Nonmagnetic Semiconductor Research Articles

Ferromagnetic Cu3N Nanoparticles Demonstrated by X-ray Magnetic Circular Dichroism (XMCD) and the Density Functional Theory (DFT) Calculations

In recent years, ferromagnetism induced by natural defects of nonmagnetic semiconductors has been widely investigated and expected to be applied in spintronics. On this basis, we report the ferromagnetic behavior of copper (I) nitride (Cu3N) nanoparticles. A robust room temperature ferromagnetism is found in Cu3N nanoparticles with the saturated magnetization of 4 memu/g (300 K). Based on the element-specific X-ray magnetic circular dichroism (XMCD) and the density functional theory (DFT) analysis, it is concluded that the ferromagnetism of Cu3N nanoparticles originate from the surface Cu vacancies. Moreover, by increasing the surface area of Cu3N, the variation of magnetism is realized, and the surface states related to ferromagnetism is further revealed.

Excited states in hydrogenated single-layer MoS2

Our calculations of the excitation spectrum of single-layer MoS2 at several hydrogen coverages, using a density-matrix based time-dependent density-functional theory (TDDFT) show that the fully hydrogenated system is metallic, while at lower coverages the spectrum consists of spin-polarized partially filled localized mid-gap states. The calculated absorption spectrum of the system reveals standard excitonic peaks corresponding to the bound valence-band hole and conduction-band electron, as well as excitonic peaks that involve the mid-gap states. Binding energies of the excitons of the hydrogenated system are found to be relatively large (few tens of meV), making their experimental detection facile and suggesting hydrogenation as a knob for tuning the optical properties of single-layer MoS2. Importantly, we find hydrogenation to suppress visible light photoluminescence, in agreement with experimental observations. In contrast, both Li and Na atoms transform the system into an n-doped non-magnetic semiconductor that does not allow excitonic states.

Thermodynamic phase diagram and thermoelectric properties of LiMgZ (Z = P, As, Bi): ab initio method study

ABSTRACT Using first principle calculations based on the density functional theory (DFT), the structural, elastic, electronic, and thermoelectric properties, and thermodynamic stability of the LiMgZ (Z = P, As, Bi) half-Heusler ternary compounds were studied. The results of structural calculations and elastic constants and investigation of the thermodynamic phase diagram represent stability for these compounds in the half-Heusler cubic structure with F4-3 m symmetry. Studying the electronic structure using generalised gradient approximation (GGA) as well as spin-orbit calculations (SOC) reveal that all three LiMgZ (Z = P, As, Bi) structures are non-magnetic semiconductors with an indirect band gap along Г-X direction with values of 1.63, 1.37 and 0.5 eV, respectively. The electron transport behaviours of these materials at various temperatures show that their dimensionless figure of merit (ZT) at the temperature of 200 K reaches its maximum value of 0.813, 0.81 and 0.78 for LiMgP, LiMgAs and LiMgBi, respectively.

Electronic and magnetic properties of 3d transition metal atom adsorbed Zr2CO2 Mxene: First-principles study

The structural, electronic and magnetic properties of the 3d transition metal (TM) atoms (from Sc to Zn) adsorbed Zr2CO2 Mxenes were investigated systematically by using first-principles calculations. The adsorption energies demonstrate that all the TM atoms are chemically adsorbed above the C atom (TC) sites of the Zr2CO2 Mxenes except for the Zn atom. Furthermore, the analyses of bond length, charge transfer and vDW interaction ratio demonstrate that the interaction between adatoms (from Sc to Cu) and under-coordinated atoms is mainly covalent bonding features. Cu adsorbed Zr2CO2 Mxene exhibits nonmagnetic metal, and Ni and Zn adsorbed systems show nonmagnetic semiconductor. While Sc, V, Cr, Mn and Co adsorbed Zr2CO2 Mxenes exhibit magnetic metal. Interestingly, the Fe adsorbed system shows bipolar magnetic semiconductor character. Especially, the Ti adsorbed system exhibits half-metallic character. Our results demonstrate that the 3d TM atom adsorbed Zr2CO2 Mxene could provide various potential application in nanoelectronics and spintronics.

Strain-improved electronic and magnetic properties of V-, Cr-, Mn- and Fe-doped α- and β-tellurene

The effect of biaxial strain on the electronic and magnetic properties of V-, Cr-, Mn- and Fe-doped α- and β-tellurene was researched by the first-principles calculations. Under the strains from −6% to 6%, the bandgap of α- and β-tellurene changes obviously, and the strained α- and β-tellurene systems keep the characteristic of non-magnetic semiconductors. Unstrained α- and β-tellurene structures doped with V, Cr, Mn and Fe atoms show different electronic properties and magnetic moments. The transition metal (TM) doped tellurene structures undergo the transformations with magnetic half-metallic-magnetic metal-magnetic semiconductor under −6% to 6% strains. Except for Mn-doped β-tellurene, the magnetic moments of other TM-doped tellurene have little change under different strains. The magnetic moment of Mn-doped β-tellurene suddenly decreases from 5μB to 2μB under the −4% and −6% strains. This is mainly due to decreasing TM-Te bond lengths and increasing interaction between TM and Te atoms under the large strains. In conclusion, TM doping and strain-engineering can effectively inject magnetic and change the magnetic moment and electronic properties of tellurene. These results have important guiding significance for the further experimental research of tellurene-based spintronics and magnetic devices.

Antiferromagnetism-induced second-order nonlinear optical responses of centrosymmetric bilayer CrI3

Antiferromagnetism (AF) in AB′-stacked centrosymmetric bilayer (BL) CrI3 breaks both spatial inversion (P) and time-reversal (T) symmetries but maintains the combined PTsymmetry, thus inducing novel second-order nonlinear optical (NLO) responses such as second-harmonic generation (SHG), linear electric-optic effect (LEO) and bulk photovoltaic effect (BPVE). In this work, we calculate AF-induced NLO responses of the BL CrI3 based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Coulomb correlation (U), i.e., the GGA+U method. Interestingly, we find that the magnetic SHG, LEO and photocurrent in the AF BL CrI3 are huge, being comparable or even larger than that of the well-known nonmagnetic noncentrosymmetric semiconductors. For example, the calculated SHG coefficients are in the same order of magnitude as that of MoS2 monolayer (ML), the most promising 2D material for NLO devices. The calculated LEO coefficients are almost three times larger than that of MoS2 ML. The calculated NLO photocurrent in the CrI3BL is among the largest values predicted so far for the BPVE materials. On the other hand, unlike nonmagnetic semiconductors, the NLO responses in the AF BL CrI3 are nonreciprocal and also switchable by rotating magnetization direction. Therefore, our interesting findings indicate that the AF BL CrI3 will not only provide a valuable platform for exploring new physics of low-dimensional magnetism but also have promising applications in magnetic NLO and LEO devices such as frequency conversion, electro-optical switches, and light signal modulators as well as high energy conversion efficiency photovoltaic solar cells.

Great enhancement of Curie temperature and magnetic anisotropy in two-dimensional van der Waals magnetic semiconductor heterostructures

In two-dimensional (2D) magnetic systems, large magnetic anisotropy is needed to stabilize the magnetic order according to Mermin-Wagner theorem. Based on density functional theory (DFT) calculations, we propose that the magnetic anisotropic energy (MAE) of 2D ferromagnetic (FM) semiconductors can be strongly enhanced in van der Waals heterostructures by attaching a nonmagnetic semiconductor monolayer with large spin-orbit coupling. We studied Cr2Ge2Te6/PtSe2 bilayer heterostructures, where each layer has been realized in recent experiments. The DFT calculations show that the MAE of Cr2Ge2Te6/PtSe2 is enhanced by 70%, and the Curie temperature TC is increased far beyond room temperature. A model Hamiltonian is suggested to analyze the DFT results, showing that both the Dzyaloshinskii-Moriya interaction and the single-ion anisotropy contribute to the enhancement of the MAE. Based on the superexchange picture, we find that the decreased energy difference between 3d orbitals of Cr and 5p orbitals of Te contributes partially to the increase of TC. Our present work indicates a promising way to enhance the MAE and TC by constructing van der Waals semiconductor heterostructures, which will inspire further studies on the 2D magnetic semiconductor systems.

Design of half-heusler thermoelectric compound TiFe0.5Ni0.5Sb with special quasi-random structure using 18-electron rule

The double half-Heusler compound TiFe0.5Ni0.5Sb with special quasi-random structure (SQS) has been modeled by using 18-electron rule, which can simulate the inevitable atomic occupation disorder between two sublattices. We studied the electronic structure and thermoelectric (TE) properties of this compound based on first-principle calculations and semi-classical Boltzmann transport theory. The calculated band structure indicates that this compound belongs to a nonmagnetic indirect band gap semiconductor with a band gap of 0.45 eV. The lattice thermal conductivity κl at room temperature is 11.24 WK−1m−1, which was lower than 18.7 WK−1m−1 for the 18-electron TiCoSb, and decreases as the temperature increased. The reason is that the lattice vibration is enhanced while thermal conductivity is suppressed when the temperature rises. The dimensionless figure of merit ZT of p-type TiFe0.5Ni0.5Sb at 1390 K is evaluated to be 0.35, which is higher than 0.1 for its parent compound 17-electron TiFeSb. The theoretical prediction for TiFe0.5Ni0.5Sb can simulate corresponding experimental results because of the use of SQS and will increase the experimental research of other disordered alloys using 18-electron rule toward designing and improving TE performance.

Investigation of the mono vacancy effects on the structural, electronic and magnetic properties of graphene hexagonal-boron nitride in-plane hybrid embracing diamond shaped graphene island

The preparation of an atomically thin sheet of graphene/h-BN (GBN) hybrid with lack of defects is almost unachievable as in three dimensional crystals. Here, we first discussed the stability and electronic properties of GBN hybrid nanosheet which is formed by implanting a diamond shaped graphene (G) island into the hexagonal boron nitride (h-BN) layout. We further investigated the effects of mono vacancy on the electronic and magnetic properties of defective GBN nanosheet. The band gap of pristine hybrid decreases with growing G island as expected and the island induces flat (dispersionless) energy bands near the Fermi energy of the h-BN nanosheet. We searched for the energetics of 7 distinct C, B and N mono vacancies created at various places such as h-BN region, G region and interface of the optimized pristine hybrid. The G-BN interface is found as the energetically most favourable place for vacancy formation. Depending on the size of the graphene island, the C vacancy is the most favourable defect type in our DFT calculations. The N vacancy is energetically preferred over the B vacancy due to its lower formation energy with the exception of biggest G island. Although pristine hybrids are non-magnetic semiconductors, the GBN hybrid can become magnetic with a reasonable amount of magnetic moment depending on the type of vacancy and vacancy site. The vacancy defected hybrids also show the properties of semiconductor except for the hybrid involving the smallest size island with the B vacancy in the h-BN layout (VBL). Defected hybrids involving the smallest G island have the highest amount of band gaps due to high ratio of carbon atoms at the interface to the island interior. Accordingly, the band gap tends to decrease with expanding G island. However, the band gap of VBL defected structures broadens with increasing size of island interestingly. Moreover, this defected hybrid introduces the greatest amount of magnetic moment (3 μB) as the island expands. The C vacancies in the G island and around the N atom introduce a magnetic moment of 2 μB as the G island expands as well. The magnetic moment of the N vacancy defected hybrid is 1 μB irrespective of island size. The vacancy defect leads local in-plane/out-of-plane distortions rather than structural reconstructions in our calculations. In general, no correlation was observed between the island size and the magnetic moment of a defective hybrid.

(Invited) Manipulating Carrier Polarization in Semiconductor Nanocrystals

Degenerately-doped semiconductor nanocrystals exhibiting tunable localized surface plasmon resonance (LSPR) have attracted significant attention in recent years due to their unique optoelectronic properties. Unlike noble metal nanoparticles, colloidal plasmonic semiconductor nanocrystals have LSPR frequencies tunable in the infrared region, which makes them appealing for terahertz imaging, heat-responsive devices, and surface-enhanced infrared spectroscopic measurements. Besides expanding the LSPR frequency range, plasmonic semiconductor nanocrystals could potentially bring about numerous other opportunities related to single-phase plasmon-exciton interactions. However, non-resonant nature of the LSPR and exciton in semiconductors has been a major obstacle toward realizing such opportunities. In this talk I will discuss the results of our recent work on the structure and composition dependent plasmonic properties of colloidal semiconductor nanostructures. I will particularly focus on generating robust excitonic splitting in degenerately-doped transparent metal oxide nanocrystals, enabled by non-resonant plasmon-exciton coupling in an external magnetic field. This phenomenon allows for controlling carrier polarization in semiconductor nanocrystals using circularly polarized light. Furthermore, the ability to control carrier localization and nanocrystal morphology allows for further manipulation of the excitonic splitting pattern and quantum states in these non-magnetic semiconductor nanostructures. Possible applications of this emerging class of multifunctional materials for new electronic and quantum information technologies will also be discussed.

Oxygen Vacancies in the Single Layer of Ti2CO2 MXene: Effects of Gating Voltage, Mechanical Strain, and Atomic Impurities

Herein, using first‐principles calculations the structural and electronic properties of the Ti2CO2 MXene monolayer with and without oxygen vacancies are systematically investigated with different defect concentrations and patterns, including partial, linear, local, and hexagonal types. The Ti2CO2 monolayer is found to be a semiconductor with a bandgap of 0.35 eV. The introduction of oxygen vacancies tends to increase the bandgap and leads to electronic phase transitions from nonmagnetic semiconductors to half‐metals. Moreover, the semiconducting characteristic of O‐vacancy Ti2CO2 can be adjusted via electric fields, strain, and F‐atom substitution. In particular, an electric field can be used to alter the nonmagnetic semiconductor of O‐vacancy Ti2CO2 into a magnetic one or into a half‐metal, whereas the electronic phase transition from a semiconductor to metal can be achieved by applying strain and F‐atom substitution. The results provide a useful guide for practical applications of O‐vacancy Ti2CO2 monolayers in nanoelectronic and spinstronic nanodevices.

Quasi-nondegenerate pump–probe magnetooptical experiment in GaAs/AlGaAs heterostructure based on spectral filtration

We report on a quasi-nondegenerate pump–probe technique that is based on spectral-filtration of femtosecond laser pulses by a pair of mutually-spectrally-disjunctive commercially available interference filters. The described technique enables to obtain pump and probe pulses with wavelengths that are spectrally close but distinct. These contradictory requirements, which are dictated, for example, by a suppression of stray pump photons from the probe beam in spin-sensitive magneto-optical experiments in non-magnetic semiconductors, can be fulfilled at very low cost and basically no requirement on space. Especially the second feature is important in pump–probe microscopy where collinear propagation of pump and probe pulses is dictated by utilization of a microscopic objective and where the setups are typically quite complex but suffer from a limited size of optical breadboards. Importantly, this spectral-filtration of 100 fs long laser pulses does not affect considerably the resulting time-resolution, which remains well below 500 fs. We demonstrate the practical applicability of this technique by performing spin-sensitive magnetooptical Kerr effect (MOKE) experiment in GaAs/AlGaAs heterostructure, where a high-mobility spin system is formed after optical injection of electrons at wavelengths close to the MOKE resonance. In particular, we studied the time- and spatial-evolutions of spin-related (MOKE) and charge-related (reflectivity) signals. We revealed that they evolve in a similar but not exactly the same way which we attributed to interplay of several electron many-body effects in GaAs.

Sn replacement influence on magnetic, electronic, thermodynamic, thermoelectric and transport properties in shandite compounds of Co3In2−xSnxS2

In this paper, we have investigated some physical properties of Co3In2−xSnxS2 (x = 0, 1, and 2) compounds. The doping in Co3In2S2, through chemical substitution of indium by tin as a low-cost neighboring element, affects their structural, electronic, magnetic, thermodynamic, and thermoelectric properties. The density functional theory (DFT) calculations show that indium substitution leads to a transition from weak-ferromagnetic metal (x = 0), to nonmagnetic semiconductor with low band gap energy at x = 1, and to a ferromagnetic half-metal at x = 2. The thermal properties, obtained by using the Gibbs code, were evaluated with temperature at various pressures from 0 to 20 GPa. The results demonstrated that chemical substitution in the studied materials affects their physical properties leading to an interest candidate for thermoelectric uses at ambient or at low temperature.

Ab initio identification of two-dimensional square-octagonal bismuthene doped with 3d transition metals as potential spin gapless semiconductor, bipolar magnetic semiconductor, and quantum anomalous Hall insulator

The electronic structures and magnetic properties of square-octagonal bismuthene embedded with 3 d transition elements were studied systematically by the first-principles calculations. The formation energies of all the cases are negative, indicating the feasibility and practicability for these doped monolayer bismuthene. No considerable spin splitting appears in Sc-, Co-, and Zn-doped compounds. After doping, Sc- and Co-doped compounds are semiconductors while Zn-doped compound is metal. Ti-, V-, Cr-, Mn-, Fe-, Ni-, and Cu-doped systems behave as magnetic semiconductors. Among them, Cu-doped system exhibits both desirable spin gapless and bipolar magnetic semiconducting characteristics. In addition, Fe-doped system is a wonderful candidate for realizing quantum anomalous Hall insulating state. • Sc- and Co-doped square-octagonal bismuthene are nonmagnetic semiconductors. • Zn-doped square-octagonal bismuthene is nonmagnetic metal. • Fe-doped square-octagonal bismuthene is a ferromagnetic Chern insulator for realizing quantum anomalous Hall insulating state. • Cu-doped square-octagonal bismuthene is either spin gapless or bipolar magnetic semiconductor.

Study of the structural and electronic properties of three- and two-dimensional transition-metal dioxides using first-principles calculations

The structural, electronic and magnetic properties of three- and two-dimensional transition-metal dioxides (VO2, CrO2, MoO2 and WO2) have been studied based on density functional theory. We found that the equilibrium structural parameters of monolayers change slightly with respect to the structural parameters of their counterparts in bulk, and that the cohesion energies calculated for dioxides in bulk and their corresponding monolayers are all positive and with the 5.0–7.0 eV/atom range. Additionally, the calculated formation energies of the bulk are greater than the energies of its receptive monolayer, indicating that the compounds in bulk are more stable than their corresponding monolayers, as expected. For the monolayers, the exfoliation energies obtained were 16.25, 18.69, 19.21 and 62.55 meV/A2 for VO2, CrO2, MoO2 and WO2 monolayers respectively. These values are close to the exfoliation energy of graphene (21 meV/A2), except for the WO2 monolayer and exfoliation energy values are not available theoretically nor experimentally. Finally, the study of the band structure and the density states shows that VO2 exhibits a magnetic metallic behavior both in bulk and in monolayer, reaching a magnetic moment of 0.22 μβ/atom which is produced by the hybridization and polarization between V-d and O-p. Meanwhile, the CrO2, MoO2 and WO2 materials have a non-magnetic semiconductor behavior both in bulk and in monolayer.

The structure, and electronic and magnetic properties of MX (M=GA, IN; X=S, SE, TE) nanoribbons

We used the first principle of density functional theory to perform detailed calculations regarding the structure, and the electronic and magnetic properties of MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se, Te) nanoribbons. The armchair nanoribbons (ARNs) are nonmagnetic semiconductors, which have even or odd oscillations of bandgaps. All small-sized zigzag nanoribbons (ZRNs) were found to break the six-membered ring structure and move to the center, thereby exhibiting nonmagnetic semiconductor behavior owing to the quantum confinement effect. However, among the large ZRNs, which are all metals, MTe ZRNs are nonmagnetic; this differs from the case of graphene, MoS2 and Ti2CO2 nanoribbons. MX (M[Formula: see text]=[Formula: see text]Ga, In; X[Formula: see text]=[Formula: see text]S, Se) ZRNs exhibited ferromagnetism owing to the presence of the unpaired electrons on the metal-edge side and the magnetic moment of each pair of molecules, which was controlled by the size of the nanoribbons. The results provided a theoretical reference that can be used in the future to produce MX materials for application in low-dimensional semiconductor devices, spin electron transport devices and new magnetoresistance devices.

Spin-Dependent Phenomena in Semiconductor Micro-and Nanoparticles—From Fundamentals to Applications

The present overview of spin-dependent phenomena in nonmagnetic semiconductor microparticles (MPs) and nanoparticles (NPs) with interacting nuclear and electron spins is aimed at covering a gap between the basic properties of spin behavior in solid-state systems and a tremendous growth of the experimental results on biomedical applications of those particles. The first part of the review represents modern achievements of spin-dependent phenomena in the bulk semiconductors from the theory of optical spin orientation under indirect optical injection of carriers and spins in the bulk crystalline silicon (c-Si)—via numerous insightful findings in the realm of characterization and control through the spin polarization—to the design and verification of nuclear spin hyperpolarization in semiconductor MPs and NPs for magnetic resonance imaging (MRI) diagnostics. The second part of the review is focused on the electron spin-dependent phenomena in Si-based nanostructures, including the photosensitized generation of singlet oxygen in porous Si and design of Si NPs with unpaired electron spins as prospective contrast agents in MRI. The experimental results are analyzed by considering both the quantum mechanical approach and several phenomenological models for the spin behavior in semiconductor/molecular systems. Advancements and perspectives of the biomedical applications of spin-dependent properties of Si NPs for diagnostics and therapy of cancer are discussed.

Two-dimensional gallium and indium oxides from global structure searching: Ferromagnetism and half metallicity via hole doping

There has been tremendous research effort in hunting for novel two-dimensional (2D) materials with exotic properties, showing great promise for various potential applications. Here, we report the findings about a new hexagonal phase of 2D Ga2O3 and In2O3, with high energetic stability, using a global searching method based on an evolutionary algorithm, combined with density functional theory calculations. Their structural and thermal stabilities are investigated by the calculations of their phonon spectra and by ab initio molecular dynamics simulations. They are predicted to be intrinsically non-magnetic stable semiconductors, with a flatband edge around the valence band top, leading to itinerant ferromagnetism and half-metallicity upon hole doping. Bilayer Ga2O3 is also studied and found to exhibit ferromagnetism without extra hole doping. The Curie temperature of these materials, estimated using Monte Carlo simulations based on the Heisenberg model, is around 40–60 K upon a moderate hole doping density.

Electron beam-induced nanopores in Bernal-stacked hexagonal boron nitride

Controlling the size and shape of nanopores in two-dimensional materials is a key challenge in applications such as DNA sequencing, sieving, and quantum emission in artificial atoms. We here experimentally and theoretically investigate triangular vacancies in (unconventional) Bernal-stacked AB-h-BN formed using a high-energy electron beam. Due to the geometric configuration of AB-h-BN, triangular pores in different layers are aligned, and their sizes are controlled by the duration of the electron irradiation. Interlayer covalent bonding at the vacancy edge is not favored, as opposed to what occurs in the more common AA′-stacked BN. A variety of monolayer, concentric, and bilayer pores in the bilayer AB-h-BN are observed in high-resolution transmission electron microscopy and characterized using ab initio simulations. Bilayer pores in AB-h-BN are commonly formed and grow without breaking the bilayer character. Nanopores in AB-h-BN exhibit a wide range of electronic properties, ranging from half-metallic to non-magnetic and magnetic semiconductors. Therefore, because of the controllability of the pore size, the electronic structure is also highly controllable in these systems and can potentially be tuned for particular applications.

Ferromagnetism and half-metallicity in two-dimensional MO (M=Ga,In) monolayers induced by hole doping

Identification of two-dimensional (2D) materials with magnetic properties has received strong research attention in the development of advanced spin-based devices. By means of first-principles calculations, we investigate the stability, electronic properties, and the hole-doping-induced magnetic properties of metal oxide ($M\mathrm{O}, M=\mathrm{Ga},\mathrm{In}$) monolayers. They are intrinsically nonmagnetic stable semiconductors, with high energetic, vibrational, and thermal stability. Hole doping can switch them from nonmagnetic to ferromagnetic and turn them into half-metals over a wide range of hole densities. Monte Carlo simulations predict that the highest Curie temperature of the GaO monolayer can reach \ensuremath{\sim}125 K. Our results indicate that monolayer $M\mathrm{O}$ could be eligible candidate materials for 2D spintronic devices.