Substitutional Impurity Atoms Research Articles

Acceptable Concentrations of Cu and Ni in Corrosion Resistance of Mg–6 mass% Zn Alloy

Cu and Ni impurities in Mg alloys are deleterious contaminants that reduce the corrosion resistance of the alloy. Mg2Cu and Mg2Ni precipitates can cause significant anodic dissolution of the Mg matrix, owing to their potential difference. Suppression of these phases can prevent the deterioration of corrosion resistance. The neutralization of these impurities through the formation of Mg–Zn intermetallic phases has been studied, because the atomic radii of Cu and Ni are similar to that of Zn. As a result, the MgZn2 phase may precipitate during the rapid cooling that occurs during the solidification of the Mg–6 mass% Zn alloy, and introduce substitutional impurity atoms in the crystal lattice. Mg(Zn, Cu)2 and Mg(Zn, Ni)2 phases can be formed instead of Mg2Cu and Mg2Ni, in the presence of both of Zn and these impurities. The microstructures and corrosion properties of the Mg–6 mass% Zn alloy with various Cu or Ni concentrations are investigated in this work. The Cu and Ni impurities are concentrated into MgZn2 phase in the Mg–6 mass% Zn alloy without Mg2Cu or Mg2Ni formation when the concentrations of these impurities are within acceptable limits. Consequently, the corrosion rate of the Mg–6 mass% Zn alloy with 1.4 mass% Cu or 0.25 mass% Ni is almost the same as that of the alloy without Cu and/or Ni contaminations.

Single-spin resonance in a van der Waals embedded paramagnetic defect.

A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials1-8. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below 10 MHz. The photokinetics of one type of defect is compatible with ground-state electron-spin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of in-plane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, out-of-plane delocalized π-orbital electron, probably originating from substitutional impurity atoms. We extracted spin-lattice relaxation times T1 of 13-17 μs with estimated spin coherence times T2 of less than 1 μs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride.

Diffusion mechanism in cutting Ni-based alloy containing γ′ phase (Ni3Al) with CBN tool based on MD simulation

Ni-based alloys are widely used in aerospace because of their high strength and high temperature oxidation resistance. CBN tool is suitable for precision machining of Ni-based alloy. Diffusion wear is an important wear form of CBN tool in the process of cutting Ni-based alloy. Therefore, it is of great significance to study the diffusion phenomenon in the process of cutting Ni-based alloy with CBN tool. In this paper, the cutting model of Ni-based alloy containing γ′ phase (Ni3Al) with CBN tool is established based on the molecular dynamics (MD) simulation method. The self diffusion activation energy of all kinds of atoms in the workpiece and the formation energy of several point defects in the tool are calculated, so as to study in depth the atom diffusion mechanism according to the simulation results. The results show that the atoms in the crystal boundary of the workpiece are the most easily diffused, followed by the atoms in the phase boundary, and the atoms in the lattice are the most difficult to diffuse. When the workpiece atoms diffuse into the tool, it is easier to diffuse into the tool grain boundary than to form interstitial impurity atoms or displacement impurity atoms. It is more difficult to form the substitutional impurity atom than to form the interstitial impurity atom.

Behavior of the Energy Spectrum and Electric Conduction of Doped Graphene.

We consider the effect of atomic impurities on the energy spectrum and electrical conductance of graphene. As is known, the ordering of atomic impurities at the nodes of a crystal lattice modifies the graphene spectrum of energy, yielding a gap in it. Assuming a Fermi level within the gap domain, the electrical conductance diverges at the ordering of graphene. Hence, we can conclude about the presence of a metal–dielectric transition. On the other hand, for a Fermi level occurring outside of the gap, we see an increase in the electrical conductance as a function of the order parameter. The analytic formulas obtained in the Lifshitz one-electron strong-coupling model, describing the one-electron states of graphene doped with substitutional impurity atoms in the limiting case of weak scattering, are compared to the results of numerical calculations. To determine the dependence of the energy spectrum and electrical conductance on the order parameter, we consider both the limiting case of weak scattering and the case of finite scattering potential. The contributions of the scattering of electrons on a vapor of atoms to the density of states and the electrical conductance of graphene with an admixture of interstitial atoms are studied within numerical methods. It is shown that an increase in the electrical conductance with the order parameter is a result of both the growth of the density of states at the Fermi level and the time of relaxation of electron states. We have demonstrated the presence of a domain of localized extrinsic states on the edges of the energy gap arising at the ordering of atoms of the admixture. If the Fermi level falls in the indicated spectral regions, the electrical conductance of graphene is significantly affected by the scattering of electrons on clusters of two or more atoms, and the approximation of coherent potential fails in this case.

Effect of impurities ordering in the electronic spectrum and conductivity of graphene

By carrying out a computation in the Lifshitz tight-binding one-electron model, we obtain the energy spectrum and electrical conductance of graphene, in the presence of substitutional impurity atoms, thus assessing the influence of the latter. In the weak-scattering approximation, we study specific features of the electron energy spectrum in the gap region having width η|δ| and centered at the point yδ, arising because of the ordering of substitutional impurity atoms on nodes of the crystal lattice. Here η is the parameter of ordering, δ is the difference of the scattering potentials of impurity atoms and carbon atoms, and y is the impurity concentration. It is shown that if the ordering parameter η is close to ηmax=2y,y<1/2, the plot of the density of electron states has peaks on the edges of the energy gap. Those peaks correspond to impurity levels. As the ordering parameter η decreases, the impurity levels split into the impurity bands. The regions of localization of electron impurity states, which arise at the edges of the spectrum and edges of the energy gap, are investigated.

Atomistic T -matrix theory of disordered two-dimensional materials: Bound states, spectral properties, quasiparticle scattering, and transport

In this work, we present an atomistic first-principles framework for modeling the low-temperature electronic and transport properties of disordered two-dimensional (2D) materials with randomly distributed point defects (impurities). The method is based on the $T$-matrix formalism in combination with realistic density-functional theory (DFT) descriptions of the defects and their scattering matrix elements. From the $T$-matrix approximations to the disorder-averaged Green's function (GF) and the collision integral in the Boltzmann transport equation, the method allows calculations of, e.g., the density of states (DOS) including contributions from bound defect states, the quasiparticle spectrum and the spectral linewidth (scattering rate), and the conductivity/mobility of disordered 2D materials. We demonstrate the method by examining these quantities in monolayers of the archetypal 2D materials graphene and transition metal dichalcogenides (TMDs) contaminated with vacancy defects and substitutional impurity atoms. By comparing the Born and $T$-matrix approximations, we also demonstrate a strong breakdown of the Born approximation for defects in 2D materials manifested in a pronounced renormalization of, e.g., the scattering rate by the higher-order $T$-matrix method. As the $T$-matrix approximation is essentially exact for dilute disorder, i.e., low defect concentrations ($c_\text{dis} \ll 1$) or density ($n_\text{dis}\ll A_\text{cell}^{-1}$ where $A_\text{cell}$ is the unit cell area), our first-principles method provides an excellent framework for modeling the properties of disordered 2D materials with defect concentrations relevant for devices.

Interaction of Phosphorus with Impurity Atoms in BCC Iron

The paper presents the results of modelling of phosphorus interaction with substitutional (Cr, Mn, P) and interstitial (C) impurity atoms in bcc iron in the framework of density functional theory using WIEN2k software. It is found that a repulsion exists of a phosphorus atom in the three first spheres of coordination of carbon, chromium and phosphorus atoms, while for manganese such repulsion of phosphorus takes place only in the second sphere. This repulsion is a consequence of an abrupt change of magnetic moment of manganese atom, so the solution energy of phosphorus almost does not change. On the contrary, chromium decreases phosphorus solubility in iron, in agreement with other data.

Electronic and magnetic properties of B, Al, N and P impurities in SnC nanoribbons: first-principles study

The structural, electronic and some magnetic behavior of the tin carbide (SnC) nanoribbons with the substitutional impurity atoms of boron, aluminum, nitrogen and phosphorus are investigated by using density functional theory (DFT) framework. The armchair and zigzag SnC nanoribbons are considered with the 7 and 4 atoms width, respectively. We found that all the four impurities prefer to substitute the Sn atom sites which located at the edge of nanoribbon or close to it. Also, the band structures analysis demonstrated that the impurities create extra bands in the region of nanoribbons band gap. So, the spin-down band gap values of the doped 7-ASnCNRs are reduced to 0.218, 0, 1.043 and 1.233 eV for B, Al, N and P atoms, respectively. Moreover, the half metal behavior is observed for the Al-doped 7-ASiCNR. For the 4-ZSnCNR, the impurity atoms could increase the band gap, especially in the spin-up channel. Finally, they introduce a total magnetic moment of 1 μB into the nonmagnetic 7-ASnCNR and they affect on the magnetic moment of the 4-ZSnCNR.

Influence of the ordering of impurities on the appearance of an energy gap and on the electrical conductance of graphene

In the one-band model of strong coupling, the influence of substitutional impurity atoms on the energy spectrum and electrical conductance of graphene is studied. It is established that the ordering of substitutional impurity atoms on nodes of the crystal lattice causes the appearance of a gap in the energy spectrum of graphene with width η|δ| centered at the point yδ, where η is the parameter of ordering, δ is the difference of the scattering potentials of impurity atoms and carbon atoms, and y is the impurity concentration. The maximum value of the parameter of ordering is {{boldsymbol{eta }}}_{{boldsymbol{max }}}{boldsymbol{=}}{bf{2}}{boldsymbol{y}}{boldsymbol{,}},{boldsymbol{y}}{boldsymbol{le }}{bf{1}}/{bf{2}}. For the complete ordering of impurity atoms, the energy gap width equals {bf{2}}{boldsymbol{y}}{boldsymbol{|}}{boldsymbol{delta }}{boldsymbol{|}}. If the Fermi level falls in the region of the mentioned gap, then the electrical conductance {{boldsymbol{sigma }}}_{{boldsymbol{alpha }}{boldsymbol{alpha }}}{boldsymbol{to }}{bf{0}} at the ordering of graphene, i.e., the metal–dielectric transition arises. If the Fermi level is located outside the gap, then the electrical conductance increases with the parameter of order η by the relation {{boldsymbol{sigma }}}_{{boldsymbol{alpha }}{boldsymbol{alpha }}}{boldsymbol{ sim }}{{boldsymbol{(}}{{boldsymbol{y}}}^{{bf{2}}}{boldsymbol{-}}frac{{bf{1}}}{{bf{4}}}{{boldsymbol{eta }}}^{{bf{2}}}{boldsymbol{)}}}^{{boldsymbol{-}}{bf{1}}}. At the concentration {boldsymbol{y}}{boldsymbol{=}}{bf{1}}{boldsymbol{/}}{bf{2}}, as the ordering of impurity atoms η →1, the electrical conductance of graphene {{boldsymbol{sigma }}}_{{boldsymbol{alpha }}{boldsymbol{alpha }}}{boldsymbol{to }}{boldsymbol{infty }}, i.e., the transition of graphene in the state of ideal electrical conductance arises.

Substrate engineering of graphene reactivity: towards high-performance graphene-based catalysts

Graphene-based solid-state catalysis is an emerging direction in research on graphene, which opens new opportunities in graphene applications and thus has attracted enormous interests recently. A central issue in graphene-based catalysis is the lack of an effective yet practical way to activate the chemically inert graphene, which is largely due to the difficulties in the direct treatment of graphene (such as doping transition metal elements and introducing particular type of vacancies). Here we report a way to overcome these difficulties by promoting the reactivity and catalytic activity of graphene via substrate engineering. With thorough first-principles investigations, we demonstrate that when introduce a defect, either a substitutional impurity atom (e.g. Au, Cu, Ag, Zn) or a single vacancy, in the underlying Ru (0001) substrate, the reactivity of the supported graphene can be greatly enhanced, resulting in the chemical adsorption of O2 molecules on graphene. The origin of the O2 chemical adsorption is found to be the impurity- or vacancy-induced significant charge transfer from the graphene–Ru (0001) contact region to the 2π* orbital of the O2 molecule. We then further show that the charge transfer also leads to high catalytic activity of graphene for chemical reaction of CO oxidation. According to our calculations, the catalyzed CO oxidation takes place in Eley-Rideal (ER) mechanism with low reaction barriers (around 0.5 eV), suggesting that the substrate engineering is an effective way to turn the supported graphene into an excellent catalyst that has potential for large-scale industrial applications.

Electronics properties of ZnSe nanotube with substitutional impurity atoms - A first-principles investigation

The band structure of pristine zinc selenide (ZnSe) nanotube, gallium, chlorine, nitrogen and arsenic substituted ZnSe nanotube are studied using density functional theory (DFT) with GGA/PBE exchange correlation functional. The state of the art of this work is to study the electronic properties of ZnSe nanotubes with substitution impurities. The ZnSe nanotube electronic properties are studied in terms of band structures and density of states spectrum. The band structure of pure ZnSe nanotube shows a semiconducting nature. The gallium, chlorine, nitrogen and arsenic substituted ZnSe structured results in metallic behavior. The density of states provides the insight for the localization of charges in the valence band and conduction band. The substitution of chlorine and nitrogen enhance the density of charges in valence band. We found that nitrogen is the most efficient acceptor impurity for p-type doping, while chlorine is the most suitable impurity for n-type doping. The results of the present work provide a clear vision to tailor the band structure of ZnSe nanotube with substitution impurity. This study is useful to researchers investigating p- and n-type doping as well as optoelectronic device manufacturers.

Electronic modes in carbon nanotubes with single and double impurity sites

A theoretical study of isolated and doubly-clustered impurities is presented for the electronic excitations in a carbon nanotube lattice. Using a matrix operator formalism and a tight-binding model where the interactions between atoms take place via nearest-neighbor hopping, the properties of the excitations are deduced. A geometry consisting of long, single-walled carbon nanotubes is assumed with the defects introduced in the form of substitutional impurity atoms, giving rise to the localized electronic modes of the nanotube as well as the propagating modes of the pure (host) material. The impurities are assumed to be in a low concentration, having the form of either a single, isolated defect or a small cluster of two defects close together. A tridiagonal matrix technique is employed within a Green’s function formalism to obtain the properties of the discrete modes of the system, including their frequencies and localization. The numerical examples show a dependence on the nanotube diameters and on the relative spatial configurations of the impurities. The results contrast with the previous studies of line impurities since there is no translational symmetry along the longitudinal axis of the nanotubes in the present case.

Lattice site specific diffusion properties for substitutional and interstitial impurity atoms in ZnO crystals

Fundamental understanding of impurity diffusion in crystals remains a challenge due to lack of experimental capabilities for measuring the diffusion properties of atoms according to their substitutional and interstitial lattice locations. With examples of indium and silver in ZnO crystals, we demonstrate an ion beam based method to experimentally determine the energetics and entropy changes in diffusion of substitutional and interstitial impurity atoms. While the interstitial Ag diffuses much faster than the substitutional Ag, as normally expected, the interstitial In migrates slower than the substitutional In, which is attributed to a large negative entropy change (∼−10 kB), possibly caused by the large atomic size of In. The activation energy and the diffusivity pre-exponential factor for the interstitial Ag are significantly enhanced, being more than a factor of two and ∼13 orders of magnitude, respectively, relative to the case for the interstitial In. This implies two different diffusion mechanisms between these two types of interstitial atoms in ZnO crystals: the direct interstitial diffusion mechanism for the interstitial In and the kick-out diffusion mechanism for the interstitial Ag. In addition, the activation energies and the diffusivity prefactors follow the Meyer-Neldel relationship with an excitation energy of ∼92 meV.

The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper

We investigate the changes in grain boundary sliding (GBS) and intergranular decohesion in copper (Cu), due to the inclusion of bismuth (Bi), lead (Pb) and silver (Ag) substitutional impurity atoms at a symmetric tilt grain boundary (GB), using a first-principles concurrent multiscale approach. We first study the segregation behavior of the impurities by determining the impurity segregation energy in the vicinity of the GB. We find that the energetically preferred sites are on the GB plane. We investigate the intergranular decohesion of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities by considering the work of separation, and the tensile strength, . Both and decrease in the presence of Bi and Pb impurities, indicating their great propensity for intergranular embrittlement, whilst the presence of Ag impurities has only a small effect. We consider GBS to assess the mechanical properties in nanocrystalline metals and find that all three impurities strongly inhibit GBS, with Ag having the biggest effect. This suggests that Ag has a strong effect on the mechanical properties of nanocrystalline Cu, even though its effect on the intergranular decohesion properties of coarse-grained Cu is not significant.

Microscopic mechanism responsible for radiation-enhanced diffusion of impurity atoms

Modelling of radiation-enhanced diffusion (RED) of boron and phosphorus atoms during irradiation of silicon substrates respectively with high- and low-energy protons was carried out. The results obtained confirm the previously arrived conclusion that impurity diffusion occurs by means of the ‘impurity atom – intrinsic point defect’ pairs and that the condition of the local thermodynamic equilibrium between substitutional impurity atoms, nonequilibrium point defects created by irradiation, and the pairs is valid. It is shown that using RED, one can form a special impurity distribution in the semiconductor substrate including retrograde profiles with increasing impurity concentration in the bulk of the semiconductor. In addition, modelling of radiation-induced segregation of nitrogen implanted in stainless steel modified by titanium is carried out. It is shown that vacancy-impurity complexes are responsible for nitrogen diffusion in an implanted layer excluding the ‘tail’ region. The calculations performed give clear evidence in favour of further investigation of various doping processes based on RED, especially the processes of plasma doping, to develop a cheap method for forming specific impurity distributions in the near surface region.

Localization of the electronic excitations in single-walled carbon nanotubes with embedded line impurities

A matrix operator formalism is used to study the excitations in long, single-walled carbon nanotubes with the dynamic electronic properties described by a tight-binding model where the interactions between atoms take place via nearest-neighbour hopping. Defects in the form of substitutional impurity atoms are introduced to study the localized electronic modes of the nanotube as well as the propagating modes of the pure (host) material. The impurities are assumed to have the form of one or more line defects parallel to the nanotube axis. Two geometric configurations are investigated corresponding to the longitudinal axis of the nanotube being parallel to either a zigzag or an armchair direction of the graphene lattice. A tridiagonal matrix technique is employed to solve the electronic operator equations that provide a description of the frequencies of the discrete modes of the system and their spatial amplitudes. Numerical examples are presented for different nanotube diameters and spatial configurations of the impurity lines.

Localization of magnetic and electronic excitations in nanotubes with line defects

A matrix Green's function formalism is employed to study the excitations in long nanotubes where the dynamics are governed by nearest-neighbor interactions between atoms. Examples of the excitations, which can be characterized in terms of the tube circumference and a one-dimensional wave number along the length, include ferromagnetic spin waves in a Heisenberg exchange model and electronic modes in a tight-binding model with hopping. It is assumed that the system is a single-walled nanotube of negligible thickness and that the atoms are arranged on a simple square lattice. Defects in the form of substitutional impurity atoms are introduced to study localized modes as well as the propagating modes of the pure (host) material. The impurities have the form of one or more line defects parallel to the nanotube axis. The derived Green's functions provide a description of the frequencies of the discrete modes of the system and their spectral intensities. Numerical examples are presented for different mode types (magnetic and electronic), nanotube diameters and arrangements of impurity lines.

Influence of intrinsic point defects and antimony impurity on the electronic structure and photoelectric properties of tin monosulfide

The electronic structure calculations of defect-free tin monosulfide SnS as well as SnS with existing intrinsic point defects (vacancies in tin (VSn) and sulfur (VS) sublattices), substitutional impurity atoms SbSn and complexes of {VSn–SbSn} type were performed using ab initio density functional theory method in the supercell model. The temperature dependence of stationary photoconductivity and spectral distribution of photosensitivity of SnS crystals doped by antimony were measured in the temperature range of 100–400 K. The results of non-empirical calculations enabled us to analyze the influence of defect formation processes on the macroscopic properties of SnS crystals. It is shown that Sb impurity improves the photoelectric characteristics of SnS crystals.

Electron transport channels and their manipulation by impurity in armchair-edge graphene nanoribbons

Under the scheme of the nonequilibrium Green’s function combined with the tight-binding approximation, we study electron transport properties in different atomic chains of an armchair-edged graphene nanoribbon (AGNR) and their manipulation using a single substitutional impurity atom. By calculation and analysis of the local bond currents between nearest atom sites in the AGNR, we find that electron transport along two armchair-edged chains is more active than that along other chains for any clean AGNR. For a metallic AGNR, interestingly, there exists a series of parallel distributed major channels for the low-energy electron transport. Further, the transport properties of these channels can be manipulated by a single substitutional impurity atom with different strength and locating position, e.g., a suitable impurity can cause a selected channel to be closed completely while others still open. However, in the high-energy regime these independent channels disappear, and a metallic AGNR becomes entirely metallic in this case. The findings here suggest that an AGNR may be used as a multi-channel plane material in the future nanoelectronic technology.

Grain Boundary Segregation of Interstitial and Substitutional Impurity Atoms in Alpha-Iron

The macroscopic behavior of polycrystalline materials is influenced by the local variation of properties caused by the presence of impurities and defects. The effect of these impurities at the atomic scale can either embrittle or strengthen grain boundaries within. Thus, it is imperative to understand the energetics associated with segregation to design materials with desirable properties. Here, molecular statics simulations were employed to analyze the energetics associated with the segregation of various elements (He, H, C, P, and V) to four <100> (Sigma 5 and 13 GBs) and six <110> (Sigma 3,9,and 11 GBs) symmetric tilt grain boundaries in alpha-Fe. This knowledge is important for designing stable interfaces in harsh environments. Simulation results show that the local atomic arrangements within the GB region and the resulting structural units have a significant influence on the magnitude of binding energies of the impurity (interstitial and substitutional) atoms. This data also suggests that the site-to-site variation of energies within a boundary is substantial. Comparing the binding energies of all ten boundaries shows that the Sigma 3(112) boundary possesses a much smaller binding energy for all interstitial and substitutional impurity atoms among the boundaries examined here. Additionally, based on the Rice-Wang model, our total energy calculations show that V has a significant beneficial effect on the Fe grain boundary cohesion, while P has a detrimental effect on grain boundary cohesion, much weaker than H and He. This is significant for applications where extreme environmental damage generates lattice defects and grain boundaries act as sinks for both interstitial and substitutional impurity atoms. This methodology provides us with a tool to effectively identify the local as well as the global segregation behavior which can influence the GB cohesion.