Three-Dimensional Germanium Oxide Nanowire Networks Zhanjun Gu,†,‡ Feng Liu,†,‡ Jane Y. Howe,§ M. Parans Paranthaman,| and Zhengwei Pan*,†,‡ Faculty of Engineering, Department of Physics and Astronomy, UniVersity of Georgia, Athens, Georgia 30602, and Materials Sciences and Technology DiVision, Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 35–39
ReceiVed October 2, 2008; ReVised Manuscript ReceiVed December 3, 2008
ABSTRACT: Three-dimensional (3D) GeO2 nanowire networks were fabricated by using germanium as both a source material and catalyst in an oxygen-rich growth environment. The branches within one network show very regular orientation relationships: either perpendicular or parallel to each other. The nanowires follow a base-growth model and the growth direction is along . The GeO2 nanowire networks exhibit strong violet emission peaked at 404 nm under a 265 nm UV light excitation. Nanowires represent key building blocks for bottom-up assembly of complex functional architectures for future electronic and optoelectronic devices.1-4 Growth of one-dimensional (1D) nanowire building blocks of controlled size and morphology can now be readily realized,5-7 but the assembly of individual building blocks into complex functional devices is still very challenging work even though breakthroughs have been made, most notably by Lieber and colleagues.8,9 An alternative to the after-growth bottom-up assembly is direct growth through branching, and significant progress has been made in the past few years. Successful examples include the growth of comb-like structures,10,11 tetrapod-shaped structures,12,13 dendritic structures,14-18 nanowire-tree arrays,19 and complex three-dimensional (3D) nanowire networks.20,21 Since these branched nanowires offer increased structural complexity and enable greater functionality, they may drive some of the most unique and exciting applications possible from the bottom-up approach to nanotechnology.1 In 3D nanowire networks, the nanowires show very regular orientation relationships: either perpendicular or parallel to each other.20,21 So far, two regularly ordered 3D nanowire networks were reported: one is WO3-δ nanowire networks synthesized by thermal evaporation of W powders via a vapor-solid (VS) mechanism20 and the other one is PbSe nanowire networks synthesized by coevaporation of PbSe and In2Se3 powders via a vapor-liquid-solid (VLS) mechanism with In as a catalyst.21 In this communication, we report the direct growth of 3D GeO2 nanowire networks by thermal evaporation of Ge powder in the presence of a controllable amount of oxygen gas via a VLS process. In this synthesis, Ge acts as both the source material and a catalyst. The GeO2 nanowire networks show strong violet emission peaked at 404 nm under a 265 nm ultraviolet (UV) light excitation. Experimental Section. The growth of GeO2 nanowires was conducted in a tube furnace system similar to that described in ref 22 In a typical run, about 0.2 g of Ge powder was placed at the center of an alumina tube that was inserted in a horizontal tube furnace. A small amount of Al powders (∼0.1 g) was spread on an alumina substrate which was located about 5 cm downstream of the Ge powder. After the alumina tube was evacuated to ∼2 × 10-3 Torr, ∼150 sccm (standard cubic centimeter per minute) flowing argon was introduced into the reaction chamber and the furnace was quickly heated to 1000 °C. 5-10 sccm of oxygen was then added through a long, thin alumina tube to a position right above the growth zone. The introduction of oxygen at the * To whom correspondence should be addressed. Tel.: +1-706-5424657; fax: +1-706-542-8806; e-mail:
[email protected]. † Faculty of Engineering, University of Georgia. ‡ Department of Physics and Astronomy, University of Georgia. § Materials Sciences and Technology Division, Oak Ridge National Laboratory. | Chemical Sciences Division, Oak Ridge National Laboratory.
Figure 1. XRD pattern of GeO2 nanowire network.
downstream of Ge source can avoid the preoxidization of Ge powder, while generating an efficient, wide oxidizing region over the growth zone to facilitate the oxide nanowire growth. At 1000 °C, Ge vapor evaporated from the Ge powder was transported by the argon carrier gas to the growth zone, where the temperature was in the range of 700-850 °C, to feed the GeO2 nanowire growth. The pressure of the reaction chamber was held at 300 Torr, and the growth time was varied from 10 to 30 min. The furnace was then cooled down naturally to room temperature in a flowing Ar atmosphere. The as-synthesized products are characterized and analyzed by X-ray diffractometer (XRD; PANalytical X’Pert PRO diffractometer with Cu KR radiation), scanning electron microscope (SEM; FEI Inspec F FEG SEM at 15 kV), transmission electron microscope (TEM; Hitachi HF-3300 FEG TEM/STEM at 300 kV), and energydispersive X-ray spectroscope (EDS) attached to the SEM and TEM. The photoluminescence (PL) properties were studied with a Horiba Jobin Yvon FluoroLog3-2iHR320 spectrofluorometer using a Xe lamp as the excitation source. Results and Discussion. The GeO2 nanowire networks were synthesized by the thermal evaporation of Ge powders in a horizontal tube furnace. After the growth, the alumina substrate was covered with a thick layer of white, wool-like products. XRD analysis (Figure 1) shows that the products are pure, crystalline GeO2 with hexagonal lattice constants of a ) 4.987 Å and c ) 5.652 Å (JCPDS card No. 85-473). The peaks are strong and narrow, indicating good crystallinity of the GeO2 nanowires. Figure 2a is a typical low-magnification SEM image of the assynthesized products, showing the high yield of GeO2 nanowire networks. Each nanowire network resembles in structure of an
10.1021/cg8010986 CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
36
Crystal Growth & Design, Vol. 9, No. 1, 2009
Communications
Figure 2. SEM images of 3D GeO2 nanowire networks. (a) Low-magnification SEM image showing high-yield growth of GeO2 nanowire networks. (b) One individual network resembling an architectural scaffold. (c, d) Side-view of nanowire networks showing the orientation relationship of the nanowires: either perpendicular or parallel to each other. (e) Top-view of a network showing the presence of a large amount of Ge particles inside the network. (f) SEM image of the edge part of a network showing that Ge particles were deposited onto the preformed nanowires to nucleate and grow the new branches. The 3D coordinators in (b), (c), and (d) indicate the nanowire growth directions.
architectural scaffold (Figure 2b), which is constructed of straight GeO2 nanowires that are intercrossed with each other to form the 3D nanowire network (Figure 2c,d). There are several key characteristics that can be identified from SEM (Figure 2) and TEM (Figure 3) studies. First, a lot of micrometer-sized particles exist inside the nanowire network (Figure 2c,e,f). The diameters of the particles are in the range of 0.5-2 µm. EDS analyses reveal that the particles are Ge with a trace of oxygen, while the nanowires are stoichiometric GeO2. The Ge particles are formed on the preformed GeO2 nanowires because of the continuous supply of Ge species from the vapor in the subsequent growth process. New GeO2 nanowires (branches) are nucleated and grown from these Ge particles, resulting in the formation of branched nanowire structures (Figures 2f and 3a). Repeating this branching growth process on the newly
formed branches leads to the formation of complex 3D nanowire networks, as shown in Figure 2b-d. These results indicate that Ge particles act as an efficient catalyst for GeO2 nanowire growth via the VLS process. This growth practice, together with the recently reported Ge-catalyzed ZnO nanowire growth,23 indicates that semiconductor Ge can potentially function as a universal catalyst for VLS growth of high quality oxide nanowires (our other experiments show that Ge can also be used to catalyze the growth of Al2O3 and SiO2 nanowires). Like the Ge-catalyzed ZnO nanowires, the diameter of the Ge particles (0.5-2 µm) is much larger than that of the associated GeO2 nanowires (100-300 nm), which is distinct from the conventional Au-catalyzed nanowires, for which the diameters of the nanowires are similar to those of the catalyst particles.5,24,25
Communications
Figure 3. TEM images of Ge-catalyzed GeO2 nanowires. (a) TEM image of part of a network clearly showing the position relationship between the Ge particles and nanowires. The Arabic numbers represent the generation of the branches. (b) One Ge particle catalyzes growth of three GeO2 nanowire arms. The bottom right inset is the electron diffraction pattern taken from the top right arm along the [001] zone. The bottom left inset is the electron diffraction pattern taken from the amorphous Ge particle.
Second, Figures 2f and 3a clearly show that the Ge-catalyzed GeO2 nanowires follow a base-growth model, in which the Ge catalyst particles stay on the branches during the growth process. This is distinct from the conventional Au-catalyzed nanowires5,24,25 and nanowire branches,15-19 in which the growth follows a tipgrowth model; that is, the catalyst particles are lifted from the growth substrates or branches by the growing nanowires. This may be caused by the strong connection between the viscous Ge particles and GeO2 nanowires. One Ge particle usually catalyzes one GeO2 nanowire (Figure 3a), but multiple nanowire growth can occasionally be observed, as shown in Figure 3b, in which three GeO2 nanowires were grown symmetrically from one Ge particle and the angle between the nanowires is approximately 120°. Third, the GeO2 nanowires are very straight and have a uniform diameter along their long axes. The branches within the same nanowire network show a preferred orientation and appear to be perpendicular or parallel to each other, which is responsible for the formation of perfectly organized 3D nanowire networks, as shown in Figure 2b-d. Electron diffraction analyses reveal that
Crystal Growth & Design, Vol. 9, No. 1, 2009 37 the GeO2 nanowires have a preferential growth direction along (Figure 3b, bottom right inset). Owing to the extreme sensitivity of GeO2 to electron beam irradiation,26,27 we were unable to obtain lattice images from the GeO2 nanowires for further microstructural studies. Electron diffraction patterns recorded from the Ge particles surprisingly shows that the Ge particles are amorphous (Figure 3b, bottom left inset). The Ge-catalyzed GeO2 nanowire growth is very sensitive to the growth conditions, especially to oxygen. If the oxygen gas is introduced upstream of the Ge source, the Ge powders will be oxidized into stable GeO2 powders at the source part, and therefore no GeO2 nanowires will be grown. When the oxygen flow rate is less than 4 sccm, the only product is Ge-catalyzed Al2O3 nanowires (results not shown here), probably because under that oxygen concentration the larger Gibbs free energy of formation for Al2O3 over GeO228 facilitates the formation of Al2O3 nanowires instead of GeO2 nanowires. If the oxygen flow rate is over 10 sccm, however, the Ge vapor will be fully oxidized in the vapor phase, and as a result a large amount of GeO2 nanoparticles are deposited on the growth substrates. The Al powders also play a crucial role in the growth of 3D GeO2 nanowire networks. When bare Al2O3 plate was used as the substrate, no 3D GeO2 nanowire networks were obtained even though branched GeO2 nanowire structures were formed. The function of Al powder may be that before the addition of oxygen gas (note that oxygen gas is introduced only when the furnace temperature reaches at 1000 °C), the melting Al at elevated temperature tends to act as flux to absorb Ge species from the vapor to form small pieces of Ge-Al alloy films on which a large amount of Ge particles were formed (Figure 4a). When oxygen is introduced, these Ge-Al alloy films facilitate the growth of aligned GeO2 nanowires along one (Figure 4b), two, or three perpendicular directions (Figure 4c), forming the backbones of the 3D networks (the Al was then oxidized into Al2O3). Since the Ge vapor is continuously supplied during the whole growth process, new Ge particles will be formed on the preformed nanowires to nucleate and grow new branches, new subbranches and so on, thus forming 3D networks (Figure 4d). The formation of 3D nanowire network is schematically shown in Figure 5. In the conventional VLS nanowire growth processes, a eutectic alloy of catalyst and nanowire material is formed and nanowires are nucleated and grown when the nanowire material is supersaturated in the alloy.5,24,25 For the present Ge-catalyzed GeO2 nanowire process, however, no Ge-GeO2 eutectic alloy exists. The growth that the catalyst and the nanowire material contain the same elements, such as Sn-catalyzed SnO2 nanowires29 and Ga-catalyzed GaN nanowires,30 has been explained by a so-called “self-catalytic growth” mechanism. This self-catalytic growth mechanism may be used to phenomenally explain the Ge-catalyzed GeO2 nanowire growth. For the Ge-catalyzed 3D GeO2 nanowire networks present here, however, several experimental phenomena are not fully understood at current stage. These include (i) why are the Ge particles not oxidized in the oxygen-rich growth environment? (ii) how are the GeO2 nanowires nucleated and grown into singlecrystalline nanowires from the Ge particles? (iii) how can the microscale Ge particles (instead of Ge coating layer) be formed on the preformed nanowires? (iv) why are the new branches grown perpendicularly to their parent branches? (v) how can the new branches grown from different particles and different parent branches be either parallel or perpendicular to each other? It is certain that much experimental and theoretical work is needed to answer these questions. GeO2 is an important optical material that emits strong visible light under UV light excitation and has a higher refractive index (n ) 1.63) than SiO2, making it a promising material for optical devices such as optical waveguides for integrated optical systems.31 To study the optical properties of the 3D GeO2 nanowire networks, we have carried out photoluminescence (PL) measurements using a Xe lamp as the excitation source. Figure 6 shows the room-
38
Crystal Growth & Design, Vol. 9, No. 1, 2009
Communications
Figure 4. SEM images taken at different growth stages showing the evolution of the GeO2 nanowire networks. (a) Ge nanoparticles formed on the growth substrates before the introduction of oxygen gas. (b, c) Aligned GeO2 nanowires grown at the early growth stage (∼5 min after the introduction of oxygen gas). (d) A well-organized 3D GeO2 nanowire network after 20 min growth. The 3D coordinator indicates the nanowire branch growth directions.
Figure 5. A schematic diagram showing the growth process of 3D GeO2 nanowire network. The Arabic numbers represent the generation of the branches.
Figure 6. Photoluminescence spectra of the GeO2 nanowire networks.
temperature excitation and emission spectra of GeO2 nanowire networks. A strong violet emission peaked at ∼404 nm is obtained under a 265 nm UV light excitation. The violet emission was previously acquired from GeO2 powders32 and GeO2 thin films,33 whereas for GeO2 nanowires blue emissions peaked at ∼450-480 nm were commonly reported.34,35 The violet emission from GeO2 thin films was attributed to the oxygen-deficient luminescence center,32 while the blue emission from GeO2 nanowires were ascribed to the radiative recombination between the electrons in oxygen vacancies and the holes on germanium-oxygen vacancies
centers.34,35 Since our GeO2 nanowire networks were synthesized in an oxygen-rich environment, the oxygen-deficient centers would unlikely be the dominant factor that causes the very strong violet emission. Further work is necessary in order to understand the underlying luminescence mechanism. Conclusions. In summary, large-scale, single-crystalline GeO2 nanowire networks have been fabricated by using Ge as both source material and catalyst in an oxygen-rich growth environment. The branches within one network show very regular orientation relationships: either perpendicular or parallel to each other. The nanowires
Communications
Crystal Growth & Design, Vol. 9, No. 1, 2009 39
follow a base-growth model and the growth direction is along . The GeO2 nanowire networks exhibit strong violet emission peaked at 404 nm under a 265 nm UV light excitation. The Gecatalyzed GeO2 nanowire growth process expands the conventional VLS nanowire growth mechanism in which metals were mainly used as the catalysts. With their complex, specific structures and strong violet emission, the 3D GeO2 nanowire networks might serve as promising candidates for advanced photonics and optoelectronics.
Acknowledgment. This work was supported by the University of Georgia Research Foundation, the US Office of Naval Research (under contract No. N004315578), and the Oak Ridge National Laboratory (ORNL). The research in ORNL was conducted through the support from the U.S. Department of Energy, Office of Basic Energy Sciences - Division of Materials Sciences and Engineering (DMSE). The TEM characterization work of this research was conducted at the ORNL’s SHaRE User Facility, which is sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Wang, D. L.; Lieber, C. M. Nat. Mater. 2003, 2, 355–356. Lieber, C. M. MRS Bull. 2003, 28, 486–491. Samuelson, L. Mater. Today 2003, 6, 22–31. Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353–389. Morales, A. M.; Lieber, C. M. Science 1998, 279, 208–211. Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947–1949. Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. Huang, Y.; Duan, X. F.; Wei, Q.; Lieber, C. M. Science 2001, 291, 630–633. Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255– 1259. Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. Am. Chem. Soc. 2003, 125, 4728–4729. Pan, Z. W.; Mahurin, S. M.; Dai, S.; Lowndes, D. H. Nano Lett. 2005, 4, 723–727. Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382–385.
(13) Yan, H. Q.; He, R. R.; Pham, J.; Yang, P. D. AdV. Mater. 2003, 15, 402–405. (14) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287–1291. (15) Gao, P. X.; Wang, Z. L. J. Phys. Chem B 2002, 106, 12653–12658. (16) Wang, D. L.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871–874. (17) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 598–602. (18) Jung, Y.; Ko, D. K.; Agarwal, R. Nano Lett. 2007, 7, 264–268. (19) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380–384. (20) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2005, 17, 2107–2110. (21) Zhu, J.; Peng, H. L.; Chan, C. K.; Jarausch, K.; Zhang, X. F.; Cui, Y. Nano Lett. 2007, 7, 1095–1099. (22) Pan, Z. W.; Dai, Z. R.; Xu, L.; Lee, S. T.; Wang, Z. L. J. Phys. Chem. B 2001, 105, 2507–2514. (23) Pan, Z. W.; Dai, S.; Rouleau, C. M.; Lowndes, D. H. Angew. Chem., Int. Ed. 2005, 44, 274–278. (24) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89–90. (25) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298–302. (26) Bai, Z. G.; Yu, D. P.; Zhang, H. Z.; Ding, Y.; Wang, Y. P.; Gai, X. Z.; Hang, Q. L.; Xiong, G. C.; Feng, S. Q. Chem. Phys. Lett. 1999, 303, 311–314. (27) Hu, J. Q.; Li, Q.; Meng, X. M.; Lee, C. S.; Lee, S. T. AdV. Mater. 2002, 14, 1396–1399. (28) Kubaschewski, O.; Alcock, C. B. Metallurgical Thermochemistry, 5th ed.; Oxford: Pergamon, 2008. (29) Orlandi, M. O.; Leite, E. R.; Aguiar, R.; Bettini, J.; Longo, E. J. Phys. Chem. B 2006, 110, 6621–6628. (30) Stach, E. A.; Pauzauskie, P. J.; Kuykendall, T.; Goldberger, J.; He, R. R.; Yang, P. D. Nano Lett. 2003, 3, 867–869. (31) Lin, Z. Y.; Garside, B. K. Appl. Opt. 1982, 21, 4324–4328. (32) Wu, X. C.; Song, W. H.; Zhao, B.; Sun, Y. P.; Du, J. J. Chem. Phys. Lett. 2001, 349, 210–214. (33) Fitting, H. J.; Barfels, T.; Trukhin, A. N.; Schmidt, B. J. Non-Cryst. Solids 2001, 279, 51–59. (34) Jiang, Z.; Xie, T.; Wang, G. Z.; Yuan, X. Y.; Cai, W. P.; Meng, G. W.; Li, G. H.; Zhang, L. D. Mater. Lett. 2005, 59, 416–419. (35) Kim, H. W.; Shim, S. H.; Lee, J. W. Appl. Surf. Sci. 2007, 253, 7207–7215.
CG8010986
