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A hybrid structure of reduced graphene oxide (rGO) sheets/ZnO nanorods was prepared and its photoluminescence intensity ratio between the UV and defect emission was enhanced up to 14 times. By controlling the reduction degree of rGO on the surface of ZnO nanorods, the UV emission was tuned with the introduction of localized surface plasmons resonance of rGO sheets. The suppression of the defect emission was ascribed to the charge transfer and decreased with the distance between the rGO and ZnO nanorods.
© 2014 Optical Society of America
1. Introduction
Wurtzite ZnO is the most promising material with wide band gap (3.37eV) and high exciton binding energy at room temperature (60meV). ZnO nanorods have attracted extensive attention as the fundamental materials for generating microwaves with short wavelengths. However, some intrinsic defects and impurities in ZnO nanorods result in low ultraviolet (UV) emission efficiency and restrict its application in LEDs. Hence, to obtain highly efficient UV emission from the near band edge is one of the most important issues for photonic applications of ZnO nanorods. Numerous studies have been conducted to improve the bandgap emission on ZnO [1–5]. Graphene, a stable monolayer of Graphite, has become a rising star in material science recently [6,7]. Due to its unusual physical properties such as high electron mobility, high thermal conductivity, and optimal mechanical properties, its applications in optoelectronic devices, nanocomposites, solar cells and sensors have attracted significant interest [8–11]. Very recently, rapidly increasing interest is focused on the plasmonic effect of graphene similar to that of the surface plasmon (SP) induced by metal. Since the SP-mediated emission from metal or metal alloy capped emitter structures has been proven as an effective way to improve the quantum efficiency of light emitting materials and light emission diodes (LEDs) [12], this provides the possibility for graphene to improve the efficiency of optoelectronic devices. It was reported by Hwang et al. that graphene plasmon generated at the interface between graphene and ZnO structures is responsible for the enhanced radiation recombination of ZnO [13]. Kim et al. found that the band edge and defect emission of ZnO are simultaneously enhanced by plasmonic resonance of reduced graphene oxide (rGO) [14].
Here we present a hybrid structure of rGO sheets/ZnO nanorods whose defect emission was significantly suppressed and the UV emission was largely enhanced. The intensity ratio between the UV and defect emission is improved by a factor of up to 14 times. rGO sheets are obtained by heat treatment of the graphene oxide (GO) under argon protection. The hydrothermal method is used to produce ZnO nanorods since it is a process of low cost, low temperature, large area uniformity and environmentally friendly method.
2. Experiment
ZnO nanorods were deposited on silicon substrates using a hydrothermal method from an aqueous solution of Zn(NO3)2 • 6H2O and C6H12N6. The detailed growth process of ZnO nanorods has been reported [15]. The diameter and length of the rod were controlled by adjusting the concentrations of the seed layer and growth solutions as 13.3 mmol/L and 0.075 mol/L, respectively. The growth time was 2.5 h and the temperature of 90 °C was maintained. GO powders were synthesized using the Hummers’ method with some modifications [9, 16]. The GO powders obtained thereby were admixed with methanol and deionized (DI) water (volume ratio of 5:1) at the concentration of 0.2 mg/ mL. The GO suspensions were sonicated for a uniform dispersion and deposited onto the ZnO nanorods by using Langmuir-Blodgett (LB) technique with the surface pressure of 1 mN/m [17–19]. Rapid thermal processing (RTP) of the samples was conducted with the controlled vacuum and gas flow at 200, 500 and 800 °C for an hour, respectively. A vacuum of 0.1 Pa was held before heat treatment. The samples were heated with a continuous flow of ultrapure argon at the rate of 10 °C/min and cooled to room temperature with the RTP furnace. After the thermal treatment, the GO/ZnO hybrid structures were reduced to rGO/ZnO. The control samples consisting only of ZnO nanorods annealed at various temperatures were also prepared.
LB assembly of GO films was carried out on a commercial LB system (KSV-Minimicro 2000, KSV NIMA, Finland). Atomic force microscope (AFM) (Innova, Veeco, USA) and scanning electron microscope (SEM) (SU-8010, Hitachi, Japan) were employed respectively to characterize the surface morphology of the samples. X-ray photoelectron spectroscopy (XPS) (AXIS ultra DLD, Shimadzu, Japan) was used to characterize the graphitization of GO sheets. The photoluminescence (PL) emission spectra were recorded by Time-resolved fluorescence spectroscopy (QM4, PTI, USA).
3. Results and discussion
The typical thickness of GO thin films with wrinkled edges was about 1-3 nm as measured in the line scans shown in Fig. 1(b) from AFM image in Fig. 1(a). The thickness of GO monolayer around 1 nm is reported previously [20, 21]. It is indicated that the as-prepared GO films have been flaked into some layers of GO sheets (FLGO). As measured using AFM, the thicknesses of four different GO layers from left to right were recorded. No significant disparities of surface morphology occurred after the thermal treatment of GO films. Figures 1(c) and 1(d) show the top and side views of as-prepared ZnO nanorods respectively, from which the diameter of the rods was ~100 nm and the length was ~1.5 um. Clearly, the bare ZnO nanorods were vertical aligned and had a smooth surface. After preparation of GO/ZnO hybrid structure shown in Fig. 1(e), the majority of ZnO nanorods were well covered by large area GO films except for little gaps between GO sheets due to the hydrophilic surface of nanorods. As shown in Fig. 1(e), the 20° tilted views of the uncovered and covered ZnO nanorods were illustrated in the inset Figs. 1(e1) and 1(e2), respectively.
Fig. 1 (a) An AFM image of GO sheets on a silicon wafer obtained using typical tapping mode measurement. (b) A depth profile in a line scan of the area shown in (a). (c) A top and (d) a side views of as-prepared ZnO nanorods. (e) A top view of GO/ZnO hybrid structure. A 20° tilted view SEM images of the uncovered (e1) and covered (e2) ZnO nanorods.
XPS analysis is used to confirm the reduction degree of rGO. Figures 2(a)-2(d) show the typical C1s XPS spectra of as-prepared GO and GO annealed at 200, 500 and 800 °C in Argon, respectively. Curve fitting of the C1s spectra was performed using commercial software Avantage, indicative of the C1s signals of the original GO deconvoluted into several signals for C-C bond (~284.5 eV), C-O bond (~286.2 eV), C = O bond (~287.8 eV), O-C = O bond (~288.9 eV), and O-CO-O bond (~290.8 eV). The effect of increased reduction temperature on (C-C) % content and the values of C/O ratio are illustrated in Fig. 2(e). The (C-C) % content corresponding to sp2 carbon fraction in as-synthesized GO (57.93%) was greatly improved to 73.44% with annealing at 200 °C and then 84.43% at 800 °C. In addition, with the increased reduction temperature from 200 °C to 800 °C, C/O ratio values increased to five times. The best reduction process can be obtained by the largest decomposition of oxygen functional groups and the simultaneous restoration of sp2 C–C bonds as complete as possible. It was decided by higher (C-C) % content (84.43%) and meanwhile larger C/O ratio value (20) that efficient reduction of GO thin films can be therefore achieved upon 800 °C. The remaining oxygen-containing groups in reduced GO, the majority of which consisted of C-O bonds, were difficult to be removed by thermal annealing [22].
Fig. 2 Typical C1s XPS spectra for GO films at different reduction temperatures: (a) unreduced GO, (b) 200 °C, (c) 500 °C, (d) 800 °C. (e) (C-C) % content and the values of C/O ratio as a function of the annealing temperature, respectively.
Since the rods exhibited dramatic change in the defect emission for different heat treatment, a detailed study of the PL spectra dependence on annealing temperature was conducted, as shown in Fig. 3(a).The main features of the PL spectra can be divided into two categories: the UV near band-edge emission around 385 nm and a broad visible emission. For the as-grown ZnO nanorods prepared by the hydrothermal method, the chemical component of the ZnO nanorods is nonstoichiometric and usually consisted of excess Zn atoms and oxygen vacancies, indicative of many lattice defects and surface defects contained in the as-grown ZnO nanorods. These defects act as nonradiative centers and reduce light emission from the ZnO. Thus weak UV emission and strong visible emission is observed from the as-grown ZnO nanorods. Thermal annealing leads to a stronger band edge UV emission attributed to exciton-related recombination of ZnO with no peak shift. The inset shows Gaussian fitting of green-yellow (~570 nm) and yellow-orange (~630 nm) defect emission in as-grown ZnO. The redshift of defect transition from green-yellow to yellow-orange is attributed to reducing and restructuring of crystal structure in ZnO with the increasing annealing temperatures. Yellow defect emission represents a common characteristic in samples prepared from aqueous solutions of zinc nitrate hydrate and is typically owing to an oxygen interstitial [23, 24]. Although the type of defect responsible for the green emission has not been conclusively identified, there is convincing evidence that it is located at the surface. The possible presence of Zn(OH)2 at the surface, especially for nanorods synthesized by solution methods, can influence the green defect emission in ZnO [25]. In addition, Orange defect emission around 630 nm also results from oxygen interstitials [26].
Fig. 3 (a) PL spectra of ZnO nanorods upon different heating temperatures. (b) Integrated PL intensity ratio as a function of annealing temperature.
In order to investigate the effectiveness of different annealing temperatures, relative PL peak intensity ratio P (IUV/IDefect), as indicated in Fig. 3(b), was adopted as a figure of merit. The optimal P value was obtained for annealing at 800 °C, reflecting an enhanced UV emission with a substantial reduction in yellow related defect emission. The enhancement obtained for the UV emission after annealing at 200 °C was related to desorption of hydroxyl groups, since the desorption rate of the hydroxyl groups peaks at ~150 °C [27]. Owing to desorption of hydrogen from Zn vacancy site, a fall in the P value was observed with annealing at 500 °C. With an increased annealing temperature at 800 °C, an increased P value was ascribed to possible bond breaking of the interstitial oxygen at the stable octahedral site for deep acceptor transition, which also resulted in a decrease in the UV emission.
GO is transformed from modified-graphene with oxygen functional groups in the form of epoxy and hydroxy groups on the basal plane and carboxyl and acetate groups at the edges [28]. This unique atomic and electronic structure of GO [29], consisting of variable sp2 and sp3 hybridized carbon atoms by reduction treatment, makes a finite band gap as opposed to the zero band gap of graphene. It has shown interesting steady-state PL emission, ranging from near-infrared (NIR) to blue fluorescence [30–35]. It can be seen in Fig. 4 that near-UV blue PL, resulting from geminate recombination of electron-hole pairs localized within sp2 carbon clusters embedded within a sp3 matrix, were observed from GO and rGO thin films on silicon when excited at 325 nm [35]. However, due to the low emission efficiency from GO and rGO, the intrinsic PL intensities were much weaker than that from ZnO nanorods, which is consistent with the previous reports [30,31,35]. The inset of Fig. 4 shows the influence of reduction temperature on the PL intensity and peak position, respectively. It is worthwhile to note that the peak position was blue-shifted as the GO thin films were reduced to rGO. The blue-shifted emission in PL spectra suggests that sp2 clusters are embedded in a sp3 matrix that acts as a tunnel barrier, causing a strong fluctuation in the local band gap [36]. Even though the PL intensity of rGO was slightly enhanced after moderate reduction under heat treatment, it was negligible when compared to that of rGO/ZnO hybrid structures. Moreover, the PL was weaker upon further reduction due to the formation of non-fluorescent graphene.
Fig. 4 (a) PL spectra of GO thin films upon different heating temperatures. The inset is the influence of reduction temperature on the PL intensity and peak position, respectively.
The PL spectra in the UV region obtained from experiments include emissions from both ZnO and rGO. In order to find the exclusive UV emission from ZnO, the intrinsic emission of rGO was subtracted from the hybrid structure to eliminate the effects from rGO emission. Figures 5(a)-5(d) show the corrected PL spectra of rGO/ZnO hybrid structures annealed at different temperatures and their control samples from measurements. To understand the influence of rGO on the field emission of the ZnO nanorods, the ratio of the corrected PL peak intensity of rGO/ZnO hybrid structures (I) to that of the corresponding control sample (I0) as the enhancement factor was proposed. The corrected intensities of the UV and defect emissions as a function of the annealing temperature were illustrated in Fig. 5(e). In all the measurements, no significant variation in PL intensity was observed when the detection angle was changed. It can be seen in Fig. 5(e) that the UV emission at about 385 nm is sensitive to the annealing treatment, namely, sp2 carbon fraction in rGO. As shown in Fig. 5(f), the UV enhancement factor varied with the changes of (C-C) % content. The PL peak of the hybrid structure did not shift with respect to that of the reference ZnO nanorods. The strongest UV peak intensity from the hybrid structure was approximately 3.26 times larger than that of bare ZnO nanorods. In addition, the enhancement factor of the defect emission around 500-750 nm was less than 1 for all the samples, indicative of great quenching of broad visible emission in ZnO nanorods capped with rGO. The most quenching reduced to 0.23 times smaller than that of ZnO. Accordingly, the proposed unique hybrid structure is able to enhance the intensity ratio between the UV and defect emission by a factor of up to 14 times. It appears that the presence of rGO sheets has significant influence on the UV emission and defect emission of ZnO nanorods.
Fig. 5 Corrected PL spectra of GO/ZnO hybrid structures annealed at (a) unannealed, (b) 200 °C, (c) 500 °C, (d) 800 °C and their control samples, respectively. (e) Intensity of UV and defect emission vs the annealing temperature. (f) The UV enhancement factor vs (C-C) %.
To further study the effect of rGO sheets on the corrected PL intensities of ZnO nanorods, MgO spacer layers of various thicknesses were sputtered between ZnO nanorods and rGO annealed at 800 °C as shown in Fig. 6. The yellow-orange defect emissions located at ~630 nm were observed in all the samples. The variation of corrected PL intensity with MgO thickness was plotted in the inset. A decrease in the UV emission and simultaneous a rise in the defect emission were discovered when the MgO layer was introduced. In fact, the band edge emission of the rGO/ZnO hybrid structure was quenched to almost half of the ZnO nanorods band edge emission upon introduction of the 20 nm MgO spacer. In contrast, the defect emission of the rGO/ZnO hybrid structure was monotonically enhanced by inserting the increased MgO layer between ZnO nanorods and rGO. It was noted that the direct covering of rGO on the surface of ZnO nanorods greatly affected the PL intensity of ZnO. This observed effectiveness can be explained by the fact that the rGO sheets produced LSP resonance and simultaneously transformed the PL emission of ZnO nanorods due to its metallic properties.
Fig. 6 Corrected PL spectra of rGO covered samples annealed at 800 °C according to the various spacer thicknesses. The inset shows integrated PL intensities of UV and defect emission vs the spacer thickness.
The corrected PL enhancement of band edge emission at about 385 nm was due to the coupling of the light emission with the localized surface plasmons (LSP) resonance of rGO sheets. The properties of rGO sheets prepared by the reduction of GO are much similar compared to that of graphene. According to previous reports, the resonant excitation of graphene plasmon in graphene/ZnO films was attributed to enhanced PL intensity, coincided well with that of metal/ZnO films [13]. Alternatively, abnormally increased absorption of graphene in the UV region was proposed in several reports [35,37]. The absorption in the UV region, corresponding to the plasmon band similar to that of plasmonic metal, can result in strong LSP at the surface of rGO/ZnO hybrid structure in the resonance wavelength, thus the enhanced UV light emission. After reduction of GO, the plasmon band in the resultant rGO broadens in the longer wavelength spectra region, indicative of more spectra overlap with the resonance band of ZnO. Therefore, the higher reduction degree of rGO results in the higher metallic property and the more overlaps between plasmon band of rGO and the absorption band of ZnO, thus much easier to lead to LSP resonance in rGO/ZnO hybrid structures, which is consistent with the enhancement ratio of UV emission as a function of (C-C) % content as indicated in Fig. 5(f). UV intensities in rGO/ZnO hybrid structures were stronger than that in ZnO nanorods capped with GO. This slightly smaller intensity in GO capped ZnO nanorods compared to that of bare ZnO was probably due to a reduced transmission in GO and a mismatch of LSP resonance domain. In addition, the UV intensities in rGO/ZnO hybrid structures were weaker by inserting the thicker MgO layer shown in Fig. 6, resulting from the exponential decay of LSP resonance in the vertical depth of penetration.
For the broad and intense defect emission around 500-750 nm in ZnO nanorods, an important issue is the suppression of defect emission by many treatments after identifying their origins. According to previous reports, the green emission can be suppressed by surfactant coating on the surface [38]. The yellow emission can be reduced by annealing in a reducing condition (hydrogen/argon mixture) [23]. As for UV-to-defect emission intensity ratio in ZnO nanorods, it has been enhanced with hydrogen plasma treatment [39]. Most interestingly, it was found that the broad defect emission consisting of green, yellow and orange was more or less quenched in rGO/ZnO hybrid structure. It was reported by Guo et al. that graphene is an important factor for quenching the fluorescence of QDs [40], resulting from the photo-excited electron or energy in QDs transferred to graphene. This suggests that the decreased charge carriers are trapped in the vacancy within ZnO and thus the defect emissions are quenched. It provides a general mechanism that an additional pathway is available for interfacial charge transfer from ZnO nanorods to GO and rGO [41]. As shown in Fig. 6, after inserting MgO spacer layer, charge transfer became difficult and the broad visible emission resulted from the superposition of two PL intensities, which further verifies the hypothesis.
4. Conclusions
The UV-to-defect emission intensity ratio in ZnO nanorods was strongly enhanced when the ZnO surface was covered with rGO. It was found that rGO sheets with more sp2 carbon clusters lead to stronger enhancement of UV emission, while the defect emission was suppressed by covering GO and rGO. Inserting different thickness of MgO spacer layer between ZnO nanorods and rGO can tune the UV emission changing from enhancement to quenching, otherwise, the defect emission from quenching from enhancement.
Acknowledgments
The authors would like to thank the financial supports by National Natural Science Foundation of China (No.51175418), National Natural Science Foundation of China Major Research Program on Nanomanufacturing (No.91323303), Program for New Century Excellent Talents in University (No.93JXDW02000006), 111 Program (No.B12016), and the Fundamental Research Funds for the Central Universities.
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