Carrier Dynamics In Two-dimensional Semiconductors Studied By Ultrafast Laser Spectroscopy

One key challenge in developing electronic technology is to find ultrathin channel materials with high charge mobilities and sizeable band gaps. More than 70 years ago, Landau and Peierls argued that strictly two-dimensional (2D) crystals were thermodynamically unstable and could not exist. However, since it was discovered, graphene is a rapidly rising star on the horizon of materials science and condensed matter physics. Graphene has attracted much attention due to its extraordinary electronic and optical properties, which accommodates a large potential in optoelectronic applications. Approximately ten thousand papers are published every year on a wide range of graphene-related topics. Therefore, it is probably fair to say that research on ’simple graphene’has already passed its zenith. Indeed, the focus has shifted from studying graphene itself to other graphene-like two-dimensional materials and the use of the material in applications.It is well known that graphene can offer extremely high charge mobilities. In my dissertation, we show that black phosphorus also has room-temperature charge mobilities on the order of 104 cm2 V-1 s-1, which are about 1 order of magnitude larger than silicon. We also demonstrate strong anisotropic transport in black phosphorus, where the mobilities along the armchair direction are about 1 order of magnitude larger than in the zigzag direction. A photocarrier lifetime as long as 100 ps is also determined. These results illustrate that black phosphorus is a promising candidate for future electronic and optoelectronic applications.However, lack of band gaps presents a significant barrier in graphene. Transition nmetal dichalcogenides can be candidates in optoelectronic applications due to their sizable and thickness-tunable band gaps. In my dissertation, spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22±1 and 110±10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 ±0 and 3.5±0.5 cm2s-1 are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications. However, it is noted that their charge mobilities are relatively low compared to graphene or black phosphorus.One way to expand these materials is to fabricate their alloys. By varying the ratio of each component of the alloy, the optical and electronic properties can be tuned, which is significant to applications, and also boost the ideas in research. In experiment, shorter lifetimes are observed due to relatively lower crystalline quality. Similar to heterostructures, with the development of new materials, there will be more materials to be chosen. Hence, we can fabricate other alloys based on our needs.The newly discovered two-dimensional materials can be used to form atomically thin and sharp can der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements:electron transfer across the interface and interlayer coupling. In my dissertation, we show, using graphene-tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarries excited in tungsten disulfide transfer to grapheme in 1 ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processer required for using van der Waals hetreostructures in electronics and photonics.In summary, the results obtained in my dissertation provide solid foundation for understanding the optical and electronic properties of two dimensional materials and their optoelectronic applications.