M. W. Murphy

Time-resolved x-ray excited optical luminescence from SnO2 nanoribbons : Direct evidence for the origin of the blue luminescence and the role of surface states

Time-resolved x-ray excited optical luminescence (XEOL) and x-ray absorption near edge structures have been employed to study the origin of the multicolor luminescence from SnO2 nanoribbons. The authors find that the yellow-green luminescence has a long lifetime while the blue luminescence a short one. The luminescence is attributed to the radiative decay of trapped electrons in oxygen vacancies just below the conduction band and electrons in the conduction band to intrinsic surface states in the band gap.

Electronic structure of TiO2 nanotube arrays from X-ray absorption near edge structure studies

We report an X-ray absorption near edge structure (XANES) investigation of several TiO2nanotube arrays, including the as-prepared nanotube arrays from electrochemical anodic oxidation of Ti foil (as-prepared ATNTA), as-prepared nanotube arrays after annealing at 580 °C (annealed ATNTA) and annealed ATNTA after electrochemical intercalation with Li (Li-intercalated ATNTA). XANES at the O K-edge and Ti L3,2 and K edges shows distinctly different spectral features for the as-prepared and the annealed ATNTA, characteristic of amorphous and anatase structures, respectively. Intercalation of Li into annealed ATNTA induces a surprising, yet spectroscopically unmistakable, anatase to rutile transition. XANES at the Li K-edge clearly shows ionic features of Li in ATNTA. The charge relocation from Ti 3d to O 2p at the conduction band in TiO2 was also observed when Li ions were intercalated into annealed ATNTA albeit no noticeable reduction of Ti4+ to Ti 3+ was observed. The O K-edge shows a distinctly enhanced feature in the multiple scattering regime, indicating a close to linear O–Li–O arrangement in Li-intercalated ATNTA. These results show bonding changes between Ti and O resulting from the interaction of Li ions in the TiO2 lattices. Such bonding variation has also been supported by X-ray excited optical luminescence (XEOL), which suggests Li+-defect interactions. The implications of these results are discussed.

Optical emission of biaxial ZnO–ZnS nanoribbon heterostructures

The electronic structure and optical properties of biaxial ZnO-ZnS heterostructure nanoribbons (NRs) have been investigated using x-ray absorption near-edge structures (XANES) and x-ray excited optical luminescence (XEOL). The XANES were recorded in total electron yield and wavelength-selected photoluminescence yield across the K- and L(3,2)-edges of zinc and sulfur and the K-edge of oxygen. The XEOL from the NRs exhibit a very weak band-gap emission at 392 nm and two intense defect emissions at 491 and 531 nm. The synchrotron x-ray pulse ( approximately 100 ps, 153 ns repetition rate) was used to track the optical decay dynamics from ZnO-ZnS NR, which can be described by two lifetimes (7.6 and 55 ns). Comparison with similar measurements for ZnO and ZnS nanowires reveals that the luminescence from ZnO-ZnS NRs was dominated by the ZnO component of the NR as the ZnS component contributes little. The implication of this observation is discussed.

Ionic nature of Ge(II)-centered dications: a germanium K-edge X-ray absorption near edge structures study

Measurement of the ionic nature of [Ge(cryptand[2.2.2])](2+) by XANES has provided direct experimental evidence that the germanium center is best described as a nearly-naked dication encased within an electron rich cryptand cage.

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.

Luminescence properties of defects in nanocrystalline ZnO

Formation of intrinsic acceptor defects in ZnO is rare due to the low formation energy of donors. Understanding this phenomenon is of interest for the fabrication of high quality light emitting diodes. Herein, we examine the temperature dependent formation of defects in nanocrystalline ZnO through a combination of X-ray excited optical luminescence (XEOL) together with X-ray absorption near edge structures (XANES) and electron spin resonance (ESR). Certain defects are shown to form under low temperature and are unstable above 700 °C. These defects have high g-values characteristic of acceptors and short spin-lattice relaxation times. XEOL measurements show that acceptor defects with a characteristic red luminescence are also formed under these conditions. Low g-value (donor) defects forming at temperatures >700 °C are shown to have spin-lattice relaxation characteristic of nonradiative recombination centers.

An X‐Ray Excited Optical Luminescence (XEOL) Analysis of Mn2+ Doped ZnS Nanostructures

ZnS:Mn nanoribbons and ZnS:Mn,Eu triangular nanoprisms grown by thermal evaporation of ZnS powder mixtures containing MnCl2, and MnCl2 + EuCl3 powders respectively were investigated using X‐ray excited optical luminescence (XEOL) together with X‐ray absorption fine structure spectroscopy (XAFS) in total electron yield (TEY) and wavelength selected photoluminescence yield (PLY). XEOL and XAFS measurements were performed by tuning the X‐rays to the Zn, and Mn L2,3‐edge absorption regions, selectively exciting the respective elements. The dominant emissions resulted from the characteristic 4T1 − 6A1 transitions of the Mn2+ ion (576 nm) in the ZnS:Mn nanoribbons and ZnS:Mn,Eu nanoprisms. The results provide insight into the energy transfer mechanisms that occur and how they affect the overall luminescence of the Mn doped ZnS nanostructures.

Pt L3,2-edge whiteline anomaly and its implications for the chemical behaviour of Pt 5d5/2 and 5d3/2 electronic states – a study of Pt-Au nanowires and nanoparticles

We report the L3,2-edge whiteline anomaly observed in PtAu nanowire, PtAu and Pt nanoparticles deposited on Si nanowire, and their comparison with that of Pt metal. It is found that charge redistribution upon the formation of these materials can indeed be tracked with the L3,2 whiteline intensity. The implications of these findings are discussed.

Microstructure and Electronic Behavior of PtPd@Pt Core-shell Nanowires

PtPd{at}Pt core-shell ultrathin nanowires were prepared using a one-step phase-transfer approach. The diameters of the nanowires range from 2 to 3 nm, and their lengths are up to hundreds of nanometers. Line scanning electron energy loss spectra showed that PtPd bimetallic nanowires have a core-shell structure, with a PtPd alloy core and a Pt monolayer shell. X-ray absorption near edge structure (XANES) spectra reveal that a strong Pt-Pd interaction exists in this nanowire system in that there is PtPd alloying and/or interfacial interaction. Extended x-ray absorption fine structures (EXAFS) further confirms the PtPd@Pt core-shell structure. The bimetallic nanowires were determined to be face-centered cubic structures. The long-chain organic molecules of n-dodecyl trimethylammonium bromide and octadecylamine, used as surfactants during synthesis, were clearly observed using aberration-corrected TEM operated at 80 KV. The interaction of Pt and surfactants was also revealed by EXAFS.

XAFS studies of Pt nanoparticles deposition on Si nanowires

We have studied X-ray absorption fine structures (XAFS) of Pt nanoparticles (PtNPs) which were deposited on silicon nanowires (SiNWs). SiNWs were fabricated via an electroless chemical etching method and served as the template for the immobilization of PtNPs. PtNPs electrolessly reduced from their ionic solution by HF-treated SiNWs were found to deposit on the tips of the SiNWs. The electronic structures of Pt were studied using X-ray absorption near-edge structures (XANES) at Pt L3,2-edge. For comparison, we also examined the Pt L3,2-edge and the Au L3-edge XANES of Pt-Au bimetallic nanoparticles co-deposited on SiNWs. We found that the PtNPs showed slightly increased whiteline intensity compared to that of Pt foil. When Au was deposited together with Pt, the resonance peaks of the NPs were slightly, but systematically shifted due to the formation of Pt-Au alloy.