Highly facetted metallic zinc nanocrystals fabricated by thermal evaporation

Highly facetted metallic zinc nanocrystals fabricated by thermal evaporation

Materials Letters 60 (2006) 2423 – 2427 www.elsevier.com/locate/matlet Highly facetted metallic zinc nanocrystals fabricated by thermal evaporation K...

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Materials Letters 60 (2006) 2423 – 2427 www.elsevier.com/locate/matlet

Highly facetted metallic zinc nanocrystals fabricated by thermal evaporation K.Y. Ng a , Amol Muley a , Y.F. Chan b , A.C.M. Ng c , A.B. Djurišić c , A.H.W. Ngan a,⁎ a

Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China b Electron Microscope Unit, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China c Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China Received 16 November 2005; accepted 11 January 2006 Available online 13 February 2006

Abstract We report the fabrication of highly facetted, hexagonal shaped metallic Zn nanocrystals by a simple catalyst-free thermal evaporation technique on Si (001) substrate using Zn pellets as source material. The Zn nanocrystals were characterized by scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The size of the crystals is of the order of 100nm and they are highly facetted along {0001} and {101¯0} planes. SAD analyses revealed the possibility of the presence of a thin ZnO layer on the surface of the as-deposited Zn nanocrystals. Formation of an epitaxial core-shell structure due to surface oxidation was further confirmed by exposing the Zn nanocrystals to air, which led to the formation of crystalline ZnO having same crystallographic orientation as that of the underlying metallic Zn. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Growth from vapor; Nanomaterials; Metallic Zn

1. Introduction Nanocrystals are of immense interest and increasing importance for future technological applications due to their peculiar and fascinating properties compared to their bulk counterparts [1]. In nanometer-scale structures, finite size effects give rise to novel electronic, magnetic, optical, and mechanical properties, and the desire to identify, understand, and exploit these size-dependent properties motivates the intensive activities on nanocrystals [2]. Semi-conducting nanocrystals such as ZnO can exhibit optical and electronic properties significantly different from bulk counterparts due to quantum confinement effects on their band gap and excitation binding energy [2–8], as well as different surface-to-volume ratios. Metallic nanocrystals, on the other hand, are known to exhibit mechanical strengths one to two orders of magnitude higher than bulk counterparts [9,10]. This immense size effect on strength is thought to be due to the shortage of dislocations [9] and is a phenomenon which must be thoroughly understood before nano- and micro-machineries can be successfully designed.

⁎ Corresponding author. E-mail address: [email protected] (A.H.W. Ngan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.01.069

Various techniques, including thermal decomposition, vaporphase transport and metallorganic chemical vapor deposition (MOCVD) [11–13], are routinely used to synthesize nanostructures. Among these, evaporation is of special interest for the fabrication of nanostructures in bulk quantities. In this article, we report the successful synthesis of novel metallic zinc nanocrystals by a simple thermal evaporation technique on Si (100) wafers. In the literature, many different morphological forms of 1D nanostructures and nano-particles of ZnO, including tetrapod nanorods, nanowires, nanobelts, and hierarchical nanostructures, have been reported [14,15], but there is so far no report on the successful synthesis of highly facetted nanorods of metallic Zn. 2. Experimental procedure The zinc nanocrystals were fabricated by an AST PEVA 350T thermal evaporator using 99.99% pure zinc pellets as the source material. Before deposition, the evaporator chamber was thoroughly cleaned and baked, and the inner wall of the chamber was lined with aluminum foil. The source holder was an alumina crucible that was heated by a tungsten filament. The substrate was n-type (100) single-crystal silicon wafer, which was pre-cleaned by touluene, acetone, pure ethanol followed by

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3. Results and discussion

Fig. 1. SEM images of the as-deposited zinc nanocrystals.

de-ionized water in an ultrasonic bath. Drying of the substrate is carried out for about 5 min at 100°C. The substrates were then placed on a substrate holder inside the evaporation chamber. The substrate was not heated during the evaporation, and the distance from it to the source was about 12 cm. After the chamber was evacuated to a base pressure of 4 × 10−6 Torr, the evaporation of Zn started with a constant supply current of 23 A. The deposition rate was set to 1Å/s and the deposition time was set to 1000s typically. Morphological, microstructural and chemical investigations of the products were performed in a Bruker D8 Advance X-ray diffractometer (XRD), LEO1530 FEGSEM scanning electron microscope (SEM) and a Philips Tecnai 20 transmission electron microscope (TEM) operating at 200 kV. A Si substrate uniformly coated with nanocrystallites was used for grazing incidence X-ray diffraction (wavelength = 0.1541 Å, and diffraction angle 2θ was in the range from 30–65°). X-ray diffraction under a grazing incident angle of 0.05° helps to minimize the Xray penetration depth and to get enhanced surface signal. The samples for the SEM and TEM examinations were prepared by adding several drops of pure methanol onto the as-deposited substrate surface, followed by transferring the nanocrystalcontaining methanol solution to carbon films supported on 300mesh copper TEM grids by means of a wire loop, and allowing the methanol to evaporate.

Fig. 1(a,b) shows the SEM micrographs at two different magnifications of the as-deposited state on the Si (100) wafer. Highly facetted crystals with sizes of the order of 0.1 μm can be seen. From the higher magnification image in Fig. 1(b), many of the nanocrystals have a “hat-like” shape in which a flatter hexagonal base of diameter 100–200 nm connects to a higher-aspect-ratio hexagonal cylinder with a smaller diameter of around 100 nm and a length of about 100 nm. Some nanocrystals have a simpler hexagonal prismatic shape, and others have zig-zag shapes such as the one near the centre in Fig. 1(b). X-ray grazing incidence diffraction was performed on a sample with the as-deposited nanocrystallites covering the whole Si substrate, and Fig. 2 shows the XRD pattern. The XRD pattern reveals two strong diffraction peaks at 2θ values at 36.35° and 43.3°, corresponding to the {0002} and {101¯1} of hexagonal zinc, respectively [16]. Also, two minor peaks can be observed at 39.05° and 54.35° which correspond to {101¯0} and {101¯2}, respectively, of zinc. Detailed structural characterization of the nanocrystals was performed by TEM. Fig. 3(a) shows the typical morphology of the nanocrystals as seen in the TEM. HRTEM imaging was attempted at the edge of the nanocrystals, as depicted in Fig 2(a). In the area shown in the inset of Fig. 3(a), lattice fringes with spacing 2.53 Å and parallel to the base of the hexagonal nanocrystal can be seen. The lattice spacing here corresponds quite well to the known value of 2.48 Å for the {0002} planes in metallic Zn. Fig. 3(b–c) shows two major-pole selected area diffraction (SAD) patterns of the same nanocrystals viewed at the same zone axis of [01¯10] and [0001], respectively. The ¯10]-zone SAD pattern in Fig. 3(b) can be seen to consist of two [01 sets of diffractions spots, one of which is brighter than the other. This suggests that there are two lattice systems present in the nanocrystals. The weaker set of spots is well aligned with the primary, brighter set of spots, and this suggests an epitaxial relationship between the two lattice systems. The primary set of spots in the SAD pattern in Fig. 3 (b) was identified to be the [01¯10] zone in a hexagonal closed pack (HCP) structure. From the diffraction pattern, the spacing of the ¯110} and {0002} atomic planes are 0.135 and 0.253 nm, res{2 pectively, which agree very well with the known values of 0.133 and 0.248nm, respectively, for pure metallic zinc [16]. The calculated c / a from the primary set of diffraction pattern is 1.86, which matches exactly the known c / a value for metallic Zn [16]. The dimmer set of diffraction consists elongated spots. The interplanar spacing of the ¯110)Zn spot is 0.165 nm, which elongated reflection closest to the (2

Fig. 2. XRD grazing incidence diffraction pattern of as-deposited Zn nanocystals.

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¯10]-zone SAD pattern from the nanocrystal in (a). Fig. 3. (a) TEM image of a Zn nanocrystal. Inset shows HRTEM image taken near the base of the nanocrystal. (b) [01 Note that the SAD in (b) and the image in (a) are not shown at the same orientation. (c) [0001]-zone SAD pattern from the nanocrystal in (a). (d) Energy dispersive Xray spectroscopy results on Zn nanocrystals.

¯110} agrees rather well with the known spacing of 0.163 nm of the {2 reflection in the HCP ZnO structure [16]. The dimmer set of spots is therefore thought to arise from the [01¯10] zone in the HCP ZnO structure, with an epitaxial relationship between the ZnO and Zn lattices, so that (2¯110)Zn||(2¯110)ZnO and (0001)Zn||(0001)ZnO. The known spacing of the {0002}ZnO planes is 0.260 nm, and this corresponds into a diffraction vector length of 3.84 nm− 1 in the reciprocal space, which is very close to the diffraction vector length of 4.04 nm− 1 for the {0002}Zn reflection. Judging from the diffused size of the diffraction spots in Fig. 3(b), the two diffraction spots {0002}ZnO and {0002}Zn are likely to overlap in Fig. 3(b). The SAD pattern shown in Fig. 3(c) also contains two sets of diffraction spots corresponding to the [0001] zone of the Zn and ZnO lattices, together with satellite spots arising from double diffraction. It should be noted that Ding et al. [17] have recently studied ZnO-covered Zn nanobelts, and they concluded from their [0001]-zone SAD that apart from the orientation relation (2¯110)Zn||(2¯110)ZnO and (0001)Zn||(0001)ZnO, ¯110)ZnO and (0001)Zn||(0001)ZnO, also another relation, (2¯110)Zn||(2 exists simultaneously. The present SAD in Fig. 3(c) only indicates existence of the first orientation relation; the second orientation relation would have given rise to extra spots incommensurate with the hexagonal arrangement of the satellite spots in the [0001] zone [17], but no such incommensurate spots are visible in Fig. 3(c). The energy dispersive X-ray spectroscopy (EDX) results shown in Fig. 3(d) indicate that the chemical composition of the nanocrystals is mainly zinc, but there is a minor amount of oxygen in the nanocrystals. The

Cu signal is most likely due to the copper grid supporting the specimen. These results suggest that the nanocrystals are mainly elemental zinc, and that the crystallographic orientation of the HCP structure coincides with the orientation of the macroscopic, hexagonal crystal, so that the cylindrical axes of the nanorods are along 〈0001〉, and the facetted, prismatic planes are {101¯0}. The SAD results in Fig. 3(b–c) indicate that partial oxidation into HCP ZnO happens on the nanorod surfaces, and the ZnO layer has epitaxial relation to the underlying metallic Zn. To further investigate the oxidation behavior of these nanocrystals, they were exposed to air for one week, before being examined again. After exposure to air, a crystalline layer was found to have grown on the surface of these hexagonal nanocrystals, as shown in Fig. 4(a). EDX measurements after the exposure also revealed increased oxygen content on the nanocrystal surfaces. HRTEM imaging of the edge the oxide layer, as shown in the top-right inset in Fig. 4(b), reveals lattice fringes with spacing of 0.279nm, which agrees well with the known spacing of 0.282 nm for the {101¯0} plane in ZnO. From the HRTEM image, the crystallographic orientation of the oxide layer is the same as the macroscopic hexagonal shape of the nanocrystal, and so the oxide layer has epitaxial relationship with the underlying Zn metal. The HRTEM image of the thicker part of the oxidized nanocrystal shown in the top-left inset in Fig. 4(b) revealed Moiré fringes which exhibit much larger periodicity than that of the ZnO lattice fringes. These Moiré fringes, as discussed by Ding et al. [17], are due to beating effects between the lattice images of the ZnO layer and the underlying

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metallic Zn. SAD performed on the oxidized nanocrystals, as shown in Fig. 4(c), also shows an additional set of diffraction spots similar to that observed in the as-deposited nanocrystals in Fig. 3(b). The evidence here is that deliberate oxidation of the Zn nanocrystals produces epitaxial HCP ZnO on their surfaces, forming an epitaxial core-shell structure of Zn–ZnO [17]. From Fig. 1(a,b), it can be seen that the Zn nanorods are all highly facetted but their orientations and sizes are rather random on the substrate in the as-deposited state. This suggests that these hexagonal nanorods were already formed during the journey from the target to the substrate, and were then deposited randomly onto the substrate. If the nanocrystals were formed on the substrate after Zn deposits have arrived at the latter, it would be difficult for them to develop into such a highly facetted morphology and yet with such a random orientation distribution. The SEM micrographs in Fig. 1(a,b) also do not reveal any wetting of the Zn deposits on the substrate, and so the formation and growth mechanism of the Zn nanocrystals cannot be explained by the conventional vapour–liquid–solid model. The nanocrystals were evidently obtained only during an early stage of the deposition. The fact that the substrate was unheated is also another factor for the retaining of the nanocrystals after deposition. The formation of the highly facetted shapes is probably due to the anisotropy in the specific surface energy in the Zn HCP structure. The computation by Matysina [18] shows that in HCP structures with a wide range of c / a ratio covering the ideal value of 1.63, the energies of ¯0} planes exhibit local minima, although in {0001}, {101¯1} and {101 ¯0} are the specific case of Zn, the energies of {101¯1} and {101 predicted to be higher than that of the {0001} planes by more than 50%. The hexagonal nanocrystals shown in Figs. 1, 3, and 4 are clearly facetted along the {0001} and {101¯0} planes. In the “hat-like” crystals shown in Fig. 1(b), the sloping sides of the bases of the hat shape are inclined by ∼63.4°, which agrees quite well with the interplanar angle of 65° between the {0001} and {101¯1} planes in Zn. The sloping faces in the bases are therefore likely to be {101¯1} planes in the HCP Zn structure. The present Zn nanocrystals with a flatter hexagonal base are ideal structures for studying the size effect on mechanical properties of HCP structures at the nanoscale, by mechanical testing by nanoindentation, for example. Zn nanocrystals can also be used as templates for the synthesis of other nanostructures [19]. In addition, metallic nanostructures are of interest for potential applications as nanoscale contacts or interconnects [20]. Zn nanocrystals may also be useful as catalysts. Also, we have demonstrated the possibility of the formation of a core-shell structure with a crystalline ZnO layer covering the underlying Zn metallic core. Coreshell semiconductor structures in general have attracted a lot of interest in recent years, as they exhibit a strong and stable band-edge luminescence with an increase in quantum yield at room temperature [21–25]. ZnO/Zn powders are commonly used as phosphors [26], so that the fabricated core-shell may also be of interest for display applications. As future work, the factors that influence the size and morphologies of the zinc nanocrystals should be explored in a systematic way.

4. Conclusions

Fig. 4. (a) TEM image showing thick crystalline oxide layer formed on the surface of a Zn nanocrystal after exposure to air. (b) HRTEM image of the oxide layer. (c) SAD pattern of the oxidized zinc nanocrystal. Satellite spots due to double diffraction can be seen.

We have synthesized Zn nanorods with a novel hexagonal shape by a simple catalyst-free evaporation technique. The nanorods are in the range of 100–200nm in size. Surface oxidization of these nanorods take place gradually after exposure to air, leading to the formation of the HCP ZnO phase which bears the same crystallographic orientation as the underlying Zn metal.

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Acknowledgment The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region (Project no. HKU7169/05E), a grant from the University Research Committee of the University of Hong Kong (10206180.16180.14500.302.01), as well as an equipment grant by the University of Hong Kong University Development Fund. The provision of the XRD facility by the Department of Chemistry at the University of Hong Kong is gratefully acknowledged. References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [2] C.B. Murray, S. Sun, W. Gaschler, H. Doyle, T.A. Betley, C.R. Kagan, IBM J. Res. Develop. 45 (2001) 47. [3] Y. Li, G.S. Tompa, S. Liang, C. Gorla, Y. Lu, J. Doyle, J. Vac. Sci. Technol., A, Vac. Surf. Films 15 (1997) 1063. [4] U. Rau, D. Braunger, H.W. Schock, Solid State Phenom. 67–8 (1999) 409. [5] B. Rech, O. Kluth, T. Repmann, T. Roschek, J. Springer, J. Muller, F. Finger, H. Stiebig, H. Wagner, Sol. Energy Mater. Sol. Cells 74 (2002) 439. [6] J.A. Rodriguez, T. Jirsak, J. Dvorak, S. Sambasivan, D. Fischer, J. Phys. Chem., B 104 (2000) 319. [7] K. Hara, Sol. Energy Mater. Sol. Cells 64 (2000) 115. [8] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.

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