TMD Materials Research Articles

Boosted electrochemical ammonia synthesis by high-percentage metallic transition metal dichalcogenide quantum dots.

The electrochemical method can directly convert N2 into the high-value-added NH3 under ambient conditions and is considered to be a green and sustainable alternative to the traditional Haber-Bosch process. However, the electrochemical nitrogen reduction reaction (NRR) suffers from a low ammonia yield rate over the reported electrocatalysts. Herein, we have developed a general strategy to boost the NRR performance, enabled by the metallic 1T phase dominated transition metal dichalcogenide quantum dots (TMD QDs). Impressively, the obtained MoSe2 QDs achieved a superior ammonia yield rate of 340 μg mg-1 cat. h-1 with excellent ammonia generation sustainability. Experimental and theoretical studies revealed that the excellent catalytic activity of MoSe2 QDs mainly originates from the ultra-small quantized size (high surface area and high-density active edge/defect sites) and high-percentage metallic 1T phase (the N2 adsorption on the 1T phase is spontaneous, and the energy barrier of the potential determining step on the 1T phase is very low). Most importantly, our concept is universal for TMD materials (i.e., MoS2, WSe2, WS2 and NbSe2) that also exhibit a much-enhanced ammonia yield rate as compared to other electrocatalysts.

Length- and Thickness-Dependent Optical Response of Liquid-Exfoliated Transition Metal Dichalcogenides

Because of their reduced dimensionality, two-dimensional materials show intriguing optical properties and strong light-matter interaction. In particular, group VI transition metal dichalcogenides have been extensively studied and proof-of-principle optical applications have been demonstrated. Most studies to date focus on individual mono- or bilayered micromechanically exfoliated samples, which often display significant variations between flakes. In this work, we study size-dependent optical properties of four group VI TMD materials: WS2, MoS2, WSe2, and MoSe2, each consisting of ensembles of nanosheets suspended in the liquid environment. Samples were produced by liquid-phase exfoliation and size-selected using cascade centrifugation with size and layer number distributions quantified by statistical atomic force microscopy. Differences in lateral size and layer number are reflected in systematic changes in the optical extinction and absorbance spectra, which we exploit to establish quantitative spectroscopic metrics to facilitate the measurement of nanosheet dimensions for each of the four materials. The lowest energy resonance, referred to as A-exciton, is analyzed in more detail. In all cases, an exponential red shift with increasing layer number is observed. Our experimental data, backed up with first-principle calculations, reveal that the magnitude of the shift is dependent on the molecular mass of the central metal atom (W, Mo), while the rate at which the peak shifts from monolayer to bulk depends on the band gap of the semiconductor.

Ab initio investigation of structural stability and exfoliation energies in transition metal dichalcogenides based on Ti-, V-, and Mo-group elements

In this work, we report an ab initio investigation based on density functional theory of the structural, energetic and electronic properties of 2D layered chalcogenides compounds based in the combination of the transition-metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and chalcogenides (S, Se, Te) in three polymorphic phases: trigonal prismatic (2H), octahedral (1T) and distorted octahedral (1T$_{\text{d}}$). We determined the most stable phases for each compound, verifying the existence of the 1T$_{\text{d}}$ phase for a small number of the compounds and we have also identified the magnetic compounds. In addition, with the determination of the exfoliation energies, we indicated the potential candidates to form one layer material and we have also found a relation between the exfoliation energy and the effective Bader charge in the metal, suggesting that when the materials present small exfoliation energy, it is due to the Coulomb repulsion between the chalcogen planes. Finally, we analyzed the electronic properties, identifying the semiconductor, semimetal and metal materials and predicting the band gap of the semiconductors. In our results, the dependence of the band gap on the $d$-orbital is explicit. In conclusion, we have investigated the properties of stable and metastable phases for a large set of TMD materials, and our findings may be auxiliary in the synthesis of metastable phases and in the development of new TMDs applications.

Efficient Ultrathin Liquid Junction Photovoltaics Based on Transition Metal Dichalcogenides.

Ultrathin photovoltaics made of MoS2 or WSe2 have the potential to convert solar energy to electricity with high efficiency because all photogenerated carriers are produced at a charge-collecting interface. However, solid-state monolayer photovoltaic devices typically require that charge carriers travel parallel to, instead of perpendicular to, the three atom-thick material toward charge-collecting contacts. Parallel charge transport across long distances decreases energy conversion efficiency. Here we demonstrate proof-of-concept monolayer and bilayer TMD|I-,I3-|Pt photoelectrochemical solar cells that use a conformal liquid electrolyte junction for efficient perpendicular charge transport over significantly larger active areas than solid state systems. Efficient perpendicular charge transport is evidenced by high peak internal quantum efficiencies (IQE) of 44.2, 9.1, and 10.5% for 0.4 mm2 MoS2, WSe2, and MoS2/WSe2 domains in a predominantly monolayer MoS2/WSe2 film. The monochromatic energy conversion efficiencies are competitive with state-of-the-art solid-state monolayer heterojunction photovoltaics. However, inefficient light absorption limits the overall power conversion efficiency to 0.19% for this planar geometry monolayer photovoltaic system. Interestingly, correlated Raman and scanning photoelectrochemical microscopy measurements revealed a nonlinear scaling relation between IQE and layer thickness for MoS2, WSe2, and MoS2/MoS2/WSe2 domains within the heterojunction film. Specifically, the monochromatic energy conversion efficiency of bilayer MoS2 is an order of magnitude greater than monolayer MoS2 and MoS2/MoS2/WSe2and itexceeds MoS2/WSe2 by a factor of 4. The structure/function relationships are hidden in ensemble-level photoelectrochemical measurements. Although nanostructured or plasmonic electrode architectures are still needed to enhance overall light absorption, our study shows that the liquid junction approach represents a simple and rapid strategy to screen ultrathin TMD materials combinations, tune interfacial energetics, and make conformal electrical contacts in photoelectrochemical energy conversion systems for electricity or solar fuels production.

Quantum-electrodynamical approach to the exciton spectrum in transition-metal dichalcogenides

Manipulation of intrinsic electron degrees of freedom, such as charge and spin, gives rise to electronics and spintronics, respectively. Electrons in monolayer materials with a honeycomb lattice structure, such as the Transition-Metal Dichalcogenides (TMD’s), can be distinguished according to the region (valley) of the Brillouin zone to which they belong. Valleytronics, the manipulation of this electron’s property, is expected to set up a new era in the realm of electronic devices. In this work, we accurately determine the energy spectrum and lifetimes of exciton (electron–hole) bound-states for different TMD materials, namely WSe2, WS2 and MoS2. For all of them, we obtain a splitting of the order of 170 meV between the exciton energies from different valleys, corresponding to an effective Zeeman magnetic field of 1400 T. Our approach, which employs quantum-field theory (QFT) techniques based on the Bethe–Salpeter equation and the Schwinger–Dyson formalism, takes into account the full electromagnetic interaction among the electrons. The valley selection mechanism operates through the dynamical breakdown of the time-reversal (TR) symmetry, which originally interconnects the two valleys. This symmetry is spontaneously broken whenever the full electromagnetic interaction vertex is used to probe the response of the system to an external field.

Moiré-templated strain patterning in transition-metal dichalcogenides and application in twisted bilayer MoS2.

To take full advantage of the electronic properties of transition-metal dichalcogenides and their vdW layered structures, it will be necessary to control the local electronic structure, on which the effect of lattice deformation is significant. Nevertheless, a general approach to programming nanoscale morphology in TMD materials, which would permit local strain engineering, has proven elusive. In this work, we propose a general moiré-templated nanoscale morphology engineering method based on bilayer TMDs. The moiré superlattice plays the key role in enforcing in-plane periodical variations in local interlayer spacing and potential energy. Upon global in-plane compression, the high-energy, large-interlayer-separation stacking domains serve as periodic buckling initiation sites. The buckled features can be thus precisely correlated to the moiré periodicity. The spatial profile of the buckled morphology and strain field are possible to be pre-determined, providing a bridge to the electronics and optoelectronics design. We take twisted bilayer MoS2 to demonstrate our approach. We further demonstrate how the morphology can modulate band gap and optical absorption of a MoS2 monolayer, envisioned as a potential constituent layer in a Moiré-templated, strain-engineered vdW heterostructure of TMDs. The robustness and programmable nature of our approach arise from superlattice symmetry, energetics and mechanics. Our approach provides a new strategy for on-demand design of morphology and local strain in TMDs under mechanical deformation.

Recent progress in two-dimensional transition metal dichalcogenides: Material synthesis for microelectronics

The size of modern electronic and optoelectronic devices has been shrunk down to nanoscale and this effort still continues. A lot of concerns focus on whether current silicon-based complementary metal oxide semiconductor (CMOS) technologies can overcome the physical limit at nanoscale. A promising alternative choice of semiconductor material, two-dimensional layered materials (2DLMs) have emerged recently, including graphene, 2D transition-metal dichalcogenides (2D TMDs), black phosphors (BPs), etc. Due to their unique atomic thickness nature, 2DLMs are likely to have the greatest potential to overcome the dilemma of geometric scaling. 2DLMs also exhibit a wide range of unique properties owing to their unusual atomically thin defined structures. Most 2D TMDs have a MX2 structure, where M represents transition metal (Mo, W, Ti, Pt, Hf, etc.) and X is the chalcogen (S, Se, or Te). Contrary to graphene which displays many superior properties but lacks a bandgap, 2D TMDs exhibit a wide range of electronic properties, from semiconducting to metallic or even superconducting, depending on the combination of M and X. For most Mo and W based 2D TMDs, the intrinsic band gap is greater than 1 eV so that the high on/off-current ratio is more easily achieved at room temperature. Moreover, 2D TMDs possess band structures that are known to be highly sensitive to various effects that arise from surfaces and interfaces, thus providing versatile knobs for tuning 2D TMD devices. Various modification methods have been applied to further physically or chemically tune 2DLMs based on their unique intrinsic properties. To date these include dimensional sizing, ion-intercalation, application of an external field, tuning the stacking order, and strain engineering, etc. These fundamental modification methods provide useful engineering tools to improve TMD electronic devices. So far, most research results of 2D TMDs have been based on mechanically exfoliated sheets which are single crystalline with few defects. But the exfoliation method has poor reproducibility, relatively low yield, and gives rise to a lateral size no more than hundreds of microns. Thus, the main bottleneck for practical application of 2D TMDs in electronic devices still remains at the stage of wafer scale uniform growth. Compared to chemical vapor deposition (CVD) synthesized graphene film with high carrier mobility and large grain size, the existing synthetic methods of 2D TMDs can only produce films with limited spatial uniformity and fair electrical performance. Thus, there is still significant room for improving current CVD methods, or exploring new synthetic methods. In this review, we summarize and compare various preparation methods of 2D TMD materials. All these methods can be actually classified by two strategies: ″Top-down″ methods, where the bulk forms are exfoliated into few-layer or monolayer sheets; and the ″bottom-up″ methods, that chemically synthesize 2D TMDs using CVD, atomic layer deposition (ALD), molecular beam epitaxy (MBE), etc. We first introduce several important ″Top-down″ methods including mechanical exfoliation, liquid phase exfoliation, ion intercalation and laser thinning. We then focus on the CVD approaches that are suitable for wafer-scale synthesis of high-quality 2D TMD films. We also introduce the latest development of 2D TMDs application in electronic devices, and provide a look at the future of this field.

Probing Interface Defects in Top-Gated MoS2 Transistors with Impedance Spectroscopy.

The electronic properties of the HfO2/MoS2 interface were investigated using multifrequency capacitance-voltage (C-V) and current-voltage characterization of top-gated MoS2 metal-oxide-semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5-10) MoS2 MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO2 gate oxide layers formed by atomic layer deposition after in-situ UV-O3 surface functionalization. The impedance response of the HfO2/MoS2 gate stack indicates the existence of specific defects at the interface, which exhibited either a frequency-dependent distortion similar to conventional Si MOSFETs with unpassivated silicon dangling bonds or a frequency dispersion over the entire voltage range corresponding to depletion of the HfO2/MoS2 surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (Dit) was extracted from the C-V responses by the high-low frequency and the multiple-frequency extraction methods, where a Dit peak value of 1.2 × 1013 cm-2 eV-1 was extracted for a device (7-layer MoS2 and 13 nm HfO2) exhibiting a behavior approximating to a single trap response. The MoS2 MOSFET with 4-layer MoS2 and 8 nm HfO2 gave Dit values ranging from 2 × 1011 to 2 × 1013 cm-2 eV-1 across the energy range corresponding to depletion near the HfO2/MoS2 interface. The gate current was below 10-7 A/cm2 across the full bias sweep for both samples indicating continuous HfO2 films resulting from the combined UV ozone and HfO2 deposition process. The results demonstrated that impedance spectroscopy applied to relatively simple top-gated transistor test structures provides an approach to investigate electrically active defects at the HfO2/MoS2 interface and should be applicable to alternative TMD materials, surface treatments, and gate oxides as an interface defect metrology tool in the development of TMD-based MOSFETs.

The Preparation of Large-Area Single Crystal MoS2 on Quartz Substrate for Photodetector

Controllable synthesis of high quality large area monolayer MoS2 is still a challenge for its practice application. In this work, we report an Au foil assistant synthesis large size (400 μm) single crystals MoS2 on quartz substrate via chemical vapor deposition (CVD). The Raman and photoluminescence spectroscopy studies indicate the synthesized monolayer MoS2 has a high homogenous optical characteristic. The optoelectronic performance of MoS2 photodetector were also fabricated which shows a high photo-response of 550mA/W and a fast photo-response time of 230 ms. The growth techniques described here will beneficial for other atomically thin TMD materials growth.

3R phase of MoS2 and WS2 outperforms the corresponding 2H phase for hydrogen evolution.

Herein, we compare the bulk, 2H and 3R phases of two most prevalent TMD materials: MoS2 and WS2. The 3R phase outperforms its 2H phase counterpart in hydrogen evolution reaction catalysis and is even comparable with the exfoliated, 1T phase in the case of MoS2.

Phase Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage.

Achieving homogeneous phase transition and uniform charge distribution is essential for good cycle stability and high capacity when phase conversion materials are used as electrodes. Herein, we show that chemical lithiation of bulk 2H-MoS2 distorts its crystalline domains in three primary directions to produce mosaic-like 1T' nanocrystalline domains, which improve phase and charge uniformity during subsequent electrochemical phase conversion. 1T'-LixMoS2, a macroscopic dense material with interconnected nanoscale grains, shows excellent cycle stability and rate capability in a lithium rechargeable battery compared to bulk or exfoliated-restacked MoS2. Transmission electron microscopy studies reveal that the interconnected MoS2 nanocrystals created during the phase change process are reformable even after multiple cycles of galvanostatic charging/discharging, which allows them to play important roles in the long term cycling performance of the chemically intercalated TMD materials. These studies shed light on how bulk TMDs can be processed into quasi-2D nanophase material for stable energy storage.

Three-Dimensional CuO Branched Nanowires for Efficient Photoelectrochemical Water Splitting

1. Objective Recently photoelectrochemical (PEC) water splitting has considered as one of the most promising method for clean and renewable energy. In an effort to find proper materials for water splitting, many metal oxides and semiconductors were studied in recent years. Among various materials, copper oxide (CuO) have attracted a lot of interest due to non-toxicity, abundance, facile, low cost, and scalable production. In particular, CuO could absorb a wide range of the solar light including near IR region due to its narrow band gap (Eg = 1.2 eV). However, its short diffusion length and low conductivity cause the recombination of photo-generated carriers. As a result, it could lead to decrease of PEC performance immediately. To prevent this problems in water splitting, several approaches have been developed including a three-dimensional (3D) branched nanowires (b-NWs) design [1], [2] or 3D networks structure design [3] or TiO2 coating on the NWs surfaces [4], or a TMD materials used as co-catalyst [5]. In this work, we report the fabrication of 3D CuO b-NWs for efficient water splitting. Highly dense b-NWs are grown on copper substrates and it used as photocathodes directly. Using these 3D CuO b-NWs, we demonstrated improved PEC performance compared to conventional CuO NWs. 2. Experiments To improve the PEC performances, three-dimensional (3D) CuO branched nanowires (b-NWs) were fabricated by combining several solution-based process such as wet chemical oxidation method [6] and chemical precipitation methods [7]. Then, highly dense 3D CuO b-NWs were grown on copper mesh substrates and it used as photocathodes directly. PEC performance was measured in a 3-electrode system under 100 mA/cm2 (1 sun) light intensity. Furthermore, Ag/AgCl electrode in saturated KCl was used as a reference electrode, Pt mesh was used as a counter electrode, and 0.25 M Na2SO4 at pH 5.68 was used as electrolytes. The current density-potential (J-V) characteristics of the electrodes were measured with a potentiostat (Iviumstat, Ivium Technologies) to apply to external potential and scan rate for measurement was 10 mV/s. 3. Results and Discussion Fig. PEC performance of conventional CuO NWs and 3D CuO b-NWs photocathodes. Diameters ranging from 50 to 500 nm and over 10 µm in lengths CuO NWs were observed. In particular, bending of CuO NWs were observed due to H2O molecule released during annealing process. The transformation of Cu(OH)2 nanowires to CuO nanowires as follows: Cu(OH)2 → CuO + H2O. Highly dense branches were attached to CuO NWs backbone. Branches ranging from 100 to 500 nm in length and ranging from 10 to 200 nm in diameter 3D CuO b-NWs are observed. Figure 2 shows PEC measurement, dark current and photo current could be observed simultaneously. The b-NWs provide a higher photocathodic current density compared to conventional CuO NWs due to effective charge transport and increased surface area. In particular, maximum current density 1.37 mA/cm2 is observed at ~0.05 V under 100 mW/cm2 (1 sun) illumination when 3D CuO b-NWs grown on copper gauze substrates. In case of conventional CuO NWs on copper gauze substrates, maximum current density 0.77 mA/cm2 was observed at ~0.05 V under 100 mW/cm2 (1 sun) illumination. 4. Conclusion We fabricated the three-dimensional (3D) CuO branched nanowires (b-NWs) by simple process. This fabrication process required short growth time and low growth temperature. Copper mesh substrates with highly dense NWs were used as photocathode directly. In particular, the highest PEC performance showed when CuO b-NWs grown on copper gauze substrates. This report presents a facile, low cost, and scalable 3D CuO b-NWs fabrication route that is widely applicable to photo-electrochemical devices. 5. Reference [1] A. Kargar, et al. ACS NANO. 2013, 7, 11112. [2] A. Kargar, et al. ACS NANO. 2013, 7, 9407. [3] A. Kargar, et al. Nanotechnology. 2014, 25, 205401. [4] Savio J. A. et al. ChemCatChem. 2015, 7, 1659. [5] Z. Yin. et al. Small. 2014, 10, 3537. [6] Zhang. W, Adv. Mater. 2003, 15, 822. [7] Q. Zhang, Progress in Materials Science. 2014, 60, 208. Figure 1

(Invited) TMD Materials Challenges: Defects, Impurities, Passivation and Interfaces

Layered Transition Metal Dichalcogenides are under scrutiny for potential applications in electronics, catalysis, sensing, photonics, and other technologies. For many applications, superior control of the materials quality and the resultant interface is essential for practical, reliable device applications. This talk will describe our recent studies investigating TMD materials surfaces using in-situ and ex-situ analysis to establish the quality of both geological and synthesized TMDs, and the challenges associated with enabling useful device technologies. This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA, by the SWAN Center, a SRC center sponsored by the Nanoelectronics Research Initiative and NIST, and the US/Ireland R&D Partnership (UNITE) under the NSF award ECCS-1407765.

Interpenetrated Graphene/WSe2 Lateral Heterostructures for Barrierless Ohmic-Contacted p-FETs

High contact resistance between TMDs and metals has been identified as an outstanding issue. TMDs tend to form a substantial Schottky barrier (SB) with commonly used metals. Air-stable graphene electrodes stacked on the WSe2 by transfer technique have been reported. However, unipolar conduction has not been achieved in FETs based on TMD materials and graphene electrodes. Here we report a synthetic process to directly grow WSe2 monolayers along the edge of patterned few-layered graphene, forming the lateral heterojunction between the graphene and monolayer WSe2. The few-layered WSe2 bridges the few-layer graphene and monolayer WSe2, and an interpenetrating layered structure is formed between graphene and WSe2. As a result, the WSe2 monolayer channel and the junction to the graphene can be formed at the same time by the synthesis method, where graphene serves as source and drain. Graphene is in-situ heavily p-doped by H2 with covent bond during WSe2 growing process. Such a unique junction structure leads to a barrier less ohmic contact between doped graphene and WSe2. Unipolar WSe2 PFET with laterally contacted p-doped graphene source/drain (p-G/WSe2 PFET) can be achieved for the first time. With Pd contacted source/drain metal, the unipolar PFET perform Ion/Ioff > 105 and field effect mobility ~20 cm2/Vˑs. Moreover we can still have unipolar PFET with Ti source/drain metal exceeds 103 Ion/Ioff ratio even Ti work function is low (4.3 eV) and close to the conduction band edge of WSe2 (electron affinity of WSe2~ 4.03 eV).

Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers.

In this work, we demonstrate that gas adsorption is significantly higher in edge sites of vertically aligned MoS2 compared to that of the conventional basal plane exposed MoS2 films. To compare the effect of the alignment of MoS2 on the gas adsorption properties, we synthesized three distinct MoS2 films with different alignment directions ((1) horizontally aligned MoS2 (basal plane exposed), (2) mixture of horizontally aligned MoS2 and vertically aligned layers (basal and edge exposed), and (3) vertically aligned MoS2 (edge exposed)) by using rapid sulfurization method of CVD process. Vertically aligned MoS2 film shows about 5-fold enhanced sensitivity to NO2 gas molecules compared to horizontally aligned MoS2 film. Vertically aligned MoS2 has superior resistance variation compared to horizontally aligned MoS2 even with same surface area exposed to identical concentration of gas molecules. We found that electrical response to target gas molecules correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. Density functional theory (DFT) calculations corroborate the experimental results as stronger NO2 binding energies are computed for multiple configurations near the edge sites of MoS2, which verifies that electrical response to target gas molecules (NO2) correlates directly with the density of the exposed edge sites of MoS2 due to high adsorption of gas molecules onto edge sites of vertically aligned MoS2. We believe that this observation extends to other 2D TMD materials as well as MoS2 and can be applied to significantly enhance the gas sensor performance in these materials.

2H → 1T phase transition and hydrogen evolution activity of MoS2, MoSe2, WS2 and WSe2 strongly depends on the MX2 composition.

We investigate herein the four most widely considered TMD materials: MoS2, MoSe2, WS2 and WSe2. A 2H → 1T phase transition occurs during the chemical exfoliation with different efficiencies among the four materials.