Bilayer of Terbium Double-Decker Single-Molecule Magnets

Control of the Magnetic Interaction between Single-Molecule Magnet TbPc2 and Superconductor NbSe2 Surface by an Intercalated Co Atom

We demonstrate that an intercalated Co atom in superconductor NbSe2 could control the magnetic interaction between the adsorbed magnetic molecule of TbPc2 and the NbSe2 substrate. An intercalated Co atom enhances the magnetic interaction between the NbSe2 and the TbPc2 spin to cause Kondo resonance at the TbPc2 position, a spin-singlet state formed by the itinerary electron. By applying a surface-normal magnetic field, we change the molecule's spin direction from the initial one directed to the Co atom to the surface normal. The change appears as a split Kondo resonance at the TbPc2, one of which is enhanced at the Tb site, which disappears when the outer magnetic field normal to the surface is applied and never appears, even if we return B to 0 T. The phenomenon suggests that the intercalated magnetic atoms can control the magnetic interaction between a magnetic molecule and the superconductor NbSe2.

Adsorption of lanthanide double-decker phthalocyanines on single-walled carbon nanotubes: structural changes and electronic properties as studied by density functional theory

CONTEXT

Molecular modeling of carbon nanotubes and lanthanide double-decker phthalocyanines hybrids is challenging due to the presence of 4f-electrons. In this paper, we analyzed the trends in structural changes and electronic properties when a lanthanide (La, Gd, and Lu) bisphthalocyanine molecule is adsorbed on the surface of two single-walled carbon nanotubes (SWCNTs) models: armchair and zigzag. The density functional theory (DFT) computations showed that the height of bisphthalocyanines complexes (LnPc₂) when adsorbed on a nanotube (LnPc₂+SWCNT) is the structural feature which is most affected by the nanotube model. The formation energy of the LnPc₂+SWCNT hybrid depends on the metal atom and the nanotube chirality. LaPc₂ and LuPc₂ bind stronger to the zigzag nanotube, while for GdPc₂, bonding to the armchair nanotube is the stronger one. The HOMO-LUMO gap energy (Egap) shows a correlation between the nature of lanthanide and the nanotube chirality. In the case of adsorption on armchair nanotube, E_gap tends to match the gap of isolated LnPc₂, whereas for adsorption on the zigzag nanotube, it is closer to the value for the isolated nanotube model. The spin density is localized on the phthalocyanines ligands (plus on Gd in the case of GdPc₂), when the bisphthalocyanine is adsorbed on the surface of the armchair nanotube. For bonding to zigzag nanotube (ZNT), it extends over both components, except for LaPc₂+ZNT, where spin density is found on the nanotube only.

METHOD

All DFT calculations were carried out using the DMol³ module of Material Studio 8.0 software package from Accelrys Inc. The computational technique chosen was the general gradient approximation functional PBE in combination with a long-range dispersion correction developed by Grimme (PBE-D2), the double numerical basis set DN, and the DFT semi-core pseudopotentials.

Magnetic molecules as local sensors of topological hysteresis of superconductors

Superconductors and magnetic materials, including molecules, are key ingredients for quantum computing and spintronics. However, only a little is known about how these materials interact in multilayer nanostructures like the hybrid architectures nowadays under development for such advanced applications. Here, we show that a single layer of magnetic molecules, Terbium(III) bis-phthalocyaninato (TbPc₂) complexes, deposited under controlled UHV conditions on a superconducting Pb(111) surface is sensitive to the topology of the intermediate state of the superconductor, namely to the presence and evolution of superconducting and normal domains due to screening and penetration of an external magnetic field. The topological hysteresis of the superconducting substrate imprints a local evolution of the magnetisation of the TbPc₂ molecules in the monolayer. Element and surface selective detection is achieved by recording the X-ray magnetic circular dichroism of the Tb atoms. This study reveals the impressive potential of magnetic molecules for sensing local magnetic field variations in molecular/superconductor hybrid devices, including spin resonators or spin injecting and spin filtering components for spintronics applications.

Substrate mediated interaction of terbium(III) double-deckers with the TiO2(110) surface

A terbium(iii)-bis(phthalocyaninato) neutral complex was deposited on the rutile TiO2(110) surface, and their interaction was studied by Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the TiO2 rutile surface favours the adsorption of isolated molecules adopting a lying down configuration with the phthalocyanine planes tilted by about 30° when they lie in the first layer. The electronic and chemical properties of the molecules on the surface were studied by XPS as a function of the TiO2(110) substrate preparation. This study evidences that strong molecule-substrate interactions are present and a charge transfer process occurs from the molecule to the surface.

Selective crystallization of four bis(phthalocyaninato)lanthanoid(iii) polymorphs

Four polymorphs of the bis-phthalocyanine lanthanoid(iii) double-decker complexes are crystallized by solution evaporation methods, and their structures determined by single-crystal X-ray crystallography.

Manipulation of Molecular Spin State on Surfaces Studied by Scanning Tunneling Microscopy

The adsorbed magnetic molecules with tunable spin states have drawn wide attention for their immense potential in the emerging fields of molecular spintronics and quantum computing. One of the key issues toward their application is the efficient controlling of their spin state. This review briefly summarizes the recent progress in the field of molecular spin state manipulation on surfaces. We focus on the molecular spins originated from the unpaired electrons of which the Kondo effect and spin excitation can be detected by scanning tunneling microscopy and spectroscopy (STM and STS). Studies of the molecular spin-carriers in three categories are overviewed, i.e., the ones solely composed of main group elements, the ones comprising 3d-metals, and the ones comprising 4f-metals. Several frequently used strategies for tuning molecular spin state are exemplified, including chemical reactions, reversible atomic/molecular chemisorption, and STM-tip manipulations. The summary of the successful case studies of molecular spin state manipulation may not only facilitate the fundamental understanding of molecular magnetism and spintronics but also inspire the design of the molecule-based spintronic devices and materials.

Screening the 4f-electron spin of TbPc2 single-molecule magnets on metal substrates by ligand channeling

Bis(phthalocyaninato)lanthanide (LnPc₂) double-decker-based devices have recently attracted a great deal of interest for data encoding purposes. Although the 4f-electrons of lanthanide ions play a key role in the experimental methodology, their localized character, deeper in energy compared to the 3d electrons of transition metals, hampers a detailed investigation. Here, our approach consists of the follow-up of the entanglement process with other molecules and with the substrate electrons by means of space-resolved detection of the Kondo resonance by scanning tunneling spectroscopy (STS), using different substrates (from weak to strong interaction). It is found that TbPc₂ molecules firstly interact with their environment by means of the π-radicals of the ligand. The radical spin of TbPc₂ can be identified by STS on a weakly interacting substrate such as Au(111). In the case of a Ag(111) substrate, we are able to analyze the effect of an electron transfer on the molecule (pairing-up of the radical spin) and the subsequent quenching of the Kondo resonance. Finally, on a strongly interacting substrate such as Cu(111), a significant rearrangement of electrons takes place and the Kondo screening of the 4f electrons of the Tb ion of TbPc₂ is observed. By comparative STS measurements on YPc₂, that has empty 4d and 4f shells, we prove that the Kondo resonance measured in the center of the TbPc₂ molecule indeed stems from the 4f-electrons. At the same time, we provide evidence for the hybridization of the 4f states with the π electron.

Magnetic properties of on-surface synthesized single-ion molecular magnets

We perform on-surface synthesis of single-ion molecular magnets on an Ag(111) surface and characterize their morphology, chemistry, and magnetism. The first molecule we synthesize is TbPc₂ to enable comparison with chemically synthesized and subsequently surface adsorbed species. We demonstrate the formation of TbPc₂ with a yield close to 100% and show that on-surface synthesis leads to identical magnetic and morphological properties compared to the previously studied chemically synthesized species. Moreover, exposure of the surface adsorbed TbPc₂ molecules to air does not modify their magnetic and morphological properties. To demonstrate the versatility of our approach, we synthesize novel Tb double deckers using tert-butyl-substituted phthalocyanine (tbu-2H-Pc). The Tb(tbu-Pc)₂ molecules exhibit magnetic hysteresis and therefore are the first purely on-surface synthesized single ion magnet.

We perform on-surface synthesis of single-ion molecular magnets on an Ag(111) surface and characterize their morphology, chemistry, and magnetism.

Impressive near-infrared brightness and singlet oxygen generation from strategic lanthanide-porphyrin double-decker complexes in aqueous solution

Although lanthanide double-decker complexes with hetero-macrocyclic ligands as functional luminescent and magnetic materials have promising properties, their inferior water solubility has negated their biomedical applications. Herein, four water-soluble homoleptic lanthanide (Ln = Gd, Er, Yb and La) sandwiches with diethylene-glycol-disubstituted porphyrins (DD) are reported, with their structures proven by both quantum chemical calculations and scanning tunneling microscopy. Our findings demonstrate that the near-infrared emission intensity and singlet oxygen (¹O₂) quantum yields of YbDD and GdDD in aqueous media are higher than those of the reported capped lanthanide monoporphyrinato analogues, YbN and GdN; the brightness and luminescence lifetime in water of YbDD are greater than those of YbN. This work provides a new dimension for the future design and development of molecular theranostics-based water-soluble double-decker lanthanide bisporphyrinates.

On-surface structural and electronic properties of spontaneously formed Tb2Pc3 single molecule magnets

The single molecule magnet (SMM) bis(phthalocyaninato)terbium(iii) (TbPc2) has received significant and increasing attention as an exemplar system for realizing molecule-based spin electronics. Attaining higher nuclearity via multi-decker TbPc systems has remained an outstanding challenge, as known examples of Tb2Pc3 systems are only those containing Pc rings with substituents (e.g. alkyl, alkoxyl). Here we report on the spontaneous formation of Tb2Pc3 species from TbPc2 precursors via sublimation in ultrahigh vacuum (UHV) onto an Ag(111) surface. The presence of Tb2Pc3 molecules on the surface are inspected using scanning probe microscopy with submolecular resolution supported by density functional theory (DFT) calculations and additional chemical analysis. We observe the selective presence of a Kondo resonance (30 K) in the Tb2Pc3 species, that we attribute to differences in the orientation of the internal molecular ligands. Formation of triple-decker complexes offers new possibilities to study and control magnetic interactions not accessible with standard TbPc2 molecules.

Magnetic bistability of a TbPc2 submonolayer on a graphene/SiC(0001) conductive electrode

The alteration of the properties of single-molecule magnets (SMMs) due to the interaction with metallic electrodes is detrimental to their employment in spintronic devices. Conversely, herein we show that the terbium(iii) bis-phthalocyaninato complex, TbPc₂, maintains its SMM behavior up to 9 K on a graphene/SiC(0001) substrate, making this alternative conductive layer highly promising for molecular spintronic applications.

Role of π-Radicals in the Spin Connectivity of Clusters and Networks of Tb Double-Decker Single Molecule Magnets

When single molecule magnets (SMMs) self-assemble into 2D networks on a surface, they interact via the π-electrons of their ligands. This interaction is relevant to the quantum entanglement between molecular qubits, a key issue in quantum computing. Here, we examine the role played by the unpaired radical electron in the top ligand of Tb double-decker SMMs by comparing the spectroscopic features of isolated and 2D assembled entities on surfaces. High-resolution scanning tunneling microscopy (STM) is used to evidence experimentally the Kondo resonance of the unpaired radical spins in clusters and islands and its quenching due to up-pairing at orbital overlaps. The presence or the absence of the Kondo feature in the dI/dV maps turns out to be a good measure of the lateral interaction between molecules in 2D networks. In a 2D cluster of molecules, the π-orbital lobes that are linked through the orbital overlap show paired-up electron wave function (one singly occupied molecular orbital (SOMO) with spin-up and the other with spin-down) and therefore do not experience the Kondo resonance in the experiment. As a result, small clusters built by STM-assisted manipulation of molecules show alternating Kondo features of quantum mechanical origin, from the monomer to the dimer and the trimer. On the other hand, when the TbPc₂ molecular clusters grow larger and form extended domains, a geometric rearrangement occurs, leading to the quenching of the Kondo signal on one lobe out of two. The even distribution of overlapping (SOMO) lobes on the perimeter of the molecule is induced by the square symmetry of the semi-infinite lattice and clearly distinguishes the lattice from the clusters.

Changing Single-Molecule Magnet Properties of a Windmill-Like Distorted Terbium(III) α-Butoxy-Substituted Phthalocyaninato Double-Decker Complex by Protonation/Deprotonation

Synthesis, structures, and magnetic properties of α-butoxy-substituted phthalocyaninato double-decker complexes Tb(α-obPc)₂ (1^-) (α-obPc: dianion of 1,4,8,11,15,18,22,25-octa(n-butoxy)phthalocyaninato) with protonated (1H), deprotonated (1[HDBU]), and diprotonated forms (1H₂⁺) are discussed. X-ray analysis was used to confirm the position of the proton in 1H, and it was revealed that the protonation induced asymmetric distortion in 1H. In contrast, 1[HDBU] was distorted in a highly symmetric windmill-like fashion. 1H is arranged in a slipped column array in the crystal packing, whereas 1[HDBU] is arranged in a one-dimensional fashion, in which the magnetic easy axes of 1[HDBU] lie along the same line. From direct-current (dc) magnetic measurements, ferromagnetic Tb-Tb interactions occur in both 1H and 1[HDBU], and magnetic hysteresis was observed. However, the area of the magnetic hysteresis in 1[HDBU] is larger than that in 1H, meaning that magnetic relaxation time (τ) is longer in 1[HDBU]. In addition, the results of alternating-current magnetic measurements in a zero dc magnetic field indicate that τ of 1[HDBU] is longer as compared to 1H. In other words, protonation/deprotonation affects not only the molecular structures and crystal packing but also the single-molecule magnet properties.