Structural Changes in 2D BiSe Bilayers as n Increases in (BiSe)_1+δ(NbSe₂)_n (n = 1–4) Heterostructures

Abstract

(BiSe)(NbSe) heterostructures with n = 1-4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with n = 1 to a square basal plane for n = 2-4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the n = 1 compound, is dramatically reduced for the n = 2-4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d, 5d doublet peaks narrowed toward higher binding energies as n increased from 1 to 2, also consistent with a reduction in the number of antiphase boundaries. Temperature-dependent electrical resistivity and Hall coefficient measurements of nominally stoichiometric samples in conjunction with structural refinements and XPS data suggest a constant amount of interlayer charge transfer independent of n. Constant interlayer charge transfer is surprising given the changes in the BiSe in-plane structure. The structural flexibility of the BiSe layer may be useful in designing multiple constituent heterostructures as an interlayer between structurally dissimilar constituents.

Kinetically Controlled Formation and Decomposition of Metastable [(BiSe)1+δ]m[TiSe2]m Compounds

“…The [(BiSe) 1+δ ] 1 [TiSe 2 ] 1 region is remarkably defect free considering the low annealing temperature and short annealing times required to transform from [(BiSe) 1+δ ] 2 [TiSe 2 ] 2 . Antiphase boundaries are present in the BiSe layer, as reported previously. ,, The visible zone axis changes orientations layer to layer, consistent with turbostratic disorder. The Bi 2 Se 3 region contains more defects, including regions where it appears that BiSe monolayers separate Bi 2 Se 3 layers.…”

Correlation of Reduced Interlayer Charge Transfer with Antiphase Boundary Formation in BixSn1–xSe–NbSe2 Heterostructures

“…This is consistent with previous studies, which concluded that antiphase boundary formation in BiSe localized the Bi valence electron, inhibiting interlayer charge transfer . However, a recent study of (BiSe) 1+ δ (NbSe 2 ) n compounds with thicker NbSe 2 layers suggests that the situation is more complicated . The BiSe lattice changes from rectangular to square for values of n greater than 1, correlated with a significant reduction in the number of antiphase boundaries, but the magnitude of interlayer charge transfer does not appear to change.…”

Misfit phase (BiSe)1.10NbSe2 as the origin of superconductivity in niobium-doped bismuth selenide

“…The thickness of the blocks and their [100] orientation were constant in the whole area of investigation. The synthesis technique used here leads to a more uniform structure than the layer-bylayer vapour deposition employed by Mitchson et al 42,43 The structure of (BiSe)1.10NbSe2, refined (Fig. 4a) using the (BiSe)1.09TaSe2 35,40 model as a starting point, is overlaid on the images.…”