Fabrication and Characterization of Macroscopic Graphene Layers on Metallic Substrates

Abstract

In 2004 there were newly conditions for a scientific revolution, with very important technology implications and yet to be completely developed. It is the isolation of atomic carbon layers, better known as graphene, whose extreme mechanical and electronic properties stand out above all known materials: it presents the highest electron mobility, ambipolarity, it supports large current densities, the highest elastic modulus, increased thermal conductivity, it shows high impermeability, and reconciles fragility and ductility. The study of graphene was about to be the next step to the boom in nanotechnology. In this case, the system to consider is purely two-dimensional, being the thinnest structure known to date. In 2010, this discovery was acknowledged with the Nobel Prize to the scientists A. Geim and K. Novoselov from the Manchester University (UK).

The goal of the project, in which this thesis was framed, is to develop new materials in ultrathin structures of few monoatomic layers, based on graphene, with extreme surface properties (very high wear resistance, ultra low friction and surface energy, extreme chemical resistance, biocompatibility) for biomedical applications. The scope of this objective is to overcome the limitations of current techniques in terms of the growth surface of graphene (of some .m2) and to extend the possible applications of graphene to systems and devices requiring macroscopic size surfaces. For this purpose, the thesis had different tasks consisting of:

a) The design and construction of a new high vacuum reactor in the Clean Room of the Universitat de Barcelona that works with high-temperature chemical vapor deposition (CVD) and magnetron sputtering.

b) Study of the principles of a CVD process of graphene on metals, and development of a modified CVD method based on pulses of gas (methane), which will reduce the current deposition times to 1-10 s, pressure of 10-4 Pa, quantity of precursor gas needed, and temperature (900-1000 °C), to grow high quality graphene.

c) Fabrication of graphene layers on metal substrates, sputtered copper/nickel and copper foils, of large area and high quality by this modified CVD technique, focusing on obtaining the material as effectively as possible towards an implementation on applications.

d) Characterization of the graphene obtained through different techniques in order to optimize its physical and surface properties, as well as the CVD method; such as structural and morphological studies by Raman spectroscopy, SEM and Optical Microscopy. In addition to a transfer process from copper to silicon.

e) Fabrication and characterization of graphene layers by means of mechanical exfoliation of graphite on silicon in order to characterize the electrical properties to fabricate a graphene-based FET. Exfoliated samples were also used to study the effect of the SHI irradiation with glancing angles on the surface.

The thesis is then, divided into four main parts. The Part I will introduce the state of the art of graphene as novel material and its outstanding properties, its discovery, and all the technologies that triggered its development during these years until the first applications.

The Part II will describe the experimental setups used throughout this work, regarding the fabrication and characterization of substrates by magnetron sputtering, the growth mechanisms of the new developed CVD system, and a brief explanation of the fundamentals of each technique.

In the part III, a complete review of all the results of the samples obtained by mechanical exfoliation of graphite and CVD on copper will be exposed; together with their characterization.

Part IV includes the main conclusions of the work, which are summarized. Afterwards, a list of the scientific results published is shown, as well as the contributions in conferences and meetings.

In the end of the manuscript, an Appendix with three sections is shown: a complete CVD review, the abstract of the patent developed during this thesis, and a complete list of all the samples produced.

En 2004, A. Geim y K. Novoselov (Universidad de Manchester, UK), aislaron por primera vez una capa atómica de carbono (la estructura más fina hasta la fecha), más conocido como grafeno, cuyas propiedades mecánicas y electrónicas extremas permanecen por encima de todos los materiales conocidos: la más alta movilidad electrónica, ambipolaridad, soporta grandes densidades de corriente, el más alto módulo elástico, más conductividad térmica, alta impermeabilidad, y reconcilia fragilidad y ductilidad. El estudio del grafeno estaba a punto de ser el siguiente paso en el boom de la Nanotecnología cuando en 2010, este descubrimiento recibió el Premio Nobel.

El objetivo de la tesis es superar las limitaciones de las técnicas actuales en cuanto a la superficie de crecimiento del grafeno (de algunas micras) y extender las posibles aplicaciones a sistemas y dispositivos que requieran de superficies macroscópicas. Para este propósito, esta tesis tuvo diferentes fases que consistieron en:

a) El diseño y construcción de un nuevo reactor de alto vacío, que lleva a cabo procesos de Depósito Químico de Vapor (CVD) a altas temperaturas y pulverización catódica.

b) Estudio de los principios del crecimiento del grafeno en metales, y desarrollo de un método CVD modificado basado en pulsos de gas (metano), que reduce el tiempo de depósito a 1-10 s, presión de 10-4 Pa, cantidad de gas precursor necesario, y la temperatura (900-1000 °C).

c) Fabricación de grafeno de gran área y alta calidad en sustratos metálicos, cobre/níquel depositado y láminas de cobre, mediante este CVD modificado.

d) Caracterización del grafeno obtenido mediante diferentes técnicas con el fin de optimizar sus propiedades físicas y superficiales, así como el método CVD; estudios estructurales y morfológicos con espectroscopia Raman, microscopía electrónica y óptica. Además de un proceso de transferencia de cobre a silicio.

e) Fabricación y caracterización de grafeno obtenido mediante exfoliación mecánica de grafito sobre silicio para caracterizarlo eléctricamente y fabricar un transistor de Efecto Campo. Las muestras exfoliadas fueron usadas también para estudiar el efecto de la radiación de SHI rasante sobre la superficie.