Charge Transfer between PbSe and NbSe₂ in [(PbSe)_1.14]_m(NbSe₂)₁ Ferecrystalline Compounds

Abstract

[(PbSe)1.14]m(NbSe2)n compounds with 1 ≤ m ≤ 6 and n = 1 were synthesized using the modulated elemental reactants (MER) method. X-ray diffraction patterns (XRD) showed that the desired compounds self-assembled during annealing of the precursors with their c-axis crystallographically aligned normal to the substrate. The c-axis lattice parameter increased by 0.612 nm as m, the number of PbSe bilayers, increased by one. Analysis of the in-plane diffraction patterns indicated that the a-lattice parameters remained constant as m was varied. Reciprocal space maps along hkl (h, k ≠ 0; l ≠0) indicate very short coherence lengths in mixed-index directions, consistent with the rotational disorder between layers observed in electron microscopy cross sections. The in-plane electrical resistivity and Hall coefficients were measured for each ferecrystal from 22 to 295 K. The resistivity systematically increased as m increased, but the magnitude of the increase is greater than predicted assuming independent layers. Assuming the metallic conduction results from a single band in the NbSe2 layer, the carrier concentrations determined from the Hall coefficients decreases as m increases, suggesting increased charge transfer from PbSe to NbSe2 with increasing values of m. First-principles electronic-structure calculations based on the generalized gradient approximation to density functional theory suggest that the PbSe valence band overlaps the empty bands in NbSe2, supporting the idea of interlayer charge transfer from PbSe to NbSe2.