Nano- as a prefix refers to the order of magnitude involved in a specific discussion. A nanometer is one billionth of a meter. Comparing a nanometer to a meter is like comparing the size of a marble to the size of the earth.
At 100 nanometers and less, quantum effects become pronounced, and this is one reason that particular distance is considered the upper size limit for objects of interest in the field of nanotechnology. The lower size limit is one nanometer. For perspective, consider the fact that the bond length between two carbon atoms in a molecule is between 0.12 and 0.15 nm. The diameter of a DNA double-helix is about two nanometers. The smallest single-cell bacteria measure roughly 200 nm in length, well above the upper size limit for nanotechnology. This size-window is of interest, in addition to its quantum significance, because the ratio of surface area to diameter (and hence mass) is vast compared to the corresponding ratio in objects that are visible. For this reason, catalytic effects are more relevant, and that fact motivates a lot of current research.
In the not too distant past, it was believed that this realm would be inaccessible. We could have a certain amount of knowledge about atoms and molecules, but we could not touch or directly manipulate them. But now there are tools that enable researchers and technicians to actually manipulate individual atoms to create nanoscale machines including motors with spinning rotors.
The startling concept of nanotechnology, which implies manipulation and assembly on a submolecular scale, was discussed as early as 1986 by K. Eric Drexler in his book Engines of Creation: The Coming Era of Nanotechnology. He foresaw nanoscale assemblers that would be capable of copying themselves and other entities. We are not there yet, but progress has been noteworthy. The scanning tunneling microscope had been invented in 1981 and by 1983 it was being used to manipulate individual atoms.
Nanotechnologists say nano-sized structures could be built via what are called positional assembly techniques. The term refers to the idea of molecule-sized robot arms that pick up, move, and place molecules one at a time under the control of a computer. Get enough molecular robot arms and you could build everyday objects from the ground up.
Positional assembly can be contrasted with techniques used for molecular assembly today, self-assembly. Self-assembly is the term used to describe the process by which ordinary chemical reactions produce particular molecular structures. For example, when chemists mix solutions, the intrinsic attraction and repulsion of specific atoms and molecules produce conditions that allow atoms to spontaneously form specific molecular structures.
Researchers in molecular electronics use self-assembly to produce batches of molecular materials. They typically use photolithography to create submicroscopic holders and contacts that serve as test-bed connections to the material. They’ll usually test the resulting molecular electronic material via analytical techniques such as mass spectrometry to confirm various properties.
Though the concept of nanoassembly has been around for more than 30 years, scientists are still taking baby steps in the field. In 2010, for example, researchers at New York University created several DNA-like robotic nanoscale assembly devices. One is a process for creating 3D DNA structures using synthetic sequences of DNA crystals that can be programmed to self-assemble using “sticky ends” and placement in a set order and orientation.
But self-assembly techniques are limited. The problem with self-assembly techniques is that they only produce engineered molecular material in 2D films. The Holy Grail for nanoassembly is to create materials engineered one molecule at a time and built into 3D structures. The idea is to hold molecular parts in the right position so that they join with other molecules in exactly the right way. That’s tricky to do.
To position single molecules with respect to one another would require a nanoscale equivalent to computer-controlled robot arms and grippers. In this scenario, molecular robotic arms would move back and forth, withdrawing atoms from “feedstock” to build any structure desired.
There is, in fact, technology that can place individual atoms. Scanning-probe microscopes, invented in Zurich by IBM researchers during the 1980s, can not only map molecules deposited on a surface, but also deposit individual atoms and molecules in desired patterns. But their error rates are high enough to necessitate relatively sophisticated error detection and correction methods. Moreover, they are much too slow to manipulate one-by-one the uncountably large numbers of molecules needed to make every-day physical objects.
Consequently, there have only been a few halting steps taken toward creating 3D structures in nanoscale. In 2010, IBM used a silicon tip measuring only a few nanometers at its apex (similar to the tips used in atomic force microscopes) to chisel away material from a substrate. The result was a complete nanoscale 3D relief map of the world one-one-thousandth the size of a grain of salt. And IBM researchers generated the whole map in only two minutes and 23 seconds. Researchers say this activity demonstrated a way to generate nanoscale patterns and structures as small as 15 nm at greatly reduced cost and complexity, opening up new prospects for fields such as electronics, optoelectronics, and medicine.
Researchers have modified both atomic-force and scanning tunneling microscopes to move around molecules in experiments, so it might be useful to explain the difference between the two. Both of these instruments only record images in normal use. Specifically, an AFM captures images by moving a nanometer-sized tip across the surface of material to be imaged. The image is formed by measuring the small force between the surface and the tip. In contrast, an STM captures images using quantum tunneling. The STM images indirectly by calculating the quantum degree tunneling between the probe and sample.
Thus the tip of an AFM (carefully) touches the surface of the sample. But the tip of an STM is kept a short distance away from the sample surface.
Finally, an STM is normally used only with conductors and only in a high vacuum. An AFM can work with both conductors and insulators, and it doesn’t need a vacuum. Generally speaking, the AFM also tends to have a higher resolution.
Nanoassembly is certainly a gee-whiz application for AFMs, but it is atypical of their normal uses. A more usual scenario can be found at the University of Calif. at Irvine, where researchers use an AFM to extract messenger RNA, a process that could benefit cancer and stem cell
research. The AFM probe acts like a needle that penetrates cell membranes and pulls out RNA. It then transports and deposits the molecules elsewhere.
All in all, nanoassembly will probably remain more of an academic curiosity than a practical manufacturing method for a long time to come. The typical developments in this area have the same character as that unveiled by researchers at Rice University in 2006. They built a nanoscale car made of oligo(phenylene ethynylene) with alkynyl axles and four spherical C60 fullerene (buckyball) wheels. As temperature rose, the buckyball wheels turned and the nanocar traveled around a gold surface. At temperatures above 300°C the car moved around too fast for the chemists to keep track of it.