NANOTECHNOLOGIES — what are they and what is there for the chemical engineerby Jacob ZabickyInstitutes for Applied Research, Ben-Gurion University of the Negev

The title of this article refers to scientific and technological developments applying to systems in the nanometric scale, that is systems of size in the approximate range of 1 to 100 nm. The unit nanometer (1 nm = 1x10 -9 m) is of the size of a few atoms aligned in a row. The prefix “nano” is derived from the Greek nanos = dwarf. Nanotechnologies are developing at present in various directions, covering a variety of disconnected fields. It will suffice at this point to say that many graduate theses are being written on the various aspects of “Nanotechnology,” and if one were to attend all the international symposia, workshops, and conferences on these fields taking place around the world, practically no time would be left to do some work on the subject. To get an idea (or perhaps to become confused) about the amplitude of this complex field, the following main subjects were addressed in the Fifth Foresight Conference on Molecular Nanotechnology, that recently took place in Palo Alto, California:

* supramolecular chemistry and self-assembly* proximal probes (e.g. STM, AFM)* biochemistry and protein engineering* computational chemistry and molecular modeling* computer science (e.g. computational models, system design issues)* natural molecular machines* materials science* mechanical engineering (CAD) and robotics

A brief description of some of the main trends will be helpful before turning to the particular interests of chemical engineers.

First should be mentioned some fundamental studies, bordering on the philosophical, where the essential characteristics of nanometric systems are being analysed for their feasibility and possible impact on our way of living. Conceptual questions are considered about the design, assembly and functions of “nanomachines,” self-replicating “nanosystems,” quantum mechanics [1]. An excerpt from a recent comment by Gavin Wheeler [2] in the sci.nanotech newsgroup may be helpful to amplify in this respect:

... As for nanotech in the wider sense, many real physicists have taken a very detailed look at the problem, and there is even a very serious technical journal (Nanotechnology, from the Institute of Physics) dedicated to their work. Two such physicists are Ralph Merkle (see http://www.merkle.com/) and Eric Drexler, author of Engines of Creation and Nanosystems. The work of such scientists, and those two books, would give you a much better idea of what is and is not possible with nanotechnology than will the popular press. Engines of Creation is available, free, online, at http://www.asiapac.com/EnginesOfCreation/, while Nanosystems (published by Wiley) should be available from any biggish library. 'Nanosystems' is the one that really gives the detailed, technical side of the proof-of-concept. You might also want to look at http://www.nanosource.org/library/index.html

Important fields considered within the domain of the nanotechnologies are diverse types of microscopies, with resolution in the nanometric range. Transmission electron microscopy (TEM) and the newer developments of the scanning electron microscopy (SEM) are well established and widely used microscopic techniques for observation of nanoscale objects. Scanning probe microscopies (SPM) of various types can be applied to very high resolution studies of the surface of samples, often “showing” individual atoms. The most widespread among the SPM instruments is the scanning tunneling microscope (STM), that can be used with conducting samples. It is based on the measurement of the small tunneling current between the sample and a tip that scans the sample at a distance of a few angstroms from its surface. Another variation†of SPM is atomic force microscope (AFM), also called scanning force microscopy, that can be used for nonconducting samples too. It operates using a so-called cantilever of microscopic dimensions that scans the surface of the sample. The cantiliver, of typical dimensions 0.1 mm length and 0.001 mm width, looks like the arm of a record player. A recent prize-winning thesis on the subject was entitled “Atomically Precise, 3D Organic Nanofabrication: Reactive Lattice Subunit Design for Inverse AFM/STM Positioning” [3].

Nanotechnologies include the fabrication of nanoscale objects. Modified SPM techniques may serve as machining methods for submicron down to nanometric size litography. Concepts such as nanowires and quantum dots ceased to be only theoretical. These techniques allowed the development of electronic circuits with components of nanoscale dimensions. In some of such circuits single-electron phenomena can be examined [4].

Chemists and chemical engineers have long been familiar with certain systems in the nanometric scale. Colloidal systems are probably the most widely known and applied in technology, including surfactants, enzymes, hormones, proteins, soluble polymers, and many others. A concept that recently apperared in the literature is “colloidal epitaxy”¨ applying to colloidal particles that were left to agglomerate in an ordered pattern [5]. A field of utmost importance is heterogeneous catalysis, involving substances with very large specific surfaces, on which the “active sites” are often a few angstom across. In the present article we will not deal with such nanoscale systems, but with others that until recently were more in the realm of solid state physics and materials engineering. Perusal of recent issues of journal specialised in materials science can provide many illustrations of the ongoing research.

A subject that is being intensively investigated at present is mechanical alloying, applied mainly, though not exclussively, to metals. When metals are milled together they tend to lose their crystallinity, yielding the so-called amorphous metals. It has been demonstrated by microscopic observation that in many cases the ‘amorphous’ metal consists in fact of nanometric crystals, too small to offer a coherent domain for X-ray powder diffraction [5,6].

Fine ceramic oxide powders have size in the range of 1 micrometer, and are ireducible by further milling. Furthermore, the milling operation causes erosion of the mill and contamination of the comminuted sample. A physical method for size reduction down to the nano scale consists of impinging micron-sized particles on a non-adhesive wall, after melting with a laser beam. The original droplets break down into nanoscale ones and solidify into tiny speres. A frequently applied chemical technique consists of hydrolyzing metal alkoxides dissolved in organic solvents. When correctly carried out a sol of metal hydroxides is formed, which gradually thickens into a gel as the hydrolysis conversion advances; thus the name sol-gel technique. The gel is colloidal, and after separating the solvent, drying and calcinating the product at a relatively low temperature to eliminate water, an oxide powder is obtained consisting of nanoscale oxide crystals. Besides the small particle size, an important feature of the process is the possibility of preparing ceramic precursor powders of ultrahigh purity, limited by the purity of the initial alkoxide. Such purity level is required in biomedical appliances, electronics and investigation of material properties. The technique can be applied†to mixtures of several alkoxides, to obtain the nanocrystalline precursors of multicomponent ceramics. An alternative to the sol-gel technique recently developed consists of decomposing alkoxide vapors at high temperature and reduced pressure. The nanocrystalline powders prepared in the vapor phase are much less agglomerated than the sol-gel ones.

Ceramic materials derived from nanocrystalline precursor frequently possess highly improved mechanical and functional properties, as compared to those derived from conventional ceramic powders. These benefits notwithstanding, products derived from nanometric precursors are entering the market at a rather slow rate. The main reason that can be adduced for this is the high costs of production. It is a challenge to be met by chemical engineers to reduce the costs of these potentially useful products, by investigating the operations and processes involved and introducing adequate modifications to the established methods.

[1] D. W. Noid, R. E. Tuzun, and B. G. Sumpter, “On the Importance of Quantum Mechanics for Nanotechnology,” Nanotechnology, in press.[2] wheeler@lpbc.jussieu.fr[3] John M. Michelsen, University of California at Irvine; see http://sugar.ps.uci.edu/~jmichels[4] Examples of nanoscale microscopy and its applications can be seen in issue of the Journal of Vacuun Science and Technology B, 15 [July-August] (1997).[5] A van Blaaderen, R Ruel and P Wiltzius, “Template-directed colloidal crystallization,” Nature, 385. 321 (1997).[6] For example: Nastructured MaterialsÆ

A better idea of what are nanoscience and nanotechnology and where are they aiming to may be obtained from the subjects treated in dozens of conferences held all over the world. These are a few examples that can be examined in the course webpage:

• 11th International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials

• Nano and Microtechnologies in the Food and HealthFood Industries

• Nanoparticles for European Industry –Manufacture, Scale-Up, Stabilization, Characterization and Toxicology

• New Technologies and Smart Textiles for Industry and Fashion

Questions for thought . . .

1. Look at the programs of recent conferences on various nanotechnological subjects. What features make these subjects different from parallel technologies related to conventionally sized objects?

2. Say you found a “supergrowth hormone” by which advanced organisms may expand in bulk size, without affecting the size and functions of present cells. What problems do you expect in a five-fold linear expansion, for example, of a cow.

3. Say you found a “supershrink hormone” by which advanced organisms may decrease in bulk size, without affecting the size and functions of present cells. What problems do you expect in a linear contraction to a one-hundredth, for example, of a cow.