CVD of MoS2 single layer flakes using Na2MoO4 - impact of oxygen and temperature-time-profile.

Two-dimensional (2D) materials are of great interest in many fields due to their astonishing properties at an atomic level thickness. Many fundamentally different methods to synthesize 2D materials, such as exfoliation or chemical vapor deposition (CVD), have been reported. Despite great efforts and progress to investigate and improve each synthesis method, mainly to increase the yield and quality of the synthesized 2D materials, most approaches still involve some compromise. Herein, we systematically investigate a chemical vapor deposition (CVD) process to synthesize molybdenum disulfide (MoS2) single layer flakes using sodium molybdate (Na2MoO4), deposited on a silica (SiO2/Si) substrate by spin-coating its aqueous solution, as the molybdenum source and sulfur powder as sulfur source, respectively. The focus lies on the impact of oxygen (O2) in the gas flow and temperature-time-profile on reaction process and product quality. Atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to investigate MoS2 flakes synthesized under different exposure times of O2 and with various temperature-time-profiles. This detailed study shows that the MoS2 flakes are formed within the first few minutes of synthesis and elaborates on the necessity of O2 in the gas flow, as well as drawbacks of its presence. In addition, the applied temperature-time-profile highly affects the ability to detach MoS2 flakes from the growth substrate when immersed in water, but it has no impact on the flake.

Local strain and tunneling current modulate excitonic luminescence in MoS2 monolayers.

The excitonic luminescence of monolayer molybdenum disulfide (MoS2) on a gold substrate is studied by scanning tunneling microscopy (STM). STM-induced light emission (STM-LE) from MoS2 is assigned to the radiative decay of A and B excitons. The intensity ratio of A and B exciton emission is modulated by the tunneling current, since the A exciton emission intensity saturates at high tunneling currents. Moreover, the corrugated gold substrate introduces local strain to the monolayer MoS2, resulting in significant changes of electronic bandgap and valence band splitting. The modulation rate of strain on A exciton energy is estimated as -69 ± 5 meV/%. STM-LE provides a direct link between exciton energy and local strain in monolayer MoS2 on a length scale of 10 nm.