https://par.nsf.gov/servlets/purl/10178865

Dynamics of polymer segments, polymer chains, and nanoparticles in polymer nanocomposite melts: A review

Poly(methyl methacrylate) (PMMA)−C60 nanocomposites, with compositions in the range 0 ≤ φC60wt ≤ 0.05, are shown to exhibit systematic increases in dynamic shear moduli, in glass transition temperature (Tg), and in the longest relaxation time of the polymer (τR) with increasing fullerene concentration. We show that while the φC60wt dependence of the plateau modulus can be reconciled with a conventional “filler” effect, the systematic increases in Tg and in τR are associated with specific interactions between the C60 and the polymer segments. In the melt, these segment−C60 interactions are proposed to reduce polymer segmental mobility in the vicinity of the particle surface and ultimately suppress polymer dynamics, as measured mechanically, in a manner consistent with an increase in the polymer segmental friction coefficient.

http://absimage.aps.org/image/MAR07/MWS_MAR07-2006-002202.pdf

Origin of dynamical properties in PMMA-C$_{60}$ nanocomposites

Understanding interfacial influence on properties of polymer nanocomposites

Nanorod diffusion and polymer dynamics in nanocomposites were investigated by means of molecular dynamics simulations. We show that thin nanorods (spherocylinders) can diffuse much faster than that predicted by a Stokes–Einstein continuum model, similar to experiments, in the dilute nanorod regime. In entangled polymer matrices, nanorod diffusion reaches a plateau region. Increase of the nanorod diameter or aspect ratio (AR) slows its diffusion. The nanorod diffusion, at long time scales, displays a non-Gaussian behavior as is depicted by the several Gaussian peaks present in the self-part of the van Hove function. We also show that the unentangled polymer dynamics decreases in the nanocomposite due to the nanorods interfacial area.

Nanorod Diffusion in Polymer Nanocomposites by Molecular Dynamics Simulations

Recently, a new two-dimensional allotrope of carbon named biphenylene has been experimentally synthesized. First-principles calculations are preformed to investigate the electronic properties of biphenylene and the doping effect is also considered to tune its electronic, magnetic, and catalytic properties. The metallic nature with an n-type Dirac cone is observed in the biphenylene. The magnetism can be induced by Fe, Cl, Cr, and Mn doping. More importantly, the doping position dependence of hydrogen evolution reaction (HER) performance of biphenylene is addressed, which can be significantly improved by atomic doping. In particular, the barrier for HER of Fe doping case is only −0.03 eV, denoting its great potential in HER catalysis.

Tuning electronic, magnetic and catalytic behaviors of biphenylene network by atomic doping

A Family of Li B Monolayers with a Wide Spectrum of Potential Applications

Recently, a two-dimensional (2D) heterostructure has been widely investigated as a photocatalyst to decompose water using the extraordinary type-II band structure. In this work, the MoTe2/PtS2 van der Waals heterostructure (vdWH) is constructed with different stacking structures. Based on density functional calculations, the stacking-dependent electronic characteristic is explored, so that the MoTe2/PtS2 vdWH possesses type-I and type-II band structures for the light-emitting device and photocatalyst, respectively, with decent stacking configurations. The band alignment of the MoTe2/PtS2 vdWH is also addressed to obtain suitable band edge positions for water-splitting at pH 0. Furthermore, the potential drop is investigated, resulting from charge transfer between the MoTe2 and PtS2, which is another critical promotion to prevent the recombination of the photogenerated charges. Additionally, the MoTe2/PtS2 vdWH also demonstrates a novel and excellent optical absorption capacity in the visible wavelength range. Our work suggests a theoretical guide to designing and tuning the 2D heterostructure using photocatalytic and photovoltaic devices.

/pdf/stacking-mediated-type-i-type-ii-transition-in-two-3a7wuc6b.pdf

Stacking-Mediated Type-I/Type-II Transition in Two-Dimensional MoTe2/PtS2 Heterostructure: A First-Principles Simulation

Recently, the synthesis of biphenylene inspires the substantial attention on the two-dimensional allotrope of carbon. Although elastic, thermal, and electronic properties of biphenylene have been reported, phonon modes and the origin of anisotropy in biphenylene are still unclear. In this work, combining the first-principles calculations and theoretical analysis, we investigate the properties of optical and acoustic phonons in monolayer biphenylene. There are nine Raman-active and five infrared-active modes which can be excited by the Raman or infrared laser. Interestingly, a Raman-active single phonon mode ([Formula: see text]) is observed, and its frequency is up to 49.67 THz at the Brillouin zone-center point. This provides promising potential for biphenylene monolayer in the application of phonon lasers, quantum nonlinear elements, and quantum mechanical resonators. Meantime, the Grüneisen constant of an [Formula: see text] mode is up to 2.07 at the zone-center point, suggesting that its Raman spectroscopy can be used to identify the lattice strain and temperature of biphenylene. To explore the origin of anisotropy in biphenylene, we calculate the covalency and cophonicity and find that the inconsistent speed of motion and different intensities of hybridization between these inequivalent carbon atoms should take responsibility for the direction dependent thermal and elastic properties in biphenylene.

Phonon properties of biphenylene monolayer by first-principles calculations

Auxetic materials are highly desirable for advanced applications because of their negative Poisson's ratios, which are rather scarce in two-dimensional materials. Motivated by the elemental mutation method, we predict a new class of monolayer IV-VI semiconductors, namely, δ-IV-VI monolayers (GeS, GeSe, SiS and SiSe). Distinctly different from the previously predicted IV-VI monolayers, the newly predicted δ-MX (X = Ge and Si; M = S and Se) monolayers exhibit a puckered unit cell with a space group of Pca21. Their stabilities were confirmed by first-principles lattice dynamics and molecular dynamics calculations. In particular, all these MX monolayers possess a large bandgap in the range of 2.08-2.65 eV and pronounced anisotropic mechanical properties, which are demonstrated by direction-dependent in-plane Young's moduli and Poisson's ratios. Furthermore, all these 2D MX monolayers possess negative Poisson's ratios (even up to about -0.3 for SiSe). Strong optical absorption is observed in these δ-IV-VI monolayers. These interesting physical properties will stimulate the development of 2D flexible devices based on IV-VI semiconductor monolayers.

Kai-Xin Ren

Papers

Tuning electronic, magnetic and catalytic behaviors of biphenylene network by atomic doping

A Family of Li B Monolayers with a Wide Spectrum of Potential Applications

Stacking-Mediated Type-I/Type-II Transition in Two-Dimensional MoTe2/PtS2 Heterostructure: A First-Principles Simulation

Phonon properties of biphenylene monolayer by first-principles calculations

Prediction of 2D IV-VI semiconductors: auxetic materials with direct bandgap and strong optical absorption.