Resonance Control for Dynamic Force Microscopy and Spectroscopy

Abstract

Dynamic force microscopy has proven to be a powerful tool for the study of delicate systems by virtually removing lateral forces that occur during imaging in contact mode. However when used in liquid environments the cantilever has a quality factor of approximately 1 and can apply high normal forces to the sample. We present a method based on positive feedback that can control the resonance characteristics, both quality factor and spring constant, of the cantilever. An effective quality factor of over 1000 could be obtained and increased force sensitivity and phase contrast were observed. Molecular processes are of a dynamic and energetic nature and thus exhibit elastic (conservative) and viscous (dissipative) forces. Using resonance controlled dynamic force spectroscopy it is possible to measure the complex response of a single molecule as a function of extension. This technique has been applied to the polysaccharide dextran, which undergoes a conformational change of the glucose ring. Extra information not available with conventional DC force spectroscopy has been accessed giving greater insight into the molecular dynamics of this transition. Imaging capabilities have been enhanced using resonance controlled dynamic force microscopy. Two different gels, 1% agarose and 30% isotactic polystyrene, and single DNA molecules have been imaged showing enhanced spatil resolution and phase contrast.