J. Enrique Ortega

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.

Multi-Component Organic Layers on Metal Substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well-defined structural and electronic properties. These issues have been studied and reviewed extensively in single-component layers, but the focus is gradually shifting towards more complex and functional multi-component assemblies such as donor-acceptor networks. These blends show different properties from those of the corresponding single-component layers, and the understanding on how these properties depend on the different supramolecular environment of multi-component assemblies is crucial for the advancement of organic devices. Here, our understanding of two-dimensional multi-component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self-assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecules supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self-assembly is summarized.

Tunable Band Alignment with Unperturbed Carrier Mobility of On-Surface Synthesized Organic Semiconducting Wires

The tunable properties of molecular materials place them among the favorites for a variety of future generation devices. In addition, to maintain the current trend of miniaturization of those devices, a departure from the present top-down production methods may soon be required and self-assembly appears among the most promising alternatives. On-surface synthesis unites the promises of molecular materials and of self-assembly, with the sturdiness of covalently bonded structures: an ideal scenario for future applications. Following this idea, we report the synthesis of functional extended nanowires by self-assembly. In particular, the products correspond to one-dimensional organic semiconductors. The uniaxial alignment provided by our substrate templates allows us to access with exquisite detail their electronic properties, including the full valence band dispersion, by combining local probes with spatial averaging techniques. We show how, by selectively doping the molecular precursors, the product’s energy level alignment can be tuned without compromising the charge carrier’s mobility.

Non-Covalent Interactions in Supramolecular Assemblies Investigated with Electron Spectroscopies

A new view: A picture of the different non-covalent interactions relevant for the self-assembly of organic layers and their spectroscopic fingerprints is provided (see figure). In particular, state-of-the-art spectroscopic measurements are performed for supramolecular assemblies, comparing the electronic structure of single-component layers with that of binary organic layers.

Π Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor

Surface-confined dehalogenation reactions are versatile bottom-up approaches for the synthesis of carbon-based nanostructures with predefined chemical properties. However, for devices generally requiring low-conductivity substrates, potential applications are so far severely hampered by the necessity of a metallic surface to catalyze the reactions. In this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO2(110) via a dehalogenative homocoupling of 4,4″-dibromoterphenyl precursors. The supramolecular phase is clearly distinguished from the polymeric one using low-energy electron diffraction and scanning tunneling microscopy as the substrate temperature used for deposition is varied. X-ray photoelectron spectroscopy of C 1s and Br 3d core levels traces the temperature of the onset of dehalogenation to around 475 K. Moreover, angle-resolved photoemission spectroscopy and tight-binding calculations identify a highly dispersive band characteristic of a substantial overlap between the precursor’s π states along the polymer, considered as the fingerprint of a successful polymerization. Thus, these results establish the first spectroscopic evidence that atomically precise carbon-based nanostructures can readily be synthesized on top of a transition-metal oxide surface, opening the prospect for the bottom-up production of novel molecule–semiconductor devices.

Interplay between Steps and Oxygen Vacancies on Curved TiO2(110)

A vicinal rutile TiO2(110) crystal with a smooth variation of atomic steps parallel to the [1-10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1-11] zigzag faceting to straight [1-10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (Obr vacs) vanish. The [1-10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (St) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of Obr vacs plus St protrusions at steps, suggesting that both Obr vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti(3+) polaron near steps and at terraces.

Graphene Tunable Transparency to Tunneling Electrons: A Direct Tool To Measure the Local Coupling

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems.

Co nanodot arrays grown on a GdAu2 template: substrate/nanodot antiferromagnetic coupling.

Controlling anisotropy and exchange coupling in patterned magnetic nanostructures is the key for developing advanced magnetic storage and spintronic devices. We report on the antiferromagnetic interaction between a Co nanodot array and its supporting GdAu2 nanotemplate that induces large anisotropy values in individual Co nanodots. In clear contrast with nonmagnetic Au substrates, GdAu2 triggers an earlier switch from out-of-plane anisotropy in monatomic high dots to in-plane when the dot height becomes biatomic.

Tuning the Graphene on Ir(111) adsorption regime by Fe/Ir surface-alloying

A combined scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory study of graphene on a Fe-Ir(111) alloy with variable Ir concentration is presented. Starting from an intercalated Fe layer between the graphene and Ir(111) surface we find that graphene-substrate interaction can be fine-tuned by Fe-Ir alloying at the interface. When a critical Ir-concentration close to 0.25 is reached in the Fe layer, the Dirac cone of graphene is largely restored and can thereafter be tuned across the Fermi level by further increasing the Ir content. Indeed, our study reveals an abrupt transition between a chemisorbed phase at small Ir concentrations and a physisorbed phase above the critical concentration. The latter phase is highly reminiscent of the graphene on the clean Ir(111) surface. Furthermore, the transition is accompanied by an inversion of the graphenes induced magnetization due to the coupling with the Fe atoms from antiferromagnetic when chemisorbed to weakly ferromagnetic in the physisorption regime, with spin polarizations whose magnitude may be tuned with the amount of Fe content.

A chemically inert Rashba split interface electronic structure of C60, FeOEP and PTCDA on BiAg2/Ag(111) substrates

The fields of organic electronics and spintronics have the potential to revolutionize the electronics industry. Finding the right materials that can retain their electrical and spin properties when combined is a technological and fundamental challenge. We carry out the study of three archetypal organic molecules in intimate contact with the BiAg2 surface alloy. We show that the BiAg2 alloy is an especially suited substrate due to its inertness as support for molecular films, exhibiting an almost complete absence of substrate–molecular interactions. This is inferred from the persistence of a completely unaltered giant spin-orbit split surface state of the BiAg2 substrate, and from the absence of significant metallic screening of charged molecular levels in the organic layer. Spin-orbit split states in BiAg2 turn out to be far more robust to organic overlayers than previously thought.