Emulsion and Hydrothermal

8/11/2019 Emulsion and Hydrothermal

1/59

Synthesis ofNanoparticles


2/59

Hydrothermal/Solvothermal SynthesisIn a sealed vessel (bomb, autoclave, etc.), solvents can be broughtto temperatures well above their boiling points by the increase in

autogenous pressures resulting from heating. Performing achemical reaction under such conditions is referred to assolvothermal processing or, in the case of water as solvent,hydrothermal processing.

Yu, J. C. et al. J. Solid State Chem. 2005 , 178, 321; Cryst. Growth Des. 2007 , 7 , 1444

TiO2 ZnIn 2S4


3/59

Microwave-Assisted Synthesis

Microwaves are a form of electromagnetic energywith frequencies in the range of 300 MHz to 300

GHz. The commonly used frequency is 2.45G Hz.

Interactions between materials and microwavesare based on two specific mechanisms: dipoleinteractions and ionic conduction. Bothmechanisms require effective coupling betweencomponents of the target material and the rapidlyoscillating electrical field of the microwaves.

Dipole interactions occur with polar molecules. Thepolar ends of a molecule tend to re-orientatethemselves and oscillate in step with the oscillatingelectrical field of the microwaves. Heat isgenerated by molecular collision and friction.Generally, the more polar a molecule, the moreeffectively it will couple with the microwave field.


4/59

Conventional Heating by Conduction

conductive heat

heating byconvection currents

slow and energyinefficient process

The temperature on the outside surface is

in excess of the boiling point of liquid


5/59

Heating by Microwave Irradiation

inverted temperature gradients !

Solvent/reagentabsorbs MW energy

Vessel walltransparent to MW

Direct in-core heating

Instant on-off


6/59

Microwave (MW) rapid heating hasreceived considerable attention as a newpromising method for the one-potsynthesis of metallic nanostructures insolutions.

In this concept, advantageous applicationof this method has been demonstrated byusing some typical examples for thepreparation of Ag, Au, Pt, and AuPdnanostructures. Not only sphericalnanoparticles, but also single crystalline

polygonal plates, sheets, rods, wires,tubes, and dendrites were preparedwithin a few minutes under MW heating .Morphologies and sizes of nanostructurescould be controlled by changing variousexperimental parameters, such as theconcentration of metallic salt andsurfactant polymer, the chain length ofthe surfactant polymer, the solvent, andthe reaction temperature. In general,nanostructures with smaller sizes,narrower size distributions, and a higherdegree of crystallization were obtained

under MW heating than those inconventional oil-bath heating.

Tsuji M. et al.


7/59

Example: Microwave-assisted synthesis of ZnO nanoparticles

OH 2

ZnH2O

OAcOAc

O

ZnO

O

O

ZnO

OODEG

Microwave ( )Nucleation Aggregation

Cluster NanocrystalCrystal structure

Schematic representation and transmission electron microscope (TEM) imagesof ZnO-cluster nanoparticles prepared by microwave irradiation

Yu, J. C. et at., Adv. Mater . 2008, in press.

1 mm 100 nm


8/59

Sonochemical Synthesis

Ultrasound irradiation causes acoustic cavitation -- the formation,growth and implosive collapse of the bubbles in a liquid

The implosive collapse of the bubbles generates a localized hot

spots of extremely high temperature (~5000K) and pressure(~20MPa).

The sonochemical method is advantageous as it is nonhazardous,rapid in reaction rate, and produces very small metalparticles.


9/59

Examples: sonochemical synthesis of mesoporous TiO 2 particles

Mesoporous TiO 2

20 kHz sonochemical processor


10/59

Formation of mesoporous TiO 2 by sonication

UIA

TIP

Hydrolysis/Condensation

))))

UIA: Ultrasound Induced Agglomeration

UIA

Titanium Oxide Sol Particle

UIAAcetic acid modified TIP

Hydrolysis/

Condensation))))

TIP : Titanium isopropoxide

Yu J. C. et al., Chem. Commun. 2003 , 2078.


11/59

MicroemulsionMicroemulsions are clear, stable, isotropic liquidmixtures of oil, water and surfactant, frequently in

combination with a cosurfactant.The aqueous phase may contain salt(s) and/or otheringredients, and the "oil" may actually be a complexmixture of different hydrocarbons and olefins.

The two basic types of microemulsions are direct (oildispersed in water, o/w) and reversed (water dispersedin oil, w/o).

Nanosized CdS-sensitized TiO 2 crystalline photocatalyst prepared by microemulsion.(Yu, J. C. et al. Chem. Commun . 2003 , 1552.)


12/59

What happens at the surfaces?

The physical properties of molecules at the interface or surface are different

than those of the molecules in bulk.

Surface molecules have surface free energy (unbalanced)

Bulk molecules very stable


13/59

Interface

When two phases meet, the boundary between them is calledan interface.

Surface or interfacial tension can be defined as the force

needed to oppose the natural pull of the molecules in the

surface or interface to minimize the size of that surface or

interface.


14/59


15/59

Cohesion and Adhesion forces

Cohesion Force hold two similar molecules together

Adhesion Force hold two different molecules together

The difference in strength between cohesive forces and adhesive forces

determine the behavior of a liquid in contact with a solid surface.


16/59

The cohesive (strong intermolecular attractive forces)

forces between liquid molecules are responsible for

the phenomenon known as surface tension.

The surface tension of a liquid results from an

imbalance of intermolecular attractive forces,

the cohesive forces between molecules.


17/59

Water does not wet waxed surfaces because the cohesive

forces within the drops are stronger than the adhesive forces

between the drops and the wax.

Water wets glass and spreads out on it because the adhesive

forces between the liquid and the glass are stronger than the

cohesive forces within the water.


18/59

Different Interfacial phases

Phases in contactExamples fromcommon use

Gas - Gas No interface possible

Gas - liquidSurface of your drink (foams

and aerosels)

Gas - SolidTop of your desk ( tabletsand capsules)

Liquid - liquidOil and Vinegar Saladdressing (emulsion)

Liquid - solid Water on lotus leaf

Solid - SolidPowder particle are incontact


19/59

Contact Angle


20/59

Factors affecting surface tension

The surface energy may altered by the addition

of solutes that migrate to the surface and

modify the molecular forces there.

The surface tension decrease with increase

temperature


21/59

Surfactants

A surfactant is briefly defined as a material that can greatly reduce the

surface tension of water when used in very low concentrations.


22/59

Introduction to surfactants

The name surfactant refers to molecules that are surface -active, usually in

aqueous solutions. Surface-active molecules adsorb strongly at the water air

interface and, because of this, they substantially reduce its surface energy.

Surfactant molecules are amphiphilic , that is, they have both hydrophi l ic and

hydrophobic moieties , and it is for this reason that they adsorb so effectively at

interfaces.


23/59


24/59

How it works? Surfactants reduce the surface tension of water by adsorbing at the liquid-

gas interface. They also reduce the interfacial tension between oil and water by adsorbing

at the liquid-liquid interface.

Surfactants may act as: detergents, wetting agents, emulsifiers, foamingagents, and dispersants.


25/59

Surface activity

Materials such as short-chain fatty acids and alcohols are soluble in both water and

oil (e.g. paraffin hydrocarbon) solvents. The hydrocarbon part of the molecule is

responsible for its solubility in oil, while the polar COOH or -OH group has

sufficient affinity to water to drag a short-length non-polar hydrocarbon chain intoaqueous solution with it.

If these molecules become located at an air-water or an oil-water interface, they are

able to locate their hydrophilic head groups in the aqueous phase and allow the

lipophilic hydrocarbon chains to escape into the vapour or oil phase


26/59

Adsorption of surface-active molecules as an orientated monolayerat air-water and oil-water interfaces.


27/59

The strong adsorption of such materials at surfaces or interfaces in the form of an

orientated monomolecular layer (or monolayer) is termed sur face activity .

Surface-active materials (or surfactants) consist of molecules containing both polar and

non-polar parts (amphiphilic).


28/59

Classification

Dependent on the nature of the hydrophilic part the surfactants are

classified as an-ionic, non-ionic, cat-ionic or amphoteric.


29/59


30/59

Nonionic surfactants

A surfactant with a non-charged hydrophilic part, e.g. ethoxylate, is non-

ionic.

These substances are well suited for cleaning purposes and are not sensitive

to water hardness.

They have a wide application within cleaning detergents and include

groups like fatty alcohol polyglycosides, alcohol ethoxylates etc.


31/59

Cationic surfactants

For this category the hydrophilic part is positively charged e.g. with a

quaternary ammonium ion.

This group has no wash activity effect, but fastens to the surfaces where

they might provide softening, antistatic, soil repellent, anti bacterial or

corrosion inhibitory effects.

The most typical applications are for softeners and antistatics.


32/59

Chemical Vapor

Deposition (CVD)


33/59

Family of CVD Technologies


34/59

What is CVD? Chemical vapor deposition (CVD) is a

chemical process used to produce high-purity, high-performance solid materials. Theprocess is often used in the semiconductorindustry to produce thin films . In a typicalCVD process, the wafer (substrate) isexposed to one or more volatile precursors ,which react and/or decompose on the

substrate surface to produce the desireddeposit. Frequently, volatile by-products arealso produced, which are removed by gasflow through the reaction chamber.
http://en.wikipedia.org/wiki/Chemical_processhttp://en.wikipedia.org/wiki/Semiconductor_industryhttp://en.wikipedia.org/wiki/Semiconductor_industryhttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Wafer_(electronics)http://en.wikipedia.org/wiki/Volatility_(chemistry)http://localhost/var/www/apps/conversion/tmp/scratch_10//en.wiktionary.org/wiki/precursorhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Chemical_decompositionhttp://en.wikipedia.org/wiki/By-producthttp://en.wikipedia.org/wiki/By-producthttp://en.wikipedia.org/wiki/By-producthttp://en.wikipedia.org/wiki/By-producthttp://en.wikipedia.org/wiki/Chemical_decompositionhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://localhost/var/www/apps/conversion/tmp/scratch_10//en.wiktionary.org/wiki/precursorhttp://en.wikipedia.org/wiki/Volatility_(chemistry)http://en.wikipedia.org/wiki/Wafer_(electronics)http://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Thin_filmhttp://en.wikipedia.org/wiki/Semiconductor_industryhttp://en.wikipedia.org/wiki/Semiconductor_industryhttp://en.wikipedia.org/wiki/Semiconductor_industryhttp://en.wikipedia.org/wiki/Chemical_processhttp://en.wikipedia.org/wiki/Chemical_processhttp://en.wikipedia.org/wiki/Chemical_process


35/59

CVD Process Applications Microfabrication processes widely use CVD

to deposit materials in various forms,including: monocrystalline , polycrystalline ,amorphous , and epitaxial . These materialsinclude: silicon , carbon fiber , carbonnanofibers , filaments , carbon nanotubes ,SiO 2, silicon-germanium , tungsten , siliconcarbide , silicon nitride , silicon oxynitride ,

titanium nitride , and various high-k dielectrics .The CVD process is also used to producesynthetic diamonds .
http://en.wikipedia.org/wiki/Microfabricationhttp://en.wikipedia.org/wiki/Monocrystallinehttp://en.wikipedia.org/wiki/Polycrystallinehttp://en.wikipedia.org/wiki/Amorphoushttp://en.wikipedia.org/wiki/Epitaxyhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Carbon_nanofibershttp://en.wikipedia.org/wiki/Carbon_nanofibershttp://en.wikipedia.org/wiki/Electrical_filamenthttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Silicon-germaniumhttp://en.wikipedia.org/wiki/Tungstenhttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_oxynitridehttp://en.wikipedia.org/wiki/Titanium_nitridehttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/Synthetic_diamondhttp://en.wikipedia.org/wiki/Synthetic_diamondhttp://en.wikipedia.org/wiki/Synthetic_diamondhttp://en.wikipedia.org/wiki/Synthetic_diamondhttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/High-k_dielectrichttp://en.wikipedia.org/wiki/Titanium_nitridehttp://en.wikipedia.org/wiki/Titanium_nitridehttp://en.wikipedia.org/wiki/Titanium_nitridehttp://en.wikipedia.org/wiki/Silicon_oxynitridehttp://en.wikipedia.org/wiki/Silicon_oxynitridehttp://en.wikipedia.org/wiki/Silicon_oxynitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Tungstenhttp://en.wikipedia.org/wiki/Silicon-germaniumhttp://en.wikipedia.org/wiki/Silicon-germaniumhttp://en.wikipedia.org/wiki/Silicon-germaniumhttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Electrical_filamenthttp://en.wikipedia.org/wiki/Carbon_nanofibershttp://en.wikipedia.org/wiki/Carbon_nanofibershttp://en.wikipedia.org/wiki/Carbon_nanofibershttp://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Epitaxyhttp://en.wikipedia.org/wiki/Amorphoushttp://en.wikipedia.org/wiki/Polycrystallinehttp://en.wikipedia.org/wiki/Monocrystallinehttp://en.wikipedia.org/wiki/Microfabrication


36/59


37/59

Chemical Vapor Deposition

CVD => Chemical Vapor Deposition PE-CVD => Plasma Enhanced CVD MO-CVD => Metal Organic CVD Atmospheric pressure CVD (AP-CVD) Low-pressure CVD (LP-CVD) Ultrahigh vacuum CVD (UHV-CVD) Aerosol assisted CVD (AA-CVD) Direct liquid injection CVD (DLICVD)


38/59


39/59

Horizontal APCVD Reactor

CVD Reactors


40/59

Thermal CVD Reactor

Chemical Vapor Deposition Apparatushttp: / /en.wikipedia.org/wiki/Chemical_vapor_deposit ion


41/59

Epitaxy is a kind of interface between a thin film and asubstrate. The term epitaxy (Greek; epi "above" and taxis "inordered manner") describes an ordered crystalline growth ona monocrystalline substrate. Epitaxial films may be grownfrom gaseous or liquid precursors. Because the substrate

acts as a seed crystal, the deposited film takes on a latticestructure and orientation identical to those of the substrate.This is different from other thin-film deposition methodswhich deposit polycrystalline or amorphous films, even on

single-crystal substrates. If a film is deposited on a substrateof the same composition, the process is called homoepitaxy;otherwise it is called heteroepitaxy.

Epitaxy


42/59

This technology is quite similar to what happens inCVD processes, however, if the substrate is anordered semiconductor crystal (i.e. silicon, galliumarsenide), it is possible with this process to continuebuilding on the substrate with the samecrystallographic orientation with the substrate actingas a seed for the deposition. If anamorphous/polycrystalline substrate surface is used,the film will also be amorphous or polycrystalline.


43/59

Homoepitaxy is a kind of epitaxy performed withonly one material. In omoepitaxy, a crystalline film isgrown on a substrate or film of the same material.This technology is applied to growing a more purifiedfilm than the substrate and fabricating layers withdifferent doping levels.

Heteroepitaxy is a kind of epitaxy performed with materialsthat are different from each other. In heteroepitaxy, acrystalline film grows on a crystalline substrate or film ofanother material. This technology is often applied to growing

crystalline films of materials of which single crystals cannot beobtained and to fabricating integrated crystalline layers ofdifferent materials. Examples include gallium nitride (GaN) onsapphire or aluminium gallium indium phosphide (AlGaInP) on

gallium arsenide (GaAs).


44/59

Methods

Epitaxial silicon is usually grown using VaporPhase Epitaxy (VPE). A modification ofChemical Vapor Deposition.

Molecular-beam and liquid-phase epitaxy(MBE and LPE) are also used, mainly forcompound semiconductors.

Metal Organic CVD (MOCVD)


45/59

Epitaxial Growth

Deposition of a layer on asubstrate which matchesthe crystalline order of thesubstrate

Homoepitaxy Growth of a layer of the same

material as the substrate Si on Si

Heteroepitaxy Growth of a layer of a

different material than thesubstrate

GaAs on Si

Ordered,crystallinegrowth;NOTepitaxial

Epitaxialgrowth:


46/59

Motivation

Epitaxial growth is useful for applications that placestringent demands on a deposited layer:

High purity Low defect density Abrupt interfaces Controlled doping profiles High repeatability and uniformity Safe, efficient operation

Can create clean, fresh surface for device fabrication


47/59

General Epitaxial DepositionRequirements

Surface preparation Clean surface needed Defects of surface duplicated in epitaxial layer Hydrogen passivation of surface with water/HF

Surface mobility High temperature required heated substrate Epitaxial temperature exists, above which deposition is ordered Species need to be able to move into correct crystallographic

location

Relatively slow growth rates result Ex. ~0.4 to 4 nm/min., SiGe on Si


48/59


49/59

Kinetics Growth rate controlled by kinetic

considerations Mass transport of reactants to surface Reactions in liquid or gas Reactions at surface Physical processes on surface

Nature and motion of step growth Controlling factor in ordering

Specific reactions depend greatly on methodemployed


50/59

Kinetics Example

Atoms can bond to flat surface, steps,or kinks.

On surface requires some critical radius Easier at steps Easiest at kinks

As-rich GaAs surface

As only forms two bonds to underlyingGa Very high energy

Reconstructs by forming As dimers Lowers energy Causes kinks and steps on surface

Results in motion of steps on surface

If start with flat surface, create steponce first group has bonded Growth continues in same way

http://www.bnl.gov/nsls2/sciOps/chemSci/growth.asp


51/59

Vapor Phase Epitaxy Specific form of chemical vapor deposition (CVD) Reactants introduced as gases Material to be deposited bound to ligands Ligands dissociate, allowing desired chemistry to reach

surface Some desorption, but most adsorbed atoms find proper

crystallographic position Example: Deposition of silicon

SiCl4 introduced with hydrogen

Forms silicon and HCl gas Alternatively, SiHCl 3, SiH2Cl2 SiH4 breaks via thermal decomposition


52/59

Precursors for VPE

Must be sufficiently volatile to allowacceptable growth rates

Heating to desired T must result in pyrolysis Less hazardous chemicals preferable

Arsine highly toxic; use t-butyl arsine instead

VPE techniques distinguished by precursorsused


53/59


54/59

Doping of Epitaxial Layers

Incorporate dopants during deposition Theoretically abrupt dopant distribution Add impurities to gas during deposition Arsine, phosphine, and diborane common

Low thermal budget results High T treatment results in diffusion of dopant

into substrate Reason abrupt distribution not perfect


55/59

Properties of Epitaxial Layer

Crystallographic structure of film reproduces that ofsubstrate

Substrate defects reproduced in epi layer

Electrical parameters of epi layer independent ofsubstrate Dopant concentration of substrate cannot be reduced Epitaxial layer with less dopant can be deposited

Epitaxial layer can be chemically purer than substrate Abrupt interfaces with appropriate methods


56/59

Applications

Engineered wafers Clean, flat layer on top of less

ideal Si substrate On top of SOI structures Ex.: Silicon on sapphire Higher purity layer on lower

quality substrate (SiC) In CMOS structures

Layers of different doping Ex. p- layer on top of p +

substrate to avoid latch-up


57/59

More applications

Bipolar Transistor Needed to produce buried

layer

III-V Devices Interface quality key Heterojunction Bipolar

Transistor LED Laser

https://reader010.{domain}/reader010/html5/0612/5b1fdddcce8e1/5b1fddf3ecc8f.jpg

http://www.search.com/reference/Bipolar_junction_transistor


58/59

VPE Advantages/Disadvantages

Low temperature process High purity (low defect density) material Readily automated for mass production Ability to grow thin layers with precise

composition, doping density, thickness O onan atomic scale for advanced systems)

Well suited to research has opened newphysics

Di d


59/59

Disadvantages

Toxic gases are used must have gas monitors andstainless steel plumbing.

The exhaust pump system includes a scrubber thatbreaks down toxic end products before atmosphericrelease.

Research systems are expensive, as are many of theprecursors (purchased as pressurized gases incylinders or as bubblers VPE works well with Si andGaAs (usually not used) and related elemental andcompound semiconductors

Documents

Emulsion and Hydrothermal