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Langmuir 2001, 17, 349-360
349
Interpretation of Contrast in Tapping Mode AFM and
Shear Force Microscopy. A Study of Nafion
P. J. James, M. Antognozzi,* J. Tamayo, T. J. McMaster, J. M. Newton,† and
M. J. Miles
University of Bristol, H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL, U.K.
Received March 6, 2000. In Final Form: November 1, 2000
The origin of phase contrast in tapping-mode atomic force microscopy has been investigated using two
complementary scanning probe microscopy techniques, atomic force microscopy and shear force microscopy,
which can be classified as a transverse dynamic force microscopy. The sample chosen for this study was
Nafion, and specifically the membrane in different hydration states by virtue of its cation form. Differences
in probe-sample adhesion throughout a sample, caused by an inhomogeneous distribution of surface
water, were an important phase-contrast mechanism. A new variant in three-dimensional force imaging,
phase-volume imaging has been a useful tool in the interpretation of phase contrast. With the use of
transverse dynamic force microscopy, approach curves were obtained while the frequency spectrum around
resonance was measured. This enabled the damping of the probe oscillation amplitude and the shift in
its resonant frequency to be decoupled. Knowing the true oscillation amplitude of the probe, it was also
possible to determine quantitatively the elastic and dissipative parts of the probe-sample interaction.
Distinct regimes were found at different probe-sample separations.
Introduction
Tapping-mode atomic force microscopy (AFM) is more
suitable than contact mode for imaging delicate samples
because of the lower lateral forces. It has been applied to
many polymer systems.1-4 It also has the added advantage
of being able to obtain phase images and topographical
data. Tapping-mode phase imaging is a relatively new
AFM technique. It can differentiate between areas with
different properties regardless of their topographical
nature.5-7 The phase angle is defined as the phase lag of
the cantilever oscillation relative to the signal sent to the
piezo driving the cantilever.
Transverse dynamic force microscopy (TDFM) is a
dynamic probe microscopy in which the detected force is
perpendicular to the probe, hence “transverse”. The first
use of this technique has been the shear force microscope
(ShFM) as a distance control mechanism in scanning nearfield optical microscopy (SNOM).8,9 In the past few years,
† Present address: National Power Innogy, Harwell International
Business Centre, Harwell, Didcot, OX11 0QA, U.K.
(1) McMaster, T. J.; Hobbs, J. K.; Barham, P. J.; Miles, M. J. AFM
Study of in situ Real Time Polymer Crystallization and Spherulite
Structure. Probe Microscopy 1997, 1(1), 43-56.
(2) Hobbs, J. K.; McMaster, T.J.; Miles, M. J.; Barham, P. J. Direct
Observations of the Growth of Spherulites of Poly(hydroxybutyrateco-valerate) Using Atomic Force Microscopy. Polymer 1998; 39(12),
2437-2446.
(3) Ratner, B.; Tsukruk, V. V. Scanning Probe Microscopy in Polymers.
ACS Symposium Series; American Chemical Society: Washington, DC,
1998.
(4) James, P. J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. In Situ
Rehydration of Perfluorosulphonate Ion-exchange Membrane Studied
by AFM. Polymer 2000, 41(11), 4223-4231.
(5) Leclere, Ph.; Lazzaroni, R.; Bredas, J. L.; Yu, J. M.; Dubois, Ph.;
Jerome, R. Microdomain Morphology Analysis of Block Copolymers bt
Atomic Force Microscopy with Phase Detection Imaging. Langmuir
1996, 12, 4317-4320.
(6) Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B.
Energy Dissipation in Tapping-Mode Atomic Force Microscopy. Appl.
Phys. Lett. 1998, 72(20), 2613-2615.
(7) Tamayo, J.; Garcia, R. Relationship between Phase Shift and
Energy Dissipation in Tapping-Mode Scanning Force Microscopy. Appl.
Phys. Lett. 1998, 73(20), 2926-2928.
(8) Betzig, E.; Finn, P. L.; Weiner, J. S. Combined Shear Force and
Near-field Scanning Optical Microscopy. Appl. Phys. Lett. 1992, 60(20),
2485-2486.
this particular experimental setup has been applied in
the study of different samples in which the term “shear
force” was not appropriate. (It recalls the idea of shear
between surfaces that is not true in all cases.) For this
reason, a more general description was required. In TDFM,
the cantilever is oriented perpendicularly to the sample
and oscillates parallel to its surface. The interaction
between the tip and the sample can be measured at
different separations by observing the change in amplitude
and the relative phase of the cantilever oscillation. The
shear force is often used in TDFM to obtain topographic
images of the surface. The oscillation amplitude of the
probe decreases monotonically when approaching the
surface; by using the amplitude signal in a feedback loop
it is therefore possible to scan the surface at constant
height. If the system is monitoring amplitude and phase
at the same time, it is also possible to record phase
information while keeping the amplitude constant. All
the TDFM images in the present work were obtained using
this technique.
TDFM has also been used for force spectroscopy. In this
case the probe is held over one point of the sample surface,
and its amplitude and phase are recorded in a series of
approach and retract cycles. The experimental quantity
that characterizes the cantilever and its interaction with
the specimen is the frequency spectrum across the
resonance peak. An original technique that records this
information at different tip-sample distances (real-time
frequency spectra) will be described in this article. To
finally evaluate forces from these measurements the
dynamics of the vibrating probe has to be modeled and
assumptions made on the actual interaction force. In our
analysis the force is considered as a combination of
(9) Yang, P. C.; Chen, Y.; Vaeziravani, M. Attractive-mode Atomic
Force Microscopy with Optical-Detection in an Orthogonal Cantilever
Sample Configuration. J. Appl. Phys. 1992, 71(6), 2499-2502.
(10) Davy, S.; Spajer, M.; Courjon, D. Influence of the Water Layer
on the Shear Force Damping in Near-field Microscopy. Appl. Phys. Lett.
1998, 73(18), 2594-2596.
(11) Brunner, R.; Marti, O.; Hollricher, O. Influence of Environmental
Conditions on Shear-Force Distance Control in Near-field Optical
Microscopy. J. Appl. Phys. 1999, 86(12), 7100-7106.
10.1021/la000332h CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/21/2000
350
Langmuir, Vol. 17, No. 2, 2001
dissipative and elastic restoring components. This assumption is based on the experimental evidence of a
viscoelastic interaction between the probe and the specimen in normal humidity conditions.10,11
Nafion is a commercially available perfluorosulfonate
cation-exchange membrane (CEM) manufactured by E I
du Pont de Nemours & Co. Inc. It is generally used as a
perm-selective separator in chlor-alkali electrolyzers12,13
and as the electrolyte in solid polymer fuel cells (SPFC).
Perfluorosulfonate cation-exchange membranes are used
in these applications because of their high ionic conductivity and their high mechanical, thermal, and chemical
stability. The industrial applications of Nafion have
prompted considerable research summarized by Eisenberg
and Yeager in 198214 and more recently by Tant et al. in
1997.15 Structurally, Nafion consists of a hydrophobic
tetrafluoroethylene (TFE) backbone with pendant side
chains of perfluorinated vinyl ethers terminated by
hydrophilic ion-exchange groups. The difference in probespecimen adhesion between the hydrophobic backbone
and hydrophilic side-group regions of the polymer allows
the spatial distribution of these two regions to be observed
using tapping-mode phase imaging.4
The two complementary scanning probe microscopy
(SPM) techniques of AFM and TDFM have been used to
investigate the difference in phase contrast exhibited by
two Nafion samples differing only in cation form (H+ and
Cs+). For each ion form of Nafion, the same probes were
used under identical imaging conditions for both AFM
and TDFM imaging. Initially standard AFM techniques
were applied to Nafion and a test sample before investigating the membrane further with many more novel
techniques, such as phase-volume imaging and the
collection of a real-time frequency spectrum.
Modeling of Probe-Sample Interaction
To understand fully the reasoning behind the experiments performed, it is essential that the nature of the
probe-sample interaction for these two complementary
techniques be understood.
Atomic Force Microscopy. In standard tapping-mode
scanning force microscopy (TMSFM), the tip intermittently
contacts the surface, resulting in a minimization of the
destructive lateral forces. This allows the study of soft
surfaces and/or weakly adsorbed molecules on a substrate.
Alternatively, the measurement of the phase lag of the
cantilever oscillation with respect to the excitation force
contains information about the interaction between the
tip and the sample, allowing compositional contrast on
heterogeneous surfaces.16
The origin and nature of the phase contrast has been
a subject of debate and discussion for the past few
years.17-34 During each oscillation cycle beginning with
the tip furthest from the specimen surface, the tip feels
a negligible force, then a long-range attractive interaction,
(12) Yeager, H. L.; Steck, A. Cation and Water Diffusion in Nafion
Ion Exchange Membranes: Influence of Polymer Structure. J. Electrochem. Soc. 1981, 128, 1880-1884.
(13) Yeager, H. L.; O’Dell, B.; Twardowski, Z. Transport Properties
of Nafion Membranes in Concentrated Solution Environments. J.
Electrochem. Soc. 1982, 129, 85-89.
(14) Eisenberg, A.; Yeager, H. L. Perfluorinated Ionomer Membranes.
ACS Books: Washington, DC; 1982.
(15) Tant, M. R.; Mauritz, K. A.; Wilkes, G. L. Ionomers: Synthesis,
Structure, Properties and Applications. Chapman & Hall: London, 1997.
(16) Tamayo, J.; Garcia, R. Deformation, Contact Time and Phase
Contrast in Tapping Mode Scanning Force Microscopy. Langmuir 1996,
12, 4430-4435.
(17) Chen, G. Y.; Warmack, R. J.; Huang, A.; Thundat, T. Harmonic
Response of Near-Contact Scanning Force Microscopy. J. Appl. Phys.
1995, 78(3), 1465-1469.
James et al.
and finally a repulsive force as it approaches and contacts
the sample. Despite the complexity of the interaction and
its effect on the cantilever dynamics, theoretical simulations and experiments of the cantilever dynamics in air
have shown that phase contrast arises from differences
in the energy dissipation between the tip and the
sample.7,22 This relationship is due to the surprisingly
simple harmonic cantilever response. In fact, calculations
and experiments show a sinusoidal movement of the
cantilever for the usual cantilever parameters in air, that
is, spring constant and quality factor on the order of 10
N/m and 100, respectively. This allows the phase shift to
be related analytically to the energy dissipated in the tipsample interaction.6,7,35
sin ψ )
( )
QED
ω A
+
ω0 A0
πkAA0
(1)
where ψ is the phase angle, ω/ω0 is the working frequency/
resonance frequency, A/A0 is the setpoint amplitude/free
amplitude, Q is the quality factor, ED is the energy
dissipation, and k is the cantilever spring constant.
(18) Anczykowski, B.; Kruger, D.; Babcock, K. L.; Fuchs, H. Basic
Properties of Dynamic Force Spectroscopy with the Scanning Force
Microscope in Experiment and Simulation. Ultramicroscopy 1996, 66(34), 251-259.
(19) Anczykowski, B.; Kruger, D.; Fuchs, H. Cantilever Dynamics in
Quasinoncontact Force Microscopy: Spectroscopic Aspects. Phys. Rev.
B Condens. Matter 1996, 53(23), 15485-15488.
(20) Burnham, N. A.; Behrend, O. P.; Oulevey, F.; Gremaud, G.; Gallo,
P. J.; Gourdon, D.; Dupas, E.; Kulik, A. J.; Pollock, H. M.; Briggs, G.
A. D. How Does a Tip Tap? Nanotechnology 1997, 8(2), 67-75.
(21) Kuhle, A.; Sorensen, A. H.; Bohr, J. Role of Attractive Forces in
Tapping Tip Force Microscopy. J. Appl. Phys. 1997, 81(10), 6562-6569.
(22) Tamayo, J.; Garcia, R. Effects of Elastic and Inelastic Interactions
on Phase Contrast Images in Tapping-Mode Scanning Force Microscopy.
Appl. Phys. Lett. 1997, 71(16), 2394-2396.
(23) Whangbo, M. H.; Magonov, S. N.; Bengel, H. Tip-Sample Force
Interactions and Surface Stiffness in Scanning Probe Microscopy. Probe
Microscopy 1997, 1(1), 23-42.
(24) Bar, G.; Brandsch, R.; Whangbo, M. H. Description of the
Frequency Dependence of the Amplitude and Phase Angle of a Silicon
Cantilever Tapping on a Silicon Substrate by the Harmonic Approximation. Surf. Sci. 1998, 411(1-2), L802-L809.
(25) Behrend, O. P.; Oulevey, F.; Gourdon, D.; Dupas, E.; Kulik, A.
J.; Gremaud, G.; Burnham, N. A. Intermittent Contact: Tapping or
Hammering? Appl. Phys. A Materials Sci. Proc. 1998, 66(Pt1SS), S219S221.
(26) Garcia, R.; Tamayo, J.; Calleja, M.; Garcia, F. Phase Contrast
in Tapping-Mode Scanning Force Microscopy. Appl. Phys. A Solids Surf.
1998, 66(Pt1SS), S309-S312.
(27) Hunt, J. P.; Sarid, D. Kinetics of Lossy Grazing Impact
Oscillators. Appl. Phys. Lett. 1998, 72(23), 2969-2971.
(28) Whangbo, M. H.; Bar, G.; Brandsch, R. Description of Phase
Imaging in Tapping Mode Atomic Force Microscopy by Harmonic
Approximation. Surf. Sci. 1998, 411(1-2), L794-L801.
(29) Bar, G.; Brandsch, R.; Whangbo, M. H. Correlation between
Frequency-Sweep Hysteresis and Phase Imaging Instability in Tapping
Mode Atomic Force Microscopy. Surf. Sci. 1999, 436(1-3), L715-L723.
(30) Bar, G.; Brandsch, R.; Whangbo, M. H. Effect of Tip Sharpness
on the Relative Contributions of Attractive and Repulsive Forces in the
Phase Imaging of Tapping Mode Atomic Force Microscopy. Surf. Sci.
1999, 422(1-3), L192-L199.
(31) Haugstad, G.; Jones, R. R. Mechanisms of Dynamic Force
Microscopy on Poly(vinyl alcohol): Region-specific Noncontact and
Intermittent Contact Regimes. Ultramicroscopy 1999, 76(1-2), 7786.
(32) Nony, L.; Boisgard, R.; Aime, J. P. Nonlinear Dynamical
Properties of an Oscillating Tip-Cantilever System in the Tapping Mode.
J. Chem. Phys. 1999, 111(4), 1615-1627.
(33) Bar, G.; Brandsch, R.; Bruch, M.; Delineau, L.; Whangbo, M. H.
Examination of the Relationship between Phase Shift and Energy
Dissipation in Tapping Mode Atomic Force Microscopy by Frequency
Sweep and Force-Probe Measurements. Surf. Sci. 2000, 444(1−3), L11−
L16.
(34) Delineau, L.; Brandsch, R.; Bar, G.; Whangbo, M. H. Harmonic
Responses of a Cantilever Interacting with Elastomers in Tapping Mode
Atomic Force Microscopy. Surf. Sci. 2000, 448(1), L179-L187.
(35) Tamayo, J. Energy Dissipation in Tapping-Mode Scanning Force
Microscopy with Low Quality Factors. Appl. Phys. Lett. 1999, 75(22),
3569-3571.
Phase Contrast
Langmuir, Vol. 17, No. 2, 2001 351
Figure 1. A graphical representation of the two solutions to
eq 1, demonstrating the two possible imaging regimes and the
phase lags associated with them. A more energy-dissipative
feature appears light in the noncontact regime and dark in the
intermittent contact regime. (DI convention has been used.)
This expression allows experimental phase curves to
be interpreted. When the cantilever is far enough from
the sample, the tip oscillates freely (A ) A0, ED ) 0) and
the phase shift is 90° (0° in the Digital Instruments (DI)
software). As the cantilever oscillates in the proximity of
the sample, the oscillation is damped (A < A0) as a
consequence of the interaction between the tip and the
sample, and a linear decrease of the damped amplitude
is produced as the probe approaches the specimen. If it
is assumed that this interaction is conservative and no
energy is dissipated, eq 1 has two solutions, producing
the two branches shown in Figure 1. As the cantilever
approaches the sample, one branch increases until the
phase shift is 180° (-90° in the DI software), whereas the
other branch goes toward 0° (90° in the DI software). The
first solution corresponds to the noncontact regime in
which an attractive interaction is responsible for the
damping of the cantilever oscillation. This interaction
shifts the cantilever resonance to lower frequencies
producing a phase shift lower than 90°. The second solution
is associated with the intermittent-contact regime, in
which the repulsive force produced during the tip-sample
contact displaces the resonance to higher frequencies. The
DI phase convention will be adopted for the remainder of
the article.
The effect of a tip-sample interaction, which involves
energy dissipation, is the displacement of the noncontact
solution to higher phase shifts and the intermittentcontact solution to lower phase shift values. The more
dissipative features will appear lighter in the noncontact
regime, whereas they will appear darker in the intermittent-contact regime. An experimental curve is a combination of both solutions. As the cantilever approaches, the
attractive force is responsible for the damping of the
oscillation, and the cantilever oscillates in the noncontact
regime. As the cantilever approaches further, the tip
strikes the surface intermittently and a sudden change
in the phase shift is observed as a consequence of the
transition of the cantilever oscillation from noncontact to
the intermittent-contact regime.
Transverse Dynamic Force Microscopy. The dynamics of an oscillating cylindrical probe can be modeled
using the simple harmonic oscillation theory36 but, in this
case, the continuum mechanics model for a cylindrical
bar has been used.37 The equation describing the trans(36) Sarid, D. Scanning Force Microscopy; Oxford University Press;
Oxford, 1991.
(37) Drummond-Roby, M. A.; Wetsel, G. C. Measurement of Elastic
Force on a Scanned Probe Near a Solid Surface. Appl. Phys. Lett. 1996,
69(24), 3689-3691.
Figure 2. (a) The two possible mechanisms for a decrease in
the oscillation amplitude of a probe as it is brought into contact
with the sample; a damping or a change in resonance frequency.
These two mechanisms are indistinguishable using conventional
feedback methods. (b) The experimental setup required to
perform a real-time frequency spectrum, enabling the two
components of the probe-sample interaction to be separated.
verse oscillation of a cylindrical bar is:
∂2
∂u
∂2u
∂2
EI
u
+
γ
+
Aσ
)0
∂t
∂z2
∂z2
∂t2
(
)
(2)
where u(z,t) is the displacement of the bar, E is the Young’s
modulus, I is the second moment of inertia, γ is the internal
friction coefficient, and σ is the density of the bar. The
boundary conditions for the clamped end (z ) 0) are: u(0)
) d0; ∂u/∂z ) 0 (d0 is the applied vibration to the probe),
and for the free end (z ) L) EI∂2u/∂z2 ) 0; EI∂2u/∂z3 ) 0.
The second and third derivatives are proportional to the
external torque and the external force, respectively. The
oscillation amplitude and resonance frequency of the probe
change when it is interacting with the surface. To model
the experimental data, the interacting force is described
by an elastic and a dissipative component. The first term
is responsible for the decrease in the oscillation amplitude,
whereas the second term takes into account the shift in
resonance frequency. The boundary conditions can be
therefore rewritten as:
EI(∂3u/∂z3)z)L ) ν(∂u/∂t)z)L + ku(L)
(3)
where ν is a dissipative coefficient and k is an elastic
constant.
The importance of a liquid film between the sample
and the probe for the TDFM contrast mechanism has
already been reported.10,11 When the probe is close to the
surface (∼10 nm) the liquid film becomes confined and
displays solidlike behavior.38,39 This gives rise to a
viscoelastic shear force,40 which is dependent on the sample
(38) Israelachvili, J. Intermolecular and Surface Forces; Academic
Press: London, 1985.
(39) Granick, S. Motions and Relaxations of Confined Liquids. Science
1991, 253, 1374-1379.
352
Langmuir, Vol. 17, No. 2, 2001
James et al.
Figure 3. Images of different cation forms of Nafion obtained using the same cantilever under identical imaging conditions. (a)
A 1-µm tapping-mode AFM topography image of Nafion 115 H+ imaged under ambient conditions. (b) A phase image corresponding
to part a (Z-scale, 25 nm and 10°, respectively). (c) A 1-µm tapping-mode AFM topography image of Nafion 115 Cs+ imaged under
ambient conditions. (d) A phase image corresponding to part c (Z-scale, 25 nm and 60°, respectively).
surface via the chemical bonding between the first liquid
layer and the surface, and on the amount of surface water,
which is humidity-dependent. A force with an elastic and
a dissipative component can simulate a viscoelastic
interaction and for this reason it has been implemented
in the described model.
Experimental Method
Sample Preparation. The membrane will rehydrate readily
if it is exposed to an environment of high relative humidity (RH).41
Careless handling can result in the membrane ion-exchanging
from the acid (H+) to a salt form (e.g., Na+ or K+). All Nafion
samples therefore were routinely prepared by refluxing with
concentrated nitric acid and deionized water (50/50 v:v), then
deionized water alone, to ensure that the membrane was in the
H+ form and free from any chemical impurities. Strips of Nafion
H+ were then converted to the Cs+ form by immersion in a 0.1
M CsNO3 solution for a week.
Atomic Force Microscopy. H+ and Cs+ samples were
mounted on magnetic stainless steel sample stubs and placed
inside a Digital Instruments Extended Multi Mode AFM using
version 4.22 of the Nanoscope software (DI-Veeco, S. Barbara,
CA). The samples were imaged using tapping-mode phase
imaging and a standard silicon cantilever (∼40 N/m) to provide
topographic and corresponding phase images. The samples were
imaged using the same cantilever under identical imaging
conditions: relative humidity, free amplitude of oscillation, and
ratio of setpoint to free amplitude. The relative humidity was
controlled by placing the AFM inside a purpose-built environmental chamber allowing the humidity to be kept constant by
(40) Hu, H.-W.; Carson, G. A.; Granick, S. Relaxation Time of Confined
Liquids under Shear. Phys. Rev. Lett. 1991, 66(21), 2758-2761.
(41) Dreyfus, B.; Gebel, G.; Aldebert, P.; Pineri, M.; Escoubes, M.;
Thomas, M. Distribution of the Micelles in Hydrated Perfluorinated
Ionomer Membranes from Sans Experiments. J. Phys. 1990, 51(12),
1341-1354.
Figure 4. Variation of IR absorption with cation form. ATR
data obtained for the two ion forms of Nafion. The spectra were
labeled with aid of reference spectra for Nafion and Teflon50
and normalized to the CF2 peaks. Comparison of the two curves
clearly illustrates the greater water content of the H+ form.
passing nitrogen gas through molecular sieve material.4 The
properties of the two ion forms of Nafion were investigated further
by obtaining attenuated total reflection Fourier transform
infrared spectra using a Nicolet 510P spectrometer.
The effect of surface water on tip-sample adhesion was
investigated by using a specimen surface prepared to have
hydrophobic and hydrophilic domains. The test sample was
prepared by cleaning glass with detergent, rinsing with water,
dipping in a mixture of chromic and sulfuric acid, rinsing with
water, dipping in 5 M NaOH, rinsing with water, and then drying
vertically in an oven. When dry, the slide was placed onto a
Phase Contrast
Langmuir, Vol. 17, No. 2, 2001 353
Figure 5. A mixed hydrophilic/hydrophobic test sample consisting of silane evaporated onto cleaned glass. (a) An optical image
of the test sample. Water droplets are clearly visible on the right-hand side of the interface running from middle top to bottom
left. (b) A force distance curve obtained to the left of the interface in part a, the energy required to break free ∼1.2f J. (c) A force
distance curve obtained to the left of the interface in part a, the energy required to break free was ∼7.6f J.
small jar of aminopropyltriethoxysilane, which was allowed to
evaporate onto the slide for 5 min. The adhesive properties of the
test sample were then studied by obtaining multiple-force
distance curves over the two regions.
Force volume imaging allows an image to be built up of the
tip-sample interaction at each pixel from an array of force
curves.42 It has been applied primarily to the study of elastic and
adhesive properties of nonhomogeneous substrates.43-46 Although
the importance of phase-distance curves has been recognized,47
the next logical progression, phase-volume imaging, had yet to
be taken. At each pixel in the image, the phase contrast can be
obtained for any tip-sample separation, hence the word volume.
This is a novel AFM technique on which no reports have been
published to date. In practice, it works in exactly the same way
as the force-volume imaging except that amplitude is used for
the feedback loop and it is the phase signal rather than the
deflection signal that is monitored.
Phase-volume (PV) images of the different cation forms of
Nafion, consisting of 64 × 64 pixels with 64 data points per
approach and retract curve, were obtained. At each pixel the
cantilever approached the surface until such time as the
oscillation amplitude decreased to the preset trigger before
(42) Hoh, J. H.; Heinz, W. F.; Hassan, E. A. Force Volume Support
Note No. 240. Digital Instruments: 1997.
(43) Cappella, B.; Dietler, G. Force-distance Curves by Atomic Force
Microscopy. Surf. Sci. Rep. 1999, 34, 1-104.
(44) Reynaud, C.; Sommer, F.; Quet, C.; El Bounia, N.; Duc, T. M.
Quantitative Determination of Young’s Modulus on a Biphase Polymer
System Using Atomic Force Microscopy. Surf. Interface Anal. 2000,
30(1), 185-189.
(45) Raiteri, R.; Butt, H. J.; Beyer, D.; Jonas, S. Heterogeneous
Polymer-Containing Films: A Comparison of Macroscopic Properties
with Microscopic Properties Determined by Atomic Force Microscopy.
Phys. Chem. Chem. Phys. 1999, 1(20), 4881-4887.
(46) Beake, B. D.; Leggett, G. J.; Shipway, P. H. Frictional, Adhesive
and Mechanical Properties of Polyester Films Probed by Scanning Force
Microscopy. Surf. Interface Anal. 1999, 27(12), 1084-1091.
(47) Chen, X.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams,
P. M.; Davies, J.; Dawkes, A. C.; Edwards, J. C. Interpretation of Tapping
Mode Atomic Force Microscopy Data Using Amplitude-Phase-Distance
Measurements. Ultramicroscopy 1998, 75(3), 171-181.
(48) Antognozzi, M.; Haschke, H.; Miles, M. J. A New Method to
Measure the Oscillation of a Cylindrical Cantilever: “The Laser
Reflection Detection System.” Rev. Sci. Instrum. 2000; in press.
withdrawing and moving onto the next pixel. All the images
were obtained with the cantilever initially at resonance to simplify
interpretation.
A Nafion 115 H+ sample was imaged under ambient conditions
using phase-volume imaging at several different free amplitudes
and ratios of set point to free amplitude. A scan size of 500 × 500
nm2 coupled with an array of 64 × 64 pixels was used to ensure
that features comparable with the cluster size could be detected.
The images could then be analyzed by taking slices through the
images at specific tip-sample distances. Phase images of Nafion
115 Cs+ were then obtained using the same cantilever under the
same conditions with a variety of different free amplitudes and
ratios of set points to free amplitude to determine the optimum
conditions. These were then used to obtain the PV image.
Transverse Dynamic Force Microscopy. All the experiments detailed below were performed using an in-house built
transverse dynamic force microscope. The probe was mounted
on a piezoelectric actuator which drove it at one of its resonant
modes. The oscillation amplitude was detected using the laser
reflection detection system (LRDS)48 that provides the true
measurement of the vibration amplitude necessary to quantify
the tip-sample interaction. Typical values for the oscillation
amplitude are about 10 nm.
An uncoated optical fiber probe was used rather than a metallic
probe49 owing to the similarity of its surface chemistry to that
of a silicon tapping-mode cantilever. The probe was prepared
using the same method as that for SNOM probes.50
To test the importance of the relative humidity in the TDFM
contrast mechanism the microscope was placed inside an inhouse built environmental chamber. A Nafion H+ sample was
mounted onto a 1.5-cm-diameter magnetic stainless steel sample
stub and placed in the transverse dynamic force microscopy. The
relative humidity could be reduced using nitrogen gas passed
through molecular sieve material or increased by bubbling
nitrogen gas through water and into the chamber. Once the
desired humidity had been reached, the sample was allowed to
(49) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. Benign Making
of Sharp Tips for STM and FIM: Pt, Ir, Au, Pd, and Rh. J. Vac. Sci.
Technol. 1995, B13(4), 1556-1559.
(50) Williamson, R. L.; Miles, M. J. Melt-drawn Scanning Near-field
Optical Probe Profiles. J. Appl. Phys. 1996, 80(9), 4804-4812.
354
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James et al.
Figure 6. (a) A 500-nm tapping mode AFM topography image of Nafion H+ imaged under ambient conditions obtained in bistable
regime. Depressions can be seen in the topographic image; these artifacts correspond to the transitions from the noncontact to the
intermittent-contact regime. (b) A phase image corresponding to part a. A much larger phase range can be observed in a bistable
regime than can be obtained working in any one regime. The Z-scales are 20 nm and 180°. (c) A line profile of the topographic image
(a) over four transitions between regimes, at each of which a depression of up to 5 nm can be observed. (d) A line profile of the
phase image (b) over four transitions between regimes, at each of which phase shifts in excess of 90° can be observed.
equilibrate. Topographic and corresponding phase images were
obtained at high (∼50%) and low (∼10%) humidities. In addition
to the images, amplitude-distance curves were obtained at each
of the humidities. This process was then repeated for the Cs+ ion
form of the membrane.
In tapping-mode AFM and TDFM, the feedback mechanism
uses the probe oscillation amplitude signal to control the probesample separation. Unfortunately, using conventional feedback
methods, it is impossible to differentiate between a damping of
the amplitude and a shift in resonance frequency (Figure 2a). As
described previously the liquid confined between the probe and
the surface may be responsible for a decrease in the oscillation
amplitude and a resonance frequency shift. These effects can be
detected by measuring the frequency spectrum of the probe while
it is approaching the surface. In this way the two components
of the interaction are separated and measured as a function of
tip-sample distance. Using the transverse dynamic force
microscopy it is possible to perform a real-time frequency
spectrum by simultaneously exciting two modes of the probe.
The first frequency is kept constant at the first (or second)
resonant peak and is used to monitor the oscillation amplitude
during the approach and retract cycle. The second frequency is
swept from just below to just above the second (or first) resonance
frequency of the free probe, by using the sweep mode of the Philips
5192 signal generator. This mode of operation will sweep the
driving frequency up and down continuously, and the sweep time
can be as low as 50 ms. The amplitude and the phase spectra are
monitored continuously as the probe approaches the sample
surface (Figure 2b) using a Labview recording system.
Results and Discussion
Atomic Force Microscopy. Tapping-mode topography
and corresponding phase images of Nafion H+ obtained
under ambient conditions are shown in Figure 3 a and b,
respectively. The images of Nafion Cs+ obtained using
the same cantilever under identical imaging conditions
are shown in Figures 3c and 3d, respectively. There is a
marked difference in the phase contrast between the two
ion forms. The phase range is significantly larger in the
Cs+ ion, 60° as opposed to 10° for the H+ form. There is
no significant difference in the topography images,
therefore topographic coupling is not responsible for the
change in phase contrast.
Attenuated total reflection (ATR) data obtained for the
two ion forms of Nafion are shown in Figure 4. The spectra
were labeled with the aid of reference spectra for Nafion
and Teflon51 and normalized to the CF2 peaks. Comparison
of the two curves clearly illustrates the greater water
content of the H+ form. This combined with the greater
charge density associated with the Cs+ ion, which would
be screened less effectively, points toward an electrostatic
force being responsible for the difference in phase contrast.
An optical image of the mixed hydrophobic/hydrophilic
test sample is shown in Figure 5a. Water droplets are
clearly visible on the right-hand side of the interface
between the silanized glass and cleaned glass, running
diagonally from top right to bottom left. The interface is
not particularly sharp because of the way in which the
silane was evaporated onto the glass. Force curves
obtained to the left and right of the interface are shown
in Figures 5b and 5c, respectively. The energy required
for the cantilever to break free from the surface was
considerably higher to the right of the interface where the
water droplets are clearly visible. The energy can be
calculated from the area under the curve, if it is assumed
(51) Kuptsov, A. H.; Zhizhin, G. N. Handbook of Fourier Transform
Raman and Infrared Spectra of Polymers; Elsvier Science: Amsterdam,
1998.
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Langmuir, Vol. 17, No. 2, 2001 355
Figure 7. Phase images corresponding to slices through the phase-volume image of Nafion Cs+ for cantilever positions from 0
to 48 nm at intervals of 3.7 nm showing the wide range of possible contrasts (Z-scale, 60°). Note that the cantilever position is
arbitrary and it does not indicate the cantilever-sample distance. The zero position is determined as the cantilever position in which
the amplitude is damped by 90%.
that Hooke’s law is obeyed and that the cantilever spring
constant is ∼0.12 N/m.
The pull-off energy was ∼1.2f J and ∼7.6f J for areas
left and right of the interface, respectively. It is clear from
these results that any surface water dramatically increases
the adhesive force between the tip and sample. Although
these features are over an order of magnitude larger than
the proposed cluster size in Nafion, this contrast mechanism would still apply when imaging at higher resolutions. The elastic response of Nafion increases with
frequency,52 at the frequencies at which the cantilever is
being driven ∼250 kHz, and so the viscoelastic energy
loss would be negligible. Therefore any energy dissipation
in the tapping interaction is primarily the result of tipsample adhesion rather than a viscoelastic energy loss.
Differences in surface adhesion over a sample caused by
an inhomogeneous distribution of surface water are
probably a very important phase-contrast mechanism.
Normal tapping-mode topography and phase images of
Nafion, obtained in a bistable regime are shown in Figures
6a and 6b, respectively. In a bistable regime, the cantilever
(52) Eisenberg, A.; King, M. Ion-Containing Polymers; Physical
Properties and Structure, Vol. 2; Academic Press: London, 1977; p 164.
(53) Kuhle, A.; Sorensen, A. H.; Zandbergen, J. B.; Bohr, J. Contrast
Artifacts in Tapping Tip Atomic Force Microscopy. Appl. Phys. A Solids
Surf. 1998, 66(Pt1 SS), S329-S332.
356
Langmuir, Vol. 17, No. 2, 2001
can oscillate in both noncontact and intermittent-contact
regimes.53-56
From the mathematical point of view, the state of the
oscillation depends on the initial conditions. In the
practice, the cantilever oscillates randomly in noncontact
and intermittent contact during the scanning, producing
a flipping of the phase shift between negative and positive
values (DI software). The sudden transitions from noncontact to intermittent-contact regime are accompanied
by artifacts in the form of depressions in the topography
image. Line profiles across both of the images are shown
in Figures 6c and 6d. The line profiles cover four transitions
between the noncontact and intermittent-contact regimes.
At each of these transitions, corresponding to the intermittent-contact regime, a depression of up to ∼5 nm (25%
of the Z-scale) and a phase shift of move than 90° is
observed in the topographic and phase image, respectively.
This result illustrates the importance of avoiding a bistable
regime in phase imaging of a surface.
Fourteen slices have been taken through a phase-volume
image of Nafion Cs+ at intervals of 3.7 nm. The oscillation
amplitude of the cantilever when free was in excess of 50
nm. Each of the slices shows the phase contrast that would
have been obtained in traditional tapping mode phase
imaging at different set points (Figure 7). At cantileversample distances slightly larger than the free amplitude
(cantilever position ≈ 37 nm), the interaction is too weak
to produce any meaningful phase contrast. A significant
higher phase shift (∆φ > 60°) is observed in certain regions
for a cantilever position from 22 to 33 nm. In these regions,
the cantilever oscillates in the intermittent-contact regime.
Once below a tip-sample separation of about 15 nm, the
images are predominantly in the intermittent-contact
regime and the phase contrast is reduced to about 20°.
However, a few points, which show up as white squares
in the dark regions, are still in the noncontact regime,
even for a damped amplitude lower than 10%. If slices
taken in the noncontact (cantilever position, 44.3 nm) and
intermittent-contact regimes are compared, the same
features are clearly visible in both images, there has been
a contrast reversal, however. This is consistent with eq
1 and implies energy dissipation in the noncontact regime.
To interpret the evolution of the phase contrast with
the damped amplitude, a topography and corresponding
phase image and the three types of amplitude-distance
curves, obtained during phase volume imaging of a Nafion
Cs+ sample using a free amplitude in excess of 5V and a
set point of 2.5V, are shown in Figure 8. The location from
which the phase-distance curves were taken has been
clearly marked in the phase image (Figure 8b). In one
case (Figure 8c), the phase distance curve starts off in the
noncontact regime before swiftly moving into the intermittent contact regime where the phase angle tends to
∼90°, indicating that there is little energy dissipation.
The second type of curve (Figure 8d) takes noticeably more
force to move from the noncontact to intermittent-contact
regimes because of damping caused by an attractive force,
which is probably electrostatic. Once in the intermittent
contact regime the phase angle is considerably lower, ∼60°,
indicating that more energy is being dissipated. Inside
regions made up of the second type of curve; a third type
(54) Behrend, O. P.; Odoni, L.; Loubet, J. L.; Burnham, N. A. Phase
Imaging: Deep or Superficial? Appl. Phys. Lett. 1999, 75(17), 25512553.
(55) SanPaulo, A.; Garcia, R. High-Resolution Imaging of Antibodies
by Tapping-Mode Atomic Force Microscopy: Attractive and Repulsive
Tip-Sample Interaction Regimes. Biophys. J. 2000, 78(3), 1599-1605.
(56) Garcia, R.; SanPaulo, A. Amplitude Curves and Operating
Regimes in Dynamic Atomic Force Microscopy. Ultramicroscopy 2000,
82(1-4), 79-83.
James et al.
Figure 8. Phase-volume images (500 nm) of Nafion Cs+ imaged
under ambient conditions. (a) A topography image (Z-scale, 20
nm); (b) a phase image corresponding to part a (Z-scale, 60°).
The phase-distance curves can be divided into three types. (c)
A curve that moves quickly from the noncontact to intermittentcontact regime. (d) A curve that requires more force to move
into the intermittent-contact regime. (e) A third type that
remains in the noncontact regime can be found within regions
made up of the second type of curve. Note that the data have
been plotted with an offset x-axis to not obscure the data.
of curve (Figure 8e) can be found where, despite the very
high free amplitude and low set point, the cantilever never
actually made contact with the surface because of particularly strong damping. The corresponding points in the
topography image (Figure 8a) appear high, because these
points are in the noncontact regime, whereas the remainder of the image is in the intermittent-contact regime.
The behavior of the first type of curve (Figure 8c) can
be attributed to the hydrophobic backbone of the membrane, whereas that of the second (Figure 8d) and third
(Figure 8e) type of curve can be attributed to the ion-rich
regions of the membrane. The inability to image the Cs+
ion form completely in the intermittent-contact regime,
using the same cantilever as that used for the H+ form,
despite using double the free amplitude, can again be
explained by the greater charge density associated with
the Cs+ ion, which is screened less effectively because of
the lower water content. This produces a strong longrange attractive interaction that shifts the resonance
frequency of the cantilever, therefore reducing its oscillation amplitude until the set point is reached. The second
type of curve (Figure 8d) corresponds to the hydrophilic
region that surrounds the Cs+ ions. These regions show
a lower phase shift with respect to the hydrophobic regions
in the intermittent-contact regime, associated to a higher
energy dissipated between the tip and the sample. The
energy dissipation would be due to preferential water
adsorption to the hydrophilic regions. The unbalance
between the lower adhesion when the water neck forms
and the needed force to break the meniscus should be the
responsible factor of energy dissipation. The contrast
reversal observed in the phase volume in the noncontact
regime supports this energy dissipation model. Only the
Phase Contrast
Langmuir, Vol. 17, No. 2, 2001 357
Figure 9. A comparison of AFM and TDFM images. (a) A 1-µm tapping-mode AFM topography image of a Nafion H+ sample imaged
under ambient conditions (Z-scale, 15 nm). (b) A phase image corresponding to part a (Z-scale, 30°). (c) A 1-µm TDFM topography
image of Nafion H+ imaged under ambient conditions (Z-scale, 400 nm). The resolution of the TDFM is lower than that of the AFM
owing to the size of the probe and thermal drift, which is more of a problem for the TDFM because of the longer scan times, 30
min compared with 4 min for the AFM. (d) Corresponding TDFM phase image (Z-scale, 30°).
rupture of a water meniscus between the tip and the
sample could produce energy dissipation in the noncontact
regime.
Although terminology such as soft and hard tapping23
is useful, what is really important is the tapping regime
in which the experiment is performed, that is whether it
is noncontact or intermittent contact. It is particularly
important not to work in a bistable regime because of the
height artifacts that can be produced. If the phase contrast
observed is greater than 90°, it is certain that this is the
case.
Several articles cite phase ranges of 90° or higher23; it
is likely that the images were obtained in a bistable regime.
Consequently, any topography data across the bistable
regime is at least partly artifactual. Many of the effects
observed in other articles57-59 including contrast reversal
in the topography and phase images may be attributed to
working in a bistable regime or moving from one regime
to another.
To use phase imaging successfully, it is essential to
establish the tip-sample interaction regime by obtaining
a phase-distance curve and adjusting the free amplitude
(57) McLean, S. R.; Sauer, B. B. Nano-deformation of Crystalline
Domains during Tensile Stretching Studied by Atomic Force Microscopy.
J. Polym. Sci. Part B Polym. Phys. 1999, 37(8), 859-866.
(58) McLean, S. R.; Sauer, B. B. Tapping-Mode AFM Studies Using
Phase Detection for the Resolution of the Nanophases in Segmented
Polyurethanes and Other Block Copolymers. Macromolecules 1997,
30(26), 8314-8317.
(59) Sauer, B. B.; McLean, R. S.; Thomas, R. R. Tapping Mode AFM
Studies of Nano-phases on Fluorine-containing Polyester Coatings and
Octadecyltrichlorosilane Monolayers. Langmuir 1998, 14(11), 30453051.
(60) Hsu, W. Y.; Gierke, T. D. Ion-Transport and Clustering in Nafion
Perfluorinated Membranes. J. Membr. Sci. 1983, 13(3), 307-326.
and set point accordingly. During the study it was observed
that different cantilevers required different amounts of
force to move from the noncontact to intermittent-contact
regimes. This is consistent with a recent study into the
effects of tip sharpness on the contrast in phase imaging.30
The transition occurred more easily with a sharper tip.
A higher attractive force appears with blunter tips, as a
consequence of the larger effective contact area for
interaction.
Transverse Dynamic Force Microscopy. A 1-µm
tapping-mode AFM topography image, its corresponding
phase image and TDFM topography, and phase images
of Nafion are shown in Figure 9. It is apparent from the
topography images that the resolution of the TDFM on
this sample is somewhat lower than that of the AFM.
This is probably the result of two main effects: first, the
size of the end of the particular TDFM probe, and second,
thermal drift, because scan times are about 30 min, rather
than the 4 min for the AFM. The nature of the sample
may also have a bearing on the ultimate resolution,
because the cluster-network model of Nafion60 postulates
a large-scale organization of clusters with transient
connective tubes which are in constant flux.
Four amplitude-distance curves obtained using Nafion
H+ and Cs+ samples across a range of humidities from 10
to 52 are shown in Figure 10. The effect of humidity is
evident for both the samples, but it is easier to explain in
Nafion H+ probably because of its higher hydrophilicity.
The difference between the points at which the amplitude
starts to be damped and drops to zero indicates the
thickness of the water layer over the surface (∼6 nm for
the H+ from at 46%RH). The hysteresis in the retract
curve is probably caused by the presence of a capillary
358
Langmuir, Vol. 17, No. 2, 2001
James et al.
Figure 10. The effect of relative humidity on surface water thickness. Four TDFM amplitude-distance curves obtained using
Nafion H+ and Cs+ samples at range of humidities. (a) Nafion H+ at 46%RH, (b) Nafion H+ at 10%RH, (c) Nafion Cs+ at 52%RH,
(d) Nafion Cs+ at 10%RH. As the humidity decreases, the slope of the approach curves becomes steeper, the evidence of a liquid-neck
formation is no longer detectable and, in the Nafion H+, the hysteresis between the approach and retract curves decreases. The
depth of the surface water layer was lower for the Cs+ ion form than for the H+ form.
neck. At lower humidities the approach curves become
steeper, the neck formation is not clearly detectable, and
the hysteresis in the curves decreases, which indicates a
decrease in the thickness of the water layer. Comparison
of the approach and retract curves for the Cs+ and H+
samples at 10% humidity shows a steeper approach curve,
which indicates that the surface is drier. This would be
consistent with the infrared data in Figure 4, which clearly
demonstrated the differences in water content of the ion
forms of the membrane. In Nafion Cs+, the noncontinuous
nature of the surface liquid film causes differences in the
force curves.
Topography and corresponding phase images of Nafion
H+ and Cs+ were obtained during the investigation. The
phase range obtained using TDFM was significantly lower
than that observed with tapping mode AFM because of
the different contrast mechanism involved. In tappingmode AFM, the phase contrast was caused by the tip going
in and out of the water layer, whereas the TDFM probe
is always inside the water layer moving from side to side.
Differences in the hydrophilicity of the sample, resulting
in preferential water adsorption to some areas, would still
be detectable because of the increased drag on the probe
in these areas. A change in phase contrast with humidity
was observed; the phase contrast is lower at the lower
humidity, which is consistent with a recent AFM study.4
The phase contrast for the Cs+ form was lower than that
of the H+ again indicating that there is less surface water
for this ion form.
A real-time frequency spectrum surface is shown in
Figure 11. The surface is obtained from the frequency
spectra taken at different tip-sample distances. The
vertical gray plane represents the reference frequency
used to measure the approach amplitude curve (white
line). It is clear that, when the probe approaches the
surface, the resonance frequency (dotted line) does not
change in a monotonic way on a nanometric scale, as
usually expected.37 Looking at the approach curve it is
possible to distinguish four regions, which are separated
by black lines. Fitting this surface with the model described
previously (eqs 2 and 3), the elastic and dissipative parts
of the interaction can be calculated. The results of this
analysis are shown in Figure 12 in which the four regions
marked in Figure 11 have been emphasized using black
vertical lines.
In region A, the probe is free, until it reaches a distance
of 16 nm from the surface when the oscillation amplitude
of the probe drops as a consequence of the formation of
a capillary condensation neck. In region B, the dissipative
force remains almost constant while the elastic component
increases. This is responsible for the decrease in the
approach-curve amplitude. (A small shift of the frequency
peak produces a significant change in the amplitude at
resonance.) In region C, the dissipative component
dominates the elastic component and is responsible for
the large decrease in the oscillation amplitude. In region
D, the slope of the elastic force is steeper than the
dissipative force, and the elastic component will eventually
become predominant for z smaller than 4 nm. These results
clearly show that the information provided by the approach
curve alone is insufficient to determine the kind of probesample interaction. The real-time frequency spectrum is
therefore a unique tool to decouple and quantify the two
components giving a better insight into the nature of the
interaction.
Phase Contrast
Langmuir, Vol. 17, No. 2, 2001 359
Figure 11. A real-time frequency spectrum obtained using the TDFM. The black curves on the surface are frequency spectra taken
at different tip-sample distances. The vertical gray plane represents the reference frequency used to measure the approach
amplitude curve (white line), and a dotted line represents the resonant frequency. Using the mathematical model highlighted in
the text, it is possible to evaluate the elastic and dissipative components of the tip probe from these data.
Figure 12. Analysis of real-time frequency spectrum obtained using the TDFM. The approach curve (thick line), the dissipative
component (dotted line), and the elastic component (continuous line) of the probe-sample interaction. The slope of the force curves
in the different regions (a, b, c, d) is added to emphasize the predominant component.
Conclusions
The origin of phase contrast in Nafion has been
investigated using the two complementary SPM techniques of AFM and TDFM. A variety of standard and new
techniques, namely phase-volume imaging and a realtime frequency spectrum were used.
Force curves obtained over a mixed hydrophobic/
hydrophilic test sample showed a much larger adhesive
force over the water-rich regions. An increase in relative
humidity resulted in an increase in the thickness of the
surface water layer and the phase contrast observed with
both SPM techniques. Therefore differences in probesample adhesion, caused by an inhomogeneous distribution of surface water, are an important phase-contrast
mechanism.
Phase-volume imaging has been a useful tool in the
interpretation of phase contrast. It has clearly demonstrated the wide range of phase contrasts that can be
observed on the same sample. Moving from the noncontact
to intermittent-contact regime resulted in a contrast
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Langmuir, Vol. 17, No. 2, 2001
inversion. The most dissipative features were light in the
noncontact and dark in intermittent-contact regimes.
When working in a bistable regime, height artifacts were
produced in the topographic images at the points where
the transition occurred. The anomalous results for the
different cation forms of Nafion and those in many
published studies can be attributed to working in a bistable
regime or moving from one regime to another.
The importance of phase-distance curves has been
highlighted as a prerequisite to imaging to ensure that
imaging takes place in any one regime, rather than relying
on a standard set of operating conditions. The sharpness
of the tip influenced the phase contrast observed, altering
the force required to move from one regime to another. It
is therefore necessary to obtain a phase-distance curve if
a cantilever is damaged or changed to ensure that imaging
continues in the same regime.
The phase-volume images of Nafion consisted of two
main types of phase-distance curves. The first curve moved
quickly from the noncontact to the intermittent-contact
regime, and once there the phase angle of ∼90° indicated
little energy dissipation. These regions were attributed
to the hydrophobic backbone. The second type of curve
required considerably more force to enter the intermittentcontact regime, and once there the phase angle of ∼60°
indicated more energy dissipation. These regions were
attributed to the ion-rich regions that would damp the
cantilever oscillation with an attractive electrostatic force
at longer distances, then, once in contact, dissipate more
energy owing to their greater affinity for water. A greater
force was required to image the Cs+ ion form in the
intermittent-contact regime compared with the H+ form
because of the lower water content and therefore reduced
screening of the Cs+ ions charge.
When compared with AFM (dynamic mode), it is clear
that TDFM differs in two main aspects: the shape and
the orientation of the probe with respect to the specimen
James et al.
surface. In TDFM the cantilevers have a cylindrical
tapered shape and are mounted perpendicular to the
specimen surface, which allows accurate control of the
tip-sample distance, ecause the probe is not extensible
in the vertical direction. This characteristic makes the
TDFM a more suitable tool for force spectroscopy: the
probe does not jump to contact during the approach and
a constant load rate can be applied by just keeping the
approach or retract speed constant. (Both these aspects
are problematic in AFM.)
Thus far efforts have been concentrated mainly on
determining the energy loss mechanism in the tip-sample
interaction and have neglected the effect of resonance
frequency shifts. A real-time frequency spectrum was
obtained to decouple the two effects of change in resonance
frequency and damping of the oscillation. It was also
possible to determine quantitatively the elastic and
dissipative parts of the interaction by accurately modeling
the dynamic of the TDFM probes. Distinct regimes were
found at different probe-sample separations.
Although tapping-mode phase imaging remains a very
useful tool for identifying and mapping regions of different
properties regardless of their topographical nature, the
interpretation is not always trivial. There have been
several pit falls for the unwary, namely phase inversion
and height artifacts.
Acknowledgment. The authors would like to thank
Anna Halter for her assistance with the ATR analysis
and Andy Humphris for his help with the “real time
frequency spectrum” technique. This work was supported
financially by the EPSRC and National Power PLC as
part of their ongoing research into regenerative fuel cell
technology.
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