ADVANCED MATERIALS

AFM-Based Nanomechanics

Objective

Local mechanical-property information is essential to evaluate emerging 

micro- and nanoscale materials, which many manufacturers would like to 

leverage for their unique properties. Existing methods, however, lack 

spatial resolution and do not visualize the distribution in properties. Our 

goal is to develop and apply new atomic force microscope (AFM) tools to 

rapidly map mechanical properties and features such as modulus, damping, adhesion and defects. Measuring localized variations in properties 

reveals material homogeneity and manufacturability and provides sizeappropriate data critical for predictive modeling of device reliability and 

performance. Our methods can also be exploited for new types of AFM 

sensing to accelerate nanotechnology research and development.

Approach

Originally developed for topography information, the AFM offers many advantages 

for materials characterization. Most notably, the small radius of the AFM tip (~5 

nm to 50 nm) provides true nanoscale spatial resolution. Several AFM methods 

have been developed to assess mechanical properties, but typically only qualitative images of relative contrast can be obtained. In comparison, contact resonance 

force microscopy (CR-FM) enables quantitative mapping of mechanical properties. 

CR-FM involves measuring the frequency of the vibrating AFM cantilever while its 

tip is in contact with a sample. From these “contact resonance” frequencies, information is obtained about the interaction forces between the tip and the sample 

(e.g., contact stiffness). Models for the tip-sample contact mechanics are used to 

relate the contact stiffness to mechanical properties such as elastic modulus. Additional measurements of the contact resonance peak width (quality 

factor) allows the tip-sample damping interaction, and ultimately the 

sample’s viscoelastic properties, 

to be determined. In systems with 

known mechanical properties, contact resonance signals are a rapid, 

sensitive probe of sub-nanometer 

changes in the tip-sample contact.

Impact and Customers

Nanocomposites are poised for explosive growth, particularly in the packaging, 

building and automotive industries. Enhanced composite performance requires optimal interfaces between the polymer matrix and the nanofiller. Nanomechanical 

mapping enables unprecedented levels of interface characterization and, ultimately, 

control.

Development of new polymer products ranging from consumer goods to bioreplacement materials continues to accelerate. Many involve strongly viscoelastic, 

compliant materials with nano- and microscale heterogeneities. Evaluating the mechanical robustness of these materials is critical for qualifying future product reliability and performance. 

New contact-based processes could revolutionize nanomanufacturing, but they 

cannot yet be sufficiently controlled to produce high-volume, low-cost products. 

Sensitive, real-time measurements of nanoscale contact interactions lead to enhanced nanofabrication process control and, hence, dramatic improvements in repeatability, yield and speed.

oak cell walls

1 mm

height stiffness

1 mm

carbon fibers in epoxy

500 nm
In recent activities, we have continued to 

expand the measurement capabilities of 

CR-FM. With academic and industrial partners, we developed methods to determine 

viscoelastic properties from the frequency 

and quality factor of the contact resonance peak. We demonstrated this new 

approach on a blend containing polystyrene (PS) domains in a polypropylene (PP) 

matrix. Figure 1 shows viscoelastic CR-FM 

maps of the storage modulus (E’) and loss 

modulus (E’’) normalized to the mean PP 

values. There is relatively little difference in 

the stiffness (storage modulus) of the two 

polymers, but the damping (loss modulus) 

shows greater contrast. The maps in Figure 1 agree well with results obtained from 

dynamic mechanical analysis techniques on 

bulk samples. These results are promising 

for characterizing a wide range of industrially important, compliant materials.

In further work, we applied viscoelastic 

CR-FM methods to a biomaterial region 

containing stiff bone connected to compliant cartilage via mineralized cartilage. 

Mechanical-property information about 

this osteochondral interface is essential to 

reveal its functionality, understand its disease states, and develop reliable replacement materials. The CR-FM maps in Figure 

2 of storage modulus and viscoelastic loss 

tangent reveal the spatial distribution of 

mechanical properties within the interface. 

With better spatial resolution than nanoindentation, CR-FM measurements indicated 

an interface width of a few micrometers and 

a gradual increase in loss tangent from the 

mineralized cartilage to the polymer-embedded cartilage. Our results not only provide insight into this specific interface region, but they also demonstrate the utility 

of CR-FM for a variety of systems containing interfaces between dissimilar materials.

We have also begun to exploit contact 

resonance concepts for other nanotechnology research needs. For example, we 

have demonstrated how nanoscale wear of 

the AFM tip can be monitored in situ and 

in real time with CR-FM. As indicated in 

Figure 3, our approach provides continuous feedback as the tip is scanned across 

a smooth, uniform surface. In contrast, conventional approaches require intermittently 

interrupting the experiment to perform 

time-consuming imaging in the scanning 

electron microscope (SEM). This simple 

example represents a much broader class 

of nanomanufacturing operations that involve contacting, sliding interfaces. Use of 

CR-FM and related AFM methods to better understand these contact interactions 

will ultimately lead to nanofabrication processes with higher yield, better quality and 

improved commercial viability.

This work was publicized in over 20 invited 

and contributed presentations at national 

and international conferences and workshops in the last 18 months. We have written ten journal articles and two book chapters on our results in the same time period. 

In addition to reporting our technical results, we are engaged in efforts to transition CR-FM methods to a wider community. 

Technology transfer efforts include development of two software platforms for data 

analysis and substantial modification of our 

SPRITE module for improved data acquisition. We also taught lecture and laboratory 

modules in a three-day class on AFM nanomechanical measurements.

Learn More

Donna Hurley, Jason Killgore, Sara 

Campbell, Tony Kos (PML)

Donna Hurley

303-497-3081

donna.hurley@nist.gov

http://www.nist.gov/mml/acmd

Publications

JP Killgore and DC Hurley, Pulsed contact resonance for AFM nanomechanical measurements, Appl. 

Phys. Lett. 100 (5), 053104 (2012).

JP Killgore, DG Yablon, AH Tsou, A Gannepalli, PA Yuya, JA Turner, R Proksch, and DC Hurley, Viscoelastic property mapping with contact resonance force microscopy, Langmuir 27 (23), 13983-13987 

(2011).

JP Killgore, RH Geiss, and DC Hurley, Continuous measurement of AFM tip wear by contact resonance force microscopy, Small 7 (8), 1018-1022 (2011).

DC Hurley, Contact resonance force microscopy techniques for nanomechanical measurements, in 

Applied Scanning Probe Methods Vol. XI, eds. B. Bhushan and H. Fuchs, Springer-Verlag, Berlin 

(2009), pp. 97-138.

ADVANCED MATERIALS

Accomplishments

Figure 1. Maps of (top) relative storage modulus E’/E’PP

and (bottom) relative loss modulus E’’/E’’PP for a PS/PP 

polymer blend obtained with viscoelastic CR-FM methods.

Figure 2. Maps of (a) topography, (b) storage modulus M’, 

and (c) loss tangent (tan δ) of bone-cartilage interface region obtained with CR-FM.

Figure 3. In-situ CR-FM measurements of contact radius (black 

line) continuously monitor the wear of the AFM tip during scanning and agree with intermittent AFM adhesion measurements 

(red dots) and ex-situ SEM measurements of tip shape (images).

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