Photocurrent study of two dimensional materials
Date of Issue2018
School of Physical and Mathematical Sciences
With the discovery of graphene, two dimensional materials have drawn great interest among researchers thanks to their unique properties. Research in graphene optoelectronics is one of the fastest developing platforms to study light-matter interaction. Beyond the active research on new fundamental aspects of this and related systems, the optical and electronic properties of graphene can be employed in numerous applications, such as highly sensitive bolometers, photodetectors, plasmonic or photovoltaic devices, which leverage on photo-induced thermal and electronic effects. Due to the lack of non-trivial band gap in graphene, researchers have explored various two-dimensional materials, among which transition metal dichalcogenide monolayers have shown various interesting properties such as charge density waves, unusual strain and thermal energy dependence, tunable light emission by chemical control and fast photoresponse, which can be used in interdisciplinary device applications. To reveal the intrinsic optical and electronic behavior of the materials mentioned above, photocurrent study turns out to be a powerful tool. This thesis covers three of my major contributions: 1. Helicity dependent photocurrent in biased bilayer graphene In this part, we report the experimental determination of the PC response of bilayer graphene as a function of light intensity and state of polarization, as well as carrier density and polarity. The data shows qualitative features in common with the PC that is expected to arise from the photon-drag and the circular photogalvanic effects, as seen in monolayer graphene. In addition, we identify a non-negligible contribution to the PC of different nature, with anomalous dependence on light polarization. These results highlight the richness of bilayer graphene photoresponse, providing an opportunity to establish light helicity as a means to manipulate the photoconductive behaviour of future optoelectronic graphene devices. 2. Low temperature photoresponse of monolayer tungsten disulphide High photoresponse can be achieved in monolayers of transition metal dichalcogenides. However, the response times are inconveniently limited by defects. Here, we report low temperature photoresponse of monolayer WS2 prepared by exfoliation and chemical vapour deposition method. The exfoliated device exhibits n-type behaviour; while the CVD device exhibits intrinsic behaviour. In off state, the CVD device has a photoresponse ratio for laser on/off four times larger than that of the device based on exfoliated WS2. And the photoresponse decay-rise time is 0.1s (limited by our setup), compared to few seconds in the exfoliated device. These findings are discussed in terms of charge trapping and localization. 3. Dichroic spin-valley PC in monolayer molybdenum disulphide The aim of valleytronics is to exploit confinement of charge carriers in local valleys of the energy bands of semiconductors as an additional degree of freedom in optoelectronic devices. Thanks to strong direct excitonic transitions in spin-coupled K valleys, monolayer MoS2 is a rapidly emerging valleytronic material, with high valley polarization in photoluminescence. Here we elucidate the excitonic physics of this material by light helicity-dependent PC studies of phototransistors. We demonstrate that large PC dichroism (up to 60%) can also be achieved in high-quality MoS2 monolayers grown by CVD, due to the circular photogalvanic effect on resonant excitations. This opens up new opportunities for valleytronic applications in which selective control of spin-valley-coupled PCs can be used to implement polarization-sensitive light-detection schemes or integrated spintronic devices, as well as biochemical sensors operating at visible frequencies.