Se 3d Research Articles

Investigation on Surface Properties of Mn-Doped CdSe Quantum Dots Studied by X-ray Photoelectron Spectroscopy

In this work, we report on the effects of incorporating manganese (Mn) dopant into different sizes of cadmium selenide (CdSe) quantum dots (QDs), which improves the electronic and optical properties of the QDs for multiple applications such as light-emitting diodes, lasers, and biological labels. Furthermore, the greener inverse Micelle method was implemented using organic ligand, which is oleic acid. This binding of the surface enhanced the QDs’ surface trap passivation of Mn-doped CdSe, which then increased the quantity of the output. In addition, the inverse Micelle technique was used successfully to dope Mn into CdSe QDs without the risk of Mn dopants being self-purified as experienced by wurtzite CdSe QDs. Also, we report the X-ray photoelectron spectroscopy (XPS) results and analysis of zinc blended manganese-doped cadmium selenide quantum dots (Mn-doped CdSe QDs), which were synthesized with physical sizes that varied from 3 to 14 nm using the inverse Micelle method. The XPS scans traced the existence of the Se 3d and Cd 3d band of CdSe crystals with a 54.1 and 404.5 eV binding energy. The traced 640.7 eV XPS peak is proof that Mn was integrated into the lattice of CdSe QDs. The binding energy of the QDs was related to the increase in the size of the QDs.

Metal organic framework UiO-66 and activated carbon composite sorbent for the concurrent adsorption of cationic and anionic metals

A composite sorbent for the simultaneous removal of both Hg2+ and SeO32− from aqueous media was produced from the solvothermal synthesis of a zirconium metal organic framework, UiO-66, in the presence of activated carbon. The composite sorbent has a large surface area of 1051 m2 g−1 with crystalized porous structures and has strong thermal stability up to 600 °C. The contaminant uptake of the sorbent follows a Langmuir adsorption isotherm with maximum sorption capacity of 205 mg g−1 and 168 mg g−1 for Hg2+ and SeO32−, respectively. Scanning electron microscopy-energy dispersive spectroscopy results show that the Se regions overlap exclusively with Zr-rich regions suggesting that SeO32− adsorption depends entirely on the exposed UiO-66 surface. In addition, X-ray photoelectron spectroscopy spectra of Se 3d and Hg 4f showed the association of SeO32− and Hg2+ on the UiO-66 and carbon surfaces, respectively. The sorbent could facilitate the development of a single process for the simultaneous removal of cationic Hg and anionic Se as well as other similar ionic metals with opposite charges from aqueous media.

Reversible Modification of Nitrogen-Doped Graphene Based on Se-N Dynamic Covalent Bonds for Field-Effect Transistors.

Temperature-dependent modification is an effective way to reversibly tailor graphene's electronic properties. We present the reversible modification of a uniform monolayer nitrogen-doped graphene (NG) film by the formation and cleavage of temperature-dependent Se-N dynamic covalent bonds. The increasing binding energy in X-ray photoelectron spectroscopy (XPS) indicates that phenylselenyl bromine (PhSeBr) bonds with pyridinic N and pyrrolic N rather than graphitic N by accepting the lone pair of electrons. The temperature dependence of Raman spectra (the increasing D band and the shifts of the 2D band) and XPS spectra (Se 3d and N 1s) indicates that the Se-N dynamic covalent bond is gradually cleaved by treatment at increasing temperatures and is also recovered by the reversible modification. Field-effect transistors (FETs) based on Se-NG exhibit a temperature-dependent change from n-type to p-type conduction and tunable electron and hole mobilities owing to the reversible formation or cleavage of Se-N dynamic covalent bonds. This result opens up opportunities for reversibly controlling electrical properties of FETs by optimizing dynamic covalent bonds.

In vacuo XPS investigation of Cu(In,Ga)Se2 surface after RbF post-deposition treatment

Latest record efficiencies of Cu(In,Ga)Se2 (CIGSe) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF-PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X-ray photoelectron spectroscopy (XPS) study on CIGSe thin films as-deposited by a three-stage co-evaporation process and after the consecutive RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu- and Ga-content at the surface of RbF-treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 eV binding energy, which appears after the RbF-PDT is similar to the one detected on a co-evaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb-In-Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF-PDT. This proves the ion-exchange mechanism in the absence of moisture and air/oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1 s is observed after etching the vacuum transferred RbF-CIGSe sample, indicating that Rb and/or F are not as much a driving force for Na as oxygen usually is.

Reversible structural changes of in situ prepared As40Se60 nanolayers studied by XPS spectroscopy

As40Se60 nanolayers, as-deposited, annealed and in situ illuminated by green (532 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy. Changes in composition and local atomic coordination occurring in the irradiated region of As40Se60 films were monitored by analysis of As 3d and Se 3d core levels. It was found that the thermal treatment causes a decrease of the concentration of homopolar (As–As and Se–Se) bonds. On the other hand, an increasing concentration of both As-rich and Se-rich structural units (s.u.) with homopolar As–As and Se–Se bonds was observed under in situ green laser illumination of As40Se60 nanolayers. This process appeared to be reversible for a few sequences of annealing and illuminating of the sample. After a few cycles storing at ambient conditions the As40Se60 film composition was gradually changing, i.e. the aging effect was detected due to a drastic loss of As under ambient conditions. The surface local structure of the As40Se60 nanolayers and their photoinduced transformation are discussed in detail.

2D tungsten diselenide analyzed by XPS

Tungsten diselenide (WSe2) was analyzed using x-ray photoelectron spectroscopy. A freshly exfoliated, oxygen-free flake was analyzed. Spectral regions for O 1s, W 4d, W 4f, W 5p, Se 3d and an extended region encompassing Se 3p, Se 3s, W 4d, Se LMM, and C 1s were acquired along with the valence band. Bulk quantitative analyses by x-ray fluorescence and x-ray diffraction indicated the material was stoichiometric and phase pure, respectively.

Artificial Intelligence Based Optimization for the Se(IV) Removal from Aqueous Solution by Reduced Graphene Oxide-Supported Nanoscale Zero-Valent Iron Composites.

Highly promising artificial intelligence tools, including neural network (ANN), genetic algorithm (GA) and particle swarm optimization (PSO), were applied in the present study to develop an approach for the evaluation of Se(IV) removal from aqueous solutions by reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO) composites. Both GA and PSO were used to optimize the parameters of ANN. The effect of operational parameters (i.e., initial pH, temperature, contact time and initial Se(IV) concentration) on the removal efficiency was examined using response surface methodology (RSM), which was also utilized to obtain a dataset for the ANN training. The ANN-GA model results (with a prediction error of 2.88%) showed a better agreement with the experimental data than the ANN-PSO model results (with a prediction error of 4.63%) and the RSM model results (with a prediction error of 5.56%), thus the ANN-GA model was an ideal choice for modeling and optimizing the Se(IV) removal by the nZVI/rGO composites due to its low prediction error. The analysis of the experimental data illustrates that the removal process of Se(IV) obeyed the Langmuir isotherm and the pseudo-second-order kinetic model. Furthermore, the Se 3d and 3p peaks found in XPS spectra for the nZVI/rGO composites after removing treatment illustrates that the removal of Se(IV) was mainly through the adsorption and reduction mechanisms.

Se-C Cleavage of Hexane Selenol at Steps on Au(111).

Selenols are considered as an alternative to thiols in self-assembled monolayers, but the Se-C bond is one limiting factor for their usefulness. In this study, we address the stability of the Se-C bond by a combined experimental and theoretical investigation of gas-phase-deposited hexane selenol (CH3(CH2)5SeH) on Au(111) using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory (DFT). Experimentally, we find that initial adsorption leaves atomic Se on the surface without any carbon left on the surface, whereas further adsorption generates a saturated selenolate layer. The Se 3d component from atomic Se appears at 0.85 eV lower binding energy than the selenolate-related component. DFT calculations show that the most stable structure of selenols on Au(111) is in the form of RSe-Au-SeR complexes adsorbed on the unreconstructed Au(111) surface. This is similar to thiols on Au(111). Calculated Se 3d core-level shifts between elemental Se and selenolate in this structure nicely reproduce the experimentally recorded shifts. Dissociation of RSeH and subsequent formation of RH are found to proceed with high barriers on defect-free Au(111) terraces, with the highest barrier for scissoring R-Se. However, at steps, these barriers are considerably lower, allowing for Se-C bond breaking and hexane desorption, leaving elemental Se at the surface. Hexane is formed by replacing the Se-C bond with a H-C bond by using the hydrogen liberated from the selenol to selenolate transformation.

Analysis of MRI by fractals for prediction of sensory attributes: A case study in loin

This study investigates the use of fractal algorithms to analyse MRI of meat products, specifically loin, in order to determine sensory parameters of loin. For that, the capability of different fractal algorithms was evaluated (Classical Fractal Algorithm, CFA; Fractal Texture Algorithm, FTA and One Point Fractal Texture Algorithm, OPFTA). Moreover, the influence of the acquisition sequence of MRI (Gradient echo, GE; Spin Echo, SE and Turbo 3D, T3D) and the predictive technique of data mining (Isotonic regression, IR and Multiple Linear regression, MLR) on the accuracy of the prediction was analysed. Results on this study firstly demonstrate the capability of fractal algorithms to analyse MRI from meat product. Different combinations of the analysed techniques can be applied for predicting most sensory attributes of loins adequately (R > 0.5). However, the combination of SE, OPFTA and MLR offered the most appropriate results. Thus, it could be proposed as an alternative to the traditional food technology methods.

XPS and Ag L3-edge XANES characterization of silver and silver–gold sulfoselenides

Gold and silver sulfoselenides are of interest as materials with high ionic conductivity and promising magnetoresistive, thermoelectric, optical, and other physico-chemical properties, which are strongly dependent on composition and structure. Here, we applied X-ray photoelectron spectroscopy and Ag L3 X-ray absorption near-edge structure (XANES) to study the electronic structures of low-temperature compounds and solid solutions Ag2SxSe1–x (0 < x < 1), AgAuS, and Ag3AuSxSe2–x (x = 0, 1, 2). Upon substitution of Se with S, a steady increase in the positive charge at Ag(I) sites and only minor changes in the local charge at chalcogen atoms were found from the photoelectron Ag 3d, S 2p, Se 3d, and Ag M4,5VV Auger spectra. The intensity of the Ag L3-edge peak, which is known to correlate with hole counts in the Ag 4d shell having a formal d10 configuration, was enhanced by 20–25% from Ag2Se to Ag2S and from Ag3AuSe2 to Ag3AuS2. The effect of gold is more pronounced, and the number of Ag d holes and the negative charge of S and Se notably decreased for Au-containing compounds; in particular, the Ag L3-edge peak is about 35% lower for AgAuS relative to Ag2S. At the same time, the Au 4f binding energy and, therefore, charge at Au(I) sites increase with increasing S content due to the transfer of electron density from Au to Ag atoms. It was concluded that the effects mainly originate from shortening of the metal–chalcogen and especially the AuAg interatomic distances in substances having similar coordination geometry.

Self-Supported Ternary Ni-S-Se Nanorod Arrays as Highly Active Electrocatalyst for Hydrogen Generation in Both Acidic and Basic Media: Experimental Investigation and DFT Calculation.

In this study, a novel three-dimensional self-supported ternary NiS-Ni9S8-NiSe nanorod (NR) array cathode has been successfully in situ constructed by a two-step hydrothermal route. When applied to hydrogen evolution, the synthesized NiS-Ni9S8-NiSe-NR electrode demonstrates optimized electrocatalytic activity and long-term durability, only requiring overpotentials as low as 120 and 112 mV to drive 10 mA cm-2 for hydrogen evolution reaction in 0.5 M H2SO4 and 1.0 M KOH, respectively. Density functional theory calculation reveals that after Se doping Se 3d orbitals are bonded to Ni 3d orbitals and S p orbitals near Fermi level, attesting a significant electron transfer between nickel and selenium atoms. The success of enhancing the electrocatalytic performance via introducing the Se dopant holds great promise for the potential optimization of other transition-metal compounds in highly efficient electrochemical water splitting for large-scale hydrogen production.

Prediction of pork quality parameters by applying fractals and data mining on MRI

This work firstly investigates the use of MRI, fractal algorithms and data mining techniques to determine pork quality parameters non-destructively. The main objective was to evaluate the capability of fractal algorithms (Classical Fractal algorithm, CFA; Fractal Texture Algorithm, FTA and One Point Fractal Texture Algorithm, OPFTA) to analyse MRI in order to predict quality parameters of loin. In addition, the effect of the sequence acquisition of MRI (Gradient echo, GE; Spin echo, SE and Turbo 3D, T3D) and the predictive technique of data mining (Isotonic regression, IR and Multiple linear regression, MLR) were analysed. Both fractal algorithm, FTA and OPFTA are appropriate to analyse MRI of loins. The sequence acquisition, the fractal algorithm and the data mining technique seems to influence on the prediction results. For most physico-chemical parameters, prediction equations with moderate to excellent correlation coefficients were achieved by using the following combinations of acquisition sequences of MRI, fractal algorithms and data mining techniques: SE-FTA-MLR, SE-OPFTA-IR, GE-OPFTA-MLR, SE-OPFTA-MLR, with the last one offering the best prediction results. Thus, SE-OPFTA-MLR could be proposed as an alternative technique to determine physico-chemical traits of fresh and dry-cured loins in a non-destructive way with high accuracy.

Electrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological Insulator

We report the electrical (angular magnetoresistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron) characterization of a bulk Bi2Se3 topological insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950. degrees C) melt and slow cooling (2. degrees C/h) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5 K. Magnetotransport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular and parallel field directions, respectively. We noticed that the MR of Bi2Se3 is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. The Hall coefficient (RH) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (C-p) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (Cp =gamma T + beta T-3) at low temperature with a calculated Debye temperature (theta(D)) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm(-1) corresponding to A(1g)(1), E-g(2) and A(1g)(2), respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f(7/2) and Bi 4f(5/2), the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grown Bi2Se3 crystal are reported here.

(Invited) Synthesis, Kinetics, Characterization and Application of Sodium and Potassium (poly)Selenides for Chalcogenide Semiconductor Doping

Alkali elements are essential for batteries and solar cells. In batteries, alkali ions are typically intercalated in layered 2D structures, such as transition metal sulfides and selenides, in a reversible fashion 1–4. In solar cells based on Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 semiconductors, Na2Sex and K2Sex compounds are sources of extrinsic doping and act as surfactants during or after the growth, ending up as segregations at the grain boundaries, where they are also thought to passivate electronic defects 5–10. Alkali (poly)chalcogenide solutions are also traditionally employed for photoelectrochemical characterization of semiconductors because they offer a wide range of redox states 11,12 and stabilize against photoanodic dissolution 13,14. More recently, they have also been adopted as electrolytes in quantum dot sensitized solar cells 15–17. However, despite their technological relevance, little is known about the single alkali selenide compounds beyond the pioneering works of Zintl and Klemm 18,19 and subsequent structural characterizations 20–24. In this presentation, our recent work on (i) synthesis, (ii) characterization (iii) reactivity to oxygen and (iv) utilization of alkali (poly)selenides is reviewed. The synthesis is performed in liquid ammonia at atmospheric pressure and -55 °C. The progress of the reaction is monitored electrochemically, in order to unveil the kinetics. The decrease of solvated electrons concentration after selenium addition is measured directly by means of cyclic voltammetry at the surface of a Pt microelectrode. The compounds are characterised by X-ray photoelectron, Raman and electronic spectroscopies. Broadening of the Se 3d XPS peaks with the increase of the number of Se atoms in the chain suggests a limited extent of negative charge delocalization among the Se atoms in the solid (poly)selenides. A direct band-gap of 1.83 eV and pseudo-direct band-gap of 2.05 eV are identified for Na2Se and K2Se, respectively. The full width at half maximum of the fluorescence emissions is on the order of 300 meV, and the emission peaks occur at more than 0.5 eV below the respected bandgaps, revealing a pronounced defect-assisted radiative recombination. The extreme sensitivity of the compounds to oxidation is studied and discussed with respect to chalcogenide grain boundary passivation. Additionally, extrinsic doping of epitaxial and polycrystalline CuInSe2 films is investigated by means of a novel gas-phase route 25 that utilizes the synthesised compounds. The resulting films are characterised by photoluminescence spectroscopy, revealing the emergence of a distinct peak at ca. 100 meV higher energy than typical 1 eV peak of Cu-poor CuInSe2, irrespective of the alkali source. Lastly, some of the species likely involved in the alkali gas-phase transport and incorporation in CuInSe2are identified for the first time by Knudsen effusion mass spectrometry.

Distinct effects of Cr bulk doping and surface deposition on the chemical environment and electronic structure of the topological insulator Bi2Se3

In this report, it is shown that Cr doped into the bulk and Cr deposited on the surface of Bi2Se3 films produced by molecular beam epitaxy (MBE) have strikingly different effects on both the electronic structure and chemical environment. Angle resolved photoemission spectroscopy (ARPES) shows that Cr doped into the bulk opens a surface state energy gap which can be seen at room temperature; much higher than the measured ferromagnetic transition temperature of ≈10K. On the other hand, similar ARPES measurements show that the surface states remain gapless down to 15K for films with Cr surface deposition. In addition, core-level photoemission spectroscopy of the Bi 5d, Se 3d, and Cr 3p core levels show distinct differences in the chemical environment for the two methods of Cr introduction. Surface deposition of Cr results in the formation of shoulders on the lower binding energy side for the Bi 5d peaks and two distinct Cr 3p peaks indicative of two Cr sites. These striking differences suggests an interesting possibility that better control of doping at only near surface region may offer a path to quantum anomalous Hall states at higher temperatures than reported in the literature.

Hollow selenium encapsulated into 3D graphene hydrogels for lithium–selenium batteries with high rate performance and cycling stability

Hollow selenium nanospheres encapsulated within 3D graphene hydrogels were prepared and researched as Li–Se battery cathode materials. It was shown that the hollow Se structure and 3D graphene were beneficial to the application Li–Se batteries.

Experimental characterization of the tensile strength of ABS parts manufactured by fused deposition modeling process

Acrylonitrile–butadiene–styrene (ABS) is one of the most widely usedindustrial thermoplastic and it is the most common material used in fused deposition modeling (FDM) technology. While FDM is one of the most used additive manufacturing (AM) technology commonly employed formodeling, prototyping, and manufacturing applications, the major research issues have been to balance ability to produce aesthetically appealinglooking products with functionality. It is therefore critical to know the mechanical properties of these parts, which, is as expected different from their nominal values.A FDM manufactured part has an internal channel referred to as infill. This internal build pattern has different mechanical properties than the same shaped part without the internal channel. This paper aims to define the effect of specimen mesostructure on the monotonic tensile behavior of ABS parts manufactured by FDM. The experiment uses the uPrint SE 3D printer to produce samples from ABS thermoplastic. The samples are tested following a modified form of ASTM D638 standard for plastic. The printed pieces are tested to failure using an Instron testing machine, and a strain gage extensometer that measures the amount of elongation in the pieces until failure. These standardized pieces are compared to know values of traditionally manufactured materials. This test shows the strength loss of materials through the 3D printing process compared to traditional manufacturing methods. The experiment for plastics is designed around two key criteria, orientation of the pieces, and the percent of infill during the printing process. It was found that a value of maximum strength, of 3D printed ABS, was approximately 92% of accepted values for the material, with a certainty of 5%. The study also shows that the bonds between layers are only 79% as strong as an axil loading along the layers.

Direct observation of ring-opening dynamics in strong-field ionized selenophene using femtosecond inner-shell absorption spectroscopy.

Femtosecond extreme ultraviolet transient absorption spectroscopy is used to explore strong-field ionization induced dynamics in selenophene (C4H4Se). The dynamics are monitored in real-time from the viewpoint of the Se atom by recording the temporal evolution of element-specific spectral features near the Se 3d inner-shell absorption edge (∼58 eV). The interpretation of the experimental results is supported by first-principles time-dependent density functional theory calculations. The experiments simultaneously capture the instantaneous population of stable molecular ions, the emergence and decay of excited cation states, and the appearance of atomic fragments. The experiments reveal, in particular, insight into the strong-field induced ring-opening dynamics in the selenophene cation, which are traced by the emergence of non-cyclic molecules as well as the liberation of Se+ ions within an overall time scale of approximately 170 fs. We propose that both products may be associated with dynamics on the same electronic surfaces but with different degrees of vibrational excitation. The time-dependent inner-shell absorption features provide direct evidence for a complex relaxation mechanism that may be approximated by a two-step model, whereby the initially prepared, excited cyclic cation decays within τ1 = 80 ± 30 fs into a transient molecular species, which then gives rise to the emergence of bare Se+ and ring-open cations within an additional τ2 = 80 ± 30 fs. The combined experimental and theoretical results suggest a close relationship between σ* excited cation states and the observed ring-opening reactions. The findings demonstrate that the combination of femtosecond time-resolved core-level spectroscopy with ab initio estimates of spectroscopic signatures provide new insights into complex, ultrafast photochemical reactions such as ring-opening dynamics in organic molecules in real-time and with simultaneous sensitivity for electronic and structural rearrangements.

Migration of 75Se(IV) in crushed Beishan granite: Effects of the iron content

The diffusion of selenite (labeled with 75Se) in compacted Beishan granite (BsG) was investigated using the in-diffusion capillary method at pH values from ∼2.0 to ∼11.0 under oxic and anoxic conditions. The results indicate that the apparent diffusion coefficient (Da) values of selenite in BsG always reached the minimum at approximately pH 5. Unexpectedly, the Da values under oxic conditions are nearly one order of magnitude lower than those under the anoxic conditions. Further characterization reveals the existence of redox-sensitive Fe(II)-containing components, which can be responsible for the great difference in Da values. Fe(2p) X-ray photoelectron spectroscopy (XPS) results show that more Fe(III)-oxyhydroxide coating is formed on the granite’s surface under aerobic conditions than is formed under anaerobic conditions. Correspondingly, Se(3d) spectra indicate that more selenium is sorbed under oxic conditions, and the sorbed amount always reached the maximum at pH values from ∼4 to ∼5. A linear combination fit of X-ray absorption near edge structure (XANES) spectroscopy data revealed that Se(0) was formed under anoxic condition and that selenite preferred to form inner-sphere complexes with Fe(III)-oxyhydroxide. Overall, this study indicates that natural Fe-bearing minerals can greatly attenuate selenite diffusion and the retardation would be enhanced under aerobic conditions.

Surface oxidation of the topological insulator Bi2Se3

An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors contribute new experimental observations about the aging and oxidation process. The authors find that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a series of ∼3.2 Å high islands are observed by atomic force microscopy over approximately 10% of the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ∼120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d and Bi 4d peaks, are clearly present after ∼190 min of ambient exposure. Variable angle spectroscopic ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time dependent changes observed in the surface crystal structure by second harmonic generation. In addition to providing the ability to nondestructively measure oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify the thickness of Bi2Se3 flakes. With these methods, the authors have constructed a consistent, experimentally based model of aging process at the surface of Bi2Se3.