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sapphire films by experimental study and electromagnetic simulation
Journal of Alloys and Compounds 625 (2015) 175–181
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Investigation of the photoluminescence properties of Au/ZnO/sapphire and ZnO/Au/sapphire films by experimental study and electromagnetic simulation Yong Zeng, Yan Zhao, Yijian Jiang ⇑ Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 27 October 2014 Accepted 28 October 2014 Available online 7 November 2014 Keywords: Electro-optical materials Photoluminescence Thin films Electromagnetic field simulation
a b s t r a c t Photoluminescent properties from Au/ZnO/sapphire and ZnO/Au/sapphire structures have been investigated. It is found that due to the co-interaction between the incident light and local surface plasmons, the ultraviolet (UV) emissions from the two structures were both enhanced and the visible emissions related to the defects were suppressed. By the means of electromagnetic simulation, it indicates that the enhancement of UV intensity is a result of the enhanced electric field intensity of the 325 nm excitation light, which is induced by localized surface plasmons resonance (LSPR). On the other hand, electron transfer which is induced by the local surface also account for the enhancement of UV emissions. The suppression of the visible emissions might be due to the flowing of electrons in the defect states to the Au, which caused the reduction of the electrons in the defect states. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, ZnO has attracted a great deal of attention for applications in ultraviolet light-emitting devices (LED) [1,2], solar cells [3,4] and photodiodes [5,6] due to its large band gap (3.37 eV) and high excitonic binding energy (60 meV). The character of photoluminescence (PL) is an important index to measure the quality of ZnO. However, visible light emissions related to defects or impurities dominate its luminescence spectra in most cases. Hence, it is important to obtain highly efficient UV emission from the near band edge and reduce the visible emitting efficiency. Recently, surface plasmons have been widely utilized to enhance the luminescence efficiency of ZnO. For example, Lei et al. and Cheng et al. have shown two- and threefold band edge emission enhancements from ZnO films by using Al and Ag cap layers, respectively [7,8]. Ábrahám et al. observed a 12-fold enhancement in the emission intensity on gold-coated substrates [9]. The mechanism of the enhancement of the band edge emission by metal capping is not yet completely understood. Li et al. consider the increase of electron density induced by localized surface plasmons (LSPs) in conduction band causes enhanced UV emission [10]. LSPs are the oscillation of charge density at the interface between metal and dielectric. Appropriate light excitation will stimulate LSRP ⇑ Corresponding author. E-mail addresses: [email protected] (Y. Zeng), [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.jallcom.2014.10.197 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
with the presence of micro-structures (such as nanosphere, nanocylinder) in the metal nanoparticles and dielectric interface. In addition, Song et al. indicate that surface passivation effect maybe contribute to the enhancement of band edge emission of ZnO structures [11,12]. Liu et al. suggest the surface modification by Pt film may be accounted for the enhancement of the bandgap emission from ZnO thin films [13]. So far, most of the literatures report the surface plasmon enhanced PL by experimental analysis. However, little literature reported the investigation of PL enhancement based on simulation. In this work, we have studied the bandedge emission enhancement and visible emission quenching from Au/ZnO/sapphire and ZnO/Au/sapphire films by using PL spectroscopy. By the means of electromagnetic simulation, the mechanisms of changes of UV and visible emissions have been presented. 2. Experiment In this experiment, we used two different types of samples, Au/ZnO/sapphire and ZnO/Au/sapphire films, as is shown in Fig. 1(a) and (b). For the growth of Au/ZnO/sapphire films, the ZnO films were first grown on sapphire substrates by pulsed laser deposited (PLD) technique. Prior to deposition, all the substrates were cleaned ultrasonically using ethanol and acetone for 5 min, respectively. The ceramic target was placed on a rotating disk. The distance between target and sapphire substrate was 5 cm. The ultrahigh vacuum chamber was evacuated to a base pressure of 6 10 4 Pa. Then we used the silicon wafer to heat the substrate, and maintained the temperature of 450 °C. After heating, oxygen was entered into the furnace and maintained at 40 Pa. KrF excimer laser (248 nm, pulse width of 23 ns) was used for ablation. Repetition rate, laser energy and pulse number were set to 3 Hz, 300 mJ and 6000, respectively. We cut a piece of ZnO film into several
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parts to make Au/ZnO/sapphire structure. The Au powder sputtering system (DC sputtering) was used to deposit gold nanoparticles in the films for different sputtering times at room temperature. A current of 5 mA was used at the pressure of 8 Pa. Another piece of ZnO film was cut into several parts to make ZnO/Au/sapphire structure. The growth of ZnO/Au/sapphire films is similar with that of Au/ZnO/ sapphire films. Firstly, the Au nanoparticles were deposited on the sapphire substrates, and then ZnO film was prepared on the Au coated sapphire substrate. The structural quality of the films was characterized by Raman spectra (HORIBA, T64000). The surface morphologies and surface roughness of films were characterized by Atomic Force Microscope (AFM) (MI, Pico Scan TM 2500). The PL spectra were carried out by using the He–Cd laser with a wavelength of 325 nm (Kimmon Koha Co., Ltd, IK3301R-G). WYKO NT1100 Profiler (3D, Non-Contact) was used to investigate the thickness of the films. The thickness of ZnO film and Au film are about 100 nm and 20 nm, respectively. The propagation of electromagnetic waves was simulated by CST MICROWAVE STUDIO (Computer Simulation Technology, Germany).
3. Results and discussions Considering in the ZnO/Au/sapphire set of samples the ZnO layer is grown at 450 °C, the high temperature maybe give rise to the size increase and coalescence of the Au particles. Therefore, AFM is used to characterize the size and morphology of Au particles. The AFM images of Au/ZnO/sapphire (sputtering time of 14 s) and Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C) are shown in Fig. 2(a) and (b), respectively. The annealing time is set to 1 h, which is equal to the time, that ZnO film grown at 450 °C. The Au/ZnO/sapphire (sputtering time of 14 s) sample shows a dense surface arrangement of gold nanoparticles. The average grain size of the gold particles is about 20 nm. Fig. 2(b) shows a lager gold particles in Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C). It means that the high temperature causes the gold particles agglomeration, leading to gold particle size bigger. In order to further study the morphology of the gold particles, we tested SEM. Fig. 3 reveal that the Au/sapphire (sputtering time of 14s, annealing in oxygen for
1 h at 450 °C) shows a dense surface arrangement of gold nanoparticles than which in Au/sapphire (sputtering time of 8s, annealing in oxygen for 1 h at 450 °C). There are gaps between the gold particles. The average grain size of the gold particles in annealed samples is about 80 nm. It is well known that Raman spectrum can sensitively reveal the changes of microstructure of the materials, leading to the analysis of crystal quality. Fig. 4(a)–(f) suggest a good wurtzite crystal structure because of the intense Raman peaks of 99 and 436 cm 1, which are the typical peaks of ZnO at the backscattering configuration and assigned to E2 (low) and E2 (high), respectively [14]. The peaks located at 417 cm 1 is attributed to sapphire substrate [15]. The small peaks in the range of 50–150 cm 1 are due to the rotational spectrum of air molecules [16]. It reveals that the introduction of gold does not impact on the crystal structure of the zinc oxide film. The room PL spectra for Au/ZnO/sapphire and ZnO/Au/sapphire films are shown in Figs. 4(a) and 5(b). Due to the influence of defects, the PL spectra of as-grown ZnO films are slightly different in Fig. 5(a) and (b). For the as-grown ZnO films, a strong UV emission peak at around 3.25 eV and a broad visible emission band at 1.5–2.5 eV were observed in the PL spectra. It is well known that the UV emission belongs to the band gap emission, while the visible emission is due to defects, such as oxygen vacancies and zinc interstitial. It can be seen in Fig. 5 that the band edge emission at 3.25 eV is enhanced and the visible emission is invariably descended in both sets of Au-decorated samples. The insets in Fig. 5(a) and (b) show the enhancement factor of the band gap emission as a function of the sputtering time of Au nanoparticles. As shown, the UV enhancement factor of Au/ZnO/sapphire films first increases with the sputtering time, reaches a maximum value of 2.5 for a sputtering time of 14 s, and then decreases. The UV emission of ZnO/Au/sapphire films also first increases with the
Fig. 1. Samples schematic diagram of (a) Au/ZnO/sapphire and (b) ZnO/Au/sapphire.
Fig. 2. AFM images for (a) Au/ZnO/sapphire (sputtering time of 14 s), (b) Au/sapphire (sputtering time of 8s, annealing in oxygen for 1 h at 450 °C) and (c) Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C).
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Fig. 3. SEM images for (a) Au/sapphire (sputtering time of 8 s, annealing in oxygen for 1 h at 450 °C), and (b) Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C).
Fig. 4. Raman spectra of (a) the as-grown film and the ZnO/Au/sapphire films with different sputtering time of (b) 6 s, (c) 8 s, (d) 10 s, (e) 12 s and (f) 14 s.
sputtering time, reaches a maximum value of 3 for a sputtering time of 8s, and then starts to decrease gradually. The band edge emission enhancement factor is sensitive to the sputtering time
of Au. SEM results show that the increase of the sputtering time, the gold nanoparticles density increases. It means that the spacing between the gold nanoparticles will decrease. As we know, if a laser beam is irradiated with the two closely spaced metal nanoparticles, hot spots will forms between the nanoparticles. The incident electric field intensity in the region (hot spots) will be significantly enhanced by the surface plasmon resonance and it will gradually increases as the spacing between the nanoparticles decreases. In our experiments, SEM results show that the Au nanoparticles inter-distances become smaller as the sputtering time increases. When laser irradiate the Au/ZnO/sapphire or ZnO/Au/ sapphire structure, whether the increase of the PL is due to the enhancement of electric field intensity of the incident light increase in the Zinc oxide surface or the interior? With continued increase of the sputtering time, the gold particles become dense and are connected together. Most of the gold particles no longer exists a gap. Whether the decrease of the PL is due to the reduction of electric field intensity of the incident light increase in the Zinc oxide surface or the interior? In order to verify our conjecture, we use electromagnetic simulation software to simulate the distribution of the electric field intensity of the incident light in the Zinc oxide surface and the interior. It should be noted that the evident changes in PL of ZnO films is attributed to the introduction of Au nanoparticles. To further
Fig. 5. The PL spectra of (a) Au/ZnO/sapphire and (b) ZnO/Au/sapphire film with different Au sputtering times. The inset is the enhancement factor of the band gap emission after normalization with the as-grown sample.
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understand the PL enhancement mechanisms for the UV emission, we establish a model to study the electromagnetic wave propagation through the gold particles. SEM results show that gold particles are not regular sphere. However, since the incident laser beam has a few hundred microns diameter, so gold particles act like some small sphere over the entire range of the incident radiation. So in electromagnetic simulation, we set the shape of the gold particles become spherical. Fig. 6(a) shows the model of Au/ZnO/ sapphire film. The thickness of ZnO film is set to be 100 nm. From AFM results, the diameter of the Au sphere is set to be 20 nm. The distance between the two Au spheres center is labeled as D. The electric field intensity of the plane wave is set to 1 and propagates along the z direction. Au particles side is the incident side. The interface between Au particles and ZnO film is labeled as M plane. Fig. 6(b) and (c) shows the propagation of 325 nm incident light in ZnO and M plane, respectively. As can be seen from the simulation results, the electric field intensity of the plane wave has changed significantly after the plane wave pass through the gold spheres. The electric field intensity in ZnO and M plane has been enhanced. Fig. 6(d) shows the maximum electric field intensity in the ZnO films and in the M plane for a distance of these two metal nanoparticles centers, respectively. From Fig. 5(d), it can be seen that the electric field intensity increases with the decrease of D. The electric field intensity reaches a maximum value when D is 20 nm, then decreases with the continued reduction of D. Fig. 7(a) shows the model of ZnO/Au/sapphire film. The thickness of ZnO film is set to be 100 nm. From SEM results, due to high temperature give rise to the size increase, the diameter of the Au sphere is set to be 80 nm. The distance between the two Au spheres center is labeled as D. The electric field intensity of the plane wave is set to 1 and propagates along the z direction. ZnO side is the incident side. The interface between Au particles and ZnO film is labeled as N plane. Fig. 7(b) and (c) shows the propagation of 325 nm incident light in ZnO and N plane, respectively. The electric field intensity in ZnO and N plane has been enhanced. Fig. 7(d) shows the maximum electric field intensity in the ZnO films and
in the N plane for a distance from these two metal nanoparticles centers, respectively. From Fig. 7(d), it can be seen that the electric field intensity increases with the decrease of D. The electric field intensity reaches a maximum value when D is 82 nm, then decreases with the continued reduction of D. The PL intensity can be adjusted by electric field intensity. Park et al. investigated the effect of an applied electric field on the PL intensity of single CdSe nanocrystals. Simultaneous intensity and frequency resolved PL showed that the PL intensity modulation was in fact due to an electric field effect on the PL quantum yield. The PL intensity can be modulated by the coaction of the applied electric field and the internal electric field induced by deeply trapped surface charges. As the field strength increased, the PL of CdSe was enhanced [17]. McNeill et al. used electric field to modulate the near-field PL of thin films of the conjugated polymer MEH-PPV. They found that the modulation of PL intensity provided a sensitive measure of changes in the local carrier density induced by the applied electric field [18]. Korsunska et al. investigated the influence of electric field on PL in visible wavelength range in nominally undoped ZnO single crystals. They found that the action of direct electric field of about 100 V/cm at 600–700 °C resulted in the increase of green band intensity near the cathode and its decrease near the anode [19]. Aleshin et al. reported on the effect of electric field on the PL from a composite consisting of a BEHP-co-MEH-PPV conjugated polymer mixed with ZnO nanoparticles. They found that the generation of excited states in the BEHP-co-MEH-PPV-ZnO structure included the formation of ‘‘exciplex’’ states and charge transfer from the polymer to nanoparticles which can be controlled by an electric field [20]. In order to further investigate the change of electric field, electric energy density (EED) has been simulated. Fig. 8(a) shows the EED of Au/ZnO structure. The plot of EED as a function of the distance between two metal nanoparticles in Au/ZnO is showed in Fig. 8(b). It reveals that EED increases with the decrease of D. EED reaches a maximum value when D is 20 nm, then decreases with the continued reduction of D. Fig. 8(c) shows the EED of
Fig. 6. (a) Model of Au/ZnO/sapphire film, the propagation of 325 nm incident light in (b) ZnO and (c) M plane, and (d) the maximum electric field intensity in the ZnO films and in the M plane for a distance of two metal nanoparticles centers.
Fig. 7. (a) Model of ZnO/Au/sapphire film, the propagation of 325 nm incident light in (b) ZnO and (c) N plane, (d) the maximum electric field intensity in the ZnO films and in the N plane for a distance of two metal nanoparticles centers.
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Fig. 8. (a) The EED of Au/ZnO structure, (b) EED as a function of the distance between two metal nanoparticles in Au/ZnO, (c) EED of ZnO/Au structure, and (d) EED as a function of the distance between two metal nanoparticles in ZnO/Au.
ZnO/Au structure. The dependence of EED on D for ZnO/Au is shown on Fig. 8(d). It can be seen that EED increases with the decrease of D. The electric field intensity reaches a maximum value when D is 82 nm, then decreases with the continued reduction of D. The results of EED have the same trend of the change of electric field intensity. The enhancement of electric field intensity is due to the cointeraction between the incident light and local surface plasmons. LSP of nanoparticles can induce short-range interactions within the distance of a few nanometers. Localized surface plasmon can be excited through near-field interactions at the interface between Au nanoparticles and the surface of ZnO films. Generally, local surface plasmon enhanced fluorescence mechanism can be summed up in three aspects: (1) the excitation rate is enhanced [21,22]. (2) The electron–hole recombination rate increase [23,24]. (3) Charge transfer [25,26]. In our simulation, we consider that the enhanced UV emission is due to the enhancement of the incident light intensity. As shown in Fig. 5(a), the electric field intensity of the incident light which irradiate in the ZnO film is enhanced
obviously. When 325 nm incident light irradiates gold nanoparticles, the oscillating electromagnetic field will cause the collective oscillations of free electrons in the Au nanoparticles. On this occasion, Au nanoparticles behave as an electric dipole that can induce a plasmon resonance. Local surface plasmon oscillation can enhance the electric field intensity of 325 nm incident light. Therefore, the electric field intensity of incident light which radiate into the ZnO film is enhanced. The enhanced electric field intensity will cause radiative recombination of electron at the conduction band and hole at the valence band. It will lead to the enhancement of the near-band-edge exciton transition. Therefore, the intensity of UV emission increased. In order to more clearly understand the simulation results, a schematic diagram about the band alignment for ZnO films was shown in Fig. 9. For the as-grown ZnO film, a certain intensity of excitation light is irradiated onto the surface to produce ultraviolet light, and the ZnO film has certain external quantum efficiency. When increasing the intensity of the incident light, more electron–hole pairs would produce radiation recombination, resulting
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Fig. 9. The schematic of the band alignment for (a) as-grown and (b) Au-decorated ZnO films.
in the increase of the external quantum efficiency of ZnO films. This leads to the enhancement of the UV emission. On the other hand, the resonant electrons in the Au may be flow to the CBM (conductor band bottom) due to the local surface plasmons resonance, which will increase the electron density in the CBM. This may be another reason of the enhancement of the UV emission (see Fig. 9). PL results show that the defect emission of all Au- decorated films have declined. As we known, abundant surface defects exist within the ZnO films. These surface defects can trap the valence band hole, and then result in the broad band visible emissions [27–29]. Lin et al. proposed the quenching of defect emission is possibly attributed to the reduction in surface defects by Pt coating [30]. Song et al. found that the capping layers of nonmetal materials (such as TiO2 and AlN) can passivate the dangling bonds and fill the surface defects. As a result, less surface defects are able to trap electrons. Accordingly, the emissions related to the defects are decreased [11]. However, the visible emission is declined for both Au/ZnO/sapphire and ZnO/Au/sapphire films in our experiments. Especially, for the ZnO/Au/sapphire film, the gold nanoparticles are located below the ZnO in the ZnO/Au/sapphire films. Therefore, the surface defect states have not been ameliorated. In addition, the surface of the ZnO/Au/sapphire film is bare, there is no passivation effect. So, we believe that surface defect passivation is not the only reason for the decline of visible light by metal capping. It is shown in Fig. 6(b) that the energy level of defect states in ZnO films (2.25 and 1.91 eV) is closed to the Au Fermi level [31]. So electrons of the defect states can flow to the Au Fermi level. This leads to a reduction in the number of electrons in the defect states. Therefore, it is suggested that is the reasons for the reduction of the visible emission.
4. Conclusion In summary, we have shown the enhanced UV emission and decreased visible emission of Au/ZnO/sapphire and ZnO/Au/sapphire films. AFM results show that the size of Au nanoparticles are 20 nm in Au/ZnO. And SEM results show that the Au is 80 nm
in ZnO/Au By the means of electromagnetic simulation, the electric field intensity spatial distributions over the ZnO film (in the xz plane) and ZnO surface (in the xy plane) have been investigated. The electric field intensity in ZnO film and ZnO surface has been enhanced. The simulation of EED shows the same trend of the change of electric field intensity. Simulation results show that the value of D can significantly affect the electric field intensity distribution of the incident light. Since the PL intensity can be adjusted by electric field intensity. It indicates that the enhancement of UV intensity is a result of the enhanced electric field intensity of the 325 nm stimulate light which induced by LSPR. Due to the LSPR, the electric field intensity of incident light which radiate into the ZnO film is enhanced. The enhanced electric field intensity will increase the radiative recombination of electron at the conduction band and hole at the valence band. On the other hand, electron transfer which induced by the local surface can also affect the enhancements of UV emissions. From the PL results of ZnO/Au/ sapphire films, surface passivation is not the main reason for the decline of the visible emission. The suppression of the visible emissions might be due to flowing of the electrons from defects states to Au Fermi level, which caused the reduction of the electrons in the defect states. Acknowledgements This work was supported by the National Science Foundation of China (51475014), the Beijing Natural Science Foundation (1132014), Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201210005004) and Scientific Research Project of Beijing Educational Committee (Grant No. KM201210005021). References [1] L.C. Zhang, Q.S. Li, L. Shang, F.F. Wang, C. Qu, F.Z. Zhao, Opt. Express 21 (2013) 16578. [2] J.Y. Lee, J.H. Lee, H.S. Kim, C.H. Lee, H.S. Ahn, H.K. Cho, Y.Y. Kim, B.H. Kong, H.S. Lee, Thin Solid Films 517 (2009) 5157. [3] Y.T. Shi, K. Wang, Y. Du, H. Zhang, J.F. Gu, C. Zhu, L. Wang, W. Guo, A. Hagfeldt, N. Wang, T.L. Ma, Adv. Mater. 25 (2013) 4413.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Investigation of the photoluminescence properties of Au/ZnO/sapphire and ZnO/Au/sapphire films by experimental study and electromagnetic simulation Yong Zeng, Yan Zhao, Yijian Jiang ⇑ Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 27 October 2014 Accepted 28 October 2014 Available online 7 November 2014 Keywords: Electro-optical materials Photoluminescence Thin films Electromagnetic field simulation
a b s t r a c t Photoluminescent properties from Au/ZnO/sapphire and ZnO/Au/sapphire structures have been investigated. It is found that due to the co-interaction between the incident light and local surface plasmons, the ultraviolet (UV) emissions from the two structures were both enhanced and the visible emissions related to the defects were suppressed. By the means of electromagnetic simulation, it indicates that the enhancement of UV intensity is a result of the enhanced electric field intensity of the 325 nm excitation light, which is induced by localized surface plasmons resonance (LSPR). On the other hand, electron transfer which is induced by the local surface also account for the enhancement of UV emissions. The suppression of the visible emissions might be due to the flowing of electrons in the defect states to the Au, which caused the reduction of the electrons in the defect states. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, ZnO has attracted a great deal of attention for applications in ultraviolet light-emitting devices (LED) [1,2], solar cells [3,4] and photodiodes [5,6] due to its large band gap (3.37 eV) and high excitonic binding energy (60 meV). The character of photoluminescence (PL) is an important index to measure the quality of ZnO. However, visible light emissions related to defects or impurities dominate its luminescence spectra in most cases. Hence, it is important to obtain highly efficient UV emission from the near band edge and reduce the visible emitting efficiency. Recently, surface plasmons have been widely utilized to enhance the luminescence efficiency of ZnO. For example, Lei et al. and Cheng et al. have shown two- and threefold band edge emission enhancements from ZnO films by using Al and Ag cap layers, respectively [7,8]. Ábrahám et al. observed a 12-fold enhancement in the emission intensity on gold-coated substrates [9]. The mechanism of the enhancement of the band edge emission by metal capping is not yet completely understood. Li et al. consider the increase of electron density induced by localized surface plasmons (LSPs) in conduction band causes enhanced UV emission [10]. LSPs are the oscillation of charge density at the interface between metal and dielectric. Appropriate light excitation will stimulate LSRP ⇑ Corresponding author. E-mail addresses: [email protected] (Y. Zeng), [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.jallcom.2014.10.197 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
with the presence of micro-structures (such as nanosphere, nanocylinder) in the metal nanoparticles and dielectric interface. In addition, Song et al. indicate that surface passivation effect maybe contribute to the enhancement of band edge emission of ZnO structures [11,12]. Liu et al. suggest the surface modification by Pt film may be accounted for the enhancement of the bandgap emission from ZnO thin films [13]. So far, most of the literatures report the surface plasmon enhanced PL by experimental analysis. However, little literature reported the investigation of PL enhancement based on simulation. In this work, we have studied the bandedge emission enhancement and visible emission quenching from Au/ZnO/sapphire and ZnO/Au/sapphire films by using PL spectroscopy. By the means of electromagnetic simulation, the mechanisms of changes of UV and visible emissions have been presented. 2. Experiment In this experiment, we used two different types of samples, Au/ZnO/sapphire and ZnO/Au/sapphire films, as is shown in Fig. 1(a) and (b). For the growth of Au/ZnO/sapphire films, the ZnO films were first grown on sapphire substrates by pulsed laser deposited (PLD) technique. Prior to deposition, all the substrates were cleaned ultrasonically using ethanol and acetone for 5 min, respectively. The ceramic target was placed on a rotating disk. The distance between target and sapphire substrate was 5 cm. The ultrahigh vacuum chamber was evacuated to a base pressure of 6 10 4 Pa. Then we used the silicon wafer to heat the substrate, and maintained the temperature of 450 °C. After heating, oxygen was entered into the furnace and maintained at 40 Pa. KrF excimer laser (248 nm, pulse width of 23 ns) was used for ablation. Repetition rate, laser energy and pulse number were set to 3 Hz, 300 mJ and 6000, respectively. We cut a piece of ZnO film into several
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parts to make Au/ZnO/sapphire structure. The Au powder sputtering system (DC sputtering) was used to deposit gold nanoparticles in the films for different sputtering times at room temperature. A current of 5 mA was used at the pressure of 8 Pa. Another piece of ZnO film was cut into several parts to make ZnO/Au/sapphire structure. The growth of ZnO/Au/sapphire films is similar with that of Au/ZnO/ sapphire films. Firstly, the Au nanoparticles were deposited on the sapphire substrates, and then ZnO film was prepared on the Au coated sapphire substrate. The structural quality of the films was characterized by Raman spectra (HORIBA, T64000). The surface morphologies and surface roughness of films were characterized by Atomic Force Microscope (AFM) (MI, Pico Scan TM 2500). The PL spectra were carried out by using the He–Cd laser with a wavelength of 325 nm (Kimmon Koha Co., Ltd, IK3301R-G). WYKO NT1100 Profiler (3D, Non-Contact) was used to investigate the thickness of the films. The thickness of ZnO film and Au film are about 100 nm and 20 nm, respectively. The propagation of electromagnetic waves was simulated by CST MICROWAVE STUDIO (Computer Simulation Technology, Germany).
3. Results and discussions Considering in the ZnO/Au/sapphire set of samples the ZnO layer is grown at 450 °C, the high temperature maybe give rise to the size increase and coalescence of the Au particles. Therefore, AFM is used to characterize the size and morphology of Au particles. The AFM images of Au/ZnO/sapphire (sputtering time of 14 s) and Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C) are shown in Fig. 2(a) and (b), respectively. The annealing time is set to 1 h, which is equal to the time, that ZnO film grown at 450 °C. The Au/ZnO/sapphire (sputtering time of 14 s) sample shows a dense surface arrangement of gold nanoparticles. The average grain size of the gold particles is about 20 nm. Fig. 2(b) shows a lager gold particles in Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C). It means that the high temperature causes the gold particles agglomeration, leading to gold particle size bigger. In order to further study the morphology of the gold particles, we tested SEM. Fig. 3 reveal that the Au/sapphire (sputtering time of 14s, annealing in oxygen for
1 h at 450 °C) shows a dense surface arrangement of gold nanoparticles than which in Au/sapphire (sputtering time of 8s, annealing in oxygen for 1 h at 450 °C). There are gaps between the gold particles. The average grain size of the gold particles in annealed samples is about 80 nm. It is well known that Raman spectrum can sensitively reveal the changes of microstructure of the materials, leading to the analysis of crystal quality. Fig. 4(a)–(f) suggest a good wurtzite crystal structure because of the intense Raman peaks of 99 and 436 cm 1, which are the typical peaks of ZnO at the backscattering configuration and assigned to E2 (low) and E2 (high), respectively [14]. The peaks located at 417 cm 1 is attributed to sapphire substrate [15]. The small peaks in the range of 50–150 cm 1 are due to the rotational spectrum of air molecules [16]. It reveals that the introduction of gold does not impact on the crystal structure of the zinc oxide film. The room PL spectra for Au/ZnO/sapphire and ZnO/Au/sapphire films are shown in Figs. 4(a) and 5(b). Due to the influence of defects, the PL spectra of as-grown ZnO films are slightly different in Fig. 5(a) and (b). For the as-grown ZnO films, a strong UV emission peak at around 3.25 eV and a broad visible emission band at 1.5–2.5 eV were observed in the PL spectra. It is well known that the UV emission belongs to the band gap emission, while the visible emission is due to defects, such as oxygen vacancies and zinc interstitial. It can be seen in Fig. 5 that the band edge emission at 3.25 eV is enhanced and the visible emission is invariably descended in both sets of Au-decorated samples. The insets in Fig. 5(a) and (b) show the enhancement factor of the band gap emission as a function of the sputtering time of Au nanoparticles. As shown, the UV enhancement factor of Au/ZnO/sapphire films first increases with the sputtering time, reaches a maximum value of 2.5 for a sputtering time of 14 s, and then decreases. The UV emission of ZnO/Au/sapphire films also first increases with the
Fig. 1. Samples schematic diagram of (a) Au/ZnO/sapphire and (b) ZnO/Au/sapphire.
Fig. 2. AFM images for (a) Au/ZnO/sapphire (sputtering time of 14 s), (b) Au/sapphire (sputtering time of 8s, annealing in oxygen for 1 h at 450 °C) and (c) Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C).
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Fig. 3. SEM images for (a) Au/sapphire (sputtering time of 8 s, annealing in oxygen for 1 h at 450 °C), and (b) Au/sapphire (sputtering time of 14 s, annealing in oxygen for 1 h at 450 °C).
Fig. 4. Raman spectra of (a) the as-grown film and the ZnO/Au/sapphire films with different sputtering time of (b) 6 s, (c) 8 s, (d) 10 s, (e) 12 s and (f) 14 s.
sputtering time, reaches a maximum value of 3 for a sputtering time of 8s, and then starts to decrease gradually. The band edge emission enhancement factor is sensitive to the sputtering time
of Au. SEM results show that the increase of the sputtering time, the gold nanoparticles density increases. It means that the spacing between the gold nanoparticles will decrease. As we know, if a laser beam is irradiated with the two closely spaced metal nanoparticles, hot spots will forms between the nanoparticles. The incident electric field intensity in the region (hot spots) will be significantly enhanced by the surface plasmon resonance and it will gradually increases as the spacing between the nanoparticles decreases. In our experiments, SEM results show that the Au nanoparticles inter-distances become smaller as the sputtering time increases. When laser irradiate the Au/ZnO/sapphire or ZnO/Au/ sapphire structure, whether the increase of the PL is due to the enhancement of electric field intensity of the incident light increase in the Zinc oxide surface or the interior? With continued increase of the sputtering time, the gold particles become dense and are connected together. Most of the gold particles no longer exists a gap. Whether the decrease of the PL is due to the reduction of electric field intensity of the incident light increase in the Zinc oxide surface or the interior? In order to verify our conjecture, we use electromagnetic simulation software to simulate the distribution of the electric field intensity of the incident light in the Zinc oxide surface and the interior. It should be noted that the evident changes in PL of ZnO films is attributed to the introduction of Au nanoparticles. To further
Fig. 5. The PL spectra of (a) Au/ZnO/sapphire and (b) ZnO/Au/sapphire film with different Au sputtering times. The inset is the enhancement factor of the band gap emission after normalization with the as-grown sample.
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understand the PL enhancement mechanisms for the UV emission, we establish a model to study the electromagnetic wave propagation through the gold particles. SEM results show that gold particles are not regular sphere. However, since the incident laser beam has a few hundred microns diameter, so gold particles act like some small sphere over the entire range of the incident radiation. So in electromagnetic simulation, we set the shape of the gold particles become spherical. Fig. 6(a) shows the model of Au/ZnO/ sapphire film. The thickness of ZnO film is set to be 100 nm. From AFM results, the diameter of the Au sphere is set to be 20 nm. The distance between the two Au spheres center is labeled as D. The electric field intensity of the plane wave is set to 1 and propagates along the z direction. Au particles side is the incident side. The interface between Au particles and ZnO film is labeled as M plane. Fig. 6(b) and (c) shows the propagation of 325 nm incident light in ZnO and M plane, respectively. As can be seen from the simulation results, the electric field intensity of the plane wave has changed significantly after the plane wave pass through the gold spheres. The electric field intensity in ZnO and M plane has been enhanced. Fig. 6(d) shows the maximum electric field intensity in the ZnO films and in the M plane for a distance of these two metal nanoparticles centers, respectively. From Fig. 5(d), it can be seen that the electric field intensity increases with the decrease of D. The electric field intensity reaches a maximum value when D is 20 nm, then decreases with the continued reduction of D. Fig. 7(a) shows the model of ZnO/Au/sapphire film. The thickness of ZnO film is set to be 100 nm. From SEM results, due to high temperature give rise to the size increase, the diameter of the Au sphere is set to be 80 nm. The distance between the two Au spheres center is labeled as D. The electric field intensity of the plane wave is set to 1 and propagates along the z direction. ZnO side is the incident side. The interface between Au particles and ZnO film is labeled as N plane. Fig. 7(b) and (c) shows the propagation of 325 nm incident light in ZnO and N plane, respectively. The electric field intensity in ZnO and N plane has been enhanced. Fig. 7(d) shows the maximum electric field intensity in the ZnO films and
in the N plane for a distance from these two metal nanoparticles centers, respectively. From Fig. 7(d), it can be seen that the electric field intensity increases with the decrease of D. The electric field intensity reaches a maximum value when D is 82 nm, then decreases with the continued reduction of D. The PL intensity can be adjusted by electric field intensity. Park et al. investigated the effect of an applied electric field on the PL intensity of single CdSe nanocrystals. Simultaneous intensity and frequency resolved PL showed that the PL intensity modulation was in fact due to an electric field effect on the PL quantum yield. The PL intensity can be modulated by the coaction of the applied electric field and the internal electric field induced by deeply trapped surface charges. As the field strength increased, the PL of CdSe was enhanced [17]. McNeill et al. used electric field to modulate the near-field PL of thin films of the conjugated polymer MEH-PPV. They found that the modulation of PL intensity provided a sensitive measure of changes in the local carrier density induced by the applied electric field [18]. Korsunska et al. investigated the influence of electric field on PL in visible wavelength range in nominally undoped ZnO single crystals. They found that the action of direct electric field of about 100 V/cm at 600–700 °C resulted in the increase of green band intensity near the cathode and its decrease near the anode [19]. Aleshin et al. reported on the effect of electric field on the PL from a composite consisting of a BEHP-co-MEH-PPV conjugated polymer mixed with ZnO nanoparticles. They found that the generation of excited states in the BEHP-co-MEH-PPV-ZnO structure included the formation of ‘‘exciplex’’ states and charge transfer from the polymer to nanoparticles which can be controlled by an electric field [20]. In order to further investigate the change of electric field, electric energy density (EED) has been simulated. Fig. 8(a) shows the EED of Au/ZnO structure. The plot of EED as a function of the distance between two metal nanoparticles in Au/ZnO is showed in Fig. 8(b). It reveals that EED increases with the decrease of D. EED reaches a maximum value when D is 20 nm, then decreases with the continued reduction of D. Fig. 8(c) shows the EED of
Fig. 6. (a) Model of Au/ZnO/sapphire film, the propagation of 325 nm incident light in (b) ZnO and (c) M plane, and (d) the maximum electric field intensity in the ZnO films and in the M plane for a distance of two metal nanoparticles centers.
Fig. 7. (a) Model of ZnO/Au/sapphire film, the propagation of 325 nm incident light in (b) ZnO and (c) N plane, (d) the maximum electric field intensity in the ZnO films and in the N plane for a distance of two metal nanoparticles centers.
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Fig. 8. (a) The EED of Au/ZnO structure, (b) EED as a function of the distance between two metal nanoparticles in Au/ZnO, (c) EED of ZnO/Au structure, and (d) EED as a function of the distance between two metal nanoparticles in ZnO/Au.
ZnO/Au structure. The dependence of EED on D for ZnO/Au is shown on Fig. 8(d). It can be seen that EED increases with the decrease of D. The electric field intensity reaches a maximum value when D is 82 nm, then decreases with the continued reduction of D. The results of EED have the same trend of the change of electric field intensity. The enhancement of electric field intensity is due to the cointeraction between the incident light and local surface plasmons. LSP of nanoparticles can induce short-range interactions within the distance of a few nanometers. Localized surface plasmon can be excited through near-field interactions at the interface between Au nanoparticles and the surface of ZnO films. Generally, local surface plasmon enhanced fluorescence mechanism can be summed up in three aspects: (1) the excitation rate is enhanced [21,22]. (2) The electron–hole recombination rate increase [23,24]. (3) Charge transfer [25,26]. In our simulation, we consider that the enhanced UV emission is due to the enhancement of the incident light intensity. As shown in Fig. 5(a), the electric field intensity of the incident light which irradiate in the ZnO film is enhanced
obviously. When 325 nm incident light irradiates gold nanoparticles, the oscillating electromagnetic field will cause the collective oscillations of free electrons in the Au nanoparticles. On this occasion, Au nanoparticles behave as an electric dipole that can induce a plasmon resonance. Local surface plasmon oscillation can enhance the electric field intensity of 325 nm incident light. Therefore, the electric field intensity of incident light which radiate into the ZnO film is enhanced. The enhanced electric field intensity will cause radiative recombination of electron at the conduction band and hole at the valence band. It will lead to the enhancement of the near-band-edge exciton transition. Therefore, the intensity of UV emission increased. In order to more clearly understand the simulation results, a schematic diagram about the band alignment for ZnO films was shown in Fig. 9. For the as-grown ZnO film, a certain intensity of excitation light is irradiated onto the surface to produce ultraviolet light, and the ZnO film has certain external quantum efficiency. When increasing the intensity of the incident light, more electron–hole pairs would produce radiation recombination, resulting
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Fig. 9. The schematic of the band alignment for (a) as-grown and (b) Au-decorated ZnO films.
in the increase of the external quantum efficiency of ZnO films. This leads to the enhancement of the UV emission. On the other hand, the resonant electrons in the Au may be flow to the CBM (conductor band bottom) due to the local surface plasmons resonance, which will increase the electron density in the CBM. This may be another reason of the enhancement of the UV emission (see Fig. 9). PL results show that the defect emission of all Au- decorated films have declined. As we known, abundant surface defects exist within the ZnO films. These surface defects can trap the valence band hole, and then result in the broad band visible emissions [27–29]. Lin et al. proposed the quenching of defect emission is possibly attributed to the reduction in surface defects by Pt coating [30]. Song et al. found that the capping layers of nonmetal materials (such as TiO2 and AlN) can passivate the dangling bonds and fill the surface defects. As a result, less surface defects are able to trap electrons. Accordingly, the emissions related to the defects are decreased [11]. However, the visible emission is declined for both Au/ZnO/sapphire and ZnO/Au/sapphire films in our experiments. Especially, for the ZnO/Au/sapphire film, the gold nanoparticles are located below the ZnO in the ZnO/Au/sapphire films. Therefore, the surface defect states have not been ameliorated. In addition, the surface of the ZnO/Au/sapphire film is bare, there is no passivation effect. So, we believe that surface defect passivation is not the only reason for the decline of visible light by metal capping. It is shown in Fig. 6(b) that the energy level of defect states in ZnO films (2.25 and 1.91 eV) is closed to the Au Fermi level [31]. So electrons of the defect states can flow to the Au Fermi level. This leads to a reduction in the number of electrons in the defect states. Therefore, it is suggested that is the reasons for the reduction of the visible emission.
4. Conclusion In summary, we have shown the enhanced UV emission and decreased visible emission of Au/ZnO/sapphire and ZnO/Au/sapphire films. AFM results show that the size of Au nanoparticles are 20 nm in Au/ZnO. And SEM results show that the Au is 80 nm
in ZnO/Au By the means of electromagnetic simulation, the electric field intensity spatial distributions over the ZnO film (in the xz plane) and ZnO surface (in the xy plane) have been investigated. The electric field intensity in ZnO film and ZnO surface has been enhanced. The simulation of EED shows the same trend of the change of electric field intensity. Simulation results show that the value of D can significantly affect the electric field intensity distribution of the incident light. Since the PL intensity can be adjusted by electric field intensity. It indicates that the enhancement of UV intensity is a result of the enhanced electric field intensity of the 325 nm stimulate light which induced by LSPR. Due to the LSPR, the electric field intensity of incident light which radiate into the ZnO film is enhanced. The enhanced electric field intensity will increase the radiative recombination of electron at the conduction band and hole at the valence band. On the other hand, electron transfer which induced by the local surface can also affect the enhancements of UV emissions. From the PL results of ZnO/Au/ sapphire films, surface passivation is not the main reason for the decline of the visible emission. The suppression of the visible emissions might be due to flowing of the electrons from defects states to Au Fermi level, which caused the reduction of the electrons in the defect states. Acknowledgements This work was supported by the National Science Foundation of China (51475014), the Beijing Natural Science Foundation (1132014), Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201210005004) and Scientific Research Project of Beijing Educational Committee (Grant No. KM201210005021). References [1] L.C. Zhang, Q.S. Li, L. Shang, F.F. Wang, C. Qu, F.Z. Zhao, Opt. Express 21 (2013) 16578. [2] J.Y. Lee, J.H. Lee, H.S. Kim, C.H. Lee, H.S. Ahn, H.K. Cho, Y.Y. Kim, B.H. Kong, H.S. Lee, Thin Solid Films 517 (2009) 5157. [3] Y.T. Shi, K. Wang, Y. Du, H. Zhang, J.F. Gu, C. Zhu, L. Wang, W. Guo, A. Hagfeldt, N. Wang, T.L. Ma, Adv. Mater. 25 (2013) 4413.
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