Introduction to Raman spectroscopy for nano material characterization

1. Raman for nano material
Prof. V. Krishnakumar
Professor and Head
Department of Physics
Periyar University
Salem – 636 011, India

2. So, if Raman spectroscopy is so powerful
and has been around for 70 years
- why is it not used more often?
•Normal Raman scattering,is extremely inefficient for nano
particles, only 1 in 107 incident photons are Raman
scattered. So Raman scattering efficiency is low for
nanoparticles.
• A limitation of normal Raman Spectroscopy is low
sensitivity.

3. There are two ways of truly isolating the Raman signal coming from nano-particles.
One is by having a nanoparticle to be the only one of its kind in the
laser's path (SERS) while the other involves a breaking of the λ/2 diffraction limit
of optical microscopes (nano-Raman).

4. SSuurrffaaccee EEnnhhaanncceedd RRaammaann SSccaatttteerriinngg
In 1977, Jeanmaire reported an interesting finding, sliver
surfaces give Raman enhancements in the range of 103~108
Surface Enhanced Raman Spectroscopy, or Surface
Enhanced Raman Scattering, often abbreviated SERS, is a
surface sensitive technique that results in the enhancement of
Raman scattering by nanoparticles adsorbed on rough metal
surfaces
In the vicinity of a rough metal surface the Raman cross
section can drastically be enhanced, by a factor of up to 106.
This allows very sensitive measurements of adsorbates on
surfaces.

5. Surface enhanced Raman spectroscopy
Au or Ag NPs
The enhancement mechanisms are roughly divided into chemical enhancement
and electromagnetic enhancement

6. Electromagnetic Theory
Two mechanisms are responsible for
the enhancement.
(1) Enhancement of the local
electromagnetic field at the surface of
a metal. When the wavelength of the
incident electromagnetic field is close
to the plasma wavelength of the metal,
electrons can be excited into an
extended surface electronic state
(surface plasmon resonance). This
leads to exceptionally large local fields.
(2) The formation of charge transfer
complexes between adsorbate and
surface (resonance
enhancement).

7. Chemical Theory
• When molecules are adsorbed
to the surface, their electronic
states can interact with the
states in the metal and produce
new transitions
• True nature of this still not fully
understood

8. Experimental Setup
Surface-enhanced Raman
spectroscopy required:
specific metals (e.g. Au,
Ag, Cu, Pt, ...)
• surfaces with roughness
on the nanometer
scale
• certain molecules
provided much higher
Raman intensities (mostly
molecules with
carbon double-bonds) N,
S. Benzene.

9. SERS Applications
• Can use SERS techniques to
– Identify molecules using the “molecular fingerprint”
provided by the Raman signal
– Perform single molecule detection due to high signal
amplification
• Besides roughened metal surfaces, you may
also use nanoparticles as SERS substrate
– Colloidal nanoparticles
– Microsphere lithography

10. Nanoparticle Advantages
• Using a “resonant” nanoparticle provides
several advantages for SERS
– Large absorption cross section - bright
– Surface can be modified – linking to molecular
probes
– No photobleaching – long term monitoring
– Tuning of resonance possible – optimize for
environment or spectral multiplexing

11. Single Nanoparticles
Recall the extinction coefficient for gold
nanoparticles
• Need to excite the
nanoparticles at the
absorption peak for best
enhancement – 517nm for
30nm gold particles
• Argon laser line at 514.5nm
J. Chem. Phys B 103, 8410 (1999).

12. Single Nanospheres
• Single nanospheres are normally
deposited on surface and probed
• Single nanospheres yield relatively small
SERS signals

13. Aggregated Nanospheres
• When NaCl is added to nanosphere
colloid, the particles aggregate
• Aggregates are found to produce much
larger SERS signals
Micheals, J. Am. Chem. Soc. 121, 9932 (1999)

14. Nanoshells
• Metal nanoshells can also produce
SERS signal
• Can push the resonance into the body’s
optical window

15. Observations
1.The absorption and scattering is greatly enhanced in
metallic nanoparticles.
2. Substrate with nanometer roughness can greatly
enhance the Raman signals, (SERS)
3. Certain molecules provided much higher Raman
intensities (mostly molecules with carbon double-bonds)
N, S. Benzene.
Unfortunately, getting the right conditions for SERS
requires much sample preparation and additional
measurements are often necessary to interpret the SERS
data collected

16. Nano- Raman
• Even under the most favourable operating conditions, the
excitation is reduced by the optical fibre cut-off and only a
faint signal is collected from the small volume that is
excited.
Optical Microscope
Atomic Force Microscope
Scanning Tunneling Microscopes (AFM/STM)
Optical Microscope + Raman Spectrometer Nano-Raman
Tip Enhanced Raman Spectroscopy (TERS)

17. Tip Enhanced Raman spectrometer
laser illuminated metal tip
Theory: (Giant) enhanced electric field confined to tip apex
Mechanism: Lightning rod and antenna effect, plasmon resonances

18. tip has to be very
close to the sample
raster-scanning the sample and
point-wise detection of the sample
Objective of the Raman system and AFM head
2 mm response
Image of an AFM tip through the Raman microscope

19. Confocal
microscope
Optical Images and Spectra
+ Tip-sample distance control
a sharp metal tip is held at constant
height (~2nm) above the sample
using a tuning-fork feedback
mechanism. F~10 pN
K. Karrai et al., APL 66, 1842 (1995)
2 nm
Topography of the sample

20. Tip-enhanced Microscopy
Þ Spatial resolution < 15 nm
Þ Signal amplification
Þ Tip as nanoscale „light source“

21. Signal Enhancement

22. Signal Enhancement
tip-enhanced signal > signal * 2500
Hartschuh et al. Phil. Trans. R. Soc. Lond A, 362 (2004)

23. Raman Spectroscopy for
nanomaterials

24. VIBRATIONAL SPECTRA OF
NANOMATERIALS
The translational symmetry of crystalline materials
is broken at grain boundaries, which results in the
appearance of specific surface and interface
vibrational contributions. Besides, the grains outer
atomic layers often react with neighboring species
(lattice reconstruction, passivation/corrosion
layers, contamination) and experience steep
thermochemical gradients during processing,
which generates new phases, with their own
spectral contributions

25. Phase Identification and Phase Transitions
in Nanoparticles
• Phase transitions can be characterized (transition
temperature, transition pressure, transition order)
through mode variation, much the same way as in bulk
materials

26. Polyaniline (PANI) Structures
Pernigraniline Base (PNB)
Violet
Emeraldine Base (EB)
Blue
NH NH N N
n
NH NH NH NH n
H2
N NH N N
n
A H A
N N N N
n
oxidation reduction
base acid oxidation reduction
Protonated Emeraldine Salt (ES)
Green
Leucoemeraldine Base (LEB)
Pale Yellow
MacDiarmid and Epstein, Synth. Met. 29, E409 (1989)

27. 1189
400 600 800 1000 1200 1400 1600 1800
Raman shift (cm-1)
1511
1513
1620
1581
1621 1622
1515
1582
RRaammaann SSppeeccttrraa ffoorr
nnaannoo AAuu--HH22OO//PPAANNII
Benzoid ring
C=C stretch.
Quinoid ring
C=C stretch.
C-N stretch.
EB
PNB ß
PNB
LEB ß
541 544 542
1339
1344
1196 1186
1542
1243
1240
1269
1208
1542
1270
1444
1473
599
1335
598
1445
801
596
802
(a) PANI
(b) Au-H2O/PANI before reaction
(c) Au-H2O/PANI after reaction

28. Size Determination in Nanomaterials
– the Phonon Confinement Model (PCM)
– the Elastic Sphere Model (ESM)

29. Flow Field Plate - Graphite
Nanocrystalline graphite has graphitic (g)
and disorder (d) peaks. The characteristic
dimension of graphitic domains is given by:
= 17. 5 nm

30. Conclusion
• Nano Raman is a useful tool to analyse
materials for photonic and micro-electronic
applications.
• Biological samples can also be probed