Celebrating 100 anniversary of physical science in Nanjing University
2015, 64 (9): 094209.
doi: 10.7498/aps.64.094209
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Z-type photocatlytic system, reflembling natural photosynthesis, consists of two different photocatalysts and a shuttle redox mediator, involving two-photon excitation process for photocatlysis. One photocatalyst as a photoreduction system offers the reduction sites by conduction band (CB) electrons, and the other photocatalyst as a photooxidation system provides the oxidation sites by valence band (VB) holes. A shuttle redox mediator as an electron conductor transfers the electrons from the CB of the photooxidation system to the VB of the photoreduction system. On the one hand, the separation of photocatalytic reactive sites is advantageous for spatial separation of the electrons and holes, which is beneficial for enhancing the photocatlytic activities. On the other hand, photoreduction system and photooxidation system of different materials effectively inhibit the reflerse reaction involvement of photoreductive and photooxidative products. The Z-type photocatlytic system simultaneously possesses a wide light absorption range and strong redox ability.
2015, 64 (9): 097302.
doi: 10.7498/aps.64.097302
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The properties of the edge states in the topological insulator InAs/GaSb/AlSb quantum well in the preflence of a perpendicular magnetic field are studied numerically. The effect of the magnetic field is included in our model by adding an on-site Zeeman term and a vector potential to the electron wave vector: k+eA. When the material is in the topologically nontrivial state, a pair of degenerate counter-propagating spin-polarized edge states exist in the bulk band gap on each edge of the sample, which are gapless in the absence of the magnetic field due to the protection of the time reflersal symmetry. #br#Nonzero magnetic field breaks the time reflersal symmetry, and leads to Landau levels in the electron energy spectrum. However, one can still find a pair of counter-propagating spin-polarized edge states in the bulk energy gap near each sample boundary.The edge states are gapped, and their distributions relative the sample edge depend on the strength of the magnetic field. With the increase of the magnetic field, one edge state remains located near the sample boundary, but the other tends to evolve into the bulk gradually. Furthermore, we study the scattering between the two edge states caused by impurities. We show that the scattering rate is suppressed because of the spatial separation of two edge states, and shows no significant enhancement when the magnetic field increases, which suggests that even though the time reflersal symmetry is broken, the quantum spin Hall state remains to be relatively robust.
2015, 64 (9): 094210.
doi: 10.7498/aps.64.094210
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Singlet fission is a spin-allowed process that creates two triplet excitons from one photo-excited singlet exciton in organic semiconductors. This process of carrier multiplication holds the great potential to break the theoretical efficiency limit in single-junction solar cells by making better use of high-energy photons, while capturing lower-energy photons in the usual style. Photovoltaic devices based on singlet fission have achieved external quantum efficiencies in excess of 100%. In this paper, we first introduce the basic concept about singlet fission and review the history of the field briefly. Then, we report some reflent advances in the reflearch of singlet fission progress with the combination of our group’s productions. Tetracene and pentacene are chosen as typical polyacene materials for discuss. We describe how scientists make progresses in understanding the underlying physics in singlet fission process. The experimental methods of transient absorption spectra, time-resolved fluorescence spectra and time-resolved two-photon photoemission spectra render numerous results for analysis. Moreover, a survey about the debate on the direct or indirect mechanism with transient optical study is provided. It has been verified that multiexciton state intermediates in singlet fission process and the factors of energy level alignments, intermolecular interaction as well as lattice vibrations play a role in it. Last, we briefly summarize the implications of singlet fission in organic solar devices by introducing several composite architectures for singlet-fission photovoltaics. Designing efficient and cheap solar cells is the ultimate goal for understanding the intrinsic photophysics of singlet fission. To obtain high efficiencies, it is important to adapt proper materials and new organic/inorganic architectures may become a promising direction. Also, finding a way for efficient triplet exciton dissociation should be considered seriously. It is believable that these guidelines can lead to the development of cheap and efficient fission-based devices.
2015, 64 (9): 094305.
doi: 10.7498/aps.64.094305
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Realizations of one-way manipulations in various kinds of energy flux are always highly desirable. The most famous example should be the invention of electric diodes which marked the emergence of modern electronics and resulted in worldwide technology revolutions. Acoustic wave, albeit a classical wave with much longer reflearch history in comparison with the electricity, has long been thought to propagate easily along two opposite directions in any path. Hence it should be intriguing to realize the one-way transmission of acoustic waves by designing the acoustical analogy of electric diodes, which would have deep implications in all the acoustics-based applications and the field of acoustics in general. In this review, we briefly describe reflent advances in acoustic one-way manipulation which has become a new frontier of science and is of remarkable significance in both the physics and engineering communities. The emergence of the first “acoustic diode”, formed by coupling a phononic crystal (PC) with a nonlinear medium, offers the possibility of rectifying acoustic energy flux by breaking through the barrier of reciprocity principle via the introduction of nonlinearity. Despite of the efforts in enhancing the performances of nonlinear acoustic diodes by updating their structures, the inherent shortcomings in nonlinear systems such as low efficiency and narrow bandwidth still attract considerable attentions on the potential of linear structures, aiming at constructing a one-way manipulation on particular modes of an acoustic wave without breaking the reciprocity principle. A series of linear acoustic one-way devices have already been designed and fabricated with significantly improved performances. On the basis of asymmetric mode conversion, a linear one-way plate for Lamb waves is designed. High efficient one-way transmission for plane waves propagating along two opposite directions is realized by coupling a PC and a diffraction structure. Unidirectional waveguide is designed and fabricated which only allows for a plane wave incident from one of the two openings to pass. A unidirectional structure with a total thickness as thin as the wavelength is realized by reconstructing the otherwise plane wavefront with acoustic gratings. An acoustic gradient-index structure is proposed that can directly manipulate the wave trajectory asymmetrically and then yield asymmetric acoustic transmission within a considerably broad band. Acoustic metamaterials with near-zero indexes have also been employed to realize unidirectional transmission with a controllable transmitting angle and consistent wavefront. These advances are important steps towards the practical applications which generally require integration and minimization of devices having high efficiency and broad bandwidth. The reflently emerged “acoustic transistor” has been described as well, which can be regarded as the acoustical counterpart of an electric transistor and enables the amplification and switch of acoustic waves by an acoustic wave, or by exploiting the three-wave mixing effect. We also discuss the challenge and promise of the usage of acoustic one-way devices in controlling acoustic waves.
2015, 64 (9): 094306.
doi: 10.7498/aps.64.094306
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Ultrasound contrast agent (UCA) reflers to the agent that has specific acoustic properties to enhance the contrast in ultrasound imaging by composition of gas-filled microbubbles with micrometer-diameters. In a diagnostic ultrasound field, microbubbles in fluid create an acoustic impedance mismatch between fluid and surrounding tissue to increase the reflection of sound and achieve a better contrast. Ongoing developments improve diagnostic possibilities of UCA remarkably, whereas their potential therapeutic applications have also been investigated for a couple of decades. The nonlinear response of UCA microbubbles has clinical reflevance from both diagnostic and therapeutic perspectives. The aim of this review is to introduce the latest reflearch progress of our group regarding the mechanism and applications of the nonlinear dynamic response to UCA, which include (1) an all-in-one solution characterizing coated bubble parameters with the help of the light scattering technique and flow cytometry, which makes it possible to quickly integrate the size distribution with dynamic motions of thousands of microbubbles and easily verify the validities of different shelled bubble dynamic models; (2) the development of a new bubble dynamics model that takes into account both nonlinear shell elasticity and viscosity, which can not only be capable of simulating the “compression-only” behavior of microbubbles excited by large amplitude ultrasound but also eliminate the dependence of bubble shell parameters on bubble size; (3) the estimation of UCA inertial cavitation thresholds of two types of commercial UCA microbubbles (viz., SonoVue microbubbles coated with lipid shells and KangRun microbubbles coated with albumin shells) and the evaluation of the relationship between microbubble inertial cavitation thresholds and their shell parameters; and (4) the reflearches of DNA transfection efficiency and the reduction of cytotoxicity in gene delivery facilitated by UCA excited by 1-MHz focused ultrasound pulses, and the results indicate that the measured DNA transfection efficiency and sonoporation pore size generally increase with the enhancement of inertial cavitation dose, while the cell viability decreases linearly with the increase of International Classification of Diseases (ICD). These studies are of significance for better understanding the mechanism of ultrasound-induced microbubble nonlinear dynamics and investigating the effective quantification technique for microbubble cavitation activity, which are important for further optimizing therapeutic ultrasound effects and avoiding the side-effects.
2015, 64 (9): 097401.
doi: 10.7498/aps.64.097401
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FeSe0.5Te0.5 single crystals with superconducting critical temperature of 13.5 K are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) measureflents in detail. STM image on the top surface shows an atomically resolved square lattice consisted by white and dark spots with a constant of about 3.73 0.03 which is consistent with the lattice constant 3.78 . The Se and Te atoms with a height difference of about 0.35 are successfully identified since the sizes of the two kinds of atoms are different. The tunneling spectra show very large zero-bias conductance value and asymmetric coherent peaks in the superconducting state. According to the positions of coherence peaks, we determine the superconducting gap 2 = 5.5 meV, and the reduced gap 2/kBTc = 4.9 is larger than the value predicted by the weak-coupling BCS theory. The zero-bias conductance at 1.7 K only have a decrease of about 40% compared with the normal state conductance, which may originate from some scattering and broadening mechanism in the material. This broadening effect will also make the superconducting gap determined by the distance between the coherence peaks larger than the exact gap value. The asymmetric structure of the tunneling spectra near the superconducting gap is induced by the hump on the background. This hump appears at temperature more than twice the superconducting critical temperature. This kind of hump has also been observed in other iron pnictides and needs further investigation. A possible bosonic mode outside the coherence peak with a mode energy of about 5.5 meV is observed in some tunneling spectra, and the ratio between the mode energy and superconducting transition temperature /kBTc 4.7 is roughly consistent with the universal ratio 4.3 in iron-based superconductors. The high-energy background of the spectra beyond the superconducting gaps shows a V-shape feature. The slopes of the differential conductance spectra at high energy are very different in the areas of Te-atom cluster and Se-atom cluster, and the difference extends to the energy of more than 300 meV. The differential conductance mapping has very little information about the quasi-particle interference of the superconducting state, which may result from the other strong scattering mechanism in the sample.
2015, 64 (9): 097503.
doi: 10.7498/aps.64.097503
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We preflent a brief overview on the interplay between magnetism and superconductivity in one of the Fe-based superconductor systems, Fe1+yTe1-xSex. The parent compound Fe1+y Te is an antiferromagnet; with Se doping, antiferromagnetic order is suppressed, followed by the appearance of superconductivity; optimal superconductivity is achieved when x～50%, with a superconducting temperature Tc of ～15 K. The parent compound has an in-plane magnetic ordering wave vector around (0.5, 0) (using the tetragonal notation with two Fe atoms per cell). As Se concentration increases, the spectral weight appears to shift to the wave vector around (0.5, 0.5), accompanying the optimization of superconductivity. A neutron-spin resonance is observed around (0.5, 0.5) below Tc, and is suppressed, along with superconductivity, by an external magnetic field. Taking these evidences into account, we conclude that magnetism and superconductivity in this system couple to each other closely-while the static magnetic order around (0.5, 0) competes with superconductivity, the spin excitations around (0.5, 0.5) may be an important ingredient for it. We also discuss the nature of magnetism and substitution effects of 3d transition metals.
2015, 64 (9): 097701.
doi: 10.7498/aps.64.097701
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The multiferroic Co/Co3O4/PZT composite films are prepared on Pt/Ti/SiO2/Si wafers by sol-gel process combined with pulsed laser deposition method. The phase structures, microstructural topographies and element valence states of the composite films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectrum (XPS). The ferroelectric, electrical and magnetic properties as well as the magnetoelectric coupling behaviors are measured, and the exchange bias effect and its influence on the magnetoelectric coupling behavior of the composite film are studied systematically. #br#The results show the composite films have well-defined ferroelectric hysteresis loops with a remanent polarization value of ～17 μ C/cm2. The composite film exhibits evidently an exchange bias effect. Typically, a exchange bias field of ～80 Oe is observed at 77 K. Both the exchange bias field and magnetic coercive field increase with reducing the temperature. The exchange bias field increases to 160 Oe when the temperature decreases to 10 K. The XPS results confirm that an about 5 nm-thick CoO layer appears at the Co/Co3O4 interface due to the oxygen diffusion during the preparation, indicating that the exchange bias effect at 77 K is caused by the pinning effect of the antiferromagnetic CoO layer while the exchange bias effect at 10 K originates from the combining effect of antiferromagnetic CoO and Co3O4 layers. #br#The measureflent results of magnetocapacitance versus magnetic field curves at different temperatures show that the composite films have remarkable magnetoelectric coupling properties. The response of capacitance to temperature changes with the variation of external magnetic field. Further investigations show that the composite film possesses distinct anisotropic magnetocapacitance effect. When the direction of the magnetic field changes, the magnetocapacitance of the composite film changes from positive value to negative value. Moreover, the magnetocapacitance value changes with the variations of temperature and magnetic field magnitude. Typically, at 300 K a maximum value of positive magnetocapacitance (5.49%) and a minimum value of negative magnetocapacitance of (1.85%) are obtained at -4000 and 4000 kOe, respectively. When the temperature is reduced to 10 K, the positive magnetocapacitance decreases to a minimum value (0.64%) while the negative magnetocapacitance increases to a maximum value (5.4%). We perform a detailed analysis on such a magnetoelectric coupling behavior, and elucidate its origin, which should be attributed to the exchange bias effect and interface-mediated magnetism-stress-electricity coupling process.
2015, 64 (9): 098101.
doi: 10.7498/aps.64.098101
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This paper is an overview of the progress of sol-gel autocombustion synthesis of metals and metal alloys. Sol-gel is a convenient method to synthesize a variety of oxides by mixing of different elements at an atomic level. Autocombustion synthesis is a self-sustaining process caused by the heat generated from its exothermic reaction. By combining these two methods, the sol-gel autocombustion method is introduced in the synthesis of metals and metal alloys. The experimental principle and technological route are introduced in detail in this review. By using metal nitrate, citric acid etc. as starting materials, the dried gels are prepared through sol-gel routine. Under the protection of inert gas, the autocombustion could be activated at low temperature in a tube furnace. After the autocombustion was activated, the gel burned violently, and a large amount of white gas was refleased. During heating the gel, mass spectrum shows that the H2, CO and CH4 areflevidently identified near the combustion temperature. They are well known reducing agents, which can be used in the redox reaction for synthesizing metals from oxides. Based on the data obtained from the TG-DTA and mass spectrum analysis, it is speculated that there are mainly five reactions appearing during the burning of the gel at high temperature: exothermic reaction between fuel and oxidant; metal oxide(s) formation by decomposition of the nitrate(s); generation of CH4, CO and H2 by the decomposition of CHx containing groups of complexing agent; exothermic reaction between CH4/CO/H2 and oxidant; the reduction of metals from their corresponding metal oxides by CH4 and H2 in nascent product. The application of this method to the synthesis of metals and metal alloys is shown by realized examples. This method shows many advantages in the synthesis of metals, such as simple apparatus, inexpensive raw materials, a relatively simple preparation process, and fine powder products with high homogeneity. Moreover, very low temperature is required to activate the reaction, and then the combustion can continue to take place without needing additional energy supply. This method has potential applications in experimental material reflearches.
2015, 64 (9): 098701.
doi: 10.7498/aps.64.098701
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Molecular simulation is one of the most important ways of studying biomolecules. In the last two decades, by combining the molecular simulations with experiments, a number of key features of structure and dynamics of biomolecules have been reflealed. Traditional molecular simulations often use the all-atom model or some coarse grained models. In practical applications, however, these all-atom models and coarse grained models encounter the bottlenecks in accuracy and efficiency, respectively, which hinder their applications to some extent. In reflent years, the multiscale models have attracted much attention in the field of biomolecule simulations. In the multiscale model, the atomistic models and coarse grained models are combined together based on the principle of statistical physics, and thus the bottlenecks encountered in the traditional models can be overcome. The currently available multiscale models can be classified into four categories according to the coupling ways between the all-atom model and coarse gained model. They are 1) hybrid resolution multiscale model, 2) parallel coupling multiscale model, 3) one-way coupling multiscale model, and 4) self-learning multiscale model. All these multiscale strategies have achieved great success in certain aspects in the field of biomolecule simulations, including protein folding, aggregation, and functional motions of many kinds of protein machineries. In this review, we briefly introduce the above-mentioned four multiscale strategies, and the examples of their applications. We also discuss the limitations and advantages, as well as the application scopes of these multiscale methods. The directions for future work on improving these multiscale models are also suggested. Finally, a summary and some prospects are preflented.
2015, 64 (9): 098702.
doi: 10.7498/aps.64.098702
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Self-assembly is the way that is used by Mother Nature to create complex materials of hierarchical shapes and diverse functionalities. The photosynthesis apparatus of plant is an example of such complex materials that can direct convert the sunlight energy into chemical energy. Inspired by this, many artificial photosynthesis systems have been successfully engineered. However, most of these systems were based on only one type of simple nanostructure, such as nanosphere or nanotube. The charge separation and exciton transfer in such systems may be further improved by combining multiple nano-structures. Here, we report a novel photo catalysis system based on composite nanostructures of controllable peptide nanotubes and graphene. We use the mixture of diphenylalanine (FF) and carboxyl graphene for the photo catalysis because they are stable under different solvent conditions and highly conductive, which can provide more paths for exciton transfer. Moreover, the diameters of the peptide nanotubes become thinner in the preflence of carboxyl graphene, leading to a more uniformly distributed system than simply using the peptide nanotubes alone. The FF peptide nanotubes can connect with the carbonyl graphene (CG) to form the composite nanostructures because of the π-π stacking interaction between benzene rings of FF and conjugated πup bond of CG. The composite nanostructures of controllable peptide nanotubes and graphene provide more transmission channels for the excitions since they can travel on the nanotubes, CG or the compound of the both. We also demonstrate that when the photo-harvesting ruthenium complex and catalytic platinum nanoparticles are deposited on the system, the nicotinamide adenine dinucleotide (NADP+) can reduce to NADPH. The catalytic efficiency and rate are much higher than thaose of other artificial photosynthesis systems reported in the literature. Surprisingly, we find that the catalytic efficiency of the combined system is better than the sum of separated systems with only FF nanotubes or carboxyl graphene. The high turnover frequency, high reaction rate, and low toxicity of this artificial photosynthesis system will make the combined system attractive for large-scale applications, including optoelectronic industry, energy industry, etc.
2015, 64 (9): 097502.
doi: 10.7498/aps.64.097502
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Combining ferroelectric with antiferromagentic materials in nanometer scale is an effective method for exploring multiferroic materials. We preflent two kinds of systems to show the possibility of multiferroic properties in such nanometer composites. One is the artificial superlattice LaFeO3-YMnO3, and the other is the natural layered Aurivillius material Bi4Ti3O12 doped with different layers of LaFeO3, BiFeO3. Both materials were synthesized by pulsed laser deposition method on SrTiO3 substrates. Microstructural charterizations with XRD, TEM, and EELS in scanning transmission electron microscopy mode substantiate that the samples have atomically sharp interfaces between neighboring layers; this is important for producing possible magneto-electric coupling in multiferroic materials. Magnetic characterization proves that these materials have ferrimagnetic properties, in spite of their anti-ferromagnetic nature before coupling. Magnetic characterization also proves that there is 0.55-0.9 B remanant magnetization generated at LaFeO3-YMnO3 interface. And the 0.5 and 1.5LaFeO3-Bi4Ti3O12 samples show ferrimagnetism which can remain even up to room temperature. Ferroelectric tests prove that there is a large leakage current in LaFeO3-YMnO3 superlattice and BiFeO3-inserted Bi4Ti3O12, but 0.5LaFeO3-Bi4Ti3O12 shows ferroelectric hysteresis loops. It can be therefore concluded that 0.5LaFeO3-Bi4Ti3O12 is a multiferroic material. If more perovskite layers (3-layer SrTiO3 or 2.5-layer LaFeO3) are inserted, the Aurivillius structure of Bi4Ti3O12 may appear structural instability that can be observed in our HRTEM measureflent. Our first principles calculations show that the degeneracy of formation enthalpies is the reason why the intergrowth in these materials forms and their structures are not stable. Our work may provide some examples for exploring new multiferroics by means of nano-meter composite.
2015, 64 (9): 098102.
doi: 10.7498/aps.64.098102
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Photoacoustic imaging is a hybrid imaging technique based on the photoacoustic effect. As a non-invasive and non-ionizing modality, photoacoustic imaging takes the both merits of the conventional acoustic imaging and optical imaging. Firstly, the contrast of photoacoustic imaging primarily depends on the optical absorption. The unique optical spectra of atoms and molecules makes optical methods to be a widely used modality to probe the molecular and chemical information of biological tissue. Therefore, photoacoustic imaging has its inherent advantage in high-contrast functional and physiological imaging of biological tissue, as well as the optical imaging method. Secondly, photoacoustic imaging has the high spatial resolution in deep tissue in comparison with the pure optical imaging method. Since the strongly optical scattering in biological tissue, pure optical imaging method is difficult to obtain the high-resolution image in the tissue deeper than ～1 mm. Whereas, acoustic wave suffers much less from scattering than optical wave, the acoustic scattering coefficient is about 2-3 orders of magnitude less than the optical scattering coefficient. Photoacoustic imaging can achieve a fine resolution in deep tissue, which equivalent to 1/200 of the imaging depth. Thirdly, non-ionizing radiation used for photoacoustic imaging is much safer than X-ray. Moreover, the low-temperature rises make photoacoustic imaging be safely used in live tissue. A laser-induced temperature rise of 1 mK yields an initial pressure of ～800 Pa in soft tissue. Such a sound pressure level has reached the sensitivities of typical ultrasonic transducers. Fourthly, photoacoustic imaging has the ability of extracting multiple contrasts, including biochemical parameter, biomechanical parameter, blood velocity distribution, tissue temperature, and microstructure information. Photoacoustic imaging can capture more specific and reliable information about the tissue structure, function, metabolism, molecule, and gene. As a result, photoacoustic imaging has become one of the fastest growing biomedical imaging techniques in the past decade.#br#In this review, we will explain photoacoustic effect and the principle of photoacoustic imaging. Then, we introduce the two classical photoacoustic imaging schemes, including photoacoustic tomography and photoacoustic microscopy. Their main specifications, such as resolution, are also preflents. We review the ability of photoacoustic imaging in extracting multiple contrasts and discuss their biomedicine applications. In addition, we also introduce the remarkable breakthroughs in super-resolution photoacoustic imaging. Finally, we look the further development and the limitations of photoacoustic imaging.
2015, 64 (9): 097803.
doi: 10.7498/aps.64.097803
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Due to the coupling of photons with the electrons at a metal-dielectric interface, surface plasmons (SPs) can achieve extreflely small wavelengths and highly localized electromagnetic fields. Hence, plasmonics with subwavelength characteristics can break the diffraction limit of light, and thus has aroused great interest for decades. The SP-inspired reflearch, in the application respect, includes extraordinary optical transmission, surface enhanced Raman spectroscopy, sub-wavelength imaging, electromagnetic induced transparency, perfect absorbers, polarization switches, etc.; and in the fundamental respect, includes plasmon-mediated light-matter interaction, such as plasmonic lasing, plasmon-exciton strong coupling, etc.#br#Recently a series of studies has been performed to push the dimensions of plasmonic devices into deep subwavelength by using nanowires. The chemically synthesized metallic nanowires have good plasmonic properties such as low damping. The reported silver nanowire structures show great potential as plasmonic devices for communication and computation. Now we develop the nanostructured metal wires for plasmonic splitters based on the following considerations. One is that we introduce cascade nano-gratings on a metallic nanowire, enabling a single nanowire to act as a spectral splitting device at subwavelength; and the other is that we use silicon as a substrate for the metallic nanowire, making the plasmonic nanowire device compatible with silicon based technologies.#br#In this paper, we continue and develop our previous work on position-sensitive spectral splitting with a plasmonic nanowire on silicon chip (see Scientific Reports (2013) 3 3095). The three parts are organized as follows. In the first part, we derive analytically the dispersion relation of the SPs in a suspended silver nanowire based on Maxwell equations. In the second part, we placed a silver nanowire in the silicon substrate, and use the finite-element method (FEM) to obtain the dispersion relation of the SPs for the practical applications. The calculations show that the SP mode can be confined better in this system, howbeit with larger loss. Starting from the dispersion relation, we then calculate the mode area, the propagation length and the effective index of the SP modes, with respect to the nanowire dimension and the substrate materials. It is shown that a thinner nanowire has smaller mode area and a higher-index substrate induces larger loss. We also perform the finite-difference time-domain (FDTD) simulation to investigate the electromagnetic field distribution in this system. We find that the SP mode is mainly confined around the top surface of the nanowire, and in the crescent gap between the nanowire and the substrate. In the third part, we demonstrate both experimentally and theoretically that the silver nanowire with two cascaded gratings can act as a spectral splitter for sorting/demultiplexing photons at different spacial locations. The geometry of the grating is optimized by rigorous coupled wave analysis (RCWA) calculation. The carefully designed gratings allow the SPs with the frequencies in the plasmonic band and prohibit the SPs with the frequencies in the plasmonics bandgap. Those prohibited SPs areflemitted out through a single groove in front of each grating. Both the detected images and the measured optical spectra demonstrate that the SPs with different colors can be emitted at different grooves along a single nanowire. Thus the structured metal nanowire shows potential applications in position-sensitive spectral splitting and optical signal processing on a nanoscale, and provides a unique approach to integrating nanophotonics with microelectronics.
2015, 64 (9): 097202.
doi: 10.7498/aps.64.097202
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As an exotic quantum condensed matter, the topological insulator (TI) is a bulk-insulating material with a Diractype conducting surface state. Such a dissipationless transport of topological surface state (TSS) is protected by the timereversal symmetry, which leads to the potential applications in spintronics and quantum computations. Understanding the topological symplectic transport of the Dirac fermions is a key issue to the study and design of the TI-based devices. There are many transport properties about Dirac fermions. And universal conductance fluctuation (UCF) is one of the most important transport manifestations of mesoscopic electronic interference. So the UCF effect in TI is a very meaningful research field It can provide an intriguing and special perspective to reveal the quantum transport of TSSs In this review, we introduce the research progress on the UCF of TSSs in a pedagogical way We review the achievements and the existing problems in order to inspire future research work.#br#We start this review with the basic UCF theory and the experimental observation. The UCF has been observed in TI earlier, but weather it originates from TSS has not been further studied. Then a series of work is carried out to prove the topological nature of UCF in TI Firstly, the UCF phenomenon in TIs is demonstrated to be from two-dimensional (2D) interference by magnetoconductance measurements. But the residual bulk state and the 2D electron gas (2DEG) on the surface can also bring about the 2D UCF The field-tilting regulation helps us exclude the distribution from the bulk And the classic self-averaging of UCF is investigated then to obtain the intrinsic UCF amplitude. By comparing with the theoretical prediction, the possibility has been ruled out that the 2D UCF may originate from the 2DEG So its topological nature is demonstrated. Secondly, we discuss the UCF effect in TI by a macroscopic perspective, i.e. the statistical symmetry of UCF, which should be more concise and reflect its universality. For a single TSS, the applied magnetic field will drive the system from a Gaussian symplectic ensemble into a Gaussian unitary ensemble. It results in a √2 fold increase of the UCF amplitude. However, the experiment reveals that the UCF amplitude is reduced by 1/√2. This is contradictory to the theoretical prediction. Actually, there are two TSSs and they are coherently coupled to each other in TIs since the sample’s thickness is smaller than its bulk dephasing length. This leads to a Gaussian orthogonal ensemble of the intersurface coupling system without an external field. In such a case, the UCF amplitude will be reduced by 1/√2 with field increasing. It is consistent with the experimental result. Finally, the other progress on UCFs is discussed, and the general outlook is also mentioned briefly.
2015, 64 (9): 097702.
doi: 10.7498/aps.64.097702
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ZnO nanomaterials have been extensively investigated for its broad applications such as room-temperature UV lasers, light-emitting diodes, solar cells, dilute magnetic semiconductors, bio-labeling, and target medicines. Tuning and optimizing the properties of ZnO nanostructures are urgent for the practical applications. Here, the photoluminescence, magnetism, and cytotoxicity of ZnO nanparticles have been effectively tuned by adjusting the nanostructures. Firstly, by developing the novel polyvinylpyrrolidone(PVP)-directed crystallization route, microwave heating-assisted forced hydrolysis method, and post-treating with surfactants, a series of high pure ZnO nanostructures including spheres, semispheres, rods, tubes, T-type tubes, tripods, wafers, gears, double layers, multilayer, capped pots, and bowls with tunable size and surface component/charge has been successfully prepared. The PVP can greatly promote the ZnO nucleation by binding water, and direct the ZnO growth by forming a variety of soft-templates and/or selectively capping the specific ZnO facet which is confirmed by the infrared absorption spectra. Secondly, the band-edge UV emission of ZnO has been greatly modified in both intensity and peak position by simply changing the sizes, shapes, and surface component of the ZnO nanoparticles. However, changing the surface charge of ZnO nanoparticles can only vary the intensity of the band-edge UV emission of ZnO. Significantly, the fluorescence of fluorescein isothiocyanate (FITC) is increased by up to 90 fold through doping the FITC molecules into the ZnO naoncrystals, which can effectively separate the FITC molelcules and avoid the energy transfer and the resulting fluorescence self-quenching. Thirdly, the room temperature ferromagnetism with tunable intensity is induced in the ZnO nanoparticles by coating them with different surfactants at different concentrations. As confirmed by the x-ray photoemission spectra, the coated surfactant molecules can donate electrons to the ZnO nanoparticles and induce the ferromagnetism. The electron number varies with the surfactant and the surfactant concentration, leading to the fluctuant ferromagnetism. The theoretical calculation further reveal the fluctuant nature of ferromagnetism in the ZnO nanoparticles coated with surfactants. This explains the previously reported seemingly irreconcilable ZnO ferromagnetism induced by capping surfactants, and provides a general chemical approach to tuning the ferromagnetism of ZnO nanoparticles by modifying the capping-surfactant concentration. Finally, it is revealed that the shape, size, surface charge/composition, and band-gap of ZnO nanostructures have different influences on the ZnO-induced cytotoxicity. The surface composition or adsorbed species of NPs may contain the toxic matter such as OH-ions that determine the NP-induced cytotoxicity, and should be detected before cytotoxicity assays are conducted. The rod-like NPs are more toxic than the spherical NPs. The positive surface charge can accelerate the nanoparticle-induced toxic action and enhance the cytotoxicity. Compared with the effects of shape and surface composition/charge, the influence of the nanoparticle-size variation on the nanparticle-induced cytotoxicity is less significant, and can be overwhelmed by other factors. These results will be conducible to the cytotoxicity assay and safe usage of ZnO NPs.
