Preventing sintering of nanoclusters on graphene by radical adsorption.

Metal nanoclusters, supported on inert substrates, exhibiting well-defined shapes and sizes in a broad range of temperatures are a major object of desire in nanotechnology. Here, a technique is presented that improves the thermal stability of monodisperse and crystalline transition metal nanoclusters grown in a regular array on metal-supported graphene. To stabilize the clusters after growth under ultrahigh vacuum the system composed of the aggregates and the graphene/metal interface is exposed to radicals resulting from the dissociation of diatomic gases. As a model system we have used Pt as the metal element for cluster growth and the template consisting of the moiré pattern resulting from the lattice mismatch between graphene and the Ir(111) surface. The study has been performed for deuterium and oxygen radicals, which interact very differently with graphene. Our results reveal that after radical exposure the thermally activated motion of Pt nanoclusters to adjacent moiré cells and the subsequent sintering of neighbor aggregates are avoided, most pronounced for the case of atomic O. For the case of D the limits of the improvement are given by radical desorption, whereas for the case of O they are defined by an interplay between coalescence and graphene etching followed by Pt intercalation, which can be controlled by the amount of exposure. Finally, we determined the mechanism of how radical adsorption improves the thermal stability of the aggregates.

PTCDA growth on Ge(111)-[Formula: see text] surfaces: a scanning tunneling microscopy study.

The initial stages of growth of PTCDA (3,4,9,10 perylene tetracarboxylic dianhydride) at room temperature (RT) on Ge(111)-[Formula: see text] surfaces have been studied by means of scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The results show that PTCDA molecules have a high mobility at RT on the well ordered areas of the semiconductor substrate, since nucleation is only observed in domain walls, steps and surface defects. However, no molecular ordering has been detected at submonolayer coverage. For higher coverages, the formation of three-dimensional (3D) molecular islands has been observed. These 3D islands present a crystalline nature as demostrated by molecularly resolved STM images. According to these STM measurements, PTCDA molecules are ordered in a herringbone structure, similar to the one observed in PTCDA bulk crystals. Moreover, the 3D crystallites are grown on top of a disordered molecular layer, which acts as a passivating layer.

Oxygen orders differently under graphene: new superstructures on Ir(111).

Using scanning tunneling microscopy, the oxygen adsorbate superstructures on bare Ir(111) are identified and compared to the ones formed by intercalation in between graphene and the Ir(111) substrate. For bare Ir(111) we observe O-(2 × 2) and O-(2 × 1) structures, thereby clarifying a persistent uncertainty about the existence of these structures and the role of defects for their stability. For the case of graphene-covered Ir(111), oxygen intercalation superstructures can be imaged through the graphene monolayer by choosing proper tunneling conditions. Depending on the pressure, temperature and duration of O2 exposure as well as on the graphene morphology, O-(2 × 2), O-(√3×√3)-R30°, O-(2 × 1) and O-(2√3 × 2√3)-R30° superstructures with respect to Ir(111) are observed under the graphene cover. Two of these structures, the O-(√3 × âˆš3)-R30° and the (2√3 × 2√3)-R30° structure are only observed when the graphene layer is on top. Phase coexistence and formation conditions of the intercalation structures between graphene and Ir(111) are analyzed. The experimental results are compared to density functional theory calculations including dispersive forces. The existence of these phases under graphene and their absence on bare Ir(111) are discussed in terms of possible changes in the adsorbate-substrate interaction due to the presence of the graphene cover.

Tug-of-war between corrugation and binding energy: revealing the formation of multiple moiré patterns on a strongly interacting graphene-metal system.

The formation of multidomain epitaxial graphene on Rh(111) under ultra-high vacuum (UHV) conditions has been characterized by scanning tunnelling microscopy (STM) measurements and density functional theory (DFT) calculations. At variance with the accepted view for strongly interacting graphene-metal systems, we clearly demonstrate the formation of different rotational domains leading to multiple moiré structures with a wide distribution of surface periodicities. Experiments reveal a correlation between the STM apparent corrugation and the lattice parameter of the moiré unit cell, with corrugations of just 30-40 pm for the smallest moirés. DFT calculations for a relevant selection of these moiré patterns show much larger height differences and a non-monotonic behaviour with the moiré size. Simulations based on non-equilibrium Green's function (NEGF) methods reproduce quantitatively the experimental trend and provide a detailed understanding of the interplay between electronic and geometric contributions in the STM contrast of graphene systems. Our study sheds light on the subtle energy balance among strain, corrugation and binding that drives the formation of the moiré patterns in all graphene/metal systems and suggests an explanation for the success of an effective model only based on the lattice mismatch. Although low values of the strain energy are a necessary condition, it is the ability of graphene to corrugate in order to maximize the areas of favourable graphene-metal interactions that finally selects the stable configurations.

Towards scalable nano-engineering of graphene.

By merging bottom-up and top-down strategies we tailor graphene's electronic properties within nanometer accuracy, which opens up the possibility to design optical and plasmonic circuitries at will. In a first step, graphene electronic properties are macroscopically modified exploiting the periodic potential generated by the self assembly of metal cluster superlattices on a graphene/Ir(111) surface. We then demonstrate that individual metal clusters can be selectively removed by a STM tip with perfect reproducibility and that the structures so created are stable even at room temperature. This enables one to nanopattern circuits down to the 2.5 nm only limited by the periodicity of the Moiré-pattern, i.e., by the distance between neighbouring clusters, and different electronic and optical properties should prevail in the covered and uncovered regions. The method can be carried out on micro-meter-sized regions with clusters of different materials permitting to tune the strength of the periodic potential.

Point defects on graphene on metals.

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural, and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in freestanding graphene layers.