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Crystallization, glass transition, and molecular dynamics in PDMS of low molecular weights: A calorimetric and dielectric study
摘要： In this work linear polydimethylsiloxane (PDMS) of five low molecular weights, Wm ～2–8 kg/mol, i.e. mainly below the entanglement threshold, is studied employing differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), a special dielectric technique in the temperature domain, and broadband dielectric spectroscopy (BDS). It is shown mainly via raw data that the crystallization and glass transition for the larger Wms follow expected trends, however, for Wm ～2 k (corresponding to ～22 monomers/chain) crystallization is severely suppressed and slower. Moreover, it is demonstrated that upon isothermal annealing the crystalline fraction (CF) is maximized for all Wms with the exception of the lower one. For the latter, results indicate that during the annealing only the first stages of crystallization (nucleation, ordering of the polymer around the nuclei) take place, while the main crystallization event (formation of crystals) is recorded only upon heating, via the cold crystallization process. The suppressed crystallizability originates from the retarded nucleation. In particular for the lower Wm, the presence of large numbers of chain ends should be at the origins of hindered short range ordering of the polymer chains, as well as in relation to the difficult formation of physical crosslinks. At the same time, in TSDC and BDS uniquely for Wm ～2 k upon annealing and CF～0, next to the relaxation peaks related with the glass transition of the bulk-like polymer (α and αc), an additional quite strong peak (αnew) is recorded at higher temperatures. The overall characteristics of αnew relaxation resemble those of PDMS chains constrained and ordered in the interfacial layer in amorphous silica/PDMS nanocomposites (αint relaxation). Thus, combing the overall results with results from the literature, it is proposed that αnew is a direct recoding of retarded dynamics of the polymer chains being ordered around the crystallization nuclei or around extremely small crystals.
关键词： Polymer chains ordering,Polymer crystallization,Glass transition,Polydimethylsiloxane,Thermally stimulated depolarization currents,Differential scanning calorimetry
更新于2025-09-10 09:29:36
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Fabrication of 2D surface-functional polymer platelets via crystallization-driven self-assembly of poly(ε-caprolactone)-contained block copolymers
摘要： Fabrication of self-supported 2D organic materials with controllable surface properties is a challenge due to their thermodynamic instability. Herein, a series of crystalline-coil block copolymers poly(ε-caprolactone)-b-poly(acrylic acid) (PCL-b-PAA) with different compositions were synthesized for this purpose by the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and the subsequent atom transfer radical polymerization (ATRP) of tert-butyl acrylate, followed by treatment of trifluoroacetic acid (TFA). Two 2D polymer platelets with the spindle shape and the elongated truncated lozenge-like shape, respectively, were successfully fabricated via crystallization-driven self-assembly of the as-synthesized PCL-b-PAA. Effect of the properties of solvents and block copolymer compositions on the morphologies of 2D polymer platelets were carefully studied. The resultant 2D polymer platelets can quickly respond to environmental pH values due to the PAA chains tethered on surface of PCL crystals can undergo a different degree of ionization as pH changes. Moreover, Ag nanoparticles were also successfully loaded on the 2D polymer platelets by in situ reduction of AgNO3.
关键词： PCL,Crystallization-driven self-assembly,Surface-functional,2D polymer platelets
更新于2025-09-10 09:29:36
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Kinetics of liquid-mediated crystallization of amorphous Ge from multi-frame dynamic transmission electron microscopy
摘要： The kinetics of laser-induced, liquid-mediated crystallization of amorphous Ge thin films were studied using multi-frame dynamic transmission electron microscopy (DTEM), a nanosecond-scale photo-emission transmission electron microscopy technique. In these experiments, high temperature gradients are established in thin amorphous Ge films with a 12-ns laser pulse with a Gaussian spatial profile. The hottest region at the center of the laser spot crystallizes in (cid:2)100 ns and becomes nano-crystalline. Over the next several hundred nanoseconds crystallization continues radially outward from the nano-crystalline region forming elongated grains, some many microns long. The growth rate during the formation of these radial grains is measured with time-resolved imaging experiments. Crystal growth rates exceed 10 m/s, which are consistent with crystallization mediated by a very thin, undercooled transient liquid layer, rather than a purely solid-state transformation mechanism. The kinetics of this growth mode have been studied in detail under steady-state conditions, but here we provide a detailed study of liquid-mediated growth in high temperature gradients. Unexpectedly, the propagation rate of the crystallization front was observed to remain constant during this growth mode even when passing through large local temperature gradients, in stark contrast to other similar studies that suggested the growth rate changed dramatically. The high throughput of multi-frame DTEM provides gives a more complete picture of the role of temperature and temperature gradient on laser crystallization than previous DTEM experiments.
关键词： liquid-mediated crystallization,dynamic transmission electron microscopy,amorphous Ge,kinetics,laser-induced crystallization
更新于2025-09-09 09:28:46
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Shape Controllable Synthesis of Silver Particles by Selecting the Crystallization Routes
摘要： Classic crystallization describes a burst nucleation followed by a layer-by-layer atom deposition. The non-classic crystallization refers to particle mediated crystallization process. Different crystallization routes lead to the formation of diverse structured materials. Here we report a rational synthesis of silver particles by selecting the crystallization routes. Silver particles were synthesized by a solution reduction approach. The crystallization routes were regulated by adding amino acids to stabilize silver ions which leads to the decrease of the reduction rate. Without amino acids, silver dendrites were largely formed. With the addition of amino acids, flower-like (low concentration of amino acids) and spherical silver (high concentration of amino acid) particles were synthesized. Three kinds of amino acids were tested and the similar results were obtained. The time-dependent characterization on the evolution of silver particles showed that silver dendrites were formed by the classic atom deposition while the other two morphologies were formed by the combination of classic and non-classic crystallization. The silver particles synthesized were evaluated for ethylene epoxidation and the dendritic particles demonstrated a high selectivity.
关键词： catalytic selectivity,silver particles,reaction kinetics,shape control,crystallization
更新于2025-09-09 09:28:46
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Semiconducting Polymer Spherulites-From Fundamentals to Polymer Electronics
摘要： The control of the morphology of semiconducting semicrystalline polymers is crucial to the performance of various electronic devices. Among other superstructures in semicrystalline polymers, spherulites stand out for various reasons. They are highly ordered, relatively easy to grow, and their underlying molecular structure provides anisotropic optical and electronic properties of the resulting polymeric material. In this feature article, a short overview of classical crystallization theory is given as well as a brief introduction to spherulites as supermolecular structures. The article then focuses on semicrystalline polymers with semiconducting properties. From classical melt crystallization toward solvent vapor annealing, different strategies of growing spherulites of these types of polymers are presented and examples of the corresponding polymers and their resulting morphology are given. Eventually, the potential application of spherulitic thin films in organic electronics such as field-effect transistors is demonstrated. Conductivity and mobility measurements are shown, particularly focusing on the anisotropy of the latter.
关键词： optical properties,spherulites,conducting polymers,crystallization,electronic properties
更新于2025-09-09 09:28:46
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The Variation of Crystalline Structure Induced by Gas Dilution and Thermal Annealing in Silicon Layers Deposited by PECVD Technique
摘要： We prepared hydrogenated thick silicon film by plasma enhanced chemical vapor deposition (PECVD) method using SiH4 and H2 gas mixture and we investigated the effect of the hydrogen dilution ratio defined as R = H2/SiH4 on the as-deposited and annealed films. With increase in hydrogen dilution ratio, amorphous to microcrystalline transition has been observed. The crystallization has been confirmed from Raman spectroscopy, UV reflectance, low angle X-ray diffraction (XRD), spectroscopic ellipsometry and atomic force microscopy (AFM) analysis. Tauc band gap shows a decreasing trend with increasing H2 dilution of silane. It decreases from 1.8 to 1.57 eV. It has been concluded that H2 dilution of silane in PECVD enhances the crystallinity of the film and affects its optical and structural properties.
关键词： Silicon nanocrystallites,Hydrogen dilution,Thermal annealing,PECVD,Crystallization
更新于2025-09-09 09:28:46
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Mathematical Modeling of Droplet Formation, Evaporation, and Film Growth to Study Crystallite Size and Film Thickness of Spray Pyrolysis Deposited TiO<sub>2</sub> Thin Films
摘要： The pure TiO2 thin films were successfully deposited onto a glass substrate by using modified spray pyrolysis system. The TiO2 thin films were deposited at temperature 350?C using Titanium trichloride precursor solutions of concentration 0.075 M, 0.1 M, and 0.125 M. Also, TiO2 thin films with precursor solutions of concentration 0.1 M were deposited at different deposition temperature 350?C, 400?C, and 450?C. The prepared samples were further annealed at 500?C for 2 h to improve crystallinity. The XRD study revealed that the deposited TiO2 thin films were polycrystalline and showed anatase phase with pre-dominant (101) peak. The crystallite size calculated from XRD was found to increase with increase in precursor solution concentration. The mathematical model to calculate crystallite size and film thickness were developed and the predicted results were compared with experimental results. The experimentally calculated crystallite size and thickness was in good agreement with the predicted results. The details of mathematical model and calculations TiO2 thin films for were discussed in detail.
关键词： Growth,Liquid phase epitaxy,Crystallization,Titanium oxide,X-ray diffraction,Semiconducting films
更新于2025-09-09 09:28:46
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Single crystal hybrid perovskite field-effect transistors
摘要： The fields of photovoltaics, photodetection and light emission have seen tremendous activity in recent years with the advent of hybrid organic-inorganic perovskites. Yet, there have been far fewer reports of perovskite-based field-effect transistors. The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Here, we demonstrate a spatially-confined inverse temperature crystallization strategy which synthesizes micrometre-thin single crystals of methylammonium lead halide perovskites MAPbX3 (X = Cl, Br, I) with sub-nanometer surface roughness and very low surface contamination. These benefit the integration of MAPbX3 crystals into ambipolar transistors and yield record, room-temperature field-effect mobility up to 4.7 and 1.5 cm2 V?1 s?1 in p and n channel devices respectively, with 104 to 105 on-off ratio and low turn-on voltages. This work paves the way for integrating hybrid perovskite crystals into printed, flexible and transparent electronics.
关键词： hybrid perovskites,field-effect transistors,single crystals,ambipolar transport,inverse temperature crystallization
更新于2025-09-09 09:28:46
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[IEEE 2018 Iranian Conference on Electrical Engineering (ICEE) - Mashhad (2018.5.8-2018.5.10)] Electrical Engineering (ICEE), Iranian Conference on - Fabrication of P-Type Microcrystalline Silicon Thin Film by Magnetron Sputtering and Copper Induced Crystallization
摘要： P-type micro-crystalline Silicon thin film was realized by magnetron sputtering and copper-induced crystallization for photovoltaic applications. Firstly, amorphous Silicon film was deposited by direct current magnetron sputtering from highly-doped single crystalline Si target. Then it was crystallized by copper-induced crystallization in nitrogen atmosphere with the annealing temperatures ranges from 450 to 950 °C. The micro-crystalline Silicon thin film was characterized by X-ray diffraction and Ramon spectrometry. Its grain size and crystallization ratio were approximately 20 nm and 93%, respectively. Finally, a PN junction solar cell was fabricated by creating the P-type microcrystalline Si thin film (as the P region) on a highly-doped N-type Silicon wafer (as N region). The fabricated device showed the good rectification characteristics of a typical diode where under dark condition it represented the rectification ratio of 150 and reverse saturation current density of 9 μA.cm-2. The fabricated solar cell showed a significant photovoltaic effect under AM 1.5G illumination conditions. The highest photovoltaic conversion efficiency of 2.1%, with the open-circuit voltage of 416 mV and short-circuit current density of 13.3 mA/cm2, was measured from the sample fabricated by the optimal process.
关键词： magnetron sputtering,microcrystalline silicon thin film,copper induced crystallization,characterization
更新于2025-09-09 09:28:46
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Bi L<sub>III</sub>-Edge XAFS Study of GeBiTe Phase-Change Recording Material Using Actual Media
摘要： The crystallization of GeBiTe (GBT) is faster than that of the well-known phase-change recording material GeSbTe (GST). Therefore, the investigation of GBT structure as well as its crystallization process is attractive. Accordingly, the high-speed crystallization of GBT is due to Bi. Thus, it was necessary to know the local structure around Bi within the amorphous GBT. Note that an interface layer, which is a very thin dielectric film adjacent to recording film, assists the crystallization of phase-change material in the optical recording media. The local structure around Bi in GBT within an actual media was analyzed using XAFS. Bi LIII-edge XAFS spectra of crystalline and amorphous GBT respectively, both with and without interface layer in the media, were obtained. As result of this analysis on GBT, nearest neighbor atom of Bi was found to be Ge. Moreover, within the amorphous GBT, the interatomic distance around Bi is larger than that around Ge. These are the differences between GST and GBT. We speculate that these factors contribute to the improvement of the GBT crystallization speed. On the other hand, the interface layer doesn’t influence the local structure of GBT; however, it does have an electric effect on the recording layer.
关键词： interatomic distance,higher crystallization speed,rewritable optical recording media,Bi LIII-edge,GeSbTe(GST),phase-change material,actual media,EXAFS,interface layer,GeBiTe(GBT)
更新于2025-09-04 15:30:14