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Highly stable ultrathin films of large unsaturated room-temperature magnetoresistance (MR) are essential for the next-generation real-time magnetoelectric devices. A large-area, transfer-free, highly crystalline, and phase-engineered ultrathin film of Td-Mo0.27W0.71Te2.02 or 2H- & Td-Mo0.22W0.89Te1.89 on a hexagonal boron nitride (h-BN) substrate was synthesized using an atmospheric-pressure chemical vapor deposition (APCVD) method. The Td-Mo0.27W0.71Te2.02 with average mobility of 725 cm2 V−1 s−1 possesses non-saturating MR of 18% at 5 K and 11% at room temperature. Quantum correction to the magnetotransport study suggests the existence of a weak anti-localization effect dominated by the electron–electron interaction to render the non-saturating linear MR in a wide temperature range. Moreover, the spin–orbit interaction in Td-Mo0.27W0.71Te2.02 was found valid till an applied field of 0.05 T with an interaction length of 18 nm at 300 K. In this alloy system, the weak localization effect was evidenced unprecedentedly by the Te-deficient 2H- & Td-Mo0.22W0.89Te1.89 thin film with unusual co-existence of two crystal phases, which exhibit a suppressed MR caused by the recurring inelastic scattering with a reduced phase coherence length. This work explores the production of phase-engineered large-area Weyl semi-metallic 2D materials for the realization of magnetoelectrics in the near future. [ABSTRACT FROM AUTHOR]

Bentonite is considered for use as a buffer material in the final disposal repositories of radioactive waste. Long-lived 135Cs with a half-life of 2.3 × 106 years is a key radionuclide in high-level waste, and lots of 137Cs with a half-life of 30.2 years exists in low-level waste. Therefore, the adsorption of Cs on bentonite is a critical issue in evaluating the long-term safety of radioactive waste disposal. In this study, EXAFS techniques were used to characterize the time-dependent process from the beginning of adsorption to equilibrium. From the results of this study, we found changes including to the Cs adsorption sites, the Cs–O distance between Cs and the oxygen atom, and that the adsorption of Cs ions occurred before the reaction reached equilibrium. The fraction of OS complexes when Cs was adsorbed on bentonite can refer to the CN (Cs–O1st)/CN (Cs–O2nd) ratio of coordination numbers, and this study found that the OS complex was the major adsorption species when Cs adsorbed onto bentonite. In addition to the ratio CN (Cs–O1st)/CN (Cs–O2nd) providing information on the adsorption site, we also discussed the change of Cs–O1st and Cs–O2nd bond distances to identify the adsorption sites at different times. Comparing the XRD patterns of montmorillonite and bentonite, we found that the interlayer collapsed after Cs was adsorbed onto montmorillonite, but it expanded after Cs was adsorbed onto bentonite. From the results of EXAFS fitting, we found that the movement of Cs ions was from regular interlayer sites to expanded interlayer sites, which caused the interatomic distance of Cs–O2nd to decrease with an increase in time. It was revealed that the adsorption of Cs on bentonite occurred in two steps. The first step includes the rapid uptake of Cs by attachment to the oxygen atoms of the H2O molecules at the regular interlayer sites, especially for the OS complexes. The second step includes a slower process where dehydrated Cs ions move from the regular interlayer sites to the expanded interlayer sites. In this study, Cs L3-edge EXAFS spectroscopy was conducted for the Cs adsorbed on bentonite to identify the Cs adsorption sites over time, as this is important in evaluating the mobility of Cs in the environment. These results are beneficial in finding the process of Cs adsorption on bentonite, which could be used for the design of the final disposal of spent nuclear fuel. [ABSTRACT FROM AUTHOR]

Cyclopentyl methyl ether (CPME), a green solvent, can be used to replace toxic halogenated solvents in the production of efficient molecular solar cells. With CPME alone as the processing solvent, a power conversion efficiency (PCE) of 3.13% was achieved when using a two-dimensional conjugated small molecule (SMPV1) as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor to form the bulk heterojunction (BHJ) organic photovoltaic (OPV) device. This low PCE arose from the low solubility of PC61BM in this green solvent. Accordingly, toluene (Tol) was introduced in various amounts as a co-solvent for CPME. The greater solubility of PC61BM in these mixtures led to significant improvements in the short-circuit current density (Jsc) and fill factor (FF) of the device, achieving a PCE of 7% after processing in the optimized green solvent mixture of CPME : Tol (60 : 40). Furthermore, thermal annealing (TA), at 80 °C for 10 min, of the active layers processed from the 60 : 40 green solvent mixture enhanced the PCE to 8.10%―the highest ever reported for a molecular solar cell processed from a green solvent mixture. Large-area devices fabricated this way, having areas of 1 and 5.5 cm2, exhibited PCEs of 6.20 and 3.73%, respectively. The morphological changes that occurred when applying the co-solvent and TA played key roles in achieving such high PCEs for molecular solar cells processed from green solvent mixtures―a promising step toward the upscaling of OPVs. [ABSTRACT FROM AUTHOR]

Multiferroic YMn1−xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn–Teller effect of Mn3+ on Fe substitution and also the local octahedral-site distortion of Fe3+ in an environment of Jahn–Teller-active Mn3+ ions in YMn1−xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn3+ ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y2−r2 orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe3+ ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe3+ ion inside the YMnO3 lattice was similar to that of the Mn3+ ion, whereas the Jahn–Teller distortion and GdFeO3-type distortion of the Mn3+ ion were decreased by the spherical high-spin Fe3+ ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn–Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1−xFexO3 thin films. [ABSTRACT FROM AUTHOR]

The development of earth-abundant and efficient electrocatalysts for the hydrogen evolution reaction (HER) is one of the keys to success for future green energy systems using hydrogen fuel. Nanostructuring of electrocatalysts is a promising way to enhance their electrocatalytic performance in the HER. In this study, pure pyrite-type beaded stream-like cobalt diselenide (CoSe2) nanoneedles are directly formed on flexible titanium foils through treating a cobalt oxide (Co3O4) nanoneedle array template with selenium vapor. The beaded stream-like CoSe2 nanoneedle electrode can drive the HER at a current density of 20 mA cm−2 with a small overpotential of 125 mV. Moreover, the beaded stream-like CoSe2 nanoneedle electrode remains stable in an acidic electrolyte for 3000 cycles and continuously splits water over a period of 18 hours. The enhanced electrochemical activity is facilitated by the unique three-dimensional hierarchical structure, the highly accessible surface active sites, the improved charge transfer kinetics and the highly attractive force between water and the surface of the nanoneedles that exceeds the surface tension of water. [ABSTRACT FROM AUTHOR]

The main issues in planar perovskite solar cells are the coverage and crystallinity of the perovskite film on the PEDOT:PSS layer. To enhance these features, we introduced alkali metal halides (salts) as additives into the perovskite precursor solution used in a two-step preparation method. These alkali metal halides chelate with Pb2+ ions and enhance the crystal growth of PbI2 films, resulting in nanostructured morphologies. The nanostructured PbI2 films promote homogeneous nucleation and larger crystallite sizes, thereby enhancing the morphology and crystallinity of the perovskite films. The alkali metal halides recrystallize the small grains and passivate the grain boundaries and interface states, allowing effective charge generation and dissociation in perovskite films. Photoluminescence measurements indicated that perovskite films prepared with salt additives featured fewer charge traps and defects. The power conversion efficiency of the device incorporating a small amount of a salt additive increased by approximately 33%―from 11.4 to 15.08%. This device was more stable than a corresponding device prepared without the additive, with only 16.5% degradation occurring over a period of 50 days. [ABSTRACT FROM AUTHOR]

A non-toxic sol–gel spin coating approach is one of the attractive routes to achieve high atom economy, good quality Cu2ZnSnS4 (CZTS) thin films. In this paper, we introduce 1,3-dimethyl-2-imadazolidinone as a solvent for the preparation of highly viscous, homogeneous, nontoxic CZTS ink that eliminates the need for the use of additional binders or additives to disperse the precursors. In addition, we further report the annealing of the spin coated CZTS thin film in 6% diluted H2S gas with an externally supplied tin and sulfur environment to suppress the loss of tin from the thin-film surface and to enhance the device performance. CZTS grain sizes greater than 0.7 μm have been achieved with no detectable presence of carbon rich layers or layers containing fine grain sizes at the Mo/CZTS interface. An efficiency of 5.67% for the champion device fabricated here has been achieved with an open circuit voltage of 0.58 V, a short current density of 18.48 mA cm−2, and a fill factor of 53.14%. [ABSTRACT FROM AUTHOR]

Solution-processed organometal trihalide-based perovskites have attracted attention in the field of solar energy due to their high absorption and low temperature fabrication. We demonstrated the droplet-assisted two-step process of spin coating lead iodide (PbI2) followed by spray coating methylammonium iodide (CH3NH3I) to prepare a continuous lead iodide perovskite film. A simple airbrush gun was used to control the volume of CH3NH3I, in order to attain a uniform, stoichiometric and continuous perovskite film. An insufficient or excess volume of CH3NH3I gives poor crystallinity and morphology, which gradually reduces the device performance. A power conversion efficiency (PCE) of 11.66% was achieved for 100 nm of PbI2 followed by 300 μl of CH3NH3I and annealing at 100 °C for 120 min. To address the reproducibility of the device performance, 50 devices were fabricated for statistical analysis and 80% of the devices showed the average PCE of 10–11% with reproducible JSC, VOC and FF. [ABSTRACT FROM AUTHOR]

We demonstrate the significance of ions with an ordered arrangement for enhancing the electron injection in polymer light-emitting diodes (PLEDs) and the electron extraction in polymer solar cells (PSCs) by using tetraoctylammonium bromide (TOAB) as a model compound. The spin-coated films of TOAB are prepared on the active layers of both PLEDs and PSCs without any annealing (TOAB-No) as well as with annealing at 60 °C (TOAB-60) and 100 °C (TOAB-100). Subsequently, they are investigated by atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD) as well as simultaneously employed as the interfacial layers in PLEDs and PSCs. The AFM results show that the active layers are well covered with TOAB-No and TOAB-100, but poorly covered with TOAB-60 due to the formation of large islands. The XRD results show that TOAB-No and TOAB-60 are polycrystalline with the (002) lattice planes parallel to the surface of the active layers while TOAB-100 is amorphous. Of note, the electron injection in PLEDs and the electron extraction in PSCs are significantly enhanced using TOAB-No and TOAB-60, but both are worsened using TOAB-100. Evidently the enhanced electron injection/extraction is mainly attributed to the ordered arrangement of ions in the TOAB crystals. This study indicates that with an ordered arrangement of ions, all ionic compounds should be excellent for enhancing the electron injection/extraction in polymer optoelectronic devices. [ABSTRACT FROM AUTHOR]

A maximal thermoelectric figure-of-merit ZT for p-type Ag(Sb1−xInx)Te2 samples with x = 0.07 is 1.35 at 650 K, yielding an enhancement of greater than 40% compared with that of an undoped AgSbTe2 compound at the same temperature. This ZT enhancement can be primarily attributed to both the greatly enhanced power factor resulting from an increase in Seebeck coefficient because of the increase in the effective mass and the substantial decrease in thermal conductivity which could be ascribed to the enhancement of the phonon scattering mechanism by dopants with different atomic weights. These results indicate that doping with In is effective for enhancing the thermoelectric performance of the p-type AgSbTe2 compound. [ABSTRACT FROM AUTHOR]

Multiferroic YMn1-xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn-Teller effect of Mn(3+) on Fe substitution and also the local octahedral-site distortion of Fe(3+) in an environment of Jahn-Teller-active Mn(3+) ions in YMn1-xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn(3+) ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y(2)-r(2) orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe(3+) ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe(3+) ion inside the YMnO3 lattice was similar to that of the Mn(3+) ion, whereas the Jahn-Teller distortion and GdFeO3-type distortion of the Mn(3+) ion were decreased by the spherical high-spin Fe(3+) ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn-Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1-xFexO3 thin films.