We introduce Plasmene- in analogy to graphene-as free-standing, one-particle-thick, superlattice sheets of nanoparticles ("meta-atoms") from the "plasmonic periodic table", which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75,000. In conjunction with top-down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.

Recently, there has been a drive to design and develop fully tunable metamaterials for applications ranging from new classes of sensors to superlenses among others. Although advances have been made, tuning and modulating the optical properties in real time remains a challenge. We report on the first realization of a reversible electrotunable liquid mirror based on voltage-controlled self-assembly/disassembly of 16 nm plasmonic nanoparticles at the interface between two immiscible electrolyte solutions. We show that optical properties such as reflectivity and spectral position of the absorption band can be varied in situ within ±0.5 V. This observed effect is in excellent agreement with theoretical calculations corresponding to the change in average interparticle spacing. This electrochemical fully tunable nanoplasmonic platform can be switched from a highly reflective 'mirror' to a transmissive 'window' and back again. This study opens a route towards realization of such platforms in future micro/nanoscale electrochemical cells, enabling the creation of tunable plasmonic metamaterials.

Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.

We systematically investigated the size- and shape-dependent SERS activities of plasmonic core-shell nanoparticles towards detection of the pesticide thiram. Monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) were synthesized and their Ag shell thickness was precisely adjusted from ∼1 nm to ∼16 nm. All these nanoparticles were used as SERS substrates for thiram detection, and the Raman intensities with three different lasers (514 nm, 633 nm and 782 nm) were recorded and compared. Our results clearly show that: (1) the excitation wavelength discriminated particle shapes regardless of particle sizes, and the maximized Raman enhancement was observed when the excitation wavelength approaches the SERS peak (provided there is significant local electric field confinement on the plasmonic nanostructures at that wavelength); (2) at the optimized laser wavelength, the maximum Raman enhancement was achieved at a certain threshold of particle size (or silver coating thickness). By exciting particles at their optimized sizes with the corresponding optimized laser wavelengths, we achieved a detection limit of roughly around 100 pM and 80 pM for NCs and NBs, respectively.

COMMUNICATIONstronger than those obtained from a commercial Klarite SERS substrate, which was rigid, opaque, and could not be used for direct chemical identifi cation on banknotes or coins.A combination of polymer-ligand-based strategy and dryingmediated air-water interfacial self-assembly was utilized for the fabrication of plasmene nanosheets with Au@Ag nanocubes (NCs) as model building blocks. [7][8][9] The physical steps towards obtaining a free-standing plasmene nanosheet are demonstrated in Figure 1 a. In brief, a concentrated drop of polystyrene (PS)-capped NCs is spread and allowed to solidify into a silvery refl ection on the surface of a water droplet. Subsequent slow evaporation resulted in plasmene nanosheets that covered almost the entire grid. One striking feature of our plasmene nanosheets is their high mechanical fl exibility, which allows them to be transferred using a high-fi delity, polydimethylsiloxane (PDMS) elastomer. This enables their use as powerful "SERS adhesive" for chemical identifi cation of trace amount of chemicals on solids of different materials with complex surface structures.To prove this exciting capability, 4-aminothiophenol (4-ATP) was used as the model target analyte because of its strong affi nity to silver and its apparent, large SERS signal. For this 40 µL of 4-ATP (1 µM, corresponding to around 4 ng cm −2 ) drops were deposited and allowed to dry onto the surface of respectively a Malaysian banknote (fi brous), an Australian banknote (polymer), and an Australian coin (rough metal) before PDMS-mediated stamping of the plasmene nanosheets. The transferred sheets had intimate contact with these complex surfaces because of their high fl exibility and robustness, and they enabled a substantial signal enhancement in the SERS detection of the trace amounts of 4-ATP molecules (Figure 1 b-d). The characteristic Raman peaks for 4-ATP could not be observed on the plain and ATP-covered solid surfaces (blue curves and black curves), but they became evident for the surfaces with attached plasmene nanosheets (red curves). This demonstrates a simple non-destructive manner for chemical identifi cation on non-planer surfaces and we believe that the observed signalenhancing capability can be potentially extended to any other complex solid surfaces.We further systematically investigated how the size and shape of the constituent nanoparticles could infl uence the SERS enhancement. We deliberately synthesized six plasmonic building blocks with different geometries (Supporting Information, Section 3.1) -small-(s-), medium-(m-), large-(l-) nanocubes (NCs) and nanobricks (NBs), which were used to produce six different sheets with the same capping ligands of identical molecular lengths. As opposed to NC building blocks, which tend to pack side by side with a regularity imposed by its highly ordered symmetrical nature, isotropic NBs are aligned horizontally with random packing directions to form unique, Plasmonic nanoparticles have become a prominent research subject in the fi eld of chemical ide...

Understanding the structure and assembly of nanoparticles at liquid|liquid interfaces is paramount to their integration into devices for sensing, catalysis, electronics and optics. However, many difficulties arise when attempting to resolve the structure of such interfacial assemblies. In this article we use a combination of X-ray diffraction and optical reflectance to determine the structural arrangement and plasmon coupling between 12.8 nm diameter gold nanoparticles assembled at a water|1,2-dichloroethane interface. The liquid|liquid interface provides a molecularly flat and defect-correcting platform for nanoparticles to self-assemble. The amount of nanoparticles assembling at the interface can be controlled via the concentration of electrolyte within either the aqueous or organic phase. At higher electrolyte concentration more nanoparticles can settle at the liquid|liquid interface resulting in a decrease in nanoparticle spacing as observed from X-ray diffraction experiments. The plasmonic coupling between the nanoparticles as they come closer together is observed by a red-shift in the optical reflectance spectra. The optical reflectance and the X-ray diffraction data are combined to introduce a new 'plasmon ruler'. This allows extraction of structural information from simple optical spectroscopy techniques, with important implications for understanding the structure of self-assembled nanoparticle films at liquid interfaces.

COMMUNICATIONtable" of plasmonic atoms. [ 9 ] Under the guidance of plasmon hybridization theory, [ 12 ] these artifi cial plasmonic atoms can be assembled into periodic nanostructure arrays with unparalleled optical signatures, opening unlimited possibilities for engineering Raman hot spots and their distribution. Consequently, this platform of combining plasmonic nanoparticle assemblies and their tunable SERS signatures allows selective coding encryption capabilities, which hold a great promise as an advanced suite of next-generation anticounterfeit security labels.We have previously developed a robust self-assembly strategy to synthesize a new class of 2D plasmonic nanomaterialssoft plasmene nanosheets. [13][14][15] Despite the exciting advances achieved for designing of such plasmonic nanosheets, successful self-assembly mechanisms are generally limited to simple nanoparticle shapes such as nanospheres, nanorods, and nanocubes. [ 13,14,16,17 ] In comparison, complex anisotropic shapes such as nanostars may exhibit unique novel properties. [ 18,19 ] Herein, we demonstrate that two new plasmonic elements-gold rhombic dodecahedrals (RD) and gold nanostars (NStr)-could self-assemble to form high-quality plasmene nanosheets. More importantly, we show that these nanosheets could be dual-coded with plasmonic signatures and SERS fi ngerprints, enabling them to serve as a unique anticounterfeit security label for banknotes. Nine different plasmonic codes were created using gold nanospheres, gold rhombic dodecahedrals, and gold nanostars as building blocks, each with three different sizes. With the same plasmonic code, fi ve additional SERS fi ngerprint barcodes were demonstrated. The facile adjustment of plasmonic codes by fi ne-tuning size and shapes in conjunction with choices of Raman dyes makes our system an ideal dual-coded currency label with virtually unlimited coding capacity.Free-standing RD-based plasmene nanosheets were fabricated using a previously developed two-step polymer-mediated self-assembly strategy. [13][14][15] High-quality and monodispersed RD nanoparticles with a nominal edge length of ≈26.3 ± 2.1 nm were fi rst functionalized with thiolated-polystyrene, followed by an evaporation induced self-assembly process at air-water interface into plasmene nanosheets. TEM characterization revealed the assembled nanosheet to be monolayered, with the RDs lying fl at into an elongated hexagon-like shape ( Figure 1 a) and exhibiting a 2D hexagonally close-packed (hcp) ordering (Figure 1 b). As a proof of our dual coding concept, a droplet (5 µL, 1 × 10 −3 M ) of 4-aminothiophenol (4-ATP) solution, which is a model Raman dye with well-established characteristic vibrational fi ngerprints, was deposited on a banknote surface, followed by stamping of nanosheets on the deposited region and spin-coated (3000 rpm,

Mirror-on-mirror platforms based on arrays of metallic nanoparticles, arranged top-down or self-assembled on a thin metallic film, have interesting optical properties. Interaction of localized surface-plasmons in nanoparticles with propagating surface-plasmons in the film underpins the exotic features of such platforms. Here, we present a comprehensive theoretical framework which emulates such a system using a five-layer-stack model and calculate its reflectance, transmittance, and absorbance spectra. The theory rests on dipolar quasi-static approximations incorporating image-forces and effective medium theory. Systematically tested against full-wave simulations, this simple approach proves to be adequate within its obvious applicability limits. It is used to study optical signals as a function of nanoparticle dimensions, interparticle separation, metal film thickness, the gap between the film and nanoparticles, and incident light characteristics. Several peculiar features are found, e.g., quenching of reflectivity in certain frequency domains or shift of the reflectivity spectra. Schemes are proposed to tailor those as functions of the mentioned parameters. Calculating the system's optical responses in seconds, as compared to much longer running simulations, this theory helps to momentarily unravel the role of each system parameter in light reflection, transmission, and absorption, facilitating thereby the design and optimisation of novel mirror-on-mirror systems.