Breaking Kasha's rule: Scientists find unique luminescence in tetrapod nanocrystals

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Berkeley Lab researchers have developed unqiue semiconductor tetrapods that under illumination break Kashas rule for photoluminescence by emitting two colors of light. Credit: courtesy of Alivisatos research group

Observation of a scientific rule being broken can sometimes lead to new knowledge and important applications. Such would seem to be the case when scientists with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) created artificial molecules of semiconductor nanocrystals and watched them break a fundamental principle of photoluminescence known as "Kasha's rule."

Named for chemist Michael Kasha, who proposed it in 1950, Kasha's rule holds that when light is shined on a molecule, the molecule will only emit light (fluorescence or phosphorescence) from its lowest energy excited state. This is why photoluminescent molecules emit light at a lower energy than the excitation light. While there have been examples of organic molecules, such as azulene, that break Kasha's rule, these examples are rare. Highly luminescent molecular systems crafted from quantum dots that break Kasha's rule have not been reported ? until now.

"We have demonstrated a semiconductor nanocrystal molecule, in the form of a tetrapod consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, that breaks Kasha's rule by emitting light from multiple excited states," says Paul Alivisatos, director of Berkeley Lab and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley. "Because this nanocrystal system has much higher quantum yield and is relatively more photostable than organic molecules, it holds promising potential for optical sensing and light emission-based applications, such as LEDs and imaging labels."

Alivisatos, an internationally recognized authority on nanochemistry, is one of two corresponding authors, along with Sanjeevi Sivasankar of DOE's Ames Laboratory and Iowa State University, on a paper describing this work in the journal Nano Letters. The paper is titled "Spatially Indirect Emission in a Luminescent Nanocrystal Molecule." Co-authoring the paper were Charina Choi, Prashant Jain and Andrew Olson, all members of Alivisatos' research group, plus Hui Li, a member of Sivasankar's research group.

Semiconductor tetrapods make exceptionally good subjects for the study of electronically coupled nanocrystals as Charina Choi, lead author of the Nano Letters paper, explains.

"For the study of nanocrystal molecules, it is important to be able to grow complex nanocrystals in which simple nanocrystal building blocks are connected together in well-defined ways," Choi says. "Although there are many versions of electronically coupled nanocrystal molecules, semiconductor tetrapods feature a beautiful symmetry that is analogous to the methane molecule, one of the fundamental units of organic chemistry."

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Artificial molecules consisting of a cadmium-selenide quantum dot core and four cadmium sulfide arms, with the fourth arm sticking out of the plane and appearing as a black dot in the center of each tetrapod. Credit: courtesy of Alivisatos research group

In this study, Choi, Alivisatos and their co-authors designed a cadmium-selenide (CdSe)and cadmium-sulfide (CdS) core/shell tetrapod whose quasi-type-I band alignment results in high luminescence quantum yields of 30- to 60-percent. The highest occupied molecular orbital (HOMO) of this tetrapod involves an electron "hole" within the cadmium-sulfide core, while the lowest unoccupied molecular orbital (LUMO) is centered within the core but is also likely to be present in the four arms as well. The next lowest unoccupied molecular orbital (LUMO+1) is located primarily within the four CdS arms.

Through single particle photoluminescence spectroscopy carried out at Ames, it was determined that when a CdSe/CdS core/shell tetrapod is excited, not only is a photon emitted at the HOMO-LUMO energy gap as expected, but there is also a second photon emitted at a higher energy that corresponds to a transition to the HOMO from the LUMO+1.

"The discovery that these CdSe/CdS core/shell tetrapods emit two colors was a surprise," Choi says. "If we can learn to control the frequency and intensity of the emitted colors then these tetrapods may be useful for multi-color emission technologies."

For example, says co-author Prashant Jain, "In the field of optical sensing with light emitters, it is impractical to rely simply on changes in emission intensity as emission intensity can fluctuate significantly due to background signal. However, if a molecule emits light from multiple excited states, then one can design a ratiometric sensor, which would provide more accurate readouts than intensity magnitude, and would be more robust against fluctuations and background signals."

Another promising possibility for CdSe/CdS core/shell tetrapods is their potential application as nanoscale sensors for measuring forces. Previous work by Alivisatos and Choi showed that the emission wavelengths of these tetrapods will shift in response to local stress on their four arms.

"When a stress bends the arms of a tetrapod it perturbs the electronic coupling within the tetrapod's heterostructure, which in turn changes the color of the emitted light, and also likely alters the ratio of emission intensity from the two excited states," Choi says. "We are currently trying to use this dependence to measure biological forces, for example, the stresses exerted by a beating heart cell."

By adjusting the length of a CdSe/CdS core/shell tetrapod's arms, it is possible to tune band alignment and electronic coupling within the heterostructure. The result would be tunable emissions from multiple excited states, an important advantage for nano-optic applications.

"We've demonstrated that the oscillator strength of LUMO+1 to HOMO light emissions can be tuned by changing the arm length of the tetrapod," Choi says. "We predict that the lifetime and energy of the emissions can also be controlled through appropriate structural modifications, including arm thickness, number of arms, chemical composition and particle strain."

Provided by Lawrence Berkeley National Laboratory (news : web)

Electronic life on the edge: Scientists discover the edge states of graphene nanoribbons

A scanning tunneling microscope determines the topography and orientation of the graphene nanoribbons on the atomic scale. In spectroscopy mode, it determines changes in the density of electronic states, from the nanoribbon's interior to its edge. Credit: Crommie et al, Lawrence Berkeley National Laboratory

As far back as the 1990s, long before anyone had actually isolated graphene ? a honeycomb lattice of carbon just one atom thick ? theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the "edge states" of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide (a nanometer is a billionth of a meter). Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn't have.

"Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge," says Michael Crommie of Berkeley Lab's Materials Sciences Division (MSD) and UC Berkeley's Physics Division, who led the research. "We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope."

The team's research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication.

The well-tempered nanoribbon

"Making flakes and sheets of graphene has become commonplace," Crommie says, "but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity" ? typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University's Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of "unzipping" carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or "chiral vector," of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as "armchair" edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab's MSD was the research team's theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

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By "unzipping" carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration. Credit: Hongjie Dai, Stanford University, and Michael Crommie et al, Lawrence Berkeley National Laboratory

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature ? not the artifact of a folded ribbon or a flattened nanotube ? the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

"Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there's no band gap," Crommie says. "Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up."

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

"In the quantum world, electrons can be described as waves in addition to being particles," Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

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Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy. Credit: Crommie et al, Lawrence Berkeley National Laboratory

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, "The optimist says, 'Wow, look at all the ways we can control these states ? this might allow a whole new technology!' The pessimist says, 'Uh-oh, look at all the things that can disturb a nanoribbon's behavior ? how are we ever going to achieve reproducibility on the atomic scale?'"

Crommie himself declares that "meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon's edge."

Although getting there won't be simple -- "The edges have to be controlled," Crommie emphasizes -- "what we've shown is that it's possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries."

Scientists discover dielectron charging of water nano-droplet

 

This image shows the surface attachment mode of two excess electrons to a water cluster (average diameter of close to 2nm) comprised of 105 molecules. The shown configuration was obtained from first-principles quantum simulations. The two wave functions each occupied by one excess electron, depicted in blue and green, are localized at opposite sides of the cluster and they are shown superimposed on the water molecules. The oxygens and hydrogens of the water molecules are represented by red and gray spheres, respectively. Credit: Uzi Landman/Georgia Tech

Scientists have discovered fundamental steps of charging of nano-sized water droplets and unveiled the long-sought-after mechanism of hydrogen emission from irradiated water. Working together at the Georgia Institute of Technology and Tel Aviv University, scientists have discovered when the number of water molecules in a cluster exceeds 83, two excess electrons may attach to it ? forming dielectrons ? making it a doubly negatively charged nano droplet. Furthermore, the scientists found experimental and theoretical evidence that in droplets comprised of 105 molecules or more, the excess dielectrons participate in a water-splitting process resulting in the liberation of molecular hydrogen and formation of two solvated hydroxide anions. The results appear in the June 30 issue of the Journal of Physical Chemistry A.

It has been known since the early 1980s that while single electrons may attach to small water clusters containing as few as two molecules, only much larger clusters may attach more than single electrons. Size-selected, multiple-electron, negatively-charged water clusters have not been observed ? until now.

Understanding the nature of excess electrons in water has captured the attention of scientists for more than half a century, and the hydrated electrons are known to appear as important reagents in charge-induced aqueous reactions and molecular biological processes. Moreover, since the discovery in the early 1960s that the exposure of water to ionizing radiation causes the emission of gaseous molecular hydrogen, scientists have been puzzled by the mechanism underlying this process. After all, the bonds in the water molecules that hold the hydrogen atoms to the oxygen atoms are very strong. The dielectron hydrogen-evolution (DEHE) reaction, which produces hydrogen gas and hydroxide anions, may play a role in radiation-induced reactions with oxidized DNA that have been shown to underlie mutagenesis, cancer and other diseases.

"The attachment of multiple electrons to water droplets is controlled by a fine balancing act between the forces that bind the electrons to the polar water molecules and the strong repulsion between the negatively charged electrons," said Uzi Landman, Regents' and Institute Professor of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech.

"Additionally, the binding of an electron to the cluster disturbs the equilibrium arrangements between the hydrogen-bonded water molecules and this too has to be counterbalanced by the attractive binding forces. To calculate the pattern and strength of single and two-electron charging of nano-size water droplets, we developed and employed first-principles quantum mechanical molecular dynamics simulations that go well beyond any ones that have been used in this field," he added.

Investigations on controlled size-selected clusters allow explorations of intrinsic properties of finite-sized material aggregates, as well as probing of the size-dependent evolution of materials properties from the molecular nano-scale to the condensed phase regime.

In the 1980s Landman, together with senior research scientists in the CCMS Robert Barnett, the late Charles Cleveland and Joshua Jortner, professor of chemistry at Tel Aviv University, discovered that there are two ways that single excess electrons can attach to water clusters ? one in which they bind to the surface of the water droplet, and the other where they localize in a cavity in the interior of the droplet, as in the case of bulk water. Subsequently, Landman, Barnett and graduate student Harri-Pekka Kaukonen reported in 1992 on theoretical investigations concerning the attachment of two excess electrons to water clusters. They predicted that such double charging would occur only for sufficiently large nano-droplets. They also commented on the possible hydrogen evolution reaction. No other work on dielectron charging of water droplets has followed since.

That is until recently, when Landman, now one of the world leaders in the area of cluster and nano science, and Barnett teamed up with Ori Chesnovsky, professor of chemistry, and research associate Rina Giniger at Tel Aviv University, in a joint project aimed at understanding the process of dielectron charging of water clusters and the mechanism of the ensuing reaction - which has not been observed previously in experiments on water droplets. Using large-scale, state-of-the-art first-principles dynamic simulations, developed at the CCMS, with all valence and excess electrons treated quantum mechanically and equipped with a newly constructed high-resolution time-of-flight mass spectrometer, the researchers unveiled the intricate physical processes that govern the fundamental dielectron charging processes of microscopic and the detailed mechanism of the water-splitting reaction induced by double charging.

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This image shows the internal attachment mode of two electrons to a water cluster (average diameter of close to 2nm) comprised of 105 molecules. The shown configuration was obtained from first-principles quantum simulations. The wave function of the two excess electrons is depicted in pink and it is shown superimposed on the water molecules of the cluster. The compact dielectron distribution is localized in a hydration cavity in the interior of the cluster. The configuration shown here corresponds to the start of the dielectron hydrogen-evolution reaction. The protons of the two reacting neighboring water molecules, located approximately in the middle of the figure, are depicted by smaller blue spheres. Together with the dielectron these protons would form, in the course of the reaction, a hydrogen molecule. The oxygens and hydrogens of the water molecules are represented by red and gray spheres, respectively. Credit: Uzi Landman/Georgia Tech

The mass spectrometric measurements, performed at Tel Aviv, revealed that singly charged clusters were formed in the size range of six to more than a couple of hundred water molecules. However, for clusters containing more than a critical size of 83 molecules, doubly charged clusters with two attached excess electrons were detected for the first time. Most significantly, for clusters with 105 or more water molecules, the mass spectra provided direct evidence for the loss of a single hydrogen molecule from the doubly charged clusters.

The theoretical analysis demonstrated two dominant attachment modes of dielectrons to water clusters. The first is a surface mode (SS'), where the two repelling electrons reside in antipodal sites on the surface of the cluster (see the two wave functions, depicted in green and blue, in Figure 1). The second is another attachment mode with both electrons occupying a wave function localized in a hydration cavity in the interior of the cluster ? the so-called II binding mode (see wave function depicted in pink in Figure 2). While both dielectron attachment modes may be found for clusters with 105 molecules and larger ones, only the SS' mode is stable for doubly charged smaller clusters.

"Moreover, starting from the II, internal cavity attachment mode in a cluster comprised of 105 water molecules, our quantum dynamical simulations showed that the concerted approach of two protons from two neighboring water molecules located on the first shell of the internal hydration cavity, leads, in association with the cavity-localized excess dielectron (see Figure 2), to the formation of a hydrogen molecule. The two remnant hydroxide anions diffuse away via a sequence of proton shuttle processes, ultimately solvating near the surface region of the cluster, while the hydrogen molecule evaporates," said Landman.

"What's more, in addition to uncovering the microscopic reaction pathway, the mechanism which we discovered requires initial proximity of the two reacting water molecules and the excess dielectron. This can happen only for the II internal cavity attachment mode. Consequently, the theory predicts, in agreement with the experiments, that the reaction would be impeded in clusters with less than 105 molecules where the II mode is energetically highly improbable. Now, that's a nice consistency check on the theory," he added.

As for future plans, Landman remarked, "While I believe that our work sets methodological and conceptual benchmarks for studies in this area, there is a lot left to be done. For example, while our calculated values for the excess single electron detachment energies are found to be in quantitative agreement with photoelectron measurements in a broad range of water cluster sizes ? containing from 15 to 105 molecules ? providing a consistent interpretation of these measurements, we would like to obtain experimental data on excess dielectron detachment energies to compare with our predicted values," he said.

"Additionally, we would like to know more about the effects of preparation conditions on the properties of multiply charged water clusters. We also need to understand the temperature dependence of the dielectron attachment modes, the influence of metal impurities, and possibly get data from time-resolved measurements. The understanding that we gained in this experiment about charge-induced water splitting may guide our research into artificial photosynthetic systems, as well as the mechanisms of certain bio-molecular processes and perhaps some atmospheric phenomena."

NRL scientists elevate warfighter readiness against invisible threats

NRL scientists elevate warfighter readiness against invisible threats

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A 100,000 times magnified view of the gold coated, silicon nanopillars that make up the arrays used in the SERS measurements. Each array consists of 250,000 nanopillars with up to 400 arrays per wafer. Credit: U.S. Naval Research Laboratory

In asymmetric warfare, early detection and identification of trace level chemical and biological agents and explosive compounds is critical to rapid reaction, response, and survivability. While there are many methods currently being used that can detect these threats, none allow for the unique fingerprinting of threat agents at trace levels.

A research team, led by Drs. Joshua Caldwell and Orest Glembocki, scientists at the U.S. Naval Research Laboratory, Electronic Science and Technology Division, has overcome this limitation with surface enhanced Raman scattering (SERS) using optically stimulated plasmon oscillations in nanostructured substrates.

Shown to provide enhancements of the Raman signal, large-area gold (Au) coated silicon (Si) nanopillar arrays are over 100 million times more sensitive than Raman scattering sensing alone, while maintaining a very uniform response with less than 30 percent variability across the sensor area.

"These arrays are over an order-of-magnitude more sensitive than the best reported SERS sensors in the literature and the current state-of-the-art large-area commercial SERS sensors," said Caldwell. "These arrays can be a key component of fully integrated, autonomously operating chemical sensors that detect, identify and report the presence of a threat at trace levels of exposure."

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Scientists Orest Glembocki (holding a DeltaNu ExamineR hand-held Raman spectrometer) and Joshua Caldwell, display one of the silicon wafers they fabricated for the SERS sensing using ion etching and e-beam lithography. Several square arrays of gold-coated silicon nanopillar were fabricated on the wafer to perform surface-enhanced Raman scattering testing. Credit: U.S. Naval Research Laboratory/Jamie Hartman

Raman devices use laser light to excite molecular vibrations, which in turn causes a shift in the energy of the scattered laser photons, up or down, creating a unique visual pattern. In the case of trace amounts of molecules in gases or liquids, detection through ordinary Raman scattering is virtually impossible. However, the Raman signal can be enhanced via the SERS effect using metal nanoparticles.

Despite surface-enhanced Raman scattering being first observed in the late 1970s, efforts to provide reproducible SERS-based chemical sensors has been hindered by the inability to make large-area devices with a uniform SERS response. The ability to reproducibly pattern nanometer-sized particles in periodic arrays has finally allowed this requirement to be met.

"While many tools are currently available to detect trace amounts of chemical warfare and biological agents and explosive compounds, a device using SERS can be used to identify these minute quantities of the chemicals of interest by providing a 'fingerprint' of the material, which all but eliminates the prevalence of false alarms," says Glembocki.

SERS offers several potential advantages over other spectroscopic techniques because of its measurement speed, high sensitivity, portability, and simple maneuverability. SERS can additionally be used to enhance existing Raman technologies, such as the hand held and standoff units that are already in use in field applications.

Seeing the light: Scientists bring plasmonic nanofields into focus

Seeing the light: Scientists bring plasmonic nanofields into focus

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By imaging fluorescence from gold within a bowtie-shaped plasmonic device, Berkeley Lab researchers gleaned the position of plasmonic modes just a few nanometers apart.

In typical plasmonic devices, electromagnetic waves crowd into tiny metal structures, concentrating energy into nanoscale dimensions. Due to coupling of electronics and photonics in these metal nanostructures, plasmonic devices could be harnessed for high-speed data transmission or ultrafast detector arrays. However, studying plasmonic fields in nanoscale devices presents a real roadblock for scientists, as examining these structures inherently alters their behavior.

“Whether you use a laser or a light bulb, the wavelength of light is still too large to study plasmonic fields in nanostructures. What’s more, most tools used to study plasmonic fields will alter the field distribution—the very behavior we hope to understand,” says Jim Schuck,  a staff scientist with Lawrence Berkeley National Laboratory (Berkeley Lab) who works in the Imaging and Manipulation of Nanostructures Facility at the Molecular Foundry.

Light microscopy plays a fundamental role in a scientist’s repertoire: the technique is easy to use and doesn’t inflict damage to a carefully crafted electronic circuit or delicate biological specimen. However, a typical nanoscale object of interest—such as a strand of DNA or a quantum dot—is well below the wavelength of visible light in size, which means the ability to distinguish one such object from another when they are closely spaced is lost.  Scientists are now challenging this limit using ‘localization’ techniques, which count the number of photons emanating from an object to help determine its position.

In previous work, Schuck and colleagues at the Molecular Foundry, a U.S. Department of Energy (DOE) Nanoscale Science Research Centers,  engineered bowtie-shaped plasmonic devices designed to capture, filter and steer light at the nanoscale. These nano-color sorter devices served as antennae to focus and sort light in tiny spaces to a desired set of colors or energies—crucial for filters and other detectors.

In this latest advance, Schuck and his Berkeley Lab team used their innovative imaging concept to visualize plasmonic fields from these devices with nanoscale resolution. By imaging fluorescence from gold within the bowtie and maximizing the number of photons collected from their bowtie devices, the team was able to glean the position of plasmonic modes—oscillations of charge that result in optical resonance—just a few nanometers apart.

“We wondered whether there was a way to use light already present in our bowties—localized photons—to probe these fields and serve as a reporter,” says Schuck. “Our technique is also sensitive to imperfections in the system, such as tiny structural flaws or size effects, suggesting we could use this technique to measure the performance of plasmonic devices in both research and development settings.”

In parallel with Schuck’s experimental findings, Jeff Neaton, Director of the Molecular Foundry ’s Theory of Nanostructured Materials Facility and Alex McLeod, an undergraduate student working at the Foundry, developed a web-based toolkit, designed to calculate images of plasmonic devices with open-source software developed at Massachusetts Institute of Technology. For this study, the researchers simulated adjusting the structure of a double bowtie antenna by a few nanometers to study how changing the size and symmetry of a plasmonic antenna affects its optical properties.

“By shifting their structure by just a few nanometers, we can focus light at different positions inside the bowtie with remarkable certainty and predictability,” said McLeod. “This work demonstrates that these nanoscale optical antennae resonate with light just as our simulations predict.”

Useful for researchers studying plasmonic and photonic structures, this toolkit will be available for download on nanoHUB, a computational resource for nanoscience and technology created through the National Science Foundation’s Network for Computational Nanotechnology.

“This work really exemplifies the very best of what the Molecular Foundry is about,” said Neaton, who is also Acting Deputy Director of Berkeley Lab’s Materials Sciences Division. “Three separate Foundry facilities—Imaging, Nanofabrication and Theory—collaborated on a significant advance in our understanding of how visible light can be localized, manipulated, and imaged at the nanoscale.”

A paper reporting this research titled, “Non-perturbative visualization of nanoscale plasmonic field distributions via photon localization microscopy,” appears in Physical Review Letters and is available to subscribers online. Co-authoring the paper with Schuck, McLeod and Neaton were Alexander Weber-Bargioni, Zhaoyu Zhang, Scott Dhuey, Bruce Harteneck and Stefano Cabrini.

Portions of this work at the Molecular Foundry were supported by DOE’s Office of Science.  Support for this work was also provided by the National Science Foundation through the Network for Computational Nanotechnology.

Fast, easy way to make hydrogen nanosensors found by scientists

Fast, easy way to make hydrogen nanosensors found by scientists

A team of Northern Illinois University scientists, with a major role played by NIU Ph.D. students, has discovered a new, convenient and inexpensive way to make high performance hydrogen sensors using palladium nanowires.

The technology could help enable a scale-up for potential industrial applications, such as safety monitors in future hydrogen-powered vehicles.

Highly flammable hydrogen gas cannot be odorized like natural gas. The new technology produces nanoscale sensors that work extremely fast and would allow for closing of safety valves before dangerous concentrations of the gas could be reached.

Scientists have known that palladium nanowires demonstrated promise as hydrogen gas sensors in speed, sensitivity and ultra-low power consumption. But the utilization of single palladium nanowires faced challenges in several areas, including nanofabrication.

“We report on hydrogen sensors that take advantage of single palladium nanowires in high speed and sensitivity and that can be easily and cheaply made,” said lead author Xiaoqiao (Vivian) Zeng, a Ph.D. student in chemistry and biochemistry at NIU. The new research is published in the January edition of the American Chemical Society's prestigious journal Nano Letters.

“The new types of hydrogen sensors are based on networks of ultra-small (< 10 nanometers) palladium nanowires achieved by sputter-depositing palladium onto the surface of a commercially available and inexpensive filtration membrane,” Zeng said.

The research was conducted at both Northern Illinois University and Argonne National Laboratory. The scientists also found that the speed of the sensors increases with decreasing thicknesses of the palladium nanowires. The sensors are 10 to 100 times faster than their counterparts made of a continuous palladium film of the same thickness.

“The superior performance of the ultra-small palladium nanowire network-based sensors demonstrates the novelty of the fabrication approach, which can be used to fabricate high-performance sensors for other gases,” said NIU Presidential Research Professor of Physics Zhili Xiao, leader of the research team and co-adviser to Zeng.

Xiao noted that Zeng’s exceptional contribution to the research is particularly impressive for a Ph.D. candidate. Zeng came to NIU in the fall of 2008 after earning her master’s degree from the University of Science and Technology Beijing. She is now a recipient of the NIU Nanoscience Fellowship, jointly supported by the university and Argonne.

“It is extremely competitive to publish an article in Nano Letters, which has a very high impact factor that is better even than the traditionally prestigious chemical and physical journals,” Xiao said. “We’re proud of Vivian’s achievements and grateful for her creativity and diligence.

“Nanoresearch is truly interdisciplinary,” Xiao added. “Chemists have undoubtedly demonstrated advantages in nanofabrication by utilizing methods of chemical synthesis to obtain extreme nanostructures, while physicists have strengths in exploration of new physical properties at the nanoscale. This research benefitted tremendously from Vivian’s expertise in chemistry. In fact, the substrates used to form the novel networks of palladium nanowires are common filtration members known to chemists.”

Scientists identify spontaneously chain-reacting molecule

Scientists identify spontaneously chain-reacting molecule

In the burgeoning field of nano-science there are now many ways of 'writing' molecular-scale messages on a surface, one molecule at a time. The trouble is that writing a molecule at a time takes a very long time.

"It is much better if the molecules can be persuaded to gather together and imprint an entire pattern simultaneously, by themselves. One such pattern is an indefinitely long line, which can then provide the basis for the ultimately thin molecular 'wire' required for nano-circuitry," says John Polanyi of the University of Toronto's Department of Chemistry, co- author of the paper to be published on Nature Chemistry this week.

The paper describes, for the first time, a simple molecule that each time it chemically reacts with a surface prepares a hospitable neighbouring site at which the next incoming molecule reacts. Accordingly, these molecules, when simply dosed (blindly) on the surface, spontaneously grow durable 'molecular-chains'. These molecular chains are the desired prototypes of nano-wires.

The experiments were conducted by graduate student Tingbin Lim in the John Polanyi Scanning Tunneling Microscopy laboratory at U of T, in conjunction with theory performed by postdoctoral fellow Dr. Wei Ji in the Hong Guo laboratory in the Department of Physics, McGill University. The experiments in Toronto yielded visual evidence of the chains, and the theory at McGill explained why the chains spontaneously grew.

"Early-on, far-sighted Xerox Research Centre Canada (XRCC) recognized this opportunity for imprinting patterns at the molecular scale, thereby persuading Ontario Centres of Excellence (OCE) and the federal Natural Sciences and Engineering Research Council (NSERC), through its Strategic Grant program, to fund the bulk of the research costs in our lab," says Polanyi.

"The experiments constituted the doctoral work of a recent PhD student in the Toronto laboratory, Dr. Tingbin Lim an outstanding student who came from Singapore to join our group and now makes his home as a scientist in Canada."

Scientists create highly ordered artificial spin ice using nanotechnology

Scientists create highly ordered artificial spin ice using nanotechnology

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(PhysOrg.com) -- An international team of researchers has succeeded in creating artificial spin ice in a state of thermal equilibrium for the first time, allowing them to examine the precise configuration of this important nanomaterial.

Scientists from the University of Leeds, the US Department of Energy's Brookhaven National Laboratory and the UK Science and Technology Facilities Council's Rutherford Appleton Laboratory say the breakthrough will allow them to study in much greater detail a scientific phenomenon known as 'magnetic monopoles', which are thought to exist in such structures. Their findings are published today in the journal Nature Physics.

Artificial spin ice is built using nanotechnology and is made up of millions of tiny magnets, each thousands of times smaller than a grain of sand. The magnets exist in a lattice in what is known as a 'frustrated' structure. Like water ice, the geometry of the structure means that all of the interactions between the atoms cannot be satisfied at the same time.

"It's like trying to seat alternating male and female diners around a table with an odd number of seats – however much you re-arrange them you will never succeed," said Dr Christopher Marrows from the University of Leeds, co-author of the paper.

In spin ice, magnetic dipoles with a north and south pole are arranged in tetrahedron structures. Each dipole has magnetic moments, similar to the protons on H2O molecules in water ice, which attract and repel each other. Consequently, the dipoles arrange themselves into the lowest possible energy state, which is two poles pointing in and two pointing out.

Dr Marrows said: "Spin ices have created a lot of excitement in recent years as it has been realised that they are a playground for physicists studying magnetic monopole excitations and Dirac string physics in the solid state. However, until now all of the samples of these artificial structures created in the lab have been what we call 'jammed'.

"What we have done is find a way to un-jam spin ice and get it into a well-ordered ground state known as thermal equilibrium. We can then freeze a sample into this state, and use a microscope to see which way all the little magnets are pointing. It's the equivalent of being able take a picture of every atom in a room as it allows us to inspect exactly how the structure is configured."

Jason Morgan, PhD student at the University of Leeds and lead author of the paper, was the first member of the team to observe the sample in equilibrium. He said: "Getting the sample to self-order in such a way has never been achieved experimentally before and for a while had been considered impossible. But when we looked at the sample using magnetic force microscopy and saw this beautiful periodic structure we knew instantly that we had achieved an ordered ground state."

The researchers have also been able to observe individual excitations out of this ground state within their sample, which they say is evidence for monopole dynamics within the lattice.

Magnetic monopoles – magnets with only a single north or south pole ¬¬– are former hypothetical particles that are now thought to exist in spin ice. There is hope among scientists that understanding these monopoles in more detail could lead to advances in a novel technology field known as 'magnetricity' – a magnetic equivalent to electricity.

Co-author Sean Langridge, a Science and Technology Facilities Council (STFC) Fellow and visiting Professor at the University of Leeds, added: "In the naturally occurring spin-ice systems this ground state is predicted but has not been experimentally observed.

"Now that is has been observed in an artificial system the next step is to observe dynamically the excitations from this ground state. We can only do this by controlling the interactions with state of the art lithographic techniques. This level of control will provide an even greater level of understanding in this fascinating system."

The team created "artificial" spin ice samples at Brookhaven using a state-of-the-art nanotechnology tool called an electron beam writer. A similar £4 million facility is shortly to be opened at the University of Leeds which will be unique to the UK and will allow continued collaboration with the researchers at Brookhaven.

Scientists crack materials mystery in vanadium dioxide

Scientists crack materials mystery in vanadium dioxide

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Theoretical research at Oak Ridge National Laboratory can help explain experimental results in vanadium dioxide, such as the formation of thin conductive channels (seen in white) that can appear under strain in a nanoscale vanadium dioxide sample.

(PhysOrg.com) -- A systematic study of phase changes in vanadium dioxide has solved a mystery that has puzzled scientists for decades, according to researchers at the Department of Energy's Oak Ridge National Laboratory.

Scientists have known that vanadium dioxide exhibits several competing phases when it acts as an insulator at lower temperatures. However, the exact nature of the phase behavior has not been understood since research began on vanadium dioxide in the early 1960s.

Alexander Tselev, a research associate from the University of Tennessee-Knoxville working with ORNL's Center for Nanophase Materials Sciences, in collaboration with Igor Luk'yanchuk from the University of Picardy in France used a condensed matter physics theory to explain the observed phase behaviors of vanadium dioxide, a material of significant technological interest for optics and electronics.

"We discovered that the competition between several phases is purely driven by the lattice symmetry," Tselev said. "We figured out that the metallic phase lattice of vanadium oxide can 'fold' in different ways while cooling, so what people observed was different types of its folding."

Vanadium dioxide is best known in the materials world for its speedy and abrupt phase transition that essentially transforms the material from a metal to an insulator. The phase change takes place at about 68 degrees Celsius.

"These features of electrical conductivity make vanadium dioxide an excellent candidate for numerous applications in optical, electronic and optoelectronic devices," Tselev said.

Devices that might take advantage of the unusual properties of VO2 include lasers, motion detectors and pressure detectors, which could benefit from the increased sensitivity provided by the property changes of vanadium dioxide. The material is already used in technologies such as infrared sensors.

Researchers said their theoretical work could help guide future experimental research in vanadium dioxide and ultimately aid the development of new technologies based on VO₂.

"In physics, you always want to understand how the material ticks," said Sergei Kalinin, a senior scientist at the CNMS. "The thermodynamic theory will allow you to predict how the material will behave in different external conditions."

The results were published in the American Chemical Society's Nano Letters. The research team also included Ilia Ivanov, John Budai and Jonathan Tischler at ORNL and Evgheni Strelcov and Andrei Kolmakov at Southern Illinois University.

The team's theoretical research expands upon previous experimental ORNL studies with microwave imaging that demonstrated how strain and changes of crystal lattice symmetry can produce thin conductive wires in nanoscale vanadium dioxide samples.