241 resultados para low energy Fe ion beams
Resumo:
Methyl radicals are generated by pyrolysis of azomethane, and the condition for achieving neat adsorption on Cu(110) is described for studying their chemisorption and reaction characteristics. The radical-surface system is examined by X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, temperature-programmed desorption, low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy under ultrahigh vacuum conditions. It is observed that a small fraction of impinging CH3 radicals decompose into methylene possibly on surface defect sites. This type of CH2 radical has no apparent effect on CH3(ads) surface chemistry initiated by dehydrogenation to form active CH2(ads) followed by chain reactions to yield high-mass alkyl products. All thermal desorption products, such as H-2, CH4, C2H4, C2H6, and C3H6, are detected with a single desorption peak near 475 K. The product yields increase with surface coverage until saturation corresponding to 0.50 monolayer of CH3(ads). The mass distribution is, however, invariant with initial CH3(ads) coverage, and all desorbed species exhibit first-order reaction kinetics. LEED measurement reveals a c(2 x 2) adsorbate structure independent of the amount of gaseous exposure. This strongly suggests that the radicals aggregate into close-packed two-dimensional islands at any exposure. The islanding behavior can be correlated with the reaction kinetics and is deemed to be essential for the chain propagation reactions. Some relevant aspects of the CH3/Cu(111) system are also presented. The new results are compared with those of prior studies employing methyl halides as radical sources. Major differences are found in the product distribution and desorption kinetics, and these are attributed to the influence of surface halogen atoms present in those earlier investigations.
Resumo:
Changes in statistics (mean, sorting, and skewness) describing grain-size distributions have long been used to speculate on the direction of sediment transport. We present a simple model whereby the distributions of sediment in transport are related to their source by a sediment transfer function which defines the relative probability that a grain within each particular class interval will be eroded and transported. A variety of empirically derived transfer functions exhibit negatively skewed distributions (on a phi scale). Thus, when a sediment is being eroded, the probability of any grain going into transport increases with diminishing grain size throughout more than half of its size range. This causes the sediment in transport to be finer and more negatively skewed than its source, whereas the remaining sediment (a lag) must become relatively coarser and more positively skewed. Flume experiments show that the distributions of transfer functions change from having a highly negative skewness to being nearly symmetrical (although still negatively skewed) as the energy of the transporting process increases. We call the two extremes low-energy and high-energy transfer functions , respectively. In an expanded sediment-transport model, successive deposits in the direction of transport are related by a combination of two transfer functions. If energy is decreasing and the transfer functions have low-energy distributions, successive deposits will become finer and more negatively skewed. If, however, energy is decreasing, but the initial transfer function has a high-energy distribution, successive deposits will become coarser and more positively skewed. The variance of the distributions of lags, sediment in transport, and successive deposits in the down-current direction must eventually decrease (i.e., the sediments will become better sorted). We demonstrate that it is possible for variance first to increase, but suggest that, in reality, an increasing variance in the direction of transport will seldom be observed, particularly when grain-size distributions are described in phi units. This model describing changes in sediment distributions was tested in a variety of environments where the transport direction was known. The results indicate that the model has real-world validity and can provide a method to predict the directions of sediment transport
Resumo:
In this article, a two-dimensional (2D) nanoplate and a 3D hierarchical structure of BiOCl were synthesized through a simple sonochemical route. Compared with previous preparation methods, the 2D nanoplates can be prepared at a relatively short time (about 30 min) with low energy used. Additionally, these 2D nanoplates can easily assemble into a 3D hierarchical structure with the surfactant reagents. The obtained products were well crystallized and subsequently characterized by a range of methods, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM), selected area electron diffraction (SAED) and Raman spectroscopy.
Resumo:
A series of donor-acceptor low-bandgap conjugated polymers, i.e., PTnBT (n = 2-6), composed of alternating oligothiophene (OTh) and 2,1,3-benzothiadiazole (BT) units were synthesized by Stille cross-coupling polymerization. The number of thiophene rings in OTh units, that is n, was tuned from 2 to 6. All these polymers display two absorption bands in both solutions and films with absorption maxima depending on n. From solution to film, absorption spectra of the polymers exhibit a noticeable red shift. Both high- and low-energy absorption bands or P'F5BT and PT6BT films locate in the visible region, which are at 468 and 662 nm for PT5BT and 494 and 657 nm for PT6BT.
Resumo:
By incorporating a new building block, 7,7,15,15-tetraoctyldinaphtho-s-indacene (NSI), into the backbone of poly(9,9-dioctylfluorene) (PFO), a novel series of blue light-emitting copolymers (PFO-NSI) have been developed. The insertion of the NSI unit into the PFO backbone leads to the increase of local effective conjugation length, to form low-energy fluorene-NSI-fluorene (FNF) segments that serve as exciton trapping sites, to which the energy transfers from the high-energy PFO segments. This causes these copolymers to show red-shifted emissions compared with PFO, with a high efficiency and good color stability and purity. The best device performance with a luminance efficiency of 3.43 cd . A(-1), a maximum brightness of 6 539 cd . m(-2) and CIE coordinates of (0.152, 0.164) was achieved.
Resumo:
We report a heteroleptic ruthenium complex (007) featuring the electron-rich 5-octyl-2,2'-bis(3,4-ethylenedioxythiophene) moiety conjugated with 2,2-bipyridine and exhibiting 10.7% power conversion efficiency measured at the AM1.5G conditions, thanks to the enhanced light-harvesting that is closely related to photocurrent. This C107 sensitizer has an extremely high molar extinction coefficient,of 27.4 x 10(3) M-1 cm(-1) at 559 nm in comparison to its analogue C103 (20.5 x 10(3) M-1 cm(-1) at 550 nm) or Z907 (12.2 x 10(3) M(-1)cm(-1) at 521 nm) with the corresponding 5-hexyl-3,4-ethylenedioxythiopliene- or nonyl-substituted bipyridyl unit. The augmentation of molar extinction coefficients and the bathochromic shift of low-energy absorption peaks along with the pi-conjugation extension are detailed by TD-DFT calculations. The absorptivity of mesoporous titania films grafted with Z907, C103, or C107 sublinearly increases with the molar extinction coefficient of sensitizers, which is consistent with the finding derived from the surface coverage measurements that the packing density of those sensitizers decreases with the geometric enlargement of ancillary ligands.
Resumo:
Polyfluorene (PF) is a class of typical blue electroluminescent (EL) material, but it exhibits undesired feature in the green spectral region under operation condition. We investigated the spectral properties of different device structures of poly(9,9-dioctylfluorene) (PFO)-based light-emitting diodes, and found that the interaction between cathode and PFO is the main origination of green emission in EL devices. The general method of inserting a buffer layer between the PFO and cathode can decrease the low energy band emission to purify the color and improve the EL performance of devices.
Resumo:
The first and second generation carbosilane dendrimers with silicon hydride terminated were synthesized, and then reacted with bis(imino)pyridyl containing allyl [4-CH2==CHCH2-2,6-(Pr2C6H3N)-Pr-i==CMe(C5H3N)MeC==N(2,6-'Pr2C6H3)], in the presence of H2PtCl6 as a hydrosilylation catalyst, to afford the first and second generation carbosilane supported ligands. Complexation reactions with FeCl(2)(.)4H(2)O give rise to iron-containing carbosilane dendrimers with FeCl2 moieties bound on the periphery. The metallodendrimers were used as catalyst precursors, activated with modified methylaluminoxane, for the polymerization of ethylene. In the case of low Al/Fe molar ratio, the metallodendrimers display much higher catalytic activity towards ethylene polymerization and produce much higher molecule weight polyethylenes than the corresponding single-nuclear complex under the same conditions.
Resumo:
ZnO and ZnO: Zn powder phosphors were prepared by the polyol-method followed by annealing in air and reducing gas, respectively. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectra (XPS), electron paramagnetic resonance (EPR), and photoluminescence (PL) and cathodoluminescence ( CL) spectra, respectively. The results indicate that all samples are in agreement with the hexagonal structure of the ZnO phase and the particle sizes are in the range of 1-2 mu m. The PL and CL spectra of ZnO powders annealed at 950 degrees C in air consist of a weak ultraviolet emission band ( similar to 390 nm) and a broad emission band centered at about 527 nm, exhibiting yellow emission color to the naked eyes. When the sample was reduced at the temperatures from 500 to 1050 degrees C, the yellow emission decreased gradually and disappeared completely at 800 degrees C, whereas the ultraviolet emission band became the strongest. Above this temperature, the green emission ( similar to 500 nm) appeared and increased with increasing of reducing temperatures.
Resumo:
Four new iridium(III) complexes 1-4, with 1,3,4-oxadiazole derivative as cyclometalated ligand for the first time, have been synthesized and structurally characterized by NMR, EA, MS and X-ray diffraction analysis (except 1). The stronger ligand field strength of the dithiolate ancillary ligands results in higher oxidation potentials and lower HOMO energy levels of complexes than acetylacetone. The absorption spectra of these complexes display low-energy metal-to-ligand charge transfer transition ranging from 350 to 500 nm. Complexes with dithiolate ancillary ligand emit at maximum wavelengths of ca. 500 nm, blue shifting 17 and 11 nm with respect to their counterpart with acetylacetone ligand. The electrophosphorescent devices with 2-4 as phosphorescent dopant in emitting layer have been fabricated. All devices have a low turn-on voltage in the range of 4.5 and 4.9 V. A high-efficiency green emission with maximum luminous efficiency of 5.28 cd/A at current density of 1.37 mA/cm(2) and a maximum brightness of 2592 cd/m(2) at 15.2 V has been achieved in device using 2 as emitter.
Resumo:
The fragmentation mechanism of aconitine-type alkaloids in the flowers of Aconitum kusnezoffii (FAK) was investigated using electrospray ionization tandem mass spectrometry (ESI-MSn) firstly. The analysis of the collision-induced dissociation (CID) spectra of three purified aconitine standards and six previously reported aconitines indicated that the fragmentation of the protonated aconitines at low-energy CID follows a similar pathway. The elimination of a C-8-substituent such as an acetic acid or a fatty acid is the dominant fragmentation mode in MS2. Successive losses of CH3COOH, CH3OH, H2O, BzOH, and CO are the main fragmentation pathways of aconitine-type alkaloids in MS3 spectra. Based on these features, a rapid method for the direct detection and characterization of alkaloids from an ethanolic extract of FAK is described. All the known aconitum alkaloids are detected and a series of lipo-aconitines has been found for the first time in this plant.
Resumo:
Procedures that allow the realization of resonance electron capture (REC) mode on a commercial triple-quadrupole mass spectrometer, after some simple modifications, are described, REC mass spectrometry (MS) and tandem mass spectrometry (MS/MS) experiments were performed and spectra for some compounds were recorded. In particular, the charge-remote fragmentation (CRF) spectra of [M - H](-) ions of docosanoic and docosenoic acids under low-energy collisionally activated dissociation (CAD) conditions were obtained, and showed that there were no significant differences for [M - H](-) ions produced at different resonances (i,e. for [M - H](-) ions with different structures). This observation was explained on the basis of results obtained from deuterium-labeled fatty acids, which showed that different CRF ions (but with the same m/z value in the absence of labels) could be produced by different mechanisms, and all of them were obviously realized under CAD conditions that made spectra practically indistinguishable. The other example, which compared the REC-MS/MS spectrum of [M - H](-) ions and EI-MS/MS spectrum of M+. ions of daidzein, demonstrated the potential of the REC-MS/MS technique for more complex structure elucidation. Copyright (C) 2000 John Wiley & Sons, Ltd.
Resumo:
Based on analyses of more than 600 surface sediment samples together with large amounts of previous sedimentologic and hydrologic data, the characteristics of modern sedimentary environments and dynamic depositional systems in the southern Yellow Sea (SYS) are expounded, and the controversial formation mechanism of muddy sediments is also discussed. The southern Yellow Sea shelf can be divided into low-energy sedimentary environment and high-energy sedimentary environment; the low-energy sedimentary environment can be further divided into cyclonic and anticyclonic ones, and the high-energy environment is subdivided into high-energy depositional and eroded environments. In the shelf low-energy environments, there developed muddy depositional system. In the central part of the southern Yellow Sea, there deposited the cold eddy sediments under the actions of a meso-scale cyclonic eddy (cold eddy), and in the southeast of the southern Yellow Sea, an anticyclonic eddy muddy depositional system (warm eddy sediment) was formed. These two types of sediments showed evident differences in grain size, sedimentation rate, sediment thickness and mineralogical characteristics. The high-energy environments were covered with sandy sediments on seabed; they appeared mainly in the west, south and northeast of the southern Yellow Sea. In the high-energy eroded environment, large amounts of sandstone gravels were distributed on seabed. In the high-energy depositional environment, the originally deposited fine materials (including clay and fine silt) were gradually re-suspended and then transported to a low-energy area to deposit again. In this paper, the sedimentation model of cyclonic and anticyclonic types of muddy sediments is established, and a systematic interpretation for the formation cause of muddy depositional systems in the southern Yellow Sea is given.
Resumo:
The Nihewan Basin is a key area for research into human occupation at high northern latitudes in northeast Asia after the initial expansion of early humans out of Africa. Well-developed late Cenozoic lacustrine deposit sequence in this basin offers a unique opportunity to address this issue. In this thesis, detailed magnetostratigraphic investigation coupled with mineral magnetism was conducted on the Donggutuo and Cenjiawan sections in the eastern basin, where lacustrine deposits sequences containing the Donggutuo, Maliang and Cenjiawan Paleolithic sites are well developed. The sequences are mainly composed of grayish-white clays, grayish-green clayey silts, grayish-yellow silts and fine-grained brown sands, which have recorded reliable polarity variations of geomagnetic field.Characteristics of the anisotropy of magnetic susceptibility show that the sediments have preserved typical original magnetic fabric for sediments, indicating that the strata were developed in a low-energy lake environment and were never perturbed by tectonic stress since deposition. High-temperature magnetic susceptibility measurements (x-T) of representative specimens and demagnetization experiments indicate that the dominant magnetic mineral and remanent carrier at the two sections is magnetite. In addition, hematite and possibly minor maghemite exists in some portions of the sequences. The majority of the samples have relatively simple demagnetization behaviors. After removal of soft magnetic components, the stable characteristic remanent magnetizations (ChRMs) are isolated, which can represent the original remanences.The Donggutuo section mainly records the Brunhes chron, the Matuyama chron and the Jaramillo subchron. The Maliang stone artifact layer occurs just below the Brunhes/Matuyama boundary; and the Donggutuo artifact layer, just below the Jaramillo onset. Accordingly, the Maliang and Donggutuo sites can be dated at about 0.78 Ma and 1.1 Ma, respectively. The Cenjiawan section has recorded a portion of the Matuyama chron. After correlations with the magnetic polarity sequence of the Majuangou section adjacent to this section, the Cenjiawan stone artifact layer is determined below the Jaramillo onset, with an estimated age of 1.1 Ma.To establish the magnetic stratigraphy framework for the lacustrine sediments in the eastern Nihewan Basin, this thesis draws on the magnetic polarity sequences of the Donggutuo and Cenjiawan sections as well as previously obtained results from the Majuangou, Haojiatai, Xiaochangliang and Donggou sections for magnetostratigraphic correlations. The accumulation of the lacustrine sequences at the east margin of the basin commenced from about 2.0 Ma. These sequences record not only the coarse magnetostratigraphy of the Brunhes normal chron and the middle to late Matuyama reverse chron (that is, the Jaramillo and Olduvai subchrons) but also some of the fine structure (that is, the Kamikatsura, Santa Rosa, Punaruu and Cobb Mountain geomagnetic events). The development of the Nihewan paleolake experienced at least twice large expansion periods, split by a large-scale shrinking event in the middle period of the paleolake development. The accumulation of the lacustrine strata was controlled by fault activities.After temporal control for the Donggutuo, Maliang and Cenjiawan Paleolithic sites were established, the three sites along with other well-dated Paleolithic/hominin sites of the Early Pleistocene in North China were combined to construct a chronological sequence of early human occupation in northeastern Asia. Furthermore, after incorporation of paleoclimate changes retrieved from Chinese loess/paleosol sequences and marine sediments, it could be possibly proposed that human groups of the Early Pleistocene in North China might have survived repeated warm/humid interglacials and cold/dry glacials, which were paced by earth orbital variations of the Eastern paleomonsoon.
