Hot electrons in metal nanoparticles thermalize with the lattice via electron−phonon coupling. Size dependency is controversially discussed in literature. Here we investigate poly-and monocrystalline gold nanoparticles via transient absorption spectroscopy. As reported earlier, electron−phonon coupling in polycrystalline particles is not size-dependent. However, we clearly observe a size-dependent electron−phonon coupling in monocrystalline particles. Larger monocrystalline particles show slower electron−phonon coupling due to the decreasing effect of electron-surface scattering, with electron−phonon coupling constants approaching the values reported for bulk gold. In polycrystalline particles, size dependencies are outweighed by effective electron scattering at grain boundaries. Linear absorption spectra indicate that plasmon damping is also enhanced in polycrystalline particles by grain boundaries.

Cation exchange is known to occur during the synthesis of colloidal semiconductor heteronanoparticles, affecting their band gap and thus altering their optoelectronic properties. It is often neglected, especially when anisotropic heterostructures are discussed. We present a study on the role of cation exchange inevitably occurring during the growth of anisotropic dot-in-rod structures consisting of a spherical ZnSe core enclosed by a rod-shaped CdS shell. The material combination exhibits a type-II band alignment. Two reactions are compared: the shell-growth reaction of CdS on ZnSe and an exchange-only reaction of ZnSe cores to CdSe. Transmission electron microscopy and a comprehensive set of optical spectroscopy data, including linear and time-resolved absorption and fluorescence data, prove that cation exchange from ZnSe to CdSe is the dominant process in the initial stages of the shell-growth reaction. The degree of cation exchange before significant shell growth starts was determined to be about 50%, highlighting the importance of cation exchange during the heteronanostructure growth.

Optical-pump-terahertz-probe (OPTP) experiments are widely employed to study the dynamics of photoexcited carriers in semiconductors. In these experiments, due to the long wavelength nature of terahertz (THz) probe radiation, the probe beam can be focused only to a spot size in the mm range. To ensure homogeneous excitation of the probed sample region, a significantly larger optical pump beam spot size must be used, which is often difficult to implement in the experiment. Frequently used experimental geometries employ beam paths that result in small pump beam spot sizes, leading to spectral distortions of the sample response, translating to uncertainties in calculated THz conductivities and fitted Drude conductivity models, for example. We investigate the influence of pump beam spot size on benchmark OPTP experiments and evaluate model calculations to estimate the induced deviations. We demonstrate the impact of this effect on the acquired data with different dependencies on the investigated sample and the employed experimental configuration. We can provide guidelines for optimal configurations for the most commonly employed experiments.

Mobile charge carriers in heterostructured nanoparticles are relevant for applications requiring charge separation and extraction. We investigate the benchmark systems CdSe–CdS core–shell quantum dots and quantum dots in quantum rods by optical and THz pump–probe spectroscopy. We relate photoconductivity and carrier location and observe that only shell-located electrons in quantum rods contribute to an observable photoconductivity. Despite the shallow electron confinement in the quasi-type II heterostructures, core-located carriers are bound into immobile excitons that respond on external electrical fields by polarization.