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Physics

Conductor polymers under control

Brazilians dominate the new area of materials technology which gained the Nobel chemistry prize in 2000

Brazilian scientists already dominate one of the most promising areas of materials technology: that of conductor and semi-conductor polymers, also called synthetic metals. These plastics which conduct electricity – and whose discovery resulted in the Nobel chemistry prize of 2000 for three scientists – are destined to turn into one of the principal prime material of the components of equipment and electronic gadgets.

To disclose their potential, a group of researchers worked on the thematic project Polyanilin and Poly (p-Phenylene Vinylene) as Active Elements of Electronic Devices and Optoelectronics, financed by FAPESP. Coordinated by Dr. Roberto Tendon da Fair, of the Physics Institute of São Carlos of the University of São Paulo (IFSC-USP), the project has the participation of the Polytechnic School of the Faculty of Science and Letters of Ribeirão Preto, both of USP.

Professor Faria belongs to the Group of Polymers Bernhard Gross (GPBG), which for more than 20 years has been researching the electrical and physical-chemical properties of polymeric materials, and, since the beginning of the 90s, has been studying conjugate polymers (conductors and semiconductors). The GPBG, which has collaborated with one of the Nobel chemistry prize winners of 2000 – the North American Dr. Alan G. MacDiarmid, with whom Dr. Faria jointly published work -, dominates from the synthesis of these electronic polymers until the manufacture of appliances (transistors, light emission diodes, etc.), passing to the technology of the production and characterization of ultrafine organic films. Among the possible applications of these products is the manufacture of flat screens for televisions and monitors, in substitution of the century old cathode ray tube, and the use of these organic products in medicine.

Chemistry changes physics
Until the middle of the 70s, nobody would have heard it said that any organic compound was capable of being a good conductor of electricity. The discovery which rocked the concept of the tight world of material physics came from a chemical laboratory. Those responsible for the change of position – the chemists Dr. Alan G. MacDiarmid, of the University of Pennsylvania (United States), and Dr. Hideki Shirakawa, of the University of Tsukuba (Japan), along with the American physicist Dr. Alan J. Heeger, of the University of Santa Barbara – received the Nobel chemistry prize of 2000. “Proof that science doesn’t live at the one address.” remembered Dr. Adnei Melges Andrade, professor at the Polytechnic School who participated in the project.

The discovery happened by accident. The Chinese assistant of Dr. Shirakawa carried out a synthesis of polyacetylene, a simple polymer formed from carbon and hydrogen. Since he didn’t understand very well the Japanese of his boss, he ended up making a mistake while following the recipe given by Dr. Shirakawa. Instead of the expected result – an infusible powder – the assistant produced a strange polymeric film.

Intrigued by that material with a metallic shine, Dr. Shirakawa decided to carefully guard it. Sometime later, he didn’t hesitate to show it to Dr. MacDiarmid, who was passing through Japan. There began a partnership which would result in a new line of research, followed today in many countries. At the invitation of Dr. MacDiarmid, Dr. Shirakawa went to Pennsylvania and both, joined by Dr. Heeger, began to investigate the physical – chemical properties of polyacetylene.

Chemical doping
On disclosing the mystery of that film, they came up against a new technique, capable of transforming a material, originally an insulator, into an excellent conductor of electricity. Known as chemical doping, this technique is very simple: molecules of acid – called doping molecules – are injected into the polymer and exchange electrical charges with the molecules of the polymer. “The method is reversible – that is, one can dope and undope a material -, which permits the total control of the degree of conductivity which one wants to confer to it.” explained Dr. Roberto Faria.

For many years the conducting polymers have been attracting scientists, who quickly found applications. The first of them was the lining of an aircraft invisible to radar – the North American fighter aircraft F-117 Stealth, used in the Gulf War. Another was the blinding of electronic equipment. Contrary to what one might think, it was not the high conductivity but the properties of semi conductivity – obtained through the interruption of the process of doping right at the initial stage – which augment the market potential of these polymers. For their optoelectronic properties – conduct electricity and as well emit light if stimulated by an electric current or luminous radiation -, they promise to be the stars of electronics of the 21st century.

Colors of the rainbow
The major objective is, with the support of a technology company, to implant a prototype of a production line of the devices available within the laboratories involved. The researchers already have control of all of the processes of synthesis and manufacture of the ultra thin polymer films. Another stage of the research, coordinated by Dr. Adnei Andrade, is in the micro electronic laboratory of the Polytechnic school: part of the preparation and characteristics of the devices with transistors, diodes and solar cells.

The project has three stages. The first is the production of the films, which begins with the synthesis of the polymers coordinated by the chemist Dr. Debora Tereza Balogh. She works with the basic polymer polyparaphenylene, which is submitted to the chemical reactions needed to turn it more or less into a conductor. Also it is through the chemical substitutions that it acquires various colors, depending on the choice of the lateral groups of the chains.

Diluted in solvent in a test tube, the primary material already shows a beautiful luminous effect: one only needs to switch on a light or pass an electric current for the liquid to shine like a lamp. This happens because the structure of the polymeric semiconductor materials contain “centers” where the carriers, positives and negatives, stay “localized”. In these “centers” there is a recombination between the negative carriers (electrons) and the positive carriers (holes), in a process called electronic transition.

This transition results in the liberation of energy in the form of light. The colors emitted – red, orange, yellow and blue – vary according to the polymer which is made or derived depending on the laterally substituted groups.In the chemistry laboratory of GPBG, students taking their masters and doctorates are researching new products and new methods of synthesis with the objective of obtaining all the colors of the rainbow.

Very fine and aseptic
After synthesis comes the preparation of the film. There are various methods for this. In the laboratories of GPBG there are methods available from the production of films with disordinated structures to those whose structure has a high degree of molecular order. The most sophisticated is the Langmuir-Blodgett method, which allows the making of films with a thickness of a molecular layer – that is, of a few nanomters or millionths of a millimeter. This monomolecular thickness is already sufficient to confer on the material the optoelectronic properties desired.

Though the work requires expensive state of the art equipment – a Langmuir-Blodgett cube with its accessories can cost US$ 60,000 -, the production follows a simple principle. In contact with the water, the diluted polymer spreads along the surface in a fine film like oil in suspension. The equipment permits the control of all the conditions necessary for the formation of the film, as well as its dimensions. Then the film is transferred to a substrate – a glass sheet or other material which permits perfect adhesion – a condition essential for the next steps.

To be correct, this delicate operation has to be done in a totally controlled atmosphere. If it wasn’t for the equipment, the “clean room” as the laboratory is called, could be confused for a surgical center. “The water used in the cubes is super purified and any ‘dust’ could compromise the work of an entire day.” explained Dr. Jose Alberto Giacometti, a researcher at GPBG. For this reason, the scientists work completed covered, with overall, hood, mascara and slippers.

Examinations and products
The films produced them pass through a battery of examinations. The molecular structure is characterized in the microscope laboratory of GPBG, equipped with an electronic microscope (SEM) and of atomic force microscope (AFM), which have a resolution of 0.1 nanometer. Also optical studies of the material are carried out, coordinated by a specialist in photo luminosity Dr. Francisco E. G. Guimarães, and then a check on its magnetic properties at the laboratory of Dr. Carlos F. O. Graeff, in the faculty of Philosophy, Science and Letters of Ribeirão Preto.

Finally, the film is ready for use in the production of the devices which are carried out in collaboration with the Polytechnic school, where the physicist Dr. Adnei Andrade coordinates the work. A specialist in electrical engineering with published works in international magazines, Dr. Andrade has already constructed various light emission devices (LEDs), transistors for field effects and photovoltaic diodes.

The development of these devices helps the researchers to better understand the electronic mechanics involved, so that they may be improved. However, this know-how, principally for the construction of displays, has a large chance of becoming a commercial product, assured Dr. Andrade.

Many publications
Over the last five years, the GPBG produced close to 50 international articles on polymer conductors and semi-conductors, as well as papers in congresses and symposiums. At the last International Conference on Science and Technology of Synthetic Metals, which took place in July in the Austrian city of Badgastein, close to 30 of the 1,000 papers presented were Brazilian and of those almost 20 were from GPBG.

“Brazil no longer passes unnoticed on the international scientific stage.” emphasized Dr. Faria. So much so that at one of these conferences almost ten years ago, a close collaboration was formed between GPBG and Dr. MacDiarmid. The first visit of Dr. Faria to the University of Pennsylvania in 1990, was fundamental for the partnership which resulted in the publication of various works. The last of them, which involved the doctorate of Dr. Jose Eduardo Albuquerque and the participation of two other Brazilians – Dr. Luiz Henrique Mattoso and Dr. Debora Terezinha Balogh -, was fundamental for the sequence of researches of the group into polyanilins.

Directly orientated by Dr. MacDiarmid, the at that time student Luiz Mattoso brought to Brazil the method of synthesis of polyanilins, which would substitute with advantages the polymers used until that time. More stable when in contact with the environment, the polyanilins are more easily synthesized and processed.

On the visit which Dr. MacDiarmid made to the laboratories of IFSC, Dr. Faria confirmed the impression which he had of the scientist: “A man with energy and a capacity for work beyond the norm. In this way he is able to extract the maximum from his students and leaves them almost exhausted, but, he is always so polite, that nobody ever refuses as request from him.” he completed.

Smaller TVs and computers

The researchers of GPBG highlight lots of applications for semi conductor polymers. As they are capable of converting electricity without generating heat – the so called cold light -, they are ideal for luminous warning devices such as the so called LEDs (light emitting diodes) used in the instrument panels of cars for example. “By improving their efficiency in the emission of light, these materials could be used even in the illumination of larger areas, substituting the actual headlamps.” guaranteed Dr. Faria.

Another field of application is in paints: in the place of common polymer resins, the paints would have luminescent resins, for example. “In this way, you could obtain a painted wall which emits light.” added the researcher. Malleable, the polymers could be injected, blown or used as a coating in the form of skins or ultra fine films, which is indeed the focus of the research of GPBG.

These films are also being studied for the manufacture of flat television screens, which permit the transformation of the equipment into a type of picture which would hang from the wall. There are still obstacles towards making these televisions commercially viable: the definition and the speed of the image, but Dr. Faria remembers that large companies within the sector are investing in research to overcome this. An example is Dupont, one of the largest in the area of polymers, who bought over Uniax – a company created by Dr. Heeger through the University of Santa Barbara – to produce LEDs and flat screens. Philips as well is buying a company generated through the University of Cambridge (England) and Sony is investing in products originating from the University of Nagoia (Japan).

As an extension to this, computers as well should become a lot smaller in size and cheaper: the flat screens should decrease the price of the monitors, which today compose close to 50% of the cost of a micro. Even the laptops will be perfected, with the elimination of the inconvenient fact of the emission of direct light, which makes the image go out of focus depending on the angle of vision. Another important application are the visors or displays, such as those of the cellular phone.

As well, medicine has already predicted important advances based on these polymers, which could be used for the manufacture of implants such as pace makers or in the reconstitution of muscles which have lost their capacity of movement. “As they are organic material, they offer less risk of rejection.” concluded Dr. Faria.

The project
Polyanilin and Poly (p-Phenylene Vinylene) as Additive Elements of Electronic and Optoelectronic Appliances (nº 99/05701-8); Modality
Thematic project; Coordinator Dr. Roberto Mendonca Faria – The Institute of Physics of São Carlos of the University of São Paulo; Investment R$ 150,000.00 and US$ 110,000.00

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