Jiada Wu

Arsenic doping for synthesis of nanocrystalline p-type ZnO thin films

Nanocrystalline p-type arsenic-doped ZnO (ZnO:As) films have been synthesized on (0001) sapphire substrates by pulsed laser deposition using a ZnO target mixed with 6.6wt% As2O3. The process of synthesizing p-type ZnO:As films was performed in an ambient gas of ultrapure (<99.99%) oxygen. The ambient gas pressure was 5Pa with the substrate temperature in the range of 350–500°C. The ZnO:As films grown at 500°C are p type, and the acceptor concentration in ZnO:As films is about 1.9×1018at.∕cm3 as determined by Hall effect measurements. The concentration of As in ZnO:As films is estimated to be about 1.7% from the x-ray photoemission spectroscopy (XPS) spectrum. Guided by the XPS analysis and a model for large-sized-mismatched group-V dopant in ZnO, an AsZn–2VZn complex was thought to be the most possible acceptor.Nanocrystalline p-type arsenic-doped ZnO (ZnO:As) films have been synthesized on (0001) sapphire substrates by pulsed laser deposition using a ZnO target mixed with 6.6wt% As2O3. The process of synthesizing p-type ZnO:As films was performed in an ambient gas of ultrapure (<99.99%) oxygen. The ambient gas pressure was 5Pa with the substrate temperature in the range of 350–500°C. The ZnO:As films grown at 500°C are p type, and the acceptor concentration in ZnO:As films is about 1.9×1018at.∕cm3 as determined by Hall effect measurements. The concentration of As in ZnO:As films is estimated to be about 1.7% from the x-ray photoemission spectroscopy (XPS) spectrum. Guided by the XPS analysis and a model for large-sized-mismatched group-V dopant in ZnO, an AsZn–2VZn complex was thought to be the most possible acceptor.

Enhanced photoelectrochemical activity of vertically aligned ZnO-coated TiO2 nanotubes

Vertically aligned ZnO-TiO2 hetero-nanostructures constructed of anatase TiO2 nanotubes (NTs) and wurtzite ZnO coatings are fabricated by atomic layer deposition of ZnO coatings on electrochemical anodization formed TiO2 NTs, and their photoelectrochemical activities are studied through photoelectrochemical and electrochemical characterization. Compared with bare TiO2 NTs, the transient photocurrent increases to over 1.5-fold for the annealed ZnO-coated TiO2 NTs under visible illumination. The ZnO-coated TiO2 NTs also show a longer electron lifetime, a lower charge-transfer resistance and a more negative flat-band potential than the bare TiO2 NTs, confirming the improved photoelectrochemical activity due to the enhanced charge separation.

Growth of ZnSe nanowires by pulsed-laser deposition

Stoichiometric ZnSe nanowires have been grown by pulsed-laser deposition on GaAs (100) substrates coated with gold layers. The gold layer plays a key role as catalyst in the deposition of ZnSe nanowires. The thickness of the gold film greatly affected the density of the ZnSe nanowires synthesized on the substrate. No ZnSe nanowires were synthesized on the bare GaAs (100) substrate. The microstructures and the chemical compositions of the as-synthesized nanowires were investigated by scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The results reveal that the as-grown thin films consist of ZnSe nanowires with diameters ranging from 20to40nm, and the nanowires appear to be randomly oriented on the Au-coated substrate. The as-grown nanowires were also observed to be elongated along different crystallographic directions.

Manipulations from oxygen partial pressure on the higher energy electronic transition and dielectric function of VO2 films during a metal–insulator transition process

Optical properties and metal–insulator transition (MIT) of vanadium dioxide (VO2) films grown by pulsed laser deposition with different oxygen pressures (5 to 50 mTorr) have been investigated by temperature dependent transmittance spectra. Three interband critical points (E1, E2 and E3) can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E1 and E2 is presented. The VO2 film grown at optimized oxygen pressure exhibits the well-defined resistivity drop (∼103 Ω cm) across the MIT process. It is found that the metal–insulator transition temperature (TMIT) increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure. It is believed that the oxygen pressure can lead to lattice defects, which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies, respectively. The donor level provides electrons for higher empty π* bands, which can make the energy barrier lower and decrease critical temperature. On the contrary, electrons jumping from the d∥ band can be recombined by holes on the acceptor, impeding the MIT occurrence. It is claimed that the electronic orbital occupancy is closely related to oxygen pressure, which changes the energy barrier and manipulates the phase transition temperature. The present results are helpful to understand the fundamental mechanism of VO2 films and practical applications for VO2-based optoelectronic devices.

Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods

Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.

High Visible Photoelectrochemical Activity of Ag Nanoparticle-Sandwiched CdS/Ag/ZnO Nanorods

We report on the sensitizing of CdS-coated ZnO (CdS/ZnO) nanorods (NRs) by Ag nanoparticles (NPs) embedded between the CdS coating and the ZnO nanorod and the improved optical and photoelectrochemical properties of the Ag NP-sandwiched nanostructure CdS/Ag/ZnO NRs. The CdS/Ag/ZnO NRs were fabricated by growing Ag NPs on hydrothermally grown ZnO NRs and subsequently depositing CdS coatings followed by subsequent N2 annealing. The structure of the fabricated CdS/Ag/ZnO NRs was characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman backscattering, revealing that the ZnO NRs and the CdS coatings are both structured with hexagonal wurtzite and the Ag NPs contact well with ZnO and CdS. Optical properties were evaluated by measuring optical absorption and photoluminescence, showing that the Ag NPs behave well as sensitizers for optical property improvement and the CdS/Ag/ZnO NRs exhibit better photoresponse in a wide spectral region than CdS/ZnO because of plasmon-enhanced absorption due to the embedment of Ag NPs. The Ag NPs also serve as electron relays from CdS to ZnO, facilitating electron transfer from the CdS coatings to the ZnO NRs. The excellent photoresponse and efficient electron transfer make the CdS/Ag/ZnO NRs highly photoelectrochemically active. The CdS/Ag/ZnO NRs fabricated on indium-tin oxide present much better photoelectrochemical performance as photoanodes working in the visible region than CdS/ZnO NRs without Ag NPs. Under visible illumination, a maximum optical-to-chemical conversion efficiency of 3.13% is obtained for CdS/Ag/ZnO NR photoanodes against 1.35% for CdS/ZnO NR photoanodes.

Enhanced Photoelectrochemical Activity of ZnO-Coated TiO2 Nanotubes and Its Dependence on ZnO Coating Thickness.

One-dimensional heterogeneous nanostructures in the form of ZnO-coated TiO2 nanotubes (ZnO/TiO2 NTs) were fabricated by atomic layer deposition of an ultrathin ZnO coating on electrochemical anodization-formed TiO2 nanotubes (NTs) with the thickness of ZnO coating being precisely controlled at atomic scale, and the photoelectrochemical activity of the fabricated ZnO/TiO2 NTs and the influence of ZnO coating and its thickness were studied. The structures of TiO2 NTs and ZnO coatings were characterized by X-ray diffraction, Raman backscattering spectroscopy, and transmission electron microscopy. The photoelectrochemical activity was studied through the measurements of electrochemical impendence, flat-band potential, and transient photocurrent density. The TiO2 NTs exhibit anatase structure, and the ZnO coatings are structured with hexagonal wurtzite. The photoelectrochemical activity of the ZnO/TiO2 NTs is strongly dependent on the thickness of ZnO coating. ZnO/TiO2 NTs with a thinner rather than a thicker ZnO coating exhibit better photoelectrochemical activity with reduced charge transfer resistance, increased negative flat-band potentials, and enhanced photocurrent densities. Under visible illumination, an increase of about 60 % in the photoelectrochemical activity is obtained for ZnO/TiO2 NTs with an about 2-nm-thick ZnO coating.

Optoelectronic properties of ZnO film on silicon after SF6 plasma treatment and milliseconds annealing

Zinc oxide thin film is one of the most promising candidates for the transparent conductive layer in microelectronic and photovoltaic applications, due to its low resistivity and high transmittance in the visible spectral range. In this letter, we present optoelectronic and structural properties of fluorine doped ZnO films deposited at low temperature on a silicon substrate. The fluorine doping was made by post-deposition SF6 plasma treatment and activation by the millisecond range flash lamp annealing. Both the microstructural and optical investigations confirm the formation of a high-quality, highly doped n-type ZnO layer. The current-voltage characteristics show a heterojunction between n+-ZnO and Si. Moreover, it is shown that the SF6 plasma treatment efficiently passivates the surface state and bulk defects in the ZnO film.

Enhanced charge separation of vertically aligned CdS/g-C3N4 heterojunction nanocone arrays and corresponding mechanisms

Vertical-aligned CdS/g-C3N4 heterojunction nanocone arrays have been fabricated on silicon(100) substrates by a plasma sputting reaction deposition and pulsed laser deposition, successively. The morphology and structure of the as-fabricated nanocone arrays were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy, revealing a single-crystalline hexagonal g-C3N4 structure of the g-C3N4 nanocone arrays and a poly-crystalline hexagonal wurtzite-structure of the CdS shells. The optical properties of the CdS/g-C3N4 nanocone arrays were studied by steady-state and time-resolved photoluminence and optical absorption measurements, showing that the CdS/g-C3N4 heterojunction could efficiently promote the separation of the electron–hole pairs generated in the nanocone arrays, which indicates the attractive prospects of the CdS/g-C3N4 nanocone arrays in the field of photocatalysis.

Characterization of carbon nitride deposition from CH4∕N2 glow discharge plasma beams using optical emission spectroscopy

The properties of plasmas in a CH4∕N2 dc abnormal glow discharge with the percentage of methane from 1% to 20% have been studied in order to understand the effect of precursor incorporation into the carbon nitride (CNx) films. The appearance of CN radicals as well as C2, CH, and NH has been revealed by optical emission spectroscopy (OES). The evolution of CN, N2, N2+, and C2 emission lines from mixed CH4∕N2 and pure N2 plasma on changing mixture ratio and polarity of discharge-field has been studied. The possible mechanisms behind their variations have been discussed. Besides, a CH4∕N2 ratio of 1∕50 and a top-needle anode is considered to be the best conditions for synthesis of β-C3N4, which has been confirmed in the as-deposited carbon nitride films with quite good crystalline features by XRD analyses.