interaction that 2D materials have demonstrated has made them highly attractive for practical device applications. Graphene, the archetype 2D material was explored extensively for a wide array of photonic applications 1 . However, due to the lack of direct bandgap in graphene, considerable attention has shifted towards 2D materials known as layered transition metal dichalcogenides (TMDs) 2,3 . These TMDs are a group of naturally abundant material with a MX 2 stoichiometry, where M is a transition metal element from group VI (M = Mo, W); and X is a chalcogen (M = S, Se, Te). One of the most intriguing aspect of TMDs is the emergence of fundamentally distinct electronic and optoelectronic properties as the material transitions from bulk to the 2D limit (monolayer) [4][5][6][7][8] . For example, the TMDs evolve from indirect to direct bandgap semiconductors spanning the energy range of 1.1 to 1.9 eV in the 2D limit 4-6 .Among the TMDs, molybdenum disulphide (MoS 2 ) is one of the most widely studied systems used to demonstrate 2D light emitters 9 , transistors 10,11 , valleytronics 12-15 and photodetectors 16,17 . The novel excitonic properties of 2D MoS 2 that make it very interesting for both fundamental studies and applications include: (i) the enhanced direct band gap photoluminescence (PL) quantum yield of the monolayer compared with the bulk counterpart 7,8 , (ii) the small effective exciton Bohr radius (0.93 nm) and associated large exciton binding energy (0.897 eV) 18,19 providing the opportunity for excitonic devices that operate at room temperature (RT) and (iii) the 2D nature of the dipole orientation making the excitonic emission highly anisotropic 20 . 3The interaction of a dipole with light can be modified by altering the surrounding dielectric environment. The most widely studied and technologically relevant phenomenon in this context is the Purcell enhancement wherein the spontaneous emission rate of the dipole is enhanced using an optical cavity by altering the photon density of states. Here the coupling between the dipole and the cavity photon is defined to be in the weak coupling regime since the interaction strength is weaker than the dissipation rates of the dipole and the photon. This regime has been demonstrated in the 2D materials using photonic crystal cavities coupled to 2D layers of MoS 2 21 , and WSe 2 22 . It resulted in an enhancement of the spontaneous emission rate and highly directional photon emission.When the interaction between the dipole and the cavity photons occur at a rate that is faster than the average dissipation rates of the cavity photon and dipole, one enters the strong coupling regime resulting in the formation of new eigenstates that are half-light -half-matter bosonic quasiparticles called cavity polaritons. Since with the pioneering work of Weisbuch et al. 23 there have been numerous demonstrations of cavity polariton formation and associated exotic phenomena in solid state systems using microcavities and quantum wells that support quasi 2D excitons [24][25][26] . H...

Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

Recently, monolayers of layered transition metal dichalcogenides (LTMD), such as MX2 (M = Mo, W and X = S, Se), have been reported to exhibit significant spin-valley coupling and optoelectronic performances because of the unique structural symmetry and band structures. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics, and optoelectronics. However, most studies to date are hindered by great challenges on the synthesis and transfer of high-quality LTMD monolayers. Hence, a feasible synthetic process to overcome the challenges is essential. Here, we demonstrate the growth of high-quality MS2 (M = Mo, W) monolayers using ambient-pressure chemical vapor deposition (APCVD) with the seeding of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS). The growth of a MS2 monolayer is achieved on various surfaces with a significant flexibility to surface corrugation. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. We also demonstrate a robust technique in transferring the MS2 monolayer samples to diverse surfaces, which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with LTMD monolayer and functional materials.

Atomically thin molybdenum disulfide (MoS(2)) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS(2) atomic thin layers is still lacking. Here we report that wafer-scale MoS(2) thin layers can be obtained using MoO(3) thin films as a starting material followed by a two-step thermal process, reduction of MoO(3) at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS(2) films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.