| 1. |
- Dreos, Ambra, 1987, et al.
(author)
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Liquid Norbornadiene Photoswitches for Solar Energy Storage
- 2018
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In: Advanced Energy Materials. - : Wiley. - 1614-6832 .- 1614-6840. ; 8:18
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Journal article (peer-reviewed)abstract
- Due to high global energy demands, there is a great need for development of technologies for exploiting and storing solar energy. Closed cycle systems for storage of solar energy have been suggested, based on absorption of photons in photoresponsive molecules, followed by on-demand release of thermal energy. These materials are called solar thermal fuels (STFs) or molecular solar thermal (MOST) energy storage systems. To achieve high energy densities, ideal MOST systems are required either in solid or liquid forms. In the case of the latter, neat high performing liquid materials have not been demonstrated to date. Here is presented a set of neat liquid norbornadiene derivatives for MOST applications and their characterization in toluene solutions and neat samples. Their synthesis is in most cases based on solvent-free Diels-Alder reactions, which easily and efficiently afford a range of compounds. The shear viscosity of the obtained molecules is close to that of colza oil, and they can absorb up to 10% of the solar spectrum with a measured energy storage density of up to 577 kJ/kg corresponding to 152 kJ mol(-1) (calculated 100 kJ mol(-1)). These findings pave the way towards implementation of liquid norbornadienes in closed cycle energy storage technologies.
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| 2. |
- Erhart, Paul, 1978, et al.
(author)
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Microscopic Origin of Thermal Conductivity Reduction in Disordered van der Waals Solids
- 2015
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 27:16, s. 5511-5518
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Journal article (peer-reviewed)abstract
- Films of layered substances like WSe2 can exhibit a reduction in the out-of-plane thermal conductivity of more than 1 order of magnitude compared to that of the bulk, effectively beating the glass limit (Science 2007, 315, 351). Here, we investigate the microscopic contributions that govern this behavior within the framework of Boltzmann transport theory informed by first-principles calculations. To quantitatively reproduce both the magnitude and the temperature dependence of the experimental data, one must account for both phonon confinement effects (softening and localization) and interlayer scattering. Both stacking order and layer spacing are shown to have a pronounced effect on the thermal conductivity that could be exploited to tune the balance between electrical and thermal conductivity.
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| 3. |
- Gray, Victor, 1988, et al.
(author)
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Loss channels in triplet-triplet annihilation photon upconversion: importance of annihilator singlet and triplet surface shapes
- 2017
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 19:17, s. 10931-10939
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Journal article (peer-reviewed)abstract
- Triplet-triplet annihilation photon upconversion (TTA-UC) can, through a number of energy transfer processes, efficiently combine two low frequency photons into one photon of higher frequency. TTA-UC systems consist of one absorbing species (the sensitizer) and one emitting species (the annihilator). Herein, we show that the structurally similar annihilators, 9,10-diphenylanthracene (DPA, 1), 9-(4-phenylethynyl)10-phenylanthracene (2) and 9,10-bis(phenylethynyl) anthracene (BPEA, 3) have very different upconversion efficiencies, 15.2 +/- 2.8%, 15.9 +/- 1.3% and 1.6 +/- 0.8%, respectively (of a maximum of 50%). We show that these results can be understood in terms of a loss channel, previously unaccounted for, originating from the difference between the BPEA singlet and triplet surface shapes. The difference between the two surfaces results in a fraction of the triplet state population having geometries not energetically capable of forming the first singlet excited state. This is supported by TD-DFT calculations of the annihilator excited state surfaces as a function of phenyl group rotation. We thereby highlight that the commonly used "spin-statistical factor'' should be used with caution when explaining TTA-efficiencies. Furthermore, we show that the precious metal free zinc octaethylporphyrin (ZnOEP) can be used for efficient sensitization and that the upconversion quantum yield is maximized when sensitizer-annihilator spectral overlap is minimized (ZnOEP with 2).
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| 4. |
- Jevric, Martyn, 1973, et al.
(author)
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Norbornadiene-Based Photoswitches with Exceptional Combination of Solar Spectrum Match and Long-Term Energy Storage
- 2018
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In: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 24:49, s. 12767-12772
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Journal article (peer-reviewed)abstract
- Norbornadiene-quadricyclane (NBD-QC) photoswitches are candidates for applications in solar thermal energy storage. Functionally, they rely on an intramolecular [2+2] cycloaddition reaction, which couples the S0 landscape on the NBD side to the S1 landscape on the QC side of the reaction and vice-versa. This commonly results in an unfavourable correlation between the first absorption maximum and the barrier for thermal back-conversion. This work demonstrates that this correlation can be counteracted by using steric repulsion to hamper the rotational motion of the side groups along the back-conversion path. It is shown that this modification reduces the correlation between the effective back-conversion barrier and the first absorption maximum and also increases the back-conversion entropy. The resulting molecules exhibit exceptionally long half-lives for their metastable forms without significantly affecting other properties, most notably solar spectrum match and storage density.
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| 5. |
- Kuisma, Mikael Juhani, 1984, et al.
(author)
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Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage
- 2016
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:7, s. 3635-3645
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Journal article (peer-reviewed)abstract
- Molecular photoswitches that are capable of storing solar energy, so-called molecular solar thermal storage systems, are interesting candidates for future renewable energy applications. In this context, substituted norbornadiene-quadricyclane systems have received renewed interest due to recent advances in their synthesis. The optical, thermodynamic, and kinetic properties of these systems can vary dramatically depending on the chosen substituents. The molecular design of optimal compounds therefore requires a detailed understanding of the effect of individual substituents as well as their interplay. Here, we model absorption spectra, potential energy storage, and thermal barriers for back-conversion of several substituted systems using both single-reference (density functional theory using PBE, B3LYP, CAM-B3LYP, M06, M06-2x, and M06-L functionals as well as MP2 calculations) and multireference methods (complete active space techniques). Already the diaryl substituted compound displays a strong red shift compared to the unsubstituted system, which is shown to result from the extension of the conjugated pi-system upon substitution. Using specific donor/acceptor groups gives rise to a further albeit relatively smaller red-shift. The calculated storage energy is found to be rather insensitive to the specific substituents, although solvent effects are likely to be important and require further study. The barrier for thermal back-conversion exhibits strong multireference character and as a result is noticeably correlated with the red-shift. Two possible reaction paths for the thermal back-conversion of diaryl substituted quadricyclane are identified and it is shown that among the compounds considered the path via the acceptor side is systematically favored. Finally, the present study establishes the basis for high-throughput screening of norbornadiene-quadricyclane compounds as it provides guidelines for the level of accuracy that can be expected for key properties from several different techniques.
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| 6. |
- Kuisma, Mikael Juhani, 1984, et al.
(author)
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Optimization of Norbornadiene Compounds for Solar Thermal Storage by First-Principles Calculations
- 2016
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In: ChemSusChem. - : Wiley. - 1864-5631 .- 1864-564X. ; 9:14, s. 1786-1794
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Journal article (peer-reviewed)abstract
- Molecular photoswitches capable of storing solar energy are interesting candidates for future renewable energy applications. Here, using quantum mechanical calculations, we carry out a systematic screening of crucial optical (solar spectrum match) and thermal (storage energy density) properties of 64 such compounds based on the norbornadiene-quadricyclane system. Whereas a substantial number of these molecules reach the theoretical maximum solar power conversion efficiency, this requires a strong red-shift of the absorption spectrum, which causes undesirable absorption by the photoisomer as well as reduced thermal stability. These compounds typically also have a large molecular mass, leading to low storage densities. By contrast, single-substituted systems achieve a good compromise between efficiency and storage density, while avoiding competing absorption by the photo-isomer. This establishes guiding principles for the future development of molecular solar thermal storage systems.
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| 7. |
- Lin, Yuan-Chih, 1987, et al.
(author)
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Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y₃₋ₓCeₓAl₅O₁₂: Design Principles for Phosphors Based on 5d-4f Transitions
- 2018
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 30:6, s. 1865-1877
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Journal article (peer-reviewed)abstract
- The oxide garnet Y 3 Al 5 O 12 (YAG), when a few percent of the activator ions Ce 3+ substitutes for Y 3+ , is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent per formance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce 3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y 3-x Ce x Al 5 O 12 . Increasing the Ce 3+ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO 8 moieties. Decreasing the Ce 3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce 3+ -doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO 8 moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence.
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| 8. |
- Lin, Yuan-Chih, 1987, et al.
(author)
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Vibrationally induced color shift tuning of photoluminescence in Ce³⁺-doped garnet phosphors
- 2019
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In: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 7:41, s. 12926-12934
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Journal article (peer-reviewed)abstract
- A critical challenge in the field of phosphor converted white light emitting diodes (pc-WLEDs) pertains to understanding and controlling the variation of emission color with device temperature. Here we, through a combined photoluminescence (PL) and Raman spectroscopy study of the three garnet type phosphors Ce3+-doped Y3Al5O12 (YAG:Ce3+), Ca3Sc2Si3O12 (CSS:Ce3+), and Sr3Y2Ge3O12 (SYG:Ce3+), show that the color of the PL is systematically shifted upon changing the operation temperature of the phosphor. A general trend is observed that the PL exhibits a red-shift as a function of increasing temperature, until the point at which the vibrational modes of the CeO8 moieties, which induce dynamical tetragonal distortions of the CeO8 dodecahedra, are fully activated. Upon further temperature increase, the PL turns to a blue-shift because of a counteracting and predominating effect of thermal lattice expansion that progressively makes the CeO8 dodecahedra more cubal like. Since this behavior is the result of the symmetry relations intrinsic to the garnet structure, the present mechanism can be generally applicable to materials of this type. It thereby provides a route for tuning the PL of this important class of phosphor materials.
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| 9. |
- Lindroth, Daniel, 1978, et al.
(author)
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Thermal conductivity in intermetallic clathrates: A first-principles perspective
- 2019
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 100:4
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Journal article (peer-reviewed)abstract
- Inorganic clathrates such as Ba8GaxGe46-x and Ba8AlxSi46-x commonly exhibit very low thermal conductivities. A quantitative computational description of this important property has proven difficult, in part due to the large unit cell, the role of disorder, and the fact that both electronic carriers and phonons contribute to transport. Here, we conduct a systematic analysis of the temperature and composition dependence of low-frequency modes associated with guest species in Ba8GaxGe46-x and Ba8AlxSi46-x ("rattler modes"), as well as thermal transport in stoichiometric Ba8Ga16Ge30. To this end, we account for phonon-phonon interactions by means of temperature-dependent effective interatomic force constants, which we find to be crucial in order to achieve an accurate description of the lattice part of the thermal conductivity. While the analysis of the thermal conductivity is often largely focused on the rattler modes, here it is shown that at room temperatures modes with ω 10meV account for 50% of lattice heat transport. Finally, the electronic contribution to the thermal conductivity is computed, which shows the Wiedemann-Franz law to be only approximately fulfilled. As a result, it is crucial to employ the correct prefactor when separating electronic and lattice contributions for experimental data.
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| 10. |
- Lindroth, Daniel, 1978, et al.
(author)
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Thermal transport in van der Waals solids from first-principles calculations
- 2016
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 94:11
-
Journal article (peer-reviewed)abstract
- The lattice thermal expansion and conductivity in bulk Mo and W-based transition metal dichalcogenides are investigated by means of density functional and Boltzmann transport theory calculations. To this end, a recent van der Waals density functional (vdW-DF-CX) is employed, which is shown to yield excellent agreement with reference data for the structural parameters. The calculated in-plane thermal conductivity compares well with experimental room-temperature values, when phonon-phonon and isotopic scattering are included. To explain the behavior over the entire available temperature range one must, however, include additional (temperature independent) scattering mechanisms that limit the mean free path. Generally, the primary heat carrying modes have mean free paths of 1 mu m or more, which makes these materials very susceptible to structural defects. The conductivity of Mo- and W-based transition metal dichalcogenides is primarily determined by the chalcogenide species and increases in the order Te-Se-S. While for the tellurides and selenides the transition metal element has a negligible effect, the conductivity of WS2 is notably higher than for MoS2, which may be traced to the much larger phonon band gap of the former. Overall, the present study provides a consistent set of thermal conductivities that reveal chemical trends and constitute the basis for future investigations of van der Waals solids.
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