Wide Tunable Band Research Articles

Planar Multilayered 2D GeAs Schottky Photodiode for High-Performance Visible-Near-Infrared Photodetection.

Novel group IV - V 2D semiconductors (e.g., GeAs and SiAs) have arisen as an attractive candidate for broad-band photodetection and optoelectronic applications. This 2D family has a wide tunable band gap, excellent thermodynamic stability, and strong in-plane anisotropy. However, their photonic and optoelectronic properties have not been extensively explored so far. This work demonstrates a broadband back-to-back metal-semiconductor-metal (MSM) Schottky photodiode with asymmetric contact geometries based on multilayered 2D GeAs. The photodetector exhibited a Schottky barrier height (SBH) in the range of 0.40-0.49 eV. Additionally, it showed a low dark current of 1.8 nA with stable, reproducible, and excellent broadband spectral response from UV to optical communication wavelengths. The highest measured responsivity in the visible is 905 A/W at 660 nm wavelength and 98 A/W for 1064 nm near-infrared at an applied voltage of -3 V and zero back gate. Most notably, the planner configuration of this GeAs photodetector showed a low detector capacitance below 1.2 pf and low voltage operation (<1 V). The stability and broadband response of the device are promising for this 2D material's application in advanced optoelectronic devices.

Wavelength-Tunable Interlayer Exciton Emission at the Near-Infrared Region in van der Waals Semiconductor Heterostructures.

The wavelength-tunable interlayer exciton (IE) from layered semiconductor materials has not been achieved. van der Waals heterobilayers constructed using single-layer transition metal dichalcogenides can produce continuously changed interlayer band gaps, which is a feasible approach to achieve tunable IEs. In this work, we design a series of van der Waals heterostructures composed of a WSe2 layer with a fixed band gap and another WS2(1-x)Se2x alloy layer with continuously changed band gaps. The existence of IEs and tunable interlayer band gaps in these heterobilayers is verified by steady-state photoluminescence experiments. By tuning the composition of the WS2(1-x)Se2x alloy layers, we realized a very wide tunable band gap range of 1.97-1.40 eV with a wavelength-tunable IE emission range of 1.52-1.40 eV from the heterobilayers. The time-resolved photoluminescence experiments show the IE emission lifetimes over nanoseconds.

Point defects: key issues for II-oxides wide-bandgap semiconductors development

II-oxides wide-bandgap semiconductor, including the beryllium oxide (BeO), magnesium oxide (MgO), zinc oxide (ZnO), have large exciton binding energy (ZnO 60 meV, MgO 80 meV), high optical gain (ZnO 300 cm&lt;sup&gt;–1&lt;/sup&gt;) and wide tunable band gap (3.37 eV ZnO, MgO 7.8 eV, BeO 10.6 eV), which are the advantages of achieving low-threshold laser devices in the ultraviolet wavelength. It is also one of the important candidates to replace the traditional gas arc lamp (such as mercury lamp, deuterium lamp, excimer lamp, xenon lamp etc.) as the source of deep ultraviolet and even vacuum ultraviolet. Although, during the past decades, the ZnO-based pn homojunction devices have made great progress in the near-UV electroluminescence, but as the band gap broadens, the acceptor (or donor) ionization energy becomes higher (On the order of hundreds meV), which causing the room temperature equivalent thermal energy (26 meV) cannot make the impurities ionizing effectively. In addition, the self-compensation effect in the doping process further weakens the carrier yield. These above drawbacks have become the bottleneck that hinders II-oxides wide-bandgap semiconductor from achieving ultraviolet laser devices and expanding to shorter wavelengths, and are also a common problem faced by other wide-bandgap semiconductor materials. The regulation of the electrical and luminescent properties of materials often depends on the control of critical defect states. The rich point defects and their combination types make the II-oxides wide-bandgap semiconductors an important platform for studying defect physics. For the identification and characterization of specific point defects, it is expected to discover and further construct shallower defect states, which will provide a basis for the regulation of electrical performance. In this paper, recent research results of II-oxides wide-bandgap semiconductors will be described from three aspects: high-quality epitaxial growth, impurity and point defects, p-type doping and ultraviolet electroluminescence. Through the overview of related research works, II-oxides wide-bandgap semiconductors are clarified as deep ultraviolet light sources materials. Meanwhile, indicates that the key to the regulation of electrical performance in the future lies in the regulation of point defects.

Topology optimization of dielectric elastomers for wide tunable band gaps

Dielectric elastomer composites exhibit band gaps—ranges of frequencies at which elastic waves cannot propagate—that are tunable by electrostatically-controlled deformations. We show how topology optimization of such composites can widen these gaps and improve their tunability. Our case study focuses on anti-plane shear waves in fiber composites, across a designated frequency range. Employing a genetic algorithm approach, we maximize the gap width when the composite is actuated by prescribed electric fields, as well the relative change in the gap width with respect to an unactuated composite. We present optimization results for a composite whose constituents agree with commercial products. We compare these results with the performance of a composite of the same constituents arranged in circular fibers, to demonstrate the improvement achieved by the optimization. We expect that the performance of dielectric elastomer composites can be further improved, by employing a larger design space than the exemplary space in this study.

A Nanoscale Fano Resonator by Graphene-Gold Dipolar Interference

A nanoscale Fano resonator composed of a hybrid graphene disk-gold ring combination is reported in this letter. The inner narrow dipolar resonance of a discrete state induced by graphene interferes with the outside broad dipolar resonance of a continuum state induced by gold, thus forming an asymmetric Fano transparency within the absorption window. The metastructure exhibits a wide tunable band along with an excellent refractive index sensing capability of 2344 nm/RIU. The geometry adjustment modulates the spectral response giving chances to the equivalent of electromagnetically induce transparency. Moreover, the group index exceeds 760 within the transparency window enabling a potential use in slow light or light storage applications. The analytic analysis is in accordance with the numerical simulation results.

Dependence of electronic and optical properties of multilayer SiC and GeC on stacking sequence and external electric field

The electronic and optical properties of different stacked multilayer SiC and GeC are investigated with and without external electric field (EEF). The band gaps of multilayer SiC and GeC are found smaller than that of monolayer SiC and GeC due to the interlayer coupling effect. When EEF is applied, the direct band gaps (ΔK–M) of multilayer SiC and direct band gaps (ΔK–K) of multilayer GeC all turn to indirect band gaps (ΔK–G) as the band at the G point drops dramatically toward zero. The imaginary part ε2(ω)s of multilayer SiC and GeC show that new absorption peaks between 2–5eV appear when the polarized direction is perpendicular to the layer plane, and new absorption peaks in infrared region appear as the EEF is higher than a certain point when the polarized direction is parallel to the layer plane. Our calculations reveal that different stacking sequences and EEF can provide a wide tunable band structures and optical properties for multilayer SiC and GeC.

A frequency-reconfigurable triple-band antenna with lumped components for wireless applications

In this research, a new frequency-reconfigurable triple-band antenna for wireless applications achieved using lumped inductor is proposed. The each three working bands can be separately and continuously reconfigured by changing the values of the lumped inductors with the radiation patterns nearly unchanged. Besides, the working state can also be modulated to make the antenna operate at single-band/dual-band by selecting certain values of the lumped inductors. By introducing a via hole connecting the top layer patch with the bottom layer patch, a miniaturized size of 29 × 15 × 1.6 mm3 can be achieved. The designed antenna has wide tunable band and can effectively covering 2.4/5.2/5.8 GHz Wireless Local Area Network and 3.5/5.5 GHz Worldwide Interoperability for Microwave Access bands. Besides, the designed antenna can also be applied for GSM850, GSM900, DCS1800, PCS1900, GPS, and 3G bands. The measured results have good agreement with the simulated results. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1374–1379, 2015

Preparation and Wide Tunable Band-Gap Energies of PbTe Nanocrystals Using Glycerin as a Capping Ligand

PbTe nanocrystals (NCs) have been synthesized by a novel method at low temperature. Pb acetate being solved in glycerol and Te in TOP solution were used as the precursors for the preparation of PbTe nanocrystals. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UVvis absorption, Fourier transform infrared spectroscopy (FTIR). These data, together with analysis of the absorption spectra, allowed us to observe the size dependence of the peaks in the absorption spectrum. The size-dependent optical spectra of the PbTe nanocrystals exhibits wide tunable band gap energies varying from UV visible range to near IR region, which corresponds to a huge blue shift of 3.0 eV in comparison to the bulk counterpart. Keywords: 1. PbTe nanocrystals, 2. optical properties, 3. wide tunable band gap, 4. microstructure