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Instrumentationand
Operation
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STM Instrumentation
• sharp metal tip• scanning system and
control electronics• feedback electronics
(keeps tunneling currentconstant)
• image processing systemdata points → image
• (off-line data analysis)
COMPONENTS
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STM Tipsetched W
cut Pt/Ir80:20
50 µm
profile
profile
α
α
→ for wide scansgood forimaging ofsteep edges
→ good foratomic resolution
not perfect for atomic resolution
cutting with e.g. scissors→ no universal recipe !
(possible)
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Tip Preparation• cutting• cleaning
– heating, oscillation(may produce blutness or lead to melting)
– voltage pulses– continuous scanning
• e.g. increasing voltage while scanning withAu tip → 25 Å elongation→ higher resolution
• sputtering• however: good results also without
cleaning !
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Chemical Composition of STM Tip Surface (1)
resolution depends on polarity→ due to adsorbed impurity atom
_ +
Na
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STM Tip Composition• theoretically better resolution for
d – elements→ more localized electrons (e.g. Na ↔ W)
• however Au (s – band) also high resolution→ identity of tip atom not known
• noble metals less susceptible to contaminationse.g. W→ always layer of oxide +
other contaminations→ but high activation energy for surface
diffusion (1,8 eV)
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Quality Control of STM Tipscourse → optical microscope:
bad performance can beexpected !
STM experiment → image quality(check different tips)[field ion microscope]
→ more information → AFM
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Single Tube Scannermaterial: lead zirconium titanate
ceramics (PZT)
metal electrodes by vapor deposition
νres ∼ 12 – 20 kHz (bar: 1 – 8 kHz)
non linear
cross talk
translation of tip or sample
Y X-X
Z
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Scan Parameters• X – frequency: 0,1 – 122 Hz (NanoScope, DI)• Y – frequency according to number of lines per
image– 128 x 128 data points per image– 256 x 256 data points per image– 512 x 512 data points per image
• ⇒ 1 second to 1,5 hours for 1 imagetypically 30 seconds to 1 minute
• scan angle, size, frequency, ...can be varied
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Feedback System• bias voltages: some 10 mV (up to several
volts)• tunneling current: some nA• modes of operation
– constant height mode→ small areas→ high scan rates possible⇒ elimination of thermal drifts, high resolution
imaging– constant current mode→ low scan rates→ wide area scans→ lower risk for tip – crashes
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Image Processing System• interface: analogue → digital (data to PC)
– lateral resolution128 x 128 pixels256 x 256 pixels512 x 512 pixels
– z – scale: 64 k resolution• off – line analysis
– image analysis– filtering– zooming– etc.
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AFM Instrumentation
• sharp tip + soft spring• scanning system and
control electronics• detection for spring defl.• feedback electronics
(keeps force constant)• image processing system
data points → image• (off-line data analysis)
COMPONENTS
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Tips and Springs (Cantilevers)• historical: gold foil (Binnig)• first developments:
fine wires (e.g. W) with diamond tips (glued ontowire)
• today:microfabricated cantilevers (not to scale !)
W – wire
diamond tip
100 µm200 µm
1,5 mm4 m
m
Si3N4
integratedpyramidal tip
fabrication: photolithographicmulitstep process starting froma silicon wafer
k = 0,1 – 1 N/mν0 = 10 – 100 kHz
glasssubstrate
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Vibration Problem
• typical vibrations of buildings: ν < 20 Hz• damping factor = (ν/ν0)2 for ν << ν0
• ⇒ amplitude of tip < 0,01 Å• ⇒ forces from 10-7 – 10-11 N can be
detected
• high frequency vibrations and noisemust be eliminated !
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Vibration Isolationheavy stone supporton bungy cords
low resonance frequencies
optional noise isolation
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Si3N4 Cantilever Wafer
approximately 500 cantilever substrates
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Cantilever Break-Off
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Cantilever Mount (2)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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SEM Image of AFM Tip and Cantilever
radius of curvature ∼ 20 nm
pyramidalsilicon nitride tip
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Si Cantilevers• different apex angles• important parameters:• spring constant
(0,01 – 300 N/m)• apex angle• resonance frequency
(6 – 600 kHz)• length (100 – 450 µm)• thickness (1 – 7 µm)
k = 0,1 N/m + ∆d = 0,1 Å ⇒ Fmin = 10-12 N
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Tip Quality Controlat the atomic level → AFM experiment
HOPG (graphite) withatomic resolution
(two types of atoms visible)
→ optical microscope→ simulations
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AFM Image Simulation (2)142 pm
20 pm
images for differenttwo – atomic tips
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Detection of Spring Deflection (5)optical lever scheme
• in commercial instruments• detector far away from
measuring cell• measurements under liquids• stable against influences
from outside• atomic resolution
z – resolution ∼ 0,1 Å
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PDPD
PDPD
IIII
+
−⇒ signal for deflection (= force)
→ further systems: e.g. piezolever
24Institute of Analytical ChemistryVienna University of Technology
Force Detection by Optical LeverX,Y adjusting knobsfor laser position
photodiode adjusting knob
laserfilter
cantileversplit photo diodedetector
mirror
prism
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25Institute of Analytical ChemistryVienna University of Technology
Commercial Liquid Cell
sample
glass cover
silicone o-ring
in/out
piezoscanner
cantilever
laser
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AFM Modes of Operation• constant height mode
height position of sample unchangedvariation of cantilever deflection is detected→ small areas→ high scan rates possible⇒ elimination of thermal drifts, high resolution
imaging (atomic resolution !)• constant force mode
cantilever deflection kept constant by feedback loop→ low scan rates→ wide area scans→ lower risk for tip – crashes
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Other AFM Components• scan system: piezo – elements→ see STM
• feedback loop→ deflection from sub – Ångstrøm range to
several micrometers→ modes of operaton analogeous to STM
→ constant height mode→ constant force mode
• image processing system→ see STM
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Commercial Instrument (3)NanoScopeMultimode SPM
Digital Instruments – VeecoSanta Barbara, CAUSA
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AFM Head (2)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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Optical Head
Digital Instruments – Veeco, Santa Barbara, CA, USA
cantilever
sample
photodetector
laser diode
mirror
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Cantilever Mount (2)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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Laser Alignment with Paper Method (2)
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Laser Alignment with Paper Method (1)
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Laser Alignment with Paper Method (3)
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Force Optimization
also additionalinformationfromforce – distance –curves
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NanoScope III System
Digital Instruments – Veeco, Santa Barbara, CA, USA
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NanoScope III TM-AFM (1)
Digital Instruments – Veeco,Santa Barbara, CA, USA
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NanoScope III TM-AFM (2)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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NanoScope III Optical Head
Digital Instruments – Veeco, Santa Barbara, CA, USA
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NanoScope III AFM Scanner
Digital Instruments – Veeco, Santa Barbara, CA, USA
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NanoScope III Cantilever Holder (2)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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NanoScope III Cantilever Holder (4)
Digital Instruments – Veeco, Santa Barbara, CA, USA
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Liquid Cell
Digital Instruments – Veeco, Santa Barbara, CA, USA
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Analytical PropertiesAnalytical Aspects
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STM – Properties forAnalytical Chemistry (1)
• atomic resolution, but also microscopic range• real space imaging → local probe• topography with direct depth information
(above atomic level)• on atomic scale → electronic structure⇒ LDOSEF
• in-situ measurements in gases or liquids possible→ chemical reactions in-situ→ electrochemistry (potential control !) at
electrode surfaces (in-situ)
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STM – Properties forAnalytical Chemistry (2)
• in many cases simple sample preparation(„as it is“ or cleavage)
• also local spectroscopy possibleScanning Tunneling Spectroscopy (STS)→ local barrier height imaging→ l-V curves
• DRAWBACKS:→ no direct element specific information→ possible artefacts by asymmetric tips
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AFM – Properties forAnalytical Chemistry (1)
• atomic resolution, but also microscopic range• real space imaging → local probe• topography with direct depth information• both conductors and insulators
(also organic and biological samples !)• in-situ measurements in gases or liquids possible→ chemical reactions in-situ→ electrochemistry (potential control !) at
electrode surfaces (in-situ)
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AFM – Properties forAnalytical Chemistry (2)
• in many cases simple sample preparation(„as it is“ or cleavage, no coating necessary !)
• additional information from force – distance –curves• further material properties by special techniques
(e.g. elasticity by force modulation, friction by LFM)• DRAWBACKS:→ no direct element specific information→ possible artefacts by asymmetric tips
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Artefacts and Solutions (1)from tip:objects sharper than tip → image of tip
e.g.
edges parallel to scan direction
pyramids representing theimage of the tip
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Artefacts and Solutions (1a)
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Artefacts and Solutions (2)
from tip:asymmetric tips → bad resolution along one direction
e.g.
→ check e.g. rotation of scan direction
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Artefacts and Solutions (3)
from tip:multiple tips → multiple images
e.g.
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Artefacts and Solutions (4)from tip:convolution of tip and sample geometry
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Artefacts and Solutions (5)vibrations:→ e.g. line structures, not from sample
e.g.
→ check image changes with:scan rate → constant imagescan angle → image rotationscan size → correct distances
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Artefacts and Solutions (6)
thermal drifts:
e.g.
→ wait for stabilization
Y – scan disabled
α α
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Artefacts and Solutions (7)impurities:
e.g.
→ clean samples→ imaging under liquids
particles can disturb motionof tip
⇒ distortion of single scan lines
particleperturbation
