| Asst Prof Sum Tze Chien
Assistant Professor
Division of Physics & Applied Physics
School of Physical & Mathematical Sciences
College of Science
Email: TZECHIEN@ntu.edu.sg
Phone: (+65)63162971
Office: SPMS-PAP-03-05 |
| Education |
- PhD National University of Singapore 2005
- MSc National University of Singapore 2001
- BSc(Hons) National University of Singapore 1999
- BSc National University of Singapore 1998
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| Biography |
Tze Chien graduated with a B.Sc. (1st Class Hon), a M.Sc. (Accelerated Masters Program) and a Ph.D. degree in Physics from the National University of Singapore, Singapore in 1999, 2001 and 2005, respectively, where he had worked on the development of a novel direct-write lithographic technique (proton beam writing) for photonic applications.
In 2005, he joined the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University as a Lecturer where he switched his research field from applied nuclear physics (proton beam writing and accelerator-based nuclear spectroscopy) to Femtosecond Laser Spectroscopy and its applications. Subsequently, he established the Femtosecond Dynamics Laboratory (xC-Lab), the Organic Electronics Laboratory and a new research group at the Division of Physics & Applied Physics. He is presently an Assistant Professor and is interested in the development and application of time-resolved and time-integrated optical spectroscopic techniques to interrogate the fundamental and applied properties of materials – i.e. probing the energy and charge transfer mechanisms in semiconducting nano-heterostructures & light harvesting systems; as well as investigating the nonlinear optical properties of nano-scale systems.
Tze Chien has received a total of 11 teaching awards from both the National University of Singapore (while he was a teaching assistant at the Department of Physics then) and the Nanyang Technological University, including the Nanyang Award for Excellence in Teaching in 2006 and the SPMS Teaching Excellence Award – Honour Roll 2010. He was also credited with the establishment of the 1st Year Physics Teaching Laboratory, the 3rd Year Physics Teaching Laboratory and the NTU H3 Physics Teaching Labs for the MOE H3 program. |
| Research Interests |
My research interests are on the development and application of time-resolved and time-integrated optical spectroscopy techniques to study a broad range of emergent nanoscale and light harvesting systems. Specifically, I focus on investigating light matter interactions; energy and charge transfer mechanisms; and probing carrier and quasi-particle dynamics in these systems. Broadly, I can categorize my group’s efforts in three main areas: (a) nanomaterials nanophotonics; (b) mixed dimension heterostructures; and (c) organic photovoltaics (OPV) and plasmonic OPV.
1. Nanomaterials Nanophotonics
One of the longest on-going project is on investigating the carrier and quasi-particle dynamics in Semiconducting Nanostructures Nanophotonics. We have worked on a range of systems – ZnO nanowires, CdS nanowires and nanobelts, ZnSe nanowires, ZnTe nanowires and nanobelts etc. New insights were gained in these studies – for example, (1) in one study involving ZnO nanowires, we established that the fast trapping of carriers to the green-emission related centers is through an ultrafast excitonic Auger recombination mechanism; (2) in another project on Cu-doped ZnO, we verified the charge transfer times from the ZnO host to the Cu subsystem occurs within 39 ps to form an “intermediately bound exciton” that is reponsible for the Green Emission in Cu-doped ZnO; (3) A depiction of semiconductor nanowires under lasing action with single photon excitation (SPE) vs two photon excitation (TPE). The commonly accepted paradigm that the higher pump intensities needed for TPE would correspond to higher exciton density threshold (nth) for two-photon pumped lasing is re-examined by Tze-Chien Sum and co-workers using femtosecond time-resolved spectroscopy. Our results show that a much lower nth is needed to achieve lasing in single ZnSe nanowires with TPE compared to SPE. This finding has significant implications for the photo-stability and durability of nanowire lasing. (4) recently, in a project involving CdS nanowires, we developed a new method for tailoring the nanowire lasing modes utilizing the intrinsic self absorption via the Urbach tail states etc. This work is published in Nanoletters.
2. Mixed Dimension Heterostructures
In this project, we focus on controlling the light harvesting and light emission properties of novel CdSe dot/CdS nanorod heterostructures (a system of mixed dimensionality). This is achieved through a good understanding the optical properties and charge transfer dynamics in these nanoheterostructures. Utilizing the CdS shell as an antenna for light harvesting and the quantum confinement afforded by the size of the dot, our group has been able to achieve ultralow threshold lasing over a range of emission wavelengths. This ultralow threshold is achieved through the large absorption cross-sections and suppressed Auger recombination rates in these nano-heterostructures. This work is published in ACSNano.
3. Organic Photovoltaics (OPV) and Plasmonic OPV
In this work, we seek to understand energy transfer mechanisms in hybrid organic photovoltaic devices, in particular that of the exciton and polaron dynamics resulting from interchain effects under device conditions (field-induced changes in the optical behaviour). Systems include hybrid Au nanowire/P3HT-PCBM photovoltaic devices and fiber-P3HT-PCBM systems. The recombination pathways will be investigated using both time-resolved photoluminescence and transient absorption spectroscopy. The knowledge gained would serve to guide the design of such organic materials inimproving the PCE through chemical and structural modifications. One recent highlight is the acceptance of our paper on investigating the loss mechanisms in Ag nanoparticle blended OPV cells in Nature Communications. |
| Research Grant |
- Academic Research Fund Tier 1 (2009-)
- Academic Research Fund Tier 2 (2012-)
- NRF CREATE (2012-)
- NRF Competitive Research Program (2010-)
- SPMS Collaborative Research Award (2011-)
- Start Up Grant (2009-)
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| Current Projects |
- Coupling between Surface Plasmons and Excitons: Towards Sub-wavelength Photonics
- Energy Transfer Mechanisms in Doped Nanoparticle Systems
- Investigating Energy & Charge Transfer Mechanisms in Hybrid Metal-Semiconductor Nano-Heterostructures
- Magneto-Optical Spectroscopy of Tunable Highly Spin Polarized Ferromagnetic Systems
- Organic Solid Solar Cell Fabricated via Water-based Processing Routes ? Studies on Materials Selection and Characterization, Fundamental Photo-physics and Device Physics
- SinBeRISE Subproject 5 - Characterization
- Toward Efficient Sunlight Harvesting
- Towards Efficient Sunlight Harvesting
| Selected Publications | - B. Wu, X. Wu, C. Guan, K. F. Tai, E. K. L. Yeow, H. J. Fan, N. Mathews* and T. C. Sum*. (2013). Uncovering Loss Mechanisms in Silver Nanoparticle-Blended Plasmonic Organic Solar Cells. Nature Communications, , DOI: 10.1038/ncomms3004.
- M. J. Li, G. C. Xing, G. Z. Xing, B. Wu, T. Wu, X. H. Zhang and T. C. Sum*. (2013). Origin of green emission and charge trapping dynamics in ZnO nanowires. Physical Review B, 87, 115309.
- X. Liu, Q. Zhang, Q. Xiong, and T. C. Sum. (2013). Tailoring the lasing modes in semiconductor nanowire cavities using intrinsic self-absorption. Nano Letters, 13, 1080 - 1085.
- Guichuan Xing, Jingshan Luo, Hongxing Li, Bo Wu, Xinfeng Liu, Cheng Hon Alfred Huan, Hong Jin Fan, and Tze Chien Sum*. (2013). Ultrafast Exciton Dynamics and Two-Photon Pumped Lasing from ZnSe Nanowires. Advanced Optical Materials, 1, 319.
- Guozhong Xing, Guichuan Xing, Mingjie Li, Edbert Jarvis Sie, Dandan Wang, Arief Sulistio, Quan-lin Ye, Cheng Hon Alfred Huan, Tom Wu, and Tze Chien Sum*. (2011). Charge transfer dynamics in Cu-doped ZnO nanowires. Applied Physics Letters, 98(10), 102105.
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