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Nanochemistry: an introduction

Special states of matter

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Few areas of chemistry-related materials science have blossomed and grown so rapidly as nano-scale matter. With new discoveries and applications appearing every week, it is impossible to provide a comprehensive view of this rapidly-expanding area here. Instead, we will limit ourselves to indicating why this state of matter is so important, how nanomaterials are prepared, and how they are visualized and studied. We will also briefly look at the carbon-based nanomaterials, and at a very small selection of recent applications.

### What are nanomaterials?

Nanomaterials or nanopowders are tiny bits of solid (usually discrete crystals) whose grain sizes fall into the 1-100 nm range in at least one direction, and often in all three dimensions. Many such materials have been known for a very long time, although it is only relatively recently that they have been thorougly characterised. Two examples are carbon-black, used in printing inks and car tires, and fumed silica. The small dimensions of nanomaterials can result in exceptional properties. For example, nanoparticulate oxides of silicon and germanium can undergo elongations by factors of 100-1000 without failing (superplasticity) because the particles are able to slide along independently without interfering with each other as happens in conventional bulk solids.

Similarly, exceptional degrees of ductility, hardness, and strength, even at high temperatures, have been observed. One method of generating super-hard materials is to embed nanoparticles in a non-crystalline matrix. For example, nanocrystals of titanium nitride embedded in thin films of silicon nitride have approached the hardness of diamonds.

Nanomaterials tend to be highly active chemically, owing to the larger fraction of atoms or molecules exposed at surface. For this reason, such materials often need to be protected or stabilized in order to maintain their integrity.

### Carbon nanomaterials

The first carbon nanostructure was the soccer-ball C60 molecule known as fullerene. Shortly after this came the C70 molecule which resembles a rugby ball. Since that time, fullerenes up to and beyond C120 have been prepared. Other atoms can be placed inside the fullerene ball (endohedral) or attached to the outside (exohedral). Fullerene salts can also be prepared in which metal cations balance the electric charge resulting from addition of electrons to the carbon structure. Alkali metal salts of C60 can markedly increase the electrical conductivity of films of this substance, which by itself is only semiconducting.

Carbon nanotubes have gradually become more important than fullerenes. These structures are similar to graphite, except that the sheets of hexagonal rings are rolled into cylindrical tubes. When bundled together, the tubes make only tangential contact with one another, and therfore act very much as individual molecules. The tubes can have single- or multiple walls and can have end caps made of half-fullerene balls. The ability of the graphite sheets to be rolled along various axes offers additional structural possibilities which affect their physical and electrical properties. The latter can range from semiconducting to metallic.

The first fullerenes were prepared by laser vaporization of graphite, and this method has been developed to the point where gram-quantities can be made. Plasma arcing has been a much more widely-employed and practical method. The fullerenes concentrate in the soot formed by the arc, while the nanotubes are deposited on the cathode surface. Instead of pure graphite, electrodes can be made of coal, which is not only cheaper but has been found to produce a large variety of other carbon nanostructures owing to the presence of hydrogen and other elements.

Plasma arcing in the presence of cobalt yields web-like structures made of long strings of single-walled nanotubes. Other elements, especially sulfur and iron, and even molecules such as naphthalene, have been found to influence the structural mix of products.

These two “classical” methods suffer from problems of product uniformity and scaleability. The reduction of hydrocarbons such as acetylene, ethylene, or methane on a catalytic surface has shown some promise as an improved way of generating carbon nanostructures.

### How nanoparticles are made

Nanoparticles are almost never the most thermodynamically stable forms of a substance, so most methods of preparation require the expenditure of energy in order to reduced ordinary matter to nano dimensions. Some of the major methods are outlined here.

Plasma arcing
  An electric arc is a plasma (ionized gas) created by vaporizing atoms in an electric field. Removal of electrons at the anode creates positive ions which are transported across the arc to the cathode where they pick up electrons and are deposited as neutral atoms. This is the method by which carbon nanotubes were first made, using electrodes made of amorphous carbon.
Chemical vapor deposition
  The material is vaporized by heating in a vacuum and allowed to deposit on a surface. The material is either deposited directly, or reacts on the surface to form a new substance. The same general method can also be used to make metal nanoparticles by microwave production of a metal-ion plasma. CVD is often used to make nanoparticles of oxides and carbides of metals in the presence of carbon or oxygen. The first layer of the new deposit can often act as a template for the deposition of further layers, resulting in what amount to a self-assembled structure. It is possible to engineer this structure by employing electron-beam lithography to modify the first layer before further layers are deposited.
Sol-gel (colloidal) deposition
  Colloids are much larger (20-100 um) than nanoparticles, but dispersions of such particles in liquid (usually water) can be agglomerated into extended networks known as gels. Addition of a suitable microstructured agent such as silica, aluminosilicates, or zirconia can impose an ordered structure on the gel, after which solvent molecules can be carefully removed while retaining the individual components of the original colloid. There are many variants of this method. One practical application is the production of commercial sunscreen products which remain encapsulated within glass-like nanoparticles. These have the advantage that when the photo-active component absorbs ultraviolet light, any reactive by-products remain separated from the skin.
Electrodeposition
  Electrochemical deposition of metals has long been used to produce finely-divided materials such as zinc powders and the platinum blacks that are made to increase the effective surface area of platinum electrodes. More recently, techniques involving deposition from liquid crystal mixtures have made it possible to deposit flat, uniform, and shiny deposits of metals including Pt, Pd, Ni, and Au, as well as certain semiconducting organic polymers and other materials. The technique has been used to deposit gold inside nanoholes produced by ion bombardment of polymer films, making them into controllable electrical, optical, or ion-sensing devices.
Ball milling
  This seemingly brute-force method has been known for more than a century as a way of producing very finely-divided powders and pigments. In this process, small balls of steel or other hard materials are placed in a rotating drum containing the solid to be pulverized. Ball milling is the preferred way of preparing nano-sized metal oxides whose uses range from specialized coatings and abrasives to structural ceramics and catalysts. Surprisingly, ball milling has also been used to prepare nanotubes of carbon, boron nitrides, as well as particles of non-stoichiometric intermetallic compounds of nickel and aluminum.
Using nanoporous materials
  Aluminosilicates such as zeolites, which can be prepared by conventional chemistry, contain nm-scale pores that can be used to fabricate nanoparticles. Treatment with surfactants, Insertion of ions within these pores or intercalation of other substances between the layer lattices followed by heating can be used to build up interesting structured and composite materials.

### Visualizing and manipulating nanoparticles

It is obviously impossible to use an ordinary microscope to resolve particles whose dimensions may only be ten percent of that of visible light. Here are brief descriptions of some of the current methods.

#### Transmission electron microscope (TEM)

This resembles the electron microscope technology that was first developed in the 1930s, in which a beam of electrons replaced the light beam employed in conventional microscopy. The sample thickness must be small enough to allow electrons to pass through. The emergent beam is enlarged with electromagnetic lenses and transmitted to a detector and imaging device.

TEM magnifications are usually at least 400,000 and go as high as 15 million, which can image individual atoms. As in SEM (see below), emission of secondary electrons from the sample can yield information about the sample composition. One limitation is that the electron beam can damage the sample; this can often be reduced by cooling the sample.

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More on transmission electron microscopes: Wikipedia article

#### Scanning electron microscope (SEM)

When an electron from the beam encounters a nucleus in the speciman, it is scattered at an angle that depends largely on the atomic number of the nucleus and on the surface topology of the sample. An additional interaction can occur between the incident electtrons and those of the atoms close to the sample surface, often resulting in emission o secondary electrons. By combining these two effects, SEM can yield much information about the surface and its composition.

[image] | [image]

A narrow (<20 nm) electron beam, focussed by an electromagnetic lens, scans across the specimen in a raster pattern. Secondary particles emitted by the specimen are picked up by a detector and their inerference patterns are deconvoluted by a computer and transmitted to a CRT display which follows the scanning pattern, thus reproducing the varying levels of secondary emission.

More on scanning electron microscopes: Wikipedia article

#### Atomic force microscopy (AFM)

This technique measures the force between a probe (known as a cantilever) and the surface being investigated. This force is attracive at greater distances (van der Waals or dispersion forces) and repulsive at short distances (the universal repulsive force). AFMs can usually work in both modes, known as contact and non-contact modes

| The AFM moves a sharp tip across a surface, which can be in contact with air or a liquid. The tip, whose radius is around 5 nm, is attached to a cantilever which bends according to the force exerted between the surface and the tip as it travels along the surface. The cantilever deflection is detected by a laser beam reflected from its surface to a position-sensitive detector whose resolution can be less than 1 nm. images: left - right |

If the cantilever surface in contact with the sample is coated with one-half of a DNA helix, the device can recognize a complementary sequence of nucleotides by its degree of hydrogen bonding, providing a rapid method of DNA analysis.

It can equally well be used to pick out individual proteins that bind to an antibody. In this artist's depiction from Arizona State U., an antibody is tethered to the AFM probe by a fine polymer thread. It is then pulled across the protein-coated surface until it binds to a complementary site as indicated by a change in the

[image]

Specialized AFM systems can measure lateral forces, magnetic forces, electrochemical processes, and certain mechanical properties of the surface.

#### Scanning tunneling microscope (STM)

The STM was the first instrument to generate real-space images of surfaces with atomic resolution. Its development in 1981 led to the 1986 Nobel prizes for its inventors at the IBM-Zurich laboratory.

The operation of the STM is similar to that of the AFM except that the scanning tip must be conductive so that a bias voltage can be maintained between the tip and the sample (which must also be conductive). When the tip is moved to within 1 nm of the sample, electrons can tunnel between the two surfaces. The resulting current varies with the spacing between the sample and the tip. Most devices can either by maintaining a constant distance, or by varying the distance in order to maintain a constant current. Although the STM nominally measures surface topography, it really measures electron-tunneling profiles which can be advantageous in studies in which electron densities are of interest.

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STMs can be used to study adsorbed molecules (including water) on a surface, and also on surfaces immersed in a liquid, including ionic solutions in which electric double layers are of interest. STM probes which act as electrochemical electrodes can image electronic and structural properties of electrochemically-active surfaces, and are useful for studies of adsorption, phase formation, and corrosion.

#### Nanomanipulators and nanotweezers

A suitably-modified STM can exert a force on a sample surface which results in a varying current as the surface is scanned. This current can be translated into a force applied to a device that fits on the operator’s finger tip, thus allowing one to “feel” objects as small as viruses. It will soon be possible for an operator to pick up and move objects on the surface by means of finger and hand motions.

The earliest nanotweezer consists of two carbon nanotube tips attached to a gold surface. These tips can be opened and closed like chopsticks by varying their electrical potentials. These are presently quite crude compared to what will likely become available.

Single-atom manipulation is presently done by applying a small voltage pulse to an STM tip. Atoms or molecules can be selective lifted, moved, and re-deposited on the surface, as most famously shown in the nano-sized IBM logo made with argon atoms or the “carbon monoxide man”.

### Gallery

### References

Carbon nanotube radio receiver (Scientific American, March 2009)