Self-assmebly of molecules on nanostructured graphene
Texto completo
(2) ! ! UNIVERSIDAD AUTÓNOMA DE MADRID! Department of Condensed Matter Physics. ! ! SELF-ASSEMBLY OF MOLECULES ON NANOSTRUCTURED GRAPHENE. ! ! ! Tesis doctoral presentada por Flavio Pendolino!. !. Para optar al título de Doctor en Ciencias Físicas Departamento de Física de la Materia Condensada Universidad Autónoma de Madrid. ! ! !. Directores de tesis: Prof. Amadeo López Vázquez de Parga. Madrid, June 2014.
(3) 1. 2. 3. Contents. Abstract. Page iii. Resumen. Page v. Motivation and Outline Introduction 1.1. Nano the New Route. Page vii Page 1 1. Nanostructured Surfaces. 2. Self-Assembly and Self-Organization. 2. Graphene. Page 9. 2.1. The Properties of Graphene. 10. 2.2. Graphene on Metals. 11. 2.3. Growth Methods. 14. Graphene/Ru(0001) Surface. 14. Cu(111) Surface. 17. Experimental Setup. Page 19. 3.1. Ultra High Vacuum System (UHV). 19. 3.2. VT-STM. 20. 3.3. Basic Principles on Theory of STM. 20. 3.4. Sample and Tip. 23. i.
(4) TCxQ on Cu(111). Page 25. 4.1. Background. 25. 4.2. Description of TCxQ Molecule. 26. 4.3. Results and Discussion. 31. 4.4. TCNQ on Cu(111). 31. TCAQ on Cu(111). 32. TCTQ on Cu(111). 35. TCPQ on Cu(111). 39. Summary. TCxQ on Graphene/Ru(0001) 5.1. 5.2. Results and Discussion. 44. Page 45 45. TCNQ on Graphene/Ru(0001). 45. TCAQ on Graphene/Ru(0001). 46. TCTQ on Graphene/Ru(0001). 53. TCPQ on Graphene/Ru(0001). 57. Summary. Caffeine on Cu(111). 63. Page 65. 6.1. Background. 65. 6.2. Results and Discussion for Caffeine Deposited on Cu(111). 67. 6.3. Summary. 76. General Conclusion. Page 77. Conclusiones. Page 79. References. Page 81. ii.
(5) Abstract In this thesis the growth and morphological characterisation for the self-assembly of various electron acceptor molecules were investigated by mean of Scanning Tunnelling Microscope (STM) in Ultra High Vacuum. Molecules, from the quinone and xanthine classes, such as 11,11, 12,12 - tetracyano-9,10-anthraquino dimethane (TCAQ), 13,13,14,14 - tetracyano-5,12 -tetracenequino dimethane (TCTQ), 15,15, 16,16 -tetracyano -6,13-pentacenequino dimethane (TCPQ) and 1,3,7-Trimethyl xanthine (caffeine), were chosen. Two different substrate, graphene/Ru(0001) and Cu (111), were used to study the influence of the substrate molecule interaction in the self-assembly processes. The metallic substrate allows the hybridisation of the molecular states and the metallic DOS. On the other hand, the graphene decouples the molecules from the metal providing a good playground to study the moleculemolecule interactions. These molecules aggregate and eventually self-assembly on the surface and the aggregation process is influenced by the number of the rings in the backbone of the molecule, the interaction with the substrate and the coverage of the surface. Basically, the TCxQ molecules show on Cu(111) a very limited aggregation character with the formation of small clusters, such as chains or dimers, and eventually, disordered structures. A qualitative explanation for the cluster formation can be given using the molecular potential maps, where the negative potential, associated with the cyano groups, interacts with the positive potential, located on the hydrogens in the molecular backbone, forming a hydrogen bond network. Analysing the apparent size in the STM images on the energetics in theoretical calculations it can be concluded that all TCxQ molecules adopt the socalled X-configuration upon adsorption on Cu(111). Unlike, the deposition of those molecules on graphene/Ru(0001) brings to a very different structure which is characterized by a formation of an extensive hydrogen bond network and a very weak interaction with the substrate. Each of the quinone derived molecules self-assemblies in a different way depending on the number of the aromatic rings in the backbone of the molecule and the coverage of the surface. At low coverage, only symmetric molecules stay on the surface and occupy the low area of the graphene moiré with unordered clusters. By increasing the coverage TCAQ molecules self-assembly into representative “railroad tracks” structure covering low and top area of the moiré . By contrast, TCTQ and TCPQ molecules formed a porous network filling the low area only. When the monolayer conditions are reached, a compact self-assembly structure is obtained only for the symmetric molecules. To investigate the influence of the rigid molecular structure on the surface, caffeine molecules were deposited iii.
(6) on Cu(111). Caffeine is basically adsorbed flat-lying on Cu(111) and forms dimers arranged into parallel rows. The molecules are easy to move indicating the absence of a preferential adsorption position and the important of the intermolecular interactions in the formation of the self-assembled structure.. iv.
(7) Resumen En esta tesis fueron investigadas el crecimiento y la caracterización morfológica para el self-assembly de diferentes moléculas aceptoras de electrones por medio de Scanning Tunnelling Microscope (STM) en Ultra Alto Vacío. Fueron elegidas moléculas que van desde las clases quinona y xantina, tales como 11,11, 12,12 - tetraciano 9,10 - Dimethane anthraquino ( TCAQ ), 13,13,14,14 - tetraciano - 5, Dimethane 12 tetracenequino ( TCTQ ), 15,15, 16,16 tetraciano Dimethane -6,13 - pentacenequino ( TCPQ ) a la 1,3,7- trimetil xantina (cafeína). Para estudiar la influencia de la interacción molécula-sustrato en los procesos de self-assembly han sido utilizados dos diferentes sustratos, grafeno / Ru(0001) y Cu(111). El sustrato metálico permite la hibridación de los estados moleculares y el metálico DOS. Por otro lado, el grafeno desacopla las moléculas del metal proporcionando una buena zona para estudiar las interacciones molécula-molécula. Estas moléculas se agregan y, al final, se autoensamblan en la superficie. En concreto el proceso de agregación está influenciado por el número de anillos en la estructura de la molécula, la interacción con el sustrato y el recubrimiento de la superficie. Básicamente, las moléculas TCxQ muestran en Cu (111) un carácter de agregación muy limitado con la formación de pequeños grupos, tales como cadenas o dímeros, y, al final, estructuras desordenadas. Una explicación cualitativa para la formación del clúster se puede hallar utilizando los mapas de potencial molecular, donde el potencial negativo, asociado con los grupos ciano, interactúa con el potencial positivo, que se encuentra en los átomos de hidrógeno en el esqueleto molecular, formando una red de enlace de hidrógeno. Analizando el tamaño que aparece en las imágenes de STM en la energética en cálculos teóricos, se puede concluir que todas las moléculas TCxQ adoptan la denominada configuración-X de la adsorción en Cu(111). Por otro lado, la deposición de las moléculas en el grafeno / Ru(0001) lleva a una estructura muy diferente, caracterizada da la formación de una extensa red de enlaces de hidrógeno y una muy débil interacción con el sustrato. Cada una de las moléculas quinonas derivadas se autoensabla de manera diferente en función del número de los anillos aromáticos en la cadena principal de la molécula y del recubrimienti de la superficie. En condiciones de bajo recubrimiento, sólo las moléculas simétricas permanecen en la superficie y ocupan la zona baja del moiré de grafeno con clusters desordenados. A medida que aumenta el recubrimiento las moléculas de TCAQ se autoensemblan en estructuras representativas “railroad tracks”cubriendo la parte del área superior y inferior del moiré. Por el contrario, las moléculas de TCTQ y TCPQ forman una red porosa que llena sólo la zona inferior. Se comprobó tambien que en el caso de tener una v.
(8) monocapa, la estructura self-assembly compacta se obtiene sólo para las moléculas simétricas. Por último, fueron depositadas moléculas de cafeína en superficies de Cu(111) para poder investigar la influencia de la estructura molecular rígida en la superficie. La cafeína es básicamente adsorbida en manera planar con respecto al Cu(111) y forma dímeros dispuestos en filas paralelas. En concreto, las moléculas presentan facilidad de movimientos, que indica la ausencia de una posición de adsorción preferencial y la importancia de las interacciones intermoleculares en la formación de la estructura self-assembly.. vi.
(9) Motivation and Outline In the last decade the advance in nanoscience has grown exponentially due to the potential use in technology reinforced by industrial demand. Nowadays, the challenge of the research is an advanced control of the global properties of a system by engineering and designing novel superstructures at the nanoscale. Atomic resolution of scanning tunnelling microscopy (STM) opened a route to manipulate atoms individually, however, this approach is restrained by a practical employment for mass production. A complementary way is to take advantage of the promising properties of self-assembly organic molecules. The formation of long range order and adsorbate-substrate interactions play a crucial role in designing a novel material which can be use in a real applications by choosing carefully the substrate and the adsorbed molecules. It is against this background that this thesis is framed with an investigation of the self-assembly properties of electron acceptor organic molecules deposited on a chosen substrates, such as Cu(111) and graphene/Ru(0001). It is proven that the organic molecule organisation is strongly affected by the substrate and the lateral interactions between molecules. Moreover, the molecular structure and the conformation, that the molecules acquired on the surface, result in the modification of the self-assembly behaviour. The thesis is organised as described in the following. Chapter 1 provides a general overview for the behaviour of molecular self-assembly on surfaces. The second section is dedicated to illustrate the properties of graphene and the method to grow it on transition metallic substrates.Finally, the main properties of the graphene/Ru(0001) and Cu(111) surfaces are discussed in detail. The chapter ends with the description of the instruments and the experimental techniques used in this work. Chapter 2 is dedicated to the study of the deposition of electron acceptor molecules TCAQ, TCTQ and TCPQ on Cu(111) surface. At beginner of the chapter, the properties of these molecules are described. The experimental results indicate the influence of the substrate on the order of the superstructures. Interaction with copper surface restricts the molecular interactions which limit and/or suppress the selfassembly structure. Chapter 3 is focused on the self-assembly of TCAQ, TCTQ and TCPQ on graphene/ Ru(0001). The extension of the backbone plane in TCxQ molecule governs the lateral interactions which bring molecules to form a different bidimentional structures vii.
(10) according to the number of aromatic rings. Chapter 4 delineates the self-assembly of caffeine on Cu(111). Due to the rigid structure of the backbone plane in caffeine, weak interactions occur between molecules and substrate. The rigid structure of caffeine implies that a weak interaction occurs with the substrate and the ordered structure is more favourable than in TCxQ systems. Finally, general conclusion and bibliography end this thesis.. viii.
(11) 1 Introduction. 1.1. Nano the New Route. There is a strong interrelation between science and technology. Very often new science brings new technology and, reciprocally, new technology creates new opportunities to advance in science. The possible influence of the “nano world” on technology was forecasted by Richard Feynman in his famous lecture at Caltech in 1959 [1]. Using the phrase “there is plenty of room at the bottom”, he discussed the possibility of manipulating individual atoms. The talk is recognized to be a pioneering event in the history of nanoscience, and it inspired the conceptual background of the field. In the past few decades, the rapid growth of nanoscience and technology took place primarily because of the availability of new synthesis of nanomaterials and, next, because of new tools for characterisation and manipulation [2, 3]. It was in this context that technology helped the progress of science with the invention of an instrument for imaging surfaces at the atomic level, the scanning tunneling microscopy (STM). The instrument was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory [4, 5]. In 1986 they share the Nobel Prize in Physics with Ernest Ruska who developed the scanning electron microscope [5, 6]. The STM opened a route to study and manipulate individual atoms and molecules. Its capability originates the so called nanoscience. A short definition of Nanoscience is the following: 1.
(12) 1. Introduction Nanoscience is the manipulation of matter of an individual atom or molecule. Thus, nanoscience deals with physical, chemical and biological processes and is employed to describe materials with structural features between atomic size and macro-molecular length scale. The main goal of nanoscience is to investigate, control and develop new classes of materials establishing new architecture nanosystems, mastering synthesis of isolated nanostructures, modifying desired properties at atomic level and, finally, connecting nanosystems to macroscopic technology for future devices.. 1.1.1. Nanostructured Surfaces. Two basic approaches can be employed to prepare a nanostructured surface: topdown and bottom-up [7, 8]. The top-down method is based in the concept of miniaturisation; this means that a macroscopic system is broken down into microscopic units to gain insight its compositional sub-systems where each subsystem is then refined in yet greater detail. The top-down approach often uses the traditional workshop or microfabrication methods where externally controlled tools are used to cut, mill, and shape materials into the desired shape and order. The morphology of the surface is modified by physical (e.g. lithography) or chemical (e.g. imprinting) process on meso or nanoscale or on the direct manipulation of entities (atoms, molecules). The bottom-up method uses the intrinsic physical and chemical properties of single molecules to drive them into organized structures. This approach relies on concepts like molecular self-assembly and/or molecular recognition. However, a necessary prerequisite in order to take advantage of this method is a full understanding of the processes which produce the formation of nanostructures on surfaces. Each entity (building block) can be arranged on the surface to obtain the desired product in a spontaneous or constrained way. Widely, two classes of processes can be categorized: self-assembly and self-organization.. 1.1.2. Self-Assembly and Self-Organization. One of the early definitions for the self-assembly and self-organization processes was reported by Whitesides and coworkers [9, 10]. They described self-assembly as a process where molecules “spontaneously assembly into structures, stable, noncovalentely joined aggregates”. By contrast, in the self-organization process, the ordering of molecules occurs in open systems driven away from thermal equilibrium. In simple words, the main difference between both processes is the driving mechanism that for self-organization is kinetic and for the self-assembly is thermodynamic. A more precise definition is found in John and Bär’s paper [11], where 2.
(13) 1. Introduction they studied a formation of protein patterns. They stated that Self-assembly implies spatial structuring as a result of minimization of the free energy in a closed system. Hence, a self-assembled structure corresponds to a thermodynamic equilibrium and Self-organization requires a situation far away from thermodynamic equilibrium and is possible only in open systems with an external energy source. Although Whitesides and John and Bär’s definitions are concrete, those definitions were more suitable for bulk processes. In fact, when the process occurs on a surface a further and more precise description is needed to include the surface influence. A step forward, in improving self-assembly and self-organization definition, is described in a review by Becker and Wandelt [7]. They started from the John and Bär’s explanation but including additional details concerning the behaviour of the system. In both definitions “only the intrinsic properties of a system such as interactions between the constituents of the system” are implied. They stressed the fact that a surface could act as template and could impose, an extrinsic interaction on the building blocks. This means that not only does thermodynamics play a role in the process but also in the nature of each part of the system (substrate and building blocks). In order to better describe the influence of the surface, Becker and Wandelt added two new definitions: templets-assisted assembly and templets-controlled assembly. In the templates-assisted assembly process, the surface acts as template that contains spatial information to guide the building blocks but does not control the final aggregation of molecules because the interactions between building blocks also play an important role. An example for this process is found in the Zou’s paper where PTCDA is adsorbed on Ag(111) and Ag(110) forming a self-assembly structure assisted by the substrate [14]. On the contrary, when the template imposes and determines the formation of the structure despite the interaction of the building blocks, then a templates-controlled assembly process occurs. Under this category is placed the work for 2D structure proposed by Li [12] where C60 is evaporated on Gr/Ru(0001) surface (Fig 1.1). The growth of C60 layer follow the substrate pattern in closely packed hexagonal and the authors claim that interaction between C60 molecules and the substrate is rather weak, but by no means negligible. The Becker and Wandelt definition are mostly focused at the adsorbate-adsorbate or adsorbate-substrate interaction (see the scheme in Fig 1.2). The importance of the role the surface plays in the final structure was emphasized by Koepf et al. [13]. They assert that the surface must be taken into account in a self-assembled 3.
(14) 1. Introduction. Figure 1.1. STM topography of (a)C60 molecules adsorbed on G/Ru(0001). (b) a zoomed in area of image showing the supramolecular structure: molecules are arranged with closely packed hexagonal growth and on atop sites of the graphene moiré pattern. Molecular lattice are outlined by rhombuses. Adapted from Li [12].. structure, they conclude that the role of the surface can be described as either inert, template or active. For the so called inert surfaces, the aggregation is controlled by molecular interaction only. When the surface is a template, the building blocks are constrained by specific directions of growth, e.g. along specific symmetry axes or surface heterogeneities. In addition Koept and coworkers consider another category where the surface is an active participant of the self-assembly process. In this case, the surface provides active constituents, adatoms and specific adsorption sites that can alter the electronic structure of the building blocks and therefore plays an active role in the final structure. In summary we can conclude that self-assembly processes are an important aspect in nanotechnology and can be used to engineer complex molecular architectures choosing dedicated shapes and functional groups in the building blocks. Adsorption, mobility, and intra- or intermolecular interactions, all of which depends on the substrate crystalline lattice, chemical nature, and symmetry are key parameters that govern the formation of molecular architectures. A balancing of lateral inter-molecular and surface-molecular interaction is at the origin of the formation of supramolecular order.. Figure 1.2. The evolution of process class proposed by Becker and Wandelt [7].. Energy scale of the interactions. There are different types of molecular interactions that can drive self-assembly processes. Typically, the interactions involved 4.
(15) 1. Introduction imply non-covalent interactions such as hydrogen bonds, van der Waals forces, π-π interactions, and/or electrostatic interaction. Covalent bonds are also involved in the formation of self-assembly structures on surface, however, these are rare cases and will not describe in this thesis. Self-assembled networks depend strongly on temperature variations and often are unstable even at moderate temperatures. The most relevant characteristics of the non-covalent interactions involved in the formation of self-assembly structures are described in the following paragraphs. • Van der Waals. Van der Waals is an attractive interaction that falls in the category of the dispersion forces. It occurs when two atoms or molecules approach to each other and a temporary induced dipoles are created which lead to intermolecular attraction. This bond usually takes place at distances greater than 3 Å. In terms of energy, van der Waals interactions are the weak (0.02-0.17 eV) [14, 15] and can be difficult to measure. Even though van der Waals are a weak interaction, they are important because they represents the main attractive forces between non polar molecules at surface. In general van der Waals forces are not very directional, hence, the formation of a pattern depends on several factors i.e. stoichiometry [16] or geometry [17] and may form competing meta-stable molecular structures which are close in energy [18, 19]. Van der Waals forces can exist between the substrate and the deposited molecules or between the molecules on the surfaces. • Hydrogen Bond. Hydrogen bond is an attractive interaction which is ranged from weak to strong energy (0.04-1.7 eV) (see Fig 1.3) and the bondlength of 1-3 Å [15, 20, 21]. According to IUPAC [22], a hydrogen bond is defined as “an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X-H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation”. Hydrogen bonding is a complex interaction, with mixed contributions from electrostatic interactions and dispersion forces. It is known that C-H group in organic molecules, e.g. benzene phenol, acetylene pyridine, can be involved in hydrogen bonds [23] with an electronegative element such as O, S, N, F, Cl, F [20, 23]. These bonds are often encountered in the selfassembly of process organic molecules due especially to the CH2 groups. For the presence of hydrogen and heteroatoms in organic molecules, the hydrogen bond is a primary driving force for each unit to an ordered aggregation in the surface. • π-π Interaction. The π-π interaction takes place between organic compounds and originates from the partial intermolecular overlap of p-orbitals in π-conjugated 5.
(16) 1. Introduction. Figure 1.3. Hydrogen bonds energy diagram, where hydrogen bond strength is correlated with functional chemical groups. Adapted from Desiraju [21].. systems. In comparison with the hydrogen bond, the π-π interaction is weaker and has no strong directionality, which makes them more difficult to rank in importance in the case of adsorption and self-organization on surfaces [24] and the contribution in the formation of ordered structures. • Electrostatic Interaction. This interaction involves attractive or repulsive forces between atoms and/or groups of atoms and/or molecules that are due to the presence of ionized entities and to their electronegative properties. The system will always try to arrange itself in such a way that it minimizes these effects. Electrostatic interactions are stronger than the previous interactions discussed and energies involved are of the order of 2 eV and 3 eV [15].. Taking into account interactions described above, organic molecules are able to create a variety of different supramolecular structures on surfaces. It is worth noting that not the only do the interactions between molecules play a dominant role but also the properties of the surface are important as well. In the following, we describe a number of experiments to show how intermolecular interactions and the surfaces influence the formation of self-assembly structure. Surface Molecule Interaction. Roos et al. [25] investigated a large organic molecule, 2,40 -bis(terpyridine) (2,40 -BTP), a multiaromatic molecule with peripheral nitrogen atoms, on graphene/Ru(0001). This molecule self-assembles into a open structure with pores where C−H· · · X interaction is responsible for the hydrogen bond network (Fig 1.4). The authors reported that the molecules only lay down between the ripples of graphene and form chains where 2,40 -BTP are arranged in an antiparallel 6.
(17) 1. Introduction way. Finally, they stated that 2,40 -BTP do not cover the ripples because the adsorption potential exceeds the intermolecular interaction. The conclusion of Roos et al. is that weak interactions between the molecules are competing with the lateral variation of the molecule-substrate interaction in the formation of the ultimate ordered structure.. Figure 1.4. (a) STM image (V= 0.97V, I= 30 pA, T=115K) and (b) scaled model of 2,40 -BTP on graphene/Ru(0001). Adapted from Roos [25].. Molecule Substrate Interaction. Tseng et al. [26] show the adsorption of the electron acceptor molecule TCNQ on Cu(100). Here, molecular self-assembly produces the formation of rectangular islands. They found that tetracyanoquinodimethane (TNCQ) interacts with the surface strongly via the lone pairs of the nitrogen atoms and the dz orbital from the atoms of the copper substrate, establishing a bond with a binding energy of about 2.2eV. This strong interaction enables the binding of the dicyano groups with the copper atoms from surface. The copper atoms, attracted to the molecules, are lifted from their equilibrium position. This is possible due to the sizeable charge transfer from the substrate to the molecule that aromatise the central ring and allows the molecule to bend. This bending, in turn, helps in the self-assembly process.. Figure 1.5. (A)-(B) Structural rearrangement for the adsorption of TCNQ deposited on Cu(100). (C) Calculated STM image showing the self-assembly structure and the schematic illustration of TCNQ monolayer. Adapted from Tseng[26].. 7.
(18) 1. Introduction Intermolecular Interaction. An example of self-assembly of molecules on graphene is reported by Barja [27]. The authors deposited TCNQ on graphene/Ir(111) at 300K (Fig 1.6) and the molecules form an ordered structure that completely covers the substrate. Due to the presence of the graphene layer, the TCNQ molecules are electronically decouple from the metallic substrate and only the intermolecular interactions are important. They show that the partial negative charge of cyan groups interact with the positive charge of hydrogen in the aromatic rings. Thus, the aggregation of molecules create a self-assembled structure driven by the hydrogen bond network.. Figure 1.6. (A) STM topography of TCNQ deposited on graphene/Ir(111) at 4.6K (89 × 45 nm2 ,V=-0.7V, I= 100pA). (B) High resolution image ( 7 × 7 nm2 ) of HOMO and LUMO is shown at bias voltage -1.5V and +1.5V, respectively. TCNQ molecule has been superimposed to show the molecular orientation. Adapted from Barja [27].. 8.
(19) 2 Graphene. Carbon has a unique role in nature, and in fact, it can be considered the fundamental constituent of life. The ability of carbon to form a number of different and complex structures is crucial in many scientific fields e.g. organic chemistry, physics, biology. Carbon presents several allotropic forms such as diamond and graphite (3D), nanotubes (1D) and fullerenes (0D). Until 2004, the 2D form, named graphene, was missing but plays an important role, especially in understanding the electronic properties of nanotubes and graphite.. Figure 2.1. Crystal structures of carbon allotropes. (Left to right) Three-dimensional structure diamond and graphite (3D); two-dimensional graphene (2D); one- dimensional nanotubes (1D); and zero-dimensional buckyballs (0D). Adapted from [28].. In 2004 Geim, Novoselov and coworkers were able to isolate small graphene flakes on SiO2 and to measure their electronic properties [29], a discovery for which they subsequently won the Nobel Prize in Physics. Graphene is a 2D material made by a single layer of carbon atoms arranged in a honeycomb network (Fig 2.2-A). The word graphene is constructed from the word graphite and the suffix -ene which is 9.
(20) 2. Graphene used for polycyclic aromatic hydrocarbons [30]. Graphene can be thought of as a monolayer of graphite. The first study about graphene was the calculation of its band structure conducted by Wallance in 1947 [31]. He found that graphene has a unusual linear electronic dispersion around the Fermi level with the behaviour of a gap zero semiconductor. In the 70’s, different experimental groups obtained graphene as secondary product during the preparation of clean metallic surfaces in UHV, as shown by van Bommel et al. [32]. A documented graphene synthesis of a carbon monolayer is reported by Oshima in 1977 on lanthanum hexaboride via segregation of dissolved carbon in the bulk [33]. Later, some studies on “graphite monolayer” or ultra-thin graphite film were performed on ruthenium [34], iridium [35] and nickel [36]. In those works the thin carbon films (monolayer) are grown via cracking of hydrocarbons using the hot metallic surface as catalyst. All those studies were limited by the experimental techniques available during those years and were unable to detect experimentally the unique electronic structure of graphene. Since its discovery by Geim, Novoselov and co-workers, graphene has attracted enormous attention because of its unique physical and chemical properties like high thermal and electronic conductivity, high mechanical rigidity, transparency, and high mobility of charge carriers [37–39].. Figure 2.2. (A) Graphene lattice is made by two carbon atoms a (blue) and b (yellow) equivalent which form two sublattices. (B) Graphene honeycomb where the pz orbitals (red) are perpendicular to the basal plane, contribute to the electronic conductivity. The σ bands (blue) connect two adjacent carbon atoms and it is responsible of the structure skeleton and the flexibility of graphene.. 2.1. The Properties of Graphene. Graphene consists of a planar honeycomb lattice of carbon atoms, as shown in Fig 2.2-A. Carbon has two electrons in the s shell and two in the p shell, a trigonal planar structure is formed thanks to the sp2 hybridization with a formation of a σ band (valence) and σ ∗ band (conduction) between the three neighbouring carbon 10.
(21) 2. Graphene atoms which are separated by a length of 1.42 nm (Fig 2.2-B). The σ bands confer to the graphene its structural properties becoming the strongest material known but without losing flexibility. The unaffected pz orbitals are perpendicular and mirror symmetric to the graphene plane. This orbital leads to the formation of the π band (valence) and π∗ band (conduction) that contribute to the electronic transport in graphene. As we can see in Fig 2.3, the π and π∗ band have a linear dispersion in a energy range ±1eV near the Fermi level at K and K0 of the Brillouin zone. At those points, named Dirac points, conduction and valence π bands touch with a consequence that the electronic band structure has a characteristic gapless spectrum and renders graphene a zero-gap semiconductor [38, 40, 41]. In the case the symmetry is broken, e.g. defects on the surface or interaction with another graphene layer, a gap is opened [42, 43]. Graphene is an inert material because of the chemical inertness of the sp2 carbon orbitals and it is stable in air up to 200◦ C [44–46]. Therefore, a high energetic species are needed to break the graphene structure. However, the π bands (perpendicular to the graphene plane) allow weak interactions with adsorbates. This means that graphene is a convenient material to decouple molecules from the metallic substrates but at the same time, allow effective charge transfer to the molecules deposited on the surface [47].. Figure 2.3. (Left) Electronic band structure for σ and π band of graphene. At K-point (Dirac point) a linear dispersion is shown near the Fermi level. (Right) Electronic dispersion in the honeycomb lattice and zoom in of the energy bands close to one of the Dirac points. Adapted from [48].. 2.2. Graphene on Metals. As shown in Fig 2.4, the superposition of two lattices with different periodicities (Left), or relative rotation of the two lattices (Right) produces a new periodicity called a moiré pattern [49]. Due to this new superperiodicity a variation of chemical and electronic properties of the graphene overlayer is observed, which arises from 11.
(22) 2. Graphene the corrugation of the moiré pattern and the modulation in the interaction with the substrate. From the point of view of the interaction between graphene and the metallic substrate we can distinguish weak and strong interactions. In fact, when graphene is grown on Ir, Pt, or Al, a weak interaction is found and a charge transfer occurs but the graphene holds the main properties of free-standing graphene. However, if the graphene is grown on Ru, Rh, Co, Pd, or Ni. the interaction with the underlying metal is strong and the electronic structure is modified. In the latter case, the π band (perpendicular to the basel plane of the graphene) can hybridize with the d orbitals of the metal, holding the graphene skeleton unmodified thanks to the sp2 bonds and, as a consequence, a modulation of the chemical interaction between graphene and the metal is expected.. Figure 2.4. Superstructure of the moiré resulting in a superimposed layers with (left) different lattice parameters and (right) rotation of a plane with same periodicity.. Preobrajenski et al. [50] demonstrated experimentally the different degrees of interaction between graphene and different transitional metals. Fig 2.5 shows XPS spectra of C-1s for graphene grown on Pt(111), Ir(111), Ru(0001) and Rh(111) compared with highly oriented pyrolytic graphite (HOPG). A slight decrease in peak position is observed in the spectra of Gr/Pt(111) and Gr/Ir(111) against the binding energy for HOPG. By contrast, the signal for graphene on Ru(0001) and Rh(111) show a peak which is split into two subcomponents (C1 and C2). The subcomponent with lower energy (C1) reflects the carbons that do not interact with the metal, while, the subcomponent at higher energy (C2) exhibits an quasi chemical bond character. Those aspects are in accordance with the measured STM images [51, 52]. Therefore, we can distinguish two kind of systems: 4d-metals, that interact with graphene using a covalent-like bonds, and 5d-metals, that have a weak interaction. A wide range of metals have thus been investigated to understand how graphene 12.
(23) 2. Graphene. Figure 2.5. Photoelectron spectra C-1s for graphene/metal substrates. HOPG is reported for comparison. On the left, scheme illuminates graphene corrugation. Adapted from [50].. interacts with the substrate and how the properties can be tailored in choosing an appropriate metal substrate. Graphene has been synthesized onto Ru(0001) [50, 51, 53–61], Ir(111) [27, 52, 62–65], Pt(111) [49, 66–70], Pd(111) [71], Re(0001) [72], Rh(111) [50], Ni(111) [73–75], Co(0001) [76], Cu(111) [77–83], Au(111) [84, 85]. The strength in the interaction between graphene and the metallic substrate not only affects the electronic properties of the graphene overlayer but also its geometric characteristic. As a general rule for weak graphene-metal interaction, the distance measured between graphene and the metal is large (about 3.3 Å) i.e. similar to the inter-planar distance in HOPG (3.35 Å). On the contrary, with strong graphene-metal interactions, a smaller distance is found to be about 2.2 Å. Simultaneously, for the weak interaction systems, the moiré corrugation is smaller than for strong interaction systems. An exception is found for the graphene on Ni(111) where a strong interaction takes place but no moiré pattern is present due to the almost perfect match between the lattice parameters of graphene and Ni(111). The existence of the moiré pattern in graphene induces a corrugation that can be used to induce an ordered structure of materials deposited on top [51]. If the moiré pattern presents also a spatial modulation in the interaction of the graphene layer and the metallic substrate the electronic properieties of the graphene layer are nanostructured and can be used to tailor, for example, the self-organisation of the electronic acceptor molecules on the surface [86].. 13.
(24) 2. Graphene. 2.3. Growth Methods. In the last decade, a number of different approaches have been developed to grow graphene. These methods include carbon segregation [74, 87, 88], mechanical exfoliation of graphite [29], chemical exfoliation of graphite [89], unzipping of carbon nanotubes [90], epitaxial growth on silicon carbide (SiC) surfaces [91], reduction of graphene oxide [92–96] and even exotic synthesis from food, waste and insects as starting material [97]. For the aim of this thesis, chemical vapor deposition (CVD) in UHV is the most reliable method to produce defect-free graphene both for academic and industrial purposes [98, 99]. Basically, the method involves the use of hydrocarbons, such as ethylene or methane, which are decomposed on a metallic surface with sufficient catalytic activity [77, 100, 101]. Finally, for those metals where the catalytic activity is low, such as gold, the direct decomposition of elemental carbon by means of low energy ethylene ions irradiated on the surcafe of the metal kept at temperature above room temperature [84, 102].. 2.3.1. Graphene/Ru(0001) Surface. Epitaxial graphene, grown on ruthenium, presents a moiré pattern with a periodicity of about 3 nm due to the difference in lattice parameters between Ru(0001) (aRu =2.71 Å) and graphene (aGr =2.46 Å). The system can be described by a superstructure of 11 hexagon by 10 lattice parameters of ruthenium, i.e. C(11 × 11)/Ru(10 × 10) from STM measurements [51]. However, various periodicities for the moiré have been proposed such as C(25 × 25)/Ru(23 × 23) by means of X−ray diffraction[103] or the one proposed by Marchini C(12 × 12)/Ru(11 × 11) from LEED experiments [53]. The different values for the periodicity of the moiré pattern for Gr/Ru(0001) may be explained by the fact that there is a small rotation (0.5◦ ) between the graphene atomic lattice and the ruthenium lattice [59]. In the X−ray diffraction experiments, the periodicity is obtained from the atomic positions of the ruthenium atoms on the last ruthenium layer, while STM only measures the corrugation of the graphene overlayer. As was previously discussed, STM images measured after the graphene formation shows a hexagonal array of the protrusions separated by 3 nm. In Fig 2.7 the unit cell of the Gr/Ru(0001) is shown. Three areas can be distinguished due to the different registry between carbon atoms and the ruthenium atoms. In the registry shown in Fig 2.6-a, all the carbon atoms are place in the threefold hollow sites, while for the registry shown in Fig 2.6-b and Fig 2.6-c one every two carbon atoms are place on top a ruthenium atom while the other carbon atom is place in a face centred cubic (FCC) or hexagonal close packed (HCP) threefold hollow site, respectively. 14.
(25) 2. Graphene. Figure 2.6. Four basic arrangement of graphene on metallic substrate. Here the configuration is known as (a) ATOP, (b) face centered cubic FCC and (c) hexagonal close packed HCP .. The different areas of the moiré pattern are named following the above description. The high area of the protrusion is called atop and all the carbon atoms are in threefold hollow sites. The low area of the moiré is divided into FCC and HCP areas with the registry described in Fig 2.6.. Figure 2.7. STM image 13.8 × 16.5 nm2 of Gr/Ru(0001) at RT. Scanning parameters: VS = -1 V, It = 0.1 nA. The blue rhombus outlines the unit cell where we can distinguish three regions. The blue circle is the atop, while green and yellow triangles are, respectively, the configuration FCC and HCP that first two ruthenium atomic layers.. The stronger interaction between graphene and ruthenium is found at the low areas of the moiré (chemical-like bond), while a weak interaction (van der Waals) is found at the atop area, as described in reference [50]. The variation of the chemical interaction between graphene and ruthenium modifies the surface dipole that in turn produces a difference in the value of the surface potential when moving from the high to the low areas of the moiré of the order of 0.25 eV (Fig 2.8) [104]. On the other hand, the apparent corrugation of the moiré pattern in the STM im15.
(26) 2. Graphene. Figure 2.8. Schematic representation of the graphene ripples on Ru(0001). Different behavior is observed between atop and low area with a difference of local work function of about ∆φ = 25 meV. Apparent corrugation is function of bias voltage [60].. ages reflects not only the geometric corrugation, but also the strong influence of the electronic structure. As can be seen in Fig 2.9-c, the apparent corrugation remains almost constant for negative bias voltage with a value of the order of 0.1 nm. When the sign of the bias voltage goes from negative to positive the apparent corrugation decreases and get inverted for bias voltages greater than +2.6V. The origin of this inversion has been traced back to the modification of a Ru surface resonance, by the presence of the graphene overlayer [105].. Figure 2.9. STM images for Gr/Ru(0001) at the bias voltages of -1V (A) and +2.8V (B) where the contrast invention is displayed. We can see from the blue marks that the ripples height for negative bias voltage appears as a hole when the bias is turned to positive. The plot shows the change of apparent ripple hight as function of bias voltage for many different experimental conditions (tip, tunneling current, samples and temperature). The data clearly show a contrast inversion for bias voltages larger than +2.6 eV. Adapted form [60].. 16.
(27) 2. Graphene. 2.3.2. Cu(111) Surface. Copper is a transition metal of the XI group of periodic table with an FCC crystal structure. The FCC (111) surface shows a hexagonal arrangement in the plane and the subsequent planes of atoms are placed in the ABABAB stacking sequence. The lateral distance between Cu atoms is 2.56 Å and the distance between planes is 2.1 Å [106–109]. Figure 2.10 illustrates the dispersion relation E(kk ) of the surface state along with the projected bulk continuum measured along the high symmetry direction in the Brillouin zone Γ M and Γ K [110]. The close-pack Cu(111) surface is less. Figure 2.10. Bands of the copper (111) measured along the high symmetry direction in the Brillouin zone Γ M and Γ K.. reactive then the more open Cu(100) and Cu(110) surfaces, as reported in previous theoretical and experimental studies [107, 111–114]. Nevertheless, molecularsubstrate interactions are stronger on copper than on graphene where only van der Waals forces drives the adsorption process. In addition, the molecules investigated in this thesis contain nitrogen atoms, which have a tendency of forming bands with copper.. 17.
(28) 2. Graphene. 18.
(29) 3 Experimental Setup. 3.1. Ultra High Vacuum System (UHV). The results described in this thesis were measured at the Department of Condensed Matter Physics at Universidad Autonoma de Madrid (UAM). All experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of 5 × 10−11 torr. The chamber is equipped with two type of evaporator, one specifically designed to evaporate molecules and the other to evaporate metals, ion gun to clean tip and samples and leak valves to dose controlled amount of gasses. A variety of surface analytical tools are available such as variable temperature scanning tunneling microscope (VT-STM), low-energy electron diffraction (LEED) and quadrupole mass-spectrometer (QMS). The key instrument for this thesis is the STM which is a powerful tool to investigate the adsorbed molecular layers with atomic resolution. Our system is mounted on a steel frame which is supported by four pneumatic suspension legs to provide an environment with sufficient vibrational insulation. The system is equipped with a fast laid entry that is pumped with a turbomolecular pump and a diafram rough pump. The fast load entry is connected with the ultra high vacuum chamber with a gate valve. Because of the small volume of load-lock chamber, UHV conditions are reached easily and quickly allowing the transfer from air to UHV or viceversa in few hours without breaking the UHV in the main and secondary chamber. The ultra high vacuum chamber consists in two chambers con19.
(30) 3. Experimental Setup nected through a CF40 flange with a a gate valve. The first chamber is devoted to the tip preparation by heating the apex of the tip or evaporating different metals on it. This chamber is pumped low by a turbomolecular pump [115] and rotary pump [116]. This pump system leads to a base pressure of 5 × 10−11 torr and 5 × 10−10 torr, respectively for the load-lock chamber and the main chamber. The pressure is measured using Bayard-Alpert gauges [117]. The main chamber is essentially used for preparing samples and STM measurements. This chamber is physically composed of two interconnected chambers (preparation and measurement). In the preparation chamber, a manipulator is used to transfer a sample between the preparation and measurement chamber and as well as between main and secondary chamber. Evaporators for metals (Fe, Cr, Mn, Pb, Cu) and molecules are attached to preparation chamber. Metal evaporators are home-made while the epitaxial molecule evaporator is commercial [118]. An ion gun [119] is placed to facilitate the sputtering of tips and samples. Four gas lines (O2 , Ar, C2 H4 , CO) are connected to the main chamber with leak valves. Residual gases are checked by means of quadrupole mass spectrometer [120].. 3.2. VT-STM. The measurement chamber contains a commercial OMICRON VT-STM [121]. Samples and tips can be transferred from the STM to the manipulator using a wobblestick. There is a parking that can accommodate up to 12 samples or tips, which is placed between the preparation and measurement chamber in order to be reachable by the manipulator and the wabble-stick. A helium flow cryostat enables cooling down the sample to 90K. When a sample is cooling, liquid He is transferred from a Dewar container to the cryostat through a vacuum-shielded flexible transfer line by the overpressure that builds up in the He container and a rotatory pump that pumps the He gas produced inside the cryostat.. 3.3. Basic Principles on Theory of STM. A scanning tunneling microscope (STM) is an instrument capable of imaging surfaces with atomic resolution [5, 122–125]. Thanks to its ultimate resolution, an enormous diversity of surface phenomena can be investigated in physics, chemistry and biology. STM was invented by Gerard Binnig y Heinrich Rohrer en 1981 and it is based on the concept of quantum mechanical tunnelling. According to classical mechanics, an object that hits a potential barrier higher than the total energy of the object will not pass through it. By contrast, in quantum theory a particle, 20.
(31) 3. Experimental Setup i.e. electron, can go through a energy potential barrier U(z) via the tunnelling phenomenon. When a metal tip is moved very close (0.5 nm) to a conducting or semiconducting sample surface and a potential (between few mVolts and few Volts) is applied between the tip and the surface, electrons can tunnel through the vacuum and a tunneling current will flow. The tunnelling current depends on tip sample distance, voltage, applied between tip and sample, and local density of states (LDOS) of the electrodes in the energy window defined by the applied bias voltage. Surface information can be acquired by monitoring the tunnelling current or the tip’s height as is scanned across the sample surface. Generally, an exact theoretical treatment of the tunnelling process in STM is virtually impossible because it requires a full description of the sample and tip electronic states. For that reason, models and approximations are needed in the theoretical description of the STM process. The simplest apparatus that contains the most important characteristic of the process is describing the problem in 1D. In this case, a particle with a energy E, mass m and wavefunction Ψ (z) in a potential landscape U(z) can be described by the Schrödinger’s equation: ~2 ∂2 Ψn (z) + U (z)Ψn (z) EΨn (z) = 2m ∂z2. (3.1). where ~ is the reduced Planck’s constant, z is the position, and m is the rest±ikz where k = ing √ mass of the electron. A solution of Eq. 3.1 is ψn (z) = ψn (0)e 2m(E−U (z)) . ~. The wave function is real inside the tip or sample when E>U(z). On the other hand, when E<U(z), the wave function decays exponentially inside the potential barrier. With a bias voltage applied between the tip and the sample a net tunnelling current is established and the probability P that an electron at z=0 (left edge of barrier) can be found at z=d (right edge of barrier) is proportional to the squared wave function P ∝ |ψn (0)|2 exp(−2κd). (3.2). If the applied voltage is small compared with the tunnelling barrier height, (U (z)−E) can be approximated to the work function φ (typically values for a metal lie between 4.0 and 5.5 eV). The density of the states in the energy window defined by the applied bias voltage (ETF - ESF = eV) determines the tunneling current between sample and tip (Fig. 3.1). The tunneling current is proportional to the total number of the states available. If the density of states of the tip does not vary significantly within (ETF - ESF ), the result can be expressed in term of the amplitude of the local density of states ρS (LDOS) of the sample to the tip position z = d. Bardeen [126] found a method to circumvent the problems connected with a theoretical description of the 21.
(32) 3. Experimental Setup. φ. Evacuum. S. φ. T. S. EF eV. T. EF. d Sample. Vacuum. Tip. Figure 3.1. Energy diagram for tunneling junction sample-vacuum-tip. EF represents Fermi’s energy, φ is the work function and ρ is the density of states. Subscripts indicated with S (red) and T (blu) for sample and tip, respectively. The applied voltage is the energy window eV between the Fermi levels of sample and tip.. complete tip-sample system. He obtained the electronic wave functions Ψ (T ) and Ψ (S) for the separate subsystems of the tip and the sample by solving the stationary Schrödinger equation and calculating the rate of electron transfer. The probability of electron transfer, the tunneling matrix element M, is determined by the overlap of the surface wave functions of the two subsystems at a separation surface place within the tunneling barrier. In this way, the total tunneling current is evaluated by summing over all states and can be written as follow 4πe I= ~. Z. +∞ −∞. [f (EFT + ) − f (EFS + )]ρs (EFT + )ρT (EFT + )|M|2 d. (3.3). where f () is the Fermi function, ρS and ρT are the sample and tip density of states, respectively. Bardeen’s theory was developed to study planer tunneling junctions. In the STM one of the electrode is at the tip with an unknown shape making the calculation more complex. The difficulty of evaluating the tunneling matrix M in Eq. 3.3 was tackled by Tersoff and Hamann [127]. They reconsidered Bardeen’s formalism adding and approximation that considers the tip with a spherical symmetry with a radius of curvature. In fact they considered that the tip can simply be described by a symmetric s-wave function. An advantage of the Tersoff and Hamann approach is that, assuming an s-wave for the tip, the current can be related to a property of the surface alone and the interpretation of (low voltage) constant-current STM images is straightforward: they reflect the contour of constant LDOS close to the Fermi level. 22.
(33) 3. Experimental Setup. 3.4. Sample and Tip. Single crystals of Cu(111) and Ru(0001) are used to perform the experiments. A heating system for each sample is incorporated into the sample holder. The sample holder for Cu (111) is commercial from Omicron. the heating element is a pyrolytic boron nitride (PBN) plate which heats the crystal. The maximum current that can go through the PBN heater is 3A and the heat up the sample at a maximum temperature of 1000K. The Ru(0001) crystal needs to be heated to a higher temperature for the cleaning procedure and a home-made sample holder is used which is based on the Omicron design. Here, the single crystal is placed directly in contact with two tungsten wires. The home-made system extends the temperature range up to about 1500K using 23A. Sample Cleaning. The cleaning cycles of samples and tips are carried out in the preparation chamber. A sample is placed in the manipulator which has the electrical contacts to provide current to the internal heating system of the samples. The Cu(111) single crystal was cleaned by repeated cycles of Ar+ sputtering followed by annealing to 1000K for 5 minutes. A minimum of 4 cleaning-cycles are used applying a pressure of argon to about 1-2 × 10−6 torr and a current of about 10 µA. A decreasing value of high voltage is applied in each cycle in the range 1.5-1.0 KeV. For Ru(0001) the surface was cleaned by 4-6 cycles of Ar+ sputtering (1.5 keV, 11µA, 1 × 10−6 torr, 10min), O2 adsorption (∼3 × 10−7 torr, 4min) with the sample kept hot (∼ 1000K) and a final flash-annealing up to 1300K (20A, 5min). The cleanliness of the surface was checked by STM, no significant contamination was found on the STM images. Temperature is measured by means of an optical pyrometer during the sample preparation. Tip Preparation. In our case, tip preparation can be divided in two main steps, a first one in air and the second one in UHV. The tip preparation in air consists in a electrochemical etching of a tungsten wire, with a diameter of 0.25 mm, in a bath of potassium hydroxide and a subsequence bath with distilled water. The major disadvantage with etching techniques is that afterwards the tip is covered with a thin layer of oxide. For reproducible results, the oxide layer needs to be subsequently removed in vacuum. In the second step the tips are introduced inUHV via the fast load entry. Once in the UHV chamber the tips are cleaned by frontal Ar+ sputtering (2.5 keV, 12µA, 1 × 10−6 torr, 40min) to remove oxides from the apex and to make it sharper. Depending on the subsequent exposure, the tip can be recrystallised their apex ftp STS experiments by electron bombardment. Evaporation of Molecules. Molecules were placed into a quartz crucible and filled up 1/3 of the total capacity. Typically evaporation temperatures in UHV are 200◦ C 23.
(34) 3. Experimental Setup for 15,15,16,16tetra-cyano-6,13-pentacenequinodimethane (TCPQ), 140◦ C for 13,13,14,14tetracyano -5,12-tetracenequinodimethane (TCTQ), 130◦ C for 11,11,12,12-Tetracyano9,10-anthra-quinodimethane (TCAQ) and 200◦ C for Caffeine. The crucibles are heated to the evaporation temperature with the evaporator shutter close during several minutes to allow the stabilisation of the crucible temperature. Then the sample is placed in front of the evaporator and the shutter is opened. The molecular coverage is controlled by the evaporation. A maximum pressure inside the main chamber was about 7 × 10−10 torr during the evaporation. Graphene Growth . In order to growth a defect free graphene layer on a metallic single crystal, it is necessary to reduce as much as possible the defects on the metallic surface. Defects in the metal surfaces are problematic as they can be nucleation seeds for 3D aggregates and, most importantly, they stop the formation of a homogenous graphene layer. Additionally, defects can further interact with organic molecules which, adsorbed on the surface, can form aggregates in an uncontrolled way. For those reasons an extremely clean supporting surface is needed. Graphene is typically produced on a clean surface of Ru(0001) by CVD. In the preparation chamber, ethylene (C2 H4 ) gas is introduced by a leak valve with a maximum pressure of 5 × 10−8 torr for about 8 minutes with the Ru(0001) sample, at 1000K (18A, 2min). Ethylene thermally decomposes on the surface and the hydrogen is lost. The carbon atoms diffuse along the surface arranging themselves in hexagons. A complete layer of graphene produced in this way is shown in Fig 3.2. If the amount of (C2 H4 ) is too low, a submonolayer of graphene is made and different islands on the surface may be found. A final flash annealing is carried on the so prepared graphene to eliminate any possible physisorbed C2 H4 molecules and to reconstruct any uncompleted patches.. Figure 3.2. STM image of graphene grown on Ru(0001) (A) 52 × 46 nm2 and (B) 183 × 172 nm2 at RT. Scanning parameters: VS = -1 V, It = 0.1 nA. The superstructure of the moiré pattern is shown as bright ripples with a corrugation of 3 nm, due to the mismatching of lattice parameters of graphene and ruthenium.. 24.
(35) 4 TCxQ on Cu(111). In this chapter, we will discuss the properties of electron acceptor molecules derived from the TCNQ. We will study the growth and self-assembly of these molecules deposited on Cu(111).. 4.1. Background. The molecules TCAQ, TCTQ and TCPQ (named from here on as TCxQ) are electron acceptor molecules and they are derived from TCNQ where the π-system is extended by adding benzene rings to the central ring of the TCNQ molecule (see Fig 4.1). TCNQ is considered as the prototypical electron acceptor molecule and is widely used in the formation of charge transfer compounds like TCNQ-TTF [86, 128–131]. TNCQ has been investigated upon adsorption on metallic surface [132– 135] alone or mixed with TTF. When TCNQ alone is adsorbed on Cu(100), the molecule gains more than one electron and the subsequent interaction of the central ring has important consequences in the self-assembly process [26]. When TCNQ is deposited in gr/Ru(0001), the TCNQ molecules gain one electron and became magnetic impurities on a metallic surface [86]. When a monolayer of TCNQ molecules is formed, a spin polarised band is formed and the TCNQ monolayer presents long range magnetic order [86] . Over the past two decades, efforts have been made at synthesis of derived TCNQ products that focused on extending the π-system in or25.
(36) 4. TCxQ on Cu(111) der to modify its electron affinity [136–140]. It was expected that increasing the number of aromatic rings would lead to an enhancement in the electron affinity and a decrease in the intermolecular coulomb repulsion in the charged species due to the delocalization of the charge in the extended π-system. However, Martin et al. [138] concluded from their voltammetric results that the redox potential became more negative when the number of aromatic rings increases, showing a trend opposite from what was expected. The conclusion was that the presence of a larger π-system is not necessarily correlated with an enhancement of acceptor properties, and in fact, the electron affinity slightly decreases in the modified TCNQ. Despite this fact, the TCxQ molecules still function as good electron acceptors and, in addition, the extended π-system in the molecules may modify the self-assembly process.. 4.2. Description of TCxQ Molecule. The first approach for extending the π-system was the synthesis of TCAQ, which has an additional aromatic ring on each side of TCNQ (see Fig 4.1) and resulted in high electrical resistivity for the bulk compound ascribed to the nonplanarity of the molecular structure and confirmed by spectroscopic measurements [138]. Orti et al. [140] reported a theoretical study of TCAQ where the authors described that increasing in the number of rings over TCNQ results in a loss of planarity. In fact, the calculated energy for various optimized structures reveal that the butterfly conformation is the most stable followed by armchair, and finally X conformation, which is planar and less stable in the gas phase (Fig 4.1). Those calculations were experimentally confirmed by Schubert et al. [141] using X-ray analysis. The same strategy of the synthesis has resulted in an asymmetric derivative containing four rings, i.e. TCTQ, and a symmetric analogue with five rings, i.e. TCPQ (see Fig 4.1). Viruela et al. [142] theoretically investigated the structure and the HOMO-LUMO orbitals for TCPQ. Like in the case of TCAQ, they found that TCPQ has a non-planar conformation with the butterfly-type structure being the most stable. Bader et al. [139] experimentally investigated the structure of TCTQ and TCPQ, and they found that, those molecules are non-planar and adopt the butterfly conformation, as predicted by theory (see Fig 4.1). From the macroscopic point of view, TCAQ, TCTQ and TCPQ are yellow powders with an evaporation temperature in UHV conditions of 130◦ C, 140◦ C and 200◦ C, respectively. As discussed earlier, the aromatic backbone in these molecules is composed of 3 rings for TCAQ (symmetric), 4 rings for TCTQ (asymmetric) and 5 rings for TCPQ (symmetric). Figure 4.1 shows the structure and lateral view for the planar, butterfly and X conformations for the three molecules. The size of the molecules are determined from the geometrical optimization for a 26.
(37) 4. TCxQ on Cu(111) single molecule in the gas phase. The calculated width is 8.1 Å for all conformations and the lengths are 9.2 Å, 11.6 Å and 14 Å for TCAQ, TCTQ and TCPQ, respectively, when a planar conformation is taken into account. When the butterfly conformation is considered, the length of the molecule is reduced to 7.8 Å (TCAQ), 10.9 Å (TCTQ) and 13.2 Å (TCPQ) due to the deformation of the backbone plane and the molecular height is now 2.5 Å (TCAQ), 2.7 Å (TCTQ) and 3.2 Å (TCPQ), calculated by measuring the distance from the plane of the cyano group to the plane of the extreme of the backbone.. Figure 4.1. (Left) Ball and stick structures for TCAQ, TCTQ and TCPQ. (Right) lateral view of planar, X and butterfly conformations and the calculated sizes. Hyperchem software with PM3 semiempirical method is used to obtain a geometrical optimisation structures and size values.. 27.
(38) 4. TCxQ on Cu(111) Figure 4.2 shows the electrostatic potential map for neutral TCxQ having a planar, X and butterfly conformation. The higher positive potential (blue) appears in the region around the hydrogen tips in the backbone, while the most negative (red) is localised on the nitrogen of the four cyano groups. An exception is the butterfly conformation for TCTQ and TCPQ, which shows a more positive potential character for the entire molecule comparing with the neutral one with planar and X conformations. Figure 4.3 shows the calculated HOMO and LUMO orbitals for TCAQ,. Figure 4.2. Potential maps for TCAQ, TCTQ and TCPQ molecules. Planar, X and butterfly conformations are drawn using a colour scale that identifies positive, neutral and negative potential, respectively with blue (+0.4 e/a0 ), green (0 e/a0 ) and red (-0.15 e/a0 ).. TCTQ and TCPQ. The HOMO and LUMO are localised on the central ring and on the dicyanomethyl groups of the TCNQ [86]. An exception occurs when HOMO1 is calculated for the planar and X conformations of the TCPQ molecule, where the orbitals are predominately concentrated on the backbone without involving the cyano groups. Orti et al. [140] and Viruela et al. [142] have calculated the atomic orbitals and energy levels for the planar conformation of TCNQ, TCAQ and TCPQ using ab-initio 6-31G∗ and PM3 algorithms. They found that adding rings to the TCNQ, i.e. extending the π-system, produces a destabilisation of the LUMO level that moves up in energy. This explains the increase in the redox potential (Ered) values measured for TCNQ (0.08 V) < TCAQ (-0.285 V) < TCTQ (-0.44 V) < TCPQ (-0.57 V) [143]. In a similar way the calculated electron affinity gets smaller as the number of rings increases. By using the linear relationship Ered = Ea+const reported by Lobach [144], Ea is calculated for TCNQ (2.8 eV) > TCAQ (2.4 eV) > TCTQ (2.3 28.
(39) 4. TCxQ on Cu(111) eV) > TCPQ (2.1 eV). This tendency predicts a decrease in the ability of TCxQ to accept electrons relative to TCNQ. Despite this reduction, TCxQ molecules are still good electron acceptors with the extra charge distributed among the four peripheral cyano’s groups and the central 6 membered ring, as reported in [145]. It has. Figure 4.3. HOMO and LUMO orbitals calculated for TCAQ, TCTQ and TCPQ molecules using planar, X and butterfly conformations by means of Hyperchem software and PM3 method.. 29.
(40) 4. TCxQ on Cu(111) been discussed for the neutral TCNQ that the central ring is not aromatic and the structure appears rigid due to the double bonds that connect the central ring to the dicyanomethylene groups in the 9,10 positions, as we can see in the scheme in Fig 4.4. The same configuration is true of all TCxQ molecules. When extra negative charge is placed on the molecules, the central ring became aromatic and the double bond acquires the character of a single bond. Simultaneously, one of the peripheral dicyanomethylene group turns into a radical while the second one becomes negatively charged (Fig 4.4-B). Importantly, the changing from the C-C double to a single bond allows either rotation or the ability to further bond, yielding in a change in molecular structure e.g. X and butterfly conformations [26].. Figure 4.4. (A) Chemical structure for TCAQ and its relative radical and dianion. In the neutral form TCAQ (or TCxQ) is not aromatic and the 6 membered ring is bonded with dicyanomethylene groups through a double bond. (B) If molecule is charged with 1e the system became aromatic and the double bond between the central ring and the cyano groups, acquired becomes a single bond with flexibility to rotate or to permit further bonding. (C) Dianion counterpart of TCAQ.. 30.
(41) 4. TCxQ on Cu(111). 4.3 4.3.1. Results and Discussion TCNQ on Cu(111). The growth and self-assembly of TCNQ on Cu(111) have been investigated by DFT calculations and STM experiments by Stradi et al. [146]. The authors have demonstrated that different supramolecular structures can be obtained by tuning the temperature of the substrate during the deposition of the TCNQ molecules. When the substrate is held at room temperature during the molecular deposition and annealing process, a well ordered structure (called complex structure) is found. Fig 4.5-A shows a large STM image of the complex structure. Fig 4.5-B panel shows a high resolution image with in combination with a model showing the molecular arrangement of the rhombus-shape unit cell containing five molecules is highlighted. On the other hand, if the temperature of the Cu(111) surface is held at 350K during the molecular deposition, a new structure (called orthogonal structure) is formed in which two neighbour molecules are rotated 90◦ with respect to each other (Fig 4.5C,D). In both cases no isolated TCNQ molecules are present on the surface, a clear indication of the high mobility of the molecules on this surface. Theoretical DFT calculations reproduce the experimental STM data and conclude that the stabilisation of the structure is not only due to the lateral interaction between molecules but also due to the molecule/substrate interaction as in the case of TCNQ on Cu(100) [26]. In the following we will investigate the influence of the increasing number of aromatic rings when TCxQ molecules are deposited on Cu(111) in comparison with the TCNQ.. Figure 4.5. (A) STM image(500Å × 500Å)acquired with Vs =+1.4V displaying highly ordered structure of TCNQ/Cu(111). (B) High resolution STM image of the complex structure (Vs =+1 V). (C) STM overview (400Å× 400Å) acquired withVs =-1 V displaying ordered compact structure of TCNQ/Cu(111), holding the substrate at 350 K. (D) High resolution STM image (Vs =+1 V) of the orthogonal compact structure. Adapted from Stradi et al. [146].. 31.
(42) 4. TCxQ on Cu(111). 4.3.2. TCAQ on Cu(111). At low coverage (0.2 ML), TCAQ is adsorbed purely along the step edges on Cu(111) as shown in Fig 4.6-A. When the coverage is increased to 0.8 ML, the step edges are completely covered by molecules and molecular island are formed in the middle of the terraces. a close inspection of the structure of the islands show no preferential shape or size. Furthermore, there are no indications of a well adored structure inside the islands (Fig 4.6-B). However, we can distinguish two different shapes, i.e. irregular island and molecular rows as marked by the arrows in Fig 4.6-B. The molecular rows cross the terraces without following any high symmetry directions of the Cu(111) substrate.. Figure 4.6. STM topographic images of TCAQ adsorbed on Cu(111) and recorded at 100K. Scanning parameters: VS =-1V, It = 0.1 nA. (A)125 × 125 nm2 low coverage (0.2 ML), TCAQ molecules are adsorbed along the step edges only. (B) 183 × 172 nm2 high coverage (0.8 ML), TCAQ forms inhomogeneous clusters randomly distributed on the surface. Arrows mark circular and row clusters that cross a terrace.. Figure 4.7 shows a close-up STM image measured at a bias voltage of -3V. The lack of long range order in the structures is clear in these images and can be attributed to an anisotropy in the interactions between the molecules deposited on the surface. Even if TCAQ does not aggregate into an ordered structure, clusters are formed indicating that a weak attractive interaction does occur between molecules. From the potential maps (see Fig 4.2), electrostatic interactions, between the positive charge located on the hydrogens of the backbone rings and the negative charge of the nitrogen in the four cyano groups are expected. Indeed, the formation of hydrogen bonds between molecules can partially justify the formation of clusters. 32.
(43) 4. TCxQ on Cu(111) When TNCQ is deposited on Cu(100) [26] and Cu(111) [146, 147], it forms islands with long range order on both surfaces. The formation of islands presenting long range order has been ascribed to the formation of hydrogen bonds between the hydrogens of the central ring of one molecule with the cyano groups of another molecule. By means of theoretical calculations it has been shown that the precise arrangement of the molecules is also mediated by the distortion introduced in the substrate. The interaction of nitrogen atoms with surface copper atoms distorts the neighbouring copper atoms facilitating the binding of an additional molecule. The presence of extra rings in the backbone of the TCAQ molecule does not allow the molecules to get close enough to take advantage of such a surface deformation that then over compensates the electrostatic repulsion between the cyano groups of both molecules and reducing in this way the possibility of self-assembly. From the image show in Fig 4.6-B and Fig 4.7 is also clear that the mobility of the TCAQ molecule is smaller than that of TCNQ where no individual molecules were observed after molecular deposition at room temperature on Cu(100) [26] and Cu(111) [146]. Due to the hybridisation with the electrons from the metal substrate,. Figure 4.7. STM topographic image of TCAQ adsorbed on Cu(111) and recored at 100K. Scanning parameters: VS = -3 V, It = 0.1 nA, (A) 91 × 79 nm2 , (B) 32 × 30 nm2 . TCAQ molecules are distributed randomly and form clusters growing from step edges.. the TCAQ molecules appeared as bright ellipsoid protrusions without any detail observed inside the molecule. The measured size of the protrusions in the STM images is 1.2 nm for the long axis and 0.8 nm for the short axis. These values agree well with the expected values obtained in the calculations for the single molecules in gas phase. The measured apparent height from the STM images is 0.12 nm. From these data is difficult to conclude which conformation is adopted by the adsorbed 33.
(44) 4. TCxQ on Cu(111) molecule. To get some clues about which is the most probable conformation of the TCAQ on Cu(111), geometry optimisation and frequency calculations by means of DFT were performed on an isolated TCAQ molecule and its anions [148]. Figure 4.8 shows the variation of the torsion angle with the charge added to the TCAQ molecule. The torsion angle τ is the angle between the methylcyano groups and the plane of the central ring. The plot shows the molecular geometry that gives the minimum energy. The butterfly configuration is the most stable conformation for either the neutral molecule or a molecule charged with one electron . The plot also show a decrease in the value of the torsion angle when one electron is transferred to the molecule.. Figure 4.8. (A) Plot of the torsion angle τ versus the negative charge. Geometrical optimisation structures illustrate butterfly and X conformations using DFT calculations [148]. (B) Front and lateral view that describe the torsion angle which represent the angle measured between the plane (green) through the cyano groups and the plane (red) through the central ring.. 34.
(45) 4. TCxQ on Cu(111). 4.3.3. TCTQ on Cu(111). Fig 4.9-A shows the deposition of TCTQ on Cu(111) recorded at 100K with a molecular coverage of 0.2 ML. At this coverage the terraces are free of molecules and the TCTQ molecules decorate the step edges. When the coverage is increased to 0.4. Figure 4.9. STM topographic image of TCTQ adsorbed on Cu(111) and recored at 100K. Scanning parameters: VS =-3V, It = 0.1 nA. (A) 92 × 86 nm2 , at low coverage (0.2 ML) TCTQ are adsorbed on the step edges only. (B) 64.0 × 60.0 nm2 , molecules are randomly distributed on the surface, forming small clusters such as dimers, esamers and chains.. ML, some TCTQ molecules are observed on the terraces, as can be seen in Fig 4.9-B. Molecules are randomly distributed and small clusters not aligned along the high symmetry directions of the Cu(111) are observed. Compared with the same coverage of TCAQ (Fig 4.6), there are two clear differences. Firstly, in this case, we have individual molecules in the middle of the terraces, contrary to what happen with TCAQ. The individual molecules have ellipsoidal shape secondly the clusters have a more organised internal structure, despite the absence of long range order, as described below and seen in Fig 4.10. The presence of isolated molecules may be due to either a reduced mobility of TCTQ or a very weak attractive interaction between the molecules. Fig 4.10-A shows a high resolution STM image where the typical cluster structures of TCTQ on Cu(111) are shown. Isolated molecules and ¯ dimers appear to be aligned along the [101] direction. A possible arrangement of a dimer is shown in Fig 4.10-B where one of the molecules is flipped with respect to the other. This local arrangement can be understood considering the potential maps for the TCTQ molecules (see Fig 4.2) allows the formation of hydrogen bonds between the nitrogen atoms from cyano’s groups in one molecule with the hydro35.
Documento similar
The spontaneous self-assembly of molecules leading to well-ordered nano- or micro-structures is a hot topic in supramolecular chemistry with interesting applications in
The UV-Visible absorption spectra at different temperatures of metalloporphyrin Zn-( R,R )-2 that contain the chiral amide groups in the 5, 15 meso positions of
1 Departamento de Fı´sica Teo´rica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Auto´noma de Madrid, E-28049, Spain; 2 NANOTEC, Istituto
a) Study and exploit the possibilities of non-covalent interactions for the construction of supramolecular architectures based on SubPcs. b) Explore the host-guest
2. The Interim Agreement shall specify, among other things, the structure of the Council, the number of its members, and the transfer of powers and responsibilities from the
classified by a color code.. 172 The resolution in Figure 3 23 is so good that we have tried to determine the exact atomic position of all the atoms in the graphene substrate
While in the kinetic regime each phonon contributes independently to the heat flux, in the collective regime the momentum is conserved and shared among the
We have also shown that the stability of the di fferent moiré patterns is the result of a subtle energy balance between the energy required to corrugate the gra- phene, that