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Paul O’Brien

Paul 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

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Paul O’Brien

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  1. Paul 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

  2. 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

  3. 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

  4. CdS Thin Films

  5. 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

  6. 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

  7. CdS Thin Film: Deposition LP-MOCVD

  8. 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

  9. CdS Thin Film

  10. CdS Thin Film • Band Gap = 2.39 eV (2.42 eV) • Deposition Temperature (300oC v. 370oC) • Deposition rate (1.06mmh-1 v. 0.20mmh-1) • Major decomposition product = Et2NC(O)SC(O)NEt2

  11. 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

  12. CuInS2, CuInSe2, CuGaS2 • Precursor synthesis • CS2 or CSe2 + NaOH + N-MHN  solution • MxSO4/MxCl + solution + (solvent at T)  (1,2, 3, or 4)

  13. CuInS2, CuInSe2, CuGaS2 • Precursor synthesis Ga(S2CNMenHex)3 (5) • Na(S2CNMenHex) (dry benzene) + GaCl3 (hexane)  Ga(S2CNMenHex)3

  14. 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)

  15. Deposition of CuIn(S,Se)2, GaInS Thin Films • AACVD

  16. 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) a. glass; b. ITO glass; c.InP(100); d.GaAs(100); e.InP(111); f.Si(111)

  17. CuInS2 thin films by AACVD • 1:1 mix of 1 and 2 • Lower Tsub: 350oC • Morphology different than LP-MOCVD • Thinner flakes (0.2mm v. 1mm) • Horizontal • After 2 hr. 1mm thick film

  18. CuInS2 thin films • On InP(100), 112 peak missing

  19. CuInSe2 by LP-MOCVD • 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

  20. CuInSe2 thin films by AACVD • 1:1 ratio of 3 and 4 • T = 425-475oC • Several different morphologies

  21. CuInSe2 thin films

  22. CuGaS2 thin film by LP-MOCVD, AACVD • 1:1 ratio of 1 and 5 • Tpre = 250oC; Tsub = 500oC LP-MOCVD • T = 400-450oC

  23. CuGaS2 thin film

  24. 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

  25. Chalcogenide Quantum Dots • Bulk: band gap specific to chemical composition • Quantum dots: band gap tuned by altering size

  26. 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

  27. 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

  28. Chalcogenide Quantum Dots • Precursor: • (Cd/Zn)[R2(dtc/dsc)]2 • Growth: • Precursor decomposed in a high boiling point coordinating solvent, TOPO

  29. Chalcogenide Quantum Dots • Synthesis of Precursor

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

  31. Chalcogenide Quantum Dots

  32. Chalcogenide Quantum Dots

  33. Chalcogenide Quantum Dots Fig.1 XRD of CdSe Fig.2 XRD of CdS

  34. Chalcogenide Quantum Dots

  35. 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

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