Charge transfer vs. dimensionality: what affects the transport properties of ferecrystals?

Abstract

A series of ([SnSe]1+δ)m(NbSe2)2 compounds with two layers of NbSe2 separated by m bilayers of SnSe, where 1 ≤ m ≤ 20, were prepared from modulated precursors by systematically changing the number of SnSe layers in the repeating unit. A change in the c-lattice parameter of 0.579(3) nm per SnSe bilayer was observed. The thickness of the NbSe2 layer was determined to be 1.281(4) nm: twice the value of a single NbSe2 layer. HAADF-STEM images revealed the presence of extensive rotational disorder and the lack of any epitaxial relationship among the constituent layers. Two different coordination environments for the Nb in NbSe2 (trigonal prismatic and octahedral) were observed. The electrical resistivity increases and the carrier concentration decreases in the ([SnSe]1+δ)m(NbSe2)2 compounds with increasing number of SnSe bilayers. The temperature dependence of the resistivity suggests localization of carriers for higher m values. The decline in carrier concentration as a function of m implies the presence of charge transfer from SnSe to NbSe2. The transport properties of the ([SnSe]1+δ)m(NbSe2)2 compounds and the previously reported ([SnSe]1+δ)m(NbSe2)1 compounds both have unusually temperature independent resistivity compared to bulk NbSe2. Compounds with similar m/n ratios exhibit similar transport properties. Consequently, the dominant effect on the transport properties of ([SnSe]1+δ)m(NbSe2)2 is charge transfer, and there are only subtle differences between a monolayer and a bilayer of NbSe2.

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

“…The Hall coefficients for all samples are positive, suggesting p-type majority charge carriers consistent with conduction through the NbSe 2 layers, as was previously observed in other NbSe 2 -containing misfit compounds. 26,37,45 The Hall coefficients decrease systematically as n increases. For the compounds with n = 2−4, the room temperature Hall coefficients are similar to bulk NbSe 2 (4−5 × 10 −4 cm 3 /C), 46 but the n = 1 compound's Hall coefficient is a factor of 2 larger.…”

“…The systematic change in Hall coefficient with m and n has been reported previously in [(SnSe) 1+δ ] m (NbSe 2 ) n and [(PbSe) 1+δ ] m (NbSe 2 ) n ferecrystals ,, and has been explained via charge donation from the monochalcogenide bilayers to the NbSe 2 layers. In bulk NbSe 2 , the Nb d z 2 conduction band is half-filled, leading to approximately 1 hole/Nb atom .…”

The Influence of Interfaces on Properties of Thin-Film Inorganic Structural Isomers Containing SnSe–NbSe2 Subunits

“…For example, the 3: 3 :1: 1 isomer can be broken into either 3: 3 + 1: 1 or 1: 3 + 3: 1 blocks. We calculated electrical resistivity, carrier concentration and the carrier mobility at room temperature for each building block from the measured values for the corresponding m : n compounds, listed in Table . ,, For the possible combinations for each isomer, we weighted the values for each building block based on its volume ratio within the isomer, essentially assuming they conduct independently in parallel. The calculated values for the isomers are listed together with the actually measured ones in Table .…”

Superlattices based on van der Waals 2D materials

“…The dimensionality (m layers of material 1 and n layers of material 2 per unit cell, i.e. their ratio m/n) of these materials is taken into account to control the properties of both materials: the electrical transport [39][40][41] , the charge transfer between the layers 42,43 or the presence/strength of a charge density wave state 44 . Another strategy followed to modulate the electrical properties is by substitution or doping in specific sites of the layers [45][46][47] .…”