- Email: [email protected]
Nucleation and growth of ZnO micro- and nanobelts during thermal evaporation
ARTICLE IN PRESS
Journal of Crystal Growth 277 (2005) 321–329 www.elsevier.com/locate/jcrysgro
Nucleation and growth of ZnO micro- and nanobelts during thermal evaporation Z.Q. Zhanga, C.B. Jianga,, S.X. Lia, S.X. Maob a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 7 June 2004; accepted 5 January 2005 Available online 11 February 2005 Communicated by J.M. Redwing
Abstract Nucleation and growth of semiconducting ZnO microbelts and nanobelts during thermal evaporation have been investigated. It is found that the ZnO micro- and nanobelts synthesized via thermal evaporation method are mostly nucleated from the attached nanorods and formed by the combination of growth on f2¯ 1 1 2g planes and f0 1 1¯ 0g planes. The combination of the growth on f2¯ 1 1 2g planes is enough for the formation of [0 0 0 1] and ½2 1¯ 1¯ 0 oriented nanobelts; but the combination of the growth on f2¯ 1 1 2g and f0 1 1¯ 0g planes is necessary for the formation of ½0 1 1¯ 0 oriented nanobelts. In all cases, the growth of ZnO 1D nanostructural materials during thermal evaporation process is accomplished by the packing of ZnO units on different close packed planes. The alteration of growth velocity on different growth direction is the basic reason for the formation of different kinds of ZnO 1D nanostructural materials. r 2005 Elsevier B.V. All rights reserved. PACS: 73.61.Tm; 81.10.Aj; 78.40.Fy Keywords: B1. Nanomaterials; B1. Zinc compounds; B2. Semiconducting materials
1. Introduction The need for blue and ultraviolet solid-state emitters and detectors has propelled investigations into various wide-band-gap semiconducting mateCorresponding author. Tel.:+86 24 83978270;
fax: +86 24 23891320. E-mail address: [email protected] (C.B. Jiang).
rials in recent years. ZnO is known to be the brightest emitter among all wide-band-gap semiconducting materials, because its exciton has a binding energy of 60 meV. The ZnO one-dimensional (1D) nanostructural materials have attracted considerable interest due to their potential applications as nanometer-scale electronics and optoelectronics in functional components [1]. For example, ZnO nanowire-based
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.01.050
ARTICLE IN PRESS 322
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
short-wavelength nanolaser has been fabricated, which could have a myriad of applications in optical computing, information storage and microanalysis [2]. Among all the nanostructural ZnO oxides, belt (ribbon)-shaped nanostructure differs from conventional nanowires and exhibits increased nanoelectronic and nanooptoelectronic performance because of the absence of dislocations and other line defects [3]. The ZnO nanobelt-based cantilevers [4], resonators [5], nanolaser [6], and gas sensors are being investigated. Moreover, ZnO nanobelt is regarded as an ideal system to fully understand dimensionally confined transport phenomena and may act as valuable unit to construct nanodevices owing to their well-defined geometry [5]. Recent progress has shown that functional design and controlled growth of 1D nanostructural materials and synthesis of doped-nanostructured materials are crucially important for the development of nanodevices. For example, selforganized ZnO nanowires and nanorods were synthesized on sapphire substrates [2,7]. Doped Si nanowires have been used to build the functional nanoscale electronic devices [8]. However, there have been a few reports on how to control the growth of 1D ZnO nanobelts and to synthesize p-type ZnO nanobelts. Similar to ZnO films, the successful synthesis of the p-type nanobelts will also open the door to fabricate ZnO nanobelt-based building blocks used in nanoelectronic and nanooptoelectronic devices [4,9]. On the other hand, although the synthesis of ZnO nanobelts was widely reported, there is still a lack of understanding of the growth mechanism which is thought to be more important in the synthesis of complex ZnO nanobelts. In the present work, the nucleation and growth of ZnO micro- or nanobelts during thermal evaporation process were investigated, and the possible mechanism was proposed.
2. Experimental procedure The ZnO micro- and nanobelts were synthesized by thermal evaporation process, which is similar to
the approach reported by Wang et al. [3] The synthesis was carried out in an alumina tube that was mounted inside a high-temperature horizontal tube furnace. The ZnO powder (99.99%) was placed in an alumina boat and then loaded into the central region of the alumina tube. The alumina substrates (40 75 mm2) were placed at the downstream end of the alumina tube. After the tube had been evacuated by a mechanical rotary pump to a pressure of 5 Pa, a carrier gas of high-purity Ar was kept flowing at a rate of 200 sccm. The pressure inside the tube was kept at 400 Torr. until the temperature increased to 1400 1C. Thermal evaporation was performed at 1400 1C for 3.5 h under 100 Torr. The temperature gradient was established by using a movable stainless cooling finger at the outlet end of the alumina tube and, was measured by a movable thermocouple inside the tube. The as-synthesized white wool-like products were characterized and analyzed by XRD (Rigaku DMAX/2400), SEM (Cambridge S-360) and TEM (JEOL 2010).
3. Results and discussion Fig. 1 shows typical scanning electron microscopy (SEM) images of the ZnO micro- and nanobelts. The microbelts were synthesized without the cooling finger. The temperature gradient under this condition was estimated to be about 10 1C/mm along the horizontal direction in the temperature range from 1100 to 850 1C. The microbelts were 2–15 mm in width and 50–100 mm in length. It is found that most of them grew from the attached ZnO microrods (Fig. 1A) and bigger microbelts exhibited a flat form with a little curvature. When a cooling finger was used, ZnO nanobelts were formed instead of microbelts (Fig. 1B). As the hot Ar gas flow passed over the surface of the alumina substrates which was put on the cooling finger, the temperature gradient was estimated to be more than 200 1C/mm in the vertical direction in the region of about 1000 1C. The length of the nanobelts was about several hundred micrometers, and the width was about 0.2–1 mm (Fig. 1B). X-ray diffraction taken from
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 1. SEM images of the ZnO microbelts (A), and nanobelts (B) synthesized by the thermal evaporation method. Inset shows the high magnification of the nanobelts.
these belt-shaped products demonstrated the wurtzite ZnO crystal structure. Transmission electron microscopy (TEM) images of the microand nanobelts showed that most nanobelts grew along the ½2 1¯ 1¯ 0 direction enclosed by ð0 1 1¯ 0Þ and 7(0 0 0 1) facets, and some of them grew along the ½0 1 1¯ 0 enclosed by 7(0 0 0 1) and ð2 1¯ 1¯ 0Þ facets, which are similar to the results reported by Wang and co-workers [3] and Yang et al. [6]. Regardless of the dimensional difference between the microbelts and the nanobelts, their nucleation and growth mechanism should be the same. However, the exact growth mechanism of these belt-shaped nanostructures is still not clear at this stage. In order to understand the growth
323
process of the nanobelts, the nucleation of the nanobelts was observated in detail by SEM and TEM, some typical images are shown in Fig. 2. It can be seen that most nanobelts grew from the attached nanorods (Figs. 2A–C) and some of them grew from the ZnO particles (Fig. 2D). As shown in Figs. 2A and B, the microbelts grew from the top end facets of the microrods. In Fig. 2C, there were many kinks on an individual microrod, and the nodes of the intersected kinks were the sources from which the nanobelts grew out along ½0 1 1¯ 0 direction. A slight fluctuation of the growth direction would leave a node, which is favourable for the ½0 1 1¯ 0 oriented nanobelts to grow with the wide surfaces of 7(0 0 0 1) and narrow sufaces of ð2 1¯ 1¯ 0Þ: The orientation of the wide facets was identical to the growth direction of the microrod. The branching growth of ½0 1 1¯ 0 nanobelts was frequently observed. Fig. 2A also shows that some of the belt-shaped structures grew from the side facets of the microrods, but the growth mode was different from those nanobelts which grew from the top end facet. In this case, the end facet of the microrod did not exhibit hexagonal shape and the microbelts grew from the bulges on the side facets of microrods (marked by circles in Fig. 2A). The orientation of the wide facets was vertical to the growth direction of the microrods. In Fig. 2D, the belt-shaped structures growing from a particle of ZnO were also observed. In addition, the microbelts growing from the top facet of a microrod were seen, as shown in Fig. 2E. The growth direction of the microbelt was along the ½2 1¯ 1¯ 0 direction with the wide and narrow surfaces of 7(0 0 0 1) and ð0 1 1¯ 0Þ; respectively, as evidenced by the selected area electron diffraction (SAED) results. A nanobelt growing from a nanorod was clearly seen in Fig. 2F. Further observation showed that the nanobelt was not a straight belt. In fact, there were many slight kinks along the nanobelt as shown in the inset of Fig. 2F. Based on the above results, it could be concluded that the nucleation of belt-shaped structures from ZnO rods is an important mechanism in thermal evaporation process. The growth of belt-shaped structures should involve a 2D growth mechanism and exhibit anisotropy in growth kinetics along
ARTICLE IN PRESS 324
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 2. SEM and TEM images of the ZnO microbelts and nanobelts nucleated from the ZnO nanorods (A–C and E, F) or ZnO particles (D). Insets E and F show the corresponding selected area electron diffraction (SAED) pattern and high magnification of a part of the nanobelts, respectively.
different crystallographic directions, since the particular growth condition is responsible for this kind of peculiar growth [10]. Fig. 3 showe that the microbelt and nanowire could also be nucleated from other micro- or nanobelt. In Fig. 3A, a branch microbelt initially grew from the base microbelt, and then a secondary microbelt grew from the branch microbelt. The SAED patterns showed that the base microbelt grew along ½1 2¯ 1 0 direction with the wide and narrow surfaces of ð0 0 0 1Þ and ð0 2¯ 1 0Þ; while the branch microbelt grew along [0 0 0 1] direction with the wide and narrow surfaces of ð1 1 2¯ 0Þ and ð1 1¯ 0 0Þ; respectively.
The growth direction of the base and branch microbelts could be easily identified from the contrast of traces along the ½1 0 1¯ 0; ½1 1¯ 0 1 and ½0 1¯ 1 1 in the root of the branch microbelt in the ¯ The longitudinal direcbeam direction of ½1 2¯ 1 3: tion of the microbelt growing from the branch microbelt was determined to be ½1 1 2¯ 0 with wide surface of 7(0 0 0 1). Fig. 3B shows the image in the beam direction after the specimen is titled 101 ¯ to ½ 2¯ 1 3¯ 1 around the ½1 1¯ 0 1 axis. from ½1 2¯ 1 3 The inset indicates the details of the [0 0 0 1] oriented microbelt in which teeth shape along the side surface of this microbelt were observed. It is seen in Fig. 3C, a nanowire grew from a nanobelt.
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 3. TEM images of the ZnO microbelts (A,B) and nanowire (C) nucleated from the ZnO nanobelts. Insets show the corresponding SAED pattern and high magnification of marked parts in the figures. The SAEDs of base microbelt and second nanobelt are marked by numbers 1 and 3. The beam ¯ (2) and ½0 1 1¯ 0; directions in A and C are [0 0 0 1] (1), ½1 2¯ 1 3 respectively.
325
The SAED pattern showed that the nanowire was growing along the [0 0 0 1] direction, while the nanobelt was growing along the [2¯ 1 1 2] direction with the wide and narrow surfaces of ð0 1 1¯ 0Þ and 7(2 1¯ 1¯ 3), respectively. The growth plane of the nanobelt was only one of the {2¯ 1 1 2} planes, which are one kind of close packed planes in wurtzite (hexagonal) structures. Packing the ZnO units on the {2¯ 1 1 2} planes would be a possible mechanism for the growth of this nanobelt. The traces shown in the inset 3 of Fig. 3A were possible intersections between the {2¯ 1 1 2} planes with the viewing surfaces. Saw-teeth structures along the two sides of the [2 1¯ 1¯ 0] oriented microbelts were observed in Fig. 4A. The side surfaces of a saw-tooth were not normal to the wide surface of the microbelt and the angle between the two edges of a tooth on the (0 0 0 1) plane was about 1201. Fig. 4B shows the saw-teeth of another [2 1¯ 1¯ 0] oriented microbelt. When the microbelt was rotated from [0 0 0 1] to [0 1¯ 1 2] (SAED pattern) close to the [0 2¯ 2 3] around the [2 1¯ 1¯ 0] axis, the surfaces of the teeth were nearly perpendicular to the beam direction. Comparing the pattern of [0 2¯ 2 3] with that of [0 1¯ 1 2] (Figs. 4D and C), it was found that the surfaces of a tooth was (1 1 1¯ 2) and (1¯ 2 1¯ 2), respectively. This result gave a direct evidence for the above-mentioned growth mechanism. The saw-teeth formed along two sides of the belts was a typical feature in [2 1¯ 1¯ 0] oriented microbelts, but the morphology of [0 1 1¯ 0] oriented nanobelts is different as shown in Figs. 2F and 5. The kinks were formed along the longitudinal direction of the [0 1 1¯ 0] oriented nanobelts. In Fig. 5, the kinks on a [0 1 1¯ 0] oriented nanobelt were observed in [0 0 0 1] direction and another direction titled about 401 from [0 0 0 1] along the [0 1 1¯ 0] axis. The surface of the nanobelt remained flat after the growth direction of the nanobelt deviates about 301 from the longitudinal direction. In addition, the side surfaces of the nanobelt were not perpendicular to the wide surface (Fig. 5A) and the nanobelt was also not uniform in thickness (the inset in Fig. 5B). Based on observations from Figs. 3 and 4, it was reasonable to conclude that the growth of the [0 1 1¯ 0] oriented nanobelts proceeded by the repeated alteration of the growth planes from {0 1 1¯ 0} to {2¯ 1 1 2} planes.
ARTICLE IN PRESS 326
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 4. TEM images of the [2 1¯ 1¯ 0] oriented ZnO microbelts (A, B), SAED pattern of [0 1¯ 1 2] (C) and sketched SAED pattern of [0 2¯ 2 3] (D). The beam directions in A, B are [0 0 0 1], [0 1¯ 1 2] and [0 0 0 1] for the inset in B, respectively. The inset shows that the teeth in this nanobelts are same to those in Fig. 4A. Fig. 5. TEM images of [0 1 1¯ 0] oriented ZnO nanobelts. Insets show the corresponding SAED pattern and high-magnification of some parts in the figures. The beam directions in A and C are [0 0 0 1].
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
327
Fig. 6. A geometrical schematic diagram showing the formation of the different kinds of nanobelts. The [2 1¯ 1¯ 0] and [0 0 0 1] oriented ZnO nanobelts are formed by the combination of the growth on the different (1 2 1 2¯ ) planes, while the [0 1 1¯ 0] oriented ZnO nanobelts are formed by the combination of the growth on the (0 1 0 1) and (1 2 1 2¯ ) planes together.
The end of this [0 1 1¯ 0] oriented nanobelt was found to be composed of two planes (Fig. 5B). Fig. 5C shows a nanobelt with turning corner of 1201. Similar to the nanobelt in Fig. 5A, the side surface was also smooth, that is, without saw-tooth shape. Above observations show that the formation of [0 1 1¯ 0] oriented nanobelts was not only by the growth of (2¯ 1 2 1) planes, but also by the combination of (0 1 1¯ 0) and (2¯ 1 2 1) planes. The smooth side surface should be a typical feature of [0 1 1¯ 0] and [2¯ 1 1 2] oriented nanobelts (Fig. 3C), in which the longitudinal direction of the nanobelts was perpendicular to the growth plane. It is known that the lattice constants of ZnO are ( and c ¼ 5:205 A: ( The value of c=a is a ¼ 3:249 A 1.602, less than 1.633. The typical slip planes for
ZnO are (0 0 0 1), {1 0 1¯ 1}, {0 1 1¯ 0} and {2¯ 1 1 2}. The longitudinal directions of nanowires or nanorods are [0 0 0 1], the growth habit planes of both the nanowires and the nanorods are (0 0 0 1). Because of the fast growth velocity of /0 0 0 1S direction, the nanorod with a sharp hexahedral pyramidal tip and the self-thinning growth of a nanowire formed from a nanorod were often observed. In addition, the alternation of the growth plane between (0 0 0 1) and other close packed planes was thought to be the basic reason for the change of the growth direction on the nanowires or nanorods, sometimes (Fig. 2C). On the other hand, the typical longitudinal directions are [0 1 1¯ 0], [2 1¯ 1¯ 0] or [0 0 0 1] for nanobelts, but the growth process was composed of the growth
ARTICLE IN PRESS 328
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 7. SEM and TEM images of comb-shaped ZnO nanostructures. The beam direction in B is [2 1¯ 1¯ 0].
on {2¯ 1 1 2} and {0 1 1¯ 0} planes. The growth of different kinds of nanobelts are schematically shown in Fig. 6, in which six different {2¯ 1 1 2} planes are labeled 1–6 as shown in the bottom of the figure. It was found that both [2 1¯ 1¯ 0] oriented nanobelts with the wide surfaces of 7(0 0 0 1) and 7(0 1 1¯ 0) could be formed via the growth on different {2¯ 1 1 2} planes. The combination of 1, 2 and 6 planes were equivalent to the combination of 4, 5 and 3. The growth plane in longitudinal direction was also preferred to {2¯ 1 1 2} rather than {2 1¯ 1¯ 0} for the {2¯ 1 1 2} oriented nanobelt, as shown in Figs. 6 and 3C. For the [0 0 0 1] oriented nanobelts with the wide surfaces on 7(0 1¯ 1 0) or 7(1 2¯ 1 0), they could be formed via the growth of 4 and 5 or 4 and 6, respectively, and so to the combination of 1 and 2 or 1 and 3. The evidence for this growth mode could be obtained from the formation of the root part on the [1 2¯ 1 0] oriented nanobelt in the inset in Fig. 3A and
the saw-teeth in the inset in 3B, in which the nucleation of [0 0 0 1] oriented nanobelts formed by the combination of growth on different {2¯ 1 1 2} planes was clearly observed. On the contrary, it was found that [0 1 1¯ 0] oriented nanobelts were formed via the combination of growth on both {2¯ 1 1 2} and {0 1 1¯ 0} planes. Among the various ZnO nanostructures reported, the comb-shaped nanostructure [11–14] is mostly similar to the saw-tooth nanobelts, which was observed in the present work. In most combshaped nanostructures, the rod-shaped, wireshaped or belt-shaped teeth were grown from one-side or both-sides of rod-shaped or beltshaped spine of the ZnO nanocombs (the one-side belt-shaped ZnO comb-shaped nanostructures are shown in Fig. 7). For all the one-side teeth ZnO nanocombs, the growth direction of the spine was [0 1 1¯ 0], while the growth direction of the teeth is [0 0 0 1]. There were many saw-teeth along the spine of the nanobelt (Fig. 7B), but only a part of them were the possible sources for the longer teeth of the nanocomb to grow. In addition, the top facet of the comb tooth was (0 1 1¯ 1), rather than (0 0 0 1). As a result, the teeth of the nanocomb would be formed by the growth of the combination on {0 1 1¯ 1} type facets along [0 0 0 1] direction. The polarization of ZnO [15,16] is thought to be an important effect, which is also the basic reason for the formation of two-side comb-shaped ZnO nanostructure [14]. Comparing the results in Figs. 3 and 4, the source for the formation of the sawteeth along the [0 0 0 1] and [2 1¯ 1¯ 0] oriented nanobelts was different to that of the teeth of nanocombs, but which would be related to the polarization of ZnO.
4. Conclusion The main results of this study may be summarized as follows: 1. The ZnO micro- or nanobelts synthesized by thermal evaporation method are mostly nucleated from the attached nanorods. 2. The growth of the micro- or nanobelts are formed via the combination of growth on
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
different {2¯ 1 1 2} planes and {0 1 1¯ 0} planes. The combination of the growth on {2¯ 1 1 2} planes is enough for the formation of [0 0 0 1] and [1¯ 1¯ 2 0] oriented nanobelts; but the combination of the growth on {2¯ 1 1 2} and {0 1 1¯ 0} planes is necessary for the formation of [0 1 1¯ 0] oriented nanobelts. 3. The growth of ZnO 1D nanostructural materials during thermal evaporation process is accomplished by the packing of ZnO units on different close packed planes. The polarization of ZnO is the basic reason for formation of different kinds of ZnO 1D nanostructural materials. Our recent work also shows that similar mechanism can also be used to interpret the growth of ZnS nanobelts during the thermal evaporation process similar to the results of Wang and co-workers [17].
Acknowledgements This work was supported by the Shenyang Research Center for Interfacial Materials Foundation and partially supported by the Special Funds for the Major State Basic Research Project 2004CB619306.
329
References [1] S.C. Minne, S.R. Manalis, C.F. Quate, Appl. Phys. Lett. 67 (1995) 3918. [2] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] W.L. Hughes, Z.L. Wang, Appl. Phys. Lett. 82 (2003) 2886. [5] X.D. Bai, P.X. Gao, Z.L. Wang, E.G. Wang, Appl. Phys. Lett. 82 (2003) 4806. [6] H.Q. Yan, J. Johnson, M. Law, R.R. He, K. Knutsen, J.R.M. Kinney, J. Pham, R. Saykally, P. Yang, Adv. Mater. 15 (2003) 1907. [7] W.I. Park, G.C. Yi, J.W. Kim, S.M. Park, Appl. Phys. Lett. 82 (2003) 4358. [8] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [9] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [10] W.S. Shi, H.Y. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Am. Chem. Soc. 123 (2001) 11095. [11] J.G. Wen, J.Y. Lao, D.Z. Wang, T.M. Kyaw, Y.L. Foo, Z.F. Ren, Chem. Phys. Lett. 372 (2003) 717. [12] H.Q. Yan, R.R. He, J. Johnson, M. Law, R.J. Sayjally, P.D. Yang, J. Am. Chem. Soc. 125 (2003) 4728. [13] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829. [14] Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502. [15] X.Y. Kong, Z.L. Wang, Nano. Lett. 3 (2003) 1625. [16] X.Y. Kong, Z.L. Wang, Appl. Phys. Lett. 84 (2004) 975. [17] C. Ma, D. Moore, J. Li, Z.L. Wang, Adv. Mater. 15 (2003) 228.
Journal of Crystal Growth 277 (2005) 321–329 www.elsevier.com/locate/jcrysgro
Nucleation and growth of ZnO micro- and nanobelts during thermal evaporation Z.Q. Zhanga, C.B. Jianga,, S.X. Lia, S.X. Maob a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China b Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 7 June 2004; accepted 5 January 2005 Available online 11 February 2005 Communicated by J.M. Redwing
Abstract Nucleation and growth of semiconducting ZnO microbelts and nanobelts during thermal evaporation have been investigated. It is found that the ZnO micro- and nanobelts synthesized via thermal evaporation method are mostly nucleated from the attached nanorods and formed by the combination of growth on f2¯ 1 1 2g planes and f0 1 1¯ 0g planes. The combination of the growth on f2¯ 1 1 2g planes is enough for the formation of [0 0 0 1] and ½2 1¯ 1¯ 0 oriented nanobelts; but the combination of the growth on f2¯ 1 1 2g and f0 1 1¯ 0g planes is necessary for the formation of ½0 1 1¯ 0 oriented nanobelts. In all cases, the growth of ZnO 1D nanostructural materials during thermal evaporation process is accomplished by the packing of ZnO units on different close packed planes. The alteration of growth velocity on different growth direction is the basic reason for the formation of different kinds of ZnO 1D nanostructural materials. r 2005 Elsevier B.V. All rights reserved. PACS: 73.61.Tm; 81.10.Aj; 78.40.Fy Keywords: B1. Nanomaterials; B1. Zinc compounds; B2. Semiconducting materials
1. Introduction The need for blue and ultraviolet solid-state emitters and detectors has propelled investigations into various wide-band-gap semiconducting mateCorresponding author. Tel.:+86 24 83978270;
fax: +86 24 23891320. E-mail address: [email protected] (C.B. Jiang).
rials in recent years. ZnO is known to be the brightest emitter among all wide-band-gap semiconducting materials, because its exciton has a binding energy of 60 meV. The ZnO one-dimensional (1D) nanostructural materials have attracted considerable interest due to their potential applications as nanometer-scale electronics and optoelectronics in functional components [1]. For example, ZnO nanowire-based
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.01.050
ARTICLE IN PRESS 322
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
short-wavelength nanolaser has been fabricated, which could have a myriad of applications in optical computing, information storage and microanalysis [2]. Among all the nanostructural ZnO oxides, belt (ribbon)-shaped nanostructure differs from conventional nanowires and exhibits increased nanoelectronic and nanooptoelectronic performance because of the absence of dislocations and other line defects [3]. The ZnO nanobelt-based cantilevers [4], resonators [5], nanolaser [6], and gas sensors are being investigated. Moreover, ZnO nanobelt is regarded as an ideal system to fully understand dimensionally confined transport phenomena and may act as valuable unit to construct nanodevices owing to their well-defined geometry [5]. Recent progress has shown that functional design and controlled growth of 1D nanostructural materials and synthesis of doped-nanostructured materials are crucially important for the development of nanodevices. For example, selforganized ZnO nanowires and nanorods were synthesized on sapphire substrates [2,7]. Doped Si nanowires have been used to build the functional nanoscale electronic devices [8]. However, there have been a few reports on how to control the growth of 1D ZnO nanobelts and to synthesize p-type ZnO nanobelts. Similar to ZnO films, the successful synthesis of the p-type nanobelts will also open the door to fabricate ZnO nanobelt-based building blocks used in nanoelectronic and nanooptoelectronic devices [4,9]. On the other hand, although the synthesis of ZnO nanobelts was widely reported, there is still a lack of understanding of the growth mechanism which is thought to be more important in the synthesis of complex ZnO nanobelts. In the present work, the nucleation and growth of ZnO micro- or nanobelts during thermal evaporation process were investigated, and the possible mechanism was proposed.
2. Experimental procedure The ZnO micro- and nanobelts were synthesized by thermal evaporation process, which is similar to
the approach reported by Wang et al. [3] The synthesis was carried out in an alumina tube that was mounted inside a high-temperature horizontal tube furnace. The ZnO powder (99.99%) was placed in an alumina boat and then loaded into the central region of the alumina tube. The alumina substrates (40 75 mm2) were placed at the downstream end of the alumina tube. After the tube had been evacuated by a mechanical rotary pump to a pressure of 5 Pa, a carrier gas of high-purity Ar was kept flowing at a rate of 200 sccm. The pressure inside the tube was kept at 400 Torr. until the temperature increased to 1400 1C. Thermal evaporation was performed at 1400 1C for 3.5 h under 100 Torr. The temperature gradient was established by using a movable stainless cooling finger at the outlet end of the alumina tube and, was measured by a movable thermocouple inside the tube. The as-synthesized white wool-like products were characterized and analyzed by XRD (Rigaku DMAX/2400), SEM (Cambridge S-360) and TEM (JEOL 2010).
3. Results and discussion Fig. 1 shows typical scanning electron microscopy (SEM) images of the ZnO micro- and nanobelts. The microbelts were synthesized without the cooling finger. The temperature gradient under this condition was estimated to be about 10 1C/mm along the horizontal direction in the temperature range from 1100 to 850 1C. The microbelts were 2–15 mm in width and 50–100 mm in length. It is found that most of them grew from the attached ZnO microrods (Fig. 1A) and bigger microbelts exhibited a flat form with a little curvature. When a cooling finger was used, ZnO nanobelts were formed instead of microbelts (Fig. 1B). As the hot Ar gas flow passed over the surface of the alumina substrates which was put on the cooling finger, the temperature gradient was estimated to be more than 200 1C/mm in the vertical direction in the region of about 1000 1C. The length of the nanobelts was about several hundred micrometers, and the width was about 0.2–1 mm (Fig. 1B). X-ray diffraction taken from
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 1. SEM images of the ZnO microbelts (A), and nanobelts (B) synthesized by the thermal evaporation method. Inset shows the high magnification of the nanobelts.
these belt-shaped products demonstrated the wurtzite ZnO crystal structure. Transmission electron microscopy (TEM) images of the microand nanobelts showed that most nanobelts grew along the ½2 1¯ 1¯ 0 direction enclosed by ð0 1 1¯ 0Þ and 7(0 0 0 1) facets, and some of them grew along the ½0 1 1¯ 0 enclosed by 7(0 0 0 1) and ð2 1¯ 1¯ 0Þ facets, which are similar to the results reported by Wang and co-workers [3] and Yang et al. [6]. Regardless of the dimensional difference between the microbelts and the nanobelts, their nucleation and growth mechanism should be the same. However, the exact growth mechanism of these belt-shaped nanostructures is still not clear at this stage. In order to understand the growth
323
process of the nanobelts, the nucleation of the nanobelts was observated in detail by SEM and TEM, some typical images are shown in Fig. 2. It can be seen that most nanobelts grew from the attached nanorods (Figs. 2A–C) and some of them grew from the ZnO particles (Fig. 2D). As shown in Figs. 2A and B, the microbelts grew from the top end facets of the microrods. In Fig. 2C, there were many kinks on an individual microrod, and the nodes of the intersected kinks were the sources from which the nanobelts grew out along ½0 1 1¯ 0 direction. A slight fluctuation of the growth direction would leave a node, which is favourable for the ½0 1 1¯ 0 oriented nanobelts to grow with the wide surfaces of 7(0 0 0 1) and narrow sufaces of ð2 1¯ 1¯ 0Þ: The orientation of the wide facets was identical to the growth direction of the microrod. The branching growth of ½0 1 1¯ 0 nanobelts was frequently observed. Fig. 2A also shows that some of the belt-shaped structures grew from the side facets of the microrods, but the growth mode was different from those nanobelts which grew from the top end facet. In this case, the end facet of the microrod did not exhibit hexagonal shape and the microbelts grew from the bulges on the side facets of microrods (marked by circles in Fig. 2A). The orientation of the wide facets was vertical to the growth direction of the microrods. In Fig. 2D, the belt-shaped structures growing from a particle of ZnO were also observed. In addition, the microbelts growing from the top facet of a microrod were seen, as shown in Fig. 2E. The growth direction of the microbelt was along the ½2 1¯ 1¯ 0 direction with the wide and narrow surfaces of 7(0 0 0 1) and ð0 1 1¯ 0Þ; respectively, as evidenced by the selected area electron diffraction (SAED) results. A nanobelt growing from a nanorod was clearly seen in Fig. 2F. Further observation showed that the nanobelt was not a straight belt. In fact, there were many slight kinks along the nanobelt as shown in the inset of Fig. 2F. Based on the above results, it could be concluded that the nucleation of belt-shaped structures from ZnO rods is an important mechanism in thermal evaporation process. The growth of belt-shaped structures should involve a 2D growth mechanism and exhibit anisotropy in growth kinetics along
ARTICLE IN PRESS 324
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 2. SEM and TEM images of the ZnO microbelts and nanobelts nucleated from the ZnO nanorods (A–C and E, F) or ZnO particles (D). Insets E and F show the corresponding selected area electron diffraction (SAED) pattern and high magnification of a part of the nanobelts, respectively.
different crystallographic directions, since the particular growth condition is responsible for this kind of peculiar growth [10]. Fig. 3 showe that the microbelt and nanowire could also be nucleated from other micro- or nanobelt. In Fig. 3A, a branch microbelt initially grew from the base microbelt, and then a secondary microbelt grew from the branch microbelt. The SAED patterns showed that the base microbelt grew along ½1 2¯ 1 0 direction with the wide and narrow surfaces of ð0 0 0 1Þ and ð0 2¯ 1 0Þ; while the branch microbelt grew along [0 0 0 1] direction with the wide and narrow surfaces of ð1 1 2¯ 0Þ and ð1 1¯ 0 0Þ; respectively.
The growth direction of the base and branch microbelts could be easily identified from the contrast of traces along the ½1 0 1¯ 0; ½1 1¯ 0 1 and ½0 1¯ 1 1 in the root of the branch microbelt in the ¯ The longitudinal direcbeam direction of ½1 2¯ 1 3: tion of the microbelt growing from the branch microbelt was determined to be ½1 1 2¯ 0 with wide surface of 7(0 0 0 1). Fig. 3B shows the image in the beam direction after the specimen is titled 101 ¯ to ½ 2¯ 1 3¯ 1 around the ½1 1¯ 0 1 axis. from ½1 2¯ 1 3 The inset indicates the details of the [0 0 0 1] oriented microbelt in which teeth shape along the side surface of this microbelt were observed. It is seen in Fig. 3C, a nanowire grew from a nanobelt.
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 3. TEM images of the ZnO microbelts (A,B) and nanowire (C) nucleated from the ZnO nanobelts. Insets show the corresponding SAED pattern and high magnification of marked parts in the figures. The SAEDs of base microbelt and second nanobelt are marked by numbers 1 and 3. The beam ¯ (2) and ½0 1 1¯ 0; directions in A and C are [0 0 0 1] (1), ½1 2¯ 1 3 respectively.
325
The SAED pattern showed that the nanowire was growing along the [0 0 0 1] direction, while the nanobelt was growing along the [2¯ 1 1 2] direction with the wide and narrow surfaces of ð0 1 1¯ 0Þ and 7(2 1¯ 1¯ 3), respectively. The growth plane of the nanobelt was only one of the {2¯ 1 1 2} planes, which are one kind of close packed planes in wurtzite (hexagonal) structures. Packing the ZnO units on the {2¯ 1 1 2} planes would be a possible mechanism for the growth of this nanobelt. The traces shown in the inset 3 of Fig. 3A were possible intersections between the {2¯ 1 1 2} planes with the viewing surfaces. Saw-teeth structures along the two sides of the [2 1¯ 1¯ 0] oriented microbelts were observed in Fig. 4A. The side surfaces of a saw-tooth were not normal to the wide surface of the microbelt and the angle between the two edges of a tooth on the (0 0 0 1) plane was about 1201. Fig. 4B shows the saw-teeth of another [2 1¯ 1¯ 0] oriented microbelt. When the microbelt was rotated from [0 0 0 1] to [0 1¯ 1 2] (SAED pattern) close to the [0 2¯ 2 3] around the [2 1¯ 1¯ 0] axis, the surfaces of the teeth were nearly perpendicular to the beam direction. Comparing the pattern of [0 2¯ 2 3] with that of [0 1¯ 1 2] (Figs. 4D and C), it was found that the surfaces of a tooth was (1 1 1¯ 2) and (1¯ 2 1¯ 2), respectively. This result gave a direct evidence for the above-mentioned growth mechanism. The saw-teeth formed along two sides of the belts was a typical feature in [2 1¯ 1¯ 0] oriented microbelts, but the morphology of [0 1 1¯ 0] oriented nanobelts is different as shown in Figs. 2F and 5. The kinks were formed along the longitudinal direction of the [0 1 1¯ 0] oriented nanobelts. In Fig. 5, the kinks on a [0 1 1¯ 0] oriented nanobelt were observed in [0 0 0 1] direction and another direction titled about 401 from [0 0 0 1] along the [0 1 1¯ 0] axis. The surface of the nanobelt remained flat after the growth direction of the nanobelt deviates about 301 from the longitudinal direction. In addition, the side surfaces of the nanobelt were not perpendicular to the wide surface (Fig. 5A) and the nanobelt was also not uniform in thickness (the inset in Fig. 5B). Based on observations from Figs. 3 and 4, it was reasonable to conclude that the growth of the [0 1 1¯ 0] oriented nanobelts proceeded by the repeated alteration of the growth planes from {0 1 1¯ 0} to {2¯ 1 1 2} planes.
ARTICLE IN PRESS 326
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 4. TEM images of the [2 1¯ 1¯ 0] oriented ZnO microbelts (A, B), SAED pattern of [0 1¯ 1 2] (C) and sketched SAED pattern of [0 2¯ 2 3] (D). The beam directions in A, B are [0 0 0 1], [0 1¯ 1 2] and [0 0 0 1] for the inset in B, respectively. The inset shows that the teeth in this nanobelts are same to those in Fig. 4A. Fig. 5. TEM images of [0 1 1¯ 0] oriented ZnO nanobelts. Insets show the corresponding SAED pattern and high-magnification of some parts in the figures. The beam directions in A and C are [0 0 0 1].
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
327
Fig. 6. A geometrical schematic diagram showing the formation of the different kinds of nanobelts. The [2 1¯ 1¯ 0] and [0 0 0 1] oriented ZnO nanobelts are formed by the combination of the growth on the different (1 2 1 2¯ ) planes, while the [0 1 1¯ 0] oriented ZnO nanobelts are formed by the combination of the growth on the (0 1 0 1) and (1 2 1 2¯ ) planes together.
The end of this [0 1 1¯ 0] oriented nanobelt was found to be composed of two planes (Fig. 5B). Fig. 5C shows a nanobelt with turning corner of 1201. Similar to the nanobelt in Fig. 5A, the side surface was also smooth, that is, without saw-tooth shape. Above observations show that the formation of [0 1 1¯ 0] oriented nanobelts was not only by the growth of (2¯ 1 2 1) planes, but also by the combination of (0 1 1¯ 0) and (2¯ 1 2 1) planes. The smooth side surface should be a typical feature of [0 1 1¯ 0] and [2¯ 1 1 2] oriented nanobelts (Fig. 3C), in which the longitudinal direction of the nanobelts was perpendicular to the growth plane. It is known that the lattice constants of ZnO are ( and c ¼ 5:205 A: ( The value of c=a is a ¼ 3:249 A 1.602, less than 1.633. The typical slip planes for
ZnO are (0 0 0 1), {1 0 1¯ 1}, {0 1 1¯ 0} and {2¯ 1 1 2}. The longitudinal directions of nanowires or nanorods are [0 0 0 1], the growth habit planes of both the nanowires and the nanorods are (0 0 0 1). Because of the fast growth velocity of /0 0 0 1S direction, the nanorod with a sharp hexahedral pyramidal tip and the self-thinning growth of a nanowire formed from a nanorod were often observed. In addition, the alternation of the growth plane between (0 0 0 1) and other close packed planes was thought to be the basic reason for the change of the growth direction on the nanowires or nanorods, sometimes (Fig. 2C). On the other hand, the typical longitudinal directions are [0 1 1¯ 0], [2 1¯ 1¯ 0] or [0 0 0 1] for nanobelts, but the growth process was composed of the growth
ARTICLE IN PRESS 328
Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
Fig. 7. SEM and TEM images of comb-shaped ZnO nanostructures. The beam direction in B is [2 1¯ 1¯ 0].
on {2¯ 1 1 2} and {0 1 1¯ 0} planes. The growth of different kinds of nanobelts are schematically shown in Fig. 6, in which six different {2¯ 1 1 2} planes are labeled 1–6 as shown in the bottom of the figure. It was found that both [2 1¯ 1¯ 0] oriented nanobelts with the wide surfaces of 7(0 0 0 1) and 7(0 1 1¯ 0) could be formed via the growth on different {2¯ 1 1 2} planes. The combination of 1, 2 and 6 planes were equivalent to the combination of 4, 5 and 3. The growth plane in longitudinal direction was also preferred to {2¯ 1 1 2} rather than {2 1¯ 1¯ 0} for the {2¯ 1 1 2} oriented nanobelt, as shown in Figs. 6 and 3C. For the [0 0 0 1] oriented nanobelts with the wide surfaces on 7(0 1¯ 1 0) or 7(1 2¯ 1 0), they could be formed via the growth of 4 and 5 or 4 and 6, respectively, and so to the combination of 1 and 2 or 1 and 3. The evidence for this growth mode could be obtained from the formation of the root part on the [1 2¯ 1 0] oriented nanobelt in the inset in Fig. 3A and
the saw-teeth in the inset in 3B, in which the nucleation of [0 0 0 1] oriented nanobelts formed by the combination of growth on different {2¯ 1 1 2} planes was clearly observed. On the contrary, it was found that [0 1 1¯ 0] oriented nanobelts were formed via the combination of growth on both {2¯ 1 1 2} and {0 1 1¯ 0} planes. Among the various ZnO nanostructures reported, the comb-shaped nanostructure [11–14] is mostly similar to the saw-tooth nanobelts, which was observed in the present work. In most combshaped nanostructures, the rod-shaped, wireshaped or belt-shaped teeth were grown from one-side or both-sides of rod-shaped or beltshaped spine of the ZnO nanocombs (the one-side belt-shaped ZnO comb-shaped nanostructures are shown in Fig. 7). For all the one-side teeth ZnO nanocombs, the growth direction of the spine was [0 1 1¯ 0], while the growth direction of the teeth is [0 0 0 1]. There were many saw-teeth along the spine of the nanobelt (Fig. 7B), but only a part of them were the possible sources for the longer teeth of the nanocomb to grow. In addition, the top facet of the comb tooth was (0 1 1¯ 1), rather than (0 0 0 1). As a result, the teeth of the nanocomb would be formed by the growth of the combination on {0 1 1¯ 1} type facets along [0 0 0 1] direction. The polarization of ZnO [15,16] is thought to be an important effect, which is also the basic reason for the formation of two-side comb-shaped ZnO nanostructure [14]. Comparing the results in Figs. 3 and 4, the source for the formation of the sawteeth along the [0 0 0 1] and [2 1¯ 1¯ 0] oriented nanobelts was different to that of the teeth of nanocombs, but which would be related to the polarization of ZnO.
4. Conclusion The main results of this study may be summarized as follows: 1. The ZnO micro- or nanobelts synthesized by thermal evaporation method are mostly nucleated from the attached nanorods. 2. The growth of the micro- or nanobelts are formed via the combination of growth on
ARTICLE IN PRESS Z.Q. Zhang et al. / Journal of Crystal Growth 277 (2005) 321–329
different {2¯ 1 1 2} planes and {0 1 1¯ 0} planes. The combination of the growth on {2¯ 1 1 2} planes is enough for the formation of [0 0 0 1] and [1¯ 1¯ 2 0] oriented nanobelts; but the combination of the growth on {2¯ 1 1 2} and {0 1 1¯ 0} planes is necessary for the formation of [0 1 1¯ 0] oriented nanobelts. 3. The growth of ZnO 1D nanostructural materials during thermal evaporation process is accomplished by the packing of ZnO units on different close packed planes. The polarization of ZnO is the basic reason for formation of different kinds of ZnO 1D nanostructural materials. Our recent work also shows that similar mechanism can also be used to interpret the growth of ZnS nanobelts during the thermal evaporation process similar to the results of Wang and co-workers [17].
Acknowledgements This work was supported by the Shenyang Research Center for Interfacial Materials Foundation and partially supported by the Special Funds for the Major State Basic Research Project 2004CB619306.
329
References [1] S.C. Minne, S.R. Manalis, C.F. Quate, Appl. Phys. Lett. 67 (1995) 3918. [2] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] W.L. Hughes, Z.L. Wang, Appl. Phys. Lett. 82 (2003) 2886. [5] X.D. Bai, P.X. Gao, Z.L. Wang, E.G. Wang, Appl. Phys. Lett. 82 (2003) 4806. [6] H.Q. Yan, J. Johnson, M. Law, R.R. He, K. Knutsen, J.R.M. Kinney, J. Pham, R. Saykally, P. Yang, Adv. Mater. 15 (2003) 1907. [7] W.I. Park, G.C. Yi, J.W. Kim, S.M. Park, Appl. Phys. Lett. 82 (2003) 4358. [8] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [9] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [10] W.S. Shi, H.Y. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Am. Chem. Soc. 123 (2001) 11095. [11] J.G. Wen, J.Y. Lao, D.Z. Wang, T.M. Kyaw, Y.L. Foo, Z.F. Ren, Chem. Phys. Lett. 372 (2003) 717. [12] H.Q. Yan, R.R. He, J. Johnson, M. Law, R.J. Sayjally, P.D. Yang, J. Am. Chem. Soc. 125 (2003) 4728. [13] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829. [14] Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502. [15] X.Y. Kong, Z.L. Wang, Nano. Lett. 3 (2003) 1625. [16] X.Y. Kong, Z.L. Wang, Appl. Phys. Lett. 84 (2004) 975. [17] C. Ma, D. Moore, J. Li, Z.L. Wang, Adv. Mater. 15 (2003) 228.