Straight Indium Antimonide Nanowires with Twinning Superlattices via

Nov 8, 2017 - Voznyy, Levina, Fan, Walters, Fan, Kiani, Ip, Thon, Proppe, Liu, and Sargent. 2017 17 (12), pp 7191–7195. Abstract: Stokes shift, an e...
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Straight Indium Antimonide Nanowires with Twinning Superlattices via a Solution Route Yinyin Qian, and Qing Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b01266 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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Straight Indium Antimonide Nanowires with Twinning Superlattices via a Solution Route



Yinyin Qian,†,‡,§, Qing Yang*,†, ‡,§,



†Hefei Na onal Laboratory of Physical Sciences at the Microscale (HFNL), ‡Department of Chemistry, §Laboratory of Nanomaterials for Energy Conversion (LNEC), ∥Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, Anhui, P. R. China

ABSTRACT: Indium antimonide (InSb) enables diverse applications in electronics and optoelectronics. However, to date, there has not been a report on the synthesis of InSb nanowires (NWs) via a solution-phase strategy. Here, we demonstrate for the first time the preparation of high-quality InSb NWs with twinning superlattices from a mild solution-phase synthetic environment from the reaction of commercial triphenylantimony with tris(2,4-pentanedionato)-indium(III). This reaction occurs at low temperatures from 165 to 195 °C (optimized at ~180 °C), which is the lowest temperature reported for the growth of InSb NWs to date. Investigations reveal that the InSb NWs are grown via a solution–liquid–solid (SLS) mechanism due to the catalysis of the initially formed indium droplets in the mild solution-phase reaction system. The twinning superlattices in the InSb NWs are determined with a pseudo-periodic length of ~42 monolayers, which result from an oscillating self-catalytic growth related to the periodical fluctuation between reduction rate of In and Sb sources in the route. The optical pump-terahertz probe spectroscopic measurement suggests that the InSb NWs have potential for applications in high-speed optoelectronic nanodevices. KEYWORDS: indium antimonide (InSb), twinning superlattice, twin-plane superlattice, group III-V 1

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semiconductor nanowire, solution-phase synthetic route, growth mechanism, solution–liquid–solid growth model, oscillatory growth

The realization of semiconductors with heterostructures and/or superlattices could enable diverse applications in electronics and optoelectronics. However, semiconducting nanowires (NWs) with superlattices and even kinked superstructures have been less frequently reported to date, despite being very promising in nanoelectronics and photonics. This potential is typically demonstrated in the systems of GaAs/GaP, n-type Si/p-type Si and Si/SiGe nanowire superlattice structures1–3 and InAs/InP, InAs/GaSb and InAs/InSb heterostructured NWs,4–7 in addition to the kinked superstructures of Si, Ge, CdS and GaN NWs8,9 that are fabricated by various vapor processes, mainly via the vapor-liquid-solid (VLS) growth mechanism. In particular, the VLS growth strategy can be utilized to fabricate twinning superlattices in InP NWs by using impurity dopants in addition to varying the nanowire diameter and growth temperature.10 The VLS process is also available for the growth of InAs NWs with highly reproducible polytypic and twin-plane superlattices in addition to GaAs and InP NWs with frequent quasi-periodic twinning or periodic side facet modulation fabricated by varying nanowire diameter and temperature without adding any impurities and/or dopant atoms.11,12 However, it is often difficult to obtain semiconducting NWs with perfect twinning superlattices due to the difficult kinetic control of the reaction.13 To the best of our knowledge, there has not been a report on the growth of InSb and other group III-V NWs with twinning superlattices at temperatures below approximately 300 °C, especially via a solution-phase route. The difficulty of such growth at low temperatures would largely hamper the follow-up study of their properties and potential applications. InSb, with a four-coordinate zincblende configuration, is an important binary group III–V 2

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compound semiconductor with a wide range of potential applications, including high-speed field-effect transistors,14,15 thermoelectric power generation,16,17 magnetic field sensors (magnetoresistors),18,19 and infrared detectors.20–22 Bulk InSb has a narrow direct band-gap energy of 0.18 eV (300 K) and possesses an extremely high room temperature mobility in part due to the small effective electron mass.23,24 InSb has a very large exciton Bohr radius (∼60 nm) and large g factors, consequently making it an ideal candidate for quantum-effect and spin-related studies.25 Compared with bulk materials, InSb nanostructures have received considerable attention owing to their novel size and dimensional-dependent physical and chemical properties.26,27 Moreover, the solid-state solution nanostructures and/or heterostructures including InAsSb and GaSb-InAsSb wires have been employed as long- and mid-wavelength infrared photodetectors,28,29 as well as high current density Esaki tunnel diodes.30 However, InSb NWs have been less reported in recent decades compared to other group III–V semiconductors,31–36 including their solid-state solutions.28–30 However, InSb NWs could be achieved via several relatively high-temperature vapor-phase epitaxy approaches, typically including metal−organic vapor phase epitaxy (MOVPE)37,38 and chemical and/or molecular beam epitaxy (CBE, MBE),39,40 in addition to vapor–liquid–solid (VLS) growth catalyzed by gold alloy droplets via a chemical vapor deposition (CVD)41 or a pulsed-laser CVD process.42 Among these vapor-phase routes,25–44 it is especially noted that the CBE process is available for the fabrication of completely stacking fault-free, freestanding InSb NWs with diameters from 50 to 200 nm via the catalysis of Au colloids at temperatures above 410 °C from precursors of trimethylindium (TMIn) and triethylantimony (TESb)43/tert-dimethylaminoantimony (TDMASb).39 In contrast, relatively low growth temperatures (320–400 °C) would result in extensive parasitic InSb thin film deposition and stacking faults within the wires.43,44 Meanwhile, it has been found that the CVD process can be extended to grow ultra-small InSb NWs with diameters down to 4.5 nm by using 3

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a smaller size of Au colloids as catalysts,45 as well as the fabrication of InP and InAs NWs finitely with twinning superlattices.10–12 In contrast to the vapor strategies,1,3–13,25–45 solution-phase synthesis has also been developed for the growth of group III–V NWs (e.g., InP, InAs, GaP and GaAs) typically by a solution–liquid–solid (SLS) mechanism using Au, In, Ga and Bi as catalysts at relatively low temperatures.46–48 However, the report on the fabrication of InSb NWs via a solution route is rare in the literature. The difficulty in the fabrication of InSb nanostructures via a solution route is mainly due to the following reasons. Firstly, InSb is not easily produced compared with InP and InAs according to thermodynamics, although its formation is generally feasible.49 Secondly, it is rather difficult to produce pure InSb in solution due to the intensive formation tendency of stable In(III) and In2O3, thermodynamically.49 That is, solution thermodynamics is generally not favorable for the formation of InSb (nanostructures) in the solution phase, even though InSb NWs can be controllably produced by varied vapor strategies.25–45 Thirdly, this difficulty is due to the lack of effective In and Sb sources with equivalent reactivity in a suitable medium. In fact, when the employed In and Sb sources have considerably different reactivity in a solution reaction system, the formation and/or growth of InSb would be easily terminated due to the early consumption of one source before the other(s), and in such a case, it is not available for the continuous growth of the nanostructured wires in a subsequent step. In addition, there may be some unreacted impurities generated and mixed with the sample. Fourthly, both In and Sb atoms (in addition to InSb itself) have large atomic/molecular masses, which is not generally favorable for the mass transfer in the growth of InSb in both vapor and solution processes compared with the other group III-V semiconductors containing light elements. These limitations would restrict the fabrication of InSb NWs in all processes but especially in relatively low-temperature solution routes, which suggests that it is important and valuable to 4

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study the growth of InSb nanostructures at low temperatures in both theoretical and experimental contexts. Fifthly, though probably not the last reason, the difference between In (1.78) and Sb (2.05) in electronegativity is relatively small (similar to GaSb),50 and the bonding tendency of InSb is covalent with very weak ionicity (electrostatic interaction) compared with the other group III-V semiconductors. As investigated via theoretical stimulations in the literature, the four-coordinate compound with high ionicity favors a wurtzite structure, while the compound with low ionicity favors zincblende form,51–53 even though the size would sometimes influence its phases and microstructures.54 The two predicted extremes have been experimentally supported by the VLS growth of wurtzite GaN and zincblende InSb with less or without any stacking fault or twin-plane structure,9,35,37,38,41,42,45 while comparatively, InAs, GaAs, GaP and InP NWs with moderate ionicity tend to grow with stacking segments or twinning structures.1,4–13,28–34,36–38 Due to the very weak ionic bonding and very low ionicity, InSb is favored to be a zincblende configuration without planar defects and polymorphisms, in theory,51–53 and as a result, it is difficult to grow InSb nanostructures with polytypisms and possible planar defects in practical processes.35,37,38,41,42,45 In particular, there has been lack of reports on the growth of InSb and even other group III-V NWs with twinning superlattices in solution phase to date, although InSb NWs have been recently obtained with stacking faults and even some ordered twins via the vapor CBE process through the VLS growth mechanism at 320–400 °C.43 Luckily, cost-effective colloidal routes have been developed for the delicate synthesis of InSb nanocrystals and/or quantum dots from the selection of some effective Sb sources. In a typical process, Westmoreland, Rheingold and their co-workers have established that a solution-phase synthetic approach to InSb nanocrystals relies upon single-source precursors of indium-antimony 5

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complexes.55,56 Raffaelle and his co-workers have developed a useful synthetic pathway to the InSb quantum dots using tris(trimethylsilyl)antimony (Sb(TMS)3) as the Sb source in trioctylphosphine reacting with indium acetate in octadecene (with trace stearic acid).57 The Talapin group has achieved monodisperse InSb nanocrystals from a different Sb source of Sb[N(Si(Me)3)2]3 that was prepared by a metathesis of SbCl3 with Li[N(Si(Me)3)2] in the medium of diethyl ether.58,59 Almost at the same time, Yarema and Kovalenko reported a solution synthetic procedure for growing zincblende/wurzite exhibiting polymorphism and polytypism of InSb nanocrystals/short nanorods using Sb[NMe2]3 as the Sb source.60 The above explorations promote fabrication techniques and will extend continuous investigations for InSb nanocrystals. However, these solution-phase strategies have not been available for the growth of InSb NWs with twinning superlattices and even ordinary wires to date. Therefore, it is still a challenge to develop an effective method, especially in a solution system, which favors the growth of the InSb nanostructured wires with twinning superlattices easily at relatively low temperatures. Here, we try to understand the crystal growth thermodynamics and kinetics based on control experiments and report for the first time the synthesis of high-quality straight InSb NWs with twinning superlattices via a mild solution-phase synthetic route from reaction of commercial triphenylantimony (Sb(Ph)3) with tris(2,4-pentanedionato)-indium(III) (In(acac)3) at relatively low temperatures of approximately 180 °C in a medium containing 1-octadecene (ODE), oleylamine (OAm), dibenzyl ether, tri-n-octylphosphine (TOP) and hexadecylamine (HDA) in the presence of borane tert-butylamine (BTB) complex as the reductant, as seen detailed procedures in Supporting Information. The present route is based on the SLS growth mechanism46–48 at low temperatures rather than the VLS mechanism from a CBE39,43,44 process and other vapor processes35,37,38,41,42,45 at relatively high temperatures. Figure 1a is a typical X-ray diffraction (XRD) pattern for the sample synthesized at 180 °C for 30 6

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min in the current solution-phase synthetic route, and this pattern is well indexed as a face-centered cubic (fcc) zincblende phase with space group F-43m compared to standard JCPDS Card, No. 89-4299, for InSb. The intensity of the (220) diffraction peak is stronger than expected, suggesting that the zincblende InSb sample grew anisotropically. Meanwhile, a trace amount of metallic indium was detected in the sample due to the observation of a weak diffraction peak located at approximately 32° (2θ, labeled with a *) that could be clearly indexed in the corresponding magnified pattern (Figure S1, Supporting Information). Additionally, stacking faults associated with twinning structures in InSb can be clearly detected due to an additional diffraction peak observed at 22.47° (2θ, marked with # in Figure S1). This finding was further confirmed by wide angle X-ray scattering (WAXS) detection, performed at the BL14B station of Shanghai Synchrotron Radiation Facility (SSRF) with X-ray photon energy of 18 keV (λ = 0.6887 Å), as shown in Figure 1b, even though it is relative weak in Figure 1a. Figure 1c is a scanning electron microscopy (SEM) image for the zincblende InSb sample that presents wire-like nanostructures with lengths exceeding 10 μm.

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Figure 1. (a) XRD pattern of the as-synthesized InSb NWs (‘*’ indicating metallic indium) along with the standard JCPDS Card No. 89-4299 for zincblende InSb, (b) magnified wide angle X-ray scattering pattern (WAXS) performed at the BL14B station of Shanghai Synchrotron Radiation Facility (SSRF) with X-ray photon energy of 18 keV, for detecting the stacking faults associated with the twinning structures in the NWs, and (c) SEM images for the NWs. Figure 2a shows a representative low-magnification transmission electron microscopy (TEM) 8

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image for the InSb NWs, and it is found that the NWs are straight in high quality and exhibit a relatively narrow size distribution centered at approximately 40-60 nm in diameter, as shown in the large-area TEM image (Figure S2). Very interestingly, the commonly kinked and/or curved NWs, easily created in SLS and even in VLS models, could be successfully eliminated in the present work. Additionally, it has been observed that there are periodic twinning superlattices1,8,10,11 in the NWs, since the twins are evident with bright contrast, appearing in a periodic manner along the axial direction of the nanowire (Figure 2a, Figure S2, S3), and the typical twinning structures are illustrated in more detail in a corresponding high-magnification TEM image (Figure 2b). In Figure 2c, the clear lattice fringes reveal that the InSb NWs with twinning superlattices are single crystalline, although they have twin-plane defects. In detail, the pseudo-periodicity in twinning occurs in relatively narrow distributions in segment lengths, and the segment lengths are approximately 36 to 42 monolayers for the as-synthesized InSb NWs in most cases based on the direct observations from TEM images (Figure 2, S2c, S3). This result is very consistent with the pseudo-periodicity of 156.49 Å (~42 monolayers) detected by small angle X-ray scattering (SAXS) detection, performed at the BL16B station of SSRF with X-ray photon energy of 10 keV (λ = 1.2398 Å), as shown in Figure S4. The observed d-spacing (approximately 0.367 nm) in ‘segment a’ and ‘segment b’ corresponds to the (111) planes of zincblende InSb (Figure 2c), which indicates that the InSb NWs are grown along the [111] direction, even though the zincblende segments are separated by twinning planes periodically (Figure 2a, 2b, S2, S3). Figure 2d displays the corresponding selected area electron diffraction (SAED) pattern for the typical NWs (Figure 2b, 2c, S3) recorded along the [0-11] zone axis. There are two sets of symmetric electron diffraction spots observed in the pattern, which reveals that the NWs have a twin relationship and share a common (111) plane along the [111] growth direction. 9

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Figure 2. (a) Representative TEM image for the InSb NWs, (b) magnified TEM image, (c) HRTEM image, and (d) corresponding SAED pattern, recorded along the [0-11] zone axis for a typical singular nanowire with twinning superlattices. Short arrows in (b) indicate the locations of twin planes in the nanowire. Figure 3a shows a high-angle annular dark-field image in the scanning TEM (HAADF−STEM) image for a typical individual InSb nanowire along the [0-11] projection. The superimposed atomic arrays in the image clearly resolve the locations of In and Sb atoms, although In has a comparable atomic number (Z = 49) to Sb (Z = 51). Additionally, the locations of two atoms can be effectively detected in the line-intensity profile (Figure 3b). Meanwhile, the atom-resolution HAADF−STEM image reveals that the twinning structures are high quality. The STEM energy dispersive X-ray spectroscopic (STEM−EDX) elemental mappings demonstrate that both In and Sb co-exist in the InSb nanowire stem with homogeneous distributions (Figure 3c), and the molar ratio is determined to be 50.61:49.39 for In:Sb, highly consistent with the stoichiometric composition of InSb with 1:1 for In:Sb (Figure S5a). However, the terminus in the nanowire end is dominated by elemental indium (Figure 1a, 3d, S5b, S6) compared with the nanowire stem (Figure 3a-c, S5a). These results reveal and confirm that the InSb NWs are grown by the self-catalysis of indium formed early in situ in the 10

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reaction system (Figure 3d, S5b, S6) via the SLS mechanism (Scheme S1), which has been developed for the growth of different group III–V46–48 and other semiconducting NWs.47,61 As detected by EDX (Figure S5b), the terminus of the In droplet contains 83.5% In and 16.5% Sb, which suggests that there is some InSb dissolved in the In droplets.

Figure 3. (a) HAADF−STEM image of an InSb nanowire projected along the [0-11] direction showing sequences of In and Sb columns, (b) line intensity profile from the atom columns denoted by the light blue rectangle along the arrow direction indicated in (a), and STEM image and EDX elemental mappings of In and Sb for a typical InSb nanowire stem (c) with In rich terminus (d). Different colors were applied to distinguish the between positions of In and Sb, and the areas of EDX elemental mappings correspond to the yellow boxes in the STEM images in (c) and (d). In the present solution synthetic route, there are two typical reductions, In(III) + 3e– → In(0) and Sb(III) + 3e– → Sb(0), involved in the system with the injection of the strong reductant of BTB, and InSb is synthesized by the combination of in situ reduced metallic In with Sb from the corresponding

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sources in stock solutions, according to the reaction of In(0) + Sb(0) → InSb; this material further grows along the direction to form nanowires with the self-catalysis of the In droplets via the SLS model46–48 due to the direct detection of In nanocrystals frozen in the tips of the InSb NWs (Figure 1, 2, S1, S5–S7 and S9–S14). To investigate the growth of the InSb nanostructures in solution media clearly, we performed some more control syntheses under different conditions via varying reaction temperature, time, source mole ratio and solvent. Investigations demonstrated that reaction temperature plays an important role in the growth of the InSb NWs since temperature generally impacts on the growth of the wires, as demonstrated in both literature1,4–13,28–45 and Supporting Information (Figure S7). There is only a very narrow reaction temperature range from ~165 °C to 195 °C for the controllable growth of the NWs in the current reaction system (Figure S7a–f, S8), and the optimized temperature was determined to be approximately 180 °C (Figure 1,2). It is noted that there are not any InSb NWs obtained at temperatures under 165 °C and/or over 195 °C (Figure S7g, h, S8), even though the formation of InSb is thermodynamically feasible at such temperatures.49 While reaction time does not obviously influence the growth of the InSb NWs, the InSb NWs can be fabricated in this route from approximately 1 min (Figure S11) to 30 min (Figure 1,2), which suggests that the growth of the InSb NWs is relative fast after the reduction of the source materials. Through careful investigation, the InSb NWs can be generally synthesized from a wide mole ratio of source materials (Figure 1, 2, S7a-f, S9), and interestingly the wires are only generated in the presence of metallic indium (rather than Sb), based on the XRD measurements (Figure 1a, S8, S9, S10). In such cases, the In nanoparticles frozen at the nanowire tips (Figure 3d, S5b, S6, S7a–f, S9a–c) confirm the self-catalyzed growth of the InSb NWs by the early formed In via the SLS mechanism at relatively low reaction temperature, similar to the growth of different group III–V semiconducting 12

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wires catalyzed by indium droplets in the literature,46–48 since the reduced metallic In would facilitate the nucleation and growth of the solid wires from a supersaturated state of source materials in the present reduction system. The self-catalyzed SLS growth of the InSb NWs is also an analogue to the gold or indium droplets-catalyzed growth of InP, InAs, InSb, GaP, GaAs, GaSb, etc., in addition to their solid-state solutions and heterostructured NWs via the VLS model, intensively investigated by different groups, including the ones in Cambridge, Lund, Eindhoven, Yorktown Heights, etc.1,4–7,10–13,28–45,62–66 Meanwhile, the NWs show a trend to grow straight at relatively low In:Sb ratios (Figure S7a–f, S9a–c) and short and curved (with less planar defects) at high In:Sb ratios, as shown for typical examples in Figure S9a and S12. The mole ratio influences the size of the In droplets (frozen as particles after reaction) and further affects the growth of the InSb nanostructures. In detail, when the In:Sb mole ratio was set to 1.5:1, the diameter of the In nanoparticles was approximately 150±15 nm, and the value of the equilibrium contact angle between the wires and the tip droplets was approximately 150° (Figure S12). When the In:Sb mole ratio was reduced to 1.25:1 and 1:1, the diameter of the In nanoparticles changed to approximately 130±10 and 55±5 nm, respectively, and the contact angle was approximately 130° and 120°, respectively, as shown in Figure S13 and S14. The trend is obvious since the increase of the In:Sb mole ratio could increase the size of In nanoparticles and the contact angle between the wires and the tip droplets in general. Furthermore, we determined the sizes of the InSb NWs and found that the wires have a wide range of diameters from approximately 20 to 100 nm (Figure S2–S4, S7, S9, S11–S24), which are smaller than those of the In droplets (Figure S6, S9, S12–S14). Typically, the wire diameters are centered at 40 to 60 nm when synthesized at the optimized temperature of 180 °C, and the straight InSb NWs obtained at low In:Sb mole ratios have a relatively small diameter (Figure S15–S19) and highly periodic 13

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coherence of the twinning superlattices (Figure S20–S24). This finding is highly consistent with the early investigations for other four-coordinate semiconductors11,67,68 although the process of mass transfer in the present solution route is more complicated than that of a vapor route due to a large diffusion resistance. As for the growth of group III–V NWs with stacking and/or twin-plane defects, including twin-plane superlattices, there are many investigations reported on the topics from both theoretical and experimental perspectives.1,3–13,28–34,36–40,51–54,62–64,67–69 However, the growth of perfect twinning superlattices is very limited in the literature. The periodic twinning structure in the InP and InAs wires10,11 is considered an alternated stacking series of octahedron segments12 isolated by {111} facets, and the appearance of such stacking segments is determined by the twin plane nucleation energy, which is lower compared to normal nucleation.11,12 The block-by-block mechanism with a relatively low-energy state is available for the fabrication of various kinked or zigzag superstructures via a ‘nanotectonic’ approach that provides iterative control over the nucleation and growth of nanowires.8 This tendency towards low-energy configuration growth is thermodynamically available for the understanding of the formation of polytypic and even twin-plane superlattices in the NWs, while it is often difficult to realize the controllable growth of the wires with perfect twinning superlattices in practice due to both a lack of dedicated tuning in thermodynamics and reducing limitations in kinetics, associated with the factors of components in the reaction system (concentration/mole ratio8–12), reaction condition (reaction temperature,3–12,39,42,43,68 time,68 dopant/media,9,10 and substrate39,43,44,66), crystal properties (ionicity,51–53 surface and diameter54), etc.3,67 These factors reported in the literature for the above VLS processes are sometimes different and/or not fully consistent with each other; however, these factors are at least partially (one or more) relevant to thermodynamics and kinetics in the growth system, and in principle, they could 14

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be tuned for wires with an oscillation and/or coherent feature during crystal growth.8,10–12,67,69 Therefore, it is convincing that the wires with perfect twinning superlattices could be achieved in an optimally given reaction system once an oscillation of growth can be generated in the synthetic process. In the present solution synthetic route at low temperatures (typically at 180 °C), we introduce the strong reductant BTB to promote the reaction. As noted in the system, In(III) has a smaller tendency to be reduced to metallic In(0) compared with the Sb(III) source to Sb(0) thermodynamically, since the standard electrode potential of E°indium (–0.338 V) is smaller than that of E°(Sb(III)/Sb) (0.152 V) in Sb2O3 + 6H+ + 6e → 2Sb + 3H2O.70 However, it is observed that In(III) is easily reduced to In(0) in practical experiments compared with Sb(III) to Sb(0), since metallic In(0) rather than Sb(0) is observed early in the sample (Figure 1a, 3d, S1, S5, S6), which suggests that the reduction rate of In(III) is kinetically faster than that of Sb(III). In short, at the optimized temperature of 180 °C, the In reduction is favored in kinetics, while the Sb reduction is relatively limited, although their reduction trends are opposite in thermodynamics (even feasible), where the difference in reduction kinetics between In and Sb is supported by certain short-time control experiments, since the Sb reduction is actually hysteretic compared with the In reduction (Table S1). In contrast, when the reaction temperature is increased to 200 °C from 180 °C, Sb(III) is easily reduced to Sb(0) with irregular shapes without any In(0) in the sample (Figure S7h, S8), suggesting that the Sb reduction kinetics versus those of In can be enhanced by the increase in temperature. To grow InSb effectively, it is better to have an equivalent reactivity between In and Sb sources, and in fact, the reductions of In and Sb are generally tuned to be equivalent to each other via mainly controlling reaction temperatures and mole ratios (Figure S7 to S14). In the present self-catalytic route, besides the abovementioned In and Sb reductions and InSb 15

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combination, there are at least several other reactions involved in the system. These reactions mainly include the dissolution of generated InSb into the catalyst of the In droplet via InSb + x In → (InSb:Inx), based on EDX detection (Figure S5b, S6); the further reduction of Sb by In(0) from Sb(III) + In(0) → Sb(0) + In(III), according to thermodynamics70 and further reaction via (InSb:Inx) + x Sb → (x + 1) InSb over the reaction thermodynamics,49 leading to the continuous catalytic growth. As noted, InSb is nominally synthesized by the combination of equivalent reduced metallic In with Sb from the corresponding sources in stock solutions, while the actual process of combination is not fully complete at a punctual point in time with the injection of BTB due to the hysteretic reduction of Sb(III) to Sb(0), based on the experimental investigations (Figure S7–S10,S12–S14, Table S1); however, the wires with ordered structures/twinning superlattices often form spontaneously without apparent modulation of feedstock or growth parameters68 in a macroscopic scale viewpoint, since the final thermodynamic equilibrium between the sources and the grown wires does not change in a given system with fixed factors, but the catalyst could change/increase the kinetic rate of approach to the equilibrium. In the present detailed growth process, metallic In is promptly reduced to be the self-catalyst, and part of it reacts with the slowly reduced Sb to quickly form InSb in the initial step to grow InSb wire via In-catalytic SLS mechanism as it reaches supersaturation. Accompanied by the reduction and combination, the In(III) concentration at the initial reaction site drops intensively to the bottom level as a consequence. It is also noted that metallic In is more active than Sb in thermodynamics,70 and the remaining quickly reduced In can also reduce Sb(III) to Sb(0) from the reaction of Sb(III) + In(0) → Sb(0) + In(III) at the same me. As a result, besides the consumption of In for the initial combination of InSb, the reduction of Sb leads to the increased consumption of In. In such a case, In would decrease with Sb increase. At this time, InSb grows ahead from (InSb:Inx) with Sb, reduced hysteretically in a subsequent step, with the Sb(III) 16

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concentration dropping to the low point, and a new round of reactions and growth proceeds forward repeatedly at a new site from the initial site, as illustrated in Scheme S2 with brief discussion. In short, the self-catalytic growth of the InSb NWs with twinning superlattices results from an oscillating growth, triggered by the periodic fluctuation between reaction rates from the incongruous reductions of In and Sb sources as the reductant BTB is injected in the system on the basis on the above investigations. This process is very analogous to the one between the diffusion rate inside the catalytic droplet and the growth rate on the liquid-solid interface for the VLS growth of ZnS67 and GaP68 with periodic twinning superlattices, as seen in the Supporting Information with some more discussions based on control experiments (Figure S12–S14). Meanwhile, the oscillatory mass transport has been directly observed in the self-catalytic VLS growth of sapphire nanowires71 and the growth of Si NWs catalyzed by AuSi alloyed droplets due to a mismatch between catalyst size and wire diameter.72 Moreover, the low-energy configuration growth trend8–12 is also favorable for the solution growth of the InSb wires in high quality, which can be tuned under the optimized conditions based on the experimental investigations of the temperature (Figure S7, S8), the time (Figure S11), the mole ratios (Figure S9, S10, S12–S14) and the media (Figure S25–S27). In addition, the tendency of twinning superlattices in the InSb NWs to grow along the direction is due to the properties of InSb with four-coordinate structure, despite being weak in ionicity.51–53

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Figure 4. (a) Raman spectrum of the InSb NWs detected at room temperature, and (b) pump-induced change in terahertz electric field (ΔE/E) at different pump-probe delays. The photoexcitation pump fluence was 50 μJ cm−2. Red empty dots and blue line represent the experimental and monoexponential fits, respectively.

Figure 4(a) depicts a representative Raman spectrum for the InSb NWs typically obtained at 180 °C for 30 min. The intense and broad peak centered at 174.6 cm−1 is attributed to the transverse optical (TO) phonon mode for the NWs while the longitudinal optical (LO) phonon mode is weak, observed as a very small shoulder at 184.6 cm−1. Interestingly, both InSb TO and LO phonon frequencies are downshifted compared to the bulk counterpart.73 The downshift of the Raman phonon wavenumber is probably due to the twinning structures with relative small length of segments in the NWs, since similar phenomena have been reported for InAs/InSb heterostructured 18

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NWs, InAs, and Si NWs.74–76 Meanwhile, the downshift of Raman signal may also be due to a laser-induced heating effect based on the early investigations of InSb and Si at relatively high temperatures.77,78 The high intensity of the InSb TO phonon frequency is thought to be relevant to a polarized first-order Raman scattering that stems from the interplay of photon confinement.79 This spectral polarization confirms that the anisotropic InSb NWs are high quality. Meanwhile, the intense TO scattering may also suggest that there is a high carrier concentration in the near surface region of the InSb NWs,80 suggesting that they may be appropriate for highly sensitive optoelectronic applications. In addition, the low phonon frequencies at 113.7 and 138.9 cm−1 can be attributed to In2O3 resulting from the partial surface oxidation of the NWs exposed in air,81 although the oxide impurities cannot be detected by XRD (Figure 1a, S1). In addition, we measured the photoconductivity of the InSb NWs using optical pump-terahertz probe (OPTP) spectroscopy at room temperature. It is noted that the OPTP detection is an ultrafast probe of photoconductivity with ultrafast resolution, and is a noncontact method that could avoid many difficulties in making a device with Ohmic contacts to the nanoscale wires. As displayed in Figure 4b, the decays of terahertz electric field (ΔE/E) are presented as a function of time after photoexcitation. They can be fitted with a monoexponential function of ΔE/E = A exp(-τ/τc) to the decays, yielding a carrier lifetime (τc) of just 9.58 ps. This is the first measurements for InSb NWs with ultrafast carrier dynamics on a picosecond time scale. Generally, it is suggested that the InSb NWs are high quality. However, we also noted that the ultrafast conductivity dynamic for the InSb NWs is probably due to the carrier trapping at surface defects since similar phenomena have been detected in GaAs NWs.82 In the present work, the surface defects mainly result from the capping effect of the solvents used in the solution strategy that can be detected in the TEM images (Figure 1d, 2a). Hopefully this result did not hinder the photoconductive property of the wires. There is no 19

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doubt that the ultrafast responses of the InSb NWs in photoconductive measurements suggest that they have potential for technical applications in high-speed optoelectronic nanodevices.82–84 In conclusion, we have demonstrated, for the first time, a facile solution-based growth strategy for the synthesis of high-quality zincblende InSb NWs with twinning superlattices, employing In(acac)3 and Sb(Ph)3 as the precursors. The InSb NWs are grown along the {111} direction by the catalysis of the early formed indium droplets via the SLS mechanism in the route. Investigations reveal that reaction temperature, source and medium selection, in addition to precursor molar ratio, play important roles in regulating the purity, shapes and twinning superlattice of the InSb NWs. Investigations suggest the reduction competition of In and Sb sources with oscillatory mode influences results in the growth of the InSb NWs in twinning superlattices in the present solution synthetic route. The growth of the InSb NWs with twining superlattices would have profound potential for the fabrication of high-performance nanostructured electronic and optoelectronic devices.

 ASSOCIATED CONTENT Supporting Information Additional information includes experimental procedures, detailed analysis of XRD patterns, TEM, HRTEM images, EDX patterns and HAADF−EDX line scans of the as-formed nanostructures, XRD patterns, WAXS and SAXS detection and TEM images of controllably synthesized nanocrystals, additional TEM images and histogram data of the InSb nanowire products. This material is available free of charge via the Internet at http://pubs.acs.org.

 AUTHOR INFORMATION 20

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Corresponding Author *Tel.: +86−551−63600243. Fax: +86−551−63606266. E−mail: [email protected] Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (21571166, 51271173) and the National Basic Research Program of China (2012CB922001). We thank the staff at the beamlines BL14B and BL16B of Shanghai Synchrotron Radiation Facility (SSRF) for their supports. We also acknowledge Dr. Zhenlin Luo and Mr. Jiangtao Zhao for assistance with and discussion on SAXS/WAXS, and Dr. Qiuping Huang and Mr. Honglei Cai in Prof. Yalin Lu’s group for assistance with and discussion of OPTP.

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(73) Aoki, K.; Anastassakis, E.; Cardona, M. Phys. Rev. B 1984, 30, 681–687. (74) Hörmann, N. G.; Zardo, I.; Hertenberger, S.; Funk, S.; Bolte, S.; Döblinger, M.; Koblmüller, G.; Abstreiter, G. Phys. Rev. B 2011, 84, 155301. (75) Patra, A.; Panda, J. K.; Roy, A.; Gemmi, M.; David, J.; Ercolani, D.; Sorba, L. Appl. Phys. Lett. 2015, 107, 093103. (76) Adu, K. W.; Gutierrez, H. R.; Kim, U. J.; Sumanasekera, G. U.; Eklund, P. C. Nano Lett. 2005, 5, 409–414. (77) Balkanski, M.; Wallis, R. F.; Haro, E. Phys. Rev. B 1983, 28, 1928–1934. (78) Liarokapis, E.; Anastassakis, E. Phys. Rev. B 1984, 30, 2270–2272. (79) Wu, J.; Zhang, D. M.; Lu, Q. J.; Gutierrez, H. R.; Eklund, P. C. Phys. Rev. B 2010, 81, 165415. (80) Frost, F.; Lippold, G.; Schindler, A.; Bigl, F. J. Appl. Phys. 1999, 85, 8378–8385. (81) Berengue, O. M.; Rodrigues, A. D.; Dalmaschio, C. J.; Lanfredi, A. J. C.; Leite, E. R.; Chiquito, A. J., J. Phys. D: Appl. Phys. 2010, 43, 045401. (82) Parkinson, P.; L.-H., J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Johnston, M. B.; Herz, L. M. Nano Lett. 2007, 7, 2162–2165. (83) Smith, F. W.; Le, H. Q.; Diadiuk, V.; Hollis, M. A.; Calawa, A. R.; Gupta, S.; Frankel, M.; Dykaar, D. R.; Mourou, G. A.; Hsiang, T. Y. Appl. Phys. Lett. 1989, 54, 890–892. (84) Baig, S. A.; Boland, J. L.; Damry, D. A.; Tan, H. H.; Jagadish, C.; Joyce, H. J.; Johnston, M. B. Nano Lett. 2017, 17, 2603−2610.

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