Carbon-fiber tips for scanning probe microscopes and molecular electronics experiments
© Rubio-Bollinger et al.; licensee Springer. 2012
Received: 10 January 2012
Accepted: 15 May 2012
Published: 15 May 2012
We fabricate and characterize carbon-fiber tips for their use in combined scanning tunneling and force microscopy based on piezoelectric quartz tuning fork force sensors. An electrochemical fabrication procedure to etch the tips is used to yield reproducible sub-100-nm apex. We also study electron transport through single-molecule junctions formed by a single octanethiol molecule bonded by the thiol anchoring group to a gold electrode and linked to a carbon tip by the methyl group. We observe the presence of conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.
KeywordsSingle-molecule junction Carbon electrodes Carbon electronics STM break junction Carbon tip Quartz tuning fork. PACS 07.79.-v, scanning probe microscopes and components 68.37.Ef, scanning tunneling microscopy (including chemistry induced with STM) 73.63.-b, electronic transport in nanoscale materials and structures 85.65. + h, molecular electronic devices 73.40.-c, electronic transport in interface structures PACS 07.79.-v, scanning probe microscopes and components 68.37.Ef, scanning tunneling microscopy (including chemistry induced with STM) 73.63.-b, electronic transport in nanoscale materials and structures 85.65. + h, molecular electronic devices 73.40.-c, electronic transport in interface structures
Understanding electron transport through a single molecule is a basic goal in molecular electronics . A primary goal is to find reliable ways to form a stable mechanical and electrical connection between the molecule and macroscopic electrodes. The mechanical and electrical properties of a molecular junction are not only determined by the molecular structure but also by the chemical nature of the electrodes .
Here, we have explored the use of carbon-based tips as contact electrodes to form molecular junctions . Using the scanning tunneling microscope (STM) break-junction technique, we have measured the electrical conductance of several hundreds of octanethiol-based single-molecule bridges (CH3-C7H14-SH) in which the thiol anchoring group is bound to a gold electrode, and the methyl group is linked to a carbon electrode.
In order to form single-molecule junctions with a carbon electrode, we provide an STM with a carbon-fiber tip [4, 5]. The microscopic structure of the tip is composed by graphitic planes aligned parallel to the fiber longitudinal axis, yielding high electrical conductivity σ = 7.7 × 104 S/m. Carbon-fiber tips are prepared from freshly cut individual carbon fibers obtained from a commercially available carbon-fiber rope and are mounted in a home-built STM .
Although the use of mechanically fabricated tips (by simply cutting a metallic wire) is rather common in STM, the atomic force microscope (AFM) resolution strongly depends on the tip sharpness because of long range interactions between the tip and the sample. We have developed an electrochemical procedure to etch carbon-fiber tips that yields sharp carbon-fiber tips.
Results and discussion
We have fabricated and characterized carbon-fiber tips for their use in combined STM/AFMs based on quartz tuning fork force sensors. We develop an electrochemical procedure to etch carbon-fiber tips which yields sub-100-nm tip apex radius of curvature in a reproducible way, increasing the lateral resolution in AFM measurements. We show that carbon-fiber tips mounted on quartz tuning fork force sensors can be reliably used in force and/or tunnel current vs. distance measurements and simultaneous STM/AFM microscopy.
In addition, we have used carbon fiber tips as electrodes in an STM-break junction configuration to form single-molecule junctions with octanethiol molecules deposited on a gold surface. We find that carbon tips provide a stable mechanical linking to the methyl group allowing to form single-molecule bridges. Therefore, carbon tips can be suitable candidates to contact a variety of organic molecules, and they can also be combined with other substrate materials including carbon itself to form purely organic single-molecule devices.
atomic force microscope
scanning tunneling microscope.
This work was supported by MICINN (Spain) through the programs MAT2008-01735, MAT2011-25046 and CONSOLIDER-INGENIO-2010 ‘Nanociencia Molecular’ CSD-2007-00010; Comunidad de Madrid through program Nanobiomagnet S2009/MAT-1726; and European Union through programs BIMORE (MRTN-CT-2006-035859) and ELFOS (FP7).
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