We present a review on the emerging materials for novel plasmonic colloidal nanocrystals. We start by explaining the basic processes involved in surface plasmon resonances in nanoparticles and then discuss the classes of nanocrystals that to date are particularly promising for tunable plasmonics: nonstoichiometric copper chalcogenides, extrinsically doped metal oxides, oxygen-deficient metal oxides and conductive metal oxides. We additionally introduce other emerging types of plasmonic nanocrystals and finally we give an outlook on nanocrystals of materials that could potentially display interesting plasmonic properties.

The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electron-phonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu(2-x)Se nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu(2-x)Se compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission.

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via 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 10(5) GM (1 GM = 10(-50) cm(4) 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 k·p 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, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-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.

The aim of this study is to determine the minimum amount of dopant that prevents the occurrence, near room temperature, of a JahnÈTeller (JÈT) transition in the M-doped lithium manganese spinel of composition with 0.00 \ x O 0.06 and M \ Ni2`, Co3`, Cr3`or Ti4`. EPR spectra and magnetic Li 1.02 M x Mn 1.98~x O 4 susceptibility data are related to the valence state of M and Mn, and the homogeneous distribution of the dopant. We Ðnd that the spinel framework is remarkably sensitive to displaying low electronic and magnetic changes in its cationic sublattice due to cation substitution. The JÈT distortion, which is associated with a sudden drop in conductivity with decreasing temperature, is suppressed by substituting 3% of Mn with Co3ò r Cr3`, or by adding an even smaller amount of Ni2`(x \ 0.02, or 1% substitution). However, this inhibition occurs only in samples with a ratio r \ [Mn4`]/[Mn3`] P 1.18, i.e., a value larger than the ratio r \ 1.106 we have with no doping (x \ 0). As a consequence, doping with the tetravalent cation Ti4`, which always decreases the r value, does not suppress the JÈT transition. We suggest that both the dopant ion and the Li`in excess over the stoichiometric composition are located in 16d sites. The removal of the JÈT transition in the Co3`(x \ 0.06) sample is also due to local disorder.