G. Haugstad
Univ. of Minnesota,
United States
Keywords: AFM, polymer, biomedical, spectroscopy
Summary:
AFM-IR modalities are explored within the context of depth sensitivity. We focus on polymeric/organic coatings ranging from tens of nanometers to ~1 micron in thickness. In our core AFM-IR methodologies we utilize pulsed IR irradiation, and resultant AC photothermal expansion (from absorption), to excite (i) the fundamental contact resonance while under contact-mode Z feedback or (ii) either the fundamental or next higher free eigenmodal resonance while under AC Z feedback, the latter implemented at either the fundamental flexural eigenfrequency or the next higher eigenfrequency. Method (ii) further utilizes heterodyning, by pulsing the IR laser at the difference of the two eigenfrequencies and taking advantage of the nonlinear tip-sample interaction, which causes frequency mixing. In separate submethods of (ii), the IR laser is either (a) pulsed to excite the next highest eigenmode while the fundamental eigenmode is mechanically driven for Z feedback (the latter being traditional AC/“tapping” mode) – what we dub “forward heterodyning”; OR (b) pulsed to excite the fundamental eigenmode while the next higher eigenmode is mechanically driven for Z feedback – what we dub “reverse heterodyning”. The depth integration of signal can range from micron-scale at the high end down in method (i) to tens of nanometers in (ii), albeit with accompanying differences in s/n. There are also differences in the availability of multimodal tribo-mechanical contrast via images of friction, contact resonance frequency (whereby stiffness), and AC phase (i.e., under conventional tapping), with friction and phase provided the greatest surface sensitivity. Further considerations include the perturbative character of tip-sample interaction. Selection of method (i) or (ii) can be affected by the presence or absence of sliding friction: highly useful in some cases, thus favoring method (i), while deleterious in other cases such as very soft materials, thus favoring method (ii). A further consideration within method (ii) is the greater propensity for tip contamination in the net-repulsive AC regime (though yielding higher s/n) compared to the net-attractive regime. Thus the increasing range of applications for AFM-IR includes a further broadening of parameter space and operational strategies.