Process developed for ultrathin carbon membranes

Scientists at the Max Planck Institute for Polymer Research (MPI-P) and the University of Bielefeld have succeeded in jointly developing a process for producing ultra-thin membranes based on carbon.

This novel method enables the production of membranes which are far thinner than was possible up until now with established procedures.

Atomic structure of a membrane. The insert in shows a magnified grain boundary where arrangements of carbon atoms into pentagons (green) and heptagons (blue) are marked. © University of Ulm 2013

They consist on average of a single molecular layer. Such monolayers can expand in the plane to the region of square meters. Long-term they should be used as a filter for the specific separation of gases.

This method was developed by scientists at the University of Bielefeld based on specific molecules prepared at the MPIP in Mainz by chemists in the group of Director Klaus Müllen. To do this, the physicists in Bielefeld required polyaromatic thiol, which are. special sulphur-based molecules, composed of a thiol group which fixes the membrane to the substrate and a hydrocarbon, which provides the carbon necessary for the membrane.

Initially, the molecules are brought onto a gold-coated substrate. The sulphur atoms combine with the gold and form self-organised monolayers. Subsequently, the researchers irradiate the electrons. Thus, the carbon parts of the monolayers begin to cross-link among themselves.

A membrane - still irregularly conditioned - develops horizontally. During the pyrolysis afterwards the membrane is exposed to temperatures of up to 900 degrees Celsius.

Under these conditions the sulphur is separated and the membrane detaches itself from the surface of the substrate.

At the same time the pyrolysis prompts further cross-linking of the carbons. In the course of this, the molecules align themselves horizontally and provide the membrane with a more homogenous and firm structure. With the aid of helium-ion-microscopy provided in Bielefeld, recording on a single-digit nanometer scale is possible. This showed film-like, ultrathin membranes with defined pores.

Scientists at the MPI-P have synthesised a number of custom-made molecules whose various properties like density, elasticity and permeability can respectively be transferred to the membranes and whose generation can meet these demands.

The membranes made by these molecules are especially interesting for filter application as they are already equipped with pores which act as a passing-through place for the separation of the particles.