Propiedades mecánicas de membranas de grafeno: consecuencias de la inducción controlada de defectos

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 22-09-201

Exfoliated graphite flakes as soft-electrodes for precisely contacting nanoobjects

This is the post-peer reviewed version of the following article: P. ares et al. “Exfoliated graphite flakes as soft-electrodes for precisely contacting nanoobjects”. 2D Matererials, 2015, 2(3): 035008. Which has been published in final form at:http://dx.doi.org/10.1088/2053-1583/2/3/035008We introduce a simple, clean and reliable method to transfer exfoliated graphite flakes as soft-electrodes for the electrical contacts of nano-objects. The microelectrodes thus produced exhibit extremely well-defined and thin edges and can be placed at any sample location with sub-micrometer precision. The procedure is carried out under ambient conditions and does not require chemical agents. We present electrical characterization of relevant examples including carbon nanotubes, metal-organic MMX nanoribbons, reduced graphene sheets and damaged circuit repair. The quality of the electrical contacts thus obtained is as good as those fabricated with conventional techniques. This technique is particularly relevant for conductive atomic force microscopy (AFM) studiesThis work was supported by Consolider CSD2010-0024, MAD2D-CM, S2013/MIT-3007 and MAT2013-46753-C2-1 and

Neutral and charged excitons interplay in non-uniformly strain-engineered WS2

We investigate the response of excitons in two-dimensional semiconductors to nonuniformity of mechanical strain. In our approach to non-uniform strain-engineering, a WS2 monolayer is suspended over a triangular hole. Large (>2%), strongly non-uniform (>0.28% µm–1), and in-situ tunable strain is induced in WS2 by pressurizing it with inert gas. We observe a pronounced shift of the spectral weight from neutral to charged excitons at the center of the membrane, in addition to well-known strain-dependent bandgap modification. We show that the former phenomenon is a signature of a new effect unique for non-uniform strain: funneling of free carriers towards the region of high strain followed by neutral to charged exciton conversion. Our result establishes non-uniform strain engineering as a novel and useful experimental 'knob' for tuning optoelectronic properties of 2D semiconductors

Age and date for early arrival of the Acheulian in Europe (Barranc de la Boella, la Canonja, Spain)

The first arrivals of hominin populations into Eurasia during the Early Pleistocene are currently considered to have occurred as short and poorly dated biological dispersions. Questions as to the tempo and mode of these early prehistoric settlements have given rise to debates concerning the taxonomic significance of the lithic assemblages, as trace fossils, and the geographical distribution of the technological traditions found in the Lower Palaeolithic record. Here, we report on the Barranc de la Boella site which has yielded a lithic assemblage dating to ,1 million years ago that includes large cutting tools (LCT). We argue that distinct technological traditions coexisted in the Iberian archaeological repertoires of the late Early Pleistocene age in a similar way to the earliest sub-Saharan African artefact assemblages. These differences between stone tool assemblages may be attributed to the different chronologies of hominin dispersal events. The archaeological record of Barranc de la Boella completes the geographical distribution of LCT assemblages across southern Eurasia during the EMPT (Early-Middle Pleistocene Transition, circa 942 to 641 kyr). Up to now, chronology of the earliest European LCT assemblages is based on the abundant Palaeolithic record found in terrace river sequences which have been dated to the end of the EMPT and later. However, the findings at Barranc de la Boella suggest that early LCT lithic assemblages appeared in the SW of Europe during earlier hominin dispersal episodes before the definitive colonization of temperate Eurasia took place.The research at Barranc de la Boella has been carried out with the financial support of the Spanish Ministerio de Economı´a y Competitividad (CGL2012- 36682; CGL2012-38358, CGL2012-38434-C03-03 and CGL2010-15326; MICINN project HAR2009-7223/HIST), Generalitat de Catalunya, AGAUR agence (projects 2014SGR-901; 2014SGR-899; 2009SGR-324, 2009PBR-0033 and 2009SGR-188) and Junta de Castilla y Leo´n BU1004A09. Financial support for Barranc de la Boella field work and archaeological excavations is provided by the Ajuntament de la Canonja and Departament de Cultura (Servei d’Arqueologia i Paleontologia) de la Generalitat de Catalunya. A. Carrancho’s research was funded by the International Excellence Programme, Reinforcement subprogramme of the Spanish Ministry of Education. I. Lozano-Ferna´ndez acknowledges the pre-doctoral grant from the Fundacio´n Atapuerca. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Atomic Force Microscopy In High Vacuum: Experiments On Graphitic Surfaces

Trabajo presentado en la conferencia Fuerzas y Túnel (FyT2014), celebrada en San Sebastián del 27 al 29 de agosto de 2014.Atomic Force Microscopy (AFM) working in high vacuum (HV) conditions is a valuable technique due to its sensitivity and versatility. In this work we present different experiments in carbon-based materials. We use the HV-AFM to perform different experiments on graphitic surfaces like graphene and graphene oxide (GO). Graphene can be described as one-atom thick layer of graphite. For many applications, the interaction of graphene and GO with the supporting substrate and with adjacent layers plays a relevant role in properties such as adhesion, charge transfer [1], doping level etc. Furthermore, it is important to take into account the presence of atmospheric contaminants for a correct interpretation of experimental data [2]. Finally, we have studied the influence of a pressure difference onto the mechanical properties of suspended graphene membranes [3]. We have studied the diffusion through these membranes as a function of the kinetic diameter of the gas molecules [4] (Figure 1).N

The influence of strain on the elastic constants of graphene

Recent advances in the understanding of graphene elasticity have shown that suspended graphene does not behave as a conventional thin plate but has a more complex behavior where flexural modes play a significant role. Among other effects, out-of-plane thermal fluctuations modify the in-plane elastic properties. Here we report indentation experiments on graphene subjected to strain. This strain is achieved by applying pressure difference across suspended graphene drumheads. Our indentation curves show an increased mechanical response at strains larger than 0.3%. Finite element simulations of the indentation curves on pressurized membranes show that this observation can be ascribed to a twofold increase of the in-plane elastic modulus and the Poisson ratio. Based in the thermodynamic theory of elastic membranes, this increase is attributed to the suppression of out-of-plane fluctuations by strain. This result reinforces the idea that suspended graphene behaves as a fluctuating membrane and points out that a careful analysis should be done when analyzing indentation curves on atomic thick materials.Financial support was received from P2013/MIT-3007, MAT2016-77608-C3-3-P, MAD2D-CM, S2013/MIT-3007, CSD2010-0024, FIS2014-58445-JIN and the ERC Advanced Grant, #290846