CAREER: Raman Spectroscopy of Interlayer Phonons in van der Waals Heterostructures

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Non-technical abstract:Van der Waals heterostructures are a new class of materials that could lead to new novel electronic and optoelectronic devices. These materials are formed by vertically stacking atomic layers such as graphene, hexagonal boron nitride, and transition metal dichalcogenide (e.g. molybdenum disulfide) layers. The forces between these layers are relatively weak. However, they can dramatically change the properties of the system and induce phenomena that are absent in individual layers. By stacking or epitaxially growing different atomic layers on one another, one may design and fabricate artificial materials with unprecedented optical, electronic, and vibrational properties that cannot be achieved in natural crystals. Raman spectroscopy, which is a direct, noninvasive, and sensitive optical probe, is used to explore the interlayer coupling, the interface effect, and the interaction between vibrations and charge carriers in these heterostructures. This research provides crucial guidance in designing novel electronic and optoelectronic devices using stacked atomic layers. The research activities have a powerful impact on the undergraduate students at the University of Northern Iowa. The principal investigator integrates the research topics into two existing courses. Undergraduates at all levels, including freshman students, are encouraged to participate. The principal investigator also uses departmental contacts with area high schools and collaborates with Physics Education faculty colleagues and the director of the Classic Upward Bound program to involve teachers and students at regional high schools. Engaging high school students and undergraduates in research is very important in advancing STEM (Science, Technology, Engineering, and Mathematics) education in young people.Technical abstract:This research explores fundamental physical properties of various van der Waals heterostructures using Raman spectroscopy of interlayer phonons (typically at low frequencies) and photoluminescence. The research projects include: (i) Develop an interlayer-phonon-based Raman technique to characterize the rotational angle and Moire wavelength of heterostructure superlattices. This study offers a simple method of characterizing Moire pattern features without using scanning tunneling microscopy; (ii) Explore the electronic, optical, and vibrational properties of complex multilayer van der Waals heterostructures and the indirect-direct gap transition in MoS2 bilayers intercalated with different middle layer materials. This research provides crucial guidance in designing novel electronic and optoelectronic devices using stacked van der Waals multilayers; (iii) Study van der Waals heterostructures that involve Dirac materials. In particular, the research team investigates the change in the electronic band structure of twisted bilayer graphene after it is placed or sandwiched between hexagonal boron nitride. This study probes the change in the electronic and vibrational properties of graphene at the hetero-interface; (iv) Probe the interaction between carriers and interlayer phonons in twisted bilayer graphene devices with a dual-gate. This study explores the Coulomb interaction between two charged graphene layers and the interlayer potential formed upon gating. The techniques and methods established in these research activities can be applied to heterostructures formed from any two-dimensional crystals stacked in any desired sequence.