Transfer of Cl Ligands between Adsorbed Iron Tetraphenylporphyrin Molecules

Conformational Switching of a Nano-Size Urea-Bridged Zn(II)Porphyrin Dimer by External Stimuli

For the first time, explicit stabilization of all the three conformers, viz. (cis,cis), (cis,trans) and (trans,trans), of a 'nano-sized' highly-flexible urea-bridged Zn(II)porphyrin dimer have been achieved via careful manipulations of external stimuli such as solvent dielectrics, temperature, anionic interactions, axial ligation and surface-induced stabilization. The conformers differ widely in their structures, chemical and photophysical properties and thus have vast potential applicability. X-ray structural characterizations have been reported for the (cis,cis) and (cis,trans)-conformers. While (cis,cis) conformer stabilized exclusively in dichloromethane, more polar solvents resulted in the stabilization of (cis,trans) and (trans,trans)-conformers. Low temperature promotes the stabilization of (cis,trans)-conformer while rise in temperature facilitates flipping to the (cis,cis) one. Significantly, exclusive stabilization of the (trans,trans)-isomer has been illustrated using acetate anion which facilitates H-bonding with the two amide linkages of the urea spacer. Remarkably, HOPG surface facilitates stabilization of the energetically challenging (trans,trans)-conformer via CH⋅⋅⋅π and π⋅⋅⋅π interactions with the solid surface to the porphyrinic cores. DFT calculations demonstrate that the relative stability of the conformers can be modulated upon slight external perturbations as also observed in the experiment. Several factors contributing towards the conformational landscape for the highly flexible urea-bridged porphyrin dimers have been mapped.

Spin-state switching of indium-phthalocyanine on Pb(100)

Indium(iii) phthalocyanine chloride deposited on Pb(100) is studied by scanning tunnelling spectroscopy at cryogenic temperatures. The Cl ions are dissociated and the remaining indium phthalocyanine (InPc) is observed in two states with the metal ion pointing to (↓) or away (↑) from the substrate. Isolated molecules and islands with a superstructure and a unit cell of four inequivalent molecules, namely one InPc↑ and three InPc↓ in different sites, are observed. Using atomic resolution images of the substrate the adsorption sites and azimuthal orientation of InPc are determined and a structure model is proposed. Conductance spectra of the lowest unoccupied molecular orbital reveal differences that depend on the adsorption sites and azimuthal orientations of the complexes. Only InPc↑ molecules exhibit Shiba states, indicating the presence of a localized spin. By electron extraction isolated complexes as well as molecules in islands are converted from InPc↑ to InPc↓. At the same time, their spin state changes, as indicated by the disappearance of the Shiba states.

A p-orbital honeycomb-Kagome lattice realized in a two-dimensional metal-organic framework

The experimental realization of p-orbital systems is desirable because p-orbital lattices have been proposed theoretically to host strongly correlated electrons that exhibit exotic quantum phases. Here, we synthesize a two-dimensional Fe-coordinated bimolecular metal-organic framework which constitutes a honeycomb lattice of 1,4,5,8,9,12-hexaazatriphenylene molecules and a Kagome lattice of 5,15-di(4-pyridyl)-10,20-diphenylporphyrin molecules on a Au(111) substrate. Density-functional theory calculations show that the framework features multiple well-separated spin-polarized Kagome bands, namely Dirac cone bands and Chern flat bands, near the Fermi level. Using tight-binding modelling, we reveal that these bands are originated from two effects: the low-lying molecular orbitals that exhibit p-orbital characteristics and the honeycomb-Kagome lattice. This study demonstrates that p-orbital Kagome bands can be realized in metal-organic frameworks by using molecules with molecular orbitals of p-orbital like symmetry.

Making and breaking of chemical bonds in single nanoconfined molecules

Nanoconfinement of catalytically active molecules is a powerful strategy to control their chemical activity; however, the atomic-scale mechanisms are challenging to identify. In the present study, the site-specific reactivity of a model rhenium catalyst is studied on the subnanometer scale for complexes confined within quasi-one-dimensional molecular chains on the Ag(001) surface by scanning tunneling microscopy. Injection of tunneling electrons causes ligand dissociation in single molecules. Unexpectedly, while half of the complexes show only the dissociation, the confined molecules show also the reverse reaction. On the basis of density functional theory calculations, this drastic difference can be attributed to the limited space in confinement. That is, the split-off ligand adsorbs closer to the molecule and the dissociation causes less structural disruption. Both of these facilitate the reverse reaction. We demonstrate formation and disruption of single chemical bonds of nanoconfined molecules with potential application in molecular data storage.

Extending the Spin Excitation Lifetime of a Magnetic Molecule on a Proximitized Superconductor

Molecular spins on surfaces potentially used in quantum information processing and data storage require long spin excitation lifetimes. Normally, coupling of the molecular spin with the conduction electrons of metallic surfaces causes fast relaxation of spin excitations. However, the presence of superconducting pairing effects in the substrate can protect the excited spin from decaying. In this work, we show that a proximity-induced superconducting gold film can sustain spin excitations of a FeTPP-Cl molecule for more than 80 ns. This long value was determined by studying inelastic spin excitations of the S = 5/2 multiplet of FeTPP-Cl on Au films over V(100) using scanning tunneling spectroscopy. The spin lifetime decreases with increasing film thickness, along with the decrease of the effective superconducting gap. Our results elucidate the use of proximitized gold electrodes for addressing quantum spins on surfaces, envisioning new routes for tuning the value of their spin lifetime.

Three-Bit Digital Comparator Based on Intracell Diffusion of Silver Single Atom

Using a single atom to construct electronic components is a promising route for the microminiaturization of electronic instruments. However, effective control of the intrinsic property in a molecular/atomic prototype component is full of challenges. Here, we present that the intracell diffusion behavior of a target Ag single atom within a unit cell of Si reconstruction is controllably modulated by constructing Ag nanoclusters and arrays in the neighboring cells. Moreover, a three-bit digital comparator device is fabricated on the basis of the diffusion time of a Ag single atom that can be effectively regulated by using the intercoupling between the target Ag monomer and the surrounding metal arrays.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

On-Surface Synthesis of One-Dimensional Coordination Polymers with Tailored Magnetic Anisotropy

One-dimensional (1D) metalloporphyrin polymers can exhibit magnetism, depending on the central metal ion and the surrounding ligand field. The possibility of tailoring the magnetic signal in such nanostructures is highly desirable for potential spintronic devices. We present low-temperature (4.2 K) scanning tunneling microscopy and spectroscopy (LT-STM/STS) in combination with high-resolution atomic force microscopy (AFM) and a density functional theory (DFT) study of a two-step synthetic protocol to grow a robust Fe-porphyrin-based 1D polymer on-surface and to tune its magnetic properties. A thermally assisted Ullmann-like coupling reaction of Fe(III)diphenyl-bromine-porphyrin (2BrFeDPP-Cl) on Au(111) in ultra-high vacuum results in long (up to 50 nm) 1D metal-organic wires with regularly distributed magnetic and (electronically) independent porphyrins units, as confirmed by STM images. Thermally controlled C-H bond activation leads to conformational changes in the porphyrin units, which results in molecular planarization steered by 2D surface confinement, as confirmed by high-resolution AFM images. Spin-flip STS images in combination with DFT self-consistent spin-orbit coupling calculations of porphyrin units with different structural conformations reveal that the magnetic anisotropy of the triplet ground state of the central Fe ion units drops down substantially upon intramolecular rearrangements. These results point out to new opportunities for realizing and studying well-defined 1D organic magnets on surfaces and demonstrate the feasibility of tailoring their magnetic properties.

Intraconfigurational Transition due to Surface-Induced Symmetry Breaking in Noncovalently Bonded Molecules

The interaction of molecules with surfaces plays a crucial role in the electronic and chemical properties of supported molecules and needs a comprehensive description of interfacial effects. Here, we unveil the effect of the substrate on the electronic configuration of iron porphyrin molecules on Au(111) and graphene, and we provide a physical picture of the molecule-surface interaction. We show that the frontier orbitals derive from different electronic states depending on the substrate. The origin of this difference comes from molecule-substrate orbital selective coupling caused by reduced symmetry and interaction with the substrate. The weak interaction on graphene keeps a ground state configuration close to the gas phase, while the stronger interaction on gold stabilizes another electronic solution. Our findings reveal the origin of the energy redistribution of molecular states for noncovalently bonded molecules on surfaces.

Reversible coordination-induced spin-state switching in complexes on metal surfaces

Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces^1,2, which is relevant for molecular spintronics devices^3-5. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions^6-9. Robust metal-organic platforms, on the other hand, rely on external axial ligands to induce spin switching^10-14. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces¹⁵.

Switching the Spin on a Ni Trimer within a Metal-Organic Motif by Controlling the On-Top Bromine Atom

Controlling the spin of metal atoms embedded in molecular systems is a key step toward the realization of molecular electronics and spintronics. Many efforts have been devoted to explore the influencing factors dictating the survival or quenching of a magnetic moment in a metal-organic molecule, and among others, the spin control by axial ligand attachments is the most promising. Herein, from the interplay of high-resolution scanning tunneling microscopy imaging/manipulation and scanning tunneling spectroscopy measurements together with density functional theory calculations, we successfully demonstrate that a Ni trimer within a metal-organic motif acquires a net spin promoted by the adsorption of an on-top Br atom. The spin localization in the trimetal centers bonded to Br was monitored via the Kondo effect. The removal of the Br ligand resulted in the switch from a Kondo ON to a Kondo OFF state. The magnetic state induced by the Br ligand is theoretically attributed to the enhanced Br 4p_z and Ni 3d_z² states due to the charge redistribution. The manipulation strategy reported here provides the possibility to explore potential applications of spin-tunable structures in spintronic devices.

Implementing Functionality in Molecular Self-Assembled Monolayers

The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on a Ag(111) metal substrate show different charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric fields, whereas their charge state is fixed otherwise. This enables the activation of selected molecules inside islands by vacancy creation from scanning-probe-based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on one hand, and the implementation of specific functionality into otherwise homogeneous monolayers, on the other. Activated molecules in the direct neighborhood influence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which offers the prospect of implementing complex functionality in such structures in the future.

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.

Precise Control of Local Spin States in an Adsorbed Magnetic Molecule with an STM Tip: Theoretical Insights from First-Principles-Based Simulation

The precise tuning of local spin states in adsorbed organometallic molecules by a mechanically controlled scanning tunneling microscope (STM) tip has become a focus of recent experiments. However, the underlying mechanisms remain somewhat unclear. We investigate theoretically the STM tip control of local spin states in a single iron(II) porphyrin molecule adsorbed on the Pb(111) substrate. A combined density functional theory and hierarchical equations of motion approach is employed to simulate the tip tuning process in conjunction with the complete active space self-consistent field method for accurate computation of magnetic anisotropy. Our first-principles-based simulation accurately reproduces the tuning of magnetic anisotropy realized in experiment. Moreover, we elucidate the evolution of geometric and electronic structures of the composite junction and disclose the delicate competition between the Kondo resonance and local spin excitation. The understanding and insight provided by the first-principles-based simulation may help to realize more fascinating quantum state manipulations.

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.

Desorption of CO from individual ruthenium porphyrin molecules on a copper surface via an inelastic tunnelling process

The coordination of CO to metalloporphyrins changes their electronic and magnetic properties. Here we locally desorb CO molecules from a single ruthenium tetraphenylporphyrin carbonyl (CO-RuTPP) on Cu(110) using STM. The desorption is triggered by the injection of holes into the occupied states of the adsorbate using an unusual two-carrier process.

Survival of spin state in magnetic porphyrins contacted by graphene nanoribbons

We report on the construction and magnetic characterization of a fully functional hybrid molecular system composed of a single magnetic porphyrin molecule bonded to graphene nanoribbons with atomically precise contacts. We use on-surface synthesis to direct the hybrid creation by combining two molecular precursors on a gold surface. High-resolution imaging with a scanning tunneling microscope finds that the porphyrin core fuses into the graphene nanoribbons through the formation of new carbon rings at chemically predefined positions. These ensure the stability of the hybrid and the extension of the conjugated character of the ribbon into the molecule. By means of inelastic tunneling spectroscopy, we prove the survival of the magnetic functionality of the contacted porphyrin. The molecular spin appears unaffected by the graphenoid electrodes, and we simply observe that the magnetic anisotropy appears modified depending on the precise structure of the contacts.

Adsorption of iron tetraphenylporphyrin on (111) surfaces of coinage metals: a density functional theory study

The adsorption of the iron tetraphenylporphyrin (FeTPP) molecule in its deckchair conformation was investigated on Au(111), Ag(111) and Cu(111) surfaces by performing spin-polarized density functional theory (DFT) calculations taking into account both van der Waals (vdW) interaction and on-site Coulomb repulsion. The deckchair conformation of the molecule favours intermolecular π-π-type interactions in a less densely packed monolayer than the saddle conformation. The activation barrier between the two stable magnetic states (high spin, S = 2 and intermediate spin, S = 1) of the molecule in vacuum disappears upon adsorption on the metal surfaces. The high-spin state of physisorbed FeTPP is stable on all adsorption sites. This result reveals that an external permanent element such as a STM tip or an additional molecule is needed to use FeTPP or similar molecules as model system for molecular spin switches.

Evidence for Ultralow-Energy Vibrations in Large Organic Molecules

The quantum efficiency or the rate of conversion of incident photon to free electron in photosynthesis is known to be extremely high. It has long been thought that the origin of this efficiency are molecular vibrations leading to a very fast separation of electrons and holes within the involved molecules. However, molecular vibrations are commonly in the range above 100 meV, which is too high for excitations in an ambient environment. Here, we analyze experimental spectra of single organic molecules on metal surfaces at ∼4 K, which often exhibit a pronounced dip. We show that measurements on iron(II) [tetra-(pentafluorophenyl)]porphyrin resolve this single dip at 4 K into a series of step-shaped inelastic excitations at 0.4 K. Via extensive spectral maps under applied magnetic fields and corresponding theoretical analysis we find that the dip is due to ultralow-energy vibrations of the molecular frame, typically in the range below 20 meV. The result indicates that ultralow energy vibrations in organic molecules are much more common than currently thought and may be all-pervasive for molecules above a certain size.

Designing a molecular magnetic button based on 4d and 5d transition-metal phthalocyanines

The field of molecular spintronics exploits the properties of organic molecules possessing a magnetic moment, either native in the form of radicals or induced by the insertion of transition metal magnetic ions. To realize logic or storage molecular spin-tronics devices, molecules with stable different magnetic states should be deposited on a substrate, and switching between the states controllably achieved. By means of a first-principles calculations, we have devised a functional molecule exhibiting different magnetic states upon structural changes induced by current injection. We investigate the prototypical case of non-planar M-Phthalocyanine (MPc), where M is a transition-metal ion belonging to the 4d and 5d series. We find that for ZrPc and HfPc deposited on a graphene decorated Ni(111) substrate, two different structural conformations could be stabilized, for which the molecules attain different magnetic states depending on the position of the M ion - whether above the Pc or between the Pc and the substrate -, acting therefore as molecular magnetic button. Our work indicates an intuitive way to engineer a magnetic molecular switch with tailored properties, starting from the knowledge of the basic atomic properties of elements and surfaces.

Diels-Alder attachment of a planar organic molecule to a dangling bond dimer on a hydrogenated semiconductor surface

Construction of single-molecule electronic devices requires the controlled manipulation of organic molecules and their properties. This could be achieved by tuning the interaction between the molecule and individual atoms by local "on-surface" chemistry, i.e., the controlled formation of chemical bonds between the species. We demonstrate here the reversible attachment of a planar conjugated polyaromatic molecule to a pair of unpassivated dangling bonds on a hydrogenated Ge(001):H surface via a Diels-Alder [4+2] addition using the tip of a scanning tunneling microscope (STM). Due to the small stability difference between the covalently bonded and a nearly undistorted structure attached to the dangling bond dimer by long-range dispersive forces, we show that at cryogenic temperatures the molecule can be switched between both configurations. The reversibility of this covalent bond forming reaction may be applied in the construction of complex circuits containing organic molecules with tunable properties.

Switching of an Azobenzene-Tripod Molecule on Ag(111)

The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

Axial Ligand Effects on the Structures of Self-Assembled Gallium-Porphyrin Monolayers on Highly Oriented Pyrolytic Graphite

Monolayers of five-coordinate gallium octaethylporphyrin complexes (Ga(OEP)X; X = Cl, Br, I, O3SCF3, CCPh) on highly oriented pyrolytic graphite were studied at the solid-liquid (1-phenyloctane) interface using scanning tunneling microscopy (STM) to probe the dependence of their properties on the nature of the axial X ligand. Density functional theory calculations of the gas-phase structures of the free molecules reveal that the gallium atom is positioned above the plane of the porphyrin macrocycle, with this pyramidal distortion increasing in magnitude according to X = O3SCF3 (displacement = 0.35 Å) < Cl, Br, I (∼0.47 Å) < CCPh (0.54 Å). All compounds exhibit pseudohexagonal close-packed structures in which the porphyrin is oriented coplanar with the surface and the axial ligand is oriented perpendicular to it, and with unit-cell parameters that are within experimental error of each other (a, b = 1.34 (3)-1.39 (2) nm, Γ = 66 (2)-68 (1)°). In contrast to these close similarities, the stabilities of the monolayers are sensitive to the nature of the axial ligand: the monolayers of Ga(OEP)(O3SCF3) and Ga(OEP)(CCPh) exhibit damage during the STM experiment upon repeated scanning and upon toggling the sign of the bias potential, but monolayers of Ga(OEP)Cl, Ga(OEP)Br, and Ga(OEP)I do not. A second important ligand-influenced property is that Ga(OEP)I forms bilayer structures, whereas the other Ga(OEP)X compounds form monolayers exclusively under identical conditions. The top layer of the Ga(OEP)I bilayer is oriented with the iodo ligand directed away from the surface, like the bottom layer, but the molecules pack in a square, lower-density geometry. The comparatively large polarizability of the iodo ligand is suggested to be important in stabilizing the bilayer structure.

Charging single Co atoms on ultrathin NaCl films

Single Co adatoms adsorbed on a double-layer NaCl film were negatively charged after applying a positive voltage pulse by STM.

Shifting the Voltage Drop in Electron Transport Through a Single Molecule

A Mn-porphyrin was contacted on Au(111) in a low-temperature scanning tunneling microscope (STM). Differential conductance spectra show a zero-bias resonance that is due to an underscreened Kondo effect according to many-body calculations. When the Mn center is contacted by the STM tip, the spectrum appears to invert along the voltage axis. A drastic change in the electrostatic potential of the molecule involving a small geometric relaxation is found to cause this observation.

Tuning the Magnetic Anisotropy of Single Molecules

The magnetism of single atoms and molecules is governed by the atomic scale environment. In general, the reduced symmetry of the surrounding splits the d states and aligns the magnetic moment along certain favorable directions. Here, we show that we can reversibly modify the magnetocrystalline anisotropy by manipulating the environment of single iron(II) porphyrin molecules adsorbed on Pb(111) with the tip of a scanning tunneling microscope. When we decrease the tip-molecule distance, we first observe a small increase followed by an exponential decrease of the axial anisotropy on the molecules. This is in contrast to the monotonous increase observed earlier for the same molecule with an additional axial Cl ligand ( Nat. Phys. 2013 , 9 , 765 ). We ascribe the changes in the anisotropy of both species to a deformation of the molecules in the presence of the attractive force of the tip, which leads to a change in the d level alignment. These experiments demonstrate the feasibility of a precise tuning of the magnetic anisotropy of an individual molecule by mechanical control.

Towards single molecule switches

Scanning tunneling microscope (STM) controlled reversible switching of a single-dipole molecule imbedded in hydrogen-bonded binary molecular networks on graphite.

Change of the magnetic coupling of a metal-organic complex with the substrate by a stepwise ligand reaction

The surface-assisted intramolecular ligand reaction of a porphyrin molecule adsorbed on Au(111) is studied by scanning tunneling microscopy and spectroscopy. The temperature-induced stepwise transformation of iron octaethylporphyrin proceeds via a concentric electrocyclic ring closure, with the final product iron tetrabenzoporphyrin being identified by its characteristic Kondo resonance. Along with the transformation of the organic ligand, changes in the magnetic fingerprint are observed, indicating an increasing coupling of the iron spin with the substrate electrons.