Size-Dependent Melting Behavior of Colloidal In, Sn and Bi Nanocrystals

Minglu Liu,Robert Y Wang

doi:10.1038/srep16353

Abstract

Colloidal nanocrystals are a technologically important class of nanostructures whose phase change properties have been largely unexplored. Here we report on the melting behavior of In, Sn, and Bi nanocrystals dispersed in a polymer matrix. This polymer matrix prevents the nanocrystals from coalescing with one another and enables previously unaccessed observations on the melting behavior of colloidal nanocrystals. We measure the melting temperature, melting enthalpy, and melting entropy of colloidal nanocrystals with diameters of approximately 10 to 20 nm. All of these properties decrease as nanocrystal size decreases, although the depression rate for melting temperature is comparatively slower than that of melting enthalpy and melting entropy. We also observe an elevated melting temperature during the initial melt-freeze cycle that we attribute to surface stabilization from the organic ligands on the nanocrystal surface. Broad endothermic melting valleys and very large supercoolings in our calorimetry data suggest that colloidal nanocrystals exhibit a significant amount of surface pre-melting and low heterogeneous nucleation probabilities during freezing.

Highlights

IntroductionThe nanoscale dimensions of the nanocrystals degraded so fast that bulk melting temperatures were observed during the first melting cycle[23,24]
We prepared our samples by synthesizing colloidal nanocrystals and dissolving them with PI resin in a shared solvent
We synthesized our colloidal nanocrystals using hot-injection techniques reported by Kravchyk et al.[27] and Yarema et al.[28,29,30], and controlled the nanocrystal diameter between 10 and 20 nm by varying the reaction temperature and time (Figure S1)

Summary

Introduction

The nanoscale dimensions of the nanocrystals degraded so fast that bulk melting temperatures were observed during the first melting cycle[23,24]. We demonstrate that this nanocrystal coalescence problem can be completely mitigated by isolating the colloidal nanocrystals from one another via dispersion within a polymer matrix. We use polyimide resin (PI resin) for the polymer matrix because it has a glass transition temperature above 305 °C. The use of the stabilizing PI resin matrix allows us to observe highly repeatable melting behavior throughout numerous melt-freeze cycles. We observe signatures of surface pre-melting and low heterogeneous nucleation probabilities in our samples that manifest themselves as broad endothermic melting valleys and very large supercooling in the calorimetry data

Methods

Results

Conclusion