result in an improvement in device performance, revealing that Me-LPPP, in addition to MEH-PPV, is hole-limited with a Ca cathode. For conjugated polymer films, such as PAni, the improvement in quantum efficiency is due to an increase in the anode work function to 5.1 ± 0.1 eV, which results in a nearly ohmic contact. For nanoparticle monolayers, the improvement is due to an increase in the local electric field across the interface. This accelerating local electric field is induced by a net negative charge on the nanoparticle surface which results either from silicon hydroxyl groups on the SiO 2 surface or from electrons which are trapped at the interface between the conjugated polymer and nanoparticle.In conclusion, we have shown that modification of the ITO electrode with SiO 2 nanoparticles can dramatically improve electroluminescence properties of polymer lightemitting devices (PLED). The charged nanoparticle surface, which serves as a carrier trap at low current densities, can induce a dipole moment across the electrode interface, effectively increasing the local electric field and promoting carrier injection. This effect enables the ability to improve PLED efficiency with a single monolayer without including additional polymer layers or modifying the electrode work function. Understanding the nature of the nanoparticle surface will clearly be critical to controlling and optimizing the performance of polymer/nanoparticle composite materials, offering further promise for innovative optoelectronic applications.

We describe the deformation behavior of polymer brushes and mushrooms compressed by finite-sized particles for the cases where the chains are fixed or mobile on the grafting surface. When the size of the particle is large compared to the grafting distance of the chains, the force on the particle is the same to lowest order in compression for both the fixed and surface mobile chains. Compression of a single mushroom can lead to a first order like escape transition, where part of the chain escapes from under the particle. These transitions can be seen either in the chain radius or in the compressional force law behavior. For surface mobile mushrooms, the force is considerably smaller because of the evacuation of chains from under the particle. The force law in this case also exhibits a maximum at a certain compression, indicating that the system undergoes a collapse transition above a critical pressure or yield stress P c ≈ kTσ0 R F3 -1, where σ0 is the grafting density (chains/area) and R F3 is the unperturbed mushroom size. Finally, we consider the case of bending of stiff chains grafted to a solid surface. In the case of a single chain, the force is a constant for weak compressions. In the multichain case, the force can be substantially lower because of the escape of chains from under the particle.

PACS. 36.20Ey -Macromolecules and polymer molecules: conformation (statistics and dynamics). PACS. 87.15 -v -Molecular biophysics. PACS. 46.30Lx -State buckling and instability.