Patterned nucleation control in vacuum deposition of organic molecules

We report a generally applicable method to pattern organic molecules on mesoscopic scales. In our method, organic molecular beam deposition was conducted on substrate surfaces prepatterned with materials to which the organic molecules have larger binding energies in comparison to the substrate. Fully uniform nucleation control at these predefined locations can be achieved by an appropriate selection of the growth parameters including temperature and deposition rate. The physical mechanisms involved are studied by Monte Carlo simulations and stand in good agreement with the experimental findings.     

Needles and clusters of zinc porphyrin molecules on mica

Molecular monolayers of zinc porphyrins deposited from a liquid solution on mica substrates have been characterised using atomic force microscopy (AFM). At room temperature, needle-shaped single layer porphyrin islands are formed due to an anisotropic growth rate. The average island size depends on the concentration of the porphyrin solution, but the length to width aspect ratio is rather independent of the island size. Annealing the porphyrin monolayers to 450 K leads to the reduction of the size of the monolayer islands in two different ways depending on the heating rate. With slow heating rate, the island size was found to shrink by losing molecules from the edge of the island. With high heating rate, nanoscale molecular clusters were found to form.     

Effect of one-dimensional diffusion on void nucleation

Nucleation of voids via the stochastic accumulation of vacancies is considered when one-dimensionally migrating self-interstitials are present. A system instability signaling a non-equilibrium phase transition is found to occur when the mean free path of the one-dimensionally moving self-interstitials becomes comparable with the average distance between the voids at a sufficiently high void-number density. At this point, due to the exponential dependence of void nucleation probability on the net vacancy flux, the nucleation of voids is much more favored at the void lattice positions. At the same time, voids initially nucleated at positions where neighboring voids are non-aligned will shrink away. The shrinkage of non-aligned voids is caused entirely by the stochastic fluctuations in point-defect fluxes received by the voids. These two processes leave the aligned voids to form a regular lattice. The formation of the void lattice in this way can be accomplished at a void swelling of below 1%, in agreement with experimental observation. PACS 61.80.Az; 61.72.Cc; 05.65.+b     

Predictions of a spiral diffusion path for nonspherical organic molecules in carbon nanotubes

The diffusive behavior of ethane and ethylene in single-walled carbon nanotubes is investigated using classical molecular dynamics simulations and density functional theory calculations. At low molecular densities, these nonspherical molecules follow a spiral path inside nanotubes with diameters of 13-22 A, which maximizes the interaction of molecular C-C bonds with the C-C bonds in the nanotubes. Spherical molecules, such as methane, are not predicted to follow a spiral diffusion path. This result quantifies the manner in which molecular shape and chemical bonding affects molecule-nanotube interactions and indicates the generality of spherical transport through nanotubes.     

Dynamics of kinks: nucleation, diffusion, and annihilation

We investigate the nucleation, annihilation, and dynamics of kinks in a classical (1+1)-dimensional straight phi(4) field theory at finite temperature. From large scale Langevin simulations, we establish that the nucleation rate is proportional to the square of the equilibrium density of kinks. We identify two annihilation time scales: one due to kink-antikink pair recombination after nucleation, the other from nonrecombinant annihilation. We introduce a mesoscopic model of diffusing kinks based on "paired" and "survivor" kinks and antikinks. Analytical predictions for the dynamical time scales, as well as the corresponding length scales, are in good agreement with the simulations.     

Laplacian growth and diffusion limited aggregation: different universality classes

It had been conjectured that diffusion limited aggregates and Laplacian growth patterns (with small surface tension) are in the same universality class. Using iterated conformal maps we construct a one-parameter family of fractal growth patterns with a continuously varying fractal dimension. This family can be used to bound the dimension of Laplacian growth patterns from below. The bound value is higher than the dimension of diffusion limited aggregates, showing that the two problems belong to two different universality classes.     

Effect of diffusion length on the nucleation of chemical vapour deposition diamond

Atomic steps have been suggested as preferential sites for nucleation. However, evidence from recent experiments on diamond growth using faceted sapphire as well as reconstructed silicon substrates shows that atomic steps alone do not always enhance nucleation in the chemical vapour deposition environment. The comparison of the diffusion length of the nucleation precursors and the width of the terraces between the surface steps provides further insights into this nucleation mechanism.     

On kinetics of reactions limited by surface diffusion

An analysis is given of the unsteady-state kinetics of the catalytic reaction limited by surface diffusion of reactants to active sites. The following assumptions are used: the surface is uniform, adsorption/desorption processes are absent, the diffusion coefficient is dependent on coverage. It is shown that the reaction rate is determined primarily by the maximal value of the diffusion coefficient, the dependence of the diffusion coefficient on coverage leads to nonexponential reaction kinetics.     

Kinetic effects in the self-assembly of pure and mixed tetradecyl and octadecylamine molecules on mica

The self-assembly of tetradecylamine (C14) and of mixtures of tetradecyl and octadecylamine (C18) molecules from chloroform solutions on mica has been studied using atomic force microscopy (AFM). For pure components self-assembly proceeds more slowly for C14 than for C18. In both cases after equilibrium is reached islands of tilted molecules cover a similar fraction of the surface. Images of films formed by mixtures of molecules acquired before equilibrium is reached (short ripening time at room temperature) show only islands with the height corresponding to C18 with many pores. After a long ripening time, when equilibrium is reached, islands of segregated pure components are formed.     

One-dimensional organic nanostructures: A novel approach based on the selective adsorption of organic molecules on silicon nanowires

Scanning tunneling microscopy (STM) is used to investigate the formation of one-dimensional organic nanostructures by chemisorption of specific molecules on silicon nanowires. STM data demonstrate that depending on the molecular functional groups, the molecules adsorb either randomly on the substrate or preferentially on the nanowires. In the latter case, chemisorption of suitable organic molecules on the nanowires leads to a well-defined one-dimensional aggregation and changes the metallic character of the nanowires to a semi-conducting one.     

Crystal growth of small-molecule organic semiconductors with nucleation additive

In this study, we employ a nucleation additive 4-octylbenzoic acid (OBA) with an optimized solvent evaporation method to regulate crystal orientation and grain width of small-molecule organic semiconductors. When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was utilized as a benchmark material to mix with the additive, a self-assembled OBA interfacial layer was formed and promoted uniform deposition of nucleation seeds. As a result, the TIPS pentacene/OBA blend crystalline film exhibited crystal alignment in long range order, attributing to a 11-fold reduction of the crystal misorientation angle and a 4-fold increase of the grain width. We further discussed the important correlation between the effective hole mobility, grain boundaries, grain width and length, and nucleation sites. Organic thin film transistors were fabricated to test charge transport, yielding a hole mobility of up to 0.17 cm²/V. This work provides a new pathway to modulate the nucleation and crystallization events of organic semiconductors, and can potentially be applied to optimize the thin film morphology and electrical performance of organic semiconducting materials in general.     

Nonlinear diffusion of water molecules in glycerol

The dependence of the concentration of water molecules in water vapor brought into contact with glycerol surface was measured in a time window of 10⁻¹–10³ s. An obvious nonlinearity of water molecule diffusion was found in a short time approximation (within a minute). A proposal is made with regard to the convective character of diffusion due to heat release during absorption.     

Adsorption and diffusion of F_2 molecules on pristine graphene

The adsorption and diffusion of F_2 molecules on pristine graphene are studied by using first-principles calculations.For the diffusion of F_2 from molecular state in gas phase to the dissociative adsorption state on graphene surface, a kinetic barrier is identified, which explains the inertness of graphene in molecular F_2 at room temperature, and its reactivity with F_2 at higher temperatures. Study of the diffusion of F_2 molecules on graphene surface determines the energy barrier along the optimal diffusion pathway, which conduces to the understanding of the high stability of fluorographene.     

Surface diffusion and entrapment of simple molecules on porous silica

Interactions of simple molecules with the surface of porous silica have been investigated using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. A monolayer of water diffuses into pores at temperatures higher than 110 K. Multilayers of water are also incorporated in pores via sequential surface diffusion. In contrast, a methanol monolayer tends to stay on the surface up to 150 K, and carbon dioxide diffuses into pores rather gradually. Results can be explained as the contribution of hydrogen bonds between the adsorbate–substrate and adsorbate–adsorbate interactions. The predominance of the former (latter) might be responsible for single-molecule migration of methanol and carbon-dioxide (collective diffusion of water molecules) on the surface. These molecules are entrapped at higher coordination sites in pores, as revealed from thermal desorption peaks appearing at higher temperatures than those from non-porous silica. However, no significant difference is observed in desorption kinetics of CF₂Cl₂, Kr, CH₄, and N₂ molecules between the porous and non-porous silica substrates.