ZnO nanowire (NW) growth mechanism was investigated in a nonvapor and noncatalytic approach for the controlled NW synthesis in a second time scale. The experimental results showed what ZnO NW growth was determined by migration of zincinterstitials and vacancies in a ZnO layer, which should be also considered in other synthesis techniques and mechanisms. The mechanism of the ZnO NW growth was explained as due to the advantageous diffusion through grain boundaries in ZnO layer and crystal defects in NWs. Additionally, on the basis of photoluminescence measurements, a feasible application of as-produced wires for optoelectronic devices was demonstrated.Peer reviewe

Jiang, Hua

Kauppinen, Esko I.

Lipsanen, Harri

Nasibulin, Albert G.

Rackauskas, Simas

Shandakov, Sergey D.

Statkute, Gintare

Tian, Ying

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Rackauskas, Simas & Nasibulin, Albert G. & Jiang, Hua & Tian, Ying& Statkute, Gintare & Shandakov, Sergey D. & Lipsanen, Harri &Kauppinen, Esko I.Title: Mechanistic investigation of ZnO nanowire growthYear: 2009Version: Final published versionPlease cite the original version:Rackauskas, Simas & Nasibulin, Albert G. & Jiang, Hua & Tian, Ying & Statkute, Gintare& Shandakov, Sergey D. & Lipsanen, Harri & Kauppinen, Esko I. 2009. Mechanisticinvestigation of ZnO nanowire growth. Applied Physics Letters. Volume 95, Issue 18. P.183114/1-3. ISSN 0003-6951 (printed). DOI: 10.1063/1.3258074.Rights: © 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other userequires prior permission of the author and the American Institute of Physics.http://scitation.aip.org/content/aip/journal/japAll material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Mechanistic investigation of ZnO nanowire growthSimas Rackauskas, Albert G. Nasibulin, Hua Jiang, Ying Tian, Gintare Statkute, Sergey D. Shandakov, HarriLipsanen, and Esko I. Kauppinen  Citation: Applied Physics Letters 95, 183114 (2009); doi: 10.1063/1.3258074 View online: http://dx.doi.org/10.1063/1.3258074 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/18?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Defects in ZnO J. Appl. Phys. 106, 071101 (2009); 10.1063/1.3216464  Effect of high-energy electron beam irradiation on the properties of ZnO thin films prepared by magnetronsputtering J. Appl. Phys. 105, 123509 (2009); 10.1063/1.3149783  A comparative analysis of deep level emission in ZnO layers deposited by various methods J. Appl. Phys. 105, 013502 (2009); 10.1063/1.3054175  Secondary growth of small ZnO tripodlike arms on the end of nanowires Appl. Phys. Lett. 91, 013106 (2007); 10.1063/1.2752605  Femtosecond laser assisted growth of ZnO nanowires Appl. Phys. Lett. 87, 133115 (2005); 10.1063/1.2061858    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 07:21:29Mechanistic investigation of ZnO nanowire growthSimas Rackauskas,1,a Albert G. Nasibulin,1,a Hua Jiang,1 Ying Tian,1 Gintare Statkute,2Sergey D. Shandakov,3 Harri Lipsanen,2 and Esko I. Kauppinen1,41Department of Applied Physics and Center for New Materials, NanoMaterials Group,Helsinki University of Technology, P.O. Box 5100, 02015 Espoo, Finland2Department of Micro and Nanosciences, Micronova, Helsinki University of Technology,02015 Espoo, Finland3Department of Physics, Laboratory of Carbon NanoMaterials, Kemerovo State University,Kemerovo 650043, Russia4VTT Biotechnology, Biologinkuja 7, 02044 Espoo, FinlandReceived 5 August 2009; accepted 12 October 2009; published online 6 November 2009ZnO nanowire NW growth mechanism was investigated in a nonvapor and noncatalytic approachfor the controlled NW synthesis in a second time scale. The experimental results showed whatZnO NW growth was determined by migration of zinc interstitials and vacancies in a ZnO layer,which should be also considered in other synthesis techniques and mechanisms. The mechanism ofthe ZnO NW growth was explained as due to the advantageous diffusion through grain boundariesin ZnO layer and crystal defects in NWs. Additionally, on the basis of photoluminescencemeasurements, a feasible application of as-produced wires for optoelectronic devices wasdemonstrated. © 2009 American Institute of Physics. doi:10.1063/1.3258074Nanostructured ZnO materials have received broad at-tention due to their distinguished performance in electronics,optics, and photonics.1 In particular, ZnO nanowires NWsare regarded as promising candidates for optoelectronicapplications such as room temperature UV laser diodes dueto their wide band gap energy of 3.37 eV and high excitonbinding energy of about 60 meV.2,3 Common ZnO NWsynthesis methods include metal-organic chemical vapordeposition,4 vapor synthesis,5 hydrothermal,6,7 and chemicalsolution route.8 Typically, the NW growth has been ex-plained by vapor-liquid-solid9 or vapor-solid10 mechanisms.Here, we synthesized ZnO NWs and investigated thegrowth mechanism on the basis of proposed synthesismethod.11,12 High crystallinity ZnO NWs are produced with-out catalyst and dominant role of Zn and ZnO vapors, in asecond time scale. Based on experimental results we showthat ZnO NW growth is determined by zinc interstitial mi-gration in a ZnO layer. This growth mechanism should bealso considered in other synthesis techniques.13,14 Addition-ally, on the basis of photoluminescence PL measurements,we demonstrated feasible application of the as-producedNWs for optoelectronic devices.The detailed description of the ZnO NW synthesismethod is given elsewhere.11 ZnO NW synthesis was carriedout by resistive heating of a Zn wire under ambient air con-ditions in the range of 673–1123 K, i.e., at temperatures nearor higher than the melting point of zinc 692 K. During thesynthesis a protective 0.5–1 m thickness of ZnO layerforms on the surface of Zn wire Fig. 1a, which preservesthe shape of the liquefied wire. ZnO NWs grow on the sur-face of the ZnO layer and have typical diameters in the rangeof 6–30 nm and lengths between 1 and 5 m. At specificconditions, when Zn wire is heated too quickly, the ZnOlayer breaks, and the NWs can grow up to 40 m, by vaporgrowth mechanism Fig. 1b. In order to avoid this, synthe-sis conditions were reached by gradually heating zinc wireand carefully controlling the temperature by an IR pyrom-eter.Transmission TEM and scanning SEM electron mi-croscopic studies were performed. Figures 1c and 1dshow the results of TEM investigations of the structures ofthe ZnO NWs grown at 803 K. Importantly, ZnO NWs syn-thesized at higher temperatures had fewer defects, while lowtemperature samples usually contained stacking faults Fig.1c. An inset of Fig. 1c is an electron diffraction patternrevealing extra weak spots resulting from the stacking faultsthat periodically appear along 11¯0 direction. Figure 1dshows a crystalline ZnO NW without stacking faults, as de-aAuthors to whom correspondence should be addressed. Electronic ad-dresses: simas.rackauskas@tkk.fi and albert.nasibulin@tkk.fi.FIG. 1. Color online Electron microscopic studies of ZnO NWs: SEMimage of a typical ZnO NW forest inset shows ZnO layer formed on thesurface of Zn wire at 1023 K and b NWs up to 40 m long. c and dHigh resolution TEM images of ZnO NWs, insets are corresponding elec-tron diffraction patterns.APPLIED PHYSICS LETTERS 95, 183114 20090003-6951/2009/9518/183114/3/$25.00 © 2009 American Institute of Physics95, 183114-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 07:21:29termined by rotation of the NW in TEM. The electron dif-fraction pattern from the ZnO NW was indexed as hexagonalZnO lattice structure with the crystallographic parameters ofa=0.325 and c=0.521 nm. In all cases the growth directionof the ZnO NW was determined to be 110.Chemical composition of the wire surface after thesynthesis was examined by Raman spectroscopy measure-ments Fig. 2a, using Wintech alpha300 spectrometer =532 nm power 160 W. The peak centered at 438 cm−1corresponds to E2 mode of ZnO and the peak at 561 cm−1 isrelated to the oxygen deficiency in ZnO.15,16Room temperature PL measurements were made using aHe–Cd laser =325 nm, power 10 mW. PL spectra Fig.2b of the ZnO NW samples reveal two peaks with a vari-able intensity. One peak, referred to as the excitonic emissionor band gap emission, is located around 380 nm. The otherdeep trap emission peak in the visible region, centered at510 nm, is due to impurities or defects, which have elec-tronic energy levels within the band gap of ZnO.17,18 Typi-cally, both peaks are present in the synthesis temperaturerange of 723–923 K Fig. 2. In samples grown at higherprocess temperature 923–1023 K, the band gap emissiondominates. This can be attributed to higher crystallinity andlower amount of defects at higher temperatures.Low temperature PL measurements Fig. 2 inset at 10Klaser =266 nm, 2nd harmonics power 6 W showed emis-sion in UV region consisting of up to four peaks, which wereconsidered to originate from excitons, acceptor-exciton com-plexes, and phonon replicas.19 The dominating bound excitonpeak is located at 375 nm, free exciton emission peak isobserved at 368 nm. Other peaks on the lower energy side,can be attributed to phonon replicas of either donor or accep-tor bound excitons.19In order to investigate the kinetics of the NW growth, weprepared samples at different temperatures with a fixedgrowth time of 1 min. The lengths of the NWs obtained fromSEM images were used for the calculations of the averagegrowth rate Fig. 2c. Two regions can be clearly distin-guished in this plot: kinetic and constant growth rate. In thekinetic region, the average NW growth rate increases withthe temperature rise until 803 K. At higher temperatures NWgrowth rate remains nearly constant. It is worth noting thatthe surface density of the NWs significantly decreases athigher temperatures. ZnO NW shape also undergoes achange from rodlike at temperatures 803 K to swordlikeshape at temperatures above 850 K.Also, we examined the kinetics of ZnO NW growth atthe fixed temperature of 803 K, where the growth rate is thehighest. It is known that during Zn oxidation ZnO layerthickens according to the parabolic law,20,21 which is associ-ated with diffusion of Zn vacancies and interstitials.22 Thevacancy and interstitial migration rate rm can be expressed asrm = A−0.5, 1where A is a coefficient of proportionality,  is time. Plottingthe kinetic data of the ZnO NW growth rate rg at 803 K inthe coordinates ln rg−ln  gives a similar linear dependenceFig. 3a,FIG. 2. Color online a Raman spectrum of ZnO NWs grown at 773 K; b Room temperature PL spectra of ZnO nanowires. Inset shows low temperaturePL spectrum of ZnO NWs grown at 773 K; c growth rate dependence on the process temperature and SEM images of ZnO NWs at indicated temperaturesscale bar 500 nm.FIG. 3. Color online Kinetic data of ZnO NW growth: a growth rate m /s dependence on time s in a double logarithmic scale; b kinetic region673–803 K data of the growth rate in the Arrhenius coordinates; c schematics of ZnO NW growth mechanism.183114-2 Rackauskas et al. Appl. Phys. Lett. 95, 183114 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 07:21:29ln rg = − 0.4721 – 0.5095 ln  . 2Consequently it can be concluded that the NW growth rate rghas also parabolic time dependence. This indicates that ZnONW growth is determined by migration of Zn vacancies orinterstitials.The rate of ZnO NW growth can be expressed byArrhenius dependence,rg =dLdt= k0 exp− EaRT , 3where L is the length of NW, k0 is the pre-exponential coef-ficient, Ea is the activation energy of the NW growth, R is thegas constant, and T is the absolute temperature. Furthermore,plotting the kinetic region 673–803 K data of the growthrate in the Arrhenius coordinates ln rg−1 /T gives a lineardependence Fig. 3b, from which the activation energywas found to be Ea=83.8 kJ /mol. This value can be attrib-uted to the migration energy of Zn interstitials and vacancies77 and 88 kJ/mol, respectively.23 Migration of oxygen in-terstitials is less probable because of larger oxygen size. In-deed, oxygen interstitial and vacancy migration energies are118 and 124 kJ/mol, respectively.23 The activation energy forZnO lattice diffusion is as high as 305 kJ/mol20,22 and, as aresult, cannot be attributed to the limiting stage of the NWgrowth. Therefore, it can be deduced that the growth of ZnONWs complies with the parabolic law and determined byzinc interstitial and vacancy migration.Based on the oxidation mechanism and our investiga-tions, ZnO NW growth mechanism can be proposed. Accord-ing to the metal oxidation mechanism20,23,24 zinc oxidationinvolves Zn diffusion from Zn–ZnO interface to the surfaceFig. 3c. Advantageous diffusion through grain boundariescomparing to lattice diffusion20,23 creates sites on the surface,from which ZnO NWs grow. Rodlike shape of ZnO NW iscreated through anisotropic Zn diffusion in the direction ofenergetically favorable crystal growth 110. The optimumgrowth conditions for dense ZnO NW “forest” occur at803 K. At temperatures 850 K, the size of ZnO grainsbecomes larger,25 so that there is less grain boundary spacefor rapid diffusion to occur. This is confirmed from SEMimages as density of ZnO NW on the surface at temperaturesabove 803 K is lower.The growth of the NWs is also determined by the pres-ence of defects such as stacking faults. The amount of thesynthesized NWs was drastically decreased when the tem-perature was above 803 K, which is likely explained by theformation of the NWs with higher crystallinity. TEM and PLinvestigations confirmed the presence of fewer defects andless surface NW density at higher synthesis temperatures.However, the annealing of ZnO NWs at high temperaturesshould not be neglected, since NWs appear on the surfacealready after 10 s.To conclude, ZnO NW growth mechanism was investi-gated in a NW growth method based on resistive heating inthe temperature range of 673–1123 K. The highest ZnO NWgrowth rate was observed at 803 K. It was found that thedirection of the ZnO NWs occurs along 110. The growthrate had parabolic dependence, which was explained by Zninterstitial and vacancy migration. ZnO NW growth activa-tion energy Ea=83.8 kJ /mol, obtained from the Arrheniusplot, confirmed this result. Zinc interstitial and vacancy mi-gration were deduced to be the limiting stage of the ZnONWs growth. The mechanism of the ZnO NW growth wasexplained as due to the advantageous diffusion through grainboundaries in ZnO layer and crystal defects in NWs. Roomtemperature PL spectra show band gap emission locatedaround 380 nm and deep trap emission peak centered at510 nm. High synthesis temperature ZnO NWs show lowerintensity of deep emission, which confirms higher NW crys-tallinity. Low temperature PL spectrum shows peaks, origi-nating from excitons, acceptor-exciton complexes and pho-non replicas.This work was supported by the Academy of FinlandProject Nos. 128445, 124283, and 128495.1Z. L. Wang, Mater. Sci. Eng., R. 64, 33 2009.2Z. L. Wang, J. Phys.: Condens. Matter 16, R829 2004.3M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber,R. Russo, and P. D. Yang, Science 292, 1897 2001.4W. I. Park, D. H. Kim, S. W. Jung, and G. C. Yi, Appl. Phys. Lett. 80,4232 2002.5Z. R. Dai, Z. W. Pan, and Z. L. Wang, Adv. Funct. Mater. 13, 9 2003.6K. Elen, H. Van den Rul, A. Hardy, M. K. Van Bael, J. D’Haen, R.Peeters, D. Franco, and J. Mullens, Nanotechnology 20, 055608 2009.7J. Wang and L. Gao, Solid State Commun. 132, 269 2004.8H. Zhang, X. Y. Ma, J. Xu, J. J. Niu, and D. R. Yang, Nanotechnology 14,423 2003.9R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 1964.10Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 2001.11S. Rackauskas, A. G. Nasibulin, H. Jiang, Y. Tian, V. I. Kleshch, J. Sainio,E. D. Obraztsova, S. N. Bokova, A. N. Obraztsov, and E. I. Kauppinen,Nanotechnology 20, 165603 2009.12A. Nasibulin, S. Rackauskas, H. Jiang, Y. Tian, P. Mudimela, S. Shanda-kov, L. Nasibulina, S. Jani, and E. Kauppinen, Nano Res. 2, 373 2009.13Z. Gu, M. P. Paranthaman, J. Xu, and Z. W. Pan, ACS Nano 3, 2732009.14N. Boukos, C. Chandrinou, K. Giannakopoulos, G. Pistolis, and A. Trav-los, Appl. Phys. A: Mater. Sci. Process. 88, 35 2007.15X. Wang, Q. Q. Li, Z. B. Liu, J. Zhang, Z. F. Liu, and R. M. Wang, Appl.Phys. Lett. 84, 4941 2004.16H. Pan, Y. W. Zhu, Z. H. Ni, H. Sun, C. Poh, S. H. Lim, C. Sow, Z. X.Shen, Y. P. Feng, and J. Y. Lin, J. Nanosci. Nanotechnol. 5, 1683 2005.17A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, Phys. Rev. B61, 15019 2000.18S. Eustis, L. H. Robins, and B. Nikoobakht, J. Phys. Chem. C 113, 22772009.19H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn,and H. Kalt, Appl. Phys. Lett. 91, 181112 2007.20D. Young, High Temperature Oxidation and Corrosion of MetalsElsevier, Oxford, 2008, Vol. 1, p. 592.21I. E. Geguzin, Diffusion Zone [in Russian] Nauka, Moscow, 1979, p.344.22W. W. Smeltzer and D. J. Young, Prog. Solid State Chem. 10, 17 1975.23G. W. Tomlins, J. L. Routbort, and T. O. Mason, J. Appl. Phys. 87, 1172000.24C. Wagner, Prog. Solid State Chem. 10, 3 1975.25K. K. Khristoforov, B. K. Avdeenko, I. G. Portnova, Y. A. Omel’chenko,and A. V. Chernykh, Glass Ceram. 43, 171 1986.183114-3 Rackauskas et al. Appl. Phys. Lett. 95, 183114 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 07:21:29

Mechanistic investigation of ZnO nanowire growth

ZnO nanowire (NW) growth mechanism was investigated in a nonvapor and noncatalytic approach for the controlled NW synthesis in a second time scale. The experimental results showed what ZnO NW growth was determined by migration of zinc interstitials and vacancies in a ZnO layer, which should be also considered in other synthesis techniques and mechanisms. The mechanism of the ZnO NW growth was explained as due to the advantageous diffusion through grain boundaries in ZnO layer and crystal defects in NWs. Additionally, on the basis of photoluminescence measurements, a feasible application of as-produced wires for optoelectronic devices was demonstrated

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This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Rackauskas, Simas & Nasibulin, Albert G. & Jiang, Hua & Tian, Ying
& Statkute, Gintare & Shandakov, Sergey D. & Lipsanen, Harri &
Kauppinen, Esko I.
Title: Mechanistic investigation of ZnO nanowire growth
Year: 2009
Version: Final published version
Please cite the original version:
Rackauskas, Simas & Nasibulin, Albert G. & Jiang, Hua & Tian, Ying & Statkute, Gintare
& Shandakov, Sergey D. & Lipsanen, Harri & Kauppinen, Esko I. 2009. Mechanistic
investigation of ZnO nanowire growth. Applied Physics Letters. Volume 95, Issue 18. P.
183114/1-3. ISSN 0003-6951 (printed). DOI: 10.1063/1.3258074.
Rights: © 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use
requires prior permission of the author and the American Institute of Physics.
http://scitation.aip.org/content/aip/journal/jap
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Powered by TCPDF (www.tcpdf.org)
Mechanistic investigation of ZnO nanowire growth
Simas Rackauskas, Albert G. Nasibulin, Hua Jiang, Ying Tian, Gintare Statkute, Sergey D. Shandakov, Harri
Lipsanen, and Esko I. Kauppinen 
 
Citation: Applied Physics Letters 95, 183114 (2009); doi: 10.1063/1.3258074 
View online: http://dx.doi.org/10.1063/1.3258074 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/18?ver=pdfcov 
Published by the AIP Publishing 
 
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130.233.216.27 On: Mon, 04 May 2015 07:21:29
Mechanistic investigation of ZnO nanowire growth
Simas Rackauskas,1,a Albert G. Nasibulin,1,a Hua Jiang,1 Ying Tian,1 Gintare Statkute,2
Sergey D. Shandakov,3 Harri Lipsanen,2 and Esko I. Kauppinen1,4
1Department of Applied Physics and Center for New Materials, NanoMaterials Group,
Helsinki University of Technology, P.O. Box 5100, 02015 Espoo, Finland
2Department of Micro and Nanosciences, Micronova, Helsinki University of Technology,
02015 Espoo, Finland
3Department of Physics, Laboratory of Carbon NanoMaterials, Kemerovo State University,
Kemerovo 650043, Russia
4VTT Biotechnology, Biologinkuja 7, 02044 Espoo, Finland
Received 5 August 2009; accepted 12 October 2009; published online 6 November 2009
ZnO nanowire NW growth mechanism was investigated in a nonvapor and noncatalytic approach
for the controlled NW synthesis in a second time scale. The experimental results showed what
ZnO NW growth was determined by migration of zinc interstitials and vacancies in a ZnO layer,
which should be also considered in other synthesis techniques and mechanisms. The mechanism of
the ZnO NW growth was explained as due to the advantageous diffusion through grain boundaries
in ZnO layer and crystal defects in NWs. Additionally, on the basis of photoluminescence
measurements, a feasible application of as-produced wires for optoelectronic devices was
demonstrated. © 2009 American Institute of Physics. doi:10.1063/1.3258074
Nanostructured ZnO materials have received broad at-
tention due to their distinguished performance in electronics,
optics, and photonics.1 In particular, ZnO nanowires NWs
are regarded as promising candidates for optoelectronic
applications such as room temperature UV laser diodes due
to their wide band gap energy of 3.37 eV and high exciton
binding energy of about 60 meV.2,3 Common ZnO NW
synthesis methods include metal-organic chemical vapor
deposition,4 vapor synthesis,5 hydrothermal,6,7 and chemical
solution route.8 Typically, the NW growth has been ex-
plained by vapor-liquid-solid9 or vapor-solid10 mechanisms.
Here, we synthesized ZnO NWs and investigated the
growth mechanism on the basis of proposed synthesis
method.11,12 High crystallinity ZnO NWs are produced with-
out catalyst and dominant role of Zn and ZnO vapors, in a
second time scale. Based on experimental results we show
that ZnO NW growth is determined by zinc interstitial mi-
gration in a ZnO layer. This growth mechanism should be
also considered in other synthesis techniques.13,14 Addition-
ally, on the basis of photoluminescence PL measurements,
we demonstrated feasible application of the as-produced
NWs for optoelectronic devices.
The detailed description of the ZnO NW synthesis
method is given elsewhere.11 ZnO NW synthesis was carried
out by resistive heating of a Zn wire under ambient air con-
ditions in the range of 673–1123 K, i.e., at temperatures near
or higher than the melting point of zinc 692 K. During the
synthesis a protective 0.5–1 m thickness of ZnO layer
forms on the surface of Zn wire Fig. 1a, which preserves
the shape of the liquefied wire. ZnO NWs grow on the sur-
face of the ZnO layer and have typical diameters in the range
of 6–30 nm and lengths between 1 and 5 m. At specific
conditions, when Zn wire is heated too quickly, the ZnO
layer breaks, and the NWs can grow up to 40 m, by vapor
growth mechanism Fig. 1b. In order to avoid this, synthe-
sis conditions were reached by gradually heating zinc wire
and carefully controlling the temperature by an IR pyrom-
eter.
Transmission TEM and scanning SEM electron mi-
croscopic studies were performed. Figures 1c and 1d
show the results of TEM investigations of the structures of
the ZnO NWs grown at 803 K. Importantly, ZnO NWs syn-
thesized at higher temperatures had fewer defects, while low
temperature samples usually contained stacking faults Fig.
1c. An inset of Fig. 1c is an electron diffraction pattern
revealing extra weak spots resulting from the stacking faults
that periodically appear along 11¯0 direction. Figure 1d
shows a crystalline ZnO NW without stacking faults, as de-
aAuthors to whom correspondence should be addressed. Electronic ad-
dresses: simas.rackauskas@tkk.fi and albert.nasibulin@tkk.fi.
FIG. 1. Color online Electron microscopic studies of ZnO NWs: SEM
image of a typical ZnO NW forest inset shows ZnO layer formed on the
surface of Zn wire at 1023 K and b NWs up to 40 m long. c and d
High resolution TEM images of ZnO NWs, insets are corresponding elec-
tron diffraction patterns.
APPLIED PHYSICS LETTERS 95, 183114 2009
0003-6951/2009/9518/183114/3/$25.00 © 2009 American Institute of Physics95, 183114-1
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 07:21:29
termined by rotation of the NW in TEM. The electron dif-
fraction pattern from the ZnO NW was indexed as hexagonal
ZnO lattice structure with the crystallographic parameters of
a=0.325 and c=0.521 nm. In all cases the growth direction
of the ZnO NW was determined to be 110.
Chemical composition of the wire surface after the
synthesis was examined by Raman spectroscopy measure-
ments Fig. 2a, using Wintech alpha300 spectrometer 
=532 nm power 160 W. The peak centered at 438 cm−1
corresponds to E2 mode of ZnO and the peak at 561 cm−1 is
related to the oxygen deficiency in ZnO.15,16
Room temperature PL measurements were made using a
He–Cd laser =325 nm, power 10 mW. PL spectra Fig.
2b of the ZnO NW samples reveal two peaks with a vari-
able intensity. One peak, referred to as the excitonic emission
or band gap emission, is located around 380 nm. The other
deep trap emission peak in the visible region, centered at
510 nm, is due to impurities or defects, which have elec-
tronic energy levels within the band gap of ZnO.17,18 Typi-
cally, both peaks are present in the synthesis temperature
range of 723–923 K Fig. 2. In samples grown at higher
process temperature 923–1023 K, the band gap emission
dominates. This can be attributed to higher crystallinity and
lower amount of defects at higher temperatures.
Low temperature PL measurements Fig. 2 inset at 10K
laser =266 nm, 2nd harmonics power 6 W showed emis-
sion in UV region consisting of up to four peaks, which were
considered to originate from excitons, acceptor-exciton com-
plexes, and phonon replicas.19 The dominating bound exciton
peak is located at 375 nm, free exciton emission peak is
observed at 368 nm. Other peaks on the lower energy side,
can be attributed to phonon replicas of either donor or accep-
tor bound excitons.19
In order to investigate the kinetics of the NW growth, we
prepared samples at different temperatures with a fixed
growth time of 1 min. The lengths of the NWs obtained from
SEM images were used for the calculations of the average
growth rate Fig. 2c. Two regions can be clearly distin-
guished in this plot: kinetic and constant growth rate. In the
kinetic region, the average NW growth rate increases with
the temperature rise until 803 K. At higher temperatures NW
growth rate remains nearly constant. It is worth noting that
the surface density of the NWs significantly decreases at
higher temperatures. ZnO NW shape also undergoes a
change from rodlike at temperatures 803 K to swordlike
shape at temperatures above 850 K.
Also, we examined the kinetics of ZnO NW growth at
the fixed temperature of 803 K, where the growth rate is the
highest. It is known that during Zn oxidation ZnO layer
thickens according to the parabolic law,20,21 which is associ-
ated with diffusion of Zn vacancies and interstitials.22 The
vacancy and interstitial migration rate rm can be expressed as
rm = A−0.5, 1
where A is a coefficient of proportionality,  is time. Plotting
the kinetic data of the ZnO NW growth rate rg at 803 K in
the coordinates ln rg−ln  gives a similar linear dependence
Fig. 3a,
FIG. 2. Color online a Raman spectrum of ZnO NWs grown at 773 K; b Room temperature PL spectra of ZnO nanowires. Inset shows low temperature
PL spectrum of ZnO NWs grown at 773 K; c growth rate dependence on the process temperature and SEM images of ZnO NWs at indicated temperatures
scale bar 500 nm.
FIG. 3. Color online Kinetic data of ZnO NW growth: a growth rate m /s dependence on time s in a double logarithmic scale; b kinetic region
673–803 K data of the growth rate in the Arrhenius coordinates; c schematics of ZnO NW growth mechanism.
183114-2 Rackauskas et al. Appl. Phys. Lett. 95, 183114 2009
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ln rg = − 0.4721 – 0.5095 ln  . 2
Consequently it can be concluded that the NW growth rate rg
has also parabolic time dependence. This indicates that ZnO
NW growth is determined by migration of Zn vacancies or
interstitials.
The rate of ZnO NW growth can be expressed by
Arrhenius dependence,
rg =
dL
dt
= k0 exp− EaRT , 3
where L is the length of NW, k0 is the pre-exponential coef-
ficient, Ea is the activation energy of the NW growth, R is the
gas constant, and T is the absolute temperature. Furthermore,
plotting the kinetic region 673–803 K data of the growth
rate in the Arrhenius coordinates ln rg−1 /T gives a linear
dependence Fig. 3b, from which the activation energy
was found to be Ea=83.8 kJ /mol. This value can be attrib-
uted to the migration energy of Zn interstitials and vacancies
77 and 88 kJ/mol, respectively.23 Migration of oxygen in-
terstitials is less probable because of larger oxygen size. In-
deed, oxygen interstitial and vacancy migration energies are
118 and 124 kJ/mol, respectively.23 The activation energy for
ZnO lattice diffusion is as high as 305 kJ/mol20,22 and, as a
result, cannot be attributed to the limiting stage of the NW
growth. Therefore, it can be deduced that the growth of ZnO
NWs complies with the parabolic law and determined by
zinc interstitial and vacancy migration.
Based on the oxidation mechanism and our investiga-
tions, ZnO NW growth mechanism can be proposed. Accord-
ing to the metal oxidation mechanism20,23,24 zinc oxidation
involves Zn diffusion from Zn–ZnO interface to the surface
Fig. 3c. Advantageous diffusion through grain boundaries
comparing to lattice diffusion20,23 creates sites on the surface,
from which ZnO NWs grow. Rodlike shape of ZnO NW is
created through anisotropic Zn diffusion in the direction of
energetically favorable crystal growth 110. The optimum
growth conditions for dense ZnO NW “forest” occur at
803 K. At temperatures 850 K, the size of ZnO grains
becomes larger,25 so that there is less grain boundary space
for rapid diffusion to occur. This is confirmed from SEM
images as density of ZnO NW on the surface at temperatures
above 803 K is lower.
The growth of the NWs is also determined by the pres-
ence of defects such as stacking faults. The amount of the
synthesized NWs was drastically decreased when the tem-
perature was above 803 K, which is likely explained by the
formation of the NWs with higher crystallinity. TEM and PL
investigations confirmed the presence of fewer defects and
less surface NW density at higher synthesis temperatures.
However, the annealing of ZnO NWs at high temperatures
should not be neglected, since NWs appear on the surface
already after 10 s.
To conclude, ZnO NW growth mechanism was investi-
gated in a NW growth method based on resistive heating in
the temperature range of 673–1123 K. The highest ZnO NW
growth rate was observed at 803 K. It was found that the
direction of the ZnO NWs occurs along 110. The growth
rate had parabolic dependence, which was explained by Zn
interstitial and vacancy migration. ZnO NW growth activa-
tion energy Ea=83.8 kJ /mol, obtained from the Arrhenius
plot, confirmed this result. Zinc interstitial and vacancy mi-
gration were deduced to be the limiting stage of the ZnO
NWs growth. The mechanism of the ZnO NW growth was
explained as due to the advantageous diffusion through grain
boundaries in ZnO layer and crystal defects in NWs. Room
temperature PL spectra show band gap emission located
around 380 nm and deep trap emission peak centered at
510 nm. High synthesis temperature ZnO NWs show lower
intensity of deep emission, which confirms higher NW crys-
tallinity. Low temperature PL spectrum shows peaks, origi-
nating from excitons, acceptor-exciton complexes and pho-
non replicas.
This work was supported by the Academy of Finland
Project Nos. 128445, 124283, and 128495.
1Z. L. Wang, Mater. Sci. Eng., R. 64, 33 2009.
2Z. L. Wang, J. Phys.: Condens. Matter 16, R829 2004.
3M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber,
R. Russo, and P. D. Yang, Science 292, 1897 2001.
4W. I. Park, D. H. Kim, S. W. Jung, and G. C. Yi, Appl. Phys. Lett. 80,
4232 2002.
5Z. R. Dai, Z. W. Pan, and Z. L. Wang, Adv. Funct. Mater. 13, 9 2003.
6K. Elen, H. Van den Rul, A. Hardy, M. K. Van Bael, J. D’Haen, R.
Peeters, D. Franco, and J. Mullens, Nanotechnology 20, 055608 2009.
7J. Wang and L. Gao, Solid State Commun. 132, 269 2004.
8H. Zhang, X. Y. Ma, J. Xu, J. J. Niu, and D. R. Yang, Nanotechnology 14,
423 2003.
9R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 1964.
10Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 2001.
11S. Rackauskas, A. G. Nasibulin, H. Jiang, Y. Tian, V. I. Kleshch, J. Sainio,
E. D. Obraztsova, S. N. Bokova, A. N. Obraztsov, and E. I. Kauppinen,
Nanotechnology 20, 165603 2009.
12A. Nasibulin, S. Rackauskas, H. Jiang, Y. Tian, P. Mudimela, S. Shanda-
kov, L. Nasibulina, S. Jani, and E. Kauppinen, Nano Res. 2, 373 2009.
13Z. Gu, M. P. Paranthaman, J. Xu, and Z. W. Pan, ACS Nano 3, 273
2009.
14N. Boukos, C. Chandrinou, K. Giannakopoulos, G. Pistolis, and A. Trav-
los, Appl. Phys. A: Mater. Sci. Process. 88, 35 2007.
15X. Wang, Q. Q. Li, Z. B. Liu, J. Zhang, Z. F. Liu, and R. M. Wang, Appl.
Phys. Lett. 84, 4941 2004.
16H. Pan, Y. W. Zhu, Z. H. Ni, H. Sun, C. Poh, S. H. Lim, C. Sow, Z. X.
Shen, Y. P. Feng, and J. Y. Lin, J. Nanosci. Nanotechnol. 5, 1683 2005.
17A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, Phys. Rev. B
61, 15019 2000.
18S. Eustis, L. H. Robins, and B. Nikoobakht, J. Phys. Chem. C 113, 2277
2009.
19H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn,
and H. Kalt, Appl. Phys. Lett. 91, 181112 2007.
20D. Young, High Temperature Oxidation and Corrosion of Metals
Elsevier, Oxford, 2008, Vol. 1, p. 592.
21I. E. Geguzin, Diffusion Zone [in Russian] Nauka, Moscow, 1979, p.
344.
22W. W. Smeltzer and D. J. Young, Prog. Solid State Chem. 10, 17 1975.
23G. W. Tomlins, J. L. Routbort, and T. O. Mason, J. Appl. Phys. 87, 117
2000.
24C. Wagner, Prog. Solid State Chem. 10, 3 1975.
25K. K. Khristoforov, B. K. Avdeenko, I. G. Portnova, Y. A. Omel’chenko,
and A. V. Chernykh, Glass Ceram. 43, 171 1986.
183114-3 Rackauskas et al. Appl. Phys. Lett. 95, 183114 2009
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Mon, 04 May 2015 07:21:29


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Mechanistic investigation of ZnO nanowire growth

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