Working Group Quantum Control

Tailor made functionalized tripodal molecules:
Towards molecular electronics

We use a home-built 5 K STM to address functional tripodal molecules on surfaces as to control and monitor their state. We therefore developed various novel experimental methodologies in order to controllably position, characterize, address and switch single molecular functional units.

One example is an electrically actuated molecular toggle switch where we used a spirobifluorene variant with a well-defined spatial arrangement of the nitrile head group. The polar group can be acted on by electrostatic forces by application of electric fields in the tunneling junction [1]. We recently extended this research with the realization of molecular rotors [2] and electrically driven molecular motors (see Fig. 1). Manipulation of individual molecules within a self-assembled structure allows writing and reading information on the nanoscale [3] and to study the intermolecular interactions (see Fig. 2) [4].

Fig.1: Top: Schematics of the relative position of the STM tip with respect to a tripodal molecule featuring a rotatable head group with three metastable states labelled A, B, C. Bottom: Time traces of the current show three distinct current levels according to the rotational state of the head group.

Fig.2: STM images at negative voltage (a,c) show different orientations of the molecular head group before (a) and after (c) scanning at positive voltage (b) for switching . The switching of individual head groups that carry an electric dipole is restricted but correlated (d,e) [4].

Optimized molecular light emitters:
Increased quantum yield and single-photon sources

The 1K scanning tunneling microscope (STM) developed in the group is equipped with a micro fabricated parabolic mirror with the STM tip in the focus that then guides the light into an optical fiber [5]. The mirror tip structure manufactured with 3D lithography gives us a significantly increased light collection efficiency in comparison with lens systems. The light emitted from the STM junction can then be guided into a grating spectrometer for spectral analysis or to a Hanbury-Brown Twiss interferometer that allows to determine the time correlation of the photon train (see Fig. 3).

The energy of the tunneling electrons can lead to various excitations in the STM junction and potentially to light emission. STM-induced luminescence (STML) allows to investigate light emission properties of atomic scale materials regions as defined by the tunnel contact area in combination with the high lateral and energy resolution of STM [6,7].

Our main focus is the investigation of electroluminescence of single molecules in the STM junction. STML combines photon spectroscopy from single molecules with the high lateral and energy resolution of STM. In this way, the adsorption configuration of single molecules and the shape and energy of the molecular orbitals can be identified, and correlated with the light emission spectrum.

In contrast to conventional photoluminescence spectroscopy, STML also allows to selectively inject holes/electrons into specific orbitals and to charge individual molecules and by this to observe not only charge neutral transitions but also transitions involving charged excited states [8],[9].

Electroluminescence from single molecules contacted by metallic leads imposes conflicting demands for the coupling between the molecule and the metal: To conduct the necessary tunneling current, a certain hybridization is necessary. At the same time, however, the chromophore needs to be decoupled to prevent fluorescence quenching. While typically a thin insulating layer of NaCl between the metal surface and the molecule is used [8], we also aim for an alternative approach where the insulating spacer is integrated into the molecular design [7,9].

Fig.3: Home-built STM operating in ultra-high vacuum and at temperatures down to 1.5 K. The STM tip is integrated in a parabolic mirror manufactured with 3D lithography. An optical fiber allows to guide light from the STM junction to a grating spectrometer or a two photon counting modules of a Hanbury-Brown Twiss interferometer.

Fig.4: Electrically-induced light emission from single molecules: STM allows inject the tunneling current at specific energies and into specific orbitals of the molecule (a). Light emission from positively charged Phthalocyanine molecules (b) is boosted by a factor of 20 compared to light emission from the neutral molecule [8]. (c) NDI chromophores in a self-assembled monolayer deposited on a Au(111) surface can be activated with the STM tip to decouple the relevant frontier orbitals. (d) In this way, individual molecules can be driven from a strongly hybridized state with quenched luminescence to a light-emitting state [9].