Jingyun Gao

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.

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.

High-quality CdTe nanowires: Synthesis, characterization, and application in photoresponse devices

High-quality straight and multiply kinked CdTe nanowires (NWs) were synthesized by the facile chemical vapor deposition method at 600 °C. The as-synthesized NWs were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and photoluminescence (PL) spectroscopy. The straight CdTe NWs have single crystalline zinc blende structure with growth direction along the ⟨111⟩ direction. Their PL spectra consist only sharp near band edge emission around 824.3 nm. The multiply kinked CdTe NWs contain one or more fixed (∼125.2°) angle joints; each arm of the kinked NWs is single crystalline with similar selected area electron diffraction pattern as that of the straight CdTe NWs. The two growth directions in the multiply kinked CdTe NWs are ⟨200⟩ and ⟨111⟩. Single straight and kinked CdTe NW photoresponse devices were fabricated and testified to have high photocurrent decay ratio, high responsivity, fast response time, and no decay tail unde...

Compensation mechanism in N-doped ZnO nanowires.

N-doped ZnO nanowires are synthesized at a relatively low growth temperature of 500 degrees C by directly heating zinc powder using NH(3) as the dopant. The incorporation of N into the ZnO nanowires is experimentally confirmed by x-ray photoelectron spectroscopy, Raman spectra and photoluminescence measurements. By combining post annealing experiments after growth with first-principles calculations, the detailed migration mechanism of N and compensation mechanism in N-doped ZnO nanowires are systematically studied. The larger aspect ratio of nanowires favors the formation of oxygen vacancy and out-diffusion of substitutional N (N(O)), making N(O) in ZnO nanowires always compensated by hydrogen interstitials (H(I)). Our results can help to explain the challenge in getting p-type ZnO and shed new light on the possible realization of p-type doping of ZnO in the future.

Effects of annealing on the ferromagnetism and photoluminescence of Cu-doped ZnO nanowires

Room temperature ferromagnetic Cu-doped ZnO nanowires have been synthesized using the chemical vapor deposition method. By combining structural characterizations and comparative annealing experiments, it has been found that both extrinsic (CuO nanoparticles) and intrinsic (Zn(1-x)Cu(x)O nanowires) sources are responsible for the observed ferromagnetic ordering of the as-grown samples. As regards the former, annealing in Zn vapor led to a dramatic decrease of the ferromagnetism. For the latter, a reversible switching of the ferromagnetism was observed with sequential annealings in Zn vapor and oxygen ambience respectively, which agreed well with previous reports for Cu-doped ZnO films. In addition, we have for the first time observed low temperature photoluminescence changed with magnetic properties upon annealing in different conditions, which revealed the crucial role played by interstitial zinc in directly mediating high T(c) ferromagnetism and indirectly modulating the Cu-related structured green emission via different charge transfer transitions.

Ultrahigh field emission current density from nitrogen-implanted ZnO nanowires

An ultrahigh field emission current density of 10.3 mA cm(-2) was obtained from nitrogen-implanted ZnO nanowires. The sample was characterized and clearly showed a nitrogen doping signal. Field emission properties of the ZnO nanowires were considerably improved after N-implantation with lower turn-on field and a much higher current density. Removal of an amorphous layer, the presence of nanoscale protuberances, and surface-related defects were found to be responsible for the significantly enhanced field emission. Our work is important for the possible applications of ZnO nanowires in flat panel displays and high brightness electron sources.

In situ observation of ZnO nanowire growth on zinc film in environmental scanning electron microscope

In situ uniform growth of ZnO nanowires was realized and monitored at real time by heating zinc film in an environmental scanning electron microscope. Better controllability and repeatability were obtained by using zinc film as source material compared to traditionally used zinc powder. Morphology of the as-grown ZnO nanowires was found to depend on both the growth temperature and holding time. Low temperature (500 degrees C) and short growth time (approximately 20 min) favor one-dimensional nanowire growth, whereas longer holding time (>40 min) or higher temperature (700 degrees C) lead to nanosheet growth. The results suggest that the zinc vapor partial pressure is vital in determining the final morphology. These results help to give more insights into the mechanism of ZnO nanowire synthesis.

A Novel Way for Synthesizing Phosphorus-Doped Zno Nanowires

We developed a novel approach to synthesize phosphorus (P)-doped ZnO nanowires by directly decomposing zinc phosphate powder. The samples were demonstrated to be P-doped ZnO nanowires by using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction spectra, X-ray photoelectron spectroscopy, energy dispersive spectrum, Raman spectra and photoluminescence measurements. The chemical state of P was investigated by electron energy loss spectroscopy (EELS) analyses in individual ZnO nanowires. P was found to substitute at oxygen sites (PO), with the presence of anti-site P on Zn sites (PZn). P-doped ZnO nanowires were high resistance and the related P-doping mechanism was discussed by combining EELS results with electrical measurements, structure characterization and photoluminescence measurements. Our method provides an efficient way of synthesizing P-doped ZnO nanowires and the results help to understand the P-doping mechanism.

Size-dependent mechanical properties of PVA nanofibers reduced via air plasma treatment

Organic nanowires/fibers have great potential in applications such as organic electronics and soft electronic techniques. Therefore investigation of their mechanical performance is of importance. The Youngs modulus of poly(vinyl alcohol) (PVA) nanofibers was analyzed by scanning probe microscopy (SPM) methods. Air plasma treatment was used to reduce the nanofibers to different sizes. Size-dependent mechanical properties of PVA nanofibers were studied and revealed that the Youngs modulus increased dramatically when the scales became very small (<80 nm).

Growth mechanism study viain situ epitaxial growth of high-oriented ZnO nanowires

In situ epitaxial growth of high-oriented ZnO nanowires on annealed zinc film was realized in an environmental scanning electron microscope. Energy dispersive spectra, X-ray diffraction and photoluminescence measurements demonstrated that a thin layer of [001] ZnO was formed on the surface of zinc film after annealing. Formation of a ZnO layer on annealed zinc film caused dense [001] oriented nanowires compared to the sparse random nanowires grown on unannealed zinc film. Possible growth mechanism for the nanowires grown on annealed and unannealed film was discussed. This work offers a novel approach for effective epitaxial growth of high-oriented ZnO nanowires and helps to understand the growth mechanism of ZnO nanowires.