Single-layer MoS2 Research Articles

MoS2-supported Au31 for CO hydrogenation: A first-principle study

While the basal plane of single-layer molybdenum disulphide (MoS2) is inert, attempts have been made to functionalize it chemically through the creation of defects and/or addition of dopants. With nanoparticles as dopants, the authors present density functional theory-based calculations of the hydrogenation of CO on a 31-atom, bilayer Au cluster supported on single-layer MoS2 (Au31/MoS2). Not surprisingly, the adsorption and reaction of all species involved in the hydrogenation occur at the edges of the cluster—the regions at which the interaction between MoS2 and the Au cluster is tracked to be the strongest. The authors find two possible mechanisms that lead to the formation of methanol: (1) CO* → CHO* → CH2O* → CH3O* → CH3OH* and (2) CO* → CHO* → HCOH* → CH2OH* → CH3OH*, where * indicates the adsorbed species. Detailed analysis of the reaction pathways, however, does not result in favoring one mechanism over the other. Rather, both mechanisms are facile. In addition, the rate-limiting step in each mechanism is found to be the formation of the formyl radical group (CO* + H* → CHO*).

Investigation of Double-Gate Ferroelectric FET Based on Single-Layer MoS2 with Consideration of Contact Resistance

A ferroelectric field-effect transistor (Fe-FET) with two-dimensional MoS2 as the channel material with and without contact resistance is explored and compared. A top-of-the-barrier model along with the ferroelectric model is used to investigate the device performance. The contact resistance can strongly affect the current–voltage characteristic. The Fe-FET with contact resistance requires a higher voltage to reach saturation. Increasing the ferroelectric thickness to a specified value decreases the output resistance, but further increase can compensate this, resulting in high output resistance. Increasing the ferroelectric thickness decreases the mean subthreshold swing in the upward and downward regime. This effect is greater for the downward regime, and the contact resistance can intensify it.

Homogeneously mixed heterostructures of BiVO4/MoS2/RGO with improved photoelectrochemical performances

In this work, homogeneously mixed heterostructures of BiVO4 and MoS2/RGO have been successfully obtained by a simple calcination method. Different from the traditional composite, ultrathin MoS2/RGO nanosheet was uniformly dispersed in the inside of BiVO4 nanoparticles, which should provide more electron/hole transport channels due to the enhanced contact area. MoS2/RGO nanocomposites were firstly prepared by a hydrothermal process, and then were dispersed uniformly in the precursor solution of BiVO4. A homogeneously mixed BiVO4/MoS2/RGO heterostructure was obtained after annealing treatment. It is noted that RGO can not only stable single-layer MoS2, but also provide the transport channels of photo-excited electrons between MoS2 and BiVO4. Moreover, loading co-catalyst, such as FeOOH/NiOOH, could further increase the photoelectrochemical performances of BiVO4/MoS2/RGO heterostructures through capturing the photo-generated holes. Hence, homogeneously mixed BiVO4/MoS2/RGO-S heterostructures exhibited higher photoelectrochemical performances than the one obtained through depositing MoS2/RGO on the surfaces of BiVO4 owing to the improved electron-hole pairs separation efficiency.

Study of the Properties of Two-Dimensional MoS2 and WS2 Films Synthesized by Chemical-Vapor Deposition

Molybdenum- and tungsten-disulfide films are synthesized by chemical-vapor deposition. The set of optimal synthesis parameters (temperature, time, and amount and ratio of precursors) is established at which MoS2 domains with maximum lateral sizes of up to 250 μm on sapphire and MoS2 and WS2 domains up to 80 μm in size on SiO2 can be grown. Domain intergrowth leads to the formation of homogeneous single-layer MoS2 films. The Raman spectra of the synthesized films contain two characteristic peaks corresponding to the atomic vibrations in MoS2 and WS2. Photoluminescence of the single-layer and bilayer MoS2 films with a maximum intensity of 670 ± 2 nm and of the single-layer WS2 films with a maximum intensity of 630 ± 2 nm is detected. The photoluminescence spectral maps (the dependences of the photoluminescence intensity on the luminescence and excitation-light wavelengths) are measured. According to the measured data, the photoluminescence excitation spectrum of MoS2 has a maximum at 350 ± 5 nm and the photoluminescence excitation spectrum of WS2 has a maximum at 330 ± 5 nm. The I–V characteristics of the synthesized films are photosensitive in the visible spectral range.

Strongly Coupled Coherent Phonons in Single-Layer MoS2.

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20 fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single-layer (1L)-MoS2, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A'1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a time scale of few tens of fs. We observe an enhancement by almost 2 orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in the 1L-MoS2 band structure induced by the A'1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrate that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process.

Tweaking the Physics of Interfaces between Monolayers of Buckled Cadmium Sulfide for a Superhigh Piezoelectricity, Excitonic Solar Cell Efficiency, and Thermoelectricity.

Interfaces of heterostructures are routinely studied for different applications. Interestingly, monolayers of the same material when interfaced in an unconventional manner can bring about novel properties. For instance, CdS monolayers, stacked in a particular order, are found to show unprecedented potential in the conversion of nanomechanical energy, solar energy, and waste heat into electricity, which has been systematically investigated in this work, using DFT-based approaches. Moreover, stable ultrathin structures showing strong capabilities for all kinds of energy conversion are scarce. The emergence of a very high out-of-plane piezoelectricity, |d33| ≈ 56 pm/V, induced by the inversion symmetry broken in the buckled structure helps to supersede the previously reported bulk wurzite GaN, AlN, and Janus multilayer structures of Mo- and W-based dichalcogenides. The piezoelectric coefficients have been found to be largely dependent on the relative stacking between the two layers. CdS bilayer is a direct band gap semiconductor, with its band edges straddling the water redox potential, thereby making it thermodynamically favorable for photocatalytic applications. Strain engineering facilitates its transition from type I to type II semiconductor in CdS bilayer stacked over monolayer boron phosphide, and the theoretically calculated power conversion efficiency (PCE) in the 2D excitonic solar cell exceeds 27% for a fill factor of 0.8, which is much higher than that in ZnO/CdS/CuInGaSe solar cell (20% efficiency). Thermoelectric properties have been investigated using semi classical Boltzmann transport equations for electrons and phonons within the constant relaxation time approximation coupled to deformation potential theory, which reveal ultralow thermal conductivity (κl ≈ 0.78 W m-1 K-1) at room temperature because of the presence of heavy element Cd, strong anharmonicity (high mode Gruneisen parameter at long wavelength, phonon lifetime <5 ps), low phonon group velocity (4 km/s), and low Debye temperature (260 K). Such a low thermal conductivity is lower than that of dumbbell silicene (2.86 W m-1 K-1), SnS2 (6.41 W m-1 K-1) and SnSe2 (3.82 W m-1 K-1), and SnP3 (4.97 W m-1 K-1). CdS bilayer shows a thermoelectric figure of merit (ZT) ≈ 0.8 for p-type and ∼0.7 for n-type doping at room temperature. Its ultrahigh carrier mobility (μe ≈ 2270 cm2 V-1 s-1) is higher than that of single-layer MoS2 and comparable to that in InSe. The versatile properties of CdS bilayer together with its all-round stability supported by ab initio molecular dynamics simulation, phonon dispersion, and satisfaction of Born-Huang stability criteria highlight its outstanding potential for applications in device fabrication and applications in next-generation nanoelectronics and energy harvesting.

Influence of atomic-scale defect on thermal conductivity of single-layer MoS2 sheet

Abstract Recently, single-layer MoS2 has increasingly promised a great potential in both thermoelectric and thermal management application due to its high Seebeck coefficient and intrinsic band gap. Herein, we investigate thermal conductivity of single-layer MoS2 sheet and the effect of atomic-scale lattice defects and temperature on thermal conductivity using non-equilibrium molecular dynamics simulations (NEMD). Simulations results indicate the thermal conductivity can be suppressed by lattice defects and the increase in the range of temperature from 100 K to 400 K. Moreover, the physical mechanism of the thermal conductivity reduction was analyzed based on the phonon group velocity, participation ratio and phonon spectral transmission coefficient. Interestingly, it is revealed that the reduction of thermal conductivity results from the decreased phonon group velocity induced by defects, and phonon localization around lattice defects. Furthermore, the contributions from various frequency range of phonons to the thermal conductivity of pristine and defective structure were quantified. Especially, it is found that the coupling strength of between low-frequency and high-frequency phonons gradually increases due to the introduction of atomic-scale defects. This study is beneficial for thermal management of nanodevices and optimizing the thermoelectric properties of MoS2 based materials.

Ag@MoS2 Core-Shell Heterostructure as SERS Platform to Reveal the Hydrogen Evolution Active Sites of Single-Layer MoS2.

Understanding the reaction mechanism for the catalytic process is essential to the rational design and synthesis of highly efficient catalysts. MoS2 has been reported to be an efficient catalyst toward the electrochemical hydrogen evolution reaction (HER), but it still lacks direct experimental evidence to reveal the mechanism for MoS2-catalyzed electrochemical HER process at the atomic level. In this work, we develop a wet-chemical synthetic method to prepare the single-layer MoS2-coated polyhedral Ag core-shell heterostructure (Ag@MoS2) with tunable sizes as efficient catalysts for the electrochemical HER. The Ag@MoS2 core-shell heterostructures are used as ideal platforms for the real-time surface-enhanced Raman spectroscopy (SERS) study owing to the strong electromagnetic field generated in the plasmonic Ag core. The in situ SERS results provide solid Raman spectroscopic evidence proving the S-H bonding formation on the MoS2 surface during the HER process, suggesting that the S atom of MoS2 is the catalytic active site for the electrochemical HER. It paves the way on the design and synthesis of heterostructures for exploring their catalytic mechanism at atomic level based on the in situ SERS measurement.

Formation of Defects in Two-Dimensional MoS2 in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations.

Production of defects under electron irradiation in a transmission electron microscope (TEM) due to inelastic effects has been reported for various materials, but the microscopic mechanism of damage development in periodic solids through this channel is not fully understood. We employ non-adiabatic Ehrenfest, along with constrained density functional theory molecular dynamics, and simulate defect production in two-dimensional MoS2 under electron beam. We show that when excitations are present in the electronic system, formation of vacancies through ballistic energy transfer is possible at electron energies which are much lower than the knock-on threshold for the ground state. We further carry out TEM experiments on single layers of MoS2 at electron voltages in the range of 20-80 kV and demonstrate that indeed there is an additional channel for defect production. The mechanism involving a combination of the knock-on damage and electronic excitations we propose is relevant to other bulk and nanostructured semiconducting materials.

Substitutional transition metal doping in MoS2: a first-principles study

Single-layer MoS2 is a direct-gap semiconductor whose band edges character is dominated by the d-orbitals of the Mo atoms. It follows that substitutional doping of the Mo atoms has a significant impact on the material’s electronic properties, namely the size of the band gap and the position of the Fermi level. Here, density functional theory is used along with the G0W0 method to examine the effects of substituting Mo with four different transition metal dopants: Nb, Tc, Ta, and Re. Nb and Ta possess one less valence electron than Mo does and are therefore p-type dopants, while Re and Tc are n-type dopants, having one more valence electron than Mo has. Four types of substitutional structures are considered for each dopant species: isolated atoms, lines, three-atom clusters centered on a S atom (c3s), and three-atom clusters centered on a hole (c3h). The c3h structure is found to be the most stable configuration for all dopant species. However, electronic structure calculations reveal that isolated dopants are preferable for efficient n- or p-type performance. Lastly, it is shown that photoluminescence measurements can provide valuable insight into the atomic structure of the doped material. Understanding these properties of substitutionally-doped MoS2 can allow for its successful implementation into cutting-edge solid state devices.

Interplay of charge transfer and disorder in optoelectronic response in Graphene/hBN/MoS2 van der Waals heterostructures

Strong optoelectronic response in the binary van der Waals heterostructures of graphene and transition metal dichalcogenides (TMDCs) is an emerging route towards high-sensitivity light sensing. While the high sensitivity is an effect of photogating of graphene due to inter-layer transfer of photo-excited carriers, the impact of intrinisic defects, such as traps and mid-gap states in the chalcogen layer remain largely unexplored. Here we employ graphene/hBN (hexagonal boron nitride)/MoS2 (molybdenum disulphide) trilayer heterostructures to explore the photogating mechanism, where the hBN layer acts as interfacial barrier to tune the charge transfer timescale. We find two new features in the photoresponse: First, an unexpected positive component in photoconductance upon illumination at short times that preceeds the conventional negative photoconductance due to charge transfer, and second, a strong negative photoresponse at infrared wavelengths (up to 1720 nm) well-below the band gap of single layer MoS2. Detailed time and gate voltage-dependence of the photoconductance indicates optically-driven charging of trap states as possible origin of these observations. The responsivity of the trilayer structure in the infrared regime was found to be extremely large (> 108 A/W at 1550 nm using 20 mV source drain bias at 180 K temperature and ≈ − 30 V back gate voltage). Our experiment demonstrates that interface engineering in the optically sensitive van der Waals heterostructures may cast crucial insight onto both inter- and intra-layer charge reorganization processes in graphene/TMDC heterostructures.

Piezotronic spin and valley transistors based on monolayer MoS2

Piezotronics and piezo-phototronics based on the third generation semiconductors are two novel fields for low power consumption, self-powered technology and internet of things. Strain-induced piezoelectric field plays a key role to modulate not only charge-carrier transport but also quantum transport properties in piezoelectric semiconductors, especially for two-dimensional materials which can withstand large strain. In this paper, we theoretically study piezotronic effect on the modulation of spin and valley properties in single-layered MoS2. Spin- and valley-dependent conductance, electronic density distribution and polarization ratio are investigated by quantum transport calculation. Because of piezotronic effect, strain-gated spin and valley transistors have excellent quantum state selectivity using by strain. Our work provides not only piezotronic effect on spin and valley quantum states, but also a guidance for designing novel quantum piezotronic devices.

Preparation of high-performance natural rubber/carbon black/molybdenum disulfide composite by using the premixture of epoxidized natural rubber and cysteine-modified molybdenum disulfide

Molybdenum disulfide (MoS2) is a potential additive for rubber enhancement. However, its poor dispersion and weak interaction with matrix limit its further application. To improve the dispersion of MoS2 in natural rubber (NR) and enhance the interaction of MoS2 with matrix, single-layered MoS2 nanosheets are prepared and further modified by cysteine. The modified MoS2 nanosheets and epoxidized natural rubber (ENR) are premixed and then added into a NR/carbon black compound to obtain a NR/carbon black/ENR-modified MoS2 nanosheets composite. The reaction between modified MoS2 nanosheets and ENR is proved by FT-IR spectra. The modified MoS2 nanosheets can improve the dispersion of carbon black in NR, and ENR can significantly improve the dispersion of modified MoS2 nanosheets in the composite. The addition of ENR-modified MoS2 nanosheets premixture can improve the abrasion resistance, increase the crosslink density and control the dynamic mechanical properties of NR. These improved properties can be attributed to the good filler dispersion and enhanced filler–matrix interaction in NR matrix.

Electrochemical fabrication of reduced MoS2-based portable molecular imprinting nanoprobe for selective SERS determination of theophylline.

A new portable molecular imprinting polymer (MIP)-SERS nanoprobe is fabricated by a convenient electrochemical method. Single-layered MoS2 is electrochemically reduced on ascreen-printed electrode as the scaffold. Functional monomers o-phenylenediamine (oPD), template theophylline (THP), and SERS-active Au nanoparticles (AuNPs) are then one-step electropolymerized on the scaffold. The morphology of the nanoprobe is found to be a three-dimensional and porous structure. The abundant AuNPs with the size of 45~50nm are trapped within the growing MIP instead of being confined to the surface. The thickness of MIP film is calculated to 25.1nm. The nanoprobe displays a strong SERS effect for THP using 532nm as excitation wavelength with adetection limit (LOD) of 0.01nM. The SERS peak intensity at 1487cm-1 increases linearly with the concentration of THP in the range0.1 nM to 0.1mM. After the template is removed, the imprint-removed nanoprobe is generated for selective binding of THP. The re-binding kinetics study implies the portable MIP-SERS nanoprobe can reach the adsorption equilibrium within 8min. This nanoprobe exhibits low SERS interference for structural analogues theobromine (THB) and caffeine (CAF). The nanoprobe was employed to THP determination in tea drink samples, with recoveries ranging from 99.0 to 102.0% and relative standard deviations of < 5.0%. Graphical abstractSchematic representation of a portable molecular imprinting SERS nanoprobe used for selective and sensitive theophylline recognition. The nanoprobe is fabricated by one-step electropolymerized o-phenylenediamine (oPD), theophylline, and electroreduced Au nanoparticles (AuNPs) on reduced MoS2 (rMoS2) modified screen-printed electrode (SPE).

Surface Plasmon Resonance-Enhanced Near-Infrared Absorption in Single-Layer MoS2 with Vertically Aligned Nanoflakes.

The development of MoS2 with two- or three-dimensional heterostructures can provide a significant breakthrough for the enhancement of photodetection abilities such as increase in light absorption and expanding the detection ranges. Till date, although the synthesis of a MoS2 layer with three-dimensional nanostructures using a chemical vapor deposition (CVD) process has been successfully demonstrated, most studies have concentrated on electrochemical applications that utilize structural strengths, for example, a large specific surface area and electrochemically active sites. Here, for the first time, we report spectral light absorption induced by plasmon resonances in single-layer MoS2 (SL-MoS2) with vertically aligned nanoflakes grown by a CVD process. Treatment with oxygen plasma results in the formation of a substoichiometric phase of MoOx in the vertical nanoflakes, which exhibit a high electron density of 4.5 × 1013 cm-2. The substoichiometric MoOx with a high electron-doping level that is locally present on the SL-MoS2 surface induces an absorption band in the near-infrared (NIR) wavelength range of 1000-1750 nm because of the plasmon resonances. Finally, we demonstrate the enhancement of photodetection ability by broadening the detection range from the visible region to the NIR region in oxygen-treated SL-MoS2 with vertically aligned nanoflakes.

Contact Engineering of Layered MoS2 via Chemically Dipping Treatments

AbstractThe performance of electronic/optoelectronic devices is governed by carrier injection through metal–semiconductor contact; therefore, it is crucial to employ low‐resistance source/drain contacts. However, unintentional introduction of extrinsic defects, such as substoichiometric oxidation states at the metal–semiconductor interface, can degrade carrier injection. In this report, controlling the unintentional extrinsic defect states in layered MoS2 is demonstrated using a two‐step chemical treatment, (NH4)2S(aq) treatment and vacuum annealing, to enhance the contact behavior of metal/MoS2 interfaces. The two‐step treatment induces changes in the contact of single layer MoS2 field effect transistors from nonlinear Schottky to Ohmic behavior, along with a reduction of contact resistance from 35.2 to 5.2 kΩ. Moreover, the enhancement of ION and electron field effect mobility of single layer MoS2 field effect transistors is nearly double for n‐branch operation. This enhanced contact behavior resulting from the two‐step treatment is likely due to the removal of oxidation defects, which can be unintentionally introduced during synthesis or fabrication processes. The removal of oxygen defects is confirmed by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. This two‐step (NH4)2S(aq) chemical functionalization process provides a facile pathway to controlling the defect states in transition metal dichalcogenides (TMDs), to enhance the metal‐contact behavior of TMDs.

Molecular Nanowire Bonding to Epitaxial Single-Layer MoS2 by an On-Surface Ullmann Coupling Reaction.

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self-assembled organic nanowire-TMDC system, a method to link and interconnect epitaxial single-layer MoS2 flakes with organic molecules is demonstrated. Whereas pristine epitaxial single-layer MoS2 has no affinity for molecular attachment, it is found that single-layer MoS2 will selectively bind the organic molecule 2,8-dibromodibenzothiophene (DBDBT) in a surface-assisted Ullmann coupling reaction when the MoS2 has been activated by pre-exposing it to hydrogen. Atom-resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig-zag edges available in a hexagonal single layer MoS2 sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic-TMDC hybrid materials.

Speed dependence of friction on single-layer and bulk MoS2 measured by atomic force microscopy

We perform atomic force microscopy (AFM) experiments on mechanically exfoliated, single-layer and bulk molybdenum disulfide (MoS2) in order to probe friction forces as a function of sliding speed. The results of the experiments demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe, manifesting in the form of varying contact stiffness and tip-sample interaction potential parameters in the thermally activated Prandtl–Tomlinson model. Our study contributes to the formation of a mechanistic understanding of the speed dependence of nanoscale friction on two-dimensional materials.

Broken adiabaticity induced by Lifshitz transition in MoS2 and WS2 single layers

The breakdown of the adiabatic Born-Oppenheimer approximation is striking dynamical phenomenon, however, it occurs only in a handful of layered materials. Here, I show that adiabaticity breaks down in doped single-layer transition metal dichalcogenides in a quite intriguing manner. Namely, significant nonadiabatic coupling, which acts on frequencies of the Raman-active modes, is prompted by a Lifshitz transition due to depopulation and population of multiple valence and conduction valleys, respectively. The outset of the latter event is shown to be dictated by the interplay of highly non-local electron-electron interaction and spin-orbit coupling. In addition, intense electron-hole pair scatterings due to electron-phonon coupling are inducing phonon linewidth modifications as a function of doping. Comprehending these intricate dynamical effects turns out to be a key for mastering characterization of electron doping in two-dimensional nano-devices by means of Raman spectroscopy.

In situ bioimaging of Lactobacillus by photoluminescence of MoS2

Due to unique properties arising from its 2D nature, molybdenum disulfide (MoS2) has been studied widely toward its application for biosensing. While MoS2 field effect transistor has been utilized for electrical detection of biological events in many studies, photoluminescence (PL) characteristics of MoS2 have been poorly employed. MoS2 PL can provide not only information of interactions between biological moieties and the surface, but also their spatial distribution. In this work, we utilized PL of single-layer MoS2 as a label-free bioimaging sensor. To fabricate the imaging device, we synthesized MoS2 by chemical vapor deposition and transferred it on a glass substrate with patterned electrodes. We employed Lactobacillus known as a probiotic microorganism to demonstrate the ability of bioimaging. We found that the MoS2 PL can be modulated by the concentration of lactic acid. Finally, we succeeded in monitoring PL image of MoS2 modulated by Lactobacillus, which can be correlated with the production of lactic acid by Lactobacillus.