Using a scanning tunneling microscope, we measured high-bias conductance of single polyporphyrin molecular wires with lengths from 1.3 to 13 nm. We observed several remarkable transport characteristics, including multiple sharp conductance peaks, conductances as high as 20 nS in wires with lengths of >10 nm, and nearly length-independent conductance (attenuation <0.001 Å(-1)). We carried out first-principles simulations on myriad metal-molecule-metal junctions. The simulations revealed that the measured conductance is coherent resonant transport via a delocalized molecular orbital.
We present an investigation of two-dimensional coordination networks formed by 5,10,15,20-tetra(4-pyridyl)porphyrin and iron atoms on a Au(111) surface. The coordination bonds are very robust as evidenced by STM manipulated lateral displacement of an entire network of islands consisting of hundreds of molecules and atoms. We also applied vertical manipulation to detach and attach single Fe atoms at the coordination sites. Moreover, low-temperature tunneling spectroscopy reveals a Kondo resonance at the Fe coordination center. These findings evidence that the network structure is stabilized by a coordination motif in which a pair of vertically aligned Fe atoms is ligated by four equatorial pyridyl groups. Such out-of-plane dinuclear coordination centers provide potential functions, such as catalytic, adsorption, and template for growing three-dimensional framework architectures.
We apply supramolecular assembly to control the adsorption configuration of Co-porphyrin molecules on Au(111) and Cu(111) surfaces. By means of cryogenic scanning tunneling microscopy, we reveal that the Kondo effect associated with the Co center is absent or present in different supramolecular systems. We perform first-principles calculations to obtain spin-polarized electronic structures and compute the Kondo temperatures using the Anderson impurity model. The switching behavior is traced to varied molecular adsorption heights in different supramolecular structures. These findings unravel that a competition between intermolecular interactions and molecule-substrate interactions subtly regulates the molecular Kondo effect in supramolecular systems.
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