Semiconductor Energy Band Research Articles

Synthesis of recyclable core/shell CoFe2O4@CoWO4 photocatalysts for efficient visible-light photocatalytic degradation of environmental pollutants

In this paper, a novel core/shell heterostructure of CoFe2O4@CoWO4 was synthesized as efficient recyclable magnetic photocatalysts. The results showed that CoFe2O4@CoWO4 displays an enhanced MB dye photocatalytic degradation activity (96.3%) compared to that of pure CoFe2O4 (10.8%) and pure CoWO4 (48.9%) after visible light irradiation for 2 h. The enhanced photocatalytic activity of CoFe2O4@CoWO4 can be attributed to the synergistic effect of magnetic core CoFe2O4 and CoWO4 shell. The CoFe2O4@CoWO4 nanocomposite was confirmed to be an effective in reducing e-/h+ recombination through the identical interaction among the energy bands of CoFe2O4 and CoWO4 semiconductors. Besides, the result shown that the magnetic CoFe2O4@CoWO4 photocatalyst was stable and separated easily by magnet with excellent recyclability.

Lotus-leaf-like Bi2O2CO3 nanosheet combined with Mo2S3 for higher photocatalytic hydrogen evolution

The exploration of multiple composite with heterojunction structures to replace single catalysts with insufficient capacity is significant for the photocatalytic evolution of high-yield hydrogen. In this case, the Mo2S3/Bi2O2CO3 composite with a unique n-n heterojunction and two-dimensional (2D) spatial structure was successfully prepared for the first time using the hydrothermal-physical mixture method. The percentage of Bi2O2CO3 in the composite can be easily adjusted by changing the amount of Bi2O2CO3 introduced into the physical mixing process. With a mass percentage of Bi2O2CO3 attained 3% to Mo2S3, the composite Mo2S3/Bi2O2CO3 with n-n heterojunction exhibited the highest photocatalytic effect among all as-prepared samples, with a photocatalytic effect 5 times higher than pure Mo2S3. The presence of Bi2O2CO3 and the synergistic interactions of n-n heterojunction significantly reduced the dispersity of short-rod shaped Mo2S3, as well as the recombination of photogenerated charge carriers. As a result, electron circulation was improved, and photocatalytic hydrogen evolution activity under visible light was improved. In addition to improving the photocatalytic hydrogen evolution effect, the composite Mo2S3/Bi2O2CO3 has been found to have excellent stability, which is a noteworthy feature. A series of characterization results and semiconductor energy band structure were used to investigate the possible photocatalytic hydrogen evolution mechanism in the Eosin-Y (EY) sensitized Mo2S3/Bi2O2CO3 system.

CQDs modulating Z-scheme g-C3N4/BiOBr heterostructure for photocatalytic removing RhB, BPA and TC and E. coli by LED light

To improve the photocatalytic performance, it is necessary to reduce the combination of photo-induced charges and improve the absorbance intensity of visible light. Herein, we have reported novel heterojunctions of indirect Z-scheme CQDs/g-C3N4/BiOBr composites (noted as CN@BOB-CQDs). Various techniques proved the successful preparation of heterojunction structure between CQDs, g-C3N4 and BiOBr. Compared with g-C3N4 and BiOBr, the composites presented excellent performance in removing Rhodamine B (RhB), bisphenol A (BPA) and tetracycline (TC) and E. coli by LED light illumination. The improvement of photocatalytic properties was assigned to its good separation efficiency of generated charges and strong redox ability of its photo-excited electrons (e) and holes (h+), which realized from the Z-type photo-excited charges transition in CN@BOB-CQDs composite. Besides, CQDs could promote the transmission of electrons and enhance the absorbance intensity of visible light. Finally based on the radicals’ trapping experiments and the semiconductor energy band position, the photocatalytic mechanism on the degradation of RhB was illustrated.

Revealing the correlation between gas selectivity and semiconductor energy band structure derived from off-stoichiometric spinel CdGa2O4

Herein, the spinel CdGa2O4 and off-stoichiometric spinel CdGa2O4 (CdO-CdGa2O4) with 3D inverse opal spherical structure have been synthesized by the combination of ultrasonic spray pyrolysis technology and self-assembled hard-template method. The CdGa2O4, with band gap of 4.14 eV and baseline resistance of Mohm-level, exhibited excellent selectivity and high response to 1 ppm formaldehyde. On the other hand, the CdO-CdGa2O4 composites (Cd:Ga atom ratio = 1.2:2), with band gap of 2.46 eV and baseline resistance of kohm-level, exhibited best sensing performance to 1 ppm NO2 with negligibly low cross-responses (Rair/Rgas < 3.5) to any VOCs. Due to the semiconductor band gap is narrowed, and the conducting channel depending on the agglomeration of surface loaded CdO nanocrystals, the gas selectivity has been changed and the baseline resistance of off-stoichiometric spinel CdGa2O4 was reduced by many orders of magnitude. Meanwhile, the improvement of gas response can be attributed to ordered bimodal-porous structure, and the n-n heterojunctions induced decrease in work function and enhancement in chemisorbed oxygen. Accordingly, the proposed correlation between gas selectivity change and band gap modulation can open new route for solving the shortcoming of poor selectivity of metal oxide semiconductor gas sensor.

Two‐step pyrolysis preparation of co‐doped porous g‐C3N4 with Co–N coordination bond for dye efficient degradation driven by visible light

AbstractBACKGROUNDIn this study, porous g‐C3N4 (PCN) doped with different masses of cobalt (1%, 3%, 5%, 10%) (Co‐PCN) were obtained by a simple two‐step calcination method. Herein, choosing Rhodamine B (RB) as target pollutant, RB degradation efficiency of Co‐PCN (5%) driven by visible light was as high as 98.1%, which is 1.28 and 2.18 times that of PCN and g‐C3N4, respectively.RESULTSCharacterization results confirmed that Co‐PCN showed abundant pore structures and was doped with cobalt by Co–N coordination bond without destroying its crystal structure. In addition, the band gap of Co‐PCN was adjusted from 2.44 eV (g‐C3N4) to 2.28 eV, and its photocurrent intensity was 1.5 times that of g‐C3N4. The capture experiments showed that • O2− and h+ are the main active species in the reaction.CONCLUSIONCombined with analysis of the semiconductor energy band theory, the excellent photocatalytic performance of Co‐PCN is attributable to the synergistic effect of its stronger visible light absorption ability obtained by adjusting band gap energy and the host‐guest interaction between cobalt and g‐C3N4. In addition, Co‐PCN possessed the satisfactory stability and photocatalytic degradation ability in actual water matrix, which is expected to provide a new strategy for actual wastewater treatment. © 2021 Society of Chemical Industry (SCI).

The Development of Color Display Technology

Light-emitting devices play a vital role in people’s lives, and their extremely wide application has won the attention and research of many scientists for a long time. Light emitting diode based on semiconductor energy band and electro-optical conversion is the mainstream light-emitting devices. Furthermore, control the light by nano optical technology such as plasmon effect and the quantum dot is also attractive. This article reviews the latest research progress of the above three light-emitting devices and their principles and characteristics. Finally, we briefly summarized the advantages and disadvantages of these devices, as well as the possible development directions and potential applications of the next generation light-emitting devices.

Environmental-friendly synthesis of heterojunction photocatalysts g-C3N4/BiPO4 with enhanced photocatalytic performance

Heterojunction photocatalysts g-C3N4/BiPO4 with varying loadings of g-C3N4 were synthesized by ball milling process in different time, the synthetic method of which was a simple, rapid, economical, and easy-controlled way. The evaluation for the performance of as-prepared g-C3N4/BiPO4 was conducted by the degradation of tetracycline (TC) in stimulated sunlight condition. The superior performance was achieved from g-C3N4/BiPO4 heterostructure photocatalyst with 5 wt% g-C3N4 and ball milling time of 18 h, which could decompose 97% TC after 120 min illumination. The enhanced photocatalytic activity could be ascribed to the synergistic effects, including changes in morphology and structure after ball milling, increased specific surface area, outstanding visible-light absorption, and efficient separation of electron-hole pairs. The possible degradation mechanism was clarified by semiconductor energy band theory. This work may present a novel strategy for preparing heterojunction photocatalysts with high photoactivity and provide the analysis method of mechanism by combining theoretical calculations and experimental observations.

Photo-stimulated anoxic reduction of birnessite (δ-MnO2) by citrate and its fine structural responses: Insights on a proton-promoted photoelectron transfer process

The photo-enhanced dissolution of semiconducting birnessite by organic reductants prevalently occurs in the euphotic zones of natural aquatic systems. However, the underlying photoelectrons transfer mechanisms and kinetic factors are not fully understood. In this study, the anoxic photoreduction of δ-MnO2 (a phase analogue for natural birnessite) by citrate under simulated solar irradiation at pH 4.0, 5.5 and 7.0 was investigated, and the structural evolution of δ-MnO2 during the reaction was examined. The photoreduction of δ-MnO2 was observed at all three pHs. ~86.0% and ~32.7% Mn2+ was released into the solution at pH 4.0 and 5.5, respectively; while negligible amount of dissolved Mn2+ was produced at pH 7.0. Mn average oxidation state (AOS) in both the bulk and surface of δ-MnO2 lowered significantly with decreasing pH (3.39/2.56, 3.57/3.07 and 3.78/3.19 in bulk/surface at pH 4.0, 5.5 and 7.0, respectively). The increase in the photoreduction rate and extent of δ-MnO2 at lower pHs indicated the photoelectron transfer was pH-dependent. From the amount of reduced Mn(III/II) in structure and Mn2+ in solution, the average photoelectron transfer amount per mol Mn was calculated as 1.81, 0.95 and 0.22 at pH 4.0, 5.5 and 7.0, respectively. The retention of Mn(II) (~13% in content) in the structure of reacted δ-MnO2 was only observed at pH 4.0 rather than higher pHs. The photoreduction of Mn(IV) involved two successive steps of single-electron transfer to produce Mn(III) and further Mn(II), and the above results suggested the second-step photoelectron transfer was more effective under acidic conditions. The potential difference between the conduction band (CB) and MnO2/Mn2+ half-reaction was enlarged as pH decreased, which conduced to the optimal effectiveness of photoelectron transfer and thorough reduction of Mn(IV/III) under lower pH conditions. The pH-dependent Mn photoreduction occurred throughout the structure of δ-MnO2, whereas for the dark reaction, the influence of pH on the reduction extent only took effect on the surface. These observations implied the dark-reaction electron transfer predominantly occurred at the surface sites. By contrast, according to the semiconductor energy band theory, the excited CB photoelectrons were shared by all atoms in the lattice of δ-MnO2, and each Mn atom had equal chance to be reduced by photoelectrons. Furthermore, the accumulation of photo-reduced and large-size Mn(III/II) in the layer caused the spatial expansion of the in-plane crystallization, and could possibly alter the layer symmetry or even promote the long-term structural transformation towards tunneled Mn oxides. This work provides insights into the photocatalytic geochemical performance of the naturally-abundant birnessite, which is influenced by varied pH conditions on Earth's surface.

Modeling of the electrical characteristics of an organic field effect transistor in presence of the bending effects

Abstract An analytical model incorporating the density of trap states for a bendable organic field effect transistor (OFET) is presented in this paper. The aim of this work is to propose a novel modeling framework to quantitatively characterize the bending effects on the electrical properties of an OFET in the linear and saturation regimes. In this model, the exponentially distributed shallow trap states are introduced into the Poisson equation to describe the carrier transports in the channel. The carrier mobility takes into account the low field mobility enhancement under gradual channel approximation and high field degradation. As a result, the generalized current-voltage transistor equations are derived for the first time to reflect the transconductance relationships of the OFET with trap states. In addition, an electro-mechanical coupling relationship is established per the metaphorical analogy between inorganic and organic semiconductor energy band models to quantify the stress-induced variations of the carrier mobility, and the threshold voltage. It is revealed that the before- and after-bending transconductances, predicted from the derived analytical model, are in good agreement with the experimental data measured from DNTT-based OFET bending tests.

Van der Waals Mixed Valence Tin Oxides for Perovskite Solar Cells as UV-Stable Electron Transport Materials.

Stable electron transport materials (ETMs) with fewer surface defects and proper energy level alignments with halide perovskite active layers are required for efficient perovskite solar cells (PSCs) with long-term durability. Here, two-dimensional van der Waals mixed valence tin oxides Sn2O3 and Sn3O4 are controllably synthesized and applied as ETMs for planar PSCs. The synthesized Sn2O3 and Sn3O4 have size of 5-20 nm and disperse well in water as stable colloids for months. Both Sn2O3 and Sn3O4 exhibit typical n-type semiconductor energy band structures, low trap density, and suitable energy level alignments with halide perovskites. Steady-state power conversion efficiencies (PCEs) of 22.36% and 21.83% are obtained for Sn2O3-based and Sn3O4-based planar PSCs. In addition, the half cells without hole transport materials and back electrodes show good UV-stability with average PCE of 99.0% and 95.7% for Sn2O3-based and Sn3O4-based devices remaining after 1000 h of ultraviolet soaking with an intensity of 70 mW cm-2.

Direct Electrical Measurements of Energy Bands in Organic Semiconductors by Device-Based Ballistic Carrier Emission Spectroscopy

Understanding the electronic structure of organic/molecular semiconductors and the energetics at their interfaces with metals is critical for organic-based electronics applications. Here we demonst...

Application of La0.3Sr0.7Fe0.7Ti0.3O3-δ/GDC electrolyte in LT-SOFC

Recently semiconductor-ionic electrolytes have capture much attentions for its promising application in low temperature solid oxide fuel cells. In this paper, semiconductor La0.3Sr0.7Fe0.7Ti0.3O3-δ (LSTF) and ionic Ce0.9Gd0.1O2-δ (GDC) composites are investigated. The new composite consisting of LSTF and GDC are used as ionic-semiconductor electrolytes for LT-SOFC. It was found that when the ratio of LSTF to GDC was 5:5, the performance of the fuel cell is the best. The open circuit voltage is 0.92 V, and the power density is 654 mW cm−2 at 600 °C. Its electrical performance is much higher than that of traditional SOFC at low temperature. The energy band of different semiconductor is discussed by using optical equipment.

The semiconductor SrFe0.2Ti0.8O3-δ-ZnO heterostructure electrolyte fuel cells

Highly ion-conducting properties in heterostructure composites and semiconductors have drawn significant attention in recent years for developing new electrolytes in low-temperature solid oxide fuel cells (LT-SOFCs). In this work, a new semiconductor heterostructure composite SrFe0.2Ti0.8O3-δ (SFT)-ZnO consisting of p-type SFT and n-type ZnO is proposed and evaluated as an electrolyte in LT-SOFCs. Electrochemical studies reveal that the prepared SFT-ZnO is a mixed ion-electron conductor possessing a high ionic conductivity of 0.21 S cm−1 at 520 °C and the assembled SFT-ZnO fuel cell can achieve a favorable peak power output of 650 mW cm−2 along with high open-circuit voltage (OCV) of 1.06 V at 520 °C. By referring the semiconductor conduction types and energy band parameters of SFT and ZnO, a p-n bulk-heterojunction effect is proposed to describe the electronic blocking and ionic promotion processes of SFT-ZnO electrolyte in a fuel cell. Our work suggests a new insight into the design of effective LT-SOFC electrolytes by using semiconductor heterostructure material.

Efficient Gate Modulation in a Screening-Engineered MoS2/Single-Walled Carbon Nanotube Network Heterojunction Vertical Field-Effect Transistor.

In this report, a screening-engineered carbon nanotube (CNT) network/MoS2/metal heterojunction vertical field effect transistor (CNT-VFET) is fabricated for an efficient gate modulation independent of the drain voltage. The gate field in the CNT-VFET transports through the empty space of the CNT network without any screening layer and directly modulates the MoS2 semiconductor energy band, while the gate field from the Si back gate is mostly screened by the graphene layer. Consequently, the on/off ratio of CNT-VFET maintained 103 in overall drain voltages, which is 10 times and 1000 times higher than that of the graphene (Gr) VFET at Vsd = 0.1 (ratio = 81.9) and 1 V (ratio = 3), respectively. An energy band diagram simulation shows that the Schottky barrier modulation of CNT/MoS2 contact along the sweeping gate bias is independent of the drain voltage. On the other hand, the gate modulation of Gr/MoS2 is considerably reduced with increased drain voltage because more electrons are drawn into the graphene electrode and screens the gate field by applying a higher drain voltage to the graphene/MoS2/metal capacitor.

Determination of physical mechanism responsible for the capacitance-voltage weak inversion “hump” phenomenon in n-InGaAs based metal-oxide-semiconductor gate stacks

Weak inversion capacitance-voltage (C-V) “hump” is a widely observed phenomenon at n-InGaAs based metal oxide semiconductor (MOS) structures. The mechanism responsible for this phenomenon is still under discussion. The C-V hump can be explained as an interaction of interface states with either one or both semiconductor energy bands. Each of the proposed mechanisms leads to a different interpretation of C-V hump. Simulating the mechanisms by relevant equivalent circuits, the capacitance and conductance characteristics of the MOS structure were calculated and compared with experimental results. The mechanism responsible for the C-V hump was determined.

Gravitational Model of a p-n Junction

When students first encounter energy bands in semiconductors, and how the properties of simple semiconductor devices such as the p-n junction diode depend on them, the concept can be a bit difficult to grasp. As part of an undergraduate physics course for non-STEM majors that includes semiconductors and simple devices based on the p-n junction, a gravitational analog of the device, using an inclined plane and ball bearings (Fig. 1), has helped students understand the concept. The model has also proved useful in introductory solid-state physics and semiconductor device courses.

Conductive CaSi2 transparent in the near infra-red range

The methods of heteroepitaxial growth of Si/CaSi2/Si(111) double heterostructures (DHS) at 500 °C have been developed. Thin CaSi2 layers with the thicknesses of 14–40 nm have been successfully embedded in the silicon matrix. The hR6-CaSi2(001)||Si(111) with hR6-CaSi2[100]||Si[1ī0] epitaxial relationship has been conserved for the embedded CaSi2 layer regardless of its thickness and the Si overgrowth mode (molecular beam epitaxy or solid phase epitaxy). The embedded CaSi2 layers are characterized by the lattice parameter distortion of about ±4% due to the difference in the thermal expansion coefficients of the silicide and silicon. Two types of Si overgrowth atop CaSi2(001) planes have been observed: (i) {111}-twinned Si crystals were found onto the CaSi2(001) surface in the DHS with CaSi2 thickness of 32–40 nm, which have preserved the {111} planes parallel to the Si(111) ones of the substrate; (ii) a polycrystalline twinned Si capping layer with a variable thickness has been formed in the samples with the smallest CaSi2 thickness (14–16 nm). Experimentally determined optical functions for the CaSi2 layer embedded in the silicon matrix have shown the presence of degenerate semiconducting properties with strong absorbance at the photon energies higher than 2.3 eV and small contribution from the free carrier absorption at 0.4–1.2 eV. Ab initio calculations within the generalized gradient approximation and screened hybrid functional of the hR-6 CaSi2 bulk with and without lattice distortion (by ±3%) have demonstrated the metal or gapless semiconductor energy band structure, because the Fermi level crosses several bands also assuming a huge free carrier concentration. The low-temperature Hall measurements and magnetoresistance measurements have proved that CaSi2 films on silicon are a gapless semiconductor with two types of carrier “pockets” (holes and electrons) that determine the resulting conductivity, concentration and mobility as a function of the Fermi level shift with the temperature increase. Mechanisms of the experimentally observed optical transparency of CaSi2 in the infra-red range are discussed.

Highly efficient photocatalytic degradation of naphthalene by Co3O4/Bi2O2CO3 under visible light: A novel p–n heterojunction nanocomposite with nanocrystals/lotus-leaf-like nanosheets structure

Exploring visible-light-driven nanomaterials to substitute crucial bulk composite under UV-light is of great significance in photodegradation of hardly-decomposed persistent pollutants in the environment. Herein, the novel p–n heterojunction photocatalyst p-Co3O4/n-Bi2O2CO3 with zero dimensional(0D)-two dimensional(2D)nanocrystals/lotus-leaf-like nanosheets structure was successfully obtained for the first time via a facile hydrothermal method, in which the density of Co3O4 loaded on the Bi2O2CO3 was easily tuned by the concentration of Co3O4 precursor in the solution. The outstanding feature of the photocatalyst is that the resultant hybrid nanocomposite exemplified the p–n heterojunction visible-light materials for degrading the naphthalene efficiently and effectively. As the presence of Co3O4 and the synergistic interactions of p–n heterojunction, the recombination of photogenerated charge carriers has been suppressed, and the visible light absorbance has been improved. Consequently, the visible light photocatalytic performance has been enhanced. The p-Co3O4/n-Bi2O2CO3 sample with Co3O4/Bi2O2CO3 mass ratio 1:6 exhibited the highest photocatalytic activities among all the as-prepared samples with the degradation efficiency of about 91.02% naphthalene in 150 min. The photocatalytic mechanism of p-Co3O4/n- Bi2O2CO3 for degrading naphthalene was analyzed based on the semiconductor energy band theory and the formation of an internal electrostatic field. The probable degradation intermediates and products of naphthalene were identified by GCMS. The proposed photodegradation pathways of naphthalene were described by combining the frontier electron density calculation and GCMS results.

Proton-Controlled Organic Microlaser Switch.

Microscale laser switches have been playing irreplaceable roles in the development of photonic devices with high integration levels. However, it remains a challenge to switch the lasing wavelengths across a wide range due to relatively fixed energy bands in traditional semiconductors. Here, we report a strategy to switch the lasing wavelengths among multiple states based on a proton-controlled intramolecular charge-transfer (ICT) process in organic dye-doped flexible microsphere resonant cavities. The protonic acids can effectively bind onto the ICT molecules, which thus enhance the ICT strength of the dyes and lead to a red-shifted gain behavior. On this basis, the gain region was effectively modulated by using acids with different proton-donating ability, and as a result, laser switching among multiple wavelengths was achieved. The results will provide guidance for the rational design of miniaturized lasers with performances based on the characteristic of organic optoelectronic materials.

Probing Vacancy Behavior in Complex Oxide Heterostructured Films

Oxygen vacancies (Vo•• ) play a critical role in the transport mechanisms within complex oxides, analogous to electrons and holes within semiconductors. Systems including memristors, all-oxide electronics, and electrochemical cells comprise substrate-supported thin films either in metal-insulator-metal or complex oxide heterostructure configurations. As well-studied defect chemistry dictates mixed electronic/ionic functionality, improving oxide-oxide interfaces necessitates a direct, spatial understanding of vacancy distributions that define electrochemically active regions. Here we show that vacancies deplete over large, micron-level distances within single crystalline perovskite Nb-doped SrTiO3 substrate (Nb:SrTiO3) substrates after typical vacuum film deposition and post-annealing processes. We demonstrate the conversion of the surface potential across a four-layer strontium titanate / yttria-stabilized zirconia (STO/YSZ) heterostructured film to spatially defined (< 100 nm) [Vo•• ] profiles within STO through a unique combination of high temperature (500 °C), in situ scanning probes and classic semiconductor energy band diagram model analysis. Further comparison between room temperature and high temperature potential profiles clearly distinguishes between electronic-dominant and activated, ionic-dominant transport characteristics within the oxide layers. Consequently, we determined that oxygen scavenging by deposited films during pulsed laser deposition significantly reduce the Nb:STO, which is then partially reoxidized in the ambient environment during cooling. The results presented herein i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films, and ii) pose the mechanism by which oxide thin film getters both enhance then deplete vacancies within the underlying substrate.