Direct Visualization of Dark Interlayer Exciton Transport in Moiré Superlattices.

Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide heterostructures are expected to achieve high-temperature exciton condensation. However, the exciton degeneracy state is usually optically inactive due to the finite momentum of interlayer excitons. Experimental observation of dark interlayer excitons in moiré potentials remains challenging. Here we directly visualize the dark interlayer exciton transport in WS2/h-BN/WSe2 heterostructures using femtosecond transient absorption microscopy. We observe a transition from classical free exciton gas to quantum degeneracy by imaging temperature-dependent exciton transport. Below a critical degeneracy temperature, exciton diffusion rates exhibit an accelerating downward trend, which can be explained well by a nonlinear quantum diffusion model. These results open the door to quantum information processing and high-precision metrology in moiré superlattices.

High-Precision Intelligent Cancer Diagnosis Method: 2D Raman Figures Combined with Deep Learning.

Raman spectroscopy, as a label-free detection technology, has been widely used in tumor diagnosis. However, most tumor diagnosis procedures utilize multivariate statistical analysis methods for classification, which poses a major bottleneck toward achieving high accuracy. Here, we propose a concept called the two-dimensional (2D) Raman figure combined with convolutional neural network (CNN) to improve the accuracy. Two-dimensional Raman figures can be obtained from four transformation methods: spectral recurrence plot (SRP), spectral Gramian angular field (SGAF), spectral short-time Fourier transform (SSTFT), and spectral Markov transition field (SMTF). Two-dimensional CNN models all yield more than 95% accuracy, which is higher than the PCA-LDA method and the Raman-spectrum-CNN method, indicating that 2D Raman figure inputs combined with CNN may be one reason for gaining excellent performances. Among 2D-CNN models, the main difference is the conversion, where SRP is based on the structure of wavenumber series with the best performances (98.9% accuracy, 99.5% sensitivity, 98.3% specificity), followed by SGAF on the wavenumber series, SSTFT on wavenumber and intensity information, and SMTF on wavenumber position information. The inclusion of external information in the conversion may be another reason for improvement in the accuracy. The excellent capability shows huge potential for tumor diagnosis via 2D Raman figures and may be applied in other spectroscopy analytical fields.

Effects of polyacrylic acid molecular weights on V2C-MXene nanocoatings for obtaining ultralow friction and ultralow wear in an ambient working environment.

Two modified V2C-MXene nanocoatings are prepared through different molecular weights of polyacrylic acid (polyacrylic acid with ∼4 50 000 is marked as LPAA, and polyacrylic acid with ∼4 000 000 is marked as HPAA) and two-dimensional V2C-MXene. Their properties are characterized using a ball-on-disc tribometer, three-dimensional white-light interferometry topography images, optical microscope, Raman spectrometer, focused ion beam/scanning electron microscope and high-resolution transmission electron microscope/energy dispersive X-ray spectrometer (HRTEM/EDS). As a result, an ultralow friction (µ ≈ 0.073 ± 0.024) and an ultralow wear (3.41 × 10-7 mm3 N-1 m-1 for ball scar, and 7.49 × 10-8 mm3 N-1 m-1 for disc track) are achieved for the LPAA@V2C vs. steel ball system tested under 4 N in the air through tribo-physicochemical interactions. During the rubbing process, the LPAA@V2C nanocoating is transferred onto counter-bodies to form mixed-phase lubricative tribofilms. Monitoring via a HRTEM/EDS, the mixed-phase lubricative tribofilms are found to be mainly composed of amorphous carbon phases containing O and V and layered nano-debris along the sliding surface. The tribofilm's stable structure is the key to realizing ultralow friction and ultralow wear through the LPAA modification. These findings disclose that MXene-based nanomaterials can be applied for material engineering and mechanical engineering under common working conditions.

Raman spectroscopy as a potential diagnostic tool to analyse biochemical alterations in lung cancer.

Patient survival remains poor even after diagnosis in lung cancer cases, and the molecular events resulting from lung cancer progression remain unclear. Raman spectroscopy could be used to noninvasively and accurately reveal the biochemical properties of biological tissues on the basis of their pathological status. This study aimed at probing biomolecular changes in lung cancer, using Raman spectroscopy as a potential diagnostic tool. Herein, biochemical alterations were evident in the Raman spectra (region of 600-1800 cm-1) in normal and cancerous lung tissues. The levels of saturated and unsaturated lipids and the protein-to-lipid, nucleic acid-to-lipid, and protein-to-nucleic acid ratios were significantly altered among malignant tissues compared to normal lung tissues. These biochemical alterations in tissues during neoplastic transformation have profound implications in not only the biochemical landscape of lung cancer progression but also cytopathological classification. Based on this spectroscopic approach, classification methods including k-nearest neighbour (kNN) and support vector machine (SVM) were successfully applied to cytopathologically diagnose lung cancer with an accuracy approaching 99%. The present results indicate that Raman spectroscopy is an excellent tool to biochemically interrogate and diagnose lung cancer.

Influence of elastic property on the friction between atomic force microscope tips and 2D materials.

The relationship between the elastic property of solid materials and friction has been discussed and studied by theoretical calculation and analysis. In the present work, we perform an experimental study concerning this relationship. Atomic force microscope (AFM) scanning of four different transition metal dichalcogenides is conducted under different experimental conditions. It is found that materials with smaller vertical interlayer force constant, which also means smaller elasticity modulus, have larger friction. We attribute this phenomenon to larger elastic deformation in softer materials, which results in a larger obstacle to the motion of AFM tips.

Electrical friction modulation on MoS2 using electron beam radiation without electrostatic interactions.

Nanoscale friction under different electronic states and the corresponding friction controlling methods are both scientifically interesting and technologically important. However, friction measurements under electrical modulation are severely hampered by electrostatic forces induced by the charge-trapping effect. Therefore, in this study, we developed a new modulation method free from the charge-trapping effect through electron beam radiation; this method successfully modulated the friction between few-layer MoS2 and the silicon tip on atomic force microscopy. Friction on monolayer MoS2 increased under electron beam radiation. Strong correlations between the accelerating voltage, beam current, and friction force were found, and constant adhesion force demonstrate that the influence of static electricity was eliminated in this method. Excited electron states caused by electron injection could be possible mechanisms for friction modulation. However, the electron beam radiation had a negligible influence on the friction of bilayer MoS2. This study is the first of its kind, revealing the effect of electron beam radiation and electronic states on friction, which is important for the development of tribological theories and nanoelectromechanical systems, and offers a new electrical modulation method for friction tuning.

Superlubricity between a silicon tip and graphite enabled by the nanolithography-assisted nanoflakes tribo-transfer.

Superlubricity between a cone-shaped (sharp) silicon tip and graphite remains a challenge in the nanotribology field. In this paper, an efficient method of achieving superlubricity between a cone-shaped silicon tip and graphite was proposed. Graphite nanoflakes were transferred onto the silicon tip by repeatedly rubbing against the scratches produced by nanolithography on a graphite surface. The superlubricity between the graphite nanoflakes-wrapped tip and highly oriented pyrolytic graphite (HOPG) was attained, and the friction coefficient was reduced to 0.0007. At low normal loads, the frictional force was small and showed a strong correlation with the sliding angle, but as the normal load increased, this dependence gradually decreased. It was firstly found that the transferred graphite nanoflakes on the contact zone of the silicon tip could be transformed into amorphous carbon layers induced by the shear force and high pressure during the superlubricity test process.

Asymmetric Modulation on Exchange Field in a Graphene/BiFeO3 Heterostructure by External Magnetic Field.

Graphene, having all atoms on its surface, is favorable to extend the functions by introducing the spin-orbit coupling and magnetism through proximity effect. Here, we report the tunable interfacial exchange field produced by proximity coupling in graphene/BiFeO3 heterostructures. The exchange field has a notable dependence with external magnetic field, and it is much larger under negative magnetic field than that under positive magnetic field. For negative external magnetic field, interfacial exchange coupling gives rise to evident spin splitting for N ≠ 0 Landau levels and a quantum Hall metal state for N = 0 Landau level. Our findings suggest graphene/BiFeO3 heterostructures are promising for spintronics.

Thickness dependent friction on few-layer MoS2, WS2, and WSe2.

Nanoscale friction on two-dimensional (2D) materials is closely associated with their mechanical, electronic and photonic properties, which can be modulated through changing thickness. Here, we investigated the thickness dependent friction on few-layer MoS2, WS2, and WSe2 using atomic force microscope at ambient condition and found two different behavior. When a sharp tip was used, the regular behavior of decreasing friction with increasing thickness was reproduced. However, when a pre-worn and flat-ended tip was used, we observed an abnormal trend: on WS2 and WSe2, friction increased monotonically with thickness, while for MoS2, friction decreased from monolayer to bilayer and then subsequently increased with thickness. As suggested by the density functional theory calculation, we hypothesize that the overall frictional behavior is a competition between the puckering effect and the intrinsic energy corrugation within the compressive region. By varying the relative strength of the puckering effect via changing the tip shape, the dependence of friction on sample thickness can be tuned. Our results also suggest a potential means to measure intrinsic frictional properties of 2D materials with minimum impact from puckering.

Enhancing the interlayer adhesive force in twisted multilayer MoS2 by thermal annealing treatment.

Few-layer MoS2 has recently gained great attention owing to its remarkable mechanical and photoelectric properties, which are strongly influenced by the interactions and relative orientations between layers. Here, we report on Raman scattering measurements of twisted MoS2 flakes prepared by exfoliation and nondestructive transfer. Thermal annealing treatment can effectively enhance the interlayer coupling of twisted MoS2 and lead to a van der Waals (vdW) interaction between two stacked layers. We have roughly calculated the interlayer coupling force by a diatomic chain model (DCM) and found that the interlayer adhesive force increased by ∼20% compared with no-treatment samples. We additionally found that the non-Bernal stacking structure of MoS2 induces a weakening in the interlayer coupling. This study could promote the development of novel semiconductors, optoelectronic devices, and superlubricity materials.

Enhanced double resonance Raman scattering in multilayer graphene with broadband coherent anti-Stokes Raman spectroscopy.

Graphene's unique gapless band structure and remarkably large third-order optical susceptibility have drawn significant attention to its nonlinear optical response, particularly in the context of coherent anti-Stokes Raman scattering (CARS). Under the combined influence of phononic and electronic resonances, the CARS response of graphene has been observed to exhibit a distinctive feature of time-resolved dip-to-peak evolution. Here, we report a greatly enhanced double resonance Raman mode beyond the G mode of multi-layer graphene with broadband CARS measurements. The significant difference in the intensity ratio between CARS and SR for this mode may be attributed to the preferential activation of low-frequency phonons in the impulsive stimulated Raman scattering process (ISRS) and a lower dephasing rate. Our results build on a foundation towards a deeper exploration of the coherent Raman response of two-dimensional materials.

Out-of-Plane Ferroelectricity in Two-Dimensional 1T‴-MoS2 Above Room Temperature.

Two-dimensional (2D) molybdenum disulfide (MoS2), one of the most extensively studied van der Waals (vdW) materials, is a significant candidate for electronic materials in the post-Moore era. MoS2 exhibits various phases, among which the 1T‴ phase possesses noncentrosymmetry. 1T‴-MoS2 was theoretically predicted to be ferroelectric a decade ago, but this has not been experimentally confirmed until now. Here, we have prepared high-purity 2D 1T‴-MoS2 crystals and experimentally confirmed the room-temperature out-of-plane ferroelectricity. The noncentrosymmetric crystal structure in 2D 1T‴-MoS2 was convinced by atomically resolved transmission electron microscopic imaging and second harmonic generation (SHG) measurements. Further, the ferroelectric polarization states in 2D 1T‴-MoS2 can be switched using piezoresponse force microscopy (PFM) and electrical gating in field-effect transistors (FETs). The ferroelectric-to-paraelectric transition temperature is measured to be about 350 K. Theoretical calculations have revealed that the ferroelectricity of 2D 1T‴-MoS2 originates from the intralayer charge transfer of S atoms within the layer. The discovery of intrinsic ferroelectricity in the 1T‴ phase of MoS2 further enriches the properties of this important vdW material, providing more possibilities for its application in the field of next-generation electronic devices.

Control of Hybrid Exciton Lifetime in MoSe2/WS2 Moiré Heterostructures.

Hybrid excitons, characterized by their strong oscillation strength and long lifetimes, hold great potential as information carriers in semiconductors. They offer promising applications in exciton-based devices and circuits. MoSe2/WS2 heterostructures represent an ideal platform for studying hybrid excitons, but how to regulate the exciton lifetime has not yet been explored. In this study, layer hybridization is modulated by applying electric fields parallel or antiparallel to the dipole moment, enabling us to regulate the exciton lifetime from 1.36 to 4.60 ns. Furthermore, the time-resolved photoluminescence decay traces are measured at different excitation power. A hybrid exciton annihilation rate of 8.9 × 10-4 cm2 s-1 is obtained by fitting. This work reveals the effects of electric fields and excitation power on the lifetime of hybrid excitons in MoSe2/WS2 1.5° moiré heterostructures, which play important roles in high photoluminescence quantum yield optoelectronic devices based on transition-metal dichalcogenides heterostructures.

Solvatochromic effect on the photoluminescence of MoS2 monolayers.

The effect of surrounding solvents on the photoluminescence (PL) of MoS2 monolayers on Si/SiO2 substrates is studied. A redshift (up to -60 meV) is observed for MoS2 monolayers with nonhalogenated solvent surroundings. A blueshift (up to 60 meV) and intensity increase (2-50 times) are observed for monolayers with halogenated solvent surroundings.

Inspection of Line Defects in Transition Metal Dichalcogenides Using a Microscopic Hyperspectral Imaging Technique.

The line defects of two-dimensional (2D) transition metal dichalcogenides (TMDs) play a vital role in determining their device performance. In this work, a microscopic hyperspectral imaging technique based on differential reflectance was introduced for the online inspection of line defects in TMDs. Upon comparison of the measurement results of imaging and spectra, the relationship between optical contrast and differential reflectance spectra was established. A light selection method was proposed to optimize the optical contrast of line defects. Via application of an image processing algorithm, an automatic detection of the line defects with a classification accuracy of 95% was achieved for WS2, MoS2, and MoSe2. This work not only provides a microscopic hyperspectral imaging technique for detecting 2D material defects but also introduces a versatile design strategy for developing an advanced machine vision spectroscopic system.

Study on the Electrical Control Mechanism of Gas Occurrence in a Microscale Coal Matrix.

The study of the gas occurrence mechanism in a microscale coal matrix is the basis of coalbed methane (CBM) reservoir formation mechanism analysis and its exploration and development scheme design, which has important scientific and engineering significance. Currently, many researchers are focusing on a specific coal type to explore the macroscopic adsorption characteristics of gas occurrence. However, the research on the microscale gas-solid coupling mechanism is relatively rare and the electrical control mechanism of gas occurrence is not reported in detail. This study focuses on the electrical mechanism of microscale gas occurrence using physical simulation experiments and molecular dynamics analysis. This study clarifies the "gas adsorption-electrical properties-functional group" linkage mechanism and explores the macroscopic performance of the microscale gas occurrence mechanism using electrical properties. The study reveals the following: (1) the coal reservoirs exhibit a weak negative potential at the nanoscale, and the trends of surface potential (SP) and surface electrical charging density (SECD) are fluctuated with the degree of coal rank increases; (2) there is a good correlation between the SP, SECD values, and the relative content of functional groups; and (3) the charge density on the coal's microscopic surface influences their gas molecule attraction capacity, affecting the gas adsorption capacity of coal reservoirs at the macroscale. This study presents a theoretical foundation for establishing the molecular force field superposition mechanism of gas occurrence in microscale coal matrix and has broad application prospects in the macroscale numerical simulation of CBM development.

Imaging of Defect-Accelerated Energy Transfer in MoS2/hBN/WS2 Heterostructures.

Engineering energy transfer (ET) plays an important role in the exploration of novel optoelectronic devices. The efficient ET has been reasonably regulated using different strategies, such as dielectric properties, distance, and stacking angle. However, these strategies show limited degrees of freedom in regulation. Defects can provide more degrees of freedom, such as the type and density of defects. Herein, atomic-scale defect-accelerated ET is directly observed in MoS2/hBN/WS2 heterostructures by fluorescence lifetime imaging microscopy. Sulfur vacancies with different densities are introduced by controlling the oxygen plasma irradiation time. Our study shows that the ET rate can be increased from 1.25 to 6.58 ns-1 by accurately controlling the defect density. Also, the corresponding ET time is shortened from 0.80 to 0.15 ns, attributing to the participation of more neutral excitons in the ET process. These neutral excitons are transformed from trion excitons in MoS2, assisted by oxygen substitution at sulfur vacancies. Our insights not only help us better understand the role of defects in the ET process but also provide a new approach to engineer ET for further exploration of novel optoelectronic devices in van der Waals heterostructures.

Visualizing Ultrafast Defect-Controlled Interlayer Electron-Phonon Coupling in Van der Waals Heterostructures.

Engineering ultrafast interlayer coupling provides access to new quantum phenomena and novel device functionalities in atomically thin van der Waals heterostructures. However, due to all the atoms of a monolayer material being exposed at the interfaces, the interlayer coupling is extremely susceptible to defects, resulting in high energy dissipation through heat and low device performance. The study of how defects affect the interlayer coupling at ultrafast and atomic scales remains a challenge. Here, using femtosecond transient absorption microscopy, a new defect-induced ultrafast interlayer electron-phonon coupling pathway is identified in a WS2 /graphene heterostructure, involving a three-body collision between electrons in WS2 and both acoustic phonons and defects in graphene. This interaction manifests as the reduced defect-related Raman resonant activity and the accelerated electron-phonon scattering time from 7.1 to 2.4 ps. Furthermore, the ultrafast interlayer coupling process is directly imaged. These insights will advance the fundamental knowledge of heat dissipation in nanoscale devices, and enable new ways to dynamically manipulate electrons and phonons via defects in van der Waals heterostructures.

Friction-Induced Clustered Rearrangement at a PbS Quantum Dot Nanocoating via Long-Term Lubrication under an Atmosphere Environment.

We report a long-term lubrication for a PbS QD nanocoating sliding against bearing steel balls in the air. Through tribo-physchemical interactions, ultralow friction (µ ≈ 0.078 ± 0.0026) is achieved for the system tested under 1 N for 60 min. During the rubbing process, the tribo-film of the counterfacing ball is covered by a degraded PbS QD layer and amorphous mixed phase. Meanwhile, the disc track surface is composed of degraded PbS QD layers, clustered rearranged PbS QD districts, induced decomposed Pb-enriched multilayers, and an amorphous mixed phase via friction-induced structural transformation. The PbS QDs are transferred onto the sliding contacts to form a robust tribo-film, which is the key to realizing ultralow friction. Consequently, a long-term lubrication mechanism is attributed to the synergetic tribo-physchemical interaction along sliding interfaces upon shift, redirection, and decomposition of nanoparticles. These discoveries reveal QD-based nanolubricants in common working conditions for mechanical engineering.

Twist-angle-controlled neutral exciton annihilation in WS2 homostructures.

Exciton-exciton annihilation (EEA), as typical nonradiative recombination, plays an unpopular role in semiconductors. The nonradiative process significantly reduces the quantum yield of photoluminescence, which substantially inhibits the maximum efficiency of optoelectronic devices. Recently, laser irradiation, introducing defects and applying strain have become effective means to restrain EEA in two-dimensional (2D) transition metal dichalcogenides (TMDCs). However, these methods destroy the atomic structure of 2D materials and limit their practical applications. Fortunately, twisted structures are expected to validly suppress EEA through excellent interface quality. Here, we develop a non-destructive way to control EEA in WS2 homostructures by changing the interlayer twist angle, and systematically study the effect of interlayer twist angle on EEA, using fluorescence lifetime imaging measurement (FLIM) technology. Due to the large moiré potential at a small interlayer twist angle, the diffusion of excitons is hindered, and the EEA rate decreases from 1.01 × 10-1 cm2 s-1 in a 9° twisted WS2 homostructure to 4.26 × 10-2 cm2 s-1 in a 1° twisted WS2 homostructure. The results reveal the important role of the interlayer twist angle and EEA interaction in high photoluminescence quantum yield optoelectronic devices based on TMDC homostructures.