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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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a brief history of CD
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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a brief history of CD
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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a brief history of CD
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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What is chemical deposition?
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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What is chemical deposition?
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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What is chemical deposition?
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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What is chemical deposition?
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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What is chemical deposition?
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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What is chemical deposition?
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