Electric-field control of single-molecule tautomerization
2020-03-18, Mangel, Shai, Skripnik, Maxim, Polyudov, Katharina, Dette, Christian, Wollandt, Tobias, Punke, Paul, Li, Dongzhe, Urcuyo, Roberto, Pauly, Fabian, Kern, Klaus
The electric field is an important parameter to vary in a single-molecule experiment, because it can directly affect the charge distribution around the molecule. Yet, performing such an experiment with a well-defined electric field for a model chemical reaction at an interface has proven to be extremely difficult. Here, by combining a graphene field-effect transistor and a gate-tunable scanning tunneling microscope (STM), we reveal how this strategy enables the intramolecular H atom transfer of a metal-free macrocycle to be controlled with an external field. Experiments and theory both elucidate how the energetic barrier to tautomerization decreases with increasing electric field. The consistency between the two results demonstrates the potential in using electric fields to engineer molecular switching mechanisms that are ubiquitous in nanoscale electronic devices.
Large Conductance Variations in a Mechanosensitive Single-Molecule Junction
2018-09-12, Stefani, Davide, Weiland, Kevin J., Skripnik, Maxim, Hsu, Chunwei, Perrin, Mickael L., Mayor, Marcel, Pauly, Fabian, van der Zant, Herre S. J.
An appealing feature of molecular electronics is the possibility of inducing changes in the orbital structure through external stimuli. This can provide functionality on the single-molecule level that can be employed for sensing or switching purposes if the associated conductance changes are sizable upon application of the stimuli. Here, we show that the room-temperature conductance of a spring-like molecule can be mechanically controlled up to an order of magnitude by compressing or elongating it. Quantum-chemistry calculations indicate that the large conductance variations are the result of destructive quantum interference effects between the frontier orbitals that can be lifted by applying either compressive or tensile strain to the molecule. When periodically modulating the electrode separation, a conductance modulation at double the driving frequency is observed, providing a direct proof for the presence of quantum interference. Furthermore, oscillations in the conductance occur when the stress built up in the molecule is high enough to allow the anchoring groups to move along the surface in a stick–slip-like fashion. The mechanical control of quantum interference effects results in the largest-gauge factor reported for single-molecule devices up to now, which may open the door for applications in, e.g., a nanoscale mechanosensitive sensing device that is functional at room temperature.
Magnetic switching of nanoscale antidot lattices
2016-05-24, Wiedwald, Ulf, Gräfe, Joachim, Lebecki, Krzysztof, Skripnik, Maxim, Haering, Felix, Schütz, Gisela, Ziemann, Paul, Goering, Eberhard, Nowak, Ulrich
We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices.
Ptychographic imaging and micromagnetic modeling of thermal melting of nanoscale magnetic domains in antidot lattices
2020, Gräfe, Joachim, Skripnik, Maxim, Dieterle, Georg, Haering, Felix, Weigand, Markus, Bykova, Iuliia, Träger, Nick, Stoll, Hermann, Tyliszczak, Tolek, Nowak, Ulrich
Antidot lattices are potential candidates to act as bit patterned media for data storage as they are able to trap nanoscale magnetic domains between two adjacent holes. Here, we demonstrate the combination of micromagnetic modeling and x-ray microscopy. Detailed simulation of these systems can only be achieved by micromagnetic modeling that takes thermal effects into account. For this purpose, a Landau–Lifshitz–Bloch approach is used here. The calculated melting of magnetic domains within the antidot lattice is reproduced experimentally by x-ray microscopy. Furthermore, we compare conventional scanning transmission x-ray microscopy with resolution enhanced ptychography. Hence, we achieve a resolution of 13 nm. The results demonstrate that ptychographic imaging can also recover magnetic contrast in the presence of a strong topological variation and is generally applicable toward magnetic samples requiring ultimate resolution.
Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions
2018-09-12, Miao, Ruijiao, Xu, Hailiang, Skripnik, Maxim, Cui, Longji, Wang, Kun, Pedersen, Kim G. L., Pauly, Fabian, Meyhofer, Edgar, Reddy, Pramod, Linke, Heiner
Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring ( para-OPE3) and meta-connected central ring ( meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.
Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy
2016-03-28, Gräfe, Joachim, Weigand, Markus, Träger, Nick, Schütz, Gisela, Goering, Eberhard J., Skripnik, Maxim, Nowak, Ulrich, Haering, Felix, Ziemann, Paul, Wiedwald, Ulf
While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media.
Charge transport in a single molecule transistor probed by scanning tunneling microscopy
2018, Bouvron, Samuel, Maurand, Romain, Graf, Alexander, Erler, Philipp, Gragnaniello, Luca, Skripnik, Maxim, Wiedmann, Dirk, Engesser, Clara, Pauly, Fabian, Fonin, Mikhail
We report on the scanning tunneling microscopy/spectroscopy (STM/STS) study of cobalt phthalocyanine (CoPc) molecules deposited onto a back-gated graphene device. We observe a clear gate voltage (Vg) dependence of the energy position of the features originating from the molecular states. Based on the analysis of the energy shifts of the molecular features upon tuning Vg, we are able to determine the nature of the electronic states that lead to a gapped differential conductance. Our measurements show that capacitive couplings of comparable strengths exist between the CoPc molecule and the STM tip as well as between CoPc and graphene, thus facilitating electronic transport involving only unoccupied molecular states for both tunneling bias polarities. These findings provide novel information on the interaction between graphene and organic molecules and are of importance for further studies, which envisage the realization of single molecule transistors with non-metallic electrodes.
Combined FORC and X-Ray Microscopy Study of Magnetisation Reversal in Antidot Lattices
2015, Gräfe, Joachim, Haering, Felix, Stahl, Christian, Weigand, Markus, Skripnik, Maxim, Nowak, Ullrich, Ziemann, Paul, Wiedwald, Ulf, Schütz, Gisela, Goering, Eberhard