Nonlinear Viscoelasticity of Highly Ordered, Two-Dimensional Assemblies of Metal Nanoparticles Confined at the Air/Water Interface

Abstract

In this study, we investigated the viscoelastic properties of metal nanoparticle monolayers at the air/water interface by dilational rheology under periodic oscillation of surface area. Au nanoparticles capped with oleylamine form a stable, dense monolayer on a Langmuir film balance. The stress response function of a nanoparticle monolayer was first analyzed using the classical Kelvin–Voigt model, yielding the spring constant and viscosity. The obtained results suggest that the monolayer of nanoparticles is predominantly elastic, forming a two-dimensional physical gel. As the global shape of the signal exhibited a clear nonlinearity, we further analyzed the data with the higher modes in the Fourier series expansion. The imaginary part of the higher mode signal was stronger than the real part, suggesting that the dissipative term mainly causes the nonlinearity. Intriguingly, the response function measured at larger strain amplitude became asymmetric, accompanied by the emergence of even modes. The significance of interactions between nanoparticles was quantitatively assessed by calculating the potential of mean force, indicating that the lateral correlation could reach up to the distance much larger than the particle diameter. The influence of surface chemical functions and core metal has also been examined by using Au nanoparticles capped with partially fluorinated alkanethiolate and Ag nanoparticles capped with myristic acid. The combination of dilational rheology and correlation analyses can help us precisely control two-dimensional colloidal assembly of metal nanoparticles with fine-adjustable localized surface plasmon resonance.

Interfacial Rheology of Charged Anisotropic Cellulose Nanocrystals at the Air–Water Interface

“…31,32 However, interfacial elasticity is also enhanced by attractive particle interactions. 32,33 van den Berg et al 34,35 demonstrated higher interfacial elasticity for CNCs with an increasing degree of esterification, although the adsorption angle increasingly diverged from 90°. We observed a sharp increase in E′ upon salt addition beyond 10 mM NaCl, in good agreement with NaCl concentrations known to allow for CNC attractive interactions.…”

Tuning the Emission Colors of Self-Assembled Quantum Dot Monolayers via One-Step Heat Treatment for Display Applications

“…30−32 To clarify the mechanism of the blueshift, we conducted an EDX microanalysis with HAADF-STEM imaging to investigate the size, morphology, and element distribution in QDs before and after thermal treatment. The QD-SAMs are soft compared with the self-assembled metal nanoparticles 18 because of weak van der Waals interactions between core crystals (smaller size and smaller Hamaker constants), and it was not easy to maintain the free-standing film structure on the Si 3 N 4 TEM grid. In addition, the damage to the organic ligands under the high-power electron beam of the HAADF-STEM system was unavoidable, so the images obtained were blurry, and the elemental line scan was noisy; however, we could still obtain several pieces of important information.…”

“…Over the past decade, our group has been studying the self-assembly of metal nanoparticles at the centimeter scale to create plasmonic metasurfaces or metamaterials, − utilizing the Langmuir–Schaefer (LS) method (transferring of a self-assembled film formed at the air–water interface by stamping). This technique is well established for molecular assembly, but is not as widely employed for nanomaterials not only because of the difficulty of obtaining a hexagonally packed local structure but also the difficulty of controlling the viscoelastic properties of the film to realize a homogeneous and continuous film in centimeter-scale through a compression process . The interaction between ligand molecules and metal cores determines not only the local packing structure but also the plasticity of the film.…”

“…This technique is well established for molecular assembly, but is not as widely employed for nanomaterials not only because of the difficulty of obtaining a hexagonally packed local structure but also the difficulty of controlling the viscoelastic properties of the film to realize a homogeneous and continuous film in centimeter-scale through a compression process. 18 The interaction between ligand molecules and metal cores determines not only the local packing structure but also the plasticity of the film. The self-assembled nanomaterials are often too rigid even when covered with organic ligands to show plasticity under compression; however, it can be improved by an optimal selection of ligand molecules.…”

Influence of Perfluorohexane‐Enriched Atmosphere on Viscoelasticity and Structural Order of Self‐Assembled Semifluorinated Alkanes at the Air‐Water Interface

“…As shown in Figure S5 we confirmed that the fitting up to the 5th mode was sufficient. Following our previous accounts [34,35] we focus first on the linear contribution and thus analyze only the phase separation and viscoelastic moduli obtained from the first mode (indicated by the index 1) of the Fourier series expansion, which represents~89 % of the signal. [36,37] As presented in Figure S6, the phase separation between stress and strain exhibits a minor increase from ϕ 1 � 0.2 to 0.6 following the increase in the strain frequency from f = 1 mHz to 150 mHz.…”