ARTICLE pubs.acs.org/crystal
Self-Assembled Highly Uniform ZnO Submicrometer Rods on Metal Grid Grown by VaporLiquidSolid Method Satyaprakash Sahoo,*,† J. F. Scott,‡ A. K. Arora,§ and Ram S. Katiyar† †
Department of Physics, University of Puerto Rico, United States Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom § Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, India ‡
bS Supporting Information ABSTRACT: Synthesis of ZnO nanostructures of different shape and size is of fundamental important for nanoscale device fabrication. Here, we report a new class of three-dimensional highly uniform ZnO hexagonal submicrometer rods. This unique morphology has achieved by suitably selecting a metal grid as the substrate in a vaporliquidsolid technique. The growth mechanism has been explained by analogy with that of metal fractals formed by electrochemical deposition. We believe that mimicking the naturally occurring complex structure will provide vital evidence to understand nonequilibrium growth processes occurring in physical, chemical, and biological sciences. Finally, this threedimensional morphology may be an alternative to nanowires and nanorods in nanomechanical reinforcement.
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n the last few decades, ZnO nanostructures of simple morphologies such as quantum dots, nanowires, and nanobelts have attracted considerable research interest due to their unique physical and chemical properties for application in optoelectronic devices, solar cells, UV detectors, gas sensors, logic-gate devices, and solidstate lasers.13 ZnO is a wide band gap material with a band gap of ∼3.37 eV and high exciton binding energy (60 meV) at room temperature. Progress in nanotechnology demands the synthesis of new complex nanostructures with improved physical and chemical properties, and in the future, more and more possible applications will emerge from these complex nanostructures. Moreover, shapecontrolled syntheses of complex nanostructures such as hierarchical and branched arrays are emerging as new challenges in the fabrication of ZnO due to difficulties in controlling the growth process and limitations in processing techniques. However, some special modifications in synthesis processes, both in solution and in vapor liquidsolid (VLS) techniques, have been reported, which result in self-assembled complex structures such as multipod, nanocomb, nanopine, nanoflower, and dendritic structures. The building blocks of these structures are either nanorods or nanowires.48 The growth of these branched structures is anisotropic in nature. In some of these structures, growth starts spontaneously from a central nucleus in a single reaction stage, whereas in some other structures, sequential nucleation results in secondary nucleation, which in turn gives rise to secondary branches on the primary structure. Complex branched structures in a nonequilibrium growth process results in either a symmetrically branched dendrite structure or a randomly branched fractal structure, depending upon the growth kinetics and boundary conditions.9 The synthesis r 2011 American Chemical Society
of nanowires or nanorodes of ZnO by VLS technique follows a metal catalyst-driven growth process.10 In the case of complex branched and hierarchical structures where a single-stage reaction occurs at a single nucleation point, resulting in multipod branched structures, or from a primary structure, secondary nucleation gives rise to highly oriented secondary branches. Here, we want to mention that in the case of the simplest multipod branched structures, each nanorod that constitutes the structure is of the same shape and size.4 However, as the structural complexity increases, for example, in nanocombs or nanopines, the primary growth structure is of larger dimension; subsequent nucleation over it results in nanowires or nanorods of relatively smaller dimension.7,8 Here, we report for the first time evidence of a nonequilibrium growth process in ZnO resulting in complex three-dimensional structure consisting of highly uniform hexagonal submicrometer rods. The fractal analysis reveals these structures to be fractal in nature. The growth mechanism has been explained by an analogy with that of metal fractals formed by electrochemical deposition. We also discuss the special mechanical stability of tiny nanopoint contacts, which in turn supports a large and complex microarchitecture. Until today, most of the complex structures studied have been restricted to naturally occurring crystal growth and biological systems. The synthesis of complex inorganic nanomaterials requires some special modification of the growth Received: May 25, 2011 Revised: June 20, 2011 Published: June 22, 2011 3642
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Figure 1. SEM image of ZnO microstructures with different magnifications.
process either in solution or in vapor transport solid methods.5,6,9,10 For example, in solution methods, precise control of pH and addition of specific chemicals to stimulate secondary nucleation results in different complex structures.5 The simple branched tetrapod nanostructures have been observed in a variety of semiconductor crystals including ZnO. Similarly, few novel ZnO hierarchical nanostructures have been synthesized by VLS methods by varying experimental conditions.1113 Usually in VLS methods, Si or Al2O3 is used as a substrate for growth of ZnO nanocrystals. We synthesized ZnO nanostructures by modifying the experimental procedure in a VLS method, which is described as follows: First, equal mole percentages of ZnO powder (purity 99.999%) was well mixed with graphite powder (purity 99.99%). This mixture was placed at one corner of a ceramic boat. Here, graphite acts as a catalyst at high temperature and helps in the reduction of ZnO in to Zn vapor. As a catalyst, it only speeds up the reduction reaction process and remains unreached with the final product. The crucial step was choosing a commercially available transmission electron microscopy (TEM) copper grid as the substrate. The advantage of the copper grid as substrate is a special effect on growth kinematics. Copper grids were coated with gold (both sides) of the desired nanometer thickness using a homemade DC sputtering unit. This process was crucial since at elevated temperature the copper grid may become folded, resulting in exposure of the bottom side. These gold-coated Cu grids were kept on an Al2O3 substrate. The Al2O3 substrate with Cu grids was placed in a ceramic boat at a distance of 5 cm from the ZnO/graphite mixture. The boat was placed at the center of a quartz furnace in such a way that the temperature of the mixture can reach 950 °C. The quartz tube
was heated at 350 °C for 30 min under argon gas flow (20 scsc), and then, the temperature of the furnace was increased to 950 °C at a rate of 60 °C/min. This temperature was maintained for about 30 min, after which the system was allowed to cool down to room temperature under constant argon gas flow. A thick white substance was seen throughout the surface of the copper grids. Raman measurements of the sample were carried out using a Horiba-Yobin T64000 micro-Raman system, and a 514.5 nm line of an argon ion laser was used as excitation source. The room temperature microphotoluminescence was carried out 325 nm line of a HeCd laser using the above Raman system. The Cu grid containing ZnO submicrometer rods (on the Al2O3 substrate) was focused by laser for Raman and PL measurements. The laser spot size was about 1.5 μm. Morphological characterizations were carried out using scanning electron microscopy (SEM) (JEOL) and TEM. For TEM study, we take the Cu grid containing ZnO sample was directly loaded on the sample holder. Figure 1 shows the typical SEM image of the as grown sample at different places on the Cu grid and with different magnifications (low and medium). From the low magnification SEM image (Figure 1a,b), highly dense and treelike ZnO microstructures can be seen on the TEM grid, which are projecting out in to free space. A magnified SEM image of ZnO nanostructures is shown in Figure 1c. From these images, it can be seen that treelike microstructures are the major structural representatives throughout the Cu grid. The height of the treelike microstructure varies from 10 to 50 μm. As expected, at high temperature, we found that one of the Cu grids got folded, and growth took place on the exposed surface (see Figure S1 in the Supporting Information). However, from these images, it is not clear about 3643
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Figure 2. FESEM images of ZnO showing the hexagonal submicrometer rods. These images show the random nature of growth process. Panel d shows a tetrapod structure attached to a nanorod.
the structural units that assemble to give rise to such morphology, and it is also not clear if any specific symmetry in the growth pattern is followed or not. For this reason, high magnification SEM studies were carried out. Figure 1d shows a high magnification SEM image of a single treelike structure. From this image, it can be seen that highly uniform ZnO submicrometer rods have branched out, which resembles a diffusion-limited aggregate. The most conspicuous feature in these macroscopic morphology is that the constituent submicrometer rods do not show any preferred growth pattern, which is also supported by FESEM and TEM images (discussed below); instead, a random growth pattern is followed. When a system is under nonequilibrium growth, it can result in either a patterned structure (dendrite structure) or a random pattern (fractal structure).14 The present macroscopic morphology can thus be treated as a fractal structure, since no preferential growth pattern of submicrometer rods is followed. We will also discuss the fractal analysis of these structures more later. Here, we point out that ZnO nanostructures of complex morphologies such as hierarchical, comblike, or pine treelike structures are made up of nanorods or nanowires. These complex morphologies have common structural features: Secondary growth structures are formed over a primary backbone. However, in the present work, the self-assembly of ZnO submicrometer rods into a three-dimensional random chain structure is unique. The composition of the ZnO submicrometer rods was characterized by energy dispersive spectrometry (EDS), which clearly showed the presence of Zn and O (see Figure S2 in the Supporting Information). Further structural characterization was carried out using field-emission scanning electron microscopy (FESEM). Figure 2
represents a typical FESEM. It can be seen from Figure 2 that the treelike microstructures are made up of submicrometer rods. A remarkable feature observed in our three-dimensional structure is that each nanorod has almost the same size and same morphology. Each nanorod is 12 μm in length and 300 nm in diameter with a hexagonal rod structure. From the FESEM images, it can also be seen that these highly monodisperse ZnO submicrometer rods during growth process have undergone a self-assembly process that results in a complex structure. Each nanorod has grown from the previous nanorod from an arbitrary point; no preferred sequential nucleation growth pattern is followed. Another significant feature about this fractal is that each nanorod is connected to the next via a by tiny point contact. We discuss how these nanopoint contacts are mechanically stable enough to support a large-scale macrostructure in the TEM section. It is important to note that in an anisotropic and equilibrium growth process, the sequential nucleation and growth process induces secondary nucleation of branched structure over a primary structure. Here, a clear deviation from this kind of growth process has taken place. A close examination of the surface of the nanorod shows that the surfaces of the hexagonal nanorod are smooth without having nanoscale outgrowth as observed in some complex ZnO nanostructures reported previously. This indicates that the rate of growth is very fast and spontaneous. It can also be seen from these FESEM images that the fractal structure is the major structural outcome in the present synthesis process; however, we also encounter a few tetrapod nanostructures. The latter morphology is quite rare in the present study. From Figure 2d, a tetrapod branching of ZnO 3644
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Figure 5. Graph of log(R) vs logN(R). The absolute value of the slope represents the fractal dimension, which is 1.21 ( 0.06 for the fractal structure shown in the inset. Figure 3. TEM image of a three-dimensional fractal structure. The dotdashed square encloses a large microstructure that is connected to a nanorod by a nanopoint contact. The contact point is circled by a solid line. Panels b and c are the images of the contact point with higher magnifications.
5 5 μm2 (shown within a dotted square boundary in Figure 3) evolves from a previously grown nanorod. The contact point of this structure on the nanorod is shown by the circle over it. A magnified TEM image of this contact point is shown in Figure 3b,c. From these images, it can be seen that the dimension of the contact point from which the secondary fractal arises is about 200 nm. The whole structure is a three-dimensional array supported by a very small (nano) point contact. This shows the mechanical stability of nanoscale contacts. Similarly, Figure 4 shows another TEM image of nanorod contact structure, where secondary nucleation took place from the different faces of a hexagonal rod. This further shows that in the present growth process, no preferred direction of nucleation is followed. The magnified TEM image of this structure shows that contact points are at a nanometer scale. From the TEM and FESEM study, secondary nucleation occurs mostly either from one of the faces or from the tip of the previously existing rod. From these TEM data, the fractal feature was confirmed by measuring the fractal Hausdorff dimension (Df), which can be expressed as,15 Df ¼ lim
R f0
Figure 4. TEM image showing the growth of submicrometer rod from the side walls of a hexagonal rod. Panels b and c are the images of the contact point with higher magnifications.
submicrometer rods can be seen, with one of its arms attached to a nanorod. Figure 3 shows the TEM image of one of the fractal structures. It is evident from the image that growth of this three-dimensional structure is a consequence of random growth of submicrometer rods. The length of this particular fractal structure is around 10 μm. A careful observation of this structure shows that after 5 μm of random growth a large secondary fractal of effective area of
log NðRÞ logðRÞ
ð1Þ
where R is the nanorod length and the sum over R is the total size of the object. The fractal analysis was performed on Figure 3 following a method reported for peptide fractal structure.16 Several numeric values of the length R (ranging from 0.4 to 1.66 μm) have been used, and corresponding N(R) values have been calculated. Figure 5 shows the plot of log(R) versus log N(R), which can be fitted by a straight line; the inset shows the TEM image on which analysis was carried out. The absolute value of the slope gives the fractal dimension (Df), which is measured to be 1.21 ( 0.06. Note that Df is a statistical quantity that gives an idea about how completely the fractal fills the space. There are possible growth mechanisms that might give rise to such fractal structures: In naturally occurring growth processes, it has been theoretically argued that the resulting structure usually maintains the minimum free energy.17 In the case of a nonequilibrium growth process that takes place by the addition of materials by diffusion, a rich variety of complex structures are expected. Conditions, such as rate of reaction, temperature, and concentration, of the solution are the key factors in determining the growth kinetics of the complex structures.18 To understand the present three-dimensional fractal ZnO submicrometer rods, 3645
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Figure 6. Schematic energy diagram showing the work function (ϕ) of Cu, electron affinity (χ) of ZnO, and nearly Ohmic contact with contact barrier (ϕs) of 0.37 eV. Cu and ZnO are separated by a dash-dot line. The flow of electron from Cu to ZnO is shown by the arrow. Figure 8. Room temperature photoluminescence of a ZnO sample, which is fitted with two Gaussian peaks.
Figure 7. Raman spectra of ZnO sample synthesized on a copper grid.
we need to understand the boundary conditions and their consequences in the growth process. The substrate that we used to grow ZnO is metallic Cu, whose work function is about 4.7 eV. This value is close to the electron affinity of ZnO, which is 4.35 eV. Thus, the contact between Cu and ZnO submicrometer rods forms a nearly Ohmic contact with a small barrier height of 0.35 eV. The energy diagram has been shown schematically in Figure 6, and the barrier height is expected to lower further at high temperatures. Metals like Zn and Cu are known to form disordered two-dimensional fractals with no apparent symmetry under electrochemical deposition.1921 These fractals are similar to the structures observed in the present case. It has also been reported that the increase in the potential of the electrochemical cell and concentration of the solution favors fractal structures. A similar analogy in growth kinetics can be correlated between electrochemically grown metal fractals and our ZnO fractals. ZnO is known to exhibit pyroelectric effect in which a pyrovoltage is developed. This effect is due to change in polarization of ZnO at high temperatures. As the contact between copper and ZnO is nearly Ohmic, electrons from Cu can flow easily into ZnO along the pyroelectric voltage gradient. Previously ZnO nanorods have been synthesized on flat insulating substrates such as Al2O3 or Si (SiO2), which were coated with few nanometers of metal catalysts. In a VLS process, a eutectic alloy droplet forms at each gold catalytic site, followed by the nucleation and growth of the solid ZnO nanorod. The ZnO submicrometer rods then grow in the direction of a vertical thermal gradient. In our case, the copper grid provides two favorable conditions: At the first site of
nucleation on the copper grid, the ZnO submicrometer rods form a nearly Ohmic contact with the copper as described above; next, because of the presence of an air gap (50 50 μm2) confined laterally by copper wires of 30 μm width, a large thermal gradient is developed in the air space. Finally, the temperature of the reaction is achieved rapidly (60 °C/min), and thus, in such a short time, a larger concentration of reactant vapor is available at the nucleation site. All of these boundary conditions probably favor growth kinetics of a fractal structure. This process is very similar to that of the diffusion-limited aggregation (DLA) process, which is known to result in such fractal structures. Raman spectroscopy is one of the most widely used techniques to investigate the phase formation and crystallinity of nanomaterials. Wurtzite ZnO has C6v4 point group symmetry with two formula units per primitive cell.22 A group theoretical calculation gives a total of six zone center (q = 0) optical phonon branches, which can be represented by the irreducible representations Γ = A1 + E1 + 2B1 + 2E2. The B1 are silent modes (neither Raman- nor infrared-allowed), and the polar E1 and A1 modes are both Raman- and infrared-active. The two E2 modes are generally labeled “low” and “high” energy. Figure 7 shows the Raman spectra of an as-grown sample. Apart from two distinct peaks at about 98 and 437 cm1, there are other 10 phonon modes with weaker intensity. The observed Raman peaks have been assigned based on the other reported results in single-crystal ZnO.22 The presence of all predicted Raman modes confirms the formation of ZnO. It is important to note that these observed phonons involve both first- and second-order Raman scattering. The peaks at 98, 380, 408, 438, 535, and 590 cm1 are the first-order Raman modes, and the others are second-order. It has been mentioned earlier that the zone center B1 mode is not Raman-active. The phonon selection rules do not allow B1 first-order Brillouin zone center (k = 0) phonons to participate in Raman scattering; however, phonons other than zone center can participate in second order Raman scattering. Thus, one expects 2B1low phonons to appear in the Raman spectra of ZnO. The peak at 535 cm1 has been assigned as 2B1low.22 A second-order Raman peak involving E2high + E2low can also appear at around 535 cm1. These peaks arise not from two vibrations each at wave vector q = 0 but from an integration over the whole Brillouin zone of all (+q, q) pairs, so it will be difficult to assign exactly the peak to 2B1low or E2high + E2low. A room temperature photoluminescence study has been carried out on these ZnO macrostructures, and Figure 8 shows the measured spectrum. It shows a distinct peak in the violet 3646
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Crystal Growth & Design region whose position is at about 3.06 eV and a broad peak at about 2.5 eV in the visible region. ZnO is a wide bandgap material with bandgap 3.37 eV; however, the presence of various intrinsic defects can give rise to deep acceptors or shallow donors. The photoluminescence emission at 3.06 eV is associated with Zn vacancies.23 The peak at 2.5 eV has been assigned to oxygen vacancies. Here, we want to point out that introduction of carbon impurity increases the intensity of PL emission at 3 eV. However, in the present case, we do not expect carbon impurity in our ZnO submicrometer rods as the reaction mixture; that is, graphite and ZnO powder are of high purity (99.999). In conclusion, by simply choosing suitable metal grid as substrate, we fabricate three-dimensional fractal structures of ZnO using a VLS method. From FESEM and TEM studies, it has been found that the fractal structures are formed by highly uniform (monodisperse) hexagonal submicrometer rods that share point contacts. On the basis of the structural characterization, the possible growth mechanism of these fractal structures has been explained by analogy with that of metal fractals formed by electrochemical deposition. Raman scattering and room temperature photoluminescence studies confirmed high crystallinity of ZnO submicrometer rod assembles. We believe that synthesis of more complex structures on metal substrates is now one step closer.
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(13) Mishra, Y. K.; Mohapatra, S.; Singhal, R.; Avasthi, D. K.; Agarwal, D. C.; Ogale, S. B. Appl. Phys. Lett. 2008, 92, 043107. (14) Sander, L. M. Nature 1986, 322, 789. (15) Beck, C.; Sch€ogl, F. Thermodynamics of Chaotic Systems; Cambridge University Press: Cambridge, 1993. (16) Lomander, A.; Hwang, W.; Zhang, S. Nano Lett. 2005, 5, 1255. (17) Pilipenko, D.; Brener, E. A.; H€uter, C. Phys. Rev. E 2008, 78, 060603. (18) Liu, X. Y.; Strom, C. S. Chem. Phys. 2000, 113, 4408. (19) Jacob, E. B.; Garik, P. Nature 1990, 343, 523. (20) Sawada, Y.; Dougherty, A.; Gollub, J. P. Phys. Rev. Lett. 1986, 56, 1260. (21) Grier, D.; Jacob, E. B.; Clarke, R.; Sander, L. M. Phys. Rev. Lett. 1986, 56, 1264. (22) Cusco, R.; Llado, E. A.; Iba~ nez, J.; Artus, L. Phys. Rev. B 2007, 75, 165202. (23) McCluskey, M. D.; Jokela, S. J.V. J. Appl. Phys. 2009, 106, 071101.
’ ASSOCIATED CONTENT
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Supporting Information. SEM image of the ZnO microstructures on a folded Cu grid and EDS spectrum of ZnO submicrometer rods formed on Cu grid. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge financial support from DoE through Grant No. DE-FG02-08ER46526. Support from UPR Materials Characterization Centre (MCC) is acknowledged. ’ REFERENCES (1) Pang, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) 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. (3) Yan, H.; He, R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang., P. J. Am. Chem. Soc. 2003, 125, 4728. (4) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (5) Sounart, T. L.; Liu, J.; Voigt, J. A.; Huo, M.; Spoerke, E. D.; McKenzie, B. J. Am. Chem. Soc. 2007, 129, 15786. (6) Jiang, P.; Zhou, J. J.; Fang, H. F.; Wang, C. Y.; Wang, Z. L.; Xie, S. S. Adv. Funct. Mater. 2007, 17, 1303. (7) Shen, G.; Bando, Y.; Liu, B.; Golberg, D.; Lee, C. J. Adv. Funct. Mater. 2006, 16, 410. (8) Yu, W.; Li, X.; Gao, X. Cryst. Growth Des. 2005, 5, 151. (9) Zhang, D. F; Sun, L. D.; Zhang, J; Yan, Z. G.; Yan, C. H. Cryst. Growth Des. 2008, 8 (10), 3609. (10) Zhou, X. M.; Wei, X. W. Cryst. Growth Des. 2009, 9 (1), 7. (11) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (12) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. 3647
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