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Toward a Single ZnO Nanowire Homojunction Ye Sun,*,†,§ Neil A. Fox,‡ Gareth M. Fuge,‡ and Michael N. R. Ashfold*,‡ Department of Physics, Harbin Institute of Technology, Harbin, China, 150001, and School of Chemistry, UniVersity of Bristol, Bristol, U.K., BS8 1TS ReceiVed: September 23, 2010; ReVised Manuscript ReceiVed: NoVember 5, 2010
ZnO nanowires were synthesized by wet chemical methods on various ZnO templates, including commercial ZnO phosphor microparticles, [002]-textured ZnO thin films, and well-aligned ZnO nanorod arrays with c-axis preferred orientation. In all cases, the resulting nanowires were observed to grow on the (002) surfaces of the ZnO templates, along the [002] direction. A dissolution/recrystallization growth mechanism is proposed for the nanowires produced in this experiment. This improved understanding of ZnO nanowire growth has enabled demonstration of a route to forming ZnO homojunctions, using wet chemical methods to extend the lengths of individual pre-existing nanorods grown by pulsed laser deposition. 1. Introduction The remarkable progress in nanoscience and nanotechnology has stimulated worldwide interest in the fabrication of nanodevices based on novel one-dimensional (1-D) nanomaterials.1-5 ZnO, a wide band-gap semiconductor, is widely touted as one of the most promising functional materials with many attractive properties and a large family of demonstrated 1-D nanostructures, including nanowires (NWs), nanorods (NRs), nanobelts, and nanotubes.6-14 The formation of heterojunctions and homojunctions of ZnO will be critical for the fabrication of p-n junction devices based on ZnO nanomaterials. n-Type ZnO NRs/ NWs have already been synthesized on p-type substrates/films to form heterojunctions or homojunctions, thereby enabling the fabrication of prototype nanodevices.15-17 For example, electroluminescence (EL) devices based on heteroepitaxial growth of vertically aligned n-type ZnO nanorods on p-type GaN thin films15 and ZnO homojunction light-emitting diodes formed by growing a quasi-array of (unintentionally doped) n-type ZnO NWs on an In-N codoped p-type ZnO film have been reported.16 The fabrication of single NW p-n junctions has aroused intense recent interest because these could be integrated into electronics as NW diodes (nanodiodes) and/or find application in a range of nanodevices. Single NW p-n junctions of Si, GaN, and InP have been reported,18-20 but reports of successful fabrication of p-n junctions inside or between individual ZnO NWs are rare.21 Progress has been hampered by the scarcity of high-quality, reproducible p-type ZnO nanomaterials. There has been much progress with regard to growth of p-type ZnO thin films and nanomaterials, however. p-Type ZnO thin films,22-24 NRs,25,26 and NW arrays27 have been produced, by a variety of growth methods, and the time is now ripe to consider the formation of homojunctions between individual ZnO NWs. Here, we report careful studies of the hydrothermal growth of ZnO NWs on P15 (ZnO-based) phosphor particles, on pre* To whom correspondence should be addressed. E-mail: yesun@ chem.au.dk (Y.S.),
[email protected] (M.N.R.A.). Tel: (+44) 1179288312 (M.N.R.A.). † Harbin Institute of Technology. ‡ University of Bristol. § Current address: Centre for Energy Materials, Department of Chemistry and iNANO, University of Aarhus, DK-8000 Aarhus C, Denmark.
existing ZnO thin films, and on pre-existing arrays of ZnO NRs, which reveal details of the growth mechanism and allow exploration of the influence of the various templates on ZnO NW growth. The investigation culminates in our demonstrating a route to forming single ZnO NR homojunctions by hydrothermal extension of pre-existing NRs grown by pulsed laser deposition (PLD). 2. Experimental Section Three different ZnO templates were employed in this work for seeding the hydrothermal growth of ZnO NWs: (1) Commercial ZnO:Zn P15-GG phosphor particles (Phosphor Technology Ltd., U.K.). After ultrasonic treatment, an aqueous suspension of the phosphor particles was poured onto 1 cm2 sections of Si wafers contained in a Petri dish. Drying in air at 80 °C (in a laboratory oven) yielded phosphor-particlecoated Si substrates. (2) [002]-textured ZnO thin films produced on Si wafers by PLD, using an apparatus that has been described previously.9 These films were grown by ablation of a ZnO target (Cerac, 99.999% purity) and deposited at substrate temperatures of 100 e Tsub e 300 °C for times in the range of 3 e tPLD e 15 min in a background pressure of O2 (p(O2) = 10 mTorr). The crystal structure and orientation of the as-grown films were examined by X-ray diffraction (XRD, Bruker AXS D8 Advance powder diffractometer with Cu KR radiation). (3) Well-aligned ZnO NR arrays with c-axis preferred orientation, also produced by PLD using Tsub ) 600 °C, p(O2) = 10 mTorr, and a growth time of tPLD ) 45 min. Scanning electron microscope (SEM) images, XRD patterns, and photoluminescence (PL) spectra of the PLD ZnO NR arrays have been presented previously.9,28 For the hydrothermal growth of ZnO NWs, 50 cm3 aliquots of zinc nitrate (Zn(NO3)2, 0.001-0.004 mol dm-3, Alfa Aesar, 99%) and hexamethylenetetramine (HMT, also termed methenamine, 0.001-0.004 mol dm-3, Alfa Aesar, 99+%) solutions were prepared with deionized water and stored in separate sealed Schott bottles (volume, V ) 135 cm3) that were then placed in an oil bath maintained at T ) 90.0 ( 0.3 °C. After reaching thermal equilibrium, the two solutions were mixed and the ZnOcoated Si wafer immediately inserted into the resulting mixture. As before, the Si substrate was attached to the underside of a
10.1021/jp109108f 2010 American Chemical Society Published on Web 11/23/2010
Toward a Single ZnO Nanowire Homojunction
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21339 after immersion for 3 h. The particle morphologies have obviously changed; most now present “hexagonal-like” morphologies with identifiable polar surfaces, indicating that some dissolution and/or recrystallization has occurred. The evolving morphology of these micrometer-sized ZnO particles implies that some ZnO crystal faces are unstable in the reactive solution, consistent with many previous observations that ZnO adopts a hexagonal-like morphology under hydrothermal growth conditions. Some small nanostructures are evident on the polar faces in Figure 1b, whereas the orthogonal faces are flat and clean. Extending the growth time to 24 h leads to the formation of many ZnO NWs on the phosphor particles, with diameters, d, in the range of 20-250 nm and lengths, l, of up to 1.5 µm, as shown in Figure 1c,d. Most of these NWs are observed to grow vertically from the polar surfaces of the phosphor particles, indicating that they grow along the [002] direction. Key reactions in the present wet chemical growth of ZnO nanostructures are the thermal decomposition of HMT to formaldehyde and ammonia (with the latter acting as a base in aqueous solution)
Figure 1. SEM images of commercial ZnO-based P15 phosphor particles: (a) as supplied, (b) after immersion in the reactive solution for t ) 3 h, and (c, d) after serving as a template in the reactive solution for t ) 24 h. With increasing time, the particle morphology is seen to evolve from irregular, to hexagonal-like particles, to hexagonal-like particles with ZnO NWs sprouting from the polar surfaces.
glass slide (with its ZnO-coated side facing downward) and the slide tilted at ∼80° to locate the substrate in the center of the reactive solution and to protect it from any precipitation. The solution was kept sealed and maintained at T ) 90 °C throughout the entirety of the reaction (t e 24 h). After the growth process, the samples were rinsed rigorously with water and then dried in air in a laboratory oven at 80 °C. The asdeposited products were characterized and analyzed by SEM (JEOL 6300LV) and transmission electron microscopy (TEM, JEOL 1200EX). 3. Results and Discussion We have previously reported the formation of ZnO NW arrays on PLD-grown ZnO buffer layers using wet chemical methods and low precursor concentrations.29 The resulting ZnO NWs were characterized by SEM and TEM and their PL properties studied. The earlier work demonstrated that the ZnO buffer layer served not just as a template to guide ZnO NW growth but was actually critical for both the growth and the alignment of the NWs. No ZnO nanostructures were produced under equivalent hydrothermal growth conditions on a bare Si wafer or on borosilicate glass or Pt-coated glass substrates. The method thus offers a straightforward route for the selective area deposition of ZnO NW arrays, simply by use of a suitably patterned ZnO template layer. Here, we report the use of similar wet chemical methods to form ZnO NWs on three different ZnO templates, focusing particularly on the NW properties and their growth mechanism. 3.1. Growth on Phosphor Particles. The SEM image of the P15 ZnO(Zn) phosphor sample shown in Figure 1a reveals micrometer-sized particles, presenting a variety of surface crystal planes. A layer of such phosphor particles was used as a template for subsequent hydrothermal growth of ZnO from an active solution formed by mixing 0.004 M solutions of HMT and Zn(NO3)2. Figure 1b shows an SEM image of such a sample
C6H12N4 + 10H2O T 6HCHO + 4NH4+ + 4OH-
(1) and the precipitation of Zn2+ ions
Zn2+ + 2OH- T ZnO + H2O
(2)
Zn2+ + 2OH- T Zn(OH)2 T ZnO + H2O
(3)
or
Equations 2 and 3 provide a rational explanation for the observed morphological evolution of the phosphor particles and the growth of c-axis aligned NWs on their polar surfaces: ZnO has dissolved from unstable crystal planes of the phosphor particles and grown, as NWs, along the [002] direction.30,31 The mechanisms and habits of ZnO crystals grown by hydrothermal methods have been investigated extensively, by many groups; dissolution/recrystallization and oriented aggregation are the most frequently proposed growth mechanisms.32,33 The SEM images presented in Figure 1 indicate that the present NW growth on ZnO phosphor particles involves dissolution/recrystallization and that, under these conditions, hexagonal ZnO is the most stable crystal habit and NWs grow on the polar surfaces of the ZnO template particles only, and along the [002] direction. Studies involving micrometer-sized ZnO phosphor particles can help in understanding the growth mechanism of ZnO NWs under the present experimental conditions, but the dissolution of these irregularly faceted and defective ZnO particles will act as a continuous source of zinc ions into the reactive solution near the substrate surface, which might well influence the subsequent ZnO NW growth. Dissolution can be expected to reveal new sites for nucleating NW growth, but at random times during the overall immersion period, resulting in formation of NWs with a broad distribution of l and d (as shown in Figure 1d). Such behavior should be contrasted with that observed when using PLD-grown [002]-textured ZnO thin film templates. Dissolution is much less in this case, and all NWs nucleate at similar (early) times, leading to uniform arrays of ZnO NWs. As part of the present study of the growth properties of ZnO
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Figure 2. XRD patterns of PLD-ZnO films grown at Tsub ) 300 °C for tPLD ) 3 and 15 min. Both spectra are dominated by the ZnO(002) peak. The small peak at 2θ = 33.1° (marked with an asterisk) is associated with the Si substrate.
Figure 3. Top-view and 30° tilt-view SEM images of NW samples grown on PLD-ZnO thin film templates from a solution of 0.002 M Zn(NO3)2 and HMT for t ) 3 h: tPLD ) 3 min (a, b) and 15 min (c, d). The scale bar applies to all four images.
NWs, we investigated the effect of the growth time of the ZnO thin film template on the subsequent wet chemical growth of ZnO NWs. 3.2. Growth on [002]-Textured ZnO Films. ZnO films produced by PLD at Tsub ) 300 °C for tPLD ) 3 and 15 min were used as templates for the subsequent wet chemical growth of ZnO NWs. Figure 2 shows XRD patterns of the respective PLD films. Apart from a weak peak attributable to the Si substrate, the ZnO(002) peak dominates both spectra, consistent with formation of [002]-textured ZnO film templates. The ZnO(002) peak clearly increases in intensity with increasing deposition time, whereas the full width at half-maximum (fwhm) of the ZnO(002) peak declines, from 0.39° (tPLD ) 3 min) to 0.16° (tPLD ) 15 min), all of which data imply that the crystal quality of, and average crystallite size within, the ZnO template film increases with tPLD. Figure 3a,b shows top-view and 30° tilt-view SEM images of a sample grown on a PLD-ZnO (tPLD ) 3 min) film in an active solution containing 0.002 M Zn(NO3)2 and HMT for t )
Sun et al. 3 h. The resulting NWs have relatively uniform lengths (l = 500 nm) and diameters (d ) 20-50 nm) and display a density of ∼5.0 × 109 cm-2. All of the NWs grew vertically from the template layer, resulting in a well-aligned NW array. SEM images of the corresponding sample grown on a PLD-ZnO (tPLD ) 15 min) template are shown in Figure 3c,d. The NW density in the latter samples is about twice that found with the PLDZnO (tPLD ) 3 min) template. HRTEM confirms that the NWs have grown along the [002] direction.29 Thus, the studies of NW growth on PLD-ZnO film templates reinforce the foregoing discussion that, at least under the present wet chemical conditions, ZnO NWs only grow on polar surfaces of the ZnO template and only along the [002] direction. The PLD-ZnO template films used in these studies supply nucleation sites for the subsequent growth of ZnO NWs. Template films formed under different PLD conditions could exhibit different crystal quality, orientation, and/or crystalline sizes, and thus present a range of nucleation conditions for the subsequent wet chemical growth of ZnO NWs. Thus, for example, in addition to tPLD, Tsub is also found to influence the eventual ZnO NW density. An ability to control the density (and thus, conceivably, the field enhancement factor) of wellaligned ZnO NW arrays could be relevant in selected field emission applications. 3.3. Growth on PLD Nanorods. Having demonstrated that thin film PLD-ZnO templates can be used to control the subsequent wet chemical growth of ZnO NWs, it is rational to assume that we could also control NW growth using a template consisting of an array of PLD-ZnO NRs. The hexagonal NRs involved in these arrays are of sufficient crystal quality that they should be stable in the mixture used for subsequent NW growth, and we should anticipate NW growth only on the top polar surfaces of the NRs. We have previously observed wet chemical growth of multiple NWs from the polar surface of large ZnO NRs (d = 100-500 nm) grown on Si in an active solution formed by mixing 0.1 M solutions of Zn(NO3)2 and HMT. We now demonstrate that, by reducing the diameters of the template NRs to ∼50 nm (i.e., similar to those of the NWs grown with the present wet chemical conditions), it is possible to form just one ZnO NW on a single PLD-ZnO template NR and thus realize the formation of a single ZnO NW homojunction. The growth of well-aligned PLD-ZnO NR arrays along the [002] direction, on Si substrates, has been reported previously.9,28 Figure 4a shows a top-view SEM image of a PLD-ZnO NR sample produced at Tsub ) 600 °C for tPLD ) 45 min. The asgrown NRs have hexagonal morphologies (shown in the inset of Figure 4a), l = 300 nm, and d = 50 nm. Such PLD-ZnO NR arrays were then used as a template for the subsequent hydrothermal growth of ZnO NWs from a 0.002 M solution of Zn(NO3)2 and HMT. Figure 4b,c shows top- and 30° tilt-view SEM images of samples formed after wet chemical growth for t ) 3 h. NWs have very obviously grown on the top of many of the PLD-ZnO NRs; the extensions have grown parallel to the template NRs, extending their lengths to l = 500-600 nm, but maintaining the diameters at d = 20-50 nm. The similarity in the diameters of the newly grown NWs and the template NRs ensures that a single NW forms on a single template NR. Some ZnO NW-on-NR samples were scratched into ethanol solution and then dropped onto a TEM grid for observation; Figure 4d shows a TEM image from such a sample. Two NW homojunctions where a single NW has grown epitaxially on a single NR are clearly observable. The diameters of the wet chemical NWs (which present in lighter gray) are measured as
Toward a Single ZnO Nanowire Homojunction
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21341 Riley, K. N. Rosser, and J. A. Smith and for their many and varied contributions to the work described herein. References and Notes
Figure 4. (a) Top-view SEM image of well-aligned NR arrays produced by PLD at Tsub ) 600 °C for tPLD ) 45 min. (b) Top-view and (c) 30° tilt-view SEM images of ZnO NWs grown on the PLDZnO NR array from a solution of 0.002 M Zn(NO3)2 and HMT for t ) 3 h. (d) TEM image of a selection of NW-on-NRs. The inset in (a) illustrates the hexagonal top faces of the PLD-ZnO NRs.
30-40 nm, just a little smaller than those of the PLD NRs (which appear as a darker gray). ZnO typically exhibits n-type conductivity due to intrinsic defects, such as oxygen vacancies. The difficulty of producing p-type material is one factor hampering fabrication of a ZnO p-n junction. Growth of p-type ZnO NRs has now been reported by a number of groups,25-27 and PLD (and several other growth techniques) have been used to explore the growth of p-type ZnO films.22,23,34-37 It should, therefore, be practical to form p-type ZnO NRs that can then serve as a template to guide the subsequent growth of n-type ZnO NWs and thereby fabricate ZnO single NW p-n junctions. Such single NW p-n junctions could find potential application in nanosized devices (e.g., photodetectors, gas sensors, or light-emitting diodes). The current study demonstrates a strategy for forming ZnO NW homojunctions by controlling NW growth on a single NR. Further studies aimed at improving the growth of ZnO NW homojunctions and, eventually, growing and characterizing single NW p-n junctions are planned. 4. Conclusion The influence of different ZnO templates (commercial ZnO phosphor microparticles, [002]-textured ZnO thin films, and well-aligned ZnO NR arrays with c-axis preferred orientation) on the hydrothermal growth of ZnO NWs has been studied carefully, yielding new insights into the growth mechanism and growth behavior of such NWs. The NWs are shown to grow only from the (002) surfaces of the ZnO templates, along the [002] direction. Using thin ZnO NRs as a template, we have demonstrated the growth of a single ZnO NW on top of a single ZnO NR, yielding a single ZnO NW homojunction. The present study suggests a route to fabricating single NW p-n junctions, which could find application in future nanodevices. Acknowledgment. The authors are most grateful to EPSRC, for financial support, and to D. Cherns, R. P. Doherty, D. J.
(1) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (2) Bao, J. M.; Zimmler, M. A.; Capasso, F.; Wang, X. W.; Ren, Z. F. Nano Lett. 2006, 6, 1719. (3) Chaste, J.; Lechner, L.; Morfin, P.; Feve, G.; Kontos, T.; Berroir, J. M.; Glattli, D. C.; Happy, H.; Hakonen, P.; Placais, B. Nano Lett. 2008, 8, 525. (4) Fei, P.; Yeh, P. H.; Zhou, J.; Xu, S.; Gao, Y. F.; Song, J. H.; Gu, Y. D.; Huang, Y. Y.; Wang, Z. L. Nano Lett. 2009, 9, 3435. (5) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (6) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (7) Pan, Z. X.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Mensah, S. L.; Kayastha, V. K.; Yap, Y. K. J. Phys. Chem. C 2007, 111, 16092. (9) Sun, Y.; Fuge, G. M.; Ashfold, M. N. R. Chem. Phys. Lett. 2004, 396, 21. (10) Vayssieres, L. AdV. Mater. 2003, 15, 464. (11) Sun, Y.; Fuge, G. M.; Fox, N. A.; Riley, D. J.; Ashfold, M. N. R. AdV. Mater. 2005, 17, 2477. (12) Weintraub, B.; Deng, Y. L.; Wang, Z. L. J. Phys. Chem. C 2007, 111, 10162. (13) Pal, U.; Santiago, P. J. Phys. Chem. B 2005, 109, 15317. (14) Wang, M.; Ye, C. H.; Zhang, Y.; Wang, H. X.; Zeng, X. Y.; Zhang, L. D. J. Mater. Sci.: Mater. Electron. 2008, 19, 211. (15) Park, W. I.; Yi, G. C. AdV. Mater. 2004, 16, 87. (16) Sun, H.; Zhang, G.; Zhang, J.; Deng, T.; Wu, B. J. Appl. Phys. 2008, 90, 543. (17) Chen, C.-H.; Chang, S.-J.; Chang, S.-P.; Li, M.-J.; Chen, I.-C.; Hsueh, T.-J.; Hsu, A.-D.; Hsu, C.-L. J. Phys. Chem. C 2010, 114, 12422. (18) Rangineni, Y.; Qi, C.; Goncher, G.; Solanki, R.; Langworthy, K. J. Nanosci. Nanotechnol. 2008, 8, 2419. (19) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (20) Cheng, G. S.; Kolmakov, A.; Zhang, Y. X.; Moskovits, M.; Munden, R.; Reed, M. A.; Wang, G. M.; Moses, D.; Zhang, J. P. Appl. Phys. Lett. 2003, 83, 1578. (21) Pradhan, B.; Batabyal, S. K.; Pal, A. J. Appl. Phys. Lett. 2006, 89, 233109. (22) Look, D. C.; Reynolds, D. C.; Litton, C. W.; Jones, R. L.; Eason, D. B.; Cantwell, G. Appl. Phys. Lett. 2002, 81, 1830. (23) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42. (24) Tay, C. B.; Chua, S. J.; Loh, K. P. J. Phys. Chem. C 2010, 114, 9981. (25) Hsu, Y. F.; Xi, Y. Y.; Tam, K. H.; Djurisic, A. B.; Luo, J.; Ling, C. C.; Cheung, C. K.; Ng, A. M. C.; Chan, W. K.; Deng, X.; Beling, C. D.; Fung, S.; Cheah, K. W.; Fong, P. W. K.; Surya, C. C. AdV. Funct. Mater. 2008, 18, 1020. (26) Fang, X.; Li, J. H.; Zhao, D. X.; Shen, D. Z.; Li, B. H.; Wang, X. H. J. Phys. Chem. C 2009, 113, 21208. (27) Xiang, B.; Wang, P.; Zhang, X.; Sayeh, S. A.; Aplin, D. P. R.; Soci, C.; Yu, D.; Wang, D. Nano Lett. 2007, 7, 323. (28) Sun, Y.; Doherty, R. P.; Warren, J. L.; Ashfold, M. N. R. Chem. Phys. Lett. 2007, 447, 257. (29) Sun, Y.; Ndifor-Angwafor, N. G.; Riley, D. J.; Ashfold, M. N. R. Chem. Phys. Lett. 2006, 431, 52. (30) Ashfold, M. N. R.; Doherty, R. P.; Ndifor-Angwafor, N. G.; Riley, D. J.; Sun, Y. Thin Solid Films 2007, 515, 8679. (31) Cho, S.; Jung, S. H.; Lee, K. H. J. Phys. Chem. C 2008, 112, 12769. (32) Govender, K.; Boyle, D. S.; Kenway, P. B.; O’Brien, P. J. Mater. Chem. 2004, 14, 2575. (33) Oliveira, A. P. A.; Hochepied, J.-F.; Grillon, F.; Berger, M.-H. Chem. Mater. 2003, 15, 3202. (34) Kim, H.; Cepler, A.; Cetina, C.; Knies, D.; Osofsky, M. S.; Auyeung, R. C. Y.; Pique, A. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 593. (35) Ohshima, T.; Ikegami, T.; Ebihara, K.; Asmussen, J.; Thareja, R. Thin Solid Films 2003, 435, 49. (36) Wang, B.; Min, J.; Zhao, Y.; Sang, W.; Wang, C. Appl. Phys. Lett. 2009, 94, 192101. (37) Nam, K. H.; Kim, H.; Lee, H. Y.; Han, D. H.; Lee, J. Surf. Coat. Technol. 2008, 202, 5463.
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