Experimental Realization of a Quaternary Bi-Chalcogenide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Experimental Realization of a Quaternary Bi-Chalcogenide Topological Insulator with Smaller Effective Mass Pedro Henrique Rezende Gonçalves, Luan Calil, Igor Antoniazzi, Thais Chagas, Angelo Malachias, Edmar Avellar Soares, Vagner E de Carvalho, Douglas Rodrigues Miquita, Rogerio Magalhaes-Paniago, and Wendell S. Silva J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 26, 2019

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The Journal of Physical Chemistry

Experimental Realization of a Quaternary Bi-Chalcogenide Topological Insulator with Smaller Effective Mass

Pedro H. R. Goncalves1, Luan Calil1, Igor Antoniazzi1, Thais Chagas1, Ângelo Malachias1, Edmar A. Soares1, Vagner E. de Carvalho1, Douglas R. Miquita2, Rogério Magalhães-Paniago1,* and Wendell S. Silva3 1Department

of Physics, Federal University of Minas Gerais, Av. Antonio Carlos 6627, CEP 31270-

901, Belo Horizonte, Brazil 2Microscopy

Center, Federal University of Minas Gerais, Av. Antonio Carlos 6627, CEP 31270-

901, Belo Horizonte, Brazil 3Brazilian

Synchrotron Light Laboratory (LNLS), National Center for Research in Energy and

Materials (CNPEM), CEP 13083-100, Campinas, Brazil

*Address

to corresponding authors: [email protected]

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; 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 have used X-Ray Photoelectron Spectroscopy to 1

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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 higher electron group velocity for the electrons in the topological states comparing with Bi2Se3.

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 properties5-7. 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 gap8. They have spin-polarization9 and are topologically protected by timereversal symmetry10-11. They exhibit conduction without dissipation and robustness with respect to defects at their surface10-11. The first generation of topological insulators was composed of CdTe/HgTe/CdTe quantum wells4,12 as well as the bulk material BixSb1-x13. 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 hexagonal in-plane structure14. The bonds between two QLs are weak, of vander-Walls type, whereas the atoms in individual QLs are bond by strong covalence forces14. 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 be produced15 and present the main features of 2


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topological insulators (i.e., a Dirac-cone, defect robustness and spin-polarization). The main binary compounds formed are Bi2Se3, Bi2Te3 and Sb2Te3. Sb2Se3 presents an orthorhombic atomic structure with no surface states and a 50 meV gap8. The dependence of the topological behavior with 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, lead to the presence of holes in the material2223.

Studies on some ternary compounds of the type Bi2Se3-xTex evidenced a strong dependence of

the Dirac cone position on the Se/Te ratio24. 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 structure (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 orthorhombic16,26. Theoretical studies indicated that a small amount of Tellurium would also play a major role to stabilize the hexagonal structure27. Whereas intrinsic vacancies heighten the Fermi level of these materials28-29, 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 atoms28, but such fine tuning of the Fermi level can also be realized by changing the Bi/Sb and Se/Te ratio.

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 3



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ramped up to 1000 K from room temperature at a rate of 5 K/min and remained at the final temperature for 30 hours. After natural cooling back to room temperature in 24 hours, 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 Spectroscopy (EELS) spectra were taken by a Gatan Image Filter (GIF) system, controlled by Digital Micrograph®. The 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 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 (300K) with a pressure better than 6.0 x 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 better than 5.0 x 10-10 mbar. 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 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.

RESULTS

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After the growth process of BiSbSe2.5Te0.5, the confirmation of the formation of a single phase was achieved by carrying out X-ray Diffraction measurements of powdered pieces of the sample. The result is shown in Fig. 1(a) and evidences the formation of a single phase structure. We have compared our Diffraction data to a Bi2Se3-like structure, where 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 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. Fig. 1(b) shows a Transmission Electron Microscopy (TEM) image of a single grain, while fig. 1(c) shows a Selected Area Electron Diffraction (SAED) pattern. The red circle in fig 1(b) defines the area where the pattern was taken. The pattern shows a perfect hexagonal lattice, however, due to stacking effects, the 2nd order reflection is more intense than the 1st order. In multilayers with AB stacking, computational studies have shown that the intensity relation between the 1st and 2nd order peaks depends on the number of stacked layers30. In our experiment the 1st order got obscured by the transmitted spot. One can also see the internal hexagonal arrangement of the more intense points, which are regularly repeated (see Supporting Information). These two tools yielded complementary information about our sample, providing both statistical and local data.

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Fig. 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 fig. 1(b), exhibiting a single crystal pattern.

The uniform distribution of all four chemical elements was observed by Electron Energy Loss Spectroscopy (EELS). Figures 2(a) and 2(b) show TEM and Energy Filtered Transmission Electron Microscopy (EFTEM) images, respectively, of a grain with the same crystallographic feature of the grain in Fig. 1(b). EFTEM images were recorded at the energy border of all four chemical elements: Bismuth, Antimony, Selenium and Tellurium, and one observe that these elements are uniformly distributed in the crystal. The exact atomic composition of 6


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Bi0.9±0.1Sb1.1±0.1Se2.5±0.1Te0.5±0.1 was determined by X-ray Photoelectron Spectroscopy (XPS) (see Fig. 2(c)). 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 Supporting Information). One can see a small difference between XPS and WDS data, more prominently in the Se/Te composition. While WDS shows a statistical result from at least 1μm depth into 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 arise to different effects on the surface. The complete XPS analysis is depicted in Table I and it is shown 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 is shown for Antimony in inset of the fig. 2(c).

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Fig. 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 I).

Element – Peak

Position (eV)

Bi – Bi 4f7/2

157

Position in literature (eV) ± 0.5 eV 159.2

Deviation (eV)

Nominal atomic percentage (%)

+2.2

Atomic percentage (%) ±2% 18.9

Sb – Sb 3d5/2

528

530.8

+2.8

21.6

20.0

Se – Se 3d5/2

54

56.0

+2.0

49.1

50.0

Te – Te 3d5/2

573

576.6

+3.6

10.4

10.0

20.0

Table 1 – XPS results of the BiSbSe2.5Te0.5 sample. 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.

Fig. 3(a) shows the X-ray powder diffraction pattern of our sample. Figs. 3(b) to 3(d) show 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 nm, 3.0375 nm and 3.0476 nm, respectively32-34. 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-of-plane 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. 8


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Fig. 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)

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Bi2Se3, (c) Bi2Te3 and (d) Sb2Te3, showing the differences among their out-of-plane lattice parameter C.

The electronic properties of the sample were studied by Angle-Resolved Photoemission Spectroscopy (ARPES). Figure 4(a) shows the energy dispersion relation of the Bi2Se3 sample. A 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 literature9,28-29. Fig. 4(b) 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 in a 0.15 eV difference if compared to Bi2Se3.

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Fig. 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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DISCUSSION

The doping of Topological Insulators with Antimony was previously studied in Bi225,39

xSbxTe3

and Bi2-xSbxSe322. In both cases it leads to the shift of the Dirac point in the direction of

the Fermi level (p-type doping). In Bi2-xSbxTe339, the increase of Antimony content (from Bi2Te3 to BiSbTe3) leads to the movement of the Dirac point (towards the Fermi level) by 0.09 eV. For values of x above 1.5, the material becomes a conventional conductor. In Bi2-xSbxSe322, 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. 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 band24. 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. 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 of 0.09 eV. Figures 4(c) and 4(d) show energy cuts at 0.15 eV (yellow dotted line) above each Dirac point of the compounds of Figs. 4(a) and 4(b), respectively. Using our ARPES results we have calculated the electron group velocity and effective mass for the topological states of BiSbSe2.5Te0.5, as well as for the measure Bi2Se3 reference sample. The Fermi momentum (kF) obtained 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) x 105 m/s and (0.08 ± 0.02) me, respectively (values for Bi2Se3 are (6.2 ± 0.1) x 105 m/s and (0.16 ± 0.02) me. Figure 5 (a, b) depicts the detailed information about the analysis that leads to these values for BiSbSe2.5Te0.5, in agreement with 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 12


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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 Fig. 4(b) exhibits a different shape, without the wellknown M-state, in accordance with the results from similar systems21. This is possibly due to the atomic disorder of both Bi/Sb and Se/Te pairs (random occupation of their sites).

Fig. 5 – (a) ARPES measurement of the BiSbSe2.5Te0.5 band structure, evidencing the Dirac cone. (b) Momentum Distribution Curves (MDC) 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).

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CONCLUSION

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 statistical and local data of the single phase 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 effective mass and higher electron group velocity.

ACKNOWLEDGMENTS

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

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SUPORTING INFORMATION

A supporting information file is provided for this paper containing 4 figures as described: Fig. S1 - Detailed look on the Selected Area Electron Diffraction image, indicating the hexagonal lattice character of our sample. Fig. S2 - Complete set of XPS spectra used for the surface composition analysis. Fig. S3 - Complete set of WDS spectra used for the bulk composition analysis. Fig. S4 - 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.

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(23) Scanlon, D. O.; King, P. D. C.; Singh, R. P.; de la Torre, A.; Walker, S. M.; Balakrishnan, G.; Baumberger, F.; Catlow, C. R. A. Controlling Bulk Conductivity in Topological Insulators: Key Role of Anti-Site Defects. Adv. Mat. 2012, 24, 16. (24) Lei, T.; Jin, K.-H.; Zhang, N.; Zhao, J.-L.; Liu, C.; Li, W.-J.; Wang, J.-O.; Wu, R.; Qian, H.-J.; Liu, F.; Ibrahim, K. Electronic structure evolution of single bilayer Bi(111) film on 3D topological insulator Bi2SexTe3−x surfaces. J. Phys.: Condens. Matter. 2016, 28, 255501. (25) Zhou, B.; Liu, Z. K.; Analytis, J. G.; Igarashi, K.; Mo, S. K.; Lu, D. H.; Moore, R. G.; Fisher, I. R.; Sasagawa, T.; Shen, Z. X.; Hussain, Z.; Chen, Y. L. Controlling the carriers of topological insulators by bulk and surface doping. Semicond. Sci. Technol. 2012, 27, 124002. (26) Li, J.; Wang, B.; Liu, F.; Liu, J.; Jia, M.; Lai, Y.; Li J.; Liu, Y. Structural and Optical Properties of Electrodeposited Bi2–xSbxSe3 Thin Films. ECS Solid State Lett. 2012, 1, 29-31. (27) Xuelai, L.; Zhimei, S.; Zhitang, S.; Feng, R.; Liangcai, W.; Weili, L. Ab initio study of Sb2SexTe3-x (x = 0, 1, 2) phase change materials. Solid State Sci. 2011, 13, 131-134. (28) Hor, Y. S.; Richardella, A.; Roushan, P.; Xia, Y.; Checkelsky, J. G.; Yazdani, A.; Hasan, M. Z.; Ong, N. P.; Cava, R. J. p-type Bi2Se3 for topological insulator and low-temperature thermoelectric applications. Phys. Rev. B 2009, 79, 195208. (29) Dai, J.; West, D.; Wang, X.; Wang, Y.; Kwok, D.; Cheong, S.-W.; Zhang, S. B.; Wu, W. Toward the Intrinsic Limit of the Topological Insulator Bi2Se3. Phys. Rev. Lett. 2016, 117, 106401. (30) Horiuchi, S.; Gotou, T.; Fujiwara, M.; Sotoaka, R.; Hirata, M.; Kimoto, K.; Asaka, T.; Yokosawa, T.; Matsui, Y.; Watanabe, K.; Sekita, M. Carbon Nanofilm with a New Structure and Property. Jpn. J. Appl. Phys. 2003, 42, 1073-1076. (31) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy. Physical Electronics Division, Perkin-Elmer Corp. 1995.

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(32) CIF from ICSD, database code: 165226 – Pérez Vicente, C.; Tirado, J. L.; Adouby, K.; Jumas, J. C.; Abba Touré, A.; Kra, G. X-ray Diffraction and 119Sn Mössbauer Spectroscopy Study of a New Phase in the Bi2Se3-SnSe System: SnBi4Se7. Inorg. Chem. 1999, 38, 2131-2135. (33) CIF from ICSD, database code: 193332 – Mansour, A. N.; Wong-Ng, W.; Huang, Q.; Tang, W.; Thompson, A.; Sharp, J. Structural characterization of Bi2Te3 and Sb2Te3 as a function of temperature using neutron powder diffraction and extended X-ray absorption fine structure techniques. J. Appl. Phys. 2014, 116, 083513. (34) CIF from ICSD, database code: 192780 – Kokh, K. A.; Atuchin, V. V.; Gavrilova, T. A.; Kuratieva, N. V.; Pervukhina, N. V.; Surovtsev, N. V. Microstructural and vibrational properties of PVT grown Sb2Te3 crystals. Solid State Commun. 2014, 177, 16-19. (35) Ritter, J. J.; Maruthamuthu, P. Synthesis of Fine-Powder Polycrystalline Bi-Se-Te, Bi-Sb-Te, and Bi-Sb-Se-Te Alloys. Inorg. Chem. 1997, 36, 260-263. (36) Lostak, P.; Novotny, R.; Benes, L.; Civis, S. Preparation and some physical properties of Sb2Te3-xSex single crystals. J. Cryst. Growth 1989, 94, 656-662. (37) Zhang, C.; Peng, Z.; Li, Z.; Yu, L.; Khorc, K. A.; Xiong, Q. Controlled growth of bismuth antimony telluride BixSb2-xTe3 nanoplatelets and their bulk thermoelectric nanocomposites. Nano Energy 2015, 15, 688-696. (38) Hong, M.; Chen, Z. G.; Yang, L.; Zou, J. BixSb2-xTe3 nanoplates with enhanced thermoelectric performance due to sufficiently decoupled electronic transport properties and strong wide-frequency phonon scatterings. Nano Energy 2016, 20, 144-155. (39) Kong, D.; Chen, Y.; Cha, J. J.; Zhang, Q.; Analytis, J. G.; Lai, K.; Liu, Z.; Hong, S. S.; Koski, K. J.; Mo, S.-K.; Hussain, Z.; Fisher, I. R.; Shen, Z.-X.; Cui, Y. Ambipolar field effect in the ternary topological insulator (BixSb1–x)2Te3 by composition tuning. Nat. Nanotechnol. 2011, 6, 705709.

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(40) Yin, J.; NS Krishnamoorthy, H.; Adamo, G.; Dubrovkin, A. M.; Chong, Y.; Zheludev, N. I.; Soci, C. Plasmonics of topological insulators at optical frequencies. NPG Asia Mater. 2017, 9, 425.

TOC Figure

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Fig. 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 fig.1(b), exhibiting a single crystal pattern. 179x150mm (300 x 300 DPI)


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Fig. 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 I). 181x111mm (300 x 300 DPI)



Fig. 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-of-plane lattice parameter C.


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Fig. 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). 515x559mm (96 x 96 DPI)



Fig. 5 - (a) ARPES measurement of the BiSbSe2.5Te0.5 band structure, evidencing the Dirac cone. (b) Momentum Distribution Curves (MDC) 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). 170x106mm (300 x 300 DPI)


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Experimental Realization of a Quaternary Bi-Chalcogenide

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