The interactions between spin wave and stacked domain walls.

In this study, the interactions between spin wave (SW) and stacked domain walls in a magnetic nanostrip are investigated via micromagnetic simulation. It is found that under the excitation of SW, the metastable TWVW structure consisting of a transverse wall (TW) and a vortex wall (VW) may transform into a 360° wall or may completely annihilate depending on the frequency and amplitude of the SW. In contrast, stacked TWs (STWs) structure shows good robustness. Similar to a single TW, the STWs can be moved by SW and the inside TWs exhibit coherent motions. Notably, the frequency dependence of STWs' velocity demonstrates obvious emergence, shift and disappearance of the resonant peaks. Such changes are found to be in accordance with SW reflection, which thus agrees with the mechanism of linear momentum transfer torque (LMTT). In concern with the SW transmission through STWs, we show that by varying TWs number and SW frequency, a wide range of transmission efficiency η can be obtained. At certain frequencies, η may increase with TWs number and may go beyond 100%, which indicates a lowered attenuation by STWs. On the other hand, the phase shift of the transmitted SW always increases linearly with the TWs number and can be resonantly enhanced at frequencies same as that of TWs normal modes. Mapping of SW reveals that the phase shift is a result of fast propagation of SW through TWs. The fast propagation and the low attenuation of SW through STWs suggests that STWs may serve as an excellent SW channel. Meanwhile, the induced STWs motion and the controlled SW transmission and phase shift by STWs also promises great uses of STWs in future magnonic devices and domain wall devices.

The origin of spin wave pulse-induced domain wall inertia.

The fundamental problem of domain wall (DW) inertia-the property that gives to inertial behaviors remains unclear in the physics of magnetic solitons. To understand its nature as well as to achieve accurate DW positioning and efficient manipulation of domain wall motion (DWM), spin wave (SW) pulse-induced DW transient effect is studied both numerically and theoretically in a magnetic nanostrip. It is shown for the first time that there occurs inevitable deceleration/automotion after SW pulse, which indicates nonzero DW inertia. The induced DWM is revealed to relate to two factors: energy storing within DW and out-of-plane tilting of DW. To explain the DWM dynamics, a one-dimensional collective model is developed to account for the excitation of spin wave pulse. The model successfully bridges DW energy, DW tilting and DW displacement and provides descriptions in accordance with numerical findings. It is made clear that the DW automotion hence DW inertia originate from the process of DW relaxation toward equilibrium. The DW inertia is expressed in terms of effective mass and turns out to be a time-dependent function with damping constant α as the governing parameter, which opposes the nature of intrinsic mass. For case containing multiple DWs, the total effective mass is shown to concern the reached velocity and stored energy of DWs instead of the number of DWs, which is against common intuition.

Source Apportionment of Volatile Organic Compounds (VOCs) by Positive Matrix Factorization (PMF) supported by Model Simulation and Source Markers - Using Petrochemical Emissions as a Showcase.

This study demonstrates the use of positive matrix factorization (PMF) in a region with a major Petrochemical Complex, a prominent source of volatile organic compounds (VOCs), as a showcase of PMF applications. The PMF analysis fully exploited the quality and quantity of the observation data, sufficed by a cluster of 9 monitoring sites within a 20 km radius of the petro-complex. Each site provided continuous data of 54 speciated VOCs and meteorological variables. Wind characteristics were highly seasonal and played a decisive role in the source-receptor relationship, hence the dataset was divided into three sub-sets in accordance with the prevailing wind flows. A full year of real-time data were analyzed by PMF to resolve into various distinct source types including petrochemical, urban, evaporative, long-range air parcels, etc., with some sites receiving more petro-influence than others. To minimize subjectivity in the assignment of the PMF source factors, as commonly seen in some PMF works, this study attempted to solidify PMF results by supporting with two tools of spatially/temporally resolved air-quality model simulations and observation data. By exploiting the two supporting tools, the dynamic process of individual sources to a receptor were rationalized. Percent contributions from these sources to the receptor sites were calculated by summing over the occurrence of different source types. Interestingly, although the Petro-complex is the single largest local VOC source in the 20 km radius study domain, all monitoring sites in the region received far less influence from the Petro-complex than from other emission types within or outside the region, which together add up to more than 70% of the total VOC abundance.

Direct imaging of the interaction between spin wave and transverse wall in magnetic nanostrip.

By micromagnetic simulations, the dynamical interaction between spin wave (SW) and a transverse wall (TW) in a magnetic nanostrip is studied. We find the dynamical interaction can be directly demonstrated by SW-induced TW oscillation, which can be obtained by calculating the total magnetic moment within the area of TW as a function of time. Two cases of the initial TW, in equilibrium state and in metastable state, are investigated and compared. Before SW reaches TW, the metastable TW oscillates naturally with a constant frequency, whereas the equilibrium TW does not oscillate. After SW acts on TW, both the metastable TW and the equilibrium TW will oscillate with a frequency that always equal to the frequency of the applied SW. The amplitude of the SW-induced TW oscillation for both the metastable case and the equilibrium case strongly depends on the frequency of the applied SW. Through tuning the frequency of the applied SW, we confirm that the natural oscillation of the metastable TW, which is independent of the applied SW, will not affect the amplitude of the SW-induced TW oscillation and the velocity of the TW motion in compare with those of the equilibrium TW. Interestingly, the frequency-response curves of the SW-induced TW oscillations display multiple resonance peaks. Moreover, we find the frequency-response curves of the SW-induced TW oscillation, SW reflection coefficient and TW velocity driven by SW share the same multiple-resonance property. It may suggest the SW-induced TW oscillation in the domain wall plays an important role in the SW-driven TW motion.

Steering Photoelectrons Excited in Carbon Dots into Platinum Cluster Catalyst for Solar-Driven Hydrogen Production.

In composite photosynthetic systems, one most primary promise is to pursue the effect coupling among light harvesting, charge transfer, and catalytic kinetics. Herein, this study designs the reduced carbon dots (r-CDs) as both photon harvesters and photoelectron donors in combination with the platinum (Pt) clusters and fabricated the function-integrated r-CD/Pt photocatalyst through a photochemical route to control the anchoring of Pt clusters on r-CDs' surface for solar-driven hydrogen (H2) generation. In the obtained r-CD/Pt composite, the r-CDs absorb solar photons and transform them into energetic electrons, which transfer to the Pt clusters with favorable charge separation for H2 evolution reaction (HER). As a result, the efficient coupling of respective natures from r-CDs in photon harvesting and Pt in proton reduction is achieved through well-steered photoelectron transfer in the r-CD/Pt system to cultivate a remarkable and stable photocatalytic H2 evolution activity with an average rate of 681 µmol g-1 h-1. This work integrates two functional components into an effective HER photocatalyst and gains deep insights into the regulation of the function coupling in composite photosynthetic systems.

Remarkably enhanced current-driven 360° domain wall motion in nanostripe by tuning in-plane biaxial anisotropy.

By micromagnetic simulations, we study the current-driven 360° domain wall (360DW) motion in ferromagnetic nanostripe with an in-plane biaxial anisotropy. We observe the critical annihilation current of 360° domain wall can be enhanced through such a type of anisotropy, the reason of which is the suppression of out-of-plane magnetic moments generated simultaneously with domain-wall motion. In details, We have found that the domain-wall width is only related to K y - K x , with K x(y) the anisotropy constant in x(y) direction. Taking domain-wall width into consideration, a prior choice is to keep K y ≈ K x with large enough K. The mode of domain-wall motion has been investigated as well. The traveling-wave-motion region increases with K, while the average DW velocity is almost unchanged. Another noteworthy feature is that a Walker-breakdown-like motion exists before annihilation. In this region, though domain wall moves with an oscillating behavior, the average velocity does not reduce dramatically, but even rise again for a large K.

Room temperature ferromagnetism and photoluminescence in Cu-doped ZnO nanocrystals.

The Zn(1-x)Cu(x)O (x = 0.0-3.5%) nanocrystals have been synthesized by a simple sol-gel method. X-ray diffraction, optical absorption and photoluminescence measurements were employed to validate consistently the incorporation of Cu ions into the ZnO wurtzite lattice without formation of secondary phases for Zn(1-x)Cu(x)O (x < 2.0%). Meanwhile, it was found that the substituted Cu-doping leads to the reduction of the band gap and the appearance of the structured green emission. Magnetization measurement showed that the low Cu-doping (x < 1.0%) develops the ferromagnetism, but the high Cu-doping destroys sharply the ferromagnetism due to the formation of the antiferromagnetic coupling among the neighboring Cu ions. It is indicated that the rational Cu-doping can tune optical and magnetic properties in ZnO.

Full-range analysis of ambient volatile organic compounds by a new trapping method and gas chromatography/mass spectrometry.

This study investigated the feasibility of analyzing a full range of ambient volatile organic compounds (VOCs) from C(3) to C(12) using gas chromatograph mass spectrometry (GC/MS) coupled with thermal desorption. Two columns were used: a PLOT column separated compounds lighter than C(6) and a DB-1 column separated C(6)-C(12) compounds. An innovative heart-cut technique based on the Deans switch was configured to combine the two column outflows at the ends of the columns before entering the MS. To prevent the resolved peaks from re-converging after combining, two techniques were attempted (hold-up vs. back-flush) to achieve the intended "delayed" elution of heavier components. Thus, the resulting chromatogram covering the full range of VOCs is a combination of two separate elutions, with the heavier section following the lighter section. With the hold-up method, band-broadening inevitably occurred for the delayed C(6)-C(7) DB-1 compounds while the light compounds eluted from the PLOT column. This broadening problem resulted in peak tailing that was largely alleviated by adding a re-focusing stage while the DB-1 compounds were back-flushed, and this modified technique is referred to as the back-flush method. With this modification, the separation of the C(6)-C(7) compounds improved dramatically, as revealed by the decrease in peak asymmetry (As) and increase in resolution. Linearity and precision for these peaks also improved, yielding R(2) and RSD values better than 0.9990 and 2.8%, respectively.

Mesoporous silicate MCM-48 as an enrichment medium for ambient volatile organic compound analysis.

This study investigated the sorption/desorption properties of MCM-48 and its applicability as a sorbent for on-line gas chromatographic analysis of ambient volatile organic compounds (VOCs). To establish a valid comparison, commercially available carbon sorbents were evaluated under similar analytical conditions. Two trapping temperatures of 30 degrees C and -20 degrees C, representing ambient and sub-ambient temperatures, were tested by trapping a full range of VOCs from C(2)-C(12). At ambient temperatures, due to the mesoporosity, the MCM-48 showed considerably limited trapping efficiency compared to microporous carbon sorbents on the highly volatile section of VOCs and only began to show effective trapping for compounds larger than C(7). Cooling to sub-ambient temperatures (e.g., -20 degrees C) extended the effective trapping down to C(4) VOCs, drastically increasing the applicability of MCM-48 as an in-line enrichment medium for gas chromatographic analysis of VOCs. The mesoporosity of MCM-48 also aided desorption. Much lower desorption temperatures (100-180 degrees C) were required for full desorption as compared to the temperatures (greater than 200 degrees C) required for carbon sorbents. Moreover, the easy desorption was accompanied by a low memory effect, as the large pores of MCM-48 can clean up more efficiently after desorption, with little residue left behind.

Construction of an automated gas chromatography/mass spectrometry system for the analysis of ambient volatile organic compounds with on-line internal standard calibration.

An automated sampling and enrichment apparatus coupled with a gas chromatography/mass spectrometry (GC/MS) technique was constructed for the analysis of ambient volatile organic compounds (VOCs). A sorbent trap was built within the system to perform on-line enrichment and thermal desorption of VOCs onto GC/MS. In order to improve analytical precision, calibration accuracy, and to safe-guard the long-term stability of this system, a mechanism to allow on-line internal standard (I.S.) addition to the air sample stream was configured within the sampling and enrichment apparatus. A sub-ppm (v/v) level standard gas mixture containing 1,4-fluorobenzene, chloropentafluorobenzene, 1-bromo-4-fluorobenzene was prepared from their pure forms. A minute amount of this I.S. gas was volumetrically mixed into the sample stream at the time of on-line enrichment of the air sample to compensate for measurement uncertainties. To assess the performance of this VOC GC/MS system, a gas mixture containing numerous VOCs at sub-ppb (v/v) level served as the ambient air sample. Various internal standard methods based on total ion count (TIC) and selective ion monitoring (SIM) modes were attempted to assess the improvement in analytical precision and accuracy. Precision was improved from 7-8% RSD without I.S. to 2-3% with I.S. for the 14 target VOCs. Uncertainties in the calibration curves were also improved with the adoption of I.S. by reducing the relative standard deviation of the slope (Sm%) by an average a factor of 4, and intercept (Sb%) by a factor of 2 for the 14 target VOCs.