No abstract

transition metal dichalcogenides (TMDs) have caught much interest due to their applications in various fields like optoelectronics, [1] sensors, [2] strain engineering, [3,4] valleytronics, [5] spintronics, [6] etc. 2D materials because of its reduced dimension experience reduced dielectric screening. [7,8] The electric field lines between an electron and hole in a single layer of a 2D material extend above and below the surface, with no material available to screen the field lines. [9] This results in a large exciton binding energy [7,8,[10][11][12][13] as well as increased quasiparticle band gap [14] and make the electrical and optical properties of the material susceptible to its dielectric environment. [15,16] A dielectric environment modifies the electron-hole interactions in 2D materials and helps in controlling the band gap of 2D materials [14,17] without disturbing the pristine quality of these materials. Previous theoretical and experimental studies have shown that a higher dielectric environment could reduce the fundamental band gap and exciton binding energy in 2D materials. [13,15,[18][19][20][21][22] The dielectric environment also plays a crucial role in determining the impurity scattering of charge carriers in 2D materials. [23][24][25] This effect is primarily responsible for the enhanced mobility of 2D materials in high dielectric medium. [23,24] Depending on the dielectric mismatch between the 2D semiconducting materials and the surrounding dielectric environment, the effective potential experienced by a mobile electron due to an ionized impurity at the surface of the membrane enhances or diminishes. [23] This effect also depends on the thickness of the semiconducting membrane. When the thickness increases above a critical limit, dielectric mismatch has no effect on Coulomb scattering. [23] All these effects show that controlling the dielectric environment of 2D materials is particularly interesting because of its profound impact on the electronic band gap, screening, exciton and trion binding energies, exciton transport, and mobility modulation, which can help toward the efficient design of various optoelectronic devices.Even though research on 2D TMDs is growing faster with the discovery of more exciting properties, its production and application have not yet started at an industrial scale. So, it isThe reduced dielectric screening in the out of plane direction, makes 2D materials sensitive to the surrounding environment, offering a unique platform with greatly tunable optoelectronic properties. Large exciton binding energy in 2D materials limits their photogeneration efficiency. The strong electric field generated at a p-n junction will help in separating these strongly bound electron hole pairs. Here, the present study demonstrates how engineering the surrounding dielectric environment would facilitate a mixed dimensional van der Waals p-n junction to improve the photoresponse to a great extent. A 3D silicon-2D monolayer MoS 2 heterostructure is fabricated as a model system. Nearly three ord...

Photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers is strongly influenced by the dielectric environment. The defect states present in the substrate induces uncontrollable doping in the TMD monolayer and thereby modifies the PL spectra. There have been enormous efforts to tune and overcome the effect of inevitable substrate defects in PL spectra, but a proper understanding and a convenient way are still lacking. Here, we systematically studied the effect of surface defects by gradually increasing the separation between WS2 monolayer and substrate. Hence, we could precisely modulate the exciton and trion contribution in the PL spectra of WS2. The excitation power dependant measurements on dielectric engineered and patterned substrates helped us to shed light on the mechanism of PL modulation in monolayer WS2. We have also studied the influence of the nature of the charge carried by substrate defects on the PL spectra. These results open a new pathway to modulate and obtain the desired PL spectra of TMDs by engineering the substrates. Our findings will be useful for fabricating excitonic interconnects, valleytronic, and single-photon devices.

Influencing spectral and directional features of exciton emission characteristics from 2D transition metal dichalcogenides by coupling it to plasmonic nanocavities has emerged as an important prospect in nanophotonics of 2D materials. Herein, the directional photoluminescence emission from a tungsten disulfide (WS2) monolayer sandwiched between a single‐crystalline plasmonic silver nanowire (AgNW) waveguide and a gold (Au) mirror is experimentally studied, thus forming a AgNW–WS2–Au cavity. Using polarization‐resolved Fourier‐plane optical microscopy, the directional emission characteristics from the distal end of the AgNW–WS2–Au cavity are quantified. Given that the geometry simultaneously facilitates local field enhancement and waveguiding capability, its utility in 2D material‐based, on‐chip nanophotonic signal processing is envisaged, including nonlinear and quantum optical regimes.

Chemical vapour deposition (CVD) is one of the most promising methods to synthesize monolayers of 2D materials like transition metal dichalcogenides (TMDs) over a large area with high film quality. Among many parameters that determine the growth of 2D materials, flow of precursor near the surface is one of the most sensitive conditions. In this study, we show how subtle changes in the flow near the substrate surface can affect the quality and coverage of the MoS2 monolayer. We fine tune the flow of the carrier gas near the substrate under two extreme conditions to grow large area and clean monolayer. In the first study, we grew several centimetres long continuous monolayer under the condition, which generally produces monolayers of few tens of micrometres in size without tuning the flow on the substrate surface. In the second case, we got monolayer MoS2 under the conditions meant for the formation of bulk MoS2.We achieved this by placing blockades on the substrate surface which helped in modifying the flow near them. Through simulation, we showed how the flow is affected near these blockades and used it as a guiding rule to grow patterned continuous MoS2 monolayers. Detailed electrical and optical measurements were done to determine the quality of the as-grown samples. Our studies provide a way to obtain clean, large area monolayer of desired pattern by tuning the flow of precursor on the vicinity of the substrate surface even when the growth conditions in CVD are far from optimum.