Mechanical Characterization of Nanocomposite CdSe Quantum Dot – MEH-PPV Polymer Thin Films via Nanoindentation

摘要

Progress in the burgeoning field of organic electronics is enabling the development of novel technologies such as low-cost, printable solar cells and flexible, high-resolution displays. One exciting avenue of research in this field is nanostructured hybrid organics such as quantum dot (QD)-polymer devices. The incorporation of QDs can greatly improve a device’s efficiency and gives one the ability to tune its electrical and optical characteristics. In order for such technologies to be commercially viable, it is important to classify their mechanical integrity and reliability. Surprisingly little is known about the mechanical properties of QD-polymer thin films (u3c100 nm). This is in part due to challenges of: (1) isolating the mechanical response of a thin film from the underlying substrate, (2) obtaining a homogeneous dispersion of QDs in the film, and (3) the sensitivity of mechanical properties to the inherent rate dependence of polymer deformation (i.e., viscoelasticity). All of these challenges can introduce significant errors in the measurement of mechanical properties. Furthermore, the deformation mechanisms in nanocomposites are not well understood, so it is difficult to predict the effect of adding QDs on the mechanical behavior of films. In this thesis, these challenges are addressed for characterizing the mechanical properties of thin films of CdSe QD-poly[2-methoxy-5-2(2΄-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) nanocomposites using quasi-static nanoindentation testing. Elastic modulus, hardness, and creep are measured as a function of QD concentration and loading and unloading rates. The QDsu27 ligands are removed by pyridine treatment prior to mixing with MEH-PPV to improve dispersion. The films are prepared via spin-coating onto glass substrates and subsequent annealing in air. Efforts are taken in the mechanical testing to minimize errors due to viscoelastic creep and interference from the substrate. Transmission electron microscopy reveals that the QDs are relatively well-dispersed in the polymer matrix. It is observed that adding QDs increases the elastic modulus (E) and hardness (H) of the films, while reducing the viscoelastic creep. Both E and H increase linearly with the volume percent of QDs. E ranges from 14.5 GPa to 52.7 GPa for pure MEH-PPV (0% QDs) and 100% QD films, respectively, while H ranges from 220 MPa to 1430 MPa for the same films, respectively. The films behave viscoelastically at lower QD loading, but assume a more granular character as the loading approaches 100%.