Emergence of On-Surface Magnetochemistry

The control of exchange coupling across the molecule–substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or “surface ligand” are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established “trans effect” to the notion of the “surface-trans effect”. As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, spin-switching has been demonstrated to be controlled collectively via the interaction with a ligand (chemical selectivity) and individually via local addressing at the interface

Investigating magneto-chemical interactions at molecule-substrate interfaces by X-ray photo-emission electron microscopy

The magneto-chemical interaction of spin-bearing molecules with substrates is interesting from a coordination chemistry point of view and relevant for spintronics. Unprecedented insight is provided by X-ray photo-emission electron microscopy combined with X-ray magnetic circular dichroism spectroscopy. Here the coupling of a Mn-porphyrin ad-layer to the ferromagnetic Co substrate through suitably modified interfaces is analyzed with this technique

Magnetic exchange coupling of a synthetic Co(II)-complex to a ferromagnetic Ni substrate

On-surface assembly of a spin-bearing and non-aromatic porphyrin-related synthetic Co(II)-complex on a ferromagnetic Ni thin film substrate and subsequent magnetic exchange interaction across the interface were studied by scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and density functional theory +U (DFT + U) calculations

Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate

We report the discovery of an antiferromagnetic coupling of the magnetic moment of chromium(II) tetraphenyl-porphyrin (CrTPP) molecules to the magnetization of the clean ferromagnetic Co(001) substrate. We assign this unusual molecule-substrate exchange coupling to the less than half-filled chromium 3d orbitals interacting with Co valence band electrons via porphyrin-ligand molecular orbitals. X-ray magnetic circular dichroism, x-ray photoelectron spectroscopy, and scanning tunneling microscopy are combined with DFT+U calculations and provide evidence for a surprising type of antiferromagnetic 90∘ indirect magnetic exchange coupling

Reusable plasmonic substrates fabricated by interference lithography: a platform for systematic sensing studies

Surface-enhanced Raman scattering (SERS) has become increasingly popular in the scientific and industrial communities because of its analytical capabilities and potential to study fundamentals in plasmonics. Although under certain conditions extremely high sensitivity is possible, the practical use of SERS is frequently limited by instability and poor reproducibility of the enhancement factor. For analytical applications or for comparative measurements to enable the distinction between electromagnetic and chemical enhancement, the development of standardized and recyclable SERS substrates, having uniform and persistent performance, is proposed. To this end, we have fabricated periodic nanoslit arrays using extreme ultraviolet lithography that provide average large (2*106) and homogeneous SERS enhancement factors with a spot-to-spot variability of less than 3%. In addition, they are reusable without any degradation or loss of enhancement. The fabrication of such arrays consists of two steps only, lithographic patterning followed by metal evaporation. Both processes may be performed over areas of several square mm on any planar substrate. The sensor capabilities were demonstrated by substrates with monomolecular films of several different thiols. The concept of reusable SERS substrates may open a powerful platform within an analytical tool and in particular for systematic SERS studies for the investigation of fundamental parameters such as chemical enhancement, surface selection rules, and molecular alignmen

Self-Assembly and Magnetic Order of Bi-Molecular 2D Spin Lattices of M(II,III) Phthalocyanines on Au(111)

Single layer low-dimensional materials are presently of emerging interest, including in the context of magnetism. In the present report, on-surface supramolecular architecturing was further developed and employed to create surface supported two-dimensional binary spin arrays on atomically clean non-magnetic Au(111). By chemical programming of the modules, different checkerboards were produced combining phthalocyanines containing metals of different oxidation and spin states, diamagnetic zinc, and a metal-free 'spacer'. In an in-depth, spectro-microscopy and theoretical account, we correlate the structure and the magnetic properties of these tunable systems and discuss the emergence of 2D Kondo magnetism from the spin-bearing components and via the physico-chemical bonding to the underlying substrate. The contributions of the individual elements, as well as the role of the electronic surface state in the bottom substrate, are discussed, also looking towards further in-depth investigations

Comparative studies on Al₂O₃ based nanocomposites of some speciality polymers and binary polymers with the corresponding SiO₂, MnO₂ and ZrO₂ based nanocomposites

292-300Al2O3, based nanocomposites of speciality polymers such as poly-N-vinylcarbazole (PNVC), polypyrrole (PPY) and polythiophene (PTP) and of binary polymers PNVC-PPY, PNVC-PTP or PNVC-PANI were prepared and characterized by FTIR spectroscopic analyses. General bulk properties such as thermogravimetric stability, morphological features (SEM) and particle sizes (TEM) were investigated and compared with those reported with other nano-oxide (SiO2, ZrO2, MnO2) based polymers prepared by similar procedures. Thermal stabilities of the polymer-metal oxide composites and of the binary polymer- Al2O3 based composites were appreciably improved compared to the same for the unmodified base polymers. SEM analyses revealed the presence of lumpy aggregates of smaller particles forming a ‘raspberry’ morphology pattern. TEM studies showed formation of spherical particles of sizes varying between 24-500 nm depending on the nature of the oxides and monomers used. The d.c. conductivities of the composites were in the range of 10-5-10-7 S/cm for the PNVC based systems including the binary copolymer composites, 10-2-10-4 S/cm for PPY -based systems, 10-3 S/cm for PTP-based systems and (0.03-6.1)×10-2 S/cm for PANI-based systems. The nature of the metal oxide counterpart did not influence the conductivity values. These composites were dispersible in aqueous medium-the dispersion stabilities being different from system-to-system depending on the polymers and the nature of the oxide counterpart

On-surface coordination chemistry: direct imaging of the conformational freedom of an axial ligand at room temperature

The on-surface ligation of nitric oxide (NO) with Co-tetraphenylporphyrin (CoTPP) sublimed onto oxygen-reconstructed Ni(001) is studied using room-temperature scanning tunneling microscopy (STM) and complementary photoemission spectroscopies. On the oxygen-reconstructed substrates, the porphyrins are observed to form well-ordered, self-assembled layers. STM directly images the NO ligand as a characteristic feature in the center of the molecule. Under certain STM imaging conditions the dynamicity of this feature can be related to the temperature-activated conformational flexibility of the NO ligand. This provides an indirect confirmation of the bending of the Co-NO bond, as predicted from classical coordination chemistry