proaches based on averaging the optical properties of bulk materials are not applicable to these systems.During the preparation of this paper, a work by Kooij et al. [17] was brought to our attention. In that work, the properties of gold nanoparticle films were studied by spectroscopic ellipsometry, extremely small k values (< 0.15) were found for their nanoparticle films. It is believed that the main reason for the difference between the optical constant reported in that work and those in this paper is that Kooij and co-workers used colloidal gold, and the films studied were of lower coverage compared with those studied here.In summary, thin films of gold nanoparticles were fabricated on a gold substrate by a LBL self-assembly approach using a dithiol as the linking agent. The optical properties of these nanoparticle thin films have been investigated using a variable angle spectroscopic ellipsometer. The refractive indices were obtained from the ellipsometric data by fitting to a Lorentz oscillator dispersion model. The modeled data show good agreement with the experimental results. The refractive indices of nanoparticle films undergo dramatic variation in the range close to the plasmon band. It is suggested that this dramatic variation of refractive index could cause difficulties for studies using conventional ellipsometers at a fixed wavelength and is possibly responsible for the significant discrepancies in refractive indices reported in the literature. Furthermore, it is found that nanoparticle films show unique optical constants different from those of typical metals or insulators. ExperimentalSolutions of gold nanoparticles in toluene were prepared as described previously [4]. Particle sizes measured by TEM were in the range 5 ± 0.6 nm. Gold substrates were prepared by thermal deposition of 150 nm gold layers on to glass microscope slides covered with a 10 nm Cr adhesion layer. The substrates were then cleaned thoroughly with hot piranha solution (30:70 H 2 SO 4 /H 2 O 2 ) for 5 min, then rinsed with water and methanol and dried in a nitrogen stream. The cleaned substrates were then left in a 1 mM 1,9-nonanedithiol toluene solution overnight. The dithiol-functionalized gold substrates were first rinsed with copious amounts of toluene and subsequently ultrasonicated in toluene for 1 min, prior to being immersed in the gold nanoparticle solution for 1 h to form the first nanoparticles layer. Subsequent layers were deposited by repeated alternate immersion in solutions of dithiol and gold nanoparticles, respectively. Rinsing and ultrasonication with toluene were performed between each immersion to remove any nonchemical adsorption.A spectroscopic Beaglehole Picometer ellipsometer was used to study the optical properties of the nanoparticle film and to follow the thickness changes as a function of the number of assembled layers. Refractive indices of the gold substrate were evaluated from a clean gold sample, and were found to be in good agreement with previous reports [14]. For each nanoparticle film, spectr...

To search for sublimable squaraine (SQ) dyes for fabricating thin films under vacuum, we have synthesized a series of 2,4-bis[4-(N,N-dialkylamino)-2,6-dihydroxyphenyl]squaraines [SQ(OH) 4 ] and studied their properties. The investigation of the behavior of their Langmuir films at the air-water interface revealed that SQ(OH) 4 molecules with branched N-alkyl groups such as sec-butyls and isobutyls have larger limiting areas and tend to form J-aggregates in the monolayers, whereas molecules with straight N-alkyl chains have smaller limiting areas and are apt to form H-aggregates. This behavior is attributable to the much larger steric hindrance of the branched N-butyl groups than that of the straight ones. The thermal stability of these dyes was investigated by differential thermal analysis (DTA) and thermogravimetry (TG), and their sublimation ability was evaluated through heating under vacuum. As a result, we verified that the four hydroxyls at the 2′,6′-positions of the two phenyl rings significantly enhanced the thermal stability and the sublimation ability of an anilino SQ dye molecule, and the introduction of branched N-butyls further promoted the sublimation ability of the target SQ(OH) 4 molecules. These phenomena may be attributed to the intramolecular hydrogen bonding between the hydroxyls and the CO groups of the four-membered ring, and the much larger intermolecular repulsive force between branched N-alkyls, respectively. In particular, the SQ(OH) 4 dye with four N-isobutyls could be sublimed without any decomposition. These results suggest that SQ(OH) 4 molecule with branched N-butyls is the most effective structure for realizing a high sublimation ability. Furthermore, pure SQ dye thin films have been successfully fabricated by molecular beam deposition of the SQ(OH) 4 dye with four N-isobutyls. The vacuum-deposited thin films of such SQ dyes have potential applications in various fields such as electrophotography, solar energy conversion, optical recording, and nonlinear optics.

During the process of developing organic molecules that match the wavelengths used in optical communications, we have discovered a new class of dyes, with absorption maxima at 1.1 mum, and clarified their molecular structures. The synthesis of this class of dyes was supposed to involve unexpected intramolecular tandem reactions, and the effect of alkyl substituents on such a synthesis was investigated. As a result, an effective alkyl-substituent structure for realizing a high synthetic yield and good solubility in organic solvents was found. Furthermore, thin films made of such a highly soluble dye were fabricated by solution-coating, and they exhibited a red-shifted absorption band with a maximum at approximately 1.3 mum, indicating the formation of J-like aggregates. The third-order nonlinear optical susceptibility chi(3) of these thin films at 1.3 mum, which is the wavelength used in optical communications, was measured by the Z-scan technique. The magnitude of chi(3) reaches approximately 10-7 esu. These results suggest that the solution-coated thin films of this type of dye are potential materials for optical switching at 1.3 mum and for other nonlinear optical applications.

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