A Controllable Solid-Source CVD Route To Prepare Topological

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A controllable solid-source CVD route to prepare topological Kondo insulator SmB6 nanobelt and nanowire arrays with high activation energy Haibo Gan, Bicong Ye, Tong Zhang, Ningsheng Xu, Hongtao He, Shaozhi Deng, and Fei Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01412 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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Crystal Growth & Design

(Submitted to Crystal Growth & Design)

A controllable solid-source CVD route to prepare topological Kondo insulator SmB6 nanobelt and nanowire arrays with high activation energy Haibo Gan1,†, Bicong Ye2,†, Tong Zhang1, Ningsheng Xu1, Hongtao He2,*, Shaozhi Deng1, *, Fei Liu1,*

1State

Key Laboratory of Optoelectronic Materials and Technologies,

Guangdong Province Key Laboratory of Display Material and Technology, and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China 2Institute

for Quantum Science and Engineering and Department of Physics,

Southern University of Science and Technology, Shenzhen 518055, China Corresponding

email: [email protected], [email protected], [email protected]

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract As a typical topological Kondo insulator (TKI), Samarium hexaboride (SmB6) is an ideal platform to investigate the interaction between strongly-correlated electron states and topological surface states. Compared with their corresponding bulk counterparts, low-dimensional SmB6 nanostructures have larger surface-volume ratio, and thus they should possess more abundant surface states and remarkable quantum behaviors. But until now, few researches have been focused on the controllable growth techniques and electrical transport properties of the SmB6 nanostructures, which keep them from rapid developments. In this study, we report a simple chemical vapor deposition (CVD) technique to prepare single crystalline SmB6 nanobelts and nanowires under control based on a solid-source route. The formation of the SmB6 nanobelts is attributed to the synergistic effect of the vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms whereas the VLS mechanism should be responsible for the formation of the SmB6 nanowires. Electrical transport studies of these nanostructures reveal that their transportation curves have four different temperature regions due to the competition of the surface and bulk conduction. Because of the larger surface to volume ratio, these SmB6 nanobelts and nanowires have much smaller residual resistance ratio than their bulk counterparts, and thus the temperature region dominated by the surface conduction also extends to higher temperatures up to 10 K. Moreover, the as-grown SmB6 nanobelts (4.4 meV) and nanowires (3.1 meV) via our solid-source route are found to have much higher activation energy than the reported SmB6 nanowires (about 2.5


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meV). Under this circumstance, the surface conduction can play a bigger role in the electrical transport of SmB6, which is much beneficial for their future applications. This controllable solid-source synthesis route to fabricate the SmB6 nanostructures may shed new light on modulating the morphology and electrical transport of other metal boride nanostructures.

KEYWORDS: SmB6 nanobelts and nanowires, solid-source CVD route, the synergistic effect of the VLS and VS mechanisms, topological Kondo insulator (TKI), electrical transport



Introduction Topological insulators (TIs) are very fascinating materials because they have insulating bulk states but metallic surface states protected by the time-reversal symmetry1-4. Different from conventional TIs possessing residual bulk conduction, topological Kondo insulators (TKIs) are more exotic and significant since their bulk phase is really insulating due to the strong electron correlation effects5-7. As a typical TKI, samarium hexaboride (SmB6) with three surface Dirac cones has gained considerable interests in recent years because it may possess unconventional superconductivity, unique transition from Kondo lattice to small-gap topological insulators at low temperature and high saturation temperature of electrical resistance610.

Nowadays, much effort has been devoted to the research on the physical properties

of bulk SmB6 crystals, revealing the unique energy-band structure9, 11-13, the intriguing quantum oscillations10, 14 and the dominant surface conduction6. Moreover, bulk SmB6 was found to exhibit weak antilocalization effect15-17, linear magnetoresistance17, surface ferromagnetism and quantized one-dimensional edge state transport16. In comparison with their bulk counterparts, SmB6 nanostructures (such as nanobelts and nanowires) have larger surface-volume ratio, and thus their surface should have higher density of state (DOS) and more distinctive quantum behaviors. But to our knowledge, only a few groups18-23 reported the synthesis of one-dimensional SmB6 nanowires in experiments until now. Nearly in all the known synthesis ways, high-cost and toxic or flammable gases (BCl3 or B10H14) were used as the boron sources for preparing the SmB6 nanowires18-22. Moreover, most of the reports were concentrated on the growth of SmB6 nanowires whereas few studies focused on the SmB6 nanobelts. At the same time, the studies on the surface transport in SmB6 nanowires are very deficient21-23 because the Hall measurements are very hard to perform on the nanowires with circular cross-section. As a result, the absence of the controllable growth technique of SmB6 nanostructures with large planar size strongly prevents people from accessing


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their intrinsic physical properties and mechanisms. On the other hand, the morphology and crystallinity of the SmB6 nanostructures will have much effect on their gap diagram and surface states21-25, so the controllable growth of SmB6 nanostructures with different morphology will be beneficial to better modulate their electrical transport behaviors. Therefore, finding an effective way to controllably fabricate different morphologies of SmB6 nanostructures remains a large challenge for all the researchers. In this paper, we report an easy and moderate solid-source synthesis route to fabricate single crystalline SmB6 nanobelt and nanowire arrays under control. And we propose the formation mechanisms for the SmB6 nanobelts and nanowires based on the experimental results, respectively. Finally, the surface transport properties of the SmB6 nanobelts and nanowires are compared together to explore their intrinsic physical mechanisms.

Experimental section Synthesis of the SmB6 nanowires and nanobelts SmB6 nanobelts and nanowires were grown through a solid-source synthesis route in a low-pressure tube furnace, as depicted in our previous works26-28. The B, B2O3 and anhydrous SmCl3 powders were placed into the reaction vessel as the source materials. Two Si (100) wafers covered by a 10 nm thick Ni catalyst film were used as the substrates for the formation of the SmB6 nanostructures, which respectively kept a distance of about 1 and 3 cm to the SmCl3 powders. The growth procedures of the SmB6 nanostructures can be seen as follows. Firstly, the furnace was heated to 700 ℃ and kept here for about 60 min under the mixed gas of Ar and H2, whose flow rate ratio was 285 to 200 sccm. Secondly, the furnace was raised to 1100℃ and maintained here for 30 ~ 60 min. In this step, the flow rate ratio of Ar to H2 was adjusted to be 285 to 30 sccm and the growth pressure ranged from 0.1 kPa to 76 kPa. Finally, the furnace was cooling down to room temperature, in which the flow rate ratio of Ar to H2 kept



unvaried. After the reaction process was over, dark-grey and dark-blue films were respectively found to be deposited on the surface of two Si substrates. Morphology and structure characterizations The morphology of the sample was characterized by scanning electron microscope (SEM) (Zeiss, SUPER-60). X-ray diffractometer (Rigaku, D-MAX 2200 VPC) and Raman spectroscope (Renishaw, Invia Reflex) were used to confirm the phase compositions of the products, respectively. And the crystalline structure and chemical compositions of the sample were investigated by transmission electron microscope (TEM) (FEI, Titan3 G2 60-300). Measurements of the electrical transport properties The e-beam lithography (EBL) technique was applied to fabricate the SmB6 nanostructure devices. To form the nice ohmic contact between the electrode and the nanostructures, the Cr (10 nm)/Au (100 nm) film was deposited on both sides of the SmB6 nanostructures as the electrodes by thermal evaporation way. The electrical transport measurements of the SmB6 nanobelts and nanowires were carried out in a 14T Quantum Design physical property measurement system (PPMS), in which the base temperature is 2 K. And the lock-in technique was used to measure the four-probe electrical resistivity of individual nanostructures. In the measurements, the amplitude and frequency of the typical excitation ac current were kept at 100 nA and 375 Hz, respectively.

Results and discussion SmB6 nanobelts and nanowires have been successfully fabricated on Si substrates by controlling the location of substrates. To prepare the nanobelts, the substrate was adjusted to keep a 1 cm distance to the SmCl3 source powders. As seen in Figs. 1(a-c), large-scale nanobelt arrays distribute over the substrate and stand to the substrate with an average angle of 70o. And their growth density reaches over 105 cm-2. Moreover, the nanobelts are found to have a mean length of 130 μm and an average width of several


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micrometers as well as their thickness varies from 40 nm to 500 nm. No catalyst nanoparticles can be found for the nanobelts. As referred by the white ellipses in Fig. 1(b), the nanobelts usually have two typical morphologies, which were respectively sword-like (>80 % of the total) or boomerang-like (80%, this work) Boomerang-like SmB6 nanobelt (98 at.%). In addition, there are tiny Cu, Si and O elements existing in the EDX spectra. The Cu and

Fig. 4 (a, b) EDX spectra of a single SmB6 nanobelt and nanowire. (c, d) Their corresponding EDX mapping results.

Si signals should come from the TEM grid and the Si substrate, respectively. And the O element may result from the surface amorphous layer of nanostructures or the surface oxygen adsorbents. Subsequently, the EDX mapping results are given in Figs. 4(c, d) to further ascertain the element distribution of nanobelt or nanowire. As seen in Figs. 4(c, d), Sm and B elements distribute uniformly throughout the whole nanobelt or nanowire whereas O element mainly exists in the sheath of nanobelt or nanowire. The EDX mapping results of SmB6 nanostructures are in good agreement with their



HRTEM images (Fig. 3), which reveals that the as-grown nanowires or nanobelts are SmB6 single crystals with a thin surface oxide layer. Although Ni catalysts are essential to the formation of both SmB6 nanobelts and nanowires in our experiments, we prefer two different growth mechanisms to illustrate their formations. As shown in Figs. 5(a-d), the possible growth process of SmB6 nanobelts can be depicted as follows. Firstly, the SmCl3 and B2O2 vapors generated by the reaction of B and B2O3 powders are transferred to the substrate region at high temperature. With the help of catalysts and H2, the SmCl3 vapors react with the B2O2 vapors and form the SmB6 molecules, which dissolve into the catalyst droplets. The chemical equation can be written as: 18 B2O2 ( g )  2 SmCl3 ( g )  3H 2 ( g )  2 SmB6 ( s )  12 B2O3 ( g )  6 HCl ( g ) [1]

Secondly, more and more SmB6 molecules are produced with the proceeding of the above reaction and continuously dissolve into the catalyst droplets. When the solubility of SmB6 in catalyst droplets arrives at supersaturation, the SmB6 solids will separate out from the surface of the catalyst droplets and act as the nucleus of the growth of the nanobelts. As explained in these two steps, the classical VLS mechanism should dominate over the initial stages of the nanobelts. Thirdly, the amount of the precipitated SmB6 solids is so large that they wrap or bury the catalyst particles thoroughly because of the oversupply of the source vapors, as observed in Fig. 5(b). As a result, these SmB6 solids turn into the new seeds for the following SmB6 molecules instead of the original catalyst droplets, which suggests that the growth mechanism of the nanobelts changes from the VLS mechanism into the VS mechanism. Similar to LaB631, {100} planes should have lower surface energy than {110} planes, which is bound to be the energetically-favorable growth plane of SmB6 nanobelts based on the crystal growth theory32. Fourthly, the resultant SmB6 molecules persistently attach on the {100} lattice planes (such as (100) and (010) planes) followed by the progression of the reaction. Afterwards, they diffuse over the steps and form the two-dimensional (2D) nucleus of


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the SmB6 nanobelts. Finally, the steps covered by 2D nucleus quickly move forwards along [100] and [010] directions and form the sword-like SmB6 nanobelts.

Fig.5 (a-d) Schematic diagrams of the growth mechanism of SmB6 nanobelts. (e) Typical SEM image of the root of nanobelt. (f, g) Lattice configuration of SmB6 nanobelts in experiments. (h) Another possible plane configuration of SmB6 nanobelts by the [110] monoaxial direction in theory, which can’t be found in all TEM results.

Based on the lattice configuration diagram of SmB6 in Fig. 5(f), the actual growth orientation of SmB6 nanobelts should be [110], which originates from the combination of two [100] and [010] growth directions. Moreover, the width of the SmB6 nanobelts turns narrower and narrower with the exhaustion of the source materials during the later growth stage, as shown in Figs. 1(a-c). The growth-direction change of the boomerang-



like nanobelts from [110] to [100] may be attributed to the different deposition rate of source vapors between [100] and [010] planes, as explained in Fig. 5(g). Accompanying with the fluctuation of chamber pressure, temperature or the vapor concentration, the adsorption energy of source vapor on the (100) surface possibly turns higher than that of the (010) surface for a few nanobelts at special situation. Correspondingly, the growth rate of the SmB6 molecules on the (100) surface become far higher than that of the (010) surface, which induces the growth direction change of the boomerang-like nanobelts from [110] to [100]. Figure 5(h) gives another possible crystal configuration of SmB6 nanobelts, which is only formed by the [110] monoaxial direction rather than the resultant direction composed of [100] and [010]. But this crystal configuration has never been found during the TEM characterizations of all the SmB6 nanobelts, which further proves the reasonability of our growth model. Therefore, the formation of the SmB6 nanobelts should be attributed to the synergistic effect of the VLS and VS mechanisms. The VLS mechanism is believed to be responsible for the formation of the SmB6 nanowires because their ends are seen to have a catalyst particle in Fig. 1(f). The possible growth process of the nanowires can be depicted as follows. Similar to the nanobelts, the formed SmB6 molecules dissolve in the catalyst droplets with high surface energy in the first step, in which the catalysts act as the nucleus for the nanowires. Secondly, when the solubility of SmB6 reach the oversaturation in catalyst droplets, the solids will precipitate from the catalyst particles and grow along the energy-favorable [100] axis. Considering that the weak interaction between catalyst particles and substrate, the catalyst particles exist at the top of the nanowires. At last, the SmB6 solids gradually grow along the [100] direction and form the nanowires with the continuous dissolution of the following SmB6 molecules into the catalyst droplets, as found in Fig. 1(f). The electrical transport behaviors of individual SmB6 nanostructures were investigated in a 14T Quantum Design PPMS system, in which the measurement


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Fig.6 (a, b) Typical photographs of individual SmB6 nanobelt and nanowire devices. (c) Temperature-dependent resistance curves of individual SmB6 nanobelts and nanowires, and the inset gives the high-magnification curves at low temperature range. (d, e) Their activation energy fitting curves.

temperature (T) ranged from 2 K to 310 K. Figures 6(a) and 6(b) give the typical photographs of individual SmB6 nanobelt and nanowire device, respectively. The thickness of the sword-like nanobelt is 125 nm, and the separation between electrodes 1 and 3 and the distance between electrodes 1 and 2 are respectively about 2.8 μm and 3.8 μm for the nanobelt device. And the nanowire diameter is about 52 nm and the distance between two measurement electrodes (electrodes 8 and 9) is about 8.7 μm for individual SmB6 nanowire device. As shown in Fig. 6(c), both the SmB6 nanowire and nanobelt have a similar change tendency with the temperature (T), which can be divided



into four sections in their characteristic curves. Section I: When T decreases from 310 K to about 50 K, the resistances (R) of the SmB6 nanobelt and nanowire are observed to increase slowly. Section II: As T is lower than about 50 K, the electrical resistance of both SmB6 nanobelt and nanowire exhibits a sharp increase with the decrease of T, which differs from their variation tendency at T higher than about 50 K. Section III: When T is below some critical temperature (TC), the increasing rate of electrical resistance becomes slower with the decrease of T for both nanobelt and nanowire. In our discussion, TC is defined as the critical temperature for temperature dependence of the electrical resistance, at which the variation tendency of the electrical resistance with the temperature occurs to a sudden change (Fig. S3 of Supporting Information). It is also seen that TC is different for the nanobelt and nanowire, which is respectively 9 K and 18 K. Section IV: When T further decreases to lower than the saturation temperature (TS), the electrical resistance of both SmB6 nanobelt and nanowire tends to be saturated and keeps almost unvaried with T. Correspondingly, TS is the saturation temperature of electrical resistance (Fig. S3 of Supporting Information), which is equal to the fluctuation of electrical resistance (FER) per unit temperature (K). As described above, the R-T curves of SmB6 nanostructures are composed of four segments, and their characteristic temperatures are different for individual nanobelt and nanowire. The possible explanations are given as follows. The electrical transport behaviors of the SmB6 nanostructures should be determined by the competition between two conduction mechanisms, which are respectively bulk conduction and topologically-protected surface conduction. When T ranges from 310 K to about 50 K, the bulk conduction dominates over the electrical transport of individual SmB6 nanostructure, which behaves as bad metals33-35. At this temperature range, topological surface conduction can be ignored because the Kondo screening effect is weak at high temperature36. Under this circumstance, the resistances of both SmB6 nanobelt and nanowire increase slowly with the decrease of temperature, as seen in Section I (Fig. 6(c)). When T reduces to lower than about 50 K, the 4f flat band and 5d conduction


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band of SmB6 begin to hybridize and lead to the emergence of a small energy-gap around the Fermi level34. As a result, the SmB6 nanobelt and nanowire undergo a transition from metal to insulator, which induces the sharp increase of the resistance, as found in Section II. Correspondingly, topological surface states are produced because of the band inversion triggered by the strong spin-orbit interaction37, 38. Although the topological surface states occur, the surface electron density is far lower than bulk electron density, which suggests bulk conduction still governs the electrical transport of the SmB6 nanostructures. When T decreases from TC to TS, both bulk conduction and topological surface conduction still coexist in both SmB6 nanobelt and nanowire. The activation energies can be deduced based on the following equation [2]34:

R  R0 exp( / k BT )

[2]

, where R represents the bulk resistance of individual SmB6 nanobelt or nanowire, R0 is a fitting constant, Δ stands for the activation energy of the SmB6 nanostructure, T is the temperature and kB is the Boltzmann constant. As shown in Figs. 6(d, e), the activation energies of individual SmB6 nanobelt and nanowire are deduced to be respectively 4.4 and 3.1 meV by calculating the slopes of the linearly fitting curves at high temperature, which are larger than those (2.3 and 2.7 meV) of SmB6 nanowires in other reports21, 23 and close to that of bulk SmB6 (3.5 meV)39. It suggests that the asgrown nanobelts and nanowires may have a higher energy-gap due to their lower impurity content in lattice (Fig. 3). When T further decreases to below TC, the freezeout of bulk carriers leads to less bulk conduction in SmB6, which in turn makes the surface conduction more important below TC. Therefore, the increasing rate of the resistance turns slower obviously with the decrease of T, as found in Section III of Fig. 6(c). As the temperature further reduces to below TS, the resistances of both SmB6 nanobelt and nanowire tend to be saturated, indicating that the 2D topological surface transport dominates over the electrical properties instead of the bulk conduction. The electrical properties of SmB6 materials are compared in Table II to better comprehend their physical mechanisms. As shown in Table II, TS are respectively 6 K



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and 10 K for the as-grown nanobelt and nanowire, which are higher than those of bulk SmB6 (3.5 ~ 5 K)34, 36, 39, 40 and agree with those (6 K) of SmB6 nanowires in previous reports23. Moreover, the residual resistance ratio, defined by (R(2 K)/R(300 K)), of the SmB6 nanobelt and nanowire are respectively 23.2 and 7.3, which are far smaller than that of bulk SmB6 (104) 41. The SmB6 nanostructures are observed to have higher TS and smaller residual resistance ratio than their bulk counterparts34, 36, 39, 40, which can be attributed to their larger surface-volume ratio (nanobelt: 17 μm-1, nanowire: 38 μm1)

and more abundant surface states. Our research results also coincide with other

reports21, 23, 42. In addition, different surface-volume ratio should be responsible for the discrepancy of low-temperature electrical transport behaviors and characteristic temperature (TS or TC) between individual SmB6 nanobelt and nanowire. Further researches on the magnetoresistance of the SmB6 nanostructures with different magnetic doping are still undergoing to explore their intrinsic transport mechanisms.

Table II. The comparable table of electrical properties of SmB6 materials Parameters Sample

Saturation

Activation

temperature (K)

energy (meV)

Bulk SmB639

4

Bulk SmB640 Bulk SmB636 34

Bulk SmB6

Residual resistance ratio R(2K)/R(300K)

Surface – volume ratio (μm-1)

3.47

—

—

3.5

—

~ 6 × 104

—

5

—

~ 1 × 103

—

104

—

5

3.0

SmB6

nanowire21

6

2.3

4.9

20

SmB6

nanowire23

6

2.67

3.86

18

10

3.1

7.3

38

6

4.4

23.2

17

SmB6 nanowire (This work) SmB6 nanobelt (This work)

~6×

Considering that the as-synthesized SmB6 nanobelts can have a very thin thickness down to 40 nm by adjusting the growth parameters, better gate tunability of the surface transport can thus be achieved in our SmB6 nanobelts in comparison with their


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corresponding single crystals. Moreover, the SmB6 nanobelts can be handily transferred onto the SiO2/Si substrates. Under this circumstance, we can fabricate the back-gated or even double-gated MOSFET device to modulate the chemical potential of SmB6. Therefore, our nanobelts are expected to exhibit much better gate-tunable surface transport than their bulk counterpart.

Conclusion In this paper, we preferred a simple solid-source CVD route to fabricate highdensity TKI SmB6 nanobelt and nanowire arrays under control. The as-grown SmB6 nanobelts and nanowires are found to be single crystals with cubic structure, and their growth direction are respectively [110] and [100]. The growth mechanisms of SmB6 nanobelts and nanowires are attributed to the VLS-VS synergistic effect and the classical VLS mechanism, respectively. The R-T curves of nanobelts and nanowires exhibit the similar four-section variation characteristics, which may result from the mutual competition of bulk conduction and surface conduction. Moreover, the resistances of both SmB6 nanostructures arrive at the saturation at low temperature of 6 K or 10 K, which reveals that the topological surface states dominate over their electrical transport at this situation. The as-grown SmB6 nanobelts and nanowires are found to have higher activation energy, which reveals the SmB6 nanostructures via the solid-source CVD route have better crystallinity and lower impurity content. Considering that the SmB6 nanostructures have larger surface-volume ratio and higher surface electron density than their bulk counterparts, they should be more ideal platforms for studying the interaction between strong electron correlation and topological surface states. Our synthesis method may provide a helpful reference to modulate the morphology and electrical transport of other metal boride nanostructures.



Acknowledgments H. B. Gan and B. C. Ye contributed to this work equally. The authors are very thankful for the support of the National Project for the Development of Key Scientific Apparatus of China (2013YQ12034506), National Science Foundation of China (Grant Nos. 51872337, 51290271, 11474364 and 11574129), National Key Basic Research Program of China (Grant No. 2013CB933601), the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2014A030306017), the Guangdong Special Support Program, the Fundamental Research Funds for the Central Universities of China, the Science and Technology Department of Guangdong Province and the Education Department of Guangdong Province.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The structure analysis of the boomerang-like SmB6 nanobelts, the definition of the critical temperatures (TC) for temperature dependence of the electrical resistance and the definition of the saturation temperature (TS) of electrical resistance are all supplied in Supporting Information.

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For Table of Contents Use Only A controllable solid-source CVD route to prepare topological Kondo insulator SmB6 nanobelt and nanowire arrays with high activation energy Haibo Gan, Bicong Ye, Tong Zhang, Ningsheng Xu, Hongtao He, Shaozhi Deng, Fei Liu

Single crystalline SmB6 nanobelts and nanowires have been respectively grown by our solid-source CVD route. The quasi two-dimensional SmB6 nanobelts are found to have more surface states and higher activation energy than the SmB6 nanowires, and thus they should be more ideally research platform for modulating their surface electron transport.


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ToC figure 88x34mm (300 x 300 DPI)


A Controllable Solid-Source CVD Route To Prepare Topological

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