Mechanistic Insights into Bioengineered Antibiofilm Enamel Pellicles

Dental caries remains the most widespread chronic disease worldwide. Basically, caries originates within biofilms accumulated on dental enamel. Despite the nonrenewable nature of the enamel tissue, targeted preventive strategies are still very limited. We previously introduced customized multifunctional proteinaceous pellicles (coatings) for controlling bacterial attachment and subsequent biofilm succession. Stemmed from our whole proteome/peptidome analysis of the in vivo acquired enamel pellicle, we designed these pellicles using hybrid mixtures of the most abundant and complementary-acting antimicrobial and antifouling proteins/peptides for synergetic suppression of early biofilms. In conjugating these domains synthetically, their bioinhibitory efficacy was remarkably boosted. Herein, we sought to explore the key structure-function relationship of these potent de novo hybridized conjugates in comparison with their individual domains, solely or in physical mixtures. Specifically, we interrelated the following facets: physicochemical and 3-dimensional folding characteristics via molecular dynamics simulations, adopted secondary structure by circular dichroism, immobilization capacity on enamel through high-spatial resolution multiphoton microscopy, and biofilm suppression potency. Our data showed consistent associations among the increased preference for protein folding structures, α-helix content, and enamel-immobilization capacity; all were inversely correlated with the attached bioburden. The expressed phenotypes could be explained by the adopted strongly amphipathic helical conformation upon conjugation, mediated by the highly anionic and acidic N-terminal pentapeptide shared region/motif for enhanced immobilization on enamel. In conclusion, conjugating bioactive proteins/peptides is a novel translational approach to engineer robust antibiofilm pellicles for caries prevention. The adopted α-helical conformation is key to enhance the antibiofilm efficacy and immobilization capacity on enamel that are promoted by certain physicochemical properties of the constituent domains. These data are valuable for bioengineering versatile therapeutics to prevent/arrest dental caries, a condition that otherwise requires invasive treatments with substantial health care expenditures.

Predicted crystal structures of xenon and alkali metals under high pressures

The pressure-induced reaction between xenon (Xe) and other non-inert gas elements and the resultant crystal structures have attracted great interest. In this work, we carried out extensive simulations on the crystal structures of Xe-alkali metal (Xe-AM) systems under high pressures. Among all predicted compounds, KXe and RbXe are found to become stable at a pressure of ∼16 GPa by adopting a cubic symmetry of space group Pm3̄m. The stabilization of KXe and RbXe requires slightly lower pressure compared with that of previously reported CsXe (25 GPa), interestingly, which is in contrast to the electronegativity order of the AMs and unexpected. Our simulations also indicate that all predicted Xe compounds contain negatively charged Xe. Moreover, our in-depth analysis indicates that the occupation of AM d-orbitals plays a critical role in stabilizing these Xe-bearing compounds. These results shed light on the understanding of the reaction between Xe and AMs and the formation mechanism of the resultant crystal structures.

High-Pressure Nonequilibrium Dynamics on Second-to-Microsecond Time Scales: Application of Time-Resolved X-ray Diffraction and Dynamic Compression in Ice

The study of nonequilibrium transition dynamics on structural transformation from the second to microsecond regime, a time scale between static and shock compression, is an emerging field of high-pressure research. There are ample opportunities to uncover novel physical phenomena within this time regime. Herein, we briefly review the development and application of a dynamic compression technique based on a diamond anvil cell (DAC) and time-resolved X-ray diffraction (TRXRD) for the study of time-, pressure-, and temperature-dependent structural dynamics. Applications of the techniques are illustrated with our recent investigations on the mechanisms of the interconversions between different high-pressure ice polymorphs. These examples demonstrate that a combination of dynamic compression and TRXRD is a versatile approach capable of providing information on the kinetics and thermodynamic nature associated with structural transformations. Future improvement of rapid compression and TRXRD techniques and potentially interesting research topics in this area are suggested.

57Fe Mössbauer isomer shift of pure iron and iron oxides at high pressure-An experimental and theoretical study

The ⁵⁷Fe isomer shift (IS) of pure iron has been measured up to 100 GPa using synchrotron Mössbauer spectroscopy in the time domain. Apart from the expected discontinuity due to the α → ε structural and spin transitions, the IS decreases monotonically with increasing pressure. The absolute shifts were reproduced without semi-empirical calibrations by periodic density functional calculations employing extensive localized basis sets with several common density functionals. However, the best numerical agreement is obtained with the B1WC hybrid functional. Extension of the calculations to 350 GPa, a pressure corresponding to the Earth's inner core, predicted the IS range of 0.00 to -0.85 mm/s, covering the span from Fe(0) to Fe(VI) compounds measured at ambient pressure. The calculations also reproduced the pressure trend from polymorphs of prototypical iron oxide minerals, FeO and Fe₂O₃. Analysis of the electronic structure shows a strong donation of electrons from oxygen to iron at high pressure. The assignment of formal oxidation to the Fe atom becomes ambiguous under this condition.

Mechanisms for Pressure-Induced Isostructural Phase Transitions in EuO

We study pressure-induced isostructural electronic phase transitions in the prototypical mixed valence and strongly correlated material EuO using the global-hybrid density functional theory. The simultaneous presence in the valence of highly localized d- and f-type bands and itinerant s- and p-type states, as well as the half-filled f-type orbital shell with seven unpaired electrons on each Eu atom, have made the description of the electronic features of this system a challenge. The electronic band structure, density of states, and atomic oxidation states of EuO are analyzed in the 0-50 GPa pressure range. An insulator-to-metal transition at about 12 GPa of pressure was identified. The second isostructural transition at approximately 30-35 GPa, previously believed to be driven by an oxidation from Eu(II) to Eu(III), is shown instead to be associated with a change in the occupation of the Eu d orbitals, as can be determined from the analysis of the corresponding atomic orbital populations. The Eu d band is confined by the surrounding oxygens and split by the crystal field, which results in orbitals of e_{g} symmetry (i.e., d_{x^{2}-y^{2}} and d_{2z^{2}-x^{2}-y^{2}}, pointing along the Eu-O direction) being abruptly depopulated at the transition as a means to alleviate electron-electron repulsion in the highly compressed structures.

New-phase retention in colloidal core/shell nanocrystals via pressure-modulated phase engineering

Core/shell nanocrystals (NCs) integrate collaborative functionalization that would trigger advanced properties, such as high energy conversion efficiency, nonblinking emission, and spin-orbit coupling. Such prospects are highly correlated with the crystal structure of individual constituents. However, it is challenging to achieve novel phases in core/shell NCs, generally non-existing in bulk counterparts. Here, we present a fast and clean high-pressure approach to fabricate heterostructured core/shell MnSe/MnS NCs with a new phase that does not occur in their bulk counterparts. We determine the new phase as an orthorhombic MnP structure (B31 phase), with close-packed zigzagged arrangements within unit cells. Encapsulation of the solid MnSe nanorod with an MnS shell allows us to identify two separate phase transitions with recognizable diffraction patterns under high pressure, where the heterointerface effect regulates the wurtzite → rocksalt → B31 phase transitions of the core. First-principles calculations indicate that the B31 phase is thermodynamically stable under high pressure and can survive under ambient conditions owing to the synergistic effect of subtle enthalpy differences and large surface energy in nanomaterials. The ability to retain the new phase may open up the opportunity for future manipulation of electronic and magnetic properties in heterostructured nanostructures.

Temperature- and Rate-Dependent Pathways in Formation of Metastable Silicon Phases under Rapid Decompression

High-pressure metallic β-Sn silicon (Si-II), depending on temperature, decompression rate, stress, etc., may transform to diverse metastable forms with promising semiconducting properties under decompression. However, the underlying mechanisms governing the different transformation paths are not well understood. Here, two distinctive pathways, viz., a thermally activated crystal-crystal transition and a mechanically driven amorphization, were characterized under rapid decompression of Si-II at various temperatures using in situ time-resolved x-ray diffraction. Under slow decompression, Si-II transforms to a crystalline bc8/r8 phase in the pressure range of 4.3-9.2 GPa through a thermally activated process where the overdepressurization and the onset transition strain are strongly dependent on decompression rate and temperature. In comparison, Si-II collapses structurally to an amorphous form at around 4.3 GPa when the volume expansion approaches a critical strain via rapid decompression beyond a threshold rate. The occurrence of the critical strain indicates a limit of the structural metastability of Si-II, which separates the thermally activated and mechanically driven transition processes. The results show the coupled effect of decompression rate, activation barrier, and thermal energy on the adopted transformation paths, providing atomistic insight into the competition between equilibrium and nonequilibrium pathways and the resulting metastable phases.

Synthesis of Atomically Thin Hexagonal Diamond with Compression

Atomically thin diamond, also called diamane, is a two-dimensional carbon allotrope and has attracted considerable scientific interest because of its potential physical properties. However, the successful synthesis of a pristine diamane has up until now not been achieved. We demonstrate the realization of a pristine diamane through diamondization of mechanically exfoliated few-layer graphene via compression. Resistance, optical absorption, and X-ray diffraction measurements reveal that hexagonal diamane (h-diamane) with a bandgap of 2.8 ± 0.3 eV forms by compressing trilayer and thicker graphene to above 20 GPa at room temperature and can be preserved upon decompression to ∼1.0 GPa. Theoretical calculations indicate that a (-2110)-oriented h-diamane is energetically stable and has a lower enthalpy than its few-layer graphene precursor above the transition pressure. Compared to gapless graphene, semiconducting h-diamane offers exciting possibilities for carbon-based electronic devices.

Possibility of realizing superionic ice VII in external electric fields of planetary bodies

In a superionic (SI) ice phase, oxygen atoms remain crystallographically ordered while protons become fully diffusive as a result of intramolecular dissociation. Ice VII's importance as a potential candidate for a SI ice phase has been conjectured from anomalous proton diffusivity data. Theoretical studies indicate possible SI prevalence in large-planet mantles (e.g., Uranus and Neptune) and exoplanets. Here, we realize sustainable SI behavior in ice VII by means of externally applied electric fields, using state-of-the-art nonequilibrium ab initio molecular dynamics to witness at first hand the protons' fluid dance through a dipole-ordered ice VII lattice. We point out the possibility of SI ice VII on Venus, in its strong permanent electric field.

A chemical perspective on high pressure crystal structures and properties

The general availability of third generation synchrotron sources has ushered in a new era of high pressure research. The crystal structure of materials under compression can now be determined by X-ray diffraction using powder samples and, more recently, from multi-nano single crystal diffraction. Concurrently, these experimental advancements are accompanied by a rapid increase in computational capacity and capability, enabling the application of sophisticated quantum calculations to explore a variety of material properties. One of the early surprises is the finding that simple metallic elements do not conform to the general expectation of adopting 3D close-pack structures at high pressure. Instead, many novel open structures have been identified with no known analogues at ambient pressure. The occurrence of these structural types appears to be random with no rules governing their formation. The adoption of an open structure at high pressure suggested the presence of directional bonds. Therefore, a localized atomic hybrid orbital description of the chemical bonding may be appropriate. Here, the theoretical foundation and experimental evidence supporting this approach to the elucidation of the high pressure crystal structures of group I and II elements and polyhydrides are reviewed. It is desirable and advantageous to extend and apply established chemical principles to the study of the chemistry and chemical bonding of materials at high pressure.

Hard BN Clathrate Superconductors

The search for hard superconductive materials has attracted a great deal of attention due to their fundamentally interesting properties and potentially practical applications. Here we predict a new class of materials based on sodalite-like BN frameworks, X(BN)₆, where X = Al, Si, Cl, etc. Our simulations reveal that these materials could achieve high superconducting critical temperatures ( T_c) and high hardness. Electron-phonon calculations indicate that T_c of these compounds varies with the doping element. For example, the superconducting T_c of sodalite-like Al(BN)₆ is predicted to reach ∼47 K, which is higher than that in the renowned MgB₂ (39 K). This phase and a series of other sodalite-based superconductors are predicted to be metastable phases but are dynamically stable as well. These doped sodalite-based structures are likely to become recoverable as potentially useful superconductors with high hardness. Our current results present a new strategy for searching for hard high- T_c materials.

Mechanism for the Structural Transformation to the Modulated Superconducting Phase of Compressed Hydrogen Sulfide

A comprehensive description of crystal and electronic structures, structural transformations, and pressure-dependent superconducting temperature (Tc) of hydrogen sulfide (H2S) compressed from low pressure is presented through the analysis of the results from metadynamics simulations. It is shown that local minimum metastable crystal structures obtained are dependent on the choice of pressure-temperature thermodynamic paths. The origin of the recently proposed 'high-Tc' superconducting phase with a modulated structure and a diffraction pattern reproducing two independent experiments was the low pressure Pmc21 structure. This Pmc21 structure is found to transform to a Pc structure at 80 K and 80 GPa which becomes metallic and superconductive above 100 GPa. This structure becomes dynamically unstable above 140 GPa beyond which phonon instability sets in at about a quarter in the Γ to Y segment. This explains the transformation to a 1:3 modulation structure at high pressures proposed previously. The pressure trend of the calculated Tc for the Pc structure is consistent with the experimentally measured 'low-Tc phase'. Fermi surface analysis hints that pressurized hydrogen sulfide may be a multi-band superconductor. The theoretical results reproduced many experimental characteristics, suggesting that the dissociation of H2S is unrequired to explain the superconductivity of compressed H2S at any pressure.

Pancakes under Pressure: A Case Study on Isostructural Dithia- and Diselenadiazolyl Radical Dimers

The isostructural dimers of the 1,4-phenylene-bridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0-10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E-E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap Δ E between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap E_g occasioned by pressure-induced spreading of the valence and conduction bands. Consistent with the predicted consequences of this competition between intra- and interdimer interactions, variable temperature high pressure conductivity measurements reveal at best an order-of-magnitude increase in conductivity with pressure for the two compounds over the pressure range 0-10 GPa. While a small reduction in the thermal activation energy E_act with increasing pressure is observed, extrapolation of the rate of decrease suggests a projected onset of metallization ( E_act ≈ 0) in excess of 20 GPa.

Venture into Water's No Man's Land: Structural Transformations of Solid H_{2}O under Rapid Compression and Decompression

Pressure-induced formation of amorphous ices and the low-density amorphous (LDA) to high-density amorphous (HDA) transition have been believed to occur kinetically below a crossover temperature (T_{c}) above which thermodynamically driven crystalline-crystalline (e.g., ice I_{h}-to-II) transitions and crystallization of HDA and LDA are dominant. Here we show compression-rate-dependent formation of a high-density noncrystalline (HDN) phase transformed from ice I_{c} above T_{c}, bypassing crystalline-crystalline transitions under rapid compression. Rapid decompression above T_{c} transforms HDN to a low-density noncrystalline (LDN) phase which crystallizes spontaneously into ice I_{c}, whereas slow decompression of HDN leads to direct crystallization. The results indicate the formation of HDA and the HDN-to-LDN transition above T_{c} are results of competition between (de)compression rate, energy barrier, and temperature. The crossover temperature is shown to have an exponential relationship with the threshold compression rate. The present results provide important insight into the dynamic property of the phase transitions in addition to the static study.

Carbon-Quantum-Dots-Loaded Ruthenium Nanoparticles as an Efficient Electrocatalyst for Hydrogen Production in Alkaline Media

Highly active, stable, and cheap Pt-free catalysts for the hydrogen evolution reaction (HER) are facing increasing demand as a result of their potential use in future energy-conversion systems. However, the development of HER electrocatalysts with Pt-like or even superior activity, in particular ones that can function under alkaline conditions, remains a significant challenge. Here, the synthesis of a novel carbon-loaded ruthenium nanoparticle electrocatalyst (Ru@CQDs) for the HER, using carbon quantum dots (CQDs), is reported. Electrochemical tests reveal that, even under extremely alkaline conditions (1 m KOH), the as-formed Ru@CQDs exhibits excellent catalytic behavior with an onset overpotential of 0 mV, a Tafel slope of 47 mV decade^-1 , and good durability. Most importantly, it only requires an overpotential of 10 mV to achieve the current density of 10 mA cm^-2 . Such catalytic characteristics are superior to the current commercial Pt/C and most noble metals, non-noble metals, and nonmetallic catalysts under basic conditions. These findings open a new field for the application of CQDs and add to the growing family of metal@CQDs with high HER performance.

Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λ_max = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3 c and P2₁/ c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc2₁). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc2₁ phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σ_RT ranging from 10^-5 S cm^-1 for the P2₁/ c phase to 10^-3 S cm^-1 for the R3 c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy E_act. For the R3 c phase, band gap closure to yield an organic molecular metal with a σ_RT of ∼10² S cm^-1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

Polymer-Passivated Inorganic Cesium Lead Mixed-Halide Perovskites for Stable and Efficient Solar Cells with High Open-Circuit Voltage over 1.3 V

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss ) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂ Br absorber and polythiophene hole-acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂ Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (V_OC ) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss .

Superconducting Hydrogen Sulfide

The recent discovery of superconductivity above 200 K in hydrogen sulfide under high pressure marks a milestone in superconductor research. Not only does its critical temperature T_c exceed the previous record in cuprates by more than 50 K, the superconductivity in hydrogen sulfide also exhibits convincing evidence that it is of conventional phonon-mediated type. Moreover, this is the first time that a previously unknown high-T_c superconductor is predicted by theory and afterwards verified by experiment. In this Minireview, we survey the progress made in the last three years in understanding this novel material, and discuss unsolved problems and possible developments to encourage future investigations.

Uptake and speciation of uranium in synthetic gypsum (CaSO4•2H2O): Applications to radioactive mine tailings

Phosphogypsum formed from the production of phosphoric acid represents by far the biggest accumulation of gypsum-rich wastes in the world and commonly contains elevated radionuclides, including uranium, as well as other heavy metals and metalloids. Therefore, billions-of-tons of phosphogypsum stockpiled worldwide not only possess serious environmental problems but also represent a potential uranium resource. Gypsum is also a major solid constituent in many other types of radioactive mine tailings, which stems from the common usage of sulfuric acid in extraction processes. Therefore, management and remediation of radioactive mine tailings as well as future beneficiation of uranium from phosphogysum all require detailed knowledge about the nature and behavior of uranium in gypsum. However, little is known about the uptake mechanism or speciation of uranium in gypsum. In this study, synthesis experiments suggest an apparent pH control on the uptake of uranium in gypsum at ambient conditions: increase in U from 16 μg/g at pH = 6.5 to 339 μg/g at pH = 9.5. Uranium L₃-edge synchrotron X-ray absorption spectroscopic analyses of synthetic gypsum show that uranyl (UO₂)²⁺ at the Ca site is the dominant species. The EXAFS fitting results also indicate that uranyl in synthetic gypsum occurs most likely as carbonate complexes and yields an average U-O distance ∼0.25 Å shorter than the average Ca-O distance, signifying a marked local structural distortion. Applications to phosphogypsum from the New Wales phosphoric acid plant (Florida, USA) and uranium mine tailings from the Key Lake mill (Saskatchewan, Canada) show that gypsum is an important carrier of uranium over a wide range of pH and controls the fate of this radionuclide in mine tailings. Also, development of new technologies for recovering U from phosphogypsum in the future must consider lattice-bound uranyl in gypsum.

Oxygen Packing Fraction and the Structure of Silicon and Germanium Oxide Glasses

The recently proposed relationship between the oxygen volume fraction and topological ordering in solid and liquid oxide glasses at high pressure is examined with Bader's atoms-in-molecules (AIM) theory using glass structures generated from first principles molecular dynamics calculations. It is shown that the atomic (O/Si and O/Ge) volume ratio derived from AIM theory is not constant with pressure. This finding is due to the continuous change in the electron topology under compression. Unlike crystalline solids, there is no distinctive transition pressure for Si-O and Ge-O coordination in a glass; instead, the changes are gradual and continuous over a broad pressure range. Therefore, relating a unique Si-O or Ge-O coordination number to the properties of the glass at a given pressure is difficult.

optPBE-vdW density functional theory study of liquid water and pressure-induced structural evolution in ice Ih

The accuracy of several local and non-local van der Waals (vdW) corrected exchange correlation functionals on the description of the effect of pressure on ice has been investigated. In a preliminary survey, the non-local vdW correction used in conjunction with the optPBE functional was shown to provide the best overall agreement on the structural parameters of ice Ih with experiments. More importantly, this combination reproduced correctly the recently observed crystal → crystal transformation in ice Ih at 80 K prior to amorphisation. The predicted transition pressure of 1.9 GPa is somewhat higher, showing that the current generation of vdW functionals are still not sufficiently accurate for the ice system. The existence of an intermediate crystalline state with a shear-hexagonal structure confirms the earlier prediction that the collapse of crystalline structure under compression originates from the softening of phonon modes in ice Ih’s basal plane.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and ∼1  Pa, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

Melting and High P-T Transitions of Hydrogen up to 300 GPa

High P-T Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300 GPa and 900 K using externally heated diamond anvil cell techniques. A new melting line determined from the disappearance of lattice mode excitations was measured directly for the first time above 140 GPa. The results differ from theoretical predictions and extrapolations from lower pressure melting relations. In addition, discontinuities in Raman frequencies are observed as a function of pressure and temperature indicative of phase transition at these conditions. The appearance of a new Raman feature near 2700  cm^{-1} at ∼300  GPa and 370 K indicates the transformation to a new crystalline phase. Theoretical calculations of the spectrum suggest the new phase is the proposed Cmca-4 metallic phase. The transition pressure is close to that of a recently reported transition observed on dynamic compression.

Crystal Structures of CaB3N3 at High Pressures

Using global structure searches, we have explored the structural stability of CaB₃N₃, a compound analogous to CaC₆, under pressure. There are two high-pressure phases with space groups R3c and Amm2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC₆. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the R3c phase is a semiconductor with a band gap of 2.21 eV and that the Amm2 phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.

Piezochromic Carbon Dots with Two-photon Fluorescence

Piezochromic materials, which show color changes resulting from mechanical grinding or external pressure, can be used as mechanosensors, indicators of mechano-history, security papers, optoelectronic devices, and data storage systems. A class of piezochromic materials with unprecedented two-photon absorptive and yellow emissive carbon dots (CDs) was developed for the first time. Applied pressure from 0-22.84 GPa caused a noticeable color change in the luminescence of yellow emissive CDs, shifting from yellow (557 nm) to blue-green (491 nm). Moreover, first-principles calculations support transformation of the sp² domains into sp³ -hybridized domains under high pressure. The structured CDs generated were captured by quenching the high-pressure phase to ambient conditions, thus greatly increasing the choice of materials available for a variety of applications.

Unusual Li-Ion Transfer Mechanism in Liquid Electrolytes: A First-Principles Study

Liquid electrolytes play an important role in commercial lithium-ion (Li-ion) batteries as a conduit for Li-ion transfer between anodes and cathodes. It is generally believed that the Li-ions move along with the salt ions; thus, Li-ion diffusion is only affected by the viscosity and salt concentration in the liquid electrolytes based on the Stokes-Einstein equation. In this study, a novel and faster Li-ion diffusion mechanism in electrolytes containing a cyanogen group is identified from first-principles molecular dynamics (FPMD) simulations. In this mechanism, the Li-ions are first detached from the Li-salt and then diffuse along with the solvent molecules, and the Li-ion diffusion does not obey the traditional Stokes-Einstein equation. The ionic conductivity of the electrolyte systems with this "solvent-assisted Li-ion diffusion" mechanism is further enhanced through Li-ion hopping. This novel Li-ion diffusion process explains recent findings of high Li-ion conductivity in electrolytes with cyanogen groups and furnishes a new paradigm for the design of fast-charging liquid electrolyte for Li-ion batteries.

New Raman measurements for H2O ice VII in the range of 300 cm-1 to 4000 cm-1 at pressures up to 120 GPa

Raman spectroscopic measurements for H₂O ice VII have been conducted to 120 GPa at 300 K in the spectroscopic range of 300-4000 cm^-1. Both moissanite and diamond anvils were used for the experiments. This overcomes the problems of overlapping spectra between the diamond anvil and sample, which had prevented the observation of the stretching modes at pressures higher than ∼23 GPa in all previous measurements. The new results reveal many bands which have not been reported before. The pressure dependences of the Raman modes show anomalous changes at 13-15, ∼27, ∼44, ∼60, and 90 GPa, implying possible structural changes at these pressures. The new results demonstrate that the predicted symmetric hydrogen bond phase X transition does not occur below 120 GPa.

pH-Dependent Synthesis of Novel Structure-Controllable Polymer-Carbon NanoDots with High Acidophilic Luminescence and Super Carbon Dots Assembly for White Light-Emitting Diodes

We use a pH-dependent solubility equilibrium to develop a one-pot aqueous synthesis of polymer carbon nanodots with novel structures. The chemical structure and photoluminescence (PL) were heavily influenced by the synthesis pH, with cross-linked polymer-carbon film (pH > 7), polymer carbon nanosheets (3 < pH < 7), and amorphous carbon structures (1 < pH < 3) achieved by altering the initial pH. Although pH-dependent structures frequently occur in typical semiconductors and supramolecular architectures involving metal, this is the first experimental work describing it in carbon nanodots. Supersmall carbon nanodots (SCNDs, ∼0.5 nm) were obtained at pH < 1; their direct white emission can be easily applied as an inexpensive color-changing layer in white LEDs. Investigation of the PL mechanism of the SCNDs revealed an uncommon multilevel highly emissive recombination channel, which could be possibly derived from the wide distributions of surface-state PL centers. Theoretical calculation of the single layer of the carbon dots further explored their band gap changes.

Electronic structures of greigite (Fe3S4): A hybrid functional study and prediction for a Verwey transition

Greigite (Fe₃S₄) is a ferrimagnetic mineral with vital functions in both the bio-geochemical cycle and novel technological applications. However, the ground state electronic structure of this material has not been fully characterized by either experiment or theory. In the present study, ab initio calculations using the hybrid functional method have been performed to investigate the electronic structure and magnetic properties. It is found that the cubic structure observed under ambient temperature is a half metal and is metastable. A more stable monoclinic structure slightly distorted from the cubic form is found. The structural distortion is induced by charge ordering and associated with a metal-to-insulator transition, resulting in a semiconductive ground state with a bandgap of ~0.8 eV and a magnetic moment of 4 μB per formula unit. The results predict, similar to the magnetite (Fe₃O₄), a Verwey transition may exist in greigite, although it has not yet been observed experimentally.

Tunable Excitonic Processes at Organic Heterojunctions

Nonradiative Auger recombination and radiative exciplex emissions at organic-organic heterojunctions are found to be mediated by the nature of the majority charge carrier. The Auger recombination is dominant when the heterojunction is electron rich, the exciplex emission is dominant when the heterojunction is hole rich, and both Auger recombination and exciplex emission are present when the heterojunction is charge balanced.

Pushing TC to 27.5 K in a heavy atom radical ferromagnet

In the solid state the iodo-substituted bisdiselenazolyl radical 1c orders as a bulk ferromagnet with T_C = 10.5 K. With the application of pressure T_C rises rapidly, reaching a value of 27.5 K at 2.4 GPa.

Low-density superhard materials: computational study of Li-inserted B-substituted closo-carboranes LiBC11 and Li2B2C10

Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with relatively low density: LiBC₁₁ and Li₂B₂C₁₀.