Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

Two-dimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution

Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.

Elastic bending modulus of single-layer molybdenum disulfide (MoS2): finite thickness effect

We derive, from an empirical interaction potential, an analytic formula for the elastic bending modulus of single-layer MoS2 (SLMoS2). By using this approach, we do not need to define or estimate a thickness value for SLMoS2, which is important due to the substantial controversy in defining this value for two-dimensional or ultrathin nanostructures such as graphene and nanotubes. The obtained elastic bending modulus of 9.61 eV in SLMoS2 is significantly higher than the bending modulus of 1.4 eV in graphene, and is found to be within the range of values that are obtained using thin shell theory with experimentally obtained values for the elastic constants of SLMoS2. This increase in bending modulus as compared to monolayer graphene is attributed, through our analytic expression, to the finite thickness of SLMoS2. Specifically, while each monolayer of S atoms contributes 1.75 eV to the bending modulus, which is similar to the 1.4 eV bending modulus of monolayer graphene, the additional pairwise and angular interactions between out of plane Mo and S atoms contribute 5.84 eV to the bending modulus of SLMoS2.

Controlling the spontaneous emission rate of monolayer MoS2 in a photonic crystal nanocavity

We report on controlling the spontaneous emission (SE) rate of a molybdenum disulfide (MoS₂) monolayer coupled with a planar photonic crystal (PPC) nanocavity. Spatially resolved photoluminescence (PL) mapping shows strong variations of emission when the MoS₂ monolayer is on the PPC cavity, on the PPC lattice, on the air gap, and on the unpatterned gallium phosphide substrate. Polarization dependences of the cavity-coupled MoS₂ emission show a more than 5 times stronger extracted PL intensity than the un-coupled emission, which indicates an underlying cavity mode Purcell enhancement of the MoS₂ SE rate exceeding a factor of 70.

Tuning the bandgap of exfoliated InSe nanosheets by quantum confinement

Strong quantization effects and tuneable near-infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ-rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution

Defect-rich MoS2 ultrathin nanosheets are synthesized on a gram scale for electrocatalytic hydrogen evolution. The novel defect-rich structure introduces additional active edge sites into the MoS2 ultrathin nanosheets, which significantly improves their electrocatalytic performance. Low onset overpotential and small Tafel slope, along with large cathodic current density and excellent durability, are all achieved for the novel hydrogen-evolution-reaction electrocatalyst.

Solvothermal-induced conversion of one-dimensional multilayer nanotubes to two-dimensional hydrophilic VOx nanosheets: synthesis and water treatment application

Ultrathin 2D nanostructures have shown many unique properties and are attractive for various potential applications. Here, we demonstrated a strategy to synthesize ultrathin VOx nanosheets. The as-obtained ultrathin VOx nanosheets showed a large Brunauer-Emmett-Teller (BET) surface area of 136.3 m2 g(-1), which is much larger than that of 1D multilayer VOx nanotubes. As a proof of concept, these hydrophilic ultrathin nanosheets were applied in water treatment and exhibited excellent absorption capability to remove Rhodamine B (RhB) in wastewater owing to their large specific surface area, good hydrophilic property, and more negative zeta potential. In addition, this method could be generalized to prepare other 2D nanostructures with great potential for various attractive applications.

Fabrication and transfer of flexible few-layers MoS2 thin film transistors to any arbitrary substrate

Recently, transition metal dichalcogenides (TMDCs) have attracted interest thanks to their large field effective mobility (>100 cm(2)/V · s), sizable band gap (around 1-2 eV), and mechanical properties, which make them suitable for high performance and flexible electronics. In this paper, we present a process scheme enabling the fabrication and transfer of few-layers MoS2 thin film transistors from a silicon template to any arbitrary organic or inorganic and flexible or rigid substrate or support. The two-dimensional semiconductor is mechanically exfoliated from a bulk crystal on a silicon/polyvinyl alcohol (PVA)/polymethyl methacrylane (PMMA) stack optimized to ensure high contrast for the identification of subnanometer thick flakes. Thin film transistors (TFTs) with structured source/drain and gate electrodes are fabricated following a designed procedure including steps of UV lithography, wet etching, and atomic layer deposited (ALD) dielectric. Successively, after the dissolution of the PVA sacrificial layer in water, the PMMA film, with the devices on top, can be transferred to another substrate of choice. Here, we transferred the devices on a polyimide plastic foil and studied the performance when tensile strain is applied parallel to the TFT channel. We measured an electron field effective mobility of 19 cm(2)/(V s), an I(on)/I(off)ratio greater than 10(6), a gate leakage current as low as 0.3 pA/μm, and a subthreshold swing of about 250 mV/dec. The devices continue to work when bent to a radius of 5 mm and after 10 consecutive bending cycles. The proposed fabrication strategy can be extended to any kind of 2D materials and enable the realization of electronic circuits and optical devices easily transferrable to any other support.

Controlled growth of high-quality monolayer WS2 layers on sapphire and imaging its grain boundary

Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2). Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.

Ultrafast saturable absorption of two-dimensional MoS2 nanosheets

Employing high-yield production of layered materials by liquid-phase exfoliation, molybdenum disulfide (MoS2) dispersions with large populations of single and few layers were prepared. Electron microscopy verified the high quality of the two-dimensional MoS2 nanostructures. Atomic force microscopy analysis revealed that ~39% of the MoS2 flakes had thicknesses of less than 5 nm. Linewidth and frequency difference of the E(1)2g and A1g Raman modes confirmed the effective reduction of flake thicknesses from the bulk MoS2 to the dispersions. Ultrafast nonlinear optical (NLO) properties were investigated using an open-aperture Z-scan technique. All experiments were performed using 100 fs pulses at 800 nm from a mode-locked Ti:sapphire laser. The MoS2 nanosheets exhibited significant saturable absorption (SA) for the femtosecond pulses, resulting in the third-order NLO susceptibility Imχ((3)) ~ 10(-15) esu, figure of merit ~10(-15) esu cm, and free-carrier absorption cross section ~10(-17) cm(2). Induced free carrier density and the relaxation time were estimated to be ~10(16) cm(-3) and ~30 fs, respectively. At the same excitation condition, the MoS2 dispersions show better SA response than the graphene dispersions.

Gate-tunable carbon nanotube-MoS2 heterojunction p-n diode

The p-n junction diode and field-effect transistor are the two most ubiquitous building blocks of modern electronics and optoelectronics. In recent years, the emergence of reduced dimensionality materials has suggested that these components can be scaled down to atomic thicknesses. Although high-performance field-effect devices have been achieved from monolayered materials and their heterostructures, a p-n heterojunction diode derived from ultrathin materials is notably absent and constrains the fabrication of complex electronic and optoelectronic circuits. Here we demonstrate a gate-tunable p-n heterojunction diode using semiconducting single-walled carbon nanotubes (SWCNTs) and single-layer molybdenum disulfide as p-type and n-type semiconductors, respectively. The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that can be tuned with an applied gate bias to achieve a wide range of charge transport behavior ranging from insulating to rectifying with forward-to-reverse bias current ratios exceeding 10(4). This heterojunction diode also responds strongly to optical irradiation with an external quantum efficiency of 25% and fast photoresponse <15 μs. Because SWCNTs have a diverse range of electrical properties as a function of chirality and an increasing number of atomically thin 2D nanomaterials are being isolated, the gate-tunable p-n heterojunction concept presented here should be widely generalizable to realize diverse ultrathin, high-performance electronics and optoelectronics.

High-performance top-gated monolayer SnS2 field-effect transistors and their integrated logic circuits

Two-dimensional (2D) layered semiconductors are very promising for post-silicon ultrathin channels and flexible electronics due to the remarkable dimensional and mechanical properties. Besides molybdenum disulfide (MoS2), the first recognized 2D semiconductor, it is also important to explore the wide spectrum of layered metal chalcogenides (LMCs) and to identify possible compounds with high performance. Here we report the fabrication of high-performance top-gated field-effect transistors (FETs) and related logic gates from monolayer tin disulfide (SnS2), a non-transition metal dichalcogenide. The measured carrier mobility of our monolayer devices reaches 50 cm(2) V(-1) s(-1), much higher than that of the back-gated counterparts (~1 cm(2) V(-1) s(-1)). Based on a direct-coupled FET logic technique, advanced Boolean logic gates and operations are also implemented, with a voltage gain of 3.5 and output swing of >90% for the NOT and NOR gates, respectively. The superior electrical and integration properties make monolayer SnS2 a strong candidate for next-generation atomic electronics.

Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2

Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in molybdenum disulfide (MoS2) were recently found to exhibit unique thickness dependence due to the interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2, isoelectronic compounds of MoS2, in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic A1g and E2g(1) modes exhibit stiffening and softening with increasing number of layers, respectively, with a small shift of less than 3 cm(-1) due to large mass of the atoms. Thickness dependence is also observed in a series of multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are found to be absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.

Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field

: Using first-principles calculations, we investigate the adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO) on monolayer MoS2. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS2 surface and act as charge acceptors for the monolayer, except NH3 which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS2 can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS2 as a potential material for gas sensing application.

Electromechanics in MoS₂ and WS₂: nanotubes vs. monolayers

The transition-metal dichalcogenides (TMD) MoS₂ and WS₂ show remarkable electromechanical properties. Strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition. Providing strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16%. We show that Raman signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain, showing that Raman spectroscopy is an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments in situ. TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter.

Highly flexible and transparent multilayer MoS2 transistors with graphene electrodes

A highly flexible and transparent transistor is developed based on an exfoliated MoS2 channel and CVD-grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (∼74%), and current on/off ratio (>10(4)) with an average field effect mobility of ∼4.7 cm(2) V(-1) s(-1), all of which cannot be achieved by other transistors consisting of a MoS2 active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (∼22 meV) forms at the MoS2 /graphene interface, which is comparable to the MoS2 /metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.

Graphene analogues of inorganic layered materials

The discovery of graphene has created a great sensation in chemistry, physics, materials science, and related areas. The unusual properties of graphene have aroused interest in other layered materials, such as molybdenum sulfide and boron nitride. In the last few years, single- as well as few-layer as well as chalcogenides and other inorganic materials have been prepared and characterized by a variety of methods. These materials possess interesting properties, and some have potential applications. This Review provides an up-to-date account of these emerging two-dimensional nanomaterials. Not only are the synthesis and characterization covered, but also important aspects such as spectroscopic and optical properties, magnetic and electrical properties, as well as applications. Salient features of the composites formed from the layered inorganic structures with graphene and polymers are presented along with a brief description of borocarbonitrides.

Spontaneous ripple formation in MoS(2) monolayers: electronic structure and transport effects

The spontaneous formation of ripples in molybdenum disulfide (MoS2 ) monolayers is investigated via density functional theory based tight-binding Born-Oppenheimer molecular dynamics. Monolayers with different lengths show spontaneous rippling during the simulations. The density of states reveals a decrease in the bandgap induced by the stretching of the MoS2 units due to ripple formation. Significant quenching in electron conductance was also observed. The ripples in the MoS2 monolayers have an effect on the properties of the material and could impact its application in nanoelectronics.

Layer-by-layer thinning of MoS₂ by thermal annealing

By thermal annealing, few-layer MoS₂ flakes can be thinned down. In one hour, the upper layer is peeled off due to sublimation. Eventually, monolayer MoS₂ is achieved. We have characterized the process by optical contrast, Raman spectroscopy and atomic force microscopy (AFM), and observed a mixture of surfaces of N and N- 1 layers.

Bending-induced delamination of van der Waals solids

Although sheets of layered van der Waals solids offer great opportunities to custom-design nanomaterial properties, their weak interlayer adhesion challenges structural stability against mechanical deformation. Here, bending-induced delamination of multilayer sheets is investigated by molecular dynamics simulations, using graphene as an archetypal van der Waals solid. The simulations show that delamination of a graphene sheet occurs when its radius of curvature decreases roughly below Rc = 5.3 nm × (number of layers)(3/2) and that, as a rule, one-third of the layers get delaminated. These clear results are explained by a general and transparent model, a useful future reference for guiding the design of nanostructured van der Waals solids.

Self-assembly of large-scale floating TiO2 nanorod arrays at the gas-liquid interface

This paper describes a facile one-step synthesis of large-scale floating TiO2 nanorod arrays via a hydrothermal reaction without using any surfactants, which opens a new way to prepare floating photocatalysts for photodecomposition of floating organics and free-standing nanorod arrays for flexible dye-sensitized solar cells. A general model is developed to analyze the driving force for the floating TiO2 film, which may be also useful for other two-dimensional materials to determine the obtainable size of an arbitrary shaped film floated by surface tension and buoyancy.

Fabrication of luminescent monolayered tungsten dichalcogenides quantum dots with giant spin-valley coupling

A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Three-dimensional mesoscale heterostructures of ZnO nanowire arrays epitaxially grown on CuGaO2 nanoplates as individual diodes

We report a three-dimensional (3D) mesoscale heterostructure composed of one-dimensional (1D) nanowire (NW) arrays epitaxially grown on two-dimensional (2D) nanoplates. Specifically, three facile syntheses are developed to assemble vertical ZnO NWs on CuGaO2 (CGO) nanoplates in mild aqueous solution conditions. The key to the successful 3D mesoscale integration is the preferential nucleation and heteroepitaxial growth of ZnO NWs on the CGO nanoplates. Using transmission electron microscopy, heteroepitaxy was found between the basal planes of CGO nanoplates and ZnO NWs, which are their respective (001) crystallographic planes, by the observation of a hexagonal Moiré fringes pattern resulting from the slight mismatch between the c planes of ZnO and CGO. Careful analysis shows that this pattern can be described by a hexagonal supercell with a lattice parameter of almost exactly 11 and 12 times the a lattice constants for ZnO and CGO, respectively. The electrical properties of the individual CGO-ZnO mesoscale heterostructures were measured using a current-sensing atomic force microscopy setup to confirm the rectifying p-n diode behavior expected from the band alignment of p-type CGO and n-type ZnO wide band gap semiconductors. These 3D mesoscale heterostructures represent a new motif in nanoassembly for the integration of nanomaterials into functional devices with potential applications in electronics, photonics, and energy.

Carrier control of MoS2 nanoflakes by functional self-assembled monolayers

Carrier doping of MoS2 nanoflakes was achieved by functional self-assembled monolayers (SAMs) with different dipole moments. The effect of SAMs on the charge transfer between the substrates and MoS2 nanoflakes was studied by Raman spectroscopy, field-effect transistor (FET) measurements, and Kelvin probe microscope (KFM). Raman data and FET results verified that fluoroalkyltrichlorosilane-SAM with a large positive dipole moment, acting as hole donors, significantly reduced the intrinsic n-doping characteristic of MoS2 nanoflakes, while 3-(trimethoxysilyl)-1-propanamine-SAMs, acting as electron donors, enhanced the n-doping characteristic. The additional built-in electric field at the interface between SiO2 substrates and MoS2 nanoflakes induced by SAMs with molecular dipole moments determined the charge transfer process. KFM results clearly demonstrated the charge transfer between MoS2 and SAMs and the obvious interlayer screening effect of the pristine and SAM-modified MoS2 nanoflakes. However, the KFM results were not fully consistent with the Raman and FET results since the externally absorbed water molecules were shown to partially shield the actual surface potential measurement. By eliminating the contribution of the water molecules, the Fermi level of monolayer MoS2 could be estimated to modulate in a range of more than 0.45-0.47 eV. This work manifests that the work function of MoS2 nanoflakes can be significantly tuned by SAMs by virtue of affecting the electrostatic potential between the substrates and MoS2 nanoflakes.

Spin-orbit splitting in single-layer MoS2 revealed by triply resonant Raman scattering

Although new spintronic devices based on the giant spin-orbit splitting of single-layer MoS(2) have been proposed, such splitting has not been studied effectively in experiments. This Letter reports the valence band spin-orbit splitting in single-layer MoS(2) for the first time, probed by the triply resonant Raman scattering process. We found that upon 325 nm laser irradiation, the second order overtone and combination Raman modes of single-layer MoS(2) are dramatically enhanced. Such resonant Raman enhancement arises from the electron-two-phonon triple resonance via the deformation potential and Fröhlich interaction. As a sensitive and precise probe for the spin-orbit splitting, the triply resonant Raman scattering will provide a new and independent route to study the spin characteristics of MoS(2).

Remarkable magnetism and ferromagnetic coupling in semi-sulfuretted transition-metal dichalcogenides

Motivated by recent investigations of semi-decorated two dimensional honeycomb structures, we demonstrated, via spin-polarized molecular-dynamics simulations and density-functional-theory calculations, that semi-sulfuretted transition-metal dichalcogenides of MX type (M = V, Nb, Ta; X = S, Se, Te) are stable and display remarkable magnetism. The unpaired d electron of the transition-metal atom arising from the breakage of the M-X bond is the mechanism behind the induction of the magnetism. The remarkable magnetism of the transition-metal atoms is caused by ferromagnetic coupling due to the competitive effects of through-bond interactions and through-space interactions. This implies the existence of an infinite ferromagnetic sheet with structural integrity and magnetic homogeneity. The estimated Curie temperatures suggest that the ferromagnetism can be achieved above room temperature in the VS, VSe, VTe, NbTe and TaTe sheets. Depending on the species of the M and X atoms, the MX sheet can be a magnetic metal, magnetic semiconductor or half-metal. Furthermore, in contrary to the recently reported semi-hydrogenated and semi-fluorinated layered materials consisting of B, C, N, etc., the MX sheets with many unpaired d electrons can offer a much stronger spin polarization and possess a more stable ferromagnetic coupling, which is critical for practical nanoscale device applications.

Low-frequency electronic noise in single-layer MoS2 transistors

Ubiquitous low-frequency 1/f noise can be a limiting factor in the performance and application of nanoscale devices. Here, we quantitatively investigate low-frequency electronic noise in single-layer transition metal dichalcogenide MoS2 field-effect transistors. The measured 1/f noise can be explained by an empirical formulation of mobility fluctuations with the Hooge parameter ranging between 0.005 and 2.0 in vacuum (<10(-5) Torr). The field-effect mobility decreased, and the noise amplitude increased by an order of magnitude in ambient conditions, revealing the significant influence of atmospheric adsorbates on charge transport. In addition, single Lorentzian generation-recombination noise was observed to increase by an order of magnitude as the devices were cooled from 300 to 6.5 K.

Intrinsic electronic transport properties of high-quality monolayer and bilayer MoS2

We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS2. Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS2 down to 4 K at high carrier densities. At lower carrier densities, low-temperature four probe transport measurements show a metal-insulator transition in both monolayer and bilayer samples. In the metallic regime, the high-temperature behavior of the mobility showed strong temperature dependence consistent with phonon-dominated transport. At low temperature, intrinsic field-effect mobilities approaching 1000 cm(2)/(V·s) were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.

MoS2 nanocrystals confined in a DNA matrix exhibiting energy transfer

We report the wet chemical synthesis of MoS2 nanocrystals (NCs), a transition-metal dichalcogenide, using DNA as a host matrix. As evidenced from transmission electron microscopy (TEM), the NCs are highly crystalline, with an average diameter of ~5 nm. Ultraviolet-visible (UV-vis) absorption studies along with band gap calculations confirm that NCs are in quantum confinement. A prominent red shift of the optical absorption bands has been observed upon formation of the thin film using hexadecyltrimethylammonium chloride (CTAC), i.e., in the case of MoS2@DNA-CTAC. In the thin film, strong electron-phonon coupling arises because of the resonance effect, which is reflected from the emergence of intense first-, second-, and third-order Raman peaks, whenever excited with the 488 nm line. We have established that our as-synthesized MoS2 NCs quench the fluorescence of a well-known DNA minor groove binding probe, Hoechst 33258. Unprecedented fluorescence quenching (94%) of donor (Hoechst 33258) emission and efficient energy transfer (89%) between Hoechst 33258 and MoS2 NCs (acceptor) are obtained. The donor-acceptor distance of these conjugates has been described by a Förster resonance energy transfer (FRET)-based model. Furthermore, employing a statistical method, we have estimated the probability of the distance distribution between the donor and acceptor. We believe that the study described herein may enable substantial advances in fields of optoelectronics, photovoltaics, catalysis, and many others.

2D single- or double-layered vanadium oxide nanosheet assembled 3D microflowers: controlled synthesis, growth mechanism, and applications

A facile one-pot solvent-thermal method was developed to synthesize a unique 3D microflower structure assembled from single- or double-layered 2D nanosheets of V4O9 (F-VO). Simply by controlling the precursor concentration, yolk-shelled V4O9 (YS-VO) or bulk V4O9 (B-VO) can be produced instead. The precursor-concentration dependent growth mechanism is proposed. The exceptional catalytic/electrochemical properties and large specific surface area of F-VO promise a wide range of applications. As a proof-of-concept demonstration, we investigate its use in high-performance supercapacitors (~392 F g(-1)), and for sensitive detection of H2O2 (with a low detection limit of ~0.1 μM) and methanol (with a low detection limit of ~60 μM). Furthermore, we show that F-VO greatly outperforms its counterparts (YS-VO and B-VO) presumably owing to its unique structure and crystal plane orientation.

Mobility engineering and a metal-insulator transition in monolayer MoS₂

Two-dimensional (2D) materials are a new class of materials with interesting physical properties and applications ranging from nanoelectronics to sensing and photonics. In addition to graphene, the most studied 2D material, monolayers of other layered materials such as semiconducting dichalcogenides MoS₂ or WSe₂ are gaining in importance as promising channel materials for field-effect transistors (FETs). The presence of a direct bandgap in monolayer MoS₂ due to quantum-mechanical confinement allows room-temperature FETs with an on/off ratio exceeding 10(8). The presence of high- κ dielectrics in these devices enhanced their mobility, but the mechanisms are not well understood. Here, we report on electrical transport measurements on MoS₂ FETs in different dielectric configurations. The dependence of mobility on temperature shows clear evidence of the strong suppression of charged-impurity scattering in dual-gate devices with a top-gate dielectric. At the same time, phonon scattering shows a weaker than expected temperature dependence. High levels of doping achieved in dual-gate devices also allow the observation of a metal-insulator transition in monolayer MoS₂ due to strong electron-electron interactions. Our work opens up the way to further improvements in 2D semiconductor performance and introduces MoS₂ as an interesting system for studying correlation effects in mesoscopic systems.

Exceptional tunability of band energy in a compressively strained trilayer MoS2 sheet

Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

Bandgap engineering of strained monolayer and bilayer MoS2

We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Grüneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is approximately linear with strain, ~45 meV/% strain for monolayer MoS2 and ~120 meV/% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.

High frequency MoS2 nanomechanical resonators

Molybdenum disulfide (MoS2), a layered semiconducting material in transition metal dichalcogenides (TMDCs), as thin as a monolayer (consisting of a hexagonal plane of Mo atoms covalently bonded and sandwiched between two planes of S atoms, in a trigonal prismatic structure), has demonstrated unique properties and strong promises for emerging two-dimensional (2D) nanodevices. Here we report on the demonstration of movable and vibrating MoS2 nanodevices, where MoS2 diaphragms as thin as 6 nm (a stack of 9 monolayers) exhibit fundamental-mode nanomechanical resonances up to f0 ~ 60 MHz in the very high frequency (VHF) band, and frequency-quality (Q) factor products up to f0 × Q ~ 2 × 10(10)Hz, all at room temperature. The experimental results from many devices with a wide range of thicknesses and lateral sizes, in combination with theoretical analysis, quantitatively elucidate the elastic transition regimes in these ultrathin MoS2 nanomechanical resonators. We further delineate a roadmap for scaling MoS2 2D resonators and transducers toward microwave frequencies. This study also opens up possibilities for new classes of vibratory devices to exploit strain- and dynamics-engineered ultrathin semiconducting 2D crystals.

Two-dimensional crystals: managing light for optoelectronics

Semiconducting two-dimensional (2D) crystals such as MoS2 and WSe2 exhibit unusual optical properties that can be exploited for novel optoelectronics ranging from flexible photovoltaic cells to harmonic generation and electro-optical modulation devices. Rapid progress of the field, particularly in the growth area, is beginning to enable ways to implement 2D crystals into devices with tailored functionalities. For practical device performance, a key challenge is to maximize light-matter interactions in the material, which is inherently weak due to its atomically thin nature. Light management around the 2D layers with the use of plasmonic nanostructures can provide a compelling solution.

PLD Grown Polycrystalline Tungsten Disulphide (WS2) Films

Polycrystalline WS2 films were grown by pulsed laser deposition (PLD) system at relatively low temperature. The main objective of this study is to optimize the growth conditions for polycrystalline WS2 films at relatively low temperature to use them for photovoltaics (PVs). Different growth conditions and substrates are used and examined systematically. It is found out that films grown on strontium titanate SrTiO3 (STO) substrate have the best structural properties when compared to other substrates examined in this work. X-ray diffraction and optical characterizations of these films reveal crystallographic growth and very promising optical properties for PVs. Furthermore, it was observed that higher growth temperature (>300°C) has an unfavorable effect on the layers by creating some tungsten metallic droplets.

Measuring the lateral size of liquid-exfoliated nanosheets with dynamic light scattering

We have developed an in situ method to estimate the lateral size of exfoliated nanosheets dispersed in a liquid. Using standard liquid exfoliation and size-selection techniques, we prepared a range of dispersions of graphene, MoS2 and WS2 nanosheets with different mean lateral sizes. The mean nanosheet length was measured using transmission electron microscopy (TEM) to vary from ∼40 nm to ∼1 μm. These dispersions were characterized using a standard dynamic light scattering (DLS) instrument. We found a well-defined correlation between the peak of the particle size distribution as outputted by the DLS instrument and the nanosheet length as measured by TEM. This correlation is consistent with the DLS instrument outputting the radius of a sphere with volume equal to the mean nanosheet volume. This correlation allows the mean nanosheet length to be extracted from DLS data.

Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets

Promising catalytic activity from molybdenum disulfide (MoS2) in the hydrogen evolution reaction (HER) is attributed to active sites located along the edges of its two-dimensional layered crystal structure, but its performance is currently limited by the density and reactivity of active sites, poor electrical transport, and inefficient electrical contact to the catalyst. Here we report dramatically enhanced HER catalysis (an electrocatalytic current density of 10 mA/cm(2) at a low overpotential of -187 mV vs RHE and a Tafel slope of 43 mV/decade) from metallic nanosheets of 1T-MoS2 chemically exfoliated via lithium intercalation from semiconducting 2H-MoS2 nanostructures grown directly on graphite. Structural characterization and electrochemical studies confirmed that the nanosheets of the metallic MoS2 polymorph exhibit facile electrode kinetics and low-loss electrical transport and possess a proliferated density of catalytic active sites. These distinct and previously unexploited features of 1T-MoS2 make these metallic nanosheets a highly competitive earth-abundant HER catalyst.

Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures between 175 and 300 K show quenching of almost 80% for an ~10% decrease in sulfur content.