Researchers have experimentally confirmed strong in-plane anisotropy in thermal conductivity along the zigzag (ZZ) and armchair (AC) directions of single-crystal black phosphorous nanoribbons. Click to enlarge.

Black phosphorous (BP) is a natural semiconductor with an energy bandgap that allows its electrical conductance to be switched “on and off.” It has been theorized that in contrast to graphene, black phosphorous has opposite anisotropy in thermal and electrical conductivities—i.e., heat flows more easily along a direction in which electricity flows with more difficultly.

Such anisotropy would be a boost for designing energy-efficient transistors and thermoelectric devices, but experimental confirmation proved challenging because of sample preparation and measurement requirements.

In a study described in an open-access paper published in Nature Communications, the researchers directly measured the in-plane thermal conductivity of single-crystal BP nanoribbons along the ZZ and AC lattice directions. The measurements were carried out in the condition of steady-state longitudinal heat flow, using suspended-pad micro-devices , over a wide temperature range from 30 to 350K (-243 ˚C to 77 ˚C).

Our results reveal a high anisotropy in thermal conductivity up to a factor of two at temperatures greater than ~100K. The high anisotropy is attributed mainly to the anisotropic phonon dispersion, and partially to the phonon–phonon scattering. A size effect in the thermal conductivity was also observed from ~50- to ~300-nm-thick BP nanoribbons in which thinner nanoribbons show lower thermal conductivity. These discoveries not only shed light on phonon physics in this interesting material but also provide important design guidelines in its device applications.

—Lee et al.

Both phonon dispersion and phonon-phonon scattering rate are orientation-dependent.

The anisotropy we discovered in the thermal conductivity of black phosphorous nanoribbons indicates that when these layered materials are patterned into different shapes for microelectronic and optoelectronic devices, the lattice orientation of the patterns should be considered. This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation. For example, these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance.

—Junqiao Wu, corresponding author

Wu and his colleagues plan to use their experimental platform to investigate how thermal conductivity in black phosphorous nanoribbons is affected under different scenarios, such as hetero-interfaces, phase-transitions and domain boundaries. They also want to explore the effects of various physical conditions such as stress and pressure.

Resources

• Sangwook Lee, Fan Yang Joonki Suh, Sijie Yang, Yeonbae Lee, Guo Li, Hwan Sung Choe, Aslihan Suslu, Yabin Chen, Changhyun Ko, Joonsuk Park, Kai Liu, Jingbo Li, Kedar Hippalgaonkar, Jeffrey J. Urban, Sefaattin Tongay & Junqiao Wu (2015) “Anisotropic in-plane thermal conductivity of black phosphorus nanoribbons at temperatures higher than 100 K” Nature Communications 6, Article number: 8573 doi: 10.1038/ncomms9573