Browsing by Author "Kong, Jing"

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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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    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.