Raman spectroscopy of diamond and doped diamond

The optimization of diamond films as valuable engineering materials for a wide variety of applications has required the development of robust methods for their characterization. Of the many methods used, Raman microscopy is perhaps the most valuable because it provides readily distinguishable signatures of each of the different forms of carbon (e.g. diamond, graphite, buckyballs). In addition it is non-destructive, requires little or no specimen preparation, is performed in air and can produce spatially resolved maps of the different forms of carbon within a specimen. This article begins by reviewing the strengths (and some of the pitfalls) of the Raman technique for the analysis of diamond and diamond films and surveys some of the latest developments (for example, surface-enhanced Raman and ultraviolet Raman spectroscopy) which hold the promise of providing a more profound understanding of the outstanding properties of these materials. The remainder of the article is devoted to the uses of Raman spectroscopy in diamond science and technology. Topics covered include using Raman spectroscopy to assess stress, crystalline perfection, phase purity, crystallite size, point defects and doping in diamond and diamond films.     

Raman spectroscopy of fullerenes and fullerene-nanotube composites

The discovery of fullerenes in 1985 opened a completely new field of materials research. Together with the single-wall carbon nanotubes (SWCNTs) discovered later, these curved carbon networks are a playground for pure as well as applied science. We present a review of Raman spectroscopy of fullerenes, SWCNTs and composite materials. Beginning with pristine C(60), we discuss intercalated C(60) compounds and polymerized C(60), as well as higher and endohedral fullerenes. Concerning SWCNTs, we show how the diameter distribution can be obtained from the Raman spectra and how doping modifies the spectra. Finally, the Raman response of C(60) encapsulated into SWCNTs (C(60) peapods) is discussed.     

Raman spectroscopy of graphite

We present a review of the Raman spectra of graphite from an experimental and theoretical point of view. The disorder-induced Raman bands in this material have been a puzzling Raman problem for almost 30 years. Double-resonant Raman scattering explains their origin as well as the excitation-energy dependence, the overtone spectrum and the difference between Stokes and anti-Stokes scattering. We develop the symmetry-imposed selection rules for double-resonant Raman scattering in graphite and point out misassignments in previously published works. An excellent agreement is found between the graphite phonon dispersion from double-resonant Raman scattering and other experimental methods.     

Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond

Raman spectroscopy is a standard characterization technique for any carbon system. Here we review the Raman spectra of amorphous, nanostructured, diamond-like carbon and nanodiamond. We show how to use resonant Raman spectroscopy to determine structure and composition of carbon films with and without nitrogen. The measured spectra change with varying excitation energy. By visible and ultraviolet excitation measurements, the G peak dispersion can be derived and correlated with key parameters, such as density, sp(3) content, elastic constants and chemical composition. We then discuss the assignment of the peaks at 1150 and 1480 cm(-1) often observed in nanodiamond. We review the resonant Raman, isotope substitution and annealing experiments, which lead to the assignment of these peaks to trans-polyacetylene.     

Resonant Raman spectroscopy of nanotubes

Single and double resonances in Raman scattering are introduced and six criteria for the observation and identification of double resonances stated. The experimental situation in carbon nanotubes is reviewed in view of these criteria. The evidence for the D mode and the high-energy mode is found to be overwhelming for a double-resonance process to take place, whereas the nature of the radial breathing-mode Raman process remains undecided at this point. Consequences for the application of Raman scattering to the characterization of nanotubes are discussed.     

High-temperature and pressure Raman spectroscopy of diamond

This study presents the results of the first use of the moissanite anvil cell (MAC) for the in-situ high-temperature and pressure Raman spectroscopy measurement of diamond. It is observed that the T_2g vibrational mode of diamond shifts toward low frequency with increasing temperature; on the other hand, the vibration band shifts toward high frequency with increasing pressure. In the high-temperature and pressure process, the slope (∂υ/∂T) becomes zero which indicates the thermal expansion. It resulted from the temperature effect, and the pressure caused by MAC will reach a balanced state about 1.6 GPa and 175 °C. The behavior of diamonds at high temperature and high pressure is mainly dominated by temperature effects for the temperature below 320 °C and the pressure less than 5 GPa.     

Thickness of diamond-like carbon coatings quantified with Raman spectroscopy

A model is developed for quantifying the thickness of thin coatings and wear scars using Raman spectroscopy. The model, which assumes that both incident and Raman light obey Beer's law, was applied to Raman spectra from a diamond-like carbon (DLC) coating containing Si and O, known as DLN (diamond-like nanocomposite). The coatings ranged in thickness from 10 nm to 2 μm, according to stylus profilometry. Systematic variations in the Raman carbon (G band) and Si (1st order) peak intensities vs. thickness were found. Fits to the model gave an optical mean free path of λ250 nm for DLN. This value is in good agreement with optical absorption coefficient values of other DLC films. Thickness profiles of wear tracks in the coatings determined by the model compared well with depths determined by profilometry.     

Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.     

Dielectric and Raman spectroscopy of MWCVD diamond thin films

The dielectric properties of diamond thin films obtained on silicon substrates by microwave plasma-assisted chemical vapour deposition (MWCVD) have been measured in the frequency range from 0.1 to 10³ kHz at different temperatures up to 150 C. The experimental results have been discussed in terms of the many body theory for dielectric relaxation in solids. Dielectric parameters as well as the d.c. conductivity of the samples have been correlated with the morphology and diamond content in the films, respectively detected from scanning electron microscopy (SEM) and Raman spectroscopy. The calculated activation energies for the dielectric relaxation mechanism agree with those obtained from other measurement techniques used in the electrical characterization of diamond films.     

Determination of nanotubes properties by Raman spectroscopy

The basic concepts and characteristics of Raman spectra from single-wall carbon nanotubes (SWNTs, both isolated and bundled) are presented. The physical properties of the SWNTs are introduced, followed by the conceptual framework and characteristics of their Raman spectra. Each Raman feature, namely the radial breathing mode, the tangential G band, combination modes and disorder-induced bands are discussed, addressing their physical origin, as well as their capability for characterizing SWNT properties.     

Raman spectroscopy of single-wall boron nitride nanotubes

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A1 tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E(2g) mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction.     

Electrodes from diamond and diamond-like materials for electrochemical applications

Generalized results are given of the investigation into the electrochemical behavior of electrodes made from monocrystals of dielectric and semiconducting synthetic diamonds, polycrystalline diamond films, amorphous carbon and hydrogenated amorphous carbon films, as well as of compacts from nano-and microdispersed diamond powders in aqueous solutions. It is shown that the decisive role in the use of these electrodes is played by the value and type of electroconductivity of diamond materials, quality (continuity) of films and their resistance to corrosion. Our findings have shown that the use of the studied materials in electrochemical processes has considerable promise.     

Defect characterization in graphene and carbon nanotubes using Raman spectroscopy

This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic experiments with microscopy studies and from the development of new theoretical models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amounts of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, respectively. Raman spectroscopy is used to study the 'atomic structure' of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be observed by near-field optical measurements on the G' feature for doped single-walled carbon nanotubes.     

Raman spectroscopy as a probe of graphene and carbon nanotubes

Progress in the use of Raman spectroscopy to characterize graphene samples for the number of graphene layers and doping level they contain is briefly reviewed. Comparisons to prior studies on graphites and carbon nanotubes are used for inspiration to define future promising directions for Raman spectroscopy research on few layer graphenes.     

Resonant Raman spectroscopy of individual strained single-wall carbon nanotubes

Resonance Raman spectra of individual strained ultralong single-wall carbon nanotubes (SWNTs) are studied. Torsional and uniaxial strains are introduced by atomic force microscopy manipulation. Torsional strain strongly affects the Raman spectra, inducing a large downshift in the E2 symmetry mode in the G+ band, but a slight upshift for the rest of the G modes and also an upshift in the radial breathing mode (RBM). Whereas uniaxial strain has no effect on the frequency of either the E2 symmetry mode in the G+ band or the RBM, it downshifts the rest of the G modes. The Raman intensity change reflects the effect of these strains on the SWNT electronic band structure.     

Surface-enhanced Raman spectroscopy using gold-coated horizontally aligned carbon nanotubes

Gold-coated horizontally aligned carbon nanotube (Au-HA-CNT) substrates were fabricated for surface-enhanced Raman spectroscopy (SERS). The Au-HA-CNT substrates, which are granular in nature, are easy-to-prepare with large SERS-active area. Enhancement factors (EFs) of ～10(7) were achieved using the Au-HA-CNTs as substrates for rhodamine 6G (R6G) molecules. Maximum enhancement was found when the polarization direction (E-field) of the incident laser beam was parallel to the aligned direction of the HA-CNTs. Simulations using the finite-difference time-domain (FDTD) method were carried out for the granular Au-HA-CNT samples. Enhancement mechanisms and determination of EFs were analyzed. Biological samples, including (13)C- and deuterium (D)-labeled fatty acids and Coccomyxa sp. c-169 microalgae cells, were also measured using this SERS substrate. The limits of detection (LODs) of D- and (13)C-labeled fatty acids on the SERS substrate were measured to be around 10 nM and 20 nM, respectively. Significantly enhanced Raman signals from the microalgae cells were acquired using the SERS substrate.     

Influencing factors for diamond formation from several starting carbons

The possible influencing factors for diamond formation which prevent non-graphitic carbons from transforming to diamond in the presence of nickel as solvent-catalyst were pursued. The relative amount of nickel to carbon did not affect the behaviour of each starting carbon on diamond formation. The existence of a graphitic structure in the starting carbon was not the major prerequisite for diamond formation. Adsorbed gases on the starting carbon and atmospheric gases in the high pressure cell were found to be the most important influencing factors for diamond formation. Hydrogen and chemical species containing hydrogen atoms were the most harmful.     

Chirality changes in carbon nanotubes studied with near-field Raman spectroscopy

We report on the direct visualization of chirality changes in carbon nanotubes by mapping local changes in resonant RBM phonon frequencies with an optical resolution of 40 nm using near-field Raman spectroscopy. We observe the transition from semiconducting-to-metal and metal-to-metal chiralities at the single nanotube level. Our experimental findings, based on detecting changes in resonant RBM frequencies, are complemented by measuring changes in the G-band frequency and line shape. In addition, we observe increased Raman scattering due to local defects associated with the structural transition. From our results, we determine the spatial extent of the transition region to be Ltrans approximately 40-100 nm.     

Gas phase sorting of fullerenes, polypeptides and carbon nanotubes

We discuss the Stark deflectometry of micro-modulated molecular beams for the enrichment of biomolecular isomers as well as single-wall carbon nanotubes, and we demonstrate the working principle of this idea with fullerenes. The sorting is based on the species-dependent susceptibility-to-mass ratio χ/m. The device is compatible with a high molecular throughput, and the spatial micro-modulation of the beam permits one to obtain a fine spatial resolution and a high sorting sensitivity.     

Defect structure of multiwalled carbon nanotubes studied by Raman spectroscopy

The defect structure of multiwalled carbon nanotubes has been studied by transmission electron microscopy and Raman spectroscopy, with particular attention to the shape and intensity of the defect band D and its overtone D*. Electron-microscopic results demonstrate that multiwalled nanotubes typically have multiple bends. The associated short- and long-range disorder influences the Raman spectrum of the nanotubes. The presence of several defect species, differing in scattering probability, results in a stochastic relationship between the intensities of the D*- and D-bands. This relationship is qualitatively interpreted in terms of general mechanisms of elastic/inelastic interactions of π-electrons with phonons and defects.