Mengjing Hou

Silver Nanorods Wrapped with Ultrathin Al2O3 Layers Exhibiting Excellent SERS Sensitivity and Outstanding SERS Stability.

Silver nanostructures have been considered as promising substrates for surface-enhanced Raman scattering (SERS) with extremely high sensitivity. The applications, however, are hindered by the facts that their morphology can be easily destroyed due to the low melting points (~100 °C) and their surfaces are readily oxidized/sulfured in air, thus losing the SERS activity. It was found that wrapping Ag nanorods with an ultrathin (~1.5 nm) but dense and amorphous Al2O3 layer by low-temperature atomic layer deposition (ALD) could make the nanorods robust in morphology up to 400 °C, and passivate completely their surfaces to stabilize the SERS activity in air, without decreasing much the SERS sensitivity. This simple strategy holds great potentials to generate highly robust and stable SERS substrates for real applications.

Ag Nanorods Coated with Ultrathin TiO2 Shells as Stable and Recyclable SERS Substrates

TiO2-coated Ag nanorods (Ag@TiO2 NRs) have been fabricated as multifunctional surface-enhanced Raman scattering (SERS) substrates. Uniform TiO2 shells could sufficiently protect the internal Ag NRs against oxidation and sulfuration, thus the temporal stability of SERS substrates was markedly improved. Meanwhile, due to the synergetic effect between crystalline TiO2 and Ag, the nanocomposites could clean themselves via photocatalytic degradation of the adsorbed molecules under ultraviolet irradiation and water dilution, making the SERS substrates renewable. Such Ag@TiO2 NRs were shown to serve as outstanding SERS sensors featuring high sensitivity, superior stability and recyclability.

Hybridized plasmon modes and near-field enhancement of metallic nanoparticle-dimer on a mirror

For the attractive plasmonic structure consisting of metal nanoparticles (NPs) on a mirror, the coexistence of near-field NP-NP and NP-mirror couplings is numerically studied at normal incidence. By mapping their 3D surface charge distributions directly, we have demonstrated two different kinds of mirror-induced bonding dipole plasmon modes and confirmed the bonding hybridizations of the mirror and the NP-dimer which may offer a much stronger near-field enhancement than that of the isolated NP dimers over a broad wavelength range. Further, it is revealed that the huge near-field enhancement of these two modes exhibit different dependence on the NP-NP and NP-mirror hot spots, while both of their near-field resonance wavelengths can be tuned to the blue exponentially by increasing the NP-NP gaps or the NP-mirror separation. Our results here benifit significantly the fundamental understanding and practical applications of metallic NPs on a mirror in plasmonics.

Tunable Lattice Coupling of Multipole Plasmon Modes and Near-Field Enhancement in Closely Spaced Gold Nanorod Arrays

Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.

Universal Near-Field Interference Patterns of Fano Resonances in Two-Dimensional Plasmonic Crystals

AbstractWe have demonstrated directly that the physical origin of Fano resonances in two-dimensional (2D) plasmonic crystals (PCs) is a wave-interference phenomenon. This is achieved by mapping the near-field interference patterns for low and high diffraction orders via three-dimensional finite element method (FEM) calculations. We show the near-field constructive and destructive resonances center at the corresponding Rayleigh anomaly (RA) wavelength of the lattice and lead to greatly enhanced and suppressed local electric fields around the nanoparticle, respectively. These collective resonances are generated by the coupling of localized surface plasmon resonances (LSPRs) with diffraction orders perpendicular or titled to the direction of the incident polarization, but not parallel to that. Further, it is revealed that such near-field interference patterns can apply to arbitrary 2D PCs. In addition, both the near- and far-field Fano-type spectral features are found to be continuously tuned by the lattice constants, which particularly benefit applications of PCs in enhanced sensing and emission. Graphical abstractBoth the constructive and destructive near-field interference patterns of the diffraction order (1, 1) in two-dimensional plasmonic crystals are mapped numerically

Near-field mapping of three-dimensional surface charge poles for hybridized plasmon modes

We describe a new computational approach to mapping three-dimensional (3D) surface charge poles and thus to determine complicated and hybridized plasmon modes in metallic nanostructures via finite element method (FEM) calculations. 3D surface charge distributions at the near-field resonance energies are calculated directly using Gauss’ law. For a nanosphere dimer, we demonstrate that higher-order hybridized plasmon modes can be addressed clearly. As an improvement to conventional mapping approaches, this new approach provides a better understanding of comprehensive physical image of plasmonic systems necessary for fundamental studies and spectroscopy applications.

Pinhole Effect on the Melting Behavior of Ag@Al2O3 SERS Substrates.

High-temperature surface-enhanced Raman scattering (SERS) sensing is significant for practical detections, and pinhole-containing (PC) metal@oxide structures possessing both enhanced thermal stability and superior SERS sensitivity are served as promising SERS sensors at extreme sensing conditions. Through tuning the Al2O3 precursors’ exposure time during atomic layer deposition (ALD), Al2O3 shells with different amount of pinholes were covered over Ag nanorods (Ag NRs). By virtue of these unique PC Ag@Al2O3 nanostructures, herein we provide an excellent platform to investigate the relationship between the pinhole rate of Al2O3 shells and the melting behavior, high-temperature SERS performances of these core-shell nanostructures. Pinhole effect on the melting procedures of PC Ag@Al2O3 substrates was characterized in situ via their reflectivity variations during heating, and the specific melting point was quantitatively estimated. It is found that the melting point of PC Ag@Al2O3 raised along with the decrement of pinhole rate, and substrates with less pinholes exhibited better thermal stability but sacrificed SERS efficiency. This work achieved highly reliable and precise control of the pinholes over Al2O3 shells, offering sensitive SERS substrates with intensified thermal stability and superior SERS performances at extreme sensing conditions.

Compositional Analysis of Ternary and Binary Chemical Mixtures by Surface-Enhanced Raman Scattering at Trace Levels

Surface-enhanced Raman scattering has been proven a powerful means in the fast detection and recognition of chemicals at trace levels, while quantitative analysis especially the compositional analysis of trace chemical mixtures remains a challenge. We report here a “triangle-rule” based on the principal component analysis (PCA) of surface-enhanced Raman scattering spectra, to calculate the composition of individual component of ternary chemical mixtures at trace levels, which can be simplified into the “balance-rule” for binary mixtures. We demonstrated the validity of the triangle-rule and balance-rule in estimating the composition of ternary and binary mixtures of methyl orange, methylene blue, and crystal violet with different molecular structures, and the validity for ternary and binary mixtures of three isomers of monochlorobiphenyl with very similar molecular structures. This idea might be also applicable to mixtures of more components at the trace levels.

Semi-quantitative analysis of multiple chemical mixtures in solution at trace level by surface-enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) technology combines with chemometric method of principal component analysis (PCA) was used to calculate the composition of chemical mixtures in solution. We reported here that there exists composition discrepancy between molecules in solution and molecules adsorbed on Ag@Al2O3 nanorods substrates due to difference in adsorption kinetics of each component. We proposed here a way to calculate the adsorption kinetics factor for each component using a standard sample as the reference, with which one could correct the predictions given by PCA. We demonstrate the validity of this approach in estimating the compositions of mixtures with two, three and four components of 1, 4-Benzenedithiol, 2-Naphthalenethiol, 4-Mercaptobenzoic acid, and 4-Mercaptopyridine molecules, with acceptable errors. Furthermore, a general formula applied to more complex mixtures was proposed to calculate compositions in solution.