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Jesper Wallentin

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.

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:

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].


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]. Recently, 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.

Phase contrast tomography


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. The image above shows part of a blueberry seed. We have recently published our first results [Dierks 2020]

Research group

 

Zhaojun Zhang, postdoc
Lucas Marcal, postdoc
Lert Chayanun, PhD student
Susanna Hammarberg, PhD student
Hanna Dierks, PhD student
Nils Lamers, PhD student

 

 

 

Publications

For an updated and complete list of publications, please see Google Scholar. A selected list of recent work:

  1. L Chayanun et al., "Direct Three-Dimensional Imaging of an X-ray Nanofocus Using a Single 60 nm Diameter Nanowire Device" Nano Letters In press https://doi.org/10.1021/acs.nanolett.0c03477
  2. L. A. B. Marcal et al., "In Situ Imaging of Ferroelastic Domain Dynamics in CsPbBr3 Perovskite Nanowires by Nanofocused Scanning X-ray Diffraction" ACS Nano In press
    https://doi.org/10.1021/acsnano.0c07426
  3. H. Dierks and J. Wallentin, "Experimental optimization of X-ray propagation-based phase contrast imaging geometry"  Opt. Express 28 (20), 29562-29575 (2020) dx.doi.org/10.1364/OE.399819
  4. S Hammarberg et al., "High resolution strain mapping of a single axially heterostructured nanowire using scanning X-ray diffraction", Nano Res. 13 (9), 2460 (2020) http://doi.org/10.1007/s12274-020-2878-6
  5. L. Chayanun et al.: "Combining Nanofocused X-Rays with Electrical Measurements at the NanoMAX Beamline"  Crystals 9 (8), 432 (2019) http://dx.doi.org/10.3390/cryst9080432
  6. L Chayanun et al.: Nanoscale mapping of carrier collection in single nanowire solar cells using X-ray beam induced current J. Synchrotron Radiat. 26 (1) (2019) doi.org/10.1107/S1600577518015229
  7. A. Troian et al, Nanobeam X-ray fluorescence dopant mapping reveals dynamics of in situ Zn-doping in nanowires Nano Lett. 18 (10), 6461 (2018) dx.doi.org/10.1021/acs.nanolett.8b02957
  8. H. Wallander and J. Wallentin, Simulated sample heating from a nanofocused X-ray beam
    J. Synchrotron Radiat. 24 (5) (2017) doi.org/10.1107/S1600577517008712
  9. J. Wallentin et al., Bending and twisted lattice tilt in strained core-shell nanowires revealed by nanofocused X-ray diffraction Nano Lett. 17 (7) (2017) dx.doi.org/10.1021/acs.nanolett.7b00918
  10. J. Wallentin, M. Osterhoff, and T. Salditt, In operando X-ray diffraction reveals electrically induced strain and bending in single nanowire device Adv. Mater. 28 (9), 1788 (2016) http://dx.doi.org/10.1002/adma.201504188