Midgap States Research Articles

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability.

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δ containing Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ (A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δ precursors with CaH2 at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δ are due to the simultaneous shift of the midgap states composed of Sn2+ with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δ precursors were essential to obtain H--doped products that have a low density of defects.

Deep machine learning unravels the structural origin of mid‐gap states in chalcogenide glass for high‐density memory integration

AbstractThe recent development of three‐dimensional semiconductor integration technology demands a key component—the ovonic threshold switching (OTS) selector to suppress the current leakage in the high‐density memory chips. Yet, the unsatisfactory performance of existing OTS materials becomes the bottleneck of the industrial advancement. The sluggish development of OTS materials, which are usually made from chalcogenide glass, should be largely attributed to the insufficient understanding of the electronic structure in these materials, despite of intensive research in the past decade. Due to the heavy first‐principles computation on disordered systems, a universal theory to explain the origin of mid‐gap states (MGS), which are the key feature leading to the OTS behavior, is still lacking. To avoid the formidable computational tasks, we adopt machine learning method to understand and predict MGS in typical OTS materials. We build hundreds of chalcogenide glass models and collect major structural features from both short‐range order (SRO) and medium‐range order (MRO) of the amorphous cells. After training the artificial neural network using these features, the accuracy has reached ~95% when it recognizes MGS in new glass. By analyzing the synaptic weights of the input structural features, we discover that the bonding and coordination environments from SRO and particularly MRO are closely related to MGS. The trained model could be used in many other OTS chalcogenides after minor modification. The intelligent machine learning allows us to understand the OTS mechanism from vast amount of structural data without heavy computational tasks, providing a new strategy to design functional amorphous materials from first principles.image

Light-induced modification of the Schottky barrier height in graphene/Si based near-infrared photodiodes

The impact of light on the Schottky barrier height (SBH) in p-type graphene/n-type Si (p-Gr/n-Si) based near-infrared photodiodes is investigated. Hall effect and optoelectronic transport measurements carried out under illumination of 905 nm wavelength light showed that zero-bias SBH in such photodiodes can be effectively tuned in a range between 0.7 and 0.9 eV consistent with the variation in their open-circuit voltage. Shockley-Read-Hall model, which considers the charge recombination through mid-gap and interface states at the p-Gr/n-Si heterojunction, is used to explain the experimentally observed nonlinear dependence of SBH on the incident light. Light induced tunability of SBH at the graphene/semiconductor heterojunction is of great importance especially for the development of new generation optically driven devices in which graphene acts as a functioning element.

Precise carbon doping regulation of porous graphitic carbon nitride nanosheets enables elevated photocatalytic oxidation performance towards emerging organic pollutants

• Short carbon chain-incorporated g-C 3 N 4 are prepared by carbon atom-self doping route. • Visible-light harvesting ability and charge transfer dynamics of g-C 3 N 4 are promoted. • As-prepared HCN-C x show C doping level-dependent photocatalytic oxidation activity. • Abundant ROS such as •O 2 − , 1 O 2 and •OH are generated in HCN-C x photocatalytic system. • •OH and 1 O 2 are particularly important to elevated photocatalytic oxidation activity. To improve the photocatalytic oxidation performance of graphitic carbon nitride (g-C 3 N 4 ) towards organic pollutants, the present work develops a homogeneous carbon atom-self doping strategy to prepare porous carbon-rich g-C 3 N 4 nanosheets (HCN-C x ). The preparation process of HCN-C x includes preorganization of L-cysteine and urea under hydrothermal environment followed by thermal copolymerization, and carbon doping level can be precisely adjusted by changing initial urea/L-cysteine molar ratio from 5000, 1667, 1000 to 500. The characteristic results combined with theory calculations confirm that C C C skeleton from L-cysteine are introduced into the heptazine framework of g-C 3 N 4 by the replacement of some C N C units; additionally, the introduction of C C C skeleton can generate defect-induced midgap states in the band structure of g-C 3 N 4 . The HCN-C x nanosheets exhibit carbon doping level-dependent and notably elevated photocatalytic oxidation activity to three emerging organic pollutants including acetaminophen (APAP), levofloxacin (LEV) and methylparaben (MPB), in which the HCN-C 0.5 performs the best. After visible-light irradiation of the HCN-C 0.5 for 10, 8 and 90 min, the removal efficiency of APAP, LEV and MPB reaches up to nearly 100%. The excellent photocatalytic oxidation performance of HCN-C x is dominated by carbon atom-self doping, which can not only enhance the visible-light harvesting efficiency but also boost photoexcited charge carrier transfer dynamics. Consequently, abundant reactive oxygen species including •O 2 − , 1 O 2 and •OH are generated, and they are responsible for the elevated photocatalytic oxidation performance of HCN-C x .

Atomically Precise Delineation of As Antisite Defect States from Undoped Gallium Arsenide Host Lattice by Scanning Tunneling Microscopy and Spectroscopy Measurements and Density Functional Theory Calculations

Using a combination of scanning tunneling microscopy (STM) and spectroscopy with density functional theory calculations, the electronic properties of the subsurface arsenic antisite defect (AsGa) are unambiguously delineated from those of the surrounding As atoms in undoped gallium arsenide (GaAs) lattice with atomic precision. In the GaAs(110) surface with AsGalocated at the second layer (2‐AsGa), it is found that the midgap state induced by 2‐AsGamanifests as a bright contrast at the AsGaAs bond site. Furthermore, it is shown that STM images taken at large magnitudes of negative sample bias are dominated by the local density of states of neighboring surface As atoms. These states lead to a four‐lobe symmetric contrast in the filled‐state STM image around the 2‐AsGadefect. These results provide insights for surface/subsurface defect engineering at the atomic scale.

Electronic properties of germanene on pristine and defective MoS2 : A first-principles study

The synthesis of germanene on semiconducting substrates such as ${\mathrm{MoS}}_{2}$ has revealed that it exhibits a V-shaped density of states around the Fermi level, indicating the presence of Dirac electrons. Further experiments demonstrated that charge inhomogeneities of germanene on ${\mathrm{MoS}}_{2}$ appear as $n$- and $p$-type doped regions, the origin of which is not well understood. In this paper, by means of density functional theory calculations, we study germanene deposited on ${\mathrm{MoS}}_{2}$ considering various defect types and a variety of stacking configurations. We find that some native defects typical to ${\mathrm{MoS}}_{2}$ samples lead to the charge transfer between germanene and ${\mathrm{MoS}}_{2}$. Unlike vacancies and antisite defects, substitution of molybdenum by group IV-V and VII transition metal atoms does not lead to any midgap states, and appears as a plausible explanation for the experimentally observed charge puddles. Our results shed light on the mechanism of breaking the charge neutrality in Dirac materials without altering their electronic properties, which could be important for the realization of lateral $p\text{\ensuremath{-}}n$ junctions based on two-dimensional materials.

Inducing chiral superconductivity on honeycomb lattice systems

Superconductivity in graphene-based systems has recently attracted much attention, as either intrinsic behavior or induced by proximity to a superconductor may lead to interesting topological phases and symmetries of the pairing function. A prominent system considers the pairing to have chiral symmetry. The question arises as to the effect of possible spin–orbit coupling on the resulting superconducting quasiparticle (QP) spectrum. Utilizing a Bogolyubov–de Gennes (BdG) Hamiltonian, we explore the interplay of different interaction terms in the system, and their role in generating complex Berry curvatures in the QP spectrum, as well as non-trivial topological behavior. We demonstrate that the topology of the BdG Hamiltonian in these systems may result in the appearance of edge states along the zigzag edges of nanoribbons in the appropriate regime. For suitable chemical potential and superconducting pairing strength, we find the appearance of robust midgap states at zigzag edges, well protected by large excitation gaps and momentum transfer.

Фотокаталитический нанореактор на основе низкоразмерных структур оксидов титана

Synthesized titanium oxide nanotubes from titanium raw materials are efficient nanoreactors for photocatalysis in comparison with macroanalogues. The dependence of the geometry of titanium oxide structures on the redistribution of the density of electronic states was studied. The presence of natural iron ligands leads to the appearance of midgap states inside the bandgap of titanium oxide.

Modulating the Midgap States of 3D–2D Hybrid ZnO by Codoping and Its Effect on Visible Photocatalysis

Modulating the Midgap States of 3D–2D Hybrid ZnO by Codoping and Its Effect on Visible Photocatalysis

The activation of bridged N atoms based on the structure engineering of PCN to boosts the release of visible-light photocatalytic hydrogen

The inherent electronic structure of the bridged N atoms of polymeric carbon nitride (PCN) weakens the carriers transport process, which seriously reduced the activity of PCN. How to modulate the electronic structure of bridged N and break its limitation of charge transfer is a major challenge. Herein, PCN with bridged N atoms activated (PCNG) was designed and synthesized by the thermal copolymerization of glyoxal and melamine. Experimental and DFT calculation results show that the strong hydrogen evolution ability of PCNG is due to the wide visible absorption range, which is contributed by the mid-gap states. More importantly, the LUMO of PCNG shows great delocalization on the all-heptazine units and forms a polarized electric field between heptazine units. Therefore, the optimal PCNG-1 performs so far high H2 evolution rate (261 umol·h−1) and apparent quantum yield (AQY = 5.32%, λ = 420 nm), under the visible light.

Unravelling a new many-body large-hole polaron in a transition metal oxide that promotes high photocatalytic activity

A many-body large polaron, which is important for both fundamental physics and technological applications, has been predicted to occur in bismuth vanadate (BiVO4). Herein, using a combination of high-resolution spectroscopic ellipsometry, X-ray absorption spectroscopy at the V L3,2- and O K-edges, and high-resolution X-ray diffraction supported by theoretical calculations, we reveal a new many-body large-hole polaron in W-doped BiVO4 films and the interplay of the large-hole polaron and indirect bandgap when determining the photocatalytic activity. With various W doping concentrations and temperatures, anomalous spectral weight transfers in the complex dielectric function are observed, revealing electronic correlations, particularly the on-site Coulomb interactions of O p (Upp) and V d (Udd), and screening in BiVO4. Due to the distortion of BiO8 dodecahedra and Udd, Bi 6 s is lifted to the top of the valance band, which results in the formation of an indirect bandgap and a large-hole polaron. The large-hole polaron is found to form as a localized midgap state, consisting of O p hybridized with the V d and Bi sp orbitals, and this is important when determining the high photocatalytic activity of BiVO4. Our results show the importance of the interplay among the charge, orbital, and lattice degrees of freedom in forming the many-body large-hole polaron, which improves the conductivity and results in a transition metal oxide with high photocatalytic activity.

Protective Coating Interfaces for Perovskite Solar Cell Materials: A First-Principles Study.

The protection of halide perovskites is important for the performance and stability of emergent perovskite-based optoelectronic technologies. In this work, we investigate the potential inorganic protective coating materials ZnO, SrZrO3, and ZrO2 for the CsPbI3 perovskite. The optimal interface registries are identified with Bayesian optimization. We then use semilocal density functional theory (DFT) to determine the atomic structure at the interfaces of each coating material with the clean CsI-terminated surface and three reconstructed surface models with added PbI2 and CsI complexes. For the final structures, we explore the level alignment at the interface with hybrid DFT calculations. Our analysis of the level alignment at the coating–substrate interfaces reveals no detrimental mid-gap states but rather substrate-dependent valence and conduction band offsets. While ZnO and SrZrO3 act as insulators on CsPbI3, ZrO2 might be suitable as an electron transport layer with the right interface engineering.

Role of Intra‐Band Relaxation of Holes and Tunneling of Electrons in Carrier Relaxation in AlGaAs/GaAs Quantum Well

The role of intra‐band hole relaxation and tunneling of electrons in the carrier relaxation in Al0.7Ga0.3As/GaAs quantum well (QW) is studied by performing pump‐probe transient reflectivity measurements with a femtosecond laser at room temperature. The ultrafast optical response of the QW sample is measured with an aim of understanding various ultrafast processes occurring on the time scale of ∼100 fs to 10 ps. Carriers are injected into the QW layer via a resonant photoexcitation of electron–hole pairs enabling a sharp rise of the signal. Carrier relaxation inside QW is dominated by non‐radiative processes like phonon‐mediated intra‐band relaxation of holes and tunneling of electrons out of the QW plane and their subsequent capture by the midgap states in the AlGaAs barrier. Subsequent to the initial thermalization within 100 fs, carrier relaxation is mainly driven by the tunneling of electrons and trapping by the midgap states over a time scale of 3–8 ps. In between, photoexcited holes are seen to relax from light to heavy hole states of QW within 1–2 ps, leading to a delayed rise of transient reflectivity signal. Systematic efforts are made to isolate various contributions which showed phonon re‐absorption leading to saturation effect in intraband relaxation.

Reinforced upconversion and charge separation via mid-gap states in WO3 nanosheet with infrared light driven tetracycline degradation

• Upconversion performance over WO 3 nanosheet was modulated by atomic doping. • Mobilized electron states increased after the atomic doping of Cu element. • NIR responsive catalytic performance was achieved over modified WO 3 . • The rate constant after modulation reached 14.4 times over that of pristine WO 3 . • Mineralization degree of TC reached 92.4% under NIR light irradiation. It is of crucial importance to fabricate near-infrared responsive (NIR) catalysts to maximize the utilization of solar energy. And achieving efficient charge separation under near-infrared light was also a great challenge. Herein, the synergy of both improved upconversion and charge separation performances of WO 3 was realized by atomic doping of Cu element. The induced mid-gap states can decrease the required excitation energy of photogenerated electrons to improve the upconversion ability, which enhanced the light absorption capacity of WO 3 nanosheet. Moreover, the introduced Cu element not only acted as the trapping sites of photogenerated charges, but also led to the charge redistribution of the crystal structure over WO 3 , which efficiently enhanced the charge separation process. As a result, excellent tetracycline photodegradation performance under NIR light (mineralization degree reaching 92.4%) was realized, where the rate constant of 1 wt% Cu-WO 3 (2.92 × 10 -3 min −1 ) reached 14.4 times over that of pristine WO 3 (2.02 × 10 -4 min −1 ).

Ab initio screening of refractory nitrides and carbides for high temperature hydrogen permeation barriers

Density functional theory was used to screen eleven refractory materials – two pure metals, six nitrides, and three carbides–as high-temperature hydrogen permeation barriers to prevent hydrogen embrittlement. Activation energies were calculated for atomic hydrogen (H) diffusion into the first subsurface layer from the lowest energy surface of the high-temperature phase of each candidate material. The candidate barrier materials with the highest activation energies are h-BN, c-BN, HfN, and ZrN with predicted barriers of 3.25 eV, 3.23 eV, 3.14 eV, and 2.76 eV, respectively. Strain energies, Bader charges, and density of states were calculated for the diffusing H at the relaxed initial state and the transition state to provide insight into contributing factors to high energy barriers. The diffusing H atom in materials with the highest predicted barriers are protic. In addition, interstitial H atoms induce mid-gap states in the density of states of both BN polymorphs. The nitrogen retention of each nitride material at high temperatures was predicted using nitrogen vacancy formation energies with respect to gaseous nitrogen. Experimental evaluation of nitrogen retention in h-BN, ZrN, and TiN confirmed their resistance to nitrogen loss at 1773 K. However, of these nitrides, TiN is predicted to be the least stable. This work identifies multiple promising materials that are predicted to be effective hydrogen barriers at high temperatures and that are stable at temperatures above 2700 K, with BN predicted to perform best.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts.

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials’ morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material’s improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

Effect of impurities from deposition precursors on the electronic properties of Si/SiO2 interfaces

In this study, we investigated the effect of impurities—carbon (C), nitrogen (N), and fluorine—on the electronic properties of a Si/SiO2 interface by measuring the atomic layer-resolved partial local density of states using first-principles simulations. Our results show that C and N impurity atoms with dangling bonds (DBs) induce midgap states at the interface. If DBs are terminated by hydrogen atoms, the formation of midgap states is effectively suppressed even in the presence of impurity atoms at the interface. If DBs are terminated by an oxygen (O) atom, both π bonding and π∗ antibonding orbitals of C=O are occupied and such occupied antibonding orbital forms a midgap state at the interface, increasing the leakage current. Based on these observations, we propose that preparation of the SiO2 substrate with oxygen-rich conditions prior to Si deposition is important in realizing a defect-tolerant interface. In addition, hydrogen treatment after Si deposition recovers the pure band structure by terminating the midgap states, thereby preventing leakage current.

Covalent adsorption of functional groups on [N]-carbophenes

Starting from the planar molecule 1,3,5-trihydroxybenzene, Du et al reported synthesizing one of a couple of possible 2D materials: graphenylene or 3-carbophene []. 3-carbophene is a member of a novel class of two-dimensional covalent organic framework, [N]-carbophenes (carbophenes). Using a high throughput method, we computed the formation energies and conduction properties of 3-, 4-, 5-, and 6-carbophenes with hydroxyl (OH), carbonyl (CO), nitro (NO2), amine (NH2), carboxyl (COOH) functional groups replacing hydrogen terminating agents. Five hundred and nine structures with randomly picked motifs, with functionalizations from a single functional group per cell to fully functionalized were studied. Our results demonstrate a negatively sloped linear relationship between the degree of functionalization and formation energy when the type of functional group and type of carbophene are held constant. The decrease in formation energy with functionalization makes Du’s synthesis of functionalized 3-carbophene more creditable. The type of carbophene, type of functional group, and the degree of functionalization all play a role in the band structure of the materials. For example, CO functional groups may lead to a mid-gap state pinned to the Fermi level, whereas the other functional groups studied keep the semiconducting nature of pristine carbophene. Thus, because carbonyl functional groups are often present in defected carbon systems, care should be taken to limit the amount of oxygen in carbophene devices where the band gap is important. Thus, this work strengthens the hypothesis of Junkermeier et al's hypothesis that Du et al synthesized 3-carbophene and not graphenylene [].

Unraveling the reinforced photoelectrocatalytic activity and stability via unique configuration of P3+-Ov-Ti3+ in TiO2-x nanotube array

Blue defective TiO2 is particularly attractive compared to TiO2 owing to its high electron conductivity and oxygen vacancies (Ov). However, it remains a challenge to overcome short-time deactivation in that active site of Ti3+ coordinated with high-electronegative oxygen atom usually aggravates the tendency to high valence state. Herein, we design a unique configuration of P3+-Ov-Ti3+ in TiO2-x nanotube array via utilizing PH3 to synchronize the reaction during the transformation from amorphous TiO2 to anatase detective TiO2 (TiO2-x). The performance results and DFT simulations reveal that the 3 s orbital (M shell) of P3+ in PT1 has a delocalized electron pair and significantly changes the valence electron configuration in 3d orbital of Ti3+ in the configuration P3+-Ov-Ti3+, thus regulating its electronic structure and forming a midgap state at the band gap. The unique structure of P3+-Ov-Ti3+ allows Ti3+ to exist in a more stable form via modulating electronic structure and constructing a new channel for electron transfer, resulting in a remarkable photoelectrocatalytic performance and an unprecedented stability with the lifetime of over 39632 h in 1 M NaClO4 electrolyte at 1 mA/cm2. This work not only provides an efficient way to prepare the stable photoelectrodes but paves the way to identify the roles of phosphorus with different valence states in catalytic activity and stability.

Absence of midgap states due to excess electrons donated by adsorbed hydrogen on the anatase TiO2(101) surface

Hydrogen is involved in various important chemical processes on metal-oxide surfaces. Due to its electron-donating character, hydrogen tends to form polaronic states, where the electron is spatially separated from the proton, which exerts significant impact on the electronic properties of metal-oxide surfaces. By individually probing the proton with nuclear reaction analysis and the electron with ultraviolet photoelectron spectroscopy and in combination with density functional theory calculations, we clarify the electronic state of hydrogen on the anatase ${\mathrm{TiO}}_{2}(101)$ surface. Hydrogen is found to adsorb with a saturation coverage of $0.48\ifmmode\pm\else\textpm\fi{}0.12$ monolayers, donating an excess electron to the substrate without forming a midgap state, which is in contrast to the behavior of oxygen vacancies.