Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
J Chem Phys. 2012 Sep 28;137(12):124701. doi: 10.1063/1.4754602.
In electric force microscopy, a charged atomic force microscope tip in vacuum senses a fluctuating electrical force generated by the sample. Such measurements can in principle probe electrical noise generated by moving charge carriers in an organic semiconductor. We present a theory of cantilever frequency fluctuations in electric force microscopy, driven by coupled charge carrier dynamics and dielectric fluctuations. The connection between observable frequency fluctuations in electric force microscopy and the Casimir-Lifshitz force is described. This classical electrodynamic calculation is based on Maxwell's equations coupled to diffusive carrier transport. The effects of carrier transport and inter-carrier interactions on the spectrum of cantilever frequency noise are elucidated. We find that a simplified model of freely diffusing carriers can overestimate cantilever frequency noise by several orders of magnitude because of the neglect of interactions. Electric force microscopy measurements on an organic field effect transistor are reported and qualitatively interpreted in terms of the suppression of electrical noise from charge carriers by Coulomb interactions.
在电力显微镜中，真空环境下带有电荷的原子力显微镜的探针可以感应到样品产生的波动电场力。这样的测量原则上可以探测到有机半导体中移动电荷载流子产生的电噪声。我们提出了一种由耦合电荷载流子动力学和介电涨落驱动的电力显微镜悬臂梁频率波动的理论。描述了在电力显微镜中观察到的频率波动与卡西米尔-利夫希茨力之间的关系。这个经典的电动力学计算是基于麦克斯韦方程与扩散载流子输运的耦合。研究了载流子输运和载流子间相互作用对悬臂梁频率噪声谱的影响。我们发现，由于忽略了相互作用，自由扩散载流子的简化模型可能会使悬臂梁频率噪声高估几个数量级。报道了对有机场效应晶体管的电力显微镜测量，并根据库仑相互作用对电荷载流子电噪声的抑制作用进行了定性解释。
