Fig. 2.

Thermal and optical properties associated with radiative heat transfer. a Dependence of the radiative heat transfer power (HTP) on the separation distance d between two graphene nanodisks (solid curves) compared with two gold nanodisks (dashed curves, disk thickness t = 2 nm). All disks are 20 nm in diameter. The HTP is plotted for different values of T ₁ (see legend), while the cold disk is at ambient temperature T ₂ = 300 K. The arrows indicate the HTP between two blackbodies of an area equal to that of the present disks and placed at temperatures T ₁ and T ₂. Both graphene nanodisks are assumed to be doped with the same Fermi energy E _F1 = E _F2 = 0.2 eV and described by the local-RPA conductivity (see Methods). b Optical absorption cross-section σ _abs normalized to the graphene area for one of the graphene disks considered in a as a function of photon energy $\hslash \omega$ and temperature T. The dashed line corresponds to Wien’s law, $\hslash \omega \approx 2.82k_{\mathrm{B}}T$. c Temperature dependence of the electronic heat capacity for one of the graphene (blue curve, see Methods) and gold (red curve, taken from ref. 91) nanodisks considered in a. d Illustrative example of the femtosecond dynamics of the electronic thermal energy in two graphene nanodisks under the conditions of a for a separation d = 1 nm, with initial temperatures T ₁ = 1,000 K and T ₂ = 300 K. The electronic thermal energy is shown for both the initially hot (orange curve) and cold (cyan curve) nanodisks, as well as their sum (black curve)