Controllable Synthesis and Charge Density Wave Phase Transitions of Two-Dimensional 1T-TaS Crystals.

1T-TaS has attracted much attention recently due to its abundant charge density wave phases. In this work, high-quality two-dimensional 1T-TaS crystals were successfully synthesized by a chemical vapor deposition method with controllable layer numbers, confirmed by the structural characterization. Based on the as-grown samples, their thickness-dependency nearly commensurate charge density wave/commensurate charge density wave phase transitions was revealed by the combination of the temperature-dependent resistance measurements and Raman spectra. The phase transition temperature increased with increasing thickness, but no apparent phase transition was found on the 2~3 nm thick crystals from temperature-dependent Raman spectra. The transition hysteresis loops due to temperature-dependent resistance changes of 1T-TaS can be used for memory devices and oscillators, making 1T-TaS a promising material for various electronic applications.