| 1. |
- Dreos, Ambra, 1987, et al.
(författare)
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Liquid Norbornadiene Photoswitches for Solar Energy Storage
- 2018
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Ingår i: Advanced Energy Materials. - : Wiley. - 1614-6832 .- 1614-6840. ; 8:18
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Tidskriftsartikel (refereegranskat)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. |
- Eklöf, Johnas, 1988, et al.
(författare)
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Understanding Interactions Driving the Template-Directed Self-Assembly of Colloidal Nanoparticles at Surfaces
- 2020
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:8, s. 4660-4667
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Tidskriftsartikel (refereegranskat)abstract
- Controlled deposition of colloidal nanoparticles using self-assembly is a promising technique for, for example, manufacturing of miniaturized electronics, and it bridges the gap between top-down and bottom-up methods. However, selecting materials and geometry of the target surface for optimal deposition results presents a significant challenge. Here, we describe a predictive framework based on the Derjaguin-Landau-Verwey-Overbeek theory that allows rational design of colloidal nanoparticle deposition setups. The framework is demonstrated for a model system consisting of gold nanoparticles stabilized by trisodium citrate that are directed toward prefabricated sub-100 nm features on a silicon substrate. Experimental results for the model system are presented in conjunction with theoretical analysis to assess its reliability. It is shown that three-dimensional, nickel-coated structures are well suited for attracting gold nanoparticles and that optimization of the feature geometry based on the proposed framework leads to a systematic improvement in the number of successfully deposited particles.
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| 3. |
- Erhart, Paul, 1978, et al.
(författare)
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Microscopic Origin of Thermal Conductivity Reduction in Disordered van der Waals Solids
- 2015
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 27:16, s. 5511-5518
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Gray, Victor, 1988, et al.
(författare)
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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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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 19:17, s. 10931-10939
-
Tidskriftsartikel (refereegranskat)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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| 5. |
- Jain, Titoo, et al.
(författare)
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Additional Article Notification: Anisotropic growth of gold nanoparticles using cationic gemini surfactants: effects of structure variations in head and tail groups
- 2014
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Ingår i: Journal of Materials Chemistry C. - 2050-7534 .- 2050-7526. ; 2:17, s. 3476-3485
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A library of gemini surfactants is employed to study surfactant directed anisotropic growth of gold nanoparticles. The surfactants are modified with respect to the length and type of the tails, as well as of the spacer group. By analyzing the structure of the anisotropic nanoparticles, it is possible to extract information on how the structure of the surfactants influences the anisotropic gold nanocrystal growth. We find that the tail length of the surfactants has a greater influence on the resulting nanoparticle aspect ratio compared to the chemical nature of the spacer group. While clear trends between the aspect ratio and the tail as well as spacer length remain elusive, we observe that surfactants with a critical micelle concentration of similar to 1 mM produce particles with the highest aspect ratio. A crystallographic analysis of nanorods obtained using gemini surfactants reveals that they grow along 100 and are bound by {310} facets. This observation, which is specific for gemini surfactants, is explained by taking into account the preferential alignment of gemini surfactants with surface steps as suggested by electronic structure calculations.
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| 6. |
- Jain, T., et al.
(författare)
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Anisotropic growth of gold nanoparticles using cationic gemini surfactants: effects of structure variations in head and tail groups
- 2014
-
Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 2:6, s. 994-1003
-
Tidskriftsartikel (refereegranskat)abstract
- A library of gemini surfactants is employed to study surfactant directed anisotropic growth of gold nanoparticles. The surfactants are modified with respect to the length and type of the tails, as well as of the spacer group. By analyzing the structure of the anisotropic nanoparticles, it is possible to extract information on how the structure of the surfactants influences the anisotropic gold nanocrystal growth. We find that the tail length of the surfactants has a greater influence on the resulting nanoparticle aspect ratio compared to the chemical nature of the spacer group. While clear trends between the aspect ratio and the tail as well as spacer length remain elusive, we observe that surfactants with a critical micelle concentration of similar to 1 mM produce particles with the highest aspect ratio. A crystallographic analysis of nanorods obtained using gemini surfactants reveals that they grow along and are bound by {310} facets. This observation, which is specific for gemini surfactants, is explained by taking into account the preferential alignment of gemini surfactants with surface steps as suggested by electronic structure calculations.
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| 7. |
- Jevric, Martyn, 1973, et al.
(författare)
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Norbornadiene-Based Photoswitches with Exceptional Combination of Solar Spectrum Match and Long-Term Energy Storage
- 2018
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 24:49, s. 12767-12772
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Klein, Andreas, et al.
(författare)
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The Fermi energy as common parameter to describe charge compensation mechanisms: A path to Fermi level engineering of oxide electroceramics
- 2023
-
Ingår i: Journal of Electroceramics. - 1573-8663 .- 1385-3449. ; 51
-
Tidskriftsartikel (refereegranskat)abstract
- Chemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.
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| 9. |
- Kuisma, Mikael Juhani, 1984, et al.
(författare)
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Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage
- 2016
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:7, s. 3635-3645
-
Tidskriftsartikel (refereegranskat)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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| 10. |
- Kuisma, Mikael Juhani, 1984, et al.
(författare)
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Optimization of Norbornadiene Compounds for Solar Thermal Storage by First-Principles Calculations
- 2016
-
Ingår i: ChemSusChem. - : Wiley. - 1864-5631 .- 1864-564X. ; 9:14, s. 1786-1794
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Tidskriftsartikel (refereegranskat)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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| 11. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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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
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 30:6, s. 1865-1877
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Tidskriftsartikel (refereegranskat)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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| 12. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Vibrationally induced color shift tuning of photoluminescence in Ce³⁺-doped garnet phosphors
- 2019
-
Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 7:41, s. 12926-12934
-
Tidskriftsartikel (refereegranskat)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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| 13. |
- Lindroth, Daniel, 1978, et al.
(författare)
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Thermal conductivity in intermetallic clathrates: A first-principles perspective
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 100:4
-
Tidskriftsartikel (refereegranskat)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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| 14. |
- Lindroth, Daniel, 1978, et al.
(författare)
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Thermal transport in van der Waals solids from first-principles calculations
- 2016
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 94:11
-
Tidskriftsartikel (refereegranskat)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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| 15. |
- Löfgren, Joakim, 1989, et al.
(författare)
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Understanding the Phase Diagram of Self-Assembled Monolayers of Alkanethiolates on Gold
- 2016
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:22, s. 12059-12067
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Tidskriftsartikel (refereegranskat)abstract
- Alkanethiolate monolayers on gold are important both for applications in nanoscience as well as fundamental studies of adsorption and self-assembly at metal surfaces. While considerable experimental effort has been put into understanding the phase diagram of these systems, theoretical work based on density functional theory (DFT) has long been hampered by the inability of conventional exchange-correlation functionals to describe dispersive interactions. In this work, we combine dispersion-corrected DFT calculations using the new vdW-DF-CX functional with the ab initio thermodynamics method to study the stability of dense standing-up and low-coverage lying-down phases on Au(111). We demonstrate that the lying-down phase has a thermodynamic region of stability starting from thiolates with alkyl chains consisting of n ? 3 methylene units. This phase emerges as a consequence of a competition between dispersive chain-chain and chain-substrate interactions, where the strength of the latter varies more strongly with n. A phase diagram is derived under ultrahigh-vacuum conditions, detailing the phase transition temperatures of the system as a function of the chain length. The present work illustrates that accurate ab initio modeling of dispersive interactions is both feasible and essential for describing self-assembled monolayers.
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| 16. |
- Manso, Mads, 1991, et al.
(författare)
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Molecular solar thermal energy storage in photoswitch oligomers increases energy densities and storage times
- 2018
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular photoswitches can be used for solar thermal energy storage by photoisomerization into high-energy, meta-stable isomers; we present a molecular design strategy leading to photoswitches with high energy densities and long storage times. High measured energy densities of up to 559 kJ kg(-1) (155 Wh kg(-1)), long storage lifetimes up to 48.5 days, and high quantum yields of conversion of up to 94% per subunit are demonstrated in norbornadiene/quadricyclane (NBD/QC) photo-/thermoswitch couples incorporated into dimeric and trimeric structures. By changing the linker unit between the NBD units, we can at the same time fine-tune light-harvesting and energy densities of the dimers and trimers so that they exceed those of their monomeric analogs. These new oligomers thereby meet several of the criteria to be met for an optimum molecule to ultimately enter actual devices being able to undergo closed cycles of solar light-harvesting, energy storage, and heat release.
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| 17. |
- Mansø, Mads, et al.
(författare)
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Dithiafulvene derivatized donor-acceptor norbornadienes with redshifted absorption
- 2019
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 21:6, s. 3092-3097
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Tidskriftsartikel (refereegranskat)abstract
- Photoisomerization of norbornadiene (N) to its metastable isomer quadricyclane (Q) has attracted interest as a strategy for harvesting and storing solar energy. For this strategy to mature the absorption maximum of N has to be moved from the UV to the visible region. Here we show that functionalization of the system with dithiafulvene (DTF) electron donors causes remarkable redshifts of various N derivatives. Thus, some derivatives were found to absorb light with an absorption onset up to 556 nm. The incorporation of DTF units comes, however, with a drawback with regard to achieving reversible N-to-Q and Q-to-N isomerizations. For some derivatives, the photoisomerization was completely quenched. The compounds were subjected to a computational study to shed light on the underlying reason for this reluctance to undergo photoisomerization. The computational study revealed that in these systems, the first excited state (S 1 ) is positioned close to or lower than the transition state for photoconversion, effectively blocking a possible conversion to Q, thus revealing a practical challenge for the future design of N-Q energy storage systems with an improved solar spectrum match.
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| 18. |
- Quant, Maria, 1985, et al.
(författare)
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Low Molecular Weight Norbornadiene Derivatives for Molecular Solar-Thermal Energy Storage
- 2016
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 22:37, s. 13265-13274
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Tidskriftsartikel (refereegranskat)abstract
- Molecular solar-thermal energy storage systems are based on molecular switches that reversibly convert solar energy into chemical energy. Herein, we report the synthesis, characterization, and computational evaluation of a series of low molecular weight (193-260 g mol(-1)) norbornadiene-quadricyclane systems. The molecules feature cyano acceptor and ethynyl-substituted aromatic donor groups, leading to a good match with solar irradiation, quantitative photo-thermal conversion between the norbornadiene and quadricyclane, as well as high energy storage densities (396-629 kJ kg(-1)). The spectroscopic properties and energy storage capability have been further evaluated through density functional theory calculations, which indicate that the ethynyl moiety plays a critical role in obtaining the high oscillator strengths seen for these molecules.
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| 19. |
- Quant, Maria, 1985, et al.
(författare)
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Solvent effects on the absorption profile, kinetic stability, and photoisomerization process of the norbornadiene-quadricyclanes system
- 2019
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:12, s. 7081-7087
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Tidskriftsartikel (refereegranskat)abstract
- Molecular photoswitches based on the norbornadiene-quadricyclane (NBD-QC) couple can be used to store solar energy and to release the stored energy as heat on demand. In this context, the energy storage time as well as the quantum yield of the energy storing reaction are important parameters. Here, we explore for the first time solvent effects on these processes for a series of four NBD-QC compounds in four different solvents with different polarity (acetonitrile, tetrahydrofuran, toluene, and hexane). We show that the energy storage time of the QC forms can vary by up to a factor of 2 when going from the most to the least polar solvent. Moreover, we show that for the norbornadiene derivatives with an asymmetric 1,2 substitution pattern, the quantum yield of conversion is highly solvent dependent, whereas this is not the case for the symmetrically substituted compounds. The spectroscopic observations are further rationalized using classical molecular dynamics (MD) simulations and time-dependent density functional theory (TDDFT) calculations. These demonstrate the importance of vibrational and rotational excitations for obtaining broad-band absorption, which is a prerequisite for capturing a wide range of the solar spectrum.
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| 20. |
- Rahm, Magnus, 1990, et al.
(författare)
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A Library of Late Transition Metal Alloy Dielectric Functions for Nanophotonic Applications
- 2020
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 30:35
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Tidskriftsartikel (refereegranskat)abstract
- Accurate complex dielectric functions are critical to accelerate the development of rationally designed metal alloy systems for nanophotonic applications, and to thereby unlock the potential of alloying for tailoring nanostructure optical properties. To date, however, accurate alloy dielectric functions are widely lacking. Here, a time-dependent density-functional theory computational framework is employed to compute a comprehensive binary alloy dielectric function library for the late transition metals most commonly employed in plasmonics (Ag, Au, Cu, Pd, Pt). Excellent agreement is found between electrodynamic simulations based on these dielectric functions and selected alloy systems experimentally scrutinized in 10 at% composition intervals. Furthermore, it is demonstrated that the dielectric functions can vary in very non-linear fashion with composition, which paves the way for non-trivial optical response optimization by tailoring material composition. The presented dielectric function library is thus a key resource for the development of alloy nanomaterials for applications in nanophotonics, optical sensors, and photocatalysis.
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| 21. |
- Wang, Zhihang, 1989, et al.
(författare)
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Storing energy with molecular photoisomers
- 2021
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Ingår i: Joule. - : Elsevier BV. - 2542-4351. ; 5:12, s. 3116-3136
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Forskningsöversikt (refereegranskat)abstract
- Some molecular photoisomers can be isomerized to a metastable high-energy state by exposure to light. These molecules can then be thermally or catalytically converted back to their initial state, releasing heat in the process. Such a reversible photochemical process has been considered for developing molecular solar thermal (MOST) systems. In this review, we introduce the concept, criteria, and state-of-the-art of MOST systems, with an emphasis on the three most promising molecular systems: norbornadiene/quadricyclane, E/Z-azobenzene, and dihydroazulene/vinylheptafulvene. After discussing the fundamental working principles, we focus on molecular design strategies for improving solar energy storage performance, remaining challenges, and potential focus areas. Finally, we summarize the current molecular incorporation into functional devices and conclude with a perspective on challenges and future directions.
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| 22. |
- Ångqvist, Mattias, 1989, et al.
(författare)
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Optimization of the Thermoelectric Power Factor: Coupling between Chemical Order and Transport Properties
- 2016
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 28:19, s. 6877-6885
-
Tidskriftsartikel (refereegranskat)abstract
- Many thermoelectric materials are multicomponent systems that exhibit chemical ordering, which can affect both thermodynamic and transport properties. Here, we address the coupling between order and thermoelectric performance in the case of a prototypical inorganic clathrate (Ba8Ga16Ge30) using a combination of density functional and Boltzmann transport theory as well as alloy cluster expansions and Monte Carlo simulations. The calculations describe the experimentally observed site occupancy factors and reproduce experimental data for the transport coefficients. By inverting the cluster expansion, we demonstrate that the power factor can be increased by more than 60% for certain chemical ordering patterns that involve reducing the number of the trivalent species on the 6c Wyckoff site. This enhancement is traced to specific features of the electronic band structure. The approach taken in the present work can be readily adapted to other materials and enables a very general form of band structure engineering. In this fashion, it can guide the computational design of compounds with optimal transport properties.
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| 23. |
- Aurino, Pier Paolo, 1985, et al.
(författare)
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Reversible metal-insulator transition of Ar-irradiated LaAlO3/SrTiO3 interfaces
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:15
-
Tidskriftsartikel (refereegranskat)abstract
- The conducting state of a quasi-two-dimensional electron gas (q2DEG), formed at the heterointerface between the two wide-bandgap insulators LaAlO3 (LAO) and SrTiO3, can be made completely insulating by low-energy, 150-eV, Ar+ irradiation. The metallic behavior of the interface can be recovered by high-temperature oxygen annealing. The electrical transport properties of the recovered q2DEG are exactly the same as before the irradiation. Microstructural investigations confirm that the transition is not due to physical etching or crystal lattice distortion of the LAO film below its critical thickness. They also reveal a correlation between electrical state, LAO film surface amorphization, and argon ion implantation. The experimental results are in agreement with density functional theory calculations of Ar implantation and migration in the LAO film. This suggests that the metal-insulator transition may be caused by charge trapping in the defect amorphous layer created during the ion irradiation.
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| 24. |
- Bancerek, Maria, et al.
(författare)
-
Origin of Macroscopic Observables of Strongly Coupled Metal Nanoparticle-Molecule Systems from Microscopic Electronic Properties
- 2024
-
Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:23, s. 9749-9757
-
Tidskriftsartikel (refereegranskat)abstract
- Strongly coupled light-matter systems are becoming a ubiquitous platform for investigating an increasing number of physical phenomena from modifying charge transport, altered emission, and relaxation pathways to selective or enhanced chemical reactivity. Such systems are investigated across a large length scale from few-nanometer-sized particles to macroscopic cavities encompassing many interacting moieties. Describing these numerous and varied physical systems is attempted in various ways from classical coupled harmonic oscillator models through quantum Hamiltonians to ab initio modeling. Here, by combining time-dependent density functional theory modeling and analysis with macroscopic models, we elucidate the origin of modifications of effective interaction parameters in terms of microscopic changes to the electronic density and Kohn-Sham transitions of the plasmonic particle and its coupled molecular counterpart. Specifically, we demonstrate how the emergence of mixed metal-molecular states and transitions modifies the effective resonances of the underlying plasmon and molecule in the regime of strong coupling and how these changes subsequently lead to the formation of mixed light-matter polaritons.
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| 25. |
- Berghäuser, Gunnar, 1983, et al.
(författare)
-
Inverted valley polarization in optically excited transition metal dichalcogenides
- 2018
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
-
Tidskriftsartikel (refereegranskat)abstract
- Large spin-orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory-experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.
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| 26. |
- Bernal, Ivan, 1984, et al.
(författare)
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Exciton broadening and band renormalization due to Dexter-like intervalley coupling
- 2018
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:2
-
Tidskriftsartikel (refereegranskat)abstract
- A remarkable property of atomically thin transition metal dichalcogenides (TMDs) is the possibility to selectively address single valleys by circularly polarized light. In the context of technological applications, it is very important to understand possible intervalley coupling mechanisms. Here, we show how the Dexter-like intervalley coupling mixes A and B states from opposite valleys leading to a significant broadening γB 1s of the B 1s exciton. The effect is much more pronounced in tungsten-based TMDs, where the coupling excitonic states are quasi-resonant. We calculate a ratio γB B 1s /γA B 1s ≈ 4.0, which is in good agreement with the experimentally measured value of 3.9 ± 0.7. In addition to the broadening effect, the Dexter-like intervalley coupling also leads to a considerable energy renormalization resulting in an increased energetic distance between A 1s and B 1s states.
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| 27. |
- Brem, Samuel, 1991, et al.
(författare)
-
Tunable Phases of Moiré Excitons in van der Waals Heterostructures
- 2020
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:12, s. 8534-8540
-
Tidskriftsartikel (refereegranskat)abstract
- Stacking monolayers of transition metal dichalcogenides into a heterostructure with a finite twist-angle gives rise to artificial moiré superlattices with a tunable periodicity. As a consequence, excitons experience a periodic potential, which can be exploited to tailor optoelectronic properties of these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moiré exciton resonances has not been fully clarified yet. Here, we combine first-principles calculations with the excitonic density matrix formalism to study transitions between different moiré exciton phases and their impact on optical properties of the twisted MoSe2/WSe2 heterostructure. At angles smaller than 2°, we find flat, moiré-trapped states for inter- and intralayer excitons. This moiré exciton phase changes into completely delocalized states at 3°. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moiré-trapped and delocalized exciton phases, respectively.
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| 28. |
- Brorsson, Joakim, 1988, et al.
(författare)
-
Efficient Calculation of the Lattice Thermal Conductivity by Atomistic Simulations with Ab Initio Accuracy
- 2022
-
Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 5:2
-
Tidskriftsartikel (refereegranskat)abstract
- High-order force constant expansions can provide accurate representations of the potential energy surface relevant to vibrational motion. They can be efficiently parametrized using quantum mechanical calculations and subsequently sampled at a fraction of the cost of the underlying reference calculations. Here, force constant expansions are combined via the hiphive package with GPU-accelerated molecular dynamics simulations via the GPUMD package to obtain an accurate, transferable, and efficient approach for sampling the dynamical properties of materials. The performance of this methodology is demonstrated by applying it both to materials with very low thermal conductivity (Ba8Ga16Ge30, SnSe) and a material with a relatively high lattice thermal conductivity (monolayer-MoS2). These cases cover both situations with weak (monolayer-MoS2, SnSe) and strong (Ba8Ga16Ge30) pho renormalization. The simulations also enable to access complementary information such as the spectral thermal conductivity, which allows to discriminate the contribution by different phonon modes while accounting for scattering to all orders. The software packages described here are made available to the scientific community as free and open-source software in order to encourage the more widespread use of these techniques as well as their evolution through continuous and collaborative development.
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| 29. |
- Brorsson, Joakim, 1988, et al.
(författare)
-
First-Principles Study of Order-Disorder Transitions in Pseudobinary Clathrates
- 2021
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:41, s. 22817-22826
-
Tidskriftsartikel (refereegranskat)abstract
- It has been recently demonstrated that the pseudoternary Ba8AlxGayGe46-x-y clathrate undergoes an order-disorder transition with increasing temperature that can be observed via site occupation factors (SOFs) and manifests itself, e.g., in electrical transport properties. Here, we generalize this result and analyze the characteristics of this order-disorder transition in the pseudobinary clathrates Ba8GaxGe46-x, Ba8GaxSi46-x, Ba8AlxGe46-x, and Ba8AlxSi46-x. To this end, we employ atomistic simulations that combine alloy cluster expansions trained against density functional theory calculations with Wang-Landau and ensemble Monte Carlo simulations. The simulations show that all four systems studied here display order-disorder transitions for at least some composition range. Based on an extensive literature survey, we also provide evidence for signatures of the transition in earlier experimental studies that to the best of our knowledge have hitherto not been related to such transitions. The predicted transition temperatures are lower for Ba8GaxGe46-x and Ba8GaxSi46-x than for Ba8AlxGe46-x and Ba8AlxSi46-x, although it appears that the simulations underestimate the transition temperatures for Ga-containing systems compared to the experiment. This nonetheless provides a sensible explanation for why the experimentally determined Al SOFs agree better with the simulated higherature disordered configurations, while the Ga SOFs more closely agree with the simulated ground-state configurations. As a result of stronger interactions, the SOFs vary substantially, especially near the stoichiometric 16:30 composition, providing an indication of why it has proved difficult to synthesize Ba8AlxGe46-x and Ba8AlxSi46-x samples at this ratio. The present study thereby yields detailed atomic-scale insights into the ordering in inorganic clathrates that, given the connection to transport properties established earlier, are not only useful from a fundamental perspective but also relevant for applications.
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| 30. |
- Brorsson, Joakim, 1988, et al.
(författare)
-
Order-Disorder Transition in Inorganic Clathrates Controls Electrical Transport Properties
- 2021
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 33:12, s. 4500-4509
-
Tidskriftsartikel (refereegranskat)abstract
- Inorganic clathrates have been extensively investigated owing to their unique and intriguing atomic structure as well as their potential as thermoelectric materials. The connection between the chemical ordering and the physical properties has, however, remained elusive. Here, this relation is uncovered through a combination of first-principles calculations, atomistic simulations, and experimental measurements of thermodynamic as well as electrical transport properties. This approach is, specifically, used to reveal the existence of an order-disorder transition in the quaternary clathrate series Ba8AlxGa16-xGe30. The results, furthermore, demonstrate that this phenomenon is responsible for the discontinuity in the heat capacity that has been observed previously. Moreover, the unusual temperature dependence of both Seebeck coefficient and electrical conductivity can be fully explained by the alterations of the band structure brought about by the phase transformation. It is finally argued that the phenomenology described here is not limited to this particular material but should be present in a wide range of inorganic clathrates and could even be observed in other materials that exhibit chemical ordering on at least one sublattice.
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| 31. |
- Brorsson, Joakim, 1988, et al.
(författare)
-
Strategic Optimization of the Electronic Transport Properties of Pseudo-Ternary Clathrates
- 2022
-
Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X .- 2199-160X. ; 8:3
-
Tidskriftsartikel (refereegranskat)abstract
- While alloying is a powerful handle for materials engineering, it is an ongoing challenge to navigate the large and complex parameter space of these materials. This applies in particular for thermoelectrics and even more so clathrates. Here, a combination of density functional theory calculations, alloy cluster expansions, Monte Carlo simulations, and Boltzmann transport theory calculations is used to identify compositions that yield high power factors in the pseudo-ternary clathrates Ba8AlxGayGe46−x−y and Ba8GaxGeySi46−x−y, while accounting for weight and raw material costs. The results show how a cost-efficient performance can be achieved by reducing the number of Al and Ga atoms per unit cell, while compensating the resulting increase in the carrier concentration via an extrinsic dopant. The approach used in this study is transferable and can be a useful tool for mapping the thermodynamic and transport properties of other multinary systems.
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| 32. |
- De Knoop, Ludvig, 1972, et al.
(författare)
-
Electric-field-controlled reversible order-disorder switching of a metal tip surface
- 2018
-
Ingår i: Physical Review Materials. - 2475-9953. ; 2:8
-
Tidskriftsartikel (refereegranskat)abstract
- While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.
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| 33. |
- Diaz de Zerio Mendaza, Amaia, 1986, et al.
(författare)
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High-Entropy Mixtures of Pristine Fullerenes for Solution-Processed Transistors and Solar Cells
- 2015
-
Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 27:45, s. 7325-7331
-
Tidskriftsartikel (refereegranskat)abstract
- The solubility of pristine fullerenes can be enhanced by mixing C60 and C70 due to the associated increase in configurational entropy. This "entropic dissolution" allows the preparation of field-effect transistors with an electron mobility of 1 cm2 V-1 s-1 and polymer solar cells with a highly reproducible power-conversion efficiency of 6%, as well as a thermally stable active layer.
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| 34. |
- Ekborg-Tanner, Pernilla, 1994, et al.
(författare)
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Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen
- 2022
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 5:8, s. 10225-10236
-
Tidskriftsartikel (refereegranskat)abstract
- Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical" sensitivity) is approximately constant at 180 nm/c(H) for nanodisk diameters of greater than or similar to 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic" sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures.
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| 35. |
- Erhart, Paul, 1978
(författare)
-
A first-principles study of helium storage in oxides and at oxide-iron interfaces
- 2012
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:11
-
Tidskriftsartikel (refereegranskat)abstract
- Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides, He migration is significantly slower and He-He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe-MgO, Fe-FeO-MgO). In both cases, the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.
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| 36. |
- Erhart, Paul, 1978, et al.
(författare)
-
Dopants and dopant-vacancy complexes in tetragonal lead titanate: A systematic first principles study
- 2015
-
Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 103, s. 224-230
-
Tidskriftsartikel (refereegranskat)abstract
- A systematic investigation of dopants in tetragonal lead titanate is presented by screening elements from the third period including K, Ca and all 3d transition metals. Formation energies and equilibrium transition states are determined by means of density functional theory calculations for both cation sites in the perovskite lattice, which allows us to discriminate between donor and acceptor type behavior. The stability of defect dipoles is determined by calculating the binding energy of transition metal-vacancy complexes. The results reveal that the tendency to substitute the Pb-site rather than the Ti-site monotonically increases going from Ti to Zn. The transition from Ti to Pb substitution depends both on the chemical equilibrium conditions and the position of the Fermi energy. This is most evident for Sc and Zn dopants that in principle can occupy both Pb-and Ti-sites depending on preparation conditions. Except for V all acceptor dopants form defect complexes with oxygen vacancies and thus can form defect dipoles causing hardening as well as aging effects. Defect dipoles involving Pb substitution and oxygen vacancies are found to be unfavorable for all dopants considered here.
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| 37. |
- Erhart, Paul, 1978, et al.
(författare)
-
Efficacy of the DFT plus U formalism for modeling hole polarons in perovskite oxides
- 2014
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 90:3, s. 035204-
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the formation of self-trapped holes (STH) in three prototypical perovskites (SrTiO3, BaTiO3, PbTiO3) using a combination of density functional theory (DFT) calculations with local potentials and hybrid functionals. First we construct a local correction potential for polaronic configurations in SrTiO3 that is applied via the DFT + U method and matches the forces from hybrid calculations. We then use the DFT + U potential to search the configuration space and locate the lowest energy STH configuration. It is demonstrated that both the DFT + U potential and the hybrid functional yield a piecewise linear dependence of the total energy on the occupation of the STH level, suggesting that self-interaction effects have been properly removed. The DFT + U model is found to be transferable to BaTiO3 and PbTiO3, and STH formation energies from DFT + U and hybrid calculations are in close agreement for all three materials. STH formation is found to be energetically favorable in SrTiO3 and BaTiO3 but not in PbTiO3, which can be rationalized by considering the alignment of the valence band edges on an absolute energy scale. In the case of PbTiO3 the strong coupling between Pb 6s and O 2p states lifts the valence band minimum (VBM) compared to SrTiO3 and BaTiO3. This reduces the separation between VBM and STH level and renders the STH configuration metastable with respect to delocalization (band hole state). We expect that the present approach can be adapted to study STH formation also in oxides with different crystal structures and chemical compositions.
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| 38. |
- Erhart, Paul, 1978, et al.
(författare)
-
First-principles study of codoping in lanthanum bromide
- 2015
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 91:16, s. Art. no. 165206-
-
Tidskriftsartikel (refereegranskat)abstract
- Codoping of Ce-doped LaBr3 with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes this enhancement on the basis of first-principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral V-Br-Sr-La complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different Ce-La-Sr-La-V-Br triple complex configurations. Codoping with other alkaline as well as alkaline-earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline-earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline-earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that codoping of wide-gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but also the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and recombination in other wide-gap materials.
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| 39. |
- Erhart, Paul, 1978, et al.
(författare)
-
Formation and switching of defect dipoles in acceptor-doped lead titanate: A kinetic model based on first-principles calculations
- 2013
-
Ingår i: Physical Review B. - 1098-0121. ; 88:2, s. artikel nr 024107-
-
Tidskriftsartikel (refereegranskat)abstract
- The formation and field-induced switching of defect dipoles in acceptor doped lead titanate is described by a kinetic model representing an extension of the well established Arlt-Neumann model [Ferroelectrics 76, 303 (1987)]. Energy barriers for defect association and reorientation of oxygen vacancy-dopant (Cu and Fe) complexes are obtained from first-principles calculations and serve as input for kinetic coefficients of the rate equation model. The numerical solution of the model describes the time evolution of the oxygen vacancy distribution at different temperatures and dopant concentrations in the presence or absence of an alternating external field. We predict the characteristic time scale for the alignment of all defect dipoles with the spontaneous polarization of the surrounding matrix. In this state the defect dipoles act as obstacles for domain wall motion and contribute to the experimentally observed aging. Under cycling conditions the fully aligned configuration is perturbed and a dynamic equilibrium is established with defect dipoles in parallel and antiparallel orientation relative to the spontaneous polarization. This process can be related to the deaging behavior of piezoelectric ceramics.
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| 40. |
- Erhart, Paul, 1978, et al.
(författare)
-
Low-Temperature Criticality of Martensitic Transformations of Cu Nanoprecipitates in alpha-Fe
- 2013
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 111:2, s. 025701-
-
Tidskriftsartikel (refereegranskat)abstract
- Nanoprecipitates form during nucleation of multiphase equilibria in phase segregating multicomponent systems. In spite of their ubiquity, their size-dependent physical chemistry, in particular, at the boundary between phases with incompatible topologies, is still rather arcane. Here, we use extensive atomistic simulations to map out the size-temperature phase diagram of Cu nanoprecipitates in alpha-Fe. The growing precipitates undergo martensitic transformations from the body-centered cubic (bcc) phase to multiply twinned 9R structures. At high temperatures, the transitions exhibit strong first-order character and prominent hysteresis. Upon cooling, the discontinuities become less pronounced and the transitions occur at ever smaller cluster sizes. Below 300 K, the hysteresis vanishes while the transition remains discontinuous with a finite but diminishing latent heat. This unusual size-temperature phase diagram results from the entropy generated by the soft modes of the bcc-Cu phase, which are stabilized through confinement by the alpha-Fe lattice.
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| 41. |
- Erhart, Paul, 1978, et al.
(författare)
-
Quasiparticle spectra, absorption spectra, and excitonic properties of NaI and SrI2 from many-body perturbation theory
- 2014
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 89:7, s. art no 075132 -
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the basic quantum-mechanical processes behind the nonproportional response of scintillators to incident radiation responsible for reduced resolution. For this purpose, we conduct a comparative first-principles study of quasiparticle spectra on the basis of the G(0)W(0) approximation as well as absorption spectra and excitonic properties by solving the Bethe-Salpeter equation for two important systems, NaI and SrI2. The former is a standard scintillator material with well-documented nonproportionality, while the latter has recently been found to exhibit a very proportional response. We predict band gaps for NaI and SrI2 of 5.5 and 5.2 eV, respectively, in good agreement with experiment. Furthermore, we obtain binding energies for the ground state excitons of 216 meV for NaI and 195 +/- 25 meV for SrI2. We analyze the degree of exciton anisotropy and spatial extent by means of a coarse-grained electron-hole pair-correlation function. Thereby, it is shown that the excitons in NaI differ strongly from those in SrI2 in terms of structure and symmetry, even if their binding energies are similar. Furthermore, we show that quite unexpectedly the spatial extents of the highly-anisotropic low-energy excitons in SrI2 in fact exceed those in NaI by a factor of two to three in terms of the full width at half maxima of the electron-hole pair-correlation function.
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| 42. |
- Erhart, Paul, 1978, et al.
(författare)
-
The Wulff construction goes low-symmetry
- 2023
-
Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-4660 .- 1476-1122. ; 22:8, s. 941-942
-
Tidskriftsartikel (refereegranskat)
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| 43. |
- Erhart, Paul, 1978, et al.
(författare)
-
Thermodynamic and mechanical properties of copper precipitates in alpha-iron from atomistic simulations
- 2013
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 88:2
-
Tidskriftsartikel (refereegranskat)abstract
- Precipitate hardening is commonly used in materials science to control strength by acting on the number density, size distribution, and shape of solute precipitates in the hardened matrix. The Fe-Cu system has attracted much attention over the last several decades due to its technological importance as a model alloy for Cu steels. In spite of these efforts several aspects of its phase diagram remain unexplained. Here we use atomistic simulations to characterize the polymorphic phase diagram of Cu precipitates in body-centered cubic (BCC) Fe and establish a consistent link between their thermodynamic and mechanical properties in terms of thermal stability, shape, and strength. The size at which Cu precipitates transform from BCC to a close-packed 9R structure is found to be strongly temperature dependent, ranging from approximately 4 nm in diameter (similar to 2700 atoms) at 200 K to about 8 nm (similar to 22 800 atoms) at 700 K. These numbers are in very good agreement with the interpretation of experimental data given Monzen et al. [Philos. Mag. A 80, 711 (2000)]. The strong temperature dependence originates from the entropic stabilization of BCC Cu, which is mechanically unstable as a bulk phase. While at high temperatures the transition exhibits first-order characteristics, the hysteresis, and thus the nucleation barrier, vanish at temperatures below approximately 300 K. This behavior is explained in terms of the mutual cancellation of the energy differences between core and shell (wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The proposed mechanism is not specific for the Fe-Cu system but could generally be observed in immiscible systems, whenever the minority component is unstable in the lattice structure of the host matrix. Finally, we also study the interaction of precipitates with screw dislocations as a function of both structure and orientation. The results provide a coherent picture of precipitate strength that unifies previous calculations and experimental observations.
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| 44. |
- Eriksson, Fredrik, 1992, et al.
(författare)
-
The Hiphive Package for the Extraction of High-Order Force Constants by Machine Learning
- 2019
-
Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 2:5
-
Tidskriftsartikel (refereegranskat)abstract
- The efficient extraction of force constants (FCs) is crucial for the analysis of many thermodynamic materials properties. Approaches based on the systematic enumeration of finite differences scale poorly with system size and can rarely extend beyond third order when input data is obtained from first-principles calculations. Methods based on parameter fitting in the spirit of interatomic potentials, on the other hand, can extract FC parameters from semi-random configurations of high information density and advanced regularized regression methods can recover physical solutions from a limited amount of data. Here, the HIPHIVE Python package, that enables the construction of force constant models up to arbitrary order is presented. HIPHIVE exploits crystal symmetries to reduce the number of free parameters and then employs advanced machine learning algorithms to extract the force constants. Depending on the problem at hand, both over and underdetermined systems are handled efficiently. The FCs can be subsequently analyzed directly and or be used to carry out, for example, molecular dynamics simulations. The utility of this approach is demonstrated via several examples including ideal and defective monolayers of MoS2 as well as bulk nickel.
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| 45. |
- Eriksson, Fredrik, 1992, et al.
(författare)
-
Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering
- 2023
-
Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 17:24, s. 25565-25574
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Tidskriftsartikel (refereegranskat)abstract
- It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.
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| 46. |
- Fan, Zheyong, et al.
(författare)
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GPUMD: A package for constructing accurate machine-learned potentials and performing highly efficient atomistic simulations
- 2022
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 157:11
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Tidskriftsartikel (refereegranskat)abstract
- We present our latest advancements of machine-learned potentials (MLPs) based on the neuroevolution potential (NEP) framework introduced in Fan et al. [Phys. Rev. B 104, 104309 (2021)] and their implementation in the open-source package gpumd. We increase the accuracy of NEP models both by improving the radial functions in the atomic-environment descriptor using a linear combination of Chebyshev basis functions and by extending the angular descriptor with some four-body and five-body contributions as in the atomic cluster expansion approach. We also detail our efficient implementation of the NEP approach in graphics processing units as well as our workflow for the construction of NEP models and demonstrate their application in large-scale atomistic simulations. By comparing to state-of-the-art MLPs, we show that the NEP approach not only achieves above-average accuracy but also is far more computationally efficient. These results demonstrate that the gpumd package is a promising tool for solving challenging problems requiring highly accurate, large-scale atomistic simulations. To enable the construction of MLPs using a minimal training set, we propose an active-learning scheme based on the latent space of a pre-trained NEP model. Finally, we introduce three separate Python packages, viz., gpyumd, calorine, and pynep, that enable the integration of gpumd into Python workflows.
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| 47. |
- Fant, Magnus, et al.
(författare)
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To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule
- 2021
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Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 60:10, s. 5132-5135
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Tidskriftsartikel (refereegranskat)abstract
- Loewenstein's rule, which states that Al−O−Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.
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| 48. |
- Fojt, Jakub, 1996, et al.
(författare)
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Dipolar coupling of nanoparticle-molecule assemblies: An efficient approach for studying strong coupling
- 2021
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 154:9
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Tidskriftsartikel (refereegranskat)abstract
- Strong light-matter interactions facilitate not only emerging applications in quantum and non-linear optics but also modifications of properties of materials. In particular, the latter possibility has spurred the development of advanced theoretical techniques that can accurately capture both quantum optical and quantum chemical degrees of freedom. These methods are, however, computationally very demanding, which limits their application range. Here, we demonstrate that the optical spectra of nanoparticle-molecule assemblies, including strong coupling effects, can be predicted with good accuracy using a subsystem approach, in which the response functions of different units are coupled only at the dipolar level. We demonstrate this approach by comparison with previous time-dependent density functional theory calculations for fully coupled systems of Al nanoparticles and benzene molecules. While the present study only considers few-particle systems, the approach can be readily extended to much larger systems and to include explicit optical-cavity modes.
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| 49. |
- Fojt, Jakub, 1996, et al.
(författare)
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Hot-Carrier Transfer across a Nanoparticle-Molecule Junction: The Importance of Orbital Hybridization and Level Alignment
- 2022
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 22:21, s. 8786-8792
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Tidskriftsartikel (refereegranskat)abstract
- While direct hot-carrier transfer can increase photocatalytic activity, it is difficult to discern experimentally and competes with several other mechanisms. To shed light on these aspects, here, we model from first-principles hot-carrier generation across the interface between plasmonic nanoparticles and a CO molecule. The hot-electron transfer probability depends nonmonotonically on the nanoparticle-molecule distance and can be effective at long distances, even before a strong chemical bond can form; hot-hole transfer on the other hand is limited to shorter distances. These observations can be explained by the energetic alignment between molecular and nanoparticle states as well as the excitation frequency. The hybridization of the molecular orbitals is the key predictor for hot-carrier transfer in these systems, emphasizing the necessity of ground state hybridization for accurate predictions. Finally, we show a nontrivial dependence of the hot-carrier distribution on the excitation energy, which could be exploited when optimizing photocatalytic systems.
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| 50. |
- Fojt, Jakub, 1996, et al.
(författare)
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Tailoring Hot-Carrier Distributions of Plasmonic Nanostructures through Surface Alloying
- 2024
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Ingår i: ACS Nano. - 1936-086X .- 1936-0851. ; 18:8, s. 6398-6405
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Tidskriftsartikel (refereegranskat)abstract
- Alloyed metal nanoparticles are a promising platform for plasmonically enabled hot-carrier generation, which can be used to drive photochemical reactions. Although the non-plasmonic component in these systems has been investigated for its potential to enhance catalytic activity, its capacity to affect the photochemical process favorably has been underexplored by comparison. Here, we study the impact of surface alloy species and concentration on hot-carrier generation in Ag nanoparticles. By first-principles simulations, we photoexcite the localized surface plasmon, allow it to dephase, and calculate spatially and energetically resolved hot-carrier distributions. We show that the presence of non-noble species in the topmost surface layer drastically enhances hot-hole generation at the surface at the expense of hot-hole generation in the bulk, due to the additional d-type states that are introduced to the surface. The energy of the generated holes can be tuned by choice of the alloyant, with systematic trends across the d-band block. Already low surface alloy concentrations have a large impact, with a saturation of the enhancement effect typically close to 75% of a monolayer. Hot-electron generation at the surface is hindered slightly by alloying, but here a judicious choice of the alloy composition allows one to strike a balance between hot electrons and holes. Our work underscores the promise of utilizing multicomponent nanoparticles to achieve enhanced control over plasmonic catalysis and provides guidelines for how hot-carrier distributions can be tailored by designing the electronic structure of the surface through alloying.
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