Daning Shi

Surface-Enhanced Raman Scattering from Individual Au Nanoparticles on Au Films

We investigated the effect of optical thick metal films on the surface-enhanced Raman scattering (SERS) activity of individual Au nanoparticle (NP) monomers and dimers. The film presence is revealed to be positive for the SERS activity of individual NP monomers, while it is not always positive for the electromagnetic enhancement at hot spots for SERS of the dimer, which is explained well by our numerical simulations. The polarized SERS signals from the NP dimer are elucidated well in terms of the plasmon hybridization of the dimer. SERS contributions both from individual NP surfaces and the junction between the NP and its supporting substrate were discussed as well.

End-to-end and side-by-side assemblies of gold nanorods induced by dithiol poly(ethylene glycol)

The assemblies of gold nanorods (Au NRs) exhibit unique properties distinct from the isolated Au NR. We report an effective and simple method for the end-to-end (E-E) and side-by-side (S-S) assemblies of Au NRs with a molecularly defined nanogap (1–2 nm) only in the presence of dithiol poly(ethylene glycol) (HS-PEG-SH). The assembled methods need neither the pH value adjustments nor the addition of other organic solvent. With increasing amount of dithiol molecules, assembled modes of Au NRs experience an interesting procedure, changing from E-E to S-S orientation. The experimental results indicate that when the concentration of HS-PEG-SH is less than 0.25 μM, electrostatic repulsion of positive-charged CTA+ is stronger than the affinity of the Au-S binding, resulting in the E-E oriented assembly. Otherwise, the S-S oriented mode is predominated. The current assembled method will be potentially useful for the optoelectronics and biomedical engineering.

Numerically investigating the enhanced Raman scattering performance of individual Ag nanowire tips

The enhanced Raman scattering (ERS) performance of individual Ag nanowire (NW) tips with the shape of a bulb, a crown, a pencil, and a nanoparticle randomly decorated crown was numerically investigated by the finite element method. Their polarized surface ERS is revealed to be either anisotropic or isotropic, which is affected by the incident light wavelength and polarization and the shape of the tips. In our results, the pencil tip presents strong ERS effects with optimized ERS enhancement and lateral spatial resolution of about 0.3×109 and 1.3 nm, respectively. Effects of the pencil tip geometry (including the shape, angle, length, and size) on its tip ERS behavior are discussed as well. The present work also holds promise for individual Ag NW tip applications in microscope and spectroscope imaging and improvement, etc.

Plasmonic Properties of the End-to-End and Side-by-Side Assembled Au Nanorods

Well-defined assemblies of Au nanorods (NRs) exhibit outstanding optical and electric properties, including surface plasmon resonance (SPR) of an individual nanorod and the coupling SPR among the assemblies. We present a direct approach for end-to-end (E-E) and side-by-side (S-S) assembly of Au NRs using poly (ethylene glycol) dithiol (HS-PEG-SH) and cysteine (Cys), respectively. The coupling SPR properties between the neighboring Au NRs are studied through experimental measurements and finite-different time-domain (FDTD) simulations. The simulated SPR tunability over the assembly agree with that of the experimental results, and both of the longitudinal SPR shifts for E-E and S-S assemblies are well fitted with the exponential function. The present assembled method provides a way for directing anisotropic nanostructures into well-defined orientations, and will be potentially useful in optoelectronics and biomedical engineering.

Plasmon Peak Sensitivity Investigation of Individual Cu and Cu@Cu2O Core–Shell Nanoparticle Sensors

It is crucial to reveal the plasmon peak sensitivity responses of individual Cu nanoparticles, which provide another kind of plasmon sensors besides Au/Ag ones. In this paper, such responses to both the bulk and local refractive index (RI) of individual Cu nanosphere sensors are theoretically investigated by Mie theory. Both of them are revealed to be quadratic. The underlying mechanisms are elucidated well in terms of Rayleigh approximation. The corresponding sensitivity factors are demonstrated to increase with the RI of the nanospheres’ bulk and local surrounding mediums linearly. The plasmon peak sensitivities and sensitivity factors of experimentally encountered Cu@Cu2O core–shell nanoparticles are calculated as well, which reveals that appropriate dielectric encapsulations to Cu nanospheres are favored for their potential plasmonic sensing and detection applications.

Dielectric Nanocup Coating Effect on the Resonant Optical Properties of Individual Au Nanosphere

By finite element method (FEM), dielectric nanocup coating effect on the resonant optical properties of individual Au nanosphere was investigated. It is demonstrated not deleterious to the sensing signals of the nanosphere. The proposed nanocomposite provides an interesting localized surface plasmon resonance (LSPR) sensor with quadratic response, which refractive index (RI) sensitivity is revealed to increase with the RI both of its surrounding and local environment. The differences between the LSPR peak positions of the nanocomposite measured from far-field and near-field spectra are discussed, too. It is believed to shed light on the future applications in surface enhanced Raman spectroscopy, biochemical sensing, and detections.

Synthesis of high-purity silver nanorods with tunable plasmonic properties and sensor behavior

Through anisotropic Ag overgrowth on the surface of Au nanobipyramids (AuNBPs), high-purity and size-controlled Ag nanorods (Au/AgNRs) are obtained by a simplified purification process. The diameters of the Au/AgNRs are determined by the size of the as-prepared AuNBPs, and the lengths of the Au/AgNRs are tunable using different amounts of Ag precursor in the growth solution. Surface-enhanced Raman scattering (SERS) studies using Rhodamine-6G (R6G) as a test molecule indicate that the Au/AgNRs have excellent sensing potential. The tunable optical properties and strong electromagnetic effect of the Au/AgNRs, along with their superior SERS signal enhancement, show that Au/AgNRs are promising for further applications in plasmon sensing and biomolecular detection.

Synthesis and Plasmonic Property of Ag Nanorods

In this paper, Ag nanorods (AgNRs) with different aspect ratios (ARs) were prepared by a seed-mediated fast growth approach. The possible growth mechanism of Ag nanostructures was proposed. With a strong interaction between CTAB and Ag seeds, the reduced Ag atoms agglomerated and attached to the performed Ag nanoparticles, where CTAB molecule layer plays a role of rod-like micelles template, leading to one dimensional growth of Ag atoms into AgNRs. The influences of the reaction conditions were discussed on the yield of AgNRs. The surface plasmon resonance (SPR) of AgNRs was studied by finite-difference time-domain (FDTD) simulations. The simulated results indicate that the transverse surface plasmon resonance (SPRT) has no obvious shifting, whereas the longitudinal surface plasmon resonance (SPRL) shows a significant redshift with the increase of AR of AgNRs, which agrees well with the experimental variation trend. It is also found that the absorption and scattering of AgNRs are stronger than that of Au nanorods (AuNRs), which is in accordance with the Raman signal enhancement by AgNRs compared with that of AuNRs, indicating that AgNR is a promising candidate in bio-molecular detection.

Plasmonic Coupling Effects on the Refractive Index Sensitivities of Plane Au-Nanosphere-Cluster Sensors

Plasmonic coupling effects (between neighboring components) are able to red shift the peak wavelengths of dipolar-localized surface plasmon resonances (LSPRs) and increase the corresponding refractive index sensitivity of nanoparticle sensors. The coupling effects on plane Au-nanosphere-cluster (including nanosphere dimer, trimer, pentamer, and heptamer) sensors are numerically investigated by finite element method (FEM). We found that the coupling does not violate the quadratic response characteristics of LSPR peak wavelengths, hence the linear responses of the sensitivities to the bulk refractive index of Au cluster sensors. Yet, for nanosphere dimer sensors, they contribute to the exponential decrease of sensitivities with their gap distances, which follow the universal plasmon ruler behavior. The amplitude of their fractional sensitivity shift is revealed to be bulk refractive index independent, which is different from that of their fractional LSPR peak wavelength shift. These are analytically explained well in terms of an effective nanoparticle model. The present work also gives an upper sensitivity limit for Au nanosphere dimer systems and provides a method to estimate the interparticle separation between the two component nanospheres of the dimer.

Facile synthesis and heteroepitaxial growth mechanism of Au@Cu core–shell bimetallic nanocubes probed by first-principles studies

Bimetallic Au@Cu core–shell nanocrystals have been synthesized by a two-step seed-mediated growth method through controlled heteroepitaxial growth of Cu shells on Au nanorods serving as seeds. The final crystal structures have cubic morphology with edge lengths from 50 to 100 nm due to a strong affinity of hexadecylamine, which is a selective capping agent for the {100} facets of Cu. Various characterizations reveal that the epitaxial growth mechanism is based on block-type growth mode, rather than layer-by-layer growth mode, owing to the larger mismatch in the different lattice constants between Au core and Cu shell. Moreover, contrastive experiments using Au nanobipyramids as seeds were carried out to gain further insight into the heteroepitaxial growth mechanism. Combining first-principles density-functional theory with experimental results, the binding energies of Cu atoms at different sites on Au surfaces are explored to clarify the aspects of nanostructure formation. This study provides useful insights into the effective synthesis method and the further investigation of the proposed growth mechanism.