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Micron 43 (2012) 910–915

Contents lists available at SciVerse ScienceDirect

Micron

j o ur nal homep age: www.elsev ier .com/ locate /micron

hort communication

ssessment of misorientation in metallic and semiconducting nanowires usingrecession electron diffraction

onia Estradéa,b,∗ , Joaquim Portillob,c , Joan Mendozab , Ivette Kostad , Maria Serretd , Carlos Müllerd ,rancesca Peiróa

LENS, MIND-IN2UB, Departament d’Electrònica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, SpainTEM-MAT, CCiT, Universitat de Barcelona, Solé i Sabarís 1, 08028 Barcelona, SpainNANOMEGAS SL, Blvd Edmond Machtens 79, B-1080 Bruxelles, BelgiumDep. Química Física, Fac. Química, Univ. de Barcelona, Martí Franquès 1, ES-08028 Barcelona, Spain

r t i c l e i n f o

rticle history:eceived 24 October 2011

a b s t r a c t

Precession electron diffraction (PED) allows for diffraction pattern collection under quasi-kinematicalconditions. The combination of PED with fast electron diffraction acquisition and pattern matching soft-

eceived in revised form 5 March 2012ccepted 5 March 2012

eywords:recessionappinganowires

ware techniques is used for the high magnification ultra-fast mapping of variable crystal orientationsand phases, similarly to what is achieved with the Electron Backscattered Diffraction technique in Scan-ning Electron Microscopes at lower magnifications and longer acquisition times. Here we report, forthe first time, the application of this PED-based orientation mapping technique to both metallic andsemiconducting nanowires.

© 2012 Elsevier Ltd. All rights reserved.

. Introduction

1D nanostructures, such as nanowires or nanorods, havettracted a high amount of attention in recent years due to theirotential applications to nano-magnetism, nano-electronics, nano-lectrophotonics or nano-sensors (Kou et al., 2011; Huang et al.,001; Duan et al., 2003; McAlpine et al., 2007). For all of thesepplications, an accurate control of the crystal orientation and ofhe degree of polycrystallinity of these nanostructures is critical tonsure an adequate performance.

Selected area electron diffraction (SAED) and convergent beamlectron diffraction (CBED) in the transmission electron microscopeTEM) are the most widely used techniques for the determina-ion of crystallographic orientation of nanowires, in addition toigh resolution transmission electron microscopy (HRTEM), wherehe crystal lattice can be directly visualized (Xiong et al., 2006;ohansson et al., 2006; Arbiol et al., 2009).

All of the aforementioned techniques require an accurate ori-

ntation of the nanowire (i.e. a tilting of the specimen until theonsidered nanowire is in zone axis conditions). This process cane difficult if the nanowire is of particularly reduced dimensions

∗ Corresponding author at: LENS, MIND-IN2UB, Departament d’Electrònica, Uni-ersitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain.el.: +34 934021695; fax: +34 934021148.

E-mail address: sestrade@ub.edu (S. Estradé).

968-4328/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.oi:10.1016/j.micron.2012.03.003

(with no Kikuchi lines to guide the tilting process) or if it is not pos-sible to find an isolated nanowire and a bunch of them contributeto the diffraction pattern. Also, nanowire orientations happen to berather time consuming, which can be a critical point if nanowiresare sensitive to the electron beam.

As an alternative, it has been reported the use of electronbackscatter diffraction (EBSD) in the scanning electron micro-scope (SEM) for the crystal orientation mapping of nanowires:Nikoobakht et al. (2004) used individual EBSD patterns for thedetermination of the growth directions of ZnO NWs; Motayed et al.(2006) determined the growth directions of GaN nanowires; andPrikhodko et al. (2008) identified the phase and crystal orientationof GasAs NWs.

Precession electron diffraction (PED) allows for diffraction pat-tern collection under quasi-kinematical conditions (White et al.,2010). Diffraction patterns are collected whilst the TEM electronbeam is precessing on a cone surface; in this way, only a few reflec-tions are simultaneously excited and, therefore, dynamical effectsare strongly reduced (Vincent and Midgley, 1993; Weirich et al.,2006; Avilov et al., 2007).

The combination of PED with fast electron diffraction acquisi-tion and pattern matching software techniques can be used for thehigh magnification ultra-fast mapping of variable crystal orienta-

tions and phases, similarly to what is achieved with the ElectronBackscattered Diffraction (EBSD) technique in Scanning ElectronMicroscopes (SEM) at lower magnifications and longer acquisitiontimes (Rouvimov et al., 2009).

dx.doi.org/10.1016/j.micron.2012.03.003http://www.sciencedirect.com/science/journal/09684328http://www.elsevier.com/locate/micronmailto:sestrade@ub.edudx.doi.org/10.1016/j.micron.2012.03.003

S. Estradé et al. / Micron 43 (2012) 910–915 911

Fig. 1. Screen captures corresponding to the indexation process. (a) Best fitting indexation of a particular diffraction pattern according to hexagonal Co. (b) Indexation of thesame diffraction pattern according to hexagonal Co at 180◦ from the best fitting orientation. (c) Best fitting indexation of the same diffraction pattern according to cubic Co.

912 S. Estradé et al. / Micron 43 (2012) 910–915

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The obtained diffraction patterns were then indexed by meansof the A-star software, considering both hexagonal and cubic struc-tures. Fig. 1 displays several screen captures corresponding to the

ig. 2. Screen capture displaying the phases map, the index map and the combinathe web version of the article.)

In the present work, we will demonstrate the use of PED-basedEM orientation mapping to characterize both metallic and semi-onducting nanowires, which does not only allow to determinehe orientation and growth direction of the nanowires in a fastnd automated manner, but also to determine its degree of poly-rystallinity and the orientation of the different grains within theanowire when it does happen to be polycrystalline.

. Material and methods

In order to demonstrate the application of PED-based orienta-ion mapping to 1D nanostructures, two kinds of nanowires wereonsidered: P:Co (99% Co, 1% P) nanowires and Ge nanowires.

P:Co and Ge nanowires characterization was carried out byltra-fast recording of precessed electron diffraction pattern with

point spread resolution of 5 nm. ED spot patterns were obtainedn a JeolJ2100 LaB6 TEM operating at 200 kV and in a J2010F FEGEM operating also at 200 kV, respectively, illuminating in focusedanobeam diffraction (NBD) mode with a 5 nm spot precessed at.6◦.

The beam scanning, precessing and counter-precessing, and theast recording were controlled by an external device, “Astar”, man-factured by NanoMEGAS (Rouvimov et al., 2009). The computeroftware associated with this technique allowed indexation andrientation assignation by optimized matching with a templateatabase of P:Co/Ge kinematical diffraction patterns using cross-orrelation.

For the Ge nanowires, the space group used to generate the tem-late database was the usual 225 (fm3m) cubic space group. For the:Co nanowires, both 194 (p63/mmc) hexagonal and 227 (fd3m)

ubic structures were considered, with best results obtained for theexagonal structure. Both for Ge and cubic P:Co 1326 templatesere used, with an angular resolution of 1◦; for hexagonal P:Co,

000 templates were used, with an angular resolution of 1◦ as well.

both. (For interpretation of the references to color in text, the reader is referred to

3. Results and discussion

Thin (about 50 nm in diameter) P:Co (99% Co, 1% P) nanowireswere first considered. These nanowires were found by preliminarySAED characterization to be polycrystalline. A 204 nm × 211 nmarea containing part of one of these nanowires was chosen, fromwhich a precessed diffraction pattern was obtained, in an auto-mated way, every 5 nm.

Fig. 3. Reliability map between values 0 – black colour – and 100 – white colour.

S. Estradé et al. / Micron 43 (2012) 910–915 913

ur key

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Fig. 4. (a) z orientation map, (b) colo

ndexation process. In Fig. 1(a), we show the best fitting indexa-ion of a particular diffraction pattern according to hexagonal Co.n Fig. 1(c), we show the best fitting indexation of the same diffrac-ion pattern according to cubic Co – notice how both index andeliability figures are now much lower. Fig. 1(b) illustrates how the80◦ ambiguity is avoided, by giving the indexation of the sameiffraction pattern according to hexagonal Co at 180◦ from the besttting orientation – notice how the index figure is much lower than

n the best fitting case.According to best fitting indexation, several maps can be

btained. In Fig. 2, another screen capture is given. In this case,e show a phases map, indicating where in the nanowire hexag-

nal (green colour) and cubic (red colour) phases are found. Noturprisingly, all of the nanowire is determined to be best fittedo hexagonal Co. Unfortunately, the phases map is observed toe rather noisy. In order to improve the data representation, thehases map can be then multiplied by the index map, so that theegions outside the nanowire are not contributing.

A reliability map between values 0 – black colour – and 100 –hite – for the whole nanowire is given in Fig. 3. Reliability was

ound to be high enough everywhere in the nanowire, except for amall central fringe, right where a slight bending occurs.

The z, x, and y orientation maps (multiplied by index map)re displayed in Fig. 4, together with the colour key nec-ssary to interpret the orientations. Interestingly, the whole

anowire is in the same zone axis (z orientation) but there is

plane rotation between two well-differentiated regions. Theimit between these two regions corresponds to the low reli-bility fringe in the nanowire, as both orientations contribute

, (c and d) x and y orientation maps.

to the diffraction patterns there, making it difficult to indexthem.

In a second experiment, extremely thin (about 10 nm in diame-ter) Ge nanowires were also considered. These nanowires were alsofound by preliminary SAED characterization to be polycrystalline.

A 100 nm × 100 nm area was chosen in this case, containing atwice-bent nanowire, from which a precessed diffraction patternwas obtained, in an automated way, every 2 nm. The obtaineddiffraction patterns were then indexed, and assigned an orien-tation, and these assigned orientations were used to build theorientation maps of the whole area, just as in the previous case.

The z, x, and y orientation maps (multiplied by index map) aredisplayed in Fig. 5, together with the colour key necessary to inter-pret the orientations. Notice that the crystal orientation changesafter the second bending point, but not after the first one.

The corresponding reliability map, between values 49 – blackcolour – and 409 – white – is given in Fig. 6. The values yielded by thereliability map are much higher in this case, and no low reliabilityzone is found between the different orientation regions. This is sobecause a FEG instrument was used to carry out this experiment,as stated in the previous section.

In summary, an analogue of the SEM-EBSD analysis in the TEMwas considered for the characterization of two kinds of nanowires.It was successfully applied to determine the changes in crystal ori-entation in 50 nm thick P:Co nanowires and in heavily bent Ge

nanowires only 10 nm thick.

The use of precession, even if with a precession angle as lowas 0.6◦, is critical. Without precession, there would be a contribu-tion of Kikuchi lines to the pattern (Kikuchi lines disappear when

914 S. Estradé et al. / Micron 43 (2012) 910–915

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Fig. 5. (a) z orientation map, (b) colo

orking with precession angles over >0.5). Also, if precession is not

sed, there exists a 180◦ ambiguity in the assignation of the zonexis to each individual diffraction pattern. Finally, and most criti-ally, without precession the intensities of the excited reflections

ig. 6. Reliability map between values 49 – black colour – and 409 – white colour.

, (c and d) x and y orientation maps.

do not relate, by far, to the square modulus of the structure factorsof the considered crystals. In fact, the higher the precession angle,the higher the resemblance. Yet, increasing precession angle doesnot come without a price. On the one hand, it decreases the spa-tial resolution of the map, as the sharpness of the focalized beamgets lower (Rauch et al., 2010). Most importantly, the orientationresolution (i.e. the distance between the assigned direction and theclosest possible direction) has to lie between 0.5◦ and 1◦, in orderto recover all the textures, deformations and orientations in a givensample. This means that the number of generated templates has tobe sufficient, so that between a given template and the closest onethere is between 0.5◦ and 1◦ of orientation separation, but it alsomeans that it is necessary to use precession angles lower than 1◦,in order to avoid duplicities in orientation assignation (we couldget two templates separated 1◦, which would happen to be equallyprobable or with the same orientation index if we used a 2◦ preces-sion angle, for instance). This is the main reason to limit precessionangle to 0.6◦ in the present work.

4. Conclusions

We have shown that it is possible to apply an analogue ofthe SEM-EBSD analysis in the TEM to the characterization of

nanowires, by ultra-fast collection of precessed electron diffractionpatterns with a point spread resolution well within the nanometerrange. This technique allows determining the orientation of thenanowires, its degree of polycrystallinity and the orientation of the

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ifferent grains within the nanowire when it does happen to beolycrystalline.

cknowledgements

The authors would like to thank for the support offered by Dr.tavros Nicolopoulos (Nanomegas Inc.). The authors also thank thenancial support by the Spanish Government (IMAGINE-Consolidernd SOLEMN projects) and the TEM facilities of Science and Tech-ical Centers of Universitat de Barcelona (CCiT-UB). One of uscknowledges support from contract CTQ2010-20726 of MICINNnd the FEDER fund.

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Assessment of misorientation in metallic and semiconducting nanowires using precession electron diffraction1 Introduction2 Material and methods3 Results and discussion4 ConclusionsAcknowledgementsReferences