Sub-Diffraction Raman imaging by Near-Field Optical Microscopy

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Sub-Diffraction Raman imaging by Near-Field
Optical Microscopy
P. G. Gucciardi, S. Trusso, C. Vasi Istituto per
i Processi Chimico-Fisici, sez. MESSINA, CNR,
Via La Farina 237, I-98123 MESSINA, Italy
S. Patanè I.N.F.M., Dipartimento di Fisica della
Materia e Tecnologie Fisiche Avanzate,Università
di Messina, Salita Sperone 31, I-98166 Messina,
Italy.
M. Allegrini I.N.F.M., Dipartimento di Fisica,
Università di Pisa, Via Buonarroti 2, I-56127
Pisa, Italy.
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Abstract
Motivations
Difficulties

• High spatial resolution 100 nm.
• Added value simultaneous sample topography and
  elastic scattering images.
• Exploitation of Surface enhancement effects
• New Physics Gradient-Field Raman Effect.

• Low efficiency of the Raman scattering.
• Low throughput of the SNOM Fiber probes.
• Very long acquisition times for Imaging purposes.
• High mechanical and thermal stability are
  required.

• Investigated Samples
• Tetracyanoquinodimethane (TCNQ) crystal showing
  surface defects.
• Localized Cu-TCNQ complexes embedded in a TCNQ
  thin film.

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Far-Field Vs Near-Field Microscopy
Near-Field Microscopy
Far-Field Microscopy

• Both the light source and the antenna are placed
  at several wavelengths from the sample.
• The Lateral resolution is determined by the Abbe
  diffraction limit.

• The sample is illuminated by a nanoscopic light
  source located close to the surface (10 nm).

• The resolution is limited by the source diameter

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Schematic of the Experiment

• The sample is raster scanned by means of a
  piezo-tube under the probe.

• SNOM probes commercial single mode optical
  fibers, tapered and coated by a thin CrAl film.
• The apical aperture is 100 nm

• Non-optical shear-force detection is
  accomplished for probe/sample distance
  stabilization, by means of a quartz tuning-fork.

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Experimental setup

• Excitation Ar laser line 514.5 nm.

• Collection Nikon 50X objective, NA 0.5, WD
  10.6 mm.

• Notch Filter Rejection Ratio 10-6.

• Spectrometer Triax 190, single grating, 1200
  lines/mm, 190 mm focal.

• Detector PMT in photon counting regime, 200-300
  dark cts/sec.

• Shear-Force tuning-fork with etherodyne
  detection.

• Signals Topography, Elastic, Raman.

• Modes Illumination or Collection.

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TCNQ
7,7,8,8 Tetracyanoquino-dimethane (TCNQ).

• High Raman Efficiency.

• The organometallic salt complexes can be
  discriminated based on the Raman shift of the
  vibrational peaks.

• Can be deposited in thin film form or as a
  monocrystal.

C-H Bending 1202 cm-1
C?N Stretch
2225 cm -1
? 2208 cm -1
CC ring Stretch 1620 cm-1
CC wing Stretch 1445 cm -1
? 1380 cm -1
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NanoRaman imaging of defects in TCNQ crystals

• Surface defects are visible in the topography
  map.

Topography

• Both the micro and nano Raman analysis evidence
  a corresponding scattering incerase.

• The nanoRaman map shows sub-diffraction length
  details.

NanoRaman _at_ 1445 cm-1
MicroRaman _at_ 1445 cm-1
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Another sample a CuTCNQ thin film

• A thin TCNQ film (yellow) was deposited on a KBr
  substrate in vacuum conditions.

• The sample was kept into contact with Cu powders
  giving rise to localized spots of Cu-TCNQ (blue)
  organometallic compounds.

• Areas in which the film is scratched out evidence
  the presence of the substrate (white).

MicroRaman Spectra
Optical Microphotograph
TCNQ
Scratch
Cu-TCNQ
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Our Target

• Localization of
• TCNQ.
• CuTCNQ Local spots.
• Scratches evidencing the KBr substrate.

On different length scales
Contrast mechanism
Absorption

• Millimeter ? Microphotograph.
• Micrometer ? MicroRaman.
• Nanometer ? SNOM.

Raman Activity
Both
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Localization of damaged areas by SNOM
Localization by Reflectivity
Topography
Reflectivity

• Scratched areas can be localized through the
  analysis of the surface topography.

• The elastic scattering signal is locally
  enhanced because of the higher reflectivity of
  the KBr substrate.

Localization by Raman Scattering
Topography
Raman _at_ 1445 cm-1

• Two holes appear in the topography.

• A vanishing Raman activity is found therein.

• Lateral resolution 300 nm.

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Localization of CuTCNQ by SNOM
Topography
Elastic Scattering
Topography shows no features ? NO SCRATCHES.
The stronger absorption of the CuTCNQ is evident
in the elastic scattering map.

• Only 100 ms of integration time are required to
  get a Raman spectrum of TCNQ.

• The CuTCNQ shows a Raman activity strongly
  reduced. Integration time 5 s.

The Raman map at 1445 cm-1 (Tint 100 ms per
point) shows the presence of areas of depleted
intensities which can be attributed to CuTCNQ.
Raman Spectra
Raman Map _at_ 1445 cm-1
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Sub-diffraction Raman Imaging
RAMAN Map _at_1445 cm -1
Elastic Scattering
Raman Map

• Integration time 100 ms per point. Total image
  acquisition time 1 hour.
• The zoom was carried out in the TCNQ zone.
• The dark clusters can be attributed to the
  presence of CuTCNQ complexes localized on
  sub-micron length scales.
• The line profile allows to assess a lateral
  resolution better than 200 nm.

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CuTCNQ
TOPOGRAPHY
A different locations shows bump-like features.
Scan width 10 10 ?m2
The bumps turn out to be TCNQ-rich zones.
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Resolution assessment in NanoRaman on Cu-TCNQ
TOPOGRAPHY
ELASTIC SCATTERING
RAMAN 1445 cm -1

• Scan width 2.5 2.5 ?m2

• Simultaneous maps of topography, Elastic and
  Raman scattering show correlated features.

• Raman imaging confirms the spectral information
  on the chemical nature of the bumps.

• A resolution of 240 nm can be assessed.

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Difficulties in Illumination-mode SNOM NanoRaman
experiments
Metal-Phosphorus trichalcogenides NanoRaman
TCNQ NanoRaman Spectrum

• Raman emission of the SNOM fiber probe in the
  100 500 cm-1 region.
• High efficiency materials are a must.

• Limited Spectral Resolution 25 cm-1
• Image acquisition times of 1h
• Variations of the baseline.

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during the last year

• A very sensitive, versatile and performant SNOM
  setup was developed for spectroscopy
  applications.
• NanoRaman imaging has been demonstrated on
  organic materials, within reasonable acquisition
  times.
• Sub-diffraction resolution has been achieved.
• Topography, Elastic and Raman scattering signals
  can be acquired simultaneously.
• Critical points and limits of illumination mode
  Near-Field Raman experiments have been
  identified.
• A class of materials suitable for NanoRaman
  investigations has been identified.

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Whats next ?

• Materials Calchogenides, Nanotubes, Silicon.
• Tecnhiques SERS.
• Instrumentation Apertureless SNOM, Transmission
  mode, Different collection angles.
• Upgrades Better spectral resolution (5 cm-1),
  better spatial (10 nm) resolutions.

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The New Concept

• Different Probes
• Aperture SNOM
• Scattering SNOM
• AFM, STM

• Olympus Microscope
• Localization
• Light Collection

• Micrometer screws
• Coarse positioning

Thanks for the kind attention !

• Clearence
• Transmission Mode