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RESEARCH NEWS RESEARCH NEWS Solid-state catalysis is one of the cornerstones of the modern chemical industry, yet knowledge of the underlying processes at play are often surprisingly limited, partly because the catalytic efficiency depends directly on the nanoscale morphology of the catalyst itself. Traditional assay techniques commonly fail to resolve details of this size, and report averages over a wide range of morphologies instead. The only way to establish a precise relation between particle structure and catalytic efficiency is to image the individual catalyst particles directly, and precisely such an approach has now been demonstrated by an Australian research group [Novo et al, Nat. Nanotechnology (2008) doi:10.1038/nnano.2008.246]. At the heart of their study is the use of dark field microscopy. “To date most of this work has focused on understanding the optical properties of such small objects, yet we wanted to measure how fast redox reactions take place at individual catalytic particles,” explains Paul Mulvaney of the University of Melbourne. Through this technique the researchers were able to measure redox reactions of just a few hundred electrons per second, and showed that the growth of a single nanocrystal can be monitored in real-time. “We hope that this method will enable scientists to study catalysts in much more detail and to establish which types of crystal are the most efficient,” Paul continues. “In the longer term, we believe that these measurements demonstrate that electrical modulation of the nanocrystal scattering provides a new method for developing plasmonic-based circuitry. Right now we are developing a method to inject electrons into the particle electrochemically, which is much more controlled than the chemical method, and will enable reversible and defined surface plasmon modulation. This will be the first step towards plasmonic switching.” Peter Dedecker The research and development of light scattering in nanowire materials is of large importance for the optimization of the external efficiency of nanowire-based photovoltaic devices [Muskens et al. 10.1021/nl0808076], by matching the wavelength of the relevant light to the nanowire diameter and the refraction index of the filler medium, more light is absorbed. The optical properties of nanowire layers relevant for solar cell applications are governed by strong multiple scattering of light. Nanowire layers were synthesised on substrates of InP, Si, and GaP, which are strongly photonic and produce multiple scattering. Using a supercontinuum white-light source and visible and near-infrared fiber spectrometers, both specular and diffuse reflectance spectra of the nanowire layers were measured over the spectral region 0.7 to 2.2 eV (1650-560 nm). Diffuse reflectance as a function of energy was calculated for the three nanowire materials. The measurement of hemispherical diffuse reflectance for different thicknesses of nanowire in the Si and GaP materials showed that light scattering contributes significantly as the wavelength becomes shorter and more closely matches the diameter of the nanowires, whilst for InP nanowires diffuse reflectance decreases sharply towards shorter wavelengths. Successful suppression of the hemispherical diffuse reflectance of InP nanowires was demonstrated, to below that of the corresponding transparent effective medium. The control and design of light scattering in nanowire-based devices is vital for increasing solar energy collection performance, where maximum spectral irradiance occurs in the green spectral region. Jon Hobden Nanowires in the sun ENERGY Learning mechanical principles from ancient fish Over 500 million years ago, in the Ordovician period, the first dermal armour among vertebrates developed with the rise of the now long-dead Ostracoderms. Today, researchers from the Departments of Materials Science and Engineering as well as Mechanical Engineering, both of the Massachusetts Institute of Technology, are investigating more recent types of fish body armour with regard to their mechanical properties (Nature Materials, doi:10.1038/nmat2231). In particular, they performed an experimental and computational study of materials design principles in the armour of the ‘living fossil’ fish Polypterus senegalus , a member of the Polypteridae family which appeared in the Cretaceous and has not changed much since. Benjamin Bruet and her group examined the features of both the individual multi-layered, mineralized ganoid scales as well as the array of interlocking scales as a whole. Evolution saw to it that this body armour developed into a system both rigid and flexible – an approach that Bruet and colleagues try to learn from. “This could enable pathways to improved bioinspired human body armour, and may provide clues to the evolutionary origins of mineralized tissues,” says Bruet. They found that many parameters, such as material variations, the number and thickness of layers, the construction of junctions, the prevailing geometries etc. significantly influence the overall properties. Each individual scale possesses a quad-layered structure, each with its own quite unique set of properties, optimized for their respective biological function. In addition, the scientists found that the junctions between material layers possess a gradual change in properties. These junctions are capable of transferring load and redistribute stress, thus relieving mechanical strain of the individual scales and significantly enhancing the performance of the system as a whole. Researchers hope to gain insight into general principles that may be applied in the design of improved engineered biomimetic structural materials. Michel Fleck BIOMATERIALS Dark field microscopy image of individual gold nanocrystals of various shapes and sizes. Single catalysts show their true colors NANOSCOPIC MATERIALS OCT-DEC 2008 | VOLUME 3 | NUMBER 5-6 8

Single catalysts show their true colors: Nanoscopic materials

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RESEARCH NEWSRESEARCH NEWS

Solid-state catalysis is one of the cornerstones of the modern chemical industry, yet knowledge of the underlying processes at play are often surprisingly limited, partly because the catalytic efficiency depends directly on the nanoscale morphology of the catalyst itself. Traditional assay techniques commonly fail to resolve details of this size, and report averages over a wide range of morphologies instead.The only way to establish a precise relation between particle structure and catalytic efficiency is to image the individual catalyst particles directly, and precisely such an approach has now been demonstrated by an Australian research group [Novo et al, Nat. Nanotechnology (2008) doi:10.1038/nnano.2008.246].At the heart of their study is the use of dark field microscopy. “To date most of this work has focused on understanding the optical properties of such small objects, yet we wanted to measure how fast redox reactions take place at individual catalytic particles,” explains Paul Mulvaney of the University of Melbourne. Through this technique the researchers were able to measure redox reactions of just a few hundred electrons per second, and showed that the growth of a single nanocrystal can be monitored in real-time.“We hope that this method will enable scientists to study catalysts in much more detail and to establish which types of crystal are the most efficient,” Paul continues. “In the longer term, we believe that these

measurements demonstrate that electrical modulation of the nanocrystal scattering provides a new method for developing plasmonic-based circuitry. Right now we are developing a method to inject electrons into the particle electrochemically, which is much more controlled than the chemical method, and will enable reversible and defined surface plasmon modulation. This will be the first step towards plasmonic switching.”

Peter Dedecker

The research and development of light

scattering in nanowire materials is of

large importance for the optimization

of the external efficiency of

nanowire-based photovoltaic devices

[Muskens et al. 10.1021/nl0808076],

by matching the wavelength of

the relevant light to the nanowire

diameter and the refraction index

of the filler medium, more light is

absorbed.

The optical properties of nanowire

layers relevant for solar cell

applications are governed by strong

multiple scattering of light. Nanowire

layers were synthesised on substrates

of InP, Si, and GaP, which are strongly

photonic and produce multiple

scattering. Using a supercontinuum

white-light source and visible and

near-infrared fiber spectrometers, both

specular and diffuse reflectance spectra

of the nanowire layers were measured

over the spectral region 0.7 to 2.2 eV

(1650-560 nm). Diffuse reflectance as

a function of energy was calculated for

the three nanowire materials.

The measurement of hemispherical

diffuse reflectance for different

thicknesses of nanowire in the Si

and GaP materials showed that light

scattering contributes significantly

as the wavelength becomes shorter

and more closely matches the

diameter of the nanowires, whilst

for InP nanowires diffuse reflectance

decreases sharply towards shorter

wavelengths. Successful suppression of

the hemispherical diffuse reflectance

of InP nanowires was demonstrated,

to below that of the corresponding

transparent effective medium.

The control and design of light

scattering in nanowire-based devices

is vital for increasing solar energy

collection performance, where

maximum spectral irradiance occurs in

the green spectral region.

Jon Hobden

Nanowires in the sunENERGY

Learning mechanical principles from ancient fish

Over 500 million years ago, in the Ordovician period, the

first dermal armour among vertebrates developed with the

rise of the now long-dead Ostracoderms. Today, researchers

from the Departments of Materials Science and Engineering

as well as Mechanical Engineering, both of the Massachusetts

Institute of Technology, are investigating more recent types of

fish body armour with regard to their mechanical properties

(Nature Materials, doi:10.1038/nmat2231). In particular,

they performed an experimental and computational study of

materials design principles in the armour of the ‘living fossil’

fish Polypterus senegalus , a member of the Polypteridae

family which appeared in the Cretaceous and has not changed

much since.

Benjamin Bruet and her group examined the features of both

the individual multi-layered, mineralized ganoid scales as well

as the array of interlocking scales as a whole. Evolution saw

to it that this body armour developed into a system both

rigid and flexible – an approach that Bruet and colleagues

try to learn from. “This could enable pathways to improved

bioinspired human body armour, and may provide clues to the

evolutionary origins of mineralized tissues,” says Bruet. They

found that many parameters, such as material variations, the

number and thickness of layers, the construction of junctions,

the prevailing geometries etc. significantly influence the

overall properties.

Each individual scale possesses a quad-layered structure, each

with its own quite unique set of properties, optimized for their

respective biological function.

In addition, the scientists found that the junctions between

material layers possess a gradual change in properties. These

junctions are capable of transferring load and redistribute

stress, thus relieving mechanical strain of the individual scales

and significantly enhancing the performance of the system as

a whole.

Researchers hope to gain insight into general principles

that may be applied in the design of improved engineered

biomimetic structural materials.

Michel Fleck

BIOMATERIALS

Dark field microscopy image of individual gold

nanocrystals of various shapes and sizes.

Single catalysts show their true colorsNANOSCOPIC MATERIALS

OCT-DEC 2008 | VOLUME 3 | NUMBER 5-68

NTv3n5-6p8_11.indd 8 11/11/2008 17:06:08