Frontiers in Magnetic Resonance

The advantages of using multifrequency Electron Paramagnetic Resonance (EPR) in studying carbon-based materials are discussed. The details of designing continuous-wave EPR spectrometers operating at different frequencies are presented. Designs of CW and pulse Electron Nuclear Double Resonance (ENDOR) spectrometers, which are very important techniques for studying precisely hyperfine interactions and local environment of paramagnetic ions in carbon-based materials are included. Analysis of EPR spectra, spin Hamiltonians, EPR lineshapes, evaluation of spin-Hamiltonian parameters, and simulation of single-crystal and powder spectra are also explained. A short review of carbon-based materials studied by EPR is given.

The focus of this chapter is set on the application of EPR methods to carbon-based materials, from nanographites to graphene-based materials, for the resolution and characterization of the different signals, related to the presence of specific species, or structures. Because of the intrinsic heterogeneity of the samples, this goal is not simple: most of the signals coming from different types of structures have similar spectroscopic features and are overlapping in the cw-EPR spectra with very different relative intensities. It is then necessary to use all possibilities that EPR offers, from the cw-EPR techniques to pulse EPR methods, to disentangle ideally all contributions. Our analysis of the EPR spectra considers the presence of three types of paramagnetic contributions: conduction electrons, edge states and molecular states. This interpretation framework has been shown to be effective for the considered materials, characterized by the presence of finite-dimension graphene layers, eventually stacked one above the other. In our analysis, we investigated different experimental parameters, like the variation in the temperature of the EPR intensity, the values of the g-tensors and the homogeneous lineshapes of the spectra to obtain further structural information. Pulse EPR methods were used to study and characterize species with long relaxation times (molecular states). Echo-detected EPR enabled to obtain their spectral lineshapes. Hyperfine spectroscopies, ESEEM, ENDOR and HYSCORE, determined the electron hyperfine couplings of unpaired electrons with magnetic nuclei, thus allowing the evaluation of the extent of the π-system and the presence of different types of nuclei.

The usefulness of electron spins in quantum information technologies such as spintronics or quantum computation is determined by the spin-lattice (T1) and spinspin (T2) relaxation times. These relaxation times should be long relative to the characteristic times required for spin control in order to allow for controlled information manipulation. Despite the central importance of T1 and T2 in modern information technologies, direct experimental access to these quantities is scarce. Electron spin resonance (ESR) spectroscopy is one of the few-experimental methods, offering direct access to both T1 and T2 of electrons. In this chapter, we present recent advancements in pulsed and continuous wave ESR spectroscopy of conducting carbon nanomaterials that have emerged with the potential for practical applications.

Electron paramagnetic resonance (EPR) spectroscopy can be fruitfully applied to study the interplay of localized and itinerant spins for carbon nanomaterials, including carbon nanotubes (CNTs), and thus provides a unique spectroscopic probe of their electronic properties upon integration as active components in composite materials. In this chapter, EPR spectroscopy is exploited to investigate the magnetic properties of double-walled carbon nanotubes (DWCNTs) and composites of oxidized multi-walled carbon nanotubes (MWCNTs) embedded in an elastomeric poly(etherester) block copolymer. In the case of DWCNTs, an asymmetric resonance line was observed that could be accurately analyzed in terms of two independent metallic lineshapes with similar g-factors, a narrow and a broad one, related to the distinct contributions of defect spins located on the inner and outer DWCNTs layers, respectively. Analysis of the spin susceptibilities indicated a ferromagnetic phase transition at low temperatures, alike metallic single wall CNTs. Interlayer coupling between the DWCNT layers is accordingly suggested to enhance exchange interactions between localized spins via conduction electrons. Conversely, in the case of MWCNTs-polymer composites, EPR spectra in combination with static magnetization measurements revealed a drastic reduction of orbital diamagnetism and g-anisotropy along with a marked enhancement of spin susceptibility, with respect to the anisotropic EPR spectrum of pristine MWCNTs. These effects indicate considerable hole doping by oxygen functional groups on the MWCNTs’ surface and an excessive increase of the density of paramagnetic defects, which are sensitive to the polymer relaxation and to the underlying MWCNT-polymer interfacial coupling.

The electron paramagnetic resonance (EPR) spectroscopy is a powerful and sensitive method to detect intrinsic and extrinsic paramagnetic point defects in a material system. EPR has recently been proven an effective tool for studying the lattice defect of nanostructured carbon materials. In particular, EPR can be used to elucidate the spin properties, including unpaired spins, conduction electrons, and dangling bonds as well as the electronic states of different carbon nanostructures. EPR studies on point-defects of carbon materials such as graphene and carbon nanotubes help to unearth several electronic and optical features of the materials. Though the magnetic feature of graphene has been studied intensively, EPR research on graphene and graphene-like structures is still a new field. This chapter focuses on discussing EPR investigations on graphene oxide, functional reduced graphene oxide, and carbon nanotubes. In that, EPR has demonstrated as a suitable tool to detect spin density changes in different functionalized nanocarbon materials. A novel approach to studying the charge transfer within quantum dots-graphene hybrids, using continuous wave EPR, will be discussed. It also enables the study of the change in the electronic properties of graphene before and after attaching of quantum dots. This contributes to improved understanding of electronic coupling effects in nanocarbon-nanoparticle hybrid materials which are promising for various electronic and optoelectronic applications.

Carbon nanotubes (CNTs) have been extensively studied due to their excellent properties such as ballistic transport. Electrically induced charge carriers in CNTs and the relation between the spin states and the ballistic transport, however, have not yet been microscopically investigated owing to experimental difficulties. Here we review electron spin resonance (ESR) spectroscopy of semiconductor single-walled CNT (SW-CNT) thin films and their transistors. We have investigated the spin states and the electrically induced charge carriers in the SW-CNTs by utilizing a transistor structure under device operation. The electrically induced ESR method is useful for the microscopic investigation into CNTs because it is capable of directly observing the spins in CNTs. We have observed a clear reverse correlation between the ESR intensity and the transistor current under high charge-density conditions. This result directly demonstrates electrically induced ambipolar spin vanishment in CNTs, providing a first clear evidence of antiparallel spin fillings of the electrically induced charges’ spins and the vacancies’ spins in CNTs. The ambipolar spin vanishment is considered to improve the transport properties of CNTs because it seems to greatly reduce carrier scatterings. Similar spin vanishment has been observed in single-layer graphene transistors. Thus, this result suggests that the electrically induced ambipolar spin vanishment is a universal phenomenon for carbon materials.

Carbon-based materials are highly diverse in terms of their mechanical, electronic, and thermodynamic properties, and as such can be used in a vast array of applications in numerous industries. These materials contain various types of carbon and oxygen radicals. Oxidation processes that influence the composition of these radical populations can have substantial effects on the materials’ electronic properties. Therefore, it is important to gain a systematic understanding of oxidation processes in carbonaceous materials at various temperatures and pressures. Electron paramagnetic resonance (EPR) spectroscopy is routinely used to characterize defects in carbon-based materials and the nature of the radicals they contain. Specifically, spin concentrations and the composition and distribution of radicals can be correlated to the electronic properties of carbon-based materials. Accordingly, over the last few years, our group has been using EPR spectroscopy to develop methodologies to explore the oxidation properties of carbon-based materials. Our overarching goal is to produce a toolkit that can correlate between the physical properties of specific carbon-based materials and these materials’ sensitivity to oxidation processes. In this chapter, we will describe our findings regarding the oxidation processes of coal and graphene oxide materials. Our data are derived from in-situ EPR experiments in which carbon-based materials were exposed to various atmospheric environments. Our findings have clear practical implications with regard to identifying appropriate storage conditions for carbon-based materials.

The two-temperature measurement method has found use in electron paramagnetic resonance (EPR) spectroscopy for determining the fraction of two kinds of paramagnetic species of different nature. The method can particularly be applied when the temperature dependence of EPR line intensity shows deviations from the Curie law. By applying this method it was possible to determine the fractions of paramagnetic centres responsible for the Curie- and the Pauli-like paramagnetism in carbon-based materials. The method has also found application when the origin of the EPR line was originated due to paramagnetic centres in spin doublet states (S = 1/2) and in excited spin triplet states (S = 1).

Spin properties of defects in carbon nanostructures are one of the fundamental directions in the physics of nanomaterials. The problem of doping nanostructures and creating intrinsic and extrinsic defects in such structures as a result of various actions (heat treatment, ionizing radiation, chemical action, etc.) is playing the central role in the further implementation of these nanostructures in real devices. Electron paramagnetic resonance (EPR) is known to be one of the most informative methods to study the intrinsic and extrinsic defects at the molecular level. The use of different frequency bands (low frequency X-band or high frequency W-band) and different regimes of the EPR signal detection (continuous or pulsed) allows one to get an access not only to the identification an electronic and microscopic structure of the defects in the crystalline matrix, but also to study coherence properties of the defects' spin. In this chapter, by means of EPR, we provide the direct observation of paramagnetic impurities in the crystalline core of nanodiamonds and we also show that nitrogen impurities in nanodiamonds interact with the diamond lattice in a similar way as in the bulk diamond crystals. We also present the results of observation of highdensity NV defect ensembles created directly by high-pressure high-temperature (HPHT) sintering procedure of the detonation nanodiamonds and show that the spin ensemble of the NV defects is characterized by the long spin-lattice and spin-spin relaxation times. The latter is important for bioimaging and quantum sensing applications.

Silicon nitro carbide, SiCN, exhibits excellent high-temperature properties. It can withstand temperatures of up to 1800° C, which is superior to those of Si, SiC and Si3N4. Magnetic composites, as well as electrically conductive ceramics on the basis of SiCN, can be developed. Therefore, SiCN constitutes a new class of materials for high-temperature electronics. SiCN ceramics, doped with the transition metal ions exhibiting superparamagnetic features are promising in building high-temperature magnetic and pressure sensors. EPR (electron paramagnetic resonance) and FMR (ferromagnetic resonance) techniques can provide important information on the properties of SiCN and its magnetic derivatives, in conjunction with structural, magnetic and electric measurements. In the present work, EPR signals due to sp2–hybridized carbon-related dangling bonds were recorded over the 4 - 300 K range. SiCN ceramics consist of nanoparticles of SiCN and a free carbon phase. The two EPR signals, which were only resolved at the higher frequencies of W (95 GHz) and G (170 GHz) bands are due to carbon-related dangling bonds present as (i) defects on the freecarbon phase and (ii) within the bulk of SiCN ceramic network. SiCN magnetic ceramics, doped with the Fe ions were synthesized at different pyrolysis temperatures in the range 600° - 1600°C. Several magnetic phases in SiCN/Fe composite are detected by EPR/FMR technique. The main sources of magnetism in these samples are: (i) superparamagnetic nanoparticles of Fe3Si, (TC = 800°C), (ii) nanoparticles of Fe5Si3 (TC = 393°C), which appear above 1000°C in single-domain state and (iii) nanoparticles of Fe70SixC30-x (620°C).

In this chapter, we present the study of SiC nanoparticles obtained by pyrolysis and self-propagating high temperature synthesis (SHS) method using multiapproaches electron paramagnetic resonance (EPR) methods, including continuous wave EPR, field swept electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and four-pulse electron spin echo envelope modulation ESEEM (hyperfine sublevel correlation, HYSCORE) spectroscopy. Three paramagnetic defects were observed in SiC nanoparticles. Two of them with giso = 2.0029(3) and giso = 2.0043(3) were assigned to carbon vacancy VC localized in the cubic (β) and hexagonal (α) phase of the SiC nanoparticles, respectively. The paramagnetic defect with giso = 2.0031(3) was attributed to the sp3-coordinated carbon dangling bonds (CDB) located in the carbon excess phase of the SiC nanoparticles. The paramagnetic defect with giso= 2.0037(3), which was observed only in SiC nanoparticles obtained by SHS method was attributed to the bulk intrinsic defect having a Si-NSi2 configuration and located in α-Si3N4 phase of the SiC nanoparticles. A high delocalization of the electronic wavefunction of the unpaired electron for the carbon vacancy VC localized in the cubic crystalline phase of the SiC nanoparticles was found from the detail study of the VC ligand structure by pulse ENDOR and HYSCORE methods.

The great potential of the silicon carbide (SiC) and diamond nanoparticles for future applications in spintronics initiates detailed investigation of the effects of impurities and defects in their electronic characteristics. Among impurities, nitrogen doped SiC nanoparticles are an important item to be studied, because nitrogen donors are common contaminations of an n-type SiC bulk material. The first information about the shallow donor state of nitrogen in SiC nanoparticles and influence of the hydrogen as well as intrinsic defects on electronic properties of nitrogen was presented in this chapter. The delocalization of the nitrogen wave function was observed with the reduction in the nanoparticle size with the onset of about d < 50 nm. The delocalization of the nitrogen wave function gives rise to the overlap between wavefunctions of the neighboring donors and transformation of the nitrogen triplet line into one single exchange EPR line. The size-dependent effect was also observed for paramagnetic substitutional nitrogen defects (P1) in nanodiamonds representing free electron interacting with the 14N nuclear spin (I = 1). The decrease of size of nanoparticle down to d < 80 nm led to a transformation of the hyperfine structure of the P1 defect into a one EPR line caused by dipole-dipole and/or exchange couplings of P1 spins with the rising amount of surface spins, which becomes more effective in nano-sized particles.

In this chapter, the nature of the defects and their relation to the incorporation of carbon, hydrogen, nitrogen and thermal treatment were investigated by electron paramagnetic resonance (EPR) spectroscopy for the fundamental insight of the electronic, optical and magnetic characteristics of the amorphous hydrogenated carbonrich silicon-carbon (a-Si1-xCx:H) and amorphous silicon carbonitride (a-SiCxNy) thin films. The paramagnetic defects due to the silicon dangling bonds (SiDBs), carbonrelated defects (CRDs) and K-center with Si-N2Si configuration were revealed in a-Si1-xCx:H films. The observed strong rise of the CRD spin density in annealed a-Si1-xCx:H films is caused by the hydrogen effusion process that takes place at Tann > 400°C. The rise of the CRD density was occurring with the exchange narrowing of its EPR linewidth owing to the appearance of carbon clusters with ferromagnetic ordering. The temperature variation of g-tensor anisotropy, measured at 37 GHz and 140 GHz frequencies for the CRD EPR line in the a-Si1-xCx:H film annealed at 950°C, was interpreted by the existence of graphite-like sp2-hybridized carbon clusters and demagnetization field. Examination of the temperature variation of the integrated intensity of the SiDB and CRD EPR lines was demonstrated that their spin systems reveal superparamagnetic and ferromagnetic features, correspondingly. The CDB and Si-related surface defects were observed in a-SiCxNy. It was found that the CDB spin concentration significantly increases with the increase of the nitrogen content.Due to the temperature variation of the linewidth and integrated intensity of the CDB EPR line, it has been supposed that the antiferromagnetic ordering takes place in the spin system.