Single- and few-layer WTe2 and their suspended nanostructures: Raman signatures and nanomechanical resonances

Hybrid mixed-dimensional WTe2/CsPbI3perovskite heterojunction for high-performance photodetectors

Perovskites have showed significant potential for the application in photodetectors due to their outstanding electrical and optical properties. Integrating two-dimensional (2D) materials with perovskites can make full use of the high carrier mobility of 2D materials and strong light absorption of perovskite to realize excellent optoelectrical properties. Here, we demonstrate a photodetector based on the WTe₂/CsPbI₃heterostructure. The quenching and the shortened lifetime of photoluminescence (PL) for CsPbI₃perovskite confirms the efficient charge transfer at the WTe₂/CsPbI₃heterojunction. After coupled with WTe₂, the photoresponsivity of the CsPbI₃photodetector is improved by almost two orders of magnitude due to the high-gain photogating effect. The WTe₂/CsPbI₃heterojunction photodetector reveals a large responsivity of 1157 A W^-1and a high detectivity of 2.1 × 10¹³Jones. The results pave the way for the development of high-performance optoelectronic devices based on 2D materials/perovskite heterojunctions.

Electrically Tunable MXene Nanomechanical Resonators Vibrating at Very High Frequencies

As an emerging class of two-dimensional (2D) layered nanomaterial, MXene exhibits a number of intriguing properties, such as good electrical conductivity and high elastic modulus, and has witnessed continued growth in related device research. However, nanoscale MXene devices which leverage both the intrinsic electrical and mechanical properties of these 2D crystals have not been experimentally studied. Here we demonstrate nanoelectromechanical resonators based on 2D MXene crystals, where Ti₃C₂T_x drumheads with a wide range of thickness, from more than 50 layers all the way down to a monolayer, exhibit robust nanomechanical vibrations with fundamental-mode frequency f₀ up to >70 MHz in the very high frequency (VHF) band, a displacement noise density down to 52 fm/Hz^1/2, and a fundamental-mode frequency-quality factor product up to f₀ × Q ≈ 6.85 × 10⁹ Hz. By combining experimental results with theoretical calculations, we independently derive the Young's modulus of 2D Ti₃C₂T_x crystals to be 270-360 GPa, in excellent agreement with nanoindentation measurements, based on which we elucidate frequency scaling pathways toward microwave frequencies. We further demonstrate electrical tuning of resonance frequency in MXene resonators and frequency-shift-based MXene vacuum gauges with responsivity of 736%/Torr and detection range down to 10^-4 Torr. Our study can lead to the design and creation of nanoscale vibratory devices that exploit the intrinsic electrical and mechanical properties in 2D MXene crystals.

Frequency Scaling, Elastic Transition, and Broad-Range Frequency Tuning in WSe2 Nanomechanical Resonators

Nanomechanical resonators based on atomic layers of tungsten diselenide (WSe₂) offer intriguing prospects for enabling novel sensing and signal processing functions. The frequency scaling law of such resonant devices is critical for designing and realizing these high-frequency circuit components. Here, we elucidate the frequency scaling law for WSe₂ nanomechanical resonators by studying devices of one-, two-, three-, to more than 100-layer thicknesses and different diameters. We observe resonant responses in both mechanical limits and clear elastic transition in between, revealing intrinsic material properties and devices parameters such as Young's modulus and pretension. We further demonstrate a broad frequency tuning range (up to 230%) with a high tuning efficiency (up to 23% V^-1). Such tuning efficiency is among the highest in resonators based on two-dimensional (2D) layered materials. Our findings can offer important guidelines for designing high-frequency WSe₂ resonant devices.

Wafer-scale, highly uniform, and well-arrayed suspended nanostructures for enhancing the performance of electronic devices

Suspended nanostructures play an important role in enhancing the performance of a diverse group of nanodevices. However, realizing a good arrangement and suspension for nanostructures of various shapes remains a significant challenge. Herein, a rapid and simple method for fabricating wafer-scale, highly uniform, well-arrayed suspended nanostructures via nanowelding lithography is reported. Suspended nanostructures with various shapes (nanowires, nanoholes, nanomesh, and nanofilms) and materials (gold, silver, and palladium metals) were employed to demonstrate the applicability of our method. Moreover, gas sensors and thermoacoustic speakers with suspended nanowires outperformed those with unsuspended nanostructures. The proposed method is expected to help advance the development of future nanodevices based on suspended nanostructures.

Light-Tunable Charge Density Wave Orders in MoTe_{2} and WTe_{2} Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure, and band gap. Moreover, optical pumping of the semimetallic 1T^{'} phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

Experimental Adhesion Energy in van der Waals Crystals and Heterostructures from Atomically Thin Bubbles

The formation of gas-filled bubbles on the surface of van der Waals crystals provides an ideal platform whereby the interplay of the elastic parameters and interlayer forces can be suitably investigated. Here, we combine experimental and numerical efforts to study the morphology of the bubbles at equilibrium and highlight unexpected behaviors that contrast with the common assumptions. We exploit such observations to develop an accurate analytical model to describe the shape and strain of the bubbles and exploit it to measure the adhesion energy between a variety of van der Waals crystals, showing sizable material-dependent trends.

Mechanical Properties of Atomically Thin Tungsten Dichalcogenides: WS2, WSe2, and WTe2

Two-dimensional (2D) tungsten disulfide (WS₂), tungsten diselenide (WSe₂), and tungsten ditelluride (WTe₂) draw increasing attention due to their attractive properties deriving from the heavy tungsten and chalcogenide atoms, but their mechanical properties are still mostly unknown. Here, we determine the intrinsic and air-aged mechanical properties of mono-, bi-, and trilayer (1-3L) WS₂, WSe₂, and WTe₂ using a complementary suite of experiments and theoretical calculations. High-quality 1L WS₂ has the highest Young's modulus (302.4 ± 24.1 GPa) and strength (47.0 ± 8.6 GPa) of the entire family, overpassing those of 1L WSe₂ (258.6 ± 38.3 and 38.0 ± 6.0 GPa, respectively) and WTe₂ (149.1 ± 9.4 and 6.4 ± 3.3 GPa, respectively). However, the elasticity and strength of WS₂ decrease most dramatically with increased thickness among the three materials. We interpret the phenomenon by the different tendencies for interlayer sliding in an equilibrium state and under in-plane strain and out-of-plane compression conditions in the indentation process, revealed by the finite element method and density functional theory calculations including van der Waals interactions. We also demonstrate that the mechanical properties of the high-quality 1-3L WS₂ and WSe₂ are largely stable in air for up to 20 weeks. Intriguingly, the 1-3L WSe₂ shows increased modulus and strength values with aging in the air. This is ascribed to oxygen doping, which reinforces the structure. The present study will facilitate the design and use of 2D tungsten dichalcogenides in applications such as strain engineering and flexible field-effect transistors.

Biaxial strain modulated the electronic structure of hydrogenated 2D tetragonal silicene

Silicene-based materials have attracted great attention due to their easier incorporation into silicon-based devices and components. In addition to the reported hydrogenated 2D tetragonal silicene (γ-SiH), we propose two stable atomic configurations of hydrogenated 2D tetragonal silicene (α-SiH and β-SiH) based on first-principles calculation. The calculated results indicate hydrogenation can effectively open the band gap of 2D tetragonal silicene, α-SiH is a semiconductor with a direct band gap of 2.436 eV whereas β-SiH is indirect band gap of 2.286 eV. We also find that the electronic band structure of α-SiH, β-SiH and γ-SiH can be modulated via biaxial strain. By applying biaxial strain in the range of -10% to 12%, the band gap of α-SiH, β-SiH and γ-SiH can be tuned in a range of 1.732-2.585 eV. Furthermore, direct-indirect or indirect-direct transition can be induced under biaxial strain, showing a high degree of flexibility in electronic band structure. The research not only broadens the diversity of hydrogenated 2D tetragonal silicenes, but also provides more possibilities of their applications in spintronic devices.

Atomic layer MoS2-graphene van der Waals heterostructure nanomechanical resonators

Heterostructures play significant roles in modern semiconductor devices and micro/nanosystems in a plethora of applications in electronics, optoelectronics, and transducers. While state-of-the-art heterostructures often involve stacks of crystalline epi-layers each down to a few nanometers thick, the intriguing limit would be hetero-atomic-layer structures. Here we report the first experimental demonstration of freestanding van der Waals heterostructures and their functional nanomechanical devices. By stacking single-layer (1L) MoS₂ on top of suspended single-, bi-, tri- and four-layer (1L to 4L) graphene sheets, we realize an array of MoS₂-graphene heterostructures with varying thickness and size. These heterostructures all exhibit robust nanomechanical resonances in the very high frequency (VHF) band (up to ∼100 MHz). We observe that fundamental-mode resonance frequencies of the heterostructure devices fall between the values of graphene and MoS₂ devices. Quality (Q) factors of heterostructure resonators are lower than those of graphene but comparable to those of MoS₂ devices, suggesting interface damping related to interlayer interactions in the van der Waals heterostructures. This study validates suspended atomic layer heterostructures as an effective device platform and provides opportunities for exploiting mechanically coupled effects and interlayer interactions in such devices.

Controllable Synthesis of Atomically Thin Type-II Weyl Semimetal WTe2 Nanosheets: An Advanced Electrode Material for All-Solid-State Flexible Supercapacitors

Compared with 2D S-based and Se-based transition metal dichalcogenides (TMDs), Te-based TMDs display much better electrical conductivities, which will be beneficial to enhance the capacitances in supercapacitors. However, to date, the reports about the applications of Te-based TMDs in supercapacitors are quite rare. Herein, the first supercapacitor example of the Te-based TMD is reported: the type-II Weyl semimetal 1Td WTe₂ . It is demonstrated that single crystals of 1Td WTe₂ can be exfoliated into the nanosheets with 2-7 layers by liquid-phase exfoliation, which are assembled into air-stable films and further all-solid-state flexible supercapacitors. The resulting supercapacitors deliver a mass capacitance of 221 F g^-1 and a stack capacitance of 74 F cm^-3 . Furthermore, they also show excellent volumetric energy and power densities of 0.01 Wh cm^-3 and 83.6 W cm^-3 , respectively, superior to the commercial 4V/500 µAh Li thin-film battery and the commercial 3V/300 µAh Al electrolytic capacitor, in association with outstanding mechanical flexibility and superior cycling stability (capacitance retention of ≈91% after 5500 cycles). These results indicate that the 1Td WTe₂ nanosheet is a promising flexible electrode material for high-performance energy storage devices.

Large-area synthesis of high-quality monolayer 1T'-WTe2 flakes

Large-area growth of monolayer films of the transition metal dichalcogenides is of the utmost importance in this rapidly advancing research area. The mechanical exfoliation method offers high quality monolayer material but it is a problematic approach when applied to materials that are not air stable. One important example is 1T'-WTe2, which in multilayer form is reported to possess a large non saturating magnetoresistance, pressure induced superconductivity, and a weak antilocalization effect, but electrical data for the monolayer is yet to be reported due to its rapid degradation in air. Here we report a reliable and reproducible large-area growth process for obtaining many monolayer 1T'-WTe2 flakes. We confirmed the composition and structure of monolayer 1T'-WTe2 flakes using x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy and aberration corrected transmission electron microscopy. We studied the time dependent degradation of monolayer 1T'-WTe2 under ambient conditions, and we used first-principles calculations to identify reaction with oxygen as the degradation mechanism. Finally we investigated the electrical properties of monolayer 1T'-WTe2 and found metallic conduction at low temperature along with a weak antilocalization effect that is evidence for strong spin-orbit coupling.

Magnetoresistance and Hall resistivity of semimetal WTe2 ultrathin flakes

This article reports the characterization of WTe₂ thin flake magnetoresistance and Hall resistivity. We found it does not exhibit magnetoresistance saturation when subject to high fields, in a manner similar to their bulk characteristics. The linearity of Hall resistivity in our devices confirms the compensation of electrons and holes. By relating experimental results to a classic two-band model, the lower magnetoresistance values in our samples is demonstrated to be caused by decreased carrier mobility. The dependence of mobility on temperature indicates the main role of optical phonon scattering at high temperatures. Our results provide more detailed information on carrier behavior and scattering mechanisms in WTe₂ thin films.

Intrinsic Phonon Bands in High-Quality Monolayer T' Molybdenum Ditelluride

The topologically nontrivial and chemically functional distorted octahedral (T') transition-metal dichalcogenides (TMDCs) are a type of layered semimetal that has attracted significant recent attention. However, the properties of monolayer (1L) T'-TMDC, a fundamental unit of the system, are still largely unknown due to rapid sample degradation in air. Here we report that well-protected 1L CVD T'-MoTe₂ exhibits sharp and robust intrinsic Raman bands, with intensities about 1 order of magnitude stronger than those from bulk T'-MoTe₂. The high-quality samples enabled us to reveal the set of all nine even-parity zone-center optical phonons, providing reliable fingerprints for the previously elusive crystal. By performing light polarization and crystal orientation resolved scattering analysis, we can effectively distinguish the intrinsic modes from Te-metalloid-like modes A (∼122 cm^-1) and B (∼141 cm^-1), which are related to the sample degradation. Our studies offer a powerful nondestructive method for assessing sample quality and for monitoring sample degradation in situ, representing a solid advance in understanding the fundamental properties of 1L-T'-TMDCs.

Environmental Instability and Degradation of Single- and Few-Layer WTe2 Nanosheets in Ambient Conditions

The ambient environmental instability and degradation mechanism of single- and few-layer WTe₂ are investigated. Oxidation of W and Te atoms appears to be a main reason for degradation. Single-layer samples' Raman signals disappear within 20 min in air. Few-layer WTe₂ exhibits saturating degradation behavior: only the top layer WTe₂ is oxidized; the degraded layer can protect inner layers from further degradation.

The In-Plane Anisotropy of WTe2 Investigated by Angle-Dependent and Polarized Raman Spectroscopy

Tungsten ditelluride (WTe₂) is a semi-metallic layered transition metal dichalcogenide with a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various materials properties. We have systemically studied the in-plane anisotropy of Raman modes in few-layer and bulk WTe₂ by angle-dependent and polarized Raman spectroscopy (ADPRS). Ten Raman modes are clearly resolved. Their intensities show periodic variation with sample rotating. We identify the symmetries of the detected modes by quantitatively analyzing the ADPRS results based on the symmetry selection rules. Material absorption effect on the phonon modes with high vibration frequencies is investigated by considering complex Raman tensor elements. We also provide a rapid and nondestructive method to identify the crystallographic orientation of WTe₂. The crystallographic orientation is further confirmed by the quantitative atomic-resolution force image. Finally, we find that the atomic vibrational tendency and complexity of detected modes are also reflected in the shrinkage degree defined based on ADPRS, which is confirmed by corresponding density functional calculation. Our work provides a deep understanding of the interaction between WTe₂ and light, which will benefit in future studies about the anisotropic physical properties of WTe₂ and other in-plane anisotropic materials.

Large-scale arrays of single- and few-layer MoS2 nanomechanical resonators

We report the fabrication of large-scale arrays of suspended molybdenum disulfide (MoS2) atomic layers, as two-dimensional (2D) MoS2 nanomechanical resonators. We employ a water-assisted lift-off process to release chemical vapor deposited (CVD) MoS2 atomic layers from a donor substrate, followed by an all-dry transfer onto microtrench arrays. The resultant large arrays of suspended single- and few-layer MoS2 drumhead resonators (0.5-2 μm in diameter) offer fundamental resonances (f0) in the very high frequency (VHF) band (up to ∼120 MHz) and excellent figures-of-merit up to f0 × Q ≈ 3 × 10(10) Hz. A stretched circular diaphragm model allows us to estimate low pre-tension levels of typically ∼15 mN m(-1) in these devices. Compared to previous approaches, our transfer process features high yield and uniformity with minimal liquid and chemical exposure (only involving DI water), resulting in high-quality MoS2 crystals and exceptional device performance and homogeneity; and our process is readily applicable to other 2D materials.

The polarization-dependent anisotropic Raman response of few-layer and bulk WTe2under different excitation wavelengths

WTe₂, which is an orthorhombic semimetal crystallized in Td phase, exhibts distinct in-plane anisotropy. The Raman modes depict different anisotropic response by rotating the incident polarization under different excitation wavelengths.