David Geohegan1 Alexander Puretzky1 Kai Xiao1 Kai Wang1 Mina Yoon1 Xiaoming Liu1 Raymond Unocic1 Bernadeta Srijanto1 Juan Carlos Idrobo1 Christopher Rouleau1 Akinola Oyedele1 Liangbo Liang1 Bobby Sumpter1 Gyula Eres2 Mengkun Tian3 Gerd Duscher3 Feng Ding4 Henry Yu5 Nitant Gupta5 Boris Yakobson5 Masoud Mahjouri-Samani6 Xufan Li7 Abdelaziz Boulesbaa8

1, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, University of Tennessee, Knoxville, Knoxville, Tennessee, United States
4, 4) Ulsan National Institute of Science and Technology (UNIST), Ulsan, , Korea (the Republic of)
5, Rice University, Houston, Texas, United States
6, Auburn University, Auburn, Alabama, United States
7, Honda Research Institute, Columbus, Ohio, United States
8, California State University, Northridge, Northridge, California, United States

Two-dimensional (2D) layered materials have emerged as potential platforms for novel electronic and optical devices, especially the semiconducting 2D transition metal dichalcogenides (TMDs) and their heterostructures. However, significant synthesis and processing challenges will be reviewed that limit their development, including wafer-scale, bottom-up synthesis of uniform layers of crystalline 2D materials that are comparable in quality to exfoliated flakes of bulk materials. Few-layered, as-synthesized crystals of 2D TMDs display remarkable heterogeneity on both the atomistic level, including vacancies, dopants, and edge terminations, and on the mesoscopic length scale involving misoriented grains, layer orientations, and interactions with substrates and adsorbates. This heterogeneity can strongly influence the structure and electronic properties in 2D materials, offering at the same time a serious challenge to synthesis control for reliable properties and a tremendous opportunity to tailor functionality.
Here we will present recent developments in both vapor-transport and laser-based synthesis and processing approaches for the synthesis of a variety of atomically-thin 2D crystals (e.g., MoSe2, WS2, Mo[1-x]WxSe2, GaSe) and understanding of mechanisms for the introduction of heterogeneity during growth, including the incorporation of defects and dopants, the role of topology-induced strain, and the orientation of layers during heterostructure growth by van der Waals epitaxy (e.g., GaSe/MoSe2). To provide rapid assessment of the effects of synthesis on the properties of 2D materials used for optoelectronic applications, laser spectroscopy-based characterization methods such as low-frequency Raman spectroscopy, low-temperature photoluminescence, and ultrafast pump-probe spectroscopy techniques are being developed to reveal different aspects relating synthesis and function, such as atomistic stacking configurations between layers, band gap shifts due to doping, the nature of defects, and quasiparticle dynamics. The prospects for use of these spectroscopic techniques as remote, real-time diagnostics will be discussed. These measurements are correlated with atomic-resolution electron microscopy to understand the exact nature, concentration, and density of defects as well as the evolution of preferred edges associated with changing crystal shapes during growth and etching. Associated theory and modeling is used to infer the responsible synthetic driving forces. Transport measurements with prototype devices are correlated to understand the effects on functionality.
Research sponsored by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science). Throughout the presentation, facilities available for collaboration at the Center for Nanophase Materials Sciences (CNMS) user facility will be presented.