Charge Transition Research Articles

Defect physics investigations in bulk NaBiO3 photocatalysts via Heyd-Scuseria-Ernzerhof hybrid density functional theory calculations.

This study employs Heyd-Scuseria-Ernzerhof hybrid density functional theory calculations to thoroughly investigate the n-type and p-type conductivity mechanisms of NaBiO3 photocatalysts. The results reveal that the intrinsic interstitial defect Na1+i is dominant under most growth conditions because of its lower formation energy. It is an excellent donor because of its shallower charge transition level. This makes it easily reach and even exceed the significant concentration of 1021 cm-3 with Na chemical potential regulation. Thus, in most circumstances, the intrinsic n-type conductivity of NaBiO3 found in experiments should primarily originate from the contribution of the interstitial defect Na1+i. The anti-site defect Bi2+Na also contributes to the unintentional n-type conductivity behavior. Especially under Na-poor and Bi-rich growth conditions, Bi2+Na becomes the dominant defect and is most responsible for the intrinsic n-type conductivity. The two major intrinsic defects, including Na1+i and Bi2+Na defects, can act as the photocatalytic reaction active sites or as a hole capture center (Bi2+Na) rather than as the recombination centers of the photo-generated electrons and holes in NaBiO3. On the other hand, based on thermodynamic simulation, the study examines the impacts of n-type and p-type doping at a fixed donor D+ or acceptor A- concentration on the conductive properties of NaBiO3 under different chemical potential conditions. It is indicated that p-type doping can convert the intrinsic n-type NaBiO3 into a p-type semiconductor only under non-thermal equilibrium growth conditions (quenching method). In contrast, n-type doping can easily enhance its n-type carrier concentration. Our results can guide optimizing the growth conditions to achieve high donor doping and high photocatalytic performance in NaBiO3 or NaBiO3-based materials.

Merging and separation of polarization singularities in complex lattices

Abstract Bound states in the continuum (BICs) and circularly polarized states (CPSs), as far-field polarization singularities, by controlling geometric parameters of photonic crystals, exhibit their evolution in momentum space that can be observed. This offers significant potential in optics and photonics. Here, we reveal that in complex lattices, far-field polarization singularities can be flexibly manipulated while preserving structural symmetry. The change of the topological charge for the at-Γ BIC can generate new BICs or CPSs. At an off-Γ point, a BIC can spawn from the collision of two CPSs. As the structure thickness increases, this BIC will meet the at-Γ BIC. The merging of BICs can induce the topological charge transition and yield a large wavevector space around Γ point with ultrahigh quality (Q) factors. Our findings provide a novel degree of freedom for manipulating polarization singularities, which holds great promise in radiation modulation and singular optics.

QDarts: A quantum dot array transition simulator for finding charge transitions in the presence of finite tunnel couplings, non-constant charging energies and sensor dots

QDarts: A quantum dot array transition simulator for finding charge transitions in the presence of finite tunnel couplings, non-constant charging energies and sensor dots

On the breakdown of Förster energy transfer theory due to solvent effects: atomistic simulations unveil distance-dependent dielectric screening in calmodulin.

Förster resonance energy transfer (FRET) is a powerful technique used to investigate the conformational preferences of biosystems, and molecular simulations have emerged as an ideal complement to FRET due to their ability to provide structural models that can be compared with experiments. This synergy is however hampered by the approximations underlying Förster theory regarding the electronic coupling between the participating dyes: a dipole-dipole term attenuated by a simple dielectric screening factor 1/n 2 that depends on the refractive index of the medium. Whereas the limits of the dipole approximation are well-known, detailed insights on how environment effects deviate from the 1/n 2 assumption and modify the R -6 distance dependence that characterizes FRET as a spectroscopic ruler are still not well understood, especially in biosystems characterized by significant structural disorder. Here we address this using a rigorous theoretical framework based on electrostatic potential-fitted transition charges coupled to an atomistic polarizable classical environment, which allows investigation of dielectric screening in atomic detail in extended simulations of disordered systems. We apply this strategy to investigate the conformational preferences of calmodulin, a protein that plays a major role in the transmission of calcium signals. Our results indicate that dielectric screening displays an exponential decay at donor/acceptor separations below 20 Å, significantly modifying the R -6 distance dependence widely adopted in FRET studies. Screening appears to be maximized at separations ∼15 Å, a situation in which the fluorophores are partially excluded from the solvent and thus screening is dictated by the more polarizable protein environment.

Anti-Site Defect-Induced Cascaded Sub-Band Transition in CuInS2 Enables Infrared Light-Driven CO2 Reduction.

Photocatalytic CO2 conversion is a promising approach to simultaneously mitigate climate change and alleviate the energy crisis. However, infrared light, which constitutes nearly half of the solar energy, has not been effectively utilized yet. In this work, we discover a photogenerated charge transition mechanism in CuInS2 with intrinsic InCu antisite defects for synergistic utilization of full-spectrum photons. Femtosecond transient absorption spectroscopy and DFT calculation unveil an intermediate band induced by the intrinsic antisite defects, where cascaded sub-band transition could be realized by high-energy photons (UV-vis) and low-energy (IR), thus improving the absorption range of infrared light as well as the utilization efficiency of photogenerated carriers. In situ Kelvin probe force microscopy demonstrates that the generation of photoexcited electrons could be greatly enhanced through this synergistic utilization of full spectrum light. Moreover, in situ X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy reveal that infrared photons could also enhance the adsorption and activation of CO2 and H2O on the catalyst surface. As a result, the CO production rate under full spectrum light reaches 19.9 μmol g-1 h-1, which is more than a 7-fold increase over that under UV-vis irradiation.

Optical Centers in Cr-, Mn-, and O-Doped AlN and Their Thermodynamic Stability Designed by a Multiscale Computational Approach.

The optical centers in AlN can frequently exist in various charge states and can be accompanied by many coexisting defect species, creating a complex environment where mutual interactions are inevitable. Therefore, it is an immediate quest to design AlN crystal growth protocols that can target a specific optical center of interest and tune its concentration while preventing the formation of other unwanted point defects. Here, we provide a powerful workflow for point defect engineering in wide band gap, binary semiconductors that can be readily used to design optimal crystal growth protocols through combining CALPHAD-based phase analysis, and ab initio defect calculations. We investigate technologically relevant chromium- and manganese-induced optical centers in AlN, followed by studying the impact of oxygen that can be unintentionally incorporated during crystal growth. We present the dominant defects in all three cases as a function of process parameters along with the optical signatures. In the case of both Cr and Mn doping, the CrAl and MnAl defects are most likely, and increasing nitrogen partial pressure tends to enhance their concentration. We show that it is possible to use nitrogen fugacity as a tool for tuning the intensity of optical signatures. We calculate the CrAl charge transition levels with respect to the valence-band maximum at 2.60 eV (E+/-), 3.83 eV (E0/-), and 5.41 eV (E-/2-) and electron and hole capture transitions with luminescence bands centered at 2.82, 1.91, and 3.15 eV. Unlike the Cr doping, Mn aggregation is unlikely, and the MnAl-VN is the most abundant defect after MnAl under most synthesis conditions. Oxygen tends to form complexes with VAl, and ON-VAl is a prominent defect following ON, with near-UV emission bands at 3.17, 3.26, and 3.81 eV. Our results agree with the available experimental optical signatures of Cr-, Mn-, and O-related centers and provide pathways on how to tune the luminous intensity of these centers through changes in growth conditions.

Universal Polaronic Behavior in Elemental Doping of MoS2 from First-Principles.

Elemental doping of two-dimensional (2D) semiconductors is crucial for manipulating their electrical and optical properties and enhancing the performance of advanced 2D devices. However, doping methods, such as ion implantation and chemical vapor deposition, can produce various outcomes extensively, depending on the chemical environment. We systematically study the elemental doping of the monolayer MoS2 by using density-functional theory calculations, which identify thermally stable sites among atomic substitutions, surface adsorption, and lattice interstitials of 27 elemental dopants, along with their formation energies and charge transition levels. By adopting the Koopmans-compliant hybrid functionals, the hydrogenic states predicted by semilocal functionals transform into localized polaronic states, which universally exhibit deep transitions located 1.0 eV away from the band edges. This polaronic behavior persists even in bulk MoS2, which suggests impurity conduction as the predominant carrier conduction mechanism. Our study offers fundamental insights into elemental doping in MoS2, which could be essential for doping transition metal dichalcogenides and similar 2D semiconductors.

Efficient optical property screening of :Ce (RE = , and Y): Based on activator level position in host band gap

Property screening is essential to identify optical materials with desirable luminescent and thermal properties for specific applications under various conditions. As promising candidates for optical applications across diverse fields, Ce-doped ▪ (RE = ▪, and Y) are studied by utilizing first-principles calculations based on density-functional theory. By employing a simple yet effective approach to analyze the electronic structures, we gain insights into the absorption edge characteristics of ▪, and evaluate the luminescent and thermal properties from the 4f and 5d positions of Ce in the host band gaps. Consequently, it is evident that the absence of luminescence in Ce-doped ▪ is due to the 5d excited states of ▪ being situated in the conduction band of ▪, regardless of its space group. While the 4f and 5d states of ▪ reside within the band gap of other ▪ hosts, potentially leading to luminescence. Excluding certain lanthanides due to potential charge transfer issues with ▪, our focus is directed towards the luminescent properties of Ce-doped ▪, ▪, ▪, and ▪. Based on schematic configuration coordinate diagrams, we conclude more significant thermal quenching resulting from 5d ionization in ▪:Ce and ▪:Ce compared to ▪:Ce and ▪:Ce. Furthermore, we provide valuable insights into the experimental phenomenon that the energy barrier for thermal quenching is smaller than that for thermal ionization in ▪:Ce. Additionally, we assess the photoelectric and thermoelectric performance based on the thermal and optical charge transition levels of Ce in ▪. Based on our study of the luminescent properties and thermal behaviors among Ce-doped ▪, we offer suggestions for screening Ce-activated optical materials, which provide valuable guidance for the design and optimization of specific materials.

Understanding the Excited Properties and the Luminescence Mechanisms of ns2 Type Ion Doped Phosphors: A First-Principles Study

Luminescent materials doped with ns2-type ions, such as Bi3+, Sb3+ ions, exhibit several excited states that can provide optical transitions, including the optical transitions within the electronic configuration of ns2 ions, the valence band to ns2 ions charge transfer (CT) transitions, the metal-to-metal charge transfer (MMCT) transition between the ns2 ions and the conduction band, and the intervalence charge transition (IVCT) between the dopant pair. The luminescence properties of these materials are significantly influenced by the environment, making the ns2 ions excellent activators for phosphors. Our talk will present the first-principles studies that revealed the geometric and electronic structures of the ground and excited states of ns2 ions, constructed the configuration coordinate diagrams, and analyzed the dynamic processes of excitation, relaxation, and emission. We will also briefly present some work on ns2 ions doped phosphors studied by the combination of first-principles and experimental methods, demonstrating the importance of theoretical calculations for the design and performance improvement of novel luminescent materials. Figure 1

(Invited) Low Contact Resistance on Buried Interfaces in Oxide Thin-Film Transistors by Selective-Area Reduction Using Hydrogenation Catalyst Electrode

As the focus on amorphous oxide semiconductor (AOS) in memory applications deepens, there is a push for higher packing densities and more efficient read-write efficiencies, leading to increasingly complex AOS transistor structures with dimensions scaled down to the nanometer size. In pursuit of high density for AOS-based dynamic random-access memory (DRAM), 3D-stacked vertical IGZO transistor devices have been employed, wherein the contact interfaces of these vertical devices are buried and located internally within the device, and these vertical or buried interfaces are prone to causing contact issues. However, conventional methods to addressing contact resistance in thin-film transistors (TFTs) used for flat-panel displays are not applicable to buried interfaces. The development of novel methods to resolve this issue is a current challenge for the future development of complex structured AOS memory devices.In this work, the contact resistance of a-IGZO TFT was reduced by three orders of magnitude by utilizing hydrogenation catalyst electrodes for a selective area reduction reaction at the interface and constructing a hydrogen transfer pathway1. The key to effective hydrogenation on the internally buried interface is the selection of the optimal catalyst metal electrode and passivation materials that satisfy the following requirements. 1) High hydrogen diffusion rate and moderate hydrogen solubility for reversible hydrogen transfer. 2) Catalytic hydrogen dissociation to highly active H0 atoms on/in the metal electrode for low-temperature reaction [Fig. 1(a)]. 3) Dense oxide passivation layer with hydrogen tolerant electronic structure to hydrogen, in which a shallower CBM than the charge transition level of E(H+/H−), to isolate the effective channel from hydrogen. Pd was selected as the best electrode for this strategy owing to its flexible lattice and unique properties of hydrogen permeability without brittleness to hydrogen. Also, amorphous Zn–Si–O (ZSO x ), which can be easily deposited by sputtering, was used as the passivation layer.Detection through hard X-ray photoelectron spectroscopy indicates that the high reactivity of H0 reduces metal oxides at the interface, forming a metallic interface layer. Fig. 1(b) shows the hydrogen annealing condition dependence of the contact resistance and effective channel length deviation estimated by TLM measurements. By optimizing hydrogen annealing conditions, the company succeeded in achieving both a small effective channel length deviation of ~40 nm and contact resistance that was reduced by approximately three orders of magnitude. The field-effect mobility was enhanced from 3.2 to ~20 cm2 V–1 s–1 as a consequent effect of the improved contact resistance. Furthermore, as shown in Fig. 1(c), the prepared a-IGZO TFTs also demonstrated excellent bias-stability, further proving the reliability of using catalytic electrodes and hydrogen tolerant passivation layer in the process.

Spontaneous Phase Transition Charge of Liquid Metals Induced NH3 Decomposition To Achieve GaN Nanomaterials at Room Temperature

Spontaneous Phase Transition Charge of Liquid Metals Induced NH₃ Decomposition To Achieve GaN Nanomaterials at Room Temperature

Atom-Vacancy-Defect-Derived Electric Hysteresis Loops and Stochastic Low-Frequency Noises in Few-Atom Layer MoS2.

Atom-vacancy-defects present in various materials yield numerous interesting physical phenomena, even obstructing high performance in some cases. On the other hand, their valuable applications to novel devices, such as nitrogen vacancy centers in diamond for quantum bits, have gathered significant attention. In particular, these tendencies become more substantial in two-dimensional (2D) (atomically) thin van der Waals layers. However, correlations with various kinds of atom defects are still under exploration. Herein, we find the stochastic behaviors of large hysteresis loops with strong photoresponse in the static electrical properties in few-atom layer semiconductors, molybdenum disulfide (MoS2). The temperature dependence and transmission electron microscopy reveal that they arise from pairs of two neighboring in-plane S-vacancy defects, which predominantly present only around the interface at the MoS2 flake/substrate, with activation energies ∼0.35 eV. The low-frequency (f) (LF) noise measurements clarify a high f shift in the two 1/f2-dependent regimes, implying stochastic behaviors of electric charges through the S-vacancy pairs with high-speed charge(spin) transitions across low kinetic energy barriers between narrow discrete states. The shallow energy sates are formed from the highly uniform S-vacancy pairs interacting with Mo atoms, which act like quantum dots. The observed stochastic operation holds promise for various application, particularly for probabilistic neuromorphic computation in artificial intelligence.

Thermoluminescence characteristics of UV-irradiated natural hackmanite

Hackmanite has received extensive attention from scholars in recent years. However, there is still no report on the thermoluminescence properties of natural hackmanites under UV irradiation, as well as its relative electron trap and luminescence center. In this paper, the structural defects and thermoluminescence spectroscopic properties of hackmanite are comprehensively characterized by XRD, TL spectroscopy, SEM, EPR, XPS and Raman spectroscopy. The traps depth of TL peaks are calculated by computerized glow curve deconvolution (CGCD) techniques. And the charge transition energy levels of intrinsic defects in Na8Al6Si6O24(Cl,S)2 are calculated by spin-polarized density-functional theory (DFT) in VASP. It shows that the low-temperature TL peaks of hackmanite are associated with Cl vacancies and photochromic properties. The high-temperature peak is caused by marginal oxygen vacancies. The results are conducive to deepening the understanding of structural defects of the hackmanites and linking the thermoluminescence with the phototropy.

Excitation-wavelength-dependent persistent luminescence from single-component nonstoichiometric CaGaxO4:Bi for dynamic anti-counterfeiting

Materials capable of dynamic persistent luminescence (PersL) within the visible spectrum are highly sought after for applications in display, biosensing, and information security. However, PersL materials with eye-detectable and excitation-wavelength-dependent characteristics are rarely achieved. Herein, a nonstoichiometric compound CaGaxO4:Bi (x < 2) is present, which demonstrates ultra-long, color-tunable PersL. The persistent emission wavelength can be tuned by varying the excitation wavelength, enabling dynamic color modulation from the green to the orange region within the visible spectrum. Theoretical calculations, in conjunction with experimental observations, are utilized to elucidate the thermodynamic charge transitions of various defect states, thereby providing insights into the relationship between Bi3+ emitters, traps, and multicolored PersL. Furthermore, the utility of color-tunable PersL materials and flexible devices is showcased for use in visual sensing of invisible ultraviolet light, multicolor display, information encryption, and anti-counterfeiting. These discoveries create new opportunities to develop smart photoelectric materials with dynamically controlled PersL for various applications.

The origin of broadband blue emission in zero-dimensional organic lead iodine perovskites: A first-principles study.

Broadband blue emission in zero-dimensional perovskites has received considerable attention, which is very important for the realization of stable blue-light emitters; however, the underlying formation mechanism remains unclear. Based on first-principles calculations, we have systematically studied the self-trapped excitons (STEs) behavior and luminescence properties in 0D-(DMA)4PbI6 perovskite. Our calculations show that there is a significant difference between the intrinsic STE luminescence mechanism (∼2.51eV) and experimental observations (∼2.70eV). In contrast, we found that the iodine vacancy (VI) is energetically accessible and exhibits a shallow charge transition level at ∼2.69eV (0/+1) above the valence band maximum, which provides the initial local well for the STEs formation. Moreover, the low electronic dimension synergistic Jahn-Teller distortion facilitates the formation of extrinsic excitons self-trapping. Further excited state electronic structure analysis and configuration coordinate diagram calculations confirmed that the broadband blue emission in 0D-(DMA)4PbI6 is the origin of VI-induced extrinsic STEs instead of intrinsic STEs. Therefore, our simulation results rationalize the experimental phenomena and provide important insights into the formation mechanism of STEs in low-dimensional perovskite systems.

Hierarchical Engineering on Built‐In Electric Field of Bimetallic Zeolitic Imidazolate Derivatives Towards Amplified Dielectric Loss

AbstractConstruction of built‐in electric field (BIEF) in nanohybrids has been demonstrated as an efficacious strategy to boost the dielectric loss by facilitating oriented transfer and transition of charges, thus optimizing the electromagnetic wave absorption property. However, the specific influence of BIEF on interface polarization needs to explore thoroughly and the BIEF strength should be further augmented. Herein, several dielectric systems incorporated Mott–Schottky heterojunctions and hollow structures are designed and constructed, where bimetallic zeolitic imidazolate framework are employed to derive Cu‐ZnO Mott–Schottky heterojunctions, and hierarchical structures and BIEF are further enriched by introducing hollow structure and reduced graphene oxide. The well‐established “double” BIEF verified by theoretical calculation and hollow engineering can regulate the conductivity, and enhance the polarization relaxation effectively. Especially, there always coexisted both enhanced charge separation and reversed charge distribution in this “double” BIEF, boosting the interface polarization. Attributing to the synergy of well‐matched impedance and amplified dielectric loss, the obtained hybrids exhibited superior absorption (reflection loss of −46.29 dB and an ultra‐wide effective absorption bandwidth of 7.6 GHz at only 1.6 mm). This work proves an innovative model for dissecting dielectric loss mechanisms and pioneers a novel strategy to explore advanced absorbers through enhancing BIEF.

Interface engineering on single-atom Cu-modified BiOBr1−xClx for efficient artificial photosynthesis of methanol

Converting CO2 into value-added fuels was an attractive way to alleviate the energy crisis. Cu-based catalysts were recognized as the most effective candidates for catalyzing CO2 to solar fuels due to their specific electronic structure. However, selective production of desired fuels from photocatalytic CO2 reduction remained a grand challenge. Herein, the interface engineering of single-atom Cu with BiOBr1−xClx nanosheets (Cu-SA/BOBC) was demonstrated to address this challenge. The single-atom Cu in BOBC favored charge transition, carrier separation, and photon utilization, with a superior performance of CO2 photoreduction yielding CH3OH of 8.21 μmol·g−1·h−1, 94.3 % electron-based CH3OH selectivity. Meanwhile, it could lower the CO2 activation energy barrier, and be conducive to strong CO* adsorption and subsequent hydrogenation of CO* to CHO*, which was critical for the high selectivity of CH3OH. This work highlighted a new insight into regulating the photoreduction of CO2 to CH3OH by interfacial interaction.

Effect of Electric Fields on the Decomposition of Phosphate Esters.

Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri-n-butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon-oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule-surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation energy with increasing electric field strength. Single-molecule nudged-elastic band (NEB) calculations also show a linear reduction in the energy barrier for carbon-oxygen bond breaking with electric field strength, due to stabilization of the charged transition state. The simulation results are consistent with experimental observations of enhanced and asymmetric tribofilm growth under electrostatic fields.

Phase diagram of the Hubbard chain with symmetric density-dependent hopping

By the bosonization and renormalization-group schemes, a half-filled Hubbard chain with density-dependent hopping is investigated at weak coupling. The model involved has an electron–hole symmetry and an SO(4) symmetry. The phase diagram consists of three different phases divided by a charge transition at on-site repulsion U=Uc and a spin transition at U=Us, with Us=−Uc=16π(tc−1), including a Mott insulator with the SDW correlation, a dimerized insulator with the BCDW correlation, and a Luttinger metal with the coexisting TS and BSDW correlations. The effective three-body repulsion (tc−1>0) and attraction (tc−1<0) induce the dimerized and metallic phases, respectively.

Optical and electrical studies on the TS defect in 4H-SiC

Abstract When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers&amp;#xD;occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence&amp;#xD;analyses on the single defect level. They reveal that the three occurring spectral&amp;#xD;signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose&amp;#xD;three different crystallographic orientations in the basal plane, which relate to the projections of&amp;#xD;the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies,&amp;#xD;when applying mechanical strain. This dependency is quantitatively calibrated. A complementary&amp;#xD;electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS&amp;#xD;defect at 0.6 eV above the valence band. For a future identification, this accurate characterization&amp;#xD;of its optical and electronic properties along with their response to mechanical strain is a milestone.