Two-Dimensional Mineral [Pb₂BiS₃][AuTe₂]: High-Mobility Charge Carriers in Single-Atom-Thick Layers

Two-dimensional (2D) electronic systems are of wide interest due to their richness in chemical and physical phenomena and potential for technological applications. Here we report that [Pb₂BiS₃]­[AuTe₂], known as the naturally occurring mineral buckhornite, hosts 2D carriers in single-atom-thick layers. The structure is composed of stacking layers of weakly coupled [Pb₂BiS₃] and [AuTe₂] sheets. The insulating [Pb₂BiS₃] sheet inhibits interlayer charge hopping and confines the carriers in the basal plane of the single-atom-thick [AuTe₂] layer. Magneto-transport measurements on synthesized samples and theoretical calculations show that [Pb₂BiS₃]­[AuTe₂] is a multiband semimetal with a compensated density of electrons and holes, which exhibits a high hole carrier mobility of ∼1360 cm²/(V s). This material possesses an extremely large anisotropy, Γ = ρ_c/ρ_ab ≈ 10⁴, comparable to those of the benchmark 2D materials graphite and Bi₂Sr₂CaCu₂O_6+δ. The electronic structure features linear band dispersion at the Fermi level and ultrahigh Fermi velocities of 10⁶ m/s, which are virtually identical to those of graphene. The weak interlayer coupling gives rise to the highly cleavable property of the single crystal specimens. Our results provide a novel candidate for a monolayer platform to investigate emerging electronic properties

Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi₂Te₃ Doping

As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (<i>ZT</i>) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high <i>ZT</i> of ∼1.9 can be achieved at 773 K in Ge_0.87Pb_0.13Te upon the introduction of 3 mol % Bi₂Te₃. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge_0.87Pb_0.13Te + 3 mol % Bi₂Te₃ was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well