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Chemists create tiny gold nanoparticles that reflect nature's patterns
Our world is full of patterns, from the twist of a DNA molecule to the spiral of the Milky Way. New research from Carnegie Mellon chemists has revealed that tiny, synthetic gold nanoparticles exhibit some of nature's most intricate patterns. Unveiling the kaleidoscope of these patterns was a Herculean task, and it marks the first time that a nanoparticle of this size has been crystallized and its structure mapped out atom by atom. The researchers report their work in the March 20 issue of ("Structural patterns at all scales in a nonmetallic chiral Au133(SR)52 nanoparticle"). "As you broadly think about different research areas or even our everyday lives, these kinds of patterns, these hierarchical patterns, are universal," said Rongchao Jin, associate professor of chemistry. "Our universe is really beautiful and when you see this kind of information in something as small as a 133-atom nanoparticle and as big as the Milky Way, it's really amazing." 
The x-ray crystallographic structure of the gold nanoparticle is shown. Gold atoms = magenta; sulfur atoms = yellow; carbon atoms = gray; hydrogen atoms = white. (Image: Carnegie Mellon) Gold nanoparticles, which can vary in size from 1 to 100 nanometers, are a promising technology that has applications in a wide range of fields including catalysis, electronics, materials science and health care. But, in order to use gold nanoparticles in practical applications, scientists must first understand the tiny particles' structure. "Structure essentially determines the particle's properties, so without knowing the structure, you wouldn't be able to understand the properties and you wouldn't be able to functionalize them for specific applications," said Jin, an expert in creating atomically precise gold nanoparticles. With this latest research, Jin and his colleagues, including graduate student Chenjie Zeng, have solved the structure of a nanoparticle, Au133, made up of 133 gold atoms and 52 surface-protecting molecules--the biggest nanoparticle structure ever resolved with X-ray crystallography. While microscopy can reveal the size, shape and the atomic lattice of nanoparticles, it can't discern the surface structure. X-ray crystallography can, by mapping out the position of every atom on the nanoparticles' surface and showing how they bond with the gold core. Knowing the surface structure is key to using the nanoparticles for practical applications, such as catalysis, and for uncovering fundamental science, such as the basis of the particle's stability. The crystal structure of the Au133 nanoparticle divulged many secrets. "With X-ray crystallography, we were able to see very beautiful patterns, which was a very exciting discovery. These patterns only show up when the nanoparticle size becomes big enough," Jin said. During production, the Au133 particles self-assemble into three layers within each particle: the gold core, the surface molecules that protect it and the interface between the two. In the crystal structure, Zeng discovered that the gold core is in the shape of an icosahedron. At the interface between the core and the surface-protecting molecules is a layer of sulfur atoms that bind with the gold atoms. The sulfur-gold-sulfur combinations stack into ladder-like helical structures. Finally, attached to the sulfur molecules is an outer layer of surface-protecting molecules whose carbon tails self-assemble into fourfold swirls. "The helical features remind us of a DNA double helix and the rotating arrangement of the carbon tails is reminiscent of the way our galaxy is arranged. It's really amazing," Jin said. These particular patterns are responsible for the high stability of Au133 compared to other sizes of gold nanoparticles. The researchers also tested the optical and electronic properties of Au133 and found that these gold nanoparticles are not metallic. Normally, gold is one of the best conductors of electrical current, but the size of Au133 is so small that the particle hasn't yet become metallic. Jin's group is currently testing the nanoparticles for use as catalysts, substances that can increase the rate of a chemical reaction.
The x-ray crystallographic structure of the gold nanoparticle is shown. Gold atoms = magenta; sulfur atoms = yellow; carbon atoms = gray; hydrogen atoms = white. (Image: Carnegie Mellon) Gold nanoparticles, which can vary in size from 1 to 100 nanometers, are a promising technology that has applications in a wide range of fields including catalysis, electronics, materials science and health care. But, in order to use gold nanoparticles in practical applications, scientists must first understand the tiny particles' structure. "Structure essentially determines the particle's properties, so without knowing the structure, you wouldn't be able to understand the properties and you wouldn't be able to functionalize them for specific applications," said Jin, an expert in creating atomically precise gold nanoparticles. With this latest research, Jin and his colleagues, including graduate student Chenjie Zeng, have solved the structure of a nanoparticle, Au133, made up of 133 gold atoms and 52 surface-protecting molecules--the biggest nanoparticle structure ever resolved with X-ray crystallography. While microscopy can reveal the size, shape and the atomic lattice of nanoparticles, it can't discern the surface structure. X-ray crystallography can, by mapping out the position of every atom on the nanoparticles' surface and showing how they bond with the gold core. Knowing the surface structure is key to using the nanoparticles for practical applications, such as catalysis, and for uncovering fundamental science, such as the basis of the particle's stability. The crystal structure of the Au133 nanoparticle divulged many secrets. "With X-ray crystallography, we were able to see very beautiful patterns, which was a very exciting discovery. These patterns only show up when the nanoparticle size becomes big enough," Jin said. During production, the Au133 particles self-assemble into three layers within each particle: the gold core, the surface molecules that protect it and the interface between the two. In the crystal structure, Zeng discovered that the gold core is in the shape of an icosahedron. At the interface between the core and the surface-protecting molecules is a layer of sulfur atoms that bind with the gold atoms. The sulfur-gold-sulfur combinations stack into ladder-like helical structures. Finally, attached to the sulfur molecules is an outer layer of surface-protecting molecules whose carbon tails self-assemble into fourfold swirls. "The helical features remind us of a DNA double helix and the rotating arrangement of the carbon tails is reminiscent of the way our galaxy is arranged. It's really amazing," Jin said. These particular patterns are responsible for the high stability of Au133 compared to other sizes of gold nanoparticles. The researchers also tested the optical and electronic properties of Au133 and found that these gold nanoparticles are not metallic. Normally, gold is one of the best conductors of electrical current, but the size of Au133 is so small that the particle hasn't yet become metallic. Jin's group is currently testing the nanoparticles for use as catalysts, substances that can increase the rate of a chemical reaction.Multimetal nanoframes improve catalyst performance
A team of researchers has synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of solid Pt-Ni bimetallic nanocrystals into porous cage-like structures or nanoframes ("Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces"). This novel material significantly enhanced catalytic activity for the oxygen reduction reaction -- the splitting of an O2 molecule into two oxygen ions -- that is critical to fuel cells and potentially other electrochemical applications. 
Illustration of the catalytic oxygen reduction reaction on the surface of platinum-nickel nanoframes with multilayered platinum skin structure. The Impact This approach to synthesizing the material is a significant advance towards realizing electrocatalysts with superior catalytic properties and lower cost. The open structure of the nanoframes addresses some of the major design criteria for advanced nanoscale electrocatalysts, namely, high surface-to-volume ratio, three-dimensional surface accessibility to reactants, and optimal precious metal use. Researchers are optimistic that the approach can be readily applied to other multimetallic catalysts, potentially lowering the cost of catalytic material production. Summary Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement of both activity and durability. A team of researchers from Argonne National Laboratory, Lawrence Berkeley National Laboratory, and the University of Wisconsin synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of solid Pt-Ni bimetallic nanocrystals into porous cage-like structures or nanoframes. The material was synthesized by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals into cage-like structures with a self-assembled Pt skin structure on the interior and exterior surfaces. The starting material, crystalline PtNi3 nanoparticles, are transformed in solution and at mild temperatures into Pt3Ni nanoframes with surfaces that have three-dimensional molecular accessibility. The Pt-rich edges of the starting PtNi3 nanoparticles are maintained in the final Pt3Ni nanoframes. Both the interior and exterior surfaces of this open framework structure are composed of a Pt-rich skin structure that exhibits enhanced oxygen reduction reaction activity. The Pt3Ni nanoframe catalysts achieved a more than 36-fold and 22-fold enhancement in two different measures of catalytic activity (mass and specific activities, respectively) for the oxygen reduction reaction in comparison to state-of-the-art carbon-supported Pt catalysts (Pt/C) during prolonged exposure to reaction conditions. This work is a significant advance towards developing more efficient electrocatalysts for water-splitting reactions and fuel generation. These electrocatalyst structures were applied to the hydrogen evolution reaction (HER), which is the crucial cathodic reaction in water-alkali electrolyzers, which generate hydrogen by splitting water. The HER activity for highly crystalline Pt3Ni–Pt-skin nanoframe surface was enhanced by almost one order of magnitude relative to Pt/C. Utilizing the spontaneous structural evolution of a bimetallic nanoparticle from solid polyhedra to hollow nanoframes with controlled size, structure, and surface composition should be readily applicable to other multimetallic catalysts.
Illustration of the catalytic oxygen reduction reaction on the surface of platinum-nickel nanoframes with multilayered platinum skin structure. The Impact This approach to synthesizing the material is a significant advance towards realizing electrocatalysts with superior catalytic properties and lower cost. The open structure of the nanoframes addresses some of the major design criteria for advanced nanoscale electrocatalysts, namely, high surface-to-volume ratio, three-dimensional surface accessibility to reactants, and optimal precious metal use. Researchers are optimistic that the approach can be readily applied to other multimetallic catalysts, potentially lowering the cost of catalytic material production. Summary Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement of both activity and durability. A team of researchers from Argonne National Laboratory, Lawrence Berkeley National Laboratory, and the University of Wisconsin synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of solid Pt-Ni bimetallic nanocrystals into porous cage-like structures or nanoframes. The material was synthesized by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals into cage-like structures with a self-assembled Pt skin structure on the interior and exterior surfaces. The starting material, crystalline PtNi3 nanoparticles, are transformed in solution and at mild temperatures into Pt3Ni nanoframes with surfaces that have three-dimensional molecular accessibility. The Pt-rich edges of the starting PtNi3 nanoparticles are maintained in the final Pt3Ni nanoframes. Both the interior and exterior surfaces of this open framework structure are composed of a Pt-rich skin structure that exhibits enhanced oxygen reduction reaction activity. The Pt3Ni nanoframe catalysts achieved a more than 36-fold and 22-fold enhancement in two different measures of catalytic activity (mass and specific activities, respectively) for the oxygen reduction reaction in comparison to state-of-the-art carbon-supported Pt catalysts (Pt/C) during prolonged exposure to reaction conditions. This work is a significant advance towards developing more efficient electrocatalysts for water-splitting reactions and fuel generation. These electrocatalyst structures were applied to the hydrogen evolution reaction (HER), which is the crucial cathodic reaction in water-alkali electrolyzers, which generate hydrogen by splitting water. The HER activity for highly crystalline Pt3Ni–Pt-skin nanoframe surface was enhanced by almost one order of magnitude relative to Pt/C. Utilizing the spontaneous structural evolution of a bimetallic nanoparticle from solid polyhedra to hollow nanoframes with controlled size, structure, and surface composition should be readily applicable to other multimetallic catalysts.New understanding of electromagnetism could enable 'antennas on a chip'
A team of researchers from the University of Cambridge have unravelled one of the mysteries of electromagnetism, which could enable the design of antennas small enough to be integrated into an electronic chip. These ultra-small antennas – the so-called ‘last frontier’ of semiconductor design – would be a massive leap forward for wireless communications. In new results published in the journal ("Electromagnetic Radiation Under Explicit symmetry Breaking"), the researchers have proposed that electromagnetic waves are generated not only from the acceleration of electrons, but also from a phenomenon known as symmetry breaking. In addition to the implications for wireless communications, the discovery could help identify the points where theories of classical electromagnetism and quantum mechanics overlap. 
The radiation pattern from a dipole antenna showing symmetry breaking of the electric field. (Image: Generated using Mathematica from Wolfram Inc) The phenomenon of radiation due to electron acceleration, first identified more than a century ago, has no counterpart in quantum mechanics, where electrons are assumed to jump from higher to lower energy states. These new observations of radiation resulting from broken symmetry of the electric field may provide some link between the two fields. The purpose of any antenna, whether in a communications tower or a mobile phone, is to launch energy into free space in the form of electromagnetic or radio waves, and to collect energy from free space to feed into the device. One of the biggest problems in modern electronics, however, is that antennas are still quite big and incompatible with electronic circuits – which are ultra-small and getting smaller all the time. “Antennas, or aerials, are one of the limiting factors when trying to make smaller and smaller systems, since below a certain size, the losses become too great,” said Professor Gehan Amaratunga of Cambridge’s Department of Engineering, who led the research. “An aerial’s size is determined by the wavelength associated with the transmission frequency of the application, and in most cases it’s a matter of finding a compromise between aerial size and the characteristics required for that application.” Another challenge with aerials is that certain physical variables associated with radiation of energy are not well understood. For example, there is still no well-defined mathematical model related to the operation of a practical aerial. Most of what we know about electromagnetic radiation comes from theories first proposed by James Clerk Maxwell in the 19th century, which state that electromagnetic radiation is generated by accelerating electrons. However, this theory becomes problematic when dealing with radio wave emission from a dielectric solid, a material which normally acts as an insulator, meaning that electrons are not free to move around. Despite this, dielectric resonators are already used as antennas in mobile phones, for example. “In dielectric aerials, the medium has high permittivity, meaning that the velocity of the radio wave decreases as it enters the medium,” said Dr Dhiraj Sinha, the paper’s lead author. “What hasn’t been known is how the dielectric medium results in emission of electromagnetic waves. This mystery has puzzled scientists and engineers for more than 60 years.” Working with researchers from the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova Ltd, the Cambridge team used thin films of piezoelectric materials, a type of insulator which is deformed or vibrated when voltage is applied. They found that at a certain frequency, these materials become not only efficient resonators, but efficient radiators as well, meaning that they can be used as aerials. The researchers determined that the reason for this phenomenon is due to symmetry breaking of the electric field associated with the electron acceleration. In physics, symmetry is an indication of a constant feature of a particular aspect in a given system. When electronic charges are not in motion, there is symmetry of the electric field. Symmetry breaking can also apply in cases such as a pair of parallel wires in which electrons can be accelerated by applying an oscillating electric field. “In aerials, the symmetry of the electric field is broken ‘explicitly’ which leads to a pattern of electric field lines radiating out from a transmitter, such as a two wire system in which the parallel geometry is ‘broken’,” said Sinha. The researchers found that by subjecting the piezoelectric thin films to an asymmetric excitation, the symmetry of the system is similarly broken, resulting in a corresponding symmetry breaking of the electric field, and the generation of electromagnetic radiation. The electromagnetic radiation emitted from dielectric materials is due to accelerating electrons on the metallic electrodes attached to them, as Maxwell predicted, coupled with explicit symmetry breaking of the electric field. “If you want to use these materials to transmit energy, you have to break the symmetry as well as have accelerating electrons – this is the missing piece of the puzzle of electromagnetic theory,” said Amaratunga. “I’m not suggesting we’ve come up with some grand unified theory, but these results will aid understanding of how electromagnetism and quantum mechanics cross over and join up. It opens up a whole set of possibilities to explore.” The future applications for this discovery are important, not just for the mobile technology we use every day, but will also aid in the development and implementation of the Internet of Things: ubiquitous computing where almost everything in our homes and offices, from toasters to thermostats, is connected to the internet. For these applications, billions of devices are required, and the ability to fit an ultra-small aerial on an electronic chip would be a massive leap forward. Piezoelectric materials can be made in thin film forms using materials such as lithium niobate, gallium nitride and gallium arsenide. Gallium arsenide-based amplifiers and filters are already available on the market and this new discovery opens up new ways of integrating antennas on a chip along with other components. “It’s actually a very simple thing, when you boil it down,” said Sinha. “We’ve achieved a real application breakthrough, having gained an understanding of how these devices work.”
The radiation pattern from a dipole antenna showing symmetry breaking of the electric field. (Image: Generated using Mathematica from Wolfram Inc) The phenomenon of radiation due to electron acceleration, first identified more than a century ago, has no counterpart in quantum mechanics, where electrons are assumed to jump from higher to lower energy states. These new observations of radiation resulting from broken symmetry of the electric field may provide some link between the two fields. The purpose of any antenna, whether in a communications tower or a mobile phone, is to launch energy into free space in the form of electromagnetic or radio waves, and to collect energy from free space to feed into the device. One of the biggest problems in modern electronics, however, is that antennas are still quite big and incompatible with electronic circuits – which are ultra-small and getting smaller all the time. “Antennas, or aerials, are one of the limiting factors when trying to make smaller and smaller systems, since below a certain size, the losses become too great,” said Professor Gehan Amaratunga of Cambridge’s Department of Engineering, who led the research. “An aerial’s size is determined by the wavelength associated with the transmission frequency of the application, and in most cases it’s a matter of finding a compromise between aerial size and the characteristics required for that application.” Another challenge with aerials is that certain physical variables associated with radiation of energy are not well understood. For example, there is still no well-defined mathematical model related to the operation of a practical aerial. Most of what we know about electromagnetic radiation comes from theories first proposed by James Clerk Maxwell in the 19th century, which state that electromagnetic radiation is generated by accelerating electrons. However, this theory becomes problematic when dealing with radio wave emission from a dielectric solid, a material which normally acts as an insulator, meaning that electrons are not free to move around. Despite this, dielectric resonators are already used as antennas in mobile phones, for example. “In dielectric aerials, the medium has high permittivity, meaning that the velocity of the radio wave decreases as it enters the medium,” said Dr Dhiraj Sinha, the paper’s lead author. “What hasn’t been known is how the dielectric medium results in emission of electromagnetic waves. This mystery has puzzled scientists and engineers for more than 60 years.” Working with researchers from the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova Ltd, the Cambridge team used thin films of piezoelectric materials, a type of insulator which is deformed or vibrated when voltage is applied. They found that at a certain frequency, these materials become not only efficient resonators, but efficient radiators as well, meaning that they can be used as aerials. The researchers determined that the reason for this phenomenon is due to symmetry breaking of the electric field associated with the electron acceleration. In physics, symmetry is an indication of a constant feature of a particular aspect in a given system. When electronic charges are not in motion, there is symmetry of the electric field. Symmetry breaking can also apply in cases such as a pair of parallel wires in which electrons can be accelerated by applying an oscillating electric field. “In aerials, the symmetry of the electric field is broken ‘explicitly’ which leads to a pattern of electric field lines radiating out from a transmitter, such as a two wire system in which the parallel geometry is ‘broken’,” said Sinha. The researchers found that by subjecting the piezoelectric thin films to an asymmetric excitation, the symmetry of the system is similarly broken, resulting in a corresponding symmetry breaking of the electric field, and the generation of electromagnetic radiation. The electromagnetic radiation emitted from dielectric materials is due to accelerating electrons on the metallic electrodes attached to them, as Maxwell predicted, coupled with explicit symmetry breaking of the electric field. “If you want to use these materials to transmit energy, you have to break the symmetry as well as have accelerating electrons – this is the missing piece of the puzzle of electromagnetic theory,” said Amaratunga. “I’m not suggesting we’ve come up with some grand unified theory, but these results will aid understanding of how electromagnetism and quantum mechanics cross over and join up. It opens up a whole set of possibilities to explore.” The future applications for this discovery are important, not just for the mobile technology we use every day, but will also aid in the development and implementation of the Internet of Things: ubiquitous computing where almost everything in our homes and offices, from toasters to thermostats, is connected to the internet. For these applications, billions of devices are required, and the ability to fit an ultra-small aerial on an electronic chip would be a massive leap forward. Piezoelectric materials can be made in thin film forms using materials such as lithium niobate, gallium nitride and gallium arsenide. Gallium arsenide-based amplifiers and filters are already available on the market and this new discovery opens up new ways of integrating antennas on a chip along with other components. “It’s actually a very simple thing, when you boil it down,” said Sinha. “We’ve achieved a real application breakthrough, having gained an understanding of how these devices work.”
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