Estudo das alterações das propriedades elétricas e morfológicas do grafeno na presença de gases, nanopartículas e líquidos

Exportado OPUSMade available in DSpace on 2019-08-13T05:11:13Z (GMT). No. of bitstreams: 1 phdive_posdefesa_final.pdf: 22034591 bytes, checksum: 88847b4e55a4696fd78e36f66daa4457 (MD5) Previous issue date: 4Nesta tese foi investigada a capacidade que os materiais grafeno e bicamadas de grafeno têm de modicarem suas propriedades elétricas e morfológicas quando colocados em diferentes ambientes e sob diferentes condições. Os estudos podem ser agrupados em três direções distintas. A primeira delas foca a exposição de bicamadas de grafeno a gases e são apresentados os resultados obtidos para um transistor de efeito de campo composto de uma bicamada. Os estudos envolvem a exposição dispositivo a atmosferas de hidrogênio e oxigênio, sob diferentes condições de temperatura. Sao tambem investigados os mecanismos de espalhamento de cargas presentes na amostra e para isso foram analisadas as alteracoes na posicao do ponto de neutralidade de cargas (CNP), no valor do mínimo de condutividade e nas mobilidades de eletrons e buracos. As analises levam em consideracao os conceitos da teoria de transporte difusivo de Boltzmann. A segunda delas foca a modicacao de monocamadas de grafeno (esfoliado e CVD), visando a producao de nanocanais do material. Sao apresentados dois diferentes metodos de producao de nanocanais de grafeno. O primeiro foi desenvolvido no Brasil e consiste do uso de nanopartículas termicamente ativadas para a producao de cortes em grafenos esfoliados mecanicamente. O segundo foi desenvolvido durante o período de Doutorado Sanduíche, na Universidade de Cornell (Ithaca,NY-EUA) e consiste do esculpimento do grafeno CVD utilizando-se uma fonte de luz branca acoplada a um sistema optico. Finalmente serao apresentados resultados do desenvolvimento de uma plataforma de grafeno CVD suspenso que permite a passagem de líquidos sob as janelas de grafeno. O conceito do projeto é baseado em uma estrutura de microfluídica, tambem desenvolvida na Universidade de Cornell.This thesis involves the changes on the electrical and morphological properties of graphene and bilayer graphene when in contact with diferent environments and subject to diferent conditions. The studies can be grouped in three distinct directions. The frst one focuses on the exposure of a bilayer graphene to gases in a feld efect transistor conguration. The studies consist of a graphene device under the exposure of hydrogen and oxygen atmospheres as a function of temperature. The charge scattering mechanisms presented in the system are also investigated by analysing the changes in the charge neutrality point (CNP), in the minimum of conductivity and in the electron and hole mobilities. The analyses are based on the Boltzmann theory of difusive electronic transport where the short-range (ressonant) and long-range (Coulomb) scattering mechanisms are identified. The second one is related to the modification of monolayer graphene (exfoliated and CVD graphene) with the purpose of producing graphene nanochannels by two diferent methods. One of them was developed in Brazil and consists of a catalytic process that etches exfoliated graphene by thermally activated nanoparticles. The other one was developed at Cornell University (USA) and consists of sculpting CVD graphene by using a pulsed laser ablation technique.Finally, the process and the design to build a platform for achieving suspended CVD graphene windows in contact with diferent liquids are presented. The platform was designed based on a buried channel microuidic structure. This project was also developed at Cornell University (USA)

Apparent softening of wet graphene membranes on a microfluidic platfor.

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes

Asymmetric Effect of Oxygen Adsorption on Electron and Hole Mobilities in Bilayer Graphene: Long- and Short-Range Scattering Mechanisms

We probe electron and hole mobilities in bilayer graphene under exposure to molecular oxygen. We find that the adsorbed oxygen reduces electron mobilities and increases hole mobilities in a reversible and activated process. Our experimental results indicate that hole mobilities increase due to the screening of long-range scatterers by oxygen molecules trapped between the graphene and the substrate. First principle calculations show that oxygen molecules induce resonant states close to the charge neutrality point. Electron coupling with such resonant states reduces the electron mobilities, causing a strong asymmetry between electron and hole transport. Our work demonstrates the importance of short-range scattering due to adsorbed species in the electronic transport in bilayer graphene on SiO₂ substrates

Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes