Insights into thiocyanate-enhanced photoluminescence in CsPbBr3 nanocrystals by ultrafast two-dimensional infrared spectroscopy.

Functionalization of perovskite nanocrystal surfaces with thiocyanate anions presents a transformative approach to enhancing stability and photoluminescence quantum yield (PLQY) through surface defect passivation. This study investigates the role of thiocyanate ligands in modifying the optoelectronic properties of CsPbBr3 nanocrystals. We employed ultrafast two-dimensional infrared spectroscopy to investigate the nature of the dynamic interaction of thiocyanate ligands with nanocrystal surfaces, providing insights into the mechanisms underlying the observed increase in PLQY and stability. Our analysis reveals that the thiocyanate ligands efficiently passivate the surface defects, thereby enhancing the PLQY and the stability of the treated nanocrystals. The spectroscopic evidence supports a model where thiocyanate binds to under-coordinated lead atoms, contributing to a stable nanocrystal surface with enhanced optoelectronic performance. This ligand-induced passivation mechanism advances our understanding of surface chemistry's role in optimizing nanomaterials for solar cell and LED applications.

Noble Metal Nanoparticles with Nanogel Coatings: Coinage Metal Thiolate-Stabilized Glutathione Hydrogel Shells.

Developing biocompatible nanocoatings is crucial for biomedical applications. Noble metal colloidal nanoparticles with biomolecular shells are thought to combine diverse chemical and optothermal functionalities with biocompatibility. Herein, we present nanoparticles with peptide hydrogel shells that feature an unusual combination of properties: the metal core possesses localized plasmon resonance, whereas a few-nanometer-thick shells open opportunities to employ their soft framework for loading and scaffolding. We demonstrate this concept with gold and silver nanoparticles capped by glutathione peptides stacked into parallel ß-sheets as they aggregate on the surface. A key role in the formation of the ordered structure is played by coinage metal(I) thiolates, i.e., Ag(I), Cu(I), and Au(I). The shell thickness can be controlled via the concentration of either metal ions or peptides. Theoretical modeling of the shell's molecular structure suggests that the thiolates have a similar conformation for all the metals and that the parallel ß-sheet-like structure is a kinetic product of the peptide aggregation. Using third-order nonlinear two-dimensional infrared spectroscopy, we revealed that the ordered secondary structure is similar to the bulk hydrogels of the coinage metal thiolates of glutathione, which also consist of aggregated stacked parallel ß-sheets. We expect that nanoparticles with hydrogel shells will be useful additions to the nanomaterial toolbox. The present method of nanogel coating can be applied to arbitrary surfaces where the initial deposition of the seed glutathione monolayer is possible.

Spontaneous Raman scattering from vibrational polaritons is obscured by reservoir states.

Vibrational strong coupling results from the interaction between optically allowed molecular vibrational excitations and the resonant mode of an infrared cavity. Strong coupling leads to the formation of hybrid states, known as vibrational polaritons, which are readily observed in transmission measurements and a manifold of the reservoir states. In contrast, Raman spectroscopy of vibrational polaritons is elusive and has recently been the focus of both theoretical and experimental investigations. Because Raman measurements are frequently performed with high-numerical aperture excitation/collection optics, the angular dispersion of the strongly coupled system must be carefully considered. Herein, we experimentally investigated vibrational polaritons involving dispersive collective lattice resonances of infrared antenna arrays. Despite clear indications of the strong coupling to vibrational excitations in the transmission spectrum; we found that Raman spectra do not bear signatures of the polaritonic transitions. Detailed measurements indicate that the disappearance of the Raman signal is not due to the polariton dispersion in our samples. On the other hand, the Tavis-Cummings-Holstein model that we employed to interpret our results suggests that the ratio of the Raman transition strengths between the reservoir and the polariton states scales according to the number of strongly coupled molecules. Because the vibrational transitions are relatively weak, the number of molecules required to achieve strong coupling conditions is about 109 per unit cell of the array of infrared antennas. Therefore, the scaling predicted by the Tavis-Cummings-Holstein model can explain the absence of the polariton signatures in spontaneous Raman scattering experiments.

Using mirrors to control molecular dynamics.

An optical cavity mixes molecular vibrations with light and changes chemical reactivity.

Vibrational Polaritons in Disordered Molecular Ensembles.

Disorder is an intrinsic attribute of any realistic molecular system. It is known to lead to localization, which hampers efficient transport. It was recently proposed that in molecular ensembles strongly coupled to photonic cavities, moderate disorder leads to delocalization and increases of the transport and chemical reaction rates. Vibrational polaritons involve molecular vibrations hybridized with an infrared cavity. When the coupling strength largely exceeds the molecular inhomogeneity, polaritons are unaffected by disorder. However, in many experiments, such a homogeneous limit does not apply. We investigated vibrational polaritons involving molecular ensembles with systematically modified disorder. Counterintuitively, moderate disorder leads to an increase in Rabi splitting and the modification of the polariton bandwidths. Experimental spectroscopic data agree with a Tavis-Cummings-like model that suggests enhanced delocalization of the reservoir states occurs via the admixture of the cavity mode. Our results provide new insights into the paradigm of disorder-induced cavity-assisted delocalization in molecular polaritons.

Ultrafast vibrational excitation transfer on resonant antenna lattices revealed by two-dimensional infrared spectroscopy.

High-quality lattice resonances in arrays of infrared antennas operating in an open-cavity regime form polariton states by means of strong coupling to molecular vibrations. We studied polaritons formed by carbonyl stretching modes of (poly)methyl methacrylate on resonant antenna arrays using femtosecond 2DIR spectroscopy. At a normal incidence of excitation light, doubly degenerate antenna-lattice resonances (ALRs) form two polariton states: a lower polariton and an upper polariton. At an off-normal incidence geometry of 2DIR experiments, the ALR degeneracy is lifted and, consequently, the polariton energies are split. We spectrally resolved and tracked the time-dependent evolution of a cross-peak signal associated with the excitation of reservoir states and the unidirectional transfer of the excess energy to lower polaritons. Bi-exponential decay of the cross-peak suggests that a reversible energy exchange between the bright and dark lower polaritons occurs with a characteristic transfer time of ∼200 fs. The cross-peak signal further decays within ∼800 fs, which is consistent with the relaxation time of the carbonyl stretching vibration and with the dephasing time of the ALR. An increase in the excitation pulse intensity leads to saturation of the cross-peak amplitude and a modification of the relaxation dynamics. Using quantum-mechanical modeling, we found that the kinetic scheme that captures all the experimental observations implies that only the bright lower polariton accepts the energy from the reservoir, suggesting that transfer occurs via a mechanism involving dipole-dipole interaction. An efficient reservoir-to-polariton transfer can play an important role in developing novel room-temperature quantum optical devices in the mid-infrared wavelength region.

Infrared Open Cavities for Strong Vibrational Coupling.

Arrays of subwavelength plasmonic nanoparticles exhibiting narrowband lattice resonances are referred to as open cavities because of their ability to strongly couple with electronic excitations in molecular chromophores. However, realization of these ideas in the mid-infrared spectral region has been limited. We demonstrated a dramatic reduction in the bandwidth of lattice resonances in large-area arrays of half-wavelength mid-infrared antennas, reaching resonance quality factors above 200. By tuning the wavelength of the antenna-lattice resonances (ALR) to match the transition frequency of the molecular vibrational modes, we achieved a strong coupling between the ALR and the carbonyl stretching excitation in a thin film of (poly)methyl methacrylate (PMMA) polymer deposited on the array. Splitting of the polaritonic transitions, reduction of their bandwidth below that of the bare molecular transition, and characteristic dispersion confirmed the strong coupling regime. Our results pave the way for exciting research on the many-body correlated dynamics of vibrational polaritons.

Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity.

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on "Naked" Silver Nanoparticles.

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the ß-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using 13C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native ß-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Surface-enhanced ultrafast two-dimensional vibrational spectroscopy with engineered plasmonic nano-antennas.

Development of noble metal nanostructure substrates that provide strong near-field enhancements enables applications of linear and nonlinear infrared (IR) spectroscopies to study minute sample quantities, such as nanometer thick films and molecular monolayers. Large near-field enhancements of the electric fields used for spectroscopic interrogation of molecules at the nanostructure surface result in enhancement of the spectroscopic signatures. This enhancement scales with the nonlinear order of the method, providing particularly large signal gains for third- and fifth-order IR methods, reaching 106 and 108 raw enhancement factors, not adjusted to the amount of interrogated sample. In this perspective, we overview the advances in the development of nano-arrays of antenna-like nanostructures for mid-IR measurements and illustrate their use in linear and especially nonlinear two-dimensional IR approaches. We discuss how studies of the interaction mechanisms between light, plasmonic antennas, and molecular excitations benefit from the nonlinear two-dimensional time-resolved methods, which involve high-order scaling of the signal with the excitation field, high sensitivity to signal localization, and coherence of the excitation over a broad bandwidth. On the other hand, we demonstrate how studies of molecular structure and ultrafast dynamics by these advanced spectroscopic methods benefit from surface enhancement of signals by plasmonic antennas.

Intramolecular hydrogen bonding protects the hydroxyl group from attack by fluctuating solvent forces.

Ultrafast spectroscopy of molecular systems involving hydrogen- (H-) bonding has been at the forefront of fundamental chemical and physical research for several decades. Among the spectroscopic observables of the ultrafast dynamics is the pure dephasing of vibrationally excited molecules. Using third-order nonlinear vibrational spectroscopy, including polarization-selective transient grating measurements of vibrational lifetime and orientational diffusion as well as two-dimensional infrared spectroscopy, we determined different individual line shape components of hydroxyl stretching (νOH) excitations in a homologous series of chlorophenols and obtained the corresponding pure dephasing rates. The pure dephasing rates are correlated with vibrational anharmonicity of the νOH mode, which is tuned remotely from the hydroxyl site by changing the position of the chlorine substituents on the phenol ring. We found that in molecules where the hydroxyl group is in its free form, the pure dephasing rates scale linearly with the mode's anharmonicity such that assuming it is dominated by the third-order diagonal term, the ultrafast dynamics follow the prediction of the Kubo-Oxtoby theory. However, in the intramolecularly H-bonded ortho-chlorophenols, this trend is reversed, and the pure dephasing slows down by ∼50% for an increase in anharmonicity of only a few wavenumbers. Because the νOH mode's anharmonicity is known to reflect the H-bonding strength, our results suggest that intramolecular H-bonding can serve as a mechanism of protection from fluctuating forces exerted by the solvent. Such an effect can be relevant for ultrafast dynamics in biomolecules, where H-bonding plays a central role.

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.

Two-Dimensional Infrared Spectroscopy Reveals Molecular Self-Assembly on the Surface of Silver Nanoparticles.

The conformation of molecules, peptides, and proteins, self-assembled into structured monolayers on the surface of metal nanoparticles (NPs), can strongly affect their properties and use in chemical or nanobiomedical applications. Elucidating molecular conformations on the NP surface is highly challenging, and the microscopic details mostly remain elusive. Using polarization-selective third-order two-dimensional ultrafast infrared spectroscopy, we revealed the highly ordered intermolecular structure of γ-tripeptide glutathione on the surface of silver NPs in aqueous solution. Glutathione is an antioxidant thiol abundant in living cells; it is extensively used in NP chemistry and related research. We identified conditions where the interaction of glutathione with the NP surface facilitates formation of a ß-sheet-like structure enclosing the NPs. A spectroscopic signature associated with the assembly of ß-sheets into an amyloid fibril-like structure was also observed. Remarkably, the interaction with the metal surface promotes formation of a fibril-like structure by a small peptide involving only two amino acids.

Quantifying conformations of ester vibrational probes with hydrogen-bond-induced Fermi resonances.

Solvatochromic shifts of local vibrational probes report on the strength of the surrounding electric fields and the probe's hydrogen bonding status. Stretching vibrational mode of the ester carbonyl group is a popular solvatochromic reporter used in the studies of peptides and proteins. Small molecules, used to calibrate the response of the vibrational probes, sometimes involve Fermi resonances (FRs) induced by inter-molecular interactions. In the present work, we focus on the scenario where FR does not appear in the infrared spectrum of the ester carbonyl stretching mode in aprotic solvents; however, it is intensified when a hydrogen bond with the reporter is established. When two molecules form hydrogen bonds to the same carbonyl oxygen atom, FR leads to strong hybridization of the involved modes and splitting of the absorption peak. Spectral overlap between the Fermi doublets associated with singly and doubly hydrogen-bonded carbonyl groups significantly complicates quantifying different hydrogen-bonded conformations. We employed a combination of linear and third-order (2DIR) infrared spectroscopy with chemometrics analysis to reveal the individual line shapes and to estimate the occupations of the hydrogen-bonded conformations in methyl acetate, a model small molecule. We identified a hydrogen-bond-induced FR in complexes of methyl acetate with alcohols and water and found that FR is lifted in larger molecules used for control experiments-cholesteryl stearate and methyl cyanoacetate. Applying this methodology to analyze acetonitrile-water solutions revealed that when dissolved in neat water, methyl acetate occupies a single hydrogen-bonding conformation, which is in contrast to the conclusions of previous studies. Our approach can be generally used when FRs prevent direct quantification of the hydrogen bonding status of the vibrational probe.

Two-Dimensional Infrared Spectroscopy with Local Plasmonic Fields of a Trimer Gap-Antenna Array.

Half-wavelength plasmonic antennas tuned to resonance with molecular vibrational excitations have been demonstrated to enhance 2DIR signals by multiple orders of magnitude. We design doubly degenerate in-plane plasmonic normal modes of the symmetric trimer gap-antenna, which have orthogonal dipole moments excited by light of the appropriate polarization, to localize the enhanced field into the antenna's gap. Vibrational excitations serve as sensitive probes of the plasmonic fields. 2DIR spectroscopy of thin molecular films indicates that molecules emitting enhanced signals experience an electric field with a direction independent of the excitation laser pulse polarization. Our results illustrate the trade-off between the large signal amplification in molecules close to the antenna surface by resonant plasmons, where the direction of the enhanced fields follows metal surface boundary conditions, and the associated limitations for the polarization-selective spectroscopy. The ultrafast quantum dynamics reported by the enhanced signals is not affected by its interaction with plasmonic excitation.

Artificial Plasmonic Molecules and Their Interaction with Real Molecules.

Plasmonic molecules are small assemblies of nanosized metal particles. Interactions between the particles modify their optical properties and make them attractive for multiple applications in spectroscopy and sensing. In this review, we focus on basic properties rather than on applications. Plasmonic molecules can be created using either nanofabrication methods or self-assembly techniques in solution. The interaction of plasmonic molecules with light leads to excitations that are classified using the concept of normal modes. The simplest plasmonic molecule is a dimer of particles, and its lowest energy excitation takes the form of a symmetric dipolar mode. More complex excitations take place when a larger number of particles is involved. The gaps between particles in a plasmonic molecule form hotspots in which the electromagnetic field is concentrated. Introducing molecules into these hotspots is the basis of a vast spectrum of enhanced spectroscopies, from surface-enhanced Raman scattering to surface-enhanced fluorescence and others. We show in this review how these spectroscopic methods can be used to characterize the fields around plasmonic molecules. Furthermore, the strong fields can be used to drive new phenomena, from plasmon-induced chemical reactions to strong coupling of quantum emitters with the plasmonic fields. We systematically discuss these phenomena, introducing in each case the theoretical basis as well as recent experimental realizations.

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas.

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule-plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions.

Communication: Probing the interaction of infrared antenna arrays and molecular films with ultrafast quantum dynamics.

Narrowband vibrational molecular transitions interacting with the broadband resonance of infrared plasmonic antennas lead to Fano lineshapes observed in linear (FTIR) and third-order (transient absorption and 2DIR) spectroscopic experiments. Both molecular and plasmonic components are inherently dissipative, and the effects associated with their coupling can be observed, in principle, when measuring the corresponding ultrafast quantum dynamics. We used 2DIR spectroscopy to study the waiting time evolution of quantum coherence excited in the carbonyl stretching modes of rhodium (acetylacetonato) dicarbonyl molecules, which were embedded in an 80 nm-thick polymer film spin-coated on an array of infrared half-wavelength gold antennas. Despite the pronounced Fano lineshapes obtained for the molecular transitions, and up to a four order of magnitude enhancement of the third-order signals, which taken together, indicate the coupling between the plasmonic and molecular transitions, the dynamics of the quantum coherence were identical to that obtained with 3 µm-thick film without the interaction with the plamson mode. This suggests that the coupling rate between the molecular and plasmonic excitations is significantly smaller than the relaxation rates of the molecular excitations monitored in the experiment. Here, the Fano lineshape, observed at the frequency of the molecular transition, can result from the mutual radiation damping of the molecular and plasmon modes.

Two-Dimensional Fano Lineshapes in Ultrafast Vibrational Spectroscopy of Thin Molecular Layers on Plasmonic Arrays.

Two-dimensional femtosecond infrared (2DIR) spectroscopy routinely provides insights into molecular structure and ultrafast dynamics in 1-100 µm thick bulk samples. Confinement of molecules to surfaces, gaps, crevices, and other topographic features, frequently encountered on the nanometer length scale, significantly alters their structure and dynamics, affecting physical and chemical properties. Amplification of 2DIR signals by the plasmon-enhanced fields around metal nanostructures can permit structural and dynamics measurements of the confined molecules. Fano resonances, induced by the interaction between laser pulses, plasmon, and vibrational modes significantly distort 2D lineshapes. For different detuning from plasmon resonance, the interference between multiple signal components leads to different line shape asymmetry, which we demonstrate on a set of linear absorption, transient absorption, and 2DIR spectra. An intuitive model used to describe experimental data points to the interference's origin. Our results will facilitate the application of surface-enhanced 2DIR spectroscopy for studies of molecular structure and dynamics in a nanoconfined environment.

Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit.

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light-matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.