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1. The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air

   https://doi.org/10.1002/smtd.202300568  

   Shamsabadi, Ahmad A. ; Fang, Hui ; Zhang, Danzhen ; Thakur, Anupma ; Chen, Cindy Y. ; Zhang, Aixi ; Wang, Haonan ; Anasori, Babak ; Soroush, Masoud ; Gogotsi, Yury ; et al ( July 2023 , Small Methods)

   MXenes, a family of 2D transition‐metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes’ thermal stability can enable deeper insights into their structure–property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti₃C₂T_x, Mo₂TiC₂T_x, and Ti₂CT_x) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near‐IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti₃C₂T_xand Ti₂CT_x, respectively, have the highest and lowest thermal stability, indicating the role of transition‐metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure–property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.

    

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2. Highly Conductive Optical Quality Solution‐Processed Films of 2D Titanium Carbide

   https://doi.org/10.1002/adfm.201600357  

   Dillon, Andrew D. ; Ghidiu, Michael J. ; Krick, Alex L. ; Griggs, Justin ; May, Steven J. ; Gogotsi, Yury ; Barsoum, Michel W. ; Fafarman, Aaron T. ( April 2016 , Advanced Functional Materials)

   MXenes comprise a new class of solution‐dispersable, 2D nanomaterials formed from transition metal carbides and nitrides such as Ti₃C₂. Here, it is shown that 2D Ti₃C₂can be assembled from aqueous solutions into optical quality, nanometer thin films that, at 6500 S cm⁻¹, surpass the conductivity of other solution‐processed 2D materials, while simultaneously transmitting >97% of visible light per‐nanometer thickness. It is shown that this high conductivity is due to a metal‐like free‐electron density as well as a high degree of coplanar alignment of individual nanosheets achieved through spincasting. Consequently, the spincast films exhibit conductivity over a macroscopic scale that is comparable to the intrinsic conductivity of the constituent 2D sheets. Additionally, optical characterization over the ultraviolet‐to‐near‐infrared range reveals the onset of free‐electron plasma oscillations above 1130 nm. Ti₃C₂is therefore a potential building block for plasmonic applications at near‐infrared wavelengths and constitutes the first example of a new class of solution‐processed, carbide‐based 2D optoelectronic materials.

    

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3. TiN–Fe Vertically Aligned Nanocomposites Integrated on Silicon as a Multifunctional Platform toward Device Applications

   https://doi.org/10.3390/cryst12060849  

   Kalaswad, Matias ; Zhang, Di ; Rutherford, Bethany X. ; Lu, Juanjuan ; Barnard, James P. ; He, Zihao ; Liu, Juncheng ; Wang, Haohan ; Xu, Xiaoshan ; Wang, Haiyan ( June 2022 , Crystals)

   Transition metal nitrides such as titanium nitride (TiN) possess exceptional mechanical-, chemical-, and thermal-stability and have been utilized in a wide variety of applications ranging from super-hard, corrosion-resistive, and decorative coatings to nanoscale diffusion barriers in semiconductor devices. Despite the ongoing interest in these robust materials, there have been limited reports focused on engineering high-aspect ratio TiN-based nanocomposites with anisotropic magnetic and optical properties. To this end, we explored TiN–Fe thin films with self-assembled vertical structures integrated on Si substrates. We showed that the key physical properties of the individual components (e.g., ferromagnetism from Fe) are preserved, that vertical nanostructures promote anisotropic behavior, and interactions between TiN and Fe enable a special magneto-optical response. This TiN–Fe nanocomposite system presents a new group of complex multifunctional hybrid materials that can be integrated on Si for future Si-based memory, optical, and biocompatible devices. 

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4. Functional two-dimensional high-entropy materials

   https://doi.org/10.1038/s43246-023-00341-y  

   Nemani, Srinivasa Kartik ; Torkamanzadeh, Mohammad ; Wyatt, Brian C. ; Presser, Volker ; Anasori, Babak ( February 2023 , Communications Materials)

   Multiple principal element or high-entropy materials have recently been studied in the two-dimensional (2D) materials phase space. These promising classes of materials combine the unique behavior of solid-solution and entropy-stabilized systems with high aspect ratios and atomically thin characteristics of 2D materials. The current experimental space of these materials includes 2D transition metal oxides, carbides/carbonitrides/nitrides (MXenes), dichalcogenides, and hydrotalcites. However, high-entropy 2D materials have the potential to expand into other types, such as 2D metal-organic frameworks, 2D transition metal carbo-chalcogenides, and 2D transition metal borides (MBenes). Here, we discuss the entropy stabilization from bulk to 2D systems, the effects of disordered multi-valent elements on lattice distortion and local electronic structures and elucidate how these local changes influence the catalytic and electrochemical behavior of these 2D high-entropy materials. We also provide a perspective on 2D high-entropy materials research and its challenges and discuss the importance of this emerging field of nanomaterials in designing tunable compositions with unique electronic structures for energy, catalytic, electronic, and structural applications.

    

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5. Polymer‐Derived High‐Temperature Nonoxide Materials: A Review

   https://doi.org/10.1002/adem.202200967  

   Zhou, Yue ; Lu, Kathy ( January 2023 , Advanced Engineering Materials)

   This review is focused on an attractive class of polymer‐derived high‐temperature ceramics, namely, polymer‐derived nonoxide materials. With a brief introduction of high‐temperature nonoxides, the origin of using polycarbosilane (PCS) polymer melt spinning to synthesize silicon carbide (SiC) fibers is traced back. For SiC formation, the four stages for the conversion from polymer precursors to microcrystalline ceramics are examined first: crosslinking, polymer decomposition, ceramic formation, and crystallization. Also, the important parameters related to PCS pyrolysis are explained, and polymer‐derived SiC microstructures and compositions are evaluated. Solid‐solution carbides and transition metal carbides are further reviewed. For boride materials, the discussion is focused on transition metal borides and boride composites. Similar to PCS conversion to SiC, nitride materials mostly start with polycarbosilazane (PSZ) precursors and form into the final materials through pyrolysis. With different carbide and nitride precursors mixed and pyrolyzed together, high‐temperature nonoxide composites are formed. Such molecular‐level intermixing and versatile capability of forming different shapes enable many exciting properties. Among these are mechanical and thermal properties, along with electrical conductivity, electromagnetic shielding, and charge storage capability. An overview of applications of polymer‐derived nonoxides is provided, followed by a summary and outlook.

    

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