Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy

Significance 

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.

Growth oscillation of MoSe2 monolayers observed by differential reflectance spectroscopy - Advances in Engineering

About the author

Lidong Sun received his Ph.D. degree in Technical Physics from Johannes Kepler University Linz, Austria, in 2002. He is currently an Assoc. Professor of Institute of Experimental Physics, Johannes Kepler University Linz, Austria. He has more than 40 contributions to international conferences and more than 60 peer reviewed publications.

Research topics
Characterization of solid surfaces and thin films using optical probes, low and variable temperature scanning tunneling microscopy (STM) and fluorescence microscopy/laser scanning microscopy; physisorption and chemisorption at surfaces; structure and dynamics of adsorbed layers; phase transitions in low-dimensional systems; structural and orientational ordering of adsorbed molecules; fabrication and characterization of nanostructured surfaces; MBE and CVD growth of 2D semiconductors.

About the author

Yanning Li received his Ph.D. Degree in 1997 from Tianjin University, China, majored in Precision Measurement Technology and Instrumentation. From 1998 to 2000, he worked as an STA fellow in National Institute of Advanced Industrial Science and Technology-AIST, Japan. From 2000 to 2001, he worked as a postdoctoral fellow in Institute of Experimental Physics, Johannes Kepler University Linz, Austria. In 2001, he became an associate professor in School of Precision Instruments and Opto-electronics Engineering, Tianjin University, and in 2009 he got professorship. He has more than 30 author/co-authored publications and more than 10 patents.

Research interests
MBE growth of 2D materials&characterizations; Micro-nano measurement technology & instrumentation; scanning probe microscopy& instrumentation; Modern analytical instrumentation; Thermal conductivity Measurement Technology & Instrumentation.

About the author

Yaxu Wei received his B.S. degree in Measurement and Control Technology and Instrument from Zhengzhou University, China, in 2012. He continued his studies as a PhD candidate in School of Precision Instruments and Opto-electronics Engineering, Tianjin University, China, majored in Instrumentation Science and Technology. From Sep. 2017 to Feb. 2018 and from Mar. 2019 to Feb. 2020, he worked as a visiting PhD student in Institute of Experimental Physics, Johannes Kepler University Linz, Austria. He has 8 author/co-authored publications.

Research interests:
MBE/CVD growth of 2D materials & characterizations; Micro-nano measurement technology & instrumentation; Characterization of thin films using optical probes.

About the author

Xiaotang Hu received his Ph.D. Degree in 1986 from Tianjin University, China, majored in Precision Measuring Technologies and Instrumentation. From 1988 to 1990, he worked as a postdoctoral fellow in National Institute of Standards and Technology (NIST), USA. In 1990, he became an associate professor in School of Precision Instruments and Opto-electronics Engineering, Tianjin University, and in 1993 he got professorship. He has more than 200 author/co-authored publications and more than 20 patents.

Research interests:
Micro-nano measurement technology & instrumentation; scanning probe microscopy & instrumentation.

Reference

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.

Go To Journal of Physics: Condensed Matter

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