Discovering Novel Properties in Few-Layer MXenes Using Analytical, In-Situ Scanning Transmission Electron Microscopy

University Of Illinois At Chicago, Chicago IL

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Materials known as MXenes are part of a growing family of two-dimensional structures that have attracted significant research interests due to their wide range of properties. These layered transition metal carbides and nitrides have the chemical formula of Mn+1XnTx (n=1-3), where M is a transition metal, X is either carbon or nitrogen, and Tx is either O, OH, F, Cl, or other surface groups that are bonded to the transition metal of the two-dimensional MXene layer. This project focuses on developing an atomic-scale understanding of the optical properties of MXenes, including their tunable plasmon resonances in the visible and near-infrared regime, which can be used for applications in nano-photonics or plasmonics. It is the PI’s hypothesis that different functional groups, Tx, control not only the layer-to-layer spacings of the two-dimensional MXene layers, but also provide the ability to tune the interactions between MXene layers as well as the density of free charges in each layer. Ti3C2 and Ti2C-based MXenes with a single type of surface termination will be used to study the atomic, electronic and plasmonic structures high-resolution electron microscopy and spectroscopy. Access to unique instrumentation at University of Illinois - Chicago (UIC) will be leveraged to study these materials over a wide range of temperatures and with unprecedented spatial and energy resolution. The project’s research activities center around hands-on research and learning experiences for science and engineering undergraduate and graduate students at the UIC, a Research-1 Hispanic-serving institution. Recruitment and training focuses on the next generation of researchers, in particular involving students from underrepresented groups and minorities through a student exchange program with Tecnologico de Monterrey, Latin@s Gaining Access to Networks for Advancement in Science and the DuSable Scholars programs. The participation of undergraduate students in active research projects is fostered through the PI’s Journal of Undergraduate Research at the University of Illinois - Chicago. Graduates of this program will contribute to academia, National Laboratories or industry, including semiconductor companies. TECHNICAL DESCRIPTION: The objectives of this research project are to develop an atomic-scale understanding of the nano-photonic and plasmonic properties of Ti3C2Tx and Ti2CTx MXenes, functionalized with a single, well-controlled species, using atomic-resolution scanning transmission electron microscopy (STEM) imaging, conventional and monochromated electron energy loss spectroscopy (EELS) combined with in-situ cooling to liquid nitrogen temperatures. Twisted bi-layer MXenes exhibiting Moiré lattice effects will be studied for the emergence of hybridized plasmons or excitons, as well as other emerging, potentially strongly correlated phenomena. Using autonomous anomaly detection in multi-dimensional data cubes via machine learning (ML), the correlation between structural defects and their impact on the plasmonic and photonic properties will be studied. This research project will consist of two tasks, i) atomic-resolution analysis of MXenes and ii) development of convolutional neural networks to identify crystal defects in atomic-resolution STEM images and identify the correlated changes in the low-loss or valence EELS. The insights from both tasks will be combined, yielding the ability to measure and control the optical or electronic, and possibly even magnetic properties of single-functionalized MXenes. The integration of research and education through the training of undergraduate and graduate students in state-of-the-art in-situ scanning transmission electron microscopy and state-of-the-art ML approaches is an integral feature of this project. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.