CRYSTAL GROWTH & DESIGN
ZnO Nanostructured Microspheres and Elongated Structures Grown by Thermal Treatment of ZnS Powder L.
Khomenkova,†
P. Ferna´ndez,* and J. Piqueras
Departamento de Fı´sica de Materiales, Facultad de Ciencias Fı´sicas, UniVersidad Complutense de Madrid, 28040 Madrid, Spain
2007 VOL. 7, NO. 4 836-839
ReceiVed NoVember 7, 2006; ReVised Manuscript ReceiVed December 8, 2006
ABSTRACT: ZnO microspheres, with surfaces covered with nanowalls, as well as nano- and microrods, have been grown by thermal treatment of ZnS powder. The structures have been grown at temperatures in the range of 750-900 °C in a catalyst-free process. Previous mechanical treatment of the ZnS powder by ball milling has been found to influence the morphology and size of the obtained structures. Cathodoluminescence in the scanning electron microscope has shown that spheres and elongated structures have an inhomogeneous luminescence emission. Introduction ZnO is a material of interest for ultraviolet optoelectronic applications, due to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV), enabling optical lasing at room temperature.1-3 Fabrication of low dimensional elongated ZnO structures such as nanowires, nanotubes, and nanorods is of interest for optoelectronics in future light-emitting nanodevices. However, there are many other applications, such as sensor devices or drug delivery systems, in which morphologies different from the elongated structures could be more suited. It is also important to note that not only size but also morphology may affect the properties of the material.4-6 Thermal evaporation of the semiconductor powder and deposition on a substrate under a gas flow is an often reported method for growing low dimensional structures. In some cases, the substrate is covered with a film, usually of gold, which acts as a catalyst. Nanowires, rods, or tubes of ZnO have been grown by deposition on different substrates, for example, without catalyst7-12 or with catalyst13-15 as well as by oxidation of Zn under oxygen flow.16-18 Sintering of compacted powder under argon flow has recently been reported to lead to the growth of elongated nanostructures of semiconductors19-24 directly on the sample surface, which acts as a source as well as a substrate. In the case of ZnO,21 the method requires high temperatures and long annealing times. Recently, an alternative route to grow ZnO nanostructures, using ZnS as a precursor, has been reported,25,26 which requires lower temperatures. In this work, ZnS compacted powder has been used as a precursor to grow ZnO nano- and microstructures by the above-mentioned vaporsolid method under argon flow without a catalyst or foreign substrate. The starting ZnS powder has been ball milled for different times to study the effect of the initial state of the precursor, particle size, and strain degree on the morphology and size of the obtained ZnO structures. The morphology and the luminescence of the ZnO nano- and microstructures have been investigated by scanning electron microscopy (SEM) and cathodoluminescence (CL) in the SEM, respectively. Experimental Section The starting material was commercially available ZnS powder of nominal purity (99.999%). The powder was compacted under a * To whom correspondence should be addressed. E-mail: arana@ fis.ucm.es. † Permanent address: V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences, 45 Prospekt Nauky, 03028 Kyiv, Ukraine.
compressive load to form disc-shaped samples of about 7 mm in diameter and 2 mm thickness. The samples were then placed on an alumina boat inside a tubular furnace and annealed under argon flow at temperatures between 700 and 900 °C for 5 or 10 h. A continuous Ar flux of 0.4 or 1 L/m was maintained during the thermal treatment. To study the influence of the initial state of the precursor, samples were also prepared from milled powder. The milling was performed in a centrifugal ball mill (Retsch S100) operating at 180 rpm with 20 mm agatha balls. The milling time was either 5 or 20 h. The morphology and size of the structures obtained after the thermal treatment were investigated by SEM in a Leica 440. X-ray diffraction (XRD) was used to control the crystal structure of the samples. Luminescence properties were investigated by means of CL in the SEM. The CL measurements were carried out at liquid nitrogen temperature on a Leica 440 or a Hitachi S-2500 SEM at an accelerating voltage of 20 kV. CL images were recorded by using a Hamamatsu R928 photomultiplier, while CL spectra were obtained with a Hamamatsu PMA-11 CCD camera and corrected for the response of the collection system.
Results and Discussion XRD spectra from the starting ZnS powder show that crystal structure is mainly cubic, although some diffraction peaks corresponding to the hexagonal phase are also observed. XRD of the thermal treated samples shows that they are composed of ZnO. Thermal treatments below 750 °C do no not yield elongated structures, which are first observed, with rod shape and lengths of a few micrometers, after treatments at this temperature for 5 h. Increasing the temperature to 900 °C produces, after 5 h of annealing, longer structures, as those shown in Figure 1a, which corresponds to the sample grown under an argon flow of 1 L/m. A decrease of the gas flow to 0.4 L/m leads to structures with larger cross-sectional dimensions and crystal habits closer to those expected for bulk material (Figure 1b). In most of the samples, the ZnO exciton peak at about 3.4 eV is observed; however, the relative intensity of this peak as compared to the deep level band varies from sample to sample. Figure 2 shows the low-energy part of the CL spectra of the samples treated at 750 and 900 °C. This low-energy emission corresponds to the green band, including several components usually ascribed to deep levels related to different point defects, among which oxygen vacancies and related centers27-30 as well as Zn interstitials31 play a major role. In the present study, this luminescent behavior is consistent with the fact that oxidation occurs in an atmosphere with a low oxygen partial pressure; hence, stoichiometry deviations toward the Zn-rich conditions should be expected.
10.1021/cg060789a CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007
ZnO Nanostructured Microspheres and Structures
Figure 1. Structures obtained from unmilled ZnS. Annealing treatments were performed for 5 h at a temperature of 900 °C under Ar flow: (a) 1 and (b) 0.4 L/min.
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Figure 3. ZnO structures obtained from ZnS powder milled for 20 h. Thermal treatments were performed at 900 °C for 5 h. Part b of this figure shows a detail of the nanowalls.
Figure 4. Nanowires grown at the junctions of the nanowalls.
Figure 2. CL spectra obtained from samples prepared from unmilled powder. Only the deep level band is shown.
XRD analysis of the milled ZnS powders reveals an average particle size below 20 nm. After milling for 5 or 20 h, no hexagonal phase is further observed. A similar structural phase transition upon milling has already been observed in CdSe.32 In samples prepared from milled powder as the starting material, higher temperatures are needed for the formation of the ZnO nano- and microstructures than in the case of nonmilled powder, and different morphologies are often obtained. For all milling conditions, treatments at 900 °C are required. The size and shape of the structures obtained from the milled powder depend on the experimental conditions and have, in general, wire or rod shapes. However, in samples prepared from powder milled for 20 h and treated at 900 °C, microspheres covered with nanowalls of about 100 nm thickness are observed (Figure 3). The growth of ZnO nanowalls has previously been reported.4,33,34 In refs 4 and 33, the nanowalls were grown by thermal evaporation of ZnO on sapphire covered with a thin layer of gold. The growth of walls took place by a vapor-liquid-solid mechanism, and no growth of walls was found in a catalyst-free surface. At the junctions of the nanowalls, nanowires were reported to grow
vertically, at a later stage, giving rise to an aligned nanowire array. In ref 34, the ZnO nanowalls were grown by molecular beam epitaxy. In this work, the use of ZnS as a starting material gives rise, under specific experimental conditions, to the spherical arrangement of nanowalls. The spheres would be of interest in applications related to high surface to volume ratios. Also, the spherical shape is considered of interest in drug delivery applications. As in the case of the nanowalls of refs 4 and 32, nanowires grow in the nanowalls, preferentially at the junctions, which generates in our samples a spherical nanowire distribution (Figure 4). After further growth of the wires, the structure has the appearance shown in Figure 5, where the nanowalls are no longer observed. The CL emission from the inner surface of the walls is in general more intense than that from the upper edges (Figure 6). This effect is similar to that observed in tubes of ZnO21 and SnO220 and reveals an inhomogeneous distribution of defects in the structures. According to the spectra shown in part b of this figure, the emission corresponds mainly to the green band, so it appears that the internal walls contain a higher defect density, probably related to oxygen vacancies. Milled semiconductor powders have been previously found to show a recovery effect during aging. This effect has been described in ref 30 for the case of ZnO. To check the influence of the strain relaxation, samples prepared from milled powder, which was stored for 1 month, were also studied. The lowest temperature required now to obtain elongated structures, nanow-
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Khomenkova et al.
Figure 5. Final stage of the nanowire growth with no visible nanowalls.
Figure 7. CL distribution in ZnO rods. (a) Emissive mode image, (b) CL image from the region shown in panel a, and (c and d) CL images of elongated structures with different morphologies.
Figure 6. (a) CL image of microspherical structures as those shown in Figure 3. (b) CL spectra recorded from structures grown on the surface (dashed line) and from the surface background (solid line).
ires, and rods, is reduced, approaching the value of 750 °C, corresponding to the unmilled powder. If the treatments are performed at 900 °C, the amount of structures obtained is much higher than without the aging process. CL spectra obtained from
these samples reveal the existence of recovery. A marked decrease of the relative intensity of the deep level band is now observed, which is indicative of strain recovery and of defect annealing. The spatial distribution of the CL emission of the elongated structures is not homogeneous. As observed in Figure 7, corresponding to a sample prepared from powder milled for 20 h and aged, the CL is strongly localized in some regions, as the bright rings perpendicular to the axes of the rods. A similar distribution of CL intensity has been previously reported for CdSe needles.29 We suggest that the ring contrast is due to the inhomogeneous distribution of recombination centers, related to point defects, and possibly stacking faults, during the growth. In addition, the rods terrminated in a basal plane show a radial fluctuation of the CL intensity as shown in Figure 7c. Rods terminated in a pointed tip show an intense CL emission in the upper region (Figure 7d) associated to the accumulation of radiative centers in nonbasal planes. The influence of the crystal face on the CL emission has been previously reported for the case of ZnO bulk single crystals.35 Conclusions ZnO nano- and microstructures with shapes of wires, rods, and spheres have been obtained by thermal treatment of
ZnO Nanostructured Microspheres and Structures
compacted ZnS, in a vapor-solid process. The particle size and strain of the starting powder, modified by ball-milling treatments, influence the morphology and size of the final structures. Microspheres with a rough surface formed by a dense distribution of nanowalls of about 100 nm thickness, as well as spherical arrangements of nanowires, are some of the structures obtained when ball-milled powder is used as the precursor. CL microscopy of the structures reveals an inhomogeneous emission, which is higher in the cavities of the nanostructured surface of the spheres and in crystallography-related specific regions of the rods. Acknowledgment. This work has been supported by MEC (Project MAT 2003-00455). L.K. is thankful for a research grant from UCM and Grupo Santander. References (1) Reynolds, D. C.; Look, D. C.; Jogai, B. Solid State Commun. 1996, 99, 73. (2) Reynolds, D. C.; Look, D. C.; Jogai, B.; Morkoc¸ , H. Solid State Commun. 1997, 102, 643. (3) Zu, P.; Tang, Z. K.; Wong, G. K. L.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Solid State Commun. 1997, 103, 459. (4) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F.; Steeves, D.; Kimball, B.; Porter, W. Appl. Phys. A 2004, 78, 539. (5) Wei, Q.; Meng, G.; An, X.; Hao, Y.; Zhang, L. Nanotechnology 2005, 16, 2561. (6) Ding, S.; Guo, J.; Yan, X.; Lin, T.; Xuan, K. J. Cryst. Growth 2005, 284, 142. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Hu, J. Q.; Bando, Y. Appl. Phys. Lett. 2003, 82, 1401. (9) Xing, Y. J.; Xi, Z. H.; Xue, Z. Q.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 1689. (10) Lee, J.-S.; Park, K.; Kang, M.-I.; Park, I.-W.; Kim, S.-W.; Cho, W. K.; Han, H. S.; Kim, S. J. Cryst. Growth 2003, 254, 423. (11) Liu, F.; Gao, P. J.; Zhang, H. R.; Li, J. Q.; Gao, H. J. Nanotechnology 2004, 15, 949. (12) Zhang, Y.; Jia, H.; Yu, D. J. Phys. D: Appl. Phys. 2004, 37, 413. (13) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897.
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