Electronic Properties of Curved Few-Layers Graphene: A Geometrical Approach

Rafael N. Gontijo, Marcus V. O. Moutinho, Ariete Righi, Po-Wen Chiu, Pedro Venezuela, and Marcos A. Pimenta. 2024. “Resonant enhancement of the 2G Raman band in twisted bilayer graphene.” Materials Chemistry and Physics, Pp. 129279. Publisher's Version Abstract

Raman spectroscopy is an extremely useful tool to characterize graphene systems. The strongest Raman features are the first-order G band and the second-order 2D and 2D′ bands, which are the overtones of the double resonance D and D’ bands. However, the 2G band, which is the overtone of the G band, is not usually observed in the spectra of monolayer graphene and of crystalline graphite. In this work, we present an experimental and theoretical investigation of the resonance Raman spectra in twisted bilayer graphene (TBG) with different twisting angles and using several laser excitation energies in the NIR and visible ranges. We observed that the 2G band is enhanced when the incident photons are in resonance with the transition between the van Hove singularities in the density of states of the TBG. We show that the 2G band has three contributions (2G1, 2G2 and 2G3), that are not dispersive by changing the laser excitation energy. We also present theoretical calculations showing that the 2G1 and 2G2 bands are related to combinations of the in-phase (IP) and out-of-phase (OP) vibrations of the atoms in the different layers. The Raman excitation profiles (REPs) of the 2G peaks are upshifted in comparison with the REP of the G band. This behavior was confirmed theoretically using a graphene tight binding model. We conclude that the different resonance behavior comes from the fact that the G band is a first-order process whereas the 2G band is second-order processes giving rise to overall different resonance conditions.

Camila Cavalini, Cesar Rabahi, Caique S. de Brito, Eunji Lee, José R. Toledo, Felipe F. Cazetta, Raphael B. Fernandes de Oliveira, Marcelo B. Andrade, Mohamed Henini, Yuhao Zhang, Jeongyong Kim, Ingrid D. Barcelos, and Yara Galvão Gobato. 2024. “Revealing localized excitons in WSe2/β-Ga2O3.” Applied Physics Letters, 124, 14, Pp. 142104. Publisher's Version Abstract

We have investigated the optical and magneto-optical properties of monolayer (ML) WSe2 on flakes of β-Ga2O3 under high magnetic fields. Remarkably, sharp emission peaks were observed and associated with localized excitons related to point defects. A detailed study of low-temperature photoluminescence (PL) and magneto-PL under high perpendicular magnetic field up to 9 T was carried out. Several sharp emission peaks have shown valley g-factors values close to −4, which is an unusual result for localized excitons in WSe2. Furthermore, some PL peaks have shown higher g-factor values of ≈−7 and ≈−12, which were associated with the hybridization of strain localized dark excitons and defects. The reported results suggest that β-Ga2O3 is, indeed, a promising dielectric substrate for ML WSe2 and also to explore fundamental physics in view of possible applications in quantum information technology.

Marília I. F. Barbosa Javier Ellena Alzir A. Batista André V. Alves, Rafael G. Silveira and Rodrigo S. Corrêa. 2024. “cis and trans isomers of Ru(III)-based complexes with lapachol: X-ray diffractions and DFT theoretical insights.” Molecular Crystals and Liquid Crystals, 768, 5, Pp. 168-179. Publisher's Version

Gabriel H. A. Jorge, Filipe A. Couto, Juliana M. P. Almeida, Victor A. S. Marques, Marcelo B. Andrade, and Cleber R. Mendonça. 2024. “Active Optical Tuning of Azopolymeric Whispering Gallery Mode Microresonators for Filter Applications.” Photonics, 11, 2. Publisher's Version Abstract

Light confinement provided by whispering gallery mode (WGM) microresonators is especially useful for integrated photonic circuits. In particular, the tunability of such devices has gained increased attention for active filtering and lasering applications. Traditional lithographic approaches for fabricating such devices, especially Si-based ones, often restrict the device’s tuning due to the material’s inherent properties. Two-photon polymerization (2PP) has emerged as an alternative fabrication technique of sub-diffraction resolution 3D structures, in which compounds can be incorporated to further expand their applications, such as enabling active devices. Here, we exploited the advantageous characteristics of polymer-based devices and produced, via 2PP, acrylic-based WGM hollow microcylinders incorporated with the azoaromatic chromophore Disperse Red 13 (DR13). Within telecommunication range, we demonstrated the tuning of the microresonator’s modes by external irradiation within the dye’s absorption peak (at 514 nm), actively inducing a blueshift at a rate of 1.2 nm/(Wcm−2). Its thermo-optical properties were also investigated through direct heating, and the compatibility of both natural phenomena was also confirmed by finite element simulations. Such results further expand the applicability of polymeric microresonators in optical and photonic devices since optically active filtering was exhibited.

Ana L.S Moura, Pedro H. Machado, and R.S. Corrêa. 2024. “2,5-diphenyloxazole and 4-pyrrolidinopyridine as conformers to picric acid: Structural characterization coupled with Multivariate Statistical Analysis.” Journal of Molecular Structure, Pp. 137670. Publisher's Version Abstract

In this study, 4-pyrrolidinopyridine (PPY) and 2,5-diphenyloxazole (DPO) were employed as N-heterocyclic derivatives to synthesize novel picrate salts. These salts were subsequently subjected to characterization using infrared spectroscopy, ultraviolet spectroscopy, and single-crystal X-ray diffraction. Comparison of these structures with entries in the Cambridge Structural Database (CSD) and the application of two multivariate statistical analyses, namely Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA), facilitated the differentiation of salts and co-crystals based on the structural behaviors of the picrate molecule. Additionally, Hirshfield Surface Analysis and 2D fingerprint plots were employed to investigate intermolecular contacts. The fluorescence emission spectra revealed that compound 2 has the potential to serve as a probe for turn-off detection of picric acid (PA).

A Gómez, E Chavarriaga, D Coral, VDN Bezzon, Y Pham, SC Yu, and O Morán. 2024. “Assessment of electronic states and local structure of Mn-atoms in nanometric-sized La0. 7Ca0. 3Mn1- x Ni x O3 manganites by means of X-ray-absorption fine structure measurements.” Applied Physics A, 130, 1, Pp. 54.

Mirela R. Valentim, Matheus J. S. Matos, and Mario S. C. Mazzoni. 2023. “B2N2O2: A wide-bandgap two-dimensional semiconductor featuring a zigzag nitrogen array of bonds.” Applied Physics Letters, 123, 9, Pp. 093103. Publisher's Version Abstract

In this work, we apply a combination of theoretical techniques to characterize a two-dimensional material with formula B2N2O2, featuring a zigzag array of nitrogen atoms. We predict its energetic, thermal, and dynamic stability and determine its electronic properties, including band structure and mobility evaluation for a phonon-mediated mechanism. We show that the compound is a wideband-gap semiconductor, with parabolic band edges and with large electron and hole mobilities within the deformation potential approach. We ascribe this result to the existence of electronic channels defined by the zigzag array of nitrogen bonds, which define the edges of both conduction and valence bands. We also propose a mechanism to synthesize the compound based on oxygen functionalization and application of pressure. Finally, we show that the results can be generalized to represent a family of 2D compounds.

Joelma R.C. Souza, Juliana A. Torres, Lucas S. Ribeiro, Jose B. G. Filho, Fabiana L. Santos, Nicholas Malgioglio, Luiz Fernando Gorup, Alexandre H. Pinto, and André E. Nogueira. 2023. “The Facile Microwave-Assisted Coprecipitation Route to Obtain Polyoxoniobate (Na7(H3O)Nb6O19·14H2O) Nanorods Modified with Copper for CO2 Photoreduction.” AppliedChem, 3, 2, Pp. 320–333. Publisher's Version Abstract

The CO2 reduction by solar means has been discussed as an alternative to emission abatement, a fundamental topic for sustainable, carbon-free production in the future. However, the choice of efficient systems, starting with the catalysts, is still a critical issue, especially due to the poor activity of available options. Polyoxometalates have been extensively studied as promising photocatalysts due to their semiconducting properties. Nevertheless, the synthetic conditions of polyoxoniobate are stringent due to the low reaction activity of Nb species, the lack of soluble precursors, and the narrow pH range. Unlike the literature, in the present study, we report a simple polyoxoniobate synthesis method. This synthesis method has some remarkable features, such as low processing time and temperature and good activity and selectivity in the CO2 photoreduction process. The results revealed an outstanding efficiency for the CO2 reduction reaction with a high selectivity of CO2 to CO conversion (92.5%). Furthermore, C2 compounds (e.g., acetate) were produced in the liquid phase of the reaction system. Our findings are significant for indicating the potential of polyoxoniobate for CO2 photoreduction, which opens a way to control competitive reactions with synthesis, leading to higher selectivity.

Luiz G. Pimenta Martins, David A. Ruiz-Tijerina, Connor A. Occhialini, Ji-Hoon Park, Qian Song, Ang-Yu Lu, Pedro Venezuela, Luiz G. Cançado, Mário S. C. Mazzoni, Matheus J. S. Matos, Jing Kong, and Riccardo Comin. 2023. “Pressure tuning of minibands in MoS2/WSe2 heterostructures revealed by moiré phonons.” Nature Nanotechnology. Publisher's Version Abstract

Moiré superlattices of two-dimensional heterostructures arose as a new platform to investigate emergent behaviour in quantum solids with unprecedented tunability. To glean insights into the physics of these systems, it is paramount to discover new probes of the moiré potential and moiré minibands, as well as their dependence on external tuning parameters. Hydrostatic pressure is a powerful control parameter, since it allows to continuously and reversibly enhance the moiré potential. Here we use high pressure to tune the minibands in a rotationally aligned MoS2/WSe2 moiré heterostructure, and show that their evolution can be probed via moiré phonons. The latter are Raman-inactive phonons from the individual layers that are activated by the moiré potential. Moiré phonons manifest themselves as satellite Raman peaks arising exclusively from the heterostructure region, increasing in intensity and frequency under applied pressure. Further theoretical analysis reveals that their scattering rate is directly connected to the moiré potential strength. By comparing the experimental and calculated pressure-induced enhancement, we obtain numerical estimates for the moiré potential amplitude and its pressure dependence. The present work establishes moiré phonons as a sensitive probe of the moiré potential as well as the electronic structures of moiré systems.

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Electronic Properties of Curved Few-Layers Graphene: A Geometrical Approach

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