Introduction
Molecular Nano-technology
Molecular nanotechnology or Nanotechnology is the name given to a specific sort of manufacturing technology to build things from the atom up, and to rearrange matter with atomic precision. The nanoscale structures can be prepared, characterized, manipulated, and even visualized with tools.
"Nanotechnology is a tool-driven field."
Other terms, such as molecular engineering or molecular manufacturing are also often applied when describing this emerging technology. This technology does not yet exist. But, scientists have recently gained the ability to observe and manipulate atoms directly. However, this is only one small aspect of a growing array of techniques in nanoscale science and technology. The ability to make commercial products may yet be a few decades away.
"Nanotechnology is Engineering, Not Science."
The central thesis of nanotechnology is that almost any chemically stable structure that is not specifically disallowed by the laws of physics can in fact be built. Theoretical and computational models indicate that molecular manufacturing systems are possible — that they do not violate existing physical law. These models also give us a feel for what a molecular manufacturing system might look like. Today, scientists are devising numerous tools and techniques that will be needed to transform nanotechnology from computer models into reality.
The key aspect of nanotechnology is that nanoscale materials offer different chemical and physical properties than the bulk materials, and that these properties could form the basis of new technologies. For example, scientists have learned that the electronic--and hence optical--properties of nanometer-size particles can be tuned by adjusting the particle size. According to a recent study by a group at Georgia Institute of Technology, when gold metal is reduced to nano size rods, its fluorescence intensity is enhanced over 10 million-fold. The study found that the wavelength of the emitted light increases linearly with the rod length, while the light intensity increases with the square of the rod length.
A Molecular Differential Gear
Macroscopic machines often use rotating shafts and gears to drive motion. Molecular machines can do likewise, sometimes using parts in ways that follow conventional engineering practice.
A differential gear links two shafts through a casing, constraining the sum of the rotational angles of the shafts to equal the rotational angle of the casing. In a car, a differential gear lets the engine drive the wheels while letting them roll different distances in cornering. A molecular differential gear like that shown above can serve this vital automotive function, but in a volume a few nano-meters on a side, containing mere thousands of atoms.
This cutaway reveals the two cylindrical shafts and their facing bevel (conical) gears, along with two of the four casing-mounted side-gears that mesh with both shaft-gears. Holding the casing fixed, clockwise rotation of the top shaft drives the side-gears, in turn driving counterclockwise rotation of the bottom shaft. Geometry and symmetries ensure relative smooth motion. For example, the shaft-gears have 14-fold symmetry, while the casing has 4-fold symmetry; if one side-gear is exactly opposite a shaft-gear tooth, its 90-degree partners will be opposite shaft-gear grooves. Thus, energy fluctuations at the tooth-meshing frequency will cancel, leaving only higher-frequency, lower-amplitude fluctuations as barriers to rotation. The shafts rotate in the casing on standard sliding-interface bearings, using the same principle to achieve smooth motion.
The chief constituents of this structure are hydrogen, carbon, silicon, nitrogen, phosphorus, oxygen, and sulphur. The larger size of second-row atoms helps in constructing tapered gears and reduces the number of atoms needed to construct the outer cylinder of the casing. Such structures are far beyond the state of the art of chemical synthesis today, but their design and modeling is becoming straightforward.
A Fine-Motion Controller for Molecular Assembly
A general-purpose molecular assembler arm must be able to move its "hand" by many atomic diameters, position it with fractional-atomic-diameter accuracy, and then execute finely controlled motions to transfer one or a few atoms in a guided chemical reaction. Our arms use large muscles and joints for large motions, but more finely controlled finger motions for precision. Assembler mechanisms will likely be designed along similar lines.
This illustration shows a structure resembling a Stewart platform that results from a long sequence of designs and redesigns aimed at specifying the atomic structure of a molecular-scale fine-motion controller. Its core consists of a shaft linking two hexagonal endplates, sandwiching a stack of eight rings. In a complete system, each ring would be rotated by a lever, which is driven by a cam mechanism. Each ring supports a strut linked to a central platform (here shown raised, displaced, and twisted). Rotating a ring moves a strut; moving a strut moves the platform; positioning all eight rings (over-) determines a platform position in x, y, z, roll, pitch, and yaw. (If the struts were rigid, six would do the job; here, two struts have been added to increase stiffness and decrease elegance.) The chief design problem is to enable an adequate range of motion without mechanical interference or unacceptable bond strains, and within the size constraints set by available modeling tools and patience. The illustrated structure can execute precise motions over several atomic diameters with associated 90-degree rotations, and contains fewer than 3,000 atoms.
Simple Pump Selective for Neon
The pump and segment of chamber wall pictured here contain 6165 atoms.
The deep grooves separating the bearing cylinders from the middle segment are partly illusory--realistic atomic radii would show these grooves as filled with what usually passes for solid matter. By the same token, the effective shape of the structure has a smoother surface, and ridges formed by lines of atoms on the surface are less knobby.
In operation, rotation of the shaft moves a helical groove past longitudinal grooves inside the pump housing. Only where facing grooves cross is there room for even a small gas molecule, and these crossing points move from one side to the other as the shaft turns. It is hoped that simulation will show this to be an effective pump, with substantial selectivity for different chemical species; the design target was an effective, selective pump for neon.
Nanowires as Switches
A very different approach to building a molecular computer from the bottom up is being pursued at Rice University's Center for Nanoscale Science & Technology in Houston. There, a group of chemistry coworkers have been synthesizing and studying molecular wires and molecular devices of a different sort. Their nanowires are conjugated chains in which, for example, functionalized benzene rings alternate with acetylene groups. The wires bear specific functional groups at either end that serve as "molecular alligator clips" for attaching the wires to gold or other electrodes. Using several techniques, scientists have measured small electrical currents flowing through these wires.
When a steadily increasing voltage is applied to the monolayer, the molecules do not pass any significant current until a threshold voltage is reached. The current then zooms up, peaks, and turns off sharply as the voltage continues ramping up. This switching behavior, known as negative differential resistance (NDR), in a related molecule at room temperature, although in that case the magnitude of the effect is not as impressive. Because these molecules can be switched between two stable oxidation states, they can store information in the form of a "0" (insulating state) or "1" (conducting state) and thus also serve as a molecular memory device.
Nano-scale Control of Electric Dipoles in Organic Molecules as 'bits'
The nanoscopic ferroelectric domains could be formed in thin films by applying electric pulses with a conductive atomic force microscope (AFM). They can be detected by using piezoelectric response, revealing that the directions of electric dipoles in organic molecules can be controlled in nano-scale. By changing the polarity of the applied pulses, temporally stable binary information could be "written" in these films.
Copolymer of vinylidene fluoride and trifluoroethylene with a molar ratio of 73/27 has electric dipoles perpendicular to the molecular axis originated from the large difference in the electron affinity of H and F atoms. The copolymer samples were deposited on Pt substrate by a spin-coating method. The Pt substratewas sputtered on silicon dioxide. The polarization alignment and switching were conducted by applying electric pulses to the film sample by using an Au-coated conductive AFM tip as shown in figure (next page). Changing the polarity of the applied electric pulses did the write and erase procedure.
Detection:
While, the ferro-electric domains were detected by utilizing piezo response. When we locate the tip on a polarization domain with applying the oscillating weak electric field, piezoelectric vibration is caused and the tip in contact with the film is consequently vibrated. The vibration is detected using the optical lever method and then the signal is demodulated by lock-in 9/30/97amplifier so that both in-phase and quadrature phase signals, i.e., both amplitude and phase information can be obtained. In this setup the oscillation frequency used must be located between the bandwidth of the feedback loop and the resonant frequency of the cantilever. When the tip is scanned, it follows the topography by feedback electronics but does not follow the oscillation. Then both topographic image and piezoelectric response can be simultaneously taken. From the detected phase information, we can determine the polarity of the domains, too. The Au coated tips and cantilevers had a spring constant of 0.75 N/m and a resonant frequency of 88 kHz, and the modulation signal applied to the tip had a frequency of 70 kHz and a voltage of 0.5 Vrms.
