Digital Pulsed Force Mode by Oxford Instruments WITec

Pulsed Force Mode (PFM) is a non-resonant, intermediate contact mode for Atomic Force Microscopy that allows the characterization of material properties such as adhesion and local stiffness along with the sample topography. Additionally, lateral forces are virtually eliminated. Therefore high-resolution mapping of delicate samples in air and fluids is easily attainable while maintaining a scanning speed comparable to contact-mode AFM.

In contrast to most other intermediate contact techniques, the normal forces on the sample (introduced by the AFM tip) are controlled by the feedback loop. The PFM electronics introduce a sinusoidal modulation to the z-piezo of the AFM with an amplitude of 10-500 nm at a user-selectable frequency of between 100 Hz and 2 kHz: far below the resonant frequency of the cantilever. A complete force-distance cycle is carried out at this rate, resulting in the force signal as shown in the figure above.

The PFM is specifically suited for delicate and soft samples investigated in air or liquids at a high scanning speed. Digital Pulsed Force Mode (DPFM) is ideal for users with advanced surface properties characterization requirements, providing material properties analysis beyond adhesion and stiffness.

With DPFM a wide variety of sample properties can be extracted from force-distance curves; such as adhesion, stiffness, viscosity, energy dissipation, contact-time, long range forces, and many more. These properties can be analyzed and imaged simultaneously along with topography. Real-time processing of digitized force curves allows immediate online monitoring of the resulting image and facilitates the adjustment of the parameters for successful imaging results. Additionally, storage of the complete measurement provides unlimited access to all data through extensive post-processing data evaluation. 

Digital Pulse Force Mode Features:

• High-resolution imaging of surface properties such as adhesion and stiffness along with the topography
• Especially suited for delicate and soft samples in air and liquids
• Lateral forces are virtually eliminated
• Fully programmable piezo modulation-frequency (131,072 points, 10 Hz-2 kHz)
• 16 Bit/5 MHz analog to digital converter
• Real-time hardware evaluation (DPFM)
• Storage of complete PFM curves over entire scan range (DPFM)
• Bidirectional linkage between imaged pixels and the corresponding stored force curves (DPFM)