Enhancement in photoluminescence performance of carbon-decorated T-ZnO

The facile preparation of ZnO possessing high visible luminescence intensity remains challenging due to an unclear luminescence mechanism. Here, two basic approaches are proposed to enhance the luminescent intensity based on the theoretical analysis over surface defects. Based on the deduction, we introduce a methodology for obtaining hybrid tetrapod-like zinc oxide (T-ZnO), decorated by carbon nanomarterials on T-ZnO surfaces through the catalytic chemical vapor deposition approach. The intensity of the T-ZnO green emission can be modulated by topography and the proportion of carbon. Under proper experiment conditions, the carbon decorating leads to dramatically enhanced luminescence intensity of T-ZnO from 400 to 700 nm compared with no carbon decorated, which elevates this approach to a simple and effective method for the betterment of fluorescent materials in practical applications.

Deep level transient spectroscopy studies of n-type ZnO single crystals grown by different techniques

In the present study single-crystalline ZnO samples grown from the vapor phase, the melt, and a high-temperature aqueous solution (hydrothermal growth) are investigated before and after hydrogen plasma treatments, by means of deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS. Dominant DLTS peaks are found to appear in the range of 120-350 K for all materials. The DLTS spectra depend on the procedure of growth of the ZnO. The thermal stabilities of the defects in an oxygen atmosphere and in an oxygen-lean atmosphere are analyzed. The origin of the DLTS peaks is discussed.

Electronic Barriers Produced at Zno Surfaces By Ion Implantation And Annealing

Modern varistors are made of polycrystalline ZnO, doped to create Schottky-like barriers at the grain boundaries. To allow the study of tailored single barrier junctions, we have implanted ZnO crystals with Bi, Sb and transition metals. The electrical properties and the distribution of Bi and Sb were measured in the implanted crystals as a function of annealing temperature. After implantation and before heating, no barriers can be observed. After heating to 600–800°C, a high voltage (insulating) barrier is created. Heating to above 1000°C produces a 2-1/2 Volt barrier with high nonlinearity. The temperatures at which these barriers appear correspond, respectively, to temperatures at which motion of Sb and Bi first can be observed by ion backscattering techniques, and at which these elements diffuse freely.

Ab initio cluster calculations on the electronic structure of oxygen vacancies at the polar ZnO(0001̄) surface and on the adsorption of H2, CO, and CO2 at these sites

Oxygen vacancies at the polar O terminated (0001) surface of ZnO are of particular interest, because they are discussed as active sites in the methanol synthesis. In general, the polar ZnO surfaces are stabilized by OH groups, therefore O vacancies can be generated by removing either O atoms or OH or H2O groups from the surface. These defects differ in the number of electrons in the vacancy and the number of OH groups in the neighborhood. In the present study, the electronic structure and the adsorption properties of four different types of oxygen vacancies have been investigated by means of embedded cluster calculations. We performed ab initio calculations on F+ like surface excitations for the different defect types and found that the transition energies are above the optical band-gap, while F+ centers in bulk ZnO show a characteristic optical excitation at 3.19 eV. Furthermore, we studied the adsorption of CO2 and CO at the different defect sites by DFT calculations. We found that CO2 dissociates at electron rich vacancies into CO and an O atom which remains in the vacancy. At the OH vacancy which contains an unpaired electron CO2 adsorbed in the form of CO2-, while it adsorbed as a linear neutral molecule at the H2O defect. CO adsorbed preferentially at the H2O defect and the OH defect, both with a binding energy of 0.3 eV.

Ab initio cluster calculations for the absorption energies of F and F+ centers in bulk ZnO

We present the results of a series of ab initio calculations on the ground states and the low lying excited states of the F and F+ centers in bulk ZnO. Both types of F centers are oxygen vacancies, causing rather strong distortions of the local geometries. The calculations were performed by means of wave function based methods, mostly at the CASSCF level. Dynamic correlation was included for the first two coordination shells of the F centers. The calculated absorption energy for the F+ center (3.19 eV) is in excellent agreement with the experimental value of 3.03 eV. For the emission from the 3T2 state of the F center to the 1A1 ground state we obtained a transition energy of 2.73 eV. Experimentally, a green photoluminescence is observed at 2.38-2.45 eV. We estimated that the errors in our calculation should be even smaller in the latter case than for the F+ state, where the calculated transition energy differs by less than 0.2 eV from the experimental value. Therefore, we assume that the 3T2 to 1A1 transition is not the origin of the green luminescence.