Abstract
We have investigated the emission properties of N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide thin films by the tunneling-electron-induced light emission technique. A fluorescence peak with vibronic progressions with large Stokes shifts was observed on both highly ordered pyrolytic graphite (HOPG) and Au substrates, indicating that the emission was derived from the isolated-molecule-like film condition with sufficient π-π interaction of the perylene rings of perylenetetracarboxylic diimide molecules. The upconversion emission mechanism of the tunneling-electron-induced emission was discussed in terms of inelastic tunneling including multiexcitation processes. The wavelength-selective enhanced emission due to a localized tip-induced surface plasmon on the Au substrate was also obtained.
Highlights
Control of molecular emission from organic materials has attracted much attention owing to its potential applications in basic molecular science and in research on soft material devices such as organic light-emitting diodes (OLEDs) and biosensors [1,2,3,4]
Homogeneous emissions were observed from the entire scanned area in both Figure 1b,d, so that homogeneous and smooth Perylenetetracarboxylic diimide (PTCDI)-C7 thin films were formed on both the substrates, which showed a good correspondence of the STM topographic images
In the Scanning-tunneling-microscope-induced light emission (STM-LE) measurement in this study, the tip was placed in contact with the thin film under our high-current condition; as a result, the tips might have swept molecules during the scan, in which tunneling electrons directly passed through the thin film to the substrate without an air gap between the tip and the sample
Summary
Control of molecular emission from organic materials has attracted much attention owing to its potential applications in basic molecular science and in research on soft material devices such as organic light-emitting diodes (OLEDs) and biosensors [1,2,3,4]. Scanning-tunneling-microscope-induced light emission (STM-LE) spectroscopy is highly effective for characterizing the optical and electronic properties of nanoscale materials such as organic single molecules or thin films at the atomic scale. It involves serious analytical difficulties in receiving extremely weak signals from the objective materials. We have studied the STM-LE from N,N’-diheptyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C7) thin films on HOPG and Au substrates. We elucidated the intrinsic optical properties of PTCDI-C7 in terms of the STM-LE spectra on the HOPG substrate compared with the absorption and photoluminescence (PL) spectra, and demonstrated the wavelength control of enhanced. The absorption and photoluminescence (PL) spectra of PTCDI-C7 were obtained using a UV-visible/NIR spectrophotometer (Hitachi HighTechnologies Co., Japan, U-3010) and a custom-built system with an argon-ion laser (Edmond Optics, USA, Multi-Line 150 mW) at 514 nm, respectively
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