/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Functionalized acenes and heteroacenes for organic electronics.

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/SAM_str_grwth_ProgSurfSci00_151.pdf

Structure and growth of self-assembling monolayers

The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.

/pdf/engineering-atomic-and-molecular-nanostructures-at-surfaces-2sqr7sjd15.pdf

Engineering atomic and molecular nanostructures at surfaces

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

The growth of molecular adlayers on solid substrates is reviewed with aspecial emphasis on molecules of relevance for organic electronics. In particular,we will consider planar molecules with extended π-systems, namely acenes (anthracene, tetracene, pentacene), perylene, coronenes, diindenoperylene, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride, poly-phenylenes, oligothiophenes, and phthalocyanines. Special consideration is given to the importance of the formation of ordered molecular overlayers, which are compared with the structure of the corresponding bulk crystals. Whenever possible, aspects relevant for device fabrication (morphology of deposited films, mobilities of charge carriers) will be addressed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400094474

Growth of aromatic molecules on solid substrates for applications in organic electronics

The formation of one-dimensional, unidirectional nanoscopic molecular structures by self-assembly on metal substrates with a long range ordering up to several hundred nanometers has been achieved by a novel mechanism, an oscillatory modulation of the adsorption energy due to charge-density waves related to a surface state. For the case of pentacene $(\mathrm{C}{}_{22}\mathrm{H}{}_{14})$ adsorbed on Cu(110), annealing at 400 K produces a regular pattern of molecular wires one molecule wide $(15.6\AA{})$ and a wire-to-wire distance of $28\ifmmode\pm\else\textpm\fi{}2\AA{}$. Results obtained for similar compounds suggest that the underlying mechanism is rather general and can be applied also for other linear aromatic molecules.

Novel mechanism for molecular self-assembly on metal substrates: unidirectional rows of pentacene on Cu(110) produced by a substrate-mediated repulsion.

The electronic level alignment of various organic molecules on metal surfaces has been determined by a combined experimental and theoretical effort. Using ab initio electronic structure calculations, it is demonstrated that the commonly observed interface dipole is largely due to a quantum-mechanical phenomenon resulting from exchange repulsion. Surprisingly, this physical effect, also referred to as Pauli repulsion dominates even in the case of aromatic molecules on Cu and Au surfaces, i.e., on interfaces that are of key importance in molecular electronics.

Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole

The growth and evolution of pentacene films on gold substrates have been studied. By combining complementary techniques including scanning tunneling microscopy, atomic force microscopy, scanning electron microscopy, near-edge x-ray-absorption fine structure, and x-ray diffraction, the molecular orientation, crystalline structure, and morphology of the organic films were characterized as a function of film thickness and growth parameters (temperature and rate) for different gold substrates ranging from Au(111) single crystals to polycrystalline gold. Moreover, the influence of precoating the various gold substrates with self-assembled monolayers (SAM's) of organothiols with different chemical terminations has been studied. On bare gold the growth of pentacene films is characterized by a pronounced dewetting while the molecular orientation within the resulting crystalline three-dimensional islands depends distinctly on the roughness and cleanliness of the substrate surface. After completion of the first wetting layer where molecules adopt a planar orientation parallel to the surface the molecules continue to grow in a tilted fashion: on Au(111) the long molecular axis is oriented parallel to the surface while on polycrystalline gold it is upstanding oriented and thus parallels the crystalline orientation of pentacene films grown on $\mathrm{Si}{\mathrm{O}}_{2}$. On SAM pretreated gold substrates the formation of a wetting layer is effectively suppressed and pentacene grows in a quasi-layer-by-layer fashion with an upstanding orientation leading to rather smooth films. The latter growth mode is observed independently of the chemical termination of the SAM's and the roughness of the gold substrate. Possible reasons for the different growth mechanism as well as consequences for the assignment of spectroscopic data of thin pentacene film are discussed.

Growth of pentacene on clean and modified gold surfaces

The growth of well-oriented crystalline films of rubrene (C42H28) on SiO2 and Au(111) substrates is achieved by employing “hot wall” deposition whereas organic molecular beam deposition (OMBD) only yields rather amorphous layers or poly-crystalline dendritic networks at elevated temperature. This pronounced difference in film growth is related to the conformational change of rubrene molecules involving a loss of chirality upon crystallization and the enhanced diffusion which becomes possible at high temperature and large vapor pressure. Moreover, it is demonstrated that the crystalline rubrene films reveal an enhanced thermal and chemical stability as compared to the OMBD grown films.

Gregor Witte

Papers

Growth of aromatic molecules on solid substrates for applications in organic electronics

Novel mechanism for molecular self-assembly on metal substrates: unidirectional rows of pentacene on Cu(110) produced by a substrate-mediated repulsion.

Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole

Growth of pentacene on clean and modified gold surfaces

Growth of crystalline rubrene films with enhanced stability