Thioether polymers are fundamental for a variety of applications. Their synthesis is, however, more challenging than that of other metal-catalyzed reactions due to the reported detachment of the S ...atom during thermal activation. In this study, it has been demonstrated unambiguously that thermal annealing results in the thioetherification of the 4-bromo-4-mercaptobiphenyl molecule (Br-MBP) adsorbed on the surface of Au(111). Through complementary techniques, such as scanning tunneling microscopy, spectroscopy, and first-principle calculations, we have identified four reaction steps, involving sulfhydryl or bromine molecular functional groups and leading to the formation of intermolecular C–S bonds. To form the thioether polymer and to overcome the competitive formation of C–C bonds, two reaction steps, the dehalogenation, and dissociation of the S–Au bond, must occur simultaneously. We detail the electronic properties of the phenyl–sulfur bond and the polymer as a function of the ligand length. This result suggests a wider perspective of this chemical synthesis.
Scanning probe microscopy and spectroscopy, and more recently, single-atom electron spin resonance, have allowed the direct observation of electron dynamics at the atomic limit. The interpretation of ...data is strongly dependent on model Hamiltonians. However, fitting effective spin Hamiltonians to experimental data lacks the ability to explore a vast number of potential systems of interest. By using plane-wave density functional theory as starting point, we build a multiplet Hamiltonian making use of maximally localized Wannier functions. The Hamiltonian contains spin–orbit and electron–electron interactions needed to obtain the relevant spin dynamics. The resulting reduced Hamiltonian is solved by exact diagonalization. We compare three prototypical cases of 3d transition metals Mn (total spin S = 5/2), Fe (S = 2), and Co (S = 3/2) on MgO with experimental data and find that our calculations can accurately predict the spin orientation and anisotropy of the magnetic adatom. Our method does not rely on experimental input and allows us to explore and predict the fundamental magnetic properties of adatoms on surfaces.
The ability of molecules to maintain magnetic multistability in nanoscale-junctions will determine their role in downsizing spintronic devices. While spin-injection from ferromagnetic leads gives ...rise to magnetoresistance in metallic nanocontacts, nonmagnetic leads probing the magnetic states of the junction itself have been considered as an alternative. Extending this experimental approach to molecular junctions, which are sensitive to chemical parameters, we demonstrate that the electron affinity of a molecule decisively influences its spin transport. We use a scanning tunneling microscope to trap a meso-substituted iron porphyrin, putting the iron center in an environment that provides control of its charge and spin states. A large electron affinity of peripheral ligands is shown to enable switching of the molecular S = 1 ground state found at low electron density to S = 1/2 at high density, while lower affinity keeps the molecule inactive to spin-state transition. These results pave the way for spin control using chemical design and electrical means.
We report on a theoretical study of adsorption of 4-acetylbiphenyl molecule and its diffusion properties in the main directions of the Au(111) surface. Structural changes of the molecule, which are ...induced by adsorption, lead to stronger conjugation of the π-system. The molecule is adsorbed in a flat configuration on the surface with roughly the same binding energy along the 11̅0 and 112̅ directions, in good agreement with experiments. Furthermore, the diffusion barriers imply an important directionality of the molecule–surface interactions. This is somewhat surprising because our calculations show that the prevailing interaction is the long-range molecule–surface van der Waals interaction. Despite its weakness, the van der Waals interaction discriminates the preferential adsorption sites as well as imposes a molecular geometry that needs to be considered when rationalizing the diffusion barriers.
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.
We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic ...and vibrational symmetries in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states--namely those belonging to the most transmitting eigenchannels--need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.
We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intragap spectral features. Chromium atoms on the superconductor β−Bi2Pd surface display ...Yu-Shiba-Rusinov bound states, detected as pairs of intragap excitations in tunneling spectra. By means of atomic manipulation with a scanning tunneling microscope’s tip, we form Cr dimers with different arrangements and find that their intragap features appear either shifted or split with respect to single atoms. These spectral variations are associated with the magnetic coupling, ferromagnetic or antiferromagnetic, of the dimer, as confirmed by density functional theory simulations. The striking qualitative differences between the observed tunneling spectra prove that intragap Shiba states are extremely sensitive to the magnetic ordering on the atomic scale.
The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in ...which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a Cu2N surface with a scanning tunneling microscope. The atoms couple antiferromagnetically via superexchange interaction through the nitrogen atom network of the surface. The emergent Kondo resonance is spatially distributed along the chain. Its strength can be controlled by mixing atoms of different transition metal elements and manipulating their spatial distribution. We show that the Kondo screening of the full chain by the electrons of the nonmagnetic substrate depends on the interatomic entanglement of the spins in the chain, demonstrating the prerequisites to build and probe spatially extended strongly correlated nanostructures.
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 tetrabenzoa,c,j,hphenazine 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.
The structure of molecular adlayers is of great interest for surface functionalization. As molecular complexity increases, the subtle interplay of the relevant interactions becomes more difficult to ...unravel. Here, we present a scanning tunneling microscope (STM) and atomic force microscope study along with free-energy calculations using density functional theory on two closely related NDI-cyclophane molecules. These three-dimensional double-decker molecules are designed to attach to the substrate with one subunit while the other functional moiety is exposed to the environment. The molecular arrangements obtained on Ag(111) are rationalized by the inclusion of adatoms from the substrate into the structure. The presence of adatoms is identified by a drastic change in corrugation of the STM images that takes place at moderate bias voltages. Our calculations using density functional theory of the system’s free-energy yield that two adatoms favorably coadsorb with the molecules.
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