2D metal-organic frameworks have been recently proposed as a flexible platform for realizing new functional materials including quantum phases. Here, we present a method to create metal-organic dimer complexes by on-surface assembly on a metal substrate using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). We demonstrate that a dimer of Mn-Phthalocyanine (MnPc)2 on a Ag(111) surface can be switched between two stable configurations upon a small conformational change controlled by STM manipulation. By means of density-functional theory calculations, it is found that the two conformations correspond to an antiferromagnetic (AFM) and a ferromagnetic (FM) state respectively. Directly coordinated Mn atoms of the dimer lead to an AFM-coupling whereas indirectly coordinated (shifted) Mn atoms lead to a FM-coupling. Rarely seen in a molecular-dimers with transition-metal atoms, the FM-AFM-FM transition is thus readily on-surface accessible. Furthermore, the two configurations of the switch are easily identified by their Kondo states, opening interesting routes in terms of both, writing (FM versus AFM states) and reading. These results pave the experimental route toward dimer-based materials with complex magnetic structures of potential interest for application in spintronics, logics and computing. On-surface assembled dimers of Mn-Phthalocyanine (MnPc)2 can be reversibly switched between an antiferromagnetic and a ferromagnetic state upon a small conformational change controlled by STM manipulation. The two states of the molecular switch are easily identified spectroscopically by their Kondo resonance, opening interesting routes for applications in molecular spintronics, logics and computing. image

Nanostructures, fabricated by locating molecular building blocks in well-defined positions, for example, on a lattice, are ideal platforms for studying atomic-scale quantum effects. In this context, STM data obtained from self-assembled Bis(phthalocyaninato) Terbium (III) (TbPc2) single-molecule magnets on various substrates have raised questions about the conformation of the TbPc2 molecules within the lattice. In order to address this issue, molecular dynamics simulations were carried out on a 2D assembly of TbPc2 molecules. The calculations are in excellent agreement with the experiment, and thus improve our understanding of the self-assembly process. In particular, the calculated electron density of the molecular assembly compares well with STM contrast of self-assembled TbPc2 on Au(111), simultaneously providing the conformation of the two Pc ligands of the individual double-decker molecule. This approach proves valuable in the identification of the STM contrast of LnPc(2) layers and could be used in similar cases where it is difficult to interpret the STM images of an assembly of molecular complexes.

We have investigated the morphological and optical properties of α- and β-phase Zinc Phthalocyanine (ZnPc) thin films for application to organic photovoltaic cells (OPVs). It was found that the α-phase is completely converted to the β-phase by thermal annealing at 220 °C under ultrahigh vacuum conditions. When the α- to β-phase transition takes place, the surface roughness of the ZnPc film became flat uniformly with a nanometer order of unevenness by anisotropic growth of crystalline grains along a lateral direction to substrates. Correspondingly, the optical absorbance of the β-phase film became greater by 1.5-2 times than that of the α-phase one in an ultraviolet-visible-near infrared (UV-vis-NIR) wavelength range, which plays a role in increasing the number of photogenerated excitons. On the contrary, time-resolved photoluminescence measurements showed that the average lifetime of excitons for the β-phase film became shorter by 1/6-1/7 than that for the α-phase one, which plays a role in decreasing the number of excitons achieving the donor/acceptor interface where excitons are separated to carriers (holes and electrons). Both the increase in the number and the shortening in the average lifetime have a trade-off relationship with each other for contribution to the photoelectric conversion efficiency of OPVs. Then, we examined an external quantum efficiency (EQE) of OPVs using the α- and β-phase films as a donor and obtained that the former OPV (α-phase) exhibits a higher EQE by ∼2 times than the latter one (β-phase) in the wavelength range of 400 nm-800 nm.

The active material of optoelectronic devices must accommodate for contacts which serve to collect or inject the charge carriers. It is the purpose of this work to find out to which extent properties of organic optoelectronic layers change close to metal contacts compared to known properties of bulk materials. Bottom-up fabrication capabilities of model interfaces under ultrahigh vacuum and single-atom low temperature (LT)-STM spectroscopy with density functional theory (DFT) calculations are used to detect the spatial modifications of electronic states such as frontier-orbitals at interfaces. The system under consideration is made of a silver substrate covered with a blend of C-60 and ZnPc molecules of a few monolayers. When C-60 and ZnPc are separately adsorbed on Ag(111), they show distinct spectroscopic features in STM. However, when C-60 is added to the ZnPc monolayer, it shows scanning tunneling spectra similar to ZnPc, revealing a strong interaction of C-60 with the ZnPc induced by the substrate. DFT calculations on a model complex confirm the strong hybridization of C-60 with ZnPc layer upon adsorption on Ag(111), thus highlighting the role of boundary layers where the donor-acceptor character is strongly perturbed. The calculation also reveals a significant charge transfer from the Ag to the complex that is likely responsible for a downward shift of the molecular LUMO in agreement with the experiment.

The vibrational excitation related transport properties of a manganese phthalocyanine molecule suspended between the tip of a scanning tunneling microsope (STM) and a surface are investigated by combining the local manipulation capabilities of the STM with inelastic electron tunneling spectroscopy. By attachment of the molecule to the probe tip, the intrinsic physical properties similar to those exhibited by a free standing molecule become accessible. This technique allows one to study locally the magnetic properties, as well as other elementary excitations and their mutual interaction. In particular a clear correlation is observed between the Kondo resonance and the vibrations with a strong incidence of the Kondo correlation on the thermopower measured across the single-molecule junction.

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

Two-dimensional networks of spins are fascinating both for the study of low-dimensional magnetism and for the prospects in molecular quantum devices. Here we have fabricated a 2D supramolecular lattice consisting of manganese phthalocyanine (MnPc) on Ag(111). Low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations are applied to study the magnetic state and the electronic structure evolution from the isolated molecule to the fully 2D self-assembled molecular network. It is found that the magnetic Kondo resonance on the Mn ion is not affected by the increasing molecular 2D coordination, whereas an unusual extension of the Kondo resonance over the MnPc molecules provides additional evidence for the magnetic polarization of the ligand. Both STS and ab initio electronic structure calculations demonstrate the formation of an underscreened 2D Kondo lattice of MnPc molecules that are prone to long-range antiferromagnetic order. A checkerboard configuration of the molecular spin density is formed above the Ag(111) surface as a result of an indirect exchange interaction mediated by the silver substrate.

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

In this work, we have focused on the elaboration and characterization of nanostructured L1(0)-CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of plane single domain state. The VSM measurements have indicated a correlated super-spin glass state (CSSG) collective behavior. Consequently, the magnetic properties are mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions. (C) 2017 Elsevier B.V. All rights reserved.

We have examined the structural, electronic, and optical properties of zinc-octaethylporphyrin [Zn(OEP)]/C60 co-deposited films to elucidate the donor (D)-acceptor (A) interactions at the D/A interface of heterojunction organic solar cells (OSCs), using Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectroscopy in combination with first-principles and semi-empirical calculations. The FT-IR and XRD results indicated that Zn(OEP) and C60 were mixed with each other at the molecular level in the co-deposited film. The theoretical calculations suggested that in the interfacial region, it is energetically preferable for the C60 molecule to face the center of the planar structure of Zn(OEP) at a distance of 2.8 Å rather than the edge of the structure at a distance of 5.0 Å. After consideration of the C60 solvent effects, this coordination model for C60-Zn(OEP) adequately explained the line shift of the UV-vis peaks with respect to the proportion of C60 in the co-deposited films. A comparison of the energy level diagrams of Zn(OEP) before and after the interaction with C60 revealed that the LUMO, HOMO, and HOMO-1 were significantly affected by the interaction with C60. In particular, the HOMO-1 wave function became spread over a portion of C60, although the charge transfer between Zn(OEP) and C60 was almost negligible. Since no PL peaks (S1 → S0) from the excited Soret band of Zn(OEP) were observed for the Zn(OEP)/C60 co-deposited films, the D/A mixing layers played a crucial role in completely dissolving the photogenerated excitons to electrons-hole pairs that cause the short-circuit current, which is relevant to improving the energy conversion efficiency of OSCs.

•Nanostructured magnetic dot arrays were fabricated by depositing CoPt clusters.•The MFM observations have revealed an out-of-plane single domain state.•The VSM measurements have indicated a correlated super-spin glass (CSSG) state.•The magnetic behavior is governed by a CSSG state with strong dipolar interaction.

In this chapter, we review the recent progress in the synthesis of phthalocyanine compounds at metallic surfaces under ultra-high vacuum conditions. Starting with tetra-carbonitrile-benzene molecules and magnetic atoms such as iron and manganese, we show that metal–organic coordination networks are formed at room temperature; then annealing at 500–600 K leads to the on-surface formation of phthalocyanine compounds. This reaction has been studied step-by-step by scanning tunneling microscopy and spectroscopy. The last part of this chapter is dedicated to the study of larger precursors functionalized with tetra-carbonitrile groups that react with copper atoms to form original polymers linked by phthalocyanine cores.

Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope (STM) tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au(111) at a temperature of 4.6 K in the presence of H2. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H2 pinning at an atomic cluster.

Scanning tunneling spectroscopy (STS) has become a key tool for accessing properties of organometallic molecules adsorbed on surfaces. However, the rich variety of signatures makes it sometimes a difficult task to find out which feature is intrinsic to the molecule, i.e., relevant for a metal ligand interaction or related to the interaction of the molecule with the substrate. Here we study the prototype covalent self-assembly of FePc and probe how electronic/magnetic properties at the local scale change as a function of temperature-induced step-by-step assembly, starting from TCNB (1,2,4,5- Tetracyanobenzene) molecular and Fe atomic precursors. Intermediate complexes with tetracoordinated Fe atoms are then used both, as synthons for the FePc and as identifiers of specific features of the STS. As observed by STS and confirmed by spin-polarized DFT calculations, the occupied d(pi) states of Fe are present in both the FePc and Fe(TCNB)(2) on Au(111). The main difference appears in the d(z)(2) states, which play a key role in magnetism as confirmed by the presence/absence of the Kondo resonance. A comprehensive picture is obtained by following with STS the hybridization of the d(z)(2) orbital of Fe to various substrates (Cu, Au and Co). Finally it is demonstrated that FePc units can be created by on-surface polymerization from the Fe(TCNB)(2) network upon thermal annealing.

A study of the surface assisted self-assembly of 1, 2, 4, 5-tetracyanobenzene (TCNB) acceptor molecules and Fe atoms on an Au(111) surface is presented. While conditions to get the two-dimensional arrays of stable Fe(TCNB)4 complexes are clearly identified, ultrahigh vacuum scanning tunneling microscopy and spectroscopy (STM/STS) coupled with first-principles calculations reveals that situations may occur where Fe and TCNB survive on the surface (as Fe–4TCNB entities) at a higher density than the original molecular monolayer without forming coordination bonds with each other. It is found that the square planar coordination of the Fe(TCNB)4 monomer complexes cannot fully develop in the presence of lateral strain due to growth-induced confinement. A phenomenon similar to steric hindrance involving a strongly modified chirality with a Fe–N–C bond angle of 120° compared to the 180° for the stable complex may then explain why the Fe atom keeps its metallic bond with the surface. The competition between steric and electronic effects, not reported before, may arise elsewhere in surface chemistry involved in the synthesis of new and potentially useful organic nanomaterials.

A study of the surface assisted self-assembly of 1,2,4,5-tetracyanobenzene (TCNB) acceptor molecules and Fe atoms on an Au(111) surface is presented. While conditions to get the two-dimensional arrays of stable Fe(TCNB)4 complexes are clearly identified, ultrahigh vacuum scanning tunneling microscopy and spectroscopy (STM/STS) coupled with first-principles calculations reveals that situations may occur where Fe and TCNB survive on the surface (as Fe-4TCNB entities) at a higher density than the original molecular monolayer without forming coordination bonds with each other. It is found that the square planar coordination of the Fe(TCNB)4 monomer complexes cannot fully develop in the presence of lateral strain due to growth-induced confinement. A phenomenon similar to steric hindrance involving a strongly modified chirality with a Fe-N-C bond angle of 120° compared to the 180° for the stable complex may then explain why the Fe atom keeps its metallic bond with the surface. The competition between steric and electronic effects, not reported before, may arise elsewhere in surface chemistry involved in the synthesis of new and potentially useful organic nanomaterials.