Phosphine Functionalization of GaAs(111)A Surfaces

Phosphorus-functionalized GaAs surfaces have been prepared by exposure of Cl-terminated GaAs(111)A surfaces to triethylphosphine (PEt3) or trichlorophosphine (PCl3), or by the direct functionalization of the native-oxide terminated GaAs(111)A surface with PCl3. The presence of phosphorus on each functionalized surface was confirmed by X-ray photoelectron spectroscopy. High-resolution, soft X-ray photoelectron spectroscopy was used to evaluate the As and Ga 3d regions of such surfaces. On PEt3 treated surfaces, the Ga 3d spectra exhibited a bulk Ga peak as well as peaks that were shifted to 0.35, 0.92 and 1.86 eV higher binding energy. These peaks were assigned to residual Cl-terminated Ga surface sites, surficial Ga2O and surficial Ga2O3, respectively. For PCl3-treated surfaces, the Ga 3d spectra displayed peaks ascribable to bulk Ga(As), Ga2O, and Ga2O3, as well as a peak shifted 0.30 eV to higher binding energy relative to the bulk signal. A peak corresponding to Ga(OH)3, observed on the Cl-terminated surface, was absent from all of the phosphine-functionalized surfaces. After reaction of the Cl-terminated GaAs(111)A surface with PCl3 or PEt3, the As 3d spectral region was free of As oxides and As0. Although native oxide-terminated GaAs surfaces were free of As oxides after reaction with PCl3, such surfaces contained detectable amounts of As0. Photoluminescence measurements indicted that phosphine-functionalized surfaces prepared from Cl-terminated GaAs(111)A surfaces had better electrical properties than the native-oxide capped GaAs(111)A surface, while the native-oxide covered surface treated with PCl3 showed no enhancement in PL intensity

Effects of Interfacial Energetics on the Effective Surface Recombination Velocity of Si/Liquid Contacts

Photoconductivity decay data have been obtained for NH_4F_((aq))-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene^(+/0) (Fc^(+/0)), [bis(pentamethylcyclopentadienyl)iron]^(+/0) (Me_(10)Fc^(+/0)), iodine (I_2), or cobaltocene^(+/0) (CoCp_2^(+/0)). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc^(+/0), I_2/I^-) or relatively negative (CoCp_2^(+/0)) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N_2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me_(10)Fc^(+/0)) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si−OCH_3 bonds, produced by reaction of H-terminated Si with CH_3OH−Fc^(+/0), showed lower surface recombination velocities in contact with N_(2(g)) or in contact with CH_3OH−Me_(10)Fc^(+/0) solutions than did NH_4F_((aq))-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH_3OH−Fc^(+/0)-treated surfaces showed lower surface recombination velocities than surfaces containing Si−I bonds, which were formed by the reaction of H-terminated Si surfaces with CH_3OH−I_2 or THF−I_2 solutions. These results can all be consistently explained through reference to the electrochemistry of Si/liquid contacts. In conjunction with prior measurements of the near-surface channel conductance for p^+−n−p^+ Si structures in contact with CH_3OH−Fc^(+/0) solutions, the data reveal that formation of an inversion layer (i.e., an accumulation of holes at the surface) on n-type Si, and not a reduced density of surface electrical trap sites, is primarily responsible for the long charge carrier lifetimes observed for Si surfaces in contact with CH_3OH or THF electrolytes containing I_2 or Fc^(+/0). Similarly, formation of an accumulation layer (i.e., an accumulation of electrons at the surface) consistently explains the low effective surface recombination velocity observed for the Si/CH_3OH−CoCp_2 and Si/CH_3CN−CoCp_2 contacts. Detailed digital simulations of the photoconductivity decay dynamics for semiconductors that are in conditions of inversion or depletion while in contact with redox-active electrolytes support these conclusions

Laserska desorpcijsko–ionizacijska masena spektroskopija potpomognuta sa C60

An application of C60-assisted laser desorption/ionization mass spectrometry of two organic samples including insulin (5734 Da), bovine albumin (66 000 Da) and a heteropoly inorganic system: phosphotungstic acid H3PO4(WO3)12×H2O, is presented. A nitrogen laser (337 nm) on a linear tandem time-of-flight mass spectrometer is used for the observation of the laser-power dependence of mass spectra.Opisuje se primjena laserske desorpcijsko–ionizacijske masene spektroskopije sa dva organska pripravka, insulin (5734 Da) sa crijevnim albuminom (66 000 Da), te heteropolni anorganski sistem od fosforovolframske kiseline, H3PO4(WO3)12×H2O. Primijenjen je dušikov laser pri linearnom tandem masenom spektrometru s mjerenjem vremena proleta i proučavana je ovisnost masenih spektara o snazi laserskog snopa. Pokazano je da se tehnika slojeva sa C60 kao substratom za desorpciju organskih ili neorganskih molekula, može primijeniti kao alternativna dobro poznatoj MALDI metodi

Which Depressive Symptoms and Medication Side Effects Are Perceived by Patients as Interfering Most with Occupational Functioning?

Background. Major depressive disorder (MDD) is associated with significant impairment in occupational functioning. This study sought to determine which depressive symptoms and medication side effects were perceived by patients with MDD to have the greatest interference on work functioning. Methods. 164 consecutive patients with MDD by DSM-IV criteria completed a standard assessment that included a self-rated questionnaire about the degree to which symptoms and side effects interfered with work functioning. Results. The symptoms perceived by patients as interfering most with work functioning were fatigue and low energy, insomnia, concentration and memory problems, anxiety, and irritability. The medication side effects rated as interfering most with work functioning were daytime sedation, insomnia, headache, and agitation/anxiety. There were no differences between men and women in symptoms or side effects that were perceived as interfering with work functioning. Limitations. This was a cross-sectional study; only subjective assessments of work functioning were obtained; the fact that patients were using varied medications acts as a potential confound. Conclusions. Specific depressive symptoms and medication side effects were perceived by patients as interfering more with occupational functioning than others. These factors should be considered in treatment selection (e.g., in the choice of antidepressant) in working patients with MDD

Theoretical Investigation of the Structure and Coverage of the Si(111)−OCH_3 Surface

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)−OCH_3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)−CH_3 and Si(111)−C_2H_5. The calculations indicate that 100% coverage is feasible for Si(111)−OCH_3 (similar to the methylated surface), as compared to only ∼80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)−OCH_3 surface from Si(111)−H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)−OCH_3, and such conditions are contrasted with the behavior of Si(111)−CH_3 and Si(111)−CH_2CH_3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface

Chlorination of hydrogen-terminated silicon (111) surfaces

Infrared absorption spectroscopy was used to investigate the chlorination of hydrogen-terminated Si(111) surfaces by three different methods: (a) exposure to a saturated solution of phosphorus pentachloride (PCl5) in chlorobenzene; (b) exposure to chlorine gas, Cl2(g), and (c) exposure to Cl2(g) under UV illumination. X-ray photoelectron spectroscopy and first principles model (clusters) calculations were used to explore the structure and dynamics of these surfaces. The infrared spectra exhibited sharp chlorine-related vibrations at 586 and 527 cm^–1. The narrow full width at half maximum of these vibrations for all three preparation methods indicated that all functionalization schemes produced a nearly complete monolayer of Cl with little surface roughening or introduction of step edges. The 527 cm^–1 mode was at a much higher frequency than might be expected for the bending vibration of Si monochloride. Theoretical calculations show, however, that this vibration involves the displacement of the top Si atom parallel to the surface, subject to a relatively stiff potential, shifting its frequency to a value fairly close to that of the Si–Cl stretching mode on a Si(111) surface