Effects Of Impurity Doping Research Articles

Fe doping induced cluster/spin glass state and metamagnetic transition in phase separated La0.5Sr0.5Mn1−xFexO3

We report the impurity doping effects in manganites La0.5Sr0.5Mn1−xFexO3 (0 ≤ x ≤ 0.2) and the field-induced metamagnetic transition in which the antiferromagnetic insulating phase transforms into ferromagnetic metallic phase. The Rietveld refined results of XRD data display no obvious structure change with a significant degree of Fe3+ doping on Mn sites. Significant ferromagnetism suppression and magnetic inhomogeneity tendency upon Fe3+-doping are evidenced, phase separated and spin-glass like states are identified. With applying a pulsed high magnetic field up to 50 T, a metamagnetic transition was induced, especially clear for x ≥ 0.1 compounds, which exhibited an irreversible phase transition from antiferromagnetic to ferromagnetic phase, and a saturation magnetization was obtained. The replacement of Mn3+ by Fe3+ ions eliminates the effects of lattice structure change on the magnetic and electronic properties because of the very similar ionic size of Mn3+ and Fe3+, but reduces the Mn3+/Mn4+ ratio and depopulates the number of available hopping sites and the hopping electrons, thus weakens the double exchange interaction. The occurrence of the AFM superexchange interaction via Fe3+–O2−–Fe3+ and Fe3+–O2−–Mn3+ bonds may bring about the competition between the antiferromagnetic ordering and the ferromagnetic ordering, leading to a cluster/spin-glass behavior in the samples with high Fe dopants concentration.

Impurity doping effects and ecological friendly synthesis of cerium sulfide powders for enhanced chromaticity and color tunability

Red non-toxic and environmentally friendly γ-phase cerium sulfide powders have promising applications in the fields of coatings, outdoor displays and smart buildings. However, the development of an environmentally friendly preparation process and the improvement of production efficiency of cerium sulfide powders remain major challenges. In this study, eco-friendly γ-Ce2S3 powders with excellent color properties have been produced by adding ammonium thiocyanate to the precursor of cerium without H2S or CS2. The effect of impurities (mainly Ce2O2S) with different amounts on spectral properties of cerium sulfide powders has been studied through chemical and physical methods. In addition, the significance of impurity doping on the reaction synthesis was studied by combining detailed experimental characteristics with density functional theory calculations. The results show that the reduction of impurity content can greatly improve the chromaticity value of γ-Ce2S3 and change color from dark red to bright red.

Finite temperature effects on the structural stability of Si-doped HfO2 using first-principles calculations

The structural stabilities of the monoclinic and tetragonal phases of Si-doped HfO2 at finite temperatures were analyzed using a computational scheme to assess the effects of impurity doping. We proposed a method that the finite temperature effects, i.e., lattice vibration and impurity configuration effects, are considered. The results show that 6% Si doping stabilizes the tetragonal phase at room temperature, although a higher concentration of Si is required to stabilize the tetragonal phase at zero temperature. These data indicate that lattice vibration and impurity configuration effects are important factors determining structural stability at finite temperatures.

Segregation behaviors of nonmetallic impurities on grain boundary of austenitic steel

Grain boundary (GB) always takes a critical role in the mechanical properties of most engineering materials, and austenitic steel (γ-Fe) is no exception. However, the microscopic mechanism of impurity segregation on γ-Fe GB is still unclear. In this work, we explored the stabilities of eight γ-Fe GBs by density functional theory (DFT) and chose the Σ5 (310) GB as a representative. Here, the separation energy, GB energy, strengthening energy, and theoretical tensile stress of Σ5 (310) GB doped by eight common nonmetallic elements (H, B, C, N, O, Si, P, and S) were analyzed to evaluate the effect of impurities segregation comprehensively. The results show that except for H and Si, impurities of B, C, N, O, P and S all can stabilize Σ5 (310) GB. However, only the B element can strengthen Σ5 (310) GB. Finally, the calculated electron transfer of doped Σ5 (310) GB unveils the superiority of the boron element and the segregation effect of other impurities. This work provides a systematical insight into the effect of impurity doping on Σ5 (310) GB, which will be a useful reference for γ-Fe processing research.

Electric field and charged impurity doping effects on the Schottky anomaly of β12-borophene

Due to the coexistence of Dirac and triplet fermions, monolayer β12-borophene has recently attracted both experimental and theoretical researchers. In particular, various phase transitions have been recently reported in the structure, in the presence of dilute charged impurity and a perpendicular electric field, leading to interesting electronic heat capacity (HC). In this paper, we systematically examine the effects of charged impurity doping and electric field on the HC of monolayer β12-borophene. To do this, we utilize the five-band tight-binding Hamiltonian model, the Green's function, T-matrix, and the Born approximation for different models considering the substrate effects. Numerical analysis reveals that the inversion symmetric model is the proper model in the pristine and perturbed metallic β12-borophene, leading to a regular reduction of HC with both charged impurity and electric field. Moreover, the pristine and perturbed Schottky anomaly alterations are fully addressed. Unforeseeably, HC irregularly fluctuates with impurity in the homogeneous model. We believe that our results provide new physical insights into the thermal properties of monolayer β12-borophene.

In-Plane Dual-Gated Spin-Valve Device Based on the Zigzag Graphene Nanoribbon

Using the nonequilibrium Green's function combined with density-functional theory, we systematically study the transport properties of the zigzag graphene nanoribbon (ZGNR). When an external transverse electric field is applied, a ZGNR can exhibit half-metallic characteristics with its conductive channel localized at the edge positions. Accordingly, an in-plane dual-gated spin-valve device is proposed. Thanks to its unique design, the proposed device overcomes the bottleneck of current leakage and avoids contact issues. Remarkably, a 100% spin injection efficiency and a giant tunnel magnetoresistance of up to ${10}^{7}$ are realized, which represent much better performance than that of traditional magnetic tunnel junctions. In addition, we also explore the effect of architecture reconstruction and impurity doping on spin-polarized transport. It is found that, in general, the tunneling process is hindered if disorder occurs at the conductive edge. By contrast, if we put the deformation or impurities away from the conductive edge, the transport properties may be barely degraded or even improved, depending on the type of disorder. It should be possible to apply these ubiquitous underlying principles to other materials, which could inspire the design of alternative spintronic devices with high performance.

Effect of Auger recombination induced by donor and acceptor states on luminescence properties of silicon quantum Dots/SiO2 multilayers

Impurity doping is a key factor that needs to be addressed to control the optical properties of silicon quantum dots (Si QDs) for their applications in optoelectronic devices. Although there have been several studies to understand the effect on n-type and p-type dopant on the luminescence properties of Si QDs, the exact influence of the dopant impurities on the optical properties and the underlying mechanism are yet to be understood. For this purpose, we investigate the luminescence properties of Si QDs/SiO2 multilayer structure doped with B and P. Using rf-magnetron sputtering and ion implantation techniques, we prepared three kinds of samples: undoped, B-doped and P-doped. Results from photoluminescence measurements indicate that the significant luminescence suppression in the doped samples can be mainly attributed to the Auger non-radiative process occurring between photoexcited excitons and free carriers. Our interpretation should serve to resolve the controversy around the effect of impurity doping on the luminescence properties of Si QDs.

Effects of Impurity Doping on the Luminescence Performance of Mn4+-Doped Aluminates with the Magnetoplumbite-Type Structure for Plant Cultivation.

Mn4+ activated LaMgAl11O19 (LMA/Mn4+) with red emitting phosphor was obtained by sintering under air conditioning. The X-ray diffraction pattern Rietveld refinement results reveal that three six-fold coordinated Al sites are substituted by Mn4+ ions. Furthermore, the chemical valence state of manganese in the LMA host was further confirmed through X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Photoluminescence emission (PL) and excitation (PLE) spectra of LMA/Mn4+ as well as the lifetime were measured, and the 663 nm emission is ascribed to the 2Eg→4A2g from the 3d3 electrons in the [MnO6]8− octahedral complex. The emission spectrum matches well with the absorption of phytochrome. Temperature-dependent PL spectra show that the color changes of the phosphor at 420 K are 0.0110 for Δx and −0.0109 for Δy. Moreover, doping Zn2+ and Mg2+ ions in the host enhances the emission intensity of Mn4+ ions. These results highlight the potential of LMA/Mn4+ phosphor for a light-emitting diode (LED) plant lamp.

Effects of Ga doping on magnetic and ferroelectric properties of multiferroic delafossite CuCrO2 : Ab initio and Monte Carlo approaches

The effects of nonmagnetic impurity doping on magnetic and ferroelectric properties of multiferroic delafossite ${\mathrm{CuCrO}}_{2}$ are investigated by means of density functional theory calculations and Monte Carlo simulations. Density functional theory calculations show that replacing up to $30%$ of ${\mathrm{Cr}}^{3+}$ ions by ${\mathrm{Ga}}^{3+}$ ones does not significantly affect the remaining Cr-Cr superexchange interactions. Monte Carlo simulations show that ${\mathrm{CuCr}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{2}$ preserves its magnetoelectric properties up to $x\ensuremath{\simeq}0.15$ with a spiral ordering, while it becomes disordered at higher fractions. Antiferromagnetic transition shifts towards lower temperatures with increasing $x$ and eventually disappears at $x\ensuremath{\ge}0.2$. Our simulations show that ${\mathrm{Ga}}^{3+}$ doping increases the Curie-Weiss temperature of ${\mathrm{CuCr}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{2}$, which agrees well with experimental observations. Moreover, our results show that the incommensurate ground-state configuration is destabilized by ${\mathrm{Ga}}^{3+}$ doping under zero applied field associated with an increase of frustration. Finally, coupling between noncollinear magnetic ordering and electric field is reported for $x\ensuremath{\le}0.15$ through simulating $P\text{\ensuremath{-}}E$ hysteresis loops, which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model.

Magnonic heat transport in the Lieb lattice

In this article we report the effect of charged impurity doping on the thermal spin-wave excitations of 2D Lieb lattice using the longitudinal/transverse and in-plane magnonic heat transport properties in a systematically Green’s function approach and the Born approximation. We present numerical results for XY Heisenberg model. Accordingly, the flow of magnonic current is enhanced depending on the impurity concentration, in which (i) at very low and very high concentrations, in-plane magnon conductivity becomes negative, (ii) the magnon Seebeck coefficient decreases (increases) before (after) a critical impurity concentration for both longitudinal/transverse and in-plane components, (iii) the longitudinal/transverse magnon thermal conductivity increases slightly with impurity concentration, while the in-plane one becomes flat, and (iv) both longitudinal/transverse and in-plane magnonic heat transport efficiencies decrease (increase) at low (high) temperatures with low impurity concentrations, whereas become flat (decrease) at high impurity concentrations. Our results are useful for the implementation of a new spin energy conversion and capable spin-filter devices for the spintronic community.

Effects of surface nanostructuring and impurity doping on ultrafast carrier dynamics of silicon photovoltaic cells: a pump-probe study

We present femtosecond optical pump-terahertz probe studies on the ultrafast dynamical processes of photo-generated charge carriers in silicon photovoltaic cells with various nanostructured surfaces and doping densities. The pump-probe measurements provide direct insight on the lifetime of photo-generated carriers, frequency-dependent complex dielectric response along with photoconductivity of silicon photovoltaic cells excited by optical pump pulses. A lifetime of photo-generated carriers of tens of nanosecond is identified from the time-dependent pump-induced attenuation of the terahertz transmission. In addition, we find a large value of the imaginary part of the dielectric function and of the real part of the photoconductivity in silicon photovoltaic cells with micron length nanowires. We attribute these findings to the result of defect-enhanced electron–photon interactions. Moreover, doping densities of phosphorous impurities in silicon photovoltaic cells are also quantified using the Drude–Smith model with our measured frequency-dependent complex photoconductivities.

Effect of impurity doping on AFM magnetization in graphene: A tight-binding model approach

We report here a tight-binding theoretical study of anti-ferromagnetism in graphene taking electron hopping up to third nearest neighbors. The graphene on substrate introduces inequivalence in the two sublattices of the honeycomb lattice. The on-site Coulomb interactions at two sublattices of graphene are treated within mean-field approximation and the electron occupancies at two sublattices include spin moments in opposite directions giving rise to anti-ferromagnetism in graphene. The AFM sublattice magnetizations are calculated by using Zubarev’s Green’s function technique and computed self-consistently for different ranges of Coulomb potential, substrate-induced gap and impurity concentrations. It is observed that [Formula: see text]-site magnetization is suppressed by [Formula: see text]-site impurity concentration, while [Formula: see text]-site magnetization remains unaffected. Similar effect has been observed at [Formula: see text]-site when doped with pentavalent dopant, while [Formula: see text]-site magnetization remains unaffected. Furthermore, substrate-induced AFM gap has also been observed at different ranges of temperature.

The Effect of Dilute Charged Impurity on the Electronic Heat Capacity and Magnetic Susceptibility of Ferromagnetic MoS2

In this paper, the effects of dilute charged impurity doping on electronic heat capacity (EHC) and magnetic susceptibility (MS) of a two-dimensional material ferromagnetic gapped graphene-like, MoS2, are investigated within the Green’s function approach by using the Kane-Mele Hamiltonian and self-consistent Born approximation (SCBA) at Dirac points. Our findings show that there is a critical impurity concentration (IC) and scattering strength (ISS) for each valley in EHC and MS curves. Also, we have found that the spin band gap decreases with impurity only for valley K, ↑ and \(K^{\prime }, \downarrow \) due to the existence of inversion symmetry between valleys. On the other hand, a magnetic phase transition from ferromagnetic to antiferromagnetic and paramagnetic has been observed. The increase of scattering rate of carriers in the presence of impurity is the main reason of these behaviors.

The Dilute Charged Impurity Effects on Electronic Heat Capacity and Magnetic Susceptibility of Ferromagnetic Silicene Sheet

Starting from the Kane-Mele Hamiltonian, Dirac cone approximation and self-consistent Born approximation (SCBA), the effects of dilute charged impurity doping on electronic heat capacity (EHC) and magnetic susceptibility (MS) of a two-dimensional material ferromagnetic graphene’s silicon analog, silicene, are investigated within the Green’s function approach that allows the accurate assessment of dynamic of carriers. Also, we have studied the behavior of these quantities in the presence of applied external electric field (AEEF). Our results show that the inversion symmetry is broken by impurity doping. According to EHC behaviors, the band gap is decreased with impurity concentration (IC), impurity scattering strenght (ISS), and AEEF. As a remarkable point, a phase transition from ferromagnetic to paramagnetic and antiferromagnetic has been observed. On the other hand, there is a critical impurity concentration for maximum response to the temperature and magnetic field. The effect of AEEF leads to the decrease of band gap and phase transition depends on the direction of AEEF.

Impurity doping effects on the orbital thermodynamic properties of hydrogenated graphene, graphane, in Harrison model

Using the Harrison model and Green's function technique, impurity doping effects on the orbital density of states (DOS), electronic heat capacity (EHC) and magnetic susceptibility (MS) of a monolayer hydrogenated graphene, chair-like graphane, are investigated. The effect of scattering between electrons and dilute charged impurities is discussed in terms of the self-consistent Born approximation. Our results show that the graphane is a semiconductor and its band gap decreases with impurity. As a remarkable point, comparatively EHC reaches almost linearly to Schottky anomaly and does not change at low temperatures in the presence of impurity. Generally, EHC and MS increases with impurity doping. Surprisingly, impurity doping only affects the salient behavior of py orbital contribution of carbon atoms due to the symmetry breaking.

Superconducting Gap Symmetry of LaFeP(O,F) Observed by Impurity Doping Effect

We have investigated Mn, Co and Ni substitution effects on polycrystalline samples of LaFePO0.95F0.05 by resistivity and magnetoresistance measurements. In LaFe1-xMxPO0.95F0.05 (M = Mn, Co and Ni), the superconducting transition temperature (Tc) monotonously decreases with increasing the impurity doping level of x. There is a clear difference of Tc suppression rates among Mn, Co and Ni doping cases, and the decreasing rate of Tc by Mn doping as a magnetic impurity is larger than those by the nonmagnetic doping impurities (Co/Ni). This result indicates that in LaFePO0.95F0.05, Tc is rapidly suppressed by the pair-breaking effect of magnetic impurities, and the pairing symmetry is a full-gapped s-wave. In the nonmagnetic impurity-doped systems, the residual resistivity in the normal state has nearly the same value when Tc becomes zero. The residual resistivity value is almost consistent with the universal value of sheet resistance for two-dimensional superconductors, suggesting that Tc is suppressed by electron localization in Co/Ni-doped LaFePO0.95F0.05.

On the Crystal Structural Control of Sputtered TiO2 Thin Films.

In this study, we focused on the origin on the selective deposition of rutile and anatase TiO2 thin films during the sputtering process. The observation on microstructural evolution of the TiO2 films by transmission electron microscopy revealed the coexistence of rutile and anatase TiO2 phases in the initial stage under the preferential growth conditions for the anatase TiO2; the observations further revealed that the anatase phase gradually dominated the crystal structure with increasing film thickness. These results suggest that the bombardment during the sputtering deposition did not obviously affect the TiO2 crystal structure, and this was also confirmed by off-axis magnetron sputtering experiments. We also investigated the mechanism of the effect of Sn impurity doping on the crystal structure using first-principles calculations. It is found that the formation energy of Sn-doped rutile TiO2 is lower than that of Sn-doped anatase TiO2; this suggests that the Sn-doped TiO2 favours the rutile phase. These results offer a guideline for the utilization of selective deposition of rutile and anatase TiO2 thin films in various industrial applications.

The effects of impurity doping on the optical properties of biased bilayer graphene

We address the optical conductivity of doped AA-stacked bilayer graphene in the presence of a finite bias voltage at finite temperature. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Green's function approach has been implemented to find the behavior of optical conductivity of bilayer graphene within linear response theory. We have found the frequency dependence of optical conductivity for different values of concentration and scattering strength of dopant impurity. Also the dependence of optical conductivity on the impurity concentration and bias voltage has been investigated in details. A peak appears in the plot of optical conductivity versus impurity concentration for different values of chemical potential. Furthermore we find optical conductivity reduces with frequency for any impurity concentration and scattering strength.

Effect of impurity doping in gapped bilayer graphene

Impurity doping plays a pivotal role in semiconductor electronics. We study the doping effect in a two-dimensional semiconductor, gapped bilayer graphene. By employing in situ deposition of calcium on the bilayer graphene, dopants are controllably introduced. Low temperature transport results show a variable range hopping conduction near the charge neutrality point persisting up to 50 K, providing evidence for the impurity levels inside the gap. Our experiment confirms a predicted peculiar effect in the gapped bilayer graphene, i.e., formation of in-gap states even if the bare impurity level lies in the conduction band. The result provides perspective on the effect of doping and impurity levels in semiconducting bilayer graphene.

Theoretical analysis of structure and formation energy of impurity-doped Mg2Si: Comparison of first-principles codes for material properties

We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg2Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green’s function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg2Si.