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Hybrid materials pass on the excitement
RESEARCH NEWS
Route to engineering nanoparticle crystallinity NANOTECHNOLOGY
There is a myriad of uses for nanoparticles and the list is growing. The name of the game, then, is to control their properties to match the application. Each week sees more methods to control nanoparticle size and size distribution, shape, and composition. But the crystalline purity of nanoparticles remains a tough nut to crack. Researchers from the University of Maryland have devised a means of engineering nanoparticle crystallinity that is as straightforward as it is promising. The ratio of single-crystal to multiplytwinned nanoparticles can be reliably controlled by simply adjusting the available ligands in the precursors [Tang and Ouyang, Nat. Mater. (2007) doi: 10.1038/ nmat1982]. Starting with phosphine complexes (PPh3)3Ag–R and using either chlorine or nitrate as the monovalent ligand R, the researchers found that chlorine inhibits the growth of twinned clusters, leading to pure, single-crystal Ag nanoparticles. The authors suggest that controlling the rate of this inhibition with temperature could provide precise control of the ratio of single to twinned crystals. The idea also opens up the floodgates for others to study the process with a wide variety of precursors. Yun Tang and Min Ouyang also undertook a number of two-color pump-probe experiments to characterize the samples. This femtosecond-scale all-optical method shows that the electron-phonon coupling constant in single-crystal samples is just one quarter that of multiply-twinned samples. The method also
OPTICAL PROPERTIES
between the two structures. In the first test, QDs with an emission peak at 548 nm and J-aggregates with a 594 nm emission peak were used. The excitation was principally into the QDs, prompting energy transfer to the J-aggregates. In a second test, QDs with an emission peak at 653 nm were used to reverse the direction of energy transfer. The efficiency of the transfer is 22%, and the researchers also demonstrate a modified J-aggregate leading to a value as high as 38%. The tuning of the transfer is a promising result for a number of potential applications, believes Qiang Zhang of Brown University. “The large light-harvesting antenna effect of the J-aggregate could be harnessed in novel solar cell designs, where the excitation will be efficiently transferred to the QDs where the charge can be separated,” he says. “Organic lightemitting diodes with QDs as the active fluorophore could also benefit from the efficient energy transfer pathway from surrounding organics to circumvent charge-injection barriers.”
A new kind of plasmonic structure based on ferromagnetic nanowire arrays could be ideal for biosensor applications [Gonzáles-Díaz et al., Adv. Mater. (2007) doi: 101002/ adma.200602938]. Localized surface plasmons play a key role in the optical properties of metallic structures, particularly nanoscale ones such as nanoparticles embedded in dielectric matrices. The size, shape, and concentration of the nanoparticles, as well as the refractive index of the matrix, determine the intensity and spectra of the resulting surface plasmonic resonances. Typically, Au or Ag nanoparticles are used for such systems, but other metals such as Fe, Co, and Ni possess spontaneous magnetization. Although these metals give rise to, in some respects, inferior surface plasmonic resonances, the addition of magneto-optical properties enables the design of new kinds of plasmonic structures. Previous work, although showing that the surface plasmon resonance of magnetic metallic nanoparticles can enhance magneto-optical properties compared with a continuous medium, has not fully considered the effect of the size of those nanoparticles and their interaction. The Spanish and German researchers used hexagonal arrays of Ni nanowires embedded in alumina membranes 5 µm thick to investigate the dependence of the magneto-optical properties on the size of the nanowires and the dielectric environment. “This material exhibits a strong enhancement of the magneto-optical properties due to the plasmon resonance of the nanowires,” says Gaspar Armelles of the Instituto de Microelectrónica de Madrid. “The magneto-optical response can be controlled by changing the structural parameters of the material.”
D. Jason Palmer
Cordelia Sealy
15 nm single-crystal Ag nanoparticles. Inset: a single particle shows perfect lattice fringes. (Courtesy of Min Ouyang.)
yields precious experimental data on the differing nanomechanics of the samples. Single-crystal nanoparticles show a 37% rise in elastic modulus. Ouyang suggests that the method is a step toward the defect engineering that lies at the heart of semiconductor technology. “It is a trickier business,” Ouyang notes, “but one that will be essential for the development of nanoscience and nanotechnology.” D. Jason Palmer
Hybrid materials pass on the excitement NANOTECHNOLOGY The prediction of resonant energy transfer between organic semiconductors (with their high oscillator strengths) and their inorganic cousins (with attractive nonlinear properties) has waited some seven years to be demonstrated. It remains, however, to make the process efficient by increasing the light/ matter interactions within hybrid composite materials. A group of researchers at Brown University and Massachusetts Institute of Technology have done just this, with a selfassembled, layered composite [Zhang et al., Nat. Nanotechnol. (2007) doi: 10.1038/nnano.2007.253]. The composite comprises colloidal CdSe-ZnS quantum dots (QDs), layered with so-called ‘J-aggregates’ of a cyanine dye called TDBC. These large assemblies of spontaneously aligned molecules have a high oscillator strength. The J-aggregates are produced as monolayers, separated from the QD structures with a layer of the polyelectrolyte PDDA. Steady-state and transient photoluminescence experiments show a continuous and efficient excitonic energy transfer
8
A new kind of plasmonics
OCTOBER 2007 | VOLUME 10 | NUMBER 10
Route to engineering nanoparticle crystallinity NANOTECHNOLOGY
There is a myriad of uses for nanoparticles and the list is growing. The name of the game, then, is to control their properties to match the application. Each week sees more methods to control nanoparticle size and size distribution, shape, and composition. But the crystalline purity of nanoparticles remains a tough nut to crack. Researchers from the University of Maryland have devised a means of engineering nanoparticle crystallinity that is as straightforward as it is promising. The ratio of single-crystal to multiplytwinned nanoparticles can be reliably controlled by simply adjusting the available ligands in the precursors [Tang and Ouyang, Nat. Mater. (2007) doi: 10.1038/ nmat1982]. Starting with phosphine complexes (PPh3)3Ag–R and using either chlorine or nitrate as the monovalent ligand R, the researchers found that chlorine inhibits the growth of twinned clusters, leading to pure, single-crystal Ag nanoparticles. The authors suggest that controlling the rate of this inhibition with temperature could provide precise control of the ratio of single to twinned crystals. The idea also opens up the floodgates for others to study the process with a wide variety of precursors. Yun Tang and Min Ouyang also undertook a number of two-color pump-probe experiments to characterize the samples. This femtosecond-scale all-optical method shows that the electron-phonon coupling constant in single-crystal samples is just one quarter that of multiply-twinned samples. The method also
OPTICAL PROPERTIES
between the two structures. In the first test, QDs with an emission peak at 548 nm and J-aggregates with a 594 nm emission peak were used. The excitation was principally into the QDs, prompting energy transfer to the J-aggregates. In a second test, QDs with an emission peak at 653 nm were used to reverse the direction of energy transfer. The efficiency of the transfer is 22%, and the researchers also demonstrate a modified J-aggregate leading to a value as high as 38%. The tuning of the transfer is a promising result for a number of potential applications, believes Qiang Zhang of Brown University. “The large light-harvesting antenna effect of the J-aggregate could be harnessed in novel solar cell designs, where the excitation will be efficiently transferred to the QDs where the charge can be separated,” he says. “Organic lightemitting diodes with QDs as the active fluorophore could also benefit from the efficient energy transfer pathway from surrounding organics to circumvent charge-injection barriers.”
A new kind of plasmonic structure based on ferromagnetic nanowire arrays could be ideal for biosensor applications [Gonzáles-Díaz et al., Adv. Mater. (2007) doi: 101002/ adma.200602938]. Localized surface plasmons play a key role in the optical properties of metallic structures, particularly nanoscale ones such as nanoparticles embedded in dielectric matrices. The size, shape, and concentration of the nanoparticles, as well as the refractive index of the matrix, determine the intensity and spectra of the resulting surface plasmonic resonances. Typically, Au or Ag nanoparticles are used for such systems, but other metals such as Fe, Co, and Ni possess spontaneous magnetization. Although these metals give rise to, in some respects, inferior surface plasmonic resonances, the addition of magneto-optical properties enables the design of new kinds of plasmonic structures. Previous work, although showing that the surface plasmon resonance of magnetic metallic nanoparticles can enhance magneto-optical properties compared with a continuous medium, has not fully considered the effect of the size of those nanoparticles and their interaction. The Spanish and German researchers used hexagonal arrays of Ni nanowires embedded in alumina membranes 5 µm thick to investigate the dependence of the magneto-optical properties on the size of the nanowires and the dielectric environment. “This material exhibits a strong enhancement of the magneto-optical properties due to the plasmon resonance of the nanowires,” says Gaspar Armelles of the Instituto de Microelectrónica de Madrid. “The magneto-optical response can be controlled by changing the structural parameters of the material.”
D. Jason Palmer
Cordelia Sealy
15 nm single-crystal Ag nanoparticles. Inset: a single particle shows perfect lattice fringes. (Courtesy of Min Ouyang.)
yields precious experimental data on the differing nanomechanics of the samples. Single-crystal nanoparticles show a 37% rise in elastic modulus. Ouyang suggests that the method is a step toward the defect engineering that lies at the heart of semiconductor technology. “It is a trickier business,” Ouyang notes, “but one that will be essential for the development of nanoscience and nanotechnology.” D. Jason Palmer
Hybrid materials pass on the excitement NANOTECHNOLOGY The prediction of resonant energy transfer between organic semiconductors (with their high oscillator strengths) and their inorganic cousins (with attractive nonlinear properties) has waited some seven years to be demonstrated. It remains, however, to make the process efficient by increasing the light/ matter interactions within hybrid composite materials. A group of researchers at Brown University and Massachusetts Institute of Technology have done just this, with a selfassembled, layered composite [Zhang et al., Nat. Nanotechnol. (2007) doi: 10.1038/nnano.2007.253]. The composite comprises colloidal CdSe-ZnS quantum dots (QDs), layered with so-called ‘J-aggregates’ of a cyanine dye called TDBC. These large assemblies of spontaneously aligned molecules have a high oscillator strength. The J-aggregates are produced as monolayers, separated from the QD structures with a layer of the polyelectrolyte PDDA. Steady-state and transient photoluminescence experiments show a continuous and efficient excitonic energy transfer
8
A new kind of plasmonics
OCTOBER 2007 | VOLUME 10 | NUMBER 10