Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance ( ), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers -the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multi-dimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.

Screening due to surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasi-particle bandgap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence (PL) peak shift at low temperature for higher energy excitons (2s, 3s, 4s, 5s) in monolayer MoSe2, while the 1s exciton peak position remains unaltered -a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasiparticle bandgap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be . % ( . % for 2s, . % for 3s, . Table I. * Model predictedIn Table I, we have also shown the measured A 1 − trion binding energy: Δ 1 − = 1 0 − 1 − , which also exhibits 15.1% reduction in sample M2. Note that, Δ 1 − being estimated from the separation between two PL peaks, does not involve the quasi-particle bandgap, and provides an independent direct evidence of the modulation of binding energy in presence of larger dielectric screening.In conclusion, we explored new perspective of environment screening on two-dimensional monolayers by exploiting the idea of increasing mismatch between quasiparticle bandgap renormalization and modification in exciton binding energy for increasing quantum number of the exciton state. The proposed technique allows us to unambiguously estimate all the necessary information about the excitonic series and quasi-particle bandgap change in twodimensional monolayer embedded in different dielectric media. Our results clearly demonstrate the prominent effect of substrate and environment induced screening in two-dimensional system, making this effect crucial to be taken into account while analyzing results in existing devices based on 2D materials. For example, the band structure of the 2D material in the region underneath the contact material or in the presence of a gate dielectric is expected to be modified locally as a result of this effect, and is expected to play an important role in determining the device performance. Similar revisit will also be required in analyzing the performance of 2D material based photodetectors as this screening induced unintentionally created built-in field at the source junction will support efficient electron-hole separation. Finally, planar heterojunctions in two-dimensional crystals are generally difficult to achieve due to stringent growth conditions. The results described in this work open up the possibility of a new class of twodimensional planar heterostructure devices by only spatially modifying the substrate or top dielectric constant.ACKNOWLEDGMENT

MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied with very strong exciton binding energy -resulting in complex photoresponse characteristics. We study the electrical response to scanning photoexcitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. Strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii) excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/multi-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/multi-layer heterojunction. Finally, we investigate laser position dependent transient response of photocurrent to reveal trapping of carriers in SiO2 at the source junction is the critical factor to determine the transient response in 2D photodetectors, and also show that, by systematic device design, such trapping can be avoided in the heterojunction devices, resulting in fast transient response. The insights obtained will play an important role in designing fast 2D TMDs based photodetector and related optoelectronic and thermoelectric devices.2 Introduction:

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultra-thin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through inter-layer dipole-dipole coupling induced non-radiative Förster resonance energy transfer (FRET) from SnSe2 underneath which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the inplane orientation of dipoles in these layered materials. The FRET driven uniform single-and twophoton luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers, and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications. KEYWORDS: MoS2, WS2, SnSe2, van der Waals heterostructure, photoluminescence enhancement, two-photon luminescence, Förster Resonance Energy Transfer (FRET), charge transfer.transfer across WS2/MoSe2 hetero-bilayer stack through FRET across higher order exciton transitions. Nonetheless, the donor's absorption is still constrained by its physical thickness at the monolayer limit despite the high efficiency of FRET at the closest possible physical separation.Here, we demonstrate enhanced PL of monolayer MoS2 (and WS2) across its vdW heterojunction with multi-layer SnSe2 via FRET with single and two-photon excitation. Counteracting the charge transfer across highly staggered conduction bands of MoS2 and SnSe2, MoS2 single-photon luminescence (1P-PL) shows ~14-fold enhancement at room temperature with resonant excitation and ~5-fold enhancement with non-resonant excitation, while two-photon luminescence (2P-PL) of MoS2 shows up to ~7.5-fold enhancement with non-resonant excitation. Even with the insertion of few-layer hBN between MoS2 and SnSe2, the 1P-PL enhancement persists up to 5 times with resonant excitation. We demonstrate modulation of the degree of the PL enhancement by systematic parameter variation, including donor material, acceptor material, their thickness, physical separation between donor and acceptor, sample temperature, and excitation wavelength which corroborate FRET aided PL enhancement. We emphasize the intrinsic advantage of realizing FRET with SnSe2 as a donor and elucidate the impact of multiple parameters on the luminescence enhance...