Surface chemistry of porphyrins and phthalocyanines

The development of a variety of nanoscale applications requires the fabrication and control of atomic or molecular switches that can be reversibly operated by light, a short-range force, electric current or other external stimuli. For such molecules to be used as electronic components, they should be directly coupled to a metallic support and the switching unit should be easily connected to other molecular species without suppressing switching performance. Here, we show that a free-base tetraphenyl-porphyrin molecule, which is anchored to a silver surface, can function as a molecular conductance switch. The saddle-shaped molecule has two hydrogen atoms in its inner cavity that can be flipped between two states with different local conductance levels using the electron current through the tip of a scanning tunnelling microscope. Moreover, by deliberately removing one of the hydrogens, a four-level conductance switch can be created. The resulting device, which could be controllably integrated into the surrounding nanoscale environment, relies on the transfer of a single proton and therefore contains the smallest possible atomistic switching unit.

/pdf/a-surface-anchored-molecular-four-level-conductance-switch-153gggsr03.pdf

A surface-anchored molecular four-level conductance switch based on single proton transfer

Although the local environment of a molecule can play an important role in its chemistry, rarely has it been examined experimentally at the level of individual molecules. Here we report the precise control of intramolecular hydrogen-transfer (tautomerization) reactions in single molecules using scanning tunnelling microscopy. By placing, with atomic precision, a copper adatom close to a porphycene molecule, we found that the tautomerization rates could be tuned up and down in a controlled fashion, surprisingly also at rather large separations. Furthermore, we extended our study to molecular assemblies in which even the arrangement of the pyrrolic hydrogen atoms in the neighbouring molecule influences the tautomerization reaction in a given porphycene, with positive and negative cooperativity effects. Our results highlight the importance of controlling the environment of molecules with atomic precision and demonstrate the potential to regulate processes that occur in a single molecule.

/pdf/controlling-intramolecular-hydrogen-transfer-in-a-porphycene-45hrudelsu.pdf

Controlling intramolecular hydrogen transfer in a porphycene molecule with single atoms or molecules located nearby

We studied the dynamics of a single cobalt (Co) atom during lateral manipulation on a copper (111) surface in a low-temperature scanning tunneling microscope. The Co binding site locations were revealed in a detailed image that resulted from lateral Co atom motion within the trapping potential of the scanning tip. Random telegraph noise, corresponding to the Co atom switching between hexagonal close-packed (hcp) and face-centered cubic (fcc) sites, was seen when the tip was used to try to position the Co atom over the higher energy hcp site. Varying the probe tip height modified the normal copper (111) potential landscape and allowed the residence time of the Co atom in these sites to be varied. At low tunneling voltages (less than ∼5 millielectron volts), the transfer rate between sites was independent of tunneling voltage, current, and temperature. At higher voltages, the transfer rate exhibited a strong dependence on tunneling voltage, indicative of vibrational heating by inelastic electron scattering.

http://absimage.aps.org/image/MAR05/MWS_MAR05-2004-004404.pdf

Controlling the Dynamics of a Single Atom in Lateral Atom Manipulation

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single ${\mathrm{D}}_{2}$ molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultrahigh geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor and transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.

/pdf/imaging-pauli-repulsion-in-scanning-tunneling-microscopy-vppnjel014.pdf

Imaging Pauli repulsion in scanning tunneling microscopy.

Scanning tunneling spectroscopy of the differential conductance is performed at constant current and at constant distance. These modes of operation significantly affect peak positions and line shapes in spectra as well as patterns in spatial maps of the differential conductance. A normalization procedure for constant-current data, which relies on experimental current-distance data, is shown to yield spectral information on the local density of states.

Local density of states from constant-current tunneling spectra

Single lead-phthalocyanine molecules adsorbed to ultrathin lead islands on Ag(111) are demetalized by transferring the lead atom of the inner macrocycle to the tip of a scanning tunneling microscope. Reactants and products are discriminated by their images and spectroscopic fingerprints.

Demetalation of a Single Organometallic Complex

Using a cryogenic scanning tunneling microscope, the dynamics of lateral translations of a single silver atom adsorbed on Ag(111) were investigated with a time resolution of 10 ns. Elevated tunneling currents passed through the tip close to the adsorbed atom induce sudden adsorption site changes, which are reflected by two-level fluctuations of the tunneling conductance. The voltage dependence of the fluctuation rate reveals different local heating of the adsorbed atom in different positions relative to the tip.

Direct observation of conductance fluctuations of a single-atom tunneling contact

Silver dimers were fabricated on Ag(111) by single-atom manipulation using the tip of a cryogenic scanning tunnelling microscope. An unoccupied electronic resonance was observed to shift toward the Fermi level with decreasing atom–atom distance as monitored by spatially resolved scanning tunnelling spectroscopy. Density functional calculations were used to analyse the experimental observations and revealed that the coupling between the adsorbed atoms is predominantly direct rather than indirect via the Ag(111) substrate. While the substrate influence is small owing to the surface-projected sp band gap, the direct interaction is most likely due to the large extension of the p wave functions at the adsorbate atoms contributing to the resonance.

http://iopscience.iop.org/article/10.1088/1367-2630/11/6/063020/pdf

Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)

Silver dimers were fabricated on Ag(111) by single-atom manipulation using the tip of a cryogenic scanning tunnelling microscope. An unoccupied electronic resonance was observed to shift toward the Fermi level with decreasing atom-atom distance as monitored by spatially resolved scanning tunnelling spectroscopy. Density functional calculations were used to analyse the experimental observations and revealed that the coupling between the adsorbed atoms is predominantly direct rather than indirect via the Ag(111) substrate.

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Local density of states from constant-current tunneling spectra

Demetalation of a Single Organometallic Complex

Direct observation of conductance fluctuations of a single-atom tunneling contact

Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)

Evolution of unoccupied resonance during the synthesis of a silver dimer on Ag(111)