Navegando por Autor "Carvalho, Bruno Ricardo de"
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Item Electronic band tuning and multivalley raman scattering in monolayer transition metal dichalcogenides at high pressures.(2022) Martins, Luiz Gustavo Pimenta; Carvalho, Bruno Ricardo de; Occhialini, Connor A.; Neme, Natália Paz; Park, Ji-Hoon; Song, Qian; Venezuela, Pedro Paulo de Mello; Mazzoni, Mário Sérgio de Carvalho; Kong, Jing; Comin, RiccardoTransition metal dichalcogenides (TMDs) possess spin-valley locking and spin-split K/K′ valleys, which have led to many fascinating physical phenomena. However, the electronic structure of TMDs also exhibits other conduction band minima with similar properties, the Q/Q′ valleys. The intervalley K−Q scattering enables interesting physical phenomena, including multivalley superconductivity, but those effects are typically hindered in monolayer TMDs due to the large K−Q energy difference (ΔEKQ). To unlock elusive multivalley phenomena in monolayer TMDs, it is desirable to reduce ΔEKQ, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we use high pressure to tune the electronic properties of monolayer MoS2 and WSe2 and probe K−Q crossing and multivalley scattering via double-resonance Raman (DRR) scattering. In both systems, we observed a pressure-induced enhancement of the double-resonance LA and 2LA Raman bands, which can be attributed to a band gap opening and ΔEKQ decrease. First-principles calculations and photoluminescence measurements corroborate this scenario. In our analysis, we also addressed the multivalley nature of the DRR bands for WSe2. Our work establishes the DRR 2LA and LA bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. Conversely, their intensity could potentially be used to monitor the presence of compressive or tensile strain in TMDs. Furthermore, the ability to probe K−K′ and K−Q scattering as a function of strain shall advance our understanding of different multivalley phenomena in TMDs such as superconductivity, valley coherence, and valley transport.Item Investigation of spatially localized defects in synthetic WS2 monolayers.(2022) Rosa, Bárbara Luiza Teixeira; Fujisawa, Kazunori; Cruz, Joyce Cristina da; Zhang, Tianyi; Matos, Matheus Josué de Souza; Sousa, Frederico Barros de; Barbosa, Tiago Campolina; Fonseca, Lucas Lafetá Prates da; Ramos, Sérgio L. L. M.; Carvalho, Bruno Ricardo de; Chacham, Helio; Neves, Bernardo Ruegger Almeida; Terrones, Mauricio; Moreira, Leandro MalardWhile the spatially nonhomogeneous light emission from synthetic WS2 monolayers is frequently reported in the literature, the nature of this phenomenon still requires thoughtful investigation. Here, we combine several characterization techniques (optical imaging, scanning probe and electron microscopy) along with density func- tional theory to investigate the presence of substitutional doping localized at narrow regions along the S zigzag edge of WS2 monolayers. We verified that photoluminescence quenching along narrow regions is not related to grain boundaries but to substitutional impurities of lighter metals at the W sites, which modify the radiative and nonradiative decay channels. We also found potential candidates for occupying the W site through ADF-STEM analysis and discussed their impact on photoluminescence quenching by performing density functional theory calculations. Our findings shed light on how atomic defects introduced during WS2 monolayer’s synthesis impact the crystalline quality and, therefore, the development of high-performance optoelectronic devices based on semiconducting 2D materials.