Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2 Monolayers

First-principles calculations are performed to explore the geometry, bonding, and electronic structures of six ultrathin photovoltaic heterostructures consisting of pristine and B- or N-doped fullerenes and MoS2 or WS2 monolayers. The fullerenes prefer to be attached with a hexagon parallel to the monolayer, where B and N favor proximity to the monolayer. The main electronic properties of the subsystems stay intact, suggesting weak interfacial interaction. Both the C60/MoS2 and C60/WS2 systems show type-II band alignments. However, the built-in potential in the former case is too small to effectively drive electron-hole separation across the interface, whereas the latter system is predicted to show good photovoltaic performance. Unfortunately, B and N doping destroys the type-II band alignment on MoS2 and preserves it only in one spin channel on WS2, which is unsuitable for excitonic solar cells. Our results suggest that the C60/WS2 system is highly promising for excitonic solar cells.

Preparation and applications of mechanically exfoliated single-layer and multilayer MoS₂ and WSe₂ nanosheets

Although great progress has been achieved in the study of graphene, the small current ON/OFF ratio in graphene-based field-effect transistors (FETs) limits its application in the fields of conventional transistors or logic circuits for low-power electronic switching. Recently, layered transition metal dichalcogenide (TMD) materials, especially MoS2, have attracted increasing attention. In contrast to its bulk material with an indirect band gap, a single-layer (1L) MoS2 nanosheet is a semiconductor with a direct band gap of ~1.8 eV, which makes it a promising candidate for optoelectronic applications due to the enhancement of photoluminescence and high current ON/OFF ratio. Compared with TMD nanosheets prepared by chemical vapor deposition and liquid exfoliation, mechanically exfoliated ones possess pristine, clean, and high-quality structures, which are suitable for the fundamental study and potential applications based on their intrinsic thickness-dependent properties. In this Account, we summarize our recent research on the preparation, characterization, and applications of 1L and multilayer MoS2 and WSe2 nanosheets produced by mechanical exfoliation. During the preparation of nanosheets, we proposed a simple optical identification method to distinguish 1L and multilayer MoS2 and WSe2 nanosheets on a Si substrate coated with 90 and 300 nm SiO2. In addition, we used Raman spectroscopy to characterize mechanically exfoliated 1L and multilayer WSe2 nanosheets. For the first time, a new Raman peak at 308 cm(-1) was observed in the spectra of WSe2 nanosheets except for the 1L WSe2 nanosheet. Importantly, we found that the 1L WSe2 nanosheet is very sensitive to the laser power during characterization. The high power laser-induced local oxidation of WSe2 nanosheets and single crystals was monitored by Raman spectroscopy and atomic force microscopy (AFM). Hexagonal and monoclinic structured WO3 thin films were obtained from the local oxidization of single- to triple-layer (1L-3L) and quadruple- to quintuple-layer (4L-5L) WSe2 nanosheets, respectively. Then, we present Raman characterization of shear and breathing modes of 1L and multilayer MoS2 and WSe2 nanosheets in the low frequency range (<50 cm(-1)), which can be used to accurately identify the layer number of nanosheets. Magnetic force microscopy was used to characterize 1L and multilayer MoS2 nanosheets, and thickness-dependent magnetic response was found. In the last part, we briefly introduce the applications of 1L and multilayer MoS2 nanosheets in the fields of gas sensors and phototransistors.

In situ catalytic growth of large-area multilayered graphene/MoS2 heterostructures

Stacking various two-dimensional atomic crystals on top of each other is a feasible approach to create unique multilayered heterostructures with desired properties. Herein for the first time, we present a controlled preparation of large-area graphene/MoS₂ heterostructures via a simple heating procedure on Mo-oleate complex coated sodium sulfate under N₂ atmosphere. Through a direct in situ catalytic reaction, graphene layer has been uniformly grown on the MoS₂ film formed by the reaction of Mo species with S pecies, which is from the carbothermal reduction of sodium sulfate. Due to the excellent graphene “painting” on MoS₂ atomic layers, the significantly shortened lithium ion diffusion distance and the markedly enhanced electronic conductivity, these multilayered graphene/MoS₂ heterostructures exhibit high specific capacity, unprecedented rate performance and outstanding cycling stability, especially at a high current density, when used as an anode material for lithium batteries. This work provides a simple but efficient route for the controlled fabrication of large-area multilayered graphene/metal sulfide heterostructures with promising applications in battery manufacture, electronics or catalysis.

Facile synthesis of ultrathin lepidocrocite nanosheets from layered precursors

Ultrathin two-dimensional nanosheets have been widely studied because of their peculiar properties and promising applications. As a typical layered material, successful exfoliation of freestanding ultrathin lepidocrocite (γ-FeOOH) nanosheets from the bulk material has not been reported to date. Herein, we report a facile synthetic route to prepare ultrathin lepidocrocite nanosheets with a thickness of approximately 2-3 nm from FeOx -propanediol layered precursors through weakening of the hydrogen bonds during the crystallization process. The ultrathin morphology and single-crystal structure of the nanosheets were confirmed by transmission electron microscopy, X-ray diffraction, and atomic force microscopy. The formation process of these nanosheets demonstrated simultaneous exfoliation and crystallization of lepidocrocite in basic aqueous solution. The obtained ultrathin nanosheets exhibited a much lower Néel temperature (18.3 K) than bulk lepidocrocite and weak ferromagnetic behavior below this temperature.

MoS2 nanoresonators: intrinsically better than graphene?

We perform classical molecular dynamics simulations to examine the intrinsic energy dissipation in single-layer MoS2 nanoresonators, where the point of emphasis is to compare their dissipation characteristics with those of single-layer graphene. Our key finding is that MoS2 nanoresonators exhibit significantly lower energy dissipation, and thus higher quality (Q)-factors by at least a factor of four below room temperature, than graphene. Furthermore, this high Q-factor endows MoS2 nanoresonators with a higher figure of merit, defined as frequency times Q-factor, despite a resonant frequency that is 50% smaller than that of graphene of the same size. By utilizing arguments from phonon-phonon scattering theory, we show that this reduced energy dissipation is enabled by the large energy gap in the phonon dispersion of MoS2, which separates the acoustic phonon branches from the optical phonon branches, leading to a preserving mechanism for the resonant oscillation of MoS2 nanoresonators. We further investigate the effects of tensile mechanical strain and nonlinear actuation on the Q-factors, where the tensile strain is found to counteract the reductions in Q-factor that occur with higher actuation amplitudes. Overall, our simulations illustrate the potential utility of MoS2 for high frequency sensing and actuation applications.

Synthesis of PbI(2) single-layered inorganic nanotubes encapsulated within carbon nanotubes

The template assisted growth of single-layered inorganic nanotubes is reported. Single-crystalline lead iodide single-layered nanotubes have been prepared using the inner cavities of carbon nanotubes as hosting templates. The diameter of the resulting inorganic nanotubes is merely dependent on the diameter of the host. This facile method is highly versatile opening up new horizons in the preparation of single-layered nanostructures.

Gate-induced electron-state tuning of MoS2: first-principles calculations

The electronic structure of electrostatically doped MoS2 thin films is investigated on the basis of first-principles total-energy calculations. We find that electron injection leads to a rapid downward shift in the energy of the unoccupied nearly free electron (NFE) state relative to other conduction bands. The NFE state finally crosses the Fermi level at an electron density of 0.81 × 10(14) cm(-2) that is attributable to the strong local electric field induced by charge accumulation near the surface. Electrons accommodated in the NFE state play an important role in determining the conducting properties of MoS2 thin films.

Solar-energy conversion and light emission in an atomic monolayer p-n diode

The limitations of the bulk semiconductors currently used in electronic devices-rigidity, heavy weight and high costs--have recently shifted the research efforts to two-dimensional atomic crystals such as graphene and atomically thin transition-metal dichalcogenides. These materials have the potential to be produced at low cost and in large areas, while maintaining high material quality. These properties, as well as their flexibility, make two-dimensional atomic crystals attractive for applications such as solar cells or display panels. The basic building blocks of optoelectronic devices are p-n junction diodes, but they have not yet been demonstrated in a two-dimensional material. Here, we report a p-n junction diode based on an electrostatically doped tungsten diselenide (WSe2) monolayer. We present applications as a photovoltaic solar cell, a photodiode and a light-emitting diode, and obtain light-power conversion and electroluminescence efficiencies of ∼ 0.5% and ∼ 0.1%, respectively. Given recent advances in the large-scale production of two-dimensional crystals, we expect them to profoundly impact future developments in solar, lighting and display technologies.

Transformation optics scheme for two-dimensional materials

Two-dimensional optical materials, such as graphene, can be characterized by surface conductivity. So far, the transformation optics schemes have focused on three-dimensional properties such as permittivity ϵ and permeability μ. In this Letter, we use a scheme for transforming surface currents to highlight that the surface conductivity transforms in a way different from ϵ and μ. We use this surface conductivity transformation to demonstrate an example problem of reducing the scattering of the plasmon mode from sharp protrusions in graphene.

Irradiation effects of high-energy proton beams on MoS2 field effect transistors

We investigated the effect of irradiation on molybdenum disulfide (MoS2) field effect transistors with 10 MeV high-energy proton beams. The electrical characteristics of the devices were measured before and after proton irradiation with fluence conditions of 10(12), 10(13), and 10(14) cm(-2). For a low proton beam fluence condition of 10(12) cm(-2), the electrical properties of the devices were nearly unchanged in response to proton irradiation. In contrast, for proton beam fluence conditions of 10(13) or 10(14) cm(-2), the current level and conductance of the devices significantly decreased following proton irradiation. The electrical changes originated from proton-irradiation-induced traps, including positive oxide-charge traps in the SiO2 layer and trap states at the interface between the MoS2 channel and the SiO2 layer. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nanoelectronic devices.

Defect-dominated doping and contact resistance in MoS2

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.

Light generation and harvesting in a van der Waals heterostructure

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p-n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

Atomically thin molybdenum disulfide nanopores with high sensitivity for DNA translocation

Atomically thin nanopore membranes are considered to be a promising approach to achieve single base resolution with the ultimate aim of rapid and cheap DNA sequencing. Molybdenum disulfide (MoS2) is newly emerging as a material complementary to graphene due to its semiconductive nature and other interesting physical properties that can enable a wide range of potential sensing and nanoelectronics applications. Here, we demonstrate that monolayer or few-layer thick exfoliated MoS2 with subnanometer thickness can be transferred and suspended on a predesigned location on the 20 nm thick SiNx membranes. Nanopores in MoS2 are further sculpted with variable sizes using a transmission electron microscope (TEM) to drill through suspended portions of the MoS2 membrane. Various types of double-stranded (ds) DNA with different lengths and conformations are translocated through such a novel architecture, showing improved sensitivity (signal-to-noise ratio>10) compared to the conventional silicon nitride (SiNx) nanopores with tens of nanometers thickness. Unlike graphene nanopores, no special surface treatment is needed to avoid hydrophobic interaction between DNA and the surface. Our results imply that MoS2 membranes with nanopore can complement graphene nanopore membranes and offer potentially better performance in transverse detection.

Transient absorption microscopy of monolayer and bulk WSe2

We present an experimental investigation on the exciton dynamics of monolayer and bulk WSe2 samples, both of which are studied by femtosecond transient absorption microscopy. Under the excitation of a 405 nm pump pulse, the differential reflection signal of a probe pulse (tuned to the A-exciton resonance) reaches a peak rapidly that indicates an ultrafast formation process of excitons. By resolving the differential reflection signal in both time and space, we directly determine the exciton lifetimes of 18±1 and 160±10 ps and the exciton diffusion coefficients of 15±5 and 9±3 cm2/s in the monolayer and bulk samples, respectively. From these values, we deduce other parameters characterizing the exciton dynamics such as the diffusion length, the mobility, the mean free path, and the mean free length. These fundamental parameters are useful for understanding the excitons in monolayer and bulk WSe2 and are important for applications in optoelectronics, photonics, and electronics.

Transport properties of monolayer MoS2 grown by chemical vapor deposition

Recent success in the growth of monolayer MoS2 via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS2. The devices show low temperature mobilities up to 500 cm(2) V(-1) s(-1) and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of optical phonons as a limiting factor is discussed.

Second harmonic generation from artificially stacked transition metal dichalcogenide twisted bilayers

Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.

Molybdenum disulfide (MoS₂) as a broadband saturable absorber for ultra-fast photonics

The nonlinear optical property of few-layered MoS₂ nanoplatelets synthesized by the hydrothermal exfoliation method was investigated from the visible to the near-infrared band using lasers. Both open-aperture Z-scan and balanced-detector measurement techniques were used to demonstrate the broadband saturable absorption property of few-layered MoS₂. To explore its potential applications in ultrafast photonics, we fabricated a passive mode locker for ytterbium-doped fibre laser by depositing few-layered MoS₂ onto the end facet of optical fiber by means of an optical trapping approach. Our laser experiment shows that few-layer MoS₂-based mode locker allows for the generation of stable mode-locked laser pulse, centered at 1054.3 nm, with a 3-dB spectral bandwidth of 2.7 nm and a pulse duration of 800 ps. Our finding suggests that few-layered MoS₂ nanoplatelets can be useful nonlinear optical material for laser photonics devices, such as passive laser mode locker, Q-switcher, optical limiter, optical switcher and so on.

Establishing water-soluble layered WS₂ nanosheet as a platform for biosensing

Layered WS2 nanosheet is a kind of two-dimensional (2D) covalent-network solid material with remarkable structural and electronic properties that has attracted increasing interest in recent years. In this work, we propose a one-step sonication-assisted exfoliation method to prepare water-soluble WS2 nanosheet and demonstrate its application as a biosensing platform. The synthesis route is simple and straightforward. We reveal that single-strand DNA (ssDNA) chains can readily be adsorbed on the nanosheet, leading to complete and fast quenching of a fluorescent dye tagged to the DNA chain. The adsorbed ssDNA is detachable from the nanosheet upon the interaction with other biomolecules, resulting in the restoration of the fluorescence. The 2D WS2 nanosheet thus acts as an efficient platform for assembling of bioprobes. Because of the extraordinarily high quenching efficiency, which is the synergic result of both excited-state energy transfer and static quenching, the WS2 platform affords minimal background and high sensitivity. Our attempt will extend the application of this material to biosensing and probing areas.

Single-layer transition metal dichalcogenide nanosheet-assisted assembly of aggregation-induced emission molecules to form organic nanosheets with enhanced fluorescence

Single-layer transition metal dichalcogenide nanosheets including MoS2, TiS2 and TaS2 can be used as novel platforms for the fluorescence enhancement of aggregation-induced emission fluorophores. The small organic AIE unit can be assembled into a two-dimensional sheet structure via a simple precipitation technique assisted by these monolayers. The resultant organic sheets possess a size of 0.2-2 μm and a thickness of 9-20 nm.

Band offset and negative compressibility in graphene-MoS2 heterostructures

We use electron transport to characterize monolayer graphene-multilayer MoS2 heterostructures. Our samples show ambipolar characteristics and conductivity saturation on the electron branch that signals the onset of MoS2 conduction band population. Surprisingly, the carrier density in graphene decreases with gate bias once MoS2 is populated, demonstrating negative compressibility in MoS2. We are able to interpret our measurements quantitatively by accounting for disorder and using the random phase approximation (RPA) for the exchange and correlation energies of both Dirac and parabolic-band two-dimensional electron gases. This interpretation allows us to extract the energetic offset between the conduction band edge of MoS2 and the Dirac point of graphene.

Phonon modes in single-walled molybdenum disulphide nanotubes: lattice dynamics calculation and molecular dynamics simulation

We study the phonon modes in single-walled MoS₂ nanotubes via lattice dynamics calculation and molecular dynamics simulation. The phonon spectra for tubes of arbitrary chiralities are calculated from a dynamical matrix constructed by the combination of an empirical potential with the conserved helical quantum numbers (κ, n). In particular, we show that the frequency (ω) of the radial breathing mode is inversely proportional to the tube diameter (d) as ω = 665.3/d cm⁻¹. The eigenvectors of the twenty lowest-frequency phonon modes are illustrated. Based on these eigenvectors, we demonstrate that the radial breathing oscillation is initially disturbed by phonon modes of three-fold symmetry, then eventually the tube is squashed by modes of two-fold symmetry . Our study provides fundamental knowledge for further investigations of the thermal and mechanical properties of MoS₂ nanotubes.

Electronic structure of a quasi-freestanding MoS₂ monolayer

Several transition-metal dichalcogenides exhibit a striking crossover from indirect to direct band gap semiconductors as they are thinned down to a single monolayer. Here, we demonstrate how an electronic structure characteristic of the isolated monolayer can be created at the surface of a bulk MoS2 crystal. This is achieved by intercalating potassium in the interlayer van der Waals gap, expanding its size while simultaneously doping electrons into the conduction band. Our angle-resolved photoemission measurements reveal resulting electron pockets centered at the K̅ and K' points of the Brillouin zone, providing the first momentum-resolved measurements of how the conduction band dispersions evolve to yield an approximately direct band gap of ∼1.8 eV in quasi-freestanding monolayer MoS2. As well as validating previous theoretical proposals, this establishes a novel methodology for manipulating electronic structure in transition-metal dichalcogenides, opening a new route for the generation of large-area quasi-freestanding monolayers for future fundamental study and use in practical applications.

Molybdenum sulfide/N-doped CNT forest hybrid catalysts for high-performance hydrogen evolution reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Theoretical prediction of electronic structure and carrier mobility in single-walled MoS₂ nanotubes

We have investigated the electronic structure and carrier mobility of armchair and zigzag single-walled MoS₂ nanotubes using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that armchair nanotubes are indirect bandgap semiconductors, while zigzag nanotubes are direct ones. The band gaps of single-walled MoS₂ nanotubes are along with the augment of their diameters. For armchair nanotubes (5 ≤ Na ≤ 14), the hole mobility raise from 98.62 ~ 740.93 cm(2)V(-1)s(-1) at room temperature, which is about six times of the electron mobility. For zigzag nanotubes (9 ≤ Na ≤ 15), the hole mobility is 56.61 ~ 91.32 cm(2)V(-1)s(-1) at room temperature, which is about half of the electron mobility.

Synthesis of wafer-scale uniform molybdenum disulfide films with control over the layer number using a gas phase sulfur precursor

We describe a method for synthesizing large-area and uniform molybdenum disulfide films, with control over the layer number, on insulating substrates using a gas phase sulfuric precursor (H2S) and a molybdenum metal source. The metal layer thickness was varied to effectively control the number of layers (2 to 12) present in the synthesized film. The films were grown on wafer-scale Si/SiO2 or quartz substrates and displayed excellent uniformity and a high crystallinity over the entire area. Thin film transistors were prepared using these materials, and the performances of the devices were tested. The devices displayed an on/off current ratio of 10(5), a mobility of 0.12 cm(2) V(-1) s(-1) (mean mobility value of 0.07 cm(2) V(-1) s(-1)), and reliable operation.

Composition-dependent Raman modes of Mo(1-x)W(x)S2 monolayer alloys

Two-dimensional (2D) transition-metal dichalcogenide alloys with tunable band gaps have promising applications in nanoelectronics and optoelectronics. Characterization of structures of 2D alloys, such as composition and atom mixing, is of fundamental importance to their applications. Here, we have conducted systematic Raman spectroscopic studies on Mo1-xWxS2 monolayers (0 ≤x≤ 1). First-order Raman modes and second-order Raman modes have been observed in the range of 100-480 cm(-1) in the 2D alloys. The out-of-plane A1' modes and in-plane E' modes showed one-mode and two-mode behaviors, respectively. The broadening of A1' and E' modes in the alloys has been observed. The disorder-related Raman peaks at ∼360 cm(-1) were only observed in the 2D alloys but not in the two end materials. Modified random-element-isodisplacement (MREI) model has been adopted to successfully predict mode behaviors of A1' and E' modes in the monolayer alloys. Further, composition-dependent A1' and E' frequencies can be well fitted by the MREI model, giving composition-dependent force constants.

New first order Raman-active modes in few layered transition metal dichalcogenides

Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E', E″ and E2g modes shift to lower frequencies, and the A'1 and A1g modes shift to higher frequencies. Second, new high frequency first order A'1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A'1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.

Sulfur-doped molybdenum oxide anode interface layer for organic solar cell application

Efficient organic solar cells (OSCs) based on regioregular of poly (3-hexylthiophene):fullerene derivative [6,6]-phenyl-C61butyric acid methyl ester composites have been fabricated on indium tin oxide (ITO) coated glass substrates by using a sputtered sulfur-doped molybdenum oxide (S-MoO3) film as anode interface layer (AIL). With the help of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, we find that oxygen flow ratio control can modulate the amount of sulfur doping into MoO3, then further tune the Mo(+4)/Mo(+5)/Mo(+6) composition ratios, Fermi level, electron affinity, valence band ionization energy and band gap of MoO3. A partially occupied Mo 4d-bands of Mo(5+) and Mo(4+) states modulated by sulfur doping are the main factor which influences the valence electronic structure of S-MoO3.These orbitals overlap interrelation push the valence band close to S-MoO3's Fermi level, thus make it into a p-type semiconductor. S-MoO3 with smaller ionization energy and electron affinity is better suitable as an efficient AIL. On the basis of these AILs, a photovoltaic power conversion efficiency up to 3.69% has been achieved, which is 12% higher than that in pure MoO3 AIL case. The result thus shows that sulfur doping is a useful method to modify anode interface layer for improving the hole-transport properties of MoO3, which can improve the device performances.

Outlook and emerging semiconducting materials for ambipolar transistors

Ambipolar or bipolar transistors are transistors in which both holes and electrons are mobile inside the conducting channel. This device allows switching among several states: the hole-dominated on-state, the off-state, and the electron-dominated on-state. In the past year, it has attracted great interest in exotic semiconductors, such as organic semiconductors, nanostructured materials, and carbon nanotubes. The ability to utilize both holes and electrons inside one device opens new possibilities for the development of more compact complementary metal-oxide semiconductor (CMOS) circuits, and new kinds of optoelectronic device, namely, ambipolar light-emitting transistors. This progress report highlights the recent progresses in the field of ambipolar transistors, both from the fundamental physics and application viewpoints. Attention is devoted to the challenges that should be faced for the realization of ambipolar transistors with different material systems, beginning with the understanding of the importance of interface modification, which heavily affects injections and trapping of both holes and electrons. The recent development of advanced gating applications, including ionic liquid gating, that open up more possibility to realize ambipolar transport in materials in which one type of charge carrier is highly dominant is highlighted. Between the possible applications of ambipolar field-effect transistors, we focus on ambipolar light-emitting transistors. We put this new device in the framework of its prospective for general lightings, embedded displays, current-driven laser, as well as for photonics-electronics interconnection.

Quantitative chemistry and the discrete geometry of conformal atom-thin crystals

When flat or on a firm mechanical substrate, the atomic composition and atomistic structure of two-dimensional crystals dictate their chemical, electronic, optical, and mechanical properties. These properties change when the two-dimensional and ideal crystal structure evolves into arbitrary shapes, providing a direct and dramatic link among geometry and material properties due to the larger structural flexibility when compared to bulk three-dimensional materials. We describe methods to understand the local geometrical information of two-dimensional conformal crystals quantitatively and directly from atomic positions, even in the presence of atomistic defects. We then discuss direct relations among the discrete geometry and chemically relevant quantities--mean bond lengths, hybridization angles, and σ-π hybridization. These concepts are illustrated for carbon-based materials and ionic crystals. The pyramidalization angle turns out to be linearly proportional to the mean curvature for relevant crystalline configurations. Discrete geometry provides direct quantitative information on the potential chemistry of conformal two-dimensional crystals.

Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides

With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.

Graphene plasmonics for terahertz to mid-infrared applications

In recent years, we have seen a rapid progress in the field of graphene plasmonics, motivated by graphene's unique electrical and optical properties, tunability, long-lived collective excitation and its extreme light confinement. Here, we review the basic properties of graphene plasmons: their energy dispersion, localization and propagation, plasmon-phonon hybridization, lifetimes and damping pathways. The application space of graphene plasmonics lies in the technologically significant, but relatively unexploited terahertz to mid-infrared regime. We discuss emerging and potential applications, such as modulators, notch filters, polarizers, mid-infrared photodetectors, and mid-infrared vibrational spectroscopy, among many others.

Improving the stability and optical properties of germanane via one-step covalent methyl-termination

Two-dimensional van der Waals materials have shown great promise for a variety of electronic, optoelectronic, sensing and energy conversion applications. Since almost every atom in these two-dimensional crystals is exposed to the surface, covalent surface termination could provide a powerful method for the controlled tuning of material properties. Here we demonstrate a facile, one-step metathesis approach that directly converts CaGe₂ crystals into mm-sized crystals of methyl-terminated germanane (GeCH₃). Replacing --H termination in GeH with --CH₃ increases the band gap by ~0.1 eV to 1.7 eV, and produces band edge fluorescence with a quantum yield of ~0.2%, with little dependence on layer thickness. Furthermore, the thermal stability of GeCH₃ has been increased to 250 °C compared with 75 °C for GeH. This one-step metathesis approach should be applicable for accessing new families of two-dimensional van der Waals lattices that feature precise organic terminations and with enhanced optoelectronic properties.

Toward low-power electronics: tunneling phenomena in transition metal dichalcogenides

In this article, we explore, experimentally, the impact of band-to-band tunneling on the electronic transport of double-gated WSe2 field-effect transistors (FETs) and Schottky barrier tunneling of holes in back-gated MoS2 FETs. We show that by scaling the flake thickness and the thickness of the gate oxide, the tunneling current can be increased by several orders of magnitude. We also perform numerical calculations based on Landauer formalism and WKB approximation to explain our experimental findings. Based on our simple model, we discuss the impact of band gap and effective mass on the band-to-band tunneling current and evaluate the performance limits for a set of dichalcogenides in the context of tunneling transistors for low-power applications. Our findings suggest that WTe2 is an excellent choice for tunneling field-effect transistors.

Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications

Heterogeneous photocatalysis that employs photo-excited semiconductor materials to reduce water and oxidize toxic pollutants upon solar light irradiation holds great prospects for renewable energy substitutes and environmental protection. To utilize solar light effectively, the quest for highly active photocatalysts working under visible light has always been the research focus. Layered BiOX (X = Cl, Br, I) are a kind of newly exploited efficient photocatalysts, and their light response can be tuned from UV to visible light range. The properties of semiconductors are dependent on their morphologies and compositions as well as structures, and this also offers the guidelines for design of highly-efficient photocatalysts. In this review, recent advances and emerging strategies in tailoring BiOX (X = Cl, Br, I) nanostructures to boost their photocatalytic properties are surveyed.

Few-layer MoSe₂ possessing high catalytic activity towards iodide/tri-iodide redox shuttles

Due to the two-dimensional confinement of electrons, single- and few-layer MoSe₂ nanostructures exhibit unusual optical and electrical properties and have found wide applications in catalytic hydrogen evolution reaction, field effect transistor, electrochemical intercalation, and so on. Here we present a new application in dye-sensitized solar cell as catalyst for the reduction of I₃⁻ to I⁻ at the counter electrode. The few-layer MoSe₂ is fabricated by surface selenization of Mo-coated soda-lime glass. Our results show that the few-layer MoSe₂ displays high catalytic efficiency for the regeneration of I⁻ species, which in turn yields a photovoltaic energy conversion efficiency of 9.00%, while the identical photoanode coupling with “champion” electrode based on Pt nanoparticles on FTO glass generates efficiency only 8.68%. Thus, a Pt- and FTO-free counter electrode outperforming the best conventional combination is obtained. In this electrode, Mo film is found to significantly decrease the sheet resistance of the counter electrode, contributing to the excellent device performance. Since all of the elements in the electrode are of high abundance ratios, this type of electrode is promising for the fabrication of large area devices at low materials cost.

Gate-voltage-controlled spin and valley polarization transport in a normal/ferromagnetic/normal MoS₂ junction

Two-dimensional (2D) materials are extensively explored due to the remarkable physical property and the great potential for post-silicon electronics since the landmark achievement of graphene. The monolayer (ML) MoS2 with a direct energy gap is a typical 2D material and promising candidate for a wide range of device applications. The extensive efforts so far have focused on the optical valley control applications of ML MoS2 rather than the electrical control of spin and valley transport. However, the electrical manipulation of spin injection and transport is essential to realize practical spintronics applications. Here, we theoretically demonstrated that the valley and spin transport can be electrically manipulated by a gate voltage in a normal/ferromagnetic/normal monolayer MoS2 junction device. It was found that the fully valley- and spin-polarized conductance can be achieved due to the spin-valley coupling of valence-band edges together with the exchange field, and both the amplitude and direction of the fully spin-polarized conductance can be modulated by the gate voltage. These findings not only provided deep understanding to the basic physics in the spin and valley transport of ML MoS2 but also opened an avenue for the electrical control of valley and spin transport in monolayer dichalcogenide-based devices.

Ion-driven photoluminescence modulation of quasi-two-dimensional MoS2 nanoflakes for applications in biological systems

Quasi-two-dimensional (quasi-2D) molybdenum disulfide (MoS2) is a photoluminescence (PL) material with unique properties. The recent demonstration of its PL, controlled by the intercalation of positive ions, can lead to many opportunities for employing this quasi-2D material in ion-related biological applications. Here, we present two representative models of biological systems that incorporate the ion-controlled PL of quasi-2D MoS2 nanoflakes. The ion exchange behaviors of these two models are investigated to reveal enzymatic activities and cell viabilities. While the ion intercalation of MoS2 in enzymatic activities is enabled via an external applied voltage, the intercalation of ions in cell viability investigations occurs in the presence of the intrinsic cell membrane potential.

Role of the seeding promoter in MoS2 growth by chemical vapor deposition

The thinnest semiconductor, molybdenum disulfide (MoS2) monolayer, exhibits promising prospects in the applications of optoelectronics and valleytronics. A uniform and highly crystalline MoS2 monolayer in a large area is highly desirable for both fundamental studies and substantial applications. Here, utilizing various aromatic molecules as seeding promoters, a large-area, highly crystalline, and uniform MoS2 monolayer was achieved with chemical vapor deposition (CVD) at a relatively low growth temperature (650 °C). The dependence of the growth results on the seed concentration and on the use of different seeding promoters is further investigated. It is also found that an optimized concentration of seed molecules is helpful for the nucleation of the MoS2. The newly identified seed molecules can be easily deposited on various substrates and allows the direct growth of monolayer MoS2 on Au, hexagonal boron nitride (h-BN), and graphene to achieve various hybrid structures.

Metal-semiconductor barrier modulation for high photoresponse in transition metal dichalcogenide field effect transistors

A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS₂ and ambipolar WSe₂ FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS₂ FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe₂ FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications.

Selective conversion of biorefinery lignin into dicarboxylic acids

The emerging biomass-to-biofuel conversion industry has created an urgent need for identifying new applications for biorefinery lignin. This paper demonstrates a new route to producing dicarboxylic acids from biorefinery lignin through chalcopyrite-catalyzed oxidation in a highly selective process. Up to 95 % selectivity towards stable dicarboxylic acids was obtained for several types of biorefinery lignin and model compounds under mild, environmentally friendly reaction conditions. The findings from this study paved a new avenue to biorefinery lignin conversions and applications.