arXiv subjects

Ramiro M. dos Santos

Publications and source records attributed to Ramiro M. dos Santos.

7 recordsLinked to original sources

arXiv2026

Symmetry Engineering Enables Deterministic Ensemble Polarization Control in Hexagonal Boron Nitride

Optical polarization is a key degree of freedom in quantum photonic and sensing technologies, enabling efficient light--matter coupling and directional emission. However, in defect ensemble-based quantum systems, orientational averaging across many defects suppresses optical anisotropy and eliminates a deterministic polarization axis. Here, symmetry engineering is introduced as a strategy to restore collective optical anisotropy in negatively charged boron vacancy (V_B^-) ensembles hosted in hexagonal boron nitride (h-BN). By imposing anisotropic in-plane tensile strain through lithographically defined nano-ridge arrays, in-plane symmetry is broken and the ensemble emission dipole is aligned with the ridge-defined strain symmetry axis. Polarization-resolved photoluminescence reveals that the polarization visibility scales with the magnitude of strain anisotropy, while the emission orientation rigidly follows the engineered symmetry axis across devices with varying strain direction. This deterministic alignment is independent of crystal orientation, establishing the optical foundation for polarization-defined ensemble spin readout and programmable photonic integration in two-dimensional quantum materials

cond-mat.mtrl-sci↗

arXiv2021

On the Adsorption Mechanism of Caffeine on MAPbI3 Perovskite Surfaces: A Combined UMC--DFT Study

Recently, it was experimentally shown that the performance and thermal stability of the perovskite MAPbI$_3$ were improved upon the adsorption of a molecular layer of caffeine. In this work, we used a hybrid methodology that combines Uncoupled Monte Carlo (UMC) and Density Functional Theory (DFT) simulations to carry out a detailed and comprehensive study of the adsorption mechanism of a caffeine molecule on the surface of MAPbI$_3$. Our results showed that the adsorption distance and energy of a caffeine molecule on the MAPbI$_3$ surface are 2.0 Å and -0.3 eV, respectively. The caffeine/MAPbI$_3$ complex presents a direct bandgap of 2.38 eV with two flat intragap bands distanced 1.15 and 2.18 eV from the top of valence bands. Although the energy band levels are not significantly shifted by the presence of caffeine, the interaction MAPbI$_3$/perovskite is enough to affect the bands' dispersion, particularly the conduction bands.

cond-mat.mtrl-sci↗

arXiv2020

Electronic and Structural Properties of Janus SMoSe/MoX$_2$ (X=S,Se) In-plane Heterojunctions: A DFT Study

The electronic and structural properties of Janus MoSSe/MoX$_2$ (X=S,Se) in-plane heterojunctions, endowed with single-atom vacancies, were studied using density functional theory calculations. The stability of these structures was verified from cohesion energy calculations. Results showed that single-atom vacancies induce the appearance of flat midgap states, and a substantial amount of charge is localized in the vicinity of these defects. As a consequence, these heterojunctions presented an intrinsic dipole moment. No bond reconstructions were noted by removing an atom from the lattice, regardless of its chemical species. Our calculations predicted indirect electronic bandgap values between 1.6-1.7 eV.

cond-mat.mtrl-sci↗

arXiv2020

A DFT Study on the Electronic Structure of In-Plane Heterojunctions of Graphene and Hexagonal Boron Nitride Nanoribbons

The structural similarity between hexagonal boron nitride (h-BN) and graphene nanoribbons allows for the formation of heterojunctions with small chain stress. The combination of the insulation nature of the former and the quasi-metallic property of the latter makes this kind of heterostructure particularly interesting for flat optoelectronics. Recently, it was experimentally demonstrated that the shapes of the graphene and h-BN domains can be controlled precisely, and sharp graphene/h-BN interfaces can be created. 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 zigzag termination were studied. Results showed that the charge density is localized in the edge of the heterojunctions, regardless of the domain size. These materials presented a metallic nature with a ferromagnetic behavior, which can be useful for magnetic applications at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2020

On the Electronic Structure of a Recently Synthesized Graphene-like BCN Monolayer from bis-BN Cyclohexane: 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 mu_B. Our calculations predicted a direct electronic bandgap value of about 1.14 eV, which is in good agreement with the experimental one. Importantly, this bandgap value is intermediate between gapless graphene and insulating h-BN.

cond-mat.mtrl-sci↗

arXiv2020

Tuning Magnetic Properties of Penta-Graphene Bilayers Through Doping with Boron 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$^2$ and sp$^3$ carbon planes, it consists of a flexible candidate for functionalization tuning of electromagnetic properties. In this work, we perform density functional theory simulations to investigate how the electronic and structural properties of PG bilayers can be tuned as a result of substitution doping. Specifically, we observed the emergence of different magnetic properties when boron was used as dopant species. On the other hand, in the case of doping with oxygen, the rupture of bonds in the sp$^2$ planes does not induce a magnetic moment in the material.

cond-mat.mtrl-sci↗

arXiv2020

Predicting the Energetic Stabilization of Janus-MoSSe/AlN Heterostructures: A DFT Study

The packing mechanisms between Janus-MoSSe and Aluminum-Nitride (AlN) sheets were systematically investigated by using Density Function Theory calculations. Results show that the stabilization (packing) energies vary from -35.5 up to -17.5 meV depending on the chemical species involved in the interface. The packing energies were obtained using the improved Lennard-Jones (ILJ) potential. The SeMoS/AlN heterostructures, when the MoS face is interacting with the AlN sheet, presented the lowest packing energies due to the sulfur's higher degree of reactivity. Importantly, the calculated bandgap values ranged within the interval 1.61-1.87 eV, which can be interesting for photovoltaic applications.

cond-mat.mtrl-sci↗