ZnO Bilayer Films Research Articles

B and Au doped ZnO and ZnO/Cu(111) bilayer films: A DFT investigation

AbstractsThe structural and electronic properties of B and Au doped graphite‐like ZnO and ZnO/Cu(111) bilayer films have been investigated by using the DFT + U scheme including dispersion correction and compared to that of undoped materials. The differences in electronic properties of the B and Au doped ZnO and ZnO/Cu(111) films compared to undoped materials were probed by CO adsorption. The same oxidation state of B (+3) was observed for B doped ZnO and ZnO/Cu(111) bilayer films. However, an excess electron associated with B doping is delocalized over the conduction band of ZnO, while it transfers to underlying Cu support of ZnO/Cu(111). This is evidenced by a much large redshift of CO frequency on B doped ZnO versus the small redshift of CO frequency on B doped ZnO/Cu(111). At variance, the oxidation state of Au dopant was reduced from +2 (5d9) for Au doped ZnO to +1 (5d10) for Au doped ZnO/Cu(111) films. This is due to the charge transfer from Cu support to doped Au which results in the stronger binding of CO on Au doped ZnO/Cu(111) compared to ZnO bilayer films. However, a slight difference in CO frequency was found for CO bound to Au doped ZnO and ZnO/Cu(111).

DFT study of electronic properties of N-doped ZnO and ZnO/Cu(111) bilayer films

The electronic properties of N-doped ZnO/Cu(111) bilayer films have been studied by using the DFT+U method including dispersion contributions and compared to that of free-standing ZnO bilayer films. The substitution of a lattice O ion in free-standing ZnO bilayer by a N atom results in oxidation state of N of -2, while the oxidation state of N becomes -3 when it replaces a lattice O in a ZnO/Cu(111) bilayer film. The extra negative charge of N in N:ZnO/Cu(111) compared to freestanding N:ZnO is due to an electron transfer from the Cu support. The difference in oxidation state of the N dopant is demonstrated by the analysis of the Bader charge, of the magnetization, and of the density of states. The changes in electronic properties of the doped ZnO films were evaluated by adsorbing CO and H2 molecules. The results show that the adsorption energies are significantly enhanced in N-doped ZnO/Cu(111) compared to the unsupported N:ZnO bilayer counterpart. This reflects the effect of the ZnO/Cu(111) interface in improving adsorption and possibly catalytic properties and the role of the N-dopant in activating the oxide film.

Controlled growth of uniform two-dimensional ZnO overlayers on Au(111) and surface hydroxylation

Ultrathin ZnO nanostructures present interesting two-dimensional (2D) graphene-like structure in contrast to wurtzite structure in bulk ZnO. Growth on Au(111) has been regarded as a well-established route to the 2D ZnO layers while controlled growth of uniform ZnO nanostructures remains as a challenge. Here, reactive deposition of Zn in O3 and NO2 was employed, which is investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). We demonstrate that uniform ZnO monolayer nanoislands and films can be obtained on Au(111) using O3 and uniform ZnO bilayer nanoislands and films form on Au(111) using NO2, respectively. Formation of atomic oxygen overlayers on Au(111) via exposure to O3 is critical to the formation of uniform ZnO monolayer nanostructures atop. Near ambient pressure XPS studies revealed that nearly full hydroxylation occurs on monolayer ZnO structures upon exposure to near ambient pressure water vapor or atomic hydrogen species, while partial surface hydroxylation happens on bilayer ZnO under the same gaseous exposure conditions.

Electronic structure of Al, Ga, In and Cu doped ZnO/Cu(111) bilayer films.

The effect of doping with group-III metals (Al, Ga and In) and Cu free standing and Cu(111) supported ZnO bilayer films has been investigated computationally by using the DFT+U method including dispersion contributions. The changes in the electronic properties of doped ZnO and ZnO/Cu(111) films have been tested by adsorbing CO probe molecules. The replacement of a lattice Zn ion in a free-standing ZnO bilayer by a group-III element generates an extra electron whose distribution depends on the dopant. In particular, while the excess electron is delocalized over the conduction band for Al or Ga doping, it is localized on the dopant in the case of In. The situation is different on the supported ZnO/Cu(111) film, where the extra electron is transferred to the underlying Cu support. While the CO adsorption energy at the doped sites in the ZnO bilayer is the same as in the ZnO/Cu(111) ultrathin films, CO exhibits a larger red-shift in the unsupported ZnO bilayer. The oxidation state of Cu replacing Zn in the unsupported ZnO films is 2+, Cu(3d9) state, while it is 1+, Cu(3d10) state, in the ZnO/Cu(111) supported films where a charge transfer from the supporting Cu metal to the Cu impurity occurs. Cu doping results in a stronger interaction with CO and a large red-shift of the CO stretching frequency. In this respect, Cu doping of ZnO/Cu(111) bilayer films could have interesting consequences in gas adsorption while doping with group-III elements does not lead to major changes of the adsorption properties when the free-standing ZnO films are compared to the supported ZnO/Cu(111) counterparts.

Oxygen Vacancy in Wurtzite ZnO and Metal-Supported ZnO/M(111) Bilayer Films (M = Cu, Ag and Au)

Using spin-polarized DFT+U calculations we have studied the nature of the O vacancy in graphitic-like ZnO bilayer films supported on Cu, Ag, and Au (111) surfaces and compared it with the same defe...

Porous/dense ZnO bilayer films grown by thermal oxidation of ZnS film with gallium

Porous/dense ZnO bilayer films were grown by thermal oxidation of ZnS films deposited with gallium in air. The microstructures and photoluminescent properties of the as-grown ZnO bilayer films were investigated by XRD, SEM, HRTEM, XPS and PL. It was found that Ga doping contributed to the growth of ZnO dense layer, which covers the ZnO porous layer formed due to the evaporation of sulfur oxides. The diffusion-limited Ga gradient distribution along the thickness of ZnO films was observed. The remained Ga on the surface region led to the formation of ZnGa2O4 layer covering the ZnO film. The growth mechanism and morphology of the ZnO film covered by ZnGa2O4 grown by physical vapor deposition of metal gallium are quite different from that of the ZnO film covering ZnGa2O4 grown by manual application of molten gallium. Meanwhile, the formation of ZnGa2O4 was prohibited by altering Ga deposition distance. Higher Ga doping decreased the concentration of oxygen vacancy, which reduced the visible emission from ZnO film in combination with existence of ZnGa2O4 layer.

CO Adsorption on Graphite-like ZnO Bilayers Supported on Cu(111), Ag(111), and Au(111) Surfaces

Graphitic-like ZnO bilayer films deposited on coinage metals Cu(111), Ag(111), and Au(111) have been studied by density functional theory calculations including dispersion corrections. The scope is to compare on an equal footing the properties of the three systems and in particular the nature of the metal/oxide interface. To this end we have considered the adsorption of a CO probe molecule and the vibrational shifts induced by adsorption on ZnO/Cu(111), ZnO/Ag(111), and ZnO/Au(111) compared to adsorption on the unsupported ZnO bilayer and on the wurtzite ZnO (1010) surface. We find that while the interaction of ZnO with Ag and Au supports is dominated by dispersion interactions with little or no charge transfer at the interface, in the case of Cu a moderate electron transfer occurs toward the ZnO bilayer. As a consequence, while the stretching frequency of CO on ZnO/Au is blue-shifted, that on ZnO/Cu is red-shifted compared to free CO. CO on ZnO/Ag is intermediate. In all three cases, however, the ZnO bi...

Defect and its dominance in ZnO films: A new insight into the role of defect over photocatalytic activity

Influence of defect on photocatalyst is in extensive concern. In the present contribution, by cold plasma treatment (CPT) technique and dip-coating method, we successfully prepared ZnO bilayer films with oxygen vacancies and interstitial zinc atoms. The defective films showed promoted photocatalytic activity, with enhanced transportation and good separation of photogenerated charge carriers being the main reasons. More importantly, combination of CPT and dip coating method allowed us to investigate the dominance of defect and its stability. Our work may not only provide helpful information for better understanding of relationship between defect and photocatalysis, but also offer a new strategy for designing stabilized defect-modulated photocatalyst.

Tunable resistive switching behaviour in ferroelectric–ZnO bilayer films

Pb(Zr0.52Ti0.48)O3/ZnO bilayer films with various ZnO-layer thicknesses were prepared by a sol–gel process, and their phase structures, electric conduction and polarization behaviour were measured. The results showed that the preferential crystal orientation of the ZnO layer changed with a change in its thickness. The strong dependence of both asymmetric current–voltage and polarization–voltage characteristics on the ZnO-layer thickness was observed. The resistance ratio of the high-resistance state (HRS) to the low-resistance state (LRS) increased with increasing ZnO-layer thickness, and a high rectification ratio was obtained in the bilayer film with an optimized ZnO-layer thickness. The combined effects of interface polarization coupling and energy band structure on the resistive switching behaviour of the bilayer films were revealed, and the electric conduction mechanisms of the bilayer films at both HRS and LRS were analysed in detail. This work presents an effective method to modulate the resistive switching behaviour of ferroelectric–ZnO heterostructures, which is significant in designing high-performance ferroelectric–semiconductor heterostructures for actual applications.

Effect of the ZnO Buffer Layer Thickness on AZO Film Properties

To evaluate the influence of the ZnO buffer layer thickness on structural, electrical and optical properties of ZnO: Al (AZO)/ZnO bi-layer films, a series of AZO/ZnO films were deposited on the quartz substrates by electron beam evaporation. X-ray diffraction measurement shows that the crystal quality of the films is improved with the increase of the film thickness. The electrical properties results show that the resistivity decreases initial and then increases. However, optical transmittance of all the films is >80% regardless of the buffer layer thickness in the visible region. The results illustrate that the insertion of ZnO buffer layer can improve the film performance.

Effects of the ZnO buffer layer and Al proportion on AZO film properties

To evaluate the influence of the ZnO buffer layer and Al proportion on the properties of ZnO: Al (AZO)/ZnO bi-layer films, a series of AZO/ZnO films are deposited on the quartz substrates by electron beam evaporation. The X-ray diffraction measurement shows that the crystal quality of the films is improved with the increase of the film thickness. The electrical properties of the films are investigated. The carrier concentration and Hall mobility both increase with the increase of buffer layer thickness. However, the resistivity reaches the lowest at about 50 nm-thick buffer layer. The lowest resistivity and the maximum Hall mobility are both obtained at 1 wt% Al concentration. But the optical transmittance of all the films is greater than 80% regardless of the buffer layer thickness with Al concentration lower than 5 wt% in the visible region.

New Generation Transparent LPCVD ZnO Electrodes for Enhanced Photocurrent in Micromorph Solar Cells and Modules

ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process step by controlling the differential doping of the nucleation and bulk parts of the layers. The resulting 2-μm-thick films are characterized by low free-carrier absorption and electron mobility over 40 cm2/Vs. They, therefore, combine high transparency in the infrared region and moderate sheet resistance that can be lowered below 20 Ω/sq. These properties make ZnO bilayers ideal candidates as electrodes for the development of micromorph thin-film solar cells with enhanced photogenerated current. The potential of such bilayer front electrodes for a further power improvement and cost reduction of industrial micromorph tandem modules is currently investigated at Oerlikon Solar. The first experiments already show a promising gain in the bottom μc-Si:H cell photogenerated current compared with the current generated with modules deposited on standard uniformly doped ZnO single-layer front contacts.

Highly transparent ZnO bilayers by LP-MOCVD as front electrodes for thin-film micromorph silicon solar cells

ZnO bilayer films were deposited by a low-pressure metalorganic chemical vapor deposition technique in a single process step, by doping with boron only the nucleation stage of the growth. The resulting 2μm thick layers are characterized by low free carrier absorption and electron mobilities over 40cm2/Vs. They therefore combine high transparency in the infrared region and moderate sheet resistances of 30Ω/sq. It is thought that the controlled introduction of boron in the early nucleation stage enables its diffusion through the layer, resulting in a very efficient reduction of the sheet resistance compared to samples deposited without doping, while preserving a strong light-scattering interface. These properties make ZnO bilayers ideal electrodes for the development of micromorph thin-film solar cells with enhanced photo-generated current.

Numerical investigation on the thermal conductive characteristics of the TiO<SUB align=right>2/ZnO bilayer films

The aim of this article is to provide a numerical approach to perform thermal conductive design or evaluation for TiO 2 /ZnO bilayer films. Based on the investigation of the structure characteristics of TiO 2 /ZnO bilayer films, the key performance about the thermal conductive characteristics of the TiO 2 /ZnO bilayer films is analysed by using equilibrium molecular dynamics (EMD) and Buckingham potential function. The results show that the temperature and the thickness of the film have a great impact on the thermal conductive characteristics of the TiO 2 /ZnO bilayer films. The simulation builds a basis for future investigation about thermal design of TiO 2 /ZnO for micro-nano manufacturing.

Surface-plasmon-mediated emission enhancement from Ag-capped ZnO thin films

Ag/ZnO bilayer thin films are fabricated on Si substrates via two-step approach of ZnO sputtering + Ag evaporation. The enhancement of the near band edge (NBE) emission of the ZnO film is realized through coupling between the surface plasmon resonating energy at Ag/ZnO interface and the photonic energy of ZnO NBE emission. The dependence of the emission enhancement ratio of ZnO on the thickness and the growth temperature T of Ag cap-layers are investigated. By evaporating Ag(8 nm) cap-layer onto ZnO(100 nm) film at high substrate temperatures (T300 ℃), the value reaches about 18,i.e., 18, which is more than twice that of Ag(8 nm)/ZnO(100 nm) bilayer films grown at low temperatures (T200 ℃). It is found that the realization of the larger can be ascribed to the bigger surface roughness of Ag/ZnO bilayer samples prepared under higher growth temperatures.

Deposition and characterization of zinc oxide and tin oxide ultrafine particle bilayer thin films

A d.c. gas discharge activating reaction evaporation technique was used to prepare gas sensitive ZnO and SnO2 bilayer films. The bilayers were grown successfully onto glass substrates and the optimum processing conditions were obtained. Here, the main crystal phases of ZnO/SnO2 and SnO2/ZnO are SnO2 and SnO were with preferential growth in the (002) and (111) directions, while ZnO phase exhibits an amorphous phase. The bilayers were characterized by X-ray diffraction, scanning electron microscope showing ultrafine particle (UFP), and their gas sensing properties were also investigated. The UFP bilayers with average surface particle size of 45–80 nm exhibited a good sensitivity of 7.8 at 330°C (SnO2/ZnO), 9.0 at 290°C (ZnO/SnO2) to 2000 ppm C2H5OH, a very good selectivity for detecting C2H5OH in the presence of gasoline gas and acetone gas, a decrease of operating temperature, and also a fast response below 15 and 20 s to C2H5OH at their optimum operating temperature, respectively. The effects of annealing on topography and crystal phase of the bilayers were discussed. Meanwhile, their interface was observed by a scanning electron microscope method, and the results show that the bilayers have an obvious diffusion and clear columnar structure.