Catalyst-free Growth of Single Crystalline Bi2Se3 Nanostructures for

Article pubs.acs.org/crystal

Catalyst-free Growth of Single Crystalline Bi2Se3 Nanostructures for Quantum Transport Studies Christian Nowka,*,† Louis Veyrat,† Sandeep Gorantla,†,‡ Udo Steiner,⊥ Barbara Eichler,† Oliver G. Schmidt,† Hannes Funke,† Joseph Dufouleur,† Bernd Büchner,†,§ Romain Giraud,†,∥ and Silke Hampel† †

Leibniz Institute for Solid State and Materials Research (IFW), Helmholtzstraße 20, D-01069 Dresden, Germany Department of Physics/Center for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway ⊥ Hochschule für Technik und Wirtschaft (HTW), Friedrich-List-Platz 1, D-01069 Dresden, Germany § Department of Physics, TU Dresden, D-01062, Dresden, Germany ∥ CNRS-Laboratoire de Photonique et de Nanostructures, Route de Nozay, F-91460, Marcoussis, France ‡

ABSTRACT: In this work we report on the growth of single crystalline Bi2Se3 nanostructures (nanoribbons, nanoflakes, and nanowires) by catalyst-free decomposition sublimation in sealed silica ampules. The nanostructures directly grow on Si/ SiO2 substrates by a vapor−solid growth mechanism and show high degree of crystallinity with dimensions of >10 μm in length and simultaneously 10−5 atm for a transport relevant species.33 However, a lower partial pressure in the range 10−5 and 10−6 atm is better to realize nanostructure growth. According to Figure 2a only the gas species BiSe and Se2 show partial pressures with the needed transport efficiency (>10−6 atm) only at a temperature of ∼530 and 450 °C, respectively. Hence, BiSe and Se2 are mainly responsible for the vapor transport and thus for the VS growth. The transport efficiency w eq 1 describes the contribution of a compound to the chemical vapor transport and is helpful if several gaseous species act as transport agent (w < 0) or as transport efficient species (w > 0).33

EXPERIMENTAL SECTION

Calculations of vapor−solid equilibria were realized with the software package Tragmin using the thermodynamic data enthalpy, entropy, and heat capacity of the gaseous species Bi(g), Bi2(g), BiSe(g), Sey(g) (1 ≤ y ≤ 8) and the condensed phases Bi, BiSe, Bi2Se3, Se.32 The synthesis of the Bi2Se3 nanostructures was realized by catalyst-free decomposition sublimation in sealed silica ampules with Bi2Se3 powder (Sigma-Aldrich 99,9%, 1−4 mg per synthesis) and Si/SiO2-substrates (B-doped, p-type, 300 nm oxid-layer) as starting materials. The ampule is divided into two chambers (see Figure 1a). The Bi2Se3 powder was placed in one chamber (decomposition zone) and the silicon substrate in the other one (deposition zone). The ampule was sealed under a vacuum (10−3 mbar) and positioned in a horizontal tube furnace. The exact position of the Bi2Se3 powder and the Si/SiO2 substrate was measured by means of the temperature profile of the horizontal tube and is illustrated in Figure 1b for several temperatures. For crystal growth the oven was heated up to the selected temperature in the hot zone between T2 = 500−600 °C and in the cold part at T1 =

w(i) = (p(i)/p*(L))Source − (p(i)/p*(L))Sink B

(1)

DOI: 10.1021/acs.cgd.5b00566 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 2. (a) Calculated composition of the Bi−Se gas phase presented by the partial pressures pi in the temperature range 400−650 °C; n(Bi) = 10 mmol, n(Se) = 15 mmol, scale basis N2 = 0.01 mmol, V = 20 mL. The blue dashed line present the partial pressure of 10−5 atm. (b) Transport efficiency for the decomposition sublimation of Bi2Se3; T2 = 550 °C, ΔT = 150 K. (c) Calculated migration rates at T1 for pure Bi2Se3.

Figure 3. SEM images of the crystal distribution on Si/SiO2 substrates: (a) Agglomeration of crystal sheets at ∼7 cm away from Bi2Se3 (b) isolated crystal at >7 cm away from Bi2Se3. SEM images of characteristic Bi2Se3 nanostructures (c) nanoflake (hexagon), (d) nanoribbon. (e) Bi2Se3 nanowire (width ≈ 40 nm, height ≈ 90 nm) illustrated by AFM images and SEM image as an inset. (f) AFM images of a thin Bi2Se3 nanoribbon (height ≈ 9 nm).

C



Article

Table 1. Variation of the Temperature at a Constant Reaction Time of 4 ha

a

sample

T2 (Bi2Se3) [°C]

T1 (Si/SiO2) [°C]

m [mg]

counted crystals

av length (ribbons) [μm]

av height (ribbons) [nm]

1 2 3 4 5 6

523−501 560 613−602 514−501 564−553 613−602

365−341 414−381 465−435 362−331 419−368 469−430

4 4 4 1 1 1

14 18 6 11 20 3

5−10 11−48 7−11 15−25 6−18 13

20−32 26−100 30−100 27−49 10−49 32

av = average.

w(i) transport efficiency of compound i, p(i) partial pressure of the gaseous species i, p*(L) balance pressure of the solvent (gas phase). Calculations of the transport efficiency for the Bi−Se-system in the temperature range between 550 and 400 °C (Figure 2b) confirm the conclusions that only the gaseous species BiSe and Se2 act as transport efficient species (w > 0). The negative transport efficiency for Se3−Se6 (w < 0) is a result of possible reactions of selenium molecules in the vapor phase. Se7 and Se8 have no impact to the vapor transport due to their very low partial pressures. Therefore, the chemical vapor transport can be defined as a decomposition sublimation with a congruent deposition of Bi2Se3 described in reaction eq 2: 2Bi 2Se3 (solid) ⇆ 4BiSe (gaseous) + Se2 (gaseous)

significant changes in the crystal length (Table 1) by increasing temperature. Nanostructures were observed in all samples with a length ≈ 10 μm. The thickness of the nanostructures of sample 5 with an initial weight of around 1 mg has a height of ∼10 nm only at a temperature T1 = 419−368 °C (sample 5). For this growth condition, besides nanoribbons with a length of ∼ 10 μm and a height of ∼10 nm (Figure 3f), we also found nanowires, which are of much interest, for instance, for studying spin-chiral Dirac−Fermions by the measurement of Aharonov− Bohm (AB) oscillations.36 Moreover, the deposited nanostructures are not agglomerated with other crystalline material and exhibit a smooth, homogeneous, and large surface, which is of high importance for contacting the crystal for electrical transport. In sample 5, the increased temperature leads to an adequate number of viable nucleus and higher mass flow rate. The initial weight and reaction time is low enough to enable a growth of the generated nucleus mainly in the x, y-direction. A growth in the z-direction was observed by increasing the growth temperature. Indeed, a higher mass flow rate leads to a higher amount of material for VS growth. So the generated nucleus can be overlapped by particles and a growth in the zdirection occurs for the nanostructures. At higher temperature (samples 3 and 6), we found much less nanostructures on the substrates, independently of the selected initial weight of Bi2Se3. The thermodynamic calculations have indicated that the partial pressure of Se2 at this growth temperature (T1 ≈ 460−430 °C) is around 10−6 atm. Investigations of the Bi2Se3 vapor pressure have shown decomposition in the range of 747−896 K (474− 623 °C)34 as well. Therefore, it is possible that volatilization occurs in the temperature range 460−430 °C, which leads to a lower nanostructure growth. Additionally, the temperature profile of the furnace can show not only a temperature gradient between our selected T2 and T1 but also with smaller temperature gradients on the silicon substrate itself, which could result in further material transports. Importantly, thin nanostructures were found only in samples with a low amount of initial weight (≈ 1 mg), while a growth in the z-direction occurs mainly for samples with a higher initial weight (≈ 4 mg). A higher initial weight also leads to longer crystals. In this case, the large initial amount of Bi2Se3 will deliver permanently material to the vapor phase and will lead to an increased crystal size. The investigation of the reaction time shows an increase of the crystal size in the x-, y-, and zdirection. A long reaction time and a high initial weight can promote the crystal growth, because material is continuously available and contributes to an enlargement of the crystal size. If the selected initial weight is too low, the critical size of nucleus cannot be achieved and not enough material will be available for the growth. Finally, we find the optimized parameter to obtain nanostructures with a length of >10 μm and a simultaneous height of ∼10 nm, i.e., T2 (Bi2Se3) = 564−

(2)

The decomposition behavior of Bi2Se3 was experimentally investigated, for instance, by Knudsen effusion techniques in the temperature range of 747−896 K34 and 735−879 K35 and confirms a decomposition of Bi2Se3 in gaseous species Se2 and BiSe. The calculations of the mass flow rate for Bi2Se3 (ṅBi2Se3) show an increasing amount of transported condensed phase by increasing temperature (Figure 2c). At the sink temperatures T1 = 350 and 400 °C, low mass flow rates of ṅBi2Se3 ≈ 0.01 mg/h and 0.1 mg/h were calculated. Altogether our calculations show very low mass flow rates and partial pressures of transport relevant species. Thus, we assumed a very gentle crystal growth and a preferred VS growth of nanostructures over deposition of large bulk crystals. Also the vapor transport of Bi2Se3 is congruent, and a deposition of other phases like BiSe or Bi4Se3 is not expected. Crystal Growth. The temperature area of the Bi2Se3 decomposition T 2 ≈ 500−600 °C derives from the thermodynamic calculations. The temperature area of Bi2Se3 deposition and the VS growth is around 150 K below the decomposition temperature27 at T1(Si/SiO2) = 350−450 °C. In our experiments we observed different areas of crystal deposition. We found in the hot part of the substrate (∼7 cm away from Bi2Se3) predominantly an agglomeration of crystalline material (Figure 3a) and in the colder part of the substrate (>7 cm away from Bi2Se3) individual and well distributed nanostructures (Figure 3b). The nanostructures have different morphologies such as nanoflakes (triangles, hexagons and rhombohedrons), nanoribbons, and nanowires (Figure 3c−e). Our goal was to synthesize individual nanostructures with a length of >10 μm and a simultaneous height of Bonset ∼ 6.5 T) longitudinal measurement exhibits oscillations of the resistivity that are periodic with inverse magnetic field (see Figure 6d). These are SdH oscillations, that develop for B > Bonset = 1/μ, where μ is the carrier mobility. The onset of the SdH oscillations hence gives access to the mobility of the electronic population. Here we find a mobility of approximatively μ ∼ 1500 cm2·V−1·s−1, which confirms the good crystal quality of our nanostructures. Moreover, from the periodicity of SdH oscillations in inverse magnetic field one can extract the carrier density of the electronic population. In our case we find a single band contribution (see Figure 6c) with a magnetic frequency f = 128

Figure 4. Thicknesses of Bi2Se3 nanostructures deposited by VS growth with the optimized parameter T2 (Bi2Se3) = 564−553 °C, T1 (substrate) = 420−370 °C, t = 4 h. Nanowires are classified by a width of ≤200 nm (h ≈ 70 nm) and ≤300 nm (h ≈ 170 nm), respectively. Several samples were considered for this statistic.

Nanoribbons are the most common morphology with dimensions mostly between 10 and 20 nm or higher than 40 nm. Very thin Bi2Se3 nanostructures can be synthesized with our optimized catalyst-free vapor transport as shown in Figure 3f. The thickness of nanoflakes is found to be mostly between 30 and 40 nm and >40 nm. TEM studies provide important information about the quality of our deposited Bi2Se3 nanostructures and are illustrated in Figure 5a,b. The HRTEM images show the atom stacks in Bi2Se3 nanostructures with a lattice spacing of ∼2.1 Å for the (110) atomic plane and 3.6 Å for the (101) atomic plane, which is in agreement with reference (PDF 00003-2014). Also the hexagonal symmetry of Bi2Se3 (spacegroup R3̅m) is indicated by the corresponding fast Fourier transform (FFT) diffractogram (insets of Figure 5a,b). These results confirm the single crystalline trigonal nature of our Bi2Se3 nanostructures. The growth directions were determined to be [101] for triangular nanostructures as well as [−210] and [110]

Figure 5. TEM images of Bi2Se3 nanotriangle (a) and nanoribbon (b) with the corresponding SAED pattern (insets) and HRTEM images. The red dashed circles represent the investigated area. E



Article

Figure 6. (a) SEM picture of a 30 nm thin Bi2Se3 flake. (b) Temperature dependence of the longitudinal resistance, showing a metallic behavior. (c) Magnetic field dependence of the transverse resistance at 15 K, for two different gate voltages. (d) Inverse-field dependence of the transverse resistance after removing a linear background. Inset: FFT showing the single-band contribution to Shubnikov−de Haas oscillations.

T−1. Depending on the assumed 2D or 3D origin of SdH oscillations one could extract a typical carrier density: n2D‑SdH = e·f/h = 3.9(0.1) × 1012 cm−2 or n3D‑SdH = 7.4(0.5) × 1018 cm−3. The discrepancy between the densities extracted from SdH and Hall measurements underlines the fact that several electronic populations, namely, bulk and topological surface states, with different mobilities and densities contribute to the charge transport. A back-gate voltage up to −110 V was applied on the thermal SiO2 (200 nm)/p-Si++ substrate. As can be seen in Figure 6c, the Hall effect is modified by the electric field, with a 10% change in the total carrier density at −110 V, whereas the SdH oscillations are not affected by the gate voltage (see Figure 6d). Since topological states located at the interface between Bi2Se3 and SiO2 are mostly affected by the gate, this result suggests that these states have a lower mobility than bulk states and topological states located at the surface, possibly due to charged traps at the interface. This interpretation is confirmed by transport measurements in high magnetic fields.38 Although those data do not decide between bulk and topological surface state origin for the SdH oscillations, they however demonstrate the existence of a topological interface state, which only contributes to the Hall resistance.39 Therefore, we can conclude that electrons from bulk states alone cannot account for the charge transport, but also the electrons from the topological surface states make an effective contribution to the conductivity. Moreover, detailed studies of Aharonov−Bohm oscillations and Universal Conductance Fluctuations at very low temperature were already performed on nanowires grown by CVT.36,40 Those studies demonstrate without ambiguity the

existence of topological surface states and the possibility to thoroughly study them in nanostructures.

■

CONCLUSIONS

■

AUTHOR INFORMATION

In this work we have shown an alternative synthesis route for the preparation of nanostructures. Starting from thermodynamic calculations we derived and tuned parameters for optimal catalyst-free CVT and the VS growth of Bi2Se3 nanostructures in a sealed ampule. With this approach thin, single crystalline Bi2Se3 nanostructures with different morphologies can be synthesized, measured, and characterized by quantum transport studies. The catalyst-free growth is shown to produce crystals with a high quality and offers a very convenient technique for the measurement of the transport properties of 3D-TI’s surface states. SdH oscillations, in combination with Hall measurements, allow one to infer the existence of topological surface states in those structures. Although electrostatic top-gating is necessary to obtain more information about the respective contribution of surface and bulk states to classical transport,41 those nanostructures already allowed the performance of studies of topological surface states and are well-suited for further quantum transport studies.36

Corresponding Author

*E-mail: [email protected]. Phone: +49 (351) 4659548. Fax: +49 (351) 4659313. F



Article

Notes

(28) Alegria, L. D.; Schroer, M. D.; Chatterjee, A.; Poirier, G. R.; Pretko, M.; Patel, S. K.; Petta, J. R. Nano Lett. 2012, 12, 4711−4714. (29) Oppermann, H.; Schmidt, M.; Schmidt, P. Z. Anorg. Allg. Chem. 2005, 631, 197−238. (30) Schöneich, M.; Schmidt, M. P.; Schmidt, P. Z. Anorg. Allg. Chem. 2010, 636, 1810−1816. (31) Krabbes, G.; Bieger, W.; Sommer, K.-H.; Söhnel, T.; Steiner, U. Software Package TRAGMIN. Calculation of Transport Equilibria by Minimization of the free Enthalpy; version 5.0; IFW Dresden, Universität Dresden, HTW Dresden, 2008. (32) Knacke, O.; Kubaschewski, O.; Hesselmann, K. Thermochemical Properties of Inorganic Substances, 2nd ed.; Springer−Verlag: Düsseldorf, 1991. (33) Binnewies, M.; Glaum, R.; Schmidt, M.; Schmidt, P. Chemical Vapor Transport Reactions; Walter de Gruyter Verlag: Berlin, 2012. (34) Krestovnikov, A. N.; Gorbov, S. I. Russ. J. Phys. Chem. 1967, 41, 376−378. (35) Boncheva-Mladenova, Z.; Pashinkin, A. S.; Novoselova, A. V. Inorg. Mater. 1968, 4, 904−907. (36) Dufouleur, J.; Veyrat, L.; Teichgräber, A.; Neuhaus, S.; Nowka, C.; Hampel, S.; Cayssol, J.; Schumann, J.; Eichler, B.; Schmidt, O. G.; Büchner, B.; Giraud, R. Phys. Rev. Lett. 2013, 110, 186806. (37) Horák, J.; Starý, Z.; Lošták, P.; Pancír, J. J. Phys. Chem. Solids 1990, 51, 1353−1360. (38) Veyrat, L.; Iacovella, F.; Dufouleur, J.; Nowka, C.; Funke, H.; Yang, M.; Escoffier, W.; Goiran, M.; Büchner, B.; Hampel, S.; Giraud, R. Band bending inversion in Bi2Se3 nanostructures. arXiv: condmat.mes-hall/1508.01881. ArXiv.org e-print archive, 2015; http:// arxiv.org/abs/1508.01881 (Accessed Aug. 11, 2015). (39) Because of the very strong bulk doping, charge accumulation gas are prevented in those structures by an upward band bending.11,42 (40) Dufouleur, J.; Veyrat, L.; Xypakis, E.; Bardarson, J. H.; Nowka, C.; Hampel, S.; Eichler, B.; Schmidt, O. G.; Büchner, B.; Giraud, R. Pseudo-ballistic transport in 3D topological insulator quantum wires. arXiv: cond-mat.mes-hall/1504.08030. ArXiv.org e-print archive, 2015; http://adsabs.harvard.edu/abs/2015arXiv150408030D (accessed June 03, 2015). (41) Sacépé, B.; Oostinga, B. J.; Li, J.; Ubaldini, A.; Couto, N. J. G.; Giannini, E.; Morpurgo, A. F. Nat. Commun. 2011, 2, 575. (42) Brahlek, M.; Koirala, N.; Bansal, N.; Oh, S. Solid State Commun. 2015, 215-216, 54−62.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully thank Dr. Sabine Wurmehl and Markus Gellesch for fruitful discussions and also Katrin Wruck, Alexander Schubert, and Dr. Ingolf Mönch for helpful support in the experimental work. J.D. acknowledges the support of the German Research Foundation DFG through the SPP 1666 “Topological Insulators” program.

■

REFERENCES

(1) Zhang, H.; Liu, C.-X.; Qi, X.-L.; Dai, X.; Fang, Z.; Zhang, S.-C. Nat. Phys. 2009, 5, 438−442. (2) Hasan, M. Z.; Kane, C. L. Rev. Mod. Phys. 2010, 82, 3045−3067. (3) Checkelsky, J. G.; Hor, Y. S.; Cava, R. J.; Ong, N. P. Phys. Rev. Lett. 2011, 106, 196801. (4) Fu, L.; Kane, C. L.; Mele, E. J. Phys. Rev. Lett. 2007, 98, 106803. (5) Moore, J. E.; Balents, L. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 121306. (6) Fu, L.; Kane, C. L. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 045302. (7) Xia, Y.; Qian, D.; Hsieh, D.; Wray, L.; Pal, A.; Lin, H.; Bansil, A.; Grauer, D.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z. Nat. Phys. 2009, 5, 398−402. (8) Chen, Y. L.; Analytis, J. G.; Chu, J.-H.; Liu, Z. K.; Mo, S.-K.; Qi, X. L.; Zhang, H. J.; Lu, D. H.; Dai, X.; Fang, Z.; Zhang, S. C.; Fisher, I. R.; Hussain, Z.; Shen, Z.-X. Science 2009, 325, 178−181. (9) Cheng, P.; et al. Phys. Rev. Lett. 2010, 105, 076801. (10) Cho, S.; Butch, N. P.; Paglione, J.; Fuhrer, M. S. Nano Lett. 2011, 11, 1925−1927. (11) Analytis, J. G.; McDonald, R. D.; Riggs, S. C.; Chu, J.-H.; Boebinger, G. S.; Fisher, I. R. Nat. Phys. 2010, 6, 960−964. (12) Ren, Z.; Taskin, A. A.; Sasaki, S.; Segawa, K.; Ando, Y. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 241306. (13) Webb, R. A.; Washburn, S.; Umbach, C. P.; Laibowitz, R. B. Phys. Rev. Lett. 1985, 54, 2696−2699. (14) Kuroda, K.; Arita, M.; Miyamoto, K.; Ye, M.; Jiang, J.; Kimura, A.; Krasovskii, E. E.; Chulkov, E. V.; Iwasawa, H.; Okuda, T.; Shimada, K.; Ueda, Y.; Namatame, H.; Taniguchi, M. Phys. Rev. Lett. 2010, 105, 076802. (15) Steinberg, H.; Gardner, D. R.; Lee, Y. S.; Jarillo-Herrero, P. Nano Lett. 2010, 10, 5032−5036. (16) Teweldebrhan, D.; Goyal, V.; Balandin, A. A. Nano Lett. 2010, 10, 1209−1218. (17) Zhang, G.; Qin, H.; Teng, J.; Guo, J.; Guo, Q.; Dai, X.; Fang, Z.; Wu, K. Appl. Phys. Lett. 2009, 95, 053114−053114−3. (18) Li, Y.-Y.; et al. Adv. Mater. 2010, 22, 4002−4007. (19) He, L.; Xiu, F.; Wang, Y.; Fedorov, A. V.; Huang, G.; Kou, X.; Lang, M.; Beyermann, W. P.; Zou, J.; Wang, K. L. J. Appl. Phys. 2011, 109, 103702. (20) Kong, D.; Randel, J. C.; Peng, H.; Cha, J. J.; Meister, S.; Lai, K.; Chen, Y.; Shen, Z.-X.; Manoharan, H. C.; Cui, Y. Nano Lett. 2010, 10, 329−333. (21) Yan, Y.; Liao, Z.-M.; Zhou, Y.-B.; Wu, H.-C.; Bie, Y.-Q.; Chen, J.-J.; Wu, X.-S.; Yu, D.-P.; Meng, J. Sci. Rep. 2013, 3, 1−5. (22) Fang, L.; Jia, Y.; Miller, D. J.; Latimer, M. L.; Xiao, Z. L.; Welp, U.; Crabtree, G. W.; Kwok, W.-K. Nano Lett. 2012, 12, 6164−6169. (23) Peng, H.; Dang, W.; Cao, J.; Chen, Y.; Wu, D.; Zheng, W.; Li, H.; Shen, Z.-X.; Liu, Z. Nat. Chem. 2012, 4, 281−286. (24) Jacobs-Gedrim, R. B.; Durcan, C. A.; Jain, N.; Yu, B. Appl. Phys. Lett. 2012, 101, 143103. (25) Mlack, J. T.; Rahman, A.; Johns, G. L.; Livi, K. J. T.; Marković, N. Appl. Phys. Lett. 2013, 102, 193108. (26) Cha, J. J.; Koski, K. J.; Cui, Y. Phys. Status Solidi RRL 2013, 7, 15−25. (27) Kong, D.; Dang, W.; Cha, J. J.; Li, H.; Meister, S.; Peng, H.; Liu, Z.; Cui, Y. Nano Lett. 2010, 10, 2245−2250. G


Catalyst-free Growth of Single Crystalline Bi2Se3 Nanostructures for

Recommend Documents