| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 92 === We report nanoscale morphological influences on optoelectronic properties investigated by conducting atomic force microscopy (CAFM) and scanning near-field optical microscopy (SNOM). The CAFM is operated with positive tip bias to get synchronously topography and current images. Furthermore, we use CAFM to measure local I-V curve and transform these data into Poole-Frenkel plot by space charge limited current (SCLC) theory and Poole-Frenkel (P-F) law. The SNOM is used to acquire synchronously topography and optical transmission intensity images.
The CAFM scanning results indicate the current distribution is not homogeneous. It should be noted that some bumps with high current comparing to other region. We measure the local I-V curve and figure out the zero-field mobility and electric coefficient by P-F plot. The calculated electric filed coefficient is consistent to many reports of macroscopic measurement. However, the zero-field mobility is 3 orders of magnitude to macroscopic results. It can be attributed to two reasons: one is that the energy deviation is smaller on nanoscale; the other is that the high current bumps have higher extent of inter-chain interaction. These two reasons make carriers conduct more easily from chain to chain which makes mobility enhanced.
Besides, we prepared MEH-PPV in the different solvents and spin coated them on gold films. As repeat scanning on chloroform prepared sample, the current becomes higher with the times of scanning increasing. But the current of toluene prepared sample becomes smaller. These phenomena indicate the good solvent prepared and gold film based samples have higher extent of inter-chain interaction.
The synchronously optical transmission intensity and topography images are obtained by SNOM. We prepare MEH-PPV in chloroform on two kinds of transparent substrate. The one is ITO glass and the other is mica. The results show that the high bumps on mica based sample have low optical transmission rate. But some high region on ITO based sample has high optical transmission rate.
Our experimental results indicate that different solvents and different substrates generate different morphology. And this makes polymer chain conformation altered accompanying conjugation length elongated or shortened. The conjugation length of electroluminescent polymer influences optoelectronic properties very much. Thus, by this technique, we can change local optoelectronic properties on nanoscale.
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