Propiedades electrónicas, mecánicas y ópticas de cristales bidimensionales de espesor atómico

Resumen

The experimental realization of graphene, just a single atomic layer of graphite obtained by mechanical exfoliation on SiO2 surfaces [Novoselov'04, Novoselov'05], has triggered a revolution in the design of electronic devices [Lin'08, Xia'09] and chemical sensors [Chen'09a, Chen'09b, Zhang'10] because of the unique electronic properties of graphene [Castro Neto'09] and its high sensitivity to the electrochemical environment. It also worth mentioning that these pioneering works have paved the way to study a very interesting family of two-dimensional crystals almost unexplored so far. These crystalline atomically thin sheets are very attractive from the fundamental point of view because the properties of materials just one layer thick may differ from their bulk counterparts. For instance, while graphite is a semimetal, graphene is an exotic zero gap semiconductor with a linear energy-momentum dispersion relation and the charge carriers behave as massless Dirac fermions. This fact made it possible to experimentally observe very unusual effects such as the anomalous quantum Hall effect [Du'09], the Klein tunnelling [Young'09], ballistic electron transport [Du'08], and bipolar supercurrent [Heersche'07]. Another remarkable example is the semiconducting transition metal chalcogenide MoS2 whose band structure drastically changes with the number of layers [Mak'10, Ramakrishna Matte'10, Splendiani'10]. Moreover, these two-dimensional materials are promising candidates for future microelectronic and sensing applications such as: flexible microelectronic components, transparent electrodes or ultrasensitive chemical sensors. Indeed these materials have already demonstrated their potential in some experimental devices like flexible field effect transistors [Ayari'07], ultrafast electronic devices [Lin'08, Xia'09] or mercury sensors [Zhang'10]. Despite the interest aroused by these atomically thin crystals, just few groups have been able to study their local electronic properties by scanning probe microscopies. The reason is that these experiments require the development of nonstandard procedures and protocols for producing, identifying and contacting electrically these crystals and specific scanning probe microscopy instrumentation. The work carried out during this thesis has initiated a research line in the Low Temperature Laboratory of the Universidad Autónoma de Madrid, devoted to the study of electronic and mechanical properties of crystalline atomically thin two-dimensional sheets, such as graphene, MoS2, NbSe2 and mica by scanning probe microscopy. We present the procedures developed to produce and identify twodimensional crystals of very diverse materials such as graphene, MoS2, NbSe2 and mica (Chapter 4). Further, we have studied the mechanical and optical properties of these novel materials. For instance, we have measured the elastic properties of suspended nanomembranes of MoS2 (Chapter 6) and the refractive index of graphene, MoS2, NbSe2 and mica crystals as thin as just one layer thick (Chapter 5). In order to explore the local electronic properties of atomically thin crystals, we have implemented a combined scanning tunnelling microscope / atomic force microscope (Chapters 1 ¿ 3), optimized to study small conductive nanopatches deposited on insulating surfaces. We have employed this new tool to probe the spatial variations of the electronic properties of graphene on SiO2 (Chapter 7). We have found that the presence of charged impurities in the substrate induces localized inhomogeneities in the electronic properties of graphene.