Quasi-hexagonal Phase Research Articles

Contact engineering for two-dimensional metal/qHP C60 van der Waals heterostructure

The fabrication of two-dimensional (2D) quasi-hexagonal phase (qHP) C60 semiconductor material offers a promising candidate for high-performance electronic devices. Selecting appropriate metals is crucial for achieving Ohmic contact (OhC) to enhance carrier injection efficiency. In this Letter, we used first-principles calculations to study the contact properties of seven 2D metal/qHP C60 van der Waals heterostructures. Metals with suitable work functions can form p-type Schottky contacts (p-ShCs), n-type Schottky contacts (n-ShCs), and OhCs. Differences in work function affect interface charge transfer, creating interface dipoles and causing band alignment deviations from the ideal Schottky–Mott limit. The calculated Fermi level pinning factors for n-type and p-type 2D metal/qHP C60 vdWh are 0.528 and 0.521, respectively. By regulating Φn and Φp based on electrostatic potential difference ΔV, we have achieved the ideal Schottky–Mott limit. We also studied the Schottky barrier height of the germanene/qHP C60 vdWh, finding that using electric field is an effective way to convert n-ShC to OhC or p-ShC. These findings provide theoretical guidance for constructing efficient 2D qHP C60 electronic devices.

Strain engineering of electronic structure and thermoelectric properties of quasi-hexagonal fullerene monolayer

As a newly synthesized two-dimensional (2D) carbon material, monolayer quasi-hexagonal phase fullerene (qHP C60) has an excellent electronic structure and low thermal conductivity. qHP C60 attracted significant attention from scientists because it has potential applications in thermoelectric materials. Thermoelectric properties of 2D materials significantly depend on the transport of carriers (such as electrons and phonons), and strain engineering is an essential method for modulating the transport of electrons and phonons in 2D materials. However, the strain engineering method for the modulation of the thermoelectric properties of monolayer qHP C60 has not been reported yet. In the present paper, the first-principles combined with the non-equilibrium Green's function method are used to investigate the ballistic transport properties of electrons and phonons in monolayer qHP C60. The effects of temperature, chemical potential, and biaxial tensile strain on the thermoelectric transport parameters (including conductivity, Seebeck coefficient, power factor, and thermal conductivity) as well as the figure of merit (ZT) of monolayer qHP C60 are presented, compared, discussed, and analyzed. We found that monolayer qHP C60 exhibits anisotropic characteristics in electron and phonon transport properties, showcasing outstanding thermoelectric properties. The distinctive quasi-hexagonal phase fullerene network structure offers a novel platform for exploring innovative 2D thermoelectric materials in research. This study provides crucial theoretical insights to guide the designing and implementation of 2D thermoelectric materials based on fullerenes.

Electron Localization and Mobility in Monolayer Fullerene Networks.

The novel 2D quasi-hexagonal phase of covalently bonded fullerene molecules (qHP C60), the so-called graphullerene, has displayed far superior electron mobilities, if compared to the parent van der Waals three-dimensional crystal (vdW C60). Herein, we present a comparative study of the electronic properties of vdW and qHP C60 using state-of-the-art electronic-structure calculations and a full quantum-mechanical treatment of electron transfer. We show that both materials entail polaronic localization of electrons with similar binding energies (≈0.1 eV) and, therefore, they share the same charge transport via polaron hopping. In fact, we quantitatively reproduce the sizable increment of the electron mobility measured for qHP C60 and identify its origin in the increased electronic coupling between C60 units.

Highly in-plane anisotropic optical properties of fullerene monolayers

Both the intrinsic anisotropic optical materials and fullerene-assembled 2D materials have attracted much interest in fundamental science and potential applications. The synthesis of a monolayer (ML) fullerene makes the combination of these two features plausible. In this work, using first-principles calculations, we systematically study the electronic structure and optical properties of quasi-hexagonal phase (qHP) ML and quasi-tetragonal phase (qTP) ML fullerenes. The calculations of qHP ML show that it is a semiconductor with small anisotropic optical absorption, which agrees with the recent experimental measurements. However, the results for qTP ML reveal that it is a semimetal with highly in-plane anisotropic absorption. The dichroic ratio, namely the absorption ratio of x- and y-polarized light αxx/αyy, is around 12 at photon energy of 0.29 eV. This anisotropy is much more pronounced when the photon energy is between 0.7 and 1.4 eV, where αxx becomes nearly zero while αyy is more than two orders of magnitude larger. This indicates qTP ML as a candidate for long-pursuit lossless metal and a potential material for atomically thin polarizer. We hope this will stimulate further experimental efforts in the study of qTP ML and other fullerene-assembled 2D materials.

Stability and Elasticity of Quasi-Hexagonal Fullerene Monolayer from First-Principles Study

As a newly synthesized two-dimensional carbon material, the stability study of monolayer fullerene networks or quasi-hexagonal phase fullerenes (qhp-C60) is timely desirable. We have investigated the stabilities of qhp-C60, including thermal, structural, mechanical, and thermodynamic stabilities, as well as the bonding characteristics, ductility, and mechanical properties, via first-principles calculations. The results show that qhp-C60 is energetically, mechanically, and thermodynamically stable. The thermodynamic stability of qhp-C60 at 300 K and 600 K is verified. The bonding characteristics of qhp-C60 are analyzed from the bond length, and it has sp2 and sp3 hybridization. The Pugh ratio (B/G) and Poisson’s ratio (v) indicate similar ductility with graphite and graphene. We also found that qhp-C60 has the lowest hardness and the anisotropy of the material. In addition, the electronic characteristics, including electron localization function (ELF), crystal orbital Hamiltonian population (COHP), and density of states (DOS) at different temperatures, are analyzed to verify the thermal stability of the material. Our results might be helpful in the material design of qhp-C60-related applications.

Stability and Strength of Monolayer Polymeric C60.

Two-dimensional fullerene networks have been synthesized in several forms, and it is unknown which monolayer form is stable under ambient conditions. Using first-principles calculations, I show that the believed stability of the quasi-tetragonal phases is challenged by mechanical, dynamic, or thermodynamic stability. For all temperatures, the quasi-hexagonal phase is thermodynamically the least stable. However, the relatively high dynamic and mechanical stabilities suggest that the quasi-hexagonal phase is intrinsically stronger than the other phases under various strains. The origin of the high stability and strength of the quasi-hexagonal phase can be attributed to the strong covalent C-C bonds that strongly hold the linked C60 clusters together, enabling the closely packed hexagonal network. These results rationalize the experimental observations that so far only the quasi-hexagonal phase has been exfoliated experimentally as monolayers.

Structural investigation of caffeine monolayers on Au(111)

The asymmetric and achiral character of caffeine (${\mathrm{C}}_{8}{\mathrm{H}}_{10}{\mathrm{N}}_{4}{\mathrm{O}}_{2}$) leads to two on-surface chiralities which has an impact on its on-surface formation. An analysis of its on-surface behavior reveals new insights of its crystallite growth. In this study the structural formation of caffeine monolayers on a Au(111) surface was analyzed by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The monolayers were prepared by molecular beam epitaxy (MBE) and analyzed at room temperature. Caffeine molecules self-assemble in a quasihexagonal phase on Au(111) similar to the high-temperature $\ensuremath{\alpha}$ phase. Two mirrored hexagonal domains are present with respect to the surface. Within the XPS measurements, no strong surface interaction was found. Therefore, a theoretical analysis of a hypothetical free-standing caffeine monolayer structure was performed by ab initio simulations. We found that a caffeine monolayer with three molecules per unit cell is preferable to one with just a single molecule, as could be expected from the LEED pattern.

Iodine Adsorption on Ni(110): 2D-Phase Transitions and NiI2 Growth

Scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and density functional theory calculations have been used to investigate I2 adsorption on the Ni(110) surface. Dissociative adsorption takes place rapidly to form a c(2 × 2) layer at an iodine coverage of 0.5 ML. DFT calculations show that hollow sites are energetically preferable for iodine adsorption on the Ni(110) surface. Increase of iodine coverage above 0.5 ML leads to the commensurate–incommensurate phase transition via nucleation of loops of domain walls and to their subsequent ordering into the striped superstructure. At saturation, a striped domain-wall phase degenerates into the uniaxially compressed quasi-hexagonal phase. Further iodine dosing leads to the nucleation and growth of 2D nickel iodide islands.

<b>Fiber Structure Development of Poly(<i>p</i>-phenylene sulfide) in Laser-heated Drawing Process</b>

The structural development of poly(p-phenylene sulfide) (PPS) fibers was analyzed through in-situ wide angle X-ray diffraction and small angle X-ray scattering during CO2 laser heated drawing. The in-situ measurements were carried out as a function of elapsed time after necking, achieving a high time resolution of 0.25 ms. The quasi-hexagonal phase appeared just after neck-drawing, and developed with increasing crystal orientation factor and decreasing long period and d-spacing within less than 1 ms after drawing. The crystallization induction time and the crystallization rate of the quasi-hexagonal phase obtained from the equatorial intensity profiles were 0.02 ms and 3.0 ms-1. The crystal orientation factor continued to increase, and saturated about 0.95 at about 8 ms after necking. And after then, the quasi-hexagonal phase started to transform into orthorhombic crystal in several decades of milliseconds with the decreasing of fiber temperature.

Molecular dynamics simulation of chains mobility in polyethylene crystal

The mobility of polymer chains in perfect polyethylene crystal was calculated as a function of temperature and chain length through Molecular dynamics in united atom approximation. The results demonstrate that the chain mobility drastically increases in the vicinity of the phase transition from the orthorhombic to quasi-hexagonal phase. In the quasi-hexagonal phase, the chain mobility is almost independent on temperature and inversely proportional to the chain length.

LEED and DFT Study of the Quasihexagonal TiO2 Structure on Cu(001)

Reactive deposition of Ti on the Cu(001) surface saturated with chemisorbed oxygen results in the formation of an ordered ultrathin film with a slightly distorted hexagonal (quasihexagonal) unit cell. We find that, under appropriate substrate temperature, evaporation rates, and O2 pressure, it is possible to cover the whole substrate surface with the TiO2 quasihexagonal phase. An O−Ti−O trilayer model for the (quasi) hexagonal structure is consistent with angle scanned XPD data relative to the Ti 2p signal and with the LEED I−V experimental intensity curves. From a careful inspection of the LEED pattern, we derive a distortion of the hexagonal unit cell corresponding to a p(2 × 7) coincidence mesh with the substrate. The p(2 × 7) coincidence was used to model XPD data. For the analysis of LEED-IV data and for DFT calculation, the p(2 × 7) was approximated by a c(2 × 6) coincidence mesh. The c(2 × 6) approximation is very close to a p(2 × 7) in terms of lattice parameters but contains a reduced number of n...

Mechanism of growth and structure of titanium oxide ultrathin films deposited on Cu(0 0 1)

The growth mechanism, composition and structure of ultrathin films of titanium oxide deposited on the Cu(001) surface were investigated by means of XPS, LEIS, LEED and STM. Titanium oxide films were deposited on the Cu(001) surface previously saturated with a (√2×2√2)R45° structure of chemisorbed oxygen. The oxide films were prepared by evaporation of titanium in O2 atmosphere (pO2 in the 10−6mbar range) while the substrate temperature was kept at 573K. The Cu LEIS signal versus the amount of deposited titanium (as determined by means of XPS) indicates the growth of 2D islands in the early stages of deposition. Upon increasing the amount of deposited titanium multilayer islands begin to grow. The XPS results indicate that the oxide phases formed for Ti coverages above 1ML have a TiO2 stoichiometry. At very low coverages, a LEED pattern with a centred rectangular unit cell is observed. STM measurements show that at this stage of the growth the oxide islands are incorporated in the outermost layer of the substrate and the removed copper atoms form islands around the oxide regions. The very early stages of titanium oxide growth corresponding to the formation of this rectangular phase were also investigated by Ti deposition on the oxygen chemisorbed phase under UHV conditions at 573K. In this way it is possible to study the reaction of Ti with chemisorbed oxygen. Upon increasing the Ti coverage above 0.5ML, an oxide film with a slightly distorted hexagonal unit cell begins to grow. The quasi-hexagonal phase of titanium oxide can also be formed by annealing at 773K the rectangular phase. At higher coverages, when the substrate surface is completely covered by the oxide, the film exhibits a LEED pattern with a regular hexagonal unit cell.

Growth and Reactivity of Titanium Oxide Ultrathin Films on Ni(110)

Soft X-ray photoelectron spectroscopy (SXPS) and X-ray absorption near-edge structure (XANES) have been combined with low-energy electron diffraction (LEED) to examine the growth of titanium dioxide thin films on Ni(110). Depending on the initial titanium coverage, the formation of two different films is observed, a quasi-hexagonal phase and a film with rutile (110) rods. Spectroscopy (SXPS and XANES) results indicate that all films consist of fully oxidized titanium. Furthermore, the reactivity of the higher coverage phase, consisting partly of rutile TiO2(110), was investigated after exposure to molecular water at 190 K. The formation of molecular water and hydroxyls was observed for low-pressure exposure (less than 10(-7) mbar). High-pressure exposure (on the order of 10(-6) mbar) resulted in hydroxylation of the thin film, which was found to be reversible upon annealing.

Are liquid crystalline properties of nucleosomes involved in chromosome structure and dynamics?

Nucleosome core particles correspond to the structural units of eukaryotic chromatin. They are charged colloids, 101 Angstrom in diameter and 55 Angstrom in length, formed by the coiling of a 146/147 bp DNA fragment (50 nm) around the histone protein octamer. Solutions of these particles can be concentrated, under osmotic pressure, up to the concentrations found in the nuclei of living cells. In the presence of monovalent cations (Na(+)), nucleosomes self-assemble into crystalline or liquid crystalline phases. A lamello-columnar phase is observed at 'low salt' concentrations, while a two-dimensional hexagonal phase and a three-dimensional quasi-hexagonal phase form at 'high salt' concentrations. We followed the formation of these phases from the dilute isotropic solutions to the ordered phases by combining cryoelectron microscopy and X-ray diffraction analyses. The phase diagram is presented as a function of the monovalent salt concentration and applied osmotic pressure. An alternative method to condense nucleosomes is to induce their aggregation upon addition of divalent or multivalent cations (Mg(2+), spermidine(3+) and spermine(4+)). Ordered phases are also found in the aggregates. We also discuss whether these condensed phases of nucleosomes may be relevant from a biological point of view.

Segmented liquid-crystalline polyesters with allyl groups as lateral substituents. Synthesis and characterization

The synthesis and the characterization of segmented liquid-crystalline [ P( n)] made by interfacial polycondensation of 3-allyl-4,4′-dihydroxybiphenyl and the acid chlorides of α,ω-dicarboxy-alkanes are reported. The flexible spacers contain even numbers of methylene units ( n=6–14). All polymers are enantiotropic showing a nematic phase with isotropization temperature in the range of about 95–175°C. At room temperature all polymers show a mesophasic structure except for P(8) which is crystalline. Both P(12) and P(14) show a quasi-hexagonal phase with six chains per unit cell.

Liquid-phase adsorption process of β-picoline on stilbite (010) observed by atomic force microscopy

Liquid-phase adsorption process of β-picoline molecules on a (010) surface of a natural zeolite stilbite was observed by atomic force microscopy (AFM). Initially, the surface was covered with excess multilayer β-picoline molecules. Removal of this excess layer by AFM scanning exposed a possible random-adsorbed monolayer phase. A well-ordered two-dimensional quasihexagonal phase developed slowly, with an estimated rate of ∼6 nm/min. The 2D unit cell dimension of the latter ordered phase was 0.55±0.04 nm. The ordered phase is incommensurate with the substrate surface, although one of the 2D lattice unit vectors of the adsorbed phase makes an angle of ∼50° and ∼100° with respect to the a and c axes of the substrate (010) plane, respectively. From the position of the methyl group in the β-picoline molecule it is estimated that the molecules are adsorbed on the surface with their C–N bond parallel to the surface.

Reversible changes in the solid state of HBA/HNA liquid crystalline copolyesters studied by X-ray diffraction

Real time X-ray diffraction patterns were recorded as a function of heating and cooling cycles using a synchrotron radiation source for copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with monomer ratios 75 25 , 58 42 and 30 70 . The angular positions of the two main interchain X-ray diffraction reflections 110 and 200 were obtained at temperatures below the melting point with a powder diffractometer, and the unit cell dimensions of the copolymers were measured. For the 30 70 and 58 42 compositions the cell dimensions vary linearly and reversibly with temperature. For the 75 25 copolymer the cell dimensions show a clear bend with temperature, which can be associated with a transformation from an orthorhombic to a quasi-hexagonal phase. It is further shown that at higher temperature the dimensions of the D 110 and D 200 coherently diffracting domains increase while the scattering intensity of the diffraction peaks decreases with temperature. These changes are reversible with temperature. Results are discussed in terms of partial melting of smaller crystallites contributing in a reversible manner to the occurrence of a mobile high temperature quasi-hexagonal phase.

Monte Carlo studies of an idealized model of a lipid-water system

Employing Monte Carlo dynamics, the equilibrium and dynamic properties of lipid-water systems are studied in the context of a diamond lattice realization. The model faithfully describe the lipid molecule geometry and reproduces the essential physical properties of real membranes. These include the phase transitions from quasi-crystalline phase {leftrightarrow} liquid bilayer {leftrightarrow} nonbilayer, quasi-hexagonal phase {leftrightarrow} dissolved liquid solution. Furthermore, the structure, ordering, and dynamics of the model liquid bilayer are in good accord with experiment, and the values of segmental ordering parameters are close to those obtained from NMR data. Due to the diamond lattice representation and a very efficient simulation algorithm, the intermediate distance scale dynamic features of the water-lipid system could be examined; these include lateral diffusion in the bilayer, the formation of a nonbilayer phase, the transbilayer diffusion of the lipid molecule, and the diffusion of a lipid molecule in the water phase. The present model can be also used to generate initial configurations for more detailed molecular (or Brownian) dynamics studies of lipid-water systems. 33 refs., 19 figs., 1 tab.