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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - SFB 951 - HIOS

Humboldt-Universität zu Berlin | Mathematisch-Naturwissen­schaft­liche Fakultät | Institut für Physik | SFB 951 - HIOS | Old Homepage Backup | Projects | Topic Area A | A7 | A7: Computersimulations of the film formation and morphology of polar and anisotropic molecules at organic/inorganic interfaces

A7: Computersimulations of the film formation and morphology of polar and anisotropic molecules at organic/inorganic interfaces

 

a) Collective behavior of anisotropic molecules in presence of inhomogeneous electrostatic or topological surface fields

 

b) Advancement of coarse-graining strategies for specific HIOS from the atomistic to the mesoscale

 

Objectives

We aim to create coarse-grained models between organic molecules in a bulk-crystal based on information from atomistic detailed models (project A1) and quantum-chemical calculations (projects A4, B4). When this is accomplished the coarse-graining procedure will be advanced to organic molecules interacting with an inorganic semiconductor surface.

Moreover coarse-grained models are applied to simulate growth processes and equilibrated surfaces, whereby we focus on organic-inorganic interfaces and the structures they form. These structures are of interest in nano-scale fabrication and to understand structural effects influencing the conductivity and other characteristica of the resulting HIOS system.

 

Which molecules/surfaces?

At first we concentrate on the molecules coronene, C60, diindenoperylene (DIP) and p-sexyphenyl. We use ZnO and GaN as surfaces.

 

FIRST

 

 

 

 

 

 

 

                             

a) coronene                  b)DIP                                        c) 6P

 

Coarse-graining

Coarse-graining is a procedure where “irrelevant” degrees of freedom are integrated out to give effective interaction potentials. Our coarse-graining strategy is based on the following two steps.

 

Step 1:

We rewrite the microscopic Hamiltonian of the system in terms of a few “relevant” degrees of freedom such as the position of the  molecule and the orientation of its principal axis. This yields the so-called effective Hamiltonian. We can calculate its two-particle-part (molecule-molecule contribution) with help of umbrella sampling simulations with restrained pulling of two molecules (project A1-Dzubiella). To test the quality of our choice of relevant variables we compare thermodynamic quantities and structure parameters from a mesoscopic simulation of this molecular pair potential with those from a MD-simulation (A1) of the atomistic system. This procedure is also applied to the interaction between a molecule and the surface.

 

Step 2:

To get an analytic expression of the two-particle effective potential we employ a fitting procedure. An ad hoc model will be created and stepwise refined by comparing the model with the numerical one from step 1 monitoring thermodynamic quantities and structure parameters. To adjust the model after each step, we use combinatorial optimization techniques. Further ingredients to adjust the model parameters are molecular quantities like the van der Waals shape, electric multipole moments or the polarizability. These quantities are provided by projects A4 (Heimel) and B4 (Knorr/Scheffler/Rinke), who use quantum-mechanical approaches.

 

TWO

 

This image shows an estimate for a coarse-grained coronene bulk system composed of Gay-Berne particles with an additional linear quadrupole pointing to the orientational axis.

 

Growth simulations

We start our surface analysis by using a kinetic Monte-Carlo (KMC) simulation. This method allows us to treat surface growth of crystals as a non-equilibrium process, considering large time scales (seconds to minutes) and large length scales (micrometers). Further, the KMC method allows us to monitor the formation of various structures.

Process parameters determine whether a surface grows as a sequence of closed layers or whether it forms islands. These parameters may include a free diffusion energy barrier, neighbour binding energies, the Ehrlich-Schwoebel-barrier, the adsorbtion rate and the temperature. They can be gained from the coarse-graining approach, but also from quantum density functional theory or experiments. A further important factor is the lattice geometry.

As a first application of the model we currently investigate the epitaxial growth of C60 on mica.

Besides this collaborative study the next step in our project will be to expand the model to include anisotropic particles, whereby we intend to focus on dipolar and quadrupolar particles. This will allow us to simulate a broad variety of molecules and to directly transfer knowledge from the coarse-graining into the growth simulation.

 

THREE

                               

 

Sample of a grown surface.

 

Planned workflow

FOUR


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