Dr. Tobias Gokus June 2nd, 2015 – AMC Workshop, Urbana-Champaign

2

NeaSNOM – A ready-to-use

Scattering-type Near-field Optical Microscope

To overcome diffraction limit of conventional spectroscopy NeaSNOM is

based on s-SNOM technology

10nm optical resolution

near-field spectroscopy

(nano-FITR)

high-speed near-field imaging

Simultaneous optical amplitude

(reflection) and phase (absorption)

measurement

VIS–IR–THz spectral range

Suitable for any ‘AFM-ready’ sample

(ambient conditions)

3

Optical (Infrared) spectroscopy is a

highly sensitive materials research tool

IR is highly sensitive to:

Molecular vibrations → Chemical composition / material identification

Crystal lattice vibrations → Structural properties

Plasmons in doped semiconductors → Electron properties

… but with conventional technology the spatial

resolution is limited to ca. l/2 (IR 10 µm)

4

scattering-type Scanning Near-field Optical Microscopy

employs a nanofocus for near-field measurements

A focused laser-beam illuminates a

commercial AFM tip

The tip generates a nano-focus, size of the

tip-radius of 10-20 nm (Lightning Rod Effect)

The near-field interaction between the tip and

the sample modifies the back-scattered light

By scanning the sample surface with the tip,

an optical image with 10nm spatial resolution

is created

Simultaneous measurement of mechanical topography and optical near-fields

s-SNOM measures the near-field optical interaction between tip and

sample which is determine by the refractive index of the sample

5

Optical Near-field signal depends on

optical/electronic properties of sample material

Dipole above surface

(dipole model)

e (w) = complex valued dielectric function of the sample

p

p´ 1

1

e

ep

Ein

Esca = s Ein AFM tip

Sample

B. Knoll, F. Keilmann, Nature 399, 134-137 (1999)

R. Hillenbrand, F. Keilmann, Phys. Rev. Lett. 85, 3029-3032 (2000)

A. Cvitkovic et al., Opt. Exp. 15, 8550 (2007)

Scattering coefficient s contains material specific

information about the sample:

1)(

1)(

we

wews

in

sca

E

E

s-SNOM is highly

sensitive to changes of

the dielectric properties

6

Metals Si (undoped) Dielectrics

- 1500 - 1000 - 500

0

2

4

6

PS

Au

Cu SiO2

Al

0

2

4

6

10 0 -10

Re(e)

-500

Im(e) = 1

w = 1000cm-1

-1000 ≈-2

Si

Knoll et. al, Nature 399, 134 (1999)

Hillenbrand et. al, Nature 418, 159 (2002)

Si

SiO2

2 µm

Near-field amplitude

Material specific optical

near-field contrast

based on difference in

refractive index

Optical Near-field signal depends on

optical/electronic properties of sample material

7

near-field amplitude min

max

Dielectric Metal

s-SNOM achieves 10nm spatial resolution

• cw laser (l=10.8µm)

• probing tip: Mikromasch DPER14

(nominal tip radius <20nm)

8

0

Ne

ar-

fie

ld a

mp

litu

de

[a

.u.]

0.5

1.0

Heig

ht z[n

m]

4

0

0

Position x [nm] 500

250

8

70%

30%

8nm

near-field amplitude min

max

Dielectric Metal

• cw laser (l=10.8µm)

• probing tip: Mikromasch DPER14

(nominal tip radius <20nm)

• near-field signal change

within <10nm

Raw data

s-SNOM achieves 10nm spatial resolution

< 1nm

9

n Interferogram

Nano-FTIR spectroscopy

FT

Am

plit

ud

e s

n

Ph

ase

jn

Nano-FTIR laser output spectra

d

Detector

BS

AFM-

Cantilever

RM

S.Amarie, et al., Phys. Rev. B 83, 045404 (2011)

F. Huth, et al., Nature Mater. 10, 352 (2011)

NeaSNOM enables

near-field spectroscopy (nano-FTIR)

Reflection

Absorption

10

Tunable QCL based midIR near-field

illumination sources cover 4-11µm

Near-field imaging of Graphene

Ne

ar-

fie

ld a

mp

litu

de

Ne

ar-

fie

ld p

ha

se

1mm

NeaSNOM enables

2D near-field imaging (chemical mapping)

Ne

ar-

fie

ld a

mplit

ud

e

Near-field imaging of minerals

(Murchison meteorite)

G. Dominguez et al., Nature Comm., 5, 5445 (2014)

11

Time resolved near-field measurements

• near-IR pump, mid-IR probe of i.e. Graphene, InAs

• Excitation of free-carriers, observation of spectral

changes

Integral Ds/s(%)

Time delay (ps)

neaSNOM

sample

stage

pump

parabolic

mirror

probe

Time-resolved near-field

measurements at up to ca. 10nm

spatial and <10fs temporal

resolution

neaSNOM supports two optical beam paths for tip illumination and

signal detection for i.e. Pump-Probe measurements

‚Dual beam design‘ of neaSNOM

M. Wagner et al., Nano Lett. 14, 894 (2014)

M. Wagner et al., Nano Lett. 14, 4529 (2014)

M. Eisele et al., Nature Phot 8, 841 (2014)

12

NeaSNOM signal at 1600cm-1 is determined by

refractive index of Si and SiO

0.0 rad

2.4 rad

w=1600cm-1

Topography

min

max

40nm

0nm

SiO

Si

2mm

Amplitude

Phase

13

0.0 rad

2.4 rad

w=1600cm-1

Topography

min

max

40nm

0nm

SiO

Si

2mm

nano-FTIR measures characteristic near-field spectrum of SiO

Spectroscopy parameters:

- spectral range 750-1450cm-1

- Spectral resolution: 8cm-1

- Measuring time: 30s

Near-field amplitude spectrum

Near-field phase spectrum Ref

SiO

x

x

Amplitude

Phase

SiO

14

NeaSNOM enables material-specific imaging of SiO

(‘ON/OFF’ resonance)

w=1130cm-1 w=1250cm-1

Imaging parameters:

- 250x125pix, 10x5mm

- Scan speed: 20.0mm/s

- Time constant: 0.5ms/pix

- Scan time ca. 2 min per image

Amplitude

Phase

0.0 rad

3.0 rad

‘ON’ resonance ‘ON’ resonance ‘OFF’ resonance

w=1600cm-1

Topography

min

max

40nm

0nm

SiO

Si

2mm

15

NeaSNOM allows spectroscopic mapping of SiO at nanoscale

Spectroscopy

parameters:

- 100 spectra

- Distance 1mm

- Spectral

resolution: 8cm-1

- 30 sec/spectrum

- Pixelsize 10nm

Amplitude

Phase

Signal change

from Si to SiO

within 2-3 pixels

(20-30nm)

0.0 rad

3.0 rad

w=1600cm-1

Topography

min

max

40nm

0nm

SiO

Si

2mm

Amplitude

Phase

16

F. Huth et al., Nano Lett. 8, 3973 (2012)

Signal in nano-FTIR: Scattering coefficient

Si

Einc

PMMA

Esca = σ(w)Einc

nano-FTIR yields broadband spectra of organic sample materials

1900 1700 1500 1300 1100 900 700

0.2

0.1

0

Nano-FTIR

(90nm PMMA film) Im[s]

Na

no-F

TIR

Ab

so

rptio

n

Frequency ω [cm-1]

17

nano-FTIR provides near-field spectra of organic materials

similar to far-field FTIR spectra

PMMA

Iinc

It

Si

Einc

PMMA

Esca = σ(w)Einc 0.2

0.1

0

FT

IR

Ab

so

rptio

n

0

0.2

0.4

0.6 C=O

C-O-C

CH2

C-C-O

1900 1700 1500 1300 1100 900 700

FTIR

(~ 5µm PMMA film)

Im[s]

Na

no

-FT

IR A

bso

rptio

n

Frequency ω [cm-1]

Nano-FTIR

(90nm PMMA film)

nano-FTIR absorption Im[σ] correlates well with far-field absorption A

18

0.2

0.1

0

Nano-FTIR

(90nm PMMA film)

Im[s]

Na

no

-FT

IR A

bso

rptio

n

Si

Einc

PMMA

Esca = σ(w)Einc

Topography Near-field Amplitude Near-field Phase

1µm PMMA PS

PS

53nm

0nm

0.8rad max

min 0rad

Scan parameters:

w=1740cm-1 (λ=5.75µm)

Time constant (Lock-In): 0.52ms

Single-frequency imaging at selected wavelengths allows for

material specific chemical mapping

19

neaSNOM achieves nanoscale spatial resolution

for polymer thin film near-field imaging

min

max

Nano-FTIR Absorption

min

max

200 nm

0nm

3.8nm

Topography

0nm

2.3nm

100 nm

1730 cm-1

ON Resonance

1650 cm-1

OFF Resonance

neaSNOM characterizes

PMMA-PS block

copolymer films (52nm)

20

nano-FTIR enables to study biological nanostructures

Ferritin molecule:

na

no

-FT

IR

IR-I

ma

gin

g

I. Amenabar et al.

Nature Comm. 4, 2890 (2013)

• single particle with 12 nm

diameter

• nano-FTIR spectroscopy in

reflection mode

• nano-FTIR broad band light

source

• in combination with 2D mapping

21

nano-FTIR enables to study biological nanostructures

Ferritin molecule:

na

no

-FT

IR

IR-I

ma

gin

g

I. Amenabar et al.

Nature Comm. 4, 2890 (2013)

• single particle with 12 nm

diameter

• nano-FTIR spectroscopy in

reflection mode

• nano-FTIR broad band light

source

• in combination with 2D mapping

Ferritin characteristics:

- Ferrihydrite core

- 24 proteins/subunits

- 5000 C=O bonds

- 1 attogram mass

12 nm

9 nm

core

22

nano-FTIR enables to study biological nanostructures

Ferritin molecule:

na

no

-FT

IR

IR-I

ma

gin

g

I. Amenabar et al.

Nature Comm. 4, 2890 (2013)

• single particle with 12 nm

diameter

• nano-FTIR spectroscopy in

reflection mode

• nano-FTIR broad band light

source

• in combination with 2D mapping

nano-FTIR of single

bio-nanoparticles

23

NeaSNOM enables development of more efficient energy

storage devices, i.e. Li batteries

AFM Topography

IR nano-imaging @ 1042cm-1

Engineering of more efficient Li-based batteries necessiates detailed understanding

of charge/discharge processes in device electrode active materials

NeaSNOM provides:

Noninvasive tool to access nanoscale Li phase

Analysis of dynamic charge/discharge processes

LiFePO crystals as used in commercially available Li

battery electrodes

NeaSNOM resolves different LixFePO4 phases of single-

crystal microparticles

Determination of Li concentration

Lucas, et al.,

Nano Lett. (2014)

24

IR s-SNOM image

l 10 mm

NeaSNOM enables real-space mapping

of mid-IR plasmons in graphene

J. Chen et al.,

Nature (2012) 487, 77

Z. Fei et al.,

Nature (2012) 487, 82

ω = 1087 cm-1, λ = 9200 nm

IR s-SNOM image

1 μm

25

IR s-SNOM image

l 10 mm

NeaSNOM enables real-space mapping

of mid-IR plasmons in graphene

J. Chen et al.,

Nature (2012) 487, 77

Z. Fei et al.,

Nature (2012) 487, 82

1.

2.

3.

Plasmon-Interference Mapping

1. Near-field at tip apex excites graphene plasmon

2. Plasmons are backreflected at graphene edge (or other defects)

3. Tip scatteres interfering fields at tip apex

ω = 1087 cm-1, λ = 9200 nm

IR s-SNOM image

1 μm

26

The plasmon wavelength on

graphene is dramatically shorter than

the free-space wavelength

ω = 1087 cm-1, λ = 9200 nm

IR s-SNOM image

1 μm

lP/2 lP ≈ 230 nm = l/40

l 10 mm

NeaSNOM enables real-space mapping

of mid-IR plasmons in graphene

J. Chen et al.,

Nature (2012) 487, 77

Z. Fei et al.,

Nature (2012) 487, 82

27

neaSNOM enables to characterize Graphene

(and other 2D materials) via Surface-Polaritons

• Graphene plasmon excitation

via tip or antenna structures

• local conductivity (carrier

concentration and mobility)

• Effects of grain boundaries

and defects

J. Chen et al.,

Nature (2012) 487, 77

Z. Fei et al.,

Nature Nanotechn.

(2012) 8, 821

Z. Fei et al.,

Nature (2012) 487, 82

P. Alonso-González

et al., Science (2014)

344, 1369

J. Chen et al.,

Nano Lett.

(2013) 13, 6210

A. Y. Nikitin et al.,

Nano Lett.

(2014) 14, 2896

S. Dai et al.,

Science (2014)

343, 1125

P. Li et al.,

Nano Lett.

(2014) 14, 4400

M. Wagner et al.,

Nano Lett.

(2014) 14, 894

Graphene Plasmonics: M. Schnell et al.,

Nature Comm.

(2013) 5, 3499

Electronic properties of Graphene

New insights into graphene

properties & physics by

applying new analysis

technique

28

neaSNOM can probe free carriers in nanowires

1 µm

IR

C

P2

P1

Au

A

B*

B

C

A

B*

B

P2

P1

n-type

undoped

Near-field amplitude

Topography

l=11.2 µm

Near-field phase

• Modulation doped InP nanowires

• ca. 1µm long highly conductive

segment in wire center

• Detection of doping gradient

between to adjacent segments

• Detection of doped

subsurface layer

J. M. Stiegler et al.,

Nano Lett (2010) 10, 1387

Contact-free

quantification of free

carrier concentration

29

Tailored design of THz-TDS system allows

easy integration into NeaSNOM

Imaging and spectroscopy in THz spectral range with

nanoscale spatial resolution

Technical details of

NeaSNOM / THz-TDS system:

- Compact system design

- Broadband parabolic mirror objective

- Precise alignment to NeaSNOMs

optical beam paths is essential

- Fiber-coupled THz-TDS system

- Tailored THz-TDS specifications

(beam diameter, pilot laser, polarization)

- Based on commercial AFM probing tips

THz-source

THz-detector

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NeaSNOM enables THz near-field spectroscopy

THz near-field spectra:

- Higher harmonics are detected

(with respect to tip oscillation frequency)

- Spectral range from ca. 0.5 – 2.5 THz

- Dynamic range ca. 50dB (O1)

- Spectral features are related to water

absorption lines in atmosphere

Detection of higher

demodulation orders

for tip-scattered THz signal

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Demonstration of nanoscale THz field confinement for tip-

sample near-field interaction

THz tip-sample interaction

confined to <15nm (!)

THz approach curves:

- Spectrally integrated THz signal

(fixed delay)

- Recording THz signal as a

function of tip-sample separation

- Detection of higher demodulation

orders for THz signal (O1 – O4)

- 1/e decay within 15nm

(ca. l/20000 !!)

z

Au film on Si

Tip-sample distance [nm]

Norm

. T

Hz s

ignal

0 30 60 180 0.0

0.5

1.0

90 120 150

32

NeaSNOM achieves nanoscale imaging at THz frequencies

500nm min

max

0nm

50nm

Au SiC

0nm

200nm

Au Doped Si 500nm

SiO

Si

0nm

30nm

500nm

Topography Spectrally integrated THz near-field signal (O2)

0nm

6nm

Multilayer

graphene SiO

500nm 500nm

33

NeaSNOM achieves extreme subwavelength-scale

THz near-field imaging

SiO

Si

0nm

30nm

500nm

Topography

Spectrally integrated THz near-field signal (O2)

He

igh

t [n

m]

TH

z n

ea

r-fi

eld

sig

na

l O

2 [

a.u

.]

0

10

30

40

0.0 1.0 Distance [mm]

0.25 0.75 0.5

1.0

25nm min

max

Lineprofile reveals 25nm

(ca. l/12000) spatial resolution

34

Summary

NeaSNOM nano-FTIR technology is a powerful tool for cutting-edge nanoanalytic

applications, allowing for

and

with outstanding spatial resolution down to 10 nm.

Thank you for your attention!

nanoscale imaging

Spectroscopy (nano-FTIR)