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Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric field. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T–P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T–P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric fields., Water can crystallize in different ice polymorphs according to temperature and pressure conditions. Here the authors predict by molecular dynamics simulations a new ice phase spontaneously forming at room temperature under high pressure and high electric field.

Both carbon dioxide (CO 2 ) and water (H 2 O) are triatomic molecules that are ubiquitous in nature, and both are among the five most abundant gases in the Earth’s atmosphere. At low temperature and ambient pressure, both CO 2 and H 2 O form molecular crystals––dry ice I and ice I h . Because water possesses distinctive hydrogen bonds, it exhibits intricate and highly pressure-dependent phase behavior, including at least 17 crystalline ice phases and three amorphous ice phases. In contrast, due to its weak van der Waals intermolecular interactions, CO 2 exhibits fewer crystalline phases except at extremely high pressures, where nonmolecular ordered structures arise. Herein, we show the molecular dynamics simulation results of numerous 2D polymorphs of CO 2 molecules in slit nanopores. Unlike bulk polymorphs of CO 2 , 2D CO 2 polymorphs exhibit myriad crystalline and amorphous structures, showing remarkable polymorphism and polyamorphism. We also show that depending on the thermodynamic path, 2D solid-to-solid phase transitions can give rise to previously unreported structures, e.g., wave-like amorphous CO 2 structures. Our simulation also suggests intriguing structural connections between 2D and 3D dry ice phases (e.g., Cmca and PA-3) and offers insights into CO 2 polyamorphic transitions through intermediate liquid or amorphous phases.

Akin to bulk water, water confined to an isolated nanoslit can show a wealth of new 2D phases of ice and amorphous ice, as well as unusual phase behavior. Indeed, 2D water phases, such as bilayer hexagonal ice and monolayer square ice, have been detected in the laboratory, confirming earlier computational predictions. Herein, we report theoretical evidence of a hitherto unreported state, namely, bilayer very low density amorphous ice (BL-VLDA), as well as evidence of a strong first-order transition between BL-VLDA and the BL amorphous ice (BL-A), and a weak first-order transition between BL-VLDA and the BL very low density liquid (BL-VLDL) water. The diffusivity of BL-VLDA is typically in the range of 10−9 cm2/s to 10−10 cm2/s. Similar to bulk (3D) water, 2D water can exhibit two forms of liquid in the deeply supercooled state. However, unlike supercooled bulk water, for which the two forms of liquid can coexist and merge into one at a critical point, the 2D BL-VLDL and BL high-density liquid (BL-HDL) phases are separated by the highly stable solid phase of BL-A whose melting line exhibits the isochore end point (IEP) near 220 K in the temperature−pressure diagram. Above the IEP temperature, BL-VLDL and BL-HDL are indistinguishable. At negative pressures, the metastable BL-VLDL exhibits a spatially and temporally heterogeneous structure induced by dynamic changes in the nanodomains, a feature much less pronounced in the BL-HDL.

Despite recent experimental evidence of the two-dimensional (2D) square ice in graphene nanocapillaries, based on transmission electron microscopy (TEM) imaging, the AA-stacked bilayer square ice structure has not been observed in all previous classical molecular dynamics (MD) simulations nor found in recent unbiased first-principles structure searches. Herein, we report the MD simulations of 2D bilayer ice formation for water confined between two parallel hydrophobic walls (nanoslit). We find a bilayer ice whose simulated TEM imaging resembles that of bilayer squarelike ice. This bilayer ice also demonstrates dynamical stability in first-principles phonon computations. The realistic structure of this bilayer ice, however, consists of two hexagonal monolayers with the AB-stacking order, where the hexagonal rings are slightly elongated with two unequal inner angles, 107 and 146° (rather than 120°). The phase diagram of the nanoslit width versus temperature exhibits a solid–liquid–solid triple point, where ...

Possible transition between two phases of supercooled liquid water, namely the low- and high-density liquid water, has been only predicted to occur below 230 K from molecular dynamics (MD) simulation. However, such a phase transition cannot be detected in the laboratory because of the so-called "no-man's land" under deeply supercooled condition, where only crystalline ices have been observed. Here, we show MD simulation evidence that, inside an isolated carbon nanotube (CNT) with a diameter of 1.25 nm, both low- and high-density liquid water states can be detected near ambient temperature and above ambient pressure. In the temperature-pressure phase diagram, the low- and high-density liquid water phases are separated by the hexagonal ice nanotube (hINT) phase, and the melting line terminates at the isochore end point near 292 K because of the retracting melting line from 292 to 278 K. Beyond the isochore end point (292 K), low- and high-density liquid becomes indistinguishable. When the pressure is increased from 10 to 600 MPa along the 280-K isotherm, we observe that water inside the 1.25-nm-diameter CNT can undergo low-density liquid to hINT to high-density liquid reentrant first-order transitions.

Using molecular dynamics simulation, we investigate the phase behavior of water confined in graphene nanocapillaries at room temperature (300 K). Here, the lateral pressure Pzz is used as the primary controlling variable, and its effect on the behavior of trilayer water is systematically studied. Three (meta)stable trilayer (TL) crystalline/amorphous ice phases, namely, TL-ABAI, TL-ABA, and TL-AAAI, are observed in our simulations with the lateral pressure in the range of 1.0 GPa ≤ Pzz ≤ 6.0 GPa. TL-ABAI exhibits a square lattice in every layer, and the three layers exhibit the ABA stacking pattern; i.e., the oxygen atoms in the two outer layers are in registry. This new trilayer ice structure can also be viewed as a bilayer clathrate hydrate with water molecules in the middle layer serving as the guest molecules. With increasing lateral pressure, typically, the solid-to-liquid-to-solid phase transition occurs, during which the structural transformation from triangular to square-like in the ice layer is a...

Water, when constrained between two graphene sheets and under ultrahigh pressure, can manifest dramatic differences from its bulk counterparts such as the van der Waals pressure induced water-to-ice transformation, known as the metastability limit of two-dimensional (2D) liquid. Here, we present result of a new crystalline structure of bilayer ice with the AB-stacking order, observed from molecular dynamics simulations of constrained water. This AB-stacked bilayer ice (BL-ABI) is transformed from the puckered monolayer square-like ice (pMSI) under higher lateral pressure in the graphene nanocapillary at ambient temperature. BL-ABI is a proton-ordered ice with square-like pattern. The transition from pMSI to BL-ABI is through crystal-to-amorphous-to-crystal pathway with notable hysteresis-loop in the potential energy during the compression/decompression process, reflecting the compression/tensile limit of the 2D monolayer/bilayer ice. In a superheating process, the BL-ABI transforms into the AB-stacked bilayer amorphous ice with the square-like pattern.

We report molecular dynamics (MD) simulation evidence for a new family of two-dimensional (2D) clathrate hydrates. Particular attention is placed on the effect of the size and hydrophilicity of guest-molecules on the formation of 2D clathrate hydrates. Among the MD simulations undertaken, the spontaneous formation of bilayer (BL) clathrate hydrates in nanoslits are found with five different hydrophobic guest molecules, namely, ethane (C2H6), ethene (C2H4), allene (C3H4), carbon dioxide (CO2) and hydrogen (H2) molecules. Our simulations suggest that the host cages in a water framework are likely BL-hexagonal cages with single occupancy for H2 or BL-heptagonal cages for CO2. With a further increase in guest size, the host cages for C2H6, C2H4, and C3H4 are BL-octagonal cages with single occupancy, and their long molecular axis tends to be normal to the surface of clathrate hydrates. In addition, for hydrophilic guest molecules, such as NH3 and H2S, which can form strong hydrogen bonds with water, we find that most guest molecules can preferentially displace water molecules from the lattice sites of the water framework, instead of being separately trapped within the water cages. Structural analogy between the 2D and 3D clathrates enlightens us to predict the stability of several bulk gas hydrates, namely, “ethane clathrate III”, “CH4 ice-i” and “H2 ice-i”. Our findings can not only enrich clathrate structures in the hydrate family but may also improve the understanding of hydrate formation in microporous media.

Two new phases of water, the mid-density hexagonal monolayer ice and the high-density flat rhombic monolayer ice, are observed in our molecular dynamics simulations of monolayer water confined between two smooth hydrophobic walls. These are in addition to the two monolayer ices reported previously, namely, the low-density 4·82 monolayer ice and the high-density puckered rhombic monolayer ice (HD-pRMI). Stabilities of the structures are confirmed by ab initio computation. Importantly, both new phases and the HD-pRMI are predicted to be ferroelectric. An in-plane external electric field can further stabilize these ferroelectric monolayer ices.

We devise a new computational approach to compute solid-liquid phase equilibria of confined fluids. Specifically, we extend the multibaric-multithermal ensemble method with an anisotropic pressure control to achieve the solid-liquid phase equilibrium for confined water inside slit nanopores (with slit width h ranging from 5.4 Å to 7.2 Å). A unique feature of this multibaric-multithermal ensemble is that the freezing points of confined water can be determined from the heat-capacity peaks. The new approach has been applied to compute the freezing point of two monolayer ices, namely, a high-density flat rhombic monolayer ice (HD-fRMI) and a high-density puckered rhombic monolayer ice (HD-pRMI) observed in our simulation. We find that the liquid-to-solid transition temperature (or the freezing point) of HD-pRMI is dependent on the slit width h, whereas that of HD-fRMI is nearly independent of the h.

A distinctive physical property of bulk water is its rich solid-state phase behavior, which includes 15 crystalline (ice I–ice XIV) and at least 3 glassy forms of water, namely, low-density amorphous, high-density amorphous, and very-high-density amorphous (VHDA). Nanoscale confinement adds a new physical variable that can result in a wealth of new quasi-2D phases of ice and amorphous ice. Previous computer simulations have revealed that when water is confined between two flat hydrophobic plates about 7–9 Å apart, numerous bilayer (BL) ices (or polymorphs) can arise [e.g., BL-hexagonal ice (BL-ice I)]. Indeed, growth of the BL-ice I through vapor deposition on graphene/Pt(111) substrate has been achieved experimentally. Herein, we report computer simulation evidence of pressure-induced amorphization from BL-ice I to BL-amorphous and then to BL-VHDA 2 at 250 K and 3 GPa. In particular, BL-VHDA 2 can transform into BL-VHDA 1 via decompression from 3 to 1.5 GPa at 250 K. This phenomenon of 2D polyamorphic transition is akin to the pressure-induced amorphization in 3D ice (e.g., from hexagonal ice to HDA and then to VHDA via isobaric annealing). Moreover, when the BL-ice I is compressed instantly to 6 GPa, a new very-high-density BL ice is formed. This new phase of BL ice can be viewed as an array of square ice nanotubes. Insights obtained from pressure-induced amorphization and crystallization of confined water offer a guide with which to seek a thermodynamic path to grow a new form of methane clathrate whose BL ice framework exhibits the Archimedean 4⋅8 2 (square-octagon) pattern.

Excitations of precursor molecules by different laser powers in laser-assisted growth of diamond films using a wavelength-tunable CO2 laser were studied. The wavelength of the CO2 laser was tuned to 10.532 μm to match a vibration mode of a precursor molecule, ethylene. The density of the incident laser power was adjusted to modify diamond crystal orientation, optimize diamond quality, and achieve high-efficiency laser energy coupling. It was observed that at incident laser power densities between 5.0 × 103 and 1.0 × 104 W/cm2, (100)-faceted diamond crystals were grown uniformly in the center areas of the diamond films. Higher incident laser powers, although further promoted growth rate, suppressed the uniformity of the diamond (100) facets. Best diamond quality was obtained within a laser power density range of 5.0 × 103 to 6.7 × 103 W/cm2, whereas the highest energy efficiency was achieved within a laser power density range of 3.3 × 103 to 6.7 × 103 W/cm2. The effects of the resonant laser energy couplin...

We present molecular dynamics simulation evidence for a freezing transition from liquid silicon to quasi-two-dimensional (quasi-2D) bilayer silicon in a slit nanopore. This new quasi-2D polymorph of silicon exhibits a bilayer hexagonal structure in which the covalent coordination number of every silicon atom is four. Quantum molecular dynamics simulations show that the stand-alone bilayer silicon (without the confinement) is still stable at 400 K. Electronic band-structure calculations suggest that the bilayer hexagonal silicon is a quasi-2D semimetal, similar to a graphene monolayer, but with an indirect zero band gap. Open image in new window

We performed first-principles calculation to show that a host–guest silicon nanostructure can exhibit half-metallic properties, wherein the host is a single-walled hexagonal silicon nanotube while the guest is a hybrid atomic chain of Mn and Co (encapsulated in the host nanotube). The calculated electronic band structures indicate that the Fermi level intersects only in the spin-up band, whereas the spin-down band exhibits semiconducting characteristics.

Evaluation of the tensile/compression limit of a solid under conditions of tension or compression is often performed to provide mechanical properties that are critical for structure design and assessment. Algara-Siller et al. recently demonstrated that when water is constrained between two sheets of graphene, it becomes a two-dimensional (2D) liquid and then is turned into an intriguing monolayer solid with a square pattern under high lateral pressure [ Nature , 2015 , 519 , 443 - 445 ]. From a mechanics point of view, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of the 2D monolayer water. Here, we perform a simulation study of the compression limit of 2D monolayer, bilayer, and trilayer water constrained in graphene nanocapillaries. At 300 K, a myriad of 2D ice polymorphs (both crystalline-like and amorphous) are formed from the liquid water at different widths of the nanocapillaries, ranging from 6.0 to11.6 Å. For monolayer water, the compression limit is typically a few hundred MPa, while for the bilayer and trilayer water, the compression limit is 1.5 GPa or higher, reflecting the ultrahigh van der Waals pressure within the graphene nanocapillaries. The compression-limit (phase) diagram is obtained at the nanocapillary width versus pressure (h-P) plane, based on the comprehensive molecular dynamics simulations at numerous thermodynamic states as well as on the Clapeyron equation. Interestingly, the compression-limit curves exhibit multiple local minima.

Results of a combined photoelectron spectroscopy and first-principles density-functional study of SiN- clusters in the size range 20or= Nor= 45 are reported and discussed. Evidence for a prolate-to-near-spherical shape transition at N = 27 is presented. It is shown that the tricapped-trigonal-prism (TTP) structural motif Si9 found in most low-lying clusters SiN-, 9or= Nor= 19, is replaced or augmented by a series of structural motifs consisting of a bulklike "adamantane" fragment plus a magic-number cluster (Si6, Si7, Si10) or TTP Si9 in low-lying prolate clusters SiN-, Nor= 20. For 28or= Nor= 45, almost all low-lying near-spherical clusters SiN- adopt "stuffed-cage"-like structures where the cages are homologous to carbon fullerenes in the sense that they are composed of only five- and six-membered rings. However the arrangement of the "stuffing" atoms is not yet diamondlike.

We performed molecular dynamics simulations of water confined to hydrophobic slit nanopores. Two five-site potential models of water were employed: the ST2 model and the TIP5P model. The simulations confirm our previous simulation results on basis of the four-site TIP4P model of water, that is, upon cooling the confined liquid water may undergo a first-order phase transition to either a bilayer crystalline ice phase or to a bilayer amorphous ice. The selection of the bilayer crystalline phase occurs at the constant normal pressure condition whereas the selection of the bilayer amorphous ice phase is likely to occur at the fixed pore-width condition. This computer-simulation generated ice form, if confirmed by crystallographic or spectroscopic experiments, may be a candidate for “ice XIII”, provided that purely quasi two-dimensional ice forms (metastable in vacuum) can be viewed as a stand-alone solid phase of ice.

Ab initio plane-wave total-energy calcuation is carried out to study the relative stability of the quasi-one-dimensional (Q1D) pentagon and hexagon ice nanotubes. Electronic structure calculations indicate the two Q1D ice nanotubes have nearly the same band structures and energy bandgap as those of proton-ordered bulk ice Ih. Ab initio molecular-orbital and density-functional theory calculations, as well as three classical potential models of water, are also employed to investigate the relative stability of the pentagon and hexagon water clusters (H2O)30, (H2O)60, and (H2O)120. Clusters of this kind can serve to bridge the gap between the small polygonal water rings and the infinitely long Q1D polygon ice nanotubes. It is found that the polygon water prisms with the size (H2O)120 begin to show the relative energetic behavior of the infinitely long polygon ice nanotubes.

Understanding phase behavior of highly confined water, ice, amorphous ice, and clathrate hydrates (or gas hydrates), not only enriches our view of phase transitions and structures of quasi-two-dimensional (Q2D) solids not seen in the bulk phases but also has important implications for diverse phenomena at the intersection between physical chemistry, cell biology, chemical engineering, and nanoscience. Relevant examples include, among others, boundary lubrication in nanofluidic and lab-on-a-chip devices, synthesis of antifreeze proteins for ice-growth inhibition, rapid cooling of biological suspensions or quenching emulsified water under high pressure, and storage of H2 and CO2 in gas hydrates. Classical molecular simulation (MD) is an indispensable tool to explore states and properties of highly confined water and ice. It also has the advantage of precisely monitoring the time and spatial domains in the sub-picosecond and sub-nanometer scales, which are difficult to control in laboratory experiments, and yet allows relatively long simulation at the 10(2) ns time scale that is impractical with ab initio molecular dynamics simulations. In this Account, we present an overview of our MD simulation studies of the structures and phase behaviors of highly confined water, ice, amorphous ice, and clathrate, in slit graphene nanopores. We survey six crystalline phases of monolayer (ML) ice revealed from MD simulations, including one low-density, one mid-density, and four high-density ML ices. We show additional supporting evidence on the structural stabilities of the four high-density ML ices in the vacuum (without the graphene confinement), for the first time, through quantum density-functional theory optimization of their free-standing structures at zero temperature. In addition, we summarize various low-density, high-density, and very-high-density Q2D bilayer (BL) ice and amorphous ice structures revealed from MD simulations. These simulations reinforce the notion that the nanoscale confinement not only can disrupt the hydrogen bonding network in bulk water but also can allow satisfaction of the ice rule for low-density and high-density Q2D crystalline structures. Highly confined water can serve as a generic model system for understanding a variety of Q2D materials science phenomena, for example, liquid-solid, solid-solid, solid-amorphous, and amorphous-amorphous transitions in real time, as well as the Ostwald staging during these transitions. Our simulations also bring new molecular insights into the formation of gas hydrate from a gas and water mixture at low temperature.

Crystallographic orientations determine the optical, electrical, mechanical, and thermal properties of crystals. Control of crystallographic orientations has been studied by changing the growth parameters, including temperature, pressure, proportion of precursors, and surface conditions. However, molecular dynamic mechanisms underlying these controls remain largely unknown. Here we achieved control of crystallographic orientations in diamond growth through a joint experimental and theoretical study of laser resonant vibrational excitation of precursor molecules (ethylene). Resonant vibrational excitation of the ethylene molecules using a wavelength-tunable CO2 laser steers the chemical reactions and promotes proportion of intermediate oxide species, which results in preferential growth of {100}-oriented diamond films and diamond single crystals in open air. Quantum molecular dynamic simulations and calculations of chemisorption energies of radicals detected from our mass-spectroscopy experiment provide an in-depth understanding of molecular reaction mechanisms in the steering of chemical reactions and control of crystallographic orientations. This finding opens up a new avenue for controlled chemical vapor deposition of crystals through resonant vibrational excitations to steer surface chemistry.

Three-dimensional (3D) gas clathrates are ice-like but distinguished from bulk ices by containing polyhedral nano-cages to accommodate small gas molecules. Without space filling by gas molecules, standalone 3D clathrates have not been observed to form in the laboratory, and they appear to be unstable except at negative pressure. Thus far, experimental evidence for guest‐free clathrates has only been found in germanium and silicon, although guest‐free hydrate clathrates have been found, in recent simulations, able to grow from cold stretched water, if first nucleated. Herein, we report simulation evidence of spontaneous formation of monolayer clathrate ice, with or without gas molecules, within hydrophobic nano-slit at low temperatures. The guest-free monolayer clathrate ice is a low-density ice (LDI) whose geometric pattern is identical to Archimedean 4·8 2 -truncated square tiling, i.e. a mosaic of tetragons and octagons. At large positive pressure, a second phase of 2D monolayer ice, i.e. the puckered square high-density ice (HDI) can form. The triple point of the LDI/liquid/HDI three-phase coexistence resembles that of the ice-I h /water/ice-III three-phase coexistence. More interestingly, when the LDI is under a strong compression at 200 K, it transforms into the HDI via a liquid intermediate state, the first direct evidence of Ostwald’s rule of stages at 2D. The tensile limit of the 2D LDI and water are close to that of bulk ice-I h and laboratory water.

Structural, electronic, and magnetic properties of the golden cage doped with a transition-metal atom, $M{\text{Au}}_{16}^{\ensuremath{-}}$ ($M=\text{Fe}$,Co,Ni), are investigated using trapped ion electron diffraction, photoelectron spectroscopy, and density-functional theory. The best agreement to experiment is obtained for endohedral $M@{\text{Au}}_{16}^{\ensuremath{-}}$ structures but with considerable distortions to the parent ${\text{Au}}_{16}^{\ensuremath{-}}$ cage. $\text{Fe}@{\text{Au}}_{16}^{\ensuremath{-}}$ and $\text{Co}@{\text{Au}}_{16}^{\ensuremath{-}}$ are found to have similar structures with ${C}_{2}$ symmetry while a ${C}_{1}$ structure is obtained for $\text{Ni}@{\text{Au}}_{16}^{\ensuremath{-}}$. The $4s$ electrons are observed to transfer to the ${\text{Au}}_{16}$ cage, whereas atomiclike magnetism due to the unpaired $3d$ electrons is retained for all the doped clusters.

Nano-crystalline films of pure cubic ZrO2 have been produced by ion beam assisted deposition (IBAD) processes which combine physical vapor deposition with the concurrent ion beam bombardment in a high vacuum environment and exhibit superior properties and strong adhesion to the substrate. Oxygen and argon gases are used as source materials to generate energetic ions to produce these coatings with differential nanoscale (7 to 70 nm grain size) characteristics that affect the wettability, roughness, mechanical and optical properties of the coating. The nanostructurally stabilized chemically pure cubic phase has been shown to possess hardness as high as 16 GPa and a bulk modulus of 235 GPa. We examine the mechanical properties and the phase stability in zirconia nanoparticles using first principle electronic structure method. The elastic constants of the bulk systems were calculated for monoclinic, tetragonal and cubic phases. We find that calculated bulk modulus of cubic phase (237GPa) agrees well with the measured values, while that of monoclinic (189GPa) or tetragonal (155GPa) are considerably lower. We observe considerable relaxation of lattice in the monoclinic phase near the surface. This effect combined with surface tension and possibly vacancies in nanostructures are sources of stability of cubic zirconia at nanoscale.

Structures and magnetic properties of gold-coated transition-metal clusters [Mn13@Au20]- and [Co13@Au20]- are studied within the framework of gradient-corrected density functional theory. The gold coating can result in either an enhanced or an attenuated effect on the magnetism of the core clusters. For [Mn13@Au20]-, the total magnetic moment is greatly enhanced to as high as 44 μB, more than 20-fold higher than that of the bare Mn13- cluster. For [Co13@Au20]-, however, the gold coating gives rise to an attenuation effect which leads to a reduction of the magnetic moment to 20μB from 30μB of the bare Co13- cluster.

We report six phases of high-density nano-ice predicted to form within carbon nanotubes (CNTs) at high pressure. High-density nano-ice self-assembled within smaller-diameter CNT (17,0) exhibits a double-walled helical structure where the outer wall consists of four double-stranded helixes, which resemble a DNA double helix, and the inner wall is a quadruple-stranded helix. Four other double-walled nano-ices, self-assembled respectively in two larger-diameter CNTs (20,0 and 22,0), display tubular structure. Within CNT (24,0), the confined water can freeze spontaneously into a triple-walled helical nano-ice where the outer wall is an 18-stranded helix and the middle and inner walls are hextuple-stranded helixes.

Liquid-solid and solid-solid phase transitions of a monolayer water confined between two parallel hydrophobic surfaces are studied by molecular dynamics simulations. The solid phase considered is the high-density rhombic monolayer ice. Based on the computed free energy surface, it is found that at a certain width of the slit nanopore, the monolayer water exhibits not only a high freezing point but also a low energy barrier to crystallization. Moreover, through analyzing the oxygen-hydrogen-oxygen angle distribution and oxygen-hydrogen radial distribution, the high-density monolayer ice is classified as either a flat ice or a puckered ice. The transition between a flat ice and a puckered ice reflects a trade-off between the water-wall interactions and the electrostatic interactions among water molecules.

In this report, we introduce new organic photorefractive composites consisting of charge transporting den-drimers highly doped with a stilbene nonlinear optic chromophore, The purpose of making these composites is to improve charge transport, by reducing inhomogeneity when compared to ordinary polymer-based systems. Because the structure of this material gives us freedom to control the orientation of charge transport agents synthetically, we can study the charge transport mechanism more systematically than in polymers. We discuss this point and present the characterization results for this material.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Possible lowest-energy structures of Si21 and Si25 are found on the basis of the starting structures obtained via the global search for nearly identical low-energy Stillinger-Weber (SW) and modified-SW structures. The fact that the lowest-energy structures are spherical-like may suggest that the prolate-to-spherical-like structural transition for the silicon cluster Sin is likely to occur in the range of 21 < n < 25.

Nanostructurally stabilized zirconium oxide (NSZ) hard transparent films were produced without chemical stabilizers by the ion beam assisted deposition technique (IBAD). A transmission electron microscopy study of the samples produced below 150 °C revealed that these films are composed of zirconium oxide (ZrO2) nanocrystallites of diameters 7.5 ± 2.3 nm. X-ray and selected-area electron diffraction studies suggested that the as-deposited films are consistent with cubic phase ZrO2. Rutherford backscattering spectroscopy (RBS) indicated the formation of stoichiometric ZrO2. The phase identity of these optically transparent NSZ films was in agreement with cubic ZrO2, as indicated by the matching elastic modulus values from the calculated results for pure cubic zirconium oxide and results of nanoindentation measurements. Upon annealing in air for 1 h, these NSZ films were found to retain most of their room temperature deposited cubic phase x-ray diffraction signature up to 850 °C. Size effect and vacancy stabilization mechanisms and the IBAD technique are discussed to explain the present results.