force microscopy experiments: fromnanotribology to molecular electronics

8/7/2019 Force Microscopy Experiments: FromNanotribology to Molecular Electronics

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Ultrahigh vacuum force microscopy


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Ultra-sensitive non-contact force microscope

combined with STM


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FN

v

FL

Friction in Every-day Life


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Friction on the Nanometer-scale: Atomic-Stick Slip

FN = 0.44 nN

Ediss = 1.4 eV(per slip)

Atomic stick-slip Friction loop

KBr(001)-crystal


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Velocity dependence of atomic friction

1 2 3 4 5 6 70,35

0,40

0,45

0,50

0,55

F

L

(nN)

ln (v / 1 nm /s)

E. Gnecco et al., Phys. Rev. Lett., 84, 1172 (2000)

Friction increases with the logarithm of velocity

The slope of the curve increases with the applied load


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Interpretation of velocitydependence

1v

vln)(

TkFvF BLL +=

Tomlinson model: thermal activation:


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Transition to Ultralow Friction on NaCl(001)UHV FFM with sharp tip vs. Prandtl-Tomlinson model

kx = 29 N/m, kz = 0.05 N/m, vx = 3 nm/s, const. zScans along [100] showing maximum variation

Stick-slip Continuous sliding in contact! > 1 < 1 mean load FN = Fz + 0.7 nNA. Socoliuc et al., Phys. Rev. Lett. 92, 134301 (2004)


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1d-Prandtl-Tomlinson-Model

20 )(212cos

2st

t xxkaxEU +

= Potential energy:

Stability criterion:

< 1: unique sliding solution > 1: instabilities

2

22

02

2

0

2

ka

E

ka

E

==

02cos2

02

2

2

2

>+

=

k

a

xE

ax

U t

t

> 1 < 1

= 3 = 1


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Instability Criterium

1d: Effective spring constant equals 2nd derivative ofadiabatic potential between tip and sample

Lateralforce

x


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Dependence of Tomlinson Parameters

max

0 L

aFE =

1

2

exp

max

= akFL

exp

1kk

+=

maxLF

expk

E0: linear increasewith normal forces

k: rather independent(contact area const.?)


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Lateral contact stiffness

contacttiplever kkkk

1111

++=

R. Carpick et al, Appl. Phys. Lett. 70, 1548-1550 (1997)

Here: klever=29N/m k kcontact 1-2N/m

Continuum model :

kcontact= 8 a G a < 1 ?

Atomistic model needed


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Dependence of Tomlinson Parameters

max

0LaF

E =

1

2

exp

max

= akFL

exp

1kk

+=

maxLF

expk

E0: linear increasewith normal forces

k: rather independent(contact area const.?)


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Lateral contact stiffness

contacttiplever kkkk

1111

++=

R. Carpick et al, Appl. Phys. Lett. 70, 1548-1550 (1997)

Here: klever=29N/m k kcontact 1-2N/m

Continuum model :

kcontact= 8 a G a < 1 ?

Atomistic model needed


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MD-simulations on Copper:Shear along (111)- face

M.R. Soerensen et al. Phys. Rev. B 53, 2101 (1996)

Typical lateral forces =0.5-2nN

5-30 atoms in contact
http://localhost:8080/diploma/results/tip_setups/cube-111


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Simulations: KBr cluster tips against KBr(001)10x10x6 slab (fixed boundaries), SciFi code (L.N. Kantorovich et al.)

Buckhingham short-range + shell model Coulomb pair potentials

U. Wyder, A. Baratoff, E. Gnecco, T. Trevethan and L. N. Kantorovich

[001]

[111]

single

[011]

Tip top layer(s) frozen; scans at constant z (corrugation unaffectedby van der Waals attraction which, together with soft cantilever kz causesjump to/from contact in experiment

cube orstub

st

ab

stab pyramid

ion [100] edge

flat bottom
http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111http://localhost:8080/diploma/results/tip_setups/cube-111


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Quasi-static atomistic simulation using pair potentials

L. Kantorovich, T. Trevethan, Kings College London

U. Wyder, A. Baratoff, University Basel

Atomistic Simulation of theTip-Sample Interaction


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[111] K-terminated Tip

-1 0 1 2 3 4 5

-0.2

-0.1

0.0

0.1

0.2

forward

backward

Lateralforce[nN]

y - displacement (units of a)

perpendicular to tip edge

-1 0 1 2 3 4 5 6

-0.3

-0.2

-0.1

0.0

0.1

forward

backward

Lateralforce[nN]

x - displacement (units of a)

parallel to tip edge Reasonable contact stiffness ofabout 1-2N/m


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Commensurate [001] Tip: [100] scans

0.0 0.5 1.0 1.5 2.0-1.0

-0.5

0.0

0.5

1.0

forward

backward

Lateralforce[nN]


0.0 0.5 1.0 1.5 2.0-1.0

-0.5

0.0

0.5

1.0

Lateralforce[nN]


MD (Berendsen)

filtered

Pyramid with 2 x 2 apex in contact (Fz 0)a model forcreation and pickup

of wear debris Apparent stick slipbehaviour while clusterpasses under tip

Low-pass filtered traces from MolecularDynamics simulation at 300 K under thesame conditions reproduce the resultsof energy minimizations at closely spaced

x intervals + fluctuations between succesivestick-slip events.


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Complex motions: 110-Tip

Nano-Walker

C


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Along direction

100nm x 100nmf0=155.2kHz, df=-20.4Hz, Q=25067 A=7nm

Heteroepitaxial growth of KBr on NaCl

Homogenously distributed islands, two and threemonolayer high, but none with one monolayer

->No carpet growth

Superstructure with a periodicity of 3.95nm along the-direction is found, which fits well to the 6:7 ratioof the lattice constants .

Corrugations between 0.4 and 1.3 are observed

10nmx10nmf0=152.6kHz,df=-28.4 Hz, Q=38000 A=11nm200nm x 200nmf0=153.6kHz df=-5Hz, A=11nm,Q=38000

Contact mode imaging of hetero structures:


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Contact mode imaging of hetero-structures:KBr/NaCl(001) super-structure

Contact mode imagingof a super-structure

(Moire-pattern)

Super-Periodicty of 6x6 KBr-units observedCorrugation of about 0.1nm


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Monte Carlo Simulations of KBr/NaCl(001)

J. Baker and P.A. Lindgard, Phys. Rev. B54, R 11137 (1996)

17% misfit leads to superstrucutre6 KBr-units fit on 7 NaCl-units

Rumpling of NaCl-interface is observed on the KBr-surface (0.01nm for 2ML)

Agreement with Helium scattering data; Duan et al. Surf. Sci. 272, 220 (1992)


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Atomic friction of KBr/NaCl(001)

Super-structure is observed in atomic frictionAtomic-scale defects are observed!

Loading dependence of atomic friction:


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FN=0.01 nN

FN=-0.31 nNClose to jump-off

0 2 4 6 8 10

-0.1

0.0

0.1

friction

force(nN)

distance (nm)

0 2 4 6 8 10

-0.1

0.0

0.1

frictionforce(nN)

distance (nm)

Loading dependence of atomic friction:KBr/NaCl(001) superstructure

Superstructure is less pronounced at lower loads

10x10nm2:Friction force map at 2 loads

Variations of the slope of the sticking phase ithin the nit


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Variations of the slope of the sticking phase within the unitcell: Effect of(FN)/E0(FN) or contact stiffness k?

kk1

exp +=

2

22

02

2

0

2

ka

E

ka

E

==

0 1 2 3 4 5

-0.1

0.0

0.1

0.2

S. Maier et al., unpublished results

frictionforce(nN)

distance (nm)

0.33 N/m

0.41 N/m

0.37 N/m

0.47 N/m0.49 N/m

0.34 N/m

0.0 0.4 0.8 1.2 1.6

0

50

100

150

200

numberofpoint

s

stiffness (N/m)

=0.36 N/m=0.1 N/m

C it h f i ti d ff?


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Can we switch friction on and off?

AC voltages were applied across thin KBr and NaCl crystals:

Capacitive interaction between lever and sample holder U2

Coulomb interaction UB

B

Controlling Friction: Actuation of Nanometer-


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Controlling Friction: Actuation of NanometerSized Contacts

A. Socoliuc, E. Gnecco, S. Maier, O. Pfeiffer, A. Baratoff, R. Bennewitz, E. Meyer, Science313, 207 (2006)

F d d f f i ti


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Frequency dependence of friction

Friction:

Thermalnoise:

Friction is switched off only if fexc = fnorm or (1/2) fnorm

No effect when fexc = ftors !

F d d f f i ti


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Frequency dependence of friction

Reduction of friction at and resonanceFrequencies of the the bending modes f and f/2

Voltage dependence of friction


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Voltage dependence of friction

Friction goes down to zero increasing the excitation amplitude !

I t t ti f D i S l b i it


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Interpretation of Dynamic Superlubricity

In the Tomlinson model: We replace E0 with E0 (1+ cos t) The parameter increases with the applied voltage

Friction decreaseswhen 1

= 7

= 5

= 3

= 1

E0

New parameter


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New parameter min

The parametermin = (1-) determines superlubricity

Static case

Phase diagram of friction


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Phase diagram of friction

A "phase diagram" in the - plane can be drawn:

"Static" SL

=1

1cr

A. Socoliuc et al., Science 2006

Modulation of the Energy barrier


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gyby actuation of the nano-contact

Standard parameters for the Tomlinson model with excitation:

0.5 times critical dampedParameters: eta=4, alpha=0.9, f=567Hz, v=10e-9m/s, gamma=1e-6kg/s,m=8e-13kg, a=0.5e-9m, c=1N/m

Ultralow friction on the macroscopic scale?


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Ultralow friction on the macroscopic scale?

Normal force per asperity is limited to 1nN

Macroscopic weight of 1g 10mN has to be distributed

to 107 mini-tips ( array of 3000 x 3000 tips)

10mm

Tips with aspacing of 3m


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IBM Zurich1000 AFMs

MEMS Devices


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MEMS-Devices

Courtesy of Sandia National Laboratories, SUMMiTTM Technologies,www.mems.sandia.gov"

Gecko uses nanometer-sized contacts


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to climb walls

Gecko is able to control thecontact area on all length scales

From B. Persson and S. GorbJCP, 119, 11437 (2003)

Other forms of "superlubricity "


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Other forms of superlubricity

Thermolubricity (Krylov et al., PRE 2005):

(taking "backjumps" into account friction vanishes at low speed)

Other forms of "superlubricity"


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Other forms of superlubricity

Structural lubricity (Dienwiebel et al., PRL 2004):

(two mismatched graphite flakes sliding past each other)

Nanomotor with Multiwalled Carbon Nanotubes:f


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Low friction and negligible wear

A. Zettl, University of California Berkeley

A. Fennimoore et al., Nature424, 408 (2003).Sliding of rolled graphene!

Transitions to negligible friction in differnt dry contacts


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M. Dienwiebel et al. PRL 92, 126101 (2004) A. Socoliuc et al. PRL 92, 134301 (2004)

0.0 0.5 1.0 1.5 2.0 2.5 3.0-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

stick-slip

"superlubricity"

FL

(nN)

x (nm)

FN

Effective corrugation on graphite reduced at low velocities at fixed room T

S.Yu Krylov et al. PRE 71, 065101(R) (2005)

Graphite against rotated flake picked up by tip NaCl(001) cleaved in UHV

Thermal activation:

small velocity ~ high temperature

Like creep of dislocationsor vortices in type II superconductors

Conclusions


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Conclusions

Observation of ultralow friction without atomic-stick slip

Continuum elasticity theory is not applicableModelling needing

Superlubricity is observed for low loads, low velocity orincommensurate structures

Wear can be avoided at low loads

Control of friction: Switch off/on friction

force microscopy experiments: fromnanotribology to molecular electronics

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