Yanhe Xiao

Highly sensitive humidity sensor based on amorphous Al2O3 nanotubes

Amorphous Al2O3 nanotubes were obtained by the transformation of NH4Al(OH)2CO3 nanotubes annealed at 600 °C. The tube-like nanostructures not only increase efficient sites for gas adsorption, but also promote the dissociation of water absorbed onto the surfaces of the nanotube walls. Moreover, they also provide effective and fast channels for vapor and liquid transport. Therefore, the sensors based on Al2O3 nanotubes show high sensitivity and fast response/recovery time to humidity. The impedance changes approximately four orders of magnitude as relative humidity (RH) varies from 11% to 95% at the measured frequency of 40 Hz. Additionally, the sensor also presents relatively small hysteresis and long-term stability. For low RH levels, the protonic conductor is dominant, whereas for increasing RH levels the ionic contribution becomes prevalent. This study demonstrates that Al2O3 nanotubes have promising applications in environmental monitoring.

Power- and energy-dependent photoluminescence of Eu3+ incorporated and segregated ZnO polycrystalline nanobelts synthesized by a facile combustion method followed by heat treatment

A simple and efficient combustion method has been developed for pure and doped ZnO polycrystalline nanobelts. Eu3+ can form a supersaturated solid solution in the nanobelts annealed at 600 °C. Photogenerated electrons cannot transfer effectively from the ZnO conduction band (CB) to the excited states of incorporated Eu3+ ions and exchange energy. However, incorporated Eu3+ can form a trap level occurring at about 90 meV below the CB minimum, thus photogenerated electrons can relax rapidly from the CB to the shallow level trap and then recombine with the free-holes of the valence band edge, becoming a dominant emission with increasing excitation power density. The heavy doping not only results in excitonic localization, but induces Auger quenching and Fano effects at higher excitation densities. Due to the solid solubility limit, however, excess Eu3+ can segregate from the ZnO matrix as Eu2O3 precipitates and form a ZnO/Eu2O3 hybrid structure upon annealing at 1000 °C. Moreover, the overlap between ZnO emission bands and Eu2O3 absorption lines results in an efficient energy transfer from ZnO host to Eu2O3 clusters and an appearance of a dominant 5D0–7F2 emission line in PL spectra.

Individual Ohmic contacted ZnO/Zn2SnO4 radial heterostructured nanowires as photodetectors with a broad-spectral-response: injection of electrons into/from interface states

ZnO and Zn2SnO4 nanowires (NWs) have relatively high sensitivities as ultraviolet (UV) photodetectors, while their bandgaps are an important limitation in their applications in the visible (VIS) and near-infrared (NIR) ranges. In this paper, we demonstrate the promising applications of Ohmic contacted individual Zn2SnO4-sheathed ZnO core/shell radial heterostructured NWs as high performance solar blind UV-VIS-NIR photodetectors with a relatively high sensitivity, stability, and reproducibility. The dominant mechanism for the excellent photoconductivity is attributed to the presence of interface states in the II-type heterostructure, which prevents the movement of charge at the heterointerface. Upon applying a negative bias voltage at one end of the detector, where the interface states will be filled, potential barriers will decrease and be eliminated. Therefore, photogenerated electron–hole pairs will be separated efficiently and electrons can migrate towards the Zn2SnO4 shell, resulting in a huge decrease in shell resistance. The absorption of the heterointerface and charged oxygen vacancies (V+O and V++O) in the depletion region can induce VIS and NIR photoresponses. These results demonstrate that individual heterostructured NWs composed of wide bandgap semiconductors can indeed serve as high-performance photodetectors in the solar blind UV-VIS-NIR range.

Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity.

ZnO nanowires have relatively high sensitivity as ultraviolet (UV) photodetectors, while the bandgap of 3.37 eV is an important limitation for their applications in solar-blind UV (SBUV), visible (VIS) and near infrared (NIR) range. Besides UV response, in this study, we demonstrate the promising applications of individual undoped ZnO NWs as high performance SBUV-VIS-NIR broad-spectral-response photodetectors, strongly depended on applied bias voltage and illumination intensity. The dominant mechanism is attributed to the existence of surface states in nanostructured ZnO. At a negative bias voltage electrons can be injected into surface states from electrode, and moreover, under light illumination photogenerated electron-hole pairs can be separated efficiently by surface built-in electric field, resulting into a decrease of potential barrier height and depletion region width, and simultaneously accompanying a filling of oxygen vacancy and a rise of ZnO Fermi level.

General synthesis of rare-earth orthochromites with quasi-hollow nanostructures and their magnetic properties

Rare-earth orthochromites are extremely interesting because of their potential applications as multifunctional materials. However, it is still a great challenge for the general synthesis of nanostructured full rare-earth orthochromites series. Here, a facile and versatile solvothermal reduction strategy is successfully employed in the preparation of rare-earth chromites with quasi-hollow nanostructures. X-ray diffraction data show that all the products have the orthorhombic perovskite structure. The electron microscopy analysis reveals that the morphology of the product is seriously affected by the rare-earth ionic radius. Tube-like and vesicle-like structures can be formed for the larger and smaller rare-earth cationic radii, respectively. The experimental results suggest that the room-temperature precursors of potassium rare-earth chromates serve as a self-template for the in situ reduction and formation of rare-earth orthochromites hollow structures. The magnetization studies demonstrate that all the products, as it would be expected, undergo a magnetic transition from paramagnetic to antiferromagnetic phase at the Neel temperature (TN1) attributed to Cr3+–Cr3+ exchange and this critical temperature goes up linearly with an increase in the rare-earth ionic radius. Additionally, some samples exhibit a variety of fancy magnetic properties, including thermal hysteresis suggesting a first-order magnetic transition, magnetization reversal due to the antiparallel polarization of the R3+ paramagnetic moments by the Cr3+ canted antiferromagnetic ones, and magnetic exchange bias related to the spin reorientation transition of the Cr3+ magnetic moments.

Self-template formation and properties study of Cr2O3 nanoparticle tubes

As an important semiconductor, Cr2O3 exhibits many attractive properties and significant industrial applications. Nevertheless, it remains a challenge to develop a simple synthetic methodology for the fabrication of Cr2O3 1D nanostructures. In this paper, Cr2O3 sub-microtubes, consisting of nanoparticles, have been successfully prepared by the in situreduction of SrCrO4 nanorods in a microemulsion system. X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy were employed to study the crystal structure and morphology of the products. Experiments showed that the solvent, reaction temperature and reaction time were critical for the formation of the Cr2O3 nanoparticle tubes. Magnetic properties of the products were investigated by the electron spin resonance and superconducting quantum interference device magnetometer. These magnetic results revealed a weak ferromagnetic behavior induced by the uncompensated surface spin below TB. The UV-Vis absorption spectrum showed three broad absorption peaks at 360, 450 and 600 nm, respectively. The nitrogen adsorption-desorption measurement was used to determine the surface area and pore size distribution of the as-obtained polycrystalline tubes. A possible growth mechanism for the Cr2O3 nanoparticle tubes was also proposed.

Effects of Interface States on Photoexcited Carriers in ZnO/Zn2SnO4 Type-II Radial Heterostructure Nanowires

Type-II band alignment of heterostructure contributes to spatially separate electrons and holes leading to an increase in minority carrier lifetime, which has much more advantages in photocatalytic activities and photovoltaic device applications. Here, Zn2SnO4-sheathed ZnO radial heterostructure nanowires were constructed to investigate systematically interfacial charge separation. The lattice mismatch between ZnO and Zn2SnO4 induces interface states to exist at their heterointerface. At low pump fluence, photoexcited charges are localized within the ZnO core rather than separated due to the large interface barrier. Correspondingly, only ZnO-related bandedge ultraviolet (UV) and green emissions are dominated in photoluminescence spectra. At high pump fluence, however, impurities are ionized and electrons trapped in interface states are excited, resulting in a decrease in interface barrier, which makes photogenerated charges efficiently separated at their heterointerface by direct tunneling, and, consequently, an additional blue-violet emission, attributed to the heterointerface recombination of electrons in Zn2SnO4 conduction band (CB) and holes in ZnO valence band. Additionally, the heterointerface can separate effectively photoexcited carriers and form a photovoltaic effect. Our results provide the localization/separation condition of photogenerated charges for the type-II band alignment of core/shell heterostructure, which should be very useful for the realization of underpinned mechanism of the developed optoelectronic devices.

Individual Zn2SnO4-sheathed ZnO heterostructure nanowires for efficient resistive switching memory controlled by interface states

Resistive switching (RS) devices are widely believed as a promising candidate for next generation nonvolatile resistance random access memory. Here, Zn2SnO4-sheathed ZnO core/shell heterostructure nanowires were constructed through a polymeric sol–gel approach followed by post-annealing. The back-to-back bipolar RS properties were observed in the Ohmic contact two-terminal devices based on individual core/shell nanowires. With increasing bias to about 1.5 V, it changes from high-resistance states (HRS) to low-resistance states, and however, it can be restored to HRS by reverse bias. We propose a new mechanism, which is attributed to the injection of electrons into/from interfacial states, arising from the lattice mismatch at ZnO/Zn2SnO4 heterointerface. Upon applying negative/positive voltage at one end of devices, where interfacial states are filled/emptied, barrier will be eliminated/created, resulting into symmetric RS characteristics. The behavior of storage and removal charges demonstrates that the heterostructures have excellent properties for the application in resistance random access memory.

SrAlxOy:Eu2+, Dy3+ (x = 4) nanostructures: Structure and morphology transformations and long-lasting phosphorescence properties

Long-lasting phosphor SrAlxOy:Eu2+, Dy3+ (x = 4) nanostructures were synthesized through a facile but efficient combustion technique followed by a post-annealing reaction at different temperatures. During the synthesis process, triethanolamine (TEA) can not only be used as an organic fuel, but also can control the morphology of nanostructured combustion products. With an increase in post-annealing temperature, their morphology transforms from nanosheets into semi-curled nanobelts due to the crystal growth and the edge curl effect, and moreover, the products with various crystallographic structures can be obtained, whose transition presents the following rule: amorphous → hexagonal β-SrAl2O4 (1000 °C) → monoclinic SrAl4O7 (1100 °C) → orthorhombic Sr4Al14O25 (1300 °C). Excess Al facilitates the stabilization of the β-SrAl2O4 high-temperature phase at room-temperature. Due to the difference of Eu2+ surroundings in various crystallographic structures, their excitation and emission spectra exhibit some discrepancy in peak position and intensity. Surface and boundary defects, originating from small size and poor crystallinity, can both increase trap densities and depth levels; while Sr vacancies (VSr), donated by the deficiency of Sr2+ ions in β-SrAl2O4 and Sr4Al14O25, mainly increase the concentration of relatively shallow traps. Thus, β-SrAl2O4 shows a relatively small afterglow decay constant compared to SrAl4O7. Moreover, the afterglow decay constant of Sr4Al14O25 is initially much smaller than those of β-SrAl2O4 and SrAl4O7, then increases gradually, and finally exceeds them. Additionally, the maximum value of initial brightness and effective afterglow time emerges in the SrAl4O7 and Sr4Al14O25 composite phosphor annealed at 1200 °C.

PMMA interlayer-modulated memory effects by space charge polarization in resistive switching based on CuSCN-nanopyramids/ZnO-nanorods p-n heterojunction

Resistive switching (RS) devices are commonly believed as a promising candidate for next generation nonvolatile resistance random access memory. Here, polymethylmethacrylate (PMMA) interlayer was introduced at the heterointerface of p-CuSCN hollow nanopyramid arrays and n-ZnO nanorod arrays, resulting in a typical bipolar RS behavior. We propose the mechanism of nanostructure trap-induced space charge polarization modulated by PMMA interlayer. At low reverse bias, PMMA insulator can block charges through the heterointerface, and and trapped states are respectively created on both sides of PMMA, resulting in a high resistance state (HRS) due to wider depletion region. At high reverse bias, however, electrons and holes can cross PMMA interlayer by Fowler-Nordeim tunneling due to a massive tilt of energy band, and then inject into the traps of ZnO and CuSCN, respectively. and trapped states are created, resulting in the formation of degenerate semiconductors on both sides of PMMA. Therefore, quantum tunneling and space charge polarization lead to a low resistance state (LRS). At relatively high forward bias, subsequently, the trapped states of and are recreated due to the opposite injection of charges, resulting in a recovery of HRS. The introduction of insulating interlayer at heterointerface, point a way to develop next-generation nonvolatile memories.