Stability, electronic and magnetic properties of embedded triangular graphene nanoflakes

Electronic and magnetic properties of triangular graphene nanoflakes embedded in fluorographene

The structural, electronic, and optical properties of an amorphous SiO2 (a-SiO2) model is investigated by using first-principles calculation. Most research works used beta-cristobalite glass structure as a reference to amorphous silica structure. However, only the electronic properties were been presented without any link towards the optical properties. Here, we demonstrate simultaneous electronic and optical properties, which closely matched to a-SiO2 properties by generating small sample of amorphous quartz glass. Using the Rietveld refinement, amorphous silica structure was generated and optimized using density functional theory in CASTEP computer code. A thorough analysis of the amorphous quartz structure obtained from different thermal treatment was carried out. The structure of amorphous silica was validated with previous theoretical and experimental works. It is shown that small sample of amorphous silica have similar structural, electronic and optical properties with a larger sample. The calculated optical and electronic properties from the a-SiO2 glass match closely to previous theoretical and experimental data from others. The a-SiO2 band gap of 5.853 eV is found to be smaller than the experimental value of ~9 eV. This is due to the underestimation and assumption made in DFT. However, the band gap value is in good agreement with the other theoretical works. Apart from the absorption edge at around 6.5 eV, the refractive index is 1.5 at 0eV. Therefore, this atomic structure can served as a reference model for future research works on amorphous structures.

Ab-initio investigation of the fundamental properties of metals (X = Be, Mg, and Ca) encapsulated CsXO3 tin-based perovskite materials

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices.

Multi-functional lead-free Ba2XSbO6 (X = Al, Ga) double perovskites with direct bandgaps for photocatalytic and thermoelectric applications: A first principles study

Structural stability, electronic and thermodynamic properties of VOPO4 polymorphs from DFT+U calculations

Two-dimensional covalent organic frameworks (2D COFs) are a class of modular polymeric crystals with high porosities and large surface areas. Their tunable microstructure (with a wide array of choices for the molecular building block) provides the opportunity for their bottom-up design and potentially tailorable physical properties. In this work, through combined density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we study the influence of different molecular functional groups and varying porosities on the electronic and thermal properties of 2D COFs. More specifically, by performing DFT calculations on 24 different 2D COFs, we demonstrate that one of the main descriptors dictating their band gaps are their mass densities or network porosities. Furthermore, we also find that specific functional groups forming the nodes can lead to larger localization of charge densities resulting in wider band gaps. By performing MD simulations to investigate their thermal properties, we show that (similar to their electronic properties) mass density is also one of the main factors dictating heat conduction, where higher densities are associated with relatively higher thermal conductivities along the 2D sheets. Our spectral energy density calculations provide insights into the highly anharmonic nature of these materials. We find that increasing porosities lead to larger anharmonic interactions and thus reduced thermal conductivities in these materials. Similar to their electronic band gaps, the nodes forming the 2D COFs also have a significant contribution in dictating their thermal conductivities with bigger nodes (accompanied by higher densities) generally resulting in relatively higher thermal conductivities in 2D COFs. Taken together, the resulting changes in the electronic and thermal properties from variations in the building blocks in 2D COFs lend insights into fundamental changes in the microscopic thermodynamics that arise from systematically changing their molecular structure. Therefore, our study provides a blueprint for the strategic syntheses of 2D COFs with “user-defined” electronic and thermal properties that will ultimately aide in their incorporation into various applications.

First-principles calculations of the structural, electronic and optical properties of In1−xBxAsyP1−y quaternary alloys lattice matched to InP and BeS

Silicene, a two-dimensional hexagonal silicon layer, exhibits exceptional electronic and thermoelectric properties. However, its application in semiconductors is hindered by its zeroband gap, which could be overcome by modifying its electronic properties through doping. In this paper, Density Functional Theory (DFT) calculations were performed to investigate the band gap opening in silicene by studying the effect of magnesium and sulphur doping on its electronic, structural and thermal properties. Pristine silicene has a lattice constant of 3.86 Å and a zero-band gap. Upon doping with 12.5% S and Mg atoms, the lattice constant modifies to 3.45 Å and 3.93 Å, respectively, resulting in a direct band gap opening. For 25% Mg and S doping, the result shows that Mg and S effectively alter the band structure and the band gap of silicene monolayer at various configurations. The maximum band gap was 0.98 eV and 1.22 eV for Mg and S doping into the meta position to the reference point R, respectively. The power factor significantly increases with doping, reaching 1.20 x 1011 WK-2m-1 and 1.40 x 1011 WK- 2 m-1 for 12.5% Mg and S doping compared to 7.4 x 1010 WK-2m-1 for pristine silicene. This substantial enhancement indicates improved thermoelectric performance, making silicene a promising candidate for thermoelectric applications. Results demonstrate that tuning the band gap through doping can simultaneously enhance the power factor, highlighting the potential of Mg/S-doped silicene for efficient energy harvesting and conversion.

Formation of boron nitride islands in the graphene nanoflakes: A DFT study

Considerable attentions have been drawn towards modulation of organic dyes with the goal of realizing effectual dye-sensitized solar cells using density functional theory (DFT). In this respect, a series of D–A1–A2 dyes containing 2-vinyl-5-(5-vinyl-2-thienyl) thiophene bridged with C=S group (A1), 2-Cyano-2-pyran-4-ylidene-acetic acid (A2) and tertiary aromatic amines as donor (D) were designed and theoretically investigated for dye-sensitized solar cells (DSSCs). These dyes were simulated using DFT and time-dependent density functional theory to calculate their electronic and optical properties, molecular reactivity indices, natural population analysis, maximum open circuit voltage (VOC) and light harvesting efficiency (LHE). The results showed that the position as well as number of fluorine atoms on Cyano-2-pyran-4-ylidene-acetic acid unit have effect on the electronic properties without necessary change the band gap energy of the dyes; therefore tuned/modulated molecular properties of the dyes. Also, dyes with donor unit containing N,N-diphenylaniline (DA-7) presented lowest band gap than those containing N-(2-pyridyl)pyridin-2-amine (DA-8) and carbazole (DA-9); thus N-phenyl-aniline unit is a better electron donor.

Hybrid functional calculations of optoelectronic properties of ultra-wide bandgap LiSmO2: A first-principle study

Polyfluorenes (PFs) have been widely studied recently due to their potential applications in light-emitting diodes, thin-film transistors, and photovoltaic cells. Manipulation of their electronic structures and morphology of polyfluorenes play key roles for the above applications. In this thesis, both theoretical and experimental investigations were conducted on various fluorene-acceptor copolymers to investigate the effects of acceptor strength on their electronic and optoelectronic properties. Variation on the surface structures and photophysical prperties of fluorene-based rod-coil block copolymer brushes with selective solvents were investigated theoretically and experimentally. In the first part of this thesis (Chapter 2), the theoretical geometries and electronic properties of fluorene-based alternating donor-acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e. HOMO, LUMO level, and band gap. It was found that the geometries of fluorene-based donor-acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene-acceptor conjugated copolymers could be tuned by the geometries or acceptor strength. In the second part of this thesis, fluorene-based donor-acceptor copolymers was synthesized and utilized as the emissive layer of light-emitting diodes. In Chapter 3, a series of novel light-emitting copolymers consisted of 9,9-dihexylfluorene (F) and different acceptor segments, including quinoxaline (Q), 2,1,3-benzothiadiazole (BT) and thieno[3,4-b]-pyrazine (TP), were synthesized by the palladium-catalyzed Suzuki coupling reaction. Three fluorene-acceptor alternating copolymers (PFQ, PFBT, PFTP), six F-TP (PFTP0.5~PFTP35) random copolymers, four random copolymers with three emitting units (PFQTPs and PFBTTPs) were investigated and compared with the parent polyfluorene (PF). The experimental results suggest that the acceptor strength or content significantly affect the electronic and optoelectronic properties. The luminescence characteristics based on the prepared polymers depend on the Forster energy transfer or the intramolecular charge transfer, or heavy-atom fluorescence quenching. The tuning of the electronic and optoelectronic properties could be achieved by incorporating various acceptors or content into the polyfluorenes. The emissive color of these LEDs covers the entire visible region, including red, green, blue, and white. In Chapter 4, the photoluminescence and electroluminescence characteristics of a series of polymer blends based on fluorene-acceptor alternating copolymers were investigated. The dependences of energy transfer and quantum efficiency on the chemical structure of component and on the composition of polymer blend were rationalized. Precise control of the composition of polymer blends resulted in incomplete Forster energy transfer from donor to acceptors. Emission from all the components simultaneously resulted in efficient white emission. In the third part of this thesis, a theoretical model based on dissipative particle dynamics was established to simulate the surface structure of rod-coil block copolymer brushes. The effects of different solvent polarities, block ratios, and grafting densities of brushes on the surface structures were investigated. For the justification of our theoretical model, a novel multifunctional amphiphilic rod-coil block copolymer of the type of PF-b-PPEGMA-b-PPOPS was synthesized by ATRP and the functional rod-coil polymer brush was successfully prepared using reactivity of the silanol-carrying PPOPS block. The comparison between simulated surface structures and AFM images shows highly similar surface structures indicating the feasibility of this theoretical model.

Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

The electronic properties and optical properties of Cr-doped monolayer WS2 under uniaxial compressive deformation have been investigated based on density functional theory. In terms of electronic structure properties, both intrinsic and doped system bandgaps decrease with the increase of compression deformation, and the values of the bandgap under the same compression deformation after Cr doping are reduced compared with the corresponding intrinsic states. When the compressive deformation reaches 10%, both the intrinsic and doped system band gaps are close to zero. New electronic states and impurity energy levels appear in the WS2 system when doped with Cr atoms. For the optical properties, the calculation and analysis of the dielectric function under each deformation regime of monolayer WS2 show that the compression deformation affects the dielectric function, and when the compression deformation is 10%, the un-doped and Cr-doped regimes show a decrease in ε1(ω) compared to the compression deformation of 8%. For each deformation system, the peak reflections occur in the ultraviolet region. Near the position where the second peak of the absorption spectrum appears, it can be seen that the ability of each system to absorb light gradually decreases with the increase of the amount of deformation and appears to be red-shifted to varying degrees. This study follows the initial principles of the density functional theory framework and is based on the CASTEP module of Materials-Studio software GGA and PBE generalizations are used to perform computations such as geometry optimization of the model. We have calculated the energy band structure of monolayer WS2 with intrinsic and compressive deformations of 2% and 4% using PBE and HSE06, respectively. The band gap values calculated using PBE are 1.802 eV, 1.663 eV, and 1.353 eV, respectively, and the band gap values calculated with HSE06 are 2.267 eV, 2.034 eV, 1.751 eV. The results show that the bandgap values calculated by HSE06 are significantly higher than those calculated by PBE, but the bandgap variations calculated by the two methods have the same trend, and the shape characteristics of the energy band structure are also the same. However, it is worth noting that the computation time required for the HSE06 calculation is much longer than that of the PBE, which is far beyond the capability of our computer hardware, and the purpose of this paper is to investigate the change rule of the effect of deformation on the bandgap value, so to save the computational resources, the next calculations are all calculated using the PBE. The Monkhorst-Pack special K-point sampling method is used in the calculations. The cutoff energy for the plane wave expansion is 400 eV, and the K-point grid is assumed to be 5 × 5 × 1. Following geometric optimization, the iterative precision converges to a value of less than 0.03 eV/Å for all atomic forces and at least 1 × 10-5 eV/atom for the total energy of each atom. The vacuum layer's thickness was selected at 20 Å to mitigate the impact of the interlayer contact force.