Dielectric Fluctuations over Polymer Films Detected Using an Atomic Force Microscope

We present a systematic study of the frequency noise experienced by a charged atomic force microscope cantilever due to thermal dielectric fluctuations in a thin-film sample of poly(vinyl acetate). Here, the tip of the commercial atomic force microscope cantilever oscillates in the conventional dire...

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Bibliographic Details
Published in:The journal of physical chemistry. B 2011-12, Vol.115 (49), p.14493-14500
Main Authors: Hoepker, Nikolas, Lekkala, Swapna, Loring, Roger F, Marohn, John A
Format: Article
Language:English
Subjects:
Atomic force microscopes
B: Surfactants, Membranes
Dielectrics
Electric potential
Fluctuation
Noise
Separation
Vibration
Voltage
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Summary:We present a systematic study of the frequency noise experienced by a charged atomic force microscope cantilever due to thermal dielectric fluctuations in a thin-film sample of poly(vinyl acetate). Here, the tip of the commercial atomic force microscope cantilever oscillates in the conventional direction, normal to the surface of the film, complementing our previous studies of dielectric fluctuations carried out using an ultrasensitive custom-fabricated cantilever oscillating parallel to the film surface. We show that frequency noise induced by mechanical vibrations can be distinguished from frequency noise resulting from thermal dielectric fluctuations by the dependence on applied voltage and tip-sample separation, allowing molecular information to be unambiguously extracted. A linear response theory for cantilever frequency noise over a molecular material correctly reproduces the observed dependences on frequency, voltage, and tip-sample separation. The technique is shown to measure primarily fluctuations in the electric field gradient over the surface, which in these measurements are generated by orientational relaxation of polar polymer segments.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp207387d