Band Gap Semiconductor Research Articles

A Full‐Scale Analysis of Absorption Edges in Dye‐Doped Nonlinear Optical Polymers

AbstractA full‐scale analysis of the absorption edges by modified Tauc‐Lorentz models is essential in determining the optical bandgap and Urbach energy of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This technique has not yet been applied to analyzing organic nonlinear optical (NLO) materials. This problem is tackled by preparing high‐quality films of guest–host NLO polymers with a wide thickness range from sub‐micron to 200 microns, allowing accurate measurement of full‐spectral absorption coefficients of NLO materials over four orders of magnitude by the UV‐VIS‐NIR spectroscopy. The Tauc model and a new Monolog–Lorentz model are used to study the optical absorption edge of guest‐host NLO polymers containing various push‐pull chromophores and the dependence of optical bandgap and Urbach edge on the structure and composition of materials is analyzed. The results reveal the critical transition of the Urbach exponential tail to a low energy tail that overlaps with vibrational overtones of materials at the telecom wavelengths. Determining the fundamental absorption region of organic NLO films in this study provides quantitative insight into the research to harness the resonance‐enhanced nonlinear coefficients of materials by operating at the wavelengths near the band edge with the control of optical loss.

Spontaneous and tunable valley polarization in two-dimensional single-layer LaCl2

Applying the valley contrasting properties of valleytronic materials to logical operations is the foundation of valleytronic device manufacturing. It is predicted that single-layer (SL) LaCl2 is an ferrovalley material with intrinsic and tunable valley polarization through first-principles calculations. It is a ferromagnetic semiconductor (bandgap 0.767 eV) with roughly 1.0 μ B per unit cell as well as out of plane magnetization, and the Curie temperature is about 149 K. The tight-binding model considering five orbitals as well as next nearest neighboring hopping get a consistent band structure with the first-principles calculation. The valley polarization changes from 40.49 to 98.51 meV under the biaxial strain of 5% ∼ −5%. Therefore, the biaxial strain can be a means to tune the valley polarization. In addition, the valley polarization of the double-layer (DL) structure (∼80 meV) is much greater than that of the SL structure (∼59 meV) due to the increased magnetic moment of the DL structure, indicating the potential tunable character by stacking few layers. We believe that SL LaCl2 has great potential for device manufacturing and application in the field of valley electronics.

Stereo-Structural Fine Tuning of Chromaticity.

Lanthanoid carboxylates were synthesized and in situ self-assembled to illustrate temperature-driven evolution in chromaticity. Evolution in structure (crystallinity), composition, luminosity, and chromaticity were investigated revealing the coupled role of divergence in order/structure (spatial organization), and composition in tuning observed color. Loss of crystallinity or increase in residual carbon leads to decrease in luminosity even with increase in hue. Comparing Ho and Er congeners shows that the density of accessible transition states relates to shifts in low and high wavelength components of color. This work demonstrates that, just as interface dipoles can lead to change in semiconductor band gap, structure and composition can analogously alter observed color.

Synthesis, characterization, and effective removal of dye pollutants from water bodies using a new ZnO nanocomposite

Biogenic zinc oxide nanocomposites were fabricated through an eco-friendly method by employing Boerhaavia diffusa leaf extract in combination with zinc nitrate. The nanocomposites underwent a comprehensive characterization process involving uv–visible spectroscopy (UV-DRS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), selected area diffraction (SAED), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDAX), high-resolution transmission electron microscopy (HRTEM), inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF), and zeta potential studies. The XRD analysis substantiated the existence of a ZnO nanocomposite possessing a hexagonal wurtzite crystalline structure. Furthermore, extended rod-shaped formations were identified in conjunction with clusters of sodium and potassium.The effectiveness of the ZnO nanocomposite in photocatalytic degradation was assessed for various types of dyes: cationic and anionic like Methylene Blue (MB), Malachite Green (MG), Congo Red (CR), and Methyl Orange (MO) in aqueous medium. The nanocomposites demonstrated remarkable photodegradation capabilities for both cationic and anionic dyes. Notably, 50 mg of the catalyst effectively removed Congo Red (15 ppm) and Methylene Blue (15 ppm) within 40–60 min under direct sunlight and 15–30 min under UV illumination. The samples are more active on UV light illumination rather than exposure to sunlight due to.the favorable semiconductor band gap (3.26 eV). As a result, this research suggests an uncomplicated, budget-friendly, and environmentally sustainable approach for manufacturing a ZnO-based photocatalyst. Moreover, the inclusion of potassium and sodium elements in the nanocomposite showcased a beneficial synergistic impact on its photoactivity. This necessitates additional scrutiny for its potential application in treating diverse organic pollutants in water resources. Additionally, high efficiency of the catalyst under direct sunlight holds potential for applications in solar cells.

ZnO/TiO2 heterojunction nanorod array for efficient photoelectrochemical water splitting

This study presents a novel approach utilizing a simple, stable, reproducible CV (Cyclic Voltammetry) electrodeposition method to deposit varying amounts of ZnO onto ordered TiO2 (Rutile) nanorod arrays. Comprehensive investigations were conducted on the morphology, semiconductor band-gap structure of different ZnO/Rutile composites, and their performance in photocatalytic (PC) and photoelectrochemical (PEC) hydrogen production. Results demonstrate that the ZnO/TiO2 photoanodes fabricated in this manner exhibit outstanding PC/PEC hydrogen production capabilities. Particularly noteworthy is the Z/R-15 photoanode, showcasing superior PEC performance with a photocurrent density 3.5 times higher than that of the pristine TiO2 photoanode under equivalent conditions. Furthermore, it was observed that depositing ZnO onto highly crystalline, annealed TiO2 nanorods yields optimal photoelectrical properties. Incorporating oxygen-rich vacancies in ZnO significantly enhances the photoelectrocatalytic performance of TiO2 photoanodes. The CV electrodeposition method offers valuable insights for developing efficient photocatalytic hydrogen production catalysts.

Machine Learning‐Enhanced Prediction of Inorganic Semiconductor Bandgaps for Advancing Optoelectronic Technologies

AbstractA pivotal challenge in advancing inorganics optoelectronic technologies, is the precise characterization of materials' electronic attributes, with the bandgap being a critical property. Conventional approaches, heavily reliant on time‐intensive and financially demanding experimental and computational methods, such as density functional theory (DFT) calculations, face limitations due to inherent estimation errors. Machine learning methodologies are developed for the prediction of bandgaps of inorganic semiconductors but most of them are employed for datasets created by DFT calculations, hence limiting their performance. Addressing this, the study leverages machine learning methodologies, harnessing both compositional and structural features, to predict the band gaps of inorganic semiconductors with enhanced accuracy. This advancement is reinforced by the employment of an experimental bandgap dataset, which, when integrated with structural descriptors obtained from the Materials Project, significantly improves prediction capabilities. This is evidenced by the model's exceptional performance across two distinct benchmark datasets. Furthermore, the model's adeptness in predicting formation energies underscores its versatility and applicability to a broad spectrum of electronic properties. These findings suggest that the predictive accuracy of this model can be further augmented through the inclusion of additional experimental bandgap measurements and the refinement of structural descriptors. This approach offers a promising and efficient alternative to traditional methodologies, potentially accelerating the development of optoelectronic technologies.

Band Gap Engineering of MoxW1-xS2 Alloy Monolayers with Wafer-Scale Uniformity.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

Sol-gel synthesis of tetragonal BaTiO3 thin films under fast heating

BaTiO3 thin films with a thickness of 400 nm were prepared by spin coating onto a quartz plate and subsequently calcined at 700 °C. The effect of fast (540 K min−1) and slow (10 K min−1) heating on morphology, unit cell parameter and dielectric constant was studied. Fast heating yields tetragonal BaTiO3 films with a semiconductor band gap of 3.54 eV, dielectric constant of 685 and a surface roughness of 12 nm. In contrast, slow heating produces tetragonal BaTiO3 films with a larger band gap of 3.78 eV, a dielectric constant of 219 and a surface roughness of 35 nm. A kinetic constant of 0.0061 min−1 was obtained in the decomposition of methylene blue with UV light over BTO films at 20 °C.

First-principle investigation of lead-free double perovskites Cs2MScCl6 (M = Na, K) for optoelectronic and thermoelectric applications

Double perovskite optoelectronic devices are gaining popularity due to their advantageous features, such as an efficient and easy-to-manage crystalline structure. In this study, we looked into the optoelectronic, mechanical, and thermoelectric attributes of Cs2MScCl6 (with M being Na or K) using density functional theory. The obtained results show that both compounds can exist stably in cubic double perovskite structure, space group Fm3¯m . The electronic band structures of Cs2NaScCl6 and Cs2KScCl6 demonstrate a direct semiconducting band gap of 3.829 eV and 3.996 eV, respectively. As the pressure rises up to 200 GPa, Cs2KScCl6 reveals extraordinary tunability along with an indirect band gap of 2.000 eV. Moreover, optical analysis—comprising dielectric constant calculations, absorption coefficient measurements, refractive indices checks, reflectivity assessments, energy loss estimations, and electrical conductivity appraisals—shows promising results too. Specifically, the absorption coefficient and conductivity predominantly fall within the ultraviolet range under normal atmospheric pressure. The thermoelectric characteristics suggest a noteworthy Seebeck coefficient, low thermal conductance and good figure of merit implying it could be used in the domain of sustainable energy.

Indirect control of band gaps by manipulating local atomic environments using solid solutions and co-doping

The ability to tune band gaps of semiconductors is important for many optoelectronics applications including photocatalysis. A common approach to this is doping, but this often has the disadvantage of introducing defect states in the electronic structure that can result in poor charge mobility and increased recombination losses. In this work, density functional theory calculations are used to understand how co-doping and solid solution formation can allow tuning of semiconductor band gaps through indirect effects. The addition of ZnS to GaP alters the local environments of the Ga and P atoms, resulting in shifts in the energies of the P and Ga states that form the valence and conduction band edges, and hence changes the band gap without altering which atoms form the band edges, providing an explanation for previous experimental observations. Similarly, N doping of ZnO is known from previous experimental work to reduce the band gap and increase visible-light absorption; here we show that, when co-doped with Al, the Al changes the local environment of the N atoms, providing further control of the band gap without introducing new states within the band gap or at the band edges, while also providing an energetically more favourable state than N-doped ZnO. Replacing Al with elements of different electronegativity is an additional tool for band gap tuning, since the different electronegativities correspond to different effects on the N local environment. The consistency in the parameters identified here that control the band gaps across the various systems studied indicates some general concepts that can be applied in tuning the band gaps of semiconductors, without or only minimally affecting charge mobility.

What's behind their various activities? Discerning the importance of surface terminations in Ag3PO4 semiconductor from DFT calculations

The surface energy and band gap of inorganic semiconductors are essential properties for a wide variety of applications in materials science, and studying morphologies holds significant importance. However, it is challenging to find a correlation between them at the atomic scale when these materials are multifunctional. This motivates the development of predictive computational models that can optimize these materials based on these quantities. Present findings shed light on the structure and electronic properties of (100), (110), and (111) surfaces of Ag3PO4 by using density functional theory calculations. By introducing two concepts, namely, polyhedron energy and polyhedron band gap, we provide compelling evidence for the effects of surface termination of Ag3PO4 photocatalyst. We compare our predictions to experimental crystal habits, and our results not only allow us to explain their evolution for obtaining a given morphology but also offer a coherent explanation, based on atomic-scale insights, for the discrepancies of morphology-photocatalytic activity relationship of Ag3PO4 in the recent literature. Overall, this work opens a novel avenue for first-principles calculations of morphology and provides a roadmap for the design of more effective inorganic semiconductors with desirable physicochemical properties for potential applications.

Theoretical exploration of band gap error dependency on band gap size in density functional theory Calculations: CdTe and GeTe as representative cases of two band structure semiconductor types

This study addresses the prevalent band gap error problem in density functional theory (DFT) with local density or generalized gradient approximation (LDA/GGA), widely considered as the “standard model” for investigating the band structure of semiconductors. Despite previous DFT efforts focusing on improving the exchange correlation energy to mitigate the underestimation of the band gap, the correlation between the band gap error and the intrinsic electronic structure of semiconductors has been overlooked. From two principal observations: (i) traditional semiconductors typically have a p-s type band structure, and (ii) a smaller band gap is linked to less underestimation, we derive two crucial questions: (i) How is band gap error manifested in s-p type semiconductors? (ii) Does the error vanish as the band gap approaches zero, akin to metals? Through the examination of CdTe and GeTe, representing p–s and s–p type semiconductors with strain-engineered band gaps, our calculations indicate that for CdTe, the band gap is underestimated, whereas for GeTe, it is overestimated. Significantly, these errors persist even as the band gap of semiconductors tends to zero, unlike in metals. This persistent discrepancy stems from the discontinuity in wavefunctions between the semiconductors' VBM and CBM.

Evolution of structural and electronic properties standardized description in rhenium disulfide at the bulk-monolayer transition

The structural and electronic properties of ReS2 different forms — three-dimensional bulk and two-dimensional monolayer — were studied within density functional theory and pseudopotentials. A method for standardizing the description of bulk unit cells and "artificial" slab unit cells for DFT research has been proposed. The preference of this method for studying zone dispersion has been shown. The influence of the vacuum layer thickness on specified special high-symmetry points is discussed. Electron band dispersion in both classical 3D Brillouin zones and transition to 2D Brillouin zones in the proposed two-dimensional approach using the Niggli form of the unit cell was compared. The proposed two-dimensional approach is preferable for low-symmetry layered crystals such as ReS2. It was established that the bulk ReS2 is a direct gap semiconductor (band gap of 1.20 eV), with the direct transition lying in the X point of the first Brillouin zone, and it is in good agreement with published experimental data. The reduction in material dimension from bulk to monolayer was conducted with an increasing band gap up to 1.45 eV, with a moving direct transition towards the Brillouin zone center. The monolayer of ReS2 is a direct-gap semiconductor in a wide range of temperatures, excluding only a narrow range at low temperatures, where it comes as a quasi-direct gap semiconductor. The transition, situated directly in the Γ-point, lies 3.3 meV below the first direct transition located near this point. The electronic density of states of ReS2 in the bulk and monolayer cases of ReS2 were analyzed. The molecular orbitals were built for both types of ReS2 structures as well as the electron difference density maps. For all types of ReS2 structures, an analysis of populations according to Mulliken and Voronoi was carried out. All calculated data is discussed in the context of weak quantum confinement in the 2D case.

Tunability of the photoelectric properties of CdSe thin films through doping with manganese toward photovoltaic applications

Tandem solar cells have been demonstrated to surpass the Shockley-Queisser limit, promising the transition to third-generation solar cell technology. Tandem solar cells comprise thin film layers of various band gap semiconductors, and CdSe is a promising candidate for the top cell. In this study, we explore the properties of thermally evaporated CdSe thin films doped with 0 to 5 wt% Mn and report a significant five-fold increase in carrier concentration. Furthermore, our optoelectrical studies indicate a band gap energy of 1.681eV, experiencing up to a 28 meV shift with 5 wt% Mn doping and identifies 1 wt% as the optimal doping concentration, offering the best electrical properties and photo-response while maintaining a stable band gap.

A review: Comprehensive investigation on bandgap engineering under high pressure utilizing microscopic UV–Vis absorption spectroscopy

Bandgap engineering plays a vital role in material development and device optimization due to its significant impact on the photovoltaic and photoelectricity properties of materials. Nevertheless, it is still a great challenge to accurately control the bandgap of semiconductors to achieve the targeted properties of materials. Recently, pressure-induced bandgap regulation has emerged as a novel and effective tool to regulate bandgap, reveal the intrinsic band nature, and construct the in-depth structure–property relationships therein. In this review, the unique techniques of microscopic in situ steady-state UV–Vis absorption spectroscopy and high-pressure diamond anvil cell are introduced. This technique provides a powerful method to monitor the bandgap behaviors at high pressure. Then, the pressure-triggered bandgap responses are outlined based on several typical semiconductors, including metal halide perovskites, inorganic quantum dots, piezochromic molecular compounds, and two-dimensional semiconductor materials. The summarized structural effects on bandgap evolution and the general principles for bandgap engineering under high pressure are expected to provide guidance for further material design under ambient conditions. Microscopic absorption spectroscopy detection under high pressure is proven to be an ideal platform for developing functional materials and high-performance devices.

Schottky infrared detectors with optically tunable barriers beyond the internal photoemission limit

Internal photoemission is a prominent branch of the photoelectric effect and has emerged as a viable method for detecting photons with energies below the semiconductor bandgap. This breakthrough has played a significant role in accelerating the development of infrared imaging in one chip with state-of-the-art silicon techniques. However, the performance of these Schottky infrared detectors is currently hindered by the limit of internal photoemission; specifically, a low Schottky barrier height is inevitable for the detection of low-energy infrared photons. Herein, a distinct paradigm of Schottky infrared detectors is proposed to overcome the internal photoemission limit by introducing an optically tunable barrier. This device uses an infrared absorbing material-sensitized Schottky diode, assisted by the highly adjustable Fermi level of graphene, which subtly decouples the photon energy from the Schottky barrier height. Correspondingly, a broadband photoresponse spanning from ultraviolet to mid-wave infrared is achieved, with a high specific detectivity of 9.83×1010 cm Hz1/2 W-1 at 2,700nm and an excellent specific detectivity of 7.2×109 cm Hz1/2 W-1 at room temperature under blackbody radiation. These results address a key challenge in internal photoemission and hold great promise for the development of the Schottky infrared detector with high sensitivity and room temperature operation.

Feature-Assisted Machine Learning for Predicting Band Gaps of Binary Semiconductors.

The band gap is a key parameter in semiconductor materials that is essential for advancing optoelectronic device development. Accurately predicting band gaps of materials at low cost is a significant challenge in materials science. Although many machine learning (ML) models for band gap prediction already exist, they often suffer from low interpretability and lack theoretical support from a physical perspective. In this study, we address these challenges by using a combination of traditional ML algorithms and the 'white-box' sure independence screening and sparsifying operator (SISSO) approach. Specifically, we enhance the interpretability and accuracy of band gap predictions for binary semiconductors by integrating the importance rankings of support vector regression (SVR), random forests (RF), and gradient boosting decision trees (GBDT) with SISSO models. Our model uses only the intrinsic features of the constituent elements and their band gaps calculated using the Perdew-Burke-Ernzerhof method, significantly reducing computational demands. We have applied our model to predict the band gaps of 1208 theoretically stable binary compounds. Importantly, the model highlights the critical role of electronegativity in determining material band gaps. This insight not only enriches our understanding of the physical principles underlying band gap prediction but also underscores the potential of our approach in guiding the synthesis of new and valuable semiconductor materials.

Revolutionizing Energy Harvesting: Harnessing Ambient Solar Energy for Enhanced Electric Power Generation

The escalating demand for sustainable and renewable energy sources has propelled the exploration of ambient solar energy as a pivotal avenue for cleaner power generation. This review paper delves into the intricacies of solar energy harvesting technologies, providing a comprehensive analysis of principles, efficiency metrics, material advancements, and versatile applications. Focusing on the photovoltaic effect as the cornerstone of solar cells, the critical impact of semiconductor material selection and bandgap engineering on efficiency is thoroughly examined. Key performance measures such as fill factor, open-circuit voltage, and short-circuit current are emphasized for their pivotal role in evaluating system efficiency. The adaptability of solar energy solutions is exemplified through diverse applications, spanning from portable electronics to large-scale solar farms. Looking towards the future, this review envisions a promising trajectory for solar energy harvesting, driven by continual advancements in technology, material science, and efficiency measures. The insights gleaned from this comprehensive examination are poised to catalyze the development of more efficient and accessible solar energy harvesting technologies. This, in turn, promises to play a significant role in the global transition towards a greener and more sustainable energy landscape, contributing substantively to sustainability, climate mitigation, and an enhanced quality of life worldwide. Keywords: Solar energy harvesting, solar cells, material selection, panel orientation, storage techniques, system integration, case studies.

Engineering piezoelectricity at vdW interfaces of quasi-1D chains in 2D Tellurene

Low-dimensional piezoelectrics have drawn attention to the realization in nano-scale devices with high integration density. A unique branch of 2D Tellurene bilayers formed of weakly interacting quasi-1D chains via van der Waals forces is found to exhibit piezoelectricity due to the semiconducting band gap and spatial inversion asymmetry. Various bilayer stackings are systematically examined using density functional theory, revealing optimal piezoelectricity when dipole arrangements are identical in each layer. Negative piezoelectricity has been observed in two of the stackings AA′ and AA″ while other two stackings exhibit the usual positive piezoelectric effect. The layer-dependent 2D piezoelectricity ( ∣ e 222D ∣ ) increases with an increasing number of layers in contrast to the odd–even effect observed in h-BN and MoS2. Notably, the piezoelectric effect is observed in even-layered structures due to the homogeneous stacking in multilayers. Strain is found to enhance in-plane piezoelectricity by 4.5 times (−66.25 × 10−10 C m−1 at −5.1% strain) due to the increasing difference in Born effective charges of positively and negatively charged Te-atoms under compressive biaxial strains. Moreover, out-of-plane piezoelectricity is induced by applying an external electric field, thus implying Tellurene is a promising candidate for piezoelectric sensors.

Two-dimensional monolayer from organic molecules F4-TCNQ via DFT calculations

Conjugated microporous polymers is a class of organic nanoporous polymers in which the pores can be easily controlled by molecule design. Here we present a new class of 2D structure based on F4-TCNQ molecules. Using ab initio calculations, the possibilities for the formation of such 2D materials based on F4-TCNQ molecules are studied as well. Among considered structures only two were selected as the most preferable for 2D layer formation. Predicted monolayers were analyzed on thermal stability, electronic, optical and mechanical properties. Combination of stability, elasticity and semiconducting band gap make suggested monolayers promising candidate for fabrication of elements of flexible and controllable optoelectronics.