The origin of sub-bands in the Raman D-band of graphene

In Raman spectroscopy investigations of defective suspended graphene, splitting in the D band is observed. Four double resonance Raman scattering processes: the outer and inner scattering processes, as well as the scattering processes with electrons first scattered by phonons (“phonon-first”) or by defects (“defect-first”), are found to be responsible for these features of the D band. The D sub-bands associated with the outer and inner processes merge with increasing defect concentration. However a Stokes/anti-Stokes Raman study indicates that the splitting of the D band due to the separate “phonon-first” and “defect-first” processes is valid for suspended graphene. For graphene samples on a SiO₂/Si substrate, the sub-bands of D band merge due to the increased Raman broadening parameter resulting from the substrate doping. Moreover, the merging of the sub-bands shows excitation energy dependence, which can be understood by considering the energy dependent lifetime and/or scattering rate of photo-excited carriers in the Raman scattering process.     

Fano resonance in Raman scattering of graphene

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG. The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG.     

Electronic structure study of ordering and interfacial interaction in graphene/Cu composites

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.     

Thermal Effects on Electron-Phonon Interactions in Silicon Nanostructures

Raman spectroscopy of silicon nanostructures, recorded using an excitation laser power density of 1.0 kW/cm ² , was employed here to reveal the dominance of thermal effects at temperatures higher than room temperature. The room temperature Raman spectrum showed only phonon confinement and Fano effects. Raman spectra recorded at higher temperatures showed an increase in FWHM and a decrease in asymmetry ratio with respect to its room temperature counterpart. Experimental Raman scattering data were analyzed successfully using theoretical Raman line-shapes generated by incorporating the temperature dependence of a phonon dispersion relation. The experimental and theoretical temperature dependent Raman spectra are in good agreement. Although quantum confinement and Fano effects persist, heating effects start dominating at temperatures higher than room temperature.     

Raman and conductivity studies of boron-doped microcrystalline diamond,facetted nanocrystalline diamond and cauliflower diamond films

We present a large amount of data showing how the electrical conductivity and Raman spectra of boron-doped CVD diamond films vary as a function of both B content and film type — in particular, diamond crystallite size. Three types of film have been investigated: microcrystalline diamond (MCD), faceted nanocrystalline diamond (f-NCD) and ‘cauliflower’ diamond (c-NCD). For the same B content (measured by SIMS), the conductance of MCD films was much higher than those for the two types of smaller grained films. Multi-wavelength laser Raman spectroscopy showed that Fano interference effects were much reduced for the smaller grain-sized material. The position of the Lorentzian contribution to the 500 cm^− 1 Raman feature was used to estimate the B content in each type of film, and compared to the value measured using SIMS. We found that the Raman method overestimated the concentration of B by a factor of ~ 5 for the f-NCD and c-NCD films, although it remains reasonably accurate for MCD films. The shortfall may be explained if only a small fraction of the B found in the small-grained films is being incorporated into substitutional sites. We conclude that in diamond films with a high concentration of grain boundaries, the majority of the B (80% in some cases) must be present at sites that do not contribute to the continuum of electronic states that give rise to metallic conductivity and the Fano effects. Such sites may include (a) interstitials, (b) the surface of the crystallites, or (c) bonded within the non-diamond carbon impurities present at the grain boundaries. This suggests that heavy doping of nanograined diamond films will give rise to a material with many different conducting regions, and possibly different conducting pathways and mechanisms.     

Hierarchical Si/ZnO trunk-branch nanostructure for photocurrent enhancement

Hierarchical Si/ZnO trunk-branch nanostructures (NSs) have been synthesized by hot wire assisted chemical vapor deposition method for trunk Si nanowires (NWs) on indium tin oxide (ITO) substrate and followed by the vapor transport condensation (VTC) method for zinc oxide (ZnO) nanorods (NRs) which was laterally grown from each Si nanowires (NWs). A spin coating method has been used for zinc oxide (ZnO) seeding. This method is better compared with other group where they used sputtering method for the same process. The sputtering method only results in the growth of ZnO NRs on top of the Si trunk. Our method shows improvement by having the growth evenly distributed on the lateral sides and caps of the Si trunks, resulting in pine-leave-like NSs. Field emission scanning electron microscope image shows the hierarchical nanostructures resembling the shape of the leaves of pine trees. Single crystalline structure for the ZnO branch grown laterally from the crystalline Si trunk has been identified by using a lattice-resolved transmission electron microscope. A preliminary photoelectrochemical (PEC) cell testing has been setup to characterize the photocurrent of sole array of ZnO NR growth by both hydrothermal-grown (HTG) method and VTC method on ITO substrates. VTC-grown ZnO NRs showed greater photocurrent effect due to its better structural properties. The measured photocurrent was also compared with the array of hierarchical Si/ZnO trunk-branch NSs. The cell with the array of Si/ZnO trunk-branch NSs revealed four-fold magnitude enhancement in photocurrent density compared with the sole array of ZnO NRs obtain from VTC processes.     

A comparison of Raman signatures and laser-induced incandescence with direct numerical simulation of soot growth in non-premixed ethylene/air flames

The predictions of “soot” concentrations from numerical simulations for nitrogen-diluted, ethylene/air flames are compared with laser-induced incandescence and Raman spectra observed from samples thermophoretically extracted using a rapid insertion technique. In some flame regions, the Raman spectra were obscured by intense, radiation that appeared to peak in the near infrared spectral region. There is a good agreement between spatial profiles of this ex situ laser-induced incandescence (ES-LII) and the “traditional” in situ laser-induced incandescence (IS-LII). Raman signatures were observed from low in the flame and extended into the upper flame regions. The spectra consisted of overlapping bands between 1000 and 2000 cm⁻¹ dominated by the “G” band, near ≈1580 cm⁻¹, and the “D” band in the upper 1300 cm⁻¹ range. Several routines are explored to deconvolve the data including 3- and 5-band models, as well as a 2-band Breit–Wigner–Fano (BWF) model. Because the Raman signals were observed at heights below those where in situ LII was observed, we postulate that these signals may be attributable to smaller particles. The results suggest that the observed Raman signals are attributable to particulate with modest (≈1 nm) crystallite sizes. This observation is discussed in the context of current models for nascent particle formation.     

Excitation energy-dependent nature of Raman scattering spectrum in GaInNAs/GaAs quantum well structures

The excitation energy-dependent nature of Raman scattering spectrum, vibration, electronic or both, has been studied using different excitation sources on as-grown and annealed n- and p-type modulation-doped Ga_1 − xIn_xN_yAs_1 − y/GaAs quantum well structures. The samples were grown by molecular beam technique with different N concentrations (y = 0%, 0.9%, 1.2%, 1.7%) at the same In concentration of 32%. Micro-Raman measurements have been carried out using 532 and 758 nm lines of diode lasers, and the 1064 nm line of the Nd-YAG laser has been used for Fourier transform-Raman scattering measurements. Raman scattering measurements with different excitation sources have revealed that the excitation energy is the decisive mechanism on the nature of the Raman scattering spectrum. When the excitation energy is close to the electronic band gap energy of any constituent semiconductor materials in the sample, electronic transition dominates the spectrum, leading to a very broad peak. In the condition that the excitation energy is much higher than the band gap energy, only vibrational modes contribute to the Raman scattering spectrum of the samples. Line shapes of the Raman scattering spectrum with the 785 and 1064 nm lines of lasers have been observed to be very broad peaks, whose absolute peak energy values are in good agreement with the ones obtained from photoluminescence measurements. On the other hand, Raman scattering spectrum with the 532 nm line has exhibited only vibrational modes. As a complementary tool of Raman scattering measurements with the excitation source of 532 nm, which shows weak vibrational transitions, attenuated total reflectance infrared spectroscopy has been also carried out. The results exhibited that the nature of the Raman scattering spectrum is strongly excitation energy-dependent, and with suitable excitation energy, electronic and/or vibrational transitions can be investigated.     

Raman study on zinc-blende single InAs nanowire grown on Si (111) substrate

We report polarized Raman scattering studies on single InAs nanowires (NWs). The NWs were grown by metalorganic chemical vapor deposition on Si (111) substrates without external catalyst and showed a zinc-blende crystal structure. The single NWs were studied for different polarization excitation of the incident laser beam relative to the NW axis. The transverse optical (TO) mode exhibits maximum intensity when both the incident and analyzed light polarizations are parallel to the NW axis. The TO mode of InAs NWs is found to act like a nearly perfect dipole antenna, which can be attributed to the one-dimensional NW geometry and Raman selection rules.     

Noble metal nanocrystals: plasmon electron transfer photochemistry and single-molecule Raman spectroscopy

The excited electronic states of noble metal Au and Ag nanocrystals are very different than those of molecules. Ag and Au nanocrystal optical transitions (plasmons) in the visible can be so intense that they significantly modify the local electromagnetic field. Also, coherent elastic Rayleigh light scattering is stronger than normal electronic absorption of photons for larger nanocrystals. These two facts make Au and Ag nanocrystals ideal nanoantennas, in that they focus incident light into the local neighborhood of subwavelength size. Surface-enhanced Raman scattering (SERS), in which the Raman scattering rate of nearby molecules increases by many orders of magnitude, is a consequence of this nanoantenna effect. Metallic nanocrystals also have no band gap; this makes them extraordinarily polarizable. Their electronic transitions sense the local environment. An extreme case is the interaction of two 30 nm Ag nanocrystals separated by a 1 nm gap. Their mutual polarization completely transforms the nature of the metallic excited electronic state. Single particles have an excited state uniformly distributed throughout the interior, while the nanocrystal dimer has its excited state localized on the metal surface in the junction. This creates an electromagnetic "hot spot" in the junction, enabling the observation of single-molecule SERS. The fact that surface molecules are typically chemisorbed and exchange electrons with the metal has interesting chemical consequences. First, the enhanced Raman intensities are controlled by quantum mechanical coupling of the molecular lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) with the optically excited electrons in the metal. Second, charge-transfer photochemistry can result from metal plasmon excitation. In crystalline Ag nanocrystals the photochemistry quantum yield can be high because the nanocrystal surface dominates plasmon nonradiative relaxation. Colloidal Ag nanocrystals stabilized by sodium citrate build up a photovoltage under visible excitation, caused by irreversible "hot hole" photo-oxidation of adsorbed citrate anion. This creates a driving force for photochemical transformation of round 8 nm Ag seeds into 70 nm single-crystal disk prisms under room lights, in a novel type of light-driven Ostwald ripening.     

A novel graphene-like titanium carbide MXene/Au–Ag nanoshuttles bifunctional nanosensor for electrochemical and SERS intelligent analysis of ultra-trace carbendazim coupled with machine learning

A new bifunctional intelligent nanosensing platform based on graphene-like titanium carbide MXene (Ti₂C MXene)/Au–Ag nanoshuttles (NSs) for both electrochemical and surface-enhanced Raman scattering (SERS) intelligent analysis of ultra-trace carbendazim (CBZ) residues in tea and rice coupled with machine learning (ML) was successfully designed. Ti₂C MXene was synthesized by selectively etching Al layers of Ti₂AlC with hydrofluoric acid and high-temperature calcination. Ti₂C MXene/Au–Ag NSs prepared by the ultrasonic dispersion of graphene-like Ti₂C MXene into Au–Ag NSs solution under dark conditions displayed large and rough surface, enhanced conductivity, excellent electrochemical response, prominent Raman enhancement, and high stability. The ML via different algorithms such as artificial neural network, support vector machine, and relevance vector machine (RVM) for the intelligent analysis of CBZ was contrasted and discussed. RVM displayed more superiority for the electrochemical analysis of CBZ in a wide linear range of 0.006 – 9.8 μM with low limit of detection (LOD) of 0.002 μM and SERS detection of CBZ in the wide linear range of 0.033 – 10 μM with low LOD of 0.01 μM. This will provide a new bifunctional intelligent sensing platform via different ML algorithms for improving accuracy of sensor via mutual verification of two or more methods of detection and a new bifunctional nanosensing platform based on the development of graphene-like nanohybrid for food and agro-products safety.     

Analysis of the Fano in diamond

The Fano appears above a critical percolation threshold corresponding to the onset of the metallic conductivity on the boron impurity band. Its parameters are deduced from the fit of the Raman curves of heavily boron doped homoepitaxial diamond films. The large (10 cm⁻¹) widening and red shift of the phonon originates from its strong coupling with the continuum of electronic transitions. The strong deformation |q|≈2.2 of its Lorentzian shape is ascribed to the Raman cross-sections relatively large for the continuum of electronic interband transitions T_e and small for the phonon T_p whose negative value induces q<0.     

Micro-Raman Mapping of 3C-SiC Thin Films Grown by Solid–Gas Phase Epitaxy on Si (111)

A series of 3C-SiC films have been grown by a novel method of solid–gas phase epitaxy and studied by Raman scattering and scanning electron microscopy (SEM). It is shown that during the epitaxial growth in an atmosphere of CO, 3C-SiC films of high crystalline quality, with a thickness of 20 nm up to few hundreds nanometers can be formed on a (111) Si wafer, with a simultaneous growth of voids in the silicon substrate under the SiC film. The presence of these voids has been confirmed by SEM and micro-Raman line-mapping experiments. A significant enhancement of the Raman signal was observed in SiC films grown above the voids, and the mechanisms responsible for this enhancement are discussed.     

Enhanced photoluminescence of multilayer Ge quantum dots on Si(001) substrates by increased overgrowth temperature

Four-bilayer Ge quantum dots (QDs) with Si spacers were grown on Si(001) substrates by ultrahigh vacuum chemical vapor deposition. In three samples, all Ge QDs were grown at 520 °C, while Si spacers were grown at various temperatures (520 °C, 550 °C, and 580 °C). Enhancement and redshift of room temperature photoluminescence (PL) were observed from the samples in which Si spacers were grown at a higher temperature. The enhancement of PL is explained by higher effective electrons capturing in the larger size Ge QDs. Quantum confinement of the Ge QDs is responsible for the redshift of PL spectra. The Ge QDs’ size and content were investigated by atomic force microscopy and Raman scattering measurements.     

Size-dependent visible absorption and fast photoluminescence decay dynamics from freestanding strained silicon nanocrystals

In this article, we report on the visible absorption, photoluminescence (PL), and fast PL decay dynamics from freestanding Si nanocrystals (NCs) that are anisotropically strained. Direct evidence of strain-induced dislocations is shown from high-resolution transmission electron microscopy images. Si NCs with sizes in the range of approximately 5-40 nm show size-dependent visible absorption in the range of 575-722 nm, while NCs of average size <10 nm exhibit strong PL emission at 580-585 nm. The PL decay shows an exponential decay in the nanosecond time scale. The Raman scattering studies show non-monotonic shift of the TO phonon modes as a function of size because of competing effect of strain and phonon confinement. Our studies rule out the influence of defects in the PL emission, and we propose that owing to the combined effect of strain and quantum confinement, the strained Si NCs exhibit direct band gap-like behavior.     

Interaction mechanism of poly (o-ethoxyaniline) and collagen blends

Blend films of poly (o-ethoxyaniline) (POEA) and collagen were fabricated by casting under optimized conditions and characterized by Raman scattering and UV-vis absorption spectroscopies. The UV-vis spectra showed that the addition of collagen in the aqueous solution of POEA promotes a dedoping of the POEA. This effect was also observed for the blend films as supported by Raman scattering and a mechanism for the chemical interaction between POEA-collagen is proposed. The influences of different percentage of collagen as well as the pH of stock solutions during the fabrication process of the blend films were also investigated. It was found that the preparation method plays an important role in the flexibility and freestanding properties of the films. Complementary, the surface morphology was studied by atomic force microscopy and the conductivity by dc measurements.     

A surface-enhanced Raman spectroscopy study of thin graphene sheets functionalized with gold and silver nanostructures by seed-mediated growth

We describe a simple method for decorating graphene (1–5 layers) with Au and Ag nanostructures (nanoparticles, nanorods, and nanoplates). We deposit graphene electrostatically from highly-oriented pyrolytic graphite onto Si/SiO₂ surfaces functionalized with (aminopropyl)trimethoxysilane and grow the metal nanostructures by a seed-mediated growth method from hexanethiolate-coated Au monolayer-protected cluster “seeds” that are attached to graphene by hydrophobic interactions. Scanning electron microscopy reveals the selective growth of Au or Ag nanostructures on the graphene surface. In the case of Au, the low pH 2.8 growth solution causes etching of the graphene and formation of scroll-like structures. For Ag, the high pH 9.3 solution does not seem to affect the graphene. Raman spectroscopy is consistent with the graphene morphology and reveals that the presence of Au and Ag nanostructures increases the Raman scattering from the graphene by a factor of about 45 and 150, respectively. This work demonstrates a simple method for decorating graphene with noble metal nanostructures that may have interesting optical, electronic, and chemical properties for applications in nanoelectronics, sensing, and catalysis.     

Reconstruction and electronic properties of silicon nanosheets as a function of thickness

We have shown, using density functional theory calculations, that the properties of Si nanosheets change as a function of thickness. While Si(111) oriented nanosheets that are 0.56 nm thick (2-layers) display a novel reconstruction, classified as Si(111)-2 × 2 on both surface layers (T. Morishita, M. J. S. Spencer, S. P. Russo, I. K. Snook and M. Mikami, Chem. Phys. Lett., 2011, 506, 221), nanosheets that are up to a thickness of 1.42 nm show the Si(111)-2 × 1 surface reconstruction, that is seen on the bulk Si(111) surface, on both sides of the nanosheet. For these thicker nanosheets, the relative orientation of the π-chain structure on each surface of the nanosheet can either be the same or different, resulting in unique electronic properties. When the orientation is the same, there is a widening of the band gap, indicating that the interaction between the surface π-chains is not present when they are oriented in different directions. The electronic properties of the nanosheets approach those of the bulk by 1.42 nm thick. The variation in structural and electronic properties of Si nanosheets with different thicknesses, as shown in this study, highlights the novelty of these materials and their significance for applications in electronic device technologies.     

Field emission and structure of aligned carbon nanofibers deposited by ECR-CVD plasma method

Aligned carbon nanofibers and hollow carbon nanofibers were grown by MW ECR-CVD method using methane and argon mixture gas at a temperature of 550°C. The carbon nanofibers and the hollow carbon nanofibers were deposited perpendicularly on Si substrates and on Si substrates coated with Ni catalyst, respectively. From TEM analysis the diameter and length of the nanofibers are approximately 60 nm and 15 μm, respectively. Raman spectra of these aligned carbon nanofibers showed new bands of 1340 and 1612 cm⁻¹ of the first-order Raman scattering and 2660, 2940 and 3220 cm⁻¹ of the second-order Raman scattering. The second-order Raman scattering bands were assigned to two overtone and one combination bands on the basis of a similar assignment of micro-crystal graphite by Nemanich and Solin. By the measurement of XPS C1s band energies of 284.6 eV for the carbon nanofiber and 284.7 eV for the hollow carbon nanofiber indicate mainly sp² carbon component in the inclusion of a small amount (<5%) of oxygen in a high binding energy region (∼288 eV). Field emission characteristics of the well-aligned carbon nanofibers and hollow carbon nanofibers were investigated and the current densities were 7.25 and 0.69 mA/cm² at 12.5 V/μm, respectively.     

Raman scattering by confined optical phonons in Si and Ge nanostructures

A microscopic theory of the Raman scattering based on the local bond-polarizability model is presented and applied to the analysis of phonon confinement in porous silicon and porous germanium, as well as nanowire structures. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green's function and the Born model, including central and non-central interatomic forces. For the porous case, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p--type (001)-oriented crystalline Si wafers in a hydrofluoric acid bath.