The Morphology and Electronic Properties of Si Nanoscale Structures on a CaF2 Surface

Technical Physics - The surface morphology, crystal structures, and band-energy parameters have been studied for nanofilms and regularly arranged nanoscale Si phases with a thickness of 1–2...     

Low-temperature specific heats of bromine—graphite intercalation compounds

Specific heats of bromine—graphite intercalation compounds were measured in a temperature range between 1.5 and 6 K, as a function of the bromine concentration. The electronic specific heats were very small. The lattice and the Einstein-type specific heats revealed that the lattice was softened and dispersionless optical mode emerged upon the bromine intercalation.     

Superconducting nanofilms: molecule-like pairing induced by quantum confinement

Quantum confinement of the perpendicular motion of electrons in single-crystalline metallic superconducting nanofilms splits the conduction band into a series of single-electron subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the energetic position of each subband can vary significantly with changing nanofilm thickness, substrate material, protective cover and other details of the fabrication process. It can occur that the bottom of one of the available subbands is situated in the vicinity of the Fermi level. We demonstrate that the character of the superconducting pairing in such a subband changes dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known crossover from the Bardeen-Cooper-Schrieffer regime to Bose-Einstein condensation (BCS-BEC) observed in trapped ultracold fermions. For Pb nanofilms with thicknesses of 4 and 5 monolayers (MLs) this will lead to a spectacular scenario: up to half of all the Cooper pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few nanometers. As a result, the superconducting condensate will be a coherent mixture of almost molecule-like fermionic pairs with ordinary, extended Cooper pairs.     

Rapid initiation of reactions in Al/Ni multilayers with nanoscale layering

Research into nanoenergetic materials is enabling new capabilities for controlling exothermic reaction rates and energy output, as well as new methods for integrating these materials with conventional electronics fabrication techniques. Many reactions produce primarily heat, and in some cases it is desirable to increase the rate of heat release beyond what is typically observed. Here we investigate the Al-Ni intermetallic reaction, which normally propagates across films or foils at rates lower than 10 m/s. However, models and experiments indicate that local heating rates can be very high (10⁷ K/s), and uniform heating of such a multilayer film can lead to a rapid, thermally explosive type of reaction. With the hopes of using a device to transduce electrical energy to kinetic energy of a flyer plate in the timescale of 100's of nanoseconds, we have incorporated a Ni/Al nanolayer film that locally heats upon application of a large electrical current. We observed flyer plate velocities in the 2-6 km/s range, corresponding to 4-36 kJ/g in terms of specific kinetic energy. Several samples containing Ni/Al films with different bilayer thicknesses were tested, and many produced additional kinetic energy in the 1.1-2.3 kJ/g range, as would be expected from the Ni-Al intermetallic reaction. These results provide evidence that nanoscale Ni/Al layers reacted in the timescale necessary to contribute to device output.     

Effect of temperature on inhomogeneous elastic deformation and negative stiffness of NiAl and FeAl alloy nanofilms

The uniaxial tension of NiAl and FeAl intermetallic alloy nanofilms at different temperatures has been investigated by the molecular dynamics method. It was previously shown that nanofilms at 0 K are elastically deformed by almost 40% and that, under strain-controlled tension, there is a region in the stress—strain curves, where an increase in the strain is accompanied by a decrease in the tensile stress, i.e., the stiffness of nanofilms is negative. Deformation of the films in the thermal instability region is associated with the appearance of domains with different elastic strains. The influence of the temperature on these effects is investigated. Particularly, it is shown that as the temperature increases, both the elastic strain and the negative stiffness of nanofilms decrease. The inhomogeneous elastic strain and negative stiffness for FeAl films are observed in a broader temperature range (to 1000 K) than for NiAl films (to 300 K), which constitutes 0.16 and 0.65 of the melting point of these materials, respectively.     

INFLUENCE OF RELATIVE DENSITY AND PARTICLE SIZE ON THE SPECIFIC HEAT OF NANOCRYSTALLINE IRON

The specific heats of two nanocrystalline iron samples with different structures have been measured in the temperature range from 2 to 26 K. Experimental results show that the specific heat of nanocrystalline iron samples exhibits abnormal characteristic compared with that of standard bulk iron. The contributions of quantum size effect, surface mode and Einstein mode have been observed in the measurement. Through analyses, it is discovered that the relative density and particle size have a great influence on specific heat of nanocrystalline iron at low temperature.     

X射线辐照后BaFC1中缺陷性质的研究

未掺Eu2+的BaFCl粉末样品在80K下经X射线辐照后,在不同的温度条件下(从80K副273K),用580nm的光波激励样品时观测到310nm和380nm的发光峰.经分析认为,样品经X射线辐照后,在晶体中形成两种卤素离子的F-H''心对.当用F心吸收带的光波激励样品时,两种F—H心对衰变成相应的自陷激子态并复合发光.310nm和380nm分别对应于F-离子和Cl-离子的自陷激子的发光,并且它们的发光强度强烈地依赖于温度的变化。     

Elektronennachemission und Photoemission von Alummiumoberflächen

Zeitschrift für Physik A Hadrons and nuclei - The specific heats of Zr-Rh alloys have been measured between 0.9 and 12 °K, and at atomic percent Rh concentrations between 0 and 8. The...     

Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

Ballistic–diffusive phonon transport and size induced anisotropy of thermal conductivity of silicon nanofilms

The effective thermal conductivity of nanofilms is size dependent due to the diffusive–ballistic transport of phonons. In this paper, we investigate the cross-plane phonon transport from the viewpoint of the phonon Boltzmann equation. A predictive model for the size dependent thermal conductivity is proposed and agrees well with the results of molecular dynamics simulation for silicon nanofilms. The ballistic transport has different effects on the heat conduction in the in-plane or cross-plane directions, which causes the anisotropy of thermal conductivity of nanofilms. Such anisotropy is also size dependent and vanishes with the increase of film thickness.     

Mo纳米薄膜热力学性质的分子动力学模拟

采用改进嵌入原子法(MAEAM),通过经典的分子动力学(MD)模拟计算了高熔点过渡金属体心立方(bcc) Mo块体Gibbs自由能和表面能. 对于纳米薄膜的热力学数据,比如Gibbs自由能等,可以看成是薄膜内部原子和表面原子两部分数据之和,然后根据薄膜的体表原子之比就可以直接计算出总的自由能,并由此可以得到热力学性质与薄膜尺寸及温度的定量关系式. 分别计算了bcc Mo块体及其纳米尺寸薄膜的自由能和热容,结果表明,Mo纳米薄膜的热力学性质具有尺寸效应,并且在薄膜尺寸小于15—20nm时,这种效应变得非常明 关键词： 改进嵌入原子法 Mo纳米薄膜 表面自由能 热容     

Numerical analysis of nanocluster formation within ns-laser ablation plume

The CIP method is used to calculate macroscopic plume expansion combined with the Zeldovich–Raiser theory for the cluster-formation process such as nucleation and growth. The effect of background gas and latent heat is examined in the one-dimensional case. The latent heats keep the plume temperature at 2500 K for a long period and this explains the delayed photoluminescence. Two contradicting experiments on the size dependence on ambient pressure are clearly explained and are attributed to the difference of laser energy. In two-dimensional simulation, a ‘mushroom-like’ plume shape is replicated, consistent with experimental results. Received: 17 January 2003 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-3-5237-2860, E-mail: tohkubo@es.titech.ac.jp     

Elektronische spezifische Wärme und Antiferromagnetismus in Chromlegierungen

The specific heats of 20 alloys of Cr with Mo, W, V, Ru and Os within the range of the body-centered cubic phase have been measured between 1.5 and 4.2°K and separated in their electronic and lattice parts. From the temperature dependence of the electrical resistance between 70 and 340°K the Néel temperatures of Cr-alloys with 0–16 at.-% Mo could be determined. A comparison of the electronic specific heats and the Néel temperatures of the investigated Cr-alloys reveals a considerable reduction of the electronic density of states within the range of concentrations with high Néel temperatures.     

Assembly of γ-Fe2O3/polyaniline nanofilms with tuned dipolar interaction

The internal morphology and magnetic properties of layer-by-layer assembled nanofilms of polyaniline (PANI) and maghemite (γ-Fe₂O₃—7.5-nm diameter) were probed with cross-sectional transmission electron microscopy (TEM) and magnetization measurements (magnetic hysteresis loops, magnetization using zero-field cooled/field-cooled protocols, and ac magnetic susceptibility). Additionally, simulations of the as-produced samples were performed to assess both the nanofilm’s morphology and the corresponding magnetic signatures using the cell dynamic system (CDS) approach and Monte Carlo (MC) through the standard Metropolis algorithm, respectively. Fine control of the film thickness and average maghemite particle–particle within this magnetic structure was accomplished by varying the number of bilayers (PANI/γ-Fe₂O₃) deposited onto silicon substrates or through changing the concentration of the maghemite particles suspended within the colloidal dispersion sample used for film fabrication. PANI/γ-Fe₂O₃ nanofilms comprising 5, 10, 25 and 50 deposited bilayers displayed, respectively, blocking temperatures (T _B) of 30, 35, 39 and 40 K and effective energy barriers (ΔE/k _B) of 1.0 × 10³, 2.3 × 10³, 2.8 × 10³ and 2.9 × 10³ K. Simulation of magnetic nanofilms using the CDS model provided the internal morphology to carry on MC simulation of the magnetic properties of the system taking into account the particle–particle dipolar interaction. The simulated (using CDS) surface–surface particle distance of 0.5, 2.5 and 4.5 nm was obtained for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. The simulated (using MC) T _B values were 33.0, 30.2 and 29.5 K for nanofilms with thicknesses of 36.0, 33.9 and 27.1 nm, respectively. We found the experimental (TEM and magnetic measurements) and the simulated data (CDS and MC) in very good agreement, falling within the same range and displaying the same systematic trend. Our findings open up new perspectives for fabrication of magnetic nanofilms with pre-established (simulated) morphology and magnetic properties.     

Thermophysical properties of stable and metastable liquid copper and nickel by molecular dynamics simulation

Molecular dynamics simulation combined with an embedded atom method (EAM) potential was applied to the calculation of the specific heat and the diffusion coefficient for superheated and undercooled liquid copper and nickel as functions of temperature. The system contains 108,000 atoms. The calculated results show that the enthalpy increases linearly with the rise of temperature. There are no breaks at their melting temperatures of 1356 and 1726 K. It is found that the calculated specific heats of Cu and Ni are 32.75 and 36.11 J/mol/K respectively. The calculated mean square displacements increase linearly with calculated time. The diffusion coefficients are exponentially dependent on temperature. Moreover, the calculated results are in good agreement with the reported experimental results for the specific heat and diffusion coefficient.     

Applying low-energy ion implantation in the creation of nanocontacts on the surface of ultrathin semiconductor films

A new way for obtaining nanocontacts on the surface of semiconductors using Si (111) thin films as an example is proposed. The essence of the method lies in the fact that, first, cobalt-silicide nanofilms with a thickness of 45–50 Å are formed by ion implantation in combination with annealing; then, atoms of the contact metal (Al) are sputtered. The specific resistance of the CoSi₂ nanofilms is (2–3) × 10^?5 Ω × cm.     

宏观长Ag2S纳米线簇的制备及其温度电导特性和光电导特性

以固态离子学方法制备的宏观长银(Ag)纳米线簇为基础,采用气-固反应法制备出宏观长硫化银(Ag₂S)纳米线簇.利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、能量色散谱(EDS)和X射线光电子能谱(XPS)等手段对样品形貌和成分进行表征.将厘米长的Ag₂S纳米线簇两端涂敷金胶作为电极,并与外电路连接.在不同温度或采用不同波长的光束辐照下,测试了样品的输运性质.无光照时,在144—380 K的温度范围内,样品的电导随温度上升而非线性增大.室温下,Ag关键词： 硫化银纳米线 温度电导 光电导     

Thermophysical properties of undercooled liquid Co-Mo alloys

Using electromagnetic levitation in combination with the oscillating drop technique and drop calorimeter method, the surface tensions and specific heats of undercooled liquid Co-10 wt% Mo, Co-26.3 wt% Mo, and Co-37.6 wt% Mo alloys were measured. The containerless state during levitation produces substantial undercoolings up to 223 K (0.13 T _L), 213 K (0.13 T _L) and 110 K (0.07 T _L) respectively for these three alloys. In their respective undercooling ranges, the surface tensions were determined to be 1895 m0.31(T m1744), 1932 m0.33(T m1682), and 1989 m0.34(T m1607) mN m^?1. According to the Butler equation, the surface tensions of these three Co-Mo alloys were also calculated, and the results agree well with the experimental data. The specific heats of these three alloys are determined to be 41.85, 43.75 and 44.92 J mol^?1 K^?1. Based on the determined surface tensions and specific heats, the changes in thermodynamics functions such as enthalpy, entropy and Gibbs free energy are predicted. Furthermore, the crystal nucleation, dendrite growth and Marangoni convection of undercooled Co-Mo alloys are investigated in the light of these measured thermophysical properties.     

Meniscus-Induced Thinning of Smectic Nanofilms

JETP Letters - A new mechanism of thinning of smectic nanofilms is discovered, differing from that previously described in experimental and theoretical works. The size and shape of a meniscus...     

Kalorimetrische Bestimmung der Übergangstemperatur zur Supraleitung von verdünnten Titan-Rhodium-Legierungen

The specific heats of Ti-Rh alloys have been measured between 0.4 and 4 °K for Rh concentrations between 0 and 3 atomic percent. The measurements have been carried out in a simple He³ cryostat. The transition temperature and the electronic specific heat are increasing functions of the Rh concentration. In quenched alloys the α′-phase can be stabilized up to an average Rh-concentration of 2.5 At-% for the whole sample.