Journal of the Korean Physical Society, Vol. 67, No. 9, November 2015, pp. 1634∼1638
Spatially resolved Optical Properties of ZnO Sub-microstructures on a Graphene Monolayer Hye Min Oh,∗ Jaesu Kim, Hyun Jeong, Yong Hwan Kim and Mun Seok Jeong† Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea
Hong Seok Lee∗ Department of Physics, Research Institute of Physics and Chemistry, Chonbuk National University, Jeonju 54896, Korea
Jin Soo Kim Division of Advanced Materials Engineering, Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 54896, Korea (Received 22 September 2015, in final form 6 October 2015) We investigated the carrier-gas flows-ratio-dependent structure evolution of ZnO submicrostructures grown on a graphene/SiO2 /Si substrate without a catalyst or seed layer by using thermal chemical vapor deposition. The surface morphology and the crystalline quality of the ZnO sub-microstructures measured by using field-emission scanning electron microscopy, X-ray diffraction, and Raman spectroscopy showed a clear dependence on the carrier-gas flow ratio. The spatially resolved photoluminescence property of the ZnO structure was investigated by using confocal scanning photoluminescence microscopy, and ZnO grown at an oxygen flow rate of 50 sccm showed relatively good optical properties. PACS numbers: 78.47.+p, 78.66.Hf, 81.05.Dz, 81.15.Gh Keywords: ZnO sub-microstructures, Graphene, Carrier gas rate, Photoluminescence DOI: 10.3938/jkps.67.1634
I. INTRODUCTION Zinc oxide (ZnO) is a direct wide band-gap semiconductor (3.37 eV) with a large exciton binding energy (60 meV). As one of the most important II−IV group semiconductors, ZnO nanomaterials have been investigated extensively due to their attractive optical and electrical properties [1]. Especially, ZnO nanostructures have received considerable attention for use in optoelectronic applications, such as UV light emitters, UV sensors, and so on [2–4]. High-quality ZnO nanostructures have been deposited by various techniques, such as magnetron sputtering, chemical vapor deposition (CVD), pulsed laser deposition, molecular beam epitaxy, and the solgel method [5–9]. ZnO growth using CVD is very attractive because it is a simple and cost-effective method compared to other methods. Graphene, a monolayer of sp2 hybrid carbon atoms, is a two-dimensional (2D) material with unique electrical and mechanical properties [10–12]. Graphene∗ These
based hybrid nanostructures may enable a new generation of nanoelectronics, functional devices, bio-molecule sensors, and optoelectronic devices [13–15]. Recently, there have been reports on the growth of one-dimensional ZnO nanorods and nanowires on graphene by using high-temperature metal-organic vapor-phase epitaxy, a photolithography-patterned ZnO seed layer based on a solution method, and CVD [16–18]. Other studies have reported ZnO structures grown on graphene without a seed layer or catalyst [19, 20]. If the crystal quality of ZnO structures is to be improved, a systematic study of various growth conditions is essential. In this study, we investigated the carrier-gas flows-rate-dependent structural evolution of ZnO submicrostructures grown on a graphene/SiO2 /Si substrate without a catalyst or seed layer by using thermal CVD. The optical and the structural properties of the ZnO submicrostructures were investigated by using field emission scanning electron microscopy (FE-SEM), high-resolution X-ray diffraction (HR-XRD), confocal scanning photoluminescence microscopy (CSPM), and Raman spectroscopy.
authors contributed equally to this work.
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Spatially resolved Optical Properties of ZnO · · · – Hye Min Oh et al.
Fig. 1. (a) Raman spectrum of monolayer graphene and (b) FE-SEM image and (c) FE-SEM image of the ZnO structure grown on graphene.
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Fig. 3. (Color online) XRD spectra for the ZnO structures grown on graphene/SiO2 /Si substrate at oxygen flow rates of 50, 100, and 200 sccm.
Fig. 2. Surface morphologies of ZnO grown at oxygen flow rates of (a) 50, (b) 100, and (c) 200 sccm under an Ar flow rate of 100 sccm.
II. EXPERIMENTAL DETAILS Graphene was synthesized on a Ni-coated SiO2 /Si substrate by using thermal CVD. A thin Ni layer with a thickness of 300 nm was deposited onto a SiO2 /Si substrate using an electron beam evaporator. After the Nicoated substrate had been placed in the thermal CVD chamber, the temperature was increased from room temperature to 950 ◦ C. The C2 H2 /H2 gas mixing ratio was optimized (2/45 sccm) for the giver reactor, and the growth time of the graphene was 1 min. After the growth had been completed, the chamber was cooled down to room temperature. The as-grown graphene sheet was separated from the underlying Ni layer by using a Ni etchant. The separated floating graphene sheet was transferred to SiO2 /Si substrate by using the conventional wet transfer method. The FE-SEM images of the prepared graphene and its Raman spectrum are shown in Fig. 1. Figure 1(a) shows that the graphene had a uniform structure. The Raman spectrum a Fig. 1(b) reveals that monolayer graphene with ∼0.4 intensity ratio of the G-band (∼1587 cm−1 ) and G band (∼2680 cm−1 ) was obtained [21]. Next, the graphene on a SiO2 /Si substrate was used as a platform for the growth of ZnO sub-microstructures,
Fig. 4. (Color online) Raman spectra of the ZnO structures grown on graphene/SiO2 /Si substrate at oxygen flow rates of 50, 100, and 200 sccm.
which were synthesized by using seed-layer-free and catalyst-free CVD. The ZnO sub-microstructures were grown by using thermal CVD as follows: The graphene/SiO2 /Si substrates were cleaned with acetone in a sonication bath for 1 h. After the chemical cleaning process, equal amounts of ZnO powder (99.99%, Aldrich Chem. Co., Inc., USA) and graphite powder (99.99%, Aldrich) were ground together and transferred to an alumina boat. The graphene/SiO2 /Si substrates and the alumina boat were placed in a small quartz tube. The distance between the graphene/SiO2 /Si substrate and the alumina boat was approximately 9 cm in the upstream direction. Argon and oxygen were used simultaneously as the carrier gas, the flow of argon was fixed at 100 sccm, and the flow rate of oxygen was varied from 50 to 200 sccm. The furnace was heated to the desired growth temperature of 1000 ◦ C. The deposition time of ZnO was 60 min. Afterwards, the tube was cooled to 25
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Journal of the Korean Physical Society, Vol. 67, No. 9, November 2015
Fig. 5. (Color online) Confocal PL images of ZnO sub-microstructures grown at oxygen flow rates of (a) 50, (b) 100, and (c) 200 sccm. The local PL spectra of samples grown at oxygen flow rates of (d) 50, (e) 100, and (f) 200 sccm. ◦
C. The FE-SEM image of the sample after growth process [Fig. 1(c)] reveals that the ZnO structure grown on the graphene monolayer only, except for SiO2 /Si substrate. The morphology and the size distribution of the ZnO structures were characterized by using FE-SEM. The structural quality of the ZnO was investigated by using HR-XRD. The optical properties of the ZnO were investigated by using CSPM, in which light with a 355nm wavelength from a diode pumped solid state laser was used as an excitation source at room temperature. Raman spectra were obtained by using light with a wavelength of 633 nm excitation wavelength from a He-Ne laser for excitation at room temperature.
III. RESULTS AND DISCUSSION The carrier-gas flow rate is one of the most important parameters as it has a strong impact onto the surface morphologies of ZnO sub-microstructures. To investigate the effect of the carrier-gas flow rate on the growth of ZnO sub-microstructures, we used thermal CVD synthesis at different carrier-gas flow ratios. Figure 2 shows FE-SEM images for the ZnO structures formed at oxygen flow rates of 50, 100, and 200 sccm, respectively. The growth temperature was fixed at 1000 ◦ C, and the total deposition time was 60 min. At the oxygen gas
flow rate of 50 sccm, the ZnO sub-microstructures were rod-shaped, with diameters ranging from 0.5 to 1.0 μm. When the oxygen flow was increased to 100 sccm, the single rods merged with neighboring rods, resulting in ZnO rods with irregular diameters [Fig. 2(b)]. With a further increase in the oxygen flow rate, the formation of ZnO needles was observed, as shown in Fig. 2(c). The variation of the ZnO sub-microstructures can be explained by using the growth rate [22]. From this result, the density of the ZnO sub-microstructures grown on the graphene/SiO2 /Si substrate was assumed to depend strongly on the carrier-gas flow rate. The crystal structure and orientation of the ZnO submicrostructures were investigated by using XRD. Figure 3 shows the XRD spectra of ZnO sub-microstructures for different O2 /Ar gas flow ratios. From the XRD spectra of the ZnO sub-microstructures, peaks are observed at 31.84, 34.48, 36.32, 47.52, 56.68, 62.93, and 72.55◦ , corresponding to the (100), (002), (101), (102), (110), (103), and (004) directions, of the ZnO sub-microstructures. The ZnO (002) and (004) peaks imply that the ZnO sub-microstructures were preferentially grown along the direction. Particularly for the oxygen flow rates of 50 and 100 sccm, the intensity of the ZnO (002) peak increased owing to the relatively higher degree of c-axis orientation. Figure 4 shows the Raman spectra of the ZnO sub-microstructures for different gas flow ratios. The peaks at 437 and 580 cm−1 were assigned as E2 (high) and A1 (LO), respectively, and the second-order
Spatially resolved Optical Properties of ZnO · · · – Hye Min Oh et al.
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broadband visible emission. In order to further investigate the effect of different carrier gas ratios, we analyzed the relative PL intensity ratio (IN BE /IDef ect ) for various the flow rates of oxygen ranging from 50 to 200 sccm (Fig. 6). The 50-sccm sample shows a very high IN BE /IDef ect . From this result, the intensity of the emission is found to depend strongly on the growth ambient. That is, the increase in defect peaks for the ZnO structures can be explained by using the amount of excess oxygen and the number of oxygen interstitial or zinc interstitial defects. This research provides fundamental supporting data for finding the optimum growth conditions for growing high-quality ZnO sub-microstructures on graphene substrates.
IV. CONCLUSION Fig. 6. (Color online) The variation of N BE /IDef ect with O2 flow rate under a 100-sccm Ar flow rate.
peak at 332 cm−1 was assigned to the E2 (high)-E1 (low) phonon mode [23]. The peaks at 377 and 410 cm−1 were assigned as A1 (TO) and E1 (TO), respectively [23]. Among these peaks, the strong E2 (high) mode indicated a wurtzite lattice and good crystal quality [24]. From these results, among the various gas flow conditions, the ZnO grown at an oxygen flow rate of 50 sccm seemed to have the best quality. To investigate the spatially resolved photoluminescence (PL) properties of ZnO sub-microstructures formed at oxygen flow rates of 50, 100, and 200 sccm, were performed confocal PL measurements at room temperature. Figures 5(a), (b), and (c) show the PL images of ZnO sub-microstructures grown at oxygen flow rates of 50, 100, and 200 sccm, respectively. Figures 5(d), (e), and (f) show the local PL spectra obtained from points I, II, and III, marked in Figs. 4(a), (b), and (c), respectively. All local PL spectra showed UV emission at ∼383 nm, corresponding to the near-band-edge (NBE) emission of ZnO attributed to band-to-band transitions, excitonic emissions, and donor-acceptor pair transitions [25]. The visible emission at 420−640 nm originated from the deep gap state induced by various structural defects: ionized charge states of intrinsic defects, oxygen vacancies, Zn interstitials, zinc vacancies, oxygen antisites, etc. [26–30]. The detailed emission mechanism of the defect-related broad luminescence centered at ∼530 nm is reported to be the recombination of holes in the valence band with electrons trapped at oxygen vacancies or complex defects, including oxygen vacancies and zinc interstitials [26–30]. According to Fig. 5, the NBE emission intensity decreased, but the intensity of the defectrelated emission increases with increasing oxygen flow rate. This phenomenon seems to be related to the oxygen vacancy concentration, which is regarded as the key parameter for the recombination mechanism causing the
ZnO sub-microstructures were grown on graphene/ SiO2 /Si substrates without a catalyst or a seed layer by using CVD. We observed the morphology and the crystal quality of the ZnO sub-microstructures on graphene by using FE-SEM, XRD, and Raman spectroscopy. The spatially resolved optical properties of the ZnO submicrostructures were investigated by using CSPM. We found that the structure and the optical properties of the ZnO sub-microstructures grown on graphene were related to the oxygen flow rate. We believe that our results provide support for the synthesis of ZnO submicrostructures on graphene without a catalyst or seed layer.
ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A3A01015595).
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