Our research concerns the intersection of nanoscience and X-ray science. We use X-rays to investigate nanostructured devices, and we develop nanostructures as X-ray detectors. We have a strong collaboration with the Nanomax beamline at MAX IV, and we also visit other synchrotrons for experiments.
Open PhD position! https://lu.varbi.com/en/what:job/jobID:321537/
We can offer many kinds of different MSc and BSc thesis projects, focusing on X-ray analysis, data analysis or nanofabrication. Please contact Jesper for more information. You can find a non-exhaustive list of ideas for projects under the headline "Semiconductor nanostructure analysis" here.
Some ongoing research projects are described below:
X-ray beam induced current
X-ray beam induced current (XBIC) in a single nanowire
X-rays that are absorbed in a semiconductor excite electrons over the bandgap, and in the presence of an internal or external electric field the electrons will generate a measurable current. With a nanofocused X-ray beam, we can locally probe the electronic properties of semiconductor devices. We have shown that X-rays can be used to image the carrier collection within single nanowire solar cells [Chayanun 2019]. Recently, we also demonstrated that scanning X-ray fluorescence can be used for mapping Zn dopants in InP nanowires with 50 nm resolution [Troian 2018].
Coherent X-ray diffraction
3D reconstruction of a single nanowire within a device
X-ray diffraction can be used to study strain, piezoelectricity and heating in crystalline samples. Modern X-ray optics can reach below 100 nm focus size, which we have used to study core-shell nanowires [Wallentin 2017]. We have shown that the shape of bent nanowires can be reconstructed in 3D with nanometre precision (Figure above). Hard X-rays can penetrate through thick samples, allowing measurements of operational devices [Wallentin 2016]. The intensity of focused X-rays can lead to beam damage, and we have studied beam induced heating of nanostructured samples [Wallander 2017]. We are developing novel methods for coherent diffraction methods, which use phase retrieval to overcome the limit of the focusing optics.
Nanostructured X-ray detectors
The nanofocus at the P10 beamline imaged with a single nanowire
Traditional X-ray detectors use bulk crystals, which limits their resolution. We have shown that X-rays can be detected by single nanowires, with a stronger response expected [Wallentin 2014]. In this project, financed by an ERC Starting Grant, we are developing vertical arrays of nanowires as high-resolution X-ray detectors.
Phase contrast imaging
Traditional X-ray imaging is based on absorption contrast, which has poor contrast for small and weakly absorbing samples. Much better contrast can be achieved using phase contrast. In this project, we are building a phase contrast tomograph based on a microfocus Cu source.