Browsing by Subject "Density functional theory (DFT)"

Now showing 1 - 5 of 5

Open Access
Engineering of the high-power laser-induced synthesis of Ni-based metal-organic framework: investigation of its optical properties, computational methodology, electrocatalytic performances, and glucose-sensing ability
(Wiley-VCH Verlag GmbH & Co. KGaA, 2024-10-16) Mutlu, Saliha; Ortaç, Bülend; Karatutlu, Ali; Görkan, Taylan; Durgun, Engin; Söyler, D.; Söylemez, S.; Arsu, N.; Savaşkan Yılmaz, Sevil
Metal-organic frameworks (MOFs) are porous materials with numerous chemical and structural possibilities. Due to their ease of modification, well-organized structure, and diverse guest molecule chemistry, MOFs are ideal platforms for uncovering improved functional material design characteristics. Quantitative analysis of glucose is crucial, especially in some food products, for quality control as well as evaluation of the glucose levels helps diagnose and treat diabetes. Recent glucose sensing devices have relied heavily on MOFs and other nanomaterials to enable user-friendly and safe non-invasive sensing methods. Nevertheless, the conventional synthesis methods involve multi-day reactions, cooling, and depressurization processes. This study demonstrates the unprecedented high-power laser-induced rapid synthesis (LIRS) of Ni-based MOF nanospheres with interconnected nano-rods and noncentrosymmetric primitive triclinic crystalline structure, highlighting their multifunctional usage in sensing and gas sorption applications. Ab initio simulations show excellent agreement with the experimental physical and gas sorption properties. Furthermore, the Ni-MOF-based biosensor accurately measures glucose real-life beverage samples, yielding promising glucose detection biosensor results with a low limit of the detection (LOD) of 13.96 µM and high sensitivity of 120.606 µA mM$^{−1}$ cm$^{−2}$.
Open Access
First principles study of 2D gallium nitride and aluminium nitride in square-octagon structure
(2017-07) Gürbüz, Emel
This thesis, deals with the planar free-standing, single-layer, square-octagon (SO) structures of GaN and AlN. We investigated single-layer and multilayer so-GaN and so-AlN structures, their stability, electronic properties and functionalization; adatom and vacancies using rst principles pseudopotential plane wave calculations. We performed an extensive analysis of dynamical and thermal stability in terms of ab-initio nite temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single layer square-octagon structures of GaN and AlN display directional mechanical properties, and have fundamental indirect band gaps, which are smaller than their hexagonal counter parts. These DFT band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain the fundamental band gaps decrease and can be closed. The energetics, binding and resulting electronic structure of bilayer, trilayer and 3D layered structures constructed by stacking of the single layers were examined. In contrast to the van der Waals solids, a signi cant chemical bonding between layers a ects the binding and transforms the planar geometry by inducing buckling. Depending on the stacking sequence and geometry, energetics, number of weak vertical bonds and direct band gap electronic structure display interesting variations promising a wide range of tunability. Furthermore, electronic and magnetic properties of these single-layer structures can be modi ed by adsorption of various adatoms, or by creating neutral cation-anion vacancies. As a future work, in-plane and vertical heterostructures of single layer so-GaN and so-AlN structures could be considered.
Open Access
Functionalization of group V monolayers and their compounds: alloying, doping and surface modification
(2020-11) Kanlı, Muammer
There has been growing interest during the last decade in two-dimensional (2D) materials due to their important roles in various scientific and technological applications such as detectors, lasers and light emitting diodes. In this thesis we present a theoretical investigation of a couple of such 2D materials from group V monolayers and their compounds. Firstly, ordered alloys of GaxAl1−xN hexagonal monolayer are studied and the effect of Al content on mechanical, electronic, thermal and optical properties are investigated. The optimized lattice constants and band gaps change in accordance to Vegard’s Law. Low barrier energies and favorable substitution of Ga by Al may show feasibility of fabrication. Segregation is also checked with mixing energy calculations. The dynamical stability of alloys is shown by phonon spectrum analysis and MD simulations. GaxAl1−xN alloys give lower in-plane stiffness than h-BN or graphene, but higher Poisson’s ratio than most realized 2D systems. Heat capacity of alloys delivers a decrease with Al content at low temperatures but it converges to the classical limit at high temperatures. The absorption onset of GaxAl1−xN alloys remain in the near UV range and prominent absorption peaks blue-shifts with increasing x in compliance with the variation of the band gap. The considered systems, in regard to their stability and tunable fundamental properties seem to be very promising 2D semiconductors for wide range of applications at reduced scales. Then, the interaction of alkali metal atoms (Li, Na, and K) with single layer and periodic structures of hb-As and sw-As phases are revealed by first-principles methods. Arsenene phases are considered to be used as electrodes (anode) for ion-batteries. Strong alkali-electrode binding and low diffusion energy barriers gives out better cycling stability and faster diffusion, respectively. hb-As shows better storage capacity than sw-As. However, deviations from ordered pattern and decline of formation energy with increasing doping level point out a possible structural transformation. By MD calculations, crystalline to amorphous phase transition is seen even for low concentrations level at ambient temperature. The average open-circuit voltages of 0.68-0.88 V (0.65-0.98 V) with specific capacity up to 715 mAhg−1 (358 mAhg−1) are calculated for single layer (periodic) configurations. Overall, non-crystalline phases are calculated to offer more favorable structures than crystalline configurations and they provide more coherent results when compared with experimental data. The obtained voltage profile together with low diffusion barriers and strong metal-electrode binding suggests arsenene as a promising anode material to be used in for alkali-ion battery applications. Lastly, the formation of dumbbell (DB) geometry upon adsorption of Ga, N adatoms to GaN monolayer is investigated. While Ga-N DBs are unstable, Ga-Ga and N-N DB geometries are predicted to form in an exothermic and spontaneous scheme. Cohesive energy of hexagonal GaN monolayer decreases when a DB is formed on its surface. Electronic structures for Ga-Ga DBs for 2×2, 3×3, 4×4 and 5×5 phases show spinpolarized and degenerate bands mainly contributed by p-orbitals of the atoms in impurity zone. Degenarated bands are not observed for N-N dumbbell for HDP, TDP, 2×2 and 3×3 phases. Upon DB formation, semiconductor GaN monolayer become spin-polarized semiconductor with varying band gap, where this functionalization allows electronic structure to be tuned substantionally. This would be highly desired for nanoscale electronic and optical devices. These Ga-Ga and N-N DB geometries may also be used for the synthesis of layered GaN structures.
Open Access
Structural and electronic properties of monolayer and multilayer gallium nitride crystals
(2016-09) Önen, Abdullatif
Three-dimensional (3D) Gallium Nitride (GaN) is a III-V compound semiconductor with direct band gap. It is widely used in light emitting diodes (LED) and has potential to be used numerous optoelectronic applications. In this thesis, rstly 3D GaN in wurtzite and zincblende structures are revisited and structural, mechanical, and electronic properties are studied and compared with the literature. Next, the mechanical and electronic properties of two-dimensional (2D) single-layer honeycomb structure of GaN (g-GaN), its bilayer, trilayer and multilayer van der Waals solids are investigated using density functional theory. Based on phonon spectrum analysis and high temperature ab initio molecular dynamics calculations, rst it is showed that g-GaN is stable and can preserve its geometry even at high temperatures. Then a comparative study is performed to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct band gap semiconductor, g-GaN in 2D has relatively wider indirect band gap. Moreover, 2D g-GaN displays higher Poisson's ratio and slightly less charge transfer from cation to anion. It is also showed that the physical properties predicted for freestanding g-GaN are preserved when g-GaN is grown on metallic, as well as semiconducting substrates. In particular, 3D layered blue phosphorus being nearly lattice matched to g-GaN is found to be an excellent substrate for growing g-GaN. Bilayer, trilayer and van der Waals crystals can be constructed by special stacking sequence of g-GaN and they can display electronic properties which can be controlled by the number of g-GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with increasing number of g-GaN layers. It is hoped that the present work will provide helpful insights for growing g-GaN which can be widely used in nanoelectronics applications in low dimensions.
Open Access
Theoretical simulations of UV-Vis and UP spectra for conjugated systems
(2009) Alkan, Fahri
Due to their unique electro-optical properties, there has been a great deal of scientific interest in electronic structure of conjugated systems. In order to reveal the complete map of their electronic structure, several experimental investigations are done using UV-Vis and ultraviolet photoelectron spectroscopy (UPS). The experimental findings are usually interpreted by the results of quantum chemical calculations. In this study, we present the theoretical simulations of UV-Vis and UP spectra of conjugated systems by using density functional theory (DFT). In UV-Vis simulations, we investigated the excited states of oligothiophene anions and cations and almost identical UV spectra were obtained for these systems. This similarity in excitation energies are explained by the resemblance in energy levels and nature of excited states in anions and cations. In UPS simulations, the energy levels of conjugated systems were calculated by using ∆SCF/TDDFT and DFT orbital eigenvalues. It is shown that there is a good agreement between ∆SCF/TDDFT and experiment, especially for the investigated oligomers. In contrast, DFT orbital energies are considerably lower than the experiment. However, spacing of energy levels is consistent with both experiment and ∆SCF/TDDFT.