Spatially controlled CdSe nanocrystal distribution in phase separated polymer blend films
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Spatially controlled CdSe nanocrystal distribution in phase separated polymer blend films Harumi Asami, Soichiro Saita, Itaru Kamiya, and Kenichi Yoshie Yokohama Research Center, Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan ABSTRACT
We report the fabrication and characterization of the self-assembled structure of tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals in binary polymer phase separated films of polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinylpyridine) (P2VP). These structures are formed via demixing of CdSe nanocrystals and binary polymer during spin coating. The nanocrystals preferentially segregate to the PS-rich phase in phase separated PS/PMMA and to the P2VP-rich phase in phase separated PS/P2VP, as shown by optical microscopy and photoluminescence images. For CdSe/PS/PMMA, we attribute the driving force for CdSe segregating to the PS-rich domain to the stronger attractive interaction of TOPO on PS with respect to PMMA due to polarity, and in the case of CdSe/PS/P2VP, segregating to the P2VP-rich domain to the interaction such as coordinate bonds being formed between pyridine groups and surface Cd atoms. INTRODUCTION
Semiconductor nanocrystals, or colloidal quantum dots, are small molecular clusters whose size is ca. 1 to 10 nm in diameter and exhibit unique optical properties arising from 3-dimensional quantum confinement of carriers, which are different from bulk crystals[1–4]. Quantum confinement leads to formation of discrete and blue-shifted absorption bands and increase of oscillator strength due to electron-hole overlap, giving enhanced emission efficiency and optical nonlinearity, and the emission wavelengths can be changed by simply controlling the nanocrystal size. Quantum dots of cadmium selenide is of particular interest since its luminescence is in the visible regime[5,6]. Thin films that consist of nanocrystal-polymer composites hold great promise as hybrid organic/inorganic materials that can be applied for example to electroluminescent devices, photovoltaic cells, and nonlinear optical devices[7–10]. The nanocrystals can be incorporated into a variety of matrices such as polymers, glasses, and/or ferroelectric materials. While there are various methods for preparing nanocrystal-polymer composites, these thin films can be fabricated simply by spin coating or casting nanocrystals/polymer solution onto a substrate. In this case, the film structure formed via self-assembly depends on the polymer-nanocrystal interaction. The fabrication methods which utilize self-assembly such as polymer phase separation provide variety of organized organic/inorganic composite structures whose size range from nanometer to micrometer[11–14]. It is also of technological importance to create structures with dimensions comparable to the wavelength of visible light wherein many opportunities exist in various optoelectronic applications such as waveguides, photonic band gap materials and electroluminescent devices[14–16]. In this l
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