Nanomechanics of CdSe quantum dot-polymer nanocomposite films

We report the mechanical characterization of a nanocomposite thin film consisting of CdSe quantum dots (QDs) and the electroluminescent polymer poly[2-methoxy-5-2(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV). The electrical and optical properties of this nanocomposite have been studied intensely for organic electronics research. However, the mechanical behaviour-which depends on several variables, such as the concentration of QDs, the interfacial surface area, deformation mechanisms, and the mechanical properties of the QDs and polymer-is not well understood. In this paper, thin films of CdSe QDs blended with MEH-PPV are prepared at different QD:polymer ratios. The QDs' surface ligands are removed to promote dispersion and to more realistically mimic QD-polymer devices. QD dispersion is verified using transmission electron microscopy, while the films' morphology and roughness are observed using atomic force microscopy. Finally, quasi-static nanoindentation is used to measure the elastic modulus, hardness, and creep of the films. The incorporation of QDs into the polymer matrix is seen to increase the elastic modulus and hardness by factors of 4 and 5, respectively, both of which scale linearly as a function of QD volume fraction. Furthermore, the QDs have the effect of suppressing the viscoelastic behaviour of the polymer, which is observed by studying the creep under a constant load. These results may have profound implications for future nanocomposite devices, such as increased stiffness, damage resistance, and long-term stability.