Ligand Crosslinking Boosts Thermal Transport in Colloidal Nanocrystal Solids.

ABSTRACT

The ongoing interest in colloidal nanocrystal solids for electronic and photonic devices necessitates that their thermal-transport properties be well understood because heat dissipation frequently limits performance in these devices. Unfortunately, colloidal nanocrystal solids generally possess very low thermal conductivities. This very low thermal conductivity primarily results from the weak van der Waals interaction between the ligands of adjacent nanocrystals. We overcome this thermal-transport bottleneck by crosslinking the ligands to exchange a weak van der Waals interaction with a strong covalent bond. We obtain thermal conductivities of up to 1.7 Wm-1 K-1 that exceed prior reported values by a factor of 4. This improvement is significant because the entire range of prior reported values themselves only span a factor of 4 (i.e., 0.1-0.4 Wm-1 K-1 ). We complement our thermal-conductivity measurements with mechanical nanoindentation measurements that demonstrate ligand crosslinking increases Young's modulus and sound velocity. This increase in sound velocity is a key bridge between mechanical and thermal properties because sound velocity and thermal conductivity are linearly proportional according to kinetic theory. Control experiments with non-crosslinkable ligands, as well as transport modeling, further confirm that ligand crosslinking boosts thermal transport.