Electronic Impurity States Research Articles

Formation of a combined electron-hole emission state in the LiRbSO4 – Eu phosphor

In the irradiated phosphor 4 LiRbSO Eu , the mechanisms of formation of the induced or combined electron-emitting state at 3.1-2.94 eV were studied using optical and thermal activation spectroscopy methods. It has been shown experimentally that the combined electron-emitting state of the phosphor is formed from the electron states of impurity and intrinsic electron and hole trapping centers of 24Eu SO   and 34 4 SO SO . Electron and hole trapping centers are created by irradiating the phosphor with photons exceeding the width of the forbidden band of the matrix, where free electrons are created in the conduction band and a hole in the valence band. The trapping center is formed by the capture of free electrons by impurities and anionic complexes according to the reaction 3 2 Eu e Eu      ,2 34 4 SO e SO    . In one process with electron centers, holes in the form of 24SO are localized. Thus, impurity and intrinsic 2 4Eu SO   and 34 4 SO SO  electron-hole trapping centers are formed. Similarly, trapping centers are formed as a result of charge transfer from the excited anion of the 24SO complex to the 3Eu  impurities and to the neighboring 2 4 SO anions according to the reaction ( 2 3 O Eu   ) and ( 2 24O SO  ), and localized holes are also formed in one act along with it. Combined electron-emitting states consisting of impurity and intrinsic electron states are excited by photons with energies of ~4.0 eV and ~4.5 eV.

Synthesis and characterization of absorption-enhanced alumina solid particle materials for direct irradiation solar-thermal conversion

Based on solar-thermal application more than 1000 °C, a novel combination of sol-gel synthesis and stepwise calcination to prepare metal-ion doped alumina composite particles as heat transfer media were proposed, enhancing solar energy absorption and lowering preparation temperature. Investigations focused on the elemental distribution, phase composition, oxidation states, optical properties and thermal stability of the composites, which were doped with Cr, Co, Cu, Fe, and Mn metal ions in a 100:5 M ratio. The first-principles calculations elucidate the mechanism behind the impact of transition metal ions on alumina’s optical properties. Results show that introducing a small quantity of metal atoms results in new impurity energy levels and electron states around EF = 0. Specifically, the band gap energy of Cu- and Mn–Al2O3 decreases from 6.017 eV to 4.464 eV and 2.170 eV, respectively. Correspondingly, the absorptivity of these composite particles increases from around 20 %–76.6 %. And Tauc analysis indicates a decrease in the optical band gap of the particle materials from 5.37 eV to a range of 2.88–4.90 eV. Additionally, doping with Mn, Fe, and Cu accelerates the in-situ formation of α-Al2O3 crystals, with a phase transition temperature below 1000 °C and a crystallinity exceeding 93 %. Thermal stability tests show that the particles are highly stable, exhibiting a mere 0.49 % mass change between 30 °C and 1200 °C. The stepwise inert annealing strategy and metal-iron doping positively influence the optical performance of alumina, synthesizing long-term high-temperature stable alumina ceramic particles with absorptance enhancement and a low α-Al2O3 phase transition temperature. These attributes make a favorable choice for solar-thermal applications in direct irradiation.

Intrinsic ferromagnetism in Hg1-xFexSe (0.00012 ≤ x ≤ 0.0013) diluted magnetic semiconductor bulk single crystals at room temperature: Role of hybridization of impurity electronic states

We investigate room-temperature structural properties and spontaneous magnetism in a series of Hg1-xFexSe (0.00012 ≤ x ≤ 0.0013) diluted magnetic semiconductor bulk single crystals grown by the Bridgman method. In order to elucidate the experimentally observed high-temperature magnetism in the studied systems, the dependencies of magnetization on magnetic field strength for the entire sample series were studied. Their detailed analysis revealed impurity contributions typical of ferromagnetic materials, which are described by magnetization curves with the parameters characterizing the spontaneous spin magnetism of the donor conduction electrons of the outer d-shells of the impurity atoms. According to the previously-developed theoretical concept, such a magnetic ordering mechanism stems from the direct exchange coupling of donor conduction electrons due to hybridization of impurity states, rather than from inter-impurity interactions. The results of optical emission spectroscopy and X-ray diffraction analysis confirmed the observed ferromagnetism to be intrinsic and linked directly to the d-electrons of iron impurity atoms.

The influence of the electric and magnetic fields on donor impurity electronic states and optical absorption coefficients in a core/shell GaAs/Al$$_{0.33}$$Ga$$_{0.67}$$As ellipsoidal quantum dot

The influence of the electric and magnetic fields on donor impurity electronic states and optical absorption coefficients in a core/shell GaAs/Al$$_{0.33}$$Ga$$_{0.67}$$As ellipsoidal quantum dot

A Theoretical Study on Photocatalytic Water Splitting of Boron Doped Defective Zirconium Dioxide

At present, environmental crisis and energy problems are becoming more and more serious. Finding a sustainable green energy source is urgent. Solar energy has attracted widespread attention. Here, we have meticulously investigated the application of boron atom-doped defective zirconia catalysts for photocatalytic water splitting through theoretical calculations. We know that doping boron atom greatly promotes the generation of oxygen vacancy defects in the zirconia catalyst, and the boron atom-doped defective zirconia catalyst has better photocatalytic water splitting performance than the pristine zirconia catalyst. In addition, boron atom-doped defective zirconia catalysts have higher stability. Through electronic structure analysis, the photo-response in the photocatalytic water splitting system and the fundamental reasons for the improvement of catalyst stability were explored. The introduction of boron atom promoted the generation of defect impurity electronic states, which made the antibonding state of the catalyst more stable cling to the Fermi level. Finally, the great photocatalytic water splitting performance of the boron-doped defective zirconia catalyst was confirmed with the difference in Gibbs free energy in photocatalytic water splitting process.

Temperature-Related Electronic Low-Lying States in Different Shapes In.1Ga.9N/GaN Double Quantum Wells under Size Effects

In this paper, we report the electronic states of hydrogenic impurity in InGaN/GaN double quantum wells (DQWs) with different shapes using a numerical procedure within the effective mass approximation. The effects of temperature, impurity position and size on the 1S-, 2S- and 2P-low-lying states are investigated for rectangular, parabolic and triangular finite potential confinements. Our results reveal that the binding energy versus the well width displays a maximum value around the effective Bohr radius and a larger value is obtained for rectangular compared to parabolic and triangular profiles. Moreover, regardless the size and impurity’s position, it is found that the main impact of the temperature is to shrink the binding energy. It is obtained that the binding energy drop is about 3.48 meV for rectangular and 4.26 meV for both parabolic and triangular profiles. Moreover, as the impurity is moved from the right barrier center to the structure center, the 1S-binding energy improvement is about 77.6 % in rectangular whereas it exceeds 91.4 % for other profiles. It is established that the binding energy can be easily modulated by adjusting the temperature, structure size and impurity's position. The results we obtained agree quite well with the findings.
 HIGHLIGHTS
 
 The maximum binding energy is obtained around the effective Bohr radius
 The binding energy is higher for rectangular shape compared to parabolic and triangular ones
 The main impact of the temperature is to shrink the binding energy
 The binding energy is improved with displacing the impurity toward the structure center

Quantum embedding methods for correlated excited states of point defects: Case studies and challenges

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density functional theory (DFT) is inherently a ground-state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$), the negatively charged nitrogen-vacancy center in diamond (${\mathrm{NV}}^{\ensuremath{-}}$), and an Fe impurity on the Al site in wurtzite AlN (${\text{Fe}}_{\text{Al}}$). In the context of these test-case defects, we demonstrate that crucial considerations of the methodology include convergence of the bulk screening of the active-space Coulomb interaction, the choice of exchange-correlation functional for the initial DFT calculation, and the treatment of the ``double-counting'' correction. For ${\mathrm{C}}_{\text{B}}{\mathrm{C}}_{\text{N}}$ we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and double-counting correction. For the ${\mathrm{NV}}^{\ensuremath{-}}$ center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in ${\text{Fe}}_{\text{Al}}$.

Impurity states of electrons on the surface of a nanotube in a magnetic field

The localization of electrons in the field of isolated nonmagnetic impurity atoms on the surface of a nanotube in a magnetic field is considered. A model of a Gaussian separable impurity potential capable of localizing an electron at any intensity is used. The positions of the local level are found in the regime of strong and weak localizations. The positions of the resonances are found, their widths are estimated. They experience Aharonov-Bohm oscillations when the magnetic flux in the tube changes. It is shown that the positions of the resonances experience oscillations similar to the de Haas-van Alphen oscillations, which are not associated with a magnetic field.

Atomic insights of electronic states engineering of GaN nanowires by Cu cation substitution for highly efficient lithium ion battery

Electronic engineering of gallium nitride (GaN) is critical for enhancement of its electrode performance. In this work, copper (Cu) cation substituted GaN (Cu-GaN) nanowires were fabricated to understand the electronically engineered electrochemical performance for Li ion storage. Cu cation substitution was revealed at atomic level by combination of X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), density functional theory (DFT) simulation, and so forth. The Cu-GaN electrode delivered high capacity of 813.2 mA h g−1 at 0.1 A g−1 after 200 cycles, increased by 66% relative to the unsubstituted GaN electrode. After 2000 cycles at 10 A g−1, the reversible capacity was still maintained at 326.7 mA h g−1. The DFT calculations revealed that Cu substitution introduced the impurity electronic states and efficient interatomic electron migration, which can enhance the charge transfer efficiency and reduce the Li ion adsorption energy on the Cu-GaN electrode. The ex-situ SEM, TEM, HRTEM, and SAED analyses demonstrated the reversible intercalation Li ion storage mechanism and good structural stability. The concept of atomic-arrangement-assisted electronic engineering strategy is anticipated to open up opportunities for advanced energy storage applications.

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.

Mechanism analysis of Au, Ru noble metal clusters modified on TiO2 (101) to intensify overall photocatalytic water splitting.

Accelerating the separation and migration of photo-carriers (electron–hole pairs) to improve the photo-quantum utilization efficiency in photocatalytic overall water splitting is highly desirable. Herein, the photo-deposition of Ru or Au noble metal clusters with superior electronic properties as a co-catalyst on the (101) facet of anatase TiO2 and the mechanism of intensifying the photocatalysis have been investigated by calculation based density functional theory (DFT). As a result, the as-synthesized Ru/TiO2 and Au/TiO2 exhibit high hydrogen evolution reaction (HER) activity. Such a greatly enhanced HER is attributed to the interfacial interactivity of the catalysts due to the existence of robust chemical bonds (Ru–O–Ti, Au–O–Ti) as electron-traps that provide the photogenerated electrons. In addition, the formation of new degenerate energy levels due to the existence of Ru-4d and Au-5d electronic impurity states leads to the narrowing of the band gap of the catalysts. In addition, the as-synthesized Au/TiO2 exhibits more faster HER rate than Ru/TiO2, which is attributed to the effects of surface plasmon resonance (SPR) as a synergistic effect of plasmon-induced ‘hot’ electrons that enhance the harvesting of the final built-in electric field and promote the migration and separation of the photo-carriers, which efficiently facilitates hydrogen evolution from the photocatalytic overall water splitting reaction.

The effects of the electric and intense laser field on the binding energies of donor impurity states (1s and 2p±) and optical absorption between the related states in an asymmetric parabolic quantum well

We have calculated the effects of electric and intense laser fields on the binding energies of the ground and some excited states of conduction electrons coupled to shallow donor impurities as well as the total optical absorption coefficient for transitions between 1s and 2p± electron-impurity states in a asymmetric parabolic GaAs/Ga1−x AlxAs quantum well. The binding energies were obtained using the effective-mass approximation within a variational scheme. Total absorption coefficient (linear and nonlinear absorption coefficient) for the transitions between any two impurity states were calculated from first- and third-order dielectric susceptibilities derived within a perturbation expansion for the density matrix formalism. Our results show that the effects of the electric field, intense laser field, and the impurity location on the binding energy of 1s-impurity state are more pronounced compared with other impurity states. If the well center is changed to be Lc<0 (Lc>0), the effective well width decreases (increases), and thus we can obtain the red or blue shift in the resonant peak position of the absorption coefficient by changing the intensities of the electric and non-resonant intense laser field as well as dimensions of the well and impurity positions.

Spin ordering contribution of iron, cobalt, and nickel impurity electron states, to the low-temperature magnetic susceptibility of mercury selenide crystals

A study of the magnetic susceptibility of electron systems of hybridized states of iron, cobalt, and nickel impurities in mercury selenide crystals at low temperatures, in connection with newly discovered evidence of spontaneous spin polarization in the anomalous Hall effect, at room temperature. It is shown that in the measured paramagnetic susceptibility of electrons in hybridized states, there exists a temperature-independent portion, and that there is enough justification to identify it using spontaneous polarization. The other portion demonstrates the Curie law with decreasing temperature, and corresponds to contributions from nonpolarized electrons. Based on findings relating to the magnetization of the examined impurity systems, we identify a part of the spontaneously polarized electron density in the localized component of hybridized states. We give quantitative estimates of the aforementioned portions of each impurity.

A DFT study on WO3 nanowires with different orientations for NO2 sensing application

The adsorption of NO2 molecule on the surface of WO3 nanowires with [001] and [010] orientations were investigated using density functional theory (DFT) calculations with aim to explore their potential sensing abilities theoretically. It is found that NO2 molecule can adsorb in multiple stable configurations for the WO3 nanowires with the two orientations, and the NO2 adsorption on the surface tunes the electronic structures of WO3 nanowires, giving rise to new impurity electronic states and then changing the band gap and the Fermi level position. The sensitive electronic structures as well as the multiple stable configurations to NO2 adsorption highlight the potential of WO3 nanowires as a sensor to detect NO2 gas. Further calculations about the atomic Mulliken charge population, planar averaged charge density differences, and electron localization functions (ELF) indicate that NO2 adsorption on [010]-oriented WO3 nanowire induces much stronger adsorbate-surface interaction and much more electrons extracted from the surface than on [001] nanowire, hinting a much higher sensitivity expectable for a NO2 gas sensor based on WO3 nanowire with [010] orientation.

Effects of impurity states on exchange coupling in Fe/Fe3O4junctions

Exchange coupling (EC) in Fe/Fe${}_{3}$O${}_{4}$ junctions containing magnetic impurities and in-gap states at the interface is calculated using a formula obtained by a cleaved layer method. The model for EC is constructed by performing first-principles calculations of the electronic and magnetic states of Co, Mn, and Cr impurities on the Fe surface and those of in-gap states in a bulk $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$, which has the same lattice structure as Fe${}_{3}$O${}_{4}$ but contains Fe defects. We show that the effect of Co impurities on EC is opposite to that of Cr and Mn impurities and that in-gap states tend to cause parallel magnetization alignment of two ferromagnets. These results are attributed to the change in electronic states caused by the presence of impurities. Further, we compare calculated results with experimental ones obtained in Fe/Fe${}_{3}$O${}_{4}$ junctions and suggest that doping magnetic impurities at the interface could be a useful way to control the magnitude and sign of the EC.

Picometer registration of zinc impurity states in Bi2Sr2CaCu2O8+δ for phase determination in intra-unit-cell Fourier transform STM

Direct visualization of electronic-structure symmetry within each crystalline unit cell is a new technique for complex electronic matter research (Lawler et al 2010 Nature466 347–51, Schmidt et al 2011 New J. Phys.13 065014, Fujita K et al 2012 J. Phys. Soc. Japan81 011005). By studying the Bragg peaks in Fourier transforms of electronic structure images and particularly by resolving both the real and imaginary components of the Bragg amplitudes, distinct types of intra-unit-cell symmetry breaking can be studied. However, establishing the precise symmetry point of each unit cell in real space is crucial in defining the phase for such a Bragg-peak Fourier analysis. Exemplary of this challenge is the high-temperature superconductor Bi2Sr2CaCu2O8+δ for which the surface Bi atom locations are observable, while it is the invisible Cu atoms that define the relevant CuO2 unit-cell symmetry point. Here we demonstrate, by imaging with picometer precision the electronic impurity states at individual Zn atoms substituted at Cu sites, that the phase established using the Bi lattice produces a ∼2%(2π) error relative to the actual Cu lattice. Such a phase assignment error would not diminish reliability in the determination of intra-unit-cell rotational symmetry breaking at the CuO2 plane (Lawler et al 2010 Nature466 347–51, Schmidt et al 2011 New J. Phys.13 065014, Fujita K et al 2012 J. Phys. Soc. Japan81 011005). Moreover, this type of impurity atom substitution at the relevant symmetry site can be of general utility in phase determination for the Bragg-peak Fourier analysis of intra-unit-cell symmetry.

Electronic structure and impurity states of S-doped cBN: A first-principle study

The electronic structure and impurity states of S-doped cBN were investigated by first-principle approaches. Our calculation shows that S substituted for an N atom creates shallow donor levels merged to the states at the conduction band edge, S substituted for a B atom creates deep donor levels within the band gap, both forming n-type cBN, and band gap of S-doped cBN decreases with the increase of dopant concentration. Moreover, from the view of energy, S is more likely to be substituted for a B atom than an N atom.

Search for Light-Induced Intrinsic Localized Modes: Negative Result

We have attempted to induce the theoretically predicted anharmonic lattice modes called intrinsic localized modes in the NaI single crystal lattice by UV excitation of the impurity electronic states of Tl+ substitution centers. Comparison of low-temperature phonon density of states measured by inelastic neutron scattering with and without simultaneous UV illumination of the sample did not reveal significant difference. Results are discussed in the context of possible improvements of the proposed experiment.

Local and quasilocal energy levels of electrons on nanotube surfaces and in rings in magnetic fields

The Lifshitz local perturbation technique is used to examine impurity states of electrons on nanotube surfaces and in rings in magnetic fields. It is shown that an impurity in the form of a thin ring on a tube can form local and quasilocal electron states. The positions and widths of the impurity levels are found. These quantities on tubes and rings are calculated numerically for the parameters of semiconductor structures.

Anomalies in the temperature dependence of the contribution to the speed of sound from hybridized electronic states of transition element impurities

The temperature dependence of the speed of sound in crystalline mercury selenide with low concentrations of iron impurities is studied. Experiments are conducted in the ranges of concentration and temperature where hybridized electronic states in iron impurities have been observed previously. It is found that at temperatures below 10 K the speed of slow transverse ultrasonic waves has an anomalous nonmonotonic segment of its temperature variation that is related to the influence of the impurities and reflects the existence of hybridized states. The observed anomalies in the sound speed are described in terms of a theory for the electron contribution to the elastic moduli that includes hybridization of impurity states and electron-electron interactions. Fits of the theoretical dependences to the experimental data yield quantitative information on the parameters of the hybridized states and of the Fermi-liquid interaction.