raman spectroscopy for nanomaterials

Raman for nano material

Prof. V. KrishnakumarProfessor and Head

Department of PhysicsPeriyar University

Salem – 636 011, India

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.

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).

Surface Enhanced Raman ScatteringSurface Enhanced Raman Scattering

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.

In 1977, Jeanmaire reported an interesting finding, sliver surfaces give Raman enhancements in the range of 103~108

Au or Ag NPs

Surface enhanced Raman spectroscopy

The enhancement mechanisms are roughly divided into chemical enhancement and electromagnetic enhancement

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 (resonanceenhancement).

Electromagnetic 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

Chemical Theory

Experimental Setup

Surface-enhanced Raman spectroscopy required:

specific metals (e.g. Au, Ag, Cu, Pt, ...)• surfaces with roughness on the nanometerscale• certain molecules provided much higherRaman intensities (mostly molecules withcarbon double-bonds) N, S. Benzene.

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

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

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).

• Single nanospheres are normally deposited on surface and probed

• Single nanospheres yield relatively small SERS signals

Single Nanospheres

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)

Nanoshells• Metal nanoshells can also produce

SERS signal

• Can push the resonance into the body’s optical window

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

Nano- Raman

Optical Microscope + Raman Spectrometer Nano-Raman

Optical Microscope

Atomic Force Microscope

Scanning Tunneling Microscopes (AFM/STM)

• 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.

Tip Enhanced Raman Spectroscopy (TERS)

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

tip has to be very close to the sample

raster-scanning the sample andpoint-wise detection of the sampleresponse2 µm

Objective of the Raman system and AFM head

Image of an AFM tip through the Raman microscope

Confocal microscope

Optical Images and Spectra

a sharp metal tip is held at constant height (~2nm) above the sampleusing a tuning-fork feedback mechanism. F~10 pN

Topography of the sample

2 nm

K. Karrai et al., APL 66, 1842 (1995)

Tip-sample distance control+

⇒ Spatial resolution < 15 nm⇒ Signal amplification

⇒ Tip as nanoscale „light source“

Tip-enhanced Microscopy

Signal Enhancement

Signal Enhancement

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

Raman Spectroscopy for nanomaterials

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

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

Phase Identification and Phase Transitions in Nanoparticles

Pernigraniline Base (PNB)Violet

Emeraldine Base (EB)Blue

NH NH NH NHn

NH NH N Nn

H2N NH N N

n

H AA

N N N Nn

reductionoxidation

reductionoxidationacidbase

Protonated Emeraldine Salt (ES)Green

Leucoemeraldine Base (LEB)Pale Yellow

Polyaniline (PANI) Structures

MacDiarmid and Epstein, Synth. Met. 29, E409 (1989)

400 600 800 1000 1200 1400 1600 1800

Raman shift (cm-1)

1513

1622

1621

1511 16

2015

81

1515

1582

Raman Spectra for Raman Spectra for nano Au-Hnano Au-H22O/PANIO/PANI

Benzoid ring C=C stretch.

Quinoid ring C=C stretch.

C-N stretch.

EB

PNB⇓

PNB

LEB⇓

544

541

542

134412

43

1269

1208

1189

1542

1270

1196

1186

1542

1444

599

1473

1335

598

1445

1339

801

596

802

1240

(a) PANI

(b) Au-H2O/PANI before reaction

(c) Au-H2O/PANI after reaction

Size Determination in Nanomaterials

– the Phonon Confinement Model (PCM)

– the Elastic Sphere Model (ESM)

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

Conclusion

• Nano Raman is a useful tool to analyse materials for photonic and micro-electronic applications.

• Biological samples can also be probed