Browsing by Subject "Electronic structure"

Now showing 1 - 20 of 71

Open Access
Ab initio study of hydrogenic effective mass impurities in Si nanowires
(Institute of Physics Publishing, 2017-01) Peelaers, H.; Durgun, Engin; Partoens, B.; Bilc, D. I.; Ghosez, P.; Van De Walle C. G.; Peeters, F. M.
The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations.
Open Access
Ab-initio electron transport calculations of carbon based string structures
(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, Salim
The new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.
Open Access
An ab initio study of vertical heterostructures formed by CdO and SnC monolayers
(Elsevier, 2024-01-30) Seyedmohammadzadeh, Mahsa; Mobaraki, Arash; Tanatar, B.; Gülseren, Oğuz
Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.
Open Access
Anatase TiO2 nanowires functionalized by organic sensitizers for solar cells: a screened Coulomb hybrid density functional study
(American Institute of Physics Inc., 2015) Ünal, H.; Gunceler, D.; Gülseren, O.; Ellialtıoğlu, S.; Mete, E.
The adsorption of two different organic molecules cyanidin glucoside (C21O11H20) and TA-St-CA on anatase (101) and (001) nanowires has been investigated using the standard and the range separated hybrid density functional theory calculations. The electronic structures and optical spectra of resulting dye-nanowire combined systems show distinct features for these types of photochromophores. The lowest unoccupied molecular orbital of the natural dye cyanidin glucoside is located below the conduction band of the semiconductor while, in the case of TA-St-CA, it resonates with the states inside the conduction band. The wide-bandgap anatase nanowires can be functionalized for solar cells through electron-hole generation and subsequent charge injection by these dye sensitizers. The intermolecular charge transfer character of Donor-π-Acceptor type dye TA-St-CA is substantially modified by its adsorption on TiO2 surfaces. Cyanidin glucoside exhibits relatively stronger anchoring on the nanowires through its hydroxyl groups. The atomic structures of dye-nanowire systems re-optimized with the inclusion of nonlinear solvation effects showed that the binding strengths of both dyes remain moderate even in ionic solutions.
Open Access
Atomic and electronic structure of carbon strings
(IOP Publishing Ltd., 2005) Tongay, S.; Dag, S.; Durgun, Engin; Senger, R. T.; Çıracı, Salim
This paper presents an extensive study of various string and tubular structures formed by carbon atomic chains. Our study is based on first-principles pseudopotential plane wave and finite-temperature ab initio molecular dynamics calculations. Infinite- and finite-length carbon chains exhibit unusual mechanical and electronic properties such as large cohesive energy, axial strength, high conductance, and overall structural stability even at high temperatures. They are suitable for structural and chemical functionalizations. Owing to their flexibility and reactivity they can form linear chain, ring, helix, two-dimensional rectangular and honeycomb grids, three-dimensional cubic networks, and tubular structures. Metal-semiconductor heterostructures and various quantum structures, such as multiple quantum wells and double-barrier resonant tunnelling structures, can be formed from the junctions of metallic carbon and semiconducting BN linear chains. Analysis of atomic and electronic structures of these periodic, finite, and doped structures reveals fundamentally and technologically interesting features, such as structural instabilities and chiral currents. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies the chemical, mechanical, and electronic properties.
Open Access
Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations
(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, Ekmel
In present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.
Open Access
CdSe/CdSe1-xTex core/crown heteronanoplatelets: tuning the excitonic properties without changing the thickness
(American Chemical Society, 2017) Kelestemur Y.; Guzelturk, B.; Erdem, O.; Olutas M.; Erdem, T.; Usanmaz, C. F.; Gungor K.; Demir, Hilmi Volkan
Here we designed and synthesized CdSe/CdSe1-xTex core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe1-xTex crown region from pure CdTe (x = 1.00) to almost pure CdSe doped with several Te atoms (x = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of ∼570 nm (for CdSe1-xTex crown with x = 0.02) and ∼660 nm (for CdSe1-xTex crown with x = 1.00). Furthermore, with the formation of the CdSe1-xTex crown region, we observed substantially improved photoluminescence quantum yields (up to ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up to ∼326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and -harvesting applications.
Open Access
Characterization of platinum nitride from first-principles calculations
(Institute of Physics Publishing, 2009) Yıldız, A.; Akıncı, Ü.; Gülseren, O.; Sökmen, İ.
We have performed a systematic study of the ground state properties of the zinc-blende, rock-salt, tetragonal, cuprite, fluorite and pyrite phases of platinum nitride by using the plane wave pseudopotential calculations within the density functional theory. The equilibrium structural parameters and bulk moduli are computed within both the local density approximation (LDA) and generalized gradient approximation (GGA). The comparison of the equation of state (EOS) calculated within the LDA for the pyrite structure with the experimental results demonstrates an excellent agreement, hence the use of the LDA rather than the GGA is essential. Complete sets of elastic moduli are presented for cubic forms. The analysis of the results reveal that the pyrite phase with PtN2 stoichiometry leads to the formation of a hard material with the shear modulus G = 206 GPa. The electronic structure of pyrite PtN2 is given, which shows a narrow indirect gap. The vibrational properties of platinum nitride are investigated in detail from lattice dynamical calculations. The calculations show that fluorite and pyrite structures are dynamically stable as well. However, the calculated vibrational modes of pyrite PtN2 do not show complete agreement with experimental Raman frequencies.
Open Access
Correlation effects in a one-dimensional electron gas with short-range interaction
(Pergamon Press, 1999) Demirel, E.; Tanatar, Bilal
We study the correlation effects in a one-dimensional electron gas with repulsive delta-function interaction. The correlation effects are described by a local-field correction which takes into account the short-range correlations. We find that the ground state energy is in good agreement with the exact result up to intermediate coupling strengths, showing an improvement over the STLS approximation. The compressibility, the static structure factor and the pair-correlation function are also calculated within the present approximation.
Open Access
Deformed octagon-hexagon-square structure of group-IV and group-V elements and III-V compounds
(American Physical Society, 2019) Görkan, T.; Aktürk, E.; Çıracı, Salim
We report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.
Open Access
Design of nanoscale capacitors based on metallic borophene and insulating boron nitride layers
(American Physical Society, 2021-12-13) Mogulkoc, Y.; Mogulkoc, A.; Guler, H. E.; Durgun, Engin
In alignment with the efforts on miniaturizing the components of electronic devices with enhanced performance, we investigate a dielectric nanocapacitor (DNC) based on metallic borophene electrodes separated with insulating hexagonal boron nitride (h-BN) monolayers (n=1–5). The capacitive performance of the proposed DNC as a function of applied electric field (→E) and thickness of the dielectric material is examined by using ab initio methods. The borophene plates and h-BN monolayers are commensurate and coupled only with van der Waals interaction, which constitutes an ideal configuration as a DNC. It is found that a single h-BN layer is not thick enough as a spacer to hinder quantum tunneling effects, and similar to the case with no insulating layer, borophene electrodes are shorted. Being effective from two h-BN layers, the charge separation on borophene plates is attained via →E in the vertical direction. The capacitance of the DNC rapidly saturates at →E≥0.1V/Å and reaches its maximum value of 0.77μF/cm2 for n=2. The capacitance decreases with an increasing number of insulating layers as the distance between electrodes enlarges and shows a similar trend that is expected from the classical Helmholtz model. Our results suggest metallic and lightweight borophene and insulating h-BN monolayers as ideal constituents for the DNC design.
Open Access
Disorder-free localization around the conduction band edge of crossing and kinked silicon nanowires
(A I P Publishing LLC, 2015) Keleş, Ü.; Çakan, A.; Bulutay, C.
We explore ballistic regime quantum transport characteristics of oxide-embedded crossing and kinked silicon nanowires (NWs) within a large-scale empirical pseudopotential electronic structure framework, coupled to the Kubo-Greenwood transport analysis. A real-space wave function study is undertaken and the outcomes are interpreted together with the findings of ballistic transport calculations. This reveals that ballistic transport edge lies tens to hundreds of millielectron volts above the lowest unoccupied molecular orbital, with a substantial number of localized states appearing in between, as well as above the former. We show that these localized states are not due to the oxide interface, but rather core silicon-derived. They manifest the wave nature of electrons brought to foreground by the reflections originating from NW junctions and bends. Hence, we show that the crossings and kinks of even ultraclean Si NWs possess a conduction band tail without a recourse to atomistic disorder.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? 1. theoretical investigation of poly(3-cyano-3′-hydroxybithiophene)
(American Chemical Society, 2002) Salzner, U.
Homo- and copolymers of hydroxythiophene and cyanothiophene have been investigated by employing density functional theory with the aim of determining the effect of donor-acceptor substitution on the electronic structure. The band gap of the copolymer is 0.11 eV smaller than that of polythiophene. Bandwidths of valence and conduction bands are reduced by 0.22 and 0.36 eV compared to polybithiophene. Conductivity after p- and n-doping could therefore be less than that of polythiophene. All properties of the copolymer are averages between those of the homopolymers. The charge separation between hydroxy- and cyano-substituted rings is 0.12 e in the neutral state and 0.13 e and the dication. The ionization potential and electron affinity of poly(hydroxythiophene) are 1.78 and 1.63 eV smaller than those of poly(cyanothiophene). According to the donor-acceptor concept, a decrease in band gap and an increase in bandwidths compared to the homopolymers should have resulted: We rationalize the absence of band broadening with reduced interaction between fragments with very different energies in agreement with perturbation theory.
Open Access
Double perovskite structure induced by Co addition to PbTiO3: Insights from DFT and experimental solid-state NMR spectroscopy
(American Chemical Society, 2019) Mete, E.; Odabaşı, S.; Mao, H.; Chung, T.; Ellialtıoğlu, Ş.; Reimer, J. A.; Gülseren, Oğuz; Üner, D.
The effects of Co addition on the chemical and electronic structure of PbTiO3 were explored both by theory and through experiment. Cobalt was incorporated into PbTiO3 during the sol–gel process with the X-ray diffraction (XRD) data of the resulting compounds confirming a perovskite structure for the pure samples. The XRD lines broadened and showed emerging cubic structure features as the Co incorporation increased. The changes in the XRD pattern were interpreted as double perovskite structure formation. 207Pb NMR measurements revealed a growing isotropic component in the presence of Co. Consistent with the experiments, density functional theory (DFT)-calculated chemical-shift values corroborate isotropic coordination of Pb, suggesting the formation of cubic Pb2CoTiO6 domains in the prepared samples. Hybrid functional first-principles calculations indicate formation of Pb2CoTiO6 with cubic structure and confirm that Co addition can decrease oxygen binding energy significantly. Experimental UV–vis spectroscopy results indicate that upon addition of Co, the band gap is shifted toward visible wavelengths as confirmed by energy band and absorption spectrum calculations. The oxygen binding energies were determined by temperature-programmed reduction (TPR) measurements. Upon addition of Co, TPR lines shifted to lower temperatures and new features appeared in the TPR patterns. This shift was interpreted as weakening of the oxygen–cobalt bond strength. The change in the electronic structure by the alterations of oxygen vacancy formation energy and bond lengths upon Co insertion is determined by DFT calculations.
Open Access
Effect of Molecular and Electronic Structure on the Light-Harvesting Properties of Dye Sensitizers
(American Chemical Society, 2007-05-24) Mete, E.; Uner, D.; Çakmak, M.; Gulseren, O.; Ellialtoğlu, Ş.
The systematic trends in structural and electronic properties of perylenediimide (PDI)-derived dye molecules have been investigated by DFT calculations based on the projector-augmented wave (PAW) method including gradient-corrected exchange−correlation effects. Time-dependent density functional theory (TDDFT) calculations have been performed to study the visible absorbance activity of these complexes. The effect of different ligands and halogen atoms attached to PDI were studied to characterize the light-harvesting properties. The atomic size and electronegativity of the halogen were observed to alter the relaxed molecular geometries, which in turn influenced the electronic behavior of the dye molecules. The ground-state molecular structure of isolated dye molecules studied in this work depends on both the halogen atom and the carboxylic acid groups. DFT calculations revealed that the carboxylic acid ligands did not play an important role in changing the HOMO−LUMO gap of the sensitizer. However, they serve as an anchor between the PDI and substrate TiO2 surface of the solar cell or photocatalyst. A commercially available dye sensitizer, ruthenium bipyridine [Ru(bpy)3]2+ (RuBpy), was also studied for electronic and structural properties in order to make a comparison with PDI derivatives for light-harvesting properties. Results of this work suggest that fluorinated, chlorinated, brominated, and iodinated PDI compounds can be useful as sensitizers in solar cells and in artificial photosynthesis.
Open Access
Effects of silicon and germanium adsorbed on graphene
(A I P Publishing LLC, 2010) Aktürk, E.; Ataca, C.; Çıracı, Salim
Based on the first-principles plane wave calculations, we studied the adsorption of Si and Geon graphene. We found that these atoms are bound to graphene at the bridge site with a significant binding energy, while many other atoms are bound at the hollow site above the center of hexagon. It is remarkable that these adatoms may induce important changes in the electronic structure of graphene even at low coverage. Semimetallic graphene becomes metallized and attains a magnetic moment. The combination of adatom orbitals with those of ππ- and π∗π∗-states of bare graphene is found responsible for these effects.
Open Access
Elastic and optical properties of sillenites: First principle calculations
(Taylor & Francis, 2020-04) Koç, H.; Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, Ekmel
In the present paper, we have investigated the electronic structure of some sillenites - Bi12MO20 (M = Ti, Ge, and Si) compounds based on the density functional theory. The mechanical and optical properties of Bi12MO20 have also been computed. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The band gap trend in Bi12MO20 can be understood from the nature of their electronic structures. The obtained electronic band structure for all Bi12MO20 compounds is semiconductor in nature. Similar to other oxides, there is a pronounced hybridization of electronic states between M-site cations and anions in Bi12MO20. Based on the obtained electronic structures, we further calculate the frequency-dependent dielectric function and other optical functions.
Open Access
Electron spectroscopy and the electronic structure of KNbO3: First principle calculations
(Taylor & Francis Online, 2014) Simsek S.; Koc, H.; Trepakov, V. A.; Mamedov, A. M.; Özbay, Ekmel
The electronic structures of KNbO3were calculated within the density functional theory, and their evolution was analyzed as the crystal-field symmetry changes from cubic to rhombohedral via tetragonal phase. We carried out electron-energy loss spectroscopy experiments by using synchrotron radiation and compared the results with the theoretical spectra calculated within Density Functional Theory. The dominant role of the NbO6 octahedra in the formation of the energy spectra of KNbO3compound was demonstrated. The anomalous behavior of plasmons in ferroelectrics was exhibited by the function representing the characteristic energy loss in the region of phase transition. © 2014 Copyright Taylor & Francis Group, LLC.
Open Access
Electronic band structure of rare-earth ferroelastics: theoretical investigations
(National Institute of Optoelectronics, 2018) Şimşek, Ş.; Uğur, G.; Uğur, Ş.; Mamedov, Amirullah M.; Özbay, Ekmel
In the present work, the electronic band structure and optical properties of RE2(MoO4)3 are investigated. The ground state energies and electronic structures were calculated using density functional theory (DFT) within the generalized-gradient approximation (GGA). The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant were also calculated. The main structure element in all our of compounds is the MoO4 tetrahedron. The presence of the MoO4 tetrahedra in the lattice of Gd2(MoO4)3, the similarity of the band structure and optical spectra of Gd2(MoO4)3 to those other tetraoxyanions of molybdenium demonstrate an important role of the MoO4 tetrahedra in the formation of the energy spectrum of Gd2(MoO4)3and other RE2(MoO4)3 compounds. This means that the MoO4 tetrahedra determine the lower edge of the conduction band and the upper edge of the valence band, and the conduction band is split into two subbands. The optical properties of RE2(MoO4)3 are in good agreement with this conclusion and previous experimental data.
Open Access
Electronic structure and optical properties of silicon nanocrystals along their aggregation stages
(Elsevier B.V., 2007) Bulutay, C.
The structural control of silicon nanocrystals is an important technological problem. Typically, a distribution of nanocrystal sizes and shapes emerges under the uncontrolled aggregation of smaller clusters. The aim of this computational study is to investigate the evolution of the nanocrystal electronic states and their optical properties throughout their aggregation stages. To realistically tackle such systems, an atomistic electronic structure tool is required that can accommodate about tens of thousand nanocrystal and embedding lattice atoms with very irregular shapes. For this purpose, a computationally efficient pseudopotential-based electronic structure tool is developed that can handle realistic nanostructures based on the expansion of the wavefunction of the aggregate in terms of bulk Bloch bands of the constituent semiconductors. With this tool, the evolution of the electronic states as well as the polarization-dependent absorption spectra correlated with the oscillator strengths over their aggregation stages is traced. The low-lying aggregate nanocrystal states develop binding and anti-binding counterparts of the isolated states. Such information may become instrumental with the maturity of the controlled aggregation of these nanocrystals.