Monolayer MoS2 Research Articles

Perceiving the Spectrum of Pain: Wavelength-Sensitive Visual Nociceptive Behaviors in Monolayer MoS2-Based Optical Synaptic Devices

Perceiving the Spectrum of Pain: Wavelength-Sensitive Visual Nociceptive Behaviors in Monolayer MoS₂-Based Optical Synaptic Devices

Bi-MoSe2 Contacts in the Ultraclean Limit: Closing the Theory-Experiment Loop.

Achieving robust electrical contacts is crucial for realizing the promise of monolayer 2D semiconductors such as semiconducting transition metal dichalcogenides (s-TMDs) in electronics. Despite recent breakthroughs, a gap remains between the experimental and theoretical understanding of metal-s-TMDs contacts. This study explores bismuth semimetal contacts to monolayer MoSe2, using a platform that minimizes experimental sources of uncertainty; we combine contact-front and contact-end measurements to measure key parameters like specific resistivity (ρc) and transfer length (Lt). We find that the resistivity of MoSe2 under the contacts is enhanced due to charge transfer that can be modeled using a self-consistent approach. In contrast, ab initio calculations of the interlayer charge transfer rate are inconsistent with the measured value of ρc, highlighting the need for new theoretical approaches.

Ultrafast‐Laser‐Induced Nanostructures with Continuously Tunable Period on Au Surface for Photoluminescence Control in Monolayer MoS2

Ultrafast‐Laser‐Induced Nanostructures with Continuously Tunable Period on Au Surface for Photoluminescence Control in Monolayer MoS₂

Theoretical study on the electrochemical CO2 reduction performance of MoS2-supported Ni single atoms with transition metal substrate doping

In the context of increasing energy issues and climate change, the development of electrochemical CO2 reduction strategies using single atom catalysts (SACs) presents a viable solution by converting CO2 into high-value-added products. Transition metal dichalcogenides (TMDs) have emerged as the main candidates for SACs in electrocatalytic CO2 reduction reaction (CO2RR) catalysts. The regulation of the second coordination sphere of SACs is known to modulate their electronic structure and catalytic behavior. This study delves into the effect of transition metal-doped substrates on SACs. We employed spin-polarization density functional theory (DFT) to design a series of Ni/TM-MoS2 monolayer MoS2 materials, with transition metals (including Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt and Au) substituting the Mo atoms and anchoring single atom Ni. Among them, six Ni/TM-MoS2 catalysts doped with Mn, Fe, Co, Ru, Rh, and Ir demonstrate enhanced CO2RR activity and selectivity when compared to unmodified Ni/MoS2. The Ni/Ru-MoS2 SAC exhibits the best catalytic activity and selectivity for formic acid production, with a limiting potential (UL) of -0.06 V. The incorporation of the metal-doped substrate lowers the work function of the catalyst, facilitating electron transfer and reaction progression. Furthermore, a volcanic relationship was observed between the work function and UL. This study provides strategic insights into the design and development of SACs, highlighting the potential for future modifications and applications.

Doping Strategy of Monolayer MoS2 to Realize the Monitoring of Environmental Concentration of Desflurane: A First-Principles Study.

Desflurane is a new volatile inhalation anesthetic that is widely used in medical operation. However, various diseases can be caused by chronic exposure to desflurane, which is also a greenhouse gas. Therefore, it is urgent to find a suitable method for monitoring desflurane. In this paper, the process of doping of Pd, Pt, and Ni on the MoS2 surface is simulated to determine the stability of the doping structure based on first-principles. The adsorption properties and sensing properties of Pd-MoS2, Pt-MoS2, and Ni-MoS2 on desflurane are explored by parameters including independent gradient model based on Hirshfeld partition (IGMH), electron localization function (ELF), and density of states (DOS), sensibility, and recovery time, subsequently. The doping results show that the three doping systems (Pd-MoS2, Pt-MoS2, and Ni-MoS2) are structurally stable, and the chemical bonds are formed with MoS2. The adsorption results show the best chemisorption between Pt-MoS2 and desflurane with the chemical bonds between them. The results of IGMH, ELF, and DOS also confirm it. The sensing characterization results show that the recovery time of Pt-MoS2 ranges between 85.27 and 0.027 s, and the sensitivity ranges from 99.26 to 25.69%, all of which can meet the requirements of the sensor. Considering the adsorption effect and sensing characteristics, Pt-MoS2 can be used as a gas-sensitive material for detecting the concentration of desflurane.

Light-Driven Ionic and Molecular Transport through Atomically Thin Single Nanopores in MoS2/WS2 Heterobilayers.

Nanofluidic ionic and molecular transport through atomically thin nanopore membranes attracts broad research interest from both scientific and industrial communities for environmental, healthcare, and energy-related technologies. To mimic the biological ion pumping functions, recently, light-induced and quantum effect-facilitated charge separation in heterogeneous 2D-material assemblies is proposed as the fourth type of driving force to achieve active and noninvasive transport of ionic species through synthetic membrane materials. However, to date, engineering versatile van der Waals heterostructures into 2D nanopore membranes remains largely unexplored. Herein, we fabricate single nanopores in heterobilayer transition metal dichalcogenide membranes with helium ion beam irradiation and demonstrate the light-driven ionic transport and molecular translocation phenomena through the atomically thin nanopores. Experimental and simulation results further elucidate the driving mechanism as the photoinduced near-pore electric potential difference due to type II band alignment of the semiconducting WS2 and MoS2 monolayers. The strength of the photoinduced localized electric field near the pore region can be approximately 1.5 times stronger than that of its counterpart under the conventional voltage-driven mode. Consequently, the light-driven mode offers better spatial resolution for single-molecule detection. Light-driven ionic and molecular transport through nanopores in van der Waals heterojunction membranes anticipates transformative working principles for next-generation biomolecular sequencing and gives rise to fascinating opportunities for light-to-chemical energy harvesting nanosystems.

Unveiling thermally driven photoluminescence in CVD grown MoS[formula omitted] dendritic flake

High-quality MoS2 nanostructures were fabricated via chemical vapor deposition technique on SiO2/Si substrate. In this paper, the effect of temperature on the photoluminescence behavior of MoS2 dendritic flake is addressed. We scrutinized the photoluminescence spectra of monolayer and multilayer regions of MoS2 in the temperature range 300 K–680 K. Monolayer and multilayer behavior of MoS2 flakes are confirmed by Raman and Photoluminescence spectroscopy. The excitonic peaks from the multilayer regime become less intense and show a red shift compared to the monolayer PL spectra. Thermally-induced bandgap modulation of MoS2 is demonstrated. The excitonic intensity and peak positions reveal pronounced temperature-dependent changes. These changes are explicable through the increased electron–phonon interaction and lattice rearrangements. Furthermore, first principle calculations are employed to glean insight into the impact of atomic rearrangements on the band gap behavior of MoS2. Our research presents a detailed understanding of thermally driven band gap modulation of monolayer and multilayer MoS2, which is essential for designing optoelectronic devices.

Deterministic grayscale nanotopography to engineer mobilities in strained MoS2 FETs

Field-effect transistors (FETs) based on two-dimensional materials (2DMs) with atomically thin channels have emerged as a promising platform for beyond-silicon electronics. However, low carrier mobility in 2DM transistors driven by phonon scattering remains a critical challenge. To address this issue, we propose the controlled introduction of localized tensile strain as an effective means to inhibit electron-phonon scattering in 2DM. Strain is achieved by conformally adhering the 2DM via van der Waals forces to a dielectric layer previously nanoengineered with a gray-tone topography. Our results show that monolayer MoS2 FETs under tensile strain achieve an 8-fold increase in on-state current, reaching mobilities of 185 cm²/Vs at room temperature, in good agreement with theoretical calculations. The present work on nanotopographic grayscale surface engineering and the use of high-quality dielectric materials has the potential to find application in the nanofabrication of photonic and nanoelectronic devices.

Modulation of Electrostatic Potential in 2D Crystal Engineered by an Array of Alternating Polar Molecules.

The moiré potential in rotationally misfit two-dimensional (2D) heterostructures has been used to build artificial exciton and electron lattices, which have become platforms for realizing exotic electronic phases. Here, we demonstrate a different approach to create a superlattice potential in 2D crystals by using the near field of an array of polar molecules. A bilayer of titanyl phthalocyanine (TiOPc), consisting of alternating out-of-plane dipoles, is deposited on monolayer MoS2. Time-resolved two-photon photoemission spectroscopy reveals a pair of interlayer exciton states with an energy difference of ∼0.1 eV, which is consistent with the electrostatic potential modulation induced by the TiOPc bilayer as determined by density functional theory calculations. Because the symmetry and the period of this potential superlattice can be changed readily by using molecules of different shapes and sizes, molecule/2D heterostructures can be promising platforms for designing artificial exciton and electron lattices.

(Invited) Transferable Organic Semiconductor Nanosheets for Van Der Waals Heterojunction Devices

The combination of organic semiconductors (OSCs) and two-dimensional materials provides a new basis to harness the advantages of both material systems, for example to fabricate unipolar pentacene thin-film transistors using graphene contacts [1] and p-n junctions using pentacene and molybdenum disulfide [2]. However, the fabrication of such devices by vapor deposition of OSCs is a challenge because growth is limited by the surface-dependent diffusion of small molecules. In this talk, I will present a novel method for the preparation of transferable OSC nanosheets that overcomes this problem. Highly crystalline, wafer-scale (3") nanosheets as thin as 20 nm can be obtained by precisely delaminating the OSC layer from a water-soluble sacrificial film in an appropriate wetting geometry. These free-floating films are then deposited on arbitrary substrates while maintaining morphology and crystallinity. Consequently, we demonstrate functional OSC nanosheets on top of prefabricated electrodes and monolayer MoS2, resulting in devices such as unipolar, ambipolar, and anti-ambipolar field effect transistors [3].

Photo-Modulation of Atomically Thin Molybdenum Diselenides Functionalized with Photochromic Molecules

Two-dimensional (2D) transition metal dichalcogenide semiconductors have garnered significant attention due to their extraordinary electrical and optical properties. It has been increasingly important to develop strategies for modulating their properties. A number of methods have been introduced, including doping, electric and magnetic fields, mechanical strain, etc. Herein, we report a novel approach of using photochromic molecules to modulate optoelectronic properties of monolayer MoSe2 with light. We have synthesized photochromic diarylethene (DAE) molecules that can switch between two isomeric forms, referred to as open and closed, under UV and visible light, respectively. The DAE molecules formed a uniform layer with a thickness of approximately 2 nm on monolayer MoSe2. Our density functional theory (DFT) calculations suggest that the TMD and DAE will align their band energy levels such that UV irradiation moves the lowest unoccupied molecular orbital (LUMO) energy of DAE in between the conduction band minimum (CBM) and valence band minimum (VBM) of MoSe2. This will facilitate photoexcited electron transfer from MoSe2 to DAE LUMO. Experimentally, we have confirmed our predictions by observing a strong quenching of photoluminescence (PL) and an increase of electrical conductivity in MoSe2. In contrast, visible light induced isomerization to the open form of DAE and expanded its energy levels, moving the LUMO level above the MoSe2 CBM. Therefore, the PL was retained and there was no significant change in electrical conductivity. We showed that this photoswitching can dynamically modulate the optoelectronic properties by demonstrating multiple cycles of PL emission and quenching, by alternating UV and visible light repeatedly. Lastly, our Kelvin probe force microscopy (KPFM) revealed that the potential of MoSe2 monolayers can also be modulated with photochromic molecules and external light irradiation. Our work elucidates photo-processes and exciton mechanisms at the hybrid interfaces and lays the foundation for novel applications including new phototransistors and other 2D optoelectronic devices.

(Invited) 2D Semiconductors from Spin-on Molecular Chemistries for Direct Large-Scale Integration

In this talk, I will discuss our recent developments centered on a process to produce fully soluble solutions from a unique molecular chemistry. This enables the creation of wafer-scale monolayers of MoS2, a prominent 2D semiconductor for logic and memory devices. This molecule can be dissolved in common solvents and spin-coated onto various substrates, including crystalline substrates, printed electrodes, and polymers. It can also be integrated into gate-all-around (GAA) 3D transistor structures for logic devices. Our method of rapidly synthesizing large-area MoS2 films, particularly in monolayer form via spin-coating, represents a novelty, as it hasn't been demonstrated previously with a single-source chemistry. This advancement is a significant step toward scalable synthesis of wafer-scale 2D semiconductor thin films, eliminating the need for post-growth wafer transfer techniques, a requirement with films grown by chemical vapor deposition (CVD) methods. Our fabrication process highlights the potential of using a single-source chemical precursor to develop monolayer 2D semiconductors that exhibit commendable transport characteristics and optical properties, which enhance with rising crystallization temperatures. Consequently, it holds promise as a semiconductor for microelectronic transistor channels in both back-end-of-line (BEOL) and front-end-of-line (FEOL) architectures.

(Invited) Enabling Bio-Realistic Artificial Intelligence Hardware with Neuromorphic Nanoelectronics

The exponentially improving performance of digital computers has recently slowed due to the speed and power consumption issues resulting from the von Neumann bottleneck. In contrast, neuromorphic computing aims to circumvent these limitations by spatially co-locating logic and memory in a manner analogous to biological neuronal networks [1]. Beyond reducing power consumption, neuromorphic devices provide efficient architectures for image recognition, machine learning, and artificial intelligence [2]. This talk will explore how low-dimensional nanoelectronic materials enable gate-tunable neuromorphic devices [3]. For example, by utilizing self-aligned, atomically thin heterojunctions, dual-gated Gaussian transistors have been realized, which show tunable anti-ambipolarity for artificial neurons, competitive learning, spiking circuits, and mixed-kernel support vector machines [4,5]. In addition, field-driven defect motion in polycrystalline monolayer MoS2 enables gate-tunable memristive phenomena that serve as the basis of hybrid memristor/transistor devices (i.e., ‘memtransistors’) that concurrently provide logic and data storage functions [6]. The planar geometry of memtransistors further allows multiple contacts and dual gating that mimic the behavior of biological systems such as heterosynaptic responses [7]. Moreover, control over polycrystalline grain structure enhances the tunability of potentiation and depression, which enables unsupervised continuous learning in spiking neural networks [8]. Finally, the moiré potential in asymmetric twisted bilayer graphene/hexagonal boron nitride heterostructures gives rise to robust electronic ratchet states. The resulting hysteretic, non-volatile injection of charge carriers enables room-temperature operation of moiré synaptic transistors with diverse bio-realistic neuromorphic functionalities and efficient compute-in-memory designs for low-power artificial intelligence and machine learning hardware [9].[1] V. K. Sangwan, et al., Matter, 5, 4133 (2022).[2] V. K. Sangwan, et al., Nature Nanotechnology, 15, 517 (2020).[3] M. E. Beck, et al., ACS Nano, 14, 6498 (2020).[4] M. E. Beck, et al., Nature Communications, 11, 1565 (2020).[5] X. Yan, et al., Nature Electronics, DOI: 10.1038/s41928-023-01042-7 (2023).[6] X. Yan, et al., Advanced Materials, 34, 2108025 (2022).[7] H.-S. Lee, et al., Advanced Functional Materials, 30, 2003683 (2020).[8] J. Yuan, et al., Nano Letters, 21, 6432 (2021).[9] X. Yan, et al., Nature, DOI: 10.1038/s41586-023-06791-1 (2023).

The Impact of ALD Precursor Choice on Nucleation and Growth of Dielectrics on 2D Materials

Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have received significant attention due to their unique optical and electrical properties, lending themselves to exciting applications such as microelectronics, sensing, catalysis, photonics, and more. The properties of TMDs can be acutely impacted by their environment due to their atomic thinness. For instance, in a TMD-based transistor with a high-k dielectric surrounding the TMD semiconductor, the dielectric could dope the channel via charge transfer; it can improve charge carrier mobility by screening the impact of charged impurities on scattering; or it can worsen charge carrier mobility by introducing defect states or interfacial roughness. As such, the quality and nature of the dielectric is critical, and ultimately those properties are determined by the means of dielectric deposition. Atomic layer deposition (ALD) is a common thin film deposition technique used for depositing high-k dielectrics for transistor devices. ALD relies on sequential self-limiting reactions between a precursor or co-reactant and a surface in order to deposit a material in a layer-by-layer manner with a high degree of control over thickness, composition, and uniformity. However, the ideal basal planes of 2D materials possess no dangling bonds or reactive functional groups, making their surfaces largely inert to ALD chemistry. Hence, when conventional ALD is performed on TMDs, nucleation and growth proceeds only at defect sites, grain boundaries, and the more reactive edge sites. This sparse nucleation leads to poor quality films of low density, high surface roughness, and pinholes.In this work, we employ physisorption-assisted ALD processes and study the nucleation and quality of the deposited dielectrics. We deposit AlOx on atomically thin monolayer MoS2 (itself on SiO2/Si) using a series of ALD precursors in order to investigate the impact of the ALD precursor on nucleation. Using scanning electron microscopy (SEM), we study film nucleation and continuity as a function of ALD precursor, cycle number, and temperature. After optimizing each process for each precursor in terms of temperature and purge time for maximized nucleation, we fabricate transistor devices in which the deposited AlOx serves as a seed layer for high-k HfO2 deposition to create the gate stack. Au contacts and a Pd top gate were used to complete the devices. Using x-ray photoelectron spectroscopy (XPS) and electrical testing, we investigate the character of the dielectric/MoS2 interface. We observe that the precursor used in the ALD process can lead to a wide range of coverages, with more conventional precursors resulting in sparse coverage, and novel precursors introduced in this study leading to nearly full coverage after just 3 nm of AlOx is deposited. We hypothesize that the improved coverage for the novel precursor is the result of enhanced interactions between the precursor and the 2D material due to its particular chemical structure. The reduced nucleation delay leads to a denser, smoother film with fewer defects at the 2D/3D material interface. The devices fabricated using the novel precursor and improved dielectric show excellent performance such as good on/off ratios (106), small device-to-device variation (ΔV T < 1 V), and low effective oxide thickness (~1 nm). This work provides useful insights into how ALD precursors could be designed to improve the quality of dielectrics on 2D materials, potentially improving the viability of 2D materials for wide ranging applications.

(Invited) Disentangling the Nonequilibrium Dynamics of Excitations in 2D Material Interfaces Under Bias: Leveraging Theory, Electrochemistry, and Time-Resolved Spectroscopy

A fundamental challenge in elucidating, controlling, and exploiting nonequilibrium relaxation in condensed phase systems is the ability to find and employ physically transparent models. Through such models, one can develop a compelling assignment and interpretation of the spectral signatures of processes spanning charge and energy transfer, quasiparticle formation, and even chemical reactivity. Two-dimensional materials, such as transition metal dichalcogenides (TMDs), offer one such challenge, especially under applied fields. In this talk, I will describe our recent work combining theory, electrochemical measurements, and ultrafast spectroscopy to demonstrate hot carrier extraction in photoelectrochemical cells composed of monolayer MoS2 on an ITO electrode exposed to electrolyte solution. In addition, I will discuss our critical assessment of the ability of a physically intuitive Hamiltonian, consisting of an infinitely heavy exciton immersed in a fermi sea of conduction band electrons, to capture and offer an interpretation of the spectral features in the linear and transient absorption optical signals of this material under an applied bias. Importantly, we leverage our analysis to identify, physically, how trion formation moves, broadens, and resizes the “A exciton” absorption of our working device. We further unify the interpretation over various sources of such TMD spectral shifts: applied bias, fluence, and (TA) delay time. Our work thus demonstrates hot carrier extraction in 2D photoelectrochemical cells, outlines a path to exploit this phenomenon in semiconductor devices, delineates open questions that are only now becoming possible to address with theory and experiment, and identifies the underlying physical process responsible for previously misidentified spectral features in 2D materials that changed with applied voltage, fluence, and time.

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.

Accelerated Electron-Hole Separation at the Organic-Inorganic Anthracene/Janus MoSSe Interface.

Organic light-absorbing materials with two-dimensional semiconductor layers as contact electrodes are promising for efficient and flexible low-cost solar cells. Considering anthracene as an absorber and a MoSSe Janus monolayer, we use non-adiabatic molecular dynamics to show that electron transfer from anthracene to MoSSe is faster on the Se side than the S side. The transfer from anthracene to MoS2 and MoSe2 monolayers takes intermediate times. As a rule, we find that a shorter adsorption distance produces a stronger donor-acceptor coupling. The smaller distance on the Se side is rationalized by the attractive force between the intrinsic dipole moment of the Janus structure and that of the molecule induced due to adsorption. Quantum coherence also affects the transfer time. The study provides detailed insights into adsorption of molecules on Janus structures and the resulting electronic and electron vibrational interactions. The results suggest that the dipole interaction plays an important role in the thermodynamic stability, alignment of electronic levels, and electron vibrational dynamics.

Valley Spin-Polarization of MoS2 Monolayer Induced by Ferromagnetic Order in an Antiferromagnet.

Transition metal dichalcogenide (TMD) monolayers exhibit unique valleytronics properties due to the dependency of the coupled valley and spin state at the hexagonal corner of the first Brillouin zone. Precisely controlling valley spin-polarization via manipulating the electron population enables its application in valley-based memory or quantum technologies. This study uncovered the uncompensated spins of the antiferromagnetic nickel oxide (NiO) serving as the ferromagnetic (FM) order to induce valley spin-polarization in molybdenum disulfide (MoS2) monolayers via the magnetic proximity effect (MPE). Spin-resolved photoluminescence spectroscopy (SR-PL) was employed to observe MoS2, where the spin-polarized trions appear to be responsible for the MPE, leading to a valley magnetism. Results indicate that local FM order from the uncompensated surface of NiO could successfully induce significant valley spin-polarization in MoS2 with the depolarization temperature approximately at 100 K, which is relatively high compared to the related literature. This study reveals new perspectives in that the precise control over the surface orientation of AFMs serves as a crystallographic switch to activate the MPE and the magnetic sustainability of the trion state is responsible for the observed valley spin-polarization with the increasing temperature, which promotes the potential of AFM materials in the field of exchange-coupled Van der Waals heterostructures.

Double-band perfect absorber based on MoS2 monolayer for sensing applications

Double-band perfect absorber based on MoS2 monolayer for sensing applications

Tuning the electronic structure of monolayer MoS2 towards metal like via vanadium doping

Doping of two-dimensional layered semiconducting materials is becoming pivotal in tailoring their electronic properties, enabling the development of advanced electronic and optoelectronic devices, where the selection of dopant is important. Here, we demonstrate the potential of substitutional vanadium (V) doping in monolayer molybdenum disulfide (MoS2) lattice in different extents leading to tunable electronic and optoelectronic properties. We found that low-level V doping (∼1 at.%) induces p-type characteristics in otherwise n-type monolayer MoS2, whereas medium-level doping (∼5 at.%) leads to an ambipolar semiconductor. Degenerately doped MoS2 (∼9 at.%) facilitates a transition from semiconducting towards metallic (metal-like) with reduced electrical resistivity (∼4.5 Ωm of MoS2 to ∼2.2×10−5Ωm), low activation energy for transport (∼11 meV), and electric field independent drain current in field effect transistor–based transfer characteristics. A detailed temperature- and power-dependent photoluminescence study along with density functional theory–based calculations in support unravels the emergence of an excitonic transition at ∼850 nm with its intensity dependent on the amount of vanadium. This study shows the potential of V doping in MoS2 for generating multifunctional two-dimensional materials for next generation electronics, optoelectronics, and interconnects with systematic control over its electronic structure in a wide range. Published by the American Physical Society 2024