Celia Rogero

Fullerenes from aromatic precursors by surface-catalysed cyclodehydrogenation

Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C60 from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C60 and the triazafullerene C57N3 from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.

Label-free detection of DNA hybridization based on hydration-induced tension in nucleic acid films

The properties of water at the nanoscale are crucial in many areas of biology, but the confinement of water molecules in sub-nanometre channels in biological systems has received relatively little attention. Advances in nanotechnology make it possible to explore the role played by water molecules in living systems, potentially leading to the development of ultrasensitive biosensors. Here we show that the adsorption of water by a self-assembled monolayer of single-stranded DNA on a silicon microcantilever can be detected by measuring how the tension in the monolayer changes as a result of hydration. Our approach relies on the microcantilever bending by an amount that depends on the tension in the monolayer. In particular, we find that the tension changes dramatically when the monolayer interacts with either complementary or single mismatched single-stranded DNA targets. Our results suggest that the tension is mainly governed by hydration forces in the channels between the DNA molecules and could lead to the development of a label-free DNA biosensor that can detect single mutations. The technique provides sensitivity in the femtomolar range that is at least two orders of magnitude better than that obtained previously with label-free nanomechanical biosensors and with label-dependent microarrays.

Resolution of site-specific bonding properties of C60 adsorbed on Au(111)

We have performed a careful study of the adsorption of C60 molecules on a Au(111) surface by using scanning tunneling microscopy and spectroscopy at room temperature. In coincidence with results from other techniques, differential conductance spectra give a value of 2.3 eV for the HOMO–LUMO gap of a monomolecular layer, with the LUMO level located at 0.6 eV above the Fermi level as a consequence of electronic charge transfer from the substrate into the molecule. Small differences in position (and shape) of the LUMO-derived resonance, in the order of 0.1 eV, are found on molecules adsorbed at step edges. We consider the Smoluchowski effect, i.e., the interaction of the molecules with a charge-depleted region, to explain the observed differences in their bonding nature. On some molecules forming part of bidimensional fullerene islands, similar differences were also detected with spatially resolved scanning tunneling spectroscopy, giving rise to a 2×2 commensurate structure of the molecular adlayer with resp...

Understanding energy-level alignment in donor-acceptor/metal interfaces from core-level shifts

The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor-acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor-acceptor stoichiometry in the contact blend.

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling.

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [Phys. Rev. Lett. 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y(3)Fe(5)O(12) bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.

Substrate-Independent Growth of Atomically Precise Chiral Graphene Nanoribbons

Contributing to the need for new graphene nanoribbon (GNR) structures that can be synthesized with atomic precision, we have designed a reactant that renders chiral (3,1)-GNRs after a multistep reaction including Ullmann coupling and cyclodehydrogenation. The nanoribbon synthesis has been successfully proven on different coinage metals, and the formation process, together with the fingerprints associated with each reaction step, has been studied by combining scanning tunneling microscopy, core-level spectroscopy, and density functional calculations. In addition to the GNR’s chiral edge structure, the substantial GNR lengths achieved and the low processing temperature required to complete the reaction grant this reactant extremely interesting properties for potential applications.

Tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo)dehydrogenation of heteroaromatics.

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest.

Interplay between Fast Diffusion and Molecular Interaction in the Formation of Self-Assembled Nanostructures of S-Cysteine on Au(111)

We have studied the first stages leading to the formation of self-assembled monolayers of S-cysteine molecules adsorbed on a Au(111) surface. Density functional theory (DFT) calculations for the adsorption of individual cysteine molecules on Au(111) at room temperature show low-energy barriers all over the 2D Au(111) unit cell. As a consequence, cysteine molecules diffuse freely on the Au(111) surface and they can be regarded as a 2D molecular gas. The balance between molecule-molecule and molecule-substrate interactions induces molecular condensation and evaporation from the morphological surface structures (steps, reconstruction edges, etc.) as revealed by scanning tunnelling microscopy (STM) images. These processes lead progressively to the formation of a number of stable arrangements, not previously reported, such as single-molecular rows, trimers, and 2D islands. The condensation of these structures is driven by the aggregation of new molecules, stabilized by the formation of electrostatic interactions between adjacent NH(3)(+) and COO(-) groups, together with adsorption at a slightly more favorable quasi-top site of the herringbone Au reconstruction.

Resistive switching dependence on atomic layer deposition parameters in HfO2-based memory devices

Resistance random access memory (ReRAM) is considered a promising candidate for the next generation of non-volatile memory. In this work, we fabricate Co/HfO2/Ti devices incorporating atomic-layer-deposited HfO2 thin films as the active material grown under different atomic layer deposition (ALD) conditions. We focus on analyzing the effect of ALD conditions on the resistive switching behaviour of the devices. Electrical characterization reveals a particular non-crossing current–voltage curve and bipolar resistive switching behaviour. Device memory properties were confirmed by stability and retention measurements as well as voltage pulses, by which logical computational processes were conducted. X-ray photoelectron spectroscopy combined with electrical measurements demonstrates that the presence of Hf sub-oxides at the interface with the underlying Ti layer is required in order to achieve a stable switching device. The ability of Ti to scavenge oxygen from the HfO2 is shown to be affected by the ALD conditions.

Understanding charge transfer in donor-acceptor/metal systems: A combined theoretical and experimental study

We develop an effective potential approach for assessing the flow of charge within a two-dimensional donor-acceptor/metal network based on core-level shifts. To do so, we perform both density functional theory (DFT) calculations and x-ray photoemission spectroscopy (XPS) measurements of the core-level shifts for three different monolayers adsorbed on a Ag substrate. Specifically, we consider perfluorinated pentacene (PFP), copper phthalocyanine (CuPc) and their 1:1 mixture (PFP+CuPc) adsorbed on Ag(111).