Dynamical behavior of multi-state on-surface molecular switching units

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BAUER, Anja Constanze, 2021. Dynamical behavior of multi-state on-surface molecular switching units [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Bauer2021Dynam-55233, title={Dynamical behavior of multi-state on-surface molecular switching units}, year={2021}, author={Bauer, Anja Constanze}, address={Konstanz}, school={Universität Konstanz} }

eng 2021 2021-10-13T05:51:08Z Bauer, Anja Constanze Dynamical behavior of multi-state on-surface molecular switching units terms-of-use A precise understanding of surface-supported molecular switching units is of great importance to further the realization of molecular active units in future information processing and storage devices in the field of molecular electronics. Regardless of whether they are excited by light, current or mechanical forces, their switching characteristics are determined by the shape of the adsorption potential, which may be influenced by the presence of the tip of a scanning tunneling microscope, by molecule-substrate and molecule-molecule interactions if they are embedded in two- dimensional (2D) networks. Thus, the ultimate ambition is to fully understand the response of the molecular switching unit to the induced changes of the shape of its adsorption potential to after all specifically adress and manipulate the system with having at best uprecendeted control. Individually adsorbed triazatruxene (TAT) molecules on a Ag(111) surface depict a perfect model system for molecular switching units, which may be incorporated in 2D molecular networks as molecular logic devices or storage cells. In this thesis TAT moleucles were electrospray deposited on a Ag(111) surface and investigated in detail by means of low-temperatur scanning tunneling microscopy (STM). High-resolution STM imaging of individually adsorbed TAT molecules yields four possible adsorption configurations caused by the formation of two surface enantiomers (S, R) and by the presence of two different orientations with respect to the Ag(111) lattice per surface enantiomer (atop-fcc and atop-hcp). Furthermore, three molecular types exist on the Ag(111) surface, which are characterized by differences in their molecule-substrate interactions being due to three possible ethyl configurations. The configuration with three upstanding ethyl moieties is labeled as TAT type 1. The configuration characterized by two/one upstanding and one/two surface coupled ethyl moieties are labeled as TAT type 2 and TAT type 3, respectively. The TAT type 1 molecules are most frequently represented (∼60%), followed by type 2 (∼37%) and type 3 molecules (∼3%). Time-dependent current-/z-traces of single TAT type 1 molecules show a tunneling current excited switching behavior between three conductance states, which are attributed to three energetically degenerate bonding configurations. The molecular switching may be driven in a particular direction, which is opposing for the two surface enantiomers at similar tunneling parameters, reaching values close to 100% directionality. The enantiomer dependent directionality implies that the chirality of the energy landscape of the adsorption potential is a vital component for directional switching. Tuning the tunneling parameters enables the manipulation and control of the adsorption potential’s symmetry to suppress or inverse the switching directionality. Modifications of the molecule-substrate interaction concerning the number of surface coupled ethyl moieties led to a stepwise suppression of the molecular switching. The switching behavior of free-standing TAT type 2 molecules is characterized by transitions between two non-degenerate conductance states, with one of both being clearly preferred. The pinning of one of the ethyl moieties to the substrate just leaves two free tails of the molecule to perform molecular switching. A two-fold pinning of the molecule, which is realized for type 3, completely suppresses the molecular switching. The required energy to induce the modifications of the molecule-substrate interaction and thus to strongly manipulate the shape of the adsorption potential may be either provided by the acceleration of ionized droplets during the electrospray deposition process or by tip-induced bias voltage ramps. The latter method allows for the controlled and targeted transition between the molecular types, and therefore enables to reversibly program the switching units by changing their molecule-substrate interaction. A further step towards the integration into complex electronic circuits was shown by the incorporation of TAT molecules in 2D honeycomb networks on Ag(111). The switching characteristics of TAT type 1 molecules in the honeycomb lattice are strongly affected by the intermolecular coupling. Regarding a probed active TAT (type 1) molecule surrounded by three nearest neighboring molecules in the honeycomb-structure four possible configurations concerning the type of the adjacent molecules are possible. The coupling of an active to solely inactive TAT molecules (type 2 is defined as an inactive switching unit if incorporated in the honeycomb-structure, type 3) leaves the number of states as three but affects the switching characteristics depending on the number of adjacent molecules. The coupling of at least two active TAT molecules leads to significant changes of the switching behavior, including the formation of modified current/z-states. The number of modified conductance states measured at an active TAT is proposed to be 3<sup>n</sup> with n being the number of involved active TAT switching units. Seven of them were identified for the coupling of two active TAT molecules. The arrangement of molecules in the honeycomb-structure reveals a further adsorption geometry per enantiomer which even enhances the probability of tip induced transitions between the molecular types. With the general ability of tip induced type transitions the targeted preparation of 2D configurations is possible, thus approaching the realization of molecular logic operations and the control of the number and characteristics of modified conductance states. Preliminary experiments on further dense layer structures and on derivatives of the TAT molecule (tris-triarylamine-TAT, tris-ferrocene-TAT) confirms the great potential of manipulation and control of molecular switching units provided by the TAT based molecules adsorbed on Ag(111). Bauer, Anja Constanze 2021-10-13T05:51:08Z

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