Improved graphene blisters by ultrahigh pressure sealing

Graphene is a very attractive material for nanomechanical devices and membrane applications. Graphene blisters based on silicon oxide micro-cavities are a simple but relevant example of nanoactuators. A drawback of this experimental set up is that gas leakage through the graphene-SiO2 interface contributes significantly to the total leak rate. Here we study the diffusion of air from pressurized graphene drumheads on SiO2 micro-cavities and propose a straightforward method to improve the already strong adhesion between graphene and the underlying SiO2 substrate, resulting in reduced leak rates. This is carried out by applying controlled and localized ultrahigh pressure (> 10 GPa) with an Atomic Force Microscopy diamond tip. With this procedure, we are able to significantly approach the graphene layer to the SiO2 surface around the drumheads, thus enhancing the interaction between them allowing us to better seal the graphene-SiO2 interface, which is reflected in up to ~ 4 times lower leakage rates. Our work opens an easy way to improve the performance of graphene as a gas membrane on a technological relevant substrate such as SiO2.Comment: pages 19, 4 figures + supplementary informatio

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

Increasing the elastic modulus of graphene by controlled defect creation

The extraordinary strength, stiffness and lightness of graphene have generated great expectations of its application in flexible electronics and as a mechanical reinforcement agent. However, the presence of lattice defects, unavoidable in sheets obtained by scalable routes, might degrade its mechanical properties. Here we report a systematic study on the elastic modulus and strength of graphene with a controlled density of defects. Counter-intuitively, the in-plane Youngâ €™ s modulus increases with increasing defect density up to almost twice the initial value for a vacancy content of â 1/40.2%. For a higher density of vacancies, the elastic modulus decreases with defect inclusions. The initial increase in Youngâ €™ s modulus is explained in terms of a dependence of the elastic coefficients on the momentum of flexural modes predicted for two-dimensional membranes. In contrast, the fracture strength decreases with defect density according to standard fracture continuum models. These quantitative structure-property relationships, measured in atmospheric conditions, are of fundamental and technological relevance and provide guidance for applications in which graphene mechanics represents a disruptive improvement.This work was supported by MAT2013-46753-C2-2-P, Consolider CSD2010-0024, FIS2011-23713 and the European Research Council Advanced Grant, #290846. MINECO/ICTI2013-2016/MAT2013-46753-C2-2-

Tailoring the thermal expansion of graphene via controlled defect creation

Contrary to most materials, graphene exhibits a negative thermal expansion coefficient (TEC), i.e it contracts when heated. This contraction is due to the thermal excitation of low energy out–of–plane vibration modes. These flexural modes have been reported to govern the electronic transport and the mechanical response of suspended graphene. In this work, we systematically investigate the influence of defects in the TEC of suspended graphene membranes. Controlled introduction of low densities of mono-vacancies reduces the graphene TEC, up to one order of magnitude for a defect density of 5 × 10 cm. Our molecular dynamics simulations reproduce the observed trend and show that TEC reduction is due to the suppression of out–of–plane fluctuations caused by the strain fields created by mono-vacancies in their surrounding areas. These results highlight the key role of defects in the properties of “real-life” graphene, and pave the way for future proposals of electronic and mechanical defect engineering.We acknowledge financial support from the Spanish MINECO (projects CSD2010-00024, MAT2011-023627, MAT2016-77608-C3-3-P, MAT2014-54484-P, MDM-2014-0377, and FIS2015-69295-C3) and from the MAD2D-CM Program (project S2013/MIT-3007) of the Comunidad de Madrid