Melting of polymer single crystals studied by dynamic Monte Carlo simulations

We report dynamic Monte Carlo simulations of lattice polymers melting from a metastable chain-folded lamellar single crystal. The single crystal was raised and then melted in an ultrathin film of polymers wetting on a solid substrate, mimicking the melting observations made by using Atomic Force Microscopy. We observed that the thickness distribution of the single crystal appears quite inhomogeneous and the thickness increases gradually from facetted edges to the center. Therefore, at low melting temperatures, melting stops at a certain crystal thickness, and melting-recrystallization occurs when allowing crystal thickening; at intermediate temperatures, melting maintains the crystal shape and exhibits different speeds in two stages; at high temperatures, fast melting makes a melting hole in the thinnest region, as well as a saw-tooth-like pattern at the crystal edges. In addition, the linear melting rates at low temperatures align on the curve extrapolated from the linear crystal growth rates. The temperature dependence of the melting rates exhibits a regime transition similar to crystal growth. Such kinetic symmetry persists in the melting rates with variable frictional barriers for c -slip diffusion in the crystal as well as with variable chain lengths. Visual inspections revealed highly frequent reversals upon melting of single chains at the wedge-shaped lateral front of the lamellar crystal. We concluded that the melting kinetics is dominated by the reverse process of intramolecular secondary crystal nucleation of polymers.     

2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩(C8-BTBT)与MoS_2界面的能级匹配与薄膜生长

结合紫外光电子能谱(UPS)、X射线光电子能谱(XPS)和原子力显微镜(AFM)等实验手段系统研究了C8-BTBT沉积在层状Mo S2基底上的界面能级匹配、薄膜生长和分子取向。研究发现C8-BTBT分子竖直生长在Mo S2上,生长过程中界面的真空能级(VL)、最高占据态轨道(HOMO)和电离能(IP)都出现了非常规的弯曲现象。这种能级弯曲行为可归因于直立分子从界面相到体相的转变过程中,其分子倾斜角(θ)存在一定的渐变,这种渐变会在沿表面法线方向诱导出一系列的层间电偶极,最终导致能级的弯曲。同时θ的变化也会改变薄膜的表面极化强度,引起IP的逐渐减小。能级的弯曲在界面处形成类P-N结的效应会对C8-BTBT基电子器件的性能有很大的影响。     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

ZnS:Tm薄膜电致发光的激发过程

利用电子束蒸发方法制得ITO-Y2O3-ZnS:Tm3+-Y2O3-Al结构的薄膜可以得到蓝色电致发光.本文首次报导了ZnS:Tm3+薄膜与ZnS:Er3+、ZnS:Tb3+薄膜电致发光的激发过程有所不同.Tm3+离子的激发可以通过某些杂质中心到Tm3+离子的能量传递来实现.     

溶胶-凝胶法制备氧化锌薄膜的压电行为

"采用溶胶-凝胶技术在单晶硅Si(111)上制备了ZnO压电薄膜,并以扫描电镜、X射线衍射仪(XRD)和原子力显微镜(AFM)进行了表征．XRD衍射实验表明ZnO薄膜随着膜厚的增大,其(002)取向逐渐增强;AFM研究了薄膜的表面形貌、粗糙度与晶粒大小的结果表明,ZnO压电薄膜的粗糙度与晶粒寸随着薄膜厚度的增大而减小．粗糙度为2.188~0.914 nm．利用PFM研究压电系数,发现随着薄膜厚度的增加,(002)生长方向增强,压电系数逐渐增大;当力参数小于薄膜的表面粗糙度时,压电系数测量不准确且在较大幅度     

基于XRD的镀锌钝化膜残余应力试验研究

利用XRD技术测试了镀锌钝化膜结合界面的残余应力，同时通过电解抛光法检测了其厚度方向残余应力的分布规律，分析了残余应力对镀锌钝化膜结合强度的影响. 试验结果表明，镀锌钝化膜的残余应力均表现为压应力，并随着基体表面残余应力的增大而减小；钝化膜在2—10μm厚度方向的残应力为-274.5—-428.3MPa，其应力为梯度分布；镀锌钝化膜与基体的界面结合强度与其残余应力成反比，减小薄膜残余应力，有利于提高镀锌钝化膜与基体的结合强度. 关键词： X射线衍射法(XRD) 镀锌钝化膜 结合强度 残余应力     

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness d_t , while they were constant for the films thicker than d_t . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l _min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.     

Fracture force analysis at the interface of Pd and SrTiO3

The objective of this work is to develop an experimental indentation based method to determine the fracture force at the interface of Pd thin films and SrTiO₃ perovskite substrate. This paper reports on the results obtained for indentation into Pd thin films which were deposited in various thicknesses from 20 nm to 200 nm under vacuum and 300 °C substrate temperature by an electron beam physical vapor deposition. Initially, the relation between grain size, elastic module and hardness was considered as a function of film thickness. Thereafter, in developing new method, oscillating indentation was performed with different applied forces and oscillating times in order to measure the critical fracture force in each thickness. The effect of oscillating time on plastically deformed regions surrounding an indentation was schematically explained in conjunction with variation of oscillating time to determine the interfacial properties of the Pd thin film. Furthermore, the accuracy of the critical fracture force was ensured by applied force versus piling up height plot. The method is validated experimentally for the soft thin films over the hard substrate. However, further study would be essential to measure the film adhesion by means of fracture force at the interface.     

分数维方法研究GaAs薄膜中的极化子

本文采用分数维方法, 在讨论Al_0.3Ga_0.7As衬底上GaAs薄膜的分数维基础上, 计算了GaAs薄膜中的极化子结合能和有效质量. 随着薄膜厚度的增加, 极化子结合能和质量变化单调地减小. 当薄膜厚度L_w<70 Å并且衬底厚度L_b<200 Å时, 衬底厚度的变化对薄膜中极化子的结合能和质量变化的影响比较显著, 随着衬底厚度的增加, 薄膜中极化子的结合能和质量变化逐渐变大; 当薄膜厚度L_w>70 Å或者衬底厚度L_b>200 Å时, 衬底厚度的变化对薄膜中极化子的结合能和质量变化的影响不显著. 研究结果为GaAs薄膜电子和光电子器件的研究和应用提供参考. 关键词： 分数维方法 GaAs薄膜 极化子 低维异质结构     

准分子激光增强高分子材料粘着力的机理研究

 由热方程计算激光与液体薄膜材料相互作用时的温度场,分析高分子材料表面熔化及材料表面液体膜汽化产生的材料的表面粗化;近一步分析高分子的断裂、F原子 替代等光化学反应;从而提出激光增强高分子材料的粘着力的表面粗化和光化学反应的机理。     

低温制备微晶硅薄膜生长机制的研究

采用热丝化学气相沉积技术制备了一系列处于不同生长阶段的薄膜样品，用原子力显微镜系 统地研究生长在单晶硅衬底和玻璃衬底上薄膜表面形貌的演化.按照分形理论分析得到：在 玻璃衬底上的硅薄膜以零扩散随机生长模式生长；而在单晶硅衬底上，薄膜早期以有限扩散 生长模式生长，当膜厚超过某一临界厚度时转变为零扩散随机生长模式.岛面密度与膜厚的 依赖关系表明，在临界厚度时硅衬底和玻璃衬底上的岛面密度均出现了极大值.Raman谱的测 量证实，玻璃衬底上薄膜临界厚度与非晶/微晶相变之间存在密切的关系.不同的衬底材料直 接影响反应 关键词： 生长机制 微晶硅薄膜 表面形貌 热丝化学气相沉积     

Near-Field Birefringence Response of Liquid Crystal Molecules in Thickness Direction of Liquid Crystal Thin Film Orientated by Shear Force

Information of molecular orientation in nematic liquid crystal （LC） is attractive and important for applications in the field of display devices. We demonstrate a novel method using a birefringence scanning near-field optical microscope （Bi-SNOM） with a probe which is inserted into the LC thin film to detect the molecular orientation from its birefringence responses in the thickness direction of the LC thin film. The probe is laterally vibrated when going forward into the LC thin film, and the retardation and azimuth angle are recorded as the probe going down. Firstly, the thickness of the LC thin film is measured by the shear force detection. Since the shear force acts as a stimulation to reorientate the LC molecules above the substrate surface, we can detect the molecular orientation caused by a polyimide alignment substrate and the effect to molecular orientation caused by vibration of fibre probe. As a result, the orientation profiling of the LC film in depth direction is obtained in both the cases that the direction of probe vibrating is vertical/parallel to the rubbing direction of the alignment film. Furthermore, the thickness of completely orientated layers just above the substrate surface can also be obtained by either vibrating probe or no-vibrating probe. Ultimately, the LC thin film can be modelled in thickness direction from all the results using this method.     

Photochemical surface modification of PET by excimer UV lamp irradiation

UV irradiation has interesting potential for the photochemical modification of polymers. In order to study cross-linking effects and/or thin-layer deposition following a treatment in the presence of bi-functional media or in inert atmosphere, irradiation of PET in various atmospheres was performed using a KrCl excimer lamp. Surface properties were investigated by atomic force microscopy, nanoindentation, micro-thermal analysis, and X-ray photo-electron spectroscopy. The studies reveal that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus are strongly affected by UV irradiation in the presence of bi-functional media. Films treated in octadiene and argon show an increase of surface modulus, much less expansion, and lower soft/melt temperatures, which is an indication of the surface cross-linking effect and a decrease of crystallinity within the near-surface layer. In the case of a diallylphthalate-treated film, depending on the local structure, either a strong decrease of melting temperature or no melting point is found, which is attributed to the irregular cross linking and thickness of the modified layer associated with a decrease of surface modulus. A significant increase of the alkali resistance is found after irradiation, as a result of both wetting and cross-linking effects on the polymer surface.     

Non-monotonic dependence of temperature of Au nanometer films dissociation into droplets on their thickness on Al2O3 surface

The process of melting with consequent disintegration into droplets of various thickness Au thin films has been studied. An unexpected character of the melting–disintegration temperature dependence on thin-film thickness has been found: a reduction of the process temperature has been observed with thickness in the range from 100 to 20 nm; however, the disintegration temperature starts to increase for Au film thicknesses lower than 20 nm. It is supposed that the observed non-monotonic behavior of the process temperature is caused by the influence of the substrate interaction with the film, resulting in an entropy change of the system.     

A Thin Liquid Film and Its Effects in an Atomic Force Microscopy Measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy （AFM） measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height yoc, at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of yoc with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H^＊ physically defining what a thin film is; namely, once the film thickness H is the same order as H^＊, the effect of film thickness should be taken into account. The value of H^＊ is dependent on Hamaker constants and liquid surface tension as well as tip radius.     

Li~+对硅酸盐玻璃中Yb~(3+)、Tm~(3+)上转换效率的影响

利用高温熔融法制备了不同Li~+含量的掺杂Yb~(3+)、Tm~(3+)的硅酸盐玻璃样品,玻璃样品在980 nm半导体激光器的泵浦下能够产生477 nm的蓝光和654 nm的红光。由上转换的荧光强度和泵浦功率的双对数拟合直线得到,添加Li~+能够增加上转换过程中Tm~(3+)向上跃迁的几率,Tm~(3+)的3F2和1G4能级的粒子数布居增加,从而提高辐射跃迁的几率。红外光谱和拉曼光谱表明Li~+的添加对玻璃基质的声子能量影响不大,却使得声子态密度减小,使上转换效率提高,从而提高Yb~(3+)、Tm~(3+)在硅酸盐玻璃中的发光强度。     

Exchange bias of Co/CoO multilayers deposited on nanosphere array

The [Co/CoO]₅ multilayer nanocap arrays are fabricated on the colloidal sphere arrays which are prepared on the Si substrate by the self-assembly technology. Compared to bilayer film, the H_EB of multilayer film is larger than that of the bilayer film. The increase of H_EB for multilayer should be ascribed to the interface increase between FM and AFM layers. In the multilayer film structure, H_EB of the nanocap array is bigger than that of the flat film, which is attributed to the decrease of FM layer thickness, the decrease of grain size, and the increase of structural defects caused by curved substrate. And the typical step is observed in flat, and the step is reduced significantly in the nanocap array due to the enhancement of interfacial coupling between neighbor FM layers. For the [Co/CoO]₅ multilayer nanocap array, H_EB increases first, and then decreases when CoO sublayer thickness changes. When CoO sublayer thickness is 15?nm, H_EB reaches its maximum. This result may be related to the topography of nanostructure multilayer on curved substrate.     

Inelastic neutron scattering study of low energy excitations in polymer thin films

We report inelastic neutron scattering measurements on polystyrene thin films in a glassy state in the meV region. We found in elastic scattering that the mean square displacement decreased with film thickness, and hence the corresponding force constant f increased. In inelastic and quasielastic scattering, we observed the so-called boson peak at around 1.5 meV and the picosecond fast process for the first time in thin films, both of which decreased in intensity with film thickness. These results were discussed in terms of the potential hardening due to the confinement of polymer chains and/or the interfacial dead layer.     

含活性剂液膜去润湿演化的稳定性特征

针对含非溶性活性剂的液膜在固体基底上的去润湿过程,基于润滑理论建立了基态和扰动态下液膜厚度和表面活性剂浓度的演化模型,应用非模态理论分析了演化过程的稳定性特征,探讨了分子间力对液膜去润湿过程的影响. 研究表明,微扰动波的引入（k=1）有利于液膜去润湿过程的稳定进行,扰动能量逐渐衰减,然而,该效果随着扰动波数的增加而显著改变,k ≥ 2时,液膜演化的稳定性反而恶化,扰动能量被逐步放大,演化呈现出非稳定特征. 增大初始液膜厚度可以有效改善液膜流动的稳定性. 范德华力放大了液膜表面的微扰动,使得液膜演化的稳定性下降;相反,Born斥力和静电斥力具有增强去润湿稳定性的作用. 关键词： 活性剂 去润湿 分离压 稳定性     

The preparation and evaluation of graded multilayer ta-C films deposited by FCVA method

In this study, a series of graded multilayer ta-C films were investigated by varying their sublayer thickness ratios, in which each film sublayer was prepared at different substrate bias by filtered cathode vacuum arc (FCVA) method. The experimental results show that the graded multilayer film structure can effectively decrease the internal stress level of deposited ta-C film, and meanwhile the graded multilayer ta-C films still have high sp³ fractions. The applied substrate bias voltage and sublayer thickness ratio can apparently influence the microstructure characteristics and internal stress of the graded multilayer ta-C films. The graded multilayer ta-C film has larger sp³ fraction when applying a larger negative substrate bias voltage and having a thicker outer sublayer during the film deposition process. However, the internal stress in the as-deposited film also increases with larger thickness of the outer sublayer, and the optimal ratio of sublayer thicknesses is 1:1:1:1 for graded ta-C film with four sublayers.