Efficient immobilization and detoxification of gaseous elemental mercury by nanoflower/rod WSe2/halloysite composite: Performance and mechanisms

Gaseous mercury pollution control technologies with low stability and high releasing risks always face with great challenges. Herein, we developed one halloysite nanotubes (HNTs)-supported tungsten diselenide (WSe2) composite (WSe2/HNTs) by one-pot solvothermal approach, curing Hg0 from complicated flue gas (CFG) and reducing second environment risks. WSe2 as a monolayer with nano-flower structure and HNTs with rod shapes in the as-prepared sorbent exhibited outstanding synergy efficiency, resulting in exceptional performance for Hg0 removal with high capture capacity of 30.6 mg·g-1 and rate of 9.09 μg·g-1·min-1, which benefited from the high affinity of selenium and mercury (1 ×1045) and the adequate exposure of Se-terminated. The adsorbent showed beneficial tolerance to high amount of NOx and SOx. An online lab-built thermal decomposition system (TPD-AFS) was employed to explore Hg species on the used-sorbent, finding that the adsorbed-mercury species were principally mercury selenide (HgSe). Density functional theory calculations indicated that the hollow-sites were the major adsorption sites and exhibited excellent selectivity for Hg0, as well as HgSe generation needed to overcome the 0.32 eV energy barrier. The adsorbed mercury displayed high environmental stability after the leaching toxicity test, which significantly decreased its secondary environmental risks. With these advantages, WSe2/HNTs possess enormous potential to achieve the effective and permanent immobilization of gaseous mercury from CFG in the future.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e²/h in addition to quarter plateaus such as 0.25 × 2e²/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

Biolayer interferometry-SELEX for Shiga toxin antigenic-peptide aptamers & detection via chitosan-WSe2 aptasensor

We report biolayer interferometry based in-vitro selection technique (BLI-SELEX) for fishing out specific aptamers against E. coli Shiga toxin subtypes viz., stx1 & stx2 via epitopic peptides. BLI-SELEX is a one-step technique for rapidly generating aptamers against protein biomarkers in a microtiter plate format, obliterating the need for multiple enrichment rounds to harvest high-affinity aptamers as in conventional SELEX. Two unique aptamers selected against stx1 & stx2 with picomolar K_d (~47 pM & ~29 pM, respectively) were successfully used to fabricate voltammetric diagnostic assay via immobilization onto chitosan exfoliated 2D tungsten diselenide (WSe₂) nanosheet platform. These aptamers modified nanosensors showed high sensitivity of ~ 5.0 μA ng^-1 mL, a dynamic response range from 50 pg mL^-1 to 100 ng mL^-1, with a detection limit of 44.5 pg mL^-1 & 41.3 pg mL^-1 for stx subtypes, respectively and showed low cross-reactivity in spiked urine, serum and milk samples. The synergistic effect of selective aptamers & high sensitivity imparted by 2D transition metal dichalcogenide (TMD) highlights the superior potential of a fabricated nanosensor for bacterial toxin detection.

Light-matter interactions in two-dimensional layered WSe2 for gauging evolution of phonon dynamics

Phonon dynamics is explored in mechanically exfoliated two-dimensional WSe₂ using temperature-dependent and laser-power-dependent Raman and photoluminescence (PL) spectroscopy. From this analysis, phonon lifetime in the Raman active modes and phonon concentration, as correlated to the energy parameter E ₀, were calculated as a function of the laser power, P, and substrate temperature, T. For monolayer WSe₂, from the power dependence it was determined that the phonon lifetime for the in-plane vibrational mode was twice that of the out-of-plane vibrational mode for P in the range from 0.308 mW up to 3.35 mW. On the other hand, the corresponding relationship for the temperature analysis showed that the phonon lifetime for the in-plane vibrational mode lies within 1.42× to 1.90× that of the out-of-plane vibrational mode over T = 79 K up to 523 K. To provide energy from external stimuli, as T and P were increased, peak broadening in the PL spectra of the A-exciton was observed. From this, a phonon concentration was tabulated using the Urbach formulism, which increased with increasing T and P; consequently, the phonon lifetime was found to decrease. Although phonon lifetime decreased with increasing temperature for all thicknesses, the decay rate in the phonon lifetime in the monolayer (1L) material was found to be 2× lower compared to the bulk. We invoke a harmonic oscillator model to explain the damping mechanism in WSe₂. From this it was determined that the damping coefficient increases with the number of layers. The work reported here sheds fundamental insights into the evolution of phonon dynamics in WSe₂ and should help pave the way for designing high-performance electronic, optoelectronic and thermoelectric devices in the future.

All-Optical Tuning of Light in WSe2-Coated Microfiber

The tungsten diselenide (WSe₂) has attracted considerable interest owing to their versatile applications, such as p-n junctions, transistors, fiber lasers, spintronics, and conversion of solar energy into electricity. We demonstrate all-optical tuning of light in WSe₂-coated microfiber (MF) using WSe₂'s broad absorption bandwidth and thermo-optic effect. The transmitted optical power (TOP) can be tuned using external incidence pump lasers (405, 532, and 660 nm). The sensitivity under 405-nm pump light excitation is 0.30 dB/mW. A rise/fall time of ~ 15.3/16.9 ms is achieved under 532-nm pump light excitation. Theoretical simulations are performed to investigate the tuning mechanism of TOP. The advantages of this device are easy fabrication, all-optical control, high sensitivity, and fast response. The proposed all-optical tunable device has potential applications in all-optical circuitry, all-optical modulator, and multi-dimensionally tunable optical devices, etc.

Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe2 Transistors

We investigate the valley Hall effect (VHE) in monolayer WSe₂ field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n -doped MoS₂, we observe the VHE in WSe₂ in both the n - and p -doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p -doped regime. Assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

Spin-Conserving Resonant Tunneling in Twist-Controlled WSe2-hBN-WSe2 Heterostructures

We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe₂) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe₂ monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe₂ monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling. In heterostructures where the two WSe₂ monolayers have a 180° relative twist, such that the Brillouin zone of one layer is aligned with the time-reversed Brillouin zone of the opposite layer, the resonant tunneling between the layers is suppressed. These findings provide evidence that, in addition to momentum, the spin-valley degree of freedom is also conserved in vertical transport.

Steep-Slope WSe2 Negative Capacitance Field-Effect Transistor

P-type two-dimensional steep-slope negative capacitance field-effect transistors are demonstrated for the first time with WSe₂ as channel material and ferroelectric hafnium zirconium oxide in gate dielectric stack. F4-TCNQ is used as p-type dopant to suppress electron leakage current and to reduce Schottky barrier width for holes. WSe₂ negative capacitance field-effect transistors with and without internal metal gate structures and the internal field-effect transistors are compared and studied. Significant SS reduction is observed in WSe₂ negative capacitance field-effect transistors by inserting the ferroelectric hafnium zirconium oxide layer, suggesting the existence of internal amplification (∼10) due to the negative capacitance effect. Subthreshold slope less than 60 mV/dec (as low as 14.4 mV/dec) at room temperature is obtained for both forward and reverse gate voltage sweeps. Negative differential resistance is observed at room temperature on WSe₂ negative capacitance field-effect-transistors as the result of negative capacitance induced negative drain-induced-barrier-lowering effect.

Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe₂ domains on c-plane sapphire for the epitaxial growth of large area monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in H₂Se to promote surface diffusion of tungsten-containing species to form oriented WSe₂ islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe₂ islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe₂ monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe₂ domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe₂ monolayers in a scalable process that is suitable for large area device fabrication.

Long-Lived Hole Spin/Valley Polarization Probed by Kerr Rotation in Monolayer WSe2

Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

Thermal Oxidation of WSe2 Nanosheets Adhered on SiO2/Si Substrates

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe2 nanosheets on SiO2/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe2-SiO2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.

Photovoltaic and photothermoelectric effect in a double-gated WSe2 device

Tungsten diselenide (WSe2), a semiconducting transition metal dichalcogenide (TMDC), shows great potential as active material in optoelectronic devices due to its ambipolarity and direct bandgap in its single-layer form. Recently, different groups have exploited the ambipolarity of WSe2 to realize electrically tunable PN junctions, demonstrating its potential for digital electronics and solar cell applications. In this Letter, we focus on the different photocurrent generation mechanisms in a double-gated WSe2 device by measuring the photocurrent (and photovoltage) as the local gate voltages are varied independently in combination with above- and below-bandgap illumination. This enables us to distinguish between two main photocurrent generation mechanisms, the photovoltaic and photothermoelectric effect. We find that the dominant mechanism depends on the defined gate configuration. In the PN and NP configurations, photocurrent is mainly generated by the photovoltaic effect and the device displays a maximum responsivity of 0.70 mA/W at 532 nm illumination and rise and fall times close to 10 ms. Photocurrent generated by the photothermoelectric effect emerges in the PP configuration and is a factor of 2 larger than the current generated by the photovoltaic effect (in PN and NP configurations). This demonstrates that the photothermoelectric effect can play a significant role in devices based on WSe2 where a region of strong optical absorption, caused by, for example, an asymmetry in flake thickness or optical absorption of the electrodes, generates a sizable thermal gradient upon illumination.