Electron acceptor molecules deposited on epitaxial graphene studied by means of low temperature scanning tunneling microscopy/spectroscopy

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 22-07-3013En esta tesis se presenta el estudio de dos moléculas aceptoras de electrones bien conocidas, TCNQ y F4-TCNQ, sobre grafeno epitaxial. El crecimiento y caracterización de sus propiedades electrónicas y magnéticas se realizó mediante Microscopía y Espectroscopia Túnel de Barrido (STM/STS) en Ultra Alto Vacío y a baja temperatura. El grafeno epitaxial se crece sobre Ru(0001) y Ir(111) mediante descomposición térmica de moléculas. A pesar de que en ambos casos la superficie de grafeno presenta un patrón de moiré, estos dos sistemas son complementarios en lo que se refiere a la interacción entre el grafeno y el metal. El balance entre las interacciones molécula-molécula y molécula-substrato es estudiada para ambas moléculas a diferentes recubrimientos mediante STM. El grafeno epitaxial nos permite la pasivación de la superficie altamente reactiva del metal permitiendo el estudio de moléculas individuales prácticamente inalteradas así como la formación de diferentes estructuras moleculares autoensambladas. La espectroscopia túnel de barrido a baja temperatura es utilizada para dar a conocer la estructura electrónica de estas capas moleculares. En el caso de las TCNQ sobre grafeno/Ir(111), las moléculas se mantienen prácticamente neutras debido a la débil interacción con el sustrato. Por otra parte, en el caso de las TCNQ adsorbidas en grafeno/Ru(0001), estos experimentos, en concordancia con cálculos DFT+D2, mostraron la ocupación parcial del LUMO de la molécula de TCNQ neutra. Este orbital se divide en dos orbitales con espines opuestos, SOMO y SUMO. El papel de la capa de grafeno es modular la hibridización entre el electrón desapareado transferido a la molécula y los electrones de conducción del Ru, dando lugar a un efecto Kondo dependiente de la posición de absorción. Esta dependencia es confirmada para recubrimientos mayores de TCNQ y para moléculas de F4-TCNQ en la misma superficie. Además, el efecto combinado de la interacción atractiva entre las moléculas de TCNQ y la ocupación parcial del LUMO da lugar a la formación de bandas intermoleculares extendidas espacialmente que permiten a las moléculas deslocalizar la carga adquirida. Estas bandas están divididas en espín, con sólo la banda de espín mayoritaria ocupada, como revelaron los cálculos DFT. La existencia del correspondiente orden magnético a largo alcance es detectado mediante experimentos de STM polarizados en espín a 4.6 K. Estos resultados son la primera evidencia experimental y teórica de la existencia de orden magnético a largoThis thesis presents a study of two well-known electron acceptor molecules, TCNQ and F4- TCNQ, deposited on epitaxial graphene. The growth and characterization of the electronic and magnetic properties of these systems have been performed by means of Low Temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS) in Ultra High Vacuum. A graphene layer is grown by chemical vapor deposition on Ru(0001) and Ir(111) surfaces. Although in both cases the graphene surface presents a moiré superstructure, these two systems are completely different from the graphene-metal interaction point of view. The relative strength between the molecule-molecule and molecule-substrate interactions is systematically studied for both molecules at different molecular coverages by means of STM. The epitaxial graphene passivates the highly reactive metallic surfaces allowing the study of almost unperturbed single molecules and the formation of self-assembly molecular structures. Low temperature scanning tunneling spectroscopy is used to unveil electronic structure of these molecular layers. In the case of TCNQ on graphene/Ir(111), the weak moleculesubstrate interaction keeps the molecules practically neutral. On the other hand, in the case of TCNQ deposited on graphene/Ru(0001), these experiments, in agreement with DFT+D2 calculations, reveal the partial occupation of the LUMO of the neutral TCNQ molecule. This orbital splits into a spin-up SOMO and a spin-down SUMO. We find that the graphene layer modulates the hybridization between the transferred unpaired electron and the Ru conduction electrons leading to a site dependent Kondo effect. This dependence is further confirmed for higher TCNQ coverages and for F4-TCNQ molecules on the same surface. In addition to that, the combined effect of the attractive interaction between TCNQ molecules and the partial occupation of the LUMO gives rise to the formation of spatially extended intermolecular bands, that allow the molecules to delocalize the charge acquired. The bands are spin split, with only the majority spin band being occupied as revealed in the DFT calculations. The existence of the corresponding long-range magnetic order is detected by spin polarized STM at 4.6 K. These findings are the first experimental and theoretical evidence for the existence of long-range magnetic order in delocalized bands in a purely organic molecular monolayer

Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001

We have performed low temperature scanning tunnelling spectroscopy (STS) measurements on graphene epitaxially grown on Ru(0001). An inelastic feature, related to the excitation of a vibrational breathing mode of the graphene lattice, was found at 360 meV. The change in the differential electrical conductance produced by this inelastic feature, which is associated with the electron-phonon interaction strength, varies spatially from one position to other of the graphene supercell. This inhomogeneity in the electronic properties of graphene on Ru(0001) results from local variations of the carbon-ruthenium interaction due to the lattice mismatch between the graphene and the Ru(0001) lattices.Comment: 6 Pages, 3 figure

Charge state control of F16CoPc on h-BN/Cu(111)

International audienceThe use of molecular materials in solar cells and nano-electronics demands a fundamental understanding and control of their electronic properties. Particularly relevant is the molecular response to the environment, that is, the interaction with the support and adjacent molecules, as well as the influence of electrostatic gating. Here, the control of molecular level alignment and charge states of fluorinated cobalt phthalocyanines (F16CoPc) on atomically thin hexagonal boron nitride (h-BN) sheets on Cu(111) is reported using scanning tunneling microscopy (STM) and spectroscopy (STS), as well as atomic force microscopy (AFM) and complementary density functional theory (DFT) calculations. Three parameters that govern the electronic level alignment of F16CoPc orbitals are investigated: i) template-induced gating by the work function variation of the h-BN/Cu(111) substrate, ii) gating by the STM tip, and iii) screening by neighboring molecules. The interplay of these parameters influences the charge distribution in the studied molecular arrangements and thus provides the possibility to tune their physicochemical behavior, for instance, the response toward electronic or optical excitation, charge transport, or binding of axial adducts

BN-patterning of metallic substrates through metal coordination of decoupled borazines

Read the full text ePDF PDF ePDFPDF PDF Tools Share We report on the synthesis of pyridine‐terminated borazine derivatives, their molecular self‐assembly as well as the electronic properties investigated on silver and copper surfaces by means of scanning tunneling microscopy and X‐ray photoelectron spectroscopy. The introduction of pyridine functionalities allows us to achieve distinct supramolecular architectures with control of the interdigitation of the molecules by surface templating. On silver surfaces, the borazine derivatives arrange in a dense‐packed hexagonal structure through van der Waals and H‐bonding interactions, whereas on Cu(111), the molecules undergo metal coordination. The porosity and coordination symmetry of the reticulated structure depends on the stoichiometric ratio between copper adatoms and the borazine ligands, permitting an unusual three‐fold coordinated Cu–pyridyl network. Finally, spectroscopy measurements indicate that the borazine core is electronically decoupled from the metallic substrate. We thus demonstrate that BNC‐containing molecular units can be integrated into stable metal‐coordination architectures on surfaces, opening pathways to patterned, BN‐doped sheets with specific functionalities, for example, regarding the adsorption of polar guest gases

Electronic and Geometric Corrugation of Periodically Rippled, Self-nanostructured Graphene Epitaxially Grown on Ru(0001)

Graphene epitaxially grown on Ru(0001) displays a remarkably ordered pattern of hills and valleys in Scanning Tunneling Microscopy (STM) images. To which extent the observed "ripples" are structural or electronic in origin have been much disputed recently. A combination of ultrahigh resolution STM images and Helium Atom diffraction data shows that i) the graphene lattice is rotated with respect to the lattice of Ru and ii) the structural corrugation as determined from He diffraction is substantially smaller (0.015 nm) than predicted (0.15 nm) or reported from X-Ray Diffraction or Low Energy Electron Diffraction. The electronic corrugation, on the contrary, is strong enough to invert the contrast between hills and valleys above +2.6 V as new, spatially localized electronic states enter the energy window of the STM. The large electronic corrugation results in a nanostructured periodic landscape of electron and holes pockets.Comment: 16 pages, 6 figure

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 materialsThis work was supported by Ministerio de Ciencia, Innovación y Universidades through grants, PID2021-128011NB-I00 and PID2019-105458RBI00. Ministerio de Ciencia e Innovación and Comunidad de Madrid through grants “Materiales Disruptivos Bidimensionales (2D)” (MAD2DCM)-UAM and “Materiales Disruptivos Bidimensionales (2D)” (MAD2DCM)-IMDEA-NC funded by the Recovery, Transformation and Resilience Plan, and by NextGenerationEU from the European Union. Comunidad de Madrid through grants NMAT2D-CM P20128/NMT-4511 and NanoMagCost. IMDEA Nanoscience acknowledges support from the “‘Severo Ochoa”’ Programme for Centres of Excellence in R&D CEX2020-001039-S. IFIMAC acknowledges support from the “‘María de Maeztu”’ Programme for Units of Excellence in R&D CEX2018-000805-M. M.G. thanks Ministerio de Ciencia, Innovación y Universidades “Ramón y Cajal” Fellowship RYC2020-029317-I. Allocation of computing time at the Centro de Computación Científica at the Universidad Autónoma de Madrid, the CINECA Consortium INF16_npqcd Project, and Newton HPCC Computing Facility at the University of Calabria (MP

Understanding the self-assembly of TCNQ on Cu(111): a combined study based on scanning tunnelling microscopy experiments and density functional theory simulations

Two polymorphic structures of TCNQ on Cu(111) can be formed by varying the deposition conditions.</p

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 TMD