A Change of Phase Morphology in Poly Lactic Acid/Poly Methyl Methacrylate Blends Induced by Graphene Nano Sheets

Blending of polymeric materials is an effective way to obtain materials with specific properties, since the properties of these multiphase polymeric materials are not only affected by the properties of the component polymers but also by the morphology formed. The research described here was focused on investigation of the morphology of polymer blends of poly lactic acid (PLA) and poly methyl methacrylate (PMMA) and the PLA/PMMA blends containing various amounts of graphene nano plates, (GNP). In this work, the blends were prepared by solution casting and the morphologies of these nano filled polymer blends were studied. By adding graphene nano plates into the PLA/PMMA blends, the morphology changed for all compositions. It was very interesting to note that the GNP were found to be preferentially located in one of the polymer phases, different for the different loadings, and its location determined the final morphology of the PLA/PMMA blends. The morphology of the blends was observed by SEM and the composition-morphology dependence responses were investigated using a Fourier transform infra-red (FTIR) spectroscopy technique.     

A new function of graphene oxide emerges: inactivating phytopathogenic bacterium Xanthomonas oryzae pv. Oryzae

The main goal of the present work is to examine the effect of graphene layers on the structural and dynamical properties of polymer systems. We study hybrid poly(methyl methacrylate) (PMMA)/graphene interfacial systems, through detailed atomistic molecular dynamics simulations. In order to characterize the interface, various properties related to density, structure and dynamics of polymer chains are calculated, as a function of the distance from the substrate. A series of different hybrid systems, with width ranging between 2.60 and 13.35 nm, are being modeled. In addition, we compare the properties of the macromolecular chains to the properties of the corresponding bulk system at the same temperature. We observe a strong effect of graphene layers on both structure and dynamics of the PMMA chains. Furthermore, the PMMA/graphene interface is characterized by different length scales, depending on the actual property we probe: density of PMMA polymer chains is larger than the bulk value, for polymer chains close to graphene layers up to distances of about 1.0–1.5 nm. Chain conformations are perturbed for distances up to about 2–3 radius of gyration from graphene. Segmental dynamics of PMMA is much slower close to the solid layers up to about 2–3 nm. Finally, terminal-chain dynamics is slower, compared to the bulk one, up to distances of about 5–7 radius of gyration.     

用于聚合物介电材料的双硅氧烷链接的环化聚合物

We studied the dielectric properties of organosilicon-containing helical cyclopolymer PbMA which consists of PMMA main chains and tetramethyldisiloxane side rings. PbMA formed films with excellent uniformity through spin-coating onto highly n-doped silicon (n-Si) wafers for constructing devices of dielectric measurements, on which the dielectric properties and I-V characteristics of PbMA were studied. PbMA has a much lower dielectric constant (lower than 2.6) in the frequency range of 10-10⁵ Hz, and better thermal stability than PMMA does. I-V data showed that the metal/PbMA/n-Si devices have different conducting directions, depending on whether Au or Al deposited over PbMA layers.     

Self-assembly between graphene sheets and cationic poly(methyl methacrylate) (PMMA) particles: preparation and characterization of PMMA/graphene composites

In this study, we presented a simple approach to prepare poly(methyl methacrylate) (PMMA)/graphene composites based on the self-assembly between graphene oxide (GO) sheets and cationic PMMA emulsion particles. Briefly, cationic PMMA emulsion particles were first synthesized by a soap-free emulsion polymerization process, in which methacryloyloxyethyl trimethyl ammonium chloride was used as the emulsifier, and then blended with the aqueous solution of GO. Through electrostatic attraction, the exfoliated GO sheets were tightly adhered on the PMMA particles. The GO sheets could be reduced in situ into graphene sheets by a chemical method, without the aggregation. The structure of the prepared composites and the influences of GO and graphene sheets on the properties of PMMA were investigated. Both GO and graphene sheets can increase the glass transition temperature and storage modulus of PMMA. Moreover, graphene sheets provided a more significant reinforcement effect.     

Preparation of graphene/PMMA composites with assistance of ultrasonic wave under supercritical CO2 conditions

The two-dimensional material graphene has many excellent physicochemical properties such as large specific surface area, high electron migration rate, good chemical properties, good thermal conductivity, high elastic modulus and mechanical strength that make it very valuable for theoretical research and application in the preparation of graphene/polymer composites. In this paper, the effects of ultrasonic intensity and reaction time on the molecular weight and yield of PMMA under supercritical CO₂ conditions were investigated. It was found that there are threshold and optimal values of ultrasonic intensity for initiating the reaction in supercritical CO₂ system. The threshold value is 150 W/cm² and the optimal ultrasonic intensity value is 225 W/cm². There is also an optimal value of ultrasonic initiation time for ultrasonic initiation polymerization. Combining the reaction yield and the molecular weight of the product, 2 h of ultrasonic initiation is a suitable initiation reaction time. Based on the synthesis of PMMA by ultrasonic excitation, the preparation of Graphene/PMMA composites by ultrasound assistance was also investigated. The TG and DTG characterization of PMMA and complex materials prepared by ultrasonic excitation showed that the radicals generated by ultrasonic excitation were uniformly distributed and did not generate unsaturated double bonded polymers. The electrical conductivity of the Graphene/PMMA composites prepared by ultrasonic excitation at a graphene content of 1 wt% increased to 1.13 × 10^-1 S/cm, which is better than that of the Graphene/PMMA prepared by in situ polymerization. This may be attributed to the ultrasound-assisted supercritical CO₂ fluid action that resulted in a more uniform distribution of Graphene mixed with PMMA in the prepared composites. Therefore, it is of important practical significance for the preparation of Graphene/PMMA composites by ultrasound-induced polymerization.     

Graphene based heterostructures

The two dimensional charge carriers in monolayer and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. These two-dimensional materials are predicted to exhibit a wide range of behavior, etc. However, graphene devices on a typical three-dimensional insulating substrates such as SiO₂ are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. We have developed a novel technique for substrate engineering of graphene devices using layered dielectric materials to build graphene based vertical heterostructures. We employ hBN, an insulating isomorph of graphite, as a substrate and gate dielectric for graphene electronics. In this review, we describe the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal hBN substrates, using a mechanical transfer process. Graphene devices on hBN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO₂. We use the enhanced mobility of electrons in hBN supported graphene to investigate the effects of electronic interactions. We find that interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of non-trivial integer and fractional quantum Hall states. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.     

Temperature dependence of the noise amplitude in graphene and graphene oxide

Graphene and related materials such as carbon nanotubes and graphene oxide are promising materials for future applications in chemical sensing and electronics. Electronic noise in these materials is typically very high due to the low number of carriers and the inverse dependence of 1/f noise on the number of carriers. We have investigated the changes in 1/f noise amplitude with temperature in exfoliated graphene and reduced graphene oxide devices. We show that using reduced graphene oxide results in an intriguing environmental coupling to noise amplitude. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of chemical solvents on the properties of CVD graphene

The ultra‐clean and defect‐free transfer of chemical vapor deposition (CVD) graphene is essential for its application in electronic devices. Here, we study the influence of commonly used etching solvents during the transfer process, i.e. ammonium persulfate, ferric chloride, and ferric nitrate, on the properties of CVD graphene by Raman spectroscopy. Obvious blue shift and broadening of Raman G and 2D peaks were observed for graphene transferred by ferric chloride and ferric nitrate, as compared to that transferred by ammonium persulfate. These changes are attributed to p‐doping as well as reduction of phonon lifetime of graphene in the presence of residue iron compounds during the transfer process. The latter would also introduce a great reduction of thermal conductivity of graphene, e.g. with 76% reduction for graphene transferred by ferric nitrate as compared to that transferred by ammonium persulfate. This work would provide valuable information for the transfer of high‐quality CVD graphene. Copyright © 2014 John Wiley & Sons, Ltd.     

硅基底石墨烯器件的现状及发展趋势

石墨烯是一种由单层碳原子紧密排列而形成的具有蜂窝状结构的二维晶体材料,特殊的结构赋予了其优异的性能,如高载流子迁移率、电导率、热导率、力学强度以及量子反常霍尔效应.由于石墨烯优异的特性,迅速激起了人们对石墨烯研究以及应用的热情.石墨烯沉积或转移到硅片后,其器件构建与集成和传统硅基半导体工艺兼容.基于石墨烯的硅基器件与硅基器件的有机结合,可以大幅度提高半导体器件的综合性能.随着石墨烯制备工艺和转移技术的优化,硅基底石墨烯器件将呈现出潜在的、巨大的实际应用价值.随着器件尺寸的纳米化,器件的发热、能耗等问题成为硅基器件与集成发展面临的瓶颈问题,石墨烯的出现为解决这些问题提供了一种可能的解决方案.本文综述了石墨烯作为场效应晶体管研究的进展,为解决石墨烯带隙为零、影响器件开关比的问题,采用了量子限域法、化学掺杂法、外加电场调节法和引入应力法.在光电器件研究方面,石墨烯可以均匀吸收所有频率的光,其光电性能也受到了广泛的关注,如光电探测器、光电调制器、太阳能电池等.同时,石墨烯作为典型的二维材料,其优越的电学性能以及超高的比表面积,使其作为高灵敏度传感器的研究成为纳米科学研究的前沿和热点领域.     

Morphology,miscibility and mechanical properties of PMMA/PC blends

This study deals with some results on morphology, miscibility and mechanical properties for polymethyl methacrylate/polycarbonate (PMMA/PC) polymer blends prepared by solution casting method at different concentration between 0 and 100 wt%. Dynamic storage modulus and tan δ were measured in a temperature range from 30 to 180°C using dynamical mechanical analyzer (DMA). The value of the storage modulus was found to increase with the addition of the PC in the matrix. Transition temperature of pure PMMA and pure PC is found to be 83.8 and 150°C, respectively. The result shows that the two polymers are miscible for whole concentration of PC in PMMA. The distribution of the phases in the blends was studied through scanning electron microscopy (SEM). Also the mechanical properties like elongation at break and fracture energy of the PMMA/PC blends increase with the increase in concentration of PC in PMMA.     

基于石墨烯的太赫兹器件研究进展

石墨烯是一种零带隙二维的半导体材料, 具有极高的载流子迁移率, 优异的机械、电学、热学和光学等性能. 在太赫兹辐射源、调制器和探测器件的研究中, 石墨烯材料具有独特的优势. 本文以石墨烯材料在太赫兹辐射源、调制器以及探测器等器件方面的应用为主, 综述了石墨烯太赫兹器件的最新研究进展.     

Preparation and optical constants of the nano-crystal and polymer composite Bi4Ti3O12/PMMA thin films

Bismuth titanate, Bi₄Ti₃O₁₂ (BTO), is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices. Its nano-crystals were compounded by the chemical solution decomposition technique. Its structure and size were analyzed by X-ray diffraction and transmissive electron microscopy. The composite thin film of BTO nano-crystals and high transparency polymethylmethacrylate (PMMA) polymer was prepared by spin coating. The transmitted spectrum of BTO/PMMA composite thin film in 300–1500 nm was measured. The film thickness d and the optical constants of the film, such as the refractive index n, the absorption coefficient α, and the extinction coefficient κ were obtained using the data from the transmitted spectrum.     

化学气相沉积石墨烯薄膜的洁净转移

石墨烯因其优异的性能在很多领域具有广阔的应用前景.目前石墨烯薄膜主要是以铜作为催化基底,通过化学气相沉积法制备.这种方法制备的石墨烯薄膜需要被转移到目标基底上进行后续应用,而转移过程则会对石墨烯造成污染,进而影响石墨烯的性质及器件的性能.如何减少或避免污染,实现石墨烯的洁净转移,是石墨烯薄膜转移技术研究的重要课题,也是本综述的主题.本综述首先简单介绍了石墨烯的转移方法;进而重点讨论由于转移而引入的各种污染物及其对石墨烯性质的影响,以及如何抑制污染物的引入或如何将其有效地去除;最后总结了石墨烯洁净转移所存在的挑战,展望了未来的研究方向和机遇.本综述不仅有助于石墨烯薄膜转移技术的研究,对整个二维材料器件的洁净制备也将有重要参考价值.     

Effect of electrical contact on performance of WSe2 field effect transistors

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe₂) have spead many interesting physical properties, which may become ideal candidates to develop new generation electronic and optoelectronic devices. In order to reveal essential features of 2D TMDCs, it is necessary to fabricate high-quality devices with reliable electrical contact. We systematically analyze the effect of graphene and metal contacts on performance of multi-layered WSe₂ field effect transistors (FETs). The temperature-dependent transport characteristics of both devices are tested. Only graphene-contacted WSe₂ FETs are observed with the metal-insulator transition phenomenon which mainly attributes to the ultra-clean contact interface and lowered contact barrier. Further characterization on contact barrier demonstrates that graphene contact enables lower contact barrier with WSe₂ than metal contact, since the Fermi level of graphene can be modulated by the gate bias to match the Fermi level of the channel material. We also analyze the carrier mobility of both devices under different temperatures, revealing that graphene contact can reduce the charge scattering of the device caused by ionized impurities and phonon vibrations in low and room temperature regions, respectively. This work is expected to provide reference for fabricating 2D material devices with decent performances.     

Dependence of the solar cell performance on nanocarbon/Si heterojunctions

Solar cells that combine single-crystalline silicon(Si) with graphene(G) have been widely researched in order to develop next-generation photovoltaic devices. However, the power conversion efficiency(PCE) of G/Si solar cell without chemical doping is commonly low due to the relatively high resistance of graphene. In this work, through combining graphene with carbon nanotube(CNT) networks, we fabricated three kinds of hybrid nanocarbon film/Si heterojunction solar cells in order to increase the PCE of the graphene based Si solar cell. We investigated the characteristics of different nanocarbon film/Si solar cells and found that their performance depends on the heterojunctions. Specifically, a doping-free G-CNT/Si solar cell demonstrated a high PCE of 7.9%, which is nearly equal to the combined value of two individuals(G/Si and CNT/Si). This high efficiency is attributed to the synergistic effect of graphene and CNTs, and can be further increased to 9.1% after applying a PMMA antireflection coating. This study provides a potential way to further improve the Si based heterojunction solar cells.     

二硫化钨/石墨烯异质结的界面相互作用及其肖特基调控的理论研究

采用第一性原理方法研究了二硫化钨/石墨烯异质结的界面结合作用以及电子性质,结果表明在二硫化钨/石墨烯异质结中,其界面相互作用是微弱的范德瓦耳斯力.能带计算结果显示异质结中二硫化钨和石墨烯各自的电子性质得到了保留,同时,由于石墨烯的结合作用,二硫化钨呈现出n型半导体.通过改变界面的层间距可以调控二硫化钼/石墨烯异质结的肖特基势垒类型,层间距增大,肖特基将从p型转变为n型接触.三维电荷密度差分图表明,负电荷聚集在二硫化钨附近,正电荷聚集在石墨烯附近,从而在界面处形成内建电场.肖特基势垒变化与界面电荷流动密切相关,平面平均电荷密度差分图显示,随着层间距逐渐增大,界面电荷转移越来越弱,且空间电荷聚集区位置向石墨烯层方向靠近,导致费米能级向上平移,证实了肖特基势垒随着层间距的增加由p型接触向n型转变.本文的研究结果将为二维范德瓦耳斯场效应管的设计与制作提供指导.     

Quantum transport in double-gated graphene devices

Double-gated graphene devices provide an important platform for understanding electrical and optical properties of graphene. Here we present transport measurements of single layer, bilayer and trilayer graphene devices with suspended top gates. In zero magnetic fields, we observe formation of pnp junctions with tunable polarity and charge densities, as well as a tunable band gap in bilayer graphene and a tunable band overlap in trilayer graphene. In high magnetic fields, the devices’ conductance are quantized at integer and fractional values of conductance quantum, and the data are in good agreement with a model based on edge state equilibration at pn interfaces.     

Artificially controlled synthesis of graphene intramolecular heterojunctions for phonon engineering

Because phonons are the main carriers for graphene heat transfer, modifying the dynamic properties of the crystal lattice by isotopes modulates the phonon behavior and alters the thermal properties. Here we demonstrate an artificially controlled texture synthesis of ¹²C‐graphene/¹³C‐graphene heterostructures via chemical vapor deposition and an O₂ plasma etching. The electrical and thermal properties of the graphene across the heterojunction show that ¹²C‐graphene and ¹³C‐ graphene are electronically connected as resistors in series, while the thermal conductivity across the junction is dramatically reduced due to the suppressed phonon propagation, which causes the conductivity across the junction to be lower than that of graphene sheets with randomly mixed isotopes. These findings should help realize novel two‐dimensional graphene thermoelectric devices where phonon modulation controls the electrons and heat transport independently. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of Nanoclay on the Morphology and Properties of Nylon 6 and Poly(Methyl Methacrylate) (PMMA) Blends

The effect of organomodified nanoclay on the morphology and properties of a (70/30 w/w) nylon 6/poly(methyl methacrylate) (PMMA) blend prepared by a melt processing method was investigated. The number average domain diameter (D_n ) of the dispersed PMMA phase was found to decrease with the addition of a small amount [0.5 per hundred resin (phr)] of clay in the blend. A much finer dispersion of the minor phase in the presence of a higher amount (5 phr) of clay indicated better mixing efficiency and improved morphology in the blend. X-ray diffraction indicated the exfoliation of the clays in the nylon 6 matrix, whereas PMMA chains only intercalated into the clay layers. However, the same effect of the clay was not observed in a (30/70 w/w) nylon 6/PMMA blend when nylon 6 became the dispersed domains. In the (30/70 w/w) nylon 6/PMMA blend, the addition of organomodified nanoclay (up to 2 phr) increased the D_n of the nylon 6 domains by preferential location of the clays inside the nylon 6 domains. Addition of styrene-maleic anhydride (SMA) copolymer effectively reduced the D_n of disperse phases in both compositions of the nylon 6/PMMA blends. Thus, in nylon 6/PMMA blends, clay platelets could prevent the coalescence of dispersed domains during melt mixing as long as it was dispersed in the matrix phase of the blend. Mechanical properties and thermal stability of the blends were also improved in the presence of clay.     

Adhesion,morphology, and heat resistance properties of polyurethane coated poly(methyl methacrylate)/fullerene-C60 composite films

Novel polyurethane (PU) adhesive was prepared and coated on poly(methyl methacrylate) (PMMA) and poly(methyl methacrylate)/fullerene (PMMA/Full-C₆₀) composite. Dip-coating technique was employed as facile and cost-effective procedure to coat polyurethane on film substrate. The properties of PU/PMMA and PU/PMMA/Full-C₆₀ composite were studied using Fourier transform infrared spectroscopy, Field Emission Scanning Electron Microscopy, tensile, adhesion, thermal and flammability measurement. Testing polyurethane-coated PMMA exhibited crumpled surface while fullerene addition formed unique pattern of dispersed spherical structures. Fullerene nanofiller loading improved the adhesion and mechanical properties of composite films due to polymer–carbon interaction. In PU/PMMA/Full-C₆₀ 0.5 composite with 0.5 wt.% nanofiller, tensile strength (71.4 MPa) was increased by 18.6% while tensile modulus was increased by 143.85% compared with PU/PMMA. In PU/PMMA/Full-C₆₀ 0.5, T₀ of 473 °C and T_max of 655 °C were observed. Increasing the fullerene content up to 0.5 wt.% decreased the peak heat release rate to 131 kW/m². Novel polyurethane-coated PMMA/Full-C₆₀ composite have potential applications as adhesive coatings in electronic and automotive appliances.