To generate nanoscale biochemical patterns for fundamental biophysical studies as well as practical biosensors, there remains a need for a high quality and versatile substrate. We show that chemically synthesized gold microplates on indium tin oxide are an ideal substrate that combines several desirable characteristics, including low cost, single crystallinity, optical transparency, electrical conductivity, and ease in chemical functionalization. We have developed a convenient one-pot method that allows us to synthesize plates of desired dimensions and surface coverage directly on indium tin oxide. We have used electrochemical desorption to strip the capping agents, allowing reliable functionalization with alkanethiol self-assembled monolayers. These plates can serve as nanoscale "lab benches" that allow high-resolution scanning probe lithography, high-resolution imaging, and electrical manipulation. Two applications are demonstrated here: nanoshaved self-assembled monolayers (SAMs) on the single crystalline microplates serve as a high-resolution etching resist; AFM nanografting on the plates generates SAM patterns with tailored terminal chemical functionalities.
Wet etching of metal substrates with patterned self-assembled monolayers (SAMs) is an inexpensive and convenient method to produce metal nanostructures. For this method to be relevant to the fabrication of high precision plasmonic structures, the kinetics of nanoscale etching process, particularly in the lateral direction, must be elucidated and controlled. We herein describe an in situ atomic force microscopy (AFM) study to characterize the etching process within patterned SAMs with nanometer resolution and in real time. The in situ study was enabled by several unique elements, including single crystalline substrates to minimize the variability of facet-dependent etch rate, high-resolution nanoshaved SAM patterns, electrochemical-potential-controlled etching, and AFM kymographs to improve temporal resolution. Our approach has successfully quantified the extent of both lateral etching and vertical etching at different potentials. Our study reveals the presence of an induction period prior to the onset of significant lateral etching, which would be difficult to observe with the limited time resolution and sample-to-sample variation of ex situ studies. By increasing the vertical etch rate during this induction period with higher potentials, gold was etched up to 40 nm in the vertical direction with minimal lateral etching. High-resolution etching was also demonstrated on single crystal gold microplates, which are high quality gold thin films suitable for plasmonics studies.
Precise spatial organization and electronic coupling between quantum dots are pivotal for many potential applications. Typical spherical quantum dots in assemblies are separated by organic ligands and hence weakly coupled. GaSe nanoparticles are disk-like particles that are four atoms thick with tunable lateral dimensions. Previous spectroscopic investigations indicate the formation of nanoscale aggregates in which the quantum dots are strongly coupled. In this report, we show that the anisotropic properties of these particles may be exploited to assemble surface-stabilized superstructures with well-defined distances between the quantum dots. By changing the ligands adsorbed on the nanoparticle edges, three distinct aggregate morphologies can be produced. The surface chemistry of GaSe orients the nanoparticles on a gold surface and induces stacking in the surface normal direction. The discrete heights of such stacked aggregates suggest that the layers are held together by van der Waals interactions with a regular spacing. Such structures, with their well-defined electronic coupling, have potential implications in fundamental studies of photoinduced charge transfer and transport, as well as device fabrications.
Here we describe a new method for preparing multiple arrays of parallel gold nanowires with dimensions and separation down to 50 nm. This method uses photolithography to prepare an electrode consisting of a patterned nickel film on glass, onto which a gold and nickel nanowire array is sequentially electrodeposited. After the electrodeposition, the nickel is stripped away, leaving behind a gold nanowire array, with dimensions governed by the gold electrodeposition parameters, spacing determined by the nickel electrodeposition parameters, and overall placement and shape dictated by the photolithography.
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