We are interested in light-matter interaction, with a particular focus on quasi-particles, such as polarons, excitons, and polaritons. These quasiparticles are important to a range of optoelectronic processes, including light emission, lasing, and solar energy conversion. A main research thrust in the latter is to explore hot carrier or multi-exciton harvesting to increase solar cell efficiency beyond the Shockley-Queisser limit. We employ state-of-the-art experimental techniques, many of them based on ultrafast lasers, to study interesting material systems (organic semiconductors, hybrid semiconductors, quantum confined materials, and 2D van der Waals semiconductors and interfaces, etc.). Our favorite tools/toys include, among others, time-resolved two-photon photoemission spectroscopy, femtosecond nonlinear optical spectroscopies, transient absorption and time-resolved fluorescence spectroscopies/microscopies, 2D electronic spectroscopies, and scanning probe spectroscopies. As examples, recent discoveries in our lab showed how an electron is protected in lead halide perovskite as a large polaron, how an exciton can split into two in organic semiconductors via the singlet fission process, and how an electron-hole pair can escape the Coulomb potential at a van der Waals interface.
Haiming Zhu, Kiyoshi Miyata, Yongping Fu, Jue Wang, Prakriti P. Joshi, Daniel Niesner, Kristopher J. Williams, Song Jin, X.-Y. Zhu, “Screening in crystalline liquids protects energetic carriers in hybrid perovskites,” Science, 2016, 353, 1409-1413.
Nicholas R. Monahan, Dezheng Sun, H. Tamura, Kristopher W. Williams, Bolei Xu, Yu Zhong, Bharat Kumar, Colin Nuckolls, Avetik R. Harutyunyan, Gugang Chen, Hai-Lung Dai, D. Beljonne, Yi Rao, X.-Y. Zhu, “Dynamics of the triplet pair state reveals the likely co-existence of coherent and incoherent singlet fission in crystalline hexacene,” Nature Chem. 2016, 8, DOI: 10.1038/NCHEM.2665
Haiming Zhu, Yongping Fu, Fei Meng, Xiaoxi Wu, Zizhou Gong, Qi Ding, Martin V. Gustafsson, M. Tuan Trinh, Song Jin, X.-Y. Zhu, “Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors,” Nature Mater. 2015, 14, 636-642.
Jailaubekov, A. P. Willard, J. Tritsch, W.-L. Chan, N. Sai, R. I. Gearba, L. G. Kaake, K. J. Williams, K. Leung, P. J. Rossky, X.-Y. Zhu, “Hot charge transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics,” Nature Mater. 2013, 12, 66-73
W.-L. Chan, M. Ligges, X.-Y. Zhu, “The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain,” Nature Chem. 2012, 4, 840-845.
W.-L. Chan, M. Ligges, A. Jailaubekov, L. Kaake, L. Miaja-Avila, X.-Y. Zhu, “Observing the Multi-Exciton State in Singlet Fission and Ensuing Ultrafast Multi-Electron Transfer,” Science 2011, 334, 1541-1545.
A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, X.-Y. Zhu, “Hot electron transfer from semiconductor nanocrystals,” Science 2010, 328, 1543-1547.