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Amplitude or frequency modulation-detection in Kelvin probe force microscopy

机译：开尔文探针力显微镜中的振幅或频率调制检测

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Kelvin probe force microscopy in ultrahigh vacuum is a sensitive tool to determine the absolute work function of sample surfaces. For the detection of the electrostatic force, an a.c.-voltage with a frequency omega is applied between tip and sample. Two different methods can be employed for the measurement: the frequency modulation (FM) mode or amplitude modulation (AM) mode detection. In FM-mode the oscillation of the frequency shift at omega is measured, which is proportional to the gradient of the electrostatic force. In AM-mode the amplitude of the cantilever oscillation at omega is measured which is proportional to the electrostatic force itself. A detailed comparison between AM and FM detection modes is presented, using measurements of gold islands on highly oriented pyrolytic graphite. The effect of tip geometry was studied for both detection modes. In the FM-mode mainly the tip apex contributes to the measured signal due to the detection of the force gradient, however, a.c.-voltages of around 2 V are necessary to perform stable measurements. A much stronger influence of the tip shape was determined for AM detection, reducing the contrast in the measured contact potential difference images. Nevertheless, the main advantage of the AM-mode is the possibility to use a.c.-voltages as low as 100 mV, which allows absolute work function measurements also on semiconductor surfaces. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 22]
机译：超高真空下的开尔文探针力显微镜是确定样品表面绝对功函数的灵敏工具。为了检测静电力，在尖端和样品之间施加频率为ω的交流电压。可以使用两种不同的方法进行测量：频率调制（FM）模式或幅度调制（AM）模式检测。在FM模式下，测得的ω频移振荡与静电力的梯度成正比。在AM模式下，测得的欧米伽悬臂振荡幅度与静电力本身成正比。通过在高度取向的热解石墨上测量金岛，对AM和FM检测模式进行了详细的比较。研究了两种检测模式下尖端几何形状的影响。在FM模式下，由于检测到了力梯度，主要是尖端顶点对被测信号有所贡献，但是，执行稳定的测量需要2 V左右的交流电压。确定了尖端形状对AM检测的更强影响，从而降低了所测得的接触电势差图像中的对比度。然而，AM模式的主要优点是可以使用低至100 mV的交流电压，这也允许在半导体表面上进行绝对功函数测量。 （C）2002 Elsevier Science B.V.保留所有权利。 [参考：22]

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