Momentum-Space Imaging of Ultra-Thin Electron Liquids in δ-Doped Silicon.

Two-dimensional dopant layers (δ-layers) in semiconductors provide the high-mobility electron liquids (2DELs) needed for nanoscale quantum-electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one-electron spectral function that can be measured by angle-resolved photoemission spectroscopy (ARPES). Here, buried 2DEL δ-layers in silicon are measured with soft X-ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device-relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX-ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the δ-layer electronic confinement. The measurements are made on ambient-exposed samples and yield extremely thin (< 1 nm) and dense (≈1014  cm-2 ) 2DELs. Critically, this method is used to show that δ-layers of arsenic exhibit better electronic confinement than δ-layers of phosphorus fabricated under identical conditions.

In operando charge transport imaging of atomically thin dopant nanostructures in silicon.

Novel approaches to materials design, fabrication processes and device architectures have accelerated next-generation electronics component production, pushing device dimensions down to the nano- and atomic-scale. For device metrology methods to keep up with these developments, they should not only measure the relevant electrical parameters at these length-scales, but ideally do so during active operation of the device. Here, we demonstrate such a capability using the full functionality of an advanced scanning microwave/scanning capacitance/kelvin probe atomic force microscope to inspect the charge transport and performance of an atomically thin buried phosphorus wire device during electrical operation. By interrogation of the contact potential, carrier density and transport properties, we demonstrate the capability to distinguish between the different material components and device imperfections, and assess their contributions to the overall electric characteristics of the device in operando. Our experimental methodology will facilitate rapid feedback for the fabrication of patterned nanoscale dopant device components in silicon, now important for the emerging field of silicon quantum information technology. More generally, the versatile setup, with its advanced inspection capabilities, delivers a comprehensive method to determine the performance of nanoscale devices while they function, in a broad range of material systems.

Ultra-shallow dopant profiles as in-situ electrodes in scanning probe microscopy.

The application of nano materials to control advanced functionality in semiconductor devices has reached the atomic scale. At this dimension the exact chemical and structural composition of a device is crucial for its performance. Rapid inspection techniques are required to find the optimal combination among numerous materials. However, to date the earliest electrical inspection is carried out after multiple fabrication processes. This delay makes the fabrication of atomically designed components very challenging. Here, we propose a sample system to chemically characterize nanoscale devices in-operando. We introduce ion-implanted contacts which embedded in the sample serve as additional electrodes to carry out scanning gate experiments. We demonstrate that the presence of these electrodes does not deteriorate the surface quality. The potential of this approach is highlighted by controlling the charge state of single dangling bonds on the silicon surface. Apart from our novel sample holder, the experimental setup was not modified making this approach compatible to most commercial low-temperature scanning probe microscopes. For silicon based devices, the versatility of this method is a promising avenue to gain a detailed and rapid understanding of functionalized atomic devices and quantum interactions at the atomic level.

Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy.

Over the past two decades, prototype devices for future classical and quantum computing technologies have been fabricated by using scanning tunneling microscopy and hydrogen resist lithography to position phosphorus atoms in silicon with atomic-scale precision. Despite these successes, phosphine remains the only donor precursor molecule to have been demonstrated as compatible with the hydrogen resist lithography technique. The potential benefits of atomic-scale placement of alternative dopant species have, until now, remained unexplored. In this work, we demonstrate the successful fabrication of atomic-scale structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we pattern a monolayer hydrogen mask to selectively place arsenic atoms on the Si(001) surface using arsine as the precursor molecule. We fully elucidate the surface chemistry and reaction pathways of arsine on Si(001), revealing significant differences to phosphine. We explain how these differences result in enhanced surface immobilization and in-plane confinement of arsenic compared to phosphorus, and a dose-rate independent arsenic saturation density of 0.24 ± 0.04 monolayers. We demonstrate the successful encapsulation of arsenic delta-layers using silicon molecular beam epitaxy, and find electrical characteristics that are competitive with equivalent structures fabricated with phosphorus. Arsenic delta-layers are also found to offer confinement as good as similarly prepared phosphorus layers, while still retaining >80% carrier activation and sheet resistances of <2 kΩ/square. These excellent characteristics of arsenic represent opportunities to enhance existing capabilities of atomic-scale fabrication of dopant structures in silicon, and may be important for three-dimensional devices, where vertical control of the position of device components is critical.

Nondestructive imaging of atomically thin nanostructures buried in silicon.

It is now possible to create atomically thin regions of dopant atoms in silicon patterned with lateral dimensions ranging from the atomic scale (angstroms) to micrometers. These structures are building blocks of quantum devices for physics research and they are likely also to serve as key components of devices for next-generation classical and quantum information processing. Until now, the characteristics of buried dopant nanostructures could only be inferred from destructive techniques and/or the performance of the final electronic device; this severely limits engineering and manufacture of real-world devices based on atomic-scale lithography. Here, we use scanning microwave microscopy (SMM) to image and electronically characterize three-dimensional phosphorus nanostructures fabricated via scanning tunneling microscope-based lithography. The SMM measurements, which are completely nondestructive and sensitive to as few as 1900 to 4200 densely packed P atoms 4 to 15 nm below a silicon surface, yield electrical and geometric properties in agreement with those obtained from electrical transport and secondary ion mass spectroscopy for unpatterned phosphorus δ layers containing ~1013 P atoms. The imaging resolution was 37 ± 1 nm in lateral and 4 ± 1 nm in vertical directions, both values depending on SMM tip size and depth of dopant layers. In addition, finite element modeling indicates that resolution can be substantially improved using further optimized tips and microwave gradient detection. Our results on three-dimensional dopant structures reveal reduced carrier mobility for shallow dopant layers and suggest that SMM could aid the development of fabrication processes for surface code quantum computers.

Composition and depth distribution of hydrocarbons in Barataria Bay marsh sediments after the Deepwater Horizon oil spill.

In 2010, an estimate 4.1 million barrels of oil were accidentally released into the Gulf of Mexico (GoM) during the Deepwater Horizon (DWH) Oil Spill. One and a half years after this incident, a set of subtidal and intertidal marsh sediment cores were collected from five stations in Barataria Bay, Louisiana, USA, and analyzed to determine the spatial and vertical distributions and source of hydrocarbon residues based on their chemical composition. An archived core, collected before the DWH oil spill from the same area, was also analyzed to assess the pre-spill hydrocarbon distribution in the area. Analyses of aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and stable carbon isotope showed that the distribution of petroleum hydrocarbons in Barataria Bay was patchy and limited in areal extent. Significant TPH and ΣPAH concentrations (77,399 µg/g and 219,065 ng/g, respectively) were detected in the surface sediments of one core (i.e., core A) to a depth of 9 cm. Based on a sedimentation rate of 0.39 cm yr(-1), determined using (137)Cs, the presence of anthropogenic hydrocarbons in these sediment core deposited ca. 50 to 60 years ago. The historical background hydrocarbon concentrations increased significantly at the sediment surface and can be attributed to recent inputs. Although the oil present in the bay's sediments has undergone moderate weathering, biomarker analyses performed on core A samples likely indicated the presence of hydrocarbons from the DWH oil spill. The effects of oiling events on Barataria Bay and other marsh ecosystems in this region remain uncertain, as oil undergoes weathering changes over time.

Pediatric Triage in a Severe Pandemic: Maximizing Survival by Establishing Triage Thresholds.

OBJECTIVES: To develop and validate an algorithm to guide selection of patients for pediatric critical care admission during a severe pandemic when Crisis Standards of Care are implemented. DESIGN: Retrospective observational study using secondary data. PATIENTS: Children admitted to VPS-participating PICUs between 2009-2012. INTERVENTIONS: A total of 111,174 randomly selected nonelective cases from the Virtual PICU Systems database were used to estimate each patient's probability of death and duration of ventilation employing previously derived predictive equations. Using real and projected statistics for the State of Ohio as an example, triage thresholds were established for casualty volumes ranging from 5,000 to 10,000 for a modeled pandemic with peak duration of 6 weeks and 280 pediatric intensive care beds. The goal was to simultaneously maximize casualty survival and bed occupancy. Discrete Event Simulation was used to determine triage thresholds for probability of death and duration of ventilation as a function of casualty volume and the total number of available beds. Simulation was employed to compare survival between the proposed triage algorithm and a first come first served distribution of scarce resources. MEASUREMENTS AND MAIN RESULTS: Population survival was greater using the triage thresholds compared with a first come first served strategy. In this model, for five, six, seven, eight, and 10 thousand casualties, the triage algorithm increased the number of lives saved by 284, 386, 547, 746, and 1,089, respectively, compared with first come first served (all p < 0.001). CONCLUSIONS: Use of triage thresholds based on probability of death and duration of mechanical ventilation determined from actual critically ill children's data demonstrated superior population survival during a simulated overwhelming pandemic.

Concentrations and sources of polycyclic aromatic hydrocarbons in surface coastal sediments of the northern Gulf of Mexico.

BACKGROUND: Coastal sediments in the northern Gulf of Mexico have a high potential of being contaminated by petroleum hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs), due to extensive petroleum exploration and transportation activities. In this study we evaluated the spatial distribution and contamination sources of PAHs, as well as the bioavailable fraction in the bulk PAH pool, in surface marsh and shelf sediments (top 5 cm) of the northern Gulf of Mexico. RESULTS: PAH concentrations in this region ranged from 100 to 856 ng g-1, with the highest concentrations in Mississippi River mouth sediments followed by marsh sediments and then the lowest concentrations in shelf sediments. The PAH concentrations correlated positively with atomic C/N ratios of sedimentary organic matter (OM), suggesting that terrestrial OM preferentially sorbs PAHs relative to marine OM. PAHs with 2 rings were more abundant than those with 5-6 rings in continental shelf sediments, while the opposite was found in marsh sediments. This distribution pattern suggests different contamination sources between shelf and marsh sediments. Based on diagnostic ratios of PAH isomers and principal component analysis, shelf sediment PAHs were petrogenic and those from marsh sediments were pyrogenic. The proportions of bioavailable PAHs in total PAHs were low, ranging from 0.02% to 0.06%, with higher fractions found in marsh than shelf sediments. CONCLUSION: PAH distribution and composition differences between marsh and shelf sediments were influenced by grain size, contamination sources, and the types of organic matter associated with PAHs. Concentrations of PAHs in the study area were below effects low-range, suggesting a low risk to organisms and limited transfer of PAHs into food web. From the source analysis, PAHs in shelf sediments mainly originated from direct petroleum contamination, while those in marsh sediments were from combustion of fossil fuels.

Biotic and abiotic controls on sediment aggregation and consolidation: implications for geochemical fluxes and coastal restoration.

This study examined the influence of particle size and organic matter on aggregation and compaction of 3 hydraulically dredged sediments from coastal Louisiana (clay, silt loam, sandy loam) saturated under a range of salinity regimes (1 and 5 PSU, 5 and 10 PSU, and 15 and 25 PSU) for 4 time periods (1, 8, 16, and 26 weeks). Particle sizes were determined using a laser diffraction particle size analyzer, which allowed us to develop high-resolution results indicating changes in aggregate size across a spectrum of experimental conditions. The sediments with greater organic matter content exhibited approximately 60% aggregation, as indicated by fewer aggregates in the clay size fraction, and subsequently more aggregates in the sand size fraction, when organic matter remained in the sediment. Additionally, the sandy sediment compacted more than the organic sediments in the first 16 weeks. These findings suggest that sediments with greater clay and organic matter content behave as larger particles and may undergo particle rearrangement and compaction over longer time scales than sandy sediments with low organic matter. For coastal wetland restoration, models should include the effect of organic matter on particle aggregation to understand sediment dynamics over geologic time.

Impacts of diverted freshwater on dissolved organic matter and microbial communities in Barataria Bay, Louisiana, U.S.A.

Here we present results of an initial assessment of the impacts of a water diversion event on the concentrations and chemical composition of dissolved organic matter (DOM) and bacterioplankton community composition in Barataria Bay, Louisiana U.S.A, an important estuary within the Mississippi River Delta complex. Concentrations and spectral properties of DOM, as reflected by UV/visible absorbance and fluorescence, were strikingly similar at 26 sites sampled along transects near two western and two eastern areas of Barataria Bay in July and September 2010. In September 2010, dissolved organic carbon (DOC) was significantly higher (568.1-1043 µM C, x=755.6+/-117.7 µM C, n=14) than in July 2010 (249.1-577.1 µM C, x=383.7+/-98.31 µM C, n=14); conversely, Abs254 was consistently higher at every site in July (0.105-0.314) than in September (0.080-0.221), averaging 0.24±0.06 in July and 0.15±0.04 in September. Fluorescence data via the fluorescence index (FI450/500) revealed that only 30% (8 of 26) of the July samples had an FI450/500 above 1.36, compared to 96% (25 of 26) for the September samples. This indicates a more terrestrial origin for the July DOM. Bacterioplankton from eastern sites differed in composition from bacterioplankon in western sites in July. These differences appeared to result from reduced salinities caused by the freshwater diversion. Bacterioplankton communities in September differed from those in July, but no spatial structure was observed. Thus, the trends in bacterioplankton and DOM were likely due to changes in water masses (e.g., input of Mississippi River water in July and a return to estuarine waters in September). Discharge of water from the Davis Pond Freshwater Diversion (DPFD) through Barataria Bay may have partially mitigated some adverse effects of the oil spill, inasmuch as DOM is concerned.

Mucin gene 19 (MUC19) expression and response to inflammatory cytokines in middle ear epithelium.

Mucin gene 19 (MUC19) has been identified as a major gel-forming mucin in the human middle ear (ME). The objectives of this investigation were to characterize the expression and assess the regulation of MUC19 in the ME cell culture models utilized in the study of otitis media (OM). Findings demonstrate that MUC19 is expressed in both human immortalized cell culture (HMEEC) and chinchilla primary epithelial culture (CMEEC). ME exposure to inflammatory cytokines TNF-alpha, IL-1beta, IL-6 and IL-8 up-regulate MUC19 transcription, most robustly after exposure to TNF-alpha. Kinetic experiments suggest a relative early response in MUC19 transcription and a down-regulation after prolonged exposure. Glycoprotein production was increased in response to the increased transcription as well. Similar to other mucin genes in the ME, MUC19 is differentially regulated after exposure to inflammatory cytokines. The large size, gel-forming properties and up-regulation in response to important inflammatory cytokines of MUC19 suggest that it has significant potential to play a role in both physiology and pathophysiology of the ME.

Process modeling of ICU patient flow: effect of daily load leveling of elective surgeries on ICU diversion.

Despite the considerable number of publications on ICU patient flow and analysis of its variability, a basic and practically important question remained unanswered: what maximum number of elective surgeries per day should be scheduled (along with the competing demand from emergency surgeries) in order to reduce diversion in an ICU with fixed bed capacity to an acceptable low level, or prevent it at all? The goal of this work was to develop a methodology to answer this question. An ICU patient flow simulation model was developed to establish a quantitative link between the daily load leveling of elective surgeries (elective schedule smoothing) and ICU diversion. It was demonstrated that by scheduling not more than four elective surgeries per day ICU diversion due to 'no ICU beds' would be practically eliminated. However this would require bumping 'extra' daily surgeries to the block time day of another week which could be up to 2 months apart. Because not all patients could wait that long for their elective surgery, another more practical scenario was tested that would also result in a very low ICU diversion: bumping 'extra' daily elective surgeries within less than 2 weeks apart, scheduling not more than five elective surgeries per day, and strict adherence to the ICU admission/ discharge criteria.

Process modeling of emergency department patient flow: effect of patient length of stay on ED diversion.

A discreet event simulation methodology has been used to establish a quantitative relationship between Emergency Department (ED) performance characteristics, such as percent of time on ambulance diversion and the number of patients in queue in the waiting room, and the upper limits of patient length of stay (LOS). A simulation process model of ED patient flow has been developed that took into account a significant difference between LOS distributions of patients discharged home and patients admitted into the hospital. Using simulation model it has been identified that ED diversion could be negligible (less than approximately 0.5%) if patients discharged home stay in ED not more than 5 h, and patients admitted into the hospital stay in ED not more than 6 h Using full factorial design of experiments with two factors and the model's predicted percent diversion as a response function, other combinations of LOS upper limits have been determined that would result in low ED percent diversion as well. It has also been determined that if the number of patients exceeds 11 in queue in ED waiting room then the diversion percent is rapidly increasing.