Giant enhancement of band edge emission based on ZnO / TiO 2 nanocomposites

Enhancement of band edge emission of ZnO nanorods up to a factor of 120 times has been observed in the composite consisting of ZnO nanorods and TiO2 nanoparticles, while the defect emission of ZnO nanorods is quenched to noise level. Through a detailed investigation, it is found that the large enhancement mainly arises from fluorescence resonance energy transfer between the band edge transition of ZnO nanorods and TiO2 nanoparticles. Our finding opens up new possibilities for the creation of highly efficient solid state emitters. ©2007 Optical Society of America OCIS codes: (240.6675) Surface photoemission and photoelectron spectroscopy; (250.5230) Photoluminescence; (260.5740) Resonance. References and links 1. J. B. Baxter, F. Wu, and E. S. Aydil, ‘‘Growth mechanism and characterization of zinc oxide hexagonal columns,’’ Appl. Phys. Lett. 83, 3797-3799 (2003). 2. L. S. -Mende and J. L. MacManus-Driscoll, ‘‘ZnO-nanostructures, defects, and devices,’’ Mater. Today 10, 40-48 (2007). 3. N. E. Hsu, W. K. Hung, and Y. F. Chen, ‘‘Origin of defect emission identified by polarized luminescence from aligned ZnO nanorods,’’ J. Appl. Phys. 96, 4671-4673 (2004). 4. C. C. Lin, H. P. Chen, H. C. Liao, and S. Y. Chen, ‘‘Enhanced luminescent and electrical properties of hydrogen-plasma ZnO nanorods grown on wafer-scale flexible substrates,’’ Appl. Phys. Lett. 86, 183103 (2005). 5. Y. G. Wang, S. P. Lau, X. H. Zhang, H. H. Hng, H. W. Lee, S. F. Yu, and B. K. Tay, ‘‘Enhancement of near-band-edge photoluminescence from ZnO films by face-to-face annealing,’’ J. Cryst. Growth 259, 335-342 (2003). 6. J. M. Lin, H. Y. Lin, C. L. Cheng, and Y. F. Chen, ‘‘Giant enhancement of bandgap emission of ZnO nanorods by platinum nanoparticles,’’ Nanotechnology 17, 4391-4394 (2006). 7. Y. Zhang, Z. Zhang, B. Lin, Z. Fu, and J. Xu, ‘‘Effect of Ag doping on the photoluminescence of ZnO films grown on Si substrate,’’ J. Phys. Chem. 109, 19200-19203 (2005). 8. L. Duan, W. Zhang, S. Zhong, and Z. Fu, ‘‘Enhancement of ultraviolet emissions from ZnO films by Ag doping,’’ Appl. Phys. Lett. 88, 232110 (2006). 9. C. W. Lai, J. An, and H. C. Ong, ‘‘Surface-plasmon-mediated emission from metal-capped ZnO thin films,’’ Appl. Phys. Lett. 86, 251105 (2005). 10. H. Y. Lin, C. L. Cheng, Y. Y. Chou, L. L. Huang, and Y. F. Chen, ‘‘Enhancement of band gap emission stimulated by defect loss,’’ Opt. Exp. 14, 2372-2379 (2006). 11. J. M. Lin, C. L. Cheng, H. Y. Lin, and Y. F. Chen, ‘‘Giant enhancement of band edge emission in ZnO and SnO nanocomposites,’’ Opt. Lett. 31, 3173-3175 (2006). 12. C. Berney and G. Danuser, ‘‘FRET or No FRET: A quantitative comparison,’’ Biophys. J. 84, 3992-4010 (2003). 13. J. Shi, J. Chen, Z. Fang, T. Chen, Y. Lian, X. Wang, and C. Li, ‘‘Photoluminescence characteristics of TiO2 and their relationship to the photoassisted reaction of water/methanol mixture,’’ J. Phys. Chem. C 111, 693-699 (2007). 14. H. Nakajima, T. Mori, and M. Watanabe, ‘‘Influence of platinum loading on photoluminescence of TiO2 powder,’’ J. Appl. Phys. 96, 925-927 (2004). 15. G. Wakefield, J. Stott, and J. Hock, ‘‘Sunscreens and Cosmetics Containing Manganese Doped Titanium Oxide Nanoparticles,’’ SÖFW J. 131, 46-51 (2005). 16. T. Ohsaka, F. Izumi, and Y. Fujiki, ‘‘Raman spectrum of anatase, TiO2,’’ J. Raman Spec. 7, 321-324 (1978). #86670 $15.00 USD Received 21 Aug 2007; revised 6 Sep 2007; accepted 10 Sep 2007; published 5 Oct 2007 (C) 2007 OSA 17 October 2007 / Vol. 15, No. 21 / OPTICS EXPRESS 13832 17. M. Grätzel, ‘‘Photoelectrochemical cells,’’ Nature 414, 338-344 (2001). 18. K. Y. Song, Y. T. Kwon, G. J. Choi, and W. I. Lee, ‘‘Photocatalytic activity of Cu/TiO2 with oxidation state of surface-loaded copper,’’ Bull. Korean Chem. Soc. 20, 957-960 (1999).


Introduction
ZnO nanostructures have been extensively studied because of their unique optical properties in the ultraviolet (UV) region and piezoelectric properties.For example, the wide band gap (3.37 eV) and the high excitonic binding energy (60 meV) make its great potential for UV light emitting and laser devices at room temperature [1][2][3].As ZnO material is shrunk to nano-scale, effects of impurity and native defects become much more significant than their bulk counterparts.Especially, due to an inherent large surface-to-volume ratio, surface defects can now play a dominant role in the optoelectronic properties.Therefore, the nano-scale devices face challenges of high efficiency because of a large defect induced loss.
To overcome this obstacle, several methods have been attempted to improve the quantum efficiency of optoelectronic properties of ZnO nanostructures, such as hydrogen doping [4], annealing [5], or metal doping/coating [6][7][8][9][10][11].In all of these methods, the underlying mechanisms responsible for the improvement include the passivation of defects by doped atoms, such as H, N, O, or metals, and the carrier transfer due to a larger band gap of the coating semiconductor.In this paper, we provide a different approach with the physical principle based on fluorescence resonance energy transfer (FRET).FRET is an important phenomenon existing in many biological systems [12].It depends on the distance between the electronic excited states of two molecules in which excitation is transferred from a donor to an acceptor.The most important factors for the occurrence of FRET include: (1) Donor and acceptor have to be in close proximity.(2) The absorption of acceptor has to overlap with the fluorescence spectrum of donor.(3) Donor and acceptor transition dipole orientations should contain a parallel component [12].In order to illustrate our working principle, the composite consisting of ZnO nanorods and TiO 2 nanoparticles has been fabricated and studied.TiO 2 has been widely used in photocatalysts, solar cells, and cosmetics due to it's photosensitivity and thermal stability [13][14][15].The strong absorption in the UV range around 3.3 eV is well feasible for the occurrence of FRET when compounded with ZnO nanorods.Quite interestingly, based on the composite of ZnO nanorods and TiO 2 nanoparticles, the band edge emission of ZnO nanorods can be enhanced up to 120 times.Our approach may serve as a general strategy for many other nanocomposites to create highly efficient solid state emitters.

Experiment
The detailed growth of ZnO nanorods by vapor-liquid-solid (VLS) process has been described in our previous reports [3,10].After the ZnO nanorods were grown, TiO 2 nanoparticles were prepared by using a sputtering system (JFC-1600, JEOL).Photoluminescence (PL) spectra were excited by a 325 nm He-Cd laser, and a photomultiplier tube (PMT) was used as a detector.Photoluminescence excitation (PLE) spectra were performed by using a Jobin Yvon Fluorolog 3 spectroscope.Cathodoluminescence (CL) signal was obtained using a Gatan MonoCL3 system with an electron acceleration voltage at 5 keV.Raman scattering spectrum was measured by a Jobin Yvon T64000 spectroscope with spectral resolution of 2 cm -1 .

Results and discussions
Figure 1 shows SEM images, PL and CL spectra of ZnO nanorods with and without TiO 2 nanoparticles.As shown in Fig. 1(a), at the top end of the ZnO nanorod, there exists a hemisphere Au catalyst, and the small spots on the surface of ZnO nanorod shown in Fig. 1(b) represent the deposited TiO 2 nanoparticles.Quite remarkably, before ZnO nanorods were compounded with TiO 2 nanoparticles, the spectra consist of a weak band edge UV emission at 3.26 eV, and a relatively strong defect emission at 2.48 eV due to oxygen vacancy and zinc interstitials [3,6].After ZnO nanorods were compounded with TiO 2 nanoparticles, we can see that the defect emission in both of the CL and PL spectra was dramatically decreased, while the band edge emission was greatly enhanced.In order to have a more detailed understanding of the influence of TiO 2 nanoparticles, the TiO 2 coating time dependence of the PL intensity ratio between the band edge and defect emissions was investigated as shown in Fig. 2. With increasing TiO 2 coating time, the PL enhancement factor of the band edge emission can be increased up to 120 times, while the defect emission can be suppressed to noise level.To clarify the PL enhancement mechanism in ZnO nanorods, the characteristic of TiO 2 nanoparticles was examined by Raman scattering, PL, and PLE spectra.Raman scattering spectrum shown in Fig. 3 clearly identifies the anatase phase of the deposited TiO 2 nanoparticles on the basis of its Raman band at 144 cm -1 of E g mode [16].Here, the E g mode corresponds to O-Ti-O bending type vibration.The absence of other peaks characterized rutile and brookite phases suggests that the deposited TiO 2 has a pure anatase phase.In addition, Fig. 4(a) gives the corresponding PL and PLE spectra, in which the emission band at 3.4 ± 0.2 eV is related to the band edge emission of anatase phase of TiO 2 [13,15].The blue shift of the emission from the band gap energy of 3.2 eV of anatase TiO 2 is possiblely due to the quantum confinement of TiO 2 nanoparticles.As shown in the SEM image of Fig. 4(b), the diameter of TiO 2 nanoparticles deposited on Si (100) with a coating time of 700 s is less than 10 nm.It is worth noting that the emission band of TiO 2 nanoparticles covers the full spectrum of the band edge emission of ZnO nanorods.The PLE spectrum of TiO 2 nanoparticles shows a wide and strong absorption at the UV range from 3.6 eV to 5 eV with peaks at 4.1 eV and 4.5 eV.Particularly, as shown by the dashed line, when the excitation photon energy is lower than 3.40 eV, the defect emission is weak, but still detectable.However, after the excitation energy exceeds the band gap energy (3.40 eV) of TiO 2 nanoparticles, the defect emission of ZnO nanorods completely disappears.This intriguing behavior implies that after relaxing to the band edge, the excited electrons and holes in TiO 2 nanoparticles directly contribute to the band edge emission of ZnO nanorods without passing through the defect channel.It therefore can be used to exclude the possibility that the carrier transfer from TiO 2 nanoparticles to ZnO nanorods is responsible for the observed enhancement.
The above results can be well interpreted if we adapt the optical process based on the intrinsic properties of FRET.In order to have a clear physical picture of the FRET process, a band alignment diagram is shown in Fig. 6 [17,18].The conduction and valence band edges of ZnO are located at -0.2 V and 3.0 V vs normal hydrogen electrode (NHE), respectively [17,18].Both energy positions are well covered by the broad band edge of TiO 2 nanoparticles.Thus, after the excitation of electron-hole pairs in TiO 2 nanoparticles, through the resonance effect, the energy is easily transferred to ZnO nanorods, and the band edge emission of ZnO nanorods is greatly enhanced.Because the resonance condition is only satisfied by the band edge emission and the incident photon energy is fully utilized through the band edge recombination, the defect emission is therefore reduced to noise level.When the incident photon energy is below the band gap energy of TiO 2 nanoparticles, the excited electron-hole pairs are created in ZnO nanorods, as usual, they can recombine through the band edge as well defect states, and the defect emission is observable as shown in Fig. 5(b).Therefore, we can see that based on the FRET process, all of the above intriguing results can be well accounted for.Finally, we like to point out that except the FRET process mentioned here, it is possible that surface passivation due to TiO 2 nanoparticles may partially contribute to the enhanced band edge emission and the suppressed defect emission.However, it does not play a decisive role, because according to our measurement the enhanced band edge emission in ZnO nanorods mainly arises from the absorption in TiO 2 nanoparticles as shown in Fig. 5.

Conclusion
It is found that based on the composite of ZnO nanorods and TiO 2 nanoparticles, the band gap emission of ZnO nanorods can be enhanced up to 120 times, while the defect emission can be suppressed to noise level.We demonstrate that the underlying mechanism responsible for the enhancement arises from the FRET process, which is a well known phenomenon in many biological systems.It is believed that our approach can be used as a general strategy for many other nanocomposites to create highly efficient solid state emitters.

Fig. 1 .
Fig. 1.Scanning electron microscopy images of ZnO nanorods with (b) and without (a) TiO 2 nanoparticles.(c) Photoluminescence and cathodoluminescence spectra of ZnO nanorods with and without TiO 2 nanoparticles.

Fig. 2 .
Fig. 2. TiO 2 coating time dependence of UV and green emissions of ZnO nanorods.The triangle denotes the PL intensity ratio between the UV emission of ZnO nanorods with (I) and without (I 0 ) TiO 2 nanoparticles.The diamond denotes the PL intensity ratio between the green emission with (I') and without (I' 0 ) TiO 2 nanoparticles.#86670 -$15.00USD Received 21 Aug 2007; revised 6 Sep 2007; accepted 10 Sep 2007; published 5 Oct 2007

Fig. 5 .
Fig. 5. (a).Photoluminescence excitation spectra of ZnO nanorods with and without TiO 2 nanoparticles monitored at 3.25 eV.The intensity (dashed line) for ZnO nanorods without TiO 2 nanoparticles has been multiplied by 20 times.(b) Photoluminescence excitation spectra of ZnO nanorods with and without TiO 2 nanoparticles monitored at 2.48 eV.

Fig. 6 .
Fig.6.Energy band alignment of ZnO/TiO 2 composite.As these two materials are brought together through the resonance effect, the excited electron-hole pairs in TiO 2 nanoparticles can easily transfer their energy into ZnO nanorods and contribute to the band edge emission.