shear force versus tapping - tuning fork afm

Interpretation of Contrast in Tapping Mode AFM andShear Force Microscopy. A Study of Nafion

P. J. James, M. Antognozzi,* J. Tamayo, T. J. McMaster, J. M. Newton,† andM. 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 twocomplementary 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 wasNafion, and specifically the membrane in different hydration states by virtue of its cation form. Differencesin probe-sample adhesion throughout a sample, caused by an inhomogeneous distribution of surfacewater, 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 oftransverse dynamic force microscopy, approach curves were obtained while the frequency spectrum aroundresonance was measured. This enabled the damping of the probe oscillation amplitude and the shift inits resonant frequency to be decoupled. Knowing the true oscillation amplitude of the probe, it was alsopossible to determine quantitatively the elastic and dissipative parts of the probe-sample interaction.Distinct regimes were found at different probe-sample separations.

IntroductionTapping-mode atomic force microscopy (AFM) is more

suitable than contact mode for imaging delicate samplesbecause of the lower lateral forces. It has been applied tomany polymer systems.1-4 It also has the added advantageof being able to obtain phase images and topographicaldata. Tapping-mode phase imaging is a relatively newAFM technique. It can differentiate between areas withdifferent properties regardless of their topographicalnature.5-7 The phase angle is defined as the phase lag ofthe cantilever oscillation relative to the signal sent to thepiezo driving the cantilever.

Transverse dynamic force microscopy (TDFM) is adynamic probe microscopy in which the detected force isperpendicular to the probe, hence “transverse”. The firstuse of this technique has been the shear force microscope(ShFM) as a distance control mechanism in scanning near-field optical microscopy (SNOM).8,9 In the past few years,

this particular experimental setup has been applied inthe study of different samples in which the term “shearforce” was not appropriate. (It recalls the idea of shearbetween surfaces that is not true in all cases.) For thisreason, a more general description was required. In TDFM,the cantilever is oriented perpendicularly to the sampleand oscillates parallel to its surface. The interactionbetween the tip and the sample can be measured atdifferent separations by observing the change in amplitudeand the relative phase of the cantilever oscillation. Theshear force is often used in TDFM to obtain topographicimages of the surface. The oscillation amplitude of theprobe decreases monotonically when approaching thesurface; by using the amplitude signal in a feedback loopit is therefore possible to scan the surface at constantheight. If the system is monitoring amplitude and phaseat the same time, it is also possible to record phaseinformation while keeping the amplitude constant. Allthe TDFM images in the present work were obtained usingthis technique.

TDFM has also been used for force spectroscopy. In thiscase the probe is held over one point of the sample surface,and its amplitude and phase are recorded in a series ofapproach and retract cycles. The experimental quantitythat characterizes the cantilever and its interaction withthe specimen is the frequency spectrum across theresonance peak. An original technique that records thisinformation at different tip-sample distances (real-timefrequency spectra) will be described in this article. Tofinally evaluate forces from these measurements thedynamics of the vibrating probe has to be modeled andassumptions made on the actual interaction force. In ouranalysis the force is considered as a combination of

† Presentaddress: NationalPower Innogy,Harwell InternationalBusiness Centre, Harwell, Didcot, OX11 0QA, U.K.

(1) McMaster, T. J.; Hobbs, J. K.; Barham, P. J.; Miles, M. J. AFMStudy of in situ Real Time Polymer Crystallization and SpheruliteStructure. Probe Microscopy 1997, 1(1), 43-56.

(2) Hobbs, J. K.; McMaster, T.J.; Miles, M. J.; Barham, P. J. DirectObservations of the Growth of Spherulites of Poly(hydroxybutyrate-co-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 SituRehydration of Perfluorosulphonate Ion-exchange Membrane Studiedby 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 btAtomic Force Microscopy with Phase Detection Imaging. Langmuir1996, 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 andEnergy 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 andNear-field Scanning Optical Microscopy. Appl. Phys. Lett. 1992, 60(20),2485-2486.

(9) Yang, P. C.; Chen, Y.; Vaeziravani, M. Attractive-mode AtomicForce Microscopy with Optical-Detection in an Orthogonal CantileverSample Configuration. J. Appl. Phys. 1992, 71(6), 2499-2502.

(10) Davy, S.; Spajer, M.; Courjon, D. Influence of the Water Layeron 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 EnvironmentalConditions on Shear-Force Distance Control in Near-field OpticalMicroscopy. J. Appl. Phys. 1999, 86(12), 7100-7106.

349Langmuir 2001, 17, 349-360

10.1021/la000332h CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 12/21/2000

dissipative and elastic restoring components. This as-sumption is based on the experimental evidence of aviscoelastic interaction between the probe and the speci-men in normal humidity conditions.10,11

Nafion is a commercially available perfluorosulfonatecation-exchange membrane (CEM) manufactured by E Idu Pont de Nemours & Co. Inc. It is generally used as aperm-selective separator in chlor-alkali electrolyzers12,13

and as the electrolyte in solid polymer fuel cells (SPFC).Perfluorosulfonate cation-exchange membranes are usedin these applications because of their high ionic conduc-tivity and their high mechanical, thermal, and chemicalstability. The industrial applications of Nafion haveprompted considerable research summarized by Eisenbergand Yeager in 198214 and more recently by Tant et al. in1997.15 Structurally, Nafion consists of a hydrophobictetrafluoroethylene (TFE) backbone with pendant sidechains of perfluorinated vinyl ethers terminated byhydrophilic ion-exchange groups. The difference in probe-specimen adhesion between the hydrophobic backboneand hydrophilic side-group regions of the polymer allowsthe spatial distribution of these two regions to be observedusing tapping-mode phase imaging.4

The two complementary scanning probe microscopy(SPM) techniques of AFM and TDFM have been used toinvestigate the difference in phase contrast exhibited bytwo Nafion samples differing only in cation form (H+ andCs+). For each ion form of Nafion, the same probes wereused under identical imaging conditions for both AFMand TDFM imaging. Initially standard AFM techniqueswere applied to Nafion and a test sample before inves-tigating the membrane further with many more noveltechniques, such as phase-volume imaging and thecollection of a real-time frequency spectrum.

Modeling of Probe-Sample Interaction

To understand fully the reasoning behind the experi-ments performed, it is essential that the nature of theprobe-sample interaction for these two complementarytechniques be understood.

Atomic Force Microscopy. In standard tapping-modescanning forcemicroscopy (TMSFM), the tip intermittentlycontacts the surface, resulting in a minimization of thedestructive lateral forces. This allows the study of softsurfaces and/or weakly adsorbed molecules on a substrate.Alternatively, the measurement of the phase lag of thecantilever oscillation with respect to the excitation forcecontains information about the interaction between thetip and the sample, allowing compositional contrast onheterogeneous surfaces.16

The origin and nature of the phase contrast has beena subject of debate and discussion for the past fewyears.17-34 During each oscillation cycle beginning withthe tip furthest from the specimen surface, the tip feelsa negligible force, then a long-range attractive interaction,

and finally a repulsive force as it approaches and contactsthe sample. Despite the complexity of the interaction andits effect on the cantilever dynamics, theoretical simula-tions and experiments of the cantilever dynamics in airhave shown that phase contrast arises from differencesin the energy dissipation between the tip and thesample.7,22 This relationship is due to the surprisinglysimple harmonic cantilever response. In fact, calculationsand experiments show a sinusoidal movement of thecantilever for the usual cantilever parameters in air, thatis, spring constant and quality factor on the order of 10N/m and 100, respectively. This allows the phase shift tobe related analytically to the energy dissipated in the tip-sample interaction.6,7,35

where ψ is the phase angle, ω/ω0 is the working frequency/resonance frequency, A/A0 is the setpoint amplitude/freeamplitude, Q is the quality factor, ED is the energydissipation, and k is the cantilever spring constant.

(12) Yeager, H. L.; Steck, A. Cation and Water Diffusion in NafionIon Exchange Membranes: Influence of Polymer Structure. J. Elec-trochem. Soc. 1981, 128, 1880-1884.

(13) Yeager, H. L.; O’Dell, B.; Twardowski, Z. Transport Propertiesof 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 PhaseContrast in Tapping Mode Scanning Force Microscopy. Langmuir 1996,12, 4430-4435.

(17) Chen, G. Y.; Warmack, R. J.; Huang, A.; Thundat, T. HarmonicResponse of Near-Contact Scanning Force Microscopy. J. Appl. Phys.1995, 78(3), 1465-1469.

(18) Anczykowski, B.; Kruger, D.; Babcock, K. L.; Fuchs, H. BasicProperties of Dynamic Force Spectroscopy with the Scanning ForceMicroscope in Experiment and Simulation.Ultramicroscopy1996, 66(3-4), 251-259.

(19) Anczykowski, B.; Kruger, D.; Fuchs, H. Cantilever Dynamics inQuasinoncontact 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 inTapping Tip Force Microscopy. J. Appl. Phys. 1997, 81(10), 6562-6569.

(22) Tamayo, J.; Garcia, R. Effects of Elastic and Inelastic Interactionson 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 ForceInteractions and Surface Stiffness in Scanning Probe Microscopy. ProbeMicroscopy 1997, 1(1), 23-42.

(24) Bar, G.; Brandsch, R.; Whangbo, M. H. Description of theFrequency Dependence of the Amplitude and Phase Angle of a SiliconCantilever Tapping on a Silicon Substrate by the Harmonic Ap-proximation. 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 orHammering? Appl. Phys. A Materials Sci. Proc. 1998, 66(Pt1SS), S219-S221.

(26) Garcia, R.; Tamayo, J.; Calleja, M.; Garcia, F. Phase Contrastin 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 ImpactOscillators. Appl. Phys. Lett. 1998, 72(23), 2969-2971.

(28) Whangbo, M. H.; Bar, G.; Brandsch, R. Description of PhaseImaging in Tapping Mode Atomic Force Microscopy by HarmonicApproximation. Surf. Sci. 1998, 411(1-2), L794-L801.

(29) Bar, G.; Brandsch, R.; Whangbo, M. H. Correlation betweenFrequency-Sweep Hysteresis and Phase Imaging Instability in TappingMode Atomic Force Microscopy. Surf. Sci. 1999, 436(1-3), L715-L723.

(30) Bar, G.; Brandsch, R.; Whangbo, M. H. Effect of Tip Sharpnesson the Relative Contributions of Attractive and Repulsive Forces in thePhase Imaging of Tapping Mode Atomic Force Microscopy. Surf. Sci.1999, 422(1-3), L192-L199.

(31) Haugstad, G.; Jones, R. R. Mechanisms of Dynamic ForceMicroscopy on Poly(vinyl alcohol): Region-specific Noncontact andIntermittent Contact Regimes. Ultramicroscopy 1999, 76(1-2), 77-86.

(32) Nony, L.; Boisgard, R.; Aime, J. P. Nonlinear DynamicalProperties 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 EnergyDissipation in Tapping Mode Atomic Force Microscopy by FrequencySweep and Force-Probe Measurements. Surf. Sci. 2000, 444(1−3), L11−L16.

(34) Delineau, L.; Brandsch, R.; Bar, G.; Whangbo, M. H. HarmonicResponses of a Cantilever Interacting with Elastomers in Tapping ModeAtomic Force Microscopy. Surf. Sci. 2000, 448(1), L179-L187.

(35) Tamayo, J. Energy Dissipation in Tapping-Mode Scanning ForceMicroscopy with Low Quality Factors. Appl. Phys. Lett. 1999, 75(22),3569-3571.

sin ψ ) ( ωω0

AA0

) +QED

πkAA0(1)

350 Langmuir, Vol. 17, No. 2, 2001 James et al.

This expression allows experimental phase curves tobe interpreted. When the cantilever is far enough fromthe sample, the tip oscillates freely (A ) A0, ED ) 0) andthe phase shift is 90° (0° in the Digital Instruments (DI)software). As the cantilever oscillates in the proximity ofthe sample, the oscillation is damped (A < A0) as aconsequence of the interaction between the tip and thesample, and a linear decrease of the damped amplitudeis produced as the probe approaches the specimen. If itis assumed that this interaction is conservative and noenergy is dissipated, eq 1 has two solutions, producingthe two branches shown in Figure 1. As the cantileverapproaches the sample, one branch increases until thephase shift is 180° (-90° in the DI software), whereas theother branch goes toward 0° (90° in the DI software). Thefirst solution corresponds to the noncontact regime inwhich an attractive interaction is responsible for thedamping of the cantilever oscillation. This interactionshifts the cantilever resonance to lower frequenciesproducing a phase shift lower than 90°. The second solutionis associated with the intermittent-contact regime, inwhich the repulsive force produced during the tip-samplecontact displaces the resonance to higher frequencies. TheDI phase convention will be adopted for the remainder ofthe article.

The effect of a tip-sample interaction, which involvesenergy dissipation, is the displacement of the noncontactsolution to higher phase shifts and the intermittent-contact solution to lower phase shift values. The moredissipative features will appear lighter in the noncontactregime, whereas they will appear darker in the intermit-tent-contact regime. An experimental curve is a combina-tion of both solutions. As the cantilever approaches, theattractive force is responsible for the damping of theoscillation, and the cantilever oscillates in the noncontactregime. As the cantilever approaches further, the tipstrikes the surface intermittently and a sudden changein the phase shift is observed as a consequence of thetransition of the cantilever oscillation from noncontact tothe intermittent-contact regime.

Transverse Dynamic Force Microscopy. The dy-namics of an oscillating cylindrical probe can be modeledusing the simple harmonic oscillation theory36 but, in thiscase, the continuum mechanics model for a cylindricalbar has been used.37 The equation describing the trans-

verse oscillation of a cylindrical bar is:

where u(z,t) is the displacement of the bar, E is the Young’smodulus, I is the second moment of inertia, γ is the internalfriction coefficient, and σ is the density of the bar. Theboundary 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 theexternal torque and the external force, respectively. Theoscillation amplitude and resonance frequency of the probechange when it is interacting with the surface. To modelthe experimental data, the interacting force is describedby an elastic and a dissipative component. The first termis responsible for the decrease in the oscillation amplitude,whereas the second term takes into account the shift inresonance frequency. The boundary conditions can betherefore rewritten as:

where ν is a dissipative coefficient and k is an elasticconstant.

The importance of a liquid film between the sampleand the probe for the TDFM contrast mechanism hasalready been reported.10,11 When the probe is close to thesurface (∼10 nm) the liquid film becomes confined anddisplays solidlike behavior.38,39 This gives rise to aviscoelastic shear force,40 which isdependentonthesample

(36) Sarid, D. Scanning Force Microscopy; Oxford University Press;Oxford, 1991.

(37) Drummond-Roby, M. A.; Wetsel, G. C. Measurement of ElasticForce on a Scanned Probe Near a Solid Surface. Appl. Phys. Lett. 1996,69(24), 3689-3691.

(38) Israelachvili, J. Intermolecular and Surface Forces; AcademicPress: London, 1985.

(39) Granick, S. Motions and Relaxations of Confined Liquids.Science1991, 253, 1374-1379.

Figure 1. A graphical representation of the two solutions toeq 1, demonstrating the two possible imaging regimes and thephase lags associated with them. A more energy-dissipativefeature appears light in the noncontact regime and dark in theintermittent contact regime. (DI convention has been used.)

Figure 2. (a) The two possible mechanisms for a decrease inthe oscillation amplitude of a probe as it is brought into contactwith the sample; a damping or a change in resonance frequency.These twomechanismsare indistinguishableusingconventionalfeedback methods. (b) The experimental setup required toperform a real-time frequency spectrum, enabling the twocomponents of the probe-sample interaction to be separated.

∂2

∂z2EI ∂

2

∂z2 (u + γ∂u∂t ) + Aσ ∂

2u∂t2

) 0 (2)

EI(∂3u/∂z3)z)L ) ν(∂u/∂t)z)L + ku(L) (3)

Phase Contrast Langmuir, Vol. 17, No. 2, 2001 351

surface via the chemical bonding between the first liquidlayer and the surface, and on the amount of surface water,which is humidity-dependent. A force with an elastic anda dissipative component can simulate a viscoelasticinteraction and for this reason it has been implementedin the described model.

Experimental Method

Sample Preparation. The membrane will rehydrate readilyif it is exposed to an environment of high relative humidity (RH).41

Careless handling can result in the membrane ion-exchangingfrom the acid (H+) to a salt form (e.g., Na+ or K+). All Nafionsamples therefore were routinely prepared by refluxing withconcentrated nitric acid and deionized water (50/50 v:v), thendeionized water alone, to ensure that the membrane was in theH+ form and free from any chemical impurities. Strips of NafionH+ were then converted to the Cs+ form by immersion in a 0.1M CsNO3 solution for a week.

Atomic Force Microscopy. H+ and Cs+ samples weremounted on magnetic stainless steel sample stubs and placedinside a Digital Instruments Extended Multi Mode AFM usingversion 4.22 of the Nanoscope software (DI-Veeco, S. Barbara,CA). The samples were imaged using tapping-mode phaseimaging and a standard silicon cantilever (∼40 N/m) to providetopographic and corresponding phase images. The samples wereimaged using the same cantilever under identical imagingconditions: relative humidity, free amplitude of oscillation, andratio of setpoint to free amplitude. The relative humidity wascontrolled by placing the AFM inside a purpose-built environ-mental chamber allowing the humidity to be kept constant by

passing nitrogen gas through molecular sieve material.4 Theproperties of the two ion forms of Nafion were investigated furtherby obtaining attenuated total reflection Fourier transforminfrared spectra using a Nicolet 510P spectrometer.

The effect of surface water on tip-sample adhesion wasinvestigated by using a specimen surface prepared to havehydrophobic and hydrophilic domains. The test sample wasprepared by cleaning glass with detergent, rinsing with water,dipping in a mixture of chromic and sulfuric acid, rinsing withwater, dipping in 5 M NaOH, rinsing with water, and then dryingvertically in an oven. When dry, the slide was placed onto a

(40) Hu, H.-W.; Carson, G. A.; Granick, S. Relaxation Time of ConfinedLiquids 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 PerfluorinatedIonomer Membranes from Sans Experiments. J. Phys. 1990, 51(12),1341-1354.

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 correspondingto part a (Z-scale, 25 nm and 10°, respectively). (c) A 1-µm tapping-mode AFM topography image of Nafion 115 Cs+ imaged underambient conditions. (d) A phase image corresponding to part c (Z-scale, 25 nm and 60°, respectively).

Figure 4. Variation of IR absorption with cation form. ATRdata obtained for the two ion forms of Nafion. The spectra werelabeled with aid of reference spectra for Nafion and Teflon50

and normalized to the CF2 peaks. Comparison of the two curvesclearly illustrates the greater water content of the H+ form.


small jar of aminopropyltriethoxysilane, which was allowed toevaporate onto the slide for 5 min. The adhesive properties of thetest sample were then studied by obtaining multiple-forcedistance curves over the two regions.

Force volume imaging allows an image to be built up of thetip-sample interaction at each pixel from an array of forcecurves.42 It has been applied primarily to the study of elastic andadhesive properties of nonhomogeneous substrates.43-46 Althoughthe importance of phase-distance curves has been recognized,47

the next logical progression, phase-volume imaging, had yet tobe taken. At each pixel in the image, the phase contrast can beobtained for any tip-sample separation, hence the word volume.This is a novel AFM technique on which no reports have beenpublished to date. In practice, it works in exactly the same wayas the force-volume imaging except that amplitude is used forthe feedback loop and it is the phase signal rather than thedeflection signal that is monitored.

Phase-volume (PV) images of the different cation forms ofNafion, consisting of 64 × 64 pixels with 64 data points perapproach and retract curve, were obtained. At each pixel thecantilever approached the surface until such time as theoscillation amplitude decreased to the preset trigger before

withdrawing and moving onto the next pixel. All the imageswere obtained with the cantilever initially at resonance to simplifyinterpretation.

A Nafion 115 H+ sample was imaged under ambient conditionsusing phase-volume imaging at several different free amplitudesand ratios of set point to free amplitude. A scan size of 500 × 500nm2 coupled with an array of 64 × 64 pixels was used to ensurethat features comparable with the cluster size could be detected.The images could then be analyzed by taking slices through theimages at specific tip-sample distances. Phase images of Nafion115 Cs+ were then obtained using the same cantilever under thesame conditions with a variety of different free amplitudes andratios of set points to free amplitude to determine the optimumconditions. These were then used to obtain the PV image.

Transverse Dynamic Force Microscopy. All the experi-ments detailed below were performed using an in-house builttransverse dynamic force microscope. The probe was mountedon a piezoelectric actuator which drove it at one of its resonantmodes. The oscillation amplitude was detected using the laserreflection detection system (LRDS)48 that provides the truemeasurement of the vibration amplitude necessary to quantifythe tip-sample interaction. Typical values for the oscillationamplitude are about 10 nm.

An uncoated optical fiber probe was used rather than a metallicprobe49 owing to the similarity of its surface chemistry to thatof a silicon tapping-mode cantilever. The probe was preparedusing the same method as that for SNOM probes.50

To test the importance of the relative humidity in the TDFMcontrast mechanism the microscope was placed inside an in-house built environmental chamber. A Nafion H+ sample wasmounted onto a 1.5-cm-diameter magnetic stainless steel samplestub and placed in the transverse dynamic force microscopy. Therelative humidity could be reduced using nitrogen gas passedthrough molecular sieve material or increased by bubblingnitrogen gas through water and into the chamber. Once thedesired humidity had been reached, the sample was allowed to

(42) Hoh, J. H.; Heinz, W. F.; Hassan, E. A. Force Volume SupportNote No. 240. Digital Instruments: 1997.

(43) Cappella, B.; Dietler, G. Force-distance Curves by Atomic ForceMicroscopy. 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 PolymerSystem Using Atomic Force Microscopy. Surf. Interface Anal. 2000,30(1), 185-189.

(45) Raiteri, R.; Butt, H. J.; Beyer, D.; Jonas, S. HeterogeneousPolymer-Containing Films: A Comparison of Macroscopic Propertieswith 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, Adhesiveand Mechanical Properties of Polyester Films Probed by Scanning ForceMicroscopy. 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 TappingMode Atomic Force Microscopy Data Using Amplitude-Phase-DistanceMeasurements. Ultramicroscopy 1998, 75(3), 171-181.

(48) Antognozzi, M.; Haschke, H.; Miles, M. J. A New Method toMeasure the Oscillation of a Cylindrical Cantilever: “The LaserReflection Detection System.” Rev. Sci. Instrum. 2000; in press.

(49) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. Benign Makingof 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-fieldOptical Probe Profiles. J. Appl. Phys. 1996, 80(9), 4804-4812.

Figure 5. A mixed hydrophilic/hydrophobic test sample consisting of silane evaporated onto cleaned glass. (a) An optical imageof the test sample. Water droplets are clearly visible on the right-hand side of the interface running from middle top to bottomleft. (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 forcedistance curve obtained to the left of the interface in part a, the energy required to break free was ∼7.6f J.


equilibrate. Topographic and corresponding phase images wereobtained at high (∼50%) and low (∼10%) humidities. In additionto the images, amplitude-distance curves were obtained at eachof the humidities. This process was then repeated for the Cs+ ionform of the membrane.

In tapping-mode AFM and TDFM, the feedback mechanismuses the probe oscillation amplitude signal to control the probe-sample separation. Unfortunately, using conventional feedbackmethods, it is impossible to differentiate between a damping ofthe amplitude and a shift in resonance frequency (Figure 2a). Asdescribed previously the liquid confined between the probe andthe surface may be responsible for a decrease in the oscillationamplitude and a resonance frequency shift. These effects can bedetected by measuring the frequency spectrum of the probe whileit is approaching the surface. In this way the two componentsof the interaction are separated and measured as a function oftip-sample distance. Using the transverse dynamic forcemicroscopy it is possible to perform a real-time frequencyspectrum 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 amplitudeduring the approach and retract cycle. The second frequency isswept from just below to just above the second (or first) resonancefrequency of the free probe, by using the sweep mode of the Philips5192 signal generator. This mode of operation will sweep thedriving frequency up and down continuously, and the sweep timecan be as low as 50 ms. The amplitude and the phase spectra aremonitored continuously as the probe approaches the samplesurface (Figure 2b) using a Labview recording system.

Results and DiscussionAtomicForceMicroscopy. Tapping-modetopography

and corresponding phase images of Nafion H+ obtainedunder ambient conditions are shown in Figure 3 a and b,respectively. The images of Nafion Cs+ obtained usingthe same cantilever under identical imaging conditions

are shown in Figures 3c and 3d, respectively. There is amarked difference in the phase contrast between the twoion forms. The phase range is significantly larger in theCs+ ion, 60° as opposed to 10° for the H+ form. There isno significant difference in the topography images,therefore topographic coupling is not responsible for thechange in phase contrast.

Attenuated total reflection (ATR) data obtained for thetwo ion forms of Nafion are shown in Figure 4. The spectrawere labeled with the aid of reference spectra for Nafionand Teflon51 and normalized to the CF2 peaks. Comparisonof the two curves clearly illustrates the greater watercontent of the H+ form. This combined with the greatercharge density associated with the Cs+ ion, which wouldbe screened less effectively, points toward an electrostaticforce being responsible for the difference in phase contrast.

An optical image of the mixed hydrophobic/hydrophilictest sample is shown in Figure 5a. Water droplets areclearly visible on the right-hand side of the interfacebetween the silanized glass and cleaned glass, runningdiagonally from top right to bottom left. The interface isnot particularly sharp because of the way in which thesilane was evaporated onto the glass. Force curvesobtained to the left and right of the interface are shownin Figures 5b and 5c, respectively. The energy requiredfor the cantilever to break free from the surface wasconsiderably higher to the right of the interface where thewater droplets are clearly visible. The energy can becalculated from the area under the curve, if it is assumed

(51) Kuptsov, A. H.; Zhizhin, G. N. Handbook of Fourier TransformRaman and Infrared Spectra of Polymers; Elsvier Science: Amsterdam,1998.

Figure 6. (a) A 500-nm tapping mode AFM topography image of Nafion H+ imaged under ambient conditions obtained in bistableregime. Depressions can be seen in the topographic image; these artifacts correspond to the transitions from the noncontact to theintermittent-contact regime. (b) A phase image corresponding to part a. A much larger phase range can be observed in a bistableregime 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 thephase image (b) over four transitions between regimes, at each of which phase shifts in excess of 90° can be observed.


that Hooke’s law is obeyed and that the cantilever springconstant is ∼0.12 N/m.

The pull-off energy was ∼1.2f J and ∼7.6f J for areasleft and right of the interface, respectively. It is clear fromtheseresults thatanysurfacewaterdramatically increasesthe adhesive force between the tip and sample. Althoughthese features are over an order of magnitude larger thanthe proposed cluster size in Nafion, this contrast mech-anism would still apply when imaging at higher resolu-tions. The elastic response of Nafion increases withfrequency,52 at the frequencies at which the cantilever isbeing driven ∼250 kHz, and so the viscoelastic energyloss would be negligible. Therefore any energy dissipation

in the tapping interaction is primarily the result of tip-sample adhesion rather than a viscoelastic energy loss.Differences in surface adhesion over a sample caused byan inhomogeneous distribution of surface water areprobably a very important phase-contrast mechanism.

Normal tapping-mode topography and phase images ofNafion, obtained in a bistable regime are shown in Figures6a and 6b, respectively. In a bistable regime, the cantilever

(52) Eisenberg, A.; King, M. Ion-Containing Polymers; PhysicalProperties and Structure, Vol. 2; Academic Press: London, 1977; p 164.

(53) Kuhle, A.; Sorensen, A. H.; Zandbergen, J. B.; Bohr, J. ContrastArtifacts in Tapping Tip Atomic Force Microscopy. Appl. Phys. A SolidsSurf. 1998, 66(Pt1 SS), S329-S332.

Figure 7. Phase images corresponding to slices through the phase-volume image of Nafion Cs+ for cantilever positions from 0to 48 nm at intervals of 3.7 nm showing the wide range of possible contrasts (Z-scale, 60°). Note that the cantilever position isarbitrary and it does not indicate the cantilever-sample distance. The zero position is determined as the cantilever position in whichthe amplitude is damped by 90%.


can oscillate in both noncontact and intermittent-contactregimes.53-56

From the mathematical point of view, the state of theoscillation depends on the initial conditions. In thepractice, the cantilever oscillates randomly in noncontactand intermittent contact during the scanning, producinga flipping of the phase shift between negative and positivevalues (DI software). The sudden transitions from non-contact to intermittent-contact regime are accompaniedby artifacts in the form of depressions in the topographyimage. Line profiles across both of the images are shownin Figures 6c and 6d. The line profiles cover four transitionsbetween the noncontact and intermittent-contact regimes.At each of these transitions, corresponding to the inter-mittent-contact regime, a depression of up to ∼5 nm (25%of the Z-scale) and a phase shift of move than 90° isobserved in the topographic and phase image, respectively.This result illustrates the importance of avoiding a bistableregime in phase imaging of a surface.

Fourteensliceshavebeentakenthroughaphase-volumeimage of Nafion Cs+ at intervals of 3.7 nm. The oscillationamplitude of the cantilever when free was in excess of 50nm. Each of the slices shows the phase contrast that wouldhave been obtained in traditional tapping mode phaseimaging at different set points (Figure 7). At cantilever-sample distances slightly larger than the free amplitude(cantilever position ≈ 37 nm), the interaction is too weakto produce any meaningful phase contrast. A significanthigher phase shift (∆æ > 60°) is observed in certain regionsfor a cantilever position from 22 to 33 nm. In these regions,thecantileveroscillates in the intermittent-contact regime.Once below a tip-sample separation of about 15 nm, theimages are predominantly in the intermittent-contactregime and the phase contrast is reduced to about 20°.However, a few points, which show up as white squaresin the dark regions, are still in the noncontact regime,even for a damped amplitude lower than 10%. If slicestaken in the noncontact (cantilever position, 44.3 nm) andintermittent-contact regimes are compared, the samefeatures are clearly visible in both images, there has beena contrast reversal, however. This is consistent with eq1 and implies energy dissipation in the noncontact regime.

To interpret the evolution of the phase contrast withthe damped amplitude, a topography and correspondingphase image and the three types of amplitude-distancecurves, obtained during phase volume imaging of a NafionCs+ sample using a free amplitude in excess of 5V and aset point of 2.5V, are shown in Figure 8. The location fromwhich the phase-distance curves were taken has beenclearly marked in the phase image (Figure 8b). In onecase (Figure 8c), the phase distance curve starts off in thenoncontact regime before swiftly moving into the inter-mittent 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 moreforce to move from the noncontact to intermittent-contactregimes because of damping caused by an attractive force,which is probably electrostatic. Once in the intermittentcontact regime the phase angle is considerably lower, ∼60°,indicating that more energy is being dissipated. Insideregions made up of the second type of curve; a third type

of curve (Figure 8e) can be found where, despite the veryhigh free amplitude and low set point, the cantilever neveractually made contact with the surface because of par-ticularly strong damping. The corresponding points in thetopography image (Figure 8a) appear high, because thesepoints are in the noncontact regime, whereas the remain-der of the image is in the intermittent-contact regime.

The behavior of the first type of curve (Figure 8c) canbe attributed to the hydrophobic backbone of the mem-brane, whereas that of the second (Figure 8d) and third(Figure 8e) type of curve can be attributed to the ion-richregions 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 beexplained by the greater charge density associated withthe Cs+ ion, which is screened less effectively because ofthe lower water content. This produces a strong long-range attractive interaction that shifts the resonancefrequency of the cantilever, therefore reducing its oscil-lation amplitude until the set point is reached. The secondtype of curve (Figure 8d) corresponds to the hydrophilicregion that surrounds the Cs+ ions. These regions showa lower phase shift with respect to the hydrophobic regionsin the intermittent-contact regime, associated to a higherenergy dissipated between the tip and the sample. Theenergy dissipation would be due to preferential wateradsorption to the hydrophilic regions. The unbalancebetween the lower adhesion when the water neck formsand the needed force to break the meniscus should be theresponsible factor of energy dissipation. The contrastreversal observed in the phase volume in the noncontactregime supports this energy dissipation model. Only the

(54) Behrend, O. P.; Odoni, L.; Loubet, J. L.; Burnham, N. A. PhaseImaging: Deep or Superficial? Appl. Phys. Lett. 1999, 75(17), 2551-2553.

(55) SanPaulo, A.; Garcia, R. High-Resolution Imaging of Antibodiesby Tapping-Mode Atomic Force Microscopy: Attractive and RepulsiveTip-Sample Interaction Regimes. Biophys. J. 2000, 78(3), 1599-1605.

(56) Garcia, R.; SanPaulo, A. Amplitude Curves and OperatingRegimes in Dynamic Atomic Force Microscopy. Ultramicroscopy 2000,82(1-4), 79-83.

Figure 8. Phase-volume images (500 nm) of Nafion Cs+ imagedunder ambient conditions. (a) A topography image (Z-scale, 20nm); (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 intermittent-contact regime. (d) A curve that requires more force to moveinto the intermittent-contact regime. (e) A third type thatremains in the noncontact regime can be found within regionsmade up of the second type of curve. Note that the data havebeen plotted with an offset x-axis to not obscure the data.


rupture of a water meniscus between the tip and thesample could produce energy dissipation in the noncontactregime.

Although terminology such as soft and hard tapping23

is useful, what is really important is the tapping regimein which the experiment is performed, that is whether itis noncontact or intermittent contact. It is particularlyimportant not to work in a bistable regime because of theheight artifacts that can be produced. If the phase contrastobserved is greater than 90°, it is certain that this is thecase.

Several articles cite phase ranges of 90° or higher23; itis likely that the images were obtained in a bistable regime.Consequently, any topography data across the bistableregime is at least partly artifactual. Many of the effectsobserved in other articles57-59 including contrast reversalin the topography and phase images may be attributed toworking in a bistable regime or moving from one regimeto another.

To use phase imaging successfully, it is essential toestablish the tip-sample interaction regime by obtaininga phase-distance curve and adjusting the free amplitude

and set point accordingly. During the study it was observedthat different cantilevers required different amounts offorce to move from the noncontact to intermittent-contactregimes. This is consistent with a recent study into theeffects 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 aconsequence of the larger effective contact area forinteraction.

Transverse Dynamic Force Microscopy. A 1-µmtapping-mode AFM topography image, its correspondingphase image and TDFM topography, and phase imagesof Nafion are shown in Figure 9. It is apparent from thetopography images that the resolution of the TDFM onthis sample is somewhat lower than that of the AFM.This is probably the result of two main effects: first, thesize of the end of the particular TDFM probe, and second,thermal drift, because scan times are about 30 min, ratherthan the 4 min for the AFM. The nature of the samplemay also have a bearing on the ultimate resolution,because the cluster-network model of Nafion60 postulatesa large-scale organization of clusters with transientconnective tubes which are in constant flux.

Four amplitude-distance curves obtained using NafionH+ and Cs+ samples across a range of humidities from 10to 52 are shown in Figure 10. The effect of humidity isevident for both the samples, but it is easier to explain inNafion H+ probably because of its higher hydrophilicity.The difference between the points at which the amplitudestarts to be damped and drops to zero indicates thethickness of the water layer over the surface (∼6 nm forthe H+ from at 46%RH). The hysteresis in the retractcurve is probably caused by the presence of a capillary

(57) McLean, S. R.; Sauer, B. B. Nano-deformation of CrystallineDomains 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 UsingPhase Detection for the Resolution of the Nanophases in SegmentedPolyurethanes and Other Block Copolymers. Macromolecules 1997,30(26), 8314-8317.

(59) Sauer, B. B.; McLean, R. S.; Thomas, R. R. Tapping Mode AFMStudies of Nano-phases on Fluorine-containing Polyester Coatings andOctadecyltrichlorosilane Monolayers. Langmuir 1998, 14(11), 3045-3051.

(60) Hsu, W. Y.; Gierke, T. D. Ion-Transport and Clustering in NafionPerfluorinated Membranes. J. Membr. Sci. 1983, 13(3), 307-326.

Figure 9. A comparison of AFM and TDFM images. (a) A 1-µm tapping-mode AFM topography image of a Nafion H+ sample imagedunder ambient conditions (Z-scale, 15 nm). (b) A phase image corresponding to part a (Z-scale, 30°). (c) A 1-µm TDFM topographyimage of Nafion H+ imaged under ambient conditions (Z-scale, 400 nm). The resolution of the TDFM is lower than that of the AFMowing to the size of the probe and thermal drift, which is more of a problem for the TDFM because of the longer scan times, 30min compared with 4 min for the AFM. (d) Corresponding TDFM phase image (Z-scale, 30°).


neck. At lower humidities the approach curves becomesteeper, the neck formation is not clearly detectable, andthe hysteresis in the curves decreases, which indicates adecrease in the thickness of the water layer. Comparisonof 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 beconsistent with the infrared data in Figure 4, which clearlydemonstrated the differences in water content of the ionforms of the membrane. In Nafion Cs+, the noncontinuousnature of the surface liquid film causes differences in theforce curves.

Topography and corresponding phase images of NafionH+ and Cs+ were obtained during the investigation. Thephase range obtained using TDFM was significantly lowerthan that observed with tapping mode AFM because ofthe different contrast mechanism involved. In tapping-mode AFM, the phase contrast was caused by the tip goingin and out of the water layer, whereas the TDFM probeis always inside the water layer moving from side to side.Differences in the hydrophilicity of the sample, resultingin preferential water adsorption to some areas, would stillbe detectable because of the increased drag on the probein these areas. A change in phase contrast with humiditywas observed; the phase contrast is lower at the lowerhumidity, which is consistent with a recent AFM study.4The phase contrast for the Cs+ form was lower than thatof the H+ again indicating that there is less surface waterfor this ion form.

A real-time frequency spectrum surface is shown inFigure 11. The surface is obtained from the frequencyspectra taken at different tip-sample distances. Thevertical gray plane represents the reference frequency

used to measure the approach amplitude curve (whiteline). It is clear that, when the probe approaches thesurface, the resonance frequency (dotted line) does notchange in a monotonic way on a nanometric scale, asusually expected.37 Looking at the approach curve it ispossible to distinguish four regions, which are separatedby black lines. Fitting this surface with the model describedpreviously (eqs 2 and 3), the elastic and dissipative partsof the interaction can be calculated. The results of thisanalysis are shown in Figure 12 in which the four regionsmarked in Figure 11 have been emphasized using blackvertical lines.

In region A, the probe is free, until it reaches a distanceof 16 nm from the surface when the oscillation amplitudeof the probe drops as a consequence of the formation ofa capillary condensation neck. In region B, the dissipativeforce remains almost constant while the elastic componentincreases. This is responsible for the decrease in theapproach-curve amplitude. (A small shift of the frequencypeak produces a significant change in the amplitude atresonance.) In region C, the dissipative componentdominates the elastic component and is responsible forthe large decrease in the oscillation amplitude. In regionD, the slope of the elastic force is steeper than thedissipative force, and the elastic component will eventuallybecome predominant forz smaller than 4 nm. These resultsclearly show that the information provided by the approachcurve alone is insufficient to determine the kind of probe-sample interaction. The real-time frequency spectrum istherefore a unique tool to decouple and quantify the twocomponents giving a better insight into the nature of theinteraction.

Figure 10. The effect of relative humidity on surface water thickness. Four TDFM amplitude-distance curves obtained usingNafion 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-neckformation is no longer detectable and, in the Nafion H+, the hysteresis between the approach and retract curves decreases. Thedepth of the surface water layer was lower for the Cs+ ion form than for the H+ form.


Conclusions

The origin of phase contrast in Nafion has beeninvestigated using the two complementary SPM tech-niques of AFM and TDFM. A variety of standard and newtechniques, namely phase-volume imaging and a real-time frequency spectrum were used.

Force curves obtained over a mixed hydrophobic/hydrophilic test sample showed a much larger adhesiveforce over the water-rich regions. An increase in relativehumidity resulted in an increase in the thickness of the

surface water layer and the phase contrast observed withboth SPM techniques. Therefore differences in probe-sample adhesion, caused by an inhomogeneous distribu-tion of surface water, are an important phase-contrastmechanism.

Phase-volume imaging has been a useful tool in theinterpretation of phase contrast. It has clearly demon-strated the wide range of phase contrasts that can beobserved on the same sample. Moving from the noncontactto intermittent-contact regime resulted in a contrast

Figure 11. A real-time frequency spectrum obtained using the TDFM. The black curves on the surface are frequency spectra takenat different tip-sample distances. The vertical gray plane represents the reference frequency used to measure the approachamplitude curve (white line), and a dotted line represents the resonant frequency. Using the mathematical model highlighted inthe 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 dissipativecomponent (dotted line), and the elastic component (continuous line) of the probe-sample interaction. The slope of the force curvesin the different regions (a, b, c, d) is added to emphasize the predominant component.


inversion. The most dissipative features were light in thenoncontact and dark in intermittent-contact regimes.When working in a bistable regime, height artifacts wereproduced in the topographic images at the points wherethe transition occurred. The anomalous results for thedifferent cation forms of Nafion and those in manypublished studies can be attributed to working in a bistableregime or moving from one regime to another.

The importance of phase-distance curves has beenhighlighted as a prerequisite to imaging to ensure thatimaging takes place in any one regime, rather than relyingon a standard set of operating conditions. The sharpnessof the tip influenced the phase contrast observed, alteringthe force required to move from one regime to another. Itis therefore necessary to obtain a phase-distance curve ifa cantilever is damaged or changed to ensure that imagingcontinues in the same regime.

The phase-volume images of Nafion consisted of twomain types of phase-distance curves. The first curve movedquickly from the noncontact to the intermittent-contactregime, and once there the phase angle of ∼90° indicatedlittle energy dissipation. These regions were attributedto the hydrophobic backbone. The second type of curverequired considerably more force to enter the intermittent-contact regime, and once there the phase angle of ∼60°indicated more energy dissipation. These regions wereattributed to the ion-rich regions that would damp thecantilever oscillation with an attractive electrostatic forceat longer distances, then, once in contact, dissipate moreenergy owing to their greater affinity for water. A greaterforce was required to image the Cs+ ion form in theintermittent-contact regime compared with the H+ formbecause of the lower water content and therefore reducedscreening of the Cs+ ions charge.

When compared with AFM (dynamic mode), it is clearthat TDFM differs in two main aspects: the shape andthe orientation of the probe with respect to the specimen

surface. In TDFM the cantilevers have a cylindricaltapered shape and are mounted perpendicular to thespecimen surface, which allows accurate control of thetip-sample distance, ecause the probe is not extensiblein the vertical direction. This characteristic makes theTDFM a more suitable tool for force spectroscopy: theprobe does not jump to contact during the approach anda constant load rate can be applied by just keeping theapproach or retract speed constant. (Both these aspectsare problematic in AFM.)

Thus far efforts have been concentrated mainly ondetermining the energy loss mechanism in the tip-sampleinteraction and have neglected the effect of resonancefrequency shifts. A real-time frequency spectrum wasobtained to decouple the two effects of change in resonancefrequency and damping of the oscillation. It was alsopossible to determine quantitatively the elastic anddissipative parts of the interaction by accurately modelingthe dynamic of the TDFM probes. Distinct regimes werefound at different probe-sample separations.

Although tapping-mode phase imaging remains a veryuseful tool for identifying and mapping regions of differentproperties regardless of their topographical nature, theinterpretation is not always trivial. There have beenseveral pit falls for the unwary, namely phase inversionand height artifacts.

Acknowledgment. The authors would like to thankAnna Halter for her assistance with the ATR analysisand Andy Humphris for his help with the “real timefrequency spectrum” technique. This work was supportedfinancially by the EPSRC and National Power PLC aspart of their ongoing research into regenerative fuel celltechnology.

LA000332H


shear force versus tapping - tuning fork afm

Documents