Observed Size Dependence Research Articles

Island Size and Environment Dependence of Adatom Capture: Cu/Co Islands on Ru(0001)

AbstractThe rate of capture by stable Co islands on Ru(0001) of additionally deposited Cu atoms is quantified using scanning tunneling microscopy, kinetic Monte Carlo simulations, and diffusion equation analyses. We find strong dependence of the capture rates on Co-island size, larger islands showing larger capture rates, qualitatively distinct from self-consistent mean-field predictions. The observed size dependence is shown to reflect larger island-free areas surrounding bigger islands, i.e., a strong correlation between island sizes and separations neglected in mean-field treatments.

Size Dependence of Polymer Composition in the Surface Layer of Poly(acrylamide-co-acrylic acid) Hydrogel Microspheres

Poly(acrylamide-co-acrylic acid) hydrogel microspheres of four different average sizes were prepared using a membrane emulsification technique. These four kinds of microspheres had average diameters of 1.35, 2.33, 3.09, and 7.43 μm in distilled water and were all fairly monodisperse. Their surface properties were studied by measuring the electrophoretic mobility of the microspheres. The mobility values were negative at pH 7.4 with the ionic strengths between 0.005 and 0.154 at 25°C. More negative mobility values were obtained with smaller microspheres than the larger ones. By analyzing the data with an electrokinetic theory for “soft” surfaces, it was found that smaller microspheres have higher surface charge density than the larger ones, although all kinds of microspheres were prepared from monomer solutions with the same monomer composition. The observed size dependence of the electrophoretic mobility suggests that copolymerization of acrylamide monomers and acrylic acid monomers does not proceed homogeneously within a microsphere. That is, charged acrylic acid monomers have a tendency to be localized in the microsphere core region, whereby the surface region of microspheres becomes poor in charges, reducing the mobility of larger microspheres. This is not the case for smaller microspheres, so that their mobility becomes more negative.

Cloud chemistry varies with drop size

Samples of fog and cloud drops were collected by drop size from several locations around the United States. Sites were selected to represent a variety of cloud types (coastal stratus clouds, frontal clouds, and radiation fog) as well as both clean and polluted environments. A size‐fractionating version of the Caltech Active Strand Cloudwater Collector (sf‐CASCC) and a two‐stage Eidgenössische Technische Hochschule (ETH) cloud impactor were each used to simultaneously collect small and large cloud drop fractions. The sf‐CASCC is designed to collect drops >23 μm in diameter in the large fraction and drops between 4 and 23 μm in the small fraction, while the ETH impactor is typically configured to collect drops >10 μm in diameter and drops between 3 and 10 μm in diameter in its two size fractions. Concentrations of sulfate and ammonium were typically observed to be enriched in small cloud drops. A similar pattern was observed for nitrate at several locations. Coarse mode aerosol species were often observed to be enriched in large cloud drops. These findings suggest that the size‐dependent chemical signature of the precursor aerosol particle size distribution is at least partially maintained in the cloud drop size spectrum. The observed size dependence of the cloud drop composition can enhance rates of aqueous phase sulfate production relative to expectations based on average cloud drop composition, bias efficiencies with which individual chemical species are incorporated into precipitation within clouds, and affect patterns of chemical deposition to plant canopies or within the respiratory tract.

Size effects in phase transitions of semiconductor nanoparticles embedded in glass

Phase transitions of copper halide particles have been investigated by optical means. A decrease of particle melting temperature with decrease of size has been found. It is shown that the observed size dependencies of crystallization and melting temperatures can be described with practically the same value of surface tension in the framework of homogeneous nucleation theory and particle melting model in which the idea about thermally activated formation of a liquid layer on the particle surface is used. It has been found that the lowest temperature at which high temperature modification of CuBr (β-phase) can be stabilized due to extra annealing of samples depends weakly on particle size. Decrease of melting temperature prevents stabilization of β-phase for particles having radii less than 5 nm.

Carrier recombination in CdS nanocrystals under single-electron and high-density excitation

Room-temperature green luminescence has been studied in laser-modified (darkened) CdS nanocrystals with the average radii a ranging above the exciton Bohr radius (from 3.3 nm to 100 nm). Under high picosecond-pulse excitation, the electron-hole plasma decay kinetics exhibited a decrease of lifetime with decreasing crystallite size, enabling the surface recombination with the velocity of 1.3*103 cm s-1 to be revealed. Under low cw excitation, a striking increase in the luminescence intensity of nanocrystals with one excited electron-hole pair was discovered for average radii below 10 nm. The bulk-like bimolecular recombination with the constant of 3*10-10 cm3 s-1 was shown to account for the observed size dependence.

A theory of polarization reversals in finite systems

Abstract A theory of polarization reversals in finite system is developed on the basis of the Kolmogorov-Avrami model. The observed size dependence of the switching time, which looks very peculiar at a glance, seems to be well explained by the present theory.

Melting and freezing behavior of indium metal in porous glasses

The size-dependent melting and freezing behavior of In metal in porous silica glasses with mean pore diameters between 6 and 141 nm has been studied by differential scanning calorimetry. The melting and freezing temperatures of the pore In were always less than the corresponding bulk values, and varied in inverse proportion to the diameter of the confining silica pores. In the smallest pores the latent heat of fusion was also determined, and found to be about one-third of its bulk value. The observed size dependence of the melting temperature was in good agreement with the predictions of thermodynamic treatments of melting in finite systems, and allowed the solid-liquid interfacial free energy to be estimated for several different geometrical models. The measured latent heat, however, was smaller than expected based on thermodynamic considerations. No evidence for an energy barrier separating the solid from the liquid was found.

Aromatic Hydrocarbon Binding to Cytochrome P450 and Other Enzyme Binding Sites: Are Hydrophobic Compounds Drawn into the Active Site or Pushed from the Aqueous Phase?

The subject of hydrocarbon inhibition of cytochrome P450-dependent reactions as well as data on other enzyme-catalyzed reactions from the literature was examined to determine the relationship between the "hydrophobicity" of the hydrocarbons and their ability to act as inhibitors. The compounds used in these studies (benzene, toluene, ethylbenzene, n-propylbenzene, and n-butylbenzene) behave as competitive inhibitors, with the affinity increasing as the size of the inhibiting hydrocarbon increases. A similarity was seen in the size dependence for both hydrocarbon inhibition of cytochrome P450-dependent activities (−0.6 to −0.7 kcal/mol/methylene group) and transfer of these compounds between aqueous and organic phases (−0.68 kcal/mol/methylene group), suggesting that the active site of cytochrome P450, in some ways, is comparable to an organic solvent in its ability to accommodate hydrophobic compounds. A more detailed examination of this process was initiated to separate the "hydrophobic effect" into its two component processes: (i) hydration of the hydrocarbon ligand and (ii) transfer of the unhydrated hydrocarbon onto the enzyme active site. In other words, do larger hydrocarbons bind more avidly to the active site because they are drawn more effectively into that site (pull), or is the size-dependent increase in hydrocarbon binding the result of the larger compounds being more efficiently expelled from the aqueous medium (push)? The results indicate that the predominant force involved in binding is the ability of the active site of cytochrome P450 and an impressive number of other enzymes to draw the hydrocarbon from the aqueous medium. The hydration of the hydrocarbon is much less dependent on the size of the hydrocarbon, indicating that dehydration or partial dehydration of the hydrocarbon molecule (upon leaving the solution and combining with the enzyme) contributes to the overall binding process to a much lesser extent; hydrophobic binding in the most widely used sense (entropy driven) is not the primary driving force that is responsible for the observed size dependence effects. It is pointed out that not all types of binding would be expected to follow the law which describes the size dependence for simple hydrocarbons because of heat-entropy relationships. The different temperature dependence of these heat-entropy relationships further complicates the analogy between enzyme-ligand binding and ligand partitioning between aqueous and organic phases. The maximum contribution that can be attributed to entropy driven hydrophobic binding (in the most widely used sense) is −0.1 to −0.2 kcal/mol/methylene group for the aromatic hydrocarbons examined here.

Gate size dependence of the radiation-produced changes in threshold voltage, mobility, and interface state density in bulk CMOS

It has been observed that radiation-induced threshold voltage shifts, interface trap densities, and channel mobility reductions are dependent on the drawn or coded geometrical sizes of transistors in one radiation-hardened VLSI CMOS technology. In another technology, no such dependencies were found. The authors suggest physical mechanisms responsible for the observed size dependencies, and warn that large-area transistors and capacitors for total dose evaluations may not always be suitable. Stress measurements indicate that the radiation-induced changes are correlated with higher tensile stress levels. >

Quantum size effects on the exciton energy of CdS clusters.

We report an experimental investigation of the dependence of the lowest exciton energy of CdS clusters (quantum dots) on the cluster size. The cluster diameter is varied from 60 to 10 \AA{}. X-ray diffraction provides definitive identification of the cluster phase and permits a determination of the average cluster size. We find that the observed size dependence cannot be explained by models based on the effective-mass approximation, but, for clusters larger than 20 \AA{} in diameter, it is in agreement with a recent tight-binding calculation [P. E. Lippens and M. Lannoo, Phys. Rev. B 39, 10 935 (1989)]. Below 20 \AA{}, where the cluster deviates from the bulk zinc-blende structure, some discrepancy between the tight-binding calculation and experiments becomes evident.

Modelling and measurement of crack closure and crack growth following overloads and underloads : Dexter, R.J., Hudak Jr, S.J. and Davidson, D.L.Eng. Fract. Mech. 1989 33, (6), 855–870

In the present work, the dislocation structure evolution and indentation size effect in (001) oriented STO have been studied by using sequential polishing, chemical etch-pit technique and high-resolution electron backscattered diffraction (HR-EBSD) analysis via Berkovich nanoindentation experiments. Nanoindentation load-displacement curves show multiple pop-in events, which relate to nucleation and extension of dislocation pile-ups around the residual impression. Sequential polishing and etching revealed the three-dimensional dislocation etch-pit structure at various sub-surface depths. The dislocation densities are determined for a 5 mN and 10 mN indentation below the surface via HR-EBSD and etch-pit analysis. With HR-EBSD, the lattice rotation and thereby GND densities are determined, while the etch-pit technique revealed the total dislocation density. Based on the independently measured dislocation densities, we clearly show a depth dependent dislocation density, where both total and GND densities increase with decreasing indentation depths. The higher dislocation densities below smaller indents explain the observed size dependency of the hardness in STO, which also sheds light on the extraordinary combination of high indentation hardness and low yield strength in STO.

PbS in polymers. From molecules to bulk solids

The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron-hole-in-a-box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 Å.

Finite-Size Scaling near Tλ of 4He

We have calculated the specific heat of confined helium near Tλ using realistic boundary conditions On the basis of our one-loop results we argue that a new size-dependent scaling variable should be employed. We find good agreement between our prediction and the observed size dependence of the temperature shift of the specific heat maximum. This seems to solve a longstanding problem.

Lattice contraction in small particles of SrCl 2: Gd 3+: explicit size effect in EPR and Raman spectroscopy

Under hydrostatic pressure, electron paramagnetic resonance (EPR) and Raman spectroscopy experiments have been performed on both the bulk and small particles of SrCl 2: Gd 3+. The measurements of the pressure dependence of the fourth-order crystal-field parameter b 4 in EPR and the Raman frequency ω, up to 6.2 and 2 kbar respectively, allow us to separate the size dependence of b 4 and ω in two contributions: the first, called implicit, is due to the lattice contraction as measured by high-resolution electron diffraction, the second, called explicit, is due to the direct influence of the size at constant volume of the unit cell. Our interpretation shows that the size must be considered as a new thermodynamic variable like pressure and temperature. In both studies, the explicit effect represents a similar fraction of the total size effect (60 and 50% in EPR and Raman spectroscopy respectively). This shows that it must be interpreted in the same way. In view of the observed size dependence on the Debye-Waller coefficient in small metallic particles, an interpretation is proposed in which the vibrational amplitude of the ions is treated as size dependent. The EPR results for a 100 Å particle yield a vibrational amplitude twice that of the bulk. The estimated effect of this increase on the Raman frequency is in good agrement with the explicit size effect deduced from the Raman spectroscopy experiments.

SIZE DEPENDENCE OF GROWTH RATE, RESPIRATORY ELECTRON TRANSPORT SYSTEM ACTIVITY, AND CHEMICAL COMPOSITION IN MARINE DIATOMS IN THE LABORATORY1

ABSTRACTThe dependence of growth, electron transport system activity and chemical composition on the size of diatoms was examined during the exponential phase of growth. The six different marine centric species compared ranged in volume from 7.7 μm3 to 62 × 105μm3. A size dependence was observed for growth, 14C uptake, respiration and the productivity index (14C/chl a). Although the size dependence of all parameters was similar, the results indicate that on a carbon basis, growth efficiency decreases with increasing size. The C/N and C/chl a ratios were not size dependent. The importance of the surface area to cell volume ratio, and the importance of carbon per unit volume in determining the observed size dependence are discussed.

Trends in the energy dependence of strong interaction characteristics at ultra-high energies

The trends in the energy dependence of strong interaction characteristics that reveal themselves when an attempt is made to bring about a closer agreement between experimentally observed size dependence of several properties of high energy hadrons in air showers and the corresponding Monte Carlo simulations are discussed. The discussion shows the need for an increase in the inelasticity and interaction cross section of both pions and nucleons at ultra-high energies, as also an increase in the cross section for the production of nucleon-antinucleon pairs and also the need for postulating a process similar to the gammaization process of Nikolsky according to which a considerable fraction of energy is transferred to the soft component by-passing the normal pionization. This is an extension of a previous paper (see ibid., vol.6, no.7, 1067 of 1973).