At the highest temperature (see Figure 1c), mostly small objects were found and in part sheets were growing out of the surface, along with sparsely distributed larger wires. The composition is Bi2Se3, indicating that the temperature is too high for the incorporation of Te. Figure 1 Electron micrographs of samples grown at various temperatures and their composition. (a) 480°C (left: 45° tilt-view SEM, right: TEM), (b) 506°C (top-view SEM), and (c) 545°C (side-view SEM). In the lattice-resolved TEM micrograph in (a), the indicated growth direction is along [110]. The inset reveals an interplanar distance of 0.4 nm. In (b),
mostly Bi2Te2Se platelets are observed, whereas at higher temperatures (c), the sample is composed of flakes as well as large Bi2Se3 wires. At 506°C, the planar growth increases and only a few, smaller nanowires are found as shown in Figure 1b. The X-ray powder diffraction selleck chemical pattern of a powder obtained from the as-grown material by scraping (cf. Figure 2) shows that the material is BTS with space group and the lattice parameters a=4.25 Å and c=29.95 Å [20]. The peak associated with [110]-oriented crystals is enhanced,
suggesting a preferred orientation within the sample. For two peaks, (107) and (01.11), the intensity RG-7388 clinical trial is too low to be resolved. Figure 2 X-ray powder diffraction pattern of the nanostructure sample grown at 506 ° C (black line). The pattern is assigned to Bi2Te2Se. The underlying red trace is the simulated pattern [20]. The inset shows a TEM micrograph Immune system of a hexagonal platelet, which is typical for the studied powder sample. At a substrate temperature of 480°C, the surface is uniformly covered with nanowires, indicating that the axial growth dominates over the planar and radial growth modes as can be seen in Figure 1a. TEM-based EDS analysis identifies the composition as BST. Lattice-resolved TEM imaging shows a spacing of 0.4 nm between adjacent lattice planes, consistent with a growth direction along [110]. This confirms the observation of a preferred growth orientation in the X-ray data of the sample grown at 506°C. At even lower temperatures, i.e. below the optimum BST growth temperature (results not shown),
axial and radial nanowire growth still dominates. These nanowires contain no Bi, since its vapour pressure is orders of magnitude lower than that of Se and Te at these temperatures. The composition of the nanostructures is further analysed using micro-Raman spectroscopy, which allows for a more precise study of the nanowires than EDS without the need of a large amount of sample material. The spectrum of a single nanowire grown at 480°C is shown in Figure 3a and exhibits four peaks that were assigned to the three modes of BST – note that the mode is split for certain stoichiometries. The Raman spectrum of Bi2(Te 1−x Se x )3 strongly depends on the compositional value x, as determined by Richter and Becker (data reproduced in Figure 3b) [21].