Pressure Impact on the Crystal Structure, Optical, and Transport Properties in Layered Oxychalcogenides BiCu<i>Ch</i>O (<i>Ch</i> = S, Se)

The layered oxychalcogenides BiCu<i>Ch</i>O (<i>Ch</i> = S, Se, Te) represent a unique family of two-dimensional semiconductors with extraordinary optoelectronic and thermoelectric properties. Chemical strategies such as elemental doping have been used to modify their crystal structures and electronic configurations for better photocatalytic performances. Herein, we report the pressure impact on the crystalline and electronic band structures of BiCu<i>Ch</i>O (<i>Ch</i> = S, Se) with in situ synchrotron X-ray diffraction, Raman spectroscopy, electric resistivity and photocurrent measurements, and first-principle calculations. Under pressure, the crystalline lattices shrink continuously without symmetry breaking, which enhances the crystal field splitting; on the other hand, the pressure-induced charge delocalization causes the band broadening. The competition between the crystal field and charge delocalization demonstrates an efficient tool for band gap engineering: the electrical conductivity is enhanced below 12 GPa and monotonically decreases up to 40 GPa. In addition, BiCuSeO exhibits considerable photocurrent up to ∼40 GPa, which suggests its potential application in pressure-responsive optoelectrical devices. The comprehensive studies of the pressure effect on the crystal structure and electronic properties in two-dimensional semiconductors provide and in-depth understanding for developing new optoelectronic materials under extreme conditions