Interplay between Crystal Structure and Optical Response in Plateau–Rayleigh Zn2GeO4/SnO2 Heterostructures

Hybrid semiconductor nanowire (NW) heterostructures are an ideal playground for cutting‐edge optoelectronic nanodevices. Among the several synthesis methods, Plateau–Rayleigh (PR) crystal growth is an effective route for producing decorated NWs with unprecedented properties. However, lateral variations in composition and/or crystal structure are postulated to play a central role in their optical response, but it is difficult to probe correlatively the elemental order, atomic organization, and light emission on length scales of tens to hundreds of nanometers. Usually, electron microscopies are applied to address the formation of clusters and imperfections in representative cross sections of the samples. Herein, a simultaneous spatially resolved nano‐analysis of the crystal symmetry, chemical composition, and optical properties of a whole Zn2GeO4/SnO2 NW heterostructure produced by PR instability is provided. The observations show the connection among Zn impurities, secondary phases, and asymmetrically distributed UV emissions present in the crystallites decorating the NW (Zn‐doped Sn1−xGexO2). The contributions of the elemental diffusion, crystal domains, and atomic site configurations to the light‐emission phenomena are disentangled in these hybrid architectures. The findings elucidate unknown underlying mechanisms that are critical to tailor emergent properties for rationally designing novel complex nanodevices based on 1D materials.


Introduction
Decorated semiconductor nanowires (NWs) [1] are promising architectures for next-generation nanoelectronic devices. Their widely tunable electronic, [2] optical, [3] and plasmonic [4] capabilities make them desirable hosts for a wide range of quantum, [5] sensing, [6] and energy-harvesting [7] applications. By manipulating the growth kinetics, it is now possible to produce extremely complex nanoscale systems. New electrical and photonic functionalities are generated by the addition of sequential branches with differences in composition and/or doping. [8][9][10] Among the several synthesis methods, the Plateau-Rayleigh (PR) crystal growth enables the formation of decorated NW systems. Unconventional morphological characteristics with modulations in periodicity, size, as well as crosssectional anisotropy are produced by controlling the deposition conditions. [11,12] Within the semiconductor families, wide bandgap (WBG) oxides, such as SnO 2 (E g % 3.6 eV) and Zn 2 GeO 4 (E g % 4.5 eV), exhibit relevant properties: visible-light transparency, high conductivity (%10 4 S cm À1 ), and large electron mobility (%80 cm 2 V À1 s À1 ). [13] High-power electronics, ultraviolet (UV) photodetectors as well as UV light-emitting diodes are just a few applications that can profit from the substantial breakdown electric field and short-wavelength emission induced by the wide energy bandgap. [14] Also, interfacial charge transfer for CO 2 reduction is possible by band structure engineering of WBG oxides. [15] Therefore, the application of PR growth approach to WBG oxides represents an excellent pathway to further expand the potential of hybrid NW heterostructures for advanced semiconductor nanotechnologies. Here, we report a correlative study of a single Zn 2 GeO 4 /SnO 2 NW architecture formed by PR-instability mechanism using a synchrotron nanoprobe. Since the performance of the heterostructure is intimately related to the crystal quality, it is crucial to examine their physical properties at the nanoscale. Commonly, native defects, heterogeneities, and/or structural imperfections drastically affect the optoelectronic responses. [16][17][18] So far, energy dispersive X-ray spectroscopy (EDS) and correlative high-resolution transmission electron microscopy (HRTEM) have been mostly used to assess the DOI: 10.1002/adpr.202300063 Hybrid semiconductor nanowire (NW) heterostructures are an ideal playground for cutting-edge optoelectronic nanodevices. Among the several synthesis methods, Plateau-Rayleigh (PR) crystal growth is an effective route for producing decorated NWs with unprecedented properties. However, lateral variations in composition and/or crystal structure are postulated to play a central role in their optical response, but it is difficult to probe correlatively the elemental order, atomic organization, and light emission on length scales of tens to hundreds of nanometers. Usually, electron microscopies are applied to address the formation of clusters and imperfections in representative cross sections of the samples. Herein, a simultaneous spatially resolved nano-analysis of the crystal symmetry, chemical composition, and optical properties of a whole Zn 2 GeO 4 /SnO 2 NW heterostructure produced by PR instability is provided. The observations show the connection among Zn impurities, secondary phases, and asymmetrically distributed UV emissions present in the crystallites decorating the NW (Zn-doped Sn 1Àx Ge x O 2 ). The contributions of the elemental diffusion, crystal domains, and atomic site configurations to the light-emission phenomena are disentangled in these hybrid architectures. The findings elucidate unknown underlying mechanisms that are critical to tailor emergent properties for rationally designing novel complex nanodevices based on 1D materials. structure and composition of hybrid semiconductor NW systems. [19][20][21][22][23] Local optical heterogeneities have also been identified using indirect techniques like cathodoluminescence. [24,25] However, a thorough and in-depth chemical and structural examination of the overall system is crucial. Although the creation of crystalline phases driven by PR mechanism has been proved, there are still many open questions. For example, the role of impurities on coupling formation and local atomic site configuration, [26] how these necklace arrangements are joined to form decorated architectures, [27][28][29] whether there is a full control over composition, diffusion paths, and/or structural modifications, [10,11,30,31] coexisting crystalline polytypes, [20,32] diffusiondriven effects, [33] phase separation, [34] radial or axial chemical modulations, [20,35,36] as well as atomic scale variability due to fluctuations in the growth rate. [10,37] In the present work, we address some of these issues in a representative Zn 2 GeO 4 /SnO 2 NW architecture produced by PR method. [38] For the first time, the elemental composition, structural order, and optical response of these decorated NW systems are simultaneously investigated at the nanoscale using X-ray fluorescence (XRF), X-ray-excited optical luminescence (XEOL), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). This "all-in-one" strategy, which makes use of a hard X-ray nanobeam, provides a complete correlative image of a single-decorated NW. For our experiment, we used the European Synchrotron Radiation Facility (ESRF)'s nano-analysis beamline ID16B. [39] 2. Results and Discussion

Radial and Axial Elemental Modulation
The analysis of the elemental composition in decorated NWs is a crucial step in the characterization of these hybrid systems, as it provides information on the type and number of dopants, as well as the heterogeneities present in the material. Figure 1a shows the scanning electron microscopy (SEM) image of a single PR heterostructure. It consists of a straight central wire (5 μm length and 100 nm diameter) surrounded by well-faceted crystallites (sizes between 500 and 800 nm). Figure 1b shows the XRF nanoimaging results obtained by raster-scanning the Zn 2 GeO 4 /SnO 2 heterostructure in the X-ray nanobeam, where the superimposed distributions of Zn (blue), Ge (green), and Sn (red) are displayed (average XRF spectrum and individual elemental distribution maps can be found in Figures S1 and S2, Supporting Information, respectively). Although it is predicted that the NW axis mostly consists of Zn 2 GeO 4 , the XRF maps of the crystallites show a clear and interesting sandwich-like arrangement. The creation of an alloy that may significantly alter the structural order is suggested by the considerable inclusion of Ge in the SnO 2 crystallites. However, there is no evidence of a one-toone connection between the Ge and Sn distributions. The Ge atoms are preferentially located within the conical faceted sides of the crystallites (sandwich-like structure compared to Sn), which may result in the creation of secondary phases or defects and consequently impact the spatial distribution of the carrier recombination processes. Moreover, Sn localization inside crystallites is proportional to the crystal volume, with a higher intensity in thick, fully faceted portions, whereas Ge is distributed in the regions that are not totally faceted (conical sides). Finally, in a minor fraction, the Zn signal comes mainly from the NW and slightly from the conical-faceted sides of the crystallites (see Figure S2, Supporting Information). In brief, our XRF findings spot the massive incorporation of Ge into the SnO 2 crystallites (accompanied also by Zn in a lower degree), suggesting Sn 1Àx Ge x O 2 alloy formation with a particular sandwich-like spatial pattern.
In good agreement with these findings, we recently reported the observation of a high Ge quantity within the SnO 2 crystallites (from 8% to 17%) by TEM-EDS. [38] Our results point to the formation of a Sn 1Àx Ge x O 2 solid solution, which preserves the rutile crystal structure. Likely, the faceted areas present a lower amount of Ge (and a higher amount of Sn) because the Ge out-diffusion to the Zn 2 GeO 4 NW surface (which is arranged in periodic droplets due to PR-instability) acts as nucleation sites for the Sn crystallites. The more faceted the Sn 1Àx Ge x O 2 crystallites is, the more Sn replaced Ge to form the rutile crystal phase. Recent studies have demonstrated that the NW coating in the PR-type processes is dictated by the evolution of the internal crystalline nanostructure over time, or by the exchange of free atoms between the NW surface and the enveloping near-surface layer. [20,32] Therefore, these factors could be involved in the different distribution of Ge and Sn within in the PR heterostructure, determining its physical properties. www.advancedsciencenews.com www.adpr-journal.com

Anisotropic Optical Response
The optical response of PR heterostructures is a critical parameter that determines their functionality and suitability for optoelectronic applications. Here, we have made use of the optical contrast created by XEOL to concurrently capture spectrally and spatially resolved luminescence signals and XRF data for each sample point (see Figure 2). Following primary XAS, the majority of photogenerated electrons and holes recombine radiatively via secondary XRF and XEOL pathways. Figure 2a presents the average XEOL spectrum, where a main emission dominates in the visible range with two components located at 2.0 and 2.4 eV. It should be noted that the prominent peak exhibits multiple resonances overlapping the main bands, which can be attributed to light reflections between the parallel facets of the Sn 1Àx Ge x O 2 crystallites. [38] In addition, there is a weak band in the UV range located at 3.3 eV. The dominant band corresponds to the emission related to the transition of the trapped electrons from oxygen vacancies to intrinsic surface states from the Sn 1Àx Ge x O 2 crystallites. The luminescence of SnO 2 is extremely complex, exhibiting a broad-emission band in the visible range, consisting of three components (orange, green, and blue at 2.1, 2.5, and 2.7 eV, respectively) associated to oxygen vacancies. [40,41] These components were also reported from SnO 2 nanoribbons by XEOL excited with both soft and hard X-rays. [42,43] Regarding the UV band, its origin is uncertain. This emission could come from the Zn 2 GeO 4 NW, which exhibits a wide emission associated to oxygen vacancies in the UV range. [44] However, an UV emission related to surface states exhibited by SnO 2 nanorods and nanoparticles has also been reported, [45] so it could also come from Sn 1Àx Ge x O 2 crystallites. Figure 2b,c displays the spatial distribution of the respective XEOL bands on the nanometer scale. The visible emission associated to oxygen vacancies is mostly distributed within the Sn 1Àx Ge x O 2 crystallites, which becomes especially stronger where Sn signal is higher. In contrast, the less intense UV band of unidentified origin is only emitted from one side of the Sn 1Àx Ge x O 2 crystallites (for more clarity, see Figure S4, Supporting Information), which raises the possibility of an orientation-dependent phenomenon connected to the crystal structure. Earlier investigations have reported faceted dependent cathodoluminescence in SnO 2 microtubes, which would imply different distribution of impurities and/or defects. [40,46] Alternative explanations for the UV emission include preferential incorporation of defects coming from the outdiffusion of Ge or Zn from the Zn 2 GeO 4 NW, which would produce electronic states in the energy bandgap and cause the UV emission. The inhomogeneous variation of the radiative centers could probably be associated with the different distribution of impurities and/or structural imperfections. The luminescence bands in semiconductors are widely known to be tightly correlated with the density of defect-related energy levels. [45,47]

Correlation between Optical Response and Elemental Distribution
It is possible to determine how the spatial distribution of heterogeneities affects the optical response by combining spatially resolved XRF and XEOL methods. Figure 3 represents the normalized radial line profiles obtained along the PR heterostructure (see Figure 3a). Axial line profiles are also shown in Supporting Information ( Figure S5, Supporting Information). Figure 3b-d shows the XRF and XEOL profiles data obtained from each of the three Sn 1Àx Ge x O 2 crystallites along the perpendicular direction to the core Zn 2 GeO 4 NW. The Sn signal (red lines) has a maximum intensity near the center of the Sn 1Àx Ge x O 2 crystallites and follows a similar trend in all cases. On the contrary, the Ge signal (green line) of the largest crystallite presents a different profile. In general, a tiny portion of the Ge-related photons could suffer self-absorption effects, which may marginally increase Zn and Sn signals. An increased Ge intensity can be seen coming from the structure's edges where the biggest crystallite looks to be entirely created. Contrarily, the smaller Sn 1Àx Ge x O 2 crystallites exhibit a more uniform Ge incorporation, with a profile resembling a Gaussian in accordance with their morphological shape. The Zn profiles (blue lines) also show a multipeak shape, which denotes a rather unusual and distinct behavior inside the crystallite volume. The Zn signals from the crystallite and the NW overlap in the lines can center because the Zn 2 GeO 4 NW is the source of the majority of the contribution. As a result, the findings not only demonstrate Zn's integration into Sn 1Àx Ge x O 2 crystallites but also its preferred spatial location toward the crystallite's edges. The dependency of the crystallographic orientation on the incorporation of dopants and impurities, which causes different elemental diffusion and trapping with subsequent material deposition, may be the cause of this Zn distribution. [48,49] Figure 2. X-ray-excited optical luminescence (XEOL) distribution with nanometric resolution. a) Average XEOL spectrum recorded over the PR heterostructure shown in Figure 1a. b,c) XEOL-integrated intensity maps over the energy ranges defined by the colored bands (visible and UV) in (a).
www.advancedsciencenews.com www.adpr-journal.com In addition to probing different sample depths (XEOL is considerably more surface sensitive than XRF), the radial line profiles of the XEOL signals add additional details about the three Sn 1Àx Ge x O 2 crystallites. In contrast to the XRF signal, the spatial distribution of XEOL is not only governed by the beam spot size and penetration of the X-ray nanobeam within the sample (energy dependent, leading to a finite-generation volume), but also by the diffusion of the photogenerated carriers. Several elements might have an influence on the resulting light profile like sample morphology, composition, internal absorption, and/or reflection mechanisms, as well as secondary photon emissions. The visible band profile (orange line), which has a Gaussian-like shape and is strongly associated with Sn, follows the crystal volume (albeit it has a reduced full width at half maximum owing to diffusion effects [50] ). The UV-emission profiles show a much thinner line width peaking near the left edge of the Sn 1Àx Ge x O 2 crystallite, which matches one of the maxima in the Zn profile nicely. Axial line profiles taken along the PR heterostructure ( Figure S5, Supporting Information) further illustrate the direct relationship between the UV band and Zn localization. These XEOL results could indicate a significant emission polarization anisotropy of the UV luminescence due to likely the faceted nature of the Sn 1Àx Ge x O 2 crystallites linked to the Zn 2 GeO 4 NW.
In short, the correlative XRF-XEOL analysis shows further Ge and Zn element colocalization in the sandwich-like regions of Sn 1Àx Ge x O 2 crystallites. In addition, for all the cases, the UV emission is closely related to the Zn distribution, which is mostly concentrated at one of the edges of the Sn 1Àx Ge x O 2 crystallites. This observation suggests that the anisotropic UV emission could come from recombination centers coupled to complexes associated to the presence of Zn.

Secondary Phase Formation
In addition to chemical composition, secondary phases that result from changes in crystalline structure have a significant impact on the optical characteristics and optoelectronic performance of materials. Thus, to investigate the long-range structural order of the PR heterostructure, nano-XRDs were also performed on the Sn 1Àx Ge x O 2 crystallites using the hard X-ray nanobeam. TEM measurements were previously used to pinpoint the rutile structure of the Sn 1Àx Ge x O 2 crystallites. When seen along [110], the facets of the Sn 1Àx Ge x O 2 crystallites are truncated by (001) planes (see Figures S6 and S7, Supporting Information). Figure 4b shows three representative XRD patterns of the (110) reflection, that were collected over three different zones of the PR heterostructure (see Figure 4a). A close inspection of the reflection (110) in the center (point 2) and one of the edges of the Sn 1Àx Ge x O 2 crystallite fully formed (point 3) reveals a Gaussian-like XRD peak at about 3.32 Å for both areas with similar linewidth, indicating the presence of a uniform crystalline alloy in these regions. However, there are two Gaussian contributions at around 3.31 and 3.33 Å on the opposite side of the crystallite (point 1), indicating the presence of two distinct Sn 1Àx Ge x O 2 phases. According to the first-principles calculations shown in Table S1, Supporting Information, the shift of the XRD peak to 3.33 Å may be connected to a lower concentration of Ge in the alloy. Figure 4c,d displays the spatial distribution of the respective XRD phases on the Sn 1Àx Ge x O 2 crystallite. The Sn 1Àx Ge x O 2 crystallite's volume is essentially covered by the phase that peaked at %3.31 Å. However, the Sn 1Àx Ge x O 2 crystallite's phase peaked at 3.33 is predominantly present on one side and exhibits a spatial distribution in the crystallite that is extremely comparable to that of the UV emission (see XEOL map, Figure 2c). Given that the regions of the Sn 1Àx Ge x O 2 crystallite with the lowest 3.31 Å phase signal and the greatest 3.33 Å phase signal coincide, the signals of the two phases are complimentary. The UV emission is therefore directly observed in the region of the PR heterostructure where the XRD results show a change in the long-range structure. The formation of the secondary phase www.advancedsciencenews.com www.adpr-journal.com within the Sn 1Àx Ge x O 2 crystallites might possibly be explained by a Zn impurity-assisted mechanism, which would have an impact on the degree of optical light and strain generated.

Zn Local Environment in the PR Heterostructure
Given the key role of Zn atoms within the Sn 1Àx Ge x O 2 crystallites, nano-X-ray absorption near-edge spectroscopy (nano-XANES) data were also taken around the Zn K-edge to get a deeper insight into its local structure. Figure 5a shows the spatially resolved XANES acquired over the three same areas where the nano-XRD measurements were collected. The XANES signal is analogous to the partial density of the empty states of the absorbing atoms. In particular, XANES collected at the Zn K-edge reveals the p-partial density of states in the conduction band. Here, two spectral XANES regions can be distinguished: a prominent white line assigned mainly to the 1s ! 4p dipolar transitions, and the post-edge resonances, which are due to multiple scattering effects. All nano-XANES spectra show an intense narrow peak at 9670.2 eV attributed to the white line. The postedge oscillations are quite similar in the sample areas marked by blue and green dots, indicating a similar local environment around Zn atoms. However, the smoother wiggles present in the XANES collected at the sample area indicated by the point 1 could be associated to the extra structural disorder caused by the presence of two crystal phases detected by XRD ( Figure S8, Supporting Information). One popular and empirical assumption is that the XANES spectrum from an unidentified local structural order may be interpreted as a linear superposition of the XANES spectra from two or more well-known samples. As a result, Figure 5b plots the XANES spectra of three model compounds, in addition to one acquired from the Sn 1Àx Ge x O 2 crystallites. In ZnO, Zn atoms are in tetrahedral sites. [51] In the Sn 0.96 Zn 0.04 O 2 alloy, Zn is introduced into the SnO 2 structure with no major change in the crystal structure. [52] Finally, the XANES spectrum around Sn K-edge acquired in SnO 2 rutile type is shown as well, [53] where each tin atom is expected to be coordinated by six oxygen atoms (octahedral coordination). Our results strongly suggest a substitutional  . Zn local environment in the PR heterostructure. a) X-ray absorption near-edge spectroscopy (XANES) data around the Zn K-edge taken in the same three sample areas where XRD measurements were acquired (see Figure 4a). b) XANES data recorded around the Zn or Sn K-edges of reference materials with tetrahedral (ZnO [51] ), distorted octahedral (Sn 0.96 Zn 0.04 O 2 [52] ), and octahedral (SnO 2 [53] ) coordination, in addition to the one acquired in the PR heterostructure (at the bottom of the graph). The spectra were shifted vertically for clarity. In short, XANES data indicate a substitutional incorporation of Zn dopants into Sn sites in the rutile crystal lattice with octahedral coordination inside the Sn 1Àx Ge x O 2 crystallites. These Zn-related defects might promote phase-separation phenomena, as well as electronic states within the bandgap responsible for the UV emission observed by XEOL.

Full Correlative Picture of the PR Heterostructure
According to the elemental nano-analysis previously described, the following sequence for the PR growth of the decorated NWs can be drawn (Figure 6a): 1) thermal growth of the precursors leads to the formation of Zn 2 GeO 4 NWs, on which an array of amorphous GeO 2 (a-GeO 2 ) droplets along the NW is initially produced via the PR instability; 2) the a-GeO 2 droplets act as preferential sites for Sn nucleation, giving rise to Sn 1Àx Ge x O 2 crystallites that encapsulate the NW; 3) the more faceted the Sn 1Àx Ge x O 2 crystallite is, the more Sn replaced Ge, which leads to the Ge/Sn sandwich pattern.
In addition, a 3D rough picture of the interplay between crystal structure and optical response in the decorated NW can be obtained through the "all-in-one" strategy. Figure 6b shows a schematic representation of the potential energy band diagram resulting from all our results. The Zn-doped Sn 1Àx Ge x O 2 crystallite exhibited an asymmetrically distributed UV emission, which has been collected only in the areas with the highest Zn impurity content, suggesting the existence of new electronic states assisted by Zn complexes. The XRD results supports the presence of a secondary crystalline phase right in the areas with the highest amount of Zn, which enter into the crystal lattice replacing Sn as indicated by XANES results.

Conclusion
Due to the complexity of the multiphase chemical phenomena involved at the nanoscale, it is difficult to understand the performance of novel decorated NW heterostructures produced via PR instability. In this work, we have characterized a representative Zn 2 GeO 4 /SnO 2 NW architecture, which has been produced by the PR instability. By combining multiple synchrotron nanoanalysis techniques (nano-XRF, nano-XEOL, nano-XRD, and nano-XAS), we shed light on the origin of the UV emission distributed asymmetrically across the heterostructure. Our findings reveal a connection between the Zn species and the UV radiative transitions. In addition, nano-XRD data show the formation of an additional phase, which is well correlated with the UV emission. To close, nano-XANES measurements indicate an octahedral coordination of Zn atoms in the Sn 1Àx Ge x O 2 crystallites. Thus, our results suggest that a Zn impurity-assisted process could be responsible for the UV optical emission, involving also the formation of a secondary phase within the Sn 1Àx Ge x O 2 crystallites. Our work showed that a multimodal synchrotron nanoprobe is an effective "all-in-one" method for obtaining a thorough understanding of intricate nanosized hybrid systems at the atomic and electronic levels.

Experimental Section
Synthesis: The syntheses of the NW heterostructures were carried out by a single-step thermal evaporation method. The material precursors were a compacted mixture of ZnO:Ge:C powders (2:1:2 wt%), where SnO 2 powders were added in a proportion of 10 wt% related to the total www.advancedsciencenews.com www.adpr-journal.com ZnO:Ge amount. These precursors acted as source and substrate, with no foreign catalyst. The thermal treatments were conducted at 800 ºC for 8 h under an Ar flow of 1.5 L min À1 (see more details in Supporting Information). X-ray Characterization: The hard X-ray nanoprobe of the nanoanalysis beamline ID16B of the ESRF was used in our study. Schematic of the experimental setup for the multiple X-ray techniques at the nanoscale is shown in Figure S10, Supporting Information. The characteristics XRF is recorded with an energy dispersive Si drift detector at an angle of θ = (15 AE 5)°with respect to the sample surface. The XEOL is detected normal to the sample surface by a far-field optical collection system and in backscattering configuration, which avoids lateral shadowing effects (see Figure S11, Supporting Information). The monochromatic X-ray beam was focused to 80 Â 60 nm 2 spot size (see Figure S12, Supporting Information) with about 10 11 ph s À1 . The XRF and XEOL maps were taken at 11.25 keV (excitation energy above the Ge K-edge located at 11.1031 keV) with a pixel size of 50 Â 50 nm 2 over a 3 Â 5 μm 2 sample area and 1 s/point integration time. For XRD measurements, the Sn 1Àx Ge x O 2 crystallites were illuminated by a 14 keV monochromatic nanobeam and scanned with a pixel size of 50 Â 50 nm 2 and 0.5 s per point integration time. The XRD is collected in transmission mode using a FReLoN camera with fiber-optic taper coupling to the scintillator. The nano-XAS data are collected in XRF mode using a Si(111) double-crystal monochromator. Distribution maps and normalized line profiles collected along the PR heterostructure from XRF, XEOL, and XRD signals were processed with PyMca software. XRD data were integrated using Dioptas program.
Electron Characterization: The morphology and structure of the obtained PR heterostructure were analyzed by SEM, with a LEO-1530 SEM equipment, and by HRTEM, using a JEOL 2100 microscope working at 200 kV. The cross-sectional samples were obtained by means of a JEOL 4500 focused-ion beam/SEM with a 30 kV ion column and a Ga þ ion source.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.