47 research outputs found
Synthesis and Characterization of Mixed Methyl/Allyl Monolayers on Si(111)
The formation of mixed methyl/allyl monolayers has been accomplished through a two-step halogenation/alkylation reaction on Si(111) surfaces. The total coverage of alkylated Si, the surface recombination velocities, and the degree of surface oxidation as a function of time have been investigated using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and microwave conductivity measurements. The total coverage of alkyl groups, the rate of oxidation, and the surface recombination velocities of Si(111) terminated by mixed monolayers were found to be close to those observed for CH_3−Si(111) surfaces. Hence, the mixed-monolayer surfaces retained the beneficial properties of CH_3−Si(111) surfaces while allowing for convenient secondary surface functionalization
Heck Coupling of Olefins to Mixed Methyl/Thienyl Monolayers on Si(111) Surfaces
The Heck reaction has been used to couple olefins to a Si(111) surface that was functionalized with a mixed monolayer comprised of methyl and thienyl groups. The coupling method maintained a conjugated linkage between the surface and the olefinic surface functionality, to allow for facile charge transfer from the silicon surface. While a Si(111) surface terminated only with thienyl groups displayed a surface recombination velocity, S, of 670 ± 190 cm s^(–1), the mixed CH_3/SC_4H_3–Si(111) surfaces with a coverage of θ_(SC_4H_3) = 0.15 ± 0.02 displayed a substantially lower value of S = 27 ± 9 cm s^(–1). Accordingly, CH_3/SC_4H_3–Si(111) surfaces were brominated with N-bromosuccinimide, to produce mixed CH_3/SC_4H_2Br–Si(111) surfaces with coverages of θ_(Br–Si) < 0.05. The resulting aryl halide surfaces were activated using [Pd(PPh_3)_4] as a catalyst. After activation, Pd(II) was selectively coordinated by oxidative addition to the surface-bound aryl halide. The olefinic substrates 4-fluorostyrene, vinylferrocene, and protoporphyrin IX dimethyl ester were then coupled (in dimethylformamide at 100 °C) to the Pd-containing functionalized Si surfaces. The porphyrin-modified surface was then metalated with Co, Cu, or Zn. The vinylferrocene-modified Si(111) surface showed a linear dependence of the peak current on scan rate in cyclic voltammetry, indicating that facile electron transfer had been maintained and providing evidence of a robust linkage between the Si surface and the tethered ferrocene. The final Heck-coupled surface exhibited S = 70 cm s^(–1), indicating that high-quality surfaces could be produced by this multistep synthetic approach for tethering small molecules to silicon photoelectrodes
Low pH enhances the action of maximin H5 against Staphylococcus aureus and helps mediate lysylated phosphatidylglycerol induced resistance
Maximin H5 (MH5) is an amphibian antimicrobial peptide specifically targeting Staphylococcus aureus. At pH 6, the peptide showed an increased ability to penetrate (∆П = 6.2 mN m-1) and lyse (lysis = 48 %) S. aureus membrane mimics, which incorporated physiological levels of lysylated phosphatidylglycerol (Lys-PG, 60 %) as compared to pH 7 (∆П = 5.6 mN m-1 and lysis = 40 % at pH 7) where levels of Lys-PG are lower (40 %). The peptide therefore appears to have optimal function at pH levels known to be optimal for the organism’s growth. MH5 killed S. aureus (minimum inhibitory concentration = 90 µM) via membranolytic mechanisms that involved the stabilization of α-helical structure (circa 45-50 %) and which showed similarities to the ‘Carpet’ mechanism based on its ability to increase the rigidity (Cs-1 = 109.94 mN m-1) and thermodynamic stability (∆Gmix = -3.0) of physiologically relevant S. aureus membrane mimics at pH 6. Based on theoretical analysis this mechanism may involve the use of a tilted peptide structure and efficacy was noted to vary inversely with the Lys-PG content of S. aureus membrane mimics for each pH studied (R2 circa 0.97), which led to the suggestion that under biologically relevant conditions, low pH helps mediate Lys-PG induced resistance in S. aureus to MH5 antibacterial action. The peptide showed a lack of haemolytic activity (< 2 % haemolysis) and merits further investigation as a potential template for development as an anti-staphylococcal agent in medically and biotechnically relevant areas
Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization
The band-edge positions of H-, Cl-,
Br-, methyl-, and ethyl-terminated
Si(111) surfaces were investigated through a combination of density
functional theory (DFT) and many-body perturbation theory, as well
as by photoelectron spectroscopy and electrical device measurements.
The calculated trends in surface potential shifts as a function of
the adsorbate type and coverage are consistent with the calculated
strength and direction of the dipole moment of the adsorbate radicals
in conjunction with simple electronegativity-based expectations. The
quasi-particle energies, such as the ionization potential (IP), that
were calculated by use of many-body perturbation theory were in good
agreement with experiment. The IP values that were calculated by DFT
exhibited substantial errors, but nevertheless, the IP differences,
i.e., IP<sub>R–Si(111)</sub>–IP<sub>H–Si(111)</sub>, computed using DFT were in good agreement with spectroscopic and
electrical measurements
Electrical Junction Behavior of Poly(3,4-ethylenedioxythiophene) (PEDOT) Contacts to H‑Terminated and CH<sub>3</sub>‑Terminated p‑, n‑, and n<sup>+</sup>‑Si(111) Surfaces
The electronic and photovoltaic properties
of junctions between
the conducting polymer polyÂ(3,4-ethylenedioxythiophene) (PEDOT) and
Si(111) surfaces have been investigated for a range of doping types,
doping levels, and surface functionalization of the Si. PEDOT–polyÂ(styrenesulfonate)
(PSS) formed ohmic, low resistance contacts to H-terminated and CH<sub>3</sub>-terminated p-type Si(111) surfaces. In contrast, PEDOT formed
high barrier height (0.8–1.0 V) contacts to n-Si(111) surfaces,
with CH<sub>3</sub>-terminated n-Si(111)/PEDOT contacts showing slightly
higher barrier heights (1.01 eV) than H-terminated n-Si(111)/PEDOT
contacts (0.89 V). PEDOT contacts to CH<sub>3</sub>-terminated and
H-terminated n-Si(111) surfaces both produced photovoltages under
illumination in accord with the Shockley diode limit based on bulk/recombination
diffusion in the semiconductor. Such devices produced solar energy-conversion
efficiencies of 5.7% under 100 mW cm<sup>–2</sup> of simulated
air mass 1.5 illumination. The electrical properties of PEDOT contacts
to CH<sub>3</sub>-terminated Si surfaces were significantly more stable
in an air ambient than the electrical properties of PEDOT contacts
to H-terminated Si surfaces. PEDOT films produced a low resistance,
tunnel-barrier type of ohmic contact to n<sup>+</sup>-SiÂ(111) surfaces.
Hence, through various combinations of doping type, doping level,
and surface functionalization, the PEDOT/Si contact system offers
a wide range of opportunities for integration into monolithic photovoltaic
and/or artificial photosynthetic systems
Heck Coupling of Olefins to Mixed Methyl/Thienyl Monolayers on Si(111) Surfaces
The
Heck reaction has been used to couple olefins to a Si(111)
surface that was functionalized with a mixed monolayer comprised of
methyl and thienyl groups. The coupling method maintained a conjugated
linkage between the surface and the olefinic surface functionality,
to allow for facile charge transfer from the silicon surface. While
a Si(111) surface terminated only with thienyl groups displayed a
surface recombination velocity, <i>S</i>, of 670 ±
190 cm s<sup>–1</sup>, the mixed CH<sub>3</sub>/SC<sub>4</sub>H<sub>3</sub>–SiÂ(111) surfaces with a coverage of θ<sub>SC<sub>4</sub>H<sub>3</sub></sub> = 0.15 ± 0.02 displayed a
substantially lower value of <i>S</i> = 27 ± 9 cm s<sup>–1</sup>. Accordingly, CH<sub>3</sub>/SC<sub>4</sub>H<sub>3</sub>–SiÂ(111) surfaces were brominated with <i>N</i>-bromosuccinimide, to produce mixed CH<sub>3</sub>/SC<sub>4</sub>H<sub>2</sub>Br–SiÂ(111) surfaces with coverages of θ<sub>Br–Si</sub> < 0.05. The resulting aryl halide surfaces
were activated using [PdÂ(PPh<sub>3</sub>)<sub>4</sub>] as a catalyst.
After activation, PdÂ(II) was selectively coordinated by oxidative
addition to the surface-bound aryl halide. The olefinic substrates
4-fluorostyrene, vinylferrocene, and protoporphyrin IX dimethyl ester
were then coupled (in dimethylformamide at 100 °C) to the Pd-containing
functionalized Si surfaces. The porphyrin-modified surface was then
metalated with Co, Cu, or Zn. The vinylferrocene-modified Si(111)
surface showed a linear dependence of the peak current on scan rate
in cyclic voltammetry, indicating that facile electron transfer had
been maintained and providing evidence of a robust linkage between
the Si surface and the tethered ferrocene. The final Heck-coupled
surface exhibited <i>S</i> = 70 cm s<sup>–1</sup>, indicating that high-quality surfaces could be produced by this
multistep synthetic approach for tethering small molecules to silicon
photoelectrodes