Characteristic Surface Plasmon Research Articles

Planar metal plasmon waveguides: frequency-dependent dispersion, propagation, localization, and loss beyond the free electron model

A numerical analysis of surface plasmon dispersion, propagation, and localization on smooth lossy films is presented. Particular attention is given to determining wavelength-dependent behavior of thin Ag slab waveguides embedded in a symmetric $\mathrm{Si}{\mathrm{O}}_{2}$ environment. Rather than considering Ag as a damped free electron gas, the metal is defined by the experimentally determined optical constants of Johnson and Christy and Palik. As in free electron gas models, analytic dispersion results indicate a splitting of plasmon modes---corresponding to symmetric and antisymmetric field distributions---as film thickness is decreased below $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. However, unlike free electron gas models, the surface plasmon wave vector remains finite at resonance with the antisymmetric-field plasmon converging to a pure photon mode for very thin films. In addition, allowed excitation modes are found to exist between the bound and radiative branches of the dispersion curve. The propagation characteristics of all modes are determined, and for thin films (depending upon electric field symmetry), propagation distances range from microns to centimeters in the near infrared. Propagation distances are correlated with both the field decay (skin depth) and energy density distribution in the metal and surrounding dielectric. While the energy density of most long-range surface plasmons exhibits a broad spatial extent with limited confinement in the waveguide, it is found that high-field confinement does not necessarily limit propagation. In fact, enhanced propagation is observed for silver films at ultraviolet wavelengths despite strong field localization in the metal. The surface plasmon characteristics described in this paper provide a numerical springboard for engineering nanoscale metal plasmon waveguides, and the results may provide a new avenue for integrated optoelectronic applications.

Chemical and Dielectric Manipulation of the Plasmonic Band Gap of Metallodielectric Arrays

The propagation characteristics of surface plasmons (SPs) on metallodielectric 1-D arrays are studied as a function of dielectric environment. Metallodielectric gratings are immersed in solvent media and, alternatively, functionalized with alkanethiol self-assembled monolayers (SAMs). While the two approaches induce shifts of the SP to lower energies and reduce the group velocity, the plasmonic band gap decreases with increasing refractive index but increases with SAM functionalization. This is the first reported study of chemically controlled manipulation of a plasmonic band gap.

Reflection electron energy loss spectrum of surface plasmon excitation of Ag: A Monte Carlo study

A Monte Carlo simulation method has been used to calculate the reflection electron energy loss spectrum for Ag. The differential inelastic cross-section is obtained from an inhomogeneous electron self-energy in the surface region. A comparison of the calculation result with an experiment shows a remarkable agreement on the spectrum shape. The simulation has clearly shown the component to the surface plasmon excitation due to individual scattering processes along trajectory part, i.e., the loss in vacuum before reflection, the loss in vacuum after reflection and loss in metal events. It is found that the bulk plasmon feature is overwhelmed by the intense surface plasmon feature in the measured spectrum peak at about 3.7 eV. However, 7.5 eV peak contains both surface- and bulk-excitation character.

Assembling Gold Nanoparticles as Nanostructured Films Using an Electrophoretic Approach

A three-dimensional array of gold nanoparticles has been assembled on a nanostructured TiO2 template by subjecting the colloidal gold suspension to a dc electric field (50−500 V). By controlling the concentration of gold colloids in toluene and the applied voltage, it is possible to control the thickness of nanostructured gold film without inducing aggregation effects. As indicated by Atomic Force Microscopy, these gold films are highly porous and consist of nanoparticle assembly of fairly uniform size. The surface plasmon characteristics observed in these nanostructured gold films suggest that these particles retain the identity of individual nanoparticles.

Improving the Photoelectrochemical Performance of Nanostructured TiO2 Films by Adsorption of Gold Nanoparticles

TiO2 films cast on conducting glass plates were modified by adsorbing gold nanoparticles (5 nm diameter) from a toluene solution. Selective formation of Au islands and larger particles on the TiO2 surface could be seen from transmission electron micrographs and AFM images. Although the adsorbed Au particles are larger in diameter (50−70 nm), they retain surface plasmon characteristics, similar to those observed for smaller gold nanoparticles in solutions. A 3-fold enhancement in photocurrent generation has been achieved upon modification of TiO2 films with Au nanoparticles. This improved photoelectrochemical performance is explained on the basis of improved interfacial charge-transfer kinetics of the TiO2/Au composite film. Such semiconductor−metal composite films have potential applications in improving the performance of photoelectrochemical solar cells.

PH Dependent changes in the optical properties of carboxylic acid derivatized silver colloidal particles

The optical properties of silver colloidal particles derivatized using an aromatic bifunctional molecule, 4-carboxythiophenol, are presented in this communication. The capping molecule forms a thiolate bond with the silver colloidal particle yielding a carboxylic acid terminal functionality which may then be charged to varying degrees by controlling the silver hydrosol pH. A progressive red shift together with a damping and broadening of the surface plasmon feature ( λ max) of the silver particles is observed as the pH is reduced from 10 to 3. Ag colloidal particles with high surface coverage of the bifunctional molecule showed negligible flocculation with time at high solution pH indicating good stabilization due to coulombic repulsive interactions. At low pH, considerable flocculation was observed even for high surface coverage due to lack of coulombic stabilization. A tentative explanation is put forward to explain changes in the optical properties of the colloidal particles due to variation in the pH dependent surface charge of the particles.

Formation of Alkali Metal Layers at the Surface of Electron-Irradiated Alkali Halides

AbstractScanning Auger microscopy (SAM) and reflected electron energy loss microscopy (REELM) have been used to study the chemical modifications induced by 10-keV electron bombardment at the surface of the alkali halides: LiF, NaF, NaCl and KI. Auger spectra show that the alkali-to-halogen surface ratio increases with irradiation time, although they provide no conclusive evidence as to the chemical nature of the damage layer. Conversely, REELM analysis clearly reveals the formation of alkali metal through the occurrence of characteristic bulk plasmon peaks and surface plasmon peaks, which can also be used to image the lateral distribution of the damage layer to nearly submicron spatial resolution. Some unexpected signals are observed, deriving from combination of surface plasmon excitations with bulk plasmon excitations, and tentatively interpreted as arising from the inhomogeneous morphology of the alkali metal layer.

Adsorption of Li, Cs, and O on CdTe.

The adsorption of Li and Cs on CdTe(100) single crystals was investigated using different surface sensitive electron spectroscopies. Low-energy electron diffraction shows a number of surface reconstructions of the clean surfaces, influenced by the Cd/Te ratio. Deposition of Li shows a constant sticking probability, while Cs has a variable sticking probability, characteristic of island growth coalescing at a coverage of a monolayer. Li and Cs both form amorphous monoatomic layers. Large work-function changes of the CdTe surface as a function of Li and of Cs coverage were found at the initial stages of adsorption, indicating that Cs and Li atoms are to a large extent ionized on the surface (100) plane. The initial dipole moments for Cs and for Li were calculated with the Helmholtz equation ${\mathrm{\ensuremath{\mu}}}_{\mathrm{C}\mathrm{s},\mathrm{\ensuremath{\theta}}\ensuremath{\rightarrow}0}$\ensuremath{\approxeq}26 D and ${\mathrm{\ensuremath{\mu}}}_{\mathrm{L}\mathrm{i},\mathrm{\ensuremath{\theta}}\ensuremath{\rightarrow}0}$\ensuremath{\approxeq}3.2 D and using Topping plots ${\mathrm{\ensuremath{\mu}}}_{0,\mathrm{C}\mathrm{s}}$=127.2 D and ${\mathrm{\ensuremath{\mu}}}_{0,\mathrm{L}\mathrm{i}}$=4.22 D. Adsorption of oxygen on the cesium-covered surface raises the work function by \ensuremath{\Delta}\ensuremath{\varphi}=0.5\ifmmode\pm\else\textpm\fi{}0.05 eV. Reflection-electron energy-loss spectra of the cesiated CdTe(100) surface at ${\mathit{E}}_{\mathit{p}}$=100 eV, recorded at a low temperature (96 K), show a characteristic Cs two-dimensional surface plasmon, \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{s}}$ at 2.05 eV, which disappears with rising temperature. During Cs growth a loss is observed at 26.6 eV which is assigned to Cs 5s core-level transitions.

SPR-based sensors studied by electron energy loss spectroscopy and attenuated total reflection

Electron energy loss spectroscopy (EELS) and attenuated total reflection (ATR) are employed to excite and study the surface plasmon of thin silver films. The films studied are used in surface plasmon resonance- (SPR-) based sensors, where the surface plasmon is excited using the evanescent wave from a light beam totally internally reflected from the base of a prism (Kretschmann configuration) and the resonance is observed as an attenuation of the reflected light intensity as the angle of incidence of the light in the prism is scanned. Such sensors exploit the high sensitivity of the surface plasmon frequency to the refractive index of layers of adsorbed molecules on the surface. The surface plasmon frequency, however, is also a sensitive function of the silver film preparation conditions which control the preferred crystallographic orientation and surface roughness of the silver. Surface roughness is known to strongly affect the surface plasmon frequency and there is much controversy concerning the influence of crystal orientation on the dispersion characteristics of the surface plasmon. The present work investigates the effect of varying the deposition temperature of the films on the surface plasmon characteristics (measured by EELS and ATR). The results are discussed in terms of the crystallographic orientation (studied by X-ray diffraction (XRD)) and the surface roughness (studied by scanning tunneling microscopy (STM)) of the films.

Surface plasmon immunosensors

AbstractA surface plasmon is a charge density wave propagating along the interface between a metal on dielectric medium. Since the energy is concentrated at the interface, sensors that utilize the surface plasmon are extremely sensitive to both the charges in the dielectric properties and the geometry of the interface.A surface plasmon immunosensor to detect antibody and antigen immunoresponse has been developed. Here the sensitivity of the sensor is theoretically investigated and the experimental results of sensor sensitivity, the real‐time measurements and the problem of repeated utilization of the sensors are evaluated using α‐Feto‐protein (α‐FP) and goat IgG. In addition, the problem of deciding whether to use antibody or antigen as immobilized sensing medium is discussed based on the characteristics of surface plasmon and biomolecules. As a result, it is concluded that with an antigen immobilized sensor, the sensitivity is higher and the real‐time response can be detected.

A technique for performing angle‐resolved electron energy loss spectroscopy: A study of polycrystalline tin

AbstractA method is presented for performing angle‐resolved electron energy loss spectroscopy (ARELS) using a doublepass cylindrical mirror analyzer (CMA) which contains an angle‐resolving movable aperture. Both the electron gun contained in the CMA and an off‐axis electron gun are used in these studies. The technique is illustrated by ARELS spectra taken from a cleaned polycrystalline tin surface because observing the behavior of the surface and bulk plasmon features provides a direct and simple way of assessing surface sensitivity. The spectra demonstrate that the incidence angle of the primary beam, the collection angle with respect to the sample normal and the primary beam energy all have a very large effect on surface senstivity.

A study of the sodium tungsten bronzes by high-resolution electron energy loss spectroscopy

Effective masses of conduction electrons in the sodium tungsten bronzes have been determined from the frequency of surface plasmon features in high-resolution electron energy loss spectra. Good agreement is found with effective masses from optical experiments. The results support the view that there is no major depletion in the carrier concentration at the surface of the bronzes.

Surface valence band and plasmon features on clean cleaved silicon

The L 1L 2,3V Auger transition from vacuum cleaved surfaces of silicon has been studied in detail. Features in the distribution which are not due to plasmon losses are shown to correspond with primary features in the valance band density of states. However, effects of transition probability energy dependence apparently smear the structure, which may also be due in part to particular surface region properties. Bulk plasmon losses up to 9 in number are clearly resolved and the relative intensities compared with simple theory. A mean free path for bulk plasmon emission is deduced which is approximately twice the m.f.p. for other loss processes. Background contamination produces a new peak at approximately 4.5 eV.