Wenhan Liu

Structures and magnetic properties of wurtzite Zn1- xCoxO dilute magnetic semiconductor nanocomposites

Cobalt-doped ZnO dilute magnetic semiconductor nanocomposites Zn1−xCoxO with Co concentrations from 0.02 to 0.25 were prepared by the sol-gel method. The magnetic measurement shows paramagnetic behavior for all the samples. The structures of these composites were investigated by x-ray diffraction and fluorescence x-ray absorption fine structure spectroscopy. It is indicated that at low Co concentration (x⩽0.05), the Co atoms are incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms. At higher Co doping concentration (x⩾0.10), the secondary phase Co3O4 is precipitated. Correlating the magnetic behavior with the structural properties of the Zn1−xCoxO nanocomposites, the authors interpret the paramagnetism to be intrinsic in nature as a result of the low effective doping of Co in ZnO and the lack of oxygen vacancies.

Quantitative microwave evanescent microscopy of dielectric thin films using a recursive image charge approach

A recursive image charge approach has been successfully developed for quantitative microwave microscopy of dielectric thin films using the scanning evanescent microwave microscope. With this approach and the recursion-to-circulation algorithm, frequency shift of the microscope as functions of the thickness of the film, dielectric constants of the film and the substrate can be efficiently computed in a circulation way.

High-throughput x-ray characterization system for combinatorial materials studies

We have developed a high-throughput x-ray characterization system, which can rapidly screen structure, composition and x-ray scintillation of combinatorial materials libraries using energy-dispersive x-ray diffraction, x-ray fluorescence, and x-ray photoluminescence. This system consists of an x-ray source, a polycapillary x-ray lens, one or two x-ray energy detectors, and a fiber optic spectrometer. A Windows-based software package was also developed to control the measurement system and data acquisition automatically. Examples are also demonstrated to show the applicability of this system.

Local Structures Around Co Atoms in Wurtzite ZnO Nano-Composites Probed by Fluorescence XAFS

The local structures around Co ions in the Zn1−xCoxO nano‐composites prepared by the sol‐gel method have been investigated by fluorescence X‐ray absorption fine structure (XAFS) technique. The results indicate that for dilute Co‐doped ZnO (x=0.02, 0.05), the Co2+ ions are incorporated into the ZnO lattice, and are located at the position of the substitutional Zn2+ ions. As the Co content increases to 0.10 or higher, only part of the Co ions enter the lattice of the wurtzite and the others exist in the form of a Co3O4 phase whose content increases with the doped Co concentration. In the substitutional Zn0.98Co0.02O sample, the bond length of the first shell RCo‐O and the second shell RCo‐Zn is smaller than the second shell Zn‐Zn distance in ZnO by about 0.01∼0.02 A. These results imply that only small local lattice deformation is induced by dilute Co2+ substituting into the Zn2+ sites.

Quantitative microscopy of nonlinear dielectric constant using a scanning evanescent microwave microscopy

Quantitative characterization of dielectric nonlinearity in ferroelectric materials has been successfully performed using a scanning evanescent microwave microscope, and a key calibration coefficient for quantitative microscopy of nonlinear dielectric constant is derived. This unique technique has advantages of high spatial resolution and simultaneously accessing of other related properties such as dielectric constant and magnetoelectric coefficient. Samples of LiNbO3 single crystal and PbTiO3 thin film are measured, which demonstrates that this technique can access the nonlinear dielectric constant of a microregion with a sensitivity of 1.0×10−21F∕V.

Local Structure around Iron Ions in Anatase TiO2

The local structure around iron impurity in anatase TiO2 nano‐composite with different iron content (1, 2, 5 and 10 wt %) has been studied by fluorescence XAFS. The results indicate that for the sample with low iron content (1 and 2 wt %), the iron ions are incorporated into the lattice of anatase, and substitute the Ti ions. With the iron content increasing to 5 wt %, only part of iron ions enter the lattice of anatase, and the rest form α‐Fe2O3 phase. As the iron content reaches 10 wt %, iron ions mainly exist in α‐Fe2O3 phase. It is found that, in the (1 wt %)Fe‐doped TiO2 the bond lengths of the first (Fe–O) and second (Fe–Ti) shells are RFe‐O=1.97 A and RFe‐Ti=3.01 A, respectively, which means that the RFe‐O is only slightly larger than the first shell Ti–O bond length by 0.01 A, while the RFe‐Ti is significantly smaller (0.04 A) than that of the corresponding Ti–Ti shell in anatase. These results imply that iron ions occupying the Ti sites greatly change the symmetry of the first Fe–O shell.

Recursive Image Charge Approach for Quantitative Characterization of Dielectric Thin Film Library Using Scanning Tip Microwave Near-field Microscopy

We developed a recursive image charge approach for quantitative characterizations of dielectric thin films using the scanning tip microwave near-field microscope. With this method, frequency shift of the microscope as functions of the dielectric constant and the thickness of a film can be effectively computed in a recursive way. We believe that this approach can promote the high-throughput characterization of the dielectric libraries.

Combinatorial material library synthesis from insoluble oxide suspension using a drop-on-demand inkjet delivery system

A combinatorial synthesis method was developed by ejecting insoluble oxide suspensions using a drop-on-demand inkjet delivery system. The insoluble oxide suspensions with ultrafine/nano particles were prepared by grinding the oxide power in water using a high-energy ball mill. Using luminescent materials as model systems, it was established that the technique is very well suited to combinatorial synthesis of insoluble oxides.