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Ferromagnetic Si at room temperature
RESEARCH NEWS
New light on semiconductor aerogels POROUS MATERIALS There have been many attempts to combine the optimal pore size (2-50 nm) of mesoporous oxide materials for the rapid transport of small gas-phase molecules with the optical and electronic properties of metal chalcogenides. However, conventional surfactant templating methods for oxides result in the structural collapse of the metal chalcogenides when the template is removed. Now, however, researchers at Wayne State University in Detroit have successfully prepared stable, mesoporous metal chalcogenide aerogels with average pore sizes of 15-45 nm [Mohanan et al., Science (2005) 307, 397]. The formation process has three steps: (i) PbS, CdSe, CdS, or ZnS nanoparticle synthesis; (ii) oxidation of capping groups, revealing reactive sites for nanoparticle condensation into porous gels; and (iii) drying of the gels using supercritical CO2. The aerogels have a high porosity comparable to oxide aerogels and retain the photoluminescence properties of the nanoparticles. The aerogel structure preserves the integrity of the nanoparticles by locking them into a network, while providing a pore structure through which chemical species can be introduced, either as analytes or secondary components for composite formation. The technique represents a powerful, yet simple method for assembling nanoparticles from solution into functional solid-state devices while retaining size-dependent properties. The combination of high interfacial surface area, quantum confinement effects, and photoluminescence is suited to photocatalytic, photovoltaic, and sensing applications. Mark Telford
Ordering organic crystals on liquid mercury ELECTRONIC MATERIALS
Scientists from Brookhaven National Laboratory, Harvard University, and Bar-Ilan University in Israel have grown ultrathin films of octadecanethiol molecules on the surface of liquid Hg and discovered that they form ordered structures [Ocko et al., Phys. Rev. Lett. (2005) 94, 017802]. “We chose an alkyl-thiol because one end of each molecule is terminated by a S atom that bonds strongly to metal surfaces to form self-assembled monolayers,” explains Henning Kraack of Bar-Ilan University. “Thiol molecules have been studied extensively on Au surfaces, but the exact nature of the S-Au bond has remained controversial. One of our main goals was to determine the nature of the bond between a similar pair, S and Hg.” The researchers measured how X-rays scatter off the film from different angles, while adding more alkyl-thiol. Three scattering patterns emerged, corresponding to different degrees of molecular order. At the lowest density, the molecules lie flat on the surface; at an intermediate density, they tilt so that the S end is in contact with the Hg; and at the highest density, they stand up straight. X-ray analysis of the lying-down phase shows that the molecules are disordered. However, the standing-up and tilted phases are very ordered. The molecules are arranged in crystalline patterns, despite the disordered, liquid nature of the underlying Hg. The tilted phase contains an unusual feature: the alkylthiol chain portions and S atoms line up
Benjamin Ocko. (Courtesy of Brookhaven National Laboratory.)
differently and form different patterns. “The S atoms from two neighboring chains chemically bond to one underlying Hg atom,” explains Benjamin M. Ocko. “In the tilted phase, the S-Hg bonds exhibit crystalline order. These bonds also form in the standingup phase, but, surprisingly, they appear disordered.” The researchers now plan to study the structure of the molecular layers sandwiched between two conducting surfaces, a configuration directly relevant to molecular electronics. Ultrathin organic films are also becoming important for other emerging technologies, such as flexible electronic displays and biomaterials that can, for example, mimic the function of cell membranes. Mark Telford
Ferromagnetic Si at room temperature MAGNETIC MATERIALS Researchers at the University of Albany-SUNY have, for the first time, demonstrated ferromagnetism in Si above room temperature (at 127°C, well above where conventional Si-based electronic devices operate) [Bolduc et al., Phys. Rev. B (2005) 71, 033302]. Si samples were implanted with 300 keV Mn+ ions. For the highest Mn concentration (1 at.%), saturation magnetization was 0.3 emu/g at 300 K, which increased twofold after annealing at 800°C for 5 minutes. The Curie temperature was >400 K. A large difference in the normalized temperaturedependent remnant magnetization up to 400 K between the implanted p- and n-type Si suggests
that ferromagnetic ordering is hole mediated. The results indicate that it may be possible to fabricate a Si-based diluted magnetic semiconductor with great potential for spintronic devices. Further work is needed to understand the structure (both electronic and physical) of the materials, and to determine the origin of the ferromagnetism. “These results indicate that the ferromagnetic exchange coupling in Si is very strong. Our future research will focus on understanding why this is so,” says Martin Bolduc. “We can now start to think of practical applications, possibly in 5-10 years,” adds lead researcher Vincent P. LaBella. Mark Telford
March 2005
15
New light on semiconductor aerogels POROUS MATERIALS There have been many attempts to combine the optimal pore size (2-50 nm) of mesoporous oxide materials for the rapid transport of small gas-phase molecules with the optical and electronic properties of metal chalcogenides. However, conventional surfactant templating methods for oxides result in the structural collapse of the metal chalcogenides when the template is removed. Now, however, researchers at Wayne State University in Detroit have successfully prepared stable, mesoporous metal chalcogenide aerogels with average pore sizes of 15-45 nm [Mohanan et al., Science (2005) 307, 397]. The formation process has three steps: (i) PbS, CdSe, CdS, or ZnS nanoparticle synthesis; (ii) oxidation of capping groups, revealing reactive sites for nanoparticle condensation into porous gels; and (iii) drying of the gels using supercritical CO2. The aerogels have a high porosity comparable to oxide aerogels and retain the photoluminescence properties of the nanoparticles. The aerogel structure preserves the integrity of the nanoparticles by locking them into a network, while providing a pore structure through which chemical species can be introduced, either as analytes or secondary components for composite formation. The technique represents a powerful, yet simple method for assembling nanoparticles from solution into functional solid-state devices while retaining size-dependent properties. The combination of high interfacial surface area, quantum confinement effects, and photoluminescence is suited to photocatalytic, photovoltaic, and sensing applications. Mark Telford
Ordering organic crystals on liquid mercury ELECTRONIC MATERIALS
Scientists from Brookhaven National Laboratory, Harvard University, and Bar-Ilan University in Israel have grown ultrathin films of octadecanethiol molecules on the surface of liquid Hg and discovered that they form ordered structures [Ocko et al., Phys. Rev. Lett. (2005) 94, 017802]. “We chose an alkyl-thiol because one end of each molecule is terminated by a S atom that bonds strongly to metal surfaces to form self-assembled monolayers,” explains Henning Kraack of Bar-Ilan University. “Thiol molecules have been studied extensively on Au surfaces, but the exact nature of the S-Au bond has remained controversial. One of our main goals was to determine the nature of the bond between a similar pair, S and Hg.” The researchers measured how X-rays scatter off the film from different angles, while adding more alkyl-thiol. Three scattering patterns emerged, corresponding to different degrees of molecular order. At the lowest density, the molecules lie flat on the surface; at an intermediate density, they tilt so that the S end is in contact with the Hg; and at the highest density, they stand up straight. X-ray analysis of the lying-down phase shows that the molecules are disordered. However, the standing-up and tilted phases are very ordered. The molecules are arranged in crystalline patterns, despite the disordered, liquid nature of the underlying Hg. The tilted phase contains an unusual feature: the alkylthiol chain portions and S atoms line up
Benjamin Ocko. (Courtesy of Brookhaven National Laboratory.)
differently and form different patterns. “The S atoms from two neighboring chains chemically bond to one underlying Hg atom,” explains Benjamin M. Ocko. “In the tilted phase, the S-Hg bonds exhibit crystalline order. These bonds also form in the standingup phase, but, surprisingly, they appear disordered.” The researchers now plan to study the structure of the molecular layers sandwiched between two conducting surfaces, a configuration directly relevant to molecular electronics. Ultrathin organic films are also becoming important for other emerging technologies, such as flexible electronic displays and biomaterials that can, for example, mimic the function of cell membranes. Mark Telford
Ferromagnetic Si at room temperature MAGNETIC MATERIALS Researchers at the University of Albany-SUNY have, for the first time, demonstrated ferromagnetism in Si above room temperature (at 127°C, well above where conventional Si-based electronic devices operate) [Bolduc et al., Phys. Rev. B (2005) 71, 033302]. Si samples were implanted with 300 keV Mn+ ions. For the highest Mn concentration (1 at.%), saturation magnetization was 0.3 emu/g at 300 K, which increased twofold after annealing at 800°C for 5 minutes. The Curie temperature was >400 K. A large difference in the normalized temperaturedependent remnant magnetization up to 400 K between the implanted p- and n-type Si suggests
that ferromagnetic ordering is hole mediated. The results indicate that it may be possible to fabricate a Si-based diluted magnetic semiconductor with great potential for spintronic devices. Further work is needed to understand the structure (both electronic and physical) of the materials, and to determine the origin of the ferromagnetism. “These results indicate that the ferromagnetic exchange coupling in Si is very strong. Our future research will focus on understanding why this is so,” says Martin Bolduc. “We can now start to think of practical applications, possibly in 5-10 years,” adds lead researcher Vincent P. LaBella. Mark Telford
March 2005
15