THE PREPARATION OF CRYSTALLINE NbSe2/TiSe2 SUPERLATTICES FROM MODULATED ELEMENTAL REACTANTS
David C. Johnson and Myungkeun Noh, Materials Science Institute and Department of Chemistry, University of Oregon, Eugene, Oregon 97403.
A series of crystalline superlattice compounds containing an integral number of inter grown transition metal dichalcogenide layers were prepared through controlled crystallization of Ti/Se/Nb/Se superlattice reactants with designed compositional modulation. The component elemental layer thicknesses and annealing sequence were chosen to favor interfacial nucleation of the component binary compounds. Theta-theta and rocking curve data were collected to study of the evolution of the initially layered reactants into the crystalline superlattices as a function of temperature. The initial layered reactant was found to contract in the c-axis direction upon initial annealing and the interfaces appear to become smoother during this initial interdiffusion. The growth of c-axis oriented NbSe2/TiSe2 crystal structure perpendicular to the substrate surface occurs upon annealing at temperature above 200deg.C. The gradual decrease of the (00l) diffraction linewidths of the growing compound as a function of annealing time and temperature indicating increases in the c-axis domain size. High quality c-axis oriented TiSe2/NbSe2 crystalline superlattices result from extended annealing at the relatively low annealing temperature of 500deg.C. The large number of observed 00l diffraction orders permits the crystal structure of the superlattice in the c-axis direction to be determined through a Reitveld analysis. The rational synthesis of intergrowth compounds from superlattice reactants as described herein will permit the tailoring of physical properties as a function of compositional layer thicknesses and native properties of the parent compounds. This will permit the exploration of the transition from composite behavior to that of a new compound as the lengthscale of the compositional modulation decreases.