Franziskus Heigl

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.

Effects of in situ vacuum annealing on the surface and luminescent properties of ZnS nanowires

We have monitored the changes that occur in the x-ray-excited optical luminescence, absorption, and photoemission spectra as a function of vacuum annealing time and temperature for ZnS nanowires. All measurements were done in situ. Initial heating causes desorption of surface oxides and a concurrent reduction in the intensity of all the luminescence peaks, which we attribute to the creation of surface states that quench the luminescence. Extended annealing causes diffusion of Au from the particle used to nucleate the wire growth, which results in an increase in intensity of its associated luminescent band at 520nm. Changes were also observed in the ZnL- and SK-edge x-ray absorption spectra, which are consistent with this interpretation.

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.

Determination of the local structure of luminescent sites in ZnS nanowires using x-ray excited optical luminescence

We have monitored the optical luminescence from ZnS nanowires as a function of x-ray energy at the Zn L edge (1022 eV). The x-ray absorption spectrum obtained using the 338 nm, band edge emission as a signal resembles that of the wurtzite form of ZnS, while that obtained using the 430 and 520 nm defect emissions, resembles that of the sphalerite phase. Wurtzite is the dominant phase of the wire, while sphalerite is only found at the end of the wire adjacent to the gold particle used for nucleation and in small, highly localized regions of the wire. Therefore, the present results support the idea that the defect luminescence centers are caused by Au ions (520 nm) and vacancies (430 nm), which are located in regions of sphalerite and show how x-ray excited optical luminescence may be used to probe the local environment of such centers.

The Origin and Dynamics of Soft X-Ray-Excited Optical Luminescence of ZnO

The distinct optical emission from ZnO materials, nanoneedles and microcrystallites synthesized with different sizes and morphologies by a flow deposition technique, is investigated with X-ray excited optical luminescence (XEOL) and time-resolved X-ray excited optical luminescence (TR-XEOL) from a synchrotron light source at the O K and Zn L(3,2) edges. The innovative use of XEOL, allowing site-specific chemical information and luminescence information at the same time, is fundamental to provide direct evidence for the different behaviour and the crucial role of bulk and surface defects in the origin of ZnO optical emission, including dynamics. XEOL from highly crystalline ZnO nanoneedles is characterized by a sharp band-gap emission (~380 nm) and a broad red luminescence (~680 nm) related to surface defects. Luminescence from ZnO microcrystallites is mostly dominated by green emission (~510 nm) associated with defects in the core. TR-XEOL experiments show considerably faster decay dynamics in nanoneedles compared to microcrystallites for both band-gap emission and visible luminescence. Herein we make a fundamental step forward correlating for the first time the interplay of size, crystallinity, morphology and excitation energy with luminescence from ZnO materials.

Dense and optical transparent CdWO4 films by sol-gel processing for scintillation applications

In this paper, we report the first successful fabrication of dense and opticallytransparent cadmium tungstate (CWO) films by sol-gel processing and the study oftheir optical and x-ray scintillation properties. A new sol-gel processing method wasdeveloped using tungstic acid and cadmium nitrate as precursors and hydrogenperoxide as solvent; homogeneous and stable CWO sols were aged at roomtemperature and used for the preparation of CWO films. A rapid sintering process wasinvestigated and found to be necessary to make dense and optically transparentnanocrystalline CWO films. CWO films were uniform, fully dense, and crack-free,with CWO as the only detectable crystalline phase, as determined by x-ray diffraction.The thickness, density, grain size, and crystallinity of CWO films are all found to bestrongly dependent on the sintering conditions and in turn impact the optical and x-rayscintillation properties. Sol-gel-derived dense CWO films demonstrated intensephotoluminescence and x-ray excited optical luminescence intensity. The relationshipsbetween sol-gel processing, nanostructures, and optical and x-ray scintillationproperties are discussed in detail.

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.

Communication: X-ray excited optical luminescence from TbCl3 at the giant resonance of terbium.

We have studied the optical recombination channels of TbCl(3) using x-ray excited optical luminescence at the N(4,5) absorption edge of Tb (giant resonance) in both the energy and time domain. The luminescence exhibits a relatively fast (5)D(3), and a slow (5)D(4) decay channel in the blue and green, respectively. The rather short lifetime of the (5)D(3) state indicates that the decay is mainly driven by Tb-Tb ion interaction via non-radiative energy transfer (cross-relaxation). At the giant resonance the X-ray Absorption Near Edge Structure (XANES) recorded using partial photoluminescence yield is inverted. In the pre-edge region the contrast of the spectral feature is significantly better in optical XANES than in total electron yield. Changes in the intensity of (5)D(3)-(7)F(5) (544 nm) and (5)D(4)-(7)F(6) (382 nm) optical transitions as the excitation energy is tuned across the giant resonance are also noted. The results provide detailed insight into the dynamics of the optical recombination channels and an alternative method to obtain high sensitivity, high energy resolution XANES at the giant resonance of light emitting rare-earth materials.

XANES and photoluminescence studies of crystalline GeO2 (Tb) nanowires

We report a synchrotron study of GeO2 nanowires (NWs) prepared by a vapor- liquid-solid (VLS) method on a silica glass substrate. The electronic and local structure of GeO2 NWs were studied by X-ray Absorption Near-Edge Structure (XANES) at the O K- and Ge L3,2-edges. X-ray Excited Optical Luminescence (energy and time resolved) were used to monitor the optical emission upon X-ray absorption with the excitation energy tuned across absorption edges of interest. Using this method one obtains site specific insight into the chemical origin of luminescence.

Origin of the Luminescence from SnO2 Nanoribbons

Time‐resolved X‐ray excited optical luminescence and X‐ray absorption near‐edge structures have been employed to study the origin of the multi‐color luminescence from SnO2 nanoribbons. We 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.