Visualization of nanocrystal breathing modes at extreme strains.

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.

Real-time visualization of nanocrystal solid-solid transformation pathways.

Measurement and understanding of the microscopic pathways materials follow as they transform is crucial for the design and synthesis of new metastable phases of matter. Here we employ femtosecond single-shot X-ray diffraction techniques to measure the pathways underlying solid-solid phase transitions in cadmium sulfide nanorods, a model system for a general class of martensitic transformations. Using picosecond rise-time laser-generated shocks to trigger the transformation, we directly observe the transition state dynamics associated with the wurtzite-to-rocksalt structural phase transformation in cadmium sulfide with atomic-scale resolution. A stress-dependent transition path is observed. At high peak stresses, the majority of the sample is converted directly into the rocksalt phase with no evidence of an intermediate prior to rocksalt formation. At lower peak stresses, a transient five-coordinated intermediate structure is observed consistent with previous first principles modeling.

Wurtzite to rocksalt phase transformation of cadmium selenide nanocrystals via laser-induced shock waves: transition from single to multiple nucleation.

The behavior of CdSe nanocrystals shocked to stresses of 2-3.75 GPa has been studied. Above 3 GPa a near-complete disappearance of the first excitonic feature and broadening of the low-energy absorption edge were observed, consistent with a wurtzite to rocksalt structural transformation. The transformation pressure is reduced relative to hydrostatic compression in a diamond anvil cell, and the rate increases, attributed to shock induced shear stress along the reaction coordinate. The especially rapid rate observed for a 3.75 GPa shock suggests multiple nucleation events per particle.

Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals.

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.

Multielectron ionization of CdSe quantum dots in intense femtosecond ultraviolet light.

Multielectron ionization of colloidal CdSe quantum dots under intense femtosecond UV excitation has been studied. By directly probing the absorption from the ionized electron, quantitative measurements of the yield and dynamics of the ionization have been made as a function of excitation fluence and variations of size and potential structure of quantum dots. The results have been explained by an ionization mechanism involving resonant two-photon absorption.

Normal and inverse primary kinetic deuterium isotope effects for C-H bond reductive elimination and oxidative addition reactions of molybdenocene and tungstenocene complexes: evidence for benzene sigma-complex intermediates.

The overall reductive elimination of RH from the ansa-molybdenocene and -tungstenocene complexes [Me(2)Si(C(5)Me(4))(2)]Mo(Ph)H and [Me(2)Si(C(5)Me(4))(2)]W(R)H (R = Me, Ph) is characterized by an inverse primary kinetic isotope effect (KIE) for the tungsten system but a normal KIE for the molybdenum system. Oxidative addition of PhH to [[Me(2)Si(C(5)Me(4))(2)]M] also differs for the two systems, with the molybdenum system exhibiting a substantial intermolecular KIE, while no effect is observed for the tungsten system. These differences in KIEs indicate a significant difference in the reactivity of the hydrocarbon adducts [Me(2)Si(C(5)Me(4))(2)]M(RH) for the molybdenum and tungsten systems. Specifically, oxidative cleavage of [Me(2)Si(C(5)Me(4))(2)]M(RH) is favored over RH dissociation for the tungsten system, whereas RH dissociation is favored for the molybdenum system. A kinetics analysis of the interconversion of [Me(2)Si(C(5)Me(4))(2)]W(CH(3))D and [Me(2)Si(C(5)Me(4))(2)]W(CH(2)D)H, accompanied by elimination of methane, provides evidence that the reductive coupling step in this system is characterized by a normal KIE. This observation demonstrates that the inverse KIE for overall reductive elimination is a result of an inverse equilibrium isotope effect (EIE) and is not a result of an inverse KIE for a single step. A previous report of an inverse kinetic isotope effect of 0.76 for C-H reductive coupling in the [Tp]Pt(CH(3))H(2) system is shown to be erroneous. Finally, a computational study provides evidence that the reductive coupling of [Me(2)Si(C(5)Me(4))(2)]W(Ph)H proceeds via the initial formation of a benzene sigma-complex, rather than an eta(2)-pi-benzene complex.