| 1. |
- Gürbüz, Emel
(författare)
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Computational studies on 2D and 3D amorphous solids : Investigation of structure-property relationship
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Amorphous materials represent a large and diverse family, with many questions still remaining unanswered regarding their structure-property relationship. The structural complexity of these materials poses challenges for simulations in contrast to crystalline materials where density functional theory (DFT) can be used easily by exploiting the translation symmetry. The use of DFT often becomes intractable due to the large system sizes required for simulating amorphous materials. In this thesis, amorphous metallic glasses and 2D materials were comprehensively investigated through the combined application of classical molecular dynamics simulations (CMD) and DFT. Glassy structures were successfully generated followed by the study of thermal and vibrational properties using CMD, while DFT was used to explore their electronic, magnetic, and optical characteristics. The results demonstrated that nanostructured voids in metallic glasses can decrease the lattice thermal conductivity. The relationships between structure, local orderings, and material properties such as magnetism and superconductivity are also examined and analyzed in this thesis. Furthermore, the unique and intriguing structure-dependent properties in low-dimensional (2D) and layered structures have been comprehensively studied that could be potentially used in future functional devices. In summary, this thesis opens up a number of avenues to explore functional amorphous materials for various technological applications.
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| 2. |
- Gürbüz, Emel, et al.
(författare)
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First-principles prediction of energy band gaps in 18-valence electron semiconducting half-Heusler compounds : Exploring the role of exchange and correlation
- 2023
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 134:20
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Tidskriftsartikel (refereegranskat)abstract
- The choice of exchange functional is a critical factor in determining the energy bandgap of semiconductors. Ab initio calculations using different exchange functionals, including the conventional generalized-gradient approximation (GGA) functionals, meta-GGA functionals, and hybrid functionals, show significant differences in the calculated energy bandgap for semiconducting half-Heusler compounds. These compounds, which have 18 valence electrons per unit cell, are of great interest due to their thermoelectric properties, making them suitable for energy conversion applications. In addition, accounting for electronic correlations using the GW method also affects the calculated energy bandgaps compared to standard GGA calculations. The variations in calculated energy bandgaps are specific to each material when using different functionals. Hence, a detailed investigation of the electronic properties of each compound is necessary to determine the most appropriate functional for an accurate description of the electronic properties. Our results indicate that no general rules can be established and a comparison with experimental results is required to determine the most appropriate functional.
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| 3. |
- Gürbüz, Emel, et al.
(författare)
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Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds : Modified Slater-Pauling rule for half-metallicity at the interface
- 2023
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 7:5
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Tidskriftsartikel (refereegranskat)abstract
- Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.
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| 4. |
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| 5. |
- Gürbüz, Emel, et al.
(författare)
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Systematic study of amorphous 2D graphene, silicene and silicon carbide: Investigation of structural, electronic, optical and vibrational properties
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Tidskriftsartikel (refereegranskat)abstract
- Although two dimensional (2D) solids in their crystalline form and their van der Waals (vdW)heterostructures have generated a lot of attention in recent years, the exploration of their amorphous forms has not been done to a great extent. Here, we present a detailed analysis of structural,electronic and thermal properties of 2D amorphous graphene(A-Gra), silicene(A-Si) and siliconcarbide(A-SiC) using Classical Molecular Dynamics (MD) simulations for structure generation, stability tests, thermal conductivity and vibrational analysis while we use first-principles density functional theory based calculations for the calculations of electronic structure and charge distributions.It is found that A-Gra is planar at 0 K while it gets wrinkled at 300 K similar to its crystallinecounterpart. At both temperatures, A-Gra can form stable vdW solids. A-Gra is found to bemetallic with a thermal conductivity around 55.30 W/K.m. We also observe that A-Gra can absorblight from IR to UV range and has plasmon peaks in UV range but red-shifted compared to thecrystalline graphene. A-Si prefers to create covalent bonds in layered structures while it keeps itsmetallicity. A-Si’s thermal conductivity is calculated as 2.68 W/Km which is the same value foramorphous one dimensional Si nanowire. A-Si’s optical absorption range is found from IR to UV,but red-shifted relative to crystalline silicene’s perpendicular components, whereas blue-shifted inparallel polarization relative to crystalline silicene. Its plasmon peak is found in UV range with 6times higher intensity compared to crystalline silicene. We also demonstrate that two dimensionalsilicon carbide can be stabilized not only as a single layer but also in layered structures with bondingbetween Si atoms only. A-SiC is metallic and its thermal conductivity is found to be 70.29 W/K.m,higher than A-Gra. Its absorption range is from IR to UV with a red-shifted spectrum comparedto its crystalline counterpart and plasmon peak residing in UV range. Another important finding isthat except A-Gra, A-Si and A-SiC has no local vibrational modes, and all three structures’ commonheat carriers are extendons, especially diffusions. Finally, the uneven charge distributions aroundthe local ring structures in all three systems can be exploited in future electronic, thermo-electric, opto-electronic devices with their local functionalility with the advantage of their metallicity andlow thermal conductivity.
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| 6. |
- Gürbüz, Emel, et al.
(författare)
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Tuning of lattice thermal conductivity of amorphous Fe0.85Zr0.15 by nanostructured voids, pressure and temperature
- 2023
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Ingår i: Journal of Non-Crystalline Solids. - : Elsevier. - 0022-3093 .- 1873-4812. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- Metallic glasses are known as one of the favorable amorphous materials with their remarkable stiffness, durability and low thermal conductivities. Furthermore, manipulation of lattice thermal conductivity by forming nanostructures is an important route to enhance the performance for several industrial applications. Here, equilibrium molecular dynamics simulations were performed for the generation of metallic glass (MG) Fe0.85Zr0.15 followed by the calculation of lattice thermal conductivity by Green–Kubo method. The amorphicity of our simulated MG is verified through the analysis of radial distribution functions and Voronoi tessellations. Nanostructured spherical voids with varying sphere radii were introduced. We have found that with increasing porosity, the lattice thermal conductivity decreases. We have also studied the dependence of temperature and pressure on thermal conductivity. Finally, our analysis of calculated vibrational densities of states shows interesting behavior of extended and localized modes in various situations of pressure, temperature and size of the nanostructured voids.
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| 7. |
- Li, Hu, 1986-, et al.
(författare)
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Observation of defect density dependent elastic modulus of graphene
- 2023
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Ingår i: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 123:5
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Tidskriftsartikel (refereegranskat)abstract
- The recent decade has witnessed a tremendous development of graphene applications in many fields; however, as one of the key considerations, the mechanical properties of graphene still remain largely unexplored. Herein, by employing focused ion beam irradiation, graphene with various defect levels is obtained and further investigated by using Raman spectroscopy and scanning tunneling microscopy. Specially, our atomic force microscopy based nanomechanical property measurement demonstrates a clear defect density dependent behavior in the elastic modulus of graphene on a substrate as the defect density is higher than a threshold value of 1012 cm−2, where a clear decay is observed in the stiffness of graphene. This defect density dependence is mainly attributed to the appearance of amorphous graphene, which is further confirmed with our molecular dynamics calculations. Therefore, our reported result provides an essential guidance to enable the rational design of graphene materials in nanodevices, especially from the perspective of mechanical properties.
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| 8. |
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| 9. |
- Rani, Parul, et al.
(författare)
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Structural and magnetic properties of amorphous CoxZr100-x films
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:13
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Tidskriftsartikel (refereegranskat)abstract
- Magnetometry and first-principles density functional theory-based calculations were used to investigate the concentration dependence of magnetic properties of amorphous CoxZr100-x alloys. A linear increase in saturation magnetization (Ms) is observed in both the experiments and calculations. Samples with Co content of at least 62 at.% are inferred to be ferromagnetic at 5 K. The experimentally determined ordering temperature is found to scale quadratically with Co concentration indicating a complex interplay between local structural motifs and magnetic parameters such as exchange interaction and anisotropy.
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| 10. |
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