Nanoscale Characterization by Scanning Probe Microscopy: Challenges and
Opportunities
Aleksandr Noy, Lawrence Livermore National Laboratory
Effective progress in the synthesis, processing and assembly of nanostructures requires
characterization techniques commensurate with the characteristic size of the structure. A
number of techniques can potentially provide such capabilities, yet only two of them
found widespread use - electron microscopy (EM) and scanning probe microscopy
(SPM). My main expertise is in the SPM area, therefore I will concentrate on discussing
the challenges faced by that field. I should also note that cost and sample preparation
requirements inherent in the EM techniques sometimes make them impractical, which
makes SPM the leading candidate for a true nanoscale benchtop characterization
technique.
State-of-the-Art and Challenges - SPM already provides a variety of unique
characterization capabilities with true nanometer-scale resolution on a variety of different
samples. The types of surfaces that SPM typically investigates range from semiconductor
nanostructures, to soft polymers, to biological tissues and cells. The market now offers a
wide variety of commercial SPM instruments at different price points and functionality
sets. There is also a variety of microfabricated probes available on the market that enable
common imaging protocols. In addition, progress in using SPM for semiconductor
process metrology resulted in development of the automated probe quality control
capabilities and automated tip exchange. The metrological capabilities of SPM were also
greatly enhanced by the use of closed-loop scanners which enabled hysteresis-free probe
positioning.
However, there are still some important challenges. The most important challenge faced
by the SPM techniques is that they typically provide no real surface composition
information. Other, arguably less important challenges include: variability in probe size,
shape and lifetime; inability to image high aspect ratio, fragile or very soft structures; low
throughput determined by the limitations of the scanning speed and scanning range.
Technological Approaches to Overcoming Challenges in SPM
Probe limitations. One of the most important technologies for boosting SPM capabilities
is carbon nanotube AFM. Use of carbon nanotubes as AFM probes (Figure 1A) promises
to reduce the effective size of the tip as well as unwanted adhesive interactions, and
boosts the aspect ratio of the tip. Challenges that still have to be overcome include
refining the fabrication process to produce uniform and stable probes, and adapting the
process to mass-production. I could also envision using semiconductor nanowires as SPM
tips.
Throughput. SPM is inherently a serial technique which acquires images point-by-point;
therefore the throughput gains may come from either increasing the scanning speed, or
from parallelization of imaging process. The speed improvements typically come from
using shorter cantilevers with higher resonance frequencies,1 or from other improvements
in the AFM bandwidth.2 Another promising approach involves using parallel cantilevers
which scan different parts of the surface at the same time.
Chemical composition. One approach for obtaining surface composition information is
based on controlling and monitoring the probe-sample interactions. Lieber and coworkers developed chemical force microscopy technique in which the AFM probe is
modified with distinct functional groups. They demonstrated that chemically-modified
AFM probes can reliably distinguish between hydrophobic and hydrophilic samples,
monitor local pH changes, and provide quantitative estimates of local surface energies.3
They have also showed that chemical modification strategy can be combined with carbon
nanotube AFM tip technology, which can potentially produce the “ultimate AFM
probe”.4 Researchers also developed a number of approaches that use energy dissipation
due to probe-sample interactions to map surface composition.5,6 The leading challenge
that still remain for CFM is developing of more robust chemical coatings and developing
modifications that will enhance chemical discrimination.
Figure 1. A. TEM micrograph of a carbon nanotube AFM tip. B. Force vs. distance profile for
interactions of two virus particles.
Another set of approaches tries to couple SPM with other techniques that typically excel
at chemical discrimination. The chief candidate for such technique is optical
spectroscopy. Near-Field Scanning Optical Microscopy (NSOM) was the first example of
such technique;7 however NSOM suffers from using a single probe to collect
topographical and optical information. As the result, high spatial resolution hurts optical
throughput. Another approach combines AFM with optical techniques such as confocal
microscopy.8 In this approach both techniques use their respective strengths: AFM
provides topography and confocal microscopy provides singlemolecule level optical
signature identification. However, this approach may not resolve optical signatures of the
surface features that are located within the diffraction limit. Another very promising
approach combines APM with surface-enhanced Raman spectroscopy (SERS).
Researchers have proposed placing Raman-enhancing nanoparticles on the SPM probe.9
Such probes have the potential to provide localized chemical identification of surface
features based on Raman signatures. Xie and co-workers proposed use of local field
enhancement by an SPM probe to improve resolution of the fluorescence imaging
techniques.10
Nanoscale Interaction Force Measurements - Characterization of the interactions
between nanostructures is at least as important as characterization of their surface
morphology and chemical composition. Scanning probe microscopy has already been
very successful in measuring these interaction forces on truly microscopic scale (Figure
1B).3 However, several important challenges still remain. A short-term problem is posed
by the lack of robust and accurate methods for calibration of the force measurements:
standard calibration techniques always produce about 10% error. Perhaps, the first step
for measurement standardization could be the establishment of a common molecular
scale force standard for probe calibration (perhaps even based on a biological system; one
possibility is to use the stretching transition that occurs in DNA). In the longer term, we
still need to push the force resolution of the cantilever systems down into the single
picoNewton regime, all while maintaining adequate cantilever stiffness to avoid jumps.
Drastic reductions in the instrument noise level and use of shorter cantilevers should
drive progress in that area. We will also need to expand the AFM to probe different
loading rates and regimes. Interaction force measurements often require large statistics;
therefore the throughput issues are very important. Most of the throughput increase
approaches are similar to the strategies that I have already discussed for imaging. Finally,
as a long-term challenge, I should mention the possibility of using SPM for direct
mapping of full energy landscapes (perhaps based on using thermal-noise assisted
probing). Such capability should then open up a way for a truly rational design of
nanoscale assemblies.
1
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3
A. Noy, D. V. Vezenov, C. M. Lieber, Annual Review of Materials Science 27, 381 (1997).
4
S. S. Wong, E. Joselevich, A. T. Woolley, C. L. Cheung, C. M. Lieber, Nature 394, 52 (1998).
5
J. Cleveland, B. Anczykowski, A. Schmidt, V. Elings, Appl. Phys. Lett. 72, 2613 (1997).
6
A. Noy, C. H. Sanders, D. V. Vezenov, S. S. Wong, C. M. Lieber, Langmuir 14, 1508 (Mar 31, 1998).
7
R. C. Dunn, Chemical Reviews 99, 2891 (1999).
8
A. Noy, T. R. Huser, Review of Scientific Instruments 74, 1217 (Mar, 2003).
9
S. A. Vickery, R. C. Dunn, Biophysical Journal 76, 1812 (1999).
10
E. J. Sanchez, L. Novotny, X. Sunney Xie, Physical Review Letters 82, 4014 (1999).
2