Exploring Resonant Charge Transport Through Individual Free-Standing Molecules With Atomic-Scale Precision
Niklas Friedrich
Universität Regensburg, 93053 Regensburg, Germany, niklas [dot] friedrich [at] ur [dot] de (niklas[dot]friedrich[at]ur[dot]de)
Coherent electron transport through individual molecules is a vital technique to probe their intrinsic quantum mechanical propertieslike energy level alignment, vibronic modes or spin states. A precise understanding of the physical processesinvolved in the electron transportis vital for embedding molecules in quantum technologies, like quantum computing or quantum sensing.
Using a low-temperature (4K) scanning tunneling microscope allows to manipulatesinglemolecules and their environment withatomic-scaleprecision. We performedlifting experiments with individual molecules, creating tunneling-transport junctions, and exploredthehigh-bias regime ofresonant and above-resonant transport conditions with tip-substrate distances of few nanometers.
In this seminar, I willfirstdiscuss the electronic transport through 7-armchair graphene nanoribbons (GNRs) containing a single, substitutionally embedded 2B-dimer. The 2B-dimer creates a topologically protected in-gap state inside the ribbon [1].We find that the coherent charge transport through the 2B-GNR is unipolar with the singly occupied 2B-state (O2B) enabling resonant holetunneling at either voltage polarity[2]. The unipolar transport is enabledby the exponential localization of the O2B aroundthe 2B-dimer resulting in a double tunneling barrier configuration.An intrinsic geometrical asymmetry of this tunneling junction leads to a diode-like behavior of the GNR with a rectification ratio that is mechanically tunable by six orders of magnitude.We map the I-V-transport spectrum over several nanometers tip-substrate separation and developa simple analytical model describing the resonance conditions.
Second, I will presentthe fluorescent properties of a PTCDA-functionalized tip. We find that the molecule maintains its electroluminescent character despite a direct metal-molecule contact[3]. Exploiting the picometer positioning capabilities of STM we provide a proof-of-concept of an atomic-scale luminescent sensor sensitive to the electrostatic and electrodynamic environment.
[1] N. Friedrich, P. Brandimarte et al., Phys. Rev. Lett. 125, 146801 (2020)
[2] N. Friedrich et al., Adv. Mater. 36, 2401955(2024)
[3] N. Friedrich et al., arXiv: 2311.16805