Exchange Coupling of Spin-Crossover Molecules to Ferromagnetic Co Islands

An approach for patterned molecular adsorption on ferromagnets, achieved via Moiré superstructures

We have used a scanning tunneling microscope operated under ultrahigh vacuum conditions to investigate an oxo-vanadium-salen complex V(O)salen, that has potential applications as qubits in future quantum-based technologies. The adsorption and self-assembly of V(O)salen on a range of single crystal metal surfaces and nanoislands and the influence of substrate morphology and reactivity has been measured. On the close-packed flat Ag(111) and Cu(111) surfaces, the molecules adsorb isolated or form small clusters arranged randomly on the surface, whereas structured adsorption occurs on two types of Co nanoislands; Co grown on Ag(111) and Ag capped Co islands grown on Cu(111), both forming a Moiré pattern at the surface. The adsorption configuration can by scanning tunneling spectroscopy be linked to the geometric and electronic properties of the substrates and traced back to a Co d-related surface state, illustrating how the modulated reactivity can be used to engineer a pattern of adsorbed molecules on the nanoscale.

Spin-Crossover and Fragmentation of Fe(neoim)2 on Silver and Gold

The neutral spin crossover complex Fe(neoim)2, neoim being the deprotonated form of the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline (neoimH), is investigated on the (111) surfaces of Au and Ag using scanning tunneling microscopy and density functional theory calculations. The complex sublimates and adsorbs intact on Ag(111), where it exhibits an electron-induced spin crossover. However, it fragments on Au. According to density functional theory calculations, the adsorbed complex is drastically distorted by the interactions with the substrates, in particular by van der Waals forces. Dispersion interaction is also decisive for the relative stabilities of the low- and high-spin states of the adsorbed complex. The unexpected instability of the complex on the gold substrate is attributed to enhanced covalent bonding of the fragments to the substrate.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

Observation of Exchange Interaction in Iron(II) Spin Crossover Molecules in Contact with Passivated Ferromagnetic Surface of Co/Au(111)

Spin crossover (SCO) complexes sensitively react on changes of the environment by a change in the spin of the central metallic ion making them ideal candidates for molecular spintronics. In particular, the composite of SCO complexes and ferromagnetic (FM) surfaces would allow spin-state switching of the molecules in combination with the magnetic exchange interaction to the magnetic substrate. Unfortunately, when depositing SCO complexes on ferromagnetic surfaces, spin-state switching is blocked by the relatively strong interaction between the adsorbed molecules and the surface. Here, the Fe(II) SCO complex [Fe^II (Pyrz)₂ ] (Pyrz = 3,5-dimethylpyrazolylborate) with sub-monolayer thickness in contact with a passivated FM film of Co on Au(111) is studied. In this case, the molecules preserve thermal spin crossover and at the same time the high-spin species show a sizable exchange interaction of > 0.9 T with the FM Co substrate. These observations provide a feasible design strategy in fabricating SCO-FM hybrid devices.

Binuclear spin-crossover [Fe(bt)(NCS)2]2(bpm) complex: A study using first principles calculations

The spin-crossover [Fe(bt)(NCS)₂]₂(bpm) complex is studied using spin-polarized density functional theory within the generalized gradient approximation, the Hubbard U and the weak van der Waals interactions in conjunction with the projector augmented wave method in its molecular and periodic arrangements. It is shown that the considered complex has three magnetic configurations [high spin state (HS)-HS, HS-low spin state (LS), and LS-LS] corresponding to those observed experimentally after two transition temperatures T_c ⁽¹⁾ of 163 K and T_c ⁽²⁾ of 197 K. For the HS-HS magnetic state, we found that the two Fe centers are antiferromagnetically coupled for both molecular and periodic structures in good agreement with the experimental observations. Our results show that the computed total energy difference between the magnetic state configurations of the considered Fe₂ complex is significantly smaller compared to those reported in the literature for other mono- or binuclear compounds.

Exploring the potential as molecular quantum-dot cellular automata of a mixed-valence Ru2 complex deposited on a Au(111) surface

A Ru2+ complex deposited on a Au(111) surface in the presence of a counterion presents excess charge localized on one side of the molecule. The switching can be promoted by an applied electric field, E, stronger than the critical field strength Ec.

Spin Crossover in a Cobalt Complex on Ag(111)

The Co-based complex [Co(H2 B(pz)(pypz))2 ] (py=pyridine, pz=pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with characteristic image contrasts when current is passed through them or one of their neighbors. Two molecules exhibit this bistability while the other two molecules are stable. The transition rates vary linearly with the tunneling current and exhibit an intriguing dependence on the bias voltage and its polarity. We interpret the states as being due to S=1 /2 and 3 /2 spin states of the Co2+ complex. The image contrast and the orders-of-magnitude variations of the switching yields can be tentatively understood from the calculated orbital structures of the two spin states, thus providing first insights into the mechanism of electron-induced excited spin-state trapping (ELIESST).

Spin-crossover complexes in nanoscale devices: main ingredients of the molecule-substrate interactions

Spin-crossover complexes embedded in nanodevices experience effects that are absent in the bulk that can modulate, quench and even suppress the spin-transition. In this work we explore, by means of state-of-the-art quantum chemistry calculations, different aspects of the integration of SCO molecules on active nanodevices, such as the geometry and energetics of the interaction with the substrate, extension of the charge transfer between the substrate and SCO molecule, impact of the applied external electric field on the spin-transition, and sensitivity of the transport properties on the local conditions of the substrate. We focus on the recently reported encapsulation of Fe(II) spin-crossover complexes in single-walled carbon nanotubes, with new measurements that support the theoretical findings. Even so our results could be useful to many other systems where SCO phenomena take place at the nanoscale, the spin-state switching is probed by an external electric field or current, or the substrate is responsible for the quenching of the SCO mechanism.

Spin-Crossover Molecules on Surfaces: From Isolated Molecules to Ultrathin Films

Molecular spintronics seeks to use single or few molecules as functional building blocks for spintronic applications, directly relying on molecular properties or properties of interfaces between molecules and inorganic electrodes. Spin-crossover molecules (SCMs) are one of the most promising classes of candidates for molecular spintronics due to their bistability deriving from the existence of two spin states that can be reversibly switched by temperature, light, electric fields, etc. Building devices based on single or few molecules would entail connecting the molecule(s) with solid surfaces and understanding the fundamental behavior of the resulting assemblies. Herein, the investigations of SCMs on solid surfaces, ranging from isolated single molecules (submonolayers) to ultrathin films (mainly in the sub-10 nm range) are summarized. The achievements, challenges and prospects in this field are highlighted.

Theoretical inspection of the spin-crossover [Fe(tzpy)2(NCS)2] complex on Au(100) surface

We explore the deposition of the spin-crossover [Fe(tzpy)₂(NCS)₂] complex on the Au(100) surface by means of density functional theory (DFT) based calculations. Two different routes have been employed: low-cost finite cluster-based calculations, where both the Fe complex and the surface are maintained fixed while the molecule approaches the surface; and periodic DFT plane-wave calculations, where the surface is represented by a four-layer slab and both the molecule and surface are relaxed. Our results show that the bridge adsorption site is preferred over the on-top and fourfold hollow ones for both spin states, although they are energetically close. The LS molecule is stabilized by the surface, and the HS-LS energy difference is enhanced by about 15%-25% once deposited. The different Fe ligand field for LS and HS molecules manifests on the composition and energy of the low-lying bands. Our simulated STM images indicate that it is possible to distinguish the spin state of the deposited molecules by tuning the bias voltage of the STM tip. Finally, it should be noted that the use of a reduced size cluster to simulate the Au(100) surface proves to be a low-cost and reliable strategy, providing results in good agreement with those resulting from state-of-the-art periodic calculations for this system.

Iron in a Cage: Fixation of a Fe(II)tpy2 Complex by Fourfold Interlinking

The coordination sphere of the Fe(II) terpyridine complex 1 is rigidified by fourfold interlinking of both terpyridine ligands. Profiting from an octa-aldehyde precursor complex, the ideal dimensions of the interlinking structures are determined by reversible Schiff-base formation, before irreversible Wittig olefination provided the rigidified complex. Reversed-phase HPLC enables the isolation of the all-trans isomer of the Fe(II) terpyridine complex 1, which is fully characterized. While temperature independent low-spin states were recorded with superconducting quantum interference device (SQUID) measurements for both, the open precursor 8 and the interlinked complex 1, evidence of the increased rigidity of the ligand sphere in 1 was provided by proton T2 relaxation NMR experiments. The ligand sphere fixation in the macrocyclized complex 1 even reaches a level resisting substantial deformation upon deposition on an Au(111) surface, as demonstrated by its pristine form in a low temperature ultra-high vacuum scanning tunneling microscope experiment.

Spin-Crossover Complexes in Direct Contact with Surfaces

The transfer of the inherent bistability of spin crossover compounds to surfaces has attracted considerable interest in recent years. The deposition of the complexes on surfaces allows investigating them individually and to further understand the microscopic mechanisms at play. Moreover, it offers the prospect of engineering switchable functional surfaces. We review recent progress in the field with a particular focus on the challenges and limits associated with the dominant experimental techniques used, namely near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). One of the main difficulties in NEXAFS-based experiments is to ascertain that the complexes are in direct contact with the surfaces. We show that molecular coverage determination based on the amplitude of the edge-jump of interest is challenging because the latter quantity depends on the substrate. Furthermore, NEXAFS averages the signals of a large number of molecules, which may be in different states. In particular, we highlight that the signal of fragmented molecules is difficult to distinguish from that of intact and functional ones. In contrast, STM allows investigating individual complexes, but the identification of the spin states is at best done indirectly. As quite some of the limits of the techniques are becoming apparent as the field is gaining maturity, their detailed descriptions will be useful for future investigations and for taking a fresh look at earlier reports.

Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption

Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF₃)₂)₂(phen)] (pypyr = 2-(2'-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex.

Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior

The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)₂]⁺ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu₂N/Cu(100) surface is evidenced. The deposited Fe^III spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)₂ molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Enhanced Cooperativity in Supported Spin-Crossover Metal-Organic Frameworks

The impact of surface deposition on cooperativity is explored in Au(111)-supported self-assembled metal-organic frameworks (MOFs) based on Fe(II) ions. Using a thermodynamic model, we first demonstrate that dimensionality reduction combined with deposition on a metal surface is likely to deeply enhance the spin-crossover cooperativity, going from γ_3D = 16 K for the bulk material to γ_2D^supp = 386 K for its 2D supported derivative. On the basis of density functional theory, we then elucidate the electronic structure of a promising Fe-based MOF. A chemical strategy is proposed to turn a weakly interacting magnetic system into a strongly cooperative spin-crossover monolayer with γ_MOF^Au(111) = 83 K. These results open a promising route to the fabrication of cooperative materials based on SCO Fe(II) platforms.

Deposition of a Cationic FeIII Spin-Crossover Complex on Au(111): Impact of the Counter Ion

Spin-crossover molecules on metallic substrates have recently attracted considerable interest for their potential applications in molecular spintronics. Using scanning tunneling microscopy, we evidence the first successful deposition of a charged Fe^III spin-crossover complex, [Fe(pap)₂]⁺ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato), on Au(111). Furthermore, the bulk form of the molecules is stabilized by a perchlorate counterion, which depending on the deposition technique may affect the quality of the deposition and the measurements. Finally, we evidence switching of the molecules on Au(111).

Controlling the magnetism of adsorbed metal-organic molecules

Gaining control on the size or the direction of the magnetic moment of adsorbed metal-organic molecules constitutes an important step towards the realization of a surface-mounted molecular spin electronics. Such control can be gained by taking advantage of interactions of the molecule's magnetic moment with the environment. The paramagnetic moments of adsorbed metal-organic molecules, for example, can be controlled by the interaction with magnetically ordered substrates. Metalloporphyrins and -phthalocyanines display a quasi-planar geometry, allowing the central metal ion to interact with substrate electronic states. This can lead to magnetic coupling with a ferromagnetic or even antiferromagnetic substrate. The molecule-substrate coupling can be mediated and controlled by insertion layers such as oxygen atoms, graphene, or nonmagnetic metal layers. Control on the magnetic properties of adsorbed metalloporphyrins or -phthalocyanines can also be gained by on-surface chemical modification of the molecules. The magnetic moment or the magnetic coupling to ferromagnetic substrates can be changed by adsorption and thermal desorption of small molecules that interact with the fourfold-coordinated metal center via the remaining axial coordination site. Spin-crossover molecules, which possess a metastable spin state that can be switched by external stimuli such as temperature or light, are another promising class of candidates for control of magnetic properties. However, the immobilization of such molecules on a solid surface often results in a quench of the spin transition due to the interaction with the substrate. We present examples of Fe(II) spin-crossover complexes in direct contact with a solid surface that undergo a reversible spin-crossover transition as a function of temperature, by illumination with visible light, or can be switched by the tip of a scanning tunneling microscope.

High Spin Polarization at Ferromagnetic Metal-Organic Interfaces: A Generic Property

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metal's electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.