Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

In recent years a wide range of materials including metals, semiconductors and metal oxides have been synthesized with nanometer dimensions. In particular II–VI semiconductor nanocrystals (NCs) are receiving considerable attention for fundamental studies as zerodimensional quantum confined material and for their potential application in optoelectronics and photonics. The colour tunability of the emission of semiconductor NCs as a function of size is one of their most attractive properties. A current bottle-neck of these materials is their poor compatibility with micropatterning methods in general and MEMS processes in particular. Inorganic colloidal nanoparticles encapsulated in polymer matrix may lead to versatile hybrid micro- and nanoscale devices made by simple techniques. Colloidal methods provide effective routes to prepare semiconductor NCs that are soluble in organic solvents with a narrow size distribution. Moreover the ability to manipulate their surface chemistry is a powerful tool to place the NCs in almost any chemical environment, due to the possibility of properly engineering their surface by ligand exchange techniques. Recent efforts have been directed towards the fabrication of polymer nanocomposites containing nanometer sized inorganic semiconductor. Such interest is due to the combination the processability and the structural properties of the polymeric matrix, i.e. thermoplastic or photosensitive materials, and the unique optoelectronic characteristics of the inorganic moiety, that can be thus used as emitters for many purposes such as optical switches, sensors, electro-luminescent devices, lasers and biomedical tags. In particular, microelectromechanical systems (MEMS) that are currently built mainly from Silicon, dielectrics, polymers and some selected metals will benefit from new functional polymeric nanocomposit materials, which combine simple UV processing and superior material properties such as biocompatibility, chemical stability, luminescence, photochromism and more. This paper describes the combination of synthesis and functionalization of luminescent colloidal NCs for polymer modification, with micro- and nanofabrication methods to form basic elements of advanced polymer MEMS/NEMS with increased inherent functionality. We prepared nanocomposites based on highly luminescent II-VI semiconductors colloidal CdS, CdSe, and CdSe@ZnS, core shell type. The NCs were incorporated in poly-methyl methacrylate (PMMA) a thermoplastic material, and SU-8, a photosensitive polymer with superior lithographic properties, respectively. Spectroscopical (UV-vis absorption and emission), structural (TEM) and morphological analysis were performed on both types of nanocomposite solutions as well as the thin films spin coated onto silicon and quartz substrates. The effect of NC composition, concentration, size, and surface chemistry was evaluated in order to understand the role played by these factors on the optical properties of the investigated nanocomposites. The presence of organic ligand shell at colloidal NC surface shows to be critical for their incorporation either into SU8 and PMMA. Incorporation of the different types of luminescent NCs in PMMA and SU-8 did not show any relevant change of the optical properties of the NCs. The excitonic peaks as well as the emission properties were retained. Preliminary spinning, UV exposure and development experiments were performed on the SU-8 based nanocomposites. The obtained structures were inspected by optical microscope and confirmed that the overall UVstructuring capability of the modified polymer was not macroscopically affected by the NC incorporation in the polymer.