bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and characterization of 2,2‐bis(methylol)propionic acid dendrimers with different cores and terminal groups
Authors: Malkoch Michael , Claesson Hans , Löwenhielm Peter , Malmström Eva , Hult Anders
Published: February 2004

Three sets of aliphatic polyester dendrimers based on 2,2‐bis(methylol)propionic acid (bis‐MPA) were synthesized. Two of the sets had benzylidene terminal groups and either a trimethylolpropane or triphenolic core moiety. The last set had acetonide terminal groups and a triphenolic core moiety. Benzylidene‐[G#1]‐anhydride and acetonide‐[G#1]‐anhydride were used as the reactive building blocks in the construction of all dendrimers. The large excess of building blocks used in the coupling reactions initially resulted in considerable material loss. This waste was eliminated through the development of a recycling method. 1H and 13C NMR and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) analysis were used to verify the purity of all compounds. Size exclusion chromatography (SEC) was used, as well as MALDI‐TOF, for molecular weight determinations. The SEC measurements were conducted with a universal calibration method and an online right‐angle laser light scattering detector. Measured dendrimer molecular weights were close to their theoretical molar masses. Observations were also made of the hydrodynamic radius and intrinsic viscosity for the different dendrimers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1758–1767, 2004
Synthesis and optical properties of novel blue‐light‐emitting poly(p‐phenylene vinylene) derivatives with pendant oxadiazole or cyano groups
Authors: Mikroyannidis John A. , Spiliopoulos Ioakim K.
Published: February 2004

Two novel poly(p‐phenylene vinylene) polymers, which carried side substituents with cyano groups or 1,3,4‐oxadiazole, were synthesized by Heck coupling. They consisted of alternating conjugated segments and nonconjugated aliphatic spacers. The polymers had moderate molecular weights, were amorphous, and dissolved readily in tetrahydrofuran and halogenated organic solvents. They were stable up to approximately 340 °C in N2 and 290 °C in air, and the anaerobic char yield was around 60% at 800 °C. The polymer with cyano side groups emitted blue light in solutions and thin films with identical photoluminescence (PL) maximum at 450 nm; this supported the idea that chain interactions were hindered even in the solid state. The PL maximum of this polymer in thin films was blueshifted upon annealing at 120 °C, indicating a thermochromic effect as a result of conformational changes in the polymer backbone. The polymer containing side substituents with oxadiazole rings emitted blue light in solutions with a PL maximum at 474 nm and blue‐greenish light in thin films with a PL maximum at 511 nm. The PL quantum yields of the polymers in tetrahydrofuran were 0.13–0.24. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1768–1778, 2004
Preparation and properties of silylated PTFE/SiO2 organic–inorganic hybrids via sol–gel process
Authors: Chen Yung‐Chih , Tsai Chi‐Che , Lee Yu‐Der
Published: February 2004

A study on poly(tetrafluoroethylene) (PTFE) reinforced with tetraethoxysilanes (TEOS) derived SiO2 is described. It included the manufacturing process of SiO2‐reinforced PTFE and the effects of silylation agent on the properties of the hybrid material, such as porosity, hydrophobic, thermal resistance, dielectric and mechanical properties, and microstructure. PTFE/SiO2 hybrids of 50 wt % SiO2 loading were prepared via a sol–gel process and were shaped by a two‐roll milling machine. Trimethylchlorosilane and hexamethydisilazane were used as the silylation agents. Our results showed that the water absorption and dielectric loss of PTFE/SiO2 hybrid had significantly improved with silylation agent. The silylation process replaced SiOH with SiCH3 on the surface of the TEOS‐derived silica colloidal particle. The existence of trimethylsilyl [Si(CH3)3] on the surface of the modified PTFE/SiO2 hybrid was confirmed via infrared and solid‐state 29Si magic‐angle spinning nuclear magnetic resonance spectra. Nitrogen‐sorption techniques were used to characterize the modified and unmodified PTFE/SiO2 hybrids. The microstructure of SiO2 in the matrix was also evaluated with scanning electron microscopy and transmission electron microscopy. Our results showed that the silylated sol–gel‐derived PTFE/SiO2 hybrids had exhibited high porosity (53.7%) with nanosize pores (10–40 nm) and nanosize colloidal particles (20–50 nm). This manifests itself as have the ultralow dielectric properties (Dk = 1.9 and Df = 0.0021), low coefficient of thermal expansion (66.5 ppm/°C), high tensile modulus (141 MPa), excellent thermal resistance (Td = 612 °C), and an increased hydrophobia (θ = 114°); moreover, the hydrophobic property of the PTFE/SiO2 hybrid was thermally stable up to 400 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1789–1807, 2004
Michael Polanyi and the discovery of co‐catalysis: Discussion of an autobiographical letter from Michael Polanyi, FRS to Peter H. Plesch of 17 December 1963
Authors: Plesch P. H.
Published: February 2004

The origin of this memoir was a letter from Michael Polanyi (M. P.) to the present writer (P. H. P.) about their researches in the mid‐1940s into the mechanism of what are now called cationic polymerizations, at the University of Manchester (England). M. P. analyzes his tactics and the mistakes made in directing this research.

When the Manchester‐trained researchers made little progress with what was a very recalcitrant problem, M. P. thinking that scientists from a different background might be more sucessful, got P. H. P., from Cambridge, to work with an Oxford‐trained chemist. They recognized that the likely cause of the irreproducibility of these polymerizations was the apparatus used which permitted access of atmospheric moisture to the reaction mixtures containing the moisture‐sensitive catalytic metal halides. Because the only method for following the very fast polymerizations was by monitoring the accompanying temperature rise, and the reactions had to be done below ambient temperature, the reaction vessel needed to be adiabatic, that is a Dewar (Thermos) flask; hence the problem of how to cool its contents. The solution was P. H. P.'s invention of the pseudo‐Dewar vessel, the Dewar space of which, instead of being evacuated permanently, could be filled with air or evacuated. This device permitted the reaction mixture to be made up and cooled, and the reactions to be started without contact with the atmosphere. Thus it was found that isobutene polymerizations, which had stopped unaccountably, could be restarted by water vapor. P. H. P. termed water a “co‐catalyst”. The consequent “Manchester” theory recognized the monohydrate of TiCl4 as a protonic acid and saw the initiation as due to the protonation of the monomer, with the formation of a tert‐carbenium ion, and these ions, formed repetitively, became the propagating species.

The Manchester theory was rapidly accepted because it could also explain observations on other related reactions. The involvement of ions established a link with non‐aqueous electrochemistry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1537–1546, 2004
Modeling hydrology, metribuzin degradation and metribuzin transport in macroporous tilled and no‐till silt loam soil using RZWQM
Authors: Malone Robert W , Ma Liwang , Don Wauchope R , Ahuja Lajpat R , Rojas Kenneth W , Ma Qingli , Warner Richard , Byers Matt
Published: February 2004

Due to the complex nature of pesticide transport, process‐based models can be difficult to use. For example, pesticide transport can be effected by macropore flow, and can be further complicated by sorption, desorption and degradation occurring at different rates in different soil compartments. We have used the Root Zone Water Quality Model (RZWQM) to investigate these phenomena with field data that included two management conditions (till and no‐till) and metribuzin concentrations in percolate, runoff and soil. Metribuzin degradation and transport were simulated using three pesticide sorption models available in RZWQM: (a) instantaneous equilibrium‐only (EO); (b) equilibrium‐kinetic (EK, includes sites with slow desorption and no degradation); (c) equilibrium‐bound (EB, includes irreversibly bound sites with relatively slow degradation). Site‐specific RZWQM input included water retention curves from four soil depths, saturated hydraulic conductivity from four soil depths and the metribuzin partition coefficient. The calibrated parameters were macropore radius, surface crust saturated hydraulic conductivity, kinetic parameters, irreversible binding parameters and metribuzin half‐life. The results indicate that (1) simulated metribuzin persistence was more accurate using the EK (root mean square error, RMSE = 0.03 kg ha−1) and EB (RMSE = 0.03 kg ha−1) sorption models compared to the EO (RMSE = 0.08 kg ha−1) model because of slowing metribuzin degradation rate with time and (2) simulating macropore flow resulted in prediction of metribuzin transport in percolate over the simulation period within a factor of two of that observed using all three pesticide sorption models. Moreover, little difference in simulated daily transport was observed between the three pesticide sorption models, except that the EB model substantially under‐predicted metribuzin transport in runoff and percolate >30 days after application when transported concentrations were relatively low. This suggests that when macropore flow and hydrology are accurately simulated, metribuzin transport in the field may be adequately simulated using a relatively simple, equilibrium‐only pesticide model. Published in 2004 for SCI by John Wiley & Sons, Ltd.
Metabolism of imidacloprid in Apis mellifera
Authors: Suchail Séverine , Debrauwer Laurent , Belzunces Luc P
Published: February 2004

Biotransformation of imidacloprid and the appearance of olefin and 5‐hydroxyimidacloprid metabolites in the honeybee were studied by HPLC‐MS/MS analysis. Honeybees were treated orally with imidacloprid at 20 and 50 µg kg−1 bee. Imidacloprid was metabolised relatively quickly and thoroughly. Twenty minutes after the beginning of imidacloprid ingestion, the sum of the residues from the three compounds amounted to only 70% of the actual given dose. Imidacloprid, 5‐hydroxyimidacloprid and olefin represented, respectively, 50%, 9% and 8% of the actual ingested dose. Six and 24 h, respectively, after ingestion of imidacloprid at 20 and 50 µg kg−1 bee, imidacloprid could no longer be detected in the honeybee. Imidacloprid had a half‐life ranging between 4.5 and 5 h and was rapidly metabolised into 5‐hydroxyimidacloprid and olefin. Except 5‐hydroxyimidacloprid in the 20 µg kg−1 treatment, these two metabolites presented a peak value 4 h after ingestion of the 20 and 50 µg kg−1 doses. This time fully coincided with the appearance of mortality induced by imidacloprid after acute oral intoxication. These results suggested that the immediate neurotoxicity symptoms are due to the action of imidacloprid, whereas 5‐hydroxyimidacloprid and/or olefin are involved in honeybee mortality. In addition, it was likely that the 30% of residues undetected 20 min after intoxication were imidacloprid metabolites, although not 5‐hydroxyimidacloprid or olefin. Thus, 5‐hydroxyimidacloprid and olefin could not be the major metabolites in the worker bees. Copyright © 2003 Society of Chemical Industry
Sorption of aged dicamba residues in soil
Authors: Menasseri Safya , Koskinen William C , Yen Pau Yong
Published: February 2004

The effect of aging (residence time in soil) on dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) and a major metabolite, 3,6‐dichlorosalicylic acid (3,6‐DCSA) sorption was determined in an unamended and a carbon‐amended sandy loam and in a silt loam soil. During the incubation, sequential solvent extraction with 0.01 M calcium chloride solution and aqueous acetonitrile + hydrochloric acid was used to determine the solution and sorbed concentrations of dicamba and 3,6‐DSCA, and sorption coefficients were calculated. Dicamba was weakly sorbed to soil (Kd <0.7). In contrast to some other classes of pesticides, sorption of dicamba did not significantly increase with aging, at least not until <15% of the applied dicamba remained. 3,6‐DSCA was strongly sorbed to soil (Kd >8) and the Kd‐a value increased by a factor of 2–6 during a 28‐day aging period. Addition of a carbon source to the soil had minimal effect on the strength of sorption of aged dicamba. However, it did appear to decrease 3,6‐DSCA availability to soil micro‐organisms; once formed 3,6‐DSCA was not further mineralized. While it appears that sorption can be well characterized for weakly sorbed pesticides using the batch equilibration method with freshly treated soils, this procedure may not be adequate for more strongly sorbed pesticides and their degradates. Copyright © 2003 Society of Chemical Industry
Effectiveness of twelve insecticides applied topically to diapausing larvae of the codling moth, Cydia pomonella L
Authors: Pasquier Denis , Charmillot Pierre‐Joseph
Published: February 2004

Dose–mortality curves were established for 12 insecticides administered by topical application to diapausing larvae from a susceptible codling moth strain. Toxicity varied greatly among the insecticides tested. LC50 values ranged from 0.1 mg kg−1 for fenoxycarb to over 2800 mg kg−1 for diflubenzuron and indoxacarb. Discriminating dose levels were determined from dose‐mortality reference curves for the detection of resistance in field‐collected diapausing larvae. Copyright © 2003 Society of Chemical Industry
Application of the Root Zone Water Quality Model (RZWQM) to pesticide fate and transport: an overview
Authors: Malone Robert W , Ahuja Lajpat R , Ma Liwang , Don Wauchope R , Ma Qingli , Rojas Kenneth W
Published: February 2004

Pesticide transport models are tools used to develop improved pesticide management strategies, study pesticide processes under different conditions (management, soils, climates, etc) and illuminate aspects of a system in need of more field or laboratory study. This paper briefly overviews RZWQM history and distinguishing features, overviews key RZWQM components and reviews RZWQM validation studies. RZWQM is a physically based agricultural systems model that includes sub‐models to simulate: infiltration, runoff, water distribution and chemical movement in the soil; macropore flow and chemical movement through macropores; evapotranspiration (ET); heat transport; plant growth; organic matter/nitrogen cycling; pesticide processes; chemical transfer to runoff; and the effect of agricultural management practices on these processes. Research to date shows that if key input parameters are calibrated, RZWQM can adequately simulate the processes involved with pesticide transport (ET, soil‐water content, percolation and runoff, plant growth and pesticide fate). A review of the validation studies revealed that (1) accurate parameterization of restricting soil layers (low permeability horizons) may improve simulated soil‐water content; (2) simulating pesticide sorption kinetics may improve simulated soil pesticide concentration with time (persistence) and depth and (3) calibrating the pesticide half‐life is generally necessary for accurate pesticide persistence simulations. This overview/review provides insight into the processes involved with the RZWQM pesticide component and helps identify model weaknesses, model strengths and successful modeling strategies. Published in 2004 for SCI by John Wiley & Sons, Ltd.
The pesticide module of the Root Zone Water Quality Model (RZWQM): testing and sensitivity analysis of selected algorithms for pesticide fate and surface runoff
Authors: Ma Qingli , Wauchope R Don , Rojas Kenneth W , Ahuja Lajpat R , Ma Liwang , Malone Robert W
Published: February 2004

The Root Zone Water Quality Model (RZWQM) is a one‐dimensional, numerical model for simulating water movement and chemical transport under a variety of management and weather scenarios at the field scale. The pesticide module of RZWQM includes detailed algorithms that describe the complex interactions between pesticides and the environment. We have simulated a range of situations with RZWQM, including foliar interception and washoff of a multiply applied insecticide (chlorpyrifos) to growing corn, and herbicides (alachlor, atrazine, flumetsulam) with pH‐dependent soil sorption, to examine whether the model appears to generate reasonable results. The model was also tested using chlorpyrifos and flumetsulam for the sensitivity of its predictions of chemical fate and water and pesticide runoff to various input parameters. The model appears to generate reasonable representations of the fate and partitioning of surface‐ and foliar‐applied chemicals, and the sorption of weakly acidic or basic pesticides, processes that are becoming increasingly important for describing adequately the environmental behavior of newer pesticides. However, the kinetic sorption algorithms for charged pesticides appear to be faulty. Of the 29 parameters and variables analyzed, chlorpyrifos half‐life, the Freundlich adsorption exponent, the fraction of kinetic sorption sites, air temperature, soil bulk density, soil‐water content at 33 kPa suction head and rainfall were most sensitive for predictions of chlorpyrifos residues in soil. The latter three inputs and the saturated hydraulic conductivity of the soil and surface crusts were most sensitive for predictions of surface water runoff and water‐phase loss of chlorpyrifos. In addition, predictions of flumetsulam (a weak acid) runoff and dynamics in soil were sensitive to the Freundlich equilibrium adsorption constant, soil pH and its dissociation coefficient. Published in 2004 for SCI by John Wiley & Sons, Ltd.
Herbicide leaching as affected by macropore flow and within‐storm rainfall intensity variation: a RZWQM simulation
Authors: Malone Robert W , Weatherington‐Rice Julie , Shipitalo Martin J , Fausey Norman , Ma Liwang , Ahuja Lajpat R , Don Wauchope R , Ma Qingli
Published: February 2004

Within‐event variability in rainfall intensity may affect pesticide leaching rates in soil, but most laboratory studies of pesticide leaching use a rainfall simulator operating at constant rainfall intensity, or cover the soil with ponded water. This is especially true in experiments where macropores are present—macroporous soils present experimental complexities enough without the added complexity of variable rainfall intensity. One way to get around this difficulty is to use a suitable pesticide transport model, calibrate it to describe accurately a fixed‐intensity experiment, and then explore the affects of within‐event rainfall intensity variation on pesticide leaching through macropores. We used the Root Zone Water Quality Model (RZWQM) to investigate the effect of variable rainfall intensity on alachlor and atrazine transport through macropores. Data were used from an experiment in which atrazine and alachlor were surface‐applied to 30 × 30 × 30 cm undisturbed blocks of two macroporous silt loam soils from glacial till regions. One hour later the blocks were subjected to 30‐mm simulated rain with constant intensity for 0.5 h. Percolate was collected and analyzed from 64 square cells at the base of the blocks. RZWQM was calibrated to describe accurately the atrazine and alachlor leaching data, and then a median Mid‐west variable‐intensity storm, in which the initial intensity was high, was simulated. The variable‐intensity storm more than quadrupled alachlor losses and almost doubled atrazine losses in one soil over the constant‐intensity storm of the same total depth. Also rainfall intensity may affect percolate‐producing macroporosity and consequently pesticide transport through macropores. For example, under variable rainfall intensity RZWQM predicted the alachlor concentration to be 2.7 µg ml−1 with an effective macroporosity of 2.2 E−4 cm3 cm−3 and 1.4 µg ml−1 with an effective macroporosity of 4.6 E−4 cm3 cm−3. Percolate‐producing macroporosity and herbicide leaching under different rainfall intensity patterns, however, are not well understood. Clearly, further investigation of rainfall intensity variation on pesticide leaching through macropores is needed. Published in 2004 for SCI by John Wiley & Sons, Ltd.
Photodegradation of bensulfuron‐methyl on soil surface
Authors: Si You‐Bin , Yue Yong‐De , Chen Huai‐Man , Zhou Dong‐Mei
Published: February 2004

Photolysis of bensulfuron‐methyl on soil surface was studied under sunlight and UV light. Seven photoproducts were isolated and characterised by spectroscopic methods. The major processes in the photolysis of bensulfuron‐methyl in soil are cleavage of the sulfonylurea bridge, scission of the SO2NH bond and contraction of the sulfuronylurea bridge. The rates of photodegradation of bensulfuron‐methyl on different soils followed first‐order rate kinetics with half lives of 21.9, 28.4, 36.9, 59.2 and 47.2 h (UV) and 23.1, 27.5, 29.1, 38.9 and 33.8 days (sunlight) for vertisol, alluvial, alfisol, red and laterite soils, respectively. The differences in rates of photodegradation were dependent upon the soil texture and organic matter content. Copyright © 2004 Society of Chemical Industry
Review: The strobilurin fungicides
Authors: Bartlett DW , Clough JM , Godwin JR , Hall AA , Hamer M , Parr‐Dobrzanski B
Published: February 2004

The original article to which this Correction refers was published in Pest Management Science 58(7): 649–662 (2002).Copyright © 2004 Society of Chemical Industry
Documenting the pesticide processes module of the ARS RZWQM agroecosystem model
Authors: Wauchope R Don , Rojas Kenneth W , Ahuja Lajpat R , Ma Qingli , Malone Robert W , Ma Liwang
Published: February 2004

We describe the theory and current development state of the pesticide process module of the USDA‐Agricultural Research Service Root Zone Water Quality Model, or RZWQM. Several processes which are significant in determining the fate of a pesticide application are included together in this module for the first time, including application technique, root uptake, ionic dissociation, soil depth dependence of persistence, volatilization, wicking upward in soil and aging of residues. The pesticide module requires a large number of parameters to run (as does the RZWQM model as a whole) and it is becoming clear that RZWQM will find most interest and use as part of a ‘scenario’ in which all data requirements are supplied and the predictions of the system compared with a real (usually partial) data set. Such a scenario may then be modified to examine the response of the system to changes in inputs. It also has significant potential as a technology transfer or teaching tool, providing detailed understanding of a specific agronomic system and its potential impacts on the environment. Published in 2004 for SCI by John Wiley & Sons, Ltd.
Test of the Root Zone Water Quality Model (RZWQM) for predicting runoff of atrazine, alachlor and fenamiphos species from conventional‐tillage corn mesoplots
Authors: Ma Qingli , Don Wauchope R , Ma Liwang , Rojas Kenneth W , Malone Robert W , Ahuja Lajpat R
Published: February 2004

The Root Zone Water Quality Model (RZWQM) is a comprehensive, integrated physical, biological and chemical process model that simulates plant growth and movement of water, nutrients and pesticides in a representative area of an agricultural system. We tested the ability of RZWQM to predict surface runoff losses of atrazine, alachlor, fenamiphos and two fenamiphos oxidative degradates against results from a 2‐year mesoplot rainfall simulation experiment. Model inputs included site‐specific soil properties and weather, but default values were used for most other parameters, including pesticide properties. No attempts were made to calibrate the model except for soil crust/seal hydraulic conductivity and an adjustment of pesticide persistence in near‐surface soil. Approximately 2.5 (±0.9), 3.0 (±0.8) and 0.3 (±0.2)% of the applied alachlor, atrazine and fenamiphos were lost in surface water runoff, respectively. Runoff losses in the ‘critical’ events—those occurring 24 h after pesticide application—were respectively 91 (±5), 86 (±6) and 96 (±3)% of total runoff losses for these pesticides. RZWQM adequately predicted runoff water volumes, giving a predicted/observed ratio of 1.2 (±0.5) for all events. Predicted pesticide concentrations and loads from the ‘critical’ events were generally within a factor of 2, but atrazine losses from these events were underestimated, which was probably a formulation effect, and fenamiphos losses were overestimated due to rapid oxidation. The ratios of predicted to measured pesticide concentrations in all runoff events varied between 0.2 and 147, with an average of 7. Large over‐predictions of pesticide runoff occurred in runoff events later in the season when both loads and concentrations were small. The normalized root mean square error for pesticide runoff concentration predictions varied between 42 and 122%, with an average of 84%. Pesticide runoff loads were predicted with a similar accuracy. These results indicate that the soil‐water mixing model used in RZWQM is a robust predictor of pesticide entrainment and runoff. Published in 2004 for SCI by John Wiley & Sons, Ltd.
High stability of CdZnSSe active layers in ZnSe‐based laser diodes by introducing strain‐compensating barrier layers
Authors: Gust Arne , Klude Matthias , Ueta Akio , Roventa Elena , Hommel Detlef
Published: February 2004

A series of four similar laser structures with an emission wavelength of 520 nm was grown by molecular beam epitaxy. The difference was the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25% neighboring the quantum well on the n‐, p‐ or n‐& p‐side. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating barrier layers. In lifetime measurements a large improvement was noticed by the p‐ and n‐ & p‐side barrier layers. With respect to the strain compensating effect of only one barrier layer, the p‐side layer is more effective than the n‐side one. It is assumed that the p‐side barrier layer acts not only as strain‐compensating layer, but also as blocking layer against the Cd diffusion into the p‐doped layers. A lifetime extension of one order of magnitude could be achieved. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Green monolithic II–VI vertical‐cavity surface‐emitting laser operating at room temperature
Authors: Kruse C. , Ulrich S. M. , Alexe G. , Roventa E. , Kröger R. , Brendemühl B. , Michler P. , Gutowski J. , Hommel D.
Published: February 2004

The realization of a monolithic all II–VI‐based vertical cavity surface emitting laser (VCSEL) for the green spectral region is reported. Optically pumped lasing operation was achieved up to room temperature using a planar VCSEL structure. Taking advantage of distributed Bragg‐reflectors based on MgS/Zn(Cd)Se superlattices as the low‐refractive index material and ZnS0.06Se0.94 layers as the high‐index material with a refractive index contrast of Δn = 0.6, a quality factor exceeding Q = 2000 is reached by using only 18 Bragg periods for the bottom DBR and 15 Bragg periods for the top DBR. The threshold power density is 0.32 MW/cm2 at a temperature of 10 K (emission wavelength 498.5 nm) and 1.9 MW/cm2 at room temperature (emission wavelength 502.3 nm). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Spatial breathing of the exciton distribution in ZnSe quantum wells
Authors: Dal Don B. , Zhao Hui , Schwartz G. , Kalt H.
Published: February 2004

We measured the transport dynamics of excitons in ZnSe quantum wells with help of a time‐resolved nanophotoluminescence setup. Right after picosecond excitation, the excitons move out of the laser spot, but then they reverse the direction of propagation, and finally spread out again. We attribute this “spatial breathing” of the exciton population to acoustic phonon emission, which is a predominantly backward scattering event. This permits us to detect the first inelastic scattering event after the fast excitation generation, causing the end of the coherent transport regime. Using this method, we can determine simultaneously the coherence time and length of excitons in an 8nm well to 29 ps and 800 nm, respectively. We can reproduce the results with help of a Monte‐Carlo simulation of the exciton dynamics in the well. The obtained values for coherence length and time are consistent with other, independent experiments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of linear and nonlinear optical spectra of various ZnO epitaxial layers and of bulk material obtained by different experimental techniques
Authors: Priller H. , Brückner J. , Gruber Th. , Klingshirn C. , Kalt H. , Waag A. , Ko H. J. , Yao T.
Published: February 2004

We investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton‐exciton‐scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth by MBE and characterization of metastable group II sulphides
Authors: Prior K. A. , Bradford C. , David L. , Tang X. , Cavenett B. C.
Published: February 2004

Many group II sulphides semiconductors have the rocksalt structure as their stable crystal structure. In the case of MgS and MnS it has been demonstrated that these compounds can be grown in the metastable zinc blende structure. Recently, at Heriot–Watt we have shown that a simple MBE growth procedure can be used to increase the thickness of these metastable layers to over 130 nm. In this paper, we summarise some of the results we have obtained and describe some of the structures which we have grown using these materials. The aim is to demonstrate that the growth method is simple, and that MgS in particular is a material with useful properties that can be incorporated into a variety of structures. Finally, we describe some potential uses of these compounds which we have not yet explored, and suggest other compounds which may be successfully grown with this method. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of heterostructures containing MnS grown by MBE
Authors: David L. , Tang X. , Beamson G. , Wolverson D. , Prior K. A. , Cavenett B. C.
Published: February 2004

MnS can be grown lattice matched to GaAs and ZnSe in the metastable zinc blende structure. Recently, layers of MnS over 100nm thick have been produced which are thick enough for structural characterisation. In this paper we report measurements obtained from X‐ray diffraction, Raman and X‐ray Photoelectron Spectroscopy. X‐ray diffraction has been used to obtain a value of Poisson's ratio of 0.475. This is unusually high and has implications for the stability of the material. Raman spectroscopy has been used to determine a value for the LO phonon frequency of 380 cm−1 which is in line with the value estimated from its reduced mass. XPS has been used to determine a value for the Valence Band offset. Unlike previous measurements on this compound we have found that the MnS growth must start with Mn, and not S. This presumably reflects the differences between our growth method and that used previously to grow zinc blende MnS. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MBE growth and optical properties of ZnO on GaAs(111) substrates
Authors: Matsumoto Takashi , Nishimura Kazuto , Nabetani Yoichi , Kato Takamasa
Published: February 2004

Hexagonal ZnO layers were grown by plasma assisted MBE on GaAs(111)B substrates. The substrate was chemically etched and no buffer layer was introduced. The epitaxial orientation relationships was ZnO(0001)//GaAs(111) and ZnO[11–20]//GaAs[01–1]. The layer–substrate interface was investigated by transmission electron microscope (TEM) observation, and 2–3 monolayer thick contrast band was found at the interface. The lattice constant c was smaller than that of bulk ZnO and changed systematically with the layer thickness. Low temperature photoluminescence (PL) spectra was dominated by a donor bund exciton band at 3.360 eV. Another bound exciton band was found around 3.330 eV. The relative intensity of the PL bands changed with growth conditions. PL peak energies became smaller as the lattice constant c became smaller. The lattice strain dependence of the exciton energy was derived.
MBE growth of ZnSSeO alloy using ZnS as a sulfur source
Authors: Nabetani Y. , Ito Y. , Mukawa T. , Okuno T. , Kato T. , Matsumoto T. , Hirai T.
Published: February 2004

We have grown quaternary alloy semiconductor ZnSSeO by molecular beam epitaxy. Large band gap bowing due to large electronegativity of O is expected in this alloy. The growth of ZnSSeO was proceeded by supplying RF‐excited oxygen during the growth of ZnSSe. The lattice constant of ZnSSeO decreased with increasing O2 flow rate. X‐ray diffraction peak was separated when O2 flow rate was high. However, controllable range of lattice constant without phase separation was expanded compared with ZnSeO ternary alloy. It was shown that S composition parasitically increased with O composition. We have explained the enhancement of S incorporation in terms of reduction of strain energy. The band gap energy decreased with increasing O2 flow rate regardless of phase separation. The amount of band gap bowing was slightly less than that found in ZnSeO. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Molecular beam epitaxy of MgxZn1−xO layers without wurzite‐rocksalt phase mixing from x = 0 to 1 as an effect of ZnO buffer layer
Authors: Fujita Shizuo , Takagi Tsuyoshi , Tanaka Hiroshi , Fujita Shigeo
Published: February 2004

MgxZn1−xO (0 ≤ x ≤ 1) layers without wurzite‐rocksalt phase mixing have been successfully grown by MBE on a‐plane sapphire substrates with ZnO buffer layers. The layers of x < 0.5 take wurzite (0001) structure eliminating the problem of wurzite‐rocksalt phase mixing reported previously between x = 0.35 and 0,45, while those of x > 0.5 rocksalt (001) with wurzite (0001) structure only at the initial stage of the growth (<25 nm). The rocksalt (001) rather than (111) which has been generally seen on sapphire without ZnO buffer layers can be explained by the model showing successive transition from wurzite (0001) to rocksalt (001) with the tensile and compressive deformation of the lattices along a‐ and b‐axes, respectively. The band gap energy of MgxZn1−xO has been continuously tuned without the problem of unstable phase mixing. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MBE growth and characterization of hexagonal ZnCdMgSe layers and ZnCdSe/ZnCdMgSe QW structures on GaAs (111) substrates
Authors: Suzuki S. , Kaifuchi Y. , Kumada H. , Ishitani Y. , Yoshikawa A.
Published: February 2004

ZnCdMgSe epilayers and ZnCdSe/ZnCdMgSe quantum well structures were grown on GaAs(111)A and B substrates by MBE using a low‐temperature‐grown CdS buffer layer. X‐ray reciprocal space maps demonstrated that ZnCdMgSe epilayers grown on CdS buffer layers were predominantly hexagonal structures. In the quantum well structures, in‐situ RHEED pattern also demonstrated the hexagonal structure during the growth. X‐ray diffraction results showed that the crystalline quality of the ZnCdMgSe epilayers grown on GaAs(111)A surface was much better than that on the (111)B surface. Low temperature photoluminescence of ZnCdMgSe epilayers were dominated by the deep and shallow donor–acceptor pair emissions for the case of GaAs(111)A and B, respectively. The peak energy positions of the photoluminescence spectra increased with increasing the growth temperature. Although strong emissions from ZnCdMgSe barriers were observed, emissions from the ZnCdSe QW structure were very weak. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Development of yellow‐green LEDs and LDs using MgZnCdSe‐BeZnTe superlattices on InP substrates by MBE
Authors: Che Song‐Bek , Nomura Ichirou , Kikuchi Akihiko , Kishino Katsumi
Published: February 2004

We have fabricated yellow‐green light emitting diodes (LEDs) and laser diodes (LDs) by using II–VI compounds grown on an InP substrate. A separate confinement hetero‐structure was fabricated by means of Cl‐doped MgSe/ZnCdSe SuperLattices(SLs) for n‐side layers, a ZnCdSe Single Quantum Well (SQW) for an active region, and N‐doped MgSe/BeZnTe SLs for p‐side layers. Lasing emission around 560 nm could be observed at a current density over 2.5 kA/cm2 under the pulsed current injection at 77 K. In order to obtain a lasing operation at room temperature, it is necessary to decrease the resistance of the LD and to suppress the current leakage at the interface between the active region and the p‐side region. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Epitaxial growth and optical properties for ultraviolet regionof BeMgZnSe on GaP(001) substrate
Authors: Niiyama Yuuki , Yokoyama Takeshi , Watanabe Masahiro
Published: February 2004

Epitaxial growth of high energy BeMgZnSe lattice matched to GaP(001) substrate was demonstrated using BeZnSe buffer layer with molecular beam epitaxy (MBE). BeZnSe buffer layers were introduced to suppress phase separation on GaP substrate even at high‐Mg and Be content region maintaining lattice match. Full‐width at half‐maximum (FWHM) (ω‐scan (004)) of Be0.47Mg0.15Zn0.38Se was 662.4 arcsec although we didn't observe photoluminescence. One possible explanation of this result is that the transition mechanism changes from direct to indirect for Mg content 8–15%. It was also found that the direct gap of BeMgZnSe has approximately 3.80 eV under lattice matched to GaP substrate. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of ZnCdSeTe/MgZnSeTe materials for ZnTe‐based visible optical devices
Authors: Nomura Ichirou , Ochiai Yasutomo , Toyomura Naobumi , Manoshiro Asuka , Kikuchi Akihiko , Kishino Katsumi
Published: February 2004

Bandgap energy and refractive indices of ZnTe‐based II–VI compounds such as ZnCdTe, ZnCdSeTe, and MgZnSeTe were systematically investigated. The refractive index data were approximated by the modified single effective oscillator method. Especially the approximation for MgZnSeTe made it possible to estimate the refractive index values for any Mg composition in any wavelength of the transmission region. Using the refractive index data, waveguide analysis and design of ZnCdTe/MgZnSeTe laser structures were performed to obtain confinement factors more than 4.5%. ZnCdTe/MgZnSeTe light emitting diodes were fabricated on ZnTe substrates observing yellow light emissions around 583 nm. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Proposal of a novel BeZnSeTe quaternary for II‐VI middle range visible light emitting devices on InP substrates
Authors: Takashima Yasushi , Nomura Ichirou , Nakai Yuki , Kikuchi Akihiko , Kishino Katsumi
Published: February 2004

A novel BeZnSeTe quaternary has been proposed for active layers of II–VI visible light emitting devices on InP substrates. BeZnSeTe, which lattice parameters were matched to InP, were grown on InP substrates by molecular beam epitaxy (MBE). The Be composition was changed from 0.11 to 0.35. The photoluminescence (PL) spectra of these samples at 15 K showed single peak emissions with wavelengths from 487 to 506 nm without any deep level emission. From the reflectivity and PL spectra at 15 K, the Γ–Γ direct and Γ–X indirect bandgaps of BeZnSeTe were evaluated, and it is shown that the crossover point from direct to indirect is around Be composition of 0.3. Applying the BeZnSeTe for the active layer, visible light emitting diodes (LEDs) were fabricated on InP substrates by MBE, resulting in yellow light single peak emissions around 575 nm at room temperature. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical and interface phonons of different kind in BeTe/ZnSe superlattices
Authors: Reshina I. I. , Ivanov S. V. , Kosobukin V. A. , Sorokin S. V. , Toropov A. A.
Published: February 2004

We have studied Raman scattering for a number of type II novel superlattices BeTe/ZnSe, which have no common cation or anion atoms at the interfaces. Under off‐resonant excitation LO‐phonons in BeTe and ZnSe layers were observed as well as a high‐frequency mode due probably to the “wrong bond” BeSe at the interface. Under excitation resonant with the direct exciton in ZnSe layers we observed ZnSe LO‐phonons of different orders and electrostatic interface modes of the Fuchs–Kliever type. Calculated frequencies of the interface modes are compared with the experimental ones. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Tuning the optical and magnetic properties of II–VI quantum dots by post‐growth rapid thermal annealing
Authors: Gurung T. , Mackowski S. , Jackson H. E. , Smith L. M. , Heiss W. , Kossut J. , Karczewski G.
Published: February 2004

We show that both the optical and magnetic properties of CdTe‐based quantum dots (QDs) can be tuned by post‐growth annealing. With increasing of annealing temperature, the photoluminescence of CdTe and CdMnTe QDs shifts towards high energies. At the same time, we observe a narrowing of the QD ensemble emission linewidth due to the improved homogeneity of the QD ensemble. We find that for annealed CdTe QDs the exciton diamagnetic shift is about two times larger that for the as‐grown ones, suggesting an increase of the average dot size upon annealing. In the case of CdMnTe QDs the emission lines of individual quantum dots narrow significantly from 3 meV down to 0.25 meV showing that effect of spin fluctuations is much reduced probably due to an increase of the average dot size. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optically controlled magnetization of zero‐dimensional magnetic polarons in CdMnTe self‐assembled quantum dots
Authors: Mackowski S. , Gurung T. , Nguyen T. A. , Jackson H. E. , Smith L. M. , Kossut J. , Karczewski G.
Published: February 2004

We find strong optically induced magnetization of zero‐dimensional magnetic polarons confined to CdMnTe quantum dots. The spin alignment of magnetic ions at B = 0 T is created through circularly polarized resonant excitation and it is probed by polarization‐resolved photoluminescence. This observation clearly demonstrates that in the CdMnTe quantum dots the exciton spin relaxation time is significantly longer than the formation time of magnetic polaron. Importantly, the spatial confinement characteristic of quantum dots increases the stability of magnetic polarons so that significant polarizaton of the emission is observed for temperatures higher than 120 K. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Diluted ZnMnTe oxide: a multi‐band semiconductor for high efficiency solar cells
Authors: Yu K. M. , Walukiewicz W. , Wu J. , Shan W. , Scarpulla M. A. , Dubon O. D. , Beeman J. W. , Becla P.
Published: February 2004

Using oxygen ion implantation and pulsed laser melting, we have synthesized thin films of highly mismatched ternary (y= 0) and quaternary (y= 0.12) Zn1−yMnyOxTe1−xalloys with oxygen content in excess ofx∼ 0.01. We show that incorporation of a small amount of isoelectronic oxygen leads to the formation of a narrow, oxygen‐derived band of extended states located well below the conduction band edge of the Zn1−yMnyTe matrix. The structure of the conduction band is well described by the anticrossing interaction between O localized states and the extended states of the host semiconductor matrix. As a result the conduction band splits into two subbands with distinctly non‐parabolic dispersion relations. The three absorption edges of this material (∼0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for the multi‐band, single junction, high efficiency photovoltaic devices. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Oxygen induced band‐gap reduction in ZnOxSe1−x alloys
Authors: Shan W. , Walukiewicz W. , Ager III J. W. , Yu K. M. , Wu J. , Haller E. E. , Nabetani Y.
Published: February 2004

The effect of alloying a small amount of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in MBE‐grown ZnOxSe1−x epitaxial films (0 ≤ x ≤ 0.0135) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band‐gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by the O incorporation. Both effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction band edge. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Spin‐flip Raman scattering studies of antimony‐doped ZnSe
Authors: Davies J. J. , Wolverson D. , Aliev G. N. , Zeng S. , Wang J. F. , Isshiki M.
Published: February 2004

Spin‐flip Raman scattering and photoluminescence studies of ZnSe crystals doped with antimony have provided evidence for the presence of (i) a shallow acceptor level at about 70 meV above the valence band and (ii) a previously unreported deep donor level at about 55 meV below the conduction band, with a g‐value of 1.81. As in ZnSe:N, it is possible that the formation of such deep‐donor states may be an inevitable consequence of high concentrations of p‐type dopants and the resulting compensation may be one of the reasons why attempts to obtain p‐type conductivity continue to be difficult. The deep donor g‐value fits well into the sequence of previously reported g‐values of 1.11, for shallow donor centres (at about 26 meV), of 1.38 for donors at about 45 meV and of 2.00 for deep donors at 90 meV. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Light scattering study on hybrid structures of Zn1−x−yCdxMnySe quantum wells with ferromagnetic Co wires
Authors: Sakuma M. , Hyomi K. , Souma I. , Murayama A. , Oka Y.
Published: February 2004

Sub‐micron scale hybrid structures of Zn1−x−yCdxMnySe diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co wires have been fabricated for the purpose of applying local magnetic fields to the DMS‐QW. The wire of DMS‐QW with the width down to 100 nm was sandwiched between the Co wires. The magneto‐optical properties are studied by spin‐flip light scattering of paramagnetic Mn‐ions in the DMS. The application of uniform magnetic fields higher than 0.25 T has been attained from the Co wires to the DMS‐QWs. In addition, the field application from the Co to the DMS‐QWs is switched on by weak magnetic fields of 0.03 T due to the magnetic shape anisotropy of the Co wires, which can realize the switching behavior of the spin alignment of Mn‐ions in the DMS‐QW. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Pressure measurements of TO‐phonon anharmonicity in isotopic ZnS
Authors: Tallman R. E. , Ritter T. M. , Weinstein B. A. , Cantarero A. , Serrano J. , Lauck R. , Cardona M.
Published: February 2004

We have measured the dependence on pressure of the line‐widths of the TO and LO Raman phonons of β‐ZnS. In order to enhance the phenomena observed, and to eliminate possible effects of isotopic disorder, we have measured a nearly isotopically pure crystal, 68Zn32S. The strongly structured pressure effects observed are interpreted on the basis of anharmonic decay and the corresponding two‐phonon density of states. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)