Physics ABC Seminar: by Dr. Hsun-Jen (Ben) Chu, NOVA research Inc. U.S. Naval Research Laboratory
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Speaker: Dr. Hsun-Jen (Ben) Chu, NOVA research Inc., Alexandria, VA. U.S. Naval Research Laboratory, Washington, D.C.
Title: From exfoliation to stacking: The interfaces, strain, and defects of 2D materials
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Meeting ID: 949 2258 5592
Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of layered materials with weakly bonded interlayers which enables one to create so-called van der Waals heterostructures (vdWH). Although the vdWH exhibits many fruitful exciting novel properties through interlayer coupling, the interface contamination in vdWH remains a major obstacle to achieve those properties. To clearly observe the intrinsic properties, such as defect density of monolayer TMDs (electrical), interlayer exciton in vdWH (optical), or atomic reconstruction in vdWH (mechanical), the clean interfaces are in urgent needs. In our research, we first introduced a unique AFM technic so-called “nano-squeegee” for crating clean interfaces in vdWH which paves the way for investigating intrinsic electrical, optical, and mechanical properties of the TMD vdWH.
Based on the clean interfaces in the vdWH enabled by the “nano-squeegee”, we could quantitively investigate the PL quality of the Monolayer TMD and correlated it with the defect density of the Monolayer TMDs. By using conductive AFM, we could measure the local electrical conductance which presumably indicates the defects sites on the TMD monolayer. The higher defect density of the TMD leads to lower PL emission or quantum yield. Another promising attribute of vdWH is control over the twist angle between layers, which leads to the formation of Interlayer exciton (ILE) when forming a type II band alignment heterostructure. With hBN encapsulation and a relative rotational angle close to 60 degrees, well pronounced ILE emission is observed at room temperature and further splits into two distinct peaks (ILE1 and ILE2) at low temp. Furthermore, we demonstrate that the ILE emission peaks have opposite circular polarizations when excited by circularly polarized light. Ab initio calculations provide an explanation of this unique and potentially useful property and indicate that it is a result of the indirect character of both transitions. We further investigate the atomic structure arrangement of TMDs by TEM and reveal that the heterostructure indeed reconstructs under a small twist angle ( ≤ 1°) between the TMDs in the commensurate system. With a small twist angle, periodic domains form with commensurate stacking within the domain. On the other hand, a rigid moiré structure is also observed by TEM when a larger twist angle (≥ 3°) is applied. This finding provides a significant departure from the current rigid-lattice moiré theory in such a system which only considered the constituent layers as rigid lattices and has not allowed for atomic-level reconstruction. These results may also provide fundamental insights into the mechanical and optical behavior of this exciting class of semiconductor heterostructure.