Construction and Implementation of a Scanning NanoRaman Spectrometer

1
TIP ENHANCED RAMAN
Motivation
Shadowing Effect with Silicon tips
Enhancement of signal with modified tips
Nanotechnology deals with manipulation of
materials at length scales smaller than 100 nm to
make useful devices and materials targeting
various applications. Properties show a
pronounced size and structure dependent behavior
at nano length scales, making it essential to
perform both physical as well as chemical
analysis. Size of these nanostructures is
determined using techniques like scanning probe
microscopy (SPM), transmission electron
microscopy (TEM), scanning electron microscopy
(SEM), X-Ray analysis etc. However there is no
satisfactory means of attaining information
about chemical structure at these length scales.
Tip-enhanced Raman spectroscopy meets the need
for chemical sensitivity with nanoscale
resolution.
Tip DOWN
TEM image of uncoated tip
TEM image of silver coated AFM tip
Tip Up
Tip Up
Tip DOWN
Objective

• Develop an instrument to give simultaneous
  topographical and chemical information with
  nanometer scale resolution.
• Increase intrinsically weak Raman signal in the
  near vicinity of a metal coated AFM tip
• Overcome the wavelength of light limitation with
  plasmon optics at the tip apex
• Scanning probe microscope tips, which are
  extensively used to obtain topographical
  information at nano-scales, can be used to
  achieve high lateral resolution for the optical
  signal.

Tip enhanced Raman signal from methylene blue dye
Raman signal of methylene blue with uncoated tips
shows a Decrease in intensity due to shadowing
of the laser beam by the tip
170 increase in intensity with coated tip in the
focal spot
Enhancement effect on different samples
PEDOT-PSS films
Cadmium Sulfide (CdS)
Single Walled Carbon Nanotubes
?
Raman mode from the tip
The figure above shows a metal particle attached
to a SPM tip. This combination should result in
high lateral resolution (R of the tip) for the
optical signal because of strongly localized
enhancement of the electric field by surface
plasmons in the metal particles deposited on the
tip.
600 increase in signal intensity with coated tip
150 increase in signal intensity
60 increase in signal intensity
NANO-RAMAN SETUP
Dependence of enhancement on sample thickness
Increased contrast ratio on silicon Suppressing
the far-field
Conclusions
Principle
Laser light is incident on the sample through a
long working distance objective. The incident
electric field is locally enhanced at the point
where the modified SPM tip comes in contact with
the sample. The enhanced Raman scattered signal
is collected through the same objective and
directed to the Raman spectrometer.

• Tip enhanced Raman scattering has been observed
  with silver as well as gold coated tips
• Although increase in signal intensity varies for
  different tips, all the tips give enhancement
• Enhancement has been obtained for various samples
  like dyes, SWCNT, CdS, silicon, PEDOT-PSS films
  etc.
• Polarization selection increases contrast!
• Preliminary estimates indicate localization of
  the enhancement on the length scale 20 nm.

Picture shows the microscope objective and the
AFM tip of the scanning nano-Raman spectrometer
Enhancement decays with the distance from the
tip, thus the signal intensity increases more for
thinner samples. Preliminary measurements 10nm
CdS film shows 180 increase 160nm CdS film shows
20 increase Enhancement is limited to 20 nm
from the tip
Acknowledgements
For scanning purposes, the contrast between the
far-field signal and the nearfar-field signal
should be 5-10 times. For silicon, we suppress
most of the far-field signal with an analyzer
United States Air Force National Science
Foundation (DMR-0215966) OBR Action Fund Ivan
Dolog and Dr Robert Mallik for CdS samples