Distinct Bond Lengths Research Articles

Strained behavior of monolayer HfS2, HfSe2, and HfTe2 from molecular dynamic simulations

We have employed the classical molecular dynamics (MD) simulations to investigate the mechanical properties of the HfX2 (X=S, Se, Te) monolayers (MLs) --the emerging nano-materials possessing highly attractive for functional electronic and optoelectronic systems. The studied materials have exhibited isotropic mechanical properties in armchair and zigzag directions. Both the stress-strain curves and the structural evolution around the fracture point have indicated that materials have undergone a brittle fracture once up to the ultimate stress. At the same temperature, HfS2 ML exhibits a higher Young's modulus than HfSe2, yet HfSe2 possesses slightly larger fracture strain, fracture stress, and toughness compared to HfS2. Among the three materials, HfTe2 displays the poorest mechanical performance. The calculated results may be attributed to materials’ distinct microstructures and bond lengths. It’s found that temperature can obviously affect the mechanical properties of materials. For example, as the temperature increases, the calculations show that the fracture strain, fracture stress and toughness have obviously decreased. However, the Young’s modulus decreases slightly and linearly with the increasing temperature. These results suggest temperature can induce the mechanical degradation.

Exploring the effect of the bromo- and iodo-halide(s) in the two-dimensional hybrid perovskites (H2-2AMP)PbX4 (X=Br and I) into the structural, chemical and optical properties

The influence of bromide and iodide-halides on (H2-2AMP)PbX4 (X = Br and I) compounds, focusing on their structural, chemical, and optical characteristics was investigated in this study. Synthesized via a reflux method under a nitrogen atmosphere, the compounds were analyzed using FT-IR spectroscopy and XRD analysis. Despite differing halides, both compounds share an orthorhombic crystal structure, with distinct cell parameters and bond lengths. The UV–Vis analysis reveals significant tuning of the cut edge within the visible range, offering insights into their potential of the materials in optoelectronic applications.

New insights into pertinent Fe-complexes for the synthesis of iron via the instant polyol process.

Chemically synthesized iron is in demand for biomedical applications due to its large saturation magnetization compared to iron oxides. The polyol process, suitable for obtaining Co and Ni particles and their alloys, is laborious in synthesizing Fe. The reaction yields iron oxides, and the reaction pathway remains unexplored. This study shows that a vicinal polyol, such as 1,2-propanediol, is suitable for obtaining Fe rather than 1,3-propanediol owing to the formation of a reducible Fe intermediate complex. X-ray absorption spectroscopy analysis reveals the ferric octahedral geometry and tetrahedral geometry in the ferrous state of the reaction intermediates in 1,2-propanediol and 1,3-propanediol, respectively. The final product obtained using a vicinal polyol is Fe with a γ-Fe2O3 shell, while the terminal polyol is favourable for Fe3O4. The distinct Fe-Fe and Fe-O bond lengths suggest the presence of a carboxylate group and a terminal alkoxide ligand in the intermediate of 1,2-propanediol. A large Fe-Fe bond distance suggests diiron complexes with bidentate carboxylate bridges. Prominent high-spin and low-spin states indicate the possibility of transition, which favors the reduction of iron ions in the reaction using 1,2-propanediol.

Synthesis of Cu2O Nanostructures with Tunable Crystal Facets for Electrochemical CO2 Reduction to Alcohols.

Electrochemical CO2 reduction enables the conversion of intermittent renewable energy to value-added chemicals and fuel, presenting a promising strategy to relieve CO2 emission and achieve clean energy storage. In this work, we developed nanosized Cu2O catalysts using the hydrothermal method for electrochemical CO2 reduction to alcohols. Cu2O nanoparticles (NPs) of various morphologies that were enclosed with different crystal facets, named as Cu2O-c (cubic structure with (100) facets), Cu2O-o (octahedron structure with (111) facets), Cu2O-t (truncated octahedron structure with both (100) and (111) facets), and Cu2O-u (urchin-like structure with (100), (220), and (222) facets), were prepared by regulating the content of a polyvinyl pyrrolidone (PVP) template. The electrochemical CO2 reduction performance of the different Cu2O NPs was evaluated in the CO2-saturated 0.5 M KHCO3 electrolyte. The as-synthesized Cu2O nanostructures were capable of reducing CO2 to produce alcohols including methanol, ethanol, and isopropanol. The alcohol selectivity of the different Cu2O NPs followed the order of Cu2O-t < Cu2O-u < Cu2O-c < Cu2O-o (with the total Faradaic efficiencies of alcohol products of 10.7, 25.0, 26.2, and 35.4%). The facet-dependent effects were associated with the varied concentrations of oxygen-vacancy defects, different energy barriers of CO2 reduction, and distinct Cu-O bond lengths over the different crystal facets. The desired Cu2O-o catalyst exhibited good reduction activity with the highest partial current density of 0.51 mA/cm2 for alcohols. The Faradaic efficiencies of alcohol products were 4.9% for methanol, 17.9% for ethanol, and 12.6% for isopropanol. The good electrochemical CO2 reduction performance was also associated with the surface reconstruction of Cu2O, which endowed the catalyst with abundant Cu0 and Cu+ sites for promoted CO2 activation and stabilized CO* adsorption for enhanced C-C coupling. This work will provide a new route for enhancing the alcohol selectivity of nanostructured Cu2O catalysts by crystal facet engineering.

Synthesis, local structure and optical property studies of α-SnS microrods by synchrotron X-ray pair distribution function and micro-Raman shift.

A hydrothermal synthesis method was employed for the preparation of tin sulfide (α-SnS) microrod samples (SnS-A and SnS-B) using ethylenediamine and deionized water as the surfactant at ratios from 50 : 50 to 100 : 00. The atomic structures of the α-SnS microrods were studied using atomic pair distribution function (PDF) analysis and total synchrotron X-ray scattering data. The synchrotron X-ray diffraction (ScXRD) patterns and PDF data reveal that the structure of the SnS microrods is orthorhombic. From the refinement of the PDF, the first and second peaks correspond to nearest (Sn2+–S2−) and second nearest distances (Sn2+–Sn2+) of 2.546 (0.003) Å and 4.106 (0.004) Å, and 2.527 (0.005) Å and 4.087 (0.006) Å for SnS-A and SnS-B samples, respectively. The TEM results show that samples SnS-A and SnS-B have a microrod structure, with microrod diameters of 800 nm and 500 nm with lengths of tens of micrometers, respectively. The SnS-A and SnS-B samples show a direct band gap of 1.6 eV and 2 eV, respectively, using the Kubelka–Munk transformation of the UV-visible spectra. The micro-Raman spectra of the SnS-A and SnS-B microrods exhibited an Ag mode of SnS at 228.4 and 223 cm−1, respectively. The second peaks at 306.7, and 309 cm−1 are associated with the secondary phases of the SnS2 phase, whereas the third broad peaks at 616.5, and 613 cm−1 revealed that there was a deformation mode of sulfate in the SnS-A and SnS-B samples.

Theoretical Investigations of the Spin Hamiltonian Parameters for the Mononuclear Square Pyramidal [CuO5] Groups in Two Paddle Wheel Copper Complexes

The spin Hamiltonian parameters (SHPs) (g factors and hyperfine structure constants) for the mononuclear square pyramidal [CuO5] groups in two paddle wheel copper complexes {Cu2(μ2–O2CCH3)4}(OCNH2CH3) and \({}_{\infty }^{3} [{\text{Cu}}_{ 2}^{\text{I}} {\text{Cu}}_{ 2}^{\text{II}} ( {\text{H}}_{ 2} {\text{O)}}_{ 2} {\text{L}}_{ 2} {\text{Cl}}_{ 2} ]\) are theoretically investigated from the perturbation calculations of these parameters for a rhombically elongated octahedral 3d 9 group. The slightly larger anisotropy Δg (≈ g // − g⊥) of complex 1 than complex 2 is attributed to the slightly bigger deviations of the polar angles related to the ideal value 90° and relative differences between the axial and basal Cu–O distances in the former. The axiality of the EPR signals for both systems can be illustrated as the fact that the perpendicular anisotropic contributions to X and Y components of the SHPs arising from the four basal ligands with slightly distinct bond lengths and bond angles may roughly cancel one another. The signs of hyperfine structure constants are also theoretically determined for both complexes.

Graphenylene Nanotubes

A new type of carbon nanotube, based on the graphenylene motif, is investigated using density functional and tight-binding methods. Analogous to conventional graphene-based nanotubes, a two-dimensional graphenylene sheet can be "rolled" into a seamless cylinder in armchair, zigzag, or chiral orientations. The resulting nanotube can be described using the familiar (n,m) nomenclature and possesses 4-, 6-, and 12-membered rings, with three distinct bond lengths, indicating a nonuniform distribution of the electron density. The dodecagonal rings form pores, 3.3 Å in diameter in graphenylene, which become saddle-shaped paraboloids in smaller-diameter nanotubes. Density functional theory predicts zigzag nanotubes to be small-band gap semiconductors, with a generally decreasing band gap as the diameter increases. Interestingly, the calculations predict metallic characteristics for armchair nanotubes with small diameters (<2 nm), and small-band gap semiconducting characteristics for larger-diameter ones. Graphenylene nanotubes with indices mod(n-m,3) = 0 exhibit a band gap approximately equal to that of armchair graphenylene nanotubes with comparable diameter.

Ferroelectric-Paraelectric Phase Transition of CsH2PO4studied by Static NMR and MAS NMR

The microscopic dynamics of CsH2PO4, with two distinct hydrogen bond lengths, are studied by static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR. The proton dynamics of the two crystallographically inequivalent hydrogen sites were discussed in terms of the 1 H

Local structural displacements across the structural phase transition inIrTe2: Order-disorder of dimers and role of Ir-Te correlations

We have studied local structure of ${\text{IrTe}}_{2}$ by Ir $L$${}_{3}$-edge extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature to investigate origin of the observed structural phase transition at ${T}_{s}\ensuremath{\sim}270$ K. The EXAFS results show an appearance of longer Ir-Te bond length ($\ensuremath{\Delta}R\ensuremath{\sim}0.05$ \AA{}) at $T&lt;{T}_{s}$. We have found Ir-Ir dimerization, characterized by distinct Ir-Ir bond lengths ($\ensuremath{\Delta}R\ensuremath{\sim}0.13$ \AA{}), existing both above and below ${T}_{s}$. The results suggest that the phase transition in IrTe${}_{2}$ should be an order-disorder-like transition of Ir-Ir dimers assisted by Ir-Te bond correlations, thus indicating important role of the interaction between the Ir $5d$ and Te $5p$ orbitals in this transition.

General Rolled-Up and Polyhedral Models for Carbon Nanotubes

In many computational studies of carbon nanotubes, the minimum energy configuration frequently settles on a structure for which the bond lengths are distinct. Here, we extend both the rolled-up and the polyhedral models for SWCNTs to produce general models incorporating either distinct bond lengths and the same bond angle, or distinct bond lengths and distinct bond angles. The CNTs considered here are assumed to be formed by sp2 hybridization but with different bond lengths so that the nanotube structure is assumed to comprise irregular hexagonal patterns. The polyhedral model with distinct bond lengths and distinct bond angles is based on the two postulates that all bonds lying on the same helix are equal in length and that all atoms are equidistant from a common axis of symmetry. The polyhedral model with distinct bond lengths and the same bond angle has the additional postulate that all the adjacent bond angles are equal. We derive exact formulae for the geometric parameters and we present asymptotic expansions for the polyhedral model with distinct bond lengths and distinct bond angles to the first two orders of magnitude. Good agreement is demonstrated for the predictions of the polyhedral model compared with the results obtained from other computational studies.

POLYHEDRAL MODELS AND GEOMETRIC STRUCTURES FOR NANOTUBES

In this thesis, some new polyhedral models for nanotubes are examined. The conventional rolled-up model for carbon nanotubes assumes that a flat sheet of graphene is rolled into a seamless right circular cylinder and therefore in terms of the geometric parameters, the curvature inherent in the structure of nanotubes is not taken into account. The conventional rolled-up model of nanotubes completely ignores any effects due to curvature while the existing ideal polyhedral models for single-walled carbon nanotubes and boron nitride nanotubes, which are both hexagonal structures, are known to give predictions for the geometric parameters of the tube which are in excellent agreement with computational studies (molecular dynamics simulations and ab initio calculations). In this thesis the notion of an ideal polyhedral model is extended to silicon and boron nanotubes, which adopt respectively squares or skew rhombi and flat equilateral triangles as their structure. The silicon nanotubes considered here are assumed to be formed by sp hybridization and thus the nanotube lattice is assumed to comprise only squares or skew rhombi. The boron nanotubes considered here are assumed to be formed by complex bonding type and therefore the nanotube lattice is assumed to comprise a triangular pattern. From molecular dynamics simulation results for carbon nanotubes and silicon nanotubes, the bond lengths are known to vary depending upon the bond direction. Often this aspect can not be ignored and therefore in this thesis both the conventional and the ideal polyhedral models are extended to include distinct bond lengths, and specifically for carbon, silicon and boron nanotubes. These general models are shown to be in excellent agreement with computational studies. We first present the standard geometric parameters for the conventional nanotube model. Noting again that the curvature inherent in this model is completely ignored, for the ideal polyhedral model for silicon nanotubes we begin with three

Lattice Anomalies and HTSC in Pnictides and Cuprates Studied by XAS: Polaron Resonance as a Common Clue

Temperature-dependent local lattice structures of 1111-type ReFeAsO1−x F x (Re=La, Sm) are studied by extended X-ray absorption fine structure (EXAFS) and compared with the results of impurity-doped La1.85Sr0.15CuO4. As electron carriers are introduced by doping with F ions, pnictgen coordination around Fe atom shows unusual local lattice response below T ∗, described as an anomalous temperature dependence of mean-squared relative displacement. The observed local lattice distortion (leading to two distinct bond lengths R 1 and R 2 where ΔR=R 1−R 2∼0.1 A) is a signature of localized polaron formation. The effect of magnetic impurities in La1.85Sr0.15CuO4 indicates (bond stretching-type) lattice distortion favours polaron resonance as a common prerequisite for high-temperature superconductivity.

Electronic properties of carbon nanotubes with distinct bond lengths

In band structure calculations commonly used to derive the electronic properties of carbon nanotubes, it is generally assumed that all bond lengths are equal. However, hexagonal carbon lattices are often irregular and may contain as many as three distinct bond lengths. A regular (n,m) carbon nanotube will be metallic if p=(n−m)/3 for integer p. Here we analytically derive the generalized condition for metallic irregular carbon nanotubes. This condition is particularly relevant to small radius nanotubes and nanotubes experiencing small applied strains.

Ideal Polyhedral Model for Boron Nanotubes with Distinct Bond Lengths

In this Article, we extend both the rolled-up model and the polyhedral geometric model for single-walled boron nanotubes with equal bond lengths to the corresponding models but having distinct bond lengths. The boron nanotubes considered here are assumed to be formed by sp2 hybridization and π-bonds and may have as many as three different bond lengths, and the nanotube lattice is assumed to comprise a triangular pattern. Beginning with the two postulates that all equivalent bond lengths lying on the same helix are equal and that all of the atoms are equidistant from a common axis of symmetry, we derive exact formulas for the geometric parameters such as chiral angles, bond angles, radius, and unit cell length. Results for both the rolled-up model and the prior geometric model are shown to emerge from the new geometric model in the limit of large radius and in the limit of equal bond lengths, respectively. Numerical results for the new polyhedral model are related to existing numerical results for a novel boron nanotube structure, which involves 1/9 of the atoms missing. This novel structure is believed to be more energetically favorable than the conventional model.

Silicon nanotubes with distinct bond lengths

In this paper, we extend both the rolled-up and the polyhedral models for single-walled silicon nanotubes with equal bond lengths to models having distinct bond lengths. The silicon nanotubes considered here are assumed to be formed by sp3 hybridization with different bond lengths so that the nanotube lattice is assumed to comprise only skew rhombi. Beginning with the three postulates that all bonds lying on the same helix are equal, all adjacent bond angles are equal, and all atoms are equidistant from a common axis of symmetry, we derive exact formulae for the polyhedral geometric parameters such as chiral angles, bond angles, radius and unit cell length. The polyhedral model presented here with distinct bond lengths includes both the rolled-up model with distinct bond lengths which arises from the first term of an asymptotic expansion, and an existing polyhedral model of the authors which assumes equal bond lengths. Finally, some molecular dynamics simulations are undertaken for comparison with the geometric model. These simulations start with equal bond lengths and then stabilize in such a way that two distinct bond lengths emerge.

Electronic properties of silicon nanotubes with distinct bond lengths

We analyze the band structure of a silicon nanotube with $s{p}^{3}$ bonds and variable bond lengths. This nanotube has many similarities with a carbon nanotube including a band gap at half-filling and conducting behavior which is dependent on structure. We derive a simple formula which predicts when the nanotube is metallic. We discuss our results in the context of a nanotube subject to small applied strains as this provides a means of distorting bond lengths in a predictable way and may be tested experimentally. The effects of strain on nanotube conductance have important implications for sensor technology.

Structure and properties of hydrogenated amorphous silicon carbide thin films deposited by PECVD

a-Si 1-x C x :H films are deposited by RF plasma enhanced chemical vapor deposition (PECVD) at different RF powers with hydrogen-diluted silane and methane mixture as reactive gases. The structure and properties of the thin films are measured by infrared spectroscope (IR), Raman scattering spectroscope and ultra violet-visible transmission spectroscope (UV-vis), respectively. Results show that the optical band gap of the a-Si 1-x C x :H thin films increases with increasing Si-C bond fraction. It can be easily controlled through controlling Si-C bond formed by modulating deposition power. At low deposition power, the bond configuration of the a-Si 1-x C x :H thin film is more disordered owing to the distinct different bond lengths and bond strengths between Si and C atoms. At a too high deposition power, it becomes still high disordered due to dangling bonds appearing in the a-Si 1-x C x :H thin film. The low disordered bond configuration appears in the thin film deposited with moderate deposition power density of about 2.5 W/cm 2 .

Nickel(II) and cerium(IV) ethyleneurea, ethylenethiourea and propyleneurea adducts

The NiCl2 · 6eu, NiCl2 · 6etu, Ce(SO4)2 · 6pu and Ce(SO4)2 · 6etu adducts (eu = ethyleneurea, etu = ethylenethiourea and pu = propyleneurea) were prepared by a solid state procedure, and characterized by C, H and N elemental analysis, i.r. spectroscopy, thermogravimetry (t.g.) and differential scanning calorimetry (d.s.c.). The i.r. results show that eu and pu coordinate through oxygen, whereas etu coordinates through nitrogen. The NiCl2 · 6eu adduct is green, whereas the NiCl2 · 6etu adduct is yellow. This fact suggests that the Δo separation is larger for the latter adduct. On the other hand, both cerium adducts are white, suggesting that, for pu and etu cerium sulfate adducts, a ligand-to-metal charge-transfer promotes the reduction of CeIV to CeIII. The t.g. thermal stability sequences are: etu > pu > eu and Ni > Ce. Cerium adducts release the six ligand molecules in a single mass loss step, suggesting that all ligand molecules are in equivalent coordinative positions (same bond length and strength). On the other hand, nickel adducts exhibit three mass loss steps in the thermal degradation (release of ligand molecules) process. This suggests that, for these compounds, there are three distinct coordinative positions, with distinct bond lengths and strengths, probably due to the Jahn–Teller effect. All four adducts melt before the onset of thermal degradation, with etu adducts exhibiting the higher melting temperatures and melting enthalpies, as a consequence of stronger intermolecular forces.

Local structure of random In xGa 1− xAs alloys by full-profile fitting of atomic pair distribution functions

The local structure of random In x Ga 1− x As alloys has been refined by fitting experimental high-resolution atomic pair distribution functions with the 8-atom unit cell of the zinc-blende lattice. Both metal and As atoms have been allowed to move off their ideal positions and so the local structural disorder due to the presence of two distinct bond lengths, Ga–As and In–As, has been quantified. It has been found that most of the disorder is accommodated by the As sublattice but the disorder on metal sites is important too. This new experimental information has been incorporated into a larger, statistically representative, model of In 0.5Ga 0.5As structure using widely available crystallographic software.

Local structure of In0.5Ga0.5As from joint high-resolution and differential pair distribution function analysis

High resolution total and indium differential atomic pair distribution functions (PDFs) for In0.5Ga0.5As alloys have been obtained by high energy and anomalous x-ray diffraction experiments, respectively. The first peak in the total PDF is resolved as a doublet due to the presence of two distinct bond lengths, In–As and Ga–As. The In differential PDF, which involves only atomic pairs containing In, yields chemical specific information and helps ease the structure data interpretation. Both PDFs have been fit with structure models and the way in that the underlying cubic zinc-blende lattice of In0.5Ga0.5As semiconductor alloy distorts locally to accommodate the distinct In–As and Ga–As bond lengths present has been quantified.