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We conduct a comprehensive numerical investigation on the size-dependence properties of second harmonic generation (SHG) from an epsilon-near-zero (ENZ) film-coupled plasmonic nanoparticle on-mirror (NPoM) system. The distinct size dependence of gold and indium-tin-oxide (ITO) leads to the existence of a critical point where the SHGs from these two materials are balanced. This study offers valuable guidance in the design of plasmonic systems containing ENZ materials for enhancing SHG.

Quantitative molecular detection based on surface-enhanced Raman spectroscopy (SERS) is still a great challenge because of the highly nonuniform distribution of the SERS hot spots and the nondeterministic spatial and spectral overlap of the analyte with the hot spot. Here, we report a nanoparticle-on-mirror plasmonic sensor excited by surface plasmon polaritons for quantitative SERS detection of alpha fetoprotein in serum with ultrahigh sensitivity. The uniform gaps between the nanoparticles and gold film and the alignment of the gap modes relative to the excitation electric field endow this substrate with a uniform and strong SERS enhancement. The limit of detection reaches 1.45 fM, 697 times higher than that under normal excitation and 7800 times higher than a commercial enzyme-linked immunosorbent assay kit. This approach offers a potential solution to overcome the bottleneck in the field of SERS-based biosensing.

Using finite element calculations, we investigated the near field properties of two types of commonly used metal nanostructures (a tip-substrate model representing a scanning tunneling microscope type setup (TS) and a nanoparticle-on-mirror (NPoM) configuration) with atomic sharpness and extreme focusing capabilities. The spatial confinement and electric field enhancement of the local field as well as the fluorescence quantum efficiency of a model molecule (as represented by an oscillating dipole) in the cavity region of the two models were systematically studied. It was found that the TS model tends to support higher local electric field enhancement while the NPoM model can provide a more localized plasmon electric field near the nanoparticle. Calculations with the radiating model molecule indicate that both TS and NPoM can cause significant enhancements to the non-radiative decay rates at the order of 106 in the wavelength range of 500–1000 nm. The TS model shows better performance for the radiative enhancements and the resulting emission quantum yield. These results are not only helpful to improve the understanding of such important nanocavities but also supply a reference for their further applications in different areas.

Graphene can be used as a starting material for the synthesis of useful nano-complexes for flexible, transparent electrodes, therapeutic, bio-diagnostics, and bio-sensing. In order to apply graphene in the medical field, the chemical vapor deposition (CVD) method has been mainly utilized considering its large and near-homogeneous carbon constituents. Especially, the less degree of perturbation of graphene monolayer (GM), which is followed by the underneath catalytic Cu surface morphology, is very crucial in terms of providing the suspended GM and relatively fluent lateral carrier mobility with lower sheet resistance value. In this work, we can suggest a surface-enhanced Raman spectroscopic (SERS) indicator in a quantitative way on the status of z-directional morphological corrugation of a CVD-grown GM (CVD-GM) by applying a nanoparticle-on-mirror (NPoM) system composed of Au nanoparticle (NP)/CVD-GM/Au thin film (TF) plasmonic junction structure. A new (or enhanced) radial breathing-like mode (RBLM) SERS signal around ~ 150 cm−1 from CVD-GM spaced in NPoM is clearly observed by employing a local z-polarized incident field formed at the Au NP–Au TF plasmonic gap junctions. With this observation, the value of I[out-of-plane, RBLM]/I[in-plane, [2D] at certain domains, can be suggested as a new optical nano-metrology value to relatively determine between lower z-directional morphological corrugation (or protrusion) status of a CVD-GM spaced in our NPoM system (lower I[RBLM]/I[2D] value) and a higher degree of lateral carrier mobility of the CVD-GM associated with lower sheet resistance values as a result of higher blue-shifted Raman in-plane (G, 2D) peak maximum position. Furthermore, we will also expect the bio-sensing performances by utilizing the high specific surface area and ultrahigh flexibility of the CVD-GM in one of the future prospective works such as pressure-strain, strain-to-electricity, and chemical-coupled sensor via I[RBLM]/I[2D] values. [ABSTRACT FROM AUTHOR]

The author examines the different defect effects of chemical vapor deposition graphene monolayer (CVD‐GM) relying on domain position at an individual nanoparticle‐on‐mirror (NPoM) plasmonic junction. The NPoM, composed of Au nanoparticle (NP)/CVD‐GM/Au thin film (TF), could previously show the mode‐selective Raman enhancement, in which the center domain exhibited the highest Raman enhancement of both radial breathing like mode (RBLM) and D mode considering the local z‐directional electromagnetic (EM) field enhancement effect in the NPoM. Meanwhile, the outer domain of the NPoM also displays the near‐two symmetrical deep pits via atomic force microscope (AFM) with the accompanying lower I[D]/I[D*], higher FWHM[RBLM], I[RBLM], and relatively red‐shifted (tensile strained) Max [RBLM] from surface‐enhanced Raman scattering (SERS) information than the central case. With these observations, the physical loading of Au NP on CVD‐GM/Au TF may induce the threading effect on the most outer position of CVD‐GM at the NPoM, leading to sp2 defect‐rich domains. In contrast, the central domains, where the sp3 defects are rich, exhibit the opposite SERS spectral phenomena, indicating that the near‐flat surface shape of Au NP bottom has a higher probability to exert a strong charge transfer along the z‐direction, resulting in higher I[RBLM] and I[D]/I[D*] than the most outer domain. As such, the author anticipates various chemical sensing using subtle changes in CVD‐GM morphology at both the most outer and center positions in the NPoM with the comparison of each SERS spectral signature. [ABSTRACT FROM AUTHOR]

Plasmonic nanocavities based on nanoparticle‐on‐mirror geometries draw widespread interest due to their easy‐fabricated structures and highly confined plasmon modes, enabling light–matter interaction for developing state‐of‐the‐art photonic devices. To truly be ideal building blocks for practical nanophotonic devices, the nanoparticle‐on‐mirror nanocavities are expected to be simultaneously spectral tunable and thermostable under the focused laser irradiation. Here, the in‐situ optical tuning of the nanocavity mode is achieved in the nanocube‐on‐mirror nanocavities by continuous laser irradiation. Through the photothermal‐assisted morphology changes, the in‐situ shifting of the plasmon resonances to the lower and higher energies is accomplished by the deformation of the surfactant covering the silver nanocube and reshaping the rounding curvature of the nanocubes, respectively. Furthermore, it is found that the alumina‐encapsulated nanocube on mirrors constructed by surfactant‐eliminated nanocubes are capable of enduring more intense irradiation than the normal ones. On this basis, the factors affecting the plasmonic heating effect are also systematically investigated, and the internal relationship among light absorption, temperature increases, and plasmonic resonance shifts is established. In general, these results provide ideas for designing novel stable nanocavities, and the surfactant‐eliminated nanocube‐on‐mirror nanocavities presented here are a promising platform for future applications in boosting light–matter interaction. [ABSTRACT FROM AUTHOR]

Highlights from the article: Gold nanoparticles (AuNPs) covered with the SAM of thiol molecules are kept dispersed in solution by the electrostatic repulsion or steric effect between the capping ligands.[2] The SAM structures are also useful in precisely defining the nanogaps. The AuNPs were adsorbed onto the SAMs by immersing the SAM/Au substrate in the AuNP solution, instead of dropcasting the AuNPs, to minimize the inhomogeneous distribution of AuNPs. GLO:1CJF/01aug19:bkcs11830-fig-0002.jpg PHOTO (COLOR): (a) Experimental scheme for exploring whether the thiol molecules that form stable SAMs on Au substrates are replaced by other thiols in solution. This contrasts with the general belief that 12-18 h is required to form thermodynamically stable SAMs.[6] The prepared SAMs are stable in pure ethanol solvent, but are replaced by other thiol molecules in solution in less than 3 h.

Nanoparticle-on-mirror systems are a stable, robust, and reproducible method of squeezing light into sub-nanometer volumes. Graphene is a particularly interesting material to use as a spacer in such systems as it is the thinnest possible 2D material and can be doped both chemically and electrically to modulate the plasmonic modes. We investigate a simple nanoparticle-on-mirror system, consisting of a Au nanosphere on top of an Au mirror, separated by a monolayer of graphene. With this system, we demonstrate, with both experiments and numerical simulations, how the doping of the graphene and the control of the gap size can be controlled to tune the plasmonic response of the coupled nanosphere using nitric acid. The coupling of the Au nanosphere and Au thin film reveals multipolar modes which can be tuned by adjusting the gap size or doping an intermediate graphene monolayer. At high doping levels, the interaction between the charge-transfer plasmon and gap plasmon leads to splitting of the plasmon energies. The study provides evidence for the unification of theories proposed by previous works investigating similar systems.

• Near field properties of two types of metallic cavities with atomic sharpness were calculated. • The considered tip-substrate model shows better performance for field enhancement and emission enhancements. • The nanoparticle-on-mirror structure can lead to more localized plasmon electric field near the nanoparticle. Using finite element calculations, we investigated the near field properties of two types of commonly used metal nanostructures (a tip-substrate model representing a scanning tunneling microscope type setup (TS) and a nanoparticle-on-mirror (NPoM) configuration) with atomic sharpness and extreme focusing capabilities. The spatial confinement and electric field enhancement of the local field as well as the fluorescence quantum efficiency of a model molecule (as represented by an oscillating dipole) in the cavity region of the two models were systematically studied. It was found that the TS model tends to support higher local electric field enhancement while the NPoM model can provide a more localized plasmon electric field near the nanoparticle. Calculations with the radiating model molecule indicate that both TS and NPoM can cause significant enhancements to the non-radiative decay rates at the order of 106 in the wavelength range of 500–1000 nm. The TS model shows better performance for the radiative enhancements and the resulting emission quantum yield. These results are not only helpful to improve the understanding of such important nanocavities but also supply a reference for their further applications in different areas. [ABSTRACT FROM AUTHOR]

• A special nanoparticle-fishbone structure system was proposed. • SPP coupler was incorporated into the traditional nanoparticle-on-mirror system. • Ultra-high local electric field enhancement was achieved by plasmon hybridizing. • The impact of SPP on the "hot spot" was studied by SERS experiments. Applying local surface plasmon resonance (LSPR) to a nanoparticle-on-mirror (NPOM) configuration for realizing highly sensitive molecule detection using surface-enhanced Raman scattering (SERS) typically enhances the Raman signal by 106–1010 under limiting conditions. We propose a special nanoparticle-fishbone structure (NPFS) system, which incorporates the surface plasmon polariton (SPP) coupler of the fishbone structure into the NPOM configuration, to achieve ultra-high local electric field enhancement by plasmon hybridizing the LSPR mode of metallic nanoparticles and the SPP mode of the fishbone array structure. During the numerical simulation, the NPFS system exhibited a SERS enhancement factor of 1011, which is two orders of magnitude higher than that of the NPOM system; the SERS experimental results further demonstrated its outstanding performance. These findings indicate that introducing a well-designed SPP coupler on the metallic film can further improve the advantages of various NPOM configurations. The proposed approach can open new horizons for designing nanoscale photonic devices for ultrasensitive molecule detection. [ABSTRACT FROM AUTHOR]

Most broadband metamaterial absorbers are realized by patterning periodic arrays of plasmonic nanoparticles (>100 nm) on dielectric/metallic substrates to enable both electric and magnetic resonances. These metamaterials, however, require costly nanolithographic top-down techniques for fabrication. Here, we demonstrate new-concept nanoparticle-on-mirror (NoM) metamaterial absorbers by densely packing plasmonic nanoparticles of much smaller size (∼30 nm) on metal films directly. Such a simple but rational design enables the use of all-solution-based bottom-up processes. Because of the decoupling of electric and magnetic polarizations in these ultrasmall nanoparticles, excellent impedance match and near-perfect light absorption can be achieved in a broad band over the solar spectrum with weak thermal emission. Proof-of-concept large-area NoM metamaterial absorbers that offer a solar absorptance of 94% but a low IR emittance of 2% are experimentally demonstrated. The outstanding performance, bottom-up process, and great compatibility render the design promising for efficient and large-scale solar energy harvesting.

[Display omitted] • Nanoparticle-on-mirror (NPoM) structure - robust and cost-efficient Raman substrate. • Fabricated by spraying Au nanoparticle solution directly on the aluminum foil. • Substrate can detect Rhodamine 6G without any post treatment up to 1 nM, (~480 pptw). • Beneficial for routine clinical applications, diagnostics, and chemical detection. We report a robust technique to fabricate a cost-efficient Raman substrate which is composed of polyvinylpyrrolidone (PVP) coated gold nanoparticles layer on commercial aluminum foil. The layer of metal nanoparticles on the aluminum foil, i.e., the nanoparticle-on-mirror (NPoM) structure was fabricated by spraying nanoparticle colloidal solution directly on the foil. The detection limit (LOD) of NPoM substrate is investigated by performing the SERS for Rhodamine 6G (R6G) with the concentration ranging from mM to nM without any post treatment of the substrate. The findings show that the LOD of 1 nM and maximum intensity enhancement factor of ~ 24 is accomplished. Field enhancement owing to reflection from the metallic mirror is the reason behind the signal enhancement and it would be beneficial for routine clinical applications, trace chemical detection, and disease diagnostics. [ABSTRACT FROM AUTHOR]

Hot electron chemistry is of paramount significance because of its applicability to photocatalytic reactions, solar energy conversion, and waste decomposition. The nonradiative decay of excited plasmons in gold nanoparticles (AuNPs) generates highly energetic nonthermal electrons and holes that can induce chemical reactions when transferred to nearby molecules. In this study, we explore the relationship between AuNP size (26-133 nm) and the plasmon-induced reaction yield. To isolate the size from other structural parameters, we prepare perfectly round gold nanospheres (AuNSs) with narrow size distributions. The use of a nanoparticle-on-mirror configuration, in which the reactant molecules (4-mercaptobenzoic acid) are positioned in nanogaps between the AuNSs and a Au film, promotes the generation of hot carriers and allows the highly sensitive detection of the reaction products (benzenethiol) using surface-enhanced Raman spectroscopy. We show that the reaction yield increases as the AuNS size increases up to 94 nm and then decreases for larger AuNSs. This peculiar Λ-shaped size-dependent reactivity can be explained by considering both the plasmonic absorption efficiency of AuNSs and the decay rate of plasmons via electron-surface scattering. The product of the calculated absorption cross section and the inverse of the AuNS size reproduces our experimental results remarkably well. These findings will contribute to the design of highly efficient plasmonic photocatalysts and photovoltaic devices.

Hot-electron chemistry at gold nanoparticle (AuNP) surfaces has received much attention recently because its understanding provides a basis for plasmonic photocatalysis and photovoltaics. Nonradiative decay of excited surface plasmons produces energetic hot charge carriers that transfer to adsorbate molecules and induce chemical reactions. Such plasmon-driven reactions, however, have been limited to a few systems, notably the dimerization of 4-aminobenzenethiol to 4,4'-dimercaptoazobenzene. In this work, we explore a new class of plasmon-driven reactions associated with a unimolecular bond cleavage process. We unveil the mechanism of the decarboxylation reaction of 4-mercaptobenzoic acid and extend the mechanism to account for the β-cleavage reaction of 4-mercaptobenzyl alcohol. Combining the construction of well-controlled nanogap systems and sensitive Raman spectroscopy with methodical changes of experimental conditions (laser wavelengths, interface materials, pH, ambient gases, etc.), we track the hot charge carriers from the formation to the transfer to reactants, which provides insights into how plasmon excitation eventually leads to the C-C bond cleavage of the molecules in the nanogap.