Vertical Schottky ultraviolet photodetector based on graphene and top–down fabricated GaN nanorod arrays

GaN has been widely used in the fabrication of ultraviolet photodetectors because of its outstanding properties. In this paper, we report a graphene–GaN nanorod heterostructure photodetector with fast photoresponse in the UV range. GaN nanorods were fabricated by a combination mode of dry etching and wet etching. Furthermore, a graphene–GaN nanorod heterostructure ultraviolet detector was fabricated and its photoelectric properties were measured. The device exhibits a fast photoresponse in the UV range. The rising time and falling time of the transient response were 13 and 8 ms, respectively. A high photovoltaic responsivity up to 13.9 A/W and external quantum efficiency up to 479% were realized at the UV range. The specific detectivity D^* = 1.44 × 10¹⁰ Jones was obtained at –1 V bias in ambient conditions. The spectral response was measured and the highest response was observed at the 360 nm band.     

High-operating-temperature MWIR photodetector based on a InAs/GaSb superlattice grown by MOCVD

We demonstrate a high-operating-temperature(HOT)mid-wavelength InAs/GaSb superlattice heterojunction in-frared photodetector grown by metal-organic chemical vapor deposition.High crystalline quality and the near-zero lattice mis-match of a InAs/GaSb superlattice on an InAs substrate were evidenced by high-resolution X-ray diffraction.At a bias voltage of-0.1 V and an operating temperature of 200 K,the device exhibited a 50%cutoff wavelength of~4.9μm,a dark current dens-ity of 0.012 A/cm²,and a peak specific detectivity of 2.3×10⁹ cm·Hz^1/2/W.     

Electrical properties and structural optimization of GaN/InGaN/GaN tunnel junctions grown by molecular beam epitaxy

The InGaN films and GaN/InGaN/GaN tunnel junctions (TJs) were grown on GaN templates with plasma-assisted molecular beam epitaxy. As the In content increases, the quality of InGaN films grown on GaN templates decreases and the surface roughness of the samples increases. V-pits and trench defects were not found in the AFM images. p⁺⁺-GaN/InGaN/n⁺⁺-GaN TJs were investigated for various In content, InGaN thicknesses and doping concentration in the InGaN insert layer. The InGaN insert layer can promote good interband tunneling in GaN/InGaN/GaN TJ and significantly reduce operating voltage when doping is sufficiently high. The current density increases with increasing In content for the 3 nm InGaN insert layer, which is achieved by reducing the depletion zone width and the height of the potential barrier. At a forward current density of 500 A/cm², the measured voltage was 4.31 V and the differential resistance was measured to be 3.75 × 10⁻³ Ω·cm² for the device with a 3 nm p⁺⁺-In_0.35Ga_0.65N insert layer. When the thickness of the In_0.35Ga_0.65N layer is closer to the “balanced” thickness, the TJ current density is higher. If the thickness is too high or too low, the width of the depletion zone will increase and the current density will decrease. The undoped InGaN layer has a better performance than n-type doping in the TJ. Polarization-engineered tunnel junctions can enhance the functionality and performance of electronic and optoelectronic devices.     

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics

The development of fiber-based smart electronics has provoked increasing demand for high-performance and multifunctional fiber materials. Carbon nanotube (CNT) fibers, the 1D macroassembly of CNTs, have extensively been utilized to construct wearable electronics due to their unique integration of high porosity/surface area, desirable mechanical/physical properties, and extraordinary structural flexibility, as well as their novel corrosion/oxidation resistivity. To take full advantage of CNT fibers, it is essential to understand their mechanical and conductive properties. Herein, the recent progress regarding the intrinsic structure–property relationship of CNT fibers, as well as the strategies of enhancing their mechanical and conductive properties are briefly summarized, providing helpful guidance for scouting ideally structured CNT fibers for specific flexible electronic applications.     

Crossing the blood–brain barrier with graphene nanostructures

Therapeutic approaches for the delivery of drugs to the central nervous system are hampered by the presence of the blood–brain barrier (BBB); overcoming this barrier is the clinical goal for the treatment of neurological disorders, including Alzheimer’s disease and Parkinson’s disease. The BBB is a cellular barrier of a highly impermeable nature that is predominantly formed by a tightly bound continuous layer of endothelial cells; it acts as a gatekeeper to restrict the free diffusion of bloodborne pathogens into the central nervous system. Targeted drug delivery systems have been explored over the past decade for crossing the BBB. Very recently, graphene nanostructures have shown great potential for crossing the BBB due to their exceptional features such as high electron mobility, ease of synthesis and functionalization, as well as control over size, shape, and the drug release profile. Graphene is evolving as a system not only to detect diseased lesions but also, in parallel, to treat neurological disorders while demonstrating minimal side effects. Given the rapid developments of innovative graphene-based delivery platforms, the present review sheds light on the status and prospects of graphene for crossing the BBB by improving, preserving, or rescuing brain energetics, with a specific focus on how graphene alters neuronal cell function.     

Thermal peeling stress analysis of thin-film high-TC superconductors

Thermal peeling stress between a thin film and the substrate is caused by the mismatch of thermal expansion coefficients while the film and substrate undergo a temperature change. The thermal peeling stress resulting from the temperature decrease from ambient to operating conditions (cryogenic temperatures) between a thin-film high-temperature superconductor and its substrate is calculated using finite element analysis (FEA). The superconductor thin film is idealized as a long bridge on a large substrate. A two-dimensional FEA model is applied to calculate the tensile (peeling) stress at the thin film/substrate interface. Results are obtained for different geometries and temperature conditions, and these results are compared with analytical predictions. A stress singularity is found at the very edge of the thin film which is not predicted by the analytical prediction. The peeling stress can be very high due to this stress singularity, even if the temperature change is not large. The stress singularity area depends on the local geometry of the edge, suggesting that refining the geometry of the thin-film HTS device is important.     

A facile top-down approach for constructing perovskite oxide nanostructure with abundant oxygen defects as highly efficient water oxidation electrocatalyst

Developing highly efficient electrocatalysts for oxygen evolution reaction (OER) is of significant importance for the application of many energy conversion and storage technologies. Perovskite oxides have attracted great attention as potential OER electro-catalysts. Their performance, however, are strongly limited by large particle size, owing to the high synthesis temperature. Herein, we report a facile top-down strategy for fabricating perovskite oxide nanostructures with large surface area and strongly improved intrinsic OER activity. SrNb_0.1Co_0.7Fe_0.2O_3-δ (SNCF) particles with micro size are treated by (NH₄)₂Fe(SO₄)₂ saturated solution for different time length at room temperature. The obtained catalysts exhibit significantly increased surface area with nanosheet structure in the outer layer. Furthermore, cobalt on the surface are reduced from Co³⁺ to Co²⁺, suggesting oxygen vacancy formation on the surface. The defective SNCF nanostructure exhibits significantly improved OER activity and good stability. The facile methodology reported in this work can be generally applied to other oxide electrocatalysts for energy applications.     

A comparison of the mechanical properties of fibers spun from different carbon nanotubes

Spinnable carbon nanotube (CNT) arrays with different CNT structures have been synthesized using different growth methods and carbon sources, and long and stable fibers have been produced. Parameters of the nanotubes such as tube diameter, wall thickness, tube length and level of defects were found to play a more important role in the mechanical properties of the fibers than did the initial tube arrangement. To improve the fiber strength, as well as the modulus, the tubes must be long and have a small diameter and thin walls. The strongest fiber from double- and triple-walled CNTs is 1.23 GPa in strength, and 32% and 221% higher than those from CNTs with ∼6 and ∼15 walls (932 and 383 MPa), respectively. The fiber strength can be improved by 25%, up to 1.54 GPa, after poly(vinyl alcohol) infiltration with volume fraction of ∼20%. Our study also shows that C₂H₄ is superior to C₂H₂ as the carbon source for the growth of mainly double- and triple-walled CNTs, and therefore the spinning of high-strength fibers.