The goal of this research is to improve the understanding of the formation, stability, and chemical reactivity of the nano-size holes in single sheet boron nitride (BN) for use as a filter and biodetector; and to study the effects of those holes on the electrical and mechanical properties of the entire BN sheet. Using atomic-resolution electron microscopy, new insight into the atomic structure of the holes and defects in single-atom-thick BN sheets will be provided. The project will improve the understanding of the changes in current-conduction in these BN sheets in the presence of the holes and defects, allowing evaluation of their true potential for next generation electronics applications. This research will also be instrumental in determining the possibilities of using single BN sheets for filtration and biodetection applications. In addition to training graduate students, the education component includes improving and expanding the new courses on Electron Microscopy and Computational Nanomechanics, providing summer research opportunities for undergraduate students, and participating in outreach programs with local high school students.
TECHNICAL DETAILS: A systematic study the formation, mechanical stability and chemical reactivity of the nano-scale holes in the single sheet h-BN will be undertaken by combining several experimental imaging and spectroscopy techniques in scanning transmission electron microscope (STEM) with theoretical predications based on calculations from first principles. Unlike conventional TEM imaging, the STEM probe electrons are scattered from the electrostatic potential of the atoms in the sample and recorded by a single-electron-sensitive annular dark field detector. This provides atomic-resolution images of the BN with direct and unambiguous atomic location identification. The spectroscopic analysis will be measured by electronic excitations inside single atoms of boron and nitrogen using electron energy loss spectroscopy (EELS). Theoretical predictions for the local density-of-states calculated for both atoms will be compared with EELS results; later, mechanical properties will be evaluated.
Transmission electron microscopy (TEM) is a powerful technique for imaging the spatial and electronic structure of defects at atomic resolution, thus serving as an ideal means to characterize defects in hexagonal boron nitride (h-BN) nanosheets. Various solution-based methods for preparing h-BN nanosheet samples were examined; sonication of bulk h-BN powder in isopropanol, followed by centrifugation, drop-casting onto an amorphous carbon support, and baking under high vacuum was found to reliably produce relatively pristine sheets with large thin areas less than 10 nm thick. These areas could be further thinned down to a few atomic layers by sputtering under the TEM electron beam; vacancies and large holes could also be consistently produced in the nanosheets by the same means. Experiments were performed examining the hole formation process in h-BN nanosheets, the electronic structure of hole edges in h-BN nanosheets, and the displacement damage of thick regions in h-BN nanosheets, however rapid sputtering of the atoms and formation of contamination spots under the TEM beam appears challenging and more work is underway. However, a computational paper was published comprehensively examining imaging- and diffraction-based techniques for determining the thickness of few-atomic-layer h-BN nanosheets. Additionally, fundamental computational studies of TEM electron-specimen interactions were pursued using the skills learned in the study of h-BN nanosheets. Using DFT calculation data, the effect of atomic bonding on scanning TEM (STEM) imaging of light element crystals was explored, showing that interatomic bonds with a significant net charge transfer between bonded atoms altered the image contrast for any mode of STEM imaging. Studies of STEM beam channeling in complex oxides were also performed, revealing unusual beam oscillation modes and hinting at the possibility of exploiting those modes to selectively excite different electronic orbitals in crystals. We also engaged experiments and objective molecular dynamics simulations in order to gain a fundamental understanding of the defects and mechanics of BN nanoribbons and nanosheets. We often found that the atoms disposed in parallel layers already display an intuitive mechanics, which resembles the ones of macroscopic objects. When considering the pure bending of layered structures, two contrasting mechanisms are conceivable: platelike bending, where the layers bend collectively and do not slide relative to each other, or a mechanism where layer sliding occurs. One can identify these mechanisms by simply bending a stack of paper. To answer which mechanism is applicable when bending BN nanoribbons, we performed direct bending experiments and simulations. We observed that the nanoribbons tend to bend reversibly by forming a localized kink, caused by the wrinkling of individual layers. Simulations reveal the atomistic details of bending, which is smooth and platelike at low curvatures, followed by kinking, the morphology and mechanics of which are in agreement with our experiments. Only in the platelike bending would the compression of the inner layers use them to wrinkle and a kink to arise. Thus, the occurrence of kinking itself, as well as the layer wrinkling are indications that bending commences in a platelike manner. Such fundamental findings are far-reaching as they have implications for future study of nanoscale-layered materials, including nanomechanical device design. Numerous research articles have been published in peer-reviewed journals. Two courses were developed and overhauled for the University of Minnesota undergraduate and graduate students on Electron Microscopy and Computational Nanomechanics. Two students (one female) received Ph.D. degrees, one student received M.S. and one graduate student will complete his Ph.D. degree in summer of 2015, all based on this grant. This project also enabled seven undergraduate students to gain research experience. The PIs have participated in the outreach activities of the UMN MRSEC center and interacted with local schools.
