Research
The research group Physics of Supramolecular Systems and Surfaces is focused on the fabrication, characterization, and application of functional nanomaterials, with an emphasis on two-dimensional carbon and graphene.
The group pioneered the fabrication of carbon nanomembranes (CNMs) from cross-linked, self-assembled monolayers. The group lays great effort on technological applications of CNMs and explores their use for electronics, photonics, biotechnology, and as tailored filter membranes for gas and liquid separation.
Our laboratory uses modern preparative and analytical tools: electron microscopes, photoelectron spectrometers, scanning probes, and CVD systems. We also operate the first helium ion microscope (HIM) at a German university. Our HIM is used for high-resolution imaging and nanolithography with a focused He ion beam.
Surface Studies and Self-Assembled Monolayeres
Specific functionalization of surfaces is of crucial importance in the material sciences. In particular in nanotechnology, complex functionalizations are desired for an increasing number of applications, which will require the development of novel interface modification strategies. Examples are the functionalization of electrodes for organic electronics, the remote control of surface properties by optical switching, the setup of biomimetic or biologically relevant interfaces (cell recognition, differentiation of stem cells, and the colonization of technical surfaces by microorganisms) as well as coatings of implants.
The most promising strategies for such surface functionalization are based on self-assembled monolayers (SAMs). SAMs have not only turned out to be the model system for organic surfaces, but are of high relevance to both fundamental and applied research. Above all, they serve as a technology platform for the further development of nanotechnology.
SAMs have produced a number of innovative stimuli in materials sciences and are increasingly applied as a technology platform for the (further) development of nanotechnology. Examples are:
- Functional coatings in organic electronics
- Carbon nanomembranes (CNMs) for sensors and bioengineering
- Control of protein affinity on surfaces for medical engineering
- Platforms for highest-resolution (sub-10 nm) lithography
- Tribological coatings of nanomechanical components
Owing to the unique combination of simple production and excellent structural properties, SAMs are particularly well suited as model systems for studying basic interface phenomena at both solid/gaseous and solid/liquid phase boundaries. SAMs have a small thickness, which makes them accessible by classical electron spectroscopy methods. Practically, all standard methods of surface analysis can be applied directly to these organic thin layers. Hence, the complete knowledge base established for metal and inorganic substrates in 'Interface Sciences' can be applied, including diffraction methods. As a result, SAMs are ideal to validate different types of theoretical approaches. Whereas thin polymer films, for example, also offer a large variety of options with regard to obtaining highly functional interfaces, they typically lack the high degree of perfection obtainable with SAMs. As a consequence, the application of diffraction methods is difficult, and a reliable, direct validation of theoretical calculations is hampered for such systems.
Carbon Nano Membranes (CNMs)
Helium Ion Micrograph of a CNM on copper grid.
Carbon nanomembranes (CNMs) are a special class of two-dimensional (2D) materials formed by the crosslinking of self-assembled monolayers. They have a thickness of around one nanometer. Thanks to their mechanical strength, they can be handled in a free-standing manner. Due to their high surface-to-volume ratio and remarkable stability, CNMs have a variety of potential applications, ranging from filtration technologies to electronic devices.
From Monolayers to Nanomembranes
The starting point of CNM fabrication is a self-assembled monolayer (SAM) of aromatic molecules which are linked to a surface via a functional group. When the so-assembled molecules are exposed to radiation, one obtains a ~1 nm thick mechanically stable film of cross-linked carbon. The Gölzhäuser group is working with partners in Europe, the US and Japan on the characterization of these materials for electrical, optical and mechanical purposes, and towards applications of CNMs into new products.
Mass transport through CNMs
CNMs consist of a dense network of sub-nanometer channels that efficiently block the passage of most gases and liquids. However, water passes through with an extremely high permeability of approximately 1.1 × 10^(−4) mol·m^(−2) · s^(−1) · Pa^(−1), as does helium, albeit with a permeability that is approximately 2,500 times lower. Consequently, this results in an exceptional molecular selectivity.
Several methods and instruments can be used to determine the membrane's permeance.
In the Mass Loss Method, a cup filled with water is sealed with a silicon chip whose orifice is covered by the membrane. Over time, the water in the cup evaporates and permeates across the membrane. The mass loss of the container can be recorded using a microbalance.
The group also has an ultra-high vacuum system (base pressure of 2 × 10^(-9) mbar) for measuring membrane permeance. The sample is sealed onto a copper disk for this purpose. The permeate is detected via a quadrupole mass spectrometer. This setup allows for the additional mixing of vapours and gases. Furthermore, it is possible to cool or heat the membrane.
To study the motion of ionic species, our group uses a homemade permeation cell consisting of two identical compartments with channels for solutions, as well as channels connecting both parts. CNM-covered silicon chips are assembled between the two compartments and sealed. Ion conductance measurements are performed using both DC and AC methods.
CNMs from different precurser molecules.
CNMs have significant potential for technological applications. CNM based products include support membranes for electron microscopy of nanometer scale objects, where the support structure is often thicker than the actual nano-object, which results in a poor image quality. The CNMs, however, are thinner than most nano-objects, which improves the image contrast significantly. In addition to the thickness, the chemical functionalities of the two sides of the CNM can be tailored by the choice of suitable molecules. By selecting the irradiation parameters, the degree of cross-linking and thus the elasticity of the CNM can be tuned. In summary, CNMs are a new class of materials with interesting properties that lead to innovative products in many fields
Mechanics of CNMs
The mechanical stability of a membrane is important for its use in industrial applications. To investigate the mechanical properties of CNMs, like Young’s modulus, we use different techniques, such as the bulge test or nanoindentation. Both techniques rely on an AFM instrument.
In the Bulge Test, the displacement of the free-standing membrane above a hole is measured as a function of the pressure difference.
In nanoindentation, the tip of an AFM is pressed onto the sample surface. The applied force is then correlated with the membrane’s deflection.
The breaking strength can be measured by performing a nanoindentation test until the membrane is broken.
Depending on the type of precursor molecules, the Young's modulus of carbon nanotube materials (CNTs) can be tailored to lie within the range of 10 to 20 gigapascals (GPa), which is about 50- to 100-fold smaller than that of a graphene monolayer (~1 terapascal (TPa)), but slightly higher than the upper bound of polymeric membranes. This reveals a correlation between the rigidity of the precursor molecules and the macroscopic mechanical stiffness of CNMs. Our setup can determine strain rates as low as 10^(-8) s^(-1). The creep rates of CNMs are in the range of 10^(-6) s^(-1) at room temperature (RT). The creep behaviour of CNMs appears to be stress-dependent and thermally activated, and can partially recover in the absence of an external load.
Helium Ion Microscopy (HIM)
The Zeiss Orion Plus Helium Ion Microscope in Bielefeld
The Helium Ion Microscope (HIM) utilizes charged particles for imaging. By employing helium ions instead of electrons, it offers sub-nanometer resolution, large depth of field, and high surface sensitivity, surpassing the capabilities of conventional electron microscopes. A significant advantage of the HIM is its ability to perform charge compensation, allowing for the imaging of insulating samples without the need for additional coatings. These features make the Helium Ion Microscope an invaluable tool in materials science, life science, nanotechnology, biotechnology, and other research fields that require precise imaging at high magnifications.
Further information about HIM is provided in our article ‘Helium ion microscopy’ [Link]
Free-standing CNMs
A collection of different CNMs on hexagonal copper grids is presented, exhibiting the different types of features that are visible in HIM images. From these images, one intuitively obtains an impression of the detailed shape of the copper grid and the CNM on top. In a) larger folds on the upper side of the image and one rupture in the centre are visible. b) is an example of a membrane rolling up at a rupture, showing the high flexibility of CNMs. Small folds like those in c) are frequently observed, while wrinkling of the freestanding membrane (d) is less often observed.
SARS-CoV-2 infected Vero E6 cells
The HIM pictures show the three-dimensional appearance of SARS-CoV-2 and the surface of Vero E6 cells at a multiplicity of infection of approximately 1 with great morphological detail. This project was undertaken in collaboration with the Department of Medical Virology at Justus-Liebig University Gießen and the Medical Faculty at Bielefeld University.
Soot from ethylene flames
Morphological variations of nascent soot collected from the ethylene C3 flame at heights of 0.5, 0.8 and 1.2 cm and imaged by HIM, showing representative primary and aggregate structures. Particles shown in the hexagons are in the apparent size range of 4-8 nm; those in the squares are 14-18 nm. Particles shown in the circles are apparent aggregates.
Mammalian cells
In specimens sputter coated with gold (top row) with a standard thickness of approx. 10 nm, the cell membrane is covered with clustered gold, masking the true cell surface. The effective charge compensation in HIM allows a high resolution inspection of uncoated cell surface in the native state (bottom row).
