Atomic Structure of Organic-Metallic MXenes Revealed by Cryo-STEM and EELS

MXenes (M_n+1X_nT_<i>x</i> with n=1,2) are a large family of two-dimensional transition metal carbides and nitrides [1]. What distinguishes MXenes from other 2D materials is that they can be functionalized with a variety of T_x surface groups. In 2020, <i>Kamysbayev et al</i>. demonstrated that these surface groups can drastically alter the MXenes’ properties [2]. For example, they demonstrated a surface group dependent superconducting behavior in Nb₂CT_x. In this contribution we will continue to explore the range of possible surface group terminations by studying a new family of MXenes functionalized with organic surface groups. These metallic-organic hybrid MXenes open the door to the rich and complex space of organic chemistry and combine the structural and electron properties of metallic material. Our analysis will focus on developing an atomic-scale understanding of structure-property relationships in these novel materials.<br/>This study will be conducted using atomic-resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and x-ray energy dispersive spectroscopy (XEDS). Using these tools in tandem we will gain insights into the local chemical composition and bonding environment with high spatial resolution.<br/>The focus of our study will be to examine Ti₃C₂terminated with hydrocarbon chains and aromatic benzene rings. Since organic material is well known to decompose at the electron dosages required for high resolution studies, we will utilize in-situ cooling to liquid nitrogen temperatures to stabilize the organic material. Our analysis will show that cooling sufficiently preserves the organic material so that the unique π and σ bonding for the different amines can be observed in C <i>K</i>-edge spectra. Additionally, we will demonstrate that atomic resolution images of the material can be acquired while preserving the delicate structure of hybrid MXenes. Our in-situ cooling study will also show dynamic processes that occur in hybrid MXenes as the result of interactions with the electron beam. We will show that highly localized defects can be introduced using the electron probe and that the defects lead to a dynamic reorganization of the MXenes layers. [3]<br/><b>References</b><br/>[1] Gogotsi, Yury, and Babak Anasori. "The rise of MXenes." (2019): 8491-8494.<br/>[2] Kamysbayev, Vladislav, et al. "Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes." <i>Science</i> 369.6506 (2020): 979-983.<br/>[3] This project is supported by a supported by the National Science Foundation (DMR-1831406) and made use of instruments in the Electron Microscopy Service at the UIC Research Resources Center. The acquisition of UIC JEOL JEM ARM200CF is supported by an MRI-R² grant from the National Science Foundation (Grant No. DMR-0959470) and the upgraded Gatan Continuum spectrometer was supported by a grant from the NSF (DMR-1626065).