High Passivation Performance of Cat-CVD i-a-Si:H Derived from Bayesian Optimization with Practical Constraints.

High-quality passivation with intrinsic hydrogenated amorphous Si (i-a-Si:H) is essential for achieving high-efficiency Si heterojunction (SHJ) solar cells. The formation of i-a-Si:H with a high passivation quality requires strict control of the hydrogen content and film density. In this study, we report the effective discovery of i-a-Si:H deposition conditions through catalytic chemical vapor deposition using Bayesian optimization (BO) to maximize the passivation performance. Another contribution of this study to materials science is the establishment of a practical BO scheme consisting of several prediction models in order to account for the practical constraints. By applying the BO scheme, effective minority carrier lifetime (τeff) is maximized within the deposition condition range, while being constrained by the i-a-Si:H thickness and the capabilities of the experimental setup. We achieved a high passivation performance of τeff > 2.6 ms with only 8 cycles in BO, starting with 14 initial samples. Within the investigated range, the deposition conditions were further explored over 20 cycles. The BO provided not only optimal deposition conditions but also scientific knowledge. Contour plots of the predicted τeff values obtained through the BO process demonstrated that there is a band-like high τeff condition in the parameter space between the substrate temperature and SiH4 flow rate. The high void fraction and epitaxial growth were inhibited by controlling the substrate temperature and SiH4 flow rate, resulting in a high passivation quality. This indicates that the combination of the SiH4 flow rate and substrate temperature parameters is crucial to passivation quality. These results can be applied to determine the deposition conditions for a good a-Si:H layer without a high void fraction or epitaxial growth. The research methods shown in this study, practical BO scheme, and further analysis based on the optimized results will be also useful to optimize and analyze the process conditions of semiconductor processes including plasma-enhanced chemical vapor deposition for SHJ solar cells.

Synthesis and Direct Observation of Chiral Supramolecular Polymer of Porphyrin Having Cholesteryl Groups.

We report the synthesis and microscopic investigations of two chiral helical porphyrin supramolecular polymers with different coordinating metals that are expected to be capable of serving as synthetic macromolecular motors driven by thermal fluctuations. Furthermore, based on their microscopic images, we propose a stepwise process for the formation of higher-order structures. These porphyrins formed completely different association states, and this was reflected in the marked differences in the shapes of the supramolecular polymers. The Cu-TChOAlaCPP supramolecular polymers formed H-aggregate rods in diisopropyl ether, then grew into superhelices and then into ribbons. On the other hand, Zn-TChOAlaCPP supramolecular polymers formed aggregates based on van der Waals interactions in diethyl ether, then grew into fibers and then grew into multiple-helices and ribbons. In addition, we imaged the interaction between long and short chains of the Cu-TChOAlaCPP supramolecular polymer by fast-scanning atomic force microscopy, and we indicated the availability as a macromolecular motor driven by thermal fluctuations.

Extremely fast charging lithium-ion battery using bio-based polymer-derived heavily nitrogen doped carbon.

Extremely high nitrogen doped carbon was designed by facile pyrolysis of bio-based poly(2,5-benzimidazole) as a single source of nitrogen and carbon. For the first time ever, a carbon-based anode with ∼17 wt% of nitrogen doping with extremely fast charging (XFC) capability at 18.6 A g-1 and ultralong cyclability (3000 cycles) with 90% capacity retention was investigated. Full cell studies also indicated the commercial competence of the novel material.

Facile synthesis of Mn-doped NiCo2O4 nanoparticles with enhanced electrochemical performance for a battery-type supercapacitor electrode.

We report the synthesis of manganese-doped nickel cobalt oxide (Mn-doped NiCo2O4) nanoparticles (NPs) by an efficient hydrothermal and subsequent calcination route. The material exhibits a homogeneous distribution of the Mn dopant and a battery-type behavior when tested as a supercapacitor electrode material. Mn-doped NiCo2O4 NPs show an excellent specific capacity of 417 C g-1 at a scan rate of 10 mV s-1 and 204.3 C g-1 at a current density of 1 A g-1 in a standard three-electrode configuration, ca. 152-466% higher than that of pristine NiCo2O4 or MnCo2O4. In addition, Mn-doped NiCo2O4 NPs showed an excellent capacitance retention of 99% after 1000 charge-discharge cycles at a current density of 2 A g-1. The symmetric solid-state supercapacitor device assembled using this material delivered an energy density of 0.87 µW h cm-2 at a power density of 25 µW h cm-2 and 0.39 µW h cm-2 at a high power density of 500 µW h cm-2. The cost-effective synthesis and high electrochemical performance suggest that Mn-doped NiCo2O4 is a promising material for supercapacitors.

Rapid Millifluidic Synthesis of Stable High Magnetic Moment FexCy Nanoparticles for Hyperthermia.

A millifluidic reactor with a 0.76 mm internal diameter was utilized for the synthesis of monodisperse, high magnetic moment, iron carbide (FexCy) nanoparticles by thermal decomposition of iron pentacarbonyl (Fe(CO)5) in 1-octadecene in the presence of oleylamine at 22 min nominal residence time. The effect of reaction conditions (temperature and pressure) on the size, morphology, crystal structure, and magnetic properties of the nanoparticles was investigated. The system developed facilitated the thermal decomposition of precursor at reaction conditions (up to 265 °C and 4 bar) that cannot be easily achieved in conventional batch reactors. The degree of carbidization was enhanced by operating at elevated temperature and pressure. The nanoparticles synthesized in the flow reactor had size 9-18 nm and demonstrated high saturation magnetization (up to 164 emu/gFe). They further showed good stability against oxidation after 2 months of exposure in air, retaining good saturation magnetization values with a change of no more than 10% of the initial value. The heating ability of the nanoparticles in an alternating magnetic field was comparable with other ferrites reported in the literature, having intrinsic loss power values up to 1.52 nHm2 kg-1.

Catalytic activation of peroxymonosulfate with manganese cobaltite nanoparticles for the degradation of organic dyes.

In this work, we report the facile hydrothermal synthesis of manganese cobaltite nanoparticles (MnCo2O4.5 NPs) which can efficiently activate peroxymonosulfate (PMS) for the generation of sulfate free radicals (SO4˙-) and degradation of organic dyes. The synthesized MnCo2O4.5 NPs have a polyhedral morphology with cubic spinel structure, homogeneously distributed Mn, Co, and O elements, and an average size less than 50 nm. As demonstrated, MnCo2O4.5 NPs showed the highest catalytic activity among all tested catalysts (MnO2, CoO) and outperformed other spinel-based catalysts for Methylene Blue (MB) degradation. The MB degradation efficiency reached 100% after 25 min of reaction under initial conditions of 500 mg L-1 Oxone, 20 mg L-1 MnCo2O4.5, 20 mg L-1 MB, unadjusted pH, and T = 25 °C. MnCo2O4.5 NPs showed a great catalytic activity in a wide pH range (3.5-11), catalyst dose (10-60 mg L-1), Oxone concentration (300-1500 mg L-1), MB concentration (5-40 mg L-1), and temperature (25-55 °C). HCO3 -, CO3 2- and particularly Cl- coexisting anions were found to inhibit the catalytic activity of MnCo2O4.5 NPs. Radical quenching experiments revealed that sulfate radicals are primarily responsible for MB degradation. A reaction sequence for the catalytic activation of PMS was proposed. The as-prepared MnCo2O4.5 NPs could be reused for at least three consecutive cycles with small deterioration in their performance due to low metal leaching. This study suggests a facile route for synthesizing MnCo2O4.5 NPs with high catalytic activity for PMS activation and efficient degradation of organic dyes.

Controlling the concentration gradient in sequentially deposited bilayer organic solar cells via rubbing and annealing.

We elucidate the formation mechanism of adequate vertical concentration gradients in sequentially deposited poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bilayer solar cells. Using advanced analytical techniques, we clarify the origins of the enhanced photovoltaic performances of as-deposited and annealed bilayer P3HT/PCBM organic solar cells upon P3HT layer rubbing prior to PCBM deposition. Energy-dispersive X-ray spectroscopy reveals the individual effects of rubbing and annealing on the formation of adequate concentration gradients in the photoactive layers. Repetitive rubbing of P3HT strongly affects the active layer nanomorphology, forming an intermixed layer in the as-deposited devices which is retained after the annealing process. Infrared p-polarized multiple-angle incidence resolution spectrometry measurements indicate that rubbing induces a minor reorganization of the P3HT molecules in the polymer-only thin films towards face-on orientation. However, the deposition of the upper PCBM layer reverts the P3HT molecules back to their original orientation. These findings suggest that the formation of an adequate concentration gradient upon rubbing corresponds to the dominant contribution to the improved photovoltaic characteristics of rubbed bilayer organic solar cells. Using the reference low bandgap copolymer PCDTBT, we demonstrate that rubbing can be successfully applied to increase the photovoltaic performances of PCDTBT/PCBM organic solar cells. We also demonstrate that rubbing can be an efficient and versatile strategy to improve the power conversion efficiency of non-fullerene solar cells by using the reference materials in the field, PBDB-T and ITIC.

Gram-Scale Synthesis of Tetrahedrite Nanoparticles and Their Thermoelectric Properties.

Here, we report a method for facile gram-scale synthesis of tetrahedrite (Cu12Sb4S13) nanoparticles (NPs) with high quality and good reproducibility. The obtained NPs had a well-defined tetrahedral shape with a mean edge length of ∼70 nm. We sintered the NPs by the hot press technique to fabricate a nanostructured pellet for thermoelectric measurements. The figure of merit (ZT) value of the pellet was 0.52 at 675 K, which was comparable with the ZT value of the non-nanostructured counterpart.

Nanobulk Thermoelectric Materials Fabricated from Chemically Synthesized Cu3Zn1-x Al x SnS5-y Nanocrystals.

Direct energy conversion of heat into electricity using thermoelectric materials is an attractive solution to help address global energy issues. Developing novel materials composed of earth-abundant and nontoxic elements will aid progress toward the goal of sustainable thermoelectric materials. In this study, we chemically synthesized Cu-Zn-Sn-S nanocrystals and fabricated a Cu3ZnSnS5-y thermoelectric material using nanocrystals as building blocks. The figure-of-merit (ZT) value of the Cu3ZnSnS5-y material was found to be 0.39 at 658 K. We substituted Zn with Al in the Cu3ZnSnS5-y system to form Cu3Zn1-x Al x SnS5-y (x = 0.25, 0.5, 0.75, and 1) to lower the lattice thermal conductivity of the resulting materials. Complete substitution of Al for Zn substantially decreased the lattice thermal conductivity and dramatically increased the electrical conductivity of the material. However, the ZT value could not be significantly enhanced, which could be primarily attributed to the high carrier thermal conductivity. These results highlight the production of Cu3Zn1-x Al x SnS5-y thermoelectric materials and unveil the scope for improvement of ZT values by altering transport properties.

Drying-Induced Self-Similar Assembly of Megamolecular Polysaccharides through Nano and Submicron Layering.

We propose a self-similar assembly to generate planar orientation of megamolecular polysaccharides on the nanometer scale and submicron scale. Evaporating the aqueous liquid crystalline (LC) solution on a planar air-LC interface induces polymer layering by self-assembly and rational action of macroscopic capillary forces between the layers. To clarify the mechanisms of nanometer- and submicron-scale layering, the polymer films are investigated by electron microscopy.

AuFePt Ternary Homogeneous Alloy Nanoparticles with Magnetic and Plasmonic Properties.

Combining Au and Fe into a single nanoparticle is an attractive way to engineer a system possessing both plasmonic and magnetic properties simultaneously. However, the formation of the AuFe alloy is challenging because of the wide miscibility gap for these elements. In this study, we synthesized AuFePt ternary alloy nanoparticles as an alternative to AuFe alloy nanoparticles, where Pt is used as a mediator that facilitates alloying between Au and Fe in order to form ternary alloy nanoparticles. The relationship among composition, structure, and function is investigated and it was found that at an optimized composition (Au52Fe30Pt18), ternary alloy NPs exhibit both magnetic and plasmonic properties simultaneously. The plasmonic properties are investigated in detail using a theoretical Mie model, and we found that it is governed by the dielectric constant of the resulting materials.

Ag/FeCo/Ag core/shell/shell magnetic nanoparticles with plasmonic imaging capability.

Magnetic nanoparticles (NPs) have been used to separate various species such as bacteria, cells, and proteins. In this study, we synthesized Ag/FeCo/Ag core/shell/shell NPs designed for magnetic separation of subcellular components like intracellular vesicles. A benefit of these NPs is that their silver metal content allows plasmon scattering to be used as a tool to observe detection by the NPs easily and semipermanently. Therefore, these NPs are considered a potential alternative to existing fluorescent probes like dye molecules and colloidal quantum dots. In addition, the Ag core inside the NPs suppresses the oxidation of FeCo because of electron transfer from the Ag core to the FeCo shell, even though FeCo is typically susceptible to oxidation. The surfaces of the Ag/FeCo/Ag NPs were functionalized with ε-poly-L-lysine-based hydrophilic polymers to make them water-soluble and biocompatible. The imaging capability of the polymer-functionalized NPs induced by plasmon scattering from the Ag core was investigated. The response of the NPs to a magnetic field using liposomes as platforms and applying a magnetic field during observation by confocal laser scanning microscopy was assessed. The results of the magnetophoresis experiments of liposomes allowed us to calculate the magnetic force to which each liposome was subjected.

Near-infrared-emitting Cd(x)Hg(1-x)Se nanorods fabricated by ion exchange in an aqueous medium.

Whereas CdSe nanorods that are grown in organic solution have a hexagonal wurtzite structure, which is the limiting case for exchange, HgSe is more commonly encountered as a cubic zinc blende system. An exchange process was performed at room temperature and at atmospheric pressure in an aqueous environment after phase transfer of the original CdSe nanorods, which reinforced the tendency for the endpoint of HgSe to be cubic. Consequently, we observed that under ambient conditions, the exchange process terminated with an average composition of only Cd(0.9)Hg(0.1)Se. Following the changes during the process by optical spectroscopy and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), we observed that the Hg(2+) ions diffused into the rods to a point limited by the formation of stacking faults due to the different lattice structures of the two limiting cases of zinc blende and wurtzite. HAADF-STEM and energy dispersive spectroscopy analyses also confirmed that the Hg substitution did not occur uniformly throughout the individual nanorods, as Hg-poor and Hg-rich regions coexist around the stacking faults. The formation of near-infrared-emitting alloyed Cd(x)Hg(1-x)Se nanorods in an aqueous medium highlights the subtle dependence of the ion-exchange process on the differences in the crystal structures of the two endpoint lattices.

Enhanced electronic properties of Pt@Ag heterostructured nanoparticles.

Platinum coated by silver nanoparticles was synthesized, which displays a unique structure where polycrystalline platinum particles are completely encapsulated in continuous monocrystalline silver shells. These particles display accentuated electronic properties, where the silver shells gain electron density from the platinum cores, imparting enhanced properties such as oxidation resistance. This electron transfer phenomenon is highly interfacial in nature, and the degree of electron transfer decreases as the thickness of silver shell increases. The nanoparticle structure and electronic properties are studied and the implication to creating sensing probes with enhanced robustness, sensitivity and controllable plasmonic properties is discussed.

Chemical stabilization of gold coated by silver core-shell nanoparticles via electron transfer.

Silver nanoparticles are notoriously susceptible to oxidation, yet gold nanoparticles coated in silver exhibit a unique electronic interaction that occurs at the interface of the two metals, leading to enhanced stability properties for the silver shell. In order to probe the phenomenon, the stability of gold nanoparticles coated by silver was studied in the presence of various chloride-containing electrolytes. It was found that a critical silver shell thickness of approximately 1 nm exists that cannot be oxidatively etched from the particle surface: this is in contrast to the observation of complete oxidative etching for monometallic silver nanoparticles. The results are discussed in terms of particle composition, structure and morphology before and after exposing the particles to the electrolytes. Raman analysis of the reporter molecule 3-amino-1,2,4-triazole-5-thiol adsorbed on the particle surface illustrates the feasibility of using gold coated by silver nanoparticle probes in sensing applications that require the presence of high levels of salt. The results provide insight into the manipulation of the electronic and stability properties for gold- and silver-based nanoparticles.

Enhanced Vertical Concentration Gradient in Rubbed P3HT:PCBM Graded Bilayer Solar Cells.

Graded bilayer solar cells have proven to be at least as efficient as the bulk heterojunctions when it comes to the Poly(3-hexylthiophene) (P3HT) - [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) donor-acceptor system. However, control of the vertical concentration gradient using simple techniques has never been reported. We demonstrate that rubbing the P3HT layer prior to PCBM deposition induces major morphological changes in the active layer. Using the newly introduced energy-dispersive X-ray spectroscopy element mapping technique, we found that rubbing P3HT induces the formation of an ideal vertical donor-acceptor concentration gradient. Furthermore, the P3HT crystallites undergo a molecular reorientation from edge-on to face-on configuration inducing a better charge transport in the vertical direction. The combination of these two major morphological changes leads to the fabrication of high-performance solar cells that exhibit, to date, the record efficiencies for spin-coated graded bilayers solar cells.