Here we report a novel physical approach for thiolate–protected fluorescent gold nanoparticles with a controlled size of the order of a few nanometers. This approach is based on a sputtering of gold into a liquid matrix containing thiolate ligand as a stabilizer at various concentrations, thus no reductant was used. The size of the gold nanoparticles was successfully controlled to range from 1.6 to 7.4 nm by adjusting the thiol concentrations. Surface plasmon absorption was observed in larger nanoparticles, but it was not observed in smaller ones. Such smaller nanoparticles fluoresced at around 670 nm with a small spectral shift according to their size, however, the diameter (1.6–2.7 nm) was very strange to show such red emission compared with photophysical characteristics of reported gold cluster or nanoparticles synthesized by chemical method. By detailed investigations using TEM, HAADF-STEM, XPS, and TGA, and size fractionation by size exclusion chromatography, we finally arrived at the plausible mechanism for the origin of unusual fluorescence property; the obtained gold nanoparticles are not single-crystal and are composed of aggregates of very small components such as multinuclear gold clusters or complexes.

Sputtering deposition of gold over a liquid matrix containing different ligands with thiolate, amine, or carboxy groups resulted in the formation of spherical gold nanoparticles (Au NPs) in the range of 2.1 to 6.7 nm, and the diameters gradually decreased as the affinity of the ligands for Au increases.

Although aberration-corrected scanning transmission electron microscope (STEM) enables the atomic-scale visualization of ultrathin 2D materials such as graphene, imaging of electron-beam sensitive 2D materials with structural complexity is an intricate problem. We here report the first atomic-scale imaging of a free-standing monolayer clay mineral nanosheet via the annular dark field (ADF) STEM. The monolayer clay nanosheet was stably observed under optimal conditions, and we confirmed that the hexagonal contrast pattern with a pore of ∼4 Å corresponds to the atomic structure of clay mineral that consisted of adjacent Si, Al, Mg, and O atoms by comparison with simulations. The findings offer the usefulness of ADF−STEM techniques for the atomic scale imaging of clay minerals and various 2D materials having electron-beam sensitivity and structural complexity than few-atom-thick graphene analogues.

Recent developments in electron microscopy and novel techniques for the precise control of low-dimensional substrate materials have led to the emergence of direct imaging of organic molecules. However, supramolecular assemblies constructed via non-covalent interactions are structurally unstable under electron bean irradiation. This study focused on a supramolecular assembly of well-isolated organic molecules based on multivalent electrostatic interactions, where anionically charged clay mineral nanosheets were used as a host material. Direct imaging of the single organic molecules and supramolecular assemblies on the monolayer clay mineral nanosheets was achieved via annular dark field scanning transmission electron microscopy using heavy metal atom markers. The versatility of multivalent electrostatic interactions for the stable fixation of organic molecules during electron microscopic imaging was demonstrated. This 2 strategy can be applied to a wide range of supramolecular systems comprising various guest molecules and host materials, thereby broadening the scope of atomic-scale imaging.

Among 2D materials, clay mineral nanosheets have been extensively studied owing to their specific features, such as high surface charge and large surface area. Recently, we reported a stable free-standing...