Scanning tunneling microscopy of electrodeposited ceramic superlattices

Cleaved cross sections of nanometer-scale ceramic superlattices fabricated from materials of the lead-thallium-oxygen system were imaged in the scanning tunneling microscope (STM). The apparent height differences between the layers were attributed to composition-dependent variations in local electrical properties. For a typical superlattice, the measured modulation wavelength was 10.6 nanometers by STM and 10.8 nanometers by x-ray diffraction. The apparent height profile for potentiostatically deposited superlattices was more square than that for galvanostatically deposited samples. These results suggest that the composition follows the applied potential more closely than it follows the applied current. The x-ray diffraction pattern of a superlattice produced under potential control had satellites out to the fourth order around the (420) Bragg reflection.     

应变层超晶格（ZnSe）n／（ZnS）m的激子结合能

在双量子阱近似下,用变分法计算了（ＺｎＳｅ）ｎ／（ＺｎＳ）ｍ应变层超晶格的激子结合能,为光致发光谱和光致发光振荡谱的分析提供了依据和参考数据。     

A-Axis--Oriented YBa2Cu3O7/PrBa2Cu3O7 Superlattices

A modulated structure has been fabricated from high transition temperature superconductors where the individual CuO(2) planes are composed of alternating superconducting and insulating strips. This structure is made by growing a-axis-oriented YBa(2)Cu(3)O(7)/PrBa(2)Cu(3)O(7) superlattices by 90 degrees off-axis sputtering on (100)SrTiO(3) and (100)LaAlO(3) substrates. Superlattice modulation is observed to a modulation wavelength of 24 angstroms (12 angstroms-YBa(2)Cu(3)O(7)/12 angstroms-PrBa(2)Cu(3)O(7)), both by x-ray diffraction and by cross-sectional transmission electron microscopy. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 3 percent. Quasi-one-dimensional conductivity should be obtainable in these structures.     

Spontaneous superlattice formation in nanorods through partial cation exchange

Lattice-mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route to synthesizing CdS-Ag(2)S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization of the superlattices. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.     

基于透射型体布拉格光栅衍射效率的研究

基于Kogelnik耦合波理论,推导了透射体布拉格光栅的衍射效率方程,数值仿真分析了折射率调制幅度、晶体厚度、相位失配参量、角度偏移量和波长偏移量等光栅写入参数对体光栅衍射效率的影响.结果表明:当入射波和衍射波没有发生相位失配时,衍射效率最大;当以布拉格角入射时,衍射效率将随介质的厚度和折射率的空间调制幅度的增加而增加;当入射光满足布拉格条件时,不同厚度和折射率调制幅度的体光栅衍射效率为100%;只要入射光偏离布拉格条件一个很小的量,体光栅衍射效率就会迅速下降;当偏离量超出一定范围,衍射效率就降低为0.同时,并非是体光栅折射率的空间调制幅度越深,衍射效率越高.     

Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices

Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.     

Imaging electron wave functions of quantized energy levels in carbon nanotubes

Carbon nanotubes provide a unique system for studying one-dimensional quantization phenomena. Scanning tunneling microscopy was used to observe the electronic wave functions that correspond to quantized energy levels in short metallic carbon nanotubes. Discrete electron waves were apparent from periodic oscillations in the differential conductance as a function of the position along the tube axis, with a period that differed from that of the atomic lattice. Wave functions could be observed for several electron states at adjacent discrete energies. The measured wavelengths are in good agreement with the calculated Fermi wavelength for armchair nanotubes.     

Dimensionality control of electronic phase transitions in nickel-oxide superlattices

The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO(3)) and the wide-gap insulator lanthanum aluminate (LaAlO(3)) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO(3) as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO(3) layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.     

Using hyperspectral imaging to discriminate yellow leaf curl disease in tomato leaves

This paper investigated the possibility of discriminating tomato yellow leaf curl disease by a hyperspectral imaging technique. A hyperspecral imaging system collected hyperspectral images of both healthy and infected tomato leaves. The reflectance spectra, first derivative reflectance spectra and absolute reflectance difference spectra in the wavelength range of 500–1000 nm of both background and the leaf area were analyzed to select sensitive wavelengths and band ratios. 853 nm was selected to create a mask image for background segmentation, while 720 nm from the reflectance spectra, four peaks (560, 575, 712, and 729 nm) from the first derivative spectra and, four wavelengths with higher values (586, 720 nm) and lower values (690, 840 nm) in the absolute difference spectra were selected as a set of sensitive wavelengths. Four band ratio images (560/575, 712/729, 586/690, and 720/840 nm) were compared with four widely used vegetation indices (VIs). 24 texture features were extracted using grey level co-occurrence matrix (GLCM), respectively. The performance of each feature was evaluated by receiver operator characteristic (ROC) curve analysis. The best threshold values of each feature were calculated by Yonden’s index. Mean value of correlation (COR_MEAN) extracted from the band ratio image (720/840 nm) had the best performance, whose AUC value was 1.0. The discrimination result for a validation set based on its best threshold value was 100%. This research also demonstrated that multispectral images at 560, 575 and 720 nm have a potential for detecting tomato yellow leaf curl virus infection in field applications.     

Superlattices of iron nanocubes synthesized from Fe[N(SiMe3)2]2

The reaction of the metal-organic precursor Fe[N(SiMe3)2]2 with H2 in the presence of a long-chain acid and a long-chain amine in various proportions produces monodisperse zerovalent iron nanoparticles. These Fe particles display magnetic properties that match those of bulk iron as evidenced by magnetic and M?ssbauer measurements. The nanoparticles adopt a cubic shape with edges of 7 nanometers and are incorporated into extended crystalline superlattices containing nanocubes in close proximity and with their crystallographic axes aligned. These superlattices are formed in solution, precipitate in high yield, and may be redissolved and redeposited as two-dimensional arrays.     

Hubble space telescope imaging of the north polar aurora on jupiter

The first direct images of the Jovian aurora at ultraviolet wavelengths were obtained by the Hubble Space Telescope Faint Object Camera near the time of the Ulysses spacecraft encounter with Jupiter on 8 February 1992. The auroral oval is not uniformly luminous. It exhibits a brightness minimum in the vicinity of longitude 180 degrees . In the few images available, the brightest part of the oval occurs in late afternoon Jovian time. The observed oval is not concentric with calculated ovals in the O(6) model of Connerney. The size of the oval is consistent with auroral particles on field lines with magnetic L parameter >8, indicating significant migration from lo, its torus, or both, if these are their origins.     

3种扁谷盗对不同波长光趋性研究

【目的】研究锈赤扁谷盗Cryptolestes ferrugineus成虫、土耳其扁谷盗Cryptolestes turcicus和长角扁谷盗Cryptolestes pusillus雌成虫对不同波长光的趋性。【方法】室内应用光行为反应装置测试3种扁谷盗羽化1、11、21和31日龄(雌)成虫对试验波长303、320、330、340、351、368、400、445、460、480、520、525、545、560、575和580 nm的趋光率。【结果】锈赤扁谷盗羽化1、11、21和31日龄成虫趋光率最高的刺激光波长分别为351、351、400、480 nm,对锈赤扁谷盗成虫诱集效果较好的光波长有351、400和445 nm,随着日龄的增加,对锈赤扁谷盗成虫诱集率大于20%的光波数增加;土耳其扁谷盗羽化1、11、21和31日龄趋光率分别为16%~51%、11%~46%、7%~37%和4%~28%,雌成虫趋光率最高的刺激光波长分别为351(368)、368、330和351 nm,对土耳其扁谷盗雌成虫诱集效果较好的光波长有368、351和303 nm;长角扁谷盗羽化1、11、21和31日龄趋光率分别为1.11%~75.56%、18.89%~70.00%、33.33%~62.22%和22.22%~71.11%,雌成虫趋光率最高的刺激光波长分别为575、320、400(445)和320(560)nm,对长角扁谷盗雌成虫诱集效果较好的光波长有320、575和330 nm。【结论】锈赤扁谷盗成虫的趋光性随着日龄的增加而增强,土耳其扁谷盗雌成虫的趋光性随着日龄的增加而下降,长角扁谷盗雌成虫的趋光性随着日龄的增加略为下降。     

Probing nanoscale ferroelectricity by ultraviolet Raman spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.     

Using spectral distance, spectral angle and plant abundance derived from hyperspectral imagery to characterize crop yield variation

Vegetation indices (VIs) derived from remote sensing imagery are commonly used to quantify crop growth and yield variations. As hyperspectral imagery is becoming more available, the number of possible VIs that can be calculated is overwhelmingly large. The objectives of this study were to examine spectral distance, spectral angle and plant abundance (crop fractional cover estimated with spectral unmixing) derived from all the bands in hyperspectral imagery and compare them with eight widely used two-band and three-band VIs based on selected wavelengths for quantifying crop yield variability. Airborne 102-band hyperspectral images acquired at the peak development stage and yield monitor data collected from two grain sorghum fields were used. A total of 64 VI images were generated based on the eight VIs and selected wavelengths for each field in this study. Two spectral distance images, two spectral angle images and two abundance images were also created based on a pair of pure plant and soil reference spectra for each field. Correlation analysis with yield showed that the eight VIs with the selected wavelengths had r values of 0.73–0.79 for field 1 and 0.82–0.86 for field 2. Although all VIs provided similar correlations with yield, the modified soil-adjusted vegetation index (MSAVI) produced more consistent r values (0.77–0.79 for field 1 and 0.85–0.86 for field 2) among the selected bands. Spectral distance, spectral angle and plant abundance produced similar r values (0.76–0.78 for field 1 and 0.83–0.85 for field 2) to the best VIs. The results from this study suggest that either a VI (MSAVI) image based on one near-infrared band (800 or 825 nm) and one visible band (550 or 670 nm) or a plant abundance image based on a pair of pure plant and soil spectra can be used to estimate relative yield variation from a hyperspectral image.     

Ferromagnetism in LaFeO3-LaCrO3 superlattices

Ferromagnetic spin order has been realized in the LaCrO3-LaFeO3 superlattices. Ferromagnetic coupling between Fe3+ and Cr3+ through oxygen has long been expected on the basis of Anderson, Goodenough, and Kanamori rules. Despite many studies of Fe-O-Cr-based compounds, random positioning of Fe3+ and Cr3+ ions has frustrated the observation of ferromagnetic properties. By creating artificial superlattices of Fe3+ and Cr3+ layer along the [111] direction, ferromagnetic ordering has been achieved.     

Electrodeposited defect chemistry superlattices

Nanometer-scale layered structures based on thallium(III) oxide were electrodeposited in a beaker at room temperature by pulsing the applied potential during deposition. The conducting metal oxide samples were superlattices, with layers as thin as 6.7 nanometers. The defect chemistry was a function of the applied overpotential: High overpotentials favored oxygen vacancies, whereas low overpotentials favored cation interstitials. The transition from one defect chemistry to another in this nonequilibrium process occurred in the same potential range (100 to 120 millivolts) in which the rate of the back electron transfer reaction became significant. The epitaxial structures have the high carrier density and low electronic dimensionality of high transition temperature superconductors.     

脐橙糖度光谱图像检测技术研究

对采集到的不同波长的光谱图像灰度分布进行洛伦茨分布(LD)、高斯分布(GD)、指数分布(ED)函数拟合,通过比较发现洛伦茨分布为最优灰度分布拟合函数.将脐橙的糖度与洛伦茨分布函数拟合所得参数分别进行多元线性回归,建立最佳单波长、最佳双波长组合、最佳三波长组合和最佳四波长组合的校正方程.结果表明:利用光谱图像技术无损检测脐橙糖度是可行性的.     

油桃糖度的光谱图像检测技术研究

应用光谱图像技术进行油桃内部品质无损检测技术的研究。通过采集不同波长(分别为632、650、670、780、850和900 nm)的光谱图像,对所采集的光谱图像灰度分布进行洛伦茨分布(LD)函数的拟合。将油桃的糖度与洛伦茨分布函数拟合所得参数分别进行多元线性回归,建立最佳单波长、最佳双波长组合、最佳三波长组合和最佳四波长组合的校正方程,相关系数分别是最佳单波长r为0.789;最佳双波长r为0.893;最佳三波长r为0.970;最佳四波长r为0.943。试验结果表明:利用光谱图像技术无损检测油桃糖度是可行性的,为计算机图像对水果进行内部品质的无损检测提供技术依据。     

磁性超晶格中静磁波的表面退磁效应

利用等效介质理论，在考虑磁性层表面动力学退磁场影响的前提下，首先给出了动力学磁化率，然后求出了色散方程。当坐标系的，轴垂直于层间界面，饱和磁化和外场均沿z轴方向时，数值计算结果表明：表面静磁波可以在任意磁性层的相对厚度时存在，同时表面波的传播方向处在x-z平面的第三和第四象限中。这两结论与以往的结果不同。体静磁模频率的连续区在表面模频率的上方。     

Order and Low Dimensionality in the Organic Superconductor (BEDT-TTF)2Cu(NCS)2 Revealed by STM

Single-crystal samples of [(BEDT-TTF)(2)]+[Cu(NCS)(2)]-were synthesized and studied with a scanning tunneling microscope (STM). Real-space images of the anion and cation surfaces with molecular resolution were obtained. The images show no evidence of structural disorder or stacking faults previously suggested. The presence of an additional modulation commensurate with the lattice provides evidence of a lattice distortion. The cause of this modulation is unknown. The presence of a charge density wave in the material would have implications on the dimensionality of the material that may explain the peculiar temperature dependence of the electrical conductivity. This interpretation is consistent with the calculated Fermi surface, which allows nesting of the wave vector.