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Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles
Thin Solid Films 520 (2012) 4646–4649
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles Trilok Singh ⁎, D.K. Pandya, R. Singh Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
a r t i c l e
i n f o
Available online 3 December 2011 Keywords: ZnO Surface plasmon Nanorods Sputtering Photoluminescence Electrochemical deposition
a b s t r a c t Electrochemical deposition of ZnO nanorods having a diameter of 80–150 nm and length ~ 2 μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100 s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375 nm and a broad defect-related emission band centered at ~ 596 nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50 s. The enhancement factor reached a maximum value for the Au sputtering time of 50 s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years, much attention has been devoted to the investigation of wide bandgap semiconductor nanostructures especially II-VI groups. Most research on nanostructures have devoted to obtain modulated band gap emission or improved luminescence efficiency of nanometer-sized semiconductors mostly for optical applications such as light emitting diode (LED), laser diode, and photosensors. Zinc oxide (ZnO) is one of the most important wide bandgap II-VI semiconductor materials for optoelectronics due to wide bandgap of ~3.37 eV and an exciton binding energy of 60 meV. It has considerable potential applications in the short-wavelength light sources such as laser diodes [1]. However, in most cases, visible light emissions related to defects or impurities dominate its luminescence spectra. Hence, it is important to obtain highly efficient ultraviolet (UV) emission from the near band edge for application in high efficient short wavelength light sources and optoelectronic devices. Studies have been conducted to improve the band edge emission and to suppress the visible emission [2–5]. Nanostructures of noble metals such as nanoparticles of platinum, gold and silver, have strong localized surface plasmonic effects, like strong absorption and luminescence [6,7]. At the interface between metal and dielectric localized surface plasmons are the oscillations of charge density and the enhancement of band edge emission from ZnO thin films by surface plasmons mediated by nanoparticles of
⁎ Corresponding author. E-mail address: [email protected] (T. Singh). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.11.074
silver [8], platinum [9], aluminum [10] and gold [11–14] was earlier reported. Moreover, the enhancement of UV lasing emission from ZnO films was obtained by surface plasmon resonance [15]. Recently, modifying the optical properties of metal oxide with metal nanoparticles has attracted much attention. In the present paper, we report about the enhancement of UV emission from ZnO nanorods coated by Au nanoparticles. This enhancement has been explained using the model of the transfer of electrons among various states. The change of the intensity of different defect related visible emissions demonstrated that there occurs electron transfer from the different defect states to the conduction band of ZnO through Au nanoparticles. These results are useful for the development of ZnO-based high efficiency LEDs and helpful in understanding the mechanism of surface plasmons coupling.
2. Experimental details The electrodeposition process was carried out using a CHI potentiostat/galvanostat (CHI Electrochemical Analyzer) in a specially designed closed three electrode glass cell. The indium doped tin oxide (ITO) transparent glass substrate (≈20 Ω/□) was used as a working electrode while platinum sheet (2×2 cm2) and saturated calomel electrode (SCE) were used for counter and reference electrodes, respectively. Prior to electrodeposition, the ITO glass substrate was rinsed with acetone, toluene, and deionized water and then ultasonicated in distilled water for 20 min. The electrolyte (bath) temperature was maintained at 80 °C. Electrochemical depositions of ZnO nanostructures were carried out at the deposition potential of −1.0 V (vs. SCE) for 5 h.
T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
4647
for different times 15, 30, 50 and 100 s using a dc sputtering system (JFC-1600, JEOL) with a fixed current of 10 mA. The morphology of nanorods was obtained using scanning electron microscopy (SEM, Zeiss, EVO-50) with operating voltage of 20 kV. For the structural studies, X-ray diffractometer (XRD, Philips Xpert Pro) using CuKα (λ = 1.5405 Å) radiation in 2θ range 20–80° and the X-rays were generated by applying 45 kV voltage on anode material Cu at a current of 40 mA. The photoluminescence (PL) spectra were obtained using 266 nm laser source (RPM 2000, Accent Optics, USA).
(a)
3. Results and discussion 3.1. Microstructural study
1 µm
The surface morphology of the electrodeposited nanostructures of ZnO is shown in Fig. 1. The diameter of the ZnO nanorods varied from 50 to 200 nm and length up to a few micrometers. The length of the nanorods could be increased by increasing the electrodeposition time, whereas the diameter of the nanorods did not change with electrodeposition time [17]. Au nanoparticles were sputtered for 15, 30, 50 and 100 s for investigating suface plasmon effect on the near band edge and defect-related emission peaks from grown nanostructures. Fig. 1(b) shows that the ZnO nanorods are covered by Au nanoparticles with Au sputtered on it for 50 s. Fig. 1(c) shows the Au thin films thickness with sputtering time on the ITO substrate and in the present case the nanorods are not verically aligned so the nanoparticle density will be varied.
(b)
200 nm
Au thin films
(c)
Fig. 1. (a) SEM image of ZnO nanorods grown at 80 °C for 5 h in ITO substrate, (b) SEM image of Au sputtered ZnO nanorods for 50 s and (c) sputtering time vs. Au-films thickness.
The ZnO nanostructures were grown from aqueous solution of 1 mM zinc nitrate [Zn(NO3)2 6H2O] and electrodeposition of ZnO nanorods involves the following reactions [16]: −
−
−
−
NO3 þ H2 O þ 2e →NO2 þ 2OH 2þ
Zn
−
þ 2OH →ZnO þ H2 O
ð1Þ ð2Þ
The total reaction may be written as 2þ
Zn
−
−
−
þ NO3 þ 2e →ZnO þ NO2 :
ð3Þ
After deposition the samples were removed from the electrolyte and rinsed in de-ionized water. The as-grown sample was cut into four pieces and then nanoparticles were sputtered on these samples
Fig. 2. (a) X-ray diffractogram of ZnO nanostructures grown at 80 °C; (b) X-ray diffractogram of ZnO nanostructures Au sputtered for 50 s.
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T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
3.2. Phase and structural analysis In order to investigate the structural properties of Au sputtered ZnO nanostructures, the X-ray diffraction measurements were performed and the XRD patterns are given in Fig. 2. The presence of several peaks in the XRD patterns of as-grown sample revealed that the nanostructures were polycrystalline in nature. The observed “d” values are in good agreement with standard “d” values and diffraction peaks indexed to the wurtzite phase of ZnO (JCPDF card no. 75-0591) with reflection peaks (100), (002), (101), (102), (103) and (112). Fig. 2(b) shows the XRD patterns of 50 s Au sputtered ZnO nanostructures. It shows well defined dominant reflection peaks having orientations in the (100), (002) and (101) planes corresponding to ZnO and dominant reflection peaks having orientations in the (111), (200), (220) and (311) planes appeared from Au nanoparticles, which are sputtered on the ZnO nanostructures. 3.3. Optical study In Fig. 3(a), we show the emission characteristics of Au-coated and un-coated ZnO nanostructures from PL spectra. PL measurements were performed at room temperature with an excitation wavelength of 266 nm. In Fig. 3(a), PL spectra of the un-coated ZnO nanostructures consist of a weak UV peak and a relatively strong and broad
visible peak. However, for the Au-coated ZnO nanostructures we observe a very strong UV emission at the wavelength of 373 nm and an almost complete quenching of the visible emission. The UV emission intensity of the Au coated ZnO nanostructures increased by a factor of ~9.5 (Fig. 3(b)) compared with the un-coated ZnO nanostructures, while the visible emission from the Au coated ZnO is quenched completely compared with the remarkable peak centered at 596 nm in PL of the un-coated ZnO nanostructures. Regarding the origin of the defect related emissions from ZnO, it is still controversial due to complicated nature of crystal defects in ZnO [18,19]. It is generally accepted that the deep-level emissions are closely related with the oxygen vacancies. In a recent study, the pronounced change in the PL spectra of metal (Zn, Ag, Ni, Au, Al and Ti) coated ZnO nanostructures with different deposition times was attributed to the metal–semiconductor contacts (Ohmic or Schottky contacts depending on the work function) at the interface [20]. The enhancement (Ohmic) or decrement (Schottky) in the PL intensity could be explained on the basis of metal semiconductor contacts for all the metals above mentioned except Au. Fang et al. reported that in case of Ohmic contact, when ZnO is excited by incident light the electron will move to the surface and accumulate there, while hole will move to the bulk region [20]. However electron affinity of ZnO (4.35 eV) is higher than the work function of metal, so it will be easy for electron transferring from metal to semiconductor and electron accumulation at the interface to increase substantially. This leads to the enhancement in the UVintensity as well as defect related emission. In case of Schottky contact, due to Schottky barrier at the interface the electron from metal will not easily transfer to the semiconductor. Thus when ZnO is excited by incident light the radiative recombination probability declines tremendously at the surface of ZnO, which causes the decrement of UV and defects intensities. In the present case a pronounced enhancement in the UV-emission and suppression of defect related emission is observed and this could not be explained with metal semiconductor (Schottky) contacts. Thus the enhancement in the UV-emission and suppression in defects emission is attributed to the localized
(a)
EC
Defects related band
Ef
E
(b)
V
Vacuum Level
Defects related bands
Φ m=5.47 eV
χ=4.35 eV
EC
Eg=3.37 eV
E=2.1 eV EV
ZnO
Fig. 3. (a) PL spectra of bare and Au coated ZnO nanostructures and (b) enhancement ratio of Au coated ZnO nanostructures in comparison to uncoated nanostructures.
Au
Fig. 4. (a) Energy band diagram of bare ZnO nanostructures, visible emission due to transition from defect levels. (b) Energy band diagram of Au coated ZnO nanostructures, electrons transfer to the conduction band of ZnO through plasmons for Aucoated ZnO nanostructures.
T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
surface plasmons [21]. At the interface between Au nanoparticles and the rough surface of ZnO nanostructures, localized surface plasmons can be created due to the resonant interaction between the electron-charge near the surface of the Au nanoparticles and the electromagnetic field of the incident light of 266 nm. The resonant oscillation of electrons in Au nanoparticles creates a local light field close to the particle surface that may strongly exceed the strength of the incident light field [22]. In the presence of Au nanoparticles, the giant enhancement of energy density of excitation source results in the improvement of excitation rate in excitation process and the decay rate in emission process. Fig. 4(a) shows the energy band diagrams of the un-coated ZnO nanostructures. The energy levels of the defect states in ZnO nanostructures are denoted by defect states, corresponding to the broad visible spectrum. Electron transfer processes via the coupling of localized surface plasmons at the interface between Au nanoparticles and ZnO nanostructures are shown in Fig. 4(b). The electron transfer from the defect states to the Au nanoparticles not only results in the increase of the resonant electron density, but also creates energetic electrons in higher energy state [23]. These resonant electrons are so active that they can escape from the surface of Au nanoparticles to the conduction band of the ZnO nanostructures [11]. Thus, the electron density in the conduction band of the ZnO is significantly increased, which leads to a significant increase of the intensity of UV emission from ZnO. In the present case, we obtained the UV enhancement factor of ~ 9.5. 4. Conclusions Au nanoparticles were sputtered on electrochemically synthesized ZnO nanorods for different sputtering times (for 0–100 s). The intensity of UV-emission was drastically enhanced and defects related emission was suppressed. Enhancement in UV-emission is due to the fact that the electrons in the defect state are transferred to the conduction band of ZnO via its coupling with the localized surface plasmons excited by the excitation source. The electrons in the defect state are transferred to the localized surface plasmons excited by the incident light and there is a decrease in the electrons in the defects
4649
states. The enhancement factor in the intensity of UV-emission peak was about 9.5 at the Au sputtering for 50 s. Acknowledgements The author (Trilok Singh) acknowledges IIT Delhi for fellowship and B.C. Joshi for helping in PL measurements. References [1] X. Wu, A. Yamilov, X. Liu, S. Li, V.P. Dravid, R.P.H. Chang, H. Cao, Appl. Phys. Lett. 85 (2004) 3657. [2] H.Y. Lin, Y.Y. Chou, C.L. Cheng, Y.F. Chen, Opt. Express 15 (2007) 13832. [3] Y. Zhang, W. Zhang, C. Peng, Opt. Express 16 (2008) 10696. [4] C.C. Lin, H.P. Chen, H.C. Liao, S.Y. Chen, Appl. Phys. Lett. 86 (2005) 183103. [5] N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, H. Haneda, Appl. Phys. Lett. 80 (2002) 2869. [6] C.L. Haynes, R.P.V. Duyne, J. Phys. Chem. B 107 (2003) 7426. [7] T.R. Jensen, M.D. Malinsky, C.L. Haynes, R.P.V. Duyne, J. Phys. Chem. B 104 (2000) 10549. [8] J.B. You, X.W. Zhang, Y.M. Fan, S. Qu, N.F. Chen, Appl. Phys. Lett. 91 (2007) 231907. [9] J.M. Lin, H.Y. Lin, C.L. Chen, Y.F. Chen, Nanotechnology 17 (2006) 4391. [10] W.H. Ni, J. An, C.W. Lai, H.C. Ong, J.B. Xu, J. Appl. Phys. 100 (2006) 26103. [11] H.Y. Lin, C.L. Cheng, Y.Y. Chou, L.L. Huang, Y.F. Chen, K.T. Tsen, Opt. Express 14 (2006) 2372. [12] X. Li, Y. Zhang, X. Ren, Opt. Express 17 (2009) 8735. [13] T. Chen, G.Z. Xing, Z. Zhang, H.Y. Chen, T. Wu, Nanotechnology 19 (2008) 435711. [14] C.W. Cheng, E.J. Sie, B. Liu, C.H.A. Huan, T.C. Sum, H.D. Sun, H.J. Fan, Appl. Phys. Lett. 96 (2010) 71107. [15] A.P. Abiyasa, S.F. Yu, S.P. Lau, E.S.P. Leong, H.Y. Yang, Appl. Phys. Lett. 90 (2007) 231106. [16] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wohrle, T. Sugiura, H. Minoura, Chem. Mater. 11 (1999) 2657. [17] S.P. Anthony, J. Lee, J.K. Kim, Appl. Phys. Lett. 90 (2007) 103107. [18] C. Chandrinou, N. Boukos, C. Stogios, A. Travlos, Microelectron. J. 40 (2009) 296. [19] Y.W. Heo, D.P. Norton, S.J. Pearton, J. Appl. Phys. 98 (2005) 073502. [20] Y.J. Fang, J. Sha, Z.L. Wang, Y.T. Wan, W.W. Xia, Y.W. Wang, Appl. Phys. Lett. 98 (2011) 033103. [21] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [22] B. Lamprecht, J.R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F.R. Aussenegg, Appl. Phys. B 79 (2001) 51. [23] C. Sonnichsen, T. Franzl, T. Wilk, G. Plessen, J. Feldmann, Phys. Rev. Lett. 88 (2002) 077402.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles Trilok Singh ⁎, D.K. Pandya, R. Singh Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
a r t i c l e
i n f o
Available online 3 December 2011 Keywords: ZnO Surface plasmon Nanorods Sputtering Photoluminescence Electrochemical deposition
a b s t r a c t Electrochemical deposition of ZnO nanorods having a diameter of 80–150 nm and length ~ 2 μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100 s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375 nm and a broad defect-related emission band centered at ~ 596 nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50 s. The enhancement factor reached a maximum value for the Au sputtering time of 50 s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years, much attention has been devoted to the investigation of wide bandgap semiconductor nanostructures especially II-VI groups. Most research on nanostructures have devoted to obtain modulated band gap emission or improved luminescence efficiency of nanometer-sized semiconductors mostly for optical applications such as light emitting diode (LED), laser diode, and photosensors. Zinc oxide (ZnO) is one of the most important wide bandgap II-VI semiconductor materials for optoelectronics due to wide bandgap of ~3.37 eV and an exciton binding energy of 60 meV. It has considerable potential applications in the short-wavelength light sources such as laser diodes [1]. However, in most cases, visible light emissions related to defects or impurities dominate its luminescence spectra. Hence, it is important to obtain highly efficient ultraviolet (UV) emission from the near band edge for application in high efficient short wavelength light sources and optoelectronic devices. Studies have been conducted to improve the band edge emission and to suppress the visible emission [2–5]. Nanostructures of noble metals such as nanoparticles of platinum, gold and silver, have strong localized surface plasmonic effects, like strong absorption and luminescence [6,7]. At the interface between metal and dielectric localized surface plasmons are the oscillations of charge density and the enhancement of band edge emission from ZnO thin films by surface plasmons mediated by nanoparticles of
⁎ Corresponding author. E-mail address: [email protected] (T. Singh). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.11.074
silver [8], platinum [9], aluminum [10] and gold [11–14] was earlier reported. Moreover, the enhancement of UV lasing emission from ZnO films was obtained by surface plasmon resonance [15]. Recently, modifying the optical properties of metal oxide with metal nanoparticles has attracted much attention. In the present paper, we report about the enhancement of UV emission from ZnO nanorods coated by Au nanoparticles. This enhancement has been explained using the model of the transfer of electrons among various states. The change of the intensity of different defect related visible emissions demonstrated that there occurs electron transfer from the different defect states to the conduction band of ZnO through Au nanoparticles. These results are useful for the development of ZnO-based high efficiency LEDs and helpful in understanding the mechanism of surface plasmons coupling.
2. Experimental details The electrodeposition process was carried out using a CHI potentiostat/galvanostat (CHI Electrochemical Analyzer) in a specially designed closed three electrode glass cell. The indium doped tin oxide (ITO) transparent glass substrate (≈20 Ω/□) was used as a working electrode while platinum sheet (2×2 cm2) and saturated calomel electrode (SCE) were used for counter and reference electrodes, respectively. Prior to electrodeposition, the ITO glass substrate was rinsed with acetone, toluene, and deionized water and then ultasonicated in distilled water for 20 min. The electrolyte (bath) temperature was maintained at 80 °C. Electrochemical depositions of ZnO nanostructures were carried out at the deposition potential of −1.0 V (vs. SCE) for 5 h.
T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
4647
for different times 15, 30, 50 and 100 s using a dc sputtering system (JFC-1600, JEOL) with a fixed current of 10 mA. The morphology of nanorods was obtained using scanning electron microscopy (SEM, Zeiss, EVO-50) with operating voltage of 20 kV. For the structural studies, X-ray diffractometer (XRD, Philips Xpert Pro) using CuKα (λ = 1.5405 Å) radiation in 2θ range 20–80° and the X-rays were generated by applying 45 kV voltage on anode material Cu at a current of 40 mA. The photoluminescence (PL) spectra were obtained using 266 nm laser source (RPM 2000, Accent Optics, USA).
(a)
3. Results and discussion 3.1. Microstructural study
1 µm
The surface morphology of the electrodeposited nanostructures of ZnO is shown in Fig. 1. The diameter of the ZnO nanorods varied from 50 to 200 nm and length up to a few micrometers. The length of the nanorods could be increased by increasing the electrodeposition time, whereas the diameter of the nanorods did not change with electrodeposition time [17]. Au nanoparticles were sputtered for 15, 30, 50 and 100 s for investigating suface plasmon effect on the near band edge and defect-related emission peaks from grown nanostructures. Fig. 1(b) shows that the ZnO nanorods are covered by Au nanoparticles with Au sputtered on it for 50 s. Fig. 1(c) shows the Au thin films thickness with sputtering time on the ITO substrate and in the present case the nanorods are not verically aligned so the nanoparticle density will be varied.
(b)
200 nm
Au thin films
(c)
Fig. 1. (a) SEM image of ZnO nanorods grown at 80 °C for 5 h in ITO substrate, (b) SEM image of Au sputtered ZnO nanorods for 50 s and (c) sputtering time vs. Au-films thickness.
The ZnO nanostructures were grown from aqueous solution of 1 mM zinc nitrate [Zn(NO3)2 6H2O] and electrodeposition of ZnO nanorods involves the following reactions [16]: −
−
−
−
NO3 þ H2 O þ 2e →NO2 þ 2OH 2þ
Zn
−
þ 2OH →ZnO þ H2 O
ð1Þ ð2Þ
The total reaction may be written as 2þ
Zn
−
−
−
þ NO3 þ 2e →ZnO þ NO2 :
ð3Þ
After deposition the samples were removed from the electrolyte and rinsed in de-ionized water. The as-grown sample was cut into four pieces and then nanoparticles were sputtered on these samples
Fig. 2. (a) X-ray diffractogram of ZnO nanostructures grown at 80 °C; (b) X-ray diffractogram of ZnO nanostructures Au sputtered for 50 s.
4648
T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
3.2. Phase and structural analysis In order to investigate the structural properties of Au sputtered ZnO nanostructures, the X-ray diffraction measurements were performed and the XRD patterns are given in Fig. 2. The presence of several peaks in the XRD patterns of as-grown sample revealed that the nanostructures were polycrystalline in nature. The observed “d” values are in good agreement with standard “d” values and diffraction peaks indexed to the wurtzite phase of ZnO (JCPDF card no. 75-0591) with reflection peaks (100), (002), (101), (102), (103) and (112). Fig. 2(b) shows the XRD patterns of 50 s Au sputtered ZnO nanostructures. It shows well defined dominant reflection peaks having orientations in the (100), (002) and (101) planes corresponding to ZnO and dominant reflection peaks having orientations in the (111), (200), (220) and (311) planes appeared from Au nanoparticles, which are sputtered on the ZnO nanostructures. 3.3. Optical study In Fig. 3(a), we show the emission characteristics of Au-coated and un-coated ZnO nanostructures from PL spectra. PL measurements were performed at room temperature with an excitation wavelength of 266 nm. In Fig. 3(a), PL spectra of the un-coated ZnO nanostructures consist of a weak UV peak and a relatively strong and broad
visible peak. However, for the Au-coated ZnO nanostructures we observe a very strong UV emission at the wavelength of 373 nm and an almost complete quenching of the visible emission. The UV emission intensity of the Au coated ZnO nanostructures increased by a factor of ~9.5 (Fig. 3(b)) compared with the un-coated ZnO nanostructures, while the visible emission from the Au coated ZnO is quenched completely compared with the remarkable peak centered at 596 nm in PL of the un-coated ZnO nanostructures. Regarding the origin of the defect related emissions from ZnO, it is still controversial due to complicated nature of crystal defects in ZnO [18,19]. It is generally accepted that the deep-level emissions are closely related with the oxygen vacancies. In a recent study, the pronounced change in the PL spectra of metal (Zn, Ag, Ni, Au, Al and Ti) coated ZnO nanostructures with different deposition times was attributed to the metal–semiconductor contacts (Ohmic or Schottky contacts depending on the work function) at the interface [20]. The enhancement (Ohmic) or decrement (Schottky) in the PL intensity could be explained on the basis of metal semiconductor contacts for all the metals above mentioned except Au. Fang et al. reported that in case of Ohmic contact, when ZnO is excited by incident light the electron will move to the surface and accumulate there, while hole will move to the bulk region [20]. However electron affinity of ZnO (4.35 eV) is higher than the work function of metal, so it will be easy for electron transferring from metal to semiconductor and electron accumulation at the interface to increase substantially. This leads to the enhancement in the UVintensity as well as defect related emission. In case of Schottky contact, due to Schottky barrier at the interface the electron from metal will not easily transfer to the semiconductor. Thus when ZnO is excited by incident light the radiative recombination probability declines tremendously at the surface of ZnO, which causes the decrement of UV and defects intensities. In the present case a pronounced enhancement in the UV-emission and suppression of defect related emission is observed and this could not be explained with metal semiconductor (Schottky) contacts. Thus the enhancement in the UV-emission and suppression in defects emission is attributed to the localized
(a)
EC
Defects related band
Ef
E
(b)
V
Vacuum Level
Defects related bands
Φ m=5.47 eV
χ=4.35 eV
EC
Eg=3.37 eV
E=2.1 eV EV
ZnO
Fig. 3. (a) PL spectra of bare and Au coated ZnO nanostructures and (b) enhancement ratio of Au coated ZnO nanostructures in comparison to uncoated nanostructures.
Au
Fig. 4. (a) Energy band diagram of bare ZnO nanostructures, visible emission due to transition from defect levels. (b) Energy band diagram of Au coated ZnO nanostructures, electrons transfer to the conduction band of ZnO through plasmons for Aucoated ZnO nanostructures.
T. Singh et al. / Thin Solid Films 520 (2012) 4646–4649
surface plasmons [21]. At the interface between Au nanoparticles and the rough surface of ZnO nanostructures, localized surface plasmons can be created due to the resonant interaction between the electron-charge near the surface of the Au nanoparticles and the electromagnetic field of the incident light of 266 nm. The resonant oscillation of electrons in Au nanoparticles creates a local light field close to the particle surface that may strongly exceed the strength of the incident light field [22]. In the presence of Au nanoparticles, the giant enhancement of energy density of excitation source results in the improvement of excitation rate in excitation process and the decay rate in emission process. Fig. 4(a) shows the energy band diagrams of the un-coated ZnO nanostructures. The energy levels of the defect states in ZnO nanostructures are denoted by defect states, corresponding to the broad visible spectrum. Electron transfer processes via the coupling of localized surface plasmons at the interface between Au nanoparticles and ZnO nanostructures are shown in Fig. 4(b). The electron transfer from the defect states to the Au nanoparticles not only results in the increase of the resonant electron density, but also creates energetic electrons in higher energy state [23]. These resonant electrons are so active that they can escape from the surface of Au nanoparticles to the conduction band of the ZnO nanostructures [11]. Thus, the electron density in the conduction band of the ZnO is significantly increased, which leads to a significant increase of the intensity of UV emission from ZnO. In the present case, we obtained the UV enhancement factor of ~ 9.5. 4. Conclusions Au nanoparticles were sputtered on electrochemically synthesized ZnO nanorods for different sputtering times (for 0–100 s). The intensity of UV-emission was drastically enhanced and defects related emission was suppressed. Enhancement in UV-emission is due to the fact that the electrons in the defect state are transferred to the conduction band of ZnO via its coupling with the localized surface plasmons excited by the excitation source. The electrons in the defect state are transferred to the localized surface plasmons excited by the incident light and there is a decrease in the electrons in the defects
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states. The enhancement factor in the intensity of UV-emission peak was about 9.5 at the Au sputtering for 50 s. Acknowledgements The author (Trilok Singh) acknowledges IIT Delhi for fellowship and B.C. Joshi for helping in PL measurements. References [1] X. Wu, A. Yamilov, X. Liu, S. Li, V.P. Dravid, R.P.H. Chang, H. Cao, Appl. Phys. Lett. 85 (2004) 3657. [2] H.Y. Lin, Y.Y. Chou, C.L. Cheng, Y.F. Chen, Opt. Express 15 (2007) 13832. [3] Y. Zhang, W. Zhang, C. Peng, Opt. Express 16 (2008) 10696. [4] C.C. Lin, H.P. Chen, H.C. Liao, S.Y. Chen, Appl. Phys. Lett. 86 (2005) 183103. [5] N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, H. Haneda, Appl. Phys. Lett. 80 (2002) 2869. [6] C.L. Haynes, R.P.V. Duyne, J. Phys. Chem. B 107 (2003) 7426. [7] T.R. Jensen, M.D. Malinsky, C.L. Haynes, R.P.V. Duyne, J. Phys. Chem. B 104 (2000) 10549. [8] J.B. You, X.W. Zhang, Y.M. Fan, S. Qu, N.F. Chen, Appl. Phys. Lett. 91 (2007) 231907. [9] J.M. Lin, H.Y. Lin, C.L. Chen, Y.F. Chen, Nanotechnology 17 (2006) 4391. [10] W.H. Ni, J. An, C.W. Lai, H.C. Ong, J.B. Xu, J. Appl. Phys. 100 (2006) 26103. [11] H.Y. Lin, C.L. Cheng, Y.Y. Chou, L.L. Huang, Y.F. Chen, K.T. Tsen, Opt. Express 14 (2006) 2372. [12] X. Li, Y. Zhang, X. Ren, Opt. Express 17 (2009) 8735. [13] T. Chen, G.Z. Xing, Z. Zhang, H.Y. Chen, T. Wu, Nanotechnology 19 (2008) 435711. [14] C.W. Cheng, E.J. Sie, B. Liu, C.H.A. Huan, T.C. Sum, H.D. Sun, H.J. Fan, Appl. Phys. Lett. 96 (2010) 71107. [15] A.P. Abiyasa, S.F. Yu, S.P. Lau, E.S.P. Leong, H.Y. Yang, Appl. Phys. Lett. 90 (2007) 231106. [16] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wohrle, T. Sugiura, H. Minoura, Chem. Mater. 11 (1999) 2657. [17] S.P. Anthony, J. Lee, J.K. Kim, Appl. Phys. Lett. 90 (2007) 103107. [18] C. Chandrinou, N. Boukos, C. Stogios, A. Travlos, Microelectron. J. 40 (2009) 296. [19] Y.W. Heo, D.P. Norton, S.J. Pearton, J. Appl. Phys. 98 (2005) 073502. [20] Y.J. Fang, J. Sha, Z.L. Wang, Y.T. Wan, W.W. Xia, Y.W. Wang, Appl. Phys. Lett. 98 (2011) 033103. [21] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [22] B. Lamprecht, J.R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F.R. Aussenegg, Appl. Phys. B 79 (2001) 51. [23] C. Sonnichsen, T. Franzl, T. Wilk, G. Plessen, J. Feldmann, Phys. Rev. Lett. 88 (2002) 077402.