The structure of silicene, the two-dimensional honeycomb sheet of Si, grown on Ag(111) was investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) combined with density functional theory (DFT) calculation. Two atomic arrangements of honeycomb configuration were found by STM, which are confirmed by LEED and DFT calculations; one is 4×4 and the other is √13×√13 R13.9°. In the 4×4 structure, the honeycomb lattice remains with six atoms displaced vertically, whereas the √13×√13 R13.9° takes the regularly buckled honeycomb geometry.

We demonstrate that silicene, a 2D honeycomb lattice consisting of Si atoms, loses its Dirac fermion characteristics due to substrate-induced symmetry breaking when synthesized on the Ag(111) surface. No Landau level sequences appear in the tunneling spectra under a magnetic field, and density functional theory calculations show that the band structure is drastically modified by the hybridization between the Si and Ag atoms. This is the first direct example demonstrating the lack of Dirac fermions in a single layer honeycomb lattice due to significant symmetry breaking.

Plasmon-induced chemical reactions of molecules adsorbed on metal nanostructures are attracting increased attention for photocatalytic reactions. However, the mechanism remains controversial because of the difficulty of direct observation of the chemical reactions in the plasmonic field, which is strongly localized near the metal surface. We used a scanning tunneling microscope (STM) to achieve real-space and real-time observation of a plasmon-induced chemical reaction at the single-molecule level. A single dimethyl disulfide molecule on silver and copper surfaces was dissociated by the optically excited plasmon at the STM junction. The STM study combined with theoretical calculations shows that this plasmon-induced chemical reaction occurred by a direct intramolecular excitation mechanism.

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

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The Kondo effect caused by the adsorption of iron phthalocyanine (FePc) on Au(111) was investigated by the combination of density functional theory and a numerical renormalization group calculation with scanning tunneling microscopy. We found that a novel Kondo effect is realized for a single FePc molecule on Au(111) by tuning the symmetry of the ligand field through the local coordination to the substrate. For FePc in the on top configuration where fourfold symmetry around the Fe(2+) ion is held, the orbital degrees of freedom survive, resulting in the spin+orbital SU(4) Kondo effect. In contrast, the reduced symmetry in the bridge configuration freezes the orbital degrees of freedom, leading to the spin SU(2) Kondo effect. These results provide a novel example to manipulate the many-body phenomena by tuning the local symmetry.

Given its central role in photosynthesis and artificial energy-harvesting devices, energy transfer has been widely studied using optical spectroscopy to monitor excitation dynamics and probe the molecular-level control of energy transfer between coupled molecules. However, the spatial resolution of conventional optical spectroscopy is limited to a few hundred nanometres and thus cannot reveal the nanoscale spatial features associated with such processes. In contrast, scanning tunnelling luminescence spectroscopy has revealed the energy dynamics associated with phenomena ranging from single-molecule electroluminescence, absorption of localized plasmons and quantum interference effects to energy delocalization and intervalley electron scattering with submolecular spatial resolution in real space. Here we apply this technique to individual molecular dimers that comprise a magnesium phthalocyanine and a free-base phthalocyanine (MgPc and HPc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby HPc molecule as a result of resonance energy transfer from the former to the latter. A reciprocating resonance energy transfer is observed when exciting the second singlet state (S) of HPc, which results in energy transfer to the first singlet state (S) of MgPc and final funnelling to the S state of HPc. We also show that tautomerization of HPc changes the energy transfer characteristics within the dimer system, which essentially makes HPc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.

The interaction of water with oxide surfaces has drawn considerable interest, owing to its application to problems in diverse scientific fields. Atomic-scale insights into water molecules on the oxide surface have long been recognized as essential for a fundamental understanding of the molecular processes occurring there. Here, we report the dissociation of a single water molecule on an ultrathin MgO film using low-temperature scanning tunnelling microscopy. Two types of dissociation pathway--vibrational excitation and electronic excitation--are selectively achieved by means of injecting tunnelling electrons at the single-molecule level, resulting in different dissociated products according to the reaction paths. Our results reveal the advantage of using a MgO film, rather than bulk MgO, as a substrate in chemical reactions.