Behind the Study: An Optical Nondestructive Method to Evaluate Defects in 2D Systems
Journal: Science Advances
I am working in a research field related to semiconducting materials that are atomically thin. In the Terrones’ group at Penn State, a core research focus is to control the synthesis and to understand the properties of an emerging family of two-dimensional materials called transition metal dichalcogenides. The projects we are interested in sometimes lie on the borders between physics, chemistry, and materials science. As a graduate student, working in such an interdisciplinary field is actually quite exciting and rewarding, because I can very often interact with researchers with various expertise and learn something unexpected from their different perspectives.
Tungsten disulfide monolayers, a member of transition metal dichalcogenides, exhibit exotic optical properties: at room temperature, the photoluminescence emissions from these monolayer crystal domains are spatially inhomogeneous. The group has been trying to figure out the origin of this heterogeneity, but despite many attempts, a clear picture has not been drawn. In 2014, a post-doc scholar, Victor Carozo, the leading author of the work later published in Science Advances joined the research group. After discussions with Prof. Terrones and other lab members, he was immediately attracted to the topic and decided to carry out an optical spectroscopic study. The key of Victor’s plan was to tackle the problem by doing measurements at cryogenic temperature, as his physics background led him to believe that when a condensed matter system is sufficiently cooled to near its thermodynamic ground state, its dynamics becomes simplified. Victor’s idea differed from that of chemists working in the same group, who often preferred to carry out experiments at room temperature -- a temperature of tremendous relevance to daily applications. Very soon, Victor discovered at a temperature as low as 77 Kelvin, a new peak emerged in the photoluminescence spectrum acquired only at the edges, but not the interior regions of the tungsten disulfide crystal flakes. This finding was quite exciting, since spatial heterogeneity was previously thought to be a feature observable only at room temperature!
The authors had several speculations on the origin of the peak, but a smoking-gun evidence was yet lacking. After lots of low temperature optical measurements, the authors reached a bottleneck: optical spectroscopy itself could not lead to a solid conclusion. A connection between the spectroscopic feature and the atomic structure of the materials was certainly needed. Prof. Terrones suggested in a group meeting that “we should try the Titan.” The “Titan” here is a nickname of a state-of-the-art electron microscope at Penn State’s Materials Research Institute. The name is given because of its ~3 meter height, and its “godlike” capability to looking into the atomic world, which is too tiny to be visible for human eyes as well as optical microscopes. The operation of Titan is extremely complicated, and preparing an electron microscopy sample that is free of any surface contamination is also a nontrivial job. Luckily, Kazunori Fujisawa, a research associate in the Terrones group and an electron microscopy expert, followed the suggestion and joined the project. Chanjing Zhou, a female grad student with great patience, managed to prepare a super clean sample for imaging. After weeks of hard work, electron microscopy images with a resolution down to a few angstroms were obtained. Remarkably, the images revealed that the edges of triangular monolayers have a higher concentration of synthetic defects than the central regions of triangles. These defects, being mostly missing sulfur atoms from the lattice, are atomically small, but are primarily responsible for the spectrum features.
Having established a connection between optical properties and defect concentration in the materials, the project team was further encouraged by Prof. Terrones to go beyond an experimental observation and to seek a theoretical explanation. Yuanxi Wang, a grad student (now a postdoc scholar) in Prof. Vincent Crespi’s group performed first principles calculations, worked out the detailed electronic band structures of defective samples, and explained the rules and dynamics of optical transitions theoretically.
The last step of the project was like piecing a jigsaw puzzle: optical spectroscopic data, electron microscopy images, and simulations were finally assembled. Several other co-authors, whose names are not explicitly cited here, also made their contributions to the work.
I am very excited to be involved in this work, and I believe more researchers will be attracted to similar topics. The story behind this publication shows a clear example of how intra-group and inter- group collaborations can combine expertise of individual researchers. Looking forward, fully understanding and engineering structural defects in atomically thin materials is still a long-term goal of the field, and certainly requires years and even decades of continuous efforts from various disciplines.