In-House Research Highlights 2017

In-House Research Highlights

Light and lattice vibrations in a crystal do not interact strongly in general, but both interact strongly with electrons. This three-way interaction allows a characterization method known as double resonance Raman spectroscopy to map out the low-energy electronic structure of materials and assign vibrational signatures to different microscopic processes. Double resonance in two-dimensional MoS2 reveals the dynamics of excitons – robust elementary excitations of a 2D crystal – between two sets of low-energy states known as valleys. The accurate assignment of vibrational signatures elucidates the essential physics limiting the performance of a novel class of “valleytronic” devices exploiting the selectivity of valleys to incident light carrying different polarizations.

Identifying crystal defects in a 2D crystal usually requires an electron microscope to directly resolve atomic details, a complex and time-consuming process on expensive equipment that can damage the sample under electron irradiation. By establishing a correlation between the modified optical response and certain defects, the MIP team and collaborators have demonstrated a quick and non-destructive method of identifying defects in 2D crystals. The reason for this correlation is identified through first-principles calculations: electrons trapped by sulfur vacancies – the absence of a sulfur atom – have energies that are forbidden for electrons in defect-free regions, and therefore emit light at wavelengths different from that of the latter.

Doping modulates the electronic, chemical, and mechanical properties of materials. For a two-dimensional tungsten disulfide, although an isolated molybdenum substitution only perturbs the host lattice negligibly, it couples strongly to common lattice defects such as sulfur vacancies, as verified by state-of-the-art electron microscopy and atomistic modeling techniques. This coupling can be potentially exploited to controllably segregate undesirable defects away from the active areas of 2D crystalline devices.

Grain boundaries are borders that separate crystals of distinct orientations and are generally considered as inevitable by-products of crystalline regions nucleating at different locations during growth. The MIP team has predicted how grain boundaries in two-dimensional crystals can form within a single grain by introducing bumps onto the substrate – the “floor” on which a 2D crystal grows. A 2D crystal warps itself as growth advances past a bump, so much that it eventually runs into itself at an angle determined by steepness of the bump. Although aesthetically jarring, grain boundaries can endow 2D crystals with desirable mechanical, electronic, and magnetic properties. The predicted topographic control of grain boundaries offers the opportunity to engineer grain boundaries into 2D crystals with atomic-scale control.

Ferromagnetic topological insulators (TIs) have promise for applications in spintronics, metrology, and quantum computing.

However, TI materials are fragile and often incompatible with nanofabrication techniques. We have developed a technique that enables persistent, micron-scale optical control of both magnetization and chemical potential in Cr-(Bi,Sb)2Te3 grown by MBE on SrTiO3. This system is uniquely positioned to enable arbitrary routing of the quantized edge states recently discovered in magnetic TIs. We also use Kerr and photocurrent microscopies to image magnetic inversion dynamics, p-n junctions, and magnetic recordings that we make in these materials. This work may enable dynamic, reconfigurable control of 1D quantum channels.

Atomically thin two-dimensional layers such as molybdenum disulfide, MoS2, are promising materials for nanoelectronics due to their exceptional electronic and optical properties. An inter-atomic potential has been developed that can accurately describe the thermodynamic and structural properties of MoS2 sheets, including defects and transitions between different structural phases. A new type of “ripple” defect has been identified as a favorable host for sulfur vacancy defects. A train of moving ripplocation defects may be able to “sweep out” sulfur vacancy defects from key regions within 2D devices.