Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.
We investigate electroluminescence of single molecular emitters on NaCl on Ag (111) and Au(111) with submolecular resolution in a low-temperature scanning probe microscope with tunneling current, atomic force and light detection capabilities. Role of the tip state is studied in the photon maps of a prototypical emitter, zinc phthalocyanine (ZnPc), using metal and CO-metal tips. CO-functionalization is found to have a dramatic impact on the resolution and contrast of the photon maps due to the localized overlap of the p-orbitals on the tip with the molecular orbitals of the emitter. The possibility of using the same CO-functionalized tip for tip-enhanced photon detection and high resolution atomic force is demonstrated. We study the electroluminescence of ZnPc, induced by charge carrier injection at sufficiently high bias voltages. We propose that the distinct level alignment of the ZnPc frontier orbital with the Au(111) and Ag(111) Fermi levels governs the primary excitation mechanisms as the injection of electrons and holes from the tip into the molecule, respectively. These findings put forward the importance of the tip status in the photon maps and contribute to a better understanding of the photophysics of organic molecules on surfaces.
One-dimensional metal–organic chains often possess a complex magnetic structure susceptible to modification by alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasi-particle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal–organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized under ultrahigh-vacuum conditions on a Au(111) surface. A combination of integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enables us to obtain essential information about their magnetic structures, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.
Tuning the chemical properties of graphene by controlled doping is a widely investigated strategy. The effect of a substitutional single dopant on graphene local reactivity is much less explored. To improve the understanding of the role of p-and n-type dopants in graphene's local chemical activity and quantification of its interaction with single molecules, we report an atomic-scale investigation of single boron (B) and nitrogen (N) dopants in graphene and their interactions with CO molecules by means of atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) experiments and theoretical calculations. We infer that N/B doping significantly increases/ lowers the chemical interaction of graphene with individual CO molecules as a result of weak electrostatic forces induced by distinct charge distribution around the dopant site. High-resolution AFM images allow dopant discrimination and their atomic-scale structural characterization, which may be crucial for the atomic-scale design of graphene derivatives with relevant potential applications in molecular sensing and catalysis.
Iron oxides are among the most abundant compounds on Earth and have consequently been studied and used extensively in industrial processes. Despite these efforts, concrete understanding of some of their surface phase structures has remained elusive, in particular the oxidized α-Fe2O3(0001) hematite surface. We detail an optimized recipe to produce this phase over the entire hematite surface and study the geometrical parameters and composition of its complex structure by means of atomically resolved microscopy, electron diffraction and surface-sensitive spectroscopies. We conclude that the oxidized α-Fe2O3(0001) surface is terminated by a two-dimensional iron oxide with structure, lattice parameters, and orientation different from the bulk substrate. Using total-energy density functional theory for simulation of a large-scale atomic model, we identify the structure of the surface layer as antiferromagnetic, conductive 1T-FeO2 attached on half-metal terminated bulk. The model succeeds in reproducing the characteristic modulations observed in the atomically resolved images and electron diffraction patterns.Mineral iron oxide is known to be available in many stoichiometries, polymorphs, and even mixtures. Hematite (ɑ-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4), and wüstite (Fe1-XO) are prominent representatives of this class, possessing a wide range of electronic, magnetic and catalytic properties, due to their different oxygen content and characteristic crystal structures 1 .Currently, a significant amount of research is focused on the catalytic processes that occur on the surfaces of iron oxides, such as CO oxidation, wastewater purification, liquid fuel synthesis via Fischer-Tropsch reactions, styrene production or water splitting 2 .The research on iron oxides has turned its focus to nanoparticles 3 and thin films, 4 which are economically advantageous and show versatility beyond that of bulk materials. The critical limit -2D iron oxide films have been achieved in a form of a monolayer of FeO(111) on Pt(111) 5 , Ag (111) 6 , Ru(0001) 7 and Pd(111) 8 . Nevertheless, stability of this monolayer is inherently linked to its strong hybridization with the metal substrate; it remains an open question whether it can exist independently or as termination of an iron oxide crystal 9,10 , in analogy with the recently revisited V2O3(0001) system 11,12 . Another recent survey on new possible candidates for 2D materials suggests that some trilayer structures of metal oxides (e.g. MnO2, CoO2, GeO2) may be stable after exfoliation from a layered bulk 13 .The atomic structures of bulk Fe3O4 and ɑ-Fe2O3 are well-known, but their surface terminations remain elusive 2,14 , owing to their complexity and the fact that their surface stoichiometry can be varied depending on the preparation. Specifically, by removing oxygen from the surface of ɑ-Fe2O3(0001) by selective sputtering, a stoichiometry and structure resembling Fe3O4(111) can be attained 15,16 . Conversely, the surfaces can be partially or fully reoxidized by increasing t...
Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between exchange and Coulomb interactions. The ability to precisely tune the Coulomb interaction enables the control of spin-correlated flat-band states, band gap, and unconventional magnetism in such strongly correlated materials. Here, we demonstrate a gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr3) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the interflat-band spacing and bandgap of CrBr3 can be continuously tuned by 120 and 240 meV, respectively, via electrostatic injection of carriers into the hybrid CrBr3/graphene system. This can be attributed to the self-screening of CrBr3 arising from the gate-induced carriers injected into CrBr3, which dominates over the weakened remote screening of the graphene substrate due to the decreased carrier density in graphene. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides effective strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.
We report on the formation of fullerene-derived nanostructures on Au(111) at room temperature and under UHV conditions. After low-energy ion sputtering of fullerene films deposited on Au(111), bright spots appear at the herringbone corner sites when measured using a scanning tunneling microscope. These features are stable at room temperature against diffusion on the surface. We carry out DFT calculations of fullerene molecules having one missing carbon atom to simulate the vacancies in the molecules resulting from the sputtering process. These modified fullerenes have an adsorption energy on the Au(111) surface that is 1.6 eV higher than that of C60 molecules. This increased binding energy arises from the saturation by the Au surface of the bonds around the molecular vacancy defect. We therefore interpret the observed features as adsorbed fullerene-derived molecules with C vacancies. This provides a pathway for the formation of fullerene-based nanostructures on Au at room temperature.
A promising strategy to estabilize earth-abundant transition metal oxides during water electrolysis in acidic media to compete with noble metal water oxidation catalysts.
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