Article pubs.acs.org/JPCC
Visible Emission from GeO2 Nanowires: Site-Specific Insights via X-ray Excited Optical Luminescence Lidia Armelao,*,†,‡ Franziskus Heigl,‡ Pil-Sook Grace Kim,§ Richard A. Rosenberg,∥ Tom Z. Regier,⊥ and Tsun-Kong Sham*,‡ †
CNR-ISTM and INSTM, Department of Chemistry, University of Padova, 35131 Padova, Italy Department of Chemistry, The University of Western Ontario, London N6A 5B7, Canada § Brockhouse Institute for Materials Research, McMaster University, Hamilton L8S 4M1, Canada ∥ Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA ⊥ Canadian Light Source, University of Saskatchewan, Saskatoon S7N 0X4, Canada ‡
ABSTRACT: Crystalline GeO2 nanowires synthesized by a flow deposition technique on a silica substrate emit intense and distinct luminescence. X-ray Excited Optical Luminescence (XEOL) stimulated by a synchrotron light source in the soft X-ray energy regime was used to monitor the optical emission from germanium oxide nanowires upon X-ray absorption with the excitation energy tuned across O K- and Ge L3,2-edges. XEOL spectra vary significantly at different edges. At the O Kedge, luminescence is characterized by green (∼520 nm) and red (∼780 nm) emission related to oxygen vacancies in the core of the nanowires and to structural defects on their surface. At the Ge L3,2-edge, emission is dominated by an intense violet peak at ∼400 nm mostly associated with defect centers at the interface between adjacent nanowires and with the silica substrate. The increasing attenuation lengths of soft X-rays tuned at excitation energies across the O K-edge (∼300 nm) and the Ge L3,2-edge (∼1 μm) enable the sampling of defects placed at different depths in the nanosized GeO2 structures, down to the nanowire−SiO2 interface. The use of tunable XEOL makes it possible to correlate excitation energy with the site-specific origin of luminescence and has provided direct evidence for the behavior of different structural defects in the visible emission from the GeO2 nanowires. These results are of key interest to exploit the nanodimensional oxides in practical device applications.
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INTRODUCTION The blue luminescent GeO2 material is highly sought after for optical waveguides and optoelectronic communication devices.1−3 It is thermally stable and exhibits a high dielectric constant; additionally, germanium oxide glass is thought to be more refractive than the corresponding silicate glass. The luminescence properties of nanodimensional structures of germanium oxide have been an area of active research for many years because of their potential for elucidating models and fundamental concepts regarding the role of structural defects in optical properties. In general, one-dimensional inorganic nanostructures currently attract a great deal of attention for future technological applications in optical devices.4 As such, GeO2 1D nanostructures could feasibly be used for nano interconnects in optoelectronic communication. 1D structures of GeO2 have been synthesized by various methods, including the carbon nanotube confined reaction of germanium,5 electrospinning,6 laser ablation,7 thermal oxidation,8,9 carbothermal reduction,3 and thermal evaporation.10−12 More often than not, GeO2 nanostructures and mesostructures are synthesized via sof t solution-phase approaches.13−17 GeO2, as in other semiconductor oxides, has oxygen deficiency, and this effect is more acute in nanowires due to the higher surface to volume ratio. These defects are, in many cases, responsible for optical and electrical properties and are the key factor to © 2012 American Chemical Society
exploit in practical devices. The photoluminescence intensity for nanowires was found to be more than an order of magnitude higher than for GeO2 nanopowders.18 Although a fair amount is understood about the optically active defects responsible for a variety of emissions observed in the near-UV and visible spectral region in germanium oxide nanocrystallites, either free-standing or embedded in a silicon dioxide matrix,19 the processes accompanying luminescence of the germanium oxide nanowires have in general been much less investigated,10,11,18 and a general model relating the nature of defects and emission properties is still under debate.18 In the present paper, we report a synchrotron radiation study on the optical luminescence properties of crystalline germanium oxide nanowires. The nanostructures have been prepared on a silica glass substrate by a flow deposition technique through a catalyst free vapor−liquid−solid (VLS) synthesis2,20 and present an average diameter of ∼500 nm. Detailed characterization of chemical composition, morphology, and structure of the GeO2 nanowires was performed by transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDS), scanning electron Received: April 27, 2012 Revised: May 30, 2012 Published: June 6, 2012 14163
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LEO 1540XB instrument fitted with an Oxford Instruments Xray system allowing for elemental mapping and analysis. Highresolution TEM (HRTEM) analysis and high angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) mode were performed on a JEOL 2010F instrument equipped with an X-ray detector (EDXS, Oxford Instruments Inca). XANES and Optical XANES. X-ray absorption near edge structure (XANES) spectra were recorded in total fluorescence yield (FLY), which was measured with a pair of chevron stacked microchannel plates (MCP), and in total electron yield (TEY) using the specimen drain current. Optical XANES spectra were recorded in total (zero-order) luminescence yield (PLY). Experiments were performed at the spherical grating monochromator (SGM) beamline (O K-edge, Ge L3,2-edge) of the Canadian Light Source (CLS), a third-generation 2.9 GeV ring operating with a current of ∼250 mA at injection. The photon beam size at the sample is of ∼100 × 100 μm2. All measurements were performed at room temperature and at an incidence angle of 45° with respect to the sample surface, unless otherwise specified. XEOL. XEOL experiments were performed at the SGM beamline of the CLS. XEOL was collected by a scientific-grade spectrometer (QE650000, Ocean Optics) via an optical fiber and a CCD detector so that all wavelengths from 200 to 900 nm were collected simultaneously at each and every excitation energy. XEOL spectra were also scanned with a JY optical monochromator. The optical detector was a Hamamatsu R94302 photomultiplier (PMT) with a ∼2 ns rise time.25 The scan with a JY monochromator equipped with a set of focused lenses collecting a large solid angle provides significantly higher sensitivity. The dark count was subtracted from the data. All measurements were performed at room temperature and at an incidence angle of 45° with respect to the sample surface, unless otherwise specified.
microscopy (SEM), and glancing incidence X-ray diffraction (GIXRD). The local and electronic structure of the GeO2 nanowires was investigated by X-ray absorption near-edge structure (XANES). X-ray excited optical luminescence (XEOL) was used to unravel the origin of luminescence from the GeO2 nanowires. The XEOL technique is a powerful tool for investigating the optical properties of light-emitting materials allowing for the acquisition of both site-specific chemical information and luminescence information.20−25 XEOL tracks the optical response of a light-emitting material to X-ray excitation by tuning the X-ray energy to a specific excitation channel, often from below to above the X-ray absorption edge of an element of interest, and monitoring the luminescence with an optical monochromator, typically in the UV−vis−near IR range (200−900 nm). In this study, XEOL is used to monitor the optical emission upon X-ray absorption with the excitation energy tuned across O K- and Ge L3,2-edges. We find that the XEOL spectra vary significantly as a function of X-ray excitation energy. At the O K-edge, XEOL is characterized by green and red luminescence, whereas at the Ge L3,2-edge emission is dominated by an intense violet peak. We attribute the emissions to different structural defects and oxygen vacancies located at the surface and core of the GeO2 nanowires, as well as at the interface between adjacent nanowires and with the silica substrate. The increasing penetration depth upon tuning the excitation energy across the O K- and Ge L3,2-edges contributes to identifying the role of surface and bulk defect states in the optical emission. Preliminary results have been previously reported in a conference proceeding.26
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EXPERIMENTAL SECTION Synthesis. GeO2 nanowires on glassy silica substrates were prepared in a horizontal tube furnace by a vapor phase deposition technique previously described.8,11,27−30 Ge powder (99.9%, Alfa Aesar) was adopted as the source compound and used as received. Silica slides (Heraeus, Quarzschmelze, Hanau, Germany) were used as the substrate and were cleaned by sonication in ethyl alcohol and acetone prior to use. GeO2 nanostructures were synthesized as follows. In a quartz tube, mounted inside a horizontal tube furnace, an alumina boat containing the source precursor was placed in the middle of the high-temperature zone of the furnace. The silica slides were placed at a position downstream of the carrier gas (Ar) flow, which was introduced at one end of the quartz tube at a flow rate of 55 sccm (standard cubic centimeters per minute), whereas the other tube end was open to air. The temperature of the furnace was increased to a maximum value of 1200 °C by a step by step procedure and kept for 6 h. After the reaction the system was cooled to room temperature, and the nanostructures appeared as white products homogeneously distributed on the silica slide. All measurements were performed at room temperature. Structural and Chemical Characterization. The microstructure and morphology of the as-synthesized samples were characterized by Scanning Electron Microscopy (SEM), Glancing Incidence X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM) combined with energy-dispersive X-ray spectroscopy. XRD patterns were recorded by a Bruker D8 Advance diffractometer equipped with a Göbel mirror and a Cu Kα source (40 kV, 40 mA), in a glancing incidence geometry at a fixed angle of 0.5° to reduce the contribution of the silica substrate. FE-SEM measurements were run on a Zeiss
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RESULTS AND DISCUSSION SEM and HRTEM images of the GeO2 nanowires are shown in Figure 1, along with the X-ray and selected area electron diffraction patterns. The low-magnification SEM image of the as-synthesized products shows the high yield of GeO2 nanowire networks and the homogeneous coverage of the silica substrate. The nanowires are in general uniform with a characteristic thickness of ∼500 nm and several tens of micrometers in length (cf. scale in Figure 1, D). All the diffraction peaks of the XRD pattern (2θ ∼ 20.5° (100), 25.9° (101), 36.0° (110), 38.0° (102), 39.5° (111), 41.8° (200), 44.9° (201), and 48.4° (112)) can be indexed according to the GeO2 hexagonal phase (JCPDS card # 85-473). The peaks are strong and narrow indicating a good crystallinity of the GeO2 nanowires. Bright field (BF) HRTEM images (Figure 1, A) show the formation of straight nanostructures with regular shape and uniform diameter along their long axes. Electron diffraction patterns recorded on individual wires by SAED (Figure 1, B) confirm the highly crystalline nature of the oxide nanosystems, in agreement with XRD data. Owing to the extreme sensitivity of GeO2 to electron beam irradiation,8,10,31 we were unable to obtain lattice images from the nanowires for further microstructural studies. Compositional analysis of the nanowires has been carried out by high angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) combined with an energy-dispersive X-ray detector (Figure 1, C). Cross-section 14164
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Figure 2. Total fluorescence yield (FLY) of the GeO2 nanowires at the O K- and the Ge L3-edge. The edge position has been labeled for both XANES spectra.
and that the surface and bulk structure are essentially the same from the germanium perspective. The corresponding TEY spectra show the same features, though noisier due to charging. The XEOL spectra from the GeO2 nanowires excited at the O K-edge and the Ge L3,2-edge are shown in Figure 3. The
Figure 1. XRD pattern (upper panel) of the GeO2 nanowires. SEM micrograph (lower panel, D) of the GeO2 nanowires and bright field (BF) HRTEM image (lower panel, A) of single nanowires. The corresponding electron diffraction pattern (SAED) taken along the [001] zone axis (lower panel, B), HAADF-STEM image, and crosssectional EDS profiles of germanium and oxygen (lower panel, C) are also reported.
Figure 3. Optical emission of GeO2 nanowires at different excitation energies: XEOL in the vicinity of the O K-edge (excitation energy 530 eV, full line) and of the Ge L3-edge (excitation energy 1220 eV, dotted line). Inset: XEOL spectra at excitation energies tuned across the Ge L3-edge, i.e., 1110 , 1220 (edge resonance), and 1230 eV (from bottom to top).
compositional line spectra show the presence of germanium and oxygen across the nanowire diameter. The electronic and local structure of the GeO2 nanowires have been investigated with X-ray absorption near-edge structure (XANES) and optical XANES. The total fluorescence yield (FLY) at the O K-edge and at the Ge L3,2-edge is shown in Figure 2. The peak in the O K-edge spectrum (Figure 2, left) appears as a result of 1s transitions to the O 2p σ* states hybridized with the s, p, and d metal orbitals11 mapping the unoccupied density of states (band structure). The Ge L3,2-edge XANES (Figure 2, right) probes primarily the s character of the unoccupied density of states above the Fermi level via a 2p to 4s or 4d transition. The first sharp resonance at 1219.4 eV and the absence of a pre-edge peak are characteristic for germanium in oxygen environment.11 Accordingly, the spectral features of the XANES O K-edge are identical to those of pure GeO2.11 As the germanium oxide wires are not larger than ∼500 nm in diameter, thickness effects (self-absorption) are not expected to be a problem in FLY. This confirms the assumption that the nanowires are homogeneous
startling differences in the XEOL spectra response as a function of X-ray energy are, to our knowledge, the most dramatic ever observed. In the spectra we can clearly distinguish three emission bands, marked as A, B, and C, centered at ∼410 nm (∼3.02 eV, violet), ∼525 nm (∼2.36 eV, green), and ∼780 nm (∼1.59 eV, red). Their relative intensities are obtained by leastsquares curve fitting the spectra to Gaussian components. Excited in the vicinity of the O K-edge, the emission spectrum remains substantially unaltered with the green peak being the most intense. With the excitation energy tuned to the Ge L3,2edge, the violet emission becomes the most intense, by far dominating the other components, while for excitation energies tuned around the Ge L3,2-edge we observe a steep variation in the optical emission profile (Figure 3, inset). To clarify the detailed features of the emission patterns from the GeO2 nanowires, we carried out an accurate spectral 14165
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different defect centers are to be considered to account for the observed luminescence, as schematically depicted in Figure 5. Oxygen-related defects and vacancies may be placed at the surface of the nanowires and in the core, or at the interface between adjacent nanowires and with the silica substrate.
mapping of the luminescence excited at increasing energies, from far below the O K-edge to far above the Ge L3,2-edge. The luminescence yield, proportional to the branching ratio, of the violet, green, and red components at excitation energies ranging from 400 to 1450 eV is shown in Figure 4. The yield of the
Figure 4. Partial luminescence yield (area under the Gaussian over the total intensity of the spectrum, the branching ratio, which was in turn normalized to the incoming photon flux) of the violet (full circles), green (empty circles), and red (empty squares) emission bands upon ramping excitation energy from 400 to 1450 eV. The vertical dashed lines denote the excitation energy tuned across the Ge L3-edge for the spectra depicted in the inset of Figure 3.
Figure 5. Schematic model of the GeO2 nanowires on a SiO2 substrate with surface, bulk, and interface defects. X-ray attenuation length35 vs excitation energy for GeO2 is also shown in the top panel for comparison (incidence angle 45°).
violet band is weak at low excitation energies and increases above 800 eV. Its maximum intensity is observed at an excitation energy just below the Ge L3,2-edge, resulting notably higher than at the O K-edge, and drops to the initial value when crossing the germanium edge. The opposite effect, although less pronounced, is observed for the green emission which dominates the spectrum at low energies. The strongest yield, observed at the O K-edge, gradually diminishes at increasing energies and reaches the minimum value just below the Ge L3,2edge. The yield of the red emission peak is weak and remains roughly the same except for an increase at the Ge L3,2-edge. Optical emission from GeO2 nanostructures has a multisource origin,32 and the main contributions are traced back to oxygen vacancies and doubly coordinated Ge atoms (diamagnetic −Ge− centers). Luminescence is often observed in the blue region and is sometimes found at still longer wavelengths (∼700 nm) associated to surface defects. An intense violet emission (∼400 nm) has been reported in three-dimensional networks of germanium oxide nanowires10 as well as from Ge and GeO2 nanocrystals embedded in a silica matrix.1 More recently, green luminescence (∼540 nm) has been observed in single crystalline GeO2 nanowires.11 The double oxygen vacancy associated with the diamagnetic 2-fold coordinated germanium lone pair center is deemed as the most suitable target for generating bright violet−blue photoluminescence from GeO2 nanowires,18,33 whereas green emission is often attributed to oxygen vacancy centers in defective GeO2−x oxides.1,3 Red light emission has been observed from glassy GeO234 and is ascribed to nonbridging oxygen hole centers, including triply coordinated Ge atoms with a dangling bond at the surface.32 The complex XEOL patterns observed from the GeO2 nanowires in the present case suggest that vacancies occur in several configurations and/or form complex defects. In particular,
At the surface, defects may nucleate from Ge−OH species and from Ge− dangling bonds.32,33 Such surface defects induce very shallow levels, giving rise to optical transitions in the near-infrared. 32 In the core, oxygen vacancies in substoichiometric GeO2−x oxides result in optical emissions in the 450−560 nm range.11,33 At the interface between GeO2 nanowires and the silica glass, and between adjacent nanowires, the 2-fold coordinated germanium centers (−Ge−) are the more likely oxygen vacancy defects.33,36 These are regarded as the probable source of the bright near-UV optical transition from GeO2 nanostructures,37 of which the initial state is reached by a singlet to triplet intersystem crossing process and becomes more effective as the size of the sample is reduced.38 Notably, we find that the intensity of the synchrotron X-ray induced luminescence is sensitive to the excitation energy at both O K- and Ge L3,2-edges. All emission components arise from oxygen-related vacancies, and X-ray excitation can distinguish among the different defect states. It must be noted that soft X-rays have a rather short attenuation length in condensed matter which exhibits a large contrast across an absorption edge; for example, the attenuation length for GeO2 is ∼300 nm just below the O K-edge (∼530 eV) and ∼1 μm just below the Ge L3,2-edge (∼1215 eV) but ∼150 and ∼300 nm just above the O K- (550 eV) and Ge L3,2-edge (1240 eV), respectively.35 Thus, by tuning soft X-rays at selected excitation energies, one can vary the sampling depth within the specimen and detect different defect species as the source of luminescence with a technique suited for in situ site-specific and element-specific detection. As the penetration depth of Xrays increases with increasing energy until it hits an absorption edge where the absorption coefficient increases abruptly, hence the penetration depth reduces abruptly, deeper defects become 14166
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indicates that this optical channel is correlated with oxygen defects in the outer layers of the GeO2 nanowires since the surface should not be affected by sampling depth. The green emission band is more sensitive to excitation energy. It shows a maximum yield around the O K-edge, a gradual quenching upon tuning X-rays to energy values just below the Ge L3,2-edge where the penetration depth is the highest, and a noticeable upturn above the germanium edge. This behavior correlates very well with the variation of the sampling depth compared to the characteristic size of the nanowires as a function of excitation energy. In Figure 5 we observe that the attenuation length of the soft X-ray below 900 eV is ∼500 nm in GeO2, i.e., similar to the typical nanowire thickness; it increases to ∼1 μm just below the Ge L3,2-edge and decreases to ∼300 nm above the germanium edge. From these observations, one can infer that the green light emission is correlated to oxygen vacancies in the core of the nanowires. Indeed, only if the penetration depth of the exciting X-rays is lower or comparable to the average diameter of the nanodimensional wires do these bulk defects become accessible and contribute to the optical channel. The violet component, substantially weak at excitation energies lower than 800 eV, shows a steep increment below the Ge L3,2-edge and evidently vanishes just above crossing the edge. The violet emission becomes intense and dominant over the green and the red components only across a limited excitation energy range, from above the O K-edge to below the Ge L3,2-edge, where the penetration depth of the X-ray increases gradually to a maximum. The optical response when crossing the Ge L3,2-edge provides information about the nature of the near-UV optical decay channel which again is a result of the abrupt change in X-ray penetration depth below and above the absorption edges of interest (see Figure 3 and Figure 5) when compared to the characteristic size of the nanowires. This leads us to assign the near-UV optical transition to defects placed between adjacent GeO2 nanowires or at the interface with the silica substrate, such as the double oxygen vacancy associated with 2-fold coordinated germanium centers. To investigate any possible contribution to the optical emission of the silica glass used as the substrate, XEOL spectra were recorded at excitation energies tuned across the Si K-edge. In Figure 7 the XEOL patterns recorded below and above the silicon edge are reported along with the spectrum taken just above the Ge L3,2-edge for comparison. Since at the Si K-edge the SiO2 glass will absorb most of the energy, if the violet peak were to originate from defects of the glass substrate we should be able to detect it. It is clearly seen from Figure 7 that excitation in the vicinity of the Si K-edge produces very weak emission and most importantly does not produce any violet luminescence. In fact, as we have already seen in Figure 3, the violet emission drops dramatically concomitant with the return of the green and red emission once the Ge L3,2-edge is reached.
gradually accessible and contribute to the optical spectrum. Thus, by varying the penetration depth of the incident photon, it is easier to correlate the excitation energy with site and excitation channel specificity in nanodimensional systems. This can be done by either varying the incident photon energy or varying the angle of incidence at a fixed photon energy.39 This notion is confirmed by the angular dependence of the partial optical yield of the silica-supported GeO2 nanowires when the angle of incidence is varied form normal (most penetrating, bulk sensitive) to grazing (less penetrating, most surface sensitive); the different emission at the Ge L3,2-edge for different angles of incidence is shown in Figure 6.
Figure 6. XEOL from the GeO2 nanowires as a function of the angle of incidence of the exciting beam at a fixed photon energy (1220 eV). Inset: luminescence intensity of the violet (∼400 nm), green (∼520 nm) and red (∼780 nm) emission components at variable incidence angle.
As noted above, at a fixed excitation energy, one can tune the sampling depth of the exciting X-rays by varying the incidence angle with respect to the sample surface. Again, the defects in buried layers contribute increasingly to the optical emission on increasing the angle of incidence, i.e., on increasing the penetration depth of the incident X-rays with reference to the glass substrate to which the randomly oriented GeO2 nanowires align. It is immediately apparent from Figure 6 that blue-violet emission at ∼400 nm diminishes rapidly as the probing depth becomes more surface sensitive, i.e., from a total dominance at 40° to negligible, at 5° incidence. According to the simplified model shown in Figure 5, we assume that the three types of defects in the nanowire samplesat the surface, in the core, and at the interfaceare responsible for the emission peaks observed in the optical spectra. By using this scheme, we cannot neglect that every type of defect can occur in a slightly different configuration, giving rise to an emission band centered at a mean value of distributed energies. The same caution should be given to the distribution of size of the nanowires, of which each can produce luminescence with a small deviation in the emission wavelength. Taking this into consideration, we can account for the broad linewidth of the emission. Briefly, we observe from Figure 4 that red emission exhibits little site or excitation channel specificity, only showing a small intensity jump at the Ge L3,2-edge. This observation strongly
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CONCLUSIONS The results reported herein represent significant progress toward unravelling the origin of optical luminescence from GeO2 nanowires, providing direct evidence for the different behavior of core and surface defects in the emission of visible light. Soft X-ray excited optical luminescence is characterized by a sharp and intense violet emission related to defects associated to 2-fold coordinated germanium atoms, green emission correlated to the content of oxygen vacancies in the core of 14167
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Giovanni Mattei, and Prof. Eugenio Tondello (University of Padova, Italy) are gratefully acknowledged for their valuable suggestions and helpful discussion.
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Figure 7. Optical emission of GeO2 nanowires at different excitation energies. From top to bottom: above the Ge L3-edge (1230 eV) and below (1840 eV) and above (1856 eV) the Si K-edge.
the nanowires, and red luminescence related to surface defect centers such as dangling bonds. The use of tunable soft X-ray excitation across shallow absorption edges (O K and Ge L3,2) and angular resolved analysis provides depth profiling within the specimen and allows us to detect different defect species as the source of luminescence with a technique suited for in situ site-specific and element-specific detection. This approach is of general interest and can be applied to the study of a variety of lightemitting nanodimensional systems. The site and excitation channel specific information due to excitation energy dependent penetration depth is here demonstrated. The observations presented in this work also represent significant progress for the design of GeO2 materials with stable and tunable light-emitting properties by controlling both size and defect properties during the synthesis procedure. These results demonstrate that important information concerning the electronic and optical properties of nanoscale materials, especially the electronic nature of defects, can be obtained using a combination of soft X-ray excitation and XEOL. Thus, XEOL, employing a synchrotron source, adds a new dimension in the analysis of nanosized light-emitting materials.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: (+39) 049 827 5236. Fax: (+39) 049 827 5227. E-mail:
[email protected] (L.A.). Tel.: (+1) 519 661 2111 ext. 86341. Fax: (+1) 519 661 3022. E-mail:
[email protected] (T.K.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Research at UWO is supported by NSERC, CFI, CRC, and OIT. L.A. acknowledges UWO for a visiting scholarship, and the research projects HELIOS (University of Padova, Italy), PRIN 2009, FIRB RBPR05JH2P, and FIRB RBAP114AMK (Ministry of Research MIUR, Italy) for financial support. Part of the research described in this paper was performed at the Canadian Light Source, which is supported by NSERC, NRC, CIHR, and the University of Saskatchewan. Prof. Xing-Tai Zhou (Shanghai Synchrotron Radiation Facility), Prof. 14168
dx.doi.org/10.1021/jp3040743 | J. Phys. Chem. C 2012, 116, 14163−14169
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