The Growth of Ultralong ZnTe Micro/Nanostructures: The Influenceof Polarity and Twin Direction on the Morphogenesis of Nanobeltsand NanosheetsMuhammad Iqbal Bakti Utama,†,# Maria de la Mata,‡,# Qing Zhang,† Cesar Magen,§ Jordi Arbiol,*,‡,∥

and Qihua Xiong*,†,⊥

†Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University,Singapore 637371‡Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, E-08193 Bellaterra, CAT, Spain§Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA) - ARAID and Departamento de Fisica dela Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain∥Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, CAT, Spain⊥Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

*S Supporting Information

ABSTRACT: Although ZnTe nanobelts present an intriguingplatform to study various optical properties and phenomena insemiconductors, there was very limited study regarding thecrystalline structure and defects of ZnTe nanobelts. Here, wecorrelate the structural properties and features in the crystal ofZnTe nanobelts with the resulting as-synthesized morphology.Ultralong ZnTe nanobelts were synthesized to reach thesubcentimeter length scale. Two types of nanobelts wereidentified according to whether tapering was present anddiscerned on the basis of crystallinity and polarity of thestructure. We conclude that tapered sheetlike nanobelts have Te-terminated lateral facets that induced lateral growth, whereasuntapered nanobelts have facets that are nonpolar and nonreactive. Axial and transversal twins were also observed, where thepolarity was conserved across twinning boundaries.

1. INTRODUCTION

Nanobelts, or also referred to as “nanoribbons”, are quasi one-dimensional (1D) nanostructures with a high aspect ratio thathave an unconfined length and a rectangular-like cross-section,with a thickness being in the nanometer scale.1 Compared totheir bulk counterpart, nanostructures, such as nanobelts fromsemiconductor compounds, are relatively easier to produce ingood crystallinity while improving the tunability of the material(electrical, optical, mechanical, or chemical) properties of thecompound.2,3 Nanobelts have thus deservedly become a focalpoint of intensive research in the past decade owing to theirpotential applications in optoelectronics.4 For example, amongthe materials that can be synthesized into nanobelts, proof-of-concept applications from II−VI semiconductor nanobelts areabundant, such as for field emitters, solar cells, photodetectors,and devices to probe emergent nanoscale phenomena.5−10

Recent breakthroughs of laser cooling of semiconductors usingCdS nanobelts further promulgate the applicability of suchnanomorphology owing to their strong exciton−longitudinaloptical phonon coupling strength.11

Zinc telluride (ZnTe) is a II−VI semiconductor with a 2.26eV direct band gap at room temperature. Besides, as-grown

ZnTe nanobelts commonly possess p-type conductivity,12

which makes ZnTe attractive for optoelectronic devices in thegreen wavelength. We recently showed that ZnTe nanobeltshave excellent optical properties and are an interesting platformto study electron−phonon interactions.13 However, the themeof other works on ZnTe nanobelts is still limited to thesynthesis and electrical transport measurements.12,14−19 Detailson the morphogenesis of ZnTe nanobelts in relation to theircrystallinity, presence of planar defects, such as twins, andatomic polarity are not yet reported. We believe that theunderstanding of the structural aspects of ZnTe nanobelts iscritical to achieving reliable and reproducible device fabricationand applications based on ZnTe nanobelts. Hence, we reportherein the synthesis of ZnTe nanobelts and the character-izations of their crystallinity and polarity. We aim to correlatethe structural properties and features of the nanobelts with themorphology, in the context of the synthesis environmentduring the growth. Specifically, we will show herein that ZnTe

Received: March 6, 2013Revised: April 8, 2013Published: April 9, 2013

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nanobelts may contain axial and transversal twins. We will thendiscuss the influence of the twins and the atomic polarity on theemergence of two different shapes of nanobelts.

2. EXPERIMENTAL SECTIONThe synthesis of the nanobelts was conducted with a home-built vaportransport thermal evaporation setup using a single zone furnace(Lindberg/Blue M TF55035C-1), which we have reported to beeffective in the growth of nanostructures.20,21 The source material washigh-purity ZnTe powder (99.99%, Alfa Aesar), and the substrate is a⟨100⟩ p-Si chip with native oxide. The substrate was coated using athermal evaporator (Elite Engineers) with 6 nm thick gold (Au) filmto act as the catalyst. The source powder was put at the center of thefurnace, whereas the substrate was positioned in the downstreamregion. The carrier gas was 30 sccm Ar with 5% H2. The centraltemperature of the furnace (850 °C) and the pressure inside thereactor (50 Torr) were set and stabilized for 90 min. The as-grownsample was characterized by field emission scanning electronmicroscopy (FE-SEM, JEOL JSM-7001F), and X-ray powderdiffraction (XRD, Bruker D8 advanced diffractometer, Cu Kαradiation) in θ−θ geometry.The segment of the as-grown sample containing nanobelts was

ultrasonicated in isopropanol to separate the nanobelts from thesubstrate. The resulting suspension was drop-casted to a clean Si chipfor Raman spectroscopy and atomic force microscopy (AFM, ParkSystems NX10), and drop-casted to a lacey carbon grid (ElectronMicroscopy Sciences) for scanning and high-resolution transmissionelectron microscopy (STEM and HRTEM, respectively). The Ramanspectroscopy was conducted on an individual nanobelt at roomtemperature using a micro-Raman spectrometer (Horiba-JY T64000)in a backscattering geometry. The backscattered signal was collectedby a 100× objective and dispersed by a 1800 g/mm grating in a singlemode with a spectral resolution of ∼1 cm−1, and recorded by a liquid-nitrogen-cooled charge-coupled device detector. The nanobelts in thelacey carbon grid were observed by means of HRTEM (JEOL 2010Fwith a field emission gun operated at 200 kV) and aberration-correctedhigh-angle angular dark-field STEM (HAADF-STEM; FEI Titan 60−300 keV, operated at 300 kV).

3. RESULTS AND DISCUSSIONFigure 1 shows an overview of our synthesis result. The as-grown sample carried very long structures that are visible to thenaked eye (Figure 1a). The XRD pattern from the sample(Figure 1b) reveals intense peaks that can be assigned todiffraction from a ZnTe crystal in cubic zincblende (ZB) phaseand the Si substrate. We also observed that the sample isdivided into three segments with a discernible appearance, ashas been marked with three differently colored arrows in Figure1a. Interestingly, the three segments contained vastly differentmorphologies of nanostructures, as observed with SEM: Theleft region marked with a blue arrow, which is the downstreamregion with a lower local temperature during the synthesis, haskinked micro/nanowires (Figure 1c); the middle region with agreen arrow, which occupied a relatively higher local synthesistemperature, has randomly oriented long micro/nanowires(Figure 1d); and the right region marked with a red arrow,which is the high-temperature upstream region, has very longnanobelts (Figure 1e). Given that the location at which thedifferent structures are present is already distinct, it is thuspossible to isolate a specific structure that one desires by simplycutting the respective segment of the substrate (originally ∼3cm long). For example, one could cut the first 0.5 cm of theupstream region of the substrate to get nanobelts with anegligible quantity of wires or kinked wires. Alternatively, asmaller piece of substrate can be used such that only a specificstructure will be obtained, separate from other structures.

The trend in the dependence of the ZnTe nanostructuremorphology to the local substrate temperature is well-documented and in agreement with the previously publishedliterature.14,15,18 The most commonly accepted explanation ofsuch an observation relates the local growth temperature to theextent of the accompanying lateral broadening of nanobelts viathe vapor−solid (VS) mechanism.22 The high local temperaturepromotes 2D nucleation in the lateral facet of the structure withthe VS mechanism alongside the axial elongation from the VLSmechanism, such that nanobelts will be formed. On the otherhand, lateral growth at lower temperature is not as extensive,which promotes the growth of nanowires instead. Inaccordance to this conclusion, we have also reported elsewherethe growth of CdS nanobeltsgrown with similar methodswith the present ZnTe nanobeltswhere the lateral broad-ening of the structure has a noticeable positive correlation withthe local temperature of the substrate.23 Similarly, nanobeltsfrom other compounds, such as ZnS24 and Al2O3,

25 are alsoobserved to grow at a higher local growth temperature than thatfor nanowire growth.An inset in Figure 1e clarifies the morphological distinction

between nanobelt and nanowire: while nanobelts have asubmicrometer level thickness that is comparable to thediameter of micro/nanowires, the width of nanobelts couldreach tens of micrometers. Meanwhile, the structure of a wire iselucidated in greater detail with zoomed-in images in Figure 1f.At the tip of most micro/nanowires (and also nanobelts; seeFigures 3e and 4b), we observed a spherical particle that mightbe composed of the coalesced Au film from the substrate due tothe high temperature during the synthesis process; this result istypical of a catalyzed nanowire in accordance to vapor−liquid−solid (VLS) growth.26 On the other hand, nanowiresor evennanobeltswere not produced when no Au film was coated onthe substrate, indicating the catalytic function of the Au particle.

Figure 1. As-grown ZnTe sample. (a) Photograph of the sample withvisible ultralong nanostructures next to a centimeter-scale ruler. Thewhite arrow denotes the direction of the increase in local temperatureduring growth. (b) XRD pattern of the sample in the semilogarithmicscale. (c−e) SEM images of nanostructures found on the samplecorresponding to different positions of the substate, as shown indifferently colored arrows in (a): (c, blue) kinked nanowires; (d,green) ultralong nanowires; (e, red) ultralong nanobelts. Inset in (e):edge-on view of a nanobelt. (f) Zoomed-in SEM images of anultralong wire, showing the tip and midregion of the wire.

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Whereas the body of the wire close to the Au particle appearsto be cylindrical, the region further away from the particleexhibited a faceted morphology with an aperiodic segmentlength. Radial growth is also suggestive, as the diameter farfrom the Au particle at the tip is much larger than that near thetip. Such “microfaceted” morphology is very well-known inmultitwinned wires with the ZB phase grown in the ⟨111⟩direction:27 microfacets in nanowires are caused by theemergence of segments in the shape of truncated octahedronwith {111} facets;28 a twin caused a 180° change in theorientation of microfacets. Thus, nucleation of multiple twinplanar defects caused alternating microfaceted segments, asseen in Figure 1f. Readers are referred to our previous work onthe structure of ZnTe nanorods29 and the other relevantliterature for more in-depth discussions regarding twinningphenomena in nanowires.30−34 Overall, the nanostructures inall segments of the as-grown sample can be grown to reach thecentimeter length scale, which we believe could still belengthened if the growth time was increased with an adequateamount of source powder. The high-yield synthesis of ultralongZnTe 1D nanostructures presents an opportunity for a largeupscaled production of the material, which will be suitable forindustrial purposes.Inspection of as-transferred single nanobelts (Figure 2a)

reveals that the ZnTe nanobelts could still be categorized into

two groups: (1) nanobelts with the classically expectedmorphology of a 1D nanostructure with parallel sides and (2)nanobelts with a sheetlike morphology that has pronouncedtapering and 2D growth. The strongly tapered nanobelts will becalled “nanosheets” hereafter. We then conducted intensivecharacterizations on the structural and crystalline properties foreach of these two groups of ZnTe nanobelts, which are thestructures-of-interest in this work, to provide an understandingof the difference and morphogenesis of the two morphologies.In terms of the optical properties, we characterized both

nanobelts and nanosheets with the resonant Raman scattering(RRS) spectroscopy. RRS was shown to be effective in probingthe electronic structure and electron−phonon coupling inZnTe nanorods, where strong coupling was observed.13 Undera 514 nm laser excitation (Figure 2b), the most noticeabledifference between the RRS spectrum of nanobelts andnanosheets is in terms of their photoluminescence (PL)intensity, in which the PL of nanobelts is stronger. However,the PL in both spectra had a very similar line shape and waspeaked at the identical position (546 nm ≈ 2.27 eV; cf. Figure

3d). Moreover, the overall features of spectra from bothmorphologies are identical. Both spectra show seven peaks

located at 206, 408, 615, 823, 1029, 1223, and 1440 cm−1

(albeit weakly) that arein averagespaced by (206 ± 7)cm−1, which is the phonon frequency of the longitudinal optical(LO) phonon of ZB ZnTe. Thus, we assign those seven peaksto the nth order (n = 1−7) LO Raman scattering, respectively.We believe that the ability to observe the high-frequency,seventh order LO Raman scattering is partly enabled due to thehigh quality of our samples; no other groups have reported thehigh-order phonon observation in ZnTe nanobelts yet. Detaileddiscussions regarding the implication of the multiphononRaman spectroscopy in ZnTe nanobelts and the time-resolvedPL characterization of the structure have been publishedelsewhere.13 Meanwhile, the peak at 125 cm−1 can be assignedto the mode from crystalline Te phase, similar to that observedin ZnTe and CdTe nanorods,35,36 and the peak at 520 cm−1

originates from the Si substrate.Figure 3a shows a representative appearance of an ultralong

ZnTe nanobelt with a length of ∼600 μm after beingtransferred to a Si chip. The ultrasonication process to removethe nanobelt from the substrate may also break the nanobelt,shortening the nanobelt from its as-grown length. AFMimaging of the nanobelts confirmed the rectangular crosssection of the nanobelt (Figure 3b,c). The lateral encroachmentin the nanobelt served as the distinction of a nanobelt from ananowire (Figure 1f). The cross-sectional shape of the nanobeltthroughout its length was fairly uniform, where the thicknessremained in the same order (cf. Figure 3b,c, which wereacquired a few hundred micrometers apart), unlike nanowires,

Figure 2. (a) Optical micrographs of nanobelts and nanosheets. (b)RRS spectra of nanobelts and nanosheets with 514 nm excitation.

Figure 3. Characterizations of nanobelts. (a) Mosaic opticalmicrograph of an ultralong nanobelt. (b, c) AFM images of the leftand right part of the nanobelt in (a), respectively. The height profilesare superimposed in the respective images. (d) RRS spectra with 532nm excitation from three different locations of the nanobelt: left,middle, and right, as illustrated by differently colored circles in (a). (e)STEM image of a nanobelt. (f) EDX compositional mapping of Zn,Te, and Au edges near the nanobelt tip as marked with a yellow squarein (e). (g, h) HRTEM images of a nanobelt, revealing the presence ofnearly periodic twins. White arrows in (e, g, h) denote the growthdirection of the nanobelt. (i) FFT from the nanobelt in (h).

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which experienced a conspicuous radial growth. The RRSspectra taken with a 514 nm laser excitation from threedifferent sites at the nanobelt (dashed circles in Figure 3a)exhibited nearly identical features. These results suggest thatthe properties of an ultralong nanobelt are consistent over alarge length scale.The height profiles in Figure 3b,c show that the thickness of

a long nanobelt that we observe is ∼300 nm. We remark thatthere is a significant number of similar structures within oursamples that have dimensions within the micrometer-sizedlevel, for which categorization as “microstructures” will be moreappropriate (for instance, the wire in Figure 1f). However,given that the catalyst used in our synthesis is a Au film, thecoalescence of the Au film at high temperature during thesynthesis might result in a broader size distribution of Aucatalyst particles. Thus, variation of sizes in nanowires/nanobelts is to be expected as the broad distribution of thecoalesced Au film dictates the diameter/thickness of nano-wires/nanobelts in VLS growth. Correspondingly, there arestructures within our sample with a thickness ≤ 100 nm, whichcan then be appropriately categorized as a “nanostructure” (seeFigure S1 in the Supporting Information).We used HAADF-STEM to view the general morphology of

a ZnTe nanobelt (Figure 3e). Similar to the instance of thenanowire shown in Figure 1f, we also observed a sphericalparticle at one end of the nanobelt (Figure 3e). Compositionalmapping with energy-dispersive X-ray (EDX) was conducted insitu with the STEM to analyze the composition of the particle(Figure 3f). The spherical particle contained concentrated Auatoms and some Zn and Te constituents as well, with the Zncomposition being slightly higher than Te. In comparison,almost no Au was present on the nanobelt, whereas Zn and Tewere distributed evenly (in a 1:1 ratio) within the nanobelt.This result is in agreement with the VLS growth mechanism,where Au particles, which behaved similarly to a “catalyst” ofthe crystal growth, served as the preferential adsorption sites ofvaporized precursor material.37 In the VLS model, the elevatedtemperature coalesces the Au film into liquid droplets andpromotes alloying between the continuously adsorbedprecursor vaporsZnTe in our case. Once the alloy issupersaturated, nucleation occurs at a single site and continuedwith 1D crystal growth of nanostructures.Although twin-free monocrystalline nanobelts were also

observed, we found that the nanobelt morphology mightcontain nearly periodical twin planes that are perpendicular tothe axis of the nanobelt growth direction (Figure 3g). Becauseof the twins, the nanobelt is divided into segments withalternating fringe contrast; we have placed pseudocolored twindomains side-by-side with the HRTEM image in Figure 3h forease of visualization. The transversal twins in the nanobelt arespontaneously formed, as we did not adjust the growthenvironment manually during the growth process. Althoughtransveral twins are common in nanobelts from othercompounds,38 reports of twinned nanostructures in ZnTewere limited only to nanowires and tripods;29,39 virtually allother works on ZnTe nanobelts showed a monocrystallinestructure without twins.12,14−19

The fast-Fourier transform (FFT) pattern from the nanobelt(Figure 3i) showed that the ZnTe nanobelt is of ZB phase,similar to the result in other works.12,14−19 Unlike our results inrandomly twinned nanorods,29 we did not identify any patternof the hexagonal wurtzite (WZ) phase in nanobelts nor therewas any extended WZ phase. The absence of WZ is reasonable

since ZnTe is more thermodynamically stable in the ZB phaserather than the WZ phase.40 Nanobelts from ZnTe are thusrelatively unique since nanobelts from other II−VI, III−V, andoxide compounds are either polytypic or growing in the WZphase completely. Instead, we observed the coexistence of twoZB patterns, namely, in [1 1 0]ZB and [110]ZB zone axes. Weremark that the observation of the sample in the [110] zoneaxis of the crystal is a reliable and unambiguous method toverify that the crystal is indeed of ZB phase instead of WZphase. Should the crystal be in WZ phase, one would observeno [110] zone axis and instead the crystal will exhibit a [112 0]zone axis behavior at the same direction. The primary reason inobserving the sample in the [110] or [112 0] zone axis isbecause the atomic stacking arrangement between ZB and WZphases (ABCABC and ABABAB, respectively) can beconclusively distinguished at those zone axes. Given thedifference in the stacking arrangement in these zone axes,there will be an associated difference in the diffraction patternproduced by the samples in those zone axes. Indeed, thepatterns match well with a ZB pattern in the [110] zone axiswith twinning features instead of a WZ pattern in the [112 0]zone axis.In the indexing, we also use the finding in our previous

work,21,29 where we established that Te2−-terminated (anion)planes of ZB ZnTe (i.e., 1 11 , 1 11, 111, 111 ) are more reactivethan are Zn2+-terminated (cation) planes (i.e., 111, 111 , 111 ,1 1 1). Thus, ZnTe nanostructures, such as nanorods andnanotripods, grown along the polar direction will select the Te-directed −⟨111⟩, which has a higher growth rate than Zn-directed +⟨111⟩. We, therefore, conclude that the presentnanobelt is growing in the [11 1] direction with {111} twinplanes.The annotated STEM in Figure 4a summarizes our

characterizations of the nanosheet-like morphology of ZnTeas we shall substantiate subsequently. In contrast to thenanobelt, the tapering of the nanosheet was evident. We alsosee that the tip of the nanosheet is also occupied by a particle(Figure 4b). Compositional mapping with EDX showed thatthe particle is mainly composed of Au, suggesting a Au-catalyzed VLS growth mechanism. We also noticed smallresidual traces of Zn that were mainly localized on the tipsurface. Meanwhile, the nanosheet is composed of Zn and Te ina 1:1 ratio.The FFT from the HRTEM image of the nanosheet showed

that the nanosheet contained the signature of twin features,showing two sets of ZB diffraction spots in [11 0]ZB and [110]ZBzone axes each (Figure 4c). However, dissimilar to the case inthe nanobelt, the growth direction of the nanosheet is along thenonpolar [11 2] direction, with the lateral facet on the right-hand side in Figure 4a being constructed from a {111} plane.We confirmed the presence of twins with an atomic resolutionHAADF-STEM study of the nanosheet, which revealed thepresence of axial {111} twin boundaries (Figure 4d). The twinboundaries originated at the stepped Au particle−nanobeltinterface and propagated through the entire length of thenanosheet. Here, the diffraction pattern from the particle at thetip (Figure 4d, inset) has been indexed to Au along the [110]zone axis, which is in line with the [110]ZB zone axis of theZnTe nanosheet. We noted that the size of the Au particle(∼200 nm) was similar to that found at the tip of the nanobelt(Figure 3f), which is reasonable since both structures are foundat a similar local temperature regime during growth within thesame substrate to allow coalescence of the Au film into isolated

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particles of similar size during the annealing. Hence, we believethat the tapering of the nanosheet could not be attributed tothe shrinkage of the Au particle catalyst during the growth.The Z-contrast imaging of STEM in HAADF mode made

HAADF-STEM sensitive to sample composition, where theobserved intensity from the sample image is approximatelyproportional to Z2. Hence, given that Te has a reasonablyhigher atomic number than Zn (ZTe = 52; ZZn = 30), we coulddistinguish the constituent atomic dumbbells of ZnTe viaaberration-corrected HAADF-STEM, with Te atoms appearingbrighter than Zn atoms. HAADF-STEM also allows determi-nation of the stacking arrangement of the dumbbells and theorientation of the dumbbells without phase interference. Thestacking arrangement of dumbbells at a region away from thetwin boundaries exhibits the ABCABC stacking that is adefining characteristic of the ZB phase. As was the case innanorods,41 the presence of twin boundaries along the {111}direction changes the dumbbell orientation. However, thepolarity of the dumbbells is still conserved across the twinboundaries. The ZnTe nanosheet also adhered to similarcharacteristics; details of the dumbbell orientation in thevicinity of twin boundaries are shown in the zoomed-in imagein Figure 4e. The atomic resolution magnified images in Figure4e have been improved applying a probe deconvolution byusing the STEM-CELL Software.42 The ZnTe dumbbells canbe resolved well, with the heavier Te atoms appearing brighterthan Zn atoms.Consequently, as seen in Figure 4a, the right-hand-side

lateral facet that has a steady straight shape is a Zn-terminated

surface, which can be assigned to (111) as is implied from theFFT indexing in Figure 4c. Meanwhile, the other lateral facetwas inclined from the growth direction of the nanosheet,creating the tapered structure. The inclination angle was alsochanging along the length of the nanosheet, while remainingpositive, which indicates that the lateral facet might have beenan active growth front. As the polarity of ZnTe is conservedacross the twin boundaries, we can then safely assign that theinclined lateral facet has a Te polarization.We clarify the influence of the polarity of the crystals to

explain how nanobelts and nanosheets have a distinctmorphology despite the fact that the axial growth in bothstructures is initiated by a similar VLS mechanism. To do so, weinvoke the growth model as discussed for tapered ZnS nanobeltby Hao et al.32 to explain the presence of nanosheets. Althoughthe ZnS nanobelt was partially in the WZ phase with the lateralexpansion being in the ZB phase without any clarification onwhether axial twinning was present, the nanobelt structureshared a similarity to our ZnTe nanosheet in which the lateralfacets are polar surfaces. In applying the growth model to ourcase, we use our previous result in which the Te-terminatedcation surface was assumed to be chemically and catalyticallymore active than that of the Zn-terminated surface.29 Underhigh supersaturation, the Te-terminated lateral facet could serveas a preferential diffusion direction of arriving adatoms otherthan the Au catalyst particle, whereas the Zn-terminated surfaceremained less reactive. Thus, the Te-terminated lateral facet inZnTe becomes a “sink” of adatoms and becomes an active frontfor crystal growth that is concurrent with the axial elongationgrowth of the nanobelt. The lateral encroachment of aparticular crystal plane closer to the particle is narrower thanthat further from the particle as a shorter time has elapsed eversince the plane is formed, resulting in a tapered structure. Here,we see that our assumption of a chemically active Te-terminated surface is in agreement with the structural datafrom STEM (Figure 4a,d,e), which shows that the lateralfacetthat is, Te-polarizedwas the facet experiencing lateralexpansion. Interestingly, our results demonstrate that ZnTe is aunique compound since nanobelts with polar lateral facets fromother II−VI compounds (e.g., CdO, CdS, CdSe, ZnS, ZnSe)would have the cation-terminated surface that is beingchemically reactive,4 which would give rise to a sawlikestructure at the cation-terminated lateral facet of the nanobelt.43

On the other hand, we observe that the influence of twindirections in the morphogenesis of the structures is rathersubtle and nonobvious, yet significant, since the ZnTenanobelts and nanosheets contain an appreciable number oftwins. As shown in Figures 3g,h and 4d, the twin planes in theZnTe is in the {111} planes, which are exactly parallel to (fornanobelts) or perpendicular to (for nanosheets) the growthplane. Moreover, all the twins that we observed (e.g., in Figure4e) are of the orthotwin type, since the polarity of the crystal isconserved across the twin. Therefore, the direction of twins inthe nanobelts and nanosheets is such that it is suitable topreserve the polar/nonpolarity of the lateral facets, which, inturn, will affect whether or not the structure is subjected tolateral growth (i.e., whether a nanobelt or nanosheet is beingformed). Consider the case where there exists at least a twinplane that is located on a plane that is at another arbitrary anglethat is neither perpendicular nor parallel to the growth plane.Such twin directions may change or reorient the growthdirection of the nanobelts so as to create a kinked structure.44,

Figure 4. Characterizations of a nanosheet. (a) HAADF-STEM image.(b) Zoomed-in STEM image close to the gold-capped tip. Inset: EDXcompositional mapping. (c) FFT of the nanosheet. (d) High-resolution HAADF-STEM image at the region in (b) indicated withan orange square. Yellow dashed lines mark the location of twinboundaries. Inset: FFT of gold particle. (e) High-magnificationHAADF-STEM showing individual dumbbells close to the twinboundaries (yellow dashed line) shown in blue and purple squares in(d). The images are superimposed with balls representing Zn and Teatoms to clarify the dumbbell orientations around the twins.

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With such an unsuitable twin direction, it will be impossible toobtain an ultralong straight nanobelt as we show in our article.

4. CONCLUSIONSWe have shown that ZnTe micro- and nanostructures can besynthesized in large quantities, with the length of the structuresreaching the milli- and centimeter scales. We were able todistinguish two types of nanobelts: (1) nanobelts with parallellateral facets and (2) tapered nanosheets, according to theirstructure and crystallinity. The two types of nanobelts haddifferent growth directions and twin directions (w.r.t. thegrowth plane) that result in the different facet polaritycharacteristics, which we correlate with the lateral growth thatis responsible for the tapering in nanosheets. We believe thatour characterization methodologies and line of reasoning mayalso be extended to explain the morphological phenomenon inother nanobelt materials.We also believe that the possibility to tune the crystalline and

morphological characteristics of ZnTe nanobelts may havepotential applications to adjust the electronic and optoelec-tronic properties of the structure. For example, we may use thefact that a bilayer of the WZ phase is created at every twinboundary of the ZB crystal.21,45 The nearly periodic occurrenceof twins in the ZnTe nanobelts can be considered to createperiodic ZB−WZ polytypism in the crystal. Interestingly,theoretical prediction and experimental works on ZnTe showthat the ZB and the less-stable WZ phases have dissimilaroptoelectronic properties (e.g., band gap, carrier effectivemasses, and refractive index).46,47 By rationally adjusting theemergence and direction of the ZB−bilayer WZ heterojunctionthat is formed with the creation of twins, it is possible toengineer the band structure of the nanostructure for thecreation of advanced electronic and optoelectronic structures,such as quantum wells or single electron transistorstructures.45,48

■ ASSOCIATED CONTENT*S Supporting InformationAdditional AFM images and height profiles of ZnTe nanobelts.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: arbiol@icrea.cat (J.A.), Qihua@ntu.edu.sg (Q.X.).Author Contributions#These authors contributed equally to this paper.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSQ.X. acknowledges the strong support from Singapore NationalResearch Foundation via a fellowship grant (NRF-RF2009-06)and Ministry of Education via two Tier2 grants (MOE2011-T2-2-051 and MOE2012-T2-2-086). This research is alsosupported, in part, by the start-up grant (M58110061) andthe New Initiative Fund (M58110100) from NanyangTechnological University. J.A. acknowledges the funding fromthe Spanish MICINN projects MAT2010-15138 (COPEON)and CSD2009-00013 (IMAGINE) and Generalitat de Cata-lunya (2009 SGR 770 and NanoAraCat). M.d.l.M. thanks CSICJAE Pre-Doc program. The authors thank the TEM facilities at

the Universitat de Barcelona and INA-LMA at the University ofZaragoza.

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