Browsing by Subject "Transport"

Now showing 1 - 13 of 13

Open Access
The effect of GaN thickness inserted between two AlN layers on the transport properties of a lattice matched AlInN/AlN/GaN/AlN/GaN double channel heterostructure
(Elsevier BV, 2014-01-31) Tülek, R.; Arslan, E.; Bayraklı, A.; Turhan, S.; Gökden, S.; Duygulu, Ö.; Kaya, A.; Fırat, T.; Teke, A.; Özbay, Ekmel
One AlInN/AlN/GaN single channel heterostructure sample and four AlInN/AlN/GaN/AlN/GaN double channel heterostructure samples with different values of the second GaN layer were studied. The interface profiles, crystalline qualities, surface morphologies, and dislocation densities of the samples were investigated using high resolution transmission electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. Some of the data provided by these measurements were used as input parameters in the calculation of the scattering mechanisms that govern the transport properties of the studied samples. Experimental transport data were obtained using temperature dependent Hall effect measurements (10-300 K) at low (0.5 T) and high (8 T) magnetic fields to exclude the bulk transport from the two-dimensional one. The effect of the thickness of the second GaN layer inserted between two AlN barrier layers on mobility and carrier concentrations was analyzed and the dominant scattering mechanisms in the low and high temperature regimes were determined. It was found that Hall mobility increases as the thickness of GaN increases until 5 nm at a low temperature where interface roughness scattering is observed as one of the dominant scattering mechanisms. When GaN thicknesses exceed 5 nm, Hall mobility tends to decrease again due to the population of the second channel in which the interface becomes worse compared to the other one. From these analyses, 5 nm GaN layer thicknesses were found to be the optimum thicknesses required for high electron mobility. (C) 2013 Published by Elsevier B.V.
Open Access
Enhanced efficiency of solution-processed small-molecule solar cells upon incorporation of gold nanospheres and nanorods into organic layers
(Royal Society of Chemistry, 2014) Xu, X.; Kyaw, A. K. K.; Peng, B.; Du, Q.; Hong, L.; Demir, Hilmi Volkan; Wong, T. K. S.; Xiong, Q.; Sun, X. W.
The significantly enhanced performance upon incorporation of Au nanoparticles in solution-processed small-molecule solar cells is demonstrated. Simultaneously incorporating Au nanospheres into the hole transport layer and Au-silica nanorods into the active layer results in superior broadband absorption improvement in the device with a power conversion efficiency of 8.72% with 31% enhancement.
Open Access
A hole accelerator for InGaN/GaN light-emitting diodes
(AIP Publishing, 2014) Zhang, Z. H.; Liu, W.; Tan, S. T.; Ji, Y.; Wang, L.; Zhu, B.; Zhang, Y.; Lu, S.; Zhang, X.; Hasanov, N.; Sun, X. W.; Demir, Hilmi Volkan
The quantum efficiency of InGaN/GaN light-emitting diodes (LEDs) has been significantly limited by the insufficient hole injection, and this is caused by the inefficient p-type doping and the low hole mobility. The low hole mobility makes the holes less energetic, which hinders the hole injection into the multiple quantum wells (MQWs) especially when a p-type AlGaN electron blocking layer (EBL) is adopted. In this work, we report a hole accelerator to accelerate the holes so that the holes can obtain adequate kinetic energy, travel across the p-type EBL, and then enter the MQWs more efficiently and smoothly. In addition to the numerical study, the effectiveness of the hole accelerator is experimentally shown through achieving improved optical output power and reduced efficiency droop for the proposed InGaN/GaN LED. (C) 2014 AIP Publishing LLC.
Open Access
The influence of aviation disasters on engine manufacturers: An analysis of financial and reputational contagion risks
(Elsevier BV, 2021-03) Akyıldırım, E.; Corbet, S.; O'Connell, J. F.; Şensoy, Ahmet
One of the key sub-sectors in the aviation industry includes that of engine manufacturers, who have long led technological advancement and the battle to reduce airline carbon emissions. However, these same companies have been susceptible to a number of issues that have been central to international airlines due to higher costs and competition pressures. When an aviation disaster occurs, there is widespread allocation of blame and responsibility, which has left engine manufacturers exposed until the true cause is identified. This can generate many issues with regards to reputational damage and ability to generate finance. We set out to analyse such interactions over time and region. Our results indicate that engine manufacturers have had to contend with substantial income and financial leverage issues in the aftermath of a major aviation disaster, irrespective of whether they have been identified as a causation factor in the incident itself. Further, we clearly identify that there exists an average one day loss of 1.64% in the immediate aftermath of aviation incidents. Substantial corporate instability is found to persist without the company being in any way responsible for the incident. Shortly thereafter, contagion effects increase as speculation diminishes and more factual evidence arrives. The role of social media is examined as a potential contributory factor.
Open Access
Investigation of high frequency performance limit of graphene field effect transistors
(American Institute of Physics, 2010-10) Pince, Ercag; Kocabas, Coşkun
Extremely high field effect mobility together with the high surface coverage makes graphene a promising material for high frequency electronics application. We investigate the intrinsic high frequency performance limit of graphene field effect transistors limited by the charge impurity scattering. The output and transfer characteristics of graphene field effect transistors together with the high frequency performance are characterized as a function of impurity concentration and dielectric constant of the gate insulator. Our results reveal that graphene transistors could provide power gain at radio frequency band.
Open Access
Modelling and solving an m-location, n-courier, priority-based planning problem on a network
(Palgrave Macmillan, 2012) Erdoǧan, G.; Tansel, B.; Akgün, I.
In this paper, we study an m-location, n-courier, priority-based planning problem on a network, which we refer to as the Courier Planning Problem (CPP). The CPP arises on a daily basis in the context of planning the transportation of materials and personnel in peacetime for the Turkish Armed Forces. The main issue addressed in CPP is to transport as many of deliverables as possible from their origins to their destinations via a fleet of transportation assets (couriers) that operate at fixed routes and schedules. Priorities must be taken into account and constraints on the routes, operating schedules, and capacities of the transportation assets must be obeyed. Time windows may be specified for some or all transportation requests and must be satisfied. We study the CPP as well as its two extensions, and present integer programming formulations based on the multi-commodity flow structure. The formulations are tested on real world-based data and display satisfactory computational performance. Our main contributions are to develop an effective formulation scheme for a complicated large-scale real world problem and to demonstrate that such problems are solvable via commercial general purpose solvers through meticulous modelling.
Open Access
Mutational analysis of the major proline transporter (PrnB) of aspergillus nidulans
(Taylor & Francis, 2003) Tavoularis, S. N.; Tazebay, U. H.; Diallinas, G.; Sideridou, M.; Rosa, A.; Scazzocchio, C.; Sophianopoulou, V.
PrnB, the L-proline transporter of Aspergillus nidulans, belongs to the Amino acid Polyamine Organocation (APC) transporter family conserved in prokaryotes and eukaryotes. In silico analysis and limited biochemical evidence suggest that APC transporters comprise 12 transmembrane segments (TMS) connected with relatively short hydrophilic loops (L). However, very little is known on the structure-function relationships in APC transporters. This work makes use of the A. nidulans PrnB transporter to address structure-function relationships by selecting, constructing and analysing several prnB mutations. In the sample, most isolated missense mutations affecting PrnB function map in the borders of cytoplasmic loops with transmembrane domains. These are I119N and G120W in L2-TMS3, F278V in L6-TMS7, NRT378NRTNRT and PY382PYPY in L8-TMS9 and T456N in L10-TMS11. A single mutation (G403E) causing, however, a very weak phenotype, maps in the borders of an extracellular loop (L9-TMS10). An important role of helix TMS6 for proline binding and transport is supported by mutations K245L and, especially, F248L that clearly affect PrnB uptake kinetics. The critical role of these residues in proline binding and transport is further shown by constructing and analysing isogenic strains expressing selected prnB alleles fused to the gene encoding the Green Fluorescent Protein (GFP). It is shown that, while some prnB mutations affect proper translocation of PrnB in the membrane, at least two mutants, K245E and F248L, exhibit physiological cellular expression of PrnB and, thus, the corresponding mutations can be classified as mutations directly affecting proline binding and/or transport. Finally, comparison of these results with analogous studies strengthens conclusions concerning amino acid residues critical for function in APC transporters.
Open Access
On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes
(Optical Society of America, 2014) Zhang, Z. H.; W. L.; Tan, S. T.; Ju, Z.; Ji, Y.; Kyaw, Z.; Zhang, X.; Hasanov, N.; Zhu, B.; Lu, S.; Zhang, Y.; Sun, X. W.; Demir, Hilmi Volkan
Electron overflow limits the quantum efficiency of InGaN/GaN light-emitting diodes. InGaN electron cooler (EC) can be inserted before growing InGaN/GaN multiple quantum wells (MQWs) to reduce electron overflow. However, detailed mechanisms of how the InGaN EC contributes to the efficiency improvement have remained unclear so far. In this work, we theoretically propose and experimentally demonstrate an electron mean-free-path model, which reveals the InGaN EC reduces the electron mean free path in MQWs, increases the electron capture rate and also reduces the valence band barrier heights of the MQWs, in turn promoting the hole transport into MQWs. (C) 2014 Optical Society of America
Open Access
Phytoremediation of multiply metal-contaminated environments: synergistic and competitive effects between heavy metals during uptake and transport
(Nova Science Publishers, Inc., 2014) Üçüncü, E.; Özkan, A. D.; Ölmez, T. T.; Tunca E.
Phytoremediation is a promising alternative to conventional metal treatment methods; however, most phytoremediation studies separately consider the removal of each individual metal, which may not fully reflect the situation present in real world contamination sites. Metal-contaminated environments seldom contain a single species of metal, and are instead host to several types of toxic metals and other contaminants. Consequently, the synergistic and antagonistic effects displayed between essential and non-essential metals, as well as these between metallic and non-metallic contaminants, are an important factor in determining the bioremediative efficiencies of plant species. The present chapter outlines the uptake, transport and sequestration mechanisms relevant to heavy metal accumulation, considers the potential competitive and cooperative interactions that occur between metals during these processes, details the current literature regarding bioremediation in multiply metal-contaminated environments and offers insights into the biochemical interactions underlying the trends observed for the beneficial and detrimental effects displayed between the accumulations of certain metals. We also illustrate the potential of metal remediation by aquatic macrophytes, a group known for the effective remediation of multiple metals, which possess life histories that render them particularly conductive to studies investigating the impact of multiple metals on metal uptake. © 2014 by Nova Science Publishers, Inc. All rights reserved.
Open Access
Regional transport infrastructure and trade flows in the EU
(2021-08) Özcan, Berrin
How does regional transport infrastructure affect bilateral trade flows? An extensive literature on infrastructure and trade flows has attempted to answer this question by using country level or regional data. This current thesis focuses on the European Union (EU) and investigates the effect of transport infrastructure on international and intranational trade flows using NUTS 2 (Nomenclature of Territorial Units for Statistics) level data from 200 EU regions between the years 2000-2010. It is the first study to focus on flows and infrastructure at regional level in a multi-country setting. As in the previous studies in the infrastructure literature, the gravity equation is used to explain the relationship between the regional transport infrastructure and trade in the EU. Various alternative estimation methods such as Fixed Effects, PPML, lagged variables, instrumental variables (IV) and Hausman-Taylor IV method are used in order to overcome the issues related to heteroskedasticity, reverse causality and biased estimates that are frequently encountered with gravity equation. In the presence of bilateral and time fixed effects, the results suggest an increase of 0.05 to 0.13 per cent bilateral trade as infrastructure measures increase by 1 per cent. The robustness check follows that the estimates are not sensitive to the choice of unit of measure for the infrastructure variables.
Open Access
Simulation of a Brownian particle in an optical trap
(American Association of Physics Teachers, 2013) Volpe, G.; Volpe, G.
Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations both in homogenous and complex environments. In particular, we show how active Brownian particles do not follow the Maxwell-Boltzmann distribution-a clear signature of their out-of-equilibrium nature- and how, unlike passive Brownian particles, microswimmers can be funneled, trapped, and sorted. (C) 2014 American Association of Physics Teachers.
Open Access
Speckle optical tweezers: micromanipulation with random light fields
(Optical Society of America, 2014-07-18) Volpe, G.; Kurtz, L.; Callegari, A.; Volpe, G.; Gigan, S.
Current optical manipulation techniques rely on carefully engineered setups and samples. Although similar conditions are routinely met in research laboratories, it is still a challenge to manipulate microparticles when the environment is not well controlled and known a priori, since optical imperfections and scattering limit the applicability of this technique to real-life situations, such as in biomedical or microfluidic applications. Nonetheless, scattering of coherent light by disordered structures gives rise to speckles, random diffraction patterns with well-defined statistical properties. Here, we experimentally demonstrate how speckle fields can become a versatile tool to efficiently perform fundamental optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these "speckle optical tweezers" will greatly broaden the perspectives of optical manipulation for real-life applications. (C) 2014 Optical Society of America
Open Access
Transport properties of epitaxial graphene grown on SiC substrate
(National Institute of Optoelectronics, 2017) Ağızaçmak, S.; Tülek, R.; Gökden, S.; Teke, A.; Arslan, E.; Aygar, A. M.; Özbay, Ekmel
In this study, the Hall effect measurement of graphene on SiC substrate was carried out as a function of temperature (12-300 K). Hall data were first analyzed to extract the temperature dependent mobilities and carrier densities of the bulk (3D) and two dimensional (2D) channels using a Simple Parallel Conduction Extraction Method (SPCEM) successfully. High carrier mobility 2.296 cm2/V.s from the graphene layer and low carrier mobility 813 cm2/V.s from the SiC were obtained at room temperature. By using SPCEM extracted data, 3D and 2D scattering mechanisms were analyzed and the dominant scattering mechanisms in low and high temperature regimes were determined. It was found that the transport was mainly determined by scattering processes in 2D graphene.