Understanding tapping-mode atomic force microscopy data on the surface of soft block copolymers

Abstract

In this paper, we focus on better understanding tapping-mode atomic force microscopy (AFM) data of soft block copolymer materials with regard to: (1) phase attribution; (2) the relationship between topography and inside structure; (3) contrast-reversal artifacts; (4) the influence of annealing treatment on topography. The experiments were performed on the surface of poly(styrene–ethylene/butylene–styrene) (SEBS) triblock copolymer acting as a model system. First, by coupling AFM with transmission electron microscopy (TEM) measurements, the phase attribution for AFM images was determined. Secondly, by imaging an atomically flat SEBS surface as well as an AFM tip-scratched SEBS surface, it was confirmed that the contrast in AFM height images of soft block copolymers is not necessarily the result of surface topography but the result of lateral differences in tip-indentation depth between soft and hard microdomains. It was also found that there is an enlarging effect in AFM images on the domain size of block copolymers due to the tip-indention mechanism. Thirdly, based on the tip-indention mechanism, tentative explanations in some detail for the observed AFM artifacts (a reversal in phase image followed by another reversal in height image) as function of imaging parameters were given. Last, it was demonstrated that the commonly used annealing treatments in AFM sample preparation of block copolymers may in some cases lead to a dramatic topography change due to the unexpected order-to-order structure transition.