Browsing by Author "Juntunen, Taneli"

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  • Anderson Localization Quenches Thermal Transport in Aperiodic Superlattices
    (2019-03-12) Juntunen, Taneli; Vänskä, Osmo; Tittonen, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We show that aperiodic superlattices exhibit intriguing interplay between phononic coherent wave interference effects and incoherent transport. In particular, broadband Anderson localization results in a drastic thermal conductivity reduction of 98% at room temperature, providing an ultralow value of 1.3 W m-1 K-1, and further yields an anomalously large thermal anisotropy ratio of ∼102 in aperiodic Si/Ge superlattices. A maximum in the thermal conductivity emerges as an unambiguous consequence of phonon Anderson localization at a system length scale bridging the extended and localized transport regimes. The frequency-resolved picture, combined with our lattice dynamical description of Anderson localization, elucidates the rich transport characteristics in these systems and the potential of correlated disorder for sub- to few-THz phononic engineering of heat transport in thermoelectric applications.
  • Cul p-type thin films for highly transparent thermoelectric p-n modules
    (2018-05-02) Morais Faustino, Bruno Miguel; Gomes, Diogo; Faria, Jaime; Juntunen, Taneli; Gaspar, Guilherme; Bianchi, Catarina; Almeida, Antonio; Marques, Ana; Tittonen, Ilkka; Ferreira, Isabel
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Developments in thermoelectric (TE) transparent p-type materials are scarce and do not follow the trend of the corresponding n-type materials - a limitation of the current transparent thermoelectric devices. P-type thermoelectric thin films of Cul have been developed by three different methods in order to maximise optical transparency (>70% in the visible range), electrical (sigma = 1.1 x 10(4) Sm-1) and thermoelectric properties (ZT= 0.22 at 300 K). These have been applied in the first planar fully transparent p-n type TE modules where gallium-doped zinc oxide (GZO) thin films were used as the n-type element and indium thin oxide (ITO) thin films as electrodes. A thorough study of power output in single elements and p-n modules electrically connected in series and thermally connected in parallel is inclosed. This configuration allows for a whole range of highly transparent thermoelectric applications.
  • Dedoping of Carbon Nanotube Networks Containing Metallic Clusters and Chloride
    (2024-10-31) Conley, Kevin; Mikladal, Bjorn; Gadgil, Bhushan; Etula, Jarkko; Juntunen, Taneli; Varjos, Ilkka; Karttunen, Antti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Doping carbon nanotube films improves their electrical conductivity and chemical stability. One conventional doping method is the in situ reduction of AuCl3 and yet its beneficial properties on the electrical conductivity lessen once the films are thermally annealed. Here, density functional theory and semiclassical Boltzmann transport theory calculations show a deterioration of the electrical conductivity of a doped network of (8,0) carbon nanotubes as the Aun clusters (n ≤ 10) become larger or the Cl impurity concentration decreases. Between the smallest and largest cluster sizes, the conductivity along the nanotubes and across the junction dropped by 47 and 90% in metallic networks. The declines in semiconducting networks were 50 and 13%, respectively. The maximum electrical conductivity across the junction was for networks containing Au5 clusters (193 times larger than the pristine network). Additionally, the electrical transport was reduced when Cl impurities were removed. Networks with excess Cl impurities had 9.6 and 36.7 times larger electrical conductivity along the nanotubes and across the junction than networks without impurities. Controlling the cluster size and Cl content would mitigate the dedoping of carbon nanotube films after thermal annealing.
  • Enhanced Thermoelectric Transport and Stability in Atomic Layer Deposited HfO2/ZnO and TiO2/ZnO Sandwiched Multilayer Thin Films
    (2020-10-28) Clairvaux Felizco, Jenichi; Juntunen, Taneli; Uenuma, Mutsunori; Etula, Jarkko; Tossi, Camilla; Ishikawa, Yasuaki; Tittonen, Ilkka; Uraoka, Yukiharu
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Herein, enhancements in thermoelectric (TE) performance, both the power factor (PF) and thermal stability, are exhibited by sandwiching HfO2 and TiO2 layers onto atomic layer deposited-ZnO thin films. High-temperature TE measurements from 300 to 450 K revealed an almost two-fold improvement in electrical conductivity for TiO2/ZnO (TZO) samples, primarily owing to an increase in carrier concentration by Ti doping. On the other hand, HfO2/ZnO (HZO) achieved the highest PF values owing to maintaining Seebeck coefficients comparable to pure ZnO. HZO also exhibited excellent stability after multiple thermal cycles, which has not been previously observed for pure or doped ZnO thin films. Such improvement in both TE properties and thermal stability of HZO can be attributed to a shift in crystalline orientation from the a axis to c axis, as well as the high bond dissociation energy of Hf-O, stabilizing the ZnO structure. These unique properties exhibited by HZO and TZO thin films synthesized by atomic layer deposition pave the way for next-generation transparent TE devices.
  • Fononin aaltoluonteen hyödyntäminen lämmönhallinnassa
    (2019-12-15) Peltola, Jemina
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Inkjet Printed Large-Area Flexible Few-Layer Graphene Thermoelectrics
    (2018-05-30) Juntunen, Taneli; Jussila, Henri; Ruoho, Mikko; Liu, Shouhu; Hu, Guohua; Albrow-Owen, Tom; Ng, Leonard W.T.; Howe, Richard C.T.; Hasan, Tawfique; Sun, Zhipei; Tittonen, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Graphene-based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene-based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet-printed large-area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so-called phonon-glass electron-crystal character (i.e., electrical transport behavior akin to that of few-layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all-graphene films show a room-temperature thermoelectric power factor of 18.7 µW m−1 K−2, representing over a threefold improvement to previous solution-processed all-graphene structures. The demonstration of inkjet-printed thermoelectric devices underscores the potential for future flexible, scalable, and low-cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.
  • Johdinratkaisut taipuisaan ja läpinäkyvään lämpösähköiseen kosketuspaneeliin
    (2016-12-20) Koskinen, Tomi
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Large-area implementation and critical evaluation of the material and fabrication aspects of a thin-film thermoelectric generator based on aluminum-doped zinc oxide
    (2020-03) Tappura, Kirsi; Juntunen, Taneli; Jaakkola, Kaarle; Ruoho, Mikko; Tittonen, Ilkka; Ritasalo, Riina; Pudas, Marko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A large-area thermoelectric generator (TEG) utilizing a folded thin-film concept is implemented and the performance evaluated for near room temperature applications having modest temperature gradients (<50 K). The TEGs with the area of ∼0.33 m2 are shown capable of powering a wireless sensor node of multiple sensors suitable e.g. for monitoring environmental variables in buildings. The TEGs are based on a transparent, non-toxic and abundant thermoelectric material, i.e. aluminium-doped zinc oxide (AZO), deposited on flexible substrates. After folding, both the electrical current and heat flux are in the plane of the thermoelectric thin-film. Heat leakage in the folded TEG is shown to be minimal (close to that of air), enabling sufficient temperature gradients without efficient heat sinks, contrary to the conventional TEGs having the thermal flux and electrical current perpendicular to the plane of the thermoelectric films. The long-term stability studies reveal that there are no significant changes in the electrical or thermoelectric properties of AZO over several months, while the contact resistance between AZO and silver ink is an issue exhibiting a continuous increase over time. The performance of the TEGs and technological implications in relation to a state-of-the-art thermoelectric material are further assessed via a computational study.
  • Large-area thermal distribution sensor based on multilayer graphene ink
    (2020-09-02) Koskinen, Tomi; Juntunen, Taneli; Tittonen, Ilkka
     Letter
    Emergent applications in wearable electronics require inexpensive sensors suited to scalable manufacturing. This work demonstrates a large-area thermal sensor based on distributed thermocouple architecture and ink-based multilayer graphene film. The proposed device combines the exceptional mechanical properties of multilayer graphene nanocomposite with the reliability and passive sensing performance enabled by thermoelectrics. The Seebeck coefficient of the spray-deposited films revealed an inverse thickness dependence with the largest value of 44.7 µV K−1 at 78 nm, which makes thinner films preferable for sensor applications. Device performance was demonstrated by touch sensing and thermal distribution mapping-based shape detection. Sensor output voltage in the latter application was on the order of 300 µV with a signal-to-noise ratio (SNR) of 35, thus enabling accurate detection of objects of different shapes and sizes. The results imply that films based on multilayer graphene ink are highly suitable to thermoelectric sensing applications, while the ink phase enables facile integration into existing fabrication processes.
  • Polymeeri-klorofylliohutkalvojen passivointi atomikerroskasvatuksen avulla
    (2012-12-07) Juntunen, Taneli
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Quantum transport and terahertz photoconductivity in a mesoscopic carbon nanotube film
    (2015-03-30) Juntunen, Taneli
     Sähkötekniikan korkeakoulu | Master's thesis
  • Thermal and thermoelectric transport in nanostructures for energy applications
    (2019) Juntunen, Taneli
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Issues concerning the control of thermal energy assume an ever increasing role in modern electronic technologies. In an effort to address this challenge, the rapid progress in nanotechnology has introduced new ways to control heat flow at microscopic length scales. Heat is additionally an ubiquitous energy resource and may be directly converted into clean and renewable electricity using thermoelectric materials. While the available thermoelectric conversion efficiency remains modest, nanostructures also provide means for improving the thermoelectric performance of bulk materials, concurrently promoting various desirable material attributes such as transparency and flexibility. This thesis elucidates nanoscale thermal phenomena concerning heat carrying phonons per se and as part of thermoelectric energy conversion. The main goals of the work are to explore the interplay of structural disorder and order for controlling nanoscale thermal transport in thin film multilayer and nanowire systems, and to generate new routes for fabricating economical and ecological large-area thermoelectric structures for sensing and energy harvesting applications. The thermal transport studies presented in this thesis show that thermal conductivity of amorphous nanostructures may be moderately tuned by interfaces regardless of the absence of crystalline order. In crystalline multilayers, however, interfaces may be rationally designed for an enormous reduction in thermal conductivity enabled by the wave interference of coherent thermal phonons. Coherent phonons may also contribute to thermal conductivity suppression observed in semiconductor core-shell nanowires, making them promising constituents for thermoelectric systems. Generally, manifestations of phonon coherence pave way for novel thermal design through phononic band structure engineering. The work also presents advanced nanofabrication routes for large-area thermoelectric nanocomposites. Particularly, the thermoelectric properties of zinc oxide thin films are transferred to a three-dimensional polymeric template by atomic layer deposition, allowing for a two-fold power output in reference to planar structures. In addition, scalable solution-based methods are used for facile fabrication of organic thermoelectric graphene nanocomposites. Finally, the work demonstrates new types of thermoelectric application prototypes with intriguing properties enabled by nanostructuring, including fully inkjet printed flexible thermoelectric circuits, the first planar fully transparent thermoelectric p-n modules, and a novel distributed thermocouple architecture of a transparent and flexible touch panel. The results not only provide new fundamental insight into phononic processes, but also enable new technological solutions for energy harvesting. Thus, the work has potentially profound implications on the emerging fields of nanophononic thermal engineering as well as transparent and flexible thermoelectrics.
  • Thermal characterization of III–V semiconductor core-shell nanowire arrays
    (2019-03-11) Koskinen, Tomi
     Sähkötekniikan korkeakoulu | Master's thesis
    Thermal management in electronics has become increasingly important due to the possibility of fabricating very small structures using different nanotechnologies. On the other hand, III–V semiconductor materials have gained interest especially in optoelectronics and photovoltaics due to their excellent optoelectronic properties. Each of these properties may be further engineered by introducing artificial nanostructures. A pivotal structure is the nanowire, which may be realized also as arrays consisting of countless nanowires, thus facilitating device integration. Moreover, the thermal conductivity of nanowires may be engineered by introducing thin coating layers, creating core-shell nanowires (CSNWs). Here we investigate the thermal properties of GaAs-AlAs and GaAs-AlGaAs CSNW arrays epitaxially grown by metal-organic vapor phase epitaxy (MOVPE). The nanowire array samples are embedded in spin-on-glass (SOG), chemi-mechanically polished (CMP) and deposited with Ti and Au layers acting as transducer layers prior to the transient thermoreflectance (TTR) measurement. The aim of this work is to establish the experimental thermal conductivity values for GaAs-based nanowire arrays and to evaluate the origin of heteroshell-induced effects on the thermal conductivity of the arrays. The experimental results display a reduction of thermal conductivity for 56 nm thick GaAs nanowires coated with AlAs shells of different thicknesses, the maximum reduction taking place with shell thickness of 35 nm. Additionally, a 6.59 W m^−1 K^−1 thermal conductivity for 105 nm GaAs nanowire arrays was measured, congruent with the previous publications on the subject. Addition of different shells resulted in a suppressed thermal conductivity with the average reduction amounting to 21.6%. The reduction is theorized to origin from the interplay of phonon trapping and increased scattering at the core-shell interface. Further measurements are required to determinate the interface quality and its effect on the phenomenon.
  • Thermal conductivity suppression in GaAs–AlAs core–shell nanowire arrays
    (2019) Juntunen, Taneli; Koskinen, Tomi; Khayrudinov, Vladislav; Haggren, Tuomas; Jiang, Hua; Lipsanen, Harri; Tittonen, Ilkka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Semiconductor nanowire heterostructures have been shown to provide appealing properties for optoelectronics and solid-state energy harvesting by thermoelectrics. Among these nanoarchitectures, coaxial core–shell nanowires have been of primary interest due to their electrical functionality, as well as intriguing phonon localization effects in the surface-dominated regime predicted via atomic simulations. However, experimental studies on the thermophysical properties of III–V semiconductor core–shell nanowires remain scarce regardless of the ubiquitous nature of these compounds in solid-state applications. Here, we present thermal conductivity measurements of the arrays of GaAs nanowires coated with AlAs shells. We unveil a strong suppression in thermal transport facilitated by the AlAs shells, up to ∼60%, producing a non-monotonous dependence of thermal conductivity on the shell thickness. Such translation of the novel heat transport phenomena to macroscopic nanowire arrays paves the way for rational thermal design in nanoscale applications.
  • Transparent, Flexible, and Passive Thermal Touch Panel
    (2016) Ruoho, Mikko; Juntunen, Taneli; Alasaarela, Tapani; Pudas, Marko; Tittonen, Ilkka
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This work presents a touch panel concept, which is enabled by a novel designof thin film thermocouples. The design offers a simple implementation byutilizing a single thin film to function as an array of thermocouples. Theconcept is demonstrated as a flexible, passive, and highly transparent touchpanel. The passive nature of the thermoelectric touch recognition allows theperformance of the presented sensor to be optimal at moderate sheet resistancevalues of the transparent conductive layers. Hence, the concept is highlypotential for low-cost large-area applications and does not rely on costlylow sheet resistance materials such as indium tin oxide. The demonstratorpresented in this work achieves a signal-to-noise ratio of 20 with a rise timeof 90 ms and is able to distinguish individual touches, sweeping with finger,as well as touching by multiple fingers at the same time. In addition, the conceptmay also be used in other thermal distribution mapping applications.