Transport and optoelectronic properties of single molecules investigated by STM

Orateur : Gaël REECHT (FU Berlin)

Understanding and controlling the electronic and optoelectronic properties of a single molecule directly bridging metallic electrodes is a cha llenging key issue towards molecular optoelectronics. In 2009, Lafferentz et al. demonstrated that a scanning tunneling microscope (STM) can be used to lift controllably a molecular wire, in order to study charge transport through individual molecules [1]. With this approach, I will present two different studies. First, a unique π-conjugated polymer chain of thiophen is lift with the tip of the STM . We measure simultaneously electronic current and emitted light of this molecular junction. We observe that electron induced light emission is highly voltage and polarity dependent. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Our molecular contact junction can therefore be viewed as a single molecular wire light emitting diode. With this work, we demonstrate that using 1D molecules we can overcome the quenching of fluorescence for molecules in direct contact with a metal [2-3]. Then, we study the electronic structure and the transport properties of the open and closed isomers of a sulfur-free diarylethene on a Au(111) surface [4]. The electronic structure is determined with scanning tunneling spectroscopy (STS) for the molecule lying on the surface. Between the two isomers, intriguing differences of the energy and the spatial extend of the molecule orbitals are observed. We then lift the two isomers with the tip of the STM and measure the current passing through the individual molecules. We observe an important difference of conductance between the two forms. With a simple analytical model of transport based on the results of the STS measurements, we show that the previously determined orbital characteristics are essential ingredients for the complete understanding of the transport properties.

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