Vol 59, No 4 (2018)

Graphene films were synthesized with the method of chemical vapor deposition using gaseous methane as the source of carbon and copper foil as the substrate for the deposition. The following conditions were found optimal to grow large-area high quality graphene films: preliminary annealing of the foil in the argon/hydrogen mixture at 970–990 °С for 30–40 min; simultaneous supply of the argon/hydrogen mixture (100 cm³/min) and methane (10 cm³/min) for 5–10 minutes, and subsequent cooling in an inert atmosphere. As a result, 1–10 layered graphene films were obtained to fully coat the copper foil over the area up to 50 cm². Several methods have been developed to transfer graphene to dielectric substrates such as silicon oxide and flexible polymer films. The obtained graphene films were used to create a flexible transparent conductive touch panel and a highly sensitive resistive humidity sensor exhibiting fast response-recovery time.

Carbon dots were prepared by the method of hydrothermal synthesis from carbon precursors of glucose and birch bark soot in aqueous ammonia. The distribution of lateral sizes of carbon dots testifies their average size to be 10–12 nm for glucose and 20–22 nm for soot. Infrared absorption spectra indicate oxygen groups on the surfaces of obtained carbon dots. Aqueous suspensions with glucose-based carbon dots exhibit strong absorption in the visible region from 300 nm to 500 nm. Soot-based carbon dots demonstrate strong absorption in the ultraviolet region, but are transparent in the visible region. The luminescence spectra exhibit that carbon dots synthesized from glucose and soot are luminescent in the same spectral region, their wavelengths of radiation depend on the wavelengths of excitation, and the intensity of luminescence depends on the presence of oxygen groups on the surface of carbon dots. Carbon dots synthesized from glucose and soot have great prospects in terms of their application in biology and medicine.

The paper considers the effect of SF₆ plasma-chemical treatment on the processes of defect formation and the electrical properties of graphene oxide partially reduced by heat treatment. The fluorine content on the graphene oxide surface is shown to increase as a result of SF₆ plasma treatment, depending on the plasma power and the duration of the treatment. The measured electrical parameters testify increased resistance of graphene oxide films as a result of plasma treatment. The rate of resistance change depends on the thickness of the films and is minimal for thin structures (∼10 nm). Further heating of graphene oxide decreases its resistance, but the content of surface fluorine changes insignificantly. Thin films (10-15 nm) exhibit the smallest change of their resistance as a result of annealing. The highest rate of resistance change is observed for non-fluorinated samples. The obtained data indicate that only several nanometers of nearsurface layers are subject to SF₆ plasma fluorination. The results testify the possibility of using SF₆ plasma treatment as an effective tool for selective fluorination of graphene oxide surface layers and controlled modification of its properties without changing the bulk properties of the material.

The conductivity of thermally reduced graphene oxide (GO) measured at various reduction stages shows that the samples demonstrate transition to the conducting state within a narrow range of annealing temperatures (150-170 °С). The conductivity in this temperature region increases by about five orders of magnitude to indicate that the percolation mechanism is responsible for this transition. According to this mechanism, the above temperature range is associated with formation of a certain number of conductive channels composed of GO fragments reduced to the conducting state. Thus, it is a new type of percolation transition, since increased conductivity is a result of increased conductivity of the particles due to thermal treatment rather than a result of increased concentration of conducting particles. A further temperature growth is associated with increased number of conducting GO fragments and the number of conducting channels, and some smoother conductivity increase. The XPS spectra of partially reduced GO samples indicate correlation between increased conductivity of GO fragments and plasmon oscillations revealed as XPS peaks. The current-voltage characteristics measured for the samples of partially reduced GO indicate a non-ohmic conductivity which manifests as increased conductivity versus voltage dependence. This is due to the fact that electrical resistance of the percolation channels is the sum of resistances of conductive GO fragments and contact resistances which depend on the voltage drop at the contact and, therefore, on the voltage applied.

An aqueous suspension of graphite oxide (GrO) was prepared from natural Yakut graphite by modified Hummers′ method. The lateral size of GrO flakes varied from 0.1 μm to 10 μm, their thickness was 20 nm. The fabricated suspension, GrO films (of various thickness and on various substrates), and GrO papers were studied in terms of their structural, optical, and electrophysical characteristics. The obtained GrO films are dielectrics with quite large resistance varying from 12.5.10⁶–4.6.10⁹ Ω depending on their thickness. The films are characterized by the luminescence in the region of 380–650 nm, the presence of oxygen-containing groups–ОН,–СООН,–С=О,–СОН, СОО–, and the transparency of 91% for a 20 nm thick film at the wavelength of 670 nm. The conducted study testifies high quality of Yakut graphite, which can be quite easily exfoliated. GrO films possess high resistance and transparency.

The paper considers the system of nitrogen-doped carbon nanofibers (N-CNFs) with palladium atoms deposited on their surfaces. The concentration of deposited palladium varied in the interval of 0.05-0.6 wt.%. The state of palladium was studied with the methods of quantum chemistry, electron microscopy, CO adsorption, and EXAFS. Carbon structures that contain porphyrin-like defects with four nitrogen atoms in the graphene layer interact strongly with palladium atoms and therefore can be stabilization centers of atomic palladium.

We propose a novel approach which considers the positions of defects in graphene structure to describe how electronic density of states and the type of graphene conductivity are affected by electron scattering by certain configurations of foreign atoms in graphene matrix. Despite the fact that there is still insufficient experimental data concerning the effect of short-range order on graphene physical properties, we assume that local disorder can play a decisive role in the low-temperature behavior of graphene’s electronic properties.

We present a theory to describe the interaction of electrons in gapped and gapless graphene with a strong off-resonant electromagnetic field (dressing field). This interaction (electromagnetic dressing) is shown to renormalize substantially electron velocities and the band gap in gapped graphene. Particularly, renormalized electronic parameters depend strongly on the field polarization: linearly polarized fields always reduce the gap, while circularly polarized fields break the equivalence of the valleys at various points of the Brillouin zone and can increase or decrease the corresponding band gaps. Moreover, a linearly polarized dressing field induces anisotropy of electron dispersion in the graphene plane. Consequently, dressing fields can be an effective tool to control electronic properties of graphene and be prospectively used in various optoelectronic devices.

The methods of molecular dynamics were used to study structural changes observed in bilayer silicene and thin slices of an ordinary Si crystal when transferred to the graphite substrate. The shape of the radial distribution function indicates noticeable distortion of the long-range order in the Si crystalline film. The first high peak of this function indicates honeycomb lattice in silicene. The motion the lithium ion in the silicene-graphene channel has been studied. The external strengthening of the silicene channel with graphene sheets not only increases its firmness but also improves the passage of intercalated lithium ions through this channel. The stresses in the channel walls are substantially decreased due to vacancy defects. Defects in the form of hexa-vacancies preserve their shape better than others defects when lithium ions pass through the channel.

We present a novel model for semiquantitative estimation of optimal concentrations of silver modifier necessary to increase the conductivity of graphene electrodes in actuators with electroactive polymers and to ensure long-term performance of the system. The model predicts approximately equal masses of silver and graphene in the electrodes after the reduction for the characteristic size of graphene flakes ∼10 nm. Also, the concentration is shown to depend strongly on the size of the flakes.

The paper presents the experimental results on temperature dependences of electrical resistance of disordered single-walled carbon nanotube (SWNT) films on polyethylene terephthalate (PET) substrates and discusses the piezoresistive effect studied in the films within the strain ranging from –0.15% to +0.15%. The nanotubes were prepared by catalytic disproportionation of carbon monoxide on Fe particles obtained by ferrocene vapor decomposition. SWNT films were prepared by their in situ deposition on silicon substrates and transferred to PET substrates. Electron transport properties were studied from room temperature down to 77.4 K. It is shown that the experimental data are described by the fluctuation-induced tunneling conduction model. The effective activation energy estimated by approximating experimental data varies from 175 meV to 6.5 meV for the samples with the time of nanotube deposition varying from 5 min to 120 min, respectively. The strain gauge factor measured in the film with the smallest sheet electrical resistance appeared to be negative and equal to –14.

Two coordination polymers are obtained by self-assembly reactions of Cd²⁺ with 2,3-bis((1H-1,2,4-triazol-1-yl)methyl)quinoxaline (BTMQ) and terephthalic acid at room temperature. Complex 1 exhibits a two-dimensional planar structure, which crystallizes in the monoclinic space group P2₁/c with unit cell parameters: a = 12.752(4) Å, b = 9.416(3) Å, c = 16.069(5) Å, α = 90, β = 102.146(5), γ = 90°, V = 1886.4(10) ų, Z = 2. Complex 2 shows a Z-type two-dimensional network structure, and crystallizes in the triclinic space group P-1 with unit cell parameters: a = 6.917(4) Å, b = 10.338(5) Å, c = 16.156(8) Å, α = 96.889(7), β = 93.421(8), γ = 96.161(7)°, V = 1137.2(10) ų, Z = 1. Fluorescence spectral studies of BTMQ and the two complexes are also investigated, and the results show that the fluorescence intensity of complex 1 is stronger than that of the ligand, but the fluorescence intensity of complex 2 is weaker than that of the ligand.

Two new 4-nitro-1,2-benzenedicarboxylate-bridged Cd(II) coordination polymers Cd(phen)(NPTA) 1 and Cd(im)₂(NPTA)·H₂O 2 (H₂NPTA = 4-nitro-1,2-benzenedicarboxylic acid, phen = 1,10-phenanthroline, im = iminazole) are synthesized. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations are carried out to study the vibrational frequencies and electronic properties. The results are in compliance with the experimentally obtained structural and spectral data.

Single and dual substituent correlation analyses are applied to study the transmission of substituent effects on IR stretching frequencies of N–H, C=N, B–N, and B–O bonds in 3,5-disubstituted-4,5-dihydro-1,2,4,5- oxadiazaboroles (1a-1r). For the correlation study σ, F, R, and R^– parameters are used as constants. The substituent effects are estimated based on the results of the statistical analysis. The differences among the regression coefficients are discussed in terms of the relative importance of the substituent field and resonance effects. For a better understanding of the results, density functional theory (DFT) calculations are performed to determine the preferred geometry and to calculate the theoretical stretching frequencies of N–H, C=N, B–N, and B–O bonds.

A reaction of Pd(PPh₃)₄ (complex 1) and 6,6′-dibromo-2,2′-bipyridyl C₁₀H₆N₂Br₂ in a 1:1 molar ratio or 2:1 produces a mononuclear [Pd(PPh₃)₂{κ¹-C₁₀H₆N₂Br}(Br)] complex (2) and a binuclear [Pd(PPh₃)₂Br]₂{μ,κ²-C₁₀H₆N₂}complex (3). In complex 3, the 2,2′-bipyridyl ligand is coordinated through two C and two metal (Pd) centers and bridges two Pd atoms to the trans-N of the 2,2′-bipyridyl ligand. The crystal characteristics are outlined as follows: space group P2(1)/n, Z = 2, a = 9.2008(4) Å, b = 14.5268(6) Å, c = 30.6609(13) Å, α = 90, β = 91.3415(13), and γ = 90°. Complexes 2 and 3 are characterized through elemental analyses, fast atom bombardment mass spectrometry, ¹H, ¹³C, and ³¹P nuclear magnetic resonance spectroscopy, and single crystal X-ray diffraction.

In this paper, the formation of Schiff base compounds has been explained by the templating effect of a metal ion modulated by counter anions. It is shown that a dicondensed Schiff ligand can utilize the N₂O₂ compartment to form a stable copper complex. However, in the presence of a strongly coordinating thiocyanate ion, the dicondensed ligand is converted to the corresponding monocondensed ligand to yield a square planar copper(II) complex coordinated by the NNO donor set of the Schiff base and the terminal thiocyanate coligand. This template approach provides a methodology to trap the monocondensed ligand HL² [2-((E)-(3-aminopropylimino)methyl)-6-methoxyphenol] as its Cu(II) compound [CuL²(NCS)]. We report herein the crystal structure and spectroscopic studies of this newly synthesized complex.

For the first time the crystal structure and configuration of matricaria ester 1 are determined by X-ray crystallographic analysis. Furthermore, the cytotoxic and antioxidant activities of the matricaria ester are investigated.