Structure Of Nanolayers Research Articles

Obtaining and studying the composition, structure and properties of nanophases and nanolayers of CoSi2

ABSTRACT The processes of formation of a thin film of cobalt disilicide on the silicon surface due to the formation of several nuclei of atomic thickness on the surface of the silicon substrate are presented. To create these structures, the ion implantation method was used. The dependence of the depth of penetration of ions into the substrate during ion implantation on temperature and ion dose was determined. In the case of implantation of Co+ ions into Si at room temperature silicon uniformly combines with Co atoms at a depth of 3.5–4 nm, the Co concentration in these layers is 45–50 at. %. Co has been shown to form chemical bonds with Si atoms in the sample containing 40–45 atomic % of Co. When implanting Co+ into Si at 600 K, it was found that the depth of the same added layer is 2.5–3 nm, and CCo forming bonds with Si atoms increases to 85–90%.

Trace cerium/ yttrium addition for inhibiting sulfur-induced intermediate temperature embrittlement in ultrahigh-purity copper

Sulfur (S), as the common impurity prevalent in copper (Cu) and Cu-based alloys, often induces the intergranular embrittlement at intermediate temperatures even with a ppm concentration, a phenomenon variously called “intermediate temperature embrittlement” (ITE). However, the inhibition of such an embrittlement has been rarely reported. In this study, we investigate the influence of trace cerium/ yttrium (Ce/Y) addition on the ductility of the ultrahigh-purity Cu containing minute amounts of S impurities at temperatures between room temperature (RT) and 900 °C. The results show that both Ce and Y additions can improve the ductility of the CuS alloy at any test temperature, which is attributed to the inhibition of Ce/Y on the grain boundary (GB) segregation of S by forming the RE-S precipitates. By comparison, the modification of ITE caused by S impurities is not significant after trace Ce is introduced, while the addition of small amounts of Y can completely eliminate the ITE phenomenon at temperatures of 300–750 °C. This results from the size of the RE-S precipitates and the structure of the transition nanolayers between the RE-S precipitates and Cu matrix, which is dependent on the diffusion rates of the dissolved S, Ce and Y atoms in Cu during the tensile deformation. Our findings provide a comprehensive experimental and computational understanding on the fundamental mechanisms of rare earth elements inhibiting the embrittlement induced by impurities in metallic materials.

GRAPHENE OXIDE-MODIFIED DIHYDROPYRIMIDINES

This study is devoted to the synthesis of various ensembles on the basis of graphene oxide nanolayers and dihydropyrimidines and the investigation of their antimicrobial activity. The “building blocks” of ensembles, viz dihydropyrimidines were synthesized by the Biginelli reaction in the presence of low-toxic copper triflate under microwave conditions. The positive side of the reaction is the absence of any additional purification stage, formed after cooling the solution precipitate just washed with a few amount of distilled water. The structures of synthesized compounds were determined by 1H, 13C and dept135 NMR spectroscopy methods. The second component, viz graphene oxide nanolayers, was synthesized by a modified Hummer method. The modification of the method is caused by the increasing of the oxidizing agent (H2SO4+KMnO4) concentration, which allowed to receive purier nanolayers. The structure and morphology of nanolayers were investigated by SEM and XRD methods, according to which it was determined that the thickness of nanolayers is 1 nm. Subsequently, graphene oxide was modified with dihydropyrimidines. The synthesis of ensembles was performed by non-covalent coupling of dihydropyrimidines with graphene oxide nanolayers by sonication. In addition, the antimicrobial activity of ensembles against S. aureus, Ps. aeruginosa and E. coli was performed. Obtained during the analysis results were compared with the activity of pristine antibiotic ampicillin. It was found that the addition of graphene oxide nanolayers to the dihydropyrimidine molecule allowed to improve the antimicrobial activity of dihydropyrimidines

The effects of deposition manner and rate on structure and morphology of porous ZnSe nanolayers: Modification of Phonon Confinement Model for resonant Raman conditions

Nanolayers of porous ZnSe with thickness of ∼50 nm were prepared by thermal vacuum evaporation, applying continuous and periodically interrupted deposition at different deposition rates. The surface morphology and film composition are studied by SEM and EDS methods. The XRD and Raman scattering measurements are used to confirm zinc blende ZnSe crystal structure. The Phonon confinement model is modified to analyze the Raman spectra excited by different laser lines both in non-resonant and near-resonant conditions. This analysis provides more profound insight in the ZnSe layers composition, nanocrystallite size and crystal lattice strain. The Raman results are also supported by the spectroscopic ellipsometry regarding the energy gap and porous properties of ZnSe layers. This study shows that both manner and rate of deposition significantly affect the nanolayers structure, morphology and optical properties and provide preparation of films with properties suitable for application which requires specific porous parameters important for the films chemical sensitivity.

Structure-dependent optical nonlinearity of indium tin oxide

We use post-deposition vacuum annealing of epsilon-near-zero (ENZ) indium tin oxide (ITO) nanolayers in order to modify their structural properties and enhance the third-order optical nonlinear response around the ENZ wavelength. We find that room temperature magnetron sputtering deposition results in polycrystalline thin films with an intrinsic tensile strain and a ⟨110⟩ fiber axis preferentially oriented normal to the substrate. Moreover, we demonstrate that post-deposition vacuum annealing treatments produce a secondary anisotropic phase characterized by compressive strain that increases with the annealing temperature. Finally, we use the Z-scan optical technique to accurately measure the complex nonlinear susceptibility χ(3) and the intensity-dependent refractive index change Δn for samples with different structural properties despite featuring similar ENZ wavelengths. Our intensity-dependent analysis demonstrates that an enhancement of the optical nonlinearity can be achieved by tuning the structure of ENZ nanolayers with values as large as χR(3)=(5.2 ± 0.3)×10−17m2/V2. This study unveils the importance of structural control and secondary phase formation in ITO nanolayers with ENZ optical dispersion properties for the engineering of integrated highly nonlinear devices and metamaterials that are compatible with the scalable silicon photonics platform.

Modeling the processes of atom structure formation of a superconducting spin valve

The paper considers the modeling of a multilayer nanocomposite, the combination of elements of which gives rise to a spin valve effect. The relevance and importance of effects in the field of spintronics and related materials and devices are described. We study the composition and atomic structure of individual layers of a multilayer nanocomposite, as well as the composition and morphology of the interface of nanocomposite layers. We analyzed a sample with a periodic superconductor-ferromagnet structure consisting of more than 20 alternating layers of niobium and cobalt. The deposition process took place in a deep vacuum. The simulation was carried out by the molecular dynamics method using the potential of the modified immersed atom method. The formation of layers was carried out in a stationary mode. The temperature was adjusted using the Nose-Hoover thermostat. The deposition of each nanofilm ended with a relaxation stage for the necessary stabilization and restructuring of the formed nanocomposite. Three deposition temperature regimes were considered: 300 K, 500 K, and 800 K. For these modes, we analysed the atomic structure of nanofilms and transition regions (interface) formed between the layers. A study of the atomic structure of nanofilms showed that niobium is formed by crystalline regions of different orientations. A cobalt nanofilm is characterized by a structure close to amorphous. The structural features of the interface between the superconductor-ferromagnet layers largely depend on a relief of the surface onto which the deposition is made. The smallest variation in atomic composition is observed in the first niobium-cobalt contact zone, since the formation of the first nanofilm occurs on an even plane of the substrate. An analysis of the influence of the temperature regime during the formation of the nanosystem shows the dependence of the processes of formation of multilayer nanofilm formation, the interface of nanolayers, as well as the composition and morphology of heterostructures on the temperature at which a nanocomposite is manufactured. An increased temperature leads to the formation of a more rarefied structure of nanolayers and an increase in the zones of the interface of nanolayers due to the diffusion of atoms of the sprayed materials.

Toward automated classification of monolayer versus few-layer nanomaterials using texture analysis and neural networks

The need for a fast and robust method to characterize nanostructure thickness is growing due to the tremendous number of experiments and their associated applications. By automatically analyzing the microscopic image texture of MoS2 and WS2, it was possible to distinguish monolayer from few-layer nanostructures with high accuracy for both materials. Three methods of texture analysis (TA) were used: grey level histogram (GLH), grey levels co-occurrence matrix (GLCOM), and run-length matrix (RLM), which correspond to first, second, and higher-order statistical methods, respectively. The best discriminating features were automatically selected using the Fisher coefficient, for each method, and used as a base for classification. Two classifiers were used: artificial neural networks (ANN), and linear discriminant analysis (LDA). RLM with ANN was found to give high classification accuracy, which was 89% and 95% for MoS2 and WS2, respectively. The result of this work suggests that RLM, as a higher-order TA method, associated with an ANN classifier has a better ability to quantify and characterize the microscopic structure of nanolayers, and, therefore, categorize thickness to the proper class.

Influence of laser pulse regime on the structure and optical properties of TiO2 nanolayers

TiO2 films were deposited onto glass substrates by pulsed laser deposition (PLD) in high vacuum under monopulse and multipulse excitation. After deposition, a thermal treatment on air atmosphere was done to promote crystallization. Films were studied by x-ray diffraction, energy dispersive spectroscopy, scanning electron, atomic force microscopy, UV–vis spectroscopy, and ellipsometry. After air annealing, films gain a different amount of oxygen: TiO1.9 in multipulse regime versus TiO1.7 in the monopulse one. Splashing is observed in both regimes although in the multipulse mode greater particles are found, that derived in a less compact film after annealing which could be the cause of the better oxygen diffusion. The optical band gap of the film prepared with monopulse excitation is 3.09 eV. This value increased to 3.34 eV with annealing, corresponding to that of anatase. The film made with multipulses has an Eg = 3.12 eV which was invariant upon annealing. The difference in the properties of the films grown in the different regimes was attributed to the re-excitation of the plasma during the ablation process in the multipulse ablation that leads to an increased splashing density and thereafter a less compact film and the presence of off-stoichiometry inclusions within the film bulk.

Reversible structural changes of in situ prepared As40Se60 nanolayers studied by XPS spectroscopy

As40Se60 nanolayers, as-deposited, annealed and in situ illuminated by green (532 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy. Changes in composition and local atomic coordination occurring in the irradiated region of As40Se60 films were monitored by analysis of As 3d and Se 3d core levels. It was found that the thermal treatment causes a decrease of the concentration of homopolar (As–As and Se–Se) bonds. On the other hand, an increasing concentration of both As-rich and Se-rich structural units (s.u.) with homopolar As–As and Se–Se bonds was observed under in situ green laser illumination of As40Se60 nanolayers. This process appeared to be reversible for a few sequences of annealing and illuminating of the sample. After a few cycles storing at ambient conditions the As40Se60 film composition was gradually changing, i.e. the aging effect was detected due to a drastic loss of As under ambient conditions. The surface local structure of the As40Se60 nanolayers and their photoinduced transformation are discussed in detail.

Pre-exfoliated nanoclay through two consecutive reaction systems: Silane functionalization followed by grafting of amino acid monomers

This work presents a novel way to produce a thermally stable nanoclay with an increased interlayer space, which are the essential requirements in fabricating clay-polymer nanocomposites. Sodium montmorillonite (Na-Mt) was functionalized through two separate, consecutive reaction systems. Functionalization with an aminosilane was first performed, in order to produce reactive groups at the silicate nanolayers. In the second step, amino acid monomers were employed for a further reaction with silane-modified nanoclay. X-ray diffraction results demonstrated an increased d001-value (24Å) using 10mmol γ-aminobutyric acid. The 001 reflection peak disappeared when 5mmol ω-aminoundecanoic acid/g Mt was used as the second type of amino acid. Transmission electron microscopy images of modified Mt with 10mmol ω-aminoundecanoic acid exhibited a well separated or exfoliated structure of nanolayers. Thermogravimetric measurements indicated a trend of increasing thermal stability of the modified nanoclays (350°C to 400°C) with increasing amounts of initial ω-aminoundecanoic acid. Based on these results, optimizing the modifiers and synthesis conditions can control the properties of modified nanoclay. Tensile modulus and strain at break of HDPE/2 mass% modified Mt showed 10% and 14% enhancements, respectively, due to the increased d001-value of the nanofiller, which results in greater filler-polymer interaction.

X-ray reflectometry and simulation of the parameters of SiC epitaxial films on Si(111), grown by the atomic substitution method

The structure and composition of SiC nanolayers are comprehensively studied by X-ray reflectometry, IR-spectroscopy, and atomic-force microscopy (AFM) methods for the first time. SiC films were synthesized by the new method of topochemical substitution of substrate atoms at various temperatures and pressure of CO active gas on the surface of high-resistivity low-dislocation single-crystal n-type silicon (111). Based on an analysis and generalization of experimental data obtained using X-ray reflectometry, IR spectroscopy, and AFM methods, a structural model of SiC films on Si was proposed. According to this model, silicon carbide film consists of a number of layers parallel to the substrate, reminiscent of a layer cake. The composition and thickness of each layer entering the film structure is experimentally determined. It was found that all samples contain superstoichiometric carbon; however, its structure is significantly different for the samples synthesized at temperatures of 1250 and 1330°C, respectively. In the former case, the film surface is saturated with silicon vacancies and carbon in the structurally loose form reminiscent of HOPG carbon. In the films grown at 1330°C, carbon is in a dense structure with a close-to-diamond density.

Radical coupling of maleic anhydride onto graphite to fabricate oxidized graphene nanolayers

Radical coupling was used to modify graphite with maleic anhydride (MAH). Azobisisobutyronitrile (AIBN) as radical generator activated MAH radically and it was reacted with defects at the surface of nanolayers. A set of batches with different reaction times (24, 48 and 72 h) were performed to obtain fully-modified nanolayers (GMA1, GMA2 and GMA3, respectively). Fourier transform infrared results approved the synthesis of MAH-grafted graphite. Thermogravimetric analysis showed that 5.9, 11.1 and 13.2 wt% of MAH was grafted onto the surface of GMA1, GMA2 and GMA3, respectively, and that was approved by X-ray photoelectron spectroscopy results. Also, X-ray diffraction patterns showed that d-spacing increased from 0.34 nm for graphite to 1.00 nm for all modified samples. However, GMA1 showed a weak peak related to graphite structure that disappeared when reaction time was increased. After modification with MAH, lamella flake structure of graphite was retained whereas the edges of sheets became distinguishable as depicted by scanning electron microscopy images. According to Raman spectra, modification progression resulted in more disorder structure of nanolayers due to grafting of MAH. Also, transmission electron microscopy images showed graphite as transparent layers while after modification, surface of nanolayers became folded due to the opposite effects of π-conjugated domains and electrostatic repulsion of oxygen-containing groups.

In situ investigations of laser and thermally modified As2S3 nanolayers: Synchrotron radiation photoelectron spectroscopy and density functional theory calculations

As-deposited, annealed, and in situ As2S3 nanolayers, illuminated by blue (405 nm) and red (650 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy and DFT electronic structure calculations. Changes in composition and local atomic coordination occurring in the irradiated region of As2S3 films were monitored by analysis of As 3d and S 2p core levels. These studies show that both the thermal treatment and the red laser illumination of As2S3 nanolayers decrease the concentration of homopolar (As-As and S-S) bonds. From the other hand, an increasing concentration of As-rich structural units (s.u.) with a homopolar As-As bond was observed under in situ blue laser illumination of As2S3 nanolayers. Molecular orbital energies were calculated for different As- and S-centered s.u. and used for the interpretation of the core levels and valence band spectra. The surface local structure of the As2S3 nanolayers and its photoinduced transformation are discussed in detail.

Evolution of the structure of near-surface ultrahigh molecular weight polyethylene nanolayers during orientational drawing

A comparative investigation of the surface structure of ultrahigh molecular weight polyethylene film filaments obtained with different draw ratios from xerogels prepared from 1.5 wt % polymer solutions in decaline and mineral oil has been performed using a SUPRA-55V scanning electron microscope and a nanoluminograph for recording thermoluminescence of ultrathin near-surface layers of solids. It has been found that, with an increase in the draw ratio, the luminescence intensity decreases, and the peaks responsible for the segmental mobility are shifted toward higher temperatures. It has been assumed that this is associated with the improvement of the structure of near-surface layers of the polymer (with a decrease in the number of microcavities and segments of molecules with a high degree of coiling). It has also been revealed that the peaks observed in glow curves of the oriented gel samples from polymer solutions in decaline are shifted more significantly than those of the gel samples from polymer solutions in mineral oil, and the extremely oriented films are characterized by a large discretization of kinetic units of motion.

First-principles study of optical properties of α-CuSe/klockmannite: Bulk and nano-layers

The optical properties of α-CuSe bulk and its nano-layers (NLs) have been studied by the first principles theoretical study in the framework of density functional theory. These properties are calculated with regard to dielectric function, refractive index, extinction coefficient, reflection coefficient, absorption coefficient, energy-loss function, and optical conductivity. To create NLs, two different thicknesses through CuSe bulk are chosen in the (0001) direction as the first and second thicknesses. The former thickness is divided into six different NLs with variant alternations. These NLs have the same chemical composition and are structural isomers. Among the NLs, the optical properties of the most stable NL and its double thickness are calculated and compared with the bulk state. The imaginary part of dielectric function has a main peak at low energies because α-CuSe is a conductive compound in the bulk state. The electronic structure of NLs shows that they have remained conductive in x (or y) direction, but they interestingly have a dielectric behavior with an ultra-low electrical conductivity in z direction. The optical curves in the bulk and NLs show the anisotropic feature between x and z directions. In the range of infrared to red light, the bulk refractive index, nz(ω), is very large, about 6, while nx(ω) is about 3. Results show that the NLs have wide absorption curves in the range of solar spectrum from infrared to ultraviolet.

Structure and special chemical reactivity of interface-stabilized cerium oxide nanolayers on TiO2(110)

Novel interface-stabilized ceria nanophases have been grown on TiO2(110) by physical vapor deposition. At low coverage, dumbbell nanostructures constituted by reconstructed titania and ceria clusters are formed, while long range ordered nanoxides can be obtained by increasing the ceria dose. Scanning tunneling microscopy and photoemission spectroscopy were used to characterize the electronic properties of the films, showing that the TiO2 substrate can effectively stabilize ceria in reduced form over a wide range of experimental conditions. Epitaxial coupling is a very useful tool for tuning the chemical properties of mixed oxide systems. The special electronic properties of the films have a direct counterpart in the chemical activity, which has been investigated by temperature programmed desorption using methanol as a probe molecule. The experimental results indicate an exceptional activity of the ceria-titania interface in the selective dehydration of methanol to formaldehyde at an unprecedented low temperature (330 K).

TiS2and ZrS2single- and double-wall nanotubes: First-principles study

Hybrid density functional theory has been applied for investigations of the electronic and atomic structure of bulk phases, nanolayers, and nanotubes based on titanium and zirconium disulfides. Calculations have been performed on the basis of the localized atomic functions by means of the CRYSTAL-2009 computer code. The full optimization of all atomic positions in the regarded systems has been made to study the atomic relaxation and to determine the most favorable structures. The different layered and isotropic bulk phases have been considered as the possible precursors of the nanotubes. Calculations on single-walled TiS2 and ZrS2 nanotubes confirmed that the nanotubes obtained by rolling up the hexagonal crystalline layers with octahedral 1T morphology are the most stable. The strain energy of TiS2 and ZrS2 nanotubes is small, does not depend on the tube chirality, and approximately obeys to D(-2) law (D is nanotube diameter) of the classical elasticity theory. It is greater than the strain energy of the similar TiO2 and ZrO2 nanotubes; however, the formation energy of the disulfide nanotubes is considerably less than the formation energy of the dioxide nanotubes. The distance and interaction energy between the single-wall components of the double-wall nanotubes is proved to be close to the distance and interaction energy between layers in the layered crystals. Analysis of the relaxed nanotube shape using radial coordinate of the metal atoms demonstrates a small but noticeable deviation from completely cylindrical cross-section of the external walls in the armchair-like double-wall nanotubes.

Electronic structure of undoped and doped SnOx nanolayers

Results of electronic structure investigations for tin oxide nanolayers obtained by tin magnetron sputtering and their following oxidation in air at different temperatures are presented. Using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy techniques it was shown that at the 240°С anneal a predominant phase in the sample surface layers is tin monoxide. With the annealing temperature increase up to 450°C the phase composition corresponds to tin dioxide. Rather high sorption sensitivity for the samples oxidized at 450°C was found. The compositional model of the gap structure based on XANES and XPS data matching for SnOx nanolayers revealed occurrence of the crossed transitions with the energy of ~3.7eV in the presence of two phases: the SnO and the SnO2 or SnOx with vacancies. Under surface doping of SnO2 nanolayers with palladium PdO and PdO2 is observed where PdO is the most intensive component. O2 and H2 multiple alternate exposures resulted in the disappearance of palladium dioxide and PdO recovery to the metallic Pd. Under Pd bulk doping of nanolayers PdO and PdO2 were observed in the surface layers. In this case PdO2 was presented by two types of particles different in size, one of them having the greatest binding energy of Pd 3d5/2 (339.0eV).

Surface structure of nickel oxide layers on a Rh(111) surface

The formation of nickel oxide nanolayers by oxidizing Ni overlayers on Rh(111) has been investigated and their structures are reported as a function of the nickel coverage and oxygen pressure. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and diffraction (XPD), and high-resolution electron energy loss spectroscopy (HREELS) have been applied to characterize the structure and stoichiometry of the nickel oxide nanolayers. Several different phases have been observed depending on the strain state of the metallic Ni overlayers. For the pseudomorphic Ni monolayer, two distinctly different oxide phases with (6×1)-Ni5O5 and (2√3×2)-Ni8O10 structures have been identified at oxygen-poor (p=5×10−8mbar) and oxygen-rich (p≥1×10−6mbar) conditions, respectively. Above one monolayer, where the Ni layers are relaxed, bulk-like NiO(100) films form at the O-rich conditions, whereas chemisorbed-type p(2×2)ONi(111) layers develop in the O-poor regime. X-ray photoelectron diffraction analysis has provided additional insight into the relaxation mechanism and the detailed atomic structure of the Ni-oxide nanolayers.

Self-assembly of organic-inorganic hybrid nanolayers: effect of endgroup polarity on nanostructures

Molecular self-assembly (MSA) offers a convenient way to elaborate highly ordered and lightweight nanostructures with molecular-level control of material structure, composition, and dimensions. This sub-branch of supramolecular chemistry which focuses on the spontaneous and reversible construction of ordered aggregates through weak intermolecular interactions represents a facile route to produce organic-inorganic hybrid nanolayers with long-range ordering. Over the past twenty years, many efforts have been devoted to the construction and structural optimization of well-ordered organic-inorganic nanolayers from monofunctional organosilane surfactants [1-3]. The organization process of such building blocks has been extensively investigated and is known to be governed mainly by the hydrophobic effect arising from a competition of attractive and repulsive interactions between inorganic polar heads, organic nonpolar tails, solvent and water molecules [4, 5]. Such interactions guide the nonpolar chains to aggregate and polar heads to maximize their exposure to the surrounding aqueous medium thus forming structures such as bilayers [6], micelles [7], or vesicles [8]. Finally, van der Waals forces appear between alkyl chains whereas hydrogen bonds and then stronger covalent bonds are formed between inorganic heads thus conferring thermal and mechanical stability to films [9, 10]. This study aims to investigate the effect on nanolayer structures of a second polar group located at the chain termination of alkylsilane building blocks. As intermolecular interactions which direct the assembly process are strongly dependent on the chemical composition of precursors, it is expected that the presence of a second polar endgroup will significantly affect these weak interactions thus leading to an alternative structure of nanolayers. Monofunctional hexadecyltrimethoxysilane (HDTMS) and bifunctional 11bromoundecyltrimethoxysilane (BUDTMS) were used independently as building blocks to produce organicinorganic nanolayers. Structures of these solid thin films were then sequentially characterized and compared after solution assembly and deposition on silicon substrates.