Hiroshi Imada

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.

Supramolecular Assembly through Interactions between Molecular Dipoles and Alkali Metal Ions

Establishing a way to fabricate well-ordered molecular structures is a necessary step toward advancement in organic optoelectronic devices. Here, we propose to use interactions between electric dipoles of molecules and alkali metal ions to form a well-developed homogeneous monolayer of diarylethene molecules on the Cu(111) surface with the aid of NaCl co-deposition. Scanning tunneling microscopy and density functional theory calculation results indicate that the formation of a row-type structure occurs as a result of interactions between the Na(+) ions and the diarylethene molecular dipoles, drastically changing the adsorption configuration from that without Na(+).

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.

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.

Adsorption-induced stability reversal of photochromic diarylethene on metal surfaces.

A scanning tunneling microscopy study of a photochromic diarylethene adsorbed on Au and Cu reveals the reversal of thermodynamic stability between its two isomers compared to the gas phase, in solution, and its single crystal, which could result from the molecule-substrate interaction such as charge transfer and hybridization.

Atom-Resolved Luminescence of Si(111)-7×7 Induced by Scanning Tunneling Microscopy

Photon emission induced by scanning tunneling microscopy (STM), i.e., scanning tunneling luminescence (STL), is widely believed to reveal the inherent optical properties of nanostructures with high spatial resolution comparable to that of STM. We investigated STL of Si(111)-7×7 using silver STM tips. The photon emission via inherent electronic transitions of Si(111)-7×7 is strongly enhanced by surface plasmon localized in the tunnel junction, allowing the surface atomic configurations to be clearly visualized by luminescence mapping. The STL maps suggest that the local optical properties of non-equivalent adatoms are different.