Preferential Doping Research Articles

Colossal Zero-Field-Cooled Exchange Bias via Tuning Compensated Ferrimagnetic in Kagome Metals.

Exchange bias (EB) is a crucial property with widespread applications but particularly occurs by complex interfacial magnetic interactions after field cooling. To date, intrinsic zero-field-cooled EB (ZEB) has only emerged in a few bulk frustrated systems and their magnitudes remain small yet. Here, enabled by high temperature synthesis, we uncover a colossal ZEB field of 4.95 kOe via tuning compensated ferrimagnetism in a family of kagome metals, which is almost twice the magnitude of known materials. Atomic-scale structure, spin dynamics, and magnetic theory revealed that these compensated ferrimagnets originate from significant antiferromagnetic exchange interactions embedded in the holmium-iron ferrimagnetic matrix due to supersaturated preferential manganese doping. A random antiferromagnetic order of manganese sublattice sandwiched between ferromagnetic iron kagome bilayers accounts for such unconventional pinning. The outcome of the present study outlines disorder-induced giant bulk ZEB and coercivity in layered frustrated systems.

A comprehensive investigation of alloying elements on site preferences, elastic properties and electronic structure of Mg17Al12 by first-principles calculations

The effect of elemental doping on the site preference, elastic properties and electronic structure of Mg17Al12 has been systematically investigated by first-principles calculations. And twenty-seven elements (Li, Na, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Ag, Cd, In, Sn, Sb, Pb, Bi and La) are considered in this study. The calculations of substitution formation energies indicate that the alkali metal (Li), alkaline earth (Ca, Sr) and rare earth (Sc, Y, La) group elements and Ti, Zr favor to occupy Mg sites, and transition group (Fe, Co, Ni, Cu, Ag, Zn, Cd) elements and Ga, Ge, Sn, Sb prefer to occupy Al site, while Na, K, V, Cr, Mn, In, Pb and Bi are unstable in Mg17Al12 phase. Meanwhile, the results of phonon dispersion curves for the preferentially doped systems show that all doped structures are dynamically stable except for Sr, La and Sn doped systems. The elastic constants indicate that all preferential doping models are mechanically stable, and solute atoms with high bulk modulus lead to an increase in the bulk modulus of Mg17Al12 phase. Simultaneously, the calculated shear modulus/bulk modulus ratio (G/B), Poisson's ratio (v) and Cauchy's pressure (C‾12‐C‾44) indicate that the ductility of doped systems is better than Mg17Al12 alloy. All structures are elastic anisotropic, while the doping of Fe, Co and Ni reduces the degree of anisotropy of Mg17Al12 phase. Furthermore, the electronic structures show a strong hybridization between the p-orbitals of Mg and Al and the d-orbitals of alkaline earth, rare earth and Zr elements, resulting in higher structural stability. Finally, the adsorption behavior of O atom on Mg17Al12(110) surface and the structural stability, lattice constants, and elastic modulus of Mg34Al24-xZnx random solid solution alloys are investigated.

Design of Graphene-derived Materials by Discrete Geometric Analysis and First-principles Calculations

Geometric structures of carbon networks are key in designing their material properties. In particular, optimization of curved structures through the introduction of topological defects and doping of heteroatoms in the lattice is crucial to the design of carbon-based, non-noble-metal-free catalysts. A simple and practical mathematical model based on discrete geometric analysis is proposed to describe the geometric features of carbon networks and their relationships to their material properties. This model can pre-screen candidates for novel material design, and these candidates can be further examined by the density functional theory (DFT). Inspired by observations regarding the preferential doping of heteroatoms at local curved sites, the important characteristics of the candidate material were experimentally realized, and its enhanced catalytic activity facilitated by chemical dopants was confirmed in the designed carbon network.

The Role of M3+ Substitutional Doping (M = In, Sb, Bi) in the Passivation of the α-CsPbI3(100) Surface

Photovoltaic applications based on perovskite materials still face several challenges to deliver high power conversion efficiency, in particular the chemical passivation of surfaces, which is required to remove surface electronic states. In this work, we propose a surface passivation route for the α-CsPbI3(100) surface through substitutional doping at the Pb sites by trivalent cations such as In, Sb, or Bi. Our ab initio investigation was based on density functional theory calculations. We found a preferential doping position at surface octahedrons by more than 0.35 eV/dopant in comparison with inner layers as well as a bandgap decrease due to surface states and optical bandgaps wider than fundamental ones as a result of dark transitions. The surface passivation with Bi atoms demonstrates to be the most effective one, with enhanced sunlight absorption when compared to the 3D bulk α-CsPbI3. Furthermore, we changed the doping concentration and demonstrate the possibility to combine trivalent cations with other passivating mechanisms. Therefore, our computational results disclose a promising route for saturating perovskite surfaces.

Carbon and silicon co-doping effect on microstructural and optoelectronic properties of ZnO: An ab initio study

The present study is a theoretical work of the effect of carbon and silicon co-doping on the optoelectronic properties of ZnO, by generalized gradient approximation (GGA) using the Perdew Burke Ernzerhof functional correlations (PBE) exchange. The results confirmed that O atoms acts as a preferential doping site in the crystal lattice. By introducing carbon atoms, the optoelectronic properties of ZnO change and we show a better absorption of visible light compared to other dopants. The co-insertion of C and Si atoms in ZnO matrix, leads to a smaller refractive index and the absorption coefficient increases. Furthermore, C and Si co-doping changes the band gap, the dielectric function and the loss energy. We also report in this paper the achieved results in the effect of co-doping with native defects points on electronic properties of ZnO.

Geometric model of 3D curved graphene with chemical dopants

Geometric structures of carbon networks are key in designing their material properties. In particular, optimization of curved structures through the introduction of topological defects and doping of heteroatoms in the lattice is crucial to the design of carbon-based, non-noble-metal-free catalysts. A simple and practical mathematical model based on discrete geometric analysis is proposed to describe the geometric features of carbon networks and their relationships to their material properties. This model can pre-screen candidates for novel material design, and these candidates can be further examined by the density functional theory (DFT). Inspired by observations regarding the preferential doping of heteroatoms at local curved sites, the important characteristics of the candidate material were experimentally realized, and its enhanced catalytic activity facilitated by chemical dopants was confirmed in the designed carbon network.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases. While preferential n-type doping was observed in graphene domains in nitrogen-containing plasmas, preferential p-type doping was observed at grain boundaries in boron-containing plasmas.

Exploration of highly photoluminescent first-row transition metals (manganese, iron, cobalt, nickel, copper and zinc) co-doped nano carbon dots as energy storage materials

Carbon nanodots have currently emerged as a unique and promising material focusing on advanced applications especially in the field of optoelectronic and energy storage devices. The preferential doping of nanocarbon dots with first row transition elements will enhance the photoluminescence characteristics and thereby, we can fine-tune its photophysical properties. In the present work, we have synthesized a series of first-row transition metal atoms (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) co-doped with nitrogen and nitrogen- sulfur carbon dots by a greener synthetic strategy. The formation of doped carbon dots were confirmed by its characteristic surface plasmon resonance absorptions using the Ultraviolet–visible spectroscopy. The presence of major elements and the functional group identification was done with the aid of EDS and FT-IR spectral analysis, the morphology and size were analyzed by high-resolution TEM analysis and photophysical characteristics were investigated from the fluorescence measurements. The photoluminescence studies reveal that by co-doping the transition metals with nitrogen or nitrogen- sulfur carbon dots, the emissions maxima of the nanodots can be tuned. The doped carbon dots were explored for their energy storage application and their super capacitance characteristics. The capacitance values of the carbon dots were found to be increased with the effect of co-doping with the first-row transition elements and the capacitance values were in the range of 2.6 µF/cm2 to 0.5 µF/cm2 and are in tune with the capacitance values for carbon-based materials including graphene and C-dots reported in the literature. Also the cyclic voltammetric loops were found to be sufficiently rectangular in shape, desirable for a supercapacitor, and the rectangular areas were found to be increased with increase in scan rate.

Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La-Doped InNbO4.

The preferential doping site, visible light response, and redox potential of Fe- and/or La-doped InNbO4 (INO) were investigated using first-principles density functional theory. Eight designed doping models, including Fe and/or La doping at In or/and Nb sites of INO are constructed, respectively. It was found that Fe-doping and Fe,La-codoping to substitute In into an INO cell are energetically favorable, confirming that the steric hindrance plays a vital role for the selective doping site than the charge of the dopants. Fe doping always formed two impurity bands between the conduction and valence bands, originated from Fe 3d state, inferring the well visible light response. Furthermore, the presence of La has a specific regulation effects for Fe doping although the energy levels of the single La-doped models were completely similar to those of the undoped INO. The electron exchange between La and Fe dopants results in the significant interaction for codoping INO. Importantly, by doping La into INO cell, the redox potentials of Fe-doped INO could be well-regulated. The band potential moved to the more positive energy level of the models Fe-doped at Nb sites, while it shifted to the more negative level if Fe was doped at In site of La-INO. The present investigation may provide the guidance for the designative dopants to construct the photocatalyst with stable, visible response, and good redox performance.

(Invited) The Interplay between Electronic and Ionic Transport in the Resistive Switching Process of Random Access Memory Devices

The formation of conductive filamentary structures characteristic to the “ON” state (or LRS - low resistance state) of the resistance based memory devices were studied using ab initio simulation techniques. The atomistic description of the rupturing/dissolution process into the “OFF” state of the memory operation (HRS - high resistance state) was found to involve both ionic and electronic transport processes. Moreover, electron and hole trapping effects were shown to have a significant role in the switching process under applied electrical field. While hole injection into oxygen reduced transition metal oxides containing a formed filament can favor the dissolution process, electron injection induces filament formation. As possible ways to achieve improved device characteristics, preferential impurity doping in these types of systems is proposed to favorably affect and control the “ON” - “OFF” transition process.

The Interplay between Electronic and Ionic Transport in the Resistive Switching Process of Random Access Memory Devices

In resistive random access memory devices (ReRAM), in order to achieve high endurance and good retention characteristics repetitive control over the resistance change between low resistance (LRS) and high resistance (HRS) states is desired. The binary transition metal oxides TiOx, NiOx, HfOx, AlOx, TaOx have been specifically promising to achieve this goal and are the preferential candidates for the next generation of resistance change based devices. To address the critical issues of ReRAM operation, however, atomistic modeling approaches based on quantum mechanical principles are needed to achieve the optimization of material properties and structures of ReRAM cells, as well as of selection devices and memory array configurations.To explain the process of resistance change observed experimentally, various switching mechanisms were proposed recently and discussed in the technical literature. Generally, in transition metal oxide system such as TiO2, HfO2, NiO, Al2O3, Ta2O5, the diffusion of oxygen vacancies to cluster and create filaments [1-8] and the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide, are considered the key ionic processes taking place during forming and switching. Filamentary models for transition metal oxides had been proposed theoretically and the formation energy implications of a conductive filament channel formation corresponding to the “ON” state or LRS [4-9] was investigated for TiO2, HfO2 and Al2O3 (Fig. 1). It was found that during the clustering process of oxygen vacancies, conductive channels are formed by electron delocalization trends induced by the vacancies. The accompanying electronic effects of charge trapping under applied electrical field during the switching process were also recently addressed [7-9]. Hole injection into a reduced transition metal oxide containing a formed filament were found to favor the dissolution, since isolated vacancies prefer the 2+ charge state configuration. On the other hand electron injection induces filament formation and a transition of vacancy charge state to 1+or neutral configuration. The energy barriers of oxygen vacancy out-diffusion from a filament in various charge states are shown in Fig. 2 as a function of chemical potential and Fermi level control. In addition, preferential impurity doping [10] in these types of systems can favorably affect the forming of the conductive filaments and the transition process between the “ON” and “OFF” states (Fig. 3). In conclusion, several competing processes may undergo simultaneously at the microscopic level during the switching of ReRAM devices. Atomistic simulations based on density functional theory can provide an accurate description of the role of each component and describe the mechanism of ReRAM switching.[1] B. Magyari-Köpe, S.G. Park, H.-D. Lee and Y. Nishi, J. Mater. Sci. 47, pp 7498, 2012. [2] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B, 82, 115109, 2010. [3] H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202, 2010. [4] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., 2011. [5] B. Magyari-Köpe, M. Tendulkar, S.G. Park, H.-D. Lee, and Y. Nishi, Nanotechnology 22, 254029, 2011. [6] S.G. Park, B. Magyari-Köpe, and Y. Nishi, IEEE Electron Device Letters, 32, 197, 2011. [7] K. Kamiya, M.Y. Yang, S.G. Park, B. Magyari-Köpe, Y. Nishi, M. Niwa, and K. Shiraishi, Appl. Phys. Lett.,100, 073502, 2012. [8] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, IEEE Trans. Electron Devices, vol. 60, pp. 3400, 2013. [9] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, Proc. IEEE IEDM, pp. 478, 2012. [10] L. Zhao, S.G. Park, B. Magyari-Köpe and Y. Nishi, Tech. Digest VLSI Tech. Symp. , 2013.

Study on the effect of In-2N co-doping at preferential locality on the photoelectric function of ZnO (GGA+U)

Nowadays although the study of In-N co-doping effect on the photoelectric function of ZnO is relatively common, all of the In-N co-doped ZnO are of random doping, and the preferential locality doping using the unpolarized structure of ZnO has not been considered so far. Therefore, in this paper, based on the density functional theory using first-principles plane-wave ultrasoft pseudopotential method, the un-doped and the In-N heavily co-doped Zn1-xInxO1-yNy (x= 0.0625, y=0.125) in different orientations have been set up, and band structures and density of states have been calculated respectively. The calculated results show that the In-N atoms along the c-axis orientation has the advantages of high stability over those in the vertical c-axis direction, the band gap is narrower, the effective mass is smaller, the mobility is greater, and the hole concentration is higher, so that the conductivity of ZnO is higher in the In-N heavily co-doped materials. We believe that these results may be helpful to the design and preparation of the conductivity of In-N heavily co-doped ZnO.

Acceptor-related low-energy photoluminescence from boron-doped Si nanocrystals

Boron (B) doped Si nanocrystals (Si-ncs) dispersed in hydrofluoric (HF) acid solution are prepared by dissolving borosilicate films containing B-doped Si-ncs in HF solution. We find that the etching rate of B-doped Si-ncs is much smaller than that of undoped Si-ncs. The difference of the etching rate allows us to extract only doped Si-ncs in the mixture of doped and undoped Si-ncs and observe the photoluminescence (PL) due to the transition from the conduction band to the acceptor state. The PL was very broad with the maximum around 1.15 eV. From the analysis of the PL data obtained for the samples prepared under different conditions and different etching time, preferential doping sites of B atoms are estimated. The data suggests that B-doped Si-ncs consists of intrinsic cores and heavily B-doped shells.

Electron-Beam-Induced Substitutional Carbon Doping of Boron Nitride Nanosheets, Nanoribbons, and Nanotubes

Substitutional carbon doping of the honeycomb-like boron nitride (BN) lattices in two-dimensional (nanosheets) and one-dimensional (nanoribbons and nanotubes) nanostructures was achieved via in situ electron beam irradiation in an energy-filtering 300 kV high-resolution transmission electron microscope using a C atoms feedstock intentionally introduced into the microscope. The C substitutions for B and N atoms in the honeycomb lattices were demonstrated through electron energy loss spectroscopy, spatially resolved energy-filtered elemental mapping, and in situ electrical measurements. The preferential doping was found to occur at the sites more vulnerable to electron beam irradiation. This transformed BN nanostructures from electrical insulators to conductors. It was shown that B and N atoms in a BN nanotube could be nearly completely replaced with C atoms via electron-beam-induced doping. The doping mechanism was proposed to rely on the knockout ejections of B and N atoms and subsequent healing of vacancies with supplying C atoms.

Synthesis of Ti 1− xSn xO 2 nanosized photocatalysts in reverse microemulsions

The preparation of Ti 1− x Sn x O 2 nanocrystalline photocatalysts in reverse microemulsions is reported in this work. The obtained materials have been characterised by total reflection X-ray fluorescence (TXRF), X-ray diffraction (XRD) and Raman and UV–vis spectroscopies. Very good accordance between calculated and obtained compositions is observed. Undoped TiO 2 prepared in this way crystallises in the anatase phase. Tin-doped anatase is formed with x < 0.05, while both anatase and rutile phases crystallise when x ≥ 0.05. When both phases coexist, a preferential doping of rutile seems to occur. When x = 0.10, a multiphase mixture containing TiO 2(anatase), TiO 2(rutile) and SnO 2 was formed. No significant modification of the band gap is found in any case. The photocatalytic activity of the obtained catalysts is compared employing the trichloroethylene photocatalytic degradation as a test reaction. The beneficial effect of Sn 4+ in the activity of TiO 2 appears to be related to the formation of anatase–rutile mixtures, leading to the highest specific photocatalytic activity in the sample of composition Ti 0.93Sn 0.07O 2, with anatase:rutile ratio close to 3.

A Study of Mn2+ Doping in CdS Nanocrystals

We report here the synthesis of water soluble Mn2+-doped CdS nanocrystals in which it has been possible for the first time to obtain a distinct Mn2+ d-related emission well-separated from the defect state emissions. By varying the reaction temperature systematically, we establish 55 °C as the optimum temperature to maximize the Mn2+ d emission, the existence of this optimum synthesis temperature being shown as the result of two opposing influences of the temperature. Most interestingly, present results establish that Mn2+ favors preferential doping of larger-sized particles even within the narrow size distribution achieved in the present synthesis, rendering the relatively smaller-sized nanocrystals depleted of Mn2+ for any given synthesis. One important aspect of the present approach is that the synthesized nanocrystals readily dissolve in water without any deleterious effect on the Mn2+ d-related emission intensity.

Nitrogen ion implantation in single wall carbon nanotubes

We report an X-ray photoemission and electron energy loss study on 3 keV N 2 + ion implantation in single wall carbon nanotubes. Our results show that nitrogen atoms can bind to carbon in tetrahedral sp 3, defects related pyridine-like, and triangular sp 2 configurations and such bondings are stable for annealing up to 650 K. Heating at higher temperatures results in preferential substitutional nitrogen doping. This technique opens a new channel for controlled doping in carbon nanotubes for device applications.

Chemical preferential doping in grain boundaries of melt textured YBa2Cu3Oy superconductors

Diffusion method has been developed to preferentially dope the grain boundaries of YBa2Cu3Oy bicrystals and polycrystals. Ca and Fe have been used as dopants in this study. The distribution of the dopants is highly localized in the grain boundaries. The critical current density of a YBa2Cu3Oy textured polycrystalline sample is significantly enhanced by doping Ca in grain boundaries; however, it is significantly decreased by doping Fe in grain boundaries. The Ca-doping effect has been explained by charge-carrier compensation in the grain boundaries, which reduces the grain boundary resistance and thus increases the grain boundary critical current. The Fe-doping effect has been explained by worsening the hole loss in grain boundary region, which increases the grain resistance and thus decreases the grain boundary critical current.

A two-dimensional modeling of the fine-grained polycrystalline silicon thin-film solar cells

A two-dimensional device modeling for polycrystalline silicon thin-film solar cells was performed. A ThRee-dimensional Emitter Based on Locally Enhanced diffusion (TREBLE) concept was applied to evaluate the degree of leveling of the device efficiency. The model assumes a n +-p-p + junction device of a 3-μm grain size and 10-μm-thick polycrystalline Si columnar structure unit, where the n + regions extend along the grain boundaries. The analysis was carried out using ISE-DESSIS, a two-dimensional semiconductor device simulator. It has been found that open circuit voltage could be improved by increasing the base doping level to the optimum value of 10 17 cm −3. Preferential doping has a beneficial effect on short-circuit current of the cell and a slight influence on open circuit voltage. Conversion efficiency of ∼10% could be expected at the surface, recombination velocity of 10 4 cm/s and dopant diffusion depth along the grain boundary x gb=2.5 μm At base doping ∼10 17 cm −3 and well-passivated grain boundaries ( S gb=10 3 cm/s), an efficiency of approximately 12% may be obtained when x gb=3.7 μm.

Thin-film polysilicon solar cells on foreign substrates using direct thermal CVD: material and solar cell design

The method to reach low-cost thin-film solar cells studied here involves the deposition of a thin layer of crystalline Si directly on cheap foreign substrates with CVD at high temperatures. Our investigation deals with controlling physical phenomena such as nucleation, layer growth and dopant diffusion to achieve specific properties enabling better performance, effectively designing both the polysilicon material and the device structure. The required grain size is obtained by depositing small isolated crystallites with the right surface density on the substrate and subsequently growing a closed layer from these nuclei. To reduce recombination velocity at grain boundaries the effect of the growth rate and post-deposition treatments are investigated. The high second diode current that usually plagues such devices can be reduced by improving the material quality and optimising the base doping level. We introduce the TREBLE concept in which carrier collection relies on the presence of deep peaks of preferential doping at grain boundaries.