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