Wiliam Ferreira da Cunha

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.

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.

CO2 adsorption on single-walled boron nitride nanotubes containing vacancy defects

The adsorption of a CO2 molecule on the vacancy defect type of armchair (5,5) and zigzag (10,0) single-walled boron nitride nanotubes was studied based on Density Functional Theory (DFT). Vacancy defects were studied and the geometrical modifications implemented on the original hexagonal lattice yielded a considerable level of changes in the electronic properties. These changes are reflected in a greater level of CO2 reactivity in relation to the adsorption over a pristine structure. For all types of studied CO2 molecule interaction, we have found a chemical adsorption process based on binding energy. Furthermore, the CO2 adsorption takes place on the top of the vacancy region. A decomposition state was observed when the CO2 molecule interacted with the armchair nanotube with a vacancy on the nitrogen site. By comparing the values of the adsorption energies with those from other defect approaches present in the literature, we conclude that the proposed protocol presents a possible tool to develop stable and sensible carbon dioxide sensors.

Carbon dioxide adsorption on doped boron nitride nanotubes

Boron nitride (BN) nanotubes are promising structures as far as the gas adsorption process is concerned. The electronic and vibrational properties of pristine and cobalt doped single walled boron nitride nanotubes of different chiralities interacting with a carbon dioxide molecule are investigated through the use of density functional theory (DFT) and the discrete variable representation method. When compared to similar simulations concerning carbon nanotubes, a stronger interaction is observed between the carbon dioxide molecule and the functionalized BN nanotube. A density of state investigation suggests that the doping induces major changes in the electronic structure pattern in the sense of critically reducing the original gap. From the vibrational point of view, we note that the zig-zag chirality tends to present higher values of vibrational frequencies for most of the states considered, regardless of the nanotubes being doped or not. Our results suggest that doped zig-zag BN nanotubes are among the best possible candidates for adsorption purposes.

Impurity effects on polaron-exciton formation in conjugated polymers

Combining the one-dimensional tight-binding Su-Schrieffer-Heeger model and the extended Hubbard model, the collision of two oppositely charged polarons is investigated under the influence of impurity effects using a non-adiabatic evolution method. Results show that electron-electron interactions have direct influence on the charge distribution coupled to the polaron-exciton lattice defect. Additionally, the presence of an impurity in the collisional process reduces the critical electric field for the polaron-exciton formation. In the small electric field regime, the impurity effects open three channels and are of fundamental importance to favor the polaron-exciton creation. The results indicate that the scattering between polarons in the presence of impurities can throw a new light on the description of electroluminescence in conjugated polymer systems.

Transport of Polarons in Graphene Nanoribbons

The field-induced dynamics of polarons in armchair graphene nanoribbons (GNRs) is theoretically investigated in the framework of a two-dimensional tight-binding model with lattice relaxation. Our findings show that the semiconductor behavior, fundamental to polaron transport to take place, depends upon of a suitable balance between the GNR width and the electron-phonon (e-ph) coupling strength. In a similar way, we found that the parameter space for which the polaron is dynamically stable is limited to an even narrower region of the GNR width and the e-ph coupling strength. Interestingly, the interplay between the external electric field and the e-ph coupling plays the role to define a phase transition from subsonic to supersonic velocities for polarons in GNRs.

Dynamical study of impurity effects on bipolaron-bipolaron and bipolaron-polaron scattering in conjugated polymers.

Combining the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model and the extended Hubbard model (EHM), the scattering of two oppositely charged bipolarons and a bipolaron-polaron pair is investigated under the influence of impurity effects using a nonadiabatic evolution method. These novel results for bipolarons show that the oppositely charged quasi-particles scatter into a mixed state composed of bipolarons and excitons. The excitation yield depends sensitively on the strength of the applied electric field. In the presence of an impurity, the critical electric field regime for formation of a state composed by bipolarons and excitons is increased. Additionally, we were able to obtain critical values of electric fields that played the role of drastically modifying the system dynamics. These facts suggest that the scattering between bipolarons and a bipolaron-polaron pair in the presence of impurities is crucial for the understanding of electroluminescence in optoelectronics devices, such as polymer light emitting diodes.

Molecular dynamics investigation of charge carrier density influence over mobility in conjugated polymers.

Charge carrier mobility is known to be one of the most important efficiency delimiting factors in conducting polymer-based electronic devices. As the transport mechanism in this class of material is nonconventional, many works have tried to elucidate the charge carriers interaction with temperature, external electric field, and the collective effects they present. Even though the multiple trap-and-release model is often used to describe these effects, its applicability is known to be restricted to electronic properties. In this work we make use of a modified version of the Su-Schrieffer-Heeger model, the most used method to describe the important properties of conducting polymer in general, to investigate the influence of temperature and carrier densities over the transport mechanisms. We obtained different regimes of temperature and carriers density influence over the systems mobility, consistent with most of the experimental data available.

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.

Optimally tuned functionals improving the description of optical and electronic properties of the phthalocyanine molecule

By means of Density functional theory and time-dependent density functional theory calculations, we present a comprehensive investigation on the influence of different functional schemes on electronic and optical properties of the phthalocyanine molecule. By carrying out our own tuning on the OT-LC-BLYP/6-31G(d,p) functional, we show that such a procedure is fundamental to accurately match experimental results. We compare our results to several others available in the literature, including the B3LYP/6-31+G(d,p) set, which is commonly portrayed as the best combination in order to obtain a good description of the band gap. The results obtained here present not only significant improvement of the optical properties from the conventional BLYP, but we can also objectively report an improvement of our tuned functional when compared to the current benchmark of the literature as far as optical properties are concerned. Particularly, by means of this approach, it was possible to achieve a good agreement between the theoretical and experimental optical gap as well as of the positioning of the main peaks in the absorption spectrum. Our results thus suggest that correcting the long-range term on exchange term of the Coulomb operator, by means of a tuning procedure, is a good option to accurately describe properties of the phthalocyanine molecule.