Smooth gap tuning strategy for cove-type graphene nanoribbons.

Graphene is a carbon-based material with an extensive range of promising properties. Since it does not present a bandgap, graphene is not suitable for optoelectronic applications. One possible way to open a gap is achieved by reducing graphene to its nanoribbon (GNR) form. Recently, a GNR with well defined cove-type periphery proper for large-scale production was synthesized showing an energy bandgap of 1.88 eV. In this work, we propose an edge termination strategy that allows for smoothly tuning the energy bandgap of cove-type GNRs by systematically changing the periodicity with which armchair-like and zigzag-like edges alternate. Using an extended two-dimensional Su-Schrieffer-Heeger tight-binding model we compare the effects of this edge termination process on lattice deformation with those arising from changes in nanoribbon width. Results show that modifications to the edges of cove-type GNRs are able to smoothly reduce energy bandgaps at the expense of losses in conjugation and increased morphological spreading. Energy band gap values starting from ≈3 eV to almost 0 eV were obtained. The flexibility provided by this gap tuning procedure places the cove-type GNR as an interesting candidate material for optoelectronic applications.

Effective Mass of Quasiparticles in Armchair Graphene Nanoribbons.

Armchair graphene nanoribbons (AGNRs) may present intrinsic semiconducting bandgaps, being of potential interest in developing new organic-based optoelectronic devices. The induction of a bandgap in AGNRs results from quantum confinement effects, which reduce charge mobility. In this sense, quasiparticles' effective mass becomes relevant for the understanding of charge transport in these systems. In the present work, we theoretically investigate the drift of different quasiparticle species in AGNRs employing a 2D generalization of the Su-Schrieffer-Heeger Hamiltonian. Remarkably, our findings reveal that the effective mass strongly depends on the nanoribbon width and its value can reach 60 times the mass of one electron for narrow lattices. Such underlying property for quasiparticles, within the framework of gap tuning engineering in AGNRs, impact the design of their electronic devices.

Temperature effects on the scattering of polarons and bipolarons in organic conductors.

The scattering process between an electron-polaron and a hole-bipolaron has been simulated using a version of the Su-Schrieffer-Heeger (SSH) model modified to include an external electric field, Coulomb interactions, and temperature effects in the scope of nonadiabatic molecular dynamics. The simulations reveal remarkable details concerning the polaron-bipolaron recombination reaction. It is found that there exists a critical temperature regime below which a hole-bipolaron and a mixed state composed by an electron-polaron and an exciton are formed and a hole-bipolaron and a free electron are the resulting products of the collisional process, if the temperature is higher than the critical value. In addition, it is obtained that both channels depend sensitively on the strength of the applied electric field. These significant results may provide guidance to understand processes regarding electroluminescence in polymer diodes.

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.

The H + Li2 bimolecular exchange reaction: dynamical and kinetical properties at J = 0.

For the first time in the literature, rigorous time-independent quantum scattering formalism was applied, by means of the ABC program, to the H + Li(2) → LiH + Li reaction. The state-to-state probabilities as a function of the total energy have been computed at zero total angular momentum (J = 0) allowing us to evaluate the effect of vibrational/rotational excitation on the reaction promotion/inhibition, the energetic distribution of products, and the temperature dependence of the J-shifting thermal rate coefficients.

A computational investigation of the multiple channels of the NF2 + F reaction.

We have theoretically studied the NF(3) = NF(2) + F, NF(2) + F = NF + F(2), and NF(2) + F = NF(2) + F reactive processes. More precisely, we have evaluated the thermal rate constants (TRC), with the Wigner and Eckart tunneling corrections, minimum energy path, and the intrinsic reaction coordinates of these systems. The NF(3) = NF(2) + F conventional and Wigner TRCs agree very well with experimental data available in the literature for a wide range of temperatures. This study gives a first step to understand and determine the correct decomposition path of nitrogen trifluoride (NF(3)).

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 carrier's 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.

Theoretical temperature dependence of the charge-carrier mobility in semiconducting polymers.

We present a theory of the temperature and electric field dependence on the mobility of polarons in conjugated polymers in terms of a tight-binding and stochastic approach. The polaron mobility is shown to have a strong dependence on the electric field, with two distinct regimes of temperature dependence. Lattice thermal oscillations enhance polaron mean velocity for electric fields of 1.0 mV/A or higher. In contrast, its mobility is damped by thermal oscillations under weaker electric fields. This semiconductor/metallic analogous behavior comes from the difference between the inertial content acquired by polarons under stronger/weaker electric fields. These new results and their analysis shed new light on several experimental controversies.

Quantum bits with polyacetylene.

The dynamics of a polyacetylene single chain as a system for possible physical implementations of quantum bits is determined. This novel proposition is studied by varying intensity and duration of application of an electric field as well as the intensity, number, and position in the polymer chain of impurity molecules. The behavior of soliton pairs, whose associated energy levels form the quantum bit, is analyzed. The chain is modeled by a modified Pariser-Parr-Pople Hamiltonian extended to include the effects of an external electric field and the parameters of the impurity molecules. The effect of the variation of the field and impurities on the separation of the energy levels associated with soliton pairs is analyzed by numerical integration of the equations of motion. Two different approaches for controlling the separation of levels are presented, and their features compared. First, the use of changes in the electric field to control the distance (and ultimately coupling) between two solitons moving freely on the chain or captured by the potential generated by the impurity molecules. Second, the change in the intensity of the impurities alone, with no application of an external field. We have found that the effect of the use of the field on the separation of levels is much smaller than the one obtained by changes in the parameters of the impurity molecules, which eventually led us to achieve quantum bit behavior in a polyacetylene chain. The influence of the field and impurity parameters in the energy levels is determined, as well as their role in the coupling of the two solitons on the chain. Critical values for distance between solitons, intensity of field, and impurities that determine whether a pair of solitons can work as a quantum bit are obtained.