Ferrocenyl-Substituted Triazatruxenes: Synthesis, Electronic Properties, and the Impact of Ferrocenyl Residues on Directional On-Surface Switching on Ag(111).

We report on seven new ferrocenyl-(1, 3)- and ferrocenylethynyl-modified N,N',N″-triethyltriazatruxenes (EtTATs) 4-7 as well as the dodecyl counterpart 2 of compound 1 and their use as molecular switching units when deposited on a Ag(111) surface. Such functional units may constitute a new approach to molecule-based high-density information storage and processing. Besides the five compounds 1-3, 6, and 7, where the 3-fold rotational symmetry of the triazatruxene (TAT) template is preserved, we also included 2-ethynylferrocenyl-TAT 4 and 2,2'-di(ethynylferrocenyl)-TAT 5, whose mono- and disubstitution patterns break the 3-fold symmetry of the TAT core. Voltammetric studies indicate that the ferrocenyl residues of compounds 1-7 oxidize prior to the oxidation of the TAT core. We have noted strong electrostatic effects on TAT oxidation in the 2,2',2″-triferrocenyl-TAT derivatives 1 and 2 and the 3,3',3″-isomer 3. The oxidized complexes feature multiple electronic excitations in the near-infrared and the visible spectra, which are assigned to dδ/δ* transitions of the ferrocenium (Fc+) moieties, as well as TAT → Fc+ charge-transfer transitions. The latter are augmented by intervalence charge-transfer contributions Fc → Fc+ in mixed-valent states, where only a part of the available ferrocenyl residues is oxidized. EtTAT was previously identified as a directional three-level switching unit when deposited on Ag(111) and constitutes a trinary-digit unit for on-surface information storage. The symmetrically trisubstituted compound 6 retains this property, albeit at somewhat reduced switching rates due to the additional interaction between the ferrocenyl residues and the Ag surface. In particular, the high directionality at low bias and the inversion of the preferred sense of the on-surface rocking motion with either a clockwise or counterclockwise switching sense, depending on the identity of the surface enantiomer, are preserved. Unsymmetrical substitution in mono- and diferrocenylated 4 and 5 alters the underlying ratchet potential in a manner such that a two-state switching between the two degenerate surface conformations of 4 or a pronounced suppression of switching (5) is observed.

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.

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.

Layer-by-Layer Decoupling of Twisted Graphene Sheets Epitaxially Grown on a Metal Substrate.

The electronic properties of graphene can be efficiently altered upon interaction with the underlying substrate resulting in a dramatic change of charge carrier behavior. Here, the evolution of the local electronic properties of epitaxial graphene on a metal upon the controlled formation of multilayers, which are produced by intercalation of atomic carbon in graphene/Ir(111), is investigated. Using scanning tunneling microscopy and Landau-level spectroscopy, it is shown that for a monolayer and bilayers with small-angle rotations, Landau levels are fully suppressed, indicating that the metal-graphene interaction is largely confined to the first graphene layer. Bilayers with large twist angles as well as twisted trilayers demonstrate a sequence of pronounced Landau levels characteristic for a free-standing graphene monolayer pointing toward an effective decoupling of the top layer from the metal substrate. These findings give evidence for the controlled preparation of epitaxial graphene multilayers with a different degree of decoupling, which represent an ideal platform for future electronic and spintronic applications.