Pulsed force mode

Pulsed-force mode AFM (PFM-AFM) is a method introduced for fast mapping of local stiffness and adliesion with lower required data storage than recording force-distance curves at each point on the x-y plane [115]. A sinusoidal or triangular modulation is applied between the tip and sample (either via lever or sample piezo) at a lower frequency than that of either the piezo or cantilever resonance frequency. Tip and sample then come... 

Miyatani T, Florii M, Rosa A, Fu]ihira M and Marti O 1997 Mapping of electric double-layer force between tip and sample surfaces in water with pulsed-force-mode atomic force microscopy Appl. Phys. Lett. 71 2632... 

Krolil, H.U., Stifter, T., Waschipky, H., Weishaupt, K., Hild, S. and Marti, O., Pulsed force mode a new method for the investigation of surface properties. Surf. Interface Anal, 27(5-6), 336-340 (1999). 

RosaZeiser, A., Weilandt, E., Hild, S. and Marti, O., The simultaneous measurement of elastic, electrostatic and adhesive properties by scanning force microscopy pulsed-force mode operation. Measur. Sci. Techno ., 8(11), 1333-1338 (1997). 

The modulation frequency is typically in the range from 100 Hz to 3 kHz, and thus much lower than the resonance frequencies of the cantilever and the scanner. This enables better control of the forces exerted on the sample. The z-mod-ulation amplitude can be varied between 10 nm and 1 pm to ensure that that the tip is retracted from the surface. Shear forces are reduced permitting investigation of soft samples because of the small duration of the tip-surface contact, between 10 3 and 10 4 s. Pulse force mode SFM has been used to map adhesion of heterogeneous polymers in dependence of temperature and molecular weight as well as map electrostatic double-layer interactions [158-160]. 

Fig. 5.21. AFM images (acquired in the pulsed force mode) showing the physical surface properties of MDMO-PPV PCBM blend films (1 4 by wt.) with a thickness of approximately 100 nm and the corresponding cross-sections, (a) Film spin-coated from a toluene solution, (b) Film spin-coated from a chlorobenzene solution. The pulsed force mode allows us to acquire the mechanical properties of the surface. The top left image shows the topography, the top right image shows the error picture, and the bottom left and bottom right images show the stiffness and adhesion, respectively...

Some keywords for further reading on measurements of electrical properties using the SFM are pulsed-force mode [33, 404, 405], special SFM tips [406-409], lithography assisted by electric potentials [244, 254, 410-426], measurements of polarisation and water droplet manipulation [427-429] or reading and writing of ferroelectric domains and piezoactivity [393, 430-438]. 

SFM also enables us to measure specific interaction forces between a small silicon tip and the surface. The pull-off forces between the tip and the surface estimated from Force vs Distance Curves (FDC) can be correlated to the adhesive interactions between tip and surface [9]. Recording of FDCs line-by-line allows us to image surface topography and adhesive surface properties simultaneously [10]. This technique has some disadvantages, like the requirement for a large amount of data acquisition and analysis, which have been alleviated by the invention of the Pulsed Force Mode (PFM). The PFM simplifies and accelerates the measurements of adhesive properties with high lateral resolution [11, 12]. 

The Pulsed Force Mode allows investigations of the tip-sample interactions on a more quantitative level. For PFM measurements, the Dimension 3100 SFM was equipped with a Pulsed Force Mode box (WITec Instruments, Ubn, Germany). For adhesive force measurements, a standard micro-fabricated silicon cantilever with aluminum reflex coating (Olympus, AC240TS) with spring constants of about 2 N/m was used. To choose cantilevers with similar spring constants, their resonance frequencies were measured [13]. All measurements are performed under ambient conditions using the same set point of 0.5 V. 

To simulate the toner-silica interactions, PS/PMMA blends were used as a model for polystyrene/acrylic toners. Silicon tips modified either with HMDS or with PDMS were applied to model surface-treated silica particles. The Pulsed Force Mode images of PS/PMMA films displayed... 

Fig. 2. Pulsed Force Mode images of PS/PMMA blend show no distinct changes in the tcqpography (upper row) when silylated tips are used but a clear dqiendence on the adhesive contrast (lower row). With a pure silicon tip (A), the elevated areas show higher adhesion after silylation, contrast is reversed. The HMDS-modified tip (B) shows a lower maximum force than the PDMS-treated tip (C).

Imaging of iPP Lamellae in iPP by TM-AFM Phase Imaging and Pulsed Force Mode... 

For pulsed force mode imaging of the PP sample, a modulation frequency in the range of 800 Hz is selected. The cantilever oscillations are adjusted such that the adhesive interactions between tip and surface are overcome, as monitored using an oscilloscope. The modulation must clearly show the snap-off of the tip. Next the four markers are set as shown in Fig. 3.22. 

Fig. 3.22 Schematic of the force signal in pulsed force mode AFM. Reproduced with permission from [57]. Copyright 2004. Elsevier...

In the so-called pulsed force mode AFM technique developed by Marti and coworkers [124], the data acquisition rates are increased. In this mode, as outlined in Sect. 3.2, the sample is modulated sinusoidally ( 1 kHz) during a conventional contact mode AFM scan to cause that the tip contacts, indents, and breaks free from the surface with a frequency of 1 kHz during scanning. Instead of recording the complete f-d curve, pull-off forces and stiffness are acquired parallel to sample topography at useful scan rates that become possible (Fig. 3.54). 

Fig. 3.54 Pulsed force mode measurement on a blend of P2VP and PtBMA spincoated on silicon. The pulsed force mode AFM images clearly distinguish the PtBMA islands from the surrounding P2VP in both topography (a) and in adhesion (b). (Reprinted with permission from [125]. Copyright 2000. American Institute of Physics)...

Pulsed Force Mode Imaging of Filler Distributions... 

We use the standard pulsed force mode-AFM procedures (Sect. 3.2.4) for the investigation of the filler distribution. A contact mode AFM cantilever with a spring... 

Fig. 3.66 Pulsed force mode AFM images of carbon black filled rubber (a) height, (b) stiffness, (c) adhesion image. Reproduced with permission from [142], Copyright 1998 American Chemical Society. The image contrast in the images (a. height, b. stiffness, c. pull-off force) have been scaled from dark (low values of property) to bright (high values of property) contrast...

Intermittent contact mode phase imaging is, similar to force modulation and pulsed, force mode, sensitive to differences in materials properties. In Fig. 3.67 (a), the stiffness difference between glass and a polymer-based matrix gives rise to excellent image contrast. In addition, the different components of the polymer blend can be recognized (compare schematic in Fig. 3.67 (b)). 

Pulsed force mode scans on the thin film of the phase separated blend of P2VP and PtBMA show pronounced contrast in two imaging modes, i.e., height and adhesion, while the stiffness suffers to some extent from artefacts. In a new mode called combined dynamic X mode SFM, these problems are overcome [30]. 

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