STM-induced light emission from organic multilayer structures
N. Okawa, A. Okada, K. Kanazawa, K. Hayashi, O. Takeuchi and H. Shigekawa
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba, 305-8573, Japan
http://dora.ims.tsukuba.ac.jp

Intensity[arb.units]

Intensity[arb.units][Introduction]
Scanning tunneling microscopy induced light emission (STM-LE) enables us to obtain
information about local optical properties in a sample surface. Although several mechanisms,
such as surface-plasmon-mediated processes for organic-monolayer systems formed on Au andAg surfaces, have been proposed to explain STM-LE spectra, details are still uncovered.
Furthermore, experiments have been performed, in general, for the basic simple structuresinstead of practically applicable structures.
In this paper, we present some of our recent results of STM-LE study performed onelectro-luminescent multilayer structures, aiming at further advance of this interesting method.
[Experiment]
An Alq3/rubrene/Au multilayer structure, for example, was prepared as a sample for experiment.
The organic films were grown by vacuum deposition, and the film thickness of each layer wasmonitored using a QCO. As shown by the schematic illustration in Fig. 1(a), the rubrenemolecular layer has a quantum well sandwiched by the metal substrate and the Alq3 layer, which
confines injected electrons and holes in the layer of rubrene molecules and increases theefficiency of light emission through their combination at the local quantum well. Measurements
were performed at room temperature in air and at about 20 K in vacuum. A mechanically cut PtIr
was used as a STM tip. The light emitted from the tunnel junction was focused onto an entranceedge of an optical fiber and introduced into a monochromater. A CCD was used for detection.
Optical measurements were performed after STM observation to image the topography of thearea. At about 20 K in vacuum, we could stabilize themolecular structures and succeeded in obtaining
reproducible STM-LE spectra.
(a)PL[Results and discussion]
Figure 1(a) shows a photoluminescence (PL) spectrum of
rubrene powder. For an electroluminescence (EL) of thepresent multilayer structure, photon emission of thiswavelength is expected to occur through the combinationof electrons and holes at the middle (rubrene) layer asshown in an inset of the figure.
Figure 1(b) shows STM-LE spectra obtained by repeatedmeasurementsforthesameplaceofan(b)STM-LE
Alq3/rubrene/TPD/Au sample in air at room temperature.
A prominent signal was observed at about 600 nm for theAu plasmon
first measurement (thick line). This component is close to
the PL spectrum of rubrene shown in Fig. 1(a), as
expected. The 600 nm-signal was weakened in the second
measurement, and the signal from Au surface plasmon
became dominant (thin line). This results suggests
4006008001000
desorption of molecules under this experimental condition,
Wavelength/nm
which supports that the signal is originated from therubrene molecules in the layered structure.
Fig.1 (a) A PL spectrum from rubrene
powder (Inset: Energy diagram of the layerThis is the first demonstration of the STM-LEstructure); (b) A sequential STM-LE spectra
measurement of an organic-multilayer structuresfrom Alq3/rubrene/TPD/Au obtained in air
practically designed for an optical device.
at room temperature (Bias voltage: 5.0 V,In addition, we developed a wavelength-resolved 2D
tunneling current: 3 nA, exposure time ofphoton mapping system and investigated the spectral
CCD: 128 sec/spectrum). Thick and thinlines stand for first and second
intensity distributions in detail. The results, together withmeasurements.
those from other systems, will be introduced and discussed
at the colloquium.
electronTip

Au

hole

organic
layertunnelgap