Scanning tunneling microscopy STM

and

atomic force microscopy AFM

• The components of a scanning probe microscope SPM

• The scanner

• Measurement of the distance between surface and tip

• The cantilever

• Basic principles of scanning tunneling microscopy STM

• The measurement modes

• Atomic force microscopy AFM

• The different modes for the AFM technique

• Scanner motion

• Size of the tip and resolution

• The SPM device in our laboratory

• Examples

Content

Motivation

• Digitally image a topographical surface• Determine the roughness of a surface sample or to measure the

thickness of a crystal growth layer• Image non-conducting surfaces such as proteins and DNA• Study the dynamic behavior of living and fixed cells• Nanolithography• manipulate individual atoms - nanoscience

Tunneling Microscopy

Atomic Force Microscopy

Contact Atomic Force Microscopy ⇒ C-AFM

Non-Contact Atomic Force Microscopy ⇒ NC-AFM

Intermittent Contact Atomic Force Microscopy ⇒ IC-AFM

Scanning Tunneling Microscopy ⇒ STM

Scanning Probe Microscopy ⇒ SPM

The different images methods

Principal components of a scanning probe microscope

The Scanner

Piezoelectric materiallead zirconium titanate PZTchanges dimensions when

a voltage is applied

Measurement of the distance between surface and tip

Bending of the cantilever shifts the position of the laser on the position sensitive photodetector

The cantilever

50 µm

100 -200 µm long, 10 - 40 µm width, 0.3 - 2 µm thick; spring constants f(shape, dimension, material); spring constants: n x 1000 N/m to n x 1/10 N/m

Convolution Effects

One thing to keep in mind : convolution effect

„The smaller thing images the bigger thing“

The signal is always a convolution of sample topography and tiptopography

Tips should be as sharp as possible (10nm standard)

Klassische Physik

d

Quantum mechanics

Tunneling of electrons:the charged particles can travel from occupied states

through a potential barrier to unoccupied states

IT ~ e-d

Theory

Apply a low potential between tip and surface

Illustration of the tunnel-tip surface junction

Potential: 0.5 - 10 V; current: 0.1 - 1 nA, distance: 0.3 - 1 nmconvention: negative tunnel voltage means electron emission from the sample

About 10 Å: overlap of WF

The tunneling process

Tunnel current density It (Bethe & Sommefeld):

3·e02 ·x

It = ———— ·Vtexp(-2xd) 8 ·hhhh ·ππππ2 ·d

e0 = elemental charge = 1.602 ·10-19 As; hhhh = Planck constant = 1.054 ·10-34 Jsd = distance tip to surface Å; x = √√√√2 ·m0 ·ΦΦΦΦ/ hhhh = 1/2 √Φ√Φ√Φ√Φ = decay curveof a wave function in the potential barrier; ΦΦΦΦ = average barrier height = surface potential in eVwith these units: 2 ·x is about 1.025 · √Φ√Φ√Φ√Φ eff. With ΦΦΦΦ eff = several eV, It changes by a factor of 10for every Å of d !

Quantum well

EVakuum

EFermi

Φ Austrittsarbeit

Φ sehr materialspezifisch

Work function

•Depends on material !

Determination of local work function

EisenNickel

Different metals

Contact between two metals

E Vacuum

E F

Φ WF

E Vacuum

E F

Φ tip

surface tip

a)

b)E Vacuum

E F

Φ tip

E Vacuum

E F

Φ WF

With external voltage (bias)

ΦOF

Φtip

sample tip

a)

b) U=0 U<0U>0

tiptiptip

sample

ener

gy

sample

sample

The tunnel microscope

The measurement modes

Constant height mode:faster, scanner always

at the same heightrequires very smooth

surfaces

Constant current mode:high precision, irregular

surfaces, timeconsuming

dangerous for partially oxidized surfaces !

First STM images by Binnig and Rohrer 1982

„Planar“ gold surface

First STM images by Binnig and Rohrer 1982

3D image of gold surface

Si(111) 7x7

← STM image

scheme

↓

Gold on Ni surface

NiAu

Important: Density of state around the Fermi energy

Density of states around the Fermi energy

ΦOF

Φtip

sample tip

ΦOF

Φtip

samples tip

D(E)= density of states

D(E) D(E)

D(E) D(E)

ener

gy en

erg

y

Constant current contour

Bias voltage

e- Distance s

eSample

eee

VDC

Density of state around the Fermi energy

AFM - Forces between tip and surface

• Van der Waals force: always present, attractive, outerelectrons, long distance

• contact force: repulsion, chemical, core electrons

• capillary force: attractive, water layer!

• electrostatic and magnetic force

• friction force

• forces in liquids

J. Israelachvili: Intermolecular and Surface Forces with Appl. toColloidal and Biological Systems, Academic Press (1985)

AFM

AFM: Forces vs. distance

Tip is mounted at the end of a cantilever, interaction with sample: attractive or repulsive

Tip a few Å abovesurface

Tip 10 - 100 Å above surface

C-AFM NC-AFM

AFM: the different operation modes

Repulsive mode, soft physical contact,

cantilever with low spring constant, lower than holding

atoms together; total force exerted

on the sample: 10-8 N to 10-6 N; cantilever is bended

Attractive mode, cantilever vibrates near surface,

distance to surface 10 - n x 100 Å,

total force exerted on the sample: 10-12 N; frequency

is kept constant during movement = constant

distance tip-surface

• Uses attractive forces tointeract surface with tip

• Operates within the van derWaal radii of the atoms

• Oscillates cantilever near itsresonant frequency (~ 200kHz) to improve sensitivity

• Advantages over contact: nolateral forces, non-destructive/no contaminationto sample, etc.

van der Waals forcecurve

Non-Contact Mode

• Contact mode operates inthe repulsive regime of thevan der Waals curve

• Tip attached to cantileverwith low spring constant(lower than effective springconstant binding the atomsof the sample together)

• In ambient conditions thereis also a capillary forceexerted by the thin waterlayer present(2-50 nm thick).

van der Waals force curve

Contact Mode

Using Vibrating Tips

No permanent tip-sample contact

No shear forces

Tapping Mode, Intermittent Contact Mode And Non-Contact Mode are themost successful methods for pure imaging

Advantages :

Non-contact imaging possible

Feedback parameter : Amplitude

• The cantilever is designed with avery low spring constant (easy tobend) so it is very sensitive to force.

• The laser is focused to reflect offthe cantilever and onto the sensor

• The position of the beam in thesensor measures the deflection ofthe cantilever and in turn the forcebetween the tip and the sample.

Force measurement

• The tip passes back and forth in astraight line across the sample (thinkold typewriter or CRT)

• In the typical imaging mode, the tip-sample force is held constant byadjusting the vertical position of thetip (feedback).

• A topographic image is built up bythe computer by recording thevertical position as the tip is rasteredacross the sample.

Sca

nn

ing

Tip

Ras

ter

Mo

tio

n

Top Image Courtesy of Nanodevices, Inc. (www.nanodevices.com)Bottom Image Courtesy of Stefanie Roes

(www.fz-borstel.de/biophysik/ de/methods/afm.html)

Raster the tip: Generating an Image

• Tip brought within nanometersof the sample (van der Waals)

• Radius of tip limits the accuracyof analysis/ resolution

• Stiffer cantilevers protectagainst sample damagebecause they deflect less inresponse to a small force

• This means a more sensitivedetection scheme is needed

• measure change inresonance frequency andamplitude of oscillation

Image courtesy of (www.pacificnanotech.com)

Scanning the sample

Scanner motion during data acquisition

No signal detection

Minimises line-to-line registration error due to scanner hysteresis

Area size: 10 Å - > 100 µm

STM

AFM

The size of the tip and the resolution

IT: exponential dependence of distance tip-sample,closest atom of tip interacts with surface

atomic resolution is achieved

Several atoms of the tip interact with the sample surface, every atom of the tip „sees“ a shifted lattice

with respect to the lattice seen by the neighboratom

The size of the tip and the resolution

Time = zero Best lateral resolution: about 10 Å

Interpretation of STM Image