Thermal Transport in Supported Graphene: Substrate Effects on Collective Excitations

A detailed computational analysis of thermal transport in supported graphene reveals the possibility of tuning its thermal conductivity by targeted chemical modifications of the substrate’s surface. Based on classical molecular dynamics with an accurate charge-optimized bond-order force field and a time-domain normal-mode analysis, our approach allows us to distinguish collective from single-phonon excitations. The computations reveal a disproportional reduction of the thermal conductivity, due to the two different excitations, when graphene interacts with a substrate. Deposition of graphene on a bare silica surface leads to a dramatic reduction of the thermal conductivity and a change in the heat transport mechanism. Remarkably, partial hydroxylation of the silica surface almost doubles the thermal conductivity of the collective excitations. Thus, specific surface terminations allow for control of the thermal conductivity of graphene.

Figure 1

(Top panels) Graphene hexagonal unit cell in the honeycomb lattice, and its first Brillouin zone. (A) Freestanding graphene. (C) Graphene on a partially hydroxylated amorphous silica substrate. Gray, blue, red, and yellow spheres represent carbon, hydrogen, oxygen, and silicon atoms, respectively. (Inset) Detailed view of a surface hydroxyl group interacting with the graphene sheet.

Figure 2

Top view of system C. Distribution of the hydroxyl groups, represented as large cyan spheres, at the surface of the amorphous silica substrate.

Figure 3

Spatially resolved deviation in height from a perfectly flat graphene sheet in force-minimized samples for both supported cases. The gradient is higher in the case of the hydroxylated substrate, while much smoother zones are present in graphene supported on the bare substrate.

Figure 4

Out-of-plane acoustic phonon lifetimes in the three different systems.

Figure 5

Single-mode and total modewise ZA thermal conductivity (in units of $\mathrm{W}/\mathrm{m}\mathrm{K}$) in the three systems considered. Mind the very different scales between the plots, especially for the total thermal conductivity of freestanding graphene.