Effect of Morphology on Ultrafast Carrier Dynamics in Asymmetric Gold–Iron Oxide Plasmonic Heterodimers

Understanding how nanoscale interfaces affect electrical and optical properties of multifunctional nanocrystal heterostructures is of paramount importance for their technological application. In this context, we investigated the ultrafast carrier dynamics of rodlike gold–iron oxide nanocrystal heterodimers, in a spectral region close to the surface plasmon resonance frequency, by means of broad-band transient absorption spectroscopy. We found that the electron–phonon relaxation time is independent of the morphology of the iron oxide domain. Moreover, we revealed a transient shift in the surface plasmon resonance frequency, which can be related to charge transfer at the interface between gold and iron oxide

Plasmon Bleaching Dynamics in Colloidal Gold–Iron Oxide Nanocrystal Heterodimers

Colloidal nanocrystal heterodimers composed of a plasmonic and a magnetic domain have been widely studied as potential materials for various applications in nanomedicine, biology, and photocatalysis. One of the most popular nanocrystal heterodimers is represented by a structure made of a Au domain and a iron oxide domain joined together. Understanding the nature of the interface between the two domains in such type of dimer and how this influences the energy relaxation processes is a key issue. Here, we present the first broad-band transient absorption study on gold/iron oxide nanocrystal heterodimers that explains how the energy relaxation is affected by the presence of such interface. We found faster electron–electron and electron–phonon relaxation times for the gold “nested” in the iron oxide domain in the heterodimers with respect to gold “only” nanocrystals, that is, free-standing gold nanocrystals in solution. We relate this effect to the decreased electron screening caused by spill-out of the gold electron distribution at gold/iron oxide interface

Plasmon Dynamics in Colloidal Au₂Cd Alloy–CdSe Core/Shell Nanocrystals

Metal–semiconductor nanocrystal heterostructures are model systems for understanding the interplay between the localized surface plasmon resonances in the metal domain and the relaxation of the excited carriers in the semiconductor domain. Here we report the synthesis of colloidal Au₂Cd (core)/CdSe (shell) nanocrystal heterostructures, which were characterized extensively with several structural and optical techniques, including time-resolved fluorescence and broad-band transient absorption spectroscopy (both below and above the CdSe band gap). The dynamics of the transient plasmon peak was dominated by the relaxation of hot carriers in the metal core, its spectral shape was independent of the pump wavelength, and the bleaching lifetime was about half a picosecond, comparable with the value found in the AuCd seeds used for the synthesis

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Two-Photon-Induced Blue Shift of Core and Shell Optical Transitions in Colloidal CdSe/CdS Quasi-Type II Quantum Rods

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied <i>via</i> two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10⁵ GM (1 GM = 10^–50 cm⁴ s phot^–1), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by <i>k·p</i> calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, <i>via</i> the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry <i>via</i> the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P<i></i>-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption