A plot of the (A) real, ε_r, and (B) imaginary, ε_i, components of the dielectric function of Ag, Au, and Si as a function of wavelength.

The polyol method for synthesizing Ag nanostructures. In (A) the reduction of Ag⁺ ions by ethylene glycol (EG) leads to the formation of nuclei which are highly volatile. As the nuclei grow, fluctuations cease and their structure becomes static and contains multiply twinned boundary defects, single twinned boundary defects, or single crystalline with no boundary defects. These seeds are then grown into different nanostructures like (B) spheres, (C) cubes, (D) truncated cubes, (E) right bipryamids, (F) bars, (G) spheroids, (H) triangular plates, (I) and wires. The red lines in the illustration refer to a twin plane and the dark faces are {100} and the light faces are {111}. (A, C–G, H, I) Reproduced with permission from [16]. Copyright 2007 ACS. (B) Reproduced with permission from [95]. Copyright 2009 ACS.

Lithographic nanofabrication for large-scale arrays of nanostructures. (A) Nanosphere lithography uses colloidal nanospheres crystallized on a substrate as a mask for metal deposition (left). After depositing metal to desired height, sonicating the substrate removes the nanospheres, leaving triangular islands in a regular array shown in the right AFM image. Reproduced with permission from [168]. Copyright 2001 ACS. (B) Electron beam lithography exposes a negative resist according to a predetermined pattern, here dots, triangles and squares (from left to right). After developing the resist, metal is deposited onto the substrate, and the remaining resist is lifted off, leaving arrays of metal nanoparticles. Reproduced with permission from [171]. Copyright 2008 Elsevier. (C) An array of counterfacing crescents of Ag and Si made with nanoskiving techniques. Reproduced with permission from [177]. Copyright 2010 ACS.

The effect of a glass substrate on a nearby Ag nanocube. (A) The calculated scattering spectra of a Ag nanocube with edge length 90 nm as it approached a glass substrate. The single peak associated with the dipolar LSPR (black line) begins to split as it approaches and touches the substrate (red line). The resulting peaks correspond to plasmon modes away from the substrate (1) and adjacent to the substrate (2). (B, C) The finite difference domain (FDTD) simulations shows the plasmon modes of a Ag nanocube (with edge length of 90 nm) associated with the spectra in (A). The white line in the simulation represent the glass substrate. Reproduced with permission from [200]. Copyright 2005 ACS.

(A–D) TEM images of Ag nanocubes with edge lengths of 36, 58, 99 and 172 nm respectively. In (E) the LSPR of the nanocubes red shifts as the edge length of the nanocubes increases. Reproduced with permission from [97]. Copyright 2010 ACS.

(A–D) TEM images of Ag triangular nanoplates with decreasing corner sharpness. The sharp cornered triangular plates in (A) were rounded until the particles had a circular disk shape (D). (E) The decreasing corner sharpness is correlated with a blue shift in the extinction spectra of the nanoparticles in solution.

Effects of crystallinity on plasmon propagation in Ag nanowires. Images of (A) chemically prepared, and (B) lithographically prepared Ag nanowires are shown for comparison. Both wires were 3.3 µm long and had a diameter of 90 nm. Light was focused on one end of the nanowires to excite PSP modes which scattered into space from the opposite end of the nanowire. (C) Spectra of scattered light from the ends of the two types of nanowires. The scattering spectrum from the polycrystalline nanowire (bottom trace) shows no regular signal modulation indicating weak plasmon modes on the nanowire. In contrast the single crystalline nanowire shows very strong modulation. Reproduced with permission from [244]. Copyright 2005 APS.

Effects of plasmonic coupling between two closely spaced nanostructures. (A) An energy level diagram illustrating the bonding and anitbonding plasmon hybridization modes for the dipole plasmon of nanospheres with a 2 nm (blue dotted line) and 10 nm (red solid) gap width (g). Spectra are also shown to demonstrate show the LSPR will shift upon hybridization and its sensitivity to gap width. In (B) the local field enhancements of a metal bow-tie structure are mapped using FDTD simulations. The polarization of the incident laser is parallel to the x–z plane along the structures interparticle axis. The maximum intensity enhancement factor was calculated to be 2.5 × 10⁴ at the tip of each triangle and was ~100 within 50 nm of the gap. Reproduced with permission from [333]. Copyright 2008 Nature.

(A) The FDTD calculated E-field enhancements (|E|²) of a Au nanowire on a Au film and Si substrate, respectively. The near field intensities are larger for the nanowires on the metallic film. (B) SERS measurements of the molecule benzenethiol (BT) from a single Au nanowire on a Au film (blue line) and on a Si substrate (green line). Reproduced with permission from [47]. Copyright 2008 ACS.

(A) A photograph of a mouse with a tumor that has been labeled with SERS tags for non-invasive tumor detection with SERS. A laser is focused onto the tumor site and the SERS signal from the tumor is recorded. (B) The SERS signal from the tumor and liver of the mouse. The SERS signal from the tumor is representative of the SERS tag used to target the tumor, in contrast there is little signal from the liver. Reproduced with permission from [451]. Copyright 2008 Nature. In (C) nanoparticles functionalized with pH sensitive molecules are introduced into cells. The pH can be determined by monitoring the SERS bands (at 1423 cm⁻¹ and 1076 cm⁻¹) from the nanostructures and subendosomal resolution of the pH inside a cell can be achieved in real time. Reproduced with permission from [452]. Copyright 2010 ACS.

Plasmonic waveguides based on Ag nanowires. (A) A nearfield image of a propagating surface plasmon along a Ag nanowire. At the end of the nanowire the plasmon radiates into space. The inset shows a 2 µm section of the scattering intensity. Reproduced with permission from [244]. Copyright 2005 APS. (B) Branched Ag nanowires can be used to route plasmons. By controlling the polarization of light (red arrow), the plasmons can be routed to different nanowire branches as seen in the optical image (top). The same structure is shown in the bottom SEM image, with the inset showing the nanobranch junction. Reproduced with permission from [357]. Copyright 2010 ACS. In (C) a polymer waveguide (white line running diagonally across the image) and Ag nanowires are illuminated with white light (left image). In the right image the same area is shown, but with the waveguide illuminating the nanowires. The three labeled nanowires clearly show nanoscale guiding of light from the center of the waveguide to outside it. Reproduced with permission from [17]. Copyright 2008 Nature. (D) The plasmons from a Ag nanowire detected using a field-effect transistor. In the reflection image (with color overlay indicating electrical current (I)) the violet circle (I₁), indicates the detected signal corresponding to the QD emission into the nanowire. The blue circles correspond to detection of PSPs launched by excitation from the laser at the ends of nanowire. The green circle corresponds to a QD away from the Ag nanowire. In the adjacent graph the current from the excitation of the QD (I₁) and the nanowire (I₂) is measured as a function of wavelength. The peak at 655 nm in I₁ (violet line in graph) represents the electrical detection of the emission from the QD. Reproduced with permission from [181]. Copyright 2009 Nature.

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (E_o) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons can be displaced from the lattice of positive ions (consisting of nuclei and core electrons) and collectively oscillate in resonance with the light. This is known as a localized surface plasmon resonance (LSPR). In (B) the nanostructure has one dimension much larger than the wavelength of light. In this case, light coupled to the nanostructure will excite the free electrons to create a propagating surface plasmon (PSP) that can travel along the surface of the metal nanostructure.

The quality factor (Q) of the LSPR for a metal/air interface. A higher Q denotes less damping and a stronger plasmon resonance. The shaded area represents the region of interest to many plasmonic applications. Reproduced with permission from [32]. Copyright 2009 Elsevier.

(A) Schematic diagram of the proposed basic photoconversion mechanism of Ag nanocrystal seeds into triangular nanoplates. Oxidative etching of the seeds produced Ag⁺, which was then reduced on the seeds by citrate photo-oxidation and grown into nanoplates. Using single excitation two different sizes of nanoplates were synthesized as shown in (B). In (C) dual beam excitation produces only one size of nanoplates as the secondary beam could inhibit particle growth. The scale bar is 200 nm for (B, C). Other structures of Ag can also be synthesized via laser excitation including (D) tetrahedrons, scale bar 1 µm, and (E) right bipyramids, scale bar 100 nm. (B, C) Reproduced with permission from [111]. Copyright 2003 Nature. (D) Reproduced with permission from [101]. Copyright 2008 ACS. (E) Reproduced with permission from [112]. Copyright 2009 Wiley.

(A) A typical experimental setup of a photoacoustic (PA) system used to measure the absorption cross section of metal nanoparticles. Excitation from a laser (red beam) is absorbed by a suspension of metal nanoparticles and the resulting PA signal is recorded by a transducer and displayed on an oscilloscope and is correlated to the absorption cross section. The PA signal for three concentrations (0.13, 0.07, and 0.03 nM) of Au-Ag nanocages is shown in the bottom image. The highest signal is correlated with the 0.13 nM concentration of nanocages. Reproduced with permission from [196]. Copyright 2009 ACS. (B) Dark field optical microscopy image showing Rayleigh scattering of individual and aggregated Ag nanocubes (100 nm edge length). (C) Near field image of the spatial distribution of a LSPR mode with energy 1.75 eV supported by a Ag triangular nanoplate. A high near-field intensity is indicated by red and low intensity by blue. Reproduced with permission from [183]. Copyright 2007 Nature.

(A) The extinction (black), scattering (green) and absorption (red) spectra of Ag nanospheres (A) 40 nm and (B) 140 nm in diameter calculated using Mie theory in water. The extinction spectrum for the larger spheres (B) is dominated by scattering and has a broad shoulder due to contributions from the quadropole plasmon modes. The smaller spheres have a narrow LSPR peak and the absorption and scattering cross sections are nearly equal.

A metal nanowire’s ability to guide propagating surface plasmon modes (λ_sp) is dependent on the diameter (d) of the wire. Decreasing d results in a strong increase in localization (indicated by the red area) of the PSP to the nanowire. The dashed line indicates where the PSP field decays to 1/e of its peak value. This results in an increase of energy loss of the plasmon because a larger portion of the wave’s energy is carried in the dissipative metal. Reproduced with permission from [4]. Copyright 2010 Nature.

(A–D) The experimental (black) and calculated (red) extinction spectra of (A) Ag spheres 40-nm in diameter, (B) cubes 40-nm in edge length, (C) octahedrons 40-nm in edge length, (D) and right bipyramids 75-nm in edge length. The experimental data were acquired from aqueous solution measurements and the calculated data were derived using Mie theory (spheres) and the DDA method (other shapes), respectively. (A, B, D) Modified with permission from [233]. Copyright 2006 ACS. (C) Modified with permission from [85]. Copyright 2010 ACS.

(A) SEM image of the sacrificial templates, Ag nanocubes, and (B–D) SEM images of the Ag-Au nanoboxes and nanocages obtained from sequential stages of a galvanic replacement reaction. Insets of (B) and (C) are microtomed TEM samples showing the hollow interior, and the insets of (A) is an electron diffraction pattern. The 100 nm scale bar applies to all SEM images. Reproduced with permission from [278]. Copyright 2004 ACS. (E, top panel) vials containing Au nanoboxes and nanocages prepared by reacting 5 mL of a 0.2 nM Ag nanocube (edge length ~40 nm) suspension with different volumes of a 0.1 mM HAuCl₄ solution. (E, lower panel) the corresponding UV-vis spectra of Ag nanocubes and Au-Ag nanoboxes/nanocages. Reproduced with permission from [195]. Copyright 2008 Nature.

(A,B) Two Ag dimers with different gap width (left) and their corresponding scattering spectra (right) as a function of energy (eV). The dimers were excited with light polarized along the interparticle axis (red) and perpendicular to it (green). Reproduced with permission from [378]. Copyright 2010 ACS. (C) Images of the lattice structure of Ag octahedrons made by Langmiur-Blodgett assembly and the scattering intensity of the lattice at different surface pressures (Π). Increasing the surface pressure reduced the interparticle spacing between the icosahedrons in the lattice. At higher surface pressures the strong coupling between nanostructures dominates over coupling across the lattice and new peaks associated with the collective charge polarizations of localized aggregates appear at longer wavelengths. At the highest surface pressures, the spectrum becomes a continuum. Reproduced with permission from [364]. Copyright 2007 ACS.