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國立清華大學工程與系統科學系
國立清華大學工程與系統科學系
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工科研究聚焦(論文精選) - 奈米科技

 



Nanotechnology

Nanotechnology is a technology of design, fabrication and applications of nanostructures and nanomaterials, which also includes fundamental understanding of physical properties and phenomena of nanomaterials and nanostructures. Nanomaterials are the materials with at least one dimension falling in nanometer (10-9 m) scale including nanoparticle (0-dimension), nanotubes and nanowires (1-dimension), thin films (2-dimension), and bulk materials made of nanoscale grains, nanocrystalline materials, or nanoscale structures, nanostructural materials (3-dimension).  Nanomaterials may exhibit physical properties distinctively different from their bulk counterpart.  To study new physical properties and the applications of nanomaterials, it is necessary to synthesize nanostructured materials with desired size, morphology, crystal and microstructure and chemical composition. Therefore, synthesis and processing of nanomaterials are the essential aspect of nanotechnology. Combining scanning probe microscopy (SPM) with other well-developed characterization and measurement techniques such as transmission electron microscopy (TEM) and synchrotron radiation X-ray diffraction, it is possible to study and manipulate the nanostructures and nanomaterials to a great detail and often down to the atomic level.

In the Department of Engineering and System Science (ESS), nanotechnology and energy-related studies are the two major research directions. Our research on nanotechnology includes nano-biomedical science/technology, nano characterization, nano materials, nanoelectronics, beam technology and molecular dynamics simulation. The following research classification depicts the representative research on nanotechnology in this department.

 



Nano-Biomedical Science/Technology

In Nano-Biomedical Science or technology researches, ESS department focuses on the employment of nano materials, including surface engineered nanoparticles, carbon nanotubes, or graphene for the detection and manipulation of biomolecules at very low level of concentration (even down to single molecule); and the other on the integration of those nano materials into micro/nano fluidic systems for cancer stem cells study. The final goal for those technologies will be either on early human diseases diagnosis, such as cancers and infections, or the study of stem cell functions in artificial tissue environment.

 


Au-coated polystyrene nanoparticles with high-aspect-ratio nanocorrugations via surface-carboxylation-shielded anisotropic etching for significant SERS signal enhancement

Hsin-Yi Hsieh, Jian-Long Xiao, Chau-Hwang Lee, Tsu-Wei Huang, Chung-Shi Yang, Pen-Cheng Wang, and Fan-Gang Tseng

The Journal of Physical Chemistry C, 2011, 115, 16258


Polystyrene beads with more intrinsic carboxyl groups and etched by argon plasma produce taller nanocorrugations. The Raman intensity enhancement on a 20-nm gold coated nanocorrugated polystyrene bead array is dominated by he pitch size of nanocorrugations (ranging from 6 nm to 12 nm on the surface of polystyrene beads) and measured 12 times higher than that of smooth particles..

Increasing the hot-spot area with high enhancement ability on SERS-active particles is generally acknowledged as one of the efficient ways to significantly improve the average SERS signal of nanoparticles. A method to create roughness on the surface of nanoparticles was proposed by oxygen plasma etching noncarboxylated polystyrene beads. In this study, we employed argon-based reactive ion etching (RIE) incorporated with carboxylated polystyrene nanoparticles to investigate the roles of nanocorrugations’ morphologies for SERS signal enhancement. Polystyrene beads with more intrinsic carboxyl groups and etched by argon plasma produce higher nanocorrugations. It is suggested that carbonyl groups with high bond energy become nanomasks on polystyrene bead surfaces and provides high selectivity between carboxyl and polystyrene surfaces under RIE. Raman intensity enhancement on a 20-nm gold coated nanocorrugated polystyrene bead array is summarized by three factors: (1) the effect of plasmonic coupling among neighboring particles, (2) the nanocorrugation-contributed roughness, and (3) the pitch size of nanocorrugations, through the analysis of SEM images, AFM height images, and LSPR signals. Among these factors, the pitch size of nanocorrugations (ranging from 6 nm to 12 nm on the surface of polystyrene beads) dominates the SERS enhancement. The 870 nm/120s oxygen plasma etched polystyrene beads (OPSBs) with a minimum pitch size of 6 nm provides the highest Raman intensity enhancement (measured by 632.8-nm He_Ne laser), which is 12 times greater than the intensity of nontreated (870 nm/0s) polystyrene beads (while the Au/Ti coating is 20 nm/5 nm).



Nanocapillary electrophoretic electrochemical chip: towards analysis of biochemicals released by single cells

Ren-Guei Wu, Chung-Shi Yang, Ching-Chang Cheing, and Fan-Gang Tseng

Interface Focus, 2011, 1, 744


We have developed a novel nanocapillary electrophoretic electrochemical (Nano-CEEC) chip to demonstrate the possibility of zeptomole-level detection of neurotransmitters released from single living cells. The chip integrates three subunits to collect and concentrate scarce neurotransmitters released from single PC-12 cells, including a pair of targeting electrodes for single cells captured by controlling the surface charge density; a dual-asymmetry electrokinetic flow device for sample collection, pre-concentration and separation in a nanochannel; and an online electrochemical detector for zeptomole-level sample detection. This Nano-CEEC chip integrates a polydimethylsiloxane microchannel for cell sampling and biomolecule separation and a silicon dioxide nanochannel for sample pre-concentration and amperometric detection. The cell-capture voltage ranges from 0.1 to 1.5 V with a frequency of 1–10 kHz for PC-12 cells, and the single cell-capture efficiency is optimized by varying the duration of the applied field. All of the processes, from cell sampling to neurotransmitter detection, can be completed within 15 min. Catecholamines, including dopamine and norepinephrine (noradrenaline) released from coupled single cells, have been successfully detected using the Nano-CEEC chip. A detection limit of 30–75 zeptomoles was achieved, which is close to the levels released by a single neuron in vitro.)

 



Nano-Materials

Our research on nanomaterials mainly includes synthesis and characterization of carbon nanotubes, graphene, nanocomposite, nanoparticles and multilayer thin films, which may be applied on solar cells, fuel cells, ferroelectric nonvolatile memory, photocatalysts, etc. The synthesis methods were physical and chemical vapor deposition, and electro deposition etc. The following papers depict the representative research on nanomaterials in this department in these two years.

 


Phase transition and mechanical properties of ZrNxOy thin films on AISI 304 stainless steel

Jia-Hong Huang, Tzu-chun Lin and, and Ge-Ping Yu

Surface & Coating Technology, 2011, 206, 107



The AES compositional depth profiles showing that there is an oxygen-rich interlayer between film/substrate interface, which greatly affect the adhesion of the ZrNxOy films during salt spray tests.

The phase ratio of ZrNxOy films with respect to oxygen flow rate. There are three major phases, ZrN, Zr2ON2, and m-ZrO2, with increasing oxygen flow rate.

ZrNxOy thin films were deposited on AISI 304 stainless steel (304SS) substrates by reactive magnetron sputtering.  The specimens were produced by sputtering a Zr target at 500°C and the reactive gases were N2 and O2 at various flow rates (ranging from 0 to 2 sccm). The purpose of this study was to investigate the effect of oxygen flow rate on the phase transition and accompanying mechanical properties of the ZrNxOy thin films.  The oxygen contents of the thin films increased significantly with increasing oxygen flow rate.  X-ray diffraction (XRD) revealed that the characteristics of the films can be divided into three zones according to the major phase with increasing oxygen content: Zone I (ZrN), Zone II (Zr2ON2) and Zone III (m-ZrO2).  The hardness of the ZrNxOy films decreased with increasing oxygen content due to the formation of the soft oxide phase.  Modified XRD sin2ψ method was used to respectively measure the residual stresses of ZrN, Zr2ON2 and m-ZrO2 phases. The results showed that the residual stress in ZrN was relieved as the oxygen content increased, and Zr2ON2 and m-ZrO2 were the phases with lower residual stress.   Compositional depth profiles indicated that there was a ZrO2 interlayer near the film/substrates interface for all samples except the mononitride ZrN specimen.  Contact angle was used as an index to assess the wettability of the film on substrate.  The contact angles of ZrN, Zr2ON2 and m-ZrO2 on stainless steel were indirectly measured using Owens-Wendt method. The results showed that ZrO2 possessed the lowest wettability on 304SS among the three ZrNxOy phases, indicating that the ZrO2 interlayer may account for the spallation of the ZrNxOy films after salt spray tests.

 


Growth of BiFeO3/SrTiO3 artificial superlattice structure by RF sputtering

Shang-Jui Chiu, Yen-Ting Liu, Hsin-Yi Lee, Ge-Ping Yu, and Jia-Hong Huang

Journal of Crystal Growth, 2011, 334, 90



Intensity distribution of a (002) radial scan of BFO/STO superlattice films deposited at various substrate temperatures. A narrow marks the position of the superlattice main peak and a dashed line denotes the position of the mean value of the superlattice. The inset shows a radial scan around the (111 )Bragg peak for a superlattice

Electric hysteresis loops of superlattice films deposited at varied substrate temperatures. For a single-layer film of BFO (thickness40nm) deposited at 600 °C and prepared under the same sputtering condition.

Asymmetric superlattice structures consisting of multiferroic BiFeO3 (BFO) and paraelectric SrTiO3 (STO) sublayers were grown on aNb-doped STO substrate by RF magnetron sputtering at temperatures in the range 500–800°C. For substrate temperatures less than 500 °C and at 800 °C, only poorly crystalline films resulted, whereas a BFO/STO superlattice structure film of great crystalline quality was obtained for a substrate temperature in the range 550–750 °C. The formation of a superlattice structure was confirmed through both the appearance of Bragg lines separated by Kiessig fringes in X-ray reflectivity curves and the satellite features of a (002) diffraction pattern. The clearly discernible main and satellite features on both sides of the substrate about the (002) STO Bragg peak of superlattice films indicate the high quality of the BFO/STO artificial superlattice structure. For BFO/STO superlattice films deposited at 600–700 °C, the increased crystalline quality with smaller interface roughness correlates with larger lattice strain and remanent polarization of the film. The lattice strain and interface quality might be important factors that influence the leakage and ferroelectric properties in BFO/STO artificial superlattices.

 


Effects of Pt shell thicknesses on the atomic structure of Ru-Pt core-shell nanoparticles for methanol electro-oxidation applications

Tsan-Yao Chen, Tsang-Lang Lin, Tzy-Jiun Mark Luo, Yongjae Choi, and Jyh-Fu Lee

ChemPhysChem, 2010, 11, 2383


High performance core-shell nanocatalyst, RuCore-PtShell, for applications in direct methanol fuel and dye-sensitized solar cell applications.

Cyclic voltametry sweeping curves of RuCore-PtShellBiNPs over MeOH electrooxidation reactions.

In this research, core-shell electrocatalysts comprising a Ru core covered with precisely controlled 1.5, 2.7 and 3.6 atomic layer (AL) thick Pt atoms were synthesized using Polyol redox method with specifically designed reduction sequences. Results of SAXS demonstrate that these electrocatalysts were successfully grown with nearly uniform size distribution and with precision control of Pt shell thicknesses. The sample with 1.5 ALs shows a 3.2-fold improvement in CO-tolerance and 2.4-fold current enhancement at the conventional battery operation potential (I300, at 300 mV vs Ag/AgCl) during methanol oxidation as compared with conventional Ptnanocatalysts. The origin of the enhanced performance and the atomic structure of the core-shell nanoparticles are elucidated to be mainly dominated by the lattice strain (and possibly also with a little bit of heteroatomic interactions). The combination effects of lattice strain and the heteroatom electron orbital interactions (i.e., ligand effect) at their core-shell interfaces facilitate the formation of Pt-(OH)ads or Pt-(OX)ads during MeOH oxidation reactions and therefore significantly promoted the current (I300) on the active sites of core-shell nanoparticles. The enhancement in catalytic activity decreases with increasing Pt shell thickness. The highest catalytic performance is found with the near single atomic Pt shell core-shell nanoparticles.

 



Nano-Electronics

Despite great advancement in high-κ gate dielectrics, metal gates and strain engineering, the effort to pursue devices with higher performance never slows down. Due to the superior carrier mobility against Si, germanium (Ge) has received much attention and been regarded as a potential channel material to accommodate the ever-stringent scaling requirement for future technology nodes. Recently, the research on Ge-related technology has drawn intense attraction. Since 2005, the department of Engineering and System Science has strived to devote itself to the development of epitaxial Ge film growth on Si substrate and the integration of crystalline high-κ dielectrics into the Ge film to form high-performance Ge-based devices including Ge MOSFETs and Ge nonvolatile memory devices. It is worth mentioning that in 2011 the department has successfully realized Ge MOS devices with equivalent oxide thickness (EOT) close to 1.0 nm and low leakage current by employing a crystalline high-κ dielectric, which attests to the feasibility and foresightedness of the process. In addition, the department has also accomplished the world’s first Ge-channel charge trap flash memory devices on Si substrate in 2009 and first resistive random access memory (ReRAM) on Ge layer in 2012. Besides the common applications to optoelectronic and CMOS devices, the advent of these Ge-based nonvolatile memory devices ushers in a new research era for Ge technologies.

The evolution of compact, lightweight, low-power, and high-quality displays has caused a large demand for liquid crystal display (LCD) drivers, with features such as low power dissipation, high speed, high resolution, and a large output voltage swing. An LCD driver is generally composed of column drivers, gate drivers, a timing controller, and a reference source. Column drivers are especially important, and generally include registers, data latches, digital-to-analog converters (DACs), and output buffers. Among these components, the DACs and output buffers determine the column driver’s speed, resolution, voltage swing, and power dissipation. Furthermore, DACs occupy the largest silicon area of a column driver chip. Because a single chip includes hundreds of DACs and output buffer amplifiers, the DACs and buffers should occupy a small die area, and their static power consumption should be small.

 


High-performance gate-all-around polycrystalline silicon nanowire with silicon nanocrystals nonvolatile memory

Min-Feng Hung, Yung-Chun Wu, and Zih-Yun Tang

Applied Physics Letters, 2011, 98, 162108


In recent years, nonvolatile flash memory has become very popular in portable electronics, and associated demand for memory density multiplying every year. The main research trends in flash memory are: (1) increasing density, (2) enhancing the program/erase (P/E) speed, and (3) improving the reliability. Although flash memory is aggressively scaled in the horizontal dimension for high-density applications, continuing to scale according to Moore’s Law is becoming increasingly difficult because of process and device physics limitations. Recently, three-dimensional (3D) multi-layer-stack memory that is based on poly-Si thin-film transistors (TFT) can be used as ultra-high-density memory, providing a possible solution for next generation flash memory. Regarding reliability, conventional flash memory usesconductive poly-Si thin film as a charge storage layer. With the device dimension scaling, the leakage current through thin tunneling oxide significantly degrades reliability.

Thus, the charge trapping layer device, Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) type NVM is a promising candidates for the next generation flash memory. Because of the Si3N4, numerous high-k materials films, nanocrystals (NCs) store charges in spatially isolated deep-level traps and a single leaky path does not cause serious threshold voltage variation. We first develop the nonvolatile Memory (NVM) based on gate-all-around (GAA) poly-Si nanowires (NWs) structure with silicon nanocrystals (NCs). The main contribution is first combing the poly-Si NWs and Si NCs NVM with high program/erase speed and superior reliability for 3D stacked high density flash memory application. The GAA NCs NVMs perform a large of threshold voltage shift, and are faster than the GAA SONOS NVMs do. In reliability studies, this NVM shows superior endurance after 104 program/erase (P/E) cycles, and only few charges lose after 10 years at 85 oC.

 


A 10b resistor-resistor-string DAC with current compensation for compact LCD driver ICs

Chih-Wen Lu, Ping-Yeh Yin, Ching-Min Hsiao, and Mau-Chung Frank Chang

IEEE International Solid-State Circuits Conference, 318, 2011


Die micrograph for 18 RRDACs and 18 buffers.

Achieving a higher color depth for LCD drivers requires a higher DAC resolution and a larger circuit die area. Due to the stringent requirement on uniformity, a resistor-string DAC (RDAC) is predominantly used for LCD column drivers. However, the area of the RDAC and related routing lines are prohibitively large for a high-resolution data converter, making it impractical for column driver ICs in high color depth displays. A typical RRDAC-a combination of two RDACs and two intermediate unity-gain buffers - may reduce the chip area. The unity-gain buffers can isolate these two RDACs. The buffers, however, have offset errors that can be further spread to the LCD driver output. Consequently, obtaining output uniformity for a high-color depth column driver is rather difficult. Furthermore, each output channel demands two additional buffers with increased power consumption. To reduce the area, researchers have proposed a RRDAC without unity-gain buffers. Under such condition, parallel channel resistor strings have been connected directly to the global resistor string. This, in fact, affects the reference voltages of the global resistor string. To overcome aforementioned issues, we propose to use a new type of 10-bit RRDAC with a current compensation scheme to provide good linearity and uniform channel performance, and simultaneously maintain the 10b DAC at a size smaller than that of a conventional 8-bit RDAC.

 



Beam Technology

The beam technology is gate way to understand the material structure in great detail in today’s modern world. Beam technology measuring the atomistic structure and electronic structure of condensed matters, which allow us to make a connection between the physics properties of materials and the material fabrication and processes. This is an important area for material scientist or physicist to cut short the time of trial and errors in material process and to have a great deep understanding in order to design the material properties in the future.

Four different kinds of beam technology are most popular and widely applied in the College of Nuclear Science of National Tsing Hua University, namely, the synchrotron radiation X-ray, the electron beam, the ion beam and the neutron beam.

The students and faculty members using the Taiwan synchrotron radiation (TLS, 1.5 GeV) in National Synchrotron Radiation Research Center near the campus of National Tsing Hua University to do X-ray diffraction and scattering, X-ray spectroscopy experiment, and X-ray image technology. The synchrotron high intensity X-ray diffraction allows us to measure the thin film structure down to sub-monolayer of thickness. The high resolution of synchrotron radiation allows us to do small angle X-ray scattering. The energy tunable capacity allows us to do element specific diffraction using their absorption edges. X-ray spectroscopy such as X-ray photoemission spectroscopy and X-ray absorption spectroscopy are to probe the binding energy of core electrons to understand the chemical state of each element. It also provides local environment of each atom, such as bond distance and coordination numbers. For example, bimetallic catalyst nano-particles with a few nm of size, the X-ray absorption spectroscopy method enable us to know the structure of nano-particle to be a core-shell, or mixed alloy. Transmission X-ray microscopy provides the 50 nm resolution image which is a non-destructive method for three dimensional morphology study in-situ. In 2014 when the new synchrotron radiation (3 GeV, 518 m circumference) to be commissioned, one of the most brilliant synchrotron in the world, additional new synchrotron beamlines provide a new golden opportunity to have advanced study. To name just a few, the microprobe beamline enable us to probe the micro strain in 50 nm spatial resolution will be very useful for innovative materials in the emerging electronic devices. New coherent beamline would be able to do single particle diffraction without periodical structure using speckle techniques.

For electron beam study on characterization of material include TEM, SEM, LEED, EELS, and EDX techniques. A long history development in the department of Engineering and system science, not only on the field emission TEM/SEM, sample preparation tools with FIB and microscopy provides best sample preparation method for TEM. The research group recently also is able to build a table top SEM including the tip emission, low cost electron detector and apply phase contrast technique to enhance the image capability. Ion beam technique also developed in National Tsing Hua University, two van de Graff accelerators to do routine RBS, PIXE, channeling effect, ion implantation, and charged particle activation analysis study.  Recently a high resolution RBS (MEIS) with 0.3 nm depth profile was home-made for material characterization of thin solid films. A recent developing Tear Hz free electron laser using 25 MeV electron accelerator which can produce million times far IR source for innovative research on this energy range.

Neutron beam study includes neutron diffraction and scattering, prompt gamma activation analysis, boron neutron capture therapy, depolarization neutron experiments and neutron radiography have been set up at 2 MW research reactor on campus. Neutron scattering method is a complementary tool of X-ray scattering, an isotopic sensitive tool allow us to replace H by D or by adjusting the H2O/D2O to probe the structure of biological matter, polymers and hydrogen storage materials. The spin of neutron is very useful to study the structure of magnetic materials for hard disk or MRAM for mass data storage. The study of material dynamics includes, the lattice vibration, molecular rotating and diffusion can be studied. The graduate students in our college also run neutron experiments around the worlds in Australia (OPAL), USA (IPNS, NIST,ORNL), Canada(Chalk Rivers), Swiss(PSI), Russia(JINR), Japan(KENS/JRR-3M), Germany(JCNS), Netherlands(Delft) and England(ISIS). It is truly an international collaboration among the world.

 

Prof. Tsang-Lang Lin and Prof. Chih-Hao Lee are leading a beamline project in together with people from NTHU, NTU and NSRRC to construct a new generation beamline at the 3 GeV synchrotron Taiwan Photon Source (TPS). The coherent beamline is one of the seven beamlines to be built in the first phase of beamline construction of TPS. The coherent beamline will enable the study of slow dynamics of biological materials, nanomaterials as well as energy materials. The construction of this beamline will be completed in 2014

 


Aggregation Structure of the Complex of Amyloid-b(1-40) and Sodium Dodecyl Sulfate as Revealed by Small-angle X-ray and Neutron Scattering

Jhih-Min Lin, Tsang-Lang Lin, U-Ser Jeng, Zyun-Hua Huang, and Yu-Shan Huang

Soft Matter, 2009, 5, 3913


Cartoon for the SDS/Ab complex formed in the solution with preexisting SDS micelles. The hydrophobic parts of the peptide (represented by the red blocks) hide inside the aliphatic core of SDS, whereas the hydrophilic parts represented by the blue blocks are in the anionic shell of SDS micelle (SDS headgroups are represented by solid spheres).

The aggregation of Amyloid-b (1-40), (Ab), is thought to cause the Alzheimer disease. It is important to develop methods to reduce the aggregation, such as adding surfactants, such as sodium dodecyl sulfate (SDS). Combining the small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) with contrast variation, we are able to determine the aggregation structure of Amyloid-b peptides in solutions with SDS. With the addition of 0.115 mMAb peptides into an aqueous solution with 6 mM SDS, time-dependent SAXS indicates the formation of globular SDS/Ab complex of a core-shell structure. With scattering contrast varied by SDS and deuterated SDS, SANS results reveal the coexistence of Ab aggregates and the SDS/Ab complex, which two further form clusters of a mass fractal structure. Based on the changes of the zero-angle scattering intensity with the contrast variation, a molecular association ratio, ~30, of SDS to Ab for the globular complex micelle is extracted. With a concentration (20 mM) above the critical micelle concentration (CMC) of SDS, time-dependent SAXS and CD reveal a relatively coherent adsorption of Ab peptides to the preexisting SDS micelles and change of secondary structure. Delicate differences in the structure and formation process of the two types of SDS/ Ab complexes, respectively formed in solutions with 6 mM and 20 mM SDS, are discussed in terms of the dissimilar association efficiencies between Ab with SDS monomers and that with SDS micelles.

 



Molecular Dynamics Simulations

Molecular dynamics is a methodology which calculates forces acting on atoms in a system from the laws of Newtonian mechanics. The simulation is performed by numerically solving Newton's equation of motion over time. Once the trajectories in phase space are obtained, including the positions and velocities of atoms, macroscopic quantities and dynamical properties which interest us can be calculated directly from the trajectories through the formulation of statistical mechanics. It provides detailed and valuable information at atomistic level, which is usually difficult to be obtained by experiments. It has become a powerful tool in the research of nano-scaled world. Molecular dynamics simulations have been used in a broad variety of applications in physics, chemistry, materials science, engineering, and biomedicine. It can serve as a refinement tool to determine the structure and dynamics of biomolecules such as in X-ray crystallography and nuclear magnetic resonance. The mechanical properties of materials can be also investigated by this method. The following six papers depict the representative research on Molecular dynamics simulation in this department.

 


Pore-Spanning Lipid Membrane under Indentation by a Probe Tip

C.-H. Huang, P.-Y.Hsiao, F.-G. Tseng, S.-K. Fan, C.-C. Fu, and R.-L. Pan

Langmuir, 2011, 27, 11930


This work reports a study of the indentation of a free-standing lipid membrane suspended over a nanopore on a hydrophobic substrate by means of molecular dynamics simulations. At the beginning of indentation, the membrane bends at the point of contact and the fringes of the membrane glide downward intermittently along the pore edges and stop gliding when the fringes reach the edge bottoms. Afterward, the bending continues and the large strain eventually induces an astonishing phase transition in the membrane, transformed from a bilayered structure to an interdigitated structure. The membrane is finally ruptured when the indentation goes deep enough. Several physical quantities in the pore regions are calculated, which include the tilt angle of lipid molecules, the nematic order, the included angle, and the distance between neighboring lipids. The variations of these quantities reveal many detailed, not-yet-specified local structural transitions of lipid molecules under indentation.

 


Unfolding Collapsed Polyelectrolytes in Alternating-Current Electric Fields

P.-Y. Hsiao, Y.-F. Wei, and H.-C. Chang

Soft Matter, 2011, 7, 1207


This work studies the unfolding of single polyelectrolyte (PE) chains collapsed by trivalent salt under the action of alternating-current (AC) electric fields through computer simulations and theoretical scaling. The results show that a collapsed chain can be unfolded by an AC field when the field strength exceeds the direct-current (DC) threshold and the frequency is below a critical value, corresponding to the inverse charge relaxation/dissociation time of condensed trivalent counterions at the interface of the collapsed electrolyte. This relaxation time is also shown to be identical to the DC chain fluctuation time, suggesting that the dissociation of condensed polyvalent counterion on the collapsed PE interface controls the polyelectrolyte dipole formation and unfolding dynamics under an AC electric field.

 

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