Growth of Dichalcogenide Layers on TiO₂(110)-MoSe₂ or TiSe₂

Transition-metal dichalcogenide (TMDC) islands and thin films are grown on TiO2(110) via physical vapor deposition of Mo and Se and are analyzed with electron diffraction, Auger spectroscopy, and scanning tunneling microscopy (STM). Surprisingly, large, crystalline TiSe2 patches develop on the TiO2 surface instead of the expected MoSe2 islands. They are identified by a hexagonal lattice with 3.4 Acirc;; periodicity, a unique (2 x 2) superstructure related to charge-density waves, and an empty conductance doublet in STM spectroscopy. Density-function-theory calculations assign the imaging contrast and the conductance doublet to tunneling into the 3p orbitals of the topmost Se layer and Ti 3d-related electronic states, respectively. Control experiments without Mo deposition confirm that the observed TMDC growth indeed results from a surface reaction between gas-phase Se and Ti3+ atoms, segregating out of the reduced TiO2 crystal. From the unique shape and orientation of the TiSe2 islands in the low-coverage regime, a distinct nucleation scheme is proposed, in which the Se(3 x 1) superstructure on TiO2 provides pinning centers for the TMDC. This article aims at increasing the awareness that Mo and Se co-deposition does not automatically result in MoSe2 formation, but parasitic processes may trigger the growth of other TMDCs.