Roland Resel

Bottom-up organic integrated circuits

Self-assembly—the autonomous organization of components into patterns and structures—is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels, the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular π–π coupling between the molecules in the self-assembled monolayers. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. Here we demonstrate SAMFETs with long-range intermolecular π–π coupling in the monolayer. We achieve dense packing by using liquid-crystalline molecules consisting of a π-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom-up electronics in a new perspective.

Controlled Deposition of Highly Ordered Soluble Acene Thin Films: Effect of Morphology and Crystal Orientation on Transistor Performance

Controlling the morphology of soluble small molecule organic semiconductors is crucial for the application of such materials in electronic devices. Using a simple dip-coating process we systematically vary the film drying speed to produce a range of morphologies, including oriented needle-like crystals. Structural characterization as well as electrical transistor measurements show that intermediate drying velocities produce the most uniformly aligned films.

Infrared Emitting and Photoconducting Colloidal Silver Chalcogenide Nanocrystal Quantum Dots from a Silylamide-Promoted Synthesis

Here, we present a hot injection synthesis of colloidal Ag chalcogenide nanocrystals (Ag(2)Se, Ag(2)Te, and Ag(2)S) that resulted in exceptionally small nanocrystal sizes in the range between 2 and 4 nm. Ag chalcogenide nanocrystals exhibit band gap energies within the near-infrared spectral region, making these materials promising as environmentally benign alternatives to established infrared active nanocrystals containing toxic metals such as Hg, Cd, and Pb. We present Ag(2)Se nanocrystals in detail, giving size-tunable luminescence with quantum yields above 1.7%. The luminescence, with a decay time on the order of 130 ns, was shown to improve due to the growth of a monolayer thick ZnSe shell. Photoconductivity with a quantum efficiency of 27% was achieved by blending the Ag(2)Se nanocrystals with a soluble fullerene derivative. The co-injection of lithium silylamide was found to be crucial to the synthesis of Ag chalcogenide nanocrystals, which drastically increased their nucleation rate even at relatively low growth temperatures. Because the same observation was made for the nucleation of Cd chalcogenide nanocrystals, we conclude that the addition of lithium silylamide might generally promote wet-chemical synthesis of metal chalcogenide nanocrystals, including in as-yet unexplored materials.

Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors

The mobility of self-assembled monolayer field-effect transistors (SAMFETs) traditionally decreases dramatically with increasing channel length. Recently, however, SAMFETs using liquid-crystalline molecules have been shown to have bulk-like mobilities that are virtually independent of channel length. Here, we reconcile these scaling relations by showing that the mobility in liquid crystalline SAMFETs depends exponentially on the channel length only when the monolayer is incomplete. We explain this dependence both numerically and analytically, and show that charge transport is not affected by carrier injection, grain boundaries or conducting island size. At partial coverage, that is when the monolayer is incomplete, liquid-crystalline SAMFETs thus form a unique model system to study size-dependent conductance originating from charge percolation in two dimensions.

Crystallographic studies on hexaphenyl thin films — a review

Abstract Thin films of hexaphenyl show interesting crystallographic features, which are of great importance for technological applications. The textures and the sizes of the crystallites have large influence on the electronic and optical properties of the thin films. Hexaphenyl thin films are prepared by physical vapour deposition on various types of substrates: isotropic and mechanical pre-treated substrates or single crystalline surfaces. Polymorphism, several types of pronounced textures with characteristic sizes of the crystallites but also epitaxial growth of hexaphenyl thin films are found. Recent results of different experimental methods-mainly X-ray diffraction pole figure technique, X-ray line profile analysis and transmission electron diffraction are summarised. The growth of the hexaphenyl thin films is analysed using the current knowledge of thin film formation of crystalline conjugated molecules. The observations on hexaphenyl thin films are compared with the thin film properties of other aromatic and heterocyclic molecules like oligoacene, PTCDA, sexithiophene and metal-phthalocyanines.

Growth and preferred crystallographic orientation of hexaphenyl thin films

Abstract Vacuum-evaporated hexaphenyl grows in a crystalline state showing strong fibre texture. The fibre axis of this texture is perpendicular to the surface of the substrate. Dependent on the sample preparation conditions, these types of texture (or preferred growth) could be detected: (i) at high substrate temperatures and low deposition rates (DR), the (001) plane of the crystallites develop perpendicular to the fibre axis; (ii) at low substrate temperatures and high DR, the (11-2) and/or (20-3) planes are aligned perpendicular to the fibre axis. Small influences on the preferred growth are observed by different substrate materials like glass, amorphous Indium Tin Oxide (ITO)-coated glass and epiready GaAs. The first hexaphenyl layers on the surface of the substrates are formed by dendritic growth of islands. Molecules perpendicular to the surface of the substrate are observed up to the third monolayer. Strongly structured surfaces and cracks in the films are found for films produced at high substrate temperatures and low DR on epiready GaAs substrates. Smooth surfaces are obtained at low substrate temperatures and high deposition rates on GaAs and ITO substrates. The size of the crystallites is strongly dependent on the substrate temperature, whereas big crystallites are found at high substrate temperatures.

A heating stage up to 1173 K for X-ray diffraction studies in the whole orientation space

A multi-purpose heating attachment designed primarily for X-ray four-circle diffractometers but applicable also for classical powder diffraction is presented. When working in reflection geometry, the air-cooled heating stage allows diffraction studies to be performed on plate-like samples up to 1173 K in the whole orientation space. This paper gives a detailed description of the assembly and important technical specifications for the performance of experiments. The heating characteristics of the heating stage, the displacement of the sample from the goniometer centre as a result of thermal expansion and the influence of the protecting dome on the diffraction experiment are presented. The simple technical construction, the low weight, the small size and good heating performance make this equipment a general purpose heating attachment for X-ray diffraction experiments in reflection geometry.

Epitaxy of Rodlike Organic Molecules on Sheet Silicates—A Growth Model Based on Experiments and Simulations

During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule−surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.

Surface induced crystallographic order in sexiphenyl thin films

The crystallographic order in sexiphenyl thin films on dielectric surfaces (like thermally oxidized silicon, KCl(100), TiO2(110) mica (001)) and on metallic surfaces (like Au(111), Al(111)) are summarized. The combination of the surface science studies considering the first molecular layers on the substrate surfaces with crystallographic studies on sexiphenyl films reveals the influence of the first molecular layers on the thin-film structure. The interaction strength of the molecules with the substrate has a large influence on the preferred orientation of the crystallites relative to the substrate surface. In the case of metallic surfaces, the orientation of the molecules at the first monolayer determines the alignment of the crystallites in the thin film. In the case of dielectric surfaces, other types of preferred crystal orientations are observed, which is connected with the increasing importance of the intermolecular interaction strength in the first monolayers. Generally, it is observed that the bulk crystal structure is already present in thin films with a nominal thickness slightly larger than the monolayer thickness. The orientation and alignment of the initial crystal clusters at the surface determine the layer growth. The thin films show specific morphologies: depending on the orientation of the molecules relative to the substrate surface, either terraced islands or needle-like structures appear. The crystallite size parallel to the surface varies in a range from nanometres up to several tens of microns, depending on the type of substrate and the thin-film growth conditions. In the case of terraced islands, the crystallite size perpendicular to the surface is comparable with the nominal film thickness.

Crystallisation kinetics in thin films of dihexyl-terthiophene: the appearance of polymorphic phases

The presence of surface-induced crystal structures is well known within organic thin films. However, the physical parameters responsible for their formation are still under debate. In the present work, we present the formation of polymorphic crystal structures of the molecule dihexyl-terthiophene in thin films. The films are prepared by different methods using solution-based methods like spin-coating, dip-coating and drop-casting, but also by physical vapour deposition. The thin films are characterised by various X-ray diffraction methods to investigate the crystallographic properties and by microscopy techniques (atomic force microscopy and optical microscopy) to determine the thin film morphologies. Three different polymorphic crystal structures are identified and their appearance is related to the film preparation parameters. The crystallisation speed is varied by the evaporation rate of the solvent and is identified as a key parameter for the respective polymorphs present in the films. Slow crystallisation speed induces preferential growth in the stable bulk structure, while fast crystallisation leads to the occurrence of a metastable thin-film phase. Furthermore, by combining X-ray reflectivity investigations with photoelectron spectroscopy experiments, the presence of a monolayer thick wetting layer below the crystalline film could be evidenced. This work gives an example of thin film growth where the kinetics during the crystallisation rather than the film thickness is identified as the critical parameter for the presence of a thin-film phase within organic thin films.