Stimulated Optical Emission from ZnO Nanobelts Grown with a Simple

Jan 13, 2011 - Cooperative Research Center, University of Miyazaki, Miyazaki 889-2192, Japan. ABSTRACT: This work reports on the stimulated optical...
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Stimulated Optical Emission from ZnO Nanobelts Grown with a Simple Carbothermal Evaporation Method B. Q. Cao,†,‡,* K. Sakai,§ D. Nakamura,‡ I. A. Palani,‡ H. B. Gong,† H. Y. Xu,† M. Higashihata,‡ and T. Okada‡,* †

School of Materials Science and Engineering, University of Jinan, Jinan 250022 People's Republic of China, Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 819-0395, Japan, and § Cooperative Research Center, University of Miyazaki, Miyazaki 889-2192, Japan ‡

ABSTRACT: This work reports on the stimulated optical emission characteristics of ZnO nanobelts that were grown with a simple carbothermal evaporation method without using any catalyst or dopant. Such nanobelts grew along the a-axis enclosed by {0001} and {01-10} base planes through a vapor-solid mechanism. Room temperature and temperature-dependent photoluminescence results indicated that such nanobelts were of high optical qualityand were better than those nanobelts grown from dopantassisted thermal evaporation or oxidation methods. Amplified spontaneous emission was observed from ZnO nanobelts with low-growth density while random laser was observed for high-density samples with increasing optical pumping density.

1. INTRODUCTION Research on ZnO has been renewed and intensively pushed in recent decades due to its possible application in ultraviolet optoelectronic devices, especially for micrometer-sized lasers.1 The controlled growth of different ZnO nanostructures as nanodevice building blocks and their stimulated emissions (SE) or lasing properties are two important issues in this area.2-4 Due to the high ionicity of its polar surfaces, growth of ZnO nanowires is not difficult. A common growth strategy applies a catalyst-assisted vapor phase transport and deposition method to grow ZnO nanowires, where the metal catalysts act as nucleation sites.5 But, ordered ZnO nanowire arrays can also be grown on pure sapphire substrates without any catalyst.6 In comparison with nanowires, the growth of ZnO nanobelts is much more difficult. It usually needs higher reaction temperatures (14001500 °C) and long growth times (2-3 h).7,8 In order to reduce the energy consumption with lower growth temperatures, some groups used tin9 or indium10 as catalyst (or dopant) to synthesize ZnO nanobelts. Such a process inevitably introduces impurities and reduces the nanobelt crystallinity. There are also some reports on the growth of ZnO nanobelts using zinc chloride,11 zinc sulfide,12 or zinc-copper alloy13 as source materials. However, these nanobelts are usually of much lower yield than nanowires. Lu et al.14 and Yang et al.15 also reported hydrothermal and electrochemical growth methods for ZnO nanobelts r 2011 American Chemical Society

with very low temperature (∼150 °C). Usually, the products obtained by solution methods are mixtures of nanowires and nanobelts. Until now, three kinds of lasing emissions were observed for ZnO materials. The Fabry-Perot (F-P) cavity formed by two parallel crystal planes was widely proposed to simulate the laser modes observed from ZnO microcrystalline films16 and nanowires.17 Czekalla et al.18 and Gargas et al.19 demonstrated the whispering gallery mode (WGM) lasing from ZnO microwires and nanodisks, respectively, where the light traveled around in the hexagonal structures due to the total internal reflection at the resonators boundary. The third kind of laser observed especially from ZnO nanopowders is the random laser.20 For ZnO nanobelts, until now, only a few manuscripts have reported lasing emission characteristics based on the proposed F-P cavity.21-24 Overall, the growth and optical properties of ZnO nanobelts were relatively small in comparison with numerous reports on nanowires. Moreover, the applied experimental conditions were rather diverse among different groups.7-15 In this work, a relatively simple but effective carbothermal evaporation method was reported to grow ZnO Received: October 11, 2010 Revised: December 13, 2010 Published: January 13, 2011 1702

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nanobelts and their stimulated optical emission properties were also discussed.

2. EXPERIMENTAL SECTION Growth of ZnO nanobelts was accomplished with a carbothermal evaporation system, which comprises a standard quartz tube furnace with an inner tube diameter of 50 mm (Kyouei, Japan), a vacuum gauge (MSK, USA), two mass flow controllers (Ar and O2, KOFLOC, Japan), and a pump station together with a needle valve. The source material was a mixture of ZnO and C powder (1:1 wt %) placed in a ceramic boat and located in the middle of the quartz tube. Silicon(100) wafers of ∼1 cm2 were used as substrates after a cleaning process. Argon was used as carrier gas with flow of 100 SCCM and the pressure was maintained at 100 Torr. The growth temperature was optimized between 950 to 1000 °C. When the temperature was below 950 °C, no products were obtained, while temperatures higher than 1000 °C were not necessary. The growth time at the peak temperature was 25 min and then the furnace was cooled down naturally to room temperature, although the ZnO nanostructure growth was initiated at a temperature above 875 °C. The morphology and crystalline microstructure of the asgrown samples were characterized with a scanning electron microscope (SEM, VE-7800, Keyence) and a high-resolution transmission electron microscope (HR-TEM; 1300NEF, 0.12 nm, JEM). X-ray powder diffraction (XRD) patterns were recorded on a Rigaku MultiFlex diffractometer using CuKR radiation. The photoluminescence (PL) spectra were first measured at room temperature under excitation with the third harmonic of a Q-switched Nd:YAG laser (355 nm, 5 ns, 5 Hz, New Wave). The excitation area is about 0.3 cm2. The pumping power density was varied from 13 kWcm-2 up to 30 MWcm-2 by tuning the laser power. The PL spectra were recorded by a spectrometer with 1200 lines/mm grating (Lambda Vision, Japan). For low-temperature PL measurements, an He-Cd laser (325 nm, 30 mW, Kimmon) was used as the excitation light source. The laser beam was focused on the sample through a light chopper. The sample was placed in a cryostat for temperature control (10 K-300 K). The PL signal dispersed by a monochromator was detected by a photomultiplier and was amplified through a lock-in amplifier. 3. RESULTS AND DISCUSSIONS Figure 1(a) shows the typical survey SEM image of the asgrown samples. The nanobelt morphology is clearly shown in the high-resolution SEM image of Figure 1(b). Such randomly aligned nanobelts are morphologically uniform on the whole substrate of ∼1 cm2 and almost no nanowires are observed. The belts are found to be several tens of micrometers in length with a width of a few micrometers and a thickness of a few hundreds of nanometers. The nanobelt is so thin that the nanobelts behind the top one can also be observed with SEM (not shown here). With the exception of an additional peak at 33°, all of the XRD peaks shown in Figure 1(c) can be attributed to the hexagonal wurtzite ZnO, which confirms the pure phase of the ZnO nanobelts. This additional peak is from the substrate due to the multiple X-ray scattering effect of silicon and can be identified as Si(200).25 Moreover, the ZnO(100) diffraction intensity of nanobelts is rather stronger than the standard powder XRD spectrum (JSPDS 36-1451). It means that the top and bottom

Figure 1. (a) Low-magnification SEM image of the ZnO nanobelts grown on silicon substrates; (b) high-magnification SEM image of a single nanobelt; and (c) XRD spectrum of ZnO nanobelts on a silicon wafer with a Si(200) peak from the substrate.

surfaces of the nanobelts are {01-10} planes, which is consistent with the following transmission electron microscopy studies. A further structural study of the as-prepared ZnO nanobelts was performed with an ultrahigh voltage omega-filter TEM (JEM1300NEF). The general low-magnification TEM image and selected-area electron diffraction (SAED) shown in Figure 2(a) confirms that the sample has belt morphology and single-crystal structure. However, the single-crystal nanobelt has a growth direction along the a-axis enclosed by {0001} and {01-10} base planes, as indicated by SAED and HRTEM shown in Figure 2(b), which is different from the typical c-axis growth direction of ZnO nanowires. Pan et al.7 found that ZnO nanobelts have two growth directions. One is growing along the c-axis without defects and another is along the a-axis with a single stacking fault parallel to the nanobelt. But our careful HRTEM observations did not find any stacking fault in our samples and the nanobelts were of high single crystal. The surface of 1703

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Figure 2. (a) TEM image of a ZnO nanobelt displaying its shape characteristics and inset is the corresponding SAED pattern confirming its single-crystal structure; (b) High-resolution TEM image showing clear lattice fringe. Using the Digital Microscopy software, the lattice fringe distances were measured and the crystal directions were identified.

the nanobelt is also atomically sharp and without any sheathed amorphous phase. The formerly reported ZnO nanobelts were usually grown with a catalyst/dopant-assisted thermal evaporation or oxidation method.7,8,26-28 In these cases, stacking faults,7,28 planar-defects,8,27 and twinning parts26 were always observed in the ZnO nanobelts. In comparison with these reports, the ZnO nanobelts here grown by a direct and simple carbothermal method without using any dopant or catalyst are of better crystal quality. Depending on the metal catalyst used or not in the synthesis process, Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms were proposed for the growth of ZnO nanobelts. For the VLS mechanism, it can be realized at a synthesized temperature of 950 °C21 or 1400 °C29 with gold catalyst. However, for the VS mechanism, it usually needs a higher growth temperature of about 1400 °C7 or intentionally doping the source materials with tin8 or antimony compounds.27,30 In our experiment, no extrinsic catalyst or dopant was used. Therefore, the nanobelt growth should be attributed to the VS mechanism, although it is still not fully understood.31 But, the growth temperature was already reduced from 1400 °C to 950-1000 °C. The optical properties of the ZnO nanobelts were investigated in-detail by different PL spectroscopies. Figure 3(a) shows the room temperature (RT) PL of ZnO nanobelts. The near band gap emission at 3.2 eV due to exciton recombination is much stronger than the visible emission at 2.7 eV, which is generally ascribed to deep-level defects.32 The strong excitonic emission and weak deep-level emission indicate that such nanobelts are of better optical quality than ZnO nanobelts growth by indium,10 tin,8 or antimony27,30 doping or oxidation of sulphide.12,26,33

Figure 3. (a) Room-temperature PL of ZnO nanobelts; (b) Temperaturedependent PL spectra from 10 to 150 K PL spectra of ZnO nanobelts. The PL spectra were vertically shifted for clear comparison. The energy spacing between two adjacent vertical lines of FXA and FXA-nLO is 72 meV, the energy of longitudinal optical phonon of ZnO.

Figure 3(b) shows the temperature-dependent PL spectra measured from 10 to 150 K. The PL spectrum at T = 10 K is dominated by an emission peak at 3.359 eV, which is typical for donor-bound excitons (D0X, I6).34 On the high energy side of this peak, three small shoulders are observed which can be assigned to the free-exciton recombinations as indexed with FXA,B (n = 1, 2). Two groups of phonon replicas assigned as D0X-nLO and FX-nLO (n = 1, 2, 3) are observed with an energy separation of 72 meV (LO-phonon energy of ZnO) below the D0X and FX peaks, respectively. The peak at 3.33 eV is most probably due to the two-electron satellite (TES) transition.35 It is found that the intensities of the two series of peaks associated with FX and D0X exhibit an opposite dependence on temperature. The intensities of FX and its related phonon replicas increase with increasing temperature while the intensities of D0X and its replicas decrease and are not detectable at temperatures over 70 K. The fast intensity reduction of D0X is a result of the rapid thermal ionization of bound excitons with increasing temperature. Therefore, more free excitons occupy the ground states. In conclusion, the above PL characteristics demonstrate that the ZnO nanobelts have good optical quality,36 which is comparable with high-quality bulk ZnO single crystals.37 1704

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Figure 4. (a) General SEM view of the measured ZnO nanobelt samples. (b) Room-temperature PL spectra of ZnO nanobelts on a silicon substrate under different excitation densities varied between 0.013 and 3.11MW/cm2 with 355 nm (5 ns, 5 Hz) Nd:YAG laser; (c) Dependence of the peak intensity and fwhm on the excitation density. Lines are only guides for the eye.

Figure 4(b) shows the RT PL spectra of ZnO nanobelts under different optical excitation intensity (IEx). Figure 4(a) is the general view of the measured ZnO nanobelt sample. The excitation intensities were increased from 13 kW/cm2 to 3.96 MW/cm2. For IEx < 500 kW/cm2, the PL peaks centered at 390 nm are rather broad and featureless with an fwhm (full width at half-maximum) of about 120 meV, which is the typical spontaneous emission of ZnO. When the IEx is further increased, the peak intensity increases superlinearly and the fwhm reduces

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Figure 5. (a) General SEM view of the measured ZnO nanobelt samples. (b) Room temperature PL spectra evolution of ZnO nanobelts under different excitation intensities with 355 nm (5 ns, 5 Hz) Nd:YAG laser. (c) Dependence of peak intensity and fwhm on the excitation density. Lines are only guides for the eye.

drastically to 30 meV, as both depicted in Figure 4(c). The characteristics of Figure 4, parts (b) and (c), imply the appearance of stimulated emission with a threshold over 500 kW/cm2. The SE process can be interpreted in terms of inelastic exciton-excition scattering and the energy of the emitting photon 2 energy Pn is theoretically given by Pn = Eex - Eex b (1-1/(n )) 38 (3kBT)/2. The biggest energy difference according to this equation is 97 meV. But, the SE peak centered at 3.159 eV lies 158 meV below the free exciton peak (3.317). This difference between the theoretical predictions and experimental results can be attributed to the sample temperature increase caused by the high-power laser excitation/heating.39,40 For this sample, even 1705

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The Journal of Physical Chemistry C the excitation density is further increased, no clear lasing actions characterized with sharp peaks were observed. To further investigate the optical properties of ZnO nanobelts, another typical sample was measured, as shown in Figure 5. By comparing the SEM images of Figure 4(a) and 5(a), we can find the main difference is the nanobelts growth density on substrates. For the ZnO nanobelts of Figure 5(a), at a low excitation intensity smaller than 3 MW/cm2, the PL spectra consist of a single broad spontaneous emission peak. The emission peaks became narrower when the IEX was further increased, as shown in Figure 5(b). Very narrow peaks appear in the PL spectra when the excitation intensity exceeds a threshold of about 10 MW/ cm2, as indicated in Figure 5(c). The FWHMs of these sharp peaks are about 1 nm, which is 20 times smaller than the fwhm of the emission peak below the threshold. What is the possible lasing mechanism of the ZnO nanobelts, FP cavity, WGM, or random laser? Considering the nanobelt geometry, the WGM mechanism can first be excluded as WGM laser can only form through total reflection within hexagonal ZnO wires18 or nanodisks19 with well-defined hexagonal crystal facets. Then, for an F-P cavity laser, two kinds of possible cavities can be expected in nanobelts. One is formed between two crystalline planes along the nanobelt growth or width direction. In this case, the nanobelts should have (1) a regular shape with two rectangular ends or side planes21,22 and (2) a well-defined cavity length (L). Since the shapes of the nanobelts in Figures 4 and 5 are rather irregular and there is no well-defined cavity length, it is not possible to calculate the laser mode spacing.40,41 Due to the uniform thickness of nanobelt, another possible F-P cavity is expected to be formed between the two basal enclosed planes. Accordingly, the lasing mode spacing can be calculated with Δλ = λ2/2nλL,21 where n = 2.4 for ZnO is the group refractive index and L is the effective cavity length. For the proposed F-P cavity in this case, L is a nanobelt thickness of about 200 nm, estimated from the SEM image (Figure 1b). So, the calculated laser mode spacing is ∼150 nm, which is obviously different from the PL of Figure 5(b). Therefore, the observed laser mechanism cannot be molded with an F-P cavity. On the basis of the above analysis, we suggest that the most possible lasing mechanism observed in Figure 5b should be random laser. Such a random laser is due to multiple scattering by the high-density ZnO nanobelts, where the randomly distributed nanobelts act as scatters and can provide a strong optical recurrent scattering that forms random laser cavities. The absent of random laser for nanobelts shown in Figure 4 is due to the low growth density, where the ZnO nanobelts are too sparse to form closed-loop paths of light to provide coherent feedback for random laser. But, SE can still be observed under high optical excitation. Such transition from amplified spontaneous emission to random lasing is similar to that reported by Cao et al.42 through continuously varying the concentration of ZnO nanoparticles dispersed in rhodamine solutions.

4. CONCLUSIONS In summary, we reported a simple and effective carbothermal evaporation method to grow ZnO nanobelts without using any catalyst or dopant. The growth temperature was decreased from 1400 °C to 950-1000 °C. Such ZnO nanobelts grow along the a-axis enclosed by {0001} and {01-10} base planes via the VS mechanism. The nanobelts are of high crystal quality, as

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demonstrated by TEM studies. By varying the optical pumping density, both amplified spontaneous emission and random lasers are observed from ZnO nanobelts, which are related to the nanobelt growth density. A relatively higher nanobelt growth density is necessary for a random laser to form closed-loop cavities. This demonstration of the simple growth method and optical properties of ZnO nanobelts provide possibilities for their application in optoelectronic devices.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (B.Q.C.); [email protected] (T.O.).

’ ACKNOWLEDGMENT The authors thank the Research Laboratory of High-Voltage Electron Microscopy at Kyushu University for its cooperation. B.C. is an Overseas Taishan Scholar and JSPS Fellow and is thankful for the support from the NSFC (51002065), Shandong Distinguished Middle-Aged and Young Scientist Encouragement and Reward Foundation (BS2010CL003), and UJN for a new faculty position. The work performed at Kyushu University was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS, No. 20360142) and Special Coordination Funds for Promoting Science and Technology from the Japan Science and Technology Agency are also acknowledged. ’ REFERENCES (1) Klingshirn, C. Phys. Status Solidi B 2007, 244, 3027–3073. (2) Wang, Z. L. Mater. Sci. Eng. R 2009, 64, 33–71. (3) Ursaki, V. V.; Zalamai, V. V.; Burlacu, A.; Fallert, J.; Klingshirn, C.; Kalt, H.; Emelchenko, G. A.; Redkin, A. N.; Gruzintsev, A. N.; Rusu, E. V. J. Phys. D 2009, 42, 095106. (4) Zimmler, M. A.; Capasso, F.; M€uller, S.; Ronning, C. Semicond. Sci. Technol. 2010, 25, 024001. (5) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700–717. (6) Cao, B. Q.; Zu niga-Perez, J.; Czekalla, C.; Hilmer, H.; Lenzner, J.; Boukos, N.; Travlos, A.; Lorenz, M.; Grundmann, M. J. Mater. Chem. 2010, 20, 3848–2854. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947–1949. (8) Deng, R.; Zhang, X. T.; Zhang, E.; Liang, Y.; Liu, Z.; Xu, H. Y.; Hark, S. K. J. Phys. Chem. C 2007, 111, 13013–13015. (9) Wang, X. D.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B. 2004, 108, 8773–8777. (10) Fan, H. J.; Fuhrmann, B.; Scholz, R.; Himcinschi, C.; Berger, A.; Leipner, H.; Dadgar, A.; Krost, A.; Christiansen, S.; G€osele, U.; Zacharias, M. Nanotechnology 2006, 17, S231–S239. (11) Zhang, J.; Yu, W. Y.; Zhang, L. D. Phys. Lett. A 2002, 279, 276–281. (12) Chen, Z. G.; Li, F.; Liu, G.; Tang, Y. B.; Cong, H. T.; Lu, G. Q.; Cheng, H. M. J. Nanosci. Nanotech. 2006, 6, 704–707. (13) Li, Y. B.; Bando, Y.; Sato, T.; Kurashima, K. Appl. Phys. Lett. 2002, 81, 144–146. (14) Lu, C. H.; Qi, L. M.; Yang, J. H.; Tang, L.; Zhang, D. Y.; Ma, J. M. Chem. Commun. 2006, 3551–3553. (15) Yang, J. H.; Liu, G. M.; Lu, J.; Qiu, Y. F.; Yang, S. H. Appl. Phys. Lett. 2007, 90, 103109. (16) Tang, Z. K.; Wong, G. K. L.; Yu, P.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Appl. Phys. Lett. 1998, 72, 3270–3272. (17) Zimmler, M. A.; Bao, J. M.; Capasso, F.; M€uller, S.; Ronning, C. Appl. Phys. Lett. 2008, 93, 051101. (18) Czekalla, C.; Sturm, C.; Schmidt-Grund, R.; Cao, B. Q.; Lorenz, M.; Grundmann, M. Appl. Phys. Lett. 2008, 92, 241102. 1706

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