We have studied the electronic structure and elastic properties of Y(n+1)Co(3n+5)B(2n) (space group P6/mmm), where n = 1, 2, 3 and [Formula: see text], using ab initio calculations. These ternary borides exhibit a bulk-modulus-to-C(44) ratio from 1.6 to 1.9, which is rather unusual for ceramics. This may be understood on the basis of the electronic structure: predominantly covalent-ionic YCo(3)B(2) layers are interleaved with predominantly metallic YCo(5) layers. Covalent-ionic bonding between B and Co may give rise to a large bulk modulus, while weak coupling between the YCo(3)B(2) and YCo(5) layers may be responsible for the low C(44) value. On the basis of the similarity in electronic structure and elasticity data, it is reasonable to assume that the Y(n+1)Co(3n+5)B(2n) compounds investigated here may exhibit similar properties to the so-called MAX phases (Barsoum 2000 Prog. Solid State Chem. 28 201).

With increased chemical diversity and structural complexity comes the opportunities for innovative materials possessing advantageous properties. Herein, we combine predictive first-principles calculations with experimental synthesis, to explore the origin of formation of the atomically laminated i-MAX phases. By probing (Mo_2/3 M_1/3²)₂ AC (where M² = Sc, Y and A = Al, Ga, In, Si, Ge, In), we predict seven stable i-MAX phases, five of which should have a retained stability at high temperatures. (Mo_2/3Sc_1/3)₂GaC and (Mo_2/3Y_1/3)₂GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagomé-like ordering of the A-element. We suggest that the formation of i-MAX phases requires a significantly different size of the two metals, and a preferable smaller size of the A-element. Furthermore, the population of antibonding orbitals should be minimized, which for the metals herein (Mo and Sc/Y) means that A-elements from Group 13 (Al, Ga, In) are favored over Group 14 (Si, Ge, Sn). Using these guidelines, we foresee a widening of elemental space for the family of i-MAX phases and expect more phases to be synthesized, which will realize useful properties. Furthermore, based on i-MAX phases as parent materials for 2D MXenes, we also expect that the range of MXene compositions will be expanded.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti₃AuC₂ and Ti₃Au₂C₂ phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti₃SiC₂ single-crystal thin films with simultaneous out-diffusion of Si. Ti₃IrC₂ is subsequently produced by a substitution reaction of Ir for Au in Ti₃Au₂C₂. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesizing a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition-metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between the Zn element from molten ZnCl₂ and the Al element in MAX phase precursors (Ti₃AlC₂, Ti₂AlC, Ti₂AlN, and V₂AlC), novel MAX phases Ti₃ZnC₂, Ti₂ZnC, Ti₂ZnN, and V₂ZnC were synthesized. When employing excess ZnCl₂, Cl-terminated MXenes (such as Ti₃C₂Cl₂ and Ti₂CCl₂) were derived by a subsequent exfoliation of Ti₃ZnC₂ and Ti₂ZnC due to the strong Lewis acidity of molten ZnCl₂. These results indicate that A-site element replacement in traditional MAX phases by late transition-metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to preparing MXenes through an HF-free chemical approach.

A Ti₃(Al _x Cu_1-x)C₂ phase with Cu atoms with a degree of ordering in the A plane is synthesized through the A site replacement reaction in CuCl₂ molten salt. The weakly bonded single-atom-thick Cu layers in a Ti₃(Al _x Cu_1-x)C₂ MAX phase provide actives sites for catalysis chemistry. As-synthesized Ti₃(Al _x Cu_1-x)C₂ presents unusual peroxidase-like catalytic activity similar to that of natural enzymes. A fabricated Ti₃(Al _x Cu_1-x)C₂/chitosan/glassy carbon electrode biosensor prototype also exhibits a low detection limit in the electrochemical sensing of H₂O₂. These results have broad implications for property tailoring in a nanolaminated MAX phase by replacing the A site with late transition elements.

M_n+1AX_n phases are a large family of compounds that have been limited, so far, to carbides and nitrides. Here we report the prediction of a compound, Ti₂InB₂, a stable boron-based ternary phase in the Ti-In-B system, using a computational structure search strategy. This predicted Ti₂InB₂ compound is successfully synthesized using a solid-state reaction route and its space group is confirmed as P[Formula: see text]m2 (No. 187), which is in fact a hexagonal subgroup of P6₃/mmc (No. 194), the symmetry group of conventional M_n+1AX_n phases. Moreover, a strategy for the synthesis of MXenes from M_n+1AX_n phases is applied, and a layered boride, TiB, is obtained by the removal of the indium layer through dealloying of the parent Ti₂InB₂ at high temperature under a high vacuum. We theoretically demonstrate that the TiB single layer exhibits superior potential as an anode material for Li/Na ion batteries than conventional carbide MXenes such as Ti₃C₂.

The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo_2/3Sc_1/3)₂AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo_1.33C sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm^-3, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo₂C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.

The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)(3)A1C(2) was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)(3)A1C(2) is determined to be P6(3)/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)(3)A1C(2), Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M(3)AX(2) lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)(3)A1C(2) becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)(3)A1C(2) were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)(3)A1C(2) with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd.

The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. Herein, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M'2M″C2 and M'2M″2C3, where M' and M″ are two different early transition metals. In these solids, M' layers sandwich M″ carbide layers. By synthesizing Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo2TiC2Tx and Ti3C2Tx. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.

Density functional theory (DFT) computations were performed to investigate the electronic properties and Li storage capability of Ti3C2, one representative MXene (M represents transition metals, and X is either C or/and N) material, and its fluorinated and hydroxylated derivatives. The Ti3C2 monolayer acts as a magnetic metal, while its derived Ti3C2F2 and Ti3C2(OH)(2) in their stable conformations are semiconductors with small band gaps. Li adsorption forms a strong Coulomb interaction with Ti3C2-based hosts but well preserves its structural integrity. The bare Ti3C2 monolayer exhibits a low barrier for Li diffusion and high Li storage capacity (up to Ti3C2Li2 stoichiometry). The surface functionalization of F and OH blocks Li transport and decreases Li storage capacity, which should be avoided in experiments. The exceptional properties, including good electronic conductivity, fast Li diffusion, low operating voltage, and high theoretical Li storage capacity, make Ti3C2 MXene a promising anode material for Li ion batteries.

Materials with good flexibility and high conductivity that can provide electromagnetic interference (EMI) shielding with minimal thickness are highly desirable, especially if they can be easily processed into films. Two-dimensional metal carbides and nitrides, known as MXenes, combine metallic conductivity and hydrophilic surfaces. Here, we demonstrate the potential of several MXenes and their polymer composites for EMI shielding. A 45-micrometer-thick Ti3C2Tx film exhibited EMI shielding effectiveness of 92 decibels (>50 decibels for a 2.5-micrometer film), which is the highest among synthetic materials of comparable thickness produced to date. This performance originates from the excellent electrical conductivity of Ti3C2Tx films (4600 Siemens per centimeter) and multiple internal reflections from Ti3C2Tx flakes in free-standing films. The mechanical flexibility and easy coating capability offered by MXenes and their composites enable them to shield surfaces of any shape while providing high EMI shielding efficiency.

Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors in particular, pseudocapacitors(1,2). Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium(3). The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre(4,5); hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cydability, but only in thin films(6) Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the 'MXene' family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a 'MAX' phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre(7,8). Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide 'clay' have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report(8), and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.

Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2AlC, Ti3AlC2, and Ta4AlC3. MXenes combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as conductive clays. This article reviews progressboth experimental and theoreticalon their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W_2/3 M²_1/3 )₂ AC, where M² = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two-with a monoclinic C2/c structure-are predicted to be stable: (W_2/3 Sc_1/3 )₂ AlC and (W_2/3 Y_1/3 )₂ AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W_1.33 C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo_2/3 Y_1/3 )₂ AlC, the resulting MXene can be either: i) (Mo_2/3 Y_1/3 )₂ C with in-plane elemental order through selective removal of Al atoms or ii) Mo_1.33 C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo_2/3 Y_1/3 )₂ C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm^-3 is obtained, which is 40% higher than that of its Mo_1.33 C counterpart. With H₂ SO₄ , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm^-3 ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

2D molybdenum disulfide (MoS₂ ) gives a new inspiration for the field of nanoelectronics, photovoltaics, and sensorics. However, the most common processing technology, e.g., liquid-phase based scalable exfoliation used for device fabrication, leads to the number of shortcomings that impede their large area production and integration. Major challenges are associated with the small size and low concentration of MoS₂ flakes, as well as insufficient control over their physical properties, e.g., internal heterogeneity of the metallic and semiconducting phases. Here it is demonstrated that large semiconducting MoS₂ sheets (with dimensions up to 50 µm) can be obtained by a facile cathodic exfoliation approach in nonaqueous electrolyte. The synthetic process avoids surface oxidation thus preserving the MoS₂ sheets with intact crystalline structure. It is further demonstrated at the proof-of-concept level, a solution-processed large area (60 × 60 µm) flexible Ebola biosensor, based on a MoS₂ thin film (6 µm thickness) fabricated via restacking of the multiple flakes on the polyimide substrate. The experimental results reveal a low detection limit (in femtomolar-picomolar range) of the fabricated sensor devices. The presented exfoliation method opens up new opportunities for fabrication of large arrays of multifunctional biomedical devices based on novel 2D materials.

2D transition-metal carbides and nitrides, named MXenes, are promising materials for energy storage, but suffer from aggregation and restacking of the 2D nanosheets, which limits their electrochemical performance. In order to overcome this problem and realize the full potential of MXene nanosheets, a 3D MXene foam with developed porous structure is established via a simple sulfur-template method, which is freestanding, flexible, and highly conductive, and can be directly used as the electrode in lithium-ion batteries. The 3D porous architecture of the MXene foam offers massive active sites to enhance the lithium storage capacity. Moreover, its foam structure facilitates electrolyte infiltration for fast Li⁺ transfer. As a result, this flexible 3D porous MXene foam exhibits significantly enhanced capacity of 455.5 mAh g^-1 at 50 mA g^-1 , excellent rate performance (101 mAh g^-1 at 18 A g^-1 ), and superior ultralong-term cycle stability (220 mAh g^-1 at 1 A g^-1 after 3500 cycles). This work not only demonstrates the great superiority of the 3D porous MXene foam but also proposes the sulfur-template method for controllable constructing of the 3D foam from 2D nanosheets at a relatively low temperature.

The recent development of nanoscale fillers, such as carbon nanotubes, graphene, and nanocellulose, allows the functionality of polymer nanocomposites to be controlled and enhanced. However, conventional synthesis methods of polymer nanocomposites cannot maximise the reinforcement of these nanofillers at high filler content. Approaches for the synthesis of high content filler polymer nanocomposites are suggested to facilitate future applications. The fabrication methods address the design of the polymer nanocomposite architecture, which encompasses one, two, and three dimensional morphologies. Factors that hamper the reinforcement of nanostructures, such as alignment, dispersion of the filler and interfacial bonding between the filler and polymer, are outlined. Using suitable approaches, maximum potential reinforcement of nanoscale fillers can be anticipated without limitations in orientation, dispersion, and the integrity of the filler particle-matrix interface. High filler content polymer composites containing emerging materials such as 2D transition metal carbides, nitrides, and carbonitrides (MXenes) are expected in the future.

The structural transitions of Ti3AlC2 induced by ion irradiation were investigated over a wide fluence range by transmission electron microscopy. No amorphization occurs even at the highest dose of 31 dpa, indicating a great tolerance to irradiation-induced amorphization. Dynamic electron diffraction simulations and high-resolution observations indicate that the nanolamellar structure of Ti3AlC2 is readily destroyed through the formation of antisite defects and a phase transformation from alpha-Ti3AlC2 to beta-Ti3AlC2 occurs at 2.61 dpa. A great number of stacking faults in basal planes are formed with increasing fluence, leading to the formation of nano Ti3AlC2 grains with different stacking sequences at 10.45 dpa. Serious structural damage and polygonization are observed at the highest dose of 31 dpa. Due to the similar structural transition process with some complex oxides (pyrochlore and murataite), it is assumed that the great irradiation tolerance of Ti3AlC2 results from the low formation energy of antisite defects. These findings first clarify the structural transition mechanism of Ti3AlC2 under ion irradiation and its relationship with irradiation tolerance, which is of vital importance in understanding the irradiation response of MAX phases and provides a clue in searching for materials with higher irradiation tolerance from MAX phases. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd.

Wave-particle duality is an inherent peculiarity of the quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Postselection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.

Abstract

The structural stability of Ti₃AlC₂ in Cu and the microstructure evolution of Cu–Ti₃AlC₂ composites prepared at different temperatures were investigated by high-resolution transmission electron microscopy and X-ray diffraction. A mild reaction between Ti₃AlC₂ and Cu occurred at 850–950 °C, and strong reactions occurred above 950 °C. The reaction was identified as diffusion of Al from Ti₃AlC₂ into Cu to form Cu(Al) solid solution. Ti₃AlC₂ retained its structure under the partial loss of Al. Further depletion of Al resulted in highly defective Ti₃AlC₂ accompanied by the inner diffusion of Cu into Ti₃AlC₂ along the passway left by the Al vacancies. When Al was removed, Ti₃AlC₂ decomposed and transformed into cubic TiC_x. In addition, TiC twins formed by the aggregation of C vacancies at twin boundaries. With the help of first-principles calculation and image simulation, an ordered hexagonal TiC_x was identified as a transition phase linking Ti₃AlC₂ and c-TiC_x. The effect of the reaction and phase transformation on the microstructure and properties of Cu–Ti₃AlC₂ composites was also discussed.

Stem cells reside in complex three-dimensional (3D) environments within the body that change with time, promoting various cellular functions and processes such as migration and differentiation. These complex changes in the surrounding environment dictate cell fate yet, until recently, have been challenging to mimic within cell culture systems. Hydrogel-based biomaterials are well suited to mimic aspects of these in vivo environments, owing to their high water content, soft tissue-like elasticity, and often-tunable biochemical content. Further, hydrogels can be engineered to achieve changes in matrix properties over time to better mimic dynamic native microenvironments for probing and directing stem cell function and fate. This review will focus on techniques to form hydrogel-based biomaterials and modify their properties in time during cell culture using select addition reactions, cleavage reactions, or non-covalent interactions. Recent applications of these techniques for the culture of stem cells in four dimensions (i.e., in three dimensions with changes over time) also will be discussed for studying essential stem cell processes.

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti₃ C₂ T_z and V₂ CT_z , by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.