The light emitted by an electron current which passes through a tunnel junction has been studied since several decades. The luminescence is known to occur not only for planar junctions but also at point junctions as, for example, in a scanning tunneling microscope (STM) /1/. Recently, this investigation technique was successfully applied to detect the luminescence of single molecules at surfaces /2,3/.

We use this technique to study organic molecules and semi-conductor quantum dots. In general, we are interested in systems whose size is of the same order of magnitude as their Fermi wavelength and the electronic wave functions extend over significant parts of the system. Our experimental set-up allows the investigation of such quantum mechanical systems not only by tunneling electron spectroscopy but also with respect to their properties as electrically driven light sources. Moreover, we employ molecular luminescence as a tool to investigate on a nanometer-scale the interaction of single molecules with their local environment.

The above scheme shows the principle of a Photon-STM based on tunnel-current-induced electroluminescence: The tunneling condition is defined by the applied bias voltage and by the current between tip and sample. For a given sample both parameters together define the distance between tip and surface. The surface coordinates x and y are defined with sub-Angstrom precision due to the highly localized tunneling current from the tip apex. STMs operated at liquid He temperature can reach a mechanical stability which is sufficient to stay above a chosen atom over extended time intervals.

The high spatial resolution becomes obvious from the orbital map (dI/dV-map) of a single pentacene molecule separated from the metallic substrate by an ultrathin insulating layer (KCl). Tunneling conditions: bias voltage of -0.4 V , constant tunneling current of 0.3 nA.

In addition to the sub-molecular spatial resolution achievable with the STM our instrument allows to analyse emitted luminescence photons with respect to energy (wavelength), polarization, emission direction, and emission time. We can thus combine the virtues of two complementary methods and overcome their respective limitations. For a description of the experimental set-up, please see the Photon-STM facility page.

On the left hand side we show a spectrum of C₆₀ molecules as an example for tunnel-current-induced luminescence recorded with the STM tip on top of a small C₆₀ nanocrystal. Detailed features of the electronic origin (second lowest triplet state) and its vibronic progressions are visible. The sharp molecular lines emerge on a broad (580nm to 830nm) pedestal of emission from the localized metal plasmons (ca. 20 cts in the plot).

A study of such tip-induced plasmons obtained on a clean metal surface is shown below. On the left hand side the plasmon emission intensity is presented as a function of wavelength (horizontal axis) and STM bias voltage (vertical axis) for a constant tunneling current I = 5 nA. The contour plot shows that the maximum of the plasmon emission shifts to smaller wavelengths for larger tip-sample separation (higher absolute bias) towards the maximum frequency of the free surface plasmons. This behavior corresponds to the tuning of a plasmonic antenna for a piezo-controlled variation of the gap between tip and substrate. Here we touch the physics investigated by the aperture-less SNOM in our department.

References:

/1/ for example: J.K.Gimzewski et al. Z. Physik B: Cond. Mat. 72 (1988), 497

/2/ X.H.Qiu, G.V.Nazin, W.Ho,. Science 299 (2003) 542