defects in ZnO nanorods prepared by a hydrothermal method

Defects in ZnO Nanorods Prepared by a Hydrothermal Method

K.H.Tam,†C.K.Cheung,†Y.H.Leung,‡A.B.Djurisˇic´,*,†C.C.Ling,†C.D.Beling,†

S.Fung,†W.M.Kwok,‡W.K.Chan,‡D.L.Phillips,‡L.Ding,§and W.K.Ge§

Department of Physics,The Uni V ersity of Hong Kong,Pokfulam Road,Hong Kong,Department of Chemistry,

The Uni V ersity of Hong Kong,Pokfulam Road,Hong Kong,and Department of Physics,The Hong Kong

Uni V ersity of Science and Technology,Clear Water Bay,Hong Kong

Recei V ed:May26,2006;In Final Form:July18,2006

ZnO nanorod arrays were fabricated using a hydrothermal method.The nanorods were studied by scanning

electron microscopy,photoluminescence(PL),time-resolved PL,X-ray photoelectron spectroscopy,and positron

annihilation spectroscopy before and after annealing in different environments and at different temperatures.

Annealing atmosphere and temperature had significant effects on the PL spectrum,while in all cases the

positron diffusion length and PL decay times were increased.We found that,while the defect emission can

be significantly reduced by annealing at200°C,the rods still have large defect concentrations as confirmed

by their low positron diffusion length and short PL decay time constants.

1.Introduction

Fabrication of ZnO nanostructures1-4and the optical proper-ties of ZnO3-15have been extensively studied.Among the various methods for fabrication of ZnO nanostructures,the hydrothermal method1-4is of particular interest since it is a low cost,environmentally friendly method with growth tem-peratures as low as90°C.As a natural consequence of solution-phase growth at low temperature,the as-grown nanorods have large defect concentrations and typically exhibit weak UV but prominent defect emission at room temperature.3,4While the morphology and orientation of the nanorods can be significantly improved by using the ZnO seeds prepared from zinc acetate solution instead of ZnO nanoparticles,1improvements of their optical quality have not been investigated in detail.While it has been shown that the room-temperature photoluminescence (PL)spectra of nanorods can be changed by annealing under different conditions,3,15other techniques have not been used to conduct a comprehensive investigation of the effects of anneal-ing on their properties.It is generally assumed that a large UV to visible emission ratio implies excellent quality of the ZnO nanostructures,but typically,no other techniques are used to verify whether the defect density is indeed low.In addition, the origin of the visible emission is highly controversial.16ZnO can exhibit different emissions in the visible range(violet,blue, green,yellow,and orange-red)which are associated with defects in the material.Green emission is the most commonly observed defect emission in ZnO.However,there is still no consensus on the origin of this emission,and a number of different hypotheses have been proposed.16The possibility that green emissions in different samples can have different origins despite similar position and width has also been proposed.16If this is indeed the case,then the origin of the defect emission cannot be postulated simply by noting the similarities with previously published spectra for ZnO prepared by other fabrication methods.Therefore,considerable interest exists in studying the defect emissions in ZnO in general and ZnO nanorod arrays in particular due to their great potential for practical applications. Since applications of nanorod arrays in optoelectronic devices require control of their emission properties,it is necessary to perform a comprehensive study of their optical properties. Here,we report an investigation of the properties of ZnO nanorods using field emission scanning electron microscopy (SEM),variable temperature PL,time-resolved PL,X-ray photoelectron spectroscopy(XPS),and positron annihilation spectroscopy(PAS).We show that PL spectra do not provide sufficient evidence of defect density in the material.Contrary to common assumption,it is possible to have strong UV emission and weak defect emission and still have considerable defect concentrations in the sample.Thus,to fully characterize sample properties,other measurements in addition to PL spectroscopy are necessary.

2.Experimental Section

The nanorod arrays were prepared from solutions of zinc nitrate hydrate and hexamethylene tetramine on Si substrates with a seed layer prepared from zinc acetate solution.1Poly-ethyleneimine was added to the solution to increase the nanorod aspect ratio.2The morphology of the nanorods before and after annealing was examined using a Leo1530field emission SEM. The rods were annealed in a tube furnace at temperatures200, 400and600°C.The annealing atmospheres were air(atmo-spheric pressure,relative humidity∼55-60%),argon,nitrogen, and forming gas(90%N2/10%H2).For annealing in a gas flow, the gas flow rate was0.1Lpm and the pressure was∼1Torr. For variable temperature PL measurements,samples were mounted at the coldfinger and placed in a closed-cycle He cryostat(APD Cryogenics,Inc.model HC-2).The excitation source was a HeCd laser(325nm).The spectra were dispersed by a spectrometer SPEX500M and recorded by a photo-multiplier tube R943and PDA-512-USB(Control Development Inc.)fiberoptic spectrometer.Time-resolved PL was measured by using the Kerr-gated fluorescence technique17with1ps

*To whom correspondence should be addressed.Tel:+852********. E-mail:dalek@hkusua.hku.hk.

†Department of Physics,The University of Hong Kong.

‡Department of Chemistry,The Univeristy of Hong Kong.

§Department of Physics,The Hong Kong University of Science and Technology.20865

J.Phys.Chem.B2006,110,20865-20871

10.1021/jp063239w CCC:$33.50©2006American Chemical Society

Published on Web09/20/2006