TEM Studies

Figure 3.5A presents the TEM image of the sample grown at 823 K followed by heat treatment at 1273 K. It confirms that it is one-dimensional and has an apparent open-end tubular structure with a diameter of ca. 100 nm and a wall thickness of 10 - 20 nm. SAED reveals a slightly diffused pattern with three distinctive rings in Figure 3.5B suggesting that

the sample is polycrystalline with crystallite sizes smaller than 4 nm. Starting from the most inside ring, these rings are assigned to the reflections from SiC(111), SiC(220), and SiC(311)

Figure 3.4 Characterization of a sample grown on Si wafer at 823 K followed by heat treatment at 1273 K. SEM studies: (A) low magnification surface image, (B) high magnification image of a tube end, and EDX (inset. Au was sputtered to increase conductivity), (C) cross-sectional image of the deposited layers on Si, and EDX (inset, from the squared area. Au was sputtered to increase conductivity), (D) XRD pattern. The peak marked with “*” is from the sample holder. (E) Low magnification image of tubes with ruptured ends and (F) high magnification of the circled area in (E) showing a ruptured end of a sample grown on Si wafer at 873 K followed by heat treatment at 1273 K.

Figure 3.5 TEM studies of a sample grown at 823 K followed by heat treatment at 1273 K.

(A) Low magnification image, and (B) SAED.

Figure 3.6 Characterization of a sample grown on Si wafer at 773 K followed by heat treatment at 1273 K. (A) Low magnification SEM image, (B) high magnification image of tube ends, and EDX (inset), (C) TEM image and EDX (inset), and (D) SAED, the polycrystalline diffraction rings are assigned to β-SiC.

planes. They can be assigned to cubic β-SiC with an estimated lattice parameter a = 0.44 nm.¹⁷ No reflections from crystallites of Si and Ca containing solids can be seen. The observations are in good agreement with the XRD data.

For the sample grown at 773 K followed by heat treatment at 1273 K, the TEM and ED data were comparable, showing an apparent open-end polycrystalline SiC tube with a diameter of ca. 80 nm and a wall thickness of 5 - 10 nm as shown in Figure 3.6.

Figure 3.7 TEM studies of a sample grown at 923 K followed by heat treatment at 1273 K.

(A) Low magnification image and EDX (inset, from the squared area). (B) SAED, (C) High-resolution image enlarged from a selected area in A.

Figure 3.7A shows the TEM data of a sample deposited at 923 K and annealed at 1273 K.

It is one-dimensional and open-end tubular with a diameter of ca. 300 nm and a wall thickness

of 60 - 80 nm. Only Si and C atoms can be observed in the EDX. SAED reveals a pattern with five distinctive rings in Figure 3.7B, suggesting that the sample is polycrystalline. They can be assigned to cubic Si with an estimated lattice parameter a = 0.54 nm and cubic β-SiC with an estimated lattice parameter a = 0.44 nm.^17,18 Starting from the most inside ring, these rings are assigned to the reflections from Si(111), SiC(111), Si(200), SiC(220), and SiC(311) planes. An HRTEM image, enlarged from the selected area in Figure 3.7A, is shown in Figure 3.7C. Two different lattice spacing values, 0.317 nm and 0.254 nm, are observed and assigned to Si (111) and β-SiC (111) planes, respectively.^17,18

The Si content will be estimated using the TGA result discussed below. The possible sources of the Si crystallites are the following ones. The first is the extrusion of Si from the polymeric material within the preceramic precursor. In literature, in the preparation of β-SiC via the Yajima-type routes, this was frequently observed at the high temperature processing stage.^12,20,21 Another possible source of Si is from the gas phase decomposition of MeSiHCl2.²² It was reported that at 905 K, MeSiHCl2 decomposed into CH4 and SiCl2

initially. Then, SiCl2 was transformed into the final gas phase products HSiCl3 and SiCl4. We propose that in this study, these chlorosilanes may react with Ca to form the Si nanocrystals in the tubular walls.²³

The data in Figure 3.8 suggest that the tubular structure was formed from an originally sealed heterostructure which ruptured later to allow the inner core to evaporate. The TEM image in Figure 3.8A shows a close examination of a tube with a ruptured end, which parallels to the SEM observations in Figures 3.4E and 3.4F. The presence of a core before the heat treatment is supported by the images of a rare example of a filled tube section shown in Figures 3.8B – 3.8D The TEM image in Figure 3.8B reveals that the section is composed of Si, C, Ca and Cl, as identified by the EDX in Figure 3.8C By analyzing the ED pattern in Figure 3.8D carefully, we conclude that the sample contains β-SiC, Si and CaCl2. In the ED pattern, the polycrystalline diffraction rings 2, 5, and 6 are assigned to β-SiC while the rings 1,

3, and 4 are assigned to Si. The diffraction spots from single crystalline CaCl2 are circled.

Based on this and the observations discussed above, we suggest that the as formed one-dimensional precursor material has a sealed radial heterostructure, which is composed of an inner core of CaCl2 encapsulated inside a preceramic shell of SiCxHy. Later, during the high temperature treatment at 1273 K, CaCl2 vaporizes and raises the pressure inside the heterostructure. This would cause the originally sealed tips to rupture and allows CaCl2 to evaporate. We expect that extended heat treatment should lower the CaCl2 concentration further. Meanwhile, SiC crystallizes inside the shell to form the apparent open-end tube morphology.

Figure 3.8 (A) TEM image of a sample grown at 873 K followed by heat treatment at 1273 K showing a tube end. (B) TEM image, (C) EDX, and (D) ED pattern of a sample grown at 923 K followed by heat treatment at 1273 K still retaining the CaCl2 core.