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J. Phys. Chem. C 2008, 112, 17926–17930
Role of the Vapor Composition in the Evolution from 1D to 2D ZnO Nanostructures Hyun-Wook Ra, Kwang-Sung Choi, Yoon-Bong Hahn, and Yeon-Ho Im* School of Semiconductor and Chemical Engineering, Chonbuk National UniVersity, Jeonju 561-756, Korea ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: September 22, 2008
The effect of source vapor composition on the growth of ZnO nanostructures inside a microchannel was investigated using a thermal evaporation system. 1D to 2D nanostructures were symmetrically evolved along the longitudinal axis (gas flow direction) of the microchannel. On the basis of the experimental conditions, the changes in the vapor composition along the channel length could be predicted by a simple chemical vapor diffusion modeling. It was revealed that the Zn vapor composition under a high oxygen pressure is one of the critical parameters responsible for evolution from 1D to 2D nanostructures. Introduction Zinc oxide (ZnO) has attracted a great deal of attention in the past few decades because of its superior properties, such as large piezoelectric constant, wide band gap energy (3.37 eV), large exciton binding energy (60 meV), and high thermal and mechanical stability. It is well-known that ZnO is one of the most prolific materials that can produce various nanostructures such as nanowires, nanobelts, nanocages, nanocombs, nanosheets, and so on.1-3 These nanostructures have attracted a great deal of attention because they can remarkably enhance the performances for conventional applications or novel applications. There have been numerous reports on the various device applications based on these nanostructures, such as piezoelectric transducers,4 transparent transistors,5 optoelectronic devices,6,7 and chemical and biological sensors.8-11 Therefore, the development of dimensionally controlled syntheses of such nanostructures has been strongly motivated for the desired practical applications. Various synthetic methods such as molecular beam epitaxy, metalorganic chemical vapor deposition (MOCVD), thermal evaporation, and the solution-phase process have been reported.3 Among these synthetic methods, the direct thermal evaporation of Zn/ZnO vapor has been most widely reported for the fabrication of various ZnO nanostructures. To explain the synthetic process of the various nanostructures by direct thermal evaporation, growth mechanisms such as the vapor-liquid-solid (VLS) process, vapor-solid (VS) process, and their combination process have been mostly proposed.1,2 However, it is difficult to understand the role of interdependent factors such as source vapor composition, reactor pressure, and growth temperature in the proposed mechanisms. Therefore, there have been few attempts to investigate the complex growth behavior based on the relationships of the key parameters.12,13 In an attempt to address the above issues, we report the net effect of the chemical vapor composition of the source vapor on the formation of various ZnO nanostructures without the presence of a catalyst during a simple thermal evaporation process. In the case of the thermal evaporation method, the compositional ratio of the source vapor strongly depends on the temperature, the pressure, and the amount of the source vapor consumed on the substrate or reactor wall. Therefore, we * To whom correspondence should be addressed. E-mail: yeonhoim@ chonbuk.ac.kr.
introduced a microchannel with a short length as a tool to derive the chemical composition ratio of the source vapor during the synthesis process without a temperature gradient. We found that various ZnO nanostructures such as 1D nanostructures (nanowires and nanobelts) and 2D nanostructures (nanosheets and nanocombs) could be grown in the microchannel simply by changing the chemical composition ratio. To understand its role, a theoretical analysis was performed to predict the changes in the chemical composition along the length of the microchannel. These results should lead to the development of various nanostructures for future applications. Experimental Section The ZnO nanostructures were synthesized by a vapor-phase transport process in a horizontal tube furnace, as reported elsewhere.14,15 The Zn powders in the alumina crucible were loaded in the center of the quartz tube, as shown in Figure 1a. The furnace was heated up to 900 °C and kept at this temperature for 30 min. Nitrogen and oxygen with flow rates of 320 and 20 sccm, respectively, were introduced into the system throughout the reaction. After the reaction was completed, the furnace was cooled down to room temperature. To investigate the net effect of the chemical composition ratio, a Si substrate with a microchannel of the semicylinder type was placed at the desired position in the furnace. The microchannel with a depth of 50 µm and length of 0.5 cm was patterned on the Si substrate by conventional semiconductor processing. Figure 1b shows the process sequence used to fabricate the microchannel, which corresponds to the cross-sectional schematic diagram along line 1-2 of Figure 1a. A 1 µm thermal silicon nitride (SiN) layer was deposited on a p-type silicon (100) wafer. The SiN layer was patterned using conventional lithography and etched using an ICP etching process. Finally, the semicylinder-type microchannel with a radius of 50 µm was obtained by wet etching with KOH solution using the prepatterned mask layer of SiN. In the upper region of the microchannel, the microgroove with a width of 2 µm was almost completely closed by the ZnO nanostructures during the initial step of the growth process. To investigate the as-synthesized products inside the microchannel, the upper part of the microchannel was carefully removed after the growth of the ZnO nanostructures. The as-synthesized products were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
10.1021/jp8065597 CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
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Figure 1. (a) Schematic illustration of Zn nanostructure growth inside the microchannel by the typical thermal evaporation process of Zn powders. The carrier gas flow direction is from region 1 to region 2. (b) Fabrication process of the microchannel.
Results and Discussion Figure 2a shows the cross-sectional image of the microchannel formed on the Si substrate in the initial step (10 min) of the total growth time. We could confirm that the groove of the upper region at the microchannel was clogged in the initial step, as shown in Figure 2a. The semicylinder-type microchannel with a radius of 50 µm and length of 0.5 cm was used to investigate the net effect of the chemical vapor composition. In thermal evaporations, the evolution of the nanostructure is strongly affected by the growth conditions, such as the pressure, temperature, and chemical composition of the vapor source. Since these parameters are strongly coupled with each other, it is hard to understand these effects independently. In this work, it can be expected that the total length of the microchannel will be short enough to avoid the formation of a temperature gradient. In addition, the small geometry of the microchannel can lead to changes of the chemical vapor composition due to the variation of the diffusion speeds and consumption amount of each species at the wall of the microchannel. Therefore, it is possible to observe the net effect of the chemical vapor composition on the formation of the nanostructure. Figures 2b and 2c show the as-synthesized 1D nanostructures in the top and bottom regions near the inlet of the microchannel after the completion of the growth. It was found that there was little difference in the shape of the nanostructures inside and outside the inlet of the microchannel. The morphology and structure of the as-synthesized products outside the microchannel are shown in Figure 3. The SEM image in Figure 3a shows a large quantity of product which consists of 1D nanostructures with diameters ranging from 60 to 150
Figure 2. (a) Cross-sectional SEM image near the inlet of the microchannel at the initial growth time of 10 min. (b,c) The assynthesized ZnO nanostructures in the top (b) and bottom regions (c) near the inlet of the microchannel after the completion of the growth.
nm and lengths of several tens of micrometers. We found that the 1D nanostructures were formed uniformly over the entire Si substrate, including the top region of the microchannel. It is worthwhile noting that no other 2D nanostructures were observed. Figure 3b shows a typical XRD pattern of the 1D nanostructures. All of the diffraction peaks correspond to the hexagonal wurtzite structure of ZnO with lattice constants a ) 0.3249 and c ) 0.5206 nm and agree well with the reported values. No characteristic peaks corresponding to impurities or elemental Zn were observed. Figure 4 shows the SEM images of the as-synthesized ZnO nanostructures near the bottom along the length inside the
17928 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Figure 3. (a) SEM image of as-synthesized 1D nanostructures outside of the microchannel. (b) XRD patterns of as-synthesized ZnO nanostructures.
microchannel. It was found that the 1D nanostructures were synthesized in the ranges from the inlet position to 0.15 cm away from it but that there was no significant difference in the shape and size of the nanostructures inside and outside the microchannel. However, Figure 4b shows that, surprisingly, 2D nanostructures such as nanocombs and nanosheets with widths of a few microns were generated along with the 1D nanostructures in the region 0.15 cm away from the inlet position. Furthermore, the nanostructures gradually changed from 1D nanostructures to 2D nanostructures from this region toward the center of the microchannel, as shown in Figure 4c, whereas a small amount of microstructures were observed near the center position (Figure 4d). The microstructures near the center have a very low density compared to the other positions and seemed to be the result of agglomeration or coalescence of the ZnO nanostructures. From the center position, along the gas flow direction, 2D nanostructures were again observed, as shown in Figure 4e, which is similar to that of Figure 4c. The transition trend of nanostructures toward the outer side of the microchannel is symmetrical to that of the inlet side (2D to 1D, Figure 4e-g). These results suggest strongly that there was no significant effect of advection or temperature gradients on the nanostructure growth inside the microchannel since the thermal mass of the microchannel in comparison with the system is negligible. We believe that the only symmetric changes in the chemical vapor composition due to diffusion inside the channel resulted in such structure evolution. The changes of the chemical vapor composition inside the microchannel should theoretically be determined by the diffusion speeds of each species in the gas phase and consumption rates
Ra et al.
Figure 4. SEM images of the as-synthesized ZnO nanostructures near the bottom along the longitudinal distance inside the microchannel. (a-g) SEM images of ZnO nanostructures corresponding to regions (a-g), respectively, which are labeled along the longitudinal distance (a, inlet; b, 1.5; c, 2.0; d, 2.5; e, 3.0; f, 3.5; and g, 5.0 mm) from the inlet of the microchannel based on the carrier gas flow direction.
of the reactant species at the wall of the microchannel. To obtain better insight into the experimental results, we performed a chemical vapor diffusion model based on mass transport, which enables the changes of the chemical vapor composition along the microchannel to be predicted. On the basis of the experimental observations, we ignored the advection effect and temperature gradients inside the microchannel. Since nitrogen gas with a high partial pressure was used as the carrier gas, we considered the binary diffusion of the target species (O2, Zn, and ZnO) for the nitrogen gas. Although it is not shown here, we performed the model for multicomponent diffusion based on the Maxwell-Stefan equations16 but obtained results similar to that for binary diffusion. In addition, homogeneous reactions inside the microchannel were ignored because each species had a comparable mean free path with the small geometry of the microchannel. In the diffusion model considered here, we assume that the chemical vapor can be satisfactorily approximated to an ideal gas mixture, due to the low operating pressure of 7.5 torr. For steady-state conditions, a diffusion equation without homogeneous reactions can be expressed as
DAB 2 ∇ PxA - RCA ) 0 RT
(A ) O2, Zn, ZnO, B ) N2) (1)
where xA is the mole fraction of species A; P is the total pressure; RCA is the consumption rate of species A due to the
Vapor Composition in 1D to 2D ZnO Nanostructures
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nanostructure growth; and DAB is the diffusivity for the binary mixture of A and B. The parameters DAB (DO2N2 ) 174 cm2/s, DZnN2 ) 140 cm2/s) were determined directly from the Chapman-Enskog kinetic theory.16 We assumed that the diffusivity of the mixture of ZnO and N2 was the same as the value of DZnN2 because the basic thermodynamic information of ZnO vapor is not known. Even though the diffusivity of the mixture of ZnO and N2 is likely overestimated due to the high atomic weight of ZnO compared to that of Zn, this approach was used to perform the qualitative analysis in terms of the diffusion speeds of each species. To solve eq 1, the mole fractions of each species at the inlets of the microchannel were determined from the volumetric flow rate ratio of each species which was supplied to the furnace since the consumptions of the chemical vapor species were started near the Si substrate. In the case of Zn, the volumetric flow rate was obtained from the measurement of the weight loss (2 g/hr) of the Zn powders after the growth. It can be expected that the Zn powders would be partially oxidized along the thermodynamics during the evaporation of the Zn metal at the furnace temperature of 900 °C. Anthrop et al. reported that ZnO sublimed congruently by dissociation into zinc atoms and oxygen molecules only at the evaporation temperature.17 Furthermore, they estimated that the partial pressure of gaseous ZnO molecules was very low, being 4 orders of magnitude less than the Zn partial pressure for the Zn-ZnO system at 1258 K. Therefore, it is reasonable to assume that the mole fraction of gaseous ZnO molecules can be ignored in the chemical vapor diffusion modeling. However, we include the effect of the ZnO vapor with a low partial pressure in this work, to perform the qualitative analysis for the role of the chemical vapor composition in the formation of the various nanostructures. In addition, it should be noted that the mole fraction of Zn obtained from the weight loss of the Zn powders in this work might be underestimated, due to the oxidation of the Zn powders. For the sake of simplifying eq 1, 1D steady state diffusion equations were considered in this work. To obtain the solution of eq 1, the consumption rate of the chemical species at the wall of the microchannel should be specified and is directly related to the growth rate of the various nanostructures. To the best of our knowledge, there are no attempts to obtain the expression of the surface kinetic growth rate due to the complexity of the growth behavior and its strong coupling with the key parameters such as the temperature, pressure, and chemical vapor composition ratio. For the purpose of simplifying the qualitative analysis, we considered simply the consumption rate (RC) of each species as the condensation rate based on the simple kinetic theory of gases.
RCA )
GsAPxA
√2πmkT
(A ) Zn, ZnO)
1 RCO2 ) RCZn 2
(2) (3)
where sA is the sticking coefficient of Zn and ZnO; P is the total pressure; k is the Bolzmann constant; T is the temperature; G is the geometry factor; and D is the diameter of the microchannel. In this work, the sticking coefficient of ZnO is assumed to be unity, due to its high melting point of 1975 °C. Weidenkarff et al. investigated the condensation and crystallization of zinc in the presence of oxygen and nitrogen in a tube furnace with a diameter of 3 cm having a large temperature gradient.18 They reported that the amount of zinc which condensed at temperatures between 833 and 973 K was less than 1% of the total mass of condensation under experimental
Figure 5. Modeling results for the variations of the chemical vapor composition along the microchannel. (a) The composition changes of each species as a function of the distance in the case where the fractions of all species entering into the microchannel are equimolar. (b) The partial pressure changes of Zn vapor as a function of the sticking factor. (c) The composition changes of each species as a function of the distance under the experimental conditions in this work and the 0.01 sticking factor of Zn vapor.
conditions similar to this work. Therefore, it is reasonable to assume that the ratio of the sticking factors of ZnO to Zn vapor is over 2 orders of magnitude. Figure 5 shows the variations of the chemical vapor composition along the longitudinal distance of the microchannel obtained based on the chemical vapor diffusion modeling. To obtain better insight into the effects of the chemical vapor composition, we calculated the changes of each partial pressure along the longitudinal distance assuming that the fractions of all species entering into the microchannel are equimolar, as shown in Figure 5a. It was found that the ZnO vapor was consumed near the edges of the microchannel, in spite of its high partial pressure compared to the experimental observations. In addition, it is worthwhile noting that the oxygen partial pressure is almost constant, due to the high diffusion speeds and low consumption rates in this region, where the nanostructures were changed, as shown in Figure 4. Therefore, it is clear that ZnO vapor and oxygen molecules are not responsible for the changes of the nanostructures in this work. To evaluate the effects of the Zn vapor, Figure 5b represents the changes of the partial pressure of Zn vapor normalized with respect to the inlet value along
17930 J. Phys. Chem. C, Vol. 112, No. 46, 2008 the channel distance as the sticking factor of Zn which is varied from 0.1 to 0.001. The modeling results show that the Zn vapor cannot be supplied to the formation region of the 2D nanostructures when the sticking factor is less than 0.01. Therefore, it can be concluded that the evolution of the nanostructures along the channel distance is most likely due to the decrease in the amount of Zn vapor and that its sticking factor must be more than 0.01 under our experimental conditions. Figure 5c shows the changes of the normalized partial pressure calculated from the inlet partial pressures under our experimental conditions and the 0.01 sticking factor of Zn vapor. The oxygen partial pressure does not change significantly along the entire length of the channel, but the Zn vapor pressure is drastically decreased, due to its slow diffusion speed and large consumption rate. These results suggest that our experimental environment shown in Figure 1 allows only the Zn vapor pressure to change along the channel distance while keeping the other conditions such as the temperature and partial pressure of oxygen constant. Therefore, the amount of Zn vapor alone may be responsible for the evolution of the nanostructures, as shown in Figure 4. In addition, it is believed that the low partial pressure of Zn vapor is responsible for the formation of the 2D nanostructures. Furthermore, the density of the 2D nanostructures was increased toward the center position, due to the decrease of the partial pressure of Zn vapor. Therefore, it is concluded that the composition of the Zn vapor plays a key role in the evolution from 1D nanostructures to 2D nanostructures. There have been numerous reports on the synthesis and control of nanostructures by simple thermal evaporation without the presence of a catalyst.1-3 Although the synthesis procedure of this method is simple, the synthesis of the nanostructures is attributed to the complex growth mechanism which is strongly coupled with the process conditions such as the temperature, pressure, and chemical composition rates of each species. Especially, several mechanisms have been proposed to understand the formation of 2D nanostructures such as nanosheets and nanocombs. The basic concepts of these mechanisms are the spontaneous organization of the vapor species into 1D nanostructures and their subsequent microscale assembly with preferential growth. Wang et al. reported that the surface polarity played an important role in the formation of their comblike structures by a self-catalyzed process due to the enrichment of Zn at the growth front.1 Park et al. suggested that 2D nanostructures such as nanocombs and nanosheets are created by a “1D branching and 2D filling” process based on the supersaturation state.19 In spite of their contributions, understanding the fundamental phenomena, when and why the evolution of the various 2D nanostructures occurs, still remains a significant challenge. This work shows clearly that the amount of Zn vapor in a high oxygen partial pressure environment is a critical parameter in the evolution from 1D nanostructures to 2D nanostructures. We believe that our efforts will provide the basic insights required for further experimental and theoretical
Ra et al. work to be done to capture the essential growth phenomena of the various nanostructures. Conclusion In conclusion, various ZnO nanostructures were synthesized inside a microchannel by the simple thermal evaporation of metallic Zn powder without the presence of a metal catalyst. It was found that the shapes of the synthesized nanostructures inside the microchannel were symmetric along the longitudinal direction (inlet to the outlet). These results could help us to deduce the net effect of the source vapor chemical composition on the evolution of the nanostructures in the absence of advection and/or a temperature gradient. The modeling results revealed that the composition of the Zn vapor under a high oxygen pressure is responsible for the evolution from 1D nanostructures to 2D nanostructures. This work will provide a basic understanding of the hierarchical assembly of nanoscale building blocks in the field of nanoscale science. Acknowledgment. This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MES) (KRF-2006-D00411-I02959). References and Notes (1) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (2) Yan, H.; He, R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. J. Am. Chem. Soc. 2003, 125, 4728. (3) Schmidt-Mende, L.; MacManus-Driscoll, J. Mater. Today 2007, 10, 40. (4) Lin, X.; He, X. B.; Yang, T. Z.; Guo, W.; Shi, D. X.; Gao, H.-J.; Ma, D. D. D.; Lee, S. T.; Liu, F.; Xie, X. C. Appl. Phys. Lett. 2006, 89, 43103. (5) Carsia, P. F.; McLean, R. S.; Reilly, M. H., Jr. Appl. Phys. Lett. 2003, 82, 1117. (6) Huang, M. H.; Moo, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (7) Pan, Z. W.; Mahurin, S. M.; Dai, S.; Lowndes, D. H. Nano Lett. 2005, 5, 723. (8) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (9) Wang, H. T.; Kang, B. S.; Ren, F.; Tien, L. C.; Sadik, P. W.; Norton, D. P.; Pearton, S. J.; Lin, J. Appl. Phys. Lett. 2005, 86, 243503. (10) Wang, J. X.; Sun, X. W.; Wei, A.; Lei, Y.; Cai, X. P.; Li, C. M.; Dong, Z. L. Appl. Phys. Lett. 2006, 88, 233106. (11) Kumar, N.; Dorfman, A.; Hahm, J-. I. Nanotechnology 2006, 17, 2875. (12) Dubrovskii, V. G.; Sibirev, N. V. Phys. ReV. E 2004, 70, 0316041. (13) Hejazi, S. R.; Madaah Hosseini, H. R. J. Cryst. Growth 2007, 209, 70. (14) Umar, A.; Im, Y. H.; Hahn, Y. B. J. Electron. Mater. 2006, 35, 758. (15) Ra, H. W.; Choi, K. S.; Ok, C. W.; Jo, S. Y.; Bai, K. H.; Im, Y. H. Appl. Phys. Lett. 2008, 93, 033112. (16) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena, 2nd ed.; Wiley: New York, 2002. (17) Anthrop, D. F.; Searcy, A. W. J. Phys. Chem. 1964, 68, 2335. (18) Weindenkaff, A.; Steinfeld, A.; Wokaun, A.; Auer, P. O.; Eichler, B.; Reller, A. Solar Energy 1999, 65, 59. (19) Park, J. H.; Choi, H. J.; Choi, Y. J.; Sohn, S. H.; Park, J. G. J. Mater. Chem. 2003, 14, 35.
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