The bilayer−substrate coupling in fluid bilayers of dipalmitoyl phosphatidylcholine (DPPC) on a solid support of spherical silica beads was examined by measuring the lateral diffusion of the lipids in both monolayers using a deuterium NMR relaxation technique. The results obtained at 55 °C show that the lipid diffusion constant in the monolayer facing the silica surface, D = 7.5 × 10-12 m2/s, is slower by a factor of 2 than that in the monolayer exposed to the bulk water (D = 14 × 10-12 m2/s). This indicates that the monolayer−monolayer coupling in fluid bilayers must be rather weak compared to the monolayer−substrate coupling across an ultrathin water film between the bilayer and the silica surface.
The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe metal-ZnO diodes as a function of native defect concentration, oxygen plasma processing, and metallization. The results show that resident native defects in ZnO single crystals and native defects created by the metallization process dominate metal-ZnO Schottky barrier heights and ideality factors. Results for ZnO(0001¯) faces processed with room temperature remote oxygen plasmas to remove surface adsorbates and reduce subsurface native defects demonstrate the pivotal importance of crystal growth quality and metal-ZnO reactivity in forming near-interface states that control Schottky barrier properties.
We have used micro-Auger electron spectroscopy, cathodoluminescence spectroscopy, and work function measurements in copper indium gallium diselenide polycrystalline solar cell films cleaved in ultrahigh vacuum. We establish that, relative to the grain interior, the grain boundary shows (1) a Cu composition decrease, as large as a factor of two, (2) a work function decrease of up to 480 meV, and (3) no additional radiative recombination centers despite a high concentration of grain boundary (GB) defects. These results confirm theoretical predictions that (i) polar GB interfaces are stabilized by massive (∼50%) removal of Cu atoms, leading to (ii) a valence band offset between GB and grain interiors that (iii) repels holes from the GB, thus likely reducing GB electron-hole recombination and improving photovoltaic (and other photonic) device operation.
A monolayer of 1,2-dipalmitoyl-d62-glycero-3-phosphocholine (DPPC-d62) coated onto silanized silica beads (spherical supported monolayer: SSM) is studied by 2H-NMR and DSC. The results are compared with those obtained from a single bilayer on the same solid support (spherical supported vesicles: SSV) and from multilamellar vesicles (MLV). The phase transition temperature (Tm) of the SSMs is significantly higher than that of the bilayer systems and the extent of this difference depends on the lipid density in the monolayer that is determined during its preparation. 2H-NMR reveals a gel and fluid phase coexistence in the SSM transition region. A comparison of the 2H-NMR line shapes suggests the presence of highly curved structures for the fluid phase of the SSM samples. From a comparison of SSM and SSV transverse relaxation in the fluid phase we can conclude that the lateral diffusion coefficient D1 in supported monolayers is similar to that in bilayers.
Lipid bilayers, the basis of all natural membranes, are highly dynamic, quasi two-dimensional, partially ordered systems. Their unique elastic properties lead to a number of applications in biomedical devices (e.g., biocompatibilization of surfaces, drug delivery). These properties are a result of the unique double-layer structure and of the frictional drag between the two bilayer leaflets. The C~O fullerene shows extraordinary lubrication properties['] and its diameter of 10SA is just one fifth the average bilayer thickness. Thus, intercalation of CbO molecules between the two monolayers of a lipid bilayer may give interesting new material properties and may also affect the hierarchy of motions in a bilayer, which cover nearly 12 orders of magnitude in time-from hertz to terahert~. ['-~l To overcome the solubility problem of plain c 6 0 in bilayers,l4I we modified C60 by attaching six pairs of CI2-and Cls-alkyl chains, corresponding to the chain length of lauric and stearic acid, respectively, onto octahedral sites (Th symmetry) around the fullerene cage, giving so-called lipofullerenes. Although this chemical modification certainly alters the rotational dynamics of the c60, and thus possibly its lubrication properties, it almost completely retains its unique symmetry, and may change the phospholipid bilayer to a composite system with new physical properties.Multilamellar vesicles (MLVs) of the lipid dipalmitoylsn-glycero-3-phosphatidylcholine (DPPC) and C6", to which six pairs of either dodecyl (1) or octadecyl (2) alkyl chains were covalently attached (denoted as C60-HADC12 and C60-HADC18, respectively), were studied at different RYR R R mixing ratios by differential scanning calorimetry (DSC). Figure 1 shows endotherms and exotherms of DPPC with 15 mol % of the fullerene for both C60-HADCI2 and c60-HADCIS. The effect of both lipo-fullerenes on the phase I ~~" I~" ' I "~~1~~' "~ 30 40 50 60 70 temperature rC1 Fig 1 DSC endotherms (full lines) and exotherms (broken lines) of lipofullerenes in DPPC multilamellar vesicles Top: DPPC with 15 mol % Cbo-HADC12 Bottom DPPC with 15 mol % ChO-HADCl8transition behavior of the DPPC was surprisingly low. We observed a broadening of the main phase transition at T, = 41 "C by a factor of 2-3 and a slight reduction of the phase transition temperature by 0.5 "C. Even the pretransition of DPPC which can be observed for heating scans at Tp = 30.5"C was not completely abolished by the presence of the lipo-fullerenes. In the case of Cho-HADCls (Fig. 1, bottom trace) a second large peak was observed at 71 "C (heating scan) and at 38°C (cooling scan). Since plain C60-HADCIS exhibits a transition at nearly the same temperature and with a similar hysteresis, we assign this peak to the Adv. Mater. 1997, 9, Nu. 11 0 WILEY-VCH Verlag CmhH, 0-69469 Weinheim, 1997 0935-9648/97/1109-0913 $17.50+.50/0 913
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