Bond Length Fluctuations Research Articles

Role of intrinsic point defects on electrocatalytic activity of a metal-free two-dimensional Polyaramid for enhanced hydrogen evolution reaction

Despite the current research aims at replacing expensive metals like Platinum with non-noble metals for producing hydrogen via hydrogen evolution reaction (HER), practical challenges like oxidation continue to be a significant concern. Our study aims at providing the metal-free alternative to such noble Pt-based catalysts. Taking care of the fact that the defects are inevitable during the material's growth process, we propose thermodynamically and thermally stable two-dimensional (2D) Polyaramid (2DPA) with possible intrinsic point defects as HER active materials with overpotentials comparable to that of Pt catalysts using density functional theory (DFT). Creating single carbon (VC), single nitrogen (VN), and single oxygen (VO) vacancies helps reducing its wide band gap upon creation of several defect states, thereby improving conductivity. VO-2DPA exhibits lowest ΔGH* value of −0.04 eV which is closest to thermoneutrality condition, i.e., ΔGH*≈0 while VN-2DPA is not found suitable for HER. The calculated low formation energies for generating point-vacancies under defect-poor growth conditions and the minimal bond length fluctuations within, serve as strong validation for the feasibility of their synthesis. Furthermore, underlying reaction mechanisms are also investigated for the HER process favouring the Heyrovsky reaction path over Tafel as the former requires lesser activation energy: 0.53 eV v. 0.86 eV for VC-2DPA and 0.34 eV v. 1.01 eV for VO-2DPA.

Analysis of Dynamical Peculiarities in Nanoalloys at Subsystems Level: Dynamical Degrees of Freedom, Temperature Differences and the Chameleon Effect.

A novel analysis of the dynamical behavior of nanoalloy systems, as represented by model Ni/Al 13-atom clusters, over a broad range of energies that cover the stage-wise transition of the systems from their solidlike to liquidlike state is presented. Conceptually, the analysis is rooted in partitioning the systems into judiciously chosen subsystems and characterizing the latter in terms of subsystem-specific dynamical descriptors that includedynamical degrees of freedom,root-mean-square bond-length fluctuation, andelement-specific subsystem temperature. The analysis reveals a host of new intriguing peculiarities in the dynamical behavior of the Ni/Al 13-mers, among which are what we call thechameleon effectand the difference in the temperatures of the Ni and Al subsystems at high energies, a difference that strongly depends on the cluster composition and also changes with energy. These do not have an analog in pure Ni13and Al13and are explained in terms of the coupled effects of the difference between the masses of the Ni and Al atoms (the mass effect) and of the difference in the anharmonicity of the overall interaction potential as experienced by the Ni and Al subsystems of the clusters (the potential effect).

Unpolarized laser method for infrared spectrum calculation of amide I C[dbnd]O bonds in proteins using molecular dynamics simulation

The investigation of the strong infrared (IR)-active amide I modes of peptides and proteins has received considerable attention because a wealth of detailed information on hydrogen bonding, dipole-dipole interactions, and the conformations of the peptide backbone can be derived from the amide I bands. The interpretation of experimental spectra typically requires substantial theoretical support, such as direct ab-initio molecular dynamics simulation or mixed quantum-classical description. However, considering the difficulties associated with these theoretical methods and their applications are limited in small peptides, it is highly desirable to develop a simple yet efficient approach for simulating the amide I modes of any large proteins in solution. In this work, we proposed a comprehensive computational method that extends the well-established molecular dynamics (MD) simulation method to include an unpolarized IR laser for exciting the CO bonds of proteins. We showed the amide I frequency corresponding to the frequency of the laser pulse which resonated with the CO bond vibration. At this frequency, the protein energy and the CO bond length fluctuation were maximized. Overall, the amide I bands of various single proteins and amyloids agreed well with experimental data. The method has been implemented into the AMBER simulation package, making it widely available to the scientific community. Additionally, the application of the method to simulate the transient amide I bands of amyloid fibrils during the IR laser-induced disassembly process was discussed in details.

Studying the impact of diagonal-doping on thermal stability of main-group metal clusters via Born Oppenheimer molecular dynamics

Doping with a heteroatom is considered to be an effective tool in tuning the physico-chemical properties of metal clusters. Over a period of time, several experimental and theoretical investigations have been carried out to understand the effect of doping on structure, electronic properties and reactivity of main group clusters. However, issues pertaining to thermal stabilities of metal clusters, particularly in the context of heteroatom doping remain unexplored. Filling this lacuna, this report focuses on evaluating the finite temperature behaviour of the main group metal clusters which are doped with diagonally adjacent dopant atoms. Born-Oppenheimer Molecular Dynamical (BOMD) simulations are performed on diagonally doped LiMg, BeAl and BSi () clusters (doped diagonal pairs). The thermal response of these doped diagonal pairs is also compared with the corresponding pristine clusters such as Li, Mg, Be, Al, B and Si () clusters. The simulation trajectories and ionic motions are analysed and root mean square bond length fluctuations are calculated as a function of temperature. The results indicate that the diagonally doped BSi () clusters are thermally more stable (1200 K) as compared to the other two doped clusters (LiMg, BeAl). The enhanced thermal stability is attributed to the higher binding energies noted for BSi () clusters. Moreover, it is found that for the B-Si cluster systems, diagonal doping always enhances the thermal stability, whereas both diagonal doping and the size of the cluster affect the thermal stabilities of the LiMg, and BeAl cluster systems. An in-depth analysis of thermal-structural-electronic (frontier molecular orbitals) properties revealed that the strength of dopant-to-parent cluster interactions are playing a significant role in influencing the thermal stabilities of diagonally doped clusters. Overall this study also highlights how the heuristic chemical principle of ‘diagonal relationship’ widely used in main-group chemistry may be extended to cluster science to draw new insights regarding thermal stabilities of metal clusters.

Exploration of the Structure–Property Relationship of Polymer‐Based Composites by Monte‐Carlo‐Coupled Viscoelastic Lattice Spring Model

AbstractA viscoelastic lattice spring model is applied to real polymer blend containing strong interaction, i.e., chemical bond, to investigate the structure–property correlation in combination of Monte Carlo method modified by cavity diffusion and bond length fluctuation algorithms. Impacts of inclusion ratio, chemical bonds, chain length, and interface orientation on the mechanical response along with fracture mode are quantified through simulation of uniaxial tensile test. It is confirmed that introduction of strong interaction and prolongation in chain length induces observable reinforcement in global modulus and ultimate tensile strength, respectively, and the amplification is more significant when the interface is parallel to external load.

Observing the Fluctuation Dynamics of Dative Bonds Using Two-Dimensional Electronic Spectroscopy.

We perform two-dimensional electronic spectroscopy on chlorophyll (Chl) a and b molecules in aprotic solvents of different Lewis basicity. By analyzing the ultrafast spectral diffusion dynamics of the Qy transition, we show that a certain timescale of the spectral diffusion dynamics is affected by the solvents' Lewis basicity. Control experiments with Chlorin-e6-a Chl molecule analog-and ab initio time-dependent density functional theory calculations confirm that we are directly probing the fluctuation dynamics of the dative bond between the solvent's lone pair and the Mg2+ center in Chls that is responsible for the Lewis basicity. The observation is indicative of dative bond length and angular fluctuations with timescales ranging between ∼30 and 150 ps and the dative bond-strength-dependent perturbation on the Qy transition frequency of Chls.

Synthesis, crystal structure and characterization of monocrystalline ZnCr2Se4 doped with neodymium

ZnCr2Se4 spinel single crystals doped with neodymium ions having concentrations of 0.05, 0.08 and 0.10 and occupying tetrahedral positions were synthesized using chemical vapour transport. Thermal analysis as well as electrical conductivity, magnetic isotherm, and magnetic susceptibility measurements showed thermal stability at higher temperatures, semiconducting behavior with a metal-insulator transition at TMI ≥ 330 ​K, an antiferromagnetic order with a Néel temperature TN ​= ​21.5 ​K, a short-range ferromagnetic interactions evidenced by a positive Curie-Weiss temperature θ ​= ​55, 60 and 68 ​K with increasing Nd-content in the sequence 0.05, 0.08 and 0.10, two critical fields Hc1 and Hc2 at 10 and 60 ​kOe, regardless of the Nd-content. The specific heat studies showed more complex magnetic transitions, i.e. two magnetic transitions at TN and at spin-glass temperature (TSG) for the Nd-poorer samples and only one magnetic transition at TN for Nd-richer ones. These effects are interpreted in terms of the superexchange integrals for the first two coordination spheres including chemical bond length and spin fluctuation.

Evidence of weak antilocalization in quantum interference effects of (001) oriented La0.7Sr0.3MnO3–SrRuO3 superlattices

Quantum corrections to conductivity in the ferromagnetic La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) thin films depend on the structural mismatches and interfaces accommodating ions and their spins. Here, by making interfaces of LSMO and SRO in the form of artificial superlattices, we achieve positive magnetoresistance (MR) and weak antilocalization (WAL), although the individual component shows negative MR and weak localization (WL). The [20 unit cell (u.c.) LSMO/3 u.c. SRO]×15 superlattice stabilizes in tetragonal symmetry associated with the rhombohedral and orthorhombic structures and demonstrates the occurrence of the single magnon scattering process. The low-field MR of the superlattice fit to the Hikami–Larkin–Nagaoka expression yields 595 Å phase coherence length (lϕ) with WAL of carriers. As the SRO layer thickness in the superlattice increases to 5 u.c., the value of lϕ = 292 Å decreases, and positive MR increases confirm the manifestation of WAL by SRO. The orthorhombic symmetry of the SRO is preserved in the [20 u.c. SRO/3 u.c. LSMO]×15 superlattice, which shows the existence of locally cooperative bond-length fluctuations and conduction due to the scattering of the electron by the Fermi liquid electrons, bond length, and spin fluctuations. However, as the LSMO layer thickness in the superlattice is increased to 5 u.c., the WL effect suppresses WAL at the low field. The spin–orbit coupling associated with magnetic anisotropy, i.e., spin and bond length fluctuations, modifies the WL in the superlattices and leads to WAL, thereby achieving positive MR.

Potassium doping-induced variations in the structures and photoelectric properties of a MAPbI3 perovskite and a MAPbI3/TiO2 junction.

It has been experimentally demonstrated that mixed metallic cation modification could be an effective strategy to enhance the performance and stability of perovskite-based solar cells (PSCs). However, there is limited microscopic understanding at the atomic/molecular level of the behavior of small radius alkali metal cation doping in both perovskite materials and perovskite/TiO2 junctions. Here, we perform a first-principles density functional theory study on the doping-induced variation of the geometric and electronic structures of MAPbI3 (MA = methylammonium) and the MAPbI3/TiO2 junction. The impacts of different doping methods, and different charge states and locations of the given dopants have been investigated. At first, we theoretically confirm that the structures doped by K+ are the most thermally stable compared to the structures doped by the other charge states of K, and that K+ dopants prefer to modify the perovskite lattice interstitially and stay near the MAPbI3/TiO2 interface. Meanwhile, we find that a severe geometric deformation occurs if two doped lattices come into contact directly, indicating that the lattice may rapidly collapse from the interior if the doping concentration is too high. Additionally, we observe that K+ doped interstitially near the MAPbI3/TiO2 interface causes the intensive distortion of the surface Ti-O bonds and severe bond-length fluctuations. Consequently, this results in distorted TiO2 bands of the interfacial layer and a slight decrease of the band offset of conduction bands between two phases. This work complements experiments and provides a better microscopic understanding of the doping modification of the properties of perovskite materials and PSCs.

Dynamics of Water Molecules and Ions in Concentrated Lithium Chloride Solutions Probed with Ultrafast 2D IR Spectroscopy.

Water and ion dynamics in concentrated LiCl solutions were studied using ultrafast 2D IR spectroscopy with the methyl thiocyanate (MeSCN) CN stretch as the vibrational probe. The IR absorption spectrum of MeSCN has two peaks, one peak for water associated with the nitrogen lone pair of MeSCN (W) and the other peak corresponding to Li+ associated with the lone pair (L). To extract the spectral diffusion (structural dynamics) of W and L species, we developed a method that isolates the peak of interest by subtracting the 2D Gaussian proxies of multiple interfering peaks. Center line slope data (normalized frequency-frequency correlation function) for 2D bands from the W and L are fit with triexponential functions. The fastest component (1.1-1.6 ps) is assigned to local hydrogen bond length fluctuations. The intermediate timescale (∼4.0 ps) corresponds to the hydrogen bond network rearrangement. The slowest component decays in ∼40 ps and corresponds to ion pair and ion cluster dynamics. The very similar W and L spectral diffusion indicates that the motions of the water and ions are strongly coupled. Orientational relaxations of the W and L species were extracted using a new method to eliminate the effects of overlapping peaks. The results show that MeSCN bound to water undergoes orientational relaxation significantly faster than MeSCN bound to Li+. The orientational and spectral diffusion results are compared. A Stark coupling model is used to extract the root mean square average electric field caused by the ion clouds along the CN moiety as a function of concentration.

Finite temperature properties and phase transition behavior of quasi-planar B36 nanocluster from first principles

Density-functional molecular dynamics simulations within the Car-Parrinello approach have been carried out to investigate the finite temperature properties and the thermal phase transition behavior of quasi-planar B36 nanocluster via calculating a number of representative indicators such as root-mean-square bond length fluctuations, mean square displacement, radial distribution function, and total energy at forty equilibrium temperatures ranging from zero to 3200 K. Dramatic changes in the temperature dependence of these quantities take place from 1900 to 2200 K found as the phase transition region of the cluster. A detailed analysis of the total energy curve and mean square displacements of individual atoms within this melting region yields Tm ≃ 2125 K as the melting point. Results broadly support the view that the nanocluster preserves its hexagonal structure and quasi-planarity up to the phase transition region, exhibiting a high thermal stability by taking into account its ultra-small size and the melting point, which can be more increased in the case of nanostructures made of several B36 units.

Evolution of local lattice distortion under irradiation in medium- and high-entropy alloys

A major challenge for future nuclear reactors is to design nuclear materials that can sustain extremely prolonged radiation damage. Local lattice distortion, considered as a core effect of high-entropy alloys (HEAs), can suppress the growth of radiation defects to control radiation performance. However, local lattice distortion in HEAs is rarely quantitatively measured. Here we employed total scattering technique to study the local structure of the face-centered cubic (fcc) equiatomic FeCoNiCr MEA, and FeCoNiCrMn and FeCoNiCrPd HEAs before and after ion irradiation. Their local lattice distortions were quantitatively evaluated based on the difference of lattice constant between the local structure and the average structure. We revealed that the mean local lattice distortion in the pristine samples varies in the following order: FeCoNiCr < FeCoNiCrMn < FeCoNiCrPd. Density functional theory (DFT) calculations further unveiled that the fluctuation of local distortions in FeCoNiCr and FeCoNiCrMn is less than 5%, whereas the highest bond-length fluctuation in FeCoNiCrPd can approach to 8%. Under irradiation the mean local lattice distortion in FeCoNiCr and FeCoNiCrMn evolves differently from FeCoNiCrPd by showing a relaxation behavior at low dose. And the impact of local lattice distortion on dislocation loops after a prolonged ion-irradiation was investigated by transmission electron microscope (TEM) and the underlying mechanism was discussed.

Car–Parrinello molecular dynamics study of the melting behaviors of n-atom ([formula omitted]) graphene quantum dots

First-principles molecular dynamics has been applied to inquire into the melting behaviors of n-atom (n=6,10) graphene quantum dots (GQD6 and zigzag GQD10) within the temperature range of T=0–500K. The temperature dependence of the geometry of each quantum dot is thoroughly evaluated via calculating the related shape deformation parameters and the eigenvalues of the quadrupole tensors. Examining the variations of some phase-transition indicators such as root-mean-square bond length fluctuations and mean square displacements broadly proposes the value of Tm=70K for the melting point of GQD6 while a continuous, two-stage phase transition has been concluded for zigzag GQD10.

Applicability of Quantum Thermal Baths to Complex Many-Body Systems with Various Degrees of Anharmonicity.

The semiclassical method of quantum thermal baths by colored noise thermostats has been used to simulate various atomic systems in the molecular and bulk limits, at finite temperature and in moderately to strongly anharmonic regimes. In all cases, the method performs relatively well against alternative approaches in predicting correct energetic properties, including in the presence of phase changes, provided that vibrational delocalization is not too strong-neon appearing already as an upper limiting case. In contrast, the dynamical behavior inferred from global indicators such as the root-mean-square bond length fluctuation index or the vibrational spectrum reveals more marked differences caused by zero-point energy leakage, except in the case of isolated molecules with well separated vibrational modes. To correct for such deficiencies and reduce the undesired transfer among modes, empirical modifications of the noise power spectral density were attempted to better describe thermal equilibrium but still failed when used as semiclassical preparation for microcanonical trajectories.

Bond-length fluctuation in the orthorhombic 3 x 3 x 1 superstructure of LiMn2O4 spinel

Single-crystal synchrotron X-ray diffraction experiments are conducted on spinel-type LiMn2O4 at 230 and 320 K to investigate the effect of charge disproportionation of Mn ions on phase transition near room temperature. The orthorhombic 3ac × 3ac × 1ac superstructure of the low-temperature form, where “ac” is the ideal cubic cell edge, has a network of Mn4+ ions at the vertices of a slightly distorted truncated square tessellation comprising one square and two octagonal prisms; the square prism and one type of octagonal prism house Mn3+ ions with Jahn-Teller (JT) elongated Mn–O bonds almost parallel to the c and b axes, respectively, whereas the other octagonal prism houses Mn ions with JT-induced bond-length fluctuation for the Mn–O bonds lying almost parallel to the a axis. The Mn ions in the latter octagonal prism are assumed to exchange their oxidation states dynamically between 3+ and 4+ in a time ratio of ~3:1, forming a polaron centered at a Mn4O4 heterocubane cluster with orbital and spin orders. The high-temperature cubic form contains an inherent positional disordering of oxygen ions. The effect of the molecular polarons on the phase transition mechanism is discussed on the basis of a spin blockade in the form of truncated square tessellation.

A new perspective of shape recognition to discover the phase transition of finite‐size clusters

An ultrafast shape-recognition technique was used to analyze the phase transition of finite-size clusters, which, according to our research, has not yet been accomplished. The shape of clusters is the unique property that distinguishes clusters from bulk systems and is comprehensive and natural for structural analysis. In this study, an isothermal molecular dynamics simulation was performed to generate a structural database for shape recognition of Ag-Cu metallic clusters using empirical many-body potential. The probability contour of the shape similarity exhibits the characteristics of both the specific heat and Lindemann index (bond-length fluctuation) of clusters. Moreover, our implementation of the substructure to the probability of shapes provides a detailed observation of the atom/shell-resolved analysis, and the behaviors of the clusters were reconstructed based on the statistical information. The method is efficient, flexible, and applicable in any type of finite-size system, including polymers and nanostructures.

Non-universality of the absorbing-state phase-transition in a linear chain with power-law diluted long-range connections

In this work we study the critical behavior of the absorbing state phase transition exhibited by the contact process in a linear chain with power-law diluted long-range connections. Each pair of sites is connected with a probability P(r) that decays with the distance between the sites r as 1/rα. The model allows for a continuous tuning between a standard one-dimensional chain with only nearest neighbor couplings (α→∞) to a fully connected network (α=0). We develop a finite-size scaling analysis to obtain the critical point and a set of dynamical and stationary critical exponents for distinct values of the decay exponent α>2 corresponding to finite average bond lengths and low average site connectivity. Data for the order parameter collapse over a universal curve when plotted after a proper rescaling of parameters. We show further that the critical exponents depend on α in the regime of diverging bond-length fluctuations (α<3).

Effect of bond length fluctuations on crystal nucleation of hard bead chains

We study the effect of bond length fluctuations on the nucleation rate and crystal morphology of linear and cyclic chains of flexibly connected hard spheres using extensive molecular dynamics simulations. For bond length fluctuations as small as a tenth of the bead diameter, the relaxation and crystallization resemble those of disconnected spheres. For shorter bond lengths and chains with <10 beads the nucleation rates depend sensitively on the bond length, number of beads per chain, and chain topology, while for longer chains the nucleation rates are rather independent of chain length. Surprisingly, we find a nice data collapse of the nucleation rate as a function of the number of bonds per sphere in the system, independent of bead chain composition, chain length and topology. Hence, the crystal nucleation rate of bead chains can be enhanced by adding monomers to the system. We also find that the resulting crystal morphologies of bead chains with bond length fluctuations resemble closely those of hard spheres including structures with a five-fold symmetry.

The role of Cu-O bond length fluctuations in the high temperature superconductivity mechanism

We review three different kinds of experiments that emphasize the non-BCS, inhomogeneous aspects of superconductivity in the high Tc cuprates. The first is the existence of two different energy scales in the superconducting state, initially identified by a comparison between tunneling and Andreev–Saint–James spectroscopies [Deutscher, Nature (London) 397, 410 (1999)]. The second are EXAFS measurements of the Cu-O bond length distribution, which have shown that below a temperature T* &amp;gt; Tc, it becomes broader than expected from the Debye-Waller broadening and presents a split [Bianconi et al., Phys. Rev. Lett. 76, 3412 (1996)]. The third one is the effect of frozen lattice disorder on critical current and vortex pinning, which profoundly affects the pairing landscape [Gutierrez et al., Nature Mater. 6, 367 (2007)]. We then discuss how these results fit with models in which the electron-lattice interaction plays a leading role.

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

This study presents a systematic investigation of the thermodynamic properties of free and \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (\ensuremath{\Theta}${}_{D}$, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of \ensuremath{\sim}0.8--5.4 nm. For large NP sizes (diameter $d$ &gt;1.5 nm) \ensuremath{\Theta}${}_{D}$ was found to decrease toward \ensuremath{\Theta}${}_{D}$ of bulk Pt with increasing NP size. For NPs \ensuremath{\le} 1 nm, a monotonic decrease of \ensuremath{\Theta}${}_{D}$ was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (${H}_{2}$) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$ support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the \ensuremath{\Theta}${}_{D}$ enhancement observed experimentally for the smallest NPs ($d$ \ensuremath{\le} 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations.