CRYSTAL GROWTH & DESIGN
Bundled Silicon Nitride Nanorings Weiyou Yang,*,† Xiaomin Cheng,† Huatao Wang,‡ Zhipeng Xie,‡ Feng Xing,§ and Linan An*,| Institute of Materials, Ningbo UniVersity of Technology, Ningbo, 315016, P. R. China, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua UniVersity, Beijing, 100084, P. R. China, The Key Laboratory on Durability of CiVil Engineering in Shenzhen, Shenzhen UniVersity, Shenzhen, Guangdong, 518060, P. R. China, and AdVanced Materials Processing and Analysis Center, UniVersity of Central Florida, Orlando, Florida 32816
2008 VOL. 8, NO. 11 3921–3923
ReceiVed July 3, 2008; ReVised Manuscript ReceiVed September 5, 2008
ABSTRACT: Bundled nanorings composed of several self-coiled nanowires are synthesized by catalyst-assisted pyrolysis of a polymeric precursor. Microstructural observation reveals that the nanowires have a bilayer structure consisting of a thin single-crystal Si3N4 layer and a thick amorphous layer. The thickness ratio of the two layers is ∼ 1:2, regardless of the nanowire size. The formation of the thick amorphous layer is likely due to the high Al concentration within the precursor. The self-coiling mechanism is discussed and attributed to the difference in growth rates of the two layers. A phenomena model is proposed to account for the formation of the nanoring structure. One-dimensional (1D) nanostructures with unique morphologies have attracted extensive attention because the properties and applications of these nanostructures are strongly dependent on their morphologies.1 Among typical morphologies such as nanotube,2 nanowire,3 nanobelt,4 and nanohelice,5 the nanoring is especially interesting because it can be an ideal system for fundamentally investigating morphology-related physical phenomena,6,7 as well as potential usefulness in many applications.8 Silicon nitride (Si3N4) is an important wide-band gap semiconductor with a band gap of 5.0 eV.9 It is an excellent host material with a capability of high doping concentrations,10 excellent thermo-mechanical properties, and corrosion resistance.11 It is expected that with the continuing development of nanotechnology, Si3N4 may find widespread applications in many fields. Si3N4 nanostructures with various morphologies have been synthesized.12 In this communication, we report the synthesis of bundled Si3N4 nanorings via catalyst-assisted pyrolysis of a polymeric precursor. The microstructure of the nanorings is characterized using electron microscopy and X-ray diffraction. A formation mechanism is discussed. To the best of our knowledge, such kind of free-standing bundled Si3N4 nanorings has not been reported previously. The unique morphology of the bundled structure could be useful for nanodevices, such as bundled cables for electrical/optical connections.13 The bundled Si3N4 nanowires were synthesized by pyrolysis of a polyaluminasilazane with FeCl2 powder as the catalyst. The precursor was obtained by reaction of commercially available polyureamethylvinylsilazane (Ceraset, Kion Corporation, Huntingdon Valley, PA, USA) and aluminum isopropoxide (AIP, Beijing Bei Hua Fine Chemicals Company, Beijing, China) at a weight ratio of 2:1, using the procedure reported previously.14 The obtained precursor was solidified by heat-treatment at 260 °C for 0.5 h in N2, and then crushed into fine powders by high-energy ball milling for 24 h. 3 wt% of FeCl2 powder (Beijing Bei Hua Fine Chemicals Company, Beijing, China) was added during ball-milling. The powder mixture was then heat-treated at 1300 °C for 2 h in a tube furnace under flowing ultrahigh purity nitrogen. The obtained products were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (SEM, JSM-6301F, JEOL, Tokyo, Japan) and a high-resolution transmission electron micro* Corresponding authors. E-mail:
[email protected] (W.Y.), lan@ mail.ucf.edu (L.A.). † Ningbo University of Technology. ‡ Tsinghua University. § Shenzhen University. | University of Central Florida.
scope (HRTEM, JEOL-2010F, Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS) by using the Cu-grid as the sample holder. The morphology of the obtained product was observed using scanning electron microscopy. Figure 1a is a typical low magnification SEM image of the product. Many nanorings among the relatively straight nanowires can be seen in the matrix. The diameters of these rings are varied from a few hundred nanometers to a few micrometers (Figures S1-S4, Supporting Information). Figure 1b is a high magnification SEM image of a typical nanoring. It is seen that the nanoring exhibits a fairly good circling structure and is composed of several coiled nanowires, forming bundled structures. The nanowires have a uniform diameter and a smooth surface. For this specific ring, the nanowires are ∼60 nm in diameter and the diameter of the ring is ∼4 µm. Figure 1c shows the typical XRD pattern of the synthesized sample, revealing that the product exhibits both R- and β-Si3N4 with β-Si3N4 being the dominate phase. Further characterization of the nanorings was carried out using transmission electron microscopy (TEM). Figure 2a is a typical TEM image of a nanoring composed of a single nanowire. Singlewire nanorings have not been observed by SEM, thus it must be a result of debundling of the bundled nanorings during TEM sample preparation. The diameter of this ring is ∼500 nm, and the nanowire is very thin, about ∼12 nm in diameter. Energy dispersive spectroscopy (EDS) analysis (Figure 2b) reveals that the nanowire consists of Si, N, O and 5.12 atom % Al (the Cu signal is from TEM copper grid). Note that since the nanowire is smaller than the electron beam size, the element concentration is an average of the crystalline phase and the amorphous phase; exact Al concentrations in the crystalline and amorphous phase cannot be measured at this moment due to the limit of the space resolution of the EDS. Figure 2c is a typical selected area electron diffraction (SAED) pattern taken from area A marked in Figure 2a, indicating the ring is R-Si3N4. Figure 2, panels d and e are HRTEM images of a nanowire. The images reveal that the coiled nanowire is composed of two layers: a crystalline left layer and an amorphous right layer. The lattice spacing of 0.28 nm (Figure 2e) corresponds to the [002] plane of R-Si3N4. Observation of several nanorings reveals that the nanowires are invariably coiled in such a way that the amorphous layer is a convex surface, as schematically shown in Figure 2f. It is worth noting that the amorphous layer is much thicker than the crystalline layer with the thickness ratio of the two layers being about 1:2. It is interesting to notice that such a thick amorphous layer has not been observed when the precursors containing a lower amount of Al were used, which led to the formation of nanowires instead of nanorings.15 While the detailed
10.1021/cg800708z CCC: $40.75 2008 American Chemical Society Published on Web 09/30/2008
3922 Crystal Growth & Design, Vol. 8, No. 11, 2008
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Figure 1. SEM images of the bundled Si3N4 nanorings. (a) Low magnification image of the synthesized product with many nanorings as indicated by arrows. (b) High magnification image of one nanoring. (c) A representative XRD pattern of the synthesized Si3N4 nanorings.
Figure 2. (a) A typical TEM image of a nanoring consisting of one coiled nanowire. (b) A typical EDS spectrum obtained from a single nanowire. (c) and (d) are the SAED pattern and corresponding HRTEM image of the nanowires taken from area A marked in (a). (e) An enlarged HRTEM image of the area B marked in (d). (f) A schematic model of a self-coiled nanoring.
mechanism is not clear at this moment, the formation of the thick amorphous layer is likely due to the high Al concentration of the precursor. Ring-shaped nanostructures have been reported previously in several material systems. The proposed formation mechanisms are polar charge induced self-coiling,5,6 or oriented attachment and selfassembly of nanoparticles.16 However, these mechanisms cannot be applied to the present bundled nanorings. Another possible driving force that can bend nanostructures is the so-called surface tension imbalance, which was first proposed by Cahn and Hanneman in 1964 to account for the spontaneous bending of III-V semiconducting thin crystals.17 However, the surface tension imbalance value calculated using the experimental results was unreasonably high (Supporting Information), suggesting that the surface tension imbalance was not the dominant mechanism causing the formation of the rings. We also considered the possibility of the formation of the nanorings due to thermal mismatch between the crystal phase and the amorphous phase. However, it was found that the temperature difference required to bend the nanowire into the rings of its current geometry was much higher than a reasonable
value (Supporting Information), eliminating the thermal mismatch as a driving force for the formation of the rings. The above discussion suggests that the formation of the nanorings cannot be driven by thermodynamic mechanisms. Here, we propose that the nanorings were likely formed due to the amorphous layer growing faster than the crystalline layer, resulting in the nanowires bending toward the crystalline layer. Assuming that the growth rates of the amorphous layer and the crystalline layer are r1 and r2, respectively, the strain generated by this difference can be calculated by ε ) r1/r2 - 1. This strain is similar to that generated by thermal mismatch, thereby the diameter, D of the nanoring can be calculated as18
1 ) D
(
t2 E2 1+ t1 E1
( ) )[ ( ) ] [ ( ) ] t2 3ε E2 t2 1+ 4t1 E1 t1 t1 t2 3 E2 t2 2 E2 3 1+ - 1t1 E1 4 t1 E1
2
(1)
where E1 and E2 are the Young’s modulus of the amorphous layer and crystalline layer, respectively, and t1 and t2 are the thicknesses
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Crystal Growth & Design, Vol. 8, No. 11, 2008 3923
References
Figure 3. A plot of the relationship between diameters of nanorings and nanowires. The data points are experimental results; the solid line is curve fitting by using eq 2.
for the amorphous layer and crystalline layer (t1/t2 ) 2), respectively. By inserting 80 GPa19 for the amorphous layer (assuming the amorphous layer is SiOx20) and 570 GPa21 for the crystalline Si3N4, eq 1 can be simplified as
ε 1 ) 0.77 D t
(2)
where t ) t1 + t2 is the diameter of the nanowire. Figure 3 plots the diameters of the nanorings as a function of the diameters of the nanowires. It is interesting to see that they follow the linear relationship as predicted by eq 2. The growth rate ratio can then be calculated by fitting eq 2 with the experimental results, to be r1/r2 ) 1.015. This suggests that the slight difference in the growth rate can lead to the formation of the nanorings. The novel structure of the nanorings is interesting and different from our previous results.15 In our previous study, straight nanowires covered with a thin amorphous layer have been obtained from pyrolysis of polyaluminasilazane precursors synthesized by the reaction of Ceraset and AIP at weight ratios of 16:1, 8:1, and 4:1. The only difference between the previous study and the current one is that in the current study the precursor used contains a higher Al concentration. Consequently, it is likely that the high Al concentration within the precursor induced the formation of a thick amorphous layer, which, in turn, induced the formation of the nanoring structure. In summary, we report for the first time the synthesis of bundled Si3N4 nanorings via catalyst-assisted pyrolysis of a polymeric precursor. The structure analysis reveals that the coiled nanowires have a bilayer structure consisting of a crystalline Si3N4 layer and an amorphous layer with their thickness ratio being 1:2. The formation of the amorphous layer is likely induced by high Al concentration in the precursor. A phenomena model based on the growth rate difference between the crystalline and amorphous layers has been proposed to relate the diameter of the nanoring to that of the nanowire. The bundled structure could be useful in nanodevices.
Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 50602025 and 50872058) and Natural Science Foundation of Ningbo Municipal Government (Grant Nos. 2006A610028 and 2006A610059). Supporting Information Available: Calculation of the surface tension imbalance and thermal mismatch between the crystalline and amorphous layers. These materials are available free of charge via the Internet at http://pubs.acs.org.
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