(P~SiC) and/or trigonal silicon nitride (a-SiaN^ nanowires (NWs) were prepared by ..... of silicon carbide nanorods and carbon nanotubes by chemical vapor ...
SYNTHESIS AND CHARACTERIZATION OF CUBIC SILICON CARBIDE (P-S1C) AND TRIGONAL SILICON NITRIDE (a-Si3N4) NANOWIRES Karine Saulig-Wenger, Mikhael Bechelany, David Cornu*, Samuel Bernard, Fernand Chassagneux and Philippe Miele Laboratoire des Multimateriaux et Interfaces UMR 5615 CNRS University Claude Bernard - Lyon 1 43 Bd du 11 novembre 1918 F-69622 Villeurbanne Cedex, France David. Cornu@univ-lyon 1 .fr Thierry Epicier GEMPPM UMR 5510 CNRS - INSA Lyon 20 Avenue Albert Einstein F-69621 Villeurbanne Cedex, France ABSTRACT By varying the final heating temperature in the range 1050°C - 1300°C, cubic silicon carbide (P~SiC) and/or trigonal silicon nitride (a-SiaN^ nanowires (NWs) were prepared by direct thermal treatment under nitrogen, of commercial silicon powder and graphite. Long and highly curved p-SiC NWs were preferentially grown below 1200°C, while straight and short a-SiaN4 NWs were formed above 1300°C. Between these two temperatures, a mixture of both nanowires was obtained. The structure and chemical composition of these nanostructures have been investigated by SEM, HRTEM, EDX and EELS. INTRODUCTION Numerous studies have been recently devoted to ceramic and metallic nanowires (NWs) due to their outstanding properties which can be tailored by varying their chemical composition and also their crystalline structure. The possible applications of these NWs run from nanoelectronics to composite materials. Among the series, silicon-based NWs, made of cubic silicon carbide (P-SiC), silicon dioxide (SiO2) or trigonal silicon nitride (a-SiaN^, are of particular interest due to their interesting mechanical1'2, electrical3'4 and/or optical5 properties. However, industrial applications clearly need a cheap growth method for the large-scale fabrication of NWs. Moreover, works are also devoted to coaxial nanocables (NCs) due to their possible uses as reinforcement agents for mechanical applications, the outer sheet of the NC acting as an interface between the nanowire and the matrix. Numerous routes have been reported for the fabrication of SiC NWs. The most promising ones for large scale production are (i) carbothermal reduction routes using carbon nanotubes (CNTs) as templates6"8 or a mixture of SiO2 nanoparticles and active carbon9 and (ii) methods based on VLS (Vapour Liquid Solid) growth mechanism e.g. assisted by catalytic metallic nanoparticles10"14. Only few techniques have been described for the preparation of silicon nitride nanowires. They usually
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correspond to those reported for SiC NWs but they are driven at higher temperature (1400°C to 1650°C). The three main routes are the following: (i) carbothermal reduction of silica15"19, (ii) the use of carbon nanotubes as templates20 and (iii) a VLS growth technique21. All those synthetic methods require however, either expensive starting materials such as CNTs or expensive equipment. We previously reported that the direct pyrolysis under nitrogen of commercial silicon powder in presence of graphite led to the formation of cubic silicon carbide (p-SiC) NWs22'23. These nanowires have diameters in the range 20 - 30 nm and micrometric lengths. We showed that when a mixture of argon and oxygen is used instead of nitrogen, amorphous silica nanowires were preferentially obtained24'25. As an extent to these results, we report in the present paper the influence of the final heating temperature on the structure and chemical composition of the resulting siliconbased nanostructures. EXPERIMENTAL
All the experiments were conducted following the same experimental procedure. Silicon powder (Aldrich 99.999%, 60 mesh) was placed in an alumina boat containing a piece of graphite. This boat was then placed in the alumina tube of a horizontal tubular furnace, the tube being previously degassed in vacuo before filling with nitrogen (electronic grade). Under the gas flow (5 mL.min"1), the boat was heated up to the selected temperature (heating rate 200°C min"1), held for 1 hour then allowed to cool down to room temperature. Finally, powder was scraped from the alumina boat and analysed by scanning electron microscopy (SEM, Model N°S800, Hitachi), high-resolution transmission electron microscopy (HRTEM, Field Emission Gun microscope JEOL 201 OF) and electron energy-loss spectroscopy (EELS, Digi-PEELS GATAN). RESULTS AND DISCUSSION In previous works, P-SiC nanowires have been obtained by heating silicon powder under nitrogen at 1200°C, this temperature being held during 1 hour before cooling down22'23. In order to examine the influence of the final heating temperature on the yield, the crystallographic structure and the chemical composition of the resulting nanowires, five independent experiments have been conducted up to 1050°C, 1150°C, 1200°C, 1250°C and 1300°C, respectively. In all these experiments, the final temperature was held during Ih before cooling down. Each crude product was first analysed by SEM and representative images of each sample are shown in fig. 1.
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Fig. 1: SEM images of the crude products obtained at different final heating temperature: (a) 1050°C, (b) 1150°C, (c) 1200°C, (d) 1250°C and (e) 1300°C.
The crude sample heated up to 1050°C contained only few nanowires (Fig. la), located on the surface of the residual silicon particles. These NWs are straight with scattered diameters (from -10 to -300 nm) and short lengths (below -3 |tim). When the experiment was conducted up to 1150°C, it resulted in the formation of larger amount of nanowires (Fig. Ib). Their diameters are in the same range but they are longer with lengths estimated above -8 jum. As illustrated by Fig. Ib, this result should be related with their highly curved shape. Nanowires exhibiting similar diameters and lengths were obtained at 1200°C (Fig. Ic). In that case, the yield, estimated from the SEM
images, was however significantly improved. In contrast, when the thermal treatment was driven up to 1250°C, the yield was not significantly improved (Fig. Id). At this temperature, two kinds of nanowires were observed: numerous long and highly curved nanowires similar to those obtained at 1200°C mixed with few straight nanowires (Fig. Id, white arrows). The latter are shorter and exhibit well-defined angles which can be interpreted as changes in direction of the NW axis (Fig. Id, white circles). The SEM image of the samples obtained after heating up to 1300°C shows a strong modification in the shape of the obtained NWs (Fig. le). No long and curved nauowires were detected but numerous straight NWs were observed. Their diameters are comparable to those previously obtained but their lengths are shorter and below -8 |tim. HRTEM investigations coupled with EELS analysis have been conducted in order to determine if there is a difference in structure and/or chemical composition within the two kinds of NWs observed by SEM. Figure 2a shows a HRTEM image of a typical long and curved nanowire obtained at 1200°C. On the corresponding EELS spectrum (Fig. 2b), two main features are observed at -100 eV and -284 eV corresponding to Si-L and C-K edges, respectively. As expected, further selected area electron diffraction (SAED) analysis showed that cubic silicon carbide (p-SiC) nanowires have been formed. According to HRTEM investigation, a high density of stacking faults was observed (fig. 2a) which can be related to high growth rate, as previously mentioned22'23.
Fig. 2: HRTEM image of a P-SiC NW (a) with the corresponding EELS spectrum (b).
In contrast, figure 3 a shows a typical HRTEM image of a straight and short nanowire obtained at 1300°C. The chemical composition of the observed NW has been determined by EELS analysis performed on its core (Fig. 3b). Two main features are observed at -100 eV and -400 eV corresponding to Si-L and N-K edges, respectively. Features at -284 eV and -532 eV, corresponding to C-K edge and O-K edges respectively, were not detected. A coating of-2.5 nm thickness was observed around the analysed NW. Its EELS analysis performed using a 2 nm probe revealed that it is composed of carbon. As illustrated by figure 3c, fast Fourier transformation (FFT) shows that the core of the NW is composed of the trigonal polymorph of silicon nitride (a-SiaN^. Complementary
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structural analysis showed that the carbon coating is amorphous. The obtained nanostructures can be considered as carbon sheathed silicon nitride nanocables (a-Si3N4@C).
Si3N4
carbon
010
100 Fig. 3: (a) HRTEM image of a a-Sia^ NW coated with amorphous carbon with corresponding (b) EELS spectrum and (c) fast Fourier transformation (FFT) of the core of the NW.
In previous works, we suggested that the silicon carbide nanowires were formed at 1200°C by a Vapour-Solid (VS) nucleation process, carbon being transported to silicon nanoparticles by nitrogen through the formation of CN-like derivatives22'23. For the formation of a-SiaN4 NWs e.g. at higher temperature (>1250°C), we can suggest a similar VS growth mechanism with a preferential nucleation of silicon nitride onto the surface of the silicon nanoparticles due to the higher temperature reached during the experiment. The formation of an amorphous coating of carbon onto the NWs can be interpreted by considering that carbon transportation by nitrogen was still effective at that temperature. Comparative analysis of nanowires obtained at 1300°C revealed that main aSisN4 NWs were free of carbon coating. Moreover, no SiC nanowires were detected in the sample obtained by heating up to 1300°C. This result is of primary importance for the determination of the
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growth mechanism of these nanowires because it indicates that their formation did not occurred when the temperature was increased, but either when the final temperature is reached or, less probably, during the cooling step. While only a-Si3N4 NWs were formed at 1300°C, a mixture of p-SiC and a-Si3N4 NWs were obtained when the experiment was conducted up to 1250°C. This is evidenced by the bright field (Fig. 4a) and dark field (Fig. 4b) TEM images recorded on a whole mass of nanowires. II!
Fig. 4: bright field (a) and dark field (b) TEM images of a whole mass of P-SiC and a-Si3N4 NWs.
The image in bright field reveals the presence of Si3N4 nanowires, which appear in bright in Fig. 4a. The dark field image 4b, made with a P-SiC reflection, reveals that the Si3N4 NWs were mixed with SiC NWs which appear in bright in this image. These observations clearly point out that between 1200°C and 1300°C, a mixture of P-SiC NWs and a-Si3N4 NWs were obtained. CONCLUSION The direct thermal treatment of a silicon powder under nitrogen yielded the formation of cubic silicon carbide (P-SiC) in the range 1050°C - 1200°C. As we found, when the experiment was driven in the range 1200°C - 1300°C, a mixture of P-SiC and trigonal silicon nitride (a-Si3N4) nanowires (NWs) were obtained. Moreover, when the experiment was conducted up to 1300°C, only a-Si3N4 NWs were observed within the sample. The growth mechanism for these nanowires were presumed to be a VS process, starting with the nucleation of silicon carbide or silicon nitride nuclei onto the surface of the silicon nanoparticles. REFERENCES
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