Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus (1503.06167v3)

Abstract: Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ~20 and ~40 W m$^{-1}$ K$^{-1}$ for black phosphorus films thicker than 15 nm, respectively, and decrease to ~10 and ~20 W m$^{-1}$ K$^{-1}$ as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ~2 for thick black phosphorus films and drops to ~1.5 for the thinnest 9.5-nm-thick film. Theoretical modeling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long-mean-free-path acoustic phonons.

• The paper demonstrates marked anisotropy with the zigzag direction exhibiting up to 40 W/m·K compared to 20 W/m·K along the armchair axis in thicker films.
• The study quantitatively shows a decrease in conductivity with film thinning, dropping from 20/40 W/m·K to about 10/20 W/m·K in armchair/zigzag directions.
• The research employs micro-Raman spectroscopy and computational modeling to link anisotropic thermal transport to phonon dispersion, informing device design in electronics and thermoelectrics.

Anisotropic In-Plane Thermal Conductivity in Few-Layer Black Phosphorus

The recent investigation into the anisotropic in-plane thermal conductivity of few-layer black phosphorus (BP) has provided new insights into the material's potential applications in electronics and optoelectronics. This paper utilized micro-Raman spectroscopy for measurement, with results showing marked anisotropic thermal conductivity along the armchair and zigzag axes, a phenomenon attributed primarily to the anisotropic phonon dispersion intrinsic to BP's lattice structure.

Key Findings in Thermal Conductivity

The experiments conducted in this paper measured the thermal conductivities of BP films, revealing that the zigzag direction exhibited higher thermal conductivity compared to the armchair direction. Specifically, for BP films thicker than 15 nm, thermal conductivities along the armchair and zigzag axes were approximately 20 W m$^{-1}$ K$^{-1}$ and 40 W m$^{-1}$ K$^{-1}$, respectively. As the film thickness reduced to 9.5 nm, the conductivities decreased to about 10 W m$^{-1}$ K$^{-1}$ (armchair) and 20 W m$^{-1}$ K$^{-1}$ (zigzag). This decline underscores a significant thickness-dependent anisotropic behavior with an anisotropy ratio that dips from ~2 in thicker films to ~1.5 in thinner films.

Methodological Approach

Measurements were conducted on suspended BP films prepared via mechanical exfoliation. These films were positioned on silicon nitride substrates to measure thermal conductivities using micro-Raman techniques, which involved analyzing laser-induced temperature-dependent Raman shifts. The use of polarized-Raman measurements was crucial for identifying lattice orientations, which informed the alignment of BP flakes for detailed analysis of direction-specific thermal transport properties.

Theoretical Implications

The superior thermal transport along the zigzag direction is primarily due to anisotropic phonon dispersion rather than significant direction-specific differences in intrinsic phonon scattering rates. Computational modeling supported these experimental findings by solving heat transport properties grounded in first-principles phonon dispersion calculations. The data suggest significant surface scattering of phonons contributing to this anisotropic behavior, especially for long mean-free-path acoustic phonons impacted by film surfaces.

Practical Implications and Future Directions

The observed anisotropic thermal properties have critical implications for BP's applications in thermoelectric devices where efficient thermal management is crucial. However, for field-effect transistors and photovoltaic devices, such anisotropy in thermal/phononic transport may pose challenges due to thermal management issues, particularly in directions of lower thermal conductivity. Future research could focus on strategies to manipulate phonon scattering mechanisms to further tailor these properties for specific applications.

Conclusion

This paper's in-depth analysis of anisotropic thermal conductivity in few-layer BP contributes to understanding the material's potential in various semiconductor applications. The strong dependence on layer thickness and orientation opens avenues for more nuanced device design wherein BP's anisotropic properties can be harnessed effectively. The theoretical and methodological advancements presented hold promise for future exploration of 2D materials' thermal properties, especially in thin films where traditional bulk measurements may not adequately capture anisotropic behaviors. This work underscores the importance of considering directionality in thermal conductivity measurements and interpretations for emerging 2D materials.