Department of Physics, NCU
Light at the Nanotip – Optical Imaging of Single Molecules and Nanometric Chemical Identification
Dr. Chi Chen (陳祺)
Assistant Research Fellow
Research Center for Applied Science, Academia Sinica
In this talk, two topics based on the optical enhancement of the localized surface plasmon on the tip apex of a scanning tunneling microscope (STM) will be presented.
First, submolecular resolution electroluminescence imaging of intramolecular transition probability will be discussed. Photon imaging of ring-like pattern with 4-fold symmetry is achieved by integrating a STM and optical detection systems. The 4-fold symmetric distribution is further resolved as two orthogonal vibronic transitions after incorporating spectral selectivity. This photon emission is through an intramolecular transition from two orthogonal LUMOs to a HOMO state, which is coupled and enhanced by the localized surface plasmon in the STM junction. This novel imaging technique opens an innovative approach to probe the electron-molecule-photon coupled system such as dye sensitized solar cell or organic light emitting diode.
Second, tip-enhanced Raman spectroscopic (TERS) imaging of the individual Raman modes of carbon nanotubes (CNT) is presented in real space with 1.7 nm lateral resolution based on the enhancement and positioning of the STM tip. This is the best resolution for optics, photon based spectroscopy, and chemical analysis in the ambient. With superior resolution, we start to observe beautiful underlying physical chemistry of CNTs, such as diameter effect, local defect, and bundling effect in real space and further distinguish one of the CNTs as a multiwall CNT. Comparing to TEM-EELS or STM-IETS, we do not require UHV and low temperature environment to reach 1 nm scale chemical analysis. This makes STM-TERS a unique analytical tool for molecules, soft materials, and biological sample with ultrahigh spatial resolution at ambient condition.