Feb 27 2018
In situ rhenium doping of monolayer MoS2. (Credit: Donna Deng/Penn State)
In two latest publications, teams of researchers led by Penn State provide new insight of why synthetic 2D materials repeatedly perform orders of magnitude worse than estimated, and how to enhance their performance in future electronics, memory storage, and photonics applications.
Two-dimensional materials can be defined as thin films measuring just an atom or two in thickness. Researchers make 2D materials using the exfoliation technique, which means peeling a portion of material off a bigger bulk material, or by condensing a gas precursor onto a substrate. The former technique provides a superior quality material but is not beneficial for making devices. The second technique is proven in industrial applications but helps produce low-performance 2D films.
In a paper published online in Scientific Reports, a Nature Group publication, scientists for the first time showed why the quality of 2D materials grown by the chemical vapor deposition technique exhibit poor performance compared to their theoretical predictions.
We grew molybdenum disulfide, a very promising 2D material, on a sapphire substrate. Sapphire itself is aluminum oxide. When the aluminum is the top layer of the substrate, it likes to give up its electrons to the film. This heavy negative doping (electrons have negative charge) limits both the intensity and carrier lifetime for photoluminescence, two important properties for all optoelectronic applications, such as photovoltaics and photosensors.
Kehao Zhang, Ph.D. Candidate, Penn State
Once they established that the aluminum was giving up electrons to the film, they employed a sapphire substrate that was cut in such a way as to expose the oxygen instead of the aluminum on the surface. This improved the photoluminescence intensity and the carrier lifetime by 100 times.
In an associated paper, published online recently in Advanced Functional Materials, a second team of scientists led by the same Penn State group used doping engineering that substitutes foreign atoms into the crystal lattice of the film so as to alter or improve the properties of the material.
People have tried substitution doping before, but because the interaction of the sapphire substrate screened the effects of the doping, they couldn’t deconvolute the impact of the doping.
Kehao Zhang, Ph.D. Candidate, Penn State
Taking the oxygen-terminated substrate surface from the first paper, the researchers removed the screening effect from the substrate and doped the molybdenum disulfide 2D film with rhenium atoms.
We deconvoluted the rhenium doping effects on the material. With this substrate we can go as high as 1 atomic percent, the highest doping concentration ever reported. An unexpected benefit is that doping the rhenium into the lattice passivates 25 percent of the sulfur vacancies, and sulfur vacancies are a long-standing problem with 2D materials.
Kehao Zhang, Ph.D. Candidate, Penn State
The doping solves two issues: It makes the material more conductive for applications like sensors and transistors, and at the same time enhances the quality of the materials by passivating the defects known as sulfur vacancies. The team forecasts that higher rhenium doping could totally get rid of the effects of sulfur vacancies.
The goal of my entire work is to push this material to technologically relevant levels, which means making it industrially applicable.
Kehao Zhang, Ph.D. Candidate, Penn State
Contributors to the Scientific Reports paper, “Deconvoluting the Photonic and Electronic Response of 2D Materials: The Case of MoS2,” are Zhang, Brian Bersch, Ganesh Bhimanapati, Baoming Wang, Ke Wang, Michael Labella, Teague Williams, Amanul Haque and Joshua Robinson, all of Penn State; Nicholas Borys, Edward Barnard and P. James Schuck, The Molecular Foundry, Lawrence Berkeley National Laboratory; and Ke Xu and Susan Fullerton-Shirey, University of Pittsburgh.
Contributors to the Advanced Functional Materials paper, “Tuning the Electronic and Photonic Properties of Monolayer MoS2 via In Situ Resubstitutional Doping,” are K. Zhang, B. Bersch, Natalie Briggs, Shruti Subamania and J.A. Robinson, Penn State; Rafik Addou, Christopher Cormier, Chenxi Zhang, Kyeongjae Cho and Robert Wallace, University of Texas at Dallas; Jaydeep Joshi and Patrick Vora, George Mason University; and K. Xu, Ke Wang and S. Fullerton-Shirey, University of Pittsburgh.
Joshua Robinson is Associate Director, Center for 2-Dimensional and Layered Materials (2DLM) and Co-Director, NSF-I/UCRC Center for Atomically Thin Multifunctional Coatings (ATOMIC), both at Penn State.
Portions of this research were supported by the National Science Foundation, The Semiconductor Research Corporation, DARPA, the U.S. Department of Energy, the Nanoelectronics Research Initiative and NIST.