Browsing by Author "May, Steven J."

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    Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes
    (IOP, 2020) Yang, Yizhou; Hantanasirisakul, Kanit; Frey, Nathan C.; Anasori, Babak; Green, Robert J.; Rogge, Paul C.; Waluyo, Iradwikanari; Hunt, Adrian; Shafer, Padraic; Arenholz, Elke; Shenoy, Vivek B.; Gogotsi, Yury; May, Steven J.; Mechanical and Energy Engineering, School of Engineering and Technology
    MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts.
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    Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene
    (Royal Society of Chemistry, 2020-12) Hantanasirisakul, Kanit; Anasori, Babak; Nemsak, Slavomir; Hart, James L.; Wu, Jiabin; Yang, Yizhou; Chopdekar, Rajesh V.; Shafer, Padraic; May, Andrew F.; Moon, Eun Ju; Zhou, Jun; Zhang, Qinghua; Taheri, Mitra L.; May, Steven J.; Gogotsi, Yury; Mechanical and Energy Engineering, School of Engineering and Technology
    Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr2TiC2Tx MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr2TiC2Tx at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr2TiAlC2 MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.