Richard Feynman in his famous work in 1959 speculated about the possibility of manipulation of atoms the way the scientists and biologists wanted to bring about the greatest technical and biological revolution that mankind has ever witnessed. Drexler in his famous work described a molecular assembly device as tiny as a molecule, capable of positioning elemental atoms according to engineering specifications. It would possess the ability to fabricate another assembler and each assembler would replicate again and again resulting in an army of workforce, molecular robots. These assemblies are likely to be very cheap and give rise to a culture of abundance. It would lead to a new era of molecular manufacturing. With these capabilities, nanotechnology would become a potential force.
With the assembler, one would be able to change the properties of materials as desired. This manipulation may allow people to build invisible super computers and tiny robots that may travel in the human body. While Drexler’s dream of self-assembly may take still years to realize, remarkable progress has been made in understanding nanomaterials with the development of atomic force microscopes and scanning tunneling microscopes (STM). With the proper selection of the charge (polarity), the magnitude and duration of voltage pulse applied between STM tip and sample surface, as well as at the tip to sample separation, single manipulation of atoms can be achieved. By placing a tungsten tip above silicon atoms and applying voltages of –5.5 V to the surface for 30 ms, silicon atoms can be lifted from the surface. Atoms could also be redeposited after they have been lifted. Thus the speculation of Feyman in transforming to reality with the advent of electron micro particle, nanotweezers and manipulators.
Control of Submicron Systems
Atomic Force Microscopy
The small size of nanoparticles and the possibility of manipulation of atoms raised several uncertainties in the minds of engineers and scientists. Feymann in his famous lecture was pessimistic about the limitation of the resolution of electron microscope. He wished the electron microscope was a hundred times more powerful to be able to observe the structure of RNA sequence of bases in DNA directly. The resolution in the modern microscopes is in the sub-nanometer range. TEM’s with an accelerating voltage of 400 kV and a resolution of 0.1 nm we need are now available. The development of atomic force microscope was therefore revolutionary, as it translated imagination to physical reality and allowed physical observation on an atomic scale. An AFM is one of the general class of instruments termed SPMs or scanning probe microscopes. These devices can make images of atoms in molecules to angstrom precision. The key feature is that atoms can be moved to precisely determined positions. The atomic force microscopy generates a topological image by systematically moving a sharp tip about 2 µm long held at the apex of a cantilever, across a surface with air or liquid. An optical lens measures the deflection of the cantilever. The positional sensitive diode is capable of measuring change in position of incident lens beam as small as 1 nm, thereby giving sub-nanometer resolution. Nanosize tips can be made about 50 nm long and 1 nm wide. Tips are normally made from silicon. The resolution is in the order of 10-50 nm. Other imaging modes include lateral force microscopy, magnetic force microscopy, scanning electrochemical microscopy and pulse force microscopy.
Scanning Tunneling Microscopy (STM)
Scanning tunnelling microscopy was invented by Binning and Bohrer in 1951 at IBM Zurich. A sharpened conducting tip is used and bias voltage is applied between the tip and the sample. The tunneling current is produced by movement of electrons over the energy barrier and it varies with tip to sample spacing, and it is the signal used to create and STM image. For tunneling, both tip and the sample must be conductors. STM as well as AFM can be used in systems that have a liquid environment which allows geo-topical biological and corrosion studies to be made in STM and AFM. Researchers have in recent years created a nanoscale grasping device, nanotweezers for measuring and manipulating molecular structure. The recent development of nanoscope provides advances in virtually every facet of scanning picture microscopy technology and a great freedom with manipulation of material and specimens at a nano level. Improvements in nanoparticle sizing and synthesis by disc centrifuges have been recently reported.