The self-assembly of monodisperse inorganic nanoparticles into highly ordered arrays (superlattices) represents an exciting route to materials and devices with new functions. It allows programming their properties by varying the size, shape, and composition of the nanoparticles, as well as the packing order of the assemblies. While substantial progress has been achieved in the fabrication of superlattice materials made of nanospheres, limited advances have been made in growing similar materials with anisotropic building blocks, which is particularly true for free-standing two-dimensional superlattices. In this paper, we report the controlled growth of free-standing, large-area, monolayered gold-nanorod superlattice sheets by polymer ligands in an entropy-driven interfacial self-assembly process. Furthermore, we experimentally characterize the plasmonic properties of horizontally aligned sheets (H-sheets) and vertically aligned sheets (V-sheets) and show that observed features can be well described using a theoretical model based on the discrete-dipole approximation. Our polymer-ligand-based strategy may be extended to other anisotropic plasmonic building blocks, offering a robust and inexpensive avenue to plasmonic nanosheets for various applications in nanophotonic devices and sensors.

We report on a generic approach to use polymers as ligands to fabricate the free-standing, monolayered nanoparticle superlattice sheets based on drying-mediated, entropy-driven self-assembly at the airÀliquid interface. Such superlattice nanosheets are the thinnest possible two-dimensional crystals whose structural features and functional properties can be adjusted by tailoring the sizes of their nanoparticle constituents. The monolayered superlattice nanosheets are highly stable under various conditions, transferable to any arbitrary substrates, and can be manufactured with focused ion beams. This demonstrates the potential to integrate the superlattice nanosheets into miniaturized optical and electronic devices.

Metallic nanoparticles that support surface plasmons are potential building units for future nanophotonic circuits, metamaterials, high-density optical data storage, etc. Many of these applications require the ability to 'dial-up' the desired plasmonic resonance modes and frequencies with high precision. Here, we demonstrate a thermal reshaping route that can be used to tailor longitudinal plasmon resonance energies of gold nanorods almost continuously from ~800 to ~560 nm. The longitudinal plasmon resonance wavelength exhibits an exponential decay function of the thermal annealing time at a given temperature. This correlates with the transmission electron microscopy characterization (TEM) which showed that the nanorod aspect ratio decreases exponentially with time, accompanying a gradual shape transformation from rod to sphere. The exponential decay half-time decreases with increasing annealing temperatures, with a value of 1.43 × 10(5) s at 50 °C down to 0.02 × 10(5) s at 100 °C. Our experimental results show that the shape transformation could be attributed to desorption of silver ions and side facet-binding Ag-Br-CTA ligands, which therefore promote the side growth leading to nanorod fattening. Compared to other synthetic methodologies to tune plasmonics, our thermal reshaping approach presents a straightforward paradigm for precisely tailoring plasmon resonance energy with a single parameter.

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