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Paul O’BrienPaul O’Brien
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
Novel synthetic routes to chalcogenide materials thin films quantum dots
Interest: semiconductor properties Applications:
Solar Cells Infrared detectors Photoconductors Thermoelectric generators and coolers LEDs
Chalcogenides
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 Film Synthesis and Deposition
Previously: Thin films deposited using metal alkyls O’Brien, Khan, and Frigo used Cd(Et2dtc)2
at T = 370oC as single-source precursors
New method: Single-source precursor: Cd(Et2mtc)2
Benefits: 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 Substrate: GaAs(100) or borosilicate glass
Cd(Et2mtc)2 volatilized at 150oC
Decomposed to CdS thin film on substrate at temperatures as low as 300oC
CdS Thin Film
Band Gap = 2.39 eV (2.42 eV)
Deposition Temperature (300oC v. 370oC)
Deposition rate (1.06mh-1 v. 0.20mh-1)
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)
Single – source asymmetrically substituted precursor
CuInS2, CuInSe2, CuGaS2
Precursor synthesis 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 H2O Cu(Se2CNMenHex)2 (3)
In Cl- -20 H2O In(Se2CNMenHex)3 (4)
CuInS2, CuInSe2, CuGaS2
Precursor synthesis Ga(S2CNMenHex)3 (5) Na(S2CNMenHex) (dry benzene) + GaCl3 (hexane)
Ga(S2CNMenHex)3
Deposition of CuIn(S,Se)2, GaInS2 Thin Films
LP-MOCVD P = 10-2 Torr Graphite susceptor 100mg stoichiometrically (1:1) mixed precursors Films deposited on various substrates
glass ITO glass InP(100) GaAs(100) InP(111) Si(111)
CuInS2 thin films by LP-MOCVD 1:1 mixture of 1 and 2 Optimum temperatures:
Tpre > 220oC (250oC); Tsubs >430oC (450oC)
Band Gap: 1.41 eV (1.5 eV) Oriented Growth – InP(100)
Tsubs (oC) t (hr) m Color
450-470 < 1 5 Dark yellow
480-500 > 2 8 Dark black
a. glass; b. ITO glass; c.InP(100); d.GaAs(100); e.InP(111); f.Si(111)
CuInS2 thin films by AACVD
1:1 mix of 1 and 2 Lower Tsub: 350oC Morphology different than LP-MOCVD
Thinner flakes (0.2m v. 1m) Horizontal
After 2 hr. 1m thick film
CuInSe2 by LP-MOCVD
1:1 ratio of 3 and 4 Tpre = 180 –250oC; Tsub = 400-450oC
Growth rate = 1 mh-1
Band Gap = 1.08 eV (1.0-1.1 eV) No oriented growth Morphology ITO coated glass and Si(100) more homogeneous
CuGaS2 thin film by LP-MOCVD, AACVD
1:1 ratio of 1 and 5 Tpre = 250oC; Tsub = 500oC LP-MOCVD T = 400-450oC
CuIn(S,Se)2, GaInS Thin Films
Conclusions: 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) m Growth Rate (m/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
Bulk: band gap specific to chemical composition
Quantum dots: band gap tuned by altering size
Chalcogenide Quantum Dots
Previous Synthetic Methods:1. Aqueous solution
Air sensitivity
2. Growth within host material Removal of host material
3. Anaerobic preparation using organometallics Hazardous, toxic, pyrophoric conditions
Chalcogenide Quantum Dots
New Method: Single molecular precursor Advantages:
Avoid hazardous precursors Only one non-volatile precursor involved New synthetic routes may lead to unique properties
Chalcogenide Quantum Dots
Precursor: (Cd/Zn)[R2(dtc/dsc)]2
Growth: Precursor decomposed in a high boiling point
coordinating solvent, TOPO
Chalcogenide Quantum Dots
“On-pot” synthesis of nanoparticles Cd(S2CNMenHex)2 dissolved in TOP Injected into hot TOPO/TOP >200oC
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 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