arXiv subjects

Rodrigo B. Capaz

Publications and source records attributed to Rodrigo B. Capaz.

At least 19 recordsLinked to original sources

arXiv2024

TMM$-$Sim: A Versatile Tool for Optical Simulation of Thin$-$Film Solar Cells

The Transfer Matrix Method (TMM) has become a prominent tool for the optical simulation of thin$-$film solar cells, particularly among researchers specializing in organic semiconductors and perovskite materials. As the commercial viability of these solar cells continues to advance, driven by rapid developments in materials and production processes, the importance of optical simulation has grown significantly. By leveraging optical simulation, researchers can gain profound insights into photovoltaic phenomena, empowering the implementation of device optimization strategies to achieve enhanced performance. However, existing TMM$-$based packages exhibit limitations, such as requiring programming expertise, licensing fees, or lack of support for bilayer device simulation. In response to these gaps and challenges, we present the TMM Simulator (TMM$-$Sim), an intuitive and user$-$friendly tool to calculate essential photovoltaic parameters, including the optical electric field profile, exciton generation profile, fraction of light absorbed per layer, photocurrent, external quantum efficiency, internal quantum efficiency, and parasitic losses. An additional advantage of TMM$-$Sim lies in its capacity to generate outcomes suitable as input parameters for electro$-$optical device simulations. In this work, we offer a comprehensive guide, outlining a step$-$by$-$step process to use TMM$-$Sim, and provide a thorough analysis of the results. TMM$-$Sim is freely available, accessible through our web server (nanocalc.org), or downloadable from the TMM$-$Sim repository (for \textit{Unix}, \textit{Windows}, and \textit{macOS}) on \textit{GitHub}. With its user$-$friendly interface and powerful capabilities, TMM$-$Sim aims to facilitate and accelerate research in thin$-$film solar cells, fostering advancements in renewable energy technologies.

cond-mat.mtrl-sci↗

arXiv2024

RI$-$Calc: A User Friendly Software and Web Server for Refractive Index Calculation

The refractive index of an optical medium is essential for studying a variety of physical phenomena. One useful method for determining the refractive index of scalar materials (i.e, materials which are characterized by a scalar dielectric function) is to employ the Kramers-Kronig (K-K) relations. The K-K method is particularly useful in cases where ellipsometric measurements are unavailable, a situation that frequently occurs in many laboratories. Although some packages can perform this calculation, they usually lack a graphical interface and are complex to implement and use. Those deficiencies inhibits their utilization by a plethora of researchers unfamiliar with programming languages. To address the aforementioned gap, we have developed the Refractive Index Calculator (RI-Calc) program that provides an intuitive and user-friendly interface. The RI-Calc program allows users to input the absorption coefficient spectrum and then easily calculate the complex refractive index and the complex relative permittivity of a broad range of thin films, including of molecules, polymers, blends, and perovskites. The program has been thoroughly tested, taking into account the Lorentz oscillator model and experimental data from a materials' refractive index database, demonstrating consistent outcomes. It is compatible with Windows, Unix, and macOS operating systems. You can download the RI-Calc binaries from our GitHub repository or conveniently access the program through our dedicated web server at nanocalc.org.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic control of Weyl nodes and wave packets in three-dimensional warped semimetals

We investigate the topological phase transitions driven by band warping and a transverse magnetic field, for three-dimensional Weyl semimetals. First, we use the Chern number as a mathematical tool to derive the topological phase diagram. Next, we associate each of the topological sectors to a given angular momentum state of a rotating wave packet. Then we show how the position of the Weyl nodes can be manipulated by a transverse external magnetic field that ultimately quenches the wave packet rotation, first partially and then completely, thus resulting in a sequence of field-induced topological phase transitions. Finally, we calculate the current-induced magnetization and the anomalous Hall conductivity of a prototypical warped Weyl material. Both observables reflect the topological transitions associated with the wave packet rotation and can help to identify the elusive 3D quantum anomalous Hall effect in three-dimensional, warped Weyl materials.

cond-mat.mes-hall↗

arXiv2023

Strain, Anharmonicity and Finite-Size Effects on the Vibrational Properties of Linear Carbon Chains

Linear carbon chains (LCCs) are the ultimate 1D molecular system and they show unique mechanical, optical and electronic properties that can be tuned by altering the number of carbon atoms, strain, encapsulation, and other external parameters. In this work, we probe the effects of quantum anharmonicity, strain and finite size on the structural and vibrational properties of these chains, using high$-$level density functional theory (DFT) calculations. We find strong anharmonicity effects for infinite chains, leading to ground$-$state nuclear wavefunctions that are barely localized at each of the dimerized geometries, i.e. strong tunneling occurs between the two minima of the potential energy surface. This effect is enhanced for compressive strains. In addition, vibrational C$-$band frequencies deviate substantially from experimental measurements in long chains encapsulated in carbon nanotubes. On the other hand, calculations for finite chains suggest that quantum anharmonicity effects are strongly suppressed in finite system, even in the extrapolation to the infinite case. For finite systems, vibrational C$-$band frequencies agree well with experimental values at zero pressure. However, these frequencies increase under compressive strain, in contradiction with recent results. This contradiction is not resolved by adding explicitly the encapsulating carbon nanotubes to our calculations. Our results indicate that LCCs embody an intriguing 1D system in which the behavior of very large finite systems do not reproduce or converge to the behavior of truly infinite ones.

cond-mat.mtrl-sci↗

arXiv2023

Reorganization energy from charge transport measurements in a monolithically$-$integrated molecular device

Intermolecular charge transfer reactions are key processes in physical chemistry. The electron-transfer rates depend on a few system's parameters, such as temperature, electromagnetic field, distance between adsorbates and, especially, the molecular reorganization energy. This microscopic greatness is the energetic cost to rearrange each single$-$molecule and its surrounding environment when a charge is transferred. Reorganization energies are measured by electrochemistry and spectroscopy techniques as well as at the single-molecule limit using atomic force microscopy approaches, but not from temperature$-$dependent charge transport measurements nor in a monolithically$-$integrated molecular device. Nowadays self$-$rolling nanomembrane (rNM) devices, with strain$-$engineered mechanical properties, on$-$a$-$chip monolithic integration, and operable in distinct environments, overcome those challenges. Here, we investigate the charge transfer reactions occurring within a ca. 6 nm thick copper$-$phthalocyanine (CuPc) film employed as electrode-spacer in a monolithically integrated nanocapacitor. Employing the rNM technology allows us to measure the molecules' charge$-$transport dependence on temperature for different electric fields. Thereby, the CuPc reorganization energy is determined as (930 $\pm$ 40) meV, whereas density functional theory (DFT) calculations support our findings with the atomistic picture of the CuPc charge transfer reaction. Our approach presents a consistent route towards electron transfer reaction characterization using current$-$voltage spectroscopy and provides insight into the role of the molecular reorganization energy when it comes to electrochemical nanodevices.

cond-mat.mtrl-sci↗

arXiv2023

Improved Performance of Organic Light-Emitting Transistors Enabled by Polyurethane Gate Dielectric

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that combine in a single structure the advantages of organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). However, low charge mobility and high threshold voltage are critical hurdles to practical OLETs implementation. This work reports on the improvements obtained by using polyurethane films as dielectric layer material in place of the standard poly(methylmethacrylate) (PMMA) in OLET devices. It was found that polyurethane drastically reduces the number of traps in the device thereby improving electrical and optoelectronic device parameters. In addition, a model was developed to rationalize an anomalous behavior at the pinch-off voltage. Our findings represent a step forward to overcome the limiting factors of OLETs that prevent their use in commercial electronics by providing a simple route for low-bias device operation.

cond-mat.mtrl-sci↗

arXiv2023

FRET$-$Calc: A Free Software and Web Server for F\"orster Resonance Energy Transfer Calculation

F\"{o}rster Resonance Energy Transfer Calculator (FRET$-$Calc) is a program and web server that analyzes molar extinction coefficient of the acceptor, emission spectrum of the donor, and the refractive index spectrum of the donor/acceptor blend. Its main function is to obtain important parameters of the FRET process from experimental data, such as: (i) effective refractive index, (ii) overlap integral, (iii) F\"{o}rster radius, (iii) FRET efficiency and (iv) FRET rate. FRET$-$Calc is license free software that can be run via dedicated web server (nanocalc.org) or downloading the program executables (for Unix, Windows, and macOS) from the FRET$-$Calc repository on GitHub. The program features a user$-$friendly interface, making it suitable for materials research and teaching purposes. In addition, the program is optimized to run on normal computers and is lightweight. An example will be given with the step by step of its use and results obtained.

cond-mat.mtrl-sci↗

arXiv2023

PLQ-Sim: A Computational Tool for Simulating Photoluminescence Quenching Dynamics in Organic Donor/Acceptor Blends

Photoluminescence Quenching Simulator (PLQ-Sim) is a user-friendly software to study the photoexcited state dynamics at the interface between two organic semiconductors forming a blend: an electron donor (D), and an electron acceptor (A). Its main function is to provide substantial information on the photophysical processes relevant to organic photovoltaic and photothermal devices, such as charge transfer state formation and subsequent free charge generation or exciton recombination. From input parameters provided by the user, the program calculates the transfer rates of the D/A blend and employs a kinetic model that provides the photoluminescence quenching efficiency for initial excitation in the donor or acceptor. When calculating the rates, the user can choose to use disorder parameters to better describe the system. In addition, the program was developed to address energy transfer phenomena that are commonly present in organic blends. The time evolution of state populations is also calculated providing relevant information for the user. In this article, we present the theory behind the kinetic model, along with suggestions for methods to obtain the input parameters. A detailed demonstration of the program, its applicability, and an analysis of the outputs are also presented. PLQ-Sim is license free software that can be run via dedicated webserver nanocalc.org or downloading the program executables (for Unix, Windows, and macOS) from the PLQ-Sim repository on GitHub.

cond-mat.mtrl-sci↗

arXiv2023

Emission Redshift in DCM2-Doped Alq$_{3}$ Caused by Non-Linear Stark Shifts and F\"orster-Mediated Exciton Diffusion

Organic light-emitting diodes (OLEDs) devices in the archetype small molecule fluorescent guest-host system tris(8-hydroxyquinolinato) aluminum (Alq$_{3}$) doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2) displays a redshift in light-emission frequency which is extremely sensitive to the dopant concentration. This effect can be used to tune the emission frequency in this particular class of OLEDs. In this work, a model is proposed to describe this effect using a combination of density functional theory (DFT) quantum-chemical calculations and stochastic simulations of exciton diffusion via a F\"orster mechanism. The results show that the permanent dipole moments of the Alq$_{3}$ molecules generate random electric fields that are large enough to cause a non-linear Stark shift in the band gap of neighboring DCM2 molecules. As a consequence of these non-linear shifts, a non-Gaussian probability distribution of highest-occupied molecular orbital to lowest-unoccupied molecular orbital (HOMO-LUMO) gaps for the DCM2 molecules in the Alq$_{3}$ matrix is observed, with long exponential tails to the low-energy side. Surprisingly, this probability distribution of DCM2 HOMO-LUMO gaps is virtually independent of DCM2 concentration into Alq$_{3}$ matrix, at least up to a fraction of 10%. This study shows that this distribution of gaps, combined with out-of-equilibrium exciton diffusion among DCM2 molecules, are sufficient to explain the experimentally observed emission redshift.

cond-mat.mtrl-sci↗

arXiv2023

Patterning edge-like defects and tuning defective areas on the basal plane of ultra-large MoS$_{2}$ monolayers toward hydrogen evolution reaction

The catalytic sites of MoS$_{2}$ monolayers towards hydrogen evolution are well known to be vacancies and edge-like defects. However, it is still very challenging to control the position, size, and defective areas on the basal plane of Mo$S_{2}$ monolayers by most of defect-engineering routes. In this work, the fabrication of etched arrays on ultra-large supported and free-standing MoS$_{2}$ monolayers using focused ion beam (FIB) is reported for the first time. By tuning the Ga+ ion dose, it is possible to confine defects near the etched edges or spread them over ultra-large areas on the basal plane. The electrocatalytic activity of the arrays toward hydrogen evolution reaction (HER) was measured by fabricating microelectrodes using a new method that preserves the catalytic sites. We demonstrate that the overpotential can be decreased up to 290 mV by assessing electrochemical activity only at the basal plane. High-resolution transmission electron microscopy images obtained on FIB patterned freestanding MoS$_{2}$ monolayers reveal the presence of amorphous regions and X-ray photoelectron spectroscopy indicates sulfur excess in these regions. Density-functional theory calculations provide identification of catalytic defect sites. Our results demonstrate a new rational control of amorphous-crystalline surface boundaries and future insight for defect optimization in MoS$_{2}$ monolayers.

cond-mat.mtrl-sci↗

arXiv2023

Enhancing Chemical Stability and Photovoltaic Properties of Highly Efficient Nonfullerene Acceptors by Chalcogen Substitution: Insights from Quantum Chemical Calculations

The chemical stability of nonfullerene acceptor (NFA) is the Achilles' heel of the research on state-of-the-art organic solar cells (OSC). The fragility of the NFA is essentially due to the weak bond that links the central donor core of the molecules with their acceptor moieties at the edges. Here we proposed the replacement of thiophene at the outer-core position of traditional NFAs for tellurophene, a hitherto unexplored modification. Since tellurium is a distinctive element among chalcogens, the basic features of Te compounds cannot be deduced straightforwardly from the properties of their lighter analogues, S and Se. The modeled Te-based NFAs presented interesting features like stronger intra- and intermolecular interactions induced by a distinctive secondary bond effect between the end acceptor moiety and the outer chalcogen atom. This design strategy resulted in stiffer molecules with red-shifted absorption spectra and less susceptible to degradation verified through stress tests and vibrational spectra analysis. Besides that, a weakened exciton binding energy has been found, opening the possibility of blends with a lower driving force. Our results shed light on several aspects of selenation and telluration of traditional NFAs, providing valuable insights into the possible consequences for OSCs applications.

cond-mat.mtrl-sci↗

arXiv2022

The role of functional thiolated molecules on the enhanced electronic transport of interconnected MoS$_2$ nanostructures

Molecular linkers have emerged as an effective strategy to improve electronic transport properties on solution-processed layered materials via defect functionalization. However, a detailed discussion on the microscopic mechanisms behind the beneficial effects of functionalization is still missing. Here, by first-principles calculations based on density functional theory, we investigate the effects on the electronic properties of interconnected MoS$_2$ model flakes systems upon functionalization with different thiol molecule linkers, namely thiophenol, 1,4-benzenedithiol, 1,2-ethanedithiol, and 1,3-propanedithiol. The bonding of benzene- and ethanedithiol bridging adjacent armchair MoS$_2$ nanoflakes leads to electronic states just above or at the Fermi level, thus forming a molecular channel for electronic transport between flakes. In addition, the molecular linker reduces the potential barrier for thermally activated hopping between neighboring flakes, improving the conductivity as verified in experiments. The comprehension of such mechanisms helps in future developments of solution-processed layered materials for use on 2D electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Structural metastability and Fermi surface Topology of SrAl2Si2

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, \alpha phase or a superconducting, BaZn2P2-type, \beta phase. We explore possible \alpha --Pc;Tc--> \beta transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectra calculations, and room-temperature synchrotron X-ray powder diffraction (XRPD) measurements up to 14 GPa using diamond anvil cell. Our theoretical and empirical analyses together with all baric and thermal reported events on both phases allow us to construct a preliminary P-T diagram of transformations. Our calculations show a relatively low critical pressure for the \alpha to \beta transition (4.9 GPa at 0 K, 5.0 GPa at 300 K and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure-low-temperature \alpha phase is separated from metastable \beta phase by a relatively high activation barrier. This analysis is supported by our XRPD data at ambient temperature and P < 14 GPa which shows an absence of \beta phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2-dimorphs or total chemical substitution of Ca by Sr in isomorphous \alpha CaAl2Si2; empirically, manifestation of such topology modification is evident when comparing the evolution of (magneto-)transport properties of members of SrAl2Si2-dimorphs and \alpha isomorphs.

cond-mat.mtrl-sci↗

arXiv2020

Electronic properties of substitutional impurities in graphene-like C$_2$N, $tg$-C$_3$N$_4$, and $hg$-C$_3$N$_4$

We study the electronic and structural properties of substitutional impurities of graphenelike nanoporous materials C$_2$N, $tg$-, and $hg$-C$_3$N$_4$ by means of density functional theory calculations. We consider four types of impurities; boron substitution on carbon sites (B(C)), carbon substitution on nitrogen sites (C(N)), nitrogen substitution on carbon sites (N(C)), and sulfur substitution on nitrogen sites (S(N)). From cohesive energy calculations, we find that the C(N) and B(C) substitutions are the most energetically favorable and induce small bond modifications in the vicinity of the impurity, while the S(N) induces strong lattice distortions. Though all of the studied impurities induce defect levels inside the band gap of these materials, their electronic properties are poles apart depending on the behavior of the impurity as an acceptor or a donor. It is also observed that acceptor (donor) wavefunctions are composed only of $\sigma$ ($\pi$) orbitals from the impurity itself and/or neighboring sites. Consequently, acceptor wavefunctions are directed towards the pores and donor wavefunctions are more extended throughout the neighboring atoms, a property that could further be explored to modify the interaction between these materials and adsorbates. Moreover, impurity properties display a strong site sensitivity and ground state binding energies ranging from $0.03$ to $1.13$ eV, thus offering an interesting route for tuning the optical properties of these materials. Finally, spin-polarized calculations reveal that all impurity configurations have a magnetic ground state that rises from the spin splitting of the impurity levels. In a few configurations, more than one impurity level can be found inside the gap and two of them could potentially be explored as two-level systems for single-photon emission, following similar proposals recently made on defect complexes on TMDCs.

cond-mat.mtrl-sci↗

arXiv2019

Flat bands and gaps in twisted double bilayer graphene

We present electronic structure calculations of twisted double bilayer graphene (TDBG): A tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers. The surface effect competes with an energy upshift of localized states at inner layers, giving rise to the peculiar angle dependence of the band gap, which reduces at low angles. For these low twist angles, the TDBG develops flat bands, in which electrons in the inner layers are localized at the AA regions, as in twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2019

Layer breathing and shear modes in multilayer graphene: A DFT-vdW study

In this work, we study structural and vibrational properties of multilayer graphene using density-functional theory (DFT) with van der Waals (vdW) functionals. Initially, we analyze how different vdW functionals compare by evaluating the lattice parameters, elastic constants and vibrational frequencies of low energy optical modes of graphite. Our results indicate that the vdW-DF1-optB88 functional has the best overall performance on the description of vibrational properties. Next, we use this functional to study the influence of the vdW interactions on the structural and vibrational properties of multilayer graphene. Specifically, we evaluate binding energies, interlayer distances and phonon frequencies of layer breathing and shear modes. We observe excellent agreement between our calculated results and available experimental data, which suggests that this functional has truly predictive power for layer-breathing and shear frequencies that have not been measured yet. This indicates that careful selected vdW functionals can describe interlayer bonding in graphene-related systems with good accuracy.

cond-mat.mes-hall↗

arXiv2019

Energy barriers for collapsing large-diameter carbon nanotubes

Single-wall carbon nanotubes (SWNTs) are best known in their hollow cylindrical shapes, but the ground state of large-diameter tubes actually corresponds to a collapsed dumbbell-like structure, where the opposite sides of the nanotube wall are brought in contact and stabilized by van der Waals attraction. For those tubes, the cylindrical shape is metastable and it is interesting to investigate the energy barrier for jumping from one configuration to another. We calculate the energy barrier for SWNT collapse by considering a transition pathway that consists of an initial local deformation that subsequently propagates itself along the SWNT axis. This leads to finite and physically meaningful energy barriers in the limit of infinite nanotubes. Yet, such barriers are surprisingly large (tens of eV) and therefore virtually unsurmountable, which essentially prevents the thermal collapse of a metastable cylindrical at any reasonable temperatures. Moreover, we show that collapse barriers increase counterintuitively with SWNT diameter. Finally, we demonstrate that, despite such huge barriers, SWNTs may collapse relatively easily under external radial forces and we shed light on recent experimental observations of collapsed and cylindrical SWNTs of various diameters.

cond-mat.mes-hall↗

arXiv2018

Photonic spin Hall effect in bilayer graphene Moiré superlattices

The formation of a superstructure - with a related Moiré pattern - plays a crucial role in the extraordinary optical and electronic properties of twisted bilayer graphene, including the recently observed unconventional superconductivity. Here we put forward a novel, interdisciplinary approach to determine the Moiré angle in twisted bilayer graphene based on the photonic spin Hall effect. We show that the photonic spin Hall effect exhibits clear fingerprints of the underlying Moiré pattern, and the associated light beam shifts are well beyond current experimental sensitivities in the near-infrared and visible ranges. By discovering the dependence of the frequency position of the maximal photonic spin Hall effect shift on the Moiré angle, we argue that the latter could be unequivocally accessed via all-optical far-field measurements. We also disclose that, when combined with the Goos-Hänchen effect, the spin Hall effect of light enables the complete determination of the electronic conductivity of the bilayer. Altogether our findings demonstrate that sub-wavelength spin-orbit interactions of light provide a unprecedented toolset for investigating optoelectronic properties of multilayer two-dimensional van der Waals materials.

physics.optics↗