State-Resolved Metal Nanoparticle Dynamics Viewed through the Combined Lenses of Ultrafast and Magneto-optical Spectroscopies

Abstract

Electronic carrier dynamics play pivotal roles in the functional properties of nanomaterials. For colloidal metals, the mechanisms and influences of these dynamics are structure dependent. The coherent carrier dynamics of collective plasmon modes for nanoparticles (approximately 2 nm and larger) determine optical amplification factors that are important to applied spectroscopy techniques. In the nanocluster domain (sub-2 nm), carrier coupling to vibrational modes affects photoluminescence yields. The performance of photocatalytic materials featuring both nanoparticles and nanoclusters also depends on the relaxation dynamics of nonequilibrium charge carriers. The challenges for developing comprehensive descriptions of carrier dynamics spanning both domains are multifold. Plasmon coherences are short-lived, persisting for only tens of femtoseconds. Nanoclusters exhibit discrete carrier dynamics that can persist for microseconds in some cases. On this time scale, many state-dependent processes, including vibrational relaxation, charge transfer, and spin conversion, affect carrier dynamics in ways that are nonscalable but, rather, structure specific. Hence, state-resolved spectroscopy methods are needed for understanding carrier dynamics in the nanocluster domain. Based on these considerations, a detailed understanding of structure-dependent carrier dynamics across length scales requires an appropriate combination of spectroscopic methods. Plasmon mode-specific dynamics can be obtained through ultrafast correlated light and electron microscopy (UCLEM), which pairs interferometric nonlinear optical (INLO) with electron imaging methods. INLO yields nanostructure spectral resonance responses, which capture the system's homogeneous line width and coherence dynamics. State-resolved nanocluster dynamics can be obtained by pairing ultrafast with magnetic-optical spectroscopy methods. In particular, variable-temperature variable-field (VTVH) spectroscopies allow quantification of transient, excited states, providing quantification of important parameters such as spin and orbital angular momenta as well as the energy gaps that separate electronic fine structure states. Ultrafast two-dimensional electronic spectroscopy (2DES) can be used to understand how these details influence state-to-state carrier dynamics. In combination, VTVH and 2DES methods can provide chemists with detailed information regarding the structure-dependent and state-specific flow of energy through metal nanoclusters. In this Account, we highlight recent advances toward understanding structure-dependent carrier dynamics for metals spanning the sub-nanometer to tens of nanometers length scale. We demonstrate the use of UCLEM methods for arresting interband scattering effects. For sub-nanometer thiol-protected nanoclusters, we discuss the effectiveness of VTVH for distinguishing state-specific radiative recombination originating from a gold core versus organometallic protecting layers. This state specificity is refined further using femtosecond 2DES and two-...

Anomalous phonon relaxation in Au 333 (SR) 79 nanoparticles with nascent plasmons

“…Steadystate absorption spectra revealed two absorption peaks at 442 and 498 nm at 80 K, which is different from the case of typical plasmonic nanoparticles. The laser fluence dependence of both peaks indicates that both are plasmon peaks (as opposed to excitons) (11)(12)(13)40). The dual plasmons indicate a nonspherical shape of Au 333 .…”

“…On the other hand, metal nanoclusters in nonmetallic (excitonic) state show multiple ground state-bleaching (GSB) bands (like organic molecules) upon photoexcitation (11)(12)(13)36). The photogenerated exciton experiences a relaxation process of electron-hole recombination, and its lifetime depends on the size and structure.…”

“…For example, SPR disappears when the size goes down below 2 nm due to quantum confinement effect and quantization of the conduction band (7). Unlike the conventional, plasmonic gold nanoparticles of facecentered cubic structure, the ultrasmall gold nanoparticles (socalled nanoclusters) with discrete energy levels exhibit unique geometrical (8)(9)(10) and electronic structures (11)(12)(13), and novel properties for applications in biomedicine (14,15), optoelectronics (16,17), and catalysis (18,19).…”

Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

“…Nanoclusters (Au 25 (SR) 18 as the example): Unlike homoleptic gold complexes, thiolate-protected gold nanoclusters are composed of a well-defined metal core and surface staple motifs (e.g., -S-Au-S-) [11]. The relaxation dynamics of gold nanoclusters of different structures have been investigated by several groups and the relaxation model is more complicated as a result of multiple contributions of both Au and S atoms to the orbitals [27,28,49,50]. Here, Au 25 (SR) 18 was chosen as an example to illustrate the photophysics.…”

“…Hartland and Vallee groups have extensively investigated the phonon dynamics and electron-phonon coupling of different sized AuNPs [23,24]. In recent years, there has also been research on the excited state dynamics of ligand-protected gold nanoclusters [1,2,[25][26][27]. The electron and phonon dynamics of the nanoclusters were found to be dependent on both size and structure [28][29][30].…”