Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10), high current density (1-10 μA·μm), and good field-effect transistor mobility (∼30 cm·V·s) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS 2 ) and tungsten (W) doping of molybdenum diselenide (MoSe 2 ) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of two dimensional (2D) materials. To date, the impact of the doping on the structural, electronic and photonic properties of in-situ doped monolayers remains unanswered due to challenges This article is protected by copyright. All rights reserved.2 including strong film-substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, we demonstrate in-situ rhenium (Re) doping of synthetic monolayer MoS 2 with ~1 at% Re. To limit substrate-film charge transfer r-plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n-type doping, which agrees with density functional theory calculations. Moreover, low-temperature photoluminescence (PL) indicates a significant quench of the defect-bound emission when Re is introduced, which is attributed to the Mo-O bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re.The work presented here demonstrates that Re doping of MoS 2 is a promising route towards electronic and photonic engineering of 2D materials.

The utilization of alkali salts, such as NaCl and KI, have enabled the successful growth of large single domain and fully coalesced polycrystalline two-dimensional (2D) transition metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties are not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkaliassisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in as-grown large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and a high density of nanoscale MoS2 particles (4  0.7/μm 2 ). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ~20× and degrades transistor performance.

The residue of common photo‐ and electron‐beam resists, such as poly(methyl methacrylate) (PMMA), is often present on the surface of 2D crystals after device fabrication. The residue degrades device properties by decreasing carrier mobility and creating unwanted doping. Here, MoS2 and WSe2 field effect transistors (FETs) with residue are cleaned by contact mode atomic force microscopy (AFM) and the impact of the residue on: 1) the intrinsic electrical properties, and 2) the effectiveness of electric double layer (EDL) gating are measured. After cleaning, AFM measurements confirm that the surface roughness decreases to its intrinsic state (i.e., ≈0.23 nm for exfoliated MoS2 and WSe2) and Raman spectroscopy shows that the characteristic peak intensities (E2g and A1g) increase. PMMA residue causes p‐type doping corresponding to a charge density of ≈7 × 1011 cm−2 on back‐gated MoS2 and WSe2 FETs. For FETs gated with polyethylene oxide (PEO)76:CsClO4, removing the residue increases the charge density by 4.5 × 1012 cm−2, and the maximum drain current by 247% (statistically significant, p < 0.05). Removing the residue likely allows the ions to be positioned closer to the channel surface, which is essential for achieving the best possible electrostatic gate control in ion‐gated devices.

Formation of an electric double layer (EDL) is a powerful approach for exploring the electronic properties of two-dimensional (2D) materials because of the ultrahigh capacitance and induced charge in the 2D materials. In this work, epitaxial graphene Hall bar devices are gated with an EDL using a 1 μm thick solid polymer electrolyte, poly­(ethylene oxide) and LiClO4. In addition to carrier density and mobility, ion dynamics associated with the formation and dissipation of the EDL are measured as a function of temperature over a gate bias range of ±2 V. The room temperature EDL formation time (∼1–100 s) is longer than the dissipation time (∼10 ms). The EDL dissipation is modeled by a stretched exponential decay, and the temperature-dependent dissipation times are described by the Vogel–Fulcher–Tammann equation, reflecting the coupling between polymer and ion mobility. At low temperatures, approaching the glass transition temperature of the electrolyte, the dissipation times of both cations and anions exceed several hours, and both p- and n-type EDLs can persist in the absence of a gate bias. The measured temperature-dependent relaxation times qualitatively agree with COMSOL multiphysics simulations of time-dependent ion transport in the presence of an applied field.