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Paul O’Brien
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1975 – Liverpool University
1978 – PhD, University of Wales, Cardiff
1978 – Appointed lecturer at Chelsea College of Science and Technology
1984 – Queen Mary and Westfield College lecturer
1994 – Promoted to chair
1995 – Professor of Inorganic Chemistry, Imperial College
1997-1998 - Royal Society Amersham International Research Fellow
1999 - Professor of Inorganic Materials Chemistry at University of Manchester
2001-2002 - Research Dean in the Faculty of Science and Engineering at
University of Manchester
2002 – Founded Nanoco Ltd to commercialize quantum dot synthesis
Presently, Professor of Inorg. Mat. Chem., Head of School of Chemistry at
University of Manchester
Research Interests
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Novel synthetic routes to chalcogenide materials
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thin films
quantum dots
Interest: semiconductor properties
Applications:
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Solar Cells
Infrared detectors
Photoconductors
Thermoelectric generators and coolers
LEDs
Chalcogenides
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Chalcogenide refers to a Group VI elements S, Se,
Te, or an alloy containing S, Se, Te.
O’Brien has explored chalcogenides of Cu, Pb,
Cd, Ga, In, Bi, Sb.
CdTe/CdS junction: a low cost alternative to
silicon in photovoltaic cells
CdS Thin Films
CdS Thin Film Synthesis and Deposition
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Previously:
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New method:
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Thin films deposited using metal alkyls
O’Brien, Khan, and Frigo used Cd(Et2dtc)2
at T = 370oC as single-source precursors
Single-source precursor: Cd(Et2mtc)2
Benefits:
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Lower deposition temperature
Higher deposition rate
Avoidance of metal alkyls and H2S
CdS Thin Film: Synthesis of Precursor
1. COS + Et2NH (10oC)  (Et2mtc)2Et2N+
2. (Et2mtc)2Et2N+ + Cd(CO2CH3)  white
precipitate
3. Recrystallized to give colorless needles of
Cd(Et2mtc)2
CdS Thin Film: Deposition
LP-MOCVD
CdS Thin Film: Deposition
LP-MOCVD
 Substrate: GaAs(100) or borosilicate glass
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Cd(Et2mtc)2 volatilized at 150oC
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Decomposed to CdS thin film on substrate at
temperatures as low as 300oC
CdS Thin Film
CdS Thin Film
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Band Gap = 2.39 eV (2.42 eV)
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Deposition Temperature (300oC v. 370oC)
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Deposition rate (1.06mmh-1 v. 0.20mmh-1)
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Major decomposition product =
Et2NC(O)SC(O)NEt2
Cd Alternatives in Thin Films
Drive to replace Cd in thin films of solar cells:
 Cd = toxic heavy metal
Alternatives:
 CdTe  Cu(In/Ga)E2 (E = S,Se)
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Single – source asymmetrically substituted
precursor
CuInS2, CuInSe2, CuGaS2
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Precursor synthesis
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CS2 or CSe2 + NaOH + N-MHN  solution
MxSO4/MxCl + solution + (solvent at T)  (1,2, 3, or 4)
M
SO4/Cl
T (oC)
Solvent
Product
Cu
SO4-2
0
MeOH
Cu(S2CNMenHex)2 (1)
In
Cl-
0
MeOH
In(S2CNMenHex)3 (2)
Cu
Cl-
-10
H2 O
Cu(Se2CNMenHex)2 (3)
In
Cl-
-20
H2O
In(Se2CNMenHex)3 (4)
CuInS2, CuInSe2, CuGaS2
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Precursor synthesis Ga(S2CNMenHex)3 (5)
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Na(S2CNMenHex) (dry benzene) + GaCl3 (hexane) 
Ga(S2CNMenHex)3
Deposition of CuIn(S,Se)2, GaInS2 Thin Films
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LP-MOCVD
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P = 10-2 Torr
Graphite susceptor
100mg stoichiometrically (1:1) mixed precursors
Films deposited on various substrates
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glass
ITO glass
InP(100)
GaAs(100)
InP(111)
Si(111)
Deposition of CuIn(S,Se)2, GaInS Thin Films
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AACVD
CuInS2 thin films by LP-MOCVD
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1:1 mixture of 1 and 2
Optimum temperatures:
 Tpre > 220oC (250oC); Tsubs >430oC (450oC)
Tsubs (oC) t (hr) mm Color
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450-470 < 1
5
Dark yellow
480-500 > 2
8
Dark black
Band Gap: 1.41 eV (1.5 eV)
Oriented Growth – InP(100)
a. glass; b. ITO glass; c.InP(100); d.GaAs(100); e.InP(111); f.Si(111)
CuInS2 thin films by AACVD
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1:1 mix of 1 and 2
Lower Tsub: 350oC
Morphology different than LP-MOCVD
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Thinner flakes (0.2mm v. 1mm)
Horizontal
After 2 hr. 1mm thick film
CuInS2 thin films
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On InP(100), 112 peak missing
CuInSe2 by LP-MOCVD
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1:1 ratio of 3 and 4
Tpre = 180 –250oC; Tsub = 400-450oC
Growth rate = 1 mmh-1
Band Gap = 1.08 eV (1.0-1.1 eV)
No oriented growth
Morphology ITO coated glass and Si(100) more homogeneous
CuInSe2 thin films by AACVD
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1:1 ratio of 3 and 4
T = 425-475oC
Several different morphologies
CuInSe2 thin films
CuGaS2 thin film by LP-MOCVD, AACVD
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1:1 ratio of 1 and 5
Tpre = 250oC; Tsub = 500oC LP-MOCVD
T = 400-450oC
CuGaS2 thin film
CuIn(S,Se)2, GaInS Thin Films
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Conclusions:
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M(S2/Se2CNRR’)2 = good precursors for CVD
AACVD and LP-MOCVD resulted in stoichiometric CuME2 films
Morphology effected by experimental parameters
XRD patterns similar for AACVD prepared films regardless of
deposited materials
Compound
Band
Gap
T (oC)
mm
Growth
Rate (mm/h)
EDX
LP-MOCVD
CuInS2
1.41
450
5
5.0
1:1:2
AACVD
CuInS2
nr
350
1
0.5
1:1:2
LP-MOCVD
CuInSe2
1.08
450
2
1.0
1:1:2
AACVD
CuInSe2
nr
450
nr
nr
nr
LP-MOCVD
GaInS2
nr
500
nr
nr
nr
AACVD
GaInS2
nr
450
1
1.5
Cu 30% Ga 24%
S46%
Chalcogenide Quantum Dots
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Bulk: band gap specific to chemical composition
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Quantum dots: band gap tuned by altering size
Chalcogenide Quantum Dots
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Previous Synthetic Methods:
1. Aqueous solution
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Air sensitivity
2. Growth within host material
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Removal of host material
3. Anaerobic preparation using organometallics
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Hazardous, toxic, pyrophoric conditions
Chalcogenide Quantum Dots
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New Method: Single molecular precursor
Advantages:
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Avoid hazardous precursors
Only one non-volatile precursor involved
New synthetic routes may lead to unique properties
Chalcogenide Quantum Dots
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Precursor:
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(Cd/Zn)[R2(dtc/dsc)]2
Growth:
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Precursor decomposed in a high boiling point
coordinating solvent, TOPO
Chalcogenide Quantum Dots
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Synthesis of Precursor
Chalcogenide Quantum Dots
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“On-pot” synthesis of nanoparticles
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Cd(S2CNMenHex)2 dissolved in TOP
Injected into hot TOPO/TOP >200oC
Chalcogenide Quantum Dots
Chalcogenide Quantum Dots
Chalcogenide Quantum Dots
Fig.1 XRD of CdSe
Fig.2 XRD of CdS
Chalcogenide Quantum Dots
QD BG
Bulk BG
Particle Size (Å)
CdS
2.51
2.42
53-59
CdSe
2.02
1.73
54-59
ZnS
nr
nr
nr
ZnSe
3.58
2.58
35-42
References
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Paul O’Brien Materials Chemistry Group http://people.man.ac.uk/~mbdsspo2/
Crouch, David; Norager, Sebastian; O’Brien, Paul; Park, Jin-Ho; Pickett, Nigel. New Synthetic Routes
For Quantum Dots. Phil. Trans. R. Soc. Lond. A (2003) 361, 297-310.
Chunggaze, M.; Malik, M. Azad; O'Brien, P.. Deposition of cadmium sulfide thin films from the
single-source precursor bis(diethylmonothiocarbamato)cadmium(II) by low-pressure metalorganic
chemical vapor deposition. Advanced Materials for Optics and Electronics (1997), 7(6), 311-316.
O’Brien, Paul; Boyle, David S.; Govender, Kuveshni. Developing Cadmium-free Window Layers for
Solar Cell Applications: Some Factors Controlling the Growth and Morphology of B-Indium Sulfide
Thin Films and Related (In,Zn)S Ternaries. J. Mater.Chem (2003), 13, 2242-2247.
Pickett, Nigel L; O’Brien, Paul. Synthesis of Semiconductor Nanoparticles Using Single-Molecular
Precursors. The Chemical Record. (2001), 1, 467-479.
Crowell, John E. Chemical Methods of Thin Film Deposition: Chemical Deposition: Chemical Vapor
Deposition, Atomic Layer Deposition, and Related Technologies. Journal of Vacuum Science &
Technology A: Vacuum, Surfaces, and Films. (2003), 21(5), S88-S95.
Frigo, D.M.; Khan, O.F.Z.; O’Brien, P. J. Cryst. Growth, 1989, 96, 989-992.
Kodas and Hampden-Smith. Aerosol Process of Materials. 1999.
Ludolph, B.; Malik, M. O’Brien, P., Revaprasadu, N. A Novel single molecule precursor routes for the
direct synthesis of highly monodispersed quantum dots of cadmium or zinc sulfide or selenide. Chem.
Commun. 1998, 1849