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Advanced materials 35(19), 2300893 (2023). doi:10.1002/adma.202300893, Published by Wiley-VCH, Weinheim

Advanced materials 35(20), 2208485 (2023). doi:10.1002/adma.202208485 special issue: "Advanced Materials Hall of Fame", Published by Wiley-VCH, Weinheim

Science advances 8(47), eade0828 (2022). doi:10.1126/sciadv.ade0828, Published by American Association for the Advancement of Science, Washington, DC [u.a.]

Funder: Deutsche Forschungsgemeinschaft; Id: http://dx.doi.org/10.13039/501100001659, Outstanding photovoltaic (PV) materials combine a set of advantageous properties including large optical absorption and high charge carrier mobility, facilitated by small effective masses. Halide perovskites (ABX3, where X = I, Br, or Cl) are among the most promising PV materials. Their optoelectronic properties are governed by the B X bond, which is responsible for the pronounced optical absorption and the small effective masses of the charge carriers. These properties are frequently attributed to the ns2 configuration of the B atom, i.e., Pb 6s2 or Sn 5s2 (“lone‐pair”) states. The analysis of the PV properties in conjunction with a quantum‐chemical bond analysis reveals a different scenario. The B X bond differs significantly from ionic, metallic, or conventional 2c 2e covalent bonds. Instead it is better regarded as metavalent, since it shares about one p‐electron between adjacent atoms. The resulting σ‐bond, formally a 2c 1e bond, is half‐filled, causing pronounced optical absorption. Electron transfer between B and X atoms and lattice distortions open a moderate bandgap resulting in charge carriers with small effective masses. Hence, metavalent bonding explains favorable PV properties of halide perovskites, as summarized in a map for different bond types, which provides a blueprint to design PV materials.

Small : nano micro 2201753 (2022). doi:10.1002/smll.202201753, Published by Wiley-VCH, Weinheim

Nature Communications 12(1), 924 (2021). doi:10.1038/s41467-021-21175-7, Published by Nature Publishing Group UK, [London]

Advanced materials 33(39), 2102356 (2021). doi:10.1002/adma.202102356, Published by Wiley-VCH, Weinheim

The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices. Here, we employ the next-generation PCM In3SbTe2 (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale “soldering” of metallic nanoantennas. Our concepts can empower improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms., Here, the authors introduce In3SbTe2 (IST) as a programmable material platform for plasmonics and nanophotonics in the infrared. They demonstrate direct optical writing, modifying and erasing of metallic crystalline IST nanoantennas, tuning their resonances, as well as nanoscale screening and soldering.

Third-generation photovoltaic (PV) materials combine many advantageous properties, including a high optical absorption together with a large charge carrier mobility, facilitated by small effective masses. Halide perovskites (ABX3, where X = I, Br or Cl) appear to be the most promising third-generation PV materials at present. Their opto-electronic properties are governed by the B-X bond. A quantum-chemical bond analysis reveals that this bond differs significantly from ionic, metallic or covalent bonds. Instead, it is better regarded as metavalent, since it shares approximately one p-electron between adjacent atoms. The resulting s–bond is half-filled, which causes pronounced optical absorption. Electron transfer and lattice distortions open a moderate band gap, resulting in charge carriers with small effective masses. Hence metavalent bonding explains the favorable PV properties of halide perovskites. This is summarized in a map for different bond types, which provides a blueprint to design third-generation PV materials.

Advanced materials 2005533 (2020). doi:10.1002/adma.202005533, Published by Wiley-VCH, Weinheim

Lone pairs explain the structure of many molecular solids, as well as the chain-like or layered structures encountered in many chalcogenide crystals. Such chalcogenides have enabled a plethora of applications, including phase-change memories, thermoelectrics, topological insulators or photoconductors. In many of these, lone pairs also are invoked to explain the unconventional material properties. The presence of so-called van der Waals gaps in layered chalcogenides and their low thermal conductivity have also been linked to lone pairs. However, for some of these systems, a second view of bonding has been proposed, where atoms are held together across the interlayer spacing by shared electrons. To clarify this situation, herein, several systems for which lone pairs have been frequently emphasized are reinvestigated theoretically. By comparing the charge and electron localization analysis in terms of a Hartree-Fock-like pair density obtained from Kohn-Sham density functional theory (KS-DFT), it is verified that the structure of several chalcogenides is governed by the presence of lone pairs, whereas others are not. As an example, crystalline Se is demonstrated to form a structure with two covalent bonds and a lone pair, whereas metavalenty bonds are the essential characteristics of crystalline Sb, crystalline Te being an intermediate case.

Crystalline GeSb2Te4 (GST) is remarkable material, as it allows to continuously tune the electrical resistance by orders of magnitude without involving a phase transition or stoichiometric changes, just by altering the short-range order. While well-ordered specimen are metallic, increasing amounts of disorder can eventually lead to an insulating state with vanishing conductivity in the 0K limit, but a similar number of charge carriers. These observations make disordered GST one of the most promising candidates for the realization of a true Anderson insulator. While so far the low-temperature properties have mostly been studied in films of small grain size, here a sputter-deposition process is employed that enables preparation of a large variety of these GST states including metallic and truly insulating ones. By growing films of GST on mica substrates, biaxially textured samples with huge grain sizes are obtained. A series of these samples is employed for transport measurements, as their electron mean free path can be altered by a factor of 20. Yet, the mean free path always remains more than an order of magnitude smaller than the lateral grain size. This proves unequivocally that grain boundaries play a negligible role for electron scattering, while intragrain scattering, presumably by disordered vacancies, dominates. Most importantly, these findings underline that the Anderson insulating state as well as the system's evolution towards metallic conductivity are indeed intrinsic properties of the material.