Zhi-Min Liao

Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions.

Ultrafast-response (20 μs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector

time, and recovery speed of our UV detectors are 8 � 10 2 , 0.7s, and 0.5s, respectively, which are significantly improved compared to the conventional ZnO NWs photodetectors. The improved performance is attributed to the existence of Schottky barriers between ZnO NW and graphene electrodes. The graphene/ZnO NW/graphene vertical sandwiched structures may be promising candidates for integrated optoelectronic sensor devices. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4724208] ZnO, as a wide direct band gap (3.37eV) compound semiconductor with large exciton binding energy (60meV), has been widely investigated for its potential applications in optoelectronic devices, gas and chemical sensors. 1,2 Due to large surface-to-volume ratio, ZnO nanowires (NWs) exhibit highly susceptible photoelectric properties by means of electron-hole generation or recombination during ultraviolet (UV) illumination. Therefore, ZnO NWs have great potential in high sensitivity and fast-response UV sensors, 3 environmental monitors, and optical communications. 4 Recently, Hu et al. 5 reported ZnO NW based UV sensors using Schottky contact formed between ZnO and Pt electrode and the device performance such as the sensitive and UV response, is much higher than that of the traditional ZnO NW photoconductivity based UV sensors. The UV detectors based on Schottky barriers formed between ZnO NW and other metal electrodes, such as gold electrodes, have also been studied. 6,7 Nevertheless, metal electrodes are poor in transparency and can dramatically influence the absorption efficiency of the UV sensors. Graphene, a monolayer sp 2 carbon atoms with unique physical properties, such as high mobility and conductivity, 8 high optical transparency 9 and mechanical flexibility, 10 etc., has attracted great research interest recently. The high conductive and optical transparent properties make graphene an ideal candidate for the application in transparent electrode. The Schottky barrier is also expected to be existed at the interface between ZnO nanowire and graphene, and it has been utilized for light-emitting diodes 11 and transparent nanogenerators. 12 In this letter, we have fabricated a vertical sandwich structure of graphene/ZnO NW/graphene. We demonstrate the high performance of our ZnO NW based vertical UV photodetector due to the existence of Schottky barriers between graphene electrodes and ZnO NW. The current on-off ratio of the UV detector is up to 8 � 10 2 at a illumination power density of 50lw/lm 2 , the photocurrent

Strain dependent resistance in chemical vapor deposition grown graphene

The strain dependence of conductance of monolayer graphene has been studied experimentally here. The results illustrate the notable transitions: the slight increase, the dramatic decrease, and the sudden dropping of the conductance by gradually increasing the uniaxial strain. The graphene conductance behaves reversibly by tuning of the elastic tensile strain up to 4.5%, while it fails to recover after the plastic deformation at 5%. The change in conductance due to strain is surprisingly high, which indicates the potential applications in electromechanical devices.

Giant negative magnetoresistance induced by the chiral anomaly in individual Cd3As2 nanowires

Dirac electronic materials beyond graphene and topological insulators have recently attracted considerable attention. Cd3As2 is a Dirac semimetal with linear dispersion along all three momentum directions and can be viewed as a three-dimensional analogue of graphene. By breaking of either time-reversal symmetry or spatial inversion symmetry, the Dirac semimetal is believed to transform into a Weyl semimetal with an exotic chiral anomaly effect, however the experimental evidence of the chiral anomaly is still missing in Cd3As2. Here we show a large negative magnetoresistance with magnitude of −63% at 60 K and −11% at 300 K in individual Cd3As2 nanowires. The negative magnetoresistance can be modulated by gate voltage and temperature through tuning the density of chiral states at the Fermi level and the inter-valley scatterings between Weyl nodes. The results give evidence of the chiral anomaly effect and are valuable for understanding the Weyl fermions in Dirac semimetals.

Electronic and Mechanical Coupling in Bent ZnO Nanowires

A red shift of the exciton of ZnO nanowires is efficiently produced by bending strain, as demonstrated by a low-temperature (81 K) cathodoluminescence (CL) study of ZnO nanowires bent into L- or S-shapes. The figure shows a nanowire (Fig. a) with the positions of CL measurements marked. The corresponding CL spectra-revealing a peak shift and broadening in the region of the bend-are shown in Figure b.

Memory and threshold resistance switching in Ni/NiO core-shell nanowires.

We report on the first controlled alternation between memory and threshold resistance switching (RS) in single Ni/NiO core-shell nanowires by setting the compliance current (I(CC)) at room temperature. The memory RS is triggered by a high I(CC), while the threshold RS appears by setting a low I(CC), and the Reset process is achieved without setting a I(CC). In combination with first-principles calculations, the physical mechanisms for the memory and threshold RS are fully discussed and attributed to the formation of an oxygen vacancy (Vo) chain conductive filament and the electrical field induced breakdown without forming a conductive filament, respectively. Migration of oxygen vacancies can be activated by appropriate Joule heating, and it is energetically favorable to form conductive chains rather than random distributions due to the Vo-Vo interaction, which results in the nonvolatile switching from the off- to the on-state. For the Reset process, large Joule heating reorders the oxygen vacancies by breaking the Vo-Vo interactions and thus rupturing the conductive filaments, which are responsible for the switching from on- to off-states. This deeper understanding of the driving mechanisms responsible for the threshold and memory RS provides guidelines for the scaling, reliability, and reproducibility of NiO-based nonvolatile memory devices.

Electrical and Photoresponse Properties of an Intramolecular p-n Homojunction in Single Phosphorus-Doped ZnO Nanowires

The single-crystal n-type and p-type ZnO nanowires (NWs) were synthesized via a chemical vapor deposition method, where phosphorus pentoxide was used as the dopant source. The electrical and photoluminescence studies reveal that phosphorus-doped ZnO NWs (ZnO:P NWs) can be changed from n-type to p-type with increasing P concentration. Furthermore, we report for the first time the formation of an intramolecular p-n homojunction in a single ZnO:P NW. The p-n junction diode has a high on/off current ratio of 2.5 x 10(3) and a low forward turn-on voltage of approximately 1.37 V. Finally, the photoresponse properties of the diode were investigated under UV (325 nm) excitation in air at room temperature. The high photocurrent/dark current ratio (3.2 x 10(4)) reveals that the diode has a potential as extreme sensitive UV photodetectors.

Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply

Graphene has a range of unique physical properties and could be of use in the development of a variety of electronic, photonic and photovoltaic devices. For most applications, large-area high-quality graphene films are required and chemical vapour deposition (CVD) synthesis of graphene on copper surfaces has been of particular interest due to its simplicity and cost effectiveness. However, the rates of growth for graphene by CVD on copper are less than 0.4 μm s-1, and therefore the synthesis of large, single-crystal graphene domains takes at least a few hours. Here, we show that single-crystal graphene can be grown on copper foils with a growth rate of 60 μm s-1. Our high growth rate is achieved by placing the copper foil above an oxide substrate with a gap of ∼15 μm between them. The oxide substrate provides a continuous supply of oxygen to the surface of the copper catalyst during the CVD growth, which significantly lowers the energy barrier to the decomposition of the carbon feedstock and increases the growth rate. With this approach, we are able to grow single-crystal graphene domains with a lateral size of 0.3 mm in just 5 s.

Single ZnO Nanowire/p‐type GaN Heterojunctions for Photovoltaic Devices and UV Light‐Emitting Diodes

We fabricate heterojunctions consisting of a single n-type ZnO nanowire and a p-type GaN film. The photovoltaic effect of heterojunctions exhibits open-circuit voltages ranging from 2 to 2.7 V, and a maximum output power reaching 80 nW. Light-emitting diodes with UV electroluminescence based on the heterojunctions are demonstrated.

Strong Second-Harmonic Generation in Atomic Layered GaSe.

Nonlinear effects in two-dimensional (2D) atomic layered materials have recently attracted increasing interest. Phenomena such as nonlinear optical edge response, chiral electroluminescence, and valley and spin currents beyond linear orders have opened up a great opportunity to expand the functionalities and potential applications of 2D materials. Here we report the first observation of strong optical second-harmonic generation (SHG) in monolayer GaSe under nonresonant excitation and emission condition. Our experiments show that the nonresonant SHG intensity of GaSe is the strongest among all the 2D atomic crystals measured up to day. At the excitation wavelength of 1600 nm, the SHG signal from monolayer GaSe is around 1-2 orders of magnitude larger than that from monolayer MoS2 under the same excitation power. Such a strong nonlinear signal facilitates the use of polarization-dependent SHG intensity and SHG mapping to investigate the symmetry properties of this material: the monolayer GaSe shows 3-fold lattice symmetry with an intrinsic correspondence to its geometric triangular shape in our growth condition; whereas the bilayer GaSe exhibits two dominant stacking orders: AA and AB stacking. The correlation between the stacking orders and the interlayer twist angles in GaSe bilayer indicates that different triangular GaSe atomic layers have the same dominant edge configuration. Our results provide a route toward exploring the structural information and the possibility to observe other nonlinear effects in GaSe atomic layers.