Initial stage of MBE growth of MoSe2 monolayer.

An atomically thin MoSe2 layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe2 layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe2 forms in addition to a comparable quantity of 2H-MoSe2. The metastable 1T-MoSe2 transfers gradually to the stable 2H-MoSe2 before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy.

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe2 layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe2 thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

Direct observation of the CVD growth of monolayer MoS2 using in situ optical spectroscopy.

Real-time monitoring is essential for understanding and precisely controlling of growth of two-dimensional transition metal dichalcogenide (2D TMDC) materials. However, it is very challenging to carry out such studies during chemical vapor deposition (CVD). Here, we report the first, real time, in situ study of the CVD growth of 2D TMDCs. More specifically, the CVD growth of a molybdenum disulfide (MoS2) monolayer on sapphire substrates has been monitored in situ using differential transmittance spectroscopy (DTS). The growth of the MoS2 monolayer can be precisely followed by observation of the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of the MoS2 monolayer and the temperature distribution in the CVD reactor has been revealed. Our results demonstrate the great potential of real time, in situ optical spectroscopy to assist the precisely controlled growth of 2D semiconductor materials.

Resolving the optical anisotropy of low-symmetry 2D materials.

Optical anisotropy is one of the most fundamental physical characteristics of emerging low-symmetry two-dimensional (2D) materials. It provides abundant structural information and is crucial for creating diverse nanoscale devices. Here, we have proposed an azimuth-resolved microscopic approach to directly resolve the normalized optical difference along two orthogonal directions at normal incidence. The differential principle ensures that the approach is only sensitive to anisotropic samples and immune to isotropic materials. We studied the optical anisotropy of bare and encapsulated black phosphorus (BP) and unveiled the interference effect on optical anisotropy, which is critical for practical applications in optical and optoelectronic devices. A multi-phase model based on the scattering matrix method was developed to account for the interference effect and then the crystallographic directions were unambiguously determined. Our result also suggests that the optical anisotropy is a probe to measure the thickness with monolayer resolution. Furthermore, the optical anisotropy of rhenium disulfide (ReS2), another class of anisotropic 2D materials, with a 1T distorted crystal structure, was investigated, which demonstrates that our approach is suitable for other anisotropic 2D materials. This technique is ideal for optical anisotropy characterization and will inspire future efforts in BP and related anisotropic 2D nanomaterials for engineering new conceptual nanodevices.

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy.

Real-time monitoring of the growth is essential for synthesizing high quality two dimensional (2D) transition-metal dichalcogenides with precisely controlled thickness. Here, we report the first real time in situ optical spectroscopic study on the molecular beam epitaxy of atomically thin molybdenum diselenide (MoSe2) films on sapphire substrates using differential reflectance spectroscopy. The characteristic optical spectrum of MoSe2 monolayer is clearly distinct from that of bilayer allowing a precise control of the film thickness during the growth. Furthermore, the evolution of the characteristic differential reflectance spectrum of the MoSe2 thin film as a function of the thickness sheds light on the details of the growth process. Our result demonstrates the importance and the great potential of the real time in situ optical spectroscopy for the realization of controlled growth of 2D semiconductor materials.