The fragmentation of fullerenes using ions, surface collisions or thermal effects is a complex process that typically leads to the formation of small carbon clusters of variable size. Here, we show that geometrically well-defined graphene quantum dots can be synthesized on a ruthenium surface using C(60) molecules as a precursor. Scanning tunnelling microscopy imaging, supported by density functional theory calculations, suggests that the structures are formed through the ruthenium-catalysed cage-opening of C(60). In this process, the strong C(60)-Ru interaction induces the formation of surface vacancies in the Ru single crystal and a subsequent embedding of C(60) molecules in the surface. The fragmentation of the embedded molecules at elevated temperatures then produces carbon clusters that undergo diffusion and aggregation to form graphene quantum dots. The equilibrium shape of the graphene can be tailored by optimizing the annealing temperature and the density of the carbon clusters.

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Transforming C60 molecules into graphene quantum dots

Progress in the measurement and modeling of physisorbed layers

Electronic, structural and chemical effects of charge-transfer at organic/inorganic interfaces

Adsorption structures and energetics of molecules on metal surfaces: Bridging experiment and theory

We developed and implemented an algorithm to determine and correct systematic distortions in low-energy electron diffraction (LEED) images. The procedure is in principle independent of the design of the apparatus (spherical or planar phosphorescent screen vs. channeltron detector) and is therefore applicable to all device variants, known as conventional LEED, micro-channel plate LEED, and spot profile analysis LEED. The essential prerequisite is a calibration image of a sample with a well-known structure and a suitably high number of diffraction spots, e.g., a Si(111)-7×7 reconstructed surface. The algorithm provides a formalism which can be used to rectify all further measurements generated with the same device. In detail, one needs to distinguish between radial and asymmetric distortion. Additionally, it is necessary to know the primary energy of the electrons precisely to derive accurate lattice constants. Often, there will be a deviation between the true kinetic energy and the value set in the LEED control. Here, we introduce a method to determine this energy error more accurately than in previous studies. Following the correction of the systematic errors, a relative accuracy of better than 1% can be achieved for the determination of the lattice parameters of unknown samples.

Determination and correction of distortions and systematic errors in low-energy electron diffraction

The geometry of adsorbed C60 influences its collective properties. We report the first dynamical lowenergy electron diffraction study to determine the geometry of a C60 monolayer, Agð111Þ-ð2 ffiffiffi p � 2 ffiffiffi p Þ30 � -C 60, and related density functional theory calculations. The stable monolayer has C60 molecules in vacancies that result from the displacement of surface atoms. C60 bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C60 monolayers on close-packed metal surfaces.

/pdf/surface-geometry-of-c60-on-ag-111-1d00dv5yst.pdf

Surface Geometry of C60 on Ag(111)

The growth, structures and thermodynamics of Xe adsorbed on Pb(111) were studied with low-energy electron diffraction (LEED). Measurements of equilibrium adsorption isobars indicate layer-by-layer growth for at least two layers in the temperature range 60–90 K, and an isosteric heat of adsorption of 191 ± 10 meV for the first Xe layer and 158 ± 20 meV for the second Xe layer. The monolayer has an incommensurate hexagonal structure with a lattice parameter similar to that found in bulk Xe, and its thermal expansion coefficient was measured to be 9 ± 1 × 10−4 K−1. The average overlayer–substrate spacing was determined from a constant-momentum-transfer analysis of the LEED intensities to be 3.95 ± 0.10 A. Vibrational information was obtained from measuring the decay of the diffraction intensities as a function of temperature. The perpendicular root mean square displacement of the Xe atoms at 50 K is 0.12 ± 0.01 A, giving an effective Debye temperature for adsorbed Xe of 37 ± 2 K. The corresponding vibrational energy for the perpendicular mode of Xe is 3.2 ± 0.3 meV.

https://iopscience.iop.org/article/10.1088/0953-8984/19/5/056011/pdf

Structural and thermal properties of Xe on the Pb(111) surface studied by low-energy electron diffraction

Temperature dependent LEED study of Pb{1 1 1}

The multilayer relaxation of the stepped Cu(5 1 1) surface has been studied by quantitative low-energy electron diffraction and analyzed using the CLEED program package. Relaxations with respect to the bulk interlayer spacing of 0.6934 A are −9.5%, −10.4%, +8.2% and −1.8% for the first four interlayer spacings, respectively (negative sign corresponds to contraction). The relaxation sequence (− − + −...) is thus in agreement with the theoretical prediction. The deeper relaxations are damped in a non-uniform manner and the lateral relaxations are smaller than 2% of the lateral spacing. This result agrees well with theoretical studies of the same surface. The Pendry R-factor for the favored structure is 0.21.

Kelly Hanna

Papers

Surface Geometry of C60 on Ag(111)

Structural and thermal properties of Xe on the Pb(111) surface studied by low-energy electron diffraction

Temperature dependent LEED study of Pb{1 1 1}

Surface relaxation of Cu(5 1 1)