Abstract
Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 µm to 5 nm, at 300 K. At low temperatures and below ∼50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however, the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5 nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed.
Highlights
The discovery of graphene by exfoliation allowed a plethora of new physics, like the quantum Hall effect [1,2], and massless Dirac fermions [3], to be explored in van der Waals layered two-dimensional (2D) systems
2D transition metal halides (TMHs), which consist of van der Waals (vdW) layers made up of transition metals like Nb, Cr, Mo, Ru, etc., and halides (I, Cl, Br), have received much attention due to their promising potential in a wide variety of magnetic phenomena based on ferromagnetic [15], and antiferromagnetic insulators [16, 17], as well as geometric frustration linked to prospective quantum spin-liquid behavior [18, 19, 20]
We experimentally report on an unequivoqual anomalous thickness-dependent conductivity scaling, and activation energy in TMH Nb3Cl8, the exact origin is still unknown
Summary
The discovery of graphene by exfoliation allowed a plethora of new physics, like the quantum Hall effect [1,2], and massless Dirac fermions [3], to be explored in van der Waals (vdW) layered two-dimensional (2D) systems. Heterostructures of TMDCs/TMHs where consecutive monolayers are stacked at an angle with respect to the underlying layer, or interfaced with other 2D materials have shown unexpected emergent properties including a correlated insulating state [12], interfacial superconductivity [13], and anomalous Hall effects [14]. We report the temperature, and thickness dependent electrical conductivities of high-quality single crystal flakes of Nb3Cl8 (within the measurable range of the experiment) ranging from 280 μm to
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