Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudomagnetic field effects, helical flat bands, and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here we show that the Dirac surface states of the topological insulator BiSe can be reversibly tuned by an externally applied elastic strain. Performing in situ X-ray diffraction and in situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial BiSe films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of BiSe and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes.

Constructing a 2D/3D heterojunction perovskite that could remain highly efficient and improve the stability of perovskite solar cells (PSCs) is explored. In this paper, we compared 2D/3D heterojunction perovskite films passivated by organosulfur and commonly used organic amine salts. Results indicated that the perovskite film passivated by organosulfur amine salts had a lower trap state density, longer carrier lifetime, improved energy level alignment, and higher thermal and moisture stability. An approximate 21% power conversion efficiency (PCE) was obtained in the champion device. After the comparison, the passivation mechanism of the organosulfur 2D/3D heterojunction was proposed.

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