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1.	Iordanidou, Konstantina, 1989, et al. (författare) Electric Field and Strain Tuning of 2D Semiconductor van der Waals Heterostructures for Tunnel Field-Effect Transistors 2023 Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 15:1, s. 1762-1771 Tidskriftsartikel (refereegranskat)abstract Heterostacks consisting of low-dimensional materials are attractive candidates for future electronic nanodevices in the post-silicon era. In this paper, using first-principles calculations based on density functional theory (DFT), we explore the structural and electronic properties of MoTe2/ZrS2 heterostructures with various stacking patterns and thicknesses. Our simulations show that the valence band (VB) edge of MoTe2 is almost aligned with the conduction band (CB) edge of ZrS2, and (MoTe2)m/(ZrS2)m (m = 1, 2) heterostructures exhibit the long-sought broken gap band alignment, which is pivotal for realizing tunneling transistors. Electrons are found to spontaneously flow from MoTe2 to ZrS2, and the system resembles an ultrascaled parallel plate capacitor with an intrinsic electric field pointed from MoTe2 to ZrS2. The effects of strain and external electric fields on the electronic properties are also investigated. For vertical compressive strains, the charge transfer increases due to the decreased coupling between the layers, whereas tensile strains lead to the opposite behavior. For negative electric fields a transition from the type-III to the type-II band alignment is induced. In contrast, by increasing the positive electric fields, a larger overlap between the valence and conduction bands is observed, leading to a larger band-to-band tunneling (BTBT) current. Low-strained heterostructures with various rotation angles between the constituent layers are also considered. We find only small variations in the energies of the VB and CB edges with respect to the Fermi level, for different rotation angles up to 30°. Overall, our simulations offer insights into the fundamental properties of low-dimensional heterostructures and pave the way for their future application in energy-efficient electronic nanodevices.
2.	Shetty, Naveen, 1988, et al. (författare) Scalable Fabrication of Edge Contacts to 2D Materials : Implications for Quantum Resistance Metrology and 2D Electronics 2023 Ingår i: ACS Applied Nano Materials. - : American Chemical Society. - 2574-0970. ; 6:7, s. 6292- Tidskriftsartikel (refereegranskat)abstract We report a reliable and scalable fabrication method for producing electrical contacts to two-dimensional (2D) materials based on the tri-layer resist system. We demonstrate the applicability of this method in devices fabricated on epitaxial graphene on silicon carbide (epigraphene) used as a scalable 2D material platform. For epigraphene, data on nearly 70 contacts result in median values of the one-dimensional (1D) specific contact resistances ρc ∼ 67 Ω·μm and follow the Landauer quantum limit ρc ∼ n-1/2, consistently reaching values ρc < 50 Ω·μm at high carrier densityn. As a proof of concept, we apply the same fabrication method to the transition metal dichalcogenide (TMDC) molybdenum disulfide (MoS2). Our edge contacts enable MoS2 field-effect transistor (FET) behavior with an ON/OFF ratio of >106 at room temperature (>109 at cryogenic temperatures). The fabrication route demonstrated here allows for contact metallization using thermal evaporation and also by sputtering, giving an additional flexibility when designing electrical interfaces, which is key in practical devices and when exploring the electrical properties of emerging materials. © 2023 The Authors.
3.	Ambrosio, Francesco, et al. (författare) Charge Localization in Acene Crystals from Ab Initio Electronic Structure 2023 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 14, s. 3343-3351 Tidskriftsartikel (refereegranskat)abstract The performance of Koopmans-compliant hybrid functionals in reproducing the electronic structure of organic crystals is tested for a series of acene crystals. The calculated band gaps are found to be consistent with those achieved with the GW method at a fraction of the computational cost and in excellent accord with the experimental results at room temperature, when including the thermal renormalization. The energetics of excess holes and electrons reveals a struggle between polaronic localization and band-like delocalization. The consequences of these results on the transport properties of acene crystals are discussed.
4.	Ambrosio, Francesco, et al. (författare) Strong Hole Trapping Due to Oxygen Dimers in BiVO4: Effect on the Water Oxidation Reaction 2019 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 10:22, s. 7113-7118 Tidskriftsartikel (refereegranskat)abstract We present a study of hole bipolarons in BiVO4. We show that in the presence of two holes O-O dimers are formed, leading to strong charge trapping. While the formation of bipolarons in bulk BiVO4 requires overcoming a kinetic barrier, we find that these defects should be spontaneously formed at the surface of the material and its interface with water. Through molecular dynamics simulations, we study the effect of bipolarons on the water-splitting reaction and show that their presence may be especially beneficial in alkaline conditions.
5.	Barzgar Vishlaghi, Mahsa, et al. (författare) Accelerating water oxidation on BiVO 4 photoanodes via surface modification with Co dopants 2023 Ingår i: Journal of Materials Chemistry A. - 2050-7488 .- 2050-7496. ; 11:31, s. 16648-16658 Tidskriftsartikel (refereegranskat)abstract Despite the vast investigations on improving the photoelectrochemical performance of BiVO4 for water splitting, charge recombination in the near-surface region remains a challenge. In this study, we showed that the diffusion of Co2+ ions into the BiVO4 subsurface boosted the water oxidation activity and charge injection efficiency remarkably. The increase in the concentration of oxygen vacancies upon the incorporation of cobalt ions was shown by electron paramagnetic resonance (EPR) spectroscopy and confirmed by density functional theory (DFT) calculations. DFT calculations revealed that vanadium sites in the subsurface region were the most favorable sites for substitution with cobalt ions. Charge localization at surface oxygen vacancies was found less favorable in the presence of cobalt in the subsurface layer, eliminating surface recombination. This resulted in 4.25 times larger charge injection efficiency and 6.2 times higher photocurrent density at the potential of ∼0.6 V, as compared to pristine BiVO4. This enhancement was significantly larger as compared to CoOx-loaded BiVO4, indicating that the suppressed recombination at the surface and improved charge transfer kinetics obtained solely by CoOx deposition are not sufficient for enhanced activity of BiVO4. A longer charge carrier lifetime obtained upon cobalt incorporation was observed by transient absorption spectroscopy and verified the reduced rate of recombination.
6.	Baskurt, Mehmet, 1992, et al. (författare) Charge Localization in Cs 2 AgBiBr 6 Double Perovskite: Small Polarons and Self-Trapped Excitons 2023 Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 127:49, s. 23966-23972 Tidskriftsartikel (refereegranskat)abstract Halide double perovskites have gained significant interest as promising materials in optoelectronic applications. Their relatively soft structure allows deformations that can potentially trap charge carriers, leading to the formation of small polarons. In this study, we investigate hole and electron polarons in Cs2AgBiBr6, a promising double perovskite, using the PBE0(α) functional. Our results reveal that spin-orbit coupling has a significant effect on the formation energies of small polarons and must be considered for accurate modeling of localized charges in Cs2AgBiBr6. While electron localization on Bi atoms is favorable, the hole polaron localized at the Ag site is very close in energy to the delocalized state. Simultaneous localization of holes and electrons at neighboring Ag and Bi sites leads to the formation of self-trapped excitons due to the significant attraction between the polarons of opposite charges. Our results provide insights into the essential properties of Cs2AgBiBr6 and have potential implications for further research and development of Cs2AgBiBr6-based green devices.
7.	Bischoff, Thomas, et al. (författare) Nonempirical hybrid functionals for band gaps of inorganic metal-halide perovskites 2019 Ingår i: Physical Review Materials. - 2475-9953. ; 3:12 Tidskriftsartikel (refereegranskat)abstract Nonempirical hybrid functionals are investigated for band-gap predictions of inorganic metal-halide perovskites belonging to the class CsBX3, with B = Ge, Sn, Pb and X = Cl, Br, I. We consider both global and range-separated hybrid functionals and determine the parameters through two different schemes. The first scheme is based on the static screening response of the material and thus yields dielectric-dependent hybrid functionals. The second scheme defines the hybrid functionals through the enforcement of Koopmans' condition for localized defect states. We also carry out quasiparticle self-consistent GW calculations with vertex corrections to establish state-of-the-art references. For the investigated class of materials, dielectric-dependent functionals and those fulfilling Koopmans' condition yield band gaps of comparable accuracy (similar to 0.2 eV), but the former only require calculations for the primitive unit cell and are less subject to the specifics of the material.
8.	Cannelli, Oliviero, et al. (författare) Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites 2022 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 13:15, s. 3382-3391 Tidskriftsartikel (refereegranskat)abstract A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr3 nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.
9.	Cannelli, Oliviero, et al. (författare) Decoupled Structural Responses upon Light and Thermal Functional Activation in CsPbBr 3 Perovskites 2022 Ingår i: Optics InfoBase Conference Papers. Konferensbidrag (refereegranskat)abstract We combine time-resolved and temperature-dependent X-ray absorption spectroscopy with state-of-the-art first principles simulations to quantify light- and thermally-induced structural distortions in CsPbBr3 perovskite nanocrystals with atomic-level precision.
10.	Capobianco, Amedeo, et al. (författare) Electron Localization and Mobility in Monolayer Fullerene Networks 2024 Ingår i: Nano Letters. - 1530-6992 .- 1530-6984. ; In Press Tidskriftsartikel (refereegranskat)abstract The novel 2D quasi-hexagonal phase of covalently bonded fullerene molecules (qHP C60), the so-called graphullerene, has displayed far superior electron mobilities, if compared to the parent van der Waals three-dimensional crystal (vdW C60). Herein, we present a comparative study of the electronic properties of vdW and qHP C60 using state-of-the-art electronic-structure calculations and a full quantum-mechanical treatment of electron transfer. We show that both materials entail polaronic localization of electrons with similar binding energies (≈0.1 eV) and, therefore, they share the same charge transport via polaron hopping. In fact, we quantitatively reproduce the sizable increment of the electron mobility measured for qHP C60 and identify its origin in the increased electronic coupling between C60 units.
11.	Falletta, Stefano, et al. (författare) Finite-size corrections of defect energy levels involving ionic polarization 2020 Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 102:4 Tidskriftsartikel (refereegranskat)abstract We develop a scheme for finite-size corrections of vertical transition energies and single-particle energy levels involving defect states with built-in ionic polarization in supercell calculations. The method accounts on an equal footing for the screening of the electrons and of the ionic polarization charge arising from the lattice distortions. We demonstrate the accuracy of our corrections for various defects in MgO and in water by comparing with the dilute limit achieved through the scaling of the system size. The general validity of our formulation is also confirmed through a sum rule that connects vertical transition energies with the formation energies of structurally relaxed defects.
12.	Fransson, Erik, 1990, et al. (författare) Phase Transitions in Inorganic Halide Perovskites from Machine-Learned Potentials 2023 Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 127:28, s. 13773-13781 Tidskriftsartikel (refereegranskat)abstract The atomic scale dynamics of halide perovskites havea direct impactnot only on their thermal stability but also on their optoelectronicproperties. Progress in machine-learned potentials has only recentlyenabled modeling the finite temperature behavior of these materialsusing fully atomistic methods with near first-principles accuracy.Here, we systematically analyze the impact of heating and coolingrate, simulation size, model uncertainty, and the role of the underlyingexchange-correlation functional on the phase behavior of CsPbX3 with X = Cl, Br, and I, including both the perovskite andthe & delta;-phases. We show that rates below approximately 60 K/nsand system sizes of at least a few tens of thousands of atoms shouldbe used to achieve convergence with regard to these parameters. Bycontrolling these factors and constructing models that are specificfor different exchange-correlation functionals, we then assess thebehavior of seven widely used semilocal functionals (LDA, vdW-DF-cx,SCAN, SCAN+rVV10, PBEsol, PBE, and PBE+D3). The models based on LDA,vdW-DF-cx, and SCAN+rVV10 agree well with experimental data for thetetragonal-to-cubic-perovskite transition temperature in CsPbI3 and also achieve reasonable agreement for the perovskite-to-deltaphase transition temperature. They systematically underestimate, however,the orthorhombic-to-tetragonal transition temperature. All other models,including those for CsPbBr3 and CsPbCl3, predicttransition temperatures below the experimentally observed values forall transitions considered here. Among the considered functionals,vdW-DF-cx and SCAN+rVV10 yield the closest agreement with experiment,followed by LDA, SCAN, PBEsol, PBE, and PBE+D3. Our work providesguidelines for the systematic analysis of dynamics and phase transitionsin inorganic halide perovskites and similar systems. It also servesas a benchmark for the further development of machine-learned potentialsas well as exchange-correlation functionals.
13.	Fransson, Erik, 1990, et al. (författare) Revealing the Free Energy Landscape of Halide Perovskites: Metastability and Transition Characters in CsPbBr₃ and MAPbI₃ 2023 Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 8229- Tidskriftsartikel (refereegranskat)abstract Halide perovskites have emerged as a promising class of materials for photovoltaic applications. A challenge of these applications is preventing the crystal structure from degrading to photovoltaically inactive phases, which requires an understanding of the free energy landscape of these materials. Here, we uncover the free energy landscape of two prototypical halide perovskites, CsPbBr3 and MAPbI3, via atomic-scale simulations using umbrella sampling and machine-learned potentials. For CsPbBr3, we find very small free energy differences and barriers close to the transition temperatures for both the tetragonal-to-cubic and orthorhombic-to-tetragonal transitions. For MAPbI3, however, the situation is more intricate. In particular, the orthorhombic-to-tetragonal transition exhibits a large free energy barrier, and there are several competing tetragonal phases. Using large-scale molecular dynamics simulations, we explore the character of these transitions and observe the latent heat and a discrete change in the structural parameters for the tetragonal-to-cubic phase transitions in both CsPbBr3 and MAPbI3, indicating first-order transitions. We find that in MAPbI3, the orthorhombic phase has an extended metastability range, and we identify a second metastable tetragonal phase. Finally, we compile a phase diagram for MAPbI3 that includes potential metastable phases.
14.	Iordanidou, Konstantina, 1989, et al. (författare) Two-dimensional MoTe2/SnSe2 van der Waals heterostructures for tunnel-FET applications 2022 Ingår i: Physical Review Materials. - 2475-9953. ; 6:8 Tidskriftsartikel (refereegranskat)abstract Two-dimensional (2D) van der Waals heterostructures (vdWHs) are attractive candidates for realizing tunnel field-effect transistors (TFETs) for low-power applications. In this work, using first-principles calculations based on density functional theory (DFT), we explore heterostructures composed of 2D MoTe2 and SnSe2. Our calculations reveal that upon forming the heterostructures, the valence band top of MoTe2 and the conduction band bottom of SnSe2 are almost aligned, forming the nearly broken-gap or type-III band alignment which is highly promising for TFETs. Interestingly, we find that the band alignment can be tuned by applying external electric fields. For positive electric fields, MoTe2 (SnSe2) band-edge positions are shifted upward (downward) with respect to the Fermi level, and more electrons are expected to tunnel from MoTe2 to SnSe2. Overall, our simulations provide fundamental insights into the electronic properties of MoTe2/SnSe2 stacks, and pave the way for the design and fabrication of future MoTe2/SnSe2-based TFETs.
15.	Keeble, D. J., et al. (författare) Identification of lead vacancy defects in lead halide perovskites 2021 Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 12:1 Tidskriftsartikel (refereegranskat)abstract Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (VPb) defects in MAPbI3 (MA = CH3NH3+) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 1015 cm−3 was determined. There was also evidence of trapping at the vacancy complex (VPbVI)− in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, VPb2−, point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms.
16.	Liu, Yun, et al. (författare) Strong electron-phonon coupling and bipolarons in Sb2S3 2023 Ingår i: Physical Review Materials. - 2475-9953. ; 7:8 Tidskriftsartikel (refereegranskat)abstract Antimony sulfide (Sb2S3) is an Earth-abundant and nontoxic material that is under investigation for solar energy conversion applications. However, it still suffers from poor power conversion efficiency and a large open circuit voltage loss that have usually been attributed to point or interfacial defects. More recently, there has been some discussion in the literature about the role of carrier trapping in the optical properties of Sb2S3, with some reporting self-trapped excitons as the microscopic origin for the performance loss, while others have found no evidence of carrier trapping with only large polarons existing in Sb2S3. By using first-principles methods, we demonstrate that Sb2S3 exhibits strong electron-phonon coupling, a prerequisite for carrier self-trapping in semiconductors, which results in a large renormalization of 200meV of the absorption edge when temperature increases from 10 to 300K. When two electrons or holes are added to the system, corresponding to a carrier density of 1.6×1020cm-3, we find wave function localization consistent with the presence of bipolarons accompanying a significant lattice distortion with the formation of Sb and S dimers. The formation energies of the electron and hole bipolarons are -330 and -280 meV per carrier, respectively. Our results reconcile some of the controversy in the literature regarding carrier trapping in Sb2S3 and demonstrate the existence of large electron-phonon coupling and carrier self-trapping that might place a fundamental limit on the open circuit voltage and, consequently, the maximum efficiency of the photovoltaic cells.
17.	Lyu, Sai, et al. (författare) Oxygen Evolution at the BiVO4-Water Interface: Mechanism of the Water Dehydrogenation Reaction 2022 Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 12:19, s. 11734-11742 Tidskriftsartikel (refereegranskat)abstract We study the water dehydrogenation reaction at the BiVO4(010)-water interface by combining nudged-elastic-band calculations and electronic structure calculations at the hybrid functional level. We investigate the pathway and the kinetic barrier for the adiabatic reaction going from the hole polaron localized in BiVO4 to the dehydrogenation of the adsorbed water molecule at the interface. The reaction is found to involve the H2O center dot+ radical cation as an intermediate, to have a kinetic barrier of 0.7 eV, and to be initiated by the electron transfer. The calculated kinetic barrier is in good agreement with experiment and is consistent with the slow hole transfer kinetics observed at the surface of BiVO4. To characterize the structural changes occurring during this process, we analyze the O-H distances for three relevant water molecules. We also examine the Wannier functions around the O atom of the adsorbate involved in the reaction to reveal the changes in the electronic structure during the hole hopping. The projected density of states of the lowest unoccupied molecular orbitals allows us to identify the atomic orbitals that are primarily involved in the reaction. We expect that the proposed reaction mechanism generally holds when the surface coverage is dominated by molecularly adsorbed water.
18.	Morana, Marta, et al. (författare) Cubic or Not Cubic? Combined Experimental and Computational Investigation of the Short-Range Order of Tin Halide Perovskites 2023 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 14, s. 2178-2186 Tidskriftsartikel (refereegranskat)abstract Tin-based metal halide perovskites with a composition of ASnX3 (where A= MA or FA and X = I or Br) have been investigated by means of X-ray total scattering techniques coupled to pair distribution function (PDF) analysis. These studies revealed that that none of the four perovskites has a cubic symmetry at the local scale and that a degree of increasing distortion is always present, in particular when the cation size is increased, i.e., from MA to FA, and the hardness of the anion is increased, i.e., from Br- to I-. Electronic structure calculations provided good agreement with experimental band gaps for the four perovskites when local dynamical distortions were included in the calculations. The averaged structure obtained from molecular dynamics simulations was consistent with experimental local structures determined via X-ray PDF, thus highlighting the robustness of computational modeling and strengthening the correlation between experimental and computational results.
19.	Ouhbi, Hassan, 1989, et al. (författare) Ab Initio Insights into Charge Localization in Bismuth Oxyhalides BiOX (X = F, Cl, Br, I) 2022 Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:46, s. 19956-19961 Tidskriftsartikel (refereegranskat)abstract Developing efficient photocatalysts for clean energy generation is crucial to achieving net-zero emissions. To this end, we investigate the behavior of photoexcited charges in bismuth oxyhalides BiOX (X = F, Cl, Br, and I), a family of inexpensive and promising photocatalysts. To model the localization of excess electrons and holes, we use hybrid density functional theory PBE0(α). Our results indicate that electron polarons are unstable in these materials. Concurrently, we find that hole localization is favorable, and we identify two different possible configurations in which polarons are formed. One consists of two dimerized halogen atoms (VK center) and is preferentially formed in BiOBr and BiOI with binding energies that amount to-0.26 and-0.21 eV, respectively. The other corresponds to localization on a single Bi site and the surrounding oxygen and halogen atoms (BiXO). This form of polaron is favorable in BiOF and BiOCl with binding energies that amount to-0.35 and-0.23 eV, respectively. These findings highlight the behavior of photogenerated carriers and may open up avenues for future investigations on carrier transport in bismuth oxyhalides.
20.	Ouhbi, Hassan, 1989, et al. (författare) Polaron formation and hopping in tantalate perovskite oxides: NaTaO3 and KTaO3 2021 Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 104:23 Tidskriftsartikel (refereegranskat)abstract Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in NaTaO3 and KTaO3. We perform Perdew-Burke-Ernzerhof hybrid density functional PBE0(alpha) calculations, in which we define the fraction alpha of the Fock exchange by enforcing the Koopmans' condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta-O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature similar to 2.94 x 10-6 cm2/Vs and similar to 1.87 x 10-4 cm2/Vs for KTaO3 and NaTaO3, respectively.
21.	Ouhbi, Hassan, 1989, et al. (författare) Probing the Polaronic Landscape in Bi 4 TaO 8 X Perovskite Oxyhalides Photocatalysts 2023 Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 127:42, s. 20965-20970 Tidskriftsartikel (refereegranskat)abstract Bismuth perovskite oxyhalides Bi4TaO8X (X = Cl, Br) are a promising class of photocatalysts due to their resistance to self-decomposition, a property often lacking in other photocatalysts. In order for them to efficiently carry out photocatalytic reactions, it is essential that the diffusion of photogenerated excess charges is not disrupted, such as by the formation of polarons, during transport to the surface, where the reactions take place. We here use a Koopmans-compliant hybrid functional to investigate the behavior of the photogenerated holes and electrons. We first demonstrate that electron polarons are unstable in these materials. Excess holes, on the other hand, localize and alter the atomic structure locally, leading to the formation of various polaronic configurations. Our results show that hole polarons are highly stable at the perovskite block [MO4] and possess features that are similar to those found for holes in NaTaO3. Furthermore, we find that the presence of two holes results in the occurrence of bipolaronic states, which are accompanied by the formation of O-O dimers. Finally, we show that holes do not localize within the halide block [X], in contrast to oxyhalides BiOX (X = Cl, Br), suggesting that Bi4TaO8X are more resistant to self-oxidation of X-, in accordance with the higher stability reported in experimental studies.
22.	Ouhbi, Hassan, 1989, et al. (författare) Strong Electron Localization in Tin Halide Perovskites 2021 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:22, s. 5339-5343 Tidskriftsartikel (refereegranskat)abstract Tin halide perovskites (THPs) have been established as a lower-toxicity alternative to lead halide perovskites. In spite of the increasing interest, the behavior of photoexcited charges has not been well understood in this class of materials. We here investigate the behavior of excess electrons in a series of tin halide perovskites by employing advanced electronic-structure calculations. We first focus on CsSnBr3 and show that electron localization is favorable in this compound and that bipolaronic states are the most stable form of self-trapped electrons. We then extend the analysis to CsSnI3, CsSnCl3, MASnBr3, FASnBr3, and DMASnBr3 and show that electron bipolarons are stable in all these compounds, thus indicating that strong electron localization is recurrent in THPs.
23.	Ricciarelli, Damiano, et al. (författare) Electron bipolarons at the DMASnBr 3 –water interface: Effect on the photocatalytic hydrogen production 2024 Ingår i: International Journal of Hydrogen Energy. - 0360-3199. ; 58, s. 863-871 Tidskriftsartikel (refereegranskat)abstract In this article, we report on advanced molecular dynamics simulations of the atomistic DMASnBr3–water interface, which, coupled with a grand-canonical formulation of adsorbates and defects, elucidate the surface chemistry and reactivity of this novel water-stable perovskite and highlight the role of small electron bipolarons in photocatalytic hydrogen production. We find that the extremely acidic nature of the surface Br atoms does not allow for significant adsorption of protons at the interface under charge-neutral conditions. However, when electrons are accumulated on the surface, the formation of a small electron bipolaron in the form of a Sn-Sn dimer provides the required electron localization to drive adsorption of H, which is assimilated on surface Sn atoms as hydride. Finally, we estimate a favourable alignment between the bipolaron energy level and the H+/H2 redox level, which suggests the occurrence of a feasible route for hydrogen evolution, bypassing the common reaction mechanism.
24.	Ricciarelli, Damiano, et al. (författare) Reaction Mechanism of Photocatalytic Hydrogen Production at Water/Tin Halide Perovskite Interfaces 2022 Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 7, s. 1308-1315 Tidskriftsartikel (refereegranskat)abstract While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr3 (DMA = dimethylammonium) perovskite against prototypical MASnI3 and MASnBr3 compounds (MA = methylammonium), employing advanced electronic-structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction. In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist in development of more stable and efficient materials for photocatalytic hydrogen reduction.
25.	Romero, Aldo H., et al. (författare) ABINIT: Overview and focus on selected capabilities 2020 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 152:12 Forskningsöversikt (refereegranskat)abstract abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned.
26.	Wiktor, Julia, 1988, et al. (författare) Quantifying Dynamic Tilting in Halide Perovskites: Chemical Trends and Local Correlations 2023 Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 35, s. 6737- Tidskriftsartikel (refereegranskat)abstract Halide perovskiteshave emerged as one of the most interestingmaterials for optoelectronic applications due to their favorable properties,such as defect tolerance and long charge carrier lifetimes, whichare attributed to their dynamic softness. However, this softness hasled to apparent disagreements between the local instantaneous andglobal average structures of these materials. In this study, we rationalizethis situation through an assessment of the local tilt angles of octahedrain the perovskite structure using large-scale molecular dynamics simulationsbased on machine-learned potentials trained using density functionaltheory calculations. We compare structural properties given by differentdensity functionals [local density approximation, PBE, PBE + D3, PBEsol,strongly constrained and appropriately normed (SCAN), SCAN + rVV10,and van der Waals density functional with consistent exchange] andestablish trends across a family of CsMX3 perovskites withM = Sn or Pb and X = Cl, Br or I. Notably, we demonstrate strong short-rangeordering in the cubic phase of halide perovskites. This ordering isreminiscent of the tetragonal phase and provides the bridge betweenthe disordered local structure and the global cubic arrangement. Ourresults provide a deeper understanding of the structural propertiesof halide perovskites and their local distortions, which is crucialfor further understanding their optoelectronic properties.
27.	Österbacka, Nicklas, 1993, et al. (författare) Charge Localization in Defective BiVO 4 2022 Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:6, s. 2960-2970 Tidskriftsartikel (refereegranskat)abstract We study the native defects in bismuth vanadate using hybrid density functional theory. We pay special attention to where excess charges localize by considering different polaronic distortions and find that charge localization has a profound effect on the local chemical environment around certain defects. In particular, oxygen dimerization may occur in the presence of acceptor defects. On the basis of Fermi level pinning due to compensation between donors and acceptors we additionally find that intrinsic p-type conductivity is difficult to achieve in BiVO4, in good agreement with experimental observations. Our results give new insights into the defect chemistry of bismuth vanadate and act as a guide for future studies on defects in complex metal oxides.
28.	Österbacka, Nicklas, 1993, et al. (författare) Influence of Oxygen Vacancies on the Structure of BiVO 4 2021 Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:2, s. 1200-1207 Tidskriftsartikel (refereegranskat)abstract We study oxygen vacancies in the tetragonal scheelite phase of bismuth vanadate and identify stable oxygen-deficient structures. Upon subjecting these to variable-cell optimization, we find that oxygen vacancies give rise to significant structural distortions, the degree of which exhibits a vacancy concentration dependence. Furthermore, we show that these distortions give rise to splitting of powder X-ray diffraction peaks, yielding patterns similar to that of the monoclinic scheelite phase, and that these effects are also present at finite temperatures. Our results highlight the need for characterization methods beyond X-ray diffraction for identifying the phase of synthesized bismuth vanadate samples and the importance of oxygen partial pressure control during synthesis.
29.	Österbacka, Nicklas, 1993, et al. (författare) Small Electron Polarons in CsPbBr₃: Competition between Electron Localization and Delocalization 2020 Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 32:19, s. 8393-8400 Tidskriftsartikel (refereegranskat)abstract We study the nature of excess electrons in CsPbBr3 and identify several single and double polaronic states. We emphasize the importance of proper inclusion of the self-interaction corrections for the stability of small electron polarons in this material. We demonstrate that spin-orbit coupling (SOC) has a significant impact on the energetics of the polaronic states. In particular, we find that SOC disfavors electron localization and leads to different polaronic geometries. Additionally, by carrying out thermodynamic integration, we show that small electron polarons are thermally stabilized in CsPbBr3. The small energy differences between the localized and delocalized electronic states could possibly reconcile the apparently conflicting properties of high charge-carrier mobilities and low recombinations rates.
30.	Österbacka, Nicklas, 1993, et al. (författare) Spontaneous Oxygen Vacancy Ionization Enhances Water Oxidation on BiVO 4 2024 Ingår i: ACS Energy Letters. - 2380-8195. ; 9:1, s. 153-158 Tidskriftsartikel (refereegranskat)abstract The influence of surface oxygen vacancies on the oxygen evolution reaction on bismuth vanadate is studied using hybrid density functional theory. Our findings reveal that the neutral charge state is thermodynamically unfavorable, leading to spontaneous ionization of VO0 into VO2+. By investigating the oxygen evolution reaction mechanism on both stoichiometric and oxygen-deficient surfaces, we find that surface oxygen vacancies reduce the reaction’s thermodynamic overpotential but only when the defects are ionized. Moreover, the reaction pathway involves the formation of surface-bound peroxide and superoxide ions as intermediates. Our work provides insight into the nature of surface oxygen vacancies and shines new light on how they may enhance the photoelectrochemical properties of semiconductors.

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