89HIGHLIGHTS 2020

Hot electrons in a nanowire hard X-ray detector, M. Zapf (a), M. Ritzer (a), L. Liborius (b), A. Johannes (c), M. Hafermann (a), S. Schönherr (a), J. Segura-Ruiz (c), G. Martínez-Criado (d) W. Prost (b) and C. Ronning (a),

Nat. Commun. 11, 4729 (2020); https:// doi.org/10.1038/s41467-020-18384-x. (a) Institute of Solid State Physics, Friedrich Schiller University of Jena (Germany) (b) Dept. Components for High Frequency Electronics and CENIDE, University of

Duisburg-Essen (Germany) (c) ESRF (d) Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Madrid (Spain)

[1] G. Martínez-Criado et al., Nano Lett. 14, 5479-5487 (2014). [2] V.B. Ram Boppana et al., J. Mater. Chem. 20, 9787 (2010).

DISCLOSING THE CAUSES OF DEGRADATION OF CADMIUM YELLOW PAINTS IN THE SCREAM

The degradation of cadmium yellow paints in The Scream (ca. 1910) by Munch is a poorly understood phenomenon. Here, non-invasive spectroscopy techniques at the macroscale combined with synchrotron radiation X-ray methods at the submicroscale were used to pinpoint the causes of degradation, thus contributing to optimising preventive conservation strategies.

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

X-ray fluorescence (XRF), X-ray absorption near- edge spectroscopy (XANES) and X-ray beam- induced current (XBIC) measurements were simultaneously taken. The XBIC measurements revealed a direct scanning X-ray imaging resolution of ~200 nm of the device (Figure 72a). Based on this approach, direct scanning X-ray imaging with spatial resolution of tens of nanometres might be possible.

The internal electric fields can be manipulated by applying a bias voltage to the p-n junction; furthermore, the detector charge collection efficiency can be improved by adjusting the applied voltage in reverse direction. By combining nanoscale XANES and XBIC, oxidisation was discovered in the n-type segment of the nanowire for applying bias voltages above -2V, while the p-type segment remained pristine (Figure 72b). This selective oxidisation is attributed to hot electron cooling, which is the degradation mechanism of the biased nanowire device (Figure 73a shows a schematic band diagram of the process). Severe swelling was also found in the n-type section after the X-ray analytical experiments, corroborating the selective oxidisation (Figure 73b).

For nanoscale applications such as using nanowires, hot electron degeneration effects

are highly relevant due to the limited thermal conductivity and the pronounced surface-to- volume ratio. Thus, the findings presented here provide a basis for understanding and constraining hot electron effects and oxidisation in nanoscale p-n junction devices and might help towards establishing the heat management that is necessary for their applications as sensors and solar cells, as well as their integration into nanoscale optoelectronic devices.

Fig. 73: a) Schematic band diagram along the p-n junction explaining the observed oxidisation in the n-doped part due to hot electrons accelerated by the built-in voltage at the p-n junction. b) Colourised scanning electron microscopy (SEM) image of the device after all measurements (tilted by 50° with respect to the sample surface). The dashed line indicates the measurement region for the high-resolution XRF/XBIC maps.

The Scream, created in different versions by Edvard Munch between 1893 and 1916, is among the most popular series of artworks of the modern era. The two most well-known versions date back to 1893 and ca. 1910, and are owned by the National Museum in Oslo and the Munch Museum in Oslo, Norway. In these masterpieces,

Munch explored the use of a wide range of newly synthesised materials, including different kinds of pigments. One, however, is causing problems for long-term conservation.

The ca. 1910 version of The Scream (Figure 74a) shows marks of alteration in a number of areas