An official website of the United States government
Colloidal copper nanorods (NRs) display transverse and longitudinal localized surface plasmon resonances. The longitudinal localized surface plasmon modes are tunable through the near‐infrared electromagnetic radiation energies with NR aspect ratios. Visible and near‐infrared transient optical response of the copper NRs is investigated under excitation conditions spanning intraband and interband excitation (0.79−3.50 eV). In both the visible and near‐infrared regions, the spectral response of the samples under intraband excitation (<2 eV) differs substantially from their response under interband excitation (>2 eV). However, the timescale of the electron−phonon coupling estimated from pump fluence‐dependent measurements (τep) is less sensitive to excitation conditions than reports for gold.τepshortens slightly from ≈616 fs with intraband excitation (at visible probe energies) to ≈565 fs with interband excitation. The observed dynamics correspond to an average sample electron−phonon coupling parameter varying across all conditions from 4.4 × 1016to 6.4 × 1016 J m−3 K−1, which is similar to bulk copper. Furthermore, coherent acoustic phonons are observed for the longitudinal localized surface plasmon resonance with a range of oscillatory periods reflecting sample size dispersion.
more »
« less
Spin‐Polarized Photoluminescence in Au 25 (SC 8 H 9 ) 18 Monolayer‐Protected Clusters
Abstract Here, the observation of spin‐polarized emission for the Au25(SC8H9)18monolayer‐protected cluster (MPC) is reported. Variable‐temperature variable‐field magnetic circular photoluminescence (VTV‐MCPL) measurements are combined with VT‐PL spectroscopy to provide state‐resolved characterization of the transient electronic structure and spin‐polarized electron‐hole recombination dynamics of Au25(SC8H9)18. Through analysis of VTV‐MCPL measurements, a low energy (1.64 eV) emission peak is assigned to intraband relaxation between core‐metal‐localized superatom‐D to ‐P orbitals. Two higher energy interband components (1.78 eV, 1.94 eV) are assigned to relaxation from superatom‐D orbitals to states localized to the inorganic semirings. For both intraband superatom‐based or interband relaxation mechanisms, the extent of spin‐polarization, quantified as the degree of circular polarization (DOCP), is determined by state‐specific electron‐vibration coupling strengths and energy separations of bright and dark electronic fine‐structure levels. At low temperatures (<60 K), metal–metal superatom‐based intraband transitions dominate the global PL emission. At higher temperatures (>60 K), interband ligand‐based emission is dominant. In the low‐temperature PL regime, increased sample temperature results in larger global PL intensity. In the high‐temperature regime, increased temperature quenches interband radiative recombination. The relative intensity for each PL mechanism is discussed in terms of state‐specific electronic‐vibrational coupling strengths and related to the total angular momentum, quantified by Landég‐factors.
more »
« less
- Award ID(s):
- 1904876
- PAR ID:
- 10449460
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Small
- Volume:
- 17
- Issue:
- 27
- ISSN:
- 1613-6810
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The magneto-optical signatures of colloidal noble metal nanostructures, spanning both discrete nanoclusters (<2 nm) and plasmonic nanoparticles (>2 nm), exhibit rich structure-property correlations, impacting applications including photonic integrated circuits, light modulation, applied spectroscopy, and more. For nanoclusters, electron doping and single-atom substitution modify both the intensity of the magneto-optical response and the degree of transient spin polarization. Nanoparticle size and morphology also modulate the magnitude and polarity of plasmon-mediated magneto-optical signals. This intimate interplay between nanostructure and magneto-optical properties becomes especially apparent in magnetic circular dichroism (MCD) and magnetic circular photoluminescence (MCPL) spectroscopic data. Whereas MCD spectroscopy informs on a metal nanostructure's steady-state extinction properties, its MCPL counterpart is sensitive to electronic spin and orbital angular momenta of transiently excited states. This review describes the size- and structure-dependent magneto-optical properties of nanoscale metals, emphasizing the increasingly important role of MCPL in understanding transient spin properties and dynamics.more » « less
-
The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin–orbit interaction in solids composed of heavy elements. Here, we study the composite particles—chiral excitons—formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, . Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin–orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin–orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.more » « less
-
Abstract Attainment of quantum‐confined materials with remarkable stoichiometric, geometric, and structural control has been made possible by advances in colloidal nanoparticle synthesis. The quantum states of these systems can be tailored by selective spatial confinement in one, two, or three dimensions. As a result, a multitude of prospects for controlling nanoscale energy transfer have emerged. An understanding of the electronic relaxation dynamics for quantum states of specific nanostructures is required to develop predictive models for controlling energy on the nanoscale. Variable‐temperature, variable‐magnetic field ( ) optical methods have emerged as powerful tools for characterizing transient excited states. For example, magnetic circular photoluminescence (MCPL) spectroscopy can be used to calculate electronic g factors, assign spectroscopic term symbols for transitions within metal nanoclusters, and quantify the energy gaps separating electronic fine‐structure states. spectroscopic methods are effective for isolating the carrier dynamics of specific quantum fine‐structure states, enabling determination of electronic relaxation mechanisms such as electron‐phonon scattering and energy transfer between assembled nanoclusters. In particular ‐MCPL is especially effective for studying electronic spin‐state dynamics and properties. This Review highlights specific examples that emphasize insights obtainable from these methods and discusses prospects for future research directions.more » « less
-
Abstract Chirality has been a property of central importance in physics, chemistry and biology for more than a century. Recently, electrons were found to become spin polarized after transmitting through chiral molecules, crystals, and their hybrids. This phenomenon, called chirality-induced spin selectivity (CISS), presents broad application potentials and far-reaching fundamental implications involving intricate interplays among structural chirality, topological states, and electronic spin and orbitals. However, the microscopic picture of how chiral geometry influences electronic spin remains elusive, given the negligible spin-orbit coupling (SOC) in organic molecules. In this work, we address this issue via a direct comparison of magnetoconductance (MC) measurements on magnetic semiconductor-based chiral molecular spin valves with normal metal electrodes of contrasting SOC strengths. The experiment reveals that a heavy-metal electrode provides SOC to convert the orbital polarization induced by the chiral molecular structure tospinpolarization. Our results illustrate the essential role of SOC in the metal electrode for the CISS spin valve effect. A tunneling model with a magnetochiral modulation of the potential barrier is shown to quantitatively account for the unusual transport behavior.more » « less
