| 1. |
- Alekseev, A. Yu, et al.
(författare)
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Curvature-induced effects in semiconducting alkaline-earth metal silicide nanotubes
- 2021
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier BV. - 1386-9477 .- 1873-1759. ; 128
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Tidskriftsartikel (refereegranskat)abstract
- By means of ab-initio techniques we have investigated changes in the structure and electronic properties of alkaline-earth metal silicide (Ca2Si, Mg2Si and MgCaSi) nanotubes caused by the curvature-induced effects. It is revealed that the curvature-induced effects can: 1) stabilize Mg2Si nanotubes in a phase, which is metastable for the parent 2D Mg2Si; 2) lead to an energy gain as a result of 2D to nanotube structural transformation in the case of ternary MgCaSi nanotubes; 3) modify the band dispersion and band gaps for nanotubes with the diameters less than 30 angstrom. In addition, Mg2Si and MgCaSi nanotubes are found to be direct band-gap (0.5-1.2 eV) materials with appreciable oscillator strength of the first direct transitions.
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| 2. |
- Alekseev, A. Yu, et al.
(författare)
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Interplay between structural changes, surface states and quantum confinement effects in semiconducting Mg2Si and Ca2Si thin films
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 25:29, s. 19952-19962
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio techniques have been used to investigate structural changes in semiconducting Mg2Si and Ca2Si thin films (from 17 nm down to 0.2 nm corresponding to the 2D structure) along with band-gap variations due to quantum confinement. Cubic Mg2Si(111) thin films being dynamically stable at thicknesses (d) larger than 0.3 nm displayed an indirect band gap, the reduction of which with increasing d could be reasonably well described by the simple effective mass approximation. Only 2D Mg2Si has a unique structure because of the orthorhombic distortion and the direct band gap. Since the surface energy of cubic Ca2Si(111) films was lower with respect to any surface of the orthorhombic phase, which is the ground state for the Ca2Si bulk, the metastable in-bulk cubic phase in the form of thin films turned out to be preferable in total energy than any orthorhombic Ca2Si thin film for d < 3 nm. Sizable structural distortion and the appearance of surface states in the gap region of Ca2Si thin films with d < 3 nm could be the reason for an odd dependence of the band-gap variation on d.
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| 3. |
- Alekseev, A. Y., et al.
(författare)
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Structural stability and electronic properties of 2D alkaline-earth metal silicides, germanides, and stannides
- 2020
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Ingår i: Japanese Journal of Applied Physics. - : IOP PUBLISHING LTD. - 0021-4922 .- 1347-4065. ; 59
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Forskningsöversikt (refereegranskat)abstract
- We present the results of an extended theoretical study of the structure, phonon, electronic and optical properties of 2D alkaline-earth metal silicides, germanides and stannides (2D Me2X, where Me=Mg, Ca, Sr, Ba and X=Si, Ge, Sn). The performed analysis has shown the occurrence of the pseudo passivation effect and ionic chemical bonding in these 2D Me2X. In addition, the preformed investigation of their phonon spectra has shown the absence of imaginary frequencies indicating the stability of these 2D structures. The band structure calculations performed using the hybrid functional have revealed that all 2D Me2X are semiconductors with the gap varying from 0.12 to 1.01 eV. Among them Mg- and Ca-based 2D materials are direct band-gap semiconductors with the first direct transition having appreciable oscillator strength. We also propose to consider ternary 2D silicides, germanides and stannides with different Me atoms as a feasible way to modify properties of parent 2D Me2X.
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| 4. |
- Baljozovic, Milos, et al.
(författare)
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Self-Assembly and Magnetic Order of Bi-Molecular 2D Spin Lattices of M(II,III) Phthalocyanines on Au(111)
- 2021
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Ingår i: MAGNETOCHEMISTRY. - : MDPI. - 2312-7481. ; 7:8
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Tidskriftsartikel (refereegranskat)abstract
- Single layer low-dimensional materials are presently of emerging interest, including in the context of magnetism. In the present report, on-surface supramolecular architecturing was further developed and employed to create surface supported two-dimensional binary spin arrays on atomically clean non-magnetic Au(111). By chemical programming of the modules, different checkerboards were produced combining phthalocyanines containing metals of different oxidation and spin states, diamagnetic zinc, and a metal-free 'spacer'. In an in-depth, spectro-microscopy and theoretical account, we correlate the structure and the magnetic properties of these tunable systems and discuss the emergence of 2D Kondo magnetism from the spin-bearing components and via the physico-chemical bonding to the underlying substrate. The contributions of the individual elements, as well as the role of the electronic surface state in the bottom substrate, are discussed, also looking towards further in-depth investigations.
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| 5. |
- Batalovic, K., et al.
(författare)
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Origin of photocatalytic activity enhancement in Pd/Pt-deposited anatase N-TiO2 – experimental insights and DFT study of the (001) surface
- 2020
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 22:33, s. 18536-18547
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Tidskriftsartikel (refereegranskat)abstract
- In pursuit of the ideal photocatalyst, cheap and stable semiconductor TiO(2)is considered to be a good choice if one is able to reduce its band gap and decrease the recombination rate of charge carriers. The approach that offers such improvements for energy conversion applications is the modification of TiO(2)with nitrogen and noble metals. However, the origin of these improvements and possibilities for further design of single-atom catalysts are not always straightforward. To shed light on the atomic-scale picture, we modeled the nitrogen-doped (001) anatase TiO(2)surface as a support for palladium and platinum single-atom deposition. The thermodynamics of various synthesis routes for Pd/Pt deposition and nitrogen doping is considered based on density functional theory (DFT)-calculated energies, highlighting the effect of nitrogen doping on metal dimer formation and metal-support interaction. XPS analysis of the valence band of the modified TiO(2)nanocrystals, and the calculated charge transfer and electronic structure of single-atom catalysts supported on the (001) anatase TiO(2)surface provide an insight into modifications occurring in the valence zone of TiO(2)due to nitrogen doping and Pd/Pt deposition at the surface. DFT results also show that substitutional nitrogen doping significantly increases metal-support interaction, while interstitial nitrogen doping promotes only Pt-support interaction.
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| 6. |
- Cha, Gihoon, et al.
(författare)
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As a single atom Pd outperforms Pt as the most active co-catalyst for photocatalytic H-2 evolution
- 2021
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Ingår i: ISCIENCE. - : Elsevier BV. - 2589-0042. ; 24:8
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Tidskriftsartikel (refereegranskat)abstract
- Here, we evaluate three different noble metal co-catalysts (Pd, Pt, and Au) that are present as single atoms (SAs) on the classic benchmark photocatalyst, TiO2. To trap the single atoms on the surface, we introduced controlled surface vacancies (Ti3+-Ov) on anatase TiO2 nanosheets by a thermal reduction treatment. After anchoring identical loadings of single atoms of Pd, Pt, and Au, we measure the photocatalytic H-2 generation rate and compare it to the classic nanoparticle co-catalysts on the nanosheets. While nanoparticles yield the well-established the hydrogen evolution reaction activity sequence (Pt > Pd > Au), for the single atom form, Pd radically outperforms Pt and Au. Based on density functional theory (DFT), we ascribe this unusual photocatalytic co-catalyst sequence to the nature of the charge localization on the noble metal SAs embedded in the TiO2 surface.
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| 7. |
- Chesnokov, A., et al.
(författare)
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The local atomic structure and thermoelectric properties of Ir-doped ZnO : hybrid DFT calculations and XAS experiments
- 2021
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry. - 2050-7526 .- 2050-7534. ; 9:14, s. 4948-4960
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Tidskriftsartikel (refereegranskat)abstract
- We combined the hybrid density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) experiments in the study of the local atomic structure around Ir ions in ZnO thin films with different iridium content. This was then used in the first principles analysis of the thermoelectric properties of material. The emphasis has been put on the conditions for a positive Seebeck coefficient and p-type electrical conductivity as the functions of the Fermi level. We studied both computationally and experimentally several possible IrOxpolyhedra (complexes) with a different number of surrounding oxygens and Ir oxidation states, including those with the formation of peroxide ions (O22−). In particular, octahedral coordination of iridium ions was identified by reverse Monte Carlo (RMC) simulations of the Ir L3-edge EXAFS spectra of ZnO:Ir thin films as the predominant complex, which is supported by the calculated lowest interstitial oxygen incorporation energies. All the calculated IrOx(x= 4, 5, 6) complexes, regardless of Ir the oxidation state, demonstrate potential for p-type conduction if the Fermi level (μF) falls in the range of 0-0.8 eV from the valence band maximum (VBM) and the Ir concentration is high enough (12.5% in the present DFT calculations). Even though the corresponding calculated Seebeck coefficient (S) around 80-89 μV K−1slightly exceeds the experimental values, we emphasise the presence of an important plateau in the dependence ofSonμFin this range for two complexes with the formation of peroxide ions (O22−). We predicted also that peroxide ions O22−are characterized by the calculated phonon frequencies of 810-942 cm−1in agreement with our previous Raman experimental results. In this light, we discuss the high sensitivity of calculatedS(μF) dependences to the atomic and electronic structure.
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| 8. |
- Denisov, Nikita, et al.
(författare)
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Light-Induced Agglomeration of Single-Atom Platinum in Photocatalysis
- 2023
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 35:5
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Tidskriftsartikel (refereegranskat)abstract
- With recent advances in the field of single-atoms (SAs) used in photocatalysis, an unprecedented performance of atomically dispersed co-catalysts has been achieved. However, the stability and agglomeration of SA co-catalysts on the semiconductor surface may represent a critical issue in potential applications. Here, the photoinduced destabilization of Pt SAs on the benchmark photocatalyst, TiO2, is described. In aqueous solutions within illumination timescales ranging from few minutes to several hours, light-induced agglomeration of Pt SAs to ensembles (dimers, multimers) and finally nanoparticles takes place. The kinetics critically depends on the presence of sacrificial hole scavengers and the used light intensity. Density-functional theory calculations attribute the light induced destabilization of the SA Pt species to binding of surface-coordinated Pt with solution-hydrogen (adsorbed H atoms), which consequently weakens the Pt SA bonding to the TiO2 surface. Despite the gradual aggregation of Pt SAs into surface clusters and their overall reduction to metallic state, which involves >90% of Pt SAs, the overall photocatalytic H2 evolution remains virtually unaffected.
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| 9. |
- Dobrota, Ana S., et al.
(författare)
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Altering the reactivity of pristine, N- and P-doped graphene by strain engineering : A DFT view on energy related aspects
- 2020
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Ingår i: Applied Surface Science. - : ELSEVIER. - 0169-4332 .- 1873-5584. ; 514
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Tidskriftsartikel (refereegranskat)abstract
- For carbon-based materials, in contrast to metal surfaces, a general relationship between strain and reactivity is not yet established, even though there are literature reports on strained graphene. Knowledge of such relationships would be extremely beneficial for understanding the reactivity of graphene-based surfaces and finding optimisation strategies which would make these materials more suitable for targeted applications. Here we investigate the effects of compressive and tensile strain (up to +/- 5%) on the structure, electronic properties and reactivity of pure, N-doped and P-doped graphene, using DFT calculations. We demonstrate the possibility of tuning the topology of the graphene surface by strain, as well as by the choice of the dopant atom. The reactivity of (doped) strained graphene is probed using H and Na as simple adsorbates of great practical importance. Strain can both enhance and weaken H and Na adsorption on (doped) graphene. In case of Na adsorption, a linear relationship is observed between the Na adsorption energy on P-doped graphene and the phosphorus charge. A linear relationship between the Na adsorption energy on flat graphene surfaces and strain is found. Based on the adsorption energies and electrical conductivity, potentially good candidates for hydrogen storage and sodiumion battery electrodes are discussed.
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| 10. |
- Dobrota, Ana S., et al.
(författare)
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First-principles analysis of aluminium interaction with nitrogen-doped graphene nanoribbons : From adatom bonding to various
- 2022
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Ingår i: Materials Today Communications. - : Elsevier BV. - 2352-4928. ; 31
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Tidskriftsartikel (refereegranskat)abstract
- Enhancing aluminium interaction with graphene-based materials is of crucial importance for the development of Al-storage materials and novel functional materials via atomically precise doping. Here, DFT calculations are employed to investigate Al interactions with non-doped and N-doped graphene nanoribbons (GNRs) and address the impact of the edge sites and N-containing defects on the material's reactivity towards Al. The presence of edges does not influence the energetics of Al adsorption significantly (compared to pristine graphene sheet). On the other hand, N-doping of graphene nanoribbons is found to affect the adsorption energy of Al to an extent that strongly depends on the type of N-containing defect. The introduction of edge-NO group and doping with in -plane pyridinic N result in Al adsorption nearly twice as strong as on pristine graphene. Moreover, double n-type doping via N and Al significantly alters the electronic structure of Al,N-containing GNRs. Our results suggest that selectively doped GNRs with pyridinic N can have enhanced Al-storage capacity and could be potentially used for selective Al electrosorption and removal. On the other hand, Al,N-containing GNRs with pyridinic N could also be used in resistive sensors for mechanical deformation. Namely, strain along the longitudinal axis of these dual doped GNRs does not affect the binding of Al but tunes the bandgap and causes more than 700-fold change in the conductivity. Thus, careful defect engineering and selective doping of GNRs with N (and Al) could lead to novel multifunctional materials with exceptional properties. [GRAPHICS]
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