Single MoleculesSome of the most prominent examples of the synthesis of 3D molecules with a well-defined shape are spherical dendrimers, arborescent-graft polymers, monodendron-jacketed linear chains, and cylindrical brushes (Fig.1). The shape of these molecules is controlled by the steric repulsion of its branches and by the branching symmetry.
The dense grafting of these molecules gives them a well-defined shape. With a well-chosen monomer unit, the molecules can undergo conformational changes with changes in their environment. This functional behavior could be used to design stimuli-responsive objects that could work as tiny springs or even motors.
Molecular visualization provides a unique opportunity for the characterization of polymers. First, it gives direct evidence of their architecture. Second, it allows for accurate measurements of the number average molecular weight when combined with an appropriate sample preparation technique. Third, it enables size measurements of the individual branches separately from the size of the whole molecule.
As an example, images of single polymer molecules with minidendritic groups are shown in Fig. 2. Due to the minidendritic groups, the molecules become thicker (about 7 nm in diameter) so that they can be visualized using regular silicon probes (click image to enlarge).
Single polymer molecules above Tg are delicate samples that require very gentle imaging conditions in tapping mode due to small size and weak adhesion to the substrate. General shape and average dimensions of the particles can be acquired using General Purpose AFM tips with curvature radius < 10 nm. Imaging of soft delicate samples requires soft AFM cantilevers.
When fine features need to be resolved, one should use Hi'Res-C AFM probes on the same AFM cantilevers for the further reduction of tip-sample interaction forces and geometrical dilation effects. When using Hi'Res-C AFM probes, "light tapping" conditions are preferable, which implies low resonance amplitude (0.2V to 1.2V) and a set-point ratio about 0.9 to 1. Scan rate should start at below 1 Hz. Scan size should start out at 50 nm (250 nm maximum). Please refer to the High-Resolution page for details.
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High resolution imaging
Hi'Res-C AFM probes with medium spring constant