Highly anisotropic thermoelectric properties of black phosphorus crystals

Black phosphorus captures enormous research attention on the anisotropic properties due to its orthorhombic crystal structure. Here the in-plane anisotropic thermoelectric behaviors of bulk black phosphorus crystals in the temperature range from 300 K to 600 K are reported for the first time. Based on the home-grown big size and high-quality black phosphorus crystals, the electrical conductivity and Seebeck coefficient are simultaneously measured by a ZEM-3 instrument system, and the thermal conductivity is measured by time-domain thermoreflectance (TDTR). For each individual parameter, the values along the zigzag and armchair directions show the same temperature-dependent trend. However, the electrical conductivity along the armchair direction is ∼ two times of that along the zigzag direction, while the thermal conductivity along the armchair direction is only ∼ one third of that along the zigzag direction. Furthermore, the Seebeck coefficients show almost isotropic behavior. As a result, the figure of merit ZT along the armchair direction is as large as ∼5.5 times of that along the zigzag direction, exhibiting high anisotropy. The results of intrinsic, orientation-dependent thermoelectric behaviors not only shed light on the fundamental physical properties of black phosphorus but also provide guidelines in the device design for potential thermoelectric applications.