Sublimation of Li@C60

R.S. acknowledges financial support from the Scottish Funding Council thorugh SRD-Grant (HR07003) and H.J.C. from an EPSRC DTG studentship (EP/M508214/1).Experiments that probe the fundamental properties of endohedral fullerenes often require the preparation of molecular beams or thin films of the neutral molecules. It is challenging to cleanly sublime this class of molecules without producing some thermal degradation. We report combined gas phase and scanning tunnelling microscopy studies that probe the thermal decay of commercial [Li+C60]PF6- in a quartz ampoule and provide treatment conditions that will allow the sublimation of intact, neutral Li@C60 accompanied by a well-characterised component of neutral C60. The decay of the material at appropriate temperatures can be modelled with the assumption of a second order decay process in the oven yielding Arrhenius parameters that can predict the ratio of Li@C60 to C60 in the sublimed material.Publisher PDFPeer reviewe

Adsorption energies of benzene on close packed transition metal surfaces using the random phase approximation

The authors acknowledge financial support from the Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003) and from EPSRC (PhD studentship for JAGT, EP/M506631/1). Funding by the Austrian Science Fund (FWF): F41 (SFB ViCoM) is grateful acknowledged.The adsorption energy of benzene on various metal substrates is predicted using the random phase approximation (RPA) for the correlation energy. Agreement with available experimental data is systematically better than 10% for both coinage and reactive metals. The results are also compared with more approximate methods, including vdW-density functional theory (DFT), as well as dispersion corrected DFT functionals. Although dispersion corrected DFT can yield accurate results, for instance, on coinage metals, the adsorption energies are clearly overestimated on more reactive transition metals. Furthermore, coverage dependent adsorption energies are well described by the RPA. This shows that for the description of aromatic molecules on metal surfaces further improvements in density functionals are necessary, or more involved many body methods such as the RPA are required.PostprintPeer reviewe

On-demand final state control of a surface-bound bistable single molecule switch

We acknowledge financial support from the Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003) and from EPSRC (PhD studentship for JAGT, EP/M506631/1). Computational support was provided via the EaStCHEM Research Computing Facility.Modern electronic devices perform their defined action because of the complete reliability of their individual active components (transistors, switches, diodes, and so forth). For instance, to encode basic computer units (bits) an electrical switch can be used. The reliability of the switch ensures that the desired outcome (the component’s final state, 0 or 1) can be selected with certainty. No practical data storage device would otherwise exist. This reliability criterion will necessarily need to hold true for future molecular electronics to have the opportunity to emerge as a viable miniaturization alternative to our current silicon-based technology. Molecular electronics target the use of single-molecules to perform the actions of individual electronic components. On-demand final state control over a bistable unimolecular component has therefore been one of the main challenges in the past decade (1−5) but has yet to be achieved. In this Letter, we demonstrate how control of the final state of a surface-supported bistable single molecule switch can be realized. On the basis of the observations and deductions presented here, we further suggest an alternative strategy to achieve final state control in unimolecular bistable switches.PostprintPostprintPeer reviewe

Strong substrate mediation of attractive lateral interactions of CO on Cu(110)

Funding: Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003), EPSRC (PhD studentship for JAGT, EP/M506631/1), Royal Society Industry Fellowship (PBW).The mechanism of chemical reactions between adsorbed species is defined by the combined effects of the adsorbate–substrate potential landscape and lateral interactions. Such lateral interactions are therefore integral to catalytic processes, but their study is often complicated by “substrate mediation”, the regulation of a two-body potential between adsorbed particles by the surface itself. Substrate mediation can influence the sign and magnitude of lateral interactions. There are notable exceptions of ordered structures forming at low coverage, indicative of short-range attractive forces where repulsive forces are expected to dominate, suggesting a strong substrate-mediated contribution. To explore further the origins of such interactions, we have investigated the adsorption of CO on Cu(110) using a combination of low-temperature microscopy and first-principles calculations. Our studies reveal that lateral adsorbate interactions, which are constrained by the metal surface, regulate the bonding between the adsorbate and substrate. Anisotropic CO–CO coupling is seen to arise from a perfect balance between the intermolecular accumulation of charge that acts as a glue (chemical coupling) at sufficiently large distances to avoid repulsive effects (dipole–dipole coupling and Pauli’s repulsion between electron clouds).PostprintPostprintPeer reviewe

Calculating the frequencies and intensities of strongly anharmonic modes of adsorbates on surfaces : a low-cost but accurate computational approach

Funding: Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003) and from EPSRC (PhD studentship for JAGT, EP/M506631/1).We present a new method for calculating the frequencies and intensities of the vibrational modes of adsorbates on surfaces. Our method is based on density functional perturbation theory (DFPT) and provides accurate estimates of the vibrational intensities even when the vibrations are strongly anharmonic. Furthermore, it does so at a negligible additional computation cost compared to conventional DFPT calculation. We illustrate our method by calculating the vibrational spectra of three example systems — ethylidyne on Rh(111), benzene on Rh(111) coadsorbed with CO, and terephthalic acid (TPA) on Cu(100) — and comparing them to experimental measurements performed using High-Resolution Electron Energy Loss Spectroscopy (HREELS). We find excellent agreement between our predictions and the experimentally measured frequencies and intensities in all three cases.PostprintPostprintPeer reviewe

Benzene adsorption on Rh(111) : a new perspective on intermolecular interactions and molecular ordering

The authors acknowledge financial support from the Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003) and from EPSRC (PhD studentships for JAGT − EP/M506631/1, and MJT − EP/K503162/1).The adsorption of benzene on the Rh(111) substrate was investigated through scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations. Experiments were carried out at various surface coverages, with the amount of benzene adsorbed determined to influence the molecular adsorption site, the intermolecular interactions, and the interaction between the molecule and the substrate. At a sub-monolayer coverage of the surface, the molecules are disordered and kept apart by a strong inter-adsorbate repulsion, with a preference for the molecule to adsorb on a three-fold hcp hollow site. At high coverage, the preferred adsorption site becomes the two-fold symmetric bridge site, whether as part of the two dense ordered structures that form at high coverage ((2√3×3)rect or (√19×√19)R23.4°) or as part of the disordered array of benzene molecules, which are arranged in formations which resemble the “building blocks” of the ordered overlayers. Despite the adsorption energy for benzene within both dense structures being similar, the (√19×√19)R23.4° overlayer is only observed if the substrate is annealed to 363 K during or after deposition, indicating that the formation of the (√19×√19)R23.4° ordering is inhibited by an activation barrier at lower temperatures and can only be overcome by increasing the temperature of the Rh(111) support.PostprintPeer reviewe

Angle-Resolved Photoelectron Spectroscopy and Scanning Tunnelling Spectroscopy Studies of the Endohedral Fullerene Li@C60

M. S., E. B., J. O. F. T. and E. E. B. C. gratefully acknowledge financial support from the Leverhulme Foundation (RPF-298 “PES of hollow nanomaterials”). M. S. and H. J. C. acknowledge the financial support of EPSRC DTP studentships (EP/M508214/1 and EP/N509644/1). R. S. acknowledges financial support from the Scottish Funding Council through SRD-Grant (HRO7003). The work of FR and BM is supported by the Fonds de la Recherche Fondamentale Collective (#T.0132.16 and J.0012.18) and by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award # DE-SC0012628. BM and FR thank the Fonds National de la Recherche (FRS.FNRS, Belgium) for its support. Computational resources were provided by Consortium des équipements de calcul intensif (CECI, FNRS 2.5020.11).Gas phase photoelectron spectroscopy (Rydberg Fingerprint Spectroscopy), TDDFT calculations and low temperature STM studies are combined to provide detailed information on the properties of the diffuse, low-lying Rydberg-like SAMO states of isolated Li@C60 endohedral fullerenes. The presence of the encapsulated Li is shown by the calculations to produce a significant distortion of the lowest-lying S- and P-SAMOs that is dependent on the position of the Li inside the fullerene cage. Under the high temperature conditions of the gas phase experiments, the Li is mobile and able to access different positions within the cage. This is accounted for in the comparison with theory that shows a very good agreement of the photoelectron angular distributions, allowing the symmetry of the observed SAMO states to be identified. When adsorbed on a metal substrate at low temperature, a strong interaction between the low-lying SAMOs and the metal substrate moves these states to energies much closer to the Fermi energy compared to the situation for empty C60 while the Li remains frozen in an off-centre position.Publisher PDFPeer reviewe

Size-Selective Carbon Nanoclusters as Precursors to the Growth of Epitaxial Graphene

The nucleation and growth mechanisms of graphene on Rh(111) via temperature-programmed growth of C2H4 are studied by scanning tunneling microscopy and spectroscopy, and by density functional theory calculations. By combining our experimental and first principles approaches, we show that carbon nanoislands form in the initial stages of graphene growth, possessing an exclusive size of seven honeycomb carbon units (hereafter labeled as 7C6 ). These clusters adopt a domelike hexagonal shape indicating that bonding to the substrate is localized on the peripheral C atoms. Smoluchowski ripening is identified as the dominant mechanism leading to the formation of graphene, with the size-selective carbon islands as precursors. Control experiments and calculations, whereby coronene molecules, the hydrogenated analogues of 7C6 , are deposited on Rh(111), provide an unambiguous structural and chemical identification of the 7C6 building blocks.PostprintPeer reviewe