Optoelectronic Devices Based on Two Dimensional Materials.

Abstract

Two dimensional (2D) semiconductors have attracted tremendous attention due to their fascinating electrical and optical properties. Transition metal dichalcogenides (TMDs) are semiconductors with band gap ranging from 1.57 eV to 2 eV, which are suitable for optoelectronic applications with visible and near-infrared light regime. Because of the reduced physical dimensions, quantum confinement becomes a dominated factor that strongly modulates TMDs properties, and therefore triggers a series of interesting phenomenon that has never been observed in bulk TMDs, including layer dependent indirect to direct band transition, larger exciton binding energy and enhanced light matter interaction. The strong light matter interaction makes TMDs a robust platform to study the physics of quasi-particles such as exciton, trion and even high order exciton particles. In this dissertation we investigate exciton dynamics and especially exciton modulation in single layer TMDs and two type TMDs heterostructures. Then the dissertation will focus on TMDs based optoelectronic device applications. For two dimensional materials, such as transition metal dichalcogenides (TMDs) and black phosphorus, many novel properties only exist at low dimensional scale. Currently, atomic force microscopy (AFM), the most commonly used method to determine the thickness of two-dimensional (2D) materials, was known to have issues with its low scan speed and inevitable invasion. Fast and non-invasive identification of layer numbers of these 2D materials is important for future materials study. Here, in this dissertation we will first demonstrate that phase-shifting interferometry (PSI) can be used as a rapid and non-invasive method to identify the surface topography of 2D materials. The optical path length (OPL) obtained by PSI shows a good agreement with model calculations and a direct relationship between OPL and the layer number of 2D materials can be established. This technique enables a rapidly study of the surface information of various types 2D materials. Due to the quantum confinement effect and sizeable semiconductor bandgap, exciton in TMDs becomes a centre topic either for physical research or engineering applications. Many efforts have been made to study the exciton dynamics in TMDs, but the exciton modulation is still barely been touched. The first part of this dissertation will focus on the ferroelectric driven method to modulate exciton and trion behaviour in monolayer molybdenum and tungsten diselenides. By using a lithium niobate substrate with substrate domain engineering, it can achieve selectively doping of monolayer TMDs upon it. The inverted and pristine domain has been proved to be able to host two different types of ferroelectric doping as a result of the remnant polarization. The photoluminescence spectroscopy is used to spatially probe the exciton and trion dynamics, and it has demonstrated that these ferroelectric domains can significantly enhance or inhibit photoluminescence, leading to strong exciton and trion modulation. This novel modulation method opens a new routine to create optically active heterostructures that can be used for photodetectors and on-chip light sources. Based on the understanding of the exciton behaviour of TMDs, this dissertation further demonstrates two type of 2D materials optoelectronic device applications. Firstly, the high efficiency monolayer molybdenum ditelluride light emitting diode has been demonstrated. The device is driven by direct current tunnelling effect, and in the meanwhile, the device can be also used as photodetector with low dark current and fast response time. As the counterpart of direct current driven, the alternative current driven monolayer tungsten disulfide light emitting diode has also been successful demonstrated. Due to the emission only happens within the rising and falling edges of AC voltage, the unique pulsed light emission can be achieved in this structure. Show more