MoTe2 Flakes Research Articles

Polarization control of lasing from few-layer MoTe2 coupled with the optical metasurface supporting quasi-trapped modes

The development of technology for integrating optical metaresonators with two-dimensional and layered van der Waals (vdW) materials opens up broad prospects for the creation of subdiffraction concentrators of electromagnetic energy, surface-emitting lasers, laser displays, and highly efficient nonlinear converters. In this work, we develop a straightforward strategy for the design and fabrication of surface-emitting laser devices based on few-layer transition metal dichalcogenides placed on the dielectric metasurfaces in the regime of quasi-trapped mode excitation. We optimize the parameters of MoTe2 flake and Si metasurface to achieve a positive feedback and to observe the lasing, resulting from their integration, with the predicted characteristics. Promising potential for the development of vdW-metalaser platform is associated with the possibility of simple polarization control of lasing regimes by employing the features of the bianisotropic response of the metasurface's building blocks.

Spatially Controlled Phase Transition in MoTe2 Driven by Focused Ion Beam Irradiations.

Phase transitions play an important role in tuning the physical properties of two-dimensional (2D) materials as well as developing their high-performance device applications. Here, we reported the observation of a phase transition in few-layered MoTe2 flakes by the irradiation of gallium (Ga+) ions using a focused ion beam (FIB) system. The semiconducting 2H phase of MoTe2 can be controllably converted to the metallic 1T'-like phase via Te defect engineering during irradiations. By taking advantage of the nanometer-sized Ga+ ion probe proved by FIB, in-plane 1T'-2H homojunctions of MoTe2 at submicrometer scale can be fabricated. Furthermore, we demonstrate the improvement of device performance (on-state current over 2 orders of magnitude higher) in MoTe2 transistors using the patterned 1T'-like phase regions as contact electrodes. Our study provides a new strategy to drive the phase transitions in MoTe2, tune their properties, and develop high-performance devices, which also extends the applications of FIB technology in 2D materials and their devices.

Emergent layer stacking arrangements in c-axis confined MoTe2

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe2 possesses two stacking arrangements, the ferroelectric Weyl semimetal Td phase and the higher-order topological insulator 1T′ phase. However, in thin flakes of MoTe2, it is unclear if the layer stacking follows the Td, 1T′, or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe2 layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T′ and Td domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe2 stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe2 flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

Arresting the surface oxidation kinetics of bilayer 1T′-MoTe2 by sulphur passivation

MoTe2 garnered much attention among 2D materials due to stable polymorphs with distinctive structural and electronic properties. Among the polymorphs, 1T′-MoTe2 in bulk form is type—II Weyl semimetal while, in monolayer form is a quantum spin Hall insulator. Thus, it is suitable for a wide variety of applications. Nevertheless, 1T′-MoTe2 degrades within a few hours when exposed to the atmosphere and causes hindrances in device fabrication. Here the degradation kinetics of CVD-synthesized 1T′-MoTe2 was investigated using Raman spectroscopy, XPS, and microscopic characterizations. The degradation rate of as-grown 1T′-MoTe2 obtained was 9.2 × 10−3 min−1. Further, we prevented the degradation of 1T′-MoTe2 by introducing a thin coating of S that encapsulates the flakes. 1T′-MoTe2 flakes showed stability for several days when covered using sulphur, indicating 25 times enhanced structural stability.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe2, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe2 can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., − 2 to − 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe2 was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p–n diode using a 0.8 nm-thick MoTe2 flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10−4. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Laser‐Assisted Thermomechanical Thinning of MoTe2 in Nanoscale Lateral Resolution

AbstractA key feature of 2D transition metal dichalcogenides (2D‐TMDCs) is that their properties are strongly dependent on their thickness, typically appearing in ultrathin mono‐ or few‐ layers. Thus, precise control of functional nanostructure is critical for fundamental research and applications in 2D‐TMDCs. Here, atomic layer precision thinning of molybdenum ditelluride (MoTe2) at nanoscale lateral resolution by introducing laser irradiated hot tip is demonstrated. The contact of the hot tip on MoTe2 surface promotes oxidation at nanoscale resolution which is simultaneously removed by thermomechanical scribing. This process completes the atomic layer precision thinning of MoTe2 while maintaining the high crystallinity of thinned MoTe2 flake. Further, the electrical properties of the MoTe2 flake are intact after thinning, which proves that the thinned MoTe2 flake obtained by these methods can potentially be utilized for device fabrication. It is believed that the work will enable applications of 2D‐TMDCs that require nanoscale resolution with controlled thickness.

Dielectric engineering enable to lateral anti-ambipolar MoTe 2 heterojunction

Atomically two-dimensional (2D) materials have generated widespread interest for novel electronics and optoelectronics. Specially, owing to atomically thin 2D structure, the electronic bandgap of 2D semiconductors can be engineered by manipulating the surrounding dielectric environment. In this work, we develop an effective and controllable approach to manipulate dielectric properties of h-BN through gallium ions (Ga+) implantation for the first time. And the maximum surface potential difference between the intrinsic h-BN (h-BN) and the Ga+ implanted h-BN (Ga+-h-BN) is up to 1.3 V, which is characterized by Kelvin probe force microscopy. More importantly, the MoTe2 transistor stacked on Ga+-h-BN exhibits p-type dominated transfer characteristic, while the MoTe2 transistor stacked on the intrinsic h-BN behaves as n-type, which enable to construct MoTe2 heterojunction through dielectric engineering of h-BN. The dielectric engineering also provides good spatial selectivity and allows to build MoTe2 heterojunction based on a single MoTe2 flake. The developed MoTe2 heterojunction shows stable anti-ambipolar behaviour. Furthermore, we preliminarily implemented a ternary inverter based on anti-ambipolar MoTe2 heterojunction. Ga+ implantation assisted dielectric engineering provides an effective and generic approach to modulate electric bandgap for a wide variety of 2D materials. And the implementation of ternary inverter based on anti-ambipolar transistor could lead to new energy-efficient logical circuit and system designs in semiconductors.

A Complete Two-Dimensional Avalanche Photodiode Based on MoTe2−WS2−MoTe2 Heterojunctions With Ultralow Dark Current

Two-dimensional (2D)-material-based photodetectors have recently received great attention due to their potentials in developing ultrathin and highly compact devices. Avalanche photodiodes (APDs) are widely used in a variety of fields such as optical communications and bioimaging due to their fast responses and high sensitivities. However, conventional APDs based on bulk materials are limited by their relatively high dark current. One solution to tackle this issue is by employing nanomaterials and nanostructures as the active layers for APDs. In this study, we proposed and fabricated an atomically-thick APD based on heterojunctions formed by 2D transition metal dichalcogenides (TMDs). A typical device structure was formed by stacking a semiconducting monolayer WS2 onto two metallic few-layer MoTe2 flakes. Due to the Schottky barrier formed between the TMD layers and their atomic thicknesses, the dark current of the APD is greatly reduced down to 93 pA. In addition, the APD can operate through a broad spectral range from visible to near-infrared region, with a responsivity of 6.02 A/W, an external quantum efficiency of 1,406%, and an avalanche gain of 587. We believe that the 2D APD demonstrated here provides a feasible approach for developing all-2D optoelectronic devices with simultaneous high-sensitivity and low noise.

MoTe2 on ferroelectric single-crystal substrate in the dual-gate field-effect transistor operation

An exfoliated MoTe2 flake in contact with a ferroelectric single-crystal substrate was studied to examine its charge carrier modulation by neighboring ferroelectric polarization. A MoTe2 field-effect transistor was fabricated, having a hexagonal-BN (hBN) flake and a ferroelectric substrate employed as top and bottom gate dielectrics. In the dual-gate operation, the charge conduction exhibited an ambipolar behavior with large hysteresis during the gate voltage sweep. It mainly originates from the ferroelectric nature in combination with the charge trap phenomena at the interfaces. Interestingly, we found out that holes are more easily trapped than electrons, and charge carriers in MoTe2 are easily modulated through the top hBN gate when the electron conduction is predominantly set by the bottom ferroelectric field. However, the controllability becomes much weaker under opposing ferroelectric polarizations. This unbalanced controllability reveals the interfacial hole-trap effect resulting from ferroelectric polarization.

Gate‐Tunable Photodiodes Based on Mixed‐Dimensional Te/MoTe2 Van der Waals Heterojunctions

AbstractThe dangling‐bond‐free surfaces of 2D materials enable them to possess various degrees of freedom to form heterostructures with non‐2D materials. This allows for the combination of the advantages of different dimensional materials to fabricate van der Waals (vdW) heterostructures, thereby improving device performance and even bringing diversity and novelty. Herein, a mixed‐dimensional vdW heterostructure photodiode comprising a 1D tellurium (Te) nanowire and a 2D molybdenum ditelluride (MoTe2) flake is demonstrated. Forward rectifying and backward rectifying characteristics are realized by applying different gate voltage. The device displays a broad spectral response from visible to near‐infrared and exhibits ultrahigh external quantum efficiency of 7.16 × 103% for photogating effect. Moreover, the response time can be improved by controlling gate voltage and a rapid response time of 4.8 ms is achieved. These mixed‐dimensional vdW heterojunctions, which take advantages of both 1D and 2D semiconductors, will facilitate the development of next‐generation electronics and optoelectronics.

Neuro-Transistor Based on UV-Treated Charge Trapping in MoTe2 for Artificial Synaptic Features.

The diversity of brain functions depend on the release of neurotransmitters in chemical synapses. The back gated three terminal field effect transistors (FETs) are auspicious candidates for the emulation of biological functions to recognize the proficient neuromorphic computing systems. In order to encourage the hysteresis loops, we treated the bottom side of MoTe2 flake with deep ultraviolet light in ambient conditions. Here, we modulate the short-term and long-term memory effects due to the trapping and de-trapping of electron events in few layers of a MoTe2 transistor. However, MoTe2 FETs are investigated to reveal the time constants of electron trapping/de-trapping while applying the gate-voltage pulses. Our devices exploit the hysteresis effect in the transfer curves of MoTe2 FETs to explore the excitatory/inhibitory post-synaptic currents (EPSC/IPSC), long-term potentiation (LTP), long-term depression (LTD), spike timing/amplitude-dependent plasticity (STDP/SADP), and paired pulse facilitation (PPF). Further, the time constants for potentiation and depression is found to be 0.6 and 0.9 s, respectively which seems plausible for biological synapses. In addition, the change of synaptic weight in MoTe2 conductance is found to be 41% at negative gate pulse and 38% for positive gate pulse, respectively. Our findings can provide an essential role in the advancement of smart neuromorphic electronics.

Crystal quality degradation in MoTe2 monolayers by a thermal annealing and its suppression by hexagonal boron nitride encapsulation

Two dimensional transition metal dichalcogenides (TMDs) have attracted great research interests owing to a crossover to a direct gap upon reduction of the layer number down to a monolayer (1L) and exciton formation at room temperature based on extraordinarily large binding energy. In particular, a semiconducting molybdenum ditelluride (MoTe2) emits photons in the near-infrared region around 1.1 eV. Furthermore, it has a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a path to practical Si-based optoelectronic device applications. However, the 1L-MoTe2 crystal has some drawbacks such as surface oxidation within several days owing to its poor structural stability and crystal defect formation by a thermal annealing at a low temperature of 200 °C. It is reported that encapsulation by hexagonal boron nitride (hBN) prevents the oxidation of 1L-MoTe2 in air. There are few reports on effects of a thermal annealing on structural stability in hBN-encapsulated 1L-MoTe2. In this study, we investigate the effects of a thermal annealing on the crystal quality of hBN-encapsulated 1L-MoTe2 by a photoluminescence (PL) measurement at low temperatures. We demonstrate that hBN encapsulation is effective for suppression of crystal defect formation in 1L-MoTe2, improving thermal stability.Bulk hBN crystals were grown using a temperature-gradient method at a high pressure (4.0-5.5GPa) and high temperature (1500-1750 °C). Monolayer MoTe2 and multilayered hBN flakes were exfoliated from bulk crystals and were prepared on a polydimethylsiloxane (PDMS) sheet. We fabricated two types of sample structures on a 100-nm-thick SiO2/Si (100) substrate by a standard dry transfer technique using these flakes. The first sample comprised 1L-MoTe2 directly deposited on the SiO2/Si substrate. The second sample comprised 1L-MoTe2 encapsulated by hBN. The fabricated sample structures were thermally annealed at temperatures ranging from 100 to 200 °C for 15min in an atmosphere.Figures 1(a) and 1(b) show the PL spectra at 25K of the SiO2-supported and hBN-encapslated 1L-MoTe2, respectively. These PL spectra are reproduced using multi-Voigt functions to evaluate PL peak intensities in each spectrum. PL emissions attributed to neutral exciton (X0), charged exciton (T), and defect-bound excitons (XB1, XB2) are observed in both samples of SiO2-supported and hBN-encapsulated 1L-MoTe2. We identify these MoTe2 crystals as monolayers from the X0 PL peak position of 1.19 eV. Figure 1(c) shows the trion spectral weight (I T /I tot) plotted as a function of the annealing temperature for the SiO2-supported and hBN-encapsulated 1L-MoTe2. For both sample structures, the I T /I tot values decreases by the thermal annealing at 200℃, implying the decrease in excess carriers in 1L-MoTe2. The excess carriers should originate from impurities and residues adsorbed on the 1L-MoTe2. Thus, the thermal annealing makes it possible to reduce the impurities and residues, realizing high-quality sample structures. Figure 1(d) shows the PL intensity ratio (I XB1 + I XB2)/I X0 plotted as a function of the annealing temperature. Here, I XB1, I XB2, and I X0 are PL intensities of XB1, XB2, and X0 peaks, respectively. For the SiO2-supported 1L-MoTe2, the (I XB1 + I XB2)/I X0 value increases by increasing the annealing temperature while it is kept to be almost constant for the hBN-encapsulated 1L-MoTe2. It is reported that the XB1 and XB2 peaks are attributed to the formation of Te vacancies by a thermal annealing and adsorption of O2 and H2O molecules at the crystal defect sites, respectively. Therefore, an increase in the (I XB1 + I XB2)/I X0 value of the SiO2-supported samples should result from the crystal defect formation in 1L-MoTe2, indicating a large density of non-radiative recombination centers at 200 °C. In contrast, for the hBN-encapsulated samples, the Te vacancy formation and adsorption of O2 and H2O molecules at the crystal defect sites should be avoided owing to the top hBN acting as the capping layer to suppress Te atom desorption.In summary, we investigated the effect of a thermal annealing on a PL spectrum for SiO2-supported and hBN-encapsulated samples. It was found that the thermal annealing at 200 °C were able to remove residues and impurities on the 1L-MoTe2. For the SiO2-supported samples, an increase in the (I XB1 +I XB2 )/I X0 value was seen by the thermal annealing, indicating crystal defect formation due to desorption of Te atoms. In contrast, for the hBN-encapsulated samples, it was kept to be almost constant even after the thermal annealing at 200 °C. It was therefore concluded that the hBN encapsulation should improve the thermal stability in 1L-MoTe2. This work was partly supported by JSPS KAKENHI Grant Numbers JP25107003, JP25107004, JP15K21722, JP26248061, JP25107002, JP16H00982, and JP20H00354 and the CREST (JPMJCR15F3), JST. Figure 1

Thickness Trends of Electron and Hole Conduction and Contact Carrier Injection in Surface Charge Transfer Doped 2D Field Effect Transistors.

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O2 plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS2, MoSe2, MoTe2, WS2, and WSe2 flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the doping-induced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

Frequency doubler based on a single MoTe2/MoS2 anti-ambipolar heterostructure

The frequency doubler is a fundamental element in high frequency integrated circuits, which is usually implemented by a large number of devices in conventional configurations. In this article, we demonstrate a frequency doubler consisting of only one single heterostructure device by stacking two atomically thin MoTe2 and MoS2 flakes together. The resultant MoTe2/MoS2 heterostructure exhibits prominent anti-ambipolar characteristic after rapid thermal annealing treatment, which effectively modulates the p-doping level of the MoTe2, and thus, the transfer characteristic of the MoTe2/MoS2 heterojunction. The anti-ambipolar behavior gives the transfer curve of the MoTe2/MoS2 heterojunction an inverted “V” shape, based on which the source—drain current of the junction oscillates at a frequency that is twice that of the signal frequency inputted through the gate electrode. As a result, the frequency of the output voltage on the loading resistance doubles the input signal frequency. The developed frequency doubler functions well within 3 kHz, which may be improved by optimizing the device structure. This approach greatly reduces the total number of devices required to achieve the function of frequency doubling, and therefore, may achieve higher degree of integration.

Modulation of Electrical Properties with Controllable Local Doping in Multilayer MoTe2 Transistors

AbstractIt has been found that substitutional atoms, surface modification, electrostatic gating, and contact electrode engineering can induce doping effects in 2D materials. However, the doping level, area, position, and pattern shape are not well controlled; thus, it is hard to generate high‐performance, multifunctional devices and logic circuits with these doping approaches. Here, it is found that a damage‐free, highly controllable, local doping in multilayer molybdenum ditelluride (MoTe2) flakes can be realized with highly controllable laser beam scanning irradiation. The doping level, area, position, and pattern shape are well controlled by adjusting parameters of the scanning laser beam. With the controllable doping, the subthreshold swing of MoTe2‐based transistors can be decreased by one order of magnitude, well‐matched p‐type and n‐type transports can be formed, and threshold voltages can be modulated, then a complementary inverter with ultra‐high gain ≈242 is realized, which is the highest in all reported inverters based on 2D materials. It is also found that a stable negative transconductance behavior can be created for the MoTe2 transistor by controlling doping position, then a frequency doubler is successfully demonstrated. Therefore, it is proposed that the position‐controlled local doping in multilayer MoTe2 may offer exciting application potentials in future nanoelectronics.

Nonvolatile and Neuromorphic Memory Devices Using Interfacial Traps in Two-Dimensional WSe2/MoTe2 Stack Channel.

Very recently, stacked two-dimensional materials have been studied, focusing on the van der Waals interaction at their stack junction interface. Here, we report field effect transistors (FETs) with stacked transition metal dichalcogenide (TMD) channels, where the heterojunction interface between two TMDs appears useful for nonvolatile or neuromorphic memory FETs. A few nanometer-thin WSe2 and MoTe2 flakes are vertically stacked on the gate dielectric, and bottom p-MoTe2 performs as a channel for hole transport. Interestingly, the WSe2/MoTe2 stack interface functions as a hole trapping site where traps behave in a nonvolatile manner, although trapping/detrapping can be controlled by gate voltage (VGS). Memory retention after high VGS pulse appears longer than 10000 s, and the Program/Erase ratio in a drain current is higher than 200. Moreover, the traps are delicately controllable even with small VGS, which indicates that a neuromorphic memory is also possible with our heterojunction stack FETs. Our stack channel FET demonstrates neuromorphic memory behavior of ∼94% recognition accuracy.

Oxide Inhibitor-Assisted Growth of Single-Layer Molybdenum Dichalcogenides (MoX2, X = S, Se, Te) with Controllable Molybdenum Release.

Though chemical vapor deposition (CVD) methods have been widely used in the growth of two-dimensional transition-metal dichalcogenides (2D TMDCs), the controllable fabrication of 2D TMDCs is yet hard to achieve because of the great challenge of concisely controlling the release of precursors vapor, one of the most critical growth kinetic factors. To solve this important issue, here we report the utilization of oxide inhibitors covering Mo source during CVD reactions to manipulate the release of Mo vapor. In contrast to the lack of capability of conventional CVD methods, 2D molybdenum dichalcogenide (MoX2, X = S, Se, Te) monolayers were successfully fabricated through the proposed CVD protocol with the oxide-inhibitor-assisted growth (OIAG) strategy. In this way, despite the fact that only separated MoTe2 flakes were prepared, both MoS2 (continuous and clean) and MoSe2 (continuous but dotted) monolayer films at the scale of centimeter were obtained. The presented OIAG method enables a comprehensive understanding and precise control of the reaction kinetics for improved growth of 2D MoX2.

Enhancement of MoTe2 near-infrared absorption with gold hollow nanorods for photodetection

Infrared (IR) light photodetection based on two dimensional (2D) materials of proper bandgap has attracted increasing attention. However, the weak IR absorption in 2D materials, due to their ultrathin attribute and indirect bandgap in multilayer structures, degrades their performance when used as IR photodetectors. In this work, we utilize the fact that few-layer MoTe2 flake has a near-IR (NIR) bandgap and demonstrate a ~ 60-fold enhancement of NIR response by introducing a gold hollow nanorods on the surface. Such gold hollow nanorods have distinct absorption peak located also at the NIR regime, therefore induces strong resonance, benefitting NIR absorption in MoTe2, resulting in strong near-field enhancement. With the evidence from steady and transient state optical spectra, we confirm that the enhancement of NIR response originates only photon absorption, rather than electron transport at interfaces as observed in other heterostructures, therefore, precluding the requirement of high-quality interfaces for commercial applications.

Bandgap opening in MoTe2 thin flakes induced by surface oxidation

Recently, the layered transition metal dichalcogenide 1T′-MoTe2 has generated considerable interest due to their superconducting and non-trivial topological properties. Here, we present a systematic study on 1T′-MoTe2 single-crystal and exfoliated thin-flakes by means of electrical transport, scanning tunnelling microscope (STM) measurements and band structure calculations. For a bulk sample, it exhibits large magneto-resistance (MR) and Shubnikov–de Hass oscillations in ρxx and a series of Hall plateaus in ρxy at low temperatures. Meanwhile, the MoTe2 thin films were intensively investigated with thickness dependence. For samples, without encapsulation, an apparent transition from the intrinsic metallic to insulating state is observed by reducing thickness. In such thin films, we also observed a suppression of the MR and weak anti-localization (WAL) effects. We attributed these effects to disorders originated from the extrinsic surface chemical reaction, which is consistent with the density functional theory (DFT) calculations and in-situ STM results. In contrast to samples without encapsulated protection, we discovered an interesting superconducting transition for those samples with hexagonal Boron Nitride (h-BN) film protection. Our results indicate that the metallic or superconducting behavior is its intrinsic state, and the insulating behavior is likely caused by surface oxidation in few layer 1T’-MoTe2 flakes.

Reversible Electrochemical Phase Change in Monolayer to Bulk-like MoTe2 by Ionic Liquid Gating.

Transition-metal dichalcogenides (TMDs) exist in various crystal structures with semiconducting, semi-metallic, and metallic properties. The dynamic control of these phases is of immediate interest for next-generation electronics such as phase change memories. Of the binary Mo and W-based TMDs, MoTe2 is attractive for electronic applications because it has the lowest energy difference (40 meV) between the semiconducting (2H) and semi-metallic (1T') phases, allowing for MoTe2 phase change by electrostatic doping. Here, we report phase change between the 2H and 1T' polymorphs of MoTe2 in thicknesses ranging from the monolayer to bulk-like case (73 nm) using an ionic liquid electrolyte at room temperature and in air. We find consistent evidence of a partially reversible 2H-1T' transition using in situ Raman spectroscopy where the phase change occurs in the topmost layers of the MoTe2 flake. We find a thickness-dependent transition voltage where higher voltages are necessary to drive the phase change for thicker flakes. We also show evidence of electrochemical activity during the gating process by observation of Te metal formation. This finding suggests the formation of Te vacancies which have been reported to lower the energy difference between the 2H and 1T' phases, potentially aiding the phase change process. Our discovery that the phase change can be achieved on the surface layer of bulk-like materials reveals that this electrochemical mechanism does not require isolation of a single layer and the effect may be more broadly applicable than previously thought.