Two-Dimensional Phononic Crystals: Disorder Matters

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201

Impact of Oligoether Side-Chain Length on the Thermoelectric Properties of a Polar Polythiophene

This article is part of the Advanced Thermoelectric Materials and Devices special issue.Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 ± 15 S cm–1 is obtained, resulting in a thermoelectric power factor of about 21 μW m–1 K–2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm–1 and 5 μW m–1 K–2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics.We acknowledge funding from the European Union’s Horizon 2020 research and innovation programme through the Marie Skłodowska-Curie grant agreement no. 955837 (HORATES) and the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship Prolongation grant. We acknowledge financial support from the Spanish Ministerio de Ciencia e Innovacíon for its support through grant CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program, and grants PID2020-119777GBI00 (THERM2MAIN), and PDC2021-121814-I00 (COVEQ). K.X. acknowledges a fellowship (CSC201806950006) from China Scholarship Council. K.X. and J.G. thank the PhD programme in Materials Science from Universitat Autònoma de Barcelona in which they are enrolled. We thank Johanna Heimonen for help with SEC measurements and Anders Mårtensson for carrying out the AFM measurements. This project was in part performed at the Chalmers Materials Analysis Laboratory (CMAL).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Phonon transport in the gigahertz to terahertz range: Confinement, topology, and second sound

Transport of heat and hypersound with gigahertz (GHz) to terahertz (THz) phonons is crucial for heat management in electronics, mediating signal processing with microwave radiation, thermoelectrics, and various types of sensors based on nanomechanical resonators. Efficient control of heat and sound transport requires new materials, novel experimental techniques, and a detailed knowledge of the interaction of phonons with other elementary excitations. Wave-like heat transport, also known as second sound, has recently attracted renewed attention since it provides several opportunities for overcoming some of the limitations imposed by diffusive transport (Fourier's regime). The frequency-domain detection of GHz-to-THz phonons can be carried out in a remote, non-destructive, and all-optical manner. The ongoing development of nanodevices and metamaterials made of low-dimensional nanostructures will require spatially resolved, time-resolved, and anisotropic measurements of phonon-related properties. These tasks can be accomplished with Brillouin light scattering (BLS) and various newly developed variants of this method, such as pumped-BLS. In the near future, pumped-BLS is expected to become useful for characterizing GHz topological nanophononics. Finally, second-sound phenomena can be observed with all-optical methods like frequency-domain thermoreflectance.This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme (No. 101003436) and the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882). J.S.R. acknowledges financial support from the Spanish Ministerio de Economıa, Industria y Competitividad for its support through Grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and Grant No. PID2020-119777GB-I0016 (THERM2MAIN). The authors thank Jeena Varghese for her assistance with preparing the figures.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Catalytically doped semiconductors for chemical gas sensing: Aerogel-like aluminum-containing zinc oxide materials prepared in the gas phase

et al.Atmospheric contamination with organic compounds is undesired in industry and in society because of odor nuisance or potential toxicity. Resistive gas sensors made of semiconducting metal oxides are effective in the detection of gases even at low concentration. Major drawbacks are low selectivity and missing sensitivity toward a targeted compound. Acetaldehyde is selected due to its high relevance in chemical industry and its toxic character. Considering the similarity between gas-sensing and heterogeneous catalysis (surface reactions, activity, selectivity), it is tempting to transfer concepts. A question of importance is how doping and the resulting change in electronic properties of a metal-oxide support with semiconducting properties alters reactivity of the surfaces and the functionality in gas-sensing and in heterogeneous catalysis. A gas-phase synthesis method is employed for aerogel-like zinc oxide materials with a defined content of aluminum (n-doping), which were then used for the assembly of gas sensors. It is shown that only Al-doped ZnO represents an effective sensor material that is sensitive down to very low concentrations (<350 ppb). The advance in properties relates to a catalytic effect for the doped semiconductor nanomaterial.S.P. and K.H. gratefully acknowledge funding from the Carl-Zeiss Foundation (project REFINE). M.R.W. acknowledges the postdoctoral Marie Curie Fellowship (IEF) HeatProNano (Grant No. 628197)Peer Reviewe

Advanced Optical Characterization of PEDOT:PSS by Combining Spectroscopic Ellipsometry and Raman Scattering

The optical properties of various PEDOT:PSS films obtained by drop casting and blade coating are analyzed by variable-angle spectroscopic ellipsometry in the visible-UV spectral range. We discuss observed differences in the optical spectra due to PSS content and DMSO treatment and correlate them to structural changes extracted from Raman measurements. In particular, we investigate the optical anisotropy of the complex refractive indices which arises from the in-plane arrangement of the PEDOT backbones, giving rise to optically uniaxial behavior with the optic axis perpendicular to the film plane. Although this is widely accepted, most investigations disregard the anisotropy for simplicity, which sometimes leads to inaccurate conclusions. In this work, we compare the results of isotropic and anisotropic analyses to clarify which kind of errors we can expect if anisotropy is not considered. Finally, the correlation between Raman scattering and ellipsometric analyses shows that not only local structural changes of the chain conformation but also the overall morphology of the composite films are significant in the interpretation of Raman spectra.The authors acknowledge funding from the Spanish Ministerio de Ciencia e Innovación (MICINN) through grants PID2019-106860GB-I00 and PID2020-119777GB-I00 and the Spanish Severo Ochoa Centre of Excellence program CEX2019-000917-S, as well as AGAUR, Generalitat de Catalunya, grant 2017-SGR-00488. M.K. is grateful to the UAB PhD program in Materials Science in which she is enrolled and the China Scholarship Council for funding (CSC 201809370071). We thank Agustín Mihi and Mariano Campoy-Quiles for their constant support and ideas.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Nanoarchitecture effects on persistent room temperature photoconductivity and thermal conductivity in ceramic semiconductors: Mesoporous, yolk-shell, and hollow ZnO spheres

Whereas size effects have been investigated extensively and are largely understood, it is significantly more challenging to elucidate how functional properties of semiconductors can be altered and ultimately be improved by a hierarchical nanoarchitecture. For semiconductor applications, such as in photovoltaics or photocatalysis, it is of great importance to learn how to avoid the recombination of photogenerated charge carriers and how to enhance their lifetime. A gas-phase synthesis method is explored, which enables the generation of spherical zinc oxide nanostructures with compact, mesoporous, a special type of core-shell, so-called yolk-shell, or hollow character. The particles with hollow character exhibit an extraordinarily long persistence of photogenerated charge carriers. It is demonstrated that the presence of the ZnO shell and its special orientation with respect to the polar character of the wurtzite lattice represent deciding factors. After photoexcitation, electrons and holes migrate to opposite sides of the interfaces, where they are stabilized. Moreover, photoluminescence thermometry was used to determine the thermal conductivity of the samples, which is lowered by a factor of ~100 compared with bulk ZnO. The thermal conductivity of this type of nanostructure is found to be only 10 times larger than that of air, and this points toward potential applications as thermoelectrics. © 2014 American Chemical Society.SP acknowledges the German Research Foundation (Grant PO 780/8-1) and the Carl-Zeiss Foundation (project Refine) for funding. ICN2 acknowledges financial support from the Spanish MICINN projects TAPHOR (Grant MAT2012-31392), nanoTHERM (Grant No. CSD2010-0044), and the FP7 projects NANOTEG-ENIAC (Grant No. 270789-2), NANOTHERM (Grant No. 318117), and MERGING (Grant No. 309150). M.R.W. gratefully acknowledges the Marie Curie Fellowship (IEF) HeatProNano (Grant No. 628197). L.Q. acknowledges support through the China Scholarship Council.Peer Reviewe

Transparent niobium-doped titanium dioxide thin films with high Seebeck coefficient for thermoelectric applications

This work reports the production and characterization of optically transparent Nb-doped TiO2 thin films with enhanced thermoelectric properties deposited on glass and Si by reactive d.c. magnetron sputtering in high vacuum. The purpose of these films is to harvest thermal energy from the environment and convert it to electrical energy. Several process parameters, such as reactive and working gas flow rate, deposition temperature, target current density and post-annealing conditions, directly affect the morphology and crystalline structure of the thin films. The optimization of these parameters results in thin films with thickness of 120–300 nm, maximum average optical transmittance in the visible range of 73%, n-type electrical resistivity of 0.05 Ω·cm, thermal conductivity around 1.5 W·m−1·K−1 and a maximum absolute Seebeck coefficient of 223 μV·K−1. The resulting maximum thermoelectric power factor is 60 μW·K−2·m−1 and the maximum thermoelectric figure of merit is 0.014. Hence, modifying the optical, electric, thermal and thermoelectric properties of the thin films enables their suitability for applications as transparent electrodes in photovoltaic systems and touch displays, amongst other devices.Joana Ribeiro is grateful to the Fundação para a Ciência e Tecnologia (FCT, Portugal) for the Ph.D. grant SFRH/BD/147221/2019. Filipe Correia is grateful to the FCT, Portugal, for the Ph.D. grant SFRH/BD/111720/2015. Funding is also gratefully acknowledged from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020-2023 and from the Spanish Ministerio de Ciencia e Innovación (MICINN) through grants SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program, and grant PID2020-119777GB-I00 (THERM2MAIN).Peer reviewe

The effect of Bi doping on the thermal conductivity of ZnO and ZnO:Al thin films

The dissipation of heat generation has been one of the largest obstacles in the design of semiconductor devices and reducing the thermal conductivity is vital for improving thermoelectric efficiency. This work focuses on the Bi doping effect on ZnO, and ZnO:Al thin films produced by magnetron sputtering with thickness varying between 500 and 900 nm. The approach introduces Bi ions, a higher mass element, into the ZnO metal-oxide matrix, to hinder phonon-mediated heat conduction and, consequently, reduce thermal conductivity. Atom probe tomography (APT) was employed to survey Bi doping distribution in ZnO:Al:Bi and ZnO:Bi thin films and to study the morphology of the grain boundaries. The thermal properties of the thin films were measured by frequency-domain thermoreflectance. Based on thermal conductivity results, it is concluded that the doping of ZnO films with Al has a significant effect on thermal conductivity, being reduced from 6.0 W m−1 K−1 in its undoped state to 3.3 W m−1 K−1 for ZnO with ∼3 at.% of Al, mainly due to alloy scattering of phonons in the wurtzite cell. Further doping with Bi contributes to a slight reduction in the thermal conductivity of ZnO:Al.Bi films (2.9 W m−1 K−1), due to grain boundary scattering by Bi/Bi2O3 phases. This result is understood as the confluence of two counteracting effects. On the one hand, the thermal conductivity of the film decreases because Bi, unlike Al, is segregated to grain boundaries and does not substitute Zn in the wurtzite crystal lattice, which is unequivocally demonstrated by APT results. On the other hand, the simultaneous presence of Al and Bi triggers a morphological change with the film's microstructure becoming more columnar. This change in microstructure from 3D island growth in ZnO:Al and ZnO:Bi to a more regular columnar structure in ZnO:Al,Bi limits further reduction in the thermal conductivity.Filipe Correia is grateful to the Fundação para a Ciência e Tecnologia (FCT, Portugal) for the Ph.D. grant SFRH/BD/111720/2015. Joana Ribeiro is grateful to the FCT, Portugal, for the Ph.D. grant SFRH/BD/147221/2019. Funding is also gratefully acknowledged from FCT/PIDDAC through the Strategic Funds project reference UIDB/04650/2020–2023 and from the Spanish Ministerio de Ciencia e Innovación (MICINN) through grants SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program, and grant PID2020-119777 GB-I00 (THERM2MAIN). This work (proposal ID 2018-020-022469) was carried out with the support of the Karlsruhe Nano Micro Facility (KNMFi, www.knmf.kit.edu), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Enhanced Photoluminescence of Cesium Lead Halide Perovskites by Quasi-3D Photonic Crystals

Cesium lead halide perovskite nanocrystals have emerged as one of the most promising candidates for manufacturing portable lasers and light sources. In order to harness and exploit their photoluminescence more effectively, the nanocrystals are often accompanied by a photonic scheme that improves light emission. In this work, one introduces a quasi-3D photonic crystal composed of a 2D-grating on top of a distributed Bragg reflector (DBR) that provides a greater photoluminescence enhancement than the isolated architectures alone. The quasi-3D photonic crystals support both Rayleigh-Wood anomalies and guided modes that populate the photonic bandgap of the Bragg mirror, all of them capable of enhancing the outcoupling of light from the emitting layer. In order to demonstrate the benefits of the quasi-3D system, one prepares 2D-gratings, DBRs, and quasi-3D photonic crystals covered with metal halide perovskite nanocrystals and studies the photoluminescence enhancement produced in each case. Interestingly, the quasi-3D structure exhibits a photoluminescence enhancement of 16 times and an increase in spontaneous emission rate, greatly exceeding the values observed for the separate components.This project was received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 637116, Enlightment), the Generalitat de Catalunya (2017-SGR-00488), and the Spanish Ministerio de Ciencia e Innovación through grants PID2019-106860GB-I00, PID2020-119777GB-I00 and CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program. L.P. acknowledges the support from the Spanish Ministerio de Ciencia e Innovación through Ramón y Cajal grant (RYC2018-026103-I). J.M.C. acknowledges an FPI fellowship (PRE2020-09411) from MICINN cofinanced by the European Social Fund and the Ph.D. program in Materials Science from Universitat Autònoma de Barcelona.Peer reviewe