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Millipede is hot tip for memory
RESEARCH NEWS
Transistors in a spin Recent research takes significant steps towards a new generation of transistors based on quantum phenomena. Two teams of researchers have created single-molecule transistors using a similar technique. Electron-beam lithography is used to create nano-scale gold wires, which are then broken by electromigration to form a gap 1-2 nm wide. With luck, a single molecule can then be trapped in the gap between the two gold 'electrodes'. Paul McEuen and Daniel Ralph at Cornell University, together with colleagues at the University of California, Berkeley, incorporated a transition metal complex that consists of a cobalt ion with
polypyridyl ligands attached to insulating tethers of different lengths [Nature (2002) 417, 722-725]. A different molecule – containing divanadium (V2) – was used by Hongkun Park's team at Harvard University, in collaboration with other researchers at the University of California, Berkeley, [Nature (2002) 417, 725-729]. In both cases, charge and spin control the motion of electrons through the transistor. Singleelectron tunnelling requires a gate voltage to overcome the repulsive Coulomb interaction between electrons so that they can 'hop', one-by-one, on and off of the molecule. Alternatively, the Kondo effect produces an exchange of electrons between the
electrode and molecule because of differences in spin orientation. The critical parameters of the Kondo system, such as spin and orbital degrees of freedom, can be defined by chemical synthesis, say Park's team. Although this is a big step forwards for molecular electronics, Silvano De Franceschi and Leo Kouwenhoven point out, in an accompanying News and Views article [Nature (2002) 417, 701-702] that there is a long way to go before such a device could compete with silicon transistors. The technological hurdles that remain to be overcome are not insignificant, agrees McEuen, not least of all the achievement of gain.
Researchers at the Institute for Microstructural Sciences in Ottawa have also taken a step forward in spin-controlled transistor [Ciorga et al., Phys. Rev. Lett. (2002) 88 (25), 256804]. Their device consists of a quantum dot (QD) connected to spin-polarized leads, which allow the QD to be emptied and refilled with electrons one at a time. By connecting the QDs to reservoirs of spinpolarized electrons, the spins of electrons following in or out can be determined. This means of 'reading' the spin properties of a dot could also, in theory, be used to 'write'. This allows the structure to function as a gate – allowing high or low current to pass through the QD.
Millipede is hot tip for memory IBM has returned to an old concept to create a new memory technology [IEEE Transactions on Nanotechnology (2002)]. 'Millipede' uses thousands of nano-scale tips to punch indentations into a thin plastic film – just like the 'punch cards' that were developed a century ago to process data. Unlike punch cards, however, the Millipede technology is rewritable and can store a trillion bits per square inch. IBM has demonstrated the proof of concept with an array of over 1000 tips – 2 µm silicon tips on v-shaped silicon cantilevers 0.5 µm thick and 70 µm long. This structure, based on atomic force microscopy arrays, is
fabricated by surfacemicromachining techniques. When the tips are brought into contact with a thin polymer coating on a silicon substrate, bits are written by heating a resistor built into the cantilever to 400°C. The hot tip softens the polymer and sinks in, creating indentations of ~10 nm. By keeping the resistor at a lower temperature (300°C), the device can read the indentations by detecting changes in resistance. To overwrite data, a series of offset pits are created by the tips so that their edges fill in the old indentations. Over 100 000 write/overwrite cycles have already been demonstrated. A prototype is now being built consisting of a
Millipede operation: the storage medium – a thin film of organic material (yellow) deposited on a silicon 'table' – is brought into contact with an array of silicon tips (green) and moved in the x- and y-directions for reading and writing. Multiplex drivers (red) allow addressing of each tip individually.
7 mm2 array of 4000 tips and IBM has high hopes for the new technology. "Since a nanometer-scale tip can address individual atoms, we anticipate further improvements beyond even this fantastic terabit milestone,"
says Gerd Binnig. "While current storage technologies may be approaching fundamental limits, this nanomechanical approach is potentially valid for a thousandfold increase in data storage density."
September 2002
7
Transistors in a spin Recent research takes significant steps towards a new generation of transistors based on quantum phenomena. Two teams of researchers have created single-molecule transistors using a similar technique. Electron-beam lithography is used to create nano-scale gold wires, which are then broken by electromigration to form a gap 1-2 nm wide. With luck, a single molecule can then be trapped in the gap between the two gold 'electrodes'. Paul McEuen and Daniel Ralph at Cornell University, together with colleagues at the University of California, Berkeley, incorporated a transition metal complex that consists of a cobalt ion with
polypyridyl ligands attached to insulating tethers of different lengths [Nature (2002) 417, 722-725]. A different molecule – containing divanadium (V2) – was used by Hongkun Park's team at Harvard University, in collaboration with other researchers at the University of California, Berkeley, [Nature (2002) 417, 725-729]. In both cases, charge and spin control the motion of electrons through the transistor. Singleelectron tunnelling requires a gate voltage to overcome the repulsive Coulomb interaction between electrons so that they can 'hop', one-by-one, on and off of the molecule. Alternatively, the Kondo effect produces an exchange of electrons between the
electrode and molecule because of differences in spin orientation. The critical parameters of the Kondo system, such as spin and orbital degrees of freedom, can be defined by chemical synthesis, say Park's team. Although this is a big step forwards for molecular electronics, Silvano De Franceschi and Leo Kouwenhoven point out, in an accompanying News and Views article [Nature (2002) 417, 701-702] that there is a long way to go before such a device could compete with silicon transistors. The technological hurdles that remain to be overcome are not insignificant, agrees McEuen, not least of all the achievement of gain.
Researchers at the Institute for Microstructural Sciences in Ottawa have also taken a step forward in spin-controlled transistor [Ciorga et al., Phys. Rev. Lett. (2002) 88 (25), 256804]. Their device consists of a quantum dot (QD) connected to spin-polarized leads, which allow the QD to be emptied and refilled with electrons one at a time. By connecting the QDs to reservoirs of spinpolarized electrons, the spins of electrons following in or out can be determined. This means of 'reading' the spin properties of a dot could also, in theory, be used to 'write'. This allows the structure to function as a gate – allowing high or low current to pass through the QD.
Millipede is hot tip for memory IBM has returned to an old concept to create a new memory technology [IEEE Transactions on Nanotechnology (2002)]. 'Millipede' uses thousands of nano-scale tips to punch indentations into a thin plastic film – just like the 'punch cards' that were developed a century ago to process data. Unlike punch cards, however, the Millipede technology is rewritable and can store a trillion bits per square inch. IBM has demonstrated the proof of concept with an array of over 1000 tips – 2 µm silicon tips on v-shaped silicon cantilevers 0.5 µm thick and 70 µm long. This structure, based on atomic force microscopy arrays, is
fabricated by surfacemicromachining techniques. When the tips are brought into contact with a thin polymer coating on a silicon substrate, bits are written by heating a resistor built into the cantilever to 400°C. The hot tip softens the polymer and sinks in, creating indentations of ~10 nm. By keeping the resistor at a lower temperature (300°C), the device can read the indentations by detecting changes in resistance. To overwrite data, a series of offset pits are created by the tips so that their edges fill in the old indentations. Over 100 000 write/overwrite cycles have already been demonstrated. A prototype is now being built consisting of a
Millipede operation: the storage medium – a thin film of organic material (yellow) deposited on a silicon 'table' – is brought into contact with an array of silicon tips (green) and moved in the x- and y-directions for reading and writing. Multiplex drivers (red) allow addressing of each tip individually.
7 mm2 array of 4000 tips and IBM has high hopes for the new technology. "Since a nanometer-scale tip can address individual atoms, we anticipate further improvements beyond even this fantastic terabit milestone,"
says Gerd Binnig. "While current storage technologies may be approaching fundamental limits, this nanomechanical approach is potentially valid for a thousandfold increase in data storage density."
September 2002
7