Four-probe measurements of anisotropic in-plane thermal conductivities of thin black phosphorus

Phosphorene, a two-dimensional material exfoliated from black phosphorus (BP), is a promising p-type, high-mobility semiconductor. Phosphorene and BP display intrinsic in-plane anisotropic transport properties due to its puckered honeycomb lattice with distinct armchair and zigzag crystallographic orientations. The anisotropic thermal transport properties of BP and phosphorene influence the performance and reliability of functional devices made from these materials, and remain to be better understood. Here, we report the anisotropic in-plane thermal conductivities of suspended multi-layer BP samples, which are measured by a four-probe thermal transport measurement method. The measurement device consists of four microfabricated, suspended Pd/SiNx lines that act as resistive heaters and thermometers. The BP flake is suspended across the microstructure in contact with all four lines. This four-probe thermal transport measurement is equipped with the unique ability to isolate the intrinsic thermal resistance from the contact thermal resistance, which can be a major source of error in thermal conductivity measurements of nanostructures. Four BP samples were measured with thicknesses ranging from 39.2 nm to 274 nm and a peak thermal conductivity of 142 W m-1 K-1 at 80 K for a 55.6 nm thick zigzag oriented flake. The measurement results exhibit more pronounced temperature dependence with a higher peak thermal conductivity together with a weaker thickness dependence than prior reports. The results suggest the important role of defects in thermal transport in thin BP flakes, which can degrade upon exposure to air and water.