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SUM Tze-Chien
Associate Professor
NRF Investigator

Associate Chair (Research)
(65) 6316 2971


Ph.D. (National University of Singapore)

Research Area: Surface and Nanoscale Physics; Laser Physics and Spectroscopy
Research Interests

Current research interests of Tze-Chien Sum and his group are on investigating light matter interactions; energy and charge transfer mechanisms; and probing carrier and quasi-particle dynamics in a broad range of emergent nanoscale and light harvesting systems using Femtosecond time-resolved spectroscopy. Within these seemingly broad material systems – organic and inorganic semiconductors, I address the following three questions: (1) Where did the energy go? That is the interplay of carrier/quasi-particle dynamics between the host energy levels, defect energy levels and the dopant energy levels. (2) What is the underlying photo-physics and light-matter interactions that gives this system its unique characteristics? That is the various processes such as carrier-carrier scattering, carrier-phonon scattering, radiative recombination and auger recombination etc. (3) How can these properties/technologies be harnessed for practical applications? That is how the knowledge gained be used for the development of novel optoelectronic devices with enhanced efficiency and sensors with better sensitivity.

Broadly, I can categorize my group’s efforts in three main areas: (A) nanomaterials nanophotonics; (B) mixed dimension heterostructures; and (C) photovoltaics (PV) and plasmonic PV. Highlights from these works are showcased below:

(A) Nanomaterials Nanophotonics

We seek to understand the carrier dynamics and energy/charge transfer processes in semi-conducting nanostructure systems such as quantum dots, nanorods and nano-belts. Using fs time-resolved spectroscopy, we probe these energy transfer processes in these systems that comprises of the interplay of carrier dynamics between the host energy levels, defect energy levels and the dopant energy levels. The systems we have investigated include: ZnO nanowires, CdS nanowires and nanobelts, ZnSe nanowires, ZnTe nanowires and nanobelts, and even graphene/monolayer materials etc.

(B) Mixed Dimension Heterostructures
In the CdSe dot/ CdS nanorod heterostructures, we focus on controlling the light harvesting and light emission properties. The CdS shell functions as an antenna for light harvesting while quantum confinement is afforded by the CdSe dot. Such system of mixed dimensionality provides an ideal testbed to study the rich photophysics and carrier dynamics.

(C) Photovoltaics and Plasmonic Photovoltaics
Ultrafast optical spectroscopy allows us to trace the fate of the carriers and quasi-particles in photovoltaic devices from genesis to the end with timescales spanning over ten orders of magnitude. Correlated with electrical characterization techniques, new insights into the mechanisms of charge generation, transfer, trapping, recombination and transport in novel PV materials will be gained through these studies.

(a) Schematics of the Urbach tails at the band edges and the length- dependent lasing mode selections for CdS nanowires (NWs). (b) In undoped semiconductors, exciton−phonon coupling distort the electron−lattice interaction, resulting in a tailing of the states at the adsorption edges known as the Urbach tail. (c) Various output lasing colors are observed at the end facets of CdS NWs with different lengths.

Metallic nanoparticles are commonly used to facilitate light trapping in organic solar cells, but they can also reduce cell performance. A team of researchers from NTU led by Assistant Professor Sum Tze-Chien demonstrate that a trap-assisted recombination of charge carriers by the nanoparticles leads to degradation, irrespective of an initial enhanced absorption and excitation.

Selected Publications
G. C. Xing, N. Mathews*, S. Sun, S. S. Lim, Y. M. Lam, M. Graetzel, S. Mhaisalkar and T. C. Sum*, “Long-Range Balanced Electron and Hole Transport Lengths in Organic-Inorganic CH3NH3PbI3”, Science 342 (6156) 344-347 (2013)
G. C. Xing, N. Mathews*, S. S. Lim, N. Yantara, X. Liu, S. Dharani, M. Graetzel, S. G. Mhaisalkar and T. C. Sum*, Low-Temperature Solution-Processed Wide Wavelength Tunable Perovskites for Lasing, Nature Materials 13 (5) 476-480 (2014)
M. J. Li, S. Bhaumik, T. W. Goh, M. S. Kumar, N. Yantara, M. Graetzel, S. G. Mhaisalkar, N. Mathews*, and T.C. Sum*, Slow Cooling and Highly Efficient Extraction of Hot Carriers in Colloidal Perovskite Nanocrystals, Nature Communications 8 14350 (2017),
W. Chen, S. Bhaumik, A. A. Veldhuis, G. C. Xing, Q. Xu, M. Graetzel, S. Mhaisalkar, N. Mathews*, and T. C. Sum*, Giant five-photon absorption from multi-dimensional core-shell halide perovskite colloidal nanocrystals, Nature Communications 8:15198 (2017)            
J.H. Fu, Q. Xu, G.F. Han, B. Wu, C. H. A. Huan, M. L. Leek & T.C. Sum*, Hot carrier cooling mechanisms in halide perovskites, Nature Communications 8 1300 (2017),