C60 molecular chains on alpha-sexithiophene nanostripes

From high quality packing to disordered nucleation or phase separation in donor/acceptor interfaces: ClAlPc-C60 on Au(111)

The dramatic consequences that the orientation adopted by the molecular dipoles, in diverse arrays of chloroaluminum phthalocyanine (ClAlPc) on Au(111), have on the ulterior adsorption and growth of C60 are explored by means of an all scanning probe microscopy approach. The unidirectional downwards organization of the molecular dipoles at the first layer reduces charge transfer from the metal to C60. Imbalance between attractive and repulsive interactions of the fullerenes are crucial for their ordered supramolecular aggregation. The effect at the basis of such self-assembling seems to be released by the all upwards dipole orientation adopted on the ClAlPc second layer. The low electronic corrugation of the bilayer results in a higher mobility of the fullerenes which for similar coverages diffuse large distances to reach uncovered first layer regions. Density functional theory calculations corroborate the experimental observations indicating the relevance of charge transfer, potential energy surface corrugation, C60 on-surface diffusion barriers and screening. The structure of the co-adsorbed C60 and ClAlPc layers strongly depends on the deposition sequence. Phase-separation, where each molecule adopts the single-component assembly, occurs if C60 is deposited first. The present results contribute to understanding the influence of the dipolar nature of molecular layers on the electronic and structure of donor/acceptor heterojunctions, which is crucial for device design via engineering the energy level alignment at organic-organic and organic-metal interfaces.

Hydrogen bonding vs. molecule-surface interactions in 2D self-assembly of [C60]fullerenecarboxylic acids

The adsorption of C₆₀-malonic derivatives C₆₁(CO₂H)₂ and C₆₆(CO₂H)₁₂ on Au(111) and a pentafluorobenzenethiol-modified Au substrate (PFBT@Au) has been investigated using scanning tunneling microscopy (STM) at a liquid-solid interface. Monofunctionalized C₆₁(CO₂H)₂ forms a hexagonal close-packed overlayer on Au(111) and individual aligned dimers on PFBT@Au(111). The difference is attributed to the nature of the substrateC₆₁(CO₂H)₂ interaction (isotropic π-Au bonding vs. anisotropic PFBTCOOH interactions). Surprisingly, in both cases, the directionality of the COOHCOOH motif is compromised in favor of synergistic van der Waals/H bonding interactions. Such van der Waals contacts are geometrically unfeasible in hexafunctionalized C₆₆(CO₂H)₁₂ and its assembly on Au(111) leads to a 2D molecular network controlled exclusively by H bonding. For both molecules, the "free" CO₂H groups on the monolayer surface can engage in out-of-plane H bonding interaction resulting in the epitaxial growth of subsequent molecular layers.

Ground state magnetic response of two coupled dodecahedra

The antiferromagnetic Heisenberg model on the dodecahedron possesses a number of ground state magnetization discontinuities in a field at the classical and quantum level, even though it lacks magnetic anisotropy. Here the model is considered for two dodecahedra coupled antiferromagnetically along one of their faces, as a first step to determine the magnetic response of collections of fullerene molecules. The magnetic response is determined from the competition among the intra-, interdodecahedral exchange and magnetic field energies. At the classical level the discontinuities of the isolated dodecahedron are renormalized by the interdodecahedral coupling, while new ones show up, with the maximum number of ground state discontinuities being six for a specific range of the coupling. In the full quantum limit where the individual spin magnitude [Formula: see text], there are two ground state discontinuities originating in the single discontinuity of the isolated dodecahedron, and another one due to the intermolecular coupling, generating a total of three discontinuities which come one right after the other. These results show that the magnetic response of more than one dodecahedra interacting together is quite richer than the one of a single dodecahedron.

Layer-Resolved Evolution of Organic Thin Films Monitored by Photoelectron Emission Microscopy and Optical Reflectance Spectroscopy

Photoelectron emission microscopy (PEEM) and differential (optical) reflectance spectroscopy (DRS) have proven independently to be versatile analytical tools for monitoring the evolution of organic thin films during growth. In this paper, we present the first experiment in which both techniques have been applied simultaneously and synchronously. We illustrate how the combined PEEM and DRS results can be correlated to obtain an extended perspective on the electronic and optical properties of a molecular film dependent on the film thickness and morphology. As an example, we studied the deposition of the organic molecule α-sexithiophene on Ag(111) in the thickness range from submonolayers up to several monolayers.

Temperature Evolution of Quasi-one-dimensional C60 Nanostructures on Rippled Graphene

We report the preparation of novel quasi-one-dimensional (quasi-1D) C60 nanostructures on rippled graphene. Through careful control of the subtle balance between the linear periodic potential of rippled graphene and the C60 surface mobility, we demonstrate that C60 molecules can be arranged into a quasi-1D C60 chain structure with widths of two to three molecules. At a higher annealing temperature, the quasi-1D chain structure transitions to a more compact hexagonal close packed quasi-1D stripe structure. This first experimental realization of quasi-1D C60 structures on graphene may pave a way for fabricating new C60/graphene hybrid structures for future applications in electronics, spintronics and quantum information.

The growth of α-sexithiophene films on Ag(111) studied by means of PEEM with linearly polarized light

In this study, we used photo electron emission microscopy (PEEM) to investigate the growth of α-sexithiophene (α-6 T) on Ag(111) surfaces. The experiments were carried out with linearly polarized ultraviolet-light (Hg lamp with hν=4.9 eV) in order to probe the alignment of the molecules on the surface. In particular, we acquired images before, during, and after growth while changing the polarization in a stepwise manner. For the stationary states of the clean and the α-6 T covered surfaces, we monitored the local electron yield and the intensity of the ultraviolet C-light (100-280 nm) reflected from the whole sample using PEEM and a photodiode, respectively. Due to the high ionization potential (IP>5 eV), there is no direct photoelectron emission from the organic crystallites. However, the photoelectron emission of the metal/organic interface is influenced by anisotropic absorption of the incident light beam, since the adsorbed molecules act as dichroic filters with distinct orientations.

Adsorption and self-assembled structures of sexithiophene on the Si(111)-√3×√3-Ag surface

The adsorption and self-assembled structures of α-sexithiophene (α-6T) have been investigated on a Si(111)-Ag surface using scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations. The adsorbed α-6T molecules are arranged into unidirectional molecular rows with a side-by-side orientation. The molecular rows reveal three kinds of appearances in the filled-state STM images, which reflect the distinct adsorption sites. From tunneling spectroscopy, we find that the filled-state STM images of α-6T should be influenced by the surface states of Si(111)-Ag. At one monolayer coverage, sequentially ordering of the triple molecular rows results in the close-packed arrangement of the α-6T overlayer.

Rational design of two-dimensional molecular donor-acceptor nanostructure arrays

The construction of long-range ordered organic donor-acceptor nanostructure arrays over microscopic areas supported on solid substrates is one of the most challenging tasks towards the realization of molecular nanodevices. They can also be used as ideal model systems to understand light induced charge transfer, charge separation and energy conversion processes and mechanisms at the nanometer scale. The aim of this paper is to highlight recent advances and progress in this topic. Special attention is given to two different strategies for the construction of organic donor-acceptor nanostructure arrays, namely (i) molecular self-assembly on artificially patterned or pre-defined molecular surface nanotemplates and (ii) molecular nanostructure formation steered via directional and selective intermolecular interactions. The interfacial charge transfer and the energy level alignment of these donor-acceptor nanostructures are also discussed.

Steric self-assembly of laterally confined organic semiconductor molecule analogues

Self-assembly of planar molecules can be a critical route to control morphology in organic optoelectronic systems. In this study, Monte Carlo simulations were performed with polygonal disc analogues to planar semiconducting molecules under confinement. By examining statistically the molecular density and configurations of such analogues, we have observed that the symmetry of the confining medium can have a greater impact on the final densified particle configurations than the intramolecular interactions. Using the steric frustration imparted by confinement, novel self-assembled (partially) ordered phases are available. Our Monte Carlo simulations suggest new avenues to control ordering and morphology of planar molecules, which are critical for high-performance organic optoelectronic devices.

Chiral "pinwheel" heterojunctions self-assembled from C60 and pentacene

We demonstrate the self-assembly of C60 and pentacene (Pn) molecules into acceptor-donor heterostructures which are well-ordered and--despite the high degree of symmetry of the constituent molecules--chiral. Pn was deposited on Cu(111) to monolayer coverage, producing the random-tiling (R) phase as previously described. Atop R-phase Pn, postdeposited C60 molecules cause rearrangement of the Pn molecules into domains based on chiral supramolecular "pinwheels". These two molecules are the highest-symmetry achiral molecules so far observed to coalesce into chiral heterostructures. Also, the chiral pinwheels (composed of 1 C60 and 6 Pn each) may share Pn molecules in different ways to produce structures with different lattice parameters and degree of chirality. High-resolution scanning tunneling microscopy results and knowledge of adsorption sites allow the determination of these structures to a high degree of confidence. The measurement of chiral angles identical to those predicted is a further demonstration of the accuracy of the models. van der Waals density functional theory calculations reveal that the Pn molecules around each C60 are torsionally flexed around their long molecular axes and that there is charge transfer from C60 to Pn in each pinwheel.

Molecular assembly of fullerene-conjugated phthalocyanine derivative on Au(111) at single molecular level

Molecular adlayers of doubly C(60)-conjugated phthalocyanine derivatives ((C(60))(2)NiPc) were examined on bare and zinc(II) octaethylporphyrin (ZnOEP)- and coronene-modified Au(111) surfaces. Electrochemical scanning tunneling microscopy (EC-STM) has revealed the structure of the (C(60))(2)NiPc adlayer at single molecular level. The (C(60))(2)NiPc adlayer is strongly influenced by the underlying organic layers, i.e., a disordered, a packed structure of (C(60))(2)NiPc was found on a clean Au(111) surface because of the strong interaction between (C(60))(2)NiPc molecule and Au(111) substrate, whereas a single (C(60))(2)NiPc molecule was clearly distinguished both on ZnOEP and coronene adlayers at a low coverage of (C(60))(2)NiPc molecules. The obtained results in the present study suggest that the underlying organic adlayers play an important role in the formation process of the (C(60))(2)NiPc molecule adlayer.

THE ROLE OF SUBSTRATE IN PACKING STRUCTURES OF SEXITHIOPHENES ON AG (111) SURFACE: MOLECULAR DYNAMICS SIMULATIONS AND QUANTUM CHEMICAL CALCULATIONS

Packing structures and orientation of sexithiophene (6T) molecules on Ag (111) surface are investigated by molecular dynamics (MD) simulations and quantum chemical calculations. Both the cluster and the slab models are employed. The density functional theory and molecular mechanism calculations demonstrate a weak physisorption and little site-preference in thiophene/ Ag (111) system. The MD simulations show that in the first layer close to the surface, the nearly coplanar 6T strips lie parallel with long axes deviating from [Formula: see text] direction about 20° – 30° and 75° – 90°. The average adsorption height of the monolayer is about 3.2 Å with most of the sulfur atoms in thienyl rings sitting on the bridge site of Ag (111) surface. The 6T molecules tend to take tilted orientations when they are far away from the surface. The packing structures of 6T layers deposited on the surface resulted from the competition between the molecule–substrate and intermolecular interactions.

Oligothiophene template effects on packings and orientations of C60 molecules on Ag(111) surface

The packing conformations of sexithiophene (6T) and the orientations of the C(60) molecules on top of the preadsorbed 6T monolayer on Ag(111) surface have been investigated by the molecular dynamics simulations (on the basis of molecular mechanics) in conjunction with quantum mechanics calculations of the relative strength of intermolecular and interfacial interactions. It is demonstrated that the flat-lying oligothiophene (nT, n = 4 and 6) monolayer is formed on the Ag(111) surface, and the arrangement of 6T molecules is more ordered than that in 4T film. It is also shown that the underlying 6T stripes make C(60) molecules aggregate in chainlike arrays on the 6T covered Ag(111) surface, showing significant template effects on the directed self-assembly of C(60) cages. For the absorbed C(60) molecule on the 6T prepatterned Ag(111) surface, four typical orientations, hexagon, pentagon, 6:6 bond, and 5:6 bond, are found to appear with populations of 26.3%, 2.7%, 37.5%, and 18.8%, respectively. When the deposition order is changed, the 6T stripes are shown to tilt with corrugation on the underlying C(60) carpet, revealing the important role of the deposition order in modulation of the ordered supramolecular nanostructures.

Template-directed molecular assembly on silicon carbide nanomesh: comparison between CuPc and pentacene

The template-directed assembly of two planar molecules (copper phthalocyanine (CuPc) and pentacene) on SiC nanomesh has been studied by scanning tunneling microscopy and photoelectron spectroscopy, respectively. Both molecules are trapped as single molecules in the cells of SiC nanomesh at low coverage. At high coverage, CuPc forms a highly ordered single-molecular array with identical symmetry and periodicity as the substrate, whereas pentacene forms a quasi-amorphous layer due to the random mixture of three different adsorption configurations. This difference in adsorption behavior is attributed to differences in molecular geometries. The measured changes of work functions reveal weak charge transfer between the molecules and substrate. Both molecules are preferentially adsorbed on the SiC nanomesh rather than on graphene. The CuPc single-molecular array possesses good long-range order, large area coverage, and a molecular density of over 3.0 x 10(13) molecules/cm(2).

Tunable two-dimensional binary molecular networks

A novel approach to constructing tunable and robust 2D binary molecular nanostructures on an inert graphite surface is presented. The guest molecules are embedded into a host molecular matrix and constrained via the formation of multiple intermolecular hydrogen bonds. By varying the binary molecular ratio and the molecular geometry, various molecular arrays with tunable intermolecular distances are fabricated. The results suggest a promising route for the fabrication of ordered and stable molecular nanostructure arrays for molecular sensors, molecular spintronic devices, and molecular p-n nanojunctions.

Entrapment of decanethiol in a hydrogen-bonded bimolecular template

We have used scanning tunneling microscopy to investigate the deposition of 1-decanethiol onto a bimolecular self-assembled network composed of PTCDI (perylene tetracarboxylic diimide) and melamine on a Au(111) surface. A new laterally organized phase in which the pores of a parallelogram bimolecular arrangement trap two decanethiol molecules is identified. Disruption of the hexagonal PTCDI-melamine network arrangement after decanethiol deposition is also observed, providing insights about the interplay between supramolecular and substrate-adsorbate interactions.

Bottom-up growth of epitaxial graphene on 6H-SiC(0001)

We use in situ low temperature scanning tunneling microscopy (STM) to investigate the growth mechanism of epitaxial graphene (EG) thermally grown on Si-terminated 6H-SiC(0001). Our detailed study of the transition from monolayer EG to trilayer EG reveals that EG adopts a bottom-up growth mechanism. The thermal decomposition of one single SiC bilayer underneath the EG layers causes the accumulation of carbon atoms to form a new graphene buffer layer at the EG/SiC interface. Atomically resolved STM images show that the top EG layer is physically continuous across the boundaries between the monolayer and bilayer EG regions and between the bilayer and trilayer EG regions.

Programmable hierarchical three-component 2D assembly at a liquid-solid interface: recognition, selection, and transformation

Recognition and selection are of fundamental importance for the hierarchical assembly of supramolecular systems. Coronene induces the formation of a hydrogen-bonded isophthalic acid supramolecular macrocycle, and this well-defined heterocluster forces, in its turn, DBA1 to form a van der Waals stabilized honeycomb lattice, leading to a three-component 2D crystal containing nine molecules in the unit cell. The recognition and selection events enable efficient error correction and healing in redundant mixtures.

Orientationally ordered C60 on p-sexiphenyl nanostripes on Ag111

Long range orientational ordering within C(60) monolayers is observed on p-sexithiophene (6P) monolayer nanostripes on Ag(111) at 77 K. Low-temperature scanning tunneling microscopy studies reveal that the C(60)-6P intermolecular interaction constrains all C(60) molecules to adsorb on their hexagons atop 6P molecules. The orientation-dependent bond-to-bond Coulomb interaction between charge deficient single bonds and double bonds with excess charge in neighboring C(60) molecules results in the in-plane orientational ordering and contributes to the lowering of the total energy of the orientationally ordered C(60) islands.