MoTe2 Film Research Articles

Light-modulated van der Waals force microscopy

Atomic force microscope generally works by manipulating the absolute magnitude of the van der Waals force between tip and specimen. This force is, however, less sensitive to atom species than to tip-sample separations, making compositional identification difficult, even under multi-modal strategies or other atomic force microscopy variations. Here, we report the phenomenon of a light-modulated tip-sample van der Waals force whose magnitude is found to be material specific, which can be employed to discriminate heterogeneous compositions of materials. We thus establish a near-field microscopic method, named light-modulated van der Waals force microscopy. Experiments discriminating heterogeneous crystalline phases or compositions in typical materials demonstrate a high compositional resolving capability, represented by a 20 dB signal-to-noise ratio on a MoTe2 film under the excitation of a 633 nm laser of 1.2 mW, alongside a sub-10 nm lateral spatial resolution, smaller than the tip size of 20 nm. The simplicity of the light modulation mechanism, minute excitation light power, broadband excitation wavelength, and diversity of the applicable materials imply broad applications of this method on material characterization, particularly on two-dimensional materials that are promising candidates for next-generation chips.

Gamma-induced stress, strain and p-type doping in MBE-grown thin film MoTe2.

A thorough examination of the stability of a 2D MoTe2 thin film exposed to high-dose gamma radiation (γ) is addressed in this study. This study compares the film before and after irradiation (10-600 kGy dosage) to report the impact of γ radiation on the surface morphology, work function, tensile strain and charge redistribution in the MoTe2 thin film. The radiation damage to the film is monitored by optical micrographs (OM) and with atomic force microscopy (AFM) and conceptualized by thermal strain in the film. Raman spectroscopy has been used to analyze the shifting and to address the strain that arises due to irradiation in its vibrational mode. Kelvin probe force microscopy (KPFM) has been performed to evaluate the work function of the film, which increases by 0.14 eV for the 600 kGy γ-irradiated sample, implying shifting of the Fermi-level to the valence band of the spectrum and thus it results in p-type doping in the film. Owing to the reduced atomic mass and high energy of tellurium atoms, γ-irradiation causes tellurium vacancies, which lead to the formation of dangling bonds at unoccupied sites. When oxygen is adsorbed at these reactive spots, a charge-transfer mechanism takes place. This mechanism involves the transfer of electrons from the thin MoTe2 film to the adsorbed oxygen, forming oxides and causing p-type doping. Furthermore, p-doping is verified by the valence band shifting by 1.27 eV in 600 kGy in γ-irradiated samples in X-ray photoelectron spectroscopy. This comprehensive study shows how γ irradiation affects the chemical and physical characteristics of the MoTe2 thin film. Consequently, it shows that if devices integrating MoTe2 thin films are meant to be used in high-dose radiation conditions, the adsorbate concentrations, radiation shielding and required lifetimes must be carefully evaluated.

Optical Modulation of MoTe2/Ferroelectric Heterostructure via Interface Doping.

Optical modulation through interface doping offers a convenient and efficient way to control ferroelectric polarization, thereby advancing the utilization of ferroelectric heterostructures in nanoelectronic and optoelectronic devices. In this work, we fabricated heterostructures of MoTe2/BaTiO3/La0.7Sr0.3MnO3 (MoTe2/BTO/LSMO) and demonstrated opposite ultraviolet (UV) light-induced polarization switching behaviors depending on the varied thicknesses of MoTe2. The thickness-dependent band structure of MoTe2 film results in interface doping with opposite polarity in the respective heterostructures. The polarization field of BTO interacts with the interface charges, and an enhanced effective built-in field (Ebi) can trigger the transfer of massive UV light-induced carriers in both MoTe2 and BTO films. As a result, the interplay among the contact field of MoTe2/BTO, the polarization field, and the optically excited carriers determines the UV light-induced polarization switching behavior of the heterostructures. In addition, the electric transport characteristics of MoTe2/BTO/LSMO heterostructures reveal the interface barrier height and Ebi under opposite polarization states, as well as the presence of inherent in-gap trap states in MoTe2 and BTO films. These findings represent a further step toward achieving multifield modulation of the ferroelectric polarization and promote the potential applications in optoelectronic, logic, memory, and synaptic ferroelectric devices.

Molecular dynamics study on the uniaxial tensile behavior of mono-layer MoTe2 film defected by mirror twin boundary

High-density mirror twin boundaries, which exhibit fascinating electronic properties, are revealed in low-dimensional MoTe2 films in recent years. Mechanical behavior plays a key role in the application of MoTe2 with these novel defects. In this paper, a mono-layer MoTe2 crystal was constructed, and the 4|4P mirror twin boundary, which is the most easily observed in the experiment, was embedded in the center of model. The molecular dynamics method combined with Stillinger-Weber potential function was used to simulate the uniaxial tension, the effects of this defect on the fracture stress of monolayer MoTe2 were studied. Results show that the 4|4P mirror twin boundary has little effect on the Young's modulus of MoTe2, but reduces the fracture stress. Larger size of the mirror twin boundary, lower temperature and the armchair loading direction can make the decrease of fracture stress more significant, however the size effect is much weaker when the film is stretched along the zigzag direction. Turning the MoTe2 model with a specified large mirror twin boundary into ternary MoTe2(1-x)Se2x model by replacing some Te atoms with Se impurities randomly and uniformly, then setting the condition of temperature and Se concentration into 12 groups for each loading direction, the ratio of the fracture stress derived from the defected model to the value obtained from ideal model almost remains unchanged. Finally, this phenomena are analyzed based on brittle fracture theory.

Interface-enhanced superconductivity in monolayer 1T′-MoTe2 on SrTiO3(001)

Introducing superconductivity into two-dimensional (2D) films with nontrivial topology has been intensively pursued as one of the feasible scenarios to realize 1D topological superconductor. Prevailing endeavors mostly exploit the external gating or proximity effect of a traditional superconductor, by which the critical temperatures (T_{mathrm{c}}) are limited to several Kelvin range. Here, we report on the discovery of interface-enhanced superconductivity in monolayer 1T′-MoTe2 film. A thermally driven phase transition from Mo6Te6 nanowires to 1T′-MoTe2 films, grown on SrTiO3(001) surface by the molecular beam epitaxial methods, is demonstrated. A combined study of scanning tunneling microscopy/spectroscopy, electrical transport and magnetization measurements indicates the T_{mathrm{c}} of MoTe2 film is around 30 K, two orders of magnitude larger than its 3D counterpart crystal. This study shows that interfacial engineering is an efficient way to tune monolayer 1T′-MoTe2 film into superconducting states, and thus may pave the way toward higher-T_{mathrm{c}} 1D intrinsic topological superconductivity.

On the Laser Generation in Two-Dimensional Materials with Pumping by Quasitrapped Modes

A model has been proposed to describe the laser generation of two-dimensional semiconductor films with near-field pumping by quasitrapped modes excited in dielectric metasurfaces. A metastructure consisting of a Si metasurface coated with a MoTe2 film, where narrow-band resonance of a quasitrapped mode is joined with a broad exciton resonance of a two-dimensional material, has been designed. Threshold conditions for generation in the MoTe2 film with pumping by quasitrapped modes have been determined. The possibility of polarization control of the emission of the proposed metastructure has been demonstrated.

Ultrafast Modulation of THz Waves Based on MoTe2-Covered Metasurface.

The sixth generation (6G) communication will use the terahertz (THz) frequency band, which requires flexible regulation of THz waves. For the conventional metallic metasurface, its electromagnetic properties are hard to be changed once after being fabricated. To enrich the modulation of THz waves, we report an all-optically controlled reconfigurable electromagnetically induced transparency (EIT) effect in the hybrid metasurface integrated with a 10-nm thick MoTe2 film. The experimental results demonstrate that under the excitation of the 800 nm femtosecond laser pulse with pump fluence of 3200 μJ/cm2, the modulation depth of THz transmission amplitude at the EIT window can reach 77%. Moreover, a group delay variation up to 4.6 ps is observed to indicate an actively tunable slow light behavior. The suppression and recovery of the EIT resonance can be accomplished within sub-nanoseconds, enabling an ultrafast THz photo-switching and providing a promising candidate for the on-chip devices of the upcoming 6G communication.

High performance near-infrared MoTe2/Ge heterojunction photodetector fabricated by direct growth of Ge flake on MoTe2 film substrate

We demonstrated a feasible strategy to fabricate MoTe2/Ge heterojunction by direct growth of Ge flake on a MoTe2 film substrate with a two-step chemical vapor deposition method. A thin transition layer (∼4 nm) mainly composed of polycrystalline germanium at the MoTe2/Ge interface was verified during the Ge flake growth. The MoTe2/Ge heterojunction-based photodetector exhibits both the response speed with a rise/fall time of 7/4 μs and the photoresponsivity and detectivity with 4.87 A W−1 and 5.02 × 1011 Jones under zero bias in the near-infrared regime, respectively. The characteristics of device performance imply its practical applicability as building block for potential near-infrared integrated photonics.

High performance ammonia gas sensor based on electrospinned Co3O4 nanofibers decorated with hydrothermally synthesized MoTe2 nanoparticles

This paper introduces a room temperature NH3 gas sensor with excellent performance based on cobalt trioxide and molybdenum telluride (Co3O4-MoTe2) nanocomposite. Co3O4 nanofiber was synthesized by electrospinning, and MoTe2 nano particle was synthesized by simple hydrothermal method. MoTe2, Co3O4 and Co3O4-MoTe2 film sensors were prepared on interdigital electrode substrate by drop coating method. X-ray photoelectron spectroscopy (XPS), dispersive spectrometer (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to observe the morphology, microstructure, and composition of the prepared nano-materials. The gas sensing performance of the pristine Co3O4, MoTe2 and Co3O4-MoTe2 composite film sensors were tested at room temperature. The results of experimental research show that the Co3O4-MoTe2 gas sensor has better sensing performance for ammonia than the pure sensor, such as high response (56.20%@1ppm), excellent selectivity, good stability, short response/recovery time (7s/7s@1ppm) and low detection limit (26 ppb). The performance enhancement of the Co3O4-MoTe2 film sensor was mainly attributed to the p-n heterojunction formed between MoTe2 and Co3O4.

Structure and morphology of 2H-MoTe2 monolayer on GaAs(111)B grown by molecular-beam epitaxy

The structure and morphology of monolayer 2H-MoTe2 on GaAs(111)B grown by molecular-beam epitaxy have been studied using scanning tunneling microscopy, electron diffraction, and X-ray photoelectron spectroscopy. The MoTe2 film grown and annealed under the Te-rich condition is mainly composed of a pure 2H phase, while, under the Te-deficient conditions, the 2H-MoTe2 phase begins to evolve into nanowire-like structures owing to the desorption of Te. The 2H-MoTe2(0001) film on GaAs(111)B exhibits two types of energetically favorable epitaxial orientations; one is a perfect alignment of [11overline{2}0]MoTe2 // [1overline{1}0]GaAs, and the other shows a slight in-plane rotation of ± 0.77∘, which reduces the effective lattice mismatch between MoTe2 and GaAs.

Directly grown Te nanowire electrodes and soft plasma etching for high-performance MoTe2 field-effect transistors

Contact tailoring for two-dimensional (2D) transition metal dichalcogenides (TMDs) to achieve high-performance devices remains a challenge. Fermi-level pinning at 2D TMD–metal contacts leads to a Schottky barrier for contacts. In addition, when there are no dangling bonds between 2D TMDs and contacts, the contact resistance increases. In this study, Te nanowire (NW) contacts were employed for a MoTe2 p-channel of enhancement mode field-effect transistor (FET). The Te NWs were directly grown on 2D MoTe2 film by metal–organic chemical vapor deposition and selectively etched by a soft plasma etching technique for contact isolation. Using t-Te NW contacts on three-atomic-layer MoTe2, a highly effective field-effect mobility of 543.9 cm2/Vs, as well as ohmic contacts and atomic hybridization, was achieved. These results of t-Te NW electrodes provide a novel device structure for p-type 2D TMD transistors with excellent performances. Further, they offer a practical guideline for wafer-scale 2D TMD-based high-performance electronics and optoelectronics.

Atmospheric-pressure CVD growth of two-dimensional 2H- and 1 T′-MoTe2 films with high-performance SERS activity

Two-dimensional (2D) molybdenum ditelluride (MoTe2) is a member of the transition-metal dichalcogenides family, which is an especially promising platform for surface-enhanced Raman scattering (SERS) applications, due to its excellent electronic properties. However, the synthesis of large-area highly crystalline 2D MoTe2 with controllable polymorphism is a huge challenge due to the small free energy difference (∼40 meV per unit cell) between semiconducting 2H-MoTe2 and semi-metallic 1 T′-MoTe2. Herein, we report an optimized route for the synthesis of 2H- and 1 T′-MoTe2 films by atmospheric-pressure chemical vapor deposition. The SERS study of the as-grown MoTe2 films was carried out using methylene blue (MB) as a probe molecule. The Raman enhancement factor on 1 T′-MoTe2 was found to be three times higher than that on 2H-MoTe2 and the 1 T′-MoTe2 film is an efficient Raman-enhancing substrate that can be used to detect MB at nanomolar concentrations. Our study also imparts knowledge on the significance of a suitable combination of laser excitation wavelength and molecule-material platform for achieving ultrasensitive SERS-based chemical detection.

Accurate Analysis of Schottky Barrier Height in Au/2H–MoTe2 Atomically Thin Film Contact

Contact between two-dimensional (2D) transition metal dichalcogenides and certain metals (Au, Pd, Ti, or Pt) results in strong Fermi-level pinning and high resistance. Furthermore, to construct a high-performance device, accurate measurement of the contact resistance and Schottky barrier height (SBH) is crucial. In this study, the SBH and the resistance of a 2H phase few-layer molybdenum telluride (2H–MoTe2) film, which was grown by metal organic chemical vapor deposition, and a Ti/Au electrode were determined by analyzing the high-temperature thermal transmission properties and the transmission line method, respectively. The carrier mobility, ON/OFF ratio, SBH, and contact resistance of the few-layer MoTe2 device, which has p-type behavior with Ti/Au electrode, were found to be 53.7 cm2 V−1 s−1, 3.44 × 106, 163 meV, and 10.2 $${\text{M}}\Omega$$ , respectively.

Escalated Photocurrent with Excitation Energy in Dual-Gated MoTe2.

Although van der Waals-layered transition metal dichalcogenides from transient absorption spectroscopy have successfully demonstrated an ideal carrier multiplication (CM) performance with an onset of nearly 2Eg, interpretation of the CM effect from the optical approach remains unresolved owing to the complexity of many-body electron-hole pairs. We demonstrate the escalated photocurrent with excitation photon energy by fabricating the dual-gate p-n junction of a MoTe2 film on a transparent substrate. Electrons and holes were efficiently extracted by eliminating the Schottky barriers in the metal contact and minimizing multiple reflections. The photocurrent was elevated proportionately to the excitation photon energy. The boosted quantum efficiency confirms the multiple electron-hole pair generation of >2Eg, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.

Application of a 2D Molybdenum Telluride in SERS Detection of Biorelevant Molecules.

Two-dimensional (2D) transition-metal dichalcogenides have become promising candidates for surface-enhanced Raman spectroscopy (SERS), but currently very few examples of detection of relevant molecules are available. Herein, we show the detection of the lipophilic disease marker β-sitosterol on few-layered MoTe2 films. The chemical vapor deposition (CVD)-grown films are capable of nanomolar detection, exceeding the performance of alternative noble-metal surfaces. We confirm that the enhancement occurs through the chemical enhancement (CE) mechanism via formation of a surface-analyte complex, which leads to an enhancement factor of ≈104, as confirmed by Fourier transform infrared (FTIR), UV-vis, and cyclic voltammetry (CV) analyses and density functional theory (DFT) calculations. Low values of signal deviation over a seven-layered MoTe2 film confirms the homogeneity and reproducibility of the results in comparison to noble-metal substrate analogues. Furthermore, β-sitosterol detection within cell culture media, a minimal loss of signal over 50 days, and the opportunity for sensor regeneration suggest that MoTe2 can become a promising new SERS platform for biosensing.

Switchable and dual-wavelength ultrafast fibre lasers with an MoTe2-based saturable absorber

ABSTRACTBased on monolayer MoTe2 film, we have proposed a mode-locked fibre laser delivering single-wavelength and dual-wavelength solitons at the telecommunication band. The saturable absorber (SA) is synthesized by coating monolayer MoTe2 film on the pinhole of fibre pigtail. The experimental results show that our soliton lasers can work at the single- and dual-wavelength states, respectively, by appropriately adjusting the polarization controller. The proposed laser can work stably at the mode-locking state for several hours, indicating that monolayer MoTe2 film is an excellent SA material for ultrafast optics.

Electrosynthesis of MoTe2 Thin Films: A Combined Voltammetry-Electrochemical Quartz Crystal Microgravimetry Study of Mechanistic Aspects

Molybdenum telluride (MoTe2) belongs to the family of layered transition metal dichalcogenides (TMDs) with unique optical, optoelectronic, structural properties and potential applications in a wide array of technologies related to solar energy conversion, optoelectronics, lubrication, and hydrogen production. Here, electrodeposition is shown to be a facile method for the synthesis of MoTe2 in bulk (i.e., not exfoliated) form. The electrosynthesis of MoTe2 films and the underlying compound formation mechanism were investigated for the first time using linear sweep voltammetry (LSV) combined with electrochemical quartz crystal microgravimetry (EQCM). A Te-modified electrode in an electrolyte containing molybdenum precursor species, a MoOx-modified electrode in tellurium precursor-containing electrolyte and a variety of control experiments were employed to elucidate the electrodeposition mechanism of MoTe2 films. Electrogeneration of HTe− was the key step in MoTe2 film formation which occurred by the reaction of electrodeposited MoOx with HTe− generated by electroreduction of Te or HTeO2+. Thermodynamic aspects of this reaction are finally presented.

Ultrahigh Speed and Broadband Few‐Layer MoTe2/Si 2D–3D Heterojunction‐Based Photodiodes Fabricated by Pulsed Laser Deposition

Abstract2D transition metal dichalcogenides are promising candidates for high‐performance photodetectors. However, the relatively low response speed as well as the complex transfer process hinders their wide applications. Herein, for the first time, the fabrication of a few‐layer MoTe2/Si 2D–3D vertical heterojunction for high‐speed and broadband photodiodes by a pulsed laser deposition technique is reported. Owing to the high junction quality, ultrathin MoTe2 film thickness, and unique vertical n–n heterojunction structure, the photodiode exhibits excellent device performance in terms of a high responsivity of 0.19 A W−1 and a large detectivity of 6.8 × 1013 Jones. The device is also capable of detecting a broadband light with wavelength spanning from 300 to 1800 nm. More importantly, the device possesses an ultrahigh response speed up to 150 ns with a 3‐dB electrical bandwidth approaching 0.12 GHz. This work paves the way toward the fabrication of novel 2D–3D heterojunctions for high‐performance, ultrafast photodetectors.

An Investigation in Phase Transition of MoTe2 Film with Continuous Tellurization Reaction

Molybdenum ditelluride (MoTe2), as a member of two dimentional transition metal dichalcogenides (2D TMDCs), has been drawing scientists’ attention due to its susceptible phase transition. Here, we studied the phase transition process of MoTe2 with tellurization reaction step by step. In the process of tellurization reaction, the 1T’ MoTe2 would firstly convert to an intermediate phase (1T’@ 2H MoTe2) and then slowly convert to 2H MoTe2 instead of forming a direct phase transition from 1T’ MoTe2 to 2H MoTe2. This result might inspire the phase engineering of other 2D TMDCs and the exploration of potential device design.

2D MoTe2 film as a saturable absorber for a wavelength-tunable ultrafast fiber laser.

We have demonstrated the realization of wavelength-tunable pulse outputs in a mode-locked fiber laser by utilizing 2D ${{\rm MoTe}_2}$MoTe2 film as the saturable absorber (SA). The SA is synthesized by coating 2D ${{\rm MoTe}_2}$MoTe2 film on the pinhole of the fiber pigtail and it can work stably at the mode-locking state for several weeks, indicating that the 2D ${{\rm MoTe}_2}$MoTe2 film is a suitable SA for ultrafast optics. The ${{\rm MoTe}_2}$MoTe2 film mode-locked fiber laser can operate well at a wide spectral band, and the central wavelength of pulses is tunable in the range from 1530 to 1560 nm, which is attributed to the polarization-sensitive feature of the proposed SA. Our new SA will benefit high-power pulsed lasers, materials processing, and frequency comb spectroscopy.