Width effects on bilayer graphene nanoribbon polarons.

Recent progress in nanoelectronics suggests that stacking armchair graphene nanoribbons (AGNRs) into bilayer systems can generate materials with emergent quasiparticle properties. In this context, the impact of width changes is especially relevant. However, its effect on charged carriers remains elusive. In this work, we investigate the effect of width and interlayer interaction changes on polaron states via a hybrid Hamiltonian that couples the electronic and lattice interactions. Results show the rising of two interlayer polarons: the non-symmetric and the symmetric. The coupling strength needed to induce the transition between states depends on the nanoribbon width, being at the most extreme case of ≈174 meV. Electronic properties such as the coupling strength threshold, carrier size, and gap are shown to respect the AGNR width family 3p, 3p + 1, and 3p + 2 rule. The findings demonstrate that strong interlayer interaction simultaneously delocalizes the carriers and reduces the gap up to 0.6 eV. Additionally, it is found that some layers are more prone to share charge, indicating a potential heterogeneous stacking where a particular electronic pathway is favored. The results present an encouraging prospect for integrating AGNR bilayers in future flexible electronics.

Torsional fracture of carbon nanotube bundles: a reactive molecular dynamics study.

Carbon nanotubes individually show excellent mechanical properties, being one of the strongest known materials. However, when assembled into bundles, their strength reduces dramatically. This still limits the understanding of their scalability. Here, we perform reactive molecular dynamics simulations to study the mechanical resilience and fracture patterns of carbon nanotube bundles (CNTBs) under torsional strain. The results revealed that the fracture patterns of CNTBs are diameter-dependent. The larger the tube diameter, the higher the plasticity degree of the bundle sample when subjected to torsional loading. Tube chirality can also play a role in distinguishing between the CNTBs during the torsion process. Armchair-based CNTBs have higher accumulated energies and, consequently, higher critical angles for the bundle fracture when contrasted with CNTBs composed of zigzag or chiral nanotubes. Remarkably, the CNTB torsional fracture can yield nanodiamondoids.

A reactive molecular dynamics study on the mechanical properties of a recently synthesized amorphous carbon monolayer converted into a nanotube/nanoscroll.

Recently, laser-assisted chemical vapor deposition has been used to synthesize a free-standing, continuous, and stable monolayer amorphous carbon (MAC). MAC is a pure carbon structure composed of randomly distributed five, six, seven, and eight atom rings, which is different from that of disordered graphene. More recently, amorphous MAC-based nanotubes (a-CNT) and nanoscrolls (a-CNS) were proposed. In this work, we have investigated (through fully atomistic reactive molecular dynamics simulations) the mechanical properties and sublimation points of pristine and a-CNT and a-CNS. The results showed that a-CNT and a-CNS have distinct elastic properties and fracture patterns compared to those of their pristine analogs. Both a-CNT and a-CNS presented a non-elastic regime before their total rupture, whereas the CNT and CNS underwent a direct conversion to fractured forms after a critical strain threshold. The critical strain values for the fracture of the a-CNT and a-CNS are about 30% and 25%, respectively, and they are lower than those of the corresponding CNT and CNS cases. Although less resilient to tension, the amorphous tubular structures have similar thermal stability in relation to the pristine cases with sublimation points of 5500 K, 6300 K, 5100 K, and 5900 K for a-CNT, CNT, a-CNS, and CNS, respectively. An interesting result is that the nanostructure behavior is substantially different depending on whether it is a nanotube or a nanoscroll, thus indicating that the topology plays an important role in defining its elastic properties.

Tuning the electronic structure properties of MoS2 monolayers with carbon doping.

The structural and electronic properties of MoS2 sheets doped with carbon line domains are theoretically investigated through density functional theory calculations. It is primarily studied how the system's electronic properties change when different domain levels are considered. These changes are also reflected in the geometry of the system, which acquires new properties when compared to the pristine structure. We predict, both qualitative and quantitatively, how the energy gap changes as a function of domain types. Strikingly, the band structure for the doped system shows semiconducting behavior with an indirect-bandgap, which is narrower than the one for bulk MoS2. This is an important feature as far as gap tuning engineering is concerned. It has a profound impact on the applicability of these systems in electronic devices, where an indirect bandgap favors the quantum yield efficiency.

Influence of quasi-particle density over polaron mobility in armchair graphene nanoribbons.

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.

Dynamic Formation of Bipolaron-Exciton Complexes in Conducting Polymers.

The recombination dynamics of two oppositely charged bipolarons within a single polymer chain is numerically studied in the scope of a one-dimensional tight-binding model that considers electron-electron and electron-phonon (e-ph) interactions. By scanning among values of e-ph coupling and electric field, novel channels for the bipolaron recombination were yielded based on the interplay between these two parameters. The findings point to the formation of a compound species formed from the coupling between a bipolaron and an exciton. Depending on the electric field and e-ph coupling strengths, the recombination mechanism may yield two distinct products: a trapped (and almost neutral) or a moving (and partially charged) bipolaron-exciton. These results might enlighten the understanding of the electroluminescence processes in organic light-emitting devices.

Nonadiabatic dynamics of injected holes in conjugated polymers.

The dynamics of injected holes in short transient times that precede polaron formation is numerically studied in the framework of a tight-binding electron-phonon interacting approach aimed at describing organic one-dimensional lattices. In particular, the direct impact of internal and external factors on the conversion of injected holes into polarons is carefully investigated. The results show that a hole injected at levels lower than the highest occupied molecular orbital forms self-trapped bound structures that can merge spontaneously to form a polaron after, at least, one picosecond. On the other hand, the life-time of such structures substantially decreases (up to a few hundreds of femtoseconds) when the influence of external electric fields, temperature effects and impurities is considered. Importantly, the critical values of the aforementioned factors in promoting the quenching of the self-trapped structures are obtained. These findings may enlighten the understanding of the mechanism of charge carrier generation in Polymer Light Emitting Diodes when several kinds of excitations are present.

Concentration effects on intrachain polaron recombination in conjugated polymers.

The influence of different charge carrier concentrations on the recombination dynamics between oppositely charged polarons is numerically investigated using a modified version of the Su-Schrieffer-Heeger (SSH) model that includes an external electric field and electron-electron interactions. Our findings show that the external electric field can play the role of avoiding the formation of excited states (polaron-exciton and neutral excitation) leading the system to a dimerized lattice. Interestingly, depending on a suitable balance between the polaron concentration and the electric field strength, the recombination mechanism can form stable polaron-excitons or neutral excitations. These results may provide guidance to improve the electroluminescence efficiency in Polymer Light Emitting Diodes.

Critical temperature and products of intrachain polaron recombination in conjugated polymers.

The intrachain recombination dynamics between oppositely charged polarons is theoretically investigated through the use of a version of the Su-Schrieffer-Heeger (SSH) model modified to include an external electric field, an extended Hubbard model, Coulomb interactions, and temperature effects in the framework of a nonadiabatic evolution method. Our results indicate notable characteristics concerning the polaron recombination: (1) it is found that there exists a critical temperature regime, below which an exciton is formed directly and (2) a pristine lattice is the resulting product of the recombination process, if the temperature is higher than the critical value. Additionally, it is found that the critical electric field regime plays the role of drastically modifying the system dynamics. These facts suggest that thermal effects in the intrachain recombination of polarons are crucial for the understanding of electroluminescence in optoelectronic devices, such as Polymer Light Emitting Diodes.

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.

Tuning magnetic properties of penta-graphene bilayers through doping with boron, nitrogen, and oxygen.

Penta-graphene (PG) is a carbon allotrope that has recently attracted the attention of the materials science community due to its interesting properties for renewable energy applications. Although unstable in its pure form, it has been shown that functionalization may stabilize its structure. A question that arises is whether its outstanding electronic properties could also be further improved using such a procedure. As PG bilayers present both sp[Formula: see text] and sp[Formula: see text] carbon planes, it consists of a flexible candidate for functionalization tuning of electromagnetic properties. In this work, we perform density functional theory calculations to investigate how the electronic and structural properties of PG bilayers can be tuned as a result of substitutional doping. Specifically, we observed the emergence of different magnetic properties when boron and nitrogen were used as dopant species. On the other hand, in the case of doping with oxygen, the rupture of bonds in the sp[Formula: see text] planes has not induced a magnetic moment in the material.

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.

Electronic structure properties of transition metal dichalcogenide nanotubes: a DFT benchmark.

In this work, we conduct a benchmark study of bandgap energies and density of states of some transition metal dichalcogenide nanotubes by means of density functional theory (DFT) methodology within both CASTEP and DMol3 methodologies. We compare different chiralities and sizes as well as different levels of theory in order to provide the literature with extensive data regarding crucial electronic structure properties of MoS2, MoSe2, mOtE2, WS2, WSe2, and WTe2 nanotubes. Although the two methods were able to rescue experimental evidences, we observed DMol3 to perform better in terms of computational cost, whereas CASTEP has shown to provide an overall greater accuracy at the cost of higher expenditures. The data provided in this work is an important suggestion of which direction future works should follow in further description of these technological promising materials. Graphical Abstract Frontal (left) and side (right) views for the schematic represenation of a zigzag TMD nanotube.

Charge Carrier Scattering in Polymers: A New Neutral Coupled Soliton Channel.

The dynamical scattering of two oppositely charged bipolarons in non-degenerate organic semiconducting lattices is numerically investigated in the framework of a one-dimensional tight-biding-Hubbard model that includes lattice relaxation. Our findings show that it is possible for the bipolaron pair to merge into a state composed of a confined soliton-antisoliton pair, which is characterized by the appearance of states within less than 0.1 eV from the Fermi level. This compound is in a narrow analogy to a meson confining a quark-antiquark pair. Interestingly, solitons are quasi-particles theoretically predicted to arise only in polymer lattices with degenerate ground state: in the general case of non-degenerate ground state polymers, isolated solitons are not allowed.

Spin-Orbit Effects on the Dynamical Properties of Polarons in Graphene Nanoribbons.

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.

Krypton-methanol spectroscopic study: Assessment of the complexation dynamics and the role of the van der Waals interaction.

The Kr-CH3OH (Krypton-Methanol) system has several technological applications, such as the determination of diffusivity coefficients, their use in the development of detectors and combustion techniques among others. We report an extensive theoretical study concerning the stability of such complex. A mix between molecular dynamics, electronic structure calculations and solution of the nuclear Schrodinger equation lead to investigation of spectroscopic constants, lifetime of the complex and its Quantum Theory Atom in Molecules (QTAIM) properties. The study of the Potential Energy Curves (PEC) suggested three configurations to be stable as their potential well were able to harbor 9 vibrational levels. Properties from the curves also allowed us to obtain the lifetime of the complex, whose values were >1 ps regardless of the conformation. Furthermore, topological investigations of the charge density profile of the complex, in the scope of QTAIM properties, show that van der Waals type interactions takes place between the noble gas and the methanol molecule. These features are in consonance to the experimental fact that this complex is stable.

Combined UMC- DFT prediction of electron-hole coupling in unit cells of pentacene crystals.

Pentacene is an organic semiconductor that draws special attention from the scientific community due to the high mobility of its charge carriers. As electron-hole interactions are important aspects in the regard of such property, a computationally inexpensive method to predict the coupling between these quasi-particles is highly desired. In this work, we propose a hybrid methodology of combining Uncoupled Monte Carlo Simulations (UMC) and Density functional Theory (DFT) methodologies to obtain a good compromise between computational feasibility and accuracy. As a first step in considering a Pentacene crystal, we describe its unit cell: the Pentacene Dimer. Because many conformations can be encountered for the dimer and considering the complexity of the system, we make use of UMC in order to find the most probable structures and relative orientations for the Pentacene-Pentacene complex. Following, we carry out electronic structure calculations in the scope of DFT with the goal of describing the electron-hole coupling on the most probable configurations obtained by UMC. The comparison of our results with previously reported data on the literature suggests that the methodology is well suited for describing transfer integrals of organic semiconductors. The observed accuracy together with the smaller computational cost required by our approach allows us to conclude that such methodology might be an important tool towards the description of systems with higher complexity.

Optical and electronic structure description of metal-doped phthalocyanines.

Phthalocyanines represent a crucial class of organic compounds with high technological appeal. By doping the center of these systems with metals, one obtains the so-called metal-phthalocyanines, whose property of being an effective electron donor allows for potentially interesting uses in organic electronics. In this sense, investigating optical and electronic structure changes in the phthalocyanine profiles in the presence of different metals is of fundamental importance for evaluating the appropriateness of the resulting system as far as these uses are concerned. In the present work, we carry out this kind of effort for phthalocyanines doped with different metals, namely, copper, nickel, and magnesium. Density functional theory was applied to obtain the absorption spectra, and electronic and structural properties of the complexes. Our results suggest that depending on the dopant, a different level of change is achieved. Moreover, electrostatic potential energy mapping shows how the charge distribution can be affected by solar radiation. Our contribution is crucial in describing the best possible candidates for use in different organic photovoltaic applications. Graphical Abstract Representation of meta-phthalocyanine systems. All calculations of this work are based on varying metal position along z axis, considering the z-axis has its zero point matching with the center of phthalocyanine cavityconsidering.

Electron-phonon coupling effects on intrachain polaron recombination in conjugated polymers.

Based on a one-dimensional Su-Schrieffer-Heeger (SSH) model with electron correlation considered within the extended Hubbard model (EHM), we investigate the role played by electron-phonon coupling constant on intrachain polaron recombination in conjugated polymers. Our results suggest that a competition between external electric field and electron-phonon coupling on defining the behavior of the charge distribution of the system takes place. Whereas increasing electric field plays the role of destabilizing the charge carriers, an increase of the electron-phonon coupling has the opposite effect. Therefore, a suitable balance of these properties can give rise to the correct description of charge carrier dynamics in conducting polymers.