2D materials: Transition metal dichalcogenides

Two dimensional transition metal dichalcogenides (2D TMDCs) MX₂, (where M =Mo, W and X = S, Se, Te) are semiconducting two dimensional crystals with a crystal structure composed of an inner layer of metal M atoms ordered in a triangular lattice sandwiched between two layers of chalcogen X atoms. They have exceptional electronic, optical, mechanical, chemical and thermal properties. The lack of dangling bonds, high mobility, sub-nanometer confinement of carriers within the plane of the material, and unique band structure make them attractive for a myriad of electronic, optoelectronic, and quantum applications. Additionally, their electronic structure and optoelectronic properties can be precisely manipulated by electrostatic stimuli, stoichiometric defects and vacancies, strain, and chemical doping, providing unprecedented tunability of the local response of the material. Such local manipulation of the optical and electronic properties can be exploited for novel device architectures, suggesting new design principles to create energy funnels for solar energy conversion or quantum optics. Such variations in composition and structure reflect characteristics of materials growth, processing, and substrate effects and affect the local energetic structure of the 2D semiconductor. The reduced dielectric screening and large Coulomb attraction yield a rich manifold of photo-excitations that are stable at room temperature, such as excitons (with a strong binding energy of hundreds of meV), positively or negatively charged trions resulting from high carrier doping levels (from the substrate, vacancies, adsorbates etc.). The optically driven nano-manipulator will then be implemented to investigate 2D TMDCs. The gate voltages created through photo-doping of the MO NCs are expected to modulate the carrier density in the TMDC, such as MoS₂, between undoped and carrier densities in the range of 10¹¹ to 10¹² cm² causing a pronounced reduction in the local bandgap (up to hundreds of meV). This is expected to create a local potential well that would localize excitons and modulate exciton photoluminescence (PL) by 1-2 orders of magnitude. Local electrostatic fields and field gradients should also create local piezoelectric-related strain fields that would in turn act as potential wells for capturing excitons. Questions to be addressed include how localized these wells can be, and if they result in true quantum dots where ever there is a nano-gate. This would be of enormous importance for quantum optics applications, e.g. individually-addressable local single-photon sources at specified locations.