bluuebunny/crossref_metadata_2025_split huggingface dataset:
Enzymatic Synthesis of Materials — An Overview
Authors: Alper Mark 
Published: March 2011

One of the more impressive features of the living organism is the degree of control it is able to exercise over its chemistry.A set of molecules in a given environment will react predictably according to the laws of chemistry. Quite often, as the synthetic organic chemist is ruefully aware, these laws allow more than one reaction to take place, and a multitude of products to be synthesized. The set of molecules that makes up the living organism is also governed by the laws of chemistry. The organism is able to, however, limit reactivity to a single product. In many cases this is a necessity; multiple reactions can often lead to unacceptable inefficiency or death.Those involved in the synthesis of new, nonbiological materials with enhanced properties could benefit from this level of control. In the simple case, production of only the single desired product in high yields would be a great economic boon. On another level, this control could lead to the production of materials that are otherwise inaccessible. An obvious example is the modification of a polymer. Only with precise reaction control can all the monomers be modified in the same way.Organisms achieve their required degree of control of chemical reactions through a mechanism of positive regulation. Under the conditions found in an organism, virtually no reaction will take place at a rate that is sufficient to sustain the life processes. Those that do occur take place only in the presence of a catalyst. To allow a reaction to occur, the organism simply synthesizes the catalyst that is specific for that reaction and does so only at the appropriate time and place and to the required extent. Should that reaction no longer be needed, the catalyst is destroyed or inactivated.
Recent Studies on the Application of Enzymes in the Synthesis of Polyesters
Authors: Morrow Cary J. , Brazwell Eugenia M. , Filos Dianela , Mercure Juanita , Romero Rosemary , Wallace J. Shield 
Published: March 2011

AbstractEnzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures. This paper summarizes our successful use of lipase-catalyzed polycondensations to prepare both a series of achiral [AA-BB]xpolyesters from bis(2,2,2- trichloroethyl) alkanedioates and diols and of an optically active, epoxy-substituted polyester having a stereochemical purity estimated to be greater than 96%, from racemic bis(2,2,2-trichloroethyl)trans-3,4-epoxyhexanedioate and 1,4-butanediol. All of the reactions were carried out at ambient temperature in anhydrous, low to intermediate polarity, organic solvents such as ether, THF, 2-ethoxyethyl ether, dibenzyl ether,o-dichlorobenzene, or methylene choride, using porcine pancreatic lipase (PPL) as the catalyst. The molecular weight achieved by the polycondensation is limited by accumulation of the trihaloethanol that forms as the reaction progresses, probably because it frees enzyme-bound water permitting hydrolysis of the polymer to occur. This problem has been alleviated by using a high boiling solvent and removing the alcohol by placing the re'action mixture under vacuum.
Engineering Enzymes for Enhanced Thermostability
Authors: Shih Phoebe , Malcolm Bruce A. , Kirsch Jack F. 
Published: March 2011

AbstractChicken egg-white lysozyme (CEWL) is used as a model to attempt to engineer proteins for enhanced thermostability. Site-directed mutagenesis is employed for selective amino acid substitution to probe the contribution of an individual amino acid in a given sequence to thermostability. A linear correlation is found between the side-chain volume of a triplet of amino acid residues located at the interior core of the protein and its thermostability. Additional mutant constructs at the core position reveal that hyperpacking can disrupt other intramolecular contacts and offset the hydrophobic stabilization due to denser packing. Multiple substitutions at different loci of the protein are combined to analyze the additivity of thermostability mutations.
Poly(L-Lysine) Adsorption to Fluoropolymer Films
Authors: Shoichet Molly S. , McCarthy Thomas J. 
Published: March 2011

AbstractThe adsorption of poly(l-lysine) (PLL) from aqueous solution to poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) is investigated. Adsorption is controlled by choice of solvent (and PLL solution conformation) or by FEP surface chemistry. PLL adsorbs from pH 11 buffer solution to FEP: PLL adsorption is concomitant with the liberation of water molecules from the FEP-water interface resulting in a decreased interfacial free energy between organic and aqueous phases. PLL does not adsorb from an aqueous pH 7 solution to FEP unless the FEP surface is carboxylic acid-functionalized (FEPCO2H):an ionic interaction between FEP-carboxylate and PLL-ammonium enhances adsorption.The surface modifications are charaterized by attenuated total reflectance infrared spectroscopy (ATR IR), ultraviolet-visible spectrophotometry (UV-vis), contact angle, and quantitatively by x-ray photoelectron spectroscopy (XPS). The adsorbed amount of PLL to FEP (from pH 11) is proportional to molecular weight whereas the adsorbed amount of PLL to FEP-CO2H (pH 7) is independent of molecular weight and likely proportional to surface charge density. The modified film samples (relative to the unmodified film samples) show improved characteristics in terms of wettability, chemical reactivity, adhesion, cell growth.
The Role of Gas Phase Decomposition in the ALE Growth of III–V Compounds
Authors: Reid K. G. , Urdianyk H. M. , El-Masry N. A. , Bedair S. M. 
Published: March 2011

ABSTRACTThe effects of the growth temperature and exposure time to TMGa for ALE of gallium arsenide was studied using TMGa and AsH3 in a modified, vertical, atmospheric, MOCVD reactor with a rotating susceptor. It was found that the temperature range for ALE growth could -be extended from 450°C to 700°C by adjustment of the exposure time to TMGa. The maximum exposure time to TMGa was found to decrease as growth temperature increased with high temperature growth limited to exposures of only fractions of a second. Beyond a critical exposure time to TMGa, gallium droplets form on the surface. It is known that premature decomposition of TMGa in the heated gaseous boundary layer causes the formation of the gallium droplets and the consequent loss of ALE growth.
Study of As and P Incorporation Behavior in GaAsP by Gas-Source Molecular-Beam Epitaxy
Authors: Liang B. W. , Hou H. Q. , Tu C. W. 
Published: March 2011

ABSTRACTA simple kinetic model has been developed to explain the agreement between in situ and ex situ determination of phosphorus composition in GaAs1−xPx (x < 0.4) epilayers grown on GaAs (001) by gas-source molecular-beam epitaxy (GSMBE). The in situ determination is by monitoring the intensity oscillations of reflection high-energy-electron diffraction during group-V-limited growth, and the ex situ determination is by x-ray rocking curve measurement of GaAs1−xPx/GaAs strained-layer superlattices grown under group-III-limited growth condition.
The Surface Chemistry of GaAs Atomic Layer Epitaxy
Authors: Creighton J. Randall , Banse Barbara A. 
Published: March 2011

ABSTRACTIn this paper we review three proposed mechanisms for GaAs ALE and review or present data in support or contradiction of these mechanisms. Surface chemistry results clearly demonstrate that TMGa irreversibly chemisorbs on the Ga-rich GaAs(100) surface. The reactive sticking coefficient (RSC) of TMGa on the adsorbate-free Ga-rich GaAs(100) surface was measured to be ∼0.5, conclusively demonstrating that the “selective adsorption” mechanism of ALE is not valid. We describe kinetic evidence for methyl radical desorption in support of the “adsorbate inhibition” mechanism. The methyl radical desorption rates determined by temperature programmed desorption (TPD) demonstrate that desorption is at least a factor of ∼10 faster from the As-rich c(2 × 8)/(2 × 4) surface than from the Garich surface. It is this disparity in CHs desorption rates between the As-rich and Ga-rich surfaces that is largely responsible for GaAs ALE behavior. A gallium alkyl radical (e.g. MMGa) is also observed during TPD and molecular beam experiments, in partial support of the “flux balance” mechanism. Stoichiometry issues of ALE are also discussed. We have discovered that arsine exposures typical of atmospheric pressure and reduced pressure ALE lead to As coverages ≥ 1 ML, which provides the likely solution to the stoichiometry question regarding the arsine cycle.
Growth of (GaAs)1−x (Si2)x Metastable Alloys using Migration-Enhanced Epitaxy
Authors: Rao T. Sudersena , Horikoshi Y. 
Published: March 2011

ABSTRACTEpitaxial (GaAs)1−x (Si2)x metastable alloys have been grown on GaAs (100) substrates using Migration-Enhanced Epitaxy in the composition range of 0<x<0.25. The lattice constant a0 of the alloys was found to decrease with increasing Si content from 0.56543nm at x=0 to 0.5601nm at x=0.25. Double-crystal x-ray diffraction rocking curve measurements and cross-sectional transmission electron microscopy studies made on a 10 period (GaAs)1−x(Si2)x/GaAs strained layer superlattice indicated sharp and abrupt interfaces. High crystalline quality GaAs has been grown on Si substrates using (GaAs)0.80(Si2)0.20/GaAs strained layer superlattices as buffer layers.
High-Conductance GaAs Tunnel Diodes by OMVPE
Authors: Venkatasubramanian R. , Timmons M. L. , Colpitts T. S. 
Published: March 2011

ABSTRACTGaAs p+-n+ tunnel diodes have been grown by atmospheric-pressure organometallic vapor phase epitaxy (OMVPE) using zinc as the dopant for the p+ regions and either Se or Si as the dopant for the n+ regions. At a growth temperature of 700° C, using a “cycled” growth for just the Zn-doped p++-GaAs layer both the conductance and the peak current of the tunnel diode has been increased by a factor of ˜65. The conductance of the tunnel diode, maximized at a growth temperature of 650 °C with the cycled growth, is comparable to the best reported values by MBE. Cycled growths for n+ Se-doped regions reduce the tunnel-diode conductance by more than two orders of magnitude. However, the cycled growth for n+-GaAs regions formed with Si doping shows no conductance degradation. A model for these observations is presented.
UV Photostimulated Si Atomic-Layer Epitaxy
Authors: Lubben D. , Tsu R. , Bramblett T. R. , Greene J. E. 
Published: March 2011

ABSTRACTSingle-crystal Si films have been grown on Si(001)2×1 substrates by UVphotostimulated atomic-layer epitaxy (ALE) from Si2H6. The ALE deposition rate R per growth cycle remains constant at 0.4 monolayers (ML) over a wide range of deposition parameters: growth temperature (Ts= 180–400 °C), Si2H6 exposure (peak pressure during gas pulse = 0.1−5 mTorr), laser energy density ( = 250–450 mJ cm−2 where is determined by Ts), and number of UV laser pulses per cycle. A film growth mocrel, based upon the results of the present deposition experiments and Monte Carlo simulations, together with our previous adsorption/desorption measurements, Is used to describe the reaction pathway for the process. The Hterminated silylene-saturated surface formed by adsorption and desorption of disilene is thermally stable and passive to further Si2H6 exposure. ArF or KrF laser pulses (≅20 ns) are used to desorb H, following a Si2H6 exposure, and the growth cycle is repeated until the desired film thickness is obtained. At Ts < 180 °C, the growth process becomes rate limited by the surface dissociation step and R decreases exponentially as a function of 1/Ts with an activation energy of ≅0.5 eV. At Ts > 400 °C, H is thermally desorbed and pyrolytic growth competes with ALE. Transmission electron micrographs together with selected-area electron diffraction patterns show that the ALE films are epitaxial layers with no observed extended defects or strain.
Digital Process for advanced VLSI's and Surface Reaction Study
Authors: Sakaue H. , Asami K. , Ichihara T. , Ishizuka S. , Kawamura K. , Horiike Y. 
Published: March 2011

ABSTRACTDigital etching was carried out by repeating the fundamental reaction cycles of adsorption, reaction and desorption for fluorine(F) or chlorine(Cl)/Si systems. In the F/Si case, atomic layer etching of Si(100) was achieved by adsorption of F atoms produced by a remote discharge of F2/99.8%He on the cooled Si surface and subsequent Ar* ion (≅20eV) irradiation. The digital method revealed that the cryogenic etching occurred by ion bombardment on physiosorbed F atoms on the cooled Si surface. Adsorption of Cl atoms on Si at room temperature allowed self-limiting reaction with etch rate of 0.4 Å/cycle. The etching increased rapidly over 40 V of substrate voltage. Secondly, reaction of TES (triethylsilane) with hydrogen(H) atoms was also found to lead to conformal CVD (Chemical Vapor Deposition) of Si film involving organic species. Then Si oxide and nitride films were formed by digital CVD which repeated a cycle of first deposition of this film and subsequent its oxidation and nitridation. The electrically excellent multilayer stacked oxide and nitride film was filled in to deep trench. Insitu FTIR-ATR spectroscopy demonstrated that the surface reaction was predominant for the TES/H process.
Decomposition of Alkylsilanes on Silicon Surfaces Using Transmission Ftir Spectroscopy
Authors: Dillon A. C. , Robinson M. B. , Han M. Y. , George S. M. 
Published: March 2011

ABSTRACTFourier transform infrared (FTIR) transmission spectroscopy was used to monitor the decomposition of alkylsilanes such as diethylsilane (DES) [(CH3 CH2)2SiH2], di-t-butylsilane (DTBS) [((CH3)3C)2SiH2] and ethylsilane (ES) [CH3CH2SiH3 on high-surface-area porous silicon samples. The FTIR spectra revealed that tKe akylsilanes dissociatively adsorb on porous silicon at 300 K to form SiH and Si-alkyl species. As the silicon surface was progressively annealed, the Si-alkyl species decomposed and produced gas phase ethylene (DES,ES) or isobutylene (DTBS). The decomposition of the alkyl group was accompanied by the growth of additional SiH surface species. These reaction products were consistent with a [β-hydride elimination reaction. Above 700 K, the SiH surface species decreased concurrently with the desorption of H2 from the porous silicon surface. The uptake of surface species was also monitored at various adsorption temperatures to determine the optimal exposure temperatures for carbon-free silicon deposition. Carbon contamination was not detected at adsorption temperatures below 640 K prior to H2 desorption. Because the alkylsilane adsorption process is self-limiting at temperatures below 640 K, alkylsilanes may be useful molecular precursors for the atomic layer epitaxy (ALE) of silicon.
The Etching of Silicon by Oxygen Observed by in situ Tem
Authors: Ross Frances M. , Gibson J. Murray 
Published: March 2011

ABSTRACTWe describe observations made in situ in a modified UHV transmission electron microscope of the process of etching of the Si (111) surface by oxygen. Etching occurs by the motion of individual bilayer steps across the surface and by analysing the step motion we discuss the etching mechanism in the context of macroscopic parameters.
P-Type Conversion of Nitrogen Doped ZnSe Films Grown By Mocvd
Authors: Khan Babar A. , Taskar Nikhil , Dorman Donald , Shahzad Khalid 
Published: March 2011

ABSTRACTWe have obtained p-type zinc selenide films by nitrogen doping combined with a post growth anneal. These films were grown on <100> gallium arsenide substrates using a low-pressure MOCVD process. Ammonia gas was used as the source for nitrogen. The as-grown films were annealed and then studied by photoluminescence (PL) and capacitance-voltage (CV) techniques. The PL data is dominated by the acceptor-bound exciton peak and donor-acceptor pair spectrum associated with the nitrogen acceptor. The C-V data shows that the films are p-type, with the highest measured net acceptor concentration of 3×1016/cm3.
Analysis and Calibration of the Flow Characteristics of A Pressure Controlled Vapor Source For Gas Source Doping: CdTe:l
Authors: Rajavel D. , Wagner B. K. , Maruyama K. , Benz R. G. , Conte A. , Summers C. J. 
Published: March 2011

ABSTRACTA pressure controlled vapor source was developed for the gas source doping of (Hg,Cd)Te alloys. The dopant source has been subjected to extensive tests, and the flow characteristics determined. The dopant source was used to control the flow rates of ethyliodide for the n-type doping of CdTe. Highly conductive CdTe:l films were grown by molecular beam epitaxy.
Formation of CdTe and GaAs by Electrochemical Atomic Layer Epitaxy (ECALE)
Authors: Suggs D. Wayne , Villegas Ignacio , Gregory Brian W. , Stickney John L. 
Published: March 2011

ABSTRACTThe principles of Atomic Layer Epitaxy (ALE) have been applied to the formation of compound semiconductors by an electrochemical technique, referred to as Electrochemical Atomic Layer Epitaxy (ECALE). Atomic layers of the component elements are alternately electrodeposited at underpotential (UPD) from separate solutions and at separate potentials. Results are presented concerning the structures of both CdTe and GaAs deposits formed by ECALE. Studies were performed using singlecrystalline Au electrodes in a UHV surface analysis instrument coupled directly with an electrochemical cell. This instrument was used in order to prevent corruption by contact with air during transfer to the surface analysis environment.
Plasma Enhanced OMCVD Atomic Layered Growth of Y-BA-CU Oxide Films
Authors: Duray S. J. , Buchholz D. B. , Song S. N. , Richeson D. S. , Ketterson J. B. , Marks T. J. , Chang R. P. H. 
Published: March 2011

ABSTRACTWe report the results of a pulsed organo-metallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10-4 -10-2 torr) and low substrate temperature (600 to 700 C) using organo-metallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organo-metallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post-deposition oxidation and heat treatments. The growth of phase-pure, highly oriented Y-Ba-Cu-O superconducting oxide films ([Tc (R=0)=90.5K] and Jc (77K, 50K gauss)=l.l×105 A/cm2) is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in-situ processing of multilayer structures (e.g. junctions).
Growth of SrS Thin Films by Atomic Layer Epitaxy
Authors: Leskela M. , Niinistö L. , Nykänen E. , Soininen P. , Tiitta M. 
Published: March 2011

ABSTRACTThe growth of strontium sulfide thin films in a flow-type Atomic Layer Epitaxy reactor from Sr(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and H2S has been studied. The growth is independent on flow rate and duration of the purge gas (N2) pulse and it does not depend on the Sr(thd)2 and H2S pulses either provided their amounts are sufficient to saturate the surface. The variables significantly affecting the growth rate are the substrate temperature and source temperature for Sr(thd)2. The observed lower than one monolayer growth rate is mainly due to the large size of the Sr(thd)2 molecule.
Rheed Intensity Oscillation During the Epitaxial Growth of Silver and Gold Films
Authors: Mae Kazuki , Kyuno Kentaro , Kaneko Takeo , Yamamoto Ryoichi 
Published: March 2011

ABSTRACTThe intensity oscillation of reflection high-energy electron diffraction (RHEED) during the autoepitaxy of Au on Au(001) and Ag on Ag(001) and during the heteroepitaxy of Au on Ag(001) and Ag on Au(001) were measured at 310 and 330 K. The morphologies of the growing surfaces are discussed in terms of the surface energies and the surface reconstruction. RHEED intensity oscillation during Au deposition on Ag(001) at 330 K rapidly decayed at the 6th period when reconstruction of the Au(001) surface appeared. Such a rapid decay was not observed at 310 K. On the other hand, the reconstruction entirely disappeared at the first minimum of the RHEED intensity oscillation in the case of Ag deposition on Au.
In Situ Rheed and Hrem Study of Initial Stages of Interfacial Reactions of Uhv Deposited Titanium Thin Films on Silicon
Authors: Wang M. H. , Chen L. J. 
Published: March 2011

ABSTRACTThe initial stages of interfacial reactions of ultrahigh vacuum (UHV) deposited Ti thin films on silicon have been studied by in-situ reflected high energy electron diffraction (RHEED) and transmission electron microscopy (TEM).An amorphous interlayer was found to form during the deposition of the first 1.7-nm-thick Ti layer. In samples annealed at 450 °C for 30–120 min, Ti5Si3, located at the Ti/a-interlayer interface, was identified to be the first nucleated phase. Ti5Si3, Ti5Si4, TiSi and C49-TiSi2 were observed in samples annealed at 475 °C for 30 and 60 min as well as at 500 °C for 10 and 20 min. Fundamental issues in silicide formation are discussed in light of the discovery of the formation of the amorphous interlayer and as many as four
different silicide phases in the initial stages of interfacial reactions of UHV deposited Ti thin films on silicon.
Morphology of Vapor Evaporated Mo Thin Films
Authors: Cheng Y. , Stearns M. B. 
Published: March 2011

ABSTRACTStudies were made of the dependence of the morphology of Mo films, prepared by ebeam evaporation in an UHV system, on the substrate temperature and deposition angle. The main characterization techniques used were large angle x-ray scattering and cross-sectional high resolution electron microscopy.
Hreels Study of the Adsorption of Organometallics on GaAs(001) Surfaces
Authors: Närmann A. , Purtell R. J. , Yu M. L. 
Published: March 2011

ABSTRACTTrimethylgallium (TMGa), Triethylgallium (TEGa) and Diethylgalliumchloride (DEGaCl) are used in combination with arsine (AsH3) in atomic layer epitaxy of GaAs. We simulated this process in a UHV system by dosing a c(2×8) or (1×6) reconstructed GaAs(001) surface with arsine and organometallics at different temperatures. After the surface reconstruction was verfied by LEED, the sample was transferred in situ to the HREELS (high resolution electron energy loss spectroscopy) chamber. We present new studies on the adsorption and pyrolysis of organometallics on GaAs(001) surfaces.As a result we find that TMGa adsorbs molecularly on an As-rich cooled surface but decomposes upon adsorption on a Ga-stabilized (1×6) surface. These two adsorption states are identified by the geometrical position of the Gamethyl compound on the surface (Ga-C bond parallel or normal to the surface). The removal of the adsorbed species occurs on an As-rich surface at temperatures higher than 450°C and on the Ga-stabilized surface at 350°C.For TEGa and DEGaCl we found similar results.
A Study of Hydrogen Atom Adsorption on Gallium Arsenide (100) by Multiple Internal Reflection Infrared Spectroscopy
Authors: Gee Paul E. , Hicks Robert F. 
Published: March 2011

ABSTRACTWe have studied the adsorption of hydrogen atoms on GaAs (100) by multiple internal reflection infrared spectroscopy. The crystal was etched in 1:1:10 H3PO4/H2O2-/H2O solution and in 1:1 HCI/H2O solution, then annealed to 580°C in the vacuum chamber. Hydrogen adsorption was carried out at -90 and 45°C. At both temperatures, a monolayer forms giving rise to infrared bands for arsenic hydride and gallium hydride at 2105 and 1860 cm−1, respectively. The arsenic hydride vibration is polarized parallel to the surface, whereas the gallium hydride vibration is polarized normal to the surface. By monitoring the changes in the intensity of the infrared absorption bands with time during exposure to H atoms and during heating, the kinetics of hydrogen adsorption and desorption can be measured. At -90°C, the H atom sticking probability follows the Langmuir model, S/So = (1-θH). Upon heating the crystal, the arsenic hydride rapidly decomposes near 120°C, while the gallium hydride slowly decomposes between 150 and 400°C.