N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) on alkali halide(001) surfaces

The growth of N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI) on KBr(001) and NaCl(001) surfaces has been studied. Experimental results have been achieved using frequency modulation atomic force microscopy at room temperature under ultra-high vacuum conditions. On both substrates, DiMe-PTCDI forms molecular wires with a width of 10 nm, typically, and a length of up to 600 nm at low coverages. All wires grow along the [110] direction (or [1$\bar{1}$0] direction, respectively) of the alkali halide (001) substrates. There is no wetting layer of molecules: Atomic resolution of the substrates can be achieved between the wires. The wires are mobile on KBr surface but substantially more stable on NaCl. A p(2 x 2) superstructure in brickwall arrangement on the ionic crystal surfaces is proposed based on electrostatic considerations. Calculations and Monte-Carlo simulations using empirical potentials reveal possible growth mechanisms for molecules within the first layer for both substrates, also showing a significantly higher binding energy for NaCl(001). For KBr, the p(2 x 2) superstructure is confirmed by the simulations, for NaCl, a less dense, incommensurate superstructure is predicted.Comment: 5 pages, 5 figure

Organic Molecules on various surfaces: frequency modulation atomic force microscopy studies

Die vorliegende Arbeit befasst sich mit verschiedenen Untersuchungen organischer Moleküle auf Oberflächen. Diese wurden mit Hilfe der frequenzmodulierten Rasterkraftmikroskopie (FM-AFM) durchgeführt, einer Rastersondenmethode, die auch die Abbildung von Isolator-Oberflächen mit hoher Ortsauflösung erlaubt. Ein Schwerpunkt dieser Arbeit liegt auf dem gleichzeitig mit der Topographie aufgezeichneten Dämpfungssignal. Die für die Dämpfung verantwortlichen Mechanismen auf Grund einer nicht konservativen Wechselwirkung zwischen Messspitze und Probe werden diskutiert. Die Messungen werden ergänzt durch Rechnungen und Simulationen mit Hilfe empirischer Potentiale. Bei der Untersuchung eines prototypischen organischen Halbleitermoleküls, 3,4,9,10-Perylen-Tetrakarbonsäure-Dianhydrid (PTCDA) auf KBr(001) wird Inselwachstum von Molekülkristalliten auf der Oberfläche festgestellt. Die Moleküle der obersten Kristalllage können sowohl im Topographie- als auch im Dämpfungssignal einzeln aufgelöst werden. Ein Mechanismus für die Dämpfung wird vorgeschlagen und durch Rechnungen belegt. Für PTCDA wird mit Hilfe empirischer Potentiale die Ehrlich-Schwoebel-Barriere berechnet, d.h. die zusätzliche Diffusionsbarriere für ein Molekül bei Überquerung einer Stufenkante. Diese Barriere ist ein wichtiger Parameter für die Kinetik des Kristallwachstums, die gefundenen Energiebarrieren stehen im Einklang mit indirekten experimentellen Hinweisen, die einen starken Ehrlich-Schwoebel-Effekt für PTCDA vermuten lassen. Das Wachstum eines dem PTCDA ähnlichen organischen Moleküls, N,N'-Dimethyl-Perylen-3,4,9,10-bis(dicarboximid) (DiMe-PTCDI) wird auf KBr(001) und NaCl(001) untersucht. Die Moleküle arrangieren sich auf diesen Substraten zu drahtartigen Strukturen, die Drähte folgen der [110]-Substratrichtung. Auf KBr sind die Drähte sehr mobil, auf NaCl wesentlich stabiler. Rechnungen und Monte-Carlo-Simulationen werden präsentiert, sie können hier zum tiefergehenden Verständnis der experimentellen Beobachtungen beitragen. Für PTCDA auf Ag(111) kann submolekularer Kontrast im Topographie- und Dämpfungssignal erzielt werden. Insbesondere für die erhöhte Dämpfung an den funktionellen Gruppen der Moleküle wird im Licht der Experimente und Rechnungen anderer Forschergruppen eine überzeugende Erklärung gegegeben: Vorgeschlagen wird ein spitzeninduziertes Aufbrechen der chemischen Bindung der funktionellen Gruppen zum Substrat und eine kurzfristige Bindung an die Spitze.The thesis presents several studies of organic molecules on surfaces by means of frequency modulation atomic force microscopy (FM-AFM), a scanning probe technique enabling especially the imaging of insulating surfaces with high lateral resolution. This work focuses on the damping signal which can be recorded simultaneously with topography. The mechanisms responsible for the damping due to non-conservative interaction of the measuring tip and the sample are discussed. Additional information on the studied systems is achieved by calculations and simulations based on empirical potentials. For the prototypical organic semiconductor molecule, 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA), on KBr(001), island growth of molecular crystallites is found. Single molecules of the topmost molecular layer can be resolved in topography as well as in the damping signal. A mechanism for the damping is presented and supported by calculations. For PTCDA, the Ehrlich-Schwoebel barrier is calculated by means of empirical potentials, i.e., the additional energetic diffusion barrier for a molecule crossing a step edge. This barrier is an important parameter for the kinetics of crystal growth. The energy barriers found are consistent with indirect experimental evidence indicating a strong Ehrlich-Schwoebel effect for PTCDA. The growth of an organic molecule similar to PTCDA, N,N'-dimethyl perylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI), is studied on KBr(001) and NaCl(001). On these substrates, the molecules form wire-like structures aligned with the [110] substrate direction. The wires are very mobile on KBr and essentially more stable on NaCl. Calculations and Monte-Carlo simulations are presented, helping to understand the experimental observations. For PTCDA on Ag(111), sub-molecular contrast can be achieved in topography and damping signal. In particular for the enhanced damping at the functional groups of the molecules, a convincing explanation is given in the light of experiments and calculation of other groups: A tip-induced breaking of the chemical bond of the functional groups to the substrate and a temporary bond to the tip is proposed

Temperature dependence of the energy dissipation in dynamic force microscopy

The dissipation of energy in dynamic force microscopy is usually described in terms of an adhesion hysteresis mechanism. This mechanism should become less efficient with increasing temperature. To verify this prediction we have measured topography and dissipation data with dynamic force microscopy in the temperature range from 100 K up to 300 K. We used 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both materials exhibiting a strong dissipation signal at large frequency shifts. At room temperature, the energy dissipated into the sample (or tip) is 1.9 eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good agreement with an adhesion hysteresis mechanism. The energy dissipation over the PTCDA surface decreases with increasing temperature yielding a negative temperature coefficient. For the KBr substrate, we find the opposite behaviour: an increase of dissipated energy with increasing temperature. While the negative temperature coefficient in case of PTCDA agrees rather well with the adhesion hysteresis model, the positive slope found for KBr points to a hitherto unknown dissipation mechanism

A new structural class of serine protease inhibitors revealed by the structure of the hirustasin–kallikrein complex

AbstractBackground: Hirustasin belongs to a class of serine protease inhibitors characterized by a well conserved pattern of cysteine residues. Unlike the closely related inhibitors, antistasin/ghilanten and guamerin, which are selective for coagulation factor Xa or neutrophil elastase, hirustasin binds specifically to tissue kallikrein. The conservation of the pattern of cysteine residues and the significant sequence homology suggest that these related inhibitors possess a similar three-dimensional structure to hirustasin.Results: The crystal structure of the complex between tissue kallikrein and hirustasin was analyzed at 2.4 Å resolution. Hirustasin folds into a brick-like structure that is dominated by five disulfide bridges and is sparse in secondary structural elements. The cysteine residues are connected in an abab cdecde pattern that causes the polypeptide chain to fold into two similar motifs. As a hydrophobic core is absent from hirustasin the disulfide bridges maintain the tertiary structure and present the primary binding loop to the active site of the protease. The general structural topography and disulfide connectivity of hirustasin has not previously been described.Conclusions: The crystal structure of the kallikrein–hirustasin complex reveals that hirustasin differs from other serine protease inhibitors in its conformation and its disulfide bond connectivity, making it the prototype for a new class of inhibitor. The disulfide pattern shows that the structure consists of two domains, but only the C-terminal domain interacts with the protease. The disulfide pattern of the N-terminal domain is related to the pattern found in other proteins. Kallikrein recognizes hirustasin by the formation of an antiparallel β sheet between the protease and the inhibitor. The P1 arginine binds in a deep negatively charged pocket of the enzyme. An additional pocket at the periphery of the active site accommodates the sidechain of the P4 valine

The evaluation of acoustic characteristic performance on natural sound absorbing materials from cogon grass waste

In the past few decades, synthetic fibers are been used widely in the field of sound absorption due to their superior characteristics such as durable and chemical resistant. However, there are several disadvantages of synthetic fibers such as non-biodegradability and hazards to the health of human. In this research, the natural sound absorber from cogon grass was investigated. The objective of the research was to evaluate the performance of cogon grass physical characteristics on its acoustical behavior, to evaluate the effect of sodium hydroxide (NaOH) treatment times on physical and acoustical characteristics of cogon grass, to investigate the decay effects after it was left over for twelve months and lastly to compare and verify the acoustical results with theoretical models based on (Delany-Bazley and Miki Model). The measurement of acoustical characteristics which are sound absorption coefficient (SAC) and noise reduction coefficient (NRC) were done by using impedance tube method (ITM). The samples of cogon grass were tested in a way of the untreated and treated with NaOH in varied soaked hours which are one, two, three, four and five hours. Scanning electron microscope (SEM) and density kit were used to investigate physical characteristics. The research confirmed that physical characteristics of tortuosity and airflow resistivity values tend to increase with the increment of treatment times, but the density and porosity tend to decrease. Untreated samples were tested with varied thicknesses of 10, 20, 30, 40 and 50mm. The results show SAC value increases when the thickness of the sample was increased. Treated samples results show the least treated sample (1 hour) reached the maximum SAC value and indicated the highest value of NRC which is 0.50. The results also show a reduction in sound absorption value after the samples were left for twelve months. Verification parts demonstrated that Delany-Bazley and Miki Model can predict approximately pattern compared with ITM results because of the theoretical models are developed by a simple empirical model approach. Overall, cogon grass samples have the good characteristics to be an acoustic material component