Abstract
Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon dispersions obtained based on density function perturbation theory, the high anisotropy is attributed mainly to direction-dependent phonon dispersion and partially to phonon–phonon scattering. Our results revealing the intrinsic, orientation-dependent thermal conductivity of black phosphorus are useful for designing devices, as well as understanding fundamental physical properties of layered materials.
Highlights
Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications
It has been theoretically predicted that black phosphorus (BP) has opposite anisotropy in thermal and electrical conductivities: electrical conductivity is higher along the AC direction, while thermal conductivity is higher along the ZZ direction[24,25,28,29,30]
Our results reveal a high anisotropy in thermal conductivity up to a factor of two at temperatures greater than B 100 K
Summary
Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Recent theoretical studies on few-layer graphene nanoribbons expect anisotropic phonon transport along the zigzag (ZZ) and armchair (AC) lattice directions of its honeycomb structure This anisotropy is not expected in the bulk[11,12,17], because it arises mainly from different strengths of boundary scattering at the nanoribbon edges with different chiralities[11,17], that is, specular scattering at the ZZ edge while angle-dependent scattering at the AC edge[17], an effect that is reduced with increasing nanoribbon width as the material approaches the bulk. A size effect in the thermal conductivity was observed from B50- to B 300-nm-thick BP nanoribbons in which thinner nanoribbons show lower thermal conductivity These discoveries shed light on phonon physics in this interesting material and provide important design guidelines in its device applications
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