Teaching Demo - CBD

8/3/2019 Teaching Demo - CBD

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previous meeting

Molecular beam epitaxy

Metal-organic vapor phase epitaxy

Spray pyrolysis Magnetron sputtering

Photochemical deposition (PCD)

Thermal evaporation Successive ionic layer adsorption and reaction

(SILAR)


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on theChemical DepositionHananish Joy G. Odarve


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in this topic

a brief history of chemical deposition

what is chemical deposition what materials can be deposited by CD

mechanisms of the chemical deposition

substrates

exampledeposition of ZnS


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a brief history of CD

Liebig (1835): deposition of silver using a chemicalsolution technique

lsterfarben : its formation on various metals fromthiosulphate solutionsfirst reported CD but therecent studies of this process invoked an

electrochemical mechanism Beutel and Kutzelnigg : wide range of deposition from

thiosulphate solutions both CD and electrochemical


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Emerson-Reynolds (1884): deposition of PbS films,[thiourea (thiocarbamide) + alkaline lead tartrate],

the metallicsulphide . . . became firmly attached as a

specularlayer to the sides of the vessel

Wide range of substrates used for deposition


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Infrared photoconductivity in CD PbS films -

central driving force for subsequent investigations in

CD lead chalcogenide films

1961: deposition of CdS was explicitly reported 1980s:

The range of materials deposited by CDgradually extended


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CdS films by CD were shown to give superiorphotovoltaic (PV) cells compared with the previously

evaporated CdS

crystals in most as-deposited CD films are very smalland so we can make nanocrystalline films

the nanocrystals (small enough) exhibit size quantization- increase in the optical bandgap with decrease in crystalsize


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What is chemical deposition?

refers to the deposition of films on a solid substratefrom a reaction occurring in a solution (almost

always aqueous)


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Example: Cadmium Sulfide (CdS)

+

Cd salt Cd salt

S2-S2-

S2-

S2-S2-

S2- S2-

=

free sulfide ions

CdSCdS

CdS

CdS CdS CdS

CdS

CdSCdS

CdSCdS

CdS

CdS

CdS

CdS

CdS

CdSCdS


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Example: Cadmium Sulfide (CdS)

Cd-thiocomplex

Free sulfideions

Cd-thiocomplex

CdSCdS

CdS

CdS CdS CdS

CdS

CdS

CCdSCdS

CdS

CdS

Cd

CdS

CdSCdS


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TRICK!

control the rate of these reactions so that they occurslowly enough to allow the CdS to either form gradually on the substrate

diffuse there and adhere either to the substrate itself (at the earlystages of deposition) or to the growing film

we dont want it to aggregate into larger particles in

solution and precipitate out


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How do we do it?

generate the sulphide slowly in the deposition solution(reaction rate is also generated) by controlling

concentration of sulphide-forming precursor

temperature of the solution

pH of the solution


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it can be carried out in alkaline and acidic solutions for alkaline solutions, we need to prevent the

precipitation of metal hydroxides (metal ion must becomplexed)complexing agents reduced the concentration of free metal ions to prevent

rapid bulk precipitation of the desired product

not too weakto prevent bulk precipitation of OH

not too strongit may prevent deposition of desired film together


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what materials can bedeposited by CD

1. compound can be made by simple precipitation

2. compound should be relatively (and preferably highly)insoluble in the solution used

3. compound should be chemically stable in the solution

4. the reaction rate should be relatively slow either the

reaction proceeds via free anion or complex-decomposition type


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mechanisms

1. ionic reactions involving free anions

2. decomposition of metal complexes

and we can further subdivide to:

formation of isolated single molecules that cluster and

eventually form a crystal or particle

mediation of a solid phase, usually the metal hydroxide.


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mechanisms

simple ion-by-ion mechanism

simple cluster (hydroxide) mechanism

complex decomposition ion-by-ion mechanism complex decomposition cluster mechanism


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mechanisms

simple ion-by-ion mechanism


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mechanisms

If the ion product [Cd2+][S2-] exceeds the solubilityproduct, Ksp, of CdS, (10

-28), CdS will form as a solid

phase

in alkaline solution - a complex is needed to keepthe metal ion in solution and to prevent the

hydroxide from precipitating out


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mechanisms

simple cluster (hydroxide) mechanism


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mechanisms

complexation of the Cd was necessary to preventCd(OH)2 precipitation

however, its an important reaction intermediates in theCD process

if the complex concentration is not high enough to

prevent completely Cd(OH)2, there will be a smallamount of Cd(OH)2


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mechanisms

Reaction 3.5 will likely to happen the solubility product Ksp for CdS is much smaller than for

Cd(OH)2

the free energy of formation of CdS is more negative thanCd(OH)2

this cluster formation acts as a catalyst for thioureadecomposition sulphide formation will occur preferentially at the surface of

the hydroxide rather than nucleate separately in the solution


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mechanisms

mechanism may change in the course of deposition

complexate themetal

complex:metalratio increases

no solid OH- phase presentin the solution

Hydroxide cluster

ion-by-ionprocess


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substrates

films can be deposited on ANY surface obviuos exceptionsDIRTY substrates , which are

unstable in the deposition solution

adherent film on Teflon, CD has the ability to form filmson wide range of substrate

shape is not usually important, very irregulargy-shapedsubstrates can be used


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substrates

nature of the substrate rough surface, we achieve adherent film, could be due to

greater actual surface area of contact per geometricsurface

oxides (quite reactive in terms of their adsorptionproperties) due to presence of hydroxyl surface groupsthat can form strong hydrogen bonds


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exampledeposition of ZnS

ZnS is a II-VI semiconductor

Preparation of Stock solutions Preparation of Substrates

Preparation of Deposition Solution

Deposition


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variations

concentration of reactants

ratio of the concentration of metal ion with the

complexant concentrations pH of the solution

temperature


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CdS forms through a number of different possiblepathways

simple ionic reaction between Cd2+ and S2-

conversion of Cd(OH)2 to CdS by sulphide

decomposition of a complex between Cd (free ion or

compound) and the sulphide precursor

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Teaching Demo - CBD