Possible evidence for quantum-size effects in self-assembled ultrathin films containing conjugated copolymers

Abstract

We present photoluminescence (PL), UV absorption, electroluminescence and x-ray reflectivity studies of self-assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be ‘‘tuned’’ by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self-assembled ultrathin films containing thin (∼7 Å) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1/d2 where d is the thickness of the assembly, as expected for confined photogenerated electron-hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (∼40 Å) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on self-assembled multilayer films with improved efficiency and stability and with threshold voltage as low as 2.6 V could be fabricated.

Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: suitable materials, structure and properties

“…Also, their smooth surface renders them well suited for X-ray reflectivity studies 131,312,381,382,385) and for near-field microscopy. For these purposes, mica 75,103,104,201,252,253,281,316,323,348,386,387) and float glass 19,80,131,180,182,184,185,388,389) present alternatives. Gold coated surfaces enable electrochemical studies 86,196,205,209,211,340,354,390,391) , investigations by quartz crystal microbalance (QCM) 65,86,102,119,130,202,238,266,279,305,338,339,350,…”

Optical properties of multilayered Alq 3 /α‐NPD structures investigated with spectroscopic ellipsometry

“…The ellipsometry spectra of the multilayered structure were simulated using the WVASE Software [8] taking into account the previously determined optical constants of Alq 3 and α-NPD. The thickness for all Alq 3 (α-NPD) layers was considered to be the same in the stack.…”

Electroluminescent characteristics of spin-assembled multilayer films with confined layer structure

“…More specifically, the PL and EL spectra of films with an organic quantum well structure are blue-shifted by the increased exciton binding energy due to the quantum confinement effect in a potential well [18][19][20][21]33,34]. Hong et al reported that, in the case of adsorbing the relatively thick insulating layer (about 3-5 nm), PL spectra of dipassembled (PPV/PSS) n or (PPV/PAA) n multilayer films are blue-shifted because these insulating layers effectively screen the interpenetration between neighboring PPV layers [35,36]. In this view, it should be noted that the respective PMAA layers of about 1.0 nm thickness can separate PPV layers inducing the blue shift in the PL and EL spectra of spin-assembled EL devices due to the relatively low level of interpenetration between the PPV and PMAA layers (i.e., the distinct interface between the two layers).…”