bluuebunny/crossref_metadata_2025 huggingface dataset:
High frequency characteristics of dielectric-loaded grating for terahertz Smith-Purcell radiation
Authors: Cao Miao-Miao  , Liu Wen-Xin  , Wang Yong  , Zhu Jue-Yuan  , Li Ke  ,   Key Laboratory of High Power Microwave Sources and Technologies, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The research on a Smith-Purcell device becomes active since it holds promise in developing a high power, tunable, and compact terahertz radiation source. In this paper, a dielectric loaded grating for Smith-Purcell device is proposed. By investigating the interaction between the sheet electron beam and surface wave above the grating, the dispersion equation with electron beam is derived, in which the electron beam has a finite thickness. And then the growth rate of the beam-wave interaction is numerically calculated from the dispersion equation. In addition, the current threshold for oscillators, known as a start current, is carefully estimated from the dispersion equation by considering the boundary conditions of electromagnetic field. The effects of structure length, electron beam parameters and dielectric constant on start current are analyzed at length. The results reveal that the start current decreases as the structure length increases. This is because as the structure length becomes greater, the distance of the beam-wave interaction becomes longer, which can strengthen the beam-wave interaction. And with increasing beam thickness and beam-grating distance, the start current increases. Because the electric field of the surface wave decreases exponentially with the increase of distance from the grating, the electron beam far from the grating cannot be bunched by the field, which makes it harder for Smith-Purcell device to oscillate. However, as the beam voltage becomes greater, the start current decreases first quickly and then slightly. Compared with the case of metal grating, it can be seen that the use of dielectric can improve the growth rate and reduce the start current for the operation of a Smith-Purcell backward wave oscillator. The start current decreases quickly when the dielectric constant is greater than 1. Then it increases slightly when dielectric constant is between 2 and 3, and finally the start current continues to decrease. But it cannot be helpful to choose a very big value of dielectric in order to obtain a low start current, because the operation frequency decreases as dielectric constant increases. It is more appropriate to choose a dielectric constant in a required frequency range. The predictions of our theory and the results from the particle-in-cell simulation are consistent with each other, which verifies the validity and accuracy of the theory in this paper.
Synthesis, structure and spectroscopic properties of Nd3+:SrY2O4 phosphor
Authors: Peng Fang  , Zhang Qing-Li  , y Wang Xiao-Fei  , Zhang Hui-Li  , Ding Shou-Jun  , Liu Wen-Peng  , Luo Jian-Qiao  , Sun Dun-Lu  , Sun Gui-Hua  ,   Key Laboratory of Photonic Devices and Materials of Anhui Province, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In order to obtain new-type laser crystals, SrY2O4 is chosen as a host material. Because Y3+ ions in SrY2O4 occupy two non-equivalent sites, it might be possible to realize dual-wave laser and broadband emission at 1.06 m by partially replacing Y3+ with Nd3+. In this work, (3 at.%) Nd3+ doped SrY2O4 phosphor is synthesized by the conventional solid state reaction. The structure and luminescence properties in the visible and near-infrared ranges are studied. The peaks in the X-ray powder diffraction pattern of (3 at.%) Nd3+:SrY2O4 can be well indexed according to ICSD#25701. The lattice parameters, atomic coordinates, atomic temperature factors etc., are obtained by the Rietveld refinement with R_p of 4.68% and R_wp of 5.91%. According to the excitation spectra in a range of 220-380 nm, it can be seen that Nd3+:SrY2O4 is efficiently excited by 353 nm which is assigned to the 4I9/24D3/2+4D5/2+2I11/2+4D1/2 transition of Nd3+ ions. Under the 353 nm light excitation, Nd3+:SrY2O4 exhibits the strongest emission at 419 nm corresponding to the 2D15/24I9/2 transition of Nd3+ ions. What is more, Nd3+:SrY2O4 can be excited effectively by 824 nm light, which matches well with the commercial 830 nm diode laser. When excited with 824 nm, the strongest fluorescence peak is located at 1083 nm with a wide bandwidth of about 90 nm. Compared with that at 8~K, the bandwidth in the fluorescence spectrum at 300 K is broadened because of the homogeneous broadening induced by the increase of temperature. Additionally, the peaks corresponding to the 4F3/24I11/2 transition are split into two groups at 8~K, which results from the two non-equivalent sites of Nd3+ ions. Compared with Nd3+:YAG, Nd3+:SrY2O4 has more potential applications in the tunable and ultrashort lasers. The fluorescence lifetime of the 4F3/24I11/2 transition of (3 at.%) Nd3+:SrY2O4 is 281.7 s, which shows slight concentration quenching compared with that of (0.5 at.%) Nd3+:SrY2O4. The fluorescence lifetime of (3 at.%) Nd3+:SrY2O4 is much longer than that of (0.6 at.%) Nd3+:YAG which is beneficial to the energy storage. In conclusion, the wide emission band and the long decay time of 1.08 m indicate that Nd3+:SrY2O4 is a very promising new-wavelength and ultrashort laser material pumped by laser diode.
Polarization switching with low power consumption induced by optical feedback in long-wavelength vertical-cavity surface-emitting lasers
Authors: Wang Xiao-Fa  , Wu Zheng-Mao  , Xia Guang-Qiong  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China;,   School of Optoelectronics Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China;,   School of Mathematics and Statistics, Southwest University, Chongqing 400715, China
Published: December 2020

The polarization switching (PS) characteristics of vertical cavity surface emitting lasers(VCSELs) have received sustained attention for the past years. With the development of manufacturing technology, the performances of 1550 nm VCSELs have been improved, however the researches on the PS of 1550 nm VCSELs are relatively inadequate for the PS characteristics in the long-wavelength VCSELs may have wide application prospects in optical information processing and optical communications. In this paper, based on the extended spin-flip model (SFM), we theoretically investigate the PS with low power consumption induced by optical feedback in long-wavelength VCSELs. Results show that the PS, which is failed to realize in free-running long-wavelength VCSELs, can be achieved by introducing a moderate-strength polarization-rotation optical feedback. By comparing two different linear dispersion effects, some interesting phenomena have been found. For weak linear dispersion, the PS is relatively easy to realize for a low injection current level, and the range of feedback strength used to control the PS is wide. However, for strong dispersion effect, the PS cannot be obtained all the time since two mode-coexisting zones will appear, and the value of injection current where the PS happens is relatively high. Meanwhile, as observed in short-wavelength VCSELs, the polarization mode hopping and multiple PS have also been found in long-wavelength VCSELs, indicating that the physics nature thet induces the PS is similar for both long and short wavelength VCSELs. In addition, because the PS in long-wavelength VCSEL is more difficult to realize as compared with that in short-wavelength VCSELs, reasonable analyses and explanations may be as follows: since the linear dispersion effect in 1550 nm-VCSEL is much stronger than that of short wavelength VCSEL, the frequency difference between the two linear polarization modes is up to 60 GHz (or 0.48 nm), thus leading to the decrease of the correlation between two linear polarization modes. As a result, it is relatively difficult to obtain the PS phenomenon at low injection current level in long-wavelength VCSEL; while using suitable polarization-rotated optical feedback can partially compensate the deficiency of this correlation. We believe that the results obtained in this work will be helpful in investigation of low power consumption for all optical buffers by using long-wavelength VCSELs.
Structural features of critical jammed state in bi-disperse granular systems
Authors: Zhang Wei  , Hu Lin  , Zhang Xing-Gang  ,   College of Science, Guizhou University, Guiyang 550025, China
Published: December 2020

A jammed state is a common phenomenon in complex granular systems, in which the relationship between the mechanical properties and the geometric structures is very complicated. The critical jammed state in a two-dimensional particle system is studied by numerical simulation. The system is composed of 2050 particles with two different radii, whose distribution is random. Initially the particles with a smaller radius are of a looser distribution in the given space. When the radius increases, a transition from the looser state to the jammed state happens. The particle dimension-radius ratio and the percentage of large particles kB play primary roles in this system, which are discussed in detail based on the statistical analysis of the average contact number, packing fraction, and contact type. By analyzing the relationship between pressure and packing fraction of the granular system, the critical jammed point for the applied pressure to the boundary can be found. Numerical simulation result shows that no obvious connection exists between the average contact number and the percentage of large particles for the case that the particle dimension-radius ratio is less than 1.4. The average contact number approximate to 4 when = 1.4, which is consistent with previous conclusions. The average contact number first decreases and then increases when the percentage of large particles become larger in the case 1.4. A minimum value C = 0.84 is obtained when kB = 0.5. When the percentage of large particles increases, the critical packing fraction decreases first and then increases in the case 1.8, but it almost keeps constant for 1.8. When the percentage of large particles is close to either 0% or 100%, the granular system is approximately mono-disperse. In this case, the average contact number and packing fraction become constant. When the percentage is close to 50%, the critical average contact number decreases all the time with larger particles-radius ratio, while the critical packing fraction decreases first and then increases. The percentage of large-small contact type is also discussed. The value varies following a quadratic function with the increase of the percentage of large particles, while the particles-radius ratio has slight impact on this variation. Specifically, we have calculated the percentage of large-small contact type based on probabilistic method, and the result agrees well with the simulation results. We give the reason why previous researchers studied the case of = 1.4 :1 and kB = 0.5 on the basis of results in this paper, and find that the values of and kB have no influence on the power-law relation around the critical jammed state.
Effect of H and He on the mechanical properties of Ti3SiC2: the first-principles calculation
Authors: Yao Bao-Dian  , Hu Gui-Qing  , Yu Zhi-Shui  , Zhang Hui-Fen  , Shi Li-Qun  , Shen Hao  , Wang Yue-Xia  ,   School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China;,   Institute of Modern Physics, Fudan University, Shanghai 200433, China
Published: December 2020

Layered MAX phase ternary compounds (M = early transition metals, A = group A elements, and X = C or N) show promise of wide applications in many applied fields because these compounds have combined ceramic and metallic properties. As an exemple of the MAX phase family, Ti3SiC2 exhibits a high melting temperature, high electrical and thermal conductivities, and an excellent resistance to oxidation and thermal shock. Particularly, it possesses unusual mechanical properties, such as easy machinability, high Young's modulus, thus it is considered as a candidate in advanced nuclear reactors.In this work, we investigate the effect of hydrogen and helium on the cleavage fracture of Ti3SiC2 in order to evaluate the reliability of Ti3SiC2 used in nuclear industry. We have performed first-principles mechanical calculations by using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Uniaxial tensile simulations along c-axis have been done to calculate the stress-strain curve and the cleavage energy for each interlayer of Ti3SiC2. It is found that Ti3SiC2 has the cleavage characteristics, and the habit cleavage plane starts from Si-Ti interlayer because of relatively weak Si-Ti bond. Hydrogen and helium always accumulate in the Si layer. Helium decreases largely the critical stress of cleavage fracture of Ti3SiC2. In contrast, hydrogen does not efficiently affect the cleavage fracture in Ti3SiC2. The difference between helium and hydrogen behaviors in Ti3SiC2 originates primarily from the difference of electronic hybridization with lattice atoms of Ti3SiC2. For helium, the neighboring Si atoms will be ejected by helium atoms, and the Si-Ti bonds will be broken, thus resulting in the cleavage fracture. However, for hydrogen, it is primarily hybridized with the s states of neighboring Si atoms, which does not severely disturb the p-d hybridization between Si and Ti atoms. Thus, the cleavage fracture from Si-Ti interlayer is hardly aggravated in the presence of hydrogen. Fortunately, Ti3SiC2 has a self-repair ability at high temperatures. It will desorb helium atoms at high helium pressure through Si layers. This behavior will alleviate the cleavage fracture induced by helium. In summary, Ti3SiC2 may be a potential material applied in light water or other fission reactors in the future.
Three-dimensional adiabatic mode parabolic equation method and its applications
Authors: Qin Ji-Xing  , Katsnelson Boris  , Peng Zhao-Hui  , Li Zheng-Lin  , Zhang Ren-He  , Luo Wen-Yu  ,   State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences Beijing 100190, China;,   Department of Marine Geosciences, School of Marine Sciences, University of Haifa, Haifa 31905, Israel;,   Haikou Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Haikou 570105, China
Published: December 2020

Complex zone of the ocean is often characterized by horizontal variations of environmental parameters(bathymetry, sound speed profile, bottom properties etc.), initiating redistribution of the sound field in horizontal plane, which is the so-called three-dimensional (3D) effect. Based on the adiabatic mode parabolic equation method, modeling of 3D effects is carried out, in which the eigenvalues and eigenfunctions are calculated by the standard normal mode model KRAKEN, and the amplitude corresponding to each mode is computed by the wide-angle parabolic equation model RAM. The present 3D model is very efficient and can give clear physical meaning, but it can be only applied to a waveguide whose properties vary gradually with horizontal range due to the adiabatic assumption between different modes. This model is then used to analyze the horizontal refraction caused by internal waves and by a coastal wedge. The numerical results show that there are several areas in the horizontal plane, corresponding to different structures of intensity distributions. Moreover, the redistribution of the sound field in horizontal plane depends on source frequency and mode number. Frequency and modal dependences lead to variations of spectrum, distortion of signal with some spectrum, and spatiotemporal fluctuations of the sound field.
Optical bandgap and absorption spectra of Y doped ZnO studied by first-principle calculations
Authors: Qu Ling-Feng  , Hou Qing-Yu  , Zhao Chun-Wang  ,   College of Science, Inner Mongolia University of Technology, Hohhot 010051, China;,   College of Arts and Sciences, Shanghai Maritime University, Shanghai 201306, China
Published: December 2020

The studies on absorption spectra of Y-doped ZnO have presented two distinctly different experimental results, which are the red shift and blue shift on the optical bandgap and absorption spectra when the mole fraction of impurity increases from 0.0313 to 0.0625. To solve this contradiction, the calculations in this paper are carried out by the CASTEP tool in the materials studio software based on the first-principal calculations of norm conserving pseudopotential of the density functional theory, and the geometric structures of ZnO, Zn0.9687Y0.0313O, Zn0.9583Y0.0417O and Zn0.9375Y0.0625O systems are constructed. By using the method of GGA+U, we calculate the band structure, density of state, electron density difference, population, orbital charges and absorption spectrum. The results show that with the doping amount increasing from 0.0313 to 0.0625, both the lattice parameters and the volume of doping system increase: the higher the total energy of the doping system the higher the formation energy of the doping system is, thereby making doping difficult and the stability of the doping system lower Increasing Y-doping concentration weakens the covalent bond, strengthens the ionic bond; as Y doping concentration increases, the Mulliken bond populations and bond lengths of Y-O parallel and vertical to c-axis decrease for the doping system. Meanwhile, the more the Y doping content, the wider the optical bandgap of the doping system becomes and thus more significant the blue shift of absorption spectra of Y-doped ZnO systems will be. The calculation results of absorption spectra of Y-doped ZnO system are consistent with the experimental data. And the contradiction between blue shift and red shift of absorption spectra of Y-doped ZnO system is explained reasonably. These results may contribute to the improvement of the design and the preparation of short wavelength optical devices from Y-doped ZnO.
Thermoluminescence and optically stimulated luminescence characteristics of Al2O3:C films annealed at different tempeartures
Authors: Wu Li  , Wang Qian  , Li Guo-Dong  , Dou Qiao-Ya  , Ji Xu  ,   School of Physics Science and Technology, Xinjiang University, Urumqi 830046, China
Published: December 2020

-Al2O3:C crystal is a high sensitive luminescence dosemeter, and it possesses a high thermoluminescence (TL) sensitivity, approximately 40-60 times greater than LiF: Mg, Ti. However, the crystal growth requires sophisticated laboratories, high temperatures and highly reducing atmosphere. The fluorescence and TL characteristics of -Al2O3:C ceramic are similar to those of -Al2O3:C crystal, however, it shows three TL peaks. In this work, porous alumina membranes are prepared by two-step anodization in 0.5 M/L oxalic acid at 5 ℃. We investigate the influence of annealing temperature ( 600 ℃) on thermoluminescence (TL) and optically stimulated luminescence (OSL) characteristics of Al2O3:C films and discuss the influence mechanism. The scanning electron microscopy measurement reveals that Al2O3:C film possesses highly ordered nanopores with homogeneous dimensions arranged in a closed-packed hexagonal pattern. The energy dispersive X ray spectroscopy and the Fourier transform infrared spectroscopy results indicate that oxalic acid impurity is incorporated into the porous alumina membrane in the synthesis process, after the annealing treatment, the oxalic acid impurity decomposes and C2+ replaces Al3+, which leads to the formation of F+and the C content of samples increasing with elevated annealing temperature. The X-ray diffraction measurement reveals that Al2O3:C films annealed at different temperatures are amorphous. TL measurements show that the dominated peak of Al2O3:C film is centered at around 310 ℃, owing to the number of F+increasing with the annealed temperature increasing, under the same irradiation dose, the sample annealed at 600 ℃ has the greatest TL intensity. With the increase of the irradiation dose, the TL intensity increases and the dominated peak gradually shifts to high temperature, which is consistent with the general order kinetic model. The sample annealed at 600 ℃ has the greatest TL sensitivity and its TL response shows excellent linear characteristic in as dose range of 1-10 Gy, but shows super-linear behavior in a dose range of 10-120 Gy. The OSL measurements show that with the increases of the annealed temperature and the irradiation dose, the OSL initial intensity increases and each of all samples shows a typical exponential decay. Compared with the case of -Al2O3:C crystal, the fast attenuation rate of film is dramatically accelerated. In a dose range of 1-200 Gy, the OSL responses of all samples each show an excellent linear characteristic, the sample annealed at 600 ℃ has the greatest OSL sensitivity. Compared with TL response, OSL response of Al2O3:C film shows a wider range of linear dose response. In this paper we have made a beneficial exploration for Al2O3:C films as OSL dosimerer.
Enhancement mode AlGaN/GaN double heterostructure high electron mobility transistor with F plasma treatment
Authors: Wang Chong  , Zhao Meng-Di  , Pei Jiu-Qing  , He Yun-Long  , Li Xiang-Dong  , Zheng Xue-Feng  , Mao Wei  , Ma Xiao-Hua  , Zhang Jin-Cheng  , Hao Yue  ,   Key Lab oratory of Wide Band gap Semiconductor Materials and Devices, the Institute of Microelectronics, Xidian University Xi'an 710071, China
Published: December 2020

Effects of double heterostructure materials (AlGaN/GaN/AlGaN/GaN) with different GaN channel thickness values (14 nm, 28 nm, 60 nm) on the high electron mobility transistor (HEMT) are simulated by using silvaco, and furthermore, the differences in characteristic among the enhancement mode devices made from such double heterostructure materials with different F injection doses (150 W, 135 W) are also simulated. The simulation results show that the threshold voltage shifts towards positive direction and the saturation current decreases as the GaN channel thickness decreases. The two-dimensional electron gas (2 DEG) density could be reduced as GaN channel thickness decreases due to piezoelectric polarization weakened by backing AlGaN barrier. Combining F plasma treatment and double heterostructure material, the enhancement mode device with high positive threshold voltage is successfully developed. The DC characteristics of the enhancement mode devices with different GaN channel thickness values are analyzed comparatively, and the simulation results are validated by using the experimental results. The threshold voltages of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 1.1 V, 0.8 V, and 0.3 V, respectively. The maximum transconductance values of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 115 mS/mm, 137 mS/mm, and 198 mS/mm, respectively. The thinner GaN channel thickness in the double heterostructure could reduce the depth of quantum well and 2 DEG density, so that the device with a GaN channel thickness of 14 cm has a lower saturation current. The breakdown voltages and gate reverse leakage currents of the three kinds of devices are investigated, and the device with a thinner GaN channel has a lower leakage current and higher breakdown voltage due to weakened vertical electrical field in thinner channel double heterostructure. The damage of channel mobility in F plasma treatment is weakened by using a lower plasma power (135 W), and the enhancement mode device without annealing process demonstrates a better saturation current and transconductance characteristic. The results of the device with annealing confirm that the plasma damage is depressed at an F injection power of 135 W. The threshold voltage temperature stability of 14 nm GaN channel thickness device is studied, and Vth is only 0.4 V after 350 ℃ 2 min annealing process. Drain induced barrier lowering (DIBL) effects of the HEMTs with double heterostructures are investigated, and the DIBL value of the14 nm GaN channel device is 16 mV/V. The DIBL value indicates a good limiting property of the 2 DEG in double heterostructure device.
An analytical model for shielding effectiveness of double layer rectangular enclosure with inner strip-shaped metallic plate
Authors: Hao Jian-Hong  , Gong Yan-Fei  , Fan Jie-Qing  , Jiang Lu-Hang  ,   School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China
Published: December 2020

An analytical model for double-layer shielding metallic enclosure with inner strip-shaped metallic plates is proposed for analyzing the shielding effectiveness of complex enclosures containing multiple spatial structures, based on the actual situation that the circuit systems in each spatial region are inevitably interfered by the electromagnetic penetration of those in adjacent spatial regions through slots, cables, and other transmission channels. The near-field electromagnetic interference of the external layer of the enclosure is represented by the equivalent electric dipoles, and the internal layer of it is regarded as the target point of shielding effectiveness, the analytical formulas of the internal layer's electromagnetic field then are derived based on Bethe's small aperture coupling theory and generalized Green's function. The model is employed to analyze the influences of some parameters of the strip-shaped metallic plate on the shielding effectiveness. It is shown that the position and direction of the strip-shaped metallic plate have obvious influence on the shielding effectiveness of the target point of the internal layer, embodied by the shielding values over some frequency ranges and different resonant modes, and at the same time the corresponding physical mechanisms are also given in detail. Comparison with the full wave simulation software CST has verified the model over a very broad frequency range, which provides a theoretical reference for the rapid calculation for shielding effectiveness of complex enclosures.
Thickness-dependent electronic structure of the interface of 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene/Ni(100)
Authors: Zhang Hong  , Niu Dong-Mei  , Lü Lu  , Xie Hai-Peng  , Zhang Yu-He  , Liu Peng  , Huang Han  , Gao Yong-Li  ,   Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, Central South University, Changsha 410083, China;,   Department of Physics and Astronomy, University of Rochester, Rochester 14627, USA
Published: December 2020

Combining ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy and atomic force microscopy (AFM), we perform a systematic investigation on the correlation of energy level alignment, film growth and molecular orientation of 2, 7-dioctyl[1]benzothieno-[3, 2-b][1]benzothiophene (C8-BTBT) on Ni(100). The molecules lie down at the first layer and are partly devulcanized by the substrate. Chemical adsorption of reaction products of sulfur atoms on the Ni substrate and the evaporation of the hydrocarbon products into vacuum make the C/S ratio as low as 11.5 : 1 in the XPS of the initially deposited C8-BTBT film of 1-4 thickness, far less than the stoichiometric of 15 : 1. With the thickness increasing from 4 to 8 , there are sharp downward shifts of Evac, HOMO and core levels of C 1s, S 2p, and a sharp increase of C/S ratio, which can be ascribed to the change of molecular orientations from lying down at 4 to standing up at 8 . From 8 onward, the C/S ratio increases steadily till it reaches 15 : 1. The energy levels show relatively less changes when the thickness increases from 8 to 32 . When the thickness increases over 32 , the energy band starts bending downward apparently because of the charging effect during the photoelectron emission processes. The poor conductivity along the standing alkyl chain of C8-is the main cause for the charging. The standing up configurations of the C8-BTBT molecules are confirmed by the AFM investigation in which the heights of the upper layers of C8-BTBT are around 30 , close to the length of the long c-axis. AFM image also indicates that the molecules tend to grow into islands for larger thickness, which is consistent with the slower decrease of the (I/I0) of Ni 2p3/2 with the C8-BTBT film thickness. Our results suggest that a buffer layer be inserted between Ni and C8-BTBT and the thickness of the C8-BTBT film be controlled as thin as possible in related devices.
Diameter monodispersity of imogolite-like nanotube: a density functional theory study
Authors: Wang Ya-Jing  , Li Gui-Xia  , Wang Zhi-Hua  , Gong Li-Ji  , Wang Xiu-Fang  ,   Science and Information College, Qingdao Agriculture University, Qingdao 266109, China;,   Emei Campus, Southwest Jiaotong University, Emei 614202, China
Published: December 2020

The diameter monodispersity and the surface charge distribution of three imogolite-like nanotubes (not substituted (IMO), substituted by NH2 (IMO-NH2), substituted by F (IMO-F) are investigated using self-consistent periodic density functional theory, and the phenomenon of the monodispersity is explained qualitatively in terms of bond length. We assume that the axial length of the nanotube is constant and confirm it; the energetic minimum axial lengths of the three nanotubes increase in the sequence IMO_NH2 IMO IMO_F, and are respectively 8.61, 8.62 and 8.66 . Then the energies for different nanotubes and lamellar structures are calculated. A series of strain energy curves of IMO, IMO_NH2 and IMO_F are plotted based on calculations, and the results show that the energetic minimum diameters of these three nanotubes increase in the sequence of IMO IMO_NH2 IMO_F, and are respectively N= 9, 10 and 11. In order to explain the diameter monodispersity, we have calculated the bond lengths of SiO, AlO and AlOH three nanotubes and plotted the curves of length against diameter. Results show that the monodispersity can be attributed to the interaction between the energy increase resulting from the stretching of the SiO, AlO bonds in the inner wall, and the energy decreases caused by the shortening of the AlOH bond in the outer wall. In a word, with the increase of tube diameter, the SiO and AlO bonds increase while the AlOH bond decreases monotonically. Additionally, we have also calculated the Mulliken charge distributions of the three nanotubes with different diameter and analysed their surface charges. On this basis, we summarize the effect of diameter on surface charge. Results show that the main positive charges are accumulating on the outer surface while the negative charges are located on the inner region, and the outer surface charge increases gradually with the increase of the diameter of the nanotubes. The study indicates that the internal surface functional group has an effect on the axial length, diameter and surface charge of the imogolite-like nanotubes. We can control the nanotube diameter and surface charge distribution by changing different functional substitutes in the inner surface; it is significant in the molecular design and application of imogolite-like materials.
Influences of Ga content on the structure and magnetic properties of Mn2 -xNiGa1+x alloys
Authors: Liu Hong-Yan  , Liu Zhu-Hong  , Li Ge-Tian  , Ma Xing-Qiao  ,   Department of Physics, University of Science and Technology Beijing, Beijing 100083, China
Published: December 2020

The structure magnetism and ordering transition of the ferromagnetic shape memory alloy Mn2 -xNiGa1+xhave been systematically studied in this paper. With increasing Ga content, the structure of the parent phase Mn2 -xNiGa1+x is transformed from Hg2CuTi-type to Cu2MnAl-type Heusler alloy gradually. Its lattice constant increases first and then decreases, reaching its maximum at x=0.3. The sample displays both the primary phase of Heusler and the Ni2In-type hexagonal phase in precipitate form when x lies in the range of 0.3-0.8. The Curie temperature of the primary phase of Heusler alloy Mn2 -xNiGa1+x reduces gradually from 590 K for Mn2NiGa to about 220 K for Ga2MnNi with the decrease of the exchange interaction between 3d electrons in the transition metals. However, the variation of Curie temperature of Ni2In-type hexagonal phase is gentle. The separation of Curie temperatures between the Ni2In-type hexagonal phase and the primary phase of Heusler occurs when x lies in the range from 0.6 to 0.8. Substitution of Mn by Ga has a significant influence on the coupling interaction among various atoms, leading to first increasing and then decreasing of the saturated magnetization of Mn2 -xNiGa1+x at low temperatures. That is, the saturated magnetization will rise for x0.4 and drops sharply for x0.4. Results of differential scanning calorimeter show that the melting temperature decreases gradually as x increases. Meanwhile, the transition temperature from parent phase (B2) to Heusler phase decreases first and increases later.
High-sensitive off-axis integrated cavity output spectroscopy and its measurement of ambient CO2 at 2 μm
Authors: Li Zhi-Bin  , Ma Hong-Liang  , Cao Zhen-Song  , Sun Ming-Guo  , Huang Yin-Bo  , Zhu Wen-Yue  , Liu Qiang  ,   Key Laboratory of Atmospheric Composition and Optical Radiation, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 220031, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

An off-axis integrated cavity output spectroscopy (OA-ICOS) is established by using a fiber-coupled distributed feedback diode laser operating near 2 μm. Its performances are evaluated and optimized through experimental investigation via detecting the pure CH4 absorptions at different pressures. The reflectivity of the cavity mirror is measured to be 0.99865, which results in the effective total optical pathlength of up to 407.4 m based on a 55 cm cavity. It is shown that the OA-ICOS configuration can be used to obtain very long optical pathlength, leading to pretty high sensitive monitoring of atmospheric trace gases. Based on the developed OA-ICOS, the atmospheric CO2 measurements are made and its performance is improved by using the wavelet denoising approach. The CO2 absorption line at 4993.7431 cm-1 is used for measuring the concentration. The measured results are compared with the results obtaind by a commercial H2O/CO2 analyzer. Agreement and disagreement are briefly discussed, and the results show that the OA-ICOS is reliable for detecting the atmospheric trace gases. The limitation of the developed OA-ICOS and the further steps towards the improvement in precision and accuracy are also presented.
An ultra-wideband absorber based on graphene
Authors: Jiang Yan-Nan  , Wang Yang  , Ge De-Biao  , Li Si-Min  , Cao Wei-Ping  , Gao Xi  , Yu Xin-Hua  ,   Guangxi Key Laboratory of Wireless Wideband Communication & Signal Processing, Guilin 541004, China;,   School of Information and Communication Engineering, Guilin University of Electronic Technology, Guilin 541004, China;,   Guangxi Experiment Center of Information Science, Guilin University of Electronic Technology, Guilin 541004, China;,   School of Physics Optoelectronic Engineering, Xidian University, Xi'an 710071, China
Published: December 2020

Stealth technology is of great importance and significance in reducing the radar cross section and improving the survivability of the target aircraft. Absorber is one of the most important structures in stealth technology. However, the present investigations of absorbers mainly focus on the narrow band or multi-band. To extend the operation bandwidth, a graphene-based absorber structure is proposed in this paper. The proposed absorber has a periodic structure whose unit cell consists of a square and a circular graphene-based ring. The surface impedance of the periodic structure can be optimized to match the impedance of the free space in a very wide band by adjusting the electrostatic bias voltage. Then the operation band is significantly extended. By using the commercial software, CST Microwave Studio 2014, the performance of the proposed absorber is studied. The simulated results show that the proposed absorber can absorb electromagnetic (EM) waves in an ultra-wideband from 2.1 to 9.0 GHz, with an absorbing rate of up to 90%. Moreover, the proposed absorber is insensitive to the polarization of the incident wave due to the symmetry of the structure. We also find that the absorber can be tuned to work at any frequency in a range from 2.0 to 9.0 GHz for a fixed geometrical parameter. The equivalent circuit model (ECM) approach and interference theory (INF) are employed to investigate the physical mechanism of the proposed absorber. According to the ECM, we analyze the resonant characteristics of the square and circular graphene rings. Owing to the existence of two different graphene rings, two resonant frequencies are detected. By optimizing the structure parameters of the graphene rings, the two resonant frequencies are brought closer, resulting in the increase of the operation band. On the other hand, the real part of the input impedance of the equivalent circuit reaches up to about 300 Ω and the imaginary part is close to 0 Ω, which provides good matching to the free space, leading to high absorption rate. According to the interference theory, the amplitudes and phases of the direct reflection and the multiple reflections of EM waves are studied. It is found that the destructive interference between the direct reflection and multiple reflection makes the absorber have high performance in an ultra-wideband. The results obtained from ECM and INF are in good agreement with the simulation ones.
Analysis of Si/SiGe/Si double heterojunction band of a novelstructure of PIN electronic modulation
Authors: Feng Song  , Xue Bin  , Li Lian-Bi  , Zhai Xue-Jun  , Song Li-Xun  , Zhu Chang-Jun  ,   School of Science, Xi'an Polytechnic University, Xi'an 710048, China
Published: December 2020

PIN is a common structure of electrical modulation in electro-optic modulator, and the performance of the electro-optic modulator is directly affected by the carrier injection in PIN structure. In previous studies, we have invented a novel structure of PIN electronic modulation based on SOI material. In the new structure, the SiGe material is adopted in the waveguide zone, therefore the double heterojunction PIN structure is formed in the horizontal direction. The carrier injection efficiency can be enhanced in the novel structure, and the power consumption could be reduced. In order to further study the modulation mechanism of the novel structure, based on the single heterojunction band theory, the barrier heights of the double heterojunction are analyzed, and the quantitative formulas of the barrier heights of the double heterojunction are given. It is shown that the barrier heights of the double heterojunction are related to the doping concentration, the band gap of material, the temperature, and the Ge content. The bands are compared between the novel structure, SiGe-OI structure and SOI structure to analyze the reason why the carrier injection of the novel structure could be enhanced. In the same conditions, the barrier heights of Si/SiGe/Si double heterojunction are minimal values, and those of SiGe and Si materials are second minimal value and maximal value, respectively. When the PIN device is set at a forward biased voltage (P region is the anode, and N region is the cathode), the balance between the carrier diffusion and the carrier drift is broken, and the PIN device is in a non-equilibrium state. According to the quantitative formula of the barrier heights of the double heterojunction, the barrier heights of Si/SiGe/Si double heterojunction are lower than that of SiGe-OI material, and the barrier height of SiGe material is lower than that of SOI material. It is shown that the barrier heights of Si/SiGe/Si double heterojunction could be flatten at first, so its PIN structure has the higher carrier injection than those of SiGe-OI and SOI under the same conditions. Finally, the band distribution of the novel structure and the relationships between the band distribution, the modulation voltage and the carrier injection are simulated. The results show that when the modulation voltage is 1 V, the carrier density of the novel structure arrives at 8×1018 cm-3, which is 800% higher than that of SOI structure, and 340% higher than that of SiGe-OI structure. The advantages of the novel structure are further indicated, and the correctness of the theoretical analysis is also verified.
Multi-scale ordered patterns in photosensitive ternary polymer mixtures
Authors: Guo Yu-Qi  , Pan Jun-Xing  , Zhang Jin-Jun  , Sun Min-Na  , Wang Bao-Feng  , Wu Hai-Shun  ,   Macromolecular Science Research Center, School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China
Published: December 2020

Multi-scale ordered patterns of multi-component polymer mixtures can reveal many peculiar chemical and physical properties, which makes these systems have very important potential applications in materials engineering. Via computer simulation, we create interesting ordered multi-scale structures in photosensitive and immiscible polymer mixtures. The system that we employed comprises a ternary, molten A/B/C polymer blends and the three components are mutually immiscible. Polymer C is non-optically active, while polymers A and B can exhibit reversible chemical reaction A ⇆ B induced by light. Firstly, we investigate the phase behavior of the ternary blend guided by cross-stripy mask and light, and find that a chessboard-like ordered pattern forms in the mixture before removing the mask. In the illuminated regions, the A and C components gather into ellipsoidal core-shell structures in the uncrossed illuminated area, while the A and B components gather into star structures in the crossed stripes regions. When we remove the mask, the entire system becomes illuminated, and the reaction A ⇆ B occurs throughout the film: the ellipsoidal core-shell structures of A and C components turn to spherical structures, and the star structures of A and B components turn into concentric square ring structures. Then we show the influences of the number of cross stripes and the initial composition on the formation of structure. The average spatial volume fraction of C component first increases and then decreases with the stripy number increasing and the C component net lattices play an important role in the stability of ordered structures. Secondly, when the blend is covered by the annular mask, the C component gathers to the illuminated regions and the A and B components are in radial arrangement. As a result, the mixture forms an interesting dartboard-like pattern. However, when the mask is removed, the photochemical reactions occur in the A and B components of the whole region, the increasing of free energy induces the dartboard-like pattern to be broken and to change into dots-ring structure and then it forms a perfect concentric ring pattern and the target-like pattern. And also, we show the effects of initial composition ratio of C component, the distance between two adjacent rings D, the ring width d, and the illumination intensity on the evolution of ordered structure. The larger the initial composition ratio of C component, the more easily the ordered target-like pattern forms; the larger the distance D and the smaller the width d, the better the pinning effect of C component is. The illumination intensity has little influence on the ordered morphology of the ternary system. We provide a simple approach to creating multi-scale patterned films with long-range order, which could guide us in fabricating nanoscale devices.
Effects of Al-2N doping on the photoelectric properties of ZnO
Authors: Hou Qing-Yu  , Qu Ling-Feng  , Zhao Chun-Wang  ,   College of Science, Inner Mongolia University of Technology, Hohhot 010051, China;,   College of Arts and Sciences, Shanghai Maritime University, Shanghai 201306, China
Published: December 2020

In a similar range of Al-2N doping amount to that in the present paper, the absorption spectra of ZnO doped system and two kinds of experimental results have been reported in the literature. However, there is no reasonable explanation for the absorption spectra of ZnO doped system. In order to solve the problem, all calculations in the present paper are carried out by the CASTEP tool in the Materials Studio software based on the first principal ultrasoft pseudopotential of the density functional theory, and the geometric structures of ZnO, Zn0.98148Al0.01852O0.96296N0.03704 and Zn0.96875Al0.03125O0.9375N0.0625 systems are constructed by first-principal. All the models are based on the optimization of the geometry structure. And the distribution of the band structure, the density of states and the absorption spectra of the doping system are calculated by the method of GGA+U. The results indicate that in the range of the doping content restricted in the present paper, the bigger the doping amount of Al-2N, the smaller the volume of doped system is; the higher the total energy, the more the stability decreases; the higher the formation energy, the harder the doping becomes and the narrower the optical band gap of doped system. Meanwhile, the higher the Al-2N doping content, the narrower the optical bandgap of the doping system becomes, which suggests that the more significant the red shift of absorption spectrum of Al-2N doped ZnO system is. Therefore, the doped system is controlled within the doping content in experiment to obtain the narrow optical band gap and red shift in absorption spectrum in Al-2N doped ZnO, in addition to the control of lower nanoscale of Al-2N doped in ZnO. At the same time, all doping systems are p-type degenerated semiconductors. Then, the higher the Al-2N doping content, the smaller the relative concentration of free holes of doped system is; the smaller the hole effective mass, the lower the mobility is; the lower the hole conductivity, the worse the conductive property of doping system is. The calculated results are in agreement with the experimental results. The research shows that Al-2N co-doped ZnO can be a new type of semiconductor material, a functional material which is used at low temperature end of thermoelectric power generation.