Browsing by Author "Martins, Luiz Gustavo Pimenta"

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    Electromechanical modulations in transition metal dichalcogenide nanosheets : implications for environmental sensors.
    (2021) Bessa, Maurício Vasconcelos; Freitas, Wellington Damaceno de; Neme, Natália Paz; Martins, Luiz Gustavo Pimenta; Barboza, Ana Paula Moreira; Matos, Matheus Josué de Souza; Mazzoni, Mário Sérgio de Carvalho; Neves, Bernardo Ruegger Almeida
    Transition metal dichalcogenides (TMDs) are key players in the two-dimensional materials nanoarena due to their exquisite optoelectronic properties under a standard environment (room temperature and atmospheric pressure). Nevertheless, as reported in the literature, they may also portray interesting physical properties under different environments. Here, we show two distinct and significant electromechanical modulations in TMD nanosheets which are tuned by the environmental conditions (applied pressure and adsorbents). Using scanning probe microscopy techniques, we modify the environmental conditions and observe steplike rises in the electrical response of all studied TMDs (MoS2, WS2, MoSe2, and WSe2monolayers and few layers). Ab initio calculations enable full understanding of specific mechanisms behind these electromechanical modulations, which may find important applications in the design of TMD-based environmental sensors.
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    Electronic band tuning and multivalley raman scattering in monolayer transition metal dichalcogenides at high pressures.
    (2022) Martins, Luiz Gustavo Pimenta; Carvalho, Bruno R.; Occhialini, Connor A.; Neme, Natália Paz; Park, Ji-Hoon; Song, Qian; Venezuela, Pedro Paulo de Mello; Mazzoni, Mário Sérgio de Carvalho; Kong, Jing; Comin, Riccardo
    Transition 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.
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    Hard, transparent, sp3 -containing 2D phase formed from few-layer graphene under compression.
    (2021) Martins, Luiz Gustavo Pimenta; Silva, Diego L.; Smith, Jesse S.; Lu, Ang-Yu; Su, Cong; Hempel, Marek; Occhialini, Connor; Ji, Xiang; Pablo, Ricardo; Alencar, Rafael Silva; Souza, Alan Custodio dos Reis; Pinto, Alysson Alves; Oliveira, Alan Barros de; Batista, Ronaldo Junio Campos; Palacios, Tomas; Mazzoni, Mário Sérgio de Carvalho; Matos, Matheus Josué de Souza; Comin, Riccardo; Kong, Jing; Cançado, Luiz Gustavo de Oliveira Lopes
    Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO2/Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp3 -containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4e6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to z 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure (z 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Theoretical calculations and experimental data indicate a novel, surfaceto-bulk phase transition mechanism that gives hint of diamondene formation.
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    High-pressure studies of atomically thin van der Waals materials.
    (2023) Martins, Luiz Gustavo Pimenta; Comin, Riccardo; Matos, Matheus Josué de Souza; Mazzoni, Mário Sérgio de Carvalho; Neves, Bernardo Ruegger Almeida; Yankowitz, Matthew
    Two-dimensional (2D) materials and their moire superlattices represent a new frontier for quantum matter research due to the emergent properties associated with their reduced dimensionality and extreme tunability. The properties of these atomically thin van der Waals (vdW) materials have been extensively studied by tuning a number of external parameters such as temperature, electrostatic doping, magnetic field, and strain. However, so far pressure has been an under-explored tuning parameter in studies of these systems. The relative scarcity of high- pressure studies of atomically thin materials reflects the challenging nature of these experiments, but, concurrently, presents exciting oppor- tunities for discovering a plethora of unexplored new phenomena. Here, we review ongoing efforts to study atomically thin vdW materials and heterostructures using a variety of high-pressure techniques, including diamond anvil cells, piston cylinder cells, and local scanning probes. We further address issues unique to 2D materials such as the influence of the substrate and the pressure medium and overview efforts to theoretically model the application of pressure in atomically thin materials.
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    Raman evidence for pressure-induced formation of diamondene.
    (2017) Martins, Luiz Gustavo Pimenta; Matos, Matheus Josué de Souza; Paschoal, Alexandre R.; Freire, Paulo T. C.; Andrade, Nádia Ferreira de; Aguiar, Acrisio Lins de; Kong, Jing; Neves, Bernardo Ruegger Almeida; Oliveira, Alan Barros de; Mazzoni, Mario Sergio de Carvalho; Souza Filho, Antonio Gomes; Cançado, Luiz Gustavo de Oliveira Lopes
    Despite the advanced stage of diamond thin-film technology, with applications ranging from superconductivity to biosensing, the realization of a stable and atomically thick two-dimensional diamond material, named here as diamondene, is still forthcoming. Adding to the outstanding properties of its bulk and thin-film counterparts, diamondene is predicted to be a ferromagnetic semiconductor with spin polarized bands. Here, we provide spectroscopic evidence for the formation of diamondene by performing Raman spectroscopy of double-layer graphene under high pressure. The results are explained in terms of a breakdown in the Kohn anomaly associated with the finite size of the remaining graphene sites surrounded by the diamondene matrix. Ab initio calculations and molecular dynamics simulations are employed to clarify the mechanism of diamondene formation, which requires two or more layers of graphene subjected to high pressures in the presence of specific chemical groups such as hydroxyl groups or hydrogens.