Volume 58, Number 3, 2004 APPLIED SPECTROSCOPY 2570003-7028 / 04 / 5803-0257$2.00 / 0q 2004 Society for Applied Spectroscopy

submitted papers

Near-Field Infrared Imaging and Spectroscopy of a ThinFilm Polystyrene/Poly(ethyl acrylate) Blend

CHRIS A. MICHAELS,* XIAOHONG GU, D. BRUCE CHASE, andSTEPHAN J. STRANICKNational Institute of Standards and Technology, Gaithersburg, Maryland 20899 (C.A.M., X.G., S.J.S.); and DuPont CentralResearch and Development, Wilmington, Delaware 19880 (D.B.C.)

The application of broadband, near-� eld infrared microscopy to thecharacterization of the mesoscale structure of a thin � lm polymerblend is described. Key features of this instrument, which couplesthe nanoscale spatial resolution of scanning probe microscopy withthe chemical speci� city of vibrational spectroscopy, include broadtunability and bandwidth, parallel spectral detection for high imageacquisition rates, and infrared-transparent aperture probes. Near-� eld spectral transmission images of a thin � lm of polystyrene/poly(ethyl acrylate) acquired in the C–H stretching region are re-ported. An assessment of the relative importance of transmissionimage contrast mechanisms is a signi� cant aim of this work. Anal-ysis of the near-� eld infrared spectra indicates that the image con-trast in the C–H stretching region is largely due to near-� eld cou-pling and/or scattering effects. Identi� cation and differentiation ofthe operative contrast mechanisms on the basis of their relativedependence on wavelength is discussed. Analysis of the contrast at-tributed to absorption is consistent with the chemical morphologyof this sample derived from previous chemical modi� cation/atomicforce microscopy studies.

Index Headings: Near-� eld optical microscopy; Infrared microscopy;Chemical imaging.

INTRODUCTION

The ability to chemically characterize heterogeneoussystems on the nanometer scale continues to grow in im-portance and to impact key applications in the � elds ofmaterials science (phase segregated systems), nanotech-nology (molecular electronics), and catalysis (single sitecatalysts). Detailed information on this length scale isnecessary to test and evaluate advanced materials and iscritical to their continued development. A promisingstrategy for the development of a nondestructive, nano-meter-scale chemical microscopy involves coupling thehigh spatial resolution of a proximal probe microscopewith the chemical speci� city of molecular spectroscopy.Molecular spectroscopies can provide remarkably de-tailed descriptions of elementary chemical processes suchas the dynamics of bond making and bond breaking.However, they are generally restricted to length scales

Received 20 June 2003; accepted 23 October 2003.* Author to whom correspondence should be sent. E-mail: chris.

michaels@nist.gov.

governed by the asymptotic nature of light: spatial con-� nement of the imaging radiation is limited to approxi-mately one half the wavelength. This often limits the ap-plication of these techniques to homogenous sample sys-tems or systems where the regions of interest are suf� -ciently dilute. Proximal probe microscopies such asscanning tunneling microscopy and atomic force micros-copy (AFM) provide researchers with the ability to viewsample surfaces on the atomic scale. Generally, scannedprobe microscopes are not capable of revealing the iden-tity of the atoms and molecules that make up the sampleunder study. Thus, a tool that performs measurements oncomplex, heterogeneous, nanometer-scale systems withthe same exacting level of chemical detail that conven-tional spectroscopic techniques currently provide on mac-roscopic sample systems remains elusive.

In 1928, Synge recognized that the resolution limit ofconventional optics could be circumvented by illuminat-ing the sample through an aperture that was signi� cantlysmaller than the wavelength of light, while keeping thesample-to-aperture separation � xed at a distance that wasalso much smaller than the wavelength of light: a regionreferred to as the optical near-� eld.1 In Synge’s scheme,the resolution obtained is a function of the aperture sizeand its distance from the sample and is no longer gov-erned by diffractive effects. The � rst demonstrations ofnear-� eld scanning optical microscopy (NSOM) in thisexperimental form were carried out by research groupsheaded by Pohl 2 at IBM Zurich and Lewis3 at CornellUniversity. One of the many variations of NSOM in-volves illumination of a sample through a nanometer-scale aperture that is placed at the end of an optical wave-guide (optical � ber) while controlling the sample-to-ap-erture separation using shear-force feedback.4,5 This pre-cise positional control and high spatial con� nement ofthe electromagnetic � elds enables one to couple thechemical speci� city of vibrational spectroscopy with thenanometer-scale resolution of a proximal probe micro-scope.

With the advent of intense laser sources and novelmethods for generating small optical apertures, inroadshave been made in the coupling of near-� eld opticalmethods with spectroscopy. Several research groups have

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developed techniques that combine conventional spec-troscopies with NSOM. Contrast mechanisms that rely onvibrational spectroscopy (infrared absorption6–14 and Ra-man15–22 spectroscopies), dielectric spectroscopy (micro-wave dispersion), 23–27 and nonlinear spectroscopy28–30

have all been demonstrated at length scales below thediffraction limit of light. In this present study, we explorethe application of broadband, near-� eld infrared (IR)spectroscopy and microscopy as a tool for the character-ization of spatial variations in chemical composition in ath in � lm polystyrene/poly(ethyl acrylate) (PS/PEA)blend. Imaging the chemical microstructure of thin, het-erogeneous polymer � lms is a challenging application forIR NSOM because these materials often display complexmorphologies that may result in signi� cant near-� eld op-tical contrast, independent of any chemical effects. Fur-ther complication arises because the correlation betweenmorphology and chemical composition is often unknown,and thus, multiple mechanisms give rise to image con-trast. Unraveling these contrast mechanisms is critical forproper image interpretation. Robust measurements of thewavelength dependence of near-� eld images yield insightinto the sources of contrast and greatly facilitate imageinterpretation. The processes that give rise to near-� eldcontrast in these measurements, including absorption,near-� eld coupling, and sample scattering, are discussedin the context of image interpretation. Additionally, ameans for gauging the relative importance of these var-ious contrast mechanisms, based on their respectivewavelength dependences, is also described. Informationabout the chemical morphology of this sample is derivedfrom near-� eld IR transmission images and spectra andis compared to results from previous chemical modi� -cation/AFM studies.31

EXPERIMENTAL

Illumination mode NSOM, in which an aperture probeis used as a sub-wavelength-diameter light source, wasutilized in this work. This light source was fabricated bytapering and metal-coating an IR-transmitting single-mode optical � ber. Next, IR radiation is brought incidentto the aperture by coupling light from a broadband, tun-able laser into the optical � ber. The transmitted near-� eldradiation is then collected and collimated by the IR com-patible optics of a conventional far-� eld microscope. Af-ter collimation, the scattered radiation (sample beam),along with a reference beam from the IR light source, issteered into an infrared focal plane array based spectrom-eter. Collection of spectral images (parallel collection ofspectra at each (x, y) scan point) with this scheme re-quires only a single sample scan, resulting in both fastdata acquisition times and robust spectroscopic measure-ments. The elements of this infrared microscope havebeen described in considerable detail elsewhere12 but arebrie� y outlined here.

The illumination-mode near-� eld probes are fabricatedfrom single-mode � uoride glass optical � bers. The etch-ing methodology is similar to that used in the fabricationof NSOM tips from glass32,33 and chalcogenide34 � bers.The broadband infrared light source is based on a com-mercial system that includes a Ti : sapphire oscillator and250 kHz regenerative ampli� er that pumps a beta-BaB2O4

optical parametric ampli� er (OPA). Mid-infrared light isgenerated by difference frequency mixing the near-infra-red signal and idler beams from the OPA in a 1-mm-thick AgGaS2 crystal, cut for type-I mixing. This differ-ence frequency generation scheme has a demonstratedtuning range of 2.5 mm to 12 mm.35 Typical average pow-ers at 3.6 mm range from 8 mW to 12 mW with an op-timized spectral full width at half-maximum (FWHM) ofapproximately 150 cm21. The small fraction of lighttransmitted through the sub-wavelength aperture (1 to 53 1025 for typical aperture diameters of 400 nm) is col-lected by a 0.5 NA CaF2 lens located behind the sample.

The samples sit on a metal disk that is supported onthree piezoelectric transducers.36 These piezoelectric ele-ments are used to position the sample relative to the � xednear-� eld probe. The probe is mounted on a fourth pie-zoelectric transducer that is used in a shear-force detec-tion circuit to position the tip a few nanometers from thesurface.37,38 The shear-force signal typically provides atopographic map of the surface, although the adhesiveforces acting between the probe and the PEA in this sam-ple effectively precluded such a measurement. The ad-hesion between the probe and the PEA (TG ; 250 K)resulted in signi� cant perturbations of the sample rasterscan and made stable topographic image acquisition im-possible. However, it is possible to acquire near-� eld datawherein the probe–sample separation is similar to the ap-erture diameter without the use of shear-force feedback.A stable scanner design allows the probe height to be setat some � xed distance above the average sample planefor the duration of the scan. This imaging technique, so-called constant height mode (CHM) imaging,39 can resultin optical contrast related to the sample topography muchlike the more prevalent constant gap mode (CGM) im-aging. The origin of topography-induced contrast and therelative merits of these two imaging approaches havebeen discussed in detail elsewhere.39 In this case, the sam-ple properties de� ned the useful imaging mode, and con-sequently, the spectral image data was collected with anominal constant probe–sample separation of 300 nm.Careful consideration of the optical contrast related to thesample topography is necessary for proper image inter-pretation and will be discussed in detail below.

Following collection, the sample and reference beamsare steered and focused onto the entrance slit of a 0.32m monochromator containing a 75 grooves/mm diffrac-tion grating blazed at 4.5 mm. A 128 3 128 pixel InSbarray detector is positioned at the focal plane of themonochromator to detect the horizontally dispersed ref-erence and sample spectra, slightly offset from each otherin the vertical direction. The method for calculating thenormalized transmission spectra from the sample and ref-erence line data is described in detail elsewhere.12 Thespectra recorded in parallel at each sample scan pointwere acquired in only two seconds, yielding an overallspectral image (50 3 50 spectra) acquisition time of ap-proximately 1.5 hours.

The PS/PEA blend was prepared by mixing solutionsof 4% PS (M w 5 250 000, Aldrich Company) and 4%PEA (Mw 5 119 300, Aldrich Company) in toluene at amass ratio of 30/70 and spin casting the solutions onto aprecleaned glass cover slip at a speed of 2000 rpm for30 s. Fourier transform infrared transmission infrared

APPLIED SPECTROSCOPY 259

FIG. 1. (a) 8 3 8 mm tapping-mode AFM topographic image of a PS/PEA blend � lm displayed with a linear gray scale from black (minimumheight) to white (250 nm). (b) Schematic illustration of the sample structure deduced from chemical modi� cation/AFM measurements in which thespheroidal inclusions are PS rich and the matrix is PEA rich. (c) 8 3 8 mm near-� eld transmission CHM image of PS/PEA blend � lm acquiredindependently of (a) at l 5 3.36 mm with a probe–sample separation of approximately 300 nm. A, B, and C identify spatial points correspondingto the spectra displayed in Fig. 4.

spectra of the pure PEA and PS � lms were collected us-ing a Nicolet 560 Fourier transform infrared spectrometerequipped with a mercury cadmium telluride (MCT) de-tector. The atomic force microscopy (AFM) image wasrecorded with a Dimension 3100 scanning probe micro-scope from Digital Instruments operated under ambientconditions using microfabricated silicon cantileverprobes.†

RESULTS AND DISCUSSION

Figure 1a is an 8 3 8 mm tapping-mode AFM imageof the PS/PEA blend � lm displayed with a linear grayscale in which white is set to the maximum height (250nm) and black is set to the minimum. On the basis of ahydrolytic etching treatment selective for PEA, coupledwith AFM measurements, the raised domains have beenidenti� ed as being PS rich, with the matrix being PEArich.31 Note that the domain sizes range from approxi-mately 200 nm to several micrometers. AFM scratchmeasurements indicate that the � lm thickness ranges fromapproximately 300 nm (dark regions of Fig. 1a) to 550nm (bright regions of Fig. 1a). Figure 1b is a schematic,three-dimensional illustration of the likely morphology ofthis sample wherein spheroid inclusions (PS rich) of var-ious sizes are distributed throughout a matrix phase (PEArich) and the protruding portions of the inclusions can becorrelated with the raised domains in the AFM image.Figure 1c is an 8 3 8 mm un� ltered, near-� eld transmis-sion CHM image of this sample acquired at a nominalwavelength of 3.36 mm (integrated over the spectralbandwidth of the laser), with a probe–sample separationof approximately 300 nm. The transmission contrast in

† Certain commercial equipment, instruments, or materials are identi� edin this paper to specify adequately the experimental procedure. In nocase does such identi� cation imply recommendation or endorsementby the National Institute of Standards and Technology, nor does itimply that the materials or equipment identi� ed are necessarily thebest available for the purpose.

this image (de� ned here as Tmax 2 Tmin /Tmax) is 0.27,where white denotes the region of highest transmissionand black the lowest and the pixel size is 80 nm. Notethat the images shown in Figs. 1a and 1c are not of thesame region of the sample since they were acquired onseparate instruments. The near-� eld image shows severalfeatures of high transmission intensity that are presum-ably correlated with large (diameter . 1 mm) raised do-mains apparent in Fig. 1a, while no features that mightbe correlated with the small domains distributed through-out the AFM image are apparent. This absence is due tosome combination of the optical properties of the smalldomains and the large difference in the spatial resolutionof AFM (,50 nm) and aperture probe IR NSOM (300–500 nm). A topographic image acquired simultaneouslywith Fig. 1c would be useful for con� rming the connec-tion between the domains seen in the AFM image andthe regions of high transmission intensity in the near-� eldimage; unfortunately, due to the PEA tacticity this wasnot possible. Nevertheless, the sample analysis describedin this paper assumes this correlation between the regionsof high near-� eld transmission intensity and the raisedcircular domains seen in the AFM. Evaluations of ;15near-� eld transmission images of various sections of thissample consistently indicate that the regions of low trans-mission are spatially continuous while the high transmis-sion regions are bounded circular features, thus reinforc-ing the validity of the postulated correlation.

The challenge in the analysis of this sample with IRNSOM is to exploit the distinct infrared optical propertiesof PS and PEA to extract information about the spatialdistribution of these components on the sub-micrometerlength scale. Note that this type of chemical informationis dif� cult to extract from atomic force measurements;previous efforts were successful only in conjunction withthe selective (and destructive) chemical modi� cation ofone of the phases (hydrolytic acid etching of PEA).31 Fig-ure 2 shows the FT-IR absorbance spectra of a 2 mm thick

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FIG. 2. Scaled infrared absorbance spectra of PS (solid line) and PEA(dashed line) in the C–H stretching region along with the relative in-tensity spectrum of the broadband IR laser (dark gray) used to acquirenear-� eld spectral images of the PS/PEA blend � lm.

pure PS � lm (solid line) and a pure PEA thin � lm (dashedline) in the CH stretching region. The PEA spectrum hasbeen scaled to show peak absorbance equal to that of PSin order to facilitate visual comparison. There are obviousdifferences in this spectral region that offer the oppor-tunity to distinguish between these two materials, includ-ing distinct bands of PS absorbance (3025 cm21) andPEA absorbance (2980 cm21). The shaded gray region inFig. 2 represents the relative intensity spectrum of thebroadband infrared laser tuned to the center wavelength(l 5 3.36 mm) used to acquire the near-� eld spectralimages described in this paper. (Note that the slight asym-metry in the laser spectrum often results from alignmentof the laser for an optimal combination of power andbandwidth.) The sharp edges of the spectrum at 2835cm21 and 3050 cm21 occur where the dispersed light fallsoff the edges of the array detector. It is worth noting thatthe signal-to-noise (S/N) ratio varies considerably acrossthe spectrum, as the photon � ux onto the edge pixels isnominally an order of magnitude less than the � ux ontothe center pixels. Use of this center wavelength yieldsnear-� eld spectral image frames across the laser spectrumincluding those at 2922 cm21 (PS/PEA absorbance), 2980cm21 (peak PEA absorbance), and 3025 cm21 (peak PSabsorbance). It is a measure of the potential of broadbandIR NSOM that these three image frames alone might beexpected to provide signi� cant information on the spatialdistribution of the PS and PEA components should ab-sorption be an important source of image contrast.

Although the motivation to explore near-� eld micros-copy in the mid-infrared region is the potential for ex-ploiting contrast due to molecular absorption resonancesfor the purposes of chemical mapping, proper image in-terpretation clearly requires consideration of contrastmechanisms that operate in the absence of absorption.(Note that these contrast mechanisms account for a sig-ni� cant fraction of near-� eld microscopy data as, with theexception of � uorescence NSOM, the vast majority ofpublished near-� eld data has been acquired at wave-lengths for which there is no appreciable sample absorp-tion.) Spatial variations in scattering due to the geometryand refractive index of the sample give rise to imagecontrast. Near-� eld images are subject to contrast due to

spatial variations in the coupling of near-� eld radiationinto modes propagating on to the detector. Palanker et al.present a detailed discussion of this effect in the contextof IR NSOM using a phenomenological tunneling modelin which this outcoupling of radiation is shown to dependcritically on the sample topography and refractive in-dex.13 In constant height mode images, the intensity ofnear-� eld radiation incident on regions of high sampletopography is clearly higher than that incident on lowtopography regions, given the rapid fall-off in evanescent� eld intensity with increasing distance from the aperture.Thus sample topography clearly gives rise to spatial var-iations in detected intensity, independent of changes insample composition. However, since the sample refrac-tive index also in� uences this near-� eld coupling effect,spatial variations in composition also result in transmis-sion image contrast. In the context of IR NSOM, Akh-remitchev et al. have presented a particularly interestingset of images showing the in� uence of sample topogra-phy on the measured image contrast.14 Given the multi-ple, and often spatially correlated, sources of contrast, theutility of IR NSOM as a chemical imaging tool for agiven specimen will depend critically on the relativemagnitude of these effects. In the case of absorption con-trast, sample thickness, and the absorption cross-sectionsare the critical parameters, although increases in samplethickness much beyond the extent of the near-� eld will,while increasing the absorbance signal, result in a signif-icant degradation of spatial resolution. Greffet and Car-minati present a detailed discussion of this effect, de-scribing the change in resolution during the transitionfrom the far-� eld to the near-� eld.40 The key to the properinterpretation of IR NSOM images is the identi� cationand differentiation of the mechanisms of image contrast.

The most salient distinction between the scattering/near-� eld coupling effects, which convey little informa-tion regarding chemical composition, and chemically spe-ci� c absorbance contrast, is their respective dependenceon wavelength. The expected wavelength dependence ofabsorbance contrast can be easily extracted from far-� eldspectra (Fig. 2) while the wavelength dependence of thescattering/near-� eld coupling contrast is expected to beweak, particularly over a limited spectral region such asthat illustrated in Fig. 2. The only source of sharp spectralvariations in the scattering/near-� eld coupling contrast isthe presence of dispersive features in the real part of therefractive index related to absorption resonances via theKramers–Kronig relations, although this is a small effectin the range of 2800–3100 cm21, over which nPS is knownto vary by only Dn 5 0.024.41 (This effect may be muchmore important for stronger absorption resonances wherethe variations in n over a vibrational bandwidth can be-come quite large.) In any case, examination of changesin image contrast with wavelength provides a means forassessing the relative contributions of the different con-trast mechanisms and progressing from near-� eld opticalimages to high spatial resolution chemical maps.

Figure 3 is a set of near-� eld transmission images, si-multaneously acquired at 2922 cm21 (a), 2980 cm21 (b),and 3025 cm21 (c). (Note that these images are extractedfrom the same data set that was integrated over frequencyto produce Fig. 1c.) The transmission contrast in theseimages is 0.28, 0.27, and 0.23, respectively (each dis-

APPLIED SPECTROSCOPY 261

FIG. 3. 8 3 8 mm near-� eld transmission spectral frames of the PS/PEA blend � lm simultaneously acquired at (a) 2922 cm21, (b) 2980 cm21, and(c) 3025 cm21. These images are displayed using independent linear gray scales in which white is set to the highest transmission and black is setto the lowest.

FIG. 4. Scaled infrared absorbance spectra (left axis) of PS (solid line)and PEA (dashed line) in the C–H stretching region along with nor-malized near-� eld transmission spectra (right axis) corresponding to thespatial points A, B, and C indicated in Fig. 1c.

played with an independent linear gray scale). The im-mediately obvious conclusion is that, while there aresmall differences in these images, the source of the ma-jority of the image contrast is only weakly dependent onfrequency, despite the large differences in absorption overthis spectral range. Recall that both PS and PEA absorbat 2922 cm21, and that 2980 cm21 is a distinct PEA ab-sorbance peak and 3025 cm21 is a distinct PS absorbancepeak. This suggests that, at least for this sample, absorp-tion is not the major source of near-� eld image contrastin the C–H stretching region.

While inspection of spectral image frames correspond-ing to different frequencies is an effective tool for eval-uation of broadband near-� eld data, it is perhaps moreuseful to look directly at the spectra corresponding to (x,y) scan points. A signi� cant issue in looking at spectraldata is the choice of background spectrum. Note that allof the spectra are normalized in real time to a reference(no sample) spectrum. This has the effect of largely re-moving the Gaussian line shape and amplitude � uctua-tions of the laser (see Ref. 12 for further discussion ofthis normalization procedure). However, this process of-ten results in a baseline response that is suf� ciently struc-tured to obscure the identi� cation of small variations in

transmission. In these cases, it is useful to normalize thespectral response at each (x, y) scan point to some suit-able background spectrum from the image, effectively re-moving the baseline structure and focusing on spatial var-iations in the spectral response with respect to the back-ground spectrum. For this sample, the background spec-trum was calculated by averaging over all (x, y) pointsexcept those of high transmission intensity, making itnominally equivalent to an average spectrum of the ma-trix phase. Note that eliminating the relatively small num-ber of high transmission intensity pixels (largely com-posing the three bright circular regions seen in Fig. 1c)altered the background spectrum very little and analysisusing an average spectrum computed over all the imagepixels yields the same conclusions drawn with the matrixphase background spectrum.

Three representative near-� eld transmission spectra areshown in Fig. 4 along with the far-� eld PS and PEAabsorbance spectra. The transmission magnitudes are cor-rected by normalization to the aforementioned averagebackground spectrum and the spatial points A, B, and Care identi� ed in Fig. 1c. Note that the spatial separationbetween these points is much smaller than the imagingwavelength. These spectra reinforce the conclusionsdrawn from inspection of the spectral image frames inFig. 3. The spectrum from point B (moderate near-� eldtransmission intensity) is very similar to the averagespectrum and shows very little spectral structure. Thepoint C spectrum (low near-� eld transmission intensity)shows lower transmission consistently across the fre-quency range, again showing very little structure. Thepoint A spectrum (high near-� eld transmission intensity)shows a small degree of spectral structure (discussed be-low) but the increased transmission is generally indepen-dent of frequency. This is consistent with analysis of thespectral image frames, which suggests that the principalsource of image contrast is only weakly dependent onfrequency in this spectral range and thus is clearly notabsorption but rather contrast due to scattering and/ornear-� eld coupling effects. The presence of contrast dueto scattering/near-� eld coupling effects is consistent withthe known sample topography and the different refractive

262 Volume 58, Number 3, 2004

FIG. 5. Scaled near-� eld infrared absorbance spectra corresponding topoints A (solid line) and B (dotted line) indicated in Fig. 1c, along witha scaled far-� eld infrared absorbance spectrum of PS (dark gray).

indices of the blend components. (nPEA in this frequencyrange is not known but Dn(nPS 2 nPEA) 5 0.12 at 589nm.42) It is not trivial to predict the dominant source ofnear-� eld image contrast for a given sample, and thusexamination of the spectral variation in the images pro-vides a useful means for distinguishing between the op-erative contrast mechanisms, particularly when they arespatially correlated. This data clearly indicates that, insome cases, resonant absorption will not be the dominantsource of optical contrast in near-� eld IR images, partic-ularly for path lengths (sample thickness) similar to thenear-� eld decay length.

Extraction of information about the spatial variation inchemical composition for this sample is clearly quitechallenging given that the near-� eld transmission contrastis dominated by scattering/near-� eld coupling effects andany absorption contrast is a secondary effect. However,it is possible to remove the nominally frequency-inde-pendent transmission differences from the spectra and fo-cus the analysis on spectral variations. The spectra frompoints A, B, and C can easily be converted to absorbancespectra for comparison to the spectra of the componentmaterials shown in Fig. 2. Note that the different averagetransmissions appear as baseline offsets in absorbancethat can be removed for purposes of comparison by set-ting the absorbance at an off-resonance frequency (2880cm21) to zero. This highlights the differences betweenabsorption on- and off-resonance and largely removes thevariations due to scattering/near-� eld coupling effects.Figure 5 shows the absorbance spectra for points A (solidline) and B (dotted line); because the spectra from pointsB and C are very similar, the point C spectrum is ex-cluded. Also shown in dark gray is a PS absorbance spec-trum, scaled to the intensity expected for a 500-nm-thick� lm. The weak spectral structure seen at point A appearsto match the absorbance spectrum of PS reasonably well,whereas there is no indication of absorption at point C.It is worth noting the general absence of a strong absor-bance signal at the 2980 cm21 peak PEA resonance inthis region (see Fig. 2). Note that the lack of absorbancesignal at point C is not the result of the lower intensityincident at this spot since absorbance is a fractional quan-tity and, in the linear regime, is independent of incidentintensity. A striking feature of these images is that the

spatial point that corresponds to the largest absorption(relative to the 2880 cm21 off-resonance response) is alsothe point of largest integrated transmission. This is pos-sible because the absorption contribution to the overalltransmission contrast is quite small compared to the spa-tial variation in the scattering/near-� eld coupling contri-bution. This surprising result points to the inherent dif-� culty in extracting information about spatial variationsin infrared absorption, and consequently chemical com-position, from single wavelength near-� eld images. Thisability to distinguish between the absorption and scatter-ing/near-� eld coupling contributions to near-� eld con-trast, based on their respective wavelength dependences,is a clear strength of broadband IR NSOM. It is worthnoting that other groups working in this area have alsonoted the desirability of obtaining near-� eld IR imagesat multiple wavelengths in order to facilitate data inter-pretation.43 In fact, most near-� eld IR data has been ac-quired at multiple wavelengths, either through the use ofline-tunable light sources,7,9,11,14 or broadly tunable freeelectron lasers,6,8,10,13,43 and the resultant spectral depen-dence of these images has generally been exploited fordata interpretation. However, all practitioners of IRNSOM would clearly bene� t from further developmentof broadband IR light sources possessing the requisitebrightness for this application.

The absorbance magnitudes at 2922 cm21 and 3025cm21 in the point A spectrum match those expected fora 500-nm-thick PS � lm. Analysis of the sample withAFM scratch measurements suggests a continuum phasethickness of approximately 300 nm. Given an averagecircular domain feature height of 150–200 nm in theAFM images, a rough estimate of the inclusion depthbased on AFM is in good agreement with an estimate ofthe PS layer depth extracted from the near-� eld infraredabsorbance spectrum in Fig. 5. There are several issuesregarding this interpretation of the near-� eld IR data thatshould be mentioned. The near-� eld absorbance spectrareported here are differential in the sense that an averageover the matrix phase pixels de� nes the background spec-trum, and thus, characterization of the inclusion thicknessbased on absorption assumes that there is an insigni� cantamount of PS distributed throughout the matrix phase.Clearly, the S/N ratio for the spectrum in Fig. 5 is rela-tively low and this sample presumably de� nes the limitof instrumental sensitivity in this spectral region. Notealso that the origin of the small feature around 2965 cm21

is uncertain although it occurs in a spectral region of PEAabsorption. Finally, the expectation of similarity betweenthe near-� eld spectra and the far-� eld absorbance spectraignores presumably small band-shifting effects due to theKramers–Kronig variations in n around absorption reso-nances.13 Further examination of the impact of these var-iations in n on near-� eld images recorded at wavelengthswhere there is signi� cant absorption is clearly desirable.Nevertheless, this analysis of the near-� eld data does re-veal absorbance spectra that are consistent with a samplestructure consisting of nominally 500-nm-thick PS-richinclusions distributed throughout a PEA continuum, assuggested by previous chemical modi� cation/AFM mea-surements. Although the technical hurdles are signi� cant,a clear direction for future development of this techniqueis the expansion of the wavelength coverage to encom-

APPLIED SPECTROSCOPY 263

pass stronger oscillators (e.g., carbonyl stretches) whereabsorption is likely to be a more important source ofimage contrast.

CONCLUSION

The application of broadband, near-� eld infrared mi-croscopy to the analysis of a thin � lm PS/PEA blend hasbeen described. The near-� eld transmission contrast inthe C–H stretching region is dominated by scattering/near-� eld coupling effects rather than by absorption. Thecapability of broadband IR NSOM to distinguish betweenthese effects is demonstrated, as are the dif� culties ininterpreting single wavelength, infrared near-� eld imagessolely in terms of absorption. Analysis of the contrast dueto absorption is consistent with a sample structure con-sisting of nominally 500-nm-thick PS-rich inclusions dis-tributed throughout a PEA matrix in agreement with ear-lier chemical modi� cation/AFM studies. Ongoing chal-lenges reside in the development of a more quantitativeunderstanding of the factors dictating the near-� eld spec-tral response of complex, heterogeneous nanometer-scalesystems such as this polymer blend sample.

ACKNOWLEDGMENT

The authors thank J.A. Woolam for providing information regardingthe variation of nPS in the infrared.

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