Surface Layer Of Crystal Research Articles

Формирование тонкого люминесцирующего слоя в кристаллах LiF под действием излучения тлеющего разряда

AbstractThe formation of thin layers of luminescent defects on the faces of planar lithium fluoride crystals located in the positive column and Faraday dark space of a glow gas discharge was studied by time-resolved confocal scanning luminescent microscopy and time-correlated single photon counting. The formation of aggregated color centers in the surface layers of crystals was established using the spectral and kinetic characteristics of luminescence appearing after irradiation. The role of gas discharge electrons, ions, and photons in the defect formation mechanism was considered. The defects were shown to be formed under the influence of vacuum ultraviolet (VUV) photons. The VUV radiation intensity distribution in the discharge gap was measured by the method of thermostimulated luminescence. The main source of this radiation was the anodic and cathodic voltage drop regions in a glow discharge.

Интенсивность контактного взаимодействия и перенос материалов при шлифовании и микроцарапании тугоплавких металлов

Peculiarities in the contact surfaces formation and material transfer at micro-scratching and grinding of refractory metals are investigated. There is shown a connection of metal quantity transferred to the area of crystal wear, a degree of ground surface charging with silicon carbide crystals and wear of an abrasive tool with the electron structure of atoms in refractory metals. It is defined, that the intensity of metal transfer in a crystal surface layer decreases with the increase of a principle quantum number of metal valence electrons. According to the intensity of the interaction silicon carbide during grinding and micro-cutting the refractory metals are classified into adhesion-active metals of IVB, VB sub-groups and inert metals of VIB subgroup of the Periodic Table.

Strontium isotope fractionation during strontianite (SrCO3) dissolution, precipitation and at equilibrium

In this study we examine the behavior of stable Sr isotopes between strontianite [SrCO3] and reactive fluid during mineral dissolution, precipitation, and at chemical equilibrium. Experiments were performed in batch reactors at 25°C in 0.01M NaCl solutions wherein the pH was adjusted by bubbling of a water saturated gas phase of pure CO2 or atmospheric air. The equilibrium Sr isotope fractionation between strontianite and fluid after dissolution of the solid under 1atm CO2 atmosphere was estimated as Δ88/86SrSrCO3-fluid = δ88/86Sr SrCO3−δ88/86Srfluid=−0.05±0.01‰. On the other hand, during strontianite precipitation, an enrichment of the fluid phase in 88Sr, the heavy isotopomer, was observed. The evolution of the δ88/86Srfluid during strontianite precipitation can be modeled using a Rayleigh distillation approach and the estimated, kinetically driven, fractionation factor αSrCO3-fluid between solid and fluid is calculated to be 0.99985±0.00003 corresponding to Δ88/86SrSrCO3-fluid = −0.15‰. The obtained results further support that under chemical equilibrium conditions between solid and fluid a continuous exchange of isotopes occurs until the system approaches isotopic equilibrium. This isotopic exchange is not limited to the outer surface layer of the strontianite crystal, but extends to ∼7–8 unit cells below the crystal surface. The behavior of Sr isotopes in this study is in excellent agreement with the concept of dynamic equilibrium and it suggests that the time needed for achievement of chemical equilibrium is generally shorter compared to that for isotopic equilibrium. Thus it is suggested that in natural Sr-bearing carbonates an isotopic change may still occur close to thermodynamic equilibrium, despite no observable change in aqueous elemental concentrations. As such, a secondary and ongoing change of Sr isotope signals in carbonate minerals caused by isotopic re-equilibration with fluids has to be considered in order to use Sr isotopes as environmental proxies in aquatic environments.

Water-vapor clustering on the surface of β-AgI crystal in the field of defects with a disordered structure

The Monte Carlo method has been employed to simulate the nucleation of condensed water phase from vapor at 260 K on a crystalline silver-iodide surface containing a defect in the form of a nanoscopic spot with a random distribution of ions. The free energy and work of formation of a nucleus have been calculated in the bicanonical ensemble at the molecular level as functions of nucleus size; computer images and spatial correlation functions of molecules have been obtained. The presence of a defect with a disordered (amorphous) structure, on the one hand, entails local destructions of a monomolecular film, but, on the other hand, shifts the onset of the adsorption process toward lower vapor pressures by several orders of magnitude. Under the conditions of a growing condensate film, the defect leads to its thermodynamic stabilization and a decrease in the barrier of the formation of subsequent layers, thereby weakening the known effect of the hydrophobicity of monomolecular films on crystalline surfaces with hexagonal structures. The factors that predetermine the abnormally high efficiency of silver-iodide particles as stimulators for atmospheric-moisture nucleation at negative Celsius temperatures seem to be the presence of extended defects on the surface of aerosol particles in combination with the hexagonal structure of their crystal lattice, the optimum magnitude of direct interactions between water molecules with ions of the crystal surface layer, and the collective domain-formation effects that result from a relatively high polarizability of iodine ions.

Fabrication and physicochemical characterization of porous composite microgranules with selenium oxyanions and risedronate sodium for potential applications in bone tumors.

Nanocrystalline hydroxyapatite containing selenite ions (SeHA; 9.6 wt.% of selenium) was synthesized using wet method and subject to careful physicochemical analysis by powder X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, solid-state nuclear magnetic resonance, wavelength dispersive X-ray fluorescence, and inductively coupled plasma optical emission spectrometry. SeHA was then used to develop the selenium-containing hydroxyapatite/alginate (SeHA/ALG) composite granules. Risedronate sodium (RIS) was introduced to the obtained spherical microgranules of a size of about 1.1–1.5 mm in 2 ways: during the granules’ preparation (RIS solution added to a suspension of ALG and SeHA), and as a result of SeHA/ALG granules soaking in aqueous RIS solution. The analysis made using 13C and 31P cross-polarization magic angle spinning nuclear magnetic resonance confirmed the presence of RIS and its interaction with calcium ions. Then, the release of selenium (inductively coupled plasma optical emission spectrometry) and RIS (high-performance liquid chromatography) from microgranules was examined. Moreover, cytotoxicity of fabricated granules was assessed by MTT test. Selenium release was biphasic: the first stage was short and ascribed to a “burst release” probably from a hydrated surface layer of SeHA crystals, while the next stage was significantly longer and ascribed to a sustained release of selenium from the crystals’ interior. The study showed that the method of obtaining microgranules containing RIS significantly affects its release profile. Performed cytotoxicity test revealed that fabricated granules had high antitumor activity against osteosarcoma cells. However, because of the “burst release” of selenium during the first 10 h, the granules significantly reduced viability of normal osteoblasts as well.

Dependence of the structure of ion-modified NiTi single crystal layers on the orientation of irradiated surface

The composition and structure of Si layers implanted into titanium nickelide single crystals with different orientations relative to the ion beam propagation direction have been studied using Auger electron spectroscopy and transmission electron microscopy. The role of the “soft” [111]B2 and “hard” [001]B2 NiTi orientations in the formation of the structure of ion-modified surface layer, as well as the defect structure of the surface layers of the single crystals, has been revealed. Orientation effects of selective sputtering and channeling of ions, which control the composition and thickness of the oxide and amorphous layers being formed, ion and impurity penetration depth, as well as the concentration profile of the Ni distribution over the surface, have been detected.

Investigation of the stability of electrical properties of reduced LiNbO3 crystals

The instability of the electrical properties of lithium niobate single crystals of congruent composition subjected to reducing thermochemical treatment has been investigated by impedance spectroscopy. It has been shown that the subsequent heating of the reduced lithium niobate samples in dry air up to 380 K or higher is accompanied by the progressive increase in their electric resistance, which is due to the oxidization of the crystal surface layers.

Design of tunable surface mode waveguide based on photonic crystal composite structure using organic liquid**Project supported by the National Natural Science Foundation of China (Grant No. 31401136) and the School Youth Fund of Henan University of Science and Technology, China (Grant No. 2014QN045).

With the method of replacing the surface layer of photonic crystal with tubes, a novel photonic crystal composite structure used as a tunable surface mode waveguide is designed. The tubes support tunable surface states. The tunable propagation capabilities of the structure are investigated by using the finite-difference time-domain. Simulation results show that the beam transmission distributions of the composite structure are sensitive to the frequency range of incident light and the surface morphology which can be modified by filling the tubes with different organic liquids. By adjusting the filler in tubes, the T-shaped, Y-shaped, and L-shaped propagations can be realized. The property can be applied to the tunable surface mode waveguide. Compared with a traditional single function photonic crystal waveguide, our designed structure not only has a small size, but also is a tunable device.

The Schoch effect mechanism analysis and its regulation of two dimensional three components phononic crystal

In this paper, through changing the phononic crystal surface and adjusting the structure of surface layer, we observed the Schoch effect of two dimensional phononic crystal, and the Schoch shift reached 7.1 times of the lattice constant. To further understand the physical mechanism of this kind of phenomenon, the finite element method is used to calculate the band structure of the model. Because of the influences of the surface of structure, there exists a kind of waveguide mode in the passband. Through careful analysis of the band structure and the sound pressure field profile, it is found the kind of waveguide mode makes the incident wave energy go into the waveguide layer which resulting in reflection beam deformation and offset. In addition, through the finite element simulation, the change regulation of the Schoch shift in the phononic crystal is studied by changing the radius of the surface layer and further increase the layer number of the surface layer of the phononic crystal to achieve the control of the Schoch effect of the phononic crystal. The conclusion of this paper can provide some theoretical reference and give some guidance for the design of acoustic sensors based on Schoch effect.

Glass Transition Behavior in Thin Polymer Films Covered with a Surface Crystalline Layer

Thin amorphous poly(ethylene terephthalate) (PET) films covered with/without a crystallized surface layer were prepared onto silicon wafers. In the former and latter cases, the surface mobility in the film was depressed and enhanced, respectively. The glass transition temperature (Tg) of the amorphous PET film decreased with the reduction of the film thickness, exhibiting a remarkable nanoconfinement effect. However, once the surface region of the thin film was crystallized, or frozen in terms of the segmental motion, Tg of the films recovered to that of the bulk. Concurrently, the apparent activation energy of the segmental motion in the surface-crystallized film was in good accordance with the bulk value as well. These results make it clear that the mobility in the surface region plays an essential role in the glass transition of the thin films.

Double Charged Surface Layers in Lead Halide Perovskite Crystals.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

The Strength Quantum Theory for Crystals with Nanodefects

The article examines the impact of nanodefects on the reduction of the crystals strength. A summary of the authors research results is illustrated by the analysis example of the NaCl crystal destruction process from the standpoint of strength quantum theory. As the base experiment is seen testing the dry and wet a salt crystal made by academician A. Ioffe. The authors research aim was a little-studied patterns of the crystal lattice destruction quantum mechanism. The article gives a brief historical background of the research in this area. The first section discusses the rystal cell dynamic characteristics change as a result of destruction. Assesses self-resonant frequency of an atom at a lattice point of the crystal before and after the destruction. In the second section of the article examines the relationship between the natural frequencies of the atoms in the lattice and elastic energy wich accumulated in the crystal body in the force loading process. The main attention is paid to theoretical modeling of the links strength between atoms in the crystal surface layer. We study the strength dependence of these relationships to the availability of nanodefects on the crystal surface. There are modeled bond strength to clean the crystal surface, as a result of the surface lattice electrification when atoms capture free electrons and for the case when jobs of atoms in the crack are filled with the external environment atoms. The calculations confirm the validity of the proposed theoretical model of reduction or the hanging of the crystals strength by the atoms interaction with the external environment.

Visible light absorbance enhanced by nitrogen embedded in the surface layer of Mn-doped sodium niobate crystals, detected by ultra violet - visible spectroscopy, x-ray photoelectron spectroscopy, and electric conductivity tests

Sodium niobate crystals doped with manganese ions, Na(NbMn)O3, were annealed in a nitrogen N2 flow at 600, 670, and 930 K. It was verified that simultaneous doping with Mn ions and annealing in nitrogen enhanced the photocatalytic features of sodium niobate. The transmission in the ultraviolet-visible range was measured at room temperature. The absorbance edge is in the range from 3.4 to 2.3 eV. The optical band gap Egap = 1.2–1.3 eV was evaluated using the Tauc relation. Crystals annealed at 670 K and 930 K exhibited an additional shift of the absorption edge of ∼20–40 nm toward longer wavelengths. The optical energy gap narrowed as a result of the superimposed effect of Mn and N co-doping. The x-ray photoelectron spectroscopy test showed that N ions incorporated into the surface layer. The valence band consisted of O 2p states hybridized with Nb 4d, Mn 3d, and N 2s states. The disorder detected in the surroundings of Nb and O ions decreased due to annealing. The binding energy of oxygen ions situated within the surface layer was EB ≈ 531 eV. The other contributions were assigned to molecular contamination. The contribution centered at 535.5 eV vanished after annealing at 600 K and 670 K. The contribution centered at 534 eV vanished after annealing at 930 K. The N2 annealing partly removed carbonates from the surfaces of the samples. In the 480–950 K range, the electric conductivity activation energy, Ea = 0.7–1.2 eV, was comparable with the optical Egap. The electric permittivity showed dispersion in the 0.1–800 kHz range that corresponds to the occurrence of defects.

The effects of external influences on the distribution of polarization in calcium barium niobate crystals of differing composition

The pyroelectric properties and state of polarization of calcium barium niobate single crystals CaxBa1–xNb2O6 (CBN) with x = 0.28, 0.30, and 0.32 are studied. It is shown that in contrast to CBN30 and CBN32 crystals, the effect of the alternating electric fields higher than the coercive field changes the state of polarization in the surface layer of the CBN28 crystal. At the same time, thermal cycling to temperatures higher than the Curie point leads to the formation of a system of antiparallel domains in CBN30 and CBN32 crystals, and to complete depolarization of CBN28 crystals.

Features of surface layer of LiNbO3 as-received single crystals: Studied in situ on treatment samples modified by elevated temperature

The surface layer of LiNbO3 congruent crystals was studied at room temperature and the 300–1000°C temperature range. The XPS test discerned two temperature ranges with different electronic structures and chemical compositions of the crystal sample surface layer. We observed a deficiency of Li ions in surface of as-received epi-polished samples, at RT, and in range from 300 to 500°C. The concentration of Li ions in epi-polished surface did not exceed 4%, which was much lower than the Li concentration determined for cleaved crystal, at RT. We explained this Li shortage with the use of leaching procedure, which was used in the crystal sample preparation stage. The XPS spectra, which have been obtained in the temperature of 300–500°C, have shown three oxidation states of niobium Nb 3d0, 3d1, 3d3, broadening of O 1s, and occurrence of C 1s core lines. In higher temperature range (700–1000°C), we detected an increased concentration of lithium and changes in the oxidation state of niobium that indicated congruent composition in the surface of lithium niobate crystal. The conditions, which corresponded to migration of Li ions, have been checked with the use of thermogravimetric analysis, effusion test, and TOF-SIMS spectroscopy. Thermogravimetry has shown mass losses of bulk crystal and powdered samples. The powdered samples exhibited saturation of mass loss at reducing conditions. The effusion experiments confirmed that O and Li2O were the most volatile species in LiNbO3. Mass spectroscopy (effusion and TOF-SIMS) shown out-diffusion of lithium ions from the deeper part of the crystal to the surface layer. TOF-SIMS maps exhibited a non–homogeneous composition and granular structure of distribution of the Li and Nb elements on the surface of the reduced sample.

High-concentration dispersions of exfoliated MoS2 sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS2 sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS2 nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS2 nanosheets using freeze-dried silk fibroin powders. Compared to MoS2 dispersion in the absence of silk fibroin powder, sonicated MoS2 dispersions with silk fibroin powder (MoS2/Silk dispersion) show noticeably higher exfoliated MoS2 nanosheet yields, with suspended MoS2 concentrations in MoS2/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL–1, respectively. The MoS2 concentration in the MoS2/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS2 nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS2 nanosheets spin-cast from MoS2 /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS2 crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS2 nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS2 nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS2 nanosheets contents.

Structure of water adsorbed in slit-shaped pores of silver iodide crystal

The structure of the adsorbate formed from water vapor in flat nanoscopic pores having the width of 2.5, 1.25, and 0.625nm is investigated in detail by Monte Carlo simulations in terms of spatial correlation functions, angle distributions and molecular configurations. The polarization energy in the interaction with the walls of the pores is included into the Hamiltonian in explicit form. The adsorption on the wall with positive ions in surface crystal layer is considerably ahead of the adsorption on the opposite wall, with negative ions on the surface. At 260K, intensive clustering takes place on the walls at initial stage of the adsorption. Five-membered molecular cycles compete successfully with six-membered ones. Traces of five- and six-membered cycles are observable even at 400K, though not more than a tenth part of all hydrogen bonds in the adsorbate at submonomolecular stage survives when heating to this temperature. The monomolecular film growing on the walls has a pronounced hexagonal structure resembling honey comb. On both walls, the film is retained on the surface due to the interactions not with the nearest ions, but with the ions of the second crystal layer. When heating, the probability of the formation of “bridges” between the molecular layers adsorbed on opposite walls rises noticeably. In extremely narrow pores, with the width comparable to molecular sizes, the field of the wall with positive ions on the surface determines the resulting structure of the film. The deficit of hydrogen bond donors in the first molecular layer adsorbed on the wall with positive ions on the surface is the main cause of observable poor wettability of the monolayer film. On the opposite wall, the poor wettability is the result of the deficit of hydrogen bond acceptors.

The role of crystallographic anisotropy in formation of the structure of silicon-implanted layers of NiTi single crystals

The Auger electron spectroscopy and transmission electron microscopy techniques have been used to study the composition and structure of the surface layers of NiTi single crystals implanted with silicon ions at various directions of the incident ion beam. The role of “soft” [111]B2 and “hard” [001]B2 orientations of the NiTi single crystal in formation of the ion-beam-modified surface layer and the defect structure of Siion-implanted near-surface layers is revealed.

Plasmon electro-optic effect in a subwavelength metallic nanograting with a nematic liquid crystal

The electro-optic effect in hybrid structures based on subwavelength metallic nanogratings in contact with a layer of a nematic liquid crystal has been experimentally studied. Metallic gratings are fabricated in the form of interdigitated electrodes, which makes it possible to use them not only as optical elements but also for the production of an electric field in a thin surface region of the layer of the liquid crystal. It has been shown that, owing to the electric-field-induced reorientation of molecules of the liquid crystal near the surface of the grating, it is possible to significantly control the spectral features of the transmission of light, which are caused by the excitation of surface plasmons. The electro-optic effect is superfast for liquid crystal devices because a change in the optical properties of the system requires the reorientation of molecules only in a very thin surface layer of the liquid crystal.

Investigating the effects of growth rate and temperature on the B/Ca ratio and δ11B during inorganic calcite formation

To deconvolve the effect of growth rate and temperature on the boron partitioning into calcite and its isotope fractionation, seeded calcite precipitation experiments were performed at a constant temperature and various growth rates and at a constant growth rate and various temperatures. We show that boron partitioning increases with increasing growth rate and decreases with increasing temperature. The B isotope fractionation between calcite and B(OH)4− increases with increasing growth rate favoring the lighter B isotope for incorporation into calcite whereas no effect of temperature was observed within the temperature range investigated (12°C to 32°C). At the lowest temperature and growth rate δ11B of the calcite almost equals that of B(OH)4− in solution. Applying the surface entrapment model (SEMO) of Watson and Liang (1995) to our data, we demonstrate that the observed effects of temperature and growth rate on B concentration can be explained by processes in the near surface layer of the calcite crystal.