High-resolution X-ray Diffraction Research Articles

Ferroelectric behavior arising from polar topologies in epitaxially strained SrTiO3-δ ultrathin films

In this work, we show that epitaxially strained SrTiO3-δ thin films, grown by ion-beam sputtering onto (001)Nb:SrTiO3 substrates, exhibit a ferroelectric behavior. At the atomic-scale, through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, it was possible to identify the presence of polar nanoregions with a non-trivial polar topological structures in the SrTiO3-δ film, which are induced through oxygen vacancies. To further confirm the presence of strained regions with a polar structure, Raman spectroscopy and high-resolution X-ray diffraction were employed and it was possible to confirm the presence of a tetragonal structure in the SrTiO3-δ film, with a tetragonality ratio (c/a) of 1.005. Scanning probe microscopy and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior with maximum polarization of ∼1.5 μC/cm2, remnant polarization of ∼0.4 μC/cm2 and coercive field of ∼0.3 MV/cm. This work opens a window for exploring novel polar topological effects in sub-10 nm thin film materials for non-volatile memory application.

Similarities and differences in the ordering at short and long ranges in a NaNbO3 based Pb-free smart system: a key to functional properties

In this study, we have investigated the crystal structure of the solid solution of antiferroelectric NaNbO3(NN) with ferroelectric Ba0.9Ca0.1TiO3(BCT), i.e. (1−x)NN- xBCT for 0⩽x⩽1.0 using High-resolution powder x-ray diffraction (HR-XRD) data, dielectric measurements in conjunction with Raman spectroscopic studies. The investigation of long-range structural phase transitions as a function of composition(x) using x-ray diffraction is complemented by short-range structural insights from Raman spectroscopy. The incorporation of BCT disrupts the delicate antiferroelectric ordering of NN and stabilizes various ferroelectric phases with increasing BCT content(x). At x = 0.10, the simultaneous presence of two ferroelectric phases viz., Pmc21 and Amm2, results in a threefold rise in remanent polarization compared to off-boundary compositions. The composition with higher BCT content (i.e. x⩾0.50 ) has shown the anomalous existence of two long-range cubic structures, both of which are characterized by Pm 3¯ m space group. In spite of having a two-phase long-range centrosymmetric structure, we have observed a slim Polarization vs. Electric field(P-E) hysteresis loop for 0.50⩽x⩽0.80 . This contrapositive behavior (i.e. hysteresis loop in a centrosymmetric structure) has been attributed to the correlations among local static polar distortions of rhombohedral type ascertained using Raman spectroscopy. The coexistence of two ferroelectric phases at room temperature(for x = 0.10) and a slim hysteresis loop (for 0.50⩽x⩽0.80 ) make these compositions potential ingredients for applications in ferroelectric memory and energy storage devices.

Synthesis and characterization of C@CdS core-shell structures for high-performance capacitors

The present study investigates the synthesis and characterization of carbon@cadium sulphide (C@CdS) core-shell nanostructures for their potential application in electrochemical double-layer (ECDL) capacitors. Two distinct synthesis methods, hydrothermal route and co-precipitation method, were employed to fabricate C@CdS materials, each resulting in unique morphology and growth mechanisms. Detailed microstructural analyses, including high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD), revealed nanosheets in co-precipitated sample (CCSP) and core-shell structures in hydrothermally synthesized sample (CCSA). Optical investigations using UV-Vis spectroscopy provided insights into the absorbance properties of the synthesized materials. Electrical characterization, including frequency response testing and impedance analysis, demonstrated distinct capacitive behaviours for CCSP and CCSA, highlighting their potential for use in energy storage applications. Electrical characterization, including frequency response testing, impedance analysis, and galvanostatic charge-discharge (GCD) curve analysis, demonstrated distinct capacitive behaviours for CCSP and CCSA, highlighting their potential for use in energy storage applications. The integration of optical, structural, and electrical analyses provides a comprehensive understanding of the synthesized C@CdS core-shell nanostructures, offering insights into their potential application for ECDL capacitor technology and other emerging energy storage applications. Further optimization of synthesis parameters and exploration of scalable production methods are suggested for future research endeavors.

Synthesis and Physical Properties of the Misfit Layered Compound (EuS)1.13(NbSe2)2.

A new misfit layered compound with the stoichiometry (EuS)1+δ(NbSe2)2 (δ ≈ 0.13) has been successfully synthesized. High-resolution transmission electron microscopy and powder X-ray diffraction confirm the misfit structure with (EuS)-(EuS) spacing of 18.30(1) Å. Magnetization, electrical resistivity, heat capacity, and thermal transport measurements show that the material is a heavily doped semiconductor or poor metal with a low thermal conductivity of ∼1 W/m K and an antiferromagnetic ordering transition at TN = 4.7 K. In contrast to the parent materials, the misfit is neither ferromagnetic nor superconducting down to T = 0.4 K. We find evidence of a field-driven transition to a ferromagnetic state due to reorientation of ferromagnetic EuS layers at μoH = 0.5 T at T = 2 K.

Deposition time dependent structural and optical properties of polycrystalline [formula omitted] films

Sputtering-derived InGaZnO4 (IGZO) thin films were grown on quartz substrates and undergone post-deposition annealing at 700 °C for 2hr in N2 ambient. The influence of deposition time or film thickness on the morphological, structural and optical properties of polycrystalline IGZO thin films has been systematically investigated by means of characterization from field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD) and ultraviolet–visible–near infrared (UV–Vis–NIR) spectrophotometer. Results of AFM have shown that the root-mean-square (RMS) surface roughness of the IGZO films increases from 0.888 to 2.640 nm as the deposition time is extended from 50 to 250min. Analysis of XRD indicates that all the films are in polycrystalline phase, with crystallites growing into larger size accordingly to increasing deposition time. Appealingly and scarcely ever reported, the XRD diffraction planes can be separated into three domains, such as (0 0 l), (1 0 l) and (1 1 l) with different growth thicknesses. Optical measurement suggests that the optical bandgap have redshifted from 3.55 to 3.31eV at longer deposition time. Related mechanics about the change in physical properties against deposition time have been elaborated in detail. The validity of Swanepoel method, 1983 in deriving the refractive index of thick IGZO films has also been discussed.

Abrupt ternary III–V metamorphic buffers

Emerging quantum materials as well as novel traditional electronic and photonic materials may enable a new generation of information science devices if they can be synthesized on suitable substrates. Additionally, material and device designs may benefit from tunable properties through engineered epitaxial strain for the manipulation of the electronic character. In this work, three series of III–V ternary alloys, GaInSb, AlInSb, and InAsSb, are grown via molecular beam epitaxy on GaAs (001) substrates to explore the flexibility of abrupt metamorphic epitaxial layers with tunable lattice parameters between 6.135 and 6.479 Å. Their deposition on both homomorphic GaAs and pseudomorphic AlAs buffers is also explored. The structures of these alloys are characterized via reflection high-energy electron diffraction, high-resolution x-ray diffraction, atomic force microscopy, and transmission electron microscopy to assess their suitability as stable buffer layers with wide variability of accessible lattice parameters.

A process-based geochemical framework for shallow-marine carbonate sediments during burial diagenesis

A significant proportion of marine calcium carbonate sediments are comprised of metastable minerals that are susceptible to diagenetic alterations during burial. These reactions can reset the geochemical signature of sediments and pore fluids and influence elemental cycling in the ocean. However, the timing and mechanisms by which these reactions take place are poorly constrained. This study uses cores drilled on the slope of the Great Bahama Bank to provide quantitative constraints on important diagenetic reactions; namely respiration-driven dissolution, authigenic carbonate mineral formation, and conversion of aragonite to low Mg calcite (LMC). We perform detailed mineralogical characterization using newly acquired, high resolution X-ray diffraction (XRD) data and over 1000 reanalyzed XRD scans, characterize sediments texturally and elementally using electron microprobe data, calculate pore fluid saturation state with respect to aragonite using a Pitzer ion interaction approach, and use pore fluid chemistry and experimental distribution coefficients to predict authigenic carbonate compositions. These data suggest that aerobic organic matter oxidation, enabled by the advection of oxygenated seawater throughout the upper ∼ 30 m interval of sediment, causes undersaturation and thus dissolution of biogenic high Mg calcite (HMC) and aragonite. Deeper, anaerobic organic matter oxidation takes over and causes supersaturation, promoting authigenic precipitation in the form of HMC, which in turn decreases pore fluid Mg/Ca and promotes aragonite conversion to LMC. One novel aspect of this study is the identification of three types of calcites using the newly acquired XRD and electron microprobe data. Based on their unique crystallographic characteristics and chemical compositions, these types of calcites are interpreted to represent pelagic biogenic LMC, bank-derived biogenic HMC, and authigenic HMC precipitated from pore fluids. Notably, the composition of the authigenic calcite matches that predicted to precipitate from pore fluids using an empirical Mg partition coefficient in calcite. Calcites that form authigenically and from aragonite via replacement are suggested to recrystallize with burial as evidenced by the decrease in Mg content and micro-strain. This process-based geochemical framework assigns diagenetic processes to a specific depth window within the sediment column and paves the way for a mechanistic understanding of carbonate diagenesis, one that is rooted in thermodynamic and kinetic bases.

Molecular beam epitaxy growth and characterization of GePb alloys

Pb based group-IV alloys such as GePb have been gaining interest as a potential alternative for infrared detectors, quantum materials, and high-speed electronic devices. Challenges remain in their growth due to the extremely low solid solubility of Pb in the Ge–Pb system. This paper reports molecular beam epitaxy growth of GePb alloy thin films on Ge(100) substrates. Effusion cells of Ge and Pb are used to control the flux ratio independently. The optimal substrate temperature is found to be near the thermocouple temperature of 300 °C based on the characterization of the grown films using high-resolution x-ray diffraction. A large change in the Ge:Pb beam equivalent pressure ratio from 10:1 to 1:1 results in only a minimal increase of the Pb composition from 0.74% to 2.84% as estimated from Raman spectroscopy and Rutherford backscattering spectrometry. Scanning electron microscopy images show a large volume of Pb islands on the surface that form into either long trapezoidal rods or uniform droplets, with increasing Pb flux and growth time the density of Pb islands increased.

Pressure/temperature-assisted crystallographic engineering–A strategy for developing the infrared phosphors

Light-emitting diodes utilizing the near-infrared (NIR), harnessed through the doping of Cr ion with structural engineering to tune the luminescent properties, provide a compelling avenue for cutting-edge high-tech industries. Overcoming the challenge of altering the local coordination surrounding the activators is crucial for advancing novel NIR phosphors. Here, we propose a pressure-assisted crystallographic engineering strategy to design the new Cr-doped phosphors. Implementing this approach at high temperature and pressure facilitates the phase transition from β-LiGaO2:Cr to α-LiGaO2:Cr, enabling the LiGaO2:Cr material to emit the NIR light, which was initially unattainable. Additionally, we demonstrate the in-situ monitoring of the reverse phase transition from α-LiGaO2:Cr to β-LiGaO2:Cr, validated by in-situ temperature-dependent high-resolution synchrotron X-ray diffraction. Furthermore, α-LiGaO2:Cr and β-LiGaO2:Cr exhibit unexpected optical properties characterized through synchrotron X-ray absorption, photoluminescence, temperature-dependent, and pressure-dependent analyses. Moreover, the α-LiGaO2:Cr shows strong mechanoluminescence and persistent luminescence. This research paves the way for developing novel NIR phosphors and sheds light on the analyses of pressure-assisted research.

Large area MoSe2 and MoSe2/Bi2Se3 films on sapphire (0001) for near-infrared photodetection

The fabrication of heterojunction-based photodetectors (PDs) is well known for the enhancement of PDs performances, tunable nature of photoconductivity, and broadband application. Herein, the PDs based on MoSe2 and MoSe2/Bi2Se3 heterojunction on sapphire (0001) substrates were deposited using a r.f. magnetron sputtering system. The high-resolution x-ray diffraction and Raman spectroscopy characterizations disclosed the growth of the 2-H phase of MoSe2 and the rhombohedral phase of Bi2Se3 thin films on sapphire (0001). The chemical and electronic states of deposited films were studied using x-ray photoelectron spectroscopy and revealed the stoichiometry growth of MoSe2. We have fabricated metal-semiconductor–metal type PD devices on MoSe2 and MoSe2/Bi2Se3 heterojunction and the photo-response measurements were performed at external voltages of 0.1–5 V under near-infrared (1064 nm) light illumination. The bare MoSe2 PD device shows positive photoconductivity behavior whereas MoSe2/Bi2Se3 heterojunction PD exhibits negative photoconductivity. It was found that the responsivity of MoSe2 and MoSe2/Bi2Se3 heterojunction PDs is ~ 1.39 A W−1 and ~ 5.7 A W−1, respectively. The enhancement of photoresponse of MoSe2/Bi2Se3 PD nearly four-fold compared to bare MoSe2 PD shows the importance of heterojunction structures for futuristics optoelectronic applications.

Geological and Crystallochemical Characterization of the Margaritasite–Carnotite Mineral from the Uranium Region of Peña Blanca, Chihuahua, Mexico

Margaritasite is a mineral compound discovered in the early 1980s in Chihuahua, Mexico. It is a natural cesium uranyl vanadate found only, so far, in the Margaritas mine of the Peña Blanca highlands. In this work, a thorough characterization of the aforementioned mineral is presented. The portfolio of the techniques employed includes high-resolution X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, transmission electron microscopy in selected area electron diffraction (SAED) mode, and X-ray absorption spectroscopy (XAS). After extensive data analysis and modeling, new information on the mineral has been retrieved. Its phase composition is margaritasite–carnotite: a solid solution of cesium and potassium uranyl vanadate [(Cs,K)2(UO2)2(VO4)2·nH2O], and margaritasite, which is practically pure cesium uranyl vanadate [Cs2(UO2)2(VO4)2·nH2O]. The crystal structure of both components presents the space group P 1 21/c 1. Yet, each phase has similar, but appreciably different, lattice parameters. The mineral has a lamellar tabular and prismatic morphology. SAED patterns confirm the crystal structure of margaritasite. XAS spectra of Cs, V, and U confirm the elemental composition, oxidation states, and interatomic distances of this structure. These findings are consistent with the presence of cesium in this unique mineral from the paragenesis point of view.

Dependence of the Structural and Magnetic Properties on the Growth Sequence in Heterostructures Designed by YbFeO3 and BaFe12O19.

The structure and the chemical composition of individual layers as well as of interfaces belonging to the two heterostructures M1 (BaFe12O19/YbFeO3/YSZ) and M2 (YbFeO3/BaFe12O19/YSZ) grown by pulsed laser deposition on yttria-stabilized zirconia (YSZ) substrates are deeply characterized by using a combination of methods such as high-resolution X-ray diffraction, transmission electron microscopy (TEM), and atomic-resolution scanning TEM with energy-dispersive X-ray spectroscopy. The temperature-dependent magnetic properties demonstrate two distinct heterostructures with different coercivity, anisotropy fields, and first anisotropy constants, which are related to the defect concentrations within the individual layers and to the degree of intermixing at the interface. The heterostructure with the stacking order BaFe12O19/YbFeO3, i.e., M1, exhibits a distinctive interface without any chemical intermixture, while an Fe-rich crystalline phase is observed in M2 both in atomic-resolution EDX maps and in mass density profiles. Additionally, M1 shows high c-axis orientation, which induces a higher anisotropy constant K1 as well as a larger coercivity due to a high number of phase boundaries. Despite the existence of a canted antiferromagnetic/ferromagnetic combination (T < 140 K), both heterostructures M1 and M2 do not reveal any detectable exchange bias at T = 50 K. Additionally, compressive residual strain on the BaM layer is found to be suppressing the ferromagnetism, thus reducing the Curie temperature (Tc) in the case of M1. These findings suggest that M1 (BaFe12O19/YbFeO3/YSZ) is suitable for magnetic storage applications.

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial NixFe3-xO4 Films on MgO(001).

Off-stoichiometric NixFe3-xO4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe3O4 to NiFe2O4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe2+ cations are substituted by Ni2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Microstructure and optical properties of β-Ga2O3 thin films fabricated by pulsed laser deposition

β-Ga2O3 has attracted extensive attention in the fields of optoelectronics and high-power electric devices due to its excellent electrical properties and thermal stability. Growth of epitaxial β-Ga2O3 films with good crystallinity is challenging since it is difficult to effectively control the impurities in the films. Here, β-Ga2O3 thin films were epitaxially grown on MgO (100) substrates via pulsed laser deposition system. The microstructure and optical properties of the β-Ga2O3 thin films were systematically characterized by combining high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectrophotometer and advanced transmission electron microscopy. Effects of the growth temperature and O2 partial pressure on the crystallinity, valance states of Ga ions and band gap of the β-Ga2O3 thin films were investigated. The light absorption wavelength of the β-Ga2O3 thin films ranges from 220 nm to 260 nm, which is close to the theoretical value. The epitaxial orientation between β-Ga2O3 film and MgO substrate is β-Ga2O3 (100) [02¯1] // MgO (100) [010]. A thin layer of γ-Ga2O3 exists in the interfacial region.

Interfaces-engineered M-structure for infrared detectors

In this paper, the authors report strain-balanced M-structures InAs/GaSb/AlSb/GaSb superlattice growth on GaSb substrates using two kinds of interfaces (IFs): GaAs-like IFs and InSb-like IFs. The in-plane compressive strain of 60-period and 100-period InAs��/GaSb/AlSb��/GaSb with different InAs (��) and AlSb (��) monolayers are investigated. The M-structures InAs/GaSb/AlSb/GaSb represent type II superlattices (T2SL) and at present are under intensive investigation. Many authors show theoretical and experimental results that such structures can be used as a barrier material for a T2SL InAs/GaSb absorber tuned for long-wave infrared detectors (8 μm–14 μm). Beside that, M-structure can also be used as an active material for short-wave infrared detectors to replace InAs/GaSb which, for this region of infrared, are a big challenge from the point of view of balancing compression stress. The study of InAs/GaSb/AlSb/GaSb superlattice with the minimal strain for GaSb substrate can be obtained by a special procedure of molecular beam epitaxy growth through special shutters sequence to form both IFs. The authors were able to achieve smaller minimal mismatches of the lattice constants compared to literature. The high-resolution X-ray diffraction measurements prove that two types of IFs are proper for balancing the strain in such structures. Additionally, the results of Raman spectroscopy, surface analyses of atomic force microscopy, and differential interference contrast microscopy are also presented. The numerical calculations presented in this paper prove that the presence of IFs significantly changes the energy gap in the case of the investigated M-structures. The theoretical results obtained for one of the investigated structures, for a specially designed structure reveal an extra energy level inside the energy gap. Moreover, photoluminescence results obtained for this structure prove the good quality of the synthesized M-structures, as well as are in a good agreement with theoretical calculations.

Phase coexistence induced surface roughness in V2O3/Ni magnetic heterostructures

We present an investigation of the microstructure changes in V2O3 as it goes through its inherent structural phase transition. Using V2O3 films with a well-defined crystal structure deposited by reactive magnetron sputtering on r-plane Al2O3 substrates, we study the phase coexistence region and its impact on the surface roughness of the films and the magnetic properties of overlying Ni magnetic layers in V2O3/Ni hybrid magnetic heterostructures. The simultaneous presence of two phases in V2O3 during its structural phase transition was identified with high resolution x-ray diffraction and led to an increase in surface roughness observed using x-ray reflectivity. The roughness reaches its maximum at the midpoint of the transition. In V2O3/Ni hybrid heterostructures, we find a concomitant increase in the coercivity of the magnetic layer correlated with the increased roughness of the V2O3 surface. The chemical homogeneity of the V2O3 is confirmed through transmission electron microscopy analysis. High-angle annular dark field imaging and electron energy loss spectroscopy reveal an atomically flat interface between Al2O3 and V2O3, as well as a sharp interface between V2O3 and Ni.

Bismuth surfactant enhancement of surface morphology and film quality of MBE-grown GaSb(100) thin films over a wide range of growth temperatures

We investigate the surface morphologies of two series of homoepitaxial GaSb(100) thin films grown on GaSb(100) substrates by molecular beam epitaxy in a Veeco GENxplor system. The first series was grown at temperatures ranging from 290 to 490°C and serves as a control. The second series was grown using the same growth parameters with bismuth used as a surfactant during growth. We compared the two series to examine the impacts of bismuth over the range of growth temperatures on the GaSb surface morphologies using atomic force microscopy and the film properties using Raman spectroscopy and scanning electron microscopy. High-resolution x-ray diffraction was performed to confirm that bismuth was not incorporated into the films. We found that the morphological evolution of the GaSb series grown without bismuth is consistent with the standard surface nucleation theory and identified the 2D-3D transition temperature as close to 290° C. In contrast, the presence of a Bi surfactant during growth was found to significantly alter the surface morphology and prevent undesired 3D islands at low temperatures. We also observed a preference for hillocks over step morphology at high growth temperatures, antistep bunching effects at intermediate temperatures, and the evolution from step-meandering to mound morphologies at low temperatures. This morphological divergence from the first series indicates that bismuth significantly increases in the 2D Erlich–Schwöebel potential barrier of the atomic terraces, inducing an uphill adatom flux that can smoothen the surface. Our findings demonstrate that bismuth surfactant can improve the surface morphology and film structure of low-temperature grown GaSb. Bismuth surfactant may also improve other homoepitaxial III-V systems grown in nonideal conditions.

BN nanosheets/aramid nanofibers with pearl layered structure towards highly thermal conductive composites

This work reports a composite film using (3-Aminopropyl)triethoxysilane-modified boron nitride nanosheets (BNNS) incorporated in the aramid nanofibers (ANF) network via sol–gel method. High-resolution TEM, SEM, XPS, and XRD outcomes confirmed the imitation pearl layered structure—BNNSs as thermal conductive fillers were layer-by-layer overlapped on ANF. The reasonable coordination between interfaces and fillers stimulated the thermal conductivity (35.4 W (m·k)-1) of composite film i.e., 338.43 % higher than that of ANF. The composite film exhibited good cyclic thermal stability, high tensile strength (65.9 MPa), and high volume resistivity (2.424 × 1015 Ω∙cm).

The Effect of Sputtering Target Density on the Crystal and Electronic Structure of Epitaxial BaTiO3 Thin Films

Barium titanate (BaTiO3) is a promising material for silicon-integrated photonics due to its large electro-optical coefficients, low loss, high refractive index, and fast response speed. Several deposition methods have been employed to synthesize BaTiO3 films. Magnetron sputtering is one of these methods, which offers specific advantages for growing large-scale films. However, there is a scarcity of studies investigating the effect of sputtering target density on the quality of BaTiO3 films. Therefore, this study aims to uncover the effect of sputtering targets on the crystal and electronic structures of epitaxial BaTiO3 thin films. Two BaTiO3 ceramic targets were sintered at different densities by altering the sintering temperatures. The crystal structure and chemical composition of the targets were then characterized using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Subsequently, BaTiO3 epitaxial films were grown by magnetron sputtering using these two targets. The crystal and electronic structures of the BaTiO3 films were analyzed using high-resolution X-ray diffraction, X-ray photoemission spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Notably, the BaTiO3 films grown with high-density targets show superior quality but contain oxygen vacancies, whereas those films synthesized with low-density targets display high surface roughness. These findings provide insights into the effect of sputtering target density on the crystal and electronic structures of epitaxial BaTiO3 thin films.

Encapsulated cementite enhances catalytic performance of carbon nanotubes for oxygen reduction reaction

Nonprecious-metal-based materials show great potential to be highly efficient catalysts for oxygen reduction reaction (ORR). There is still room for improvement in the performances of nonprecious-metal-based catalysts for ORR. In this paper, we investigated the catalytic performances of N-doped carbon nanotubes (CNTs) with encapsulated cementite (Fe3C@CNT) for ORR. The encapsulation structure was confirmed by scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses. The catalytic performances of Fe3C@CNT for ORR were investigated by cyclic voltammetry, linear sweep voltammetry, Tafel analysis, rotating disk electrode, and rotating ring-disk electrode tests. The results showed that both the onset potential and half-wave potential of Fe3C@CNT-catalyzed ORR were 60 mV higher than those of 20 wt% Pt/C catalyst. Fe3C@CNT catalyzed ORR mainly happened through a four-electron pathway. The long-time running stability of the Fe3C@CNT was also superior to that of the 20 wt% Pt/C catalyst. The eutectic layers between the CNTs and cementite enhanced the electron transfer from the inner-layer CNT to the surface-layer N-doped CNT. Calculations revealed that the iron and carbon in the cementite were in positive and negative electronic states, respectively. The encapsulated cementite promoted the formation of equipotential circles on the surface-layer CNT, which greatly enhanced the catalytic performance of the Fe3C@CNT for ORR. N-doped CNTs with encapsulated cementite show great potential to be high-performance catalysts for ORR.