If today a compact disc (CD) can store some 240 million bytes, the equivalent, for example, of 300,000 pages of text written with double spacing, the prospect for the next few years is to increase this capacity to 10 billion bytes, 41 times more than the present one, or as much as 12.3 million pages. Besides dominating the music recording market, the CD is also the standard medium for multimedia devices, which combine text, images and sound, besides what it evolved into, the versatile digital disc or digital video disc (DVD). Accordingly, increasing the quantity of memory without enlarging the size of the disc are two strategic lines of research in this area of optical-electronic systems for the digital storage of information.
In this regard, the Photonic Materials group from the General and Inorganic Chemistry Department of the Chemistry Institute at the São Paulo State University (Unesp) in Araraquara has produced new materials and made important discoveries on the mechanisms for recording and unrecording DVDs. Under the coordination of Professors Younes Messaddeq and Sidney José Lima Ribeiro, the group is studying phenomena photoinduced by lasers in chalcogenides, special types of glass that have excellent properties – at the nanometric level – for recording digital information and holographic images.
Working with this material, they also discovered other mechanisms that explain the structural changes seen in the chalcogenides, when exposed to irradiation with light, particularly in compositions based on gallium, germanium, sulfur, arsenic sulphide and antimony. One of the phenomena observed is photoexpansion, which brings about an increase in the volume of the material, without any thermal alteration, just with the light from the laser. This technique also makes it possible to apply these special kinds of glass to the manufacturing of microlenses of the thickness of a strand of hair for security areas, in microcameras; in medicine, in invasive surgeries; and in the military area, in systems for remote guidance of missiles.
The importance of the experiments carried out in Araraquara lies in a pilot project presented last year by researchers from Matsushita Electrical Industrial, developed in Japan, which demonstrated the use of DVD in digital camera, in a way that is similar to videotapes. The raw material used in this DVD was based on chalcogenides made up of germanium, tellurium, antimony and sulfur. Nowadays, there is a worldwide technological race to develop these materials. Besides the characterization of the lasers that carry out the reading of the electrical signals that are contained in the little discs, the scientists are researching new methods and materials, to make the products more efficient and cheaper. Accordingly, fine films prepared from special kinds of glass are promising to replace, with advantage, the polymers currently used in the manufacture of CDs and DVDs.
Recording and rerecording
Chalcogenides guarantee the product three key points for the development of the technology for producing DVDs: stability, reversibility (the capacity for recording and rerecording on the same disc) and sensitiveness for the storage of data. At the moment, the principle of change of the reversible phase (which makes it possible to record, delete and rerecord) is attributed to photothermal induction, using temperature variations of lasers that change the state of these special glasses from crystalline to amorphous (non-crystalline) and vice-versa. In the amorphous phase, the material records the data; and in the crystalline phase, it reproduces it.
The researchers from Unesp believe that making the best use of chalcogenides will come from using the phenomenon of photoexpansion. Using a laser with suitable potency and exposure time, they found that the vitreous sample irradiated with ultraviolet light generated an expansion of the surface of the material in the order of 25%.
This percentage, which expands the spectrum of activity, making the radius of the lens bigger, makes it possible to store more information, in a superior manner to that noted in other kinds of chalcogenide glasses, receiving other intensities of light. These showed a maximum of variations through expansion of 0.7%. The results were presented in 2001, at the First International Workshop on Amorphous and Nanostructured Chalcogenides, in Bucharest, in Romania. “This award and the international recognition have given us the motivation to go into more depth in these studies, with the objective of revealing other properties of these materials and developing fine films to be applied in the advancement of DVD technology”, says Messaddeq.
The prizewinning research was started by Sandra Helena Peratello, studying for her doctorate, in conjunction with the Optical and Electrical Properties of Solids Laboratory of the Physics Institute of the University of São Paulo, in São Carlos, under the coordination of Professor Maximo Siu Li. “The objective was to produce chalcogenide glasses that had been little studied, based on gallium, germanium and sulfur. We found an extraordinary expansion in the irradiated surface of these materials. This discovery surprised the international community in this area”, Messaddeq explains.
From the scientific point of view, the explanations about the mechanisms that govern these chemical and physical changes in the chalcogenides, presented by several authors, were not sufficient to understand the origin of the phenomena of photoexpansion. Following structural analyses in more depth carried out at the National Synchrotron Light Laboratories (LNLS), in Campinas, the group from Araraquara observed that there is a reaction with oxygen during the exposure of the glass, demonstrating the interference of air in these phenomena. “Our analyses showed that photoexpansion disappears when the sample is irradiated in a vacuum. To the contrary, in an oxygen rich atmosphere, the phenomenon of photoexpansion is much greater. We showed that air is an important factor, particularly if we want to make reproducible materials”, the professor says. Once more, the results presented at the international congress for non-oxide glasses (oxide glasses are those used in windows and in ordinary mirrors), in September 2002, also surprised the international community.
The chalcogenides, compounds based on gallium, germanium and sulfur, developed by the group show high efficiency in diffraction, because they have a large number of grooves per millimeter, an advantage for DVD technology, as it provides more channels for storing data. However, photoexpansion shows disadvantages for the stability of the material. “In this regard, the group is endeavoring to look for alternative materials, based on the same matrix, to attain the desired responses”, says Messaddeq. Studies are being done on materials still little explored, like glasses based on antimony and tungsten oxide.
The group from Unesp has been working on high purity chalcogenide glasses since 1999. In the last few years, the majority of researches carried out in Brazil with this material has focused on glasses on the basis of arsenic sulphide, due to the facility with which it is prepared and purified. “The whole technology was developed to purify glass, leaving aside alternative materials like germanium and gallium”, he explains.
To get the technology for preparing chalcogenides, Messaddeq asked for cooperation and visited a laboratory in the Czech Republic. Since October of last year, other compounds of chalcogenides are under development, with the visit of Russian researcher Igor Scripachev, head of the Materials Purification Department of the High Purity Substances Chemistry Institute, of the Russian Academy of Science, as a visiting professor at the IQ/Unesp. “This Russian group is world leader in the area. The same technology was installed in the United States and in China at an extremely high price. We secured the cooperation of Professor Igor through a FAPESP scholarship”, says Messaddeq.
With the international repercussion of the group’s work, Messaddeq was invited to be a member of the Amorphous Chalcogenides and Nanostructure network, a group of researchers that is working in the area, with the incentive of the European Community. He was also invited to collaborate with researchers from the University of Cincinnati, in the United States, and from the University of Montpellier II, in France, besides groups in Hungary and Japan.
Study and Applications of Photosensitive Glasses; Modality Regular line of research grants; Coordinator Younes Messaddeq – IQ/Unesp; Investment R$ 87,221.00 and US$ 117,282.00