ABSTRACT N ‐Heterocyclic carbenes (NHCs) have emerged as a unique class of ligands for gold nanoparticles (Au NPs), combining strong metal binding with intrinsic electronic conductivity. Yet over the past decade, studies on Au NP@NHC systems have primarily focused on their stability, while the conductivity of NHCs has remained largely unexplored due to synthesis challenges. Here, we present a synthetic strategy that addresses this gap by employing amino‐functionalized NHC‐Au complexes with in situ oxidative polymerization of polyaniline (PANI) to yield electronically coupled Au NP@NHC‐PANI hybrids in aqueous media. This strategy enables both a controlled PANI shell growth and introduction of an electronically active NHC interlayer. Single‐particle scattering spectroscopy reveals that NHCs improve the interfacial electronic coupling as evidenced by pronounced plasmonic linewidth broadening. Conductivity measurements further confirm that NHCs enhance charge transport: conductive atomic force microscopy (C‐AFM) shows an increase in contact current from 14.6 to 99.4 pA under a 300‐mV bias, while lateral four‐probe conductance increases from 0.17 to 3.5 nS. These results provide the first direct experimental evidence of the conductive role of NHCs in hybrid NP‐polymer systems, establishing a new interface‐engineering strategy for the rational design of electronically delocalized nanostructures and their applications in nanoelectronics.

Abstract Chemical interface damping (CID) is a phenomenon of central importance in plasmonic spectroscopy, plasmonic energy transfer, and plasmon-induced hot electron chemistry. It arises from either resonant energy transfer to an absorbing surrounding or the scattering of electrons in the plasmonic nanostructure with interfacial electronic states associated with a medium, often a semiconductor or a molecular adsorbate. The broadening of the plasmon line width upon the modification of the nanoparticle surface by such a medium is a hallmark of CID. However, CID is invariably accompanied by a change in the plasmon line width caused by the increase in the optical dielectric constant around the nanoparticle. By electrodynamic simulations, we characterized this optical effect in a model system comprised of gold nanorods. Simulations show that the total plasmon line width decreases with an increase in the dielectric constant of the medium. The optical narrowing effect induced by surface coating of a nanoparticle can therefore negate the line width broadening caused by the CID effect of the surface coating. We developed an experimental approach to isolate CID from single-particle plasmon line width measurements by quantifying and subtracting out the optical effect. By using electron-blocking Al2O3-coated gold nanorods as a reference, we determined for the Au–TiO2 interface that supports interfacial charge transfer a CID rate free of convolution with the optical effect. This approach will enable more reliable measurements of CID rates for a variety of adsorbates and semiconductors coated on plasmonic nanostructure surfaces and provide quantitative insights into energy and electron transfer in these hybrid systems.

Surface plasmons are among the strongest light absorbers, 1 making them ideal for photocatalysis. 2 Plasmonic photocatalysis exploits the tunable plasmon resonance to generate ‘hot’ electrons and holes with energies matching reactant excitations, thereby enabling reaction pathways inaccessible to thermal catalysis. 3,4 However, major barriers to engineered plasmonic photocatalysis are (1) differentiating among the types of hot carriers, including initially created athermal electrons and holes; and (2) controlling their respective hot thermal distributions and relaxation pathways occurring within ~100 fs before lattice heating sets in. 5,6 Here, we distinguish athermal from thermal hot carriers, assign unique contributions to electrons and holes, and follow each carrier’s role in photo-initiated charge transfer by time-resolving luminescence from single plasmonic nanoparticles. A model that uses no free parameters and combines the microscopic Boltzmann equation with the macroscopic two-temperature model offers conclusive support. 7 We find that femtosecond thermalization of initially excited athermal d-band holes leads to the creation of thermal hot electrons and holes with picosecond lifetimes. Depending on energetics, either type of carrier (athermal vs thermal and hole vs electron) can be injected through an interfacial metal-semiconductor barrier. These results suggest that plasmonic photocatalysis can be prolonged for picoseconds. Our results provide the level of mechanistic insight that is required to rationally advance the field of plasmonic photocatalysis.

Acoustic modes in plasmonic nanostructures provide fundamental insights into their optomechanical behavior at the nanoscale, enabling emerging applications in plasmon-enhanced optomechanics, ultrasensitive sensing, and nanoscale energy transduction. Here we explore the modulation of acoustic phonon dynamics in lithographically fabricated gold nanodisks via laser-induced photothermal annealing. Using a correlated approach that utilizes both single-particle transient extinction spectroscopy and advanced electron microscopy, we directly link nanoscale structural transformations to changes in mechanical properties as probed through the coherence of the excited acoustic modes. Specifically, ultrafast pump-probe microscopy reveals an enhancement in the acoustic mode quality factor of annealed gold nanodisks, indicative of reduced damping and improved vibrational coherence. Structural characterization via scanning electron microscopy and electron backscatter diffraction confirms that photoinduced annealing results in smoother surface morphology and overall enhanced crystallinity. The improved crystalline order reduces defect and crystal boundary scattering, which we suggest as the reason underlying the lower quality factor before annealing. These findings demonstrate that targeted structural engineering at the nanoscale offers a powerful strategy for optimizing the optomechanical performance of plasmonic nanostructures, with broad implications for the design of next-generation nanophotonic and optomechanical systems.

Abstract We report the synthesis of plasmonic gold–copper nanostructures with tunable copper spatial distribution and oxidation state. Au–Cu alloy shells were deposited in aqueous solution onto colloidal gold nanorods that functioned as seeds, and the resulting shell morphology was found to depend strongly on the nanorod aspect ratio. For relatively fat gold nanorods (16 × 31 nm), Au–Cu growth was preferentially localized near the rod midsections, forming belt-like structures, whereas for skinnier nanorods (9 × 32 nm), Au–Cu growth produced patchy surface coverage around the entire rod. The shell thickness was tuned from 1 to 4 nm; upon air exposure, thicker shells exhibited a higher Cu(I) fraction compared to the initial shell. Incorporation of the Au–Cu shell led to pronounced shifts in the plasmonic features of the gold nanorods. These tunable plasmonicAu–Cu nanorods provide a materials basis for Cu-based plasmonic materials in the context of electrochemical CO2 reduction.

Magnesium nanoparticles have emerged as a promising plasmonic material due to their low cost and biocompatibility, yet their optical absorption at the single-particle level is largely uncharacterized. While ensemble extinction measurements of 170 nm Mg spheroids show a broad extinction spectrum, we demonstrate through correlated single-particle dark-field scattering and photothermal absorption spectroscopies that individual nanoparticles support well-defined plasmon resonances in absorption and scattering. We found that the peaks in absorption and scattering occur at a similar wavelength average, with absorption consistently broader than scattering. Simulations reproduce these trends and confirm that the broader absorption line width arises from the large dispersion of the real part of Mg's dielectric function. These findings provide fundamental insights into the spectral differences in absorption and scattering by Mg nanoparticles and demonstrate the necessity of single-particle measurements for understanding their optical response, crucial for optimizing performance in diverse plasmonically powered applications.

Solvated electrons are strong homogeneous reducing agents, and their generation with visible light can unlock new redox chemistry. Water imposes a high photoemission energy barrier for gold, restricting the accessible spectral window for plasmon-mediated solvated electron generation to the near-ultraviolet region. Here, we first demonstrate that by using hexamethylphosphoramide, an organic solvent that supports large applied cathodic potentials without decomposition, the photoemission threshold is lowered to provide access to the entire visible spectrum. Next, we achieve solvated electron yields up to 150-fold higher with coupled plasmon modes from clustered gold nanoparticles, as compared to a smooth gold electrode. The observed quantum yield correlates with the local electric field enhancement by gap plasmon modes for these nanostructured electrodes as identified by varying the particle density. Overall, this study offers mechanistic insights into how coupled plasmon modes and threshold optimization can be used to enhance solvated electron generation with visible light.

The physico-chemical properties of 'bottom-up' carbon dots synthesized from small molecules feature both generalities, such as sp2-networked carbon and core-surface energy transfer, and heterogeneities, due to the unpredictable location of heteroatoms and often non-crystalline structure. Here we focus our review on three aspects of these systems: (1) coupling characterization with bottom-up synthesis to identify and remove confounding byproducts such as small molecules or hydrogen-rich polymers; (2) single-particle characterization to obtain unambiguous information on carbon dots and highlight the distribution of properties around the ensemble average; (3) electronic structure of carbon dots and how it can help elucidate the origin of important properties such as optical absorption and fluorescence from a heterogeneous ensemble of carbon dots.

To enable rear illumination (e.g., TIRF), single-particle fluorescence microscopy, and scanning tunneling microscopy (STM) on the same nanoparticle sample, we investigate the smoothness limit and the thickness limit of template-stripped gold films made with a simple room-temperature deposition protocol ranging from 1 to 200 pm/s on four common substrates: mica, fused silica, silicon, and quartz. The resulting transparent conductive gold film achieves a thickness as low as 9 nm, absorbance as low as 0.2, and a root-mean-square roughness of 80 pm over a 100 × 100 nm2 area. We further assess whether such gold films enable single-particle characterization by fluorescence imaging and STM imaging on the same sample. Carbon dots, made by a top-down method, with a height as low as 1.0 nm (∼3 layers), can be resolved clearly on the gold film island surfaces by using both atomic force microscopy and STM, and the carbon dot single-particle fluorescence blinking can be measured by confocal microscopy. In this way, both optical and electronic characterization can be enabled on the same sample using a substrate that is relatively easy to make in batches.

Single-particle measurements of chiral nanostructures have the potential to offer more detailed insights compared to the ensemble-averaged signals obtained in ensemble circular dichroism spectroscopy. For instance, single-particle circular differential scattering (CDS) studies have revealed the effects of structural heterogeneity in chiral plasmonic nanostructures. However, differential light-matter interactions of 3D chiral nanostructures caused by diverse orientations on the supporting substrate remain largely unexplored. Here, the CDS of DNA origami-templated twisted gold nanorod (AuNR) dimers on a glass support is investigated. With the help of correlated scanning electron microscopy and electromagnetic simulations, it is demonstrated how the broad diversity of spectral lineshapes and signal intensities as well as sign of the CDS originates mainly from four different orientations of the chiral dimer once deposited from solution on the substrate and dried. Most surprisingly, it is found that even for the same enantiomer a sign reversal is possible depending solely on the dimer orientation when deposited on the support. These results are important for the correct interpretation of single-particle chiroptical studies and furthermore provide valuable insights into the design of substrate-supported chiral plasmonic metamaterials.

In the summer of 2023, a group of 35 researchers from across the discipline of nanochemistry gathered to discuss challenges and opportunities that will define the next 10 years of cutting edge nanochemical discovery. Over 2 days, researchers identified four areas with rich possibilities to accelerate the science and translation of nanostructures into society-enhancing tools for technology, health, and sustainability. In this Nano Focus, we summarize highlights from these discussions. A full report from the workshop is provided as Supporting Information.

Correlating a nanoparticle's morphology with its optical properties is essential and is achieved by a combination of electron microscopy and optical spectroscopy. Machine learning has gained attention for enhancing in situ measurements and enabling inverse nanoparticle design. However, new training data for each specific condition are often required when testing data differ from training data. We propose a method to adapt existing training data for predicting the size of gold nanorods (AuNRs) on different substrates. This method is based on simulated spectra of AuNRs on glass and indium tin oxide-coated glass (ITO), adapting the resonance energy between substrates. Using the adapted data, we train a decision tree regressor to predict AuNR sizes on ITO and test it with experimental data on ITO. This correction achieves comparable accuracy in predicting AuNR length to a decision tree trained directly on ITO. In addition, we apply the correction method to predict AuNR sizes on Al2O3, despite the lack of extensive training data, leading to an improvement in length prediction as well. Our analysis reveals that length prediction is more sensitive to the change in the resonance energy, suggesting that substrate differences mostly affect the length prediction. Overall, adapting training data enables real-time size determination across various environments without additional training data.

Plasmon-coupled circular dichroism enables chiral molecule detection by inducing circular dichroism at the plasmon resonance through interactions with a plasmonic sensor. Coupled nanoparticles offer potentially higher sensitivities due to stronger plasmonic fields at the junctions. However, ensemble-level sensitivity is limited by signal averaging, and the structural chirality of the sensor itself can obscure the molecular response. To overcome these issues, we combine single-particle dark-field scattering with electron microscopy. Individual gold nanosphere dimers, selected to avoid interference from structural chirality, yield unambiguous plasmon-coupled circular dichroism for hemoglobin as an analyte, while monomers give no detectable signal. We explain these results based on their difference in refractive index sensitivities with respect to hemoglobin's circular birefringence, as supported by electrodynamic simulations. This study sheds new light on the mechanism of plasmon-coupled circular dichroism by isolating the response of individual nanostructures and thereby avoiding ensemble averaging over a heterogeneous mixture of nanostructure geometries.

Here we study the controlled growth of ultrathin molybdenum dioxide (MoO2) flakes, a metallic analogue of the widely studied transition metal dichalcogenide MoS2. This study demonstrates the growth of three distinct MoO2 polymorphs (monoclinic, tetragonal, and a newly identified hexagonal phase) using physical vapor deposition. Comprehensive characterization through atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirms their unique structures and validates the newly observed hexagonal polymorph, which is also supported through simulations. Computational modeling suggests that the nucleation and coalescence of gas-phase clusters drive the polymorph formation. Optical measurements reveal that these polymorphs exhibit distinct photonic resonances, influenced by their geometry and thickness. This work opens new possibilities for integrating MoO2 in hybrid structures and photonic devices, leveraging its polymorphic diversity and close relation to MoS2, for advanced material design.

Plasmon-induced resonance energy transfer (PIRET) is a promising approach for plasmonic photocatalysis and energy conversion, but challenges include elucidating the mechanism and maximizing its efficiency, both of which are hampered by competing processes. Another challenge is demonstrating that PIRET can photoinitiate reactions that follow efficient pathways compared to bulk processes. We report a plasmon-induced route to plasmonic-polymer hybrid nanomaterials using in operando single-particle spectroelectrochemistry. An energy transfer efficiency of 40% is achievable when the spectral overlap between gold nanorod scattering and polymer absorption is maximized. We also show that PIRET-initiated polymerization proceeds through a different mechanism than bulk polymerization, supported by spectroscopic evidence and density functional theory calculations, highlighting efficient energy cascading from photon to plasmon to exciton and, lastly, to unconventional light-initiated chemistry.

Plasmonic nanostructures have the potential to revolutionize photocatalysis by harnessing hot carriers to drive novel chemical reactions. Precise control of reaction sites on nanoparticles remains crucial for advancing catalyst design. The influence of hot carriers, particularly in the interplay with surface electrochemistry, needs further exploration. Employing single-particle spectroelectrochemical methods, we identify the conditions that lead to tip-preferred versus isotropic dissolution. We investigate how the applied potential, excitation laser power density, and wavelength of illumination directly direct gold nanorod dissolution. There is a competition between hot carrier localization and electrochemistry in determining the dissolution anisotropy. We observe that higher potentials favor isotropic dissolution, whereas higher laser power densities drive tip-specific dissolution. The results provide new insights into the control of tuning the gold nanoparticle dissolution anisotropy.

Plasmon-induced interfacial charge separation is a promising way to efficiently extract energetic carriers through direct plasmon decay. This mechanism of charge transfer has been investigated by single-particle scattering spectroscopy, which measures the homogeneous plasmon line width. The line width is broadened by charge transfer, generally known as chemical interface damping. However, conflicting reports exist regarding the effect of chemical interface damping on the corresponding single-particle absorption spectrum, which is needed to accurately determine absolute light conversion efficiencies. This work aims to resolve this question by directly correlating absorption and scattering spectra of individual gold nanorods in the presence and absence of a charge-accepting interface. We find that for TiO2 coated nanorods, the absorption line width is indeed broadened due to chemical interface damping but is overall narrower than the scattering line width. Chemical interface damping is furthermore found to increase with larger resonance energies. The observed differences in line widths between absorption and scattering are elucidated within the context of an analytically tractable model describing the lowest energy optically bright and higher-order optically dark plasmon modes of the nanorod, including bulk, radiative, and chemical interface damping effects. Taken together, these results establish that single-particle absorption spectroscopy is capable of revealing interfacial charge injection by direct plasmon decay.

Carbon dots are remarkable nanomaterials with many applications, but the sources of their emission are still uncertain. Carbon dots exhibit complex behaviors such as excitation-dependent emission due to their heterogeneous composition and structure. Most studies have been carried out on the ensemble level, where sample heterogeneity remains hidden. Understanding the complex emission of carbon dots requires single-particle measurements. Here, we determined that for red-emitting carbon dots made from two bottom-up precursors, there is a significant population of dots with more than one emitting moiety. Polarization-resolved, single-dot emission microscopy revealed subpopulations of carbon dots based on their emission intensity and polarization. For the multichromophoric carbon dots, we found an average of about four emitters. Single-particle spectroscopy, acquired in parallel to the emission trajectories, and molecular dynamics simulations furthermore established that the countable chromophores in the carbon dots are chemically similar, considering the rather narrow room-temperature emission line width and the absence of significant spectral diffusion.

Carbon dots have received considerable attention due to their tunable emission. Single-particle techniques revealed that individual top-down and bottom-up green carbon dots can support several chromophores. In particular, several studies demonstrated that bottom-up synthesized carbon dots are typically made of amorphous carbon and are multichromophoric but may also just be chemically impure, with free dye in solution or polymerized in a carbon matrix. Carbon dots made by top-down precursors, however, are highly graphitic and more often single-chromophoric, begging the question if carbon dots made from bottom-up precursors could have similar optical properties compared to their top-down counterparts, if properly purified. Here, we compare green-emitting carbon dots made by two methods: top-down by chemical oxidation and bottom-up from small-molecule precursors in a solvothermal synthesis followed by rigorous purification. Such dots have cores of different crystallinity, but both types have oxidized surfaces. Just as ensemble absorption and emission spectra show only subtle differences, we find based on single-particle emission imaging that both types of carbon dots contain similar weights of carbon dots with single and multiple chromophores. Surprisingly, the carbon dots are optically similar, despite coming from opposing synthetic approaches. Although the majority of all carbon dots are single-chromophoric, top-down carbon dots are found to more likely have only one emitting chromophore, whereas bottom-up carbon dots are comparatively more multichromophoric. In the multichromophoric case, bottom-up carbon dots have on average a greater number of chromophores than top-down carbon dots. Our results showing that very differently made carbon dots with different structural properties exhibit strikingly similar emission properties reveal the important insight that out of structural heterogeneity emerges spectroscopic homogeneity.

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.