Introduction to Raman Spectroscopy Ze ev Abrams (who freel

1
Introduction toRaman Spectroscopy

• Zeev Abrams
• (who freely plagiarized from Hezy Amiel Xin
  Heng )
• Tel-Aviv University
• December 2004

2
Outline

• Introduction
• Theory of atomic vibrations and Raman scattering
• Instrumentation for Raman spectroscopy
• Raman Spectroscopy for Carbon Nanotubes
• Examples

3
Chandrasekhara Venkata Raman

• 1888-1970
• Discovered the inelastic scattering phenomenon in
  1928
• Was awarded the Nobel Prize for Physics in 1930

a.k.a. Sir Chandra
4

• Raman spectroscopy probes the vibration modes of
  materials, much like infrared (IR) spectroscopy.
• However, whereas IR bands arise from a change in
  the dipole moment, Raman bands arise from a
  change in the polarizability.
• In many cases, transitions that are allowed in
  Raman are forbidden in IR, so these techniques
  are often complementary.

5
Applications of Raman Spectroscopy

• Raman Spectroscopy is a method of
  determining modes of molecular motions,
  especially
• vibrations. It is predominantly applicable to the
  qualitative and quantitative analyses of
• covalently bonded molecules.
• Extra
• -Identification of phases (mineral inclusions,
  composition of the gas phase inclusions)
• -Anions in the fluid phase (OH-, HS-, etc.)
• -Identification of crystalline polymorphs
  (Sillimanite, Kyanite, andalusite, etc.)
• -Measurement of mid-range order of solids
• -Measurement of stress
• -High-pressure and High-temperature in situ
  studies
• -Phase transition and order-disorder transitions
  in minerals (quartz, graphite)
• -Water content of silicate glasses and minerals
• -Speciation of water in glasses

6
Energy Scheme for Photon Scattering
Virtual State
hn0hnm
hn0
Energy
hn0
hn0
hn0
hn0-hnm
E0hnm
E0
Rayleigh Scattering (elastic)
Stokes Scattering
Anti-Stokes Scattering
IR Absorption
E-hvm
Raman (inelastic)
The Raman effect comprises a very small
fraction, about 1 in 107 of the incident photons.
7
IR Spectrography - Absorption
I0(n)
I(n)
Sample
Laser
detector
Raman Spectrography - Scattering
Sample
n0 ? nM - Raman
n0
n0 - Rayleigh
detector
Laser
8
Raman Spectrum
A Raman spectrum is a plot of the intensity of
Raman scattered radiation as a function of its
frequency difference from the incident radiation
(usually in units of wavenumbers, cm-1). This
difference is called the Raman shift.
Raman Spectrum of CCl4
9
Outline

• Introduction
• Theory of atomic vibrations and Raman scattering
• Instrumentation for Raman spectroscopy
• Raman Spectroscopy for Carbon Nanotubes
• Examples

10
The simplest real vibrating systema diatomic
molecule
Just like Hookes law FkX
displacement
Reduced mass
Where
11
Scattering of radiation from a diatomic molecule
?
Induced dipole moment
For a small amplitude of vibration, the
polarizability a is a linear function of q
?
Rayleigh scattering
Stokes scattering
Anti-Stokes scattering
12
Example 1 the vibration modes of CO2
Raman Active
IR Active
IR Active
13
Example 2 the vibration modes of H2O
All the modes are both Raman IR Active
14
Normal vibrations of CH2Cl2
n stretching d bending
15
Selection rules
y0 and y1 are the wavefunctions of a molecule
before and after a vibrational transition,
respectively.
mi ( i x,y,z ) are the components of the dipole
moment. If one of the integrals Ii ? 0, than the
transition is IR active
aij ( i,j x,y,z ) are the components of the
polarizability tensor. If one of the integrals
Iij ? 0, than the transition is Raman active
16
The simplest Raman active crystal1D chain with
2 atoms in the unit cell
m1
m2
K
u2n1
u2n
Equations of motion
Assume the solutions
Obtain the frequencies
17
The phonon spectrum
IR Raman Active
18
Outline

• Introduction
• Theory of atomic vibrations and Raman scattering
• Instrumentation for Raman spectroscopy
• Raman Spectroscopy for Carbon Nanotubes
• Examples

19
A Typical Raman System
20
Typical geometries for Raman scattring
90o scattering
180o scattering
21
Spectrographs for Raman
Spex 1877 triple monochromator
Spex 1403/4 double monochromator
22
Photo-Detectors
Charge coupled device (CCD)
Photodiode array detector
23
Have a good spectrograph!
24
An example of IR spectroscopy (my own work)
25
Outline

• Introduction
• Theory of atomic vibrations and Raman scattering
• Instrumentation for Raman spectroscopy
• Raman Spectroscopy for Carbon Nanotubes
• Examples

26
A quick reminder of what CNTs look like
27
(No Transcript)
28
The first major study of CNTs using Raman
Spectroscopyin SCIENCE VOL. 275 10 JANUARY
1997 (4 years after CNTs were discovered by
Iijima)
29
From the aforementioned article These were the
original modes thought to have been in an SWNT.
Note the UNIT CELL in the upper right.
30
Symmetry groups and wavenumbers
31
Of course, I made life easier for you all by not
giving you the real article which was 19 pages
long
32
The connection between the tube diameter and the
breathing frequency
And the main symmetry groups used
33
One Example Near-field Raman of SWNT

(Hartschuh et al. High-Resolution
Near-Field Raman Microscopy of Single-Walled
Carbon Nanotubes , P.R.L.,9, 95503,(2003))

• Technique Tip-enhanced SERS (Silver tip is
  raster scanned over sample surface)
• Object single-wall nanotube (SWNT)
• The 1st paper to show
  SWNT was detected optically with lt30 nm
  resolution in Raman Spectroscopy
• Experimental Setup
• (1) Optical microscopy with x, y scan stage
• (2) Laser beam
  is focused on sample surface, polarized
  along tip axis
• (3) Sharp Ag tip (10-15 nm in radius)
  positioned near the focus of laser beam and above
  sample about 1 nm (sensitive! )
• (4) Raman scattering is collected with
  same objective, transmitted by beam-splitter and
  filtered by a long pass filter
• (5) Signals are detected by either a
  combination of a spectrograph and a
  thermoelectrically cooled CCD or by a
• 
  narrow band-pass filter (FWHM10 nm) centered
  at 760 or 700 nm followed by a photon
• 
  counting avalanche photo-diode
• Two different types of SWNTs
• (1) SWNT bundles grown by CVD on
  substrate
• (2) Purified SWNTs with diameter 1.0-1.8 nm
  produced by arc-discharge

34

• Fig 2 Simultaneous near-field Raman image (a) and
  topographic image
• (b) of SWNT grown by CVD. The Raman image is
  acquired by detecting
• Intensity of the G band upon laser excitation at
  633 nm. Cross sections
• are taken along the indicated dashed line in (c)
  Raman image and
• (d) topographic image. The height of individual
  tubes is about 1.4 nm.
• Vertical units are photon counts / second
• -Resolution of SERS-Raman is better
• than topographic image
• -No Raman scattering signal is detected
• from humidity related circular features
• present in the topographic image.
• -Vertically and horizontally oriented SWNTs are
• observed in Raman image with similar signal
• intensities even if laser is z-polarized (right
  above).
• (different from far field Raman, right)

35

• Fig (3) (a) dependence of Raman scattering
  strength of G band on the longitudinal
• separation ( ) between a single SWNT and
  the tip. The solid line is an exponential
• fit with a decay length of 11nm. The signal is
  normalized with the far-field signal.
• (b) Scanning electron microscopy of a sharp Ag
  tip fabricated by focused ion beam
• milling.
• -Fig. (3) demonstrates enhanced field confinement
• in longitudinal direction.
• --Tip is positioned above one SWNT and Raman
  scattering strength is recorded as function
• of . The curve is fitted with
  exponential and normalized with Raman strength
  without
• Ag tip. 11nm fit is consistent with 10-15nm tip
  radius.
• --High experimental enhancement is 1000, compared
  with theoretical )
• 
  
  
• 
  
  
• 
  
  Fig (4) (a) Three-dimensional topographic image
  of a SWNT grown by Arc-discharge. The
• 
  
  3 bumps with height 5 nm are presumably enclosed
  Ni/Y catalyst particles and indicate the
• 
  
  initial point of growth.

36

• Fin