P. K. Tseng

Electronic structure of the Fe-layer-catalyzed carbon nanotubes studied by x-ray-absorption spectroscopy

X-ray-absorption near edge structure (XANES) measurements have been performed to investigate the local electronic structures of the Fe-catalyzed and stabilized carbon nanotubes (CNT) with various diameters. The intensities of the π*- and σ*-band and the interlayer-state features in the C K-edge XANES spectra of these CNTs vary with the diameter of the CNT. The white-line features at the C K- and Fe L3-edges suggest a strong hybridization between the C 2p and Fe 3d orbitals, which lead to an enhancement of the C K- and reduction of the Fe L3-edge features, respectively, indicative of a charge transfer from C 2p to Fe 3d orbitals. The Fe K-edge spectra reveal a p–d rehybridization effect that reduces p-orbital occupation at the Fe site.

Charge transfer and hybridization effects in Ni3Al and Ni3Ga studies by x-ray-absorption spectroscopy and theoretical calculations

This work investigates the charge transfer and Al(Ga) p-Ni d hybridization effects in the intermetallic Ni3Al (Ni3Ga) alloy using the Ni L3,2 and K edge and Al (Ga) K x-ray absorption near edge structure (XANES) measurements. We find that the intensity of near-edge features at the Ni L3 edge in the Ni3Al (Ni3Ga) alloy decreased with respect to that of pure Ni, which implies a reduction of the number of unoccupied Ni 3d states and an enhancement of the Ni 3d state filling in the Ni3Al (Ni3Ga) alloy. Two clear features are also observed in the Ni3Al (Ni3Ga) XANES spectrum at the Al (Ga) K edge, which can be assigned to unoccupied Al 3p-(Ga 4p-) derived states in Ni3Al (Ni3Ga). The threshold at the Al K-edge XANES for Ni3Al shifts towards the higher photon energy relative to that of pure Al, suggesting that Al loses some p-orbital charge upon forming Ni3Al. On the other hand, the Ni K edge shifts towards the lower photon energy in Ni3Al (Ni3Ga) relative to that of pure Ni, suggesting a gain of charge at the ...

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 C 1s core exciton in chemical vapor deposition diamond is observed, which can be assigned to the transition from the C 1s 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.

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.

Si K and Co L3 XANES study of thin-film CoSi2

Abstract X-ray absorption near edge structure (XANES) spectra of thin-film CoSi 2 were measured at the Si K- and Co L 3 -edge. The Si K-edge spectrum for CoSi 2 showed a dramatic reduction of intensity in the first broad feature accompanied by an increase in intensity of a peaked feature at higher energies when compared to the spectrum for crystalline Si. We attribute these two features to the Si 1s photoelectron excitations to a broad Si 3p nonbonding band and a relatively narrow band of hybridized Si p - Co 3d antibonding states, respectively. The Co L 3 -edge white line spectrum for CoSi 2 reveals three distinct features which can be attributed to excitations to the unoccupied Co 3d nonbonding states and hybridized antibonding Co (3d, 4s) - Si p states.

The effect of annealing time on the electronic structure of the Fe-Cu-Nb-Si-B alloys

Abstract Three nanocrystalline (nc-) Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 (nc-FCNSB) samples were obtained from amorphous (a-) Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 (a-FCNSB) samples by annealing for 30, 60, and 120 min at a temperature of 550°C. X-ray absorption near-edge-structure (XANES) spectra of these samples were measured at the Fe L 3,2 -edge using the sample drain current mode at room temperature. The intensities in the Fe L 3,2 -edge XANES spectra of nc-FCNSBs are significantly different from that of the Fe–Si–B alloy, indicating that Cu and Nb strongly influence the Fe 3d local electronic structure. Crystallization of the FCNSB alloy under the optimum annealing condition improves the magnetic properties of the nc-FCNSB alloys.

Correlation between photoluminescence and positron annihilation spectra in porous silicon

Abstract We have performed two-dimensional angular correlation of electron-positron annihilation radiation (2D-ACAR) measurements on a series of porous silicon with photoluminescence (PL) peak energy in the range from 1.6 to 2.0 eV. The electron-positron momentum spectra of porous silicons can be well resolved into two peaks with different line width. The results shows two distinct momentum spectra for the studied samples. The samples with PL peak energy below 1.8 eV exhibit a strong positronium signal in the momentum spectra, and the obtained result shows a good correlation with PL spectra as predicted by quantum confinement. However, the samples with PL peak energy above 1.8 eV show little amount of positronium signal and the width of positronium peak remains almost constant. This behavior can be explained by the formation of silicon compounds on the surface of pores. With the result of these measurements, we suggest that the PL mechanism is different for the porous silicon with PL peak below and above 1.8 eV. For the PL peak below 1.8 eV, the emission arises from silicon nano-crystal, so that the PL spectrum highly depends on the nano-crystal size. However, for the PL above 1.8 eV, we suggest that the luminescence is dominated by the silicon compounds on the surface of the pores which block the free volume for positronium annihilation. Our suggested model can be used to resolve several difficulties in the existing PL spectra.

Electronic properties of the diamond films with nitrogen impurities: An x-ray absorption and photoemission spectroscopy study

X-ray absorption near-edge structure (XANES) measurements have been performed for nitrogen (N) containing diamond films with three different N concentrations at the C K-edge using the sample drain current mode. The C K-edge XANES spectra of these diamond films resemble that of the pure diamond regardless of the N concentration, which suggests that the overall bonding configuration of the C atom is unaltered. N impurities are found to reduce the intensities of both the sp2- and sp3-bond derived resonance features in the XANES spectra. The valence-band photoelectron spectra indicate that N atoms cause the broadening of the valence band σ- and π-bond features and the enhancement and reduction of the σ- and π-bond features, respectively.

A study of porous silicon prepared under different HF concentrations by positron annihilation

Abstract Two-dimensional angular correlation of positron-electron annihilation radiation and positron-lifetime experiments have been performed on lightly doped porous silicons prepared under different HF concentrations. Positronium formation in the etched pores and positron trapped in voids are observed in both experiments. A surprising result is found that both positron lifetime and momentum spectra show a reduction in the size of the etched pores with decreasing HF concentration in the etching solution. This trend is different from the intuitive expectation that the pore size increases with increasing porosity. Our result can be explained in terms of the formation mechanism of porous film in lightly doped silicon.