Jun Hee Yoon

Controlled Assembly and Plasmonic Properties of Asymmetric Core–Satellite Nanoassemblies

The assembly of noble metal nanoparticles offers an appealing means to control and enhance the plasmonic properties of nanostructures. However, making nanoassemblies with easily modifiable gap distances with high efficiency has been challenging. Here, we report a novel strategy to assemble gold nanoparticles (AuNPs) into Janus-type asymmetric core-satellite nanostructures. Markedly different desorption efficiency between large and small AuNPs in ethanol allows us to prepare the asymmetric core-satellite nanoassemblies in a dispersed colloidal state with near 100% purity. The resulting nanoassemblies have well-defined structures in which a core AuNP (51 nm) is covered by an average of 13 ± 3 satellite AuNPs (13 nm) with part of the core surfaces left unoccupied. Strong surface plasmon coupling is observed from these nanoassemblies as a result of the close proximity between the core and the satellites, which appears significantly red-shifted from the surface plasmon resonance frequencies of the constituting nanoparticles. The dependence of the surface plasmon coupling on a gap distance of less than 3 nm is systematically investigated by varying the length of the alkanedithiol linkers. The asymmetric core-satellite nanoassemblies also serve as an excellent surface-enhanced Raman scattering substrate with an enhancement factor of ~10(6). Finally, we demonstrate that the presented assembly method is extendible to the preparation of compositionally heterogeneous core-satellite nanoassemblies.

Probing Quantum Plasmon Coupling Using Gold Nanoparticle Dimers with Tunable Interparticle Distances Down to the Subnanometer Range

The assembly of noble metal nanoparticles is an appealing means to control the plasmonic properties of nanostructures. Dimers are particularly interesting because they are a model system that can provide fundamental insights into the interactions between nanoparticles in close proximity. Here, we report a highly efficient and facile assembly method for dimers and other forms of assemblies. Gold nanoparticles (AuNPs) adsorbed on aminosilanized glass surfaces protect the silanes underneath the nanoparticles from hydrolysis. This masked desilanization allows us to prepare AuNP homodimers on glass slides with remarkably high yield (∼90%). The interparticle distance and, accordingly, the surface plasmon coupling are readily tuned at the molecular level using self-assembled monolayers of alkanedithiols. As the interparticle distance is reduced, the resonance surface plasmon coupling progressively redshifts, following the classical electromagnetic model. When the interparticle distance enters the subnanometer regime, however, the resonance band begins to blueshift and significantly broadens. The comparison of our observations with theoretical studies reveals that quantum tunneling effects play a significant role in the plasmonic response of AuNP dimers in the subnanometer gap region. The assembly method based on the masked desilanization is extendable to the formation of various other forms of nanoassemblies and, thus, will further our understanding of plasmonic interactions in nanoassemblies.

Time-dependent and symmetry-selective charge-transfer contribution to SERS in gold nanoparticle aggregates.

We report the time- and symmetry-dependent surface-enhanced Raman scattering (SERS) of gold nanoparticle (AuNP) aggregates. The addition of p-aminothiophenol (p-ATP) instantly induces the aggregation of AuNPs, confirmed by large absorption in the near-IR region. Dynamic light scattering measurements show that the addition of p-ATP immediately assembles the AuNPs (13 nm) to form aggregates with a mean diameter of approximately 200 nm, which then further grow to a size of approximately 300 nm. Raman spectra acquired via time lapse show that the a(1)-symmetry bands of p-ATP are enhanced simultaneously with the formation of the aggregates, indicating that the electromagnetic enhancement largely contributes to the SERS of the AuNP aggregates. In contrast, the enhancement of the b(2)-symmetry bands occurs approximately 10 h after the formation of the aggregates and slowly progresses. The enhancement of the b(2) mode is attributed to the charge transfer between AuNPs and adsorbates, rather than the reorientation of the adsorbates because thiophenol and p-methylthiophenol that have surface structures and intermolecular interactions similar to those of p-ATP do not exhibit a symmetry-specific Raman enhancement pattern. To elucidate the disparity in the timescale between the charge-transfer resonance and the formation of the aggregates, we propose two models. A further close approach of the AuNPs constituting the aggregates causes the additional adsorption of the initially adsorbed p-ATP onto neighboring AuNPs, tuning the charge transfer state to be in resonance with the Raman excitation laser. Density functional theory calculations confirm the resonance charge-transfer tunneling through the bridging p-ATP in the AuNP-p-ATP-AuNP structures. Alternatively, the gradual continuing adsorption of p-ATP increases the local Fermi level of AuNPs into the region of resonant charge transfer from the Fermi level to the LUMO of the adsorbates. This model is corroborated by the faster appearance of b(2)-mode enhancement for the AuNPs with initially higher zeta potentials.

Surface Plasmon Coupling of Compositionally Heterogeneous Core–Satellite Nanoassemblies

Understanding plasmon coupling between compositionally heterogeneous nanoparticles in close proximity is intriguing and fundamentally important because of the energy mismatch between the localized surface plasmons of the associated nanoparticles and interactions beyond classical electrodynamics. In this Letter, we explore surface plasmon coupling between silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), assembled in the form of core-satellite structures. A recently developed assembly method allows us to prepare ultrapure core-satellite nanoassemblies in solution, where 50 nm AgNPs are surrounded by 13 nm AuNPs via alkanedithiol linkers. We observe changes in the plasmon coupling between the AgNP core and AuNP satellites as the core-to-satellite gap distance varies from 2.3 to 0.7 nm. Comparison with theoretical studies reveals that the traditional hybridized plasmon modes are abruptly replaced by charge-transfer plasmons at a ∼1 nm gap. Changes with the number of satellites are also discussed.

Probing interfacial interactions using core-satellite plasmon rulers.

Understanding molecular interactions at the interfaces of nanoparticles is fundamentally important because they determine the stability, affinity, functionality, and assembly of nanoparticles. However, probing the governing intermolecular forces at the interfaces, particularly for the nanoparticles dispersed in solution, remains challenging. Here, we demonstrate that the interfacial interactions between citrate-capped gold nanoparticles and various molecular functional groups can be probed using a plasmon ruler, based on a well-defined core-satellite nanoassembly structure. Different nature of the interactions causes a subtle change in the interparticle distance, and the change is sensitively measured as a shift in the plasmon coupling band of the core-satellite nanoassemblies. Molecular interactions including covalent bonding, hydrogen bonding, electrostatic interactions, and van der Waals interactions are explored.

Plasmon coupling between silver nanoparticles: Transition from the classical to the quantum regime

We explore plasmon coupling between silver nanoparticles (AgNPs) as two AgNPs approach each other within a subnanometer distance. We prepare AgNP dimers with two 21-nm AgNPs separated by alkanedithiol linkers in high yield. Changing the length of the alkanedithiol linkers enables us to control the interparticle distance down to the subnanometer level on the molecular scale. We observe that the longitudinal plasmon coupling band, which is sensitive to the interaction between AgNPs, gradually redshifts as the interparticle distance decreases. This observation is fully consistent with the classical electromagnetic model. The redshift of the plasmon coupling, however, undergoes a drastic change when the interparticle distance reaches ∼1nm. The longitudinal plasmon coupling band vanishes and a new intense band appears at a shorter wavelength. This band redshifts as the nanogap further narrows, but crosses over to a blueshift at ∼0.7nm. A comparison of our observation with finite-difference time-domain simulations reveals that this band arises from quantum effects. Controlled assembly of AgNP dimers in combination with simulations allows us to observe the transition of the plasmon coupling from the classical to the quantum regime at the ensemble level.

Creating SERS hot spots on ultralong single-crystal β-AgVO3 microribbons

β-AgVO3 ribbons are novel one-dimensional structures with excellent structural, electrical, and catalytic properties. Here we report the synthesis of ultralong β-AgVO3 microribbons and their applications as surface-enhanced Raman scattering (SERS) substrates. A hydrothermal method allows us to prepare millimeter-long β-AgVO3 ribbons with a cross-section of ∼2.5 μm × ∼1 μm in a high yield. The length of the β-AgVO3 microribbons is controlled by the amount of pyridine added to AgNO3 and NH4VO3. The perfectly smooth and single crystalline β-AgVO3 surfaces drastically change into rough and rippled surfaces upon reaction with 4-aminothiophenol. Electron microscopy studies reveal that Ag nanoparticles with a diameter of ∼7 nm are formed on the surface, rendering the ultralong β-AgVO3 microribbons highly SERS-active. Possible mechanisms of the transformation and applications of the resulting SERS-active substrates in microfluidics are discussed herein.

Spatially Controlled SERS Patterning Using Photoinduced Disassembly of Gelated Gold Nanoparticle Aggregates

Controlling the assembly of the nanoparticles is important because the optical properties of noble metal nanoparticles, such as the surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS), are critically dependent on interparticle distances. Among many approaches available, light-induced disassembly is particularly attractive because it enables spatial modification of the optical properties of nanoparticle assemblies. In this study, we prepare gold nanoparticle (AuNP) aggregates in a gel matrix. Irradiation of the gelated AuNP aggregates at 532 nm leads to the disassembly of the aggregates, changing the color (SPR) from dark blue to red and extinguishing the SERS signal along the irradiated pattern, which opens the possibility of facile fabrication of spatially controlled SERS-generating microstructures. The photoinduced disassembly of the AuNP aggregates in solution is also investigated using UV-vis spectroscopy and transmission electron microscopy.