In the present review, available MXene surface-terminating species are explored in view of current synthesis protocols. Their chemical properties are also considered in view of stoichiometry and coordination, which govern the stability of both terminations and MXene sheets. Furthermore, available post-processing methods are discussed in relation to how the termination chemistry can be further tuned, enabling bare MXene sheets as well as terminations that are not native to the MXene synthesis. Finally, this review explores the properties enabled by the MXene surface chemistry and the emerging applications they facilitate. In the conversion of three-dimensional (3D) MAX phases to two-dimensional (2D) MXene sheets, the freshly exposed and highly reactive surfaces are terminated by species that originate from the ambient environment. Accordingly, these are known as surface terminations. The MXene sheets inherit properties such as composition and structure from the parent MAX phase; however, given the reduced dimensionality of MXenes, the surface terminations decisively influence their chemistry, which ultimately governs the MXene properties.

Artificial superlattices exhibit exceptional electronic, magnetic, optical, and mechanical properties which make them unique candidates for applications in a broad range of technologies. A common key feature of superlattices is the need for atomically abrupt interfaces. However, superlattices comprised of materials with different properties, such as melting points and diffusivities, pose large challenges for achieving high crystal quality of both constituents with abrupt interfaces. By employing ion-assisted magnetron sputter epitaxy, we present an innovative solution to this problem with utilizing a unique combination of thermal radiation and kinetic energy that enable sufficient adatom mobility for epitaxial growth of both materials. The research was implemented for the case of CrB2/TiB2 heteroepitaxial superlattices, as neutron interference mirrors, wherein the constituents’ melting points differ by 1100 K. Ion-induced intermixing was avoided by commencing growth of each TiB2 and CrB2 layer by up to 3 unit cells (uc) without ion assistance, forming a buffer to protect the interface during the ion-assisted growth of the remainder of each layer. Heteroepitaxial superlattice growth with interface widths σCrB2 ∼1 uc and σTiB2 ∼2 uc was confirmed for different modulation periods. More than 3000 uc (∼1 µm) thick superlattices with abrupt interfaces were demonstrated for neutron mirror applications.

We report on the formation of the Cr2C compound using chemical etching-free methodology to extract Al from a Cr2AlC MAX phase thin film. Cr2AlC/Cu assemblies were deposited on sapphire substrates, using magnetron sputtering, and were subsequently annealed in vacuum. The Al from the MAX phase was shown to diffuse into Cu resulting in the formation of Al4Cu9 and causing the MAX phase to collapse into Cr2C grains. These carbide grains were characterized by transmission electron microscopy and the interatomic distances extracted were in good agreement with ab initio calculations predicting the equilibrium volume of the Cr2C phase.

We investigate the mobility of structural defects, adatoms and defect-adatoms combination in self-supporting graphene subjected to keV ion irradiation. In a first scenario, homogeneous irradiation using 20 keV Ar+ ions at a dose of 3 x 1014 ions cm-2 induces tensile strain of up to 0.8%. This strain diminishes with increasing defect density at a dose of 5 x 1014 ions cm-2, indicating a strain-relaxation mechanism. Contrary to the expected localized behavior, vacancies exhibit long-range interactions, contributing to global strain effects across the lattice. In a second scenario, by employing a nanopore mask, we spatially confined defect generation to periodically aligned circular regions surrounded by non-irradiated material, enabling direct observation of vacancy and adatom dynamics. Selected area electron diffraction (SAED) reveals significant structural damage in areas adjacent to irradiated regions, suggesting that single vacancies migrate over distances on the order of 100 nm from irradiated to non-irradiated zones even at room temperature. The build-up of lattice strain observed here may play a key role in lowering the migration barrier of single vacancies, thereby facilitating their diffusion into pristine lattice regions. Furthermore, our findings highlight the role of preexisting surface contaminants in preserving lattice integrity through a self-healing mechanism, where adatominduced lattice reconstruction mitigates defect-induced structural degradation.

High-entropy (HE) MAX phases represent an emerging family of multi-constituent solid solutions that provide large compositional variations and, therefore, a wide variety of properties. Here, we report the experimental realization of M2AlC (M = Ti/Ta/V/Nb/Cr) MAX phase by an energy-efficient self-propagating high-temperature synthesis, which enables facile scalability to an environmentally friendly industrial production. The HE-MAX phase was developed according to crystal size, electronegativity, and valence electron concentration of corresponding metals required to form a substitutional single-phase material. Variations in initial mixture composition, inert gas pressure, additive amount and sample diameter played a decisive role in HE-MAX formation. The combustion of the stoichiometric mixture favors the formation of the HE-carbide. Deviation from the stoichiometry has resulted in the formation of 211 and/or 413 type HE-MAX phases. Fine-tuning the aluminum and carbon content in the initial mixture, facilitated the formation of a layered structure, characteristic of MAX phases. DSC/TG analysis proved an enhanced oxidation resistance of HE-MAX phases, which outperforms conventional MAX phases and several previously studied HE-MAX phases.

The MAX phases constitute a family of nanolaminated ternary carbides and nitrides renowned for their compositional versatility, as reflected in the easy formation of solid solutions with variable chemical complexity. Synthesizing MAX phase solid solutions with intentionally tailored chemical complexity can produce materials that are able to meet the property requirements of the targeted application(s). This work presents an effective strategy specifically developed to design and fabricate highly phase-pure ceramics based on chemically complex MAX phase solid solutions by sterically stabilizing their unit cells. Steric unit cell stabilization is achieved via a judicious balance of dissimilar M- and A-elements, which targets the minimization of lattice distortions. This work produced high-purity (up to 88.7 wt %) (Zr-0.8,Ti-0.2)(2)(Al,Sn,Pb)C and (Zr-0.8,Ti-0.2)(2)(Al,Sn,Pb,Bi)C 211 MAX phase solid solutions by spark plasma sintering at 1350-1500 degrees C. Molten Zn- and/or Pb-/Bi-containing intermetallics facilitated the synthesis of soft (3-5 GPa), coarse-grained (length >20 mu m, thickness >10 mu m), and damage-tolerant ceramics. Intermetallics comprising Zn, Pb, and Bi improved (a) C/carbide dissolution, (b) Sn/C diffusion, and (c) carbide wetting, thus producing a 312 (Zr-0.8,Ti-0.2)(3)(Al,Sn,Pb,Bi)C-2 MAX phase solid solution. Forming (Zr-0.8,Ti-0.2)(3)(Al,Sn,Pb,Bi)C-2 contributed to the growth of very large platelets (length >100 mu m) with a distinct (312-core)/(211-shell) morphology. Zn did not occupy the A-site, unlike Al, Sn, Pb, and Bi. Sterically balanced A-site elemental occupancies, albeit nonequimolar, alleviated lattice distortions and aided the steric stabilization of the crystal structure, whereas the chemical complexity on the A-site increased the configurational entropy of the synthesized MAX phase compounds, despite pre-existing M-site compositional restrictions, further enhancing their thermodynamic stability.

The influence of heavy atom incorporation (in this case, tungsten, W) into scandium nitride is examined to assess its impact on the electronic structure and associated thermoelectric properties. Incorporating W, with its 5d valence electrons, is expected to shift the Fermi level into the conduction band. A solid solution of Sc1−x​Wx​Ny​ system is also expected to form as ScN exhibits the largest unit cell among the early 3d transition metal nitrides. However, phase separation is initiated at x = 0.10 and results in Sc- and W-rich regions occurring through conventional nucleation and growth. High-temperature nitrogen substoichiometry (at ∼800 °C) and formation of secondary phase is governed by inducing N vacancies in the crystal system. The N/W ratio alters the occupancy of the nonbonding t2g states in the valence band and results in phase instability. The Sc1−x​Wx​Ny​ system is found to be less covalent than a ScN reference sample indicating the presence of ionic and metallic bonds as observed through spectroscopic studies. A unique combination of a metal-like Seebeck coefficient with increased electrical resistivity is found for the Sc1−x​Wx​Ny​ system compared to the ScN reference. This study aims to elucidate the structural, microstructural, and electronic properties of the Sc1-xWxNy system and establishing a correlation with thermoelectric properties, through a combined experimental and theoretical approach.

The creation of vacancies and/or pores into two-dimensional materials, like graphene and MXenes, has shown to increase their performance for sustainable applications. However, a simple and affordable method with controlled and tailorable vacancy concentration and/or pores size remains challenging. Herein, a simple and reproducible method is presented for controlled synthesis of Mo1.74CTz MXene with randomly distributed vacancies and pores, obtained from selective etching of both Ga and Cr in the Cr-alloyed MAX-phase like precursor Mo1.74Cr0.26Ga2C. Structural and compositional analysis of the 3D alloy show approximate to 13% Cr on the metal site, homogeneously distributed between different particles and within the atomic structure. After etching, it translates to Mo1.74CTz MXene, exhibiting defect-rich sheets. Notably, the incorporation of Cr facilitates a shorter etching time with an improved yield compared to Mo2CTz. The Mo1.74CTz MXene displays excellent electrochemical properties, almost doubling the capacitance values (1152 F cm(-3) and 297 F g(-1) at 2 mV s(-1) scan rate), compared to its pristine counterpart Mo2CTz. The presented method and obtained results suggest defect engineering of MXenes through precursor alloying as a pathway that can be generalized to other phases, to further improve their properties for various applications.

Single-crystal CrB₂/TiB₂ diboride superlattices with well-defined layers are promising candidates for neutron optics. However, excess B in sputter-deposited TiB_y using a single TiB₂ target deteriorates the structural quality of CrB_x/TiB_y (0001) superlattices. We study the influence of co-sputtering of TiB₂ + Ti on the stoichiometry and crystalline quality of 300-nm-thick TiB_y single layers and CrB_x/TiB_y (0001) superlattices on Al₂O₃(0001) substrates grown by DC magnetron sputter epitaxy at growth-temperatures T_S ranging from 600 to 900 °C. By controlling the relative applied powers to the TiB₂ and Ti magnetrons, y could be reduced from 3.3 to 0.9. TiB_2.3 grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films. Close-to-stoichiometry CrB_1.7/TiB_2.3 superlattices with modulation periods Λ = 6 nm grown at 750 °C showed the highest single crystal quality and best layer definition. TiB_2.3 layers display rough top interfaces indicating kinetically limited growth while CrB_1.7 forms flat and abrupt top interfaces indicating epitaxial growth with high adatom mobility.

Soft and stretchable nanocomposites can match the mechanical properties of neural tissue, thereby minimizing foreign body reactions to provide optimal stimulation and recording specificity. Soft materials for neural interfaces should simultaneously fulfill a wide range of requirements, including low Young's modulus (<<1 MPa), stretchability (>= 30%), high conductivity (>> 1000 S cm(-1)), biocompatibility, and chronic stability (>> 1 year). Current nanocomposites do not fulfill the above requirements, in particular not the combination of softness and high conductivity. Here, this challenge is addressed by developing a scalable and robust synthesis route based on polymeric reducing agents for smooth, high-aspect ratio gold nanowires (AuNWs) of controllable dimensions with excellent biocompatibility. AuNW-silicone composites show outstanding performance with nerve-like softness (250 kPa), high conductivity (16 000 S cm(-1)), and reversible stretchability. Soft multielectrode cuffs based on the composite achieve selective functional stimulation, recordings of sensory stimuli in rat sciatic nerves, and show an accelerated lifetime stability of >3 years. The scalable synthesis method provides a chemically stable alternative to the widely used AgNWs, thereby enabling new applications within electronics, biomedical devices, and electrochemistry.