Strong enhancement of Raman scattering from a bulk-inactive vibrational mode in few-layer MoTe₂

Effective concentration ratio driven phase engineering of MBE-grown few-layer MoTe2

The polymorphic nature of ultrathin transition metal dichalcogenide (TMDC) materials makes the phase engineering of these materials an interesting field of investigation. Understanding the phase-controlling behavior of different growth parameters is crucial for obtaining large-area growth of a desirable phase. Here, we report a detailed study on the effect of growth parameters for engineering different phases of few-layer MoTe2 on sapphire using molecular beam epitaxy (MBE). Our study shows that the 2H phase of MoTe2 is stabilized in a certain regime of the flux ratio and growth temperature, while on both the lower, as well as, the higher sides of this regime, the 1T' phase is favored. The combined effect of growth parameters is explained using the effective concentration ratio of Te and Mo at the growth surface, which is found to be the primary factor governing the phase selectivity in few-layer MoTe2. XPS and KPFM investigations show the contribution of excess carrier doping in driving the phase change. The effect of the sapphire substrate on the crystallinity and phase-dependent morphological features has also been studied. This knowledge of versatile and controlled phase engineering of few-layer MoTe2 paves the way for fabricating large-scale hetero-phase-based metal-semiconductor heterostructures for future electronic and optoelectronic device applications.

2D Reconfigurable van der Waals Heterojunction for Logic Gate Circuits and Wide-Spectrum Photodetectors via Sulfur Substitution and Band Matching

The attractive physical properties of two-dimensional (2D) semiconductors in group IVA-VIA have been fully revealed in recent years. Combining them with 2D ambipolar materials to construct van der Waals heterojunctions (vdWHs) can offer tremendous opportunities for designing multifunctional electronic and optoelectronic devices, such as logic switching circuits, half-wave rectifiers, and broad-spectrum photodetectors. Here, an optimized SnSe0.75S0.25 is grown to design a SnSe0.75S0.25/MoTe2 vdWH for logic operation and wide-spectrum photodetection. Benefiting from the excellent gate modulation under the appropriate sulfur substitution and type-II band alignment, the device exhibits reconfigurable antiambipolar and ambipolar transfer behaviors at positive and negative source-drain voltage (Vds), enabling stable XNOR logic operation. It also features a gate-modulated positive and negative rectifying behavior with rectification ratios of 265:1 and 1:196, confirming its potential as half-wave logic rectifiers. Besides, the device can respond from visible to infrared wavelength up to 1400 nm. Under 635 nm illumination, the maximum responsivity of 1.16 A/W and response time of 657/500 μs are achieved at the Vds of -2 V. Furthermore, due to the strong in-plane anisotropic structure of SnSe0.75S0.25-alloyed nanosheet and narrow bandgap of 2H-MoTe2, it shows a broadband polarization-sensitive function with impressive photocurrent anisotropic ratios of 15.6 (635 nm), 7.0 (808 nm), and 3.7 (1310 nm). The direction along the maximum photocurrent can be reconfigurable depending on the wavelengths. These results indicate that our designed alloyed SnSe0.75S0.25/MoTe2 vdWH has reconfigurable logic operation and broadband photodetection capabilities in 2D multifunctional integrated circuits.

Phase transition in WSe2-xTex monolayers driven by charge injection and pressure: a first-principles study

Alloying strategies permit new probes for governing structural stability and semiconductor-semimetal phase transition of transition metal dichalcogenides (TMDs). However, the possible structure and phase transition mechanism of the alloy TMDs, and the effect of an external field, have been still unclear. Here, the enrichment of the Te content in WSe2-xTex monolayers allows for coherent structural transition from the H phase to the T' phase. The crystal orbital Hamiltonian population (COHP) uncovers that the bonding state of the H phase approaches the high-energy domain near the Fermi level as the Te concentration increases, posing a source of structural instability followed by a weakened energy barrier for the phase transition. In addition, the structural phase transition driven by charge injection opens up new possibilities for the development of phase-change devices based on atomic thin films. For WSe2-xTex monolayers with the H phase as the stable phase, the critical value of electron concentration triggering the phase transition decreases with an increase in the x value. Furthermore, the energy barrier from the H phase to the T' phase can be effectively reduced by applying tensile strain, which could favor the phase switching in electronic devices. This work provides a critical reference for controllable modulation of phase-sensitive devices from alloy materials with rich phase characteristics.

Automated statistical analysis of raman spectra of nanomaterials

Both at the academic and the industrial level, material scientists are exploring routes for mass production and functionalization of graphene, carbon nanotubes (CNT), carbon dots, 2D materials, and heterostructures of these. Proper application of the novel materials requires fast and thorough characterization of the samples. Raman spectroscopy stands out as a standard non-invasive technique capable of giving key information on the structure and electronic properties of nanomaterials, including the presence of defects, degree of functionalization, diameter (in the case of CNT), different polytypes, doping, etc. Here, we present a computational tool to automatically analyze the Raman spectral features of nanomaterials, which we illustrate with the example of CNT and graphene. The algorithm manages hundreds of spectra simultaneously and provides statistical information (distribution of Raman shifts, average values of shifts and relative intensities, standard deviations, correlation between different peaks, etc.) of the main spectral features defining the structure and electronic properties of the samples, as well as publication-ready graphical material.

Exploring the Surface Oxidation and Environmental Instability of 2H-/1T'-MoTe2 Using Field Emission-Based Scanning Probe Lithography

An unconventional approach for the resistless nanopatterning 2H- and 1T'-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3 /MoOx as well as TeO2 /TeOx . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T'-/(Td )-MoTe2 to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet.

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-Powered Programmable van der Waals Photodetectors with Nonvolatile Semifloating Gate

Tunable photovoltaic photodetectors are of significant relevance in the fields of programmable and neuromorphic optoelectronics. However, their widespread adoption is hindered by intricate architectural design and energy consumption challenges. This study employs a nonvolatile MoTe2/hexagonal boron nitride/graphene semifloating photodetector to address these issues. Programed with pulsed gate voltage, the MoTe2 channel can be reconfigured from an n+-n to a p-n homojunction and the photocurrent transition changes from negative to positive values. Scanning photocurrent mapping reveals that the negative and positive photocurrents are attributed to Schottky junction and p-n homojunction, respectively. In the p-n configuration, the device demonstrates self-driven, linear, rapid response (∼3 ms), and broadband sensitivity (from 405 to 1500 nm) for photodetection, with typical performances of responsivity at ∼0.5 A/W and detectivity ∼1.6 × 1012 Jones under 635 nm illumination. These outstanding photodetection capabilities emphasize the potential of the semifloating photodetector as a pioneering approach for advancing logical and nonvolatile optoelectronics.

Phase-dependent Friction on Exfoliated Transition Metal Dichalcogenides Atomic Layers

The fundamental aspects of energy dissipation on 2-dimensional (2D) atomic layers are extensively studied. Among various atomic layers, transition metal dichalcogenides (TMDs) exists in several phases based on their lattice structure, which give rise to the different phononic and electronic contributions in energy dissipation. 2H and 1T' (distorted 1T) phase MoS2 and MoTe2 atomic layers exfoliated on mica substrate are obtained and investigated their nanotribological properties with atomic force microscopy (AFM)/ friction force microscopy (FFM). Surprisingly, 1T' phase of both MoS2 and MoTe2 exhibits ≈10 times higher friction compared to 2H phase. With density functional theory analyses, the friction increase is attributed to enhanced electronic excitation, efficient phonon dissipation, and increased potential energy surface barrier at the tip-sample interface. This study suggests the intriguing possibility of tuning the friction of TMDs through phase transition, which can lead to potential application in tunable tribological devices.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Intrinsic 1[Formula: see text] phase induced in atomically thin 2H-MoTe2 by a single terahertz pulse

The polymorphic transition from 2H to 1[Formula: see text]-MoTe2, which was thought to be induced by high-energy photon irradiation among many other means, has been intensely studied for its technological relevance in nanoscale transistors due to the remarkable improvement in electrical performance. However, it remains controversial whether a crystalline 1[Formula: see text] phase is produced because optical signatures of this putative transition are found to be associated with the formation of tellurium clusters instead. Here we demonstrate the creation of an intrinsic 1[Formula: see text] lattice after irradiating a mono- or few-layer 2H-MoTe2 with a single field-enhanced terahertz pulse. Unlike optical pulses, the low terahertz photon energy limits possible structural damages. We further develop a single-shot terahertz-pump-second-harmonic-probe technique and reveal a transition out of the 2H-phase within 10 ns after photoexcitation. Our results not only provide important insights to resolve the long-standing debate over the light-induced polymorphic transition in MoTe2 but also highlight the unique capability of strong-field terahertz pulses in manipulating quantum materials.

Boosting internal quantum efficiency via ultrafast triplet transfer to 2H-MoTe2 film

Organic systems often allow to create two triplet spin states (triplet excitons) by converting an initially excited singlet spin state (a singlet exciton). An ideally designed organic/inorganic heterostructure could reach the photovoltaic energy harvest over the Shockley-Queisser (S-Q) limit because of the efficient conversion of triplet excitons into charge carriers. Here, we demonstrate the molybdenum ditelluride (MoTe₂)/pentacene heterostructure to boost the carrier density via efficient triplet transfer from pentacene to MoTe₂ using ultrafast transient absorption spectroscopy. We observe carrier multiplication by nearly four times by doubling carriers in MoTe₂ via the inverse Auger process and subsequently doubling carriers via triplet extraction from pentacene. We also verify efficient energy conversion by doubling the photocurrent in the MoTe₂/pentacene film. This puts a step forward to enhancing photovoltaic conversion efficiency beyond the S-Q limit in the organic/inorganic heterostructures.

Band Alignments, Electronic Structure, and Core-Level Spectra of Bulk Molybdenum Dichalcogenides (MoS2, MoSe2, and MoTe2)

A comprehensive study of bulk molybdenum dichalcogenides is presented with the use of soft and hard X-ray photoelectron (SXPS and HAXPES) spectroscopy combined with hybrid density functional theory (DFT). The main core levels of MoS₂, MoSe₂, and MoTe₂ are explored. Laboratory-based X-ray photoelectron spectroscopy (XPS) is used to determine the ionization potential (IP) values of the MoX₂ series as 5.86, 5.40, and 5.00 eV for MoSe₂, MoSe₂, and MoTe₂, respectively, enabling the band alignment of the series to be established. Finally, the valence band measurements are compared with the calculated density of states which shows the role of p-d hybridization in these materials. Down the group, an increase in the p-d hybridization from the sulfide to the telluride is observed, explained by the configuration energy of the chalcogen p orbitals becoming closer to that of the valence Mo 4d orbitals. This pushes the valence band maximum closer to the vacuum level, explaining the decreasing IP down the series. High-resolution SXPS and HAXPES core-level spectra address the shortcomings of the XPS analysis in the literature. Furthermore, the experimentally determined band alignment can be used to inform future device work.

Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe₂ by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe₂ flake that gradually transformed into n-MoTe₂ after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N₂) gas environment. Subsequently, we illuminated this n-MoTe₂ sample with DUV light in oxygen (O₂) gas and reversed its carrier polarity toward p-MoTe₂. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe₂ is challenging. The DUV + N₂ treatment significantly altered the polarity of MoTe₂ of different thicknesses from p-type to n-type under the DUV + N₂ treatment, but the DUV + O₂ treatment did not completely alter the polarity of thicker n-MoTe₂ flakes to p-type. In addition, we investigated the photoresponse of MoTe₂ after DUV light treatment in N₂ and O₂ gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe₂, we have shown that the response time of the DUV + O₂ treated p-MoTe₂ is faster than that of the pristine and doped n-MoTe₂ films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe₂ in optoelectronic devices.

Two-dimensional cobalt telluride as a piezo-tribogenerator

Two-dimensional (2D) materials have been shown to be efficient in energy harvesting. Here, we report the use of waste heat to generate electricity via the combined piezoelectric and triboelectric properties of 2D cobalt telluride (CoTe₂). The piezo-triboelectric nanogenerator (PTNG) produced an open-circuit voltage of ∼5 V under 1 N force and the effect of temperature in the range of 305-363 K shows a four-fold energy conversion efficiency improvement. The 2D piezo-tribogenerator shows excellent characteristics with a maximum voltage of ∼10 V, fast response time, and high responsivity. Density functional theory was used to gain further insights and validation of the experimental results. Our results could lead to energy harvesting approaches using 2D materials from various thermal sources and dissipating waste heat from electronic devices.

Picosecond electrical response in graphene/MoTe2 heterojunction with high responsivity in the near infrared region

Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity. Devices based on two-dimensional (2D) transition metal dichalcogenides can exhibit picosecond photoresponse speed. However, 2D materials naturally have low absorption, and when increasing thickness to gain higher responsivity, the response time usually slows to nanoseconds, limiting their photodetection performance. Here, by taking time-resolved photocurrent measurements, we demonstrated that graphene/MoTe2 van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction, which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd3As2 and implies a photodetection bandwidth as wide as 100 GHz. Furthermore, we found that an additional charge carrier transport channel provided by graphene can effectively decrease the photocurrent recombination loss to the entire device, preserving a high responsivity in the near-infrared region. Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.

Electronic and Near-Infrared-II Optical Properties of I-Doped Monolayer MoTe2: A First-Principles Study

Near-infrared-II (NIR-II, 1000-1700 nm) fluorescence imaging is widely used for in vivo biological imaging. With the unique electronic structures and capability of band-gap engineering, two-dimensional (2D) materials can be potential candidates for NIR-II imaging. Herein, a theoretical investigation of the electronic structure and optical properties of iodine (I)-doped monolayer MoTe₂ systems with different doping concentrations is carried out through simulations to explore their NIR optical properties. The results suggest that the emergence of impurity levels due to I doping effectively reduces the bandwidth of I-doped monolayer MoTe₂ systems, and the bandwidth decreases with the increase in the I doping concentration. Although the I and Mo atoms possess clear covalent-bonding features according to the charge density difference, impurity levels induced by the strong hybridization between the I 5p and Mo 4d orbitals cross the Fermi level, making the doped systems exhibit metallic behavior. In addition, with the increase in the I doping concentration, the energy required for electron transition from valence bands to impurity levels gradually decreases, which can be linked to the enhancement of the optical absorption in the red-shifted NIR-II region. Meanwhile, with a higher I doping concentration, the emission spectra, which are the product of the absorption spectra and quasi-Fermi distributions for electrons and holes, can be enhanced in the NIR-II window.

Unusually large exciton binding energy in multilayered 2H-MoTe2

Although large exciton binding energies of typically 0.6–1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe₂ with a conversion efficiency of 99%, which suggests strong Coulomb interaction. However, the origin of strong Coulomb interaction in multilayered 2H-MoTe₂, including the exciton binding energy, has not been elucidated to date. In this study, unusually large exciton binding energy is observed through optical spectroscopy conducted on CVD-grown 2H-MoTe₂. To extract exciton binding energy, the optical conductivity is fitted using the Lorentz model to describe the exciton peaks and the Tauc–Lorentz model to describe the indirect and direct bandgaps. The exciton binding energy of 4 nm thick multilayered 2H-MoTe₂ is approximately 300 meV, which is unusually large by one order of magnitude when compared with other multilayered TMD semiconductors such as 2H-MoS₂ or 2H-MoSe₂. This finding is interpreted in terms of small exciton radius based on the 2D Rydberg model. The exciton radius of multilayered 2H-MoTe₂ resembles that of monolayer 2H-MoTe₂, whereas those of multilayered 2H-MoS₂ and 2H-MoSe₂ are large when compared with monolayer 2H-MoS₂ and 2H-MoSe₂. From the large exciton binding energy in multilayered 2H-MoTe₂, it is expected to realize the future applications such as room-temperature and high-temperature polariton lasing.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF₂-mediated surface oxidation of 2H-MoTe₂ (alpha phase MoTe₂). MoTe₂ exposed to XeF₂ gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO _x ) on MoTe₂ regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe₂, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe₂ field-effect transistors (FETs) with a MoO _x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm² V^-1 s^-1 with a high on/off current ratio of ∼10⁵ than that of the MoTe₂ device with conventional metal contacts (0.07 cm² V^-1 s^-1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe₂ devices, applicable for 2D electronics.

Orbital Gating Driven by Giant Stark Effect in Tunneling Phototransistors

Conventional gating in transistors uses electric fields through external dielectrics that require complex fabrication processes. Various optoelectronic devices deploy photogating by electric fields from trapped charges in neighbor nanoparticles or dielectrics under light illumination. Orbital gating driven by giant Stark effect is demonstrated in tunneling phototransistors based on 2H-MoTe₂ without using external gating bias or slow charge trapping dynamics in photogating. The original self-gating by light illumination modulates the interlayer potential gradient by switching on and off the giant Stark effect where the d_z 2-orbitals of molybdenum atoms play the dominant role. The orbital gating shifts the electronic bands of the top atomic layer of the MoTe₂ by up to 100 meV, which is equivalent to modulation of a carrier density of 7.3 × 10¹¹ cm^-2 by electrical gating. Suppressing conventional photoconductivity, the orbital gating in tunneling phototransistors achieves low dark current, practical photoresponsivity (3357 AW^-1 ), and fast switching time (0.5 ms) simultaneously.

Optical Control of Multistage Phase Transition via Phonon Coupling in MoTe_{2}

The temporal characters of laser-driven phase transition from 2H to 1T^{'} has been investigated in the prototype MoTe_{2} monolayer. This process is found to be induced by fundamental electron-phonon interactions, with an unexpected phonon excitation and coupling pathway closely related to the nonequilibrium relaxation of photoexcited electrons. The order-to-order phase transformation is dissected into three substages, involving energy and momentum scattering processes from optical (A_{1}^{'} and E^{'}) to acoustic phonon modes [LA(M)] in subpicosecond timescale. An intermediate metallic state along the nonadiabatic transition pathway is also identified. These results have profound implications on nonequilibrium phase engineering strategies.

Low-power-consumption CMOS inverter array based on CVD-grown p-MoTe2 and n-MoS2

Two-dimensional (2D) semi-conductive transition metal dichalcogenides (TMDCs) have shown advantages for logic application. Complementary metal-oxide-semiconductor (CMOS) inverter is an important component in integrated circuits in view of low power consumption. So far, the performance of the reported TMDCs-based CMOS inverters is not satisfactory. Besides, most of the inverters were made of mechanically exfoliated materials, which hinders their reproducible production and large-scale integration in practical application. In this study, we demonstrate a practical approach to fabricate CMOS inverter arrays using large-area p-MoTe₂ and n-MoS₂, which are grown via chemical vapor deposition method. The current characteristics of the channel materials are balanced by atomic layer depositing Al₂O₃. Complete logic swing and clear dynamic switching behavior are observed in the inverters. Especially, ultra-low power consumption of ∼0.37 nW is achieved. Our work paves the way for the application of 2D TMDCs materials in large-scale low-power-consumption logic circuits.

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A practical approach to fabricate large-scale CMOS inverter arrays is demonstrated

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A method to balance the current characteristics of the channel materials is developed

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Complete logic swing and clear dynamic switching behavior are observed

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Ultra-low power consumption of ∼0.37 nW is achieved

Dual-Regulation of Defect Sites and Vertical Conduction by Spiral Domain for Electrocatalytic Hydrogen Evolution

Insufficient active sites and weak vertical conduction are the intrinsic factors that restrict the electrocatalytic HER for transition-metal dichalcogenides. As a prototype, we proposed a model of spiral MoTe₂ to optimize collectively the above issues. The conductive atomic force microscopy of an individual spiral reveals that the retentive vertical conduction irrespective of layer thickness benefits from the connected screw dislocation lines between interlayers. Theoretical calculations uncover that the regions near the edge step of the spiral structures more easily form Te vacancies and have lower ΔG_H * as extra active sites. A single spiral MoTe₂ -based on-chip microcell was fabricated to extract HER activity and achieved an ultrahigh current density of 3000 mA cm^-2 at an overpotential of 0.4 V, which is about two orders of magnitude higher than the exfoliated counterpart. Profoundly, this unusual spiral model will initiate a new pathway for triggering other inert catalytic reactions.

MoTe2 Field-Effect Transistors with Low Contact Resistance through Phase Tuning by Laser Irradiation

Due to their extraordinary electrical and physical properties, two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered promising for use in next-generation electrical devices. However, the application of TMD-based devices is limited because of the Schottky barrier interface resulting from the absence of dangling bonds on the TMDs’ surface. Here, we introduce a facile phase-tuning approach for forming a homogenous interface between semiconducting hexagonal (2H) and semi-metallic monoclinic (1T′) molybdenum ditelluride (MoTe₂). The formation of ohmic contacts increases the charge carrier mobility of MoTe₂ field-effect transistor devices to 16.1 cm² V⁻¹s⁻¹ with high reproducibility, while maintaining a high on/off current ratio by efficiently improving charge injection at the interface. The proposed method enables a simple fabrication process, local patterning, and large-area scaling for the creation of high-performance 2D electronic devices.

Photocarrier Dynamics in MoTe2 Nanofilms with 2H and Distorted 1T Lattice Structures

Molybdenum telluride (MoTe₂), an emerging layered two-dimensional (2D) material, possesses excellent phase-changing properties. Previous studies revealed its reversible transition between 2H and 1T' phases with a transition energy as small as 35 meV. Since 1T'-MoTe₂ is metallic, it can serve as an electrical contact for semiconducting 2H-MoTe₂-based optoelectronic devices. Here, the photocarrier dynamics in MoTe₂ nanofilms synthesized by a one-step method and with coexisting multiple phases are investigated by transient absorption measurements. Both the energy relaxation time and the recombination lifetime of the excitons are shorter in the 1T'-MoTe₂ compared to its 2H phase. These results provide information on the different photocarrier dynamical properties of these two phases, which is important for future 2D optoelectronic and phase-change electronic devices based on MoTe₂.

Ambipolar Channel p-TMD/n-Ga2 O3 Junction Field Effect Transistors and High Speed Photo-sensing in TMD Channel

Highly crystalline 2D/3D-mixed p-transition metal dichalcogenide (TMD)/n-Ga₂ O₃ heterojunction devices are fabricated by mechanical exfoliation of each p- and n-type material. N-type β-Ga₂ O₃ and p-type TMD separately play as a channel for junction field effect transistors (JFETs) with each type of carriers as well as materials for a heterojunction PN diode. The work thus mainly focuses on such ambipolar channel transistors with two different types of channel in a single device architecture. For more extended applications, the transparency of high energy band gap β-Ga₂ O₃ (E_g  ≈ 4.8 eV) is taken advantage of, firstly to measure the electrical energy gap of p-TMDs receiving visible or near infrared (NIR) photons through the β-Ga₂ O₃ . Next, the p-TMD/n-Ga₂ O₃ JFETs are put to high speed photo-sensing which is achieved from the p-TMD channel under reverse bias voltages on n-Ga₂ O₃ . The photo-switching cutoff frequency appears to be ≈16 and 29 kHz for visible red and NIR illuminations, respectively, on the basis of -3 dB photoelectric power loss. Such a high switching speed of the JFET is attributed to the fast transport of photo-carriers in TMD channels. The 2D/3D-mixed ambipolar channel JFETs and their photo-sensing applications are regarded novel, promising, and practically easy to achieve.

The optical response of artificially twisted MoS[Formula: see text] bilayers

Two-dimensional layered materials offer the possibility to create artificial vertically stacked structures possessing an additional degree of freedom-the interlayer twist. We present a comprehensive optical study of artificially stacked bilayers (BLs) MoS[Formula: see text] encapsulated in hexagonal BN with interlayer twist angle ranging from 0[Formula: see text] to 60[Formula: see text] using Raman scattering and photoluminescence spectroscopies. It is found that the strength of the interlayer coupling in the studied BLs can be estimated using the energy dependence of indirect emission versus the A[Formula: see text]-E[Formula: see text] energy separation. Due to the hybridization of electronic states in the valence band, the emission line related to the interlayer exciton is apparent in both the natural (2H) and artificial (62[Formula: see text]) MoS[Formula: see text] BLs, while it is absent in the structures with other twist angles. The interlayer coupling energy is estimated to be of about 50 meV. The effect of temperature on energies and intensities of the direct and indirect emission lines in MoS[Formula: see text] BLs is also quantified.

Tunable and Robust Near-Room-Temperature Intrinsic Ferromagnetism of a van der Waals Layered Cr-Doped 2H-MoTe2 Semiconductor with an Out-of-Plane Anisotropy

The intrinsically nonmagnetic feature of van der Waals (vdW) layered transition-metal dichalcogenide (TMDC) semiconductors limits the spintronic applications of these semiconductors. In this paper, we demonstrate a facile Te flux strategy to induce intrinsic ferromagnetism in the vdW layered 2H-MoTe₂ semiconductor by magnetic chromium (Cr) doping. The Curie temperature (T_c) and saturation magnetization (M_s) can be well tuned by adjusting the Cr doping concentration. A notable T_c up to 275 K can be achieved for the vdW layered Cr-doped 2H-MoTe₂ bulk crystals, which is much higher than that of recently reported van der Waals ferromagnetic semiconductors (T_c is mostly less than 70 K), in contrast to the diamagnetic feature of the pristine MoTe₂. Meanwhile, the highest M_s of the vdW layered Cr-doped 2H-MoTe₂ bulk crystals can reach 4.78 emu g^-1, which is stronger than most values reported for magnetic-element-doped van der Waals materials. In addition, all of the as-grown semiconducting Cr-doped 2H-MoTe₂ (Cr-2H-MoTe₂) single crystals display a large magnetic anisotropy with an out-of-plane easy axis of magnetization. The observed ferromagnetism in the Cr-2H-MoTe₂ has intrinsic characteristics, which can be mainly attributed to the spin polarization caused by Cr doping as confirmed by the density functional theory (DFT) calculations. Our approach offers an avenue to tune the ferromagnetism in the vdW layered semiconductor and explore its diverse spintronic and magnetoelectric applications.

Highly Sensitive, Ultrafast, and Broadband Photo-Detecting Field-Effect Transistor with Transition-Metal Dichalcogenide van der Waals Heterostructures of MoTe2 and PdSe2

Recently, van der Waals heterostructures (vdWHs) based on transition-metal dichalcogenides (TMDs) have attracted significant attention owing to their superior capabilities and multiple functionalities. Herein, a novel vdWH field-effect transistor (FET) composed of molybdenum ditelluride (MoTe2 ) and palladium diselenide (PdSe2 ) is studied for highly sensitive photodetection performance in the broad visible and near-infrared (VNIR) region. A high rectification ratio of 6.3 × 105 is obtained, stemming from the sharp interface and low Schottky barriers of the MoTe2 /PdSe2 vdWHs. It is also successfully demonstrated that the vdWH FET exhibits highly sensitive photo-detecting abilities, such as noticeably high photoresponsivity (1.24 × 105 A W-1 ), specific detectivity (2.42 × 1014 Jones), and good external quantum efficiency (3.5 × 106 ), not only due to the intra-TMD band-to-band transition but also due to the inter-TMD charge transfer (CT) transition. Further, rapid rise (16.1 µs) and decay (31.1 µs) times are obtained under incident light with a wavelength of 2000 nm due to the CT transition, representing an outcome one order of magnitude faster than values currently in the literature. Such TMD-based vdWH FETs would improve the photo-gating characteristics and provide a platform for the realization of a highly sensitive photodetector in the broad VNIR region.

Atmospheric-pressure CVD growth of two-dimensional 2H- and 1 T'-MoTe2films with high-performance SERS activity

Two-dimensional (2D) molybdenum ditelluride (MoTe₂) is a member of the transition-metal dichalcogenides family, which is an especially promising platform for surface-enhanced Raman scattering (SERS) applications, due to its excellent electronic properties. However, the synthesis of large-area highly crystalline 2D MoTe₂with controllable polymorphism is a huge challenge due to the small free energy difference (∼40 meV per unit cell) between semiconducting 2H-MoTe₂and semi-metallic 1 T'-MoTe₂. Herein, we report an optimized route for the synthesis of 2H- and 1 T'-MoTe₂films by atmospheric-pressure chemical vapor deposition. The SERS study of the as-grown MoTe₂films was carried out using methylene blue (MB) as a probe molecule. The Raman enhancement factor on 1 T'-MoTe₂was found to be three times higher than that on 2H-MoTe₂and the 1 T'-MoTe₂film is an efficient Raman-enhancing substrate that can be used to detect MB at nanomolar concentrations. Our study also imparts knowledge on the significance of a suitable combination of laser excitation wavelength and molecule-material platform for achieving ultrasensitive SERS-based chemical detection.

Modulation of MoTe2/MoS2 van der Waals heterojunctions for multifunctional devices using N2O plasma with an opposite doping effect

van der Waals layered heterojunctions have a variety of band offsets that open up possibilities for a wide range of novel and multifunctional devices. However, due to their poor pristine carrier concentrations and limited band modulation methods, multifunctional p-n heterojunctions are very difficult to achieve. In this report, we developed a highly effective N2O plasma process to treat MoTe2/MoS2 heterojunctions. This allowed us to adjust the hole and electron concentrations in the two materials independently and simultaneously. More importantly, for the first time, we were able to create opposite doping on the two sides of the junction through a single-step treatment. With a very wide doping range from pristine to degenerate levels, a MoTe2/MoS2 heterojunction can be modulated to behave as a forward rectifying diode with enhanced rectifying ratio and as a tunneling transistor with negative differential resistance at room temperature. The new approach provides an effective and generic doping scheme for heterojunctions to construct versatile and multifunctional electronic devices.

Seeded 2D epitaxy of large-area single-crystal films of the van der Waals semiconductor 2H MoTe2

The integration of two-dimensional (2D) van der Waals semiconductors into silicon electronics technology will require the production of large-scale, uniform, and highly crystalline films. We report a route for synthesizing wafer-scale single-crystalline 2H molybdenum ditelluride (MoTe₂) semiconductors on an amorphous insulating substrate. In-plane 2D-epitaxy growth by tellurizing was triggered from a deliberately implanted single seed crystal. The resulting single-crystalline film completely covered a 2.5-centimeter wafer with excellent uniformity. The 2H MoTe₂ 2D single-crystalline film can use itself as a template for further rapid epitaxy in a vertical manner. Transistor arrays fabricated with the as-prepared 2H MoTe₂ single crystals exhibited high electrical performance, with excellent uniformity and 100% device yield.

Promoting a Weak Coupling of Monolayer MoSe2 Grown on (100)-Faceted Au Foil

As a two-dimensional semiconductor with many physical properties, including, notably, layer-controlled electronic bandgap and coupled spin-valley degree of freedom, monolayer MoSe₂ is a strong candidate material for next-generation opto- and valley-electronic devices. However, due to substrate effects such as lattice mismatch and dielectric screening, preserving the monolayer's intrinsic properties remains challenging. This issue is generally significant for metallic substrates whose active surfaces are commonly utilized to achieve direct chemical or physical vapor growth of the monolayer films. Here, we demonstrate high-temperature-annealed Au foil with well-defined (100) facets as a weakly interacting substrate for atmospheric pressure chemical vapor deposition of highly crystalline monolayer MoSe₂. Low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a honeycomb structure of MoSe₂ with a quasi-particle bandgap of 1.96 eV, a value comparable with other weakly interacting systems such as MoSe₂/graphite. Density functional theory calculations indicate that the Au(100) surface exhibits the preferred energetics to electronically decouple from MoSe₂, compared with the (110) and (111) crystal planes. This weak coupling is critical for the easy transfer of monolayers to another host substrate. Our study demonstrates a practical means to produce high-quality monolayers of transition-metal dichalcogenides, viable for both fundamental and application studies.

A Library of Atomically Thin 2D Materials Featuring the Conductive-Point Resistive Switching Phenomenon

Non-volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few-layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX₂ , M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS₂ /MoS₂ ), and an atomically thin insulator (h-BN). These results indicate the universality of the phenomenon in 2D non-conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming-free characteristic. A dissociation-diffusion-adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive-point mechanism supported by first-principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

Symmetry induced phonon renormalization in few layers of 2H-MoTe2 transistors: Raman and first-principles studies

Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe₂ channel based field-effect transistors. While the [Formula: see text] and B_2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 10¹³/cm², A_1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe₂ that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B_2g modes as compared with the A_1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS₂ and those observed here in a few-layered MoTe₂ demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

Nonlinear Optical Imaging, Precise Layer Thinning, and Phase Engineering in MoTe2 with Femtosecond Laser

The control of layer thickness and phase structure in two-dimensional transition metal dichalcogenides (2D TMDCs) like MoTe₂ has recently gained much attention due to their broad applications in nanoelectronics and nanophotonics. Continuous-wave laser-based thermal treatment has been demonstrated to realize layer thinning and phase engineering in MoTe₂, but requires long heating time and is largely influenced by the thermal dissipation of the substrate. The ultrafast laser produces a different response but is yet to be explored. In this work, we report the nonlinear optical interactions between MoTe₂ crystals and femtosecond (fs) laser, where we have realized the nonlinear optical characterization, precise layer thinning, and phase transition in MoTe₂ using a single fs laser platform. By using the fs laser with a low fluence as an excitation light source, we observe the strong nonlinear optical signals of second-harmonic generation and four-wave mixing in MoTe₂, which can be used to identify the odd-even layers and layer numbers, respectively. With increasing the laser fluence to the ablation threshold (F_th), we achieve layer-by-layer removal of MoTe₂, while 2H-to-1T' phase transition occurs with a higher laser fluence (2F_th to 3F_th). Moreover, we obtain highly ordered subwavelength nanoripples on both the thick and few-layer MoTe₂ with a controlled fluence, which can be attributed to the fs laser-induced reorganization of the molten plasma. Our study provides a simple and efficient ultrafast laser-based approach capable of characterizing the structures and modifying the physical properties of 2D TMDCs.

Sustainable Nanosheet Antioxidants for Sepsis Therapy via Scavenging Intracellular Reactive Oxygen and Nitrogen Species

Sepsis is an aberrant systemic inflammatory response mediated by excessive production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Developing an efficient antioxidant therapy for sepsis via scavenging ROS and RNS remains a big challenge owing to the insufficient activity and sustainability of conventional antioxidants. Herein, biocompatible transition-metal dichalcogenide antioxidants with excellent scavenging activity and sustainability for H₂O₂, O₂^•-, OH^•, and nitric oxide are developed for effective sepsis treatment. WS₂, MoSe₂, and WSe₂ nanosheets exfoliated and functionalized with a biocompatible polymer effectively scavenge mitochondrial and intracellular ROS and RNS in inflammatory cells. Among the nanosheets, WS₂ most efficiently suppresses the excessive secretion of inflammatory cytokines along with scavenging ROS and RNS without affecting the expression levels of the anti-inflammatory cytokine and ROS-producing enzymes. The WS₂ nanosheets significantly improve the survival rate up to 90% for severely septic mice by reducing systemic inflammation. The pharmacokinetics suggests that the WS₂ nanosheets can be excreted from mice 3 days after intravenous injection. This work demonstrates the potential of therapeutic nanosheet antioxidants for effective treatment of ROS and RNS-related diseases.

Lattice vibration characteristics in layered InSe films and the electronic behavior of field-effect transistors

Understanding how temperature affects the structural and electronic properties for two-dimensional (2D) semiconductors could promote the application and development of nanoelectronic devices. Here, the temperature dependence of lattice structure for indium selenide (InSe) nanosheets and the corresponding electronic properties of 3 nm indium-deposited InSe field-effect transistors (FETs) are systematically demonstrated. Analyses of Raman spectra suggest that the difference of phonon frequency (Δω) for the A[Formula: see text] mode is found to be 3.14 cm^-1, which is larger than that of the E[Formula: see text] mode due to the stronger electron-phonon coupling for the A[Formula: see text] mode. The device performance based on indium-deposited InSe is systematically explained using Kelvin probe force microscopy (KPFM) and the predicted energy band structure. Furthermore, FETs based on temperature and variable thickness InSe flakes are designed as applicable devices. Our findings are of fundamental importance to explain the underlying physics in intrinsic InSe transistors and improve further applications.

Anisotropic electron-photon-phonon coupling in layered MoS2

Transition metal dichalcogenide, MoS2has attracted a lot of attention recently owing to its tunable visible range band gap, and anisotropic electronic and transport properties. Here, we report comprehensive inelastic light scattering measurements on both chemical vapor deposition grown (horizontally and vertically aligned) flakes, and mechanically exfoliated flakes of single crystal MoS2. We probe the anisotropic optical response by studying the polarization dependence intensity of the Raman active phonon modes as a function of different incident photon energy and flake thickness. Our polarization dependent Raman studies reveal strong anisotropic behavior reflected in the anomalous renormalization of the modes intensity as a function of flake thickness, phonons and photon energy. Our observations reflect the strong anisotropic light-matter interaction in this high crystalline symmetric layered MoS2system, especially for the in-plane vibrations, crucial for understanding as well as future applications of these materials.

Intercalation-Induced Disintegrated Layer-By-Layer Growth of Ultrathin Ternary Mo(Te1-xSx)2 Plates

Nanometer-thick transition-metal dichalcogenides (TMDs) have attracted increasing research interest because of their exotic physical properties, but their high-yield and large-scale synthesis remains a challenge for their practical device applications. In this study, we realize the high-yield synthesis of nanometer-thick single-crystalline Mo(Te_1-xS_x)₂ plates by a facile chemical vapor deposition method. Adding S powders in the precursors can result in the products varying from well-faceted MoTe₂ hexagonal plates to irregular Mo(Te_1-xS_x)₂ plates with randomly stacked nanometer-thick layer steps. Moreover, their lateral dimension increases from several μm for binary MoTe₂ to several tens of μm for ternary Mo(Te_1-xS_x)₂. More interestingly, such irregular Mo(Te_1-xS_x)₂ plates can form few layers by ultrasonic exfoliation. Our detailed electron microscopy analyses show that three kinds of S forms influence the ternary growth. In particular, elemental S₈ intercalations play an important role in the growth and exfoliation of ultrathin Mo(Te_1-xS_x)₂ plates. This study enriches the fundamental understanding of zero-valent intercalation in TMDs and provides a new insight into secure high-yield nanometer-thick TMDs, which is critical for practical applications.

Atomic-Precision Repair of a Few-Layer 2H-MoTe2 Thin Film by Phase Transition and Recrystallization Induced by a Heterophase Interface

2D semiconductors have emerged as promising candidates for post-silicon nanoelectronics, owing to their unique properties and atomic thickness. However, in the handling of 2D material, various forms of macroscopic damage, such as cracks, wrinkles, and scratches, etc., are usually introduced, which cause adverse effects on the material properties and device performance. Repairing such macroscopic damage is crucial for improving device performance and reliability, especially for large-scale 2D device arrays. Here, a method is demonstrated repair damage to few-layer 2H-MoTe₂ films with atomic precision, and its mechanism is elucidated. The repaired 2H-MoTe₂ inherits the lattice orientation of the adjacent original 2H-MoTe₂ , thereby forming an atomically perfect lattice at the repaired interface. The time-evolution experiments show that the interface between the 2H- and early formed 1T'-MoTe₂ plays an important role in the subsequent phase transition and recrystallization. Electrical measurements on the original MoTe₂ , repaired MoTe₂ , and cross-interface regions show unobservable differences, indicating that the repaired MoTe₂ has the same electrical quality as the original one and the interface does not introduce extra scattering centers for carrier transport. The findings provide an effective strategy for macroscopic damage repair of few-layer 2H-MoTe₂ , which paves the way for its practical application in advanced electronics and optoelectronics.

Polytypism in few-layer gallium selenide

Gallium selenide (GaSe) is one of the layered group-III metal monochalcogenides, which has an indirect bandgap in the monolayer and a direct bandgap in bulk unlike other conventional transition metal dichalcogenides (TMDs) such as MoX₂ and WX₂ (X = S and Se). Four polytypes of bulk GaSe, designated as β-, ε-, γ-, and δ-GaSe, have been reported. Since different polytypes result in different optical and electrical properties even with the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We performed polarized Raman measurements on GaSe and found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even with the same thickness due to different stacking sequences of the polytypes. By comparing the ultra-low-frequency Raman spectra with the theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the stacking sequences of trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.

Oxygen-Sensitive Layered MoTe2 Channels for Environmental Detection

The oxygen (O₂)-dependent resistance change of multilayered molybdenum ditelluride (MoTe₂) channels was characterized. A variation of the channel resistance could reproducibly determine relative O₂ content (denoted as the O₂ index). We found that Joule heating in a layered MoTe₂ field-effect transistor caused the O₂ index to decrease drastically from 100 to 12.1% in back gate modulation. Furthermore, Joule heating caused effective O₂ desorption from the MoTe₂ surface and repeatable O₂ detection by multilayered MoTe₂ channels was realized. This work not only explored the influence of O₂ on the electrical properties of multilayered MoTe₂ channels but also revealed that MoTe₂ channels are promising for sensing O₂ in an environmental condition.

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

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

Ultrafast carrier and phonon dynamics in few-layer 2H-MoTe2

By using femtosecond pump-probe spectroscopy with broadband detection from near-infrared to midinfrared, the carrier and phonon dynamics in few-layer 2H-MoTe₂ after ultrafast excitation have been investigated in detail. Immediately following the photoexcitation, an ultrafast relaxation of the generated hot carriers by releasing phonons is observed within hundreds of femtoseconds. The subsequent electron-hole recombination with a time constant of ∼1.5 ps is clearly identified and demonstrated to be mediated through a defect-assisted process. Furthermore, we confirm that the observed redshift of the exciton resonance energy on longer time scales arises from the ultrafast thermalization of the 2H-MoTe₂ lattice caused by the transfer of electronic excitation to the phonon system. As a result, the thermalization dynamics of the lattice within 2 ps and the following cooling process of the phonon system on the 100 ps time scale are directly monitored.

Continuously Tuning Electronic Properties of Few-Layer Molybdenum Ditelluride with in Situ Aluminum Modification toward Ultrahigh Gain Complementary Inverters

Semiconducting molybdenum ditelluride (2H-MoTe₂), a two-dimensional (2D) transition metal dichalcogenide, has attracted extensive research attention due to its favorable physical properties for future electronic devices, such as appropriate bandgap, ambipolar transport characteristic, and good chemical stability. The rational tuning of its electronic properties is a key point to achieve MoTe₂-based complementary electronic and optoelectronic devices. Herein, we demonstrate the dynamic and effective control of the electronic properties of few-layer MoTe₂, through the in situ surface modification with aluminum (Al) adatoms, with a view toward high-performance complementary inverter devices. MoTe₂ is found to be significantly electron doped by Al, exhibiting a continuous transport transition from p-dominated ambipolar to n-type unipolar with enhanced electron mobility. Using a spatially controlled Al doping technique, both p- and n-channels are established on a single MoTe₂ nanosheet, which gives complementary inverters with a record-high gain of ∼195, which stands out in the 2D family of materials due to the balanced p- and n-transport in Al-modified MoTe₂. Our studies coupled with the tunable nature of in situ modification enable MoTe₂ to be a promising candidate for high-performance complementary electronics.