The grafting of alkanethiol-terminated poly(ethylene glycol) [HS(CH2)11(OCH2CH2)n-OCH3; n ) 34-56, MW ≈ 2224 Da] onto polycrystalline gold from dilute solutions was investigated by ellipsometry, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and in situ second harmonic generation. After immersion of a gold-coated Si wafer into a 50 µM dimethylformamide solution, the thickness of the grafted layer increases in a first rapid step up to ∼20 Å. After about 10 min, the thickness rises notably again and reaches saturation after ∼2 h at ∼120 Å. The kinetics of film formation clearly deviate from Langmuir kinetics, which is normally observed for the self-assembly of nonfunctionalized alkanethiols. Our observation can be explained by a conformational transition of the grafted poly(ethylene glycol) chains from amorphous coils to a brush morphology, predominantly consisting of helices with an orientation perpendicular to the surface. The second harmonic generation experiments show that the coverage at saturation of adsorption corresponds to ∼90% that of self-assembled monolayers of alkanethiols, indicating a densely packed film.
We report markedly different transport properties of ABA- and ABC-stacked
trilayer graphenes. Our experiments in double-gated trilayer devices provide
evidence that a perpendicular electric field opens an energy gap in the ABC
trilayer, while it causes the increase of a band overlap in the ABA trilayer.
In a perpendicular magnetic field, the ABA trilayer develops quantum Hall
plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2,
whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and
10 with 4-fold spin and valley degeneracy.Comment: 4 pages, 4 figure
The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., approximately 45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from approximately 25 A in the dry state to approximately 70 A in contact with D2O, which corresponds to a total water uptake of approximately 38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (approximately 8 mass %) with a corresponding change of the layer thickness from approximately 110 to approximately 125 A. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.
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