and Interfacial Sciences of Low-Dimensional Materials
My group is interested in the surface and interfacial science of two-dimensional materials, which now also include semiconducting metal dichalcogenides (MoS2, WSe2, …), silicene, phosphorene, insulating h-BN (hexagonal boron nitride), mica, and so on. We employ various optical spectroscopies and scanning probe microscopies to explore the “Two-Dimensional Wonder Land”. Main research topics are as follows:
Interfacial Charge Transfer
Many molecular or material properties are affected by the charge state or charge density. When in contact, charge transfer may occur between any two unequal chemical entities because molecules and crystalline solids have varying affinity for an excess electron. Because of the high fraction of surface atoms, interfacial charge transfer are likely to significantly modify the electronic and possibly geometric structures of most 2D crystals. For example, chemical reactivity and electrical conductivity of graphene, and photoluminescence quantum yield of MoS2 can be greatly controlled by varying their charge density. We seek to unravel mechanisms for charge transfer processes induced by substrates and various charge dopants in order to establish chemical methods for controllable charge transfer or charge doping.
Nano Lett. 12, 648 (2012); Nano Lett. 10, 4944 (2010)
Molecular Behavior in Confined Space
Molecules may behave very differently exhibiting unique chemical properties and reactions in small nanoscopic space than in 3-dimensional free space. Molecular diffusion, for example, is known to be greatly affected by the size of the confined space and the chemical nature of the walls defining the space. In this regard, two-dimensional materials can also serve as a model system that provides two-dimensional confined space with van der Waals gap of ~0.3 nm. We are exploring diverse behaviors of molecules confined in the truly 2D space.
J. Am. Chem. Soc. 136, 6634 (2014)
Photophysics and Photochemistry of 2D Materials
We are also interested in chemical reactions and charge carrier behaviors of excited 2D materials. For this purpose, photons can provide specific amount of energy with high spatial and temporal resolution. Chemical reactions could be initiated by photo-exciting either the 2D crystals or adsorbed molecular species. Electron-hole pairs can also be created by the photoirradiation. The ensuing charge transfer, migration and chemical reactions can be monitored by various real-time pump-probe methods based on ultrafast pulsed lasers.
Spectroscopic characterization of 2D Materials
Among various optical spectroscopies, Raman and photoluminescence spectroscopies operated on an optical microscope are best used to characterize 2D materials for their geometric structures, electronic structures, chemical changes and so on. Despite their straightforward operation and excellent spectral accuracy, however, the interpretation and quantification require sophisticated analysis as demonstrated in the example below in addition to theoretical understanding on their vibrational and electronic structures. We are developing spectroscopic analytical frameworks customized for high-resolution quantification of defects, charge density and mechanical strain. In addition, the photoluminescence spectroscopy setup will be expanded for time-resolved capability based on ultrafast pulsed lasers.
Nature Commun. 3, 1024 (2012); ACS Nano 7, 1533 (2013)
Surface-Specific Nonlinear Spectroscopy & Microscopy
We are also building up a surface-specific spectroscopy and microscopy based on nonlinear optical phenomena. Since SHG (second harmonic generation) is only allowed in non-centrosymmetric systems, it will be an ideal probe for many 2D materials. For example, odd-number-layered MoS2 is SHG-active while even-number-layered is inactive. SHG spectroscopy and microscopy will visualize the difference in thickness, stacking domain, symmetry-breaking changes in various 2D materials. SFG (sum frequency generation) spectroscopy will also be realized on an optical microscope to investigate molecular behaviors in a confined space.