Acceptor States Research Articles

Visualizing chaperonin function in situ by cryo-electron tomography

Chaperonins are large barrel-shaped complexes that mediate ATP-dependent protein folding1–3. The bacterial chaperonin GroEL forms juxtaposed rings that bind unfolded protein and the lid-shaped cofactor GroES at their apertures. In vitro analyses of the chaperonin reaction have shown that substrate protein folds, unimpaired by aggregation, while transiently encapsulated in the GroEL central cavity by GroES4–6. To determine the functional stoichiometry of GroEL, GroES and client protein in situ, here we visualized chaperonin complexes in their natural cellular environment using cryo-electron tomography. We find that, under various growth conditions, around 55–70% of GroEL binds GroES asymmetrically on one ring, with the remainder populating symmetrical complexes. Bound substrate protein is detected on the free ring of the asymmetrical complex, defining the substrate acceptor state. In situ analysis of GroEL–GroES chambers, validated by high-resolution structures obtained in vitro, showed the presence of encapsulated substrate protein in a folded state before release into the cytosol. Based on a comprehensive quantification and conformational analysis of chaperonin complexes, we propose a GroEL–GroES reaction cycle that consists of linked asymmetrical and symmetrical subreactions mediating protein folding. Our findings illuminate the native conformational and functional chaperonin cycle directly within cells.

(Invited) Computational Analysis of the Excited States in the Film States of Non-Fullerene Acceptors

Non-fullerene acceptors are currently widely used as the acceptors of organic thin film solar cells due to their high efficiency. However, the molecular mechanism is still unclear. We have studied the excited states in the film states of non-fullerene acceptors together with experimental collaborators by using theoretical methods. In this presentation, we will present recent progress of our research. For example, we will talk about the excited states in the film states of ITIC. ITIC is a popular acceptor-donor-acceptor-type non-fullerene acceptor. Recently, our collaborator, Prof. Yamakata at Okayama University, Japan, found that the charge separation can take place in ITIC acceptor film even without donor molecules. We investigated this phenomenon by using MD simulations and quantum chemical calculations. MD simulations of the ITIC film showed that the acceptor parts of ITIC are stacked with each other. The dominant angle between two stacked ITIC was found to be ~180 degrees, making J-type dimer. On the other hand, it was also found that there are quite a few V-type dimers with the angles less than 90 degrees. Quantum chemical calculations revealed that the dipole moment in the excited state of J-type dimer is almost zero whereas that of V-type dimer is quite large, indicating strong charge-transfer excited state. Therefore, it is considered that the charge separation of ITIC film can take place in the V-type dimers. We also investigated the excited states in the film states of ITIC analogs. Recently, our collaborator, Prof. Ie at Osaka University, Japan, synthesized several ITIC analogs with different sidechains and found the different efficiency of solar cells. We theoretically investigated the excited states in the film sates of ITIC analogs as in the case of ITIC. It is found that the modification of side chains can change the distributions of angles between two stacked molecules and excited-state properties. These findings highlight the importance of controlling stacking structures in the film state for the efficiency of organic thin film solar cells.

(Invited) Atomic-Scale Modeling of Charge-Transfer Kinetics at LixCoO2 Cathode-Electrolyte Interfaces

In the search for Li-ion batteries with improved performance, interface engineering is a comparably underexplored area with significant opportunities for innovation. Ion transport across interfaces is, for instance, a significant bottleneck for fast charging/discharging. At electrode interfaces, it is often essential to capture the coupled motion of ions and electrons in order to provide accurate computational input for interfacial design. In this work we employ the recently proposed theory of coupled ion-electron transfer kinetics[1,2] in conjugation with constrained and constant-potential density functional theory to examine Li+ transfer across battery electrolyte-cathode interfaces at the atomic scale. Here, we focus on the common Li x CoO2 intercalation-type cathode material and liquid-state organic electrolytes. We establish trends in transport kinetics for different states-of-charge (SOC, i.e., varied degree of lithiation, x), types of electrolytes, and counterions that closely reproduces experimentally observed trends. From our modeling, we can identify a few key descriptors to predict performance metrics in Li-ion transport of a given electrolyte-cathode combination. These include the Li-ion free energy of adsorption, the free energies of reorganization of the cathode material and electrolyte upon the addition/removal of electrons, as well as the electronic coupling between electron acceptor and donor states. As these quantities can be relatively straightforwardly estimated for any given electrode-electrolyte pair, we envisage broad applications of our modeling approach in the interfacial design for batteries and related fields.[1] Fraggedakis et al., Electrochimica Acta, 367, 137432 (2021)[2] Bazant, Faraday Discussion, 246, 60 (2023)

(Invited) Wide Bandgap Semiconductor Homo- and Hetero-P-N Junctions

Wide bandgap (WBG) semiconductors, such as gallium nitride (GaN), gallium oxide (Ga2O3), aluminum nitride (AlN), and diamond, hold immense promise for power electronics applications by drastically reducing power loss, enhancing switching frequency, and reducing system volume. However, effective doping (p-type or n-type) remains a substantial challenge for these materials, given that the p-n junction is a fundamental building block for device design. Selective-area doping, crucial for high-performance electronic devices and integrated circuits, poses another significant obstacle.We report recent progress on selective-area doping for GaN homo-p-n junctions. Although GaN shows substantial development in both p-type and n-type doping, achieving selective doping, especially p-type, remains challenging through techniques such as ion implantation, diffusion, or in-situ doping. Our investigation delves into the challenges of p-GaN regrowth using metal-organic chemical vapor deposition (MOCVD). Interface contaminants post etching and regrowth, such as silicon (Si), carbon (C), and oxygen (O), have been systematically investigated. An optimized interface treatment has been identified to effectively reduce surface charge and, consequently, reverse leakage current to the level comparable to the as-grown p-n junction. Additionally, a novel hydrogen plasma treatment has been proposed to realize selective doping for GaN, successfully applied to p-GaN gated high electron mobility transistors (HEMTs) to improve the dynamic performance by avoiding the etching damage and edge termination for vertical GaN p-n diodes to realize high breakdown voltage.β-Ga2O3 has emerged as a standout in power electronics applications, surpassing Si (3000×), SiC (10×), and GaN (4×) in Baliga’s figure of merit. Its great potential for mass production, facilitated by the availability of high-quality and large-size wafers, significantly enhances its attractiveness. However, challenges, including the absence of p-type doping due to deep acceptor states and a flat valence band, as well as exceptionally low thermal conductivity, limit its performance at high power and temperatures. Overcoming these challenges necessitates hetero-integration with semiconductors possessing p-type characteristics and high thermal conductivity. P-type diamond stands out due to its exceptional thermal conductivity, high critical breakdown field, and well-established p-type characteristics. Moreover, since the n-type doping of diamond also poses challenges, fusing the p-type diamond and n-type Ga2O3 to create a diamond/Ga2O3 p-n-heterojunction can effectively overcome doping bottlenecks for both materials. Our successful construction of a diamond/β-Ga2O3 hetero-p-n junction through the mechanical integration of bulk p-type diamond and bulk n-type Ga2O3 presents robust electrical performance up to 125 ℃, with hysteresis lower than 0.7 V @ 1 μA. Impressively, the ideality factor of the p-n junction is remarkably low at 1.28, and the rectification ratio exceeds 108. This approach has also been applied to other hetero-p-n junctions, such as the diamond/GaN structure, demonstrating consistent robust performance. These findings underscore the significant potential of the mechanical integration approach, offering a promising avenue to simplify the fabrication process and facilitate the widespread application of WBG heterojunctions.

Native Defects Hybridization and Electronic Transport in Cu2Se‐CuGaSe2 Hierarchical Composites

AbstractEngineering the energy distribution of electronic defects in semiconductors can afford the coexistence of degenerate and nondegenerate transport behavior. By leveraging native defects in Cu2Se and CuGaSe2 phases, the tunability of the concentration and energy distribution of active electronic defects in hierarchical (1‐x)Cu2Se‐(x)CuGaSe2 composites is demonstrated. This study found that the electrical conductivity of various composites decreases with rising temperature indicating degenerate semiconducting behavior, whereas the carrier density increases with temperature, which is consistent with non‐degenerate semiconductivity. Remarkably, this study found that while the carrier density and electrical conductivity drop with the increasing CuGaSe2 content for samples with x ≤ 0.45 is consistent with an increase in the activation energy of “active” electronic defects, the sudden increase by 140% in the electrical conductivity and by 109% in the carrier density for near equimolar composition suggests the formation of a large density of electronic defects with lower activation energy. This unusual behavior is understood within the context of the hybridization of coexisting native electronic defects to form degenerate hybrid acceptor states with lower activation energy. The reported unique electronic transport can be leveraged for the development of more versatile electronic and optoelectronic devices with superior performance.

The Hydroxylated Carbon Nanotubes as the Hole Oxidation System in Electrocatalysis.

The hydroxylated carbon nanotubes (CNTs-OH), due to their propensity to trap electrons, are considered in many applications. Despite many case studies, the effect of the electronic structure of the CNT-OH electrode on its oxidation properties has not received in-depth analysis. In the present study, we used Fe(CN)63-/4- and Ru(NH3)63+/2+ as redox probes, which differ in charge. The CNT-OH and CNT electrodes used in the cyclic voltammetry were in the form of freestanding films. The concentration of holes in the CNTs-OH, estimated from the upshift of the Raman G-feature, was 2.9×1013 cm-2. The standard rate constant of the heterogeneous electron transfer (HET) between Fe(CN)63-/4- and the CNTs-OH electrode was 25.9×10-4 cm·s-1. The value was more than four times higher than the HET rate on the CNT electrode (ks=6.3×10-4 cm·s-1), which proves excellent boosting of the redox reaction by the holes. The opposite effect was observed for the Ru(NH3)63+/2+ redox couple. While the redox reaction rate constant at the CNT electrode was 1.4×10-4 cm·s-1, there was a significant suppression of the redox reaction at the CNT-OH electrode (ks<0.1×10-4 cm·s-1). Based on the DFT calculations and the Gerischer model, we find that the boosting of the HET from the reduced form of the redox couple to CNT-OH occurs when the reduced forms of the redox couples are negatively charged and the occupied reduced states are aligned with acceptor states of the nanotube electrode.

Fermi energy modulation by tellurium doping of thermoelectric copper(I) iodide

Copper(I) iodide (CuI) is the leading inorganic p-type transparent conductor, attracting major attention for its promising optoelectronic properties and facile growth methods, although, commercial uptake is limited due to its as-of-yet insufficient electrical conductivity. Doping CuI with the chalcogens (O, S, Se, Te) is a viable route to tune its electrical conductivity for applications such as in thin film transistors, hole transport layers in solar cells, and transparent thermoelectric generators. The heaviest chalcogen element, Te, is yet to be explored in heavily intrinsically p-type doped CuI at non-alloying concentrations, the subject of the present work. We report the effects of tellurium at the boundary between the doping and alloying regime (up to a maximum of 2.4 % Te) in CuI thin films and investigation the variation in the thermoelectric properties and electronic band structure of the material. Ion implanting tellurium into CuI led to a progressive reduction in the films' work functions from 4.9 eV to 4.5 eV while the ionization potential remained unchanged, measured through photoemission spectrometry. This signified a modulation of the Fermi energy relative to the valence band edge, having a major effect on the materials' electrical conductivity and Seebeck coefficient, the former decreasing by 3 orders of magnitude, while the latter increased by 80 %. We conducted density functional theory (DFT) calculations to elucidate the effect of tellurium doping on the band structure of CuI. Tellurium doping corroborated the shift of Fermi energy, the incorporation of impurity acceptor states deeper into the band gap, in addition to disordering the valence band maximum. This work shows that, the Fermi energy in heavily p-type doped CuI can be moved away from the valence band through Te doping in addition to introducing band disorder, useful for controlling the hosts’ transport properties.

Significantly enhanced reliability in defect-engineered BaTi1-xMgxO3 ceramics

BaTiO3-based ceramic is considered to be a fascinating dielectric material with high reliability for ultra-thin multilayer ceramic capacitors (MLCC). Here, we fabricated a series of acceptor-doped BaTiO3 (Mg: 0.2, 0.5, 1.0, 1.5, 2.0, and 5.0 mol%) ceramics with Al2O3 and SiO2 as sintering additives. With an increase in Mg concentration, the insulation resistance of the BaTi1-xMgxO3 ceramics initially increases and then decreases, in which the 0.5 mol% Mg-doped ceramic achieves superior degradation behavior. The reason is that with the incorporation of the acceptor ion Mg2+, the charge compensation mechanism in the system changes. Initially, barium vacancy is generated, which is gradually dominated by oxygen vacancy. Analysis indicates that the segregation of the acceptor state barium vacancy to the grain boundary increases the schottky barrier of the grain boundary, while the gradual increase of the donor state oxygen vacancy reduces the reliability of the ceramic. This study provides a comprehensive overview illustrating the significant role of point defects in the potential barrier of the grain boundary in acceptor-doped BaTiO3 ceramics. It identifies a pathway to achieve high reliability in BaTiO3-based MLCC.

Structural, optical, and electrical characterization of TiO2-doped yttria-stabilized zirconia electrolytes grown by atomic layer deposition

Electrolyte material optimization is crucial for electrochemical energy storage devices. The specific composition and structure have an impact on conductivity and stability, both of which are essential for efficient device performance. The effects of controlled incorporation of TiO2 into a Yttria-Stabilized Zirconia (YSZ) electrolyte using the atomic layer deposition (ALD) technique are investigated in this study. The surface chemical composition analysis reveals variations in the Ti oxidation state and a decrease in the O/(Zr + Y + Ti) ratio as TiO2 concentration increases. The formation of acceptor states near the valence band is proposed to reduce the bandgap with the Fermi level. The structural properties indicate that as TiO2 concentration increases, surface homogeneity and crystallite size increase. The contact angle with water indicates a hydrophobic behavior influenced by surface morphology and potential oxygen vacancies. Finally, electrical properties, measured in Ru/TiO2-doped YSZ/Au capacitors operated at temperatures between 100 and 170 °C, showed that the TiO2 incorporation improved the ionic conductivity, decreased the activation energy for conductivity, and improved the capacitance of the cells. This study highlights the importance of the ALD technique in solid-state electrolyte engineering for specific applications, such as energy storage devices.

Mn-doped Pt/ZnO adsorbent with enhanced synergetic Pt-sulfur acceptor interaction for effective atmospheric ultra-deep desulfurization

Developing high-activity and high-sulfur capacity adsorbent for reactive adsorption desulfurization (RADS) can be challenging when the relationship between rate-determining steps and synergetic metal-sulfur acceptor interaction is poorly understood. Herein, we report the desulfurization performance of Mn-doped Pt/ZnO with different Mn amounts and calcination temperatures under atmospheric RADS of dibenzothiophene (DBT) to understand rate-determining steps. Results suggested that the C-S cleavage of DBT and direct transfer of sulfur from PtSx to the sulfur acceptor were two rate-determining steps and relied on Pt and sulfur acceptor states. The Pt/ZnO-1MnO-400 was verified as the most outstanding adsorbent with 100 % desulfurization efficiency, 0.316 g-S/g-Ads sulfur capacity, and 55.3 h−1 TOF, attributing to form small Pt (∼2.5 nm) and agglomerate-free sulfur acceptor (∼35 nm) particles. This work provides innovative insight into the relationship between rate-determining steps and synergetic Pt-sulfur acceptor interaction, which paves the way for the design and synthesis of efficient adsorbent for RADS.

Selenium-alloyed tellurium oxide for amorphous p-channel transistors.

Compared to polycrystalline semiconductors, amorphous semiconductors offer inherent cost-effective, simple and uniform manufacturing. Traditional amorphous hydrogenated Si falls short in electrical properties, necessitating the exploration of new materials. The creation of high-mobility amorphous n-type metal oxides, such as a-InGaZnO (ref. 1), and their integration into thin-film transistors (TFTs) have propelled advancements in modern large-area electronics and new-generation displays2-8. However, finding comparable p-type counterparts poses notable challenges, impeding the progress of complementary metal-oxide-semiconductor technology and integrated circuits9-11. Here we introduce a pioneering design strategy for amorphous p-type semiconductors, incorporating high-mobility tellurium within an amorphous tellurium suboxide matrix, and demonstrate its use in high-performance, stable p-channel TFTs and complementary circuits. Theoretical analysis unveils a delocalized valence band from tellurium 5p bands with shallow acceptor states, enabling excess hole doping and transport. Selenium alloying suppresses hole concentrations and facilitates the p-orbital connectivity, realizing high-performance p-channel TFTs with an average field-effect hole mobility of around 15 cm2 V-1 s-1 and on/off current ratios of 106-107, along with wafer-scale uniformity and long-term stabilities under bias stress and ambient ageing. This study represents a crucial stride towards establishing commercially viable amorphous p-channel TFT technology and complementary electronics in a low-cost and industry-compatible manner.

Photocatalytic activity enhancement of two-step and one-pot synthesis of Pd/ZnO nanocomposites: an experimental and DFT study.

Pd/ZnO nanocomposites were successfully synthesized by means of one and two pot synthesis and applied in the photodegradation of Rh6G. The nanocomposites were characterized by XRD, SEM, TEM, FTIR and micro-Raman spectroscopies. It was found the presence of PdZn2, PdO and agglomerated particles in the support surface for the Palladium-based nanocomposites fabricated by one-pot route; the two-step method allowed the formation of spherical Pd nanoparticles, with homogeneous distribution in the nanocomposite matrix, with an average size of 2.16nm. The results show higher photocatalytic efficiency for the samples fabricated under the two-step approach compared to the one-pot synthesis. Based on experimental results, density functional theory (DFT) calculations were carried out to understand the enhancement photocatalytic of Pd/ZnO nanocomposites. To achieve it, the ZnO (001) and (101) surfaces were built and decorated by different Pd coverages. The theoretical results indicated two different photocatalytic mechanisms. In ZnO (001) case, the electrons flowed from surface to Pd, generating the superoxide radical anion (⋅O2-). Furthermore, the density of states of the ZnO (001) surface was modified by impurity Pd-d states at proximity to the conduction states, which may work as electron acceptors states. On the other hand, we found that the electrons flow from Pd to ZnO (101) surface, inducing the formation of ⋅OH and ⋅O2- for the degradation of Rh6G. The density of states of the ZnO (101) revealed a reduction in its bandgap, due to Pd-d states localized above valence states. Hence, our theoretical results suggest that the Pd-d states may facilitate the mobility of electrons and holes in (001) and (101) surfaces, respectively, reducing the rate of charge recombination.

Surface stability and morphological transformations of CsPbI3

Metal-halide perovskites, particularly inorganic cesium-lead halide perovskites, have emerged as exceptional candidates for several technological applications in the 21st century, such as photovoltaic devices, optoelectronic and photocatalysis. This study systematically investigates the CsPbI3 surfaces through density functional theory (DFT) simulations and morphological analyses. The (001), (110), and (111) surfaces were investigated in terms of their possible terminations (here named α, β, γ, δ and ε), where the relations between their outermost coordination polyhedra, bond lengths, charge distribution, electronic and morphological properties were revealed. The results demonstrate that the (001) and (110) surfaces stand out as the most stables, with Esurf001-α=Esurf110-γ=0.08J/m2. Concerning the electronic properties, it is observed that the (110) and (111) present α terminations with acceptor states, while the β with donor states, making it possible to tune the system semiconducting behavior (n or p-type) via surface termination control. The Wulff construction was employed to show that (001), (110) and (111) surface stabilizations can produce cubic, dodecahedral and octahedral nanocrystal morphologies, respectively. By probing the depths of CsPbI3 surfaces, this research advances new concepts about the design and functionalization of perovskite halide, offering a crucial direction for experimental synthesis strategies.

Origin of discrete donor–acceptor pair transitions in 2D Ruddlesden–Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

PROPERTY ENHANCEMENT OF VANADIUM DOPED SILICON CARBIDE: A COMPUTATIONAL STUDY

Dilute magnetic semiconductors (DMS) are those semi-conductor that show magnetic behavior when some impurities are doped in them. The DMS has an application in spintronic devices. Among all the materials for DMS SiC is a promising one as it shows stronger coupling and high Curie temperature so it is very useful for spintronics. One of the important polytypes of SiC is cubic silicon carbide. In this paper, we investigate when one of the TM vanadium (V) when doped with 3C-SiC studied by using the first principle energy code, Quantum Espresso which uses pseudopotential within DFT. Here we observe when V is doped with a Si substation site of 3C SiC then two deep levels will be introduced one is deep donor and another deep acceptor state. When vanadium doped with C substitution site of 3C-SiC half-metallic character introduced. Again here also calculate formation energy for both before and after relaxation. The calculation of formation energy indicates V impurity prefers the Si site to C site in cubic SiC.

Effect of the Heterovalent Sc3+ and Nb5+ Doping on Photoelectrochemical Behavior of Anatase TiO2

In this study, we explored the effect of either Nb or Sc doping at a concentration range of 0.0–1.0 at.% on the physical–chemical and photoelectrochemical behavior of TiO2 anatase electrodes. This behavior was characterized by work function, flat band potential, donor density, spectral dependence of photocurrent and stationary photocurrent measurements. All experimental results are interpreted in terms of the formation of the shallow delocalized polaron states in the case of Nb doping and deep acceptor states induced by Sc doping on TiO2 anatase.

Dielectric Screening and Charge-Transfer in 2D Lead-Halide Perovskites for Reduced Exciton Binding Energies.

Layered lead-halide perovskites have shown tremendous success as an active material for optoelectronics. This is attributed to the electronic structure of the inorganic sublattice and large exciton binding energies due to quantum and dielectric confinement. Expanding functionalities for applications that depend on free-carrier generation requires new material design routes to decrease the binding energy. Here we use electronic structure methods with model Bethe-Salpeter equation (BSE) to examine the contributions of the dielectric screening and charge-transfer excited-states to the exciton binding energy of phenylethylammonium (PEA2PbBr4) and naphthlethylammonium (NEA2PbBr4) lead-bromide perovskites. Our model BSE calculations show that NEA introduces hole acceptor states which impose charge-transfer character on the exciton along with larger dielectric screening. This substantially decreases the exciton binding compared to PEA. This result suggests the use of organic cations with high dielectric screening and hole acceptor states as a viable strategy for reducing exciton binding energies in two-dimensional halide perovskites.

Surface states modulation via vacancy engineering for creating band bending on semiconductor BiOBr photocatalysts

In microelectronic devices, surface modification could be used to introduce acceptor or donor surface states by defect engineering method to modify the alignment of energy band for desired properties, which was not used in the catalytic performance enhancement of powder photocatalysts. In this work, surface oxygen vacancies are induced to adjust the surface electrical structures and characteristics of the semiconductor bismuth oxybromide (BiOBr). In addition to being defect states to trap photoexcited electrons, surface oxygen vacancies also bring acceptor surface states, which result in an enhanced upward band bending and attract more photoexcited holes to the surface. Significantly, with a high concentration of electrons and holes accumulated on the surface, the BiOBr with surface oxygen vacancies shows good activity in the activation of O2 and oxidation of water compared to the one with bulk oxygen vacancies. Especially, it can decompose Rhodamine B (RhB) under natural sunlight in one minute, exceeding the majority of previously reported photocatalytic materials under comparable conditions. This work demonstrates that the surface oxygen vacancies could induce strengthened band bending at the BiOBr surface and remarkably improve catalytic performance, which provides a scalable strategy to practically manipulate the surface built-in electric field of powder photocatalysts.

The blue color mechanism on sapphires from different gem deposits before and after heating under oxidizing atmosphere

The blue color of sapphire is commonly related to the amount of Fe and Ti impurities replacing Al3+ in the Al2O3 structure. Generally, the color intensity on sapphires is related to the gem deposits including the basaltic-related and metamorphic-related ones. The color of sapphires has been changed after heating under oxidizing atmosphere. However, the explanation about the color mechanism from some previous research contradicted each other and it was still wondered. For this reason, this research is focused on the role of Fe and Ti oxidation states as well as the blue color mechanism on sapphires before and after heating under oxidizing atmosphere. In this study, the sapphire samples were collected from different gem deposits including basaltic-related sapphires from Kanchanaburi province, Thailand and metamorphic-related ones from Sri Lanka before and after heating at 1100 °C under oxidizing atmosphere. As a result, the blue color on sapphires before heating can be described as a hole color center assigned to Fe3+-Ti4+ mixed acceptor states inside an energy band gap that could receive an electron from the valence band for charge-balancing after excitation. After heating, the basaltic-related sapphires turned from dark blue to light blue and the metamorphic-related ones turned from light blue to colorless. The Fe3+-Ti4+ mixed acceptor states were decreased because a hole color center was filled by an electron from oxygen during the heating process instead of an electron from the valence band. Therefore, it can be concluded that the blue color mechanism on sapphires before and after heating under an oxidizing atmosphere can be explained by an energy band model involving the presence or absence of Fe3+-Ti4+ mixed acceptor states as well as a hole color center inside an energy band gap.

Density functional theory study of adsorption of organic molecules on ZnO monolayers: Implications for conduction type and electrical characteristics

Graphene-like ZnO monolayer, a typical two-dimensional material, has garnered enormous research attention due to its desirable optoelectronic properties and potential broad applications. However, the difficult in fabricating p-type ZnO material is still undergoing. Herein, the atomic and electronic properties of the adsorption of the selected organic molecules including electrophilic molecule tetracyano-p-quinodimethane (TCNQ) and nucleophilic molecule tetrathiafulvalene (TTF) on ZnO monolayers are carried out by the first-principles calculations. The results indicate that the adsorption of TCNQ on the ZnO monolayer leads to typical p-type doping, with a small gap (Ep) between the lowest unoccupied molecular and the valance band maximum with shallow acceptor states in the middle gap. However, the adsorption of TTF can obtain n-type doping of ZnO monolayer with the large gap (En) between the highest occupied molecular and the conduction band minimum with deep donor states. For TCNQ-ZnO monolayer, it exhibits the enhancement absorb in solar energy. There is a considerable charge transfer and strong non-covalent interaction between TCNQ and ZnO monolayer. The work function of TCNQ-ZnO ML is higher than that of pristine ZnO ML. However, the adsorption of TTF can significantly reduce the work function of ZnO ML, turning it into an n-type conduction semiconductor. Additionally, upon the adsorption of organic molecules, the layer thickness and the interlayer coupling as well as the adsorption concentration can modify the band gap of ZnO, and further tune the conduction type of ZnO. Remarkably, applying electric field can alter the number of hole and further greatly tune the electrical characteristic of TCNQ-ZnO ML. The current results can promote the applications of two-dimensional ZnO in nanoelectronics and optoelectronics.