Nanocarbon hybrids of graphene‐based materials and ultradispersed diamond: investigating structure and hierarchical defects evolution with electron‐beam irradiation

We report the influence of electron‐beam (E‐beam) irradiation on the structural and physical properties modification of monolayer graphene (Gr), reduced graphene oxide (rGO) and graphene oxide (GO) with ultradispersed diamond (UDD) forming novel hybrid composite ensembles. The films were subjected to a constant energy of 200 keV (40 nA over 100 nm region or electron flux of 3.9 × 10¹⁹ cm⁻²s⁻¹) from a transmission electron microscope gun for 0 (pristine) to 20 min with an interval of 2.5 min continuously – such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the modifications induced by E‐beam, the films were analyzed prior to and post‐irradiation. We focus on the characterization of hierarchical defects evolution using in situ transmission electron microscopy combined with selected area electron diffraction, Raman spectroscopy (RS) and Raman mapping techniques. The experiments showed that the E‐beam irradiation generates microscopic defects (most likely, interstitials and vacancies) in a hierarchical manner much below the amorphization threshold and hybrids stabilized with UDD becomes radiation resilient, elucidated through the intensity, bandwidth, and position variation in prominent RS signatures and mapping, revealing the defects density distribution. The graphene sheet edges start bending, shrinking, and generating gaps (holes) at ~10–12.5 min owing to E‐beam surface sputtering and primary knock‐on damage mechanisms that suffer catastrophic destruction at ~20 min. The microscopic point defects are stabilized by UDD for hybrids in the order of GO > rGO ≥ Gr besides geometric influence, i.e. the int erplay of curvature‐induced (planar vs curved) energy dispersion/absorption effects. Furthermore, an attempt was made to identify the nature of defects (charged vs residual) through inter‐defect distance (i.e. L_D). The trends of L_D for graphene‐based hybrids with E‐beam irradiation implies charged defects described in terms of dangling bonds in contrast to passivated residual or neutral defects. More importantly, they provided a contrasting comparison among variants of graphene and their hybrids with UDD. Copyright © 2015 John Wiley & Sons, Ltd.     

Temperature‐depending Raman line‐shift of silicon carbide

Silicon carbide (SiC) is often used for electronic devices operating at elevated temperatures. Spectroscopic temperature measurements are of high interest for device monitoring because confocal Raman microscopy provides a very high spatial resolution. To this end, calibration data are needed that relate Raman line‐shift and temperature. The shift of the phonon wavenumbers of single crystal SiC was investigated by Raman spectroscopy in the temperature range from 3 to 112°C. Spectra were obtained in undoped 6H SiC as well as in undoped and nitrogen‐doped 4H SiC. All spectra were acquired with the incident laser beam oriented parallel as well as perpendicular to the c‐axis to account for the anisotropy of the phonon dispersion. Nearly all individual peak centers were shifting linearly towards smaller wavenumbers with increasing temperature. Only the peak of the longitudinal optical phonon A₁(LO) in nitrogen‐doped 4H SiC was shifting to larger wavenumbers. For all phonons, a linear dependence of the Raman peaks on both parameters, temperature and phonon frequency, was found in the given temperature range. The linearity of the temperature shift allows for precise spectroscopic temperature measurements. Temperature correction of Raman line‐shifts also provides the ability to separate thermal shifts from mechanically induced ones. Copyright © 2009 John Wiley & Sons, Ltd.     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of defect evolution by TEM with in situ ion irradiation and coordinated modeling

The paper describes a novel transmission electron microscopy (TEM) experiment with in situ ion irradiation designed to improve and validate a computer model. TEM thin foils of molybdenum were irradiated in situ by 1?MeV Kr ions up to ～0.045 displacements per atom (dpa) at 80°C at three dose rates ?5?×?10^?6, 5?×?10^?5, and 5?×?10^?4?dpa/s – at the Argonne IVEM-Tandem Facility. The low-dose experiments produced visible defect structure in dislocation loops, allowing accurate, quantitative measurements of defect number density and size distribution. Weak beam dark-field plane-view images were used to obtain defect density and size distribution as functions of foil thickness, dose, and dose rate. Diffraction contrast electron tomography was performed to image defect clusters through the foil thickness and measure their depth distribution. A spatially dependent cluster dynamic model was developed explicitly to model the damage by 1?MeV Kr ion irradiation in an Mo thin foil with temporal and spatial dependence of defect distribution. The set of quantitative data of visible defects was used to improve and validate the computer model. It was shown that the thin foil thickness is an important variable in determining the defect distribution. This additional spatial dimension allowed direct comparison between the model and experiments of defect structures. The defect loss to the surfaces in an irradiated thin foil was modeled successfully. TEM with in situ ion irradiation of Mo thin foils was also explicitly designed to compare with neutron irradiation data of the identical material that will be used to validate the model developed for thin foils.     

Structural variation in a synchrotron‐induced contamination layer (a‐C:H) deposited on a toroidal Au mirror surface

A carbon layer deposited on an optical component is the result of complex interactions between the optical surface, adsorbed hydrocarbons, photons and secondary electrons (photoelectrons generated on the surface of optical elements). In the present study a synchrotron‐induced contamination layer on a 340 mm × 60 mm Au‐coated toroidal mirror has been characterized. The contamination layer showed a strong variation in structural properties from the centre of the mirror to the edge region (along the long dimension of the mirror) due to the Gaussian distribution of the incident photon beam intensity/power on the mirror surface. Raman scattering measurements were carried out at 12 equidistant (25 mm) locations along the length of the mirror. The surface contamination layer that formed on the Au surface was observed to be hydrogenated amorphous carbon film in nature. The effects of the synchrotron beam intensity/power distribution on the structural properties of the contamination layer are discussed. The I(D)/I(G) ratio, cluster size and disordering were found to increase whereas the sp²:sp³ ratio, G peak position and H content decreased with photon dose. The structural parameters of the contamination layer in the central region were estimated (thickness ? 400 Å, roughness ? 60 Å, density ? 72% of bulk graphitic carbon density) by soft X‐ray reflectivity measurements. The amorphous nature of the layer in the central region was observed by grazing‐incidence X‐ray diffraction.     

A single‐crystal Raman and infrared study of the nonlinear optical crystal MBANP

Single‐crystal Raman and polycrystalline thin‐film infrared measurements have been obtained for the polar organic nonlinear optical material 2‐(α‐methylbenzylamino)‐5‐nitropyridine (MBANP). For comparison, thin‐film polycrystalline infrared measurements were also made on 2‐(α‐methylbenzylamino)‐3,5‐dinitropyridine (MBADNP). The long wavelength electronic absorption was measured in several solvents and as a thin solid film. The Raman spectra are dominated by three intense bands attributed to vibrations of the ring, the NO₂ substituent, and the N H bond. The most intense scattering and absorption arose from the α_bb component of the polarisability tensor. This implies that the most significant contribution to the transition polarisability arises from the electronic transition near 383 nm, polarised along the b‐axis of the crystal. The strongest bands in the infrared spectra are also associated with the same three bands, consistent with the predictions of the effective conjugation coordinate (ECC) theory, implying efficient electron–phonon coupling (or electronic delocalisation) in the conjugated system. DFT calculations of vibrational wavenumbers and eigenvectors were used to assign relevant vibrational features and to derive useful information about the molecular structure. This single‐crystal material is also a strong candidate for an efficient laser Raman converter with a large wavenumber shift of 3404 cm⁻¹ and a high damage threshold. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of electron beam irradiation on structural and optical properties of α-Ag2WO4 nanoparticles

The influence of 8 MeV electron beam irradiation on the structural and optical properties of silver tungstate (α-Ag₂WO₄) nanoparticles synthesized by chemical precipitation method was investigated. The dose dependent effect of electron irradiation was investigated by various characterization techniques such as, X-ray diffraction, scanning electron microscopy, UV–vis absorption spectroscopy, photoluminescence and Raman spectroscopy. Systematic studies confirm that electron beam irradiation induces non-stoichiometry, defects and particle size variation on α-Ag₂WO₄, which in turn results changes in optical band gap, photoluminescence spectra and Raman bands.     

Micro‐Raman depth profiling of silicon amorphization induced by high‐energy ion channeling implantation

In this work, we study the silicon amorphization dependence on the crystal depth induced by 6‐MeV Al²⁺ ions implanted in the <110> and randomly oriented silicon crystal channels, which was not directly experimentally accessible in the previous similar high‐energy ion–crystal implantation cases. Accordingly, the micro‐Raman spectroscopy scanning measurements along the crystal transversal cross section of the ion implanted region were performed. The ion fluence was 10¹⁷ particles/cm². The scanning steps were 0.2 and 0.3 µm, for the channeling and random ion implantations, respectively. The obtained results are compared with the corresponding Rutherford backscattering spectra of 1.2‐MeV protons in the random and channeling orientations measured during the channeling implantation. Additionally, scanning electron microscope picture was taken on the transversal cross section of the implanted region in the channeling implantation case. We show here that the obtained silicon amorphization maxima are in excellent agreement with the corresponding estimated maxima of the aluminum concentration in silicon. This clearly indicates that the used specific micro‐Raman spectroscopy scanning technique can be successfully applied for the depth profiling of the crystal amorphization induced by high‐energy ion implantation. Copyright © 2013 John Wiley & Sons, Ltd.     

Monitoring of the microstructure of ion‐irradiated nuclear ceramics by in situ Raman spectroscopy

Raman spectroscopy is an efficient technique for studying the evolution of microstructure of materials under irradiation. For that purpose, a Raman spectrometer has been recently installed at the JANNUS‐Saclay platform. In this paper, we describe the new setup for in situ experiments. These in situ experiments allowed following the microstructural evolution of different materials (SiC, ZrO₂ and B₄C) as a function of ion fluence on a single sample (either single crystal or polycrystalline ceramics) under the same irradiation conditions. Our results show that Raman spectroscopy is a versatile non‐contact technique for studying on‐line crystalline phase changes or amorphization of irradiated iono‐covalent solids. A detailed analysis of Raman spectra is provided for the three materials (SiC, ZrO₂ and B₄C) investigated in this study, revealing quite different behaviors upon irradiation. Basically, Raman spectroscopy gives insight on these evolutions at the level of bonds given by specific phonon modes, in good agreement with Rutherford backscattering channeling (RBS/C), X‐ray diffraction (XRD) or transmission electron microscopy (TEM) data, which provide information at a long‐range scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Integration of correlative Raman microscopy in a dualbeam FIB SEM

We present an integrated confocal Raman microscope in a focused ion beam scanning electron microscope (FIB SEM). The integrated system enables correlative Raman and electron microscopic analysis combined with focused ion beam sample modification on the same sample location. This provides new opportunities, for example the combination of nanometer resolution with Raman advances the analysis of sub‐diffraction‐sized particles. Further direct Raman analysis of FIB engineered samples enables in situ investigation of sample changes. The Raman microscope is an add‐on module to the electron microscope. The optical objective is brought into the sample chamber, and the laser source, and spectrometer are placed in a module attached onto and outside the chamber. We demonstrate the integrated Raman FIB SEM function with several experiments. First, correlative Raman and electron microscopy is used for the investigation of (sub‐)micrometer‐sized crystals. Different crystals are identified with Raman, and in combination with SEM the spectral information is combined with structurally visible polymorphs and particle sizes. Analysis of sample changes made with the ion beam is performed on (1) structures milled in a silicon substrate and (2) after milling with the FIB on an organic polymer. Experiments demonstrate the new capabilities of an integrated correlative Raman–FIB–SEM. Copyright © 2016 John Wiley & Sons, Ltd.     

Raman spectroscopic study of calf thymus DNA: an effect of proton‐ and γ‐irradiation

Raman spectroscopy was applied to analyse structural changes in calf thymus double‐stranded deoxyribonucleic acid (dsDNA) after proton‐ and γ‐irradiation (0.5, 5 and 50 Gy). Characteristic Raman bands of phosphodiester linkages, nucleic bases and deoxyribose moieties were sensitive to irradiation. A significant damage of the macromolecules was observed only at the highest dose (50 Gy) of both types of radiations. Spectral changes confirmed a radiation‐induced alteration of the native structure of dsDNA. Nucleic bases, especially pyrimidines, were the most sensitive to radiation, while some alterations in the sugar–phosphate backbone were also detected. The differences in the dose‐dependent effects of proton vs γ‐irradiation on studied biomolecule are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of irradiation on ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate: an ESR study

Ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate [C₁₁H₁₅NO₂S] was synthesized by the Gewald method. Its single crystals were grown from an alcohol/ethyl acetate solution at 15 °C and characterized using IR and ¹H‐NMR. These single crystals were irradiated for 72 h at 298 K by a ⁶⁰Co gamma source with a dose speed of 0.864 kGy/h. After irradiation, electron spin resonance (ESR) measurements were carried out to study radiation‐induced radicals in the temperature range from 120 to 450 K. Additionally, for the single crystal, ESR angular dependencies were measured in the xy, xz and yz planes of the substance. This irradiated single crystal was analyzed based on the ESR spectra. Analysis of the spectra revealed that the radical was formed by a C–H bond fission at the carbon end of the substance. It was also observed that the color of the sample changed after irradiation. The hyperfine and g parameters were determined from the experimental spectra. It was inferred from these results that the hyperfine parameters and g value exhibited anisotropic behavior. The average values of these parameters were calculated as follows: g = 2.0088, A_H1=H2 = 20.70 G, A_H3=H4 = 10.80 G, A_Ha = 4.59 G, A_Hb = 3.24 G and, A_N = 6.10 G. Copyright © 2011 John Wiley & Sons, Ltd.     

Metal–metal bonding and structures of trinickel and tricobalt dipyridylamido complexes from surface‐enhanced Raman spectra

We recorded surface‐enhanced Raman scattering (SERS) spectra of metal‐string complexes Co₃(dpa)₄ Cl₂ [di(2‐pyridyl)amido (dpa)], Ni₃(dpa)₄ Cl₂ and the oxidized form of the Ni₃ complex to determine their vibrational wavenumbers and to investigate their structures. For SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and the complexes remain in thermal equilibrium. From our analysis of the vibrational normal modes we assigned the SERS lines at 242 and 276 cm⁻¹ to Ni₃ and Co₃ symmetric‐stretching modes of the symmetric form. For Co₃ (dpa)₄Cl₂ a Raman line at 383 cm⁻¹ was assigned to the Co Co stretching mode of the unsymmetric form. The wavenumber of the Ni₃ symmetric‐stretching mode of the oxidized form [Ni₃(dpa)₄]³⁺ is 274 cm⁻¹, greater than that for neutral Ni₃(dpa)₄Cl₂, in agreement with a prediction of delocalized molecular‐orbital theory that an electron is removed from an antibonding orbital after oxidation. The experimental data show that the SERS technique serves as an excellent tool to observe the variation of metal–metal bonding during an oxidation or reduction reaction. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Real‐time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low‐density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C C symmetrical stretching vibration of polyethylene (PE) at 1132 cm⁻¹ was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c‐axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (〈P₂(cosθ)〉) and fourth (〈P₄(cosθ)〉) moments of the orientation distribution function. The orientation parameters (P₂, P₄) were found to increase along the axial distance in the film line even past the frost‐line height (FLH). The P₂ values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I₁₁₃₂/I₁₀₆₄) obtained from a single, ZZ‐back‐scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright © 2008 John Wiley & Sons, Ltd.     

Resonant two‐magnon Raman scattering in high‐Tc cuprates

Resonance enhancement of one‐phonon, two‐phonon, and two‐magnon Raman scattering in a general, exactly solvable, multiband model is explained in a way that is in accordance with the general analytical properties of the total optical conductivity tensor. Using this approach, the charge‐transfer limit of the Emery three‐band model is examined to explain resonance enhancement of the two‐magnon Raman spectra of high‐T_c cuprates, which is found in experiments to be of 3 orders of magnitude. While previous Raman and optical conductivity analyzes of the cuprates, based on the single‐band Hubbard model, are found to be consistent with the picture where one hole per one CuO₂ unit is localized on the Cu ion, the present three‐band approach allows the study of the opposite, strong copper‐oxygen hybridization limit, which is found to be in agreement with the results of nuclear magnetic resonance (NMR) and one‐phonon Raman scattering experiments. Copyright © 2010 John Wiley & Sons, Ltd.     

Transmission of an X‐ray beam through a two‐dimensional photonic crystal and the Talbot effect

Results of computer simulations of the transmission of an X‐ray beam through a two‐dimensional photonic crystal as well as the propagation of an X‐ray beam in free space behind the photonic crystal are reported. The photonic crystal consists of a square lattice of silicon cylinders of diameter 0.5 µm. The amount of matter in the path of the X‐ray beam rapidly decreases at the sides of the cylinder projections. Therefore the transmission is localized near the boundaries, and appears like a channeling effect. The iterative method of computer simulations is applied. This method is similar to the multi‐slice method that is widely used in electron microscopy. It allows a solution to be obtained with acceptable accuracy. A peculiarity in the intensity distribution inside the Talbot period z_T in free space was found when the intensity is approximately equal to the initial value at a distance 0.46z_T, and it is shifted by half a period at distance 0.5z_T. The reason for this effect is the existence of a periodic phase of the wavefunction of radiation inside the intensity peaks. Simulations with zero phase do not show this effect. Symmetry rules for the Talbot effect are discussed.     

In‐plane and out‐of‐plane defects of graphite bombarded by H,D and He investigated by atomic force and Raman microscopies

Graphite samples exposed to H, D and He plasma at fluencies from 10¹⁶ to 10¹⁸ cm⁻² have been investigated by means of atomic force and Raman microscopies. The ion energy was varied between 40 and 800 eV, and the ion incidence was either perpendicular (Highly Oriented Pyrolitic Graphite) or parallel (carbon/carbon composite) to the basal plane. When increasing the impinging ion energy, the growth of nanometric domes at the surface has been observed by atomic force microscopy and the incident kinetic energy has been found as the parameter determining their height. Two different Raman signatures related to (1) a graphitic nano‐crystalline component similar to that of a 10¹⁴ cm⁻² bombarded 1‐, 2‐ and 3‐layer graphene, and to (2) an amorphous component, have been evidenced. Polarization studies have revealed that these components are related to regions with either in‐plane or out‐of‐plane disorder, coexisting in the material. These Raman studies have also revealed that both the defect–defect distance in the first case and the aromatic domain size in the second case are typically 1 nm. When the number of vacancies created in the material increases, the number of in‐plane defects decreases to the benefit of the out‐of‐plane defects. Copyright © 2014 John Wiley & Sons, Ltd.     

Observation of low‐wavenumber out‐of‐plane optical phonon in few‐layer graphene

Few‐layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low‐wavenumber Raman peak of ~120 cm⁻¹ and a phonon‐induced oscillation in the kinetic curve of electron–phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low‐wavenumber out‐of‐plane optical mode in the few‐layer graphene. The phonon band shows an asymmetric shape, a consequence of so‐called Breit‐Wigner‐Fano resonance, resulting from the coupling between the low‐wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low‐wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of simultaneous helium implantation on the microstructure evolution of Inconel X-750 superalloy during dual-beam irradiation

This study focuses on investigation into the effect of helium implantation on microstructure evolution in Inconel X-750 superalloy during dual-beam (Ni⁺/He⁺) irradiation. The 1 MeV Ni⁺ ions with the damage rate of 10^?3 dpa/s as well as 15 keV He⁺ ions using rate of 200 appm/dpa were simultaneously employed to irradiate specimens at 400 °C to different doses. Microstructure characterization has been conducted using high-resolution analytical transmission electron microscopy (TEM). The TEM results show that simultaneous helium injection has significant influence on irradiation-induced microstructural changes. The disordering of γ′ (Ni₃ (Al, Ti)) precipitates shows noticeable delay in dose level compared to mono heavy ion irradiation, which is attributed to the effect of helium on promoting the dynamic reordering process. In contrast to previous studies on single-beam ion irradiation, in which no cavities were reported even at high doses, very small (2–5 nm) cavities were detected after irradiation to 5 dpa, which proved that helium plays crucial role in cavity formation. TEM characterization also indicates that the helium implantation affects the development of dislocation loops during irradiation. Large 1/3 〈1?1?1〉 Frank loops in the size of 10–20 nm developed during irradiation at 400 °C, whereas similar big loops detected at higher irradiation temperature (500 °C) during sole ion irradiation. This implies that the effect of helium on trapping the vacancies can help to develop the interstitial Frank loops at lower irradiation temperatures.     

Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm⁻¹ is interpreted as a quasimode with mixed A₁ and E₁ symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.