Nanotechnology can only develop with appropriate analysis and measurement. The ‘Lab on a chip’ capabilities of AFM is one way of achieving this – here we get a rundown of how the technology has developed and what it can offer today
Analytical chemists have been making bulk measurements on materials for many years. Recognising the differences in performance between a surface measurement and a volume measurement has also been possible. Physical measurement techniques like thermal analysis and spectroscopy are well established in both research and quality control methods for characterisation.
[caption id="attachment_36188" align="alignright" width="200"] Figure 2: HT AFM to study a polymer blend of polystyrene and polypropylene.[/caption]
However, with the advent of scanning probe microscopy in the mid-1990s, researchers have been looking at the idea of making physical characterisation measurements at the end of the scanning probe itself. The ability to fabricate reproducible and sharp probes from silicon and silicon nitride has enabled the development of a whole family of scanning probe techniques. Usually based on the method known as atomic force microscopy (AFM), the family has included methods to measure topography, stiffness, adhesion, electrical and magnetic properties with a spatial resolution down in the 10’s of nanometers.
Microscale probe measurements became available in the mid-90s. The measurement of thermal conductivity and then microscale thermal analysis measurements were introduced by TopoMetrix using a platinum wire probe (a Wollaston wire). This was a significant breakthrough but while it offered potential for active and reactive temperature measurements on surfaces, it did not have the resolution required to satisfy scientists who wished to understand material properties of, for example, polymers where studies at the domain level, around 50 nm, was necessary.
In 2006, the development work of Professor William King at Georgia Tech led to the release of a new type of thermal probe composed of doped silicon known as the ThermaLever. These probes have a heater integrated into the end of the cantilever which allows them to be heated in a controlled fashion to around 400?C at very rapid heating rates. Another significant advance with these probes is the end radius of the tip. In previous styles of thermal probes, the tip radius is significantly larger than standard AFM probes. This is not true with these probes which provide lateral resolution in imaging very similar to other AFM probes. This probe has led to the successful development and release of a number of techniques which will be described and illustrated here.
[caption id="attachment_36186" align="alignleft" width="200"] Figure 3: Image clearly shows the penetration holes made by the probe in the core and skin layers[/caption]
Four techniques will be considered here. The first is known as heated tip AFM (HT AFM). This mode provides simultaneous thermal information with the regular modes of AFM. The heated tip AFM may be used in conjunction with the common imaging modes such as tapping, contact imaging, force curves, force volume imaging and pulsed force mode. This technique has been applied in the study of energetic materials and membrane nanocomposites.
Figure 2 shows the use of HT AFM to study a polymer blend of polystyrene and polypropylene. These images illustrate the effect of heating the sample from room temperature to 250°C returning to room temperature. The images on the left show sample topography while those on the right show the changing phase response of the sample. It clearly illustrates how the polymer morphology is changing as a function of temperature.
[caption id="attachment_36190" align="alignleft" width="200"] Figure 3: image shows the thermo-mechanical scans defining glass transition (Tg) and melt (Tm) temperatures.[/caption]
The second of the techniques is known as nano-TA meaning nanoscale thermal analysis measurements. The nano-TA probe is first used by the atomic force microscope to image with nanoscale resolution (with the normal visualisation modes of the AFM) which then helps the user identify the points where they would like to get local specific thermal property information. The probe is moved to the first point selected and placed on the surface of the sample. The temperature of the tip is then ramped linearly with time while the degree of bending is monitored. At the point of phase transition, the material beneath the tip softens and the probe penetrates into the sample, this provides the nanoscale equivalent of a bulk thermo-mechanical analysis experiment whereby you can measure the phase transition temperatures of the sample such as Tg or Tm.
To illustrate this technique, we show the local thermal measurements made on a sample of a multilayer film (BOPP – biaxially oriented polypropylene). These films have become a popular, high growth film on the world market because of a unique combination of properties such as better shrinkage, stiffness, transparency, sealability, twist retention and barrier. Applications are found in packaging, pressure sensitive tape and printing. In figure 3, the image on the left clearly shows the penetration holes made by the probe in the core and skin layers while the right image shows the thermo-mechanical scans defining glass transition (Tg) and melt (Tm) temperatures.
[caption id="attachment_36192" align="alignright" width="200"] Figure 4: Another method for mechanical property characterisation is Lorentz Contact Resonance imaging (LCR).[/caption]
Another method for mechanical property characterisation is Lorentz Contact Resonance imaging (LCR)(Figure 4). This is based is based on the Lorentz force, the force on an electrical current in a magnetic field. An oscillating current passing through the probe interacts with a magnetic field that is focused near the probe, resulting in an oscillating tip sample force. The frequency of the oscillating current can be rapidly changed to measure nanomechanical spectra of contact resonances.
Driving the tip in this fashion instead of with a piezoelectric crystal has many advantages, including no moving parts in the drive system leading to clean cantilever resonance spectra with no parasitic peaks.
[caption id="attachment_36193" align="alignright" width="200"] Figure 5: An example where wood cells have been mapped. Measurements have been made a number of frequencies.[/caption]
Figure 5 is an example where wood cells have been mapped. Measurements have been made a number of frequencies. The composite overlay (bottom left) is particularly striking. This composite image was made by overlying the LCR amplitudes collected at three different contact resonances (right hand set of three images). These resonances were selected to highlight the varying ratios of the lignin and cellulose which make up the sample.
The thermomechanical spectra may also be most revealing. In this example of a polystyrene-low density polyethylene blend, the different polymers are readily differentiated on the basis of the peak positions and/or the heights of the spectra in the LCR image (figure 6).
The final technology in this review is nanoIR technology. nanoIR breaks through resolution limits in conventional IR spectroscopy by using the tip of an atomic force microscope probe to measure infrared absorption. The sample is illuminated by a tunable IR source. When IR radiation is absorbed by a region of the sample, the region will heat up. The heat generates a rapid thermal expansion pulse that can be detected by the AFM cantilever tip.
The nanoIR system uses a pulsed, tunable IR source to excite molecular absorption in a sample that has been mounted on a ZnSe prism (Figure 7). The IR beam illuminates the sample by total internal reflection similar to conventional ATR spectroscopy. As the sample absorbs radiation, it heats up, leading to rapid thermal expansion that excites resonant oscillations of the cantilever. The induced oscillations decay in a characteristic ringdown.
[caption id="attachment_36194" align="alignleft" width="200"] Figure 6: In this example of a polystyrene-low density polyethylene blend, the different polymers are readily differentiated on the basis of the peak positions and/or the heights of the spectra in the LCR image.[/caption]
The ringdown can be analysed via Fourier techniques to extract the amplitudes and frequencies of the oscillations. Measuring the amplitudes of the cantilever oscillation as a function of the source wavelength creates local absorption spectra; the oscillation frequencies of the ringdown are related to the mechanical stiffness of the sample. The IR source can also be tuned to a single wavelength to simultaneously map surface topography, mechanical properties, and IR absorption in selected absorption bands.
While AFM has been very successful addressing problems in basic nanoscale research as well as applied problems in materials science and engineering, there is a clear gap in AFM capabilities. That is the ability to chemically characterise regions of the sample. In fact, the ability to identify material under the tip of an AFM has been identified as one of the “Holy Grails” of probe microscopy. While AFM can measure mechanical, electrical, magnetic and thermal properties of materials, it has lacked the robust ability to chemically characterise unknown materials. With a new instrument known as the nanoIR, it is now straightforward to deliver robust chemical characterisation to the AFM. The nanoIR uses infrared spectroscopy to provide chemical analysis of samples on the sub-micron length scale. Infrared spectroscopy measures the wavelength dependent absorption of radiation that results from excitations of specific molecular vibrations. The resulting absorption spectra provide rich information about the chemical content of material under the AFM tip.
[caption id="attachment_36195" align="alignright" width="200"] Figure 7: The nanoIR system uses a pulsed, tunable IR source to excite molecular absorption in a sample that has been mounted on a ZnSe prism.[/caption]
The applications for this technique are extremely varied. Users have published many papers to date on samples as diverse as skin to polymers to cellulose and pharmaceuticals. To illustrate the power of the AFM-IR method, we have chosen an example where we were able to study molecular interactions between active pharmaceutical ingredients (API) and passive carrier materials in a drug’s dispersion (figure 8). The IR images provide resolution well below the diffraction limit and hence permits chemical imaging of the API distribution.
The world of the lab-on-a-tip is growing as researchers start to publish peer-reviewed papers. When the early adopters started to make nanoIR measurements, it looked like the bulk of the work would be in the world of polymers but this has changed of late with examples starting to appear in the life sciences.
[caption id="attachment_36196" align="alignleft" width="200"] Figure 8: Molecular interactions between active pharmaceutical ingredients (API) and passive carrier materials in a drug’s dispersion.[/caption]
To learn more about this growing analytical field of techniques, readers are encouraged to consider these papers:
Spatial Differentiation of Sub-Micrometer Domains in a Poly(hydroxyalkanoate) Copolymer Using Instrumentation that Combines Atomic Force Microscopy (AFM) and Infrared (IR) Spectroscopy. Journal of Applied Spectroscopy, Volume 65, Number 10 (Oct. 2011) Page 1145-1150. Marcott, C, Lo, M, Kjoller, K, Prater, C and Noda, I.
AFM–IR: Combining Atomic Force Microscopy and Infrared Spectroscopy for Nanoscale Chemical Characterization. Journal of Applied Spectroscopy, Volume 66, Number 12 (Dec. 2012) Page 1365-1384. Dazzi, A, Prater, C, Hu, Q, Bruce Chase, D, Rabolt, J F and Marcott, C.
Atomic force microscope infrared spectroscopy on 15 nm scale polymer nanostructures. Rev. Sci. Instrum. 84, 023709 (2013); Felts, J R, Cho, H, Yu, M-F, Bergmann, L A, Vakakis, V and King W P.
Nanoscale Imaging of Plasmonic Hot Spots and Dark Modes with the Photothermal-Induced Resonance Technique. Nano Lett., 2013, 13 (7), pp 3218–3224, DOI: 10.1021/nl401284m; Lahiri, B, Holland, G, Aksyuk, V and Centrone, A.
Nanoscale infrared (IR) spectroscopy and imaging of structural lipids in human stratum corneum using an atomic force microscope to directly detect absorbed light from a tunable IR laser source. Exp Dermatol. 2013 Jun;22(6):419-21. doi: 10.1111/exd.12144. Epub 2013 May 8; Marcott, C, Lo, M, Kjoller, K, Domanov, Y, Balooch, G and Luengo, G S.
Morphology of water transport channels and hydrophobic clusters in Nafion from high spatial resolution AFM-IR spectroscopy and imaging. Electrochemistry Communications, Volume 30, May 2013, Pages 5–8; Awatani, T, Midorikawa, H, Kojima, N, Ye, J and Marcott C.