The cover image illustrates the  nanoimprinting litography process using PDMS templates, which manages to obtain 2D photonic supercrystals made of perovskite nanocrystals, as shown in the article "Templated‐assembly of CsPbBr₃ perovskite nanocrystals into 2D photonic supercrystals with amplified spontaneous emission" by Agustín Mihi et al.  (DOI: 10.1002/anie.202006152).

"CsPbBr₃ perovskite nanocrystals are readily shaped into large area 2D photonic crystals by means of nanoimprinting lithography. The periodic patterning provides efficient light coupling to the nanocrystal layer, increasing the electric field intensity within the perovskite film and resulting in amplified spontaneous emission (ASE) under lower optical excitation fluences in the near‐IR."

The following figure (Figure 1 of the mentioned article) shows the nanoimprinting litography process, and the resulting photonic architecture obtained, after the mold is removed with SEM (Scanning Electron Microscopy) images. 

Figure: a) Schematic illustration of PDMS template‐assisted fabrication of 2D photonic supercrystals made of perovskite NCs. b),c) Representative SEM images of 2D photonic supercrystals made of CsPbBr₃ arrays with lattice parameters of 600 nm and 1700 nm, respectively. The inset in (b) is a photograph of different CsPbBr₃ SC arrays on glass substrates, displaying diffraction under white light illumination.

You can read the article abstract, and the article in Open Access here!  

This article is a collaboration of ICMAB researchers from the NANOPTO group with researchers from the Ludwig-Maximilians-Universität (LMU) (Munich, Germany) and the CIC biomaGUNE (Donostia/San Sebastián, Spain). 

Reference:

Templated‐assembly of CsPbBr₃ perovskite nanocrystals into 2D photonic supercrystals with amplified spontaneous emission
Lakshminarayana Polavarapu, David Vila-Liarte, Maximilian W. Feil, Aurora Manzi, Juan Luis Garcia-Pomar, He Huang, Markus Döblinger, Luis M Liz-Marzán, Jochen Feldmann, Agustín Mihi. 
Angew. Chem. Int. Ed.. 59, 2-9, 2020
DOI:10.1002/anie.202006152
COVER DOI: 10.1002/anie.202011445