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Spin coating high-mobility films
RESEARCH NEWS
Spin coating high-mobility films FABRICATION AND PROCESSING
Solution processing of semiconductor thin films could be used to make low-cost electronic devices. (Courtesy of IBM Research.)
A group at IBM’s T. J. Watson Research Center has developed a technique for spin coating metal chalcogenide films that could enable the low-cost fabrication of thin-film devices [Mitzi et al., Nature (2004) 428, 299]. Solution processing of semiconductor films offers a cheap method for making large numbers of simple electronic devices, but has so far been largely restricted to organic systems. Metal chalcogenides provide an
opportunity for obtaining higher mobilities, but their low solubility means they are generally deposited using thermal evaporation, chemical vapor deposition, or sputtering techniques. The IBM team improved the solubility of main group chalcogenides, such as SnS2-xSex, in the strong solvent hydrazine by adding excess chalcogen (S or Se). Highly soluble hydrazinium precursors form, which can be solution processed into thin films. Lowtemperature decomposition of the precursors then yields the metal chalcogenide. Using this process, spin coating in air can form continuous, crystalline films as thin as 50 Å. Thin-film transistors based on spin-coated chalcogenide films show n-type transport, large current densities >105 A/cm2, and mobilities >10 cm2/V•s, an order of magnitude higher than previous results. “These easily processed semiconducting films could eventually be used to make circuitry for very low cost displays, high-performance smart cards, sensors, and solar cells or, for flexible electronics, coated onto a wide variety of molded or plastic shapes,” explains David B. Mitzi. Jonathan Wood
A new class of buckyballs FABRICATION AND PROCESSING Maryvonne Hervieu and colleagues from the Laboratoire de Cristallographie et Sciences des Materiaux (CRISMAT) in France have synthesized a bismuth aluminate with a three-dimensional framework consisting of fullerene-like ‘Al84’ spheres [Hervieu et al., Nat. Mater. (2004) 3, 269]. The researchers dub the new material a ‘fullerenoid oxide’. Aluminates, silicates, and other tetrahedral oxides can form complex structures, which have potential for numerous applications, including catalysis, ion exchange, and molecular sieves, as well as thermoand photoluminescent pigments. The new aluminate Sr33Bi24+δAl48O141+3δ/2, which the team isolated by solid-state reaction from a mixture of SrO, Bi2O3, and Al2O3, exhibits a remarkable structural arrangement and could find similar applications. X-ray diffraction reveals large, spherical cages of AlO4 tetrahedra. This lattice consists of 84 Al atoms connected, via O atoms, in pentagons and
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May 2004
hexagons to form spheres. The Al84 spheres share the same configuration as the D2d isomer of C84 fullerene molecules. The spheres have a diameter of 18.5 Å, much larger than the 8.5 Å diameter of C84, because of the O atoms located between adjacent Al atoms. The Al84 spheres are packed in face-centered cubic arrays. Within the Al84 spheres, a BiSrO array forms a nest of spheres, much like the layers of an onion. “There are five concentric spheres with decreasing sizes,” explains Hervieu. “First a ‘O126’ sphere; second, ‘Sr32(Bi8.25 3.75)’; third, a ‘O40’ sphere; then a ‘Bi16’ sphere; and, lastly, a smaller ‘O12’ sphere.” (A represents a vacancy.) Hervieu believes this first fullerenoid oxide opens a route to a new and promising class of materials. “The tremendous arrangement of the different species undoubtedly conceals formation mechanisms for other materials,” she says. Jonathan Wood
Strong conductors METALS AND ALLOYS Many applications require conducting materials to have both high electrical conductivity and mechanical strength. However, methods used to strengthen metals typically result in a significant decrease in conductivity. Approaches such as solid solution alloying, cold working, and grain refinement are based on the introduction of defects. But this also increases the scattering of electrons and, therefore, the resistivity of the metal. Lei Lu and coworkers at the Institute of Metal Research in Shenyang, China sidestep this trade-off between electrical conductivity and mechanical strength by synthesizing pure Cu samples with a high density of nanoscale growth twin boundaries [Lu et al., Sciencexpress, published online 18 March 2004, DOI: 10.1126/science.1092905]. Using the pulsed electrodeposition technique from an electrolyte of CuSO4, they produce Cu foils that are about ten times stronger than coarsegrained Cu but with a similar conductivity. The presence of nanoscale growth twins results in a tensile yield strength of 900 MPa and an ultimate tensile strength of 1068 MPa. The electrical resistivity of the material, across a temperature range of 4-296 K, is an order of magnitude less than nanocrystalline foils with conventional grain boundaries. The researchers reason that a high density of twin boundaries should strengthen the metal, since they are able to block dislocation motion like conventional ones. But the electrical resistivity of twin boundaries is about one order of magnitude less than highangle grain boundaries, explaining the low electrical resistivity of the Cu foils containing nanoscale growth twins. Jonathan Wood
Spin coating high-mobility films FABRICATION AND PROCESSING
Solution processing of semiconductor thin films could be used to make low-cost electronic devices. (Courtesy of IBM Research.)
A group at IBM’s T. J. Watson Research Center has developed a technique for spin coating metal chalcogenide films that could enable the low-cost fabrication of thin-film devices [Mitzi et al., Nature (2004) 428, 299]. Solution processing of semiconductor films offers a cheap method for making large numbers of simple electronic devices, but has so far been largely restricted to organic systems. Metal chalcogenides provide an
opportunity for obtaining higher mobilities, but their low solubility means they are generally deposited using thermal evaporation, chemical vapor deposition, or sputtering techniques. The IBM team improved the solubility of main group chalcogenides, such as SnS2-xSex, in the strong solvent hydrazine by adding excess chalcogen (S or Se). Highly soluble hydrazinium precursors form, which can be solution processed into thin films. Lowtemperature decomposition of the precursors then yields the metal chalcogenide. Using this process, spin coating in air can form continuous, crystalline films as thin as 50 Å. Thin-film transistors based on spin-coated chalcogenide films show n-type transport, large current densities >105 A/cm2, and mobilities >10 cm2/V•s, an order of magnitude higher than previous results. “These easily processed semiconducting films could eventually be used to make circuitry for very low cost displays, high-performance smart cards, sensors, and solar cells or, for flexible electronics, coated onto a wide variety of molded or plastic shapes,” explains David B. Mitzi. Jonathan Wood
A new class of buckyballs FABRICATION AND PROCESSING Maryvonne Hervieu and colleagues from the Laboratoire de Cristallographie et Sciences des Materiaux (CRISMAT) in France have synthesized a bismuth aluminate with a three-dimensional framework consisting of fullerene-like ‘Al84’ spheres [Hervieu et al., Nat. Mater. (2004) 3, 269]. The researchers dub the new material a ‘fullerenoid oxide’. Aluminates, silicates, and other tetrahedral oxides can form complex structures, which have potential for numerous applications, including catalysis, ion exchange, and molecular sieves, as well as thermoand photoluminescent pigments. The new aluminate Sr33Bi24+δAl48O141+3δ/2, which the team isolated by solid-state reaction from a mixture of SrO, Bi2O3, and Al2O3, exhibits a remarkable structural arrangement and could find similar applications. X-ray diffraction reveals large, spherical cages of AlO4 tetrahedra. This lattice consists of 84 Al atoms connected, via O atoms, in pentagons and
10
May 2004
hexagons to form spheres. The Al84 spheres share the same configuration as the D2d isomer of C84 fullerene molecules. The spheres have a diameter of 18.5 Å, much larger than the 8.5 Å diameter of C84, because of the O atoms located between adjacent Al atoms. The Al84 spheres are packed in face-centered cubic arrays. Within the Al84 spheres, a BiSrO array forms a nest of spheres, much like the layers of an onion. “There are five concentric spheres with decreasing sizes,” explains Hervieu. “First a ‘O126’ sphere; second, ‘Sr32(Bi8.25 3.75)’; third, a ‘O40’ sphere; then a ‘Bi16’ sphere; and, lastly, a smaller ‘O12’ sphere.” (A represents a vacancy.) Hervieu believes this first fullerenoid oxide opens a route to a new and promising class of materials. “The tremendous arrangement of the different species undoubtedly conceals formation mechanisms for other materials,” she says. Jonathan Wood
Strong conductors METALS AND ALLOYS Many applications require conducting materials to have both high electrical conductivity and mechanical strength. However, methods used to strengthen metals typically result in a significant decrease in conductivity. Approaches such as solid solution alloying, cold working, and grain refinement are based on the introduction of defects. But this also increases the scattering of electrons and, therefore, the resistivity of the metal. Lei Lu and coworkers at the Institute of Metal Research in Shenyang, China sidestep this trade-off between electrical conductivity and mechanical strength by synthesizing pure Cu samples with a high density of nanoscale growth twin boundaries [Lu et al., Sciencexpress, published online 18 March 2004, DOI: 10.1126/science.1092905]. Using the pulsed electrodeposition technique from an electrolyte of CuSO4, they produce Cu foils that are about ten times stronger than coarsegrained Cu but with a similar conductivity. The presence of nanoscale growth twins results in a tensile yield strength of 900 MPa and an ultimate tensile strength of 1068 MPa. The electrical resistivity of the material, across a temperature range of 4-296 K, is an order of magnitude less than nanocrystalline foils with conventional grain boundaries. The researchers reason that a high density of twin boundaries should strengthen the metal, since they are able to block dislocation motion like conventional ones. But the electrical resistivity of twin boundaries is about one order of magnitude less than highangle grain boundaries, explaining the low electrical resistivity of the Cu foils containing nanoscale growth twins. Jonathan Wood