scanning tunneling microscopy stm atomic force microscopy … · • basic principles of scanning...
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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