L-14 Raman Spec

7/29/2019 L-14 Raman Spec

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Nano-photonics

-Raman Spectroscopy


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Outline

- Raman effect and Raman Spectroscopy;

- Near-field Scanning Optical Microscopy (NSOM);

- Near-field Raman Spectroscopy;

- Physical Review Letter Paper.


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Raman Spectroscopy

Raman Effect : 1928 by C.V. Raman (Nobel 1930)

--Inelastic light scattering by optical phonons in materials, or more generally by other elementary

excitations (e.g. magnons, electronic excitations, etc).

Simple model: harmonic crystal

--Collection of normal modes of lattice vibrations,

--Phonons, Quasi-particles,

Conservation Laws( space-time symmetries )

1) Energy: + : anti-stokes

(one phonon process) - : stokes (creation)

Note that stokes usually much stronger!

2) Momentum:

Comments:

~ a few eV, Debye ~0.01 eV

ki

)2

1)(()( knk ii

)(kiLS

}{ Gknqqn LS

Gqq SL ,

LS , D


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-Conclusion1) collect information on and q, q obtain information on

2) sharp peaks on Raman Spectrum one-phonon process (1st-order Born Expansion)

broad backgroundhigher-order, multi-phonon processes

-Notes:1) No translation symmetry (solutions, alloys, glass): RS density of states of particular excitations

2) Quantum Wells, QWs, QDs: k in some directions, (perpendicular to size quantization)

3) Absorbing medium: k is complex.

Spontaneous and stimulated Raman scattering-Spontaneous process, weak, -Stimulated, strong

Modelby Hellwarth et. al 1963

Probability /time for photon to emit into stokes mode: (Einstein)

Stimulated Raman gain:

Briefly explain why stimulated RS is much stronger (exponential )

SL , )(ki

dt

dmmDmP SSL )1(

-16cm10~./ vol

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)Im()Im( SL qqk


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Stimulated RS described thru. nonlinear polarization (Garmire,1963)

Langevin Equation in Statistical

Describing harmonic oscillator under external field.

NL polarization: (stokes)3rd-order

Stokes-Anti stokes coupling:Assumptions: isotropic; slowly-varying-amplitude; non-depleted pump approximation

Phase-matching in the second term

These basic models for Raman effects are employed to explain spectroscopic properties of samples.

Raman Spectroscopy:

-Inelastic collision:light loses ( or gains) energy due to vibration energy-level change in sample

-Chance of photons interaction is small(~1 in million),

usually counts Raman scattered photons instead of measuring intensity on a DC meter.

-Raman-active

Polarizability changes as the molecules move during the vibrations

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Apparatus for Raman Spectroscopy:-A large (~1m) grating spectrometer is to measure the

small color changes.Great care must be taken to exact the signal in the presence of the large

Raman-scattered light

-Detection for low light levels

(1) photon counting commonly involved (2) Dark-counts, (Above: Raman Spectra ofKTP)

-Light source: Laser

(Schematic setup ref. to NCSU optics group)

C45


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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 ofcovalently 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

To visit caltech webpage: mineral.gps.caltech.edu Mineral Spectroscopy Server.


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Near-field Scanning Optical Microscopy

First proposed by Synge, S. H. in 1928 about his basic ideas of

designing sub-wavelength aperture scanned in the near-field ofsample, raster scanned images, piezoelectric position control,

opaque (metal) coatingsto confine optical field,

NSOM( orSNOM, European name)

-The interaction of light with sample close to a metal aperture, which

constrains the lateral extent of light. The aperture is held in place in

a manner similar to those used for other scanning probes.

Advantages of near-field interactions: Improved spatial resolution,

Surface enhancement,

Combined topography,

Low sensitivity (small cross section) incoherent in

conventional Raman SERS( surface enhanced Raman

Scattering)

(Graphs ref. to Hallens group at North Carolina State University)

)Sin(2

n


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NSOM Image Theory (Paesler, )Optical Field detected at distance Z be written in terms of F.T.

of the field at z=0,

Now consider

-To examine how (4.4) and (4.6) behave with regards to delivering

spatial information detected at z=Z (far-field), considering

(4.4))0,(),(

i.e.,/overintegratetoneedonlyfieldfarFor

(4.3))0,(),(

2

2

)(22

)(22

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Continued:

Notes: (4.8) fulfills all our notions about far-field microscope and its inability

to carry information beyond certain spatial frequency

(4.9) integral doesnt vanish for , such that high-frequency

elements still contribute to the signal arriving at z=Z (far field).

(4.9) collapses to (4.8) when w (aperture width) is large, i.e.

In the near-field, evanescent terms must be taken into account, due to the convolution of the tip and

the sample.

)8.4(/for0,

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Ultrasensitive Raman Spectroscopy and SERS

Advantages of Raman Spectroscopy:

1) provide extremely rich information on molecular vibrations, and material structures

2) No special requirements for sample preparations in contrast with IR spectroscopy

3) Can be applied non-invasively under almost any ambient condition.

Disadvantages:

extremely small cross section,

Low sensitivity

SERS: Surface Enhanced Raman ScatteringDiscovered in 1977, Jeanmire et al. & Albrecht et al.

--Strongly increased Raman signals from molecules attached to metal nanostructures

--SERS active substrates: metallic structures with size about 10--100 nm (e.g. colloidal Ag, Au)

General contributions:

1)Electromagnetic field enhancement

2) Chemical first layer effect

moleculecm /10

230

nowrealizabletenhancementotal1014

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1) Electromagnetic field enhancement

Excitations of EM resonances: Surface Plasmon

For isolated metallic Ag particles, enhancement is

(2) Chemical first layer effectElectronic interaction between molecule and metal

Mechanism--Dynamic charge transfer(1) Photon annihilation, and excitation of electron into a hot electron in metallic cluster;

(2) Transfer of hot electron into the LUMO of the molecule (molecular vibrations involved);(3) Transfer of hot electron from LUMO back into metal (with changed electronic state);

(4) Electron returns to initial state, and stokes and/or anti stokes photons created.

Extra Notes:(1) Excitations are not homogeneousExistence of hot spots and hot electrons. (Improvement: Tip-Enhanced SERS)

Normally

(2) Vibrational population pumping by SERS balances stokes and anti-stokes ratio

(3) Developing stable SERS-active substrates: Ag films made by vapor deposition, Ag and Au colloidal particles self

assembled, colloidal metallic particles in hydrosols

(4) Strong fluctuations in single-molecule SERS signals appear due to Brownian motion of metallic particles in and out of

the probing volume

)/exp(/ 1 TknII BasLSERSSERS

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Experimental Set-up for single-molecule SERS. Insert shows an electron micrograph of typical SERS-active colloidal clusters

(Adapted from K. Kneipp, Bioimaging 1998,6, 104-106 )


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Applications:1) Trace analysis down to approximately 100 molecule detection limit

Potential in environmental, biomedical, pharmaceutical researches

(Feature article: J.J. Laserna et al., Analytica Chimica Acta, 283,607-622,1993)

2) Raman detection of single molecules:

(To see last page)

3) DNA sequencing

The nucleotide bases show well-distinguished SERS spectra

(Feature article: K. Kneipp et al., Phys. Rev. E, 57, 6281, 1998)

____________________________________________________________________________Screening of single molecules in extremely small volumes using a combination of scanning near-field

microscopy and SERS promises exciting opportunities for future developments ofmicroinstrumentation fordetection and identification ofsingle-molecules in small volumes on the order ofatto-liters.

--Kneipp et al. Chem. Rev.99, 2957,1999

--If more interested in theoretical details, please refer to Surface Enhanced Raman Scattering, R. K. Chang, ed.,1982

detectionmolecule-singleforallows/10*4llyTheoretica 218 moleculecm


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Near-Field Raman and Various Combinations

(ABOVE and RIGHT: P.G. Gucciadi SILC-Net School, May 2002)

Comparison:(1) Illumination

(2) Resolution

(3) simultaneous topographic image and Raman scattering image

(3) Selection Rules and Rayleigh Tail (Next page)

(4) Surface Enhancement Effect (Next page)

Incident Light

Far Field Microscopy

Sample Surface

Colle

ction

Optics

Screen

Diffraction

Spot


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(Potassium titanyl phosphate (KTP), H.D. Hallen et al. SPIE Proceedings 3467 (1998))

Feature ArticlesP. G. Gucciardi, et al. " Optical Near-Field Raman imaging with sub-diffraction resolution ", Applied Optics

in press (June 2003).

C.L. Jahncke, et al, Proc. of the 7th International Conf. on near-field optics, NFO-7, August 2002, P.142


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More evolvements:

(1) Micro-Raman (Far-field)--1960s, Intensive Laser input, and output collected by a microscope

(2) Nano-Raman--1990s, Similar to SNOM, but output is directed to a spectroscopy

before being analyzed

(3) Tip-Enhanced SERS Raman:--A laser scanning con-focal microscope and an AFM capable of both tip and

sample scanning.

Cresyl blue SERS spectra. Adapted from Stckle et al., Chem. Phys. Lett.2000, 318, 131.


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One Example: Near-field Raman of SWNT(Hartschuh et al., 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


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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)


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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 of11nm. 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.

(b) Near field Raman spectra detected at the marked positions 1 to 4 in (a).

The spectra is offset for clarity.

1) G band at 1596 nm^(-1) is not shifted V.S.G band at 2619 is shifted to 2610 and doublepeaked

2)

Explanation: Variations in Raman spectrum reflect changes in the molecular structure

caused by external stress, catalyst particles, or local defect, etc.

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L-14 Raman Spec