school of science –faculty of physics institute of applied

School of Science – Faculty of Physics Institute of Applied Physics

[email protected]

Downtown Dresden

“Probing nanoscale physical propertiesin oxides by scanning probe methods”

Lukas M. EngIAP, TU DresdenGermany

5th ISOECargèse, FAugust 24 –September 3 2021

Excellence Univ. 2019 - 2025

< 2 %

ct.qmat:Complexity

and Topology in Quantum

Materials Excellence Universities

Prague

School of Science – Faculty of Physics Institute of Applied Physics

[email protected]

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

Nobel prize in physics, 1986IBM Rüschlikon

Switzerland

Scanning Tunneling Microscopy: STM

G. Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 (1982)

STM-Discovery

J. Repp et al., Phys. Rev. L.

94, 026803 (2005)

„Seeing“ wavefunctions

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)

• 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)

AC-STM

G.P. Kochanski, Phys. Rev. Lett. 62, 2285 (1989)

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

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

Sichtbar - unsichtbar

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

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


(Non) Contact SFM

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

… pick the low-hanging fruits …

#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)

M. Abplanalp et al., Appl. Phys. A

66, S231 (1998)

PFM

BTO (100)

L.M. Eng et al., Appl. Phys. Lett. 74, 233 (1999)

PFM – “X-ray”

Euler angles

dij

BaTiO3 ceramic

K. Franke and L.M. EngSurf. Sci. 600 4896 (2006)

PFM & PFS

PFM: piezoresponse force microscopyPFS: pull-off force spectroscopy

PZT (53/47)


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.


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)


PFM & PFS

Results 2:• A• A• A

•Pz , r , Uint , esurface , ebulk

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

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

Excitations

… focus on the higher-hanging fruits …

Force

Distance


• 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


Non-Contact interactions

Force

Distance h


Non-Contact interactions

1 2 3 4

1

w

W

W

h

w

W

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

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).

Dissipated Power

ko = 102 N/mwres = 105 HzQo = 105b = 0,1Aosc = 2 nmDAdrive = 10-15 m = 1 fm

with

1 aW

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• …

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)

Excitations

[m]

Au, Ag + different topologies thereof

Al, Ga, Ingraphene, TMDC, TI, 2DEG, multi-ferroics, …

#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

-

+ + + + +

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.

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

FM-KPFM


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


Frequency Spectrum

f0 = 151 kHz, fMOD = 2 kHz

1/f-noise

non-invasive FM-KPFM

UCPD = 5 mVscale x 10

UCPD = 50 mV

UCPD = 500 mV

dB

frequency [kHz]


Cu-TBPP

Cu (100)

Surface potential Cu-TBPPCu-TBPP: Copper-Tetrabuthyl-Porphyrin

fMOD

f0-fMOD f0+fMOD

f0+2fMODf0-2fMOD

sideband at f0-fMOD is measured via lock-in techniques

f0 f

Aforce gradient

FM Kelvin


f0 = 151 kHz, fMOD = 2 kHz

force gradientQuantum

capacitance

Frequency Spectrum

FM-KPFM


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

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

#3: Scanning Nonlinear Dielectric Microscopy (SNDM)

K. Honda et al., J. Phys.: Conf. Ser. 209, 012050 (2010)

Nonlinear Dielectric Microsc.

e(0 Hz)e(wp) e(2wp)


n-/p-doped semiconductors

5 layersP,n-doped

7 layersB,p-doped


DC = 10-22F

MOSFET Failure Analysis

MOSFET with 40 nm gate channels


e(w)

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

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

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

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

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

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

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

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)

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)

#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)

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)

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

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

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)

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)

The Future –

Time-resolved SPM

#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

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

• …

school of science –faculty of physics institute of applied

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