Paramagnetic Defects Research Articles

Variations of paramagnetic defects and dopants in geo-MoS2 from diverse localities probed by ESR.

Exfoliated flakes from molybdenite crystals often still serve as benchmark substrates for two-dimensional MoS2 fundamental and device-oriented research. In this article, results are reported of a multi-frequency electron paramagnetic resonance (EPR) study on a series of natural 2H MoS2 crystals taken from various (seven) geological sites with the intent to explore the variations in quality and properties in terms of occurring paramagnetic point defects, with particular focus on the assessment of the predominant type of impurity dopant. The sample set covers three types of overall doping regimes, i.e., p-type, n-type, and mixed (n-type and p-type parts in one sample). The doping type appears primarily governed by substitutional impurities as evidenced by the observed As and N acceptor (both substituting for S) and Re donor (substituting for Mo) signals. For all p-type specimens, doping is found to be ruled by As where, however, a strong variation is revealed in doping uniformity, which appears not directly correlated with the As dopant density. Without specific precautions taken, surface contamination related EPR signals are observed in virtually all As-excavated geo-MoS2 specimens. While several of these signals are of unassigned origin, two prominent ones are identified, one as concerning oxo-Mo5+ compounds and the other Mn2+ centers. The geo-MoS2 sample with the foremost n-type doping shows, besides the prime Re donor EPR signal, an intense powder-pattern signal, tentatively typified by g∥ = 2.076, g⊥ = 2.253, which is suggested to originate from intercalation-related defects. The results bear out the necessity of rigorous surface cleaning, even including invasive removal of surface layers, to obtain pristine MoS2 parent crystals suitable for enabling exfoliation of high quality flakes.

Radiation-induced paramagnetic defects in porous silicon under He and Ar ion irradiation

A study of damage and the concentration of radiation-induced paramagnetic defects by He+ and Ar+ ion beam irradiation in porous silicon is presented. Porous silicon layers were fabricated and irradiated with beams of noble gases ions. The shift in a crystalline phase line in Raman spectra was used to estimate the size of nanocrystallites in the structure of porous silicon at different irradiation fluences. Electron paramagnetic resonance measurements were conducted to obtain the fluence dependence of the spin concentration in the porous structure after ion implantation. The concentration of paramagnetic defects (dangling bonds) in porous silicon was found to be monotonically dependent on ion irradiation fluence and reached the value of 1021 cm−3. A comparison of the defect formation rate in bulk and porous silicon is shown, the extreme spin concentration in porous silicon is estimated. The potential application of ion beam irradiations to enhance the efficiency of porous silicon-based contrasts agents in magnetic resonance imaging is discussed.

Titania-based nanoheterostructured microspheres for prolonged visible-light-driven photocatalysis

Titanium dioxide is a widely used photocatalytic material possessing such advantages as safety, low cost, and high reactivity under the ultraviolet light illumination. However, its applicability in sunlight is limited due to the wide band gap and, as a consequence, the low quantum yield. Doping of titanium dioxide with metal or non-metal atoms and creating heterojunctions based on it are some of the most efficient ways to overcome this drawback. Herein we propose a new facile way of synthesis of nitrogen-doped TiO2/MoO3 and TiO2/WO3 microsphere-shaped nanocomposite photocatalysts, combining the advantages of these two methods. It is revealed that such structures are not only photo-active when exposed to visible light, but can also accumulate a photoinduced charge, thus allowing the catalytic reaction to be prolonged for a long time after the illumination is switched off (up to 48 h). With the help of EPR spectroscopy, paramagnetic defects in the samples were determined. The obtained results show good application prospects of the visible-light-driven TiO2-based nanoheterostructured microspheres in the environmental purification.

NMR and EPR study of cubic π-phase SnS semiconductor nanoparticles

We report an 119Sn nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) study of cubic π-phase SnS semiconductor nanoparticles. The data are compared with orthorhombic α-SnS. The NMR spectrum of cubic SnS reveals two inequivalent tin atoms in its crystal structure. The spin-lattice relaxation time of 119Sn nuclei in the cubic phase is much longer than that in the orthorhombic phase, indicating that the former is mainly governed by Raman scattering, while the latter is dominated by interaction of nuclear spins with unpaired electron spins of paramagnetic defects. This is supported by EPR measurements showing much larger content of paramagnetic defects in the orthorhombic phase compared with the cubic phase. The study provides novel research findings on the physical properties of the newly discovered cubic π-SnS, which is an emerging semiconducting material system for various applications.

Photoactivity under visible light of defective ZnO investigated by EPR spectroscopy and photoluminescence

The photochemical and photophysical properties of a defective zinc oxide prepared by precipitation have been investigated using mainly electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. As already reported in the literature the band gap of the oxide contains intra band gap states related to the presence of point defects (mainly cation vacancies) in the structure. The concentration and the energy levels of such defects, a fraction of them being paramagnetic, are the basis of a mechanisms of double and multiple excitations allowing visible photons with energy lower than the band gap value (hν <Eg) to promote electrons from the valence band (VB) to the conduction band (CB) similarly, though less efficiently, to what done by UV light with hν >Eg. Remarkably, some of the paramagnetic defects present in the bulk of the as prepared materials derive from excitation of VB electrons induced by ambient light illumination.The electron excitation from VB to CB has been observed upon irradiation under vacuum at 77 K with polychromatic light and applying four different filters to the emitted radiation with cut-off in the range between 400 nm and 495 nm. The process has been monitored by EPR as the photoexcitation ends up in the formation of trapped electron centers (signal at g = 1.96) and trapped hole centers (signal in the range 2.021 >g > 2.003) both clearly detected by this technique. A scheme of the energy state distribution in the band gap of the defective oxide, obtained coupling EPR and photoluminescence results, is proposed. Finally, a non-negligible fraction of the visible light generated carriers reach the surface of the nanocrystals, entailing the typical redox reactions of photocatalysis as indicated by the detection of hydroxyl radicals in solution performed by spin-trapping. For this reason, the wet chemistry prepared zinc oxide can be considered a visible light active (VLA) system.

“Gamma Rays Interactions on Optical, FTIR and ESR Spectra of Alkaline Earth Binary Borate Glasses”

Abstract Alkaline earth binary borate glasses with the composition of (100-x) B2O3 + X (BaO, CaO, SrO) where x=22, 32 or 42 wt. % were prepared by melting annealing technique. Some of their physical and spectroscopic properties have been studied before and after gamma irradiation such as Density, oxygen packing density, molar and specific volumes, as well as optical UV-visible absorption, optical energy gap (Eopt), FTIR and ESR spectroscopy. The UV-visible spectra show a shifting of the optical edge to higher wavelengths with increasing the polarizability of the introduced modifier ion and the dose of gamma radiation. FTIR spectra show the vibrational bands of the structural borate groups; the triangular BO3 and tetrahedral BO4 units at 1200-1600 cm-1 and 800-1200 cm-1 respectively. ESR spectra show a constant behavior of the glasses at the low doses of radiation up to 60 kGy but show high effect of radiation at the higher doses due to the formation of more paramagnetic defects and free radicals.

Recombination luminescence of X-ray induced paramagnetic defects in BaY2F8

Recombination luminescence (RL) and RL-detected electron paramagnetic resonance (RL-EPR) in BaY2F8 single crystal has been investigated after irradiation with X-rays at low temperature. The recombination process, which lasts for several hours at 4 K, results in several broad bands in the RL spectrum. RL-EPR spectra show pronounced angular dependences on crystal orientation relative to external magnetic field. Based on the determined spin-Hamiltonian parameter values the recombination centres have been proposed to be F-type electron and self-trapped hole (VK) centres.

Influence of the Formation Conditions and Parameters of Defects on Carrier Transport in Nanocrystalline Cobalt Oxide

The structural and electrical properties of samples made of nanostructured cobalt oxide prepared by thermal decomposition and electrospinning (as well as the characteristics of their paramagnetic defects) have been investigated. A correlation is found between the conductivity of the obtained material and its structural parameters, depending on the synthesis method. It is revealed by electron spin resonance (ESR) spectroscopy that the conductivity of nanostructured cobalt oxide is determined by both the concentration of defects and their characteristics.

A combined study of the thermoluminescence and electron paramagnetic resonance of point defects in ZrO2:Er3+

This work describes thermoluminescence of beta irradiated zirconium oxide (ZrO2) doped with erbium. The sample was synthesized by the solution combustion method. Kinetic analysis of the thermoluminescence is reported and paramagnetic defects sensed by electron paramagnetic resonance (EPR) have been investigated. Using X-ray diffraction, the phase of the sample was determined to be monoclinic. Glow curves measured at 1 °C s−1 show two peaks at 50°C (peak 1) and at 112°C (peak 2). The dose response of peak 1 is sublinear within 51–411 Gy whereas that of peak 2 is linear between 51 and 155 Gy becoming sublinear thereafter up to 411 Gy. Peak 1 fades within 1800 s of irradiation whereas peak 2 fades to 59% of its initial intensity within 14,600 s of irradiation. Kinetic analysis of the peaks was done using the initial rise, whole glow peak and curve fitting methods. The order of kinetics of both peaks was determined to be first order. The activation energy of peaks 1 and 2 were found to be 0.67 ± 0.01 eV and 0.71 ± 0.02 eV respectively. The EPR spectrum of erbium doped ZrO2 reveals that there are two types of Zr3+ ions present in the sample as distinguished by their coordination in the material. The F+ type centres are generated during sample irradiation and these centres accountable for the TL peaks.

Local structure and kinetics of paramagnetic defects, induced by γ-irradiation of the erbium doped Ag5Ga5Ge95S200 glasses

Defects induced by γ-irradiation in Er-free and Er-doped glasses Ag5Ga5Ge95S200 were studied using the Raman and EPR spectroscopies. It is shown that γ-irradiation causes an increase in the intensity of Raman bands related with formation of S3Ge-GeS3 structural groups. During the low-temperature annealing at temperatures of 95–145 °C, the concentration Ns of γ-induced paramagnetic centres (PC), having the axial symmetry of the g-tensor with gz = 2.0180 and gx = gy = 2.0088, is being strongly reduced and the symmetry of the environment of these centres becomes lower. Subsequent storage of annealed samples in the air for about 15 days leads to a gradual restoration of the Ns concentration. The kinetics of these processes is described taking into account both the high mobility of sulfur vacancy and the long-time relaxation of the glass structure. A correlation between the intensity of 257 cm−1 Raman band belonging to the S3Ge(Ga)-(Ga)GeS3 complex and PC characteristics is discussed.

Experimental and theoretical interpretation of the order/disorder clusters in CeO2:La

A spectroscopic study of nanostructured lanthanum-doped cerium oxide samples showed that a mild decrease by 5.3% in the surface concentration of Ce(III) species and, a reduction by 0.2 eV in the so-called energy-gap (EF – Ev) after lanthanum addition was sufficient to create the previously reported significant dual behavior when exposed to carbon monoxide at 653 K. The observed X-ray diffraction patterns indicated the presence of a pure fluorite-type crystalline structure. A higher presence of paramagnetic defects clusters, indicated by electron paramagnetic resonance spectroscopy measurements and confirmed by the X-ray photoelectron spectroscopy, was attributed as being responsible for its dual behavior in a CO(g) atmosphere. Theoretical studies have shown the band structure and density of state of the pure and doped material, illustrating the orbitals that participate in the valence band and conduction band. The addition of a small quantity of La3+ converted some Ce3+ into Ce4+, narrowed the “effective band-gap” by 0.2 eV, and created a singly ionized oxygen vacancy species, thus changing the total electrical resistance and creating its dual behavior. The exposure to a reducing atmosphere such as carbon monoxide resulted in a significant increase in defects and converted some Ce4+ into Ce3+ and vice-versa with oxygen atmosphere exposure.

Paramagnetic Mn Antisite Defects in Nanoceramics of Aluminum–Magnesium Spinel

The effect of structure and size parameters on the formation of intrinsic and impurity paramagnetic centers in nanoceramics of aluminum–magnesium spinel is studied. The studied samples (grain size ~30 nm) are obtained by thermobaric synthesis. Microcrystalline ceramics and MgAl2O4 single crystal are used as the reference samples. Characteristic paramagnetic centers of Mn2+ (hyperfine structure constant (HFS) A = 82 G) are present in both single crystal and microceramics. In the studied samples of nanoceramics in the initial state, both impurity Mn2+ and intrinsic F+ centers exist. Unlike the nanoceramics, the centers of F+ type in the reference sample appear only after the irradiation with accelerated electrons (130 keV). The parameters of Mn2+ centers in nanoceramics significantly differ on that in microceramics and single crystal. EPR signal of Mn2+ centers in nanoceramics is characterized by two anomalous constant HFS (A1 = 91.21 G, A2 = 87.83 G) caused by two types of octahedrally coordinated manganese ions ([Mn2+]$$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$$ antisite defects). The features of spectral parameters of manganese centers correlate with a decrease in the cell parameter of MgAl2O4 in the nanostructural state. The observed effects are interpreted based on the assumed scheme of [Mn2+]$$_{{{\text{A}}{{{\text{l}}}^{{{\text{3 + }}}}}}}$$ charge compensation by the aluminum antisite defect and F+ center.

Structural transformation and nature of defects in titanium carbide treated in different redox atmospheres.

The defect structure and phase formation processes occurring in the bulk and on the surface of titanium carbide (TiC) under thermal treatment in different ambient atmospheres (air, hydrogen, vacuum) were characterized using XRD, SEM, EPR, XPS and IR-spectroscopy. The oxidized states of both titanium (TiO2−x) and carbon, in the form of carbonate–carboxylate structures (O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O), were found on the surface of untreated TiC. Carbon vacancies were detected as paramagnetic defects in the crystalline lattice of TiC. The heat treatment of TiC in reducing conditions leads to an increase of the electrical conductivity associated with the formation of structural defects. Annealing of TiC in air causes its oxidation with the formation of an anatase-type TiO2 phase. Paramagnetic defects typical of both TiC and TiO2−x were revealed.

γ-Irradiation effects in borosilicate glass studied by EPR and UV–Vis spectroscopies

Assessing the behavior of borosilicate glass used for nuclear waste immobilization under irradiation is essential for determining the long-term behavior of the high-level waste. In this work, the γ irradiation induced structural properties of the borosilicate glass have been studied via ultraviolet–visible (UV–Vis) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. The optical band gap (Eg) and the Urbach energy (Eu) deduced from the UV–Vis spectra of borosilicate glasses. The Eg decreased and the Eu increased with the increase of absorbed dose, which indicated the increase of non-bridging oxygen and disorder in the glass network. The EPR results revealed that there formed the following paramagnetic defects: hole center associated with boron (BOHC), hole centers near alkaline ions (HC1 center), E′ centers (Si), peroxy radical (Oxy defect), non-bridging oxygen hole centers (NBOHC), BO3− center, Si − BOHC (BOSi) and B–O–B–iso center (HOHOBOH-O·-BOHOH).

Introduction of spin centers in single crystals of Ba2CaWO6−δ

Developing the field of quantum information science (QIS) hinges upon designing viable qubits, the smallest unit in quantum computing. One approach to creating qubits is introducing paramagnetic defects into semiconductors or insulators. This class of qubits has seen success in the form of nitrogen-vacancy centers in diamond, divacancy defects in SiC, and P doped into Si. These materials feature paramagnetic defects in a low nuclear spin environment to reduce the impact of nuclear spin on electronic spin coherence. In this work, we report single crystal growth of Ba$_2$CaWO$_{6-\delta}$, and the coherence properties of controllably introduced W$^{5+}$ spin centers generated by oxygen vacancies. Ba$_2$CaWO$_{6-\delta}$ ($\delta$ = 0) is a B-site ordered double perovskite with a temperature-dependent octahedral tilting wherein oxygen vacancies generate W$^{5+}$ (d$^1$), $S = \frac{1}{2}, I$ = 0, centers. We characterized these defects by measuring the spin-lattice ($T_1$) and spin-spin relaxation ($T_2$) times from T = 5 to 150 K. At T = 5 K, $T_1$ = 310 ms and $T_2$ = 4 $\mu$s, establishing the viability of these qubit candidates. With increasing temperature, $T_2$ remains constant up to T = 60 K and then decreases to $T_2$ $\approx$ 1 $\mu$s at T = 90 K, and remains roughly constant until T = 150 K, demonstrating the remarkable stability of $T_2$ with increasing temperature. Together, these results demonstrate that controlled defect generation in double perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for QIS.

Paramagnetic defects in neutron-irradiated α-quartz: Novel Al-associated E’ centers

Two new paramagnetic defects ( and ) have been revealed in neutron-irradiated natural -quartz by using electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of the center as well as a previously reported but incompletely characterized center demonstrate that their super-hyperfine structures arise from interaction with 27Al, the first-ever examples of Al-associated centers in crystalline quartz. The matrices g and , of , g and of the center, have been determined.

Location of paramagnetic defects in detonation nanodiamond from proton spin-lattice relaxation data

We developed an approach for determining location of intrinsic paramagnetic defects in nanodiamonds from the data of proton spin-lattice relaxation of the surface hydrogen atoms. The approach was applied to the detonation nanodiamond (DND) of the diameter of 5 nm. We found that dangling bonds with unpaired electron spins are located within the near-surface belt at the distance of 0.3–0.9 nm from the DND surface. The NMR data are compared with the results of EPR measurements.

Electronic g-tensors of nanodiamonds: Dependence on the size, shape, and surface functionalization.

The electronic g-tensor dependence on the size, shape, and surface functionalization of nanodiamonds (NDs) is theoretically investigated by selecting dangling bonds and single substitutional nitrogen atoms as a main source of the unpaired electrons. The performed g-tensor calculations reveal that aforementioned paramagnetic impurities introduced into octahedrally shaped ND of C84H64 size behave in a very similar manner as those embedded into a smaller octahedral model of C35H36 size. Since cubic and tetrahedral NDs-C54H48 and C51H52-demonstrate a wider range of g-shift values than octahedral systems, the g-tensor dependence on different shapes can be considered as more pronounced. However, a different surface functionalization scheme, namely, fluorination, results in a much larger variation of the g-shifts, pointing to a significant impact the F atoms have on the local environment of the unpaired electrons in C35F36. A partial surface functionalization of C35H36 with benzoic acid and aniline groups indicates that, in some special cases, these linkers might induce a noticeable spin density redistribution which in turn substantially modifies the g-shift values of the system. Additional infrared (IR) spectra calculations show that some of paramagnetic defects in C35H36 and C35F36 possess clearly expressed signatures which could be useful while analyzing the experimental IR spectra of NDs.

Electrically detected electron nuclear double resonance in 4H-SiC bipolar junction transistors

We demonstrate high signal-to-noise electrically detected electron-nuclear double resonance measurements on fully processed bipolar junction transistors at room temperature. This work indicates that the unparalleled analytical power of electron-nuclear double resonance in the identification of paramagnetic point defects can be exploited in the study of defects within fully functional solid-state electronic devices.

Energy transfer and defect formation processes in nano silicon under the influence of epithermal neutrons

The neutrons scattering and capture cross-section processes has been calculated for natural 28 Si , 29 Si , 30 Si isotopes which are main part of nanosilicon samples when irradiated for 20 hours by epithermal neutron flux. The values of energies has been determined which given to nanosilicon nuclei as a result of scattering processes in the energy intervals of investigated neutrons. The cross-sections of radiation capture process and the amount of 31 Si radioactive isotope which can be formed by by 30 Si isotope in the energy interval of epithermal neutrons, the parameters of energy supply and ionization processes has been determined by interaction between energy carried of β - particles which disseminated in evironment and silicon atoms as a result of their β - decay. The formed defects has been determined in electron structure of nanosilicon under the influence of primary and secondary electron beams. Characterized interaction processes between nanosilicon and gamma rays irradiated from radioactive isotopes in impurities up to 1% in nanosilicon which formed under the influence of neutron flux. As a result of SEM investigation, interaction between surface defects inherent to nanoscale systems and O 2 , H 2 O active components that arranged environment and increasing number of surface oxidation atoms determined under the influence of radiation from radioactive isotopes which are product of radiation capture processes when impact by neutron flux. The progression of agglomeration processes of nanosilicon particles under the influence of secondary radiation processes that caused by neutron flux has also been proved experimentally by SEM investigations. The characteristic of identification and generation processes of paramagnetic defects, that formed as a result of secondary radiation processes investigated by electron paramagnetic resonance spectroscopy method.