Molecular beam epitaxy growth of monolayer hexagonal MnTe2 on Si(111) substrate

Monolayer MnTe₂ stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe₂ bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe₂. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe₂ on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn⁴⁺ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe₂ compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe₂ monolayer is a semiconductor with a large bandgap of ~2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe₂ film provides a new platform to investigate the 2D magnetism.     

Stabilization variation of organic conductor surfaces induced by π-π stacking interactions

The structures and stabilization of three crystal surfaces of TCNQ-based charge transfer complexes(CTCs) including PrQ(TCNQ) 2,MPM(TCNQ) 2,and MEM(TCNQ) 2,have been investigated by scanning tunneling microscopy(STM).The three bulk-truncated surfaces are all ac-surface,which are terminated with TCNQ molecular arrays.On the ac-surface of PrQ(TCNQ) 2,the TCNQ molecules form a tetramer structure with a wavelike row behavior and a γ angle of about 18° between adjacent molecules.Moreover,the dimer structures are resolved on both ac-surfaces of MPM(TCNQ) 2 and MEM(TCNQ) 2.In addition,the tetramer structure is the most stable structure,while the dimer structures are unstable and easily subject to the STM tip disturbance,which results in changeable unit cells.The main reasons for the surface stabilization variation among the three ac-surfaces are provided by using the ’π-atom model’.