Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, United States.
J Phys Chem B. 2011 Dec 15;115(49):14493-500. doi: 10.1021/jp207387d. Epub 2011 Nov 11.
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.
我们对聚醋酸乙烯酯薄膜样品中热介电涨落引起的带电原子力显微镜悬臂的频率噪声进行了系统研究。在这里，商业原子力显微镜悬臂的尖端在常规方向上振荡，即垂直于薄膜表面，这补充了我们之前使用超灵敏定制悬臂在平行于薄膜表面的方向上进行的介电涨落研究。我们表明，机械振动引起的频率噪声可以通过对施加电压和尖端-样品分离的依赖性与热介电涨落引起的频率噪声区分开来，从而可以明确提取分子信息。对于在分子材料上的悬臂频率噪声的线性响应理论正确地再现了观察到的频率、电压和尖端-样品分离的依赖性。该技术被证明主要测量表面上的电场梯度的涨落，在这些测量中，由极性聚合物段的取向弛豫产生。
