DOI: 10.25549/usctheses-c40-375842
title: Synthesis, characterization, and device application of selected layered materials

description: In this dissertation, I present my work on the synthesis, characterization, and device application of layered materials including graphene, tungsten diselenide, black phosphorus, and violet antimony-phosphorus. During the past several years, an entirely new and promising field emerged in solid-state physics called two-dimensional (2D) semiconducting materials. These materials usually have a bandgap depending on the number of layers with properties very different from other three-dimensional electronic systems. As more and more 2D materials are discovered and identified, this field is expanding with unprecedented pace with multiple papers published almost daily and quickly becoming one of the hottest areas of solid-state research with extremely rich physics and great promise for applications. ❧ One important aspect for these 2D materials is the improved methods of material production, including chemical vapor deposition (CVD), physical vapor deposition (PVD), metalorganic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), solution-based synthesis, and tape-based exfoliation. Although graphene was the first to be thinned down to monolayer, many other materials existing in layered form can be thinned to a few layers and even monolayers with appropriate methods. Moreover, since these 2D materials are held up by Van der Waals force, both the assembling techniques and the disassembling techniques can be used to allow certain combinations of the interfaces, such as BN/WSe₂ heterostructure. ❧ To further explore the 2D physics in such thin materials, characterizations are always necessary, including but not limited to scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectrum, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Energy-dispersive X-ray spectroscopy (EDX), and optical microscopy (OM). Among these approaches, some are analytical techniques used for the elemental analysis or chemical characterization of a sample; some enable the instrument's user to examine fine detail of the materials; and some can provide structural details such as tension, defects, or surface roughness of the sample. By combining all these techniques, a comprehensive understanding for these 2D materials can be achieved, and can further provide help to the device fabrication. ❧ When it comes to device work, there are all kinds of parameters to describe the performance of various devices. Usually, for general field effect transistors (FETs), the most common used parameters are mobility, on/off ratio, contact resistance, and subthreshold swing (SS).  For light sensors and gas sensors, the lowest detectable limit and the current ratio (I/I₀) are also important. To improve these values and thus tuning the performance of the devices, several different methods can be adopted. One method is to use low-temperature measurement, since the carrier mobility of semiconductor materials are related to temperature. Usually 2D materials would show unique and interesting behaviors under ultra-low temperature, due to the loss of the third dimension. The second method, which makes use of phase engineering in transition metal dichalcogenide (TMDC) materials, can also improve the mobility value of such FETs. This method relies on the multiple intrinsic phases of TMDC materials and may also be applied on other layered material like phosphorene. The third method is that, by partially substitution, i.e., introducing another element into the system to perform partial replacing, it is also feasible to tune the properties of the material, which leads to different behavior of the as-made FETs. The last method is associated with the reaction between palladium metal contact and hydrogen gas molecules. As palladium can absorb hydrogen and form palladium hydride, the work function of palladium can be tuned as a function of the hydrogen concentration. Hence, it is possible to lower the contact resistance, by changing the work function of the contact materials. All these methods can result in substantial improvement for the FETs made with certain channel materials and contact materials. ❧ This dissertation is presented with 8 chapters. Chapter 1 is a general introduction regarding the structure, electrical properties, and applications of 2D materials including graphene, tungsten diselenide, black phosphorus and violet antimony-phosphorus. For these 2D materials, synthesis methods are presented in chapter 2. The synthesized materials are further used for fabrication of FETs. Chaper 3, 4, 5, 6, 7 are focused on the device work on these materials. Chapter 3 is dedicated to graphene devices, chapter 4 to tungsten diselenide phase engineering, chapter 5 to tungsten diselenide FETs with in-plane heterojunctions, chapter 6 to black phosphorus with tunable contacts, and chapter 7 to violet antimony-phosphorus photodetectors. The last chapter, chapter 8, will summarize and suggest the future directions of electronics based on 2D materials.