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Nanoscience & Microscopies SEM, AFM & STM. Block Diagram of E-beam. SEM Picts. SEM Picts. Dendritic Growth. Ostrinium Nubilialis. Miami Research Results. STM. Scanning tunneling microscope. Outside the Barrier. Wavefunction extends outside of well edge for a finite square well.

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slide4

SEM Picts

Dendritic Growth

Ostrinium Nubilialis

slide6
STM
  • Scanning tunneling microscope
slide7

Outside the Barrier

Wavefunction extends outside of well edge

for a finite square well

slide8

Two finite wells at a distance

  • The electron wavefunctions extend

into space between finite wells

http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm

slide9

Two finite wells at close approachcan look like a barrier to the electrons!

  • Electrons can tunnel into or through barriers depending on…

http://phys.educ.ksu.edu/vqm/html/qtunneling.html

slide10

Two finite wells at close approach

  • Electron wavefunctions can overlap

http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm

slide11
This is the STM image of Si(111)-7x7 surface, the white spots represents the position of the atoms.
scanning
Scanning --
  • Constant Current Mode
  • Constant Height Mode
  • http://www.surfaces.lsu.edu/STMoverview.html
slide14

Modes of Operation

  • Constant Current
  • Constant

Height

userpage.chemie.fu-berlin.de/.../html/node5.htm

use of stm chemical constrast
Use of STM- Chemical Constrast

The surface of alloys may differ in several respects from the corresponding bulk

structure. The surface may also exhibit a chemical order different from the one in

the bulk of the material. This influences processes such as gas adsorption and

catalysis, since adsorbed molecules often prefer a certain chemical environment

of substrate atoms.

It all started with an (111) oriented surface of

a PtNi alloy (bulk composition 25% Pt,

75% Ni; surface approx. 50% of each).

We found that we can distinguish between

Pt and Ni atoms on this surface with the STM!Since Pt and Ni are about 50% of the surface

atoms each, the brighter species is Ni.

The white blobs in the image are impurities

of unknown nature.

http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/chemical_resolution.html

use of stm
Excitation of different vibrational modes depending on species (C2H2 vs. C2D2)

Science, 280, 1715 (1998).

Single molecule chemical reactions

Science News, 158, 215 (2000).

Use of STM
slide17

Researchers usually compare "before" and

"after" pictures of typical regions of a material

to see how it changes during a phase transition.

This is now see on an atomic scale.

As the lead atoms in this 20 x 13 nanometer

region are warmed from 40 to 136 Kelvin,

they switch from the corrugated to the flat

arrangement at the transition temperature

of 86 Kelvin .

Vienna University of Technology

downside to stm
Requires electrical path – for tunneling

Gives information of heights of electron clouds above surface – eg. graphite structure

Some questions cannot be explored

Downside to STM?
slide19

Other cool sites…

  • http://www.almaden.ibm.com/vis/stm (Interesting STM images)
  • http://www.nobel.se/physics/educational/microscopes/scanning/ (Interesting STM images)
slide21
AFM
  • Cantilever structure on end on end of wafer holds the pyramidal tip
  • Light from a diode laser bounces of tip and strikes a split photodetector
  • Motion is detected by differences in intensity on the detector portions

http://www.stolaf.edu/depts/physics/afm/diagram.html

Let’s also look at:

http://www.weizmann.ac.il/Chemical_Research_Support/surflab/peter/afmworks/

wafer tip
Wafer & TIP
  • Si or SiN – 10 nm at end – ~100 atoms

Electron micrograph of two 100 µm long V-shaped cantilevers

(by Jean-Paul Revel, Caltech; cantilevers from Park Scientific

Instruments, Sunnyvale, CA).

  • Three common types of AFM tip.
  • normal tip (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall).
  • Electron micrographs by Jean-Paul Revel, Caltech.

http://stm2.nrl.navy.mil/how-afm/how-afm.html

afm modes
Contact

Low and High Gain

Low gain – “drag” tip across sample see deflection which is OK for relatively flat surfaces

High gain – tries to push down on sample to maintain a particular height which is OK for more bumpy surfaces, but not for soft surfaces

- Tapping mode

Good for local surface distortions to measure friction and spring forces on surfaces

Non-contact

Requires a frequency modulation technique which is good for biological samples- repulsive forces from surface

AFM Modes
metrology fabrication
Metrology -- Fabrication
  • AFM can be used to study the success of e-beam lithography and subsequent processing steps.

Nanotechnology, 13 659 (2002).

stm afm
Microscopic techniques which give us resolutions in angstroms or nanometers

Used for fabrication, chemical and biological studies

A definite “force” in the nanoworld !

STM & AFM