Continuous tip monitoring and sensing of surface mechanical properties during Atomic Force Microscopy imaging using higher harmonics

Resumen del trabajo presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.The nonlinear interaction between an AFM tip and a sample in tapping mode atomic force microscopy (TM-AFM) induces an anharmonic cantilever motion that can be expressed in terms of a Fourier series with amplitude coefficients and frequencies multiples of the fundamental frequency, known as higher harmonics. The amplitudes of such higher harmonics can be expressed in terms of both tip and sample parameters, e.g., the tip radius (R) and the sample Young’s modulus (E). Here we present a method that describes the dependence of the amplitudes of the 6th and 7th higher harmonics to with R and E. This method has been applied to rectangular silicon cantilevers with force constant around 26 N/m and fundamental frequencies in the 250-350 kHz range. During TM-AFM imaging we simultaneously acquire the topography, the phase of the 1st (fundamental) eigenmode and the amplitude of the 6th or 7th higher harmonic, respectively. The frequency of the 6th harmonic is nearly in resonance with the frequency of the 2nd flexural mode. The information extracted from the images has been compared with those from amplitude curves and with computer simulations obtained using the Virtual Environment for Dynamic AFM (VEDA) open code. in order to extrapolate a model of the 6th and 7th higher harmonic amplitude depending on the cantilever and measurement parameters and then go beyond the qualitative interpretation. The obtained results have been complemented with standard methods to calibrate the radius using tip calibration samples of gold nano-particles (5.5 nm mean radius) on mica substrates and compared to measurements performed using the Peak Force technique.Peer Reviewe