The dependence of the boson peak on the thickness of Cu50Zr50 film metallic glasses.

In this study, intensive calculations were performed to investigate the behavior of the low-temperature excess heat capacity of Cu50Zr50 ultrathin film metallic glasses. Our results show that there is a well-defined boson peak in the film metallic glasses and that the boson peak height exhibits an obvious size-dependent feature. Furthermore, there is a critical thickness dc in the curves between the boson peak height and the thickness, where the boson peak height changes abruptly. Through structural analysis, we found that the low-temperature excess heat capacity of the film metallic glasses is correlated with the density layering structure near the surface. The structural parameter S is defined by atomic density and it was found that the boson peak height is highly correlated with S. Our investigation of ultrathin film metallic glasses provides a deeper understanding about the structural origin of the boson peak in metallic glasses.

A comprehensive study of the red persistent luminescence mechanism of Y2O2S:Eu,Ti,Mg.

Y2O2S:Eu,Ti,Mg, a persistent luminescence (PersL) material that exhibits eye-sensitive red emission for longer than 4-5 h, has attracted much attention and has been intensively researched over the past decade. If it is figured out how to prolong its decay time for longer than 8 h, the amazing candle-like red PersL performance, once lit, can illuminate a room all night without electricity. However, the PersL mechanism is still confusing, since different investigators have their own unique understanding about it based on their personal experimental observations. In this work, a more comprehensive and detailed investigation of the PersL mechanism is carried out, based on the defect levels induced by Eu, Ti, and Mg impurities and anion vacancies, using first-principles calculations. Our calculated results suggest that the empty spin-down 4f levels of Eu3+ appear in the band gap, while the occupied spin-up 4f levels are just below the valence band maximum (VBM). The 3d levels of Ti4+ are located in the band gap, with the highest levels around 1.4 eV below the conduction band minimum (CBM). Positively charged anion vacancies were found to induce empty defect levels just below the CBM and so could serve as electron trap centers, which prolong the lifetimes of excited electrons and lead to the PersL of the Ti4+ ion. When Eu3+ is co-doped with Ti4+, the energy of the excited Ti4+ ions is transferred to Eu3+. This mechanism can explain well most of the experimental observations that have appeared in the literature over the past decade. The obtained PersL mechanism is very clear in terms of the roles played by most types of defect, so we hope it can provide physical understanding and create intrigue around the idea of practical guidelines for the design of new red PersL materials in the future.

Structural origin of the high glass-forming ability of Ce70Ga10Cu20 alloys.

The CeGaCu amorphous alloy has a good glass-forming ability and many special properties. However, its structure at the atomic scale is unclear. We systematically investigated the structure evolution of Ce70GaxCu30-x (x = 6, 10, 13) glass formation melts by ab initio molecular dynamics (AIMD) simulations. Based on the trajectories from the simulations, the pair-correlation function, coordination numbers, chemical short-range order, Voronoi polyhedra and electronic structures were discussed. Our results show that the concentration of Ga- and Cu-centered icosahedral (-like) clusters in Ce70Ga10Cu20 melts are larger than those in Ce70Ga6Cu24 and Ce70Ga13Cu17 melts. Furthermore, electronic analysis showed that the hybridization between Ga 4p and Cu 3d (Ce 5d) orbitals is strong and that of Cu 3d orbitals and Ga 4p orbitals was strengthened in Ce70Ga10Cu20 melts, which means that the interactions between Ga and Cu atoms nearby were enhanced in the Ce70Ga10Cu20 melts. The stability of the Ga- or Cu-centered icosahedral clusters increased accordingly, which favored their glass-forming ability. Our investigation helps people obtain an increased understanding of the glass-forming ability from the viewpoint of chemical interactions for metallic glasses.

Reaction mechanism between small-sized Ce clusters and water molecules: an ab initio investigation on Cen + H2O.

Reactions of small-sized cerium clusters Cen (n = 1-3) with a single water molecule are systematically investigated theoretically. The ground state structures of the Cen/H2O complex and the reaction pathways between Cen + H2O are predicted. Our results show the size-dependent reactivity of small-sized Ce clusters. The calculated reaction energies and reaction barriers indicate that the reactivity between Cen and water becomes higher with increasing cluster size. The predicted reaction pathways show that the single Ce atom and the Ce2 and Ce3 clusters can all easily react with H2O and dissociate the water molecule. Under UV-irradiation, the reaction of a Ce atom with a single H2O molecule may even release an H2 molecule. The reaction of either Ce2 or Ce3 with a single H2O molecule can fully dissociate the H2O into H and O atoms while it is bonded with the Ce cluster. The electronic configuration and oxidation states of the Ce atoms in the products and the higher occupied molecular orbitals are analyzed by using the natural bond orbital (NBO) analysis method, from which the high reactivity between the reaction products of Cen + H2O and an additional H2O molecule is predicted. Our results offer deeper molecular insights into the chemical reactivity of Ce, which could be helpful for developing more efficient Ce-doped or Ce-based catalysts.

Reaction mechanism between small-sized Ce clusters and water molecules II: an ab initio investigation on Cen (n = 1-3) + mH2O (m = 2-6).

Possible reactions between the products of the three independent reactions involving a small Ce cluster and a single water molecule, Cen + H2O (n = 1-3), and an additional H2O molecule are systematically investigated. The ground-state isomers of the final products and the reaction pathways involving multiple water molecules are predicted. We find that under either ambient or UV-irradiation conditions, all the reactions can entail low energy barriers. In addition, the final products of the reaction between Cen and more than two H2O molecules are also predicted through an extensive structural search. The calculated reaction energies suggest that although small-sized Ce clusters can react with more than two water molecules, the reactions with one or two water molecules are dominant. The electronic structures of all the ground-state isomers and the corresponding oxidation states of Ce atoms in these isomers are computed and determined via the natural bond orbital (NBO) method. The results indicate that a single Ce atom and a Ce2 cluster can react with a maximum of four and six water molecules, respectively, while a Ce3 cluster can react with more than six water molecules. This comprehensive study offers an improved understanding of the mechanism underlying the reactions between a single Ce atom or a small Ce cluster and two or more H2O molecules. Knowledge obtained from this study can be helpful for the development of high-performance Ce-doped or Ce-based catalysts.

Understanding the quenching nature of Mn4+ in wide band gap inorganic compounds: design principles for Mn4+ phosphors with higher efficiency.

Mn4+ doped phosphors, as an alternative to rare-earth element doped phosphors, have attracted immense attention owing to their ultrahigh quantum efficiency of red emission for potential applications in high rendering white LEDs (light-emitting diodes). Their performance can be largely affected by quenching phenomena such as thermal quenching, concentration quenching and the quenching induced by some intrinsic/extrinsic defects. However, the quenching mechanisms due to the defect levels and host band are still incompletely understood. In this work, we carry out a comprehensive first-principles study on the underlying quenching mechanisms due to the defect levels of Mn4+ and other extrinsic/intrinsic defects, using the prototype oxide Y3Al5O12 (YAG), fluorides K2TiF6 (KTF) and ZnTiF6·6H2O (ZTF) as examples. From the comparison of the defect levels of Mn4+ with the host bands, we find that it is the very small energy difference between the defect levels of Mn4+ and the valence bands maximum (VBM) of YAG that causes the lower luminescence thermal stability of YAG:Mn4+, which we name as the hole-type thermal quenching mechanism. For the concentration quenching, it is nearly impossible for the Mn4+-Mn4+ pairs, previously considered as the main quenching centers, to appear in phosphors. A new quenching nature has been discussed. For the impurity ionic effects, the hole-type defects can largely stabilize the Mn ions in +4 states, thereby enhancing the emission intensity. These proposed mechanisms can offer deeper insights into the luminescence behavior of Mn4+ and a better practical understanding of the high photoluminescence quantum yield red phosphors by adjusting their chemical components.

The influence of liquid Pb-Bi on the anti-corrosion behavior of Fe3O4: a first-principles study.

In this work, the influence of Pb and Bi atoms on the anti-corrosion behavior of the oxide film (Fe3O4) formed on steel surface is investigated based on first-principles calculations. Through calculations of the formation energies, we find that Pb and Bi atoms can promote the formation of point defects, such as interstitial atoms and vacancies in Fe3O4. Besides, the effects of the concentration of Pb (or Bi) and pressure on the formation of these defects are also studied. Our results depict that a high density of Pb (or Bi) and compression pressure can promote the formation of defects in Fe3O4 significantly. Furthermore, the energy barriers for Pb and Bi atom migration in Fe3O4 are also estimated using the climbing image nudge elastic band (CI-NEB) method, which implies that Pb and Bi can diffuse more easily in Fe3O4 compared to Fe. Our results reveal the underlying mechanism of how Pb and Bi influence the anti-corrosion ability of oxide films in an accelerate driven system (ADS). It is instructive for improving the corrosion resistance of the oxide films in the ADS.

Identifying structural distortion in doped VO2 with IR spectroscopy.

Doping VO(2) with tungsten can lower the metal-insulator transition (MIT) temperature and thus provide a controlled means for tailoring the MIT properties of VO(2) materials. Here, infrared spectroscopy has been employed as a tool for identifying structural changes in doped VO(2) as a way of lowering the MIT temperature.

Ni2+-Doped Garnet Solid-Solution Phosphor-Converted Broadband Shortwave Infrared Light-Emitting Diodes toward Spectroscopy Application.

Broadband shortwave infrared (SWIR) light-emitting diodes (LEDs), capable of advancing the next-generation solid-state smart invisible lighting technology, have sparked tremendous interest and will launch ground-breaking spectroscopy and instrumental applications. Nevertheless, the device performance is still suppressed by the low quantum efficiency and limited emission bandwidth of the critical phosphor layer. Herein, we report a high-performance Ni2+-doped garnet solid-solution broadband SWIR emitter centered at ∼1450 nm with a large full-width at half-maximum of ∼300 nm, thereby fabricating, for the first time, a directly excited Ni2+-doped garnet solid-solution phosphor-converted broadband SWIR LED device. A synergetic enhancement strategy, adding a fluxing agent and a charge compensator simultaneously, is proposed to deliver a more than 20-fold increase of the SWIR emission intensity and nearly 2-fold improvement of the thermal quenching behavior. The site occupation and mechanism behind the synergetic enhancement strategy are elucidated by a combination of experimental study and theoretical calculation. A prototype of the SWIR LED with a radiation flux of 1.25 mW is fabricated and utilized as an invisible SWIR light source to demonstrate the SWIR spectroscopy applications. This work not only opens a window to explore novel broadband SWIR phosphors but also provides a synergetic strategy to remarkably improve the performance of artificial SWIR LED light sources.

Rutile TiO2(011)-2 × 1 Reconstructed Surfaces with Optical Absorption over the Visible Light Spectrum.

The stable structures of the reconstructed rutile TiO2(011) surface are explored based on an evolutionary method. In addition to the well-known "brookite(001)-like" 2 × 1 reconstruction model, three 2 × 1 reconstruction structures are revealed for the first time, all being more stable in the high Ti-rich condition. Importantly, the predicted Ti4O4-2 × 1 surface model not only is in excellent agreement with the reconstructed metastable surface detected by Tao et al. [Nat. Chem. 3, 296 (2011)] from their STM experiment but also gives a consistent formation mechanism and electronic structures with the measured surface. The computed imaginary part of the dielectric function suggests that the newly predicted reconstructed surfaces are capable of optical absorption over the entire visible light spectrum, thereby offering high potential for photocatalytic applications.

New structures of bilayer germanium nanosheets predicted by a particle swarm optimization method.

A global search for the stable structures of bilayer Ge (BLG) is performed, and the most stable and meta-stable BLG structures are predicted for the first time. Phonon-spectrum calculations and ab initio molecular dynamics simulations confirm their dynamical and thermal stability. The computed electronic structures suggest that the most stable structure is metal while the meta-stable structure of BLG is a semiconductor with an indirect band gap (0.32 eV at the level of PBE functional and 0.81 eV at the level of HSE06). By straining the layer plane of the meta-stable BLG, we observe a phase transition from semiconductor to metal. Furthermore, the adsorption of gas molecules of CO, CO2, NH3, NO and NO2 on the meta-stable structure is also studied. Our results show that the predicted meta-stable BLG also possesses a good feature in gas sensors.

Persistent Luminescence Hole-Type Materials by Design: Transition-Metal-Doped Carbon Allotrope and Carbides.

Electron traps play a crucial role in a wide variety of compounds of persistent luminescence (PL) materials. However, little attention has been placed on the hole-trap-type PL materials. In this study, a novel hole-dominated persistent luminescence (PL) mechanism is predicted. The mechanism is validated in the night pearl diamond (NPD) composed of lonsdaleite with ultralong persistent luminescence (PL) (more than 72 h). The computed band structures suggest that the Fe ion dopant in lonsdaleite is responsible for the luminescence of NPD due to the desired defect levels within the band gap for electronic transition. Other possible impurity defects in lonsdaleite, such as K, Ca, Mg, Zn, or Tl dopants, or C vacancy can also serve as the hole-trap centers to enhance the PL. Among other 3d transition-metal-ion dopants considered, Cr and Mn ions are predicted to give rise to PL property. The predicted PL mechanism via transition-metal doping of lonsdaleite offers an exciting opportunity for engineering new PL materials by design.

Highly efficient visible-light-driven photocatalytic activities in synthetic ordered monoclinic BiVO4 quantum tubes-graphene nanocomposites.

Photocatalytic purification of polluted water is a very promising way to alleviate the increasingly serious water resources crisis. Despite tremendous efforts, developing visible-light-driven photocatalysts with high activity at low cost still remains a great challenge. Herein, we report for the first time the design and synthesis of ordered m-BiVO(4) quantum tubes-graphene nanocomposites that exhibit unprecedented visible-light-driven photocatalytic activities, over 20 times faster than that of commercial P25 or bulk BiVO(4) and roughly 1.5 times more active than that of bare m-BiVO(4) quantum tubes. Notably, the unusual photoreactivities arise from the synergistic effects between the microscopic crystal structure of m-BiVO(4) and macroscopic morphological features of ordered m-BiVO(4) quantum tubes and two-dimensional graphene sheets. These structural features help to provide increased photocatalytic reaction sites, extended photoresponding range, enhanced charge transportation and separation efficiency simultaneously. Briefly, this work not only provides a simple and straightforward strategy for fabricating highly efficient and stable graphene-based nanocomposites, but also proves that these unique structures are excellent platforms for significantly improving their visible-light-driven photoactivities, holding great promise for their applications in the field of purifying polluted water resources.

Highly depressed temperature-induced metal-insulator transition in synthetic monodisperse 10-nm V2O3 pseudocubes enclosed by {012} facets.

Monodisperse 10-nm V(2)O(3) pseudocubes enclosed by {012} facets were successfully synthesized for the first time via a novel and facile solvothermal method, offering the first opportunity to elucidate the effect of finite-size and facet on the temperature-induced reversible metal-insulator transition (MIT) behavior of V(2)O(3). Very excitingly, the transition temperature of these V(2)O(3) pseudocubes drastically depressed from 133 K to 36 K and their corresponding hysteresis width highly narrowed from 17 K to 5 K, compared to the MIT behaviors of other irregular V(2)O(3) particles with average sizes of 10 nm, 20 nm, 40 nm, 170 nm and 2 µm. Notably, the size-related surface energy, grain boundary connectivity and volume expansion could be used to account for their strong size-dependent transition temperature and hysteresis width. Moreover, the improved grain boundary connectivity associated with well-defined {012} facets enabled these 10-nm V(2)O(3) pseudocubes to display a 10 times higher resistivity jump (at the order of 10(5)) and by nearly one-half smaller hysteresis width of 5 K than the irregular 10-nm V(2)O(3) particles with randomly exposed facets, directly evidencing the pronounced influence of facets on the MIT behavior. Briefly, the present work not only develops an effective strategy for synthesizing high-quality nanocrystals but also provides an excellent platform to investigate the size- and facet-dependent temperature-induced MIT behavior, enabling to design smart electrical switching nano-devices in the rapidly developing ultra-low temperature field.