Scanning angle Raman spectroscopy measurements of thin polymer films for thickness and composition analyses

Scanning angle (SA) Raman spectroscopy was used to measure the thickness and composition of polystyrene films. A sapphire prism was optically coupled to a sapphire substrate on which 6–12% (w/v) polystyrene in toluene was spin coated. Raman spectra were collected as the incident angle of the p-polarized, 785-nm excitation laser was varied from 56 to 70°. These angles span above and below the critical angle for a sapphire/polystyrene interface. The thickness of the polystyrene film was determined using a calibration curve constructed by calculating the integrated optical energy density distribution as a function of incident angle, distance from the prism interface and polymer thickness. The calculations were used to determine the incident angle where waveguide modes are excited within the polymer film, which is the angle with the highest integrated optical energy density. The film thicknesses measured by SA Raman spectroscopy ranged from less than 400 nm to 1.8 μm. The average percent uncertainty in the SA Raman determinations for all films was 4%, and the measurements agreed with those obtained from optical interferometery within the experimental uncertainty for all but two films. For the 1270-nm and 580-nm polystyrene films, the SA Raman measurements overestimated the film thickness by 5 and 18%, respectively. The dependence of the calibration curve on excitation polarization and composition of the polymer and bulk layers was evaluated. This preliminary investigation demonstrates that scanning angle Raman spectroscopy is a versatile method applicable whenever the chemical composition and thickness of interfacial polymer layers needs to be measured.     

Surface-enhanced spectroscopy of Langmuir—Blodgett films of N-octyl,N′-isobutyl-3,4:9,10-perylene-bis(dicarboximide)

The infrared, resonant Raman and surface-enhanced resonant Raman spectra of N-octyl, N′-isobutyl-3,4:9,10-perylene-bis(dicarboximide) (PBDC) were obtained and the aggregation of PBDC molecules was studied using surface-enhanced fluorescence. Langmuir—Blodgett (LB) films of the neat dye as well as mixed layers with arachidic acid, were prepared and transferred to glass slides and surface-enhanced active surfaces for spectroscopic characterization. A strong red shifted excimer emission was characteristic of LB layers of the neat material and of the thin solid films formed by dipping a glass slide into a solution containing the dye and a volatile solvent. However, using mixed LB monolayers of PBDC and arachidic acid, the surface-enhanced fluorescence of the monomer was observed. The surface-enhanced scattering was measured for both neat and mixed LB layers.     

Vibrational spectroscopy of dimethylchlorooctadecylsilane covalently bonded to ultrathin silica films immobilized on Ag surfaces

FTIR and Raman spectroscopies have been used to characterize the structure and conformational order of dimethylchlorooctadecylsilane (DOS) covalently bonded to ultrathin silica films supported on Ag substrates. Ultrathin silica films of ca. 30 A thickness prepared from sol-gel methods are immobilized on Ag surfaces modified with a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (3MPT). This layered structure provides a unique opportunity for acquiring complementary spectral data from both FTIR and Raman spectroscopies, which are useful in elucidating alkylsilane conformation pertaining to stationary phases for reversed-phase liquid chromatography (RPLC). Characterization of octadecyltrichlorosilane (OTS) layers on thin silica films of ca. 800 A thickness on 3MPT-modified Ag surfaces has been reported previously. Differences between the ultrathin silica films used in this study and the thin silica films used in this previous study are considered. The results from both FTIR and Raman spectroscopy presented here suggest that bonded DOS alkyl chains are in a disordered, liquid-like state with close to monolayer surface coverage.     

Surface-enhanced raman scattering on dendrimer/metallic nanoparticle layer-by-layer film substrates

In this paper, the fabrication, characterization, and application of unique layer-by-layer (LBL) films of dendrimers and metallic nanoparticles is reported. Silver nanoparticles (d = approximately 20 nm) are produced in solution by sodium citrate reduction and incorporated into thin films with generation 1 and 5 DAB-Am dendrimers (polypropylenimine dendrimers with amino surface groups) by the LBL technique. The resulting nanocomposite films are characterized by UV-visible surface plasmon absorption and atomic force microscopy (AFM) measurements, and employed as substrates for surface-enhanced Raman scattering (SERS) of 2-naphthalenethiol. Through variation of the molecular size (dendrimer generation) and concentration of the cross-linker used, as well as the number of layers produced, the optical properties of several different possible architectures are studied. In the films, Ag nanoparticles are shown to be effectively immobilized and stabilized with increased control over their spacing and aggregation. Moreover, the films are shown to be excellent substrates for SERS measurements, demonstrating significant enhancement capability. As expected, large electromagnetic enhancement of Raman scattering signals is found to be strongly dependent on interparticle coupling between neighboring metallic nanoparticles. Finally, the possibility of detecting SERS signals from architectures with intervening layers between the metal nanoparticles and analyte molecules is explored. It is shown that although there are decreases in intensity with increasing number of intervening layers (as is expected from the distance dependence of SERS), electromagnetic enhancement is still able to function at these distances, thus offering the possibility of developing sensors with external layers that are chemically selective for specific analytes.     

NIR- and VIS-raman spectroscopy of CVD diamond films

Raman spectroscopy is a widely used method for the analysis of CVD diamond layers, because it enables to distinguish between different carbon phases, such as diamond, graphite, amorphous carbon and nanocrystalline carbon, which are all commonly present in CVD diamond films. A comparison of visible Raman spectroscopy and near-infrared Raman spectroscopy applied to CVD diamond layers on various substrates (Si, SiAlON, Au, Pd, Mo, W and cemented carbides) has been carried out with the result that visible Raman spectroscopy can be preferably used for characterizing the diamond quality, but for the detection of non-diamond carbon phases NIR-Raman spectroscopy exhibits certain advantages. The NIR-Raman spectra of diamond on silicon substrates are interfered by thermal activation of the Si.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

Vibrational spectroscopy of dimethylchlorooctadecylsilane covalently bonded to ultrathin silica films immobilized on Ag surfaces

FTIR and Raman spectroscopies have been used to characterize the structure and conformational order of dimethylchlorooctadecylsilane (DOS) covalently bonded to ultrathin silica films supported on Ag substrates. Ultrathin silica films of ca. 30 Å thickness prepared from sol-gel methods are immobilized on Ag surfaces modified with a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (3MPT). This layered structure provides a unique opportunity for acquiring complementary spectral data from both FTIR and Raman spectroscopies, which are useful in elucidating alkylsilane conformation pertaining to stationary phases for reversed-phase liquid chromatography (RPLC). Characterization of octadecyltrichlorosilane (OTS) layers on thin silica films of ca. 800 Å thickness on 3MPT-modified Ag surfaces has been reported previously. Differences between the ultrathin silica films used in this study and the thin silica films used in this previous study are considered. The results from both FTIR and Raman spectroscopy presented here suggest that bonded DOS alkyl chains are in a disordered, liquid-like state with close to monolayer surface coverage.     

Surface Enhanced Raman Spectroscopy on Carbon Filaments

 A method for analysis of carbon-containing thin films by using surface enhanced Raman spectroscopy (SERS) is described. Thin films of boron nitride or silicon carbide which are deposited on carbon filaments were coated additionally with silver nanoparticles. A very thin plasma polymer film was deposited on the silver particles to give a better long time stability. Using these layers, very intensive carbon band were detected.     

Fabrication, structural characterization, and applications of langmuir and langmuir-blodgett films of a poly(azo)urethane

The synthesis of a poly(azo)urethane by fixing CO(2) in bis-epoxide followed by a polymerization reaction with an azodiamine is presented. Since isocyanate is not used in the process, it is termed "clean method" and the polymers obtained are named "NIPUs" (non-isocyanate polyurethanes). Langmuir films were formed at the air-water interface and were characterized by surface pressure vs mean molecular area per mer unit (Pi-A) isotherms. The Langmuir monolayers were further studied by running stability tests and cycles of compression/expansion (possible hysteresis) and by varying the compression speed of the monolayer formation, the subphase temperature, and the solvents used to prepare the spreading polymer solutions. The Langmuir-Blodgett (LB) technique was used to fabricate ultrathin films of a particular polymer (PAzoU). It is possible to grow homogeneous LB films of up to 15 layers as monitored using UV-vis absorption spectroscopy. Higher number of layers can be deposited when PAzoU is mixed with stearic acid, producing mixed LB films. Fourier transform infrared (FTIR) absorption spectroscopy and Raman scattering showed that the materials do not interact chemically in the mixed LB films. The atomic force microscopy (AFM) and micro-Raman technique (optical microscopy coupled to Raman spectrograph) revealed that mixed LB films present a phase separation distinguishable at micrometer or nanometer scale. Finally, mixed and neat LB films were successfully characterized using impedance spectroscopy at different temperatures, a property that may lead to future application as temperature sensors. Principal component analysis (PCA) was used to correlate the data.     

Preparation of a nanocomposite film from poly(diallydimethyl ammonium chloride) and gold nanoparticles by in-situ electrochemical reduction, and its application to SERS spectroscopy and sensing of ascorbic acid

A nanocomposite film is described that is composed of alternating layers of poly(diallydimethyl ammonium chloride) and gold nanoparticles that interact through electrostatic forces. The films of varying thickness were prepared by the layer-by-layer technique, and Au-NPs were generated by electrochemical reduction of hexachloroauric acid. The composite films were characterized by UV?Cvis spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. Most nanocomposite films exhibit linear, uniform, and regular layer-by-layer growth during the process of formation. The films exhibit unique performance in terms of surface enhanced Raman scattering and electrocatalytic activitiy towards the oxidation of ascorbic acid.

Effect of layer structures of gold nanoparticle films on surface enhanced Raman scattering

Using a method of collecting nanoparticles at a water/hexane interface in a close-packed monolayer film and transferring such films onto a solid substrate, three-dimensional multilayer films of nanoparticles were formed. The packed nanoparticles were gold nanospheres (NS) with a 26 nm diameter or gold nanorods (NR) with a 31 nm diameter and 74 nm length. We investigated variations in the surface enhanced Raman scattering (SERS) intensities from such nanoparticle films as the layer compositions were changed. The films stacked with NR layers generated much higher SERS intensity than those of NS layers. The SERS intensities from both kinds of films increased as the number of layers were increased. However, when the NR layer and NS layer were stacked alternately, SERS intensity varied in a zigzag fashion. It was found that the structure of top layer plays a distinguishable role in generating strong SERS enhancement while the lower layers contribute to SERS with less dependency on structures. Interlayer coupling as well as intralayer coupling was considered in order to explain the observations.     

Photoluminescence quenching effect on porous silicon films for gas sensors application

Porous silicon (PS) films were investigated by Raman, and photoluminescence (PL) spectroscopies using different laser excitations: 488.0, 514.5, 632.8, and 782.0 nm. The analysis of the first-order and second-order Raman spectra have shown that the band gaps of the PS films are indirect as in the bulk c-Si. The Raman phonon and the PL spectra as well as the spectral distribution of the linear polarisation degree (LPD) of PS layers have shown to be dependent on the laser excitation energy. This dependence cannot be explained within the quantum confinement model. A mechanism for the PL emission in PS layers is presented in which the radiative recombination of electron-hole pairs occurs in localised centres (the Si-O-SiR moieties) at the pore/crystallite interface. These quasi-molecular centres are Jahn-Teller active, i.e. the radiative recombination is a phonon-assisted phenomena. The adsorption of gas molecules on the porous silicon surface was studied throughout photoluminescence quenching effect. The adsorption experiments were performed at 10(-6) bar of pressure using gas molecules of organic solvents. In all these cases, the PL intensity was recovered after gas desorption. The PL quenching effect was explained in the sense of electron transfer mechanism (ET).     

Preparation of the hybrid film of poly(allylamine hydrochloride)-functionalized graphene oxide and gold nanoparticle and its application for laser-induced desorption/ionization of small molecules

Hybrid films of gold nanoparticles and graphene oxides (GOs) were prepared by directly growing gold nanoparticles on supported thin layers of GO films on a glass slide. The gold/GO nanohybrid films were thoroughly investigated using various analytical methods, including Raman spectroscopy and atomic force microscopy. The hybrid film was then applied to laser desorption/ionization (LDI) of small molecules, which enabled mass spectrometric analysis of analytes. After a series of detailed mechanistic studies and systematic investigations, we found that the gold/GO hybrid films serve as a successful LDI platform for small-molecule analysis because of the high desorption efficiency of analytes from the hybrid films without inducing significant fragmentation of analytes. We suggest that the underlying GO films may effectively dissipate excess thermal energy generated by laser irradiation of Au to prevent undesirable analyte fragmentation.     

Growth and Characterization of Graphene Layers on Different Kinds of Copper Surfaces

Graphene films were grown by chemical vapor deposition on Cu foil. The obtained samples were characterized by Raman spectroscopy, ellipsometry, X-ray photoelectron spectroscopy and electron back-scatter diffraction. We discuss the time-dependent changes in the samples, estimate the thickness of emerging Cu₂O beneath the graphene and check the orientation-dependent affinity to oxidation of distinct Cu grains, which also governs the manner in which the initial strong Cu-graphene coupling and strain in the graphene lattice is released. Effects of electropolishing on the quality and the Raman response of the grown graphene layers are studied by microtexture polarization analysis. The obtained data are compared with the Raman signal of graphene after transfer on glass substrate revealing the complex interaction of graphene with the Cu substrate.     

High Resistive ZnO／Diamond／Si Films Grown via Metal-organic Chemical Vapour Deposition

Piezoelectric ZnO layers with high resistivity for surface acoustic wave applications were prepared on polycrystalline diamond/Si substrates with(Ill) orientation via metal-organic chemical vapour deposition.The characteristics of the films were optimized through different growth methods. The comparative study of the X-ray diffraction spectra and scanning electron microscopic images showed that the final-prepared ZnO films were dominantly c-axis oriented. Zn and O elements in the final prepared ZnO films were investigated through X-ray photoelectron spectroscopy. According to the statistical results, the n(Zn)/n(O) ratio is near 1. The Raman scattering was also performed in back scattering configuration. E2 mode was observed for the final films, which indicated that the better quality ZnO films had been obtained. The resistivity of the films was also enhanced via the modification of the growth methods.     

The formation of MoS<Subscript>2</Subscript> secondary phase at the Cu—Zn—Sn—S/Mo interface during the sulfurization process of Cu—Zn—Sn precursor films

We report on the influence of the sulfurization conditions on the MoS₂ secondary phase formation in Cu—Zn—Sn—S thin films synthesized by thermal evaporation method of metals and intermetallics that can be used for the formation of absorbing layers of solar cells. The dependence between photoconductivity and the intensity of the base line of MoS₂ was found in Raman spectra, which is described by a curve with characteristic maximum. It was found that the secondary phase formation on the Cu—Zn—Sn—S/Mo boundary and photoconductivity of sulfurized films are strongly dependent on the applied temperature conditions. Specifically, films without MoS₂ phase have a low photoconductivity, whereas a high photoconductivity was observed for the films with significant content of secondary phase.     

The influence of various deposition techniques on the photoelectrochemical properties of the titanium dioxide thin film

Thin sol–gel TiO₂ layers deposited on the conductive ITO glass by means of three various deposition techniques (dip-coating, inkjet printing and spray-coating) were used as photoanode in the three-compartment electrochemical cell. The thin TiO₂ films were treated at 450 °C and after calcination all samples possessed the crystallographic form of anatase. The relationship between surface structure and photo-induced conductivity of the nanostructured layers was investigated. It was found that the used deposition method significantly influenced the structural properties of prepared layers; mainly, the formation of defects and their quantity in the prepared films. The surface properties of the calcined layers were determined by XRD, Raman spectroscopy, SEM, AFM, UV–Vis analyses and by the optical microscopy. The photo-induced properties of nanoparticulate TiO₂/ITO photoanode were studied by electrochemical measurements combined with UV irradiation.     

Surface enhanced Raman scattering from layered assemblies of close-packed gold nanoparticles

A synthetic method of ordering hydrophilic gold nanoparticles into a close-packed two-dimensional array at a hexane-water interface and subsequent transferring of such structure onto a solid substrate is described. By repeating the transfer process, multilayered gold nanoparticle films are formed without need of linker molecules. Their surface enhanced Raman scattering (SERS) efficiencies are compared as a function of the number of layers. It is shown that both the number of layers and the particle size contribute to SERS phenomenon. Judging from the noticeable dependence of SERS efficiency on the nanometer scale architecture, the close-packed nanoparticle formation at an immiscible interface presents a facile route to the preparation of highly active and relatively clean SERS substrates by controlling both the particle size and the film thickness. Among the investigated samples, the gold nanoparticle film assembled with quintuple layers of 30 nm diameter particles showed the maximum SERS efficiency.     

Molecular hydrogen-induced nucleation of hydrogenated silicon nanocrystals at low temperature

Highly crystallized hydrogenated silicon layers were obtained via the treatment of hydrogenated polymorphous silicon films in a molecular hydrogen ambient. This contrasts other postdeposition studies that obtained nanocrystalline silicon films but necessitated either a plasma activation or high-temperature annealing. The structure of the samples was analyzed by Raman spectroscopy to determine the crystallite volume fraction, which was found to increase up to 80% within 1 hour of treatment. Atomic force microscopy (AFM) showed that the roughness of the surfaces was found to increase after the H₂ treatment. Optical transmission and spectroscopic ellipsometry revealed the pronounced porosity of the films characterized by a static refractive index that is below three, which is a low value for hydrogenated silicon films and a void fraction that is around 15% in the bulk of the films. The effect of the hydrogen molecules on the structure of the films was discussed in terms of the compressive stress exerted by the molecules, trapped in structural inhomogeneities, on the amorphous tissue. It is suggested that for this process to take effect, the films need to be porous and that the amorphous network needs to be in a “relaxed” state.     

Temperature study of Raman, FT-IR and photoluminescence spectra of ZnPc thin layers on Si substrate

The temperature study of zinc phthalocyanine (ZnPc) thin layers deposited on (0 0 1) Si substrate using Raman, FT-IR absorption and photoluminescence (PL) methods are reported. The Raman scattering spectra of ZnPc layers were investigated in the spectral range 1250–1650 cm⁻¹ and in the temperature range 100–500 K. The changes of spectral parameters such as the band position, integrated intensity and full width at half maximum (FWHM) of selected Raman modes while heating and cooling processes have been determined. The fast decrease of the frequency and the intensity of these modes observed with the increase of the temperature above 420 K, can be probably caused by the change of crystalline form of ZnPc thin layer. The FT-IR measurements have been performed in the temperature range 98–523 K. Our study allowed us to estimate the orientation of the molecular plane similar to these of CuPc thin films deposited on Si substrate. The Raman spectra have been compared with FT-IR spectra of ZnPc molecules in KBr pellets and thin layers of ZnPc on (0 0 1) Si substrate. The PL spectra of ZnPc layers were measured in the spectral range 350–1200 nm and in the temperature range 13–320 K. With increasing temperature from 13 to 175 K we observed the increase of PL bands at 1.76 and 1.85 eV which disappear reaching temperature above 200 K.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.