Fonin, Mikhail
0000-0003-3015-0045 Berufsbeschreibung
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Multiscale Reciprocal Space Mapping of Magnetite Mesocrystals
Mesocrystals are a class of nanostructured material, where a multiple-length-scale structure is a prerequisite of many interesting phenomena. Resolving the mesocrystal structure is quite challenging due to their structuration on different length scales. The combination of small- and wide-angle X-ray scattering (SAXS and WAXS) techniques offers the possibility of non-destructively probing mesocrystalline structures simultaneously, over multiple length scales to reveal their microscopic structure. This work describes how high dynamical range of modern detectors sheds light on the weak features of scattering, significantly increasing the information content. The detailed analysis of X-ray diffraction (XRD) from the magnetite mesocrystals with different particle sizes and shapes is described, in tandem with electron microscopy. The revealed features provide valuable input to the models of mesocrystal growth and the choice of structural motif; the impact on magnetic properties is discussed.
Influence of surface and subsurface Co–Ir alloy on the electronic properties of graphene
Using density functional theory (DFT) calculations and angle-resolved photoemission spectroscopy (ARPES) the structural and electronic properties of graphene on the surface and subsurface Co–Ir alloy are investigated upon the intercalation of Co in graphene/Ir(111). It is found computationally that the interaction strength between graphene and substrate is strongly affected by the composition and nature of an alloy, implying the large difference in the electronic structure of monolayer graphene on CoxIr1−x/Ir(111) and Ir/CoxIr1−x/Ir(111). Our theoretical results are supported by ARPES data, which demonstrate the disappearance of the Dirac cone when graphene lies on Co and its restoration upon the formation of the subsurface Co–Ir alloy.
Tip‐Induced Inversion of the Chirality of a Molecule's Adsorption Potential Probed by the Switching Directionality
The switching behavior of surface‐supported molecular units excited by current, light, or mechanical forces is determined by the shape of the adsorption potential. The ability to tailor the energy landscape in which a molecule resides at a surface gives the possibility of imposing a desired response, which is of paramount importance for the realization of molecular electronic units. Here, by means of scanning tunneling microscopy, a triazatruxene (TAT) molecule on Ag(111) is studied, which shows a switching behavior characterized by transitions of the molecule between three states, and which is attributed to three energetically degenerate bonding configurations. Upon tunneling current injection, the system can be excited and continuously driven, showing a switching directionality close to 100%. Two surface enantiomers of TAT show opposite switching directions pointing at the chirality of the energy landscape of the adsorption potential as a key ingredient for directional switching. Further, it is shown that by tuning the tunneling parameters, the symmetry of the adsorption potential can be controllably adjusted, leading to a suppression of the directionality or an inversion of the switching direction. The findings represent a molecule‐surface model system exhibiting unprecedented control of the shape of its adsorption potential.
Bulk-Like Magnetic Signature of Individual Fe4H Molecular Magnets on Graphene
Single-molecule magnets (SMMs) incorporate key properties that make them promising candidates for the emerging field of spintronics. The challenge to realize ordered SMM arrangements on surfaces and at the same time to preserve the magnetic properties upon interaction with the environment is a crucial point on the way to applications. Here we employ inelastic electron tunneling spectroscopy (IETS) to address the magnetic properties in single Fe4 complexes that are adsorbed in a highly ordered arrangement on graphene/Ir(111). We are able to substantially reduce the influence of both the tunneling tip and the adsorption environment on the Fe4 complex during the measurements by using appropriate tunneling parameters in combination with the flat-lying Fe4H derivative and a weakly interacting surface. This allows us to perform noninvasive IETS studies on these bulky molecules. From the measurements we identify intermultiplet spin transitions and determine the intramolecular magnetic exchange interaction constant on a large number of molecules. Although a considerable scattering of the exchange constant values is observed, the distribution maximum is located at a value that coincides with that of the bulk. Our findings confirm a retained molecular magnetism of the Fe4H complex at the local scale and evaluate the influence of the environment on the magnetic exchange interaction.
Imaging the Single-Electron Ln-Ln Bonding Orbital in a Dimetallofullerene Molecular Magnet
Chemically robust single-molecule magnets (SMMs) with sufficiently high blocking temperatures TB are among the key building blocks for the realization of molecular spintronic or quantum computing devices. Such device applications require access to the magnetic system of a SMM molecule by means of electronic transport, which primarily depends on the interaction of magnetic orbitals with the electronic states of the metallic electrodes. Scanning tunneling microscopy in combination with ab initio calculations allows to directly address the unoccupied component of the single-electron molecular orbital that mediates the ferromagnetic exchange coupling between two 4f ions within a lanthanide endohedral dimetallofullerene deposited on a graphene surface. The single-electron metal-metal bond provides a direct access to the molecule's magnetic system in the transport experiments, paving the way for investigation and controlled manipulation of the spin system of individual dimetallofullerene SMMs, essential for molecular spintronics.
Hydrogen-intercalated graphene on SiC as platform for hybrid superconductor devices
Moyamoya disease is a unique cerebrovascular disorder that is characterized by chronic bilateral stenosis of the internal carotid arteries and by the formation of an abnormal vascular network called moyamoya vessels. In this stury, the authors inspected whether differentiation between patients with moyamoya disease and those with atherosclerotic disease or normal controls might be possible by using deep machine learning technology.
Electrospray Deposition and Magnetic Properties of Prototypical Molecular Magnets
The controlled deposition, characterization and manipulation of single molecule magnets (SMMs) on surfaces is one of the crucial points to be addressed with regard to their possible implementation as functional units in future electronic and spintronic devices. Profound understanding of molecule-molecule and molecule-substrate interactions is required as well as unraveling their effect on the molecular electronic and magnetic properties. Local measurement techniques like scanning tunneling microscopy (STM) together with ensemble-averaging methods like X-ray absorption spectroscopy (XAS) have been proven to set up an appropriate frame to study these materials. The majority of these studies deal with SMMs that exhibit rather simple structures with mostly only one magnetic ion. The situation gets more complicated when it comes to larger polynuclear compounds that can be quite fragile with respect to surface deposition or not easy to organize on surfaces due to their bulky ligand shell. Here, we provide an overview of our results on successful deposition of polynuclear SMMs on functional surfaces by employing the electrospray ion beam deposition method. For two prototypes in the field, Mn12-ac and Fe4H, we obtain highly ordered submonolayers on functional surfaces and elucidate the electronic coupling to the respective substrates using scanning tunneling spectroscopy (STS). New results for Mn12-ac on graphene/Ir(111) and for Fe4H on Au(111) are compared to previous studies on a decoupling graphene layer. X-ray magnetic circular dichroism (XMCD) measurements on submonolayers of uniformly aligned Fe4 molecules on both substrates reveal its robust magnetism, showing magnetic anisotropy values similar to bulk.
Exceptionally high blocking temperature of 17 K in a surface-supported molecular magnet
Single-molecule magnets (SMMs) are among the most promising building blocks for future magnetic data storage or quantum computing applications, owing to magnetic bistability and long magnetic relaxation times. The practical device integration requires realization of 2D surface assemblies of SMMs, where each magnetic unit shows magnetic relaxation being sufficiently slow at application-relevant temperatures. Using X-ray absorption spectroscopy and X-ray magnetic circular dichroism, it is shown that sub-monolayers of Dy2 @C80 (CH2 Ph) dimetallofullerenes prepared on graphene by electrospray deposition exhibit magnetic behavior fully comparable to that of the bulk. Magnetic hysteresis and relaxation time measurements show that the magnetic moment remains stable for 100 s at 17 K, marking the blocking temperature TB(100) , being not only in excellent agreement with that of the bulk sample but also representing by far the highest one detected for a surface-supported single-molecule magnet. The reported findings give a boost to the efforts to stabilize and address the spin degree of freedom in molecular magnets aiming at the realization of SMM-based spintronic units.
Quantum Well States for Graphene Spin-Texture Engineering
The modification of graphene band structure, in particular via induced spin–orbit coupling, is currently a great challenge for the scientific community from both a fundamental and applied point of view. Here, we investigate the modification of the electronic structure of graphene (gr) initially adsorbed on Ir(111) via intercalation of one monolayer Pd by means of angle-resolved photoelectron spectroscopy and density functional theory. We reveal that for the gr/Pd/Ir(111) intercalated system, a spin splitting of graphene π states higher than 200 meV is present near the graphene K point. This spin separation arises from the hybridization of the graphene valence band states with spin-polarized quantum well states of a single Pd layer on Ir(111). Our results demonstrate that the proposed approach on the tailoring of the dimensionality of heavy materials interfaced with a graphene layer might lead to a giant spin–orbit splitting of the graphene valence band states.
Sacrificial Templating : A Route to Europium-II Oxide (EuO) Particles with Arbitrary Shape Prepared Indirectly by Hostile Takeover
Shape–property correlations of nanocrystals have recently moved into focus in materials science research. Magnetic properties, for instance, depend strongly on shape. Because crystal morphology is determined by a certain set of lattice planes (hkl) representing the surfaces, the achievable shapes are set by crystallographic symmetry. For instance, for a cubic crystal (90° angles) system it is very hard to realize hexagonal crystal shapes (120° angles). Breaking this paradigm is, thus, highly challenging. Here, we present a synthesis concept suitable for the synthesis of atypical Europium(II) oxide (EuO) particles. EuO is interesting as it is one of few materials known that belong to the class of intrinsic ferromagnetic semiconductors. It is shown that ZnO nanoparticles (rod-like, hexagonal platelets, spherical, and dumbbells) act as a sacrificial template and can be converted to the corresponding EuO particles by preservation of structural features, when treated with Eu-vapor. The transformation proceeds via a Kirkendall mechanism. This new reaction pathway enhances the accessibility of EuO particles tremendously and enables systematic studies on the magnetic shape property relationships of this highly functional metal oxide.