The recent discovery of the interfacial superconductivity (SC) of the Bi ₂ Te ₃ /Fe _1+y Te heterostructure has attracted extensive studies due to its potential as a novel platform for trapping and controlling Majorana fermions. Here we present studies of another topological insulator (TI)/Fe _1+y Te heterostructure, Sb ₂ Te ₃ /Fe _1+y Te, which also has an interfacial 2-dimensional SC. The results of transport measurements support that reduction of the excess Fe concentration of the Fe _1+y Te layer not only increases the fluctuation of its antiferromagnetic (AFM) order but also enhances the quality of the SC of this heterostructure system. On the other hand, the interfacial SC of this heterostructure was found to have a wider-ranging TI-layer thickness dependence than that of the Bi ₂ Te ₃ /Fe _1+y Te heterostructure, which is believed to be attributed to the much higher bulk conductivity of Sb ₂ Te ₃ that enhances indirect coupling between its top and bottom topological surface states (TSSs). Our results provide evidence of the interplay among the AFM order, itinerant carries from the TSSs, and the induced interfacial SC of the TI/Fe _1+y Te heterostructure system.

A compact solid-state UV flame sensing system is developed and tested. We have addressed two approaches for obtaining additional six orders of long-wavelength rejection power on top of that of the built-in UV sensor based on a wide-bandgap II-VI Schottky-barrier structure. An op-amp-based amplification circuit with an average gain of 16 600 is used for signal amplification. It is demonstrated that the developed sensing system could detect a standard butane-air flame with excellent solar-blind characteristics, potentially capable for monitoring industrial boilers. Several feasible approaches were discussed for further improvement in the performance of the developed sensing system targeting its general usage in fire-safety applications.

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe ₂ thin films. The as-grown MoSe ₂ thin films were characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence (PL), reflection high energy electron diffraction (RHEED) and angular resolved photoemission spectroscopy (ARPES). A new Raman characteristic peak at 1591 cm ⁻¹ was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe ₂ could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

To study the in situ doping effect upon monotonically increasing dopant concentration, a Bi₂Te₃ layer doped with Fe up to ~6.9% along the growth direction was fabricated by the molecular beam epitaxy (MBE) technique. Its resistance versus temperature curve displays a superconductivity transition at about 12.3 K. Detailed structural and chemical analysis via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) reveal that this layer consists of two types of unexpected Fe-Te nanostructures: one is FeTe thin layer formed near the surface, and the other is FeTe₂ nanorod embedded in the Bi₂Te₃ layer. Based on the results of further electrical and magnetotransport studies, it is likely that the observed superconductivity originates from the interface between the FeTe nanostructure and the neighboring Bi₂Te₃ layer. We have addressed the formation mechanisms of the observed nanostructures, which is attributed to the strong reaction between Fe and Te atoms during the growth process. The findings of this study also provide an unusual approach to synthesizing nanostructures via heavy doping if the dopant element is strongly reactive with an element in the host matrix.

We discuss the optical analogy of reflected electron diffraction from a grating with non-uniform spacing at the nanoscale, and its relationship to another grating with uniform spacing. Two analytical approaches that offer a quantitative solution for the reflected diffraction from a grating are presented. A compact and low-cost experimental kit for demonstrating the physics behind the reflected diffraction from one-dimensional surface structures is described. As an example, the convolution theorem is used to explain the reflected diffraction from a sample consisting of two individual one-dimensional surface structures overlapping orthogonally. This study will broaden students' understanding of the physics behind diffraction.