Growth of metal halide perovskite nanowires for X-ray detection applications

CsPbBr3 metal halide perovskite nanowire X-ray detectors. Left: Cross-sectional SEM. Middle: Photoluminescence [Zhang 2021]. Right: X-ray image of test pattern with 2 micron lines[Zhang 2022].

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  [Zhang 2021] showed that the nanowires have an impressive stability to air exposure, with samples exposed to air for 4 months still exhibiting comparable photoluminescence and UV stability to fresh samples. The second paper demonstrated high-resolution X-ray imaging using these structures [Zhang 2022], with excellent radiation stability.

Free-standing metal halide perovskite nanowires devices and heterostructures

Freestanding CsPbBr3 nanowires. Left: Cross-sectional SEM of as-grown nanowires. Middle: Cross-sectional optical microscopy (not false colored) of blue-green CsPbCl1.1Br1.9-CsPbBr3 heterostructured nanowires [Zhang 2022]. Single nanowire transistor [Lamers 2022].

We have discovered a method to grow free-standing vertically aligned CsPbBr3 metal halide perovskites [Zhang 2022]. Part of the nanowires can be converted to blue-emitting CsPbCl1.1Br1.9. MHPs are soluble in polar solvents, which makes normal lithography processing schemes difficult to use. However, we have found a method to perform electron beam lithography (EBL) using only non-polar solvents [Lamers 2022]

Nanostructured X-ray detectors and X-ray beam induced current (XBIC)

Left: The nanofocus at the NanoMax beamline, MAX IV, Lund [Chayanun 2020], imaged with a single nanowire. Right: X-ray beam induced current (XBIC) in a single nanowire [Chayanun 2019].

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-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].

Coherent X-ray diffraction of nanocrystals and nanoscale devices

3D strain simulation and measurement of axially heterostructured nanowire [Hammarberg 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 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].

X-ray imaging of ferroelastic domains

Left: Imaging of ferroelastic domain dynamics in a CsPbBr3 perovskite nanowire as the temperature is ramped across a phase transition [Marcal 2020]. The Bragg peak and the domain pattern change as the temperature crosses the orthorhombic to tetragonal phase transition at 80C. Right: 3D reconstruction of two ferroelastic domains in a CsPbBr3 nanoparticle [Dzhigaev 2021]

We have recently shown that it is possible to image ferroelastic domains inside nanowires of the metal halide perovskite CsPbBr3 [Marcal 2020]. A presentation by Lucas Marcal on these results can be found here. Similar methods can be used to image ferrolastic domains induced by AFM [Marcal 2021]. Recently, we demonstrated 3D imaging of ferroelastic domains in CsPbBr3 nanoparticles [Dzhigaev 2021]. A presentation about these results can be found here. Dmitry Dzhigaev has also made a longer presentation about the method, Bragg Coherent Diffraction Imaging, found here.

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]