The use of two-dimensional electronic spectroscopy (2DES) to study electron−electron scattering dynamics in plasmonic gold nanorods is described. The 2DES resolved the time-dependent plasmon homogeneous line width Γ h (t), which was sensitive to changes in Fermi-level carrier densities. This approach was effective because electronic excitation accelerated plasmon dephasing, which broadened Γ h . Analysis of Γ h (t) indicated plasmon coherence times were decreased by 20−50%, depending on excitation conditions. Electron−electron scattering rates of approximately 0.01 fs −1 were obtained by fitting the timedependent Γ h broadening; rates increased quadratically with both excitation pulse energy and frequency. This rate dependence agreed with Fermi-liquid theory-based predictions. Hot electron thermalization through electron−phonon scattering resulted in Γ h narrowing. To our knowledge, this is the first use of the plasmon Γ h (t) to isolate electron−electron scattering dynamics in colloidal metal nanoparticles. These results illustrate the effectiveness of 2DES for studying hot electron dynamics of solution-phase plasmonic ensembles.

Gold nanoparticles are well-known to exhibit size-dependent properties that are responsible for their unique catalytic, optical, and electronic applications. However, electron−phonon coupling, which is important for photocatalysis and light harvesting, is one of the rare properties of gold that is size-independent above a threshold value, e.g., for nanospheres larger than approximately 5 nm in diameter. Here, we show that when interfaced to a comparably sized Pt nanoparticle, the electron−phonon coupling constant of the hybrid material depends on the diameter of the Au domain. This is important because the electron−phonon coupling constant describes the efficiency by which hot electrons are converted to local heat by the primary electron−phonon scattering thermalization channel. We begin by synthesizing a library of Au−Pt hybrid nanoparticle heterodimers by growing size-tunable Au nanoparticles on Pt nanoparticle seeds. By systematically varying reagent concentration and reaction time, the Au domain diameter of the Au−Pt hybrid nanoparticle heterodimers can be tuned between 4.4 and 16 nm while the size of the Pt domain remains constant. Calibration curves allow us to dial in precise Au domain sizes, and microscopic analysis of the Au−Pt heterodimers provides insights into how they grow and how their morphologies evolve. Femtosecond time-resolved transient absorption spectroscopy reveals that for Au−Pt heterodimers having Au domain diameters of 8.7 to 14 nm, the electron−phonon coupling constant decreases by more than 80%, which is not observed for comparably sized Au nanoparticles. Interfacing smaller Au domains with Pt nanoparticle surfaces causes an increase in the density of states near the Fermi level of Au, which results in accelerated thermalization times through an increased number of electron−phonon interactions. The combination of precision hybrid nanoparticle synthesis and size-dependent electron−phonon coupling may be important for designing composite metals for photocatalytic and light-harvesting applications and for engineering different functions into established materials.

Recent advances in colloidal synthesis enable the generation of multicomponent metal−semiconductor nanoparticles that share a solid-state interface, thus providing a tunable platform for the tailored electronic and optical properties of nanoscale heterostructures. Here, the influence of size and material composition on electron−phonon scattering was investigated for a series of gold− metal chalcogenide (PbS, ZnS, and Cu 2−x S) hybrid nanoparticles using femtosecond time-resolved transient extinction spectroscopy. The influence of semiconductor size on electron−phonon coupling in the hybrid nanoparticles was studied using two Au−PbS systems having different PbS diameters, 6 ± 1 and 17 ± 3 nm. For Au−PbS (PbS = 6 ± 1 nm), an approximately 30% acceleration of the electron−phonon scattering rate was observed with respect to 5 ± 1 nm gold nanoparticles. In contrast, the system having the larger PbS domain size exhibited a decelerated rate when compared to gold nanoparticles. The nanostructure dependence of the electron− phonon scattering rates was attributed to differences in band edge alignment with respect to the Au Fermi level. Electron−phonon scattering was accelerated for Au−Cu 2−x S where the conduction band edge is in close alignment with the gold Fermi level. In contrast, the ultrafast response of Au−ZnS displayed no significant difference from pure AuNPs, which is consistent with minimal energy alignment between the two domains; the ZnS domain is an effective insulator in this case. These results demonstrate that controlled and selective modifications to both the size and composition of the semiconductor domain in metal−semiconductor hybrid nanoparticles impact band alignment, which in turn can be leveraged to modulate electronic thermalization in plasmonsupporting heterostructures.

Electronic relaxation dynamics of neutral Au38(SC6H13)24 monolayer-protected clusters (MPCs), following excitation of the mixed 15 875 cm−1 charge transfer resonance, were studied using femtosecond transient absorption (fsTA) and two-dimensional electronic spectroscopy (2DES). The excited carriers relax by three different mechanisms, including an ∼100 fs HOMO−12/−13 to HOMO−4/−6 hole transfer, picosecond HOMO−4/−6 to HOMO hole transfer, and subsequent electron–hole recombination that persisted beyond the hundreds of picoseconds measurement range. The fsTA data revealed two transient bleach components at 15 820 and 15 625 cm−1, where the lower frequency component exhibited a delayed first-order buildup of 80 ± 25 fs that matched the decay of the high-energy bleach component (110 ± 45 fs). These results suggested that the excited charge carriers internally relax within the exited-state manifold in ≈100 fs. 2DES resolved multiple electronic fine-structure transient peaks that spanned excitation frequencies ranging from 15 500 to 16 100 cm−1. State-to-state dynamics were understood by the analysis of time-dependent 2DES transient signal amplitudes at numerous excitation-detection frequency combinations. An off-diagonal cross peak at 15 825–15 620 cm−1 excitation-detection signified the HOMO−12/−13 to HOMO−4/−6 hole transfer process. The lowest-frequency (15 620 cm−1) 2DES diagonal fine-structure peak exhibited instantaneous amplitude but intensified following a 75 ± 10 fs buildup when compared to diagonal peaks at higher frequencies. This observation indicated that the charge transfer resonance in Au38(SC6H13)24 MPCs is comprised of several electronic transitions of unique spectral weights, which may result from different orbital contributions associated with specific cluster domains. The use of 2DES in combination with structurally precise MPCs can provide a platform for understanding structure-dependent electronic dynamics in metal nanoclusters and technologically important metal–chalcogenide interfaces.

Biological thiols are antioxidants essential for the prevention of disease. For example, low levels of the tripeptide glutathione are associated with heart disease, cancer, and dementia. Mn2+-doped wide bandgap semiconductor nanocrystals exhibit luminescence and magnetic properties that make them attractive for bimodal imaging. We found that these nanocrystals and silica-encapsulated nanoparticle derivatives exhibit enhanced luminescence in the presence of thiols in both organic solvent and aqueous solution. The key to using these nanocrystals as sensors is control over their surfaces. The addition of a ZnS barrier layer or shell produces more stable nanocrystals that are isolated from their surroundings, and luminescence enhancement is only observed with thinner, intermediate shells. Tunability is demonstrated with dodecanethiol and sensitivities decrease with thin, medium, and thick shells. Turn-on nanoprobe luminescence is also generated by several biological thiols, including glutathione, N-acetylcysteine, cysteine, and dithiothreitol. Nanoparticles prepared with different ZnS shell thicknesses demonstrated varying sensitivity to glutathione, which allows for the tuning of particle sensitivity without optimization. The small photoluminescence response to control amino acids and salts indicates selectivity for thiols. Preliminary magnetic measurements highlight the challenge of optimizing sensors for different imaging modalities. In this work, we assess the prospects of using these nanoparticles as luminescent turn-on thiol sensors and for MRI.