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 similar to two times of that along the zigzag direction, while the thermal conductivity along the armchair direction is only similar to 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 similar to 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.