Moses O. Nnaji

Rapid differential scanning calorimetry (DSC) measurements were performed on Ti, TiN, TiAl, and AlN multilayers that were magnetron sputtered at ambient temperature and had a similar overall composition to MAX-phase Ti2AlN. The DSC yielded thermal scans with exotherms indicating diffusion and crystallization processes during the annealing process, including the formation of Ti2AlN. Multilayer Ti/AlN yielded a distinctly large exotherm in the range of 500–700 °C, in contrast to the other films, and was estimated to have the highest cumulative enthalpy during annealing up to 700 °C. X-ray diffraction measurements and annealing treatments also indicated that all TiAlN films yielded Ti2AlN by 600 °C, with STEM imaging revealing the onset of Ti2AlN-like structural ordering in Ti/AlN by 475 °C. These data collectively show that multilayer films with Ti/AlN chemistry are distinctly reactive, experiencing phase transformation events that facilitate MAX-phase Ti2AlN synthesis at low substrate temperatures.

Since the initial synthesis of van der Waals 2D indium selenide is first documented in 1957, five distinct polymorphs and their corresponding polytypes are identified. In this study, a unique phase of indium selenide is reported via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large‐area films – which is named the phase. The quintuple layers of the phase, characterized by a unique zigzag atomic configuration with unequal indium‐selenium bond lengths from the middle selenium atom, are distinct from any other previously reported phase of indium selenide. Cross‐sectional STEM analysis has revealed that the layers exhibit intralayer shifting. It is found that indium selenide films with layers display electric‐field‐induced switchable polarization characteristic of ferroelectric materials, suggesting the breaking of the inversion symmetry. Experimental observations of nonlinear optical phenomena – Second Harmonic Generation (SHG) responses further support this conclusion. This study reports a phase of indium selenide showing ferroelectricity over large areas at room temperature in a low‐dimensional limit.

Mn+1AXn-phase Ti2AlN thin-films were synthesized using reactive sputtering-based methods involving the deposition of single-layer TiAlN, and Ti/AlN and TiN/TiAl multilayers of various modulation periods at ambient temperature and subsequent annealing at elevated temperatures. Ex situ and in situ x-ray diffraction measurements were used to characterize the Ti2AlN formation temperature and phase fraction. During annealing, Ti/AlN multilayers yielded Ti2AlN at a significantly lower in situ temperature of 650 °C compared to TiN/TiAl multilayers or single-layer TiAlN (750 °C). The results suggest a reactive multilayer mechanism whereby distinct Ti and AlN layers react readily to release exothermic energy resulting in lower phase transition temperatures compared to TiN and TiAl layers or mixed TiAlN. With a modulation period of 5 nm, however, Ti/AlN multilayers yielded Ti2AlN at a higher temperature of 750 °C, indicating a disruption of the reactive multilayer mechanism due to a higher fraction of low-enthalpy interfacial TiAlN within the film.