Two-dimensional 1T-HfSe2: Surface structure and air stability

The two-dimensional semiconductor HfSe2 is a promising candidate for future electronic, optical, and sensing applications due to its moderate bandgap (∼1.1 eV) and the spontaneous formation of a native high-κ dielectric in ambient conditions. In this study, a range of experimental techniques was employed to investigate the intrinsic surface properties of bulk HfSe2 grown by chemical vapor transport and to directly track oxidation in air. Scanning tunneling microscopy reveals a high defect density across the HfSe2 surface caused by Se vacancies and impurities, confirmed by XPS, inductively coupled plasma mass spectrometry, and high-resolution Rutherford backscattering spectrometry. Furthermore, XPS highlights the highly reactive nature of the HfSe2 surface in air, where the formation of HfOx is detected after a 1 min exposure to air, resulting in changes to the electronic properties, such as the work function and the valence band offset. Understanding the intrinsic surface properties, air stability, and material quality will facilitate the integration of 1T-HfSe2 in novel devices.