Systematic study of antibonding modes in gold nanorod dimers and trimers.

Using on-wire lithography to synthesize well-defined nanorod dimers and trimers, we report a systematic study of the plasmon coupling properties of such materials. By comparing the dimer/trimer structures to discrete nanorods of the same overall length, we demonstrate many similarities between antibonding coupled modes in the dimers/trimers and higher-order resonances in the discrete nanorods. These conclusions are validated with a combination of discrete dipole approximation and finite-difference time-domain calculations and lead to the observation of antibonding modes in symmetric structures by measuring their solution-dispersed extinction spectra. Finally, we probe the effects of asymmetry and gap size on the occurrence of these modes and demonstrate that the delocalized nature of the antibonding modes lead to longer-range coupling compared to the stronger bonding modes.

Tunable and broadband plasmonic absorption via dispersible nanoantennas with sub-10 nm gaps.

Plasmonic nanoparticles have traditionally been associated with relatively narrow absorption profiles. But, for many of the most exciting potential applications for these particles, such as solar energy applications, broadband absorption is desirable. By utilizing on-wire lithography, nanostructures which absorb light through the visible and near-IR portions of the electromagnetic spectrum can be synthesized.

Dispersible surface-enhanced Raman scattering nanosheets.

Ultrathin and flexible silica nanosheets, synthesized with gold nanorod dimers embedded uniformly throughout, can be dispersed in solution and deposited onto arbitrary surfaces. These novel materials conform and maintain the as-synthesized density of dimers, allowing them to be used reliably in labeling and detection applications.

Dispersible gold nanorod dimers with sub-5 nm gaps as local amplifiers for surface-enhanced Raman scattering.

We report the synthesis of solution-dispersible, 35 nm diameter gold nanorod dimers with gaps as small as ∼2 nm for surface-enhanced Raman scattering (SERS). Using on-wire lithography (OWL), we prepared tailorable dimers in high yield and high monodispersity (∼96% dimers) that produce both large and reproducible SERS signals with enhancement factors of (6.8 ± 0.7) × 10(8) for single dimers in air and 1.2 × 10(6) for ensemble-averaged solution measurements. Furthermore, we show that these structures, which are the smallest ever made by OWL, can be used to detect molecules on flat surfaces and in aqueous solutions. When combined, these attributes with respect to sensitivity, reproducibility, and tailorability lead to a novel and powerful local amplification system for SERS applications.

Synthetically programmable nanoparticle superlattices using a hollow three-dimensional spacer approach.

Crystalline nanoparticle arrays and superlattices with well-defined geometries can be synthesized by using appropriate electrostatic, hydrogen-bonding or biological recognition interactions. Although superlattices with many distinct geometries can be produced using these approaches, the library of achievable lattices could be increased by developing a strategy that allows some of the nanoparticles within a binary lattice to be replaced with 'spacer' entities that are constructed to mimic the behaviour of the nanoparticles they replace, even though they do not contain an inorganic core. The inclusion of these spacer entities within a known binary superlattice would effectively delete one set of nanoparticles without affecting the positions of the other set. Here, we show how hollow DNA nanostructures can be used as 'three-dimensional spacers' within nanoparticle superlattices assembled through programmable DNA interactions. We show that this strategy can be used to form superlattices with five distinct symmetries, including one that has never before been observed in any crystalline material.

One-dimensional nanorod arrays: independent control of composition, length, and interparticle spacing with nanometer precision.

We report the synthesis of solution dispersible, one-dimensional metal nanostructure arrays as small as 35 nm in diameter using on-wire lithography, wherein feature thickness and spacing in the arrays is tailorable down to approximately 6 and 1 nm, respectively. Using this unique level of control, we present solution-averaged extinction spectra of 35 nm diameter Au nanorod dimers with varying gap sizes to illustrate the effect of gap size on plasmon coupling between nanorods. Additionally, we demonstrate control over the composition of the arrays with Au, Ni, and Pt segments, representing important advances in controlling the ordering of sub-100 nm nanostructures that are not available with current synthesis or assembly methods.

Correlating nanorod structure with experimentally measured and theoretically predicted surface plasmon resonance.

The extinction spectra of Au nanorods electrochemically synthesized using anodic aluminum oxide templates are reported. Homogeneous suspensions of nanorods with average diameters of 35, 55, 80, and 100 nm and varying lengths were synthesized, and their resultant surface plasmon resonances were probed by experimental and theoretical methods. Experimental extinction spectra of the nanoparticles exhibit good overall agreement with those calculated using the discrete dipole approximation. We determine the dependence of the dipole plasmon wavelength on both rod length and diameter, and we then utilize these results to derive an equation for predicting longitudinal dipole resonance wavelength for nanorod dimensions beyond the quasistatic limit. On average, the equation allows one to predict plasmon resonance maxima within 25 nm of the experimentally measured values. An analysis of factors that are important in determining the plasmon width is also provided. For long rods, the width decreases with increasing length in spite of increased radiative damping due to increased frequency dispersion in the real part of the metal dielectric function.

Silver-based nanodisk codes.

We report a novel method for synthesizing silver-based nanodisk code (NDC) structures using on-wire lithography, where we employ milder synthetic and etching conditions than those used to synthesize the analogous gold structures. The silver structures exhibit stronger surface-enhanced Raman scattering signals than their Au counterparts at 633 and 532 nm excitation and, therefore, lead to lower limits of detection when used in the context of DNA-based detection assays. Finally, use of two enhancing nanostructured materials in one disk code dramatically increases the information storage density for encoding. For example, a disk code consisting of 5 gold disk pairs has 13 unique combinations of enhancing patterns, while one with 5 disk pairs that can be either gold or silver has 98.

On-wire lithography: synthesis, encoding and biological applications.

The next step in the maturing field of nanotechnology is to develop ways to introduce unusual architectural changes to simple building blocks. For nanowires, on-wire lithography (OWL) has emerged as a powerful way of synthesizing a segmented structure and subsequently introducing architectural changes through post-chemical treatment. In the OWL protocol presented here, multisegmented nanowires are grown and a support layer is deposited on one side of each nanostructure. After selective chemical etching of sacrificial segments, structures with gaps as small as 2 nm and disks as thin as 20 nm can be created. These nanostructures are highly tailorable and can be used in electrical transport, Raman enhancement and energy conversion. Such nanostructures can be functionalized with many types of adsorbates, enabling the use of OWL-generated structures as bioactive probes for diagnostic assays and molecular transport junctions. The process takes 13-36 h depending on the type of adsorbate used to functionalize the nanostructures.