Tunable Band Gap Research Articles

Flexible, photoluminescent 0D Cs4PbX6 (X = Br, Br/I)–PMMA composite films for white LED via water-induced recrystallization

3D CsPbX3 inorganic perovskite materials have attracted much attention in optoelectronic devices because of their strong absorbance, high photoluminescent quantum yield, tunable band gap, and narrow emission bandwidth. However, their practical usefulness is limited due to their poor stability in ambient conditions. Here, we created photoluminescent 0D Cs4PbX6 (X = Br, Br/I) suspensions in toluene by adding a small amount of water. The photoluminescent 0D Cs4PbX6 perovskite was mixed with polymethylmethacrylate (PMMA) forming 0D Cs4PbX6/PMMA composite films with higher PL, stability, transparency, and transmittance than that of the 3D CsPbX3/PMMA composite films prepared separately. Moreover, the PL intensity maintains 90% of the initial value after 30 days in water, showing excellent water stability. The flexible white-light LED device prepared by the composite films illustrated good luminescence performance with color rendering index 74.77, chromaticity coordinates (0.32, 0.33), and color temperature 6997 K.Graphical abstract

High responsivity VIS-NIR photodetector based on (PEA)2PbI4/P3HT heterojunction

Two-dimensional (2D) Ruddlesden–Popper (RP) perovskite is an ideal platform for building high-performance photodetectors due to its unique structure, excellent stability, and tunable bandgap. However, 2D perovskite heterojunction photodetectors are still less reported. In this work, the photodetector based on (PEA)2PbI4/P3HT heterojunction was prepared by the liquid-phase solution method under air conditions, which showed a responsivity (R) of 8.38 A W−1 and a specific detectivity (D*) of 6.53 × 1011Jones under 808 nm light. In addition, the device has a transient response time of 79.57 μs/484 μs and a high stability performance that maintains more than 95% of the photocurrent under continuous cycling tests and shows good cyclic stability after 30 days without encapsulation. This work provides a promising strategy for extending 2D perovskite photodetectors in the VIS to NIR spectral range.

Gate-Tunable Positive and Negative Photoconductance in Near-Infrared Organic Heterostructures for In-Sensor Computing.

The rapid growth of sensor data in the artificial intelligence often causes significant reductions in processing speed and power efficiency. Addressing this challenge, in-sensor computing is introduced as an advanced sensor architecture that simultaneously senses, memorizes, and processes images at the sensor level. However, this is rarely reported for organic semiconductors that possess inherent flexibility and tunable bandgap. Herein, an organic heterostructure that exhibits a robust photoresponse to near-infrared (NIR) light is introduced, making it ideal for in-sensor computing applications. This heterostructure, consisting of partially overlapping p-type and n-type organic thin films, is compatible with conventional photolithography techniques, allowing for high integration density of up to 520 devices cm-2 with a 5µm channel length. Importantly, by modulating gate voltage, both positive and negative photoresponses to NIR light (1050nm) are attained, which establishes a linear correlation between responsivity and gate voltage and consequently enables real-time matrix multiplication within the sensor. As a result, this organic heterostructure facilitates efficient and precise NIR in-sensor computing, including image processing and nondestructive reading and classification, achieving a recognition accuracy of 97.06%. This work serves as a foundation for the development of reconfigurable and multifunctional NIR neuromorphic vision systems.

Tunable large omnidirectional photonic bandgap in a one-dimensional photonic crystal comprising elliptical metamaterials based on elasto-optic effect

Tunable large omnidirectional photonic bandgap in a one-dimensional photonic crystal comprising elliptical metamaterials based on elasto-optic effect

Silver-Doped CsPbI2Br Perovskite Semiconductor Thin Films

All-inorganic perovskite semiconductors have received significant interest for their potential stability over heat and humidity. However, the typical CsPbI3 displays phase instability despite its desirable bandgap of ~1.73 eV. Herein, we studied the mixed halide perovskite CsPbI2Br by varying the silver doping concentration. For this purpose, we examined its bandgap tunability as a function of the silver doping by using density functional theory. Then, we studied the effect of silver on the structural and optical properties of CsPbI2Br. Resultantly, we found that ‘silver doping’ allowed for partial bandgap tunability from 1.91 eV to 2.05 eV, increasing the photoluminescence (PL) lifetime from 0.990 ns to 1.187 ns, and, finally, contributing to the structural stability when examining the aging effect via X-ray diffraction. Then, through the analysis of the intermolecular interactions based on the solubility parameter, we explain the solvent engineering process in relation to the solvent trapping phenomena in CsPbI2Br thin films. However, silver doping may induce a defect morphology (e.g., a pinhole) during the formation of the thin films.

Effect of Lewis acid-base additive on lead-free Cs2SnI6 thin film prepared by direct solution coating process

Abstract Inorganic Cs2SnI6 perovskite has exhibited substantial potential for light harvesting due to its exceptional optoelectronic properties and remarkable stability in ambient conditions. The charge transport characteristics within perovskite films are subject to modulation by various factors, including crystalline orientation, morphology, and crystalline quality. Achieving preferred crystalline orientation and film morphology via a solution-based process is challenging for Cs2SnI6 films. In this work, we employed thiourea as an additive to optimize crystal orientation, enhance film morphology, promote crystallization, and achieve phase purity. Thiourea lowers the surface energy of the (222) plane along the ＜111＞ direction, confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy studies, and density functional theory calculations. Varying thiourea concentration enables a bandgap tuning of Cs2SnI6 from 1.52 eV to 1.07 eV. This approach provides a novel method for utilizing Cs2SnI6 films in high-performance optoelectronic devices.

Broad band modulation of two-dimensional Mo1-x W x S2 by variational compositions

Precisely tuning bandgap enables tailored design of materials, which is of crucial importance for optoelectronic devices. Towards this end, ternary Mo1-x W x S2 monolayers with continually variational transition metal compositions are synthesized by precisely control of the precursors and growth temperatures in the chemical vapor deposition process, and thus to manipulate the bandgap of the monolayers. Energy dispersive x-ray spectroscopy demonstrates that the composition of the ternary Mo1-x W x S2 monolayers can be effectively modulated by the precursors and synthesizing temperatures. Frequency and intensity of the Raman and photoluminescent peaks of the Mo1-x W x S2 monolayers are continually modulated by the variational Mo and W compositions. The maximum of 0.148 eV bandgap modulation is achieved by modulating the transition metal compositions, which is highly consistent to the calculated 0.158 eV by first-principles theory. Photodetectors based on the Mo1-x W x S2 monolayers are fabricated and U-shape of photoresponse characteristics are demonstrated as x change from 0 to 1 under the same measurement conditions. The estimated photocurrent, photoresponsivity and external quantum efficiency show that the minimum values appear at the composition of x = 0.5, while the maximum values appear at x = 1. The results illustrate an excellent level of control on the band structure of two-dimensional ternary Mo1-x W x S2 by precisely control of the transition metal compositions.

A numerical approach to optimize the efficiency of a novel HTL-free Sr3Ti2S7 Ruddlesden-Popper perovskite solar cell

The exceptional environmental and structural stability of two-dimensional Ruddlesden-Popper perovskites has made them appealing candidates for high-performance perovskite solar cells. Besides, Ruddlesden-Popper perovskites can be synthesized using low-cost solution processing and have tunable bandgaps as compared to their conventional three-dimensional counterparts. Therefore, a Ruddlesden–Popper perovskite solar cell has been reported in this investigation and the SCAPS program has been used to theoretically study the device performance of the proposed lead-free, environmentally friendly perovskite solar cell. A novel Sr3Ti2S7 material has been utilized as the light absorber for the first time in such a theoretical study. Two different configurations, employing WS2 and C60 electron transport layers have been presented and device simulations have been performed to optimize the solar cell. The utilization of C60 as an electron transport material did not prove to be beneficial for the solar cell as it suffered from major recombination losses. It was also found that higher doping concentration of the electron transport materials could help these cells achieve improved fill factor. The diffusion length of the electrons and holes with respect to absorber defect density was also determined. Mott-Schottky analysis proved that the ITO/C60/Sr3Ti2S7/Au structure had a lower flatband potential compared to ITO/WS2/Sr3Ti2S7/Au. Additionally, from the impedance spectroscopy results, the recombination lifetime of the ITO/WS2/Sr3Ti2S7/Au configuration was found to be 46.7 μs compared to only 12.9 μs for the ITO/C60/Sr3Ti2S7/Au architecture. It was also established that if the thickness of the absorber and the bulk defect concentration were optimized the ITO/WS2/Sr3Ti2S7/Au and ITO/C60/Sr3Ti2S7/Au solar cells could achieve conversion efficiencies close to 11.0 % and 10.0 % respectively.

Band-gap tuning in Mn-doped Er2Ti2O7: Insight from the experimental and theoretical approach

We report the studies of electronic and optical properties of Er2Ti2−xMnxO7 (x = 0, 0.1, & 0.2) through combined theoretical insights from density functional theory (DFT) calculations and experimental approaches, including ultraviolet-visible absorption, photoluminescence emission spectroscopy, and Raman spectroscopy. For quantitatively correct optical bandgaps through DFT calculations, we considered the LDA-1/2 technique for the exchange-correlation interactions. The energy bandgap values obtained using theoretical formulations are in close accordance with the experimental results. The bandgap value obtained using the LDA-1/2 approach indicates the insulated ground state of the Er2Ti2−xMnxO7 (x = 0, 0.1, & 0.2) system. The band gap value obtained experimentally reduces from 3.82 eV to 2.45 eV as the level of Mn doping increases from 0 to 0.2. This reduction in the experimental band gap is attributed to the fact that inclusion of Mn atoms into the crystal lattice alters the electronic structure of the system. Substituting the Mn atom introduces additional electronic states within the band structure, leading to a modification of the band gap. Crystallographic data indicates that inclusion of Mn reduces the Ti-O bond length as in indicated in the table which reduces to 1.939(0) Å for x = 0.2 from 1.947(0) Å for x = 0. This results in the reduction of the extent of hybridization between O-2p and Ti-3d states, thereby reducing the band gap.

Recent Developments of Nanostructured Photocatalysts Based on Semiconducting Chalcogenides for Organic Reactions

Abstract:: The earth-abundant metal chalcogenide is a highly versatile semiconductor with unique electronic and optical properties that seem to outperform the photocatalytic properties of metal oxides and metal nanoparticles. For ages, researchers are reaping the benefits of its photocatalytic properties in numerous reactions. However, the high charge recombination rates, poor compositional stability, and a smaller number of catalytically active sites restrict its application. The development of different kinds of heterojunctions by the combination of metal-chalcogenides with other conducting and semiconducting materials like metal oxides, metal nanoparticles, g-C3N4, single-atom catalysts, and MOF results in superior photocatalytic activity. This review provides insight into the various classes of metal-chalcogenide-based heterostructures and their application in various organic transformations. A brief overview of the synergistic properties arising from the development of such heterostructures helps to understand the surface interactions so that highly stable, efficient, and selective metal-chalcogenide-based heterostructures can be developed for industrially important photocatalytic organic transformations. This review also describes the role of mediators in boosting the stability and catalytic efficiency of the metal chalcogenides. Moreover, a thorough emphasis on the morphological impact of photocatalysts in various reactions will help with the development of metal chalcogenide heterostructures with tunable morphology and bandgap.

Point-junction and alkali-assisted surface selenium diffusion: Unveiling a two-step method for enhancing the efficiency of VTD-SnS thin-film solar cells

Surface and interface engineering in thin-film solar cells (TFSCs) is vital for optimizing charge carrier dynamics, reducing recombination, enhancing material properties, and ultimately improving efficiency and overall performance. In this study, e-beam evaporation was used to deposit selenium (Se) onto tin sulfide (SnS) absorber layers with a thickness of 10 nm to 50 nm. The Se layer optimization was validated using various characterization techniques, which confirmed the formation of micro-sized openings via Se droplet deposition. This Se passivation-plus-local opening geometry plays a crucial role in forming point-junctions with the n-type CdS during cell fabrication, presenting a novel approach in state-of-the-art technology. The results revealed a significant enhancement in the conversion efficiency of the SLG/Mo/SnS/Se/CdS/i-ZnO/AZO/Al device to 3.01 % (VOC = 0.326 V, JSC = 20.21 mA cm−2, and FF = 0.45) with 30 nm Se layer deposition. Furthermore, a 30 nm-thick NaF-assisted Se layer was deposited to improve device performance. The diffusion of both Se and Na into the SnS substrate was validated using XRD, SEM, EDS, AFM, and XPS. The power conversion efficiency (PCE) of the SLG/Mo/SnSSe-NaF/CdS/i-ZnO/AZO/Al device was further enhanced to 3.70 % (VOC = 0.340 V, JSC = 22.56 mA cm−2, and FF = 0.48). This enhancement is attributed to the creation of localized junctions via Se surface passivation, and the concurrent SnSSe formation induces band gap tuning and increases the grain size, contributing to additional PCE improvement during Na-assisted Se diffusion.

Layer-controllable synthesis, lattice structural Determination, and fluorescence lifetime imaging of (HA)2(MA)n-1PbnI3n+1 Quasi-2D organic-inorganic perovskite microsheets

Hybrid quasi-two-dimensional (quasi-2D) perovskites have attracted great interest due to their excellent photovoltaic and light-emitting properties, which enable the rapid development of high-performance perovskite-based solar cells, light-emitting diodes, and lasers. Although many efforts have been made to improve the quality and stability of organic-inorganic perovskites, the controlled synthesis of high-quality layered perovskites remains challenging. Here, single-crystalline quasi-2D (HA)2(MA)n-1PbnI3n+1 perovskites with layer numbers of n from 1 to 3 have been synthesized in a controllable manner, exhibiting stable lattice structures and tunable optical emission. Structurally, hexylamine (HA) with longer alkyl chains was used as an organic spacer in Ruddlesden-Popper (RP) phase perovskites to improve the chemical stability of perovskites. Various ratios of precursor materials were adopted for separating pure and large-scale quasi-2D (HA)2(MA)n-1PbnI3n+1 perovskites, in which different layer-number perovskites with tunable band gaps were obtained with the modulated fluorescence lifetimes. Our lattice structural and fluorescence-lifetime characterizations of RP-phase perovskites provide critical information on the emerging quasi-2D perovskite-crystal systems and their fluorescence relaxation dynamics. The employed strategy to synthesize high-quality perovskites (HA)2(MA)n-1PbnI3n+1 is technically essential to develop the rising perovskite-based photovoltaic and light-emitting applications.

First-principles study of the effect of strain on the structural and optoelectronic properties of flexible photovoltaic material Cs2AgInBr6

The lead-free double-perovskite halide materials are promising materials for photovoltaics. Recently, Cs2AgInBr6 (CAIB) has been synthesized with the estimated direct nature of a band gap value of 1.57 eV. To cover the wide solar spectrum for photo-conversion, the applied strain is one of the promising approaches to achieve it through band gap tuning. The density functional theory is used to investigate the effect of compressive strain on the structural, electronic, and optical properties of CAIB. The elastic constants follow the Born–Huang stability criterion and show the mechanical stability of the composition even under compressive strain. The Poisson’s ratio in the range of 0.23–0.26 and B/G > 1.75 indicate the ductile and soft nature of the material. The band gap increases monotonically without changing the direct nature of the band gap by increasing the compressive strain. However, the larger value of strain reproduces more dispersive conduction band minima and valence band maxima, resulting in lower effective masses and consequently larger carrier mobilities. The variations in the optical properties of CAIB are explored under compressive strain. The structural, electronic, and good photo response of the material in the visible and ultraviolet regions indicate the suitability of the material for flexible photovoltaics.

Emission behaviors from ZnO microwire pumped CsPbBr3 perovskite microwire.

Perovskite semiconductor materials have attracted significant attention in the fields of photovoltaics and luminescence due to their excellent photoelectric properties, such as high carrier mobility, high absorption coefficient, and high fluorescence quantum yield. In particular, low-dimensional metal-halide perovskite microcrystalline materials have been reported to exhibit low-dimensional lasing phenomena and laser devices due to their high gain and widely tunable bandgap. In this Letter, one-dimensional (1-D) ZnO microwires with their ultraviolet lasing emissions are utilized as an excitation source to pump CsPbBr3 microwire on hybrid ZnO-CsPbBr3 microscale structures. At higher excitation, the amplified spontaneous emission (ASE) behaviors from CsPbBr3 microwire are realized with ultralow threshold by indirect pumping from the ZnO lasing emission for the first time, to the best of our knowledge. In comparison, the ASE behaviors from the CsPbBr3 microwire directly pumped by Nd:YAG Q-switched laser and continuous wave laser are also performed at room temperature. There are also no multimode lasing behaviors observed. The paper provides a new method to achieve a low threshold on-chip microlaser by a high-quality perovskite micro-nano structure.

Deep learning-based inverse design of lattice metamaterials for tuning bandgap

In this paper, deep learning neural networks is used to predict the band structure of metamaterial lattices, and proactive inverse design is employed in bandgap modulation. A parametric design of the metamaterial lattice is proposed to achieve a rich design space. The corresponding band structure data is calculated by finite element method (FEM) to construct the data set. We successfully bypass complex theoretical or numerical methods to establish the mapping relationship between the lattice geometry parameters of metamaterials and the band structure data by constructing and training fully connected neural networks and convolutional neural networks (CNN). By combining the trained neural network model into an inverse design method of bandgap tuning, the geometric parameters of the metamaterial lattice can be obtained directly by inputting the target band structure. Finally, three object band structures are designed and verified by finite element simulation and experiment, which verifies the effectiveness of the inverse design method. This design approach can be extended to design other metamaterial properties.

Band gap tunability in DC sputtered Ni-doped ZnO thin films for wide usage in optoelectronic gadgets

ZnO thin films have brought significant applicability in optoelectronic and energy storage devices based on their environmental friendliness. Pristine ZnO, 6.25 % and 12.5 % Ni-doped ZnO thin films were deposited on Si substrate via direct current and radio frequency magnetrons, respectively. The X-ray diffraction patterns exhibited phase purity of synthesized thin films. The micrographs showed reduction in average grain size for pure ZnO, 6.25 % and 12.5 % Ni-doped ZnO as 95 nm, 70 nm and 60 nm, respectively. The elemental purity was confirmed by energy dispersive X-ray. The ellipsometric analysis showed that Ni inclusion brought band gap variation from 3.06 to 3.08 eV for pure ZnO to 12.5 % Ni-doped ZnO thin films. The real part of dielectric constant enhanced from 3.60 to 3.75 for 12.5 % Ni-doped ZnO, simultaneously depicting reduction in dielectric loss from 0.45 to 0.40. Conclusively, proposing Ni-doped ZnO thin films with low dielectric loss as a potential candidate in optoelectronic industry.

Investigating the effect of lead substitution on the optical, electrical, and photoresponse properties of Quasi-2D double perovskites

Quasi-2-dimensional (2D) halide perovskites have recently attracted attention due to their higher operational stability as alternatives to 3-dimensional (3D) perovskites having exceptional optoelectronic and charge transport properties. To reduce the lead content, here following the double perovskite approach lead is substituted with silver and bismuth simultaneously, and three quasi-2D perovskites, with general formula (C7H10N)2Pb(1-2x)AgxBixBr4, (0 ≤ x ≤ 0.5) were prepared. The optical studies show that the partially lead substituted sample has the lowest optical band gap, aptly supported by the theoretical calculations. The powder X-ray diffraction technique along with field-emission scanning electron microscopy suggests enhancement in crystallinity along with the decrease in grain boundaries with the substitution of lead. The improvement in crystallinity with concomitant reduction in grain boundaries has led to the decrease in point defects as identified from the positron annihilation lifetime spectroscopy and coincidence Doppler broadening analysis. The tuned band gap, improved crystal quality along with lower defects jointly contributed to the enhancement in electrical properties of the perovskites with varying lead percentages. Finally, the photoresponse of all the materials was studied after fabricating metal (Al)-semiconductor (MS) junction thin film photodetector devices.

Molecular beam epitaxy and characterization of ferroelectric quaternary alloy Sc0.2Al0.45Ga0.35N

In this study, we demonstrate ferroelectricity in high-quality monocrystalline quaternary alloy ScAlGaN. Sc0.2Al0.45Ga0.35N films are grown by plasma-assisted molecular beam epitaxy and exhibit a surface roughness of 0.5 nm, limited by the roughness of the underlying molybdenum template. Polarization-electric field and positive-up-negative-down measurements reveal unambiguous ferroelectric switching with a coercive field of ∼5.5 MV cm−1 at 10 kHz and high remanent polarization of ∼150 μC cm−2. Time-dependent measurements suggest that the polarization reversal behavior adheres to the Kolmogorov–Avrami–Ishibashi model and follows a scheme of domain nucleation and growth. Detailed piezoresponse force microscopy studies further elucidate the evolution of polarity reversal domains in wurtzite nitride ferroelectrics and support the notion that the growth of inversion domains occurs via an in-plane motion of the domain walls. The realization of functional ferroelectric quaternary alloys in the wurtzite nitride family extends beyond being a technical demonstration. The additional degree of bandgap, band alignment, lattice parameter, and piezoelectric constant tunability achievable through quaternary alloys unveils a vast dimension through which wurtzite nitride ferroelectrics can be optimally engineered for a broad variety of high-performance electronic, optoelectronic, and acoustic devices and systems.

Improving ZnO ultraviolet Schottky photodetectors by inserting a MoS2 layer

ZnO Schottky junction photodetectors (PDs) has become a research focus due to their wide application, high-performance and low-cost. However, their development has been limited by the low photo-response. Two-dimensional MoS2 exhibit numerous advantages, including a tunable band gap, a strong exciton effect, and broadband absorption. In this work, we incorporate MoS2 into ZnO thin films using a simple spin-coating method to create Schottky ultraviolet (UV) PDs that show a remarkably increased responsivity (by 885%) and a substantially improved photo-current (by one order of magnitude). Furthermore, they exhibit a relatively low noise equivalent power (5.11×10−13 W) and a high normalized detectivity 2.96×1011 Jones. This outstanding performance can be attributed to the enhanced absorption and efficient charge transfer of ZnO film after introduction of MoS2 layer. In this study, an easy method is presented for creating high-performance ZnO Schottky PDs.

Plasmon-Induced Ultrafast Hot Hole Transfer in Nonstoichiometric CuxInyS/CdS Heteronanocrystals.

Plasmonic semiconductors are promising candidates for developing energy conversion devices due to their tunable band gap, cost-effectiveness, and nontoxicity. Such materials exhibit remarkable capabilities for harvesting infrared photons, which constitute half of the solar energy spectrum. Herein, we have synthesized near-infrared (NIR) active CuxInyS nanocrystals and CuxInyS/CdS heterostructure nanocrystals (HNCs) to investigate plasmon-induced charge transfer dynamics on an ultrafast time scale. Employing femtosecond transient absorption spectroscopy, we demonstrate that upon exciting the HNCs with sub-band gap NIR photons (λ = 840 nm), the hot holes are generated in the valence band of plasmonic CuxInyS and transferred to the adjacent semiconductor. The decreased signal intensity and accelerated hole phonon relaxation dynamics for HNCs reveal efficient transfer of plasmon-induced hot carriers from CuxInyS to CdS under both 840 and 350 nm laser excitations, providing a pathway for enhanced carrier utilization. These findings shed light on the potential of ternary chalcogenides in plasmonic applications, highlighting efficient hot carrier extraction to adjacent semiconductors.