oxide includeinorganic salts, such as WCl6 and organic compounds such as

W(OC2H5). The latter are called alkoxides and are the class ofprecursors most widely used in sol-gel research.

An alkoxide can be prepared by reacting an inorganic salt withan alcohol. For example, W(OC

2H

5) can be obtained by reacting

the inorganic salt WCl6 with ethanol. An acid or base catalyst such as NH4OH, HNO3 or H2SO4 is

usually added to influence the hydrolysis and condensation ratesand the structure of the condensed product.



Most gels are amorphous, even after drying, but manycrystallize when heated. Therefore high temperaturecalcination and hydrothermal processes are frequently

used to obtain crystalline nanomaterials after a sol-gelprocess.

Calcination is a thermal treatment process that affectsthermal decomposition, phase transitions, structureformation, and removal of volatile materials.

Hydrothermal processes utilize pressurized steam tocrystallize and form the desired shape, size and structure ofthe nanomaterial. It works at relatively low temperatures, issimple to operate and results in high sample homogeneity

and quality. However it takes a longer time to carry out theprocess than calcination does.

Recently hydrothermal processes carried out in amicrowave device have helped in reducing the time

without reducing the quality of the synthesized material



Vapor Transport Method: ZnO growth

Source: Ting-Jen Hsueh, Yi-Wen Chen, Shoou-Jinn Chang, Sea-Fue Wang ,Cheng-Liang Hsu, Yan-Ru Lin, Tzer-Shen Lin, I-Cherng Chen , “ ZnO nanowire-based CO sensors prepared on patterned ZnO:Ga/SiO2/Si templates” Sensorsand Actuators B 125 (2007) 498–503



Another way is to group them according to the

form of products:

1. Nanoparticles by means of colloidal processing,flame combustion and phase segregation

2. Nanorods or nanowires by template-basedelectroplating, solution-liquid-solid (SLS) and

spontaneous anisotropic growth.3. Thin film by molecular beam epitaxy (MBE) and

atomic layer deposition.



Obviously there are two approaches to synthesis ofnanomaterials and the fabrication of nanostructures:

1. TOP-DOWN, example: attrition or milling. Thebiggest problem is the imperfection of the surfacestructure.

2. BOTTOM-UP, example: colloidal process, ALD

3. COMBINATION (HYBRID) OF BOTH, example:lithography etching process is TOP-DOWN whereas thin film growth is BOTTOM-UP



Lithography Technique

SiO2/Si

PMGI resist

SiO2/Si

PMGI resistphotoresist AZ6112

SiO2/Si

PMGI resist PR AZ6112

UV light

photomask

SiO2/Si

SiO2/SiSiO2/Si

RuO2 NR

Au TiO2/Ti

(a) (b) (c)

(d) (e) (f)

PMGI resist coating and

prebaking at 185oC

imaging resist coating

and prebaking at 105oC

UV light exposing to

create patterns

imaging resist and PMGI

developing, followed by

hardbaking at 120oC

successive depositions of

Au, Ti (partially oxidized

into TiO2), and RuO2NR

resists removal, leaving

desired patterned

RuO2NR/TiO2/Ti/Au/SiO2/

Si stacks

1 mm

40 mm0.2 mm

2.4 mm0.5 mm

4 mm

3 mm

1 mm

1 mm

5 mm

5 mm

Interdigital (finger-like) electrodewith the spacing between each

finger is 40 μm. Steps in preparing the sample



(a) (b)

(c) (d)

1 mm 1 mm

1 mm 1 mm

SEM images of finger-like electrodes of 0.2 mm in

width and 20 µm in finger spacing with low

magnifications: (a) 50x, (b) 100x, and (c) 250x (d)

500x.



200 nm

200 nm

500 nm

500 nm

(a) (b)

(c) (d)

100 m 200 m

(e)(f)

SEM images of finger-likeelectrodes (a) top view ofRuO2NR-F, (b) cross-sectional

view of RuO2NR-F, (c) top view ofRuO2NR-FH and (d) cross-

sectional view of RuO2NR-FHafter pulse deposition, (e) cleanpatterns after pulse deposition,and (f ) destroyed patterns afterCV deposition.

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Ch 2 Nanomaterials Synthesis