The structural and optical properties of Volmer–Weber-type ZnO nanorods

Phys. Status Solidi A 208, No. 5, 1021–1026 (2011) / DOI 10.1002/pssa.201000054 p s sa

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applications and materials science

The structural and optical propertiesof Volmer–Weber-type ZnO nanorods

Se-Jeong Park1, Weizhen He1, Jijun Qiu2,7, Jin Woo Kim3, Ik Jae Lee4, Beomkeun Kim5, Hyung-Kook Kim1,2,6,and Yoon-Hwae Hwang*,1,2,6

1Department of Nano Fusion Technology & BK 21 Nano Fusion Technology Division, Pusan National University,

Miryang 627-706, Korea2Research Center for Dielectric and Advanced Matter Physics (RCDAMP), Pusan National University, Busan 609-735, Korea3Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea4Pohang Acceleration Laboratory, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea5School of Mechanical & Automotive Engineering, Inje University, Gimhae 621-749, Korea6Department of Nanomaterials Engineering, Pusan National University, Miryang 627-706, Korea7State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese

Academy of Sciences, Shanghai 200050, P. R. China

Received 26 March 2010, revised 18 June 2010, accepted 29 June 2010

Published online 21 March 2011

Keywords ZnO, Volmer-Weber type, nanorod, micro-Raman, photoluminescence, X-ray photoelectron spectroscopy

*Corresponding author: e-mail [email protected], Phone: þ82-55-3505274, Fax: þ82-55-3535844

Volmer–Weber-type ZnO nanorods (VW ZnO NRs) were

fabricated by using a hybrid method combined with RF

sputtering and hydrothermal methods. The structural and

optical properties of VW ZnO NRs were investigated by using

synchrotron X-ray diffraction (XRD), micro-Raman spectro-

scopy, photoluminescence (PL) spectroscopy and X-ray photo-

electron spectroscopy (XPS). It was found that VW ZnO NRs

showed wurtzite structure and were vertically standing on a

substrate with a compressive stress from the substrate that can

be reduced by annealing. In themicro-Raman study, a forbidden

E1(LO) mode in a bulk ZnOwas observed in VWZnONRs due

to the many edges in NR structures. It was also confirmed that

VWZnONRs have a wurtzite structure by observing E2 low and

E2 high peaks. In the PL spectra, VW ZnO NRs showed a broad

emission in the visible range, especially yellow emission, with a

weak UV emission. The UV emission was monotonously

enhanced on increasing the annealing temperature. The yellow

emission was enhanced in the annealed sample at 200 8C due to

an evaporation of hydroxyl groups (OH�) on the surface of VW

ZnO NRs.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction ZnO is awell-known II–IVoxide semi-conductor material that has wide bandgap (3.37 eV) and largeexciton binding energy (60meV). Recently, ZnO nanostruc-tures have attracted much attention due to their usefulproperties in many applications such as UV nanolasers [1],gas sensors [2–5], light-emitting diodes (LED) [6–8], dye-sensitized solar cells [9–11], etc. [12–16]. Among variousZnO nanostructures such as nanorods (NRs) [1–5], nanowires[6–16], nanobelts [17, 18] and many others [19–21], the NRstructure has been intensively studied due to its property ofpreferential growth along a certain direction which isespecially useful for various device fabrications [1–21].Usually, in a growth process of ZnO NRs, Stranski–Krastanow (SK)-type NRs [1–11] were formed because thedeposited atoms first form a seed layers then the ZnO NRstarted to grow. Different from SK-type ZnO NRs, also

Volmer–Weber (VW)-type [22–24] ZnO NRs [25] can growdirectly from the substrate without any seed layer. The uniquestructure of VW-type ZnO NRs might be more useful fordevice applications because NRs are separated from eachother. However, the growth of VW-type ZnO with a properlength is difficult by using sputtering or PLD method and thephysical properties of VW-type ZnO NRs have not beenintensively studied.

In this study, we fabricated VW-type ZnO NRs byusing a hybrid method combined with RF sputtering andhydrothermal methods. The structural and optical proper-ties of as-grown and annealed VW ZnO NRs wereinvestigated by using a synchrotron X-ray diffraction(XRD), micro-Raman spectroscopy, photoluminescence(PL) spectroscopy and X-ray photoelectron spectroscopy(XPS).


1022 S.-J. Park et al.: Structural and optical properties of Volmer–Weber type ZnO nanorodsp

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Figure 1 (onlinecolourat:www.pss-a.com)(a)SEMimageofVWZnO NRs with 772 nm height. (b) ZnO [002] peak of synchrotronXRD data (l¼ 1.24236 A at the 5C2 beamline at the PohangAcceleration Laboratory) of VW ZnO NRs shown in (a). A blacksolid line is the as-grown sample and a red dotted line is the 500 8Cannealed sample. The inset shows the full scan data.

2 Experimental A two-stepmethodwas introduced inthis study to grow VW ZnO NRs on Pt(111)/TiO2/SiO2/Si(100) substrates. First, VW-type ZnO nanocrystals wereformed on Pt(111)/TiO2/SiO2/Si(100) substrates as seeds byusing an RF sputtering method. The sputtering time was15min and the RF power was 50W. The growth conditionsfor the seeded ZnO nanocrystal formation on the substratewere almost identified to those in previous reports [22–24].Secondly, ZnO nanowires were synthesized on the VW-typeseeded substrates by using a hydrothermalmethod. Themoredetailed synthesis process was reported in a recentlypublished paper [25]. The grown samples were annealed inthe sputtering chamber at temperatures of 200 and 500 8C for30min in vacuum atmosphere of 5� 10�5 Torr.

The structural properties of VW ZnO NRs sample wereinvestigated by scanning electron microscopy (SEM, S-4700,HITACHI), synchrotron XRD spectroscopy (l¼ 1.24236 A at5C2 beamline in the Pohang Acceleration Laboratory),transmission electron microscopy (TEM, JEM 2011 at theBusan branch of KBSI) and micro-Raman spectroscopy(514.5nm Ar-ion laser, at the Gwang-ju branch of KBSI).The optical properties were examined by PL spectroscopy(SPEX 1403) by using a He–Cd laser (532 nm, at the Gwang-ju branch of KBSI) as a light source and XPS (ESCALAB250 XPS spectrometer at the Busan branch of KBSI) at roomtemperature.

3 Results and discussion Figure 1a shows an SEMimage of VW ZnO NRs. We could observe that a hexagonalshape VW ZnO NRs stood vertically without any ZnO layeron the substrate, which was confirmed by EDX measure-ments [25]. Figure 1b shows the synchrotron XRD data. Asshown in the inset, there was no extra peak except for a ZnO[002] peak, indicating a wurtzite structure of VW ZnO NRs.In Fig. 1b, the 2u values of theVWZnONRs for the as-grownsample and the 500 8C annealed sample were 27.539 and27.5298, respectively. Both values were smaller than that ofthe standard 2u value of bulk ZnO, 27.6098. Therefore, the c-axis lattice constants of the VW ZnO NRs from the X-raydata were 5.21964 A for the as-grown sample and 5.22150 Afor the 500 8C annealed sample and both values were largerthan that of bulk ZnO, 5.20661 A. From these results, itseems that the VW ZnO NRs acquire stress from theplatinum substrate because a relatively fast growth mechan-ism in the hydrothermal method caused the weak bindingbetween zinc and oxygen and lots of defects, such as oxygenvacancies, zinc interstitials or complexes of the defects. Thestrain caused by a stress can be described by the strain–stressrelation for the hexagonal symmetry zincite [26] as shown inEq. (1):

� 20

sXRDfilm ¼ 2C2

13�C33ðC11 þ C12Þ2C13

cfilm�c0c0

; (1)

where Cij are the elastic constants of zincite [27], s is thefilm stress that is from tensile or compressive strain, c0 andcfilm are the strain-free lattice parameter and the lattice

11 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

parameter of the film, respectively. The estimated stressfrom Eq. (1) was 0.5331GPa for the as-grown VW ZnONRs and was 0.6008GPa for the 500 8C annealed sample:i.e. the stress after annealing increased about 13% comparedto that in the as-grown VW ZnO NRs.

Due to the stress from the substrate, the lattice constantof ZnO (002) planewas larger than that of the bulk. The TEMexperimentwas conducted tomeasure the lattice constants ofa VW ZnO NR at different positions. The inset of Fig. 2ashows the image of whole VW ZnO NRs and two differentpositions were selected. As shown in Fig. 2b, one is in themiddle of the NR, ‘A’ region and the other is the tip of theNR, ‘B’ region. Figure 2b and c shows lattice image ofselected regions in Fig. 2a, and the values of lattice constantswere 0.217 nm (0.434 nm for a wurtzite structure) for the ‘A’region and 0.205 nm (0.410 nm for a wurtzite structure) forthe ‘B’ region. The two values from different positions weresmaller than that of the bulk ZnO and the value from our

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Figure 2 (online colour at: www.pss-a.com) TEM images of VWZnO NRs separated from the substrate. (a) The A region is in themiddle of theNRs and theB region is the tip of theNRs, and the inset(left down) is the whole image (the scale bar is 100 nm). (b) Thelattice imageofZnO(002) in theAregion. (c)The lattice imageof theB region.

Figure 3 (online colour at: www.pss-a.com) The nonresonantRaman spectrum of VW-type ZnO NRs at room temperature. Thebackscattering geometry was used and the incident light was unpo-larized (Ar ion laser, 514.4 nm).

previous XRD measurement. It seems that the stress at thebottom where a VW ZnO NR stands on the substrate makesthe lattice constant extended. However, the upper part of aVW ZnO NR is free from the stress, therefore the latticeswere relaxed and shrunk by the defects or dangling bondstoward the tip of VW ZnO NRs.

We studied the Raman modes in VW ZnO NRs by usinga nonresonant micro-Raman technique. A backscatteringgeometry was used and the light source (20mW, 514.5 nm)was unpolarized and was parallel to the c-axis of the sample.Wurtzite ZnO belongs to the P63mc space group and fouratoms are in the primitive unit cell. Optical phonon modesare involved in first-order Raman scattering at the Brillouinzone centre (G-point) [28–33]. According to group theory,Raman-active modes are A1(TO, LO), E1(TO, LO), andE2(high, low) andB1(high, low) is the silent, Raman-inactivemode [32, 33].

Figure 3 shows the Raman spectra of VW ZnO NRs atroom temperature. In the whole spectra, E2 low and E2 high,E1(LO) modes, and the 2nd-order peaks were distinguished.E2 low and E2 high peaks existed at 100 and 439 cm�1,respectively, and were the most dominant peaks. Theexistence of E2 peaks indicates that VW ZnO NRs have awurtzite structure. E2 low corresponds to the vibration of theheavy Zn sublattice and E2 high corresponds to the vibrationof the oxygen sublattice [34].

In Fig. 3, we could not observe A1(TO) and E1(TO)modes that are supposed to be observed with the back-scattering geometry [35]. However, we were able to observethose two modes in a longer VW ZnO NRs (data are notshown in this paper).A1(TO)was distinguishable butE1(TO)was only revealed by the deconvolution of the E2 high peak. Itseems that the relatively short distances along a- and b-axesin NR structures made the E1(TO) mode silent in VW ZnONRs.

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The peak at thewave number of 563 cm�1 corresponds totheA1(LO)mode. It only appears when the c-axis of wurtziteZnO is parallel to the sample surface [34, 36, 37]. TheE1(LO) mode, forbidden in the backscattering geometry[35], was also found at 583 cm�1. According to Ref. [35], theE1(LO) mode appeared when the incident light was focusedon a sample edge. Therefore, it seems thatmany edges ofVWZnO NRs may allow the E1(LO) mode to be displayed in thespectra. The 2nd-order peaks also showed at 333, 1105 and1157 cm�1, which are the acoustic phonon overtone andoptical phonon overtone with A1 symmetry and E1

symmetry.From the XRD data analysis in Fig. 1, it was mentioned

that VW ZnO NRs experience a compressive stress from thesubstrate. Generally, the Raman shift ofE2 high in bulk ZnO is437 cm�1 and shows a larger value, if the compressive stresstakes place in ZnO [38]. The observed larger value of theE2 high Raman shift of VW ZnO NRs (439 cm�1) alsoconfirms the existence of a compressive stress observed inXRD data.

Figure 4 shows room-temperature PL spectra of as-grown VW ZnO NRs and annealed VW ZnO NRs attemperatures of 200 and 500 8C. PL of bulk ZnO typicallyshows a UV peak (3.27 eV) that is near band-edge emission[39], and a green-yellow broad band caused by intrinsic orextrinsic defects in ZnO. However, most of the visibleemissions seem to depend on the fabrication methods of theZnO sample, growth conditions, dopants and growthenvironment and so on.

In the case of VW ZnO NRs, the PL of the as-grownsample exhibited a weak UV emission, but showed broadred-green band and dominant yellow emission at 2.16 eV,which came from complexes of oxygen vacancies and zincinterstitials (VoZni) [40]. Yellow emission is a commonfeature in samples prepared by a hydrothermal process and



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Figure 4 (online colour at: www.pss-a.com) Room-temperaturephotoluminescence spectra of as-grown VW ZnO NRs (-&-, blackline) and annealed VW ZnO NRs at 200 8C (-*-, red line), and500 8C (-D-, blue line).

Figure 5 (online colour at: www.pss-a.com)High-resolution XPSspectra of O1s of VW-type ZnO NRs with different annealingtemperatures at room temperature.

was typically attributed to oxygen interstitials [41].Regardless of the exact origin of the yellow-orange emission,oxygen interstitials can be one of the main factors that causethat emission and this luminescence is not significantlyinfluenced by the surface enhancement [41]. The deep levelresponsible for yellow emission did not come from thesurface of samples unlike green emission [40]. It is generallybelieved that a green emission of ZnO originates fromoxygen vacancies. However, several mechanisms for thegreen emission have been proposed, for example thetransition between singly ionized oxygen vacancy and aphotoexcited hole [42], the transition between an electronclose to the conduction band and a deeply trapped hole atVþþ

o [43], and surface defects [44] and so on. The variousdefects like oxygen interstitials, oxygen vacancies or zincinterstitials could make sublevels induce a nonradiativeprocess and obstruct the direct recombination of theelectrons and the holes between the conduction band andthe valence band in VW ZnO NRs. Therefore, the yellowemission predominantly appeared in the PL spectra of as-grown VW ZnO NRs instead of the UV emission. In Fig. 4,there exists a shoulder around 1.92 eV that is related to theincident laser (He–Cd laser, 325 nm).

The purpose of the annealing process is to improve thecrystal quality or to remove residual chemicals. As we canobserve in Fig. 4, the intensity of UV emission increased asthe annealing temperature increased up to 500 8C. However,the intensity of the yellow emission did not monotonouslydecrease. The yellow emission of VW ZnO NRs annealed at200 8C increased dramatically. A possible explanation is thedisappearance of the hydroxyl groups (OH�) covering thesurface of the ZnO NRs. According to Moon et al.’s report[45], the ZnO coated with hydroxyl groups (OH�) on thesurface showed the quenching effect of yellow emission, andthe emission was recovered upon reheating the samples.


While our samplewas undergoing the heat process at 200 8C,the hydroxyl groups on the surface were evaporated and thequenching effect of the visible range would disappear.Therefore, it seems that the heat treatment might be moreeffective for the elimination of the hydroxyl groups on thesurface than for the realignment of the inner NR structure.

After the sample was annealed at 500 8C in vacuum, theintensity of the yellow emission dramatically decreased andthe weak blue emission appeared along with the enhance-ment of the UV emission. It is believed that the weaklybonded zinc and oxygen elements were combined by a highthermal energy. However, the yellow emission did notdisappear completely because the defects that emittedyellow still exist in the NRs. If we had annealed theVolmer-Weber type ZnO nanorods for more than 1 hour, theyellow emission would have been much more lessened. Theyellow centre peak of 2.16 eV was divided into two peaks,2.07 and 2.19 eV, and the origins of the two peaks are notclear at this stage.

Figure 5 shows the high-resolution XPS data of O1s inas-grown, annealed at 200 8C and annealed at 500 8C ZnONRs. The XPS data of the as-grown and the 200 8C annealedsample consisted of two Gaussian curves. One (OI) peak

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originates from the O2� combining with Zn2þ [46, 47] islocated around 530 eV and the other (OII) peak is locatedaround 531 eV. There exist several possible origins of the OII

peak, which are oxygen in the hydroxyl groups [48–50], anintermediate state between O2� and dissociated oxygen[48, 51], doubly combined carbon with oxygen[52] andoxygen deficiency in the ZnO matrix [51, 53]. As we canobserve in Fig. 5, there was no peak position shift betweenas-grown and 200 8C annealed samples. The only differenceis the ratio of the OI to the OII peak area. After annealing thesample at 200 8C, the area of OI peak increased about 3%(from 31 to 34%). In contrast, that of OII peak decreasedabout 3% (from 69 to 66%). Based onXPS data, there was nosignificant change of O1s peak between the as-grown and the200 8C annealed samples.

After annealing at 500 8C, as shown in Fig. 5, the bindingenergy of the Zn–O bond (OI) shifted 400meV to a higherenergy. It is thought that Zn2p orbital and O1s orbital weremore overlapped to enhance the UV emission. The secondpeak (OII) also shifted about 700meV to a higher energy,indicating that a considerable change took place in the ZnONRs besides the evaporation of hydroxyl groups. Possibleorigins of theOII peak position change are adsorbed-oxygen-related species and suboxides of ZnO by a thermal process[54], and the partial elimination of the aforementionedorigins. InXPS data of the 500 8Cannealed sample, we couldobserve an extra peak (OIII) at 533.1 eV that corresponds tosingly bonded carbon with oxygen [52]. It also indicates thathydroxyl groups on the sample surface prevent contami-nation from air in addition to the quenching effect on yellowemission.

4 Conclusion We fabricated VWZnONRs by using ahybrid method combined with an RF sputtering method andhydrothermal method. It was found from the XRD and SEMmeasurements that the VW ZnO NRs grew vertically on thePt substrate along the [002] growth direction and has awurtzite structure. It was also found that VW ZnO NRsexperience a compressive stress from the bottom becausetherewas no seed layer inVWZnONRs.We also confirm thewurtzite structure of VW ZnO NRs from the backscatteringRamanmeasurements by observing E2 low and E2 high modes.In VW ZnO NRs, A1(TO) and E1(TO) modes that exist inbulk ZnO were absent from the Raman spectrum. However,the forbidden E1(LO) mode was observed due to theexistence of many edges in the NR structure.

We observed a weak UV emission and a strong yellowemission in as-grown VW ZnO NRs due to the existence ofhydroxyl groups on surfaces and defects. The yellowemission of the annealed sample at 200 8C was enhancedbecause the screening effect of hydroxyl groups (OH�)disappeared. The UV emission was monotonously enhancedby heating the sample up to 500 8C.

Acknowledgements This work is supported by NationalResearch Foundation of Korea (grant no. 20100027284).

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