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    Questions for review

  • What makes the SEM such a useful instrument? What can it do that a normal optical microscope cannot?
  • Explain the usage of the electron beam in the SEM.
  • What is meant by “images of high resolution"?

Scanning tunneling microscopes

A Brief Historical Note

The scanning tunneling microscope (STM) had its birth in 1981, invented by Gerd Binnig and Heinrich Rohrer of IBM, in Zurich, Germany. They won the 1986 Nobel Prize in physics for this accomplishment, but use of the microscope itself was somewhat slow to spread into the academic world. STM is used to scan surfaces at the atomic level, producing a map of electron densities; the surface science community was somewhat skeptical and resistant of such a pertinent tool coming from an outside, industrial source. There were questions as to the interpretations of the early images (how are we really sure those are the individual silicon atoms?), as well as the difficulty of interpreting them in the first place – the original STMs did not include computers to integrate the data. The older electron microscopes were generally easier to use, and more reliable; hence they retained preference over STMs for several years after the STM development. STM gained publicity slowly, through accomplishments such as IBM’s famous xenon atom arrangement feat (see fig. 3) in 1990, and the determination of the structure of “crystalline” silicon.

false-color STM image of xenon atoms arranged on a nickel background. From: D.M. Eigler, E.K. Schweizer. Positioning single atoms with a scanning tunneling microscope. Nature 344, 524-526 (1990).

How STMs Work: The Basic Ideas

  • I. the probe

    Scanning Tunneling Microscopy relies on a tiny probe of tungsten, platinum-iridium, or another conductive material to collect the data. The probe slowly “scans” across a surface, yielding an electron-density map of the nanoscale features of the surface. To achieve this resolution, the probe must be a wire with a protruding peak of a single atom; the sharper the peak, the better the resolution. A voltage difference between the tip and the sample results in an electron “tunneling” current when the tip comes close enough (within around 10 Å). This “tunneling” is a phenomenon explained by the quantum mechanical properties of particles; the current is either held constant and probe height recorded, or the probe’s height is maintained and the change in current is measured to produce the scanning data. In constant current microscopy, the probe height must be constantly adjusted, which makes for relatively slow scanning, but allows fairly irregular surfaces to be examined. By contrast, constant height mode allows for faster scanning, but will only be effective for relatively smooth sample surfaces.
  • Ii. piezoelectric scanner

    In order to make the sub-nanometer vertical adjustments required for STM, piezoelectric ceramics are used in the scanning platform on which the sample is held. Piezoelectric materials undergo infinitesimally small mechanical changes under an applied voltage; therefore in the positioning device of a STM, they provide the motion to change the tip height at small enough increments that collision with the sample surface can be avoided. A data feedback loop is maintained between probe and piezoelectric positioner, so that the tip’s height can be adjusted as necessary in constant-current mode, and can be brought close enough to the sample to begin scanning in the first place.
  • Iii. the computer

    Though the earliest STMs did not include a computer with the scanning apparatus, current models have one attached to filter and integrate the data as it is received, as well as to monitor and control the actual scanning process. Grayscale primary images can be colored to give contrast to different types of atoms in the sample; most published STM images have been enhanced in this way.

Questions & Answers

so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Nanotechnology: content and context. OpenStax CNX. May 09, 2007 Download for free at http://cnx.org/content/col10418/1.1
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