Two-dimensional molybdenum ditelluride waveguide-integrated near-infrared photodetector

Low-cost, small-sized, and easy integrated high-performance photodetectors for photonics are still the bottleneck of photonic integrated circuits applications and have attracted increasing attention. The tunable narrow bandgap of two-dimensional (2D) layered molybdenum ditelluride (MoTe2) from ∼0.83 to ∼1.1 eV makes it one of the ideal candidates for near-infrared (NIR) photodetectors. Herein, we demonstrate an excellent waveguide-integrated NIR photodetector by transferring mechanically exfoliated 2D MoTe2onto a silicon nitride (Si3N4) waveguide. The photoconductive photodetector exhibits excellent responsivity (R), detectivity (D*), and external quantum efficiency at 1550 nm and 50 mV, which are 41.9 A W-1, 16.2 × 1010Jones, and 3360%, respectively. These optoelectronic performances are 10.2 times higher than those of the free-space device, revealing that the photoresponse of photodetectors can be enhanced due to the presence of waveguide. Moreover, the photodetector also exhibits competitive performances over a broad wavelength range from 800 to 1000 nm with a highRof 15.4 A W-1and a largeD* of 59.6 × 109Jones. Overall, these results provide an alternative and prospective strategy for high-performance on-chip broadband NIR photodetectors.

Directionally-Resolved Phononic Properties of Monolayer 2D Molybdenum Ditelluride (MoTe2) under Uniaxial Elastic Strain

Understanding the phonon characteristics of two-dimensional (2D) molybdenum ditelluride (MoTe2) under strain is critical to manipulating its multiphysical properties. Although there have been numerous computational efforts to elucidate the strain-coupled phonon properties of monolayer MoTe2, empirical validation is still lacking. In this work, monolayer 1H-MoTe2 under uniaxial strain is studied via in situ micro-Raman spectroscopy. Directionally dependent monotonic softening of the doubly degenerate in-plane E2g1 phonon mode is observed with increasing uniaxial strain, where the E2g1 peak red-shifts -1.66 ± 0.04 cm-1/% along the armchair direction and -0.80 ± 0.07 cm-1/% along the zigzag direction. The corresponding Grüneisen parameters are calculated to be 1.09 and 0.52 along the armchair and zigzag directions, respectively. This work provides the first empirical quantification and validation of the orientation-dependent strain-coupled phonon response in monolayer 1H-MoTe2 and serves as a benchmark for other prototypical 2D transition-metal tellurides.

Self-Assembled Monolayer Doping for MoTe2 Field-Effect Transistors: Overcoming PN Doping Challenges in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have gained significant attention as next-generation semiconductor materials that could potentially overcome the integration limits of silicon-based electronic devices. However, a challenge in utilizing TMDs as semiconductors is the lack of an established PN doping method to effectively control their electrical properties, unlike those of silicon-based semiconductors. Conventional PN doping methods, such as ion implantation, can induce lattice damage in TMDs. Thus, chemical doping methods that can control the Schottky barrier while minimizing lattice damage are desirable. Here, we focus on the molybdenum ditelluride (2H-MoTe2), which has a hexagonal phase and exhibits ambipolar field-effect transistor (FET) properties due to its direct band gap of 1.1 eV, enabling concurrent transport of electrons and holes. We demonstrate the fabrication of p- or n-type unipolar FETs in ambipolar MoTe2 FETs using self-assembled monolayers (SAMs) as chemical dopants. Specifically, we employ 1H,1H,2H,2H perfluorooctyltriethoxysilane and (3-aminopropyl)triethoxysilane as SAMs for chemical doping. The selective SAMs effectively increase the hole and electron charge transport capabilities in MoTe2 FETs by 18.4- and 4.6-fold, respectively, due to the dipole effect of the SAMs. Furthermore, the Raman shift of MoTe2 by SAM coating confirms the successful p- and n-type doping. Finally, we demonstrate the fabrication of complementary inverters using SAMs-doped MoTe2 FETs, which exhibit clear full-swing capability compared to undoped complementary inverters.

Promoted Electronic Coupling of Acoustic Phonon Modes in Doped Semimetallic MoTe2

As a prototype of the Weyl superconductor, layered molybdenum ditelluride (MoTe2) encompasses two semimetallic phases (1T' and Td) which differentiate from each other via a slight tilting of the out-of-plane lattice. Both phases are subjected to serious phase mixing, which complicates the analysis of its origin of superconductivity. Herein, we explore the electron-phonon coupling (EPC) of the monolayer semimetallic MoTe2, without phase ambiguity under this thickness limit. Apart from the hardening or softening of the phonon modes, the strength of the EPC can be strongly modulated by doping. Specifically, longitudinal and out-of-plane acoustic modes are significantly activated for electron doped MoTe2. This is ascribed to the presence of rich valley states and equispaced nesting bands, which are dynamically populated under charge doping. Through comparing the monolayer and bilayer MoTe2, the strength of EPC is found to be less likely to depend on thickness for neutral samples but clearly promoted for thinner samples with electron doping, while for hole doping, the strength alters more significantly with the thickness than doping. Our work explains the issue of the doping sensitivity of the superconductivity in semimetallic MoTe2 and establishes the critical role of activating acoustic phonons in such low-dimensional materials.

Rydberg Excitons and Trions in Monolayer MoTe2

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS₂, MoSe₂, WS₂, and WSe₂), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe₂). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe₂ to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton-phonon coupling. Furthermore, we observe a strongly gate-tunable exciton-trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe-Salpeter equation approach calculations. Our results help bring monolayer MoTe₂ closer to its potential applications in near-infrared optoelectronics and photonic devices.

Laser-Induced Phase Transition and Patterning of hBN-Encapsulated MoTe2

Transition metal dichalcogenides exhibit phase transitions through atomic migration when triggered by various stimuli, such as strain, doping, and annealing. However, since atomically thin 2D materials are easily damaged and evaporated from these strategies, studies on the crystal structure and composition of transformed 2D phases are limited. Here, the phase and composition change behavior of laser-irradiated molybdenum ditelluride (MoTe₂ ) in various stacked geometry are investigated, and the stable laser-induced phase patterning in hexagonal boron nitride (hBN)-encapsulated MoTe₂ is demonstrated. When air-exposed or single-side passivated 2H-MoTe₂ are irradiated by a laser, MoTe₂ is transformed into Te or Mo₃ Te₄ due to the highly accumulated heat and atomic evaporation. Conversely, hBN-encapsulated 2H-MoTe₂ transformed into a 1T' phase without evaporation or structural degradation, enabling stable phase transitions in desired regions. The laser-induced phase transition shows layer number dependence; thinner MoTe₂ has a higher phase transition temperature. From the stable phase patterning method, the low contact resistivity of 1.13 kΩ µm in 2H-MoTe₂ field-effect transistors with 1T' contacts from the seamless heterophase junction geometry is achieved. This study paves an effective way to fabricate monolithic 2D electronic devices with laterally stitched phases and provides insights into phase and compositional changes in 2D materials.

Strain-Induced Indirect-to-Direct Bandgap Transition, Photoluminescence Enhancement, and Linewidth Reduction in Bilayer MoTe2

Two-dimensional (2D) layered materials provide an ideal platform for engineering electronic and optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain on various spectral features of bilayer MoTe₂ photoluminescence (PL). We found that bilayer MoTe₂ can be converted from an indirect to a direct bandgap material through strain engineering, resulting in a photoluminescence enhancement by a factor of 2.24. Over 90% of the PL comes from photons emitted by the direct excitons at the maximum strain applied. Importantly, we show that strain effects lead to a reduction of the overall linewidth of PL by as much as 36.6%. We attribute the dramatic decrease of linewidth to a strain-induced complex interplay among various excitonic varieties such as direct bright excitons, trions, and indirect excitons. Our experimental results on direct and indirect exciton emission features are explained by theoretical exciton energies that are based on first-principles electronic band structure calculations. The consistent theory-experimental trend shows that the enhancement of PL and the reduction of linewidth are the consequences of the increasing direct exciton contribution with the increase of strain. Our results demonstrate that strain engineering can lead to a PL quality of the bilayer MoTe₂ comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe₂ more suitable for silicon-photonics integration due to the reduced silicon absorption.

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe₂) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-T_d interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe₂ with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the T_d-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-T_d-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

P-P Orbital Interaction Enables Single-Crystalline Semimetallic β-MoTe2 Nanosheets as Efficient Electrocatalysts for Lithium-Sulfur Batteries

The practical implementation of lithium-sulfur batteries (LSBs) has been impeded by the sluggish redox kinetics of lithium polysulfides (LiPSs) and shuttle effect of soluble LiPSs during charge/discharge. It is desirable to exploit materials combining superior electrical conductivity with excellent catalytic activity for use as electrocatalysts in LSBs. Herein, we report the employment of chemical vapor transport (CVT) method followed by an electrochemical intercalation process to fabricate high-quality single-crystalline semimetallic β-MoTe₂ nanosheets, which are utilized to manipulate the LiPSs conversion kinetics. The first-principles calculations prove that β-MoTe₂ could lower the Gibbs free-energy barrier for Li₂S₂ transformation to Li₂S. The wavefunction analysis demonstrates that the p-p orbital interaction between Te p and S p orbitals accounts for the strong electronic interaction between the β-MoTe₂ surface and Li₂S₂/Li₂S, making bonding and electron transfer more efficient. As a result, a β-MoTe₂/CNT@S-based LSB cell can deliver an excellent cycling performance with a low capacity fade rate of 0.11% per cycle over 300 cycles at 1C. Our work might not only provide a universal route to prepare high-quality single-crystalline transition-metal dichalcogenides (TMDs) nanosheets for use as electrocatalysts in LSBs, but also suggest a different viewpoint for the rational design of LiPSs conversion electrocatalysts.

Atomic Layer MoTe2 Field-Effect Transistors and Monolithic Logic Circuits Configured by Scanning Laser Annealing

Atomically thin semiconductors such as transition metal dichalcogenides have recently enabled diverse devices in the emerging two-dimensional (2D) electronics. While scalable 2D electronics demand monolithic integrated circuits consisting of complementary p-type and n-type transistors, conventional p-type and n-type doping in desired regions, monolithically in the same semiconducting atomic layers, remains elusive or impractical. Here, we report on an agile, high-precision scanning laser annealing approach to realizing 2D monolithic complementary logic circuits on atomically thin MoTe₂, by reliably designating p-type and n-type transport polarity in the constituent transistors via localized laser annealing and modification of their Schottky contacts. Pristine p-type field-effect transistors (FETs) transform into n-type ones upon controlled laser annealing on their source/drain gold electrodes, exhibiting a mobility of 96.5 cm² V^-1 s^-1 (the highest known to date) and an On/Off ratio of 10⁶. Elucidation and validation of such an on-demand configuration of polarity in MoTe₂ FETs further enable the construction and demonstration of essential logic circuits, including both inverter and NOR gates. This dopant-free, spatially precise scanning laser annealing approach to configuring monolithic complementary logic integrated circuits may enable programmable functions in 2D semiconductors, exhibiting potential for additively manufactured, scalable 2D electronics.

Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe₂, WSe₂, and MoS₂ channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

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 2E_g, 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 >2E_g, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.

Structural Phase Transition and Interlayer Coupling in Few-Layer 1T' and Td MoTe2

We performed polarized Raman spectroscopy on mechanically exfoliated few-layer MoTe₂ samples and observed both 1T' and T_d phases at room temperature. Few-layer 1T' and T_d MoTe₂ exhibited a significant difference especially in interlayer vibration modes, from which the interlayer coupling strengths were extracted using the linear chain model: strong in-plane anisotropy was observed in both phases. Furthermore, temperature-dependent Raman measurements revealed a peculiar phase transition behavior in few-layer 1T' MoTe₂. In contrast to bulk 1T' MoTe₂ crystals, where the phase transition to the T_d phase occurs at ∼250 K, the temperature-driven phase transition to the T_d phase is increasingly suppressed as the thickness is reduced, and the transition and the critical temperature varied dramatically from sample to sample even for the same thickness. Raman spectra of intermediate phases that correspond to neither 1T' nor T_d phase with different interlayer vibration modes were observed, which suggests that several metastable phases exist with similar total energies.

Nanoscale Friction Behavior of Transition-Metal Dichalcogenides: Role of the Chalcogenide

Despite extensive research on the tribological properties of MoS₂, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS₂, MoSe₂, and MoTe₂ in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS₂ has the highest friction among these materials and MoTe₂ has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.

Homogeneous 2D MoTe2 CMOS Inverters and p-n Junctions Formed by Laser-Irradiation-Induced p-Type Doping

Among all typical transition-metal dichalcogenides (TMDs), the bandgap of α-MoTe₂ is smallest and is close to that of conventional 3D Si. The properties of α-MoTe₂ make it a favorable candidate for future electronic devices. Even though there are a few reports regarding fabrication of complementary metal-oxide-semiconductor (CMOS) inverters or p-n junction by controlling the charge-carrier polarity of TMDs, the fabrication process is complicated. Here, a straightforward selective doping technique is demonstrated to fabricate a 2D p-n junction diode and CMOS inverter on a single α-MoTe₂ nanoflake. The n-doped channel of a single α-MoTe₂ nanoflake is selectively converted to a p-doped region via laser-irradiation-induced MoO_x doping. The homogeneous 2D MoTe₂ CMOS inverter has a high DC voltage gain of 28, desirable noise margin (NM_H = 0.52 V_DD , NM_L = 0.40 V_DD ), and an AC gain of 4 at 10 kHz. The results show that the doping technique by laser scan can be potentially used for future larger-scale MoTe₂ CMOS circuits.

Self-Terminated Surface Monolayer Oxidation Induced Robust Degenerate Doping in MoTe2 for Low Contact Resistance

We introduce an effective method to degenerately dope MoTe₂ by oxidizing its surface into the p-dopant MoO_x in oxygen plasma. As a self-terminated process, the oxidation is restricted only in the very top layer, therefore offering us an easy and efficient control. The degenerate p-doping with the hole concentration of 2.5 × 10¹³ cm^-2 can be obtained by applying a ∼300 s O₂ plasma treatment. Using the degenerately doped MoTe₂, we demonstrate a record low contact resistance of 0.6 kΩ μm for MoTe₂. Our measurement highlights an excellent stability for the plasma-doped MoTe₂. The doped characteristics are robust with no significant degradation even after a one-year exposure to the air. The oxygen plasma doping technique is compatible with the conventional semiconductor processes, which can be utilized to realize high-performance MoTe₂ field-effect transistors (FETs) or tunnel FETs in the future.

Doping of MoTe2 via Surface Charge Transfer in Air

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

Automated Mechanical Exfoliation of MoS2 and MoTe2 Layers for 2D Materials Applications

An automated technique is presented for mechanically exfoliating single-layer and few-layer transition metal dichalcogenides using controlled shear and normal forces imposed by a parallel plate rheometer. A thin sample that is removed from bulk MoS₂ or MoTe₂ is initially attached to the movable upper fixture of the rheometer using blue dicing tape while the lower base plate also has the same tape to capture and exfoliate samples when the two plates are brought into contact then separated. A step-and-repeat exfoliation process is initiated using a preprogrammed contact force and liftoff speed. It was determined that atomically thin films of these materials could be obtained reproducibly using this technique, achieving single-layer and few-layer thicknesses for engineering novel 2D transistor devices for future electronic technologies. We show that varying the parameters of the rheometer program can improve the mechanical exfoliation process.

High-Efficiency Monolayer Molybdenum Ditelluride Light-Emitting Diode and Photodetector

Developing a high-efficiency and low-cost light source with emission wavelength transparent to silicon is an essential step toward silicon-based nanophotonic devices and micro/nano industry platforms. Here, a near-infrared monolayer MoTe₂ light-emitting diode (LED) has been demonstrated and its emission wavelength is transparent to silicon. By taking advantage of the quantum tunneling effect, the device has achieved a very high external quantum efficiency (EQE) of 9.5% at 83 K, which is the highest EQE obtained from LED devices fabricated from monolayer TMDs so far. When the device is operated as a photodetector, the MoTe₂ device exhibits a strong photoresponsivity at resonant wavelength 1145 nm. The low dark current of ∼5pA and fast response time 5.06 ms are achieved due to suppression of hBN tunneling layer. Our results open a new route for the investigation of novel near-infrared silicon integrated optoelectronic devices.

Novel 2D Layered Molybdenum Ditelluride Encapsulated in Few-Layer Graphene as High-Performance Anode for Lithium-Ion Batteries

Molybdenum ditelluride nanosheets encapsulated in few-layer graphene (MoTe₂ /FLG) are synthesized by a simple heating method using Te and Mo powder and subsequent ball milling with graphite. The as-prepared MoTe₂ /FLG nanocomposites as anode materials for lithium-ion batteries exhibit excellent electrochemical performance with a highly reversible capacity of 596.5 mAh g^-1 at 100 mA g^-1 , a high rate capability (334.5 mAh g^-1 at 2 A g^-1 ), and superior cycling stability (capacity retention of 99.5% over 400 cycles at 0.5 A g^-1 ). Ex situ X-ray diffraction and transmission electron microscopy are used to explore the lithium storage mechanism of MoTe₂ . Moreover, the electrochemical performance of a MoTe₂ /FLG//0.35Li₂ MnO₃ ·0.65LiMn_0.5 Ni_0.5 O₂ full cell is investigated, which displays a reversible capacity of 499 mAh g^-1 (based on the MoTe₂ /FLG mass) at 100 mA g^-1 and a capacity retention of 78% over 50 cycles, suggesting the promising application of MoTe₂ /FLG for lithium-ion storage. First-principles calculations exhibit that the lowest diffusion barrier (0.18 eV) for lithium ions along pathway III in the MoTe₂ layered structure is beneficial for improving the Li intercalation/deintercalation property.

Via Method for Lithography Free Contact and Preservation of 2D Materials

Atomically thin 2D materials span the common components of electronic circuits as metals, semiconductors, and insulators, and can manifest correlated phases such as superconductivity, charge density waves, and magnetism. An ongoing challenge in the field is to incorporate these 2D materials into multilayer heterostructures with robust electrical contacts while preventing disorder and degradation. In particular, preserving and studying air-sensitive 2D materials has presented a significant challenge since they readily oxidize under atmospheric conditions. We report a new technique for contacting 2D materials, in which metal via contacts are integrated into flakes of insulating hexagonal boron nitride, and then placed onto the desired conducting 2D layer, avoiding direct lithographic patterning onto the 2D conductor. The metal contacts are planar with the bottom surface of the boron nitride and form robust contacts to multiple 2D materials. These structures protect air-sensitive 2D materials for months with no degradation in performance. This via contact technique will provide the capability to produce "atomic printed circuit boards" that can form the basis of more complex multilayer heterostructures.

Tunable Mobility in Double-Gated MoTe2 Field-Effect Transistor: Effect of Coulomb Screening and Trap Sites

There is a general consensus that the carrier mobility in a field-effect transistor (FET) made of semiconducting transition-metal dichalcogenides (s-TMDs) is severely degraded by the trapping/detrapping and Coulomb scattering of carriers by ionic charges in the gate oxides. Using a double-gated (DG) MoTe₂ FET, we modulated and enhanced the carrier mobility by adjusting the top- and bottom-gate biases. The relevant mechanism for mobility tuning in this device was explored using static DC and low-frequency (LF) noise characterizations. In the investigations, LF-noise analysis revealed that for a strong back-gate bias the Coulomb scattering of carriers by ionized traps in the gate dielectrics is strongly screened by accumulation charges. This significantly reduces the electrostatic scattering of channel carriers by the interface trap sites, resulting in increased mobility. The reduction of the number of effective trap sites also depends on the gate bias, implying that owing to the gate bias, the carriers are shifted inside the channel. Thus, the number of active trap sites decreases as the carriers are repelled from the interface by the gate bias. The gate-controlled Coulomb-scattering parameter and the trap-site density provide new handles for improving the carrier mobility in TMDs, in a fundamentally different way from dielectric screening observed in previous studies.

New Mo6 Te6 Sub-Nanometer-Diameter Nanowire Phase from 2H-MoTe2

A novel phase transition, from multilayered 2H-MoTe₂ to a parallel bundle of sub-nanometer-diameter metallic Mo₆ Te₆ nanowires (NWs) driven by catalyzer-free thermal-activation (400-500 °C) under vacuum, is demonstrated. The NWs form along the 〈11-20〉 2H-MoTe₂ crystallographic directions with lengths in the micrometer range. The metallic NWs can act as an efficient hole injection layer on top of 2H-MoTe₂ due to favorable band-alignment. In particular, an atomically sharp MoTe₂ /Mo₆ Te₆ interface and van der Waals gap with the 2H layers are preserved. The work highlights an alternative pathway for forming a new transition metal dichalcogenide phase and will enable future exploration of its intrinsic transportation properties.

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (R_c). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS₂ and MoTe₂ by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS₂ and MoTe₂, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS₂. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that R_c is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

Synthesis of High-Quality Large-Area Homogenous 1T' MoTe2 from Chemical Vapor Deposition

High-quality large-area few-layer 1T' MoTe₂ films with high homogeneity are synthesized by the controlled tellurization of MoO₃ film. The Mo precursor plays a key role in determining the quality and morphology of the 1T' MoTe₂ . Furthermore, the amount of Te strongly influences the phase of the MoTe₂ . The growth method paves the way toward the scalable production of 1T' MoTe₂ -based applications.

Activation of New Raman Modes by Inversion Symmetry Breaking in Type II Weyl Semimetal Candidate T'-MoTe2

We synthesized distorted octahedral (T') molybdenum ditelluride (MoTe2) and investigated its vibrational properties with Raman spectroscopy, density functional theory, and symmetry analysis. Compared to results from the high-temperature centrosymmetric monoclinic (T'mo) phase, four new Raman bands emerge in the low-temperature orthorhombic (T'or) phase, which was recently predicted to be a type II Weyl semimetal. Crystal-angle-dependent, light-polarization-resolved measurements indicate that all the observed Raman peaks belong to two categories: those vibrating along the zigzag Mo atomic chain (z-modes) and those vibrating in the mirror plane (m-modes) perpendicular to the zigzag chain. Interestingly, the low-energy shear z-mode and shear m-mode, absent from the T'mo spectra, become activated when sample cooling induces a phase transition to the T'or crystal structure. We interpret this observation as a consequence of inversion-symmetry breaking, which is crucial for the existence of Weyl fermions in the layered crystal. Our temperature-dependent Raman measurements further show that both the high-energy m-mode at ∼130 cm(-1) and the low-energy shear m-mode at ∼12 cm(-1) provide useful gauges for monitoring the broken inversion symmetry in the crystal.

Structural and Electrical Properties of MoTe2 and MoSe2 Grown by Molecular Beam Epitaxy

We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.

Nonlinear Saturable Absorption of Liquid-Exfoliated Molybdenum/Tungsten Ditelluride Nanosheets

Molybdenum disulfide (MoS2 ) and tungsten disulfide (WS2 ), two representative transition metal dichalcogenide materials, have captured tremendous interest for their unique electronic, optical, and chemical properties. Compared with MoS2 and WS2 , molybdenum ditelluride (MoTe2 ) and tungsten ditelluride (WTe2 ) possess similar lattice structures while having smaller bandgaps (less than 1 eV), which is particularly interesting for applications in the near-infrared wavelength regime. Here, few-layer MoTe2 /WTe2 nanosheets are fabricated by a liquid exfoliation method using sodium deoxycholate bile salt as surfactant, and the nonlinear optical properties of the nanosheets are investigated. The results demonstrate that MoTe2 /WTe2 nanosheets exhibit nonlinear saturable absorption property at 1.55 μm. Soliton mode-locking operations are realized separately in erbium-doped fiber lasers utilizing two types of MoTe2 /WTe2 -based saturable absorbers, one of which is prepared by depositing the nanosheets on side polished fibers, while the other is fabricated by mixing the nanosheets with polyvinyl alcohol and then evaporating them on substrates. Numerous applications may benefit from the nonlinear saturable absorption features of MoTe2 /WTe2 nanosheets, such as visible/near-infrared pulsed laser, materials processing, optical sensors, and modulators.

Structural Phase Transitions by Design in Monolayer Alloys

Two-dimensional monolayer materials are a highly anomalous class of materials under vigorous exploration. Mo- and W-dichalcogenides are especially unusual two-dimensional materials because they exhibit at least three different monolayer crystal structures with strongly differing electronic properties. This intriguing yet poorly understood feature, which is not present in graphene, may support monolayer phase engineering, phase change memory and other applications. However, knowledge of the relevant phase boundaries and how to engineer them is lacking. Here we show using alloy models and state-of-the-art density functional theory calculations that alloyed MoTe2-WTe2 monolayers support structural phase transitions, with phase transition temperatures tunable over a large range from 0 to 933 K. We map temperature-composition phase diagrams of alloys between pure MoTe2 and pure WTe2, and benchmark our methods to analogous experiments on bulk materials. Our results suggest applications for two-dimensional materials as phase change materials that may provide scale, flexibility, and energy consumption advantages.

Phase-Engineered Synthesis of Centimeter-Scale 1T'- and 2H-Molybdenum Ditelluride Thin Films

We report the synthesis of centimeter-scale, uniform 1T'- and 2H-MoTe2 thin films via the tellurization of Mo thin films. 1T'-MoTe2 was initially grown and converted gradually to 2H-MoTe2 over a prolonged growth time under a Te atmosphere. Maintaining excessive Te was essential for obtaining the stable stoichiometric 2H-MoTe2 phase. Further annealing under a lower partial pressure of Te at the same temperature, followed by a rapid quenching, led to the reverse phase transition from 2H-MoTe2 to 1T'-MoTe2. The orientation of the 2H-MoTe2 film was determined by the tellurization rate. Slow tellurization was the key for obtaining a highly oriented 2H-MoTe2 film over the entire area, while fast tellurization led to a 2H-MoTe2 film with a randomly oriented c-axis.

Electrostatically Reversible Polarity of Ambipolar α-MoTe2 Transistors

A doping-free transistor made of ambipolar α-phase molybdenum ditelluride (α-MoTe2) is proposed in which the transistor polarity (p-type and n-type) is electrostatically controlled by dual top gates. The voltage signal in one of the gates determines the transistor polarity, while the other gate modulates the drain current. We demonstrate the transistor operation experimentally, with electrostatically controlled polarity of both p- and n-type in a single transistor.

Reconfigurable Ion Gating of 2H-MoTe2 Field-Effect Transistors Using Poly(ethylene oxide)-CsClO4 Solid Polymer Electrolyte

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 μF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.