Abstract
Dynamic force microscopy (DFM) has become a multifunctional and powerful technique for the study of the micro–nanoscale imaging and force detection, especially in the compositional and nanomechanical properties of polymers. The energy dissipation between the tip and sample is a hot topic in current materials science research. The out-of-plane interaction can be measured by the most commonly used tapping mode DFM, which exploits the flexural eigenmodes of the cantilever and a sharp tip vibrating perpendicular to the sample surface. However, the in-plane interaction cannot be detected by the tapping mode. Here a bimodal approach, where the first order flexural and torsional eigenmodes of the cantilever are simultaneously excited, was developed to detect the out-of-plane and in-plane dissipation between the tip and the polymer blend of polystyrene (PS) and low-density polyethylene (LDPE). The vibration amplitudes and phases have been recorded to obtain the contrast, energy dissipation and virial versus the setpoint ratio of the first order vibration amplitude. The pull-in phenomenon caused by a strong attractive force can occur near the transitional setpoint ratio value, the amplitude setpoint at which the mean force changes from overall attractive to overall repulsive. The in-plane dissipation is much lower than out-of-plane dissipation, but the torsional amplitude image contrast is higher when the tip vibrates near the sample surface. The average tip-sample distance can be controlled by the setpoint ratio to study the in-plane dissipation. Both flexural and torsional phase contrasts and torsional amplitude contrast can also be significantly enhanced in the intermediate setpoint ratio range, in which compliant heterogeneous materials can be distinguished. The experiment results are of great importance to optimize the operating parameters of image contrast and reveal the mechanism of friction dissipation from the perspective of in- and out-of-plane energy dissipation at different height levels, which adds valuable ideas for the future applications, such as compliant materials detection, energy dissipation and the lateral micro-friction measurement and so on.
Highlights
IntroductionDynamic force microscopy (DFM) has become a multifunctional and universal technique for the application of micro–nanoscale imaging and force detection, including topography imaging, and measuring modulus of elasticity, viscoelastic properties and other physical properties in the microscale and nanoscale worlds.[1,2,3,4,5,6,7] The new techniques in the dynamic force microscopy (DFM) eld, such as bimodal[8,9,10,11] or higher modes,[12,13,14] multifrequency AFM,[15,16,17] and intermodulation method,[18,19,20] have complemented the traditional amplitude modulation mode and can obtain high resolution images of heterogeneous materials, cells or DNA.[21]
The general form of bimodal AFM is the rst two excitation exural frequencies, indicating that the out-of-plane interaction can be explored by this method
Sometimes the friction, the in-plane interaction or dissipation play an important role in the detection of the heterogeneous materials and some laminated structures,[25,26] so it is necessary to excite the torsional vibration mode to detect the local mechanical and tribological properties in lateral dimension.[27,28]
Summary
Dynamic force microscopy (DFM) has become a multifunctional and universal technique for the application of micro–nanoscale imaging and force detection, including topography imaging, and measuring modulus of elasticity, viscoelastic properties and other physical properties in the microscale and nanoscale worlds.[1,2,3,4,5,6,7] The new techniques in the DFM eld, such as bimodal[8,9,10,11] or higher modes,[12,13,14] multifrequency AFM,[15,16,17] and intermodulation method,[18,19,20] have complemented the traditional amplitude modulation mode and can obtain high resolution images of heterogeneous materials, cells or DNA.[21]. Sometimes the friction, the in-plane interaction or dissipation play an important role in the detection of the heterogeneous materials and some laminated structures,[25,26] so it is necessary to excite the torsional vibration mode to detect the local mechanical and tribological properties in lateral dimension.[27,28] In previous researches, the bimodal method was o en implemented in ultra-high vacuum (UHV) or liquid for the high resolution. The ultrasensitive detection of lateral atomic-scale interactions on graphite (0001) has been completed via room-temperature dynamic force microscopy using simultaneous excitation and FM detection of the lowest exural and torsional cantilever resonance modes.[26] In addition, the frictional processes on the Br-doped NaCl (001)
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