Strain release at the graphene-Ni(100) interface investigated by in-situ and operando scanning tunnelling microscopy

Interface strain can significantly influence the mechanical, electronic and magnetic properties of low- dimensional materials. Here we investigated by scanning tunneling microscopy how the stress intro- duced by a mismatched interface affects the structure of a growing graphene (Gr) layer on a Ni(100) surface in real time during the process. Strain release appears to be the main factor governing morphology, with the interplay of two simultaneous driving forces: on the one side the need to obtain two-dimensional best registry with the substrate, via formation of moire patterns, on the other side the requirement of optimal one-dimensional in-plane matching with the transforming nickel carbide layer, achieved by local rotation of the growing Gr flake. Our work suggests the possibility of tuning the local properties of two-dimensional films at the nanoscale through exploitation of strain at a one-dimensional interface

On-Surface Synthesis of Polypyridine: Strain Enforces Extended Linear Chains

Strain-induced on-surface transformations provide an appealing route to steer the selectivity towards desired products. Here, we demonstrate the selective on-surface synthesis of extended all-trans poly(2,6-pyridine) chains on Au(111). By combining high-resolution scanning tunneling and atomic force microscopy, we revealed the detailed chemical structure of the reaction products. Density functional theory calculations indicate that the synthesis of extended covalent structures is energetically favored over the formation of macrocycles, due to the minimization of internal strain. Our results consolidate the exploitation of internal strain relief as a driving force to promote selective on-surface reactions

Temperature-Driven Changes of the Graphene Edge Structure on Ni(111): Substrate vs Hydrogen Passivation

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 \ub0C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges

Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules

Intermediate states arc elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces

Operando atomic-scale study of graphene CVD growth at steps of polycrystalline nickel

An operando investigation of graphene growth on (100) grains of polycrystalline nickel (Ni) surfaces was performed by means of variable-temperature scanning tunneling microscopy complemented by density functional theory simulations. A clear description of the atomistic mechanisms ruling the graphene expansion process at the stepped regions of the substrate is provided, showing that different routes can be followed, depending on the height of the steps to be crossed. When a growing graphene flake reaches a monoatomic step, it extends jointly with the underlying Ni layer; for higher Ni edges, a different process, involving step retraction and graphene landing, becomes active. At step bunches, the latter mechanism leads to a peculiar \u2018staircase formation\u2019 behavior, where terraces of equal width form under the overgrowing graphene, driven by a balance in the energy cost between C\u2013Ni bond formation and stress accumulation in the carbon layer. Our results represent a step towards bridging the material gap in searching new strategies and methods for the optimization of chemical vapor deposition graphene production on polycrystalline metal surfaces

Imaging on-surface hierarchical assembly of chiral supramolecular networks

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10′-dibromo-9,9′-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling

Reorganization energy and polaronic effects of pentacene on NaCl films

Due to recent advances in scanning-probe technology, the electronic structure of individual molecules can now also be investigated if they are immobilized by adsorption on nonconductive substrates. As a consequence, different molecular charge states are now experimentally accessible. Thus motivated, we investigate as an experimentally relevant example the electronic and structural properties of a NaCl(001) surface with and without pentacene adsorbed (neutral and charged) by employing density-functional theory. We estimate the polaronic reorganization energy to be E-reorg similar or equal to 0.8 - 1.0 eV, consistent with experimental results obtained for molecules of similar size. To account for environmental effects on this estimate, different models for charge screening are compared. Finally, we calculate the density profile of one of the frontier orbitals for different occupations and confirm the experimentally observed localization of the charge density upon charging and relaxation of molecule-insulator interface from ab initio calculations

Charge-Induced Structural Changes in a Single Molecule Investigated by Atomic Force Microscopy

Intramolecular structural relaxations occurring upon electron transfer are crucial in determining the rate of redox reactions. Here, we demonstrate that subangstrom structural changes occurring upon single-electron charging can be quantified by means of atomically resolved atomic force microscopy (AFM) for the case of single copper(II)phthalocyanine (CuPc) molecules deposited on an ultrathin NaCl film. Imaging the molecule in distinct charge states (neutral and anionic) reveals characteristic differences in the AFM contrast. In comparison to density functional theory simulations these changes in contrast can be directly related to relaxations of the molecule's geometric structure upon charging. The dominant contribution arises from a nonhomogeneous vertical relaxation of the molecule, caused by a change in the electrostatic interaction with the surface

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer

Control over the film-substrate interaction is key to the exploitation of graphene\u2019s unique electronic properties. Typically, a buffer layer is irreversibly intercalated \u201cfrom above\u201d to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction \u201cfrom below\u201d. By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene\u2019s electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices

Surface states characterization in the strongly interacting graphene/Ni(111) system

By combining nonlinear photoemission experiments and density functional theory calculations, we study the modification of Ni(111) surface states induced by the presence of graphene. The main result is that graphene is able to displace the Ni(111) surface states from the valence band close to the Fermi level uncovering the d-band of Ni. The shift of the surface states away from the Fermi level modifies their k-dispersion and the effective mass. The unoccupied image state of graphene/Ni(111) has been also characterized. The ab initio calculations give a theoretical insight into the electronic properties of graphene/Ni(111) in the two stable top-fcc and top-bridge phases showing that the interface properties are poorly dependent on the stacking. The screening properties to an externally applied electric field are also discussed