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doi:10.1016/j.ph

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Physica E 30 (2005) 51–54

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Effect of substrate temperature on the growth and luminescenceproperties of ZnO nanostructures

Aurangzeb Khan�, Martin E. Kordesch

Department of Physics and Astronomy and CMSS program, Ohio University, 251 B Clippinger, Athens, OH 45701, USA

Received 6 June 2005; accepted 4 July 2005

Available online 2 September 2005

Abstract

ZnO nanowires, nanorods and nanoribbons have been prepared by heating a mixture of ZnO/graphite powders using the thermal

evaporation and vapor transport on Si(1 0 0) substrates without any catalyst. The nanostructures are grown as a function of substrate

temperature ranging from 900 to 1300K. These nanostructures are of the size 100–300 nm in diameter or width and several tens of

micrometers in length. We studied the influence of the substrate temperature on the luminescent properties of these nanostructures. We

observed a strong relationship between the substrate temperature and the green emission band in ZnO, i.e., the photoluminescence study

revealed that the green emission peak of the ZnO nanostructures is suppressed relative to the band edge emission when the substrate

temperature is decreased from 1300 to 900K.

r 2005 Elsevier B.V. All rights reserved.

PACS: 78.55.Et; 78.67.Bf; 81.10.Bk

Keyword: Nanostructures; ZnO; Photoluminescence; Nanowires

1. Introduction

One-dimensional nanostructured materials such as na-nowires [1], nanorods [2] and nanobelts [3] have attractedmuch attention over the years due to their fundamentalimportance and potential applications in nanodevices [4].ZnO, a wide band gap semiconducting material (3.37 eV at300K) [5] with large exciton binding energy (60meV), hasshown promising prospects for nanoscale structures. ZnOnanowires, nanorods, etc. have been successfully synthe-sized by various methods like chemical or physical vapordeposition [6–8], Thermal evaporation [9], magnetronsputtering [10] and laser ablation [11] and have receivedmuch attention due to their novel physical properties ofnanoscale materials, and their potential application inconstructing nanoscale electronic and optoelectronic de-vices [12]. The properties of ZnO have been studied

e front matter r 2005 Elsevier B.V. All rights reserved.

yse.2005.07.005

ing author. Tel.: +1740 597 1259; fax: +1 740 593 0433.

ess: khan@helios.phy.ohiou.edu (A. Khan).

extensively using various techniques such as positronannihilation spectroscopy [13–19], electron paramagneticresonance (EPR) [20–25] and photoluminescence (PL)[26–28]. A number of theoretical calculations investigatingthe defects in ZnO have been reported [29,30]. However,there are still a number of disputes such as the shallowdonors in ZnO and the origin of the visible emission.Moreover, the position of the peaks in EPR and PL studiesof ZnO is also controversial. This may be due to the factthat the properties of ZnO depend on the fabricationconditions.Our objective was to study the luminescent properties of

the ZnO nanostructures grown at various substratetemperatures. As the non-radiative recombination centersin the ZnO vary with temperatures, ZnO nanostructuresare synthesized at different temperatures ranging from 900to 1300K, and PL study was performed at 300Kon all samples. We observed a strong relationshipbetween the substrate temperature and the green emissionband of the ZnO nanostructures. Banerjee et al. [12] have

ARTICLE IN PRESSA. Khan, M.E. Kordesch / Physica E 30 (2005) 51–5452

also shown that the growth of ZnO nanostructures isextremely temperature sensitive, which supports our basichypothesis.

2. Experimental

ZnO nanowires have been grown by thermal evaporationand vapor transport using a mixture of ZnO powders (alfaaesar 99.0% min, �325 mesh) and graphite powder (alfaaesar 99.0% , �300 mesh) mixed in equal proportions byvolume. The mixture was kept in a quartz boat and placedin a 3 inch diameter quartz tube already placed horizontallyin a conventional furnace. One end of the tubes isconnected to the Ar tank and the other end is sealed toan oil tank trap. High-purity Ar gas (99.9%) flow of10–20 sccm was used for the vapor transport inside the tubeduring thermal evaporation. The mixture was heated at1300–1400K while the substrates were kept at differenttemperatures (zones) that varied from 1300 to 900K insidethe tube for the growth of nanostructures. After 45min ofoperation the furnace power was turned off and it wascooled down to room temperature. We observed two kindsof colors for ZnO deposited material on the substrate,white and very light gray. The white color material isusually deposited on the substrates at temperatures greaterthan 1100K, while the light gray color materials usuallyappear to be deposited on the substrates at a temperaturerange of 900–1100K.

The sample material was studied with a scanningelectron microscope (SEM) [JOEL JSM 5300], an energydispersive spectrometer (EDS), a transmission electronmicroscope TEM [JOEL 1010] and an X-ray diffractometer(XRD) [Rigaku Geigerflex, 2000W] with Cu Ka (1.54 A) as

Fig. 1. SEM micrographs of ZnO nanowires grown in various conditio

the incident radiation. Photoluminescence spectra weremeasured at 300K with a xenon arc lamp-based fluores-cence spectrometer with an excitation wavelength of320 nm.

3. Results and discussion

Fig. 1 shows images of ZnO nanostructures grown atdifferent substrate temperatures on Si(1 0 0). Ar gas is usedinside the tube at a flow rate of 10–30 sccm. Nanowires andnanoribbons shown in Fig. 1a are grown at 1300Ksubstrate temperature and nanostructures shown in Fig.1b–f are grown at 1200, 1150, 1100, 1000 and 900Ksubstrate temperatures, respectively. It is observed thatmost of the nanowires grown at a substrate temperaturerange of 1300–1150K are very long compared to the onesgrown at a relatively low substrate temperature. It is alsoobserved that nanostructures grown in the substratetemperature range 900–1000K have branches and mostof them are considered to be nanorods.A representative energy dispersive X-ray (EDX) spec-

trum of the nanostructures is shown in Fig. 2. Only peaksassociated with Zn and O atoms are seen in this EDXspectrum (the Si-related peak in the spectrum comes fromthe Si substrate), leading to the fact that the nanomaterialsare indeed ZnO material. The peak comparison also showsthat there is a fair amount of nanomaterials deposited overthe substrate.Fig. 3 illustrates the XRD spectra of the ZnO powders

and nanostructures grown on the Si. The positions of XRDpeaks show good agreement with those of the hexagonalwurtzite structure for bulk ZnO with lattice constants ofa ¼ b ¼ 0:32489 nm and c ¼ 0:52062 nm [31]. No typical

ns at substrate temperature of 1300–900K. The bar scale is 10mm.

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Fig. 2. EDS spectra of as-made ZnO nanostructures.

Fig. 3. XRD spectra of the (a) ZnO powders and (b) ZnO nanostructures.

Fig. 4. TEM images of ZnO nanostructures. The insets are their selected

area electron diffraction (SAED) pattern. The bar scales for (a) and (b) are

1mm and 100 nm, respectively.

A. Khan, M.E. Kordesch / Physica E 30 (2005) 51–54 53

diffraction peaks corresponding to Zinc or other impuritieshave been found in any of our samples. The strongest peakin the XRD pattern of ZnO nanowires is (0 0 2) rather than(1 1 0) as in bulk ZnO, implying that the nanowires havepreferred orientation of [0 0 1].

Fig. 4 shows transmission electron microscope (TEM)analysis of the ZnO nanostructures. Fig. 4a shows theimage of nanowires grown at a substrate temperature of1300K. The image shows that ZnO nanosheets are alsogrown alongside nanowires at this temperature. The insetsare their selected area electron diffraction (SAED) pattern.The SAED analysis confirms that the nanostructure is asingle-crystal wurtzite structure.

PL measurements are carried out at room temperaturewith an excitation wavelength of 320 nm to examine theemission spectra of the nanostructures fabricated atvarious substrate temperatures. A relatively wide emissionband is observed, which ranges from 450 to 550 nmcentered around 480 nm corresponding to the green region.The possible candidates for the transition in this spectrarange are zinc vacancies, interstitial zinc and lattice defectsrelated to oxygen and zinc. This green transition is also

thought to be only attributed to the singly ionized oxygenvacancy in ZnO and the emission results from the radiativerecombination of a photo-generated hole with an electronoccupying the oxygen vacancy [32]. But based on certaintheoretical predictions and other experimental results, it ismore likely that the interstitial oxygen is responsible for theyellow emission [33]. As the nanostructures in ourexperiments are synthesized in a relatively low oxygen-deficient ambient, it is reasonable to believe that the greenemission band peak is related to oxygen vacancies. Wan etal. [34] have shown that the green emission peak in ZnOnanowires is oxygen vacancies dependent too.The near band edge ultraviolet (NBE UV) peak appears

at 381 nm corresponding to 3.25 eV. It is observed that theNBE UV emission is enhanced when the substratetemperature is decreased. In other words, the greenemission band is suppressed by lowering the substratetemperatures. On the other hand at 1000K substratetemperature, the near band edge emission peak is wellabove the green emission band and with the decrease insubstrate temperature the green peak intensity appears tobecome lower and lower as shown in Fig. 5. Wang et al.[28] have shown an analogous effect for the ZnO thin films(grown at 250 1C substrate temperature) annealed for 2 h inair at 700, 800 and 900 1C. The green emission peakintensity became larger when the annealing temperatureincreased compared to that for substrate temperature250 1C, at which the films were grown. This means thatvarying the temperature results in the variation of the greenband emission. From these results it can be concluded thatthe substrate temperature has a great influence on the typeand concentration of defects in ZnO nanostructures.

4. Conclusion

The dependence of the luminescence properties of ZnOnanostructures on substrate temperature was investigatedwithout annealing. The ZnO nanowires and nanorods arehighly crystalline at all temperatures of our study. PL studyshows that the green emission band peak, which is mainlydue to defects in ZnO, can be lowered by lowering thesubstrate temperature up to a certain limit. The best

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Fig. 5. PL spectra of ZnO nanomaterials grown at 900–1300K substrate temperatures.

A. Khan, M.E. Kordesch / Physica E 30 (2005) 51–5454

temperature for the low level of defects in ZnO nanos-tructures was found to be 900K.

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