Formation and Structure of Hydrolytic Methylaluminoxane Activators

Methylaluminoxane (MAO) activators have sheet structures which form ion-pairs on reaction of neutral donors such as octamethyltrisiloxane (OMTS). The ion-pairs can be detected by electrospray ionization mass spectrometry (ESI-MS) in polar media. The growth of these reactive precursors during hydrolysis of Me₃Al can be monitored using ESI-MS. Density functional theory, combined with numerical simulation of growth, indicates that this process involves rapid formation of low MW oligomers, followed by assembly of these species into low MW sheets. These can grow through further addition of low MW oligomers or by fusion into larger sheets. The mechanism of these growth processes leads to the prediction that even-numbered sheets should be favored, and this surprising result is confirmed by ESI-MS monitoring experiments of both activator growth and MAO aging.     

Mass Spectrometric Characterization of Methylaluminoxane‐Activated Metallocene Complexes

Electrospray‐ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp₂ZrMe₂], [Cp₂ZrMe(Cl)], and [Cp₂ZrCl₂] in fluorobenzene (PhF) solution are reported. The results demonstrate that alkylation and ionization are separate events that occur at competitive rates in a polar solvent. Furthermore, there are significant differences in ion‐pair speciation that result from the use of metallocene dichloride complexes in comparison to alkylated precursors at otherwise identical Al/Zr ratios. Finally, the counter anions that form are dependent on the choice of precursor and Al/Zr ratio; halogenated aluminoxane anions [(MeAlO)_x(Me₃Al)_y?z(Me₂AlCl)_zMe]^? (z=1, 2, 3…?) are observed using metal chloride complexes and under some conditions may predominate over their non‐halogenated precursors [(MeAlO)_x(Me₃Al)_yMe]^?. Specifically, this halogenation process appears selective for the anions that form in comparison to the neutral components of MAO. Only at very high Al/Zr ratios is the same “native” anion distribution observed when using [Cp₂ZrCl₂] when compared with [Cp₂ZrMe₂]. Together, the results suggest that the need for a large excess of MAO when using metallocene dichloride complexes is a reflection of competitive alkylation vs. ionization, the persistence of unreactive, homodinuclear ion pairs in the case of [Cp₂ZrCl₂], as well as a change in ion pairing resulting from modification of the anions formed at lower Al/Zr ratios. Models for neutral precursors and anions are examined computationally.     

Influence of atomic layer deposition chemistry on high-k dielectrics for charge trapping memories

In this work we report the performance of the SiO₂/Si₃N₄/HfO₂ and SiO₂/Si₃N₄/ZrO₂ stacks with emphasis on the influence of atomic layer deposition chemistry used for forming the HfO₂ and ZrO₂ blocking layers. Two Hf precursors were employed – tetrakis(ethylmethylamino)hafnium (TEMAH) and bis(methylcyclopentadienyl)methoxymethyl hafnium (HfD-04). For ZrO₂, tetrakis(ethylmethylamino)zirconium (TEMAZ) and bis(methylcyclopentadienyl)methoxymethyl zirconium (ZrD-04) were used as metal precursors. Ozone was used as the oxygen source. The structural characteristics of the stacks were examined by transmission electron microscopy and grazing incidence X-ray diffraction. The electrical properties of the stacks were studied using platinum-gated capacitor structures. The memory performance of the stacks was evaluated by write/erase (W/E) measurements, endurance and retention testing. Endurance measurements revealed the most important difference between the stacks. The films grown from TEMAH and TEMAZ could withstand a significantly higher number of W/E pulses (>3 × 10⁵ in the 10 V/?11 V, 10 ms regime), in comparison to the stacks made from HfD-04 and ZrD-04 precursors (<5 × 10³ W/E cycles). This difference in endurance characteristics is attributed mainly to the different deposition temperatures suited for these two precursors and the nature of the layer formed at the Si₃N₄/HfO₂ and the Si₃N₄/ZrO₂ interfaces.     

Investigation of environmental reliability of optical polymer waveguides embedded on printed circuit boards

In this paper, the effects of environmental stresses on the properties of polymeric optical waveguides were investigated. Optical multimode waveguides were embedded on printed circuit boards by employing commercial polymers. Three optical-PCB constructions varying in board structure and in optical build-up materials were compared. The guide systems were subjected to four different tests: damp heat-high humidity, isothermal annealing, thermal shock and environmental flowing multigas test. Isothermal annealing reduced the refractive index to greatest extent. The optical-PCB structure with optical surface build-up layer was observed to be more vulnerable under temperature shock when compared with the O-PCB with optical inner layer. The buffer layer beneath the optical build-up was found to improve the stability of the optical waveguides significantly. The results indicated of a wavelength dependence to the aging factor with a failure mechanism. The factors affecting the performance and reliability of polymer-based optical waveguides on PCBs were discussed.     

A Note on the Finite-Dimensional Distributions of Dispersing Billiard Processes

In this short note we consider the finite-dimensional distributions of sets of states generated by dispersing billiards with a random initial condition. We establish a functional correlation bound on the distance between the finite-dimensional distributions and corresponding product distributions. We demonstrate the usefulness of the bound by showing that it implies several limit theorems. The purpose of this note is to provide a tool facilitating the study of more general functionals of the billiard process.     

Influence of pyrene-labeling on fluid lipid membranes

We elucidate the influence of pyrene-labeled phospholipids on the structural properties of a fluid dipalmitoylphosphatidylcholine lipid membrane. To this end, we employ extensive atomic-scale molecular dynamics simulations with varying concentrations of pyrene-linked lipids. We find pyrene labeling to perturb the membrane structure significantly in the vicinity of the probe, the correlation length in the bilayer plane being about 1.0-1.5 nm. The local perturbations lead to enhanced ordering and packing of lipid acyl chains located in the vicinity of the probe. Surprisingly, this holds true not only for lipids that reside in the same leaflet as the pyrene-labeled probe but also for lipids in the opposite monolayer. The latter is due to substantial interdigitation of the pyrene moiety into the opposite leaflet, suggesting that occasional excimer formation may take place for probes in different leaflets. As a related issue, we also discuss the location and conformational orientation of the pyrene moieties. In particular, the orientational distribution of pyrene turns out to be more broad and diverse than the distribution of the corresponding acyl tails of nonlabeled lipids.     

Comparison between the electrical properties of atomic layer deposited thin ZrO2 films processed from cyclopentadienyl precursors

Electrical characterization of zirconium oxide (ZrO₂) based metal-oxide-semiconductor (MOS) structures has been carried out. ZrO₂ films have been atomic layer deposited (ALD) by using novel cyclopentadienyl-based precursors, which have recently revealed themselves as very adequate in terms of thermal stability and high permittivity of the dielectrics deposited. Our results demonstrate good quality of the films, especially when mixed alkylamido-cyclopentadienyl precursors are used on SiO₂/Si substrates. Conduction mechanisms in these MIS capacitors were studied, with moderately or highly-doped silicon used as substrate.     

Energy-efficient neighbor discovery protocol for mobile wireless sensor networks

Low energy consumption is a critical design requirement for most wireless sensor network (WSN) applications. Due to minimal transmission power levels, time-varying environmental factors and mobility of nodes, network neighborhood changes frequently. In these conditions, the most critical issue for energy is to minimize the transactions and time consumed for neighbor discovery operations. In this paper, we present an energy-efficient neighbor discovery protocol targeted at synchronized low duty-cycle medium access control (MAC) schemes such as IEEE 802.15.4 and S-MAC. The protocol effectively reduces the need for costly network scans by proactively distributing node schedule information in MAC protocol beacons and by using this information for establishing new communication links. Energy consumption is further reduced by optimizing the beacon transmission rate. The protocol is validated by performance analysis and experimental measurements with physical WSN prototypes. Experimental results show that the protocol can reduce node energy consumption up to 80% at 1–3 m/s node mobility.     

Tilt: major factor in sterols' ordering capability in membranes

Using extensive atomistic simulations, we show that there is a single experimentally accessible parameter--the sterol tilt--that can be used to determine a sterol's capability to induce order, and thus to promote, e.g., formation of lipid rafts. The observations also facilitate designing new effective sterols.