Direct growth of NbSe2 and TiSe2 thin flakes via deposited film selenization

We discuss the direct growth of thin flakes of NbSe2 and TiSe2 on c-plane sapphire substrate via the selenization of deposited Nb and Ti films, respectively. The influence of the cooling rate on the shapes of the flakes, such as hexagons and triangles, are studied for NbSe2 thin flakes. In particular, rapid cooling after the selenization of deposited Nb film found to be an effective process to grow triangular-shaped NbSe2 flakes. Raman spectroscopy on one sample prepared with rapid cooling process detected an unidentified vibration peak in addition to the prominent characteristic peaks of 2H-NbSe2. The selenization of Ti film can be only achieved with the use of sodium chloride (NaCl) agent, resulting in the growth of TiSe2 thin flakes in half-hexagonal shape without the incorporation of residual materials. Temperature-dependent resistances measurements, including the temperature of maximum resistance and the Rmax/R293K ratio, indicated conventional charge density wave transition in the TiSe2 thin flakes.