paul o’brien

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 Films

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

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

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)

Deposition of CuIn(S,Se)2, GaInS Thin Films

AACVD

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

CuInS2 thin films

On InP(100), 112 peak missing

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

CuInSe2 thin films by AACVD

1:1 ratio of 3 and 4 T = 425-475oC

Several different morphologies

CuInSe2 thin films

CuGaS2 thin film by LP-MOCVD, AACVD

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

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


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


New Method: Single molecular precursor Advantages:

Avoid hazardous precursors Only one non-volatile precursor involved New synthetic routes may lead to unique properties


Precursor: (Cd/Zn)[R2(dtc/dsc)]2

Growth: Precursor decomposed in a high boiling point

coordinating solvent, TOPO


Synthesis of Precursor


“On-pot” synthesis of nanoparticles Cd(S2CNMenHex)2 dissolved in TOP Injected into hot TOPO/TOP >200oC


Fig.1 XRD of CdSe Fig.2 XRD of CdS


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

paul o’brien

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