Defect-induced magnetism in neutron irradiated 6H-SiC single crystals

Accurate Measurement of Defect Generation Rates in Silicon Carbide Irradiated with Energetic Ions

In this work, we obtained the Si vacancy generation rates η in SiC nanowire samples irradiated with 1, 3 MeV protons, and 2.8 MeV helium ions using the electrical resistivity measurement, which further indicated an intuitive linear function correlation between η and the nuclear stopping power of the incident ions at a low dpa level with a coefficient of 2.15 × 10-3 eV-1. Prediction through this correlation is consistent with previous work. Besides, the measured value is about 1/2 of the simulation results with the popular SRIM code. Overall, our work provides a feasible way to get the generation rate of a certain irradiation-induced defect by electric measurements, and the correlation obtained is practically useful in various applications.

Room-temperature half-metals induced via chemical surface modification: 2D Mn2Se2 monolayer

Compared with various antiferromagnetic (AFM) materials, two-dimensional (2D) room-temperature ferromagnetic (FM) materials are rarely discovered because of the geometrically determined spin interactions. Since 2D FM materials have shown great potential in the next-generational information devices, it is quite important to design new FM materials based on the reported AFM materials. Here, in this study, we found that the Mn₂Se₂ monolayer can be converted to half-metal from AFM semiconductor at room temperature by edge modification of certain chemical groups (such as -Cl, -Br, -I, and -S) based on systematical first-principles calculations. Our results show that the adsorbed chemical groups significantly modify the electronic states of Mn ions and the resulting spin interactions. Moreover, our results indicate that the Curie temperatures (T_c) of some Mn₂Se₂ monolayer derivatives approach or even exceed room temperature, among which Curie temperatures after chemical modification by -Cl, -Br, -I, -S are 290 K, 320 K, 400 K, and 1050 K, respectively. Thus, chemical modifications can be one of the effective methods to construct 2D FM materials in experiments.

Atomic Defect Induced Saturable Absorption of Hexagonal Boron Nitride in Near Infrared Band for Ultrafast Lasing Applications

Defect-induced phenomena in 2D materials has received increasing interest among researchers due to the novel properties correlated with precise modification of materials. We performed a study of the nonlinear saturable absorption of the boron-atom-vacancy defective hexagonal boron nitride (h-BN) thin film at a wavelength of ~1 μm and its applications in ultrafast laser generation. The h-BN is with wide band gap of ~6 eV. Our investigation shows that the defective h-BN has a wide absorption band from visible to near infrared regimes. First-principle calculations based on density functional theory (DFT) indicate that optical property changes may be attributed to the boron-vacancy-related defects. The photoluminescence spectrum shows a strong emission peak at ~1.79 eV. The ultrafast Z-scan measurement shows saturable absorbance response has been detected for the defective h-BN with saturation intensity of ~1.03 GW/cm² and modulation depth of 1.1%. In addition, the defective h-BN has been applied as a new saturable absorber (SA) to generate laser pulses through the passively Q-switched mode-locking configuration. Based on a Nd:YAG waveguide platform, 8.7 GHz repetition rate and 55 ps pulse duration of the waveguide laser have been achieved. Our results suggest potential applications of defective h-BN for ultrafast lasing and integrated photonics.

Strain tuning of the Curie temperature and valley polarization in two dimensional ferromagnetic WSe2/CrSnSe3heterostructure

Magnetic proximity effect can be used to tailor the magnetic ground state and valley polarization in the monolayer of transition metal dichalcogenides. Thus, we explore the effect of biaxial tensile and compressive strain on valley polarization in the WSe₂/CrSnSe₃heterostructures with different stacking orders systematically. The indirect band gaps in the two most stable stackings; hollow (0.27 eV) and top (0.33 eV) were further enhanced to 0.35 eV under tensile strain while suppressed to almost 0.13 eV under compressive strain. The heterostructures had a FM ground state with a total magnetic moment per unit cell of 6μ_Bin pristine as well as strained structures. In hollow stacking and compressively strained structures, we obtained a perpendicular magnetocrystalline anisotropy, while the top stacking and tensile strain structures had small in-plane anisotropy. An enhancement was found in Curie temperature from 73 K in pristine to 128 K in a 6% tensile strained structure. The valley splittings found in pristine hollow (4 meV) and top (9 meV) stacked heterostructures were further enhanced to 29 meV and 22 meV at 5% compressive strain respectively. This enhancement was attributed to the increased spatial dependence of the charge density along K⁺and K^-directions of the Brillouin zone, which give rise to the different local dipolar fields at these valleys. Our results suggest that strain could be an effective way to control or tune the valley splitting in WSe₂/CrSnSe₃heterostructures.

High Curie temperature and carrier mobility of novel Fe, Co and Ni carbide MXenes

Two-dimensional (2D) magnets with room temperature ferromagnetism and semiconductors with moderate band gap and high carrier mobility are highly desired for applications in nanoscale electronics and spintronics. By performing the first-principles calculations, we investigate novel Fe, Co, Ni carbide based pristine (M₂C) and functionalized (M₂CT₂, T: F, O, OH) MXenes. Our calculations show that Fe₂C, Co₂C, Ni₂C, Fe₂CF₂, Fe₂CO₂, Fe₂C(OH)₂, Co₂CF₂, Co₂C(OH)₂ and Ni₂CF₂ are dynamically and mechanically stable. More importantly, Fe₂C, Co₂C, Fe₂CF₂ and Fe₂C(OH)₂ exhibit intrinsic ferromagnetism (magnetic moments 2-5μ_B per unit cell). Monte Carlo simulations suggest high Curie temperatures of 590 and 920 K for Fe₂C and Fe₂CF₂, respectively, at the HSE06 level owing to the large spin magnetic moments and strong ferromagnetic coupling. Based on the deformation potential theory, we predict high and anisotropic hole mobility (0.2-1.4 × 10⁴ cm² V^-1 s^-1) for semiconducting Fe₂CO₂ and Co₂C(OH)₂. Additionally, Ni₂CF₂ demonstrates highly anisotropic electron mobility together with a direct band gap. Our results further show the effectiveness of surface functionalization in modulating the electronic and magnetic properties and broadening the properties of MXenes to achieve long-range intrinsic ferromagnetism well above room temperature and high carrier mobility.

Doping Dependent Magnetic Behavior in MBE Grown GaAs1-xSbx Nanowires

Intrinsic and Te-doped GaAsSb nanowires with diameters ~100–120 nm were grown on a p-type Si(111) substrate by molecular beam epitaxy (MBE). Detailed magnetic, current/voltage and low-energy electron energy loss spectroscopy measurements were performed to investigate the effect of Te-doping. While intrinsic nanowires are diamagnetic over the temperature range 5–300 K, the Te-doped nanowires exhibit ferromagnetic behavior with the easy axis of magnetism perpendicular to the longitudinal axis of the nanowire. The temperature dependence of coercivity was analyzed and shown to be in agreement with a thermal activation model from 50–350 K but reveal more complex behavior in the low temperature regime. The EELS data show that Te doping introduced a high density of states (DOS) in the nanowire above the Fermi level in close proximity to the conduction band. The plausible origin of ferromagnetism in these Te-doped GaAsSb nanowires is discussed on the basis of d⁰ ferromagnetism, spin ordering of the Te dopants and the surface-state-induced magnetic ordering.

First-principles study the single-layer transition metal trihalide CrXSe3 (X = Sn, Ge, Si) as monolayer ferromagnetic semiconductor

Layered transition metal trihalides ABX₃ are promising candidate materials for monolayer magnets. In this paper, we investigated single-layer CrXSe₃ (X  =  Sn, Ge, Si) as monolayer ferromagnetic semiconductors (FMS). Firstly, our calculated interlayer binding energies and mechanical properties demonstrate the feasibility of obtaining the free-standing monolayer CrXSe₃ from the layered van der Waals crystal CrXSe₃ via mechanical exfoliating method. Plus, we find that the ferromagnetic (FM) super-exchange interaction dominates over the anti-ferromagnetic (AFM) direct-exchange interactions, making CrSnSe₃ and CrGeSe₃ monolayers FM with the magnetic moments of 6.0 µ _B per unit cell. Particularly, the FM configurations of CrSnSe₃ and CrGeSe₃ monolayers become more stable under the increasing tensile strain, and CrSiSe₃ converts to FM from AFM under biaxial tensile strain larger than 2%. Additionally, the three monolayers CrXSe₃ are all semiconducting with energy band gaps of 0.76, 0.87 and 1.1 eV for X being Sn, Ge and Si, respectively. Our results suggest CrXSe₃ as monolayer FMS hold promising potential in spintronics.

Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS Interfaces: Band Alignment and Magnetic Structure

Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field, and local proximitized magnetic exchange. In this work, we present lattice-matched hybrid epitaxy of semiconductor-ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure and their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the band gap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the overall ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs and magnetic moments outside the detection limits on the pure InAs side. This work presents a step toward realizing defect-free semiconductor-ferromagnetic insulator epitaxial hybrids for spin-lifted quantum and spintronic applications without external magnetic fields.

Mechanism of upconversion luminescence enhancement in Yb3+/Er3+ co-doped Y2O3 through Li+ incorporation

Li⁺ doping is a well-known, simple, yet efficient strategy to optimize the properties of upconverting materials. Nonetheless, the position of Li⁺ in the lattice and the mechanism of upconversion enhancement are still controversial, especially in Yb³⁺/Er³⁺ co-doped Y₂O₃. This paper presents a comprehensive investigation of the above issues (i.e. the position occupied by Li⁺ in the lattice and the mechanism of luminescence enhancement, in terms of decreased defects) by studying (Y_0.78-XYb_0.20Er_0.02Li_X)₂O₃ powders. Neutron powder diffraction was employed for the first time in the literature to show that Li⁺ ions are accommodated in Y sites of YO₆ octahedra, confirmed also by the content of oxygen defects, which was increased with the increase of Li⁺ concentration. FT-IR showed that there was a small change in the amount and the type of the surface-absorbed groups with the increase in the Li⁺ content, thus not supporting the prevailing conclusion that the quenching groups are decreased by doping Li⁺. Positron annihilation lifetime (PLAS) experiments showed that the total defect concentration and the large defect clusters, which are considered as quenching centers, are decreased with increasing Li⁺-content, resulting in the enhancement of the emission intensity in Yb³⁺/Er³⁺ co-doped Y₂O₃.

Strong Coupling of Folded Phonons with Plasmons in 6H-SiC Micro/Nanocrystals

Silicon carbide (SiC) has a large number of polytypes of which 3C-, 4H-, 6H-SiC are most common. Since different polytypes have different energy gaps and electrical properties, it is important to identify and characterize various SiC polytypes. Here, Raman scattering is performed on 6H-SiC micro/nanocrystal (MNC) films to investigate all four folded transverse optic (TO) and longitudinal optic (LO) modes. With increasing film thickness, the four folded TO modes exhibit the same frequency downshift, whereas the four folded LO modes show a gradually-reduced downshift. For the same film thickness, all the folded modes show larger frequency downshifts with decreasing MNC size. Based on plasmons on MNCs, these folded modes can be attributed to strong coupling of the folded phonons with plasmons which show different strengths for the different folded modes while changing the film thickness and MNC size. This work provides a useful technique to identify SiC polytypes from Raman scattering.

Defect-Enhanced Charge Separation and Transfer within Protection Layer/Semiconductor Structure of Photoanodes

Silicon (Si) requires a protection layer to maintain stable and long-time photoanodic reaction. However, poor charge separation and transfer are key constraint factors in protection layer/Si photoanodes that reduce their water-splitting efficiency. Here, a simultaneous enhancement of charge separation and transfer in Nb-doped NiO_x /Ni/black-Si photoanodes induced by plasma treatment is reported. The optimized photoanodes yield the highest charge-separation efficiency (η_sep ) of ≈81% at 1.23 V versus reversible hydrogen electrode, corresponding to the photocurrent density of ≈29.1 mA cm^-2 . On the basis of detailed characterizations, the concentration and species of oxygen defects in the NiO_x -based layer are adjusted by synergistic effect of Nb doping and plasma treatment, which are the dominating factors for forming suitable band structure and providing a favorable hole-migration channel. This work elucidates the important role of oxygen defects on charge separation and transfer in the protection layer/Si-based photoelectrochemical systems and is encouraging for application of this synergistic strategy to other candidate photoanodes.

Robust 2D Room-Temperature Dilute Ferrimagnetism Enhancement in Freestanding Ammoniated Atom-Thin [0001] h-BN Nanoplates

The author reports an anthracene vapor-assisted transport growth (∼4.8% in yield) of freestanding atomically thin pristine two-dimensional (2D) hexagonal boron nitride (h-BN) nanoplates directly from bulk powders. A room-temperature dilute magnetism was first observed in the pyrolytic 2D [0001] h-BN nanoplates, which is attributed to the missing N atom numbers (N_N) or existence of a nitrogen-vacancy (N_v) with volume fraction ∼1.46%. Upon the postannealing in ammonia, the unsupported ammoniated 2D h-BN nanoplates showed an enhanced robust ferrimagnetism with the effective magnetic moment as high as 0.024 μ_B/per N, and Néel temperature at 174.9 K. At 295 K, a symmetric electron paramagnetic resonance peak signal was experimentally measured at g ∼ 2.1267, revealing the presence of an unpaired electron trapped at a B atom site in B-rich h-BN. As a promising 2D dilute magnetic semiconductor candidate, our finding favors the ammoniated atom-thin h-BN nanoplate for realization of spintronic nanodevices operating at room temperature.

Synergistic Phase and Disorder Engineering in 1T-MoSe2 Nanosheets for Enhanced Hydrogen-Evolution Reaction

MoSe₂ is a promising earth-abundant electrocatalyst for the hydrogen-evolution reaction (HER), even though it has received much less attention among the layered dichalcogenide (MX₂ ) materials than MoS₂ so far. Here, a novel hydrothermal-synthesis strategy is presented to achieve simultaneous and synergistic modulation of crystal phase and disorder in partially crystallized 1T-MoSe₂ nanosheets to dramatically enhance their HER catalytic activity. Careful structural characterization and defect characterization using positron annihilation lifetime spectroscopy correlated with electrochemical measurements show that the formation of the 1T phase under a large excess of the NaBH₄ reductant during synthesis can effectively improve the intrinsic activity and conductivity, and the disordered structure from a lower reaction temperature can provide abundant unsaturated defects as active sites. Such synergistic effects lead to superior HER catalytic activity with an overpotential of 152 mV versus reversible hydrogen electrode (RHE) for the electrocatalytic current density of j = -10 mA cm^-2 , and a Tafel slope of 52 mV dec^-1 . This work paves a new pathway for improving the catalytic activity of MoSe₂ and generally MX₂ -based electrocatalysts via a synergistic modulation strategy.

Controllable growth of vertically aligned graphene on C-face SiC

We investigated how to control the growth of vertically aligned graphene on C-face SiC by varying the processing conditions. It is found that, the growth rate scales with the annealing temperature and the graphene height is proportional to the annealing time. Temperature gradient and crystalline quality of the SiC substrates influence their vaporization. The partial vapor pressure is crucial as it can interfere with further vaporization. A growth mechanism is proposed in terms of physical vapor transport. The monolayer character of vertically aligned graphene is verified by Raman and X-ray absorption spectroscopy. With the processed samples, d⁰ magnetism is realized and negative magnetoresistance is observed after Cu implantation. We also prove that multiple carriers exist in vertically aligned graphene.

Wetting behavior of water on silicon carbide polar surfaces

Technically important wide band-gap semiconductors such as GaN, AlN, ZnO and SiC are crystallized in polar structures.

Electron spin-polarization and spin-gapless states in an oxidized carbon nitride monolayer

A stable 2D spin-gapless honeycomb lattice oxidized carbon nitride material, C₂NO.

Carbon p electron ferromagnetism in silicon carbide

Ferromagnetism can occur in wide-band gap semiconductors as well as in carbon-based materials when specific defects are introduced. It is thus desirable to establish a direct relation between the defects and the resulting ferromagnetism. Here, we contribute to revealing the origin of defect-induced ferromagnetism using SiC as a prototypical example. We show that the long-range ferromagnetic coupling can be attributed to the p electrons of the nearest-neighbor carbon atoms around the VSiVC divacancies. Thus, the ferromagnetism is traced down to its microscopic electronic origin.

Near-infrared luminescent cubic silicon carbide nanocrystals for in vivo biomarker applications: an ab initio study

Molecule-sized fluorescent emitters are much sought-after to probe biomolecules in living cells. We demonstrate here by time-dependent density functional calculations that the experimentally achievable 1-2 nm sized silicon carbide nanocrystals can emit light in the near-infrared region after introducing appropriate color centers in them. These near-infrared luminescent silicon carbide nanocrystals may act as ideal fluorophores for in vivo bioimaging.