Two-dimensional transition metal dichalcogenides (2D-TMDCs) have exhibited potential wide range applications owing to their novel physical and chemical properties. Synthesis of high-quality TMDCs monolayers has remained a challenge to researchers, however. Microelectronic processes compatible technologies, such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), have been used to grow 2D-TMDCs. These methods particularly allow online diagnosis for more information about the film evolution and morphology during growth, even though it is not fully explored in literature. Unlike the reflection high energy electron diffraction (RHEED), online diagnosis using differential reflectance spectroscopy (DRS) and differential transmittance spectroscopy (DTS) techniques are generally preferred due to their non-destructive and non-invasive characteristics. DRS, for instance, is preferably used in the characterization of optical absorption of highly sensitive two-dimensional materials and exhibits great potential for both in situ real time and ex-situ applications.
In a recent paper published in the Journal of Physics: Condensed Matter, Yaxu Wei (PhD candidate), Associate Professor Dr. Chunguang Hu, Professor Dr. Yanning Li, and Professor Dr. Xiaotang Hu from Tianjin University in collaboration with Dr. Michael Honage and Associate Professor Lidong Sun from Johannes Kepler University Linz in Austria performed an in-situ DRS measurements during MBE of atomically thin layers of MoSe2 on mica. Their main aim was to investigate the real-time application of DRS in monitoring the synthesis and growth of TMDCs.
Results showed that during layer-by-layer growth, the authors observed an oscillation of the DRS signal. This oscillation was characterized by monolayer periodicity comprising of two-dimensional MoSe2 thin films. The edges of the TMDCs monolayers exhibited modified electronic structures whose formation was attributed to the high sensitivity of the DRS to step density. The correlation between the DRS signal and step density was successfully validated using a combination of in situ optical spectroscopy and ex-situ atomic force microscopy.
The research team noted that DRS works well in any transparent ambient. As such, the variation of the DRS signal is very useful in the detailed investigation of the morphology and evolution of the optical properties of the atomically thin MoSe2 across all the stages of growth. Besides, the growth of MoSe2 on mica was represented by a quasi-layer-by-layer mode where the new layer starts to form even without completion of the preceding one. In this way, the resulting DRS signal constitutes the sum of the step density of the topmost two layers. Even though the exact oscillation shape may vary, the correlation between the DRS signal and morphology should apply for other TMDCs thin films.
In summary, the study investigates the growth oscillation of MoSe2 monolayers by DRS. The work demonstrates useful insights about the capability of the DRS and especially its ability to work in any transparent ambient. To this note, the research team is hopeful that the study would pave way for precise realization of the controlled growth of TMDCs monolayers using both MBE and CVD techniques.
Wei, Y., Hu, C., Li, Y., Hu, X., Hohage, M., & Sun, L. (2020). Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy. Journal of Physics: Condensed Matter, 32(15), 155001.