共查询到20条相似文献，搜索用时 171 毫秒
1.
KS Johnson JH Thywissen NH Dekker KK Berggren AP Chu R Younkin M Prentiss 《Science (New York, N.Y.)》1998,280(5369):1583-1586
The spatially dependent de-excitation of a beam of metastable argon atoms, traveling through an optical standing wave, produced a periodic array of localized metastable atoms with position and momentum spreads approaching the limit stated by the Heisenberg uncertainty principle. Silicon and silicon dioxide substrates placed in the path of the atom beam were patterned by the metastable atoms. The de-excitation of metastable atoms upon collision with the surface promoted the deposition of a carbonaceous film from a vapor-phase hydrocarbon precursor. The resulting patterns were imaged both directly and after chemical etching. Thus, quantum-mechanical steady-state atom distributions can be used for sub-0.1-micrometer lithography. 相似文献
2.
A recently designed single-crystal surface calorimeter has been deployed to measure the energy difference between two solid surface structures. The clean Pt{100} surface is reconstructed to a stable phase in which the surface layer of platinum atoms has a quasi-hexagonal structure. By comparison of the heats of adsorption of CO and of C(2)H(4) on this stable Pt{100}-hex phase with those on a metastable Pt{100}-(1x1) surface, the energy difference between the two clean phases was measured as 20 +/- 3 and 25 +/- 3 kilojoules per mole of surface platinum atoms. 相似文献
3.
Tomatsu K Nakatsuji K Iimori T Takagi Y Kusuhara H Ishii A Komori F 《Science (New York, N.Y.)》2007,315(5819):1696-1698
When tin (Sn) atoms are deposited on a clean germanium (Ge) (001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge-dimer position. We identified the dimer as a heterogeneous Sn-Ge dimer by reversing its buckling orientation with a scanning tunneling microscope (STM) at 80 kelvin. An atomic seesaw switch was formed for one-dimensional electronic conduction in the Ge dimer-row direction by using the STM to reversibly flip the buckling orientation of the Sn-Ge dimer and to set up standing-wave states. 相似文献
4.
A new high-pressure polymorph with a modified spinel structure, beta-Mn(2)GeO(4), is stable in a pressure range intermediate between the field of the polymorph with the olivine structure and that of another high-pressure polymorph. Oxygen atoms are located approximately in cubic close packing with manganese and germanium atoms in octahedral and tetrahedral interstices, respectively, as in the spinel structure; however, germanium atoms form Ge(2)O(7) groups instead of isolated GeO(4) groups. 相似文献
5.
Structural transformations on elemental semiconductor surfaces typically occur above several hundred degrees Celsius, and the atomic motions involved are extremely rapid and difficult to observe. However, on the (111) surface of germanium, a few lead atoms catalyze atomic motions so that they can be observed with a tunneling microscope at temperatures below 80 degrees C. Mass transport and structural changes are caused by the creation and propagation of both vacancy-like and interstitial-like point defects within the crystal surface. The availability of dangling bonds on the surface is critical. A detailed atomic model for the observed motions has been developed and is used to explain the structural phase transition Ge(111)-c(2x8) <--> 1x1, which occurs near 300 degrees C. 相似文献
6.
A New Dense Form of Solid Germanium 总被引：1，自引：0，他引：1
A new form of solid germanium, of greater density than ordinary cubic germanium, can be formed by compressing cubic germanium at pressures exceeding 120 kilobars and reducing the pressure back to that of the atmosphere. The crystal structure is tetragonal, with a equal to 5.93 angstroms and c, to 6.98; 12 atoms per unit cell; and theoretical density, 5.91 grams per cubic centimeter. Electrically it behaves like a semiconductor. At temperatures above 200 degrees C it reverts rather rapidly to the cubic form. 相似文献
7.
With the use of scanning tunneling microscopy, it is shown that germanium atoms adsorbed on the (100) surface of silicon near room temperature form chainlike structures that are tilted from the substrate dimer bond direction and that consist of two-atom units arranged in adjoining substrate troughs. These units are distinctly different from surface dimers. They may provide the link missing in our understanding of the elementary processes in epitaxial film growth: the step between monomer adsorption and the initial formation of two-dimensional growth islands. 相似文献
8.
Boland JJ 《Science (New York, N.Y.)》1993,262(5140):1703-1706
Chlorine atoms strongly chemisorbed at dangling bond sites on the Si(100)-(2 x 1) surface are observed by scanning tunneling microscopy (STM) to hop between adjacent sites. The origin of this behavior is suggested to be an interaction between the field of the probe tip and the dipole moment of the silicon-chlorine bond. Chlorine atom migration is shown to be facilitated by the presence of a metastable chlorine bridge-bonded minimum. The STM probe was used to excite single chlorine atoms into this bridging configuration, resulting in a local population inversion. Selective application of voltage pulses between the probe tip and the surface rearranged the local bonding and induced transformations between different types of chlorine sites. In this manner, adsorbed species can be dissected and their composition and structure directly probed. 相似文献
9.
X-ray fluorescence: detection of lead in wall paint 总被引：1，自引：0，他引：1
An instrument has been developed for the in situ determination of lead on painted surfaces. It utilizes, as a source of gamma rays, radioactive cadmium-109 and its daughter silver-109 (metastable) to excite the K series x-rays of lead, and a solid-state, lithium-drifted germanium detector. The device, which is capable of detecting 0.26 milligram of lead per square centimeter of paint [approximately 3 percent (by weight) of lead in a single coat] beneath ten layers of lead-free paint, has been tested in a preliminary survey of several tenement apartments in New York City. 相似文献
10.
The lithium-drifted germanium detector enables determination of europium-155 on a routine basis in environmental samples contaminated with debris from nuclear weapons. From measurements of europium-155, cesium-144, and strontium-90 in air filters collected between 1961 and 1966, the yield of europium-155 from weapons was estimated at 1400 atoms per 10(6) fissions, which is close to the yield of europium-155 from fast fission of uranium-238. 相似文献
11.
Huismans Y Rouzée A Gijsbertsen A Jungmann JH Smolkowska AS Logman PS Lépine F Cauchy C Zamith S Marchenko T Bakker JM Berden G Redlich B van der Meer AF Muller HG Vermin W Schafer KJ Spanner M Ivanov MY Smirnova O Bauer D Popruzhenko SV Vrakking MJ 《Science (New York, N.Y.)》2011,331(6013):61-64
Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse. 相似文献
12.
Cohen ML 《Science (New York, N.Y.)》1986,234(4776):549-553
It is now possible to start with a simple model of a solid composed of atomic cores and itinerant valence electrons and compute the total energy for a given structural arrangement of atoms with enough precision to predict the existence of new solids and their properties. The application of the model based on the pseudopotential method is described with silicon chosen as a prototype material. With only information about the constituent atoms, the electronic, structural, vibrational, and even superconducting properties of solids can be calculated from first principles. The successful predictions of superconductivity in highly condensed hexagonal silicon and the existence of new high-pressure semiconductor phases are highlighted. A discussion is presented of the use of the method to discover new stable or metastable solids at high pressures. 相似文献
13.
The reaction of tetrakis(tri-tert-butylsilyl)cyclotrigermene with trityl tetraphenylborate in benzene produces tris(tri-tert-butylsilyl)cyclotrigermenium tetraphenylborate [(tert-Bu3SiGe)3+BPh4-], which can be isolated as a yellow solid that is stable in the absence of air. The crystal structure of the cyclotrigermenium ion reveals a free germyl cation with a 2pi-electron system. The three germanium atoms form an equilateral triangle similar to the carbon analog, the cyclopropenium ion. 相似文献
14.
Robert A Sirjean O Browaeys A Poupard J Nowak S Boiron D Westbrook CI Aspect A 《Science (New York, N.Y.)》2001,292(5516):461-464
We report the realization of a Bose-Einstein condensate of metastable atoms (helium in the lowest triplet state). The excitation energy of each atom with respect to the ground state is 20 electron volts, but inelastic processes that would destroy the sample are suppressed strongly enough in a spin-polarized sample to allow condensation. Our detection scheme takes advantage of the metastability to achieve detection of individual atoms as well as of the decay products of inelastic processes. This detection opens the way toward new studies in mesoscopic quantum statistical physics, as well as in atomic quantum optics. 相似文献
15.
The blending of mesoporosity with the properties of semiconductors promises new types of multifunctional nanomaterials. It would be particularly interesting to combine the shape selectivity of a mesoporous oxide with the electronic and photonic characteristics of a useful semiconductor. We demonstrated the synthesis of a mesoporous germanium semiconductor using liquid-crystals-templated chemistry. The template removal was achieved by a two-step ion-exchange thermal procedure. This semiconductive mesoporous form of germanium possesses hexagonal pore ordering with very high surface area and exhibits strongly size-dependent optical properties as well as photoluminescence. 相似文献
16.
[目的]为重金属的植物化研究提供一定的基础。[方法]以3个水稻品种的幼苗为载体,研究金属锗在水稻体内的植物化过程。[结果]各个品种的水稻幼苗对锗毒性的抵抗能力存在差异。以拌土或浇灌的形式添加锗,0.060μg/ml的锗对吉丰3号、吉丰10号幼苗的农艺性状没有影响,但对吉丰47号幼苗的生长发育具有促进作用;1.00μg/ml的锗则对水稻幼苗的生长发育具有毒害作用。锗进入土壤后,能够迅速且大量地被水稻吸收并运往地上部分,其在水稻体内的积累规律为茎叶>根>糙米,并且茎叶中锗的含量远高于根和糙米中的锗含量。锗在水稻体内的最高允许浓度为0.060μg/ml。[结论]锗在水稻体内的植物化过程因水稻品种的不同而不同,0.060μg/ml锗的植物化效果最好。 相似文献
17.
锗是具有高度生物活性的微量元素,广泛分布在植物体内。综述了有机锗对植物的影响及其作用机理,结果表明,有机锗可以抑制植物体内产生的自由基,提高保护性酶的含量及活性,促进或抑制植物生长,影响植物对其他元素的吸收等。该研究为锗植物化应用的深入研究提供依据。 相似文献
18.
19.
锗在马铃薯—土壤体系内的迁移转化规律研究 总被引：1，自引：1，他引：0
[目的]研究锗在马铃薯—土壤体系内的迁移和转化规律。[方法]用0、0.01、0.30、0.60、0.80、1.00、1.50μg/ml的GeO2溶液对不同品种马铃薯进行浸种处理,研究锗对马铃薯幼苗生长发育的影响;通过小区模拟试验,探索锗在马铃薯—土壤体系内的迁移和转化规律。[结果]各个品种的马铃薯幼苗对锗毒性的抵抗能力存在差异。0.60μg/ml的锗对东农303号、延薯4号的农艺性状没有影响,但对克新2号幼苗的生长发育具有促进作用;1.00μg/ml的锗对马铃薯幼苗的生长发育具有毒害作用。锗进入土壤后,能够迅速且大量地被马铃薯吸收并运往地上部分,其在马铃薯体内的积累规律为:叶>块茎。[结论]马铃薯对锗是主动吸收过程,锗在马铃薯中的最高允许浓度为0.60μg/ml。 相似文献
20.
电感耦合等离子体原子发射光谱法测定金花茶中的有机锗 总被引：2，自引：0，他引：2
先在6mol／L盐酸介质中通过加热去除金花茶花朵中的无机锗,再采用微波消解样品和电感耦合等离子体原子发射光谱法测定有机锗。结果表明,该方法能基本除净无机锗,无机锗回收率达6．2％,样品中的有机锗含量（0．15±0．02）mg／kg与总锗含量（0．16±0．02）mg／kg基本一致,经t检验法检验无显著性差异,证明样品中含有的锗均为有机锗,而且加热不能除去有机锗。可见,采用电感耦合等离子体原子发射光谱法测定金花茶中的有机锗,既准确、快速,又符合分析要求。 相似文献