Improving the theoretical description of charge transport in organic crystals.

Charge hopping based on Marcus theory is often used to predict charge carrier mobilities in organic crystals, although it is known to systematically underestimate the values. Here we show that this deficiency may lie on a fundamental aspect of quantum statistical averages, rather than on the approximation itself. Under adequate Boltzmann weighing procedure used to evaluate electron and hole transfer integrals, a kinetic Monte Carlo model is employed to describe mobilities in an azacene derivative. The values are in good agreement with experimental data suggesting that the evaluation of transfer integrals may be the weak link in hopping transport models.

CO2 adsorption in nitrogen-doped single-layered graphene quantum dots: a spectroscopic investigation.

In this work, we investigate the adsorption process of CO2 in graphene quantum dots from the electronic structure and spectroscopic properties point of view. We discuss how a specific doping scheme could be employed to further enhance the adsorbing properties of the quantum dots. This is evaluated by considering the depth of the potential well, the spectroscopic constants, and the lifetime of the compound. Electronic structure calculations are carried out in the scope of the density functional theory (DFT), whereas discrete variable representation (DVR) and Dunham methodologies are employed to obtain spectroscopic constants and hence the lifetimes of the systems. Our results suggest that nitrogen-doped graphene quantum dots are promising structures as far as sensing applications of CO2 are concerned. Graphical Abstract Adsorption mechanism of the CO2 molecule in (a) a pristine and (b) a nitrogendoped Graphene Quantum Dot.

Polaron Properties in Armchair Graphene Nanoribbons.

By means of a 2-D tight-binding model with lattice relaxation in a first-order expansion, we report different polaron properties depending on the armchair graphene nanoribbons width family as well as on its size. We find that representatives of the 3p+2 family do not present a polaronic-mediated charge transport. As for 3p and 3p+1 families, the polaron behavior was completely dependent on the system's width. In particular, we observed a greater degree of delocalization for broader nanoribbons; narrower nanoribbons of both families, on the contrary, typically presented a more localized polaronic-type transport. Energy levels and occupation numbers analysis are performed to rigorously assess the nature of the charge carrier. Time evolution in the scope of the Ehrenfest molecular dynamics was also carried out to confirm the collective behavior and stability of the carrier as a function of time. We were able to determine that polarons in nanoribbons of 3p family present higher mobility than those in 3p+1 nanoribbons. These results identify the transport process that takes place for each system, and they allow the prediction of the mobility of the charge carriers as a function of the structural properties of the system, thus providing guidance on how to improve the efficiency of graphene nanoribbon-based devices.

Improving the Description of the Optical Properties of Carotenoids by Tuning the Long-Range Corrected Functionals.

In this work we use gap-fitting procedure to tune the long-range corrected functionals and accurately investigate the electronic and optical properties of the five main molecules composing Buriti oil (extracted from Mauritia flexuosa L.) in the framework of density functional theory (DFT) and time-dependent (TD) DFT. The characteristic length (1/ω) was observed to be entirely system dependent, though we concluded that its determination is of fundamental importance to rescue geometrical, electronic, and optical properties with accuracy. We demonstrate that our approach of tuning characteristic length for each system resulted in an absorbance spectra in better experimental agreement when compared to the traditional methodology. Therefore, this study indicates that the tuning of the range-separation parameter is crucial to improve the description of the optical properties of conjugated molecules when TDDFT is used. For example, the wavelength of maximum absorption, λmax, for the phytofluene, obtained using B3LYP, is 381 nm, while using the gap-fitting procedure for the tuned-LC-BLYP the estimated λmax changed to 358 nm. The latter estimate is in better agreement with the experimental value of 350 nm.

Rovibrational energies and spectroscopic constants for H2O-Ng complexes.

In this work, rovibrational energies and spectroscopic constants for the water -Ng complexes (Ng = He, Ne, Ar, Kr and Xe) were calculated through two different approaches (by solving the Nuclear Schrödinger equation and by applying the Dunham's method) and using two different potential energy curves (PEC). These PEC were determined using potential parameters obtained through molecular beam scattering experiments and accurate theoretical calculation, respectively. It was found that the theoretical rovibrational energies are in a good agreement (only for the lowest numbers of vibrational states) with those obtained through experimental PEC. Another important conclusions was regarding the calculated first two rovibrational energies for the H 2 O-Ar system, that are in a good agreement with the experimental data.

A detailed reactive cross section study of X + Li2 → Li + LiX, with X = H, D, T, and Mu.

In this work we apply quasiclassical trajectory theory to the X + Li2 → Li + LiX reactions, with X standing for H, D, T, and Mu, in order to determine dynamical properties such as state-to-state reactive cross-section, rotational, vibrational, and translational product distributions. By using the literature benchmark potential energy surface, we were able to predict the aforementioned dynamical property in remarkable qualitative agreement with data in the literature for the H + Li2 → Li + LiH channel. Particularly, our results points toward the well known cross section independence with ro-vibrational excitations for high excitation regimes. Since the methodology is known to be well suited for the other species, as we considered the same PES, our results are expected to be similarly accurate for D, T, and Mu. The present work consists on a significant progress in this area of research, since previous theoretical calculations-based on known potential energy surface-deviated from the experimental results.

Vibrational and electronic structure analysis of a carbon dioxide interaction with functionalized single-walled carbon nanotubes.

Electronic and vibrational properties of different single-walled carbon nanotubes (SWNTs) interacting with a CO2 molecule are investigated through the use of density functional theory (DFT) calculations and the discrete variable representation (DVR) method, respectively. We observed a considerable geometry difference between pristine and doped nanotubes. Consequently, a greater binding energy between the former type of nanotubes and the adsorbing molecule is achieved, a fact that finds experimental support and leads us to consider cobalt-doped nanotubes as promising candidates for chemical sensors. From the vibrational point of view, we note that the zigzag chirality tends to present higher values of vibrational frequencies for most of the states considered regardless of the nanotubes being doped or not. The potential energy curves (PECs) for the interactions between CO2 and all of the considered nanotubes together with spectroscopic constants are provided, and the reliability of the performed calculations makes the data a useful source of comparison for future works.

Exciton dissociation and charge carrier recombination processes in organic semiconductors.

Exciton dissociation and charge recombination processes in organic semiconductors, with thermal effects taken into account, are described in this paper. Here, we analyzed the mechanisms of polaron-excitons dissociation into free charge carriers and the consequent recombination of those carriers under thermal effects on two parallel π-conjugated polymers chains electronically coupled. Our results suggest that exciton dissociation in a single molecule give rise to localized, polaron-like charge carrier. Besides, we concluded that in the case of interchain processes, the bimolecular polaron recombination does not lead to an usual exciton state. Rather, this type of recombination leads to an oscillating dipole between the two chains. The recombination time obtained here for these processes are in agreement with the experimental results. Finally, our results show that temperature effects are essential to the relaxation process leading to polaron formation in a single chain, as in the absence of temperature, this process was not observed. In the case of two chains, we conclude that temperature effects also help the bimolecular recombination process, as observed experimentally.

Thermal rate constant calculation of the NF + F reactive system multiple arrangements.

In this work we analyzed the multiple channels of the reaction NF+F through the evaluation of thermal rate constants with both Wigner and Eckart tunneling corrections. Minimum energy paths and intrinsic reaction coordinates of the systems were obtained and accurately studied in order to ensure the consistency of our results. Specifically, we investigated the NF + F = N + F(2), NF + F = NF + F, and NF(2) = NF + F, reactive systems. As experimental data are available for the latter reaction, we were able to conclude that our thermal rate constants are in agreement for a wide range of temperatures. The here performed study is relevant to the understanding of the decomposition process of nitrogen trifluoride (NF(3)).