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Versatile Route to the Controlled Synthesis of Multilevel Branched Silicon Submicrometer/ Nanostructures Min Sun,† Yihua Gao,*,† Jun Su,† Xiangyun Han,† Xianghui Zhang,† Qi Zhang,† Guozhen Shen,† Aiqing Zhang,‡ Lei Jin,§ and Jianbo Wang§ Wuhan National Laboratory for Optoelectronics, School of Physics, Huazhong UniVersity of Science and Technology, Wuhan 430074, China, College of Chemistry and Materials, South-Central UniVersity for Nationalities, Wuhan 430074, China, and Department of Physics and Center for Electron Microscopy, Wuhan UniVersity, Wuhan 430072, China ReceiVed: September 05, 2009; ReVised Manuscript ReceiVed: October 21, 2009
A variety of silicon based multilevel branched submicrometer/ nanostructures, such as branched nanowheatheads, big branched nanowheat-heads, and branched nanowires, have been rationally synthesized via a simple one-step, inexpensive, and catalyst-free fabrication technique. High-resolution transmission electron microscopy studies suggested that the main stem of wheat head and the nanotips of silicon branched nanowheat-heads are single crystals with the preferential growth direction along the [1j12] and [11j2] and orientation, respectively. Compared with big branched nanowheat-heads and branched nanowires, the room-temperature Raman frequency of branched nanowheat-heads is blue-shifted and its full width at half-maximum broadens. A moderately strong photoluminescence emission at 550 nm was suggested to be induced by defects, such as stacking faults or the SiOx surface in the branched nanowheat-heads, suggesting potential applications in light-emitting nanodevices. These studies shed light on new opportunities for fabricating different 3-dimensional nanostructures based on their property investigation. 1. Introduction Extensive efforts of ever-increasing control over the unique morphologies, intriguing properties, potential novel applications in mesoscopic physics, and fabrication of nanoscale devices are outstanding themes of nanotechnology, in particular, when the materials are of technological relevance.1-3 Obviously, silicon has long been considered as a promising candidate for the semiconducting material, due to its peculiar physical properties in a microelectronic field. A diversity of silicon based nano- or microscale structures, such as, porous silicon,4 nanowire,5 nanotube,6 and single-crystal silicon panel of nanostructures from wires to needles and nanotrees,7 have triggered much substantial attention as novel structures for various applications in the fields of solar cells,8,9 chemical sensors,10 or high surface electrodes for lithium batteries.11-13 Great progress has been made in the synthesis of silicon nanostructures. Nevertheless, the controllable preparation of silicon branched nanostructures, especially the preparation of complex multilevel-shaped dendrites, has become a very important technology matter and faces a serious challenge. Complex multilevel-shaped dendrites are anticipated to play a dominant role as parallel connectivity and interface compatibility of different functional elements and units in assembling mini-functional and high-integrated devices of the next generation. Conceptually, multilevel-shaped dendrites open the door to more complex three-dimensional materials and devices exploiting the unique function of nanomaterials. Lieber et al.14 reported the synthesis of branched and hyperbranched nanowire structures via a multistep nanocluster-catalyzed vapor-liquid-solid ap* Corresponding author. E-mail:
[email protected]. Phone: +86-27-87792242-806; +86-15807135274. Fax: +86-27-87792225. † Huazhong University of Science and Technology. ‡ South-Central University for Nationalities. § Wuhan University.
proach. Silicon nanotree structures, which are formed in a low pressure chemical vapor deposition reactor (LPCVD) by twostep deposition sequences and directly under the main catalyst, have also been reported.15,7 However, these studies usually consist of two or three steps and control over the nanocluster catalyst concentration to confine the density and diameter of nanoscale branches that ultimately are central to the rational design of building blocks for devices. To seek after the potential of the multilevel branched nanostructures, in this vein, we carried out the effective synthesis of large-scale silicon unique multilevel branched nanowheat-heads (BNWHs) using a simple, one-step, inexpensive, and catalyst-free fabrication technique. To the best of our knowledge, the single-step synthesis of silicon multilevel BNWHs has not yet been reported to date. This synthetic strategy by adjusting experimental parameters can be readily extended to prepare novel diverse branched nanostructures, such as big branched nanowheat-heads (BBNWHs), branched nanowires (BNWs), and much more complex structures. The microstructure feature and optical properties of the complex silicon BNWHs compared with BBNWHs and BNWs were also discussed in detail. 2. Experimental Section The novel silicon multilevel branched nanostructures were fabricated in a vertical induction furnace consisting of a fusedquartz tube and an induction-heated cylinder made of high-purity graphite coated with a carbon fiber thermo-insulating layer.16 The heated cylinder had two inlets on its top and bottom, respectively, and one outlet on its bottom. In order to prepare BNWHs, a mixture of SiO powder and graphite powder (graphite powder was not added into the synthesis of BNWs) was loaded into a graphite crucible at the center cylinder zone. After evacuation of the chamber to 3.6 × 10-3 Pa, the furnace was rapidly heated and kept at 1300 °C for 1 h. During this
10.1021/jp908797d 2010 American Chemical Society Published on Web 11/06/2009
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period, two pure N2 flows were supplied from the inlets at the top and bottom, which were maintained at a constant flow rate of 500 and 200 sccm, respectively. After the furnace was cooled to room temperature in the flowing N2, some gray-green products were collected from the internal wall of graphite crucible. The corresponding morphology and microstructure of the silicon branched structures have been systematically characterized in order to elucidate the growth procedure. The features of morphology as overall information were analyzed by means of field emission scanning electron microscope (FE-SEM, Sirion 200). The crystal structure and phase purity were determined using X-ray diffraction (XRD, X’Pert PRO) and X-ray photoelectron spectrometer (XPS, VG Multilab2000). Research on transmission electron microscopy (TEM) and selected area electron diffraction (SAED) was performed on electron microscope (Tecnai G2 20) equipped with an energy-dispersive X-ray spectrometer (EDS). The high-resolution TEM (HR-TEM) micrographs were recorded with a JEOL JEM-2010FEF (UHR) electron microscope equipped with field emission gun. Measurements of Raman spectroscopy excited by an argon-ion laser with wavelength of 514.5 nm and the photoluminescence (PL) spectroscopy excited by a He-Cd laser (λex ) 325 nm) were implemented on a LabRam HR UV spectrometer (JY-Horiba). 3. Results and Discussion 3.1. Morphology and Structural Features of Multilevel Branched Silicon Crystallized Nanowheat-heads. The representative FE-SEM images of the Si BNWHs are shown in Figure 1. Figure 1a reveals a typical low-magnification FE-SEM image of the ordered BNWHs, which shows the general morphology of the product. Due to the role of saturated vapor pressure and airflow regulation, each branch is almost independent and preferentially oriented. Magnified FE-SEM image depicted in Figure 1b clearly shows the specific characteristic of multilevel-shaped dendrites which resemble “wheat head”. Each branched nanostructure is almost uniformly composed of interesting hierarchical “wheat head-like” structures with the lengths of several tens of micrometers and the transverse sizes ranging from 15 to 70 nm. These branched nanostructures with large surface-to-bulk ratios and small branch diameters are be propitious to enhance surface energy and activity of themselves, Compared with the multistep synthetic strategy, this new singlestep preparation technology of BNWHs represents a significant step in pursuit of multilevel branched nanomaterials and nanoscale devices.17,14 Structure and surface composition of as-prepared BNWHs could be further determined according to analyzation to relevant peaks in the XRD and XPS spectra. In reference to Figure 2a, a typical XRD pattern of the product is displayed. The sharp diffraction peaks can be indexed to cubic silicon (JSPCD, No. 27-1402), and a small quantity of phase belonging to oxidation of silicon is indicated by the arrow plot. This assignment is supported by the high-resolution XPS spectrum of Si 2p from the surface of the BNWHs as shown in Figure 2b. The Si 2p curve was resolved into two peaks at 99.3 and 101.2 eV with a better symmetry, corresponding to the binding energy of elemental Si and Si2+, respectively. The results of XRD and XPS analysis indicate that the as-grown product mainly consists of single crystal Si and a minute amount of SiO originated from the surface oxidation of complex branched structures. The microstructures and composition information of these novel Si BNWHs are validated using TEM and EDS. The TEM image of the BNWHs is shown in Figure 3a,b, which reflects
Figure 1. FE-SEM images with different magnifications of the large areas of orderly silicon branched nanostructures, showing unusual wheat head-like morphology.
that a bunch of BNWH structure was composed of innumerable subbranches. One side of the subbranches grows many serrated nanotips in ordered arrangement, under the suitable germination condition, some nanotips as a new growing point begin to sprout and sent up secondary, tertiary, and quaternary dendritic crystal, which was most dense and ultimately interweaved a complex dendritic crystal system, namely BNWHs. EDS spectra (Figure 3c) recorded from the crystalline wheat heads used to check the structure chemical composition, which show the appearance of Si and O X-ray signals. Thus confirming that the crystalline structure is made of Si, and the oxygen scattering are extremely few and may originate from the oxidation of silicon covered on the surface of branched structures, whereas the Cu signals come from a Cu TEM grid. To determine the crystal structure of the sample, systematic HRTEM and SAED imaging were carried out. Figure 4a is a HRTEM image of the branched nanostructure, which shows the dendrites are crystalline Si and a small quantity of amorphous silicon oxide covered on the surface of the complex branched structure. Twins and stacking faults are clearly observed in the juncture zone between nanotip and the main stem of wheat heads. It is easy to see that stacking faults and microtwins grow along the axis of branch in 〈112〉 orientation, which consistent with the literature.18 Figure 4b,c depict the HRTEM images of the area framed in Figure 4a. The pattern taken from the nanotip indicates the clearly resolved fringes with 0.31 nm separation correspond to the {111} lattice distances in cubic Si (Figure
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Figure 4. (a) HRTEM images of the Si branched nanostructure. (b and c) Lattice-resolved TEM images of the parts framed in panel a. The interplanar spacing of 0.31 nm fits well with the {111} lattice distances in cubic Si. (d) SAED patterns taken along the [110] zone axis of Si nanobranch.
Figure 2. (a) XRD pattern recorded from the as-obtained product. (b) The high-resolution Si 2p XPS survey spectrum of silicon BNWHs with the result from peak fitting procedure.
Figure 3. (a and b) TEM images of novel Si BNWHs. (c) EDS spectra taken from the center of the wheat heads.
4b), and the d spacing of 0.31 nm in Figure 4c corresponds identically to the {111} plane separation of the main stem of dendrite. It was shown that the 〈111〉 surface, which has the lowest surface energy among the surfaces in silicon, plays an important role during nanotip and the main stem of wheat head preferential growth. The representative SAED pattern of ML-
BNS is shown in Figure 4d. All diffraction spots can be easily attributed to the [110] zone axis of single-crystal silicon. The result further suggested that the Si nanotip and the main stem of wheat head are single crystals with the preferential growth direction along the [1j12] and [11j2] orientation (see also the HRTEM image of Figure. 4a), respectively. Based on all of the above analysis, we deduce the possible formation mechanism of silicon multilevel branched nanostructures. The reaction between SiO and graphite powder results in an intense Si vapor formation. The Si vapor deposits in the form of liquid droplets on the internal wall of graphite crucible. Once the initial Si crystals are formed, the coarsening of which will develop into some branches. This branching instability causes small bumps on the branches to develop sub-branches, and thus leads to the formation of multilevel branched nanostructures. This formation mechanism is in agreement with previous analysis of fractal growth, which is nonequilibrium, diffusioncontrolled kinetic process.19-21 3.2. Structural Characterization of Unique Silicon Big Branched Nanowheat-Heads. The general morphology of unique BBNWHs obtained at 1400 °C for 50 min by SiO and graphite powder was presented by the FE-SEM image shown in Figure 5a. Because of the higher reaction temperature, the BBNWHs are much larger than the BNWHs. Furthermore, detailed microstructures of magnified BBNWHs were also revealed through Figure 5b as well as the inset of Figure 5a. Reaction temperature effect on BBNWHs demonstrated that the surface of the wheat head has many processes as seed particles, and a second growth phase initiated and develop gradually into clearly defined “wheat heads” (Figure 5b) under the suitable fabrication conditions. Similar to the above-mentioned wheat head, one side of the wheat head grows many serrated processes. The SAED pattern (inset of Figure 5b) taken from BBNWHs indicates that the intensity of the cubic Si (111), (002), and (113) diffraction rings exhibits strong texture feature of the Si crystals in the branches, which implies the crystals in the branches should have a similar orientation and growth direction. In accordance with the above-mentioned branches, the axes of the branches
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Figure 5. (a and b) FE-SEM images with different magnification of the unique silicon BBNWHs grown at 1400 °C for 50 min. The inset of panel b shows the corresponding SAED pattern of BBNWHs.
Figure 6. (a) FE-SEM images of the novel branched silicon nanowires obtained at 1300 °C for 50 min by thermally evaporating SiO powder and gallium (Ga) pellet. (b-e) Typical TEM images of the silicon branch of nanowires. (f and g) SAED pattern and HRTEM image taken from the branch of nanowire marked with a square, respectively.
are generally along the 〈112〉 direction. Therefore, a versatile route for the synthesis of diverse proportion multilevel branched structures can be controlled by varying experimental parameters. 3.3. Structural Characterization of Novel Silicon Branched Nanowires. Figure 6a shows the SEM image of the branched nanowires obtained at 1300 °C for 50 min by thermally evaporating SiO powder and gallium (Ga) pellet. Due to the catalytic ability of Ga ions at relatively high temperature, more complex branched nanowire architectures revealed through the TEM (Figure 6b-e) are much different from the abovementioned branched silicon nanostructures. The TEM images show that the branches grow within a range of angles, ca. 60°-70°, with respect to the backbone. The specific angles indicate that branch growth is epitaxial, which is similar to the literature results.14 Detailed microstructures of branched nanowires were shown in Figure 6f,g by the corresponding SAED and HRTEM pattern. A SAED pattern (Figure 6f) taken from the branch of nanowire marked with a square in Figure 6b displayed that they are amorphous. Furthermore, HRTEM images (Figure 6g) give a more direct image of these noncrystalline nanowires. The chemical composition of the branch was determined by EDS. Only silicon and small amounts of oxygen
were detected. In addition, the single step preparation method of as-prepared branched nanowires has more excellent features than those reported in the relative literature.7,14,15 The details of the preparation mechanism need to be explored in-depth, and we expect these new structures could be used to fabricate interesting functional devices. 3.4. Raman Spectra and Photoluminescence Properties of Three Kinds of Silicon Branched Nanostructures. To further study and examine the as-grown novel silicon BNWHs, BBNWHs, and BNWs, we performed Raman and photoluminescence measurements on the three samples. The Raman spectra of samples (1-3) in Figure 7a show prominent Raman features at ∼515.2, 516.9, and 519.4 cm-1, respectively. In comparison with the first-order optical phonon peak of crystalline silicon (a Raman peak at 521 cm-1, the full width at halfmaximum (fwhm) of 2.8 cm-1), the corresponding Raman peaks of three kinds of Si nanostructures have tiny Raman frequency blue-shifted and its line width broadened, which could be due to the quantum confinement effect or defects, e.g., stacking faults, observed by the HRTEM of branched nanostructure. Comparing with the other two types, the Raman frequency blueshifted and fwhm broadened of silicon BNWHs is even more obvious. This phenomenon is also different from that observed previously in randomly oriented and highly oriented SiNWs.22,5 It is known that the quantum confinement effect will significantly affect the electron and phonon properties of crystals if their size is less than the Bohr radius of silicon, which is less than 10 nm at room temperature. The dimension of some branches is less than the Bohr radius of silicon, which would bring about Raman frequency down-shift and the broadening λ of the Raman peak of silicon branched nanostructures. Finally, the typical room-temperature PL spectra excited by a He-Cd laser (λex ) 325 nm) of the above-mentioned three kinds of Si nanostructures are shown in Figure 7b. For all of the samples, a moderately strong photoluminescence emission at 550 nm was suggested to be induced by defects, such as stacking faults or the SiOx surface in the branched nanowheatheads. However, the PL of BBNWHs can be Gaussian divided into a strong emission at 550 nm (2.25 eV) and a weak violet emission at 420 nm (2.95 eV) (inset of Figure 7b), which may be ascribed to the small quantity of silicon monoxide covered on the surface of BBNWHs. This result was further demonstrated by the intrinsic photoluminescence of silicon monoxide powder shown in the inset of Figure 6b. Compared with the previous studies on the PL of Si nanostructures, three emission
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Sun et al. of Si nanostructure. Therefore, the observed results suggest that multilevel shapely dendrites open the door to more complex three-dimensional materials and further assembling minifunctional and high-integrated devices of the next generation. Acknowledgment. Financial support of this work is provided by National Natural Science Foundation of China (No. 10774053), the Hubei Province Nature Science Foundation of China (No. 2007ABB008), the High-level Talent Recruitment Foundation of Huazhong University of Science and Technology, the Basic Scientific Research Funds for Central Colleges (C2009Q045), and Programs Foundation of Ministry of Education of China (No. 20070487038). The authors thank the Analysis and Testing Center of Huazhong University of Science and Technology for support. References and Notes
Figure 7. (a) Raman and (b) corresponding photoluminescence spectra acquired from the as-grown silicon BNWHs (1), BBNWHs (2), and BNWs (3) at room temperature.
bands have blue-shifted, which may be the effect of quantum confinement and the peculiar nanostructures, the profound signification of the mechanism need to be further explored.23,24 4. Conclusions In summary, the multilevel silicon branched nanowheat-heads have been synthesized via a simple, one-step, inexpensive and catalyst-free fabrication technique. We have demonstrated this basic approach can be readily extended to prepare other silicon based novel branched nanostructures, such as BBNWHs and BNWs. Detailed TEM studies of the BNWHs and BBNWHs demonstrate that the branches grow with the preferential growth direction along the 〈112〉 orientation. In comparison with the first-order optical phonon peak of crystalline silicon, the Raman peaks of three structures are blue-shifted and their fwhm broadens, which could be due to the quantum confinement effect of nanostructures or defects, e.g., stacking faults. Also the corresponding PL results show that the strong and green emission peaks around 550 nm dominate the spectrum, which have a blue shift compared with the previous studies on the PL
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