Gabino Rubio-Bollinger

Elastic properties of freely suspended MoS2 nanosheets.

We study the elastic deformation of few layers (5 to 25) thick freely suspended MoS2 nanosheets by means of a nanoscopic version of a bending test experiment, carried out with the tip of an atomic force microscope. The Youngs modulus of these nanosheets is extremely high (E = 0.33 TPa), comparable to that of graphene oxide, and the deflections are reversible up to tens of nanometers.

Mechanical Properties and Formation Mechanisms of a Wire of Single Gold Atoms

A scanning tunneling microscope supplemented with a force sensor is used to study the mechanical properties of a novel metallic nanostructure: a freely suspended chain of single gold atoms. We find that the bond strength of the nanowire is about twice that of a bulk metallic bond. We perform ab initio calculations of the force at chain fracture and compare quantitatively with experimental measurements. The observed mechanical failure and nanoelastic processes involved during atomic wire fabrication are investigated using molecular dynamics simulations, and we find that the total effective stiffness of the nanostructure is strongly affected by the detailed local atomic arrangement at the chain bases.

Optical identification of atomically thin dichalcogenide crystals

We present a systematic study of the optical contrast of niobium diselenide and molybdenum disulfide flakes deposited onto silicon wafers with a thermally grown silicon oxide layer. We measure the optical contrast of flakes whose thickness, which is obtained by atomic force microscopy, ranges from 200 layers down to a monolayer using different illumination wavelengths in the visible spectrum. The refractive index of these thin crystals has been obtained from the optical contrast using Fresnel law. In this way the optical microscopy data can be quantitatively analyzed to determine the thickness of the flakes in a fast and nondestructive way.

Onset of energy dissipation in ballistic atomic wires.

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.

Observation of a Parity Oscillation in the Conductance of Atomic Wires

Using a scanning tunnel microscope or mechanically controllable break junctions atomic contacts for Au, Pt, and Ir are pulled to form chains of atoms. We have recorded traces of conductance during the pulling process and averaged these for a large number of contacts. An oscillatory evolution of conductance is observed during the formation of the monoatomic chain suggesting a dependence on the numbers of atoms forming the chain being even or odd. This behavior is not only observed for the monovalent metal Au, as was predicted, but is also found for the other chain-forming metals, suggesting it to be a universal feature of atomic wires.

Electric‐Field Screening in Atomically Thin Layers of MoS2: the Role of Interlayer Coupling

This work was supported by MICINN/MINECO (Spain) through the programs MAT2011-25046 and CONSOLIDER-INGENIO-2010 “Nanociencia Molecular” CSD-2007-00010, Comunidad de Madrid through program Nanobiomagnet S2009/MAT-1726 and the European Union (FP7) through the programs RODIN and ELFOS. E.C. acknowledges a Marie Curie Grant, PIEF-GA-2009-251904.

Tunneling Spectroscopy in Small Grains of Superconducting MgB 2

We report on tunneling spectroscopy experiments in small grains of the new binary intermetallic superconductor MgB(2). Experiments have been performed at 2.5 K using a low temperature scanning tunneling microscope. Good fit to the BCS model is obtained, with a gap value of 2 meV. In the framework of this model, this value should correspond to a surface critical temperature of 13.2 K. No evidence of gap anisotropy has been found.

Engineering the Thermopower of C60 Molecular Junctions

We report the measurement of conductance and thermopower of C60 molecular junctions using a scanning tunneling microscope (STM). In contrast to previous measurements, we use the imaging capability of the STM to determine precisely the number of molecules in the junction and measure thermopower and conductance continuously and simultaneously during formation and breaking of the molecular junction, achieving a complete characterization at the single-molecule level. We find that the thermopower of C60 dimers formed by trapping a C60 on the tip and contacting an isolated C60 almost doubles with respect to that of a single C60 and is among the highest values measured to date for organic materials. Density functional theory calculations show that the thermopower and the figure of merit continue increasing with the number of C60 molecules, demonstrating the enhancement of thermoelectric preformance by manipulation of intermolecular interactions.

Mechanical properties of freely suspended semiconducting graphene-like layers based on MoS2

We fabricate freely suspended nanosheets of molybdenum disulphide (MoS2) which are characterized by quantitative optical microscopy and high-resolution friction force microscopy. We study the elastic deformation of freely suspended nanosheets of MoS2 using an atomic force microscope. The Youngs modulus and the initial pre-tension of the nanosheets are determined by performing a nanoscopic version of a bending test experiment. MoS2 sheets show high elasticity and an extremely high Youngs modulus (0.30 TPa, 50% larger than steel). These results make them a potential alternative to graphene in applications requiring flexible semiconductor materials.PACS, 73.61.Le, other inorganic semiconductors, 68.65.Ac, multilayers, 62.20.de, elastic moduli, 81.40.Jj, elasticity and anelasticity, stress-strain relations.

Study of electron-phonon interactions in a single molecule covalently connected to two electrodes

Presented here is a study of electron-phonon interactions in a single molecule junction where the molecule is covalently connected to two electrodes. In this system, vibration modes in a single molecule junction are measured by sweeping the bias voltage between the two electrodes and recording the differential conductance while the strain in the junction is changed by separating the two electrodes. This unique approach allows changes in conductance to be compared to changes in the configuration of a single molecule junction. This system opens a new door for characterizing single molecule junctions and a better understanding of the relationship between molecular conductance, electron-phonon interactions, and configuration.