Orientationally ordered C60 on p-sexiphenyl nanostripes on Ag111

Explore the Structural and Electronic Properties at the Organic/Organic Interfaces of Thiophene-Based Supramolecular Architectures

The structural and electronic properties at organic/organic interfaces determine the functionality of organic electronics. Here, we investigated the structural and electronic properties at interfaces between methyl-substituted dicyanovinyl-quinquethiophenes (DCV5T-Me2) and other electron acceptor molecules, namely fullerene (C60) and tetracyanoquinodimethane (TCNQ), by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). Upon adsorption on Au(111), DCV5T-Me2 molecules self-assemble into compact islands at sub-monolayer coverage through hydrogen bonding and electrostatic interactions. A similar bonding configuration dominates in the second layer of a bilayer film, where DCV5T-Me2 possesses higher-lying LUMO (lowest unoccupied molecular orbital) and LUMO+1 in energy due to a decoupling effect. The co-deposition of DCV5T-Me2 and C60 does not result in ordered hybrid assemblies at the sub-monolayer coverage on Au(111). On the other hand, C60 molecules can self-assemble into ordered islands on top of the DCV5T-Me2 monolayer. The dI/dV spectra reveal that the LUMO of decoupled C60 is 400 mV lower in energy than the LUMO of decoupled DCV5T-Me2. This energy difference facilitates electron transfer from DCV5T-Me2 to C60. The co-deposition of DCV5T-Me2 and TCNQ leads to the formation of hybrid nanostructures. A tip-induced electric field can manipulate the charging and discharging of TCNQ by surrounding DCV5T-Me2, manifested as sharp peaks and dips in dI/dV spectra recorded over TCNQ.

Evaporation-induced self-assembly of C60 on SrTiO3(110) reconstructed surfaces

SrTiO₃(110) polar surface was treated with repeated cycles of argon ion sputtering and annealing. Three reconstructions, namely (4 × 1), (2 × 8), and (6 × 8), were identified with subsequent scanning tunneling microscopy measurements. Using the evaporation-induced self-assembly method, C₆₀ molecules deposited onto these reconstruction surfaces demonstrated a quasi-close packing growth mode with substantial differences. Influence factors are revealed from the investigation of these differences, such as the substrate structure and topography as well as the intermolecular and molecular-substrate interactions. Our study emphasizes the feasibility of controllable molecular self-assembly through choosing surface reconstructions.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Controlled Manipulation of Magic Number Gold-Fullerene Clusters Using Scanning Tunneling Microscopy

We report controlled manipulation of magic number gold-fullerene clusters, (C₆₀) _m-(Au) _n, on a Au(111) substrate at 110 K using scanning tunneling microscopy (STM). Each cluster consists of a two-dimensional gold island of nAu atoms confined by a frame of mC₆₀ molecules. Using STM, C₆₀ molecules are extracted from the molecular frame one at a time. The extraction is conducted by driving the STM tip into the cluster, leading to one of the molecules being squeezed out of the frame. Unlike at room temperature, the extracted molecules do not move away from the cluster because of the lack of thermal energy at 110 K; they are found to be attached to the outside of the frame. Reversible manipulation is also possible by pushing an extracted molecule back into the frame. This reversible manipulation is possible only for molecules from the edge of the cluster.

Rotational superstructure in van der Waals heterostructure of self-assembled C60 monolayer on the WSe2 surface

Hybrid van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials and self-assembled organic molecules are promising systems for electronic and optoelectronic applications with enhanced properties and performance. Control of molecular assembly is therefore paramount to fundamentally understand the nucleation, ordering, alignment, and electronic interaction of organic molecules with 2D materials. Here, we report the formation and detailed study of highly ordered, crystalline monolayers of C₆₀ molecules self-assembled on the surface of WSe₂ in well-ordered arrays with large grain sizes (∼5 μm). Using high-resolution scanning tunneling microscopy (STM), we observe a periodic 2 × 2 superstructure in the C₆₀ monolayer and identify four distinct molecular appearances. Using vdW-corrected ab initio density functional theory (DFT) simulations, we determine that the interplay between vdW and Coulomb interactions as well as adsorbate-adsorbate and adsorbate-substrate interactions results in specific rotational arrangements of the molecules forming the superstructure. The orbital ordering through the relative positions of bonds in adjacent molecules creates a charge redistribution that links the molecule units in a long-range network. This rotational superstructure extends throughout the self-assembled monolayer and opens a pathway towards engineering aligned hybrid organic/inorganic vdW heterostructures with 2D layered materials in a precise and controlled way.

Supramolecular heterostructures formed by sequential epitaxial deposition of two-dimensional hydrogen-bonded arrays

Two-dimensional (2D) supramolecular arrays provide a route to the spatial control of the chemical functionality of a surface, but their deposition is in almost all cases limited to a monolayer termination. Here we investigated the sequential deposition of one 2D array on another to form a supramolecular heterostructure and realize the growth-normal to the underlying substrate-of distinct ordered layers, each of which is stabilized by in-plane hydrogen bonding. For heterostructures formed by depositing terephthalic acid or trimesic acid on cyanuric acid/melamine, we have determined, using atomic force microscopy under ambient conditions, a clear epitaxial arrangement despite the intrinsically distinct symmetries and/or lattice constants of each layer. Structures calculated using classical molecular dynamics are in excellent agreement with the orientation, registry and dimensions of the epitaxial layers. Calculations confirm that van der Waals interactions provide the dominant contribution to the adsorption energy and registry of the layers.

Growth and annealing kinetics of α-sexithiophene and fullerene C60mixed films

Thin films of α-sexithiophene (6T) and C60mixtures deposited on nSiO substrates at 303 and 373 K were investigated in real time andin situduring the film growth using X-ray diffraction. The mixtures are observed to contain the well known 6T low-temperature crystal phase and the β phase, which usually coexist in pure 6T films. The addition of C60modifies the structure to almost purely β-phase-dominated films if the substrate is at 303 K. In contrast, at 373 K the low-temperature crystal phase of 6T dominates the film growth of the mixtures. Post-growth annealing experiments up to 373 K on equimolar mixtures and pure 6T films were also performed and followed in real time with X-ray diffraction. Annealing of pure 6T films results in a strong increase of film ordering, whereas annealing of equimolar 6T:C60mixed films does not induce any significant changes in the film structure. These results lend further support to theories about the important influence of C60on the growth behaviour and structure formation process of 6T in mixtures of the two materials.

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.

Positioning and switching phthalocyanine molecules on a Cu(100) surface at room temperature

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual phthalocyanine (H2Pc) molecules adsorbed on a Cu(100) surface at room temperature. Scanning tunneling microscopy (STM) imaging directly demonstrates that H2Pc molecules can be controlled to move along the [011] or [011̅] surface direction of the Cu(100) surface, and the orientation of H2Pc molecules can also be switched between two angles of ±28° with respect to the [011] surface direction by a lateral manipulation. Owing to the highly efficient control over the adsorption site and orientation of H2Pc adsorbed on the Cu(100) surface by lateral manipulation, a pyramidal array formed by 10 H2Pc molecules has been constructed on the Cu surface as a prototype of binary memory, and every molecule within such a molecular array can be individually and reversibly controlled by a STM tip.

High-fidelity self-assembly of crystalline and parallel-oriented organic thin films by π-π stacking from a metal surface

Organic semiconductor applications will significantly benefit from atomically precise, cofacial stacking of extended π-conjugated molecular systems for efficient charge transport. Surface-assisted self-assembly of poly(hetero)cyclic molecules via donor-acceptor type π-π stacking is a promising strategy to organize functional, many-layered architectures. We have employed tris(N-phenyltriazole) as a model system to achieve molecular-level structural ordering through more than 20 molecular layers from its own metal-templated monolayer. Effective charge transport through such layers enabled molecular-resolution imaging by scanning tunneling microscopy. The structure and chemical composition of the films, grown on Ag(111) or Au(100), were further analyzed by noncontact atomic force microscopy and X-ray photoelectron spectroscopy, revealing a cofacial stacking geometry of the molecular layers. Scanning tunneling spectroscopy measurements show a decrease of the band gap with increasing film thickness, consistent with π-π stacking and electron delocalization. The present study provides new strategies for the fabrication of normally inaccessible structural motifs, atomic precision in organic films, and the effective conduction of electrons through multiple organic molecular stacks.

Switching molecular orientation of individual fullerene at room temperature

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual fullerene molecules adsorbed on Si (111) surface at room temperature. Scanning tunneling microscopy imaging directly demonstrates that the orientation of individual fullerene with an adsorption geometry of 5-6 bond is rotated by integral times as 30 degree after a pulse bias is applied between the STM tip and the molecule. Dependences of the molecular rotation probability on the voltage and the process of applied bias reveal that the rotation of a fullerene molecule takes place in two successive steps: the bonding between the fullerene and the Si surface is firstly weakened via electronic excitation and then low energy electron bombardment causes the molecule to rotate by certain degree.

The role of gap states in the energy level alignment at the organic-organic heterojunction interfaces

The interface properties of organic-organic heterojunctions (OOHs), such as interface energy level alignment (ELA), interfacial charge transfer, interface nanostructuring, molecular orientation and so on, play an essential role in determining the device performance for some technologically important organic electronic devices, encompassing organic solar cells, bipolar organic field-effect-transistors, and organic light-emitting-diodes. The aim of this article is to provide a balanced assessment on the understanding of the ELA at the small-molecule based OOH interfaces with well-defined molecular orientation, with particular emphasis on the role of gap states in organic thin films. A generalized picture of gap states determined ELA at the OOH interfaces is provided and their implications in relevant organic electronic devices have been discussed.

Formation of rectangular packing and one-dimensional lines of C60 on 11-phenoxyundecanethiol self-assembled monolayers on Au(111)

The behavior of C(60) molecules deposited onto 11-phenoxyundecanethiol (phenoxy) self-assembled monolayers (SAMs) is studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and spectroscopy. We observe that after thermally annealing between 350 and 400 K in vacuum a combination of hexagonally close-packed islands, rectangularly packed islands, and isolated single lines of C(60) is observed when the C(60) is initially deposited on an unannealed phenoxy SAM. However, only rectangularly packed islands are found when they are deposited on a preannealed phenoxy SAM. We determine the rectangular packing to have a (2√3 × 4) rectangular unit cell with respect to the underlying Au(111) substrate. This type of C(60) structure has not been observed previously for multicomponent self-assemblies on a surface. We discuss the possible causes for the formation of this structure as well as the differences between starting on an unannealed SAM and an annealed one. This study demonstrates the capability of functionalized alkanethiol SAMs to control the growth and structure of C(60) islands during annealing depending on the structural changes of the SAM itself; by preannealing the SAM, the motion of the C(60) can be confined and unique structures resulting from interactions between the SAM molecules and C(60) can be produced.

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.

Template-assisted assembly: scanning tunneling microscopy study of solvent-dependent adlattices of alkyl-derivatized tetrathiafulvalene

The self-assembly of an adsorbate as a function of the strength of solvent-substrate adsorption is an important yet relatively unexplored subject. In this study, how the strength of solvent-substrate adsorption and solvent-solvent attraction affects the assembly of tetrakis(octadecylthio)tetrathiafulvalene (1) is scrutinized by scanning tunneling microscopy (STM). For solvents with strong intermolecular interactions and adsorption onto graphite, such as long n-alkanes (C(n)H(2n+2), n ≥ 13), STM reveals that the solvent molecules form lamellae which become a template to direct the assembly of 1 into one-dimensional arrays. The lengths of one of the unit cell vectors for the assemblies are increased and well correlated with the solvent sizes. In situ STM monitoring of 1 introduced onto graphite with preadsorbed n-tetradecane adlattices shows that the developed assemblies of 1 have striped features aligned parallel to the underlying template. In contrast, for solvents with weak adsorption, such as short n-alkanes (C(n)H(2n+2), n ≤ 12), toluene, and 1,2,4-trichlorobenzene, the adlattice structures of 1 are solvent-independent.

Direct patterning of a cyclotriveratrylene derivative for directed self-assembly of C60

A novel apex-modified cyclotriveratrylene (CTV) derivative with an attached thiolane-containing lipoic acid linker was directly patterned onto gold substrates via dip-pen nanolithography (DPN). The addition of a dithiolane-containing linker to the apex of CTV provides a molecule that can adhere to a gold surface with its bowl-shaped cavity directed away from the surface, thereby providing a surface-bound CTV host that can be used for the directed assembly of guest molecules. Subsequent exposure of these CTV microarrays to C60 in toluene resulted in the directed assembly of predesigned, spatially controlled, high-density microarrays of C60. The molecular recognition capabilities of this CTV template toward C60 provides proof-of-concept that supramolecular CTV scaffolds can be directly patterned onto surfaces providing a foundation for the development of organic electronic and optoelectronic materials.

Molecular-scale investigation of C60/p-sexiphenyl organic heterojunction interface

In situ low-temperature scanning tunneling microscopy (LT-STM) and ultraviolet photoelectron spectroscopy (UPS) experiments have been carried out to investigate the interface properties at the C(60)∕p-sexiphenyl (6P) organic-organic heterojunction interface, including the interfacial energy level alignment and the supramolecular packing structures. As revealed by UPS measurements, the vacuum level is almost aligned at the C(60)∕6P interface, suggesting that the interface is dominated by weak intermolecular interactions, such as van der Waals and π-π interactions. In situ LT-STM experiments also indicate the formation of a molecularly sharp C(60)∕6P interface with hexagonally-close-packed C(60) layers nucleated atop 6P layer on graphite.

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.