Danny Kojda

Temperature-Dependent Thermoelectric Properties of Individual Silver Nanowires

The thermoelectric properties of the Ag NWs are discussed in comparison to the bulk: SAg;Pt(T) was measured with respect to platinum and is in agreement with the bulk, (T) and (T) showed reduced values with respect to the bulk. The latter are both notably dominated by surface scattering caused by an increased surface-to-volume ratio. By lowering T the electron mean free path strongly exceeds the NW’s diameter of 150 nm so that the transition from diusive transport to quasi ballistic one dimensional transport is observed. An important result of this work is that the Lorenz numberL(T) turns out to be independent of surface scattering. Instead the characteristic ofL(T) is determined by the material’s purity. Moreover, (T) and L(T) can be described by the bulk Debye temperature of silver. A detailed discussion of the temperature dependence of L(T) and the scattering mechanisms is given.

The effect of a distinct diameter variation on the thermoelectric properties of individual Bi0.39Te0.61 nanowires

The reduction of the thermal conductivity induced by nano-patterning is one of the major approaches for tailoring thermoelectric material properties. In particular, the role of surface roughness and morphology is under debate. Here, we choose two individual bismuth telluride nanowires (NWs), one with a strong diameter variation between 190 nm and 320 nm (NW1) and the other of 187 nm diameter with smooth sidewalls (NW2). Both serve as model systems for which bulk properties are expected if surface properties do not contribute. We investigate the role of the diameter variation by means of a combined full-thermoelectrical, structural and chemical characterization. By transmission electron microscopy the structure, chemical composition and morphology were determined after the thermoelectrical investigation. The NWs showed an oriented growth along the direction and the same composition. The Seebeck coefficients of both NWs are comparable to each other. The electrical conductivity of both NWs exceeds the bulk value indicating the presence of a topological surface state. Whereas the thermal conductivity of NW2 compares to the bulk, the thermal conductivity of NW1 is about half of NW2 which is discussed with respect to its distinct diameter variation.

Dielectrophoretic investigation of Bi2Te3 nanowires-a microfabricated thermoelectric characterization platform for measuring the thermoelectric and structural properties of single nanowires

In this article a microfabricated thermoelectric nanowire characterization platform to investigate the thermoelectric and structural properties of single nanowires is presented. By means of dielectrophoresis (DEP), a method to manipulate and orient nanowires in a controlled way to assemble them onto our measurement platform is introduced. The thermoelectric platform fabricated with optimally designed DEP electrodes results in a yield of nanowire assembly of approximately 90% under an applied peak-to-peak ac signal Vpp = 10 V and frequency f = 20 MHz within a series of 200 experiments. Ohmic contacts between the aligned single nanowire and the electrodes on the platform are established by electron beam-induced deposition. The Seebeck coefficient and electrical conductivity of electrochemically synthesized Bi2Te3 nanowires are measured to be -51 μV K(-1) and (943 ± 160)/(Ω(-1) cm(-1)), respectively. Chemical composition and crystallographic structure are obtained using transmission electron microscopy. The selected nanowire is observed to be single crystalline over its entire length and no grain boundaries are detected. At the surface of the nanowire, 66.1 ± 1.1 at.% Te and 34.9 ± 1.1 at.% Bi are observed. In contrast, chemical composition of 64.2 at.% Te and 35.8 at.% Bi is detected in the thick center of the nanowire.

Free-standing millimetre-long Bi2Te3 sub-micron belts catalyzed by TiO2 nanoparticles.

Physical vapour deposition (PVD) is used to grow millimetre-long Bi2Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20 × 105 S m −1 indicates an excellent sample quality.

Electrical conductivity and Seebeck coefficient measurements of single nanowires by utilizing a microfabricated thermoelectric nanowire characterization platform

We demonstrate the fabrication and improvements of our next generation Thermoelectric Nanowire Characterization Platform (TNCP) that is utilized to investigate the thermoelectric properties of individual nanowires to obtain the Seebeck coefficient S, electrical conductivity σ and thermal conductivity κ from the same test specimen. Only from these data, the so-called figure of merit ZT can be obtained for a single nanowire. In order to analyze the structural composition of single nanowires the TNCP has also to fulfill the purpose of a sample holder used in Transmission Electron Microscopy. Our second generation of TNCPs has been designed for these purposes. As before, individual nanowires are assembled on the TNCP by means of dielectrophoresis. After this assembly the nanowire is merely physically contacted to the electrodes on the TNCP. Contact generation is in first place done by an electron beam-induced deposition (EBID) process of Pt and measurements of S and σ are carried out on individual nanowires and presented here. As the EBID process is very complex and difficult to handle we have developed a novel method using a shadow mask process for the local evaporation of platinum to generate ohmic contact between the nanowire and the surrounding electrodes.

The Effect of the MEMS Measurement Platform Design on the Seebeck Coefficient Measurement of a Single Nanowire

In 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.

Perfect quintuple layer Bi2Te3 nanowires: Growth and thermoelectric properties

Bi2Te3 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.Bi2Te3 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.

Thermoelectric Properties Investigation of Single nanowires by utilizing a Thermoelectric Nanowire Characterization Platform

We demonstrate the design and fabrication of a novel micromachined Thermoelectric Nanowire Characterization Platform (TNCP) which is utilized to characterize the thermoelectric properties of various nanowires. Single nanowire is assembled onto the pre-fabricated TNCP by means of dielectrophoresis (DEP) in combination with a water droplet evaporation scheme. After assembly, a reliable ohmic contact is generated between the bismuth telluride (Bi2Te3) nanowire and the underlying electrodes by means of scanning electron microscope (SEM) focused electron beam-induced deposition (EBID). Finally, the electrical conductivity and Seebeck coefficient of Silver (Ag) and Bi2Te3 nanowires are investigated and presented in this paper.