Photonics of novel two-dimensional semiconductors and heterostructures

Innumerable material systems have been developed with two-dimensional (2D) materials due to their distinctive electronic, optoelectronic, and mechanical properties. Such properties offer great functionalities that enable this classification of materials to be used for a plethora of applications.

In this thesis, the influence of light on various 2D materials is studied. The core of this work involves exploring the electronic and optoelectronic properties of chalcogenides, oxides, and elemental 2D materials. The produced 2D materials were investigated for their light-matter interaction to be used in applications such as a photodetector and an artificial optoelectronic synapse. Materials of interest include tin monosulfide (SnS) - a layered monochalcogenide, zinc oxide (ZnO) - a non-layered 2D oxide, molybdenum disulfide (MoS2) - a layered dichalcogenide, and black phosphorus (BP) - an elemental analogous of graphene.

Functional materials like SnS and ZnO have been researched extensively in their bulk form that shows intriguing optoelectronic properties. Despite this, the lack of successful synthesis techniques to prepare large-area ultra-thin materials has prevented further exploration of these materials to be used for light sensing applications. Recent progress in liquid metal chemistry offers real solutions in this area that provides the possibility to explore the untapped properties of atomically thin SnS and ZnO nanostructures. Some of the niche areas for further investigation of the fundamental electronic and optoelectronic properties of both SnS and ZnO 2D materials were identified and studied. In addition to studying the optoelectronic properties of stand-alone 2D materials, the interaction of electron-hole pairs at the heterointerface of semiconductors was also studied. Before this, various 2D-based heterostructures devised for light sensing applications were reviewed. After which, a vertically stacked heterostructure made of BP and MoS2 synthesized via mechanical exfoliation technique was investigated for field-effect transistor and photodetection performances. The aforementioned goals to study the novel optoelectronic properties of each material were implemented in three distinct stages.

The first stage of research involved experimental verification and analysis of the electronic and optoelectronic properties of wafer-scale atomically thin layers of SnS. To date, conventional synthesis strategies have prevented the exploration of the intriguing optoelectronic properties of SnS in its atomically thin form. This is mainly due to the strong interlayer coupling between adjacent SnS layers. Here, by utilizing the weak vdW forces between layered SnS, highly crystalline, large-area single and multiple-unit cell thick SnS layers have been delaminated through the liquid metal exfoliation technique. Nanolayers of SnS with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (~1.8 nm) were synthesized from the metallic liquid tin in a sulfur-rich environment, with lateral dimensions in the millimeter scale. Eventually, the as-prepared stoichiometric atomically thin SnS layers revealed broadband spectral response ranging from deep ultraviolet (UV) to near-infrared (NIR) wavelengths (i.e., 280 nm to 850 nm) with ultra-fast photodetection capabilities in the range of microseconds. The devised unit-cell thick photodetectors of SnS showed performances superior to that of commercial photodetectors. Figures of merit with responsivity the range 102-103 A.W-1, detectivity in the range 109-1010 Jones, and external quantum efficiency between 104 105 % have been recorded for unit cell thick photodetectors. The tunable bandgap nature of the material analysed experimentally and theoretically provided further insight in developing SnS layers as high-performance photodetectors.

In the second stage of research, optical sensing of few atom thick ZnO nanosheets was performed. Despite being a widely recognized functional material, there have been minimal investigations to obtain large area ultra-thin 2D ZnO nanosheets. By utilizing the recently developed liquid metal exfoliation technique, atomically thin nanolayers of ZnO were synthesized by delaminating the interfacial oxide layer from the liquid ZnO droplet. The attained non-layered ZnO nanosheets with a thickness of 5 nm were revealed to perform as a strictly visible-blind UV photodetector. At a low operating bias of 50 mV and low intensity of 0.5 mW/cm2, the devised UV photodetector demonstrated high figures of merit such as external quantum efficiency, responsivity, and detectivity of 4.3×103 %, 12.64 A.W-1 and 5.81×1015 Jones at a wavelength of 365 nm. Additionally, the available gap states in 2D ZnO nanosheets displayed bio-realistic synaptic characteristics of the human brain. Important synaptic behaviours such as short-term potentiation, long-term potentiation, paired-pulse facilitation, Hebbian's synaptic learning, and many more were demonstrated solely through UV optical stimuli. This way, critical neural functionalities were emulated by applying photonic signals with 2D ZnO nanosheets highly desirable for advanced electronic applications.

After analysing the optoelectronic properties of pristine materials, the third and final stage of research involved studying a heterostructure for field-effect transistor and photodetection applications. Different alien semiconductors such as BP and MoS2 were combined to form a vertically stacked vdW heterostructure. The heterojunction was electrically tuned using a gate voltage which achieved a remarkably low dark current observed in the order of picoamperes. Additionally, nanoflakes of BP and MoS2 synthesised via mechanical exfoliation were developed to perform as a UV-visible photodetector.

Overall, it is perceived that investigation into the optoelectronic properties of two pristine materials and a heterostructure formed with 2D materials could contribute to the development of advanced optoelectronic applications for practical purposes. Furthermore, it is strongly felt that the affordable, scalable technique implemented here could provide stimulating pathways towards miniaturizing light detection devices and neuromorphic circuits.