Dariya Savchenko

Comprehensive portrait of cholesterol containing oxidized membrane.

Biological membranes are under significant oxidative stress caused by reactive oxygen species mostly originating during cellular respiration. Double bonds of the unsaturated lipids are most prone to oxidation, which might lead to shortening of the oxidized chain and inserting of terminal either aldehyde or carboxylic group. Structural rearrangement of oxidized lipids, addressed already, is mainly associated with looping back of the hydrophilic terminal group. This contribution utilizing dual-focus fluorescence correlation spectroscopy and electron paramagnetic resonance as well as atomistic molecular dynamics simulations focuses on the overall changes of the membrane structural and dynamical properties once it becomes oxidized. Particularly, attention is paid to cholesterol rearrangement in the oxidized membrane revealing its preferable interaction with carbonyls of the oxidized chains. In this view cholesterol seems to have a tendency to repair, rather than condense, the bilayer.

The electron spin resonance study of heavily nitrogen doped 6H SiC crystals

The magnetic and electronic properties of heavily doped n-type 6H SiC samples with a nitrogen concentration of 1019 and 4 × 1019 cm−3 were studied with electron spin resonance (ESR) at 5–150 K. The observed ESR line with a Dysonian lineshape was attributed to the conduction electrons (CE). The CE ESR (CESR) line was fitted by Lorentzian (insulating phase) (T < 40 K) and by Dysonian lineshape (metallic phase) above 40 K, demonstrating that Mott insulator-metal (IM) transition takes place at ∼40 K, accompanied by significant change in the microwave conductivity. The temperature dependence of CESR linewidth follows the linear Korringa law below 40 K, caused by the coupling of the localized electrons (LE) and CE, and is described by the exponential law above 40 K related to the direct relaxation of the LE magnetic moments via excited levels driven by the exchange interaction of LE with CE. The g-factor of the CESR line (g‖ = 2.0047(3), g⊥ = 2.0034(3)) is governed by the coupling of the LE of nitrogen donors a...

Electron paramagnetic resonance study of paramagnetic centers in carbon-fumed silica adsorbent

Fumed silica A-300 was carbonized by means of pyrolysis of CH2Cl2. The obtained initial SiO2:C nanopowders of black color, with an average diameter of 14–16 nm and carbon (C) concentration 7 wt. %, subjected to the oxidation and passivation treatment were studied by electron paramagnetic resonance (EPR) in the temperature range 4–400 K. Two EPR signals of Lorentzian lineshape with nearly equal g-factors and different linewidth were observed in the initial, oxidized, and passivated SiO2:C nanopowders. The two-component EPR spectrum was explained by the presence of C in two electronic states. The intensive narrow EPR signal, which has a temperature-dependent intensity, linewidth, and resonance field position, was attributed to the carbon-related defect with non-localized electron hopping between neighboring C-dangling bonds. The striking effect is that the temperature dependence of the EPR linewidth demonstrates the motional narrowing of the EPR signal at very low temperatures from 4 K to 20 K, which is not...

Temperature dependent behavior of localized and delocalized electrons in nitrogen-doped 6H SiC crystals as studied by electron spin resonance

We have studied the temperature behavior of the electron spin resonance (ESR) spectra of nitrogen (N) donors in n-type 6H SiC crystals grown by Lely and sublimation sandwich methods (SSM) with donor concentration of 1017 cm−3 at T = 60–150 K. A broad signal in the ESR spectrum was observed at T ≥ 80 K with Lorentzian lineshape and g|| = 2.0043(3), g⊥ = 2.0030(3), which was previously assigned in the literature to the N donors in the 1s(E) excited state. Based on the analysis of the ESR lineshape, linewidth and g-tensor we attribute this signal to the conduction electrons (CE). The emergence of the CE ESR signal at T > 80 K was explained by the ionization of electrons from the 1s(A1) ground and 1s(E) excited states of N donors to the conduction band while the observed reduction of the hyperfine (hf) splitting for the Nk1,k2 donors with the temperature increase is attributed to the motional narrowing effect of the hf splitting. The temperature dependence of CE ESR linewidth is described by an exponential la...

Electrically-detected electron paramagnetic resonance of point centers in 6H-SiC nanostructures

The results of investigation of electrically-detected electron paramagnetic resonance (EDEPR) and classical electron paramagnetic resonance (EPR) (X-band) for the identification of shallow and deep boron centers, NVSi defects, and isolated silicon vacancies (VSi), which are formed directly during the preparation of planar nanostructures under conditions of silicon-vacancy injection at the SiO2/n-6H-SiC interface without any subsequent irradiation, are presented. The prepared sandwich nanostructures are an ultra-narrow p-type quantum well, confined by δ barriers heavily doped with boron on an n-6H-SiC surface, which are self-ordered during pyrolytic-oxide deposition and subsequent short-time boron diffusion. The EDEPR data of point centers in sandwich nanostructures, prepared within the framework of Hall geometry, are recorded by measuring the field dependences of the magnetoresistance without an external cavity, microwave source and detector, due to the presence of microcavities embedded in the quantum-well plane and microwave generation under conditions of a stabilized source-drain current from δ barriers containing dipole boron centers. The obtained EDEPR spectra of the shallow and deep boron centers are analyzed using the data of EPR studies in 6H-SiC bulk crystals [10]. The EDEPR spectrum of the isolated silicon vacancy reveals both the negatively charged state VSi− (S = 3/2) and the neutral state in hexagonal (VSi(h)) and quasicubic (VSi(k1, k2)) states (S = 1). In turn, NVSi defects are detected not only by the EDEPR method at 77 K, but also through the use of a Bruker ELEXSYS E580 EPP spectrometer at 9.7 GHz, in a temperature range of 5–40 K. The EDEPR and EPR spectra recorded on the same sandwich nanostructure are virtually identical and correspond to the center in the triplet state with spin S = 1. The EPR spectrum, which is a lowintensity line doublet with a splitting value equal to ΔB = 237.6 mT, is observed in the background of the EPR spectrum from donors of nitrogen, the concentration of which in the n-6H-SiC initial sample was 5 × 1018 cm−3, whereas nitrogen donor centers are not revealed in the EDEPR spectrum because of total occupation by silicon vacancies inside the 6H-SiC sandwich nanostructure.

Superhyperfine Interactions of the Nitrogen Donors in 4H SiC Studied by Pulsed ENDOR and TRIPLE ENDOR Spectroscopy

The nitrogen donors residing at quasi-cubic lattice site (Nk) in 4H SiC were investigated by field sweep electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and pulsed General TRIPLE ENDOR spectroscopy. The 29Si and 13C superhyperfine lines observed in the FS ESE and ENDOR spectra of Nk in n-type 6H SiC were assigned by pulsed General TRIPLE resonance spectroscopy to the specific carbon (C) and silicon (Si) atoms located in the nearest environment of Nk in 4H SiC. The superhyperfine interaction constants and their relative signs for Nk with 29Si and 13C nuclei located in the nearest-neighbor shells are found from the General TRIPLE ENDOR spectra to be positive for C atoms and negative for Si atoms.

Temperature behavior of the conduction electrons in the nitrogen-doped 3C SiC monocrystals as studied by electron spin resonance

The temperature behavior of the electron spin resonance (ESR) spectra of nitrogen donors in n-type bulk 3C SiC monocrystals with (ND − NA) ≈ 1017 cm−3 was studied at T = 10–50 K. The triplet lines due to the hyperfine (hf) interaction with 14N nuclei (I = 1, 99.6%) along with a single line with similar isotropic g values of 2.0050(3) were observed in the ESR spectrum of n-type 3C SiC monocrystals in the temperature interval from 10 to 35 K. The observed reduction of the hf splitting for the nitrogen donor residing cubic position (Nk) in the temperature interval from 15 to 35 K was attributed to the motion narrowing effect of the hf splitting. With further increase of the temperature up to 35 K, only one single line with a Lorentzian lineshape was observed in the ESR spectrum of n-type 3C SiC, which was previously assigned in the literature to the unknown deeper donor center. Based on the temperature behavior of the ESR linewidth, integral intensity, and g-value, we have attributed this signal to the condu...

The spin relaxation of nitrogen donors in 6H SiC crystals as studied by the electron spin echo method

We present the detailed study of the spin kinetics of the nitrogen (N) donor electrons in 6H SiC wafers grown by the Lely method and by the sublimation “sandwich method” (SSM) with a donor concentration of about 1017 cm−3 at T = 10–40 K. The donor electrons of the N donors substituting quasi-cubic “k1” and “k2” sites (Nk1,k2) in both types of the samples revealed the similar temperature dependence of the spin-lattice relaxation rate (T1−1), which was described by the direct one-phonon and two-phonon processes induced by the acoustic phonons proportional to T and to T9, respectively. The character of the temperature dependence of the T1−1 for the donor electrons of N substituting hexagonal (“h”) site (Nh) in both types of 6H SiC samples indicates that the donor electrons relax through the fast-relaxing centers by means of the cross-relaxation process. The observed enhancement of the phase memory relaxation rate (Tm−1) with the temperature increase for the Nh donors in both types of the samples, as well as ...

N-VSi-related center in non-irradiated 6H SiC nanostructure

We present the first findings of the vacancy-related centers identified by the electron spin resonance (ESR) and electrically-detected (ED) ESR method in the non-irradiated 6H-SiC nanostructure. This planar 6H-SiC nanostructure represents the ultra-narrow p-type quantum well confined by the δ-barriers heavily doped with boron on the surface of the n-type 6H-SiC (0001) wafer. The EDESR method by measuring the only magnetoresistance of the 6H SiC nanostructure under the high frequency generation from the δ-barriers appears to allow the identification of the silicon vacancy centers as well as the triplet center with spin state S=1. The same triplet center that is characterized by the larger value of the zero-field splitting constant D and anisotropic g-factor is revealed by the ESR (X-band) method. The hyperfine (hf) lines in the ESR and EDESR spectra originating from the hf interaction with the 14N nucleus allow us to attribute this triplet center to the N-VSi defect.