Ionic liquids at electrified interfaces

Until recently, “room-temperature” (<100–150 °C) liquid-state electrochemistry was mostly electrochemistry of diluted electrolytes(1)–(4) where dissolved salt ions were surrounded by a considerable amount of solvent molecules. Highly concentrated liquid electrolytes were mostly considered in the narrow (albeit important) niche of high-temperature electrochemistry of molten inorganic salts(5-9) and in the even narrower niche of “first-generation” room temperature ionic liquids, RTILs (such as chloro-aluminates and alkylammonium nitrates).(10-14) The situation has changed dramatically in the 2000s after the discovery of new moisture- and temperature-stable RTILs.(15, 16) These days, the “later generation” RTILs attracted wide attention within the electrochemical community.(17-31) Indeed, RTILs, as a class of compounds, possess a unique combination of properties (high charge density, electrochemical stability, low/negligible volatility, tunable polarity, etc.) that make them very attractive substances from fundamental and application points of view.(32-38) Most importantly, they can mix with each other in “cocktails” of one’s choice to acquire the desired properties (e.g., wider temperature range of the liquid phase(39, 40)) and can serve as almost “universal” solvents.(37, 41, 42) It is worth noting here one of the advantages of RTILs as compared to their high-temperature molten salt (HTMS)(43) “sister-systems”.(44) In RTILs the dissolved molecules are not imbedded in a harsh high temperature environment which could be destructive for many classes of fragile (organic) molecules

Full X-ray pattern analysis of vacuum deposited pentacene thin films

Pentacene thin films with thicknesses ranging from 10 nm to 180 nm are investigated by specular X-ray diffraction in the reflectivity regime and in the wide angular regime. The results of the reflectivity measurements show a clear shift of the 001 reflection of the thin film phase depending on the layer thickness. It is shown that this shift can be explained by the dynamical scattering theory. The wide angular regime measurements show the 00L of the thin film phase. Williams-Hall plots are used to extract information on the crystallite size and mean micro strain of the thin film phase. The crystallite size is in good agreement with the results obtained by the reflectivity measurements. From this it can be concluded that the thin film phase crystallites are extended over the entire film thickness down to the substrate. Additionally an increase of the micro strain with increasing film thickness is observed

Evaluation of organic sub-monolayers by X-ray based measurements under gracing incident conditions

The structural investigations of model organic systems like pentacene on silicon oxide in the monolayer regime is very important for the basic understanding of initial nucleation process together with the electronic performance of transistor devices. A method for the evaluation of the island formation and layer closing of the first monolayer is introduced. The method is based on specular X-ray reflectivity and diffuse scattering and reveal integral information on the coverage together with the size and separation of pentacene islands. The results are in good agreement with AFM investigation that encourages the use of this type of investigation in in-situ experiments

Full X-ray pattern analysis of vacuum deposited pentacene thin films

61.05.C- X-ray diffraction and scattering, 68.35.bm Polymers, organics, 61.82.Rx Nanocrystalline materials,

Surface Induced Order of Solution Processed Caffeine Needles on Silica and Muscovite Mica

In this work the orientational order of needle shaped caffeine crystallites at silica and muscovite mica surfaces is investigated by X ray diffraction experiment and optical microscopy. The investigation of thin films reveals the formation of caffeine in the anhydrous monoclinic beta polymorphic structure independent of the surface with the disclike caffeine molecules assemble in an edge on conformation. The silica surface provides isotropic properties for crystal growth which leads to a fiber texture arrangement of the needles; a majority of needles have a 010 texture with all other directions being randomly distributed. The mica surface with its highly regular surface structure induces defined textures and azimuthal order of the caffeine needles resulting in a fully defined epitaxial order. The edge on and disclike caffeine molecules align along the [100] m , [110] m , and [1 1 over bar0] m real space directions of the mica substrate and show a mirror symmetry around the [110] m The experiments on these two surfaces show that the type of surface is able to induce order during caffeine needle growt

Substrate-induced phase of a [1]benzothieno[3,2-b]benzothiophene derivative and phase evolution by aging and solvent vapor annealing

Substrate-induced phases (SIPs) are polymorphic phases that are found in thin films of a material and are different from the single crystal or "bulk" structure of a material. In this work, we investigate the presence of a SIP in the family of [1]benzothieno[3,2-b]benzothiophene (BTBT) organic semiconductors and the effect of aging and solvent vapor annealing on the film structure. Through extensive X-ray structural investigations of spin coated films, we find a SIP with a significantly different structure to that found in single crystals of the same material forms; the SIP has a herringbone motif while single crystals display layered π-π stacking. Over time, the structure of the film is found to slowly convert to the single crystal structure. Solvent vapor annealing initiates the same structural evolution process but at a greatly increased rate, and near complete conversion can be achieved in a short period of time. As properties such as charge transport capability are determined by the molecular structure, this work highlights the importance of understanding and controlling the structure of organic semiconductor films and presents a simple method to control the film structure by solvent vapor annealing

Biaxial growth of pentacene on rippled silica surfaces studied by rotating grazing incidence X-ray diffraction

Pentacene is known to grow on isotropic silicon oxide surfaces in a substrate-induced phase with fiber textured crystallites. This growth study reports on the growth of pentacene crystallites on uniaxially oriented surfaces. Silica substrates have been treated by ion beam sputtering so that ripples with a lateral corrugation length of 38 nm and a surface roughness of 1.3 nm are formed. Pentacene thin films with a nominal thickness in the range from 20 nm up to 300 nm are deposited on top of the rippled surfaces. The films are characterized by atomic force microscopy and grazing incidence X-ray diffraction. Bi-axially oriented crystals are formed due to the grooves of the substrate surface opening up the possibility of a defined in-plane alignment of the crystals. In a first stage of thin film growth, the thin film phase (TFP)of pentacene is formed, while in the later stage the bulk crystal structure (C, Campbell phase)also appears. Due to the bi-axial alignment of the crystallites the transition from the thin film phase to the bulk crystal structure can be directly investigated. An epitaxial relationship with (1 2 0)TFP || (2 1 0)C and [ 122 1 0]TFP ||[1 122 0]C is observed which can be explained by an adaption of the herringbone layers of both crystal structures. This work reveals one possible microscopic mechanism for the transition from the metastable substrate-induced phase of pentacene to its equilibrium bulk structure