Nanocrystal superlattices that exhibit improved order on heating: an example of inverse melting?
Grazing incidence small angle X-ray scattering (GISAXS) measurements reveal that superlattices of 1.7 nm diameter, gold (Au) nanocrystals capped with octadecanethiol become significantly more ordered when heated to moderate temperatures (50-60 °C). This enhancement in order is reversible and the superlattice returns to its initially disordered structure when cooled back to room temperature. Disorder-order transition temperatures were estimated from the GISAXS data using the Hansen-Verlet criterion. Differential scanning calorimetry (DSC) measurements of the superlattices exhibited exotherms (associated with disordering during cooling) and endotherms (associated with ordering during heating) near the transition temperatures. The superlattice transition temperatures also correspond approximately to the melting and solidification points of octadecanethiol. Therefore, it appears that a change in capping ligand packing that occurs upon ligand melting underlies the structural transition of the superlattices. We liken the heat-induced ordering of the superlattices to an inverse melting transition.
McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, TX 78712-1062, USA. korgel@che.utexas.edu.
Yixuan
Y
Yu
Avni
A
Jain
Adrien
A
Guillaussier
Vikas Reddy
VR
Voggu
Thomas M
TM
Truskett
Detlef-M
DM
Smilgies
Brian A
BA
Korgel
D002152:Calorimetry, Differential Scanning; D006358:Hot Temperature; D053758:Nanoparticles; D012542:Scattering, Radiation