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Cite This: J. Phys. Chem. C 2019, 123, 14398−14403

Experimental Realization of a Quaternary Bi-Chalcogenide Topological Insulator with Smaller Effective Mass Pedro H. R. Goncalves,† Luan Calil,† Igor Antoniazzi,† Thais Chagas,† Â ngelo Malachias,† Edmar A. Soares,† Vagner E. de Carvalho,† Douglas R. Miquita,‡ Rogeŕ io Magalhães-Paniago,*,† and Wendell S. Silva§

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†

Department of Physics and ‡Microscopy Center, Federal University of Minas Gerais, Av. Antonio Carlos 6627, CEP 31270-901 Belo Horizonte, Brazil § National Center for Research in Energy and Materials (CNPEM), Brazilian Synchrotron Light Laboratory (LNLS), CEP 13083-100 Campinas, Brazil S Supporting Information *

ABSTRACT: It is known that Sb2Se3 does not exhibit topological insulator behavior due to its orthorhombic structure. The introduction of a small amount of bismuth and tellurium may change its structure to hexagonal, leading to a stable topological insulator compound. We report here the synthesis and the structural, chemical, and electronic properties of the topological insulator BiSbSe2.5Te0.5. Combining X-ray and electron diffraction measurements, we demonstrate the formation of this stable quaternary hexagonal single crystal. We used X-ray photoelectron spectroscopy to determine quantitatively the exact chemical composition of the sample. The topological insulating behavior is similar to that of other bismuth chalcogenides, as probed by angle-resolved photoemission spectroscopy. A p-type doping, leading to a 0.15 eV shift of the Fermi level was found. This value compensates the intrinsically n-type doping produced by selenium vacancies. We also found a smaller effective mass and a higher electron group velocity for the electrons in the topological states compared with Bi2Se3.

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INTRODUCTION Since the theoretical prediction1,2 and experimental demonstration3,4 of the so-called topological insulators (TIs), such as Bi2Se3 and Bi2Te3, many scientific groups have started to study new materials with potential two- and three-dimensional topological properties.5−7 These materials have the remarkable property of behaving as a common insulator in the bulk, but with the presence of conducting states at the surface. Such conducting states present a linearly dispersed band inside the bulk gap.8 They have spin-polarization9 and are topologically protected by time-reversal symmetry.10,11 They exhibit conduction without dissipation and robustness with respect to defects at their surface.10,11 The first generation of topological insulators was composed of CdTe/HgTe/CdTe quantum wells4,12 as well as the bulk material BixSb1−x.13 The second generation of TIs is formed by bismuth chalcogenides. Their atomic structure is based on stacks of five atomic sheets, called quintuple layers (QLs), with a hexagonal in-plane structure.14 The bonds between two QLs are weak, van der Walls type, whereas the atoms in individual QLs are bond by strong covalence forces.14 This takes place due to the intercalation of atoms from the 15th and 16th columns of the periodic table. These Bi-chalcogenides are considerably easy to produce15 and present the main features of topological insulators (i.e., a Dirac-cone, defect robustness, and spin-polarization). The main binary compounds formed © 2019 American Chemical Society

are Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 . Sb 2 Se 3 presents an orthorhombic atomic structure with no surface states and a 50 meV gap.8 The dependence of the topological behavior on the structure of similar samples15−18 has become a central issue for the experimental realization of compounds of this material class. Considerable efforts have been made to study the topological behavior of samples containing three19,20 or four21 of these atoms. It is known that the substitution of bismuth and selenium by antimony and tellurium, respectively, leads to the presence of holes in the material.22,23 Studies on some ternary compounds of the type Bi2Se3−xTex evidenced a strong dependence of the Dirac cone position on the Se/Te ratio.24 A related dependence was found on Bi2−xSbxTe325 and Bi2−xSbxSe322 system compounds due to the Bi/Sb ratio. It has been reported that BiSbSe3 exhibits a coexistence of the two main types of structures (hexagonal and orthorhombic),16,26 which leads to the loss of the topological insulator behavior. In particular, if one increases the antimony content above 0.5−0.7 in Bi2−xSbxSe3, the resulting structure becomes orthorhombic.16,26 Theoretical studies indicated that a small amount of tellurium would also play a major role to stabilize the Received: February 25, 2019 Revised: May 18, 2019 Published: May 22, 2019 14398

DOI: 10.1021/acs.jpcc.9b01811 J. Phys. Chem. C 2019, 123, 14398−14403

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The Journal of Physical Chemistry C hexagonal structure.27 Whereas intrinsic vacancies heighten the Fermi level of these materials,28,29 an ideal topological insulator must have the Fermi level at the Dirac cone. The standard approach to control the position of the Fermi level is to p-dope these compounds, using for example calcium atoms,28 but such fine tuning of the Fermi level can also be realized by changing the Bi/Sb and Se/Te ratio.

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EXPERIMENTAL METHODS In our experiment, nominal BiSbSe2.5Te0.5 crystals were prepared in an evacuated (P < 10−5 mbar) and sealed quartz tube by mixing Bi (99.999%), Sb (99.999%), Te (99.999%), and Se (99.999%) powders at the given molar proportions. The tube was placed in a furnace that was ramped up to 1000 K from room temperature at a rate of 5 K/min and remained at the final temperature for 30 h. After natural cooling back to room temperature in 24 h, the single crystals of BiSbSe2.5Te0.5 were produced. We confirm the formation of a single phase by carrying out X-ray diffraction measurements of powdered pieces using an Empyrean Panalytical X-ray diffractometer with a Cu-tube source. The structural characterization was made by transmission electron microscopy (TEM). The TEM experiments were taken in a FEI-Termofisher Tecnai G2-20, LaB6 filament, operating at 200 kV. Electron energy loss spectra (EELS) were taken by a Gatan Image Filter (GIF) system, controlled by Digital Micrograph. The energy filtered transmission electron microscopy (EFTEM) results came from a Spectrum Imaging (SI) tool, a technique that generates a spatially resolved distribution of EELS data. The chemical analysis was made by X-ray photoelectron spectroscopy. The X-ray photoelectron spectroscopy (XPS) experiment was performed on an UHV system equipped with a SPECS PHOIBOS 100 electron analyzer and an Al X-ray source. The experiment was carried out at room temperature (300 K) with a pressure higher than 6.0 × 10−10. The energy resolution was approximately 0.8 eV. The electronic properties of the sample were studied by angle-resolved photoemission spectroscopy (ARPES). The base pressure during the experiment was maintained at higher than 5.0 × 10−10 mbar. The ARPES measurements were carried out in an UHV system of the PGM Beamline at the Brazilian Synchrotron Light Laboratory (LNLS) at 300 K using a SPECS PHOIBOS 150 electron analyzer, an incident beam with a fixed photon energy of 103.5 eV with p polarization for both Bi2Se3 and BiSbSe2.5Te0.5 (the first for comparison). The energy and angular resolutions were 0.10 eV and ∼0.2°, respectively.

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Figure 1. (a) X-ray powder diffraction pattern of BiSbSe2.5Te0.5 sample, showing the Miller indexes for all peaks. (b) Transmission electron microscopy image of one individual grain of BiSbSe2.5Te0.5, and (c) selected area electron diffraction pattern of the region highlighted in red in (b), exhibiting a single crystal pattern.

of Bi2Se3 and our sample was observed. Since the position of the peaks only depends on the lattice parameter of the material, we are only sure of the presence of a single phase but not of its chemical composition. Additional experimental techniques were employed to quantify the chemical composition. Figure 1b shows a transmission electron microscopy (TEM) image of a single grain, while Figure 1c shows a selected area electron diffraction pattern. The red circle in Figure 1b defines the area where the pattern was taken. The pattern shows a perfect hexagonal lattice; however, due to stacking effects, the second-order reflection is more intense than the first-order reflection. In multilayers with AB stacking, computational studies have shown that the intensity relation between the first- and second-order peaks depends on the number of stacked layers.30 In our experiment, the firstorder peak got obscured by the transmitted spot. One can also see the internal hexagonal arrangement of the more intense points, which are regularly repeated (see the Supporting Information). These two tools yielded complementary information about our sample, providing both statistical and local data. The uniform distribution of all four chemical elements was observed by electron energy loss spectroscopy (EELS). Figure 2a,b shows the TEM and energy filtered transmission electron microscopy (EFTEM) images, respectively, of a grain with the same crystallographic feature of the grain in Figure 1b. The EFTEM images were recorded at the energy border of all of the four chemical elements: bismuth, antimony, selenium, and tellurium; one observes that these elements are uniformly distributed in the crystal. The exact atomic composition of Bi0.9±0.1Sb1.1±0.1Se2.5±0.1Te0.5±0.1 was determined by X-ray photoelectron spectroscopy (XPS) (see Figure 2c). An overall (Bi, Sb)/(Se, Te) ratio of approximately 2:3 was found. For comparison, an analysis by wavelength dispersive spectroscopy (WDS) was made, and it shows an atomic composition of Bi1.1±0.1Sb0.9±0.1Se2.1±0.1Te0.9±0.1 (see the Supporting Information). One can see a small difference between the XPS and WDS data, more prominently in the Se/Te composition. While WDS shows a statistical result from at least 1 μm depth into

RESULTS

After the growth process of BiSbSe2.5Te0.5, the confirmation of the formation of a single phase was achieved by carrying out Xray diffraction measurements of powdered pieces of the sample. The result is shown in Figure 1a and evidences the formation of a single-phase structure. We compared our diffraction data to a Bi2Se3-like structure, where the values of the atomic scattering factor of bismuth and selenium were replaced by the average of Bi/Sb at the bismuth position and Se/Te in the selenium position (1:1 and 2.5:0.5 ratios, respectively). We found values of (2.859 ± 0.001) nm for the lattice parameter C and (0.43 ± 0.01) nm for the lattice parameter A. A small difference between the lattice parameter 14399


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The Journal of Physical Chemistry C

Figure 2. (a) TEM image of a grain of BiSbSe2.5Te0.5 used for chemical analysis. (b) EFTEM image of the same grain close to the absorption edges of bismuth (2850 eV, red), antimony (528 eV, blue), selenium (1476 eV, orange), and tellurium (572 eV, green). (c) X-ray photoelectron spectrum of the sample showing the peaks 3d of tellurium, 3d of antimony, 4f of bismuth, and 3d of selenium. An inset shows the fit of the 3d peaks of antimony (see Table 1).

the sample, XPS is more surface sensitive. Lohani et al (ref 21) have proposed a structure for BiSbTeSe2 with a unit cell with QLs of the type Se−Bi−Te−Sb−Se and calculated the band structure of this material. While the bulk composition of our sample shows an approximately BiSbTeSe2 structure, the surface composition is more selenium rich, which can result in different effects on the surface. The complete XPS analysis is depicted in Table 1 and given in the Supporting Information. The difference between the XPS peak positions and the expected values taken from the literature31 also indicates that there is no segregation of all four elements. Typical Gaussian fit used to determine the XPS peak area for antimony is shown in the inset of Figure 2c. Figure 3a shows the X-ray powder diffraction pattern of our sample. Figure 3b−d shows the calculated reference diffraction patterns of Bi2Se3, Bi2Te3, and Sb2Te3, respectively. The calculated out-of-plane lattice parameters C of the binary compounds are 2.8616, 3.0375, and 3.0476 nm, respectively.32−34 In comparison to our sample lattice parameter C of 2.859(1) nm, it differs by only by 0.1% from Bi2Se3 and about 6% for the other two compounds. The dependence of the Se/Te ratio on the lattice parameter of Bi2TexSe3−x35 and Sb2TexSe3−x27,36 are present in the literature. One can see that the replacement of selenium by tellurium increases the out-ofplane lattice parameter C. The substitution of bismuth by antimony in Bi2−xSbxTe337,38 and Bi2−xSbxSe316 also affects the lattice parameter C, decreasing its value. Both results are in accordance with our findings in BiSbSe2.5Te0.5. The electronic properties of the sample were studied by angle-resolved photoemission spectroscopy (ARPES). Figure 4a shows the energy dispersion relation of the Bi2Se3 sample. A

Figure 3. (a) X-ray powder diffraction pattern of the BiSbSe2.5Te0.5 sample produced for this work. Theoretical calculations (referenced by crystallographic data32−34) of the X-ray pattern for (b) Bi2Se3, (c) Bi2Te3, and (d) Sb2Te3, showing the differences among their out-ofplane lattice parameter C.

linear dispersion of the topological surface state, as well as the valence band below the Dirac point was observed. We found a 0.37 eV shift of the Fermi level due to the intrinsic n-type doping caused by selenium vacancies, as previously discussed in the literature.9,28,29 Figure 4b shows the same energy dispersion relation for the BiSbSe2.5Te0.5 sample. One observes a 0.22 eV down shift of the Dirac point, consisting of a 0.15 eV difference when compared to Bi2Se3.

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DISCUSSION The doping of topological insulators with antimony was previously studied in Bi2−xSbxTe325,39 and Bi2−xSbxSe3.2222 In both cases, it leads to the shift of the Dirac point in the direction of the Fermi level (p-type doping). In Bi2−xSbxTe3,39 the increase of antimony content (from Bi2Te3 to BiSbTe3) leads to the movement of the Dirac point (toward the Fermi level) by 0.09 eV. For values of x above 1.5, the material becomes a conventional conductor. In Bi2−xSbxSe3,22 the doping effect of antimony is stronger. A doping of x = 0.3 leads to a shift of the Dirac cone of ∼0.1 eV also approaching the Fermi level.

Table 1. XPS Results of the BiSbSe2.5Te0.5 Samplea element−peak

position (eV)

position in literature (eV) ± 0.5 eV

deviation (eV)

atomic percentage (%) ± 2%

nominal atomic percentage (%)

Bi−Bi 4f7/2 Sb−Sb 3d5/2 Se−Se 3d5/2 Te−Te 3d5/2

157 528 54 573

159.2 530.8 56.0 576.6

+2.2 +2.8 +2.0 +3.6

18.9 21.6 49.1 10.4

20.0 20.0 50.0 10.0

a

The deviation from the literature values is due to the oxidation state of each element, indicating the absence of segregation. The data used for this analysis can be seen in the Supporting Information. 14400


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Figure 5. (a) ARPES measurement of the BiSbSe2.5Te0.5 band structure, evidencing the Dirac cone. (b) Momentum distribution curves obtained from (a), showing the method to extract the Fermi momentum (kF) and the Dirac point energy (ED). Measuring kF and ED, we were able to calculate the slope of the cone, and use this information to obtain effective mass (m*) and group velocity (vg).

Figure 4. Angle-resolved photoemission spectroscopy of (a) Bi2Se3 and (b) BiSbSe2.5Te0.5. The white lines represent the Fermi level of each sample. The blue lines represent the Dirac point level. Energy cuts at approximately 0.15 eV above the Dirac point of (c) Bi2Se3 and (d) BiSbSe2.5Te0.5. These energies are marked with yellow lines in panels (a) and (b).

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, we have studied the quaternary topological insulator BiSbSe2.5Te0.5. We have carried out X-ray powder diffraction and transmission electron microscopy measurements to provide the statistical and local data of the singlephase character of our sample. We used electron energy loss spectroscopy to observe the uniform distribution of all four chemical elements. The exact atomic composition of Bi0.9±0.1Sb1.1±0.1Se2.5±0.1Te0.5±0.1 was determined by X-ray photoelectron spectroscopy (XPS), which also proved the absence of segregation for each of the four elements. We have also compared the reference diffraction patterns of Bi2Se3, Bi2Te3 and Sb2Te3 to BiSbSe2.5Te0.5, evidencing a dependence of the lattice parameter C on the Bi/Sb and Se/Te ratios. Angle-resolved photoemission spectroscopy was used to show the electronic properties of our sample, comparing them with those of a Bi2Se3 reference sample. For the retrieved composition, a suitable content of antimony and tellurium atoms replacing bismuth and selenium atoms was established, keeping the presence of a topological state and providing a positive doping. Our specific amount of antimony and tellurium keeps the hexagonal symmetry of the system and leads to smaller values of the effective mass and a higher electron group velocity.

The introduction of tellurium leads to a similar p-doping effect. Tellurium in Bi2Se3 (i.e., varying x from 0 to 3 in Bi2Se3−xTex) leads to a nesting of the Dirac cone inside the valence band.24 For values up to x = 2, the Dirac cone is not buried inside the valence band. For x = 2.46 (Bi2Se2.46Te0.54), the Dirac point energy is at −0.37 eV. The ARPES measurements in BiSbSe2Te21 have shown that the Dirac point is −0.31 eV below the Fermi level. The difference from our sample, BiSbSe2.5Te0.5, is 0.09 eV. Figure 4c,d shows energy cuts at 0.15 eV (yellow dotted line) above each Dirac point of the compounds of Figure 4a,b, respectively. Using our ARPES results, we calculated the electron group velocity and the effective mass for the topological states of BiSbSe2.5Te0.5, as well as for the measure Bi2Se3 reference sample. The Fermi momentum (kF) obtained for our compound was 0.05 ± 0.01 Å−1 (kF = 0.09 ± 0.01 Å−1 for Bi2Se3). Group velocity and effective mass were calculated for BiSbSe2.5Te0.5 as (6.6 ± 0.1) × 105 m/s and 0.08 ± 0.02 me, respectively (values for Bi2Se3 are (6.2 ± 0.1) × 105 m/s and 0.16 ± 0.02 me, respectively). Figure 5a,b depicts the detailed information about the analysis that leads to these values for BiSbSe2.5Te0.5, in agreement with the experimental results9 and theoretical calculations for a similar stoichiometry (BiSbSe2Te).40 The same ARPES experiment was carried out in BiSbSe2.5Te0.5 using photon energy of 21.2 eV from a He lamp, leading to the same values of kF, group velocity, and effective mass. One also observes that the valence band of BiSbSe2.5Te0.5 in Figure 4b exhibits a different shape, without the well-known M-state, in accordance with the results from similar systems.21 This is possibly due to the atomic disorder of both Bi/Sb and Se/Te pairs (random occupation of their sites).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01811. Selected area electron diffraction image, indicating the hexagonal lattice character of our sample (Figure S1); complete set of XPS spectra used for the surface composition analysis (Figure S2); complete set of WDS spectra used for the bulk composition analysis (Figure S3); ARPES measurement of BiSbSe2.5Te0.5 using energy of 21.2 eV from a He lamp, evidencing the surface nature of the linear states below the Fermi level and the same values of kF, ED, vg, and m* obtained from the synchrotron ARPES measurements with energy of 103.5 eV (Figure S4) (PDF) 14401


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AUTHOR INFORMATION

Corresponding Author

*E-mail: rogerio@fisica.ufmg.br. ORCID

Rogério Magalhães-Paniago: 0000-0002-5203-0944 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the LNLS for allocating synchrotron beamtime and the PGM beamline staff for support during the experiment. Financial support of the agencies FAPEMIG, CAPES, and CNPq is gratefully acknowledged. W.S.S. thanks Professor Yves Petroff for his critical reading.

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