Showing papers on "Raman spectroscopy published in 2002"

Structure-Assigned Optical Spectra of Single-Walled Carbon Nanotubes

Abstract: Spectrofluorimetric measurements on single-walled carbon nanotubes (SWNTs) isolated in aqueous surfactant suspensions have revealed distinct electronic absorption and emission transitions for more than 30 different semiconducting nanotube species. By combining these fluorimetric results with resonance Raman data, each optical transition has been mapped to a specific (n,m) nanotube structure. Optical spectroscopy can thereby be used to rapidly determine the detailed composition of bulk SWNT samples, providing distributions in both tube diameter and chiral angle. The measured transition frequencies differ substantially from simple theoretical predictions. These deviations may reflect combinations of trigonal warping and excitonic effects.

Handbook of vibrational spectroscopy

Abstract: VOLUME 1: THEORY AND INSTRUMENTATION Introduction to the Theory and Practice of Vibrational Spectroscopy Instrumentation for Mid- and Far-infrared Spectroscopy Instrumentation for Near-infrared Spectroscopy Instrumentation for Raman Spectroscopy Time-resolved Spectroscopy Dichroism and Optical Activity in Vibrational Spectroscopy Surface-enhanced Vibrational Spectroscopy Other Instrumental Approaches for Vibrational Spectroscopy Calibration Procedures and Standards for Vibrational Spectroscopy VOLUME 2: SAMPLING TECHNIQUES Mid- and Near-infrared Transmission Spectroscopy Mid-infrared External Reflection Spectroscopy Mid-infrared Internal Reflection Spectroscopy Diffuse Reflection Spectroscopy Other IR Sampling Techniques Raman Spectroscopy Low Temperature and High Pressure Sampling Techniques Microscopy Depth profiling by Vibrational Spectroscopy Optical Conduits for Vibrational Specroscopy Hyphenated Techniques Atmospheric VOLUME 3: SAMPLE CHARACTERIZATION AND SPECTRAL DATA PROCESSING Spectra-Structure Correlations Group Theoretical and Numerical Approaches to the Calculation of Vibrational Spectra Discrimant Analysis Two-dimensional (2D) Analysis Spectral Enhancement and Band Resolution Techniques Quantitative Analysis Anomalies, Atifacts and Common Errors in Using Vibrational Spectroscopy Techniques Glossary VOLUME 4: APPLICATIONS IN INDUSTRY, MATERIALS AND THE PHYSICAL SCIENCES Analysis and Characterization of Polymers and Rubbers Rheo-optical Measurements of Polymers and Rubbers Materials Science Spectoelectrochemistry Process Vibrational Spectroscopy Atmospheric and Astronomical Vibrational Spectroscopy Industrial Applications of Vibrational Spectroscopy Forensic Applications of Vibrational Spectroscopy Catalysis Other Applications of Vibrational Spectroscopy Vibrational Spectroscopy in Education VOLUME 5: APPLICATIONS IN LIFE, PHARMACEUTICAL AND NATURAL SCIENCES Biomedical Applications Biochemical Applications Pharmaceutical Applications Food Science Agricultural Applications Abbreviations and Acronyms, Glossary, List of Contributors and Subject Index

What vibrations tell us about proteins

Abstract: This review deals with current concepts of vibrational spectroscopy for the investigation of protein structure and function. While the focus is on infrared (IR) spectroscopy, some of the general aspects also apply to Raman spectroscopy. Special emphasis is on the amide I vibration of the polypeptide backbone that is used for secondary-structure analysis. Theoretical as well as experimental aspects are covered including transition dipole coupling. Further topics are discussed, namely the absorption of amino-acid side-chains, 1H/2H exchange to study the conformational flexibility and reaction-induced difference spectroscopy for the investigation of reaction mechanisms with a focus on interpretation tools.

Synthesis and characterization of hydroxyapatite crystals: A review study on the analytical methods

Abstract: For the synthesis of hydroxyapatite crystals from aqueous solutions three preparation methods were employed. From the experimental processes and the characterization of the crystals it was concluded that aging and precipitation kinetics are critical for the purity of the product and its crystallographic characteristics. The authentication details are presented along with the results from infrared spectroscopy, X-ray powder diffraction, Raman spectroscopy, transmission and scanning electron photographs, and chemical analysis. Analytical data for several calcium phosphates were collected from the literature, extensively reviewed, and the results were grouped and presented in tables to provide comparison with the data obtained here.

Ultralow-threshold Raman laser using a spherical dielectric microcavity

Abstract: The ability to confine and store optical energy in small volumes has implications in fields ranging from cavity quantum electrodynamics to photonics. Of all cavity geometries, micrometre-sized dielectric spherical resonators are the best in terms of their ability to store energy for long periods of time within small volumes. In the sphere, light orbits near the surface, where long confinement times (high Q) effectively wrap a large interaction distance into a tiny volume. This characteristic makes such resonators uniquely suited for studies of nonlinear coupling of light with matter. Early work recognized these attributes through Raman excitation in microdroplets-but microdroplets have not been used in practical applications. Here we demonstrate a micrometre-scale, nonlinear Raman source that has a highly efficient pump-signal conversion (higher than 35%) and pump thresholds nearly 1,000 times lower than shown before. This represents a route to compact, ultralow-threshold sources for numerous wavelength bands that are usually difficult to access. Equally important, this system can provide a compact and simple building block for studying nonlinear optical effects and the quantum aspects of light.

Surface-enhanced Raman scattering and biophysics

Abstract: Surface-enhanced Raman scattering (SERS) is a spectroscopic technique which combines modern laser spectroscopy with the exciting optical properties of metallic nanostructures, resulting in strongly increased Raman signals when molecules are attached to nanometre-sized gold and silver structures. The effect provides the structural information content of Raman spectroscopy together with ultrasensitive detection limits, allowing Raman spectroscopy of single molecules. Since SERS takes place in the local fields of metallic nanostructures, the lateral resolution of the technique is determined by the confinement of the local fields, which can be two orders of magnitude better than the diffraction limit. Moreover, SERS is an analytical technique, which can give information on surface and interface processes. SERS opens up exciting opportunities in the field of biophysical and biomedical spectroscopy, where it provides ultrasensitive detection and characterization of biophysically/biomedically relevant molecules and processes as well as a vibrational spectroscopy with extremely high spatial resolution. The article briefly introduces the SERS effect and reviews contemporary SERS studies in biophysics/biochemistry and in life sciences. Potential and limitations of the technique are briefly discussed.

Characterization of irradiated starches by using FT-Raman and FTIR spectroscopy.

Abstract: Fourier transform infrared (FTIR) and Fourier transform Raman (FT-Raman) methods were used for rapid characterization and classification of selected irradiated starch samples. Biochemical changes due to irradiation were detected using the two vibrational spectroscopic techniques, and canonical variate analysis (CVA) was applied to the spectral data for discriminating starch samples based on the extent of irradiation. The O−H (3000−3600 cm-1) stretch, C−H (2800−3000 cm-1) stretch, the skeletal mode vibration of the glycosidic linkage (900−950 cm-1) in both Raman and infrared spectra, and the infrared band of water adsorbed in the amorphous parts of starches (1550−1750 cm-1) were employed in classification analysis of irradiated starches. Spectral data related to water adsorbed in the noncrystalline regions of starches provided a better classification of irradiated starches with 5 partial least-squares (PLS) factors in the multivariate model. Keywords: Starch irradiation; FT-Raman spectroscopy; FTIR spectro...

Single-pulse coherently controlled nonlinear Raman spectroscopy and microscopy

Abstract: Molecular vibrations have oscillation periods that reflect the molecular structure, and are hence being used as a spectroscopic fingerprint for detection and identification. At present, all nonlinear spectroscopy schemes use two or more laser beams to measure such vibrations1. The availability of ultrashort (femtosecond) optical pulses with durations shorter than typical molecular vibration periods has enabled the coherent excitation of molecular vibrations using a single pulse2. Here we perform single-pulse vibrational spectroscopy on several molecules in the liquid phase, where both the excitation and the readout processes are performed by the same pulse. The main difficulty with single-pulse spectroscopy is that all vibrational levels with energies within the pulse bandwidth are excited. We achieve high spectral resolution, nearly two orders of magnitude better than the pulse bandwidth, by using quantum coherent control techniques. By appropriately modulating the spectral phase of the pulse we are able to exploit the quantum interference between multiple paths to selectively populate a given vibrational level, and to probe this population using the same pulse. This scheme, using a single broadband laser source, is particularly attractive for nonlinear microscopy applications, as we demonstrate by constructing a coherent anti-Stokes Raman (CARS) microscope operating with a single laser beam.

The Raman effect : a unified treatment of the theory of raman scattering by molecules

Abstract: Preface Part One Linear Raman Spectroscopy 1 Survey of light--scattering phenomena 2 Introduction to theoretical treatments of incoherent light scattering 3 Classical theory of Rayleigh and Raman scattering 4 Quantum mechanical theory of Rayleigh and Raman scattering 5 Vibrational Raman scattering 6 Rotational and vibration--rotation Raman scattering 7 Vibrational resonance Raman scattering 8 Rotational and vibration--rotation resonance Raman scattering 9 Normal and resonance electronic and vibronic Raman scattering 10 Rayleigh and raman scattering by chiral systems Part Two Appendices -- a compendium of mathematical and physical tools Introduction Appendix 1 The right--handed cartesian axis system and related coordinate systems Appendix 2 The summation convention Appendix 3 Direction cosines Appendix 4 Isotropic averages of products of direction cosines Appendix 5 The Euler angles and the rotation operator Appendix 6 Complex numbers and quantities Appendix 7 Some properties of matrices Appendix 8 Vectors I Appendix 9 Vectors II Appendix 10 Tensors Appendix 11 Elescrostatics Appendix 12 Appendix 13 The interaction of a system of electric charges with electric and magnetic fields Appendix 14 The polarizability tensor Appendix 15 Appendix 16 Maxwell equations in vacuum and in media Appendix 17 Monochromatic plane harmonic waves in vacuum and in a non--absorbing linear medium Appendix 18 Appendix 19 Appendix 20 Sources of electromagentic radiation Appendix 21 Polarization of electromagnetic radiation Appendix 22 Appendix 23 Clebsch--Gordan coeeficients and Wigner 3--j and 6--j symbols

Cerium oxide nanoparticles: Size-selective formation and structure analysis

Abstract: Nanoparticles of cerium oxide with a narrow size distribution (±15%) are prepared by mixing cerium nitrate solution with an ammonium reagent. High-resolution transmission electron microscopy (TEM) indicates that over 99% of the synthesized particles are single crystals. TEM and photon absorption are used to monitor particle size. The lattice parameter increases up to 0.45% as the particle size decreases to 6 nm, as observed with x-ray diffraction. Raman spectra also suggest the particle-size effect and concomitant lattice expansion. The lattice expansion can be explained by increased concentrations of point defects with decreasing particle size.

Properties of GaN and related compounds studied by means of Raman scattering

Abstract: In the last decade, we have seen very rapid and significant developments in Raman scattering experiments on GaN and related nitride compounds: the Γ-point phonon frequencies have been identified for both cubic and hexagonal structures of binary compounds of GaN, AlN, and InN. The phonon spectra of their ternary alloys, InGaN and AlGaN, were also intensively studied. On the basis of these studies, characterizations of strain, compositional fluctuation, defects, impurities, etc, are now being intensively conducted. Besides such pure lattice properties, coupled modes between a lattice vibration (LO phonon) and a collective excitation of free carriers (plasmon) in GaN have been thoroughly studied, and the results are now widely applied to characterize carrier-transport properties. Low-dimensional structures of nitrides such as quantum dots and superlattices will soon enter the most active field of Raman scattering characterization. This article briefly reviews the present status of Raman scattering experiments on GaN and related nitride compounds and presents some future prospects.

Raman amplifiers for telecommunications

Abstract: Raman amplifiers are being deployed in almost every new long-haul and ultralong-haul fiber-optic transmission systems, making them one of the first widely commercialized nonlinear optical devices in telecommunications. This paper reviews some of the technical reasons behind the wide-spread acceptance of Raman technology. Distributed Raman amplifiers improve the noise figure and reduce the nonlinear penalty of fiber systems, allowing for longer amplifier spans, higher bit rates, closer channel spacing, and operation near the zero-dispersion wavelength. Lumped or discrete Raman amplifiers are primarily used to increase the capacity of fiber-optic networks, opening up new wavelength windows for wavelength-division multiplexing such as the 1300 nm, 1400 nm, or short-wavelength S-band. As an example, using a cascade of S-band lumped amplifiers, a 20-channel, OC-192 system is shown that propagates over 867 km of standard, single-mode fiber. Raman amplifiers provide a simple single platform for long-haul and ultralong-haul amplifier needs and, therefore, should see a wide range of deployment in the next few years.

Surface-enhanced Raman spectroscopy in single living cells using gold nanoparticles

Abstract: Ultrasensitive Raman measurements in single living cells are possible through exploiting the effect of surface-enhanced Raman scattering (SERS). Colloidal gold particles (60 nm in size) that are deposited inside cells as "SERS-active nanostructures" result in strongly enhanced Raman signals of the native chemical constituents of the cells. Particularly strong field enhancement can be observed when gold colloidal particles form colloidal clusters. The strongly enhanced Raman signals allow Raman measurements of a single cell in the 400-1800 cm-1 range with 1-μm lateral resolution in relatively short collection times (1 second for one mapping point) using 3-5 mW near-infrared excitation. SERS mapping over a cell monolayer with 1-μm lateral resolution shows different Raman spectra at almost all places, reflecting the very inhomogeneous chemical constitution of the cells. Colloidal gold supported Raman spectroscopy in living cells provides a tool for sensitive and structurally selective detection of native chemicals inside a cell, such as DNA and phenylalanine, and for monitoring their intracellular distributions. This might open up exciting opportunities for cell biology and biomedical studies.

Locally enhanced raman spectroscopy with an atomic force microscope

Abstract: An atomic force microscope (AFM) tip is used to selectively produce surface enhanced Raman scattering (SERS) for localized Raman spectroscopy. Spectra of thin films, undetectable with a Raman microprobe spectrometer alone, are readily acquired in contact with a suitably gold-coated AFM tip. Similarly, an AFM tip is used to remove sample layers at the nanometer scale and subsequently serve as a SERS substrate for ultra-trace analysis. This demonstrates the interface of an AFM with a Raman spectrometer that provides increases sensitivity, selectivity and spatial resolution over a conventional Raman microprobe. An AFM guiding the SERS effect has the potential for targeted single molecule spectroscopy.

Raman spectroscopy of molybdenum oxides

Abstract: A special sample was prepared by controlled oxidation of MoO2, which contained MoO2, Mo4O11 and MoO3, in order to extend the knowledge about the resonance Raman effect in reduced molybdenum oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because technical partial oxidation catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 have been identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixture of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the molybdenum oxide phases Mo4O11 and MoO2 in dilution with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced molybdenum oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidation reactions.

Attaching Proteins to Carbon Nanotubes via Diimide-Activated Amidation

Abstract: Carbon nanotubes are functionalized by bovine serum albumin (BSA) proteins via diimide-activated amidation under ambient conditions. The nanotube-BSA conjugates thus obtained are highly water-soluble, forming dark-colored aqueous solutions. Results from characterizations using atomic force microscopy (AFM), thermal gravimetric analysis, Raman, and gel electrophoresis show that the conjugate samples indeed contain both carbon nanotubes and BSA proteins and that the protein species are intimately associated with the nanotubes. Bioactivities of the nanotube-bound proteins are evaluated using the total protein micro-determination assay (the modified Lowry procedure). The results show that the overwhelming majority (∼90%) of the protein species in the nanotube-BSA conjugates remain bioactive.

G-band resonant Raman study of 62 isolated single-wall carbon nanotubes

Abstract: We report G-band resonance Raman spectra of single-wall carbon nanotubes ~SWNTs! at the singlenanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diameters dt ranging between 0.95 nm and 2.62 nm, we have conclusively determined the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diameter dependent (v G 51591 cm 21 ), while the lower-frequency peak is given by v G 5v G 2C/dt , with C being different for metallic and semiconducting SWNTs (CM.CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1 ,I v G /I v G,0.3 for most of the isolated SWNTs (;90%). Unusually high or low Iv G /I v G ratios are observed for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44 interband transition and scattered photons are in resonance with E33 . Since the Eii values depend sensitively on both nanotube diameter and chirality, the ( n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theoretical predictions and experimental observations about these special G-band phenomena gives additional support for the (n,m) assignment from resonance Raman spectroscopy.

Quantum chemical calculation of vibrational spectra of large molecules—Raman and IR spectra for Buckminsterfullerene

Abstract: In this work we demonstrate how different modern quantum chemical methods can be efficiently combined and applied for the calculation of the vibrational modes and spectra of large molecules. We are aiming at harmonic force fields, and infrared as well as Raman intensities within the double harmonic approximation, because consideration of higher order terms is only feasible for small molecules. In particular, density functional methods have evolved to a powerful quantum chemical tool for the determination of the electronic structure of molecules in the last decade. Underlying theoretical concepts for the calculation of intensities are reviewed, emphasizing necessary approximations and formal aspects of the introduced quantities, which are often not explicated in detail in elementary treatments of this topic. It is shown how complex quantum chemistry program packages can be interfaced to new programs in order to calculate IR and Raman spectra. The advantages of numerical differentiation of analytical gradients, dipole moments, and static, as well as dynamic polarizabilities, are pointed out. We carefully investigate the influence of the basis set size on polarizabilities and their spatial derivatives. This leads us to the construction of a hybrid basis set, which is equally well suited for the calculation of vibrational frequencies and Raman intensities. The efficiency is demonstrated for the highly symmetric C(60), for which we present the first all-electron density functional calculation of its Raman spectrum.

Infrared and Raman study of interlayer anions CO32–, NO3–, SO42– and ClO4– in Mg/Al-hydrotalcite

Abstract: The difference in the local environment of CO32−, NO3−, SO42−, and ClO4− in Mg/Al-hydrotalcite compared to the free anions was studied by infrared and Raman spectroscopy. In comparison to free CO32− a shift toward lower wavenumbers was observed. A band around 3000–3200 cm−1 has been attributed to the bridging mode H2O-CO32−. The IR spectrum of CO3− hydrotalcite clearly shows the split ν3 band around 1365 and 1400 cm−1 together with weak ν2 and ν4 modes around 870 and 667 cm−1. The ν1 mode is activated and observed as a weak band around 1012 cm−1. The Raman spectrum shows a strong ν1 band at 1053 cm−1 plus weak ν3 and ν4 modes around 1403 and 695 cm−1. The symmetry of the carbonate anions is lowered from D 3 h to C 2 s resulting in activation of the IR inactive ν1 mode around 1050–1060 cm−1. In addition, the ν3 shows a splitting of 30–60 cm−1. Although NO3-hydrotalcite has incorporated some CO32− the IR shows a strong ν3 mode at 1360 cm−1 with a weak band at 827 cm−1, and the ν4 band is observed at 667 cm−1, although it is largely obscured by the hydrotalcite lattice modes. The Raman spectrum shows a strong ν1 mode at 1044 cm−1 with a weaker ν4 band at 712 cm−1. The ν3 mode at 1355 cm−1 is obscured by a broad band due to the presence of CO32−. The symmetry of NO3− did not change when incorporated in hydrotalcite. The IR spectrum of SO4-hydrotalcite shows a strong ν3 at 1126, ν4 at 614 and a weak ν1 mode at 981 cm−1. The Raman spectrum is characterized by a strong ν1 mode at 982 cm−1 plus medium ν2 and ν4 bands at 453 and 611 cm−1; ν3 cannot be identified as a separate band, although a broad band can be seen around 1134 cm−1. The site symmetry of SO42− is lowered from T d to C 2 v . The distortion of ClO4− in the interlayer of hydrotalcite is reflected in the IR spectrum with both ν3 and ν4 bands split around 1096 and 1145 cm−1 and 626 and 635 cm−1, respectively. A weak ν1 band is observed at 935 cm−1. The Raman spectrum shows a strong ν1 mode at 936 cm−1 plus ν2 and ν4 bands at 461 and 626 cm−1, respectively. A ν3 mode cannot be clearly recognized, but a broad band is visible around 1110 cm−1. These data indicative a lowering of symmetry from T d to C s .

Phonons and lattice dielectric properties of zirconia.

Abstract: We have performed a first-principles study of the structural and vibrational properties of the three low-pressure (cubic, tetragonal, and especially monoclinic) phases of ${\mathrm{ZrO}}_{2},$ with special attention to the computation of the zone-center phonon modes and related dielectric properties. The calculations have been carried out within the local-density approximation using ultrasoft pseudopotentials and a plane-wave basis. The fully relaxed structural parameters are found to be in excellent agreement with experimental data and with previous theoretical work. The total-energy calculations correctly reproduce the energetics of the ${\mathrm{ZrO}}_{2}$ phases, and the calculated zone-center phonon frequencies yield good agreement with the infrared and Raman experimental frequencies in the monoclinic phase. The Born effective charge tensors are computed and, together with the mode eigenvectors, used to decompose the lattice dielectric susceptibility tensor into contributions arising from individual infrared-active phonon modes. This work has been partially motivated by the potential for ${\mathrm{ZrO}}_{2}$ to replace ${\mathrm{SiO}}_{2}$ as the gate-dielectric material in modern integrated-circuit technology.

Preparation and Characterization of Gold Nanoshells Coated with Self-Assembled Monolayers

Abstract: This paper describes the functionalization of the surfaces of gold nanoshells, which consist of silica nanoparticles coated with a continuous thin layer of gold. Previous studies have shown that gold nanoshells exhibit optical properties similar to those of metal colloids (e.g., strong optical absorptions and large third-order nonlinear optical polarizabilities). In contrast to metal colloids, however, the plasmon resonance of the nanoshells can be tuned to specific wavelengths across the visible and infrared range of the electromagnetic spectrum by adjusting the relative size of the dielectric core and the thickness of the gold overlayer. In efforts to develop new strategies for protecting and manipulating these nanoparticles, this paper describes the functionalization of the surfaces of gold nanoshells with self-assembled monolayers derived from the adsorption of a series of alkanethiols. The nanoshells are characterized by transmission electron microscopy, UV−vis spectroscopy, FTIR spectroscopy, Raman ...

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers

Abstract: Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources.

Nitrogen-related local vibrational modes in ZnO:N

Abstract: We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition (CVD) containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy (SIMS), was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm−1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD.

Controlling the optical response of regular arrays of gold particles for surface-enhanced Raman scattering

Abstract: This paper is intended to show how the control of the optical response of regular arrays of gold nanoparticles allows to improve our understanding of surface-enhanced Raman scattering (SERS). Regular particle arrays, designed by electron-beam lithography, exhibit remarkable optical properties and appear to be suitable substrates for the deepened study of the mechanisms at the origin of the SERS effect. Indeed, the resonance of the surface plasmons localized on the particles, which are at the origin of visible to near-infrared extinction spectra and the SERS effect, can be tuned to almost any desirable wavelength by varying the particle shape, size, and spacing, thus optimizing the Raman amplification. The optical extinction spectrum of various gratings was calculated in order to gain insight into their physical sense. The SERS study of trans-1,2-bis (4-pyridyl) ethylene (BPE) adsorbed on these arrays enabled us to determine the enhancement factors G of four bands in the BPE Raman spectrum. The G values thus deduced were found to be of the same order of magnitude as those calculated using a phenomenological relation derived from the electromagnetic theory. Furthermore, a photon scanning tunneling microscope enabled us to acquire near-field optical images of the arrays and to estimate the electric near-field enhancement resulting from plasmon excitation; the Raman enhancement factor thus obtained is of the same order of magnitude as that found from Raman experiments.