Application of the method of synchronous detection for higher-harmonics imaging in tapping-mode atomic force microscopy

In tapping-mode atomic force microscopy (AFM), the interaction between the tip and the sample is in fact non-linear and consequently higher harmonics of the fundamental resonance frequency of the oscillating cantilever are generated. In this paper, we present an AFM system with a novel approach to higher-harmonics imaging. The system uses the method of synchronous detection, based on the Fourier method, which allows to record simultaneously the amplitudes and phases of the fundamental resonance frequency and of the higher harmonics. The used detection system, composed of 16-bit 100 mega-samples per second (MSPS) analog-to-digital converter (ADC) and field-programmable gate array (FPGA) device, allows to measure the amplitude and phase of the cantilever within one oscillation cycle and with good signal-to-noise ratio. As a result, good-quality images at higher harmonics could be obtained with the use of conventional cantilevers. The obtained results prove that higher-harmonics imaging can be used to distinguish between different materials.