Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low. The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QY of 0.6%, which indicates a considerable defect density. Here we report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude. The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a final QY of more than 95%, with a longest-observed lifetime of 10.8 ± 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.

Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e., SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.

Transition metal dichalcogenides (TMDs) are being considered for a variety of electronic and optoelectronic devices such as beyond complementary metal-oxide-semiconductor (CMOS) switches, light-emitting diodes, solar cells, as well as sensors, among others. Molybdenum disulfide (MoS2) is the most studied of the TMDs in part because of its availability in the natural or geological form. The performance of most devices is strongly affected by the intrinsic defects in geological MoS2. Indeed, most sources of current transition metal dichalcogenides have defects, including many impurities. The variability in the electrical properties of MoS2 across the surface of the same crystal has been shown to be correlated with local variations in stoichiometry as well as metallic-like and structural defects. The presence of impurities has also been suggested to play a role in determining the Fermi level in MoS2. The main focus of this work is to highlight a number of intrinsic defects detected on natural, exfoliated MoS2 crystals from two different sources that have been often used in previous reports for device fabrication. We employed room temperature scanning tunneling microscopy (STM) and spectroscopy (STS), inductively coupled plasma mass spectrometry (ICPMS), as well as X-ray photoelectron spectroscopy (XPS) to study the pristine surface of MoS2(0001) immediately after exfoliation. ICPMS used to measure the concentration of impurity elements can in part explain the local contrast behavior observed in STM images. This work highlights that the high concentration of surface defects and impurity atoms may explain the variability observed in the electrical and physical characteristics of MoS2.

Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.