Controlled spin switching in a metallocene molecular junction

Conductance and assembly of quasi-1D coordination chain molecular junctions with triazole derivatives

Incorporating transition metal atoms into metal-molecule-metal junctions presents opportunities for exploring the electronic properties of coordination complexes, organometallics and metal-organic materials on the single molecule level. Recent single molecule conductance studies have shown that in situ incorporation of electrode metal atoms into coordination chains formed in the junction can occur with deprotonated, negatively charged organic ligands, such as the imidazolate (Im-) anion. However, the mechanism and chemical principles, such as the role of the charge state of the ligand, for the construction of such coordination chains are still debated. Here, we probe the role of the ligand charge state and electronic structure in single-molecule conductance and formation of metal-molecule coordination chains. We perform break junction measurements with triazole isomers, which can bridge junctions both in their neutral and charged forms, and find that prior deprotonation of the ligands is not required for coordination complex assembly, but can affect the molecular conductance and junction formation probability. Our results indicate that coordination chains can form with neutral ligands, as long as the electron density in the frontier MOs is concentrated at the binding sites and along the direction of pulling, promoting ligand binding and incorporation of gold atoms into the junction during elongation. Our findings may provide insight into design principles for in situ assembled molecular wires with transition metal atoms and open the door to electronic and spintronic studies of such materials.

Single-Spin Sensing: A Molecule-on-Tip Approach

Magnetometry plays a pivotal role in addressing the requirements of ultradense storage technology and overcoming challenges associated with downscaled spin qubits. A promising approach for atomic-scale single-spin sensing involves utilizing a magnetic molecule as a spin sensor, although such a realization is still in its early stages. To tackle this challenge and underscore the potential of this method, we combined a nickelocene molecule with scanning tunneling microscopy to perform versatile spin-sensitive imaging of magnetic surfaces. We investigated model Co islands on Cu(111) of different thicknesses having variable magnetic properties. Our method demonstrates robustness and reproducibility, providing atomic-scale sensitivity to spin polarization and magnetization orientation, owing to a direct exchange coupling between the nickelocene-terminated tip and the Co surfaces. We showcase the accessibility of magnetic exchange maps using this technique, revealing unique signatures in magnetic corrugation, which are well described by computed spin-density maps. These advancements significantly improve our capacity to probe and visualize magnetism at the atomic level.

Unconventional Surface Doping Effect on the Spin State of an Adsorbed Magnetic Molecule

Magnetic molecules adsorbed on two-dimensional (2D) substrates have attracted broad attention because of their potential applications in quantum device applications. Experimental observations have demonstrated substantial alteration in the spin excitation energy of iron phthalocyanine (FePc) molecules when adsorbed on nitrogen-doped graphene substrates. However, the underlying mechanism responsible for this notable change remains unclear. To shed light on this, we employ an embedding method and ab initio quantum chemistry calculations to investigate the effects of surface doping on molecular properties. Our study unveils an unconventional chemical bonding at the interface between the FePc molecule and the N-doped graphene. This bonding interaction, stronger than non-covalent interactions, significantly modifies the magnetic anisotropy energy of the adsorbed molecule, consistent with experimental observations. These findings provide valuable insights into the electronic and magnetic properties of molecules on 2D substrates, offering a promising pathway for precise manipulation of molecular spin states.

Fano resonance in molecular junctions of spin crossover complexes

In this paper, we introduce a novel molecular switch paradigm that integrates spin crossover complexes with the Fano resonance effect. Specifically, by performing density-functional theory calculations, the feasibility of achieving Fano resonance using spin crossover complexes is demonstrated in our designed molecular junctions using the complex {Fe[H2B(pz)2]2[Bp(bipy)]} [pz = 1-pyrazolyl, Bp(bipy) = bis(phenylethynyl)(2,2'-bipyridine)]. It is further revealed that the Fano resonance, particularly the Fano dip, is most prominent in the junction with cobalt tips among all the schemes, together with the spin-filtering effect. Most importantly, this junction of cobalt tips is able to exhibit three distinct conductance states, which are controlled by the modulation of Fano resonance due to the spin-state transition of the complex and the applied gate voltage. Such a molecular switch paradigm holds potential for applications in logic gates, memory units, sensors, thermoelectrics, and beyond.

Manipulating the Spin Orientation of Co Atoms Using Monatomic Cu Chains

Harnessing the spin of single atoms is at the heart of quantum information nanotechnology based on magnetic concepts. By attaching single Co atoms to monatomic Cu chains, we demonstrate the ability to control the spin orientation by the atomic environment. Due to spin-orbit coupling (SOC), the spin is tilted by ≈58° from the surface normal toward the chain as evidenced by inelastic tunneling spectroscopy. These findings are reproduced by density functional theory calculations and have implications for Co atoms on pristine Cu(111), which are believed to be Kondo systems. Our quantum Monte Carlo calculations suggest that SOC suppresses the Kondo effect of Co atoms at chains and on the flat surface. Our work impacts the fundamental understanding of low-energy excitations in nanostructures on surfaces and demonstrates the ability to manipulate atomic-scale magnetic moments, which can have tremendous implications for quantum devices.

Tuning hydrogenation chemistry of Pd-based heterogeneous catalysts by introducing homogeneous-like ligands

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H₂ activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

Manipulation of the Magnetic State of a Porphyrin-Based Molecule on Gold: From Kondo to Quantum Nanomagnet via the Charge Fluctuation Regime

By moving individual Fe-porphyrin-based molecules with the tip of a scanning tunneling microscope in the vicinity of the elbow of the herringbone-reconstructed Au(111) containing a Br atom, we reversibly and continuously control their magnetic state. Several regimes are obtained experimentally and explored theoretically: from the integer spin limit, through intermediate magnetic states with renormalized magnetic anisotropy, until the Kondo-screened regime, corresponding to a progressive increase of charge fluctuations and mixed valency due to an increase in the interaction of the molecular Fe states with the substrate Fermi sea. Our study demonstrates the potential of utilizing charge fluctuations to generate and tune quantum magnetic states in molecule-surface hybrids.

Dynamical Screening of Local Spin Moments at Metal-Molecule Interfaces

Transition-metal phthalocyanine molecules have attracted considerable interest in the context of spintronics device development due to their amenability to diverse bonding regimes and their intrinsic magnetism. The latter is highly influenced by the quantum fluctuations that arise at the inevitable metal-molecule interface in a device architecture. In this study, we have systematically investigated the dynamical screening effects in phthalocyanine molecules hosting a series of transition-metal ions (Ti, V, Cr, Mn, Fe, Co, and Ni) in contact with the Cu(111) surface. Using comprehensive density functional theory plus Anderson's Impurity Model calculations, we show that the orbital-dependent hybridization and electron correlation together result in strong charge and spin fluctuations. While the instantaneous spin moments of the transition-metal ions are near atomic-like, we find that screening gives rise to considerable lowering or even quenching of these. Our results highlight the importance of quantum fluctuations in metal-contacted molecular devices, which may influence the results obtained from theoretical or experimental probes, depending on their possibly material-dependent characteristic sampling time-scales.

Hierarchical equations of motion approach for accurate characterization of spin excitations in quantum impurity systems

Recent technological advancement in scanning tunneling microscopes has enabled the measurement of spin-field and spin-spin interactions in single atomic or molecular junctions with an unprecedentedly high resolution. Theoretically, although the fermionic hierarchical equations of motion (HEOM) method has been widely applied to investigate the strongly correlated Kondo states in these junctions, the existence of low-energy spin excitations presents new challenges to numerical simulations. These include the quest for a more accurate and efficient decomposition for the non-Markovian memory of low-temperature environments and a more careful handling of errors caused by the truncation of the hierarchy. In this work, we propose several new algorithms, which significantly enhance the performance of the HEOM method, as exemplified by the calculations on systems involving various types of low-energy spin excitations. Being able to characterize both the Kondo effect and spin excitation accurately, the HEOM method offers a sophisticated and versatile theoretical tool, which is valuable for the understanding and even prediction of the fascinating quantum phenomena explored in cutting-edge experiments.

Unveiling the Decisive Factor for the Sharp Transition in the Scanning Tunneling Spectroscopy of a Single Nickelocene Molecule

Scanning tunneling microscopy (STM) has been utilized to realize the precise measurement and control of local spin states. Experiments have demonstrated that when a nickelocene (Nc) molecule is attached to the apex of an STM tip, the dI/dV spectra exhibit a sharp or a smooth transition when the tip is displaced toward the substrate. However, what leads to the two distinct types of transitions remains unclear, and more intriguingly, the physical origin of the abrupt change in the line shape of dI/dV spectra remains unclear. To clarify these intriguing issues, we perform first-principles-based simulations on the STM tip control process for the Cu tip/Nc/Cu(100) junction. In particular, we find that the suddenly enhanced hybridization between the d orbitals on the Ni ion and the metallic bands in the substrate leads to Kondo correlation overwhelming spin excitation, which is the main cause of the sharp transition in the dI/dV spectra observed experimentally.

Ionization of Decamethylmanganocene: Insights from the DFT-Assisted Laser Spectroscopy

Metallocenes represent one of the most important classes of organometallics with wide prospects for practical use in various fields of chemistry, materials science, molecular electronics, and biomedicine. Many applications of these metal complexes are based on their ability to form molecular ions. We report the first results concerning the changes in the molecular and electronic structure of decamethylmanganocene, Cp*₂Mn, upon ionization provided by the high-resolution mass-analyzed threshold ionization (MATI) spectroscopy supported by DFT calculations. The precise ionization energy of Cp*₂Mn is determined as 5.349 ± 0.001 eV. The DFT modeling of the MATI spectrum shows that the main structural deformations accompanying the detachment of an electron consist in the elongation of the Mn-C bonds and a change in the Me out-of-plane bending angles. Surprisingly, the DFT calculations predict that most of the reduction in electron density (ED) upon ionization is associated with the hydrogen atoms of the substituents, despite the metal character of the ionized orbital. However, the ED difference isosurfaces reveal a complex mechanism of the charge redistribution involving also the carbon atoms of the molecule.

Unravelling the robustness of magnetic anisotropy of a nickelocene molecule in different environments: a first-principles-based study

Recent scanning tunneling spectroscopy with single metallocene molecule-functionalized tips have proved to be a powerful tool to probe and control individual spins and spin-spin exchange interactions due to the robustness of the magnetic properties of the metallocene molecule in different surroundings. However, accurate prediction of such robustness at a first-principles-based level by the conventional density functional theory (DFT) has remained challenging. In this paper, we have performed a detailed investigation of the evolution of electronic and magnetic properties of a nickelocene molecule (NiCp₂) in different environments, i.e., free-standing, adsorbed on Cu(100) and as a functionalized tip apex. Using an embedding method, which combines DFT and the complete active space self-consistent field (CASSCF) method recently developed, we demonstrate that the nickelocene molecule almost preserves its spin and magnetic anisotropy upon adsorption on Cu(100), and also in the position of the tip apex. In particular, the cyclic π* orbital of the Cp rings could hybridize with the singly occupied d_π orbitals of the Ni center of the molecule, protecting these orbitals from external states. Hence the molecular spin maintains S = 1, the same as in the free-standing case, and its magnetic anisotropy is also robust with energies of 3.56, 3.34, and 3.51 meV in free-standing, adsorbed on Cu(100), and functionalized tip apex states, respectively, in good agreement with previous theoretical and experimental results. This work thus provides a first-principles-based understanding of the relevant experiments. Such agreement between theoretical simulations and experimental measurements highlights the potential usefulness of the method for investigating the local electronic and spin states of organometallic molecule-surface composite systems.

Coherent and Incoherent Tunneling into Yu-Shiba-Rusinov States Revealed by Atomic Scale Shot-Noise Spectroscopy

The pair breaking potential of individual magnetic impurities in s-wave superconductors generates localized states inside the superconducting gap commonly referred to as Yu-Shiba-Rusinov (YSR) states whose isolated nature makes them promising building blocks for artificial structures that may host Majorana fermions. One of the challenges in this endeavor is to understand their intrinsic lifetime, ℏ/Λ, which is expected to be limited by the inelastic coupling with the continuum thus leading to decoherence. Here we use shot-noise scanning tunneling microscopy to reveal that electron tunneling into superconducting 2H-NbSe_{2} mediated by YSR states is not Poissonian, but ordered as a function of time, as evidenced by a reduction of the noise. Moreover, our data show the concomitant transfer of charges e and 2e, indicating that incoherent single particle and coherent Andreev processes operate simultaneously. From the quantitative agreement between experiment and theory we obtain Λ=1  μeV≪k_{B}T demonstrating that shot noise can probe energy scales and timescales inaccessible by conventional spectroscopy whose resolution is thermally limited.

Spin-Crossover in Supramolecular Iron(II)-2,6-bis(1H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2'-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

Formation and Evolution of Metallocene Single-Molecule Circuits with Direct Gold-π Links

Single-molecule circuits with group 8 metallocenes are formed without additional linker groups in scanning tunneling microscope-based break junction (STMBJ) measurements at cryogenic and room-temperature conditions with gold (Au) electrodes. We investigate the nature of this direct gold-π binding motif and its effect on molecular conductance and persistence characteristics during junction evolution. The measurement technique under cryogenic conditions tracks molecular plateaus through the full cycle of extension and compression. Analysis reveals that junction persistence when the metal electrodes are pushed together correlates with whether electrodes are locally sharp or blunt, suggesting distinct scenarios for metallocene junction formation and evolution. The top and bottom surfaces of the "barrel"-shaped metallocenes present the electron-rich π system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on a sharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small, flat patch on a dull tip can bind more strongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of model electrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs during junction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecific interaction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from that identified for the more commonly studied, rod-shaped organic molecular wires.

Origin of Asymmetric Splitting of Kondo Peak in Spin-Polarized Scanning Tunneling Spectroscopy: Insights from First-Principles-Based Simulations

The spin-polarized scanning tunneling microscope (SP-STM) has served as a versatile tool for probing and manipulating the spintronic properties of atomic and molecular devices with high precision. The interplay between the local spin state and its surrounding magnetic environment significantly affects the transport behavior of the device. Particularly, in the contact regime, the strong hybridization between the SP-STM tip and the magnetic atom or molecule could give rise to unconventional Kondo resonance signatures in the differential conductance (dI/dV) spectra. This poses challenges for the simulation of a realistic tip control process. By combining the density functional theory and the hierarchical equations of motion methods, we achieve first-principles-based simulation of the control of a Ni-tip/Co/Cu(100) junction in both the tunneling and contact regimes. The calculated dI/dV spectra reproduce faithfully the experimental data. A cotunneling mechanism is proposed to elucidate the physical origin of the observed unconventional Kondo signatures.

Richness of molecular junction configurations revealed by tracking a full pull-push cycle

In the field of molecular electronics, the interplay between molecular orientation and the resulting electronic transport is of central interest. At the single molecule level, this topic is extensively studied with the aid of break junction setups. In such experiments, two metal electrodes are brought into contact, and the conductance is typically measured when the electrodes are pulled apart in the presence of molecules, until a molecule bridges the two electrodes. However, the molecular junctions formed in this pull process reflect only part of the rich possible junction configurations. Here, we show that the push process, in which molecular junctions are formed by bringing the electrodes towards each other, allows the fabrication of molecular junction structures that are not necessarily formed in the pull process. We also find that in the extreme case, molecular junctions can be formed only in the push process that is typically ignored. Our findings demonstrate that tracking the two inverse processes of molecular junction formation, reveals a more comprehensive picture of the variety of molecular configurations in molecular junctions.

Iron phthalocyanine on Au(111) is a "non-Landau" Fermi liquid

The paradigm of Landau’s Fermi liquid theory has been challenged with the finding of a strongly interacting Fermi liquid that cannot be adiabatically connected to a non-interacting system. A spin-1 two-channel Kondo impurity with anisotropy D has a quantum phase transition between two topologically different Fermi liquids with a peak (dip) in the Fermi level for D < D_c (D > D_c). Extending this theory to general multi-orbital problems with finite magnetic field, we reinterpret in a unified and consistent fashion several experimental studies of iron phthalocyanine molecules on Au(111) that were previously described in disconnected and conflicting ways. The differential conductance shows a zero-bias dip that widens when the molecule is lifted from the surface (reducing the Kondo couplings) and is transformed continuously into a peak under an applied magnetic field. We reproduce all features and propose an experiment to induce the topological transition.

Single molecules on metal surfaces are paradigmatic systems for the study of many-body phenomena. Here, the authors show that several spectroscopic experiments on iron phthalocyanine on Au(111) surface can be described in a unified way in terms of a strongly interacting topologically non-trivial (non-Landau) Fermi liquid.

Designing a mechanically driven spin-crossover molecular switch via organic embedding

Among spin-crossover complexes, Fe-porphyrin (FeP) stands out for molecular spintronic applications: an intricate, yet favourable balance between ligand fields, charge transfer, and the Coulomb interaction makes FeP highly manipulable, while its planar structure facilitates device integration. Here, we theoretically design a mechanical spin-switch device in which external strain triggers the intrinsic magneto-structural coupling of FeP through a purely organic embedding. Exploiting the chemical compatibility and stretchability of graphene nanoribbon electrodes, we overcome common reliability and reproducibility issues of conventional inorganic setups. The competition between the Coulomb interaction and distortion-induced changes in ligand fields requires methodologies beyond the state-of-the-art: combining density functional theory with many-body techniques, we demonstrate experimentally feasible tensile strain to trigger a low-spin (S = 1) to high-spin (S = 2) crossover. Concomitantly, the current through the device toggles by over an order of magnitude, adding a fully planar mechanical current-switch unit to the panoply of molecular spintronics.

Laser spectroscopic and computational insights into unexpected structural behaviours of sandwich complexes upon ionization

Transition-metal sandwich complexes play key roles in various fields such as fundamental and applied chemistry; many of their unique properties arise from their ability to form stable or reactive ions. The first mass-analyzed threshold ionization (MATI) spectra of mixed sandwich compounds, (Ch)(Cp)Cr and (Cot)(Cp)Ti (Ch = η⁷-C₇H₇, Cp = η⁵-C₅H₅, Cot = η⁸-C₈H₈), presented in this work provide an extremely accurate description of the electron detachment. The ionization energies of the neutrals and stabilization energies of the metal-ligand interactions upon ionization are derived from the MATI data with an accuracy of 0.0006 eV. In combination with DFT calculations, laser threshold ionization spectroscopy reveals surprisingly different structural variations accompanying the detachment of the non-bonding d_z² electron from the sandwich molecules. The geometry of (Ch)(Cp)Cr remains practically unchanged while the ionization of (Cot)(Cp)Ti causes a noticeable shortening of the inter-ring distance, similar to that resulting from the ionization of a typical antibonding orbital. Electron density analysis throws light on the nature of these amazing effects.

Superconducting Scanning Tunneling Microscope Tip to Reveal Sub-millielectronvolt Magnetic Energy Variations on Surfaces

Combining the complex ordering ability of molecules with their local magnetic properties is a little-explored technique to tailor spin structures on surfaces. However, revealing the molecular geometry can be demanding. Nickelocene (Nc) molecules present a large spin-flip excitation leading to clear changes of conductance at the excitation-threshold bias. Using a superconducting tip, we have the energy resolution to detect small shifts of the Nc spin-flip excitation thresholds, permitting us to reveal the different individual environments of Nc molecules in an ordered layer. This knowledge allows us to reveal the adsorption configuration of a complex molecular structure formed by Nc molecules in different orientations and positions. As a consequence, we infer that Nc layers present a strong noncollinear magnetic-moment arrangement.

Tuning the Magnetic Coupling of a Molecular Spin Interface via Electron Doping

Mastering the magnetic response of molecular spin interfaces by tuning the occupancy of the molecular orbitals, which carry the spin magnetic moment, can be accomplished by electron doping. We propose a viable route to control the magnetization direction and magnitude of a molecular spin network, in a graphene-mediated architecture, achieved via alkali doping of manganese phthalocyanine (MnPc) molecules assembled on cobalt intercalated under a graphene membrane. The antiparallel magnetic alignment of the MnPc molecules with the underlying Co layer can be switched to a ferromagnetic state by electron doping. Multiplet calculations unveil an enhanced magnetic state of the Mn centers with a 3/2 to 5/2 spin transition induced by alkali doping, as confirmed by the steepening of the hysteresis loops, with higher saturation magnetization values. This new molecular spin configuration can be aligned by an external field, almost independently from the hard-magnet substrate effectively behaving as a free magnetic layer.

Experimental Validation of Quantum Circuit Rules in Molecular Junctions

A series of diarylacetylene (tolane) derivatives functionalised at the 4- and 4′-positions by thiolate, thioether, or amine groups capable of serving as anchor groups to secure the molecules within a molecular junction have been prepared and characterised. The series of compounds have a general form X-B-X, Y-B-Y, and X-B-Y where X and Y represent anchor groups and B the molecular bridge. The single-molecule conductance values determined by the scanning tunnelling microscope break-junction method are found to be in excellent agreement with the predictions made on the basis of a recently proposed ‘molecular circuit law’, which states ‘the conductance of an asymmetric molecule X-B-Y is the geometric mean of the conductance of the two symmetric molecules derived from it, and .’ The experimental verification of the circuit law, which holds for systems in which the constituent moieties X, B, and Y are weakly coupled and whose conductance takes place via off-resonance tunnelling, gives further confidence in the use of this relationship in the design of future compounds for use in molecular electronics research.

The Kondo Effect of a Molecular Tip As a Magnetic Sensor

A single molecule offers to tailor and control the probing capability of a scanning tunneling microscope when placed on the tip. With the help of first-principles calculations, we show that on-tip spin sensitivity is possible through the Kondo ground state of a spin S = 1/2 cobaltocene molecule. When attached to the tip apex, we observe a reproducible Kondo resonance, which splits apart upon tuning the exchange coupling of cobaltocene to an iron atom on the surface. The spin-split Kondo resonance provides quantitative information on the exchange field and on the spin polarization of the iron atom. We also demonstrate that molecular vibrations cause the emergence of Kondo side peaks, which, unlike the Kondo resonance, are sensitive to cobaltocene adsorption.

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

The societal impact of the electronics industry is enormous—not to mention how this industry impinges on the global economy. The foreseen limits of the current technology—technical, economic, and sustainability issues—open the door to the search for successor technologies. In this context, molecular electronics has emerged as a promising candidate that, at least in the short-term, will not likely replace our silicon-based electronics, but improve its performance through a nascent hybrid technology. Such technology will take advantage of both the small dimensions of the molecules and new functionalities resulting from the quantum effects that govern the properties at the molecular scale. An optimization of interface engineering and integration of molecules to form densely integrated individually addressable arrays of molecules are two crucial aspects in the molecular electronics field. These challenges should be met to establish the bridge between organic functional materials and hard electronics required for the incorporation of such hybrid technology in the market. In this review, the most advanced methods for fabricating large-area molecular electronic devices are presented, highlighting their advantages and limitations. Special emphasis is focused on bottom-up methodologies for the fabrication of well-ordered and tightly-packed monolayers onto the bottom electrode, followed by a description of the top-contact deposition methods so far used.

Minimally invasive spin sensing with scanning tunneling microscopy

Minimizing the invasiveness of scanning tunneling measurements is paramount for observation of the magnetic properties of unperturbed atomic-scale objects. We show that the invasiveness of STM inspection on few-atom spin systems can be drastically reduced by means of a remote detection scheme, which makes use of a sensor spin weakly coupled to the sensed object. By comparing direct and remote measurements we identify the relevant perturbations caused by the local probe. For direct inspection we find that tunneling electrons strongly perturb the investigated object even for currents as low as 3 pA. Electrons injected into the sensor spin induce perturbations with much reduced probability. The sensing scheme uses standard differential conductance measurements, and is decoupled both by its non-local nature, and by dynamic decoupling due to the significantly different time scales at which the sensor and sensed object evolve. The latter makes it possible to effectively remove static interactions between the sensed object and the spin sensor while still allowing the spin sensing. In this way we achieve measurements with a reduction in perturbative effects of up to 100 times relative to direct scanning tunneling measurements, which enables minimally invasive measurements of a few-atom magnet's fragile spin states with STM.

Vibron-assisted spin excitation in a magnetically anisotropic molecule

The electrical control and readout of molecular spin states are key for high-density storage. Expectations are that electrically-driven spin and vibrational excitations in a molecule should give rise to new conductance features in the presence of magnetic anisotropy, offering alternative routes to study and, ultimately, manipulate molecular magnetism. Here, we use inelastic electron tunneling spectroscopy to promote and detect the excited spin states of a prototypical molecule with magnetic anisotropy. We demonstrate the existence of a vibron-assisted spin excitation that can exceed in energy and in amplitude a simple excitation among spin states. This excitation, which can be quenched by structural changes in the magnetic molecule, is explained using first-principles calculations that include dynamical electronic correlations.

Substituent effects on the electronic structures of sandwich compounds: new understandings provided by DFT-assisted laser ionization spectroscopy of bisarene complexes

Recent advances on substituent effects in transition metal bisarene complexes studied with high-resolution threshold ionization spectroscopy are reviewed to demonstrate new aspects of the ligand influence on electronic structures of sandwich molecules. Unprecedented accuracy in the determination of ionization energies provided by the laser techniques makes it possible to reveal and describe quantitatively such fine phenomena as isotope effects, the mutual substituent influence or variations of substituent effects on replacing the central metal atom with its Group analogues. In combination with DFT calculations, laser ionization spectroscopy unveils mechanisms of the ligand influence on unique redox properties of sandwich complexes which are of key importance for their practical use.

On-Surface Atom-by-Atom-Assembled Aluminum Binuclear Tetrabenzophenazine Organometallic Magnetic Complex

The Kondo effect results from the interactions of the conduction electrons in a metal bulk with localized magnetic impurities. While adsorbed atop a metallic surface, the on-surface nanoscale version of this effect is observed when a single magnetic atom or a single magnetic molecule (SMM) is interacting with the conduction electrons. SMMs are commonly organometallic complexes incorporating transition-metal atoms in different oxidation states. We demonstrate how a single nonmagnetic neutral tetrabenzo[a,c,j,h]phenazine molecule can be on-surface-coordinated with exactly two aluminum metal atoms (between Al(I) and Al(II) oxidation state on the Au(111) surface) by low-temperature scanning tunneling microscope (LT-STM) single-atom manipulation. It results in a Kondo measurable localized molecular magnetic moment. This opens a new way to design SMM complexes without the need for heavy transition-metal atoms and complex ligands to stabilize the molecular coordination sphere.

Detection of Spin-Vibration States in Single Magnetic Molecules

The spin states of magnetic molecules have advantageous attributes as carriers of quantum information. However, spin-vibration coupling in molecules causes a decay of excited spin states and a loss of spin coherence. Here, we detect excitations of spin-vibration states in single nickelocene molecules on Ag(110) with a scanning tunneling microscope. By transferring a nickelocene to the tip, the joint spin-vibration states with an adsorbed nickelocene were measured. Chemical variations in magnetic molecules offer the opportunity to tune spin-vibration coupling for controlling the spin coherence.

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.

Electric-field control of spin orientation of manganocene: An insight into molecule-substrate interactions

The manipulation of spin orientations in molecular nanomagnets assembled on surfaces is essential for the development of memory devices. These properties are dominated by interactions with the substrate. Here, we show that individual manganocene molecules deposited on Cu(111) exhibit different easy magnetization directions in an applied electric-field due to different contact geometries. Using Hubbard-U corrected density-functional theory to describe strong correlation effects and a non-self-consistent diagonalization method to treat spin-orbit coupling, we demonstrate that the field-induced spin reorientation transition occurs in the standing-up molecule in both high-spin (HS) and low-spin states, while the transition only occurs in the HS state for the flat-lying molecule. We propose plausible mechanisms in terms of charge polarization at the interface as well as modifications of the electronic states near the Fermi level E _F. We show that the molecule largely preserves its arrangement of 3d orbitals in the standing configuration due to the "insulating layer" (bridging ligand), whereas direct contact of the Mn ion with the substrate in the lying configuration induces an orbital degeneracy around E _F, thus preventing the electrical modulation of magnetic anisotropies.

Jahn-Teller Splitting in Single Adsorbed Molecules Revealed by Isospin-Flip Excitations

Scanning tunneling spectroscopy measurements of Mn phthalocyanine (MnPc) molecules adsorbed on (sqrt[3]×sqrt[3]) surface alloys show single inelastic steps at exclusively positive or negative bias strongly depending on the tip position. This is in contrast to conventional molecular excitation thresholds, which are independent of the current direction and therefore always occur at both positive and negative bias. This polarity selectivity is found to coincide with the spatial distribution of occupied and empty orbitals. Because of the interaction with the substrate, charge transfer into the doubly degenerate d_{π} orbitals of MnPc takes place. The resulting Jahn-Teller effect lifts the degeneracy and leads to an isospin- or pseudospin-flip excitation, the inelastic analogue of an orbital Kondo resonance.

Visualizing Intramolecular Distortions as the Origin of Transverse Magnetic Anisotropy

The magnetic properties of metal-organic complexes are strongly influenced by conformational changes in the ligand. The flexibility of Fe-tetra-pyridyl-porphyrin molecules leads to different adsorption configurations on a Au(111) surface. By combining low-temperature scanning tunneling spectroscopy and atomic force microscopy, we resolve a correlation of the molecular configuration with different spin states and magnitudes of magnetic anisotropy. When the macrocycle exhibits a laterally undistorted saddle shape, the molecules lie in a S = 1 state with axial anisotropy arising from a square-planar ligand field. If the symmetry in the molecular ligand field is reduced by a lateral distortion of the molecule, we find a finite contribution of transverse anisotropy. Some of the distorted molecules lie in a S = 2 state, again exhibiting substantial transverse anisotropy.

The Kondo resonance line shape in scanning tunnelling spectroscopy: instrumental aspects

In the scanning tunnelling microscope, the many-body Kondo effect leads to a zero-bias feature of the differential conductance spectra of magnetic adsorbates on surfaces. The intrinsic line shape of this Kondo resonance and its temperature dependence in principle contain valuable information. We use measurements on a molecular Kondo system, all- trans retinoic acid on Au(1 1 1), and model calculations to discuss the role of instrumental broadening. The modulation voltage used for the lock-in detection, noise on the sample voltage, and the temperature of the microscope tip are considered. These sources of broadening affect the apparent line shapes and render difficult a determination of the intrinsic line width, in particular when variable temperatures are involved.

Control of Oxidation and Spin State in a Single-Molecule Junction

The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations.