Cristina Gómez-Navarro

Atomic structure of reduced graphene oxide.

Using high resolution transmission electron microscopy, we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.

Elastic properties of chemically derived single graphene sheets.

The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness ( E = 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.

Electrical Conduction Mechanism in Chemically Derived Graphene Monolayers

We have performed a detailed study of the intrinsic electrical conduction process in individual monolayers of chemically reduced graphene oxide down to a temperature of 2 K. The observed conductance can be consistently interpreted in the framework of two-dimensional variable-range hopping in parallel with electric-field-driven tunneling. The latter mechanism is found to dominate the electrical transport at very low temperatures and high electric fields. Our results are consistent with a model of highly conducting graphene regions interspersed with disordered regions, across which charge carrier hopping and tunneling are promoted by strong local electric fields.

Contactless experiments on individual DNA molecules show no evidence for molecular wire behavior

A fundamental requirement for a molecule to be considered a molecular wire (MW) is the ability to transport electrical charge with a reasonably low resistance. We have carried out two experiments that measure first, the charge transfer from an electrode to the molecule, and second, the dielectric response of the MW. The latter experiment requires no contacts to either end of the molecule. From our experiments we conclude that adsorbed individual DNA molecules have a resistivity similar to mica, glass, and silicon oxide substrates. Therefore adsorbed DNA is not a conductor, and it should not be considered as a viable candidate for MW applications. Parallel studies on other nanowires, including single-walled carbon nanotubes, showed conductivity as expected.

Solvent‐Induced Delamination of a Multifunctional Two Dimensional Coordination Polymer

A coordination polymer is fully exfoliated by solvent-assisted interaction only. The soft-delamination process results from the structure of the starting material, which shows a layered structure with weak layer-to-layer interactions and cavities with the ability to locate several solvents in an unselective way. These results represent a significant step forward towards the production of structurally designed one-molecule thick 2D materials with tailored physico-chemical properties.

Intrinsic electrical conductivity of nanostructured metal-organic polymer chains

One-dimensional conductive polymers are attractive materials because of their potential in flexible and transparent electronics. Despite years of research, on the macro- and nano-scale, structural disorder represents the major hurdle in achieving high conductivities. Here we report measurements of highly ordered metal-organic nanoribbons, whose intrinsic (defect-free) conductivity is found to be 104 S m−1, three orders of magnitude higher than that of our macroscopic crystals. This magnitude is preserved for distances as large as 300 nm. Above this length, the presence of structural defects (~ 0.5%) gives rise to an inter-fibre-mediated charge transport similar to that of macroscopic crystals. We provide the first direct experimental evidence of the gapless electronic structure predicted for these compounds. Our results postulate metal-organic molecular wires as good metallic interconnectors in nanodevices.

Scanning force microscopy three-dimensional modes applied to the study of the dielectric response of adsorbed DNA molecules

We have developed a set of working modes for scanning probe microscopy?(SPM), which generalizes the usual method of acquiring data. We call these modes three-dimensional?(3D) modes. Using these modes it is possible to measure typical SPM magnitudes, such as, for example, the tunnel current, the normal force and the amplitude or frequency of the cantilever oscillation, as a function of any other two magnitudes of the system: f(x1,x2). In this paper we present different examples of 3D modes. In particular, we have applied 3D modes to the study of the electrostatic interaction of co-adsorbed single walled carbon nanotubes and individual DNA molecules with a metallic scanning force microscopy tip. The data indicate that adsorbed DNA has a dielectric constant similar to that of the glass substrate.

Topographic characterization and electrostatic response of M-DNA studied by atomic force microscopy

Modified DNA species have attracted the interest of the scientific community in the last years in a search for an appropriate molecular wire for the incoming nanotechnology. M-DNA, a complex of DNA with divalent metallic ions, is one of the candidates for a molecular wire. In this paper we describe the procedure to fabricate M-DNA using NiCl2 and CoCl2, and a simple test to check the production of the modified biomolecule. We present atomic force microscope (AFM) images of nickel and cobalt M-DNA. Our results show that the DNA modified with these metal ions suffers a fivefold reduction in length and an increment of almost one order of magnitude in height as compared to the length and height of regular B-DNA. This type of condensation of the DNA is fully reversible upon the addition of EDTA. AFM images of reversed M-DNA show no differences from regular B-DNA. Two types of electrostatic experiment performed on this modified molecule show no evidence for metallic or semiconductor behaviour.

Quantitative theory for the imaging of conducting objects in electrostatic force microscopy

A theoretical method for the imaging of metallic objects in electrostatic force microscopy is presented. The technique, based on the generalized image charge method, includes intrinsically the mutual polarization between the tip, the sample, and the metallic objects. Taking also into account the cantilever and macroscopic shape of the tip, the theory gives us a quantitative value for the electrostatic interaction between the tip and the objects over the surface. Experimental data of frequency shifts in an oscillating tip induced by grounded and isolated nanotubes are analyzed finding an excellent quantitative agreement between experimental data and numerical calculations.

Confining crack propagation in defective graphene.

Crack propagation in graphene is essential to understand mechanical failure in 2D materials. We report a systematic study of crack propagation in graphene as a function of defect content. Nanoindentations and subsequent images of graphene membranes with controlled induced defects show that while tears in pristine graphene span microns length, crack propagation is strongly reduced in the presence of defects. Accordingly, graphene oxide exhibits minor crack propagation. Our work suggests controlled defect creation as an approach to avoid catastrophic failure in graphene.