Correlated Imaging and TERS (Tip Enhanced Raman Scattering) – Combining AFM and Raman

In AFM, a sharp tip is held at close proximity with a sample using a force-based feedback loop. The instrument not only measures the force on which the primary feedback loop is based but also other quantities, including surface potential, electrical current, and specific nanomechanical properties.

By scanning the tip and sample corresponding to each other and gauging these quantities at discrete locations sequentially, it is possible to create three-dimensional images of selected sample properties.

Although AFM provides data highly useful for scientific research, it lacks the chemical specificity provided by vibrational spectroscopies.

In a Raman experiment, the detection of inelastically scattered light is performed by illuminating the sample with monochromatic light. Although majority of the scattered light is elastically scattered devoid of a frequency change called Raleigh scattered, a small portion is scattered at a different frequency due to polarizability change of the illuminated molecule.

This change is called as the Stokes shift. A plot of the calculated intensity of these shifts against the frequency is termed as a Raman spectrum. Raman spectroscopy determines molecular composition by exploring bands at characteristic frequencies, symmetry or orientation of crystals or molecules by observing peaks utilizing polarization selection for the incident and scattered light, and stress or strain measurement in a crystal by exploring the frequency shift of a typical Raman band.

A Raman microscope is a combination of a Raman spectrometer and an optical microscope to leverage the high spatial resolution offered by a confocal optical setup.

Figure 1. Atomic force microscopy and correlated optical spectroscopies can yield information about the sample composition (here a cross section of some food packaging material), shape, and various other properties, such as thermal property maps and nanomechanical maps.

Combining AFM with Raman Spectroscopy

Tip Enhanced Raman Scattering (TERS)

The use of the AFM tip as a light source brings out the full synergistic effect of AFM and optical spectroscopies. It is possible to obtain 30-50 times higher spatial resolution when the end radius of an AFM tip is less than 20 nm.

It is possible to perform tip-assisted optical spectroscopy measurements by utilizing AFM tip as a scatter light source followed by the scanning of the sample, as shown in Figure 2.

Figure 2. General setup for linear and non-linear tip-assisted Scanning Probe Microscopy (SPM) and optical spectroscopies.

After bringing a suitable AFM tip into the optical near-field, a continuous or pulsed light is shined on from the side. The backscattered light from the tip-sample gap is determined in the far-field through the use of a suitable detection scheme, thus enabling the measurement of optical signals such as infrared, Raman, or non-linear second-harmonic data with lateral resolution detected by the size of the AFM tip.

For TERS, it is essential to metalize the tip in order to excite the surface plasmon to provide the typically seen enhancement factors that make Raman spectroscopy at the nanometer scale feasible.

TERS demonstrates better surface sensitivity when compared to far-field Raman spectroscopy due to the strong localization of the electromagnetic field surrounding the tip.

In Raman, electromagnetic field’s polarization along the tip axis affects the selection rules for Raman emission. The significance of controlling the polarization of the incident beam for TERS is illustrated in Figure 3.

Figure 3. TERS spectra of graphene obtained using a gold tip and tuning fork AFM feedback. It is evident that the TERS effect only happens when the incident light was polarized along the tip axis (p-polarized) and not perpendicular to it (s-polarized). (The authors would like to thank Samuel Berweger/University of Colorado for helping acquire the spectrum.)

Tip-enhanced spectroscopies like TERS pave the way for a whole new field of research. Besides improved spatial resolution, the combination of conventional far-field spectroscopies and atomic force microscope also provides higher sensitivity to surface features.

Instrumentation for a Combined AFM-Raman System

The following design factors need to be taken into account for a combined AFM-Raman system in order to maintain the performance of both systems.

• Optical interference – It is necessary to change the AFM beam-bounce system’s wavelength to the near-IR to avoid interference with optical measurements
• Noise – Cooling systems used to cool several lasers employed in spectrometer systems may create noise and generate heat in the proximity of the AFM, thus affecting the AFM measurement.
• Measurement location – To run the system without comprising performance, it may be possible to shuttle the sample between the AFM and the Raman spectrometer as illustrated in Figure 4.

Figure 4. View of the Dimension Icon stage and optics arm of the Raman microscope for co-located AFM-Raman measurements (shown on top). The Icon stage shuttles the sample between the AFM head (left) and the Raman objective (right). The red spot emanating the objective is the Raman laser illuminating the sample during a Raman measurement. Photos of the sample-scanning Innova-IRIS and Catalyst-IRIS systems for correlated and TERS imaging are shown on bottom.

Nevertheless, it is necessary to scan the sample underneath the tip as is performed on Bruker’s Catalyst and Innova AFMs to obtain the benefits of both TERS and AFM’s high spatial resolution. For certain non-TERS co- localized AFM and Raman measurements, it is possible to employ a tipscanning AFM such as Dimension Icon from Bruker.

Co-Localized AFM and Raman Measurements

It is possible to accomplish co-located, correlated AFM and Raman data acquisition in tip-scanning mode, which is useful in a variety of applications although the analysis’ spatial resolution is diffraction-limited for the optical portion of the data.

Atomic force microscopy and Raman are often utilized for exploring the properties of semiconductors and nanomaterials such as carbon nanotubes and graphene. The combination of Raman spectroscopy and quantitative nanomechanical measurements (QNM) helps to better understand these materials.

A semiconductor structure with partially buried silicon is shown in the dataset demonstrated in Figure 5.

Figure 5. Simultaneous AFM-Raman acquisition sequence with (left) AFM sample topography, and (right) Raman map. The Raman image was created by plotting the intensity of the main silicon band at 520 cm^-1. The areas in red, yellow and green depict the areas of exposed silicon. Image size 30 µm.

AFM and Raman images of the G and D-band of a graphene flake made on silicon oxide are illustrated in Figure 6.

Figure 6. AFM topography (left) and Raman images of the G (middle) and D-band (right) of a graphene flake on silicon. Both Raman and AFM data confirm the layered structure with 300pm step height separating layers. The Raman image of the D-band also suggests an increased density of defects along the edge of the single layer (see circle). Image width is 15 µm.

Four channels such as topography, adhesion, modulus, and deformation of QNM data from a scan size of 2.5 µm is shown in Figure 7.

Figure 7. Simultaneously recorded quantitative nanomechanical AFM data of a single and double layer of the graphene flake. The wrinkles marked by arrows visible in the topography (top left) are strongly reflected in the mechanical property channels as being softer (bottom left) with less adhesion (top right) than the surrounding material. The deformation channel (bottom right) points to a strong plastic deformation of the graphene layers as they do not relax during the sub-millisecond contact time with the AFM probe.

TERS Measurements

TERS enables higher spatial resolution of analysis and shows promise in molecular studies. Dataset of Raman spectra taken at various heights of the tip above the sample, the malachite green is shown in Figure 8.

Figure 8. Tip-assisted Raman spectroscopy spectra of malachite green obtained using a gold tip illuminated by 633 nm light at varying distances above the surface. The red spectra was obtained with the tip in feedback whereas the black spectrum was collected with the sample pulled 60nm away from the tip. By comparing the peak intensities with the tip approached to the retracted spectra, one can clearly see the striking enhancement of Raman modes.

The reproducibility of the TERS signal changes at distances well below the diffraction limit is shown in Figure 9.

Figure 9. TERS spectrum of malachite green obtained using the Innova IRIS at two different locations separated by <90 nm indicated by the blue and green dots in the above SPM image. The first spectrum at location 1 is displayed in blue, the second second spectrum at location 2 in green, and the third spectrum back at location 1 is shown in red proving the sub-diffraction lateral resolution capabilities and reproducibility.

The diffraction limit restricts stress measurements on Raman scattering to typically 1 µm lateral resolution. However, TERS can extend these measurements to a few nanometer resolution. A spectral measurement taken by a model device comprising a 70 nm strained silicon layer residing on top of an ultrathin BaOx barrier layer followed by bulk Si is shown in Figure 10.

Figure 10. Tip enhanced Raman spectrum of Si device with 70nm thick strained Si layer on ultrathin BaOx followed by bulk Si. The increased signal from the strained Si layer upon tip approach is readily apparent. (Courtesy of D. Kosemura, Meiji University, Japan.)

Conclusion

Co-localized AFM and Raman instrumentation enables scientists to get data about nanoscale properties and composition by exploring samples utilizing both scanning probe methods and optical spectroscopy. Diffraction-limited AFM-Raman experiments are uncomplicated as is the data interpretation.

TERS presents chemical data on the nanometer scale. Although TERS experiments are uncomplicated to perform, it may be necessary to consider the tip-sample interaction in the optical near-field for data interpretation.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.