Modifying Thermal Transport in Colloidal Nanocrystal Solids with Surface Chemistry

Abstract

We present a systematic study on the effect of surface chemistry on thermal transport in colloidal nanocrystal (NC) solids. Using PbS NCs as a model system, we vary ligand binding group (thiol, amine, and atomic halides), ligand length (ethanedithiol, butanedithiol, hexanedithiol, and octanedithiol), and NC diameter (3.3-8.2 nm). Our experiments reveal several findings: (i) The ligand choice can vary the NC solid thermal conductivity by up to a factor of 2.5. (ii) The ligand binding strength to the NC core does not significantly impact thermal conductivity. (iii) Reducing the ligand length can decrease the interparticle distance, which increases thermal conductivity. (iv) Increasing the NC diameter increases thermal conductivity. (v) The effect of surface chemistry can exceed the effect of NC diameter and becomes more pronounced as NC diameter decreases. By combining these trends, we demonstrate that the thermal conductivity of NC solids can be varied by an overall factor of 4, from ∼0.1-0.4 W/m-K. We complement these findings with effective medium approximation modeling and identify thermal transport in the ligand matrix as the rate-limiter for thermal transport. By combining these modeling results with our experimental observations, we conclude that future efforts to increase thermal conductivity in NC solids should focus on the ligand-ligand interface between neighboring NCs.

Ligand Crosslinking Boosts Thermal Transport in Colloidal Nanocrystal Solids

“…However, this diameter difference by itself is insufficient to explain the observed difference in thermal conductivity. Our 7.9 nm diameter samples provide a more suitable comparison to the prior work on 8 nm samples by Liu et al . In this comparison, our measured value of 0.81 Wm −1 K −1 is still double that of the value reported by Liu et al …”

“…Our measured value of 1.7 Wm −1 K −1 exceeds the highest reported thermal conductivity in a colloidal NC solid by a factor of ≈4 (i.e., Liu et al . measured a thermal conductivity of 0.4 Wm −1 K −1 for 8 nm diameter PbS NCs with short ethylenediamine ligands).…”

“…Figure a,b is a schematic illustration and cross‐sectional SEM image of an NC solid sample that is ready for measurement by the 3ω method (i.e., an NC solid sandwiched by a top SiO 2 layer and a bottom Si substrate). A detailed description of our 3ω measurement system can be found in Section II of the Supporting Information and also in our previous work …”

“…showed that thermal conductivity is insensitive to this parameter. The groups of Ong and Liu both showed that the NC solid thermal conductivity is directly related to the volume fraction of the ligand matrix. Consequently, a partial explanation for this difference in thermal conductivity is our larger 20 nm NC cores (i.e., smaller volume fraction of the ligand matrix).…”

“…showed that NC solid thermal conductivity is largely insensitive to the thermal conductivity of the NC core, which indicates that the NC–ligand interface and/or the ligand matrix is limiting thermal transport. Additional experiments by our group investigated a variety of NC–ligand binding groups, ligand lengths, and NC sizes. Our studies narrowed down the source of the low thermal conductivity to the ligand matrix itself.…”