In this paper, we introduce a low-cost, expansible, and compatible measurement and control system for atomic force microscopes (AFM) based on a quartz tuning fork (QTF) self-sensing probe and frequency modulation, which is mainly composed of an embedded control system and a probe system. The embedded control system is based on a dual-core OMAPL138 microprocessor (DSP + ARM) equipped with 16 channels of a 16-bit high-precision general analog-to-digital converter (ADC) and a 16-bit high-precision general digital-to-analog converter (DAC), six channels of an analog-to-digital converter with a second-order anti-aliasing filter, four channels of a direct digital frequency synthesizer (DDS), a digital input and output (DIO) interface, and other peripherals. The uniqueness of the system hardware lies in the design of a high-precision and low-noise digital—analog hybrid lock-in amplifier (LIA), which is used to detect and track the frequency and phase of the QTF probe response signal. In terms of the system software, a software difference frequency detection method based on a digital signal processor (DSP) is implemented to detect the frequency change caused by the force gradient between the tip and the sample, and the relative error of frequency measurement is less than 3%. For the probe system, a self-sensing probe controller, including an automatic gain control (AGC) self-excitation circuit, is designed for a homemade balanced QTF self-sensing probe with a high quality factor (Q value) in an atmospheric environment. We measured the quality factor (Q value) of the balanced QTF self-sensing probes with different lengths of tungsten tips and successfully realized AFM topography imaging with a tungsten-tip QTF probe 3 mm in length. The results show that the QTF-based self-sensing probe and the developed AFM measurement and control system can obtain high quality surface topography scanning images in an atmospheric environment.

Recently, great advances had been made by using scanning probe microscopy (SPM) to quantify the relative permittivity of thin film materials on a nanometer scale. The imaging techniques of permittivity for thin film materials with SPM, especially for photoelectric materials, have not been fully researched until now. Here, we presented a method to image permittivity of thin film materials by using a scanning capacitance microscope (SCM). This method combined the quantitative measurement by using SCM with the capacitance gradient–distance fitting curve to obtain the two-dimensional (2D) permittivity image at room temperature under atmospheric conditions. For the demonstration, a 2D permittivity image of film of molybdenum oxide (MoO3), a kind of photoelectric material, was acquired. From the image, it could be found that the average values of permittivity of MoO3 film and of MoO3 film-doped NaCl were about 8.0 and 9.5, respectively. The experimental results were quantitatively consistent with other experimental results of the same material. The reported technique here could provide a novel method for imaging the relative permittivity with nanometer resolution and be helpful for the study of photoelectric materials.