Techniken der Oberflächenphysik (Techniques of Surface ...€¦ · Laser light from a solid-state...
Transcript of Techniken der Oberflächenphysik (Techniques of Surface ...€¦ · Laser light from a solid-state...
Fachgebiet Angewante Nanophysik, Institut für Physik
Contact: [email protected]
Office: Heisenbergbau (Gebäude V) 202, Unterpörlitzer Straße 38 (tel: 3748)
www.tu-ilmenau.de/nanostruk
Vorlesung: Donnerstag (U), 17:00 – 18:30, F 3001 Übung: Donnerstag (G), 17:00 – 18:30, F 3001
Prof. Yong Lei & Dr. Yang Xu, Dr. Huaping Zhao
Techniken der Oberflächenphysik (Techniques of Surface Physics)
Characterization of surfaces
An appropriate characterization will play a crucial role in determining
various surface structures and their properties (especially for nano-
surfaces.
Three broadly approved aspects of characterization are
1. Morphology
2. Crystalline structure
3. Chemical analysis
SEM: Scanning Electron Microscope; STM/AFM: Scanning Tunneling
Microscope/Atomic Force Microscope
TEM: Transmission Electron Microscope
X-Ray: X-ray Morphology; IP: Image Processing; LM: Lightweight
Morphology; RBS: Rutherford Backscattering Spectrometry (Kelsall et
al., Nanoscale science and technology. 2005)
Techniques of Surface Physic
---STM and AFM(SPM)
Visible by eyes
10-3 ~~~ mm
Optical microscope
10-6 ~~~ µm
SEM and TEM
10-9 ~~~ nm
m
SPM
The tunneling phenomenon in quantum mechanics: quantum tunneling effect: an electron's penetration of an energy barrier, even though the electron's energy is below the height of barrier.
Scanning Tunneling Microscope
Scanning Tunneling Microscope
• tunneling phenomenon:
The principle of STM
Pro
be
Sample
Gerd Binnig Heinrich Rohrer
(born 20 July 1947) German physicist (born June 6, 1933) Swiss physicist
They shared half of the 1986 Nobel Prize in Physics with for the design of the scanning tunneling microscope (STM).
The manipulation of STM
Constant current image (topography) of an antiferromagnetic atomic layer iron on W(001) with defects and atoms.
It ~ e-2kd
The structure of STM
http://www.youtube.com/watch?v=47UgMpXFVj4
• STM
The Application of STM
1. Atomic imaging – most important function of STM
Nickel (110)
Platinum (111)
cobalt sulfide "nanoflower" structure synthesized on a Au(111) surface 9nm x 9nm
Is the atoms or molecules observed by STM is 100%
the real image of atoms or molecules?
those form of the electron cloud of the atom or molecule
2. Manipulation of single atoms and single molecules
Since the invention of STM, people want to "hand" manipulate atoms by
the STM probe to achieve the dream of manipulate atoms. In the early
1990s, IBM's scientists use Xe atoms to write three letters "IBM" on Ni
(110) surface, first demonstrated the possibility of the manipulation of
individual atoms at low-temperature STM.
The dream of directly manipulate atoms
Desorption: Similar to vertical manipulation, but desorption of atom/molecule into surrounding gas phase.
Lateral manipulation: Transfer of atoms/molecules along surface using attractive/ repulsive forces between tip and atom/molecule.
Vertical manipulation: Transfer of atoms/molecules between surface and STM tip using electronic/ vibration excitation by tunneling.
positioning 48 iron atoms into a ring in order to "corral" surface state electrons and force them into "quantum" state.
3. Single-molecular chemical reactions
Synthesis: Selective bond formation between two molecular units using lateral manipulation followed by electronic excitation.
Dissociation: Selective bond breaking within a molecule by tunneling processes.
Schematic of bond formation in Au-PTCDA switch: atom and molecule are both negatively charged and stabilized by repulsive interaction (a). By tunneling out of molecular resonance, PTCDA is neutralized, and electrostatic repulsion is weakened (b). This makes Au atom moving towards molecule and form the bond.
4. Construct molecular-level electronic device
Terbium atom (red) is sandwiched between two organic molecules (grey and blue) to form a single-molecule magnet.
The advantages and disadvantages of STM
• Advantages: allows to detect many features, including roughness, defects, and to determine size and conformation of molecules. Other advantages: • obtain much more details than other microscopes, for a
better understanding of research topics at molecular level. • STM is versatile, it can be used in ultra-high vacuum, air,
water and other liquids, and gas. • can be operated in temperatures from zero Kelvin up to a few
hundred degrees.
• Disadvantages:
• It is difficult to use STM effectively. It is a very specific technique that requires a lot of skill and precision.
• STM requires very stable and clean surfaces, excellent vibration control and sharp tips. STM only can be used to scan conducting samples which are not easily oxidized.
• STM uses highly specialized equipment that is very expensive.
Atomic Force Microscope (AFM)
1986 --- Binnig, Quate and Gerber invented the first atomic force microscope
Beam-deflection measurement
The principle of AFM
When a tip is close to sample,
typically two forces operate:
Coulombic and van der Waals
interactions.
Coulombic Interaction: strong, short range
repulsive force between tip and sample.
This repulsion increases as the distance
decreases.
Van der Waals interactions: long range attractive force, which become
obvious at distance of down to 10 nm.
The combination of these interactions
results in a force-distance curve
•As tip is brought towards
the sample, van der Waals
forces cause attraction.
• As tip gets closer to the
sample this attraction
increases.
•However at very small
separations the repulsive
coulombic force becomes
dominant. The repulsive
force causes the cantilever
to bend when the tip is
getting closer to the
surface.
Laser light from a solid-state diode is reflected at back of cantilever and
collected by a position-sensitive detector (2 close-spaced photodiodes).
Angular displacement of cantilever results in 1 photodiode collecting more light
than the other photodiode, producing an output signal, which is proportional to
the deflection of cantilever.
Its high sensitivity and simple operation, and cantilevers do not require
electrical contacts or other special treatments, and can therefore be fabricated
relatively cheaply.
Beam-deflection measurement
The structure of AFM
Feedback circuit
Position Sensing Part
Force Sensing Part
Position Sensing photodetector
The manipulation of AFM
Constant-height scan
• Two scanning processes:
Constant-force scan
• Three AFM imaging modes:
• 1. Contact AFM
• < 0.5 nm probe-surface separation
• 2. Tapping mode AFM
• 0.5-2 nm probe-surface separation
• 3. Non-contact AFM
• 0.1-10 nm probe-surface separation
• 1. Contact AFM
•tip contacts the sample
surface.
•The photo detector monitors
the changing of cantilever
deflection and the force is
calculated using Hooke’s law:
•The feedback circuit adjusts the probe height to maintain a constant
force and deflection on the cantilever, i.e. deflection setpoint.
F = − k x (F = force, k = spring constant, x = cantilever deflection)
Constant-force scan
• 2. Tapping mode AFM (Intermittent contact )
•cantilever oscillates at (or
slightly below) its resonant
frequency. Oscillation
amplitude ranges in 20-100
nm. Tip lightly touch
(“taps”) on sample surface
during scanning.
•Oscillation decreases when tip is closer to surface. Hence changes of
oscillation amplitude are used for measuring tip/surface separation.
Feedback circuit adjusts probe height to maintain a constant amplitude
of oscillation. i.e. the amplitude setpoint.
• Resonant frequency of cantilever dependent on tip/surface separation.
Tapping mode in air:
A small piezoelectric crystal on AFM tip holder makes the cantilever oscillate up and down.
Tapping mode in liquids:
Tapping mode operation in liquids is a very useful tool for biologists.
• 3. Non-contact AFM
• cantilever oscillate near sample
surface, but does not contact it.
Oscillation is at slightly above
resonant frequency. Van der Waals
force decrease resonant frequency.
This decrease of resonant
frequency causes oscillation
amplitude to decrease.
•Normally adsorbed fluidic layer is much thicker than the dominant
region of van der Waals force - probe is either out of range of van der
Waals force, or becomes trapped in fluidic layer. Therefore non-
contact mode AFM works best under ultra-high vacuum conditions.
The Properties of the different operation modes in AFM.
Advantage Disadvantage
Contact Mode
- High scan speeds - “Atomic resolution” possible - Easier scanning of rough
samples with large changes in vertical topography.
• Lateral forces distort image • cause damage to soft samples.
Tapping Mode
- High lateral resolution (1 to 5 nm). - Lower forces, less damage to soft samples. - Almost no lateral forces.
• Slower scan speed than contact mode
Non-contact Mode
- Both normal and lateral forces are minimised, good for measuring very soft samples
- Can get atomic resolution in UHV environment
• Slower scan speed than tapping and contact modes • Usually only applicable in extremely hydrophobic samples with a minimium fluidic layer.
Advantages and Disadvantages of AFM Modes
Electrostatic force microscope (EFM)
Variety of AFM
EFM is a dynamic non-contact atomic force microscopy where the
electrostatic force is probed. This force arises due to the attraction
or repulsion of separated charges. It is a long-range force and can
be detected 100 nm or more from the sample.
A conductive tip and sample separated with a distance z. A bias
voltage between tip and sample is applied forming a capacitor C.
The capacitance depends on the geometry of the tip and sample.
The total energy stored in the capacitor is U = ½ C⋅ΔV2.
The z component of the force (the force along the axis connecting
the tip and sample) is thus:
Felectrostatic = ½ C/z ΔV2
Magnetic force microscope (MFM)
MFM is a variety of AFM in which a sharp magnetized tip scans a
magnetic sample; the tip-sample magnetic interactions are detected
and used to reconstruct the magnetic structure of the sample
surface. Many kinds of magnetic interactions are measured by MFM.
MFM scanning often uses non-contact AFM mode.
Tip curvature radius: no coating AFM tip
10 nm; coating AFM tip 35-50 nm.
Coating layer: normally two kinds of
metals, sometimes just one metal.
High resolution AFM tip:
Tip curvature radius: 1 nm.
• 1. Imaging
The application of AFM
AFM 3D image of a detail of the free surface of an artificial pattern
The figure illustrates 800 nm wide and 10 nm high Pd/Fe/Pd thin film dots
NCAFM image of the Ge/Si(105) surface, 4.2 nm x 4.2 nm
PMMA spheres scanning range 45x45 μm
AFM image of human plasma sample (fibrinogen)
• 2. Measuring forces (and mechanical properties) at nanoscale
Illustration of an AFM tip measuring the force to move a cobalt atom on a crystalline surface. (Credit: Image courtesy of IBM) http://www.youtube.com/watch?v=BUq2bQkL1zo
Single-molecule force microscope uses a target molecule at AFM tip end to probe surface molecules with a strong attractive force. Force measurements are mapped as an image.
• 3. As a nanoscale tool
bending, cutting and extracting soft materials (Polymers, DNA, nanotubes), at nano-scale
grab and hold a nanoparticle in a position
Manipulating nanotube on Si substrate. AFM tip creates Greek letter "theta" from a 2.5 μm long nanotube
A single nanotube (red) on an insulating substrate (SiO2, green) is manipulated in a few steps onto a W film thin wire (blue), finally stretched across an tungsten oxide barrier (yellow).
Advantages :
1) high-resolution 3-D surface images
2) not require special sample treatments (no sample's destruction)
3) Usually not require vacuum (operate both in air and liquid);
4) could be used for organic materials
Disadvantages:
1) image size is much smaller than that of electron microscopes;
2) slow scanning rate, unlike an electron microscope which
does it in almost real-time.
3) tip convolution -- not true sample topography
4) expensive tips
The advantage and disadvantage of AFM
Tip convolution----Tip Related Artifacts
protrusions (dots) appear
wider, depressions (pores) appear narrower than the reality.
http://www.youtube.com/watch?v=fivhcWYEtkQ
AFM Section Analysis of the nano-discs, the average height and size of
the nano-discs are approximately 1.5 and 80 nm, respectively.
AFM Section Analysis of the nano-hemisphere. To accurately reflect the shape
of the nanoparticles, we used the same dimension scale for the horizontal and
vertical coordinates. The average height and base diameter of the nano-
hemispheres are approximately 35-40 and 75 nm, respectively.
AFM Section Analysis of the nano-hemiellipsoids. To accurately reflect the shape
of the nanoparticles, we used the same dimension scale for the horizontal and
vertical coordinates. The average height and base diameter of the nano-
hemiellipsoids are approximately 50-55 and 65 nm, respectively.
AFM Section Analysis of the nano-conics. To accurately reflect the shape of the
nanoparticles, we used the same dimension scale for the horizontal and vertical
coordinates. The average height and base diameter of the nano-conics are
approximately 55-60 and 60 nm, respectively.
Thank you and have a nice day!