Inclined Ultrathin BiOSe Films: A Building Block for Functional van der Waals Heterostructures.

As an emerging ultrathin semiconductor material, BiOSe exhibits prominent performances in electronics, optoelectronics, ultrafast optics, However, until now, the in-plane growth of BiOSe thin films is mostly fulfilled on atomically flat mica substrates with interfacial electrostatic forces setting obstacles for BiOSe transfer to fabricate functional van der Waals heterostructures. In this work, controlled growth of inclined BiOSe ultrathin films is realized with apparently reduced interfacial contact areas upon mica flakes. Consequently, the transfer of BiOSe could be facile by overcoming weaker electrostatic interactions. From cross-sectional characterizations at the BiOSe/mica interfaces, it is found that there are no oxide buffer layers in existence for both in-plane and inclined growths, while the un-neutralized charge density is apparently decreased for inclined films. By mechanical pressing, inclined BiOSe could be transferred onto SiO/Si substrates, and back-gated BiOSe field effect transistors are fabricated, outperforming previously reported in-plane BiOSe devices transferred with the assistance of corrosive acids and adhesive polymers. Furthermore, BiOSe/graphene heterostructures are fulfilled by a probe tip to fabricate hybrid phototransistors with pristine interfaces, exhibiting highly efficient photoresponses. The results in this work demonstrate the potential of inclined BiOSe to act as a building block for prospective van der Waals heterostructures.