Nanostructured X-ray detectors

The nanofocus at the P10 beamline, PETRA-III, Germany (left, [Wallentin 2014]), and the NanoMax beamline, MAX IV, Lund (right, [Chayanun 2020]), imaged with a single nanowire.

Traditional X-ray detectors use bulk crystals, which limits their resolution. In this project, financed by an ERC Starting Grant, we are developing vertical arrays of nanowires as high-resolution X-ray detectors. We have shown that X-rays can be detected by single nanowires, with much higher spatial resolution than commercial systems [Chayanun 2020].

 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].  We also demonstrated that scanning X-ray fluorescence can be used for mapping Zn dopants in InP nanowires with 50 nm resolution [Troian 2018].

Metal halide perovskite nanowires

CsPbBr3 metal halide perovskite nanowires. Left: Cross-sectional SEM, right: Photoluminescence [Zhang 2021]

Metal halide perovskites are most famous for their rapid development in solar cells, but they are also promising materials for X-ray scintillation detectors. We are synthesizing CsPbBr3 nanowire arrays using solution growth, by using anodized aluminum oxide nanopores as templates. Our first paper in this project was recently published [Zhang 2021], where we show that the nanowires have an impressive stability to air exposure, with ith samples exposed to air for 4 months still exhibiting comparable photoluminescence and UV stability to fresh samples.

Coherent X-ray diffraction of nanocrystals

Left: 3D strain simulation and measurement of axially heterostructured nanowire [Hammarberg 2020]. Right: Imaging of ferroelastic domain dynamics in a CsPbBr perovskite nanowire as the temperature is ramped across a phase transition [Marcal 2020].

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 [Wallentin 2017] and axially hetereostructured nanowires [Hammarberg 2020]. We have imaged ferroelastic domains inside nanowires of the metal halide perovskite CsPbBr3 [Marcal 2020]. We have shown that the shape of bent nanowires can be reconstructed in 3D with nanometre precision [Wallentin 2017]. 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 also developing novel methods for coherent diffraction methods, which use phase retrieval to overcome the limit of the focusing optics. Recently, we showed how the uncontrolled rotation of 60 nm nanoparticles could be used for 3D strain imaging [Björling 2020].

Phase contrast tomography

In this project, we have built a phase contrast X-ray tomograph based on a microfocus Cu source. 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. The system is based on a microfocus X-ray source with a Cu target, a high-resolution detector and a high-precision sample stage. It's capable of X-ray absorption contrast and phase contrast imaging at a spatial resolution of about 1 micrometer in 2D and 3D. The image above shows part of a blueberry seed. We have recently published our first results: [Dierks 2020]