Strain Engineering of Monolayer MoS2 for Electrocatalysis Enhancement and Band Structure Modulation

Thesis (Ph.D.)--University of Washington, 2021In this dissertation, strain engineering is utilized to modulate the bandgap structure of monolayer MoS2 and enhance its catalytic performances. A general electron-beam lithography fabrication process is developed to fabricate a flexible electrochemical nanodevice, which is further combined with a biaxial beam bending method to apply biaxial tensile strain on the monolayer MoS2 samples. After introducing the tensile strain, the electronic states and the electrochemical capacitance of the monolayer MoS2 are characterized using two types of experiments. In the first experiment, the newly developed nanodevice is applied to measure the Density of States (DOS) of the MoS2. Measuring the intrinsic DOS and defect states of atomically thin layered materials are of profound importance for understanding their exotic physical properties and how to apply them in real life. Typically, the measurements of defect states require either ultra-high vacuum or low temperature. Here, we measured the defect states of two-dimensional (2D) materials in an ambient environment rooting on the electrochemical capacitance. The highest energy resolution of the electrochemical capacitance spectrum approaches 116 meV, close to the theoretical limit of thermal broadening at room temperature (3.5 kBT = 91 meV). Meanwhile, the absolute energy positions of the DOS can be obtained by integrating a reference electrode in our electrochemical system. With this approach, the DOS and defect states in monolayer MoS2 are mapped out by measuring its electrochemical capacitance. We can also monitor the DOS evolution during the electrochemical reactions. Our preliminary result on the mercury ions adsorption shows that the Mo defects in monolayer MoS2 dominate the chemical adsorption procedure. This work paves the way towards a new platform for measuring the intrinsic DOS and defect states of 2D materials under an ambient environment. Following the electrochemical capacitance measurements, the hydrogen evolution reaction property was determined using the monolayer MoS2 as the catalyst. The catalytic property is highly dependent on the number of their active sites and the turnover frequency (TOF) of catalysts. While increasing the number of active sites eventually approaches a saturation point, theoretically raising the TOF can improve the catalysis performance linearly. On the other side, the TOF has a strong correlation with the DOS of materials. Thus, it is critical to develop a strategy for establishing a relationship between the electronic structure and TOF. The purpose of this endeavor was to use strain engineering to uncover this missing puzzle piece. Tensile strain contributes to spin-orbital coupling in monolayer MoS2, resulting in an increased electronic state near the conduction band edge. As demonstrated experimentally, this unusual electronic structure in strained monolayer MoS2 causes an unanticipated three-zone catalysis process and an ultrahigh TOF of 27.75 s-1 in the hydrogen evolution reaction, which is superior to platinum. This study establishes a connection between the strain condition, electronic states, and catalytic property