Physical Chemistry of Nanocrystals
We develop physical-chemical methods to advance optoelectronic applications of nanostructured materials.
We are convinced that the continuous effort in the synthesis of colloidal nanostructures over the past decades has led to a level of maturity that warrants their successful implementation into every-day life technology, such as quantum dot television displays. The challenge is now to optimize selected nanostructures for specific applications. While we synthesize some of the nanomaterials investigated by us in our own labs (e.g. lead halide perovskites), we are increasingly collaborating with other chemistry groups to get first-hand access to highly complex nanostructures, such as nanoclusters, nanosheets or -platelets as well as tailored organic π-systems.
The transport properties of nanostructures are probed by us through temperature-dependent field-effect transistor measurements. To this end, we have developed a tailored micro-printing procedure that enables assessing the electric properties of individual nanostructured domains as small as few micrometers. Combining this ability with scanning electron microscopy and nano-focused X-ray diffraction allows for the correlation of local structural details with the transport behavior of nanostructured materials.
To excel in the optical application of semiconductor nanostructures, we have built a time-resolved photocurrent measurement system, which allows probing the electric response of thin films to short pulses of visible or infrared light with a time-resolution of 1.7 ns. This enables the optimization of nanostructured materials for application in fast optical data communication.
For an understanding of the electronic structure of semiconductor nanomaterials, we have developed an electrochemically modulated absorption spectroscopy set-up, which probes the absorption spectrum of thin film materials under varying electrochemical bias. With this, we are able to visualize the complex electronic structure of quantum-confined materials.
To complement our own methods, we also collaborate with several physics groups in the fields of X-ray diffraction, nanospectroscopy, X-ray photoelectron spectroscopy, neutron scattering and nanolithography.
With this toolbox of physical-chemical methods, we are currently working on three core fields (click for more details):
Nanomaterials for optical data communication
Over the past three decades, data communication rates have seen a steady increase of 50 % per year, also because of the growing utilization of optical fiber cables. When the optical data arrives at the consumer, it must be converted into electrical data, since conventional computers operate entirely with electric transistors. This crucial task is carried out in every data communication network by so-called “optical transceivers”. A growing problem in this respect is that a lot of research and development is devoted towards increasing the speed of optical data transmission in the fibers, but not in the optical transceivers. This is analogous to expanding the number of lanes on a motorway but neglecting the need for frequent and fast exits. Our goal is to develop new materials for better, faster optical transceivers to help increase data communication rates.
Lead halide perovskite nanocrystals for light emitting diodes
LEDs have revolutionized the way we light-up our world. From smart phone displays, high-definition television screens over car headlights with unprecedented range to energy-efficient and warm lighting applications at home – LEDs are ubiquitous. Nonetheless, the technology behind LEDs is still dynamically evolving. While recent years have seen the breakthrough and wide-spread economic success of organic light emitting diodes (OLEDs), a new class of materials has emerged as an attractive alternative, especially in television screens: Quantum Dots in so-called QLEDs. Quantum dots are semiconductor nanocrystals which emit extremely color-pure light, leading to a visual perception of a QLED display as extraordinarily rich in contrast and brilliant. However from a device standpoint, QLEDs are not so much diodes but rather color filters which convert a bright blue backlight into additional shade of red and green. This is because of the unsolved challenge to make QLED materials that at the same time are bright emitters and good electric conductors as required for a true diode. Our goal is to make nanomaterials for QLEDs that overcome this challenge.
Superlattices of nanocrystals and -clusters towards novel anisotropic materials
In analogy to classical crystals which are ordered arrays of atoms, three-dimensional superstructures of nanocrystals or -clusters (NCs) are referred to as “superlattices”, in which the NCs function as “quasi-atomic” building blocks. The analogy to atoms continues as many semiconductor NCs are small enough to fall into the large quantum confinement regime, such that they exhibit discrete, atom-like electronic states. An overwhelming structural diversity of NC superlattices has been reported, which has frequently served to reiterate the term “quasi-atoms”, including the associated expectations for future applications of NC superlattices as novel electronic materials with emergent properties. With our research, we seek to answer fundamental questions on these fascinating nanomaterials: Does structural order and orientation in NC superlattices change the optoelectronic properties of the array compared to a disordered ensemble of the same material? What is the value of long-range order and orientation for transport in NC superlattices? Is it possible to create novel anisotropic materials with direction-dependent electric transport, similar to graphene or black phosphorus?