Lateral structure of (TiSe₂)_n(NbSe₂)_m superlattices

The structures of a series of (TiSe₂)_n(NbSe₂)_m superlattices grown through controlled crystallization of designed multilayer reactants have been studied. X-ray diffraction of the data of the superlattices after crystallization show considerable preferred orientation, with the basal plane of the dichalcogenide structure parallel to the substrate to within 0.1°. Lattice refinement using the observed (00l) diffraction maxima yields lattice parameters along the c axis that are consistent with those expected based on the target superlattices and lattice parameters of the binary constituents. These (00l) diffraction data, however, contain no information about the crystalline structure in the ab plane of the superlattice associated with the preferred c-axis orientation. Off-specular x-ray diffraction (XRD), scanning electron microscopy, and scanning transmission x-ray microscopy (STXM) were used to explore the structure and homogeneity of the superlattices in the ab plane. XRD results rule out preferred long-range orientational order of the ab plane. Between grains, both the backscattered electron images and STXM images show grain domain structure in the ab plane with a characteristic grain domain size of approximately 50 μm. X-ray absorption microscopy in the STXM mode obtained at the Ti L_2,3 edge shows that the titanium in the superlattices is present as both octahedral Ti consistent with the TiSe₂ structure and metallic Ti. A comparison of the data obtained from these techniques highlights chemical information, which can be deduced on a submicrometer range from the space resolved spectra obtained using STXM.