Strong Trapping Research Articles

Strong Interfacial Charge Trapping in Ultrathin SrRuO3 on SrTiO3 Probed by Noise Spectroscopy.

SrRuO3 (SRO) has emerged as a promising quantum material due to its exotic electron correlations and topological properties. In epitaxial SRO films, electron scattering against lattice phonons or defects has been considered as only a predominant mechanism accounting for electronic properties. Although the charge trapping by polar defects can also strongly influence the electronic behavior, it has often been neglected. Herein, we report strong interfacial charge trapping in ultrathin SRO films on SrTiO3 (STO) substrates probed by noise spectroscopy. We find that oxygen vacancies in the STO cause stochastic interfacial charge trapping, resulting in high electrical noise. Spectral analyses of the photoinduced noise prove that the oxygen vacancies buried deep in the STO can effectively contribute to the charge trapping process. These results unambiguously reveal that electron transport in ultrathin SRO films is dominated by the carrier number fluctuation that correlates with interfacial charge trapping.

Chain-Breaking Antioxidant and Peroxyl Radical Trapping Activity of Phenol-Coated Magnetic Iron Oxide Nanoparticles.

Superparamagnetic iron oxide nanoparticles (SPION) are important materials for biomedical applications, and phenol capping is a common procedure to passivate their surface. As phenol capped SPION have been reported to behave as antioxidants, herein, we investigate the mechanism underlying this activity by studying the reaction with alkyl peroxyl (ROO•) radicals. SPION were prepared by coprecipitation of Fe(II) and Fe(III), using phenolic antioxidants (gallic acid, Trolox and nordihydroguaiaretic acid) as post-synthesis capping agents and by different purification procedures. The reactivity of ROO• was investigated by inhibited autoxidation studies, using styrene as an oxidizable substrate (solvent MeCN, 30 °C) and azo-bis(isobutyronitrile) as a radical initiator. While unprotected, bare SPION behaved as prooxidant, accelerating the O2 consumption of styrene autoxidation, phenol capping provided a variable antioxidant effect that was dependent upon the purification degree of the material. Thoroughly washed SPION, containing from 7% to 14% (w/w) of phenols, had a low reactivity toward peroxyl radicals, while SPION with a higher phenol content (46% to 55%) showed a strong radical trapping activity. Our results indicate that the antioxidant activity of phenol-capped SPION can be caused by its release in a solution of weakly bound phenols, and that purification plays a major role in determining the properties of these materials.

Efficient hybrid white organic light-emitting diodes with superior color stability and slow efficiency roll-off based on ultrathin emitting layer structures

In this work, high-performance hybrid white organic light-emitting diodes (WOLEDs) with high efficiencies, slow roll-off and excellent color stability were realized by strategically inserting ultrathin red phosphorescent emitter film into bluish-green thermally activated delayed fluorescent (TADF) light-emitting layer (EML). Experimental results demonstrated that the introduction of ultrathin phosphor film helps to harvest superfluous triplet excitons within bluish-green EML and relieve local exciton accumulation-induced quenching, thereby guaranteeing high exciton utilization probability. Besides, the strong holes' trapping of ultrathin layer leads to improved carriers balance, thus realizing stable recombination zone. Finally, the optimal hybrid WOLED obtained the maximum current efficiency (ηc), power efficiency (ηp) and external quantum efficiency (EQE) of 63.84 cd A−1, 66.06 lm W−1 and 28.16%, respectively. At the brightness of 1000 cd m−2, ηc, ηp and EQE as high as 57.14 cd A−1, 49.88 lm W−1 and 25.23%, respectively, can still be remained by the same device, manifesting slow efficiency roll-off. In addition, the resulting WOLED displayed excellent color stability with slightly shifted Commission Internationale de l’Eclairage coordinates (CIEx,y) from (0.43, 0.44) at 1000 cd m−2 to (0.41, 0.43) at 10000 cd m−2.

Applying GMRES to the Helmholtz equation with strong trapping: how does the number of iterations depend on the frequency?

We consider GMRES applied to discretisations of the high-frequency Helmholtz equation with strong trapping; recall that in this situation the problem is exponentially ill-conditioned through an increasing sequence of frequencies. Our main focus is on boundary-integral-equation formulations of the exterior Dirichlet and Neumann obstacle problems in 2- and 3-d. Under certain assumptions about the distribution of the eigenvalues of the integral operators, we prove upper bounds on how the number of GMRES iterations grows with the frequency; we then investigate numerically the sharpness (in terms of dependence on frequency) of both our bounds and various quantities entering our bounds. This paper is therefore the first comprehensive study of the frequency-dependence of the number of GMRES iterations for Helmholtz boundary-integral equations under trapping.

Microporous Cyclodextrin Film with Funnel‐type Channel Polymerized on Electrospun Cellulose Acetate Membrane as Separators for Strong Trapping Polysulfides and Boosting Charging in Lithium–Sulfur Batteries

The “shuttle effect” of polysulfides hampers the commercialization of lithium–sulfur (Li‐S) batteries. Here, a thin molecular sieve film was decorated on the surface of an electrospun cellulose acetate (CA) membrane derived from recycled cigarette filters, where the truncated cone structure β‐cyclodextrin (β‐CD) was selected as the building block to physically block and chemically trap polysulfides while simultaneously dramatically speeding up ion transport. Furthermore, on the β‐CD free side of the separator facing the cathode, graphite carbon (C) was sputtered as an upper current collector, which barely increases the thickness. These benefits result in an initial discharge performance of 1378.24 mAh g−1 and long‐term cycling stability of 863.78 mAh g−1 after 1000 cycles at 0.2 C for the battery with the β‐CD/CA/C separator, which is more than three times that of the PP separator after 500 cycles. Surprisingly, the funnel‐type channel of β‐CD generates a differential ionic fluid pressure on both sides, speeding up ion transport by up to 69%, and a 65.3% faster charging rate of 9484 mA g−1 was achieved. The “funnel effect” of a separator is regarded as a novel and high‐efficiency solution for fast charging of Li‐S and other lithium secondary batteries.

First-principles study of interface stability and behaviors of He at the W/Y2O3 interface

In this study, we investigated the stability of the W/Y2O3 interface and the behaviors of He near the interface using the first-principles method. We reported that the termination structures had an essential effect on interfacial stability by calculating the adhesion energy and interface energy. The OW-terminated interface had better stability than the YW-terminated interface when O chemical potential ΔμO was larger than − 5.1 eV. The result of the segregation energy demonstrated that the W/Y2O3 interface could act as a strong trap to capture the He of the W layer, but it was extremely difficult to capture the He of the YO layer. By examining the effect of strain on the dissolution property of He in W and Y2O3 bulks, we reported that the strain caused by lattice matching would be the primary reason to affect the solution of He near the interface. A diffusion study reported a reaction region in the W layer near the interface, the thickness approached 0.3 nm. The interface was favorable to the release of the He because the trapped He preferred to diffuse along the interface plane, rather than to be filtered in the W and YO layers across the interface. In addition, we also reported that the configuration of the He cluster at the interface roughly presented as a flat platelet structure and accommodated up to three He atomic layers. Those calculation results agreed with experimental results and provided theoretical support to deeply understand the phase interface effects on the behaviors of He in the Y2O3 dispersion strengthened W.

Graphene/Ge microcrystal photodetectors with enhanced infrared responsivity

We report on the electrical and optical properties of microcrystal arrays obtained by depositing Ge on a deeply patterned Si substrate. Finite difference time domain simulations indicate that the faceted morphology and high refractive index of Ge microcrystals lead to strong light trapping effects, enhancing infrared light absorption in the spectral window between the direct and indirect absorption edge of Ge (≈1550–1800 nm). This is experimentally confirmed by fabricating microcrystal-based Ge-on-Si photodiodes employing graphene as a top transparent contact. In these devices, the ratio between the responsivities at 1550 and 1700 nm is more than ten times larger than that of photodiodes based on conventional Ge-on-Si epilayers.

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper.

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W-1 @ 265-276nm, and high photoconductive gain of ≈2 × 103 . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.

In-situ 3D observation of hydrogen-assisted particle damage behavior in 7075 Al alloy by synchrotron X-ray tomography

We directly captured, classified, and evaluated 3D particle debonding and fracture behavior in a H-charged 7075 Al alloy throughout the entire tensile deformation using synchrotron X-ray tomography and microstructural feature tracking techniques. The effects of particle size, shape, spatial clustering and stress state on strain-dependent particle damage were identified and isolated from each other. Moreover, state-of-the-art imaging and tracking techniques enabled the establishment of spatially and time-resolved hydrogen distributions during deformation. Based on realistic hydrogen partitioning among various nanoscopic trap sites, the contributions of particles to hydrogen trapping and the hydrogen effect at individual damaged particles were assessed quantitatively. Fracturing of coarse and irregular Al7Cu2Fe particles was found to be the predominant particle damage mode due to the spatial clustering and brittleness of these particles, but a hydrogen effect was not observed. The debonding of Mg2Si particles seemed to be the result of competition between hydrogen and clustering-induced stress localization, but detrimental effects of hydrogen on ductile fracture induced by accelerating interfacial debonding were found to be limited. The quantitative evaluation of particle damage in the present model material clarified a viable strategy for mitigating hydrogen embrittlement, which involves introducing and modifying intermetallic particles with strong hydrogen trapping capacities.

Boosting the photocatalytic nitrogen reduction to ammonia through adsorption-plasmonic synergistic effects

Ammonia is one of the most essential chemicals in the modern society but its production still heavily relies on energy-consuming Haber-Bosch processes. The photocatalytic reduction of nitrogen with water for ammonia production has attracted much attention recently due to its synthesis under mild conditions at room temperature and atmospheric pressure using sunlight. Herein, we report a high-performance Au/MIL-100(Cr) photocatalyst, comprising MIL-100(Cr) and Au nanoparticles in photocatalytic nitrogen reduction to ammonia at ambient conditions under visible light irradiation. The optimized photocatalyst (i.e., 0.10Au/MIL-100(Cr)) achieved the excellent ammonia production rate with 39.9 µg gcat−1 h−1 compared with pure MIL-100(Cr) (2.73 µg gcat−1 h−1), which was nearly 15 times that on pure MIL-100(Cr). The remarkable activity could be attributed to the adsorption-plasmonic synergistic effects in which the MIL-100(Cr) and Au are responsible to the strong trapping and adsorption of N2 molecules and photo-induced plasmonic hot electrons activating and decomposing the N2 molecules, respectively. This study might provide a new strategy for designing an efficient plasmonic photocatalyst to improve the photocatalytic performance of N2 fixation under visible light irradiation.

Co3O4 modified Mn0.2Cd0.8S with different shells forms p-n heterojunction to optimize energy/mass transfer for efficient photocatalytic hydrogen evolution

The construction of an unique structure to optimize the energy/mass transfer for photocatalysts is an effective way to improve their solar-chemical energy conversion efficiency. Here, p-type Co3O4 polyhedrons and n-type Mn0.2Cd0.8S microrods were successfully coupled to construct Mn0.2Cd0.8S/Co3O4 p-n heterojunction. In order to optimize the energy/mass transfer in the Mn0.2Cd0.8S/Co3O4 p-n heterojunction photocatalyst, ZIF-67 was used as precursor to create hollow Co3O4 derivatives [Co3O4 porous polyhedrons (PPs), Co3O4 hollow double-shell polyhedrons (HDSPs), and Co3O4 hollow single-shell polyhedrons (HSSPs)]. And Co3O4 hollow single-shell polyhedrons (HSSPs) possesses these prominent advantages that strong light trapping ability, short carrier transport distance, and larger catalyst/solvent contact area in comparison with other two derivatives. More importantly, the band structures of the differently structural Co3O4 are distributed stepwise. The valence band potential of Co3O4 HSSPs is more positive meaning stronger oxidation ability, which effectively improves the separation efficiency of electron-hole pairs in Mn0.2Cd0.8S/Co3O4 p-n heterojunction system. Therefore, Mn0.2Cd0.8S/Co3O4 HSSPs showed more excellent hydrogen evolution activity (11222 μmol g−1 h−1) which was about 26 times and 2.34 times that of Mn0.2Cd0.8S and Mn0.2Cd0.8S/Co3O4 PPs, respectively. This work will provide broad prospects for the rational design of efficient photocatalysts.

Plasmonic and Graphene-Functionalized High-Performance Broadband Quasi-Two-Dimensional Perovskite Hybrid Photodetectors.

Quasi-two-dimensional (2D) layered organic-inorganic hybrid perovskites have attracted extensive attention, owing to their excellent optoelectronic tunability and moisture stability compared with three-dimensional perovskite counterparts and show great potential for application in photodetectors (PDs). However, owing to the unavoidable grain boundary defects of perovskite polycrystalline films, the photocurrent is limited by poor light absorption and charge mobility. Therefore, the preparation of quasi-2D perovskite films with strong light trapping and high charge mobility has been challenging. In this study, novel broadband quasi-2D perovskite (BA)2(FA)n-1PbnI3n+1 hybrid-structure PDs with good stability were fabricated by combining both monolayer graphene and Au square nanoarrays. The hybrid system using both graphene and Au square nanoarrays effectively improved the carrier mobility and light absorption and simultaneously maximized light trapping and light-induced carrier extraction, which resulted in PDs with greatly enhanced photocurrent in the visible and near-infrared range. The graphene-Au array-perovskite-based PDs had a low dark current of 10-10 A, large on/off ratio of 104, high responsivity of 18.71 A W-1, and detectivity of 2.21 × 1013 Jones. The responsivity and detectivity were two orders of magnitude higher than those of PDs based only on perovskites. This work demonstrates a promising and feasible device based on the coupling of a gold array, layered graphene, and quasi-2D perovskites, which shows great potential for the development of high-performance broadband perovskite PDs.

Interfacial Effect on Dielectric Properties of Self-Assembled Polythiourea-Based Copolymers for Ultrahigh Energy Storage.

Polymer dielectrics are highly desirable in capacitor applications due to their low cost, high stability, and reliability. However, there still remains a lack of feasible methods to prepare polymer dielectrics with high energy density and low dielectric loss, which severely hampers the development of compact and efficient power electronics. Here, an amphiphilic block copolymer, polythiourea-b-polydimethylsiloxane (PTU-b-PDMS), with an extraordinarily high energy density of 29.8Jcm-3 and a low loss is synthesized via polyaddition polymerization. This is highly relevant to the block molecule conformation in the interfacial region of the self-assembled PTU-b-PDMS. The block molecule in the interface adopts an extended conformation when the PTU forms nanodots, whereas the block molecule adopts a coiled conformation when the PTU forms nanostrands. The observation and characterization have proved that the coiled block molecule in the interfacial region can simultaneously induce extra strong charge trapping sites and dipolar polarization. It substantially improves the breakdown strength from 652 to 1166MVm-1 , while maintaining a high dielectric constant of 5 and a low loss of <0.01.This work offers unprecedented structural insights into the conformation-induced interfacial effect and enables rational design of self-assembled copolymers to boost their dielectric properties and energy density.

A Universal Dopant-Free Polymeric Hole-Transporting Material for Efficient and Stable All-Inorganic and Organic-Inorganic Perovskite Solar Cells.

Hole-transporting materials (HTMs) with desired properties play a crucial role in achieving efficient and stable perovskite solar cells (PSCs). However, most high-performance devices generally employ HTMs that require additional complicated doping treatments, which are harmful to the device stability. In this work, a fluorine-substituted polymer electron-donor material, PM6, is developed as a dopant-free HTM in regular all-inorganic CsPbI2Br PSCs. Benefiting from the matched energy-level alignment, high hole mobility, and effective defect passivation, a champion power conversion efficiency (PCE) of 16.06% with an ultrahigh fill factor of 82.54% is achieved for the PM6-based PSCs. Compared to doped Spiro-OMeTAD (PCE of 14.46%), PM6 significantly enhances the PCE of CsPbI2Br PSCs with negligible hysteresis owing to its more efficient charge transportation, suppressed recombination, and strong trap passivation effect. Moreover, remarkable improvements in long-term stability, thermal stability, and operational stability are all gained for the PM6-based PSCs. In addition, the successful application of PM6 as a dopant-free HTM in organic-inorganic hybrid PSCs enables an impressive PCE of 20.05% with superb device stability, manifesting the generality of the polymer donor material in various PSC systems.

Permanent magnet array-driven navigation of wireless millirobots inside soft tissues.

Creating wireless milliscale robots that navigate inside soft tissues of the human body for medical applications has been a challenge because of the limited onboard propulsion and powering capacity at small scale. Here, we propose around 100 permanent magnet array–based remotely propelled millirobot system that enables a cylindrical magnetic millirobot to navigate in soft tissues via continuous penetration. By creating a strong magnetic force trap with magnetic gradients on the order of 7 T/m inside a soft tissue, the robot is attracted to the center of the array even without active control. By combining the array with a motion stage and a fluoroscopic x-ray imaging system, the magnetic robot followed complex paths in an ex vivo porcine brain with extreme curvatures in sub-millimeter precision. This system enables future wireless medical millirobots that can deliver drugs; perform biopsy, hyperthermia, and cauterization; and stimulate neurons with small incisions in body tissues.

Investigation of Hydrogen Diffusion in High Hardness Steels

Development of high hardness steels (HHS) for use in armor, shipbuilding, construction and automotive requires investigation into their sensitivity to hydrogen embrittlement. Hydrogen embrittlement, often caused by environmental corrosion, significantly impairs the mechanical properties of steel, producing increased local hardness and increased likelihood of catastrophic brittle failure. Mitigation of embrittlement can employ the introduction of hydrogen traps or phase barriers into the steel. In this work, we employed the Devanathan-Stachurski method to investigate the diffusion of hydrogen atoms through a steel membrane. The effect of strong hydrogen trapping was observed as an apparent change in hydrogen diffusivity from the initial and subsequent diffusion trials. We compared three different high hardness steel samples and used two methods to determine the apparent diffusivity and number of strong and weak hydrogen traps. Metallographic analysis of the samples illustrates the effect of composition on the diffusivity data. Figure 1

A numerical study on the closed packed array of gold discs as an efficient dual mode plasmonic tweezers

In this report, we propose the closed pack array of gold discs on glass, as a dual mode plasmonic tweezers that benefits from two trapping modes. The first trapping mode is based on leaky surface plasmon mode (LSPM) on the gold discs with a longer penetration depth in the water and a longer spatial trapping range, so that target nanoparticles with a radius of 100 nm can be attracted toward the gold surface from a vertical distance of about 2 µm. This trapping mode can help to overcome the inherent short range trapping challenge in the plasmonic tweezers. The second trapping mode is based on the dimer surface plasmonic mode (DSPM) in the nano-slits between the neighboring gold discs, leading to isolated and strong trapping sites for nanoparticles smaller than 34 nm. The proposed plasmonic tweezers can be excited in both LSPM and DSPM modes by switching the incident wavelength, resulting in promising and complementary functionalities. In the proposed plasmonic tweezers, we can attract the target particles towards the gold surface by LSPM gradient force, and trap them within a wide half widthhalfmaximum (HWHM) that allows studying the interactions between the trapped particles, due to their spatial proximity. Then, by switching to the DSPM trapping mode, we can rearrange the particles in a periodic pattern of isolated and stiff traps. The proposed plasmonic structure and the presented study opens a new insight for realizing efficient, dual-mode tweezers with complementary characteristics, suitable for manipulation of nanoparticles. Our thermal simulations demonstrate that the thermal-induced forces does not interefe with the proposed plasmonic tweezing.

How charge trapping affects the conductivity of electrochemically doped poly(3-hexylthiophene) films

Electrochemical doping is an elegant method of controlling the doping level and charge carrier densities of conjugated polymer films and enhancing their thermoelectric figure of merit. Applying this doping technique to films of poly(3-hexylthiophene) (P3HT) results in conductivities with values as high as 200 S/cm. The stability of the doped films in the solid state can be probed by UV-vis-NIR spectroscopy. We found that the choice of the conducting salt in the liquid electrolyte exerts a strong influence over the conductivity. Using TBAPF6 and LiClO4 provides highest conductivities for P3HT films, while LiTFSI and TBABF4 show overall lower performance. This effect is also reflected in cyclic voltammetry measurements coupled with in situ spectroscopy. Overall lower reversibility upon multiplex cycling in LiTFSI and TBABF4 electrolytes suggests strong charge trapping effects, which one might attribute to a considerable fraction of charges (holes/ions) remaining in the films after charge/discharge cycles. The salts with stronger charge irreversibility in the electrochemistry experiments show the poorer solid state conductivities. Our conclusion is that one should carefully choose the electrolyte to ensure good percolation pathways and delocalized charge transport throughout doped films.

Soft-Microstructured Transparent Electrodes for Photonic-Enhanced Flexible Solar Cells

Microstructured transparent conductive oxides (TCOs) have shown great potential as photonic electrodes in photovoltaic (PV) applications, providing both optical and electrical improvements in the solar cells’ performance due to: (1) strong light trapping effects that enhance broadband light absorption in PV material and (2) the reduced sheet resistance of the front illuminated contact. This work developed a method for the fabrication and optimization of wavelength-sized indium zinc oxide (IZO) microstructures, which were soft-patterned on flexible indium tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) substrates via a simple, low-cost, versatile, and highly scalable colloidal lithography process. Using this method, the ITO-coated PET substrates patterned with IZO micro-meshes provided improved transparent electrodes endowed with strong light interaction effects—namely, a pronounced light scattering performance (diffuse transmittance up to ~50%). In addition, the photonic-structured IZO mesh allowed a higher volume of TCO material in the electrode while maintaining the desired transparency, which led to a sheet resistance reduction (by ~30%), thereby providing further electrical benefits due to the improvement of the contact conductance. The results reported herein pave the way for a new class of photonic transparent electrodes endowed with mechanical flexibility that offer strong potential not only as advanced front contacts for thin-film bendable solar cells but also for a much broader range of optoelectronic applications.

Effect of microstructure on hydrogen embrittlement susceptibility in quenching-partitioning-tempering steel

Hydrogen embrittlement (HE) restricts the application of high strength steel in sustainable energy productions. As one type of efficient hydrogen trap sites, the NbC carbide precipitation is a superior approach to mitigate the HE susceptibility. The microstructure, mechanical properties and HE susceptibility were investigated in different high strength steels treated by quenching and partitioning (Q&P), quenching-partitioning-tempering (Q-P-T) and intercritical annealing quenching and partitioning (IAQP) processes in this study. The results show that the NbC particles can significantly improve the HE resistance of high strength steel treated by Q-P-T process. The NbC carbide precipitation has four different morphologies with varied sizes, which can effectively trap a large amount of diffusible hydrogen atoms. The retained austenite phases with different morphologies and locations have different hydrogen trap ability, but ferrite phase does not show strong hydrogen trap ability. Meanwhile, the NbC carbide precipitation can enhance the yield strength of steel effectively through precipitation strengthening, but has little effect on its ductility.