Could adding defects make a good material even better? Scientists have found that linear defects in a promising thin film create one-atom-thick metallic wires. These wires cross the otherwise intact material, offering a way to channel electrons and photons, tiny packets of light. A multidisciplinary team made this discovery using resources at the Molecular Foundry and the Advanced Light Source.
The team worked with transition metal dichalcogenides (TMDs) because the materials have exceptional optical characteristics. This research found that a single TMD layer could emit as much light as an equivalent material that is 10,000 times thicker, paving the way toward smaller, more efficient devices. Further, the team found that engineering defects (purposely introducing missing or displaced atoms) into TMDs could modify their intrinsic properties. These modifications might improve the material or lead to altogether new useful properties for future energy conversion, quantum computing and communication systems.
In the world of semiconductors, impurities and defects can be a good thing. They modify the properties of materials such as silicon, and scientists can exploit these properties to develop better transistors for laptop computers, smart phones, and solar cells. Recently, scientists discovered a new class of semiconductor that is only three atoms thick and extends in a two-dimensional plane, similar to graphene. These two-dimensional semiconductors, called transition metal dichalcogenides (TMDs), have exceptional optical characteristics. They can be developed into ultra-sensitive photodetectors, and a single TMD layer emits as much light as a three-dimensional TMD crystal composed of 10,000 layers.
For the past several years, scientists have wondered if impurities and defects could also modify TMDs' intrinsic properties, perhaps in ways that improve the semiconductor or lead to new functionalities. Scientists at the Molecular Foundry, in collaboration with researchers at the Advanced Light Source, have taken a big step towards answering this question. They found—to their surprise—how substantial linear defects in TMDs create entirely new properties. Some of these properties indicate that defects in TMDs might even mediate superconducting states.
The team synthesized three-atom-thick, clean layers of molybdenum diselenide, which is a type of TMD. They then studied the material with a microscope that can visualize atoms and their electronic wave functions. They discovered a linear defect formed by a line of missing selenium atoms. This defect creates one-atom-thick metallic wires to transport electrons or photons across the otherwise intact semiconductor like veins.
Molecular Foundry, Lawrence Berkeley National Laboratory
Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under contract DE-AC02-05CH11231 (user proposal #3282) (scanning transmission microscopy (STM) imaging, STM spectroscopy, theoretical simulations, and analysis). A.W.B. and S.W. were supported by the DOE, Office of Science, Basic Energy Sciences, Scientific User Facilities Division Early Career Award. S.B. acknowledges fellowship support by the European Union under FP7-PEOPLE-2012-IOF-327581. Advanced Light Source and SIMES were supported by Office of Basic Energy Science, DOEnder contracts DE-AC02-05CH11231 and DE-AC02-76SF00515, respectively. H.R. acknowledges support from the Max Planck Korea/POSTECH Research Initiative of the National Research Foundation of Korea (NRF) under Project No. NRF-2011-0031558. M.B.S. was supported by the DOE, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering through the Chemical and Mechanical Properties of Surfaces and Interfaces Program. Portions of the computational work were done with National Energy Research Scientific Computing Center resources. M.F.C. acknowledges support from National Science Foundation grant EFMA-1542741 (sample surface preparation development).
S. Barja, S. Wickenburg, Z.F. Liu, Y. Zhang, H. Ryu, M.M. Ugeda, Z. Hussain, Z.X. Shen, S.K. Mo, E. Wong, M.B. Salmeron, F. Wang, M.F. Crommie, D.F. Ogletree, J.B. Neaton, and A. Weber-Bargioni, "Charge density wave order in 1D mirror twin boundaries of single-layer MoS2." Nature Physics 12, 751–756 (2016). [DOI: 10.1038/nphys3730]
Lawrence Berkeley National Lab news release: Berkeley Lab Scientists Discover Surprising New Properties in a 2-D Semiconductor