Nearly every specialist in the SPM community has confronted failures in their experience due to mutual displacement of probe and sample. This effect occurs due to thermal or mechanical drifts inside the AFM system. The results of the entire experiment could be disastrous, specifically for small scan areas (less than 1 nm).
Mechanical Drift Caused by Piezoceramic Properties
Even the most efficient piezoceramic devices can be affected by non-linearity, creep, and hysteresis. Applying a special software and closed-loop (CL) correction is the only way to ensure ultimate repeatability of the system. Practically, CL sensors always generate some noise in the system, and hence, majority of the commercially available SPMs cannot be used for working on fields lesser than 500 nm with closed-loop correction.
The unique design of the NTEGRA Therma measuring head enables maintaining ultra-high reproducibility and stability of probe movement. Among commercially available instruments, scanner sensors of NTEGRA Therma have the lowest noise level.
The hardware correction on regions as small as 50 nm is made possible by the engineering solutions. In fact, it is possible to image even atomic lattice with CL sensors switched on.
Thermal Drift Caused by Non-Uniform Thermal Expansion of SPM Parts
It is easy to identify a temperature noise of 3–5 K magnitude even inside a climate-controlled room. Some heat is also generated during the operation of SPM. In the case of commercially available SPMs, the typical values of thermal drift are tens of nanometers per hour. As the temperature range of experiment is widened, the thermal drift influence becomes more prominent. The drift of nearly hundreds of nanometers per Kelvin becomes customary for normal SPM.
NTEGRA Therma integrates unique design solutions to resist the thermal drift. Specific features such as precise stabilization of the scanning module temperature, thoroughly developed system geometry, and special combination of materials with similar coefficients of conductivity and thermal expansion, and various other features allow XY drifts as small as 3–5 nm/hour at room temperature, and around 10 nm/K at varying temperature.
Figure 1. Atomic lattice of HOPG obtained at extremely low scan rate. (about 1 line/sec)
Figure 2. Atomic lattice of mica as imaged with closed loop correction.
Figure 3. Nanotubes and nanoparticles in long-term experiment. Overall displacement for 7 hours is about 35 nm. Sample courtesy of Dr. H. B. Chan, Department of Physics, University of Florida, USA
AFM-Based Tomography Using the NTEGRA Tomo
AFM tomography is a technique based on ultramicrotomy as well as atomic-force microscopy (AFM). It enables the investigation of inner properties of nearly all polymer materials, including pretty hard ones. Following serial AFM imaging of the block face in combination with sectioning by an ultramicrotome, 3D reconstruction can be carried out.
Figure 4. Principle scheme of the AFM tomography setup: 1—sample; 2—sample holder; 3—movable ultramicrotome arm; 4—ultramicrotome knife; 5—AFM scanner; 6—probe holder; 7—AFM probe
Figure 5. Silica nanoparticles within polymer matrix (nanocomposite material). Each individual image size is 20 x 40 nm, spaces are 200 nm. Sample courtesy of Dr. Aliza Tzur, Technion, Israel
Figure 6. 3D Model of multicomponent polymer blend. Model size 8.0 x 5.6 x 0.6 µm, spaces between sections 40 nm. Sample courtesy of Dr. Christian Sailer, Institut f. Polymere, ETH-Honggerberg, Switzerland
Figure 7. AFM tomography of resin-embedded cyanobacteria. Photosynthetic membrane lamellae are clearly seen both on enlarged AFM image and on a 3D model (4.9 x 4.6 x 0.9 µm, spaces between sections 50 nm). Sample courtesy of Dr. N. Matsko, ETH, Zurich, Switzerland
SPM + Confocal Microscopy/Spectroscopy
Advantages of Combination
SPM in combination with confocal microscopy/spectroscopy enables simultaneously performing chemical and physical characterization of the same area on the sample surface. NTEGRA Spectra has successfully combined fluorescence microscopy and spectroscopy, Raman, SNOM (near-field optical microscopy), and AFM techniques.
Furthermore, unique nonlinear optical effects that occur as a result of the interaction of light with an SPM probe result in huge enhancement of fluorescence and Raman signals. The precise spatial coordination of a special AFM tip and focused laser spot has made tip-enhanced Raman scattering (TERS) experiments possible. It is now possible to perform optical characterization with resolution far beyond the diffraction limit.
Figure 8. Raman microscopy with ultra-high spatial resolution. A—tip-enhanced Raman scattering (TERS) experiment; B—intensity of carbon nanotube G-band increases by several orders of magnitude when the probe tip is landed; C—confocal Raman image of carbon nanotube bundle; D—TERS image of the same nanotube bundle. Note, TERS provides more than four times better spatial resolution as compared to confocal microscopy. Data courtesy of Dr. S. Kharintsev and Dr. J. Loos, TUE, the Netherlands; and Dr. P. Dorozhkin, ISSP RAS, Russia.
Figure 9. Microalgae seen by bright field microscopy (A), Raman microscopy at beta-carotene line (B), and confocal microscopy of autofluorescence (C). Sample courtesy of Dr. Don McNaughton, Monash University, Victoria, Australia.
Figure 10. SNOM image of mitochondria dyed with FITC-labeled antibodies. Note XY resolution beyond the diffraction limit.
This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.
For more information on this source, please visit NT-MDT Spectrum Instruments.