F. Z. Chien

Diameter dependence of the electronic structure of ZnO nanorods determined by x-ray absorption spectroscopy and scanning photoelectron microscopy

OK-, ZnL3, and K-edges x-ray absorption near-edge structure (XANES) spectra and scanning photoelectron microscopy (SPEM) spectra were obtained for ZnO nanorods with various diameters. The analysis of the XANES spectra revealed increased numbers of O2p and Zn4p unoccupied states with the downsizing of the nanorods, which reflects the enhancement of surface states when the diameter is decreased. Valence-band photoemission spectra show a significant narrowing of the valence band for the 45nm diameter nanorod. The Zn3d intensities in the Zn3d SPEM spectra are drastically diminished for all nanorods as compared to the ZnO reference film, which can be interpreted as a reduction in density of itinerant final states or in transition probability.

Comparison of the electronic structures of Zn1−xCoxO and Zn1−xMgxO nanorods using x-ray absorption and scanning photoelectron microscopies

X-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been performed for Zn1−xCoxO and Zn1−xMgxO to elucidate the effects of the doping of Co and Mg, which have very different electronegativities, on the electronic structures of ZnO nanorods. The intensities of O K-edge near-edge features in the XANES spectra of Zn1−xCoxO and Zn1−xMgxO nanorods are found to be lower than those of ZnO, which suggests that both Co and Mg substitutions of the Zn ions enhance the effective charge on the O ion. The valence-band SPEM measurements show that Mg doping does not increase the density of near-Fermi-level states, which implies that Mg doping will not improve field emission of ZnO nanorods. It is surprising to find that both Co and Mg substitutions of Zn increase the numbers of O 2p dominated valence-band states, despite that Co and Mg have larger and smaller electronegativities than that of Zn.

Charge transfer in nanocrystalline-Au/ZnO nanorods investigated by x-ray spectroscopy and scanning photoelectron microscopy

O K- and Zn and Au L3-edge x-ray absorption near-edge structure (XANES), x-ray emission spectroscopy (XES), and scanning photoelectron microscopy (SPEM) are performed to investigate the electronic structure of ZnO nanorods with nanocrystalline (nc)-Au particles grown on the surfaces. The XANES spectra of nc-Au∕ZnO nanorods reveal the decrease of the number of both O 2p and Zn 4s∕3d unoccupied states with the increase of the nc-Au particle size. The number of Au 6s∕5d unoccupied states increases when the size of nc-Au particle decreases, indicating that the deposition of nc-Au particles on the surface of ZnO nanorods promotes charge transfer from the ZnO nanorods to nc-Au particles. Excitation energy dependent XES and SPEM spectra show that the number of electrons in the valence band of O 2p-Zn 4sp hybridized states decreases as the nc-Au particle size increases, revealing that more electrons are excited from the valence band to the conduction band of ZnO nanorods and the storage of electrons in nc-Au part...

Mg-induced increase of band gap in Zn1−xMgxO nanorods revealed by x-ray absorption and emission spectroscopy

X-ray absorption near-edge structure (XANES) and x-ray emission spectroscopy (XES) measurements were used to investigate the effect of Mg doping in ZnO nanorods. The intensities of the features in the O K-edge XANES spectra of Zn{sub 1-x}Mg{sub x}O nanorods are lower than those of pure ZnO nanorods, suggesting that Mg doping increases the negative effective charge of O ions. XES and XANES spectra of O 2p states indicate that Mg doping raises (lowers) the conduction-band-minimum (valence-band-maximum) and increases the bandgap. The bandgap is found to increase linearly with the Mg content, as revealed by photoluminescence and combined XANES and XES measurements.

Angle-dependent x-ray absorption spectroscopy study of Zn-doped GaN

As-grown and Zn-implanted wurtzite GaN samples have been studied by angle-dependent x-ray absorption near edge structure (XANES) measurements at the N and Ga K-edges and the Ga L3-edge. The angle dependence of the XANES spectra shows that the Ga–N bonds lying in the bilayer have lower energies than bonds along the c-axis, which can be attributed to the polar nature of the GaN film. The comparison of the Ga L3-edge XANES spectra of as-grown and Zn-doped GaN reveals significant dopant induced enhancement of near-edge Ga d-derived states.

X-ray absorption of Si–C–N thin films: A comparison between crystalline and amorphous phases

X-ray absorption near edge structure (XANES) spectra of crystalline (c)- and amorphous (a)-Si–C–N thin films were measured at the C, N, and Si K edge using the fluorescence and sample drain current modes. A sharp peak similar to the Cu200a1s core exciton in chemical vapor deposition diamond is observed, which can be assigned to the transition from the Cu200a1s to sp3 hybridized states in c-Si–C–N. The C K edge XANES spectrum of a-Si–C–N contains a relatively large 1s→π* peak, implying that carbon atoms in the a-Si–C–N film are bonded largely in graphite-like sp2 configurations. A shift of the a-Si–C–N π* peak towards the lower energy by ∼0.3 eV relative to that of c-Si–C–N is observed, which can be attributed to a higher degree of disorder-induced localization of excited electrons. Both a- and c-Si–C–N N K-edge XANES spectra resemble that of α-Si3N4. The Si K-edge absorption spectra of the Si–C–N thin films indicate a proportional combination of local Si–N and Si–C bonds. The increase of the binding energies of e...

X-ray-absorption studies of boron-doped diamond films

X-ray-absorption near-edge structure (XANES) measurements have been performed for a variety of boron-doped and undoped diamond films at the C K edge using the sample drain current mode. The C K-edge XANES spectra of B-doped diamonds resemble that of the undoped diamond regardless of the B concentration, which suggests that the overall bonding configuration of the C atom is unaltered. B impurities are found to enhance both the sp3- and sp2-bond derived resonance features in the XANES spectra.

Role of valence-band Co 3d states on ferromagnetism in Zn1−xCoxO nanorods

This work investigates the electronic and ferromagnetic properties of Zn1−xCoxO nanorods using x-ray absorption, x-ray magnetic circular dichroism, and scanning photoelectron microscopy methods. The magnetic moment of Co ions in Zn1−xCoxO nanorods is found greatly reduced relative to that of the Co metal. The intensities of valence-band features near the valence-band maximum/Fermi level (EF) of ferromagnetic nanorods are substantially larger than those of weaker ferromagnetic nanorods, suggesting that the occupation of near-EF valence-band Co 3d states is important in determining the ferromagnetic behavior in Zn1−xCoxO nanorods.

Electronic and local atomic structures of (La1−xPrx)0.85Zr0.15MnO3 studied by x-ray absorption spectroscopy

Mn L3,2-, K-, O K- and Pr M5,4-edge x-ray absorption near-edge structure (XANES) spectra of (La1−xPrx)0.85Zr0.15MnO3 (LPZMO) with x = 0, 0.05, 0.1, 0.15 and 0.2 were measured to study the electronic structure and the effect of Pr substitution. An analysis of Mn L3,2- and K-edge spectra indicates that LPZMO is an electron-doped system. Pr substitution is found to increase the average effective charge of Mn ions and to induce an additional unoccupied majority spin ( spin) eg subband in the Mn L3,2-edge XANES spectrum. Mn L3,2-, O K- and Pr M5,4-edge XANES results demonstrate that the substitution of La ions by smaller Pr ions changes O 2p–Mn 3d and O 2p–Pr 4f hybridized states. Extended x-ray absorption fine structure (EXAFS) analysis at the Mn K edge reveals clear changes in local atomic structure around Mn ions due to the Pr substitution. XANES and EXAFS analyses show a small distortion in the MnO6 octahedron or a disorder in the lattice, which may be due to localization of charge carriers and/or the increased effective positive charge of Mn ions.

Electronic structure of the Fe–Cu–Nb–Si–B alloys by x-ray absorption spectroscopy

We measured x-ray absorption near-edge-structure (XANES) spectra of nanocrystalline- (nc-) and amorphous- (a-) Fe73.5Cu1Nb3Si13.5B9 (nc-FCNSB and a-FCNSB) and Fe78Si13B9 (a-FeSiB) alloys at the Fe L3,2 edge using the sample drain current mode and at the Cu L3,2, and Nb L3 edge and Si K edge using the fluorescence mode. The features in the Fe L3-edge XANES spectrum of nc-FCNSB changed shape significantly with the addition of Cu and Nb to the Fe–Si–B alloy under the optimum annealing conditions, indicating that Cu and Nb strongly influence the Fe 3d local electronic structure. Closely examining the Cu L3,2-edge XANES spectrum of nc-FCNSB reveals that the Cu clusters essentially have a body-centered-cubic structure. The white-line features at the Nb L3 edge suggest a slight increase in delocalization of Nb 4d orbits when a-FCNSB is crystallized into nc-FCNSB. The Si K-edge XANES spectrum demonstrates the dominance of Fe–Si bonds around the Si atom in nc-FCNSB.