Nanotechnology Microscopes : Scanning Tunnelling And Atomic Force Microscope
7. Microscopes:
7.a Atomic Force Microscope:
The Atomic Force Microscope (AFM ) is being used in a wide range of technologies including electronics, biological, chemical, aerospace, automotive, medical, and energy industries. The materials that normally require AFM investigation include thin and thick film coatings, ceramics, glasses, synthetic and biological membranes, metals, polymers, and semiconductors. The AFM is being used to study phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, plating, and polishing. AFM enables us to see the surface of a material in atomic resolution.
The first AFM was made by gluing a tiny shard of diamond onto one end of a tiny strip of gold foil. A small hook at the end of the cantilever was pressed against the surface while the sample was scanned beneath the tip. The force between tip and sample was measured by tracking the deflection of the cantilever. This was done by monitoring the tunneling current at a second tip positioned above the cantilever. They could delineate lateral features as small as 300 Å. The force microscope emerged in this way. Today the tip-cantilever assembly typically is microfabricated from Si or Si3N4.
The force between the tip and the sample surface is very small, usually less than 10-9 N. The detection system does not measure force directly. It senses the deflection of the microcantilever.
The beam-bounce method is a widely used technique. In this system an optical beam is reflected from the mirrored surface on the backside of the cantilever onto a position-sensitive photo detector. In this arrangement a small deflection of the cantilever will tilt the reflected beam and change the position of beam on the photo detector. Motion of the cantilever has a strong effect on the laser output, and this is exploited as a motion detector.
Operation:
An atomically sharp tip is scanned over a surface with feedback mechanisms that enable the piezo-electric scanners to maintain the tip at a constant force (to obtain height information), or height (to obtain force information) above the sample surface. Tips are typically made from Si3N4 or Si, and extended down from the end of a cantilever. The nanoscope AFM head employs an optical detection system in which the tip is attached to the underside of a reflective cantilever. A diode laser is focused onto the back of a reflective cantilever. As the tip scans the surface of the sample, moving up and down with the contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode. The photo detector measures the difference in light intensities between the upper and lower photo detectors, and then converts to voltage. Feedback from the photodiode difference signal, through software control from the computer, enables the tip to maintain either a constant force or constant height above the sample. In the constant force mode the piezo-electric transducer monitors real time height deviation. In the constant height mode the deflection force on the sample is recorded.
7.b Scanning Tunneling Microscope (STM):
The acronym STM is used for either scanning tunneling microscope or scanning tunneling microscopy. The scanning tunneling microscope (STM) is a non-optical microscope. It scans an electrical probe over a surface to be imaged to detect any weak electric current flowing between the tip and the surface. The STM allows us to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a material. The STM normally offers better resolution than the atomic force microscope (AFM). Both the STM and the AFM belong to the class of scanning probe microscopes.
The STM can obtain images of conductive surfaces at an atomic scale 2 x 10-10 m or 0.2 nanometer, and expected to be used for manipulating individual atoms, trigger chemical reactions.
STM employs principles of quantum mechanics. An atomically sharp probe (the tip) is moved over the surface of the material under study, and a voltage is applied between probe and the surface. The STM will get within a few nanometers of what it is observing. Depending on the voltage electrons will "tunnel" (this is a quantum-mechanical effect) or jump from the tip to the surface (or vice-versa depending on the polarity), resulting in a weak electric current. The size of this current is exponentially dependent on the distance between probe and the surface. For a current to flow the substrate being scanned must be conductive. Insulators cannot be scanned through the STM.
A feedback loop keeps the tunneling current constant by adjusting the distance between the tip and the surface (constant current mode). This adjustment is done by placing a voltage on the electrodes of a piezoelectric element. By scanning the tip over the surface and measuring the height (which is directly related to the voltage applied to the piezo element), it is possible to reconstruct the surface structure of the material being measured. High-quality STMs can reach sufficient resolution to show single atoms. The scanning tunneling microscope (STM) is used in industrial and fundamental research to obtain atomic-scale images of metal surfaces. It provides a three-dimensional profile of the surface, which is useful for investigating surface roughness, surface defects at molecular level.
The electron cloud associated with metal atoms at a surface extends a very small distance above the surface. When a very sharp tip which has been treated so that a single atom projects from its end is brought sufficiently close to such a surface, there is a strong interaction between the electron cloud on the surface and that of the tip atom, and an electric tunneling current flows when a small voltage is applied. At a separation of a few atomic diameters, the tunneling current rapidly increases as the distance between the tip and the surface decreases. This changes in tunneling current with distance results in atomic resolution if the tip is scanned over the surface to produce an image.
7.c Comparison of AFM and STM:
Normally, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is more complex when characteristics such as tip shape and contact force are considered. STM is generally applicable only to conducting samples while AFM is applied to both conductors and insulators. AFM offers the advantage that the writing voltage and tip-to-substrate spacing can be controlled independently, whereas with STM the two parameters are integrally linked.
