Bruker Optics

Application Note AN # 523
Combining Atomic Force Microscopy
with Polarized Raman Microscopy

Introduction
Atomic force microscopy is a powerful tool for the characterisation of surfaces down to the molecular and evenatomic level. It is based on a microscopically small tip which
is fixed to an oscillating cantilever. The tip is moved acrossthe sample to obtain topographical information.
Since the invention of atomic force microscopy (AFM) by
Gerd Binning et al. in 1986 [1], this imaging technique hasdeveloped from the area of fundamental research into a routine tool for the investigation of surface structures. Raman
microspectroscopy is recognized as a versatile tool formolecular structure analysis in many fields including materials science. More recently AFM and Raman microscopy
were combined to allow morphological/topographical aswell as chemical mapping of the same sample area. Raman
microscopy ideally complements AFM imaging since it isadding molecular spectroscopic information.
For example Raman microspectroscopy and AFM can provide better understanding of the interaction between epoxy
polymer films on a metal substrate. The adhesive interaction between the epoxy polymer and the metallic substratedepends on the molecular structures formed within thenanoscale interphase region between polymer and metalsubstrate. Epoxy-based polymers are often applied onto
metallic substrates and cured to obtain bonded joint components or painted surfaces.

Experimental
The integration of an AFM into the framework of a Raman
spectrometer which itself is coupled to an optical microscope provided some interesting challenges. In order tomaintain the excellent noise level of better than 0.1 nm inz-direction of the AFM the rigidity and physical size of theoptical Raman microscope was of crucial importance.
Bruker has developed a compact Raman microscope/AFM
system which is based on the Raman microscope SENTERRA and the platform of the N8 NEOS, a benchtop AFM
system for use with optical microscopes [2]. Optimum
stability was achieved by mounting the Raman microscope
SENTERRA on the N8 NEOS granite base (Figure 1). The
entire system is placed on an actively damped tabletop withnoise isolating enclosure (not shown).

Application example
Applications of the combined AFM/Raman system areranging from the investigation of semiconductors, composites, defect inspection, nanostructures of various materials
including carbon nano tubes and graphene monolayers.
For example AFM/Raman reveals detailed information about
the nanostructure of an epoxy polymer which is deposited
on a metal substrate. Figure 3 shows the topographical
AFM image of a polymer with nanoscale thickness on analuminium substrate.

Figure 1: SENTERRA Raman Microscope with AFM N8 NEOS.

Whilst Raman mapping and optical visualisation is realisedusing the included confocal optical microscope and integrated Raman spectrometer, topographical images areobtained on the identical sample site by the AFM N8 NEOS
without sample translation. Sample analysis is further facilitated by software controlled switching between AFM andRaman mode as well as by fast change of the AFM tip incase of necessity (see Figure 2).
The N8 NEOS AFM/SPM uses fiber-optic interferometry
for the detection of the cantilever deflection. A cantileverexchange is easily done without alignment or manual adjustment.

Figure 3: 40 µm x 40 µm topographical AFM image of the epoxy
nanostructure on an aluminum substrate with height analysis.

Compact designAccurate tip deflection
 No laser alignment
 Fast exchange of the tipFigure 2: Interferometric detection of cantilever deflection.

Besides the topographical information AFM can deliver also
insight to other properties such as adhesion and viscoelasticity. On the molecular level Raman microscopy can add
further valuable information e. g. about the steric orientation
or crystallinity.

Figure 4 shows two typical Raman spectra which wererecorded at different locations of the 40 µm x 40 µm area ofinterest. Maximum intensity of the phenyl ring vibration at
1004 cm -1 was found in the flat regions whereas weaker
relative intensities to the reference Raman line of the nonaromatic vibration at 1114 cm -1 could be observed in theregion of higher thickness.

5a: Intensity of Raman lines@ 1114 cm -1

5b: Ratio of Raman lines:
1004 cm -1/1114 cm -1

Figure 5: 2D Raman images (40 µm x 40 µm) of epoxy nanostructure
on an aluminium substrate with underlaying visual images, figure 5a:
Raman intensity distribution of a non-aromatic vibration (1114 cm -1)of the network chains, figure 5b: Raman intensity ratio of the phenylring breathing vibration (1004 cm -1) of the epoxy versus the Raman
intensity of the non-aromatic vibration.

SummaryFigure 4: Inside circle: Raman lines (1114 cm -1 and 1004 cm -1) usedfor spectral Raman imaging.

In Figure 5a the Raman intensity distribution of this nonaromatic vibration of the polymer network chains is shown.
The Raman intensity corresponds directly to the thickness
distribution (ca. 600 nm to 150 nm, see Figure 3) of thesample found in the AFM image. Figure 5b displays theRaman intensity ratio of the phenyl ring breathing vibration
at 1004 cm -1 versus the Raman intensity of the non-aromatic vibration (1114 cm -1).
Obviously the interaction of the polymer chains with thepolarized Raman laser provides information about the stericorientation. It turns out that the intensity ratio is higher inthose regions of the sample area with low thickness indicating that the aromatic entities in the network chains tend toorient parallel to the sample surface [3].

www.brukeroptics.com

In conclusion Raman microscopy and atomic force microscopy integrated in an optical microscope can provide acomprehensive picture for the study of polymer surfaces
and thin films providing more insight to the principles ofadhesion on a nanometric scale. Topographical informationfrom the AFM such as the thickness of nano layers can bedirectly related to molecular spectroscopic information in
the Raman spectra.

References[1] G. Binning, C. Quate and Ch. Gerber, Phys Rev. Lett. 56 (9), 930(1986)[2] Bruker Nano, company publication “N8 Neos SENTERRA”(2009)[3] C. Wehlack, W. Possart et al., publication in preparation

Acknowledgement
The other authors would like to thank Prof. W. Possart andC. Wehlack, Universtiy of the Saarland, Saarbruecken, Germany for courtesy of samples and figures 3,4 and 5.

Bruker Optics Inc.

Bruker Optik GmbH

Bruker Optik Asia Pacific Ltd.

Billerica, MA · USA
Phone +1 (978) 439-9899
Fax +1 (978) 663-9177
info@brukeroptics.com

Ettlingen · Deutschland
Phone +49 (7243) 504-2000
Fax +49 (7243) 504-2050
info@brukeroptics.deHong Kong
Phone +852 2796-6100
Fax +852 2796-6109
asiapacific@brukeroptics.com.hk

Bruker Optics is continually improving its products and reserves the right to change specifications without notice.
© 2010 Bruker Optics BOPT-100901-1