jurnal nanowire PVD

8/2/2019 jurnal nanowire PVD

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L o w - T e m p e r a t u r e G r o w t h o f Z n O N a n o w i r e A r r a y b y a

S i m p l e P h y s i c a l V a p o r - D e p o s i t i o n M e t h o d

Seung Chu l Lyu, Ye Zhan g, and Ch eol J in Lee*

Department of Nanotechnology, Hanyang University, Seoul 133-791 Korea

Hyun Ruh and Hwack Joo LeeMicrostructure Analysis Laboratory, Korea Research Institute of S tandard s and S cience,

Daejon 305-600, Korea

Received Au gust 14, 2002. Revised Man uscript Received April 14, 2003

Well-aligned single-cryst alline wu rzite zinc oxide (ZnO) na nowire a rr ay wa s su ccessfullyfabricated on an Al2O3 substrate by a simple physical vapor-deposition method at a lowtemperature of 450 C. The diameter and growth rate of ZnO nanowires increased as afunction of growth temperatur e. TEM observation showed th at the ZnO nan owires weresynth esized along th e c-axial direction of th e hexagonal crystal stru cture. We demonstra tethat ZnO nanowires followed the self-catalyzed growth mechanism on the ZnO nuclei. Besideshigh-qua lity ZnO n an owires, sometimes a fascinat ing hierar chically ordered ZnO stru cture

was also observed.

I n t r o d u c t i o n

Since the first discovery of carbon nanotubes, one-dimensional semiconductor materials have attractedextensive interest because they exhibit curious struc-tures and various remarkable physical, chemical, andelectrical properties distinctive from those of conven-tional bulk materials.1-6 Among the various semicon-ductor nanowires, zinc oxide (ZnO) nanowire having adirect band gap of 3.37 eV and large exciton bindingener gy of 60 meV can pr omise pra ctical ap plicat ions inthe a rea of nan oscale laser diodes and ultra violet (UV)sensors. Because of the radial qua ntum confinementeffect of nanowires, ZnO nanowires possess high densityof s t a t es a t t he band edge . Recent l y H uang e t a l .reported t hat UV lasing nan o-devices working at roomtempera tur e with a low-lasing thresh old based on ZnOnan owires would be quite potentially feasible.7,8

ZnO nan owires can be synth esized by various m eth-ods such as a rc discha rge, laser va porization, pyrolysis,electrodeposit ion, and chemical or physical vapor

deposition.7-17 To date, well-aligned ZnO nan owirear ra ys, which h ave th e well-faceted hexagona l stru cture,were synthesized on a sapphire substrate by a carbonther mal-reduction vapor tr ans port meth od at 880 C.7,8

In addit ion, ZnO nanobelts were also obtained byevaporat ing ZnO powder at a h igh temperat ur e of 1400C.18 These previously r eported ZnO nanowires ornanobelts had high purity a nd high-crystalline str uc-tur e, but t he growth tempera tur e was too high to allowuse of low-tempera tur e-endur ance substr ates. To apply

ZnO nanowires to many nanoscale devices, it is verydesirable to have synthesized those nanowires at lowgrowth temperature. Recently, ZnO nanorods weregrown directly on silica or silicon substrate at a tem-perature of 500 C in a two-temperature-zone furnaceusing evaporated zinc acetylacetonate hydrate (Zn-(C5H 7O2)2xH 2O) as a reaction source.15 But i n t ha twork, the produced ZnO nanowires had poor morphologyand a low growth rate. More recent ly, Park et al .report ed vertically well-aligned ZnO na nowires on sap-phire substrates or ZnO nanoneedles on si l icon sub-

* Corr esponding aut hor. E-mail: cjlee@han yang.ac.kr. Tel: +82-2-2293-4744. Fa x: +82-2-2290-0768.

(1) Han , W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. S cience 1997, 277,1287.(2) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheu ng, C. L.; Lieber,

C. M. Na t u r e 1998, 39 4, 52.(3) Cu i, Y.; Wei, Q. Q.; Park , H. K.; Lieber, C. M. S cience 2001,

293 , 1289.(4) Hu ang, Y.; Duan, X.; Cui, Y.; Lauh on, L. J.; Kim, K. H.; Lieber,

C. M. S cience 2001, 29 4, 1313.(5) Kim, J . R.; So, H. M.; Par k, J. W.; Kim, J . J.; Kim, J.; Lee, C. J.;

Lyu, S. C. Appl. Phys. L ett. 2002, 80 , 3548.(6) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang Y.; Ruh, H.; Lee, H. J.

Appl. Phys. Lett. 2002, 81 , 3648.(7) Hu ang, M. H .; Mao, S.; Feick, H.; Yan, H . Q.; Wu, Y. Y.; Kind,

H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292 , 1897.(8) Hu ang, M. H .; Wu, Y. Y.; Feick, H.; Tran, N .; Weber, E.; Yang,

P. D. Adv. M ater. 2001, 13 , 113.

(9) Duan, X. F.; Lieber, C. M. Adv. Mater. 2000, 12 , 298.(10) Li, Y.; Meng, G. W.; Zhang, L. D.; Phillipp, F. Appl. Phys. Lett.

2000, 76, 2011.(11) Konenkamp, R.; Boedecker, K.; Lux-Steiner, M. C.; Poschen-rieder, M.; Zenia, F.; Clement, C. L.; Wagner, S. Appl. Phys. Lett. 2000 ,77, 2575.

(12) Liu, R.; Vertegel, A. A.; Bohannan, E. W.; Sorenson, T. A.;Switzer, J. A. Chem. Mater. 2001, 13 , 508.

(13) Kong, Y. C.; Yu, D. P.; Zhan g, B.; Fang, W.; Feng, S. Q. Appl.Phys. Lett. 2001, 78 , 407.

(14) Hu, J. Q.; Li, Q.; Wong, N. B.; Lee, C. S.; Lee, S. T. Chem.Mater. 2002, 14 , 1216.

(15) Wu, J. J .; Liu, S. C. Adv. Mater. 2002, 14 , 215.(16) Par k, W. I.; Kim, D. H.; J ung, S.-W.; Yi, G.-C. Appl. Phys. Lett.

2002, 80 , 4232.(17) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. L ett. 2002, 81 ,

757.(18) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. S cience 2001, 291 , 1947.

3294 Chem. Mater. 2003, 15, 3294-3299

10.1021/cm020465j CCC: $25.00 2003 American Chemical SocietyPublished on Web 07/09/2003


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stra tes a t low-temperatu res r anging from 400 to 500 Cusing metal-organ ic chemical vapor deposition.16,19 Intheir m ethod, a very thin ZnO epitaxial layer was grownon the sapphire subst rates prior to ZnO nanowiregrowth. Despite much pr ogress on th e synth esis of ZnOnanowires, it is still difficult to produce high-quailtyZnO nanowires at low temperature using a simplegrowth technique. Curr ently, a lar ge-scale synthesis ofhigh-quali ty ZnO nanowires at low temperature re-mains a great challenge.

In this paper, we demonstrate fabrication of a high-quality single-crystalline ZnO nanowire array on theAl2O3 substrate using a simple physical vapor-depositionmeth od at a low growth temper atu re of 450 C. The ZnOnanowires were synthesized along the c-axis growthdirection and followed the catalyst-free growth mecha-nism. We empha size that our simple growth techniqueemploys a much lower growth temperature for thesynthesis of high-quality ZnO na nowires. Su ch a low-t empera t ure grow th m ethod m ay open up t he op-portunities for fabricating ZnO nanowires onto variouslow-temperatur e-enduran ce substra tes a nd extend therea lm of ZnO-based na noscale devices. We also discov-ered that some nanowires have a hierarchically orderedZnO structure, the so-called fractal nanofern. Thesefractal s t ructures of ZnO nanowires are caused bystructural defects at low growth temperature.

E x p e r im e n t a l S e c t i o n

High-quality ZnO nanowires were synthesized on NiO-nanoparticle-deposited Al2O3 substrates. The Al2O3 materialused consists of grain structure, resulting in mound morphol-ogy on th e su rface. In a t ypical pr ocess, a s olut ion (0.01 M) ofnickel nitrate (Sigma-Aldrich 99.999%) and ethanol wasdropped onto the Al2O3 substrate (10 mm 5 mm). Afterdrying the Al2O3 substrate a t 400 C in ambient a ir , NiOparticles were evenly formed on the Al 2O3 substrate . Thesubstr ate was pu t on th e top of a quartz boat loaded with meta lzinc powders (100 mesh , 99.999%, Sigma-Aldrich) and inser ted

into a quar tz tu be furn ace. The vertical distance between t hezinc source and the Al2O3 substrate was about 3-5 mm. Tosynth esize ZnO nanowires, the furna ce was heat ed to reactiontemper at ur e under an Ar flow rat e of 500 sccm. The Zn sourcewas physically vaporized to synthesize the ZnO nanowires inatmospheric pressure under an Ar (99.99%) flow ra te of 500sccm for 60 min a t th e temperat ure r ange of 450-600 C. Afterthe r eaction, the su bstra te sur face appeared as a layer of whitewaxlike m aterial. The as-deposited products were char acter-ized by scann ing electron microscopy (SEM; Hitachi, S-4700),tran smission electron microscopy (TEM; Hitachi, H-9000),energy-dispersive X-ray spectroscopy (EDX; Hitachi, S-4700),an d X-ra y diffraction (XRD; Rigaku , DMAX 2500).

R e s u l t s a n d D i s c u s s i o n

Figur e 1 shows EDX an alysis an d XRD spectra of th esynth esized white waxlike mater ial. EDX indicates th atthe products consist of zinc and oxygen elements exactlyas sh own in F igure la. The element of Pt was from thecoating layer used for SEM imaging of the products.XRD measurement demonstrates that the producedZnO m ater ials have a single-crystalline wurt zite st ruc-tur e as sh own in F igure 1b. Only (10l) peaks (l ) 1,2,3)ar e observed in the XRD spectra , implying that th e ZnO

materials have some tilted alignment degree toward thevertical direction. The str ong inten sities of ZnO diffra c-tion peaks indicate that the products have a high-purityZnO wurtzite phase.

Figur e 2 shows the SE M ima ges of th e ZnO ma ter ialssynthesized at 450-600 C. In the SEM image, high-purity ZnO nanowires are synthesized on the Al2O3substr ate a t a low growth t empera tur e of 450 C. Thisresult indicates tha t for zinc metal (mp 419 C) vaporpress ur e at 450 C is high enough to susta in the growthof ZnO na nowires. Figur e 2a-d shows low-magnificationSEM images, indicating that vertically aligned ZnOnan owires have a r adial direction on the su bstra te witha high density. It is suggested that the t ilted alignmentof ZnO nanowires is caused by the surface morphologyof the Al2O3 substra te mat erial. In th e SEM images, onecan f i nd t ha t t he a l i gnm ent of t he ZnO nanow i readvances as the growth temperature increases from 450to 600 C. Furth er study is n ecessary to elucidate thealignment dependence of ZnO nanowires on growthtemperatur e. The growth rate of ZnO nanowires alsoincreases as a function of growth temperature. Thegrowth rates of ZnO nanowire for the growth temper-at ur es of 450, 500, 550, an d 600 C ar e 2.6, 4.8, 5.4, an d7.2 m, respectively. High-magnification SEM imagesindicate th at the str aight ZnO nan owires ha ve a un i-form d iameter a nd clean sur face as shown in Figure 2e-

h. The average diameters of ZnO nanowires grown at450, 500, 550, and 600 C ar e 55, 65, 84, and 100 n m,respectively. The higher growth temperatures will bein favor of formation of ZnO nanowires with largerdiameter s. Higher growth tem pera tur e is also beneficialto synthesizing ZnO nanowires possessing perfect hex-agonal cryst a l m orphol ogy as show n in t he SEMimages.

We employed NiO nanoparticles on the Al2O3 sub-stra te to synthesize ZnO na nowires. In our experiment,NiO nanopar ticles sized several t ens of nan ometers ar eproduced on the Al2O3 substrate by adopting a simplebaking process at 400 C in the ambient air as shownin Figure 3a. The SEM image shows that the Al2O3

(19) Park, W. I.; Yi, G.-C.; Kim, M. Y.; Pennycook, S. J. Adv. Mater.2002, 14 , 1841.

F i g u r e 1 . EDX analysis of the ZnO nan owires on th e Al2O3 substra te (a) and XRD spectra (b).

L ow -T em p eratu re Grow th of Z nO N an ow ire A rray Ch em . M at er., V ol. 15, N o. 17, 2003 3295


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F i g u r e 2 . SEM images of aligned ZnO nanowires on the Al2O3 substrate: at low magnification (a-d), at high magnification

(e-h). The growth temperatures are 450 (a, e), 500 (b, f), 550 (c, g), and 600 C (d, h).

3296 Ch em . M ater., Vol. 15, N o. 17, 2003 L yu et al.


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substrate surface reveals isolated grain morphologybecause the Al2O3 material used has a bulk crystal

str uctur e. Figure 3b shows an SEM image for th e initialgrowth stage of ZnO na nowires on the NiO-nanopar-ticle-deposited Al2O3 substr ate, in which th e ZnO na no-wires were synthesized for 3 min at 550 C. The laterallygrown ZnO layer, which is originated from the NiOna nopar ticles, perfectly covers t he NiO cata lyst par ticleon the Al2O3 substr ate. As a resu lt, it would be impos-sible to supply a Zn source into th e NiO na noparticlesdur ing the ZnO na nowire growth . Figure 3c is th e cross-sectiona l SEM image of th e ZnO na nowires synth esizedon the Al2O3 substr ate a t 550 C, indicating the sur faceof the Al2O3 gra in covered with a ZnO layer. Fr om SEMobservation, we suggest that the ZnO nanowires aredirectly synthesized not on the NiO nanoparticles buton the lateral ly grown ZnO layer. Park et al .16 an-nounced th e cata lyst-free growth of ZnO nan owire, inwhich a thin ZnO epitaxial layer was grown on thesapph ire substr ates before ZnO na nowire growth. Fig-ure 3d shows the SEM image of wel l -al igned ZnOnanowires synthesized on the silicon substrate at 550C. Compared with ZnO nanowires grown on the Al2O3substra te, the synt hesized ZnO nan owires have verticald irect ion because t he used subs t ra t e has a p lanesurface.

It is well-known that transition metal oxides, suchas N iO and F eO, have a similar cata lytic effect on t he

VLS growth of semiconductor nanowires as well asmetal catalysts.20 Our experimental r esult shows th at

the NiO nanoparticles are fully covered with a laterallygrown ZnO layer and no metal catalyst particles areobserved on t he ZnO nan owire t ip. Moreover, th e ZnOnanowires could be synthesized on the Al2O3 or thesilicon substrate without catalyst nanoparticles. There-fore, we suggest that the ZnO nanowires fol low acatalyst-free growth model in our method. In the initialstage, NiO nanoparticles on the Al2O3 subst rate arecovered with th e ZnO layer; and t hen t he nu cleation canoccur at an y of the sites on t he ZnO layer. Continu ousfeeding of an evaporated Zn source and elementaloxygen int o favora ble nu cleat ion s ites of ZnO will leadto one-dimensional growth of ZnO nan owires. Theoxygen element ma y origin from the am bient gas which

is introduced into the qua rtz tu be becaus e the r eactionpressure is atmospheric.Figure 4 shows a TEM image of the ZnO nanowires

synthesized at 450 C. It indicates tha t th e ZnO nano-wires have u niform diameter an d n o catalyst particleat the t ip as shown in Figure 4a . High-resolution TEMimage an d electr on diffraction cha ra cterization indicat etha t t he ZnO nan owire is grown a long th e [2h110] zoneaxis a s shown in Figure 4b. In t he HRTEM an alysis,the ZnO nanowire is single-crystal and follows c-axialgrowth direction (c ) 5.13 ). From TEM observation,

(20) Chen, X. L.; Li, J. Y.; Cao, Y. G.; Lan, Y. C.; Li, H.; He, M.;Wang, C. Y.; Zhang, Z.; Qiao, Z. Y. Adv. Mater. 2000, 12 , 1432.

F i g u r e 3 . SEM image of NiO nanoparticles produced on the Al2O3 substra te (a) and SEM image for t he initial growth stage ofZnO nanowires on the NiO-nanoparticle-deposited Al2O3 substrate (b). Cross-sectional SEM image of the synthesized ZnO nanowiresat 550 C (c) and SEM image of well-aligned ZnO nanowires synthesized on the silicon substrate at 550 C (d).



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we consider t hat high-crystalline hexagonal st ru cturedZnO nan owires can be successfully synth esized a t lowgrowth t emperature.

During SEM observations, we could s ometimes findnovel hierar chical-structur ed ZnO fractal n anofern s onthe su rface of the ZnO nanowire a rra y grown at edge

F i g u r e 4 . TEM image of ZnO nanowires synthesized at 450 C (a) and HRTEM image of ZnO nanowire with selected areaelectron diffraction pattern (b).

F i g u r e 5 . SEM images of novel hierarchical structured ZnO fractal nanoferns on the surface of ZnO nanowire array (a-c) an dHRTEM image of a ZnO fractal nanofern with selected area electron diffraction pattern (d).

3298 Ch em . M ater., Vol. 15, N o. 17, 2003 L yu et al.


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areas of the substrate where no catalyst nanoparticleswere supplied (Figure 5). The HRTEM image indicatest ha t t he produced ZnO fract a l nanoferns have nocatalyst par ticle at the t ip an d some strain. Actually,the growth model of those curious stru ctures could notbe explained using t he catalyst-assisted VLS model. Iti s well -known that s t rains and defects , which areoften found in the crystalline materials with wurtzitest ructure, mainly influence the one-dimensional

growth of whiskers or nanowires.21

Here we suggestthat the growth of ZnO fractal nanoferns originatesfrom some structural defects within the ZnO crystal.The fractal structure of ZnO nanowires could be usedto build functional nanodevices such as the t ips ofatomic force microscopes and scanning tunneling mi-croscopes.

C o n c l u s i o n

We demonst ra ted th at s ingle-crysta lline wurt zite ZnOna nowire arr ay was successfully fabr icat ed on th e NiO-

nan oparticle-deposited Al2O3 subst rate by a simplephysical vapor-deposition method at a low temperatureof 450 C. The d iam et er and grow th ra t e of ZnOnanowires increased as the growth temperature in-creased. TEM observation showed that the ZnO nano-wires were synthesized along the c-axial direction of thehexagona l crysta l structur e. We demonstra te tha t ZnOnanowires followed the catalyst-free growth mechanism.Some fascinating hiera rchically ordered str ucture wasalso observed. Our results hold promise for the fabrica-tion of ZnO-nanowires-based nanoscale devices ontovarious low-temperature-endurance substrates.

A c k n o w l e d g m e n t . We than k J . S. J oeng for helpfuldiscussion regard ing the TEM a na lysis. This work wassupported by the Center for Na notubes and Na nostruc-tured Composites at SKKU and by the National R&Dproject for Na no Science an d Techn ology of MOST.

CM020465J(21) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.;

Gibbons, P. C.; Buhro, W. E. S cience 1995, 270 , 1791.


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