Black phosphorous could be the next big material

A team of researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory, Berkeley Lab, believes that black phosphorous nanoribbons have potential for future uses in optoelectronic, thermoelectric and electronic devices. Researchers have demonstrated strong in-plane anisotropy in thermal conductivity (directionally dependent), along armchair directions and in zigzag directions for single-crystal blackphosphorous nanoribbons. Junqiao Wu is a Berkeley Lab’s Materials Sciences Division and University of California (UC) Berkeley’s Department of Materials Science and Engineering. He said, “Imagine the black phosphorous lattice as a two-dimensional network of springs connected to balls, where the network is softer in one direction than the other.” “Our study shows how heat flow in black phosphorous nuribbons can vary in different directions.” This is a single-crystal research. Image: Nature Communications. Junqiao Wu (Berkeley Lab). “This thermal conductivity anisotropy was predicted recently for 2D Black phosphorous crystals, but has never been observed.” Professor Wu and his colleagues presented their findings in a Nature Communications article. Black phosphorous can be switched on or off to turn electrical conductance on or off. This is because it’s the most stable thermodynamically at room temperature and under pressure. This natural semiconductor has an energy bandgap which allows it to switch its electrical conductivity ‘on/off’. Researchers have proposed that black phosphorous has an opposite anisotropy of electrical and thermal conductivities to graphene. The theory is that heat moves more quickly in the direction of electricity flows with greater difficulty. This anisotropy could be an advantage in designing efficient thermoelectric devices or transistors. This theory was difficult to confirm due to sample preparation and measurement. Professor Junqiao Wu. Image: Berkeley Lab. Prof. Wu stated that the study revealed high levels of directional anisotropy for thermal conductivity above 100 Celvin. According to the researchers “This anisotropy was attributed mainly to phonon dispersion with some contribution from phonon-phonon scattering rate, both of which are orientation-dependent.” Detailed analysis showed that at 300 degK, thermal conductivity declined as the nanoribbon thickness shrank from c.300 nanometers to c.50 nanometers. The anisotropy ratio was at least two within this range of thickness. Professor Wu stated that the anisotropy ratio of this thickness range was at a factor of two. For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. “Effective thermal management could be used to lower chip temperature and improve chip performance,” Prof. Wu and his team intend to make use of their experimental platform in order to study how thermal conductivity is affected by different conditions, including domain boundaries and phase-transitions. The team would like to investigate the impact of pressure and stress on thermal conductivity. DEO Office of Science funded the study. The study was funded by the DEO Office of Science. Urban, Sefaattin Tongay & Junqiao Wu. Nature Communications. 16 October, 2015. doi: 10. 1038/ncomms9573.

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