Expanding Characterization of Materials with Kelvin Force Microscopy

Sergei Magonov

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Outline

Introduction to Kelvin Force Microscopy

Diff t KFM M d d Th i P ti l E l tiDifferent KFM Modes and Their Practical Evaluation

Applications of Kelvin Force MicroscopyMetals and SemiconductorsMolecular Self-Assemblies

Conclusions

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Atomic Force Microscopy/Scanning Force Microscopy

Hi h R l i /L L l El i Other Properties

Air Vapors Liquid Vacuum

High-Resolution/Low-Force Profilometry

Local Mechanical Properties

Local Electromagnetic Properties

High/Low Temperatures

Other Properties:Optical, Thermal, …

Air Vapors Liquid Vacuum High/Low Temperatures

Local Electromagnetic Properties

Magnetic Force Microscopy, MFM Conducting AFM, c-AFM

Force sensing Current sensing

Scanning spreading resistanceElectric Force Microscopy, EFM: dF/dZ

Kelvin Force Microscopy, KFM (SKPM) :

surface potential, dC/dZ

Scanning spreading resistance microscopy, SSRM

Capacitance/Impedance sensingsurface potential, dC/dZ

Piezoresponse Force Microscopy, PFMScanning capacitance microscopy

Scanning microwave microscopy

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Kelvin Probe Technique (macroscopic method)

l A l S hl f A 2004 8 4801

Vs = ΔWf + Vox + φs

Surface potential of semiconductor

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B. Laegel, M. D. Ayala, R. Schlaf APL 2004, 85, 4801.

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Historical Pathway of AFM-based Electrostatic modes

Electric force microscopy∼

Metalized ProbeContours of constant

force gradient

Back electrode

force gradient

Contours of constant force gradient

J E Stern B D Terris H J Mamin and DY Martin D A Abraham H K Wickramasinghe

An example of a cross-talk

J.E. Stern, B.D. Terris, H.J. Mamin, and D. Rugar, Appl. Phys. Lett. 53, 2717, 1988.

Y. Martin, D. A. Abraham, H. K. Wickramasinghe, Appl. Phys. Lett. 52, 1103 ,1988.

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Electrostatic Force in Atomic Force Microscopy

Responses at ω and 2ω can be used ∼Metalized Probe p

for detection of surface charge and capacitance/dielectric constant.Back electrode

KFM feedback is the nullifying th l t t ti f t

M. Nonnenmacher, M. P. O’Boyle, H. K.

J. M. R. Weaver and D. W. Abraham, J. Vac. Sci. Techn. 1991, B9, 1559

the electrostatic force at ω.

, y ,Wickramasinghe, Appl. Phys. Lett. 58, 2921, 1991.

Electrostatic force at 2ω is proportional to dC/dZ

Problem: the AFM probe responds to all tip-sample forces!Single Pass – the use of 2 frequencies(ωmech, ωelec ) for probing mechanical and electrostatic tip-sample forces + operation in non-contact mode

Solution:

2 Path Lift technique - the use of single frequency (ωmech ) with probing the electrostatic force in non contact mode

Y. Martin, D. A. Abraham, H. K. Wickramasinghe, Appl. Phys. Lett. 52, 1103 ,1988

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V. B. Elings, J. A. Gurley US Patent 5,308,974, 1994.electrostatic force in non-contact mode

Advanced Consideration of Electric Force Measurements with AFM

Electrostatic energy and force

J. Colchero, A. Gil, and A. M. Baro Phys Rev B 64 (2001) 245403

Cone – Metal SurfaceCantilever – Metal Surface

, , y ( )

Apex – Metal SurfaceTotal force

Force gradient vs. Force !?F

S ti l R l ti i KFMSpatial Resolution in KFM

U. Zerweck et al Phys. Rev. B 2005, 71, 125424 – UHV

S. Kitamura et al Appl. Surf. Sci. 2000, 157, 222 Atomic-scale KFM resolution on crystals in UHV

KCl Au

KFM-FM

KFM-Lift

KFM-FMKCl

F K k t l Ph R B 2008 77 235427

M. Zhao et al Nanotechnology 2008, 19, 235704

KFM-AM

800 nm

530

KFM LiftBR

185 nm

InSb

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F. Krok et al Phys. Rev. B 2008, 77, 235427 530 nm185 nmUHV

Kelvin Force Microscopy in AM-AM and AM-FM modes

AM-AMAM-AM: the amplitude changes at ωelec reflect the electrostatic force variations .AM AMAM-FM: the electrostatic force gradient causes the phase shift at ωmech which is reflected in the amplitude of “satellites”.

Amplitude-versus-Frequency (sketch)

ωmechωelec

Heterodyne procedure

ωmech− ωelecωmech+ ωelec

AM-FM

A Non-contact

Imaging in the intermittent contact!

Intermittent contact

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ZUHV: L.Eng et al Phys. Rev. B 71 (2005) 125424

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Conducting Probes for AFM-Based Electrical Modes

TEMSEM TEM

Pt coated Ol mp sPt-coated Olympus probes

SEM SEM5 N/m, ~ 70 kHz

SEM images- courtesy of Maozi Liu (Agilent Labs)

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TEM image – courtesy of Bernard Mesa (MicroStar Technologies)

F(CF2)14 - (CH2)20H F14H20 F(CF2)12- (CH2)8H F12H80.60 nm

1 93 2 48 1 67 0 98

Semifluorinated Alkanes: Self-Assembly in Thin Films

CF -δ CH +δ

0.48 nm1.93 nm 2.48 nm 1.67 nm 0.98 nm

F14H20 F14H20 F14H20 F14H20

-CF2δ – CH2

+δ -

40 nm

500 nm1 μmF14H20 P fl d li

1 μm

Substrates: Si, mica, graphite Solvent: perfluorodecalin

F14H20: Perfluorodecalin vapor

A. Mourran et al Langmuir 2005, 21, 2308

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Self-Assemblies of Semifluorinated Alkane F14H20 on Si: EFM, KFM and dC/dZ Topography X-component (5kHz), KFM servo off

AM-FM

Electric Force Microscopy, EFM

1.2 μmSurface Potential (5kHz) KFM servo on dC/dZ (10kHz)

1.2 μm

Surface Potential (5kHz)

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1.2 μm1.2 μm

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Topography Surface Potential, AM-FM (Phase)Surface Potential, AM-AM

Comparison of AM-AM and AM-FM modes in study of F14H20 self-assemblies

Surface Potential, AM-FM (Phase)

Surface Potential, AM-AM Surface Potential, AM-FM (Y)

Surface Potential, AM-FM (Y)

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Spatial resolution of KFM (AM-FM) in the Intermittent Contact: F14H20 on HOPG

Topography Surface Potential Surface Potential TopographyTopography Surface Potential Surface Potential TopographyF14H20 F14H2010 nm

Topography Surface Potential Surface Potential Surface PotentialF H

1 μm 1 μm 3 μm500 nm

F14H20

2 nm

0.1V

1 μm 1 μm 3 μm200 nm

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0

0 25

0

0 25

0 0.5 1 1.5 2 µ0

0 25

Kelvin Force Microscopy of Substrates: Au (111) & HOPGTopography Surface Potential

Topography

PhaseHOPG Au (111)

0.25

0.5

0.75

1

1 25

0.25

0.5

0.75

1

1.25

0.25

0.5

0.75

1

1.25

0

2

4

6

8

10

Topography

µm

1.25

1.5

1.75

2µm

1.25

1.5

1.75

2µm

1.25

1.5

1.75

2

12

14

16

18

2 h ft l0

2

4

6

8

10

12

0 4 8 12 16 µm0

2

4

6

8

0

0

2

4

6

8

10

µ0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10Surface P t ti l

2 hours after cleavage +15 min +30 min +45 min +60 min +75 min

µm

12

14

16

18

m

2

4

6

8

12

14

16

18

12

14

16

18

12

14

16

18

12

14

16

18

0

2

0

20

20

2

0

2

0

2

Potential

4

6

8

10

12

14

16

18

4

6

8

10

12

14

16

18

4

6

8

10

12

14

16

2

4

6

8

10

12

14

16

4

6

8

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12

14

16

18

4

6

8

10

12

14

16

18

Phase

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Kelvin Force Microscopy & Compositional Imaging of Material Heterogeneities SiGe PMMA

topography surface potential topography surface potential

topography surface potential topography surface potentialTPV (polypropylene, EPDM, carbon black) C60H120

25 μm 25 μm 8 μm 8 μm

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5 μm 5 μm 8 μm 8 μm

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Monitoring Changes of Soldering Materials: 58%Bi-42%Sn Alloy

topography surface potential1 hr after preparation Bi 4.22eV Sn 4.42 eV

8 μm 8 μm

topography surface potential15 hr after preparation

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8 μm 8 μm

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Topography Surface PotentialKFM of Semifluorinated Alkane F14H20 Assemblies on Si Substrate

Zmean(2) – Zmean(1) 0.723 V

Zmean(2) – Zmean(1) 0.757 V

F(CF2)14 - (CH2)20H-CF2

-δ – CH2+δ -

1.93 nm 2.48 nm

CF2 CH2

A. El Abed et al PRE 2002, 65, 051603

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Topography Surface Potential

KFM of Semifluorinated Alkane F14H20 Assemblies on Mica Substrate

Zmean(2)–Zmean(1) 1.51 V

-+

μ=3.1D

3 μm 3 μmTopography Surface Potential A. Mourran et al

Langmuir 2005, 21, 2308

Zmean(2)–Zmean(1)1.44 V 0

)(εε

μϕϕ⋅⋅

+−

−=Ae

V tipsi

CPD tipsi )( ϕϕ −−=

e

VoltsCPDV 39.1+=

1.5 μm 1.5 μm

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KFM of Semifluorinated Alkane F14H20 Assemblies on HOPG substrateTopography Surface PotentialPhase Zmean(2) – Zmean(1)

0.73 V

700 nm700 nm 700 nmTopography Surface Potential

111 2 2

19.0 nm11

2 22 2

200 nm200 nm

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KFM of Semifluorinated Alkane F14H20 Assemblies on Si substrate: Humid Air

Topography Topography Topography TopographySpreading of self-assemblies and conversion from spirals to toroids

Zmean(2) – Zmean(1) 0 48 V

Topography Topography

3 μm 1 μm1 μm 1 μm

Surface Potential0.48 V

1 1

400 nm400 nm400 nm

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0 min 44 min

Monitoring of Sublimation of F12H8 Self-Assemblies on Mica

TopographyTopography 115 min 148 minTopography Topography

800 nm 800 nm 800 nm 800 nm

55 minTopography Surface Potential Topography Surface Potential

800 nm 800 nm

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Monitoring of Sublimation of F12H8 Self-Assemblies on GraphiteTopography Surface Potential Topography Surface Potential

1 31 3

1.4 μm 1.4 μm 1 μm 1 μm

Topography Surface Potential Topography Surface Potential2 42 4

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1.4 μm 1 μm1.4 μm 1 μm

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Monitoring of Sublimation of F12H8 Self-Assemblies on GraphiteTopography Surface Potential Topography

Surface Potential

700 nm 700 nmTopography Surface Potential

Surface potential of FnHm (n=12, 14; m = 20, 12, 10, 8) self-assemblies does not depend on the molecular length being determined primarily by g g p y ymolecular dipoles of -CF2

-δ – CH2+δ -,

-CF3 and –CH3 groups and their orientation.

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500 nm 500 nm

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topography phase surface potential

Self-Assembly of CdTe Nanoparticles into Nanowires

After spin

Z. Tang, N. A. Kotov, M. Giersig Science 2002, 297, 237

After spin-cast in 90% humidity

5 μm 5 μm 5 μmtopography phase surface potential

Sample in 90% humidity after night

5 μm5 μm5 μmtopography phase surface potential

Dried

High luminescence quantum yields

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topography surface potential topography surface potentialSelf-Assembly of CdTe Nanoparticles into Nanowires

300 nm 300 nm

0.8 μm0.8 μmtopography (210 min later) surface potential

S. Shanbhag, N. Kotov, J. Phys. Chem. B, Let 2006, 110, 12211

Cubic NCs: ZnS, CdS, ZnSe, PbSe, and CdTe

μ

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1.2 μm1.2 μm

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ConclusionsExpansion of AFM applications strongly depends on practical implementation of a combination of high-resolution capabilities with mapping of local properties in broad range of environments. Here the unique capabilities of KFM as the characterization technique down to the sub-100 nm scale were demonstrated on a number of materials.

The progress of AFM-based local electric (as well as mechanical) studies is based on use of multiple frequencies and a broader frequencymultiple frequencies and a broader frequency range. Analysis of multifrequency responses leads to more sensitive and high-resolution mapping of these properties.

Nanotechnology 18 (2007) 065502R W Stark, N Naujoks and A Stemmer

The multifrequency approach in AFM and related techniques requires definite efforts towards optimization of experiments, which include a choice of rational detection schemes, an appropriate choice of probes and imaging conditions. There is no doubts that the payoff will be quite valuable and unique information about materials, their properties and behavior will be discovered

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their properties and behavior will be discovered.

Acknowledgements

John Alexander (Agilent Technologies) - for everyday cooperation on AFM developments and applicationsp pp

Martin Moeller (RWTH--DWI, Aachen, Germany)

Nicholas Kotov (University of Michigan Ann Arbor MI)Nicholas Kotov (University of Michigan, Ann Arbor, MI)

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