Surface Defect Density Research Articles

Facile Functionalization of Graphene to Tune Response of Printed Electrochemical Sensors to Neurotransmitters

Neurotransmitters are small molecules involved in neuronal signaling and can also serve as stress biomarkers.1 Their abnormal levels have been also proposed to be indicative of several neurological diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington disease, among others. Hence, measuring their levels is highly important for early diagnosis, therapy, and disease prognosis. In this work, we investigate facile functionalization methods to tune and enhance sensitivity of printed graphene sensors to neurotransmitters. Sensors based on direct laser scribing and screen-printed graphene ink are studied. These printing methods offer ease of prototyping and scalable fabrication at low cost.The effect of functionalization of laser induced graphene (LIG) by electrodeposition and solution-based deposition of TMDs (molybdenum disulfide2 and tungsten disulfide) and metal nanoparticles is studied. For different processing methods, electrochemical characteristics (such as electrochemically active surface area: ECSA and heterogenous electron transfer rate: k0) are extracted and correlated to surface chemistry and defect density obtained respectively using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. These functionalization methods are observed to directly impact the sensitivity and limit of detection (LOD) of the graphene sensors for the studied neurotransmitters. For example, as compared to bare LIG, it is observed that electrodeposition of MoS2 on LIG improves ECSA by 3 times and k0 by 1.5 times.3 Electrodeposition of MoS2 also significantly reduces LOD of serotonin and dopamine in saliva, enabling detection of their physiologically relevant concentrations (in pM-nM range). In addition, chemical treatment of LIG sensors is carried out in the form of acetic acid treatment. Acetic acid treatment has been shown previously to improve C-C bonds improving the conductivity of LIG sensors.4 In our work, in particular, acetic acid treatment leads to larger improvement of LOD of norepinephrine compared to MoS2 electrodeposition.In addition, we investigate the effect of plasma treatment to tune the sensor response by modifying the defect density and chemistry. For example, we find that oxygen plasma treatment of screen-printed graphene ink greatly improves LOD of norepinephrine up to three orders of magnitude, which may be attributed to the increased defects and oxygen functional groups on the surface as evident by XPS measurements. Defects are known to play a key role in enhancing the sensitivity of 2D materials to surface interactions, and have been explored in tuning/enhancing the sensor sensitivity.5 Building on our previous work,3 we apply a custom machine learning-based data processing method to further improve that sensitivity and LOD, and also to automatically benchmark different molecule-material pairs.Future work includes expanding the plasma chemistry and conditions, studying the effect of precursor mixture in laser-induced solution-based functionalization, and understanding the interplay between molecule-material system. Work is also underway to improve the machine learning model by using nonlinear learning models such as neural networks to improve the sensor sensitivity, selectivity, and robustness. References A. J. Steckl, P. Ray, (2018), doi:10.1021/acssensors.8b00726.Y. Lei, D. Butler, M. C. Lucking, F. Zhang, T. Xia, K. Fujisawa, T. Granzier-Nakajima, R. Cruz-Silva, M. Endo, H. Terrones, M. Terrones, A. Ebrahimi, Sci. Adv. 6, 4250–4257 (2020).V. Kammarchedu, D. Butler, A. Ebrahimi, Anal. Chim. Acta. 1232, 340447 (2022).H. Yoon, J. Nah, H. Kim, S. Ko, M. Sharifuzzaman, S. C. Barman, X. Xuan, J. Kim, J. Y. Park, Sensors Actuators B Chem. 311, 127866 (2020).T. Wu, A. Alharbi, R. Kiani, D. Shahrjerdi, Adv. Mater. 31, 1–12 (2019).

Irradiation effect on structural and electrical properties of YMnO3/ITO/glass thin film

In this communication, the structural, microstructural and electrical properties of YMnO3/ITO/glass thin films have been thoroughly studied. Moreover, the electrical properties of this film have been studied in the absence of magnetic field and in the vicinity of applied magnetic field of 1 T. In addition to it, the YMnO3/ITO/glass thin films were grown using sophisticated pulsed laser deposition (PLD) technique which was further irradiated by 120 MeV Ag+9 ions with different ion fluence of 5 × 1011–5 × 1013 ions/cm2. In order to calculate crystallite size of the film, Scherrer’s formula and W-H plot are plotted. For the investigation of surface morphology, Atomic force microscopy (AFM) has been performed. Variation in electrical properties have been discussed in the context of average grain size, grain boundary density, rms surface roughness, defect density and oxygen vacancies. To understand the dielectric response of pristine and all the irradiated thin films under zero and 1 T applied magnetic fields, universal dielectric response (UDR) model and relaxation model have been fitted. Frequency dependent a.c. conductivity have been fitted by Jonscher’s power law and from its calculation maximum barrier height have been estimated. Average normalized constant (ANC), which provides the grain boundary density, have been analyzed from impedance data.

Multifunctional Thiophene Cascading SnO2/Perovskite Interfaces for Efficient and Stable MAPbI3 Photovoltaics.

The power conversion efficiency (PCE) and stability of n-i-p perovskite solar cells (PSCs) are significantly affected by inherent defects of SnO2 and perovskite layers. In this work, we incorporate 2-bromo-3-thiophenic acid (BrThCOOH) as a multifunctional passivant to simultaneously passivate the defects of SnO2 surface and perovskite layer. BrThCOOH permeates evenly into the MAPbI3 and coordinates with Pb2+ and iodine vacancies (VI+) to reduce surface defect density and inhibit the decomposition of MAPbI3. Carboxylic acid effectively passives the oxygen vacancy on the surface of SnO2 through coordination bonds, reducing the probability of electron capture by SnO2 surface defects, thus contributing to electron transport in device. The interaction of BrThCOOH with MAPbI3 and SnO2 surfaces leads to an upward shift in energy levels, reducing energy loss during charge migration. The optimal MAPbI3 device with BrThCOOH-modified SnO2 (T-SnO2) reveals an improved PCE of 21.12%, much higher than that of the control one (19.12%). The hydrophobicity of BrThCOOH-modified MAPbI3 is also improved, which is beneficial to the durability of the device. After 100 h of storage in the environment, the generated PSCs maintain their initial PCE of 75%, demonstrating excellent long-term stability without any encapsulation.

Dual-phase CsPbBr3–CsPb2Br5 perovskite scintillator for sensitive X-ray detection and imaging

Cesium lead bromide and its derivatives have recently attracted the scientific community for X-ray detection and imaging due to their effective atomic number, strong scintillating luminescence, fast scintillating decay time, and tunable optical bandgap, despite their poor stability against ultraviolet (UV) illumination, moisture, and thermal degradation. Here, we report the synthesis of a highly luminescent and stable dual-phase CsPbBr3–CsPb2Br5 perovskite nanocrystal (PNCs) system prepared using the hot-injection method for scintillation application. CsPb2Br5 developed on the surface of the CsPbBr3 reduces the surface defect density and greatly enhances the photostability, water resistance, thermal stability, and luminescence properties of the CsPbBr3 nanocrystals. Based on the excellent stability of PNCs in various environments, we mixed the PNCs with polystyrene (PS) and spin-coated the glass plate to evaluate the optical and scintillation properties. The dual-phase CsPbBr3–CsPb2Br5 exhibits a strong green emission at 522 nm with a photoluminescence quantum yield of 65.2% and a fast response time of 2.93 ns under UV excitation. Under X-ray irradiation, CsPbBr3–CsPb2Br5 PNCs exhibited excellent linear response as a function of the dose rate at 50 keV and a low detection limit of 1.130 μGyair s−1, which is less than the typical medical imaging dose (5.5 μGyair s−1) and a light yield of 19,200 photons/MeV. Moreover, the CsPbBr3–CsPb2Br5 scintillating film displays excellent imaging ability with a spatial resolution of up to 8.19 lp/mm at a modulation transfer function (MTF) of 0.2 under an exposure dose of 46.96 μGyair s−1. These findings show that the dual-phase CsPbBr3–CsPb2Br5 structure can be a promising fast scintillating material for X-ray imaging in medical and industrial applications.

Effects of Chemical Valences of Sulfur on the Performance of CsFAMA Perovskite Solar Cells.

The low electrical conductivity and the high surface defect density of the TiO2 electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO2 layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO2/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO2 ETL layers treated with Na2S, Na2S2O3, and Na2SO4, respectively. Experimental results show that the Na2S and Na2S2O3 interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO2/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na2SO4 interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO2/PVK interface and device performance. These results indicate that S2- can obviously improve the quality of TiO2 and PVK layers and TiO2/PVK interfaces, while SO42- has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.

Effect of Growth Conditions on the Surface Morphology and Defect Density of CS‐PVT‐Grown 3C‐SiC

AbstractA systematic approach to determine the most crucial growth parameters and their effect on the surface morphology and defect density of sublimation grown (001) cubic silicon carbide (3C‐SiC) is conducted. Close space physical vapor transport (CS‐PVT) growth on 3C‐SiC and 4° off oriented homoepitaxial chemical vapor deposition (CVD) grown seeding layers is performed. For each growth run, one growth parameter, e.g., temperature, pressure, or N2 flux is varied, while the other parameters are kept constant. Raman spectroscopy, potassium hydroxide (KOH) etching, as well as optical microscopy and atomic force microscopy (AFM) are used for the sample characterization. Step‐flow‐controlled growth is observed on all grown samples, while the stacking fault density is generally reduced with respect to the seeding layers. Increased temperature as well as increased pressure leads to roughening of the growth surface attributed to step bunching, while their effect on the stacking fault density seems to be only minor in the analyzed parameter range. Areas of strongly enhanced step bunching are present on all samples, and this effect is more pronounced at higher temperature and pressure. Increased nitrogen doping leads to a decreased stacking fault density but also increases the length of remaining stacking faults. The surface morphology is influenced by N2 on a macroscopic level rather than on microscopic scale.

Growth mechanisms of interfacial carbides in solid-state reaction between single-crystal diamond and chromium

Interfacial bonding is one of the most challenging issues in the fabrication, and hence comprehensively influences the properties of diamond-based metal matrix composites (MMCs) materials. In this work, solid-state (S/S) interface reaction between single-crystal synthetic diamond and chromium (Cr) metal was critically examined with special attention given to unveil the role of crystal orientation in the formation and growth of interfacial products. It has been revealed that catalytically converted carbon (CCC) was formed prior to chromium carbides, which is counterintuitive to previous studies. Cr7C3 was the first carbide formed in the S/S interface reaction, aided by the relaxation of diamond lattices that reduces the interfacial mismatch. Interfacial Cr7C3 and Cr3C2 carbides were formed at 600 and 800 °C, respectively, with the growth preferred on diamond (100) plane, because of its higher density of surface defects than (111) plane. Interfacial strain distribution was quasi-quantitively measured using windowed Fourier Transform-Geometric Phase Analysis (WFT-GPA) analysis and an ameliorated strain concentration was found after the ripening of interfacial carbides. Textured morphologies of Cr3C2 grown on diamond (100) and (111) planes were perceived after S/S interface reaction at 1000 °C, which is reported for the first time. The underlying mechanisms of Cr-induced phase transformation on diamond surface, as well as the crystal orientation dependent growth of interfacial carbides were unveiled using the first-principles calculation. The formation and growth mechanisms of Cr3C2 were elucidated using TEM and XRD analyses. Finally, an approach for tailoring the interfacial microstructure between synthetic diamond and bonding metals was proposed.

Stratified Oxygen Vacancies Enhance the Performance of Mesoporous TiO2 Electron Transport Layer in Printable Perovskite Solar Cells.

The low electrical conductivity and the high surface defect density of the TiO2 electron transport layer (ETL) limit the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Here, the conductivity and defect modulation of the mesoporous TiO2 (mp-TiO2 ) ETL via oxygen vacancy (OV) management by the reduction and oxidation treatment are reported. Reduction treatment via reducing agent introduces abundant OVs into the TiO2 nanocrystalline particles on the surface and at the subsurface. The following oxidation treatment via hydrogen peroxide removes the surface OVs while remains the subsurface OVs, resulting in stratified OVs. The stratified OVs improve the conductivity of TiO2 ETL by increasing carrier donors and decrease nonradiative centers by reducing surface defects. Such synergy ensures the capability of mp-TiO2 as the well-performed ETL with improved energy level alignment, suppressed interface recombination, enhanced carrier extraction, and transport. As a result, printable hole-conductor-free carbon-based mesoscopic PSCs based on the modulated mp-TiO2 ETL demonstrate a highest reported PCE of 18.96%.

Understanding the pulsed laser‐induced modification processes of TiO2 nanomaterials in aqueous solution

AbstractThree different types of TiO2 nanoparticles in anatase and rutile phases were subject to nanosecond 532‐nm laser pulses in aqueous solution for enhanced reactivity in photocatalytic applications. Color changes observed during the laser processes resulted from laser pulse‐induced heating of TiO2 surfaces to form surface disorder with surface defects. Interestingly, TiO2 nanoparticles with different size and bulk phases undergo different changes in surface morphology, bulk phase, and defect density under the same laser pulses. The results are related with the differences in the absorption by initial defects on the TiO2 surface. Our results show that the type and density of initial defects within TiO2 matrix are important in determining subsequent thermal equilibrium‐driven changes. It is suggested that nanosecond laser pulses with energies lower than the bandgap of the semiconductor materials provide a chance for selective modification of oxide nanomaterials such as TiO2 for a new photoresponsive functionality in diverse applications.

Mitigating Surface Deficiencies of Perovskite Single Crystals Enables Efficient Solar Cells with Enhanced Moisture and Reverse‐Bias Stability

AbstractMetal halide perovskite single crystals are promising for diverse optoelectronic applications due to their outstanding properties. In comparison to the bulk, the crystal surface suffers from high defect density and is moisture sensitive; however, surface modification strategies of perovskite single crystals are relatively deficient. Herein, solar cells based on methylammonium lead triiodide (MAPbI3) thin single crystals are selected as a prototype to improve single‐crystal perovskite devices by surface modification. The surface trap passivation and protection against moisture of MAPbI3 thin single crystals are achieved by one bifunctional molecule 3‐mercaptopropyl(dimethoxy)methylsilane (MDMS). The sulfur atom of MDMS can coordinate with bare Pb2+ of MAPbI3 single crystals to reduce surface defect density and nonradiative recombination. As a result, the modified devices show a remarkable efficiency of 22.2%, which is the highest value for single‐crystal MAPbI3 solar cells. Moreover, MDMS modification mitigates surface ion migration, leading to enhanced reverse‐bias stability. Finally, the cross‐link of silane molecules forms a protective layer on the crystal surface, which results in enhanced moisture stability of both materials and devices. This work provides an effective way for surface modification of perovskite single crystals, which is important for improving the performance of single‐crystal perovskite solar cells, photodetectors, X‐ray detectors, etc.

Graded 2D/3D Perovskite Hetero-Structured Films with Suppressed Interfacial Recombination for Efficient and Stable Solar Cells via DABr Treatment.

Several strategies and approaches have been reported for improving the resilience and optoelectronic properties of perovskite films. However, fabricating a desirable and stable perovskite absorber layer is still a great challenge due to the optoelectronic and fabrication limitations of the materials. Here, we introduce diethylammonium bromide (DABr) as a post-treatment material for the pre-deposited methylammonium lead iodide (MAPbI3) film to fabricate a high-quality two-dimensional/three-dimensional (2D/3D) stacked hetero-structure perovskite film. The post-treatment method of DABr not only induces the small crystals of MAPbI3 perovskite secondary growth into a large crystal, but also forms a 2D capping layer on the surface of the 3D MAPbI3 film. Meanwhile, the grains and crystallization of 3D film with DABr post-treatment are significantly improved, and the surface defect density is remarkably reduced, which in turn effectively suppressed the charge recombination in the interface between the perovskite layer and the charge transport layer. The perovskite solar cell based on the DABr-treatment exhibited a significantly enhanced power conversion efficiency (PCE) of 19.10% with a notable improvement in the open circuit voltage (VOC) of 1.06 V and good stability, advocating the potential of this perovskite post-treatment approach.

Green-antisolvent-induced homogeneous phase distribution for efficient and stable MA-free 2D perovskite solar cells

The phase distribution of two-dimensional (2D) perovskites is critical for solar cell power conversion efficiency (PCE). Here, we report a simple green anti-solvent (ethyl acetate, EA) method for obtaining uniform phase dispersion of methylamine (MA)-free quasi-2D Dion-Jacobson (DJ) phase perovskite films (DMePDA)FA3Pb4I13 (n = 4). The films present a more homogeneous phase distribution for their rapid extraction of solvents and the interaction between Pb2+ and the carbonyl group in EA, resulting in a more shining surface, reduced surface-defect density, higher out-plane mobility, and higher conductivity. We obtain a PCE of 18.86 %, which is the highest for MA-free DJ phase 2D perovskite-based devices. In addition, the target devices maintain 85 % and 90 % of their initial values after 1008 h under 85 % relative humidity at 85 ℃ (ISOS-D-3) and continuous illumination at maximum power point (MPP) tracking at ∼ 45 ℃ (ISOS-L-1), respectively.

Copper content effects on passive film of modified 00Cr20Ni18Mo6CuN super austenitic stainless steels in acidic environment

To provide a theoretical basis for the design of super austenitic stainless steel used in flue gas desulfurization environment, by changing the Cu content in 00Cr20Ni18Mo6CuN super austenitic stainless steel to explore the influence of Cu on its corrosion resistance, by electrochemical methods, XPS and first-principle computational simulations. The results show that Cu promotes the selective dissolution of Fe, Cr and Mo in stainless steel, and the copper content changes the proportion of compounds in the passive film, as well as its surface quality, resistance and defect density. The addition of one Cu atom increases the adsorption energy and work function of NH3 on Cr2O3 surface, reduces the charge transfer and hybridization. However, when the Cu content exceeds 1 wt%, the surface of the passive film is loose and has many defects. The appearance of oxygen vacancy and two Cu atoms leads to the decrease of adsorption energy and work function, and enhances the charge transfer and hybrid effect. The optimal Cu content is obtained through research, which not only improves the corrosion resistance of 00Cr20Ni18Mo6CuN super austenitic stainless steel in flue gas desulfurization environment, prolonging the service life of 00Cr20Ni18Mo6CuN super austenitic stainless steel, but also has practical application value.

Macroscale superlubricity and durability of in situ grown hydrogenated graphene coatings

Although macroscale superlubricity reduces the friction of graphene materials, industrial application has been limited to special environments or complex strain engineering. Herein, high-quality hydrogenated graphene coatings (0 < ID/IG < 0.21) are synthesized on industrial materials in situ by hot-filament chemical vapor deposition. The ball-on-disk friction tests demonstrate that the hydrogenated graphene coatings have superior lubricating properties and long service lifetime at a macro-load of 1 N and high sliding speed (0.6 m/s) under ambient conditions (23–25 °C, 50–55 %RH). The hydrogenated graphene coatings deposited with moderate hydrogen supplies show ultra-low friction and even localized superlubricity (∼0.009) due to the critical factor of surface defect density. Long-term stability is observed after more than 24,840 laps. Molecular dynamics simulation is performed to explain the self-passivating and self-healing effects on the atomic level. The macroscale lubrication mechanism involves H atoms capturing carbon dangling σ-bonds. The high-quality and chemically inert graphene surface facilitates the formation of a thick solid tribofilm at the sliding contact interface. The chemisorbed H atoms on the graphene layer lead to continuous healing and stabilization of the defective structure during the macroscale friction test. Our new findings provide insights into the lubrication mechanism and reveal a new approach to achieving macroscale superlubricity and high durability for hydrogenated graphene coatings.

Surface quality improvement at selective laser melting AlSi10Mg by optimizing single point diamond turning parameters

Abstract Selective laser melting allows aluminum-silicon alloy mirrors further lightweight for aerospace applications. The reflective surfaces based on Selective laser melting aluminum-silicon alloy substrates are commonly machined by single point diamond turning. However, many surface defects on single point diamond turning machined surfaces may limit their direct applications in optical system. In the paper, single point diamond turning parameters (cutting depth, feed rate, and cutting speed) are optimized orderly to improve its surface quality. The single point diamond turning machined surface morphologies are measured by using white light profilometer. In our selective laser melting AlSi10Mg substrate, scratches and holes mainly damaged single point diamond turning surface. Scratches are caused by inclusions while holes are generated by gas pores and inclusions. Single point diamond turning parameters optimization reduces the density of such surface defects, but these defects cannot be eliminated totally.

Numerical Study on the Effect of Dual Electron Transport Layer in Improving the Performance of Perovskite–Perovskite Tandem Solar Cells

AbstractC Perovskite–perovskite tandem solar cells (P‐TSCs) are an attractive option to overcome the limitation of Si‐based solar cells due to their ability to deposit in both the top and bottom cells by a simple solution process. Despite these advantages, the power conversion efficiency (PCE) of P‐TSCs still falls behind that of Si‐perovskite tandem devices and is highly limited by the nonradiative recombination between the perovskite and electron transport layer (ETL) interface. In this work, with the help of numerical simulations (SCAPS 1D), the influence of the various ETLs on the performance of P‐TSCs is studied. Different ETLs such as C60, PCBM, ZnO, WO3, CdSe, and In2O3, BCP are incoroprated, which directly interface with the perovskite absorber layer in top (high bandgap absorber layer) and bottom subcell (low bandgap absorber layer) of P‐TSC. The simulation analysis shows that reducing the interface surface defect density and band offsets at the ETL/perovskite interface significantly helps achieving high‐efficiency P‐TSC over 35% . Various remediation for incorporating present simulation analysis into practical devices for better‐performing P‐TSCs is also provided. The analysis provides a pathway toward understanding the reasons behind the current trend of low PCE in 2 terminal P‐TSC and achieving high‐efficiency P‐TSC solar cells.

Influence of the defect density on the ordering of an NHC monolayer on a silicon surface

We demonstrate the formation of highly ordered cyCAAC monolayers on silicon showing strong work function reductions. The grade of ordering thereby strongly depends on the surface defect density revealing an initial adsorption on defects.

In-Situ Monitoring of Reciprocal Charge Transfer and Losses in Graphene-Silicon CCD Pixels

Charge-coupled devices (CCD) allow imaging by photodetection, charge integration, and serial transfer of the stored charge packets from multiple pixels to the readout node. The functionality of CCD can be extended to the non-destructive and in-situ readout of the integrated charges by replacing metallic electrodes with graphene in the metal-oxide-semiconductors (MOS) structure of a CCD pixel. The electrostatic capacitive coupling of graphene with the substrate allows the Fermi level tuning that reflects the integrated charge density in the depletion well. This work demonstrates the in-situ monitoring of the serial charge transfer and interpixel transfer losses in a reciprocating manner between two adjacent Gr-Si CCD pixels by benefitting the electrostatic and gate-to-gate couplings. We achieved the maximum charge transfer efficiency (CTE) of 92.4%, which is mainly decided by the inter-pixel distance, phase clock amplitudes, switching slopes, and density of surface defects. The discussion on overcoming transfer losses and improving CTE by realizing a graphene-electron multiplication CCD is also presented. The proof of the concept of the in-situ readout of the out-of-plane avalanche in a single Gr-Si CCD pixel is also demonstrated, which can amplify the photo packet in a pre-transfer manner.

Secondary Anti-Solvent Treatment for Efficient 2D Dion-Jacobson Perovskite Solar Cells.

Surface defects-mediated nonradiative recombination plays a critical role in the performance and stability of perovskite solar cells (PSCs) and surface post-treatment is widely used for efficient PSCs. However, the commonly used surface passivation strategies are one-off and the passivation defect ability is limited, which can only solve part of the defects in the topmost surface area. Here, a secondary anti-solvent strategy is proposed to further reduce surface defects based on conventional surface passivation for the first time. Based on this, the crystallization quality of 2D Dion-Jacobson perovskite is enhanced and the surface defects density is further reduced by nearly two orders. In addition, a gradient structure of perovskite with n= 2 phases located at the top of the film and 3D-like phases located at the bottom of the film can also be obtained. The modulated perovskite film boosts the efficiency of 2D perovskites (n= 5) up to 19.55%. This strategy is also very useful in other anti-solvent processed perovskite dipping systems, which paves a promising avenue for minimizing surface defects toward highly efficient perovskite devices.

High-quality N-polar GaN optimization by multi-step temperature growth process

We report growth optimization of Nitrogen (N)-polar GaN epitaxial layers by hot-wall metal–organic vapor phase epitaxy on 4H-SiC (0001̄) with a misorientation angle of 4° towards the [112̄0] direction. We find that when using a 2-step temperature process for the N-polar GaN growth, step bunching is persistent for a wide range of growth rates (7 nm/min to 49 nm/min) and V/III ratios (251 to 3774). This phenomenon is analyzed in terms of anisotropic step-flow growth and the Ehrlich–Schwöebel barrier, and their effects on the surface step height and step width. The N-polar GaN growth is further optimized by using 3-step and 4-step temperature processes and the layers are compared to those using the 2-step temperature process in terms of surface morphology and defect densities. It is shown that a significantly improved surface morphology with a root mean square of 1.4 nm and with low dislocation densities (screw dislocation density of 2.8 × 108 cm−2 and edge dislocation density of 1.3 × 109 cm−2) can be achieved for 4-step temperature process. The optimized growth conditions allow to overcome the step-bunching problem. The results are further discussed in view of Ga supersaturation and a general growth strategy for high-quality N-polar GaN growth is proposed.