Electronic Optical Properties Research Articles

Effect of Cu doping on the electronic structure, magnetic and optical properties of GaN/ZnS heterojunctions: a first-principles study

In this paper, the GaN/ZnS heterojunction systems with Cu doping and point defects coexisting are calculated by using the generalized gradient approximation in density functional theory, and the electronic structure, magnetic coupling mechanism and optical properties of each system are analysed. The results show that the magnetic source of the systems is twofold, on the one hand, it is the cationic vacancies that cause the unpaired electrons to undergo spin polarization, thus contributing magnetic moments to the system. On the other hand, Cu doping induces the GaN/ZnS heterojunction to produce bound magnetic polaron, which causes the system to undergo a magnetic phase transition, and the concentration of bound magnetic polarons constrains the total magnetic moment of the system. In terms of optical properties, the Cu-doped heterojunction system containing Ga vacancies introduces deep energy levels leading to the formation of hole-electron complex centres. Compared with the non-magnetic heterojunction system, the magnetic heterojunction system has a higher absorption intensity for visible light, and the red shift of its absorption spectrum is more obvious. In addition, the Cu-doped GaN/ZnS heterojunction system is able to achieve different types of conductivity when containing different vacancy defects.

Influence of local cation order on the electronic structure and optical properties of cation-disordered semiconductor AgBiS2

Abstract Site disorder exists in some practical semiconductors and can significantly impact their intrinsic properties both beneficially and detrimentally. However, the uncertain local order and structure poses a challenge for both experimental and theoretical research. Especially, it hinders the investigation of the effects of the diverse local atomic environments resulting from the site disorder. In this study, we employ the special quasi-random structure method to perform first-principles research on the connection between local site disorder and electronic/optical properties, using the cation-disordered AgBiS2 (rock salt phase) as an example. We predict that cation-disordered AgBiS2 has a bandgap ranging from 0.6 to 0.8 eV without spin-orbit coupling and that spin-orbit coupling reduces this by approximately 0.3 eV. We observe the effects of local structural features in the disordered lattice, such as the one-dimensional chain-like aggregation of cations that results in the formation of doping energy bands near the band edges, the formation and broadening of band-tail states, and the disturbance in the local electrostatic potential, which significantly reduces the bandgap and stability. The influence of these ordered features on the optical properties is confined to alterations in the bandgap and does not markedly affect the joint density of states or optical absorption. Our study provides a research roadmap for exploring the electronic structure of site-disordered semiconductor materials, suggests that the ordered chain-like aggregation of cations is an effective way to regulate the bandgap of AgBiS2, and provides insight into how variations in local order associated with processing can affect properties.

Influence of Ca-substituted K-induced cluster defects on the electronic structure and optical properties of KDP crystals

Influence of Ca-substituted K-induced cluster defects on the electronic structure and optical properties of KDP crystals

THEORETICAL INVESTIGATION OF NITROGEN AND BORON DOPING EFFECTS ON THE ELECTRONIC AND OPTICAL PROPERTIES OF CERIA

This study investigates the impact of nitrogen (N) and boron (B) doping on the electronic structure and optical properties of ceria (CeO2) using a combination of density functional theory plus U (DFT+U) and the GFN-xTB method. The calculations of electronic parameters and population analysis indicated that doping with N and B reduces the surface Lewis acidity of ceria, decreases the bandgap, and narrows the edges of the valence band and conduction band. These findings offer valuable insights into comprehending the photocatalytic performance of CeO2-based materials.

First-principle calculations of the electronic structure and optical properties of β-Ga2O3 with various vacancy defects

In this work, the effects of five types of vacancy defects on the electronic, optical and thermodynamic properties of β-Ga2O3 were investigated using first-principle calculations. The findings revealed that oxygen vacancies were more prevalent than gallium vacancies, with tetrahedral gallium vacancies being easier to form than octahedral ones. Vacancy defects induced a shift in the energy bands towards the lower energy end, with gallium vacancies reducing the bandgap from 4.9 eV to 3.1 eV, while the effect of oxygen vacancies on the bandgap was negligible. Oxygen vacancies introduced deep impurity levels near the Fermi level, while gallium vacancies introduced shallow impurity levels, primarily due to contributions from O-2p, Ga-4s, and Ga-4p orbital electrons. The differential charge density diagrams illustrated that gallium vacancies had a more pronounced impact on electronic distribution compared to oxygen vacancies, with VO3 exhibiting the least influence. As the temperature increases, the β-Ga2O3 with VO1 and VO2 exhibit higher thermodynamic stability compared to the intrinsic β-Ga2O3 and the one with VO3, while the intrinsic β-Ga2O3 has a superior heat capacity value compared to the β-Ga2O3 containing the five vacancy defects. The defect energy levels resulting from vacancies enhanced light absorption and conductivity, particularly oxygen vacancies, which improved the absorption capacity of β-Ga2O3 in the visible light range. These findings provide valuable insights for elucidating optical absorption experimental phenomena and photoluminescence.

Recycling of UO22+ expanded by metal-organic framework bearing pyrazinoquinoxaline tetracarboxylic acid: pH fluorescence sensing and photocatalytic activity

With the wide application of nuclear energy, the recycling of uranium resources and environmental protection have become the focus of scientific research and industrial attention. Uranyl ions have better electronic configurations and optical properties, but poor energy utilization and recycling as ionic states. Drawing on the structural features of Metal-Organic Frameworks (MOFs), the combination of UO22+ nodes with photoactive ligands should be able to effectively regulate the energy level matching between metal nodes and conjugated ligands, improve the electronic configuration and energy band structure, and thus produce better photoresponsive functional properties. In the present work, an example of a novel two-dimensional layered U-MOF was constructed by self-assembly, which utilizes pyrazinoquinoxaline tetracarboxylic acid. Due to the stable coordination structure, matched energy levels and synergistic photoresponsive behavior of UO22+ with pyrazinoquinoxaline ligands, U-MOF enables fluorescence sensing response in aqueous solution at pH=2–5, 9.5–12 and fluorescence quenching detection of a wide range of substrates. Theoretical calculations reveal that the interaction of the hydrogen ion with pyrazinozinoquinoxaline moiety extends the degree of conjugation of the moiety. Meanwhile, the interaction of hydroxide ions with pyrazinozinoquinoxaline groups elevates the highest occupied molecular orbital (HOMO) energy level, which significantly affects the photoelectric properties of U-MOF. In addition, U-MOF can be used as a photocatalyst to realize the conversion of benzylamine to N-benzylbenzylideneimine by oxidation under mild conditions. The effective electron transfer between uranyl ions and ligands ensured the photogenerated electron-hole separation efficiency and enhanced the energy utilization. Therefore, the related results of U-MOFs illustrate the practicality of introducing UO22+ nodes into MOFs to construct novel photoresponsive materials, which provides a new way for the efficient recycling and utilization of uranium resources.

Dynamical and optical properties of rare phenakite mineral: A combined experimental and computational study

The electronic structure, optical and dynamical properties of the natural phenakite Be2SiO4, are studied using ab initio calculations completed by Raman spectroscopy experiments. The calculated structural properties are in excellent agreement with experimental data and are accompanied by chemical bonds analysis using Maximally Localized Wannier Functions method. The theoretical analysis of projected density of states reveals the nature of the valence band as composed by electronic states of oxygen atoms, while conducting band is composed by electronic states of Si-atoms. Using linear response method the optical properties are calculated and compared with optical properties of quartz. It was shown the similarity of rare phenakite crystal and quartz. The calculated phonon dispersion structure reveals mostly delocalized nature of phonon states with quasilocalized phonons in high-frequency range, where the most intense band in Raman spectrum is observed. The band is ambiguously assigned to the breathing mode in SiO4 tetrahedra. Basing on such peculiarity in the spectra the rapid nondestructive diagnostic method of the sample is purposed.

A first-principles investigation on a new group of diamond-like semiconductors Cu4-II-Ge2-VI7 (II = Zn, Cd and VI = S, Se) as photovoltaic absorbers

Many diamond-like semiconductors with tetrahedral bonds, such as Si, CdTe and Cu(In,Ga)Se2, have been utilized as absorbers in photovoltaics (PV). In this work, a novel group of diamond-like compounds, namely Cu4-II-Ge2-VI7 with II = Zn/Cd and VI = S/Se, have been explored by first-principles means. From the calculations of the electronic structures and related optical properties, we have demonstrated that all these quaternaries have direct-type band structures, while their band gap energies are not suitable for PV applications. By anionic alloying, however, the width of the band gaps can be tuned to the desired energy. For example, the Cu4ZnGe2S4Se3 alloy has a suitable band gap of 1.34 eV. Our work provides the support and perspective for further experimental research on this group of semiconductors and their alloys as potential candidates in PV technologies.

An Ester-Functionalized Bidentate Titanium-based Metal-Organic Framework with Visible-Light Enhanced Activity for CO2 Photoreduction.

The limited visible light response is a critical drawback that hampers the photocatalytic efficacy of Ti-MOFs. However, study concerning the enhancement of the visible-light response of Ti-MOFs is still in its nascent stage. In this study, we employ the 'dual-ligand decrystallization strategy' to manipulate the electronic environment of Ti4+, leading to the synthesis of three ester-functionalized bidentate Ti-MOFs with enhanced visible light response. Our findings reveal that this approach not only reduces the bandgap of Ti-MOFs but also enhances their photocatalytic activity for carbon dioxide reduction. Specifically, compared to the bandgap of Ti-BPDC at 2.98 eV, the bandgap of Ti-BPDC-CA 1 : 2 has been reduced to 2.14 eV. Moreover, Ti-BPDC-CA 1 : 2 exhibits extraordinary photocatalytic activity with the formic acid (HCOOH) production rate of 617 μmol g-1 h-1 with over 99.5 % selectivity, which is 3.47 times higher than that of Ti-BPDC. Besides providing a cost-effective strategy for enhancing the visible light response of Ti-MOFs, our study also serves as an illustrative example for establishing the correlation between electronic structure and optical properties.

Theoretical insight into physical characteristics of lead-free perovskites Rb2TlSbX6 (X = Cl, Br, I) for optoelectronic devices.

Novel optoelectronic and thermoelectric properties with broad compositional range, non-toxic nature and structural stability make halide-based double perovskites fascinating for flexible optoelectronic devices. In this work, the structural electronic, optical and transport properties of Rb2TlSbX6 (X = Cl, Br, I) were studied using density functional theory for optoelectronic devices. The elastic analysis demonstrates ductile nature, mechanical stability, anisotropic behaviour and feasibility for flexible optoelectronic devices. The band structure study using Tran-Blaha-modified Becke-Johnson (TB-mBJ) potential shows that all studied materials have direct bandgap. In addition, the bandgap of Rb2TlSbCl6 is more appropriate for optoelectronic devices. The small loss and maximum absorption in visible regions make these materials prime candidates for optoelectronic devices. The transport features indicate that all the studied double perovskites reflect p-type semiconducting behaviour as highlighted by positive Seebeck coefficient values. Furthermore, the high power factor values of Rb2TlSbX6 (X = Cl, Br, I) double perovskites make them suitable for thermoelectric device applications at high temperatures. Based on electronic optical and thermoelectric properties Rb2TlSbCl6 is the best candidate for flexible optoelectronic devices. In this paper, structural optimization of Rb2TlSbX6 (X = Cl, Br, I) double perovskites was conducted utilizing the Wien2k software based on first principle calculations with Perdew-Burke-Ernzerhof's generalized-gradient approximation (PBE-sol approximation). The TB-mBJ potential was employed to compute the accurate band gap of studied materials. The thermoelectric properties are evaluated with BoltzTraP code, showing a predominance of P-type charge carriers in all studied perovskites. This methodological strategy verifies the material's remarkable stability and optical properties and offers a solid framework for examining its potential in optoelectronic devices.

Strain Engineering on the Electronic Structure and Optical Properties of Monolayer WSi2X4 (X = N, P, As)

Strain Engineering on the Electronic Structure and Optical Properties of Monolayer WSi2X4 (X = N, P, As)

Investigation of optical properties of Ce and Eu-doped Gd2SiO5 insights from GGA + U calculations

Gadolinium silicate, (Gd2SiO5) co-doped with Ce and Eu has been found to exhibit enhanced luminescence efficiency, which makes it a promising material for use in scintillators and phosphors. It has excellent scintillation properties such as high density and high Zeff. In this study, we used density functional theory (DFT) calculations within the Wien2k software to investigate the effect of Ce and Eu concentration and native defects on the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5. We utilized the DFT + U method to treat the localized 4f electrons of Ce and Eu. Our results indicate that the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5 are significantly affected by the concentration of the dopants and presence of native defects. We found that increasing the concentration of Ce and Eu dopants leads to a shift in the bandgap to lower energies, resulting in enhanced absorption and emission spectra. Moreover, our calculations reveal that presence of oxygen vacancies and Gd interstitials can induce new defect levels in the bandgap, which may affect the luminescence properties of the material. Our study provides valuable insights into the atomic-level mechanisms that govern the luminescence properties of Ce and Eu co-doped Gd2SiO5 which can aid in the design and optimization of luminescent materials for various applications.

Exploring tunable optoelectronic properties of two-dimensional GaS/PtSSe heterostructures under biaxial strain and external electric field

Vertically stacked heterostructures offer novel material options for designing tunable optoelectronic devices due to their adjustable electronic and optical properties. This work compares the electronic structure, charge transfer, and optical properties of two different contact faces of GaS/Janus PtSSe heterostructures using the density functional theory. The results show that GaS/PtSSe form a built-in electric field at the interface, directing from the monolayer GaS to the PtSSe. GaS/SePtS and GaS/SPtSe are determined to be indirect bandgap semiconductors with bandwidths of 1.51 eV and 1.11 eV utilizing the PBE. In this study, the band alignment of GaS/PtSSe can induce a semiconductor-to-metal transition. Notably, GaS/SePtS is highly sensitive to biaxial strain and external electric field, transitioning its energy band alignment from Type-I to Type-II. Applying tensile strain enhances the sunlight absorption of GaS/PtSSe, making it a promising candidatein the tunable electronic devices and photodetectors.

Optical Properties of H-Bonded Heterotriangulene Supramolecular Polymers: Charge-Transfer Excitations Matter.

H-bonded N-heterotriangulene (NHT) supramolecular polymers offer a nice playground to explore the nature and dynamics of electronic excitations in low-dimensional organic nanostructures. Here, we report on a comprehensive molecular modeling of the excited-state electronic structure and optical properties of model NHT stacks, highlighting the important role of intermolecular charge-transfer (CT) excitations in shaping their optical absorption and emission lineshapes. Most importantly, we show that the coupling between the local and CT excitations, modulated by the electric fields induced by the presence of polar amide groups forming H-bonded arrays along the stacks, significantly increases the resulting hybrid exciton bandwidth. We discuss these findings in the context of the efficient transport of singlet excitons over the μm length scale reported experimentally on individual self-assembled nanofibers with molecular-scale diameter.

Growth, spectroscopic, thermal and laser frequency doubling properties of CTGAS crystal

A novel nonlinear optical crystal Ca3Ta(Ga0.7Al0.3)3Si2O14 (CTGAS) was successfully grown by the Czochralski method. The structural, spectroscopic and thermal properties have been systematically investigated, and the laser frequency doubling process has also been explored. Excellent thermal properties have been presented, such as moderate thermal conductivity, high specific heat, and low thermal expansion anisotropy. The electronic structure and optical properties have been investigated using the XPS technique and simultaneous assisted first-principles calculations. The optimal phase matching in type I (40.45°, 30.0°) and type II (64.73°, 0°) orientations were determined, frequency-doubling output was achieved in type I and type II, with laser energies of up to 10.04 mJ and 10.14 mJ obtained @ 532 nm, and optical-to-optical conversion efficiencies of 14.04 % and 17.21 %, respectively. The experimental findings demonstrate the great potential of CTGAS crystal as a frequency-doubling material.

Comparison of electronic structures, thermodynamic, optical and mechanical properties of MgB2 and MgMB4(M=Al, Pb, W) based on first-principles

As an important superconducting material, MgB2 has a wide range of applications in electronics and communication fields. The optimization of its properties has been a hot research topic. In this paper, it uses first-principles calculations to delve into the impact of doping Type-I superconducting elements M (M = Al, Pb, W) on the properties of MgB2. The main focus is on exploring improvements in thermodynamic, optical and mechanical properties in conjunction with the analysis of electronic properties. For thermodynamic properties, MgPbB4 demonstrates significant variation in entropy and free energy as temperature increases, reaching 356.84 KJ/mol and −411.46 KJ/mol, respectively, at 2000 K. Regarding optical properties, the absorption band edge disappears and the range widens with the introduction of M elements. Specifically, MgWB4 exhibits the highest reflectivity R (0) and static dielectric constant ε1 (0) in the [001] direction, measured at 0.49 and 32.2, respectively. In terms of mechanical properties, the elastic modulus of MgAlB4 and MgWB4 enhances notably with B/G ratios less than 1.75, indicating their brittleness. MgWB4, in particular, stands out with the highest elastic modulus and the weakest elastic anisotropy AU (0.703), exhibiting a spatial distribution nearer to spherical. Conversely, MgPbB4 transforms the material into a ductile one, with reduced elastic modulus and the strongest anisotropy AU (4.963). This study can provide a more comprehensive perspective for the future design of MgB2-based materials.

Optical absorption properties in multi-layer InGaN/GaN core-shell quantum dots: The influences of ternary mixed crystal effect and hydrogen-like impurity

Based on the effective mass approximation and the compact density matrix theory, the electronic states and optical absorption properties of intersubband transitions between the conduction-band energy levels in zinc-blende InGaN/GaN/InGaN/GaN core-shell quantum dots (CSQDs) are investigated by using the finite difference method. The effects of hydrogen-like impurity and In component on the wave functions, transition energies, and optical absorption properties are discussed in detail. The results show that hydrogen-like impurity can cause the peak positions of optical absorption coefficients (OACs) and refractive index changes (RICs) to move to high energy area, and the peak values of OACs rise. Increasing In component of InGaN crystal in the core region of a CSQD has the same effect. But an opposite tendency will be found when increasing In component in the shell-well of a CSQD, i.e. optical absorption peaks move to low energy area and their values decrease. Comparing to the shell-well, the core region has more powerful confinement effect on electrons. So the better absorption properties are more likely to be obtained for this type of multi-layer InGaN/GaN CSQDs when In component in the core is larger than that in the shell-well.

Biaxial strain-modulated power conversion efficiency, electronic structures, and optical properties of type-II MoS2/BC6N vdW heterostructure: A density functional theory study

The power conversion efficiency (PCE), electronic structures, and optical properties of a novel MoS2/BC6N van der Waals heterostructure (vdWH) were systematically investigated under different biaxial strains ranging from –8–8 % by first-principles density functional theory calculations. The semiconductive MoS2/BC6N vdWH exhibited indirect bandgap (1.02 eV) with type-II band alignment. The bandgap of the MoS2/BC6N vdWH decreased to 0.57 eV with increasing tensile strain, and increased to 1.57 eV with increasing compressive strain. When the strain exceeded –4 %, the band alignment of the vdWH transformed from type-II to type-I, exhibiting a direct bandgap. Strong optical absorption of the MoS2/BC6N vdWH was observed in the visible and ultraviolet regions. The maximum absorption peak produced a redshift with increasing tensile strain and a blueshift as increasing compressive strain. The PCE of the intrinsic MoS2/BC6N vdWH was 11.5 %, which gradually increased with increasing compressive strain but decreased with increasing tensile strain, that is, PCEs of 5.6 %, 8.0 %, 16.1 %, and 22.8 % for strain of 4 %, 2 %, –2 %, and –4 %, respectively. Therefore, the biaxial strain can effectively modulate the electronic structures, optical properties, and PCE of MoS2/BC6N vdWH. Moreover, their excellent optical properties and ultrahigh PCE indicate their significant potential for use in photovoltaic devices.

Strain-bandgap quasi-linear correlation applied to modulation of NiBr2 monolayer optical properties

Recently, two-dimensional transition metal dihalides have shown diverse applications in spintronics, quantum devices, and optoelectronic devices. In this work, density functional theory (DFT) is adopted to investigate the effect of biaxial strain upon the electronic structure and optical properties of NiBr2 monolayer. The results show that NiBr2 is an indirect semiconductor with band gap 2.62 eV (DFT + U) and 3.18 eV (HSE06), which could vary quasi-linearly from 2.73 eV to 1.71 eV modulated by biaxial strain (6 % to −10 %). Combined with the analysis of the density of states (DOS), the primary reason for the alteration in the band gap is that strain significantly affects the behavior of Ni 3d electrons. When the strain changes from positive to negative, the DOS of Ni 3d shifts towards the Fermi level, resulting in a reduction in the band gap. Within this strain range, all of the optical properties exhibit a quasi-linear tendency similar to the band gap variation. The optical peaks of NiBr2 under compressive strain are not only red-shifted into the visible light (VL) region but also significantly enhanced in the VL and ultraviolet (UV) regions. Applying strain can greatly boost the electrical and optical properties of NiBr2 monolayer, making it a promising candidate in the field of ultraviolet detectors.

Change in nonlinear optical response of C24 nanocage upon doping with lithium based superalkalis

Density functional theory (DFT) calculations are used to analyze the change in nonlinear optical (NLO) response, electronic and geometric properties of the Li based superalkalis doped C24 nanocage. It was observed that the adsorption of Li4N, Li3O and Li2F superalkalis on C24 nanocage results in thermodynamically stable isomers (A-F). The energy gap between the highest occupied and the lowest unoccupied molecular orbitals (GH-L) is reduced after superalkalis doping on carbon (C24) nanocage. Density of states spectra depict the strong contribution of superalkalis in HOMOs of the considered complexes. Natural bond orbital (NBO) charge analysis showed that the charge is being transferred from superalkali toward C24 nanocage. The values of polarizability (α o ) and hyperpolarizability (β o ) showed that doping of superalkalis on C24 has a significant effect on its NLO response, resulting in a considerable increase in values of α o and β o . Li4N@C24 isomer E showed the highest β o value of 6470.74 au. Time dependent density functional theory (TD-DFT) calculations are implemented to analyze the absorption spectra. This research provides unique and highly efficient superalkalis doped C24 isomers for their applications in future electronic devices.