Physik supramolekularer Systeme und Oberflächen
Fakultät für Physik, Universität Bielefeld
Universitätsstr. 25, 33615 Bielefeld
Tel: +49-521-106 5351
Fax: +49 521 106 6002
Since 9.2016 Principal Investigator of Priority Program 1928 of German Research Foundation, Project: Dipolar molecular rotors in surface-anchored metal-organic frameworks
2014–2016 Postdoctoral Fellowship supported by Graphene Flagship, Project: Graphene nanomembranes from aromatic monolayers for nanofiltration applications
2011–2013 Postdoctoral Fellowship supported by Volkswagen Foundation, Project: Nano microphone
2010.11 Ph.D in Physics from Department of Physics, Bielefeld University, supervised by Prof. Dr. Armin Gölzhäuser.
Carbon possesses unique bonding properties, which enable it to form different allotropes, such as diamond, fullerene and carbon nanotubes, graphite and graphene. The spontaneous self-assembly of carbon-based organic molecules on solid surfaces represents a bottom-up approach to prepare carbon nanomaterials with monomolecular thickness featuring precise dimensions and desired functionalities. It has been challenging, however, to precisely stack monolayers consisting of different components on top of each other for the realization of thin film devices. The formation of covalent bonds between adjacent precursor molecules is achieved by electron irradiation induced cross-linking, forming so-called carbon nanomembranes with sufficient mechanical and thermal stability, which allows precise control of film thicknesses down to 1 nm. As carbon nanomembranes inherit physical and chemical properties from the precursor molecules, both fundamental studies on material properties and application-specific research on energy storage devices, sensors and membrane separations utilizing precisely controlled thin films are made possible on a molecular level.
My research activities are summarized as follows:
(1) Mechanical properties and vibrational modes of ultrathin carbon nanomembranes
Mechanical properties including elasticity, viscoelasticity, creep rates and ultimate tensile strength are usually part of material specifications. Mechanical characterization of CNMs are conducted by bulge testing in an AFM. Typical CNMs possess a Young’s moduli of ~10 GPa, a creep rate of 10^-6 s^-1 and an ultimate strength of 560 MPa. A general correlation between the rigidity of molecules and the mechanical stiffness of CNMs is found: membranes from rigid precursors (group II) exhibit higher elastic moduli of 15~19 GPa, while membranes from flexible oligophenyls (group I) exhibit relatively lower moduli.
The vibration was actuated by applying an AC voltage to a piezoelectric disk. The mode shapes of 1-nm-thick CNM with a size of 50×50 µm^2 were visualized with a Mirau interferometer using a stroboscopic light source, which is synchronized with the excitation voltage and has a time resolution of about 80 ns. The restoring force is either the flexural rigidity (plate regime) or the residual stress (membrane regime), depending on the thickness of the vibrating structure. The linear dispersion relation of the transverse membrane waves confirms the membrane model.
(2) Electronic transport through carbon nanomembranes and building CNM/graphene nanocapacitors
A two-terminal setup consisting of a Au substrate and a eutectic Ga-In conical tip was used to investigate the electronic transport through CNMs. The current densities versus bias voltage for CNMs from four different molecules were plotted below. We obtained a tunneling decay constant of 0.5 Å^-1 for pristine SAMs and of 0.53 Å^-1 for cross-linked monolayers. The contact resistance increases by more than 1 order of magnitude after cross-linking.
All-Carbon Capacitors (ACCs) consists of graphene electrodes and carbon nanomembrane as dielectric layer. The dielectric thickness is determined by the number of CNM layers with a precision of 1 nm. The active capacitor area is defined by the width of two graphene electrodes. A dielectric constant of 3.5 and a capacitance density of 0.3 µF/cm2, and a dielectric strength of 3.2 MV/cm^2 were determined. The energy density and power density of a graphene/CNM/graphene capacitor are 0.029 W·h/kg and 6.4×10^5 W/kg, respectively. Pyrolyed graphitic carbon (PGC) prepared from annealed carbon nanomembranes was used as electrode materials and the obtained energy density and power density are 0.135 W·h/kg and 1.4×10^7 W/kg. These results show the potential of carbon nanomembranes to be used as dielectric components in next-generation environment-friendly carbon-based energy storage devices.
(3) Molecular mechanisms of electron-induced crosslinking of aromatic monolayers
Helium ion microscope has been employed as an imaging technique with higher resolution due to its sub-nanometer sized ion probe, a high brightness and a low proximity effect. We employed helium ions to irradiate molecular monolayers. Three regimes were identified: formation of nuclei, one-dimensional and two-dimensional growth. A complete cross-linking requires a significantly lower dose, indicating a more efficient dissociative electron attachment process.
Surface-enhanced Raman Scattering spectroscopy (SERS) is a powerful surface-sensitive analytical technique that allows for detecting chemical and structural information of molecules on metallic surfaces. SERS assisted by Au nanoparticle was employed to monitor structural changes involved in the conversion of pristine monolayers to cross-linked monolayers. Statistical analysis showed that the ratio of the number of Raman-active sites to the total number of measurement sites decreases exponentially with increasing the irradiation dose.
(4) Gas/liquid transport and separation mechanisms through CNMs
Two-dimensional nanomembranes with a thickness below ~5 nm and pores tuned to act as molecular sieves are predicted to be ideal separation membranes with many advantages over bulk membranes. We used a scalable CNM with a high density of pores that combines high water permeance with high selectivity. Transport measurements showed that water passes through with an extremely high permeance of 1.1 × 10^-4 mol·m^-2·s^-1·Pa^-1, as does helium, but with a ~2500 times lower flux. The observed water permeation might be attributed to the coordinated water transport within sub-nanometer channels.
(5) Hybrid materials: van der Waals heterostructures and polymer carpets
CNMs possess different chemical functional groups on two sides, which allows them to be used as building blocks in combination with other materials to prepare novel hybrid materials. We demonstrated the preparation of heterostructures of CNMs with C60 and Au nanoparticles. This modular approach is based on the utilization of bifacial chemical functionalization and mechanical stability of carbon nanomembranes.
Ultrathin polymer membranes are being discussed as novel materials for miniaturized sensors. We also prepared so-called “polymer carpets”, i.e., a freestanding polymer brush grown by surface-initiated polymerization on a crosslinked 1-nm-thick monolayer. The carpet mechanics and the dramatic changes of the film properties (optical, wetting) upon external stimuli are investigated.
(6) Dipolar rotors as building blocks in surface-anchored metal organic frameworks for light modulation applications
Molecular rotors in analogy to macroscopic devices are molecules that consist of a stator and a rotator which rotate with respect to each other around a common axis. We use SURMOFs as the molecular frameworks that provide a modular architecture to arrange the dipoles and to control their intermolecular distances on the dimensions essential to tailor mutual dipole-dipole interactions and thereby spontaneous ordering.
SURMOFs can be built up to thicknesses where the optical extinction becomes complete for the absorption band of the rotor. These factors motivate us to pursue the modulation of light in thin film SURMOF rotor device prototype, using external electrical field to align the rotors into an orientation with low optical absorption. In the absence of applied field, the rotors will relax back into a high absorption state.