Substrate Matters: Surface-Polariton Enhanced Infrared...
Transcript of Substrate Matters: Surface-Polariton Enhanced Infrared...
M. Autore, L. Mester, M. Goikoetxea, R. HillenbrandCIC nanoGUNE BRTA, Donostia-San Sebastian, Spain
Substrate Matters:
Surface-Polariton Enhanced Infrared
Nanospectroscopy of Molecular Vibrations
molecule
substrate
CaF2
c-SiO2 + Au
1100 1200
IR s
cattering
c-SiO2
AuSi
substrate
Infrared
light
IR spectra of PEO
Nano Lett. 2019, 19, 11, 8066-807311.3.2020 Nanolight 2020, Benasque
AFM probe
Frequency w (cm-1)
Scattering-type scanning
near-field optical microscopy(S-SNOM & nano-FTIR)
Introduction to infrared nanospectroscopy (nano-FTIR)
Substrate-enhanced nano-FTIR of molecular vibrations
(non-resonant)
Phonon-polariton resonant substrates for nano-FTIR
Table of contents
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Infrared spectroscopy is a powerful tool for material analysis
Infrared light is highly sensitive to
- molecular vibrations → chemical composition
- crystal lattice vibrations → structural properties
- plasmons in doped semiconductors, graphene → electron properties
- …
w0
+
-
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Chemical mapping is possible, but
spatial resolution is diffraction limited > l/2 ≈ 10 - 100 µm
1.6µm
λ = 0.5 µm (visible) λ = 10 µm (infrared)Test Pattern
The lateral resolution in (far-field) IR spectroscopy is limited to about λ/2
Focused laser beam illuminates AFM tip
near-field image
λ = 10 µm
Nature Mat. 3, 606-609 (2004)
0
20
field
enhan
cem
ent metal
tip air
l ≈ 10 µm
E
Tip creates nano-focus
Resolution in (far-field) IR spectroscopy is limited
Near-field IR microscopy (s-SNOM or nano-FTIR)
overcomes diffraction limit by orders of magnitude
AFM
tip
Sample
in
out
IR
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25 nm
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Nanoidentification by infrared s-SNOM and nano-FTIR
DetectorAFM-
Cantilever
RM
BS
d
Fourier Transform &
normalization
Interferogram
Amplitude
frequency
Phase
F.Huth, Nano Lett., 12, 3973-3978 (2012)
s-SNOM is based on atomic force
microscopy and interferometric
detection of the tip-scattered light.
nano-FTIR spectroscopy allows
for chemical nanoidentification
Detector
signalBroadband
IR laserMirror
position d
nano-FTIR spectrum
with effective tip polarizability
where (near-field reflection coefficient)
b =e -1
e +1
Keilmann, Hillenbrand, in Nano-Optics and Near-Field Optical Microscopy (Artech House, 2008)
Approx. for Abs(b) < 1
inE
scaE
e = ¢ e + i ¢ ¢ e
p'= p×e -1
e +1
2a
z
pr
Sample properties are measured via
near- and far-field reflection coefficients (b and r)
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Esca = 1+ r( )2aeff Ein = snf e
ijnf Ein
Quasi-electrostatic dipole model of s-SNOM (bulk samples)
aeff = atip
atipb1 -
16p(z+a)³
aeff b
Far-field
contribution
Near-field
contribution
1
atip
Introduction to infrared nanospectroscopy (nano-FTIR)
Substrate-enhanced nano-FTIR of molecular vibrations
(non-resonant)
Phonon-polariton resonant substrates for nano-FTIR
Table of contents
11.3.2020 Nanolight 2020, Benasque 7
14x signal enhancementby using a highly reflective Au substrate compared to CaF2
1000 1200
w (cm-1)
1100 1300
Na
no-F
TIR
am
plit
ud
e s
3/s
3(A
u)
0
1
0.5
1000 1200
w (cm-1)
1100 1300
experiment simulation
0.03 (7%)
0.46 (100%)
Reflective substrates strongly enhance nano-FTIR signals
PEO
molecule
Au or CaF2
+
-
1
𝑟
PEO on CaF2
PEO on Au
Nano-FTIR amplitude essentially
probes near-field reflection of PEO
rCaF2 = -0.06
Sustrate reflectivities
rAu = 1
13 nm
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CaF2
Au
bAu ≈ 1
bCaF2 ≈ 0.3
Sketch of experiment
14x signal enhancementby using a highly reflective Au substrate compared to CaF2
1000 1200
w (cm-1)
1100 1300
Na
no-F
TIR
am
plit
ud
e s
3/s
3(A
u)
0
1
0.5
1000 1200
w (cm-1)
1100 1300
experiment simulation
0.03 (7%)
0.46 (100%)
Reflective substrates strongly enhance nano-FTIR signals
PEO
molecule
Au or CaF2
+
-
1
𝑟
PEO on CaF2
PEO on Au
Nano-FTIR amplitude essentially
probes near-field reflection of PEO
rCaF2 = -0.06
Sustrate reflectivities
rAu = 1
13 nm
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CaF2
Au
bAu ≈ 1
bCaF2 ≈ 0.3
Sketch of experiment
Increased
tip-substrate
coupling
Incident and scattered light
reflected via substrate
Neubrech, PRL 101, 157403 (2008)Huth, Nano Lett. 2013, 13, 1065-1072
Resonant S-SNOM tips Resonant tip-antenna coupling Resonant tip-substrate coupling
Calculations J. Aizpurua (DIPC, Spain)
SiC
Pt500
0
Strong field-enhancement can be provided e.g. by:
Resonant enhancement of near-fields at the tip apex
Escap
p’
e.g. phonon- or plasmon-
polariton resonance
= b p
b > 1 b < 1
e.g. in plasmon-resonant
gold nanowire
Near-field intensity0 2000 F
ield
enhancem
ent
e.g. tailored probe tip length
inE
scaE
Au
Si
1 µ
m
NFEx
L
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e.g. polar crystal SiC
- Sophisticated fabrication- Sophisticated fabrication
- Localized hotspots on sample
+ flat surface
+ simple (no fabrication required)
+ hotspot independent of position
l=1 l=2 l=3
Electron energy loss spectroscopy maps
Esca
E in
~12 nm
O’Callahan, J. Phys. Chem. C
2019, 123, 17505-17509
Neubrech, PRL 101, 157403 (2008)Huth, Nano Lett. 2013, 13, 1065-1072
Resonant S-SNOM tips Resonant tip-antenna coupling
Calculations J. Aizpurua (DIPC, Spain)
SiC
Pt500
0
Strong field-enhancement can be provided e.g. by:
Resonant enhancement of near-fields at the tip apex
Escap
p’
e.g. phonon- or plasmon-
polariton resonance
= b p
b > 1 b < 1
e.g. in plasmon-resonant
gold nanowire
Near-field intensity0 2000 F
ield
enhancem
ent
e.g. tailored probe tip length
inE
scaE
Au
Si
1 µ
m
NFEx
L
11.3.2020 Nanolight 2020, Benasque 11
e.g. polar crystal SiC
- Sophisticated fabrication- Sophisticated fabrication
- Localized hotspots on sample
+ flat surface
+ simple (no fabrication required)
+ hotspot independent of position
l=1 l=2 l=3
Electron energy loss spectroscopy maps
Esca
E in
~12 nm
O’Callahan, J. Phys. Chem. C
2019, 123, 17505-17509
Resonant tip-substrate coupling
Resonant tip-substrate coupling (here SiC substrate)
Dielectric function
of SiC-sample
+= i
TO LO
‘ -1
Far-field reflectivity
SiC
Reststrahlenband
Near-field spectrum
dipole model
S = Esca2
tip
SiC substrate
Esca
‘
‘‘
‘ < 0
SiC
Au
929
- 1
+ 1b =
Near-field reflection
coefficient
Hillenbrand, Nature 418, 159-162 (2002)
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Dielectric function
of SiC-sample
+= i
TO LO
‘ -1
Far-field reflectivity
SiC
Reststrahlenband
Near-field spectrum
dipole model
S = Esca2
‘
‘‘
‘ < 0
SiC
Au
929
1
0
14
929 cm-1
Au
SiC
Hillenbrand, Nature 418, 159-162 (2002)
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Resonant tip-substrate coupling (here SiC substrate)
tip
SiC substrate
Esca
Dielectric function
of SiC-sample
+= i
TO LO
‘ -1
Far-field reflectivity
SiC
Reststrahlenband
Near-field spectrum
dipole model
S = Esca2
‘
‘‘
‘ < 0
SiC
Au
929
0
1
978 cm-1
978
1
0
14
929 cm-1
Au
SiC
Hillenbrand, Nature 418, 159-162 (2002)
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Resonant tip-substrate coupling (here SiC substrate)
tip
SiC substrate
Esca
Resonant tip-substrate coupling
provides large (additional) field enhancement
Phonon-polariton
resonant substrate,
such as c-SiO2
1
Pe
rmittivity
1,2
-20
40
0
20
Na
no-F
TIR
am
plit
ud
e
s4/s
4(A
u)
1000 1200
Frequency w (cm-1)
c-SiO2
0
1
2
3
4
Tip-substrate resonance of quartz matches PEO‘s vibrational resonance
→ We aim to exploit polariton-resonance to further increase nano-FTIR signals of PEO
c-SiO2
Resonance
condition
≈ -1
Signal level
on Au
𝑟
≈ -1
Localized phonon-polariton
resonance in quartz
p
p’1
1
+
-=
p
Autore, Nano Lett. 19, 11, 8066-8073 (2019)
PEO
vibrational
resonance
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Resonant tip-substrate coupling
provides large (additional) field enhancement
Phonon-polariton
resonant substrate,
such as c-SiO2
1
Pe
rmittivity
1,2
-20
40
0
20
Na
no-F
TIR
am
plit
ud
e
s4/s
4(A
u)
1000 1200
Frequency w (cm-1)
0
1
2
3
4
c-SiO2
Signal level
on Au
𝑟
Localized phonon-polariton
resonance in quartz
PEOp
p’1
1
+
-=
p
Autore, Nano Lett. 19, 11, 8066-8073 (2019)
PEO on
c-SiO2
→ We aim to exploit polariton-resonance to further increase nano-FTIR signals of PEO
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Tip-substrate resonance of quartz matches PEO‘s vibrational resonance
Resonance
condition
≈ -1
≈ -1
PEO
vibrational
resonance
c-SiO2
Topographyt (nm)
0
23300 nm
s4/s
4(A
u)
1000 1200
Nano-FTIR amplitude
1000 1200
c-SiO2
PEO on
c-SiO2
0
1
2
3
0
1
s4/s
4(A
u)
0
1
Ds
4
4
2
PEO Spectral contrast
Resonant tip-substrate coupling boosts nano-FTIR signal beyond Au
𝜔PEO
s4 (a.u.) S-SNOM amplitude
Δs4 = Difference of curves
(c-SiO2*) – (PEO on c-SiO2)
Frequency w (cm-1) Frequency w (cm-1)
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c-SiO2*
Dip in resonance indicates large coupling between
molecule and tip-induced phonon-polariton
(Analog to SEIRA effect, but in near-field microscopy)
c-SiO2 c-SiO2
Increased tip-substrate distance
reduces tip-substrate coupling
su
btr
actio
n
Comparison of substrate-enhancements
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s4/s
4(A
u)
1000 1200
Frequency w (cm-1)
5
0
1.4 (300%)
0.03 (7%)CaF2
c-SiO2
0.46 (100%)
Si
w (cm-1)1100 1200
10 Au
0.25 (54%)
1100
15
Ds4
r = 1
r = 0.29
r = -0.06
r < 1Polariton-resonant substrate yields
further 3x enhancement
Metal substrate yields
14x higher spectral contrast than CaF2
PEO
c-SiO2
PEO
substrate
+
-
+
-
r
Comparison of substrate-enhancements
PEO
c-SiO2
PEO
substrate
PEOAu
+
-
c-SiO2
+
-
+
-
s4/s
4(A
u)
1000 1200
Frequency w (cm-1)
5
0
1.4 (300%)
0.03 (7%)CaF2
c-SiO2
0.46 (100%)
Si
w (cm-1)
1100 1200
w (cm-1)1100 1200
10
Au
3.4 (750%)
0.25 (54%)
20
1100
15
Ds4
Ds4
25
30
35
r = 1
r = 0.29
r = -0.06
r < 1
r = 1
r
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Polariton-resonant substrate
+ illumination of tip via gold surface
(+ illumination via propagating SPhP) yields
two orders of magnitude enhancement
(compared to CaF2)
Polariton-resonant substrate yields
further 3x enhancement
Metal substrate yields
14x higher spectral contrast than CaF2
Outlook
Polar crystals
strong lattice vibrations (phonons)
e.g. SiO2, SiC, Al2O3, …
Metals / doped semiconductors
collective free electron oscillations (plasmons)
plasma frequency
(longitudinal oscillation)
-1
Wide range of materials host polariton
resonances that can enhance sensing ( -1)
Strong spectral contrast Ds4 predicted
even for nm-thin molecular layers
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c-SiO2c-SiO2
Tip-substrate coupling increases
for thin molecular layers
Cc-SiO2 / CAu
En
ha
nce
ment
PEO thickness (nm)
Summary
Autore, Nano Lett. 2019,19,11, 8066-8073
Non-resonant tip-substrate coupling (e.g. Au substrate) → 14x enhancement compared to CaF2
Resonant tip-substrate coupling (Quartz) + efficient tip illumination (via Au)
→ 7.5x enhancement compared to Au
→ 14x enhancement compared to Silicon
Phonon-resonant tip-substrate coupling (e.g. Quartz substrate) → further 3x enhancement
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PEO
substrate
CaF2
+
-
c-SiO2 + Au
1100 1200
Na
no-F
TIR
am
plit
ud
e
c-SiO2
AuSi
substrate
AFM
tip
Spectra of PEO
Infrared
light
Frequency w (cm-1)
Nano-FTIR experiment
11.3.2020 Nanolight 2020, Benasque 22
Acknowledgements
Autore, Nano Lett. 2019,19,11, 8066-8073
M. AutoreM. Goikoetxea R. Hillenbrand
Grant no. 721874Projects MAT2015-65525-R, RTI2018-094830-B-100, and MDM-2016-0618
CIC nanoGUNE
Tolosa Hiribidea, 76
E-20018 Donostia-San Sebastian
+ 34 943 574 000
www.nanogune.eu
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