Quantifying ion-induced defects and Raman relaxation length in graphene

Raman scattering is used to study disorder in graphene subjected to low energy (90 eV) Ar⁺ ion bombardment. The evolution of the intensity ratio between the G band (1585 cm⁻¹) and the disorder-induced D band (1345 cm⁻¹) with ion dose is determined, providing a spectroscopy-based method to quantify the density of defects in graphene. This evolution can be fitted by a phenomenological model, which is in conceptual agreement with a well-established amorphization trajectory for graphitic materials. Our results show that the broadly used Tuinstra-Koenig relation should be limited to the measure of crystallite sizes, and allows extraction of the Raman relaxation length for the disorder-induced Raman scattering process.     

A reconsideration of the relationship between the crystallite size La of carbons determined by X-ray diffraction and Raman spectroscopy

Non-graphitic carbon materials produced by pyrolyzing wood at temperatures from 400 to 2400 °C and various types of commercial carbon fibers were examined by X-ray diffraction and Raman spectroscopy. The specimens cover a wide range of crystallite sizes L_a, in particular also very small sizes below 2 nm. The X-ray data were evaluated using the Scherrer equation and by an advanced approach using full curve fitting. The ratio of the D/G band intensities was determined from the Raman data by different evaluation techniques. A critical assessment of the classical linear relationship between 1/L_a and the D/G ratio shows that the relationship breaks down for crystallite sizes below 2 nm in accordance with recent theoretical predictions. The results are compared with data from the literature, showing that there are additional discrepancies between the data from various carbon types at large L_a due to different methods of data evaluation.     

Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.     

LaNiO3 nanopowder prepared by an ‘amorphous citrate’ route

Aqueous solution techniques are simple and easy to use technological methods for preparing single-phase ceramic powders with controlled and homogeneous grain size. This study presents the preparation of ferroelectric LaNiO₃ ceramic samples by a gel-method using low sintering temperatures and the evolution of the amorphous complex and LaNiO₃ nanocrystallites with temperature.Ferroelectric LaNiO₃ powders were prepared using an ‘amorphous citrate’ route. X-ray diffraction, Raman scattering and electron microscopy techniques were used to characterize the obtained products after thermal treatments at between 550 and 750 °C and revealed the formation of LaNiO₃ nanocrystallites of perovskite structure with homogeneous grain size after thermal treatment at 650, 700 and 750 °C, with particle sizes of ∼30, 42 and 65 nm, respectively. Raman spectra exhibit the characteristic band of the LaNiO₃ perovskite-phase at 392 cm⁻¹ and decreasing band width with temperature, an effect associated to the observed change in grain size.     

Effect of Grain Boundaries on the Lattice Thermal Transport Properties of Insulating Materials: A Predictive Model

We present two theoretical models to predict the lattice thermal conductivity degradation of insulating materials at high temperature (above one‐third of the Debye temperature). This degradation is due to the presence of grains, with known sizes and shapes, inducing thermal resistance at their boundaries. The first model is derived directly from the kinetic theory of gases (KTG). The formulation of the second is based on a localized continuum model (LCM), assuming phonon Umklapp scattering and the Debye approximation of phonon density of state. The two proposed models are purely predictive, as no experimental information related to the grain size dependence of the thermal conductivity is necessary for the parameterization of the models. The predictive accuracy of the two proposed models is tested on several different types of electrically insulating compounds. Although the model derived from the KTG is similar to the well‐known Kapitza thermal resistance formalism, it fails to predict the grain size dependence of the lattice thermal conductivity. The one derived from a LCM is a new formalism predicting, with good accuracy, the lattice thermal conductivity as a function of the average grain size. It is applicable for microstructures with a grain size typically above 20–50 nm.     

Probing inhomogeneous doping in overlapped graphene grain boundaries by Raman spectroscopy

The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB–stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements (Tsen et al., 2012). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.     

Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.     

Anharmonicity of graphite from UV Raman spectroscopy to 2700 K

First-order Raman spectra of pyrolytic graphite (PG) and highly oriented pyrolytic graphite (HOPG) were recorded in situ up to 2670 K and 2491 K, respectively, using a development of wire-loop heating cell technique attached to a UV-Raman spectrometer (244 nm). Raman shift of the E_2g in-plane stretching mode of graphite (G band) is used to discuss the anharmonicity by a comparison with calculations in the density-functional theory (DFT). High temperature Raman shifts are well described by anharmonic DFT calculations [1] up to 900 K. Anharmonicity is also determined from the temperature dependence of the Raman linewidth. The quartic term of phonon–phonon scattering process dominates at high temperature with respect to electron–phonon coupling that causes a slight decrease of linewidth with increasing temperature below 1000 K. The G band position is determined with a good reproducibility to 2700 K and can be used as a thermometer for in situ studies. Deep UV-Raman proves a viable solution for expanding significantly the temperature range for studying in situ vibrational properties of condensed matter, and particularly the monitoring of carbon-based material processing.     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

Deposition and characterization of nanocrystalline diamond films on Co-cemented tungsten carbide inserts

Nanocrystalline diamond films were deposited on Co-cemented tungsten carbides using bias-enhanced hot filament CVD system with a mixture of acetone, H₂ and Ar as the reactant gas. The effect of Ar concentration on the grain size of diamond films and diamond orientation was investigated. Nanocrystalline diamond films were characterized with field emission scan electron microscopy (FE-SEM), Atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction spectroscopy (XRD). Rockwell C indentation tests were conducted to evaluate the adhesion between diamond films and the substrates. The results demonstrated that when the Ar concentration was 90%, the diamond films exhibited rounded fine grains with an average grain size of approximately 60–80 nm. The Raman spectra showed broadened carbon peaks at 1350 cm^− 1 and 1580 cm^− 1 assigned to D and G bands and an intense broad Raman band near 1140 cm^− 1 attributed to trans-polyacetylene, which confirmed the presence of the nanocrystalline diamond phase. The full width at half maximum of the <111> diamond peak (0.8°) was far broader than that of conventional diamond film (0.28°–0.3°). The Ra and RMS surface roughness of the nanocrystalline diamond film were measured to be approximately 202 nm and 280 nm with 4 mm scanning length, respectively. The Ar concentration in the reactant gases played an important role in the control of grain size and surface roughness of the diamond films. Nanocrystalline diamond-coated cemented tungsten carbides with very smooth surface have excellent characteristics, which made them a promising material for the development of high performance cutting tools and wear resistance components.     

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.     

Raman characterization and UV optical absorption studies of surface plasmon resonance in multishell nanographite

Nanographite (NG) particles were produced by annealing of superpurified detonation nanodiamonds (grain size ~ 5 nm) at 1600 °C. The aim of this research was to provide Raman characterization of nanographites obtained and to investigate characteristic features of UV optical absorption in NG suspensions caused by the excitation of surface plasmon resonance (SPR) and its dependence on disorder and defectiveness of graphene shells during their transformation. The 1-st and 2-nd order Raman spectra of the NG samples excited at 514 nm were analyzed. Two different approaches applied for evaluation of the in-plane NG crystallite sizes by using the D- and G-band intensities ratio gave quite different results (~ 3.5 nm and ~ 5.5 nm) reflecting, most likely, a complicated NG structure. The changes in both intensity and position of SPR absorption peak for water suspension of NG particles may originate in structural imperfections and/or changes in aggregation of NG particles.     

Synthesis,calcination and characterization of Nanosized ceria powders by self-propagating room temperature method

Nanometric ceria powders with fluorite-type structure were obtained by applying self-propagating room temperature method. The obtained powders were subsequently thermally treated (calcined) at different temperatures for different times. Powder properties such as specific surface area, crystallite size, particle size and lattice parameter have been studied. Roentgen diffraction analysis (XRD), BET and Raman scattering measurements were used to characterize the as-obtained (uncalcined) powder as well as powders calcined at different temperatures.It was found that the average diameter of the as-obtained crystallites is in the range of 3–5 nm whereas the specific surface area is about 70 m²/g. The subsequent, 15 min long, calcination of as-obtained powder at different temperatures gradually increased crystallite size up to ～60 nm and reduced specific surface down to 6 m²/g. Raman spectra of synthesized CeO_2?y depicts a strong red shift of active triply degenerate F_2 g mode as well as additional peak at 600 cm^?1. The frequency of F_2 g mode increased while its line width decreased with an increase in calcination temperature. Such a behavior is considered to be the result of particle size increase and agglomeration during the calcination. After the heat treatment at 800 °C crystallite size reached value larger than 50 nm. Second order Raman mode, which originates from intrinsic oxygen vacancies, disappeared after calcination.     

Cation distribution and magnetostrictive strain in CuFe2−xGaxO4 ceramics

The structural, magnetic and magnetostriction properties in CuFe_2-xGa_xO₄ ceramics synthesized by solid state reaction route and sintered at 1000 °C and 1020 °C are reported here. As per the structural analysis based on the Rietveld refinement of XRD patterns and Raman scattering, the samples crystalized into mixed spinel structures with the coexistence of tetragonal and cubic symmetry. The samples sintered at 1000 °C contained the tetragonal structure as the major structural phase, whereas the cubic phase was dominant in the samples sintered at 1020 °C. The Ga ions were found to preferentially substitute the octahedral sites. For x = 0.15 and 0.20 samples sintered at 1020 °C, a colossal grain growth was observed with grain sizes on the millimeter scale. Analysis of magnetization data in terms of fitting to law of approach to saturation revealed an enhancement of saturation magnetization due to Ga substitution. Magnetostriction strain exhibited a correlation with the occupancy of Cu²⁺ ions in the octahedral sites, signifying local Jahn–Teller distortion of the lattice as the mechanism behind the magnetostriction. High magnetostriction strain (?88 ppm at 5 kOe) with its rapid increase in the low-magnetic field region demonstrated the potential application of Ga-doped CuFe₂O₄ for magnetoelectric cofired ceramics. The colossal grain size with relatively low density of grain boundaries played an important role in the rapid rise of magnetostriction curve in low-magnetic field region.     

Structure of the silver containing diamond like carbon films: Study by multiwavelength Raman spectroscopy and XRD

In the present study structure of silver containing diamond like carbon (DLC:Ag) films deposited by reactive magnetron sputtering was investigated by X-ray diffractometry (XRD) and multiwavelength Raman spectroscopy. In the case of the DLC:Ag films containing low amount of silver, crystalline silver oxide prevails over silver. While at higher Ag atomic concentrations formation of the silver crystallites of the different orientations was observed. Surface enhanced Raman scattering (SERS) effect was detected for high Ag content in the films. For UV excited Raman spectra sp³ bonded carbon related Raman scattering T peak at ~ 1060 cm^− 1 was detected only for the films with the highest amount of silver (34.3 at.%). The dependence of the Raman scattering spectra parameters such as position of the G peak, G peak full width at half maximum (FWHM(G)), D/G peak area ratio on Ag atomic concentration in DLC:Ag film as well as Raman scattering spectra excitation wavelength were studied. The dependence on Ag amount in film was more pronounced in the case of the Raman scattering spectra excited by higher wavelength laser beam, while in the case of the spectra excited by 325 nm and 442 nm laser beams only weak dependence (or no dependence) was observed. Overall tendency of the decrease of the dispersion of the G peak with the increase of Ag atomic concentration was found. Thus sp³/sp² bond ratio in DLC:Ag film decreased with the increase of Ag atomic concentration in the films.     

Effect of Bi contents on key physical properties of NiO NPs synthesized by flash combustion process and their cytotoxicity studies for biomedical applications

Nickel oxide has tremendous applications in the field of biomedicine. In this study, NiO nanoparticles were synthesized with different Bi contents (NiO@Bi; 0.0–7.5 wt%), and multifunctional usages were investigated. Structural confirmation was conducted through XRD and Raman studies, which revealed a monophasic cubic system. With increasing Bi content, broadening of the XRD and Raman peaks were observed, indicating a reduction in particle size. The crystallite size was found to be in the range of 10–26 nm. The decrease in particle size was confirmed through dynamic light scattering measurement. The homogeneous distribution of all elements and the presence of Bi were detected by an EDX/SEM e-mapping study. Field emission electron microscopy confirmed the formation of spherical shape nanoparticles. The grain size was reduced from 30 nm to 10 nm with Bi content, in accordance with XRD and Raman results. The Kubelka-Munk method was employed to determine the effect of Bi content on the optical band gap of NiO. The energy gap was reduced with Bi content in the range of 3.32–3.50 eV. Antimicrobial and in vitro cytotoxic characteristics of the prepared NPs were also studied. The results revealed that all NiO@Bi NPs had negligible antimicrobial activity and no cytotoxic effects on both normal and activated splenic cells. The in vivo acute cytotoxicity study indicated no cytotoxic effects on liver and kidney functions. The prepared NiO@Bi NPs were implanted in living organisms without hepatic/renal toxicity, demonstrating excellent biocompatibility, cell viability, and superior quality of nanocrystals, suggesting that the prepared NPs are ideal candidates for antibacterial and biomedical applications.     

Fabrication and application of nano–microcrystalline composite diamond films on the interior hole surfaces of Co cemented tungsten carbide substrates

Nano–microcrystalline composite diamond films are deposited on the interior hole surface of Co cemented tungsten (WC–6%Co) drawing using a squirrel-cage hot filament passing through the interior hole with large aperture by the bias-enhanced hot filament CVD. A new process is used to deposit nano–microcrystalline composite diamond coatings by a two-step hot filament chemical vapor deposition (HFCVD) procedure. Research results show that the as-deposited composite diamond films exhibit nanocrystalline diamond crystallites with grain sizes ranging from 60 to 90 nm and their surface roughness is measured as approximately Ra 220 nm with 4 mm scanning length. The Raman spectrum mainly exhibits three features near 1332, 1560 cm^? 1 (G peak), and a weak peak at approximately 1150 cm^? 1, which is attributed to the transpolyacetylene. XRD pattern indicates good crystallite quality of the composite films. The as-fabricated diamond coated dies show obvious performance enhancement in the practical application. Comparing with the WC–Co drawing die, the working lifetime of the diamond coated drawing die increases by a factor of above 15. Furthermore, the surface quality of the drawn copper pipes is greatly improved.     

Preparation and characterization of nanocrystalline and mesoporous strontium titanate thin films at room temperature

The low temperature perovskite-type strontium titanate (SrTiO₃) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol–gel route. The prepared sol had a narrow particle size distribution with hydrodynamic diameter of about 17 nm. X-ray diffraction (XRD) revealed that the synthesized powders had a perovskite-SrTiO₃ structure with preferable orientation growth along the (1 0 0) direction. TEM images showed that the average crystallite size of the powders annealed in the range 300–800°C was around 8 nm. FE-SEM analysis and AFM images revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 25 nm at 600°C. Based on Brunauer–Emmett–Taylor (BET) analysis, the synthesized powders showed mesoporous structure with BET surface area in the range 92–75 m²/g at 400–600°C. One of the smallest crystallite sizes and one of the highest surface areas reported in the literature were obtained, which can be used in many applications, such as photocatalysts.     

Synthesis and Thermal Treatment of Pd-Cr@Carbon for Efficient Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells

A nanostructured Pd-Cr catalyst was deposited on a supported carbon surface using the modified borohydride reduction method for the oxygen reduction reaction (ORR) to be utilized as an efficient catalyst in the proton-exchange membrane fuel cell. The crystal structure and feature nanostructure of the Pd-Cr@carbon were established through the use of X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Meanwhile, its catalytic activity was studied using the cyclic voltammetry and electrochemical polarization techniques. Based on the XRD analysis, it was observed that the Pd phase with the fcc crystal structure was dominant, while the Pd-Cr phase with tetragonal crystal structure was detected only for the as-prepared sample and samples calcined at 573 K. The estimated average crystallite size of the Pd phase increased from 9.66 to 37.54 nm as the calcination temperature increased to 973 K, and the calcination time had a slight effect on the crystallite size. On the other side, the average crystallite size for the formed Pd-Cr phase slightly increased from 43.74 nm for the as-prepared sample to 44.90 nm for the sample calcined at 573 K for 3 h. The TEM examination revealed the uniform distribution of the Pd and Pd-Cr nanoparticles upon the carbon surface. The calcination temperature and time played an important role in controlling the structural and morphology parameters of Pd-Cr@carbon. The adsorption/desorption potentials were found to be dependent on the calcination temperature and time and hence the particle and crystallite sizes. The optimum ORR activity and chemical stability were observed for samples calcined at 773 K for 3 h.

     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.