Examinando por Autor "Quesada, Sergio J"

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Enhanced Nonlinear Optical Coefficients of MAPbI3 Thin Films by Bismuth Doping
(ACS, 2020-02-18) Redondo-Obispo, Carlos; Suárez, Isaac; Quesada, Sergio J; Ripolles, Teresa S; Martínez-Pastor, Juan P; Álvarez, Angel L; de Andrés, Alicia; Coya, Carmen
The poor photostability under ambient conditions of hybrid halide perovskites has hindered their recently explored promising nonlinear optical properties. Here, we show how Bi3+ can partially substitute Pb2+ homogeneously in the commonly studied MAPbI3, improving both environmental stability and photostability under high laser irradiation. Bi content around 2 atom % produces thin films where the nonlinear refractive (n2) and absorptive coefficients (β), which modify the refractive index (Δn) of the material with light fluence (I), increase up to factors of 4 and 3.5, respectively, compared to undoped MAPbI3. Higher doping inhibits the nonlinear parameters; however, the samples show higher fluence damage thresholds. Thus, these results provide a road map on how MAPbI3 can be engineered for practical cost-effective nonlinear applications by means of Bi doping, including optical limiting devices and multiple-harmonic generation into optoelectronics devices.
New Concepts for Production of Scalable Single Layer Oxidized Regions by Local Anodic Oxidation of Graphene
(Wiley, 2019-08-21) Quesada, Sergio J; Borrás, Fernando; García-Vélez, Miguel; Coya, Carmen; Climent, Esteban; Munuera, Carmen; Villar, Ignacio; de la Peña O'shea, Victor; de Andrés, Alicia; Álvarez, Angel L
A deep comprehension of the local anodic oxidation process in 2D materials is achieved thanks to an extensive experimental and theoretical study of this phenomenon in graphene. This requires to arrange a novel instrumental device capable to generate separated regions of monolayer graphene oxide (GO) over graphene, with any desired size, from micrometers to unprecedented mm2, in minutes, a milestone in GO monolayer production. GO regions are manufactured by overlapping lots of individual oxide spots of thousands μm2 area. The high reproducibility and circular size of the spots allows not only an exhaustive experimental characterization inside, but also establishing an original model for oxide expansion which, from classical first principles, overcomes the traditional paradigm of the water bridge, and is applicable to any 2D-material. This tool predicts the oxidation behavior with voltage and exposure time, as well as the expected electrical current along the process. The hitherto unreported transient current is measured during oxidation, gaining insight on its components, electrochemical and transport. Just combining electrical measurements and optical imaging estimating carrier mobility and degree of oxidation is possible. X-ray photoelectron spectroscopy reveals a graphene oxidation about 30%, somewhat lower to that obtained by Hummers’ method.