Hongwen Liu

Plasmon-enhanced molecular fluorescence from an organic film in a tunnel junction

Scanning tunneling microscope (STM)-excited molecular fluorescence from H2TBP porphyrin (H2TBPP) thin films on Au (111), Ag, highly oriented pyrolytic graphite (HOPG), and indium tin oxide (ITO) surfaces has been investigated in air. Molecular fluorescence was observed from the H2TBPP films on Au and Ag, but it was extremely weak or undetectable from films on HOPG and ITO. The maximum intensity of molecular fluorescence from H2TBPP∕Ag is at least 100 times stronger than that from H2TBPP∕HOPG. Strong enhancement of molecular excitation by substrate surface plasmons is suggested for the STM-excited molecular fluorescence from H2TBPP only on the noble metal substrates.

Scanning Tunneling Microscope (STM)-Excited Molecular Fluorescence from Porphyrin Thin Films

Scanning tunneling microscope (STM)-excited molecular fluorescence from Meso-tetrakis (3,5-di-tertiarybutyl-phenyl) porphyrin (H2TBPP) thin films with 1–4 nm in thickness on gold (111) and highly oriented pyrolytic graphite (HOPG) surfaces has been respectively investigated in air at room temperature. The thin films of porphyrin were prepared by using spin-casting method. Molecular fluorescence emitted from porphyrin on Au(111) was observed, in company with plasmon-mediated light emission from gold substrate. No light is observed from porphyrin on HOPG. Mechanisms of such different luminescent properties of the STM junctions are discussed.

Topography Dependence of Tunneling-Induced Fluorescence from Porphyrin Film

We have measured spatially and spectrally resolved molecular fluorescence for H2TBPP film on Au excited by a scanning tunneling microscope (STM) in an ultrahigh vacuum (UHV) system. The tunneling-induced photon intensity map thus obtained shows a similar contrast to the STM grey-scale topography image, and indicates that the stronger tunneling luminescence comes from the higher topographies in STM image. This correlation is the same as the previously-reported correlation between the photon map of plasmon-mediated light emission for pure gold and its topography. This result supports the mechanism of surface-plasmon enhanced molecular fluorescence (SEMF), i.e., tunneling-electrons-induced molecular fluorescence in organic film is enhanced by the surface plasmons.

Study on Enhancement of Tunnelling-Induced Fluorescence from Porphyrin Film by Substrate Plasmon

We have studied the effect of the plasmon field on the STM-induced luminescence from porphyrin (H2TBPP) film on Au substrate. We have measured the spectrally resolved photon intensity map for the STM-induced luminescence, and found the similarity of their contrast for the photon maps for different spectral regions with different spectral origins, i.e. molecular fluorescence and the local plasmon induced luminescence from the substrate. This fact indicates that the molecular fluorescence is closely related with the plasmon field of the substrate, and supports that the STM-induced fluorescence from porphyrin film on Au is enhanced by the local plasmon field of the Au substrate.

Comparison of scanning tunneling microscope-light emission and photoluminescence from porphyrin films using ultra-high vacuum scanning tunneling microscopy

In order to study the interaction between molecules and photon fields, including plasmonic and external laser fields, we have carried out in situ measurements of photoluminescence (PL) from porphyrin molecules on Au substrates with and without a scanning tunneling microscope (STM) tip. Measurements were performed in a ultra-high vacuum scanning tunneling microscope chamber during irradiation by a He-Cd laser with incident power varying in the 10−3 to 10−7 W range. At an incident power of around 10−7 W, the spectra depend strongly on the presence of STM tip, which is associated with STM light emission from molecules. We estimated the ratio of quantum efficiency of scanning tunneling microscope-induced light emission (STML) from molecules to PL on the basis of the STML/PL intensity ratio observed experimentally at a laser power of 7.5 × 10−8 W, with the use of a 40 μm laser beam diameter and an effective area of 2 nm for STML. The estimated quantum efficiency for an electron in STML is roughly 1010 times la...