Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of <0.7 ns. To clarify whether C60 films can be used as electronic decoupling layers, the electronic structure of individual Ir(ppy)3 molecules is investigated on various C60 film thicknesses. While the single molecules on a C60 monolayer exhibit a vacuum level alignment, their electronic structure on C60 bi- and trilayers suggests a charge transfer to the C60 film. Hence, C60 films are less suitable as inert decoupling layers. Nevertheless, single Ir(ppy)3 molecules adsorbed on various C60 film thicknesses prove to be an ideal model system to study the influence of molecules on the SPP excitation in tunnel junctions. Surprisingly, the level of control systematically increases from one to three C60 layers due to the changing electronic structure.

Kern, Klaus
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis6733-5

 Record created 2015-08-19, last modified 2018-03-17

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