Carrier dilution in TiSe₂ based intergrowth compounds for enhanced thermoelectric performance

Abstract

Synthesis and electrical properties of kinetically stabilized (PbSe) 1+d (TiSe 2 ) n thin-film intergrowths are reported for 1 r n r 18. A linear increase in the c-lattice parameter of the intergrowth is observed as n is increased and the slope is consistent with the inclusion of an additional TiSe 2 structural unit as n is incremented by 1 and the observed intercept is consistent with the expected thickness of a PbSe bilayer. The charge donated to the TiSe 2 constituent from the PbSe is diluted across more layers as n is increased, leading to a systematic increase in the Seebeck coefficient. The room temperature resistivity values of the reported compounds are all on the order of 10 À5 O m and depend on defect densities that affect the mobility, making the magnitude of the resistivity less sensitive to n. The temperature dependence is metallic for large n, with a slight upturn at low temperatures due to localization of carriers for small n values. The power factor increases with n, including the highest reported for chalcogenide misfit layered and related compounds, showing that nanostructuring and modulation doping are an effective means of tuning the power factor of thermoelectric intergrowth materials. Since these compounds have very low thermal conductivity due to structural anisotropy and misregistration between intergrowth constituents, this suggests that varying their nanoarchitecture is a promising approach to obtain high values of zT.

Structural Changes as a Function of Thickness in [(SnSe)1+δ]mTiSe2 Heterostructures

“…The larger error for the m = 2 compound is a consequence of the overlap of the TiSe 2 (110) reflection with the (110) SnSe 2 reflection (Figure a). These a -axis lattice parameters are within error of each other and are consistent with those previously reported for other ferecrystals containing TiSe 2 layers and the binary TiSe 2 compound. ,,,− This suggests that the in-plane structure of TiSe 2 is independent of the SnSe layer thickness.…”

“…As the number of SnSe bilayers deposited in the precursor increases, there is a systematic increase in the c -axis lattice parameter of 5.79(1) Å per bilayer of SnSe in [(SnSe) 1+δ ] m TiSe 2 . This result is consistent with the 5.77(5) and 5.806(2) Å increase in the c -axis lattice parameter per SnSe bilayer reported for [(SnSe) 1+δ ] m [NbSe 2 ] n - and [(SnSe) 1+δ ] m [MoSe 2 ] n -based compounds, respectively. , Extrapolating this relationship to m = 0, the thickness of the single TiSe 2 layer is 6.25(3) Å in each compound, which is thicker than the c -axis lattice parameter of bulk TiSe 2 (6.008 Å) and the thickness per TiSe 2 layer in (PbSe) 1+δ [TiSe 2 ] n (6.03–6.04 Å) obtained from the change in c -axis lattice parameter as n is varied. ,, The larger value reflects the different species interacting across the van der Waals interface. A single TiSe 2 layer has two TiSe 2 –SnSe interfaces that are mismatched and hence cannot nest together.…”

“…For example, bilayers of bulk rock salt structured constituents between dichalcogenide layers distort significantly, with the cations moving as much as 0.2 Å toward the anion layers in the dichalcogenide. , As the thickness of rock salt structured layers is increased, the distortions evolve toward a bulk structure with a surface distortion at the interface. , These structural distortions reflect changes in the free energy landscape as the ratio of atoms at the interface relative to those interior decreases. The properties of these heterostructures have been observed to systematically change as layer thicknesses are varied, reflecting the interactions between the layers. − Understanding how structural distortions in 3D materials change as their thickness approaches the 2D limit and how these distortions impact their properties is necessary to design heterostructures with specific properties.…”

Charge Density Wave Transition in (PbSe)1+δ(VSe2)n Compounds with n = 1, 2, and 3

“…The change in thickness as n is increased yields the thickness of a VSe 2 layer from the slope and the thicknesses of the PbSe layer from the intercept. The PbSe bilayer thickness of 0.617(5) nm is slightly thicker than the 0.607–0.612 nm found in a series of [(PbSe) 1.14 ] m (NbSe 2 ) n compounds and the 0.61(1) nm found for the PbSe bilayer thickness in (PbSe) 1+δ (TiSe 2 ) n ferecrystals . The thickness of the VSe 2 trilayer is 0.610(2) nm, which is slightly thicker than the 0.596(1) nm reported for the VSe 2 subunit in (SnSe) 1.15 (VSe 2 ) n compounds …”

“…The PbSe bilayer thickness of 0.617(5) nm is slightly thicker than the 0.607−0.612 nm found in a series of [(PbSe) 1.14 ] m (NbSe 2 ) n compounds 33 and the 0.61(1) nm found for the PbSe bilayer thickness in (PbSe) 1+δ (TiSe 2 ) n ferecrystals. 35 The thickness of the VSe 2 trilayer is 0.610(2) nm, which is slightly thicker than the 0.596(1) nm reported for the VSe 2 subunit in (SnSe) 1.15 (VSe 2 ) n compounds. 13 A Rietveld refinement of the n = 1 out-of-plane XRD was performed to determine relative positions of the atomic planes along the c-axis.…”