Near-field optical imaging of enhanced electric fields and plasmon waves in metal nanostructures

In this article, studies on noble metal nanostructures using near-field optical microscopic imaging are reviewed. We show that near-field transmission imaging and near-field two-photon excitation imaging provide valuable methods for investigation of plasmon resonances in metal nanostructures. The eigenfunctions of plasmon modes in metal nanoparticles are directly visualized using these methods. For metal nanowire systems, wavevectors of the longitudinal plasmon modes can be estimated directly from the wave-function images, and the dispersion relations are plotted and analyzed. Using ultrafast transient near-field imaging, we show that the deformation of the plasmon wave function takes place after photoexcitation of a gold nanorod. Such methods of plasmon-wave imaging may provide a unique basic tool for designing plasmon-mode-based nano-optical devices. We also demonstrate that the near-field two-photon excitation probability images reflect localized electric-field enhancements in metal nanostructures. We apply this method to gold nanosphere assemblies and clearly visualize the local enhanced optical fields in the interstitial sites between particles (hot spots). We also show the contribution of hot spots to surface enhanced Raman scattering. The methodology described here may provide valuable basic information about the characteristic enhanced optical fields in metal nanostructures as well as on their applications to new innovative research areas beyond the conventional scope of materials.     

Near-field optical imaging of plasmon modes in gold nanorods

We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images.     

Plasmon mode imaging of single gold nanorods

We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The observed PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron-hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as electric field distributions around the nanorod.     

Surface plasmon near-field imaging of very thin microstructured polymer layers

We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes.     

Immobilizing shortened single-walled carbon nanotubes (SWNTs) on gold using a surface condensation method

We propose a surface condensation method for assembling single-walled carbon nanotubes (SWNTs) on gold. The as-prepared long and randomly tangled SWNTs were cut into short pipes by chemical oxidation, allowing the nanotubes to be terminated by carboxyl functionalities. A surface condensation reaction was then performed by immersing an amino self-assembled monolayer (SAM)-modified gold substrate into the dimethylformamide suspension of carboxylic nanotubes with the aid of dicyclohexylcarbodiimide condensation agent. Raman spectroscopy and atomic force microscopy (AFM) results show that a highly aligned assembly of SWNTs has been formed on gold, with the nanotubes standing on the surface stable enough for a long ultrasonication. In combination with the microcontact printing (muCP) technique, we have fabricated patterned nanotube assemblies using this surface condensation method. Moreover, we found that the "giant" carbon nanotubes tend to form bundles on an amino-terminating surface, likely following a nucleation-growth model.     

Setup of a scanning near field infrared microscope (SNIM): imaging of sub-surface nano-structures in gallium-doped silicon

We have realized a scanning near-field infrared microscope in the 3-4 microm wavelength range. As a light source, a tunable high power continuous wave infrared optical parametric oscillator with an output power of up to 2.9 W in the 3-4 microm range has been set up. Using scanning near field infrared microscopy (SNIM) imaging we have been able to obtain a lateral resolution of < or =30 nm at a wavelength of 3.2 microm, which is far below the far-field resolution limit of lambda/2. Using this "chemical nanoscope" we could image a sub-surface structure of implanted gallium ions in a topographically flat silicon wafer giving evidence for a near-field contrast. The observed contrast is explained in terms of the effective infrared reflection as a function of the sub-surface gallium doping concentration. The future use of the setup for nm imaging in the chemically important OH, N-H and C-H stretching vibration is discussed.     

Nonlinear optical detection of proteins based on localized surface plasmons in surface immobilized gold nanospheres

A new nonlinear optical method is presented to detect proteins binding to a gold surface without using fluorescent-dye labeling. After exposure of the protein-binding surface to a gold nanosphere solution, the nanospheres are immobilized above a gold surface with a nanogap supported by the protein. The gold nanospheres immobilized on the gold surface show strong localized surface plasmon (LSP) resonance, and the formation of this structure results in a marked increase in the optical second harmonic (SH) activity of the gold surface arising from a large enhancement of the electric field localized adjacent to the nanospheres on the LSP resonance. The SH image, therefore, gives a high contrast ratio, 7.0:1, of protein-binding spots to control spots. The contrast ratio is much greater than those obtained by linear reflectivity imaging.     

Nanoscale molecular patterns fabricated by using scanning near-field optical lithography

Nanometer-scale patterns have been created in self-assembled monolayers by using a scanning near-field optical microscope coupled to an ultra-violet laser emitting light at a wavelength of 244 nm. Sharp, chemically well-defined features with dimensions as small as 40 nm have been created routinely, and on occasions line widths of 25 nm (lambda/10) have been achieved. Because of the wide range of photochemical methods available for surface derivatization, this approach promises to provide a flexible and versatile route to the generation of molecular and biological nanostructures for a wide range of applications.     

Micrometer and nanometer scale patterning using the photo-fries rearrangement: toward selective execution of molecular transformations with nanoscale spatial resolution

The photolithographic modification of monolayers provides a versatile and powerful means of fabricating functionalized nanostructured surfaces. In this contribution, we present photosensitive thiol-bearing aryl ester groups which are capable of undergoing the so-called photo-Fries rearrangement to yield hydroxyketones. Phenyl 16-mercaptohexadecanoate was prepared by a three-step synthesis. This molecule undergoes a photoisomerization reaction upon illumination with UV light at ca. 250 nm. Subsequently this molecule was applied as a self-assembled monolayer on gold. Following photochemical modification, the adsorbates were selectively derivatized to yield amino-functionalized surfaces using a simple two-step reaction. This reaction was monitored by X-ray photoelectron spectroscopy and contact angle measurements and friction force microscopy. Micrometer-scale patterned surfaces were produced using a contact mask in conjunction with a frequency-doubled argon ion laser (lambda=244 nm). Near-field optical exposure was carried out by coupling the laser to a scanning near-field optical microscope and yielded nanometer-scale resolution. Following derivatization, the resulting structures were analyzed by friction force microscopy. Clear contrast was observed in the friction signal following surface modification.     

The Conformations of Amino Acids on a Gold(111) Surface

The interactions of amino acids with inorganic surfaces are of interest for biologists and biotechnologists alike. However, the structural determinants of peptide–surface interactions have remained elusive, but are important for a structural understanding of the interactions of biomolecules with gold surfaces. Molecular dynamics simulations are a tool to analyze structures of amino acids on surfaces. However, such an approach is challenging due to lacking parameterization for many surfaces and the polarizability of metal surfaces. Herein, we report DFT calculations of amino acid fragments in vacuo and molecular dynamics simulations of the interaction of all amino acids with a gold(111) surface in explicit solvent, using the recently introduced polarizable gold force field GolP. We describe preferred orientations of the amino acids on the metal surface. We find that all amino acids preferably interact with the gold surface at least partially with their backbone, underlining an unfolding propensity of gold surfaces.     

Kinetic and mechanistic investigations of the light induced formation of gold nanoparticles on the surface of TiO2

The kinetics of the formation of gold nanoparticles on the surface of pre-illuminated TiO(2) have been investigated using stopped-flow technique and steady state UV/Vis spectroscopy. Excess electrons were loaded on the employed nanosized titanium dioxide particles by UV-A photolysis in the presence of methanol serving as hole scavenger, stored on them in the absence of oxygen and subsequently used for the reduction of Au(III) ions. The formation of gold nanoparticles with an average diameter of 5 nm was confirmed after mixing of the TiO(2) nanoparticles loaded with electrons with aqueous solution of tetrachloroaureate (HAuCl(4)) by their surface plasmon absorbance band at 530 nm, as well as by XRD and HRTEM measurements. The rate of formation of the gold nanoparticles was found to be a function of the concentration of the gold ions and the concentration of the stored electrons, respectively. The effect of PVA as a stabilizer of the gold nanoclusters was also studied. The observed kinetic behavior suggests that the formation of the gold nanoparticles on the TiO(2) surface is an autocatalytic process comprising of two main steps: 1) Reduction of the gold ions by the stored electrons on TiO(2) forming gold atoms that turn into gold nuclei. 2) Growth of the metal nuclei on the surface of TiO(2) forming the gold particles. Interestingly, at higher TiO(2) electron loading the excess electrons are subsequently transferred to the deposited gold metal particles resulting in "bleaching" of their surface plasmon band. This bleaching in the surface plasmon band is explained by the Fermi level equilibration of the Au/TiO(2) nanocomposites. Finally, the reduction of water resulting in the evolution of molecular hydrogen initiated by the excess electrons that have been transferred to the previously formed gold particles has also been observed. The mechanism of the underlying multistep electron-transfer process has been discussed in detail.     

Near-field diffraction in a two-dimensional V-groove and its role in SERS

Optical field distribution in micro-nano geometries of miniaturized optical devices is often significantly different from that in identical but macroscopic geometries. Plasmon effects and near-field diffraction can enhance the local field intensity, leading to enhanced cross section for light absorption and scattering, which can be utilized in substrate-enhanced spectroscopies for the detection of trace amounts of adsorbed chemicals. A specific problem is an ingenious but only empirically described way to enhance signal intensity in Raman spectroscopy by the use of a substrate patterned with gold coated micron size pyramidal pits. While Raman enhancement on nanostructured substrates is generally attributed to surface plasmons, here the micron size, and thus the sub-wavelength to near-wavelength dimensions suggest that resonant enhancement emanating from optical near-field diffraction might also play a role. To answer this question, light diffraction in a projection of the pyramidal pit: a V-groove, was modelled with a modified Neerhoff-Mur formalism suitable to calculate electromagnetic field distribution in sub-wavelength structures. Under the boundary conditions a perfect conductor screen was assumed, which excludes plasmon effects. The calculations show that interference in the cavity causes a modest resonant increase in local intensity and that near-field diffraction strongly influences the field distribution, which is explained with the electrodynamic edge effect. The magnitude of the resonant electric field on its own cannot account for the experimentally observed Raman enhancement. However, a resonant enhancement of a similar magnitude is expected for the emitted Stokes frequencies. In this case the geometry implements the conditions for the classical electromagnetic Raman enhancement, ~E(4), in a good agreement with experimental results.     

Surface-enhanced Raman scattering of EDOT and PEDOT on silver and gold nanoparticles

Surface-enhanced Raman scattering (SERS) spectra of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and its monomer 3,4-ethylenedioxythiophene (EDOT) on Ag and Au nanoparticles presenting different morphologies and stabilizing agents have been obtained using the excitation radiation at 633 nm. The SERS spectra of the monomer and polymer are strongly dependent both on the metal and capping agent of the substrate. SERS spectra of EDOT on Au nanospheres indicates that adsorption occurs with the thiophene ring perpendicular to the metal surface. In contrast, polymerization takes place on the silver surface of Ag nanospheres. EDOT adsorption on Ag nanoprisms with polyvinylpyrrolidone (PVP) as capping agent occurs similarly to that observed on gold. Surface-enhanced resonance Raman scattering (SERRS) spectra of PEDOT on gold nanostars that present a thick layer of PVP show no chemical interaction of PEDOT with the metal surface; however, when PEDOT is adsorbed on citrate stabilized gold nanospheres, the SERRS spectra suggest that thiophene rings are perpendicular to the surface. Oxidation of PEDOT also is observed on Ag nanospheres. The investigation of the interface between PEDOT and metal surface is crucial for the development in polymer-based optoelectronic devices since this interface plays a crucial role in their stability and performance.     

Organocobalt cluster complexes XXIX. Novel silicon compounds containing the nonacarbonyl tricobaltcarbon substituent

The reaction of bromomethylidynetricobalt nonacarbonyl or, more effectively, of methylidynetricobalt nonacarbonyl with diverse silicon hydrides (R₃SiH, Ph₃SiH, Me₂(EtO)SiH, R_nCl_3-nSiH (n = 02), etc. results in formation of silylmethylidynetricobalt nonacarbonyl complexes. Silicon-functional interconversions such as SiCl → SiOH, SiCl → SiOMe, SiOH → SiF, and SiOH → SiOSiMe₃, have provided still other substituted silylmethylidynetricobalt nonacarbonyl complexes, generally in high yield. The compounds Me(HO)₂SiCCO₃(CO)₉ and (HO)₃SiCCo₃(CO)₉ have been incorporated into methylsilicone polymers by H₂SO₄-induced reactions with cyclo-(Me₂SiO)₃.     

Oxadiazole‐2‐thiol Adsorption on Gold Nanorods: A Joint Theoretical and Experimental Study by Using SERS,XPS, and DFT

The chemisorption of 1,3,4‐oxadiazole‐2‐thiol (ODT) on gold nanorods has been investigated by using surface‐enhanced Raman spectroscopy (SERS) and density functional theory (DFT). Although most of the SERS spectra have remarkable similarity to the normal Raman spectra of the pure analyte, the adsorption of ODT on a gold surface leads to a drastic change in its Raman spectrum and distinct vibrational features are obtained with gold nanorods and spherical nanoparticles. Simulated Raman spectra for hybrid systems that consist of an oxadiazole moiety coordinated to a Au₂₀ gold cluster provided valuable information about the coordination mode and enabled us to assign vibration modes.     

Near-field Raman imaging and electromagnetic field confinement in the self-assembled monolayer array of gold nanoparticles

Spatial distribution of surface enhanced Raman activity is visualized for two-dimensional (2D) nearly close-packed and well-ordered monolayer array of gold nanoparticles by using scanning near-field optical microscope. The 2D arrays exhibit highly nonuniform enhancement in Raman scattering, i.e., the regions along the edge of the 2D array are preferentially enhanced. We demonstrate that the spatial distribution of the localized electric field is also nonuniform and agrees well with that of the Raman enhancement.     

A mild etch for the fabrication of three-dimensional nanostructures in gold

A novel mild etch is reported for the fabrication of three-dimensional structures in gold films when used in conjunction with an alkylthiol resist. This etch consists of a solution of mercaptoethylamine in ethanol to which a small amount of ammonium hydroxide has been added. Using mask-based photolithography, micron-scale features have been created that exhibit good edge definition. For long-chain thiols, there is little tendency for the regions protected by thiols to be eroded on extended exposure to the etch solution. In conjunction with scanning near-field photolithography, the new etch solution enables the fabrication of nanoscale structures with dimensions significantly smaller than the conventional diffraction limit.     

Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.     

Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.     

Tailored nanoporous gold for ultrahigh fluorescence enhancement

We report molecular fluorescence enhancement of free-standing nanoporous gold in which the nanoporosity can be arbitrarily tailored by the combination of dealloying and electroless gold plating. The nanoporous gold fabricated by this facile method possesses unique porous structures with large gold ligaments and very small pores, and exhibits significant improvements in surface enhanced fluorescence as well as structure rigidity. It demonstrates that the confluence effect of improved quantum yield and excitation of fluorophores is responsible for the large fluorescence enhancement due to the near-field enhancement of nanoporous gold, which arises from the strong electromagnetic coupling between neighboring ligaments and the weakening of plasmon damping of the large ligaments because of the small pore size and large ligament size, respectively.