Kuniyuki Miwa

Real-space investigation of energy transfer in heterogeneous molecular dimers

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 H2Pc) and find that locally exciting MgPc with the tunnelling current of the scanning tunnelling microscope generates a luminescence signal from a nearby H2Pc 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 (S2) of H2Pc, which results in energy transfer to the first singlet state (S1) of MgPc and final funnelling to the S1 state of H2Pc. We also show that tautomerization of H2Pc changes the energy transfer characteristics within the dimer system, which essentially makes H2Pc a single-molecule energy transfer valve device that manifests itself by blinking resonance energy transfer behaviour.

Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System

We investigate the near-field interaction between an isolated free-base phthalocyanine molecule and a plasmon localized in the gap between an NaCl-covered Ag(111) surface and the tip apex of a scanning tunneling microscope. When the tip is located in the close proximity of the molecule, asymmetric dips emerge in the broad luminescence spectrum of the plasmon generated by the tunneling current. The origin of the dips is explained by energy transfer between the plasmon and molecular excitons and a quantum mechanical interference effect, where molecular vibrations provide additional degrees of freedom in the dynamic process.The electronic excitation of molecules triggers diverse phenomena such as luminescence and photovoltaic effects, which are the bases of various energy-converting devices. Understanding and control of the excitations at the single-molecule level are long standing targets, however, they have been hampered by the limited spatial resolution in optical probing techniques. Here we investigate the electronic excitation of a single molecule with sub-molecular precision using a localised plasmon at the tip apex of a scanning tunnelling microscope (STM) as an excitation probe. Coherent energy transfer between the plasmon and molecular excitons is discovered when the plasmon is located in the proximity of isolated molecules, which is corroborated by a theoretical analysis. The polarised plasmonic field enables selective excitation of an electronic transition between anisotropic frontier molecular orbitals. Our findings have established the foundation of a novel single-molecule spectroscopy with STM, providing an integrated platform for real-space investigation of localised excited states.

Interplay between Plasmon Luminescence and Vibrationally Resolved Molecular Luminescence Induced by Scanning Tunneling Microscopy

The effects of coupling between a molecular exciton and a surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Greens function method. Molecular absorption and enhancement by molecular electronic and vibrational modes (molecular modes) lead to dip and peak structures in the luminescence spectra of the surface plasmons. These structures will correspond to the peak and dip structures observed in a recent experiment. In addition to the molecular dynamics, it is found that the reabsorption by surface plasmons plays an important role in determining the luminescence spectral profiles.

Effects of Interference between Energy Absorption Processes of Molecule and Surface Plasmons on Light Emission Induced by Scanning Tunneling Microscopy

A theoretical analysis of plasmon and molecular luminescence induced by scanning tunneling microscopy is carried out in the case of luminescence from clean metal surfaces using a molecule-covered tip and from molecular layers. The effects of coupling between molecular exciton and surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Greens function method. The enhancement by molecular electronic and vibrational modes (molecular modes) and the energy absorption by the creation of molecular excitons lead to peak and dip structures in the luminescence spectra of surface plasmons. The re-emission of energy in surface plasmons by exciton annihilation leads to a dent structure in their luminescence spectra. It is found that the processes of energy absorption by the creation of molecular excitons and the subsequent re-emission by exciton annihilation interfere with each other, resulting in the enhancement and supp...

Atomic-scale luminescence measurement and theoretical analysis unveiling electron energy dissipation at a p-type GaAs(110) surface

Luminescence of p-type GaAs was induced by electron injection from the tip of a scanning tunnelling microscope into a GaAs(110) surface. Atomically-resolved photon maps revealed a significant reduction in luminescence intensity at surface electronic states localized near Ga atoms. Theoretical analysis based on first principles calculations and a rate equation approach was performed to describe the perspective of electron energy dissipation at the surface. Our study reveals that non-radiative recombination through the surface states (SS) is a dominant process for the electron energy dissipation at the surface, which is suggestive of the fast scattering of injected electrons into the SS.

Effects of plasmon energetics on light emission induced by scanning tunneling microscopy

A theoretical model of plasmon and molecular luminescence induced by scanning tunneling microscopy using a molecule-covered tip on clean metal surfaces is developed. The effects of coupling between molecular exciton and interface plasmon on the luminescence spectra are investigated for variable energy of plasmon modes by means of the nonequilibrium Greens function method. It is found that spectral features arising from interference between the processes of energy absorption by the molecule and interface plasmons appear near the energy of the excitonic mode. For the energy of plasmon above (below) the energy of excitonic mode, an additional peak structure appears in the energy range slightly below (above) the energy of the excitonic mode. Prominent peak and dip structures observed in recent luminescence experiments are interpreted by the developed theory whereby its utility in the fields of plasmonics and nanophotonics is demonstrated.

Vibration-assisted upconversion of molecular luminescence induced by scanning tunneling microscopy

We investigate the effects of coupling between a molecular exciton, which consists of an electron and a hole in a molecule, and a surface plasmon (exciton-plasmon coupling) on the electron transitions of the molecule using nonequilibrium Green’s function method. Due to the exciton-plasmon coupling, excitation channels of the molecule arise in the energy range lower than the electronic excitation energy of the molecule. It is found that the electron transitions via these excitation channels give rise to the molecular luminescence and the vibrational excitations at the bias voltage lower than the electronic excitation energy of the molecule. Our results also indicate that the vibrational excitations assist the emission of photons, whose energy exceeds the product of the elementary charge and the bias voltage, (upconverted luminescence).

First-principles study on oxygen ion conduction of La2GeO5 based on the density functional theory

We performed first-principles simulations based on the density functional theory for investigations on the atomic geometry of La2GeO5, which is a fast oxygen ion conductor applicable to solid oxide fuel cells. While two experimental studies have reported contradicting results about the configuration of GeO4 tetrahedral substructures, i.e., sp2- or sp3-like, we found that only the sp3-like form is stable. We confirmed that the favorability of oxygen sites for vacancy formation is fundamentally affected by this configuration. The bonding mechanisms between atoms are discussed based on analyses of atomic distances and electronic density of states.