STM and AFM Tutorial

Katie Mitchell

January 20, 2010

Overview

• Scanning Probe Microscopes

• Scanning Tunneling Microscopy (STM)

• Atomic Force Microscopy (AFM)

– Contact AFM

– Non-contact AFM

• RHK UHV350 AFM/STM

Scanning Probe Microscopes

• Sharp probe tip is scanned across surface

• Piezoelectric materials provide atomic scale control of x,y,z motion

• Detect signal that reflects topography or other spatially varying property of interest

• Good vibration isolation required

Scanning tunneling microscope

Scanning Tunneling Microscopy

Image from Will's Nanotechnology Blog

V ~ 0.1 - 2 V

I ~ pA – nA

can operate in

air, liquid or

vacuum

sample and tip

must be

conducting

Scanning Tunneling Microscopy

zw n 2exp02

2

2

m

Tip

Sample

Tip

Sample

EF

EVacuum (0 eV)

a)

b)

e-

Tip

Sample

c)

e-

EF

EF

EF

EF

EF

eVeV

z

1D Potential Barrier

Current-Distance Dependence

0.0 0.5 1.0 1.5 2.0 2.5 3.0-7

-6

-5

-4

-3

-2

-1

0

1

Natu

ral

Lo

g o

f T

un

neli

ng

Cu

rren

t

Tip Displacement (Å)

= 1.0±0.1 Å-1

W and Pt-Ir tips on Si(111)-7x7

M. Rempel, MSc Thesis, Univ. of Saskatchewan, 2003

Scanning Tunneling Microscopy3D Picture

EEMw 22

dMEeVEe

I FF

eV2

0

4

dEeVEe

I FF

eV

0

4

Transition probability

Tunneling current

Assuming s-wave tip state

STM on Au(111)

60°

120°

100 x 100 nm, 25 mV, 1.15 nA

Au(111): High Resolution

5 x 5 nm, 90 mV, 1.09 nA50 x 50 nm, 25 mV, 1.15 nA

Highly Ordered Pyrolytic Graphite (HOPG)

HOPG lattice

5 nm x 5 nm

STM image

0.1V, 1.0 nA

Pt-Ir tip

Images,

Mark Rempel

Si(111) – 7x7

Model of Si(111)-7x7

surface structure; the side

length of the surface unit

cell is 26.6 Å

-1.5 V, 0.42 nA - filled states

+1.5 V, 0.50 nA - empty states

STM images, Dale Heggie

Si(111) – 7x7

+1.5 V, 0.12 nA +1.5 V, 0.78 nA +1.5 V, 0.80 nA

+2.0 V, 0.34 nA STM images, Dale Heggie

InP(100)

-1.81 V, 0.35 nA -2.0 V, 0.23 nA

STM images, Dale Heggie

InP(100)

-1.8 V, 0.58 nA

“ghost” tip images

noise

Scanning Tunneling Spectroscopy

• Measure I(V)

• Calculate sample density of states

eVE

V

I

dV

dI

F

Si(111)-7x7

• (dI/dV)/(I/V) as a function of

sample bias, for Si(111)-7x7

• (a) – (d) W tip

• (e) – (f) Pt-Ir tip

• features can be compared

to expected density of states

for Si(111)-7x7 surface

Figure, Mark Rempel

STM Tips

• Most common tips

– Electrochemically etched W wire

– Mechanically cut Pt-Ir wire

• Desirable properties

– Conducting

– Hard

– Inert (for imaging in air)

– Best shape varies with application

a)

b)

SEM images

100 m 1 m

Tungsten STM tip imaged at 150x and 20,000x magnification

19

DC Drop-off Method for Tungsten

• Anode (W wire) and cathode submerged in aqueous solution of NaOH or KOH

• W wire etches and breaks at the air/solution interface

• Remaining wire is used as the tip

C.J. Chen, Introduction to Scanning Tunneling Microscopy. OUP (1993)

20

Etching Mechanism

AnodeW(s) + 6OH- WO3 (s) + 6e- + 3H2O

WO3(s) + 2OH- WO42-(aq) + H2O

Cathode6H2O + 6e- 3H2(g) + 6OH-

OverallW(s) + 2OH- + 2H2O 3H2(g) +WO4

2-(aq)

G. Stephen Kelsey, J. Electrochem. Soc. 124(6), 814 (1977).

21

Etching Apparatus

Atomic Force Microscopy

Image from Will's Nanotechnology Blog

measure deflection of

a cantilever of known

spring constant, due to

tip-surface interaction

forces

F ~ nN

can operate in air,

liquid or vacuum

sample and tip may be

conducting or insulating

Atomic Force Microscopy

Wikipedia

• Contact Mode– Operate in region of repulsive tip-sample forces

– Scanning at constant cantilever deflection results in contour map of constant force, which represents topography

– Twisting of cantilever is a measure of lateral (friction) forces

– Contact mode can be very useful but can also be damaging to delicate surfaces

Atomic Force Microscopy

Wikipedia

• Non-Contact Mode (NC AFM)– Operates in region of attractive (van der Waals) forces

– Cantilever is forced to oscillate at its resonant frequency (100-300 kHz)

– Change in force experienced by tip changes the resonant frequency

– Change in resonant frequency is used as the feedback signal

– Very high Q cantilevers allow very small changes in resonant frequency to be detected, and thus small changes in force

– Ideal for delicate samples, and can give atomic resolution

Force-Distance Curves

Shahin, V. et al. J Cell Sci 2005;118:2881-2889

AFM Cantilevers and Tips

• Microfabricated cantilevers with integrated tips

– Silicon nitride, silicon oxide, silicon

– Spring constants: 0.1-1 N/m (contact), 10-100 N/m (non-contact)

– Resonance frequencies: 1-50 kHz (contact), 100-300 kHz (non-contact)

– Coatings depend on application: eg. conducting, magnetic, functionalized (specific molecules)

SEM image of cantilever

Cantilever Example

• Nanosensors PPP-NCL-10– Intended for non-contact mode

– Fabricated from n-type silicon

– no coating

– Specifications• t = 7.00 microns

• w = 38 microns

• l = 229 microns

• h = 10 – 15 microns

• c = 47 N/m

• f = 184 kHz

• Tip radius ~ 7 nm

RHK UHV 350 AFM/STM

• full range of AFM acquisition modes• AFM, STM or simultaneous AFM/STM

Sample holder

Cantilever

RHK Technology Inc.

Position sensitive detector

Design Features• Sample stationary, tip/cantilever scanned

• “Beetle” or “Walker” design– Piezo legs walk down ramp due to slip/stick

• Piezo legs used for approach and scanning

• Central piezo stack used for z motion

• Laser deflection and 4-quadrant PSD

AFM, Wikibooks

Design Features

• Head accepts STM tip or AFM cantilever holders

• Tip and cantilever holders can be transferred on and off in vacuum

• Sample may be heated or cooled while scanning

• Vibration isolation

– External airlegs

– Viton stack supporting sample stage

– Scan head designed to have high resonant frequencies

– Scan head decoupled from chamber

Sample Holders

Thermocouple connections

Sapphire washer to hold sample in place

Ramps for piezo legs

Control Hardware

• SPM 1000 controller

– Primary interface

– Front-panel controls

• PLL Pro

– Digital controller for AFM

• PMC 100 Piezo Motor Control

– Controls laser lens and position sensitive detector motors for AFM

• Small piezo control box (for X,Y,Z)

Control Software

• XPMPro

– STM/AFM data acquisition, processing and analysis

– May be used free for data analysis on any computer running Windows (XP, Vista)*

• PLLPro

– Controls the AFM mode, and all setup and parameters for AFM operation

*http://www.rhk-tech.com/downloads_files_V2.php

Specifications

From RHK

product brochure