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Growth mechanisms for ZnO nanorods formed by pulsed laser deposition
Superlattices and Microstructures 39 (2006) 33–40 www.elsevier.com/locate/superlattices
Growth mechanisms for ZnO nanorods formed by pulsed laser deposition Ye Sun, Gareth M. Fuge, Michael N.R. Ashfold∗ School of Chemistry, University of Bristol, Bristol BS8 1TS, UK Available online 8 September 2005
Abstract Arrays of well-aligned ZnO nanorods have been grown on Si substrates at 600 ◦ C using a catalystfree pulsed laser deposition (PLD) method, with and without pre-deposition of a thin ZnO layer at a lower substrate temperature. Deposited products were analyzed and characterized by scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray analysis, X-ray diffraction and photoluminescence (PL) measurements. EM revealed that rods grown directly on Si exhibit needle-like morphologies, with diameters, d, typically in the range 20–60 nm, lengths ∼200–800 nm and, in most cases, capping particles of similar cross-section at their tips. HRTEM images show that these nanoparticles are ZnO also, suggesting that these derive from post-growth crystallisation of oxygen rich molten zinc droplets that cap the nanorods during growth. PLD of ZnO onto Si substrates that have been pre-coated with a thin ZnO film deposited at 300 ◦ C yields denser, more uniform arrays of longer (∼1–1.2 µm), thinner (d ∼ 6–20 nm) nanorods, without any obvious capping particle. This suggests that a ZnO buffer layer can play a useful role in providing a high density of nucleation sites for subsequent growth of smaller diameter nanorods. The respective product arrays are most readily understood in terms of vapor–liquid–solid (V–L–S) and vapor–solid (V–S) growth models, respectively. Comparative studies of the room temperature PL from the respective samples reveal much higher UV emission intensity from nanorod arrays grown on the ZnO pre-coated Si substrates. © 2005 Elsevier Ltd. All rights reserved.
∗ Corresponding author. Tel.: +44 (0)117 9288312; fax: +44 (0)117 9250612.
E-mail address: [email protected] (M.N.R. Ashfold). 0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2005.08.029
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Y. Sun et al. / Superlattices and Microstructures 39 (2006) 33–40
1. Introduction ZnO, with its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV at room temperature), has potential in many application areas, including shortwavelength ultraviolet lasers and blue–green optoelectronic devices [1]. ZnO is also noteworthy for the ease with which it forms one-dimensional (1-D) nanostructures (e.g. nanobelts, nanorods, nanowires and nanotubes). The high aspect ratios, size effects and possible quantum confinement effects presented by such nanostructures offer many potential advantages, e.g. improved sensitivity to chemical agents and much enhanced luminescence efficiency. The synthesis of ZnO nanorods has received much attention in the last few years. Many different methods for synthesizing arrays of well-aligned ZnO nanorods have been reported, including chemical vapour transport and condensation (CVTC) [2], thermal evaporation [3–5], hydrothermal methods [6–8], template-based growth [9], chemical vapour deposition [10] and pulsed laser deposition (PLD) [11]. Challenges remain, however. These include, for example, achieving further reductions in rod diameter, improving control of the nanorod morphology, and gaining a deeper understanding of the growth mechanism itself. Synthesis of well-aligned ZnO nanorod arrays by PLD onto a Si substrate maintained at 600 ◦ C has been reported previously [11]. Here we demonstrate ways of growing longer, thinner, higher density arrays of ZnO nanorods on Si by first depositing a thin ZnO film at markedly lower substrate temperatures (Tsub = 300 ◦C). Rod diameters as small as ∼6 nm are observed. Nanorod structures, morphologies and crystallinities grown under these two different deposition conditions are compared and contrasted, with a view to unraveling mechanistic details of the prevailing growth regimes, and the role of the pre-deposited thin template film of ZnO. Studies of the photoluminescence (PL) of both types of nanorod array are reported also. 2. Experimental The PLD apparatus and the experimental methods have both been described in detail elsewhere [8]. The PLD chamber was evacuated to a typical base pressure of 1 × 10−6 Torr using a turbomolecular pump. The output of an ArF excimer laser (Lambda-Physik COMPex 201, 193 nm) operating at a pulse repetition rate of 10 Hz was focused onto a rotating ZnO target (Cerac, 99.999% purity) at 45◦ to the surface normal. The fluence incident on the target was estimated to be in the range 6–10 J cm−2 . The Si substrate (mounted as the front part of a Si/Cu/Si sandwich structure, the rear layers of which act as an effective heat-spreader) was located 40–60 mm from the target, with its front face perpendicular to the target surface normal. The substrate temperature could be adjusted to any user selected value in the range 20 ≤ Tsub ≤ 750 ◦C by controlling the output of a cw CO2 laser (Synrad, model: J48-2W-3180) aligned so as to illuminate the rear surface of the heat-spreader. All samples were grown in the low background pressure of oxygen (p(O2 ) = 10−2 Torr, 99.99% stated purity, flowing at 10 sccm). A typical deposition run lasted for t = 45 min. (27 000 laser shots). Two families of samples were deposited in these experiments. The first set was grown directly on pre-cleaned pieces of Si(100) wafer maintained at 600 ◦C. The second family were grown by first depositing a thin ZnO film
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Fig. 1. (a) SEM image (viewing at a tilt angle of ∼40◦ ) of a nanorod array grown by PLD onto a Si wafer at Tsub = 600 ◦ C for t = 45 min. (b) TEM image of a selection of rods broken from this sample. (c) A tilt-view SEM image of a nanorod array grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min. (d) TEM image of selected nanorods from this sample.
by PLD on a Si substrate at Tsub = 300 ◦C for t300 = 1 min, then boosting Tsub to 600 ◦C before continuing deposition for a further t600 = 44 min. Deposited material was analysed and characterized by scanning electron microscopy (SEM, using a JEOL JSM 5600LV instrument), high-resolution transmission electron microscopy (HRTEM, JEOL 2100EX), X-ray diffraction (XRD, Bruker AXS D8 Advance powder X-ray diffractometer with Cu Kα radiation), and electron dispersive X-ray analysis (EDX). Photoluminescence spectra were measured at room temperature using a He–Cd laser (325 nm, output power ∼3 mW) as the excitation source. 3. Results and discussion Fig. 1(a) and (b) show, respectively, a SEM image (viewing at a tilt angle of ∼40◦ ) of a nanorod array grown by PLD onto a Si wafer at Tsub = 600 ◦C for t = 45 min, and a TEM image from a selection of rods broken from this sample. The sample is composed of needle-like nanorods, with lengths l in the range 200–800 nm. The root diameters of the nanorods are fairly uniform (droot ∼ 60 nm) but the top diameters (dtop) appear to decrease with increasing rod length and, as Fig. 1(b) shows, dtop can be as small as 20 nm in the case
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Fig. 2. EDX spectrum from a sample of nanorods grown for t600 = 45 min on a bare Si substrate.
of the longest rods. Nearly every nanorod examined from this sample exhibits a capping bead-like nanoparticle. In all cases, the dimension of the nanoparticle is comparable to dtop of the nanorod it caps; since the longer rods are generally thinner at their tips, the capping particle size shows an anti-correlation with the rod length. The enlarged TEM image of the tip of one nanorod shown as an inset to Fig. 1(b) reveals an apparent boundary between the capping particle and the main shaft of the rod, suggesting that the particle and the main shaft are probably not parts of the same single crystal. The use of a pre-deposited ZnO layer as a possible way of supplying nucleation sites for the subsequent epitaxial growth of ZnO nanorods is attracting much current interest [4, 7,12,13]. Here we demonstrate that nanorod arrays grown subsequent to the introduction of a thin ZnO buffer layer can exhibit very different morphology. Fig. 1(c) and (d) show a corresponding tilt-view SEM image of an extensive nanorod array and a TEM image of selected nanorods grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min. This procedure results in a much higher density of longer, thinner nanorods, most of which have l ∼ 1–1.2 µm, droot ∼ 20 nm and dtop as small as 6–10 nm. The TEM image shown in the inset within Fig. 1(d) illustrates another striking difference between these nanorods and those grown without the ZnO buffer layer: the latter rods are flat topped, with no evident capping particle. Both types of nanorod display needle-like morphologies, consistent with the rate of 1-D (longitudinal) growth rate far exceeding that of any growth transverse to the long axis. However, the very different droot and dtop values for the two kinds of nanorods, and the more gradual variation of d with l in the case of rods grown on the pre-deposited ZnO thin film, suggest that this cannot be the whole story. Energy dispersive X-ray (EDX) analysis (in the SEM) was used to test the composition of both families of nanorods; the spectra obtained from the two samples were indistinguishable. Fig. 2 shows one such spectrum, for the case of nanorods grown for t600 = 45 min on a bare Si substrate. Zinc and oxygen are the only detectable elements
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Fig. 3. (a) and (b): 2θ scans of the samples imaged by SEM in Fig. 1(a) and (c), respectively. The associated θ -rocking curves of the respective (0002) peaks are shown to the right, with their FWHM indicated, in degrees.
(apart from a Si peak from the underlying substrate), supporting the view that no other metal elements are aiding in catalyzing the observed nanorod growth. Thus we conclude that the rods and, in this case, the capping particles, must be ZnO or Zn. The crystal structure and the orientation of the ZnO nanorod arrays were investigated by XRD. Fig. 3(a) and (b) show 2θ scans of the same samples as those imaged by SEM in Fig. 1(a) and (c). Each of the displayed XRD spectra is totally dominated by the strong ZnO(002) reflection at 34.4◦. The simplicity of these XRD spectra confirms that both nanorod arrays are comprised of strongly c-axis aligned, crystalline ZnO. θ -rocking curve measurements allow further quantification of the alignment of the nanorod samples. As the right hand panels in Fig. 3 show, the θ -rocking curve of the nanorod array grown on bare Si displays a smaller FWHM value, implying that it has somewhat better overall alignment than the array of longer, thinner nanorods grown on the thin ZnO film. Fig. 4 displays three HRTEM images. Fig. 4(a) and (b) were taken from, respectively, the main shaft of a single nanorod grown on a bare Si substrate and from within its capping particle. Parallel crystal planes, aligned normal to the growth direction, are clearly evident in Fig. 4(a); the measured plane spacing (∼0.52 nm) is characteristic of wurtzite ZnO(001). The HRTEM image of a region within the capping particle (Fig. 4(b)) reveals two interleaved crystal planes, both with inter-plane spacings of ∼0.245 nm and with an intersection angle of ∼100◦. These match well with literature values for planes such ¯ in ZnO (for which the lattice parameters are 0.2471 nm, and the as (101) and (011) inter-plane angle 99.25◦), but show no obvious match with any documented Zn crystal structures. Thus we conclude that the capping particles are also ZnO. All of the growth of these samples occurs at a temperature (600 ◦C) that is substantially above the melting point of Zn (419 ◦C). Any nanosized zinc particles that exist during the growth process should thus be in the liquid phase, whereas any nanosized ZnO particles present under these conditions are more likely to merge together to form bigger crystallites [14]. The
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Fig. 4. HRTEM images from: (a) the main shaft of a single nanorod grown at Tsub = 600 ◦ C for t = 45 min on a bare Si substrate; (b) within the capping particle of the nanorod shown in (a), and (c) the tip of a nanorod grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min.
present observations regarding nanorod growth on a bare Si substrate at 600 ◦C can thus be rationalised as follows [11]: Zn and O atoms, and small metallic and oxide recombination products are transported to the substrate during the PLD process. Some are accommodated, and some post-deposition processing will occur by interaction with the background O2 at the prevailing Tsub. We propose that Zn nanodroplets formed in the earliest stages of deposition serve to nucleate the nanorod growth. Subsequent 1-D growth is fuelled by the oxygenated capping bead of molten Zn, i.e. a classic vapour–liquid–solid (V–L–S) growth mechanism [15]. Each deposition run is ended by switching off the PLD laser, then the CO2 heating laser, and allowing the sample to cool in the flowing background pressure of O2 . The capping particles are assumed to crystallize as ZnO during this postdeposition cooling stage. Broadly similar mechanisms have recently been proposed for ZnO rod growth by heating powdered metallic Zn [16] and by CVTC methods [17]. SEM images of nanorods grown on the thin ZnO template film revealed no capping particle—a view confirmed by the HRTEM image of the very top of one such rod shown in Fig. 4(c). This image clearly shows parallel crystal planes extending to the very tip of the rod. The measured plane spacing is 0.26 nm (characteristic of ZnO (002)), and entirely consistent with the view that the nanorod is one single crystal that has grown along the [001] direction. Such observations strongly suggest that, under these conditions, nanorod growth involves a vapour–solid (V–S) mechanism [18]. The experimental results provide a number of insights into the likely growth mechanism and the way in which a thin ZnO buffer layer can influence nanorod growth. We deduce that nanorod growth via a V–L–S mechanism is favoured by the provision of localized islands of molten Zn, whereas a seed layer of nanosized ZnO crystallites favours growth of a higher density of longer, thinner rods by a V–S mechanism. It is thus tempting to suggest that it may be possible to realize nanorod diameter control either by controlling the catalyst particle size (in the case of V–L–S growth) or the size of the nucleation sites (for V–S growth). The θ -rocking curve measurements show the nanorod arrays grown on the thin ZnO buffer layer to be less well aligned than those grown directly on Si. It is unclear at this stage whether this is a fundamental character of V–L–S growth under our experimental
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Fig. 5. Wavelength dispersed PL spectra (plotted on a common vertical scale) following 325 nm excitation of the ZnO nanorod arrays whose SEM images were shown in Fig. 1(a) and (c).
conditions, or simply a reflection of the different nanorod densities, diameters and lengths. Nanorods grown on both sets of substrates show needle-like morphologies, but there are clear differences in the respective d versus l dependences. We propose the following as a plausible explanation for the different relative longitudinal and transverse growth rates in the two regimes. If the rate of incorporation of fresh ablated material into the capping bead during V–L–S growth is less than the rate at which it feeds material into ZnO nanorod growth, then the size of the capping bead must inevitably decrease with increasing l, and the observed tapered needle-like geometry must inevitably result. Pre-deposition of a thin ZnO film at Tsub less than the melting point of Zn precludes formation of zinc droplets on the Si substrate, and nanorod growth necessarily follows a V–S mechanism. PL spectra of both types of sample are shown in Fig. 5. Both are dominated by a strong, narrow (E ∼ 106 meV), peak centred at ∼380 nm. PL spectra reported from most previous investigations of ZnO nanorod samples show both the 380 nm UV emission feature and a much broader emission band in the green–yellow region. The UV emission arises from the near band-edge transition of wide band gap ZnO [19], while the broad green–yellow emission has been variously attributed to a range of different defect states, including O vacancies, and both O and Zn interstitials [20]. The ratio of the peak UV and green–yellow emission intensities (IUV /IGY ) provides one indication of the material quality. The present samples both show barely detectable green–yellow emission (IUV /IGY > 150), further confirming the high crystallinity of the material contributing to both families of nanorod array. In summary, arrays of well aligned, ultra-thin, needle-like ZnO nanorods have been grown on Si substrates, with and without pre-deposition of a thin ZnO film, by catalyst-free PLD methods using a ZnO target in a low background pressure of O2 . The nanorods grown on these different substrate types show obvious morphological differences. Compared
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with growth on bare Si, pre-deposition of a ZnO thin film is found to encourage growth of longer, thinner nanorods (reflecting the much higher nucleation site density), with no capping particle at their tips. Such observations are rationalised in terms of the different growth mechanisms prevailing in the two cases: V–L–S, in the case of growth on Si at 600 ◦C, compared with a V–S growth mechanism in the case of growth on a predeposited ZnO film. XRD and PL measurements show both arrays to be composed of high quality material, potentially suitable for application in optoelectronic nanodevices. Detailed mechanistic studies of ZnO nanorod growth such suggest routes to realising selective area deposition of small diameter ZnO nanorods of controlled morphology. Acknowledgements The authors are grateful to EPSRC for financial support of this work via the portfolio partnership LASER, to the University of Bristol and the Overseas Research Scholarship (ORS) scheme for a postgraduate scholarship (Ye Sun), and to Drs J. Charmant and S.A. Davis, Prof. D. Cherns, and J.A. Jones and K.N. Rosser for their contributions to the work described herein. References [1] Y.F. Chen, D. Bagnall, T.F. Yao, Mater. Sci. Eng. B 75 (2000) 190. [2] 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. [3] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] J.S. Jie, G.Z. Wang, Y.M. Chen, X.H. Han, Q.T. Wang, B. Xu, J.G. Hou, Appl. Phys. Lett 86 (2005) 031909. [5] H. Ham, G.Z. Shen, J.H. Cho, T.J. Lee, S.H. Seo, C.J. Lee, Chem. Phys. Lett. 404 (2005) 69. [6] L. Vayssieres, K. Keis, S.E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3350. [7] Z.R. Tian, J.A. Voigt, J. Liu, B. McKenzie, M.J. McDermott, M.A. Rodriguez, H. Konishi, H. Xu, Nat. Mater. 2 (2003) 821. [8] S.J. Henley, M.N.R. Ashfold, D.P. Nicholls, P. Wheatley, D. Cherns, Appl. Phys. A 79 (2004) 1169. [9] Y.Q. Liang, C.G. Zhen, D.C. Zou, D.S. Xu, J. Am. Chem. Soc. 126 (2004) 16338 and references therein. [10] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [11] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [12] L.E. Greene, M. Law, D.H. Tan, M. Montano, J. Goldberger, G. Somorjai, P.D. Yang, Nano. Lett. 5 (2005) 1231. [13] L.S. Wang, X.Z. Zhang, S.Q. Zhao, G.Y. Zhou, Y.L. Zhou, J.J. Qi, Appl. Phys. Lett. 86 (2005) 024108. [14] A.B. Hartanto, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299. [15] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2002) 113. [16] H.Y. Dang, J. Wang, S.S. Fan, Nanotechnology 14 (2003) 738. [17] C.Y. Geng, Y. Jiang, Y. Yao, X.M. Meng, J.A. Zapien, C.S. Lee, Y. Lifshitz, S.T. Lee, Adv. Funct. Mater. 14 (2004) 589. [18] J.F. Conley, L. Stecker, Y. Ono, Nanotechnology 16 (2005) 292. [19] S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, H.W. Shim, E.K. Suh, C.J. Lee, Chem. Phys. Lett. 363 (2002) 134. [20] X. Liu, X.H. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (2004) 3141.
Growth mechanisms for ZnO nanorods formed by pulsed laser deposition Ye Sun, Gareth M. Fuge, Michael N.R. Ashfold∗ School of Chemistry, University of Bristol, Bristol BS8 1TS, UK Available online 8 September 2005
Abstract Arrays of well-aligned ZnO nanorods have been grown on Si substrates at 600 ◦ C using a catalystfree pulsed laser deposition (PLD) method, with and without pre-deposition of a thin ZnO layer at a lower substrate temperature. Deposited products were analyzed and characterized by scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray analysis, X-ray diffraction and photoluminescence (PL) measurements. EM revealed that rods grown directly on Si exhibit needle-like morphologies, with diameters, d, typically in the range 20–60 nm, lengths ∼200–800 nm and, in most cases, capping particles of similar cross-section at their tips. HRTEM images show that these nanoparticles are ZnO also, suggesting that these derive from post-growth crystallisation of oxygen rich molten zinc droplets that cap the nanorods during growth. PLD of ZnO onto Si substrates that have been pre-coated with a thin ZnO film deposited at 300 ◦ C yields denser, more uniform arrays of longer (∼1–1.2 µm), thinner (d ∼ 6–20 nm) nanorods, without any obvious capping particle. This suggests that a ZnO buffer layer can play a useful role in providing a high density of nucleation sites for subsequent growth of smaller diameter nanorods. The respective product arrays are most readily understood in terms of vapor–liquid–solid (V–L–S) and vapor–solid (V–S) growth models, respectively. Comparative studies of the room temperature PL from the respective samples reveal much higher UV emission intensity from nanorod arrays grown on the ZnO pre-coated Si substrates. © 2005 Elsevier Ltd. All rights reserved.
∗ Corresponding author. Tel.: +44 (0)117 9288312; fax: +44 (0)117 9250612.
E-mail address: [email protected] (M.N.R. Ashfold). 0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2005.08.029
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1. Introduction ZnO, with its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV at room temperature), has potential in many application areas, including shortwavelength ultraviolet lasers and blue–green optoelectronic devices [1]. ZnO is also noteworthy for the ease with which it forms one-dimensional (1-D) nanostructures (e.g. nanobelts, nanorods, nanowires and nanotubes). The high aspect ratios, size effects and possible quantum confinement effects presented by such nanostructures offer many potential advantages, e.g. improved sensitivity to chemical agents and much enhanced luminescence efficiency. The synthesis of ZnO nanorods has received much attention in the last few years. Many different methods for synthesizing arrays of well-aligned ZnO nanorods have been reported, including chemical vapour transport and condensation (CVTC) [2], thermal evaporation [3–5], hydrothermal methods [6–8], template-based growth [9], chemical vapour deposition [10] and pulsed laser deposition (PLD) [11]. Challenges remain, however. These include, for example, achieving further reductions in rod diameter, improving control of the nanorod morphology, and gaining a deeper understanding of the growth mechanism itself. Synthesis of well-aligned ZnO nanorod arrays by PLD onto a Si substrate maintained at 600 ◦ C has been reported previously [11]. Here we demonstrate ways of growing longer, thinner, higher density arrays of ZnO nanorods on Si by first depositing a thin ZnO film at markedly lower substrate temperatures (Tsub = 300 ◦C). Rod diameters as small as ∼6 nm are observed. Nanorod structures, morphologies and crystallinities grown under these two different deposition conditions are compared and contrasted, with a view to unraveling mechanistic details of the prevailing growth regimes, and the role of the pre-deposited thin template film of ZnO. Studies of the photoluminescence (PL) of both types of nanorod array are reported also. 2. Experimental The PLD apparatus and the experimental methods have both been described in detail elsewhere [8]. The PLD chamber was evacuated to a typical base pressure of 1 × 10−6 Torr using a turbomolecular pump. The output of an ArF excimer laser (Lambda-Physik COMPex 201, 193 nm) operating at a pulse repetition rate of 10 Hz was focused onto a rotating ZnO target (Cerac, 99.999% purity) at 45◦ to the surface normal. The fluence incident on the target was estimated to be in the range 6–10 J cm−2 . The Si substrate (mounted as the front part of a Si/Cu/Si sandwich structure, the rear layers of which act as an effective heat-spreader) was located 40–60 mm from the target, with its front face perpendicular to the target surface normal. The substrate temperature could be adjusted to any user selected value in the range 20 ≤ Tsub ≤ 750 ◦C by controlling the output of a cw CO2 laser (Synrad, model: J48-2W-3180) aligned so as to illuminate the rear surface of the heat-spreader. All samples were grown in the low background pressure of oxygen (p(O2 ) = 10−2 Torr, 99.99% stated purity, flowing at 10 sccm). A typical deposition run lasted for t = 45 min. (27 000 laser shots). Two families of samples were deposited in these experiments. The first set was grown directly on pre-cleaned pieces of Si(100) wafer maintained at 600 ◦C. The second family were grown by first depositing a thin ZnO film
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Fig. 1. (a) SEM image (viewing at a tilt angle of ∼40◦ ) of a nanorod array grown by PLD onto a Si wafer at Tsub = 600 ◦ C for t = 45 min. (b) TEM image of a selection of rods broken from this sample. (c) A tilt-view SEM image of a nanorod array grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min. (d) TEM image of selected nanorods from this sample.
by PLD on a Si substrate at Tsub = 300 ◦C for t300 = 1 min, then boosting Tsub to 600 ◦C before continuing deposition for a further t600 = 44 min. Deposited material was analysed and characterized by scanning electron microscopy (SEM, using a JEOL JSM 5600LV instrument), high-resolution transmission electron microscopy (HRTEM, JEOL 2100EX), X-ray diffraction (XRD, Bruker AXS D8 Advance powder X-ray diffractometer with Cu Kα radiation), and electron dispersive X-ray analysis (EDX). Photoluminescence spectra were measured at room temperature using a He–Cd laser (325 nm, output power ∼3 mW) as the excitation source. 3. Results and discussion Fig. 1(a) and (b) show, respectively, a SEM image (viewing at a tilt angle of ∼40◦ ) of a nanorod array grown by PLD onto a Si wafer at Tsub = 600 ◦C for t = 45 min, and a TEM image from a selection of rods broken from this sample. The sample is composed of needle-like nanorods, with lengths l in the range 200–800 nm. The root diameters of the nanorods are fairly uniform (droot ∼ 60 nm) but the top diameters (dtop) appear to decrease with increasing rod length and, as Fig. 1(b) shows, dtop can be as small as 20 nm in the case
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Fig. 2. EDX spectrum from a sample of nanorods grown for t600 = 45 min on a bare Si substrate.
of the longest rods. Nearly every nanorod examined from this sample exhibits a capping bead-like nanoparticle. In all cases, the dimension of the nanoparticle is comparable to dtop of the nanorod it caps; since the longer rods are generally thinner at their tips, the capping particle size shows an anti-correlation with the rod length. The enlarged TEM image of the tip of one nanorod shown as an inset to Fig. 1(b) reveals an apparent boundary between the capping particle and the main shaft of the rod, suggesting that the particle and the main shaft are probably not parts of the same single crystal. The use of a pre-deposited ZnO layer as a possible way of supplying nucleation sites for the subsequent epitaxial growth of ZnO nanorods is attracting much current interest [4, 7,12,13]. Here we demonstrate that nanorod arrays grown subsequent to the introduction of a thin ZnO buffer layer can exhibit very different morphology. Fig. 1(c) and (d) show a corresponding tilt-view SEM image of an extensive nanorod array and a TEM image of selected nanorods grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min. This procedure results in a much higher density of longer, thinner nanorods, most of which have l ∼ 1–1.2 µm, droot ∼ 20 nm and dtop as small as 6–10 nm. The TEM image shown in the inset within Fig. 1(d) illustrates another striking difference between these nanorods and those grown without the ZnO buffer layer: the latter rods are flat topped, with no evident capping particle. Both types of nanorod display needle-like morphologies, consistent with the rate of 1-D (longitudinal) growth rate far exceeding that of any growth transverse to the long axis. However, the very different droot and dtop values for the two kinds of nanorods, and the more gradual variation of d with l in the case of rods grown on the pre-deposited ZnO thin film, suggest that this cannot be the whole story. Energy dispersive X-ray (EDX) analysis (in the SEM) was used to test the composition of both families of nanorods; the spectra obtained from the two samples were indistinguishable. Fig. 2 shows one such spectrum, for the case of nanorods grown for t600 = 45 min on a bare Si substrate. Zinc and oxygen are the only detectable elements
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Fig. 3. (a) and (b): 2θ scans of the samples imaged by SEM in Fig. 1(a) and (c), respectively. The associated θ -rocking curves of the respective (0002) peaks are shown to the right, with their FWHM indicated, in degrees.
(apart from a Si peak from the underlying substrate), supporting the view that no other metal elements are aiding in catalyzing the observed nanorod growth. Thus we conclude that the rods and, in this case, the capping particles, must be ZnO or Zn. The crystal structure and the orientation of the ZnO nanorod arrays were investigated by XRD. Fig. 3(a) and (b) show 2θ scans of the same samples as those imaged by SEM in Fig. 1(a) and (c). Each of the displayed XRD spectra is totally dominated by the strong ZnO(002) reflection at 34.4◦. The simplicity of these XRD spectra confirms that both nanorod arrays are comprised of strongly c-axis aligned, crystalline ZnO. θ -rocking curve measurements allow further quantification of the alignment of the nanorod samples. As the right hand panels in Fig. 3 show, the θ -rocking curve of the nanorod array grown on bare Si displays a smaller FWHM value, implying that it has somewhat better overall alignment than the array of longer, thinner nanorods grown on the thin ZnO film. Fig. 4 displays three HRTEM images. Fig. 4(a) and (b) were taken from, respectively, the main shaft of a single nanorod grown on a bare Si substrate and from within its capping particle. Parallel crystal planes, aligned normal to the growth direction, are clearly evident in Fig. 4(a); the measured plane spacing (∼0.52 nm) is characteristic of wurtzite ZnO(001). The HRTEM image of a region within the capping particle (Fig. 4(b)) reveals two interleaved crystal planes, both with inter-plane spacings of ∼0.245 nm and with an intersection angle of ∼100◦. These match well with literature values for planes such ¯ in ZnO (for which the lattice parameters are 0.2471 nm, and the as (101) and (011) inter-plane angle 99.25◦), but show no obvious match with any documented Zn crystal structures. Thus we conclude that the capping particles are also ZnO. All of the growth of these samples occurs at a temperature (600 ◦C) that is substantially above the melting point of Zn (419 ◦C). Any nanosized zinc particles that exist during the growth process should thus be in the liquid phase, whereas any nanosized ZnO particles present under these conditions are more likely to merge together to form bigger crystallites [14]. The
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Fig. 4. HRTEM images from: (a) the main shaft of a single nanorod grown at Tsub = 600 ◦ C for t = 45 min on a bare Si substrate; (b) within the capping particle of the nanorod shown in (a), and (c) the tip of a nanorod grown by PLD for t600 = 44 min on top of a ZnO buffer layer deposited for t300 = 1 min.
present observations regarding nanorod growth on a bare Si substrate at 600 ◦C can thus be rationalised as follows [11]: Zn and O atoms, and small metallic and oxide recombination products are transported to the substrate during the PLD process. Some are accommodated, and some post-deposition processing will occur by interaction with the background O2 at the prevailing Tsub. We propose that Zn nanodroplets formed in the earliest stages of deposition serve to nucleate the nanorod growth. Subsequent 1-D growth is fuelled by the oxygenated capping bead of molten Zn, i.e. a classic vapour–liquid–solid (V–L–S) growth mechanism [15]. Each deposition run is ended by switching off the PLD laser, then the CO2 heating laser, and allowing the sample to cool in the flowing background pressure of O2 . The capping particles are assumed to crystallize as ZnO during this postdeposition cooling stage. Broadly similar mechanisms have recently been proposed for ZnO rod growth by heating powdered metallic Zn [16] and by CVTC methods [17]. SEM images of nanorods grown on the thin ZnO template film revealed no capping particle—a view confirmed by the HRTEM image of the very top of one such rod shown in Fig. 4(c). This image clearly shows parallel crystal planes extending to the very tip of the rod. The measured plane spacing is 0.26 nm (characteristic of ZnO (002)), and entirely consistent with the view that the nanorod is one single crystal that has grown along the [001] direction. Such observations strongly suggest that, under these conditions, nanorod growth involves a vapour–solid (V–S) mechanism [18]. The experimental results provide a number of insights into the likely growth mechanism and the way in which a thin ZnO buffer layer can influence nanorod growth. We deduce that nanorod growth via a V–L–S mechanism is favoured by the provision of localized islands of molten Zn, whereas a seed layer of nanosized ZnO crystallites favours growth of a higher density of longer, thinner rods by a V–S mechanism. It is thus tempting to suggest that it may be possible to realize nanorod diameter control either by controlling the catalyst particle size (in the case of V–L–S growth) or the size of the nucleation sites (for V–S growth). The θ -rocking curve measurements show the nanorod arrays grown on the thin ZnO buffer layer to be less well aligned than those grown directly on Si. It is unclear at this stage whether this is a fundamental character of V–L–S growth under our experimental
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Fig. 5. Wavelength dispersed PL spectra (plotted on a common vertical scale) following 325 nm excitation of the ZnO nanorod arrays whose SEM images were shown in Fig. 1(a) and (c).
conditions, or simply a reflection of the different nanorod densities, diameters and lengths. Nanorods grown on both sets of substrates show needle-like morphologies, but there are clear differences in the respective d versus l dependences. We propose the following as a plausible explanation for the different relative longitudinal and transverse growth rates in the two regimes. If the rate of incorporation of fresh ablated material into the capping bead during V–L–S growth is less than the rate at which it feeds material into ZnO nanorod growth, then the size of the capping bead must inevitably decrease with increasing l, and the observed tapered needle-like geometry must inevitably result. Pre-deposition of a thin ZnO film at Tsub less than the melting point of Zn precludes formation of zinc droplets on the Si substrate, and nanorod growth necessarily follows a V–S mechanism. PL spectra of both types of sample are shown in Fig. 5. Both are dominated by a strong, narrow (E ∼ 106 meV), peak centred at ∼380 nm. PL spectra reported from most previous investigations of ZnO nanorod samples show both the 380 nm UV emission feature and a much broader emission band in the green–yellow region. The UV emission arises from the near band-edge transition of wide band gap ZnO [19], while the broad green–yellow emission has been variously attributed to a range of different defect states, including O vacancies, and both O and Zn interstitials [20]. The ratio of the peak UV and green–yellow emission intensities (IUV /IGY ) provides one indication of the material quality. The present samples both show barely detectable green–yellow emission (IUV /IGY > 150), further confirming the high crystallinity of the material contributing to both families of nanorod array. In summary, arrays of well aligned, ultra-thin, needle-like ZnO nanorods have been grown on Si substrates, with and without pre-deposition of a thin ZnO film, by catalyst-free PLD methods using a ZnO target in a low background pressure of O2 . The nanorods grown on these different substrate types show obvious morphological differences. Compared
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with growth on bare Si, pre-deposition of a ZnO thin film is found to encourage growth of longer, thinner nanorods (reflecting the much higher nucleation site density), with no capping particle at their tips. Such observations are rationalised in terms of the different growth mechanisms prevailing in the two cases: V–L–S, in the case of growth on Si at 600 ◦C, compared with a V–S growth mechanism in the case of growth on a predeposited ZnO film. XRD and PL measurements show both arrays to be composed of high quality material, potentially suitable for application in optoelectronic nanodevices. Detailed mechanistic studies of ZnO nanorod growth such suggest routes to realising selective area deposition of small diameter ZnO nanorods of controlled morphology. Acknowledgements The authors are grateful to EPSRC for financial support of this work via the portfolio partnership LASER, to the University of Bristol and the Overseas Research Scholarship (ORS) scheme for a postgraduate scholarship (Ye Sun), and to Drs J. Charmant and S.A. Davis, Prof. D. Cherns, and J.A. Jones and K.N. Rosser for their contributions to the work described herein. References [1] Y.F. Chen, D. Bagnall, T.F. Yao, Mater. Sci. Eng. B 75 (2000) 190. [2] 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. [3] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [4] J.S. Jie, G.Z. Wang, Y.M. Chen, X.H. Han, Q.T. Wang, B. Xu, J.G. Hou, Appl. Phys. Lett 86 (2005) 031909. [5] H. Ham, G.Z. Shen, J.H. Cho, T.J. Lee, S.H. Seo, C.J. Lee, Chem. Phys. Lett. 404 (2005) 69. [6] L. Vayssieres, K. Keis, S.E. Lindquist, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3350. [7] Z.R. Tian, J.A. Voigt, J. Liu, B. McKenzie, M.J. McDermott, M.A. Rodriguez, H. Konishi, H. Xu, Nat. Mater. 2 (2003) 821. [8] S.J. Henley, M.N.R. Ashfold, D.P. Nicholls, P. Wheatley, D. Cherns, Appl. Phys. A 79 (2004) 1169. [9] Y.Q. Liang, C.G. Zhen, D.C. Zou, D.S. Xu, J. Am. Chem. Soc. 126 (2004) 16338 and references therein. [10] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [11] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [12] L.E. Greene, M. Law, D.H. Tan, M. Montano, J. Goldberger, G. Somorjai, P.D. Yang, Nano. Lett. 5 (2005) 1231. [13] L.S. Wang, X.Z. Zhang, S.Q. Zhao, G.Y. Zhou, Y.L. Zhou, J.J. Qi, Appl. Phys. Lett. 86 (2005) 024108. [14] A.B. Hartanto, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299. [15] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2002) 113. [16] H.Y. Dang, J. Wang, S.S. Fan, Nanotechnology 14 (2003) 738. [17] C.Y. Geng, Y. Jiang, Y. Yao, X.M. Meng, J.A. Zapien, C.S. Lee, Y. Lifshitz, S.T. Lee, Adv. Funct. Mater. 14 (2004) 589. [18] J.F. Conley, L. Stecker, Y. Ono, Nanotechnology 16 (2005) 292. [19] S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, H.W. Shim, E.K. Suh, C.J. Lee, Chem. Phys. Lett. 363 (2002) 134. [20] X. Liu, X.H. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (2004) 3141.