Effect Of Thermal Disorder Research Articles

The effect of chemistry and thermal fluctuations on charge injection barriers at aluminum/polyolefin interfaces

Charge injection at metal/polymer interfaces is a critical process in many technological devices, including high voltage capacitors and cables in which polyolefin materials, such as polyethylene (PE) and polypropylene (PP), are often used as insulation materials. We use simulations based on density-functional theory to study charge injection at aluminum/PE and aluminum/PP interfaces. Specifically, we investigate the influence of incorporating a variety of polar chemical impurities at the PE and PP chain ends on electron and hole injection barriers. Crucially, we account for the effect of thermal disorder by considering ensembles of thousands of interface structures obtained from ab initio molecular dynamics trajectories at 373 K. We show that the mean injection barrier can change by up to 1.1 eV for Al/PE and 0.6 eV for Al/PP, as compared to the pristine case, depending on which chemical impurity is introduced. We also show that the spread of injection barriers from thermal fluctuations also depends strongly on the chemistry of the impurity. The observed trends can be understood with a simple model based on thermal fluctuations of the dipole moment density associated with the chemical impurity at the interface. We further verify this model by considering larger interface models with lower impurity densities. Our results demonstrate that small chemical modifications, which may arise from oxidation, for example, have a significant influence on charge injection barriers in metal/polyolefin interfaces.

Generalization on Entropy-Ruled Charge and Energy Transport for Organic Solids and Biomolecular Aggregates.

Herein, a generalized version of the entropy-ruled charge and energy transport mechanism for organic solids and biomolecular aggregates is presented. The effects of thermal disorder and electric field on electronic transport in molecular solids have been quantified by entropy, which eventually varies with respect to the typical disorder (static or dynamic). Based on our previous differential entropy (hs)-driven charge transport method, we explore the nonsteady carrier energy flux principle for soft matter systems from small organic solids to macrobiomolecular aggregates. Through this principle, the synergic nature of charge and energy transport in different organic systems is addressed. In this work, entropy is the key parameter to classify whether the carrier dynamics is in a nonsteady or steady state. Besides that, we also propose the formulation for unifying the hopping and band transport, which provides the relaxation time–hopping rate relation and the relaxation time–effective mass ratio. The calculated disorder drift time (or entropy-weighted carrier drift time) for hole transport in an alkyl-substituted triphenylamine (TPA) molecular device is 9.3 × 10–7 s, which illustrates nuclear dynamics-coupled charge transfer kinetics. The existence of nonequilibrium transport is anticipated while the carrier dynamics is in the nonsteady state, which is further examined from the rate of traversing potential in octupolar molecules. Our entropy-ruled Einstein model connects the adiabatic band and nonadiabatic hopping transport mechanisms. The logarithmic current density at different electric field-assisted site energy differences provides information about the typical transport (whether trap-free diffusion or trap-assisted recombination) in molecular devices, which reflects in the Navamani–Shockley diode equation.

Effects of dynamic disorder on thermoelectric transport in soft materials

A model is developed that accounts for the effects of thermal disorder (both static and dynamic) in predicting the thermoelectric (TE) performance of weakly bonded semiconductors. With dynamic disorder included, the model is found to fit well with experimental results found in the literature for the density-of-states and the energy-dependent carrier mobility, which are key for assessing TE properties. The model is then used to analyze the concentration-dependent TE properties of the prototypical small molecular semiconductor rubrene. At low (e.g., intrinsic) carrier concentrations, where Fermi level pinning occurs, dynamic disorder is found to reduce electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF) to values that are much lower than those traditionally predicted by static disorder models. As carrier concentration (p) increases, S exhibits nonlinear behavior, increasing well above the conventional S vs log(p) relationship before reaching a peak value (Speak∼1550μV/K). A critical carrier concentration (pcrit.≈4.299×10−4 molar ratio) is observed near Speak at which thermoelectric transport transitions from trap-limited behavior at low concentrations to conventional band behavior at high concentrations. Above this value, σ and PF are reduced compared to the perfect crystal and static-only conditions, causing a drop in the maximum PF by factors of 3 and 2.3, respectively. This PF reduction, while not as large as the PF reduction that occurs for low carrier concentration, is found to occur in a high concentration regime (p>pcrit.) that contains the PF maximum and has remained inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.

Evidence of nickel ions dimerization in NiWO[formula omitted] and NiWO[formula omitted]-ZnWO[formula omitted] solid solutions probed by EXAFS spectroscopy and reverse Monte Carlo simulations

The existence of exchange-coupled Ni2+ ions – the so-called magnetic dimers – in wolframite-type NiWO4 and ZncNi1−cWO4 solid solutions with high nickel content was discovered by X-ray absorption spectroscopy combined with reverse Monte Carlo (RMC) simulations. Temperature- (10–300 K) and composition-dependent X-ray absorption spectra were measured at the Ni K-edge, Zn K-edge, and W L3-edge of microcrystalline NiWO4, ZncNi1−cWO4 and ZnWO4. Structural models were obtained from simultaneous analysis of the extended X-ray absorption fine structure (EXAFS) spectra at three metal absorption edges using RMC simulations. The obtained radial distribution functions for different atomic pairs made it possible to trace in detail the changes in the local environment of metal ions and the effect of thermal disorder. Dimerization of Ni2+ ions within quasi-one-dimensional zigzag chains of [NiO6] octahedra was evidenced in NiWO4 in the whole studied temperature range. It manifests itself as the splitting of the Ni–Ni radial distribution function into two separate peaks. The effect is further preserved in solid solutions ZncNi1−cWO4 for c≤0.6, which is related to the probability to find two Ni2+ ions in neighbouring positions.

Polymorphous nature of cubic halide perovskites

It has been long known that numerous halide and oxide perovskites can have non-ideal octahedra, showing tilting, rotation, and metal atom displacements. It has also been known that compounds that have at low temperatures a single structural motif ("monomorphous structures") could become disordered at higher temperatures, resulting in non-ideal octahedra as an entropy effect. What is shown here is that in many cubic halide perovskites and some oxides compounds a distribution of different low-symmetry octahedra ("polymorphous networks") emerge already from the minimization of the systems internal energy, i.e., they represent the intrinsic, preferred low temperature pattern of chemical bonding. Thermal disorder effects build up at elevated temperatures on top of such low temperature polymorphous networks. Compared with the monomorphous counterparts, the polymorphous networks have lower predicted total energies (enhanced stability), larger band gaps and dielectric constants now dominated by the ionic part, and agrees much more closely with the observed pair distribution functions. The nominal cubic perovskites (Pm-3m) structure deduced from X-Ray diffraction is actually a macroscopically averaged, high symmetry configuration, which should not be used to model electronic properties, given that the latter reflect a low symmetry local configuration.

Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped Bi_{2}Sr_{2}Y_{0.08}Ca_{0.92}Cu_{2}O_{8+δ} Driven by Midinfrared Pulse Excitations.

Optimally doped cuprate are characterized by the presence of superconducting fluctuations in a relatively large temperature region above the critical transition temperature. We reveal here that the effect of thermal disorder, which decreases the condensate phase coherence at equilibrium, can be dynamically contrasted by photoexcitation with ultrashort midinfrared pulses. In particular, our findings reveal that light pulses with photon energy comparable to the amplitude of the superconducting gap and polarized in plane along the copper-copper direction can dynamically enhance the optical response associated with the onset of superconductivity. We propose that this effect can be rationalized by an effective d-wave BCS model, which reveals that midinfrared pulses result in a transient increase of the phase coherence.

Theoretical Investigations on Molecular Packing Motifs and Charge Transport Properties of a Family of Trialkylsilylethynyl-Modified Pentacenes/Anthradithiophenes

A family of trialkylsilylethynyl (TAS)-functionalized pentacenes (PENs) and anthradithiophenes (ADTs) are of immense interest due to their good solubility and air stability for uses in optoelectronic devices. Different TAS-substituted PENs and ADTs would result in different crystal packing motifs and carrier transport properties. Quantum nuclear-enabled hopping model combined with molecular dynamics (MD) simulations was used to investigate the effects of the chemical modifications on the carrier transport properties. The disorder-free hole mobilities show that 6,13-bis(trialkylsilylethynyl)anthradithiophenes (TAS-ADTs) own better intrinsic hole transport behaviors than 6,13-bis(trialkylsilylethynyl)pentacenes (TAS-PENs). The MD simulations show that in comparison with TAS-PENs, the thermal disorder effects are less significant for TAS-ADTs; this is probably due to the C–H···S hydrogen bonds, which are thought to stabilize the molecules in crystal environments. Furthermore, the syn-TAS-ADTs show more serio...

Advanced approach to the local structure reconstruction and theory validation on the example of the W L3-edge extended x-ray absorption fine structure of tungsten

Atomistic simulations of the experimental W L3-edge extended x-ray absorption fine structure (EXAFS) of bcc tungsten at T = 300 K were performed using classical molecular dynamics (MD) and reverse Monte Carlo (RMC) methods. The MD-EXAFS method based on the results of MD simulations allowed us to access the structural information, encoded in EXAFS, beyond the first coordination shell and to validate the accuracy of two interaction potential models—the embedded atom model potential and the second nearest-neighbor modified embedded atom method potential. The RMC-EXAFS method was used for more elaborate analysis of the EXAFS data giving access to thermal disorder effects. The results of both methods suggest that the correlation in atomic motion in bcc tungsten becomes negligible above 8 Å. This fact allowed us to use the EXAFS data to determine not only mean-square relative displacements of atomic W–W pair motion but also mean-square displacements of individual tungsten atoms, which are usually accessible from diffraction data only.

Effects of thermal disorder on the electronic structure of halide perovskites: insights from MD simulations.

The effects of thermal disorder on the electronic properties of organic/inorganic halide perovskites were investigated using ab initio molecular dynamics simulations. It was generally found that band gap variations due to effects of thermal disorder are the largest in materials with the smallest lattice constant. The factors that may lead to departure from this trend include the degree of rotational and translational motion of the organic cation and the strength of its dipole. It was found that the contribution of the flexible organic part to the band gap variations is considerably smaller than the contribution of the inorganic part of the material. The results of our simulations indicate that band gap variations in halide perovskites fall within the range exhibited in inorganic semiconductors.

Doping colloidal bcc crystals \u2014 interstitial solids and meta-stable clusters

The addition of a small amount of dopant impurities to crystals is a common method to tune the properties of materials. Usually the doping grade is restricted by the low solubility of the dopants; increasing the doping concentration beyond this solubility limit leads to supersaturated solutions in which dopant clusters dominate the material properties, often leading to deterioration of strength and performance. Descriptions of doped solids often assume that thermal excitations of the on average perfect matrix are small. However, especially for bcc crystals close to their melting point it has recently become clear that the effects of thermal disorder are strong. Here we study the doping of weak bcc crystals of charged colloids via Brownian dynamics simulations. We find a complex phase diagram upon varying the dopant concentration. At low dopant concentrations we find an interstitial solid solution. As we increase the amount of dopants a complex meta-stable liquid-in-solid cluster phase emerges. Ultimately this phase becomes meta-stable with respect to macroscopic crystal-crystal coexistence. These results illustrate the complex behaviour that emerges when thermal excitations of the matrix drive impure crystals to a weak state.

Ab initio investigation of the anomalous phonon softening in FeSi

The anomalous softening of the acoustic phonon peak in FeSi has recently received considerable experimental attention. In our work, we investigate the effect of thermal disorder on the lattice dynamics and the filling of the narrow band gap of FeSi using density functional theory. We show, by comparing the phonon density of states from temperature-independent and temperature-dependent force constants, that thermal structural disorder together with thermal expansion explains the anomalously strong renormalization of the acoustic phonons. Furthermore, we find an intricate interplay between thermal disorder and volume in gap closure.

Atomistic simulations of the Fe K-edge EXAFS in FeF3 using molecular dynamics and reverse Monte Carlo methods

Atomistic simulations of the experimental Fe K-edge extended x-ray absorption fine structure (EXAFS) of rhombohedral (space group ) FeF3 at T = 300 K were performed using classical molecular dynamics and reverse Monte Carlo (RMC) methods. The use of two complementary theoretical approaches allowed us to account accurately for thermal disorder effects in EXAFS and to validate the developed force-field model, which was constructed as a sum of two-body Buckingham-type (Fe–F and F–F), three-body harmonic (Fe–F–Fe) and Coulomb potentials. We found that the shape of the Fe K-edge EXAFS spectrum of FeF3 is a more sensitive probe for the determination of potential parameters than the values of structural parameters (a, c, x(F)) available from diffraction studies. The best overall agreement between the experimental and theoretical EXAFS spectra calculated using ab initio multiple-scattering approach was obtained for the iron effective charge q(Fe) = 1.71. The RMC method coupled with the evolutionary algorithm was used for more elaborate analysis of the EXAFS data. The obtained results suggest that our force-field model slightly underestimates the amplitude of thermal vibrations of fluorine atoms in the direction perpendicular to the Fe–F bonds.

Spontaneous Polarization Induced by Side Chains in Ordered Poly(3-hexylthiophene)

We have found that ordered poly(3-hexylthiophene) (P3HT) exhibits spontaneous polarization along the backbone direction. This effect is caused by the lack of inversion symmetry due to head-to-tail side-chain arrangement. We have also shown that spontaneous polarization in ordered P3HT keeps significant values even at room temperature when the effects of thermal disorder are important. Consequently, it has a strong effect on electronic properties of the material. For example, at the interface between the crystalline and amorphous domain in P3HT, the electric field caused by spontaneous polarization confines the delocalized HOMO state to one side of the crystalline domain.

Analysis of the U L3-edge X-ray absorption spectra in UO2 using molecular dynamics simulations

Uranium L3-edge X-ray absorption spectroscopy was used to study the atomic structure of uranium dioxide (UO2). The extended X-ray absorption fine structure (EXAFS) was interpreted within the ab initio multiple-scattering approach combined with classical molecular dynamics to account for thermal disorder effects. Nine force-field models were validated, and the role of multiple-scattering contributions was evaluated.

The Effects of Thermal Disorder on the Solution-Scattering Profiles of Macromolecules

The solution-scattering profiles of macromolecules are significantly affected by the thermal motions of their atoms, especially at wide scattering angles, even when only a single conformational state is significantly populated in solution. Here it is shown that the impact thermal motions have on the molecular component of the solution-scattering profile of a single-state macromolecule can be predicted accurately if the variances and covariances of the thermal excursions of its atoms from their average positions are known.

Effects of thermal disorder on the electronic properties of ordered polymers.

The effects of thermal disorder on the electronic properties of crystalline polymers were investigated. Atomic configurations of the material were obtained using classical Monte Carlo simulations at room temperature, while electronic structure calculations were performed using the density functional theory based charge patching method and the overlapping fragment method. We investigated two different stable configurations of crystalline poly(3-hexylthiophene) (P3HT) and calculated the density of electronic states and the wave function localisation. We found that the effect of disorder in side chains is more pronounced in the more stable configuration of P3HT than in the other one due to the larger conformational freedom of side chains. The results show that disorder in main chains has a strong effect on the electronic structure and leads to the localisation of the wave functions of the highest states in the valence band, similar to localisation that occurs in amorphous polymers. The presence of such states is one possible origin of thermally activated electrical transport in ordered polymers at room temperature.

Multiscale Approach to Theoretical Simulations of Materials for Nuclear Energy Applications: Fe-Cr and Zr-based Alloys

ABSTRACTWe review basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: Fe-Cr and Zr-based alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of Fe-Cr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For Zr-Nb alloys, we demonstrate how microscopic parameters calculated from first-principles can be used in higher-level models.

Effect of thermal disorder on high figure of merit in PbTe

Effect of thermal disorder on high figure of merit in PbTe

Phonon softening and metallization of a narrow-gap semiconductor by thermal disorder

The vibrations of ions in solids at finite temperature depend on interatomic force–constants that result from electrostatic interactions between ions, and the response of the electron density to atomic displacements. At high temperatures, vibration amplitudes are substantial, and electronic states are affected, thus modifying the screening properties of the electron density. By combining inelastic neutron scattering measurements of Fe 1- x Co x Si as a function of temperature, and finite-temperature first-principles calculations including thermal disorder effects, we show that the coupling between phonons and electronic structure results in an anomalous temperature dependence of phonons. The strong concomitant renormalization of the electronic structure induces the semiconductor-to-metal transition that occurs with increasing temperature in FeSi. Our results show that for systems with rapidly changing electronic densities of states at the Fermi level, there are likely to be significant phonon–electron interactions, resulting in anomalous temperature-dependent properties.

Recent advances in the study of thermal effects on EXAFS

Understanding the effects of thermal disorder on extended X-ray absorption fine structure increases the accuracy of results and allows one to gain original insights on local dynamical properties. Some recent advances are reviewed here.