Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the University of Konstanz are working on storing and processing information on the level of single molecules to create the smallest possible components that will combine autonomously to form a circuit. As recently reported in the academic journal ("Light-Induced Switching of Tunable Single-Molecule Junctions"), the researchers can switch on the current flow through a single molecule for the first time with the help of light. Dr. Artur Erbe, physicist at the HZDR, is convinced that in the future molecular electronics will open the door for novel and increasingly smaller – while also more energy efficient - components or sensors: “Single molecules are currently the smallest imaginable components capable of being integrated into a processor.” Scientists have yet to succeed in tailoring a molecule so that it can conduct an electrical current and that this current can be selectively turned on and off like an electrical switch. 
Light on – molecule on. For the first time a light beam switches a single molecule to closed state (red atoms). At the ends of the diarylethene molecule gold electrodes are attached. This way, the molecule functions as an electrical switch. (Image: HZDR/Pfefferkorn ) This requires a molecule in which an otherwise strong bond between individual atoms dissolves in one location – and forms again precisely when energy is pumped into the structure. Dr. Jannic Wolf, chemist at the University of Konstanz, discovered through complex experiments that a particular diarylethene compound is an eligible candidate. The advantages of this molecule, approximately three nanometres in size, are that it rotates very little when a point in its structure opens and it possesses two nanowires that can be used as contacts. The diarylethene is an insulator when open and becomes a conductor when closed. It thus exhibits a different physical behaviour, a behaviour that the scientists from Konstanz and Dresden were able to demonstrate with certainty in numerous reproducible measurements for the first time in a single molecule. A computer from a test-tube A special feature of these molecular electronics is that they take place in a fluid within a test-tube, where the molecules are contacted within the solution. In order to ascertain what effects the solution conditions have on the switching process, it was therefore necessary to systematically test various solvents. The diarylethene needs to be attached at the end of the nanowires to electrodes so that the current can flow. “We developed a nanotechnology at the HZDR that relies on extremely thin tips made of very few gold atoms. We stretch the switchable diarylethene compound between them,” explains Dr. Erbe. When a beam of light then hits the molecule, it switches from its open to its closed state, resulting in a flowing current. “For the first time ever we could switch on a single contacted molecule and prove that this precise molecule becomes a conductor on which we have used the light beam," says Dr. Erbe, pleased with the results. "We have also characterized the molecular switching mechanism in extremely high detail, which is why I believe that we have succeeded in making an important step toward a genuine molecular electronic component.” Switching off, however, does not yet work with the contacted diarylethene, but the physicist is confident: “Our colleagues from the HZDR theory group are computing how precisely the molecule must rotate so that the current is interrupted. Together with the chemists from Konstanz, we will be able to accordingly implement the design and synthesis for the molecule.” However, a great deal of patience is required because it’s a matter of basic research. The diarylethene molecule contact using electron-beam lithography and the subsequent measurements alone lasted three long years. Approximately ten years ago, a working group at the University of Groningen in the Netherlands had already managed to construct a switch that could interrupt the current. The off-switch also worked only in one direction, but what couldn't be proven at the time with certainty was that the change in conductivity was bound to a single molecule. Nano-electronics in Dresden One area of research focus in Dresden is what is known as self-organization. “DNA molecules are, for instance, able to arrange themselves into structures without any outside assistance. If we succeed in constructing logical switches from self-organizing molecules, then computers of the future will come from test-tubes," Dr. Erbe prophesizes. The enormous advantages of this new technology are obvious: billion-euro manufacturing plants that are necessary for manufacturing today’s microelectronics could be a thing of the past. The advantages lie not only in production but also in operating the new molecular components, as they both will require very little energy.
read more "Researchers succeed in light-controlled molecule switching"
Light on – molecule on. For the first time a light beam switches a single molecule to closed state (red atoms). At the ends of the diarylethene molecule gold electrodes are attached. This way, the molecule functions as an electrical switch. (Image: HZDR/Pfefferkorn ) This requires a molecule in which an otherwise strong bond between individual atoms dissolves in one location – and forms again precisely when energy is pumped into the structure. Dr. Jannic Wolf, chemist at the University of Konstanz, discovered through complex experiments that a particular diarylethene compound is an eligible candidate. The advantages of this molecule, approximately three nanometres in size, are that it rotates very little when a point in its structure opens and it possesses two nanowires that can be used as contacts. The diarylethene is an insulator when open and becomes a conductor when closed. It thus exhibits a different physical behaviour, a behaviour that the scientists from Konstanz and Dresden were able to demonstrate with certainty in numerous reproducible measurements for the first time in a single molecule. A computer from a test-tube A special feature of these molecular electronics is that they take place in a fluid within a test-tube, where the molecules are contacted within the solution. In order to ascertain what effects the solution conditions have on the switching process, it was therefore necessary to systematically test various solvents. The diarylethene needs to be attached at the end of the nanowires to electrodes so that the current can flow. “We developed a nanotechnology at the HZDR that relies on extremely thin tips made of very few gold atoms. We stretch the switchable diarylethene compound between them,” explains Dr. Erbe. When a beam of light then hits the molecule, it switches from its open to its closed state, resulting in a flowing current. “For the first time ever we could switch on a single contacted molecule and prove that this precise molecule becomes a conductor on which we have used the light beam," says Dr. Erbe, pleased with the results. "We have also characterized the molecular switching mechanism in extremely high detail, which is why I believe that we have succeeded in making an important step toward a genuine molecular electronic component.” Switching off, however, does not yet work with the contacted diarylethene, but the physicist is confident: “Our colleagues from the HZDR theory group are computing how precisely the molecule must rotate so that the current is interrupted. Together with the chemists from Konstanz, we will be able to accordingly implement the design and synthesis for the molecule.” However, a great deal of patience is required because it’s a matter of basic research. The diarylethene molecule contact using electron-beam lithography and the subsequent measurements alone lasted three long years. Approximately ten years ago, a working group at the University of Groningen in the Netherlands had already managed to construct a switch that could interrupt the current. The off-switch also worked only in one direction, but what couldn't be proven at the time with certainty was that the change in conductivity was bound to a single molecule. Nano-electronics in Dresden One area of research focus in Dresden is what is known as self-organization. “DNA molecules are, for instance, able to arrange themselves into structures without any outside assistance. If we succeed in constructing logical switches from self-organizing molecules, then computers of the future will come from test-tubes," Dr. Erbe prophesizes. The enormous advantages of this new technology are obvious: billion-euro manufacturing plants that are necessary for manufacturing today’s microelectronics could be a thing of the past. The advantages lie not only in production but also in operating the new molecular components, as they both will require very little energy.
Li Tan "This is a relatively new community of research," said Tan, who co-authored a recent paper outlining the team's design. "Most of the people (in it) are inspired by the cuttlefish, whose skin changes color and texture, as well. "Changing color is relatively easy; a TV can do that. Changing texture is harder. We wanted to combine the two." To do so, the team has created a structure consisting of three layers: a base that insulates against heat, a middle that readily absorbs light, and a top made of either a liquid or solid. Paper, glass, foil, silicon and other materials have all proven suitable for the middle layer, so long as it includes a distribution of colored pixels. The middle and top layers also contain colloids: microscopic particles of soda lime, glass or copper. When a moderately intense laser strikes the middle layer, it begins warming any pixels that absorb it -- that is, those that don't share the light's color. Through the process of convection, these localized increases in temperature trigger ruptures along the surface of the top layer or volcano-like eruptions within it. In both cases, the resulting suction draws the colloids toward the heated, light-absorbing areas -- thereby reproducing the color that shines upon the surface. If the light is red, for instance, the colloids migrate to cover green pixels and leave only the red exposed. Under violet light, the particles obscure the majority of red pixels while leaving most of the green uncovered. This same photo-thermal principle allows the team to replicate or create patterns, Tan said, either by directing lights in deliberate trajectories or simply flashing them through transparent images overlaying the structure. The team has used these techniques to write words and mimic checkerboard patterns, among others. When the top layer is a solid, cooling it will obscure the word or pattern, which reappears upon reheating. According to Tan, the design's first apparent application -- camouflage -- probably won't see the light of day for a while. The more immediate potential application, Tan said, relies on the photo-thermal principle that drives the design's color and texture changes. In the same way that the technique can direct the assembly of microscopic colloids, it might also accelerate the accumulation of cells and facilitate the growth of biological tissue, he said. "Starting from small building blocks and growing them into large structures usually takes a very long time," Tan said. "In our case, it really doesn't."