image: Rice University materials scientists replace all the atoms on top of a three-layer, two-dimensional crystal to make a transition-metal dichalcogenide with sulfur, molybdenum and selenium. view more
Credit: Jing Zhang/Rice University
HOUSTON -- (Aug. 14, 2017) -- Like a sandwich with wheat on the bottom and rye on the top, Rice University scientists have cooked up a tasty new twist on two-dimensional materials.
The Rice laboratory of materials scientist Jun Lou has made a semiconducting transition-metal dichalcogenide (TMD) that starts as a monolayer of molybdenum diselenide. They then strip the top layer of the lattice and replace precisely half the selenium atoms with sulfur.
The new material they call Janus sulfur molybdenum selenium (SMoSe) has a crystalline construction the researchers said can host an intrinsic electric field and that also shows promise for catalytic production of hydrogen.
The work is detailed this month in the American Chemical Society journal ACS Nano.
The two-faced material is technically two-dimensional, but like molybdenum diselenide it consists of three stacked layers of atoms arranged in a grid. From the top, they look like hexagonal rings a la graphene, but from any other angle, the grid is more like a nanoscale jungle gym.
Tight control of the conditions in a typical chemical vapor deposition furnace -- 800 degrees Celsius (1,872 degrees Fahrenheit) at atmospheric pressure -- allowed the sulfur to interact with only the top layer of selenium atoms and leave the bottom untouched, the researchers said. If the temperature drifts above 850, all the selenium is replaced.
"Like the intercalation of many other molecules demonstrated to have the ability to diffuse into the layered materials, diffusion of gaseous sulfur molecules in between the layers of these Van der Waals crystals, as well as the space between them and the substrates, requires sufficient driving force," said Rice postdoctoral researcher Jing Zhang, co-lead author of the paper with graduate student Shuai Jia. "And the driving force in our experiments is controlled by the reaction temperature."
Close examination showed the presence of sulfur gave the material a larger band gap than molybdenum diselenide, the researchers said.
"This type of two-faced structure has long been predicted theoretically but very rarely realized in the 2-D research community," Lou said. "The break of symmetry in the out-of-plane direction of 2-D TMDs could lead to many applications, such as a basal-plane active 2-D catalyst, robust piezoelectricity-enabled sensors and actuators at the 2-D limit."
He said preparation of the Janus material should be universal to layered materials with similar structures. "It will be quite interesting to look at the properties of the Janus configuration of other 2-D materials," Lou said.
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Co-authors of the paper are graduate students Weibing Chen and Zehua Jin and postdoctoral researcher Hua Guo of Rice; research scientist Iskandar Kholmanov and professor Li Shi, the Myron L. Begeman Fellow in Engineering at the University of Texas at Austin; and graduate students Liang Dong and Dequan Er and Vivek Shenoy, a professor of materials science and engineering, of mechanical engineering and applied mechanics and of bioengineering at the University of Pennsylvania. Lou is a professor of materials science and nanoengineering.
The Air Force Office of Scientific Research, the Welch Foundation, the Army Research Office and the National Science Foundation supported the research.
Read the abstract at http://pubs.acs.org/doi/abs/10.1021/acsnano.7b03186
DOI: 10.1021/acsnano.7b03186
This news release can be found online at http://news.rice.edu/2017/08/14/2-faced-2-d-material-is-a-first-at-rice/
Follow Rice News and Media Relations via Twitter @RiceUNews
Related materials:
Lou Group: http://n3lab.rice.edu
Shenoy Research Group: http://shenoy.seas.upenn.edu/index.html
Nanomaterials and Thermo-Fluids Laboratory (Shi Group): https://faculty.engr.utexas.edu/li-shi/li-shi/research-group-members
Rice Department of Materials Science and NanoEngineering: https://msne.rice.edu
Images for download:
http://news.rice.edu/files/2017/08/0814_JANUS-1-web-2aa39jl.jpg
Rice University materials scientists replace all the atoms on top of a three-layer, two-dimensional crystal to make a transition-metal dichalcogenide with sulfur, molybdenum and selenium. (Credit: Jing Zhang/Rice University)
http://news.rice.edu/files/2017/08/0814_JANUS-2-web-2liil0f.jpg
This image shows top (left) and side views of Janus sulfur molybdenum selenium created at Rice University. Careful control of heating allows sulfur to replace just the top plane of selenium atoms in the new two-dimensional material. (Credit: Jing Zhang/Rice University) Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,879 undergraduates and 2,861 graduate students, Rice's undergraduate student-to-faculty ratio is 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for quality of life and for lots of race/class interaction and No. 2 for happiest students by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger's Personal Finance. To read "What they're saying about Rice," go to http://tinyurl.com/RiceUniversityoverview.
Journal
ACS Nano