Enhanced magnetic susceptibility in Ti3C2Tx MXene with Co and Ni incorporation.

Magnetic nanomaterials are sought to provide new functionalities for applications ranging from information processing and storage to energy generation and biomedical imaging. MXenes are a rapidly growing family of two-dimensional transition metal carbides and nitrides with versatile chemical and structural diversity, resulting in a variety of interesting electronic and optical properties. However, strategies for producing MXenes with tailored magnetic responses remain underdeveloped and challenging. Herein, we incorporate elemental Ni and Co into Ti3C2Tx MXene by mixing with dilute metal chloride solutions. We achieve a uniform distribution of Ni and Co, confirmed by X-ray fluorescence (XRF) mapping with nanometer resolution, with Ni and Co concentrations of approximately 2 and 7 at% relative to the Ti concentration. The magnetic susceptibility of these Ni- and Co-incorporated Ti3C2Tx MXenes is one to two orders of magnitude larger than pristine Ti3C2Tx, illustrating the potential for dilute metal incorporation to enhance linear magnetic responses at room temperature.

Structural Evolution of Ultrathin SrFeO3-δ Films during Oxygen Evolution Reaction Revealed by In Situ Electrochemical Stress Measurements.

We report the electrochemical stress analysis of SrFeO3-δ (SFO) films deposited on Au substrates during oxygen evolution reactions (OERs). Our in situ analysis of Au reveals conversion reactions from Au to Au(OH)3, AuOOH, and AuOx during the OER. Au reactions cause a monotonic compressive stress on surfaces assigned to the formation of Au hydroxides and oxides. Electrochemical stress analysis of SrFeO3-δ/Au shows a dramatically different behavior during the OER, which we attribute to structural evolutions and conversion reactions, such as the conversion of SFO to iron (oxy)hydroxides. Interestingly, electrochemical stress analysis of SrFeO3-δ/Au shows a tensile trend, which evolves with cycling history. Electrochemical stress analysis of SFO films before the onset of the OER shows in situ changes, which cause tensile stresses when cycling to 1.2 V. We attribute these stresses to the formation of Fe2+δOδ(OH)2-δ (0 ≤ δ ≤ 1.5)-type materials where δ approaches 1.5 at higher potentials. At potentials higher than 1.2 V and during OER, surface stress response is rather stable, which we assign to the full conversion of SFO to iron (oxy)hydroxides. This analysis provides insight into the reaction mechanism and details of in situ structural changes of iron perovskites during the OER in alkaline environments.

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO3.

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO3 has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO3 using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO3 films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO3 with free electron concentrations, n3D, from 3.3 × 1019 cm-3 to 1.6 × 1020 cm-3. The films exhibit a maximum room-temperature mobility of 42 cm2 V-1 s-1 at n3D = 3.3 × 1019 cm-3. Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO3-based field-effect transistors. The transistors show promise for CaSnO3's high-voltage capabilities by exhibiting low off-state leakage below 2 × 10-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 106. This work serves as a starting point for future studies on the semiconducting properties of CaSnO3 and many devices that could benefit from CaSnO3's exceptionally wide band gap.

Interfacial Topochemical Fluoridation of MAPbI3 by Fluoropolymers.

Herein it is demonstrated that, under conditions relevant to perovskite synthesis (>140 °C in air), fluoride can topochemically react across the interface between a halide perovskite and a fluoropolymer when in close contact, thereby creating a small quantity of strongly bonded lead fluoride species. The quantity increases with temperature and processing duration. Photoinduced charge carrier lifetime provides a metric for the resulting changes in electronic structure of the perovskite. Under short-duration and/or moderate temperature processing, fluoride transfer to the perovskite yields increased carrier lifetimes, up to 3-fold longer than control samples, which is attributed to passivation of surface defects. Under more forcing conditions, the trend reverses: excessive fluoridation leads to shortened carrier lifetimes, which is ascribed to substantial interfacial formation of PbF2. It is demonstrated that an interface with bulk crystalline PbF2 quenches perovskite photoluminescence, likely due to PbF2 serving as an electron acceptor for the conduction band of MAPbI3.

Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene.

Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr2TiC2Tx MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr2TiC2Tx at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr2TiAlC2 MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.

Tailoring Electronic and Optical Properties of MXenes through Forming Solid Solutions.

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (Mn+1Xn) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M2XTx structure (Ti2-yNbyCTx, Ti2-yVyCTx, and V2-yNbyCTx, where Tx stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Synthesis and Characterization of SrFexMn1-x(O,F)3-δ Oxide (δ = 0 and 0.5) and Oxyfluoride Perovskite Films.

We report the synthesis and characterization of as-grown SrFexMn1-xO2.5 epitaxial films, which were also subjected to postgrowth oxidation and topotactic fluorination to obtain SrFexMn1-xO3 and SrFexMn1-xO2.5-δFγ films. We show how both the B-site cation and anion composition influence the structural, electronic, and optical properties of this family of perovskite materials. The Fe substitution of Mn in SrMnO2.5 gradually expands the c-axis parameter, as indicated by X-ray diffraction. With increasing x, the F content incorporated under identical fluorination conditions increases, reaching its maximum in SrFeO2.5-δFγ. In the compounds with mixed B-site occupation, the Fe 2p photoemission peaks are shifted upon fluorination, while the Mn 2p peaks are not, suggesting inductive effects lead to asymmetric responses in how F alters the Mn and Fe bonds. Electronic transport measurements reveal all compounds are insulators, with the exception of SrFeO3, and demonstrate that fluorination increases resistivity for all values of x. Optical absorption spectra in the SrFexMn1-xO2.5 and SrFexMn1-xO3 films evolve systematically as a function of x, consistent with a physical scenario in which optical changes with Fe substitution arise from a linear combination of Mn and Fe 3d bands within the electronic structure. In contrast, the F incorporation induces nonlinear changes to the optical response, suggesting a more complex impact on the electronic structure in materials with concurrent B-site and anion site substitution.

Depth-Resolved Modulation of Metal-Oxygen Hybridization and Orbital Polarization across Correlated Oxide Interfaces.

Interface-induced modifications of the electronic, magnetic, and lattice degrees of freedom drive an array of novel physical properties in oxide heterostructures. Here, large changes in metal-oxygen band hybridization, as measured in the oxygen ligand hole density, are induced as a result of interfacing two isovalent correlated oxides. Using resonant X-ray reflectivity, a superlattice of SrFeO3 and CaFeO3 is shown to exhibit an electronic character that spatially evolves from strongly O-like in SrFeO3 to strongly Fe-like in CaFeO3 . This alternating degree of Fe electronic character is correlated with a modulation of an Fe 3d orbital polarization, giving rise to an orbital superstructure. At the SrFeO3 /CaFeO3 interfaces, the ligand hole density and orbital polarization reconstruct in a single unit cell of CaFeO3 , demonstrating how the mismatch in these electronic parameters is accommodated at the interface. These results provide new insight into how the orbital character of electrons is altered by correlated oxide interfaces and lays out a broadly applicable approach for depth-resolving band hybridization.

Physical properties of epitaxial SrMnO2.5-δ F γ oxyfluoride films.

Recently, topotactic fluorination has become an alternative way of doping epitaxial perovskite oxides through anion substitution to engineer their electronic properties instead of the more commonly used cation substitution. In this work, epitaxial oxyfluoride SrMnO2.5-δ F γ films were synthesized via topotactic fluorination of SrMnO2.5 films using polytetrafluoroethylene as the fluorine source. Oxidized SrMnO3 films were also prepared for comparison with the fluorinated samples. The F content, probed by x-ray photoemission spectroscopy, was systematically controlled by adjusting fluorination conditions. Electronic transport measurements reveal that increased F content (up to γ = 0.14) systematically increases the electrical resistivity, despite the nominal electron-doping induced by F substitution for O in these films. In contrast, oxidized SrMnO3 exhibits a decreased resistivity and conduction activation energy. A blue-shift of optical absorption features occurs with increasing F content. Density functional theory calculations indicate that F acts as a scattering center for electronic transport, controls the observed weak ferromagnetic behavior of the films, and reduces the inter-band optical transitions in the manganite films. These results stand in contrast to bulk electron-doped La1-x Ce x MnO3, illustrating how aliovalent anionic substitutions can yield physical behavior distinct from A-site substituted perovskites with the same nominal B-site oxidation states.

Control of MXenes' electronic properties through termination and intercalation.

MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes' electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes' electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

2D molybdenum and vanadium nitrides synthesized by ammoniation of 2D transition metal carbides (MXenes).

MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, Ti4N3 and Ti2N are the only nitride MXenes reported so far. Here by ammoniation of Mo2CTx and V2CTx MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting Mo2N and V2N were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that Mo2N retains the MXene structure and V2C transforms to a mixed layered structure of trigonal V2N and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of Mo2N and V2N are three and one order of magnitude larger than those of the Mo2CTx and V2CTx precursors, respectively. This study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis.

Interface-Induced Phenomena in Magnetism.

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films.

We have investigated the effect of La and Fe deficiency on the c-axis lattice parameter and optical absorption in La1-xFe1-yO3-δ films deposited by oxide MBE. La deficient films display minimal changes in optical and structural properties compared with stoichiometric LaFeO3, even with relatively large cation vacancy concentrations (∼22%). Fe deficient films show a linear increase in c-axis parameter with increasing Fe vacancy concentration. The optical absorption coefficient is reduced for Fe deficient films over the spectral range from 1.2 to 5.0 eV. This leads to a relatively simple and effective method to quantify the approximate Fe deficiency in LaFeO3. Minimal changes in optical and structural properties are observed when cation deficient films are exposed to strongly oxidizing (ozone annealing) conditions. This indicates a strong accompaniment of charge balancing oxygen vacancies to maintain a nominal Fe3+ valence state, acting to stabilize the band structure even in the presence of significant cation off-stoichiometry.

Polar Oxides without Inversion Symmetry through Vacancy and Chemical Order.

One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegard's principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca2Fe2O5 and Sr2Fe2O5. We then realize epitaxial (SrFeO2.5)1/(CaFeO2.5)1 thin film superlattices possessing both anion-vacancy order and Sr and Ca chemical order at the subnanometer scale, confirmed through synchrotron-based diffraction and aberration corrected electron microscopy. Through a detailed symmetry analysis and density functional theory calculations, we show that A-site cation ordering lifts inversion symmetry in the superlattice and produces a polar compound. Our results demonstrate how control of anion and cation order at the nanoscale can be utilized to produce acentric structures markedly different than their constituents and open a path toward novel structure-based property design.

Control of electronic properties of 2D carbides (MXenes) by manipulating their transition metal layers.

In this study, a transition from metallic to semiconducting-like behavior has been demonstrated in two-dimensional (2D) transition metal carbides by replacing titanium with molybdenum in the outer transition metal (M) layers of M3C2 and M4C3 MXenes. The MXene structure consists of n + 1 layers of near-close packed M layers with C or N occupying the octahedral site between them in an [MX]nM arrangement. Recently, two new families of ordered 2D double transition metal carbides MXenes were discovered, M'2M''C2 and M'2M''2C3- where M' and M'' are two different early transition metals, such as Mo, Cr, Ta, Nb, V, and Ti. The M' atoms only occupy the outer layers and the M'' atoms fill the middle layers. In other words, M' atomic layers sandwich the middle M''-C layers. Using X-ray atomic pair distribution function (PDF) analysis on Mo2TiC2 and Mo2Ti2C3 MXenes, we present the first quantitative analysis of structures of these novel materials and experimentally confirm that Mo atoms are in the outer layers of the [MC]nM structures. The electronic properties of these Mo-containing MXenes are compared with their Ti3C2 counterparts, and are found to be no longer metallic-like conductors; instead the resistance increases mildly with decreasing temperatures. Density functional theory (DFT) calculations suggest that OH terminated Mo-Ti MXenes are semiconductors with narrow band gaps. Measurements of the temperature dependencies of conductivities and magnetoresistances have confirmed that Mo2TiC2Tx exhibits semiconductor-like transport behavior, while Ti3C2Tx is a metal. This finding opens new avenues for the control of the electronic and optical applications of MXenes and for exploring new applications, in which semiconducting properties are required.

Towards 3D Mapping of BO6 Octahedron Rotations at Perovskite Heterointerfaces, Unit Cell by Unit Cell.

The rich functionalities in the ABO3 perovskite oxides originate, at least in part, from the ability of the corner-connected BO6 octahedral network to host a large variety of cations through distortions and rotations. Characterizing these rotations, which have significant impact on both fundamental aspects of materials behavior and possible applications, remains a major challenge at heterointerfaces. In this work, we have developed a unique method to investigate BO6 rotation patterns in complex oxides ABO3 with unit cell resolution at heterointerfaces, where novel properties often emerge. Our method involves column shape analysis in ABF-STEM images of the ABO3 heterointerfaces taken in specific orientations. The rotating phase of BO6 octahedra can be identified for all three spatial dimensions without the need of case-by-case simulation. In several common rotation systems, quantitative measurements of all three rotation angles are now possible. Using this method, we examined interfaces between perovskites with distinct tilt systems as well as interfaces between tilted and untilted perovskites, identifying an unusual coupling behavior at the CaTiO3/LSAT interface. We believe this method will significantly improve our knowledge of complex oxide heterointerfaces.

Raman scattering in La1-xSrxFeO3-δ thin films: annealing-induced reduction and phase transformation.

Raman scattering in thin film La0.2Sr0.8FeO3-δ on MgO(0 0 1) collected at 300 K after different stages of annealing at selected temperatures T (300 K < T < 543 K, to 10 h) and analysis reveal changes in spectral characteristics due to a loss of oxygen, onset of oxygen vacancy-induced disorder, and activation of Raman-inactive modes that are attributed to symmetry lowering. The interpretation is further supported by carrier transport measurements under identical conditions showing orders of magnitude increase in the resistivity induced by oxygen loss. After prolonged annealing in air, evolution of the spectrum signals the appearance of a possible topotactic transformation of the crystal structure from that of the rhombohedral ABO3 perovskites to that of Brownmillerite-like structure consisting of octahedrally and tetrahedrally coordinated Fe atoms.

Band structure and optical transitions in LaFeO3: theory and experiment.

The optical absorption properties of LaFeO(3) (LFO) have been calculated using density functional theory and experimentally measured from several high quality epitaxial films using variable angle spectroscopic ellipsometry. We have analyzed the calculated absorption spectrum using different Tauc models and find the model based on a direct-forbidden transition gives the best agreement with the ab initio band gap energies and band dispersions. We have applied this model to the experimental data and determine the band gap of epitaxial LFO to be ∼2.34 eV, with a slight dependence on strain state. This approach has also been used to analyze the higher indirect transition at ∼3.4 eV. Temperature dependent ellipsometry measurements further confirm our theoretical analysis of the nature of the transitions. This works helps to provide a general approach for accurate determination of band gaps and transition energies in complex oxide materials.

Atomic-scale characterization of oxide thin films gated by ionic liquid.

Ionic liquids (ILs) have received considerable interest for use in electrostatic gating in complex oxide systems. Understanding the ionic liquid/oxide interface, and any bias-induced electrochemical degradation, is critical for the interpretation of transport phenomena. The integrity of the interface between ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate and La1/3Sr2/3FeO3 under various biasing conditions was examined by analytical transmission electron microscopy, and we report film degradation in the form of an irreversible chemical reaction regardless of the applied bias. This results in an intermixing region of 4-6 nm at the IL/oxide interface. Electron energy loss spectroscopy shows La and Fe migration into the ionic liquid, resulting in secondary phase formation under negative bias. Our approach can be extended to other ionic liquid/oxide systems in order to better understand the electrochemical stability window of these device structures.

Control of functional responses via reversible oxygen loss in La1-xSrxFeO3-δ films.

La0.3 Sr0.7 FeO3-δ films undergo dramatic changes in electronic and optical properties due to reversible oxygen loss induced by low-temperature heating. This mechanism to control the functional properties may serve as a platform for new devices or sensors in which external stimuli are used to dynamically control the composition of complex oxide heterostructures.