Facile Electrochemical Synthesis of 2D Monolayers for High-Performance Thin-Film Transistors

Electrochemical and X-ray structural evidence of multiple molybdenum precursor candidates from a reported non-aqueous electrodeposition of molybdenum disulfide

A published report of electrodeposited molybdenum(iv) disulfide microflowers at 100 °C in the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethane)sulfonimide (PP13-TFSI) from 1,4-butanedithiol and the concentrated filtrate from a reaction mixture of molybdenum(vi) trioxide and ethylene glycol could not be reproduced reliably, affording numerous uniquely coloured reaction mixtures that precipitated a variety of crystalline molybdenum coordination complexes. Further attempts to use the same two of these filtrates to electrodeposit molybdenum(iv) disulfide from 0.1 M PP13-TFSI in tetrahydrofuran with 1,4-butanedithiol at room temperature were unsuccessful. Various crude reaction mixtures grew crystals of different identity from eight attempts to synthesize the reported molybdenum-precursor. Single crystal X-ray diffraction (SC-XRD) offered insight into a wide range of structural features from four candidate paramagnetic precursor compounds, including a novel organomolybdenum cluster. Electrochemical studies of the various molybdenum-precursor filtrates, ethylene glycol, and 1,4-butanedithiol were conducted in 0.1 M PP13-TFSI in tetrahydrofuran, offering insight into differences between preparations of the molybdenum-precursor and interference between ethylene glycol and 1,4-butanedithiol on platinum working electrodes. Molybdenum(iv) disulfide electrodeposition attempts included cyclic voltammetry and chronoamperometry on platinum and glassy carbon working electrodes, which led to either no deposited material, or molybdenum, carbon, oxygen, and sulfur containing amorphous and non-homogenous deposits, as indicated by SEM-EDS analysis.

Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS₂ (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS₂ film was grown by direct sulfurization. First, the growth of MoS₂ was observed under an optical microscope. The number of MoS₂ layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS₂. The growth of MoS₂ is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS₂ on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS₂ film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm^-1 is 19.1 cm^-1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS₂ thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS₂ continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS₂ film. This work provides a reference for growing MoS₂ in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS₂ requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS₂ monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS₂ monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS₂ monolayers are largely free of IDBs but instead have translational boundaries that arise when WS₂ domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS₂ monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS₂ field effect transistors exhibited an I_ON/_OFF of ∼10⁷ and mobility of 16 cm²/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Benchmarking monolayer MoS2 and WS2 field-effect transistors

Here we benchmark device-to-device variation in field-effect transistors (FETs) based on monolayer MoS₂ and WS₂ films grown using metal-organic chemical vapor deposition process. Our study involves 230 MoS₂ FETs and 160 WS₂ FETs with channel lengths ranging from 5 μm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of 2D FETs across 1 × 1 cm² chips owing to high quality and uniform growth of these TMDs followed by clean transfer onto device substrates. We are able to demonstrate record high carrier mobility of 33 cm² V^-1 s^-1 in WS₂ FETs, which is a 1.5X improvement compared to the best reported in the literature. Our experimental demonstrations confirm the technological viability of 2D FETs in future integrated circuits.

Graphene to Advanced MoS2: A Review of Structure, Synthesis, and Optoelectronic Device Application

In contrast to zero-dimensional (0D), one-dimensional (1D), and even their bulk equivalents, in two-dimensional (2D) layered materials, charge carriers are confined across thickness and are empowered to move across the planes. The features of 2D structures, such as quantum confinement, high absorption coefficient, high surface-to-volume ratio, and tunable bandgap, make them an encouraging contestant in various fields such as electronics, energy storage, catalysis, etc. In this review, we provide a gentle introduction to the 2D family, then a brief description of transition metal dichalcogenides (TMDCs), mainly focusing on MoS2, followed by the crystal structure and synthesis of MoS2, and finally wet chemistry methods. Later on, applications of MoS2 in dye-sensitized, organic, and perovskite solar cells are discussed. MoS2 has impressive optoelectronic properties; due to the fact of its tunable work function, it can be used as a transport layer, buffer layer, and as an absorber layer in heterojunction solar cells. A power conversion efficiency (PCE) of 8.40% as an absorber and 13.3% as carrier transfer layer have been reported for MoS2-based organic and perovskite solar cells, respectively. Moreover, MoS2 is a potential replacement for the platinum counter electrode in dye-sensitized solar cells with a PCE of 7.50%. This review also highlights the incorporation of MoS2 in silicon-based heterostructures where graphene/MoS2/n-Si-based heterojunction solar cell devices exhibit a PCE of 11.1%.

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 μm) monolayer WS₂ using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS₂ samples. The absorption spectrum of monolayer WS₂ shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS₂ gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS₂. Furthermore, the efficacy of the synthesized WS₂ crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of μ ∼ 6 cm² V^-1 s^-1, current ON/OFF ratio of ∼10⁶, and subthreshold swing (SS) of ∼641 mV decade^-1, which are comparable to those of the exfoliated monolayer WS₂ FETs. These findings suggest that our APCVD-grown WS₂ has the potential to be used for next-generation nanoelectronic and optoelectronic applications.

Wafer-Scale, Conformal, and Low-Temperature Synthesis of Layered Tin Disulfides for Emerging Nonplanar and Flexible Electronics

Two-dimensional (2D) metal dichalcogenides have drawn considerable interest because they offer possibilities for the implementation of emerging electronics. The emerging electronics are moving toward two major directions: vertical expansion of device space and flexibility. However, the development of a synthesis method for 2D metal dichalcogenides that meets all the requirements remains a significant challenge. Here, we propose a promising method for wafer-scale, conformal, and low-temperature (≤240 °C) synthesis of single-phase SnS₂ via the atomic layer deposition technique. There is a trade-off relationship between the crystallinity and orientation preference of SnS₂, which is efficiently eliminated by the two-step growth occurring at different temperatures. Consequently, the van der Waals layers of the highly crystalline SnS₂ are parallel to the substrate. Thin-film transistors (TFTs) comprising the SnS₂ layer show reasonable electrical performances (field-effect mobility: ∼0.8 cm² V^-1 s^-1 and on/off ratio: ∼10⁶), which are comparable to that of a single-crystal SnS₂ flake. Moreover, we demonstrate nonplanar and flexible TFTs to identify the feasibility of the implementation of future electronics. Both the diagonal-structured TFT and flexible TFT fabricated without a transfer process show electrical performances comparable to those of rigid and planar TFTs. Particularly, the flexible TFT does not exhibit substantial degradation even after 2000 bending cycles. Our work would provide decisive opportunities for the implementation of future electronic devices utilizing 2D metal chalcogenides.

A biomimetic 2D transistor for audiomorphic computing

In this article, we introduce a biomimetic audiomorphic device that captures the neurobiological architecture and computational map inside the auditory cortex of barn owl known for its exceptional hunting ability in complete darkness using auditory cues. The device consists of multiple split-gates with nanogaps on a semiconducting MoS₂ channel connected to the source/drain contacts for imitating the spatial map of coincidence detector neurons and tunable RC circuits for imitating the interaural time delay neurons following the Jeffress model of sound localization. Furthermore, we use global back-gating capability to demonstrate neuroplasticity to capture behavioral and/or adaptation related changes in the barn owl. Finally, the virtual source model for current transport is combined with finite element COMSOL multiphysics simulations to explain and project the performance of the biomimetic audiomorphic device. We find that the precision of the biomimetic device can supersede the barn owl by orders of magnitude.

Biomimetic audiomorphic functionalities can be implemented in solid-state devices including 2D materials. Here, the authors fabricate a device based on multiple split gates with nano-gaps on a single semiconducting MoS₂ channel that captures the neurobiological architecture and computational map inside the auditory cortex of barn owl.

Extraordinary Radiation Hardness of Atomically Thin MoS2

We demonstrate that atomically thin layered two-dimensional (2D) semiconductors are promising candidates for space electronics owing to their inherent and extraordinary resilience to radiation damage from energetic heavy charged particles. In particular, we found that ultrathin MoS₂ nanosheets can easily withstand proton and helium irradiation with fluences as high as ∼10¹⁶ and ∼10¹⁵ ions/cm², respectively, corresponding to hundreds or thousands of years of unshielded exposure to radiation in space. While radiation effects on 2D material-based field effect transistors have been reported in the recent past, none of these studies could isolate the impact of irradiation on standalone ultrathin 2D layers. By adopting a unique experimental approach that exploits the van der Waals epitaxy of 2D materials, we were able to differentiate the effects of radiation on the 2D semiconducting channel from that of the underlying dielectric substrate, semiconductor/substrate interface, and metal/semiconductor contact interface, revealing the ultimate potential of these 2D materials. Furthermore, we used a statistical approach to evaluate the effect of radiation damage on critical device and material parameters, including threshold voltage, subthreshold slope, and carrier mobility. The statistical approach lends additional credence to the general conclusions drawn from this study, overcoming a common drawback of methods applied in this area of research. Our findings do not only offer exciting prospects for the operation of modern electronics in space, but may also benefit electronics applications in high-altitude flights, military aircraft, satellites, nuclear reactors, particle accelerators, and other high-radiation environments. Additionally, they highlight the importance of evaluating the impact of damage to the substrate and surrounding materials on electrical characteristics during future radiation studies of 2D materials.

Electrochemical Polishing of Two-Dimensional Materials

Two-dimensional (2D) layered materials demonstrate their exquisite properties such as high temperature superconductivity, superlubricity, charge density wave, piezotronics, flextronics, straintronics, spintronics, valleytronics, and optoelectronics, mostly, at the monolayer limit. Following initial breakthroughs based on micromechanically exfoliated 2D monolayers, significant progress has been made in recent years toward the bottom-up synthesis of large-area monolayer 2D materials such as MoS₂ and WS₂ using physical vapor deposition and chemical vapor deposition techniques in order to facilitate their transition into commercial technologies. However, the nucleation and subsequent growth of the secondary, tertiary, and greater numbers of vertical layers poses a significant challenge not only toward the realization of uniform monolayers but also toward maintaining their consistent electronic and optoelectronic properties which change abruptly when transitioning from the monolayer to multilayer form. Chemical or physical techniques which can remove the unwanted top layers without compromising the material quality will have tremendous consequences toward the development of atomically flat, large-area, uniform monolayers of 2D materials. Here, we report a simple, elegant, and self-limiting electrochemical polishing technique that can thin down any arbitrary thickness of 2D material, irrespective of whether these are obtained using powder vapor transport or mechanical exfoliation, into their corresponding monolayer form at room temperature within a few seconds without compromising their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D transition-metal dichalcogenides owing to the stability of their basal planes, enhanced edge reactivity, and stronger than van der Waals interaction with the substrate. Our study also reveals that 2D monolayers are chemically more robust and corrosion resistant compared to their bulk counterparts in similar oxidative environments, which enables electrochemical polishing of such materials down to a monolayer.

In Situ Optical Tracking of Electroablation in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are a unique class of 2D materials possessing unique optoelectronic properties when exfoliated into mono- and few-layer sheets. Recently, electroablation (EA) has become of interest as a promising synthesis method for single-layer sheets of TMDs. Here, we introduce spectroelectrochemical micro-extinction spectroscopy (SE-MExS) as a high-throughput technique to study electrochemical thinning of TMDs as it occurs. This approach enables the parallel use of spectroscopy and imaging to nondestructively characterize 2D materials in situ. We unravel optoelectronics of the TMDs by observing changes in optical properties during EA. We find that the EA process for MoS₂, WS₂, MoSe₂, and WSe₂ occurs edge first, generating high density of edge sites. Our results show that stable monolayers of MoS₂, WS₂, and MoSe₂ can be synthesized from bulk precursors by the EA process, while conversely, no WSe₂ remains postablation.

Defect Dynamics in 2-D MoS2 Probed by Using Machine Learning, Atomistic Simulations, and High-Resolution Microscopy

Structural defects govern various physical, chemical, and optoelectronic properties of two-dimensional transition-metal dichalcogenides (TMDs). A fundamental understanding of the spatial distribution and dynamics of defects in these low-dimensional systems is critical for advances in nanotechnology. However, such understanding has remained elusive primarily due to the inaccessibility of (a) necessary time scales via standard atomistic simulations and (b) required spatiotemporal resolution in experiments. Here, we take advantage of supervised machine learning, in situ high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations to overcome these limitations. We combine genetic algorithms (GA) with MD to investigate the extended structure of point defects, their dynamical evolution, and their role in inducing the phase transition between the semiconducting (2H) and metallic (1T) phase in monolayer MoS₂. GA-based structural optimization is used to identify the long-range structure of randomly distributed point defects (sulfur vacancies) for various defect densities. Regardless of the density, we find that organization of sulfur vacancies into extended lines is the most energetically favorable. HRTEM validates these findings and suggests a phase transformation from the 2H-to-1T phase that is localized near these extended defects when exposed to high electron beam doses. MD simulations elucidate the molecular mechanism driving the onset of the 2H to 1T transformation and indicate that finite amounts of 1T phase can be retained by increasing the defect concentration and temperature. This work significantly advances the current understanding of defect structure/evolution and structural transitions in 2D TMDs, which is crucial for designing nanoscale devices with desired functionality.

High-Resolution Inkjet-Printed Oxide Thin-Film Transistors with a Self-Aligned Fine Channel Bank Structure

A self-aligned inkjet printing process has been developed to construct small channel metal oxide (a-IGZO) thin-film transistors (TFTs) with independent bottom gates on transparent glass substrates. Poly(methylsilsesquioxane) was used to pattern hydrophobic banks on the transparent substrate instead of commonly used self-assembled octadecyltrichlorosilane. Photolithographic exposure from backside using bottom-gate electrodes as mask formed hydrophilic channel areas for the TFTs. IGZO ink was selectively deposited by an inkjet printer in the hydrophilic channel region and confined by the hydrophobic bank structure, resulting in the precise deposition of semiconductor layers just above the gate electrodes. Inkjet-printed IGZO TFTs with independent gate electrodes of 10 μm width have been demonstrated, avoiding completely printed channel beyond the broad of the gate electrodes. The TFTs showed on/off ratios of 10⁸, maximum mobility of 3.3 cm² V^-1 s^-1, negligible hysteresis, and good uniformity. This method is conductive to minimizing the area of printed TFTs so as to the development of high-resolution printing displays.

Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides

Physics of monolayer and few-layer transition metal dichalcogenides (TMDs) and chemistry of few-layer TMDs have been well studied in recent years in the context of future electronic, optoelectronic, and energy harvesting applications. However, what has escaped the attention of the scientific community is the unique chemistry of monolayer TMDs. It has been demonstrated that the basal plane of multilayer TMDs is chemically inert, whereas edge sites are chemically active. In this article, we experimentally demonstrate that the edge reactivity of the TMDs can be significantly impeded at the monolayer limit through monolayer/substrate interaction, thus making the monolayers highly resistant to electrooxidation and corrosion. In particular, we found that few-layer flakes of MoS₂ and WS₂ exfoliated on conductive TiN substrates are readily corroded beyond a certain positive electrode potential, while monolayer remnants are left behind unscathed. The electrooxidation resistance of monolayers was confirmed using a plethora of characterization techniques including atomic force microscope (AFM) imaging, Raman spectroscopy, photoluminescence (PL) mapping, scanning/transmission electron microscope (S/TEM) imaging, and selected area electron diffraction (SAED). It is believed that strong substrate monolayer interaction compared to the relatively weak interlayer van der Waals interaction is responsible for the superior monolayers chemical stability in highly corrosive oxidizing environments. Our findings could pave the way for the implementation of monolayer transition metal disulfides as superior anticorrosion coating which can have a significant socioeconomic impact.