Zno Nanorods

Applied Physics A

DOI: 10.1007/s00339-003-2286-2

Materials Science & Processing

a.b. hartanto1 x. ning2 y. nakata1 t. okada1,u

Growth mechanism of ZnO nanorods from nanoparticles formed in a laser ablation plume 1 Graduate

School of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan 2 Fudan University, Shanghai, P.R. China

Received: 11 June 2003/Accepted: 1 July 2003 Published online: 23 September 2003 • © Springer-Verlag 2003

We have succeeded in synthesizing ZnO nanorods by pulsed-laser ablation at comparatively high gas pressures without using a catalyst. The nanorods had an average size of 300 nm and a length of about 6 µm. Stimulated emission was observed from the nanorods at 388 nm by optical pumping. As a catalyst was not used in our method, nanorod growth was not controlled by the vapor-liquid-solid (VLS) mechanism. We found that nanoparticles formed by condensation of ablated particles in the laser ablation plume play an important role in nanorod growth. ABSTRACT

PACS 61.46.+w;

1

81.07.Bc; 78.66.Hf; 78.67.Bf; 81.16.Mk

Introduction

In the past few years, nanosized wide-gap semiconductor materials, such as nanorods, nanowires, and nanobelts, have been of growing interest due to their importance in both scientific research and potential technological applications, including nano-optical devices. Zinc oxide (ZnO), a wide-gap compound II–VI semiconductor that has a direct band gap of about 3.37 eV at room temperature, is a well-known material suitable for generating ultraviolet (UV) light. Furthermore, a large exciton binding energy of about 60 meV in ZnO, which is significantly larger than the thermal energy at room temperature (26 meV), ensures efficient exciton emission at room temperature under low excitation energy [1]. Thus, considerable effort has been devoted to the synthesis and study of nanoscale ZnO materials. For example, ZnO nano-materials have been synthesized by various approaches, such as chemical vapor deposition [2], physical vapor deposition [3], molecular beam epi-

taxy [4], and a simple method that just involves heating a Zn powder containing catalyst nanoparticles [5]. All these approaches apply the vapor–liquid–solid (VLS) mechanism for nanowire growth, in which a metal or oxide catalyst is necessary to dissolve feeding source atoms in the molten state to initiate the growth of nano-materials. UV stimulated emission at room temperature from optically pumped nanowires has also been reported [6]. ZnO nanobelts have been successfully synthesized by simply evaporating ZnO powders without the presence of catalyst [7]. But a very high temperature, as high as the melting point of bulk ZnO, is needed in such method. Pulsed-laser ablation is also a very simple method and is widely used for the synthesis of various thin films. But this method has only been employed in a few instances to synthesize nano-scale ZnO materials such as nanowires or nanorods. In this letter, we report the growth of large quantities of ZnO nanorods by pulsed-laser ablation without using a catalyst. We

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found that nanoparticles formed by the condensation of ablated species in the high-pressure gas phase play an important role in the growth of the ZnO nanorods. 2

Experimental

In our experiment, sintered ZnO of 99.99% purity was used as the source material for the synthesis of the nanorods. This material was ablated with a KrF excimer laser, which operated at a wavelength of 248 nm, repetition rate of 20 Hz, and fluence of about 3 J/cm2 , in a chamber filled with oxygen background gas. Ablated species were then deposited on a sapphire substrate that was mounted on a heater. The target–substrate distance was fixed to be 20 mm and the chamber pressure was set to be relatively high, at more than 1 Torr. The morphology and crystallinity of the as-deposited products were observed by scanning electron microcopy (SEM) and X-ray diffractometry (XRD), respectively. 3

Results and discussion

After 30 min of deposition, a white-colored product was found to cover the substrate surface. SEM analysis of the as-deposited products showed that ZnO nanorods grew at substrate temperatures of more than 600 ◦ C. Figure 1a shows an SEM image of the top of a ZnO nanorod array grown on the sapphire substrate at 700 ◦ C and a background gas pressure of 5 Torr. Figure 1b shows an SEM image of a single ZnO nanorod taken from the ZnO rod array. Nanorods with an average size of about 300 nm and a uniform length of about

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Applied Physics A – Materials Science & Processing

6 µm were grown perpendicular to the substrate surface. The nanorods had hexagon edges and were isolated from each other. We can also see from Fig. 1b that there was a layer with a thickness of several tens of nanometers between the substrate and the nanorods. Since a catalyst was not used, it is suggested that this layer plays an important role in nanorod growth. A typical XRD pattern of the ZnO nanorod array is shown in Fig. 1c. Only diffraction lines from the (002) and (004) planes can be observed, indicating that the nanorod arrays were almost entirely caxis oriented. In order to explore possible stimulated emission from the nanorods, photoluminescence (PL) from isolated nanorods was observed by optical pumping using the third harmonic (THG) of a Nd:YAG laser operating at 355 nm. Figure 2 shows the evolution of the emission spectra as the pump energy was increased. Stimulated emission was observed in the UV region around 388 nm, with a full width at half maximum (FWHM) of approximately 7 nm, when the pump energy exceeded about 50 mJ/cm2 . In order to understand the growth mechanism of the nanorods synthesized by our method, it is important to know the initial stage of nanorod growth. Figure 3a–d shows SEM images of the film surface after 10, 60, 120, and 300 s of deposition, respectively. After 10 s, ZnO thin-film islands with thicknesses of several tens of nanometers, which almost cover the substrate surface, can be observed in Fig. 3a. XRD analysis revealed that these thin film islands were c-axis oriented. These results, which are consistent with Fig. 1b, indicate that these films correspond to the thin layer between the nanorods and the substrate. The surface morphology of the film after 60 s of deposition (Fig. 3b) shows a lot of very small ball-like crystals stacked on the film islands. Further deposition made these ball-like crystals grow horizontally, forming the base of the nanorods. At a time of 120 s after the start of deposition, the base of the nanorods was completely formed and the horizontal growth had stopped (Fig. 3c). From this stage, the crystal growth was then dominated by vertical growth and the nanorods grew longer and longer. We can see from Fig. 3d

FIGURE 1 SEM images and XRD spectrum of ZnO nanorods. a SEM image of ZnO nanorod arrays deposited at 700 ◦ C. b SEM image of a single ZnO nanorod. c XRD spectra from ZnO nanorod arrays

FIGURE 2 Photoluminescence spectra from ZnO nanorods

FIGURE 3

SEM images of films for different deposition times: a 10, b 60, c 120, and d 300 s

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Growth mechanism of ZnO nanorods from nanoparticles formed in a laser ablation plume

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FIGURE 4 SEM images of ZnO rods. a Top view of the film deposited at 600 ◦ C for 10 min (Step 1). b Cross-sectional view of the film deposited at 300 ◦ C for 1 min after Step 1 (Step 2). c Cross-sectional view of the film annealed at 700 ◦ C for 20 min after Step 2

that with only 5 min of deposition, ZnO nanorods could be produced by our methods. In a previous report we demonstrated the visualizations of particles behavior in the gas phase by the UV-Rayleigh scattering (UV-RS) method [8]. It revealed that in a high oxygen gas pressure of more than 1 Torr, nanoparticles were formed in the gas phase by the condensation of ablated species, and then reached the substrate surface. We also confirmed that nanorods could grow at such high gas pressures and that no nanorods but only thin films were formed when deposition was conducted at low gas pressure. As a consequence, we suggested that nanoparticles formed in the gas phase play a very important role in nanorod growth. An additional set of experiments was conducted to support this suggestion. Firstly, in Step 1, we synthesized ZnO nanorods for 10 min of deposition time at a substrate temperature of 600 ◦ C. The top view of the film after Step 1 can be seen in Fig. 4a, which shows that

ZnO nanorod arrays were formed. In Step 2, the substrate temperature was set to 300 ◦ C and 1 min of deposition was performed after Step 1. Figure 4b shows the cross-sectional SEM image of the film after Step 2. Nanoparticles stacked on top of the nanorods can be observed in Fig. 4b. Finally, in Step 3 we annealed this film at 700 ◦ C for 20 min. The crosssectional SEM image of the film after Step 3 can be seen in Fig. 4c, which shows that there were no particles on the top of nanorods after this step. The nanoparticles had melted at 700 ◦ C, supporting nanorod growth. These results give direct evidence that nanoparticles formed in the gas phase which reach the substrate surface play an important role in nanorod growth. In conclusion, we have demonstrated a new growth method for ZnO nanorods by pulsed-laser ablation. We have proved that nanoparticles formed in the laser ablation plume are transported onto the substrate, where they fuse with the nanorods. Stimulated emission from the ZnO nanorods was observed in the

UV region around 388 nm by optical pumping. ACKNOWLEDGEMENTS The authors gratefully acknowledge Mr. M. Kawakami for his contribution to this research. The authors would also like to thank Prof. Y.F. Lu at the University of Nebraska, Lincoln, for his valuable comments. Part of the experimental work was carried out at the Center of Advanced Instrumental Analysis, Kyushu University.

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