Vol. 65, No. 17 (2016)
SPECIAL TOPIC — Progress in Soft Matter Research
2016, 65 (17): 174201. doi: 10.7498/aps.65.174201
Liquid-crystal polymers in confined system is a fundamental issue in soft matter. Theoretical method plays animportant role in studying these systems. The intention of this work is to give a thorough reviewof the theoretical methodologies used in tackling confined liquid crystals. At first, some basic concept of liquid crystal, such as a vital order parameter for orientation, phases of liquid crystal, the uniaxial and biaxial of liquid crystal, are presented. After that, a brief review of the development of liquid-crystal theories, which include the Onsager model, the Maier-Saupe model, the McMillanmodel, the Landau-de Gennes expansion, the Frank elastic model and the self-consistent field model for liquid-crystal polymers, are given. All these theories havetheir own advantages and disadvantages. For example, the phenomenological Frank elastic model is the most widely used model due to its simplicity. In contrast, parameters in the self-consistent field model are physically meaningful, however, it is rather complicated. During recent decades, with these theories and suitable boundary treatment, plenty confined liquid crystal systems are investigated. In this review, we focus on three kinds of confined systems: 1) the surface wetting behavior in slits; 2) the two-dimensional liquid crystals confined by a boundary line and 3) defects in the orientational field of rigid rods on spherical surface. Results arrived from different At the end of this review, we give a list of frontier issues and an outlook for thecoming ten years.
From the perspective of mesoscale, the formation mechanism of crystal network structure of supramolecular gel, the influence of structure on macroscopic properties, and the design and control of supramolecular gels are reviewed Crystal network is a key character of the hierarchical structure of the gel, the formations of the basic and multi-level crystal networks are based on the crystal nucleation and growth. The engineering and controlling of the gel structure can be implemented by various stimuli, such as additives, sonication, seeding, and thermodynamic driving force, which leads to a controllable performance of the gel In addition, the methods of characterizing supramolecular gels are systematically summarized, such as, rheology, atomic force microscope, scanning tunnel microscope, scanning electron microscope, transmission electron microscope, polarizing optical microscope, X-ray diffraction, small-angle X-ray scattering, small-angle neutron scattering, nuclear magnetic resonance spectroscopy, dynamic light scattering etc. Supramolecular gel performance is determined by the hierarchy mesoscopic structures, which can significantly improve the properties of the material. Four factors can be correlated to the structure and performance of material: topology, correlation length, symmetry/ordering, and strength of association of crystal networks. According to the more in-depth understanding of mesoscopic supramolecular gels, the research and development of such a material will be pushed to a new stage.
Colloidal particles in solution exhibit rich phase behaviors and behavior like big-atom. In the past decades, as modelling systems, colloids have been widely employed in the study of nucleation, crystallization, glass transition and melting. A number of advances have been achieved. These advances to a large extent extend and complete the understanding of various phase transitions. Recently, a number of active fields are emerging with colloidal model systems. In this review, the advances and the emerging fields are summarized. At the end, the potential directions and the challenges for future studies are suggested.
Nowadays, although our understanding on liquid water have lots of progresses due to the development of experimental tools and computer simulation techniques, the molecular level structure of water, its heterogeneity, is still elusive. In the end of the nineteenth century, Rntgen proposed that the water is a mixture of two molecular complexes, which cannot be confirmed by experiments at that time. In the middle of the twentieth century, Bernal and his followers regarded the structure of liquid water as a random tetrahedral network, which was widely accepted by most scientists. With the development of computer science and the discovery of several amorphism, more and more attentions are paid on the mixture model of liquid water. In this paper, we firstly review some latest evidences about the multiple types of local structure in liquid water in both simulations and experiments. In all-atom simulation, the distributions of the local structure index obtained by minimizing the energy of samples are double peak at all temperatures. In experiment, the X-ray emission spectroscopy of liquid water at ambient pressure shows that there are two local structures in liquid water, one is order and ice-like, the other one is disorder and gas-like. Secondly, some results of our group on this topic are presented. We transformed the Raman spectra into the high-dimensional vectors and analyze the vectors with the principal component analysis method. The results show that all the end points of vectors are in a line in the high-dimensional space which implies that they can be obtained by linearly combining two basic points in that line. This means that the Raman spectra can be decomposed into two basic spectra. We also perform the same analysis on the distributions of tetrahedral order parameter in liquid water and obtained similar results. It is an obvious signal of the existence of multi-component in liquid water. Finally, we introduce the mixture model of liquid water which can be used to explain the thermodynamic properties of liquid water. In the mixture model, the form of the Gibbs free energy of liquid water is the same as the binary regular solution. The free energy is a function of the concentration of the disorder local structure. The anomalies of liquid water are directly caused by the change of concentration of the disorder local structure. In the low temperature and high pressure region, the mixture model has a critical point, which is consistent with the liquid-liquid phase transition theory.
The nanotechnology has emerged as an effective tool to fabricate next-generation microelectronics, biologically responsive materials, and structured membranes. The self-assembly of nanoscale phases has extensively been studied in thin films because of their potential applications in sub-100 nm structures. The control of the ordering of nanaoscale patterns is critical for various technological applications. A variety of approaches such as topographical and chemical patterning have resulted in an enhancement in long-range orders of nanoscale patterns. The macroscopically large areas of nanoscale domains with single-crystal order in polymer thin films can be utilized to fabricate portable ultra-high density data storages, advanced sensors and ultra-light electronic devices. However, as pattern size decreases below 100 nm, there appear many new challenges such as the cost of patterning and the precise control of the line edge roughness and line width roughness. Precisely controlling nanostructure shapes and placements in material is a continuing challenge. Measurement platform to provide accurate and detailed information about nanostructure orientations and placements is a key to this challenge. In this review, we examine the recent progress of characterization tools in polymer thin films. We highlight our efforts to control surface pattern formations of polymer thin films and our use of statistically-useful scattering techniques and real-space imaging tools to quantify the order of nanoscale patterns. In some technological applications of biological membranes, such as chemical separations, drug delivery and sensors, the orientation distribution of nanostructures is often more important. The real-space imaging methods of characterizing the orientation distribution of nanostructures, such as cross-sectional electron microscopy measurements and depth profiling by alternating etch and surface imaging steps are readily performed on thin polymer films over large areas. However, these real-space imaging techniques are destructive measures of nanostructures in polymer thin films. Also it is challenging to in-situ measure the evolution of orientation of nanoscale patterns during processing by using these destructive real-space imaging techniques. Rotational small-angle neutron scattering (RSANS) and grazing-incidence small-angle x-ray scattering (GISAXS) are effective and non-destructive measurement tools to measure the evolution of orientation distribution of nanoscale patterns during processing. In this rotational small angle neutron scattering method, the sample is rotated in the neuron beam. By accumulating the scattering density at each sample rotation angle, the three-dimensional Fourier space of the internal ordering in the nanostructured film can be mapped. By using this relatively new rotational small angle neutron scattering method and established models for nanoscale patterns, the full three-dimensional orientation distribution of nanoscale patterns can be obtained.
A review of correlative modeling for transport properties, microstructures, and compositions of granular materials in soft matter
2016, 65 (17): 178101. doi: 10.7498/aps.65.178101
The transport property of granular material that is a typical of soft matter, plays a significant role in durability and service life in a relevant practical engineering structure. Physical properties of material is generally dependent on its microstructure. Meanwhile, the formation of microstructure is directly related to compositions of granular material. Understanding the intrinsic mechanisms of composition, microstructure, and transport property are of great importance for improving mechanical properties and durability of granular material. In this article, we review the new progress of modeling transport properties of granular multiphase materials. We focus on the three main aspects involving the simulations for geometrical models of composition structures, the quantitative characterizations for microstructures of pore and interface phases, and the theoretical and numerical strategies for transport properties of granular multiphase materials. In the first aspect, in-depth reviews of realizing complex morphologies of geometrical particles, detecting the overlap between adjacent non-spherical particles, and packing randomly non-spherical particles are presented. In the second aspect, we emphasize the development progress of the interfacial thickness and porosity distribution, the interfacial volume fraction, and the continuum percolation of soft particles such as compliant interfaces and discrete pores. In the final aspect, the modeling the transport properties and the frontier issues of the effective diffusion and anomalous diffusion in granular multiphase materials are elucidated. Finally, some conclusions and perspectives for future studies are provided.
2016, 65 (17): 178103. doi: 10.7498/aps.65.178103
Heat transportation is one of the most ubiquitous phenomenon in the mother nature. Manipulating heat flow at will is of tremendous value in industry, civil life and even military. It would be a common sense that in different materials thermal properties are different. According to this knowledge people may design thermal materials to control heat conduction. One of the most common and successful example is blanket, which has been invented for thousands of years to keep us warm in cold days and keep icecream cool in summer. However, those great inventions are not powerful enough to manipulate heat flow at will. So there are still a lot of demands for designing the so-called metamaterials which have special properties that should not exist in nature. In 2006, Leonhardt and Pendry's research group (Pendry, Schurig and Smith) independently proposed a type of optical metamaterial which is also called invisible cloak. This device is well known for bending light around an object to make it invisible. Such a significant progress soon enlightened a lot of scientists in different aspects since it offers a powerful approach to design metamaterials. The principle of invisible cloak, which is concluded as transformation optics has been applied to light waves, acoustic, seismic, elastic waves, hydrodynamics and even matter waves as they all satisfy with wave equation. Although the conduction equation which governs the process of heat conduction is totally different from wave equation, from 2008 to 2012, Fan's group and Guenneau's group established the theoretical system of transformation thermotics. Since then, many thermal metamaterials with novel thermal properties have been figured out. Therefore, a boom in transformation thermotics and thermal metamaterials has begun. In this article, we will introduce some most recent achievements in this field, including novel thermal devices, simplified experimental method, macro thermal diode based on temperature dependent transformation thermotics, and the important role that soft matters play in the experimental confirmations of thermal metamaterials. These works pave the developments in transformation mapping theory and can surely inspire more designs of thermal metamaterials. What is more, some approaches proposed in this article provide more flexibility in controlling heat flow, and it may also be useful in other fields that are sensitive to temperature gradient, such as the Seebeck effect and many other domains where transformation theory is valid.
2016, 65 (17): 178106. doi: 10.7498/aps.65.178106
biocompatibility. Considering the critical role of DNA less than 150 base pairs (bp) in cellular processes such as regulated gene expression, quantifying the intrinsic bend ability of DNA on a sub-persistence length scale is essential to understanding its molecular functions and the DNA-protein interaction. From the classical point of view, double-stranded DNA is assumed to be stiff and can be treated by semi-flexible chain, but recent studies have yielded contradictory results. A lot of studies tried to prove that the worm-like chain model can be used to fully describe DNA chain. However, recent theoretical and experimental studies indicated that DNA exhibits high flexibility on a short length scale, which cannot be described by the worm-like chain model. Further studies are needed to address the extreme flexibility of DNA on a short length scale. On the basis of the predictability of the double helical structure and the Watson-Crick binding thermodynamics for DNA, a class of DNA reactions can be defined, called toehold-mediated strand-displacement reaction, in which one complementary single-stranded DNA sequence first binds to the dangling toehold domain of the substrate in a pre-hybridized double-stranded DNA, then triggers the strand-displacement reaction, and finally results in the dissociation of the third strand previously bound to the substrate with partial complementarity. In dynamic DNA nanotechnology, isothermal toehold-mediated DNA strand-displacement reaction has been used to design complex nanostructure and nanodevice for molecular computation. The kinetics of the strand-displacement can be modulated using the toehold length. In order to weaken the coupling between the kinetics of strand-displacement and the thermodynamics of the reaction, the concept of toehold exchange was introduced by Winfree et al. to improve the control of strand-displacement kinetics. More importantly, the biomolecular reaction (BM) rate constant of toehold exchange can be analytically derived using the three-step model. Through utilizing strand-displacement reactions and taking advantage of its programmable sequences and precise recognition properties, DNA can be used to build complex circuits which can proceed robustly at constant temperature, achieving specific functions. DNA strand-displacement reaction can be employed to fabricate logic gates, and large and complex circuits for DNA computing, to mimic the naturally occurring occurrence of biological systems. Based on that, DNA circuit can then be used to direct the assembly of nanodevice following the designed pathway, and modulate the chemical reaction networks on the surface of living cell or in cellular systems for biosensing, even program the cellular machinery in the future for genetic diagnostic or gene therapy. In the present paper, we reviewed the proceedings in the fields of DNA structure and conformational changes, and DNA flexibility, discussed the mechanism of DNA strand-displacement reaction at the molecular level, and introduced the recent studies in DNA computation as well as the dynamic DNA nanotechnology, such as self-assembly.
2016, 65 (17): 178201. doi: 10.7498/aps.65.178201
The continuum version of the wormlike chain model (WLC), which was initially developed by Saito, Takahashi and Yunoki in 1967, is particularly suitable for description of polymer conformational properties affected by the chain rigidity. The WLC model is capable of covering an extensive range of chain rigidity, from the flexible chains to the rigid chains, by tuning the persistence length directly. It is widely accepted as a coarse-grained model that can be used to capture the physical properties, such as conformation and structures, of a larger class of real polymers than the Gaussian chain (GSC). Recently, the WLC model attracts increasing interests because of its advantages in studying a variety of polymeric systems, including liquid crystalline polymers and conjugated polymers. This review article focuses on applications of the WLC model, incorporated in the framework of self-consistent field theory, which is an effective method in theoretical exploration of phase separation in polymer systems. The article also pays particular attention to the developments of the numerical schemes to solve the modified diffusion equation governing the probability distribution of polymers. In addition, we summarize recent applications of the self-consistent field theories based on WLC model in the following three areas: phase transitions in liquid-crystalline polymers; the influence of surface curvature on polymeric systems involving the chain orientation effects; self-assembly of wormlike block copolymers. These studies are beyond the scope of self-consistent field theories based on a GSC model, which have been utilized in a large number of theoretical studies in recent years. Finally, we propose the perspectives of theoretical developments in field-theory simulations based on the WLC model for future work. In the polymer literature, it is generally appreciated that chain-rigidity is an important factor that influences the properties of structural stabilities on the meso-scale. The theoretical studies indentify the key physical mechanisms that play crucial roles in many experimental systems with attractively promising applications in practice, for systems such as liquid crystalline polymers and organic solar cell based on the conjugated polymers.
2016, 65 (17): 178301. doi: 10.7498/aps.65.178301
Biomimetic microfluidic systemscan be obtained through multidisciplinary approaches by using bio-inspired structural and functional designsfor the microfluidic devices. This review mainly focuseson the concept of biomimetic microfluidics to improve the properties of microfluidic systemsfor breaking through the bottlenecks of the current microfluidic devices, such as antifoulingsmart, anddynamic response insidethe microchannelsunder different environments. In addition, herewe showthecurrentresearch progress ofbiomimetic microfluidic systems in applicationsrelatedtoantifoulingandsmartdevices, andbiomedicalresearch The review discusses both physical theories and critical technologies in thebiomimetic microfluidics, from biomimetic design to real-worldapplications, so as to offer new ideas for the design and application of smart microfluidics, and the authors hope this review will inspire the active interest of many scientists in the area of the development and application of soft matter, and multi-functional and smart biomimetic devices.
Predicting 3D structure of proteins from the amino acid sequences is one of the most important unsolved problems in computational biology and biophysics. This review article attempts to introduce the most recent effort and progress on this problem. After a brief introduction of the background and basic concepts involved in protein structure prediction, we went through the specific steps that have been taken by most typical structural modeling approaches, including fold recognition, model initialization, conformational search, model selection, and atomic-level structure refinement. Several representative structure prediction methods were introduced in detail, including those from both template-based modeling and ab initio folding approaches. Finally, we overview the results shown in the community-wide Critical Assessment of protein Structure Prediction (CASP) experiments that have been developed for benchmarking the state of the art of the field.
2016, 65 (17): 178702. doi: 10.7498/aps.65.178702
We investigate the dynamics of actin monomers that are assembled into long filaments via the particle-based Brownian dynamics simulations. In order to study the dynamics of long filaments containing up to several hundred protomers, a coarse-grained model for actin polymerization involving several simplifications is used. In order to overcome the large separation of time scales between the diffusive motion of the free monomers and the relatively slow polymerized and depolymerized processes at the two ends of the filaments, all polymerized and depolymerized rates are rescaled by a dimensionless parameter. Actin protomers within a filament generally possess three nucleotide states corresponding to a bound adenosine triphosphate (ATP), adenosine diphosphate with inorganic phosphate (ADP. Pi), and ADP molecules in the presence of ATP hydrolysis. Here in this paper, single nucleotide state and two nucleotide states of actin protomers are described by the simplified theoretical model, giving the dependence of the growth rate on actin concentration. The simplest case where all protomers are identical, is provided by the assembly of ADP-actins. In the simulations, the growth rate is found to increase linearly with free monomer concentration, which agrees quantitatively with in vitro experimental result. These surprised phenomena observed in the experiments, such as treadmilling processes and length diffusion of actin filaments at the steady state, are presented in detail by Brownian dynamics simulations. For free actin concentrations close to the critical concentration, cT ccr, T, the filaments undergo treadmilling, that is, they grow at the barbed end and shrink at the pointed end, leading to the directed translational motion of the filament. In the absence of ATP hydrolysis, the functional dependence of a length diffusion constant on ADP-actin monomer concentration implies that a length diffusion constant is found to increase linearly with ADP-actin monomer concentration. With the coupling of ATP hydrolysis, a peak of the filament length diffusion as a function of ATP-actin monomer concentration is observed i. e. , the length diffusion coefficient is peaked near to 35 mon2/s below the critical concentration and recovers to the expected estimate of 1 mon2/s above the critical concentration. These obtained results are well consistent with the experimental results and stochastic theoretical analysis. Furthermore, several other quantities and relations that are difficult to study experimentally but provide nontrivial crosschecks on the consistency of our simulations, are investigated in the particle-based simulations. The particle-based simulations developed in our studies would easily extend to study a variety of more complex systems, such as the assembly process of other dynamic cytoskeletons
Bacteria form a complex system. It consists of many components that cover broad size scales, including ions, small molecules, DNA, polymers, sub-micrometer sized organelles and compartments, micrometer sized cells, packs of cells in films of a few micrometers in thickness, large swarms or populations spanning plates over several centimeters in diameter, etc. The mechanisms to be explored span a wide range of time scales from micro-second or shorter for molecular interaction, to milli-second or longer times for diffusion and transport, up to minutes and hours for cellular metabolism, growth, and reproduction. An invisible colony of bacteria can grow rapidly and becomes visible to the human eye in several hours. Novel phenomena or behaviors emerge across these broad size and time scales. For example, the rotation direction and speed of a flagella motor, about 50 nm in diameter, are both tightly regulated by a signaling pathway within the cell. The fast rotation of the helical flagellum driven by the rotary motor is a key to explaining the bacterial swimming trajectory, chemo-taxis, accumulation, adhesion, or anchored body rotation near or at a solid surface. The activities of individual bacteria in response to their physicochemical environment give rise to their collective response such as quorum sensing, swarming, and growth of biofilms. The physical biology of bacteria is an interdisciplinary research covering micromechanics, micro-fluidics, non-equilibrium statistical physics, etc. This review covers several aspects of bacterial motility, including flagella motor behavior, bacterial swimming and accumulation near the surface, the self-organized patterns of bacterial swarms, and chemo-taxis regulated by the biochemical signaling network inside bacteria. Instead of presenting each aspect as a separate topic of microbiological study, we emphasize the strong relations among these topics, as well as the multidisciplinary perspective required to appreciate the strong relations among the topics covered. For instance, we point out the relevance of numerous phenomena in thin film fluid physics to bacterial swarming, such as capillary flow, surface tension reduction by surfactant, Marangoni flow, and viscous fingering. Another notable example is a recent application of a statistical mechanical theory called the first passage time theory to account for the intervals between the switches of bacterial motor rotation from clockwise to counter-clockwise, and vice versa. In concluding remarks, we point out a few open questions in the field of bacterial motility and likely advances that might transform the field. The central view conveyed through this review article is that further progress in the field demands interdisciplinary efforts. Therefore, a collaborative approach among those with both in depth knowledge and broad perspectives in biological and physical sciences will prove to be the most successful ones.
2016, 65 (17): 178704. doi: 10.7498/aps.65.178704
Cancer, as a conundrum, is currently the biggest killer of human health. The major viewpoint of carcinogenesis is owing to somatic gene mutations. Based on such a viewpoint and the development of gene sequencing technology, extensive genomic alterations in cancer genomes have been identified. How to develop a better understanding of the link between gene mutations and carcinogenesis as well as efficient clinical cancer therapy is therefore a major challenge. Weinberg and Hanahan have suggested 10 hallmarks of cancer. The hallmarks are highly regulated by the corresponding signaling pathways. Thus, cancer itself is also a disease of dysfunction of signal transduction pathways related to multiple fundamental cell processes, including proliferation, differentiation, apoptosis, invasion and so on. Despite the signaling pathways are extremely complex in cancer cells, one can still focus on the signaling networks that govern the corresponding cell processes for modeling to discuss its dynamics and regulation functions quantitatively. Systems biology provides appropriate approach to integrate the experimental data (clinical data) and signaling pathway for a comprehensive analysis, resulting in a further prediction for optimal therapy and drug discovery. In this paper, we review the recent progress of dynamical modeling of signaling networks by using systems biology approaches that help to exploring the mechanisms of carcinogenesis. We first discuss the motif dynamics of the signaling networks. The presented generic circuit model can be decomposed into two loops and the circuit can achieve tristability through four kinds of bifurcation scenarios when parameter values are varied in a wide range. Then, we show the relative well-studied core signaling networks that regulate the cell survival, apoptosis, proliferation, invasion and energy metabolism processes. For each fundamental cell process, we individually review the dynamics of corresponding signaling network based on the systems biology approaches, including the NF-B signaling pathway that regulates the cell survival process, the Ras signaling pathway that governs the cell proliferation process, the EMT and mitochondrial signaling pathway that modulate the cell invasion and apoptosis processes. Furthermore, two coupled signaling networks, i.e., the p53 and TNF- signaling networks are discussed. Lastly, we review the breast cancer and gastric cancer signaling networks which contain several fundamental cell processes. The potential contribution for cancer treatment is also suggested. These dynamical modeling based on the core signaling networks can facilitate the understanding of the mechanisms of carcinogenesis and provide us the possible clues and ideas of the cancer treatment and drug design. We believe more exciting research works in this field will be stimulated in the near future.
2016, 65 (17): 178705. doi: 10.7498/aps.65.178705
Combining precise microscopic measurement with quantitative image analysis, video microscopy has become an important, real space experiment technique to study the microscopic properties of soft matter systems. On the one hand, it provides a basic tool to observe and record the microscopic world. On the other hand, it offers a essential experiment method to study the underlying physics of the microscopic world. This paper reviews the development of the video microscopy, introduces the corresponding hardware and video processing software, and summarizes the typical applications, and recent progresses of video microscopy in colloidal suspensions. The future of the video microscopy in the soft condensed matter physics and interdisciplinary research is discussed.
2016, 65 (17): 170201. doi: 10.7498/aps.65.170201
Artificial micro-scale or nano-scale machines that are capable of converting energy to mechanical work, have long been pursued by science and engineering communities for their potential applications in microfluidics, biology and medicine. From a physics point of view, they are also ideal models to investigate fundamental statistical phenomena in non-equilibrium active matters. Inspired by bio-machines and bio-motors like ATP synthase and flagellum motors, we propose a simple design of rotary motors based on pure self-diffusiophoresis effects. The basic design of the rotor consists of three colloidal beads with different surface properties, which leads to different interactions between the beads and solvent molecules. Chemical reactions are imposed on the surface of one of the beads, which creates a source of one of the two solvent molecules and generates a local concentration gradient. The other two beads connected to the catalytic bead have different affinities to the solvent molecules, which leads to asymmetric diffusiophoretic forces on the two non-catalytic beads. A net torque is thus obtained from difference of the diffusiophoretic forces between the two non-catalytic beads. In our simulation, we employ hybrid molecular dynamics (MD) simulations and multi-particle collision dynamics (MPC) to investigate the motion of microrotors. The binary fluid is composed with A-type and B-type solvent particle whose interactions are described by multi-particle collision dynamics while beads-particle interactions are modeled by molecular dynamics. In MPC, all fluid particles execute alternating streaming and collision steps. During streaming steps, the solvents move ballistically. During collision steps, particles are sorted into square cells and only interact with particles in the same cell under a specific stochastic rotation rule. MPC algorithm locally conserves mass, linear momentum, angular momentum and energy, and properly captures thermal fluctuation, mass diffusion, dissipation and hydrodynamic interactions. In our simulation, standard MPC parameters are employed which correspond to a liquid-like behavior of fluid. In MD, beads-solvent interactions are described by Lennard-Jones(LJ) potential with different parameter combinations and the equations of motion is integrated by velocity-Verlet algorithm. To perform hybrid molecular dynamic simulations with multi-particle collision dynamics, between two MPC collision steps, 50 MD steps are implemented for the solvent particles that are in the interaction range of colloidal beads. We first investigate the solvent concentration distribution around static microrotor, and confirm that the catalytic bead generates a steady-state local concentration gradient. Net angular displacements are obtained when the rotor is allowed to rotate freely. The rotational direction and speed of the micorotor are determined by bead-solvent interactions, the rotor geometry, the solvent viscosity and the catalytic reaction ratio. We also study the scenario in which two rotors are placed in close vicinity to each other. We find that the coupling between the concentration fields around the rotors reduces the rotational speed of both rotors.
Effects of Dzyaloshinskii-Moriya interacton and decoherence on entanglement dynamics in Heisenberg spin chain system with different initial states
2016, 65 (17): 170301. doi: 10.7498/aps.65.170301
With considering the intrinsic decoherence, the dynamic behaviors of quantum entanglement in a three-qubit XXZ Heisenberg system with Dzyaloshinskii-Moriya (DM) interaction and anisotropy for different initial states are investigated. The research result shows that the anisotropy parameter does not affect the system entanglement, however, the intrinsic decoherence has obvious inhibitory effect on entanglement. When the initial state of system is an entangled state, we can obtain the stable value of entanglement by adjusting DM interaction parameters appropriately. As the system initial state is a separation state, entanglement oscillates, and the amplitude of oscillation decays with time periodically, and there will appear the death phenomenon after each oscillation, and with time going on, its concurrence will be zero. When the initial state is entangled, by choosing the proper DM parameter, the three pairs of entanglements oscillate with time and eventually approach to a steady value. The increase of accelerates the decay of concurrence. When the initial state is separated, entanglement oscillates, and the amplitude of oscillation decays with time periodically, and there will appear the death phenomenon after each oscillation, with time going on, its concurrence will be zero. Therefore, the proper initial state and DM interaction parameters can control the concurrence effectively under the intrinsic decoherence, thereby obtaining the preferable entanglement resource.
2016, 65 (17): 170501. doi: 10.7498/aps.65.170501
In this paper the synchronization problem for fractional-order chaotic system with unknown external disturbance is investigated by adaptive fuzzy control. Based on the fractional Lyapunov stability theorem, an adaptive fuzzy controller, which is accompanied with fractional adaptation law, is established. Fuzzy logic system is used to approximate an unknown nonlinear function. The fuzzy approximation error can be canceled by the proposed fractional adaptation law. Just like the stability analysis in an integer-order chaotic system, the quadratic Lyapunov function is used to analyze the stability of the fractional-order closed-loop system. The control method can realize good synchronization performances between two fractional-order chaotic systems, and the synchronization error tends to zero asymptotically. Besides, the proposed controller can also guarantee the boundedness of all signals in the closed-loop system. Finally, the numerical simulation results illustrate the effectiveness of the proposed control method for fractional-order chaotic system in the presence of the external disturbances.
2016, 65 (17): 170701. doi: 10.7498/aps.65.170701
We make a machine that can perform as an invisible hand able to write and draw smoothly accompanied with the incidental music. And this machine can be used in the commercial advertising display, artmobile poster writing, the accessory equipment of the multimedium classrooms, stage effect, new art pattern especially in dark. We present a new display application of the long lag phosphor (LLP) material in this paper. A prototype is fabricated which can be written, drawn and displayed by controlling a laser beam on the screen which is made from the LLP material. For selecting the match laser beam wavelength for different LLP material screens, the energy band structure of the LLP material Mn(H2PO4)2 of 3-4 eV band gap is calculated by VASP (Vienna abinitio simulation package) software and its Raman shift peaks of Mn(H2PO4)2 are tested at 625 nm, 769 nm, 1049 nm and in far infrared wavelength range. The intensity of powdery LLP SrAl2O4: Eu2+, Dy3+, which is tested by the instrument of UWLA(ultra-weak luminescence analyzer), can decay from 43479 to 9570 electronic counts in 5 min, and then descend slowly. The intensity decay of coated film LLP Mn(H2PO4)2, which is tested by the instrument of HANDYSCOPE HS3, can decay quickly at the beginning and then slowly after 400s. These intensity decay results can explain that LLP materials of SrAl2O4:Eu2+, Dy3+ and Mn (H2PO4)2 are suitable for displaying the image by our prototype. A prototype is successfully made by our group for writing English and Chinese words and drawing picture. Arduino Board is used to control two step motors, and X mirror and Y mirror are rotated to reflect the laser beam. An excitation dot is formed on the surface of LLP display screen. By drawing vectorgragh with Coreldraw and convert it into .nc file, the computer runs G-code in CNC (computer numerical control) automatically. Arduino controlled mirror rotation drives the laser beam. The trace of the laser dot is left on the screen and becomes article or graphs in afterglow. The whole device can be energy saving, eyes comfortable, low cost and easy to pick up.
ATOMIC AND MOLECULAR PHYSICS
Bonding nature of the amorphous structure studied by a combination of cutoff and electronic localization function
2016, 65 (17): 173101. doi: 10.7498/aps.65.173101
The analysis of the local structure of covalent glass is one of the major challenges for analyzing the amorphous structure. Usually, people use a cutoff distance to determine the coordinated atoms and relevant structural information, such as coordination number and bond angles. Recently, the electron localization function (ELF) has been used to analyze the local structure of amorphous Ge2Sb2Te5. But how to determine the EFL threshold and cutoff distance has not been reported. Here, according to the ab-initio calculations, we systematically investigate the relationship between the bond number and the ELF threshold, and also the cutoff distance in amorphous GeTe. The reasonable value of the ELF threshold and the cutoff distance are determined according to the inflection point and slope change of the bond number with ELF value respectively. Furthermore, the minimal ELF value distributions of Ge-Ge, Ge-Te and Te-Te bonds are presented. The comparison shows that the majority of removed bonds in structural analysis are weak Ge-Te bonds due to the low localization degree of electron. In contrast, the stronger Ge-Ge bonds are almost unchanged when changing the ELF threshold value from 0.58 to 0.63 because of the high localization degree of electron. The average minimal ELF value of Ge-Te bonds in crystalline GeTe is calculated, and it is close to the ELF threshold that is determined by the inflection point. t is easy to find that the Ge-Te bonds which are removed by increasing the ELF threshold are relatively weak. Therefore, these weaker bonds should be removed in structure analysis, which also means that the ELF threshold determined by the inflection point are reasonable value. Finally, based on the EFL threshold value, the coordination number and bond angle distribution of Ge in amorphous GeTe are obtained. The analysis of the coordination number of the Ge atoms shows that as the ELF threshold increases from 0.58 to 0.63, the 5- fold Ge atoms almost disappear because they are against the (8-N) rule. Furthermore, when the ELF threshold value is 0.58, the bond angle distribution analysis of Ge atoms shows that the local structure is a configuration that is mainly defectively octahedral (3-fold Ge) and distorted tetrahedral (4-fold Ge), but it remains unchanged when the threshold value increases to 0.63. It further demonstrates that all the removed chemical bonds are weaker ones as the ELF threshold increases. This approach is useful to improve the accuracy of amorphous structure analysis by obtaining the more reasonable inter-atomic bonding information. And it should be applied to the structural analyses of other systems generally.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (17): 174101. doi: 10.7498/aps.65.174101
Circulators are widely used microwave components that rely on magnetic materials. They have been a subject of extensively theoretical and experimental development for over 50 years. Nowadays, commercial circulators require ferrite and external bias magnetic field to realize circulation performance. However, ferrite circulators suffer major drawbacks: they are too heavy, incompatible with integrated circuit technologies, expensive, sensitive to temperature, etc. So, it is very hard to further improve the characteristic of traditional ferrite circulator. And it is important to overcome the major drawbacks of the traditional ferrite circulator. In this paper, the anomalous refraction feature of the phase gradient metasurface is utilized to realize nonreciprocal characteristics. Magnetless circulator based on phase gradient metasurface is proposed and then analyzed. The circulator consists of phase gradient metasurfaces and a three-port waveguide. Three metasurfaces are arranged into 60-degree angle with respect to each other. The metasurface shows high efficiency in anomalous refraction. With the help of phase gradient metamaterial, the signal can only be refracted to the next port in rotation along one direction. That makes the circulation performance. To design and optimize the circulator for better circulation performance, the numerical simulations are performed using the full-wave electromagnetic simulator CST Microwave Studio 2013. To verify the design of the circulator based on phase gradient metasurface, the circulator is fabricated using waveguide and metasurfaces. The scattering parameters of the magnetless circulator based on phase gradient metasurface are measured using a vector network analyzer (Agilent N5230 A). The measured S-parameters show that the circulator exhibits good circulation performances at a frequency of 20.8 GHz. At 20.8 GHz, the insertion loss is 0.8 dB. And the return loss and isolation degree can reach -10 dB. In this paper, a new method is used to design the circulators. This work makes it possible to reduce the weight of the device. Moreover, it is also insensitive to temperature. Therefore, we can make a conclusion that the magnetless circulator based on phase gradient metasurface has potential value in application. However, there is still lots of work to do to improve the performance of the circulator. In future work, we will use wideband metasurfaces to broaden the bandwidth, improve the isolation degree, reduce the insertion loss, and reduce the return loss. And free space can be lead into the circulator to reduce the bulk of the circulator and improve the circulation performance.
Research on coordinate transformation design of a cylinderical acoustic cloak with pentamode materials
2016, 65 (17): 174301. doi: 10.7498/aps.65.174301
The pentamode material, similar to fluid in physical properties, serves as a useful way for the physical implementation of the anisotropic fluid. Based on the similarity, a method to design cloak with the pentamode materials has been put forward by Norris. To analyze the effect factors and rules of the stealth performance of the cloak, the present article is focused on the studying of the coordinate transformation equation of the pentamode cloak design of Norris. Cloaks with different materials parameters distribution can be achieved by adjusting coordinate transformation equations. There are four kinds of the distribution of pentamode cloak material parameters: the density equation being constant, the modulus equation being constant, the density equation being, power equation and the modulus equation being power equation. The average visibility is considered as the standard of stealth effect and is calculated with different coordinate transformation equations by using the software COMSOL. The average visibility is used to analyze the relationship between stealth effect and coordinate transformation equations. The relationship between the coordinate transformation equation and the route of acoustic wave transmission, the relationship between the materials of obstacle and the stealth effect, and the relationship between the route of acoustic wave transmission and the stealth effect are studied. Two results are achieved by comparing these relationships mentioned above. The first is that the stealth effect of a cloak with aluminum obstacle is worse than one with water obstacle. The reason lies in the impedance mismatch between the aluminum and the cloak material. The second result shows that the coordinate transformation equation is related to the distribution of material parameters and the route of acoustic wave transmission and it can affect the scattering property of the cloak. When the route of acoustic wave transmission is close to inner surface of cloak, the stealth effect is relatively poor, while when the route of acoustic wave transmission is close to outer surface of cloak, the stealth effect is relatively well. The reason is that when the route of acoustic wave transmission is close to inner surface of cloak, the acoustic wave affects the obstacle which leads to the enhancement of the scattering of obstacle. So when designing the cloak, not only the physical realization of the cloak material but also the distributed situation of the route of acoustic wave transmission should be considered. And the route of acoustic wave transmission is decided by the coordinate transformation equation. Therefore the stealth performance can be improved by applying proper coordinate transformation equation.
Experimental investigation on the starting vortex induced by symmetrical dielectric barrier discharge plasma actuator
2016, 65 (17): 174701. doi: 10.7498/aps.65.174701
Flow control using plasma actuator is a promising research field of aeronautical applications. Due to its low energy consumption, rapid response and simple construction, this actuator has been investigated in various aerodynamics problems, such as boundary layer flow control, drag reduction, lift enhancement, noise reduction, and flow separation control. In order to understand the controlling mechanism of plasma actuator, many researchers have been carried out some experiments on the plasma actuator characterization in quiescent air and obtained the evolution process of starting vortex induced by plasma actuator. But the plasma actuator always works under flow condition. Therefore, understanding the interaction process between the starting vortex and incoming flow is a key to promote this technology development. In this paper, the starting vortex induced by symmetrical Dielectric Barrier Discharge (DBD) plasma actuator in quiescent air or under flow condition was investigated using Particle Image Velocimetry (PIV). Compared with the asymmetrical DBD plasma actuator, the symmetrical plasma actuator adopted the whole metal plate model as the insulated electrode. Three layers of kapton film as dielectric material covered the testing model and the thickness of each layer was 0.05 mm. The copper foil which was 2 mm in width and 0.05 mm in thickness was mounted on the trailing edge of the plate and oriented along the spanwise direction to induce a wall jet in the streamwise direction. The input AC voltage was 8 kV p-p and the frequency of the power source was 3 kHz. The wind speed was 1 m/s. The results suggested that the symmetrical actuator produced one pair of counter-rotating starting vortexes on each side of upper electrode and the trajectory of the starting vortex core was shown to scale with t0.7 in quiescent air. Compared to the evolution law of starting vortex in still air, the development evolution and life time of starting vortex under flow condition was different due to the interaction influence between incoming flow and starting vortex. The breakdown time of downstream starting vortex was earlier and the location of the starting vortex core scaled with t0.45 under flow condition. Conversely, the life time of upstream starting vortex which was in the opposite direction of incoming flow was delayed. The incoming flow enhanced the upstream starting vortex's capability of promoting mixing and entraining high-momentum fluid into boundary layer, therefore the boundary layer became more energetic and capable of withstanding adverse pressure gradient. The jet effect and mixing function could be achieved by the symmetrical plasma actuator. These investigations laid the groundwork for flow control using DBD plasma actuator at high wind speed or high Reynolds number.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Molecular dynamics simulation of effects of temperature and chirality on the mechanical properties of single-layer molybdenum disulfide
2016, 65 (17): 176201. doi: 10.7498/aps.65.176201
Recently, the effect of temperature on the mechanical property (the Young's modulus) of the single-layer molybdenum disulfide (SLMoS2) is shown to be insignificant, which is obviously incompatible with the previously published result, i. e. the Young's modulus of SLMOS2 decreases monotonically as temperature increases. Aiming at clarifying the relationships between the mechanical properties of the single-layer molybdenum disulfide (SLMoS2) along the armchair (AC) and zigzag (ZZ) directions and the temperature, classical molecular dynamics (MD) simulations are performed to stretch the SLMoS2 along the AC and ZZ directions at the temperatures ranging from 1 K to 800 K by using the Stillinger-Weber (SW) interatomic potentials in this paper. The mechanical properties of SLMoS2 at the temperatures ranging from 1 K to 800 K, including ultimate strength, ultimate strain, and Young's Modulus, are calculated based on the stress-strain results obtained from the simulations. Results are obtained and given as follows. (1) The mechanical properties of the SLMoS2, including the ultimate strength and Young's modulus, are found to monotonically decrease as temperature increases. Increasing the temperature, the ultimate strength of SLMoS2 in the AC direction drops faster than in the ZZ direction, whereas the Young's modulus of SLMoS2 in the ZZ direction decreases quicker than in the AC direction, which means that the chirality effect on the ultimate strength is remarkably different from the Young's modulus of SLMoS2. However, the ultimate strain in the ZZ direction at the temperatures in a range from 1 K to 800 K is close to that in the AC direction, which means that the effect of chirality on the ultimate strain is insignificant. (2) Unlike the published results in the literature, the phase transition of SLMoS2 is found to only occur at a temperature of 1 K and at the moment of initial crack formation as tensiled along the ZZ direction, and the new phase of quadrilateral structure keeps stable after unloading. (3) The linear thermal expansion coefficients along the ZZ and AC directions are also measured, the magnitudes of which are found to be consistent with the published experimental results. Our simulation results support the viewpoint that the effect of the temperature on the mechanical property of SLMoS2 is significant, and the SLMoS2 can be regarded as an anisotropic material as the chirality effect cannot be ignored. The linear thermal expansion coefficients obtained with MD simulation are all in good agreement with the experimental data.
2016, 65 (17): 176202. doi: 10.7498/aps.65.176202
A locally resonant stiffened plate is constructed by attaching a two-dimensional periodic array of spring-mass resonators to a traditional periodic stiffened plate. A method based on the finite element method and Bloch theorem is presented for calculating the flexural wave dispersion relation and forced vibration response of the proposed locally resonant stiffened plate. The method is validated by comparing the predictions with simulations by FEM software COMSOL. The effects of the spring-stiffness and mass ratio of local resonators on the flexural wave band gap and vibration reduction performance are analysed, which can facilitate the design of the locally resonant stiffened plate for vibration-reduction applications in engineering. The main findings of this work are as follows. 1) The local resonator can have a significant effect on the propagation of flexural wave in stiffened plate. On the one hand, the local resonator is able to create a low-frequency local resonance band gap; on the other hand, it can enhance the high-frequency Bragg band gap. Within the band gap frequency range, the vibration of the locally resonant stiffened plate can be reduced remarkably. 2) The spring-stiffness of local resonators shows a notable influence on the band gap and vibration reduction performance of the locally resonant stiffened plate. As the spring-stiffness gradually increases, the nature frequency of local resonator is gradually tuned to higher frequency, and the phenomenon of band-gap transition and band-gap near-coupling may arise. Under the near-coupling condition, the pass band between two band gaps turns narrow, and it seems that these two band gaps form a super-wide pseudo-gap (within which only a very narrow pass band exists). This behaviour is of great interest for the broad band vibration reduction applications. Moreover, the complete band gap will disappear if the nature frequency of local resonator is tuned to a higher value than a threshold frequency, which is dependent on the geometrical and material parameters. 3) The influence of the additional mass ratio of local resonator on the band gap behavior is highly relevant to the nature frequency of local resonator. If the nature frequency of resonator is lower than the band-gap near-coupling frequency, both the local resonance band gap and Bragg band gap are broadened with increasing the additional mass ratio of resonator. When the nature frequency of resonator is close to the band-gap near-coupling frequency, the phenomenon of band-gap near coupling and band-gap transition may arise or disappear as the additional mass ratio of resonator gradually changes. When the nature frequency of resonator is higher than the band-gap near-coupling frequency, on the one hand, the lower frequency band gap will disappear rapidly with increasing the mass ratio of resonator. However, it will be present again if the mass ratio of resonator increases up to a large enough value. On the other hand, the higher frequency band gap is broadened with increasing the mass ratio, but if the mass ratio is tuned to a larger value than a specific value, this band gap will transform from local resonance band gap to Bragg band gap, and the normalized gap width of this band gap will be narrowed with increasing the mass ratio.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (17): 177701. doi: 10.7498/aps.65.177701
Electronic devices are highly demanded commodities and will continue increasing in popularity in the near future, all of which require powers in one way or another. A challenge that arises in remote or inconvenient locations is access to reliable power sources. Energy harvesting technology is critical in the development of self-powered electronic devices. In this paper we present a novel approach to vibration energy harvesting, which is based on uni-polar electret film. Uni-polar electret film is of a flexible polymeric material which can exhibit permanent polarization and induce durable electric filed. In this study, real charge electret films are prepared by using the negative corona charging one-side metalized irradiation cross-linked polypropylene (IXPP) films. Vibration energy harvesters based on such electret films are designed and fabricated. The charge stability in IXPP electret film is investigated by measuring the surface potential of sample. The electromechanical properties of the energy harvester sample are tested by measuring quasi-static and dynamic sensitivities. The energy harvesting from vibrations by using the energy harvester sample, at various vibration frequencies, load resistances, and seismic mass values, is also studied. The results show that as the IXPP film is charged with a corona voltage of -13 kV, grid voltage of -2.0 kV and charging time of 60 s, the stable surface potential of -680 V is obtained after 15-day storage in the laboratory environment. The quasi-static sensitivity of energy harvester sample is 1800 pC/N at a pressure of 1.3 kPa. At an optimum load resistance of 80 M and a resonance frequency of 70 Hz, a maximum output power of 5 W is obtained for an energy harvester sample with an effective area of 13 cm2 and a seismic mass value of 42.2 g.
2016, 65 (17): 177801. doi: 10.7498/aps.65.177801
In this paper, based on the Lee-Low-Pines transformation, the ground-state properties of the bipolaron with the Rashba spin-orbit coupling effect in the quantum dot are studied by using the Pekar variational method. The expressions for the ground-state interaction energy Eint and binding energy Eb of the bipolaron are derived. The results show that Eint is composed of four parts: the electron-longitudinal optical (LO) phonon coupling energy Ee-ph, confinement potential of the quantum dot Ecouf, Coulomb energy between two electrons Ecoul and additional term in the Rashba spin splitting energy ER-ph originating from the LO phonon, where Ecouf and Ecoul are positive definite. These indicate that Ecouf and Ecoul are the repulsive potential of the bipolaron. Generally, it is unable to form the electron-electron coupling structure in the quantum dot because two electrons repel each other by means of the screened Coulomb potential and confinement potential of the quantum dot. However, the numerical results show that the ground-state binding energy of the bipolaron Eb is greater than zero under the condition of the electron-phonon strong coupling (coupling strength 6), so the condition of forming the steady bipolaron structure in quantum dots is naturally met (binding energy Eb 0). In addition, the ground-state energy of the bipolaron E is always less than zero, thus the ground-state biplaron in the quantum dot is in the steady bound state. This can be explained by the physical mechanism. Firstly, the electron-LO phonon coupling energy Ee-ph in the ground-state interaction energy of the bipolaron is always negative. Secondly, the electron-LO phonon coupling interaction in the low-dimensional structures of II-VI semiconductors is great enough (generally 6.0) so that the electron-LO phonon coupling energy Ee-ph is dominant in the ground-state energy E and, therefore the screened Coulomb potential and confinement potential of the quantum dot can be overcome and a steady electron-electron structure can be formed. The numerical results also indicate that the binding energy of the bipolaron Eb increases with increasing the confinement strength of quantum dot 0, dielectric constant ratio of medium and electronphonon coupling strength , but it shows the direct opposite cases from linear increase to decrease with increasing the Rashba spin-obit coupling strength R; the ground-state energy of the bipolaron splits into three energy levels due to the Rashba effect: E(), E() and E(), which correspond to spin orientations of two electrons respectively: up, down and antiparallel; the absolute value of ground-state energy |E| increases with increasing and , but it shows the direct opposite cases from linear increase to decrease with increasing the Rashba spin-obit coupling strength R; the electron-phonon coupling energy obviously accounts for a larger proportion than that of the Rashba spin-obit coupling energy in the ground-state energy of the bipolaron, but the electron-phonon coupling and Rashba spin-obit coupling infiltrate each other and influence each other significantly. In short, the electron in narrow-gap II-VI heterojunctions have higher Rashba spin splitting energy and larger application range. For these quantum dot structures, it is impossible and unnecessary to inhibit the formation of bipolarons. It is more accurate that the bipolaron is chosen as the elementary excitation than the single polaron when investigating the electron-phonon interaction and Rashba spin-orbit coupling, and the bipolaron has more practical significances and potential application values.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (17): 178102. doi: 10.7498/aps.65.178102
The highly desirable properties of nitrogen-doped graphene nanomaterial, such as high surface area, good hydrophilicity, and enhanced electrocatalytic activity and charge-transfer property, make it an ideal candidate for electrode materials used in the field of energy conversion and storage. Up to now, methods of synthesizing nitrogen-doped graphene nanomaterials mainly include chemical vapor deposition, thermal annealing graphite oxide with NH3, and graphene treated with nitrogen plasma. However, these methods of producing the nitrogen-doped graphene nanomaterials are either costly for practical applications or involving environmently hazardous reagents, and the full potentials of nitrogen-doped graphene materials are hard to achieve without scalable production at low cost. Therefore, a simple and cost-effective method of producing the nitrogen-doped graphene nanomaterial is desirable. In this paper, nitrogen-doped graphene nanoplatelets are prepared by a simple and eco-friendly mechanochemical pin-grinding process under N2 atmosphere through using natural graphite flake as the precursor at room temperature. The as-prepared nitrogen-doped graphene sample is characterized by X-ray photoelectron spectroscopy, Raman spectra, nitrogen adsorption, SEM, and TEM. The images of SEM and BET (Brunauer-Emmett-Teller) surface area measurements demonstrate an effective and spontaneous delamination of the starting graphite into small graphene nanoplatelets even in the solid state by pin-grinding process. The cleavage of graphitic C-C bonds by pin grinding creates numerous active carbon species, which can directly react with nitrogen. X-ray photoelectron spectroscopy measurements indicate that the active carbon species react with nitrogen to form the aromatic C-N in pyrazole and pyridazine rings at the fresh broken edges of the graphitic frameworks. Both pyrrolic nitrogen and pyridinic nitrogen are at the edge of carbon framework, which can provide chemically active sites to improve the electrochemical performance of carbon material. Electrochemical impedance spectroscopy indicvates that nitrogen-doped graphene nanoplatelets possess excellent electrocatalytic activity for the redox reaction between iodide and triiodide ions, used in dye-sensitized solar cells. The charge-transfer resistance of nitrogen-doped graphene nanoplatelet electrode is 1.1 cm2, which is comparable to that of Pt electrode. The capacitance properties of the as-prepared nitrogen-doped graphene nanoplatelets are also investigated. Cyclic voltammetry and galvanostatic charge-discharge curves show that nitrogen-doped graphene nanoplatelets have good capacitive performance. At a current density of 0.3 A/cm2, the specific capacitance of nitrogen-doped graphene nanoplatelets is 202.8 F/g. The good electrochemical performance of nitrogen-doped graphene nanolplatelet can be attributed to its high surface area and doping nitrogen at the edge. The simple and eco-friendly preparation procedure, low cost, and good electrochemical performance allow the as-prepared nitrogen-doped graphene nanoplatelets to be a promising candidate for the electrode materials in dye-sensitized solar cells and supercapacitors.
2016, 65 (17): 178104. doi: 10.7498/aps.65.178104
Polyethylene/carbon nanotube (PE/CNT) composites with high hydrogen content as a kind of structural material for space radiation shielding have extensive potential applications in future aerospace field due to their unusual radiation shielding, lightweight, and easy processing. In the space irradiation environment, the composites are sensitive to radiation damage, which changes their microstructures, directly affecting their mechanical performances and shielding effectiveness. Low energy electrons (200 keV) are important radiation environmental factors. Effects and mechanisms for mechanical damage of PE/CNTs composites induced by low energy electrons are studied, which has important academic value and practical significance. Previous research mainly involves the qualitative evaluations of the changes in the mechanical performance index of polymer nanocomposites. The inner relationship between microstructural change induced by radiation and mechanical behavior of the nanocomposites, especially in the PE/CNTs composites has not been studied in depth so far. In this paper, low-density polyethylene (LDPE)/ multi-walled carbon nanotube (MWCNT) composites are chosen as a research object. Based on 110 keV electron irradiation, tensile deformation mechanism of the LDPE/MWCNT composite is studied. The synchrotron radiation X-ray small angle scattering (SAXS) and wide angle diffraction (WAXD) are used to reveal the real-time microstructure evolutions of the nanocomposites after and before irradiation in the process of stretching. Tensile deformation mechanisms of LDPE/MWCNT composite after and before the 110 keV electron irradiation are revealed. Experimental results show that the tensile deformation behavior for the irradiated LDPE by 110 keV electrons is different from that for unirradiated sample. The electron irradiation increases the tensile strength of the LDPE/MWCNT composite and reduces the elongation at break. Moreover, with increasing the irradiation fluence, the tensile strength and the elongation at break of the LDPE/MWCNT composite significantly increases and decreases, respectively. The electron irradiation could hinder the deformations of the LDPE matrix including crystalline case and amorphous case, constrain the fragmentation of original lamellae, the directional arrangement of the MWCNTs, the formation of new crystal and the rotation of lamellae, especially in higher irradiation fluence. During tensile deformation, the main strengthening mechanism for the irradiated LDPE/MWCNT composites by the 110 keV electrons is crosslinking strengthening effect in LDPE matrix. On the other hand, enhanced interaction (mainly interface strengthening) between MWCNTs and LDPE matrix induced by irradiation is attributed to the main strengthening mechanism for the irradiated LDPE/MWCNT composites. These results could provide a theoretical basis and technical support for the reasonable design and successful application of CNT-based polymer composites as structural material for space radiation shielding.
First-principle studies of mechanical, electronic properties and strain engineering of metal-organic framework
2016, 65 (17): 178105. doi: 10.7498/aps.65.178105
Metal-organic frameworks (MOFs) have attracted a great deal of interest from both academia and industry due to their extensive potential applications. The tunable physical properties through the manipulation of composition have led to increasing attention to the exploration of the MOF applications. However, the tunability of physical property of MOF with external mechanical load, which usually steams from actual fabrication and application processes, has been rarely investigated. Here, ab initio (first-principles) density functional theory (DFT) calculations are performed to investigate the mechanical, electrical properties and strain engineering of a typical metal-organic framework, MOF-5. Preliminary calculations by using different pseudopotentials and cut-off energies are performed to verify the adopted critical parameters in subsequent simulations. Both the structural stability of MOF-5 and the effect of applied strain are investigated from an energetic point of view. With the increase of applied strain, the cohesive energy of MOF-5 decreases, inducing the reduction of structural stability. In addition, the variation of cohesive energy of MOF-5 shows an asymmetry under expansive and compressive conditions. By applying strain along different directions, the mechanical properties of MOF-5 are systematically investigated, and mechanical constants including Young's modulus, Poisson ratio and elastic constants are obtained. In addition, by analyzing the band gap of MOF-5, the intrinsic electrical property of MOF-5 is clarified. The band gap of MOF-5 is 3.49 eV, indicating that MOF-5 is a wide bandgap semiconductor, which is represented by the combination effect of both [Zn4O]6+ metal clusters and organic linkers. Analysis on the strain engineering of electrical properties of MOF-5 reveals that the applied strain induces the decrease of band gap of MOF-5, and thus leading to the increase of conductivity. This transition is induced by the decrease of conduction energy-level. Further studies on the variations of PDOS and covalent bond show that the strain engineering of electrical property of MOF-5 intrinsically originates from the variation of covalent bond in the organic linker. The applied strain apparently weakens the covalent bond, and thus inducing the relaxation and redistribution of electrons, which increases the activities of electrons, and finally leads to the overall increase of conductivity of MOF-5. This theoretical study quantitatively clarifies the tunability of electronic band gap of MOF-5 with external strain, and provides a theoretical guidance in the design optimization and property evaluation of gas sensors based on MOF-5.
2016, 65 (17): 178107. doi: 10.7498/aps.65.178107
Rare earth doped silica glass can be used as the central material of optical fiber, which can be applied to the fiber laser. It becomes a focus in the field of laser materials. Compared with different kinds of rare earth elements, ytterbium is regarded as a promising laser nuclear fusion material due to its simple level structure, strong energy conversion efficiency, long fluorescent lifetime, etc. Nowadays, the usual fabrication method of optical fiber preform is the chemical vapor deposition (CVD). However, the preform made by CVD has low doping concentration, few kinds of doping elements, low homogeneity and hard-to-make into optical fiber of large core diameter. To solve these problems, a noble method, which is called non-chemical vapor deposition (Non-CVD), is developed. Sol-Gel method is a kind of Non-CVD, which can perfectly solve the inhomogeneity in material. The glass has harmonious component since the whole process is at a liquid level.Sol-Gel method is a liquid phase synthesis method. The raw materials, including TEOS, absolute ethyl alcohol, ammonium hydroxide and deionized water, are uniformly mixed and become gel from sol through the hydrolysis and condensation. AlCl3 and YbCl36H2O are also added as the dopants. After that, heat the gel and let the hydroxyl and organic release, then we will be able to obtain the SiO2-doped powder. Combining with the laser melting technology, the ytterbium doped silica glass is made. It is known from the DSC-TG curve of xerogel that during the heating process, water and organic are expelled from the system. It needs a holding period at 500 ℃ to ensure that the water and organic are expelled adequately. Moreover, the FTIR spectrum shows that after high temperature treatment the OH- concentration in the xerogel decreases dramatically. The physical and spectrum properties of ytterbium doped silica glass are also tested. The Yb-doped silica glass which shows the amorphous state has good optical properties. The absorption spectrum and fluorescence spectrum demonstrate the typical absorption peak and emission peak of Yb3+, respectively. The density and refractive index of the glass are 2.409 g/cm3 and 1.462, respectively. The fluorescence lifetime () of the silica glass is 0.88 ms, the corresponding emission cross-section (emi) is 0.54 pm2, and the gain coefficient (emi) is 0.48 pm2m. In conclusion, the Yb-doped silica glass is successfully prepared by the Sol-Gel method combined with laser melting technology, which possesses good physical and optical properties. This work is meaningful for preparing high-performance Yb-doped fiber, and even for developing the high power laser.
Dual in-plane-gate coupled IZO thin film transistor based on capacitive coupling effect in KH550-GO solid electrolyte
2016, 65 (17): 178501. doi: 10.7498/aps.65.178501
Low-voltage electric-double-layer oxide-based thin-film transistors are of great prospect and investigative value in the fields of micro multi-state memory devices, detectors, electrochemical sensors, and biological synapses simulation, and so on. In addition, low-voltage electric-double-layer oxide-based thin-film transistors have increasingly attracted attention among researchers due to the characteristics of high mobility, high visible light transmittance and low temperature preparation. Currently, the researches about low-voltage electric-double-layer oxide-based thin-film transistors are broadly divided into two aspects. On the one hand, the researches focus on ZnO as a channel layer, source and drain electrode materials, then gradually develop into In, Sn and Ga oxides as well as complex oxides containing these elements, which has made tremendous progress. On the other hand, the development and research of the gate dielectric materials have received more attention. It is found that by adopting an organic/inorganic proton conductor film as the gate dielectric of low-voltage electric-double-layer oxide-based thin-film transistors, the protons in the gate dielectric will move in the direction away from gate, and finally accumulate on the surface of gate dielectric layer close to the channel layer, with the positive bias applied to the gate. In conclusion, though the researches about low-voltage electricdouble- layer oxide-based thin-film transistors have already made great progress, further explorations and investigations are necessary from its wide applications. Consequently, the development of new material architecture of low-voltage electric-double-layer oxide-based thin-film transistor is one way to achieve this goal. Silane coupling agents (3-triethoxysilylpropyla-mine)-graphene oxide (KH550-GO) solid electrolyte is prepared on plastic substrate by spin coating process. The electrical performances of dual in-plane-gate coupled protonic/electronic hybrid IZO thin film transistor gated by KH550-GO solid electrolyte are further studied. The results indicate that the electric-double-layer capacitance and proton conductivity of KH550-GO solid electrolyte respectively achieve 2.03 F/cm2 and 6.9910-3 S/cm, respectively. Due to high electric-double-layer capacitance and proton conductivity, protonic/electronic hybrid IZO thin film transistor gated by KH550-GO solid electrolyte has lower power consumption (its operation voltage ~2 V). Current on/off ratio of 1.23107 and field-effect mobility of 24.72 cm2/(Vs) are shown in the device. Due to the capacitive coupling effect of KH550-GO solid electrolyte, the device with the stimulus of dual in-plane-gate voltage, can effectively modulate the threshold voltage, the subthreshold swing and the field-effect mobility, and demonstrate AND logic operation successfully. Dual in-plane-gate coupled protonic/electronic hybrid IZO thin film transistors prepared in this paper have potential applications in the field of biosensors and artificial synapses.