Altermagnetic spintronics requires materials in which compensated magnetic order, symmetry-controlled electronic responses, and epitaxial tunability can be combined in experimentally accessible thin films. MnTe is a key material in this context, but experimental studies have focused mainly on the stable NiAs-type polymorph, whereas the polar wurtzite phase remains largely unexplored. Here we demonstrate molecular-beam epitaxy growth and investigate properties of nearly phase-pure wurtzite MnTe deposited directly on GaAs(111)B, and show that small changes in the growth conditions strongly modify the phase composition, from a multiphase state with endotaxial NiAs-type inclusions embedded in wurtzite MnTe matrix to an almost single-phase polar wurtzite layer.

In this study, we present the results of structural investigations of topological crystalline insulator (TCI) SnTe nanowires (NWs) grown by two different methods: autocatalytic and gold-assisted. The growth was performed using two distinct approaches: molecular beam epitaxy (MBE) and physical vapor deposition (PVD). Cross-sectional studies of oxidized NWs reveal a pronounced, unusual, anisotropic elemental distribution and a complex oxidation behavior. Taking into account the differences in the diffusion coefficients of the constituent elements and the high concentration of intrinsic cation vacancies, a two-step mechanism has been proposed. The initial step involves the formation of a thin surface oxide layer based on the Cabrera-Mott model adapted for semiconductors. This is followed by interdiffusion, described as the Kirkendall effect, in which tin migrates outward, whereas oxygen migrates inward into the NW. The process is not entirely uniform, and throughout the formation of the amorphous oxide skin, crystalline filaments are carved out as a result of ion migration. Concurrently, filaments act as channels, facilitating ion exchange through the associated vacancy diffusion mechanism. For Au-catalyzed NWs, thin Au films in the form of a ferrule extending several tens of nanometers were observed beneath the nanodroplets along the sidewalls, effectively suppressing oxygen adsorption in these regions. Chemical analysis of oxidized NWs coated with a Ti/Au bilayer, commonly employed for electrical contacts, shows that Ti binds oxygen from the oxide. However, this does not alter the previously created chemical segregation. The results clarify and advance the understanding of interface stability and oxidation in IV-VI semiconductor nanostructures, offering guidance for device integration.

This paper explores the potential for spin-triplet superconductivity in molecular beam epitaxy grown PbTe/SnTe semiconductor heterostructures. We present indications of spin-triplet pairing by soft point-contact spectroscopy experiments, using both spin-polarized and unpolarized electrons, and additionally, by a detailed analysis of the upper critical field as inferred from the four probe resistance measurements. The experimental data are described in terms of the Anderson-Brinkman-Morel model of p-wave electron pairing. Our results may be understood in terms of strain-induced topological superconductivity by Tang and Fu (2014 Nat. Phys. 10 964).

Encapsulation within single-walled carbon nanotubes (SWCNTs) provides a route to stabilize quasi-1D confined halide fragments derived from layered NiCl2 and to tune their electronic and magnetic responses. Here we combine high-resolution STEM/EDS and spin-polarized DFT + U calculations to examine NiCl2 fragments confined inside metallic (11,11) SWCNTs. Encapsulated NiCl2 forms structurally continuous confined fragments derived from the layered NiCl2 motif, consistent with the extended filling observed experimentally, in which the collinear FM configuration is energetically preferred (0 K) within the strongly confined (11,11) reference model, while larger diameters bring FM and AFM closer in energy (and can favor AFM), in contrast to antiferromagnetic bulk NiCl2 (see SI). Spin-resolved PDOS and band analyses indicate that the pronounced Ni 3d spin-down peak at the Fermi level originates from weakly dispersive states, while CNT it-derived bands contribute the dominant delocalized channels at charge neutrality. Transport metrics suggest a confinement-related DOS-velocity mismatch that suppresses spin-down transport tendencies at neutrality while enabling increased spin-down contribution under modest chemical-potential shifts (approximate to+0.1-0.2 eV). Our results suggest that NiCl2@SWCNT provides a case study for exploring how confinement tunes magnetic competition and spin-dependent transport tendencies in low-dimensional halide guests.

Bone tissue is a living tissue with an inherent capacity for self-regeneration; however, in cases of severe trauma or substantial bone loss, implantological treatment has become the standard therapeutic approach. To support healing between bone tissue and the implant surface, implant coatings are proposed to enable tight bonding of the implant surface with the bone, mimicking the natural self-regeneration processes of bone tissue. X-ray photoelectron spectroscopy and Raman spectroscopy investigations have confirmed the nucleation of calcium phosphate spheres under physiological conditions on hafnium dioxide (HfO2) films deposited by Atomic Layer Deposition (ALD). Scanning electron microscopy analysis showed that these spheres possess a flake-like structure. Moreover, transmission electron microscopy investigation confirmed that these flakes are composed of nanocrystalline, hexagonal hydroxyapatite domains. The obtained results indicate that the spheres nucleated on HfO2 films deposited by ALD show significant similarities to the carbonated hydroxyapatite observed in bone. Therefore, HfO2 films represent a promising material for implant surface modification due to their potential to enhance osseointegration.

A three-step process was developed for growing high-quality, optically uniform WSe2 monolayers by molecular beam epitaxy (MBE), taking advantage of the use of hexagonal boron nitride (hBN) as a substrate. The process was optimized to maximize the efficiency of photoluminescence and promote the formation of hexagonal WSe2 domains. Atomic force microscopy was employed to estimate the dispersion of the WSe2 hexagonal domains' orientation. Monolayer character of the film was identified using optical methods and verified with a high-resolution transmission electron microscopy cross-section. Temperature- and magnetic-field-dependent studies revealed the behavior of exciton complexes to be analogous to that of exfoliated counterparts. Direct growth on hBN, combined with a uniform optical response, proves that MBE-grown WSe2 is superior to mechanically exfoliated WSe2 in terms of the convenience of use and reproducibility. The provided results establish significant progress in the optical quality of epitaxially grown transition-metal dichalcogenide monolayers and the fabrication of large-scale functional devices.

The large-scale synthesis of van der Waals heterostructures (vdWHSs) is required to adopt these materials in electronic devices. However, the repeatable and controllable growth of vdWHSs has proven challenging. Here, we investigate the technological aspects of solid-source chemical vapor deposition (CVD) of two-dimensional heterostructures, with WS2/graphene and MoS2/graphene as examples. We show that by modification of one variable at least one another is unintentionally altered. For example, change in the growth pressure influences the evaporation rate of sulfur and shifts the position of one of the growth zones. We also perform a statistical screening of the 11 process parameters, indicating which of them impact the evaporation of the precursors. The screening indicates that the evaporation depends on weight of growth promoter (NaCl), growth temperature, precursors temperature, time difference between main and sulfur growth zones reaching the set temperatures, pressure, carrier gas flow, and process time. Finally, the five consecutive, identical growth processes show the seemingly inherent variability in synthesizing vdWHSs. We suggest that the high but limited airtightness of the CVD system or the substrate features can cause repeatability issues. Our study can facilitate future research on van der Waals heterostructures growth.

N-polar InN/In0.61Al0.39N heterostructures are grown directly on sapphire by using metalorganic chemical vapor deposition. The thickness of Mg-doped In0.61Al0.39N is 340 nm, and the root-mean-square surface roughness of 20 nm thick InN is ∼3.2 nm. An optional AlN spike grown at 710 °C for 35 s is used either as an interlayer to separate the InAlN buffer from the InN channel or as a part of InAlN nucleation after sapphire nitridation. High-resolution transmission electron microscopy reveals approximately two monolayers of AlN if used as the interlayer. In this case, the concentration of screw and edge threading dislocations in partially strained InN decreased down to 6.5 × 109 and 38 × 109 cm−2, respectively. More importantly, the interlayer inclusion suppressed remote donor and alloy disorder scatterings, providing, at room temperature, the InN free electron mobility and concentration of 620 cm2/V s and 3 × 1013 cm−2, respectively. On the other hand, omitting the AlN spike by InAlN nucleation led to structural deteriorations while buffer resistivity increased to 1.7 kΩ/□. A current density of ∼12–16 A/mm, breakdown field of ∼75 kV/cm, and electron drift velocity of ∼2 × 107 cm/s were determined in InN by applying 10 ns voltage pulses on fabricated test resistors.

Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from specific symmetry operation that connects the spin sublattices. In this report, we show with ab initio calculations that semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such a space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism.

Self-assembled, highly anisotropic nanostructures are spontaneously formed in the molecular beam epitaxy of antimony triselenide on GaAs substrates. These one-dimensional (1D) nanostripes have all the orientations parallel to the substrate surface and preserve the epitaxial relationship with the substrate. The shape of the nanostripes is directly related to the highly anisotropic stibnite structure of antimony triselenide which consists of 1D ribbons held together by weak van der Waals forces. The fabrication of well-ordered arrays of horizontal nanostripes aligned in directions defined by the orientation of the substrate may contribute significantly to the development of electronic circuits and networks composed of interconnected nanostructures leading to applications in neuromorphic devices, gas sensors and polarization-sensitive photodetectors.

Quantum dots consisting of an axial Zn0.97Mg0.03Te insertion inside a large-bandgap Zn0.9Mg0.1Te nanowire core are fabricated in a molecular-beam epitaxy system by employing the vapor-liquid-solid growth mechanism. In addition, this structure is coated with a thin ZnSe radial shell that forms a type-II interface with the dot semiconductor. The resulting radial electron-hole separation is evidenced by several distinct effects that occur in the presence of the ZnSe shell, including the optical emission redshift of about 250 meV, a significant decrease in emission intensity, an increase in the excitonic lifetime by one order of magnitude, and an increase in the biexciton binding energy. The type-II nanowire quantum dots where electrons and holes are radially separated constitute a promising platform for potential applications in the field of quantum information technology.

Van der Waals heterostructures (vdWHSs) enable the fabrication of complex electronic devices based on two-dimensional (2D) materials. Ideally, these vdWHSs should be fabricated in a scalable and repeatable way and only in the specific areas of the substrate to lower the number of technological operations inducing defects and impurities. Here, we present a method of selective fabrication of vdWHSs via chemical vapor deposition by electron-beam (EB) irradiation. We distinguish two growth modes: positive (2D materials nucleate on the irradiated regions) on graphene and tungsten disulfide (WS2) substrates, and negative (2D materials do not nucleate on the irradiated regions) on the graphene substrate. The growth mode is controlled by limiting the air exposure of the irradiated substrate and the time between irradiation and growth. We conducted Raman mapping, Kelvin-probe force microscopy, X-ray photoelectron spectroscopy, and density-functional theory modeling studies to investigate the selective growth mechanism. We conclude that the selective growth is explained by the competition of three effects: EB-induced defects, adsorption of carbon species, and electrostatic interaction. The method here is a critical step toward the industry-scale fabrication of 2D-materials-based devices.

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments.

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron-hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour-liquid-solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and micro-photoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, ΔCR. The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.

Structural analysis of epitaxial layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor (DMS), together with investigations of their magnetotransport properties, has been thoroughly performed. The obtained results are compared with those for the reference (Ga,Mn)As layers, grown under similar conditions, with the aim to reveal an impact of Bi incorporation on the properties of this DMS material. Incorporation of Bi into GaAs strongly enhances the spin-orbit coupling strength in this semiconductor, and the same has been expected for the (Ga,Mn)(Bi,As) alloy. In turn, importantly for specific spintronic applications, strong spin-orbit coupling in ferromagnetic systems opens a possibility of directly controlling the direction of magnetization by the electric current. Our investigations, performed with high-resolution X-ray diffractometry and transmission electron microscopy, demonstrate that the (Ga,Mn)(Bi,As) layers of high structural quality and smooth interfaces can be grown by means of the low-temperature molecular-beam epitaxy method, despite a large difference between the sizes of Bi and As atoms. Depending on the applied buffer layer, the DMS layers can be grown under either compressive or tensile misfit strain, which influences their magnetic properties. It is shown that even small 1% Bi content in the layers strongly affects their magnetoelectric properties, such as the coercive field and anisotropic magnetoresistance.

Nearly a 30% increase in the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry, and the first-principles calculations. The effect relies on defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires, where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs-MnAs granular system. Such a unique combination of two types of hexagonal lattices provides a possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.

Magnetic carbon activated (MAC) Fe3O4-loaded activated carbon was developed using termite feces and sulfuric acid as the carbon-modifying agent. The key point of the synthetic strategy was that the carbonization, activation and Fe3O4 loading were accomplished simultaneously, via thermic activation/magnetization. The adsorptive characteristics of the MAC for removal of Cr(VI) were investigated. Batch adsorption experiments were performed using both AC and MAC. The evaluation of the adsorption kinetics, isotherm, and thermodynamics was investigated. Adsorption of Cr(VI) onto MAC was highly pH dependent and was found to be optimum at pH 3.0. Nonlinear regression analysis revealed that the Sips isotherm model provides a better correlation for Cr (VI) adsorption onto MAC. The maximum adsorption capacities, at 25 degrees C, depends on the conditions of preparation of the adsorbent, and were noted to be 60 mg/g for AC and 35, 44 and 66 mg/g for MAC-600, MAC-700 and MAC-800, respectively. The pseudo second-order model best fit the adsorption kinetics. The magnetic property in MAC ensured easy separation of adsorbent using a magnet, after adsorption from the aqueous solution. (C) 2019 Elsevier B.V. All rights reserved.