Nearfield spectroscopy
Katrin F. Domke
Emmy Noether Group
Max Planck Institute for Polymer Research, Mainz
[email protected]
IMPRS Berlin April 2017
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Nearfield spectroscopy
Content
Fundamentals
I
I
I
I
Sub-wavelength optics, intro
Plasmons
The evanescent field
Light/plasmon coupling
Application
I
Tip-enhanced Raman spectroscopy
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Nearfield spectroscopy
Literature
I
C. Girard et al. The physics of the nearfield Rep. Prog. Phys. 63
(2000) 893
I
Nearfield optics and surface plasmon polaritons (S. Kawata, Ed.)
Springer 2001
I
Principles of nano-optics (L. Novotny and B. Hecht, Eds)
Cambridge University Press 2006
I
J.M. Pitarke et al. Theory of surface plasmons and surface-plasmon
polaritons Rep. Prog. Phys. 70 (2007) 1
I
Principles of surface-enhanced Raman spectroscopy and related
plasmonic effects (E.C. Le Ru and P. Etchegoin, Eds) Elsevier 2008
I
Plasmonics. From basics to advanced topics. (S. Enoch and N.
Bonod, Eds) Springer 2012
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Nearfield spectroscopy: fundamentals
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How small can we see?
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Optical spectroscopy
Spatial resolution is diffraction limited
resolved
Rayleigh
criterion
Abbe’s diffraction limit: d =
unresolved
λ
2nsinα
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Optical spectroscopy
Spatial resolution is diffraction limited
resolved
Rayleigh
criterion
Abbe’s diffraction limit: d =
unresolved
λ
2nsinα
How can we push spatial optical resolution?
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Optical spectroscopy beyond the diffraction limit
Strategies to improve optical resolution
I
decrease wavelength of light source
I
increase refractive index (e.g. oil immersion)
I
use nonlinear optical effects (e.g. frequency doubling)
I
increase detectable spatial frequency
−→ nearfield optics
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Nearfield vs farfield
Nearfield
I
I
emerging from sub-wavelength source (λ<< r)
decays exponentially (r−3 or r −4 ), non-propagating
Farfield
I
I
propagating far from source (λ>> r)
decays with r−1
Dipole emission
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Scanning nearfield optical microscopy (SNOM)
With sub-wavelength metal probes
d << λ
metal aperture
aperture-less
(bulk metal)
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Creating a near-field
What happens when we illuminate a small metal aperture or tip?
+
+
?
-
+
+
+
+
-
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Exciting plasmons!
Collective oscillations of conduction-electron gas
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What is a plasmon?
Collective oscillations of conduction-electron gas
I
a density wave in an electron gas (analogous to a sound wave
in a molecule gas)
I
exists mainly in metals which have weakly bound electrons
(free-electron gas model, jellium model)
I
is a collective motion of billions of electrons in sync
I
can be described, like any other wave, in terms of a specific
energy Ep and momentum p, or circular frequency ωp = Ep /~
and wavevector k = p/~.
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The plasmon resonance frequency
Electrons in an external electric field experience acceleration,
friction and restoring forces. The plasmon resonance frequency
depends on
I
element / composition (charge carrier density)
I
particle size and shape
I
dielectric environment
A crude Coulomb - harmonic oscillator approximation:
1
1 e2
= me ωp2 r 2
4π0 r
2
s
ne 2
where ωp ≈
plasmon frequency
me 0
U(r ) =
with U(r ) potential energy, r displacement, 0 vacuum permittivity, e electron
charge, me electron effective mass, n charge carrier density
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Plasmons applied: Why are metals shiny?
Electrons screen light (external electromagnetic field), i.e. electron
oscillations follow external field, up to a certain frequency when
the external field oscillates too fast for the electrons to follow.
Plasmon energy Ep = ωp ~ and momentum p = k~
What are typical energies for (volume) plasmons?
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Plasmons applied: Why are metals shiny?
Electrons screen light (external electromagnetic field), i.e. electron
oscillations follow external field, up to a certain frequency when
the external field oscillates too fast for the electrons to follow.
Plasmon energy Ep = ωp ~ and momentum p = k~
What are typical energies for (volume) plasmons?
Al
Ep
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Measuring plasmons energies with EELS
Electron energy loss spectroscopy (electron-analogue of IR spectroscopy)
Electrons (2 keV) are reflected from an Al surface and loose energy
to plasmons.
1
2
3
4 5
What are 1, 2, 3, ...?
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From bulk to surface to particle plasmons
Moving towards lower plasmon energies
+ + +
- - -
+
-
+
-
+ + +
- -
-
d << λ
ωbp =
q
ne 2
me 0
√
ωsp ≈ ωbp / 2
√
ωmp ≈ ωbp / 3
with ω plasmon frequency, n charge carrier density, me effective
mass, 0 vacuum permittivity
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From bulk to surface to particle plasmons
Moving towards lower plasmon energies
+ + +
- - -
+
-
+
-
+ + +
- -
-
d << λ
ωbp =
q
ne 2
me 0
√
ωsp ≈ ωbp / 2
√
ωmp ≈ ωbp / 3
with ω plasmon frequency, n charge carrier density, me effective
mass, 0 vacuum permittivity
Remember: oscillating charges emit a field
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The evanescent field at surfaces
Confined to the nm-regime
Solution for Maxwell’s equations at metal/dielectric interface
Propagation length: 10s of µm ([email protected] nm: 22 µm)
Skin depth: 10s of nm (Au@600 nm: 30 nm (metal), 280 nm (air))
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The evanescent field at surfaces
Confined to the nm-regime
Solution for Maxwell’s equations at metal/dielectric interface
Propagation length: 10s of µm ([email protected] nm: 22 µm)
Skin depth: 10s of nm (Au@600 nm: 30 nm (metal), 280 nm (air))
Sub-wavelength confined field (in z) can be used for nano-optics!
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Coupling of (farfield) light and plasmons
Not so easy...
I
waves can couple: photon + plasmon = plasmon polariton
I
but bulk plasmons are longitudinal oscillations (parallel to
propagation direction) - they don’t match transverse photons
(perpendicular to propagation)
I
surface plasmons are transverse oscillations - but are
mismatched in momentum at a flat metal/air interface
light: kx =
~ω √
d
c
surface plasmon: ksp
~ω
=
c
r
m d
m + d
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Photon + plasmon = plasmon polariton
The dispersion relation: ω vs k -plots
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Photon + plasmon = plasmon polariton
The dispersion relation: ω vs k -plots
How can we excite plasmons with light?
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Surface plasmon polariton excitation
Nanostructures, Otto or Kretschmann configuration
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provide missing momentum with nanostructure or grating
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slow down light in high-refractive index material
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Light incident at a metal interface
is partially reflected and partially transmitted
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The evanescent wave
is generated at the critical angle of total internal reflection
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Plasmon-based nano-optics
extraordinary transmission
Wood's anomaly
fancy world-record stuff
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Nearfield spectroscopy
Application
Tip-enhanced Raman spectroscopy
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AFM/STM sample topography
60nm
60nm
Nanoscale structure! Nanoscale chemistry?
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Wouldn’t it be nice if ...
O
guanine
N
HN
H2N
N
N
H
H
N
60nm
60nm
cytosine
O
N
NH2
Nanoscale structure and nanoscale chemistry!
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The ideal tool
Identification
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chemical specificity
I
label-free
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quantitative response
I
non-invasive
Localization
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high sensitivity
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interfacial specificity
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Raman vibrational spectroscopy
with light
excite atomic motions
Ein
Eex
Evib = Ein - Eex
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Raman vibrational spectroscopy
with light
excite atomic motions
Ein
Eex
Evib = Ein - Eex
C=O
C-O Si-O
O-H
Me-O
C-H
Evib
4000
C≡C
C=C C-C
3000
2000
1000
wavenumbers / cm-1
stretching
detect vibrational fingerprint
0
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Raman vibrational spectroscopy
Low scattering cross sections
Raman
scattering
106 photons
1 photon
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Raman vibrational spectroscopy
Low scattering cross sections
Raman
scattering
106 photons
1 photon
→ boost Raman signal
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Surface-enhanced Raman scattering (SERS)
Strong nearfield through plasmon excitation
Fleischmann et al. Chem. Phys. Lett. 26 (1974) 163
Jeanmaire and Van Duyne J. Electroanal. Chem. 84 (1977) 1
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Surface-enhanced Raman scattering (SERS)
Strong nearfield through plasmon excitation
→ limited to rough surfaces of Au, Ag, Cu
Fleischmann et al. Chem. Phys. Lett. 26 (1974) 163
Jeanmaire and Van Duyne J. Electroanal. Chem. 84 (1977) 1
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Shalaev’s hot spots
Localized surface plasmons
G(x,y): field enhancement
2 x 104
1 x 104
100
50
50
x / nm
dimer
y / nm
100
sphere above surface
Shalaev et al. Phys. Rev. B 57 (1998) 13265
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Shalaev’s hot spots
Localized surface plasmons
G(x,y): field enhancement
2 x 104
1 x 104
100
50
50
x / nm
dimer
y / nm
100
sphere above surface
Shalaev et al. Phys. Rev. B 57 (1998) 13265
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Shalaev’s hot spots
Localized surface plasmons
G(x,y): field enhancement
2 x 104
1 x 104
100
50
50
x / nm
dimer
y / nm
100
sphere above surface
Shalaev et al. Phys. Rev. B 57 (1998) 13265
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Tip-enhanced Raman scattering (TERS)
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Wessel’s prediction
1985
• nanometer optical resolution
• single-molecule sensitivity
• but complex setup
J. Wessel JOSA B 2 (1985) 1538
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... 15 years later
First TERS spectrum
Zenobi and coworkers Chem. Phys. Lett. 318 (2000) 131
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Molecular interactions
Co-porphyrin at Au(111) in air
B
A
50 x 50 nm2
B
A
KFD et al. ChemPhysChem 10 (2009) 1794
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Molecular orientation and interactions
DNA base pair formation at Au(111) in air
intensity (arbitr. units)
T
T
A
A
z
AT
Raman shift (cm-1)
!
Dai Zhang, KFD, Bruno Pettinger ChemPhysChem 11 (2010) 1662;
KFD et al. JACS 129 (2007) 6708
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Single-molecule TERS
Dye at Au(111) in air
Au(111)
KFD et al. JACS 128 (2006) 14721; JPCC 111 (2007) 8611
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Sub single-molecule TERS (UHV)
Unprecedented chemical spatial resolution
UHV-STM
UHV-TERS
Zhang et al. Nature 498 (2013) 82
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Sub single-molecule TERS (UHV)
Unprecedented chemical spatial resolution
UHV-STM
UHV-TERS
→ Raman mapping with sub-nm resolution
Zhang et al. Nature 498 (2013) 82
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Electrochemical TERS (EC-TERS)
TERS
A
B
scanning
probe
B
CV
A
Challenges: 1) TERS in liquid and 2) under potential control.
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TERS at solid/liquid interfaces in transmission
Zenobi and coworkers J. Raman Spectrosc. 40 (2009) 1392;
Van Duyne and coworkers Nano Letters 15 (2015) 7956.
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TERS at solid/liquid interfaces in transmission
Zenobi and coworkers J. Raman Spectrosc. 40 (2009) 1392;
Van Duyne and coworkers Nano Letters 15 (2015) 7956.
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TERS at solid/liquid interfaces in transmission
× opaque substrates
Zenobi and coworkers J. Raman Spectrosc. 40 (2009) 1392;
Van Duyne and coworkers Nano Letters 15 (2015) 7956.
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TERS at solid/liquid interfaces in side illumination
Bin Ren and coworkers J. Am. Chem. Soc. 137 (2015) 11928.
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TERS at solid/liquid interfaces in side illumination
× limited tip scanning
Bin Ren and coworkers J. Am. Chem. Soc. 137 (2015) 11928.
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Side-illumination EC-TERS @ MPIP
Opaque samples, potential control
hite
ht
b
STM
a
55°
Objective
b
ve
Coated
tip
STM Coated
tip
Ob
jec
ti
d
Sample
White
light
Objec
Sample
c
d
d
Martı́n Sabanés, Driessen, KFD Anal. Chem. 88 (2016) 7108
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Resonant TERS of dye/Au(111) in water
Raman enhancement of 105
air
1615 cm-1
1364 cm-1
1175 cm-1
Intensity (arb. unit)
801 cm-1
a
b
water
c
d
e
400
600
800 1000 1200 1400 1600
tip check
Raman shift (cm-1)
Martı́n Sabanés, Driessen, KFD Anal. Chem. 88 (2016) 7108
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Nonresonant TERS of PhS/Au(111) in water
Intensity (arb. unit)
Monolayer sensitivity
a air
b
x20
400
600
800
1000 1200 1400 1600
c water
d
Raman shift (cm-1)
Martı́n Sabanés, Driessen, KFD Anal. Chem. 88 (2016) 7108
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Nonresonant TERS of adenine/Au(111) in 0.01 M H2 SO4
Intensity (arbitr. units)
1.8 mW, 5 s
approached
retracted
0
500
1000
1500
Raman shift (1/cm)
2000
2500
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EC-TERS proof of principle: adenine/Au(111)
Intensity (arbitr. units)
1.8 mW, 5 s
-0.4 V vs Ptpseudo
0.5 V vs Ptpseudo
Ebias = 0.4 V
0
500
1000
1500
Raman shift (1/cm)
2000
2500
Martı́n Sabanés, KFD ChemElectroChem (2017) submitted
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EC-TERS proof of principle: adenine/Au(111)
1.8 mW, 5 s
Intensity (arbitr. units)
Seminar by
Natalia Martín Sabanés
-0.4 V vs Pt
tomorrow 11.30h
Richard Willstädter Haus
pseudo
0.5 V vs Ptpseudo
Ebias = 0.4 V
0
500
1000
1500
Raman shift (1/cm)
2000
2500
Martı́n Sabanés, KFD ChemElectroChem (2017) submitted
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Outlook: Protein structure and MoS2 electrocatalysis
Nanoscale structure and chemistry elucidated with EC-TERS
1
H 2O
T
36 ˚C
D 2O
3
2 ˚C
2
NH4Ac
2 < pH < 11
4
Au
---
+++
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Nearfield spectroscopy
Content
Fundamentals
I
I
I
I
Sub-wavelength optics, intro
Plasmons
The evanescent field
Light/plasmon coupling
Application
I
Tip-enhanced Raman spectroscopy
Thank you!
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The dielectric function of a metal
Some clues how to get there
http://emlab.utep.edu/ee5390em21/Lecture%202%20–
%20Lorentz%20and%20Drude%20models.pdf
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