Low dimensional materials
Our group has been involved in the last 15 years in the study of electronic transport phenomena in diverse low dimensional materials, both 1D and 2D.
During the the 90´s our research was focused on the creation and transport properties of one dimensional quantum contacts. these atomic necks were created with the tip of a Scanning Tunneling Microscope. The dimension of the contacts are comparable with the Fermi wavelength of the electrons and this results in clear quantum effects in the electrical transport [Pascual_93, Pascual_95]
During the last 10 years we have been studding diverse properties of Carbon Nanotubes, one dimensional tubes made exclusively out of carbon atoms. In collaboration with Prof. Reifenberger's group, we developed a technique for the realization of reliable electrical contacts to carbon nanotubes [dePablo99].
After studding in detail the formation of contacts between a conductive AFM tip and a carbon nanotube [dePablo00_1] we were able to determine the electronic transport regime of carbon nanotubes by measuring the resistance vs. length for several nanotubes by Conductive AFM. [dePablo02].
We were also able to tune the resistance of the tubes by the creation of atomic scale defects by ion irradiation [Gomez-Navarro05]. These works showed that carbon nanotubes in the presence of defects and at low voltages are in a strong localization quantum transport regime where the resistance depends exponentially with the length of the tubes. In the high voltage regime electronic transport is dominated by electron-phonon interactions [Sundqvist07]
The correlation of mechanical deformation and electronic structure also showed unexpected results such as an oscillatory behavior of the band gap. (Gomez-Navarro04, Gomez-Navarro06). The initial stages of the contact between a gold tip and a carbon nanotube have been the subject of a combined theoretical and experimental work [Gonzalez09]
We have been also involved in the polemic raised during the first years of this century regarding the electronic transport properties of double stranded DNA fibers. For this task we started by applying the technique of conductive AFM that was previously tested and applied for carbon nanotubes to DNA fibers [dePablo00_2].
These experiments, showed negative results, however, in order to reduce the influence of the electrical contacts between the electrode and the DNA fibers we also employed Electrostatic Force Microscopy to determine the intrinsic properties of DNA [Gomez-Navarro02].
Other sophisticated AFM techniques such as Jumping mode [dePablo98] were also used to characterize conductive nanostructures such as V2O5 nanoribbons[Gomez-Navarro03]. This technique allows making current maps of conductive ribbons without damaging the samples.
We are currently working in the electrical, mechanical and structural characterization of chemically derived graphene monolayer in collaboration with the Nanoscale Science department at the Max Plank Institute for Solid State Research in Stuttgart. The process to obtain thesechemically derived graphene layers starts by in the oxidation of graphite. Graphite oxide is highly hydrophilic and easily dispersed in water to single layers. Once single layers are obtained they are adsorbed on an insulating substrate and subsequently reduced to partially recover the properties of graphene. The conductivity of as-obtained graphene is still 2-3 orders of magnitude lower than pristine graphene [Gomez07]. We have been able to determine that this difference is mainly due to the presence of small 2-3 nm2 highly defective areas [Gomez 2010]. However, this conductivity can be improved by a CVD treatment that involves the healing of defects and incorporation of carbon atoms within defective areas [Lopez09]. Our current interests are further characterization of these graphene layers and their interaction with other chemical and bio species.
Our efforts have been also directed to the study of ultralong natural graphene ribbons made by mechanical cleavage of Graphite. The goal is to produce nanoribbons with edges following the crystallographic direction of the graphene in contrast with nanoribbons produced by nanolitography procedures. To this end we use a micromechanical cleavage technique based on PDMS stamps.
In the last years, in collaboration with Prof. Zamora's group we have been involved in the production and characterization of metal-organic conductive wires based on coordination polymers. One of our main achievements is a method that allows forming one dimensional conductive structures by direct sublimation of the polymer crystal [Welt09]. The electrical properties of these nanostructure have been confirmed using conducing atomic force microscopy [Welte10]