Grafeno em substratos transparentes dielétricos para aplicações fotônicas

• Autor(a) principal: Romagnoli, Priscila
• Data de Publicação: 2017
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações do Mackenzie
• Texto Completo: http://dspace.mackenzie.br/handle/10899/24279
• topic: grafeno; substrato dielétrico transparente; contraste óptico; ângulo de brewster; dopagem elétrica; poling térmico; plásmons de superfície; plásmons em grafeno; CNPQ::ENGENHARIAS::ENGENHARIA ELETRICA

Graphene is a two-dimensional material composed of carbon atoms arranged in a hexagonal lattice with excellent electronic properties, such as high electron mobility, and optical properties, such as wavelength-independent absorption due to the zero electronic bandgap [1]. Novoselov and Geim [2] isolated graphene for the rst time in 2004 and, since then, it has been proving applicable to several areas. In photonics, to date, application as saturable absorbers, polarizers and modulators, among others, have been demonstrated using the nanomaterial on transparent dielectric substrates. This type of substrate becomes necessary for photonic applications, since generally absorption is undesirable. With this requirement in mind, this thesis studies graphene on transparent dielectric substrates, with two main objectives. The rst objective is to increase the optical contrast of graphene on transparent substrates, improving its visualization. The re ectance of silicate glasses covered with graphene, for example, is 4.3% (normal incidence), while the re ectance of glass without graphene is 4.0%, which generates low values of contrast, of the order of 8%. This problem is studied and two solutions to increase the optical contrast are presented. The rst one uses re ected light at the Brewster angle of the substrate, which allows the cancellation of the substrate re ection and, thus, for the observation of re ection only in regions covered with graphene. The technique showed an increase in optical contrast of up to 380 over conventional optical microscopy. The second technique to enhance the contrast explores the unusual fact that re ection on a glass with graphene depends on from which medium light incides (from air or glass). With normal incidence, it was observed that glass substrate incidence generates an increase in optical contrast of 1.3 in relation to air incidence. The technique can be easily implemented in a conventional optical microscope. The second objective of this thesis is to investigate a new method for electrically doping graphene on glass. Doping allows adjustment of the optical and electronic characteristics of graphene. The method consists of exploring the process of thermal poling of silicate glasses. Poling is a known method for recording an electric eld close to the surface in glasses. The doping by poling is investigated by the generated spectral shifts in the Raman G band of graphene and from surface plasmon excitations. It was noted, however, that poling generated a charge concentration in graphene that was lower than expected, 2,9 1012cm2, which indicates that the recorded eld is neutralized before it reaches graphene. Although the origin of such neutralization is not yet understood, one possibility is the migration of charges from graphene to the substrate, which would screen the electric eld in graphene. The e ects of doping due to interaction with the atmosphere and the substrate are also shown to be important factors that signi cantly in uence the nanomaterials charge concentration. A signi cant reduction in these e ects is demonstrated using a substrate of exfoliated hBN on silica and in an argon atmosphere.