Materials Letters 65 (2011) 1938–1940

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Materials Letters

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Growth of self-assembled ZnO rods cellular network over a large area by thermalevaporation method

M. Senthil Kumar a, Deepak Chhikara b, K.M.K. Srivatsa b,⁎a Physics of Energy Harvesting Division, National Physical Laboratory, Council of Scientific and Industrial Research (CSIR), New Delhi, 110 012, Indiab Materials Physics and Engineering Division, National Physical Laboratory, Council of Scientific and Industrial Research (CSIR), New Delhi, 110 012, India

⁎ Corresponding author. Tel.: +91 11 45608609; fax:E-mail address: kmk_srivatsa@nplindia.org (K.M.K. S

0167-577X/$ – see front matter © 2011 Published by Edoi:10.1016/j.matlet.2011.03.069

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2011Accepted 16 March 2011Available online 23 March 2011

Keywords:Network patternThermal evaporationElectron microscopy (SEM)Photoluminescence spectroscopy

Self-assembled hexagonal wurtzite ZnO rods network has been grown on Si (100) substrate by thermaloxidation of Zn powder at 600 °C with an oxygen flow of 100 sccm. The ZnO rods are found to be grown in acellular network structure over a large substrate area in millimeter scale. They exhibit well-defined hexagonalfacets with a diameter of ~200 nm and a length of ~2.5 μm and grow along ZnO b0001N direction mostlyperpendicular to the substrate. Excellent luminescence properties of a strong UV emission peak withnegligible green band have been obtained at room temperature.

+91 11 45609310.rivatsa).

lsevier B.V.

© 2011 Published by Elsevier B.V.

1. Introduction

Synthesis of ZnO nanostructures has been under intense researchover a decade due to their excellent physical properties, such as widedirect band gap (3.37 eV) with large exciton binding energy (60 meV)and good piezoelectricity and various forms of interesting nanos-tructures have been established [1]. Subsequently, several ZnO basednano-devices have been developed in recent years for variousapplications in short-wavelength photonics, gas/chemical/bio sens-ing, electronics, and field emission [2]. Here, organizing thenanostructures as functional two-dimensional networks is a realchallenge for fabrication of functional and practical nano-devices.Researchers employ few advanced tools like focused ion beam [3],atomic force microscopy [4], etc. for post-growth handling ofnanostructures for this purpose. Though various desired structurescould be fabricated in principle through these advanced tools, thefabrication process is relatively slow and inconvenient and alsodemands a heavy budget. Therefore, growth of self aligned nanos-tructutres at designated positions is of great interest as it is aneconomic and convenient approach to fabricate nanostructuralnetworks. A number of techniques for the self-alignment ofnanostructures during growth has been developed by using metalcatalyst, substrate modification, electrical field, etc. In most cases, thesubstrate is pre-patternedwithmetal catalyst by lithography to assignthe nanostructure growth [5]. However, the growth of nanostructuresusing self-patterned metal catalyst without the complex lithography

could be a more simple and feasible approach to fabricate networknanostructures [6].

In the present work, we report self-assembled growth of well-faceted ZnO nanorods in cellular network structure on Au coated Si(100) substrate by thermal oxidation of Zn powder source.

2. Experimental

The ZnO rods network growth was performed by simple thermalevaporation technique in a horizontal quartz reactor mounted in aresistive heating furnace. A thin Au layer of about 8 nm thickness wasdeposited on Si (100) substrate by using sputtering technique at roomtemperature. Pure Zn (99.99%) powder of about 2 g was put in analumina boat and the Si substrate was placed face down direction onthe alumina boat at 5 mm above the Zn source. The alumina boat wasthen loaded into center of the horizontal quartz reactor and thereactor was flushed with N2 gas for about 20 min to clean the growthzone. Then, the reactor temperature was raised to a desired value at arate of 30 °C/min under the continuous supply of 1 SLPM of N2 gas.The O2 gas was added at the reach of growth temperature and thegrowth was carried for a period of 40 min for all the experiments. Thegrowth was performed at a furnace temperature of 600 °C and O2 gasflow of 100 sccm. After the growth, the reactor was allowed to cooldown to room temperature under the same gas flow conditions.Whitish or grayish-white colored deposition products were found onthe whole surface area of the substrate.

The morphology and composition of grown samples werecharacterized by scanning electron microscopy (SEM) equippedwith energy-dispersive X-ray (EDX) spectrometer and their crystal-line properties were investigated by x-ray diffraction (XRD). Room

Fig. 1. SEM images of self-assembled ZnO rods network grown on Si (100) substrate measured at different magnifications: (a) 200 X, (b) 1 KX, (c) 10 KX and (d) 30 KX.

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temperature photoluminescence (RT-PL) was measured with XenonLamp (at 325 nm) as an excitation source to study the opticalproperties of the grown samples.

3. Results and discussion

Surfacemorphology of the grown ZnO rods, as observed by the SEM,is shown in Fig. 1 (a–d). As seen in Fig. 1(a), the ZnOrods are observed togrow in a cellular network pattern over a large area of Si substrate. Thatis, the rods grow at the boundary of cellular structure and at very fewplaces, they are also found inside the cells entirely or partially fulfillingthe area as indicated by the arrowmarks in Fig. 1(b). The cell structuresvary from 20 to 80 μm in size and they are continuously joined to eachother forming a cellular network structure. A close SEM observation[Figs. 1(c) and (d)] clearly identifies hexagonally faceted top end andside planes of the grown rod array, which indicates that the rods growalong the b0001N direction mostly perpendicular to the Si substrate.Dimensions of the rods are highly uniformwith a diameter of ~200 nmand a length of ~2.5 μm.EDXmeasurements revealed the presence of Znand O without any impurities thus confirmed the formation of highquality ZnO rods. The cross-sectional view of the grown ZnO rodsnetwork structure is presented in Fig. 2. The ZnO rods are growndirectlyfrom the Si substrate and their base size at the interface is slightly biggerthan the diameter of the rods. Also, the bases are connected when therods grow closer to each other.

Fig. 2. Cross-sectional SEM image of self-assembled ZnO rods network grownonSi (100)substrate. The arrow marks point the interface of the rods with the substrate.

Fig. 3 gives the θ–2θ XRD scan of the self-assembled ZnO rodsnetwork structure on Si (100) substrate. The spectrum shows XRDpeaks that could be well-indexed to hexagonal wurtzite structure ofZnO. Out of the observed ZnO peaks, the ZnO (0002) is quite strong inintensity. Low intensity peaks related to Au, Zn, and AuZn are alsoobserved. The high intensity peak at (0002) implies that the grownZnO rods are of highly crystalline in nature with phase purity andpreferentially grow along c-axis from the substrate as also observedby the SEM. To the best of our knowledge, the growth of self-assembled rod network structure on Au coated Si (100) without anypre-patterning has been observed for the first time.

The self-formation of ZnO rod cellular network structures seemsunlikely andmighthave beenguidedby theAu catalyst. The SEMdidnotshow the presence of Au tip on the grown rods but EDX confirmed theexistence of Au catalyst in the grown sample. Huang et al. have dem-onstrated the selectivegrowthof hexagonally facetedZnOnanowiresonAupatterned substratewithout observing anymetal tip at the endof thenanowires [5]. In our case, it is assumed that the thin Au catalyst layer isconverted into nanoclusters during the ramping period of the reactor,and formed a cellular network structure. Au catalyst could dissolve theZn vapor in it tomake anAu–Zn alloy thatwill lead to the growth of ZnOrods [7]. Hence, the ZnO rod growth follows Au nanocluster pattern toform the reported network structure. It is observed that the formation of

Fig. 3. The θ–2θ XRD spectrum of self-assembled ZnO rods network grown on Si (100)substrate. The peaks are indexed to hexagonal wurtzite crystal structure of ZnO.

Fig. 4. Room temperature photoluminescence spectrum of ZnO rod network structureon Si (100) substrate.

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ZnO self-assembled network structure is very much sensitive to thegrowth condition and was grown only at 600 °Cwith an oxygen flow of100 sccm. Further investigations are in progress to understand theformation of self-assembled cellular network pattern.

The growth process of 1-D ZnO structures are normally describedby vapor–liquid–solid (VLS) or vapor–solid (VS) mechanisms [8,9]. Asdiscussed before, no Au particles were found at the tip of the grownrods and hence, the VS mechanism is followed to explain the growthof ZnO rods [7]. The growth of rods by VS mechanism is divided intotwo parts: nucleation and growth. At first, the Au/Zn droplets becomesupersaturated and ZnO nucleation can be promoted by reactingwith oxygen when made available. At this stage, hexagonally facetedZnO could form on the Si substrate due to the inherent property ofwurtzite ZnO crystal. In a crystal point of view, the wurtzite ZnO hasthe low energy facets of {01–10}, {2–1–10}, and {0001} and they havea tendency to form faceted hexagonal structures with {0001} topand bottom surfaces enclosed by the six crystallographic equivalent{01–10}/{2–1–10} side surfaces [10]. In the second stage, rod sproutsout from the faceted ZnO nucleation. The interface of ZnO nanorodwith Si (100) substrate has been investigated by Wu et al. by usinghigh resolution transmission electron microscopy and electronenergy-loss spectroscopy [11]. It has been found that the ZnO(0002) plane can grow parallel to Si (100) plane with formation ofa thin SiOx interfacial layer. According to the lowest surface energyprinciple, the chemically active (0002) Zn face at the top grows fasterthan other faces and forms faceted single crystalline hexagonal-

shaped 1-D nanostructure [12]. As the Zn and O fluxes arecontinuously supplied, the ZnO rods grow along vertical direction toform the aligned rod network structure.

Optical properties of the grown ZnO rods have been characterizedby PL measurements at room temperature. The typical RT-PL spec-trum is presented in Fig. 4. A UV emission is observed at 386 nm alongwith a weak, broad green emission centered at 500 nm. The UV bandemission of ZnO has been well-understood to be related to the excitonemission. The green transition has been attributed to the singlyionized oxygen vacancy in ZnO and the emission results from theradiative recombination of a photo-generated hole with an electronoccupying the oxygen vacancy [13]. The strong UV emission from ZnOrods network structure with negligibly lower green emission issuggested to be due to the low concentration of oxygen vacancies.

4. Conclusion

Self-assembled ZnO rod cellular network structure has been ob-tained on Au coated Si (100) substrate by using thermal evaporationtechnique. The grown ZnO rods showed wurtzite crystal structure withwell-constructed hexagonal facets and grow along b0001N directionpreferentially perpendicular to the Si substrate. Excellent photolumi-nescence properties were also measured.

Acknowledgements

The financial support from CSIR through network project (NWP-25) to carry out this work is gratefully acknowledged.

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