Nonpolar Substrates Research Articles

Growth and properties of GaAs/AlGaAs on nonpolar substrates using molecular beam epitaxy

Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)-oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room-temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250-Å-thick initial layer which was grown at a 0.1-μ/h growth rate at a substrate temperature of 500 °C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor-acceptor recombination at 1.479 eV, which appears to be Ge0Ga -Ge0As. These results show that high-quality GaAs/AlGaAs is obtainable on nonpolar substrates, which has important implications for the monolithic integration of III-V’s with Si.

Modification of the 14C most-probable-number method for use with nonpolar and volatile substrates.

A method was developed to allow the use of volatile and nonpolar substrates in 14C most-probable-number tests. Naphthalene or hexadecane was sorbed to filter paper disks and submerged in minimal medium. The procedure reduced the volatilization of the substrates while allowing them to remain available for microbial degradation.

GaAs/AlGaAs multiquantum wells grown on nonpolar semiconductor substrates

We report the first study of single-crystalline GaAs grown by molecular beam epitaxy (MBE) on a nonpolar substrate. In this study, GaAs was grown under various conditions on Ge(100). Surface morphologies were specular and quite strongly dependent on surface preparation. Photoluminescence spectra from such GaAs is quite intense and is dominated by donor–acceptor recombination at 1.479 eV which appears to be Ge0Ga−Ge0As. We also observe its phonon replica at 1.442 eV and the recombination of bound excitons at 1.514 eV. This and capacitance–voltage measurements indicate incorporation of Ge from the substrate into the GaAs. Photoluminescence from GaAs/AlGaAs multiple quantum well structures originates from the same processes as in bulk GaAs; in quantum wells, however, the transitions occur at higher energies due to the enhancement of the GaAs band gap. The observed energies are in good agreement with theoretical predictions. We have also studied the growth of GaAs on Si(100) and on Ge films on Si(100).

Polar semiconductor quantum wells on nonpolar substrates: (Al,Ga)As/GaAs on (100)Ge

Optical properties of GaAs bulk layers and (Al,Ga)As/GaAs quantum wells with well sizes of 80, 100, and 200 Å have been studied. The structures were grown on (100) oriented Ga doped Ge substrates using molecular beam epitaxy. In contrast to earlier less successful attempts to grow polar semiconductors on nonpolar (100) substrates, the structures prepared in this study had good surface morphologies across the entire substrate and seemed relatively free of nonradiative traps. In bulk and quantum well structures, bound excition and donor-acceptor transitions were observed. The luminescent intensity at 2 K was comparable to that of similar structures grown on GaAs substrates and dominated by donor-acceptor (Ge0Ga-Ge0As) transitions. The ability to successfully grow polar semiconductors on (100) nonpolar semiconductors should provide many opportunities in hybrid and heterojunction device applications.

Regio- and stereoselectivity of hepatic cytochrome P-450 toward polycyclic aromatic hydrocarbon substrates.

Although the cytochrome P-450 system shows broad substrate specificity toward the PAH, it also has regio- and stereoselectivity. Of the PAH substrates yet studied, phenanthrene is metabolized most efficiently, whereas chrysene and DBA are much more slowly turned over. Interestingly, the cytochrome P-450 system and epoxide hydrolase favor the formation of dihydrodiols with R,R configuration. The predominant enantiomers of the dihydrodiols with the bay-region double bond, as well as bay-region diol epoxides derived from phenanthrene, chrysene, BA and B[a]P, are superimposable when their bay-regions are aligned (Fig. 3). These metabolically favored isomers of the dihydrodiols and bay-region diol epoxides are also the more carcinogenic isomers when derived from carcinogenic polycyclic hydrocarbons.

Sensitive and Accurate Methodology for Measuring the Kinetics of Concentration-Dependent Hydrocarbon Metabolism Rates in Seawater by Microbial Communities

A method having sufficient sensitivity to resolve the kinetic constants for dissolved nonpolar substrate metabolism, together with the related rate constants in natural waters, is presented. The method is based on the rate of CO(2) recovery from radioactive dissolved substrate. Sensitivity is enhanced by using large seawater volumes, high-specific-activity isotopes, and by reducing background radioactivity. Before use, commercial isotopes are purified by mild alkaline hydrolysis followed by sublimation from base to remove CO(2) as well as interfering polar C-substrates. During sample analysis, chilled Tenax resin is used to remove volatile C-substrate from the nitrogen stream containing CO(2) recovered from substrate oxidation. Chromatographic evidence of purity, shown to be insufficient, is augmented by kinetic data from toluene utilization by mixed cultures and by rates in induced versus noninduced pure cultures. Accuracy is enhanced by using short (<10 h) incubation times and small hydrocarbon concentrations so that the metabolism rates in unamended natural water systems can be evaluated. Toluene metabolism rates in seawater as low as 1 pg/liter per h and at concentrations as low as 20 ng/liter have been determined.

Membrane-bound estrone as substrate for microsomal UDP-glucuronyltransferase.

Nonpolar substrates of microsomal UDP-glucuronyl-transferase partition between the hydrophobic phase of the microsomal membrane and the bulk aqueous phase in a suspension of microsomes in water. Partitioning of estrone into the membranes was measured in the studies presented and was extensive. Comparison of the rate of conjugation of estrone and the rate of its release from microsomes into the bulk aqueous phase showed that the pool of estrone within the membrane is the substate for UDP-glucuronyl-transferase. The rate of conjugation of estrone was 6-fold greater than the rate of release of estrone from the membrane into the aqueous phase. Several additional experiments showed that the rate of glucuronidation of estrone did not depend on the amount of estrone in the bulk aqueous phase. It is concluded that the microsomal membrane serves to concentrate nonpolar substrates of UDP-glucuronly-transferase. The phospholipid region of the microsomal membrane also may be a co-factor of UDP-glucuronyltransferase in the sense that binding of estrone to the membrane restricts its orientation in a manner that facilitates catalysis.

Effect of substrate polarity on the activity of soybean lipoxygenase isoenzymes

AbstractIn order to characterize the several isoenzymes of soybeans, they were examined with respect to the effect of the polar nature of the substrate. In general, lipoxygenase‐1 was most active when presented with charged substrates such as the anionic form of linoleic acid or of potassium linoleyl sulfate, whereas lipoxygenase‐2 and‐3 preferred nonpolar substrates such as unionized linoleic acid, methyl linoleate, linoleyl methane sulfonate, 10,13‐nonadecadieneamine, or linoleyl acetate. Linoleyl sulfate, which has been advanced as an excellent readily soluble substrate for lipoxygenase, was indeed the best substrate found for lipoxygenase‐1. Lipoxygenase‐2 and‐3 were, by contrast, totally inactive against this substrate. The favorable response of linnoleic acid to lipoxygenase‐2 and‐3 at pH 6.8 was ascribed to the anomalously high pKa value of linoleic acid compared to that of short chain carboxylic acids. The pH‐activity profile obtained with lipoxygenase acting on linoleyl sulfate (which was charged at all pH values examined) was shifted to lower pH values compared to the linoleic acid activity profile. The effect of changing from the charged to the uncharged substrate, when tested against lipoxygenase‐1, was to increase the Km by an order of magnitude.

Seedling Growth in Controlled-Nutrient Conditions

An apparatus is described in which seedlings can be grown on a non-polar substrate in con tinuously flowing nutrient solutions giving a constant and defined root environment. The apparatus, which is simple to construct, allows seedling growth studies to be begun as soon as the seeds have germinated. In an experiment in which seedlings were grown for 13 d in nutrient solutions allowing normal growth or severely restricted growth, or were transferred from the latter to the former after 3 or 6 d, root lengths and cotyledon areas differed significantly after only 3 d. After 13 d the transferred seedlings, although smaller in absolute size than the normal seedlings, resembled them in a number of over-all morphological features. Because of the rapid and substantial effects that nutrient treatments can have on seedling growth, it is suggested that seedling development studies should be carried out under carefully controlled and accurately defined nutrient conditions.

The Isolation and Primary Structure of a Peptide Containing the Oxidation-Reduction Active Cystine of Escherichia coli Lipoamide Dehydrogenase

An 11-residue peptic peptide from the catalytic center of Escherichia coli lipoamide dehydrogenase has been isolated and sequenced. This peptide contains the oxidation-reduction active disulfide of the enzyme which functions in concert with the flavin during catalysis. The peptide was isolated from a digest of a derivative in which the free sulfhydryls of the enzyme were substituted with the colored maleimide N-(4-dimethylamino-3,5-dinitrophenyl)maleimide. The sequence of the peptide was determined by a subtractive Edman degradation to be: [see PDF for sequence] This work suggests that the oxidation-reduction active disulfide of E. coli lipoamide dehydrogenase is contained in a hydrophobic pocket at the catalytic center of the enzyme, consistent with the binding of the nonpolar substrate dihydrolipoamide. Evidence is also presented that the two subunits of the enzyme are identical.

Reaction of C2F5 radicals with HCl. Determination of the bond dissociation energy D(C2F5—H)

The reaction C2F5+ HCl → C2F5H + Cl k2(2) was investigated over the range 23–152°C using perfluoropropionic anhydride as a photolytic source of C2F5 radicals. Arrhenius parameters are given by log(k2/cm3 mol–1 s–1)=(10.80 ± 0.14)–(4240 ± 230)/θ where θ= 2.303 RT/cal mol–1. The reactivity of C2F5 radicals with polar and non-polar substrates is discussed. The above value of E2 leads to a value of the bond dissociation energy D(C2F5—H)= 103.1 ± 1.5 kcal mol–1 in good agreement with previous work.

Evaluation of Pollen Viability by Enzymatically Induced Fluorescence; Intracellular Hydrolysis of Fluorescein Diacetate

Viable pollen grains immersed in a solution of fluorescein diacetate, made up in a concentration of about 10−6 M in sucrose of suitable tonicity (usually about 0.5 M), rapidly accumulate free fluorescein, which can be detected by its fluorescence. This fluorochromatic reaction (FCR) probably depends upon the entry of the nonpolar substrate into the vegetative cell where it is hydrolyzed by esterase to give the polar product, fluorescein, which is retained by the cell membrane. The test is primarily one for the integrity of the plasmalemma of the vegetative cell. Since this integrity is likely to be closely correlated with viability, the FCR provides an effective method for assessing pollen quality.

Reactions of aldehyde oils and methyl azelaaldehydate with polyols

AbstractIn exploratory tests, mono‐, di‐, and trialdehyde oils and methyl azelaaldehydate (MAZ), preferably in the form of their acetals, reacted with various polyols with few exceptions to produce potentially useful derivatives. The polyols included pentaerythritol, methyl glucoside, glucose, sorbitol and poly (vinyl alcohol) (PVA). Preliminary evaluation of some of the derivatives was carried out in several applications. Among the more promising were: films derived from MAZ and glucose or PVA; melt adhesives for nonpolar substrates from trialdehyde oil and methyl glucoside; and molding compounds derived from MAZ and PVA.

Essential Oils and their Constituents XXIII: Chemotaxonomy of the Genus Mentha

Essential oils derived from Mentha aquatica L.; M. citrata Ehrh. (bergamot mint); M. pulegium L. (European and North African pennyroyal); Hedeoma pulegioides (L.) Pers. (American pennyroyal); M. rotundifolia and M. sylvestris L. were examined by gas-liquid partition chromatography using a polar (SAIB) and nonpolar (Ucon) substrate, respectively. Quantitative data illustrating the extent of compositional differences between these products are presented and compared with similar criteria of authenticity for M. arvensis (mint); M. piperita (peppermint); M. cardiaca, M. spicata , and M. viridis (spearmints). The occurrence of isopiperitenone as a major constituent of M. rotundifolia derived from a French mutant strain and the detection of several terpenoids occurring as trace constituents in these species are reported for the first time. Correlation of the chemical data illustrates their value in plant classification.

Structure and retention time in the gasliquid chromatography of unsaturated fatty acids on polyester substrates.

IT is generally accepted that plots of log10 retention time in the gas-liquid chromatography of the longer chain normal saturated fatty acid methyl esters against the number of carbon atoms in the fatty acid chain give a straight line with both non-polar and polar substrates. On the other hand, various simple plots of similar values obtained from unsaturated fatty acid esters with polar substrates, correlating chain length or number of double bonds, yield surprisingly little information1–4. In the course of gas-liquid chromatographic examination of the methyl esters of the fatty acids of marine oils the retention-time data of a number of identified unsaturated fatty acids examined by different workers1,4–9, all using polyester substrates, were plotted on semilog paper against the number of carbon atoms in the fatty acid chain. Despite the fact that a number of differing polyester substrates were used, the resulting plots showed a surprising overall similarity. This suggested that the key to the relation of the plotted points lay in the structural position of the methylene-interrupted double bond systems in the fatty acid chains in the case of the polyethylenic fatty acids, and in the position of the isolated double bonds in the monoethylenic fatty acids, rather than in the type of polyester.

Adhesion of High Polymers. VI. The Effect of Molecular Weight of NBR Copolymers of Various Polarities on Adhesion to Polar and Nonpolar Materials

Abstract 1. The adhesion of fractions of NBR of varying polarity to a polyamide has been investigated at room and elevated contact temperatures. 2. The adhesion of each copolymer investigated has been shown to fall with the increase of molecular weight of fractions up to a certain limit (300,000– 350,000). Above this weight the magnitude of adhesion remains practically constant. 3. The adhesion of all fractions tested increases with increasing contact temperature, attaining the cohesive strength of the respective fraction. The lower the value of molecular weight, the lower is this temperature limit. 4. It was shown that at room contact temperature adhesion of nitrile co-polymers to polyamide depends only slightly on the polarity of the adhesive. The lower the polarity of the copolymers, the more the heating of the bonded specimens increases the adhesion strength. The lower adhesion of the polar nitrile copolymers to the highly polar polyamide might be explained by a purely kinetic factor, that is, the lack of flexibility of macromolecules of such polymers and the greater number of intermolecular links, which makes diffusion difficult. 5. It has been established that for bonding at room temperature the adhesion of unfractionated commercial nitrile rubbers to polyamide is determined primarily by the polydispersity of the rubber rather than by its polarity. The adhesion of copolymers at elevated temperatures depends to a high degree on their polarity. 6. It was found that the adhesion of fractions of NBR to nonpolar substrates (PIB) is very low and is independent of polarity and molecular weight, and of temperature of thermal treatment. Smaller values of adhesion in this case are determined by noncompatability of polar adhesive with nonpolar substrates. 7. The results of the work were discussed in the light of the diffusion theory of adhesion.

Gas‐liquid chromatography of fatty derivatives. IV. Quantitative analysis of n‐alcohols

AbstractThe separation of fatty alcohols has been accomplished by using a nonpolar substrate on a solid support treated with alkali to reduce adsorptivity. By this technique well‐resolved symmetrical peaks are obtained, and the stability of a nonpolar substrate is retained. Relative detector sensitivity factors have been determined for the alcohols with chain lengths of C8 through C18, permitting a correction of peak areas. The response of the detector is approximately a linear function of chain length for the n‐alcohols.The accuracy and precision of the determination have been studied by a statistical comparison of the results of replicate values with values from known compositions.

Gas‐liquid chromatography of fatty derivatives

SummaryThe separation of fatty amines has been carried out with nonpolar substrates on solid supports of Chromosorb and Chromosorb W, which were previously treated with potassium hydroxide to overcome adsorptivity. In this manner well‐resolved symmetrical peaks are obtained. Untreated supports give peaks which trail and prohibit precise quantitative measurements. Both Apiezon L on Chromosorb W and Silicone Grease on Chromosorb have proved effective, the latter at 176°C. and the former at 225°C. Relative detector sensitivity factors have been determined for the amines C8 to C18, thus permitting accurate analyses. The relative response for primary alkyl fatty amines is a linear function of molecular weight.

Gas-Liquid Chromatography of Pyridines Using New Solid Support

A solid support has been developed for the gas-liquid chromatography of pyridines, superior to Chromosorb or Celite 545; symmetric peaks were obtained when nonpolar substrates were used. The solid support was prepared from a commercial detergent by heating it and then extracting it with petroleum ether. The porous residue was used as a liquid substrate carrier. The new solid support was used to study the selectivity of several nonpolar and slightly polar liquid substrates. Decided differences in the selectivities of the liquid substrates permit a wide choice in the use of these substrates for separations of pyridines. The study on the selectivity of the liquid substrates was used to select two columns employed in series to separate a test mixture of 14 pyridines.