Plasmonic Molecules Research Articles

Ultrasonic Control of Polymer-Capped Plasmonic Molecules.

Plasmonic molecules (PMs) composed of polymer-capped nanoparticles represent an emerging material class with precise optical functionalities. However, achieving controlled structural changes in metallic nanoparticle aggregation at the nanoscale, similar to the modification of atomic structures, remains challenging. This study demonstrates the 2D/3D isomerization of such plasmonic molecules induced by a controlled ultrasound process. We used two types of gold nanoparticles, each functionalized with hydrogen bonding (HB) donor or acceptor polymers, to self-assemble into different ABN-type complexes via interparticle polymer bundles acting as molecular bonds. Post-ultrasonication treatment significantly shortens these bonds from approximately 14 to 2 nm by enhancing HB cross-linking within the bundles. This drastic change in the bond length increases the stiffness of the resulting clusters, facilitating the transition from 2D to 3D configurations in 100% yield during drop-casting onto substrates. Our results advance the precise control of PMs' nanoarchitectures and provide insights for their broad applications in sensing, optoelectronics, and metamaterials.

Electronic excitations at the plasmon–molecule interface

Electronic excitations at the plasmon–molecule interface

Two-color interferometric scattering (iSCAT) microscopy reveals structural dynamics in discrete plasmonic molecules.

Plasmonic molecules are discrete assemblies of noble metal nanoparticles (NPs) that are of interest as transducers in optical nanosensors. So far, NPs with diameters of ∼40 nm have been the preferred building blocks for plasmonic molecules intended as optical single molecule sensors due to difficulties associated with detecting smaller NPs through elastic scattering in conventional darkfield microscopy. Here, we apply 405 nm, 445 nm two-color interferometric scattering (iSCAT) microscopy to characterize polyethylene glycol (PEG) tethered dimers of 10 nm and 20 nm Ag NPs and their monomers. Dimers of both NP sizes can be discerned from their respective monomers through changes in the average iSCAT contrast. In the case of 20 nm Ag NPs, dimer formation induces a change in the sign of the iSCAT contrast, providing a characteristic signal for detecting binding events. 20 nm Ag NP dimers with 0.4 kDa and 3.4 kDa polyethylene glycol (PEG) spacers show iSCAT contrast distributions with significantly different averages on both wavelength channels. The iSCAT contrast measured for individual PEG-tethered 10 nm or 20 nm NP dimers as a function of time shows contrast fluctuations indicative of a rich structural dynamics in the assembled plasmonic molecules, which provides an additional metric to discern dimers from monomers and paves the path to a new class of interferometric plasmon rulers.

Dipole-multipole plasmonic coupling between gold nanorods and titanium nitride nanoparticles for enhanced photothermal conversion.

The plasmonic photothermal conversion efficiency can be enhanced by coupling among plasmonic atoms or plasmonic molecules due to the amplified local electric field and extinction cross-section. Recently, it has been theoretically proved that hybridization between dipolar modes and higher order modes can provide higher enhancement than that among dipolar modes in terms of both near- and far-field, which may lead to a higher photothermal conversion rate. In this work, we systematically investigated the photothermal conversion enhancement of plasmonic coupling between a dipolar mode of a titanium nitride nanoparticle (TiN NP) and a higher order mode of a gold nanorod (Au NR), which was compared to that of coupling among TiN NPs' dipolar modes. We evaluated the photothermal conversion efficiency of dipole-dipole coupling and dipole-multipole coupling in the nanocluster under the illumination of a monochromatic laser of 808 nm wavelength and simulated solar light, respectively. Both experimental tests and numerical simulations suggested that the plasmonic dipole-multipole coupling exhibited higher enhancement in photothermal conversion than dipole-dipole plasmonic coupling.

Innentitelbild: 2D Plasmonische Moleküle durch Wasserstoffbrückenbindungen zwischen polymerbeschichteten Nanopartikeln (Angew. Chem. 47/2023)

Innentitelbild: 2D Plasmonische Moleküle durch Wasserstoffbrückenbindungen zwischen polymerbeschichteten Nanopartikeln (Angew. Chem. 47/2023)

Inside Cover: 2D Plasmonic Molecules via Hydrogen Bond Interaction between Polymer‐Grafted Nanoparticles (Angew. Chem. Int. Ed. 47/2023)

Inside Cover: 2D Plasmonic Molecules via Hydrogen Bond Interaction between Polymer‐Grafted Nanoparticles (Angew. Chem. Int. Ed. 47/2023)

The Mechanism and Fine-Tuning of Chiral Plexcitons in the Strong Coupling Regime.

Chiral plexcitons, produced by the strong interaction between plasmonic nanocavities and chiral molecules, can provide a promising direction for controlling chiroptical responses on the nanoscale. Here, we reveal the chiral origin and electromagnetic hybridization process in chiral strongly coupled systems. The mechanism and unique advantages of chiral plexcitons for fine-tuning circular dichroism (CD) responses are demonstrated, providing a rule for controlling chiral light-matter interactions in complex chiral nanosystems. Furthermore, we experimentally demonstrate the fine-tuning of chiral plexcitons in hybrid systems consisting of plasmonic nanoparticles and chiral J-aggregates. Continuous and precise tuning of the CD resonance positions was successfully achieved in a given structure. Compared with the previous work, the CD spectral tuning accuracy has been improved by an order of magnitude, which can reach the level of 1 nm. Our findings provide a feasible strategy and theoretical basis for accurately controlling chirality in multiple dimensions.

2D Plasmonic Molecules via Hydrogen Bond Interaction between Polymer-Grafted Nanoparticles.

The use of macromolecular design features to regulate non-covalent bonding on the nanoscale is a young and emerging fabrication strategy for advanced nanostructures. For the first time, we describe a self-assembly method to create a series of 2D plasmonic molecules (PMs) using hydrogen-bond interaction between a pair of polymer-capped gold nanoparticles (hydrogen-bond donor and acceptor). Due to the nature of hydrogen-bond interaction, we found that polymer interaction and solvation compete with each other during the self-assembly process, which turns out to be the most important condition for controlling the coordination number of PMs. We have conducted an extensive study on the solvent effect, which has helped us to design and fabricate a series of precise PMs with high symmetry.

2D Plasmonische Moleküle durch Wasserstoffbrückenbindungen zwischen polymerbeschichteten Nanopartikeln

AbstractDie Nutzung makromolekularer Gestaltungsmittel zur Regulierung von nicht‐kovalenten Bindungen im Nanoskalenbereich ist eine junge und emporkommende Herstellungsmethode für fortgeschrittene Nanostrukturen. Zum ersten Mal beschreiben wir eine Methode der Selbstorganisation, um eine Serie 2D‐plasmonischer Moleküle (PM) herzustellen, die Wasserstoffbrückenwechselwirkungen zwischen Paaren polymerbeschichteter Goldnanopartikel ausnutzen (Wasserstoff‐brückendonator und ‐akzeptor). Wir fanden heraus, dass aufgrund der Natur von Wasserstoffbrückenwechselwirkungen die Polymerinteraktion und die Solvatisierung während des Selbstorganisationsprozesses miteinander konkurrieren. Dies erwies sich als die wichtigste Bedingung, um die Koordinationszahl der PM zu kontrollieren. Wir führten eine umfassende Studie hinsichtlich des Lösungsmitteleffekts durch, die uns half, eine Serie präziser PM mit hoher Symetrie zu entwerfen und herzustellen.

Dual-Dewetting Process for Self-Assembled Nanoparticle Clusters in Wafer Scale.

Plasmonic molecules, which are geometrically well-defined plasmonic metal nanoparticle clusters, have attracted significant attention due to their enhancement of light-matter interactions owing to a stronger electric field enhancement than that by single particles. High-resolution lithography techniques provide precise positioning of plasmonic nanoparticles, but their fabrication costs are excessively high. In this study, we propose a lithography-free, self-assembly fabrication method, termed the dual-dewetting process, which allows the control of the size and density of gold nanoparticles. This process involves depositing a gold thin film on a substrate and inducing dewetting through thermal annealing, followed by a second deposition and annealing. The method achieves a uniform distribution of particle size and density, along with increased particle density, across a 6-inch wafer. The superiority of the method is confirmed by a 30-fold increase in the signal intensity of surface-enhanced Raman scattering following the additional dewetting with an 8 nm film, compared to single dewetting alone. Our findings indicate that the dual-dewetting method provides a simple and efficient approach to enable a variety of plasmonic applications through efficient plasmonic molecule large-area fabrication.

Effective Single-Mode Methodology for Strongly Coupled Multimode Molecular-Plasmon Nanosystems.

Strong coupling between molecules and quantized fields has emerged as an effective methodology to engineer molecular properties. New hybrid states are formed when molecules interact with quantized fields. Since the properties of these states can be modulated by fine-tuning the field features, an exciting and new side of chemistry can be explored. In particular, significant modifications of the molecular properties can be achieved in plasmonic nanocavities, where the field quantization volume is reduced to subnanometric volumes, thus leading to intriguing applications such as single-molecule imaging and high-resolution spectroscopy. In this work, we focus on phenomena where the simultaneous effects of multiple plasmonic modes are critical. We propose a theoretical methodology to account for many plasmonic modes simultaneously while retaining computational feasibility. Our approach is conceptually simple and allows us to accurately account for the multimode effects and rationalize the nature of the interaction between multiple plasmonic excitations and molecules.

Enhanced Diffractive Circular Dichroism from Stereoscopic Plasmonic Molecule Array

Artificial nanostructures with large optical chiral responses have been intensively investigated recently. In this work, we propose a diffractive circular dichroism enhancement technique using stereoscopic plasmonic molecule structures. According to the multipole expansion analysis, the z-component of the electric dipole becomes the dominant chiral scattering mechanism during the interaction between an individual plasmonic molecule and the plane wave at a grazing angle. For a periodical structure with the designed plasmonic molecule, large diffractive circular dichroism can be obtained, which can be associated with the Wood-Rayleigh anomaly. Such a diffractive circular dichroism enhancement is verified by the good agreement between numerical simulations and experimental results. The proposed approach can be potentially used to develop enhanced spectroscopy techniques to measure chiral information, which is very important for fundamental physical and chemical research and bio-sensing applications.

Ostensible Steady-State Molecular Cooling with Plasmonic Gold Nanoparticles.

The optical and chemical properties of plasmonic materials have sparked extensive research in exploring their applications in various areas such as photocatalysts, chemical sensors, and photonic devices. However, complicated plasmon-molecule interactions have posed substantial obstacles for the development of plasmonic material-based technologies. Quantifying plasmon-molecule energy transfer processes is a crucial step to understand the complex interplay between plasmonic materials and molecules. Here we report an anomalous steady-state reduction in the anti-Stokes to Stokes surface-enhanced Raman spectroscopy (SERS) scattering intensity ratio of aromatic thiols adsorbed on plasmonic gold nanoparticles under continuous-wave laser irradiation. The observed reduction of the scattering intensity ratio is closely related to the excitation wavelength, the surrounding media, and component of the plasmonic substrates used. Moreover, we observed a similar extent of scattering intensity ratio reduction with a range of aromatic thiols and under different external temperatures. Our discovery implies that there are either unexplained wavelength-dependent SERS outcoupling effects, or some unrecognized plasmon-molecule interactions which lead to a nanoscale plasmon refrigerator for molecules. This effect should be taken into consideration for the design of plasmonic catalysts and plasmonic photonic devices. Moreover, it could be useful for cooling large molecules under ambient conditions.

Aspects of the interaction of pectin-coated Ag nanoparticles with methylene blue with regard to photodynamic applications

The promising future of photodynamic therapy has led to progress in the design of highly effective new-generation photosensitizers based on the combination of plasmonic metal nanoparticles and fluorescent molecules to kill cancer cells and bacteria. In this work, the details of the interaction of pectin-coated Ag nanocomposites (NCs) of different sizes (8, 10, 13, and 28 nm) and various types of pectin shells with the classical photosensitizer methylene blue (MB) were investigated. Pectin-Ag NCs were characterized by UV–vis spectroscopy, DLS, TEM, and XPS. The enhanced singlet oxygen generation effect of pectin-Ag@MB complexes was size-dependent (the smaller the size of the plasmonic particles, the higher the 1O2 generation) and associated with the local concentration of the dye in different molecular forms in the pectin shell of nanoparticles. In cell experiments, the pectin-Ag@MB complexes did not show enhanced phototoxicity or cellular uptake towards cancer cells, compared to free MB. The prolonged release of MB from the complex in a phosphate-buffered saline solution (pH 7.4) was observed. The amount of irreversibly bound MB depended on the type of pectin shell and equaled 1.13, 1.45, and 2.57 µg per mg pectin for Citrus25-Ag, Classic25-Ag, and Classic10-Ag NCs, respectively. The obtained physicochemical insights supplement the present understanding of the fundamental aspects of the interaction between plasmonic polymer-capped nanoparticles and MB and could be important for the rational design of new theranostic agents.

Mapping nanoscale electric field hotspots of a plasmon–molecule system: A theoretical study

The coherent interaction between localized surface plasmon resonance modes and excitons of a single or a collection of quantum emitters have fueled the development of novel applications in quantum optics and material science. In this work, using first-principles simulations, I analyze the modifications in absorption spectra and electric near-field enhancements in a structure consisting of an aluminum nanotriangle interacting with a varying number of pyridine molecules (placed at the nanotriangle tips) in close proximity. What is more, I find very interesting spatial variation in induced electron density and electric near-field enhancements with a remarkable dependence on the number of interacting pyridine molecules and the direction of light illumination. The obtained results may help to improve our understanding of the light–matter interaction at the sub-nanometer scale. • The coherent interaction between LSPR modes and excitons have fueled the development of novel applications in material science. • We study a nanostructure consisting of an aluminum nanotriangle interacting with a varying number of pyridine molecules. • Our results may help to improve our understanding of the light–matter interaction at the sub-nanometer scale.

Modeling and measuring plasmonic excitations in hollow spherical gold nanoparticles.

We investigate molecular plasmonic excitations sustained in hollow spherical gold nanoparticles using time-dependent density functional theory (TD-DFT). Specifically, we consider Au60 spherical, hollow molecules as a toy model for single-shell plasmonic molecules. To quantify the plasmonic character of the excitations obtained from TD-DFT, the energy-based plasmonicity index is generalized to the framework of DFT, validated on simple systems such as the sodium Na20 chain and the silver Ag20 compound, and subsequently successfully applied to more complex molecules. We also compare the quantum mechanical TD-DFT simulations to those obtained from a classical Mie theory that relies on macroscopic electrodynamics to model the light-matter interaction. This comparison allows us to distinguish those features that can be explained classically from those that require a quantum-mechanical treatment. Finally, a double-shell system obtained by placing a C60 buckyball inside the hollow spherical gold particle is further considered. It is found that the double-shell, while increasing the overall plasmonic character of the excitations, leads to significantly lowered absorption cross sections.

Centimeter-Scale Superlattices of Three-Dimensionally Orientated Plasmonic Dimers with Highly Tunable Collective Properties.

The precise organization and orientation of plasmonic molecules on substrates is crucial to their application in functional devices but still remains a grand challenge. This article describes a bottom-up strategy to efficiently fabricate centimeter-scale superlattices of three-dimensionally oriented plasmonic dimers with highly tunable collective optical properties on substrates. The in-plane (i.e., X-Y plane) and out-of-plane (i.e., along Z-axis) orientation of the constituent plasmonic dimers can be precisely controlled by a combination of directional capillary force and supporting polymer film. Our experimental measurements and numerical simulations show that the macroscopic dimer superlattices exhibit polarization-dependent plasmon Fano resonances in air and multimodal surface lattice resonances with high quality factors in a homogeneous medium, owing to the high positional and orientational ordering of the subunits. Our strategy enables the fabrication of complex plasmonic nanostructures with precise configurations for advanced plasmonic devices such as plasmon nanolasing and metamaterials.

Intermolecular Forces Dictate Vibrational Energy Transfer in Plasmonic-Molecule Systems.

Plasmonic materials are a promising category of photocatalysts for solar energy harvesting and conversion. However, there are some significant obstacles that need to be overcome to make plasmonic catalysts commercially available. One major challenge is to obtain a systematic understanding of how to design and optimize plasmonic systems from the perspective of both plasmonic materials and reagent molecules to achieve highly efficient and selective catalysis. It is well-known that the contributions of plasmon-molecule interactions such as plasmon-induced resonant energy transfer and charge transfer to the catalytic mechanism are rather complicated and possibly multifold. Observation of these phenomena is challenging due to the highly heterogeneous nature of plasmonic substrates as well as the large difference in sizes and optical cross sections between plasmonic materials and molecules. In this work, we use a molecular perspective to examine the crucial process of energy transfer between plasmons and molecules, with the goal of determining which experimental parameters can be used to control this energy flow. We employ ultrafast surface-enhanced anti-Stokes and Stokes Raman spectroscopy to investigate vibrational energy transfer in plasmonic-molecule systems. By comparing the energy transfer kinetics of five different aromatic thiols on the picosecond time scale, we find that intermolecular forces play an important role in energy distribution in molecules adsorbed to plasmonic materials, which changes the amount of energy deposited onto the molecule and the lifetime of the energy deposited. Our work implies that careful consideration of catalyst loading and molecule adsorption geometry is crucial for enhancing or suppressing the rate and efficiency of plasmon-driven energy transfer.

Plasmon Character Index: An Accurate and Efficient Metric for Identifying and Quantifying Plasmons in Molecules.

Plasmons, which are collective and coherent oscillations of charge carriers driven by an external field, play an important role in applications such as solar energy harvesting, sensing, and catalysis. Conventionally, plasmons are found in bulk and nanomaterials and can be described with classical electrodynamics. In recent years, plasmons have also been identified in molecules, and these molecules have been utilized to build plasmonic devices. As molecular plasmons can no longer be described by classical electrodynamics, a description using quantum mechanics is necessary. In this Letter, we develop a quantum metric to accurately and efficiently identify and quantify plasmons in molecules. A number, which we call the plasmon character index (PCI), can be calculated for each electronic excited state and describes the plasmonicity of the excitation. PCI is developed from the collective and coherent excitation picture in orbitals and shows excellent agreement with the predictions from scaled time-dependent density functional theory but is vastly more computationally efficient. Therefore, PCI can be a useful tool in identifying and quantifying plasmons and will inform the rational design of plasmonic molecules and nanoclusters.

Plasmon-Driven Motion of an Individual Molecule.

We demonstrate that nanocavity plasmons generated a few nanometers away from a molecule can induce molecular motion. For this, we study the well-known rapid shuttling motion of zinc phthalocyanine molecules adsorbed on ultrathin NaCl films by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced light emission. Comparing spatially resolved single-molecule luminescence spectra from molecules anchored to a step edge with isolated molecules adsorbed on the free surface, we found that the azimuthal modulation of the Lamb shift is diminished in case of the latter. This is evidence that the rapid shuttling motion is remotely induced by plasmon–molecule coupling. Plasmon-induced molecular motion may open an interesting playground to bridge the nanoscopic and mesoscopic worlds by combining molecular machines with nanoplasmonics to control directed motion of single molecules without the need for local probes.