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Compact metal probes: A solution for atomic force microscopy based tip-enhanced Raman spectroscopy
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Image of FIG. 1.
FIG. 1.

(a) Illustration (not at scale) of the TERS configuration with side illumination/collection optics. A long working distance 20× objective is used to focus the Raman laser on the tip-sample interface. The Raman excitation light comes from the 514.5 nm line of an Ar+ laser. The angle of the objective axis is 20° with respect to the sample plane. TERS with horizontal (H) and vertical (V) polarizations was performed. The sample surface shown is an actual AFM image of the test sample acquired with the silver probe shown in the scanning electron micrograph in (b); the probe consists of a 50 μm diameter silver wire flattened and bent which was glued onto a standard AFM silicon chip using a composite epoxy glue. The tip apex was obtained by mechanically cutting the end of the wire. (c) Picture of the Raman-AFM setup with side illumination/collection optics. It consists of a modified AFM 5420 from Agilent Technologies coupled to a Raman system LabRam HR-800 from HORIBA Scientific. Alignment of tip/sample/laser is performed with the help of top and side views using three digital microscope cameras (one coupled to the 20× objective of the Raman system). The original scanner of the AFM is used in the feedback loop to move the cantilever up and down as it follows the topography of the sample that is scanned in the x-y plane using a nPoint stage with 100 μm range.

Image of FIG. 2.
FIG. 2.

(a) AFM image in tapping mode of the 1D array of silicon stripes acquired with a custom-made silver cantilever. Except for a 1st order plane subtraction (flattening), no data manipulation was performed. (b) Height profile along the dashed line in (a). (c) Resonance spectrum of the cantilever used in TERS experiments ensuring compatibility of the probes for dynamic AFM (tapping mode).

Image of FIG. 3.
FIG. 3.

(a) AFM topography of the silicon stripes acquired in tapping mode. (b) Amplitude image presenting basically a differential topography. (c) Phase image (degrees units) whose contrast is discussed in the main text. (d) Sequential TERS (tip down) and far field (tip up) measurements while operating the AFM in contact and tapping modes, respectively.

Image of FIG. 4.
FIG. 4.

Spot size determination using the scanning knife-edge method. (a) AFM image of the CoPc/gold interface obtained by photolithography. The sharp interface is evidenced in the cross section shown in (b). A Raman line scan was performed across the interface; the intensity of the CoPc band at 1545 cm−1 is plotted in (c). FWHM of the derivate of the line scan gives a value of 4.7 μm. The approximate size of the laser spot gives a value of 36.2 μm ±7.8%.

Image of FIG. 5.
FIG. 5.

Wearing off of the metal coating of a conventional silicon tip with a silver layer. (a) The tip before and (b) after the AFM-TERS experiment. In the insets at the bottom are shown the AFM images of the test sample and cross sections for each tip.

Image of FIG. 6.
FIG. 6.

TERS experiments on a flat silicon substrate. Similar enhancement is seen between tapping and contact mode.

Image of FIG. 7.
FIG. 7.

(a) Imaging in contact mode of the sample topography; (b) deflection (or differential) image; (c) friction image. TERS and far field spectra were acquired for (d) horizontal polarization and (e) vertical polarization.


Generic image for table
Table I.

Results obtained from experiments.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Compact metal probes: A solution for atomic force microscopy based tip-enhanced Raman spectroscopy