| 1. |
- Abdi-Jalebi, Mojtaba, et al.
(författare)
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Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations
- 2018
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 12:7, s. 7301-7311
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Tidskriftsartikel (refereegranskat)abstract
- We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.
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| 3. |
- Ammothum Kandy, Akshay Krishna, et al.
(författare)
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Curvature Constrained Splines for DFTB Repulsive Potential Parametrization
- 2021
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 17:3, s. 1771-1781
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Tidskriftsartikel (refereegranskat)abstract
- The Curvature Constrained Splines (CCS) methodology has been used for fitting repulsive potentials to be used in SCC-DFTB calculations. The benefit of using CCS is that the actual fitting of the repulsive potential is performed through quadratic programming on a convex objective function. This guarantees a unique (for strictly convex) and optimum two-body repulsive potential in a single shot, thereby making the parametrization process robust, and with minimal human effort. Furthermore, the constraints in CCS give the user control to tune the shape of the repulsive potential based on prior knowledge about the system in question. Herein, we developed the method further with new constraints and the capability to handle sparse data. We used the method to generate accurate repulsive potentials for bulk Si polymorphs and demonstrate that for a given Slater-Koster table, which reproduces the experimental band structure for bulk Si in its ground state, we are unable to find one single two-body repulsive potential that can accurately describe the various bulk polymorphs of silicon in our training set. We further demonstrate that to increase transferability, the repulsive potential needs to be adjusted to account for changes in the chemical environment, here expressed in the form of a coordination number. By training a near-sighted Atomistic Neural Network potential, which includes many-body effects but still essentially within the first-neighbor shell, we can obtain full transferability for SCC-DFTB in terms of describing the energetics of different Si polymorphs.
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| 4. |
- Ammothum Kandy, Akshay Krishna
(författare)
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Linear models for multiscale materials simulations : Towards a seamless linking of electronic and atomistic models for complex metal oxides
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multiscale modelling approaches, connecting data from electronic structure calculations all the way towards engineering continuum models, have become an important ingredient in modern materials science. Materials modelling in a broader sense is already amply used to address complex chemical problems in academic science, but also in many industrial sectors. As far as multiscale modelling is concerned, however, many challenges remain, in particular when it comes to coupling and linking the various levels along the multiscale ladder in a seamless and efficient fashion. This thesis focusses on the development of new and efficient linear models to improve the quality and parameterisation processes of the two-body potentials used in empirical and semi-empirical methods within a multiscale materials modelling framework. In this regard, a machinery called curvature constrained splines (CCS) based on cubic splines to approximate general two-body potentials has been developed. The method is linear, and parameters can be easily solved in a least-square sense using a quadratic programming approach. Moreover, the objective function is convex, implying that global minima can be readily found. This makes the optimisation process easy to handle and requires little to no human effort. Initial tests to validate the method were performed on molecular and bulk neon systems. Later, the method was extended to incorporate long-range interactions by including atomic charges. The capability of the method was demonstrated for ZnO polymorphs, and at the same time benchmarked towards the conventional Buckingham potentials applied to the same problem. The results indicate that the CCS+Q method performs on par with the Buckingham approach, but is much faster and easier to parameterise. The merits of the method is further demonstrated with an exploration of size and shape dependent stability of CeO2 nanoparticles.Having established the framework of the CCS methodology, the method was further used to develop repulsive potentials for the semi-empirical self-consistent charge density functional tight binding (SCC-DFTB) method. The generation of the repulsive potentials is normally a tedious and time-consuming task. The CCS methodology makes this process significantly more efficient, and further provides new opportunities to explore the limits of the SCC-DFTB method. The development of repulsive potentials for bulk Si polymorphs showed that it is possible to retrieve a good description of each individual polymorph, but impossible to obtain an acceptable joint description of all polymorphs. The results indicated that a transferable repulsive potential needs to have coordination dependence, and by the use of a many-body artificial neural network representation for the repulsive potential, it was indeed possible to obtain a global transferability. The CCS methodology was finally used to model a system of considerable chemical diversity and complexity, namely reduced CeO2 within the SCC-DFTB formalism. Here, the CCS framework facilitated the development of an efficient workflow that yielded a harmonized description of Ce ions in different oxidation states. In short, the introduced CCS-based workflow proved to extend the applicability of SCC-DFTB to complex oxide systems with correlated electronic states. To conclude, the CCS methodology is demonstrated to be a versatile tool for efficient linking between (and within) electronic and atomistic models.
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| 5. |
- Arismendi Arrieta, Daniel Jose, et al.
(författare)
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H2O2(s) and H2O22H2O(s) crystals compared with ices : DFT functional assessment and D3 analysis
- 2023
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 159:19
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Tidskriftsartikel (refereegranskat)abstract
- The H2O and H2O2 molecules resemble each other in a multitude of ways as has been noted in the literature. Here, we present density functional theory (DFT) calculations for the H2O2(s) and H2O2·2H2O(s) crystals and make selected comparisons with ice polymorphs. The performance of a number of dispersion-corrected density functionals—both self-consistent and a posteriori ones—are assessed, and we give special attention to the D3 correction and its effects. The D3 correction to the lattice energies is large: for H2O2(s) the D3 correction constitutes about 25% of the lattice energy using PBE, much more for RPBE, much less for SCAN, and it primarily arises from non-H-bonded interactions out to about 5 Å.The large D3 corrections to the lattice energies are likely a consequence of several effects: correction for missing dispersion interaction, the ability of D3 to capture and correct various other kinds of limitations built into the underlying DFT functionals, and finally some degree of cell-contraction-induced polarization enhancement. We find that the overall best-performing functionals of the twelve examined are optPBEvdW and RPBE-D3. Comparisons with DFT assessments for ices in the literature show that where the same methods have been used, the assessments largely agree.
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| 6. |
- Barcaro, Giovanni, et al.
(författare)
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Dynamical and Structural Characterization of the Adsorption of Fluorinated Alkane Chains onto CeO2
- 2018
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 122:41, s. 23405-23413
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Tidskriftsartikel (refereegranskat)abstract
- The widespread use of ceria-based materials and the need to design suitable strategies to prepare eco-friendly CeO2 supports for effective catalytic screening induced us to extend our computational multiscale protocol to the modeling of the hybrid organic/oxide interface between prototypical fluorinated linear alkane chains (polyethylene-like oligomers) and low-index ceria surfaces. The combination of quantum chemistry calculations and classical reactive molecular dynamics simulations provides a comprehensive picture of the interface and discloses, at the atomic level, the main causes of typical adsorption modes. The data show that at room temperature. a moderate. percentage` of fluorine atoms (around 25%) can enhance the interaction of the organic chains by anchoring strongly pivotal fluorines to the channels of the underneath ceria (100) surface, whereas an excessive content can remarkably reduce this interaction because of the repulsion between fluorine and the negatively charged oxygen of the surface.
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| 8. |
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| 9. |
- Castleton, Christopher, 1969-, et al.
(författare)
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Benchmarking Density Functional Theory Functionals for Polarons in Oxides : Properties of CeO2
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 123:9, s. 5164-5175
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Tidskriftsartikel (refereegranskat)abstract
- We examine methods for studying polarons in metal oxides with density functional theory (DFT), using the example of cerium dioxide and the functionals, local density approximation + U (LDA+U), generalized gradient approximation + U (GGA+U) in the Perdew-Burke-Ernzerhof parametrization (PBE+U), as well as the hybrid functionals B3LYP, Heyd?Scuseria?Ernzerhof (HSE)03, HSE06, and PBE0. We contrast the four polaron energies commonly reported in different parts of the literature: formation energy, localization/relaxation energy, density-of-states level, and polaron-hopping activation barrier. Qualitatively, all these functionals predict "small" (Holstein) polarons on the scale of a single lattice site, although LDA+U and GGA+U are more effective than the hybrids at localizing the Ce 4f electrons. The improvements over pure LDA/GGA appear because of changes in the filled Ce 4f states when using LDA/GGA+U but due to changes in the empty Ce 4f states when using the hybrids. DFT is shown to have sufficient correlation to predict both adiabatic and (approximate) diabatic hopping barriers. Overall, LDA+U = 6 eV provides the best description in comparison to the experiment, followed by GGA+U = 5 eV. The hybrids are worse, tending to overestimate the gap and significantly underestimate the polaron-hopping barriers.
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| 10. |
- Colbin, Simon, et al.
(författare)
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A Halogen‐Free and Flame‐Retardant Sodium Electrolyte Compatible with Hard Carbon Anodes
- 2021
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Ingår i: Advanced Materials Interfaces. - : John Wiley & Sons. - 2196-7350. ; 8:23
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Tidskriftsartikel (refereegranskat)abstract
- For sodium-ion batteries, two pressing issues concerning electrolytes are flammability and compatibility with hard carbon anode materials. Non-flammable electrolytes that are sufficiently stable against hard carbon have—to the authors’ knowledge—previously only been obtained by either the use of high salt concentrations or additives. Herein, the authors present a simple, fluorine-free, and flame-retardant electrolyte which is compatible with hard carbon: 0.38 m sodium bis(oxalato)borate (NaBOB) in triethyl phosphate (TEP). A variety of techniques are employed to characterize the physical properties of the electrolyte, and to evaluate the electrochemical performance in full-cell sodium-ion batteries. The results reveal that the conductivity is sufficient for battery operation, no significant self-discharge occurs, and a satisfactory passivation is enabled by the electrolyte. In fact, a mean discharge capacity of 107 ± 4 mAh g−1 is achieved at the 1005th cycle, using Prussian white cathodes and hard carbon anodes. Hence, the studied electrolyte is a promising candidate for use in sodium-ion batteries.
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