Low Defect Density Research Articles

High gain NiO/ZnO heterojunction photodiodes operated in Deep-ultraviolet region

Epitaxially matched (0001) Al2O3 single substrates are utilized to design p-n heterojunctions (p-NiO/n-ZnO) as they offer low defect density for device fabrication due to small lattice mismatches. Using the radio frequency sputtering technique, the NiO/ZnO thin film-based heterojunction has been fabricated by varying the n-type film's (ZnO) thickness for ultra-violet (UV) photodiode application. It has been investigated how different ZnO film thicknesses affect junction parameters and photoresponse properties. The high photosensitivity (∼ 104), enhanced current gain (∼ 105) and fast response obtained at very low UV intensity (0.62μW/cm2) at 300 nm wavelength are attributed to the internal gain mechanism (recombination tunnelling and space-charge limited current conduction with the remarkable effect of the presence of excess free oxygen) in these devices. The prepared heterojunction photodiode with high speed and deep-UV sensitivity can be a potential solution for visible blind UV detection.

On the selective formation of cubic tetrastack crystals from tetravalent patchy particles.

Achieving the formation of target open crystalline lattices from colloidal particles is of paramount importance for their potential application in photonics. Examples of such desired structures are the diamond, tetrastack, and pyrochlore lattices. Here, we demonstrate that the self-assembly of tetravalent patchy particles results in the selective formation of cubic tetrastack crystals, both in the bulk and in the systems subjected to external fields exerted by the solid substrate. It is demonstrated that the presence of an external field allows for the formation of well-defined single crystals with a low density of defects. Moreover, depending on the strength of the applied external field, the mechanism of epitaxial growth changes. For weakly attractive external fields, the crystallization occurs in a similar manner as in the bulk, since the fluid does not wet the substrate. Nonetheless, the formed crystal is considerably better ordered than the crystals formed in bulk, since the surface induces the ordering in the first layer. On the other hand, it is demonstrated that the formation of well-ordered cubic tetrastack crystals is considerably enhanced by the increase in external field strength, and the formation of the thick crystalline film occurs via a series of layering transitions.

Performance optimization of CsPbIBr2-based perovskite solar cells through device modeling

Among all-inorganic perovskite materials, CsPbIBr2 provides the optimal equilibrium between optical bandgap and phase stability. However, notwithstanding these advantageous, interfacial defects and improper band alignment continue to diminish the photovoltaic efficacy of CsPbIBr2-based PSCs. This study used the SCAPS-1D software to undertake a thorough examination of operating mechanism of CsPbIBr2-based devices. A comprehensive analysis is conducted on a range of physical parameters pertaining to the FTO/ZnOS/CsPbIBr2/CZTS configuration, encompassing doping concentration, operating temperature, defect density, electron affinity, thickness, series and shunt resistance. The simulation outcomes revealed that PSCs characterized by a low defect density and an ideal band structure enhance the performance of the devices by facilitating the transport and separation of charge carriers. The optimized device achieved an efficiency of 16.68%, short-circuits current density (JSC) of 11.52 mA cm−2, open-circuit voltage (VOC) of 1.64 V, and Fill factor (FF) of 87.83%. These simulation findings will provide useful information for experimental fabrication of efficient CsPbIBr2-based inorganic PSC.

Implications of Defect Density and Polymer Interactions for CO2 Capture on Amine-Functionalized MIL-101(Cr).

Rising anthropogenic carbon emissions have dire environmental consequences, necessitating remediative approaches, which includes use of solid sorbents. Here, aminopolymers (poly(ethylene imine) (PEI) and poly(propylene imine) (PPI)) are supported within solid mesoporous MIL-101(Cr) to examine effects of support defect density on aminopolymer-MOF interactions for CO2 uptake and stability during uptake-regeneration cycles. Using simulated flue gas (10 % CO2 in He), MIL-101(Cr)-ρhigh (higher defect density) shows 33 % higher uptake capacity per gram adsorbent than MIL-101(Cr)-ρlow (lower defect density) at 308 K, consistent with increased availability of undercoordinated Cr adsorption sites at missing linker defects. Increasing aminopolymer weight loadings (10-50 wt.%) within MIL-101(Cr)-ρlow and MIL-101(Cr)-ρhigh increases amine efficiencies and CO2 uptake capacities relative to bare MOFs, though both incur CO2 diffusion limitations through confined, viscous polymer phases at higher (40-50 wt.%) loadings. Benchmarked against SBA-15, lower polymer packing densities (PPI>PEI), weaker and less abundant van der Waals interactions between aminopolymers and pore walls, and open framework topology increase amine efficiencies. Interactions between amines and Cr defect sites incur amine efficiency losses but grant higher thermal and oxidative stability during uptake-regeneration cycling. Finally, >25 % higher CO2 uptake capacities are achieved for aminopolymer/MIL-101(Cr)-ρhigh under humid conditions, demonstrating promise for realistic applications.

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives.

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Synthesis and Properties of Halide Perovskites Nanocrystals Toward Solar Cells Application

Halide perovskite nanocrystals (HP NCs) based perovskite solar cells (PSCs) have attracted serious attention along with relatively unstable bulk HP‐based thin‐film solar cells aiming to make more efficient and stable PSCs. HP NCs have still limited employment in PSCs despite their inherent unique properties such as strong absorption and photoluminescence, controlled transport and charge‐carrier mobility, low defect density, solution processability, and compatibility with large‐scale deposition techniques suitable for PSCs. Nanostructuring of the perovskites offers other unique characteristics such as confinement effects. These exciting characteristics together with the exceptional tunability in their shape, composition, and functionalities make HP NCs promising for PVs applications. In order to develop efficient HP NCs PSCs, major focus lies on discussing the various synthesis approaches, optoelectrical behaviors of HP NCs, and optimized device architectures. In this context, this review article deals with various approaches to HP NCs synthesis, properties, current challenges, and factors affecting the performance of the NC‐based PSCs. Based on these discussions, perspectives on promising directions for effective utilization of HP NCs toward the improvement and commercialization of the exciting promising material‐based stable and efficient PSCs in the near future are provided.

High quality a-InGaZnO and a-ZrAlO deposited at 375 °C by spray pyrolysis for low voltage operation TFTs

We report the high-performance, bottom-gate a-InGaZnO (IGZO) TFT with high-k ZrAlO (ZAO) gate insulator by spray pyrolysis (SP). The a-IGZO/ZAO TFT exhibits the average saturation mobility of 23.12 cm2 V−1 s−1, subthreshold swing of 121 mV dec-1, ION/OFFratio of ∼108 with no hysteresis and low off-state current of ∼10-19A µm−1. In addition, the TFT exhibits good interface quality with less trap density (∼9x1010 cm−2) and conduction band tail state (∼4.3x1018 cm−3) extracted from TCAD fitting. The high-performance is attributed to the formation of uniform and dense a-IGZO and ZAO thin films with low defect density. Therefore, the SP a-IGZO/ZAO TFTs have great potential to be employed in low-cost, low driving voltage TFTs for display application.

Template-Confined Oriented Perovskite Nanowire Arrays Enable Polarization Detection and Imaging.

Polarized light detection can effectively identify the difference between the polarization information on the target and the background, which is of great significance for detection in complex natural environments and/or extreme weather. Generally, polarized light detection inevitably relies on anisotropic structures of photodetector devices, while organic-inorganic hybrid perovskites are ideal for anisotropic patterning due to their simple and efficient preparation by solution method. Compared to patterned thin films, patterned arrays of aligned one-dimensional (1D) perovskite nanowires (PNWAs) have fewer grain boundaries and lower defect densities, making them well suited for high-performance polarization-sensitive photodetectors. Here, we fabricated PNWAs crystallographically aligned with variable line widths and alignment densities employing CD-ROM and DVD-ROM grating pattern template-confined growth (TCG) methods. The photodetectors constructed from MAPbI3 PNWAs achieved responsivity of 35.01 A/W, detectivity of 6.85 × 1013 Jones, and fast response with a rise time of 172 μs and fall time of 114 μs. They were successfully applied to high-performance polarization detection with a polarization ratio of 1.81, potentially applicable in polarized light detection systems.

Synchronous effect of coordination and hydrogen bonds boosting the photovoltaic performance of perovskite solar cells

Superior quality of perovskite film with perfect crystalline and low defect density is crucial for achieving excellent photovoltaic performance and durability of perovskite solar cells (PSCs), and regulation of growth of perovskite crystal and defects passivation by additive is remain challenge. Herein, two acyl molecules (Succinimide: SCI, and Hydantoin: HDT) were incorporated into perovskite precursor solution as additive. It can be found that the HDT molecule can not only passivate undercoordinated Pb2+ defects in perovskite films by the -C=O groups with coordination bonds, but also formed hydrogen bonds with I− by -NH- group, which effectively inhibited the formation of iodine vacancy defects and regulated growth rate of perovskite crystals, and the quality of perovskite films was optimized with enlarging grain size and reduced trap-density. Consequently, power conversion efficiency of HDT-based device was enhanced from 19.47 % to 21.00 %, with an enhancement of JSC and VOC. Moreover, the HDT-based device without encapsulation retained 83.7 % of its initial PCE after being aged at 10–20 % RH for 60 days. Obviously, such a study could provide a new strategy to optimize quality of perovskite film for boosting performance of PSCs.

Anomalous X-ray pulse responses in MAPbBr3 single crystal-based detectors

Organic-inorganic perovskite single crystals (SCs) are considered promising semiconductors for high-sensitivity X-ray detectors because of their strong X-ray interaction, low-temperature processability, and excellent properties such as long diffusion length, low defect density, and long carrier lifetime. However, the specific pulse response of MAPbBr3 SC-based X-ray detectors frequently exhibits a tail and over/undershoots at several conditions, which degrades the X-ray image quality; however, no detailed analysis of these characteristics has been reported thus far. For high-resolution X-ray imaging, it is necessary to obtain an ideal pulse response that is free of tails and over/undershoots. In this study, we investigated the specific X-ray responses of solution-grown MAPbBr3 SC-based X-ray detectors under various externally applied voltages. Based on the study on charge carrier dynamics, we analyzed the origin of the tail and over/undershoots in X-ray pulse response shapes. We theoretically verified that the unideal pulse response in perovskite SC-based X-ray detectors is attributed to the defect states inherent in perovskite SCs.

Few-layer graphene production through graphite exfoliation in pressurized CO2 assisted by natural surfactant

Graphene research has captivated researchers worldwide, propelling innovation across diverse industries. Through the liquid-phase exfoliation methodology of graphite powder, we have demonstrated a rapid route for obtaining few-layer and multi-layer graphene using a natural surfactant, cardanol. Aqueous phase exfoliation of graphite in the presence of cardanol as a surfactant was conducted to obtain pre-exfoliated graphite suspensions. The influence of different ultrasonication times, 10, 20, and 30 min, and contact times with the surfactant, 1 and 60 min, on the stability and concentration of dispersed exfoliated graphite was evaluated. Results indicate that ultrasonication for 20 min resulted in improved stability and reduced graphene flake sizes, making it suitable for scalable graphene production. Subsequently, the most stable dispersions of exfoliated graphite were subjected to CO2-pressurized treatment. Promising results were obtained when employing cardanol at its critical micelle concentration. The graphene exhibited good structural quality, low defect density, and small stacking, with an average size of 15 nm, where 40 % of the stacked graphene was smaller than 5 nm. The findings provide valuable recommendations for the scalable production of graphene with multilayers and a few layers (FLG/MLG), using cardanol, a friendly surfactant, and a novel method of exfoliation utilizing supercritical CO2. This technology represents an innovative approach, with potential applications in supercapacitors, solar cells, biosensors, polymer composites, and advanced materials.

Influence of growth temperature on the magnetization dynamics of sputtered CoFeB thin films on various substrates and their microwave device functionality

In the present investigation, sputtering growth of 100 nm thick Co40Fe40B20 (CoFeB) film has been investigated in a low temperature range from room temperature (RT) to 400 ℃ on silicon (Si), silicon dioxide (Si/SiO2), and sapphire (Al2O3) substrates. The film surface morphology, static and dynamic magnetization were measured as a function of growth temperature and substrate. The grown films were further tested as a potential material for microwave device functionality in terms of filter and phase shifter. Surface roughness was lowest for the film grown on Si/SiO2 substrate at 200 ℃ (0.39 nm) measured from atomic force microscopy. Static magnetic properties measured by tracing magnetic hysteresis loop suggest a soft magnetic behaviour with in-plane magnetic anisotropy. Film saturation magnetization increases with the increase in growth temperature up to 200 ℃ and then decreases from 200 ℃ to 400 ℃. This suggest that the sputtering growth from room temperature to 200 ℃ offer a moderate to low interfacial stress resulting in low defects density. Gilbert damping constant (αeff) of the thin film is obtained from the ferromagnetic resonance measurements in a broad band of microwave frequencies. Damping was lowest (5.4×10−3) in the film grown at 200 ℃ on the Si/SiO2 substrate which correlates with the roughness and magnetization data. A low value of αeff is desirable for a material to be used in energy efficient spintronics and magnonic applications. The CoFeB films were also utilized in prototype microwave band-stop filter and phase shifter device applications. The filter has been tested in the frequency range from 6GHz to 25GHz with an applied magnetic bias up to 4.0 kOe. The frequency tunability with respect to the applied magnetic bias measured to be around 3.64 GHz/kOe. The maximum attenuation of −4dB was observed for the film grown on Si/SiO2 substrate at 200 ℃. Whereas, phase shifter fabricated on Si/SiO2/CoFeB film grown at 200 ℃ shows a differential phase shift up to 75°/cm. These results indicates that the CoFeB film deposited at low growth temperature can be a potential material in spintronics and CMOS applications where low temperatures are required for device fabrications.

Characteristics of 4-inch (100) oriented Mg-doped β-Ga2O3 bulk single crystals grown by a casting method

In this Letter, 4-inch diameter (100) magnesium-doped monoclinic gallium oxide (Mg:β-Ga2O3) bulk single crystals were grown by a casting method. The crystallographic structure, electrical, optical properties, and defects of Mg-doped β-Ga2O3 was thoroughly investigated by XRD rocking curves, AFM, non-contact resistivity measurements, XPS, optical transmission spectra, Raman spectroscopy, and defect-selective chemical etching processes. Comparing to the UID samples, the larger bandgap of 4.75 eV and high resistivity of 3.50×1012 Ω∙cm indicated Mg a proper dopant for fabricating semi-insulating β-Ga2O3 substrates which could be preferred for power devices. XPS spectra exhibited the ability of Mg2+ to inhibit the formation of oxygen vacancy defects in the crystals. Meanwhile, the high crystalline quality and low density of defects demonstrated the advantages of casting method in achieving large-size and high-quality β-Ga2O3 bulk single crystals with changeable resistivity. This result indicates a high potential to achieve high-quality and low-cost substrates which satisfy the requirement of high-performance power electronic devices.

Enhancement of SrTiO3 photocatalytic efficiency by Al doping: Answers from the structure, morphology and electronic properties contributions

Our study focuses on disclosing the mechanisms standing behind the improved photocatalytic performance of SrTiO3, where Al modification of the perovskite structure boosts the photocatalytic O2 evolution activity of the Al:SrTiO3 system. By adapting the synthesis method that produces well-crystallized materials with low defect density and employing surface modification techniques, we aim to enhance the photocatalytic efficiency of SrTiO3via Al2O3 nanoceramic oxide doping at concentrations ranging from 0 to 10% and further examining of the relationship between doping process and the changes in the electronic and crystalline structure of SrTiO3. The prepared Al-based SrTiO3 perovskite samples (Al3%:SrTiO3, Al7%:SrTiO3, Al10%:SrTiO3) were thoroughly characterized to understand their structural, electronic, and morphological properties. Complementary X-ray techniques were employed to assess the stoichiometry (X-ray photoelectron spectroscopy - XPS), local environment, and chemical state (X-ray absorption spectroscopy - XAS in both total electron yield (TEY) and fluorescence yield (TFY). The comprehensive characterization enables us to understand the changes in the electronic properties and morphological features of the modified samples elucidating the surface formation mechanism while providing insights into the structural modifications induced by Al doping in the SrTiO3 perovskite lattice. Our findings give new perspectives for the development of Al-modified SrTiO3 perovskite materials with enhanced photocatalytic performance providing rich insights into the optimization of photocatalytic processes for applications in environmental remediation and sustainable energy production.

Thermally Evaporated Blue Quasi-Two-Dimensional Perovskite Light-Emitting Diodes via Low-Dimensional Phase Distribution Arrangement.

Perovskite light-emitting diodes (PeLEDs) have shown great potential in the display domain due to their wide color gamut, narrow emission, and low cost. In current PeLEDs manufacturing methods, thermal evaporation shows great competitiveness with its advantages of easy patterning, production line compatibility, and solvent-free processability. However, the development of thermally evaporated blue PeLEDs is limited by their low radiative recombination rate and high defect density. Herein, we report high-performance thermally evaporated blue PeLEDs by in situ introduction of ammonium cations. We confirm that phenethylammonium (PEA+) has lower adsorption energy, which significantly reduces the low-n phases in a quasi-2D perovskite film. The energy transfer rate is also promoted by the PEA+ addition. As a result, we fabricate blue PeLEDs with an external quantum efficiency of 1.56% by thermal evaporation. The strategy of arranging phase distribution could benefit the industrialization of full-color PeLEDs.

Suppressing Halide Segregation via Pyridine-Derivative Isomers Enables Efficient 1.68 eV Bandgap Perovskite Solar Cells.

Light-induced phase segregation is one of the main issues restricting the efficiency and stability of wide-bandgap perovskite solar cells (WBG PSCs). Small organic molecules with abundant functional groups can passivate various defects, and therefore suppress the ionic migration channels for phase segregation. Herein, a series of pyridine-derivative isomers containing amino and carboxyl are applied to modify the perovskite surface. The amino, carboxyl, and N-terminal of pyridine in all of these molecules can interact with undercoordinated Pb2+ through coordination bonds and suppress halide ions migration via hydrogen bonding. Among them, the 5-amino-3-pyridine carboxyl acid (APA-3) treated devices win the champion performance, enabling an efficiency of 22.35% (certified 22.17%) using the 1.68 eV perovskite, which represents one of the highest values for WBG-PSCs. This is believed to be due to the more symmetric spatial distribution of the three functional groups of APA-3, which provides a better passivation effect independent of the molecular arrangement orientation. Therefore, the APA-3 passivated perovskite shows the slightest halide segregation, the lowest defect density, and the least nonradiative recombination. Moreover, the APA-3 passivated device retains 90% of the initial efficiency after 985 h of operation at the maximum power point, representing the robust durability of WBG-PSCs under working conditions.

Enhanced Amplified Spontaneous Emission from Perovskite Films by a Multifunctional 4′‐Aminoacetophenone Hydrochloride Additive Assistant Engineering

AbstractMetal halide perovskites are considered as a prospective candidate as optical gain media for stimulated emission, and the advancement of amplified spontaneous emission (ASE) of perovskite materials is gradually becoming the key to realize the commercialization of perovskite lasers. Nonetheless, the ASE of solution‐processed polycrystalline perovskite films still experiences a high threshold and poor optical stability due to their high defect density and poor crystalline quality. In this scenario, a noticeably ameliorated ASE performance with decreased threshold and greater photo‐stability is demonstrated in solution‐processed (FAPbI3)0.85(MAPbBr3)0.15 polycrystalline perovskite films by introducing a neoteric 4′‐Aminoacetophenone hydrochloride (APCl) additive with cost‐effective fabrication. The ASE threshold of the APCl‐embedded perovskite films is prominently curtailed from 12.8 to 5.3 µJ cm−2, with an augmented optical gain, ASE output intensity, and optical durability. The ameliorated ASE performance can be credited to multifunctional effects of the APCl additive in the perovskite films, presenting a low defect density, an unambiguously elevated photoluminescence, a prolonged decay lifetime and thus enabling the easily achieved population inversion caused by the attenuated nonradiative carrier recombination in the optical gain media. These findings provide a significant foundation and pave the way for realizing the electrically excited lasing based on the perovskite materials.

Niobium doping of CVD-WS2 monolayers using solid precursors with and without salt-KBr as a catalyst: A comparative study

The in situ doping of tungsten disulfide (WS2) monolayers with Niobium (Nb) atoms was obtained through the atmospheric pressure chemical vapor deposition (APCVD) method by controlling the Nb2O5 and WO3 precursors mass ratios. Samples were prepared employing the same growth parameters with and without the use of KBr salt as a catalyst. In both cases, the Nb incorporation quenches the luminescence intensity of the doped samples, and it depends on the Nb2O5:WO3 mass ratio. X-ray photoelectron spectroscopy (XPS) measurements were performed, and the observed redshift at the Fermi level indicated p-type doping. Nb-doped WS2 monolayers synthesized without the use of the catalyst have lower defect density, as revealed by a careful Raman analysis, and are free of agglomerates on their monolayers’ basal plane. The use of KBr as the catalyst increases the average crystal size but also increases the disorder effects compared with samples prepared without the use of salt. The presence of agglomerates of impurities at the surface of doped samples was observed in samples prepared with the use of salt as a catalyst. The controlled synthesis of p-type WS2 with low defect density is essential for developing electronic and photonic devices based on WS2 material.

Spin-State Control of Shallow Single NV Centers in Hydrogen-Terminated Diamond.

The ability to control the charge and spin states of nitrogen-vacancy (NV) centers near the diamond surface is of pivotal importance for quantum applications. Hydrogen-terminated diamond is promising for long spin coherence times and ease of controlling the charge states due to the low density of surface defects. However, it has so far been challenging to create negatively charged single NV centers with controllable spin states beneath a hydrogen-terminated surface because atmospheric adsorbates that act as acceptors induce surface holes. In this study, we optically detected the magnetic resonance of shallow single NV centers in hydrogen-terminated diamond through precise control of the nitrogen implantation fluence. Furthermore, we found that the probability of detecting the resonance was enhanced by reducing the surface acceptor density through passivation of the hydrogen-terminated surface with hexagonal boron nitride without air exposure. This control method opens up new opportunities for using NV centers in quantum applications.

Laser Powder Bed Fusion of Hard‐Phase Containing Co‐Base Alloy

This work considers the processing of the hard‐phase containing CoCrW‐alloy Celsit F using powder bed fusion‐laser beam/metal (PBF‐LB/M), whereby a suitable process window for producing dense parts having a low‐defect density is determined. Several cuboid specimens are manufactured by varying the exposure parameters (laser power, hatch distance, scanning speed). With the help of quantitative image analysis, the samples are evaluated regarding crack density and porosity, and optimal exposure parameters are derived. Dense samples with a low defect density can be produced if the energy density is 60–70 J mm−3 and the base‐plate is preheated to 800 °C. Choosing lower energy densities leads to increased pore formation, whereas higher energy densities result in increased ablation and cracking. With a view to a later application in which additively manufactured hot work tools are made of Celsit F, the possibility of introducing internal cavities as potential cooling channels is considered. It can be shown that the hardness sideways is lower than above and below the cavity due to a changed microstructure and locally different heat dissipation. The results of this work give a first guideline for the PBF‐LB/M‐processing of wear‐resistant CoCrW alloys with a high hard phase volume content to tools for hot working.