Controlling the morphology and wavelength of self-assembled coaxial GaAs/Ga(As)Sb/GaAs single quantum-well nanowires.

Antimonide-based ternary III-V nanowires (NWs) provide a tunable bandgap over a wide range, and the GaAsSb material system has prospective applications in the 1.3-1.55 µm spectral range of optical communications. In this paper, GaAs/Ga(As)Sb/GaAs single quantum well (SQW) NWs were grown on Si(111) substrates by molecular beam epitaxy (MBE). In addition, the morphologies and tunable wavelengths of the GaAs/Ga(As)Sb/GaAs SQWs were adjusted by interrupting the Ga droplets and changing the growth temperatures and V/III ratios. The four morphologies of the GaAs/Ga(As)Sb/GaAs SQW NWs were observed by scanning electron microscopy (SEM). The microscale lattice structure related to the incorporation of Sb in GaAs/Ga(As)Sb/GaAs SQWs was studied by Raman spectroscopy. The crystal quality of the GaAs/Ga(As)Sb/GaAs SQW NWs was researched by X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, the optical properties of the GaAs/Ga(As)Sb/GaAs SQWs were investigated by photoluminescence (PL) spectroscopy. The PL spectra showed the peak emission wavelength range of ∼818 nm (GaAs) to ∼1628 nm (GaSb) at 10 K. This study provides an approach to enhance the effective control of the morphology, structure and wavelength of quantum well or core-shell NWs.

Enhancing Performance of a GaAs/AlGaAs/GaAs Nanowire Photodetector Based on the Two-Dimensional Electron-Hole Tube Structure.

Here, we design and engineer an axially asymmetric GaAs/AlGaAs/GaAs (G/A/G) nanowire (NW) photodetector that operates efficiently at room temperature. Based on the I-type band structure, the device can realize a two-dimensional electron-hole tube (2DEHT) structure for the substantial performance enhancement. The 2DEHT is observed to form at the interface on both sides of GaAs/AlGaAs barriers, which constructs effective pathways for both electron and hole transport in reducing the photocarrier recombination and enhancing the device photocurrent. In particular, the G/A/G NW photodetector exhibits a responsivity of 0.57 A/W and a detectivity of 1.83 × 1010 Jones, which are about 7 times higher than those of the pure GaAs NW device. The recombination probability has also been significantly suppressed from 81.8% to 13.2% with the utilization of the 2DEHT structure. All of these can evidently demonstrate the importance of the appropriate band structure design to promote photocarrier generation, separation, and collection for high-performance optoelectronic devices.

Influence of the depletion region in GaAs/AlGaAs quantum well nanowire photodetector.

In semiconductor nanowire (NW) photodetectors, the Schottky barrier formed by the contact between metal and semiconductor can act as a depletion layer. For NW structures with a smaller diameter, the depletion region is especially important to the carrier transport. We prepared a GaAs/AlGaAs quantum well NW photodetector with a two-dimensional electron-hole tube, in which the two-dimensional hole tube (2DHT) formed by the inner layer of GaAs and AlGaAs has the most important role in the regulation of carriers. By adjusting the bias voltage to vary the depth of the depletion region, we have confirmed the influence of the depletion region in a 2DHT. A significant inflection point was found in the responsivity-voltage curve at 1.5 V. By combining the depletion region and 2DHT, the responsivity of the fabricated device was increased by 18 times to 0.199 A W-1 and the detectivity is increased by 5 times to 5.8 × 1010 Jones, compared to the pure GaAs NW photodetector. Reasonable combination of depletion layer and 2DHT was proved to promote high-performance NW photodetector.

Ultrafast carrier relaxation dynamics of photoexcited GaAs and GaAs/AlGaAs nanowire array.

Femtosecond optical pump-probe spectroscopy is employed to elucidate the ultrafast carrier nonradiative relaxation dynamics of bare GaAs and a core-shell GaAs/AlGaAs semiconductor nanowire array. Different from the single nanowire conventionally used for the study of ultrafast dynamics, a simple spin coating and peeling off method was performed to prepare transparent organic films containing a vertical oriented nanowire array for transient absorption measurement. The transient experiment provides the direct observation of carrier thermalization, carrier cooling, thermal dissipation and band-gap energy evolutions along with the carrier relaxations. Carrier thermalization occurs within sub-0.5 ps and proceeds almost independently on the AlGaAs-coating, while the time constants of carrier cooling and thermal dissipation are increased by an order of magnitude due to the AlGaAs-coating effect. The concomitant band-gap evolutions in GaAs and GaAs/AlGaAs include an initial rapid red-shift in thermalization period, followed by a slow blue and/or red shift in carrier cooling, and then by an even slower blue shift in thermal dissipation. The evolution is explained by the competition of band-gap renormalization, plasma screening and band-filling. These findings are significant for understanding the basic physics of carrier scattering, and also for the development of flexible optoelectronic devices.

Analysis of the influence and mechanism of sulfur passivation on the dark current of a single GaAs nanowire photodetector.

Nanowire photodetectors, which have the advantages of fast response and high photoelectric conversion efficiency, can be widely applied in various industries. However, the rich surface states result in large dark current and can hinder the development of high-performance nanowire photodetectors. In this paper, the influence and mechanism of sulfur surface passivation on the dark current of a single GaAs nanowire photodetector have been studied. The dark current is significantly reduced by about 30 times after surface passivation. We confirm that the origin of the reduction of dark current is the decrease in the surface state density. As a result, a single GaAs nanowire photodetector with low dark current of 7.18 × 10-2 pA and high detectivity of 9.04 × 1012 cmHz0.5W-1 has been achieved. A simple and convenient way to realize high-performance GaAs-based photodetectors has been proposed.

[Optical Properties Investigation of p-Type ZnO Film Based on Doping by Diffusion].

The main purpose of this paper is to investigate the optical properties of p-type ZnO film based on P doping. ZnO film was grown by Atomic layer deposition (ALD) on InP subsrate in this experiment, and phosphorus diffused into ZnO lattice by annealing treatment at different temperature (500, 700 °C). The optical properties of samples were investigated by photoluminescence (PL) spectroscopy, which indicated that the annealing temperature is the important factor influencing the phosphorus diffusion doping. The low-temperature PL spectra of the sample which annealed at 700 °C for 1 h exhibited acceptor related emission peaks located at 3.351, 3.311, 3.246 and 3.177 eV, which were attributed to A °X, FA, DAP and DA-1LO, respectively. The acceptor binding energy is estimated to be about 122 meV, which is agreed with the theoretic values in phosphorus-doped ZnO films. In this paper, through thermal diffusion method to realize the p-type doped ZnO thin films, it solved the main problems which limited the development of ZnO based optoelectronic devices, and has an important significance for the development of the ZnO semiconductor materials and ZnO based photoelectric device.

[Preparation of Mg(x)Zn(1-x)O nanofibers using PVP nanofibers as templates by atom layer deposition and their optical properties].

In the present paper, Mg(x)Zn(1-x)O nanofibers with different doping concentration were prepared by atom layer deposition (ALD) using polyvinyl pyrrolidone (PVP) nanofibers as template, which were synthesized by electrospinning. The influence of different Mg doping concentration on the structure and optical properties of composite nanofibers was investigated. The samples were characterized by field emission scanning electron microscopy (FESEM), ultraviolet visible (UV-Vis) absorption spectroscopy and photoluminescence (PL) spectra. The doping of Mg did not change the morphologies of ZnO nanofibers, the morphologies of all the samples were very similar while the diameter of Mg(x)Zn(1-x)O-PVP composite nanofibers became larger after doping. With the increase in the Mg doping concentration, the absorption edge shifted to larger energy side, revealing the band gap tenability of Mg(x)Zn(1-x)O nanofibers. Meanwhile, a significant blue shift of the UV emission peak from 377 to 362 nm was observed in PL spectra. Compared with ZnO-PVP composite nanofibers, the UV emission intensity of Mg(x)Zn(1-x)O-PVP composite nanofibers was much stronger. Component control Mg(x)Zn(1-x)O nanofibers can be synthesized by this method. The doping of Mg elements in ZnO can effectively improve the band gap of ZnO-PVP nanofibers and the intensity of UV emission.

[Preparation of MgxZn1-xO/Au/MgxZn1-xO multilayer transparent conductive film and studies of its photoelectric properties].

In the present paper, MgxZn1-xO and MgxZn1-xO/Au/MgxZn1-xO multilayer structures of transparent conductive film were prepared by the simple operation of sol-gel and RF magnetron sputtering method on quartz substrate respectively and then they were annealed. The surface, electrical, crystal and optical properties of the films at different annealing temperature were determined by UV-Vis spectrophotometer, X-ray diffraction, photoluminescence and Hall effect, respectively. The influence of annealing temperature on the films was also investigated. The testing results indicated that the films with good c-axis orientation presented hexagonal wurtzite structure. With increasing Mg components, the optical band gap of ZnO thin film increased gradually. There was an obvious blue shift phenomenon in PL spectrum and absorption spectrum line. But the electrical properties of the films declined. In MgxZn1-xO/Au/MgxZn1-xO multilayer structure of thin film samples, the existence of Au interlining led to the poor optical properties of thin film, and the light transmittance in the ultraviolet region was 60%. Compared with MgxZn1-xO film, the electrical properties of MgxZn1-xO/Au/MgxZn1-xO multilayer structure of transparent conductive film were improved, the resistivity and migration rate were significantly increased. In addition, high temperature annealing treatment could effectively improve the crystal quality of thin film and further improve the electrical characteristics of the samples. After the annealing treatment at 500 °C, migration rate of the film reached to 40.9 cm2 · 1 Vs(-1) while the resistivity was 0.0057 Ω · cm. Due to the rising of temperature, the crystal size increased from 25.1 to 32.4 nm to reduce the mobility of the film. Therefore, MgxZn1-xO/Au/MgxZn1-xO multilayer structure of transparent conductive film played an important role in promoting the ZnO transparent conductive film application in deep ultraviolet devices.

Room-Temperature Near-Infrared Lasing from GaAs/AlGaAs Core-Shell Nanowires Based on Random Cavity.

Room-temperature lasing based on low-dimensional GaAs nanowires (NWs) is one of the most critical and challenging issues in realizing near-infrared lasers for nanophotonics. In this article, the random lasing characteristics based on GaAs NW arrays have been discussed theoretically. According to the simulation, GaAs/AlGaAs core-shell NWs with an optimal diameter, density, and Al content in the shell have been grown. Systematic morphological and optical characterizations were carried out. It is found that the GaAs NWs with the additional growth of the AlGaAs shell exhibit improved emission by about 2 orders of magnitude at low temperatures, which can be attributed to the suppression of crystal defects. At room temperature, lasing was observed with a threshold around 70.16 mW/cm2, and the random lasing mechanism was discussed in detail. This work is of great significance for the design of random cavities based on semiconductor NWs, which is important for optoelectronic integration.

Controlling the random lasing action from GaAs/AlGaAs axial heterostructure nanowire arrays.

Controlling the random lasing action from disordered media is important to obtain customizable lasers with unprecedented properties. In this paper, systematic investigations of random scattering based on GaAs/AlGaAs axial heterostructure nanowire (NW) arrays are presented. By manipulating the diameter and density of GaAs/AlGaAs axial heterostructure NWs during growth, different types of random lasers (Anderson localized and delocalized random lasers) have been successfully realized. The threshold, Q factor, and spatial coherence of these two types of lasers are experimentally discussed and analyzed. Finally, a proof-of-concept demonstration of speckle-free imaging based on the NW lasers has been conducted. This research enables the tunability of random lasers with exceptional performance and lays the foundation for achieving random lasing control.

The Effect of the Solution Flow and Electrical Field on the Homogeneity of Large-Scale Electrodeposited ZnO Nanorods.

In this paper, we demonstrate the significant impact of the solution flow and electrical field on the homogeneity of large-scale ZnO nanorod electrodeposition from an aqueous solution containing zinc nitrate and ammonium nitrate, primarily based on the X-ray fluorescence results. The homogeneity can be enhanced by adjusting the counter electrode size and solution flow rate. We have successfully produced relatively uniform nanorod arrays on an 8 × 10 cm2 i-ZnO-coated fluorine-doped tin oxide (FTO) substrate using a compact counter electrode and a vertical stirring setup. The as-grown nanorods exhibit similar surface morphologies and dominant, intense, almost uniform near-band-edge emissions in different regions of the sample. Additionally, the surface reflectance is significantly reduced after depositing the ZnO nanorods, achieving a moth-eye effect through subwavelength structuring. This effect of the nanorod array structure indicates that it can improve the utilization efficiency of light reception or emission in various optoelectronic devices and products. The large-scale preparation of ZnO nanorods is more practical to apply and has an extremely broad application value. Based on the research results, it is feasible to prepare large-scale ZnO nanorods suitable for antireflective coatings and commercial applications by optimizing the electrodeposition conditions.

Neuroprotective Strategies for Stroke by Natural Products: Advances and Perspectives.

Cerebral ischemic stroke is a disease with high prevalence and incidence. Its management focuses on rapid reperfusion with intravenous thrombolysis and endovascular thrombectomy. Both therapeutic strategies reduce disability, but the therapy time window is short, and the risk of bleeding is high. Natural products (NPs) have played a key role in drug discovery, especially for cancer and infectious diseases. However, they have made little progress in clinical translation and pose challenges to the treatment of stroke. Recently, with the investigation of precise mechanisms in cerebral ischemic stroke and the technological development of NP-based drug discovery, NPs are addressing these challenges and opening up new opportunities in cerebral stroke. Thus, in this review, we first summarize the structure and function of diverse NPs, including flavonoids, phenols, terpenes, lactones, quinones, alkaloids, and glycosides. Then we propose the comprehensive neuroprotective mechanism of NPs in cerebral ischemic stroke, which involves complex cascade processes of oxidative stress, mitochondrial damage, apoptosis or ferroptosis-related cell death, inflammatory response, and disruption of the blood-brain barrier (BBB). Overall, we stress the neuroprotective effect of NPs and their mechanism on cerebral ischemic stroke for a better understanding of the advances and perspective in NPs application that may provide a rationale for the development of innovative therapeutic regimens in ischemic stroke.

GaAs Nanowire Photodetectors Based on Au Nanoparticles Modification.

A high-performance GaAs nanowire photodetector was fabricated based on the modification of Au nanoparticles (NPs). Au nanoparticles prepared by thermal evaporation were used to modify the defects on the surface of GaAs nanowires. Plasmons and Schottky barriers were also introduced on the surface of the GaAs nanowires, to enhance their light absorption and promote the separation of carriers inside the GaAs nanowires. The research results show that under the appropriate modification time, the dark current of GaAs nanowire photodetectors was reduced. In addition, photocurrent photodetectors increased from 2.39 × 10-10 A to 1.26 × 10-9 A. The responsivity of GaAs nanowire photodetectors correspondingly increased from 0.569 A∙W-1 to 3.047 A∙W-1. The reasons for the improvement of the photodetectors' performance after modification were analyzed through the energy band theory model. This work proposes a new method to improve the performance of GaAs nanowire photodetectors.

Photoresponse improvement of mixed-dimensional 1D-2D GaAs photodetectors by incorporating constructive interface states.

Mixed-dimensional optoelectronic devices bring new challenges and opportunities over the design of conventional low-dimensional devices. In this work, we develop unreported mixed-dimensional GaAs photodetectors by utilizing 1D GaAs nanowires (NWs) and 2D GaAs non-layered sheets (2DNLSs) as active device materials. The fabricated photodetector exhibits a responsivity of 677 A W-1 and a detectivity of 8.69 × 1012 cm Hz0.5 W-1 under 532 nm irradiation, which are already much better than those of state-of-the-art low-dimensional GaAs photodetectors. It is found that this unique device structure is capable of converting the notoriously harmful surface states of NWs and 2DNLSs into their constructive interface states, which contribute to the formation of quasi-type-II band structures and electron wells in the device channel for the substantial performance enhancement. More importantly, these interface states are demonstrated to be insensitive to ambient environments, indicating the superior stability of the device. All these results evidently illustrate a simple but effective way to utilize the surface states of nanomaterials to achieve the high-performance photodetectors.

Emission characteristics variation of GaAs0.92Sb0.08/Al0.3Ga0.7As strained multiple quantum wells caused by rapid thermal annealing.

Rapid thermal annealing is an effective way to improve the optical properties of semiconductor materials and devices. In this paper, the emission characteristics of GaAs0.92Sb0.08/Al0.3Ga0.7As multiple quantum wells, which investigated by temperature-dependent photoluminescence, are adjusted through strain and interfacial diffusion via rapid thermal annealing. The light-hole (LH) exciton emission and the heavy-hole (HH) exciton emission are observed at room temperature. After annealing, the LH and HH emission peaks have blue shift. It can be ascribed to the variation of interfacial strain at low annealing temperature and the interfacial diffusion between barrier layer and well layer at high annealing temperature. This work is of great significance for emission adjustment of strained multiple quantum wells.

Optical characteristics of GaAs/GaAsSb/GaAs coaxial single quantum-well nanowires with different Sb components.

III-V ternary alloy quantum-wells have become a hot topic in recent years. Especially, GaAs/GaAsSb quantum wells have attracted increasing attention due to their numerous applications in the field of near-infrared optoelectronic devices. With the further reduction of dimensions, GaAs/GaAsSb nanowires show many special properties compared to their quantum well structures. In this work, GaAs/GaAs1-x Sb x /GaAs coaxial single quantum-well nanowires with different Sb composition were grown by molecular beam epitaxy. The band structure and the optical properties were investigated through power-dependent and temperature-dependent photoluminescence measurement. It has been found that a deeper quantum well is created with the increase of Sb component. Thanks to the deeper quantum well, more effective electron confinement has been realized, the emission from the sample can still be detected up to room temperature. The different trend of peak position and shape at various temperatures also supports the improved temperature stability of the samples. These results will be beneficial for the design of alloy quantum wells, and will facilitate the development of alloy quantum-well based devices.

Improved Optical Property and Lasing of ZnO Nanowires by Ar Plasma Treatment.

ZnO nanowires play a very important role in optoelectronic devices due to the wide bandgap and high exciton binding energy. However, for one-dimensional nanowire, due to the large surface to volume ratio, surface traps and surface adsorbed species acts as an alternate pathway for the de-excitation of carriers. Ar plasma treatment is a useful method to enhance the optical property of ZnO nanowires. It is necessary to study the optical properties of ZnO nanowires treated by plasma with different energies. Here, we used laser spectroscopy to investigate the plasma treatments with various energies on ZnO nanowires. Significantly improved emission has been observed for low and moderate Ar plasma treatments, which can be ascribed to the surface cleaning effects and increased neutral donor-bound excitons. It is worth mentioning that about 60-folds enhancements of the emission at room temperature can be achieved under 200 W Ar plasma treatment. When the plasma energy exceeds the threshold, high-ion beam energy will cause irreparable damage to the ZnO nanowires. Thanks to the enhanced optical performance, random lasing is observed under optical pumping at room temperature. And the stability has been improved dramatically. By using this simple method, the optical property and stability of ZnO nanowires can be effectively enhanced. These results will play an important role in the development of low dimensional ZnO-based optoelectronic devices.

Surface State Passivation and Optical Properties Investigation of GaSb via Nitrogen Plasma Treatment.

GaSb is one of the most suitable semiconductors for optoelectronic devices operating in the mid-infrared range. However, the existence of GaSb surface states has dramatically limited the performance of these devices. Herein, a controllable nitrogen passivation approach is proposed for GaSb. The surface states and optical properties of GaSb were found to depend on the N passivation conditions. Varying the plasma power during passivation modified the chemical bonds of the GaSb surface, which influenced the emission efficiency. X-ray photoelectron spectroscopy was used to quantitatively demonstrate that the GaSb oxide layer was removed via treatment at a plasma power of 100 W. After nitrogen passivation, the samples exhibited enhanced emission. Free exciton emission was the main factor leading to this enhanced luminescence. An energy band model for the surface states is used to explain the carrier radiative recombination processes. This nitrogen passivation approach can suppress surface states and improve the surface quality of GaSb-based materials and devices. The enhancement in exciton-related emission by this simple approach is important for improving the performance of GaSb-based optoelectronic devices.

Ordered and Disordered Phases in Mo1-xWxS2 Monolayer.

With special quasirandom structure approach and cluster expansion method combined with first-principle calculations, we explore the structure and electronic properties of monolayer Mo1-xWxS2 alloy with disordered phase and ordered phase. The phase transition from ordered phase to disordered phase is found to happen at 41 K and 43 K for x = 1/3 and x = 2/3, respectively. The band edge of VBM is just related with the composition x, while the band edge of CBM is sensitive to the degree of order, besides the concentration of W. Near the CBM band edge, there are two bands with the Mo-character and W-character, respectively. It is found that in disordered phase the Mo-character band is mixed with the W-character band, while the opposite happens in ordered phase. This result leads to that the splitting of two bands near CBM in ordered phase is larger than in disordered phase and gives rise to the smaller band gap in ordered phase compared to the disordered phase. The electron effective mass in ordered phase is smaller than in disordered phase, while the heavy hole effective mass in ordered phase is larger than that in disordered phase.

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy.

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration.