Exploring the Electronic Structure and Chemical Homogeneity of

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Exploring the Electronic Structure and Chemical Homogeneity of Individual BiTe Nanowires by Nano-Angle-Resolved Photoemission Spectroscopy 2

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Janina Krieg, Chaoyu Chen, José Avila, Zeying Zhang, Wilfried Sigle, Hongbin Zhang, Christina Trautmann, Maria Carmen Asensio, and Maria Eugenia Toimil-Molares Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00400 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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Exploring the Electronic Structure and Chemical Homogeneity of Individual Bi 2 Te3 Nanowires by Nano-Angle-Resolved Photoemission Spectroscopy Janina Krieg,

∗,†,‡

Hongbin Zhang,

Chaoyu Chen,

‡

¶, §

José Avila,

Christina Trautmann,

†, ‡

¶,§

Zeying Zhang,

‡, k

Maria Carmen Asensio,

Eugenia Toimil-Molares

Wilfried Sigle,

¶, §

⊥

and Maria

∗,†

Materials Research Department, GSI Helmholtz Centre for Heavy Ion Research, Planckstr. 1, 64291 Darmstadt, Germany, Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany, Synchrotron SOLEIL, L'Orme des Merisiers Saint-Aubin BP48, 91192 Gif-sur-Yvette Cedex, France, Université Paris Saclay, L'Orme des Merisiers Saint-Aubin BP48, 91192 Gif-sur-Yvette Cedex, France, School of Physics, Beijing Institute of Technology, Beijing 100081, China, and Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

E-mail: [email protected]; [email protected]

Phone: +49 6159 711896. Fax: +49 6159 713266

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Abstract Due to their high surface-to-volume ratio, cylindrical Bi2 Te3 nanowires are employed as model systems to investigate the chemistry and the unique conductive surface states of topological insulator nanomaterials. We report on nano-angle-resolved photoemission spectroscopy (nano-ARPES) characterization of individual cylindrical Bi2 Te3 nanowires with a diameter of 100 nm. The nanowires are synthesized by electrochemical deposition inside channels of ion-track etched polymer membranes. Core level spectra recorded with sub-micron resolution indicate a homogeneous chemical composition along individual nanowires, while nano-ARPES intensity maps reveal the valence band structure at the single nanowire level. First principles electronic structure calculations for chosen crystallographic orientations are in good agreement with those revealed by nano-ARPES. The successful application of nano-ARPES on single onedimensional nanostructures constitutes a new avenue to achieve a better understanding of the electronic structure of topological insulator nanomaterials.

Keywords Bi2 Te3 , nanowire, electrochemical deposition, ion track technology, topological insulator, nano-angle-resolved photoemission spectroscopy (nano-ARPES)

To whom correspondence should be addressed GSI Helmholtz Centre for Heavy Ion Research ‡ Technische Universität Darmstadt ¶ SOLEIL Synchrotron § Université Paris Saclay k Beijing Institute of Technology ⊥ Center for Electron Microscopy ∗

†

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In the last decades bismuth telluride (Bi 2 Te3 ) based materials have attracted great interest due to their potential application for thermoelectric cooling or power generation. Besides being the most ecient room-temperature bulk thermoelectric material 1 for 60 years, Bi2 Te3 was predicted to be a three-dimensional topological insulator in 2009. 2 This new class of quantum matter stands out due to the presence of conducting surface states while the bulk possesses an energy gap. 3,4 First experiments conrming the existence of such a phase of matter were conducted by transport measurements in HgTe quantum wells 5 as well as angle-resolved photoemission spectroscopy (ARPES) on bulk single-crystals of Bi 1−x Sbx 6 and Bi2 Se3 . 7 ARPES is a powerful tool to directly visualize collective excitations and low energy electronic states of a material and thus provides a direct identication of conducting surface states. In the case of Bi 2 Te3 , rst ARPES studies were performed on high quality thin lms and single crystals 810 revealing the important role of strong spin-orbit coupling and the existence of surface states, 8 as well as showing their Dirac-like nature and visualizing the band gap. 9,10 Nanowires of Bi2 Te3 have been extensively studied during the last decades, because of their predicted enhanced thermoelectric eciency arising from low-dimensionality eects. 11 Additionally, they are excellent model systems to address the surface states of topological insulators. 1216 Their large surface-to-volume ratio reduces the otherwise dominating contributions of the bulk, thus improving the access to the surface contributions. This attribute is especially interesting to optimize surface state transport measurements. To date such measurements are still very challenging, but essential on the route to develop future spintronics devices based on topological insulators. 1722 Currently, various techniques for synthesizing Bi2 Te3 nanowires are well established, amongst them vapor-liquid-solid growth 23 and template-assisted electrochemical deposition. 2428 So far, the search for topological non-trivial states in Bi 2 Te3 nano-objects such as nanowires has been lead by transport measurements providing an indirect investigation of these unique surface states. 1416,2931 However, it is well-known that a heterogeneous stoichiometry drives

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important electronic modications changing the doping, carrier density, gaps as well as massless properties and thus inuences the thermoelectric 32,33 and electrical transport properties. 34 Consequently, the direct investigation of the electronic band structure of topological insulator nanowires and their surface states requires, as in the case of two-dimensional materials, a full study using ARPES. However, ARPES measurements with enough spatial resolution and sucient signal intensity to characterize nano-objects are extremely challenging and had to be specically developed. 3539 In this letter, we report angle-resolved photoemission spectroscopy measurements with sub-micron resolution (nano-ARPES) and chemical mapping on individual Bi 2 Te3 nanowires. The implementation of this technique enabled the direct observation of the electronic structure of selected sections of individual nanowires. Bi2 Te3 nanowires employed in this work are synthesized by electrochemical deposition in ion-track membranes. First, polymer templates with cylindrical nanochannels are prepared by heavy ion irradiation and subsequent chemical etching. 40,41 Secondly, these nanochannels are electrochemically lled with Bi 2 Te3 forming nanowires. Polycarbonate foils of 30 µm thickness were irradiated with GeV Au ions (typical uence: 109 ions/cm2 ) at the Universal Linear Accelerator (UNILAC) at GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. Along their trajectory the Au ions damage the polymer, creating so-called ion tracks that are tens of micrometers long and few nanometers in diameter. 40,41 Etching the irradiated polymer foil in a suitable etchant (6 mol/L NaOH solution at 50◦ C) converts the track of each individual ion into a cylindrical channel. Their diameter is adjustable with etching time 42,43 which was set to 4 min to yield cylindrical channels with (100 ± 5) nm diameter. The subsequent electrochemical deposition of Bi 2 Te3 within the nanochannels is carried out at 30 ◦ C in a three-electrode conguration using a Gamry REF600 potentiostat. For this purpose, a conductive gold layer on one side of the membranes serves as working electrode. A platinum spiral wire serves as counter electrode. The electrolyte consists of 5 mmol/L bismuth nitrate pentahydrate and 7.5 mmol/L tellurium

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dioxide in 1 mol/l nitric acid. 44 A deposition potential of 0 V was applied in reference to a saturated calomel electrode (SCE). 45 More information on the Bi 2 Te3 nanowire growth can be found in the supporting information. For high resolution scanning (HRSEM) and transmission electron microscopy (TEM) as well as for the nano-ARPES measurements, the nanowires are released from their polymer template and transferred onto a suitable substrate. This is realized by placing the embedded nanowires on e.g. a silicon wafer or close to a TEM grid and dissolving the polycarbonate by drop casting of dichloromethane. Subsequently the samples are immersed in two fresh baths of dichloromethane for several minutes. By darkeld TEM imaging the nanowires are shown to consist of several hundred nanometer or micrometer long single-crystalline sections, separated by single grain boundaries. Figure 1 (a) displays two such single-crystalline sections and their grain boundary oriented approximately 45 ◦ with respect to the nanowire axis. In addition, the cylindrical geometry and the smooth contour of the nanowire are clearly visible. Figure 1 (b) shows a high-resolution TEM (HRTEM) image of a part of such a single crystalline section displaying its ordered structure with planes of interplanar distances of approximately 0.22 nm. This ordered structure continues to the nanowire edge and no evidence of oxidation of the surface was found. Four-circle x-ray diractometry was carried out on the nanowire array while still embedded in the polymer template. The inset in g. 1 (c) displays this arrangement schematically. In this way the average preferred crystallographic orientation perpendicular to the nanowire axis is investigated. Previous studies showed that the crystallographic orientation of Bi 2 Te3 nanowires synthesized by electrochemical deposition can be controlled by varying the growth parameters such as the deposition potential and temperature. 45,46 Figure 1 (c) shows an ω 2θ-scan of the fabricated nanowire array together with the known diraction values of Bi 2 Te3 (black) and Au powder (blue) (JCPDS les no. 08-0027 and no. 04-0784, respectively). The main reections visible at approximately 23.8 ◦ , 27.8◦ , 41.4◦ and 50.5◦ can be identied as originating from (101), (015), (110), and (205) planes of Bi2 Te3 , respectively. Reections at

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Figure 1: Crystallographic and compositional characterization of cylindrical 100 nm diameter Bi2 Te3 nanowires: (a) Darkeld TEM image of a representative 100 nm diameter nanowire, visualizing a grain boundary (red arrow) between two grains of several hundred nm length. (b) High-resolution TEM image of a single-crystalline section of a nanowire. The atomic planes parallel to the wire axis exhibit an interplanar distance of about 0.22 nm. (c) Fourcircle x-ray diraction patterns of a nanowire array embedded in the polycabonate template as shown schematically in the inset. The bottom shows standard Bi 2 Te3 and Au powder diraction values. (d) Energy-dispersive x-ray spectroscopy along two lines at the surface of two nanowires (insets) indicating close to stoichiometric chemical composition (dashed lines).

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38.4◦ and 44.5◦ as well as all reections at angles higher than 60 ◦ are attributed to the gold working electrode. X-ray diractograms on samples fabricated using the same parameters and after removal of the gold layer in KI solution showed no reections around 38 ◦ and 44◦ . The comparison of the measured intensities of the reections to those predicted for the powder sample, indicates that the nanowires are strongly textured. The two preferred orientations are the (101) and (205) planes perpendicular to the nanowire axis. The chemical composition of the nanowires was analyzed by energy dispersive x-ray spectroscopy (EDX) within a TEM. Linescans with a spatial resolution of approx. 1 nm were recorded along as well as perpendicular to the nanowire axis and revealed close to stoichiometric composition. For most nanowires the chemical composition is homogeneous within the range of the linescan measurements as shown in g. 1 (d). In few exceptional cases the surface of the nanowires was found to be locally rich in Te (see supporting information g.S2). However, it is worth mentioning that EDX is not able to provide information on the oxidation states of elements crucial for the evaluation of the surface quality. Amongst others, this information is accessed by analyzing the core level spectra obtained from nano-ARPES measurements that will be described in more detail below. Bi2 Te3 nanowires exhibiting the geometric, compositional and crystallographic properties mentioned above and summarized in g. 1 were prepared for nano-ARPES measurements at the ANTARES beamline of the SOLEIL synchrotron facility. This beamline oers a unique combination of nano-ARPES measurements and mapping by recording the core levels with extremely high spatial resolution ( ∼ 120 nm). 39 Employing both methods, we successfully identied individual wires deposited on a silicon substrate (boron p-doped) and visualized the electronic band structure of dierent sections of these nanowires. If not stated otherwise, the photon energy used in this experiment was set to 100 eV. Due to the surface sensitivity of nano-ARPES, the nanowire samples had to be carefully prepared to provide a suciently high surface quality. In this case, the nanowires were transferred onto a silicon wafer by performing the drop-casting method under argon atmosphere to avoid oxidation and hence

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surface quality loss due to exposure to ambient air. Using a transfer chamber the Ar atmosphere was maintained as the sample was transfered to the beamline and inserted into the experimental station. Additionally, the sample's surface was cleaned by argon plasma sputtering (500 V, 38 mA) within the preparation chamber available inside the experimental station under ultra-high vacuum conditions ( ∼ 10−9 mbar). The cleanness of the sample's surface was rst evaluated by survey photoemission spectra recorded using large photospots (∼ 100 µm) on the entire sample area to identify oxygen and carbon contamination which possibly originate from oxidation and polymer residues. More details on the sample preparation protocol can be found in the supporting information (g. S3 and S4). (a)

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Figure 2: Nanowire identication by nano-ARPES at 100 eV photon energy: (a) Highresolution scanning electron microscopy image and (b) Te 4d core level mapping of the Bi2 Te3 nanowires on a silicon substrate (step size 1 µm). (c) Te 4d and (d) Bi 5d core level mappings of the single nanowire marked by the yellow box in (b). (e) Core level spectra of two points on a Bi 2 Te3 nanowire (red) and the silicon substrate (blue) at the positions marked in (b) by a circle and a cross, respectively. Figure 2 (a) displays a representative HRSEM image of Bi 2 Te3 nanowires lying on the silicon wafer. The 100 nm diameter nanowires are typically around 15 µm long. To identify the Bi2 Te3 nanowires on the substrate xed inside the experimental station, the sample was mapped in a selected area taking advantage of the precise sample scanning and focused nanospot. Figure 2 (b) presents a 100 µm x 80 µm mapping recording the Te 4d core level with a step size of 1 µm recorded from a dierent position on the same substrate. The quality of the nano-ARPES mapping demonstrates that, similar to the HRSEM image (g. 2 (a)), 8


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areas of high nanowire density as well as single wires can be well resolved and identied. The small focus of the x-ray beam and the precise positioning system ensure the high resolution needed to subsequently locate single nanowires of interest. In this case the representative area to be investigated is marked by the yellow box in g. 2 (b). This area of 10 µm x 10 µm was mapped recording the Te 4d (g. 2 (c)) and the Bi 5d core level (g. 2 (d)) with a step size of 400 nm. The Te 4d mapping indicates a homogeneous Te content along the wire axis. Both images show a shadow eect separated by a darker line parallel to the nanowire axis. Repeating the mapping on a smaller area and with smaller step size, this eect vanished (see g. 3). In general, the Te 4d mapping provides a slightly better resolution and contrast of the nanowire compared to the Bi 5d data. The dierence is due to the signal obtained from the silicon substrate as obvious from the core level spectrum in Figure 2 (e). It shows two typical core level spectra recorded in highly focused nano-spot nano-ARPES mode over a binding energy range of 65 eV. Focusing the beam to about 100-150 nm, the spectra are obtained from an individual Bi 2 Te3 nanowire (yellow circle in g. 2 (b)) and from the bare silicon substrate (yellow cross in g. 2 (b)). Two doublets can be identied at 41.6 eV and 39.9 eV as well as at 27.7 eV and 24.7 eV binding energy, originating from the Te 4d and Bi 5d core level, respectively. 4749 The intense and broad peak at around 7 eV is identied as a background signal caused by the silicon substrate. Additionally, a reference spectrum measured on the pure silicon substrate exhibits a broad peak with a maximum at about 25 eV, adding a background signal to the Bi 5d core level doublet. Both substrate peaks possibly arise from the native oxide on the silicon 50 as well as from the HF treatment of the substrate prior to the nanowire deposition forming silicon mono- and dihydrides. 51 As expected, the intensity of the substrate peaks decreases for spectra obtained on the nanowire. They do not vanish completely due to the beamspot being slightly larger than the wire diameter. Additionally, the beam-to-nanowire position can be misaligned due to the limited stepsize of 100 nm scanned with a beamspot of ∼ 120 nm (see inset in g. 3). This background signal is responsible for the reduced resolution of the nanowire in the Bi 5d mapping compared

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to the one recorded using the Te 4d energy. For this reason, the mappings to identify the nanowires were always conducted using the Te 4d doublet signal. The upper image in Figure 3 shows a high resolution mapping of a section of the nanowire in the yellow box in Figures 2 (b)-(d), recorded directly after the sample had been exposed to Ar plasma for 20 min and transferred from the preparation to the measurement chamber. In this case, the mapping's step size was reduced to 100 nm. The image reveals a homogeneous diameter in agreement with the HRSEM and TEM images as well as a continuous and high Te intensity peak contrast along the nanowire axis. The numbers (1) to (4) indicate the four positions on the nanowire from which core level spectra were recorded. They are located between 500 nm and 1 µm apart from each other along the nanowires axis. For Te 4d

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Figure 3: High-resolution Te 4d core level mapping with 100 nm step size of the Bi 2 Te3 nanowire marked in g. 2 (b) and (c) in the same color code. The numbers indicate the corresponding measuring positions for the core level spectra of the Te 4d and Bi 5d doublets. Shaded curves indicate the area ts to the spectra. The black lines show the subtracted Shirley background (solid line) and the one measured on the Si substrate (dashed line and bottom curves). 10


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comparison, a core level spectrum obtained from one position on the Si substrate at a distance of approximately 3 µm from the nanowire is added (bottom black curve). Similar to Figure 2 (e), the Bi 5d signal is convoluted by the silicon substrate background resembling the shape of an exponentially smeared out Gaussian as indicated by the dashed black curve in Figure 3. The measured spectra of the Te 4d and Bi 5d doublets are summarized in Figure 3 for all four wire positions. The two peaks of the Te 4d and the Bi 5d doublets, resulting from the spin-orbit coupling, are separated by around 1.5 eV and 3 eV, respectively, which is consistent with literature values. 4749 All core level spectra were normalized to the substrate peak at around 7 eV. No peak shift could be detected for all four spectra, giving evidence for a constant chemical composition within the chosen wire section. Using a full quantitative analysis, we observe that the peak areas do not change indicating a constant Bi/Te ratio along the nanowire axis with a spatial resolution better than 120 nm. The peak areas tted by Voigt and Lorentz distributions (solid red and blue shaded curves) as well as the subtracted Shirley background (solid black line) are indicated on the core level spectra measured on position (1) of the nanowire. The Bi 5d signal was deconvoluted by keeping the same shape of an exponentially smeared out Gaussian as determined from the t to the substrate spectrum (black curve). These results show that the chemical composition stays homogeneous along the nanowire section which supports our ndings from the EDX in TEM measurements (g. 1 (d)) obtained from dierent nanowires synthesized under the same conditions within the same batch. Although the EDX in TEM provides in principle a spatial resolution of 1 nm, the signal from the wires comprises not only information from the surface, but also from the nanowire bulk. Using the core level spectra obtained at ANTARES, solely the chemical composition of the nanowire surface is probed indicating its homogeneity with a spatial resolution of better than 120 nm. Addressing the angle-resolved photoemission information, the valence band structure of two separate cylindrical nanowire sections was recorded by nano-ARPES as displayed in Figures 4 (a) and (b). The exact position of the measurements are indicated by the white

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and the red marker in the Te 4d core level mappings presented in Figures 4 (c) and (d), respectively. The nano-ARPES energy-momentum data in Figure 4 (a) was recorded at the position of a single nanowire, while the image in Figure 4 (b) was obtained from a wire within a nanowire bundle. For both positions the valence band was imaged within a range of 5 eV below the Fermi energy using an incident photon energy of 100 eV. The x-ray beamspot size of about 120 nm lies within the diameter of the nanowire and is smaller than the large single crystalline sections extending over several hundreds of nm as shown in g. 1 (a). To resolve the valence band structures a polynominal background was tted and subtracted from each of the energy-momentum spectra. A detailed description of the data processing is provided in the supporting information g. S5 and S6. The appearing features in Figure 4 (a) indicate that the Fermi level lies within the valence band and thus hint at a p-doping of this nanowire section. The comparison of Figures 4 (a) and (b) reveals dierent valence band structures at the two separate positions exhibiting the same chemical composition. Clearly, the energy-momentum data in Figure 4 (b) exhibits dierent features with the most intense bulk band having its maximum at about 1 eV below the Fermi level. We attribute this more complex and less well dened band structure to a dierent crystallographic orientation of the nanowire investigated at this position. Another possible reason is that the band structure originates from two consecutive nanowire section surfaces, which belong to dierent grains and crystallographic orientations, respectively, as discussed in g.1 (b). To get a better understanding of the nano-ARPES data, we performed rst principles electronic structure calculations for various crystallographic orientations (see the supporting information for details), which are displayed in g. 5. Following the XRD results (g. 1 (c)), the surface states of semi-innite Bi 2 Te3 projected onto planes perpendicular to the (205) and (101) planes are calculated. It is noted that there are many possibilities to choose such planes, and three of them, which are perpendicular to vectors with low Miller indices lying in either the (205) or the (101) plane, were selected. For instance, the planes perpendicular to vectors [-532] (yellow) and [-121] (red) are approximately perpendicular to the (205) and

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Figure 4: Nano-ARPES data (a) and (b) from two dierent sample positions as marked in (c) on an individual wire and in (d) on a bundle of nanowires, respectively.

(101) plane, respectively, while the plane perpendicular to vector [010] (green) is approximately perpendicular to both. Hereafter, the Miller index of the vectors are used to denote the corresponding planes, as schematically displayed in Figure 5 (a). The corresponding band structures are shown in Figure 5 (b)-(d). Obviously, for all band structures obtained in our calculations, there exist topological surface states at the fermi energy EF connecting the valence and conduction bands, due to the nontrivial topological nature of the band gap in Bi2 Te3 . Furthermore, the energy bands of Figures 5 (b) and (c) follow a similar distribution diering mainly at the zone boundaries (close to the M and K points) where the bands of the (-121) plane (red) exhibit an upturn. The energy band structure calculated for the (010) plane (green) shows a dierent band distribution with the most pronounced spectral weight around 1 eV and 3 eV below the Fermi energy. The calculated band structures along chosen crystallographic orientations are in good agreement with the nano-ARPES measurements. For instance, three parabola-shaped symmetric bands in g. 4 (a) can be compared to the most intense bands in g. 5 (b) and (c), where the slight upturn of the bands at the zone

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boundaries can be attributed to the dierent crystallographic orientations. Moreover, the nano-ARPES data shown in g. 4 (b) with intensive weight around 1 eV below the Fermi energy are similar to the surface states obtained for the (010) plane (green) in g. 5 (d). Thus, based on such calculations, the measured nano-ARPES spectra are certainly arising from dierent oriented crystal sections of the Bi 2 Te3 nanowire. E

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Figure 5: Electronic structure of Bi 2 Te3 nanowires along dierent crystallographic orientations. (a) Schematic of plane orientation for planes perpendicular to (101) (purple, left) and (205) (purple, right) planes in the hexagonal lattice. Planes perpendicular to vectors [-532], [-121] and [010] are marked in yellow, red and green, respectively. First principles electronic structure calculations for (b) (-532) (c) (-121) and (d) (0-10) planes In summary, we report electronic heterogeneities comparing the valence band structure of chemically homogeneous, individual, cylindrical Bi 2 Te3 nanowires of 100 nm diameter and 15 µm length. Despite the challenges of establishing a high quality sample surface, we were able to resolve the valence band structure of various sections of nanowires. Two dierent band structures were found, attributed to various crystallographic orientations of the nanowire sections with respect to the X-ray beam. These band structures were compared 14


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to energy band calculations for the expected crystallographic orientations. The chemical composition of the wire surface and bulk are found to remain homogeneous. The Fermi level of the nanowires resides within the valence band. Further nano-ARPES measurements on single nanowires for investigations of the band gap and possible Dirac cones resulting from the surface states of the topological insulator properties of Bi 2 Te3 are in progress. The method is also of great interest for other topological insulator nanomaterials.

Acknowledgement This work was supported by the SOLEIL synchrotron under the proposal 20130543. J.K. and M.E.T.-M. acknowledge support by the DFG Priority Program SPP 1666. J.K. acknowledges support by the graduate school HGS-HIRe. Z. Z. is funded by the International Graduate Exchange Program of Beijing Institute of Technology. We thank S. Lorcy for the technical support at the ANTARES beamline as well as F. Boui for help during sample preparation.

Supporting Information Available Fabrication of Bi2 Te3 nanowires by electrochemical deposition in etched ion-track membranes. Transfer of polymer-embedded Bi 2 Te3 nanowires onto a suitable silicon substrate. EDX in TEM linescans of the Bi 2 Te3 nanowire edge showing local Te enrichment. Core level spectra for evaluation of the cleanness and quality of the sample. Investigation of a possible ageing eect of the samples. Description of data processing of nano-ARPES spectra. Details on the DFT calculations of electronic structures for dierent crystallographic planes. This material is available free of charge via the Internet at http://pubs.acs.org/ .

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For TOC Only nano-ARPES mapping

Dichloromethane PC Membrane Bi2Te3 Nanowires Sili con

sub

stra

te

10 µm

20


Exploring the Electronic Structure and Chemical Homogeneity of

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