27 research outputs found

    Improved recursive Green's function formalism for quasi one-dimensional systems with realistic defects

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    We derive an improved version of the recursive Green's function formalism (RGF), which is a standard tool in the quantum transport theory. We consider the case of disordered quasi one-dimensional materials where the disorder is applied in form of randomly distributed realistic defects, leading to partly periodic Hamiltonian matrices. The algorithm accelerates the common RGF in the recursive decimation scheme, using the iteration steps of the renormalization decimation algorithm. This leads to a smaller effective system, which is treated using the common forward iteration scheme. The computational complexity scales linearly with the number of defects, instead of linearly with the total system length for the conventional approach. We show that the scaling of the calculation time of the Green's function depends on the defect density of a random test system. Furthermore, we discuss the calculation time and the memory requirement of the whole transport formalism applied to defective carbon nanotubes

    Electronic transport in metallic carbon nanotubes with mixed defects within the strong localization regime

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    We study the electron transport in metallic carbon nanotubes (CNTs) with realistic defects of different types. We focus on large CNTs with many defects in the mesoscopic range. In a recent paper we demonstrated that the electronic transport in those defective CNTs is in the regime of strong localization. We verify by quantum transport simulations that the localization length of CNTs with defects of mixed types can be related to the localization lengths of CNTs with identical defects by taking the weighted harmonic average. Secondly, we show how to use this result to estimate the conductance of arbitrary defective CNTs, avoiding time consuming transport calculations

    Influence of defect-induced deformations on electron transport in carbon nanotubes

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    We theoretically investigate the influence of defect-induced long-range deformations in carbon nanotubes on their electronic transport properties. To this end we perform numerical ab-initio calculations using a density-functional-based tight-binding (DFTB) model for various tubes with vacancies. The geometry optimization leads to a change of the atomic positions. There is a strong reconstruction of the atoms near the defect (called "distortion") and there is an additional long-range deformation. The impact of both structural features on the conductance is systematically investigated. We compare short and long CNTs of different kinds with and without long-range deformation. We find for the very thin (9,0)-CNT that the long-range deformation additionally affects the transmission spectrum and the conductance compared to the short-range lattice distortion. The conductance of the larger (11,0)- or the (14,0)-CNT is overall less affected implying that the influence of the long-range deformation decreases with increasing tube diameter. Furthermore, the effect can be either positive or negative depending on the CNT type and the defect type. Our results indicate that the long-range deformation must be included in order to reliably describe the electronic structure of defective, small-diameter zigzag tubes.Comment: Materials for Advanced Metallization 201

    An improved Green's function algorithm applied to quantum transport in carbon nanotubes

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    The renormalization-decimation algorithm (RDA) of L\'opez Sancho et al. is used in quantum transport theory to calculate bulk and surface Green's functions. We derive an improved version of the RDA for the case of very long quasi one-dimensional unit cells (in transport direction). This covers not only long unit cells but also supercell-like calculations for structures with disorder or defects. In such large systems, short-range interactions lead to sparse real-space Hamiltonian matrices. We show how this and a corresponding subdivision of the unit cell in combination with the decimation technique can be used to reduce the calculation time. Within the resulting algorithm, separate RDA calculations of much smaller effective Hamiltonian matrices must be done for each Green's function, which enables the treatment of systems too large for the common RDA. Finally, we discuss the performance properties of our improved algorithm as well as some exemplary results for chiral carbon nanotubes

    Synthesis and Assembly of Zinc Oxide Microcrystals by a Low‐Temperature Dissolution–Reprecipitation Process: Lessons Learned About Twin Formation in Heterogeneous Reactions

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    Cobalt-doped zinc oxide single crystals with the shape of hexagonal platelets were synthesized by thermohydrolysis of zinc acetate, cobalt acetate, and hexamethylenetetramine (HMTA) in mixtures of ethanol and water. The mineralization proceeds by a low-temperature dissolution–reprecipitation process from the liquid phase by the formation of basic cobalt zinc salts as intermediates. The crystal shape as well as twin formation of the resulting oxide phase can be influenced by careful choice of the solvent mixture and the amount of doping. An understanding of the course of the reaction was achieved by comprehensive employment of analytical techniques (i.e., SEM, XRD, IR) including an in-depth HRTEM study of precipitates from various reaction stages. In addition, EPR as well as UV/Vis spectroscopic measurements provide information about the insertion of the cobalt dopant into the zincite lattice. The Langmuir–Blodgett (LB) technique is shown to be suitable for depositing coatings of the platelets on glass substrates functionalized with polyelectrolyte multilayers and hence is applied for the formation of monolayers containing domains with ordered tessellation. No major differences are found between deposits on substrates with anionic or cationic surface modification. The adherence to the substrates is sufficient to determine the absolute orientation of the deposited polar single crystals by piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) studies

    Electronic transport through defective semiconducting carbon nanotubes

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    We investigate the electronic transport properties of semiconducting (m, n) carbon nanotubes (CNTs) on the mesoscopic length scale with arbitrarily distributed realistic defects. The study is done by performing quantum transport calculations based on recursive Green's function techniques and an underlying density-functional-based tight-binding model for the description of the electronic structure. Zigzag CNTs as well as chiral CNTs of different diameter are considered. Different defects are exemplarily represented by monovacancies and divacancies. We show the energy-dependent transmission and the temperature-dependent conductance as a function of the number of defects. In the limit of many defetcs, the transport is described by strong localization. Corresponding localization lengths are calculated (energy dependent and temperature dependent) and systematically compared for a large number of CNTs. It is shown, that a distinction by (m − n)mod 3 has to be drawn in order to classify CNTs with different bandgaps. Besides this, the localization length for a given defect probability per unit cell depends linearly on the CNT diameter, but not on the CNT chirality. Finally, elastic mean free paths in the diffusive regime are computed for the limit of few defects, yielding qualitatively same statements

    Quantum transport in graphene nanoribbon networks: complexity reduction by a network decimation algorithm

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    We study electronic quantum transport (QT) in graphene nanoribbon (GNR) networks on mesoscopic length scales. We focus on zigzag GNRs and investigate the conductance properties of statistical networks. To this end we use a density-functional-based tight-binding model to determine the electronic structure and QT theory to calculate electronic transport properties. We then introduce a new efficient network decimation algorithm that reduces the complexity in generic three-dimensional GNR networks. We compare our results to semi-classical calculations based on the nodal analysis (NA) approach and discuss the dependence of the conductance on network density and network size. We show that a NA model cannot reproduce the QT results nor their dependence on model parameters well. Thus, solving the quantum network by our efficient approach is mandatory for accurate modelling the electron transport through GNR networks

    Fabrication of a transversal multilayer thermoelectric generator with substituted calcium manganite

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    The sintering behavior and thermoelectric performance of Ca0.99Gd0.01Mn0.99W0.01O3 was studied, and a multilayer thermoelectric generator was fabricated. The addition of CuO as sintering additive was found to be effective for the reduction in the sintering temperature from 1300°C to about 1000°C-1050°C. Dense samples were obtained after firing at 1050°C, whereas some porosity remained after firing at 1000°C. Samples sintered at reduced temperature exhibit lower electrical conductivity, whereas the Seebeck coefficient S = −150 μV/K at 100°C is not affected by lowering the sintering temperature. The figure of merit is ZT = 0.12 at 700°C for samples sintered at 1300°C; ZT = 0.08 and 0.03 were obtained for multilayer laminates sintered at 1050°C and 1000°C, respectively. A transversal multilayer thermoelectric generator (TMLTEG) was built by stacking layers of substituted CaMnO3 green tapes, and printing AgPd conductor stripes onto the thermoelectric layers at an angle of 30° relative to the direction of the heat flow. The multilayer stack was co-fired at 1000°C. The TMLTEG has a power output of 2.5 mW at ∆T= 200 K in the temperature interval of 25°C-300°C. A meander-like generator with larger power output comprising six TMTEGs is also presented

    Effect of Carbon Nanotubes on Thermoelectric Properties in Zn0.98_{0.98}Al0.02_{0.02}O

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    Thermoelectric oxides can provide the advantage of high-temperature stability in oxygen-containing atmospheres. It is known that the incorporation of multiwalled carbon nanotubes (mw-CNT) can change the thermoelectric as well as the structural properties of oxides. Here, we report the influence of mw-CNT on the thermoelectric properties of Al-doped ZnO (AZO). The preparation of the mw-CNT-added AZO was done using an ultrasonic mixing of the starting materials followed by a spark plasma sintering process under vacuum. The Seebeck coefficient S, thermal conductivity λ and electrical conductivity σ were determined in the temperature range between 300 K and 900 K. It was observed that the thermal conductivity is significantly reduced by the incorporation of the mw-CNT. At the same time, the electrical conductivity is increased by a factor of 21 from 8700 S/m to 190,000 S/m. The Power factor PF=S2σ indicates that the addition of mw-CNT improves the thermoelectric properties of Al doped ZnO in comparison to the reference sample prepared with same process but without mw-CN
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