Electron-hole Research Articles

Studies on Electronic Structure and Optical Properties of MoS2/X (X = WSe2, MoSe2, AlN, and ZnO) Heterojunction by First Principles

The single-layer MoS2 is a highly sought-after semiconductor material in the field of photoelectric performance due to its exceptional electron mobility and narrow bandgap. However, its photocatalytic efficiency is hindered by the rapid recombination rate of internal photogenerated electron–hole pairs. Currently, the construction of heterojunctions has been demonstrated to effectively mitigate the recombination rate of photogenerated electron–hole pairs. Therefore, this paper employs the first principles method to calculate and analyze the four heterojunctions formed by MoS2/WSe2, MoS2/MoSe2, MoS2/AlN, and MoS2/ZnO. The study demonstrates that the four heterojunctions exhibit structural stability. The construction of heterojunctions, as compared to a monolayer MoS2, leads to a reduction in the band gap, thereby lowering the electron transition barrier and enhancing the light absorption capacity of the materials. The four systems exhibit II-type heterojunction. Therefore, the construction of heterojunctions can effectively enhance the optical properties of these systems. By forming heterojunctions MoS2/WSe2 and MoS2/MoSe2, the absorption coefficient in the visible light region is significantly increased, resulting in a greater ability to respond to light compared to that of MoS2/ZnO and MoS2/AlN. Consequently, MoS2-based heterojunctions incorporating chalcogenide components WSe2 and MoSe2, respectively, exhibit superior catalytic activity compared to MoS2 heterojunctions incorporating non-chalcogenide components ZnO and AlN, respectively. The absorption spectrum analysis reveals that MoS2/MoSe2 exhibits the highest light responsivity among all investigated systems, indicating its superior photoelectric performance.

Efficient Photocatalytic Degradation of Humic Acid in Water Using N‐Doped Ti3AlC2 MAX

AbstractHumic acid (HA) is the main precursor of carcinogenic compounds such as trichloromethane, which form during the disinfection of drinking water. Therefore, removing HA from water sources is crucial for producing safe drinking water. This study introduces a novel approach using dilute nitric acid as a nitrogen source to dope pristine Ti3AlC2 (TAC) to varying degrees, enhancing the material‘s photocatalytic degradation performance for HA. Characterisations through X‐ray diffraction, Fourier transform infrared, scanning electron microscopy, UV–vis and X‐ray photoelectron spectroscopy revealed that the surface morphology of the modified catalyst changed from an original blocky lamellar structure to a rod‐like cluster structure, which is advantageous for light trapping. In addition, the incorporation of nitrogen reduced the band gap of the material from 1.25 to 1.15 eV, considerably enhancing its overall light responsiveness and thereby improving its photocatalytic performance. The results demonstrated that the photocatalytic degradation efficiency of HA by TAC under optimal conditions reached 93.8 %, which is 4.3 times higher than the 21.8 % efficiency achieved using the undoped TAC material. The enhanced photocatalytic activity is attributed to N doping, which not only reduces the TiO2 band gap at the TAC surface but also improves the electrochemical behaviour of TAC. This results in enhanced light response and more efficient separation of photogenerated electron–hole pairs. The mechanism for CTAC–Nx photocatalytic degradation of HA was investigated using trapping agent experiments, revealing that the primary active substance responsible for the degradation is O2−.

A novel lanthanum-modified polytitanium chloride coagulant: Efficient algae removal and enhanced photocatalytic ability of recycled algal sludge

The removal of harmful cyanobacterial blooms (HCBs) and reuse of the resulting algal sludge are pressing issues in current environmental governance and ecological conservation. Aiming at tackling the above-mentioned challenges, titanium (Ti)-based coagulants are promising candidates. However, most of them suffer from poor stability and weak actual algal removal ability, and recycling of the algal sludge usually produces titanium dioxide (TiO2) with low photocatalytic ability. In this work, a lanthanum (La)-modified polytitanium chloride (La-PTC) coagulant is reported. La in the La-PTC coagulant serves a "kill two birds with one stone" strategy in algae removal and algae sludge reuse. Owing to the introduction of La ions, the La-PTC coagulant exhibits ultra-high stability and excellent algae removal capability with an efficiency of 98.71%, which is 7.25% higher than that of PTC coagulant. Moreover, recycling algae sludge can prepare high catalytic (2.45 times the commercial P25 TiO2) La/C-TiO2, where the presence of La enhances its visible light response range and inhibits electron hole recombination. The strategy of this La modified coagulant can not only achieve efficient removal of HCBs, but also transform the recovered algal sludge into photocatalysts with higher catalytic capacity.

Interface-engineered Ni(Fe/Al)-LDHs/g-C3N4 nanosheets with metallic Fe/Al[sbnd]N bonds for efficient photocatalytic degradation of tetracycline hydrochloride

To improve the photogenerated electron–hole separation efficiency and the number of reactive oxygen species of g-C3N4 in visible light. Herein, an interface-engineered Ni(Fe/Al)-LDHs/g-C3N4 nanosheets with Fe/AlN bonds were prepared, which shows highly enhanced photocatalytic degradation performance with excellent structural stability. The interfacial structure of g-C3N4 was precisely regulated by metal Fe/Al on the surface of layered double hydroxides (LDHs), which could form Fe/AlN bonds as the interfacial charge transfer channels. In addition, the metal Ni and Fe/Al on the surface of LDHs nanosheets can furnish more photocatalytic active sites and interface synergy. The obtained 7 %NiFe-LDHs/g-C3N4 owns the degradation rate of 99.23 % for tetracycline hydrochloride (TCH) in visible light. This work highlights a rational interface engineering strategy for the formation of a composite structure with interface interaction for efficient photocatalysts.

Cu-doped Bi2S3 nanorod films via chemical synthesis for improved photoelectrochemical water splitting

Bismuth sulfide (Bi2S3) is a promising photoelectrochemical (PEC) material due to its favorable optoelectronic properties. However, its overall PEC performance is limited by slow charge transport and high electron–hole recombination at the Bi2S3-electrolyte interface. In this study, we have intentionally introduced Cu into Bi2S3 photoelectrodes to enhance both their physical properties and PEC performance. Cu-doping concentrations ranging from 0 to 8.4 at.% were incorporated into Bi2S3 films synthesized via chemical bath deposition followed by annealing. The undoped Bi2S3 photoelectrodes exhibited nanorods with lengths of 50–100 nm, a direct bandgap of 1.26 eV, and a photocurrent density of 4.7 mA/cm2 at 1.5 V. As the Cu-doping concentration increased from 1.0 to 4.4 at.%, the nanorod length extended to 200–400 nm, with a corresponding increase in density, leading to an enhanced photocurrent density of 6.8 mA/cm2. However, further increasing the Cu-doping concentration to 8.4 at.% resulted in a reduction in film thickness, nanorod density, and a decrease in photocurrent to 6.0 mA/cm2. These results demonstrate that Cu-doping in Bi2S3 significantly increases nanorod density and improves both the physical and PEC properties, offering a promising approach for developing more efficient PEC water-splitting devices.

Current density–voltage characteristics of exciplex-type organic light-emitting diodes expressed by a simple analytic equation

Exciplex-type bilayer organic light-emitting diodes (OLEDs) with ohmic contacts exhibited current density–voltage (J–V) characteristics that closely matched a simplified analytical model proposed by Nikitenko and Bässler. The analytical model is based on the following key assumptions: (i) complete hole–electron recombination at the interface between a hole transport layer (HTL) and an electron transport layer (ETL), (ii) ohmic contacts at the interfaces between metal electrodes and carrier transport layers, and (iii) electric-field-independent carrier mobilities in both HTL and ETL. The excellent matching shows that the simplified analytical model is sufficient to describe the J–V characteristics of the OLEDs. We also demonstrated that if the carrier mobility of one carrier transport layer is known, that of the other transport layer can be estimated using the equation derived by the simplified analytical model. The simplified analytical model provides a useful method to estimate carrier mobilities within carrier transport layers themselves in OLEDs.

Application of TiO2 Photocatalyst for Improving Ozone Generation Efficiency

ABSTRACT With a strong oxidizing capability, ozone (O3) is widely used in industry. However, ozone generators on the market are expensive and require a lot of energy to operate, which remains the bottleneck for the wide applications. In this study, a novel and energy-efficient ozone generation system is developed via the combination of black-TiO2 photocatalyst and plasma system. Black-TiO2 catalyst is prepared by calcining commercial TiO2 catalyst at 550 °C under a nitrogen atmosphere to extend the absorption spectrum to visible light range to improve catalytic activity. The experimental results show that ozone generation efficiency can be greatly increased to 415 g O3/kWh via combined black-TiO2 catalyst and plasma with O2 as the feeding gas. This can be attributed to the fact that black-TiO2 catalyst with a smaller energy gap is more efficiently activated to form electron–hole pairs in plasma, which can enhance ozone generation. On the other hand, black-TiO2 catalyst can provide abundant active sites for active O atoms and O2 molecules to promote O3 generation. Black-TiO2 catalyst prepared in this study reveals good stability. Overall, combination of black-TiO2 catalyst with plasma reveals good ozone generation efficiency and has a good potential for practical applications.

Suppressed nonradiative electron–hole recombination in bismuth halide perovskite Cs3Bi2Cl9: Time domain ab initio analysis

Time-domain density functional theory, coupled with non-adiabatic molecular dynamics simulations, was employed to explore the defect characteristics and the associated nonradiative recombination processes in the bismuth halide perovskite Cs3Bi2Cl9. Our findings indicate that Cs3Bi2Cl9 inherently exhibits p-type semiconductor behavior, with vacancies at the Cs and Bi sites acting as predominant shallow acceptor defects. Although Cl vacancy and interstitial Cl defects introduce trap states within the bandgap of Cs3Bi2Cl9, the by-defect electron–hole (e-h) recombination is substantially impeded, which is attributed to the remarkable local structural deformations associated with the BiCl63− octahedral compression around the defects, which further results in decoupling between the defect state and the band edge state. As a result, the enhanced delocalization of defect states leads to a notably small wave function overlap between defect states and band edge states, as well as weak nonadiabatic couplings dominated by low-frequency phonons. Our study offers crucial insights into the mechanism of defect-mediated e-h recombination in bismuth-based perovskites and provides guidelines for designing efficient optoelectronic devices based on these materials.

Effect of Halide Ions on the TiO2-mediated Photocatalysis of Carbendazim in Aqueous Medium Under Near-ultraviolet Light Irradiation

The degradation of carbendazim (CBZ) through TiO2 photocatalysis, in the presence of halide ions and under near-UV light irradiation, was investigated. HPLC–MS technique was used to characterize the photoproducts. Spectrophotometric analysis showed that CBZ degraded slowly in TiO2 aqueous dispersions containing no salt (CBZ conversion of 6% after ca. 5 h of irradiation). The photodegradation efficiency increased particularly by addition of bromide salts. Indeed, CBZ reached complete degradation after ca. 30 min at the maximum concentration of NaBr used (0.05 M). Two significant aspects have emerged from the data analysis: the bromide role is to cause inhibition of the electron–hole recombination, a reaction known to be competitive with the reactive process; CBZ photodegradation is especially initiated by direct hole transfer pathway, whereas the OH• role is crucial in the catalyst regeneration process. Degradation attempts under sunlight appeared promising for a more sustainable photocatalytic process.

Low temperature recombination luminescence of Mg3Y2Ge3O12:Tb3+

A study was conducted to examine the recombination processes in persistent phosphor Mg3Y2Ge3O12:Tb3+ garnet at low temperatures. Photoluminescence (PL), recombination luminescence (RL), electron paramagnetic resonance (EPR), and EPR detected by PL or RL were measured.In samples with low Tb3+ concentration, a broad PL and RL band around 400–450 nm and characteristic Tb3+ lines were observed. However, in samples with high Tb3+ concentration, only Tb3+ lines were present. Both the broad-band and the line components exhibit long-lasting tunneling luminescence with hyperbolic decay. After 263 nm UV irradiation signals of intrinsic electron (F-type) and hole (V-type) trapping centres were observed in the EPR spectra. Such signals were also observed in RL-detected EPR spectra, indicating that the broad RL band at low Tb3+ concentrations originates from tunneling recombination between these intrinsic traps. At high Tb3+ concentrations, the RL-EPR spectrum was not observed, suggesting that intrinsic electron and Tb-related hole trapping centres probably participate in the tunneling recombination.

Research on electrical conductive properties of thulium-doped calcium zirconate solid electrolyte

To further investigate the electrochemical performance of Tm doped CaZrO3 electrolyte, the CaZr1−xTmxO3−α (x = 0, 0.025, 0.05, 0.075 and 0.1) solid electrolyte specimens were prepared by high temperature solid state method. The phase structure and microstructure of the electrolyte samples were analyzed by Raman spectrum, XRD and SEM. The electrical conductivity of the specimen was measured at the temperature of 673∼1373K in hydrogen-rich and oxygen-rich atmosphere by the two-terminal AC impedance spectroscopy method. The H/D isotope effect of the specimen at different temperature was tested to confirm the dominant conducting carrier in predetermined temperature and atmosphere. It is found that proton is the dominant charge carrier both in oxygen-rich and hydrogen-rich atmosphere at the lower temperature below 1073 K. However, at higher temperature above 1073K, the predominant charge carrier seems to be oxygen ion vacancy in hydrogen-rich, whereas to be electron hole in oxygen-rich atmosphere, based on the analysis of the atmospheric dependence of the electrical conductivity. Moreover, partial conductivities of conducting species, the active doping amount of Tm and the standard Gibbs free energy changes for interstitial proton production by dissolution of water and hydrogen in Tm doped electrolyte were estimated based on crystal defect chemistry theory.

The impact of hetero-aromatic rings on optoelectronic and charge transport properties of fused polycyclic compounds

The structural, optoelectronic, and charge-transport properties of linearly fused polycyclic hydrocarbon molecules with heteroatoms (NH, O, S, and Se) in five-membered rings have been investigated using the DFT approach. The hybrid functional B3LYP in combination with the 6–311 + G (d,p) basis set is utilized in the Gaussian 16 W software package for all the calculations. The absorption and emission energies are investigated by using time-dependent density functional theory (TD-DFT) at the same level of theory. To investigate the stability of each molecule concerning its aromaticity, nucleus-independent chemical shift (NICS) values are computed. Electron affinity (EA), hole extraction potential (HEP), ionization potential (IP), and electron extraction potential (EEP) are also reported. The simulated hole (λ h) and electron (λ e) reorganization energies for the majority of the molecules are lower than the characteristics hole and electron-transporting materials. The reported fused polycyclic compounds may have potential applications in optoelectronic devices.

Pure Hexagonal Diamond with Symmetry-Doping Properties.

The difficulties in asymmetrical doping of wide band gap materials, especially for the n-type diamond challenge, hamper their application in electronic devices. Here, we propose doping diamond polytypes with reasonable band structures to solve this problem. Various elements are doped into six diamond polytypes, and their doping behaviors are investigated. The results show that pure hexagonal (2H) diamond has the band gap with a value of 4.42 eV and the smallest carrier effective masses among six calculated diamond polytypes, exhibiting high electron mobility and hole mobility up to 4250 and 5840 cm2·V-1·s-1, respectively. Compared to cubic (3C) diamond, the impurity formation energy in 2H-diamond is reduced, which is attributed to its C3v symmetry. More importantly, 2H-diamond exhibits favorable doping symmetry with a donor level of 0.14 eV for phosphorus and an acceptor level of 0.19 eV for boron, respectively, which originate from smaller effective masses and stronger delocalization at the band edge in 2H-diamond. These ionization energies are smaller than those in 3C-diamond of 0.32 and 0.58 eV, respectively. This reveals that phosphorus and boron in 2H-diamond are more easily excited at room temperature, producing good n- and p-type conductivities. All of these make 2H-diamond a potential candidate for the replacement of 3C-diamond as a wide band gap material theoretically. These provide a way to realize high-quality n-type diamond, giving significant insights into the realization of diamond-based electronic devices. This also supplies a solution for the difficulties in asymmetric doping of other wide band gap materials.

Electron–photon–phonon interactions in Dirac semimetals: Magneto-optical absorption and mobility analysis

We study the magneto-optical properties of Dirac semimetal (DSM) slabs with particular emphasis on Cd3As2 through electron–photon–phonon interactions, focusing on the magneto-optical absorption coefficient (MOAC) and full-width at half maximum (FWHM). Studying the Landau level (LL) energy of DSMs in the (xy) plane and the z-direction revealed a unique deviation from the square root dependence on the magnetic field, distinguishing DSMs from other semiconductors. At high magnetic fields, the electron–hole symmetry in the LL spectrum is broken, indicating a topological phase in DSMs. For undoped DSMs, MOAC is driven by interband transitions, with peaks from one-photon absorption being smaller and positioned to the left of two-photon ones. Increasing the magnetic field increases peak values. FWHM for one- and two-photon processes increases with the magnetic field and follows a T dependence on temperature. In doped DSMs, both intraband and interband transitions occur, with new interband peaks emerging at higher temperatures near the Fermi energy. Increased electron density shifts the peak position slightly toward higher energy. Peaks from optical phonon emission are consistently higher and located to the right of those from optical phonon absorption, indicating a stronger emission process. The FWHM data allow for the estimation of electron mobilities, and using a reasonable broadening parameter, our predicted mobility values agree with experimental results.

Charge dynamics in the 2D/3D semiconductor heterostructure WSe2/GaAs

Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices, which integrate van der Waals 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe2 on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe2/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in GaAs and holes in WSe2. The type-II band alignment and fast photoexcited carrier dissociation shown here indicate that WSe2/GaAs is a promising junction for advanced photovoltaic and other optoelectronic devices, making use of the best properties of van der Waals (2D) and conventional (3D) semiconductors.

Synergistic Interactions of Bulk Polarization and Built-In Electric Field Inducing 2D/2D S-Scheme Homojunction Toward Enhanced Photocatalytic Performance.

The rational design of S-scheme photocatalysts, achieved by serially integrating two different semiconductors, represents a promising strategy for efficient charge separation and amplified photocatalytic performance, yet it remains a challenge. Herein, ZnIn2S4 (ZIS) and oxygen-doped ZnIn2S4 (O-ZIS) nanosheets are chosen to construct a homojunction catalyst architecture. Theoretical simulations alongside comprehensive in situ and ex situ characterizations confirm that ZIS and O-ZIS with noncentrosymmetric layered structures can generate a polarization-induced bulk-internal electric field (IEF) within the crystal. A robust interface-IEF is also created by the strong interfacial interaction between O-ZIS and ZIS with different work functions. Owing to these features, the O-ZIS/ZIS homojunction adopts an S-scheme directional charge transfer route, wherein photoexcited electrons in ZIS and holes in O-ZIS concurrently migrate to their interface and subsequently recombine. This enables spatial charge separation and provides a high driving force for both reduction and oxidation reactions simultaneously. Consequently, such photocatalyst exhibits an H2 evolution rate up to 142.9µmol h-1 without any cocatalysts, which is 4.6- and 3.4-fold higher than that of pristine ZIS and O-ZIS, respectively. Benzaldehyde is also produced as a value-added oxidation product with a rate of 146.9µmol h-1. This work offers a new perspective on the design of S-scheme systems.

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.

Performance analysis and optimization of SnSe thin-film solar cell with Cu2O HTL through a combination of SCAPS-1D and machine learning approaches

Tin selenide (SnSe) is an auspicious light-harvesting material in photovoltaic (PV) technology due to its larger absorption coefficients and earth-abundance nature. However, because of the potential difficulty of defect-free fabrication, and non-optimized electron transport layer (ETL) and hole transport layer (HTL) alignment, it could no longer achieve its realistic objectives. Therefore, designing a suitable SnSe-based PV device with an appropriate ETL and HTL is crucial to achieving optimum performance. In this research, we proposed a novel heterojunction thin-film solar cell (TFSC) configuration of Ni/Cu2O/SnSe/WS2/FTO/Al and simulated its PV performance metrics using SCAPS-1D solar cell simulation software. This research additionally provided a comparative performance analysis of the SnSe TFSC with numerous ETLs and HTLs. It is evident that the suggested TFSC with WS2 ETL and Cu2O HTL creates appropriate band alignment with the SnSe light harvesting layer, which reduces charge recombination at both the ETL/absorber and absorber/HTL interfaces. Here, the impact of absorber layer thickness, doping concentration, bulk defect density, and defect density at the ETL/absorber and absorber/HTL interfaces is investigated. Besides, the impact of radiative recombination, back contact metal work function, and working temperature are carefully examined. After optimization of all the material properties, a maximum efficiency of 29.31 % with open circuit voltage (Voc) of 0.89 V, short circuit current density (Jsc) of 38.23 mA/cm2, and fill-factor (FF) of 86.30 % were attained for the SnSe-based proposed structure. Furthermore, a supervised linear regression machine learning algorithm is employed to investigate how the behavior of the material attributes affects the solar cell's designed PV performance. Overall, our findings can be helpful to experimentalists to fabricate inexpensive, highly efficient, and Cd-free SnSe-based heterostructure solar cells in the near future.

Infrared single-photon detection with superconducting magic-angle twisted bilayer graphene.

The moiré superconductor magic-angle twisted bilayer graphene (MATBG) shows exceptional properties, with an electron (hole) ensemble of only ~1011 carriers per square centimeter, which is five orders of magnitude lower than traditional superconductors (SCs). This results in an ultralow electronic heat capacity and a large kinetic inductance of this truly two-dimensional SC, providing record-breaking parameters for quantum sensing applications, specifically thermal sensing and single-photon detection. To fully exploit these unique superconducting properties for quantum sensing, here, we demonstrate a proof-of-principle experiment to detect single near-infrared photons by voltage biasing an MATBG device near its superconducting phase transition. We observe complete destruction of the SC state upon absorption of a single infrared photon even in a 16-square micrometer device, showcasing exceptional sensitivity. Our work offers insights into the MATBG-photon interaction and demonstrates pathways to use moiré superconductors as an exciting platform for revolutionary quantum devices and sensors.

Piezo-photocatalysis synergy in γ-GeSe for highly efficient oxygen evolution reaction

Solar-driven semiconductor photocatalysts are highly appealing in applications of environmental remediation and energy conversion. However, photocatalytic reactions, particularly oxygen evolution reaction (OER), are often constrained by the swift recombination of electron–hole pairs, thereby resulting in low reaction efficiency. Although it is effective to separate charge carriers by constructing heterojunctions to form built-in electric field, the lattice mismatch and inefficient interlayer charge transfer of heterojunctions in the photocatalysts limit their further development. Here, we propose a new strategy by constructing an internal electric field for OER through an individual piezoelectric two-dimensional material. The results indicate that the piezoelectric effect regulates the electronic structure, reduces bandgap, improves light absorption efficiency, and that the displacement of positive and negative charge centers is the key factor in the enhanced OER. This research indicates the feasibility of combining piezoelectric properties of two-dimensional materials with OER (1.19 eV), providing new insights and guidance for applying the piezoelectric effect in the OER and opening up a way to promote efficient separation of charge carriers.