Deconvoluting the Photonic and Electronic Response of 2D Materials: The Case of MoS2

Influence of Substrate-Induced Charge Doping on Defect-Related Excitonic Emission in Monolayer MoS2

Many applications of transition metal dichalcogenides (TMDs) involve transfer to functional substrates that can strongly impact their optical and electronic properties. We investigate the impact that substrate interactions have on free carrier densities and defect-related excitonic (XD) emission from MoS2 monolayers grown by metal-organic chemical vapor deposition. C-plane sapphire substrates mimic common hydroxyl-terminated substrates. We demonstrate that transferring MoS2 monolayers to pristine c-plane sapphire dramatically increases the free electron density within MoS2 layers, quenches XD emission, and accelerates exciton recombination at the optical band edge. In contrast, transferring MoS2 monolayers onto inert hexagonal boron nitride (h-BN) has no measurable influence on these properties. Our findings demonstrate the promise of utilizing substrate engineering to control charge doping interactions and to quench broad XD background emission features that can influence the purity of single photon emitters in TMDs being developed for quantum photonic applications.

Dilute Rhenium Doping and its Impact on Defects in MoS2

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

Among numerous thin film synthesis methods, metalorganic chemical vapor deposition performed in a showerhead reactor is the most promising one for broad use in scalable and commercially adaptable two-dimensional material synthesis processes. Adapting the most efficient monolayer growth methodologies from tube-furnace systems to vertical-showerhead geometries allows us to overcome the intrinsic process limitations and improve the overall monolayer yield quality. Here, we demonstrate large-area, monolayer molybdenum disulphide growth by combining gas-phase precursor supply with unique tube-furnace approaches of utilizing sodium molybdate pre-seeding solution spincoated on a substrate along with water vapor added during the growth step. The engineered process yields a high-quality, 4-inch scale monolayer film on sapphire wafers. The monolayer growth coverage, average crystal size and defect density were evaluated using Raman and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and scanning transmission electron microscopy imaging. Our findings provide a direct step forward toward developing a reproducible and large-scale MoS₂ synthesis with commercial showerhead reactors.

Anticancer properties of vincristine is modulated by microRNAs in acute lymphoblastic leukemia Nalm6 cell line

Precursor B-cell acute lymphoblastic leukemia (B-ALL), a highly diverse disease, is the most widespread pediatric malignancy characterized by cytogenetic and molecular abnormalities such as altered microRNA (miR) expression signatures. MiRs are a class of short noncoding RNAs. Dysregulation in the expression of miRs plays a crucial role in different types of cancers. Vincristine is an antineoplastic drug with a broad spectrum of activity against different hematologic malignancies and is the first-line treatment for B-ALL. Previous studies have proposed miR-17 and miR-181/b as oncomirs and miR-34/a as a tumor suppressor in Nalm6 cells, thus in the current study, we investigated the effects of vincristine treatment on the expression of miR-17, miR-34/a and miR-181/b expression levels. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay was conducted to estimate the optimal concentration of vincristine in the Nalm-6 cell line. Expression of miRs was calculated using real-time PCR. Our results showed significant downregulation of miR-17 (FC = 0.226; P < 0.0004) in Nalm6 cells after vincristine treatment. Conversely, miR-34/a (FC = 4.823; P < 0.0001) was significantly upregulated. Also, the expression of miR-181/b (FC = 0.156; P < 0.3465) was not significantly different between the vincristine treated group and the control group. In conclusion, it is proposed that one of the mechanisms by which vincristine improves B-ALL is by modulating the expression of specific miRs. These specific miRs will serve as good diagnostic and prognostic biomarkers.

Spatially Resolved Persistent Photoconductivity in MoS2-WS2 Lateral Heterostructures

The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS₂-WS₂ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days. Informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS₂ domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.

Influence of the Substrate on the Optical and Photo-electrochemical Properties of Monolayer MoS2

Substrates influence the electrical and optical properties of monolayer (ML) MoS₂ in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS₂. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS₂. Here, we report spatially resolved PL, Raman, and photo-electrochemical current measurements of 5-10 μm-wide ML-MoS₂ triangles deposited on pairs of indium-doped tin oxide (ITO) electrodes that are separated by a narrow insulating quartz channel [i.e., an ITO interdigitated array (IDA) electrode]. Optical microscopy images and atomic force microscopy measurements revealed that the ML-MoS₂ triangles lie conformally on the quartz and ITO substrates. The PL spectrum of MoS₂ shifts and decreases in intensity in the ITO region, which can be attributed to differences in nonradiative and radiative exciton decay channels. Raman spectra showed no significant peak shifts on the two substrates that would have indicated a substrate-induced doping effect. We spatially resolved the photo-electrochemical current because of hole-induced iodide oxidation and observed that ML-MoS₂ produces lower photocurrents in the quartz region than in the ITO region. The correlated PL, Raman, and photocurrent mapping data show that the MoS₂/quartz interface diminishes fast nonradiative exciton decay pathways but the photocurrent response in the quartz region is likely limited by inefficient in-plane carrier transport to the ITO electrode because of carrier recombination with S vacancies in MoS₂ or charged impurities in the quartz substrate. Nonetheless, the experimental methodology presented herein provides a framework to evaluate substrate engineering strategies to tune the (photo)electrocatalytic properties of two-dimensional materials.

Recent Developments in Controlled Vapor-Phase Growth of 2D Group 6 Transition Metal Dichalcogenides

An overview of recent developments in controlled vapor-phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin-film formation mechanisms and strategies for realizing 2D TMD films with less-defective large domains are of central importance because single-crystal-like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)₂ atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD-based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor-phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.

Monolayer MoS2 Nanoribbon Transistors Fabricated by Scanning Probe Lithography

Monolayer MoS₂ is a promising material for nanoelectronics; however, the lack of nanofabrication tools and processes has made it very challenging to realize nanometer-scale electronic devices from monolayer MoS₂. Here, we demonstrate the fabrication of monolayer MoS₂ nanoribbon field-effect transistors as narrow as 30 nm using scanning probe lithography (SPL). The SPL process uses a heated nanometer-scale tip to deposit narrow nanoribbon polymer structures onto monolayer MoS₂. The polymer serves as an etch mask during a XeF₂ vapor etch, which defines the channel of a field-effect transistor (FET). We fabricated seven devices with a channel width ranging from 30 to 370 nm, and the fabrication process was carefully studied by electronic measurements made at each process step. The nanoribbon devices have a current on/off ratio > 10⁴ and an extrinsic field-effect mobility up to 8.53 cm²/(V s). By comparing a 30 nm wide device with a 60 nm wide device that was fabricated on the same MoS₂ flake, we found the narrower device had a smaller mobility, a lower on/off ratio, and a larger subthreshold swing. To our knowledge, this is the first published work that describes a working transistor device from monolayer MoS₂ with a channel width smaller than 100 nm.

Considerations for Utilizing Sodium Chloride in Epitaxial Molybdenum Disulfide

The utilization of alkali salts, such as NaCl and KI, has 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 is not fully established. In this work, we report alkali-free epitaxy of MoS₂ on sapphire and benchmark the properties against alkali-assisted 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 high density of nanoscale MoS₂ particles (4 ± 0.7/μm²). 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.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS₂) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS₂ grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS₂-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS₂, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS₂ growth on sapphire was further developed to the rational synthesis of an in-plane MoS₂-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS₂ parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS₂ was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe₂) 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 WSe₂/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10⁷), high current density (1-10 μA·μm^-1), and good field-effect transistor mobility (∼30 cm²·V^-1·s^-1) 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.