bluuebunny/crossref_metadata_2025 huggingface dataset:
The beam combination of multi-core photonic crystal fiber by using the Kagome fiber
Authors: Zhou Yu-Zhu  , Huang Li-li  , Chai Lu  , Wang Qing-Yue  , Hu Ming-Lie  ,   Key Laboratory of Opto-electronic Information Science and Technology of Ministry of Education, College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
Published: December 2020

Using multi-core photonic crystal fiber (PCF) has advantages of large-mode-area that can support high beam intensity and disperse heat. However, only when the beam profile in far-field and the focal point of in-phase super mode is Gaussian-shaped, the energy can be more concentrated as compared with other shapes. And this beam profile feature limits the applications of multi-core PCF.With the development of optics, there is a practical solution to improve the beam profile of multi-core PCF in which a Kagome fiber is used. This solution is to couple the in-phase super mode source (obtained from multi-core PCF) into Kagome fiber to achieve the beam combination of multi-core photonic crystal fiber, i. e. the beam profile remains to be Gaussian-shaped at any location in the optical field. The Kagome fibers have a novel hollow structure and thus will show some new properties, such as broad optical transmission bands with relatively low loss, no detectable surface modes, and high confinement of light at the core, and these features are suitable for beam combination.In this paper, a Ti: sapphire femtosecond pulsed fiber oscillator, with its center wavelength of 800 nm and output power of 550 mW, is used to pump a piece of seven-core nonlinear PCF, with an efficiency of 19%. EFL of the coupling lens is 18.40 mm and the NA is 0.15. Then the in-phase super mode source can be obtained from the 15 m multi-core PCF, with a broadband spectrum from 700 to 1050 nm. The beam profile of farfield and the focal point of in-phase super mode is Gaussian-shaped and there is a seven-core-shaped pattern at nearfield and other locations in the optical field. In order to combine the beams of multi-core fiber, the in-phase super mode source is coupled into a piece of Kagome fiber, 10 cm long, by using the coupling lens whose EFL is 13.86 mm. Its coupling efficiency is 71% and the output beam profile remains to be Gaussian-shaped at any locations in the optical field; this means that there is no seven-core-shaped pattern. It also transmits broadband spectrum with low loss. Moreover, this experiment also proves that the solution can be used for different multi-core PCFs and can have a higher coupling efficiency, 80%. Thus a reference can be given for high power applications of multi-core PCF, and inspiration may be given to some other frontier fields in fiber optics.
Total ionizing dose radiation effects in foue-transistor complementary metal oxide semiconductor image sensors
Authors: Wang Fan  , Li Yu-Dong  , Guo Qi  , Wang Bo  , Zhang Xing-Yao  , Wen Lin  , He Cheng-Fa  ,   Key Laboratory of Functional Materials and Devices under Special Environments, CAS.; Xinjiang Key Laboratory of Electric Information Materials and Devices; Xinjiang Technical Institute of Physics and Chemistry, CAS., Urumqi 830011, China;,   Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Radiation effects on four-transistor (4 T) active pixel sensor complementary metal-oxide-semiconductor (CMOS) image sensor induced by -ray are presented. The samples are 4 megapixels resolution CMOS image sensor using 11 upm pitch high dynamic 4 T pixels. They are manufactured with 0.18 upm specialized CMOS image sensortechnology. Three samples have been exposed to 200 krad(Si) 60Co -ray with different biasing condition (1# is static-biased, 2# dynamic-biased, and 3# is grounded during irradiation), and the dose rate is 50 rad(Si)/s. The influences of radiation on full well charge capacity, dark current, and conversion gain of the device are investigated. Experimental result shows that the conversion gain is not sensitive to the ionizing radiation, and it is mainly determined by the CMOS digital or analog circuits. It is known that the total ionizing dose for induced degradation in deep submicron MOSFET is negligible and so there is almost no radiation effect on the digital or analog circuits exposed to the ionizing radiation. Therefore, conversion gain does not have obvious degradation after irradiation. While full well charge capacity has a degradation after irradiation, which is due to the change of TG channel doping profile induced by the radiation. As the dose increases, dark current increases rapidly. The main source of dark current in 4 T CMOS image sensor is the current from STI interface and TG-PD overlap region. Experimental result also shows that different from 3 T CMOS image sensor, there is no biasing effect in 4 T CMOS image sensor. This is because for the 4 T CMOS image sensor most of the degradation come from STI interface and TG-PD overlap region, while biasing condition almost has no influence on both ofthem.
Study on temperature and electric field dependence of conductivity in polyimide
Authors: Wang Song  , Wu Zhan-Cheng  , Tang Xiao-Jin  , Sun Yong-Wei  , Yi Zhong  ,   Research Institute of Electrostatic and Electromagnetic Protection, Ordnance Engineering College, Shijiazhuang 050003, China;,   Beijing Institute of Spacecraft Environment Engineering, Beijing 100094, China
Published: December 2020

The deep dielectric charging (DDC) imposes a potential threat on spacecrafts. On the one hand, this kind of polymer insulator dielectric, represented by polyimide, is significantly dependent on temperature; on the other hand, during the charging process the high electric field (at the level of 107 V/m) will enhance the conductivity of the dielectric. Therefore, in order to make a precise assessment of DDC by computer simulation, the conductivity model should take into account the temperature and electric field dependences. In this field, two conductivity models are usually adopted for DDC simulation. One of them is proposed by Adamec. It puts emphasis on the enhanced conductivity due to high electric field, while its temperature dependence is based on the famous Arrhenius formula. Adamec model can make good performance versus electric field, but it is inappropriate in low temperatures. Another model combines the thermally assistant hopping conductivity and the variable-range hopping conductivity together, so it shows advantage in the temperature dependence, which is named as TAH VRH model. Although this model also can include the influence from electric fields, the effectiveness is not so good as that of Adamec model. In order to combine the advantages of these two models, i.e. the Adamec model and TAH VRH model, a new conductivity model is proposed with fewer parameters than those in TAH VRH. It is derived by replacing the Arrhenius formula in Adamec model with a simplified temperature model referred to as TAH VRH model. This formulation enables the new model to deal with a wider temperature range and keep the good performance versus high electric fields. The proposed model is verified partly by the measured data of a kind of polyimide. Satisfactory agreement is obtained in data fitting by using the new model, where the temperature dependence is better than that of Adamec model. In addition, to overcome the unreasonable increase in conductivity in low temperature and high electric field, a useful technique is proposed. By temperature mapping in the electric field correlated factors namely the carrier concentration and mobility enhancement factor, this technique can extend the feasible temperature range to a lower limit. This is done according to the assumption that the carrier concentration is small at low temperatures, and consequently the electric field influence should not be large. At high temperatures or in low electric fields, the temperature mapping is of little effect. Finally, analysis of the model's sensitivity versus several parameters is provided, demonstrating the advantage of applicability of the new model with fewer parameters.
First-principles investigations on structure and thermodynamic properties of CaS under high pressures
Authors: Wu Ruo-Xi  , Liu Dai-Jun  , Yu Yang  , Yang Tao  ,   College of Chemical Engineering, Sichuan University, Chengdu 610044, China
Published: December 2020

First-principles calculations of the electronic structure and thermodynamic properties of calcium sulfide (CaS) have been carried out by the plane-wave pseudopotential density functional theory method. The calculated values of lattice constant, elastic modulus and its derivative for CaS under zero pressure and zero temperature, agree well with the experimental data and some of the existing model calculations. The band structure and density of states are discussed in detail. Moreover, the dependences of the volume variation, bulk elastic modulus, thermal expansion coefficient and heat capacity on pressure have been investigated for the first time, so far as we know. It is concluded that under the condition of zero temperature (0 K) and zero pressure (0 GPa), the volume is 44.6 3 when the energy of the crystal unit cell reaches a minimum in the structural model of CaS, which is the most stable system. The energy band of CaS is mainly composed of low band gap, valence band and conduction band, the GV-XC band gap of CaS is 2.435 eV. The DOS results show that the valence band is mainly of Ca 3s and S 3p, while the conduction band is mainly of Ca 4d and a small amount of S 3p. At a certain temperature, the volume change rate, heat capacity and thermal expansion coefficient decrease with rising pressure, and the body elastic modulus B increases simultaneously. In contrast, when the pressure is constant, the volume change rate and body elastic modulus B decrease with the increase of temperature, while the thermal expansion coefficient and heat capacity increase as the temperature rises. When the temperature is higher than a certain value, the heat capacity CV is close to the Dulong-Petit limit, and the effect of temperature on the heat capacity is minimal. Furthermore, under the condition of low pressures, the influence of temperature on thermal expansion coefficient is greater than that of the pressure on it.
Ultra-thin broadband flat metasurface to focus electromagnetic waves and its application in high-gain antenna
Authors: Hou Hai-Sheng  , Wang Guang-Ming  , Li Hai-Peng  , Cai Tong  , Guo Wen-Long  ,   Air and Missile Defense College, Air Force Engineering University, Xi'an 710051, China
Published: December 2020

The phase gradient metasurface has strong abilities to manipulate electromagnetic waves on a subwavelength scale and has a potential to enhance the antenna gain. Based on the single multi-resonance metallic patch srtucture, we propose a new kind of ultra-thin broadband unit cell to manipulate electromagnetic waves and enhance the gain. It has been demonstrated that anomalous reflection can be achieved by utilizing the magnetic resonance between metallic patch and ground plane. Moreover, it is believed that resonance with low quality factor (Q factor) is useful in extending the working bandwidth. In order to extend the bandwidth of phase modulation, it is necessary to design a kind of low-Q unit cell. Besides, we need to extend the phase shift to cover the entire range [0, 360] to achieve the focusing effect. Thus we design a suitable symmetrical unit cell composed of ring and cross metallic patterns to control the phase of reflected waves. The symmetrical structure is useful for decreasing the Q factor so as to get a kind of low-Q unit cell. Theoretically, ring and cross metallic patch can be regarded as multi-resonance unit cells, which can cover the entire scope [0, 360]. The unit cell operates at 15-18 GHz with a thickness of 1 mm and the sides of 0.3 0( 0=20 mm). Furthermore, we design a phase gradient metasurface composed of the designed unit cell to verify the broadband anomalous reflection and focusing effects in CST Microwave Studio; the effect can be clearly illustrated in the simulation results obtained at 15-18 GHz. Due to the successful conversion from plane wave to quasi-spherical wave, we can place the Vivaldi antenna at the focal point of the metasurface as a feed source to transform the quasi-spherical wave to plane wave to enhance antenna gain. The simulation results are in good agreement with the theoretical analysis. Meanwhile, the designed metasurface and Vivaldi antenna have been fabricated and applied to enhance the gain of Vivaldi antenna. Both simulation and test results show that the peak gain has been averagely enhanced by 11 dB during the -1 dB gain bandwidth of 15-18 GHz and the fractional bandwidth is 18.2%. Moreover, due to the thin thickness, light weight and broad band, the designed unit cell may open up a new route for the applications of phase gradient metasurfaces in the microwave band region, and may also used as an alternative of high-gain antenna.
An improved centroid localization algorithm based on iterative computation for wireless sensor network
Authors: Jiang Rui  , Yang Zhen  ,   College of Telecommunications and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Published: December 2020

Wireless sensor network (WSN) is a basic component of internet and it plays an important role in many application areas, such as military surveillance, environmental monitoring and medical treatment. Node localization is one of the interesting issues in the field of WSN. Now, most of the existing node localization algorithms can be divided into two categories. One is range-based measurement and the other is range-free measurement. The localization algorithm of range-based measurement can achieve better location accuracy than the localization algorithm of range-free measurement. However, they are generally very energy consuming. Therefore, the range-free measurements are most widely used in practical applications. According to the application of localization algorithm in WSN by range-free measurements, an improved centroid localization algorithm based on iterative computation for wireless sensor network is proposed. In this algorithm, the position relationship of the closed area surrounded by the anchor nodes inside the unknown node's communication range and the unknown node is obtained by approximate point-in-triangulation test at first. Different position relationships determine different stopping criteria for iteration. Then, the centroid coordinates of the closed area surrounded by the anchor nodes inside the unknown node's communication range and the received signal strength (RSSI) between the centroid node and the unknown node are calculated. The anchor node with the weakest RSSI would be replaced by the centroid node. By this method, the closed area surrounded by the anchor nodes inside the unknown node's communication range is reduced. The location accuracy is increased by the cyclic iterative method. With the change of the anchor node ratio, the communication radius of the unknown node and the effect of RSSI error, the algorithm performance is investigated by using simulated data. The simulation results validate that although the improved centroid localization algorithm performance will be lost when the number of the anchor nodes inside the unknown node communication range decreases, the new approach can achieve good performance under the condition of few anchor nodes inside the unknown node communication range and this method is of strong robusticity against RSSI error disturbance.
Supersymmetry of the solution to the master equation between two-level atom and laser channel
Authors: Ren Yi-Chong  , Fan Hong-Yi  ,   University of Science and Technology of China, Hefei 200036, China
Published: December 2020

We propose a new Ket-Bra entangled state (KBES) method to solve the master equation of finite-level system. The KBES method can convert the master equation into Schrdinger-like equation which is easier to solve than the master equation, and Schrdinger equation in a certain form can also be used to solve the Schrdinger-like equation. Thus the KBES method has a wider application range. In the paper, we mainly study the master equation of the two-level atom. The corresponding master equation is solved by the KBES method, and for the first time we obtain the opera-sum solution of the atom. Furthermore, we compare this result with the well known solution that describes the laser channel. There is much analogousness between both opera-sum solutions, which show that there is some supersymmetry between Bose creation-annihilation operator and upper-down transition operators of atom. Finally, we further analyze the supersymmetry between the bose and atom system, and find that the spin-up and spin-down operator can be represented by the creation and annihilation operator repectively, which can be achieved in infinite ways. It is easy to understand that the bose operator is infinite-level while the spin operator is two-level, thus the creation-annihilation operator is super-complete for the spin operator. Thus the representation is not unique, and all of this directly shows and proves the supersymmetry.
Static bifurcation analysis of natural circulation inner evaporative cooling system in wind turbine
Authors: Wang Hai-Feng  , Li Wang  , Gu Guo-Biao  , Shen Jun  , Teng Qi-Zhi  ,   Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China;,   Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China;,   School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
Published: December 2020

Wind power is one of the most attractive renewable clean energies under development at present. On a global scale, wind power generation development was very rapid in recent years. As the wind power generation tends to develop toward large-scale and offshore, the traditional cooling methods gradually expose their own shortcomings. As the large wind turbine installation tower is high, and the installation site is dispersed, the installation and maintenance of wind turbine generator are difficult. So, the generator is required to have a small weight and less maintenance. Self-circulation inner evaporative cooling system (SCIECS) has the following advantages: self-circulation without pump, high cooling efficiency, safe and reliable operation, and basically maintain-free, etc. The self-circulation of cooling system can be realized by the 35 between wind turbine generator and the horizontal direction, and it is very suitable for being used in a large-scale wind power generator. Owing to the intrinsic nonlinearity of two-phase self-circulation system, changes of operation condition and circuit topology in a large range may lead to an instability of the cooling system, causing the system parameters to severly change. The instability of the cooling system can cause the local overheating and even burning of the generator, which provides a huge security risk for the cooling system, thus threatening the safe and stable operation of the generator. The stability of SCIECS is very important for the safe operation of wind turbine. In this paper, static stability of the SCIECS in wind power generator is studied based on the nonlinear bifurcation analysis theory and its numerical continuation method. System static bifurcation diagram is obtained to analyze the evolution characteristics of the SCIECS. Parameter effect of the system static bifurcation is analyzed at the same time. In order to verify the theoretical prediction of the static bifurcation of the small-angle two-phase natural circulation, an experimental platform is built. Static bifurcation of the SCIECS is observed experimentally. The experimental results show that the static bifurcation phenomenon exists in the natural circulation two-phase flow of small angle, and the theoretically predicted m-Q bifurcation curves are in good agreement with the experimental curves, which verifies the correctness of the theoretical analysis.
Newly exposed conflicting news based network opinion reversal
Authors: Wu Yue  , Du Ya-Jun  , Chen Xiao-Liang  , Li Xian-Yong  ,   College of Computer and Software Engineering, Xihua University, Chengdu 610039, China
Published: December 2020

Studying the process of network public opinion reversal is of great significance for guiding public opinion toward a positive direction. Currently, the research on opinion reversal mainly focuses on the construction of dynamic models and analysis of simulations and the results of which have a certain theory value. However, whether these models are applicable to the real social network environment has not been tested. For studying the process of public opinion reversal, we build a model according with the realities, and make an in-depth analysis of the typical case of opinion reversal. Some rules are found from the observation and statistics: the fundamental reason of public opinion reversal is the conflicting news. Spreading of news affects the opinions of the group. The news properties, including transmission rate, credibility, opinion polarity, publication date and the degree of message source determine the extent to reverse. Based on these rules, parameters of news properties are set, and a model of opinion reversal is proposed by combing the information dissemination with opinion evolution. Simulation results show that the transmission rate of news, the credibility of news, and the degree of message source have a positive influence on the margin of reversal. The influence of credibility is more dramatic than that of transmission rate. Moreover, the public opinion would be reversed more quickly and completely if the conflicting news is released more easily. The proposed model can fit the actual data, which is helpful for understanding and explaining the process of network public opinion reversal, and provides theoretical basis for guiding the network public opinion.
Homogenization and entropy analysis of a quadratic polynomial chaotic system
Authors: Zang Hong-Yan  , Chai Hong-Yu  ,   Mathematics and Physics School, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

In this paper, firstly we construct a quadratic chaotic system and prove that it is a topological conjugate system of Tent map. Secondly, having analyzed the probability density function of the system, we propose an anti-trigonometric function map. Additionally, the performances of the quadratic chaotic system such as information entropy, Kolmogorov entropy and discrete entropy are tested for both the original systems and the homogenized systems with different parameters. Numerical simulations show that the information entropy of the uniformly distributed sequence is close to the theoretical limit and the discrete entropy remains unchanged. This result shows that the homogenization method is effective. In conclusion, the chaotic sequence after homogenization not only inherits the diverse properties of the original sequence, but also exhibits better uniformity.
Collision reactions of CH radical on diamond and their effects on the carbon film growth
Authors: Song Qing  , Quan Wei-Long  , Feng Tian-Jun  , E Yan  ,   School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou 730070, China
Published: December 2020

The growth mechanism of hydrogenated carbon films in plasma-enhanced chemical vapor deposition (PECVD) is complicated and much attention has to be paid to it for the unique properties of carbon films. In this paper molecular dynamics simulations are carried out to illustrate the collision behaviors of CH radical on the clear and hydrogenated diamond (111) surface with varying incident energy (from 1.625 to 65 eV), aiming at the growth mechanism of hydrogenated carbon film by PECVD. Our simulations show that the behaviors of incident CH radical can be divided into adsorbing, rebounding, reaction releasing one H atom and reaction releasing two H atoms, while the reaction releasing one H2molecule rarely occurs. At low incident energy, selective adsorption of CH at unsaturated surface C site is the dominated growth mechanism since no reactions can conduct. Such growth model results in films with rough surface, high hydrogen fraction, and loose structure. As the incident energy increases, two chemical reactions that one releases one H atom and the other releases two H atoms are important. Caused by these reactions, the saturated C site in the surface will be transferred into unsaturated one, so that it can further adsorb subsequently incident CH radicals. The occurrence of these reactions makes films grow more uniformly, leading to the smoothness and dense structure of the films. The hydrogen fraction in the films will be reduced by these reactions. To confirm the above growth mechanism, the carbon film growth from CH radicals are then simulated. The film obtained with low energy (3.25 eV) CH radicals is found to be loose, rough, and have many carbon chains with adsorbed hydrogen atoms on the surfaces, while the film produced with high energy (39 eV) radicals are dense, smooth and the chains on the surfaces are short and have less hydrogens. On the other hand, most of the C atoms in the films deposited with low energy have one H atom as coordination, while for high energy most of C atoms in the films have no H atom as coordination. These observations agree well with the proposed growth mechanism. The destruction effects caused by the incident CH radicals are also analyzed based on the variation of the sp2-C and sp3-C in the films.
Theoretical studies of dielectronic recombination for Au34+ ions
Authors: Fu Yan-Biao  , Wang Xu-Dong  , Su Mao-Gen  , Dong Chen-Zhong  ,   Key Laboratory of Atomic and Molecular Physics and Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China;,   Joint Laboratory of Atomic and Molecular Physics, Northwest Normal University and Institute of Modern Physics of Chinese Academy of Sciences, Lanzhou 730070, China
Published: December 2020

Dielectronic recombination (DR) rate coefficients of complex structure ions are very important for spectral simulation in some application researches, such as nuclear fusion and extreme ultraviolet lithography. Theoretical calculations are made for dielectronic recombination rate coefficients of Au34+ ions by using a flexible relativistic atomic code. Influences of excitation and radiation channels, configuration interactions, and decays to autoionizing levels possibly followed by radiative cascades (DAC) on DR rate coefficient are analyzed. The contribution of DAC is evident. The total DR rate coefficient is greater than either the radiation recombination coefficient or three-body recombination coefficient for electron temperature greater than 1 eV. In order to facilitate simple applications, the total DR rate coefficients for the ground state and the first excited state are fitted to an empirical formula. These results should be useful for further analyzing the DR process of complex structures ions.
Cluster-plus-glue-atom model of FCC solid solutions and composition explanation of typical industrial alloys
Authors: Hong Hai-Lian  , Dong Chuang  , Wang Qing  , Zhang Yu  , Geng Yao-Xiang  ,   Key Laboratory of Materials Modification (Ministry of Education), Dalian University of Technology, Dalian 116024, China;,   School of Mechanical and Electrical Engineering, Sanming University, Sanming 365004, China
Published: December 2020

It was found previously by us that the compositions of industrial alloy specializations are related to the chemical short-range ordering in solid solution alloys, which is in accordance with the cluster-plus-glue-atom model. This model identifies short-range-ordered chemical building units in solid solutions, which the specific alloy compositions rely on. For instance, substitutional-type FCC solid solution alloys are described by cluster-based units formulated as [cluster](glue atom)16, where the bracketed cluster is the nearest-neighbor coordination polyhedral cluster, cuboctahedron in this case, and one-to-six glue atoms occupy the inter-cluster sites at the outer-shell of the cluster. In the present paper, we investigate the atomic configurations of these local units in substitutional-type FCC solid solutions by exhausting all possible cluster packing geometries and relevant cluster formulas. The structural model of stable FCC solid solutions is first reviewed. Then, solute distribution configurations in FCC lattice are analyzed by idealizing the measured chemical short-range orders within the first and second neighborhoods. Two key assumptions are made with regards to the cluster distribution in FCC lattice. First, the clusters are isolated to avoid the short-range orders from extending to longer range ones. Second, the clusters are at most separated by one glue atom to confine the inter-cluster distances. Accordingly, only a few structural unit packing modes are identified. Among them, the configurations with glue atoms 0, 1, 3, and 6 show good homogeneities which indicate special structural stabilities. Finally, compositions of FCC Cu-Zn (representative of negative enthalpy systems) and Cu-Ni (positive enthalpy ones) industrial alloys are explained by using the structure units of cluster packing and the cluster formulas, expressed as [Zn-Cu12]Zn1-6 and [Zn-Cu12](Cu, Zn)6, where the cluster is Zn-centered, shelled with Cu atoms, and glued with one to six Zn or with a mixture of six Cu and Zn. In particular, the formula [Zn-Cu12]Zn6, with the highest Zn content, corresponds to the solubility limit in Cu-Zn alpha phase zone, which is also the composition of the specification C27400. The Cu-rich Cu-Ni alloys are explained by cluster formulas [Cu-Cu12](Cu, Ni) 6, where the cluster is Cu centered and glued with a mixture of six Cu and Ni. The Ni-rich Monel alloy is explained by cluster formulas [Ni-Ni12](Cu5Ni)-[Ni-Ni12]Ni6. The present work provides a new approach to alloy composition explanation and eventually to alloy composition design from the perspective of short-range ordering in solid solutions.
Dynamical evolution study of metal nanofilms on graphite substrates
Authors: Li Yan-Ru  , He Qiu-Xiang  , Wang Fang  , Xiang Lang  , Zhong Jian-Xin  , Meng Li-Jun  ,   Faculty of Physics and Optoelectronic Engineering, Xiangtan University, Xiangtan 411105, China;,   Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, Xiangtan University, Xiangtan 411105, China
Published: December 2020

The dynamical evolution process of nanoscaled film on a solid substrate depends on many factors, such as the properties of thin film, the characteristics of the substrate, and the external environment. It is essential to elucidate the influences of these factors for our understanding self-organized growth of nanoparticles and the dewetting/detachment mechanism of nanofilm on a solid substrate. In the present paper, we investigate the dynamical dewetting/detachment of metal Au and Pt nanofilm on a graphene/graphite substrate at high temperature by using the molecular dynamics simulation technique. We discuss the influences of metal-substrate interaction, temperature and thickness of film on the dewetting dynamics. Our results reveal that the Au and Pt nanofilms with the same initial thickness on graphene substrates manifest different dewetting dynamical processes at high temperatures. Some nanoscale holes are formed randomly during the dewetting of Pt nanofilm with a thickness of less than 0.6 nm because of the strong interaction between the Pt films and substrate. In contrast, no hole is observed and a nanodroplet is formed directly by high temperature dewetting for Au nanofilm with the same initial thickness as that of Pt nanofilm. The resulting Au and Pt nanodroplets move in the vertical direction due to the surface tension and the constraint of the solid substrate. A high-temperature nanodroplet will be detached from the graphene substrate surface at a constant speed. Interestingly, the values of detachment velocity (vd) of nanodroplets show different dependences on initial thickness for Au and Pt nanofilm, respectively. In a thickness range of 0.2-2.3 nm, the vd of Pt nanodroplet increases and then decreases as the thickness of nanofilm increases. However, the vd of Au nanodroplet decreases gradually and then increases steeply as the Au nanofilm turns thicker. The different thickness dependences of vd for Au and Pt nanofilms are analyzed qualitatively by considering different metal-substrate viscous dissipations. In addition, the detachment time (td) of a dewetting metal film is also related to the temperature and the thickness of substrate. Our results demonstrate that the td decreases monotonically with the decrease of film thickness and the raise of temperature. These results provide a theoretical guideline for industrial production processes, such as metal coating, flotation, and the surface cleaning.
First-principle study of the magnetism and photocatalyticactivity of RE(La/Ce/Pr/Nd) doping anatase TiO2
Authors: Li Cong  , Zheng You-Jin  , Fu Si-Nian  , Jiang Hong-Wei  , Wang Dan  ,   Department of Physics, Mudanjiang Normal University, Heilongjiang Laboratory of New Carbon-base Functional and Superhard Material, Mudanjiang 157011, China
Published: December 2020

The models of pure TiO2 and La/Ce/Pr/Nd singly doped TiO2 are established by using the plane wave potential based on density functional theory. After geometry optimization, the stability of the mixed structure is analyzed by calculating the formation energy. Then the magnetic state of each doped model is analyzed by calculating the spin electronic state density. The analyses are verified by comparing magnetic ground state energies. Finally the influences of each rare earth element on band structure and absorption spectrum of TiO2 are discussed. The results show that La/Pr doped TiO2 presents ferromagnetism, Nd doped TiO2 exhibits anti-ferromagnetism, and Ce doped TiO2 is paramagnetic body. The band structure of TiO2 is affected less because Ce is doped and the red shift of absorption spectrum is not obvious. While visible light absorption coefficient of TiO2 is effectively improved because La/Nd is doped. Pr doped TiO2 manifests an absorption peak in the infrared region. If the electronic structure is considered in the further calculation research, one should make sure what magnetic state the system is in and whether there is spin-electron band splitting effect firstly. In order to obtain the correct results, the influence of the band structure should not be ignored.
Electrical properties of LaTiO high-k gate dielectric Ge MOS Capacitor and Ti content optimization
Authors: Xu Huo-Xi  , Xu Jing-Ping  ,   Department of Electronic Information, Huanggang Normal University, Huangzhou 438000, China;,   School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Ti is intentionally added into La2O3 to prepare LaTiO gate dielectric Ge metal-oxide-semiconductor (MOS) capacitor with both high k value and good interface quality. In order to examine the effects of Ti content on the electrical properties of the device, LaTiO films with different Ti/La2O3 ratios (10.6%, 18.4%, 25.7% and 31.5%) are deposited by reactively co-sputtering Ti and La2O3 targets. Capacitance-voltage curves, gate-leakage current properties and high-field stress characteristics of the devices are measured and analyzed. It is found that some electrical properties, such as interface-sate density, gate-leakage current, device reliability and k value, strongly depend on Ti content incorporated into La2O3. Ti incorporation can significantly increase the k value: the higher the Ti content, the larger the k value is. The relevant mechanism lies in the fact that higher Ti content leads to an increase of Ti-based oxide in the LaTi-based oxide, because Ti-based oxide has larger k value than La-based oxide. On the contrary, interface quality, gate-leakage current and device reliability deteriorate as Ti content increases because Ti-induced defects at and near the interface increase with Ti content increasing. Of the Ti/La2O3 ratios in the examined range, the largest Ti/La2O3 ratio is 31.5%, which results in the highest k value of 29.4, the largest gate-leakage current of 9.710-2 Acm-2 at Vg=1 V, the highest interface-sate density of 4.51012 eV-1cm-2 and the worst device reliability, while the La2O3 film without Ti incorporation exhibits the lowest k value of 11.7, the smallest gate-leakage current of 2.510-3 Acm-2 at Vg=1 V, the lowest interface-sate density of 3.31011 eV-1cm-2 and the best device reliability. As far as the trade-off among the electrical properties is concerned, 18.4% is the most suitable Ti/La2O3 ratio, which leads to a higher k value of 22.7, lower interface-sate density of 5.51011 eV-1cm-2, an acceptable gate-leakage current of 7.110-3 Acm-2 at Vg=1 V, and a better device reliability. In view of the fact mentioned above, excellent electrical properties could be obtained by setting Ti content to be an optimal value. Therefore, the optimization of Ti content is critical for LaTi-based oxide Ge MOS device preparation.
Theoretical analysis of the upper critical field of two-band superconductor LaNiC2
Authors: Liu Min-Xia  , He Lin  , Zhang Geng  , Ye Hai  , Huang Xiao-Yuan  , Xu Yong-Zhao  ,   School of Electronic Engineering, Dongguan University of Technology, Dongguan 523808, China
Published: December 2020

LaNiC2 is one of ternary RNiC2 compounds, where R is a rare earth or Y. Its space group is Amm2. the symmetry along the c-axis of the crystal structure lacks inversion symmetry along the c-axis. In 2009, Hillier et al. performed the muon spin relaxation experiment (upSR) which implied that time-reversal symmetry is broken in LaNiC2. As a weak correlation noncentrosymmetric superconductor, LaNiC2 has attracted wide research interest in recent years. Though a lot of theoretical and experimental studies have been carried out, the order parameter of this compound remains highly controversial. The measurements of specific heat and nuclear quadrupole relaxation suggest that LaNiC2 is normally BCS-like, which is further supported by theoretical calculations. But recently another study showed that the London penetration depth depends on T2 below 0.4 Tc indicative of nodes in the energy gap. Evidence of possible nodal superconductivity can also be inferred from the early measurements of specific heat given by Lee et al. However, the experimental results obtained by Chen et al. supported the existence of two-gap superconductivity in LaNiC2.Based on the above case, the two-band Ginzburg-Landau theory is used to study the temperature dependence of the upper critical field for the superconductor LaNiC2 in this paper. Choosing the Ginzburg-Landau theory for calculating the upper critical field is just because Ginzburg-Landau theoretical model is simple, easy to understand, low-calculation, and the clear physical meanings of the parameters. The theoretical results in this paper accord with the experimental data very well in the whole temperature range. The curve of Hc2 (T) has an obvious positive curvature near the critical temperature, which is typical feature of multi-gap superconductor. Therefore, our results show strong evidence that two-gap scenario is better to account for the superconductivity of LaNiC2, consistent with the results of Chen Jian et al. The influences of two different energy bands on the upper critical field are also studied. It is found that the relatively small coherent length has a grester influence on the upper critical magnetic field of LaNiC2. So if we want to improve the upper critical field of LaNiC2, reducing the relatively small coherence length can be achieved in theory.
In situ tuning hydrostatic pressure at low temperature using electrically driven diamond anvil cell
Authors: Ding Kun  , Wu Xue-Fei  , Dou Xiu-Ming  , Sun Bao-Quan  ,   State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Published: December 2020

Traditionally, a diamond anvil cell (DAC) operated at low temperature can be pressurized by using a helium-driven piston or remote control tightening mechanism. This approach of pressurizing DAC is not convenient for operating at low temperature. Here we develop a low-temperature pressurizing technique for in situ tuning pressure in DAC at 20 K by an electrically driven method. The improved DAC pressure apparatus is composed of traditional DAC device and a piezoelectric actuator (PZT). Here the PZT used in the experiment is the PSt 150/1010/40 supplied by the Piezomechanik. Both parts are assembled together in a red copper or stainless steel cylinder. The DAC part is thermally contacted with a low temperature holder for cooling the chamber of the DAC in the experiment. The wires of the PZT connect with the voltage source through the wiring terminals of the cryostat. As the DAC apparatus cools down, two electrodes of the PZT are connected together when a voltage difference between the electrodes is generated. When the temperature of the DAC chamber arrives at the presetting value, two electrodes of the PZT are connected with the voltage source for applying voltage to the PZT. In this paper, we find that the PZT stroke shows a linear increase with increasing voltage at 300 K, whereas it is approximately linear at 80 and 6 K. The maximum strokes are 40, 26 and 15 upm at 300, 80 and 6 K respectively when the applied voltage is 120 V. The experimental results show that the PZT-driven DAC apparatus can continuously generate pressure from 0.49 to 4.41 GPa at low temperature and applied voltage of 0-290 V, where at zero voltage an initial pressure of 0.49 GPa is generated by using driven screws of the DAC device at room temperature. The pressure in the DAC chamber is determined by the red shift of ruby florescence line. The calibrated chamber temperature in DAC is determined as a function of pressure (PZT voltage) by using the intensity ration (R2/R1) of ruby R2 and R1 fluorescence lines. We find that the chamber temperature only slightly increases with increasing pressure in a range of (19 1) K. The main difference between the present device and the other tuning DAC apparatus is that the force on the DAC can be conveniently applied by using PZT voltage. This guarantees a high pressure-tuned resolution in the experiment, e. g., we tune a single InAs quantum dot (QD) emission wavelength to match the cavity mode. Such a tuning technique is found to have applications in realizing a compact tunable single photon source or completing two-photon interference of Hong-Ou-Mandel experiments between the QD and nitrogen vacancy center in diamond or atom, respectively.
Photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe quantum dots with a core-shell structure
Authors: Lin Ying-Ying  , Li Kui-Ying  , Shan Qing-Song  , Yin Hua  , Zhu Rui-Ping  ,   State Key Laboratory of Metastable Materials Technology and Science, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

The study on photoelectronic characteristics of ZnSe quantum dots (QDs) is of significance for investigating its microelectronic structure and expanding its potential applications because ZnSe QD has low biologic toxicity. In the present paper, the surface photovoltaic and photoacoustic technologies, and laser Raman, X-ray diffraction, transmission electron microscopy and Foureier transform infrared spectroscopy spectrum are jointly used to probe the microstructures, the photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe QDs prepared by water-phase synthesis at different reflux temperatures. The results indicate that the ZnSe QDs with a mean grain size of about 3 nm has a core-shell ZnSe/ZnS/L-Cys structure, in which the sulfhydryl groups in ligand prefer reacting with Zn atom at the (220) face to form the ZnS shell layer between the core-ZnSe and ligand L-Cys. The results show that the QDs with n-type photovoltaic property display a wide range of surface photovoltaic response and weak photoacoustic signal upon the illumination of near ultraviolet to visible light as compared with others QDs with similar core-shell structures in II-VI group. Especially, the strong SPV response and the weak PA signal in a wavelength region of 350-550 nm imply that the photon energies in the range are almost all used to produce the surface photovoltaic (SPV) phenomenon instead of the thermal lattice vibration caused by non-radiative de-excitation process. This reveals the energy complementary relationship between the photoacoustic and the surface photovoltaic phenomena of the QDs. The PA signals appearing in a short wavelength range of 300-350 nm and the Raman peaks located in a high frequency ranges of 1120 cm-1, 1340 cm-1 and 1455 cm-1 are identified as relating closely to the multi-phonon vibration modes of ligand L-Cys. At low reflux temperature, the photoelectric threshold of the SPV response that relates to the core-ZnSe displays a red shift to a certain extent as compared with the bulk ZnSe. The narrowed bandgap may be attributed to quantum confinement effect of the QDs. In addition, the intensity of the SPV response that relates to the core-ZnSe gradually increases with the decrease of the reflux temperature. The results show that the above improved surface photovoltaic characteristics of the QDs may benefit from the reduced average grain size of the ZnSe QDs, thus causing its surface and small-size effects.
Structures and photoluminescence properties of Se and SeMo2 nanoflakes
Authors: Wang Bi-Ben  , Zhu Ke  , Wang Qiang  ,   College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China;,   Division of Technical Support, Institute of Physics, Chinese Academy of Science, Beijing 100190, China;,   State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China
Published: December 2020

Se and MoSe2 nanoflakes are prepared in N2 environment by hot filament chemical vapor deposition through using Se and MoO3 powders as the source materials. The structures and compositions of Se and MoSe2 nanoflakes are systemically studied by using field emission scanning electron microscope, transmission electron microscope, energy dispersive X-ray spectroscope, micro-Raman spectroscope, and X-ray photoelectron spectroscope. The results indicate that the mixing of the Se and MoO3 powders directly affects the formations and structures of Se and MoSe2 nanoflakes. When the Se and MoO3 powders are fully mixed, the Se nanoflakes are formed, however the MoSe2 nanoflakes are formed under no mixture of Se and MoO3 powders. This is due to the fact that different reactions of Se and MoO3 powders in gas environment with or without mixing the Se and MoO3 powders are generated. The study of photoluminescence properties indicates that the photoluminescence peaks are generated at about 774, 783 nm and 783, 784 nm for the Se and MoSe2 nanoflakes, respectively, which are different from the photoluminescence properties of monolayer MoSe2 nanosheet. These outcomes can enrich our knowledge of the synthesis and optical properties of two-dimensional Se-based nanomaterials and will contribute to the development of optoelectronic devices of two-dimensional Se-based nanomaterials.