Julio Gómez-Herrero

WSXM: A software for scanning probe microscopy and a tool for nanotechnology

In this work we briefly describe the most relevant features of WSXM, a freeware scanning probe microscopy software based on MS-Windows. The article is structured in three different sections: The introduction is a perspective on the importance of software on scanning probe microscopy. The second section is devoted to describe the general structure of the application; in this section the capabilities of WSXM to read third party files are stressed. Finally, a detailed discussion of some relevant procedures of the software is carried out.

Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime.

Carbon nanotubes1,2 are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge3. Defects are known to modify the electrical resistance of carbon nanotubes4; they can be present in as-grown carbon nanotubes, but controlling their density externally opens a path towards the tuning of the electronic characteristics of the nanotube. In this work, consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room-temperature resistance versus SWNT length (R(L)) along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation, and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a SWNT of 400 nm length.

Properties of Metallic Nanowires: From Conductance Quantization to Localization

Material structures of reduced dimensions exhibit electrical and mechanical properties different from those in the bulk. Measurements of room-temperature electronic transport in pulled metallic nanowires are presented, demonstrating that the conductance characteristics depend on the length, lateral dimensions, state and degree of disorder, and elongation mechanism of the wire. Conductance during the elongation of short wires (length l ∼ 50 angstroms) exhibits periodic quantization steps with characteristic dips, correlating with the order-disorder states of layers of atoms in the wire predicted by molecular dynamics simulations. The resistance R of wires as long as l ∼ 400 angstroms exhibits localization characteristics with In R(l) ∼ l2.

DNA-mediated anisotropic mechanical reinforcement of a virus

In this work, we provide evidence of a mechanism to reinforce the strength of an icosahedral virus by using its genomic DNA as a structural element. The mechanical properties of individual empty capsids and DNA-containing virions of the minute virus of mice are investigated by using atomic force microscopy. The stiffness of the empty capsid is found to be isotropic. Remarkably, the presence of the DNA inside the virion leads to an anisotropic reinforcement of the virus stiffness by ≈3%, 40%, and 140% along the fivefold, threefold, and twofold symmetry axes, respectively. A finite element model of the virus indicates that this anisotropic mechanical reinforcement is due to DNA stretches bound to 60 concavities of the capsid. These results, together with evidence of biologically relevant conformational rearrangements of the capsid around pores located at the fivefold symmetry axes, suggest that the bound DNA may reinforce the overall stiffness of the viral particle without canceling the conformational changes needed for its infectivity.

Jumping mode scanning force microscopy

In this letter, we present a new scanning probe microscopy mode, jumping mode, which allows the simultaneous measurement of the topography and of some other physical property of the sample. Essentially, at each image point first the topography of the sample is measured during a feedback phase of a cycle, and then the tip–sample interaction is evaluated in real time as the tip is moved away and towards the sample. Since the lateral motion is done out of contact the method is free, or nearly free, of shear forces. The general advantages of jumping mode are discussed. Finally, two different applications of this mode are presented. In addition to the topography, the first application measures the adhesion between the tip and the sample, while the second determines the corresponding electrostatic interaction.

Single layers of a multifunctional laminar Cu(I,II) coordination polymer

A multifunctional bidimensional mixed-valence copper coordination polymer [Cu2Br(IN)2]n (IN = isonicotinato) has been characterized in crystal phase and isolated on graphite surface as single sheets.

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.

Resolution of site-specific bonding properties of C60 adsorbed on Au(111)

We have performed a careful study of the adsorption of C60 molecules on a Au(111) surface by using scanning tunneling microscopy and spectroscopy at room temperature. In coincidence with results from other techniques, differential conductance spectra give a value of 2.3 eV for the HOMO–LUMO gap of a monomolecular layer, with the LUMO level located at 0.6 eV above the Fermi level as a consequence of electronic charge transfer from the substrate into the molecule. Small differences in position (and shape) of the LUMO-derived resonance, in the order of 0.1 eV, are found on molecules adsorbed at step edges. We consider the Smoluchowski effect, i.e., the interaction of the molecules with a charge-depleted region, to explain the observed differences in their bonding nature. On some molecules forming part of bidimensional fullerene islands, similar differences were also detected with spatially resolved scanning tunneling spectroscopy, giving rise to a 2×2 commensurate structure of the molecular adlayer with resp...

Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions

Antimonene fabricated by mechanical exfoliation is highly stable under atmospheric conditions over periods of months and even when immersed in water. Density functional theory confirms the experiments and predicts an electronic gap of ≈1 eV. These results highlight the use of antimonene for optoelectronics applications.

Highly conductive self-assembled nanoribbons of coordination polymers.

Organic molecules can self-assemble into well-ordered structures, but the conductance of these structures is limited, which is a disadvantage for applications in molecular electronics. Conductivity can be improved by using coordination polymers-in which metal centres are incorporated into a molecular backbone-and such structures have been used as molecular wires by self-assembling them into ordered films on metal surfaces. Here, we report electrically conductive nanoribbons of the coordination polymer [Pt(2)I(S(2)CCH(3))(4)](n) self-assembled on an insulating substrate by direct sublimation of polymer crystals. Conductance atomic force microscopy is used to probe the electrical characteristics of a few polymer chains ( approximately 10) within the nanoribbons. The observed currents exceed those previously sustained in organic and metal-organic molecules assembled on surfaces by several orders of magnitude and over much longer distances. These results, and the results of theoretical calculations based on density functional theory, confirm coordination polymers as candidate materials for applications in molecular electronics.