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Chemical Physics Letters 446 (2007) 370–373

Enhancement of the ultraviolet emission of ZnO nanostructures bypolyaniline modification

Ming Chang a,b,c,*, Xue Li Cao a,b, Haibo Zeng a,b, Lide Zhang a,b

a Key Lab of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Anhui, People’s Republic of Chinab The Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

c School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430063, PR China

Received 4 May 2007; in final form 24 August 2007Available online 2 September 2007

Abstract

The photoluminescence properties of polyaniline modified ZnO nanostructures electrodeposited on ITO glass have been investigated.The great enhancement of the peak intensity ratio of ultraviolet to visible emission was observed after the polyaniline modification on theZnO nanostructured films. Such enhancement can be ascribed to the passivation effect caused by the polyaniline surface modification.This simple modification method provides a chance for various ZnO nanostructures to improve their ultraviolet emission property forthe optoelectronic applications.� 2007 Elsevier B.V. All rights reserved.

1. Introduction

As one of the most promising materials for the fabrica-tion of optoelectronics devices at room temperature, thesynthesis and optical properties of ZnO structures havebeen extensively investigated in recent years [1–4].Although lots of research have been done on the visibleemissions (such as green emission [5,6], yellow emission[7–9] or blue emission [10,11]), the ultraviolet emission,which is consented to originate from the band edge emis-sion or the exciton transition, is still the most promisingone for the optoelectronics devices application [12,13].However, the ZnO nanostructures with perfect ultravioletemission are not easily obtained. Generally, visible emis-sions are occurred in some degree depending on differentsynthesis strategies for most of the ZnO nanostructures.So it is very meaningful to enhance the ultraviolet emissionproperty for those ZnO nanostructures to be used in theoptoelectronics devices. In this Letter, polyaniline was

0009-2614/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2007.08.078

* Corresponding author. Address: Key Lab of Materials Physics,Institute of Solid State Physics, Chinese Academy of Sciences, Hefei230031, Anhui, People’s Republic of China.

E-mail address: mchang@issp.ac.cn (M. Chang).

adopted to modify the surface of the electrodepositedZnO nanostructures with conventional visible emission,which resulted in the obvious enhancement of ultravioletemission accompanied with the decreasing of defectemission.

2. Experimental

Two ZnO samples were prepared by a three-electrodeelectrodeposition system in 0.1 M zinc nitrate aqueoussolution. ITO (indium-tin oxide) glass, Ag/AgCl (saturateKCl) and graphite electrode were used as working elec-trode, reference electrode and counter electrode, respec-tively. Deposition were conducted under currentstaticcondition with current density 1.5 mA/cm2 at 80 �C forsample 1 and 1 mA/cm2 at 65 �C for sample 2. The deposi-tions all proceed for 1 h. The polyaniline modification wereconducted by immerge the dried ZnO samples in 0.01 Maniline monomer aqueous solution and equivalent amountof ammonium persulfate for 24 h.

Field emission scanning electronmicroscopy (FESEM,JEOL 6700F or Sirion 200) was used to examine the mor-phology and microstructure of the samples. Phase analysis

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of the products was carried out on a Philips X’Pert powderX-ray diffractometer using Cu Ka (0.154 19 nm) radiation.Photoluminescence (PL) spectra were recorded on a LAB-RAM-HR spectrometer (Jobin-Yvon) excited with the325 nm He–Cd laser at room temperature. Fourier trans-form infrared (FTIR) spectra were recorded by FTIR spec-troscopy (Nicolet Magna-750).

3. Results and discussion

For sample 1, closely packed ZnO film with some hexag-onal structure on the substrate surface was obtained(Fig. 1a). Rod-like structure on the film can be clearlyobserved from the side view in the inset image of Fig. 1a.The corresponding XRD pattern in Fig. 1b revealed itshexagonal wurtzite structure, showing highly crystalline.The strong peak ratio of (002) to other peaks indicatesits preferential c-axis orientation. The film seems to be con-stituted by nanorods crowded and intercrossed each other.For sample 2, ZnO film with plate-like structure was clearlyobserved from Fig. 1c. Furthermore, each plate composedof some plate structures by means of stack (the inset of the

Fig. 1. (a) SEM image of the sample 1(the inset shows the side view of the samXRD pattern of the sample 2.

Fig. 1c). The corresponding XRD pattern in Fig. 1d revealsits hexagonal wurtzite structure similar to sample 1. Differ-ences in synthesis temperature and current density result inthe different growth manner. Although the XRD tests showsimilar results for the two samples, the different defectstates induced by different growth mechanisms will greatlyaffect their photoluminescence property.

For the sample 1, Fig. 2a shows the PL spectra beforeand after polynailine modification. Before the modification,strong and broad visible emission with center at 553 nm canbe observed (the solid line in Fig. 2a), as well as a weakultraviolet emission peak at 396 nm (the inset of theFig. 2a). The solid and the dash-line curve of the inset arethe local part of the original curve and the fitted peak ofthe ultraviolet emission, respectively. The peak intensityratio of ultraviolet to visible emission is calculated to be1:25.4. After the polyaniline modification, the dramaticsdecreases of the visible emissions and marked increase ofthe ultraviolet emissions were observed (the dash-line inFig. 2a). Moreover, the peak intensity ratio of ultravioletto visible emission was enhanced to 2.6:1 (about 66 timesenhancement obtained). For the as prepared sample 2,

ple). (b) XRD pattern of the sample 1. (c) SEM image of the sample 2. (d)

Fig. 2. (a) Photoluminescence spectra of the sample 1. The solid and dash-lines curve corresponding to the as-prepared sample and the polyanilinemodified sample, respectively. The inset shows the ultraviolet zone of thePL spectra of the as prepared sample, the solid curve and the dot line showthe original curve and the fitted ultraviolet peak. (b) Photoluminescencespectra of the sample 2. The solid and dash-lines curve corresponding tothe as-prepared sample and the polyaniline modified sample, respectively.The inset shows the ultraviolet zone of the PL spectra of the as preparedsample.

Fig. 3. FTIR spectra of (a) pure ZnO, (b) pure polyaniline and (c) ZnOafter polyaniline modification.

372 M. Chang et al. / Chemical Physics Letters 446 (2007) 370–373

broad and strong visible emission was observed (the solidline in Fig. 2b) at 599 nm (yellow emission) and a weakultraviolet emission was observed at 384 nm (from the insetof the Fig. 2b). The peak intensity ratio of ultraviolet to vis-ible emission was enhanced from 1:26.3 to 1:1.38 (about 19times enhancement obtained) after the surface modification(the dash-line in Fig. 2b shows the PL spectra of the modi-fied sample).

Visible emissions are commonly observed in the solutionchemical synthesized ZnO nanostructures. The green emis-sion was widely believed to originate from single negativelycharged interstitial oxygen ion (O- i (�)) which prefers toaggregate at the surface of the ZnO nanostructure andthe yellow emission can be ascribed to the intrinsic defects[6–8,14]. These defects construct some defect states andwould become nonradiative centers under the conditionof photo excitation by capture photo-induced carriers.Hence, the efficiency of ultraviolet emission decreased.

After the polyaniline modification, the peak ratio of ultra-violet emission to visible emission enhanced. This indicatesthe change of defect states by polyaniline modification. Themodification result in the decrease of nonradiative centersand more carriers participate in the band gap emissions,which represented as the increase of ultraviolet emissionsintensity.

Till now, the mechanisms to explain the enhanced photo-luminescence of semiconductor nanostructures can bedivided into two groups. The first mechanism was used inplastic light-emitting diodes constructed of a conjugatedpolymer and indium arsenide-based nanocrystals (NCs)where the emitting active region was allowed to absorbenergy from the host through neutral-excitation energytransfer [15]. The second mechanism mainly includes thequantum confinement effect [16] and the electronic passiv-ation effect [17–19]. Electronic passivation effect refers tothe surface capping which block the surface state recombi-nation and prevent a decrease in carriers. It is obviously thatthe energy transfer mechanism and the quantum confine-ment effect do not work in our experiments for polyanilineis not a fluorescence polymer and the size of the synthesizedZnO nanostructure is not small enough to exhibit the quan-tum confinement effect. So, the enhanced ultraviolet emis-sion by polyaniline modification may be the result of theelectronic passivation effect.

To further verify the deduction, FTIR spectra wererecorded to investigate the interaction between ZnO andpolyaniline. Fig. 3 shows the FTIR spetra of pure ZnO(curve a), pure PANI (curve b) and ZnO/PANI composite(curve c), respectively. Four peaks of ZnO at �3417 cm�1,�1633 cm�1, �1384 cm�1 and �879 cm�1 are observed inFig. 3a. The characteristic peaks of PANI in Fig. 3b canbe assigned as the following. The peak at 3426 cm�1 isattributable to N–H stretching mode, the peaks at1571 cm�1 and 1483 cm�1 are attributed to C@N andC@C stretching modes for the quinoid and benzenoidrings, the peaks at about 1294 cm�1 and 1243 cm�1 are

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attributed to C–N stretching mode for benzenoid ring, andthe peak at 1110 cm�1 is assigned to the plane bendingvibration of C–H (modes of N@Q @N, Q@N+H–B andB–N+H–B), which is formed during protonation [19]. Itis evident from Fig. 3c that the FTIR spectrum of the com-posite contains contributions from both the ZnO nano-structure and the polyaniline. It is also noted, bycomparing Fig. 3b and c, that some polyaniline peaks areshifted due to interactions with ZnO nanostructure. Forexample, the stretching modes of C@N, C@C, and C–Nat 1571 cm�1, 1483 cm�1, 1243 cm�1 and 1294 cm�1 allshift to higher wave numbers. Similarly, the peak at1110 cm�1, formed upon protonation, also shifts to1180 cm�1. However, N–H stretching peak at 3417 cm�1

shifts to lower wave number. These IR absorption changessuggest that the C@N, C@C, and C–N bonds becomestronger, but the N–H bond becomes weaker. This is prob-ably because of the hydrogen bonding between the surfacesof the ZnO and the N–H group in the polyaniline macro-molecule [19]. Strong surface interactions enable moreeffective electronic passivation [18]. So, it is convinced thanthe enhancement of the peak ratio can be ascribed to theelectronic passivation effect.

For the sample 1, the intensive (002) diffraction peakand the great peak ratio between (002) and other peaksreveal the strong preference orientation and good crystalli-zation (Fig. 1b). Broad and intensive green emission indi-cates the surface defects abundant state. After thepolyaniline modification, the electronic passivation effectgreatly reduced nonradiative traps, hence the nonradiativerecombination was depressed and more carriers partici-pated in the band dap emission. So the peak ratio of ultra-violet to visible emission was greatly enhanced. As to thesample 2, the nonradiative recombination at the surfacewas depressed at the same way. But as a result of its platestack growth manner, more interior defects exist in sample2. And those interior defects remain unaffected by the poly-aniline modification. As a result, the peak ratio was alsoenhanced, but the efficiency decreased relative to the sam-ple 1 and there still a strong and broad yellow emission.

Above all, after the polyaniline modification, the peakratio of ultraviolet emission to visible emission can beenhanced greatly. So this simple modification method pro-vides a chance for various ZnO nanostructures to ownexcellent ultraviolet emission property and be used in theoptoelectronics field.

4. Conclusions

By the modification of polyaniline on the ZnO nanostruc-ture, the peak ratio of ultraviolet emission to visible emis-sion was enhanced greatly. The efficiency of enhancementdepends on defect state for the enhancement originatedfrom the passivation effect, which results in the reductionof non-radioactive recombination caused by surface defects.This simple modification method provides a chance for var-ious ZnO nanostructures to enhance their ultraviolet emis-sion property and be used in optoelectronics field.

Acknowledgements

This work was supported by the Major Research Plan ofthe National Natural Science Foundation of China (grant90406008, 10604055) and the National Major Project ofFundamental Research (973 Program, Grant No.2005CB623603).

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