共查询到20条相似文献，搜索用时 15 毫秒
1.
Yin Y Rioux RM Erdonmez CK Hughes S Somorjai GA Alivisatos AP 《Science (New York, N.Y.)》2004,304(5671):711-714
Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, we show that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications. 相似文献
2.
Self-assembled devices composed of periodic arrays of 10-nanometer-diameter cobalt nanocrystals display spin-dependent electron transport. Current-voltage characteristics are well described by single-electron tunneling in a uniform array. At temperatures below 20 kelvin, device magnetoresistance ratios are on the order of 10%, approaching the maximum predicted for ensembles of cobalt islands with randomly oriented preferred magnetic axes. Low-energy spin-flip scattering suppresses magnetoresistance with increasing temperature and bias-voltage. 相似文献
3.
Bulk quantities of defect-free silicon (Si) nanowires with nearly uniform diameters ranging from 40 to 50 angstroms were grown to a length of several micrometers with a supercritical fluid solution-phase approach. Alkanethiol-coated gold nanocrystals (25 angstroms in diameter) were used as uniform seeds to direct one-dimensional Si crystallization in a solvent heated and pressurized above its critical point. The orientation of the Si nanowires produced with this method could be controlled with reaction pressure. Visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra. 相似文献
4.
The dynamics and spectroscopy of silicon nanocrystals that emit at visible wavelengths were analyzed. Size-selective precipitation and size-exclusion chromatography cleanly separate the silicon nanocrystals from larger crystallites and aggregates and provide direct evidence for quantum confinement in luminescence. Measured quantum yields are as high as 50 percent at low temperature, principally as a result of efficient oxide passivation. Despite a 0.9-electron-volt shift of the band gap to higher energy, the nanocrystals behave fundamentally as indirect gap materials with low oscillator strength. 相似文献
5.
Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals
When used as a photocatalyst, titanium dioxide (TiO(2)) absorbs only ultraviolet light, and several approaches, including the use of dopants such as nitrogen, have been taken to narrow the band gap of TiO(2). We demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO(2) through hydrogenation. We showed that disorder-engineered TiO(2) nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent. 相似文献
6.
New physics occurs in semiconductors when one or more dimensions of the crystal are reduced to a size comparable to bulk electron delocalization lengths (tens to hundreds of angstroms). The properties of "quantum dots" or semiconductor nanocrystals are now being studied, as techniques to fabricate the crystallites are developed. Temperature-dependent electron diffraction studies on nanocrystals of CdS show a large depression in the melting temperature with decreasing size, as a larger fraction of the total number of atoms is on the surface. Thermal stability may play a role in determining the uses of semiconductor nanocrystals. 相似文献
7.
Mocatta D Cohen G Schattner J Millo O Rabani E Banin U 《Science (New York, N.Y.)》2011,332(6025):77-81
Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices. 相似文献
8.
选某牧场患有慢性钴缺乏病症状的羊120只(5～6月龄)，随机分为4组，每组30只。采用3种方法对其进行治疗。结果表明：3种方法均可防治本牧场羊钴缺乏。口服硫酸钴对提高羊的生产性能(增重)效果最好；钴丸组对增强缺钴羊机体免疫力及其造血、糖代谢等生理功能上效果最佳；施钴肥的效果稍逊色于其他方法，但因其方便、实用，也可作为本牧场防治钴缺乏的一种方法。 相似文献
9.
Dubertret B Skourides P Norris DJ Noireaux V Brivanlou AH Libchaber A 《Science (New York, N.Y.)》2002,298(5599):1759-1762
Fluorescent semiconductor nanocrystals (quantum dots) have the potential to revolutionize biological imaging, but their use has been limited by difficulties in obtaining nanocrystals that are biocompatible. To address this problem, we encapsulated individual nanocrystals in phospholipid block-copolymer micelles and demonstrated both in vitro and in vivo imaging. When conjugated to DNA, the nanocrystal-micelles acted as in vitro fluorescent probes to hybridize to specific complementary sequences. Moreover, when injected into Xenopus embryos, the nanocrystal-micelles were stable, nontoxic (<5 x 10(9) nanocrystals per cell), cell autonomous, and slow to photobleach. Nanocrystal fluorescence could be followed to the tadpole stage, allowing lineage-tracing experiments in embryogenesis. 相似文献
10.
Atomic force microscopy was used to characterize the sliding of molybdenum oxide (MoO3) nanocrystals on single-crystal molybdenum disulfide (MoS2) surfaces. Highly anisotropic friction was observed whereby MoO3 nanocrystals moved only along specific directions of the MoS2 surface lattice. The energy per unit area to move the MoO3 nanocrystals along their preferred sliding direction was an order of magnitude less than required to slide macroscopic MoS2-bearing contacts. This extreme friction anisotropy was exploited to fabricate multicomponent MoO3 nanostructures. These reversibly interlocking structures could serve as the basis for devices such as mechanical logic gates. 相似文献
11.
Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO(2) single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO(2) facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers. 相似文献
12.
Matsumoto Y Murakami M Shono T Hasegawa T Fukumura T Kawasaki M Ahmet P Chikyow T Koshihara S Koinuma H 《Science (New York, N.Y.)》2001,291(5505):854-856
Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with theconcentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin. 相似文献
13.
We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world. 相似文献
14.
The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases. 相似文献
15.
Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified. 相似文献
16.
17.
氯化钴在大菱鲆饲料中的安全性评价 总被引：1，自引：0，他引：1
通过研究饲料中的氯化钴对大菱鲆幼鱼的存活、生长性能、血清生理生化指标、抗氧化酶活力、消化酶活力、血清和肝脏中VB12含量、肌肉和肝脏中钴的残留量、肝脏和肠道组织结构的影响来评估氯化钴在海水鱼饲料中的安全性。通过在基础饲料中添加氯化钴制剂来配制4种具有不同氯化钴含量(0、8、40及80 mg/kg,不计结晶水重量)的等氮等能的实验饲料,每个饲料处理设6个重复,每个重复养殖30尾大菱鲆;养殖实验为期10周。实验结果表明:饲料中添加氯化钴对试验鱼的存活没有显著影响(P0.05),但饲料中80 mg/kg的氯化钴显著降低了大菱鲆的特定生长率、终末体重、摄食率、肝体比和脏体比(P0.05);饲料中高剂量氯化钴(40和80 mg/kg)显著降低了大菱鲆血清过氧化氢酶、谷草转氨酶和碱性磷酸酶活力以及大菱鲆胃淀粉酶和胰蛋白酶活力(P0.05);大菱鲆的血糖含量、血清和肝脏中VB12含量、肝脏和肌肉中钴的残留量均随饲料中钴的添加量增加而显著升高(P0.05);饲料中添加氯化钴对大菱鲆的肠道组织结构没有产生明显影响,但饲料中添加40和80 mg/kg的氯化钴增宽了肝脏肝血窦。研究结果表明:实验条件下,氯化钴在大菱鲆饲料中的添加量8 mg/kg是安全的。 相似文献
18.
以3,5-二叔丁基水杨醛和(1R,2R)-环己二胺为原料,合成了(R,R)-N,N-二(3,5-二叔丁基水杨醛)-1,2环己二胺钴配合物(Salen-Co(Ⅱ)配合物),并以此作为化学修饰剂,利用直接混合法将化学修饰剂、粘合剂和石墨粉按一定比例研磨混合制作成西夫碱钴配合物修饰碳糊电极(Salen-Co/CPE).实验表明,该西夫碱钴配合物修饰碳糊电极对甲醛具有良好的电催化作用.利用计时电流法,在优化的实验条件下,氧化峰电流与甲醛的浓度在3.0×10-2～1.2×10-1mol/L的范围内具有良好的线性关系,检出限为3.33×10-3mol/L. 相似文献
19.
Ding Z Quinn BM Haram SK Pell LE Korgel BA Bard AJ 《Science (New York, N.Y.)》2002,296(5571):1293-1297
Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (approximately 2 to 4 nanometers in diameter) was observed by differential pulse voltammetry (DPV) in N,N'-dimethylformamide and acetonitrile. The electrochemical gap between the onset of electron injection and hole injection-related to the highest occupied and lowest unoccupied molecular orbitals-grew with decreasing nanocrystal size, and the DPV peak potentials above the onset for electron injection roughly correspond to expected Coulomb blockade or quantized double-layer charging energies. Electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redox-active coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak maximum at 640 nanometers, a significant red shift from the photoluminescence maximum (420 nanometers) of the same silicon NC solution. These results demonstrate that the chemical stability of silicon NCs could enable their use as redox-active macromolecular species with the combined optical and charging properties of semiconductor quantum dots. 相似文献
20.
Bis(3-fluorosalicylaldehyde) ethylenediimine cobalt(II), a reversible oxygen-carrying compound, has been crystallized as a tetramer with two oxygen molecules bound to the four cobalt atomns, each oxygen molecule bridging between two metal atoms. These dimers are further linked by two bonds between the oxygen atom of the salicylaldehyde and the cobalt atom. The oxygen-to-oxygen distance, 1.21 angstroms in molecular oxygen, has increased to 1.308(28) angstroms (where the number in parentheses is the estimated standard deviation) in this compound, the shortest distance yet observed in sutch a bridging arrangement. 相似文献