MoTe2 Flakes Research Articles

Waveguide-integrated van der Waals heterostructure photodetector at telecom wavelengths with high speed and high responsivity.

Intensive efforts have been devoted to explore novel optoelectronic devices based on two-dimensional (2D) transition-metal dichalcogenides (TMDCs) owing to their strong light-matter interaction and distinctive material properties [1, 2]. In particular, photodetectors featuring both high-speed and high-responsivity performance are of great interest for a vast number of applications such as high-data-rate interconnects operated at standardized telecom wavelengths [3, 4]. Yet, the intrinsically small carrier mobilities of TMDCs become a bottleneck for high-speed application use [5]. Here, we present high-performance vertical van der Waals heterostructure-based photodetectors integrated on a silicon photonics platform. Our vertical MoTe2/graphene heterostructure design minimizes the carrier transit path length in TMDCs and enables a record-high measured bandwidth of at least 24 GHz under a moderate bias voltage of −3 volts. Applying a higher bias or employing thinner MoTe2 flakes boosts the bandwidth even to 50 GHz. Simultaneously, our device reaches a high external responsivity of 0.2 A/W for incident light at 1300 nm, benefiting from the integrated waveguide design. Our studies shed light on performance trade-offs and present design guidelines for fast and efficient devices. The combination of 2D heterostructures and integrated guided-wave nano photonics defines an attractive platform to realize high-performance optoelectronic devices [6–8], such as photodetectors [9], light-emitting devices [10] and electro-optic modulators [11].

Generation and detection of coherent longitudinal acoustic waves in ultrathin 1T’-MoTe2

Layered transition metal dichalcogenides have attracted substantial attention owing to their versatile functionalities and compatibility with current nanofabrication technologies. Thus, noninvasive means to determine the mechanical properties of nanometer (nm) thick specimens are of increasing importance. Here, we report on the detection of coherent longitudinal acoustic phonon modes generated by impulsive femtosecond (fs) optical excitation. Broadband fs-transient absorption experiments in 1T’-MoTe2 flakes as a function of thickness (4–30 nm) yield a longitudinal sound speed of vL = (2990 ± 90) m s−1. In addition, temperature-dependent measurements unveil a linear decrease in the normalized Young's modulus EL/EL, 295 K with a slope of δ(EL/EL, 295 K)/δT = (–2.0 ± 0.1) 10−3 K−1 and no noticeable change caused by the Td – 1T’ structural phase transition or variations in film thickness.

Large Multidirectional Spin-to-Charge Conversion in Low-Symmetry Semimetal MoTe2 at Room Temperature.

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration, unusual configurations are expected in low-symmetry transition-metal dichalcogenide semimetals, which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe2 flakes, which are stacked in graphene lateral spin valves. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration.

Giant enhancement of superconductivity in few layers MoTe2**Project supported by the Guangdong Innovative and Entrepreneurial Research Team Program, China (Grant No. 2016ZT06D348), the National Natural Science Foundation of China (Grant No. 11874193), and the Shenzhen Fundamental Subject Research Program, China (Grant Nos. JCYJ20170817110751776 and JCYJ20170307105434022).

Recently, the layered transition metal dichalcogenide 1T′ MoTe2 has attracted considerable attention due to its non-saturating magnetoresistance, type-II Weyl semimetal properties, superconductivity, and potential candidate for two-dimensional (2D) topological insulator in the single- and few-layer limit. Here in this work, we perform systematic transport measurements on thin flakes of MoTe2 prepared by mechanical exfoliation. We find that MoTe2 flakes are superconducting and have an onset superconducting transition temperature Tc up to 5.3 K, which significantly exceeds that of its bulk counterpart. The in-plane upper critical field (Hc2||) is much higher than the Pauli paramagnetic limit, implying that the MoTe2 flakes have Zeeman-protected Ising superconductivity. Furthermore, the Tc and Hc2 can be tuned by up to 320 mK and 400 mT by applying a gate voltage. Our result indicates that MoTe2 flake is a good candidate for studying exotic superconductivity with nontrivial topological properties.

Visible-to-near-infrared photodetector based on graphene–MoTe2–graphere heterostructure**Project supported by the National Natural Science Foundation of China (Grant No. 21405109) and the Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments, China (Pilt No. 1710).

Graphene and transition metal dichalcogenides (TMDs), two-dimensional materials, have been investigated wildely in recent years. As a member of the TMD family, MoTe2 possesses a suitable bandgap of ∼ 1.0 eV for near infrared (NIR) photodetection. Here we stack the MoTe2 flake with two graphene flakes of high carrier mobility to form a graphene–MoTe2–graphene heterostructure. It exhibits high photo-response to a broad spectrum range from 500 nm to 1300 nm. The photoresponsivity is calculated to be 1.6 A/W for the 750-nm light under 2 V/0 V drain–source/gate bias, and 154 mA/W for the 1100-nm light under 0.5 V/60 V drain–source/gate bias. Besides, the polarity of the photocurrent under zero Vds can be efficiently tuned by the back gate voltage to satisfy different applications. Finally, we fabricate a vertical graphene–MoTe2–graphene heterostructure which shows improved photoresponsivity of 3.3 A/W to visible light.

Vertical Heterophase for Electrical, Electrochemical, and Mechanical Manipulations of Layered MoTe2

AbstractPhase engineering is a breakthrough for various electronic and energy device applications with transition metal dichalcogenides (TMDs). Chemical methods, such as lithium intercalation, are mostly used for phase engineering, which achieves atomically thin flakes and high catalytic performances in several group 6 TMDs including MoS2. However, chemical methods cannot be applied to MoTe2, a widely investigated group 6 TMD with intriguing semiconducting, topological, and catalytic properties. The lack of modifying MoTe2 by chemical methods remains a puzzling issue considering the small energy difference between the polymorphs of MoTe2. Here, a convection‐assisted lithium ion intercalation and phase transition is reported to achieve a vertical heterophase in a MoTe2 crystal. The vertical heterophase in MoTe2 reduces the Schottky barrier with metal electrodes down to 66 meV, enhancing the overall ion conductance for electrochemical hydrogen production. Moreover, the weakened adhesion of the 1T' phase layers on the top and bottom surfaces in the vertical heterophase, formed by the intercalation, enables a unique surface tension‐driven exfoliation of MoTe2 flakes. The heterophase chemical engineering suggests a new platform for hybrid catalysts and next‐generation electronic devices based on 2D materials.

Nucleation engineering for atomic layer deposition of uniform sub-10 nm high-K dielectrics on MoTe2

Abstract Continuous, uniform, and sub-10 nm Al2O3 high-K dielectrics upon two-dimensional exfoliated multilayer MoTe2 are realized by atomic layer deposition (ALD) based on a nucleation layer (NL) prepared by the ozone-based process, interfacial AlN, and low-temperature (low-T) physical adsorption. The NLs gives rise to significant reduction of the leakage current in the sub-10 nm Al2O3 high-K dielectrics as shown by conductive atomic force microscopy. For the ozone-based NL, X-ray photoelectron spectroscopy (XPS) reveals the oxidation of MoTe2 which is detrimental to the electrical properties of MoTe2. For the AlN NL, XPS reveals that Mo N bonds were formed without Mo O bonds and no chemical shift appeared in Te-3d XPS spectrum, indicating the AlN NL did not result in the MoTe2 oxidation but instead the formation of an MoN layer. For the low-T NL, the XPS spectra of MoTe2 are the same as those of the as-exfoliated MoTe2 flake, consistent with the absence of chemical reactions during low-T physical adsorption. The result demonstrates that the NLs prepared by the low-T physical adsorption and the interfacial AlN are effective and favorable for nucleating high-quality high-K gate dielectrics on MoTe2 transistors.

Precise Layer Control of MoTe2 by Ozone Treatment.

Transition metal dichalcogenides (TMDCs) demonstrate great potential in numerous applications. However, these applications require a precise control of layer thickness at the atomic scale. In this work, we present an in-situ study of the self-limiting oxidation process in MoTe2 by ozone (O3) treatment. A precise layer-by-layer control of MoTe2 flakes can be achieved via multiple cycles of oxidation and wet etching. The thinned MoTe2 flakes exhibit comparable optical properties and film quality to the pristine exfoliated ones. Besides, an additional p-type doping is observed after O3 oxidation. Such a p-doping effect converts the device properties of MoTe2 from electron-dominated to hole-dominated ambipolar characteristics.

Effect of thermal conductivity of substrate on laser‐induced phase transition of MoTe2

AbstractThe phase transition of molybdenum ditelluride (MoTe2) has aroused great interest for enormous potential application in two‐dimensional (2D) optoelectronic sensors and nonvolatile information storage. Many in situ synthesis such as chemical vapor deposition and tellurization of Mo as well as postgrowth methods including thermal and strain engineering have been reported to realize the 2H‐to‐1T′ phase transition in MoTe2. Laser irradiation as one of thermal engineering has been demonstrated to be efficient and convenient for phase transition in MoTe2 flakes, but the substrate effect especially the thermal conductivity of substrate has not been studied. In this work, we systematically investigate the effect of four substrates (SiO2, Si, PDMS, and GaAs) with different thermal conductivities on the laser‐induced phase transition of MoTe2. The evolutions of Raman spectra clearly reveal the phase transition from 2H to 1T′ phase in MoTe2 on four different substrates with the increase in laser power. The laser power required for the phase transition is proportional to the substrate thermal conductivity. The PDMS substrate with the lowest thermal conductivity of 0.35 W/MK requires the lowest laser power of 2 mW to complete the phase transition. We have also fabricated a micro pattern heterostructure through artificial manipulation of the laser beam. Our work may provide valuable insights on the laser‐induced phase transition in MoTe2 and be helpful for phase transition of other TMDs.

Efficient Self-Driven Photodetectors Featuring a Mixed-Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake.

Heterojunctions formed from low-dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed-dimensional van der Waals (vdW) heterojunction comprising a 1D n-type Ga-doped CdS nanowire and a 2D p-type MoTe2 flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm-2 . A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS-MoTe2 interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe2 . Such mixed-dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self-driven photodetection devices.

Controlled Layer-by-Layer Oxidation of MoTe2 via O3 Exposure.

Growing uniform oxides with various thickness on TMDs is one of the biggest challenges to integrate TMDs into complementary metal oxide semiconductor (CMOS) logic circuits. Here, we report a layer-by-layer oxidation of atomically thin MoTe2 flakes via ozone (O3) exposure. The thickness of MoO x oxide film could be tuning with atomic-level accuracy simply by varying O3 exposure time. Additionally, MoO x-covered MoTe2 shows a hole-dominated transport behavior. Our findings point to a simple and effective strategy for growing homogeneous surface oxide film on MoTe2, which is promising for several purposes in metal-oxide-semiconductor transistor, ranging from surface passivation to dielectric layers.

Second Harmonic Generation in Atomically Thin MoTe2

AbstractOptical second harmonic generations (SHGs) from atomically thin MoTe2 flakes with 2H and 1T′ phases are studied. From 2H‐MoTe2 samples with odd (even) numbers of layers, strong (negligible) SHGs are observed due to the layer‐dependent broken inversion symmetry. When pumped by a telecom‐band laser, SHG from a monolayer 2H‐MoTe2 is about one order of magnitude stronger than that from a monolayer WS2; an extremely high second‐order nonlinear susceptibility of 2.5 nm V−1 is estimated, presenting the highest value among those reported in 2D materials. SHG measurements in MoTe2 are also demonstrated as an efficient way to distinguish the 2H‐to‐1T′ phase transition. Compared with the SHG in 2H‐MoTe2, 1T′‐MoTe2's SHG has much lower efficiency and the polarization dependence is changed from a six‐fold pattern to a two‐lobe pattern.

Two-Dimensional MoTe­2 PN Diode and CMOS Inverter By Atomic Layer Deposition-Induced Hydrogen Doping

Recently, 2D transition metal dichalcogenides(TMDs) MoTe2 has shown outstanding properties for future electronic devices. Such TMD structures without surface dangling bonds make the 2D MoTe2 more favorable candidate than conventional 3D Si in a few nanometer scale. The band gap of thin MoTe2 appears close to that of Si and quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control charge carrier polarity of MoTe2, functional devices such as complementary metal–oxide–semiconductor (CMOS) inverter have not been reported. Here we demonstrate a 2D CMOS inverter in a single MoTe2 nanosheet by straightforward selective doping technique. In a single MoTe2 flake, initially p-doped channel was selectively converted to n-doped region with high electron mobility of 18 cm2/Vs by atomic layer deposition-induced H-doping. Our ultrathin CMOS inverter exhibited a high DC voltage gain of 29, AC gain of 18 at 1 kHz, and a low static power consumption of a few nW. Our results show a great potential of MoTe2 for future electronic devices based on 2D semiconducting materials Figure 1

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

AbstractSemiconducting molybdenum ditelluride (2H‐MoTe2), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe2 transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe2 layers, the carrier polarity of MoTe2 transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe2 is significantly enhanced from 0.1 to 20 cm2 V−1 s−1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of &gt;25 and photodetectors based on few‐layer MoTe2 flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide semiconductors.

Reducing the Schottky barrier between few-layer MoTe2 and gold

Schottky barriers greatly influence the performance of optoelectronic devices. Schottky barriers can be reduced by harnessing the polymorphism of 2D metal transition dichalcogenides, since both semiconducting and metallic phases exist. However, high energy, high temperature or chemicals are normally required for phase transformation, or the processes are complex. In this work, stable low-resistance contacts between few layer MoTe2 flakes and gold electrodes are achieved by a simple thermal annealing treatment at low temperature (200–400 °C). The resulting Schottky barrier height of the annealed MoTe2/Au interface is low (~23 meV). A new Raman Ag mode of the 1T′ metallic phase of MoTe2 on gold electrode is observed, indicating that the low-resistance contact is due to the phase transition of 2H-MoTe2. The gold substrate plays an important role in the transformation, and a higher gold surface roughness increases the transformation rate. With this method, the mobility and ON-state current of the MoTe2 transistor increase by ~3–4 orders of magnitude, the photocurrent of vertically stacked graphene/MoTe2/Au device increases ~300%, and the response time decreases by ~20%.

Carrier-Type Modulation and Mobility Improvement of Thin MoTe2.

A systematic modulation of the carrier type in molybdenum ditelluride (MoTe2 ) field-effect transistors (FETs) is described, through rapid thermal annealing (RTA) under a controlled O2 environment (p-type modulation) and benzyl viologen (BV) doping (n-type modulation). Al2 O3 capping is then introduced to improve the carrier mobilities and device stability. MoTe2 is found to be ultrasensitive to O2 at elevated temperatures (250 °C). Charge carriers of MoTe2 flakes annealed via RTA at various vacuum levels are tuned between predominantly pristine n-type ambipolar, symmetric ambipolar, unipolar p-type, and degenerate-like p-type. Changes in the MoTe2 -transistor performance are confirmed to originate from the physical and chemical absorption and dissociation of O2 , especially at tellurium vacancy sites. The electron branch is modulated by varying the BV dopant concentrations and annealing conditions. Unipolar n-type MoTe2 FETs with a high on-off ratio exceeding 106 are achieved under optimized doping conditions. By introducing Al2 O3 capping, carrier field effect mobilities (41 for holes and 80 cm2 V-1 s-1 for electrons) and device stability are improved due to the reduced trap densities and isolation from ambient air. Lateral MoTe2 p-n diodes with an ideality factor of 1.2 are fabricated using the p- and n-type doping technique to test the superb potential of the doping method in functional electronic device applications.

A Simple Method for Synthesis of High-Quality Millimeter-Scale 1T' Transition-Metal Telluride and Near-Field Nanooptical Properties.

The controlled synthesis of MoTe2 and WTe2 is crucial for their fundamental research and potential electronic applications. Here, a simplified ambient-pressure chemical vapor deposition (CVD) strategy is developed to synthesize high-quality and large-scale monolayer and few-layer 1T'-phase MoTe2 (length ≈ 1 mm) and WTe2 (length ≈ 350 µm) crystals by using ordinary salts (KCl or NaCl) as the growth promoter combining with low-cost (NH4 )6 Mo7 O24 ·4H2 O and hydrate (NH4 )10 W12 O41 ·xH2 O as the Mo and W sources, respectively. Atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy confirm the high-quality nature and the atomic structure of the as-grown 1T' MoTe2 and WTe2 flakes. Variable-temperature transport measurements exhibit their semimetal properties. Furthermore, near-field nanooptical imaging studies are performed on the 1T' MoTe2 and WTe2 flakes for the first time. The sub-wavelength effects of 1T'-phase MoTe2 (λp ≈ 140 nm) and WTe2 (λp ≈ 100 nm) are obtained. This approach paves the way for the growth of special transition-metal dichalcogenides materials and boosts the future polaritonic research of 2D telluride compounds.

Sensitive Phonon-Based Probe for Structure Identification of 1T' MoTe2.

In this work, by combining transmission electron microscopy and polarized Raman spectroscopy for the 1T' MoTe2 flakes with different thicknesses, we found that the polarization dependence of Raman intensity is given as a function of excitation laser wavelength, phonon symmetry, and phonon frequency, but has weak dependence on the flake thickness from few-layer to multilayer. In addition, the frequency of Raman peaks and the relative Raman intensity are sensitive to flake thickness, which manifests Raman spectroscopy as an effective probe for thickness of 1T' MoTe2. Our work demonstrates that polarized Raman spectroscopy is a powerful and nondestructive method to quickly identify the crystal structure and thickness of 1T' MoTe2 simultaneously, which opens up opportunities for the in situ probe of anisotropic properties and broad applications of this novel material.

Determination of Crystal Axes in Semimetallic T′‐MoTe2 by Polarized Raman Spectroscopy

Distorted octahedral T′ phase of MoTe2 has recently attracted significant interest due to its predicted topological states and novel charge transport properties. Here, we report a nondestructive method for determining the crystal orientation of few‐layer T′‐MoTe2 flakes by polarized Raman spectroscopy. The experimentally observed Raman modes are assigned to eigenmodes of vibrations predicted by density functional theory calculations. Polarized Raman measurements reveal four distinct types of angle‐dependent intensity variations. From group theory, it can be deduced that the intensity of the Bg mode reaches a maximum in the configuration when the polarization vector of the incident light is either parallel or orthogonal to the metal–metal zigzag chain direction. The intensity variation of the Bg mode cannot be used to unambiguously determine the crystal orientation. Using electron diffraction analysis, it is demonstrated that the intensity of the Ag mode at around 162 cm−1 reaches a maximum when the polarization vector of the incident light is parallel to the metal–metal chain direction in the configuration. Furthermore, a simple method is proposed for identifying crystal orientation in nonpolarized Raman spectroscopy.

Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe2

Monoclinic group 6 transition metal dichalcogenides (TMDs) have been extensively studied for their intriguing 2D physics (e.g. spin Hall insulator) as well as for ohmic homojunction contacts in 2D device applications. A critical prerequisite for those applications is thickness control of the monoclinic 2D materials, which allows subtle engineering of the topological states or electronic bandgaps. Local thickness control enables the realization of clean homojunctions between different electronic states, and novel device operation in a single material. However, conventional fabrication processes, including chemical methods, typically produce non-homogeneous and relatively thick monoclinic TMDs, due to their distorted octahedral structures. Here, we report on a post-patterning technique using laser-irradiation to fabricate homojunctions between two different thickness areas in monoclinic MoTe2. A thickness-dependent electronic change from a metallic to semiconducting state, resulting in an electronic homojunction, was realized by the optical patterning of pristine MoTe2 flakes, and a pre-patterned device channel of monoclinic MoTe2 with a thickness-resolution of 5 nm. Our work provides insight on an optical post-process method for controlling thickness, as a promising approach for fabricating impurity-free 2D TMDs homojunction devices.