We demonstrate that the intensity of the band edge emission and lasing threshold from ZnO microstructures can be improved via the surface plasmon effect of Au nanoparticles. The near-band-edge emission can be enhanced by 11 fold and the defect-related emission is completely suppressed due to the electron transfer between the conduction band and defect levels through the localized surface plasmon resonance. The results suggest that Au nanoparticles can effectively enhance the lasing characteristic by turning down the lasing threshold, which is attributed to the resonance coupling between the surface plasmon and the optical transition in ZnO. In addition, the formation of superior scattering species by Au nanoparticles play another important role in the random lasing mechanism.
© 2017 Optical Society of America
ZnO has attracted considerable attention due to its wide direct band gap (3.37 eV) with large exciton binding energy (60 meV) as well as being potential candidates in the field of short-wavelength semiconductor lasers and light-emitting diodes at room temperature (RT). Due to the expected low threshold required for lasing in ZnO structures in reduced dimensionality, lasing in nanostructured ZnO is of particular interest, and lasing behaviors have been demonstrated in various types of ZnO micro/nanostructures [1, 2]. Regarding the emission properties in ZnO micro/nanostructures, a large portion of excited charge carriers are trapped in the defect and/or surface states, resulting in intense defect-related emissions and reduction of ultraviolet (UV) emission efficiency [3–5]. Therefore, improving UV luminescence efficiency has become a vital issue in the field of ZnO research. The localized surface plasmon resonance has been employed to enhance the florescence emission . An enhancement in UV emission from Au-coated ZnO films was observed and there was a significant change in the visible emission compared with bare ZnO films . Surface-plasma (SP)-mediated emission from different metal-decorated ZnO nanorods and the coupling of the light from ZnO with the SPs of various metals has been intensely studied [8–14]. Furthermore, the SP effect can also be applicable to the improvement of the lasing properties. An enhancement of UV-emission Fabry–Pérot lasing property from Au-decorated ZnO micro-flowers has been demonstrated . Epitaxially noble metal nanodisks formed on ZnO nanoplate can assist in lowering the whispering-gallery-mode lasing threshold . The achievement of the improved random lasing emission from ZnO nanostructures coated with nanoparticles (NPs) of Ag , Zn  and Pt  was reported. Recently, the random lasing wavelength of the ZnO microstructure can be controlled of by the SP effect of Ag NPs .
Although the above-mentioned ZnO micro/nano-structures can be grown by various techniques, most of them are for laser applications and still were accomplished through vapor phase growth [15–20]. The vapor deposition route needs relatively expensive vacuum equipment and complex procedures, which limit use in practical applications. Compared to the vapor deposition route, a solution-processed method can easily coat on different flexible substrates and be employed as one of the potential inks for three-dimensional printing and for paint-on lasers . More recently, we demonstrated that the lasing threshold of random lasing (RL) in solution-processed ZnO micro/nano-structures could be lowered to ~0.4 MW/cm2 via the reduction of surface-trapped states . Hence, to further understand the SP resonance effect on the improvement of the random lasing performance is of high interest. However, to date, there are limited studies dedicated to the localized SP-coupling-induced UV enhancement of solution-processed ZnO luminescence, except for the investigation on enhanced UV emission of electrochemically fabricated ZnO nanorods using Au nanoparticles . Moreover, the controllability of the random lasing characteristics needs to be addressed. In this paper, the in-depth SP effect of Au nanoparticles on an improvement of band-edge emission and lasing threshold of the ZnO microstructures grown by a facile sol-gel method was investigated. The band-edge emission could be enhanced over ten times and the defect-related emission is completely suppressed due to the electron transfer between conduction band and defect levels through the SP resonance. More importantly, the lasing threshold of ZnO microstructures with an Au coating was reduced. The enhanced lasing emission property was attributed to resonance coupling between the surface plasmon and the optical transition in Au-ZnO hybrids.
2. Experimental details
ZnO microstructures were fabricated by the two-stage reaction process sol-gel method. Full details of the growth process can be found in Ref [22, 24]. After the films became dry by heating at 200 °C for 30 min, they were treated at 1,000 °C in ambient argon for 1 h. Finally, Au NPs were coated with different coating times by a DC sputtering system. The morphologies were characterized through a scanning electron microscope (SEM) and transmission electron microscopy (TEM). The micro-photoluminescence (μ-PL) measurements were carried out by a continuous- wave (CW) He-Cd laser at an excitation wavelength of 325 nm. The laser light was focused on the sample by a 40X UV objective lens and the emitted radiation was collected into the same objective lens to a spectrograph equipped with a liquid-N2 CCD array detector. To study the lasing behavior, the samples were excited by a Q-switch 4th harmonic Nd: YAG laser (266 nm, pulse width of 550 ps, repetition rate of 8.73 kHz) instead of the CW laser. All measurements were performed at room temperature.
3. Results and discussion
Figure 1(a) presents the x-ray diffraction patterns of the as-obtained ZnO and the typical Au/ZnO microstructures reveal the formation of well crystalline hexagonal wurtzite phase ZnO. When Au NPs were deposited on to ZnO, one additional peak at 38.4° was associated with the face centered cubic phase of metallic Au. In these samples, no other peak was detected, indicating the products were pure. Figure 1(b) and 1(c) shows the morphologies of ZnO microstructures before and after coating Au nanoparticles, respectively. The bare ZnO microstructures have a smooth surface as shown in Fig. 1(b). After Au sputtering, the microstructures surfaces were decorated with a discontinuous layer of nanoparticles. The observed size distribution of the Au NPs was some nanometers. The size of Au NPs increases with higher sputtering currents and/or longer sputtering times. Figure 1(d) and 1(e) shows the TEM image of the ZnO microstructures coated 30-s coated and100-s coated Au nanoparticles, respectively. The sphere-like Au NPs can be observed in the TEM images. The average size of the Au nanoparticles is 6.28 nm and 7.81nm for 30-s and 100-s sample, respectively. The inter-spacing of 30-s coated Au NPs is larger than that of the coated 100-s Au NPs.
Figure 2(a) displays the PL spectra of ZnO microstructures without and with different Au coating time. Each spectrum consists of a dominant peak in the UV region and a broad peak in the visible. The UV emission is usually ascribed to the main contribution of free exciton (FX) at near band edge (NBE), as already observed in various ZnO nanostructures . The green emission center at 2.35 eV originates from the electron transition from the conduction band edge to a trap level and the singly ionized oxygen vacancies at the particle surface, which results in strong band bending and a significant reduction in the rate of radiative NBE recombination [24–26]. After Au coating with different times, the UV peak intensity increases monotonically with increasing of deposition time of Au NPs in the range of 10-30 s. The inset of Fig. 2(a) shows the enlarged visible emission band.
At Au deposition time of 30 s, we observed 11-fold PL enhancement in Fig. 2(b). Note that the green emission is heavily suppressed in all cases. After that, the UV intensity decreases. Two mechanisms were proposed to interpret the origin of the UV enhancement. One reason is due to localized surface plasmons, where the electrons transfer between a conduction band and defect level through the SP resonance (SPR) . The metal energy between the light emission and SP resonant is important to achieve the SP enhancement. The frequency of the SP closely depends on the size and shape of nano-metals . The other reason is caused by the metal-semiconductor contacts (Schottky or Ohmic contacts depending on the work function) at the interface. In case of Ohmic contact, when ZnO is excited by incident light the electrons will move to the surface and accumulate there, while holes will move to the inner region of the material . The electron transfers from metal to semiconductor and electrons accumulate at the interface, which results in the enhancement in the UV-intensity as well as defect related emission. In our case, the defect related emission was suppressed and the enhancement in the UV-intensity was strongly correlated to Au-coating time. Therefore, we consider that the SPR plays a significant role in our hybrid Au/ZnO materials.
To clearly interpret the SPR effect for the enhancement of the NBE emission and the suppression of the defect-related emission, a schematic diagram about the band alignment is illustrated in Fig. 3. The conduction band of ZnO is located at −4.19 eV versus vacuum level, and the Femi level of Au is at −5.3eV versus vacuum level . It can be seen that since the defect level (−4.99eV) is very close to the Femi level of the Au, the defect-related electron from ZnO microstructures can be transferred to the Femi level of Au. The electron in the Femi level of Au can be used to excite the surface plasma. When this occurs on the SPR, the excited electrons are transited to high-energy states, which can transfer to the conduction band of ZnO microstructures. Consequently, the electron density in the conduction band of the ZnO microstructures is increased. This means that SPR leads to enhancement of the intensity of NBE emission and suppresses the intensity of defect-related emission from ZnO microstructures.
The morphology and spatial density of the Au NPs not only influence the local field enhancement but the change of the absorption spectra of Au NPs. Therefore, optimizing the SP-assisted coupling conditions that refers to the coating time of 30s leads to a largest enhancement factor. The decrease of enhancement ratio at longer coating time is attributed to decrease the overlapping between the absorption spectra of Au and defect emission spectra in ZnO. The shielding of the excited/emitted light by the high density of Au NPs may also result in the reduction of enhancement ratio.
Figure 4 (a) plots the typical evolution of the lasing emission spectra of 100-s Au-coated ZnO microstructures with increasing pulsed excitation power density. Upon lowering pumping power density, only broad spontaneous emission is observed. As the pumping power density reaches a threshold value, sharp peaks emerge from the single broad emission, which demonstrates that lasing action occurs. The inset of Fig. 4 (a) shows integrated intensity as a function of pumping power density. The lasing threshold (Pth) can be 0.33 MW/cm2.
The lasing is attributed to the existence of the random lasing caused by multiple scattering in the ZnO microstructures [22, 29]. The lasing thresholds and the effective slope efficiency of ZnO microstructures without and with Au coating of different coating times are plotted in Fig. 4(b). It is observed that the lasing threshold can be reduced and the slope efficiency can be increased with the presence of an Au coating. Figure 4 (c) shows the lasing threshold and the effective slope efficiency as a function of different Au coating time. The ZnO microstructures coated 30-s Au nanoparticles had the lowest lasing threshold at 0.17 MW/cm2, which is which is one order of magnitude lower than that of the random lasing from GaN nanopowders with Au NPs . With increasing the sputtering time from 30 s, the threshold value increases while the relative slope efficiency decreases. This result is consistent with the results of CW photoluminescence. Hence, the dependence of Pth for the Au-coated ZnO can also be interpreted by the SPR effects of Au nanoparticles. Furthermore, the enhancement of scattered light is expected to decrease the lasing threshold. The incident light may be absorbed by the structure of a thick Au film when the sputter time is more than 30 s. In other words, the absorption and scattering from the Au nanoparticles dominates the behavior of the light dissipation. Therefore, it can be expected to decreases the optical performance of random lasing.
In conclusion, an 11-fold enhancement of the band-edge emission from hybrid structures of Au/ZnO microstructures compared to that from bare ZnO microstructures was observed. The obtained enhancement in the PL is explained by SP resonance assisted by electron transfer to the conduction band of ZnO. PL characteristics are affected when a local incident electromagnetic field is enhanced. The band alignment can be clearly understood through the energy transfer model for the conduction band and the defect level of ZnO via SPR by the incident light. We further demonstrated that the random lasing characteristics of ZnO microstructures were controlled by the thickness of an Au coating. Controllability of random characteristics can be further used by other devices to improve various properties, such as electrical luminescence (EL). These results suggest that a hybrid Au-ZnO microstructure is a promising candidate for potential application in highly-efficient emission devices in a UV regime.
Ministry of Science and Technology (MOST) of Taiwan (MOST 105-2112-M-006-004-MY3).
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