Bottleneck For Transport Research Articles

Coherent Acoustic Phonons in Colloidal Semiconductor Nanocrystal Superlattices.

The phonon properties of films fabricated from colloidal semiconductor nanocrystals play a major role in thermal conductance and electron scattering, which govern the principles for building colloidal-based electronics and optics including thermoelectric devices with a high ZT factor. The key point in understanding the phonon properties is to obtain the strength of the elastic bonds formed by organic ligands connecting the individual nanocrystallites. In the case of very weak bonding, the ligands become the bottleneck for phonon transport between infinitively rigid nanocrystals. In the opposite case of strong bonding, the colloids cannot be considered as infinitively rigid beads and the distortion of the superlattice caused by phonons includes the distortion of the colloids themselves. We use the picosecond acoustics technique to study the acoustic coherent phonons in superlattices of nanometer crystalline CdSe colloids. We observe the quantization of phonons with frequencies up to 30 GHz. The frequencies of quantized phonons depend on the thickness of the colloidal films and possess linear phonon dispersion. The measured speed of sound and corresponding wave modulus in the colloidal films point on the strong elastic coupling provided by organic ligands between colloidal nanocrystals.

Market Power and Spatial Arbitrage between Interconnected Gas Hubs

This paper examines the efficiency of the arbitrages performed between two regional markets for wholesale natural gas linked by a capacity-constrained pipeline system. We develop a switching regime specification to (i) detect if the observed spatial arbitrages satisfy the integration notion that all arbitrage opportunities between the two markets are being exploited, and (ii) decompose the observed spatial price differences into factors such as transportation costs, transportation bottlenecks, and the oligopolistic behavior of the arbitrageurs. Our framework incorporates a test for the presence of market power and it is thus able to distinguish between the physical and behavioral constraints to marginal cost pricing. We use the case of the “Interconnector” pipeline linking Belgium and the UK as an application. Our empirical findings show that all the arbitrage opportunities between the two zones are being exploited but confirm the presence of market power.

Head‐of‐tide bottleneck of particulate material transport from watersheds to estuaries

AbstractWe measured rates of sediment, C, N, and P accumulation at four floodplain sites spanning the nontidal through oligohaline Choptank and Pocomoke Rivers, Maryland, USA. Ceramic tiles were used to collect sediment for a year and sediment cores were collected to derive decadal sedimentation rates using 137Cs. The results showed highest rates of short‐ and long‐term sediment, C, N, and P accumulation occurred in tidal freshwater forests at the head of tide on the Choptank and the oligohaline marsh of the Pocomoke River, and lowest rates occurred in the downstream tidal freshwater forests in both rivers. Presumably, watershed material was mostly trapped at the head of tide, and estuarine material was trapped in oligohaline marshes. This hydrologic transport bottleneck at the head of tide stores most available watershed sediment, C, N, and P creating a sediment shadow in lower tidal freshwater forests potentially limiting their resilience to sea level rise.

Temperature Dependence of Charge Localization in High‐Mobility, Solution‐Crystallized Small Molecule Semiconductors Studied by Charge Modulation Spectroscopy

In solution‐processable small molecule semiconductors, the extent of charge carrier wavefunction localization induced by dynamic disorder can be probed spectroscopically as a function of temperature using charge modulation spectroscopy (CMS). Here, it is shown based on combined field‐effect transistor and CMS measurements as a function of temperature that in certain molecular semiconductors, such as solution‐processible pentacene, charge carriers become trapped at low temperatures in environments in which the charges become highly localized on individual molecules, while in some other molecules the charge carrier wavefunction can retain a degree of delocalization similar to what is present at room temperature. The experimental approach sheds new insight into the nature of shallow charge traps in these materials and allows identifying molecular systems in which intrinsic transport properties could, in principle, be observed at low temperatures if other transport bottlenecks associated with grain boundaries or contacts could be removed.

Multifold Increases in Thermal Conductivity of Polymer Nanocomposites through Microwave Welding of Metal Nanowire Fillers

A 40-fold thermal conductivity increase in silver nanowire-filled polydimethylsiloxane composites is demonstrated by in situ microwave welding of nanowire fillers. Even for nanowire fractions ≤0.04, welding counteracts thermal transport bottlenecks associated with low polymer thermal conductivity and high polymer–filler interface thermal resistances. The microwave transparency of the polymer precludes thermal degradation, and the composites retain high compliance.

Spin and charge transport in graphene-based spin transport devices with Co/MgO spin injection and spin detection electrodes

In this review we discuss spin and charge transport properties in graphene-based single-layer and few-layer spin-valve devices. We give an overview of challenges and recent advances in the field of device fabrication and discuss two of our fabrication methods in more detail which result in distinctly different device performances. In the first class of devices, Co/MgO electrodes are directly deposited onto graphene which results in rough MgO-to-Co interfaces and favor the formation of conducting pinholes throughout the MgO layer. We show that the contact resistance area product (RcA) is a benchmark for spin transport properties as it scales with the measured spin lifetime in these devices indicating that contact-induced spin dephasing is the bottleneck for spin transport even in devices with large RcA values. In a second class of devices, Co/MgO electrodes are first patterned onto a silicon substrate. Subsequently, a graphene-hBN heterostructure is directly transferred onto these prepatterned electrodes which provides improved interface properties. This is seen by a strong enhancement of both charge and spin transport properties yielding charge carrier mobilities exceeding 20,000cm2/(Vs) and spin lifetimes up to 3.7ns at room temperature. We discuss several shortcomings in the determination of both quantities which complicates the analysis of both extrinsic and intrinsic spin scattering mechanisms. Furthermore, we show that contacts can be the origin of a second charge neutrality point in gate dependent resistance measurements which is influenced by the quantum capacitance of the underlying graphene layer.

Identifying Pathways for Improving Household Food Self-Sufficiency Outcomes in the Hills of Nepal.

Maintaining and improving household food self-sufficiency (FSS) in mountain regions is an ongoing challenge. There are many facets to the issue, including comparatively high levels of land fragmentation, challenging terrain and transportation bottlenecks, declining labor availability due to out-migration, and low technical knowledge, among others. Using a nonparametric multivariate approach, we quantified primary associations underlying current levels of FSS in the mid-hills of Nepal. A needs assessment survey was administered to 77 households in Lungaun (Baglung District), Pang (Parbat District), and Pathlekhet (Myagdi District), with a total of 80 variables covering five performance areas; resulting data were analyzed using Classification and Regression Trees. The most parsimonious statistical model for household FSS highlighted associations with agronomic management, including yields of maize and fingermillet within a relay cropping system and adoption of improved crop cultivars. Secondary analyses of the variables retained in the first model again focused primarily on crop and livestock management. It thus appears that continued emphasis on technical agricultural improvements is warranted, independent of factors such as land holding size that, in any case, are very difficult to change through development interventions. Initiatives to increase household FSS in the mid-hills of Nepal will benefit from placing a primary focus on methods of agricultural intensification to improve crop yields and effective technology transfer to increase adoption of these methods.

Toward Carbon Nanotube Based Thermal Interface Materials

Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (CNTs) with Ohmic back contacts are demonstrated in anodized aluminum oxide (AAO) nanopore templates on arbitrary substrates for use as thermal interface materials (TIMs). Such CNT-based TIMs are a promising alternative to conventional metal-filled epoxy TIMS, (e.g. silver paste), due to the large thermal conductivity of CNTs and the ability to eliminate epoxy in the thermal pathways, a major bottleneck for phonon transport. Without an efficient TIM between heat-producing devices and their packaging material (heat sink), the advantages of advanced power electronics are obscured due to the degradation of both performance and lifetime that occur with increasing temperatures. Briefly, we prepare templates by sputter depositing low residual stress Nd-doped Al films onto W-coated substrates, followed by electrochemical anodization to form aluminum oxide (AAO) nanopore arrays. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer that can be selectively etched away. This leaves an electrically conductive W film at the bottom of every nanopore, provides a back electrical contact for both the electrodeposition of Co catalyst nanowires and later transport measurements. These structures are then use to grow CNTs arrays via thermal chemical vapor deposition (CVD). Critical issues include (1) optimizing the crystalline quality of individual CNTs to ensure high thermal conductivity, (2) achieving high densities of CNTS to maximize the thermal pathway, and (3) planarizing the CNT array tips to enable direct contact to a heat-producing device surface with as many of the CNTs as possible. CNT crystalline quality is primarily controlled via the CVD process choices such as carbon feedstock gas, catalyst material, and growth conditions (temperature and time). The array density is primarily impacted by the initial fraction of nanopores filled with Co, and a competition during CVD between (a) CNT growth from the Co nanowires in the nanopores and (b) decomposition of carbonaceous material on the AAO template surface that chokes the pores, solved by controlling the aspect ratio of the open pore above the Co catalyst. Finally, we demonstrate the use of ultrasonication to cut all the CNTs to a uniform height as well as ohmic contacts between CNT tips and the W backplane, implying that the CNT-Co-W structure is unbroken, critical for thermal transport. These properties are studied using scanning and transmission electron microscopies, Raman spectroscopy, and thermal/electrical transport measurements. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Super-Joule heating in graphene and silver nanowire network

Transistors, sensors, and transparent conductors based on randomly assembled nanowire networks rely on multi-component percolation for unique and distinctive applications in flexible electronics, biochemical sensing, and solar cells. While conduction models for 1-D and 1-D/2-D networks have been developed, typically assuming linear electronic transport and self-heating, the model has not been validated by direct high-resolution characterization of coupled electronic pathways and thermal response. In this letter, we show the occurrence of nonlinear “super-Joule” self-heating at the transport bottlenecks in networks of silver nanowires and silver nanowire/single layer graphene hybrid using high resolution thermoreflectance (TR) imaging. TR images at the microscopic self-heating hotspots within nanowire network and nanowire/graphene hybrid network devices with submicron spatial resolution are used to infer electrical current pathways. The results encourage a fundamental reevaluation of transport models for network-based percolating conductors.

Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices

Recently, it has been shown that oxide barriers in graphene-based non-local spin-valve structures can be the bottleneck for spin transport. The barriers may cause spin dephasing during or right after electrical spin injection which limits spin transport parameters such as the spin lifetime of the whole device. An important task is to characterize the quality of the oxide barriers of both spin injection and detection contacts in a fabricated device. To address this issue, we discuss the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barriers on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices. By both simulations and reference measurements on devices with non-ferromagnetic electrodes, we demonstrate that the background signal can be caused by an inhomogeneous current flow through the oxide barriers. As a main result, we demonstrate the existence of charge accumulation besides the actual spin accumulation signal in non-local voltage measurements, which can be explained by a redistribution of charge carriers by a perpendicular magnetic field similar to the classical Hall effect. Furthermore, we present systematic studies of the phase of the low frequency non-local ac voltage signal which is measured in non-local spin measurements when applying ac lock-in techniques. This phase has so far widely been neglected in the analysis of non-local spin transport. We demonstrate that this phase is another hallmark of the homogeneity of the MgO spin injection and detection barriers. We link backgate dependent changes of the phase to the interplay between the capacitance of the oxide barrier and the quantum capacitance of graphene.

Research universities of the future: an 'aerotropolitan' perspective

A large vigorous pool of top scientists, engineers and technology managers will be key to national military and economic strength in the 21st century knowledge economy. Two critical challenges our nation faces - transportation bottlenecks reducing the productivity of busy scientists etc. and a waning pool of white male students on which US graduate schools traditionally have focused their best recruitment and mentoring for advanced educational credentials in science and technology - may become deal-breakers in the coming decades. Cutting the time spent on ground travel of business trips to improve productivity as suggested by John Kasarda's 'aerotropolis' is a promising approach. He says to locate the scientists and their laboratories, etc. at or very close to a hub airport integrated with a planned city. Although necessary, a hugely expensive network of these aerotropolises - which would require a large government investment at a time of political gridlock in the nation's capital - is not likely to be built. Retrofitting older cities with high-speed ground transportation to realise the aerotropolis' most notable advantage might be cheaper and attract political support.

Characterization of charge traps in pentacene diodes by electrical methods

The charge transport is investigated in pentacene diodes by steady-state current–voltage technique, impedance spectroscopy method, and transient capacitance spectroscopy method. The temperature dependencies were analyzed to obtain activation energies of charge transport processes. All characterization techniques observed activation energy of about 0.3eV, which represents the bottleneck for charge transport through the organic diode. This comparison of experimental techniques demonstrates their capabilities and possible resolution.

The Improvement of Lot Delivery Time in 450 mm Semiconductor Manufacturing

Semiconductor industry has tremendous development in recent decades. According to Moore's Law ( 1), semiconductor wafer manufacturers have to search improvement opportunities constantly in order to be competitive. International technology roadmap for semiconductor (2) has indicated the possible scenarios for 450 mm wafer fabs, and the conveyor will be a potential solution to realize transportation automation. How to reduce the bottleneck of transportation is a valuable issue for the highly capital intensive industry. The purpose of this paper is to focus on improving the bottleneck of conveyor loops and provide effective transport dispatching rule for 450 mm wafer fab. The results demonstrate that controlling the speed of bottleneck loops can expedite the movement of lots and reduce lot transport time with the range from 6.24% to 13.02% without more input resources. These results are also verified by statistics. So the proposed rule can effectively reduce product cycle time in 450 mm wafer fab under conveyor transportation environment.

Suppression of contact-induced spin dephasing in graphene/MgO/Co spin-valve devices by successive oxygen treatments

By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance--area products $({R}_{c}A)$ of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential $dV\text{\ensuremath{-}}dI$ curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small ${R}_{c}A$ values. With increasing ${R}_{c}A$ values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.

Lithium-Ion Transport at Electrolyte/Electrode Interface Visualized By Nanoscale Meniscus Cell Microscope

Towards the establishment of low-carbon emission society, one urgent issue is to standardize an efficient method to store unused energy. A promised candidate is an electrochemical storage such as lithium-ion batteries. However, it is still hindered as a major problem that the lithium-ion batteries do not conduct with sufficiently high energies capabilities and stable operational cycles. In particular, it is remained as an inefficient barrier of the lithium-ion (de)intercalation at the interface between electrode and electrolyte. It is therefore necessary to determine a crucial factor which influence to the homogeneity in ion transport across the interface by in-situ measurement with nanometer resolution.In this study, the homogeneity in local ion transport of LiFePO4 thin electrodes is investigated by a nanoscale meniscus cell microscope (NMCM). It is a newly established scanning probe microscopy with a nano-size pipet with electrolyte and a reference-electrode.1 As the pipet is in proximity of the LiFePO4 electrode, a meniscus is created. Then, ionic current is only measured through the meniscus when the voltage is applied between reference and LiFePO4electrodes, as shown in Figure 1(a). When the pipet is scanned through the interface in nanometer step, a mapping of the ion current distribution in lithium (de)intercalation is obtained with topographical information in Figure 1 (b). Further, as the pipet was pinned at specific area, it allows us to study the localized ion transport by cyclic voltammetry or charge /discharge process. This new analytic technique will reveal a bottleneck of the ion transport in the materials. leading to the creation of electrode with homogeneous ion transport. Reference: (1) Y. Takahashi, et al., under review.

Expression of a xylose-specific transporter improves ethanol production by metabolically engineered Zymomonas mobilis.

Zymomonas mobilis is a promising organism for biofuel production as it can produce ethanol from glucose at high rates. However, Z. mobilis does not natively ferment C5 sugars such as xylose. While it has been engineered to do so, the engineered strains do not metabolize these sugars at high rates. Previous research has identified some of the bottlenecks associated with xylose metabolism in Z. mobilis. In this work, we investigated transport as a possible bottleneck. In particular, we hypothesized that the slow uptake of xylose through the promiscuous Glf transporter may limit the efficiency of xylose metabolism in Z. mobilis. To test this hypothesis, we expressed XylE, the low-affinity xylose transporter from Escherichia coli, in a xylose-utilizing strain of Z. mobilis. Our results show that the expression of this pentose-specific transporter improves the rate of xylose utilization in Z. mobilis; however, this enhancement is seen only at high xylose concentrations. In addition, we also found that overexpression of the promiscuous Z. mobilis transporter Glf yielded similar results, suggesting that the transport bottleneck is not due to the specificity, but rather the capacity for sugar uptake.

Development of endosperm transfer cells in barley.

Endosperm transfer cells (ETCs) are positioned at the intersection of maternal and filial tissues in seeds of cereals and represent a bottleneck for apoplasmic transport of assimilates into the endosperm. Endosperm cellularization starts at the maternal-filial boundary and generates the highly specialized ETCs. During differentiation barley ETCs develop characteristic flange-like wall ingrowths to facilitate effective nutrient transfer. A comprehensive morphological analysis depicted distinct developmental time points in establishment of transfer cell (TC) morphology and revealed intracellular changes possibly associated with cell wall metabolism. Embedded inside the grain, ETCs are barely accessible by manual preparation. To get tissue-specific information about ETC specification and differentiation, laser microdissection (LM)-based methods were used for transcript and metabolite profiling. Transcriptome analysis of ETCs at different developmental stages by microarrays indicated activated gene expression programs related to control of cell proliferation and cell shape, cell wall and carbohydrate metabolism reflecting the morphological changes during early ETC development. Transporter genes reveal distinct expression patterns suggesting a switch from active to passive modes of nutrient uptake with the onset of grain filling. Tissue-specific RNA-seq of the differentiating ETC region from the syncytial stage until functionality in nutrient transfer identified a high number of novel transcripts putatively involved in ETC differentiation. An essential role for two-component signaling (TCS) pathways in ETC development of barley emerged from this analysis. Correlative data provide evidence for abscisic acid and ethylene influences on ETC differentiation and hint at a crosstalk between hormone signal transduction and TCS phosphorelays. Collectively, the data expose a comprehensive view on ETC development, associated pathways and identified candidate genes for ETC specification.

Unless changes are made in Benin, multiple storage and transport bottlenecks may prevent vaccines from reaching the population

Unless changes are made in Benin, multiple storage and transport bottlenecks may prevent vaccines from reaching the population

Many rivers to cross: the journey of zinc from soil to seed

An important goal of micronutrient biofortification is to enhance the amount of bioavailable zinc in the edible seed of cereals and more specifically in the endosperm. The picture is starting to emerge for how zinc is translocated from the soil through the mother plant to the developing seed. On this journey, zinc is transported from symplast to symplast via multiple apoplastic spaces. During each step, zinc is imported into a symplast before it is exported again. Cellular import and export of zinc requires passage through biological membranes, which makes membrane-bound transporters of zinc especially interesting as potential transport bottlenecks. Inside the cell, zinc can be imported into or exported out of organelles by other transporters. The function of several membrane proteins involved in the transport of zinc across the tonoplast, chloroplast or plasma membranes are currently known. These include members of the ZIP (ZRT-IRT-like Protein), and MTP (Metal Tolerance Protein) and heavy metal ATPase (HMA) families. An important player in the transport process is the ligand nicotianamine that binds zinc to increase its solubility in living cells and in this way buffers the intracellular zinc concentration.

Container Shipping Market Dimensions and Customer Orientation in the Czech Republic

The forthcoming research characterises and analyses the key market players (shipping lines), their customer service policy together with the implications the container transport bottlenecks have for the market in the Czech Republic with customer orientation being one of these. Evaluating companies' customer orientation and analyzing container shipping dimensions in the market of transport and logistics is a highly complex task, especially in the times of the changes in the market. To obtain the data necessary for an informed analysis, open interviews with selected customer service managers will be made and questionnaires distributed in the Czech Republic. Due to the concentration of container and logistics activity within a relatively small number of global shipping lines present in the countries with substantial market share, the approach of market characteristics will be case study based. It must be stressed this is a conceptual paper only, the data has not been collected yet; therefore, there is no conclusion on the research. It is focused primarily on methodology applied and a literature review.