(Invited) The Interplay between Thermal and Mechanical Properties in Colloidal Quantum Dot Assemblies

Abstract

The ongoing interest in colloidal quantum dot assemblies for real-world electronic and photonic devices necessitates that their thermal and mechanical properties be well understood. Large thermal conductivities are desirable because this minimizes the temperature rise during operation and thereby improves device performance and lifetime. In addition, large elastic moduli and hardness directly impact device durability.In this talk, I present my group’s findings on two separate correlation regimes between thermal and mechanical properties in colloidal quantum dot assemblies. These two correlation regimes correspond to (i) assemblies with weak inter-particle forces and (ii) assemblies with strong inter-particle forces.In the weak inter-particle force regime, thermal and mechanical properties intersect via the speed of sound. This stems from the speed of sound being linearly-related to thermal conductivity and quadratically-related to elastic modulus. In the strong inter-particle force regime, thermal and mechanical properties intersect via quantum dot diameter. We find that the quantum dot diameter primarily effects thermal conductivity via the mean free path of the thermal energy carriers. The interrelationship between diameter and mechanical properties in this regime is similar to an effective medium approximation, but with added subtleties. In particular, we find that the surface ligand properties and particle diameter can be intercoupled. This in turn has important impacts on the mechanical properties of the overall quantum dot assembly.In order to demonstrate the above thermomechanical relationships, I present my group’s combined experimental and modeling studies on three classes of colloidal particle assemblies. First, we use thermal processing methods to study the effect of native oleic acid ligands versus cross-linked oleic acid ligands. Second, we use single-domain superlattices and disordered thin films to study the effect of particle ordering. Lastly, we use ligand exchange methods to study the effect of organic versus inorganic ligands.