Tunable Band Gap Research Articles

Photoluminescence properties of two-dimensional semiconductor heterointerfaces

Two-dimensional metal-sulfur compounds have attracted much attention due to their novel physical properties, such as layered structure, ultrathin physical dimensions, and continuously tunable bandgap. The vertical stacking of different 2D semiconductors enables the heterojunction to retain the excellent properties of its constituent materials and has physical properties such as interlayer energy transfer and interlayer carrier transfer. In this paper, we utilize the carrier interlayer transfer properties of p-n heterojunctions and form heterojunctions using p-type Te and PdSe2 prepared with n-type monolayer WS2 using the microzone transfer technique. We found that the PL spectrum of monolayer WS2 is purer after heterojunction formation. The photoluminescence peaks representing exciton recombination are sharper, while the peaks represented by trions almost disappear. These phenomena indicate that we can utilize p-n junctions to capture the PL spectra of excitons in WS2, which is important for the further study of the optical properties of 2D metal-sulfur compounds.

Design strategies of cementitious metamaterials (CMs) with tunable bandgaps: Density customization and geometric optimization

The propagation of low-frequency elastic waves may exert significant stress on both human health and environmental safety. This study presents a new kind of cementitious metamaterials (CMs) characterized by tunable bandgaps for efficient absorption of low-frequency elastic waves. A dynamic system model is employed to examine and analyze the eigenfrequencies of these CMs. Herein, the porous alkali-activated municipal solid waste incineration fly ash (AAFMs) is modulated with customed density and then combined with rubber for generating CMs. Two distinct strategies are proposed to modulate the bandgap properties of the CMs: density customization and geometric optimization. The AAFMs' density is meticulously regulated by altering the foaming aluminum content, allowing the customization of CMs' bandgaps. Additionally, the CMs' bandgaps can also be tailored through geometric optimization, a process that involves a numerical method focused on examining the impact of varying inclined angles in inner and outer re-entrant structures. A transmission loss model is also developed to validate the bandgap characteristics of these CMs. Overall, the study explores the potential applications of these CMs, with a specific focus on their efficacy in mitigating low-frequency vibrations and attenuating noise. The successful utilization of these materials in such domains holds promising prospects for substantial advancements across diverse industrial sectors.

Defect Engineering of MoTe2 via Thiol Treatment for Type III van der Waals Heterojunction Phototransistor.

Molybdenum ditelluride (MoTe2) nanosheets have displayed intriguing physicochemical properties and opto-electric characteristics as a result of their tunable and small band gap (Eg ∼ 1 eV), facilitating concurrent electron and hole transport. Despite the numerous efforts devoted to the development of p-type MoTe2 field-effect transistors (FETs), the presence of tellurium (Te) point vacancies has caused serious reliability issues. Here, we overcome this major limitation by treating the MoTe2 surface with thiolated molecules to heal Te vacancies. Comprehensive materials and electrical characterizations provided unambiguous evidence for the efficient chemisorption of butanethiol. Our thiol-treated MoTe2 FET exhibited a 10-fold increase in hole current and a positive threshold voltage shift of 25 V, indicative of efficient hole carrier doping. We demonstrated that our powerful molecular engineering strategy can be extended to the controlled formation of van der Waals heterostructures by developing an n-SnS2/thiol-MoTe2 junction FET (thiol-JFET). Notably, the thiol-JFET exhibited a significant negative photoresponse with a responsivity of 50 A W-1 and a fast response time of 80 ms based on band-to-band tunneling. More interestingly, the thiol-JFET displayed a gate tunable trimodal photodetection comprising two photoactive modes (positive and negative photoresponse) and one photoinactive mode. These findings underscore the potential of molecular engineering approaches in enhancing the performance and functionality of MoTe2-based nanodevices as key components in advanced 2D-based optoelectronics.

Evolution of Electroless Cu/CuxO Nanostructured Thin Films: Study of Surface Fractal, Tuning Band Gap and Electronic Structure

Evolution of Electroless Cu/CuxO Nanostructured Thin Films: Study of Surface Fractal, Tuning Band Gap and Electronic Structure

PbI2 Passivation of Three Dimensional PbS Quantum Dot Superlattices Toward Optoelectronic Metamaterials.

Lead chalcogenide colloidal quantum dots are one of the most promising materials to revolutionize the field of short-wavelength infrared optoelectronics due to their bandgap tunability and strong absorption. By self-assembling these quantum dots into ordered superlattices, mobilities approaching those of the bulk counterparts can be achieved while still retaining their original optical properties. The recent literature focused mostly on PbSe-based superlattices, but PbS quantum dots have several advantages, including higher stability. In this work, we demonstrate highly ordered 3D superlattices of PbS quantum dots with tunable thickness up to 200 nm and high coherent ordering, both in-plane and along the thickness. We show that we can successfully exchange the ligands throughout the film without compromising the ordering. The superlattices as the active material of an ion gel-gated field-effect transistor achieve electron mobilities up to 220 cm2 V-1 s-1. To further improve the device performance, we performed a postdeposition passivation with PbI2, which noticeably reduced the subthreshold swing making it reach the Boltzmann limit. We believe this is an important proof of concept showing that it is possible to overcome the problem of high trap densities in quantum dot superlattices enabling their application in optoelectronic devices.

Band gap tuning in calcium hydroxide-nitrate nanocomposite materials

Band gap tuning in calcium hydroxide-nitrate nanocomposite materials

Electrochemical applications of CdO-Co-ZnO nanocomposites, their synthesis and characterization reveal their multifunctional abilities

In this study, we investigated the electrochemical catalysis potential of hybrid nanocomposites containing CdO, Co and ZnO nanocomposites, as opposed to Zn-O doped Co nanocomposites, have weaker Coulomb interactions due their ionic bonds. Because CdO and Co form a covalent bond, Co interacts more strongly with O than Zn. In order to reduce nanoparticle crystallinity, oxygen defects improve the interaction between −O and oxygen defects in the lattice. From SEM micrographs, it appears that CdO does not completely change under the influence of dopants. It can be seen from the SEM image that both materials have very tightly packed particles. The Co and CdO dopants in ZnO nanocomposites prevent them from absorbing a large range of visible wavelengths. It is more energy-dense for nanocomposites with 5.28 eV to compare to 5.14 eV nanocomposites. The fact that CdO matrix has a tuneable bandgap is evident since different types of dopants are used in its manufacture. There are at least three distinct absorption modes in Co nanocomposites doped with CdO, around 450, 498, and 676 cm−1. In addition to its absorption from 450 cm−1 and 498 cm−1 vibrational modes, Co-O stretching absorption along the [101] plane has also been observed at 676 cm−1. As a method of studying charge carriers, photoluminescence spectroscopy is usually used. This method can be used to analyze electron-hole pairs (e−/h+) formed by semiconducting particles. It is in the blue emission range between the luminescence band of 615 nm and the valence band of 635 nm. With increasing cobalt and zinc concentrations, CdO nanomaterials lose their remanent magnetization. CdO has been demonstrated to have significant coercive effects in both pure and additively incorporated solutions regardless of their anisotropic, morphological, porosity, and particle size distribution. Electrochemical impedance spectrum measurements were conducted between 100 kHz and 0.01 Hz. According to the Nyquist plot, purity CdO, CdO doped Co, and CdO doped ZnO nanocomposites show a high frequency resistance to charge transfer. Nanocomposites that contained CdO doped Co & ZnO were exposed to UV light for 120 min to remove the solution. The degradation of MO is virtually nonexistent when no photocatalyst is present, but with a photocatalyst, degradation can reach 92.56 %.

Modification Strategies of Hexagonal Boron Nitride Nanomaterials for Photocatalysis.

Although hexagonal boron nitride (h-BN) was initially considered a less promising photocatalyst due to its large band gap and apparent chemical inertness, its unique two-dimensional lamellar structure coupled with high stability and environmental friendliness, as the second largest van der Waals material after graphene, provides a unique platform for photocatalytic innovation. This review not only highlights the intrinsic qualities of h-BN with photocatalytic potentials, such as high stability, environmental compatibility, and tunable bandgap through various modification strategies but also provides a comprehensive overview of the recent advances in h-BN-based nanomaterials for environmental and energy applications, as well as an in-depth description of the modification methods and fundamental properties for these applications. In addition, we discuss the challenges and prospects of h-BN-based nanomaterials for future photocatalysis.

Exploring thermal and in-situ mechanical properties of flexible 2D tungsten disulfide foam-polymer composite for thermal management

In the realm of thermal management applications, there is a growing need for flexible materials that can efficiently dissipate heat. Polymer composites incorporating two-dimensional (2D) materials offer promising solutions due to their superior thermal and mechanical properties. Tungsten disulfide (WS2) is an excellent filler candidate for polymer composites due to its superior thermal conductivity, mechanical strength, and electrical properties. Unlike graphene, WS2 has a tunable bandgap, exhibits higher thermal resistance in inert atmospheres, is an effective lubricant over a wide temperature range of −190°C to over 800°C, and offers superior gamma radiation shielding. However, high-density 2D material polymer composite fabrication faces challenges due to the agglomeration tendency and sedimentation of heavy nanomaterials within the polymer matrix. This poor dispersion stability negatively impacts the performance and reliability of the composite. We developed a flexible WS2 foam-polydimethylsiloxane (PDMS) composite via freeze drying and vacuum-assisted infiltration, which not only overcomes fabrication challenges but also enables unique filler reinforcement foam designs. The thermal conductivity of WS2-PDMS foam was 1.57 times higher than that of neat PDMS. These thermal properties were modeled using the Lewis-Nielsen model. In-situ tensile tests were conducted to understand the mechanical reinforcing behavior and failure mechanisms of WS2 foam, which were further studied using the Gibson and Ashby model. The addition of WS2 into PDMS resulted in an elastic modulus 1.56 times higher than that of neat PDMS. The composite's mechanical properties were analyzed using the Halpin-Tsai model. These findings highlight the potential of WS2-PDMS composites for flexible thermal management applications.

A low-frequency pure metal metamaterial absorber with continuously tunable stiffness

To address the incompatibility between high environmental adaptability and deep subwavelength characteristics in conventional local resonance metamaterials, and overcome the deficiencies in the stability of existing active control techniques for band gaps, this paper proposes a design method of pure metal vibration damping metamaterial with continuously tunable stiffness for wideband elastic wave absorption. We design a dual-helix narrow-slit pure metal metamaterial unit, which possesses the triple advantage of high spatial compactness, low stiffness characteristics, and high structural stability, enabling the opening of elastic flexural band gaps in the low-frequency range. Similar to the principle of a sliding rheostat, the introduction of continuously sliding plug-ins into the helical slits enables the continuous variation of the stiffness of the metamaterial unit, achieving a continuously tunable band gap effect. This successfully extends the effective band gap by more than ten times. The experimental results indicate that this metamaterial unit can be used as an additional vibration absorber to absorb the low-frequency vibration energy effectively. Furthermore, it advances the metamaterial absorbers from a purely passive narrowband design to a wideband tunable one. The pure metal double-helix metamaterials retain the subwavelength properties of metamaterials and are suitable for deployment in harsh environments. Simultaneously, by adjusting its stiffness, it substantially broadens the effective band gap range, presenting promising potential applications in various mechanical equipment operating under adverse conditions.

First-principles calculation of monolayer PdSe2 with Se and Pd vacancy and its effect on quantum capacitance

Abstract The emergence of 2-D materials such as graphene has caught the attention of the scientific community. 2-D materials have a higher surface area per unit mass, which is ideal for electrodes. Another popular material among researchers is the monolayer palladium diselenide or PdSe2, a semiconductor with a tunable bandgap energy. Since there are fewer states in the fermi level of PdSe2, the quantization effect is more prevalent, hence, it would most likely draw its capacitance from its electronic configuration. In this study, the electronic properties such as band structure, density of states, and quantum capacitance of pristine monolayer PdSe2, PdSe2 with Se, and Pd vacancy were calculated based on density functional theory via Quantum Espresso. The formation energies of all systems were energetically favourable. The system with Se vacancy has the highest formation energy with a value of -3.47 eV. The density of states of all systems were observed to have a local minima at the fermi level. More occupiable states around the valence band were observed for the systems with vacancy. Bader charge analysis showed a notable decrease of charge in Se atoms near the Pd vacancy, while the Pd atoms in the Se vacated system showed stronger charge transfer between Pd and another Se atom. The quantum capacitance and surface charge values were calculated using the density of states. Higher surface charge values at the negative voltage were observed for the systems with induced vacancies. Based on the results, the systems with vacancies have improved the quantum capacitance and surface charge at the negative potential.

Mass-controlled contact variable stiffness elastic metamaterials for bandgap tuning

Mass-controlled contact variable stiffness elastic metamaterials for bandgap tuning

Automated design of hybrid halide perovskite monolayers for band gap engineering

The recent experimental confirmation of perovskite monolayers has sparked ongoing efforts in their prediction and synthesis, showcasing their flexible tunable band gap and potential in advanced functional devices. Although large-scale computational designs have been successfully performed for inorganic perovskite monolayers, the complexity introduced by organic cations hinders the same approaches applied to the hybrid halide perovskite monolayers. To address this challenge, we have proposed a high-throughput first-principles computational workflow that automates the design of hybrid halide perovskite monolayers. We strategically reduce the complexity of the configurations by analyzing the orientation of cations and the structural symmetry. Over 400 hybrid halide perovskite monolayers have been designed, and their structures and fundamental properties are stored in the database. Correlation analyses show a strong correlation between band gaps and metal-halogen-metal bond angles or metal-halogen bond lengths, consistent with prior studies for bulk and layered perovskites. The underlying physics that the band gap is modulated by the antibonding in the metal-halogen bond makes band gap engineering of hybrid halide perovskite monolayers feasible. Accordingly, initial research on lateral heterojunctions and solar cells has been conducted to explore the potential practical applications of the designed hybrid halide perovskite monolayers. Our study lays the foundation for further exploration of hybrid halide perovskite monolayers and highlights promising opportunities for their potential applications in electronic and optical devices.

Single-atom iron nanoatalysts facilitate electrochemiluminescence of graphitic carbon nitride for copper ion sensing

As an emerging electrochemiluminescence (ECL) nanomaterial, graphitic carbon nitride (g-C3N4) has attracted considerable attention from the scientific community due to its excellent optical properties, favorable biocompatibility, and tunable bandgap structure. However, the ECL intensity of g-C3N4 is subject to various influencing factors, including conductivity and electrode passivation, which pose notable challenges in improving its ECL performance. In this study, iron single atom nanocatalysts (Fe SAC) were introduced onto g-C3N4 sheets, and a novel approach was proposed to enhance the active sites, modulate the bandgap, and bolster the conductivity through synergistic mechanism. Furthermore, Fe SAC acted as the co-reactant promoter, amplifying the ECL signal significantly. Comparative analysis revealed a visible improvement in the ECL performance of Fe SAC-g-C3N4/K2S2O8 in a potential range of 0 to −0.9 V. Via the electron transfer strategy, a copper ion-sensitive ECL sensor based on Fe SAC-g-C3N4 was developed with the linear range of 0.01–100 μM and a detection limit of 0.75 nM. This sensor has been successfully applied in detecting copper ions in water environment and human serum samples, providing a new strategy for enhancing the ECL performance of g-C3N4 and expanding the utility of Fe SAC in ECL.

Efficient PbS quantum dots tandem solar cells through compatible interconnection layer

Lead sulfide quantum dots (PbS QDs) hold unique characteristics, including bandgap tunability, solution-processability etc., which make them highly applicable in tandem solar cells (TSCs). In all QD TSCs, its efficiency lags much behind to their single junction counterparts due to the deficient interconnection layer (ICL) and defective subcells. To improve TSCs performance, we developed three kinds of ICL structures based on 1.34 and 0.96 eV PbS QDs subcells. The control, 1,2-ethanedithiol capped PbS QDs (PbS-EDT)/Au/tin dioxide (SnO2)/zinc oxide (ZnO), utilized SnO2 layer to obtain high surface compactness. However, its energy level mismatch causes incomplete recombination. Bypassing it, the second ICL (PbS-EDT/Au/ZnO) removed SnO2 and boosted the power conversion efficiency (PCE) from 5.75% to 8.69%. In the third ICL (PbS-EDT/poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)/Au/ZnO), a thin layer of PTAA can effectively fill fissures on the surface of PbS-EDT and also protect the front cells from solvent penetration. This TSC obtained a PCE of 9.49% with an open circuit voltage of 0.91 V, a short circuit current density of 15.47 mA/cm2, and a fill factor of 67.7%. To the best of our knowledge, this was the highest PCE achieved by all PbS QD TSCs reported to date. These TSCs maintained stable performance for a long working time under ambient conditions.

Tunable bandgap and luminescence properties by engineering exciton coupling in Ag NP embedded Er3+ bismuth tellurite glass system

Tunable bandgap and luminescence properties by engineering exciton coupling in Ag NP embedded Er3+ bismuth tellurite glass system

Impact of boron substitution on the thermoelectric, mechanical stability, electronic and optical properties of InP alloys

Density Functional Theory (DFT) calculations, utilizing the full potential linearized augmented plane wave (FP-LAPW) method, were performed to investigate the effect of boron (B) substitution on the InP system through GGA and mBJ-GGA formalisms, aiming to improve its band gap energy. Lattice equilibrium optimization determined that the stable structure of the alloys at equilibrium energy shows a decrease in lattice constants with increasing boron concentration (x). The calculated formation enthalpies, which are negative, indicate that these systems are experimentally synthesizable. The elastic analysis confirms that the doped alloys are elastically stable and exhibit ductile properties. The optical parameter analysis reveals that the optical energy loss functions are minimal or zero in the infrared, visible, and ultraviolet ranges, suggesting their suitability for optical applications across these energy ranges. Specifically, the band gaps were found to range from 0.878 eV for x = 0.25–1.920 eV for x = 0.75 using the mBJ-GGA formalism. Furthermore, the role of B-doping in the InP system for thermoelectric technology was systematically investigated, revealing that our alloys possess good thermoelectric properties with a figure of merit (ZT) reaching up to 0.7 for x = 0.75. These results demonstrate that accurate tuning of the energy band gap through doping is an effective technique for discovering novel materials suitable for both thermoelectric and optoelectronic applications.

In situ mass spectrometric investigation to probe GeSn growth dynamics and mechanisms in the chemical vapor deposition processes

GeSn has attracted increasing attention due to its tunable bandgap from indirect to direct resulting in unique electronic and optoelectronic capabilities. Chemical vapor deposition (CVD) is well acknowledged as an advanced growth method for GeSn, demonstrating its capability with grown materials for infrared lasers and detector development. As an in situ diagnostics of the CVD process, a residual gas analyzer (RGA) could enable the detection of all gaseous species during growth, thus probing the growth dynamics and mechanism. Therefore, it is highly desirable but is lacking in the (Si)GeSn research community. This work utilized an RGA equipped with a specialized differential vacuum pumping system to analyze the mass spectra of the GeSn CVD precursors of SnCl4, GeH4, and their combination. The spectra of SnCl4 collected from gas samples at a standard base pressure of 0.3 Torr display consistent fragments with central mass-to-charge ratios (m/z) of 78, 120, 155, 225, and 260, corresponding to Sn+, SnCl2+, SnCl+, SnCl2+, SnCl3+, and SnCl4+, respectively. These profiles closely resemble those observed in the National Institute of Standards and Technology and Wiley and Matsumoto et al. The simultaneous introduction of SnCl4 and GeH4 at a combined pressure of several tens of Torr yields spectra, indicating a chemical reaction that produces GeCl4 at room temperature in the vacuum chamber and tubing. Utilizing Gaussian16 and ORCA codes, the ab initio and density functional thermochemistry computations were employed to predict potential essential reactions and validate the experimental findings.

Deciphering the energy storage and electro‐catalytic potential of the BaLa2MnS5 ternary metallic sulfide prepared via single source precursor route

AbstractThe present work elucidates the first report on the synthesis and energy applications of the novel BaLa2MnS5 prepared from single source precursor route. This metal chalcogenide expressed a tuned band gap of 3.84 eV and an average crystallite size of 20.52 nm. Functional groups explored for BaLa2MnS5 expressed strong signals for presence of the metal sulfide bonds. The thermal decomposition pattern of BaLa2MnS5 followed a two‐phased mechanism. Synthesized metal sulfide possessed an irregular morphology with particles arranged in random manner. An assessment of the chalcogenide BaLa2MnS5 for energy applications has been carried out. When employed as an electrode material in 1 M KOH, which acted as the background electrolyte, BaLa2MnS5 chalcogenide showed a specific capacitance of 597 F g−1. Furthermore, this BaLa2MnS5 chalcogenide decorated electrode has a low resistance, as shown by the Rs of 1.35 Ω, and a specific power density of 7366 W kg−1, according to the impedance investigations. The electrochemical results for the OER activity are indicative of the OER overpotential and Tafel slope values as 388 mV and 108 mV/dec. This electrode achieved the HER overpotential value of 241 mV while the obtained Tafel slope was 194 mV/dec.

Perovskite nanowire-based electrochemical sensing for selective and rapid detection of hydrogen peroxide

Nanowires possess inherent advantages in improving the reduction of boundary stacking defects and enhancing charge transfer or migration. Here, we synthesized all-inorganic perovskite nanowires CsPbBr3 (PVSK NW) and obtained CsPb1-xSnxBr3 (PVSK-Sn NW) and CsPb1-xGexBr3 (PVSK-Ge NW) nanowires through doping with different elements (Sn and Ge). Elemental doping endows the nanowires with unique narrow fluorescence spectra, high optical absorption coefficients, tunable optical bandgaps, and high carrier mobility. Introducing different perovskite nanowires into carbon-based composite materials (super p/Nafion) can overcome the challenges of hydrogen peroxide (H2O2) detection sensitivity and matrix interference. H2O2 in PBS (phosphate buffered saline) solution (pH 7.0) was detected by current analysis i-t curve mode. The results demonstrate that the addition of perovskite nanowires exhibits a high level of detection sensitivity for H2O2 electrochemical reduction. Particularly, the linear concentration range detected by PVSK-Sn NW is 1–6000 µM, with a detection limit of 0.12 µM. Even in the presence of interfering molecules (ascorbic acid, uric acid, glucose), the detection of H2O2 maintains high sensitivity and selectivity. This study overcomes the limitations of perovskite materials in water-based detection, enhancing their future commercial applications as sensing devices.