No abstract

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 (CH, 6P) as a typical representative of the class of small, rod-like conjugated molecules and rutile TiO(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 TiO(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.

Controlling the work function of transition metal oxides is of key importance with regard to future energy production and storage. As the majority of applications involve the use of heterostructures, the most suitable characterization technique is Kelvin probe force microscopy (KPFM), which provides excellent energetic and lateral resolution. In this paper, we demonstrate precise characterization of the work function using the example of artificially formed crystalline titanium monoxide (TiO) nanowires on strontium titanate (SrTiO3) surfaces, providing a sharp atomic interface. The measured value of 3.31(21) eV is the first experimental work function evidence for a cubic TiO phase, where significant variations among the different crystallographic facets were also observed. Despite the remarkable height of the TiO nanowires, KPFM was implemented to achieve a high lateral resolution of 15 nm, which is close to the topographical limit. In this study, we also show the unique possibility of obtaining work function and conductivity maps on the same area by combining noncontact and contact modes of atomic force microscopy (AFM). As most of the real applications require ambient operating conditions, we have additionally checked the impact of air venting on the work function of the TiO/SrTiO3(100) heterostructure, proving that surface reoxidation occurs and results in a work function increase of 0.9 eV and 0.6 eV for SrTiO3 and TiO, respectively. Additionally, the influence of adsorbed surface species was estimated to contribute 0.4 eV and 0.2 eV to the work function of both structures. The presented method employing KPFM and local conductivity AFM for the characterization of the work function of transition metal oxides may help in understanding the impact of reduction and oxidation on electronic properties, which is of high importance in the development of effective sensing and catalytic devices.

Organic electronics require a precise control over properties of a molecule–substrate interface as well as film growth morphology, from both fundamental points of view, when a clean vacuum environment is needed and also under ambient air conditions. In this paper, we present submonolayer molecular films of para-hexaphenyl (6P) formation on the rutile TiO2(110) substrates and ways of affecting the growth and morphology via ion-beam nanopatterning. Ultrahigh vacuum deposition and measurements are followed by the film evolution study upon air exposure. Strongly anisotropic TiO2(110) surfaces, in the form of terraced ripples with a preserved (1 × 1) structure, were controllably fabricated utilizing ion-beam bombardment and characterized by means of high-resolution scanning tunneling microscopy and low-energy electron diffraction. 6P thin films were prepared using organic molecular beam epitaxy and characterized in situ by noncontact atomic force microscopy. Ex situ characterization was performed by tapping-mode atomic force microscopy, scanning electron microscopy, and noncontact atomic force microscopy with molecular resolution. We have demonstrated that by changing the size of locally preserved (1 × 1) surface areas, determined by the ripple parameters, different 6P assemblies can be promoted. With the shrinking size of the uninterrupted (1 × 1) terminated areas, 6P changes its growth morphology from needlelike to islandlike accompanied by a reorientation of the molecules from flat-lying to upright-standing. The resulting morphology depends on the structure of a two-dimensional phase of lying molecules formed at the initial stage of deposition, which can be either a well-ordered wetting layer or a two-dimensional mobile lattice gas. The postgrowth remainders of these two-dimensional phases participate in additional nucleation processes forming small islands or clusters.