The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine (Tin+1Cn/Tin+1Nn) and functionalized MXenes (Tin+1CnT2/Tin+1NnT2, T stands for the terminal groups: -F, -O and -OH, n = 1, 2, 3) are investigated by density functional theory calculations. Carbide-based MXenes possess larger lattice constants and monolayer thicknesses than nitride-based MXenes. The in-plane Young's moduli of Tin+1Nn are larger than those of Tin+1Cn, whereas in both systems they decrease with the increase of the monolayer thickness.Cohesive energy calculations indicate that MXenes with a larger monolayer thickness have a better structural stability. Adsorption energy calculations imply that Tin+1Nn have stronger preference to adhere to the terminal groups, which suggests more active surfaces for nitridebased MXenes. More importantly, nearly free electron states are observed to exist outside the surfaces of -OH functionalized carbide/nitride based MXenes, especially in Tin+1Nn(OH)2, which provide almost perfect transmission channels without nuclear scattering for electron transport.The overall electrical conductivity of nitride-based MXenes is determined to be higher than that of carbide-based MXenes. The exceptional properties of titanium nitride-based MXenes, including strong surface adsorption, high elastic constant and Young's modulus, and good metallic conductivity, make them promising materials for catalysis and energy storage applications.

MXenes are emerging two-dimensional (2D) materials for energy-storage applications and supercapacitors. Their surface chemistry, which determines critical properties, varies due to different synthesis conditions. In this work, we synthesized TiVC solid-solution MXenes by two different synthesis methods and investigated their surface functional groups. We performed etching of the TiVAlC MAX phase using two different solutions, a highly concentrated HF (50 wt % ≈ 29 M) and a mixture of LiF and HCl (1.9 M LiF/12 M HCl). Large-scale delamination of TiVCT x to produce single-flake suspension was achieved by further intercalation of the resultant MXene from LiF/HCl with tetrabutylammonium hydroxide (TBAOH). X-ray diffraction indicates a large interlayer spacing of 2.18 nm for TiVCT x MXene flakes. To investigate the structural stability and adsorption energy of different functional groups on TiVC MXenes, density functional theory (DFT) calculations were performed and supported with X-ray photoelectron spectroscopy (XPS) measurements. A higher concentration of O and a lower concentration of −F were achieved on the TiVC synthesized by LiF/HCl, both of which provide a more favorable surface chemistry for energy-storage applications. Our results provide the first systematic study on the effect of synthesis conditions on the surface chemistry of solid-solution TiVC MXenes.

An emerging non-volatile, solid-state memory is resistance random access memory (RRAM). RRAM is based on reversible switching of resistance in semiconductor and insulator thin films. Here, unipolar resistance switching is demonstrated in electrodeposited films of [111]-textured cuprous oxide (Cu2O). The textured Cu2O is electrodeposited from a highly alkaline bath using tartrate as complexing agent. The switching is observed in a cell composed of a film of Cu2O sandwiched between Au and Au-Pd contacts. The switching is attributed to the formation and rupture of a Cu nanofilament in the Cu2O. The initial resistance of the cell is 6.5 x 10 6 Ω, and a conducting filament is formed in the film by scanning the applied electric field to 6.8 x 10 6 V m -1 . The cell is then reversibly cycled between a low resistance state of 16.6 Ω and a high resistance state of 4 x 10 5 Ω by SET and RESET processes. In the low resistance state the resistance decreases linearly with decreasing temperature, consistent with metallic behavior. The resistance temperature coefficient of 1.57 x 10 -3 K -1 is similar to that of nanoscale metallic Cu. Current-voltage (I-V) data suggests that applying a higher compliance current increases the filament size during the FORMING and the SET process, and also causes a higher RESET current. The filament diameter varies from 50 to 147 nm for compliance currents ranging from 10 to 100 mA. At high electric field in the as-deposited state, the conduction behavior follows Poole-Frenkel emission. The filament temperature is estimated from the non-ohmic behavior of the cell in the RESET step. The calculated temperature of 798 K before rupture of the Cu filament suggests Joule heating of the filament, resulting in melting, sintering, or thermal oxidation of the Cu filament. SUPPORTING INFORMATIONCalculation of the resistance of copper filament using RESET curve. Effect of compliance current on RESET current, voltage, and current density. This material is available free of charge via the internet at

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