ABSTRACT
An important aspect concerning the performance of armchair graphene nanoribbons (AGNRs) as materials for conceiving electronic devices is related to the mobility of charge carriers in these systems. When several polarons are considered in the system, a quasi-particle wave function can be affected by that of its neighbor provided the two are close enough. As the overlap may affect the transport of the carrier, the question concerning how the density of polarons affect its mobility arises. In this work, we investigate such dependence for semiconducting AGNRs in the scope of nonadiabatic molecular dynamics. Our results unambiguously show an impact of the density on both the stability and average velocity of the quasi-particles. We have found a phase transition between regimes where increasing density stops inhibiting and starts promoting mobility; densities higher than 7 polarons per 45 Å present increasing mean velocity with increasing density. We have also established three different regions relating electric field and average velocity. For the lowest electric field regime, surpassing the aforementioned threshold results in overcoming the 0.3 Å fs-1 limit, thus representing a transition between subsonic and supersonic regimes. For the highest of the electric fields, density effects alone are responsible for a stunning difference of 1.5 Å fs-1 in the mean carrier velocity.
ABSTRACT
Graphene nanoribbons are 2D hexagonal lattices with semiconducting band gaps. Below a critical electric field strength, the charge transport in these materials is governed by the quasiparticle mechanism. The quasiparticles involved in the process, known as polarons and bipolarons, are self-interacting states between the system charges and local lattice distortions. To deeply understand the charge transport mechanism in graphene nanoribbons, the study of the stability conditions for quasiparticles in these materials is crucial and may guide new investigations to improve the efficiency for a next generation of graphene-based optoelectronic devices. Here, we use a two-dimensional version of the Su-Schrieffer-Heeger model to investigate the stability of bipolarons in armchair graphene nanoribbons (AGNRs). Our findings show how bipolaron stability is dependent on the strength of the electron-phonon interactions. Moreover, the results show that bipolarons are dynamically stable in AGNRs for electric field strengths lower than 3.0 mV/Å. Remarkably, the system's binding energy for a lattice containing a bipolaron is smaller than the formation energy of two isolated polarons, which suggests that bipolarons can be natural quasiparticle solutions in AGNRs. Graphical Abstract Charge localization of bipolarons in armchair garphene nanoribbons.
ABSTRACT
Graphene nanoribbons (GNRs) are two-dimensional structures with a rich variety of electronic properties that derive from their semiconducting band gaps. In these materials, charge transport can occur via a hopping process mediated by carriers formed by self-interacting states between the excess charge and local lattice deformations. Here, we use a two-dimensional tight-binding approach to reveal the formation of bipolarons in GNRs. Our results show that the formed bipolarons are dynamically stable even for high electric field strengths when it comes to GNRs. Remarkably, the bipolaron dynamics can occur in acoustic and optical regimes concerning its saturation velocity. The phase transition between these two regimes takes place for a critical field strength in which the bipolaron moves roughly with the speed of sound in the material.
ABSTRACT
The dynamical properties of polarons in armchair graphene nanoribbons (GNR) is numerically investigated in the framework of a two-dimensional tight-binding model that considers spin-orbit (SO) coupling and electron-lattice (e-l) interactions. Within this physical picture, novel polaron properties with no counterparts to results obtained from conventional tight-binding models are obtained. Our findings show that, depending on the system's width, the presence of SO coupling changes the polaron's charge localization giving rise to different degrees of stability for the charge carrier. For instance, the joint action of SO coupling and e-l interactions could promote a slight increase on the charge concentration in the center of the lattice deformation associated to the polaron. As a straightforward consequence, this process of increasing stability would lead to a depreciation in the polaron's motion by decreasing its saturation velocity. Our finds are in good agreement with recent experimental investigations for the charge localization in GNR, mostly when it comes to the influence of SO coupling. Moreover, the contributions reported here provide a reliable method for future works to evaluate spin-orbit influence on the performance of graphene nanoribbons.