Hong-Guang Xu

Room-temperature ferromagnetism in (Mn, N)-codoped ZnO thin films prepared by reactive magnetron cosputtering

(Mn, N)-codoped ZnO films were grown on fused silica substrates by reactive magnetron cosputtering. X-ray diffraction measurements reveal that the films have the single-phase wurtzite structure with c-axis preferred orientation. X-ray photoelectron spectroscopy studies indicate the incorporation of both divalent Mn2+ and trivalent N3− ions into ZnO lattice. Acceptor doping with nitrogen partly compensates the “native donors,” which results in a low electron concentration of 3.16×1016cm−3 though p-type conductivity is not achieved. (Mn, N)-codoped ZnO films show significant ferromagnetism with Curie temperature above 300K. The mechanism of ferromagnetic coupling in codoped ZnO is discussed based on a bound magnetic polaron model.

Structural, optical, and magnetic properties of Mn-doped ZnO thin film

The Zn(1-x)Mn(x)O (x = 0, 0.16, and 0.25) thin films were grown on fused quartz substrates by reactive magnetron cosputtering. X-ray-diffraction measurement revealed that all the films were single phase and had wurtzite structure with c-axis orientation. As Mn concentration increased in the Zn(1-x)Mn(x)O films, the c-axis lattice constant and band-gap energy increased gradually. In Raman-scattering studies, an additional Mn-related vibration mode appeared at 520 cm(-1). E(2H) phonon line of Zn(1-x)Mn(x)O alloy was broadened asymmetrically and redshifted as a result of microscopic structural disorder induced by Mn(2+) random substitution. The Zn(0.84)Mn(0.16)O film exhibited a ferromagnetic characteristic with a Curie temperature of approximately 62 K. However, with increasing Mn concentration to 25 at. %, ferromagnetism disappeared due to the enhanced antiferromagnetic superexchange interactions between neighboring Mn(2+) ions.

Preparation and characterization of ZnO particles embedded in SiO2 matrix by reactive magnetron sputtering

ZnO particles embedded in SiO2 thin films were prepared by a radio-frequency magnetron sputtering technique. X-ray diffraction (XRD) and optical-absorption spectra showed that ZnO particles with hexagonal wurtzite structure had been embedded in the SiO2 matrix, and the size of ZnO particles increased with increasing annealing temperature from 773to973K. Raman-scattering and Fourier transform infrared (FTIR) spectrum measurements also confirmed the presence of ZnO particles. When the annealing temperature was lower than 973K, room-temperature photoluminescence (PL) spectra showed dominative deep-level emissions in the visible region and very weak ultraviolet emissions. As the annealing temperature increased to 973K, an emission band in the ultraviolet region besides the emissions from free and bound excitons recombination was observed in the low-temperature PL spectra. The origin of the ultraviolet emission bands was discussed with the help of temperature-dependent PL spectra. When the annealing temperatur...

Structures and photoelectron spectroscopy of Cun(BO2)m− (n, m = 1, 2) clusters: Observation of hyperhalogen behavior

The electronic structures of CuBO(2)(-), Cu(BO(2))(2)(-), Cu(2)(BO(2))(-), and Cu(2)(BO(2))(2)(-) clusters were investigated using photoelectron spectroscopy. The measured vertical and adiabatic detachment energies of these clusters revealed unusual properties of Cu(BO(2))(2) cluster. With an electron affinity of 5.07 eV which is larger than that of its BO(2) superhalogen (4.46 eV) building-block, Cu(BO(2))(2) can be classified as a hyperhalogen. Density functional theory based calculations were carried out to identify the ground state geometries and study the electronic structures of these clusters. Cu(BO(2)) and Cu(BO(2))(2) clusters were found to form chainlike structures in both neutral and anionic forms. Cu(2)(BO(2)) and Cu(2)(BO(2))(2) clusters, on the other hand, preferred a chainlike structure in the anionic form but a closed ringlike structure in the neutral form. Equally important, substantial differences between adiabatic detachment energies and electron affinities were found, demonstrating that correct interpretation of the experimental photoelectron spectroscopy data requires theoretical support not only in determining the ground state geometry of neutral and anionic clusters, but also in identifying their low lying isomers.

Structures and magnetic properties of CrSin− (n = 3–12) clusters: Photoelectron spectroscopy and density functional calculations

Chromium-doped silicon clusters, CrSi(n)(-) (n = 3-12), were investigated with anion photoelectron spectroscopy and density functional theory calculations. The combination of experimental measurement and theoretical calculations reveals that the onset of endohedral structure in CrSi(n)(-) clusters occurs at n = 10 and the magnetic properties of the CrSi(n)(-) clusters are correlated to their geometric structures. The most stable isomers of CrSi(n)(-) from n = 3 to 9 have exohedral structures with magnetic moments of 3-5μ(B) while those of CrSi(10)(-), CrSi(11)(-), and CrSi(12)(-) have endohedral structures and magnetic moments of 1μ(B).

Effect of oxygen-related surface adsorption on the efficiency and stability of ZnO nanorod array ultraviolet light-emitting diodes

Ultraviolet light-emitting diodes using MgZnO-coated and bare ZnO nanorod arrays as active layers were manufactured. Both types were exposed to ambient air over a 1-yr period to assess their stability. By monitoring the electroluminescence evolution with air-exposure time and comparing the changes of electroluminescence and x-ray photoelectron spectra before and after vacuum desorption, it is concluded that surface-adsorbed O2 and OH− species, as acceptor and donor surface states, quench ultraviolet electroluminescence, and favor undesirable surface-mediated nonradiative and deep-level recombination. The MgZnO coating prevents surface adsorption, and so the coated nanorod device shows higher efficiency and stability than the uncoated one.

Microsolvation of LiI and CsI in water: anion photoelectron spectroscopy and ab initio calculations.

In order to understand the microsolvation of LiI and CsI in water and provide information about the dependence of solvation processes on different ions, we investigated the LiI(H2O)n(-) and CsI(H2O)n(-) (n = 0-6) clusters using photoelectron spectroscopy. The structures of these clusters and their corresponding neutrals were investigated with ab initio calculations and confirmed by comparing with the photoelectron spectroscopy experiments. Our studies show that the structural evolutions of LiI(H2O)n and CsI(H2O)n clusters are very different. The Li-I distance in LiI(H2O)n(-) increases abruptly at n = 3, whereas the abrupt elongation of the Li-I distance in neutral LiI(H2O)n occurs at n = 5. In contrast to the LiI(H2O)n(-) clusters, the Cs-I distance in CsI(H2O)n(-) increases significantly at n = 3, reaches a maximum at n = 4, and decreases again as n increases further. There is no abrupt change of the Cs-I distance in neutral CsI(H2O)n as n increases from 0 to 6. Water molecules interact strongly with the Li ion; consequently, water molecule(s) can insert within the Li(+)-I(-) ion pair. In contrast, five or six water molecules are not enough to induce obvious separation of the Cs(+)-I(-) ion pair since the Cs-water interaction is relatively weak compared to the Li-water interaction. Our work has shown that the structural variation and microsolvation in MI(H2O)n clusters are determined by the delicate balance between ion-ion, ion-water, and water-water interactions, which may have significant implications for the general understanding of salt effects in water solution.

Photoelectron spectroscopy and density functional calculations of CuSin− (n = 4–18) clusters

We conducted a combined anion photoelectron spectroscopy and density functional theory study on the structural evolution of copper-doped silicon clusters, CuSi(n)(-) (n = 4-18). Based on the comparison between the experiments and theoretical calculations, CuSi(12)(-) is suggested to be the smallest fully endohedral cluster. The low-lying isomers of CuSi(n)(-) with n ≥ 12 are dominated by endohedral structures, those of CuSi(n)(-) with n < 12 are dominated by exohedral structures. The most stable structure of CuSi(12)(-) is a double-chair endohedral structure with the copper atom sandwiched between two chair-style Si(6) rings or, in another word, encapsulated in a distorted Si(12) hexagonal prism cage. CuSi(14)(-) has an interesting C(3h) symmetry structure, in which the Si(14) cage is composed by three four-membered rings and six five-membered rings.

Hydrogen bonds in the nucleobase-gold complexes: Photoelectron spectroscopy and density functional calculations

The nucleobase-gold complexes were studied with anion photoelectron spectroscopy and density functional calculations. The vertical detachment energies of uracil-Au(-), thymine-Au(-), cytosine-Au(-), adenine-Au(-), and guanine-Au(-) were estimated to be 3.37 ± 0.08 eV, 3.40 ± 0.08 eV, 3.23 ± 0.08 eV, 3.28 ± 0.08 eV, and 3.43 ± 0.08 eV, respectively, based on their photoelectron spectra. The combination of photoelectron spectroscopy experiments and density functional calculations reveals the presence of two or more isomers for these nucleobase-gold complexes. The major isomers detected in the experiments probably are formed by Au anion with the canonical tautomers of the nucleobases. The gold anion essentially interacts with the nucleobases through N-H···Au hydrogen bonds.

Structural and Magnetic Properties of CoGen− (n=2–11) Clusters: Photoelectron Spectroscopy and Density Functional Calculations

A series of cobalt-doped germanium clusters, CoGe(n)(-/0) (n=2-11), are investigated by using anion photoelectron spectroscopy combined with density functional theory calculations. For both anionic and neutral CoGe(n) (n=2-11) clusters, the critical size of the transition from exo- to endohedral structures is n=9. Natural population analysis shows that there is electron transfer from the Ge(n) framework to the Co atom at n=7-11 for both anionic and neutral CoGe(n) clusters. The magnetic moments of the anionic and neutral CoGe(n) clusters decrease to the lowest values at n=10 and 11. The transfer of electrons from the Gen framework to the Co atom and the minimization of the magnetic moments are related to the evolution of CoGe(n) structures from exo- to endohedral.