Nanoscale Analytics

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Nanoscale Analytics

• Optical Spectroscopy
• Nearfield Microscopy

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Outline

• Motivation
• Survey Material information vs. Spatial
  resolution
• The optical resolution problem
• Optical Response of Matter
• Energy levels in matter
• Dielectric function
• Spectroscopy
• Photons, energy scales, optical processes
• Linear (one-photon)
• Absorption, Fluorescence (PL), Raman
• Non-linear (two or more photons)
• SHG, Hyper-Raman, CARS
• General Microscopy
• Resolution
• Challenges in nanoscale microscopy
• Optical Nanoscale Microscopy
• Farfield concepts
• Confocal
• 4p

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Motivation

• Optical structure analysisat the nanoscale

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Chemical analysis techniques
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The Abbe diffraction limit
(E. Abbe, Arch. Mikrosk. Anat. 1873, p413)
Spatial ResolutionWhen do you see two points as
separate? (The central question in all microscopy)
0l
0.5l
1.0l
Dx
l
Rayleigh criterion Dx gt l/2
(Lord Rayleigh, Phil. Mag. 54, 1896, p167)
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Optical Response of Matter

• Classical
• Dielectric Function e(w) n2(w) Index of
  Refraction
• Quantum Mechanical
• Electron-photon interactions ?natom,nradHint
  natom,nrad? ? 0

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Energy Levels of Matter
isolated atoms
molecules
nano crystals
crystals
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One-Photon Absorption
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Dielectric FunctionIndex of Refraction

• Isolated oscillators (Gases)
• Non-metallic solids, liquids
• (high density,bound e- feelneighboring atoms)
• Metals
• free e-dominate bound e-

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Spectroscopy

• Learning about matterbyoptical interactions

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Photons and Material Science

• Photon(single quantum of the light field)
• Energy (wavelength, frequency)
• Direction (wavevector k)
• Polarization (linear, circular)
• Wavetrain(classical light beam, pulse)
• Coherence
• Poisson statistics
• Phase
• Amplitude/Intensity

Any property may be utilized for analytic
applications
Spectroscopy Energy
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Optical Energy Scales
Room temperature
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Optical Processes

• Photon in Photon out
• Scattering
• Fluorescence
• Stimulated emission
• Photon in
• Absorption
• Photo-chemistry
• Ionization
• Photon out
• Spontaneous emission
• Recombination
• Phosphorescence
• Linear 1 photon Nonlinear 2, 3,
  photons

Franck-Condon Principle
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Absorption
IR Absorption (intraband)
Vis Absorption (interband)
Transmission
S1
S1
n2 n1 n0
n2 n1 n0
S0
S0
hnabs
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Fluorescence, Luminescence
Bandstructure(Silicon)
Molecular energy levels (Jablonski diagram)
S2
Internal conversion
conduction band
S1
Intersystem crossing
T1
hnabs1
hnabs2
hnfl
direct
indirect
hnabs
hnphosph
hnPL
hnphonon
hnPL
S0
hnabs
valence band
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Raman scattering
Rayleigh Scattering (elastic)
naS nL
Stokes
anti-Stokes
naS nL nM
nS nL - nM
Nonresonant Raman Scattering (inelastic)
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Complementary Methods
Absorption
Visible
IR
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Complementary Methods
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Spectroscopy Applications

• Examples from bulk spectra

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Molecular Vibrations in Chloroform (CHCl3)
Raman spectrum
3000 cm-1
0 cm-1
IR absorption
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Fingerprinting Materials
Raman spectra in the region 100-1000 cm-1 of(a)
antimonite, (b) cobaltite, (c) chalcopyrite,
(d) enargite, (e) molybdenite, (f) kermesite
and (g) pyrite.
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Semiconductor Bandstructure
Energy
Conduction band
Eg
Heavy holes
D
Valence band
Light holes
Split-off holes
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CdSe Quantum Rods
absorption
PL
4.2 nm
20 nm
40 nm
(Hu et al., Science, 292, 2001)
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Microscopy

• Achieving spatial resolution
• Some general aspects

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Not to ConfuseResolution and Sensitivity

• SensitivityThe capacity to respond to
  stimulation
• Detecting signals fromone or two moleculesis no
  problem!(Even the naked human eye can do it)
• ResolutionSeparating something into its
  constituent parts
• We cannot tellhow many molecules do we see?

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Definitions of Resolution(Imaging Theory)
Linear System
Object
Image
Point Spread Function PSF
Line Spread Function LSF
Edge Spread Function ESF
Modulation Transfer Function MTF
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Resolution in the Eyes of an Engineer

• Vary object spatial frequency
• Fixed PSF
• Linear system gives convolution image
• Nonlinear systems improve resolution

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Resolution Limit

• Vary object spatial frequency
• Fixed PSF
• Linear system gives convolution image
• Evaluate MTF

Resolution Limit
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Noise Lowers Resolution
without
with
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Nanoscale Microscopy

• nm scale only a few atoms, small signals

Must overcome noise and parasitic signals!
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Nanoscale Microscopy

• Small signal extraction
• Selective activation
• Only in area of interest
• Whenever possible Favorable!
• Blocking/discriminatingparasitic signals

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Optical Nanoscale Microscopy

• Methods for optical lateral resolution

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Optical Microscopy

• Dimensionality of methods ? objects of interest
• 1D vertical only interfaces, thin films,
  epilayerse.g. ellipsometry
• 2D purely lateral columnse.g. conventional
  microscopy
• 3D vertical and lateral dotse.g. confocal
  microscopy

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Optical Nanoscale MicroscopyLight in/from Small
Dimensions

• Ideas for confining light
• Farfield exp(ikx)
• Focal point engineering
• Surface reflections
• Evanescent light exp(-kx)
• Total internal reflection
• Surface plasmons
• Nearfield 1/rn
• Through small apertures
• Field enhancing at small objects

How to confine a wave into a sub-wavelength
area-of-interest ???
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Nano-Optical Microscopy Techniques
4pSTED ! lt 20nm
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Confocal Microscope

• Excitation pinhole
• Detection pinhole
• Imaged in the sample at the same focus
• Hence
• Con - focal

(Nikon, www.microscopyu.com)
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Confocal Light Microscope

• Advantages
• Rejection of out-of-focus light high sensitivity
• Depth resolution (true 3-Dim Imaging)
• Spectroscopy possible
• Disadvantages
• Single channel detection
• Image construction by scanning
• Theoretical limits reached

Very mature technology
(Nikon, www.microscopyu.com)
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4p Microscope

• Conventional 2p
• Illumination from one halfspace
• Opaque transparent samples
• 4p
• Illumination from all directions
• Only for transparent samples

Pioneered 1990iesS. W. Hell, MPI Göttingen,
Germany.
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Scanning Near field Optical Microscope SNOM

• History
• Envisioned 1928 1956 Synge, Phil. Mag. 6,
  356, 1928J.A. O'Keefe, JOSA 46, 359 1956

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SNOM

• Realization
• Microwaves 1972Ash, Nicholls, Nature 237
• Visible 1984Pohl, Denk, Lanz, APL 44Lewis,
  Isaacson, Harootunian, Murray, Ultramicroscopy 13

tapered optical fiber
cut-off diameter
Al coating
aperture
near-field
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SNOM Variants
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SNOM Limits of Scaling Down

• Energy balance
• Reflection 2/3
• Absorption 1/3

• Throughput only 10-6 to 10-10(depending on
  aperture)

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Alternative to AperturesLocalized Nearfield
Enhancement

• single colloids clusters extended tips

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Model Case Single Sphere
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Model Case Single Sphere

• Optimal enhancement
• Material/wavelength matters
• Best metals with little absorption
• Diameter irrelevant
• Other shapes 2 ? f gt0

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Clusters Relation with SERS

• Surface enhanced Raman scattering SERS
• First observed 1974Fleischmann, Hendra,
  McQuillan, Chem. Phys. Lett. 26.
• Molecules at rough metal surfaces
• Stochastic hot spots at interparticle gaps
• Enhancement of scattering cross sectionby
  103-106 (even 1014 reported)
• Two main enhancements factors
• Quasi-Electrostatic
• Chemical

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Field Enhancement-Confinement_at_AFM or STM tips

• Aperture squeeze
• Sharp tip enhance
• Lightning rod effect
• infinitesimal radius infinite fields, infinitely
  confined

Smaller radius ? lower fields
Smaller radius ? higher fields
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Apertureles SNOM (aSNOM)

• Excitation from above or below the sample
• Opaque and transparent samples
• Lateral resolution (extent) apex radius
• Staticnearfield overwhelms parasitic radiation
• Dynamictip-sample distance modulated
  demodulated detection

farfield
nearfield
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aSNOM Variants
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Spectro-Microscopy
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Frontiers in Nano-Optics

• Applications in Physics, Material
  Science, Chemistry, Biology, Medical Science,

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Confocal PL partially ordered GaInP2

• A,B,Csub-bandgap emission highly localized
  states(not actually resolved)
• D bandgap excitons (long range order)

5x5 mm area
(S. Smith et al., APL 74, p706, 1999)
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Confocal Raman Diamond films

• Pixel-by-pixel analysis of full spectra for
  material science information
• Raman peaks
• FWHM amorphous material
• Center tensile/compressivestress

(www.witec.de)
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4p Microscope
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Stimulated Emission Depletion (STED)
space
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Saturated DepletionBreaks Diffraction Barrier
S.W. Hell (2003), Nature Biotechnol. 21,
1347. S.W. Hell (2004), Phys. Lett. A 326
140. S.W. Hell, M. Dyba, S. Jakobs (2004), Curr.
Opin. Neurobiol. 14, 599.
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4p-STED Microscopy
Fluorescently tagged microtubuli with an axial
resolution of 50-70 nm
M. Dyba, S. Jakobs, S.W. Hell (2003), Nature
Biotechnol. 21, 1303.
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4p Microscopy of Living Cells

• Microtubulin fibers ofa mammal cell,
  dye-labelled
• 15-times better resolution than confocal
• Monolayer of dye molecules on coverslip imaged
  as 50 nm thick

(www.4pi.de)
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4p Microscopy of Living Cells

• Mitochondrial matrixof live yeast cell (green)
• Cell wall counterstained white

(Egner, A., S. Jakobs and S. W. Hell (2002).
www.4pi.de)
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SNOM Single Molecule Detection
Rhodamine 6G dye molecules
Confocal 1x1 mm
SNOM 1x1 mm
(www.witec.de)
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SNOM Phase Mixtures
2-phase polymer mixture for LED applications
AFM Topography
SNOM Fluorescence
excitation l457nm, detection lgt490nm
(www.witec.de)
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SNOM Human Chromosomes
Human Chromosomes in the meta phase
The chromosomes consists of two chromatides
(12),tied together at the cineotocher
(C). Helical DNA is visible in the dark areas
(H).
(www.witec.de)
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Reminder Apertureless SNOM (aSNOM)

• nearfield interaction
• farfield detection
• opaque and transparent samples
• wavelength-independent resolution apex radius

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aSNOM 3d Focus Characterization
Experimental
5 um
Simulated measurement
Simulation
z
x
5 um
Gaussian focus
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aSNOM Resolution
Residue of latex evaporation mask
Confocal Au islands not resolved
aSNOM easily resolved
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aSNOM Material Specificity
Visible
Infrared
Topography
Optical
(Taubner, Hillenbrand, Keilmann, J Microscopy,
210, 2003)
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aSNOM Nano-Lithography
Influence of incident polarization
First results
(Bachelot et al., JAP, 94, 2003)
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aSNOM Nano-Raman
Confocal
Topography
aSNOM
polarization dependent

• depolarized nearfield
• resolution 23nm

(Hartschuh et al., PRL, 90, 2003)
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aSNOM CARS of DNA clusters
topography
1337 cm-1
1278 cm-1
1337 cm-1without tip
(Ichimura et al., APL, 84, 2004)
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Vertical Excitation gt TipSample
EinEout
kin -kout
dipoles?
optical amplitude
topography
k
E
Au disks on glass
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Horizontal Excitation gt Sample gt Tip
Eout
Ein
kin -kout
dipoles
optical amplitude
topography
k
E
Au disks on glass
quadrupoles
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Application 1d Plasmons
nm
nm
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Focussed Beamsin Angular Representation
Classically
Quantum mechanically
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Loss of Nearfield Information
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Nearfield Restauration
superlensing material
amplitude l10.85mm
phase l11.03mm
amplitude l9.25mm
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aSNOM coupling forbidden light into propagating
modes
40nmAg
25nmAu
(Wurtz et al., Nano Lett., 3, 2003)