CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 21, NUMBER 2 APRIL 27, 2008

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1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 21, NUMBER 2 APRIL 27, 2008 ARTICLE Growth Mechanism and Characterization of Single-crystalline -doped O 2 Nanowires and Self-organized O 2 / 2 O 3 Heterogeneous Microcomb Structures Yong Su a, Liang Xu a,b, Xue-mei Liang a, Yi-qing Chen a a. School of Materials Science and Engineering, Hefei University of Technology, Hefei , China; b. Hefei General Machinery Research Institute, Hefei , China (Dated: Received on April 20, 2007; Accepted on May 24, 2007) Single-crystalline -doped O 2 nanowires and O 2 : 2 O 3 heterogeneous microcombs were synthesized by a simple one-step thermal evaporation and condensation method. They were characterized by means of X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). FE-SEM images showed that the products consisted of nanowires and microcombs that represent a novel morphology. XRD, SAED and EDS indicated that they were single-crystalline tetragonal O 2. The influence of experimental conditions on the morphologies of the products is discussed. The morphology of the product showed a ribbon-like stem and nanoribbon array aligned evenly along one or both side of the nanoribbon. It was found that many 2 O 3 nanoparticles deposited on the surface of the microcombs. The major core nanoribbon grew mainly along the [110] direction and the self-organized branching nanoribbons grew epitaxially along [ 110] or [1 10] orientation from the (110) plane of the stem. A growth process was proposed for interpreting the growth of these remarkable O 2 : 2 O 3 heterogeneous microcombs. Due to the heavy doping of, the emission peak in photoluminescence spectra has red-shifted as well as broadened significantly. Key words: Microcomb, Nanoribbon, Photoluminescence I. INTRODUCTION In recent years, much effort has been made to fabricate one one-dimensional semiconductor nanoscale materials for further application of nanodevices and complicated nanocircuits, due to their novel properties [1-3]. Among metal oxides, tin oxide (O 2 ) represents a functional material that can be adjusted for multiple applications by incorporation of dopant ions. Because of its large bandgap (E g =3.6 ev), O 2 is transparent in the visible-light region of the spectrum, and is thus useful as a conductive electrode and an antireflective coating [4,5]. Stimulated by the novel properties of carbon nanotubes, quasi-one-dimensional nanostructures of O 2 are currently the subject of intensive research because of the potential for nanoscale electronic and optoelectronic applications [5-9]. Modification of O 2 properties by impurity incorporation is currently another important issue for possible applications in ultraviolet optoelectronics and spin electronics [10,11]. Doping in semiconductors with selective elements offers an effective method to adjust their electrical, optical, and magnetic properties, which is crucial for their practical application [12,13]. It is well known that for a wide-band-gap semiconductor, the addition of impuri- Author to whom correspondence should be addressed. suyong1963@126.com, Tel: ties often induces dramatic changes in its electrical and optical properties [11,13]. In our previous experiment, undoped O 2 asterisklike nanostructures were successfully synthesized by the thermal evaporation method [9]. In this work, the synthesis of single-crystal -doped O 2 nanowires and self-organized O 2 : 2 O 3 heterogeneous microcombs structure is studied using a simple one-step physical process. The controlled growth of self-organized microscale comb consists of a periodic array of very uniform, perfectly aligned, evenly spaced single-crystalline straight O 2 nanoribbons with periods in the range of 20 µm to 80 µm. The ribbon-like stem of the microcombs is about 150 nm thick and several tens to several hundreds of microns long. Furthermore, no extra catalyst was used in our experiment, it is discovered that the formation process of the O 2 comb-teeth follow a faceted epitaxial growth process through atom-plane stacking parallel to the [ 110] or [1 10] directions. These branched nanostructures can potentially find diverse applications in nanoscale electronic, optoelectronic and photonic devices. II. EXPERIMENTS The synthesis was based on thermal evaporation of a mixture of O mounted horizontally inside a hightemperature tube furnace. An alumina boat filled with O and with molar ratio of 2:1 was placed in the 181 c 2008

2 182 Electric couple Yong Su et al. Horizontal tube electric furnace 2O3 and O powders Substrate 1 ( o Substrate 2 o C) ( C) Ar gas flow Vacuum pump Quartz tube FIG. 1 Schematic illustration of the experimental setup applied to synthesize the -doped O2 nanostructures. As-prepared O2 :2 O3 microcombs and -doped O2 nanowires deposited on the Au-coated Si substrate 1 and substrate 2, respectively. 20 µm 1 µm FIG. 3 Morphologies of the products deposited on the Si substrates which located in the temperature region of C. FE-SEM image of the products on a silicon substrate with a 2-nm-thick gold coating. An enlarged FE-SEM image of the sample, showing a nanowire with a nanocluster catalyst at its end. FIG. 2 XRD pattern of as-deposited products, demonstrating that products consist of O2 and 2 O3 nanowires. middle of the quartz tube at the center of the furnace. Several pieces of Si(100) plates were used as substrates, which were located at downstream positions of source materials. Finally, the entire assembly was heated to 1100 C within 30 min and maintained at this temperature for 1 h. The temperature of the substrates was approximately C during the growth. Schematic illustration of the experimental setup applied to synthesize the -doped O2 nanostructures is shown in Fig.1. Ar+5%H2 was used as carrier gas and flowed constantly at a rate of 50 standard cubic centimeters per minute (sccm). The chamber pressure was kept at 26.6 kpa during the experimental process. After the furnace slowly cooled to room temperature, substrates were taken out from the furnace tube. A white color flocculent product was found on the Au-coated area of the substrates. The morphologies and structures of the as-deposited products were characterized and analyzed by field-emission scanning electron microscopy (FE-SEM) (JEOL model JSM-6700F), X-ray diffrac tion (XRD) (MAC Science, model MXPAHF), highresolution transmission electron microscopy (HRTEM) (JEOL model 2010, operating at 200 kv), and selected area electron diffraction (SAED). Their components were measured via energy-dispersive X-ray spectroscopy (EDS) attached in the HRTEM system. A photoluminescence (PL) spectrum was measured using a He-Cd laser (325 nm) as the excitation source at room temperature. III. RESULTS AND DISCUSSION A. Structural characteristics of -doped O2 nanowires The XRD pattern has been measured for assessing the overall structure and phase purity (see Fig.2). The major diffraction peaks correspond to O2 crystal faces. Some weak peaks corresponding to the 2 O3 also exist in this spectrum, revealing a small amount of 2 O3 also was synthesized during the experiment. Analysis from the XRD pattern reveals that O2 has a tetragonal rutile structure with a lattice constant of a=4.74 A and c=31.9 A and 2 O3 has a c

3 500 nm Decomposition Mechanism of Azoisobutyronitrile nm FIG. 4 TEM and HRTEM images, and EDS patterns, of -doped O2 nanowires. TEM image of a -doped O2 nanowire, showing that its diameter is very uniform. HRTEM image from the box in panel a, the corresponding electron diffraction (ED) pattern is shown in the inset, revealing that the nanowire grows along the [112] direction. (c) EDS spectrum of the -doped O2 nanowire present in ( peaks are caused by the surface of the copper grids used in the TEM measurement). (d) EDS spectrum taken from the spherical-shaped tip. wurtzite (hexagonal) structure with lattice constants of a=3.253 A and c=5.20 A, which is consistent with the standard values for bulk O2 (JCPDS ) and 2 O3 (JCPDS ). Figure 3 shows the morphologies of the products deposited on the Si substrate which was located in the temperature region of C. Scanning electron microscopy (SEM) observations reveal that the products consist of a large quantity of wirelike nanostructures with typical lengths in the range of several tens to several hundreds of micrometers and diameters normally in the range of nm. Figure 3 shows a representative enlargred FE-SEM of the as-grown products. It clearly reveals that each nanowire has a uniform diameter ranging from 100 nm to 200 nm. Detailed microstructure and composition information of as-grown -doped O2 nanowires was further characterized by TEM. A representative TEM image shown in Fig.4 reveals that the each produced -doped O2 nanowire has a uniform diameter along its entire length. The HRTEM image and corresponding SAED pattern (see Fig.4) reveal that nanowire is single-crystalline with a uniform structure. As shown in Fig.4, the spacing between adjacent lattice planes is 0.334, corresponding to the d-spacing of the (002) planes of the O2, which indicates that the growth direction of -doped O2 nanowires is parallel to the [112] crystal direction. Further compositional analysis by EDS (see Fig. 4(c)) reveals that the -doped O2 nanowire has a ::O atomic ratio of 1:4.54:9.78. According to the classical explanation, the catalyst clusters at the tip of the -doped O2 nanowires shown in TEM detection (Fig.4) are considered as the evidence for the operation of the vapor-liquid-solid (VLS) mechanism. For interpreting the experimental result, a possible growth process of the -doped O2 nanowires is proposed: It is known that O is metastable and will decompose to and O2 at a temperature higher than 300 C [14]. Moreover, the higher the reaction temperature, the faster the rate of the decomposition [15]. As-formed may be in the form of small-sized liquid clusters (: mp of 232 C, bp of 2270 C [14]) when reduced from O. The clusters are then transported by the carrier gas (Ar) to c

4 184 a lower temperature region, where they deposited in the form of liquid droplets on the Si substrate. Therefore, these liquid droplets can serve as ideal nucleation sites for the preferential absorption of the evaporated, and O2 vapor (The dosage of O2 gas into the reactor was controlled using a leak valve). Continuous dissolution of, and O atoms in --O eutectic alloy droplets will lead to the nucleation and growth of O2 nanowires through the VLS process when the alloy droplets become saturated with reactant. At the same time, doping was achieved through the process of substitute atom in O2. Continuous feeding of and atoms into the liquid droplet sustains the growth of the -doped O2 nanowires. By analyzing the EDS spectrum (Fig.4(d)) of the end particle of a single nanowire, we find that it really consists of,, and O, which also supports our above discussion to some extent. B. Structural characteristics of O2 :2 O3 heterogeneous microcombs structure Figure 5 shows a representative FE-SEM image of the as-prepared O2 microcomb structure deposited on the Si substrate which was located in the temperature region of C. It is clearly seen that a row of nanoribbons grow along one side of a nanoribbon to form a comb-like nanostructure. It is noted that the nanoribbon branches have uniform diameters and are evenly distributed along one side of stem. Highmagnification SEM images (Fig.5) show that each array usually contains several tens of very straight, perfectly aligned and evenly spaced nanoribbons with almost constant width and spacing. The ribbon-like stem of the microcombs is about 150 nm thick and several tens to several hundreds of microns long. The microcombs are mainly made of periodic arrays of straight nanoribbons that have a rectangular cross section, as shown in Fig.5. Each nanoribbon has a uniform width and thickness, and the typical widths and thicknesses are in the range of nm and nm, respectively. The spacing between the nanoribbons varies in a wide range from 200 nm to 400 nm; however, for an individual comb the nanoribbons on it are evenly spaced and such uniformity can extend to up to several tens of microns wide. Further observation reveals that there are lots of nanoparticles deposited on the surface of the microcombs. Further structural analysis indicates the growth direction of the main stem of this ribbon is parallel to the [110] crystalline orientation of O2. The corresponding SAED pattern (inset in Fig.6) can be indexed to be the [001] zone axis of the rutile structured O2 crystal. From the TEM image shown in Fig.6, it is expected that the branch grows orthogonally on the stem, resulting in comb-like structure. Furthermore, this structure can be a planar form because the SAED Yong Su et al. 5 µm 500 nm FIG. 5 FE-SEM image of the as-prepared O2 /2 O3 heterogeneous microcomb structure deposited on the Si substrate which located in the temperature region of C. High-magnification SEM image of O2 /2 O3 heterogeneous microcomb structure, showing many 2 O3 nanocrystals formed in the O2 microcomb surface. patterns recorded from the stem and branch have the same orientation as the sample tilted in the TEM observation. Due to the tetragonal structured O2, the crystalline orientation of [110] is perpendicular to the [1 10] (or [11 0]) crystalline orientation, it is apparent that the branches grow along [ 1 10] (or [11 0]) crystalline orientation. Figure 6 shows HRTEM image taken in the junction region between the stem and the branch present in revealed that formed 2 O3 nanocrystals in the O2 microcomb surface. The lattice spacing of 3.4 A between adjacent lattice planes in this image corresponds to the distance between two (110) plane of a rutile O2 lattice. EDS sprectra were also recorded of the microcomb and the attached particles in the surface of micromb (see Fig.7), they confirmed that both the comb-stem and comb-teeth are composed of, and O elements with the same atomic ratios, and the particles are elemental with trace of O and (Fig.7). The one-step growth of the unique O2 :2 O3 heterogeneous microcomb structure is a spontaneous and selforganized process. Although the synthesis procedure is very simple, the formation and assembly processes are complex and precisely self-controlled since they involve: (i) the growth of the comb-stem along [110] by vapordeposition [6]; (ii) the nucleation and epitaxial growth of evenly spaced nanoribbons arrays along [1 10] or [11 0] directions on one edge of the comb ribbon; and (iii) the c

5 Decomposition Mechanism of Azoisobutyronitrile 185 O O nm 5 10 Energy / kev FIG. 7 and EDS spectra of O2 microcomb and 2 O3 nano-crystal, respectively. 10 nm FIG. 6 Typical TEM image of part of a O2 /2 O3 heterogeneous microcomb, displaying parallel, straight, uniform nanoribbons growing perpendicularly from both edges of the comb ribbon (the corresponding SAED spectra is shown in the inset, revealing the major core nanoribbon grew mainly along [110] direction and the self-organized branching nanoribbons grew epitaxially along [1 10] or [11 0] orientation). HRTEM image of junction region of O2 microcomb, showing the 2 O3 nanocrystals formed in the O2 microcomb surface. planar filling that makes the comb ribbon widen and thicken [16,17]. At the same time, the as-grown microcombs can also be used as templates for the adsorption or deposition of the vapors in the furnace system, resulting in the formation of nuclei on the growing microcomb surface. In this process, due to the residual O2 insurmountable negligible leakage in the furnace system, the high temperature will cause oxidation of the droplet into 2 O3 nanocrystals, and thus finally form the O2 /2 O3 heterogeneous microcomb structure. The TEM analyses suggest that the growth of the microcomb-stem is a vapor-solid (VS) process rather than a VLS process because we did not find any low melting-point phases at the growth front of the nanoribbon of comb-stem. However the exact reason for the formation of 2 O3 nanocrystals was not clear at this stage, but it was most likely linked to the reaction between active vapor and the formed O2 microcombs. It was imaginable that due to the high content of in the vapor, the O2 micrombs were preferentially formed by the VS model. The content of vapor around micrombs increased at the same time. The O2 nanoparticle is provided with high surface energy, so it can absorb the atom easily. Then the fresh O2 microcomb tended to serve as a template for the adsorption or deposition of the vapors, so some part of O2 microcomb surface was eroded to form 2 O3 nanocrystal. The final size of the 2 O3 nanocrystal may be determined by the concentration in the vapor, the ratio of to in the vapor may be critical to get this structure. Several repeated experiments with different ratio of to in the source materials have been done, and the products were quite different. If this ratio was too low to form the 2 O3 nanocrystals, atoms may dissolve into O2 nanowires (surface layer) to be the impurities. If this ratio was too high, the nucleation and growth of 2 O3 nanostructure will happen preferentially. A mixture of O2 nanowires and 2 O3 nanostructure (nanowire or nanocrystals) may be obtained. Therefore, we considered that the ratio of to in the vapor can remarkably influence the morphologies of the O2 /2 O3 composite nanostructures. Further research work should be done to measure and control the ratio of to in the vapor by an efficient method. C. Luminescence properties of -doped O2 nanowires and O2 /2 O3 heterogeneous microcombs Figure 8 dispalys the PL spectra recorded from the as-prepared -doped O2 nanowires and the O2 /2 O3 heterogeneous microcombs at room temperature, respectively. As shown in Fig.8, the excitation energy is ev (325 nm) and the two nanosc

6 186 Yong Su et al. straight O 2 nanoribbons with periods in the range of 20 µm to 80 µm. The branching mechanism discussed in this work is generic and, therefore, can be applied to other materials (e.g. In in 2 O 3 and Cd in O 2 ), thus opening a new approach in synthesizing of similarly branched structures. These branched nanostructures can potentially find diverse applications in nanoscale electronic, optoelectronic and photonic devices. Due to the heavy doping of, the O 2 nanostructures exhibit a green emission peak in PL spectrum that has red-shifted as well as significantly broadened. FIG. 8 PL spectrum of -doped O 2 nanowires and the O 2/ 2O 3 heterogeneous microcombs at room temperature. The excitation wavelength of PL is the 325 nm line from a He-Cd laser. tructures have the same dominant peak, located at 584 nm (2.123 ev), which has red-shifted strongly to lower energy as well as broadened seriously as compared to the emission of the undoped O 2 nanowires [18-20]. Large charge density of will induce more defects such as oxygen vacancies which act as luminescent centers. Furthermore, the high aspect ration of the nanostructures favored the existence of large quantities of oxygen vacancies. The strong yellow emission band at 584 nm (2.123 ev) can be attributed to surface states induced by oxygen or tin vacancies as suggested by other authors [19-21]. It is interesting to note that green emission intensity in O 2 nanowires is stronger than that in the O 2 microcombs. Vanheusden proved that the stronger the intensity of green luminescence, the more singly ionized oxygen vacancies there are [21]. The weaker green light emission in O 2 microcombs might be related to the lower oxygen vacancies concentration compared to the O 2 nanowires, beause O 2 microcombs formed in the higher temperature region of C compared with that of O 2 nanowires formed, which decreased the oxygen vacancy concentration. IV. CONCLUSION In summary, large-scale single-crystalline -doped O 2 nanowires and self-assembled O 2 : 2 O 3 heterogeneous microcombs were achieved and their composition and single-crystalline structures were confirmed using XRD, TEM, and SAED. The as-synthesized doped O 2 nanowires are single-crystalline, the typical diameter of -doped O 2 nanowires is in the range of nm, and the lengths are in the range of several tens to several hundreds of micrometers. The doped O 2 nanowires were grown by an -catalyzed VLS process. The controlled growth of self-assembled microcombs consists of a periodic array of very uniform, perfectly aligned, evenly spaced single-crystalline V. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No )and the Natural Science Foundation of Anhui province, China (No ). [1] Y. i and C. M. Lieber, Science 397, 851 (2001). [2] X. F. Duan, Y. Huang, Y. i, J. Wang, D. C. Smith, and C. M. Lieber, Nature 409, 66 (2001). [3] M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith, and C. M. Lieber, Nature 415, 616 (2002). [4] G. Sberveglieri, Sens. Actuators B 6, 239 (1992). [5] Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, and C. Zhou, Adv. Mater. 15, 1754 (2003). [6] Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 (2001). [7] D. F. Zhang, L. D. Sun, J. L. Yin, and C. H. Yan, Adv. Mater. 15, 1022 (2003). [8] T. o and T. H Wang, Chem. Commun (2004). [9] D. Cai, Y. Su, and Y. Q. Chen, Mater. Lett. 59, 1984 (2005). [10] H. Kim and A. Piqué, Appl. Phys. Lett. 84, 218 (2004). [11] Q. Wan and T. H. Wang, Chem. Commun (2005). [12] I. M. Chan, T. Y. Hus, and F. C. Hong, Appl. Phys. Lett. 81, 1899 (2002). [13] N. Pho, T. N. Hou, and K. Jing, Nano Lett. 3, 925 (2003). [14] A. I. Cooper, Adv. Mater. 15, 1049 (2003). [15] S. Takeuchi, M. Iwanaga, and M. Fujii, Philos. Mag. A 69, 1125 (1994). [16] J. H. Park, H. J. Choi, Y. J. Choi, S. H. Sohn, and J. G. Park, J. Mater. Chem. 14, 35 (2004). [17] Y. H. Leung, A. B. Djurisic, J. o, M. H. Xie, Z. F. Wei, S. J. Xu, and W. K. Chan, Chem. Phys. Lett. 394, 452 (2004). [18] J. Q. Hu, J. X. L. Ma, N. G. Shang, Z. Y. Xie, N. B. Wong, C. S. Lee, and S. T. Lee, J. Phys. Chem. B 106, 3823 (2002). [19] J. Q. Hu, Y. Bando, and D. Golberg, Adv. Funct. Mater. 13, 493 (2003). [20] N. Chiodini, A. Paleari, D. DiMartino, and G. Spinolo, Appl. Phys. Lett. 81, 1702 (2002). [21] K. Vanheusden, W. L.Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, and B. E. Gnade, J. Appl. Phys. 79, 7983 (1996). c 2008