Materials in Focus
High strength gold bridge, one atom long
(State University of New York Buffalo News Center. See also the press release by Charlotte Hsu.)
Image courtesy of University at Buffalo. Click image to enlarge.
(State University of New York Buffalo News Center. See also the press release by Charlotte Hsu.)
Image courtesy of University at Buffalo. Click image to enlarge.
Image caption: A bridge made of a single atom of gold has twice the strength of bulk gold, according to new UB research.
 |
Smaller is not necessarily weaker, Harsh Deep Chopra and his colleagues at the State University of New York at Buffalo have discovered. In fact, a bridge made of just a single gold atom between a gold substrate and a gold coated cantilever tip has at least twice the modulus of the bulk, they report in a recent paper in Physical Review B. “You would think that if you make these constrictions so tiny they would become ever more fragile,” Chopra says. “Actually, they become even harder to elastically deform.” They note that when the atoms do not have the full coordination of bulk gold, the gold-gold bonds contract and strengthen, which leads to modulus enhancement. This discovery could prove to be important as nano-devices are made smaller and smaller with each generation.
To obtain these results, the researchers used a custom-built metrology instrument with unprecedented resolution and stability to make atomic-sized bridges and to measure pico-level forces and deformations. Chopra describes it as a very stable system that doesn’t require any feedback loop to be able to form samples as small as a single atom between the cantilever tip and a substrate of any metal. “We’re then able to pull and push on samples over picometer distances and measure the pico-Newton forces,” Chopra says.
Using a bottom-up approach, these scientists were able to study deformation behavior at all length scales, from the atomic level to the bulk. Starting with a single atom gold bridge and adding atoms one at a time, they discovered that pure, homogeneous shear on the close-packed plane occurs just as predicted up to about 19 atoms. Beyond this point, defects start to become visible, so an average shear distance of about 1.66 angstroms prevails for the slip distance of gold. This is still a surface-dominated effect. Somewhere between a sample diameter of 2.2 to 2.7 nm, a switch from surface- to volume-dominated deformation was expected to occur. With their picometer resolution, Chopra and his colleagues saw just that. “So with our instrumentation and method,” Chopra says, “we are able to see homogeneous shear at the atomic level, followed by the transformation to defect-dominated deformation when the sample reached about 19 atoms, and finally the transition from surface- to volume-dominated deformation as the sample reached a couple of nanometers.”
Part of Chopra’s research philosophy is being able to look at the evolution of various materials properties—be they electrical, mechanical, magnetic, or electron transport properties—as a function of size down to extreme length scales. “The Fermi wavelength of electrons in metals is the smallest relevant length scale corresponding to a single atom and the ultimate limit in device miniaturization,” Chopra explains. “We can see how these physical behaviors change as you go form one atom, two atoms, three atoms to the bulk. And when you get to the bulk, it is basically the sum total of everything that goes on at the atomic scale. So it allows you to link these atomic processes to be able to get a macroscopic description of the material.” [Physical Review B]
To obtain these results, the researchers used a custom-built metrology instrument with unprecedented resolution and stability to make atomic-sized bridges and to measure pico-level forces and deformations. Chopra describes it as a very stable system that doesn’t require any feedback loop to be able to form samples as small as a single atom between the cantilever tip and a substrate of any metal. “We’re then able to pull and push on samples over picometer distances and measure the pico-Newton forces,” Chopra says.
Using a bottom-up approach, these scientists were able to study deformation behavior at all length scales, from the atomic level to the bulk. Starting with a single atom gold bridge and adding atoms one at a time, they discovered that pure, homogeneous shear on the close-packed plane occurs just as predicted up to about 19 atoms. Beyond this point, defects start to become visible, so an average shear distance of about 1.66 angstroms prevails for the slip distance of gold. This is still a surface-dominated effect. Somewhere between a sample diameter of 2.2 to 2.7 nm, a switch from surface- to volume-dominated deformation was expected to occur. With their picometer resolution, Chopra and his colleagues saw just that. “So with our instrumentation and method,” Chopra says, “we are able to see homogeneous shear at the atomic level, followed by the transformation to defect-dominated deformation when the sample reached about 19 atoms, and finally the transition from surface- to volume-dominated deformation as the sample reached a couple of nanometers.”
Part of Chopra’s research philosophy is being able to look at the evolution of various materials properties—be they electrical, mechanical, magnetic, or electron transport properties—as a function of size down to extreme length scales. “The Fermi wavelength of electrons in metals is the smallest relevant length scale corresponding to a single atom and the ultimate limit in device miniaturization,” Chopra explains. “We can see how these physical behaviors change as you go form one atom, two atoms, three atoms to the bulk. And when you get to the bulk, it is basically the sum total of everything that goes on at the atomic scale. So it allows you to link these atomic processes to be able to get a macroscopic description of the material.” [Physical Review B]
Zeolites with micro- and mesoporosity
Korea Advanced Institute of Science and Technology (KAIST)
Zeolites are a family of materials containing approximately 200 crystalline, microporous aluminosilicate structures with controlled pore diameters of <2 nm. This controlled pore size has made zeolites excellent filters for separating or selectively adsorbing molecules by size and shape. Zeolites are also the acid catalysts of choice for many industrial reactions. Much effort has been expended in trying to extend their pore sizes into the mesoporous range (2-50 nm) to handle larger molecules, with only modest success. Now researchers at the Korea Advanced Institute of Science and Technology (KAIST) led by Ryong Rhoo have succeeded in producing crystalline, mesoporous molecular sieves (MMS) with a hierarchy of pore sizes ranging from the micro-to the mesoporous in the same sample, as reported recently in Science.
The key to the synthesis of these unique materials lies in the use of polyquaternary ammonium surfactants. “Hexagonal mesostructures were generated by aggregation of the surfactant molecules,” the researchers wrote, “whereas the crystallization of microporous frameworks was directed by quaternary ammonium groups within the mesopore walls.” By varying the number of quaternary ammonium species in the head group of the surfactant, the researchers were able to control the thickness of the mesopore walls and the structure of the zeolitelike microporous frameworks within them. Reported mean micropore diameters ranged from 0.55 to 0.65 nm, with mean mesopore diameters ranging from 3.5 to 4.7 nm. Mesopores with diameters up to 21 nm were formed using micelle swelling agents.
Functionally, these materials showed excellent properties as acid catalysts for various reactions involving bulky organic molecules that are too large to fit into traditional zeolite catalyst pores. The researchers attribute the increased catalytic activity compared to conventional zeolites or amorphous MMSs to the ease of molecular diffusion through the mesopores, the strong acidity of the crystalline zeolitelike structures, and the high concentration of surface acid sites. [Science]
Korea Advanced Institute of Science and Technology (KAIST)
Zeolites are a family of materials containing approximately 200 crystalline, microporous aluminosilicate structures with controlled pore diameters of <2 nm. This controlled pore size has made zeolites excellent filters for separating or selectively adsorbing molecules by size and shape. Zeolites are also the acid catalysts of choice for many industrial reactions. Much effort has been expended in trying to extend their pore sizes into the mesoporous range (2-50 nm) to handle larger molecules, with only modest success. Now researchers at the Korea Advanced Institute of Science and Technology (KAIST) led by Ryong Rhoo have succeeded in producing crystalline, mesoporous molecular sieves (MMS) with a hierarchy of pore sizes ranging from the micro-to the mesoporous in the same sample, as reported recently in Science.
The key to the synthesis of these unique materials lies in the use of polyquaternary ammonium surfactants. “Hexagonal mesostructures were generated by aggregation of the surfactant molecules,” the researchers wrote, “whereas the crystallization of microporous frameworks was directed by quaternary ammonium groups within the mesopore walls.” By varying the number of quaternary ammonium species in the head group of the surfactant, the researchers were able to control the thickness of the mesopore walls and the structure of the zeolitelike microporous frameworks within them. Reported mean micropore diameters ranged from 0.55 to 0.65 nm, with mean mesopore diameters ranging from 3.5 to 4.7 nm. Mesopores with diameters up to 21 nm were formed using micelle swelling agents.
Functionally, these materials showed excellent properties as acid catalysts for various reactions involving bulky organic molecules that are too large to fit into traditional zeolite catalyst pores. The researchers attribute the increased catalytic activity compared to conventional zeolites or amorphous MMSs to the ease of molecular diffusion through the mesopores, the strong acidity of the crystalline zeolitelike structures, and the high concentration of surface acid sites. [Science]
Nano Focus
Light antennas from DNA-programmed quantum dots
(University of Toronto. See also the press release from the University of Toronto "Engineering in the News" web page.)
Image courtesy of Ted Sargent, University of Toronto. Click image to enlarge.
(University of Toronto. See also the press release from the University of Toronto "Engineering in the News" web page.)
Image courtesy of Ted Sargent, University of Toronto. Click image to enlarge.
Image caption: From top left to bottom right, DNA-functionalized quantum dots with varying valencies self-assemble into a complex "light antenna" structure.
 |
Solar cells and other photonic devices might increase their efficiency if a “light antenna” were available to absorb incoming light and funnel it to a central junction in the circuit. As reported recently in Nature Nanotechnology, researchers at the University of Toronto have succeeded in synthesizing such antennas in a “one-pot” process that connects CdTe quantum dots together in controlled patterns using DNA strands as the linkage material.
“DNA was advantageous for a number of reasons,” says Ted Sargent, one of the lead researchers on the project. “It was straightforward to attach a portion of the DNA sequences to the surfaces of the quantum dots.” Also, they could easily control the length of this quantum dot binding domain, which features phosphorothioate linkages within the DNA backbone; these linkages have the highest affinity for the cations of the CdTe quantum dot. “The length determined how many DNA sequences could fit onto each quantum dot, thus allowing us to program the number of DNA strands, or valency, of each quantum dot,” Sargent says.
The variable valency—from one to five DNA strands per quantum dot—gave the researchers the building blocks needed to design the geometry of any structure. The well-known bonding specificity of the DNA bases allowed them to specify which quantum dots bonded to each other. Furthermore, by controlling the diameters of the quantum dots, the researchers were also able to control their optical properties, such as absorption and luminescence color.
In a demonstration of the light antenna concept, Sargent and his co-workers assembled a structure with one red quantum dot in the center bonded to four yellow quantum dots, each of which was bonded to one green one. “When we photoexcite the green particles, they rapidly transfer their energy downhill to the yellow, and then to the red, particles,” according to co-corresponding author Shana Kelley, also Professor at the University of Toronto. “In this way, these new structures function like the photosynthetic reaction centers in plants – they are light-harvesting antennas.”
[Nature Nanotechnology]
“DNA was advantageous for a number of reasons,” says Ted Sargent, one of the lead researchers on the project. “It was straightforward to attach a portion of the DNA sequences to the surfaces of the quantum dots.” Also, they could easily control the length of this quantum dot binding domain, which features phosphorothioate linkages within the DNA backbone; these linkages have the highest affinity for the cations of the CdTe quantum dot. “The length determined how many DNA sequences could fit onto each quantum dot, thus allowing us to program the number of DNA strands, or valency, of each quantum dot,” Sargent says.
The variable valency—from one to five DNA strands per quantum dot—gave the researchers the building blocks needed to design the geometry of any structure. The well-known bonding specificity of the DNA bases allowed them to specify which quantum dots bonded to each other. Furthermore, by controlling the diameters of the quantum dots, the researchers were also able to control their optical properties, such as absorption and luminescence color.
In a demonstration of the light antenna concept, Sargent and his co-workers assembled a structure with one red quantum dot in the center bonded to four yellow quantum dots, each of which was bonded to one green one. “When we photoexcite the green particles, they rapidly transfer their energy downhill to the yellow, and then to the red, particles,” according to co-corresponding author Shana Kelley, also Professor at the University of Toronto. “In this way, these new structures function like the photosynthetic reaction centers in plants – they are light-harvesting antennas.”
[Nature Nanotechnology]
Energy Focus
Harnessing “junk” energy
(State University of New York at Buffalo and California Insititue of Technology. See also the press release by Charlotte Hsu.)
Harnessing “junk” energy
(State University of New York at Buffalo and California Insititue of Technology. See also the press release by Charlotte Hsu.)
Image courtesy of State University of New York at Buffalo. Click image to enlarge.
Image caption: Mathematically designed grain structures from an enhanced version of Hertz's law.
 |
Researchers at the State University of New York at Buffalo and the California Institute of Technology have developed a rigorous mathematical treatment that extends Hertz’s law, which describes the repulsive force between two elastic, spherical objects that are pressed together, to objects having other shapes. As reported recently inPhysical Review E, a team led by Surajit Sen modified the law first elucidated by Heinrich Hertz in 1881 to include contact between grains of irregular shapes often found in nature. “Pointy or blunt,” Sen says, “we have found a way to calculate the force” between adjacent particles. Usually nature presents us with one force law that we are stuck with, but “now we can generate almost any kind of non-linear force law,” he says.
This discovery could make it possible to harness the “junk” energy that is commonly wasted in road vibrations or rock concerts. If a material were available with a carefully designed microstructure that would propagate this energy in a controlled manner from grain to grain, it could be possible to convert the junk energy to electricity for useful purposes. “By tweaking force propagation from one grain to another, we can potentially channel energy in controllable ways, which includes slowing down how energy moves, varying the space across which it moves and potentially even holding some of it down," Sen says.
For now the discovery remains a mathematical one only. Demonstrating this effect experimentally using the mechanical forces between grains in a material could take some time, Sen concedes. But it is possible to construct a system that converts mechanical energy into electrical energy, which might make the challenge of producing a practical device less daunting. “We could have chips that take energy from road vibrations, runway noise from airports-- energy that we are not able to make use of very well,” Sen says, “and convert it into pulses, packets of electrical energy, that become useful.” [Physical Review E]
This discovery could make it possible to harness the “junk” energy that is commonly wasted in road vibrations or rock concerts. If a material were available with a carefully designed microstructure that would propagate this energy in a controlled manner from grain to grain, it could be possible to convert the junk energy to electricity for useful purposes. “By tweaking force propagation from one grain to another, we can potentially channel energy in controllable ways, which includes slowing down how energy moves, varying the space across which it moves and potentially even holding some of it down," Sen says.
For now the discovery remains a mathematical one only. Demonstrating this effect experimentally using the mechanical forces between grains in a material could take some time, Sen concedes. But it is possible to construct a system that converts mechanical energy into electrical energy, which might make the challenge of producing a practical device less daunting. “We could have chips that take energy from road vibrations, runway noise from airports-- energy that we are not able to make use of very well,” Sen says, “and convert it into pulses, packets of electrical energy, that become useful.” [Physical Review E]
Image in Focus
Microbial Flare
Ronn S. Friedlander, Harvard-MIT Division of Health Sciences and Technology
Polarized light micrograph of an E. coli colony. Congo red staining of amyloid fibers creates the remarkable birefringence patterns seen here.
Ronn S. Friedlander, Harvard-MIT Division of Health Sciences and Technology
Polarized light micrograph of an E. coli colony. Congo red staining of amyloid fibers creates the remarkable birefringence patterns seen here.
0 comments:
Post a Comment