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1. The world of two-dimensional carbides and nitrides (MXenes)

   https://doi.org/10.1126/science.abf1581  

   VahidMohammadi, Armin; Rosen, Johanna; Gogotsi, Yury (June 2021, Science)

   null (Ed.)

   A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies. 

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2. Carbides and Nitrides of Zirconium and Hafnium

   https://doi.org/10.3390/ma12172728  

   Ushakov, Sergey V.; Navrotsky, Alexandra; Hong, Qi-Jun; van de Walle, Axel (September 2019, Materials)

   Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40–60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions. 

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3. Synthesis and chemical stability of technetium nitrides

   https://doi.org/10.1039/D1CC02525B  

   Siska, Emily; Smith, Dean; Salamat, Ashkan; Lawler, Keith V.; Lavina, Barbara; Poineau, Frederic; Forster, Paul M. (January 2021, Chemical Communications)

   null (Ed.)

   We demonstrate the synthesis and phase stability of TcN, Tc 2 N, and a substoichiometric TcN x from 0 to 50 GPa and to 2500 K in a laser-heated diamond anvil cell. At least potential recoverability is demonstrated for each compound. TcN adopts a previously unpredicted structure identified via crystal structure prediction. 

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4. Photochemistry of carbon nitrides and heptazine derivatives

   https://doi.org/10.1039/D1CC02745J  

   Hwang, Doyk; Schlenker, Cody W. (September 2021, Chemical Communications)

   We explore the photochemistry of polymeric carbon nitride (C 3 N 4 ), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (Δ E ST ). In violation of Hund's multiplicity rules, the lowest energy singlet (S 1 ) is stabilized relative to the lowest triplet (T 1 ) electronic excited state. Exploiting this unique inverted Δ E ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted Δ E ST , paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump–push–probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary “push” excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted Δ E ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance. 

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5. Pressure-stabilized hafnium nitrides and their properties

   https://doi.org/10.1103/PhysRevB.95.020103  

   Zhang, Jin; Oganov, Artem R.; Li, Xinfeng; Niu, Haiyang (January 2017, Physical Review B)