Overview: Surface and Interfacial Sciences of Low-Dimensional Materials

Surfaces and interfaces of materials have observed many interesting new scientific discoveries and lead to important fundamental principles since the demonstration of molecule-thin oil film on water by B. Franklin in 1783 and others shown in the figure below. As nanoscience expands in research activity to reach an established scientific discipline during the past three decades, the importance of surface and interface is ever increasing because of the high surface/volume ratio for nanomaterials in general. Understanding and controlling the surface and interfacial chemistry in such confined space will benefit many applications such as heterogeneous catalysis, photovoltaics, electrochemistry, semiconductor device fabrication, etc.

In addition, various low-dimensional materials such as quantum dots and nanowires exhibit unique physical, chemical, and other properties distinct from those of their bulk counterparts due to the so-called quantum confinement effect. More recently, truly two-dimensional crystals represented by graphene have revealed novel fundamental physics and supreme material properties, now considered as industrially attractive materials. Understanding surface/interface and nanomaterials themselves will be a prerequisite for the successful application of nanoscience for the above technological goals.

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”. The 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, the chemical reactivity and electrical conductivity of graphene, and the 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.

Nat. Commun. 10, 4931 (2019); 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.

Nano Lett. 17, 7267 (2017); 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 photoexcited 2D materials. For this purpose, photons can provide a 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 photoirradiation. The ensuing charge transfer, migration, and chemical reactions can be monitored by various real-time pump-probe methods based on ultrafast pulsed lasers.

Nano Lett. 19, 4043 (2019); ACS Nano 10, 8723 (2016)

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 the theoretical understanding of 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.

ACS Nano 11, 10935 (2017); Nature Commun. 3, 1024 (2012); ACS Nano 7, 1533 (2013)

Surface-Specific Nonlinear Spectroscopy & Microscopy

We are also building up 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.

Nano Lett. 20, 8825 (2020)