Harald Graaf

Nanopatterning of alkyl monolayers covalently bound to Si(111) with an atomic force microscope

Alkyl monolayers covalently bound to silicon were prepared through the reaction between 1-alkene molecules and hydrogen-terminated Si. The surfaces were anodized in nanometer scale with a contact-mode atomic force microscope (AFM) by applying positive bias voltage to the surface with respect to a conducting cantilever under ambient conditions. Following the anodization, patterned areas were selectively modified by chemical etching and coating with different molecules. The alkyl monolayers showed high resistance against chemical etching and protected Si surfaces from oxidation. AFM lithography of monolayers on Si was found to be useful for nanofabrication of organic/inorganic interfaces based on the Si–C covalent bond.

BTQBT (bis-(1, 2, 5-thiadiazolo)-p-quinobis(1, 3-dithiole)) Thin Films; A Promising Candidate for High Mobility Organic Transistors

BTQBT (bis-(1, 2, 5-thiadiazolo)-p-quinobis(1, 3-dithiole)) films have been prepared as active semiconducting layers of organic field effect transistors (FETs). BTQBT films showed a p-type semiconducting behavior. The hole mobility and on/off ratio of BTQBT films under ultrahigh vacuum conditions reached to 0.2 cm2/Vs and 108, respectively, by optimization of the growth conditions. These values are in the same order as those of pentacene thin films, which indicates that BTQBT molecules have a great potential for active layers of organic electronic devices.

Influence of Substrate Roughness on the Formation of Aliphatic Self-Assembled Monolayers (SAMs) on Silicon(100)

The peak shifts of the CH2-vibrations are an indicator of the amount of gauche-conformational disorder present in aliphatic self-assembled monolayers (SAMs). We have investigated the relationship between the surface roughness and the peak position as a function of temperature. With increasing substrate roughness, the CH2 vibration peaks shift to higher wave-numbers. The magnitude of the shift is up to 6 cm-1 which corresponds to a change from a condensed, almost all-trans conformational phase to a liquid-like, gauche-disordered phase. On polished substrates although increased temperatures lead to a slightly more ordered SAM, the layers are in almost an all-trans conformational phase independent on the coverage. From these results an island growth and annealing effects-model is proposed.

Influence of gas molecules on the charge carrier mobility in thin films of semiconducting perylene tetracarboxylic imides

Thin films (30nm) of crystalline N,N′-dimethylperylene-3,4,9,10-biscarboximide (MePTCDI) or amorphous 1,6,7,12-tetrachloro-N,N′-dimethylperylene-3,4,9,10-biscarboximide (Cl4MePTCDI) were prepared by physical vapor deposition and characterized by conductivity and field effect measurements at pristine films and under increasing partial pressure of oxygen, ethanol, acetone, or n-butane. Changes in the conductivity of the films were observed. Field effect measurements served to differentiate between changes in the charge carrier mobility μ and changes of the charge carrier concentration n. μ was found to decrease significantly, whereas rather small changes were found in n. Gas molecules that diffused into the films changed μ at least ten times more efficiently than those molecules just adsorbed on the surface.

Suppression of chromophore coupling in thin films by chemical substitution of a perylene tetracarboxylic acid diimide

Abstract Thin films (1–100 nm) of 1,6,7,12-tetrachloro- N , N ′-dimethyl perylene tetracarboxylic acid diimide (Cl 4 MePTCDI) were prepared on glass and quartz substrates by drop coating from solution and by physical vapor deposition under variation of substrate temperature and deposition rate. Optical absorption spectra were measured in situ in dependence on average film thickness during film growth under variation of deposition rate and substrate temperature and following film preparation ex situ in dependence on sample temperature. The results are compared to those obtained earlier [ J. Phys. Chem. , B 1999 , 103 , 3078 ] at thin films of unsubstituted N , N ′-dimethyl perylene tetracarboxylic acid diimide (MePTCDI). In contrast to unsubstituted MePTCDI no significant chromophore coupling could be detected in films of Cl 4 MePTCDI. A relative high optical emission signal for thin films of Cl 4 MePTCDI with characteristics quite similar to the solution spectrum clearly confirmed this interpretation. A twisted structure of the molecule as indicated earlier [ J. Phys. Chem ., 1992 , 96 , 7988] is discussed as the basis of changes in the solid state structure relative to unsubstituted MePTCDI which has a planar aromatic system and grows in crystalline films.

Atomic force microscope anodization of Si(111) covered with alkyl monolayers

Alkyl monolayers on Si were prepared through the reaction between 1-alkenes and hydrogen-terminated Si by heat treatment. The monolayers were characterized by atomic force microscopy (AFM), force curve and water contact angle measurements. It was found that surface properties were modified by the formation of highly ordered closely packed monolayers. The monolayers were anodized with a contact-mode AFM by applying voltage between the conductive cantilever and surface under ambient conditions, which resulted in nanometer-scale oxidation of surfaces. After anodization, patterned areas were modified by removing the silicon oxide and terminating the surface of the grooves with hydrogen atoms by NH4F etching, and by covering the etched surface with 1-octadecene molecules. The monolayers themselves showed high resistance to NH4F etching and air oxidation. AFM lithography of monolayers anchored covalently on Si was found to be useful for nanofabrication of organic/inorganic interfaces based on Si–C covalent bonds.

Redox reactions of acetone and ethanol with the surface of N,N′-dimethylperylene-3,4,9,10-biscarboximide (MePTCDI) thin films

Thin films of the title compound MePTCDI were deposited by physical vapour deposition on microstructured interdigital Au electrode arrays on a SiO2–Si substrate. Initial island formation followed by bulk film growth was detected by conductivity measurements during the film growth. Conductivity changes were observed as a consequence of exposure of the films to partial pressures of acetone and ethanol in the range of 10-7–10-4 mbar. Following a relative fast decrease at the beginning of exposure the conductivity reached a steady state after about 10 min in each case. The conductivity of the films in the steady state followed a Langmuir adsorption law. Analysis of the time-dependence of the conductivity before the steady state was reached revealed the presence of a redox reaction which leads to the observed changes in charge carrier concentration in the films. Assuming first order kinetics for this reaction the transients as obtained at different partial pressures could be successfully discussed and the validity of the model consisting of a surface reaction in two steps was thereby confirmed.

Force curve measurement of self-assembled organic monolayers bound covalently on silicon (111)

Self-assembled monolayers of alkyl chains were prepared on silicon (111) surfaces. The surface was characterized by atomic force microscopy (AFM). Atomically flat terraces were observed in topographic images of the films with contact mode AFM, indicating the formation of highly ordered monolayers. Force curve measurements showed that the adhesion force between organic films and gold coated cantilever was much stronger comparing to the force on hvdrogen terminated surfaces.

Design of domain size and molecular interactions in organic semiconductors to control the emission yield of thin films

Perylene- and phthalocyanine- pigment molecules were systematically modified and consequences were studied for their solid state properties. Thin films (1 - 150 nm) were prepared by physical vapor deposition. Intermolecular interactions were probed by optical measurements in absorption and emission. Atomic force microscopy served to analyze the morphology of films. Different interactions among the molecules and with the substrate surfaces allowed to prepare either crystalline or amorphous films. Crystalline films of perylene pigments were typically characterized by strong chromophore coupling leading to a characteristic splitting, well- defined shifts of the optical absorption bands and emission mainly from excimer species whereas the chromophore coupling in amorphous films was suppressed sufficiently to provide a significantly increased optical emission yield from uncoupled monomer states. Temperature-dependent optical emission experiments are presented which allow a detailed discussion of monomer vs. excimer emission. Decoupling of the chromophores could be obtained by appropriate chemical substitutions at the aromatic core system of phthalocyanines and perylene pigments that led to strong deviations from planarity. This was achieved by the introduction of bulky substituents in the bay position of the aromatic perylene core and by changes in the coordination number of the central group in phthalocyanines. The strategy led to a strongly enhanced optical emission for both classes of materials. This could be obtained, however, either in an amorphous arrangement of the molecules or under conservation of crystallinity, both offering alternative advantages.