Nucleation and 3D growth of para-sexiphenyl nanostructures from an oriented 2D liquid layer investigated by photoemission electron microscopy

Aurophilic Molecules on Surfaces. Part II. (NapNC)AuCl on Au(111)

Although aurophilicity is a well-known phenomenon in structural gold chemistry and is found in many crystals of Au(I) complexes, its potential for self-assembly in thin films is not yet explored. This paper is Part II of a study, in which we investigated the ultrathin film formation of chlorido(2-naphthyl isonitrile) gold(I) on gold surfaces. Here, we present the data for the growth of (NapNC)AuCl on isotropic Au(111) surfaces. Already during physical vapor deposition, the condensation of ultrathin films is monitored by photoelectron emission microscopy (PEEM) and incremental and spectrally resolved changes in the optical reflectance (DDRS). Additional structural data obtained by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) reveal that the "crossed swords" packing motif known from the bulk is also present in thin films.

Crystal Orientation Imaging of Organic Monolayer Islands by Polarized Light Microscopy

The initial stage of organic semiconductor film formation greatly affects the properties of films, which are used in organic devices including thin-film transistors and light-emitting diodes. Organic monolayer islands that are formed on a suitable substrate can be observed with a conventional optical microscope. Furthermore, the use of a polarized microscope allows the determination of the refractive index and crystal orientation of islands. Here, we report organic monolayer islands of 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DPh-DNTT) deposited on a Si substrate with thermally grown SiO₂ to investigate the crystal orientation of islands by polarized light microscopy. The observation of DPh-DNTT islands under polarized quasi-monochromatic light reveals that reflection intensity depends on both the crystal orientation and irradiation wavelength. A comparison between experimental and calculated reflection intensities provides an estimate of an anisotropic complex refractive index in the plane. The crossed-polarized microscopy image of a SiO₂/Si substrate with DPh-DNTT islands shows that the contrast between the islands and SiO₂ surface is sensitive to the angle between the polarizer and analyzer and depends on the direction of crystal orientation. The dependence of reflection contrast, which can be explained by the anisotropic extinction coefficient, is used to confirm crystal orientation.

Fabrication of ion bombardment induced rippled TiO2 surfaces to influence subsequent organic thin film growth

Control over organic thin film growth is a central issue in the development of organic electronics. The anisotropy and extended size of the molecular building blocks introduce a high degree of complexity within the formation of thin films. This complexity can be even increased for substrates with induced, sophisticated morphology and anisotropy. Thus, targeted structuring like ion beam mediated modification of substrates in order to create ripples, pyramids, or pit structures provides a further degree of freedom in manipulating the growth morphology of organic thin films. We provide a comprehensive review of recent work on para-hexaphenyl (C₃₆H₂₆, 6P) as a typical representative of the class of small, rod-like conjugated molecules and rutile TiO₂(1 1 0) as an example for a transparent oxide electrode to demonstrate the effect of ion beam induced nanostructuring on organic thin film growth. Starting from molecular growth on smooth, atomically flat TiO₂(1 1 0) (1  ×  1) surfaces, we investigate the influence of the ripple size on the resulting 6P thin films. The achieved 6P morphologies are either crystalline nano-needles composed of flat lying molecules or islands consisting of upright standing 6P, which are elongated in ripple direction. The islands' length-to-width ratio can be controlled by tuning the ripples' shape.

Thin film growth of aromatic rod-like molecules on graphene

Research on graphene (Gr) is a vastly expanding field due to its potential for technological applications. Its close structural and chemical relationship to conjugated organic molecules makes it a superior candidate as a transparent electrode material in organic electronics and optoelectronics. The growth of organic thin films-intensively investigated in the past few decades-has demonstrated the complexity in growth and nucleation processes arising from the anisotropy and spatial extension of the molecular building blocks. Choosing the small, conjugated rod-like molecules para-hexaphenyl and pentacene as model representatives for small organic molecules, we review recent findings in organic thin film growth on a variety of Gr substrates. Special attention is paid to the differences in the resulting growth arising from the various methods of Gr fabrication and support that affect both the Gr-molecule interfacing and the involved molecular diffusion processes.

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.

Organic surface-grown nanowires for functional devices

Discontinuous organic thin film growth on the surface of single crystals results in crystalline nanowires with extraordinary morphological and optoelectronic properties. By way of being generated at the interface of organic and inorganic materials, these nanowires combine the advantages of flexible organic films with the defectless character of inorganic crystalline substrates. The development of destruction-free transfer and direct growth methods allows one to integrate the organic nanowires into semiconductor, metallic electronic or photonic platforms. This article details the mechanisms that lead to the growth of these nanowires and exemplifies some of the linear as well as non-linear photonic properties, such as optical wave guiding, lasing and frequency conversion. The article also highlights future potential by showing that organic nanowires can be integrated into optoelectronic devices or hybrid photonic/plasmonic platforms as passive and active nanoplasmonic elements.

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects - wrinkles - are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.