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

Abstract

Scanning tunneling microscope ͑STM͒-excited molecular fluorescence from H 2 TBP porphyrin ͑H 2 TBPP͒ thin films on Au ͑111͒, Ag, highly oriented pyrolytic graphite ͑HOPG͒, and indium tin oxide ͑ITO͒ surfaces has been investigated in air. Molecular fluorescence was observed from the H 2 TBPP 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 H 2 TBPP/ Ag is at least 100 times stronger than that from H 2 TBPP/ HOPG. Strong enhancement of molecular excitation by substrate surface plasmons is suggested for the STM-excited molecular fluorescence from H 2 TBPP only on the noble metal substrates.

Tunneling-Current-Induced Light Emission from Copper Phthalocyanine Thin Films

“…The other reason is that the fluorescence of CuPc might be enhanced by the surface plasmons on the Au(111) substrate [18][19][20]. It was reported that STM induced light emission from porphyrin molecules was enhanced by plasmon emission in the same energy range [21]. In our experiments, on the Au(111) surface, plasmon-mediated light emission was clearly observed, whereas HOPG did not have plasmon mode in the energy range of our experiment [22].…”

Substrate effect of STM-induced luminescence from porphyrin molecules

“…In the thickness range from 6 to 20 ML, we have observed that for the thicker films, molecular fluorescence is generated much more due to increase of the effectively decoupled emitter from the substrate [17]. Since MF can be observed for the metal substrates with plasmons and not for HOPG etc without plasmons [9], we can say that surface plasmons at least influence 20 ML molecular layers. This work now is still going on.…”

“…By now, the light emission mechanism for metal and semiconductor is well known. Recently, for some STML of organic molecules, characteristic spectra for the molecules, molecular fluorescence (MF), have been observed [6][7][8][9]. However, how molecular fluorescence (MF) is excited by STM is still unclear.…”

“…The metallic substrates exhibited strong plasmon-mediated STM-induced light emission but the later ones did not. Thus, we proposed a model for such strong molecular light emission named surface plasmon-enhanced molecular fluorescence (SEMF) [9]. Surface plasmon-enhanced phenomenon is well known in the other optical techniques like surface-enhance Raman scattering [12], second harmonic generation [13] and near-field microscopy [14], but the understanding in STML from molecules is still limited.…”

“…In recent years, scanning tunneling microscope-induced luminescence (STML) of organic molecules [2][3][4][5][6][7][8][9] has attracted much research effort due to its potential applications in nanometer scale molecular optoelectronic device, such as organic light-emitting diodes (OLED). The new physics in STML is photon emission at low electron energies.…”