40 research outputs found
Phonons in highly crystalline mesoporous silicon The absence of phonon softening upon structuring silicon on sub 10 nanometer length scales
No full textThis article presents inelastic thermal neutron scattering experiments probing the phonon dispersion in mesoporous silicon with pores 8 nm across. Scattering studies reveal the energy momentum relation for transverse and longitudinal phonons along the high symmetry directions amp; 120548; amp; 119871;, amp; 120548; amp; 119870; and amp; 120548; amp; 119883; in the Brillouin zone. The dispersion up to phonon energies of 35 meV unambiguously proves that the phonon group velocities in highly crystalline silicon are not modified by nanostructuring down to sub 10 nanometer length scales. On these length scales, there is apparently no effect of structuring on the elastic moduli of mesoporous silicon. No evidence can be found for phonon softening in topologically complex, geometrically disordered mesoporous silicon putting it in contrast to silicon nanotubes and nanoribbon
Einfluss von Oberflächeneigenschaften auf die thermoelektrischen Transporteigenschaften einzelner einkristalliner Nanodrähte
No full textDiese Arbeit demonstriert die vollständige thermoelektrische Charakterisierung einzelner einkristalliner Bismuttellurid- und Silbernanodrähte und deren anschließende lokale strukturelle und chemische Charakterisierung mittels analytischer Transmissionselektronenmikroskopie. Die lokale strukturelle, chemische und morphologische Charakterisierung entlang der Nanodrähte trägt essentiell zum Verständnis des thermoelektrischen Transportes bei und bestätigt die Korrelation zwischen Oberflächen- und den thermoelektrischen Eigenschaften. Für durchmesservariierte Bismuttelluridnanodrähte wird der Einfluss der Morphologie auf die Wärmeleitfähigkeit bei Raumtemperatur quantifiziert. Im Vergleich zu einem glatten Referenznanodraht zeigt der durchmesservariierte Nanodraht gleicher Zusammensetzung und Kristallorientierung eine Reduktion der Wärmeleitfähigkeit um 55 %. Diese Reduktion kann durch Phononenrückstreuung an der eingekerbten Oberfläche erklärt werden. Die elektrische Leitfähigkeit und der Seebeckkoeffizient der Bismuttelluridnanodrähte deuten auf einen topologischen Oberflächenzustand hin. Für Silbernanodrähte werden die elektrische Leitfähigkeit und die Wärmeleitfähigkeit im Temperaturbereich von 1,4 K bis 300 K gemessen. Mit fallender Temperatur steigt die relative Reduktion der Wärmeleitfähigkeit im Verhältnis zur elektrischen Leitfähigkeit stärker, sodass die Lorenzzahl die klassische Wiedemann-Franz-Relation nicht erfüllt und eine Funktion der Temperatur darstellt. Der Temperaturverlauf der Lorenzzahl der Silbernanodrähte entspricht der 1938 von Makinson aufgestellten Theorie für hochreine Metalle und ist im Tieftemperaturbereich um bis zu zwei Größenordnungen zum Sommerfeldwert reduziert.This work demonstrates the full thermoelectric characterisation of individual single crystalline bismuth telluride and silver nanowires and their subsequent local structural and chemical characterisation via analytical transmission electron microscopy along the whole nanowires. Therefore, the correlation between the structure, in particular the surface morphology, and the thermoelectric transport properties is unambiguously shown. For diameter varied bismuth telluride nanowires the influence of the morphology on the thermal conductivity is quantified at room temperature. The diameter varied nanowire shows a reduction of 55 % with respect to the smooth nanowire of the same chemical composition and structural orientation. This reduction can be explained by phonon backscattering at the indents. The electrical conductivity and the Seebeck coefficient indicate the presence of a topological surface state. For silver nanowires the electrical and thermal conductivity are determined in the temperature range between 1.4 K and 300 K. With decreasing temperature the relative reduction of the thermal conductivity is higher than the reduction of the electrical conductivity resulting in a temperature-dependent Lorenz number, so that the classical Wiedemann-Franz relation is not fulfilled. The temperature characteristic of the silver nanowires'' Lorenz number is in agreement with the theory Makinson established for highly pure metals in 1938 and is reduced by two orders of magnitude with respect to the Sommerfeld value in the low temperature regime
Synthesis of organic inorganic hybrids based on the conjugated polymer P3HT and mesoporous silicon
No full textOrganic inorganic hybrids are a class of functional materials that combine favorable properties of their constituents to achieve an overall improved performance for a wide range of applications. This article presents the synthesis route for P3HT porous silicon hybrids for thermoelectric applications. The conjugated polymer P3HT is incorporated into the porous silicon matrix by means of melt infiltration. Gravimetry, sorption isotherms and energy dispersive X ray spectroscopy EDX mapping indicate that the organic molecules occupy more than 50 of the void space in the inorganic host. We demonstrate that subsequent diffusion based doping of the confined polymer in a FeCl3 solution increases the electrical conductivity of the hybrid by five orders of magnitude compared to the empty porous silicon hos
Solid phases of spatially nanoconfined oxygen A neutron scattering study
No full textWe present a comprehensive neutron scattering study on solid oxygen spatially con amp; 64257;ned in 12 nm wide alumina nanochannels. Elastic scattering experiments reveal a structural phase sequence known from bulk oxygen. With decreasing temperature cubic amp; 947; , orthorhombic amp; 946; and monoclinic amp; 945; phases are unambiguously identi amp; 64257;ed in con amp; 64257;nement. Weak antiferromagnetic ordering is observed in the con amp; 64257;ned monoclinic amp; 945; phase. Rocking scans reveal that oxygen nanocrystals inside the tubular channels do not form an isotropic powder. Rather, they exhibit preferred orientations depending on thermal history and the very mechanisms, which guide the structural transition
Crystallize it before it diffuses Thin film growth of the phosphorus rich semiconductor CuP2
No full textNumerous phosphorus rich metal phosphides containing both P amp; 8722;P bonds and metal amp; 8722;P bonds are known from the solid state chemistry literature. A method to grow these materials in thin film form would be desirable, as thin films are required in many applications and they are an ideal platform for high throughput studies. In addition, the high density and smooth surfaces achievable in thin films are a significant advantage for characterization of transport and optical properties. Despite these benefits, there is hardly any published work on even the simplest binary phosphorus rich phosphide films. Here, we demonstrate growth of single phase CuP2 films by a two step process involving reactive sputtering of amorphous CuP2 x and rapid annealing in an inert atmosphere. At the crystallization temperature, CuP2 is thermodynamically unstable with respect to Cu3P and P4. However, CuP2 can be stabilized if the amorphous precursors are mixed on the atomic scale and are sufficiently close to the desired composition neither too P poor nor too P rich . Fast formation of polycrystalline CuP2, combined with a short annealing time, makes it possible to bypass the diffusion processes responsible for decomposition. We find that thin film CuP2 is a 1.5 eV band gap semiconductor with interesting properties, such as a high optical absorption coefficient above 105 cm amp; 8722;1 , low thermal conductivity 1.1 W K m , and composition insensitive electrical conductivity around 1 S cm . We anticipate that our processing route can be extended to other phosphorus rich phosphides that are still awaiting thin film synthesis and will lead to a more complete understanding of these materials and of their potential application
Characterization and modeling of the temperature-dependent thermal conductivity in sintered porous silicon-aluminum nanomaterials
No full textNanostructured silicon and silicon-aluminum compounds are synthesized by a novel synthesis strategy based on spark plasma sintering (SPS) of silicon nanopowder, mesoporous silicon (pSi), and aluminum nanopowder. The interplay of metal-assisted crystallization and inherent porosity is exploited to largely suppress thermal conductivity. Morphology and temperature-dependent thermal conductivity studies allow us to elucidate the impact of porosity and nanostructure on the macroscopic heat transport. Analytic electron microscopy along with quantitative image analysis is applied to characterize the sample morphology in terms of domain size and interpore distance distributions. We demonstrate that nanostructured domains and high porosity can be maintained in densified mesoporous silicon samples. In contrast, strong grain growth is observed for sintered nanopowders under similar sintering conditions. We observe that aluminum agglomerations induce local grain growth, while aluminum diffusion is observed in porous silicon and dispersed nanoparticles. A detailed analysis of the measured thermal conductivity between 300 and 773 K allows us to distinguish the effect of reduced thermal conductivity caused by porosity from the reduction induced by phonon scattering at nanosized domains. With a modified Landauer/Lundstrom approach the relative thermal conductivity and the scattering length are extracted. The relative thermal conductivity confirms the applicability of Kirkpatrick's effective medium theory. The extracted scattering lengths are in excellent agreement with the harmonic mean of log-normal distributed domain sizes and the interpore distances combined by Matthiessen's rule
Water Adsorption Enhances Electrical Conductivity in Transparent P Type CuI
No full textCuI has been recently rediscovered as a p type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a 2 fold increase in the conductivity of CuI after exposure to ambient humidity for 5 h, followed by slight long term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. On the basis of the large intragrain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobilit
Perfect quintuple layer Bi2Te<sub>3</sub> nanowires: Growth and thermoelectric properties
No full textBi2Te3 nanowires are promising candidates for thermoelectric applications. Vapor-liquid-solid growth of these nanowires is straightforward, but the traditional Au-catalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications. (C) 2017 Author(s)
The Effect of the MEMS Measurement Platform Design on the Seebeck Coefficient Measurement of a Single Nanowire
No full textIn order to study the thermoelectric properties of individual nanowires, a thermoelectric nanowire characterization platform (TNCP) has been previously developed and used in our chair. Here, we report on a redesigned platform aiming to optimize performance, mechanical stability and usability. We compare both platforms for electrical conductivity and the Seebeck coefficient for an individual Ag nanowire of the previously-used batch and for comparable measurement conditions. By this, the measurement performance of both designs can be investigated. As a result, whereas the electrical conductivity is comparable, the Seebeck coefficient shows a 50% deviation with respect to the previous studies. We discuss the possible effects of the platform design on the thermoelectric measurements. One reason for the deviation of the Seebeck coefficient is the design of the platform leading to temperature gradients along the bond pads. We further analyze the effect of bonding materials Au and Pt, as well as the effect of temperature distributions along the bond pads used for the thermovoltage acquisition. Another major reason for the variation of the measurement results is the non-homogeneous temperature distribution along the thermometer. We conclude that for the measurement of small Seebeck coefficients, an isothermal positioning of voltage-probing bond pads, as well as a constant temperature profile at the measurement zone are essential
