AFM and Morphology Characterization

Background:

The nanoscale morphology of polymer based devices is a critical component of the device operation.  Both polymer based photovoltaics and polymer light emitting devices (PLEDs) require a uniform film of an appropriate thickness for efficient device operation.  Additionally, photovoltaic devices require phase separation on the nanometer scale to allow for efficient exciton dissociation and charge transport.  In contrast, PLEDs generally require a homogenous film with no aggregation or phase separation.

There are several techniques that can be used to probe the nanoscale morphology of polymer films.  The main techniques used in our group include AFM, TEM, and SEM.

Atomic Force Microscopy:

Atomic force microscopy (AFM) is based on the interaction of an extremely sharp tip (~10 nm radius) with the surface of a substrate.  The most common approach involves scanning the tip across the substrate while the tip gently taps the substrate.  The tip follows the surface of the substrate and a height image can be generated as shown in figure 1.

Figure 1. A series of height images showing the change in morphology as the ratio of polymer (PTVBT) to PCBM is varied.

The Reynolds group and the Schanze group share a Veeco Innova scanning probe microscope (SPM) with a Nanodrive controller as shown in Figure 2.

Figure 2.  The Veeco Innova SPM.

This Innova SPM has several imaging modes including tapping mode, contact mode, phase imaging, electrostatic force microscopy (EFM), scanning tunneling microscopy (STM), and conductive atomic force microscopy.  Imaging in multiple different modes helps us to gain a more complete understanding of the sample.  For example, in figure 3 the EFM image shows different electrostatic interactions with the high regions of the height image and helps to show an underlying morphology.

  

Figure 3.  Height (right) and EFM image at 6V applied bias (left) of an oligomer doped polymer film.

Transmission Electron Microscopy:

Transmission electron microscopy (TEM) is a technique which relies on the transmission of electrons through a sample to generate an image.  This technique is commonly used along with AFM to help obtain a more complete picture of the polymer film morphologies.  Figure 4 shows an AFM height image as well as a TEM bright field image of a polymer film doped with an emitting oligomer.

   

Figure 4. AFM height image and corresponding TEM bright field image of an oligomer doped polymer film.  AFM image size is 5x5 μm.  Scale bar in TEM image is 1 μm.

TEM along with scanning electron microscopy (SEM) are techniques which are available to the Reynolds group through the Major Analytical Instrumentation Center at the University of Florida <http://maic.mse.ufl.edu/>.