Clicking beyond suspensions: understanding thiol-ene chemistry on solid-supported MoS2.

Molecular functionalization of MoS2 has attracted a lot of attention due to its potential to afford fine-tuned hybrid materials that benefit from the power of synthetic chemistry and molecular design. Here, we report on the on-surface reaction of maleimides on bulk and molecular beam epitaxy grown single-layer MoS2, both in ambient conditions as well as ultrahigh vacuum using scanning probe microscopy.

Probing the Phase Transition to a Coherent 2D Kondo Lattice.

Kondo lattices are systems with unusual electronic properties that stem from strong electron correlation, typically studied in intermetallic 3D compounds containing lanthanides or actinides. Lowering the dimensionality of the system enhances the role of electron correlations providing a new tuning knob for the search of novel properties in strongly correlated quantum matter. The realization of a 2D Kondo lattice by stacking a single-layer Mott insulator on a metallic surface is reported. The temperature of the system is steadily lowered and by using high-resolution scanning tunneling spectroscopy, the phase transition leading to the Kondo lattice is followed. Above 27 K the interaction between the Mott insulator and the metal is negligible and both keep their original electronic properties intact. Below 27 K the Kondo screening of the localized electrons in the Mott insulator begins and below 11 K the formation of a coherent quantum electronic state extended to the entire sample, i.e., the Kondo lattice, takes place. By means of density functional theory, the electronic properties of the system and its evolution with temperature are explained. The findings contribute to the exploration of unconventional states in 2D correlated materials.

Metastable Polymorphic Phases in Monolayer TaTe2.

Polymorphic phases and collective phenomena-such as charge density waves (CDWs)-in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods such as molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, scanning tunneling microscopy (STM) is used to map MBE-grown monolayer (ML) TaTe2 . This work reports the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrates that their relative coverage can be controlled by adjusting synthesis parameters. Several superperiodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, this work provides theoretical insight on the delicate balance between Te…Te and Ta-Ta interactions that dictates the stability of the different phases. The findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.

Switchable molecular functionalization of an STM tip: from a Yu-Shiba-Rusinov Tip to a Kondo tip.

In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (YSR) bound states appear against the featureless density of states of a metallic graphene on Ir(111) sample. Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorption configuration of the molecule, the interaction strength between the SC tip and the charged TCNQ molecule can be reversibly controlled by tuning the tip-sample distance. The controlled transition from one coupling regime to the other allows us to verify the relation between the energy scales of the two competing many-body effects for the functionalized tip. Quenching the SC state of the Nb tip with a magnetic field switches abruptly from a tip dominated by the YSR bound states to a Kondo tip.

Phase control and lateral heterostructures of MoTe2 epitaxially grown on graphene/Ir(111).

Engineering the growth of the different phases of two-dimensional transition metal dichalcogenides (2D-TMDs) is a promising way to exploit their potential since the phase determines their physical and chemical properties. Here, we report on the epitaxial growth of monolayer MoTe2 on graphene on an Ir(111) substrate. Scanning tunneling microscopy and spectroscopy provide insights into the structural and electronic properties of the different polymorphic phases, which remain decoupled from the substrate due to the weak interaction with graphene. In addition, we demonstrate a great control of the relative coverage of the relevant 1T' and 1H MoTe2 phases by varying the substrate temperature during the growth. In particular, we obtain large areas of the 1T' phase exclusively or the coexistence of both phases with different ratios.

Efficient photogeneration of nonacene on nanostructured graphene.

The on-surface photogeneration of nonacene from α-bisdiketone precursors deposited on nanostructured epitaxial graphene grown on Ru(0001) has been studied by means of low temperature scanning tunneling microscopy and spectroscopy. The presence of an unoccupied surface state, spatially localized in the regions where the precursors are adsorbed, and energetically accessible in the region of the electromagnetic spectrum where n-π* transitions take place, allows for a 100% conversion of the precursors into nonacenes. With the help of state-of-the-art theoretical calculations, we show that such a high yield is due to the effective population of the surface state by the incoming light and the ensuing electron transfer to the unoccupied states of the precursors through an inelastic scattering mechanism. Our findings are the experimental confirmation that surface states can play a prominent role in the surface photochemistry of complex molecular systems, in accordance with early theoretical predictions made on small molecules.

Thermally Activated Processes for Ferromagnet Intercalation in Graphene-Heavy Metal Interfaces.

The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we achieve a nanoscale control of thermally activated intercalation of a homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited, in turn, onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures, depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in situ high-quality Gr-FM/HM heterostructures, enabling the realization of novel spin-orbitronic devices that exploit the extraordinary properties of Gr.

Electronic Properties of Sulfur Covered Ru(0001) Surfaces.

The structural properties of sulfur superstructures adsorbed on Ru(0001) have been widely studied in the past. However, much less effort has been devoted to determine their electronic properties. To understand the connection between structural and electronic properties, we have carried out density functional theory periodic boundary calculations mimicking the four long-range ordered sulfur superstructures identified experimentally by means of scanning tunneling microscopy (STM) techniques. Our simulations allow us to characterize the nature of the sulfur-Ru bond, the charge transfer between the Ru substrate and the sulfur adlayers, the interface states, and a parabolic state recently identified in STM experiments. A simple analysis, based on a one-dimensional model, reveals that this parabolic state is related to a potential well state, formed in the surface when the concentration of sulfur atoms is large enough to generate a new minimum in the surface potential.

High yielding and extremely site-selective covalent functionalization of graphene.

We describe a method to functionalize graphene covalently with 92% yield and 98% site-selectivity and strict spatial periodicity on the nanometer scale. This method could be extended to other functional molecules.

Organic Covalent Patterning of Nanostructured Graphene with Selectivity at the Atomic Level.

Organic covalent functionalization of graphene with long-range periodicity is highly desirable-it is anticipated to provide control over its electronic, optical, or magnetic properties-and remarkably challenging. In this work we describe a method for the covalent modification of graphene with strict spatial periodicity at the nanometer scale. The periodic landscape is provided by a single monolayer of graphene grown on Ru(0001) that presents a moiré pattern due to the mismatch between the carbon and ruthenium hexagonal lattices. The moiré contains periodically arranged areas where the graphene-ruthenium interaction is enhanced and shows higher chemical reactivity. This phenomenon is demonstrated by the attachment of cyanomethyl radicals (CH2CN(•)) produced by homolytic breaking of acetonitrile (CH3CN), which is shown to present a nearly complete selectivity (>98%) binding covalently to graphene on specific atomic sites. This method can be extended to other organic nitriles, paving the way for the attachment of functional molecules.

Controlling the spatial arrangement of organic magnetic anions adsorbed on epitaxial graphene on Ru(0001).

Achieving control over the self-organization of functional molecules on graphene is critical for the development of graphene technology in organic electronic and spintronic. Here, by using a scanning tunneling microscope (STM), we show that the electron acceptor molecule 7,7',8,8'-tetracyano-p-quinodimethane (TCNQ) and its fluorinated derivative 2,3,5,6-tetrafluoro-7,7',8,8'-tetracyano-p-quinodimethane (F4-TCNQ), co-deposited on the surface of epitaxial graphene on Ru(0001), transform spontaneously into their corresponding magnetic anions and self-organize in two remarkably different structures. TCNQ forms densely packed linear magnetic arrays, while F4-TCNQ molecules remain as isolated non interacting magnets. With the help of density functional theory (DFT) calculations, we trace back the origin of this behavior in the competition between the intermolecular repulsion experienced by the individual charged anions, which tends to separate the molecules, and the delocalization of the electrons transferred from the surface to the molecules, which promotes the formation of molecular oligomers. Our results demonstrate that it is possible to control the spatial arrangement of organic magnetic anions co-adsorbed on a surface by means of chemical substitution, paving the way for the design of two-dimensional fully organic magnetic structures on graphene and on other surfaces.

Probing the site-dependent Kondo response of nanostructured graphene with organic molecules.

TCNQ molecules are used as a sensitive probe for the Kondo response of the electron gas of a nanostructured graphene grown on Ru(0001) presenting a moiré pattern. All adsorbed molecules acquired an extra electron by charge transfer from the substrate, but only those adsorbed in the FCC-Top areas of the moiré show magnetic moment and Kondo resonance in the STS spectra. DFT calculations trace back this behavior to the existence of a surface resonance in the low areas of the graphene moiré, whose density distribution strongly depends on the stacking sequence of the moiré area and effectively quenches the magnetic moment for HCP-Top sites.

Two distinct phases of bilayer graphene films on Ru(0001).

By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.