题目：Carrier Multiplication in Semiconductor Nanocrystal Quantum Dots（一箭双雕）

报告人：Dr. John A. McGuire （Los Alamos National Laboratory, U.S.A.）
               
摘要：
Much of the interest in colloidal semiconductor nanocrystal quantum dots (NQDs) arises from the dramatic effects of confinement, which leads to enhancement of the strength of many physical phenomena compared to what is observed in bulk materials as well as numerous potential applications. One such phenomenon is carrier multiplication (CM), or multiexciton generation, a process whereby multiple electron-hole pairs (excitons) are generated by absorption of a single photon. Intense interest in CM has been driven by the potential of CM in NQDs to increase the fundamental limits of photovoltaic power conversion efficiencies. While numerous publications have presented evidence of enhanced CM quantum efficiencies (QEs) in NQDs, over the last year a vigorous controversy has emerged due to large discrepancies between the values of CM QEs reported by different groups.1, 2, 3 In this talk I will discuss our most recent studies of CM in PbSe NQDs.4 We have used transient absorption and photoluminescence upconversion spectroscopy to measure the radiative and nonradiative dynamics of multiexcitons in NQDs. I will present evidence that NQDs experience photoinduced charging under the conditions that have typically been used in studying CM in NQDs and that failure to account for photoinduced charging can lead to large variations in apparent QEs with systematic bias to larger values. Under conditions that minimize the effects of photoinduced charging, excitonic dynamics reveal that CM QEs of PbSe NQDs are enhanced compared to the bulk when the QE is parameterized by the ratio of the photon energy to the band gap of the material. However, QEs are lower than previously claimed in many publications. I will discuss the implications for exploiting CM in photovoltaics and the challenges ahead. Time permitting, I will also discuss the validity of parameterizing CM QEs by the band-gap-normalized photon energy.

1 R.D. Schaller and V.I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).
2 R.J. Ellingson, M.C. Beard, J. C. Johnson et al., Nano Lett. 5, 865 (2005).
3 G. Nair, S.M. Geyer, L.-Y. Chang, and M.G. Bawendi, Phys. Rev. B 78, 125325 (2008).
4 J.A. McGuire, J. Joo, J.M. Pietryga, R.D. Schaller, and V.I. Klimov, Accs. Chem. Res. 41, 1810 (2008).

Dr. John A. McGuire 简介：
Dr. John McGuire earned his B.S. in physics and biology from Stanford University in 1992 after which he spent two years in the People’s Republic of China as a teacher and technical editor and as an administrator for Volunteers in Asia. After returning to the United States, he resumed his studies at the University of California, Berkeley, where he obtained his Ph.D. in physics in 2004 under the tutelage of Professor Yuen-Ron Shen. At Berkeley, his research focused on nonlinear optical studies of ultrafast electronic and vibrational dynamics at surfaces. This work included the first measurements of picosecond and sub-picosecond vibrational dynamics at water interfaces.
Dr. McGuire is currently a Postdoctoral Research Associate at Los Alamos National Laboratory in the Softmatter Nanotechnology and Advanced Spectroscopy Team led by Dr. Victor Klimov. His work at Los Alamos has focused on fundamental questions about multiexciton dynamics in quasi-zero-dimensional semiconductor nanocrystal quantum dots (NQDs), engineering multiexciton interactions in heterostrucutred NQDs, and exploiting the dynamic signatures of multiexcitons to study carrier multiplication, a process of potential utility in enhancing the efficiency of NQD-based photovoltaics.

时间：2009年3月5日（星期四）上午10：00  
地点：中科院物理研究所D楼210会议室
联系人：金奎娟  研究员（Tel：82648099）
主办单位：中国科学院光物理重点实验室