Scientists have perfected a technique for very accurately measuring and controlling the electromagnetic waves within some of the shortest laser pulses ever made, says new research. Being able to fully understand and control these laser pulses represents an important step towards using them to track and manipulate electrons in leading-edge research at the sub-atomic level.

The study, published in Nature Physics, focused on extremely short laser pulses, less than 10 femtoseconds long - a femtosecond is one million-billionth of a second. These laser pulses can allow scientists to move and control the electrons in atoms and molecules, and to understand, for example, how molecules are formed. To achieve this reliably, the pulse of electromagnetic waves emitted from the laser must be controlled and measured with a precision which, until now, has been very hard to achieve.

The team of physicists from Imperial College London attained an unprecedented level of accurate measurement by firing the femtosecond laser pulse into a sample of gas, which responds by emitting an x-ray pulse which is even shorter in duration - up to 10 times shorter than the original laser pulse. The researchers found that the spectrum of the x-ray pulse has encoded within it all the information necessary to precisely reconstruct the waveform of the original laser pulse. Through careful measurements and some 'intelligent' software designed specifically for this purpose, the researchers were therefore able, for the first time, to measure the waveform of individual femtosecond pulses.

Dr John Tisch, one of the Imperial research team, said: "This measurement technique is so accurate that we can determine the position of a peak in the pulse of electromagnetic waves from the laser with a precision of a mere 0.05 femtoseconds - in other words, 50 attoseconds. Also, the measurement can be made on individual pulses rather than by looking at the average properties of many pulses, so this is an important step forwards."

Dr Tisch explains that not only will this new technique lead to a greater ability to use short laser pulses for accurate sub-atomic level research, but it also sheds new light on the extremely short x-ray pulses emitted in response: "The x-ray pulses we used in the measurement process of our research are of great interest in their own right," he says. "They are on the attosecond timescale, which is even shorter than a femtosecond - just one billion-billionth of a second. They are a new tool for scientists to probe even faster motion than the femtosecond pulses that triggered them."

The research team have recently received a four-year £2.5 million grant from the EPSRC to take this research to the next stage. Professor Jonathan Marangos explains: "Now we've perfected this technique, we are going to look into using our accurate measurements and control of these lasers to manipulate electrons and control quantum processes."

The research was funded by a Basic Technology Programme grant from RCUK.

Lawrence Livermore National Laboratory scientists for the first time have validated the idea of using extremely short and intense X-ray pulses to capture images of objects such as proteins before the X-rays destroy the sample.

At the same time, the team also established a speed record of 25 femtoseconds for flash imaging.

The new method will be applicable to atomic-resolution imaging of complex biomolecules when even more powerful X-ray lasers, currently under construction, are available. The technique will allow scientists to gain insight into the fields of materials science, plasma physics, biology and medicine.

Using the free-electron laser at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Livermore scientists, as part of an international collaboration led by LLNL's Henry Chapman and Janos Hajdu of Uppsala University, were able to record a single diffraction pattern of a nanostructured object before the laser destroyed the sample. A Livermore-developed computer algorithm was then used to recreate an image of the object based on the recorded diffraction pattern. This "lensless" imaging technique could be applied to atomic-resolution imaging because it is not limited by the need to build a high-resolution lens. The flash images could resolve features 50 nanometers in size, which is about 10 times smaller than what is achievable with an optical microscope.

Theory predicts that a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell with an ultra-short and extremely bright X-ray pulse before the sample explodes and turns into a plasma. This means that scientists could better understand the structure of macromolecular proteins without crystallizing them and thus allow rapid study of all classes of proteins.

Livermore scientists, along with colleagues at Uppsala University in Sweden, DESY, Technische Universität Berlin, the Center for Biophotonics Science and Technology at UC Davis, Stanford Synchrotron Radiation Laboratory, and private firm Spiller X-ray Optics of Livermore, conducted the first experimental demonstration of this theory.

Computer simulations based on four different models suggest that a near-atomic resolution structure could be achieved by well-thought out choice of pulse length and intensity of X-ray wavelength before the sample is stripped of its electrons and is destroyed. However, up until now, there had been no experimental verification of the technique.

The experimental demonstration of "flash diffractive imaging" uses the first soft X-ray FEL (free electron laser) in the world located at the FLASH facility at DESY. FLASH generates high-power soft X-ray pulses by the principle of self-amplification of spontaneous emission. The pulses are 10 million times brighter than today's brightest X-ray sources, synchrotrons. In addition, this experiment showed that it only takes a 25-femtosecond pulse duration to capture the image.

There has been a question whether the diffraction pattern recorded under these circumstances could be reconstructed to obtain undamaged sample information.

"These results could become a standardized method," Chapman said. "This imaging could be applied at the cellular, sub-cellular and down on to single molecule scale."

Other Livermore authors include Anton Barty, Michael Bogan, Sebastien Boutet, Matthias Frank, Stefan Hau-Riege, Stefano Marchesini, Bruce Woods, Saša Bajt, Henry Benner, Richard London, Richard Lee and Abraham Szoke.

The work was funded by a Laboratory Directed Research and Development strategic initiative proposal for "biological imaging with fourth-generation light sources."

The research appears in the Nov. 12 online edition of Nature Physics. It will appear on the cover of the December hard copy issue of Nature Physics.

This research was funded in part by the National Science Foundation's Center for Biophotonics Science and Technology (CBST) headquartered at the UC Davis Medical Center in Sacramento. CBST is a multi-institutional research center established in 2002 by LLNL and UC Davis researchers and is dedicated to the development and application of photonic instrumentation and methods to address important problems in the biomedical technology sector.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.

A new imaging technique developed by researchers at the University of Illinois overcomes the limit of diffraction and can reveal the atomic structure of a single nanocrystal with a resolution of less than one angstrom (less than one hundred-millionth of a centimeter).

Optical and electronic properties of small assemblages of atoms called quantum dots depend upon their electronic structure – not just what's on the surface, but also what's inside. While scientists can calculate the electronic structure, they need to know where the atoms are positioned in order to do so accurately.

Getting this information, however, has proved to be a challenge for nanocrystals like quantum dots. Mapping out the positions of atoms requires clues provided by the diffraction pattern. But quantum dots are so small, the clues provided by diffraction alone are not enough.

By combining two sources of information – images and diffraction patterns taken with the same electron microscope – researchers at the U. of I. can achieve sub-angstrom resolution of structures that were not possible before.

"We show that for cadmium-sulfide nanocrystals, the improved image resolution allows a determination of their atomic structures," said Jian-Min (Jim) Zuo, a professor of materials science and engineering at the U. of I., and corresponding author of a paper that describes the high-resolution imaging system in the February issue of Nature Physics.

Images from electron microscopy can resolve individual atoms in a nanocrystal, but the atoms soon suffer radiation damage, which limits the length of observations. Patterns from X-ray diffraction can be used to determine the structure of large crystals, but not for nanocrystals, which are too small and don't diffract well.

To achieve sub-angstrom resolution, Zuo and colleagues developed a reiterative algorithm that processes and combines shape information from the low-resolution image and structure information from the high-resolution diffraction pattern. Both the image and the diffraction pattern are taken with the same transmission-electron microscope.

"The low-resolution image provides the starting point by supplying missing information in the central beam and supplying essential marks for aligning the diffraction pattern," said Zuo, who also is a researcher at the university's Frederick Seitz Materials Research Laboratory. "Our phase-retrieval algorithm then reconstructs the image."

To demonstrate the technique, the researchers took a new look at cadmium-sulfide quantum dots.

"We chose cadmium-sulfide quantum dots because of their size-dependent optical and electronic properties, and the importance of atomic structure on these properties," Zuo said. "Cadmium-sulfide quantum dots have potential applications in solar energy conversion and in medical imaging."

Using the reiterative algorithm, the smallest separation between the cadmium and sulfide atomic columns was measured at 0.84 angstroms, the researchers report.

"Since low-resolution images can be obtained from different sources, our technique is general and can be applied to non-periodic structures, such as interfaces and local defects," Zuo said. "Our technique also provides a basis for imaging the three-dimensional structure of single nanoparticles."

With Zuo, co-authors of the paper are former doctoral student and lead author Weijie Huang (now at Dow Chemical Co.), U. of I. professor of materials science and engineering Moonsub Shim, former postdoctoral research associate Bin Jiang (now at FEI Co.), and former doctoral student Kwan-Wook Kwon (now at LAM Research).

The U.S. Department of Energy, the American Chemical Society and the National Science Foundation funded the work.

In the world of nanomaterials, scientists and engineers can create new structures with tiny building blocks as small as one billionth of a meter.

But in order to construct new materials and devices, researchers first need to understand how these tiny units interact with each other.

One such building block is graphite oxide, which is often used to make graphene — a hotly studied material that scientists believe could be used to produce low-cost carbon-based transparent and flexible electronics. Like graphene, graphite oxide is essentially a sheet that is only one atom thick, but can be as wide as tens of micrometers.

Jiaxing Huang, assistant professor of materials science and engineering at Northwestern University, and his research group at the McCormick School of Engineering and Applied Science set out to investigate how these graphite-oxide sheets assemble. Their results, published as the cover article in the Jan. 26 issue of the Journal of the American Chemical Society, surprised them.

“We were very curious how these extremely thin two-dimensional sheets interact with each other,” Huang says. “This knowledge can also help to prepare better graphene thin films.”

Huang and his group studied the sheets by putting them onto a water surface — a process called Langmuir-Blodgett assembly, which makes the sheets stay flat and allows scientists to move them around.

The effect reminded the researchers of water lilies on a pond, and Huang asked his sister to help to create a Chinese water painting similar to that of Claude Monet’s series of paintings “Water Lilies” to demonstrate the idea. The artwork was chosen as one of the first illustrated covers for the 130-year-old journal.

Researchers used a barrier to push the sheets together to see how they would interact and then “fished” the interacting sheets off the water surface using glass slides or silicon wafers. Huang and his colleagues expected to see that individual sheets had stacked one upon the other, like a shuffled deck of cards. Instead they found that the edges of the graphite oxide sheets rumpled as they were pushed together.

“This was quite a surprise for us,” Huang says. “Now we understand that electrostatic repulsion is the dominant interaction when these sheets are pushed together in this edge-to-edge geometry. This prevents graphite oxide layers from overlapping with each other.”

When squeezed even further, the sheets eventually formed an interlocking structure that becomes a continuous membrane.

This film — consisting of flat, non-overlapping single layers tiling over large areas — has been very difficult to achieve by conventional thin-film processing techniques such as drop casting or spraying.

This breakthrough could have two immediate technological impacts. “Because we can keep them close to each other and still keep them flat, it provides high coverage of the surface with the single layers — which in turn will translate into high successful yield in graphene device fabrication,” Huang says. “On the other hand, the continuous graphite oxide monolayer can be made into a transparent conductor after conversion to graphene.”

Now, after studying how they interact edge-to-edge, Huang hopes to study face-to-face contact of the graphene-based materials. Stacking graphene sheets directly on top of each other will form graphite and lose the advantages of the single-atom-thick graphene materials. But Huang hopes to find a way to stack graphene without making graphite, which could create functional materials for energy-related applications such as electrodes for batteries, ultracapacitors and fuel cells.

“If we are good at making these tiny building blocks and if we can control how they assemble, we will create a lot of wonderful new things,” Huang says.

In addition to Huang, co-authors of the paper include National Science Foundation graduate research fellow Laura Cote and postdoctoral fellow Franklin Kim, both of whom, according to Huang, “did a wonderful job” to create the high-quality graphite oxide sheets used in the experiment.

Materials Chemists Apply Photonic Crystals to Forensics

Photonic crystals -- materials with precise patterns of gaps that make them reflect only selected wavelengths of light -- could soon replace the traditional ink-based fingerprinting. In a new silica-based, photonic-crystal material, the spacing of the gaps changes in response to pressure applied. Corresponding changes in its color reveal fingerprints with high precision -- not only the ridges in the skin, but also the depth of the ridges, the shape of the finger, and the mechanical properties of the skin.

TORONTO -- Increased airport security ... Better police forensics work ... Even improved bridge and building safety. These are all the tremendous possibilities stemming from a new material that's 20 times thinner than a strand of human hair.

Imagine a security system that relied on something unique to every single person -- his fingerprint. Now, scientists have developed a material that makes those prints nearly impossible to forge.

At the University of Toronto inside a science lab, Materials Chemist Andre Arsenault starts from scratch making new crystals. The raw materials are a lot like opal gemstones, which reflect light.

"Opal gemstone is very nice because you get all these multi-faceted color effects," Arsenault tells DBIS.

But these crystals are microscopic. Inside a flask, they form millions of tiny silica circles. Chemists fill the spaces with a synthetic rubber and then dissolve the silica, leaving behind a thin, honeycomb-like structure called a photonic crystal. When you press on it, the holes get closer together, changing the wavelength of light that's reflected.

"As you start pressing, you're gonna gradually go through the rainbow toward the blue. So you're gonna go red, orange, yellow, green, blue, purple," Arsenault says.

With a traditional ink fingerprint, the only thing that can be seen is the ridges on the finger. Full-color prints provide so much more. "You can get information about the depth of the ridges on the people's fingers," Arsenault says. "You can get information about the shape of people's fingers, as well as even the mechanical properties of the skin."

He even made a rubber replica of his fingerprint, which might fool a traditional fingerprint scan. The new material picked up the fake.

Researchers say the photonic crystal material is inexpensive to make and could be used to improve sensors in a number of consumer products.

Researchers “tune” graphene’s properties by growing it on different surfaces.

Researchers at Rensselaer Polytechnic Institute have discovered a new method for controlling the nature of graphene, bringing academia and industry potentially one step closer to realizing the mass production of graphene-based nanoelectronics.

Graphene, a one-atom-thick sheet of carbon, was discovered in 2004 and is considered a potential heir to copper and silicon as the fundamental building blocks of nanoelectronics.

With help from an underlying substrate, researchers for the first time have demonstrated the ability to control the nature of graphene. Saroj Nayak, an associate professor in Rensselaer’s Department of Physics, Applied Physics, and Astronomy, along with Philip Shemella, a postdoctoral research associate in the same department, have determined that the chemistry of the surface on which graphene is deposited plays a key role in shaping the material’s conductive properties. The results are based on large-scale quantum mechanical simulations.

Results show that when deposited on a surface treated with oxygen, graphene exhibits semiconductor properties. When deposited on a material treated with hydrogen, however, graphene exhibits metallic properties.

“Depending on the chemistry of the surface, we can control the nature of the graphene to be metallic or semiconductor,” Nayak said. “Essentially, we are ‘tuning’ the electrical properties of material to suit our needs.”

Conventionally, whenever a batch of graphene nanostructures is produced, some of the graphene is metallic, while the rest is semiconductor. It would be nearly impossible to separate the two on a large scale, Nayak said, yet realizing new graphene devices would require that they be comprised solely of metallic or semiconductor graphene. The new method for “tuning” the nature of graphene is a key step to making this possible, he said.

Graphene’s excellent conductive properties make it attractive to researchers. Even at room temperature, electrons pass effortlessly, near the speed of light and with little resistance. This means a graphene interconnect would likely stay much cooler than a copper interconnect of the same size. Cooler is better, as heat produced by interconnects can have negative effects on both a computer chip’s speed and performance.

Results of the study were published this week in the paper “Electronic structure and band-gap modulation of graphene via substrate surface chemistry” in Applied Physics Letters, and are featured on the cover of the journal’s January 19 issue.

Large-scale quantum simulations for the study were run on Rensselaer’s supercomputing system, the Computational Center for Nanotechnology Innovations (CCNI).

Researchers received funding for the project from the New York State Interconnect Focus Center at Rensselaer.

University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature. Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals.

Their results, published online in the journal Nature Nanotechnology, indicate that graphene holds great promise for replacing conventional semiconductor materials such as silicon in applications ranging from high-speed computer chips to biochemical sensors.

A team of researchers led by physics professor Michael S. Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.

In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is "intrinsic" to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature.

"Other extrinsic sources in today's fairly dirty graphene samples add some extra resistivity to graphene," explained Fuhrer, "so the overall resistivity isn't quite as low as copper's at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper."

In semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm²/Vs in silicon, and 77,000 cm²/Vs in indium antimonide, the highest mobility conventional semiconductor known.

"Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we've shown that the mobility at room temperature is over 100,000 cm²/Vs" said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).

Mobility determines the speed at which an electronic device (for instance, a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals.

Mobility can also be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.

Graphene is therefore a very promising material for chemical and bio-chemical sensing applications. The low resitivity and extremely thin nature of graphene also promises applications in thin, mechanically tough, electrically conducting, transparent films. Such films are sorely needed in a variety of electronics applications from touch screens to photovoltaic cells.

Fuhrer and co-workers showed that although the room temperature limit of mobility in graphene is as high as 200,000 cm²/Vs, in present-day samples the actual mobility is lower, around 10,000 cm²/Vs, leaving significant room for improvement. Because graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide.

Trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility. Also, vibrations of the silicon dioxide atoms themselves can also have an effect on the graphene which is stronger than the effect of graphene's own atomic vibrations. This so-called "remote interfacial phonon scattering" effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene.

"We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering." Fuhrer said.

"Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO₂," J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer (will appear in Nature Nanotechnology online March 23, 2008)