Table of Contents
Tapping (PFT) and ScanAsyst
(SA) are two Atomic Force Microsocope (AFM) imaging techniques
recently introduced by Bruker which fits into the framework of existing
AFM modes. The crucial step in AFM is the time-consuming actual
adjustment of AFM feedback parameters by the user. ScanAsyst provides
consistent expert-quality results independent of user experience.
Tapping, like TappingMode, avoids lateral forces by intermittent
contacting of the sample. It differs from TappingMode in that it
operates in a non-resonant mode. The benefits of contact and
TappingMode imaging are combined in the oscillating system in PeakForce
Tapping and unwanted resonances at turnaround points are avoided.
The general operation is illustrated in figure 1.
Figure 1. Experimental data
of force curves for a cantilever operated in PeakForce Tapping. The
lever is driven by a sinusoidal wave and the curves are displayed as
force versus time and force versus distance.
The controlled interaction force can be lower due to the higher
force sensitivity in a soft cantilever. The typical repetition rate of
2kHz allows for imaging speeds that are comparable to TappingMode
operation (figure 2).
Figure 2. 2mm scan of a
sample of graphene obtained in PeakForce Tapping operation. Several
monoatomic steps and small islands can be clearly identified.
uses PeakForce Tapping mechanism and adjusts all
critical imaging parameters. This gives high-quality images without the
user adjusting imaging parameters and the problematic AFM user
interfaces. A basic SA interface is shown in figure 3. The user only
has to select the actual scan area. There is also an option to set the
AutoControl field by individual and choose the parameters.
Figure 3. Screen shot of
the basic SA interface. All feedback settings and the scan rate are
automatically calculated by the AFM.
The underlying calculations enabling SA happens on the fast FPGA
chips implemented in Bruker controllers. The noise threshold, the key
parameter, is automatically adjusted by the AFM after it has completed
a full frame as not all scans require the ultra-low noise that Bruker
AFMs can achieve. By selecting a discrete threshold, the AFM can
adjust feedback and imaging speed to get a certain result instead of
manipulating the feedback loop. The data that SA produces even during
first time imaging is better than could be produced by an AFM expert
Figure 4. 80nm scan of C18H38
alkane chains obtained in PFT. The inter-lamellar distance is only 2nm!
The built-in flexibility of SA allows users to fully or partially
control the PFT operation. An example of an expanded SA user interface
is shown in figure 5.
Figure 5. Screen shot of
the expanded SA interface. If desired, SA allows flexibility for
parameters to be adjusted manually.
During SA/PFT imaging the user can constantly monitor the integrity
of operation by looking at the built-in force monitor as shown in
Figure 6. Real-time shot of
the force-monitor during imaging with SA. This allows the user to
constantly monitor the integrity of the imaging process.
The issue of height data in TappingMode is resolved in PFT, since it
responds only to short-range interaction as long-range interactions are
ignored for height control, a key to high-resolution imaging. By
consistently controlling the short-range interaction forces, PFT
enables image quality control with fewer artifacts.
Key features PFT are:
- Insensitivity to effects due to resonant system geometry (figure
- Unaffected by Q-factor of cantilever (figure 8).
- Can be operated for changing environments.
- Operated at fixed frequency, hence cantilever tuning unnecessary.
- Re-tuning not required even with temperature or medium change.
- Insensitive to changes in probe resonant frequency and Q as PFT.
- Imaging forces as low as a few tens of pico-newtons are allowed
as the SA software subtracts even the background changes caused by
temperature or fluid- level fluctuations (figure 9). SA operation at
different temperatures is given in figure 10.
Figure 7. 160nm linescan of
steep trenches. The flat bottom indicates that the probe reached all
Figure 8. 30mm scan of a
Teflon membrane in PeakForce (left) and regular TappingMode (right).
Artifacts visible in TappingMode operation are not present in the
PeakForce Tapping data.
Figure 9. 1mm scan of
Origami DNA in buffer solution using SA. Single strands of DNA
comprising the square structure are clearly discernible.
Figure 10. 500nm images of C60H122
at room temperature and 70°C.
The force curves are also available to the user to extract
additional material specific information. Bruker uses the capability of
obtaining multiple force–distance curves at each image location in its
PeakForce QNM package. Figure 11 shows the resulting curves from a HSDC
of 100ms and one selected curve.
Figure 11. Result of a HSDC
during the imaging process. The force curves that enable the imaging
can be extracted and also be used for further analysis.
A variety of modes, traditionally carried out in contact mode
operation, can greatly benefit from combination with PFT. Electrical
modes such as Scanning Capacitance Microscopy (SCM) or Tunneling AFM
(TUNA) would get a performance boost. A TUNA image obtained by
combining the SA/PFT is shown in figure 12.
Figure 12. PFT-TUNA image
of carbon nanotubes. Sample topography on the left and conductivity map
on the right. Sample courtesy of Prof. Hague, Rice University.
Advantage of AFM dominated Tapping is the lack of lateral forces
inherent to contact imaging. But its complexity has prevented the
automation of the critical step and the feedback loop adjustment has
hindered the advancement of AFM This note shows that PeakForce
Tapping generate data that is equal and better than TappingMode and
the data using ScanAsyst is reliable even if obtained by a new
Nano provides Atomic Force Microscope/Scanning Probe Microscope
(AFM/SPM) products that stand out from other commercially available
systems for their robust design and ease-of-use, whilst maintaining the
highest resolution. The NANOS measuring head, which is part of all our
instruments, employs a unique fiber-optic interferometer for measuring
the cantilever deflection, which makes the setup so compact that it is
no larger than a standard research microscope objective.
This information has been sourced, reviewed and
adapted from materials provided by Bruker AXS.
For more information on this source, please visit Bruker