Spontaneous Room Temperature Elongation of CdS and Ag₂S Nanorods via Oriented Attachment

Abstract

Spontaneous elongation from nanorod to nanowire in the presence of an amine is reported for nanocrystals of cadmium sulfide and silver sulfide (cation exchanged from CdS). Elongation occurs instantaneously where the final aspect ratio is a controllable multiple of the original nanorod length.Transmission electron microscopy (TEM) data and kinetic modeling reveal the influential factors on the attachment process are the concentration of amine, duration and temperature of the reaction. The elongated nanorods are further characterized by X-Ray diffraction (XRD), photoluminescence (PL), ultraviolet-visible spectroscopy (UV-Vis) and X-ray photoelectron spectroscopy (XPS). A mechanism of oriented attachment is evidenced by the doubling in length of asymmetrically gold tipped CdS nanorods with the corresponding absence of elongation in symmetrically tipped nanorods.1

A multi-rate kinetic model for spontaneous oriented attachment of CdS nanorods

“…This is observed to be the case 25 , but the virtually degeneracy in the formation energies of the wurzite and zinc-blende structures means that the rods tend to branch rather than elongate in a measurable, one dimensional fashion. Ag 2 S, with a similar dipole moment to CdS 33 , undergoes a similar oriented attachment process 25 . The model predicts that in general materials with high surface energy and dipole moment should display oriented attachment over a wide range.…”

“…25 The TEM counting statistics have been verified by the XRD analysis. Table 1 shows the fitting parameters extracted from XRD scans 25 of the starting 35 x 5 nm rods and the elongated 140 nm x 5 nm rods. The scans are fit with pseudo-Voigt peaks using a least squares fitting method.…”

“…The role of the amine is to depassivate the rod ends by stripping them of their surface ligands (Figure 1). 25 If the amine were a reactant, reactions rates would increase with amine concentration, if even to a "saturation" concentration, above which no increase in rate would be seen. However, the existence of a peak precludes the possibility of the amine having to be treated as a reactant, and the reaction rates need only be describe in terms of i-mer concentrations.…”

“…Recently we reported the spontaneous attachment of semiconductor CdS and Ag 2 S nanorods only at the (001) growth facet following selective removal of ligands using an amine 25 . As this attachment takes place in only one dimension, a mechanism of complete oriented attachment is unambiguous with the formation of integer multiples of the original nanorod length that can be accurately confirmed by TEM and XRD.…”

Protein immobilisation on perpendicularly aligned gold tipped nanorod assemblies

“…Nanorods of 100 nm in length were obtained by controlled oriented attachement of as synthesised nanorods 35 nm in length. 7 The nanorods are bound to the substrate at the end facets and are 50 separated from each other (~ 2 nm) by interdigitated longchain octadecyl phosphonic acid ligands. The resulting As the processes of gold tip formation introduces additional insulating surfactants to the as deposited nanorods, the excess surfactant was removed using a 1:1 H 2 :O 2 plasma prior to 15 protein immobilisation.…”