Polarity Assignment in ZnTe, GaAs, ZnO, and GaN-AlN Nanowiresfrom Direct Dumbbell AnalysisMaria de la Mata,† Cesar Magen,‡,§ Jaume Gazquez,† Muhammad Iqbal Bakti Utama,∥ Martin Heiss,⊥

Sergei Lopatin,# Florian Furtmayr,&,∇ Carlos J. Fernandez-Rojas,†,○ Bo Peng,∥ Joan Ramon Morante,■

Riccardo Rurali,† Martin Eickhoff,& Anna Fontcuberta i Morral,⊥ Qihua Xiong,∥,★ and Jordi Arbiol*,†,◆

†Institut de Ciencia de Materials de Barcelona, ICMAB-CSIC, E-08193 Bellaterra, CAT, Spain‡Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragon-ARAID, Universidad de Zaragoza, 50018 Zaragoza,Spain§Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain∥Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University,Singapore 637371⊥Laboratoire des Materiaux Semiconducteurs, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland#FEI Company, Achtseweg Noord 5, 5600 KA Eindhoven, The Netherlands&I. Physikalisches Institut, Justus-Liebig-Universitat Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany∇Walter Schottky Institut, Technische Universitat Munchen, Am Coulombwall 4, DE-85748 Garching, Germany○Grupo de Física de la Materia Condensada, Departamento de Física, Facultad de Ciencias, Universidad de los Andes, La Hechicera,Merida 5101, Venezuela■Catalonia Institute for Energy Research, IREC. 08930 Sant Adria del Besos, Spain and Department d’Electronica, Universitat deBarcelona, 08028 barcelona, Spain★Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798◆Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, CAT, Spain

*S Supporting Information

ABSTRACT: Aberration corrected scanning transmission electron microscopy (STEM)with high angle annular dark field (HAADF) imaging and the newly developed annularbright field (ABF) imaging are used to define a new guideline for the polarity determinationof semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i)when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case whereone of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoreticalfundaments of these procedures allow us to overcome the main challenge in theidentification of dumbbell polarity. It resides in the separation and identification of theconstituent atoms in the dumbbells. The proposed experimental via opens new routes forthe fine characterization of nanostructures, e.g., in electronic and optoelectronic fields,where the polarity is crucial for the understanding of their physical properties (optical andelectronic) as well as their growth mechanisms.

KEYWORDS: Dumbbell, polarity, annular bright field, ABF, high angle annular dark field, HAADF,scanning transmission electron microscopy, STEM, semiconductor nanowires

The existence of internal electric fields in ionic crystals has astraightforward impact on many physical properties that

determine the electronic structure, the local charge carrierdistribution and the related electromagnetic features, amongothers.1 The polarity in compound semiconductors arises fromthe partial ionicity of their chemical bonds. The charge transferbetween the elements of the compound results in the formationof atomic pairs of opposite partial charge (ionicity), which arecommonly referred to as “dumbbells”. The oppositely signedcharge of the atoms constituting the dumbbells implies the

existence of this internal electric field in the crystal.1 Innanoscale structures such as nanowires (NWs), it turns out thatonly a particular polarity drives the unidirectional growth.2−5

Indeed, as it has been demonstrated recently, it influences thegrowth mechanism, drives the final orientation of thenanowires2,6 or can even lead to the formation of newnanostructures such as tripods.7 Polarity driven growth

Received: March 1, 2012

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mechanisms can even explain the formation of kinking in somesemiconductor heterostructures.8 Nevertheless, in spite of thisadvancement in the role played by the polarity in thenanostructures growth mechanisms, there is also an importantissue regarding the conservation of polarity along thenanostructure, and particularly through the twin boundariesthat needs to be addressed. Twins correspond to 180° rotationsof the structure along the [111] or [0001] growth axes in zinc-blende (ZB) or wurtzite (WZ) structure, respectively.9−11

Orthotwins correspond to twin defects where the polarity isconserved across the boundary, while in paratwins it is not.2,12

It is not yet clear what happens at the nanostructure level dueto the intrinsic difficulties to determine it.Reliable methods for polarity determination of compound

semiconductor NWs at the atomic scale are still scarce,although several approaches have been reported, lacking spatialresolution. The simplest method, which is not sensitive downto the atomic scale, is the homoepitaxy of the NWs on asubstrate. Here, one assumes that the polarity is conserved atthe interface and that the orientation of the nanowirecorresponds with that defined for the substrate.13−15 Inaddition to the nonuniversality of this method because itcannot be applied for the growth on nonpolar or amorphoussubstrates, it also opens the question of polarity conservation atthe interface and also in the formation of inversion boundariesand defects during the growth.16,17 Another commonly usedtechnique is the convergent beam electron diffraction (CBED).Here, the CBED patterns are compared with simulated imagesof different polarity.18−21 Although this is an establishedmethodology, especially in thin films, it has several drawbacksthat limit its use in the case of nanostructures and might causemisleading results: first, due to the large volume needed foraccurate diffraction, samples with extremely small diametercannot be easily investigated. Second, it has a low spatialresolution, which as a consequence does not allow thevisualization of polarity changes at an interface, such as at anantiphase boundary (APB). Finally, the technique becomeshighly challenging in cases where the NW exhibits a highdensity of twins or even twins occurring in the three-dimensional space.2 Another technique that offers thepossibility of obtaining compositional information at theatomic scale is the scanning tunneling microscopy (STM).22

STM is also a constrained technique which cannot be appliedto all structures. Nanowires can be investigated in twoconfigurations only: (i) by directly positioning them on aconductive substrate23−25 or (ii) by embedding them in amatrix and cleaving the substrate in situ in the microscope.While the second method is limited to the cleavage planes ofthe sample, the first method requires the removal of the firstsurface monolayers in order to leave a clean surface which canoften damage the structure.23,24 In addition, the spatialresolution is often not good enough to distinguish bothatoms in the dumbbell, which are typically separated by only0.11−0.15 nm (see Table 1). Furthermore, the visualization oflight elements (e.g., O and N), which are main components inmany compound semiconductors like metal oxides or nitrides isnot possible.The ability to determine the polarity across hexagonal/cubic

(2H/3C) faulted structures at atomic resolution was firstreported in GaN by means of HRTEM.27 Later on, the polarityof ZnO from direct structural images was also reported.28

Dumbbell images from single atomic layers of BN with Zdifferences as small as 2 were also reported and interpreted

elsewhere.29 Also by means of HRTEM, many physicalproperties such as a polar ordering in GeTe are nowadaysdirectly extracted from dumbbell images of nanostructures.30

However, although HRTEM has been demonstrated to be apowerful tool for achieving the top spatial resolutions, it hasbeen always applied to nanostructured systems with lowthicknesses (few nanometers). In the precedent cases, onlyatomically flat sample wedges, thin nanoparticles or exfoliatedultrathin layers could be analyzed, sometimes requiring anadditional computer simulation process to recognize thepatterns or either reconstruct the images.

Table 1. Atomic Distances between Constituent Atoms ofDumbbell Pairs along the [110]ZB or [11-20]WZ Projectionsfor ZnTe (ZB), GaAs (ZB), ZnO (WZ), GaN (WZ), and AlN(WZ), where ZB Refers to the Zinc-Blende Cubic Structureand WZ to the Wurtzite Hexagonal Structure26

material ZnTe GaAs ZnO GaN AlN

atomic distance betweendumbbell elements (nm)

0.152 0.141 0.115 0.109 0.109

Other techniques with a high spatial resolution and achemical contrast (sensitivity) include spectrum imaging (SI)by electron energy loss spectroscopy (EELS)31−33 and energydispersive X-ray spectroscopy (EDX),34,35 both performed in ascanning transmission electron microscope (STEM). Whileatomic resolution in EELS was already accomplished in the 90s,and with more chemical accuracy during the past decade,31,40 inthe case of EDX it has just recently been demonstrated.30 Thedrawback here is that it necessitates a long acquisition time andintense electron beams that often result in the degradation ofthe material as well as a long postobservation data processing.All of these difficulties and limitations have contributed to

the fact that up to now, reliable methods are not yet operativeand polarity data with atomic resolution for many samples isnot available (e.g., raw nanowires).In contrast, as we report in this paper, there is a better

approach for determining the polarity in semiconductornanowires; the aberration corrected scanning transmissionelectron microscopy (STEM) which can reach direct imaging atsub-Angstrom resolution.37 This is an advantageous approachwith respect to the above-mentioned requirements, additionallybenefiting from straightforward sample preparation in the caseof NWs, which are mechanically removed from the substrate,transferred to a holey or lacey carbon grid38 and placed in themicroscope (no need of extra sample preparation to achieveatomically flat wedges). We distinguish between three differentcases of compound semiconductors, according to the atomicnumber (Z) of the dumbbell constituents, for which thetechnique has to be applied in a different manner: i) high-medium Z (such as in ZnTe); ii) nearly identical Z (GaAs);and iii) low Z (GaN and ZnO). All images presented here wereobtained in STEM mode, although they were acquired usingdifferent methodologies. For the first two systems we providehigh angle annular dark field (HAADF) images, while for thelast two, involving lighter elements, we present a novelmethodology based on annular bright field (ABF)39 STEMimaging for the determination of the local polarity with atomicresolution. Finally, we will present theoretical calculations ofthe conductivity in the ballistic regime, showing that the changeof polarity in a twin perpendicular to the growth axis in a GaAsNW can dramatically decrease the carrier transport within the

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nanowire, due to scattering at the polar twin boundary. Thiswork opens the route for the investigation of polarity incompound semiconductors in a reliable and rapid manner,thereby enabling a direct correlation with the functionalproperties and enhanced understanding of the related growthmechanisms.

■ HIGH-MEDIUM ATOMIC NUMBER

Figure 1 presents polarity imaging in ZnTe NWs using twodifferent STEM methods. The first one corresponds to HAADFor Z-contrast imaging. As shown in the inset of Figure.1A, theimages are obtained by scanning the electron beam across thesample and collecting the electrons scattered elastically at highangles, mainly due to interactions with phonons (thermaldiffuse scattering or TDS), resulting in incoherent imaging withchemical contrast.40,41 The resulting images are intuitivelyinterpretable atomic resolution images where the observedcontrast scales with the atomic number (Z) of the atomiccolumns.42,43 In this way, it is possible to resolve atomiccolumns within the dumbbell and to identify the differentelements as a consequence of the Z dependence (as it is thecase for ZnTe). Nevertheless, the Z contrast makes the lightestelements almost undetectable when combined with heavieratoms.

The second method is ABF STEM, which has recently beenused successfully for the imaging of the lightest elements suchas hydrogen.44 In this case, the required set up consists of abright field detector in which the central area is covered with abeam stopper or, equivalently, an annular detector that collectsthe direct beam diffraction disk (top right inset in Figure 1C).45

With this setup, the image is formed from the wave interferenceof the elastically scattered electrons, with negligible contribu-tion from TDS (particularly for light elements).36 The resultingphase image allows the visualization of light and heavy atomiccolumns46 which will be of special interest in the case of O andN atomic columns in ZnO and GaN/AlN compounds (see theSupporting Information for more details on the experimentalsetup and properties of both techniques, S1 and S2).We first demonstrate both techniques for the case of ZnTe

NWs, composed by heavy elements with clearly differentiatedZ, an element of high atomic number (Z > 50) and anotherelement with midvalued atomic number (50 > Z > 15), i.e., theso-called “high-medium atomic number” compounds (TableS1). The ZnTe NWs were obtained by the vapor transporttechnique,47 with the layered-structured muscovite mica as thesubstrate.48,49 For this case, both HAADF and ABF approachesare valid to visualize the sites of individual atoms within adumbbell, as shown in Figure 1A,C. In Figure 1A, the dumbbell

Figure 1. Polarity determination in ZnTe NW. All yellow arrows present in the image are pointing along the growth direction of the NW. (A)HAADF STEM detail of the dumbbell pairs in a selected region of the ZnTe NW visualized on the left upper inset. The bottom left insetcorresponds to a magnified detail of a dumbbell pairs separated by a twin boundary as a raw data (left) and after temperature coloring (right). On theraw image details, we have superimposed the Zn and Te atoms with different colors to indicate the dumbbell rotation: blue and green for Zn and Te,respectively, when facing the left side (oriented along the [110] zone axis), and red and orange for Zn and Te, respectively, when facing the right side([−1−10] zone axis) after a twin boundary. A scheme of the HAADF-STEM setup is shown on the top right inset. (B) HAADF intensity profileobtained across the dumbbell Zn−Te pair on the red squared region in A. (C) ABF STEM detail of the dumbbell pairs in the same region as in A. Ascheme of the ABF-STEM setup is shown on the top right inset. (D) ABF intensity profile obtained across the dumbbell Zn−Te pair on the greensquared region in C.

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positions show bright contrast, with the most intense spotscorresponding to the heaviest element (Z-contrast images). Onthe other hand, ABF-STEM images (Figure 1C) offer aninversion of contrast with the atomic columns shown as darkspots, where the darkest correspond to the heaviest elements.These two methods are not only capable of visualizing thepolarity of the NWs but also allow unambiguous differentiationof the different polytypes (dependence on the atomic planestacking order, Figure S3)50 occurring along the NW growthdirection. It should be noted that the presence of a twin in theZB structure creates a single monolayer of WZ at the boundary.Thus, many consecutive twins may lead to the formation of WZor some other polytypes if the periodicity is lower. In the pastfew years, a great debate has been generated on the search fornew optoelectronic properties of the NWs when intermixing ofpolytypes could be synthesized to create supelattices.16,51−54

Unambiguous determination of the polytypes formed in theNW growth should conclude the debate on the existence ofhexagonal silicon (type IV) in NWs.10,11,34,55,56 It is importantto remark here that, due to the extremely small depth of fieldimplied in aberration corrected HAADF-STEM images (whichis not the case in HRTEM images), the atomically resolved areaonly includes a small thickness portion of the NW of a fewnanometers. As a consequence, the image is not interfered byother planar defects occurring above or below the focal plane,57

thus unequivocally resolving the atomic structure. Taking intoaccount that the brightest (darkest) contrast spots correspondto the heaviest atoms (Te) in HAADF-STEM (ABF-STEM), itis possible to directly visualize the polarity on the acquiredimages. In order to unambiguously differentiate the atomicpositions within the dumbbell, intensity line profiles have beenacquired along the dumbbell positions (Figure 1B,D).According to our present results, we can point out that thepolarity on ZnTe NWs is always such that Te faces toward thenanowire tip (Te-polarized, see in Table 2 for the complete listof polarities observed for every studied material). Furthermore,despite the multiple twin defects leading to the differentpolytypic structures, the polarity is maintained across the twinboundaries, thus having only orthotwins instead of the inversedpolarity paratwins.2,12,58 The distance between the Zn−Tedumbbell pairs measured by using both techniques (0.15 nm) isin good agreement with the atomic distance expected fromtheoretical calculation (Table 1).59

■ MEDIUM AND SIMILAR ATOMIC NUMBERAn example of a compound semiconductor formed by elementsof medium and similar atomic number is GaAs. Here, wepresent the first polarity study on GaAs NWs exhibiting amodulated polytypic structure, grown by the gallium-assistedtechnique with molecular beam epitaxy. The growth conditions

Figure 2. (A) TEM general view of one of the studied GaAs NWs with modulated polytypic structure. (B) Atomic resolution aberration correctedHAADF-STEM detail of the Ga−As dumbbell pairs in a selected region of the visualized GaAs NW. (C) Raw HAADF-STEM magnified detail of thesquared region in B. (D) Temperature colored detail of the same region in C to enhance the contrast. (E) HAADF intensity profile obtained acrossthe dumbbell Ga−As pair on the red squared region in C and D. The red arrow points the profile direction.

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of the nanowire (e.g., growth rate) have been varied in order tomodulate the crystal structure.16,54 In Figure 2A, a lowresolution transmission electron micrograph of the nanowireis presented. Separated extended regions of WZ and ZB as wellas the presence of some other multiple twinned regions withintermixed polytypes are indicated. The study has beenperformed on one of the highly twinned regions. In Figure2B, the HAADF STEM image unveils perfectly separated Ga−As dumbbell pairs. A detail of the raw data is shown in Figure2C. The intensity profiles have been obtained on each of thedumbbell pairs, as the one shown in Figure 2E. Here, thedifference in intensity between the Ga and As is clearlyobservable, providing the path for the elemental mapping of thedumbbells.In Figure 2D we show a temperature colored detail of the

same region shown in Figure 2C, resulting in an elementalimage. This measurement indicates that the polarity on GaAs

NWs is such that As atoms are facing the tip (As-polarized,growth direction (111)B, see Table 2), as it was found inhomoepitaxy experiments.13 Moreover, we find that similar tothe case of ZnTe, the polarity across the twin boundaries isalways kept, thus corresponding to orthotwins. The distancebetween the Ga−As dumbbell pairs corresponds to 0.14 nm, ingood agreement with the calculated values (Table 1).To further highlight the importance of twin defects in

nanowires thus the need of a reliable experimental techniqueto identify them, we have performed first-principles electrontransport calculations to assess the impact that polar (paratwin)and nonpolar (orthotwin) twin defects have on theconductance of GaAs NWs. The electronic structure iscalculated within density-functional theory as implemented inthe Siesta code60 and the conductance is calculated followingLandauer theory.61 We have found that in presence of nonpolardefects the conductance is almost entirely preserved at theband-edges. Polar twin defects, on the other hand, lead to acomplete suppression of the conductance in the upper 0.3−0.4eV of the valence band and a stronger reduction, compared tononpolar defect, elsewhere (see the Supporting Information forfull details of the calculations and a more extended discussion,see also Figures S4 and S5).

Table 2. Polarity as Observed for ZnTe, GaAs, ZnO, GaN,and AlNa

NW material ZnTe GaAs ZnO GaN AlN

polarity (the element which is facing thetip)

Te As Zn N N

aThe polarity is given by the element that faces the tip of the NW, thiselement shows the termination of the {111} or {0001} growth planes,when the NW is in ZB or WZ phase, respectively.

Figure 3. (A) HAADF-STEM general view of one of the studied ZnO NWs. (B) Atomic resolution aberration corrected HAADF-STEM detail ofthe ZnO WZ structure in a selected region of the NW in A. (C) Atomic resolution aberration corrected ABF-STEM detail obtained on the same areaas in B. (D) ABF-STEM magnified detail of the region marked with an orange rectangle in C. (E) Temperature colored detail of the same region inD. (F) ABF intensity profile obtained across the Zn−O dumbbell pair inside the red rectangle region in C. The red arrow in C and F points to theprofile direction.

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■ LOW ATOMIC NUMBERWe have demonstrated with two examples that the HAADF-STEM is an excellent tool for resolving the dumbbell pairs. Amore challenging task is the resolution of light elements such asO and N, which are in fact two of the elements of majorinterest for a wide sector of the semiconductor community, as itincludes technologically relevant materials such as ZnO orGaN/AlN. In the case of ZnO, one of the most frequently usedmaterials in NWs semiconductors research,62−64 there is a long-standing debate on how to measure its polarity. The polaritystudy illustrated in Figure 3, has been performed on a ZnO NW(Figure 3A), obtained by a vapor transport technique,47 withthe layered-structured muscovite mica as the substrate toachieve a well-crytallized heteroepitaxial growth via the van derWaals epitaxy mechanism,48 similar to those reported else-where.65

In the characterization study shown in Figure 3, we compareagain the images obtained from HAADF-STEM (Figure 3B)with that obtained from ABF-STEM (Figure 3C). Contrary tothe ZnTe example shown in Figure 1, in the present case,HAADF-STEM cannot resolve the dumbbell pairs in thestructure due to the low Z number of the oxygen atoms (Figure3B). However, in Figure 3C, the ABF-STEM image perfectlyshows the presence of a lighter contrast zigzag below the darkeratomic positions (Zn). Zoomed-in view of the nanowires withABF-STEM is shown in Figure 3D and 3E, with raw ABF-STEM signal and applied temperature coloring, respectively.The Zn atomic positions perfectly coincide in both, HAADF-STEM and ABF-STEM techniques.In addition, the lightest contrast observed on the ABF-STEM

images is placed at 0.12 nm from the Zn position (Figure 3C−F), which is in perfect agreement with the expected Zn−Odumbbell pair separation distance (Table 1). Intensity profilesobtained along the dumbbell pairs clearly confirm the 0.12 nmdumbbell pair distances (Figure 3F). Direct observation of thedumbbell pairs reveals all the required information about theNW polarity. The growth of the ZnO NWs analyzed hereoccurred along the +[0001] axis with O on the base and Znpointing to the tip. This Zn polarized growth (Table 2) is ingood agreement with previous works based on indirectmeasurements.19

Finally, in order to explore the spatial and chemicalresolution limits of the ABF STEM, we have considered thestudy of the polarity in GaN NWs grown on Si(111) as well asof polarity transfer in a GaN-AlN core−shell NW hetero-structure. This study, visualized in Figure 4, has been performedon a GaN/AlN NW heterostructure (Figure 4A), grown byplasma assisted molecular beam epitaxy (PAMBE) on Si (111)using nitrogen-rich growth conditions as reported elsewhere.66

In Figure 4B the sharp interface between the GaN core andthe AlN shell is displayed. Despite the low interatomic distancewithin the Ga−N and Al−N dumbbell pairs (barely 0.11 nm inboth cases, Table 1) and the low Z value of the N atoms (TableS1), the polarity can be resolved with atomic resolution. Thefirst important result to point out is that the GaN core has awurtzite (WZ) structure with the nitrogen atoms pointing tothe tip (N polarized (N-face polarity), Table 2), and that thispolarity is transferred to the shell material (AlN). Due to thisperfect transfer the AlN part maintains the same polarity withthe (0001) planes terminated by N atoms (N-face polarity). Aperfect epitaxial relationship occurs between both materialsresulting in a perfectly abrupt interface as shown in Figure 4B−E. The measured distances between the dumbbell pairs in both,Ga−N and Al−N, are exactly 0.11 nm, in good agreement withthe expected 0.109 nm, as calculated in Table 1. It should benoticed that the polarity of the GaN NWs can depend on thegrowth process, the substrate material and the nucleationconditions as well as the presence of buffer layers. This mightexplain the apparent contradiction to the results presented inother works. In ref 67 the N-face polarity of GaN NWs grownby PAMBE on Si (111) was determined by channeling-enhanced electron energy loss spectroscopy and CBED,whereas growth on an AlN buffer was found to result in Ga-face polarity. In contrast, N-face polarity was found forPAMBE-grown GaN NWs on both bare Si (111) substratesand AlN buffer layers using resonant X-ray diffraction.68 Ga-face polarity was found for GaN NWs grown by ammonia-molecular beam epitaxy on Si(111) using CBED69 and also forGaN NWs grown on Si(111) by PAMBE employing also

Figure 4. (A) HAADF-STEM general view of the studied GaN-AlNNW heterostructure. (B) Atomic resolution aberration corrected ABF-STEM detail of the GaN-AlN WZ heterostructure in a selected regionof the NW in A. (C) ABF-STEM magnified detail of the squaredregion in B. (D) Temperature colored detail of the same region in C.(E) ABF intensity profile obtained across the Ga−N and Al−Ndumbbell pairs inside the red rectangle region shown in B. Notice thatwe have obtained one profile for the N column (red profile) and asecond one for the Ga and Al atomic columns (blue profile), seecorresponding red and blue dashed line profile position in B.

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analysis by CBED.70 Here we show the importance of a directvisualization technique that can unambiguously determine thedumbbell composition and thus the NW polarity.As a conclusion, we have presented two powerful tools,

namely HAADF and ABF STEM imaging, which can be appliedfor the direct determination of the dumbbell polarity incompound semiconductor nanowires. Both techniques could beapplied to any other nanostructure, such as nanoparticles,interfaces, thin film heterostructures and extended to any othermaterial, such as complex oxides, where the possibility ofimaging oxygen is priceless. Up to our knowledge, it is the firsttime that polarity can be imaged directly on semiconductorNWs where one of the constituent elements of the dumbbell isa light element (e.g., O or N), which is the case for ZnO, GaN,and AlN. One of the most interesting results is that afteranalyzing several twin boundaries in different semiconductornanowires (mostly of ZnTe and GaAs) we could never find achange in polarity across twin boundaries. Fortunately, thepresence of nonpolar twin boundaries (orthotwins) do notsignificantly modify the electronic structure of the material andits conductivity, in sharp contrast to the presence of polar twins(paratwins) which would drastically reduce the conductance, asdemonstrated theoretically in the present work.Future studies should include the characterization of more

complex heterostructures such as the heterointerface betweenpolar and nonpolar materials.

■ ASSOCIATED CONTENT

*S Supporting InformationABF and HAADF STEM experimental set up and conditions,description of the polytypic species in semiconductor materialsand first-principles calculations: electronic structure andconductivity. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: arbiol@icrea.cat.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was partially supported by the Spanish ProjectConsolider Ingenio CSD2009 00013 IMAGINE and CSD200900050 MULTICAT. J.A. acknowledges the funding from theSpanish MICINN Projects MAT2010-15138 and MAT2010-21510 and Generalitat de Catalunya (2009 SGR 770,NanoAraCat and XaRMAE). M.H. and A.F.i.M. thank fundingthrough the ERC starting Grant “Upcon”, the SNF throughProject No. 129775/1, and the NCCR “Quantum Science andTechnology”. Q.X. acknowledges the funding support of thiswork from Singapore National Research Foundation throughNRF fellowship grant (NRF-RF2009-06) and additional start-up grant support (M58113004) from Nanyang TechnologicalUniversity. R.R. acknowledges funding under Contract Nos.200950I164, TEC2009-06986, FIS2009-12721-C04-03, andCSD2007-00041. M.E. and F.F. thank funding from the EUvia the Project DOTSENSE (FP7, Grant No. 224212).

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