凹槽铜基底表面与单层石墨烯的相互作用特性研究

基于石墨烯二维材料的诸多应用需要将其大面积、高质量地转移到目标基底上,迫切需要了解石墨烯在剥离和转移过程中与基底之间的相互作用特性.本文采用经典分子动力学方法探索了铜基底表面凹槽的几何特征尺寸对石墨烯吸附和剥离过程中凹槽基底对石墨烯吸附作用的影响机理和规律.结果表明:对于固定边界条件下的单层石墨烯,当基底表面的凹槽宽度固定不变时,吸附过程中基底对石墨烯的吸附力随二者间距的减小,呈现先增大后减小的趋势;其最大吸附力随凹槽深度的增加而增大,而当凹槽深度继续增大至石墨烯未能吸附进入凹槽底部的临界值时,吸附力迅速减小;剥离过程中,石墨烯完全剥离的临界作用力随凹槽深度的增加同样呈现先增大后减小的趋势,且与剥离前石墨烯与凹槽基底的相互作用面积有关;当基底表面凹槽的深度固定不变时,吸附和剥离过程中石墨烯-基底之间的吸附力随间距的变化规律取决于石墨烯在基底凹槽处的稳态吸附构型.     

不同温度条件下单层石墨烯纳米带弛豫性能的分子动力学研究

分别采用Tersoff-Brenner势和AIREBO势,对三种长宽比的单层石墨烯纳米带在不同热力学温度(0.01—4000 K)下的弛豫性能进行了分子动力学模拟.对基于两种势函数模拟的石墨烯纳米带弛豫的能量曲线和表面形貌进行了分析对比,研究了石墨烯纳米带在弛豫过程中的动态平衡过程.模拟结果表明:单层石墨烯纳米带并非完美的平面结构,边缘处和内部都会呈现一定程度的起伏和皱褶,这与已有的实验结果相符合;石墨烯纳米带的表面起伏程度随长宽比的减小而减小,并且在不同温度条件下,系统动能对石墨烯纳米带的弛豫变形的影响很大,即系统温度越高,石墨烯纳米带的弛豫变形幅度愈大;高长宽比纳米带在一定温度条件下甚至会出现卷曲现象.最后,对采用Tersoff-Brenner势和AIREBO势进行石墨烯的分子动力学模拟进行了深入分析.     

多孔石墨烯时间维度反渗透滤盐机理研究

时间维度选择性反渗透原理虽然克服了反渗透膜微孔尺寸的限制,一定程度上突破了渗透性和选择性之间的平衡,但多层反渗透膜时间维度的滤盐机理尚未明晰.本文采用分子动力学方法,揭示了多孔石墨烯反渗透膜的厚度和剪切速度对盐水反渗透特性的影响规律.结果表明,随着多孔石墨烯反渗透膜旋转速度的增加,离子截留率增加而水通量先增加后降低;反渗透膜厚度的增加会提高离子截留率,但阻碍了水通量的上升.本文创新性地对三层石墨烯反渗透膜上的纳米孔结构进行了设计研究,发现梯度孔结构在保证高选择性的同时提高了渗透性;供给端最内层纳米孔径的变化对水通量的影响最为显著,水通量随该孔径的增加而快速上升.研究结果进一步阐明并验证了时间维度反渗透滤盐机理,利用梯度孔的设计提升了相同膜厚度情况下的水通量,为大尺度滤盐设备的设计研发提供了理论基础.     

转角多层石墨烯层间耦合的共振拉曼光谱研究

我们通过共振拉曼光谱测量了转角多层石墨烯的层间振动模式:剪切模和呼吸模。根据改进的线性模型,我们发现在转角多层石墨烯界面处的层间呼吸耦合与正常Bernal堆垛多层石墨烯的强度相当。此结果明显不同于层间剪切耦合,后者在转角多层石墨烯界面处的层间剪切耦合减弱到了正常Bernal堆垛多层石墨烯的20%。另外,我们首次发现层间呼吸耦合存在着次近邻原子层之间的相互作用,其强度为最近邻的9%。我们发现当采用与界面层间旋转角度相对应的激发光时,转角多层石墨烯的拉曼信号得到极大的增强。为此,我们引入光学跃迁允许的电子联合态密度的概念,通过理论计算,我们发现这种联合态密度的极大值决定了拉曼信号共振线型的激发光能量极值。本研究表明,层间振动模式是探测二维层状异质层间耦合的有效手段,为其在器件应用方面的研究奠定了基础。     

基底支撑刚度梯度变化对石墨烯层间摩擦力的影响

基于纳米摩擦能耗理论,利用分子动力学方法建立了公度接触下支撑刚度梯度变化的石墨烯层间摩擦力模型,分析了基底质心刚度和支撑刚度梯度变化对基底和薄片各接触区摩擦能耗的贡献.结果表明:软边界区始终贡献驱动力;硬边界区贡献的摩擦力最大,且随着支撑刚度的增大,硬边界区对总摩擦的贡献比也越高.各接触区的摩擦力是薄片和基底之间的褶皱势和接触区产生的法向变形差两部分的共同作用.前者是公度接触下阻碍滑移的界面势垒和刚度梯度方向上不同刚度支撑原子热振动引起的势梯度;后者是接触边界过渡区两侧原子的非对称变形和自由度约束突变引起的非平衡边界势垒相耦合的结果.本文对研究公度接触下刚度梯度支撑的纳米器件的相对运动规律有指导意义.     

基于原子力显微镜的石墨烯表面图案化摩擦调控

摩擦可调控的石墨烯作为固体润滑剂在微/纳机电系统中具有巨大的应用潜力.本文采用导电原子力显微镜对附着在Au/SiO₂/Si基底上的石墨烯进行氧化刻蚀,比较了在不同刻蚀参数下石墨烯纳米图案的摩擦性能,并且通过开尔文力显微镜分析了不同刻蚀参数对纳米图案氧化程度的影响.结果表明:施加负偏压可以在石墨烯表面制造出稳定可调的氧化点、线等纳米级图案,氧化点的直径和氧化线的宽度都随着电压的增大而增大;增加石墨烯的厚度可以提高纳米图案的连续性和均匀性.摩擦力随着针尖电压的增大而增大,这是由于电压增大了弯液面力和静电力.利用这些加工的纳米级图案可以精确地调控石墨烯表面的摩擦大小.通过导电原子力显微镜刻蚀技术实现石墨烯表面纳米摩擦特性的可控,为石墨烯在微/纳米机电系统中的摩擦行为研究和具有图案表面的纳米器件的制备提供了新的思路和方法.     

导电石墨烯孔道内双电层结构的研究

为有效开发和利用新能源,人们迫切需要高性能的超级电容器提供能量的存储和转换.在超级电容器中双电层结构扮演着关键性的角色.本文利用分子动力学方法通过建立开放的石墨烯纳米孔道(1～2 nm),研究了KCl溶液在纳米孔道内的双电层结构,同时也比较了恒电量模拟(Q)和恒电势模拟法(U)下双电层结构的异同.结果表明在恒电势模拟法考虑了导电石墨烯壁的镜像作用使结果更符合实验中的材料系统.而石墨烯壁的镜像作用能额外吸附离子从而增强孔道内部的阴阳离子,这可能有助于电极电容的提升.通过对不同孔道高度的研究,本文发现水分子作为介电材料在水基超级电容器中发挥着决定性的作用.它能在很大程度上抵消不同离子和不同孔道高度下双电层的变化,从而在不同情况下获得了相似的电容.     

石墨烯厚度与其力-距离关系的实验和模拟研究

通过原子力显微镜高度曲线确定高度分别为0.57 nm、0.90 nm和1.30 nm三个石墨烯片,随后进行多次力-距离曲线测量.结果发现:三个石墨烯片吸附力数值的平均值分别为0.30 nN、0.32 nN和0.34 nN,脱附力的平均值分别为5.33 nN、5.66 nN和7.24 nN.实验显示吸、脱附力随石墨烯厚度...     

多层石墨烯的表面起伏的分子动力学模拟

运用经典分子动力学方法,研究了呈现不同堆积方式的多层石墨烯在不同温度下的表面起伏,并且和单层、双层石墨烯做对比,计算发现:室温下,多层石墨烯中存在着横向特征尺寸约为100 A的起伏,该尺寸会随着温度的升高而增大;同时,起伏的高度也随着温度的升高而增大,这些石墨烯的层内起伏高度关联函数都遵从幂指数标度行为G_h(q)αq~(-α),对于同一种石墨烯,温度越高幂指数越小;而在同一温度下,不同堆积方式的石墨烯的幂指数也不同,所有这些特征都来源于温度以及层间耦合作用引起的非谐效应。     

单层单晶石墨烯与柔性基底界面性能的实验研究

单晶石墨烯具有更优异的力学及电学性能,有望成为新一代柔性电子器件的核心材料.因此,有必要从实验的角度精细分析化学气相沉积法制得的大尺度单晶石墨烯与柔性基底复合结构的界面力学行为.本文通过显微拉曼光谱实验方法测量了不同长度的单层单晶石墨烯/PET(聚对苯二甲酸乙二醇酯)基底的界面力学性能参数及其在长度方向上界面边缘的尺度效应.实验给出了石墨烯在PET基底加载过程中与基底间黏附、滑移、脱黏三个界面状态的演化过程与应力分布规律.实验发现,单晶石墨烯与柔性基底间由范德瓦耳斯力控制的界面应变传递过程存在明显的边缘效应,并且与石墨烯的长度有关.界面的切应力具有尺度效应,其值随石墨烯长度的增加而减小,而石墨烯界面传递最大应变以及界面脱黏极限则不受试件尺度的影响.     

Effective boundary slip and wetting characteristics of water on substrates with effects of surface morphology

ABSTRACT

In the present study, molecular dynamics (MD) simulation was used to investigate the relationship between wetting behaviour and slip length on patterned substrates. We adopted two solid surfaces of Si(100) and graphite due to similarities in their intrinsic contact angle. Contact angle and apparent slip length were obtained using discrete simulations with the same thermodynamic states. In the present study, a number of questions regarding surface roughness and the problem of contact angle (θ) and slip length (L_s) are discussed. These questions include the relationship between θ and surface roughness, the characteristics used to describe the difference between static and dynamic fluid fields and the reason for a lack of multilayer sticking observed in the current cases. Our results indicate that the quasi-universal θ ? L_s equation proposed by Hung et al. (2008) is applicable to cases involving a Cassie-like nanoscale roughened surface. In contrast, in cases with a Wenzel-like nanostructure, the no-slip boundary conditions are independent of variations in the contact angle. The adoption of a Wenzel–Cassie hybrid model helped to verify that the fluid density inside the cavity is a critical indicator of wettability of the wall–fluid interface. Our results also demonstrate that ρ_{f, cav} is a critical property in the measurement of hydrodynamic effects and thus its importance as an indicator of the validity of the equation θ ? L_s. The average time that water molecules are trapped and the number of averaged hydrogen bonds within cavities in a dynamic fluid field were also investigated.

     

Ultrasound assisted reduction of graphene oxide to graphene in l-ascorbic acid aqueous solutions: Kinetics and effects of various factors on the rate of graphene formation

The reduction of graphene oxide (GO) to graphene (rGO) was achieved by using 20 kHz ultrasound in l-ascorbic acid (l-AA, reducing agent) aqueous solutions under various experimental conditions. The effects of ultrasound power, ultrasound pulse mode, reaction temperature, pH value and l-AA amount on the rates of rGO formation from GO reduction were investigated. The rates of rGO formation were found to be enhanced under the following conditions: high ultrasound power, long pulse mode, high temperature, high pH value and large amount of l-AA. It was also found that the rGO formation under ultrasound treatment was accelerated in comparison with a conventional mechanical mixing treatment. The pseudo rate and pseudo activation energy (E_a) of rGO formation were determined to discuss the reaction kinetics under both treatment. The E_a value of rGO formation under ultrasound treatment was clearly lower than that obtained under mechanical mixing treatment at the same condition. We proposed that physical effects such as shear forces, microjets and shock waves during acoustic cavitation enhanced the mass transfer and reaction of l-AA with GO to form rGO as well as the change in the surface morphology of GO. In addition, the rates of rGO formation were suggested to be affected by local high temperatures of cavitation bubbles.     

Molecular dynamics study of the shear strength and fracture behavior of nanoporous graphene membranes

We perform molecular dynamics (MD) simulations to study the structural response and fracture characteristics of nanoporous graphene (NPG) membranes subjected to shear loading. The effects of porosity, temperature, and shear velocity on the mechanical responses of NPG membranes are examined. The results show that the wrinkling of the membrane becomes more obvious with increasing strain. Fractures occur around holes on the long diagonal of the NPG parallelogram, and fracture stress in the NPG membrane decreases with increasing porosity. In addition, the effect of shear velocity on the shear modulus decreases with increasing porosity. The fracture strain of NPG membranes with different porosities obviously decreases with increasing temperature. The results enhance our understanding of the shear mechanical properties of NPG membranes and are helpful for the design and application of high-performance NPG membranes.     

Synthesis and characterization of graphene quantum dots and their size reduction using swift heavy ion beam

Graphene quantum dots (GQDs) are nanosized fragments of graphene displaying quantum confinement effect. They have shown to be prepared from various methods which include ion beam etching of graphene. However, recently the modification of the GQDs has garnered tremendous attention owing to its suitability for various applications. Here, we have studied the effect of swift ion beam irradiation on the properties of GQDs. The ion beam treatment on the GQDs exhibited the change in observed photoluminescence of GQDs as they exhibited a blue luminescence on excitation with longwave UV (≈365?nm) due to the reduction in size and removal of the ethoxy (–C–O–C–) groups present on the quantum dots. This was confirmed by transmission electron microscopy, particle size analysis, and Fourier transform infrared spectroscopy.     

Nitrogen doping effects on the structure of graphene

Graphene and nitrogen doped graphene have been prepared by modified Hummers’ method and the following ammonia heat-treatment process, respectively. The effects of N-doping on the structure of graphene have been systematically investigated by various characterization techniques. SEM, TEM, BET, Raman and XRD analysis were used to distinguish the difference of the microstructures; and FT-IR, XPS, especially XANES were performed to elucidate the bonding information such as C-N. The effect of nitrogen doping on the structure of graphene has been obtained. More defects are present on nitrogen doped graphene as elucidated by BET, XRD, Raman, and XANES characterizations. XANES analysis also indicates that the N-doping decreases the surface oxygen-containing groups.     

The effect of deposition temperature on the surface coverage and morphology of iron-phosphate coatings on low carbon steel

The influence of deposition temperature and concentration of NaNO₂ in the phosphating bath on the surface morphology and coverage of iron-phosphate coatings on low carbon steel was investigated. The phosphate coatings were chemically deposited on steel from phosphate bath at different temperatures (30-70 °C) and with the addition of different amounts of accelerator, NaNO₂ (0.1, 0.5 and 1.0 g dm⁻³). The morphology of phosphate coatings was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of iron-phosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique.It was shown that the increase in temperature of the NaNO₂-free phosphating bath up to 70 °C caused an increase in surface coverage. The addition of NaNO₂ in the phosphating bath significantly increased the surface coverage of phosphate coatings deposited at temperatures lower than 50 °C. The phosphate crystals were of laminated and needle-like structures for deposits obtained at temperatures lower than 50 °C, while at higher temperatures needle-like structure was transformed to laminated structure. The increase in NaNO₂ concentration in the phosphating bath from 0.1 to 1.0 g dm⁻³ did not significantly increase the surface coverage, but decreased the crystals size, consequently favouring the phosphate nucleation and better packing of the crystals.     

Theoretical and numerical studies of non-equilibrium slip effects on a catalytic surface

A new concentration slip model to describe the rarefied gas effect on the species transport in microscale chemical reactors was derived from the approximate solution of the Boltzmann equation. The present model is more general and recovers the existing models in the limiting cases. The analytical results showed that the concentration slip is dominated by two different mechanisms, the reaction induced concentration slip (RIC) and the temperature slip induced concentration slip (TIC). The magnitude of RIC slip is proportional to the product of the Damköhler number and Knudsen number. The impact of the velocity, concentration and temperature slips on the coupling between the surface catalytic reactions and the homogeneous gas phase reactions was examined using the detailed chemistry of hydrogen and methane within a wide range of accommodation coefficients in a two-dimensional microscale chemical reactor. The results showed that the impact of reaction induced concentration slip (RIC) effects on catalytic reactions strongly depends on the Damköhler number, the Knudsen number and the surface accommodation coefficient. It was found that the TIC slip had a strong effect on the fuel oxidation rates and the RIC slip dramatically changed the mass fraction distributions of radicals, especially when the mass accommodation coefficients were far less than unity.     

Molecular investigation of water adsorption on graphene and graphyne surfaces

In this paper the adsorption action of a water droplet on the graphene and graphyne externals has been examined. Conclusions received from the calculation of the water contact angle on the graphene and the graphynes surfaces have demonstrated that graphyne is more hydrophobic than graphene. Sketching the contour maps of the water interaction showed different behaviors of water droplet on these surfaces. The results show that water molecules, form a sub_layer of water on the graphyne substrate while this sub_layer does not exist on the graphene. Molecular investigations of the water on the surfaces show that the attendance of a sub_layer of water on the substrate can cause changes, such as the number of hydrogen bonds per water molecule in the water droplet, the order of molecules in different layers of water droplet, and parallel forces to the surface between surface water molecules and substrate, in the structural properties of water droplet. In this study the interaction between first layer and sub-layer of water was investigated. Water drops on surface can affect on the behavior of water sub-layer.     

Mechanical properties of thin films of graphene materials: A study on their structural quality and functionalities

Several studies have been done on physiochemical properties of thin films of graphene materials, but less on their mechanical properties. The mechanical properties such as tensile and storage modulus of films of graphene oxide (GO), different reduced graphene oxides (rGO), functionalised reduced graphene oxide (frGO) and a few layers graphene (graphene) were analysed in this study. During syntheses processes, a range of variations occurs due to different reducing agents and functionalising components used; this affects or changes the mechanical properties of the materials. In addition, it has become vital to comprehend the mechanical properties of these films as the potential applications such as sensor and electrodes demand extended life cycles or lifetime. It has been found that the ultimate tensile strength (UTS), tensile modulus, and storage modulus vary across all the samples that highly depend on nature/efficiency of reducing agent used, amount of impurities such as oxygen functional groups and defect density such as discrepancies/holes in the aromatic structure. The highest UTS and modulus have been identified with a few layers graphene and with hydroiodic acid reduced GO among the rGOs. The frGO shows almost similar properties to that of graphene.     

Effects of different parameters on thermal and mechanical properties of aminated graphene/epoxy nanocomposites connected by covalent: A molecular dynamics study

This paper is devoted to studying the thermal and mechanical properties of aminated graphene (AG)/epoxy nanocomposites connected by covalent bond using molecular dynamics (MD) simulation. The effects of crosslinking degree, mass fraction and functionalized graphene (FG) type on AG/epoxy nanocomposites are considered. The elasticity modulus (E), the glass transition temperature (T_g), the coefficient of thermal expansion (CTE) and the interfacial energy (E_int) are also investigated. The MD simulation results indicate that, when the mass fraction of AG is between 1.2% and 3.1% and crosslinking degree reaches about 70%, the E, T_g, E_int and CTE of AG/epoxy nanocomposites are significantly improved compared with those of pure epoxy and graphene/epoxy nanocomposites. The reason is that AG not only possesses some excellent thermodynamic properties of graphene, but also has the function of curing agent to crosslink with epoxy monomer to form the carbon-nitrogen (C–N) covalent bond. A better interfacial interaction between nanoparticles and epoxy is essential in enhancing the thermal and mechanical properties of nanocomposite materials, which will provide a microscopic theoretical basis for the study of epoxy nanocomposites.