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Materials Letters 62 (2

Synthesis of novel zinc oxide microphone-like microstructures

Aurangzeb Khan ⁎, Martin E. Kordesch

Department of Physics and Astronomy and CMSS program, Ohio University, Athens OH 45701 USA

Received 11 April 2007; accepted 3 May 2007Available online 10 May 2007

Abstract

Novel microphone-like ZnO microstructures were grown at a very high density via a simple thermal evaporation process using commerciallyavailable ZnO powder in ambient air at ∼1050±20 °C in 1 h. The unique as-grown microstructures were characterized in detail in terms of theirstructural and optical properties. The structural properties of the synthesized products confirmed that they were wurtzite hexagonal phase for theas-grown products. Raman-scattering spectra exhibited a strong and dominated Raman-active E2 (high) mode at 441 cm−1, confirming thewurtzite hexagonal phase for the as-grown microphone-like ZnO morphologies. The cathodoluminescence (CL) spectrum shows a suppressednear band edge emission at ∼380 nm and strong green emission at ∼500 nm.© 2007 Elsevier B.V. All rights reserved.

PACS: 71.55.Gs; 73.61.Ga; 74.25.GzKeywords: Zinc oxide; Microstructures; Raman spectroscopy; Cathodoluminescence; Thermal evaporation; X-ray diffraction

1. Introduction

ZnO is a wurtzite hexagonal phase II–VI semiconductorwith a direct wide band gap (3.37 eV), large excitation bindingenergy (60 meV), larger than other semiconductor materialssuch as GaN (25 meV) and ZnSe (22 meV). It is one of the mostimportant functional materials which exhibits near-UV emis-sion, transparent conductivity, and piezoelectricity. Due to theversatile properties of ZnO, it has been used in a variety ofapplications which include nanolasers, field effect transistors,gas and bio-sensors, nanocantilevers, nanoresonators, nano-circuits [1–3], and so on. ZnO has shown a wide diversity in itsmorphologies and hence it is believed that probably it is the

⁎ Corresponding author. 251 B Clippinger labs, Department of Physics andAstronomy, Ohio University, Athens OH 45701, USA. Tel.: +1 740 597 1259;fax: +1 740 593 0433.

E-mail address: khan@helios.phy.ohiou.edu (A. Khan).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.05.005

richest family of nanostructures. In recent years, much progresshad been made in the synthesis of various kinds ofmorphologies of ZnO. Until now, a variety of ZnO nano- andmicrostructures have been synthesized via a number offabrication techniques which include nanowires, nanorods,nanobelts, nanotubes, nanobows, nanorings, nanosprings,hexagonal nanocolumns and so on [4–13]. In addition tothese morphologies some complex morphologies of ZnO hadalso been reported in the literature, for instance, Umar et al.reported the complex star and flower-shaped morphologies bynovel cyclic feeding chemical vapor deposition process[14,15], Khan et al. and Umar et al. successfully synthesizedmicro-sized cages and spheres [16,17], Niu et al. grownmushroom-like ZnO microcrystals through a solution calcina-tion process [18], complex nanosheet networks and hexagonalnanodisks have also been prepared by thermal evaporationprocess [19], Lao et al. andWang et al. fabricated complex ZnOnanocombs on a polycrystalline Al2O3 substrate by thermalevaporation of ZnO [20,21], etc. Even though a large number ofZnO morphologies are reported in the literature, but to the bestof our knowledge, the microphone-like morphologies of ZnO

Fig. 1. SEM images of the ZnO microphone-like structures. (a) shows microphone-like structures made of nanostructures with blunt facets as enlarged image can be in(c). The needles decorated microphone-like structures are shown in the low magnification image in (b). These kinds of structures have needles both blunt and sharpcoming out of it and are shown clearly in (d) and (e).

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are not yet reported. Here, we present the successful synthesisof microphone-like morphologies of ZnO simply by thermalevaporation process using commercially available ZnO powderin ambient air at ∼1050±20 °C. Moreover, the as-grownproducts have been extensively characterized in terms of theirstructural and optical properties. The as-grown microphone-like structures may have potential applications in thefabrication of various efficient optoelectronic devices in thenear future.

2. Experimental

Microphone-like ZnO structures were synthesized viathermal evaporation process using commercially availableZnO powder (Alfa-Aesar 99% min, 325 mesh) as sourcematerial. The synthesis was done in ambient air at ∼1050±20 °C. The source material, ZnO powder was put into a quartzboat and placed at the center of the resistively heated furnace.After the desired reaction time (1 h), the reaction was terminatedand a grayish white colored powder was obtained in the quartzboat which was characterized in terms of its structural andoptical properties. The general morphologies of the as-grownproducts were characterized using the scanning electronmicroscope (SEM) [JOEL JSM 5300] equipped with the energy

dispersive X-ray (EDX) spectroscopy. The detailed propertiesof the structures were determined using high-resolutiontransmission electron microscopy (HRTEM) [Philips CM300UT instrument operated at 300 kV and providing a point-to-point resolution of 1.72 Å]. The crystallinity of the synthesizedmicrostructures were examined by X-ray diffraction (XRD)patterns [Rigaku Geigerflex, 2000 Watts] using Cu-Kα (1.54 Å)as the incident radiation. Raman spectroscopy was done with aWITec near-field scanning optical microscope operating in theRaman mode having a 10× lens with a working distance of7 mm using a frequency doubled Nd:YAG with 532-nm laserexcitation [22]. The laser intensity was controlled at 5 mW andthe spatial resolution of the microscope operating in thisconfiguration was 1.3 μm. Cathodoluminescence (CL) mea-surement of the as-grown ZnO microstructures was conductedusing a CL setup, the details of this CL setup is explainedelsewhere [23].

3. Results and discussion

Fig. 1(a)–(e) show the typical SEM images of the as-grown productsynthesized by direct heating of commercial ZnO powder in ambientair. The images clearly reveal that the as-grown are the microphone-likemorphologies grown in very high density. Fig. 1(a) and (b) shows thetypical SEM images of two different areas of the as-grown products

Fig. 2. (a) A typical XRD spectrum of the ZnO structures. All the peaks wereindexed to wurtzite hexagonal structures and (b) EDX of the as-grown ZnOmicrostructures.

232 A. Khan, M.E. Kordesch / Materials Letters 62 (2008) 230–234

obtained from the same quartz boat. These low magnifications images,(Fig. 1(a) and (b)) reveal that microphone-like structures containmainly three types of heads, i.e. microphone head consisting ofnanostructures with blunt facets (Fig. 1(c)), head containing nail-likenanorods (Fig. 1(d)) and head possessing needle-like nanorods (Fig. 1(e)). It is interesting to note that the diameter of the microphone stem isreducing from top to the bottom. The diameters of the microphonestems are in the range of 40–60 μm. The sizes of the microphone headsare in the range of 80–110 μm. Furthermore, interestingly, it was alsoobserved that the whole microphone-like structure is composed of onespecial kind of nanostructures i.e. blunt facet, nail-like or needle-likenanorods. The average diameters of these nanorods are in the range of700–900 nm, in all the cases.

The crystallinity and crystal growth direction and chemicalcompositions of the as-grown microphone-like structures wereobtained by the X-ray diffraction (XRD) pattern and energy dispersiveX-ray (EDX) spectroscopy and shown in Fig. 2. Fig. 2(a) shows thetypical XRD pattern of the as-grown products which confirms that thesynthesized microphone-like structures are single crystalline andpossessing a wurtzite hexagonal phase of ZnO. All the peaks arelabeled to wurtzite crystal phase of pure bulk ZnO, with latticeconstants a=3.25 Å and c=5.21 Å (ICDD PDF card #00-003-0888).Moreover, no other peak related to any impurities was obtained from

the XRD pattern. It is also confirmed from the EDX spectra, that thegrown products are composed of zinc and oxygen only and no impuritywas found in the as-grown products (Fig. 2(b)).

The detailed structural characterization of the as-grown productswas done with high-resolution transmission electron microscopy(HRTEM) and shown in Fig. 3. Fig. 3(a) and (b) shows the typicalHRTEM images of the blunt-shaped and needle-shaped ZnO nanorodsgrown in the microphone-like morphologies. The corresponding line-scan of each image shows that distance between two fringes is about0.52 nm, equal to the lattice constant of wurtzite hexagonal phase ZnO,confirming that the grown structures are single crystalline and grownalong the c-axis direction.

The optical properties of as-grown products were observed usingthe Raman-scattering spectrum and cathodoluminescence (CL) studies.Fig. 4 shows the Raman-scattering spectrum of the as-grown ZnOmicrophone-like structures. As for ZnO is concerned, it has a wurtzitecrystal structure, which belongs to the space group C6v

4 , having twoformula units per primitive cell, where all atoms are occupying C3v

sites [8,19]. Group theory predicts that A1+2E2+E1 are Raman-activemodes which are as follows: E2(low) at 101 cm−1, A1(TO) at380 cm−1, E1(TO) at 407 cm−1, E2 (high) at 437 cm−1, A1(LO) at574 cm−1, and E1(LO) at 583 cm

−1 [24]. The low frequency E2 Ramanmode corresponds to the heavy Zn sublattices, while the highfrequency E2 Raman mode is attributed to the oxygen atoms. In ourmicrophone-like ZnO structures, the following modes have beenobserved: A1(TO) at 380 cm−1, E1(TO) at 410 cm−1, E2(high) at441 cm−1, and A1(LO) at 576 cm−1. The rest of the vibrations areusually labeled as 2nd order vibrations [24]. The full width halfmaximum of the E2 (high) is ∼8 cm−1, showing good crystallinequality of the structures. These microstructures are formed by smallcrystallites, so there is a small shift in the Raman frequencies of theoptical phonons relative to the bulk, which usually occurs innanostructures and its origin is still under debate [25].

Fig. 5 shows a typical CL spectrum of the as-grown microphone-like ZnO structures. Mainly two peaks have been observed from thespectrum, i.e. peak centered at 381 nm in the ultraviolet (UV) region,which originated due to the recombination of free excitons throughan exciton–exciton collision process and corresponding to the nearband edge (NBE) emission of wide band gap of ZnO [8,10,19], whilethe other peak was centered at 510 nm in the green visible region.The green emission is also called as deep level emission (DLE) andoriginates from the structural defects in the corresponding products.Therefore DLE is usually called defects related peak, and is mainlybelieved to be originated from the oxygen vacancies [8,10,19]. Thegreen emission is dominated over the near band edge emission asobserved from CL spectrum which indicates that the as-grownproducts have some impurities and structural defects such as oxygenvacancies and zinc interstitials. The enhanced green emission in theformed microphone-like ZnO structures has a great interest fortypical applications of ZnO phosphors, such as field emissivedisplay technology, and etc.

4. Conclusion

In summary, novel microphone-like ZnO microstructureshave been successfully synthesized by direct heating ofcommercial ZnO powder in ambient air. The detailed structuralcharacterizations confirmed that the ZnO microphone-likestructures are single crystalline and are a hexagonal phasewurtzite. Raman-scattering spectrum exhibited a sharp andstrong peak for E2 (high) mode, again confirming the wurtzite

Fig. 3. A typical HRTEM images of the (a) crystallite and (b) needle, which form the microphone-like structures.

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hexagonal phase for the as-grown products. The room-temperature CL spectrum shows a broad green emission and asuppressed and small UV emission. The enhanced green

Fig. 4. Raman laser of the ZnO microphone-like structure with frequencydoubled Nd:YAG (532 nm).

emission in the formed microphone-like ZnO structures maybe of interest for applications of ZnO phosphors in displaytechnology, and etc.

Fig. 5. CL spectrum of the as-grown product ZnO microstructures.

234 A. Khan, M.E. Kordesch / Materials Letters 62 (2008) 230–234

Acknowledgements

We are thankful to Dr. H. H. Richardson, Department ofChemistry and Bio-chemistry for helping us with the Ramanspectra and Dr. Jadwisienczak, School of Electrical Engineeringand Computer Science, Ohio University, for the CL spectra. Weare also grateful to Dr. Saule Aldabergenova at the University ofErlangen Nuremberg, for his assistance to study the HRTEM.

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