Direct Observation of the Tunneling Channels of a Chemisorbed Molecule

Weexploit several scanning tunnelingmicroscopy (STM) techniques, such as atom manipulated scans and constant-height scans, to atomically resolve the adsorption geometry of isolated cobalt-phthalocyanine (CoPc)molecules on a copper (111) surface and to obtain proper low-temperature maps of the molecular conductance. By comparing these crucial findings to density functional calculations, we then provide fresh insight into the CoPc-metal interface. This innovative STM study should be applicable to a wide variety of molecules relevant for molecular electronics. SECTION Surfaces, Interfaces, Catalysis U nderstanding electron transport in hybrid metalorganic devices is fundamental to the emerging technology of molecular electronics. Functional molecules are in factbound to replacediodes, transistors, andswitches to answer the multiple tasks of electronic circuits. Phthalocyanine (Pc) and porphyrin derivatives have been most studied in this context, also because of their potential interest for spin-dependent electronics, and optoelectronics. The contact between a conductor and a molecule is a decisive factor for the quality of these nanoscale devices since itmodifies the properties of the molecule and therefore its functionalities. For this reason, the metal-molecule contact is one of the most urgent matters to address in molecular electronics. One way to study this is to focus on model playgrounds such as single molecules adsorbed on pristine metal surfaces. Recent scanning tunneling microscopy and spectroscopy (STM and STS) studies have evidenced how the adsorption site, the close environment, and conformational changes, of moleculescan influence theirconductanceandspin-polarization. In this letter we wish to go one step further and show how to gather crucial information on the metal-molecule interface that is usually inaccessible to STM. For this purpose we use low-temperature STM and STS to investigate a common dye molecule such as single cobalt-phthalocyanine (CoPc) on acopper (111) surface.Compared toprevious studies,weexploit several innovative techniques in relation to singlemolecules, such as atom-manipulated scans and constant-height scans. In this way we are able to determine the adsorption site of a single CoPc and identify in a reliable way its major conductance channels with a high intramolecular resolution. By comparing these crucial parameters to density functional calculations, we are then in a favorable position to accurately describe, from a chemical viewpoint, the molecule-metal contact and to identify the origin of the conductance channels observed. This experimentmay serve as an example for future investigations of single molecules at the atomic level. The measurements were carried out with a low-temperature STM operating below 5 10 mbar. After cleaning the Cu(111) surface by argon ion bombardment and annealing, CoPcwas sublimated fromacrucible containinga99.5%pure powder heated to at least 400 C, resulting in an average coverage of 0.03monolayers on the Cu(111) surface. Amonolayer corresponds to one molecule occupying an area of 140 Å. The samplewas then transferred in theSTMandcooled to4.6K. STS measurements consisted in mapping the spatial variation of the differential conductance (dI/dV)at each pixel of a given area of the surface (the bias was measured with respect to the sample). The dI/dVwasmeasured via lock-in detection by applying amodulation voltage to the bias (15mVrms in amplitude at a frequency close to 700 Hz). During the acquisition of a dI/dVmap, the feedback loop was open, and the tip was kept at a constant height relative to the copper surface. A variety of electrochemically etched W tips were employed. After a sputter/anneal cycle, the tips were treated in vacuo by soft indentations into the clean copper surface, therefore the tip apex was presumably coated with copper. Figure 1a presents an STM image of CoPcmolecules deposited on the Cu(111) surface. At this coverage, CoPcmolecules are well separated from one another. This might reflect a randomdistribution ofCoPc on the surfaceproducedbya gas-like phase existing at room temperature, but could also be related Received Date: March 16, 2010 Accepted Date: April 15, 2010