Fluid Helium in a Narrow Pore at Zero Temperature

The zero temperature adsorption isotherm of ⁴He in a rigid cylindrical pore with alkali metal walls is computed within finite-range density functional theory, with the radius of the tube as a parameter. It is shown that starting from narrow pores and increasing the radius, the adsorbed helium first becomes bound in a quasi-onedimensional phase, and finally condenses into a quasi-twodimensional configuration by going through an almost filling situation. The results are in agreement with recent calculations for carbon nanotubes based on thermodynamical approaches.     

Condensation of Fluids Confined in Non-rigid Nanopores: With a Little Help from the Substrate

Fluids adsorbed in nanopores exert a force on the substrate that produces a compression or expansion of the host. Although this effect has been observed experimentally and computationally, an overwhelming majority of the theoretical studies have assumed that the environment provides a fixed, static potential in which the adsorbate moves. In a recent paper (H.-Y. Kim, S.M. Gatica, G. Stan and M.W. Cole, J. Low Temp. Phys. (this issue) 2009) we showed that the relaxation of the substrate is not limited to affecting the capacity of uptake, but has dramatic consequences on the physical properties of the adsorbates, like phase transitions and the energetics in low dimensions. For example, ³He in a rigid 1D pore is a gas, and in a non-rigid carbon nanotube is a liquid. In this paper we explore more situations involving classical and quantum fluids in slit pores and in bundles of carbon nanotubes. We find that, due to the cooperation of the relaxing substrate, confined classical gases condense at higher temperatures compared to extremely rigid substrates and ³He has a cohesive energy of up to 9.5 K when confined within a narrow non-rigid slit pore. We observe that a compression of a bundle of nanotubes of less than 0.02% gives rise to changes of up to 9% in the critical temperature.     

Thermal Compression of Atomic Hydrogen on Helium Surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid ⁴He surface. The surface gas density is increased locally by thermal compression up to 5.5 × 10¹² cm⁻² at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.     

Optical Observation of Atomic Hydrogen on the Surface of Liquid Helium

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Base degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The optical spectrum of the adsorbed hydrogen atoms was not known a priori. It shows a resonance that is much broader than that of a hydrogen atom in vacuo, and it is shifted to lower frequencies. From the fluorescence intensity we determine that we have reached a surface density corresponding to one atom per square De Broglie wavelength. This means that our experiments take place at the edge of quantum degeneracy. In the regime where the adsorption isotherm is known we can use the measured hydrogen densities to infer the temperature of the helium surface. We use this information to determine the thermal conductance between the surface and the bulk of liquid helium. We find quantitative agreement between the measured temperature drops and the prediction of ripplon-phonon coupling theory.     

Production of nanometer-size GaAs nanocristals by nanosecond laser ablation in liquid

This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm2 in our experiments) and is as low as 2.5 nm for the highest power used. The absolute value of the average diameter also decreases significantly, from 13.7 to 8.7 nm, as the laser power increases in this interval. Due to the narrow nanoparticle size dispersion achieved at the highest laser powers two absorption band edges are clearly distinguishable at about 1.72 and 3.15 eV which are ascribed to E0 and E1 effective optical transitions, respectively. A comparison of the energies with those known for bulk GaAs allows one to conclude that an average diameter of the investigated GaAs nanoparticles is close to 10 nm, i.e., they are quantum dots. High resolution transmission electron microscopy (HRTEM) images show that the bare GaAs nanoparticles are nanocrystalline, but many of them exhibit single/multiple twin boundary defects or even polycrystallinity. The formation of the GaAs crystalline core capped with a SiO2 shell was demonstrated by HRTEM and energy dispersive X-ray (EDX) spectroscopy. Effective band edges can be better distinguished in SiO2 capped nanoparticles than in bare ones, In both cases the band edges are correlated with size quantum confinement effect.     

Electrophoretically induced aqueous flow through single-walled carbon nanotube membranes

Electrophoresis, the motion of charged species through liquids and pores under the influence of an external electric field, has been the principle source of chemical pumping for numerous micro- and nanofluidic device platforms. Recent measurements of ion currents through single or few carbon nanotube channels have yielded values of ion mobility that range from close to the bulk mobility to values that are two to seven orders of magnitude higher than the bulk mobility. However, these experiments cannot directly measure ion flux. Experiments on membranes that contain a large number of nanotube pores allow the ion current and ion flux to be measured independently. Here, we report that the mobilities of ions within such membranes are approximately three times higher than the bulk mobility. Moreover, the induced electro-osmotic velocities are four orders of magnitude faster than those measured in conventional porous materials. We also show that a nanotube membrane can function as a rectifying diode due to ionic steric effects within the nanotubes.     

ESR on adsorbed doubly spin-polarized atomic hydrogen

Studies of the ESR spectrum of spin-polarized atomic hydrogen adsorbed on a liquid helium film are presented. The absorption peaks associated with the surface atoms are displaced relative to that of the gaseous atoms due to the electronic dipole-dipole interaction, which does not average to zero for atoms moving in two dimensions. This phenomenon, first observed by Reynolds et al., allows the surface atom density to be measured directly and, through the lineshape, information on the dynamics of the 2-D gas can in principle be obtained. Here we present a more detailed study, with a better characterized substrate for the helium film.Using only the assumption that the ideal gas approximation is valid for the experimental conditions, we find that the binding energy of hydrogen atoms to the liquid helium surface is 1.03 (2) K. Although these measurements of the binding energy are not the most accurate, they are the most direct.The ESR lineshape of the absorption peak of the bulk atoms is determined by the inhomogeneity of the applied magnetic fields, whereas the resonance lineshape of the adsorbed atoms, which is very asymmetric and much broader than the main resonance, is clearly due to some other mechanism. In spite of a considerable effort to explain the observed lineshapes, we have not reached satisfactory conclusions.     

Poly(amidoamine) dendrimers as nanoscale diffusion probes in sol-gel films investigated by total internal reflection fluorescence spectroscopy

Three generations of poly(amidoamine) dendrimers were dye-labeled and chemically modified to have terminal carboxyl groups and used as variably sized probes to study diffusion in thin sol-gel films. Total internal reflection fluorescence spectroscopy experiments, both correlation and concentration-jump measurements, were employed to measure the relative populations and effective diffusion coefficients of dendrimers in the films. For films prepared from small (27-nm) silica particles, larger dendrimers could be completely excluded from penetrating the sol-gel structure. In films made of larger (150-nm) particles with correspondingly larger pores, concentration-jump experiments showed that larger dendrimers are excluded from more of the intraparticle pore space than small dendrimers. Similarly, fluorescence-correlation measurements showed that the diffusion of smaller dendrimers exhibited greater tortuosity than larger dendrimers in the interparticle pores of the film. The smaller dendrimers explore a greater volume of smaller, more convoluted pores, whereas larger dendrimers penetrate a smaller volume of larger, more open pores.     

多孔TiO2光催化纳米薄膜的制备和微观结构研究

锐钛矿型多孔ＴｉＯ_２纳米薄膜可以从含聚乙二醇（ＰＥＧ）的钛酸盐溶胶前驱体中通过溶胶-凝胶法制备．涂层的形貌,如孔的大小和孔的分布可以通过聚乙二醇的加入量和分子量来控制．当聚乙二醇的加入量和分子量越大,聚乙二醇热分解后在薄膜中产生的气孔就越多和孔径越大．随着ＴｉＯ_２薄膜中气孔孔径和数量的增加,光的散射增强,薄膜的透光率减小．通过扫描电镜（ＳＥＭ）和重量法测定了薄膜的厚度,每镀一次薄膜的厚度增加约为０．０８μｍ．通过Ｘ射线光电子能谱（ＸＰＳ）和红外光谱（ＩＲ）确定了多孔ＴｉＯ_２纳米薄膜中元素的化学组成和表面羟基含量．实验结果表明：薄膜中除含有Ｔｉ、Ｏ元素外,还有一定量来自有机前驱物中未完全燃烧的碳和少量从玻璃表面扩散到薄膜中的Ｎａ和Ｃａ元素;同时发现薄膜表面的羟基含量随聚乙二醇的加入量的增加而增加     

Capillary Condensation and Hysteresis for 4He in Aerogel

When fluids are adsorbed in a porous medium, confinement and disorder can dramatically affect their critical behavior. Silica aerogels, with their high porosities and tenuous structures, provide an opportunity to study the effects of random impurities on the liquid-vapor transition. Previous measurements with helium and nitrogen confined in a 95% porosity acrogel showed no hysteresis between cooling and warming behavior near the critical temperature, T _c, allowing equilibrium behavior to be observed. T _c was suppressed and coexistence curves were much narrower than for bulk fluids. However, recent measurements further from T _c found long time constants and hysteresis between filling and emptying. We have made high resolution isotherm measurements near the critical point of helium in a 95% porosity aerogel, using a capacitive technique to measure the density directly. We also measured the pressure and bulk density in situ and used a ballast volume to control pressure. Thermal relaxation was very slow in the coexistence region, but our technique allowed us to directly check for equilibrium at each point. T _c was suppressed, but the coexistence curve was not as narrow as in the earlier measurements. However, we observed hysteresis and finite compressibilities at all temperatures below T _c, indicating the importance of surface tension and capillary condensation.     

Theoretical Study of Quantum Gel Formation in?Superfluid 4He

Bosonic density functional theory calculations were carried out for neon, argon, and fluorine based systems in superfluid ⁴He with an emphasis on the formation of dimeric species in the liquid. These atomic species display relatively strong binding with helium and hence their solvation structures in the liquid exhibit highly localized liquid helium layers around them. These solvent layers modify the gas phase dimer potentials by inclusion of a recombination barrier, which provides stabilization for the solvated atoms. Of closed shell species neon is shown to exhibit a recombination barrier of 3 K for the dimer and up to 5.8 K for specific cluster geometries. For argon, the liquid induced potential barrier is only 0.7 K and it has a rather large amount of excess energy available along the recombination coordinate indicating that it is not possible to stabilize argon atoms in superfluid helium. Atomic fluorine shows the most pronounced effect with the recombination barrier of 26.8 K for producing ground state F₂. It is concluded that neon and fluorine atoms are good candidates to form impurity based quantum gels in bulk superfluid helium.     

Structural and luminescent characteristics of macro porous silicon

The results of structural and luminescent study of porous Si prepared by electrochemical etching and containing the pores of micron sizes are presented. The samples demonstrate bright luminescence with external quantum efficiency of 15–20%. Raman scattering spectra contain only one line corresponding to bulk silicon. Atomic force microscopy image shows the structural elements essentially larger than quantum confinement crystallites. However, X-ray diffraction measurements demonstrate the presence of strained Si quantum confinement nanowires. It is shown that the recombination through interface oxide-related centers of the carriers excited due to light absorption in quantum confinement nanowires gives an essential contribution to emission spectra.     

Specific heat of thin helium films in a cylindrical geometry

We report measurements of the specific heat of thin helium films adsorbed in the 0.2 µm pores of Anopore membranes. Over the temperature range of these studies, two specific heat anomalies, strongly dependent on helium film thickness, were found.     

ZnO nanowires by pulsed laser vaporization: synthesis and properties

We report a new pulsed-laser vaporization (PLV) technique to synthesize nanowires of single-crystal ZnO having a wurtzite structure by using colloidal gold nanoparticles as seeding catalysts. The average diameter of the nanowires is approximately 13 nm, with a very narrow range of 7 to 25 nm. The nanowires are straight for the most part, with the axes parallel to the [0001] growth direction. Raman and photoluminescence spectra from the nanowires and bulk ZnO are similar except for a approximately 510 nm band in the nanowires due to oxygen vacancies. The bulk-like vibrational and electronic properties of the nanowires is due to the diameter being larger than the threshold below which quantum confinement-induced effects are expected.     

Nitrogen Codoped Unique Carbon with 0.4 nm Ultra‐Micropores for Ultrahigh Areal Capacitance Supercapacitors

A full understanding of ion transport in porous carbon electrodes is essential for achieving effective energy storage in their applications as electrochemical supercapacitors. It is generally accepted that pores in the size range below 0.5 nm are inaccessible to electrolyte ions and lower the capacitance of carbon materials. Here, nitrogen‐doped carbon with ultra‐micropores smaller than 0.4 nm with a narrow size distribution, which represents the first example of electrode materials made entirely from ultra‐microporous carbon, is prepared. An in situ electrochemical quartz crystal microbalance technique to study the effects of the ultra‐micropores on charge storage in supercapacitors is used. It is found that ultra‐micropores smaller than 0.4 nm are accessible to small electrolyte ions, and the area capacitance of obtained sample reaches the ultrahigh value of 330 µF cm^?2, significantly higher than that of previously reported carbon‐based materials. The findings provide a better understanding of the correlation between ultra‐micropore structure and capacitance and open new avenues for design and development of carbon materials for the next generation of high energy density supercapacitors.     

前驱体对炭泡沫孔结构的影响

分别以煤沥青、石油中间相沥青和AR沥青为前驱体制备炭泡沫材料。采用GPC测定前驱体分子量,SEM观察所制炭泡沫的孔结构,光学显微镜测量所制炭泡沫的孔径及其分布。结果发现,由于煤焦油沥青不含中间相,且QI含量较高,导致在实验条件下不能直接制备出合格的炭泡沫。以石油中间相沥青和AR沥青为原料均能制备出具有分布均匀开孔结构,且微观各向异性的炭泡沫。由AR沥青制备的炭泡沫呈现平均孔径较小(212μm)、孔壁较薄、孔径分布较窄(180μm~300μm)、开孔率较高、以及韧带排列较规整等特点,表明低QI含量、低分子量且分布较窄的前驱体有利于发泡。     

Optical Study of 4He Condensation into a Silica Aerogel

We use optical methods to study condensation of ⁴He into a 95% porosity silica aerogel at temperatures below the bulk critical point. Simugtaneous pressure and optical measurements are performed along isotherms, as the cell is very slowly filled or emptied. We find that the pressure presents a quasiplateau below the bulk saturation pressure P _sat over a finite range of densities inside the aerogel. In this range, strong light scattering is observed, which shows that the helium density fluctuates on a microscopic scale. Quantitative analysis shows that the helium density is correlated over distances somewhat larger than the gel correlation length. We discuss our resugts in terms of two possible scenarios, capillary condensation and liquid-gas transition.     

Fluid States of Helium Adsorbed in Nanopores

We have studied helium adsorbed in new nanopores which have regular structures from nano-cage to three-dimensionally (3D) connected pores. Adsorption potential and layer formation of the adsorbed helium are observed by the vapor pressure for the adsorption. New paradigms of zero-D and 1D helium fluids are realized in nano-cages and nano-channels, respectively. The superfluid onsets (transitions) in the 1D and 3D nanopores show obvious dependence on the pore connections. The superfluid in the 3D pores has properties similar to the Bose-Einstein condensation of the 3D Bose atomic gas. The films of the ³He gas formed in the ⁴He preplated nanopores show the dimensional crossover depending on the pore connection: from the 2D Boltzmann gas to a 1D or 3D gas state with decreasing temperature. This gas changes to the degenerate state in each dimension at the lower temperatures. Extremely high frequency measurements of the helium adatoms on flat substrate determined the superfluid vortex parameters of the ⁴He films, and revealed a slippage (decoupling) of the helium adatoms in the non-superfluid state.     

Theory and experiments of a three-cavity wavelength-selective photodetector

We propose a novel wavelength-selective photodetector with three subcavities, i.e., a filtering cavity, a spacer cavity, and an absorption cavity, for obtaining a narrow spectral response linewidth and a high quantum efficiency simultaneously. A theoretical prediction has been made that a less than 1-nm linewidth and a quantum efficiency as high as 90% are possible. We discuss the effects of the key factors on the performance of this type of photodetector that has been designed and fabricated. A spectral response linewidth of approximately 1.4 nm (FWHM) and an external quantum efficiency higher than 50% have been achieved experimentally. Such devices are promising for wavelength-division multiplexing applications.     

Supercooled liquid foaming of a Zr-Al-Cu-Ag bulk metallic glass containing pressurized helium pores

Foaming of a Zr-based metallic glass in the supercooled liquid is successfully performed by introducing pressurized pores and subsequent isochronal annealing. Melting of a Zr₄₈Cu₃₆Al₈Ag₈ powder under 12 MPa pressurized helium atmosphere followed by water quenching introduces spherical helium pores, whose average diameter and volume fraction are estimated respectively to be 30 μm and 7%, into a fully glassy bulk Zr₄₈Cu₃₆Al₈Ag₈ alloy. The isochronal annealing of the porous alloy below the crystallization temperature under atmospheric pressure of argon enables the expansion of pores by viscous flow deformation of the supercooled liquid, resulting in a high porosity structure up to 70% with a uniform cell size and cell distribution.