Peisheng Liu

ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metal-semiconductor interface, improvement of blue emission and photocatalysis.

A weak acid selective etching strategy was put forward to fabricate oxide-based hollow nanoparticles (HNPs) using core/shell nanostructures of active metal/oxide nanoparticles as sacrificial templates. ZnO-based HNPs, including pure ZnO, Au/ZnO, Pt/ZnO, and Au/Pt/ZnO HNPs with diameter below 50 nm and shell thickness below 6 nm has been first achieved at low temperature. The diameter, thickness, and even sizes of ZnO and noble metal ultrafine crystals of HNPs can be well adjusted by the etching process. Synchronous with the formation of HNPs, the internal metal-semiconductor interfaces can be controllably eliminated (Zn-ZnO) and reconstructed (noble metal-ZnO). Excitingly, such microstructure manipulation has endued them with giant improvements in related performances, including the very strong blue luminescence with enhancement over 3 orders of magnitude for the pure ZnO HNPs and the greatly improved photocatalytic activity for the noble metal/ZnO HNPs. These give them strong potentials in relevant applications, such as blue light emitting devices, environment remediation, drug delivery and release, energy storage and conversion, and sensors. The designed fabrication procedure is simple, feasible, and universal for a series of oxide and noble metal/oxide HNPs with controlled microstructure and improved performances.

Violet photoluminescence from shell layer of Zn/ZnO core-shell nanoparticles induced by laser ablation

A strong violet photoluminescence (PL) band at 425nm (2.92eV) was observed from the ZnO shell layer of the Zn∕ZnO core-shell nanoparticles prepared by laser ablation in liquid media. Such violet PL decreases with increase of the shell thickness or annealing temperature, showing good controllability. Based on the electron paramagnetic resonance measurements, the violet emission is attributed to the electronic transition from the defect level, corresponding to high-concentration zinc interstitials, to the valence band. This study is in favor to clarify the defect-related emissions and to extend the optical and electronic applications of nanostructured ZnO.

Room temperature synthesized rutile TiO2 nanoparticles induced by laser ablation in liquid and their photocatalytic activity

TiO(2) nanoparticles were prepared by one-step pulsed laser ablation of a titanium target immersed in a poly-(vinylpyrrolidone) solution at room temperature. The products were systematically characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The results indicated that the rutile TiO(2) nanocrystalline particles were one-step synthesized at room temperature and the mean size in diameter is about 50 nm with a narrow size distribution. A probable formation process was proposed on the basis of the microstructure and the instantaneous plasma plume induced by the laser. Photocatalytic activity was monitored by degradation of a methylene blue solution. The as-prepared rutile TiO(2) nanoparticles demonstrate a good photocatalytic performance. This work shows that pulsed laser ablation in liquid media is a good method to synthesize some nanosized materials which are difficult to produce by other conventional methods.

Surface optical phonon Raman scattering in Zn∕ZnO core-shell structured nanoparticles

Surface optical (SO) phonon vibration mode predominant Raman scattering spectra were observed in the range of 545–565cm−1 in metal-semiconductor Zn∕ZnO core-shell structured nanoparticles, prepared by laser ablation of a zinc target in a surfactant aqueous solution. The SO phonon mode exhibits significant size confinement effect. Such SO dominant Raman scattering is attributed to the existence of a large number of disorderly arranged areas among ultrafine ZnO grains formed under the extreme condition of laser ablation. This study demonstrates that the properties of surface phonons can availably reflect some important physical information.

Strong localization effect in temperature dependence of violet-blue emission from ZnO nanoshells

Violet-blue photoluminescence of Zn/ZnO core/shell nanoparticles, induced by laser ablation in liquid, was observed. The emission band shows an abnormal red first and then blueshift with temperature increasing from 8 to 300 K and abnormal narrowing at the temperature range from 30 to 130 K. Such temperature-dependent behaviors can be well described by localization model on interstitial zinc defect on both peak energy and peak width evolutions, reflecting the strong localization effect from the special microstructure. This work will be beneficial to understanding of the physical origins of the violet-blue emission in nanostructured ZnO, and extending its optical and electronic applications.

Localized surface plasmon resonance of Cu nanoparticles by laser ablation in liquid media

Expanding localized surface plasmon resonance (LSPR) towards long wavelengths has been the focus of plasmonics for several decades. Compared with the most studied Au and Ag nanoparticles, Cu nanoparticles have intrinsic long-wavelength LSPR, but hard to be facially fabricated due to their sensitivity to oxidation. Here, we report on the facile and rapid fabrication of colloidal copper nanoparticles by laser ablation in liquid (LAL) operated in ambient conditions, and their LSPR properties. A formation mechanism was put forward to reveal the optical properties of colloidal copper nanoparticles. The effects of different parameters, such as laser power density, ablation time, surfactant concentration, organic solvents and aging time, on LSPR properties were investigated. Eventually, we can decrease the oxidation of colloidal copper nanoparticles by mediating disparate factors, and then tune the position and intensity of copper LSPR peaks. And our results will be beneficial to the application of copper LSPR in optical catalysis, sensitivity, detection devices, and conductive pattern making based on the printing technologies.

Rapid and High-Efficiency Laser-Alloying Formation of ZnMgO Nanocrystals.

Applications of ZnMgO nanocrystals (NCs), especially in photoelectric detectors, have significant limitations because of the unresolved phase separation in the synthesis process. Here, we propose a rapid and highly efficient ZnMgO NC alloying method based on pulsed laser ablation in liquid. The limit value of homogeneous magnesium (Mg) is pushed from 37% to 62%, and the optical band gap is increased to 3.7 eV with high doping efficiency (>100%). Further investigations on the lattice geometry of ZnMgO NCs indicate that all ZnMgO NCs are hexagonal wurtzite structures, and the (002) and (100) peaks shift to higher diffraction angles with the increase in Mg doping content. The calculated results of the lattice constants a and c slightly decrease based on Bragg’s law and lattice geometry equations. Furthermore, the relationship between annealing temperature and the limit value of homogeneous Mg is examined, and the results reveal that the latter decreases with the former because of the phase separation of MgO. A probable mechanism of zinc magnesium alloy is introduced to expound on the details of the laser-alloying process.

Threshold fields in the dc bias dependence of dielectric responses of relaxor ferroelectric terpolymer films

The dc bias dependence of dielectric properties has been investigated for poly (vinylindene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor terpolymer films. The frequency dependence of the temperature of the permittivity maximum, Tm, was analyzed using the Vogel–Fulcher (VF) relation. Two threshold fields, ∼30 and 160 MV/m were found in the electric-field dependence of Tm. Tm shows a minimum at ∼30 MV/m and a maximum at ∼160 MV/m. The VF parameters activation energy Ea and freezing temperature Tf respectively show a maximum and a minimum at ∼25 MV/m.

Effect of in-plane misfit strains on dielectric and pyroelectric response of poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymer

Using Landau–Devonshire (LD) phenomenological model, we investigate the dielectric and pyroelectric behaviors of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] ferroelectric Langmuir–Blodgett super thin films. Effects of homogeneous in-plane misfit strains and external electric field on the dielectric properties are combined to be investigated. The theoretical analysis indicates that phase transition temperature can be changed by in-plane misfit strains. Furthermore results show in-plane strains only change the position of peak but value of dielectric constant and pyroelectric coefficient. At one temperature (below phase transition temperature), compressive strain is more beneficial. Consequently the dielectric and pyroelectric responses can be optimized to satisfy various demands by the applied electric fields and tuning in-plane misfit strains.

Structural and dielectric properties of ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films with different bottom electrodes

In this work we studied 90 nm thick copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%) grown by Langmuir–Blodgett onto silicon substrates. The effects of two different bottom electrodes, namely, Al and LaNiO3 (LNO) are investigated. A Debye-like relaxation near 320 K is enlarged by LNO bottom electrode. X-ray diffraction pattern attests that two different crystallographic structures coexist below 340 K in film deposited on LNO. Compared with Al electrode, LNO electrode strongly increases defects or gauche segments in film and further weakens properties of film.