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The strain in hybrid van der Waals heterostructures, made of two distinct two-dimensional van der Waals materials, offers an interesting handle on their corresponding electronic band structure. Such strain can be engineered by changing the relative crystallographic orientation between the constitutive monolayers, notably, the angular misorientation, also known as the "twist angle". By combining angle-resolved photoemission spectroscopy with density functional theory calculations, we investigate here the band structure of the WS2/graphene heterobilayer for various twist angles. Despite the relatively weak coupling between WS2 and graphene, we demonstrate that the resulting strain quantitatively affects many electronic features of the WS2 monolayers, including the spin-orbit coupling strength. In particular, we show that the WS2 spin-orbit splitting of the valence band maximum at K can be tuned from 430 to 460 meV. Our findings open perspectives in controlling the band dispersion of van der Waals materials. [ABSTRACT FROM AUTHOR]

We report pulsed laser deposition synthesis and characterization of polycrystalline Fe1-xCoxSi thin films on Si (111). X-ray diffraction, transmission electron, and atomic force microscopies reveal films to be dense, very smooth, and single phase with a cubic B20 crystal structure. Ferromagnetism with significant magnetic hysteresis is found for all films including nominally pure FeSi films in contrast to the very weak paramagnetism of bulk FeSi. For Fe1-xCoxSi this signifies a change from helimagnetism in bulk, to ferromagnetism in thin films. These ferromagnetic thin films are promising as a magnetic-silicide/silicon system for polarized current production, manipulation, and detection. [ABSTRACT FROM AUTHOR]

We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior. [ABSTRACT FROM AUTHOR]

Copper‐based metal chalcogenides are versatile materials used for a variety of electronic devices, yet realizing the full scope of copper‐based composite materials is hindered by limited understanding of their chemical properties. In this work, copper selenide thin films (TFs) are fabricated using radio frequency (RF) magnetron co‐sputtering to investigate the effects of composition and annealing temperature on the films. The surface composition of the TFs is examined using energy dispersive X‐Ray spectroscopy (EDS) and X‐Ray photoelectron spectroscopy (XPS). In the unannealed films, an increase in Se composition leads to smaller grain sizes and smoother surfaces. The influence of higher Se content on the dispersive surface free energy (SFE) is shown through contact angle measurements. Work functions (WFs), determined by Kelvin probe (KP) and ultraviolet photoelectron spectroscopy (UPS), rose with increasing Se content. After annealing, the TFs exhibited opposite trends. As the films transitioned from an amorphous to a crystalline phase due to annealing, their roughness significantly increased. [ABSTRACT FROM AUTHOR]

CVD grown MoSe2 monolayers were electrically characterized at room temperature in a field effect transistor (FET) configuration using an ionic liquid (IL) as the gate dielectric. During the growth, instead of using MoO3 powder, ammonium heptamolybdate was used for better Mo control of the source and sodium cholate added for lager MoSe2 growth areas. In addition, a high specific capacitance (∼7 μF/cm2) IL was used as the gate dielectric to significantly reduce the operating voltage. The device exhibited ambipolar charge transport at low voltages with enhanced parameters during n- and p-FET operation. IL gating thins the Schottky barrier at the metal/semiconductor interface permitting efficient charge injection into the channel and reduces the effects of contact resistance on device performance. The large specific capacitance of the IL was also responsible for a much higher induced charge density compared to the standard SiO2 dielectric. The device was successfully tested as an inverter with a gain of ∼2. Using a common metal for contacts simplifies fabrication of this ambipolar device, and the possibility of radiative recombination of holes and electrons could further extend its use in low power optoelectronic applications. [ABSTRACT FROM AUTHOR]

Germanium telluride (GeTe) is both polar and metallic, an unusual combination of properties in any material system. The large concentration of free-carriers in GeTe precludes the coupling of external electric field with internal polarization, rendering it ineffective for conventional ferroelectric applications and polarization switching. Here we investigate alternate ways of coupling the polar domains in GeTe to external electrical stimuli through optical second harmonic generation polarimetry and in situ TEM electrical testing on single-crystalline GeTe nanowires. We show that anti-phase boundaries, created from current pulses (heat shocks), invert the polarization of selective domains resulting in reorganization of certain 71o domain boundaries into 109o boundaries. These boundaries subsequently interact and evolve with the partial dislocations, which migrate from domain to domain with the carrier-wind force (electrical current). This work suggests that current pulses and carrier-wind force could be external stimuli for domain engineering in ferroelectrics with significant current leakage. [ABSTRACT FROM AUTHOR]

A facile transfer process for transition metal dichalcogenide WS2 flakes is reported and the effect of the underlying substrate on the flake properties is investigated using Raman spectroscopy. The flakes are transferred from their growth substrate using polymethyl methacrylate (PMMA) and a wet etch to allow the user to transfer the flakes to a final substrate using a microscope and micromanipulator combined with semi-transparent Kapton tape. The substrates used range from insulators such as industry standard high-k dielectric HfO2 and 'green polymer' parylene-C, to conducting chemical vapor deposition (CVD) grown graphene. Raman spectroscopy is used first to confirm the material quality of the transferred flakes to the substrates and subsequently to analyze and separate the effects arising from material transfer from those arising from interactions with the substrate. We observe changes in the Raman spectra associated with the interactions between the substrates in the flakes. These interactions affect both in-plane and out-of-plane modes in different ways depending on their sources, for example strain or surface charge. These changes vary with final substrate, with the strongest effects being observed for WS2 transferred onto graphene and HfO2, demonstrating the importance of understanding substrate interaction for fabrication of future devices. [ABSTRACT FROM AUTHOR]

We report on transport measurements in monolayer MoS2 devices, close to the bottom of the conduction band edge. These devices were annealed in situ before electrical measurements. This allows us to obtain good ohmic contacts at low temperatures, and to measure precisely the conductivity and mobility via four-probe measurements. The measured effective mobility up to μ eff = 180 cm2 V−1 s−1 is among the largest obtained in CVD-grown MoS2 monolayer devices. These measurements show that electronic transport is of the insulating type for σ ≤ 1.4 e 2/ h and n ≤ 1.7 × 1012 cm−2, and a crossover to a metallic regime is observed above those values. In the insulating regime, thermally activated transport dominates at high temperature (T > 120 K). At lower temperatures, conductivity is driven by Efros–Schklovkii variable range hopping in all measured devices, with a universal and constant hopping prefactor, that is a clear indication that hopping is not phonon-mediated. At higher carrier density, and high temperature, the conductivity is well modeled by the Boltzmann equation for a non-interacting Fermi gas, taking into account both phonon and impurity scatterings. Finally, even if this apparent metal-insulator transition can be explained by phonon-related phenomena at high temperature, the possibility of a genuine 2D MIT cannot be ruled out, as we can observe a clear power-law diverging localization length close to the transition, and a one-parameter scaling can be realized. [ABSTRACT FROM AUTHOR]

We characterize the effect of ferromagnetic nickel nanoparticles (size ∼ 6   n m ) on the magnetotransport properties of chemical-vapor-deposited (CVD) graphene. The nanoparticles were formed by thermal annealing of a thin Ni film evaporated on top of a graphene ribbon. The magnetoresistance was measured while sweeping the magnetic field at different temperatures, and compared against measurements performed on pristine graphene. Our results show that, in the presence of Ni nanoparticles, the usually observed zero-field peak of resistivity produced by weak localization is widely suppressed (by a factor of ∼3), most likely due to the reduction of the dephasing time as a consequence of the increase in magnetic scattering. On the other hand, the high-field magnetoresistance is amplified by the contribution of a large effective interaction field. The results are discussed in terms of a local exchange coupling, J ∼ 6   m e V , between the graphene π electrons and the 3d magnetic moment of nickel. Interestingly, this magnetic coupling does not affect the intrinsic transport parameters of graphene, such as the mobility and transport scattering rate, which remain the same with and without Ni nanoparticles, indicating that the changes in the magnetotransport properties have a purely magnetic origin. [ABSTRACT FROM AUTHOR]

The adaptable nature of the SARS-CoV-2 virus has led to the emergence of multiple viral variants of concern. This research builds upon a previous demonstration of sampling human hand odor to distinguish SARS-CoV-2 infection status in order to incorporate considerations of the disease variants. This study demonstrates the ability of human odor expression to be implemented as a non-invasive medium for the differentiation of SARS-CoV-2 variants. Volatile organic compounds (VOCs) were extracted from SARS-CoV-2-positive samples using solid phase microextraction (SPME) coupled with gas chromatography–mass spectrometry (GC–MS). Sparse partial least squares discriminant analysis (sPLS-DA) modeling revealed that supervised machine learning could be used to predict the variant identity of a sample using VOC expression alone. The class discrimination of Delta and Omicron BA.5 variant samples was performed with 95.2% (±0.4) accuracy. Omicron BA.2 and Omicron BA.5 variants were correctly classified with 78.5% (±0.8) accuracy. Lastly, Delta and Omicron BA.2 samples were assigned with 71.2% (±1.0) accuracy. This work builds upon the framework of non-invasive techniques producing diagnostics through the analysis of human odor expression, all in support of public health monitoring. [ABSTRACT FROM AUTHOR]

Measuring sodium ion concentration in breast milk can provide crucial health information for both mother and infant, including early signs of low-grade infection and reduced milk supply. Traditional sensing methods are slow, bulky, expensive, and require skilled operators. Here, we develop a coverslip-sized, high-precision lab-on-a-chip device that processes and detects sodium ions in human breast milk. The device uses micro-electrodialysis to extract sodium ions into a simple acceptor solution with 92 ± 3 % efficiency and employs a graphene ion-selective sensor for high-performance quantification. We demonstrate a straightforward calibration strategy, enabling the device to measure breast-milk sodium ion levels in 141 seconds, with accuracy comparable to inductively coupled plasma mass spectrometry. Our approach offers a promising pathway to efficient, point-of-care diagnosis of conditions associated with metal-ion levels in complex liquid-biopsy samples. • Lab-on-chip device for detecting sodium ions in breast milk at lab-level accuracy. • Integration of micro electrodialysis with graphene-based biosensing. • High-efficiency isolation and high-sensitivity quantification of sodium ions. [ABSTRACT FROM AUTHOR]

Since the beginning of the COVID-19 pandemic, there has been enormous interest in the development of measures that would allow for the swift detection of the disease. The rapid screening and preliminary diagnosis of SARS-CoV-2 infection allow for the instant identification of possibly infected individuals and the subsequent mitigation of the disease spread. Herein, the detection of SARS-CoV-2-infected individuals was explored using noninvasive sampling and low-preparatory-work analytical instrumentation. Hand odor samples were obtained from SARS-CoV-2-positive and -negative individuals. The volatile organic compounds (VOCs) were extracted from the collected hand odor samples using solid phase microextraction (SPME) and analyzed using gas chromatography coupled with mass spectrometry (GC-MS). Sparse partial least squares discriminant analysis (sPLS-DA) was used to develop predictive models using the suspected variant sample subsets. The developed sPLS-DA models performed moderately (75.8% (±0.4) accuracy, 81.8% sensitivity, 69.7% specificity) at distinguishing between SARS-CoV-2-positive and negative -individuals based on the VOC signatures alone. Potential markers for distinguishing between infection statuses were preliminarily acquired using this multivariate data analysis. This work highlights the potential of using odor signatures as a diagnostic tool and sets the groundwork for the optimization of other rapid screening sensors such as e-noses or detection canines. [ABSTRACT FROM AUTHOR]

Charge transport in ferroelectric (FE) gated graphene far from the Dirac point (DP) was studied in the temperature range 300 K < T < 350 K. A non-monotonic/monotonic/non-monotonic behavior in the conductivity [σ(T)] was observed as one moved away from the DP. As the gate polarization increased, additional impurity charges were compensated, which reduced charge scattering. The uncompensated charges doped graphene and σ(T) switched to a monotonic increase with increasing T. However, far from the DP, the polarization reached saturation, which resulted in still lower impurity charge scattering. The carrier concentration increased, and a non-monotonic response in σ(T) reappeared, which was attributed to phonon scattering. A theoretical model is presented that combined impurity charge and phonon scattering conduction mechanisms. The top gate polarizable FE provided a novel approach to investigate charge transport in graphene via controlled compensation of impurity charges, and in the process revealed non-monotonic behavior in σ(T) not previously seen in SiO2 back gated graphene devices. [ABSTRACT FROM AUTHOR]

In this work, we measure and tune simultaneously the vibration of a 1-nm thick MoS2 suspended monolayer with standard electrical excitation and optical techniques. At ambient temperature, we first investigate the strong parametric coupling between two different mechanical modes (ω1 and ω2). We demonstrate a high and quasi-linear tunability of the mode frequencies with the parametric pump voltage. Then, we couple the highly tunable main vibration (ω1) to a parametric pump frequency (ωp) to obtain a high number of sidebands at frequencies ω1 ± m ωp, driving the mechanical mode with a large external electrical force. This oscillating force, applied via the gate voltage, acts as a stress onto the MoS2 sheet. The obtained frequency comb has a large spectral band and contains up to 100 harmonics, with potential applications in quantum information processing and heat or sound transport. [ABSTRACT FROM AUTHOR]

Charge transport near the Dirac point (DP) was investigated in graphene using ferroelectric (FE) gating in the temperature range of 300 < T < 350 K. We observed that the conductivity (σ) near the DP had a positive temperature gradient that switched to a negative temperature gradient with increasing temperature. The switch to a negative temperature gradient shifted to higher temperatures and gradually weakened upon moving away from the DP. Impurity charge compensation via polarization of the FE together with a temperature-dependent graphene–impurity charge separation was proposed as being responsible for the non-monotonicity in σ(T). A self-consistent theory for graphene transport with impurity charge scattering and phonon scattering was used to analyze the results. Non-monotonic charge transport was also observed in the temperature dependence of the residual conductivity (σr). Theoretical analysis of both σ and σr revealed a temperature independent contribution of ∼ 1.16 e 2 h that is probably inherent to pristine graphene. [ABSTRACT FROM AUTHOR]

Neurological disorders such as epilepsy arise from disrupted brain networks. Our capacity to treat these disorders is limited by our inability to map these networks at sufficient temporal and spatial scales to target interventions. Current best techniques either sample broad areas at low temporal resolution (e.g. calcium imaging) or record from discrete regions at high temporal resolution (e.g. electrophysiology). This limitation hampers our ability to understand and intervene in aberrations of network dynamics. Here we present a technique to map the onset and spatiotemporal spread of acute epileptic seizures in vivo by simultaneously recording high bandwidth microelectrocorticography and calcium fluorescence using transparent graphene microelectrode arrays. We integrate dynamic data features from both modalities using non-negative matrix factorization to identify sequential spatiotemporal patterns of seizure onset and evolution, revealing how the temporal progression of ictal electrophysiology is linked to the spatial evolution of the recruited seizure core. This integrated analysis of multimodal data reveals otherwise hidden state transitions in the spatial and temporal progression of acute seizures. The techniques demonstrated here may enable future targeted therapeutic interventions and novel spatially embedded models of local circuit dynamics during seizure onset and evolution. Driscoll, Rosch et al. design transparent graphene-based surface probes to achieve simultaneous electrophysiological recording and calcium imaging of epileptic seizures in mice. This method could be used to investigate circuit dynamics during seizure onset and evolution with high temporal and spatial resolution. [ABSTRACT FROM AUTHOR]

Monolayer MoS2 crystals investigated in this work were grown via chemical vapor deposition on Si/SiO2 substrates. Using a wet KOH etch, these crystals were transferred onto the edge of a freshly cleaved p-Si/SiO2 wafer where they formed mechanically robust heterojunctions at the p-Si/MoS2 interface. Electrical characterization of the device across the junction yielded an asymmetric I–V response similar to that of a p-n diode. The I–V response was electrostatically tunable via an ionic liquid gel gate. This is the first report demonstrating reversible gate control of the p-Si/MoS2 diode current by several orders of magnitude while lowering its turn-on voltage. Fermi energy level shifts within the MoS2 bandgap by the gate was believed to be responsible for the observed effects. The ease of fabrication, low operating voltages (<±2 V), and moderately high throughput currents (∼1 µA) are attractive features of this diode, especially for use in sensors and power saving electronics. [ABSTRACT FROM AUTHOR]

Due to the large surface to volume ratio of two-dimensional (2D) materials, the electrical performance of MoS2 is very sensitive to extrinsic ambient conditions. Post treatments are favorable to improve or recover the electrical performance of MoS2. In this work, the influence of post forming gas annealing (FGA) and bis(trifluoromethane) sulfonamide (TFSI) treatment on the electrical performance of monolayer top-gated MoS2 transistors is investigated. A negative shift of threshold voltage and an improvement in electron mobility are observed for both the post FGA and TFSI treatment. However, post TFSI treatment is more effective than the post FGA treatment in improving the electron mobility and gate controllability. Linear transmission-line-model measurement results indicate that the post FGA treatment is preferable in reducing the contact resistance, while post TFSI treatment is more effective in reducing the sheet resistance of MoS2. Raman and x-ray photoelectron spectroscopy reveal that water desorbs from MoS2 after the FGA and TFSI treatment and S vacancies are introduced into MoS2 after FGA. The results show that post TFSI treatment can be a simple and effective approach to improve the electrical property of MoS2 and may be used for other S-containing 2D transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]

All cells release low molecular weight organic compounds that possess finite vapor pressures at body and/or ambient temperatures. These volatile organic compounds (VOCs) may possess an odor and can be found emanating from all body fluids. As cells turn malignant, analysis of changes in these VOCs can provide insight into cancer onset and diagnosis. Previous studies have demonstrated that dogs can be trained to distinguish ovarian cancer tissues of various stages and grades from normal ovarian tissue and other gynecological malignancies with sensitivity and specificity over 95%. When trained on biopsied tissue, dogs were able to detect the VOC disturbances in peripheral blood samples with the same accuracy. Building on these earlier studies, we examined the VOCs emanating from plasma samples from primary ovarian cancer patients, patients with benign reproductive tract growths, and healthy controls. We used a three-pronged sensor approach to analyze the VOCs from plasma: canines trained on tissue and plasma samples, analysis using solid phase microextraction gas chromatography–mass spectrometry, and novel single stranded DNA-coated carbon nanotube sensor field effect transistors. Each of the three experimental approaches used in this study provided preliminary evidence that plasma from ovarian cancer patients emits a volatile odor signature that can be distinguished from the VOCs of patients with benign ovarian tumors and controls. Our results provide optimism that a diagnostic approach based on the analysis of the VOC odor signature of ovarian cancer is achievable. [ABSTRACT FROM AUTHOR]

Simpler and more rapid approaches for therapeutic drug-level monitoring are highly desirable to enable use at the point-of-care. We have developed an all-electronic approach for detection of theHIVdrug tenofovir based on scalable fabrication of arrays of graphene field-effect transistors (GFETs) functionalized with a commercially available DNA aptamer. The shift in the Dirac voltage of the GFETs varied systematically with the concentration of tenofovir in deionized water, with a detection limit less than 1 ng/mL. Tests against a set of negative controls confirmed the specificity of the sensor response. This approach offers the potential for further development into a rapid and convenient point-of-care tool with clinically relevant performance. [ABSTRACT FROM AUTHOR]

Phase-change materials undergo rapid and reversible crystalline-to-amorphous structural transformation and are being used for nonvolatile memory devices. However, the transformation mechanism remains poorly understood. We have studied the effect of electrical pulses on the crystalline-to-amorphous phase change in a single-crystalline Ge2Sb2Te5 (GST) nanowire memory device by in situ transmission electron microscopy. We show that electrical pulses produce dislocations in crystalline GST, which become mobile and glide in the direction of hole-carrier motion. The continuous increase in the density of dislocations moving unidirectionally in the material leads to dislocation jamming, which eventually induces the crystalline-to-amorphous phase change with a sharp interface spanning the entire nanowire cross section. The dislocation-templated amorphization explains the large on/off resistance ratio of the device. [ABSTRACT FROM AUTHOR]

We present experimental evidence of electronic and optical interlayer resonances in graphene van der Waals heterostructure interfaces. Using the spectroscopic mode of a low-energy electron microscope (LEEM), we characterized these interlayer resonant states up to 10 eV above the vacuum level. Compared with nontwisted, AB-stacked bilayer graphene (AB BLG), an ≈0.2  Å increase was found in the interlayer spacing of 30° twisted bilayer graphene (30°-tBLG). In addition, we used Raman spectroscopy to probe the inelastic light-matter interactions. A unique type of Fano resonance was found around the D and G modes of the graphene lattice vibrations. This anomalous, robust Fano resonance is a direct result of quantum confinement and the interplay between discrete phonon states and the excitonic continuum. [ABSTRACT FROM AUTHOR]