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Time-resolved spectroscopy of plasmonic nanoparticles is a vital technique for probing their ultrafast electron dynamics and subsequent acoustic and photothermal properties. Traditionally, these experiments are performed with spectrally broad probe beams on the ensemble level to achieve high signal amplitudes. However, the relaxation dynamics of plasmonic nanoparticles is highly dependent on their size, shape, and crystallinity. As such, the inherent heterogeneity of most nanoparticle samples can complicate efforts to build microscopic models for these dynamics solely on the basis of ensemble measurements. Although approaches for collecting time-resolved microscopy signals from individual nanoparticles at selected probe wavelengths have been demonstrated, acquiring time-resolved spectra from single objects remains challenging. Here, we demonstrate an alternate method that efficiently yields the time-resolved spectra of a single gold nanodisk in one measurement. By modulating the frequency-doubled output of a 96 MHz Ti:sapphire oscillator at 8 kHz, we are able to use a lock-in pixel-array camera to detect photoinduced changes in the transmission of a white light continuum probe derived from a photonic crystal fiber to produce broadband femtosecond transmission spectra of a single gold nanodisk. We also compare the performance of the lock-in camera for the same single nanoparticle to measurements with a single-element photodiode and find comparable sensitivities. The lock-in camera thus provides a major advantage due to its ability to multiplex spectral detection, which we utilize here to capture both the electronic dynamics and acoustic vibrations of a single gold nanodisk following ultrafast laser excitation.more » « lessFree, publicly-accessible full text available May 30, 2025
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The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating.
Free, publicly-accessible full text available July 5, 2025 -
Optical applications of plasmonic nanoparticles depend critically on particle properties such as relative proximity, composition, crystallinity, and shape. The most common nanoparticle fabrication techniques, colloidal synthesis and electron beam lithography, allow the tailoring of some of these parameters, yet do not provide control over all of them. Scanning probe block copolymer lithography (SPBCL), a technique that grows nanoparticles on substrates from precisely deposited precursor droplets, merges the advantages of colloidal synthesis and electron beam lithography, and offers high throughput, precise particle positioning, and composition control. A few challenges with the SBCL method remain: fabrication of optically relevant particle sizes on optically transparent supports, and detailed correlation of their optical and morphological properties. Here, we adapt SPBCL to fabricate large arrays of gold nanoparticles on glass supports. The resulting nanoparticles have varying shapes, and at ∼100 nm in diameter, they support strong plasmon resonances. In order to fully exploit the high-throughput fabrication method, we designed an automated dark-field microscope and correlated the optical behavior to the mechanical properties as determined through electron and pump–probe microscopy. We find that the SPBCL-synthesized nanoparticles are highly crystalline, supporting both plasmon oscillations and mechanical vibrations with lifetimes comparable to colloidal nanospheres. Our work highlights SPBCL as a promising and versatile synthesis approach for plasmonic nanoparticles, leading the way toward extensive screening capabilities for optical properties and hence improved potential applications.more » « less
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The hybridization of plasmonic energy and charge donors with polymeric acceptors is a possible means to overcome fast internal relaxation that limits potential photocatalytic applications for plasmonic nanomaterials. Polyaniline (PANI) readily hybridizes onto gold nanorods (AuNRs) and has been used for the sensitive monitoring of local refractive index changes. Here, we use single-particle spectroscopy to quantify a previously unreported plasmon damping mechanism in AuNR–PANI hybrids while actively tuning the PANI chemical structure. By eliminating contributions from heterogeneous line width broadening and refractive index changes, we identify efficient resonance energy transfer (RET) between AuNRs and PANI. We find that RET dominates the optical response in our AuNR–PANI hybrids during the dynamic tuning of the spectral overlap of the AuNR donor and PANI acceptor. Harnessing RET between plasmonic nanomaterials and an affordable and processable polymer such as PANI offers an alternate mechanism toward efficient photocatalysis with plasmonic nanoparticle antennas. This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry Letters, copyright © The American Chemical Society 2023 after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.jpclett.3c01990more » « less
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Plasmonic photocatalysis uses the light-induced resonant oscillation of free electrons in a metal nanoparticle to concentrate optical energy for driving chemical reactions. By altering the joint electronic structure of the catalyst and reactants, plasmonic catalysis enables reaction pathways with improved selectivity, activity, and catalyst stability. However, designing an optimal catalyst still requires a fundamental understanding of the underlying plasmonic mechanisms at the spatial scales of single particles, at the temporal scales of electron transfer, and in conditions analogous to those under which real reactions will operate. Thus, in this review, we provide an overview of several of the available and developing nanoscale and ultrafast experimental approaches, emphasizing those that can be performed in situ. Specifically, we discuss high spatial resolution optical, tip-based, and electron microscopy techniques; high temporal resolution optical and x-ray techniques; and emerging ultrafast optical, x-ray, tip-based, and electron microscopy techniques that simultaneously achieve high spatial and temporal resolution. Ab initio and classical continuum theoretical models play an essential role in guiding and interpreting experimental exploration, and thus, these are also reviewed and several notable theoretical insights are discussed.