Configuration Coordinate Diagram Research Articles

Insight into the damage mechanism of KH2PO4 crystals with BaK-SiP by hybrid density functional theory

The laser-induced damage threshold (LIDT) of potassium dihydrogen phosphate (KDP) crystals is one of the key factors affecting the application of KDP crystals for Inertial confinement fusion (ICF). Previous studies have found that Ba, Si impurities can form BaK-SiP defect complexes in KDP crystals. However, the microscopic nature of the defect complexes is still unclear. We precisely calculate defect formation energy (DFE) in various charge states (-1, 0, +1), configuration coordinate diagram (CCD) and optical spectra by hybrid density functional theory. We found BaK-SiP introduce two defect states in the band gap. In the transitions (BaK-SiP)✕+h∙→(BaK-SiP)∙ and (BaK-SiP)✕+e-→(BaK-SiP)′, excited state and ground state have an intersection point in the Marcus inverted region and the activation energies are nearly 0 eV. The results showed that the defect complexes could capture holes and electrons through lattice relaxation, causing significant heat deposition and resulting in crystal damage. This implies a new damage mechanism of KDP crystals is discovered.

Luminescence of oxygen vacancies in hafnium oxide, characteristics of emission bands and use for diagnostics of technological processes

Oxygen vacancies play an important role in the formation of the electrical and optical properties of hafnium oxide. Both conductivity and emission bands are determined by this intrinsic defect. The work studies the luminescence of oxygen vacancies. The spectral and temperature dependences of luminescence show the leading role of electron-phonon interaction. The work proposes an algorithm for determining the parameters of this interaction and constructs configuration-coordinate diagrams of emission centers that are associated with different charge states of oxygen vacancies in hafnium oxide.

Optical properties of Ba-impurity defect clusters in KH2PO4 crystals

Potassium dihydrogen phosphate (KDP) is widely used in electro-optical switches and inverter crystals in inertial confinement fusion (ICF). The early studies suggested that defect clusters are one of the main reasons why the laser-induced damage threshold (LIDT) is far lower than the theoretical value. However, it is not clear what type of defect clusters cause this phenomenon. The BaK defect cluster models have been determined by simulation using General Utility Lattice Programs (GULP). We found that BaK+VH and BaK+VK cause a dramatic distortion on H-O bonds with maximum values of 29.5 % and 27.12 %, respectively. By analyzing the configuration coordinate diagram and optical spectra, we believe that both defect clusters can promote ionization through defect-assisted "2+1" photon absorption, which leads to free electrons more easily exceeding the critical value for electron avalanche breakdown. Our study will offer insight into the effects of defect clusters in KDP crystals.

Unraveling Broadband Near-Infrared Luminescence in Cr3+-Doped Ca3Y2Ge3O12 Garnets: Insights from First-Principles Analysis.

In this study, we conducted an extensive investigation into broadband near-infrared luminescence of Cr3+-doped Ca3Y2Ge3O12 garnet, employing first-principles calculations within the density functional theory framework. Our initial focus involved determining the site occupancy of Cr3+ activator ions, which revealed a pronounced preference for the Y3+ sites over the Ca2+ and Ge4+ sites, as evidenced by the formation energy calculations. Subsequently, the geometric structures of the excited states 2E and 4T2, along with their optical transition energies relative to the ground state 4A2 in Ca3Y2Ge3O12:Cr3+, were successfully modeled using the ΔSCF method. Calculation convergence challenges were effectively addressed through the proposed fractional particle occupancy schemes. The constructed host-referred binding energy diagram provided a clear description of the luminescence kinetics process in the garnet, which explained the high quantum efficiency of emission. Furthermore, the accurate prediction of thermal excitation energy yielded insights into the thermal stability of the compound, as illustrated in the calculated configuration coordinate diagram. More importantly, all calculated data were consistently aligned with the experimental results. This research not only advances our understanding of the intricate interplay between geometric and electronic structures, optical properties, and thermal behavior in Cr3+-doped garnets but also lays the groundwork for future breakthroughs in the high-throughput design and optimization of luminescent performance and thermal stability in Cr3+-doped phosphors.

Thermometric properties of SrMoO4:Tb3+ phosphor based on redshift of charge transfer band edge

In this study, SrMoO4:Tb3+ phosphors were successfully synthesized for a single-band ratiometric (SBR) thermometry utilizing the redshift of CTB. The SrMoO4:Tb3+ phosphors have a tetragonal system owing to I41/a space group and irregular microparticles possessing a mean size of 20 μm. The incorporation of Tb3+ ions into the SrMoO4 matrix lattice leads to a reduction in the bandgap energy owing to occurrence of defects level in gap-band. Thermal quenching concentration of Tb3+ is 1%, due to the dipole-dipole interaction between Tb3+ and Tb3+ ions. This study observed an interesting redshift in the edge of the CTB with increasing temperature, which can be explained using the configurational coordinate diagram. This phenomenon can be utilized to construct this SBR temperature measurement strategy by selecting appropriate excitation peaks or bands with different thermal responses. The study also demonstrated that SrMoO4: 1%Tb3+ has higher Sa value of 13.31 × 10−2 K−1 @573 K and Sr value of 3.54 K-1@298 K in optical thermometry, making it a suitable candidate for optical temperature measurement.

Radiation-induced modification effects in covalent-network glass formers: Phenomenological description within unified configuration-enthalpy model

Externally-induced modification activated by high-energy excitation such as γ-irradiation from 60Co source is analyzed in chalcogenide glasses in terms of radiation-structural and glass-relaxation metastability, at the example of sulphides (including stoichiometric arsenic trisulphide, As2S3) and selenides. Unified configuration-enthalpy model evolving conjugated configuration-coordinate (free energy in dependence on configuration coordinate) and thermodynamic enthalpy (temperature deviations in enthalpy, configurational entropy or free volume) diagrams is proposed to describe metastability in these glasses under external influences, such as (1) physical ageing, (2) irradiation, (3) thermal annealing, (4) rejuvenation, and their combinations. The model predicts glass stabilization in the ground state composed of partial sub-states related to (i) irradiation, (ii) rejuvenation, (iii) annealing, as well as (iv) ideal-glass deep states, connected by over-barrier jumping and through-barrier tunneling transitions. This model allows an adequate parameterization of optical responses related to metastability in these glasses defined through blue (bleaching) or red (darkening) shift in their optical transmission spectra.

Dual-emitting Dy3+/Eu3+ co-doped SrLaGaO4 phosphor: Tunable luminescence, energy transfer and ratiometric temperature sensing

Dy3+/Eu3+ co-doped SrLaGaO4 warm white phosphor was synthesized by solid state reaction and studied for the first time. Under 352 nm excitation, the obtained tetragonal SrLaGaO4: Dy3+ phosphor exhibit blue (483 nm) and yellow (577 nm) emission and the optimal doping concentration of Dy3+ is 5 mol%. After co-doping with Dy3+ and Eu3+ ions, phosphor emits warm white light and the correlated color temperature can be adjusted from 4745 K to 3004 K. The quantum yield of SrLaGaO4: 0.05Dy3+ and SrLaGaO4: 0.05Dy3+, 0.008Eu3+ are 11.3 % and 13.27 %, respectively. In addition, temperature-dependent spectra showed that Dy3+ and Eu3+ have different emission variations in SrLaGaO4: 0.05Dy3+, 0.012Eu3+ phosphor, which was explained by thermally induced energy transfer between Dy3+ and Eu3+ in configuration coordinate diagram. The fluorescence intensity ratios of Eu3+ and Dy3+ at 298 K- 423 K were calculated to analyze the temperature sensing properties. The maximum absolute sensitivity of SrLaGaO4: 0.05Dy3+, 0.012Eu3+ phosphor was obtained as 0.63 % K–1 at 298 K. Finally, the WLED device is prepared by SrLaGaO4: Dy3+, Eu3+ phosphor, which emits warm white light with a correlated color temperature of 3550 K. In summary, SrLaGaO4: Dy3+, Eu3+ are excellent single-phase warm white component applicated for WLED and optical thermometers.

Case of the Bromine Vacancy in Cs4PbBr6.

Typically defect tolerance is equated with a lack of deleterious defects or with abundant defects creating only shallow levels. Here, we address the idea that deep defects, when unavoidable, do not guarantee harmful consequences. Using halogen vacancy as a common defect among halides, we explore its behavior in Cs4PbBr6. It is a large gap material (band gap of ∼4 eV) known for its green emission at ∼520 nm. We show that its Br-vacancy is indeed a deep defect as obtained from hybrid density functional calculations. An analysis of the configuration coordinate diagram corresponding to the defect's charge transition levels enables us to conclude that the nonradiative recombination cycle will be hampered by an extremely slow hole capture process. Therefore, Br-vacancy will not suppress light emission in Cs4PbBr6. Although this finding does not signal that all deep defects will behave similarly, it indicates that defect tolerance may be achievable despite their occurrence.

Emission color tuning and dual-mode luminescence thermometry design in Dy3+/Eu3+ co-doped SrMoO4 phosphors

The challenge of building a highly reliable contactless temperature probe with high sensitivity, good temperature-induced color discriminability, and economical synthesis has prompted the research community to work in the field of rare-earth-based luminescence thermometry. Moreover, the fast-growing market for optoelectronic devices has increased the demand for tunable color-emitting phosphors. In this study, Dy3+/Eu3+ co-doped SrMoO4 phosphors were developed as tunable color-emitting source and dual-mode luminescence thermometer. A facile and cost-effective auto-combustion method was used to synthesize the phosphors. Our work demonstrates a viable scheme for tailoring the emission of single-phase phosphors by precisely controlling the dopant concentrations and by modulating excitation wavelength. The overall emission is tuned from greenish-yellow to white and greenish-yellow to reddish-orange. A detailed energy transfer process from the host to the Ln3+ ions and between the Ln3+ ions is discussed. Further, anti-thermal quenching in the emission of Dy3+ ion is observed when excited with 297 nm. The dual-mode luminescence thermometry has been studied by analyzing the fluorescence intensity ratio of Dy3+ and Eu3+ ions upon excitation at 297 nm. The maximum relative sensitivity value for 4% Eu3+ co-doped SrMoO4:4%Dy3+ phosphor is 1.46% K−1 at 300 K. Furthermore, the configurational coordinate diagram is presented to elucidate the nature of temperature-dependent emission. Therefore, our research opens up new avenues for the development of color-tunable luminescent materials for various optoelectronic and temperature-sensing applications.

Dangling Bonds as Possible Contributors to Charge Noise in Silicon and Silicon-Germanium Quantum Dot Qubits.

Spin qubits based on Si and Si1-xGex quantum dot architectures exhibit among the best coherence times of competing quantum computing technologies, yet they still suffer from charge noise that limit their qubit gate fidelities. Identifying the origins of these charge fluctuations is therefore a critical step toward improving Si quantum-dot-based qubits. Here, we use hybrid functional calculations to investigate possible atomistic sources of charge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). We evaluate the role of global and local environment in the defect levels associated with DBs in Si, Ge, and Si1-xGex alloys, and consider their trapping and excitation energies within the framework of configuration coordinate diagrams. We additionally consider the influence of strain and oxidation in charge-trapping energetics by analyzing Si and GeSi DBs in SiO2 and strained Si layers in typical Si1-xGex quantum dot heterostructures. Our results identify that Ge dangling bonds are more problematic charge-trapping centers both in typical Si1-xGex alloys and associated oxidation layers, and they may be exacerbated by compositional inhomogeneities. These results suggest the importance of alloy homogeneity and possible passivation schemes for DBs in Si-based quantum dot qubits and are of general relevance to mitigating possible trap levels in other Si, Ge, and Si1-xGex-based metal-oxide-semiconductor stacks and related devices.

Electronic structures and optical properties of defective KDP with V-doped: A first-principles study

The effect of V single-doping on the defect formation energies, electronic structures, and optical properties of potassium dihydrogen phosphate (KH2PO4, KDP) crystals in tetragonal and orthorhombic phases has been investigated using first-principles calculations based on density functional theory (DFT). Meanwhile, we apply the HSE06 hybrid functional to correct the "band-edge" problem as well as the FNV correction method for the finite-size system to revise the defect formation energy. Our research reveals that V doping could lead to a marked expansion of the lattice at the defect sites, making the crystal susceptible to damage. The band gap of the V-KDP crystals is smaller than that of the perfect KDP crystals because the V-3d orbitals hybridize with the O-2p orbitals to create defective energy levels at the conduction band minimum (CBM). Furthermore, we used configuration coordinate diagrams (CCD) and optical spectra to analyze the optical properties of V defects and found absorption peaks at 6.01 eV (206 nm) in tetragonal KDP and 5.68 eV (218 nm) in orthorhombic KDP. The calculation results indicate that the quality of the KDP crystals can be enhanced by reducing the concentration of V-impurities, which can serve as a theoretical guide for the practical application of KDP.

Effects of framework dopants on luminescent properties of red nitride phosphors via band structure modelling

Currently, researchers have been able to manipulate the luminescent properties and thermal stability of nitride red phosphor Sr2Si5N8:Eu2+ through rare earth doping. However, there is limited research on the doping of its framework. This work investigated the crystal structure, band structure, and luminescence properties of Sr2Si5N8: Eu2+ and its framework doped systems. We selected B, C, and O as doping elements because the corresponding formation energies of these elements doped structures are relatively low. Then, we calculated the band structures of various doped systems in both the ground and excited states. This analysis aimed to investigate their luminescent properties using the configuration coordinate diagram. The results show that doping with B, C, or O has minimal effect on the emission peak width. The thermal quenching resistance of the B- or C-doped system was enhanced due to the increased energy differences between the 5d energy level of the electron-filled state in the excited state and the bottom of the conduction band, compared to the undoped system. However, the thermal quenching resistance of the O-doped system varies depending on the position of the silicon vacancy. The work indicates that framework doping can also improve the thermal quenching resistance of phosphors besides rare earth ions doping.

Bistable Boron-Related Defect Associated with the Acceptor Removal Process in Irradiated p-Type Silicon—Electronic Properties of Configurational Transformations

The acceptor removal process is the most detrimental effect encountered in irradiated boron-doped silicon. This process is caused by a radiation-induced boron-containing donor (BCD) defect with bistable properties that are reflected in the electrical measurements performed in usual ambient laboratory conditions. In this work, the electronic properties of the BCD defect in its two different configurations (A and B) and the kinetics behind transformations are determined from the variations in the capacitance-voltage characteristics in the 243–308 K temperature range. The changes in the depletion voltage are consistent with the variations in the BCD defect concentration in the A configuration, as measured with the thermally stimulated current technique. The A→B transformation takes place in non-equilibrium conditions when free carriers in excess are injected into the device. B→A reverse transformation occurs when the non-equilibrium free carriers are removed. Energy barriers of 0.36 eV and 0.94 eV are determined for the A→B and B→A configurational transformations, respectively. The determined transformation rates indicate that the defect conversions are accompanied by electron capture for the A→B conversion and by electron emission for the B→A transformation. A configuration coordinate diagram of the BCD defect transformations is proposed.

Near-Infrared Emitting of Zero-Dimensional Europium(II) Halide Scintillators: Energy Transfer Engineering via Sm2+ Doping

Near-infrared (NIR)-emitting scintillators, coupled with high quantum efficiency silicon-based photodetectors, have emerged as a promising solution for highly efficient radiation detection applications. However, the study of efficient NIR-emitting scintillators and their associated luminescence mechanism is still limited. In this work, we developed NIR-emitting zero-dimensional Cs4EuBr6 halide scintillators via Sm2+ doping. Single crystals of Cs4EuBr6 with varying Sm2+ concentrations were grown by the vertical Bridgman method. Under X-ray irradiation, the scintillation emission of highly Sm-doped Cs4EuBr6 single crystals is dominated by the Sm2+ 5d–4f emission peaking at 831 nm with a weak Eu2+ 5d–4f emission peaking at 443 nm. The energy transfer processes between Eu2+ and Sm2+ were investigated by using photoluminescence (PL) spectra, PL decay kinetics, and soft X-ray excited decay kinetics measurements. Based on the temperature-dependent PL decay results, we constructed a configurational coordinate diagram of Sm2+ in the Cs4EuBr6 host. Furthermore, we evaluated the gamma spectroscopy response of Cs4EuBr6:Sm single crystals using an avalanche photodiode detector known for its high sensitivity in the NIR wavelength region.

Finite-size corrections to defect energetics along one-dimensional configuration coordinate

Recently, effective one-dimensional configuration coordinate diagrams have been utilized to calculate the line shapes of luminescence spectra and nonradiative carrier capture coefficients via point defects. Their calculations necessitate accurate total energies as a function of configuration coordinates. Although supercells under periodic boundary conditions are commonly employed, the spurious cell size effects have not been previously considered. In this study, we have proposed a correction energy formalism and verified its effectiveness by applying it to a nitrogen vacancy in GaN.

First-Principles Investigation on the Stability, Electronic Structure, and Exciton Self-Trapping Mechanism of 0D and 1D Cs3Cu2Cl5

Copper(I) halides with broadband emission from self-trapped excitons (STEs) are promising luminescent materials due to their structural diversities and intriguing photophysical properties. Despite great efforts invested in these materials, an in-depth understanding of their structural variations and the corresponding STE emission mechanisms is still lacking. Here, first-principles calculations reveal two distinct STEs in 0D and 1D polymorphs of Cs3Cu2Cl5, which feature the same chemical composition but different crystal structures. The electronic band structure analysis reveals that the strong Cu–Cl 4s–3p coupling along the 1D [Cu2Cl5]3– chain brings a large band dispersion, resulting in an increase of optical anisotropy in the 1D structure. The STE in 0D Cs3Cu2Cl5 involves the formation of one Cu–Cu bond and the breakage of two Cu–Cl bonds within an isolated [Cu2Cl5]3– unit. In contrast, the exciton self-trapping in the 1D structure is found to be associated with the formation of one Cu–Cu bond accompanied by two Cu–Cl bonds in the [Cu2Cl5]3– chain. Despite significant structural differences in 0D and 1D Cs3Cu2Cl5, quantitative characterizations of the configuration coordinate diagrams in the two polymorphs lead to similar STE emission energies (2.38 and 2.48 eV for 0D and 1D, respectively), which theoretically explains similar green-light emission experimentally observed in both compounds. This study provides important insights for understanding complex relations between the structural dimensionality, building units, and emission properties in low-dimensional metal halides.

Impact of Ga and N Vacancies at the GaN m -Plane on the Carrier Dynamics of Micro-Light-Emitting Diodes

Micro-light-emitting diodes ($\text{\ensuremath{\mu}}\mathrm{LEDs}$) based on $\mathrm{Ga}\mathrm{N}$ are a key component for next-generation displays, but serious surface $e$-$h$ recombination deteriorates the device efficiency. To gain microscopic insights into the surface recombination, we use hybrid functional first-principles calculations to investigate $\mathrm{Ga}$ (${V}_{\mathrm{Ga}}$) and $\mathrm{N}$ (${V}_{\mathrm{N}}$) vacancies on the $\mathrm{Ga}\mathrm{N}$ $m$ plane that can be created considerably during the production of one-dimensional $\mathrm{Ga}\mathrm{N}$ structures. We find that the surface ${V}_{\mathrm{Ga}}$ is not critical for nonradiative Shockley-Read-Hall (SRH) recombination in light of the large formation energy (>2 eV) and its shallow levels. In contrast, the surface ${V}_{\mathrm{N}}$ exhibits a low formation energy (2 eV) and develops a deep defect state near the midgap, indicating the possibility of becoming useful SRH recombination centers. By constructing configuration-coordinate diagrams, we demonstrate that the energy barriers for electron and hole capture on the surface ${V}_{\mathrm{N}}$ are small enough to cause significant capture coefficients due to strong electron-phonon coupling.

Photon avalanche assisted upconversion via customizing the green emission.

The developed SnWO4 phosphors incorporated with Ho3+, Yb3+ and Mn4+ ions have been explored under 980 nm laser irradiation. The molar concentration of dopants has been optimized to 0.5 Ho3+, 3.0 Yb3+ and 5.0 Mn4+ in SnWO4 phosphors. The upconversion (UC) emission from the codoped SnWO4 phosphors has been substantially amplified up to 13 times and described based on the energy transfer and charge compensation. On incorporating the Mn4+ ions in the Ho3+/Yb3+ codoped system the sharp green luminescence shifted to reddish broadband emission due to the photon avalanche mechanism. The processes accountable for the concentration quenching have been described based on critical distance. The interaction responsible for the concentration quenching in Yb3+ sensitized Ho3+ and Ho3+/Mn4+:SnWO4 phosphors is considered to be dipole-quadrupole and exchange interaction type, respectively. The activation energy 0.19 eV has been determined, and the phenomenon of thermal quenching is discussed using a configuration coordinate diagram.

Study on the optical properties and electronic structures of Ba-doped KH2PO4 crystals

Electronic structure and one-dimensional configurational coordinate diagrams considering the electron–phonon coupling are derived to calculate the spectra.

Barrierless Exciton Self‐Trapping and Emission Mechanism in Low‐Dimensional Copper Halides

AbstractLow‐dimensional copper halides having nontoxic elements are attracting increasing attention for their peculiar emission properties. Self‐trapped excitons (STEs) account for their high photoluminescence quantum yields (PLQYs) with emission that can stretch across the entire visible spectrum. However, intrinsic factors that influence the formation or loss of the emissive species in low‐dimensional copper halides remain elusive. Here, a comprehensive study on the STE formation dynamics of one‐dimensional CsCu2I3 and zero‐dimensional Cs3Cu2I5 is presented. It is found from STE kinetic analysis that a slower STE formation demonstrated by the 1D structure is not hindered by a potential barrier, but instead related to the number of phonons released in the self‐trapping process. It is further revealed that in 1D CsCu2I3, the non‐radiative recombination of STEs mainly occurs via the intersection between the STE state and the ground state in the configuration coordinate diagram, placing an intrinsic limit on the PLQY at room temperature. These findings show that the STE formation is affected by both the self‐trapping depth and the phonon energy as opposed to a potential barrier in low‐dimensional copper halides. The better understanding of STE formation and recombination processes provide basis for improving design and performance for broadband light emitting devices.