Interstitial Positions Research Articles

Structural modulation and spin glassiness upon oxidation in oxygen storage material LnFeMnO4+x for Ln = Y, Lu, and Yb

The mixed valence multiferroic LnFe2+Fe3+O4 (where Ln = Y, Lu, and Yb) can reversibly uptake oxygen into its lattice, which is evidenced by a crystallographic phase transition along with the appearance of structural modulations. In this study, we show that the Mn-substituted version of this multiferroic can also be readily oxidized to LnFe3+Mn3+O4.5 revealing similar oxygen storage behavior. Through neutron, electron, and synchrotron x-ray diffraction studies, we observe a structural modulation that we attribute to a displacement wave in the fully oxidized compound. This wave exhibits commensurability with a wavevector q = (−2/7, 1/7, 0). Bond valence summation analysis of plausible interstitial oxygen positions suggests that oxygen insertion likely occurs at the middle of the Fe/Mn–O bipyramid layers. The structural modulation of LnFeMnO4.5 is two-dimensional, propagates along the ab-plane, and is highly symmetric as 12 identical modulation vectors are observed in the diffraction patterns. The nature of the lanthanide, Ln3+, does not seem to influence such modulations since we observe identical satellite reflections for all three samples of Ln = Y, Lu, and Yb. Both LnFeMnO4 and LnFeMnO4.5 display spin glassy behavior with 2D short-range magnetic ordering being observed in LnFeMnO4. Analysis of the neutron diffraction data reveals a correlation length of ∼10 nm. Upon oxidation to LnFeMnO4.5, the short-range magnetic order is significantly suppressed.

Formation of Zr-rich BCC phase and its relation on the hydrogen storage properties of TiVNbZr high entropy alloy

Due to the large difference in atomic size between Zr and Nb, TiVNbZr high entropy alloy possesses greater lattice distortion. The change of Zr content can affect the stability of hydrides, which can have an important impact on the hydrogen storage performance. In order to explore the effect of Zr/Nb on hydrogen storage properties, Ti32.5V27.5Nb40-xZrx (x = 7.5, 10, 12.5, 15) high entropy alloys composed of BCC phase are designed. It is found that with the increase of Zr/Nb, the dendritic microstructures first disappear and then gradually become coarse. The Zr-rich BCC phase and the Nb-rich BCC phase appear in the interdendritic and dendritic regions, respectively. Ti32.5V27.5Nb25Zr15 shows excellent activation performance, which is related to the Zr-rich BCC phase with the most catalytic phase on the surface. The hydrogen storage performance of low Zr/Nb composition is better at low temperature, and that of high Zr/Nb composition becomes better after rising to high temperature. This is because with the change of Zr/Nb, low temperature can promote the hydrogen absorption reaction and high temperature can increase the diffusion rate of hydrogen atoms in the interstitial positions of the alloy. Ti32.5V27.5Nb32.5Zr7.5 achieves the best hydrogen absorption capacity of 1.71 wt % at 150 °C. All components undergo a transition from the BCC phase to the FCC phase during the hydrogen absorption process, and then change to a single-phase BCC structure after dehydrogenation. With the increase of Zr content, the first step desorption temperature raises in turn, owing to the stronger thermal stability of hydride.

First-Principles Study of Oxygen in ω-Zr

Zirconium alloys, which are widely used as cladding materials in nuclear reactors, are prone to react with oxygen (O). Furthermore, the ω-Zr in zirconium alloys can significantly increase the strength and hardness of these alloys, but there is a lack of reports on the behavior of oxygen in ω-Zr in the current literature. To investigate their interactions, we have studied the behavior of O in ω-Zr using the first-principles approach. In this work, we examined the effects of vacancy and alloying elements (Nb, Sn) on the behavior of O in ω-Zr. The results show that O with a formation energy of −5.96 eV preferentially occupies an octahedral interstitial position in ω-Zr. A vacancy reduces the formation energy of O in a tetrahedral interstitial position in ω-Zr. Nb and Sn decrease the formation energy of O in the octahedral interstitial position by 6.16 eV and 5.08 eV. Vacancy effectively reduces the diffusion barrier of O around it, which facilitates the diffusion of O in ω-Zr. Nb and Sn preferentially occupy the 1b and 2d substitution sites in ω-Zr, respectively. Nb makes the diffusion barrier of O in ω-Zr lower and promotes the diffusion of O in ω-Zr. Moreover, Sn makes the diffusion of O around Sn difficult. It was further found that O is less prone to form clusters in ω-Zr and tends to independently occupy interstitial positions in ω-Zr. In particular, a single vacancy would make the binding energy between O atoms to be further reduced.

Exploring the influence of rare earth ions at A-site on the properties of LnBaCo2O5+δ cathode for solid oxide fuel cells

The influence of rare earth ions on layered perovskite (AA'B2O5+δ) is the key to the development of highly active layered perovskite cathode. Herein, we make a systematic explanation through experiment and first-principles calculation. The lattice constant decreases with the decrease of Ln3+ ionic radius, resulting in a smaller space of interstitial oxygen position and increasing oxygen Frenkel energy. The electrochemical activity of LnBaCo2O5+δ (Ln = Pr, Nd and Sm) electrodes decreases with the decrease of Ln3+ ionic radius, which is mainly attributed to a lower oxygen Frenkel energy. Electrochemical impedance spectra tests show that there are significant differences in the surface exchange process of oxygen because of different surface oxygen vacancy concentration for LnBaCo2O5+δ. For Ln = Pr, Nd, and Sm cathodes, the maximum power densities of the anode-supported Ni-YSZ/YSZ/GDC/LnBaCo2O5+δ single cells reach 1.48, 1.36 and 1.06 W/cm2 at 800 °C, respectively, indicating that PrBaCo2O5+δ with the maximum rare earth ion radius exhibits better oxygen reduction reaction. These findings provide guidance for the design and fabrication of highly active layered perovskite electrode materials.

Unlocking high-rate O3 layered oxide cathode for Na-ion batteries via ion migration path modulation

O3-NaNi1/3Fe1/3Mn1/3O2 is a promising layered cathode material with high specific capacity, low cost, and simple synthesis. However, sluggish kinetic hindrance is attributed to the size discrepancy between the large Na-ion and narrow tetrahedral interstitial positions, leading to inferior rate capacity and low reversible capacity. Herein, F− with light‐weight and strong electronegativity is introduced to substitute O atoms in the bulk structure, which intensifies the bond strength of transition metal and oxygen and enlarges the Na+ diffusion channel. In addition, density-functional theory (DFT) calculations demonstrate that the electrostatic interaction is weakened between Na+ in the tetrahedral site and the transition-metal cation directly below it, dramatically reducing the migration barriers of Na+ diffusion. Consequently, the as-obtained NaNi1/3Fe1/3Mn1/3O1.95F0.05 sample displays outstanding rate performance of 86.7 mA h g−1 at 10 C and excellent capacity retention of 84.1% after 100 cycles at 2 C. Moreover, a full cell configuration using a hard carbon anode reaches the energy density of 307.7 Wh kg−1. This strategy paves the way for novel means of modulating the Na-ion migration path for high-rate O3-type layered cathode materials.

Divergent patterns of meiotic double strand breaks and synapsis initiation dynamics suggest an evolutionary shift in the meiosis program between American and Australian marsupials.

In eutherian mammals, hundreds of programmed DNA double-strand breaks (DSBs) are generated at the onset of meiosis. The DNA damage response is then triggered. Although the dynamics of this response is well studied in eutherian mammals, recent findings have revealed different patterns of DNA damage signaling and repair in marsupial mammals. To better characterize these differences, here we analyzed synapsis and the chromosomal distribution of meiotic DSBs markers in three different marsupial species (Thylamys elegans, Dromiciops gliorides, and Macropus eugenii) that represent South American and Australian Orders. Our results revealed inter-specific differences in the chromosomal distribution of DNA damage and repair proteins, which were associated with differing synapsis patterns. In the American species T. elegans and D. gliroides, chromosomal ends were conspicuously polarized in a bouquet configuration and synapsis progressed exclusively from the telomeres towards interstitial regions. This was accompanied by sparse H2AX phosphorylation, mainly accumulating at chromosomal ends. Accordingly, RAD51 and RPA were mainly localized at chromosomal ends throughout prophase I in both American marsupials, likely resulting in reduced recombination rates at interstitial positions. In sharp contrast, synapsis initiated at both interstitial and distal chromosomal regions in the Australian representative M. eugenii, the bouquet polarization was incomplete and ephemeral, γH2AX had a broad nuclear distribution, and RAD51 and RPA foci displayed an even chromosomal distribution. Given the basal evolutionary position of T. elegans, it is likely that the meiotic features reported in this species represent an ancestral pattern in marsupials and that a shift in the meiotic program occurred after the split of D. gliroides and the Australian marsupial clade. Our results open intriguing questions about the regulation and homeostasis of meiotic DSBs in marsupials. The low recombination rates observed at the interstitial chromosomal regions in American marsupials can result in the formation of large linkage groups, thus having an impact in the evolution of their genomes.

Bourdieusian Boundary-Making, Social Networks, and Capital Conversion: Inequality among International Degree Holders in Hong Kong

Sociological research richly documents the many ways through which education becomes a form of convertible capital, but focuses less on the cultural schemas that graduates possess and use to respond to disruptions of capital conversion processes. Using the case of international degree holders in Hong Kong, this article draws upon Bourdieu’s theory of practice to interrogate the cultural schemas that valorize international degrees when their conversion pathways to economic capital are subjectively perceived to weaken. This article unearths the role of social networks in embedding cultural schemas and their effects on relations within the field: when faced with diminishing economic returns, international degree holders hold fast to their schemas vis-à-vis fellow international graduates and reconceptualize their degrees as symbolic capital to cope with the loss by enacting symbolic violence against domestic degree holders. Class boundaries are ultimately entrenched when international degree graduates valorize their cultural capital gains and legitimate their economic capital losses. Doing so compromises their class interests by forcing themselves into an interstitial position between different fields: though they occupy dominating homologous positions in the cultural field, they choose to overlook their dominated homologous positions in the economic field.

Superior Phosphorous Doping in Nanocrystalline Silicon Thin Films and Their Application as Emitter Layers in Silicon Heterojunction Solar Cells

Phosphorous doping in the nc-Si network induces gradually reduced crystallinity; however, preferential growth along <220>-oriented crystallites promotes the columnar-like growth morphology. At optimum doping, substitution by donor P+-atoms in the c-Si lattice contributes surplus free electrons and high carrier mobility, resulting in superior electrical conductivity in the n-nc-Si network. An elevated doping leads to incorporating elemental P0 atoms in the interstitial position or forming P–Si–H clusters and generating voids between the crystalline columns. Segregation of defects contributes to decreasing carrier mobility and reducing conductivity after diminishing crystallinity and narrowing the optical band gap. By precisely controlling the growth at 250 °C and efficient electrically active doping by P+-atoms, n-nc-Si thin films with superior dark conductivity (∼101 S cm–1) are produced in which the percolation of charge carriers through the crystalline columns could facilitate stacked-layer devices. The optimum n-nc-Si thin films are used as the emitter layers in n-nc-Si/p-c-Si heterojunction solar cells (HJSCs). Furthermore, an ultrathin a-Si:H buffer layer on the p-c-Si minimizes the junction carrier recombination loss, and subsequent postdeposition short-time H-plasma treatment (PSHPT) ensures seeds for superior nanocrystallization in the n-nc-Si emitter layer. The n-nc-Si/(PSHPT)i-nc-Si(buffer layer)/p-c-Si HJSC delivers a PV conversion efficiency, η ∼12.35%, via a reasonable fill factor of ∼0.647 and sensible JSC of ∼32.75 mA cm–2. Further improvement in the PV performance could be possible using suitably thinner p-c-Si wafers, harmonizing with the effective carrier diffusion length, and fabricating a high-quality passivation structure on the backside.

Absorption versus adsorption: high-throughput computation of impurities in 2D materials

Doping of a two-dimensional (2D) material by impurity atoms occurs via two distinct mechanisms: absorption of the dopants by the 2D crystal or adsorption on its surface. To distinguish the relevant mechanism, we systematically dope 53 experimentally synthesized 2D monolayers by 65 different chemical elements in both absorption and adsorption sites. The resulting 17,598 doped monolayer structures were generated using the newly developed ASE DefectBuilder—a Python tool to set up point defects in 2D and bulk materials—and subsequently relaxed by an automated high-throughput density functional theory (DFT) workflow. We find that interstitial positions are preferred for small dopants with partially filled valence electrons in host materials with large lattice parameters. In contrast, adatoms are favored for dopants with a low number of valence electrons due to lower coordination of adsorption sites compared to interstitials. The relaxed structures, characterization parameters, defect formation energies, and magnetic moments (spins) are available in an open database to help advance our understanding of defects in 2D materials.

Quasi-2D FCC lithium crystals inside defective bi-layer graphene: insights from first-principles calculations

Quasi-2D crystals inside bi-layer graphene have been observed in in-situ TEM experiments [Nature 564 (2018) 234]. It was also revealed that Li crystals have the FCC structure, nucleate at point defects in graphene and contain impurity atoms. Using first-principles calculations, we systematically study the interaction of isolated Li atoms and those assembled in FCC crystals with vacancy-type defects in graphene and show that quasi-2D Li crystals encapsulated between graphene sheets must indeed nucleate at the defects and that the interaction of not only isolated Li atoms, but also Li crystals with the defects in graphene is strong. We further demonstrate that a moiré pattern develops at the graphene/Li interface. Finally, we investigate the behavior of impurities most likely to be found in the encapsulated Li crystals, such as O, N, S and F and show that all impurity atoms take octahedral interstitial positions and strongly interact with atoms in Li crystals, thus impeding the de-lithiation process. Our theoretical work focused on the fundamental aspects of the behavior of Li inside bi-layer graphene should help rationalize the results of in-situ TEM experiments and shed light on the role of impurities in the degradation of anode materials during Li-ion battery operation.

Undoped and Fe-Doped Anatase/Brookite TiO2 Mixed Phases, Obtained by a Simple Template-Free Synthesis Method: Physico-Chemical Characterization and Photocatalytic Activity towards Simazine Degradation

For the first time, Fe-doping (0.05, 1.0, and 2.5 wt.% Fe) was performed on a high-surface-area anatase/brookite TiO2 by adopting a simple template-free sol-gel synthesis followed by calcination at a mild temperature. The powders’ textural and surface properties were characterized by following a multi-technique approach. XRD analysis showed that the anatase/brookite ratio slightly varied in the Fe-doped TiO2 (from 76.9/23.1 to 79.3/22.7); Fe doping noticeably affected the cell volume of the brookite phase, which decreased, likely due to Fe3+ ions occupying interstitial positions, and retarded the crystallite growth. N2 sorption at −196 °C showed the occurrence of samples with disordered interparticle mesopores, with an increase in the specific surface area from 236 m2 g−1 (undoped TiO2) to 263 m2 g−1 (2.5 wt.% Fe). Diffuse Reflectance UV-Vis spectroscopy showed a progressive decrease in the bandgap energy from 3.10 eV (undoped TiO2) to 2.85 eV (2.5 wt.% Fe). XPS analysis showed the presence of some surface Fe species only at 2.5 wt.% Fe, and accordingly, the ζ-potential measurements showed small changes in the pH at the isoelectric point. The photocatalytic degradation of simazine (a persistent water contaminant) both under UV and simulated solar light was performed as a probe reaction. Under UV light, Fe-doping improved simazine degradation in the sample at 0.05 wt.% Fe, capable of degrading ca. 77% simazine. Interestingly, the undoped TiO2 was also active both under UV and 1 SUN. This is likely due to the occurrence of anatase/brookite heterojunctions, which help stabilize the photogenerated electrons/holes.

Effect of Zn atomic diffusion due to pulsed electric field treatment on the thermoelectric properties of Zn–Sb films

Zn4Sb3 has brought into sharp focus by its low cost and excellent thermoelectric (TE) properties in the medium temperature range. Zn atoms can diffuse in the Zn4Sb3 matrix under the treatment of temperature or current. This provides a way to control its TE performance. In this study, the TE properties of the Zn4Sb3 films were improved by the Zn atomic diffusion under the treatment of periodic pulsed electric field. The Zn atomic diffusion is caused by the electrostatic interaction of the pulsed electric field and the diffusion mode is related to the pulse period. When the pulse period is smaller than 10 s, the interstitial Zn atom diffuses first. The Zn vacancy increases, leading to the increase of the carrier concentration and the decrease of the resistivity. When the pulse period increases to 100 s, more Zn(1) atoms diffuse through the interstitial position, resulting in a small amount of ZnSb phase. Thereby in the Zn4Sb3 films, the carrier concentration decreases and the Seebeck coefficient increases. This kind of the synergistic effect decreases the resistivity and increases the Seebeck coefficient. The power factor of the Zn4Sb3 films treated with a pulse period of 100 s is more than 4500 μW m−1 K−2 in the range of 373 K–573 K. This study presents a new method to control atomic diffusion by periodic pulsed electric field, and the TE properties of the Zn4Sb3 films are improved.

Physical mapping of ribosomal DNA sites and genome size in polyploid series of Urochloa humidicola (Rendle) Morrone & Zuloaga (Poaceae)

ABSTRACT Urochloa humidicola (Rendle) Morrone & Zuloaga is a species native to Africa that grows in tropical regions, especially in sites with temporary flooding. Despite its importance as forage crop, the karyotypic characterization of its cytotypes remains mainly unfinished, and its polyploid origin is unknown. Therefore, cytogenetic studies can provide information about the evolutionary history of species and to characterize the core and reserve collection of the germplasm banks. We performed the karyotype analyses, mapped ribosomal sites, and estimated genome size, confirming three ploidy levels (6×, 7×, and 9×) and one aneuploid accession H031 (2n = 6× = 36 + 1). Karyotypes are symmetrical and the number of 35S and 5S rDNA sites varied between the cytotypes. However, all sites were at terminal and interstitial positions in the chromosomes, respectively, except one chromosome of the aneuploid accession with interstitial colocalization. The polymorphism of the number and distribution of rDNA sites in U. humidicola was confirmed by (i) the variation between and within plants and in metaphases of the same meristem in hexaploids and heptaploids; (ii) the occurrence of rDNA sites in hemizygosis in all accessions, except heptaploids; (iii) the size heteromorphism in some ribosomal sites of homologous chromosomes. The karyotype data and the mapping of ribosomal sites corroborate the proposals of allopolyploidy in U. humidicola and the hybrid origin to the H031 accession.

American Public School Librarianship: A History

American Public School Librarianship: A History

Effects of alkali-metals (X = Li, Na, K) doping on the electronic, optoelectronic, thermodynamic, and X-ray spectroscopic properties of X–SnI3 halide perovskites

The effect of alkali metals (Li, Na, K) doping at X sites in tin-based X–SnI3 halide perovskite on structural, electronic, phonon, thermodynamic, and X-ray core-level spectroscopic properties was investigated and correlated with optoelectronic efficiency in the development of a stable and environmentally friendly solar technology to replace toxic lead-based perovskite solar cells (PSCs). These calculations were carried out based on the density functional theory approach using the Quantum Espresso software with the generalized gradient approximation (GGA) exchange correlation Perdew Burke Ernzerhorf functional. In addition, the phonon frequency, thermodynamics, and X-ray spectroscopy calculations were completed in Materials Studio using the CASTEP code. Our results reveal that there is a clear correlation between the alkali metal's ionic radius, changes in the lattice structural parameters, and the electronic band gap, such that the lattice constants are 5.940, 5.868, and 5.897 Å and the band gaps are 1.20, 1.14, and 1.10 eV for LiSnI3, NaSnI3, and KSnI3, respectively. The highest lattice constant obtained for LiSnI3, indicates that Li atoms occupied interstitial positions in the perovskite crystal, whereas the direct band gap decreases as cationic size increases, with KSnI3 exhibiting the least band gap. Phonon calculations show that KSnI3 has the lowest negative vibration region in its phonon dispersion plot, indicating dynamic stability. Interestingly, all the structures are found to portray high optical absorption, high extinction coefficient, high polarizability. Additionally, the free energy computation of the perovskites shows a reduction with rising temperature, but enthalpy increases significantly with increasing temperature. Yet, due to phonon lattice vibrations, the heat capacity increased as the temperature rose. Although the XSnI3 (X = Li, Na, and K) perovskites under investigation will perform efficiently as semiconductors and can be used for optoelectronic applications, the KSnl3 perovskite material has the most desirable properties for the fabrication of solar cell devices.

Influence of the superionic transition on the diffusion of cations in ThO2 nanocrystals. A molecular dynamics simulation

Molecular dynamics simulation of self-diffusion of thorium cations in the bulk of ThO2 nanocrystals with a free surface was carried out. The presence of the surface created a natural influx of cationic vacancies into the bulk. The stability of the surface was ensured by the equilibrium octahedral shape of the nanocrystals. The model systems consisted of 15,960 particles, the diffusion tracking times reached 3.5 ms. The temperature range from 3100 K to 3600 K was investigated. The upper limit of this range corresponded to the melting of model nanocrystals, the lower limit was defined by the computing performance. The superionic transition was within this temperature range, at T = 3220 K. The values of the diffusion coefficient ranged from 2 10–12 to 4 10–10 cm2/s. The simultaneous presence of several vacancy-type diffusion mechanisms was registered. Along with direct diffusion jumps of individual particles, a diffusion mechanism was described, consisting in a complex collective movement of cations towards the vacancy, including their temporary displacements to the interstitial positions. The influence of the superionic transition on the effective diffusion activation energy was studied.

Machine learning-aided Genetic algorithm in investigating the structure–property relationship of SmFe12-based structures

We investigate the correlation between geometrical information, stability, and magnetization of SmFe12-based structures using machine learning-aided genetic algorithm structure generation and first-principle calculation. In parallel with structure generation inherited using the USPEX program, a pool of structures is created for every population using the sub-symmetry perturbation method. A framework using embedded orbital field matrix representation as structure fingerprint and Gaussian process as a predictor has been applied to ranking the most potential stability structures. As a result, the original structure SmFe12 with the well-known tetragonal I4/mmm symmetry is investigated with a parabolic dependence between formation energy and its magnetization by continuous distortions of the unit-cell lattice parameter and individual sites. Notably, a SmFe12 structure with I4/mmm symmetry is found with 7.5% increasing magnetization while keeping the similar formation energy with the most stable structures in this family. With SmFe11CoN family, structures with N interstitial position in the center of Sm and Fe octahedron show outperform all other structures in both ability of stabilization and remaining high magnetization of the original structure. Finally, further investigation using metric learning embedding space brings valuable insight into the correlation between geometrical arrangement, stability, and magnetization of this structure family.

Chromosome Mapping of Repetitive DNAs in the Picasso Triggerfish (Rhinecanthus aculeatus (Linnaeus, 1758)) in Family Balistidae by Classical and Molecular Cytogenetic Techniques

This work presents the cytogenetic analysis conducted on the Picasso triggerfish (Rhinecanthus aculeatus (Linnaeus, 1758)) from Thailand. Mitotic chromosomes were prepared from the anterior kidney. The cell suspensions were harvested by in vivo colchicine treatment. The present study includes the chromosomal investigation on R. aculeatus, using conventional (Giemsa staining, Ag-NOR and C-banding) and molecular approaches (in situ mapping of five different repetitive DNA classes including 18S rDNA, 5S rDNA, (CA)15, (GA)15 and (CAA)10 as markers.) The results showed that R. aculeatus has karyotypes formed exclusively by telocentric chromosomes (44t; NF=44). The C-positive heterochromatic blocks are preferentially located in the centromeric and telomeric regions of some chromosomal pairs. The Ag-NOR sites occupy the interstitial position of the long arms of the largest telocentric pair (pair 1). The exclusive location of the major ribosomal sites in these pairs was confirmed by hybridization with 18S rDNA probes. However, the 5S rDNA genes are not located on 18S rDNA-bearing chromosomes, but instead located exclusively in the subcentromeric region of pair 4. The mapping of (CA)15, (GA)15 and (CAA)10 microsatellites are sparsely dispersed along all the chromosomes. The karyotype formula of R. aculeatus is 2n (44) = 44t.

Electronic Structures and Spectroscopic Signatures of Noble-Gas-Doped Nanodiamonds.

Fluorescent nanodiamonds, that is, those containing optically active defects, have attracted interest for their ability to be used as qubits; for in vivo imaging; and as sensors for spin, stress, and temperature. One of the most commonly studied nanodiamond color centers is the nitrogen vacancy. However, there is strong interest in discovering other impurity centers that provide localized midband gap transitions. Noble gas atoms have garnered attention since they have been discovered within nanodiamonds produced through high-pressure-high-temperature laser-heated diamond anvil cell synthesis methods, where they are commonly used as hydrostatic pressure media. Noble gas atoms that exist in macrosized natural or synthetic diamonds have been shown to be able to form color centers. This research uses ab initio density functional theory and cluster models to systematically study the localized electronic structure for group VIII impurities of nanodiamond, including helium, neon, argon, krypton, and xenon. An in-depth examination of the interaction between the noble gas atom and diamond lattice has been carried out. The changes to the vibrational and UV/vis absorption spectra have been analyzed. It was determined that the energetically preferred geometry is dependent on the atom size. Most noble gas defects are stabilized within the nanodiamond lattice and exist in tetrahedral interstitial positions, except for the largest noble gas studied in this work, Xe, which was determined to prefer a substitutional configuration. Both Kr and Xe are expected to be able to manifest visible/near-IR optical responses when included in the diamond lattice.

Electronic structure analysis of light-element-doped anatase TiO[formula omitted] using all-electron GW approach

A new structural phase of anatase TiO2 codoped with C and N with a relatively low band gap was discovered. Phase stability was first analyzed using density functional theory calculations by considering interstitial and substitutional positions and oxygen vacancies. The stable defect states were found to depend on the oxygen (O2) pressure conditions or oxygen chemical potential in C and N monodoped and codoped TiO2. Thereafter, the all-electron GW approach based on many-body perturbation theory was adopted to determine the electronic structures and understand the band gap narrowing mechanism. The band gap could be controlled by varying the oxygen pressure and doping states, and the band gap narrowing due to C, N codoping was more than that due to monodoping. Under an intermediate oxygen pressure condition, the band gap of TiO2 codoped with C and N was found to be 2.28 eV. Therefore, TiO2 codoped with C and N can be used as a visible-light-response photocatalyst.