On the calculation of effective electric field in In0.53Ga0.47As surface channel metal-oxide-semiconductor field-effect-transistors

The effective electron mobility of In0.53Ga0.47As metal-oxide-semiconductor field-effect-transistors with HfO2 gate oxide was measured over a wide range of channel doping concentration. The back bias dependence of effective electron mobility was used to correctly calculate the vertical effective electric field. The effective electron mobility at moderate to high vertical effective electric field shows universal behavior independent of substrate impurity concentration. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3588255

The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption

We exploit the high sensitivity and moderate spectral resolution of the $HST$-Cosmic Origins Spectrograph to detect far-ultraviolet spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. We measure a CO column density and rotational excitation temperature of N(CO) = 2 +/- 1 $\times$ 10$^{17}$ cm$^{-2}$ and T_rot(CO) 500 +/- 200 K for the absorbing gas. We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by ultraviolet line photons, predominantly HI LyA. All three objects show emission from CO bands at $\lambda$ $>$ 1560 \AA, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H$_{2}$, and photo-excited H2; all of which appeared as a "continuum" whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar LyA emission profile. We find CO parameters in the range: N(CO) 10$^{18-19}$ cm$^{-2}$, T_{rot}(CO) > 300 K for the LyA-pumped emission. We combine these results with recent work on photo- and collisionally-excited H$_{2}$ emission, concluding that the observations of ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio in the inner disk is ~1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.Comment: 12 pages, 7 figures, 3 tables. ApJ - accepte

A Study of Fluid-Membrane Transport Processes and Their Applications: A Molecular Dynamics Approach

Selectively permeable membranes perform important roles in a wide range of systems including naturally occurring lipid membranes in biological systems to engineered polymeric membranes in filtration and energy technologies. In order to design technologies that incorporate such membranes it is crucial to understand the behavior of these systems on the molecular level so that optimal performance and maximum efficiencies can be achieved. One of the main focuses of this thesis is how ionic hydration affects the transport of electrolyte species through porous membranes. Molecular dynamics simulations are used to examine this in detail both for model systems as well as for actual industrial membranes including zeolite materials as potential ion exchange membranes (IEMs) in energy systems such as redox flow batteries. In addition, model selective membranes are used to computationally predict the phase equilibrium behavior of various gas/liquid systems at experimentally difficult conditions. Because of their safety, capacity, and small environmental footprint, redox flow batteries (RFBs) have become an attractive form of energy storage. However, this technology is not yet widely used commercially due to inefficiencies in the ion-exchange membrane. The current technology widely utilizes polymeric membranes that have stability problems in the highly reactive environment of the RFB and tend to break down, shortening the life of the battery. Also, they present less than desirable selectivity for proton transport, which is crucial to the overall efficiency of the battery. It has been proposed that thin zeolite membranes will provide both the stability and the selectivity to improve the performance of RFBs and make their wide-scale application more feasible. A molecular dynamics study of six zeolite framework types and the ions present in the vanadium-RFB has been undertaken to determine their transport behavior and investigate at the molecular level the requirements for suitability in IEM applications. In addition to investigating different zeolite frameworks, the effect of composition was examined by introducing different levels of aluminum substitution into the crystalline structure of one specific framework, namely MFI. By investigating two characteristics, membrane loading and intramembrane diffusion, it is possible to predict the overall ion permeability with the goal of optimizing the amount of substitution for high proton permeability while maintaining selectivity to undesirable ions. Beyond these specific applications, model selective membranes were also used in molecular dynamics simulations to predict phase behavior of various gas/liquid at conditions difficult to achieve in experiments. This research was carried out to fill data gaps that are urgently needed for the design of industrial processes for gas capture/ storage or separation. By validating our models against limited experimental data available, we show that molecular modeling can be a useful predictive tool for industrial applications. Simulations are currently not widely used in industry for data prediction even though they can be carried out at a small fraction of the cost of experimental studies

Computational Molecular Modeling of Transport Processes in Nanoporous Membranes

In this report we have discussed the important role of molecular modeling, especially the use of the molecular dynamics method, in investigating transport processes in nanoporous materials such as membranes. With the availability of high performance computers, molecular modeling can now be used to study rather complex systems at a fraction of the cost or time requirements of experimental studies. Molecular modeling techniques have the advantage of being able to access spatial and temporal resolution which are difficult to reach in experimental studies. For example, sub-Angstrom level spatial resolution is very accessible as is sub-femtosecond temporal resolution. Due to these advantages, simulation can play two important roles: Firstly because of the increased spatial and temporal resolution, it can help understand phenomena not well understood. As an example, we discuss the study of reverse osmosis processes. Before simulations were used it was thought the separation of water from salt was purely a coulombic phenomenon. However, by applying molecular simulation techniques, it was clearly demonstrated that the solvation of ions made the separation in effect a steric separation and it was the flux which was strongly affected by the coulombic interactions between water and the membrane surface. Additionally, because of their relatively low cost and quick turnaround (by using multiple processor systems now increasingly available) simulations can be a useful screening tool to identify membranes for a potential application. To this end, we have described our studies in determining the most suitable zeolite membrane for redox flow battery applications. As computing facilities become more widely available and new computational methods are developed, we believe molecular modeling will become a key tool in the study of transport processes in nanoporous materials

Using Molecular Simulations To Develop Reliable Design Tools and Correlations for Engineering Applications of Aqueous Electrolyte Solutions

Many industrial processes involve processing aqueous electrolyte solutions. There is thus a need for accurate theories to predict their thermophysical properties. Recent studies have shown that the size of the hydrated ion plays an important role in determining these properties. In this study, we first used molecular dynamics simulations to estimate the effective hydrated ionic size and the free energy of solvation, and then developed correlations allowing for the prediction of these quantities. The temperature dependence of these solution properties was also investigated. Our studies have shown that the effective (hydrated) size, the charge density, and the free energy of solvation of the ions are strongly interdependent. The effective hydrated ionic size also plays an important role in determining the selectivity of membranes to remove such hydrated ions from solutions, for example, in membrane based desalination processes, and related water purification technologies

Correction to: Whole lung tissue is the preferred sampling method for amplicon-based characterization of murine lung microbiota

http://deepblue.lib.umich.edu/bitstream/2027.42/173982/1/40168_2021_Article_1121.pd

Whole lung tissue is the preferred sampling method for amplicon-based characterization of murine lung microbiota

Abstract Background Low-biomass microbiome studies (such as those of the lungs, placenta, and skin) are vulnerable to contamination and sequencing stochasticity, which obscure legitimate microbial signal. While human lung microbiome studies have rigorously identified sampling strategies that reliably capture microbial signal from these low-biomass microbial communities, the optimal sampling strategy for characterizing murine lung microbiota has not been empirically determined. Performing accurate, reliable characterization of murine lung microbiota and distinguishing true microbial signal from noise in these samples will be critical for further mechanistic microbiome studies in mice. Results Using an analytic approach grounded in microbial ecology, we compared bacterial DNA from the lungs of healthy adult mice collected via two common sampling approaches: homogenized whole lung tissue and bronchoalveolar lavage (BAL) fluid. We quantified bacterial DNA using droplet digital PCR, characterized bacterial communities using 16S rRNA gene sequencing, and systematically assessed the quantity and identity of bacterial DNA in both specimen types. We compared bacteria detected in lung specimens to each other and to potential source communities: negative (background) control specimens and paired oral samples. By all measures, whole lung tissue in mice contained greater bacterial signal and less evidence of contamination than did BAL fluid. Relative to BAL fluid, whole lung tissue exhibited a greater quantity of bacterial DNA, distinct community composition, decreased sample-to-sample variation, and greater biological plausibility when compared to potential source communities. In contrast, bacteria detected in BAL fluid were minimally different from those of procedural, reagent, and sequencing controls. Conclusions An ecology-based analytical approach discriminates signal from noise in this low-biomass microbiome study and identifies whole lung tissue as the preferred specimen type for murine lung microbiome studies. Sequencing, analysis, and reporting of potential source communities, including negative control specimens and contiguous biological sites, are crucial for biological interpretation of low-biomass microbiome studies, independent of specimen type. Video abstracthttp://deepblue.lib.umich.edu/bitstream/2027.42/173981/1/40168_2021_Article_1055.pd