Abstract
Polystyrene-b-poly(L-lactide) (PS-PLLA) diblock copolymer thin films with well-oriented cylindrical microdomains normal to the substrate were prepared by spin coating. After hydrolysis of PLLA, well-oriented hexagonal cylinder (HC) nanochannel arrays over large area were obtained; providing a simple and efficient path to prepare nanoporous templates for pore-filling with optoelectronic materials such as CdS and light-emitting polymers to examine the quantum confinement effect. To achieve efficient pore-filling process, specific treatment on the polymeric templates was carried out. There are two key features in the improvement of pore-filling efficiency: (1) controlling wetting by decreasing the contact angle of solution for substrate; (2) releasing air-block effect by air-extracting apparatus. Oxygen plasma treatment was conducted first to create hydrophilic PS templates so as to sequester the solution of functional gold nanoparticles/aqueous into porous templates by capillary force. On the other hand, in contrast to the pore-filling methods by modifying the substrate with the assistance of the air-extracting apparatus, methanol is used as co-solvent to dissolve the CdAc2．2H2O; the wettability of the methanol solution on the PS templates can also be improved without RIE treatment. To avoid air blocking effect, the filling process was carried out in a vacuum holder whereas ultrasonic process can also be applied to introduce the solution into the templates. Because of HC nanochannel arrays and the cylindrical nanochannels truly span the entire thickness of the films; a special experiment (directed capillary force) is designed for pore-filling of Cd2+ into template. The formation of CdS nanostructures were then achieved by using H2S(g) as reduction agent at which the cylinder morphology of CdS nanocrystals were obtained as evidenced by transmission electron microscopy and electron diffraction. Interesting spectroscopic results were found in ultraviolet (UV) and potoluminescence (PL) spectra; both exhibiting a higher efficiency of emission for the CdS confined in the templates as compared to the CdS nanoparticles formed directly on cadmium thin films. We speculate that the increase in intensity is attributed to the significant increase of filling CdS nanocrystals in the nanoporous template. By contrast, a blue-shift for light-emitting polymers confined in the templates as compared to the thin films of light-emitting polymers was observed. TEM image shows that hexagonal light-emitting polymers nanoarrays were obtained from the contrast by bromines (Br) loading on repeat unit of light-emitting polymers. In addition, as evidenced by cryo high resolution transmission (HR-TEM) and energy dispersive x-ray analysis (EDX), the dark contrast indicates the signal of Br element. Furthermore, the mapping image of Br element from field-emission scanning electron microscopy (FE-SEM) EDX indicates that the Br elements distributes uniformly in PS template.