Charge localization and hopping in a topologically engineered graphene nanoribbon

Graphene nanoribbons (GNRs) are promising quasi-one-dimensional materials with various technological applications. Recently, methods that allowed for the control of GNR’s topology have been developed, resulting in connected nanoribbons composed of two distinct armchair GNR families. Here, we employed an extended version of the Su-Schrieffer-Heeger model to study the morphological and electronic properties of these novel GNRs. Results demonstrated that charge injection leads to the formation of polarons that localize strictly in the 9-AGNRs segments of the system. Its mobility is highly impaired by the system’s topology. The polaron displaces through hopping between 9-AGNR portions of the system, suggesting this mechanism for charge transport in this material

On the electronic structure of a recently synthesized graphene-like BCN monolayer from bis-BN cyclohexane with single-atom vacancies : a DFT study

Since the rising of graphene, boron nitride monolayers have been deeply studied due to their structural similarity with the former. A hexagonal graphene-like boron–carbon–nitrogen (h-BCN) monolayer was synthesized recently using bis-BN cyclohexane (B2N2C2H12) as a precursor molecule. Herein, we investigated the electronic and structural properties of this novel BCN material, in the presence of single-atom (boron, carbon, or nitrogen) vacancies, by employing density functional theory calculations. The stability of these vacancy-endowed structures is verified from cohesion energy calculations. Results showed that a carbon atom vacancy strongly distorts the lattice leading to breaking on its planarity and bond reconstructions. The single-atom vacancies induce the appearance of flat midgap states. A significant degree of charge localization takes place in the vicinity of these defects. It was observed a spontaneous magnetization only for the boron-vacancy case, with a magnetic dipole moment about 0.87 μB.Our calculations predicted a direct electronic bandgap value of about 1.14 eV. Importantly, this bandgap value is intermediate between gapless graphene and insulating hexagonal boron nitride

A genetic algorithm approach to design principles for organic photovoltaic materials

The increase in the efficiency of organic photovoltaic (OPV) devices relies on understanding the underlying science of several interconnected physical mechanisms that prevent 1D optimization strategies to succeed. Here, a combination of kinetic Monte Carlo simulations of exciton dynamics with a genetic algorithm to automatically optimize the external quantum efficiency of donor–acceptor interfaces under different scenarios is employed. Simulations include phenomena from light absorption to exciton diffusion, dissociation, radiative recombination, and internal conversion, thus modeling the main physical processes that define the overall efficiency of an OPV up to charge separation. It is shown that when internal conversion is kept in check, the combination of optimal transition dipole moments and absorption energies points at low bandgap polymers as the most promising candidates for donor materials. However, when non-radiative deexcitation mechanisms are stronger, the optimization strategy shifts toward higher bandgaps, focusing rather on increasing the fluorescence quantum yield of the donor. Finally, the approach shows that adjusting the energy levels of the acceptor so that exciton transfers across the interface become negligible produces important gains in efficiency and at the same time reduces the system’s dependence on large electronic couplings. The findings indicate pathways for engineering highly efficient organic interfaces

Exciton Diffusion in Organic Nanofibers: A Monte Carlo Study on the Effects of Temperature and Dimensionality

Abstract Organic nanofibers have found various applications in optoelectronic devices. In such devices, exciton diffusion is a major aspect concerning their efficiency. In the case of singlet excitons, Förster transfer is the mechanism responsible for this process. Temperature and morphology are factors known to influence exciton diffusion but are not explicitly considered in the expressions for the Förster rate. In this work, we employ a Kinetic Monte Carlo (KMC) model to investigate singlet exciton diffusion in para-hexaphenyl (P6P) and α-sexithiophene (6T) nanofibers. Building from previous experimental and theoretical studies that managed to obtain temperature dependent values for Förster radii, exciton average lifetimes and intermolecular distances, our model is able to indicate how these parameters translate into diffusion coefficients and diffusion lengths. Our results indicate that these features strongly depend on the coordination number in the material. Furthermore, we show how all these features influence the emitted light color in systems composed of alternating layers of P6P and 6T. Finally, we present evidence that the distribution of exciton displacements may result in overestimation of diffusion lengths in experimental setups

Theoretical prediction of electron mobility in birhodanine crystals and their sulfur analogs

Molecular crystals compose the current state of the art when it comes to organic-based optoelectronic applications. Charge transport is a crucial aspect of their performance. The ability to predict accurate electron mobility is needed in designing novel organic semiconducting materials. In the present work, the Semi-Classical Marcus (SCM) and Marcus–Levich–Jortner (MLJ) hopping models are employed to numerically describe the charge mobility in six distinct birhodanine-like crystals. These materials were recently used in n-channel organic transistors as electron transporting layers. Results have revealed that the MLJ approach predicts electron mobilities in good agreement with the experiment, whereas SCM underestimates this parameter. Remarkably, we found for one of the birhodanine derivatives studied here average electron mobility of 0.14 cm V−1s−1, which agrees with the one reported in experimental investigations. Moreover, it was identified that the MLJ approach presents a strong dependency on external reorganization energy. For SCM, a change in the reorganization energy value has a small impact on mobility, while for MLJ it impacts the average electron mobility that exponentially decays by increasing the external reorganization energy. Importantly, we highlight the primary source of the differences in predicting the electron mobility presented by both approaches, providing useful details that will help the selection of one of these two models for study different species of organic molecular crystals

Evaluating the performance of NIST’s framework cybersecurity controls through a constructivist multicriteria methodology

This paper aims to show how creating a risk plan can be solved with the help of the constructivist multicriteria method. A case study using Multicriteria Decision Aid Constructivist (MCDA-C) was applied, with cybersecurity framework’s controls as a reference. The study was conducted in a large Brazilian bank in Brazil. The relevance of this work is the need to show that the application of multicriteria methods can be applied in the context of information security, which recommends the use of such methods to assist in risk analysis. The methodology used in this study was both quantitative and qualitative, obtaining primary data through brainstorming with decision-makers and forms answered by experts. The secondary data were obtained through the Framework for Improving Critical Infrastructure Cybersecurity, created by NIST - the National Institute of Standards and Technology of the United States. The problem was structured according to the constructivist method, and the data collected were processed and calculated. The study concluded that the category of Security Continuous Monitoring controls stood out compared to other categories. It also shows the importance of applying the constructivist method for the management of cyber risks by unravelling a problem and providing a basis for decision making. Our work contributes to a better understanding of risk management, encouraging the adoption of the constructivist method as a form of risk management best practice

A DFT study on the electronic structure of in-plane heterojunctions of graphene and hexagonal boron nitride nanoribbons

Accepted Manuscript.The structural similarity between hexagonal boron nitride (h-BN) and graphene nanoribbons allows forming heterojunctions with small chain stress. The insulation nature of the former and the quasi-metallic property of the latter make them attractive for flat optoelectronics. Recently, shapes of graphene and h-BN domains were precisely controlled, creating sharp graphene/h-BN interfaces. Here, we investigated the electronic and structural properties of graphene (h-BN) nanoribbon domains of different sizes sandwiched between h-BN (graphene) nanoribbons forming in-plane heterojunctions. Different domain sizes for the non-passivated zigzag edge termination were studied. Results showed that the charge density is localized in the edge of the heterojunctions, regardless of the domain size. The systems with graphene domains are metallic, presenting null band gaps. The ones with the h-BN island are small-bandgap semiconductors with the highest bandgap value around 0.2 eV. The calculated bandgap has the same magnitude of the certain threshold for DFT. As a general trend, these materials exhibit a ferromagnetic behavior, which can be useful for magnetic applications at the nanoscale

Organic Electronics from Nature: Computational Investigation of the Electronic and Optical Properties of the Isomers of Bixin and Norbixin Present in the Achiote Seeds

Organic compounds have been employed in developing new green energy solutions with good cost-efficiency compromise, such as photovoltaics. The light-harvesting process in these applications is a crucial feature that still needs improvements. Here, we studied natural dyes to propose an alternative for enhancing the light-harvesting capability of photovoltaics. We performed density functional theory calculations to investigate the electronic and optical properties of the four natural dyes found in achiote seeds (Bixa orellana L.). Different DFT functionals, and basis sets, were used to calculate the electronic and optical properties of the bixin, norbixin, and their trans-isomers (molecules present in Bixa orellana L.). We observed that the planarity of the molecules and their similar extension for the conjugation pathways provide substantially delocalized wavefunctions of the frontier orbitals and similar values for their energies. Our findings also revealed a strong absorption peak in the blue region and an absorption band over the visible spectrum. These results indicate that Bixa orellana L. molecules can be good candidates for improving light-harvesting in photovoltaics

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit

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