37 research outputs found
Determining the Electronic Structure and Thermoelectric Properties of MoSâ/MoSeâ TypeâI Heterojunction by DFT and the Landauer Approach
The electronic structure and thermoelectric properties of MoX2 (X = S, Se) Van der Waals heterojunctions are reported, with the intention of motivating the design of electronic devices using such materials. Calculations indicate the proposed heterojunctions are thermodynamically stable and present a band gap reduction from 1.8 eV to 0.8 eV. The latter effect is highly related to interactions between metallic d-character orbitals and chalcogen p-character orbitals. The theoretical approach allows to predict a transition from semiconducting to semi-metallic behavior. The band alignment indicates a type-I heterojunction and band offsets of 0.2 eV. Transport properties show clear n-type nature and a high Seebeck coefficient at 300 K, along with conductivity values (Ï/Ï) in the order of 1020. Lastly, using the Landauer approach and ballistic transport, the proposed heterojunctions can be modeled as a channel material for a typical one-gate transistor configuration predicting subthreshold values of â60 mV decâ1 and fieldâeffect mobilities of â160 cmâ2 Vâ1 sâ1
Active tuning of high-Q dielectric metasurfaces
We demonstrate the active tuning of all-dielectric metasurfaces exhibiting
high-quality factor (high-Q) resonances. The active control is provided by
embedding the asymmetric silicon meta-atoms with liquid crystals, which allows
the relative index of refraction to be controlled through heating. It is found
that high quality factor resonances () can be tuned over more than
three resonance widths. Our results demonstrate the feasibility of using
all-dielectric metasurfaces to construct tunable narrow-band filters.Comment: 4 pages, 6 figure
The Piezoresponse in WOâ Thin Films Due to Nâ-Filled Nanovoids Enrichment by Atom Probe Tomography
Tungsten trioxide (WO3) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d33 = 35 pmVâ1 on WO3 thin films ~200 nm deposited using RF-sputtering onto alumina (Al2O3) substrate with post-deposit annealing treatment of 400 °C in a 3% H2/N2-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO3 with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N2-filled nanovoids that acts as strain centers producing a local deformation of the WO3 lattice into a non-centrosymmetric structure, which is related to piezoresponse observations
Active tuning of high-Q dielectric metasurfaces
We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances (Qâ=â270â±â30) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters
MoS<sub>2</sub> Thin Films for Photo-Voltaic Applications
The low dimensional chalcogenide materials with high band gap of ~1.8 eV, specially molybdenum di-sulfide (MoS2), have been brought much attention in the material science community for their usage as semiconducting materials to fabricate low scaled electronic devices with high throughput and reliability, this includes also photovoltaic applications. In this chapter, experimental data for MoS2 material towards developing the next generation of high-efficiency solar cells is presented, which includes fabrication of ~100 nm homogeneous thin film over silicon di-oxide (SiO2) by using radio frequency sputtering at 275 W at high vacuum~10â9 from commercial MoS2 99.9% purity target. The films were studied by means of scanning and transmission electron microscopy with energy disperse spectroscopy, grazing incident low angle x-ray scattering, Raman spectroscopy, atomic force microscopy, atom probe tomography, electrical transport using four-point probe resistivity measurement as well mechanical properties utilizing nano-indentation with continuous stiffness mode (CSM) approach. The experimental results indicate a vertical growth direction at (101)-MoS2 crystallites with stacking values of 7-laminates along the (002)-basal plane; principal Raman vibrations at E12g at 378 cmâ1 and A1g at 407 cmâ1. The hardness and elastic modulus values of H = 10.5 ± 0.1 GPa and E = 136 ± 2 GPa were estimated by CSM method from 0 to 90 nm of indenter penetration; as well transport measurements from â3.5 V to +3.5 V indicating linear Ohmic behavior
Dark-State-Based Low-Loss Metasurfaces with Simultaneous Electric and Magnetic Resonant Response
The realization of metamaterials or metasurfaces with simultaneous electric and magnetic response and low loss is generally very challenging at optical frequencies. Traditional approaches using nanoresonators made of noble metals, while suitable for the microwave and terahertz regimes, fail at frequencies above the near-infrared, due to prohibitive high dissipative losses and the breakdown of scaling resulting from the electron mass contribution (kinetic inductance) to the effective reactance of these plasmonic meta-atoms. The alternative route based on Mie resonances of high-index dielectric particles normally leads to structure sizes that tend to break the effective-medium approximation. Here, we propose a subwavelength dark-state-based metasurface, which enables configurable simultaneous electric and magnetic responses with low loss. Proof-of-concept metasurface samples, specifically designed around telecommunication wavelengths (i.e., λ â 1.5 ÎŒm), were fabricated and investigated experimentally to validate our theoretical concept. Because the electromagnetic field energy is localized and stored predominantly inside a dark resonant dielectric bound state, the proposed metasurfaces can overcome the loss issue associated with plasmonic resonators made of noble metals and enable scaling to very high operation frequency without suffering from saturation of the resonance frequency due to the kinetic inductance of the electrons
Light-Driven Nanoscale Vectorial Currents
Controlled charge flows are fundamental to many areas of science and
technology, serving as carriers of energy and information, as probes of
material properties and dynamics, and as a means of revealing or even inducing
broken symmetries. Emerging methods for light-based current control offer
promising routes beyond the speed and adaptability limitations of conventional
voltage-driven systems. However, optical manipulation of currents at nanometer
spatial scales remains a basic challenge and a key step toward scalable
optoelectronic systems and local probes. Here, we introduce vectorial
optoelectronic metasurfaces as a new class of metamaterial in which ultrafast
charge flows are driven by light pulses, with actively-tunable directionality
and arbitrary patterning down to sub-diffractive nanometer scales. In the
prototypical metasurfaces studied herein, asymmetric plasmonic nanoantennas
locally induce directional, linear current responses within underlying
graphene. Nanoscale unit cell symmetries are read out via polarization- and
wavelength-sensitive currents and emitted terahertz (THz) radiation. Global
vectorial current distributions are revealed by spatial mapping of the THz
field polarization, also demonstrating the direct generation of elusive
broadband THz vector beams. We show that a detailed interplay between
electrodynamic, thermodynamic, and hydrodynamic degrees of freedom gives rise
to these currents through rapidly-evolving nanoscale forces and charge flows
under extreme spatial and temporal localization. These results set the stage
for versatile patterning and optical control over nanoscale currents in
materials diagnostics, nano-magnetism, microelectronics, and ultrafast
information science