1,721,056 research outputs found
Ab Initio Calculation of the IR Spectrum of PTFE: Helical Symmetry and Defects
No full textA detailed analysis of the structure and vibrational properties of PTFE and the assignment of its IR spectrum are
carried out by means of density functional theory simulations on infinite, one-dimensional chains. Calculations take into
consideration regular polymer chains with different conformations (157, 136, 103, 41, and 21) in order to investigate the main
features due to the peculiar helical structures in the IR spectra. In addition, also the helix-reversal defect and effects related to
conformational disorder are considered, to analyze the contributions in the spectrum due to defects and to the amorphous phase.
The present study solves the ambiguities in the interpretation of the 638−626 cm−1 doublet, assigning the lower frequency
component to a normal mode of the helix-reversal defect. This interpretation is consistent with the general belief that the PTFE
crystal contains a large concentration of defects already at low temperature and that the two crystalline transitions at room
temperatures (19 and 30 °C) are accompanied by an order−disorder transition. As an additional result, the 788 cm−1 band,
previously adopted to measure the amount of amorphous material in real samples, is confirmed as a marker of this phase
Structural and electronic properties of organo-halide hybrid perovskites from ab initio molecular dynamics
No full textThe last two years have seen the unprecedentedly rapid emergence of a new class of solar cells, based on hybrid organic–inorganic halide perovskites. The success of this class of materials is due to their outstanding photoelectrochemical properties coupled to their low cost, mainly solution-based, fabrication techniques. Solution processed materials are however often characterized by an inherent flexible structure, which is hardly mapped into a single local minimum energy structure. In this perspective, we report on the interplay between structural and electronic properties of hybrid lead iodide perovskites investigated using ab initio molecular dynamics (AIMD) simulations, which allow the dynamical simulation of disordered systems at finite temperature. We compare the prototypical MAPbI3 (MA = methylammonium) perovskite in its cubic and tetragonal structure with the trigonal phase of FAPbI3 (FA = formamidinium), investigating different starting arrangements of the organic cations. Despite the relatively short time scale amenable to AIMD, typically a few tens of ps, this analysis demonstrates the sizable structural flexibility of this class of materials, showing that the instantaneous structure could significantly differ from the time and thermal averaged structure. We also highlight the importance of the organic–inorganic interactions in determining the fluxional properties of this class of materials. A peculiar spatial localization of the valence and conduction band edges is also found, with a dynamics in the range of 0.1 ps, which is associated with the positional dynamics of the organic cations within the cubo-octahedral perovskite cage. This asymmetry in the spatial localization of the band edges is expected to ease exciton dissociation and assist the initial stages of charge separation, possibly constituting one of the key factors for the impressive photovoltaic performances of hybrid lead-iodide perovskites
Polymorphism of even nylons revisited through periodic quantum chemical calculations
Full text linkDensity functional theory calculations with periodic boundary conditions have been carried out for the 3D crystal cells of alpha and gamma polymorphs of even polyamides (ranging from nylon-4 up to nylon-12) in order to investigate the relative stability of the two forms. The infinite 1D polymer chains possessing alpha and gamma conformations have been also simulated to shed light on the intramolecular effects taking place in these systems and to discriminate the intermolecular effects in terms of hydrogen bonding and packing of CH2 units. The results of this analysis allowed to give a detailed interpretation of the polymorphism properties observed in even nylons, predicting a larger stability for the cc form in nylon-4 and nylon-6 and a stabilization of the gamma phase from nylon-8 up to nylons having a larger number of CH2 unit in the chain
Ab Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs
No full textState-of-the-art computational methods in
solid-state chemistry were applied to predict the structural
and spectroscopic properties of the α and γ crystalline
polymorphs of nylon 6. Density functional theory calculations
augmented with an empirical dispersion correction (DFT-D)
were used for the optimization of the two different crystal
structures and of the isolated chains, characterized by a
different regular conformation and described as one-dimen-
sional infinite chains. The structural parameters of both
crystalline polymorphs were correctly predicted, and new
insight into the interplay of conformational effects, hydrogen
bonding, and van der Waals interactions in affecting the
properties of the crystal structures of polyamides was obtained.
The calculated infrared spectra were compared to experimental data; based on computed vibrational eigenvectors, assignment of
the infrared absorptions of the two nylon 6 polymorphs was carried out and critically analyzed in light of previous investigations.
On the basis of a comparison of the computed and experimental IR spectra, a set of marker bands was identified and proposed as
a tool for detecting and quantifying the presence of a given polymorph in a real sample: several marker bands employed in the
past were confirmed, whereas some of the previous assignments are criticized. In addition, some new marker bands are proposed.
The results obtained demonstrate that accurate computational techniques are now affordable for polymers characterization,
opening the way to several applications of ab initio modeling to the study of many families of polymeric materials
A computational investigation on singlet and triplet exciton couplings in acene molecular crystals
No full textA density matrix based approach for studying excitons in organic crystals
No full textA theoretical analysis of molecular excitons in crystalline p-Nitro-Aniline (PNA) is presented. The approach is general and can be straightforwardly extended to the calculation of the exciton structure of other molecular crystals or aggregates. Based on the evaluation of transition density matrices (TDM), the method allows to easily classify the excited states of interacting molecules on the basis of those of the constituent molecules. Exciton couplings of selected dimers within the PNA crystal have been evaluated and the exciton dispersion has been determined. The experimental absorption spectrum of PNA powders can be analysed in details based on the exciton density of states
Chlorine Incorporation in the CH3NH3PbI3 Perovskite: Small Concentration, Big Effect
No full textpeer reviewedThe role of chlorine doping in CH3NH3PbI3 represents an important open issue in the use of hybrid perovskites for photovoltaic applications. In particular, even if a positive role of chlorine doping on perovskite film formation and on material morphology has been demonstrated, an inherent positive effect on the electronic and photovoltaic properties cannot be excluded. Here we carried out periodic density functional theory and Car-Parrinello molecular dynamics simulations, going down to ∼1% doping, to investigate the effect of chlorine on CH3NH3PbI3. We found that such a small doping has important effects on the dynamics of the crystalline structure, both with respect to the inorganic framework and with respect to the cation libration motion. Together, we observe a dynamic spatial localization of the valence and conduction states in separated spatial material regions, which takes place in the 10-1 ps time scale and which could be the key to ease of exciton dissociation and, likely, to small charge recombination in hybrid perovskites. Moreover, such localization is enhanced by chlorine doping, demonstrating an inherent positive role of chlorine doping on the electronic properties of this class of materials
: A Detailed Study Based on Density Functional Theory and Symmetry Mode Analysis
No full textFerroelectricity in halide perovskites currently represents a crucial issue, as it may have an important role for the enhancement of solar cells efficiency. Simulations of ferroelectric properties based on density functional theory are conceptually more demanding compared with "conventional" inorganic ferroelectrics due to the presence of both organic and inorganic components in the same compound. Here we present a detailed study focused on the prototypical CH3NH3PbI3 perovskite. By using density functional theory combined with symmetry mode analysis, we disentangle the contributions of the methylammonium cations and the role of the inorganic framework, therefore suggesting possible routes to enhance the polarization in this compound. Our estimate of the polarization for the tetragonal phase at low temperature is similar to 4.42 mu C/cm(2), which is substantially lower than that of traditional perovskite oxides
Effect of chemical substitutions on singlet and triplet exciton ouplings in crystalline acenes: a theoretical investigation
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