Layer and Growth Temperature Dependent Electronic Properties of VSe2 Thin Films*

TMDC provide a unique platform exhibiting ground states from CDW, to ferromagnetism and superconductivity. Among the TMDC family, bulk 1-T VSe2 is known as a typical 3D CDW material with a transition temperature of 105 K. But there have been two interesting observations in a few layers VSe2. First, while bulk VSe2 is paramagnetic, VSe2 a few layers limit was shown ferromagnetism. Second, multiple CDW patterns with different Q-vectors have been observed. In this work, we grow VSe2 thin films at two growth temperatures (Tg) by a home-built MBE and study the samples in a 4K STM. In the samples grown at low Tg, we found different 2D CDW patterns in the first and second layer. The high Tg films show stripe-type topographies in the first and second layer with different periodicity. The STM scans and RHEED images indicate that the second layer grown at high Tg is 1T’ phase VSe2. Based on this we conclude that two different Tg produce distinct polymorphs of VSe2. We further compare the dI/dV spectrums of the two polymorphs as a function of layer thickness and discuss the evolution of the electronic properties with thickness.