Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin-orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7-3.1 and 1.9-3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M and X) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo < W) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S < Se). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression

Respuesta mecánica del grafito bajo condiciones extremas

Nuestro interés fundamental es conocer la respuesta del grafito bajo condiciones extremas de estrés; aunque queda claro que, para ello, necesitaremos desarrollar nuevas metodologías de análisis o mejorar las existentes. Por ello, nos planteamos los dos grandes objetivos genéricos. En primer lugar, estudiar el comportamiento elástico de los yunques utilizados para generar el estrés, zafiro y moissanita, tanto en la superficie del yunque, que estará en contacto con la muestra, como a lo largo de todo su volumen. Esto permitirá analizar las condiciones de estrés generadas en el interior de una celda de presión y conocer en qué regiones de la celda el estrés generado es máximo. Y en segundo lugar, estudiar los efectos del estrés en el grafito, señalando las diferencias existentes entre experimentos hidrostáticos o uniaxiales y los aquí realizados, analizando la reversibilidad de los fenómenos observados y diferenciando cuáles de estos fenómenos se deben a una disminución del volumen y cuáles a la aparición de diversos componentes de estrés. Contodo ello estaremos en disposición de conocer el comportamiento elásticodel grafito bajo estrés y analizar la respuesta mecánica del grafitorecuperado tras un ciclo de estrés

Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin-orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7-3.1 and 1.9-3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M and X) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo < W) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S < Se). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression

Raman modes and Grüneisen parameters of graphite under compressive biaxial stress

We study the behaviour of the most characteristic Raman contributions of highly oriented pyrolytic graphite at biaxial stresses up to 5 GPa. We use moissanite anvils to compress the samples, which allowed us to observe for the first time the evolution with stress of the disorder-induced D band along with all the overtones and combination bands in the Raman spectrum of graphite. Theoretical calculations at different pressures, performed within the density functional theory, are provided to confirm that the D band process involves iTO phonons along the K–C direction. A complete set of Grüneisen parameters is reported for all the features of the Raman spectrum of graphite; the analysis of the ratios of such parameters provides direct information on the intrinsic coupling and stress response of each phonon branch.</p

High-resolution mapping of infraslow cortical brain activity enabled by graphene microtransistors

Recording infraslow brain signals (<0.1 Hz) with microelectrodes is severely hampered by current microelectrode materials, primarily due to limitations resulting from voltage drift and high electrode impedance. Hence, most recording systems include high-pass filters that solve saturation issues but come hand in hand with loss of physiological and pathological information. In this work, we use flexible epicortical and intracortical arrays of graphene solution-gated field-effect transistors (gSGFETs) to map cortical spreading depression in rats and demonstrate that gSGFETs are able to record, with high fidelity, infraslow signals together with signals in the typical local field potential bandwidth. The wide recording bandwidth results from the direct field-effect coupling of the active transistor, in contrast to standard passive electrodes, as well as from the electrochemical inertness of graphene. Taking advantage of such functionality, we envision broad applications of gSGFET technology for monitoring infraslow brain activity both in research and in the clinic