Development of Atomic Force Microscope Techniques for Extracting Work Function and Density of States
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In order to understand potential novel electrical device applications for a material, it is critical to understand fundamental properties about how charges can move within the material. The density of states (DOS) is a measure of available states for charge carriers at a given energy level in a material. The work function of a material is related to the energy required to remove an electron from the material, which is necessary to understand what happens at the interface between two materials. Promising techniques to measure both of these properties involve using the nanometer-scale tip of an atomic force microscope (AFM) to perform electrical measurements. This is done on devices in a transistor geometry, where the AFM tip is placed near the surface of a material and measures the electric potential at the surface in response to an applied back gate voltage; the rate of change of this response can be used to determine DOS and the value of the surface potential can be used to determine work function. DOS measurements can be improved by refining the gate voltage sweep rate necessary to avoid hysteresis. The techniques were implemented on control samples and on atomically thin two-dimensional molybdenum disulfide, a semiconductor that is attractive for potential electronic device applications. Future measurements of DOS and work function on molybdenum disulfide will allow for a better understanding of how molybdenum disulfide can be used in the development of novel transistors.