Single-molecule luminescence and absorption spectroscopy with a photon-STM

DIPC Seminars

You-Soo Kim (RIKEN, Japan)
Donostia International Physics Centre
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Single-molecule luminescence and absorption spectroscopy with a photon-STM Single-molecule luminescence and absorption spectroscopy with a photon-STM Yousoo Kim Surface and Interface Science Laboratory, RIKEN E-mail: Excitation of molecules by light irradiation triggers various important processes including luminescence, photovoltaic effect and photochemical reactions, and detailed understanding of the molecular excited states is crucial to improve organic opto-electronic devices. Absorption spectroscopy is a powerful tool to describe the molecular excitations and the combination with emission (luminescence) spectroscopy which deals with deexcitation processes is effective to investigate the excited states. Single-molecule luminescence detection has progressed rapidly and become indispensable in quantum physics, physical chemistry, and biophysics. However, despite considerable effort and progress, absorption spectroscopy is far behind; a number of molecules are still necessary to obtain an absorption spectrum. A difficulty lies in the difference between the diffraction limit of excitation light and absorption cross section of a single molecule. Here I introduce our recent progress in measurement of the single molecule luminescence and absorption spectra of a single molecule using a scanning tunnelling microscope (STM) equipped with optical detection facilities. In this talk, I will address two main issues with our experimental efforts on investigating interaction of electrons with a single molecule during excitation/deexcitation processes of a single free-base phthalocyanine (H2Pc) on the ultrathin insulating films grown on a metal substrate. The first part is assigned to single-molecule chemistry and luminescence spectroscopy. From a H2Pc, a new phthalocyanine [H0Pc]2- was produced by means of single molecule chemical reaction by injecting tunnelling electrons from the STM tip. Scanning tunnelling luminescence (STL) spectra of the H2Pc exhibit intrinsic fluorescence around 1.5-1.8 eV which agrees well with a previously reported fluorescence spectrum. STL spectra of [H0Pc]2- show a low-energy luminescence peak at 1.33 eV in addition to fluorescence peaks around 1.5-1.8 eV, which indicates that [H0Pc]2- has much different luminescence properties from H2Pc. Time-dependent density functional theory calculation of gas-phase [H0Pc]2- predicts that energy of the first triplet excited state T1 is about 1.3 eV, which suggests that the newly discovered low-energy luminescence is due to phosphorescence of [H0Pc]2-. The second part focuses on measurement of absorption spectra from a single H2Pc molecule. This is achieved by reduction in the size of the excitation source down to nanometre scale; we found that localized plasmons at the apex of the tip of STM can be considered as a point excitation source driven by the tunnelling current. Sharp dips corresponding to the molecular excitations appear in a broad spectrum of the localized plasmon emission obtained when the STM tip is placed in the proximity (2-3 nm) of the molecule, clearly indicating that the energy of the localized plasmon is absorbed by the single molecule.