Photochemical desorption from chlorinated Si(100) and Si(111) surfaces — Mechanisms and models

Understanding the mechanisms controlling the anisotropy of microetching is particularly critical as the scale of semiconductor devices shrink. Defining complex, dynamic chemical systems such as halogen etching require microscopic measurements combining kinetics, dynamics, surface layer composition and micromorphology on prototypical surfaces. This study is concerned with two important variables in addition to spontaneous chemical etching, the role of electronic defects induced by high level doping in producing site-specefic reaction and the enhancement of etching by irradiation at low fluences.

Substitutional defects introduced by selective doping significantly influence the rate of chlorine etching by forming shallow electronic states that are ionized at room temperature¹. We have shown that chlorine sticking coeficients as well as laser-assisted etching are significantly affected by doping at very high dopant levels. Enhancement for n-type doping is consistent with the simple assumption that holes at the surface should enhance Si-Si surface bond breaking and in disagreement with the fact that heavily p-doped silicon has a higher chlorine sticking coefficient than n-doped material².

Carrier effects generated by photoirradiation with above bandgap photons are considerably more complex than simple doping. A depletion layer and associated electric field are set up at the surface and minority carriers are preferentially swept to the surface. The type of photocarrier present at the surface is determined by both the doping and the photoirradiation.

Using photoinduced etching of heavily doped Si(100) and Si(111) by chlorine at low laser fluences, we studied the mechanism of photostimulated desorption using core-level photoemission and time-of-flight measurements of the photoproducts². These results will be interpreted in terms of field-modified electron-hole transport together with carrier-modified chlorine adsorption and desorption.     

A novel nanoflow interface for atmospheric pressure ionization mass spectrometry

A novel spray-ionization technique for nanoflow liquid chromatography/mass spectrometry (nLC/MS) has been developed by modifying the sonic spray ionization (SSI) technique. A solution from a tapered fused-silica capillary is sprayed by a gas flow coaxial to the capillary, and ions produced are analyzed with an ion-trap mass spectrometer. The ion intensity is shown to have a steep threshold at a low gas velocity and to be much less dependent on the gas velocity than that of conventional SSI, in which the ion intensity is strongly dependent on the gas velocity and reaches its maximum at sonic velocity. Thus, we conclude that the concentration of charge in the solution at the tapered capillary tip with an inner diameter of 15 microm is almost at saturation so that charged droplets are produced from the solution by electrical force, rather than by sheer stress due to the gas flow. The ions are readily produced from these charged droplets. Preliminary results are compared with results obtained with a miniaturized electrospray unit.     

Influence of organic ions on DNA damage induced by 1 eV to 60 keV electrons

We report the results of a study on the influence of organic salts on the induction of single strand breaks (SSBs) and double strand breaks (DSBs) in DNA by electrons of 1 eV to 60 keV. Plasmid DNA films are prepared with two different concentrations of organic salts, by varying the amount of the TE buffer (Tris-HCl and EDTA) in the films with ratio of 1:1 and 6:1 Tris ions to DNA nucleotide. The films are bombarded with electrons of 1, 10, 100, and 60?000 eV under vacuum. The damage to the 3197 base-pair plasmid is analyzed ex vacuo by agarose gel electrophoresis. The highest yields are reached at 100 eV and the lowest ones at 60 keV. The ratios of SSB to DSB are surprisingly low at 10 eV (～4.3) at both salt concentrations, and comparable to the ratios measured with 100 eV electrons. At all characteristic electron energies, the yields of SSB and DSB are found to be higher for the DNA having the lowest salt concentration. However, the organic salts are more efficient at protecting DNA against the damage induced by 1 and 10 eV electrons. DNA damage and protection by organic ions are discussed in terms of mechanisms operative at each electron energy. It is suggested that these ions create additional electric fields within the groove of DNA, which modify the resonance parameter of 1 and 10 eV electrons, namely, by reducing the electron capture cross-section of basic DNA units and the lifetime of corresponding transient anions. An interstrand electron transfer mechanism is proposed to explain the low ratios for the yields of SSB to those of DSB produced by 10 eV electrons.     

Thermoluminescence of irradiated polystyrene

Thermoluminescence of irradiated polystyrene has been studied in the temperature range 100 to 440°K. Three glow peaks with maximum at 160, 221, and 378°K have been observed. These peaks are analyzed by different methods and the activation energies which were obtained are compared. The activation energies are found to be 0.22, 0.48, and 1.45 eV for the peaks with maxima at 160, 221, and 378°K, respectively. Second-order kinetics is appropriate to all these cases. The glow peaks are attributed to the decay of the free radicals formed on irradiation and subsequent thermal stimulation. The peak with the maximum at 160°K is attributed to electron trapping by the carbonyl groups or peroxy radicals formed on irradiation. The curve with the peak at 221°K is attributed to the cyclohexadienyl radical, and the curve with the peak at 378°K is attributed to the chain radical ? CH₂? C (C₆H₅)? CH₂? . The centers responsible for the observed thermoluminescence are identified by correlation with electron spin resonance (ESR) data obtained on the same samples.     

Interaction of isomeric forms of xanthophyll pigment zeaxanthin with dipalmitoylphosphatidylcholine studied in monomolecular layers

Two-component monomolecular layers were formed with DPPC and two stereoisomers of zeaxanthin 9-cis and 13-cis at the argon-water interface. Very distinct over-additivity which represents affection of a lipid arrangement in the membrane has been observed in the case of zeaxanthin 9-cis (maximum at 20 mol%) but not in the case of zeaxanthin 13-cis. The differences in the organization of the isomers of zeaxanthin-DPPC monolayers are interpreted in terms of the different orientation of both xanthophylls at the interface observed at relatively high surface pressures (>25 mN/m) comparable to the surface pressures of biomembranes. The results are consistent with the model according to which zeaxanthin 9-cis adopts a vertical orientation at the polar-nonpolar interface in contrast to zeaxanthin 13-cis, which is oriented horizontally owing to the fact that it interacts by two hydroxyl groups with the same hydrophobic-hydrophilic interface in the monolayer. The findings are discussed in comparison with the behavior of zeaxanthin in the conformation all-trans in the same system. Zeaxanthin all-trans forms efficiently molecular aggregates in the mixed monolayers in contrast to cis isomers. Circular dichroism measurements show the formation of molecular structures by zeaxanthin 13-cis that are interpreted as dimers. FTIR measurements show that these dimers are stabilized by van der Waals interactions unlike aggregated structures formed by all-trans zeaxanthin that are stabilized by hydrogen bonding. Physiological importance of the differences in aggregation and orientation of stereoisomers of zeaxanthin in lipid environment is discussed.     

Molecular scale bioelectrochemistry

Our ability to interface with biological macromolecules continues to advance at a significant pace and brings with it potential applications that are as broad as they are powerful. Running alongside this evolution has been the refinement of methods by which interfaces can be probed at scales approaching (or being at) the molecular. Through combinations of experimental design, proximal probe technology and high sensitivity optical imaging, one can truly probe the bioelectronic interface at levels that are both startling and associated with unprecedented levels of detail.     

氯仿,乙醇,苯有关二元体系加压相平衡研究

氯仿、乙醇、苯有关二元体系加压相平衡研究马忠明，陈庚华，王琦，严新焕，韩世钧，余淑娴（浙江大学化学系，杭州，３１００２７）（江西大学化学系）关键词加压汽液平衡，醇烃体系，氯仿，乙醇，苯醇是极性分子，烃是非极性或弱极性分子，醇与醇、烃与烃分子及醇与烃分...     

Investigation of the stabilities of neutral and ionic lead and lead-antimony clusters under single and multiphoton ionization conditions

The fluence dependence of lead cluster ion distributions at 222 nm and 308 nm reveal markedly different behavior. Results obtained at 308 nm display a simple uniform increase in intensity with higher laser fluence with little change in relative intensities. At 222 nm, however, a significant transformation is found from a markedly different low fluence distribution to a high fluence pattern, which is essentially indistinguishable from that observed at 308 nm. It is concluded that mass spectra obtained at 308 nm, regardless of fluence, or at 222 nm and high fluence contain appreciable contributions from fragmentation. Hence, under these conditions the mass spectra are found to be dominated by cluster ion stabilities. Magic numbers observed at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters. This suggests the stabilities of both neutral and monovalent cationic lead clusters are largely determined by close-packing considerations, and are not appreciably influenced by electronic structure. Similar preferences for close-packed structures are also found for mixed lead-antimony clusters containing one or two antimony atoms that are ionized using high fluence 308 nm excitation.     

A Raman spectroscopic study of synthetic giniite

The mineral giniite has been synthesised and characterised by XRD, SEM and Raman and infrared spectroscopy. SEM images of the olive-green giniite display a very unusual image of pseudo-spheres with roughened surfaces of around 1-10microm in size. The face to face contact of the spheres suggests that the spheres are colloidal and carry a surface charge. Raman spectroscopy proves the (PO4)3- units are reduced in symmetry and in all probability more than one type of phosphate unit is found in the structure. Raman bands at 77K are observed at 3380 and 3186cm-1 with an additional sharp band at 3100cm-1. The first two bands are assigned to water stretching vibrations and the latter to an OH stretching band. Intense Raman bands observed at 396, 346 and 234cm-1are attributed to the FeO stretching vibrations. The giniite phosphate units are characterised by two Raman bands at 1023 and 948cm-1 assigned to symmetric stretching mode of the (PO4)3- units. A complex band is observed at 460.5cm-1 with additional components at 486.8 and 445.7cm-1 attributed to the nu(2) bending modes suggesting a reduction of symmetry of the (PO4)3- units.     

Synthetic Reactions of Metal Atoms at Temperatures of 10 to 273°K

Metal atoms, easily formed by vaporizing metals at high temperatures under vacuum, are reacted with inorganic and organic compounds in solution at 80 to 273°K or by co-condensing the atoms and substrate vapors at 10 to 80°K. Gram quantities of products containing metals in low valency states can be isolated in many cases, and some of the compounds formed are inaccessible by conventional synthetic routes.     

Numerical simulation of the solvate structures of acetylsalicylic acid in supercritical carbon dioxide containing polar co-solvents

Hydrogen-bonded complexes of acetylsalicylic acid with polar co-solvents in supercritical carbon dioxide, modified by methanol, ethanol, and acetone of 0.03 mole fraction concentration, are studied by numerical methods of classical molecular dynamics simulation and quantum chemical calculations. The structure, energy of formation, and lifetime of hydrogen-bonded complexes are determined, along with their temperature dependences (from 318 to 388 K at constant density of 0.7 g cm^?3). It is shown that the hydrogen bonds between acetylsalicylic acid and methanol are most stable at 318 K and are characterized by the highest value of absolute energy. At higher supercritical temperatures, however, the longest lifetime is observed for acetylsalicylic acid–ethanol complexes. These results correlate with the known literature experimental data showing that the maximum solubility of acetylsalicylic acid at density values close to those considered in this work and at temperatures of 318 and 328 K is achieved when using methanol and ethanol as co-solvents, respectively.     

Microdetermination of alpha-amino acids by spectrophotometric and titrimetric methods

Micro-methods for spectrophotometric and oxidimetric determination of alpha-amino-acids are described. They are based on a reaction of the acids with peri-naphthindan-2,3,4-trione hydrate at pH 2.5 to give a red precipitate of dihydroxy-peri-naphthindenone. The red product is dissolved and measured either spectrophotometrically at 243 nm or titrimetrically by reaction with iodine or N-bromosuccinimide. The results obtained by the three methods are concordant, the average recovery being 98 %.     

CHF_2Br与激发态氧原子O(~1D)的反应机理研究

利用密度泛函理论(DFT)研究了CHF2Br与O(1D)的反应机理.首先在B3LYP/6-311+G(d)水平上优化了各驻点的几何结构,并通过频率分析加以确认;然后利用内稟反应坐标理论,以相同的方法计算了反应的最小能量途径,进而确认了过渡态所连接的反应物和产物;最后在B3LYP/aug-cc-pvtz水平上对所有驻点的能量进行了校正.     

Regioselective reactivity of an asymmetric tetravalent di[dihydroxotin(IV)] bis-porphyrin host driven by hydrogen-bond templation

Local molecular environment effects on the rates of ligand exchange at an asymmetric di[dihydroxotin(IV)] bis-porphyrin 5 are examined. The host 5 possesses four non-equivalent tin(IV)-ligand binding sites that are distinguished by their position relative to a shallow cavity, by the steric environment at each binding site and by electronic-structure differences between the constituent porphyrin and quinoxalinoporphyrin macrocycles. These design features of the asymmetric host are confirmed by X-ray crystal structure analysis. Binding experiments with monodentate carboxylic acids and bidentate dicarboxylic acids show significant differences in the rate of ligand exchange at each of the four tin(IV) binding sites. For monodentate carboxylic acids, binding preferentially occurs at the exterior porphyrin site. Further addition of carboxylic acid results in sequential binding at the quinoxalinoporphyrin sites and lastly at the interior site on the porphyrin, with high regioselectivity. These selective binding outcomes are immediately apparent by NMR spectroscopy. A series of 2D NMR spectroscopy experiments allowed identification of the preferred binding sites at the host. This positively identifies that steric hindrance and electron-withdrawing functionality on the porphyrin macrocycle impede ligand exchange. However, these effects are overcome by dicarboxylic acid guests, which form ditopic hydrogen-bond interactions between the intracavity hydroxo ligands in the initial stage of ligand exchange, leading to regioselective binding between the tin(IV) sites within the cavity. It is envisaged that the factors identified herein that define regioselective ligand exchange at host 5 will find wider application in supramolecular systems incorporating tin(IV) porphyrins.     

Synthesis of highly substituted ureas and thioureas through 1,3-diaza-Claisen rearrangements

Isocyanates and isothiocyanates that are not activated by an electron withdrawing group react with azanorbornenes in benzene at reflux to afford ureas and thioureas through the corresponding 1,3-diaza-Claisen rearrangements. At higher temperatures, a triazinone byproduct is observed. Isocyanates and isothiocyanates that are activated by an electron-withdrawing group react at room temperature to give the corresponding ureas and thioureas. The reactions of the activated isocyanates and isothiocyanates are also accompanied by the formation of isoureas and isothioureas. Interestingly, while benzoyl isocyanate reacts with N-benzyl azanorbornene at room temperature to give a 2:1 mixture of urea to isourea, in benzene at reflux the only product observed is the urea. A crossover experiment rules out the possibility that the products are formed through a retro-Diels-Alder, [4+2] cycloaddition sequence instead of a 1,3-diaza-Claisen rearrangement. Competition experiments between isocyanates and isothiocyanates with limiting azanorbornene indicate that isothiocyanates react faster to give the rearrangement product. Since isocyanates are shown to be more electrophilic, these data are consistent with a fast addition step and a rate-determining rearrangement step.     

Aminolysis of crosslinked polysiloxanes: Tautomeric catalysis by 2-pyridone

Results are presented which extend earlier findings on the transformation of insoluble crosslinked polysiloxanes into soluble linear polymers by selective aminolysis at crosslink sites. The present study provides evidence of a substantial steric effect and catalysis by 2-pyridone. The results with 2-pyridone are attributed to tautomeric catalysis which, to our knowledge, had not been observed before in substitution reactions at silicon. Evidence is also presented that unhindered, difunctional primary amines effect aminolysis with both amine groups to produce insoluble crosslinked polymer, presumably with silylamine functionality at crosslink sites.     

Modeling of biological reactions

Cell membranes consist of lipid bilayers in which proteins are embedded. Many cell functions are carried out at the cell boundary which interface with water. Here we describe the response to an anesthetic of a lipid bilayer and of an enzyme separately. While both systems are markedly affected by anesthetics at appropriately high concentrations, the result at the clinical concentrations seems best explained as principally an effect on the protein somewhat accentuated by its presence in the lipid bilayer. Thus the lipoprotein complex seems to have the properties of the protein alone, except with somewhat greater induced sensitivity due to the lipid matrix in which it is immersed.     

Isomeric transitions in size-selected methanol hexamers probed by OH-stretch spectroscopy

We have measured the isomeric transition between the energetically lowest lying isomers of S6 and C2-symmetry of (CH3OH)6. The clusters are size-selected by deflection in collisions with He, and the isomers are identified by their infrared spectra of the OH-stretching vibration. The measurements are carried out at three source temperatures 253, 300 and 373 K which correspond to the cluster temperatures 93, 106 and 135 K. The latter ones are estimated by a relaxation model that accounts for the cluster formation and the energy released by the condensation. The transition takes place at a cluster temperature of about 102 K which is in agreement with the Molecular Dynamics simulation of such a transition at about 117 K using a realistic model potential.     

Separation and Direct Chemical Determination of Nitrosamines by High Performance Liquid Chromatography (HPLC)

Abstract

Volatile and non-volatile nitrosamines are determined at subparts per million levels in a continuous, inexpensive, low-temperature procedure when samples are compatible with the aqueous-alcoholic solvents. Individual nitrosamines are resolved by gradient elution on appropriate HPLC columns, converted to nitrite ion by ultra-violet light, combined with modified Griess reagent and the red-colored product measured at 540 nanometers. Inorganic nitrite may be determined and two, organic compounds are known to interfere. The detection level for N-nitrosodimethyl amine is 0.1 parts per million.     

Electrode independent chemoresistive response for cobalt phthalocyanine in the space charge limited conductivity regime

The electrical properties of 50 nm thick metallophthalocyanine films, prepared by organic molecular beam epitaxy (OMBE) on interdigitated electrodes, were studied with DC current-voltage measurements and impedance spectroscopy. The transition from Ohmic behavior at low voltages to space-charge-limited conductivity (SCLC) at higher voltages depends on the metal electrode (Pt, Pd, and Au), but does not correlate with the work function of the electrode. Impedance spectroscopy studies show the coexistence of low- and high-frequency traps in the thin film devices, and the contribution of low-frequency traps associated with Ohmic behavior diminishes at higher bias. Although device resistances are strongly influenced by the electrode material, and vary by a factor of over 300, the relative chemical sensor responses on exposure to dimethyl methylphosphonate (DMMP), methanol, water, or toluene vapors are similar for CoPc on Pt, Pd, and Au electrodes when these devices are operated in the SCLC regime at room temperature. When the devices are operated at voltages where the low-frequency interfacial traps are filled, the sensor response to analyte becomes uniform and reliable regardless of the specific interfacial electrode contact.