Surface Oxidation of Bi2(Te,Se)3 Topological Insulators Depends on

Dec 16, 2015 - To ensure that measurements made for Bi2Te2Se were relevant to Bi2Te3 and Bi2Se3, single crystals of each were cleaved in a nitrogen at...
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Surface Oxidation of Bi(Te,Se) Topological Insulators Depends on Cleavage Accuracy Conor R. Thomas, Matthew K. Vallon, Matthew G. Frith, Hikmet Sezen, Satya K. Kushwaha, Robert J. Cava, Jeffrey Schwartz, and Steven L. Bernasek Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03923 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015

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Surface Oxidation of Bi2(Te,Se)3 Topological Insulators Depends on Cleavage Accuracy Conor R. Thomas‡ 1, Matthew K. Vallon‡ 1, Matthew G. Frith1, Hikmet Sezen2, Satya K. Kushwaha1, Robert J. Cava1, Jeffrey Schwartz1, Steven L. Bernasek*1 1

Princeton University, Department of Chemistry, Princeton, NJ, 08540, USA Elettra-Sincrotrone Trieste S.C.p.A., Area Science Park, 34012 Trieste, IT

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Supporting Information Placeholder ABSTRACT: Topological insulators based on the Bi2(Te,Se)3 family of materials enable new possibilities for spintronics and quantum computation. The surface chemistry of these materials is not well understood, and controversy exists concerning aerobic oxidation of their native surfaces. Here we compare the surface oxidation of Bi2Te2Se, Bi2Te3, and Bi2Se3. We also report that synchrotron-based scanning photoelectron microscopy shows that aerobic oxidation of Bi2Te2Se is rapid along step edge defects, but not on basal planes.

Topological insulators are characterized by their unique surface electronic states, which involve a Dirac cone with linear dispersion and massless electrons that exhibit spin1,2 momentum locking. It is not surprising, therefore, that most research on TIs has focused on their physics, including 3-6 transport measurements, angle-resolved photoemission 7,8 9,10 spectroscopy, scanning tunneling microscopy, and theo2,11-13 ry. Yet, if new devices based on TIs, such as for spintron2,14 ics and quantum computation, are to be realized, addressing TIs through their surface chemistry must also be under15,16 stood. Indeed precisely controlling the surface chemistry of TIs could be a prerequisite for interfacing them with other 3,5 materials: Dirac states are highly surface-sensitive. Absent packaging, a freshly cleaved sample of the TI would be exposed to air, and there are conflicting accounts as to the effects of such exposure on members of the Bi2(Te,Se)3 family 4,13,17-21 of TIs. In particular, several reports based on X-ray photoelectron spectroscopy (XPS) and transport measurements claim rapid, general oxidation of Bi2Te3 and Bi2Se3 surfaces with concomitant degradation of their TI proper4,19 ties. Others use density functional theory, scanning probe microscopy, and XPS to claim that basal planes of these same species are resistant to oxidation, with defects proposed, but 13,18,21 not demonstrated, to be sites of oxidation. Herein we provide direct XPS comparison among Bi2Te3, Bi2Se3, and Bi2Te2Se aerobic oxidation profiles that shows Bi2Te2Se to be comparable in reactivity to Bi2Te3. No Se oxidation is observed, suggesting an oxide growth method different from 13,19 that suggested for Bi2Se3. We also address disparate oxidation reactivity claims through scanning photoelectron microscopic (SPEM) analysis, which provides direct experimental

evidence for rapid aerobic oxidation at the step edges of 13,18,21 Bi2Te2Se (BTS) TIs, but not, as predicted by calculation, on basal planes.

Figure 1. XPS spectra of a Bi2Te2Se crystal freshly cleaved (bottom traces), and exposed to ambient air for two weeks (top traces). Intensity data for the freshly cleaved crystal are divided by 2 to facilitate peak comparisons. (a) Te 3d shows growth of a Te-O species at Te 3d5/2 EB = 575.9 eV. (b)

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Growth of a Bi-Te-O peak is observed at 157.6 eV in the Bi 4f region. (c) The Se 3d does not change with oxidation of Bi2Te2Se over this time scale. (d) O 1s peaks on the freshly

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and hydroxyl species, with growth of a Te-O species and TeOH species on exposure to air.

cleaved samples are assigned to adsorbed instrumental water Figure 2 (a) A plot comparing the peak area ratio of oxidized species to metal species with air exposure on a logarithmic time axis (See Supporting Information Figures 1-3). (b,c) Plots of the binding energy separation between the oxide and TI peaks for Te (b), and Bi (c). (d) Model of Bi2Te2Se surface oxidation showing rapid oxidation of step edge Te atoms. Bi atoms are labeled by the number of adjacent Te-O ligands. This is modeled with atomic, not ionic, radii to enable visualization of the underlying layers; atomic positions are accurately located. Single crystals of Bi2Te2Se, Bi2Te3, and Bi2Se3 were grown from a melted mixture of the elements and were cleaved 6 from the large boules thus prepared. Members of the Bi2(Te,Se)3 family have a layered structure consisting of quintuple layers of Te/Se (1) – Bi – Te/Se (2) – Bi – Te/Se (1). The22 se layers are separated by a van der Waals gap, which allows in principle for facile cleavage along a basal plane leading to a terraced surface that can be visualized by atomic 16,18 force microscopy (AFM). A common method to cleave these materials is to align a sharp blade parallel to the basal (0001) plane, then apply force to separate the crystal along a van der Waals gap. Removal of the uppermost layer of a cleaved sample is accomplished by applying adhesive tape to the surface and lifting off a thin sheet of Bi2(Te,Se)3 to expose a pristine surface. These methods are not well controlled, and slight misalignment of the initial cleavage can result in a stepped defect-heavy surface. To ensure that measurements made for Bi2Te2Se were relevant to Bi2Te3 and Bi2Se3, single crystals of each were cleaved in a nitrogen atmosphere glove box (O2, H2O < 0.5 ppm) first with a clean razor blade then using adhesive tape without particular attention paid to crystallographic orientation. Each was then sealed in a vacuum transfer device and placed into the antechamber of the XPS instrument; this enables XPS analysis of the pristine crystal surfaces with no air exposure. Spectra were measured using an ESCALab Mk II hemispherical analyzer and a Phi 04-548 dual X-ray anode with Mg Kα (1253.6 eV) radiation. Scans were taken with a pass

energy (Epass) 20 eV, 0.05 eV step sizes, and 500 ms dwell time. The instrument was 2-point calibrated using binding energies for Au 4f (84.0 eV) and Cu 2p (932.7 eV). Each freshly cleaved sample was calibrated to the adventitious C 1s peak at 284.6 eV, and subsequent spectra were fit to the resulting Bi 4f peak binding energy. All peaks, except for Bi 4f, were fit with a Gaussian-Lorentzian mix. For Bi 4f, peaks were analyzed using a damped asymmetric Lorentzian fit. All peaks, except O 1s, were analyzed with Shirley background subtraction; linear background subtraction was used for O 1s. After each analysis samples were removed from the vacuum chamber and stored in the ambient laboratory environment. XP spectra for Bi2Te2Se are shown in Figure 1a-d for freshly cleaved crystal and one exposed to air for two weeks (336 hours). The Te 3d region (Figure 1a) shows two species after air exposure, Te in Bi2Te2Se at Te 3d5/2 EB = 572.5 eV and an oxidized Te-O species at Te 3d5/2 EB = 575.9 eV. The Bi 4f region also contains a high-binding energy peak at Bi 4f7/2 = 159 eV, which is assigned to Bi atoms ligated to Te-O; it is unlikely that Te-O-Bi bonds can form in such a short time scale, given the screening by Te at the surface. The O 1s region shows water and hydroxyl peaks from surface-adsorbed ambient water from the glove box or XPS instrument, as well as growth of a tellurium oxide peak at 529.7 eV. The rapid oxidation observed for a step-heavy surface is hypothesized to follow a different model from that described by Yashina et 13 al. for well-cleaved surfaces; Te or Se atoms of the top atomic layer will first oxidize at step edges forming a Te-O or Se-O

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species, then grow laterally (Figure 2a). This model assumes that the surface will not rapidly reconstruct a Te/Se-Bi bond to a Te/Se-O-Bi bond, as the close packing of Te/Se at the surface should screen the underlying Bi from reaction with O2. Surface oxidation was followed by calculating the ratio of peak areas of oxidized and unoxidized species for each element, XOx : XBi2(Te,Se)3 (X = Te, Se, or Bi, where XOx is the oxidized species, and XBi2(Te,Se)3 is the native one). Oxidation of each species followed logarithmic growth and saturated at around 2 weeks (Figure 2a); Bi2Se3 is less reactive than Bi2Te3, 13,16,19 which is comparable to Bi2Te2Se. Reports on oxidation of

Bi2Te3 and Bi2Se3 claim oxide growth throughout the top two quintuple layers; if the same occurred for Bi2Te2Se, oxidation of the Se layer beneath that of Bi would be expected, but no such oxidation was measured over the time period studied (Figure 1c). An interesting relationship was found between XOx : XBi2(Te,Se)3 (peak area ratio), and the binding energy separation (∆EB) between the high binding energy oxide peaks and low binding energy (Bi2(Te,Se)3) peaks. For tellurium (Figure 2b), ∆EB vs oxidation is nonlinear; it is hypothesized that step edge Te atoms oxidize first, which leads to the rapid initial increase in ∆EB.

Figure 3 SPEM maps of a Bi2Te2Se crystal cleaved and exposed to air for 30 minutes. Unprocessed maps of Bi 4f (a) and O 1s (b) show both morphological and chemical information, and show the terrace and step edge structure of Bi2Te2Se. The image in c is obtained after morphology correction and then dividing the O 1s map by the Bi 4f map, which shows that oxygen is strongly localized on step edges. Detailed XP spectra were taken at three specific points noted on the Bi 4f map (a). Point 1 is in an intermediate terrace, point 2 is on a large terrace, and point 3 is directly on a step edge. Detailed XP spectra for each point are shown in (d), point 1 are the red traces, 2 are blue, and 3 are green. Te 3d5/2 XP spectra show two peaks, one for Bi2Te2Se at 572.5 eV, and an oxide peak at 574.5 eV. The oxide shoulder is smallest at point 1, stronger at 2, and strongest at 3, the step edge. Bi 4f7/2 spectra show a single species at points 1 and 2, and a small shoulder appears at point 3. The O 1s spectra show the most dramatic differences, as point 3 is the only location with detectable oxygen. Next, non-adjacent Te sites oxidize, leading to the flat center region, and, finally, adjacent Te sites oxidize, and ∆EB increases. For Bi, the ∆EB increases linearly with growth of the oxide-ligated Bi peak (Figure 2c). While the Bi 4f peaks are fit to two species, there are likely multiple overlapping peaks that correspond to Bi ligated to one, two, or three adjacent [Te-O] species. As more Te atoms on the surface oxidize, the average Bi state shifts from one to three adjacent [Te-O] ligands, and thus the observed ∆EB increases. Spatially resolved determination of speciation is critical to

address apparent discrepancies concerning oxidation of members of the Bi2(Te,Se)3 family: Laboratory XPS is typically not spatially resolved, and element mapping by energydispersive X-ray microscopy probes too deeply (~50 µm) to study surface oxidation. Scanning photoelectron microscopy (SPEM) (using the ESCA Microscopy Beamline; ELETTRA Sincrotrone Trieste) addressed these limitations. In this method Fresnel zone plate optics and an order-sorting aperture focus the incoming X-ray beam (699.2 eV) diameter to 120 nm; the sample stage is rastered with respect to the fo-

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cused beam. A 48-channel hemispherical analyzer enables spectral mapping; each pixel of a SPEM image is an independent XPS spectrum. High-resolution XP point spectra then provide detailed chemical state information for a given microstructure; those spectra shown in Figure 3 were taken at Epass = 20 eV, 0.08 eV step size and 40 ms dwell time. A Bi2Te2Se crystal was cleaved and exposed to air for 30 min before introduction into the vacuum chamber. SPEM images shown in Figures 3a and b emphasize morphology: Bi 4f (Figure 3a) and O 1s (Figure 3b) XPS maps show a 20 x 20 µm region comprised of three large step edges and four flat terraces. The spatial resolution of the instrument is better than 100 nm, so the height of any resolved step edges must be greater than for cleavage at a single quintuple layer (1 nm). Morphological contributions to spectral maps were removed by dividing the photoelectron peak signal by the 23 baseline signal; features that are oriented normal to the detector will have a stronger baseline signal than features oriented at an angle, and taking the peak-to-background ratio removes this effect. This technique was used to obtain a map of the O:Bi ratio without morphology (Figure 3c), calculated from their O 1s and Bi 4f XPS maps; in this way the key observation was made that oxidation at step edges is rapid relative to oxidation at the basal plane: Almost no oxygen is present on the terraces (Figure 3b), but an O 1s peak at 530 eV binding energy is clearly visible on the step edges, which is attributed to oxidized Te. Accordingly, spectra (Figures 3b) were taken at points “1” (intermediate sized terrace), “2” (large terrace), and “3” (exposed step edge) as shown in Figure 3a. Te 3d5/2 spectra indicate two species for each of these measurements: We assign the lower binding energy peak to unoxidized Te and the higher binding energy component to surface oxidized Te (dashed vertical line); this latter component is also strongest at the step edge. It is interesting that the binding energy difference between unoxidized and oxidized Te is not the same as is measured in standard XPS: The extremely high photon flux of the focused X-ray beam relative to that of a standard spectrometer causes a partial and/or full reduction of the surface oxide, which has been 24 observed for other metal oxides studied by this technique. Finally, the Bi 4f7/2 spectra also show a single species at points “1” and “2”, but a small shoulder at higher binding energy was recorded at the step edge, point “3,” which is assigned those Bi atoms bonded to oxidized Te atoms. We have shown that scanning photoelectron microscopy can provide direct evidence for differential oxidation of step edge defects relative to basal planes of Bi2Te2Se. Our observations support theoretical studies that claim stability and inertness of flat surfaces of members of the Bi2(Te,Se)3 13,18 family. They may also explain the otherwise rapid report4,19 ed oxidation of Bi2Te2Se, Bi2Te3 and Bi2Se3: Step edge density will have a profound effect on the observed rate of oxidation. To standardize future experimental studies of these materials controlled cleavage processes will be required to expose clean surfaces of comparable planarity; use of a Laue camera to align single crystals prior to cleavage might be required to reduce the incidental structural defects that could otherwise compromise experimental interpretations.

ASSOCIATED CONTENT Supporting Information

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X-ray photoelectron spectroscopy data for Bi2Te2Se, Bi2Te3, and Bi2Se3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

[email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was funded by the Army Research Office on grant # MURI ARO W911NF-12-1-0461. The authors thank Dr. Luca Gregoratti and Dr. Matteo Amati at the ESCA Microscopy Beamline, Elettra-Sincrotrone Trieste for their assistance with SPEM measurements.

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