Variation of Kondo temperature induced by molecule-substrate decoupling in film formation of bis(phthalocyaninato)terbium(III) molecules on Au(111)

Steering Electron-Induced Surface Reaction via a Molecular Assembly Approach

Electrons not only serve as a "reactant" in redox reactions but also play a role in "catalyzing" some chemical processes. Despite the significance and ubiquitousness of electron-induced chemistry, many related scientific issues still await further exploration, among which is the impact of molecular assembly. In this work, microscopic insights into the vital role of molecular assembly in tweaking the electron-induced surface chemistry are unfolded by combined scanning tunneling microscopy and density functional theory studies. It is shown that the selective dissociation of a C-Cl bond in 4,4″-dichloro-1,1':3',1''-terphenyl (DCTP) on Cu(111) can be efficiently triggered by an electron injection via the STM tip into the unoccupied molecular orbital. The DCTP molecules are embedded in different assembly structures, including its self-assembly and coassemblies with Br adatoms. The energy threshold for the C-Cl bond cleavage increases as more Br adatoms stay close to the molecule, indicative of the sensitive response of the electron-induced surface reactivity of the C-Cl bond to the subtle change in the molecular assembly. Such a phenomenon is rationalized by the energy shift of the involved unoccupied molecular orbital of DCTP that is embedded in different assemblies. These findings shed new light on the tuning effect of molecular assembly on electron-induced reactions and introduce an efficient approach to precisely steer surface chemistry.

Kondo Effect of Co-Porphyrin: Remarkable Sensitivity to Adsorption Sites and Orientations

We investigated the Kondo effect of cobalt(II)-5-15-bis(4'-bromophenyl)-10,20-bis(4'-iodophenyl)porphyrin (CoTPPBr2I2) molecules on Au(111) with low-temperature scanning tunneling microscopy under ultrahigh vacuum conditions. The molecules exhibit four adsorption configurations at the top and bridge sites of the surface with different molecular orientations. The Kondo resonance shows extraordinary sensitivity to the adsorption configuration. By switching the molecule between different configurations, the Kondo temperature is varied over a wide range from ≈8 up to ≈250 K. Density functional theory calculations reveal that changes of the adsorption configuration lead to distinct variations of the hybridization between the molecule and the surface. Furthermore, we show that surface reconstruction plays a significant role for the molecular Kondo effect.

Magnetic Interactions in Substitutional Core-Doped Graphene Nanoribbons

The design of a spin imbalance within the crystallographic unit cell of bottom-up engineered 1D graphene nanoribbons (GNRs) gives rise to nonzero magnetic moments within each cell. Here, we demonstrate the bottom-up assembly and spectroscopic characterization of a one-dimensional Kondo spin chain formed by a chevron-type GNR (cGNR) physisorbed on Au(111). Substitutional nitrogen core doping introduces a pair of low-lying occupied states per monomer within the semiconducting gap of cGNRs. Charging resulting from the interaction with the gold substrate quenches one electronic state for each monomer, leaving behind a 1D chain of radical cations commensurate with the unit cell of the ribbon. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal the signature of a Kondo resonance emerging from the interaction of S = 1/2 spin centers in each monomer core with itinerant electrons in the Au substrate. STM tip lift-off experiments locally reduce the effective screening of the unpaired radical cation being lifted, revealing a robust exchange coupling between neighboring spin centers. First-principles DFT-LSDA calculations support the presence of magnetic moments in the core of this GNR when it is placed on Au.

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.

Observation of Yu-Shiba-Rusinov States and Inelastic Tunneling Spectroscopy for Intramolecule Magnetic Exchange Interaction Energy of Terbium Phthalocyanine (TbPc) Species Adsorbed on Superconductor NbSe2

We investigated the spin properties of the terbium phthalocyanine (TbPc) species adsorbed on the superconductor NbSe2 surface using scanning tunneling microscopy and spectroscopy. TbPc2 is a molecule in a class of single-molecule magnets (SMMs), and the use of superconductor electrodes attracts attention for the application to the devices using the spin degree of freedom. TbPc is a building block of TbPc2 and can reveal the spin component's behavior. In the experiment, TbPc species were placed on the surface of the superconductor NbSe2. We measured Yu-Shiba-Rusinov (YSR) states caused by the interaction between the superconducting state and magnetic impurity and inelastic tunneling spectroscopy (IETS) for the spin excitation, below 1 K. We also measured the Kondo state formed by the magnetic singlet formation. We detected the radical spin at the ligand position of the TbPc by the presence of the Kondo peak and demonstrated that the radical spin forms the YSR feature. In addition, the exchange interaction energy (Eex) between the spins of the radical ligand (Pc) and the center 4f metal atom (Tb3+) is determined by using the IETS technique. Eex is a critical parameter that determines the blocking temperature, below which the sample behaves as an SMM. IETS results show that the statistical distribution of Eex has peaked at 1.3, 1.6, and 1.9 meV. The energy range is comparable to the recent theoretical calculation result. In addition, we show that the energy variation is correlated with the bonding configuration of TbPc.

Rare-earth based tetrapyrrolic sandwiches: chemistry, materials and applications

This review summarises advances in chemistry of tetrapyrrole sandwiches with rare earth elements and highlights the current state of their use in single-molecule magnetism, organic field-effect transistors, conducting materials and nonlinear optics.

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.

Enhanced magnetic spin-spin interactions observed between porphyrazine derivatives on Au(111)

Magnetic molecules are of interest for application in spintronic and quantum-information processing devices. Therein, control of the interaction between the spins of neighboring molecules is the critical issue. Substitution of outer moieties of the molecule can tune the molecule-molecule interaction. Here we show a novel spin behavior for a magnetic molecule of vanadyl tetrakis (thiadiazole) porphyrazine (abbreviated as VOTTDPz) adsorbed on Au(111), which is modified from vanadyl phthalocyanine (VOPc) by replacing the inert phthalocyanine ligand with a reactive thiadiazole moiety. The magnetic properties of the molecules are examined by observing the Kondo resonance caused by the screening of an isolated spin by conduction electrons using scanning tunneling spectroscopy. The Kondo features are detected at the molecule whose shape and intensity show site-dependent variation, revealing complex spin-spin interactions due to the enhanced interaction between molecules, originating from the functionalization of the ligand with a more reactive moiety.

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.

Real space manifestations of coherent screening in atomic scale Kondo lattices

The interaction among magnetic moments screened by conduction electrons drives quantum phase transitions between magnetically ordered and heavy-fermion ground states. Here, starting from isolated magnetic impurities in the Kondo regime, we investigate the formation of the finite size analogue of a heavy Fermi liquid. We build regularly-spaced chains of Co adatoms on a metallic surface by atomic manipulation. Scanning tunneling spectroscopy is used to obtain maps of the Kondo resonance intensity with sub-atomic resolution. For sufficiently small interatomic separation, the spatial distribution of Kondo screening does not coincide with the position of the adatoms. It also develops enhancements at both edges of the chains. Since we can rule out any other interaction between Kondo impurities, this is explained in terms of the indirect hybridization of the Kondo orbitals mediated by a coherent electron gas, the mechanism that causes the emergence of heavy quasiparticles in the thermodynamic limit.

Coordination structure conversion of protonated bisporphyrinato terbium(iii) double-decker complexes and creation of a Kondo assembly by electron injection on the Au(111) surface

The first step towards the synthesis of single-molecule magnet (SMM)-based spintronics devices is the organization and manipulation of magnetic molecules on surfaces. Our previous studies on bulk crystals demonstrated that protonated porphyrinato double-decker complexes [Tb(Hoep)(oep)] (oep = 2,3,7,8,12,13,17,18-octaethylporphyrinato) are not SMMs; however, once a hydrogen is removed to produce their neutral radical forms, [Tb(oep)₂], they convert to SMMs. These intriguing properties encouraged us to examine the electronic/spin properties of these complexes and their chemical conversion ability after their transfer onto a metal substrate, similar to the environment required for the practical application of SMMs. Herein, we conducted a single-molecule-scale conversion of the protonated bis(porphyrinato)terbium(iii) double-decker complex [Tb(Hoep)(oep)], whose hepta-coordinated terbium ion changes into octa-coordinated [Tb(oep)₂] on detaching a hydrogen atom by scanning tunnelling microscopy. This conversion can be caused by the injection of tunnelling electrons of energy 1.5-2.5 eV. We confirmed the conversion by analysing the topographic image and the spin state of the molecule. The latter was achieved by examining the Kondo resonance, which originated from the screening of the molecular spin by the conduction electrons of the metal. The Kondo resonance was not observed for [Tb(Hoep)(oep)] but was observed for the converted species, which agrees well with a model containing the [Tb(oep)₂] molecule and Kondo resonance originating from the π-electron spin of the porphyrin ligand. Even though it is not possible to provide complete evidence of the SMM properties of the transferred molecule, we have demonstrated a possible path to realize the switch-on SMM properties of a single molecule.

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.

Alkyl chain length effects on double-deck assembly at a liquid/solid interface

Controlled double-deck packing is an appealing means to expand upon conventional 2D self-assembly which is critical in crystal engineering, yet it is rare and poorly understood. Herein, we report the first systematic study of double-deck assembly in a series of alkylated aminoquinone derivatives at the liquid-solid interface. The competition between the fraction of alkyl chains adsorbed on the surface and the optimal conformation of the alkyl chains near the head group leads to a stepwise structural transformation ranging from complete double-deck packing to complete monolayer packing. Alkyl chains on the bottom or top layer of the double-deck assemblies were selectively visualized by carefully tuning the scanning tunneling microscopy settings. A method to easily identify mirror image domains was discovered based on the coincidence of domain boundaries with a graphite main axis. The effect of molecular symmetry and metal complexation on the formation of the double-deck assembly was also explored. Based on 2D crystal engineering principles, this bottom-up double-deck assembly can potentially provide an essential toehold for constructing precise 3D hierarchical structures.

Detection and Manipulation of Charge States for Double-Decker DyPc2 Molecules on Ultrathin CuO Films

Charge states of lanthanide double-decker phthalocyanines complexes significantly influence their geometrical structures and magnetic properties. In this study, the charge states of single DyPc₂ molecules on an ultrathin CuO film were detected by scanning tunneling microscopy and spectroscopy in magnetic fields. Four types of adsorptions of DyPc₂ molecules on CuO were experimentally observed. Without applying voltages, two of them were positively charged with the other two at the neutral state. By controlling the sample bias, two types of neutral molecules can be switched to the positively and negatively charged states, respectively. This manipulation was not realized for the DyPc₂ cations. A way to precisely detect the molecular charge states with and without current is beneficial for the development of molecular electronics.

Tuning the spin-related transport properties of FePc on Au(111) through single-molecule chemistry

Tuning the spin-related transport properties of FePc on Au(111) through single-molecule chemistry.

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.

A Ru–Ru pair housed in ruthenium phthalocyanine: the role of a “cage” architecture in the molecule coupling with the Ag(111) surface

A combined experimental and theoretical study on the Ru–Pc dimer on Ag(111) proves surface-to-molecule charge transfer and predicts magnetic moment quenching.

Bilayer of Terbium Double-Decker Single-Molecule Magnets

We report a low-temperature scanning tunneling microscopy and spectroscopy study of the structural and electronic properties of a bilayer of terbium double-decker (bis(phthalocyaninato)terbium(III), TbPc₂) molecules on Au(111) at 5 K. The TbPc₂ molecules are found to adsorb flat on top of a first compact TbPc₂ monolayer on Au(111), forming a square-like packing similar to the underlying first layer. Their frontier-orbital electronic structure, measured by tunneling conductance spectroscopy, clearly differs from that of the underlying first monolayer. Our results of second-layer molecules indicate the absence of, both, hybrid molecule-substrate electronic states close to the Fermi level and a zero-bias Kondo resonance. We attribute these findings to a decreased electronic coupling with the Au(111) substrate.

Thermostability and photoluminescence of Dy(iii) single-molecule magnets under a magnetic field

A series of compounds [ADyL₄]·[solvent] composed of a dysprosium(iii) ion coordinated by four chelated naphthyridine-like ligands (L = 4-hydroxy-8-methyl-1,5-naphthyridine-3-carbonitrile) and an alkali metal ion (A = Na, K, Rb, Cs) were synthesized and characterized. They behave as single-molecule magnets under a zero dc field with an effective energy barrier of around 95 cm^-1. Meanwhile, the main part, [ADyL₄], of these SMMs is thermostable and sublimable. The geometric structures of three sublimed compounds are identical to the original ones without solvents, which is confirmed by X-ray diffraction using single crystal and powder samples. The static and dynamic magnetic properties remain unchanged before and after sublimation. Luminescence measurements at 5-77 K were performed to verify the energy gap between low-lying states and to understand the pathway of the thermal relaxation process of magnetization, as well as to inspect the tiny variation in magnetic sublevels for the ground term of Dy(iii). The photoluminescence spectra under a magnetic field (0-36 T) for the Dy-SMMs are investigated for the first time. The energy splitting of the two lowest sublevels of the ground term ⁶H_15/2 of Dy(iii) are analyzed using the Zeeman formula.

Low-temperature scanning tunneling microscopy study on the electronic properties of a double-decker DyPc2 molecule at the surface

Inaccessibility of the Dy 4f states in the double-decker DyPc₂ molecule, as evidenced by dI/dV mapping in the constant height mode.