Substrate Matters: Surface-Polariton Enhanced Infrared …benasque.org/2020nanolight/talks_contr/115_Mester.pdf · 2020-03-13 · Surface-Polariton Enhanced Infrared Nanospectroscopy

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

11.3.2020 Nanolight 2020, Benasque 2

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

+

-

Nanolight 2020, Benasque 311.3.2020

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


25 nm


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)


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


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


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


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


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


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)


Dielectric function

of SiC-sample

+= i

TO LO

‘ -1


SiC

Reststrahlenband

Near-field spectrum

dipole model

S = Esca2

‘

‘‘

‘ < 0

SiC

Au

929

1

0

14

929 cm-1

Au

SiC




tip

SiC substrate

Esca

Dielectric function

of SiC-sample

+= i

TO LO

‘ -1


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




tip

SiC substrate

Esca


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



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


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)


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


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


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


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


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


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

[email protected]

www.nanogune.eu

“The big Challenge of the small”

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Substrate Matters: Surface-Polariton Enhanced Infrared …benasque.org/2020nanolight/talks_contr/115_Mester.pdf · 2020-03-13 · Surface-Polariton Enhanced Infrared Nanospectroscopy