Browsing by Subject "First principles"

Now showing 1 - 5 of 5

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
High conducting nanowires obtained from uniform titanium covered carbon nanotubes
(2005) Daǧ, Sefa; Durgun, Engin; Çıracı, Salim
We have shown that Ti atoms can form continuous coating of carbon nanotubes at different amount of coverage. The circular cross section changes to a square-like form, and the semiconducting tube becomes ferromagnetic metal with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band gap closing of SWNT at the corners of the square. The magnetic properties of Ti coated tubes depend strongly on the geometry, amount of Ti coverage and also on the elastic deformation of the tube. While the magnetic moment can be pronounced significantly by the positive axial strain, it can decrease dramatically upon the adsorption of additional Ti atoms to those already covering the nanotube. Besides, electronic structure and spin-polarization near the Fermi level can also be modified by radial strain. Our results have been obtained by the first-principles, spin-relaxed pseudopotential plane wave calculations within the density functional theory. © TÜBİTAK.
Open Access
Spintronic properties of carbon and silicon based nanostructures
(2007) Durgun, Engin
In this thesis, nanostructures which may display novel spintronic behaviors are revealed and their properties are investigated by using first-principles methods. We have concentrated on three different systems, namely carbon linear chains, singe-wall carbon nanotubes and silicon nanowires. First of all, an extensive study of the electronic, magnetic and transport properties of finite and infiniteperiodic atomic chains composed of carbon atoms and 3d transition metal (TM) atoms are carried out. Finite-size, linear molecules made of carbon atomic chains caped with TM atoms, i.e. TM-Cn-TM structures are found to be stable and exhibit interesting magnetoresistive properties. The indirect exchange interaction of the two TM atoms through a spacer of n carbon atoms determines the type of the magnetic ground state of these structures. The n-dependent variations of the ground state between ferromagnetic (F) and antiferromagnetic (AF) spin configurations exhibit several distinct features, including regular alternations and irregular forms. We present a simple analytical model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. The periodically repeated TM-Cn atomic chains exhibit half-metallic properties with perfect spin polarization at the Fermi level (EF ). When connected to appropriate electrodes the TM-Cn-TM atomic chains act as molecular spin-valves in their F states due to the large ratios of the conductance values for each spin type. Secondly, a systematic study of the electronic and magnetic properties of TM atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs) is presented. The adsorption on the external and internal wall of SWNT is considered and the effect of the TM coverage and geometry on the binding energy and the spin polarization at EF is examined. All those adsorbed chains studied have F ground state, but only their specific types and geometries demonstrated high spin polarization near EF . Their magnetic moment and binding energy in the ground state display interesting variation with the number of d−electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic metal electrodes, these magnetic needles or nanomagnets can perform as spindependent resonant tunnelling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Finally, bare, hydrogen terminated and TM adsorbed Silicon nanowires (SiNW) oriented along [001] direction are investigated. An extensive analysis on the atomic structure, stability, elastic and electronic properties of bare and hydrogen terminated SiNWs is performed. It is then predicted that specific TM adsorbed SiNWs have a half-metallic ground state even above room temperature. At high coverage of TM atoms, ferromagnetic SiNWs become metallic for both spin-directions with high magnetic moment and may have also significant spin-polarization at EF . The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Most of these systems studied in this thesis appear to be stable at room temperature and promising for spintronic devices which can operate at ambient conditions. Therefore, we believe that present results are not only of academic interest, but also can initiate new research on spintronic applications of nanostructures.
Open Access
Titanium dioxide nanostructures for photocatalytic and photovoltaic applications
(2008) Çakır, Deniz
In this thesis, TiO2 nanostructures and their photocatalytic and photovoltaic ap- plications have been investigated by using the ¯rst-principles calculations based on density functional theory. We have concentrated on three di®erent systems, namely TiO2 clusters, nanowires and surfaces. TiO2 is widely used in various applications, since it is chemically stable in di®erent conditions, ¯rm under il- lumination, non toxic, and relatively easy and cheap to produce. Most of the technological applications such as photovoltaic and photocatalytic of TiO2 are mainly related to its optical properties. First of all, structural, electronic, and magnetic properties of small (TiO2)n (n=1{10) clusters have been studied. Various initial geometries for each n have been searched to ¯nd out the ground state geometries. In general, it has been found that the ground state structures (for n=1{9) have at least one dangling or pendant O atom. Only the lowest lying structure of n=10 cluster does not have any pendant O atom. In the ground state structures, Ti atoms are at least 4{fold coordinated for n ¸ 4. Clusters prefer to form three{dimensional and compact structures. All clusters have singlet ground state. The formation energy and HOMO{LUMO gap have also been calculated as a function of the number of TiO2 unit to study the stability and electronic properties. The formation energy increases with increasing size of the cluster. This means that clusters become stronger as their size grows. The interaction of the ground state structure of each (TiO2)n cluster with H2O has been investigated. The binding energy Eb of H2O molecule decreases and oscillates as the cluster size increases. The interaction of the ground state structure of n=3, 4, 10 clusters with more than one H2O molecule has also been studied. We have calculated Eb per adsorbed molecule and we have shown that it decreases with increasing number of adsorbed H2Omolecule (N). When N ¸ 2 for n=3 and N ¸3 for n=4 clusters, H2O molecules bind more strongly to n=10 cluster. The adsorption of transition metal (TM) atoms such as V, Co, and Pt on n=10 cluster has been studied as well. All these elements interact with the cluster forming strong chemisorption bonds, and the permanent magnetic moment is induced upon the adsorption of Co or V atoms. Second of all, structural, electronic and magnetic properties of very thin TiOx (x=1,2) nanowires have been presented. All stoichiometric TiO2 nanowires ex- hibit semiconducting behavior and have non{magnetic ground state. There is a correlation between binding energy (Eb) and the energy band gap (Eg) of these TiO2 nanowires. In general, Eb increases with Eg. In non-stoichiometric TiO nanowires, we have both metallic and semiconducting nanowires. In addition to non{magnetic TiO nanowires, we have also ferromagnetic nanowires. Three{ dimensional (3D) structures are more energetic than planar ones for both stoi- chiometries. The stability of TiOx nanowires is enhanced by the increase of the size and coordination number of Ti and O atoms which tend to possess at least four and two nearest neighbors, respectively. We have also studied the structural and electronic properties of rutile (110) nanowires obtained by cutting bulk ru- tile along the [110] direction with a certain cross section. The bulk nanowires are more energetic than the thin nanowires after a certain diameter. Like thin TiO2 nanowires, all bulk wires are semiconducting and Eg oscillates with the cross section of these (110) nanowires. Third of all, we have studied the interaction of perylenediimide (PDI){based dye compounds (BrPDI, BrGly, and BrAsp) with both the unreconstructed (UR) and reconstructed (RC) anatase TiO2 (001) surfaces. All dye molecules form strong chemical bonds with the surface in the most favorable adsorption struc- tures. The lowest binding energy is 2.60 eV which has been obtained in the adsorption of BrPDI dye on the UR surface. In UR{BrGly, RC{BrGly and RC{ BrAsp cases, we have observed that HOMO and LUMO levels of the adsorbed molecules appear within the band gap and conduction band regions, respectively. Moreover, we have obtained a gap narrowing upon adsorption of BrPDI on the RC surface. Because of the reduction in the e®ective band gap of the surface{dye system and possibly achieved the visible light activity, these results are valuable for photovoltaic and photocatalytic applications. We have also considered the e®ects of the hydration of surface on the binding of BrPDI. It has been found that the binding energy drops signi¯cantly for the completely hydrated surfaces.Fourth of all, we have considered the interaction of BrPDI, BrGly, and BrAsp dye molecules with defect free rutile TiO2 (110) surfaces. All dye molecules form moderate chemical bonds with surface in the most stable adsorption structures. The average binding energy of dye molecules is about 1 eV. Regardless of the type of dye molecules, HOMO and LUMO levels of the adsorbed dye appear within the gap and the conduction band region of defect free surface, respectively. The e®ect of the slab thickness on the interaction strength between the dye and the surface has also been examined. Unlike the four layers slab, BrGly and BrAsp molecules are dissociatively adsorbed on the three layers slab. The interaction between BrPDI and partially reduced rutile (110) as well as platinized surface has been also considered in order to ¯gure out the e®ects of O vacancy and preadsorbed small Ptn (n=1, 3 and 5 ) clusters on the binding, electronic, and structural properties of the dye{surface system. It has been found that BrPDI dye prefers to bind to the O vacancy site for the partially reduced surface case. Transition metal deposition on metal oxides plays a crucial role in various industrial areas such as catalysts and photovoltaic cells. Finally, an extensive study of the adsorption of small Ptn (n=1{8) and bimetallic Pt2Aum (m=1{5) clusters on partially reduced rutile TiO2 (110) has been presented. The e®ect of surface O vacancies on the adsorption and growth of Pt and bimetallic Pt{Au clusters over the defective site of the 4£2 rutile surface has been studied. Struc- tures, energetics and electronic properties of adsorbed Ptn and Pt2Aum clusters have been analyzed. The surface O vacancy site has been found to be the most active site for a single Pt monomer. Other Pt clusters, namely dimer, trimer and so on, tend to grow around this monomer. As a result, O vacancy site behaves as a nucleation center for the clustering of Pt atoms. Small Pt clusters interact strongly with the partially reduced surface. Eb per adsorbed Pt atom is 3.38 eV for Pt1 case and Eb increases as the cluster size grows due to the formation of strong Pt{Pt bonds. Pt clusters prefer to form planar structures for n = 1{6 cases. The calculated partial density of states of Ptn{TiO2 surface has revealed that the surface becomes metallic when n ¸ 3. In the case of bimetallic Pt-Au clusters, Aum clusters have been grown on the Pt2{TiO2 surface. Previously ad- sorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. This Pt2 cluster also inhibits sintering of the Au clusters on the surface. The interaction between the adsorbed Au atoms and titania surface as well as previously adsorbed Pt dimer is weak compared to Pt{TiO2 surface interactions. Since charge state of the clusters adsorbed on the oxide surfaces iscrucial for catalysis applications of these clusters, total charge on each atom of the metal clusters has also been calculated. Charge transfer among the cluster atoms and underlying oxide surface is more pronounced for Ptn clusters. Furthermore, the absolute value of total charge on the clusters is greater for Ptn than that of bimetallic Pt{Au case.