Temperature controlled 1T/2H phase ratio modulation in mono- and a few layered MoS2 films

Abstract

Along with hexagonal crystalline symmetry MoS2 (2H-MoS2), tetragonal structured MoS2 (1T-MoS2) possessing a peculiar optical and electrical properties shows attracting application in high-performance devices. On the other hand, instead of complicated and time-consuming techniques for the synthesis of 1T/2H mixed phase of MoS2, an efficient way to deposit mixed phase mono and a few layered MoS2 thin films for a very short span of time of 20 s has been realized by pulsed laser deposition. The 1T/2H phase ratio in monolayer to a few layered MoS2 films has been modulated by altering the substrate temperatures. Irrespective of deposition temperatures all the films showed a mixed phase structure while the 2H to 1T phase ratio increased from 66 to 84% with an increase in deposition temperature from 400 to 720 °C. The presence of 1T-2H mixed phase MoS2 layers was confirmed and their structural and optical properties were analyzed in nano (TEM), micro (Raman, AFM) and macro (ellipsometer) scale. The ellipsometer analysis demonstrated the A/B and C/D excitonic absorption peaks corresponding to 2H phase while the presence of 1T phase caused excess semimetal like charge density of the order of ~1020 cm−3. At low deposition temperatures, lack of sufficient energy to redistribute the impingement plasma particle to the most stable 2H-MoS2 phase led to a mixed phase of MoS2 film while sulfur deficiency induced defects and excess negative charges attributed to form 1T phase at the high deposition temperatures.