| Summary: | Using the first-principles method, an unmanufactured structure of blue-phosphorus-like monolayer CSe (<i>β</i>-CSe) was predicted to be stable. Slightly anisotropic mechanical characteristics in <i>β</i>-CSe sheet were discovered: it can endure an ultimate stress of 5.6 N/m at 0.1 along an armchair direction, and 5.9 N/m at 0.14 along a zigzag direction. A strain-sensitive transport direction was found in <i>β</i>-CSe, since <i>β</i>-CSe, as an isoelectronic counterpart of blue phosphorene (<i>β</i>-P), also possesses a wide indirect bandgap that is sensitive to the in-plane strain, and its carrier effective mass is strain-dependent. Its indirect bandgap character is robust, except that armchair-dominant strain can drive the indirect-direct transition. We designed a heterojunction by the <i>β</i>-CSe sheet covering <i>α</i>-CSe sheet. The band alignment of the <i>α</i>-CSe/<i>β</i>-CSe interface is a type-II van der Waals <i>p</i>-<i>n</i> heterojunction. An appreciable built-in electric field across the interface, which is caused by the charges transfering from <i>β</i>-CSe slab to <i>α</i>-CSe, renders energy bands bending, and it makes photo-generated carriers spatially well-separated. Accordingly, as a metal-free photocatalyst, <i>α</i>-CSe/<i>β</i>-CSe heterojunction was endued an enhanced solar-driven redox ability for photocatalytic water splitting via lessening the electron-hole-pair recombination. This study provides a fundamental insight regarding the designing of the novel structural phase for high-performance light-emitting devices, and it bodes well for application in photocatalysis.
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