school of science –faculty of physics institute of applied
TRANSCRIPT
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School of Science – Faculty of Physics Institute of Applied Physics
Downtown Dresden
“Probing nanoscale physical propertiesin oxides by scanning probe methods”
Lukas M. EngIAP, TU DresdenGermany
5th ISOECargèse, FAugust 24 –September 3 2021
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Excellence Univ. 2019 - 2025
< 2 %
ct.qmat:Complexity
and Topology in Quantum
Materials Excellence Universities
Prague
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School of Science – Faculty of Physics Institute of Applied Physics
Downtown Dresden
“Domain walls in oxides –A 2D-electron gas with exceptionalproperties”
Lukas M. EngIAP, TU DresdenGermany
5th ISOECargèse, FAugust 24 –September 3 2021
Thursday, Sept. 2, 2021
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Nobel prize in physics, 1986IBM Rüschlikon
Switzerland
Scanning Tunneling Microscopy: STM
G. Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 (1982)
STM-Discovery
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Nobel prize in physics, 1986IBM Rüschlikon
Switzerland
Scanning Tunneling Microscopy: STM
G. Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 (1982)
STM-Discovery
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J. Repp et al., Phys. Rev. L.
94, 026803 (2005)
„Seeing“ wavefunctions
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BaTiO3(111)
STM 4 oxides ?
Vsample= +2.2 VIt = 0.1 nA
vacuum-annealed @ 1500 K
(4 x 4) @ 1500 K
(3 x 3) @ 1550 K
(2 x 2) @ 1600 K
C. Hagendorf et al., Surf. Sci. 436, 121 (1999)
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• 1 - 2 u.c. BTO• 2% compressive strain• c(2x2) superstructure• out-of-plane polarization (values ?)• annealing increases oxygen deficiency
STM 4 oxides ?
BaTiO3(100) / Pt
S. Förster et al., J. Chem. Phys.
135, 104701 (2011)
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AC-STM
G.P. Kochanski, Phys. Rev. Lett. 62, 2285 (1989)
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S.J. Stranick and P. S. WeissJ. Phys. Chem. 98, 1762 (1994)
AC-STM
CuO
5 nm
Al2O3/Al
Pb-silicate-glass
G.P. Kochanski, Phys. Rev. Lett. 62, 2285 (1989)
@ ~1 GHz
100 nm
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Ch. Gerber et al.,Phys. Rev. Lett. 56, 930 (1986)
AFM-DiscoveryAtomic Force Microscopy: AFM
laserbeamdetector
sample
AFM tip
piezo scanner
tu-dresden.de/mn/physik/iap
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Sichtbar - unsichtbar
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J. Israelachvili, Academic Press (1985)D. Tabor, Cambridge Univ. Press (1979)
R
dL
a
dSNano-Tip
Chemical forces:attractive: binding & adhesion
repulsive: Pauli- & nuclear
Van der Waals forces: F.O. Goodman et al.,
Phys. Rev. B 43, 4728 (1991)
Electrostatic / magnetic forces:S. Hudlet et al.,
Euro. Phys. J. 25, (1998)
Forces in SFM – tip-model
Optical forces:induced dipolar interaction
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Force
Distance
Force-Distance-curve
• Repulsive interaction
• Fstatic (³ 10 nN)
• Adhesion; large contact area
• Attractive interaction
• Cantilever oscillates at resonance frequency
• Measure Df ~ Interaction force
• true atomic resolution possible
Dw w» - 0
2kdFdz
³ 10 nm
tu-dresden.de/mn/physik/iap
(Non) Contact SFM
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Outline
• Piezo-response Force Microscopy (PFM)
• PFM + Pull-off Force Spectroscopy (PFS)
• Kelvin-Probe Force Microscopy (KPFM)
• Scattering Near-field Optical Microscopy (SNOM)
• Metamaterials and Superlensing
• [Time-resolved Scanning Probe Microscopy (tr-SPM)]
• Wrap up
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… pick the low-hanging fruits …
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#1: PiezoresponseForce Microscopy (PFM)
K. Franke et al., Proc. IEEE Intern. Symp. Appl. Ferroelectr. & Europ. Conf. Appl. Polar Dielectr. and Piezoelectric
Force Microscopy Workshop (ISAF/ECAPD/PFM), 1 (2016) M. Abplanalp et al., Appl. Phys. A 66, S231 (1998)L.M. Eng et al., Appl. Phys. Lett. 74, 233 (1999)
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M. Abplanalp et al., Appl. Phys. A
66, S231 (1998)
PFM
BTO (100)
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L.M. Eng et al., Appl. Phys. Lett. 74, 233 (1999)
PFM – “X-ray”
Euler angles
dij
BaTiO3 ceramic
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K. Franke and L.M. EngSurf. Sci. 600 4896 (2006)
PFM & PFS
PFM: piezoresponse force microscopyPFS: pull-off force spectroscopy
PZT (53/47)
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K. Franke and L.M. EngSurf. Sci. 600 4896 (2006)
PFM & PFS
PFM: piezoresponse force microscopyPFS: pull-off force spectroscopy
• freely moveable surface charges of density r(for instance deposited by the tip of the SFM)
• charges representing the dipoles of the remanent polarization Pz
• charges screening these dipoles• charges trapped within the material, giving rise to local
electric fields and generating the internal bias voltage Uint• polarization charges influencing the SFM measurements
through the inverse piezoelectric effect.
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K. Franke and L.M. EngSurf. Sci. 600 4896 (2006)
PFM & PFS
Results 1:• Pz = ~0.2 As/m2
• r: No surface charges up to + 45 V DC bias
surface charging for U < -20 V, and linear increase with U
Pz
r
PZT (53/47)
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K. Franke and L.M. EngSurf. Sci. 600 4896 (2006)
PFM & PFS
Results 2:• A• A• A
•Pz , r , Uint , esurface , ebulk
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Iron
Ceramics
V. Veselago, 1968, „The electrodynamics of substances with simultaneouslynegative values of e and µ“, Sov. Phys. Uspekhi 10, 509
e = e(w) Dielectric Constant µ = µ(w) Magnetic Susceptibility
Material Constants
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n2 = e(w) µ(w)
e = e(w) Dielectric Constant µ = µ(w) Magnetic Susceptibilityn: complex refractive index
Moreover: e(w) and µ(w) are complex
Material Constants
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Excitations
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… focus on the higher-hanging fruits …
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Force
Distance
Force-Distance-curve
• Repulsive interaction
• Fstatic (³ 10 nN)
• Adhesion; large contact area
• Attractive interaction
• Cantilever oscillates at resonance frequency
• Measure Df ~ Interaction force
• true atomic resolution possible
Dw w» - 0
2kdFdz
³ 10 nm
tu-dresden.de/mn/physik/iap
Non-Contact interactions
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Force
Distance h
Force-Distance-curve
Non-Contact interactions
1 2 3 4
1
w
W
W
h
w
W
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nc-SFM modes
• nc-SFM: topography • MFM: magnetic texture• KPFM: surface CPD; capacitance• SDM: dissipated power (SDM: scanning dissipation microscopy)• SNOM: scattered light, Raman, local optical / IR / THz properties• …
illumination
Q-Cap
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Fdrive : driven oscillator force Fts : tip-sample interaction forceswres : angular resonance frequency
ko : spring constantmeff : effective massGo : damping coefficient
nc-SFM modes
Fts separates: i) Fcons conservative; even/mirror symmetry: cosineii) Fdiss dissipative; odd/point symmetry: sine
J. E. Sader et al., Nanotechnology 16 (2005) S94; E. Neuber, PhD thesis, TUD (2019).
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Dissipated Power
ko = 102 N/mwres = 105 HzQo = 105b = 0,1Aosc = 2 nmDAdrive = 10-15 m = 1 fm
with
1 aW
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SDM mechanismsSDM: Scanning Dissipation Microscopy
• virtual dissipation:- artifacts - inappropriate control loop parameters
• energy transfer into higher order cantilever eigenmodes• dissipation based on interactions:
- tip / sample atomic interaction (adhesion hysteresis)- tip / sample phonon interaction
• electric dissipation: tip charge interacting with - (free) electron density of sample- sample phonons- electric sample texture
• magnetic dissipation: tip magnetization interacting with- sample magnetic dipoles / textures / spins- (free) sample electrons (eddy currents)
• “stochastic” dissipation: fluctuations, random fields• …
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Skyrmions: SkLHedge hog Bloch SKY Néel SKY Anti-SKY
2 µm Mn1.4PtSnPRB 103, 184411 (2021)
GaV4S811,2 K50 mT
LT-Sky phase
Cu2OSeO3: 10 K, 80 mT
tilted conical phase[010]
[100]
Néel
Néel
npj Q-Mater. 5, 44 (2020)Sci. Rep. 7, 44663 (2017)N-Mater. 14, 1116 (2015)unpublished (2021)
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Excitations
[m]
Au, Ag + different topologies thereof
Al, Ga, Ingraphene, TMDC, TI, 2DEG, multi-ferroics, …
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#2: Kelvin probe force microscopy (KPFM)
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)T. Wagner et al., Appl. Phys. Lett. 103, 023102 (2013)
F, r, UCPD , CQ
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-
+ + + + +
Ubias
- - - - -
+ + + + +
• workfunction change• interface and molecular dipoles• charge transfer• polarizability• others
Local Potentials by FM-KPFM
200 )(2 CPDDC UUzAR
kAff -=Dpe
A: osc. Amplitude = const.z: tip position = const.
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Bias voltage modulation
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Bias voltage modulation
Noncontact AFM:
lever vibrated at its resonance frequency fo
³ 10 nm
Voltage applied between tip and sample:
Attractive electrostatic force: withF CzUel =
12
2¶¶
U U U f text DC mod mod= + cos( )2p
U U eext= - Df /
Df = workfunction difference between tip and sample (=contact potential difference CPD)
Idea: Modulation electrostatic force can be discriminated against other forces with high sensitivity
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
FM-KPFM: Frequency-modulated Kelvin force-probe-microscopy
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Bias voltage modulation
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Bias voltage modulation
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Bias voltage modulation
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FM-KPFM
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
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fMOD
f0-fMOD f0+fMOD
f0+2fMODf0-2fMOD
sideband at f0-fMOD is measured via lock-in techniques
f0 f
Aforce gradient
FM Kelvinforce AM Kelvin
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
Frequency Spectrum
f0 = 151 kHz, fMOD = 2 kHz
1/f-noise
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non-invasive FM-KPFM
UCPD = 5 mVscale x 10
UCPD = 50 mV
UCPD = 500 mV
dB
frequency [kHz]
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
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Cu-TBPP
Cu (100)
Surface potential Cu-TBPPCu-TBPP: Copper-Tetrabuthyl-Porphyrin
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fMOD
f0-fMOD f0+fMOD
f0+2fMODf0-2fMOD
sideband at f0-fMOD is measured via lock-in techniques
f0 f
Aforce gradient
FM Kelvin
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
f0 = 151 kHz, fMOD = 2 kHz
force gradientQuantum
capacitance
Frequency Spectrum
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FM-KPFM
U. Zerweck et al., Phys. Rev. B 71, 125424 (2005)
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Luminescence
0 V
0.15 V
0 nm
16 nm
10 µm 10 µm
1
2
3
5 µm
1
2
3
7 ML Optical layer ID
0 ML
KPFMTopo
1
2 3
Graphene on SiO2
T. Wagner et al., Appl. Phys.Lett. 103, 023102 (2013)
1st sideband: FM-KPFM
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Graphene on SiO2
Quantum Capacitance
# of chargecarriers
ML
BL
ML
BL
T. Wagner et al., Appl. Phys.Lett. 103, 023102 (2013)
2nd sideband: Quantum capacitance
F, r, UCPD , CQ
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#3: Scanning Nonlinear Dielectric Microscopy (SNDM)
K. Honda et al., J. Phys.: Conf. Ser. 209, 012050 (2010)
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Nonlinear Dielectric Microsc.
e(0 Hz)e(wp) e(2wp)
K. Honda et al., J. Phys.: Conf. Ser. 209, 012050 (2010)
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n-/p-doped semiconductors
5 layersP,n-doped
7 layersB,p-doped
K. Honda et al., J. Phys.: Conf. Ser. 209, 012050 (2010)
DC = 10-22F
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MOSFET Failure Analysis
MOSFET with 40 nm gate channels
K. Honda et al., J. Phys.: Conf. Ser. 209, 012050 (2010)
e(w)
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S.C. Kehr et al., Synch. Rad. News 30, 31 (2017) J. Döring et al., J. Appl. Phys. 120, 84103 (2016)
J. Döring et al., Appl. Phys. Lett. 105, 053109 (2014) R. Jacob et al., Optics Express 18, 26206 (2010)
M.P. Nikiforov et al., J. Appl. Phys. 106, 114307 (2010)
Ωilluminationread out
sample
#4: Scattering scanning near-field optical microscopy
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visibleIR
s-SNOM
1 2 3 4
1wW
W
h
wW
• based on non-contact AFM • far-field illumination and read out• tip serves as antenna• analysis of local optical properties
B. Knoll et al., Opt. Comm. 182, 321 (2000)S. Kehr et al., PRL 100, 256403 (2008)
visible, IR, THz
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Tip resonancePlasmon polariton(vis. wavelengths)
Sample resonancePhonon polariton(IR/THz wavelengths)b = es -1
es +1
p-polarization
Resonant field enhancement
ei (w)^
e1
e1
e2e1 e2 e1 e2
w: vis … THz
at tip polarizabilityh tip-sample distanceb sample responseaeff total polarizabilityes sample dielelctric
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n(λ)
T=4-300K
• local optical / IR properties• optical resolution ~ 5 nm <<• low-temperature measurements
• FELBE-SNIM (RT): polariton-based resonances
• THz-SNIM (RT): field-driven transient states
• LT-SNIM (FELBE): (structural) phase transitions
IR-s-SNIM @ FELBE/TELBEs-SNIM: scattering-type scanning near-field IR optical microscopy
• free-electron laser (FELBE): 4-250 μm; 75-1,2 THz; 310-5 meV; 2500-40 cm-1
• superradiant THz sources (TELBE): 0,1-3 mm; 3-0,1 THz; 15-0,5 meV; 100-3,3 cm-1
⇒ resonant excitation:phonons, magnons, spinons
⇒ spectroscopy + imaging / microscopy
l
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D. Lang et al., Rev. Sci. Instrum. 89, 033702 (2018)
Si/SiO2 contrast @ 7 K
- l = 9,7 μm (FEL)- near-field decay: ~30 nm
Topography Topography
Near field NF2W Near field NF2We(w) , a
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R. Jacob et al., Opt. Express 18, 26206 (2010)
Theory: (4,0 ± 1,0) x 1019 cm-3
Experiment: (3,7 ± 0,3) x 1019 cm-3
Burried p:Si
N
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Doped GaAs/InGaAs NWs
- local - contact-free- non-invasive
ωpl, gp N,m*,µ
D. Lang, et al., Nanotechnol. 30, 084003 (2019)
N, m*, µ
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LT-IR-s-SNOM spectroscopy
tetragonalorthorhombic
J. Döring et al., APL 105, 053109 (2014)J. Döring et al., JAP 120, 84103 (2016)
e(w,T)
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L. Wehmeier et al., Phys. Rev. B
100, 035444 (2019)
sample scanning
l1 = 15.3 µml2 = 15.8 µm
c
c
a
a
c
c
a
a
PFM: Piezoresponse Force MicroscopyPbZr0.2Ti0.8O3: PZT(001)
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#5 Superlensing & Metamaterials
S.C. Kehr et al., ACS Photonics 3, 20 (2016)M. Fehrenbacher et al., Nano Lett. 15, 1057 (2015)
S.C. Kehr et al., Nature Comm. 2, 249 (2011)S.C. Kehr et al., Optical Material Express 1, 1051 (2011)
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Iron
Ceramics
V. Veselago, 1968, „The electrodynamics of substances with simultaneouslynegative values of e and µ“, Sov. Phys. Uspekhi 10, 509
Material Constants
n2 = e(w) µ(w)
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LHM: left handed materialNRM: negative refractive material
RHM:
V. Veselago, 1968, „The electrodynamics of substances with simultaneouslynegative values of e and µ“, Sov. Phys. Uspekhi 10, 509.
Meta-material
Negative Refraction
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metamaterial
2d
point source
point image
Superfocusing with LHMs
J.B. Pendry, Phys. Rev. Lett. 85, 3966 (2000)
e1= e1 (w) e1= e1 (w)e2= e2 (w)
LHM: Left-Handed Material
BA A
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λ [µm]10 20
Re(
ε)
SrRuO3
0
-250
BiFeO3
PZT
SrTiO3
Im(ε
)10-2
BiFeO3
PZT
SrTiO3
SrRuO3
103
PZT: PbZrO3/PbTiO3BFO: BiFeO3STO: SrTiO3
Superlens Sample Design
d
2d
d
eA = +1
eA = +1
eB = -1
250
Possible pairs A-B-A:PZT-STO-PZTBFO-STO-BFOBFO-PZT-BFO
12 16
SL
5
-5
0
Phonon-enhanced near-field
14Re(εA)= - Re(εB)
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perovskitemetamaterial
2dd d
Ferroic superlensing
S.C. Kehr et al., Nature Comm. 2, 249 (2011)S.C. Kehr et al., Optical Mater. Express 1, 1051 (2011)
e1= e1 (w) e1= e1 (w)e2= e2 (w)
S.C. Kehr et al., ACS Photonics 3, 20 (2016)
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The Future –
Time-resolved SPM
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#6: Pump-probe Kelvin probeforce microscopy (pp-KPFM)J. Murawski et al., J. Appl. Phys. 118, 154302 (2015) J. Murawski et al., J. Appl. Phys. 118, 244502 (2015)
#7: Time-resolved SNOM (tr-s-SNOM)F. Kuschewski et al., Sci. Rep. 340, 6136 (2015)
Ch. Loppacher et al., Nanotechnology 16, S1 (2005)
#8: Optical pump – electrical probe KPFM (op/ep-KPFM)
The Future
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Summary
• Pz , r , Uint , esurface , ebulk
• dij
•F, r, UCPD , CQ
• e(w) , a(w)
• N, m*, µ
• time-resolved properties
• dissipation
• nonlinearities, higher-order SPM
• …