UC Berkeley

Layered Transition Metal Chalcogenides (LTMCs) exhibit a wealth of physical properties. Structurally, they are characterized by strong intra-layer bonding and weak inter-layer interactions. This strong structural anisotropy enables exfoliation into thin layers, sometimes even down to single unit cell thickness.

LTMCs have been studied for decades, but recent advances in nanoscale materials characterization and device fabrication have opened up new research opportunities. As the thickness of these materials go thinner and thinner, their properties may change significantly, which calls for re-exploring of their unique optical and electronic properties. During my PhD study, I have explored two categories of LTMCs: LTMC semiconductors and LTMC metals.

LTMC semiconductors such as MoS2, MoSe2, WS2 and WSe2 have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. I have studied the optical properties of few layer WSe2, and found a thermally induced direct-indirect band gap transition. The ultrathin body of these LTMC monolayer semiconductors also makes them sensitive to both the ambient and external electric field. By combining these two unique properties, I discovered a strategy for dynamically modulating the photoluminescence intensity of MoS2 by orders of magnitude. The defect free, nanometer-thick LTMC layers are ideal tunneling media. Based on this, I proposed to use LTMC as the elastic tunneling medium to construct a non-impact nano-electro-mechanical switch, which shows 4 orders of magnitude modulation in the electrical resistance by applying a mechanical force.

LTMC materials span a wide range of categories. Some LTMC show semiconducting properties, while others are metallic, or even superconducing. Bi2Te3, a metallic LTMC, commonly known as a high performance thermoelectric material, also attracts renewed attention because it is found to be topological insulator. The conduction on its surface is provided by topologically protected surface states, which has a massless Dirac like dispersion, with spin and momentum degree of freedom interlocked. It would be interesting to explore the possibility of engineering the geometry of these exotic conductive surfaces at nanoscale. During my PhD study, I introduced dense, nanosized antidot arrays into Bi2Te3 microflakes, and studied its magneto-transport properties. This modification completely altered the electrical properties of this material. I observed signature of Ahoronov-Bohm type oscillations in our device, indicating that charge carriers in topological insulators are indeed interacting with our antidot arrays, thus proved the possibility of creating new functionalities in this material via nano-structuring.