Abstract
Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. These functional properties often involve large-amplitude strains and structural modifications, and thus require an understanding of the dynamics of these processes. Here we use femtosecond X-ray scattering techniques to visualize, in real time and with atomic-scale resolution, light-induced anisotropic strains in nanocrystal spheres and rods. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrier-induced stress. These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. This work represents the first direct real-time probe of the dynamics of these large-amplitude strains and shape changes in few-nanometre-scale particles.
Highlights
Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries
Extensive investigations have been carried out probing modifications in the electronic, thermodynamic and kinetic properties of semiconductor quantum dots as a function of size and shape, the size-dependent structural dynamics exhibited by nanocrystals are not well understood
Measurement of multiple nanocrystal diffraction peaks enables the photoinduced time-dependent strains to be projected along the relevant axes of the nanostructure, independent of its orientation. For both nanosized spheres and rods, we show that intense above-bandgap photoexcitation leads to percent level radial expansions occurring on few hundred femtosecond timescales
Summary
Nanoscale dimensions in materials lead to unique electronic and structural properties with applications ranging from site-specific drug delivery to anodes for lithium-ion batteries. Strains at the percent level are observed in CdS and CdSe samples, associated with a rapid expansion followed by contraction along the nanosphere or nanorod radial direction driven by a transient carrierinduced stress These morphological changes occur simultaneously with the first steps in the melting transition on hundreds of femtosecond timescales. Measurement of multiple nanocrystal diffraction peaks enables the photoinduced time-dependent strains to be projected along the relevant axes of the nanostructure, independent of its orientation For both nanosized spheres and rods, we show that intense above-bandgap photoexcitation leads to percent level radial expansions occurring on few hundred femtosecond timescales. In this work, we resolve the structural dynamics associated with the first steps in the semiconductor nanocrystal melting process, which occur concurrently with the ultrafast strain response
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