Two-dimensional post transition metal dichalcogenides for high performance optoelectronic devices

Given the large surface-volume ratio for two-dimensional (2D) materials, the intrinsic photoexcitation behaviours are found to be exponentially enhanced, inducing strong influences on the material physiochemical properties. Therefore, with the emerging research of ultra-thin nanostructures, the exploration of the light-matter interaction based practical applications upon 2D materials has been receiving increasing attention in recent years. Apart from the well-studied graphene and black phosphorus (BP) with broadband absorption, metal dichalcogenides have been identified as new candidates for the development of high-performance optical and optoelectronic devices due to their strong optical absorption nature covering from ultra-violet (UV) to visible light spectra. However, comprehensive studies in mechanisms for those unique light-matter phenomena as well as their practical devices are still in the early stage. Thus, fully understanding the light-matter interaction properties and implementing them into the practical applications can be great challenges and research opportunities in near future. Therefore, the author of this thesis focuses on the study of 2D post transition metal dichalcogenides (PTMDs) from their synthesis methods to the fundamental analyses of their optical properties, consequently leading to high-effective applications for each chosen material. The research work started from the gallium sulphide (Ga2S3) which is exfoliated from the Ga2S3 bulk into 2D form by using the conventional liquid phase exfoliation (LPE) method. With thoroughly characterisations upon the obtained 2D Ga2S3, the nanoflakes are found to possess an ultrafast excitation relaxation behaviour in response to the excitation of visible light. Moreover, a considerable variation for refractive index of the 2D Ga2S3 is observed under the irradiation of green light. Based on that, the ultra-thin 2D Ga2S3 based all-optical switching on silicon waveguide platform is successfully realised. With a silicon-based asymmetric Mach-Zehnder interferometer (MZI) structure, the external high frequency pump signal (¿ = 532 nm) applied onto the deposited 2D Ga2S3 on one arm leads to the variation of the refractive index, which in turn changes the phase of the optical mode in the waveguide and varies the optical transmission power after combining the light (¿ = 1550 nm) from two arms. Such power variation can be detected by fast-response photodetector and the ultra-fast optical switching results can be obtained with the switching on and off of the pump signal.  Therefore, such a platform paves the way of 2D materials for the applications of the high-performance and high-compactness on-chip optoelectronic devices. After the photonic applications, the author also explores the great potential for 2D PTMDs in physisorption-base gas sensors enabled by the light-matter interactions. The tin sulphide (SnS2) is chosen to be thinned down into few layers thick using wet chemical synthesis approach. Upon comprehensive investigations of material characterisations, the 2D SnS2 shows a prolonged excitation radiative lifetime under the illumination of visible light. On the other hand, it is known that the gas sensing behaviour is dominated by a physisorption-based mechanism for most 2D materials including the 2D SnS2 nanoflakes. The photoinduced abundant electron-hole pairs are expected to be able to replace the thermal excitation owing to its strong optical absorption nature at the visible light spectrum. The SnS2 nanoflakes are thus realised in an all-optical gas sensing platform based on a D-shape fibre, in which an incident laser signal with the wavelength of 473 nm is given. Given the strong interactions with leaking light out from the D-shaped optical fibre, the significant free carries are generated upon the SnS2 deposition layer and consequently interacting with the surrounding analyte gas molecules, resulting in changes in optical absorption and scattering properties with the increasing or decreasing concentration of analyte gases. As a result, the all-optical gas sensor demonstrates an outstanding sensing performance towards the NO2 gas with an estimated limit of detection (LOD) of 464 ppt at room temperature. In the final stage of this thesis, the author focuses on the investigations of Janus crystal structure as well as its intrinsic light-matter properties. With a well-controlled LPE method, the non-layered indium sulphide (In2S3) is cleaved into 2D structures in the liquid medium. Owing to the small quantity of oxygen dissolved in the liquid solvent, the surface S atoms are partially replaced by the O upon the surface of the ultrathin In2S3 under the strong mechanical agitation. To confirm such a unique asymmetric crystal structure, the high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADAF-STEM) are utilised, in which the oxysulphide nucleation layer with a hexagonal coordination system is found to be formed upon the original tetragonal In2S3 body where the interface between two distinct layers remains in a covalent bond. Given that, a built-in electric field is expected to be formed inside the unit-cell crystal system, leading to spatially separations of electron-hole pairs between oxysulphide and sulphide phases, resulting in a significantly prolonged excitation radiative behaviour, which is further confirmed upon the time-resolved photoluminescence (TRPL) measurement. As a result, a high performance ultrasensitive visible light driven chemical sensing platform is realised with the obtained Janus indium oxysulphide (JIOS) nanoflakes. In the presence of paramagnetic NO2 gas molecules, the photo-generated excessive electrons are re-distributed onto the SnS2 surface and forming electron dipole coupling with the absorbed gas molecules, consequently, leading to the electrical conductivity variations with NO2 concentration changes. Such a JIOS sensor demonstrates an outstanding room temperature gas sensing performance (LOD=363 ppt) to NO2 gas with an impressive repeatability and long-term stability. In summary, the author successfully achieved several breakthroughs on the studies of light-matter interactions for PTMD materials from fundamental mechanisms to high performance practical applications during his PhD period. It is expected that the outcomes of this research work will open the new era for 2D materials in the development of high-performance, low-cost and robust optoelectronic devices.