11 June 2009

Gallop on nanotechnology

Ten in minus nine
Popular about nanotechnology"Popular mechanics" No. 4-2009

Recently, it is difficult to find a publication in Russia that would not mention the word "nanotechnology" about and without it. However, the real meaning of this term is not clear to everyone. In this project, prepared with the participation of RUSNANO specialists, we will try to tell you what this word really means.

What is "nano"?

The prefix "nano" (Greek for "dwarf") means "one billionth of a fraction". That is, one nanometer (1 nm) is one billionth of a meter (10-9 m). To estimate the scale, imagine a globe and a penny coin – this is about how a meter and a nanometer relate to each other.

The school ruler is marked with millimeters (thousandths of a meter), micrometers (they are also microns, millionths of a meter) are the size of what is visible in a good microscope (cells, microbes and their organs). Viruses are measured in hundreds of nanometers, large protein molecules in dozens, and recently transistors in computer processors. Simple molecules are measured in units of nanometers, atoms in tenths.

NanoscaleAt the nanoscale, it is customary to measure what fits in the size range from atoms to viruses (0.1–100 nm).

Why does the range of nanoscale sizes arouse increased interest of scientists and technologists? The fact is that researchers have learned to operate with objects of such sizes quite recently. But it is at this level that many processes of fundamental importance are observed – from chemical reactions to quantum effects. Knowledge of these processes will make it possible to create nanoscale structures that give materials and devices useful, and sometimes simply extraordinary properties.

Science and technologyThe methods of creating such nanoscale structures are called nanotechnology.

Generally speaking, nanotechnology is not an independent branch of science. Rather, it is a complex of applied technologies, the fundamental foundations of which are studied in such disciplines as colloidal chemistry, surface physics, quantum mechanics, molecular biology, etc.

Feel the nanomirThe resolution of a conventional optical microscope (about half the wavelength of light) is insufficient for nanoscale objects.

In order to see the nanomir, we had to develop other methods

The diffraction limit for visible light makes it possible to achieve approximately 1000x magnification - this corresponds to a resolution of the order of several hundred nanometers. It is impossible to see objects with the size of tens, and even more so in units of nanometers in such a microscope. Therefore, the first step to the nanomir was an electron microscope.

Electron microscopeBy its principle, it resembles an ordinary microscope, but instead of light, electrons focused by magnetic lenses work here.

A beam of electrons passing through a thin sample interacts with it, and then falls on a fluorescent screen that makes the picture visible to the human eye. Atomic layers and steps are already visible in the photos taken with a transmission electron microscope, which allows you to achieve magnification by millions of times. The atoms there have the form of dots, and in order to examine the surface in detail, more advanced tools using other principles are needed.

How the electron microscope (EM) worksThe work of EM is based on the fact that electrons, like photons, exhibit both corpuscular (inherent in particles) and wave properties at the same time.

Accelerated to high energies, they can have a de Broglie wavelength of hundredths of a nanometer (15 keV corresponds to 0.01 nm). And although electronic lenses are significantly inferior to optical focusing properties, the magnification of an electron microscope can reach millions of times, and the resolution is tenths of a nanometer.

Scanning Tunneling MicroscopeA device using the quantum tunneling effect, a scanning tunneling microscope (STM), allows us to consider individual atoms.

However, to be precise, the scanning tunneling microscope does not examine, but rather "feels" the surface under study. Not literally, of course: a very thin needle-probe with a tip one atom thick moves over the surface of the object at a distance of about one nanometer. At the same time, according to the laws of quantum mechanics, the electrons overcome the vacuum barrier between the object and the needle – tunnel, and a current begins to flow between the probe and the sample. The magnitude of this current depends very much on the distance between the tip of the needle and the surface of the sample – when the gap changes by tenths of a nanometer, the current can increase or decrease by an order of magnitude. So, by moving the probe along the surface using piezoelectric elements and tracking the change in current, you can explore its relief almost "by touch".

The creation of STM was a significant step in the development of the nanomir. In 1986, Gerd Binnig and Heinrich Rohrer, employees of the IBM Research Center in Zurich, were awarded the Nobel Prize for this achievement.

STM allows you to see surface details with a resolution of hundredths and even thousandths of a nanometer (corresponds to an increase of about 100 million times). In fact, as already mentioned, this is not a photo. This is just a graphical representation of how the gap between the probe and the surface changes to maintain a constant current value. The interaction of the STM probe with the electron shells of atoms makes it possible to study the smallest details available today.

Magnetic resonance power tomographyMagnetic resonance imaging (MR) has literally revolutionized modern medicine.

For the first time it became possible to observe biological processes in real time without disturbing their natural course. However, the highest resolution of modern tomographs is measured in fractions of a millimeter, when moving to a smaller scale, problems begin to arise. Special MR microscopes have a resolution of the order of micrometers – this is all that can be obtained using traditional magnetic resonance methods. Noise in the signal received by the coils interferes with achieving greater accuracy. But scientists have come up with a way to circumvent this limitation: recently appeared magnetic resonance force microscopes use direct measurement of the interaction force of the gradient magnetic field with the spins of hydrogen nuclei in a sample located on the tip of the cantilever. The deviation of the cantilever is measured using a laser interferometer. Using this technique, in 2007, a resolution of about 0.1 microns (on a slice of an inorganic sample) was achieved at IBM's Almaden Research Center in San Jose. And more recently, researchers there have built and demonstrated the capabilities of three-dimensional MR scanning on a sample of tobacco mosaic virus (18 nm in diameter and up to 300 nm in length). By combining MR force microscopy with three-dimensional mechanical scanning and using a special algorithm for processing the data obtained, scientists were able to achieve a spatial resolution of about 4 nm when scanning a biological sample.

Atomic Force microscopeSTM has one important limitation: the object of research can only be metals or semiconductors (recall that the effect is based on tunnel current).

Dielectrics in STM will not be "considered". For their study, the STM developers proposed another method called scanning atomic force microscopy. The principle of its operation is that at small distances between the probe and the sample, a force acts, the magnitude and direction of which depend on the gap. This force is measured by fixing the probe needle on an elastic cantilever suspension (cantilever) and determining its deviation. Atomic force microscopy can be used to study any surface, regardless of whether they are conductors or dielectrics.

One of the important advantages of the atomic force microscope (AFM) is the possibility of its use in the study of biological samples: it does not require vacuum or thin layers (unlike an electron microscope). AFM also allows you to study not only the relief of the surface, but also the interaction between specific molecular objects – it is enough to "fix" one of the studied molecules on the tip of the probe. However, AFM is much inferior to STM in resolution (on the order of units of nanometers) due to strong thermal noise affecting measurements.

See the nanometerAFM and STM are special cases of the so–called scanning probe microscopy, a very powerful research tool that allows you to study various properties of surfaces, not just the relief.

Everything is determined by what to use as a probe. For example, with the help of a conductive needle, it is possible to study the local dielectric properties of the surface with nanometer accuracy – this is electro-force microscopy (ESM). Using a ferromagnetic probe, it is possible to study the distribution of the magnetic field at nanometer scales (MSM, magnetic force microscopy).

One of the most interesting and exotic variants of probe microscopy is scanning near–field optical microscopy (BOM), developed by Dieter Pohl, an employee of the IBM Research Center in Zurich. In this case, a diaphragm with a diameter of several nanometers is used as a probe. Light with a wavelength of hundreds of nanometers is able to penetrate through such a subwavelength diaphragm according to the laws of quantum mechanics, but at a small distance comparable to the diameter of the hole (this is the so-called near field). If you place a sample there, the light reflected from it can be registered. In this case, a real image of the surface in visible light is obtained, depending on its local optical properties, and with a nanometer resolution!

Scanning Electron microscopeIf you do not shine through the sample, but scan its surface with a beam of electrons focused into a very small spot (several nanometers), the latter not only scatter on the atoms of the sample, but also generate secondary electrons, X-rays and visible radiation.

The work of the scanning electron microscope is based on the registration of these data. Unlike translucent EM, it can be used to examine "thick" samples. By registering the scattering angles, radiation intensity and secondary electron energies, it is possible to study not only the surface relief, but also the chemical composition of the sample, as well as the structure of the sample in the subsurface layer (tens and hundreds of nanometers). The resolution of a scanning electron microscope is usually somewhat less than that of a transmission microscope, and ranges from units to tens of nanometers.

Nanotechnology is all around usLike Mr. Jourdain from Moliere's "Philistine in the Nobility", who did not know that he was speaking in prose, many do not realize that some of the familiar things around us are already achievements of nanotechnology

If you think that nanotechnology is a distant future or even the lot of science fiction, then you are mistaken. Nature "invented" nanotechnology (as well as many other things) long before man, who only in the last few decades has been following the same path, trying to repeat some of her inventions. The successes so far are quite modest, but scientists have already managed to achieve some achievements in the field of nanotechnology. Here are just a few examples.

On every tablePerhaps the most famous example of successful and mass–produced nanotechnology is electronic components.

A few years ago, this area was called microelectronics, but now it can be rightfully called nanoelectronics: in 2003, Intel switched to 90-nm processor technology, which fully falls under the definition of nanotechnology (less than 100 nm). And progress in this area is very fast – currently Intel processors are already being produced using 45 nm technology. Moreover, this is a mass and serial production, which is in almost any modern computer. Such a processor consists of many hundreds of millions of transistors, each of which has a size of only a few tens of nanometers. Compared to the previous generation (65 nm technology), the clock frequency has increased (about 3 GHz), the number of transistors has also increased (almost doubled), and heat dissipation has significantly decreased. In the next few years, Intel plans to switch to 32-nm, and then to 22-nm processor manufacturing technology.

Useful dustOne of the most popular types of nanoproducts are ultrafine powders.

The grinding of substances to nanoparticles of tens or hundreds of nanometers in size often gives them new useful qualities. The fact is that such a nanoparticle consists of only a few thousand or millions of atoms, so they all end up close to the surface, on the border with the outside world, and interact energetically with it. The total surface of the particles in such a nanopowder becomes huge.

For example, silver in the form of nanoparticles becomes extremely harmful to bacteria – this property is successfully used in modern wound healing dressings, as well as in antimicrobial tissues. Nanopowder from used tires, when added to the raw material for asphalt, makes the road surface extremely wear-resistant. Clay nanopowders have been actively used in insulating coatings of power cables in recent years – such insulation burns very badly, and this is very good for the safety of buildings. Titanium dioxide nanoparticles (the bases of the well-known titanium whitewash) are a very effective photocatalyst and are used as an active element in filters of household air purifiers. And platinum nanoparticles are used in catalytic afterburners of modern cars to reduce the emission of harmful substances into the atmosphere.

NanodrugsUnfortunately, the medical nanorobot (nanobot), the description of which is so fond of flaunting in popular literature, is fantastic.

However, this does not detract from the success of nanotechnology in modern medicine. One of the main areas of work is nanocapsules for targeted drug delivery. This method allows you to affect only the affected cells without damaging healthy ones. This idea was formulated at the beginning of the XX century by the German doctor Paul Ehrlich and called by him a "magic bullet" – but only nanotechnology (for example, placing the active substance in a capsule of liposomes) made it possible to achieve its implementation. Drugs of this type (liposomal) for the treatment of certain forms of cancer and fungal infections, hepatoprotectors and even influenza vaccines have been mass-produced since the mid-1990s.