Roles of electrons on the thermal transport of 2D metallic MXenes

MXenes have many potential applications in electronics, energy storage, sensors, thermal management, and biomedical catalysis. As one of the most widely explored two-dimensional (2D) materials, their transport properties are of broad interest. Due to the difficulty in direct experimental measurements of their thermal conductivity, existing works mainly rely on theoretical approaches. However, only lattice contribution to thermal conductivity in MXenes was studied in previous works, while the role of electrons in thermal transport MXenes has never been elucidated. Herein, we investigate the electron and phonon contribution to the thermal conductivity of three typical metallic 2D MXenes ${[\mathrm{Ti}}_2{\mathrm{CF}}_2, {\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2]$ with a first-principles approach, in which the mode-level electron-phonon coupling is rigorously considered. The thermal conductivity values are predicted to be 69.1, 104.7, and 54.3 W/mK for ${\mathrm{Ti}}_2{\mathrm{CF}}_2, {\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$, respectively. The contribution of electron to total thermal conductivity (37.3–61.3%) is much larger than most existing 2D materials. We find that the relatively large electron density in metallic MXenes leads to the considerable electronic thermal conductivity. Moreover, due to large phonon-electron scattering phase space and matrix element, the phonon thermal conductivity is largely suppressed by the scattering with electrons. Our results clarify the role of electrons in the thermal transport in metallic MXenes and can provide a deeper understanding of the transport mechanism in metallic 2D materials.

Figure 1

(a) Top and side views of the crystal structure of the three metallic MXenes. The black boxes indicate the corresponding unit cells. The thicknesses of monolayer MXenes are denoted by dashed lines in the second row. (b) The calculated electronic band structure and (c) phonon dispersion of ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$.

Figure 2

Electron thermal conductivity and phonon thermal conductivity from 200 to 800 K for (a) ${\mathrm{Ti}}_2{\mathrm{CF}}_2$, (b) ${\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and (c) ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$. The hollow scatters in (a) are the data for some other typical MXenes from first-principles calculation [46] (theory) and experiment [51] (exp.).

Figure 3

(a) Electrical conductivity and (b) normalized Lorenz ratio from 200 to 800 K for ${\mathrm{Ti}}_2{\mathrm{CF}}_2, {\mathrm{Ti}}_2{\mathrm{CCl}}_2$, and ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$. (c) The electron scattering rates from phonons and the resolved scattering rates from acoustic phonons compared to that from optical phonons at 300 K for ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$.

Figure 4

(a) Phonon thermal conductivity without (solid lines) and with (dashed lines) phonon-electron scattering. (b) The phonon-phonon scattering rates (ph-ph, shown by shallow blue scatters) compared to the phonon-electron scattering rates (ph-el, shown by red scatters) at 300 K for ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$.

Figure 5

(a) The phonon-electron scattering rates at 300 K, (b) the nesting function, and (c) the electron-phonon coupling matrix elements mapping onto the three acoustic phonon branches for ${\mathrm{Ti}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$. Only the electron modes with electron energy 0.2 eV above/below the Fermi level are considered in the calculation of the electron-phonon coupling matrix elements.