Nanofluidic transistor. Imagine a valve to precisely control the flow of liquids but with dimensions so tiny that only one molecule at a time can pass through it. Controlled flow of ions in a liquid was recently demonstrated through very small nanochannels barely large enough to pass large molecules. Named “nanofluidic transistors,” the nanochannel assembly functions in a way similar to ordinary transistors where the flow of electrons can be regulated by applying a voltage. Demonstrations were carried out on a 35-nanometer channel constructed between two silicon dioxide plates; the channel was filled with water and potassium chloride salt. The flow of potassium ions could be completely stopped by applying an electric current across the channel. The regulation of the flow (or current) of charged molecules was also demonstrated. This exciting discovery now makes possible detection and separation of individual molecules in a fluid. Among the important implications of this discovery are advanced nanoscale chemical analysis with extreme sensitivity and the capability of sorting individual molecules.

Unexpected spontaneous reversal of magnetization in nanoscaled structures. New and unexpected magnetic phenomena have been discovered in ultrathin bilayers of ferromagnetic and antiferromagnetic films. Ferromagnetic materials (e.g., iron) have a positive magnetization due to the alignment of the magnetic moments. Antiferromagnet materials (e.g., nickel oxide) have no net magnetization due to the anti-parallel alignment of the magnetic moments. In bulk magnetic materials, regions of aligned magnetic moments, termed magnetic domains, are expected to align with an external applied magnetic field. The magnetic strength is determined by the degree of alignment of the magnetic domains. In contrast to naturally occurring bulk magnetic materials, an ultrathin ferromagnetic layer in close contact with an antiferromagnetic layer will spontaneously align opposite to the applied magnetic field upon cooling. The close proximity of the two different layers also results in an increase in magnetic strength. The ability to control and detect the magnetic alignment in ultrathin magnetic materials could lead to new concepts in computer data storage design. The fundamental understanding of the unexpected phenomena may also influence future research and development of magnetic based biological and chemical sensors.

Nano-electronic hydrodynamics and turbulence. Electrons moving across a nanometer-sized wire have been found to behave hydrodynamically, i.e., like a liquid flowing from one bucket to another through a small opening. This behavior is exactly contrary to expectations from a quantum mechanical prediction, and it has prompted theoretical predictions of new phenomena. Most striking is the prediction of possible turbulent electrical transport with eddy currents in nanoscale conductors that could seriously limit current flow. Such turbulent currents could then lead to extremely high electronic temperatures due to the “friction” of the electrons as they move against each other, resulting in potential premature failure at much reduced current flow. Experiments are being carried out to test these theoretical developments.

Using bioinspired methods to synthesize and assemble materials. Biological systems are renowned for synthesizing inorganic materials under mild conditions and assembling them into exquisitely shaped structures with high precision and control. Recently, by emulating the underlying chemistry and approaches of biology, several inorganic materials have been synthesized under mild conditions (room temperature, neutral pH, etc.), with a potential for significant energy savings in their large-scale manufacture. Some of the materials synthesized include semiconducting titanium dioxide, gallium oxide, and zinc oxide for solar energy conversion; ferroelectric barium titanate nanoparticles for energy storage; magnetite nanoparticles for ultra-high density magnetic information storage; and nanocrystalline palladium for hydrogen storage. Furthermore, by exploiting the ability of biological macromolecules (e.g., DNA, proteins, viruses) to self-assemble into large, well-defined structures and to nucleate the growth of inorganic materials, researchers have shown that complex electronic circuit elements and large ordered arrays of nanoparticles can be assembled with a precision that far exceeds the current top-down fabrication capabilities.

Unveiling the superconductor mystery. Understanding the phenomena of superconductivity and its mechanism has been among the most challenging issues facing the condensed matter and materials physics communities. The mystery of superconductivity is being tackled by a concerted effort, coupling synthesis and characterization with theory, modeling, and simulation. The recent discovery of superconductivity in actinide- and boron-containing materials indicates superconductivity may exist in many material systems yet to be discovered. The search for new materials is augmented by sophisticated techniques to modify the electronic properties of known superconducting materials, both chemically and electrically. Advances in new characterization tools, including proximal probes, have made possible the discovery of new phenomena, including competing phases within the superconducting phase. First principles calculations assisted by generalized density functional theory enabled accurate predictions of the electronic structure of superconducting materials. When coupled to an electron pairing mechanism, numerical models are being developed to predict the superconducting transition temperature as a first step towards a priori design of new superconductors.

Measuring the ultrafast motion within a molecule using its own electrons. Modern ultrafast lasers make it possible, in principle, to follow in real time the motions of the atoms that comprise a molecule. However, optical lasers are only indirect probes of atomic motion. This problem will be alleviated with the advent of the world’s first x-ray free-electron laser, the Linac Coherent Light Source (LCLS), since x-rays allow direct tracking of atomic positions. Until the LCLS is available, optical laser pulses can be used in clever ways to track atomic motion in molecules. In one recently demonstrated example, the molecule’s own electrons are used as the probe of atomic motion in a highly excited molecule. The electric field from an intense, optical laser pulse initially pulls electrons away from the molecule and then accelerates them back toward it. The highly energetic electrons scatter from the molecule. Rather than measure the scattered electrons, as might be done in an electron diffraction experiment, the new method exploits another phenomenon that is particularly sensitive to atomic motion. When the electrons re-collide with the molecule, they emit x-ray radiation in a process known as high-harmonic generation (HHG), and it is these x-rays that are detected. The wavelength of the re-colliding electrons is comparable to distances between atoms in a molecule; thus, the HHG x-rays emitted are highly sensitive to atomic motion within the molecule. This new method shows great promise as a way of imaging energetic molecules undergoing ultrafast structural transformations, including the fundamental action of all of chemistry, and the making and breaking of chemical bonds.

Sunlight-driven transformation of carbon dioxide into methanol. The first step in the chemical transformation of carbon dioxide into a transportable fuel such as methanol involves the interaction of light with a catalyst in a process known as photocatalysis. It has long been known that the photocatalytic formation of methanol from carbon dioxide can be initiated by high-energy ultraviolet radiation. Recent work has demonstrated that the critical first reaction that splits carbon dioxide into carbon monoxide and a free oxygen atom can also be triggered with visible light. This advance makes it feasible to consider harnessing sunlight to drive the photocatalytic production of methanol from carbon dioxide. The key to the new advance is to perform the initial photocatalytic reaction on the walls of the nanometer-sized channels of a porous silica solid through the excitation by visible light of a bimetallic catalyst. The energy from the absorption of light causes an electron to transfer from one metal in the catalyst to the other and subsequently activates the gaseous carbon dioxide to eliminate an oxygen atom to yield the carbon monoxide product. Various combinations of metals are now being explored with the goal of designing a complete and sustainable system to produce methanol.

Catalytic synthesis of alternative fuels and chemicals. Current manufacturing technologies for fuels and chemicals are often inefficient. The need to dramatically improve efficiency in fuel and chemical production is motivating the search for new chemical pathways using new catalysts tailored to guide chemical reactions with precision toward a selected product without wasteful sub-products. Recent approaches enlist different catalysts to cooperate in parallel to transform molecular intermediates. Sometimes referred to as tandem catalysis, this approach can potentially yield ultrahigh selectivity. An example is the venerable Fisher-Tropsch production of diesel fuel from carbon monoxide and hydrogen. Model catalysts for this polymerization reaction are typically unselective and yield a mixture of hydrocarbons or alcohols with carbon-chain lengths varying over a wide range. For minimum energy consumption and maximum yield, the ideal process should provide a very narrow carbon-chain range. Two recent advances may rejuvenate the Fisher-Tropsch process: the discovery of efficient metathesis catalysts, which led to the Nobel Prize in Chemistry for 2006, and the selective activation of carbon-hydrogen bonds. Two catalysts are necessary to carry out these two very different functions simultaneously on the same growing polymers. The carbon-hydrogen activation catalyst limits the yield of low-end hydrocarbons, and the metathesis polymerization catalyst simultaneously controls the high-end hydrocarbons. This can potentially lead to an ideal diesel-oil without the need for energy-intensive separations. This new tandem catalysis application is being followed intensely by researchers worldwide for its potential to revolutionize the science of alternative fuels and chemicals synthesis.

Carrier multiplication: a possible revolutionary step toward highly efficient solar cells. In a normal solar cell, a single photon from the sun is converted into a single carrier of electrical current (an electron-hole pair) in a bulk crystal material called a semiconductor. This process is inherently inefficient because much of the energy of the solar photon is wasted as excess heat in the semiconductor. Recent experiments on the interaction of photons with nanocrystalline samples of semiconductors have demonstrated a remarkable effect, known as carrier multiplication, in which a single photon creates multiple charge carriers. Recent work has demonstrated that as many as seven charge carriers can be created with a single photon and that the process is universal, i.e., it occurs in all types of nanocrystalline semiconductors. These new results suggest that nanoscale confinement plays an important role in the carrier multiplication mechanism, which is now thought to be an instantaneous excitation of multiple electrons by a single photon. Critical issues must be addressed before an operational solar cell based on carrier multiplication can be created, such as separating and harvesting the charge carriers to create electrical current. However, present estimates of the conversion efficiency for a solar cell based on carrier multiplication are as high as 50 percent, which is about twice that of the best solar cell in current operation. Doubling solar cell conversion efficiency would represent a revolutionary advance in our ability to harness renewable energy from the sun.

Visualizing chemistry: the promise of advanced chemical imaging. The emerging possibility of “chemical imaging” is transforming the way scientists follow the chemical transformation of molecules on surfaces, within cells, or immersed in other complex environments. Chemical imaging is the term given to a set of experimental techniques that use photon beams, electron beams, or proximal electromechanical probes to track molecules in two- or three-dimensional space and real time, while keeping track of chemical identity and even molecular structure. In the ideal limit, chemical imaging means nanometer spatial resolution, femtosecond temporal resolution, and “fingerprint” recognition of the molecular mass and structure. As a recent example, researchers are using focused laser beams (space and time information) coupled with mass spectrometry (chemical identification), to track specific metabolites in functioning cells. Multiplexing the mass information allows the simultaneous mapping of several species. Understanding the metabolic transformation of important biomolecules in cells is the first step toward influencing them in service of improved biochemical processes. Other examples include the use of chemical imaging to examine single-site catalysts as they influence reactions on surfaces and light-harvesting “antenna molecules” that are key participants in photochemical charge-transfer processes.

Commissioning and initial instrument results. Construction and commissioning of the Spallation Neutron Source, an accelerator-based neutron source that will provide the most intense pulsed neutron beams in the world for scientific research and industrial development, was completed, and the facility began operations in late FY 2006. The backscattering spectrometer that is part of the initial suite of instruments has unprecedented dynamic range and an energy resolution of better than 3 x 10^-6 electron volts. Initial operation of this hardware involved test measurements of excitations in picoline (a hydrocarbon), which confirmed the performance of the instrument.

Synchrotron X-Rays Demonstrate Nanoscale Ferroelectricity. Films only a few atoms thick have been made that retain the controllable electric polarization needed for next generation nanoscale devices. Such ultrathin ferroelectric films have the potential to revolutionize future electronics, sensors, and actuators. Previous studies suggested that, as devices are miniaturized, they lose their ferroelectric character. These studies showed that ferroelectricity persists in films only 6 atoms thick. This landmark success was achieved using a unique instrument to observe thin film growth with high intensity x-rays from the Advanced Photon Source. X-rays reveal in real time the film structure as it grows, atomic layer by atomic layer. The in-situ x-ray techniques developed for this study can now be used to understand the synthesis and environmental interactions of other complex materials, thus addressing a wide range of energy-related challenges.

A Superconductor that Tolerates Magnetic Fields. One of the biggest obstacles to the practical use of superconductors is the motion of magnetic flux due to an electric current in a superconductor. This motion of magnetic flux reduced the superconducting properties. A large research effort has gone into finding ways to prevent energy loss occurring from the movement of magnetic flux in copper oxide high temperature superconductors. It has been found that the magnetic flux in certain magnesium diboride films is intrinsically motionless, or “frozen,” in applied magnetic fields up to 14 Tesla. Such a complete apathy to an applied magnetic field has never been seen before in any other superconductor. While the theoretical explanation for this behavior has eluded scientists, the experimental finding has drawn a lot of attention. This behavior may make it possible to fabricate superconducting wire that can carry very large electric currents.

Using Electron Spin, not Electron Charge, to Carry Information. Today’s computers are based on resistive circuitry using the movement of charged electrons. The resistance generates heat, and the removal of this heat is a fundamental limiting factor in creating the next generation of ultra small and ultra fast circuit elements. In a remarkable discovery, theorists have determined that in certain materials a spin current can be created with the application of a suitably oriented electric field, with no dissipation of energy. The spin current could potentially be used to carry out the same logic operations with no energy loss. This has been verified recently with experiments on gallium arsenide. This discovery may lead to computers with much greater capabilities including speed and capacity due to smaller circuit elements and with a significant reduction in energy loss.

Plutonium Helps Understand Superconductivity’s Mysteries. Magnetic resonance studies of the fundamental mechanism responsible for superconductivity in PuCoGa₅ reveal strong similarities to the high-T_c copper oxide materials. These results confirm earlier theories that this unique family of plutonium superconductors is nearly magnetic. This is a new class of superconducting materials and forms a conceptual bridge between two families of magnetically mediated superconductors, the heavy fermion metals and the copper oxides. The discovery of additional classes of superconducting materials enhances our ability to understand the mechanisms responsible for high temperature superconductivity.

Ultrafast Studies of Nanocrystals. The fastest phase transition between nanocrystal structures ever recorded has been observed by ultrafast laser techniques. The reversible structural change in nanocrystals of vanadium dioxide switches the material from a semiconductor to a metallic phase, increasing the electrical conductivity by a factor of 100-10,000 depending on nanoparticle size. Correspondingly large changes from optical transparency to high reflectivity occur at the same time. Lasers with pulses as short as one ten-trillionth of a second were used to track the phase change in vanadium dioxide nanoparticles. This discovery may be key to possible applications requiring extremely rapid switching from transparent to reflective states. These include protective overlayers for sensitive infrared detectors, nonlinear optical switches, fiber-optic pressure sensors, and electrically or optically triggered transistors that could switch hundreds of times faster than conventional silicon devices.

First Direct Observations of Quasiparticles. Quasiparticles provide a convenient simplification to describe the behavior of electrons in a superconductor. A quasiparticle can be thought of as a single particle moving through a system, surrounded by a cloud of other particles either pushed away or dragged along by its motion. Prior investigations of their dynamics have been indirect. Through the use of a new optical technique it was possible to perform the first direct study of the dynamics of quasiparticles in a superconductor. It was discovered that the quasiparticles can propagate remarkably far, several hundreds of nanometers. Knowledge of the dynamics of quasiparticles, specifically their rates of diffusion, scattering, trapping, and recombination, is critical for the both the applications and fundamental understanding of superconductivity.

Confining Electrons in New Two-dimensional Materials. Transition metal oxides, like semiconductors, are materials that confine electrons to a plane. It may now be possible to construct near-perfect layered materials of two perovskite structured materials. It has been shown through computational models that a single layer of LaTiO₃ in SrTiO₃ will serve as an electron donor and positive charge layer to retain those electrons in a thin layer as a two-dimensional electron gas (2DEG). Electrons behaving like a 2DEG appear to be an exotic phenomenon, but they are not. Many semiconductor electronic devices operate by creating just such a gas by an applied electric field inducing a thin conducting region at an interface—the field effect transistor being the prime example. Such thin electron layers have become a valuable tool for scientists studying the ways in which electrons organize their collective behavior. By expanding the materials available to create 2DEGs, new, more diverse opportunities have been created to expand our knowledge of electronic behavior that in turn can produce new applications.

Inexpensive Route to Solar Cells Using Nanomaterials. New and novel semiconductor nanocrystal-polymer solar cells with surprisingly high efficiencies have been fabricated. In a solar cell, the conversion of light energy to electrical current occurs at the nanometer scale. Thus the development of methods for controlling materials on this scale creates new opportunities for more advanced solar cells. These advances are required because, although solar cells based on silicon and gallium arsenide have achieved high efficiencies and have found a variety of markets, more widespread applications remain limited by their high cost of production. These new cells are formed in an inherently inexpensive process from a colloidal solution of semiconductor nanocrystals in a semiconducting polymer. The unique features of nanosized objects are exploited to optimize the cell performance by controlling the shape of the nanocrystals. The performance of the new cells already rivals that of the best polymer-based devices. While the power conversion efficiency is still below that of current amorphous silicon and single crystal devices, there are opportunities to increase performance further by adding additional nanocrystal components to capture more of the solar spectrum. Furthermore, the same methods can be extended to address other optoelectronic applications, such as photodetectors and light emitting diodes.

Predicting Magnetism in Nanomaterials. As recording media and sensors become smaller and ever-denser, it is increasingly important to control magnetism in nanostructures. But the physical properties of magnetic nanostructures are linked in complex ways and are difficult to predict, much less control. In this work, the magnetic properties of a cobalt nano-wire next to a platinum surface step were predicted from first-principles. The results are in perfect agreement with experiment and show the importance of a proper quantum mechanical description of the interplay of different magnetic phenomena. This work, based on newly developed quantum mechanical models implemented on high-performance computers, shows that accurate predictions can be made for a nanostructure comprised of a few hundred atoms. With continued theoretical development and more powerful computers, this paves the way toward prediction and control of more complex and useful magnetic structures.   

Explaining Materials Deformation Mechanisms from Atomic-scale Measurements. Using the world's most advanced electron microscope, the first direct observations of atomic details in complex crystalline dislocation cores revealed the atomic mechanisms underlying the deformation of intermetallic compounds with complex crystal structures. It was discovered that the diffusion of chromium atoms into and out of the crystal dislocation cores hinders dislocation motion in Laves-phase Cr₂Hf, a model intermetallic compound, thus providing a clue as to the origin of the brittleness and poor low temperature ductility of these intermetallic alloys. The poor low-temperature ductility of these intermetallic alloys has prevented their fabrication and use for decades. Some of the most attractive high-strength alloys for advanced high-temperature fission and fossil energy conversion applications possess similar complicated atomic configurations and lack the low-temperature ductility required for their fabricated by conventional cold deformation processes without crack formation. This discovery provides new atomistic insight into the behavior of crystal dislocations in complex intermetallic compounds necessary to design new fabricable alloys with the required strength at high service temperatures.

Discovery of Mechanism of Surface Mass Transport. Researchers have discovered that trace concentrations of sulfur can enhance the rate of mass transport on copper surfaces by many orders of magnitude and have established the atomic scale mechanism by which this enhancement occurs. This discovery was enabled by low-energy electron microscopy measurements of the motion of singe-atom-high steps on copper exposed to calibrated doses of sulfur. By comparing observations of the motion of these steps with theoretical predictions based on calculations of the electronic structure of the surface, this research established that surface mass transport is catalyzed by the formation of a large number of mobile copper sulfide clusters. Such highly mobile clusters are believed to be a common feature of impure surfaces. The enhanced mass transport allows the formation of much flatter and more defect free surfaces. This discovery provides insight to many previous puzzling observations of anomalous surface mass transport. It is an important advance towards the capability to control the nanoscale morphology of surfaces, a critical necessity for nanoscale applications.

Superior Iron-based Alloys and Steels. Fundamental laws of alloying coupled with advanced microanalytical characterization led to the discovery that yttrium containing iron-based alloys substantially enhance the stability of the amorphous (non-crystalline) state. Two technical implications are: (1) large bulk physical dimensions of this class of amorphous alloys can be made and (2) this understanding provides a new direction for designing bulk amorphous metals for structural and functional applications. Bulk tool steel was fabricated that was twice as hard as conventional tool steel. These achievements are milestones in the science of amorphous metals and the design of functional complex metallic alloys. Even more important, this research has demonstrated that microalloying is a new approach for designing bulk amorphous alloys. Their unique atomic configurations and the absence of a crystalline lattice allow bulk amorphous metals to outperform their crystalline counterparts by exhibiting superior magnetic and mechanical properties and corrosion resistance coupled with high thermal stability.            

Fracture Resistance Mechanism in Ceramics. Structural ceramics are complex structures of micron-sized matrix grains separated by a nanoscale intergranular film. For many years it has been observed that certain additives, specifically rare-earth atoms, influence the ceramic’s fracture resistance. But detailed information about how this effect is achieved and how it can be controlled had been inaccessible with current diagnostic capabilities. Now, new scanning transmission electron microscopy (STEM) and associated chemical analysis techniques have revealed the local atomic structure and bonding characteristics of the grain boundaries with close to atomic resolution. Applied to silicon nitride ceramics containing a range of rare-earth additives, these methods together have revealed how each atom bonds at a specific location depending on atom radius, electronic configuration and the presence of oxygen; this variation in bonding sites can be directly related to the fracture resistance or toughness of the ceramic.

Better Protective Coatings. Previously unattainable insight into stress development and failure mechanisms in thermally grown surface oxides on metal alloys has been obtained by a new in-situ synchrotron x-ray technique. This technique enabled, for the first time, the uncoupling and isolation of mechanical stress contributions from oxide growth, phase transformations, and creep deformation processes. For pure thermally-grown alumina, steady state oxidation creates compressive stresses. However, when certain “reactive elements” are added to the alloy, it is found that tensile stresses develop instead. Maximizing the tensile offset can lead to dramatic improvement in performance of a protective oxide. A 10 percent shift in the tensile direction can translate to a 40 percent improvement in operating lifetime. Better control of early stage oxidation leads to thinner, and thus longer lifetime protective oxides by speeding the transformation to a stable oxide structure. These results underpin future alloy development for high-temperature nuclear and fossil energy generation technologies and more fuel efficient jet engine applications where operating lifetime has great economic value.

New Composite Materials that Respond to Magnetic Fields. Magnetic-field-structured composites are a novel class of material in which magnetic particles, dispersed in a polymerizable medium, are organized into chains and other structures by magnetic fields while the polymer solidifies. These chains of particles can be electrically conductive, and this electrical conductivity can be extremely sensitive to temperature, pressure, and chemical vapors that penetrate and swell the polymer. In the present work it was demonstrated that even modest magnetic fields produced by simple copper coils cause these materials to contract significantly, like artificial muscles. This contraction was found to be accompanied by an enormous, 50,000-fold increase in electrical conductivity. This is by far the largest “magnetoresistance” effect ever observed in such modest magnetic fields and paves the way to using magnetic fields to control heat and current transport in micro and nano machines, and to tailoring the sensing response of these materials.

The “Giant Proximity Effect.” The reproducible confirmation of the existence of a Giant Proximity Effect (GPE) has challenged experimentalists for over a decade. In the traditional Proximity Effect (PE), a very thin layer of normal metal, when placed between two thicker superconductor slices, behaves like a superconductor. That is, superconducting or paired electrons retain phase coherence even while separated by the normal metal gap. In the newly discovered GPE, the normal-metal barrier layer is as much as 100 times thicker than in the PE case, a result that stands outside of any present theories. In addition to challenging the theoretical community and providing new clues to the causes of high-temperature superconductivity, this result may lead to new advances in superconducting circuitry as it is relatively easy to prepare reproducible thick barriers which will improve device uniformity and yield.

World’s Smallest Nanomotor. The smallest synthetic motor—a 300 nanometer gold rotor on a carbon nanotube shaft—has been demonstrated. This “nanomotor” continues the dramatic advances in the miniaturization of electromechanical devices and is a key step in the realization of practical synthetic nanometer-scale electromechanical systems (NEMS). In initial testing, the rotor rotated on its nanotube shaft for thousands of cycles with no apparent wear or degradation in performance. This is attributed to the unique low-friction characteristics of the carbon nanotube shaft. The new motor design has significant potential for NEMS applications. It should be possible to fabricate arrays of orientationally-ordered nanotube-based actuators on substrates by using alignment techniques.

Magnetohydrodynamic Turbulence in Liquid Metals. Application of a strong magnetic field can completely change flow characteristics of an electrically conducting fluid. The transformation may occur in processes ranging from the generation of sunspots to crystal growth. One particular aspect of this phenomenon, the damping of flow variations along the magnetic field lines and the corresponding development of elongated or even two-dimensional flow structures, affect nearly all aspects of turbulent flow behavior, including heat transfer and mixing. In a series of high resolution numerical experiments it has been shown that the anisotropy of flow (or directionality of flow) patterns is a robust universal feature determined primarily by the strength of the magnetic field, conductivity, and kinetic energy. Furthermore, the elongation of flow patterns is approximately the same for flow structures of different size. This property can be effectively employed for accurate modeling of magnetohydrodynamic turbulence. The results of the work are relevant to technological applications, such as continuous casting of steel, crystal growth, and development of lithium breeding blankets for fusion reactors.

Nanoparticle Catalysts. Methods were developed for depositing and stabilizing nanometer-sized platinum group metals, including palladium and rhodium, on surfaces of carbon nanotubes in supercritical fluid carbon dioxide. Uniformly distributed monometallic and bimetallic nanoparticles with narrow size distributions are formed on the surfaces of the carbon nanotubes. The carbon nanotube-supported palladium and rhodium nanoparticles demonstrated improved performance over commercial carbon-based palladium and rhodium catalysts for hydrogenation of olefins and aromatic compounds. These new nanoscale catalysts are currently being tested as electrocatalysts for low temperature polymer electrode fuel cells applications.

Timing the World’s Shortest X-Ray Pulses. Light sources based on particle accelerators, such as the Linac Coherent Light Source (LCLS), will revolutionize x-ray science due to their unprecedented brightness and extremely short pulse duration. To take full advantage of x-ray pulses that last only a few femtoseconds (10^-15 seconds), they must be timed relative to equally short pulses from an optical laser. Such measurements are vital to a wide range of LCLS experiments in which a sample is excited by an optical pulse and probed by an x-ray pulse. At the Stanford Linear Accelerator Center, ultrashort x-ray pulses were generated when 80-femtosecond electron pulses from an accelerator were sent through an undulator magnet; the x-ray and electron pulses were perfectly coincident in time. A crystal placed near the path of the electron beam experienced intense electric fields that altered the optical properties of the crystal, the electro-optic (EO) effect. An optical laser beam passing through the crystal sensed the EO effect, turning the time delay between the optical pulse and the electron/x-ray pulse into a spatial displacement on a detector. The current timing resolution of 60 femtoseconds could be improved to 5 femtoseconds, matching the projected performance of accelerator-based light sources into the foreseeable future.

Molecular Fragmentation Observed in Unprecedented Detail. Researchers working at the Advanced Light Source have advanced our ability to observe the total destruction of a molecule to new levels of sophistication, challenging theoretical understanding and paving the way for research to be performed at next-generation light sources. When a hydrogen molecule is exposed to x-ray photons of the appropriate energy, the two electrons it possesses can be ejected at once, leaving behind two positively charged nuclei that rapidly explode. Thus, absorption of one x-ray photon causes the complete destruction of the molecule. Using modern techniques of three-dimensional imaging and ultrafast timing, the motions of all four particles from a single event can be related to one another. The results are surprising and challenge our current theoretical understanding of how x-rays interact with matter.

Complete Ionization of Clusters in Intense VUV Laser Fields. BES-supported researchers have developed a theory that explains recently-observed ionization behavior of xenon clusters that were exposed to intense, coherent vacuum ultraviolet (VUV) pulses from a free-electron laser (FEL). Surprisingly, at intensities that produce only single ionization of an isolated xenon atom, the clusters irradiated by the FEL showed massive ionization in which every atom in the cluster was highly ionized, producing ions with charge states up to +8. This implies that each xenon atom in the cluster absorbed about 30 VUV photons. The key difference between clusters and isolated atoms is that energetic electron-ion collisions occur within the clusters and modify the single-photon absorption cross section, thus allowing a large number of photons to be absorbed. This process is called “inverse bremsstrahlung” and, when incorporated into a simple linear absorption model, clearly reproduces the experimental observations. Theories such as this will be needed to understand the behavior of matter when it is exposed to intense, coherent X-ray pulses from next-generation light sources such as the LCLS.

The Roaming Atom: Straying from the Lowest-energy Reaction Pathway. A fundamental tenet of modern chemical reaction theory is the concept of the transition state, a transient molecular entity that lies on the most direct pathway from reactants to products and whose properties govern the rate of reaction. Recently, it was shown that in a simple chemical reaction, the decomposition of formaldehyde, a substantial fraction of the dissociating molecules avoid the region of the transition state entirely. These studies combine ion imaging experiments with theoretical trajectory calculations to reveal that the dissociation takes place via a mechanism in which one hydrogen atom begins to roam away from the molecule and nearly dissociates, then returns to react with the remaining hydrogen atom. Along with other recent findings on reactions such as O + CH₃, these results challenge conventional notions of chemical reactions and raise the question of how common such processes might be. A key question is whether such a mechanism applies only to reactions forming hydrogen, during which a light hydrogen atom may rapidly explore regions far from the conventional transition state.

New Combustion Intermediates Discovered. A complete mechanism for the combustion of simple hydrocarbon fuels includes dozens of distinct molecular species and hundreds of chemical reactions. The identification of which molecules to include in a combustion chemistry mechanism still requires experimental detection, particularly for reactive intermediates. A class of unstable molecules known as enols, which have OH groups adjacent to carbon-carbon double bonds, are not currently included in standard combustion models. In work performed at the Advanced Light Source, significant quantities of 2, 3, and 4-carbon enols were observed using photoionization mass spectrometry of flames burning representative compounds from modern fuels. Concentration profiles of the enols taken in the model flames demonstrate that their presence cannot be accounted for by isomerization reactions that convert more stable molecules into enols. This leads to the conclusion that an entire class of important reaction intermediates is absent from current combustion models, and the models will need substantial revision.

Unified Molecular Picture of the Surfaces of Aqueous Solutions. A long-term controversy exists regarding the detailed, molecular nature of the surface of an aqueous solution containing molecular ions (or electrolytes). Joint theoretical and experimental studies have led to a new, unified view of the structure of the interface between air and aqueous electrolytes. Molecular dynamics simulations have shown that in basic salt solutions positively charged ions (cations) are repelled from the interface, while negatively charged ions (anions) exhibit a propensity to migrate toward the surface that correlates with the anion’s polarizability and physical size. In acidic solution, however, there is a high propensity for cations to be located at the air/solution interface. In this case, both cations and anions are concentrated at the surface and reduce the surface tension of water. These conclusions have been verified by surface-selective nonlinear vibrational spectroscopy experiments. Understanding the behavior of ions at aqueous surfaces is important to the heterogeneous chemistry of seawater aerosols and to the tropospheric ozone destruction in the Arctic and Antarctic due to reactions on ice pack covered with sea spray.

Self-Assembled Artificial Photosynthesis. In natural photosynthesis, self-assembly of light-absorbing molecules, or chromophores, at specific distances and orientations is especially important in two parts of the overall photosynthetic system: the antenna component, where light is collected; and the reaction center, where charge is separated. Recently, a green organic chromophore was discovered that exhibits photophysical and photoredox properties similar to those of natural chlorophyll a. When conjoined with four similar chromophores, the molecules self-assemble in solution to form an antenna-reaction center complex. Self-organization of the large structure is believed due to the propensity of these similar chromophores to align in a cofacial stacking arrangement. The self-assembled organic has attributes that closely mimic the primary events in photosynthesis: efficient light energy capture over a wide spectral range, energy funneling toward a core electron-transferring unit, and excited-state symmetry breaking of a molecular pair resulting in charge separation. The structure of the new array was determined at the Advanced Photon Source.

Two-Dimensional Spectroscopy Reveals Energy Transport Pathways In Photosynthesis. Photosynthetic antennas capture solar photons and transport the absorbed energy to the photosynthetic reaction center where charge separation occurs. Energy transfer by the antenna is nearly 100 percent efficient, although the mechanism for the process has been elusive. A novel spectroscopic technique known as a two-dimensional photon echo, commonly used in the infrared, has been extended to the visible spectral region and has revealed important details about energy transfer in photosynthetic light harvesting. In antenna pigments from green sulfur bacteria, distinct energy transport pathways have been identified that depend on the spatial properties of the pigment-protein complex. Contrary to the accepted model of a sequential cascade in energy from high- to low-lying excited states, these results reveal excited states that are distributed over two or more chlorophyll molecules and a pathway in which energy levels are skipped on the way to the lowest level. The new two-dimensional electronic spectroscopic method, which measures electronic couplings and maps the flow of excitation energy, opens the door to investigation of other photoactive systems and can be applied to improving the efficiency of molecular solar cells.

How Water Networks Accommodate an Excess Electron. In bulk water an excess electron can become trapped within a cavity formed by a network of hydrogen-bonded water molecules. This “solvated electron” is a critical chemical intermediate in the radiolysis of aqueous solutions. One approach to understanding the solvated electron is to study the structure and dynamics of clusters of water containing an excess electron in the gas phase. This approach has not yet been successful because these anionic water clusters are hard to make and because an accurate theoretical description for them is lacking. Recent work has shown that anionic clusters containing four to six water molecules can be created within gas-phase matrices of inert argon clusters, where their infrared spectra can be obtained. Analysis of these spectra using density functional theory shows that the diffuse electron interacts most strongly with a single water molecule that is hydrogen bonded to two other waters in a rearranged network. The spectra also exhibit evidence for the rapid exchange of energy between the vibrations of the hydrogen atoms on the unique molecule and the excess electron. This new technique can now be extended to larger water clusters that better mimic the solvated electron in bulk water.

Gold, a Magnificent Nanoscale Catalyst. When gold atoms are assembled as tiny clusters smaller than 8 nanometers and attached to the surface of titanium oxide, they acquire the remarkable ability to dissociate oxygen at room temperature and insert that oxygen into very specific locations in molecules. The origin of such unusual reactivity—discovered some 10 years ago—has until recently evaded a widely accepted explanation. Numerous parameters in the material are important and usually cross-correlated: gold particle dimension and shape, metal oxidation state, oxide support reducibility, and interaction of the gold with the support. Separating those parameters in these materials, which are macroscopically amorphous, would demand special analytical techniques that are able to focus on the detailed properties of individual chemical bonds in the solid. Therefore, researchers pursued a different route using existing and well-known surface science techniques:  they accurately synthesized and stacked one-atom-thick layers of gold extended in two dimensions, and supported them on top of perfect oxide crystals of known structure. They demonstrated that the nanoscale properties of gold metal are achievable by controlling the layer thickness to between 2 and 3 atoms. Such knowledge can now be extended to the manipulation of selective oxidation chemistry or the discovery and assembly of new catalysts.

Theory Guides Scientists on How to Extract Hydrogen from Natural Sources and Store it Efficiently. Two of the keys to a hydrogen economy are having an abundant supply of hydrogen and having materials that can store such hydrogen in a readily accessible form. Both of those challenges can be addressed by designing materials—chemical catalysts—that bind atomic hydrogen with medium strength and release molecular or gaseous hydrogen with very little heating. A random or systematic search for such catalysts, even with high-throughput techniques, would be very expensive and take many years. Scientists resorted to so-called density-functional theory, which is an electronic structure theory of matter, and other theories that describe chemical reactivity to design the ideal bimetallic catalysts, combinations of two metals, in special atomic arrangements that would result in solids with the desired properties. They arrived at a new theoretical construct called near-surface alloys of metals, such as a crystal of platinum containing a single layer of nickel atoms in its second row, that possesses the unique catalytic behavior sought. Having by now mapped entire families of such new theoretical materials—a feat unachievable by direct experimental means—these scientists have embarked on the challenge of fabricating these new structures and have already demonstrated their concept with a few successful examples.

Devising the Next-Generation Wonder Molecules—Fine Chemistry inside Nano Cages. In the future drugs, fibers, fuel additives, molecular electronics devices, solar energy conversion dyes, and flavors may be synthesized in a similar manner using sets of discrete cavities to contain and isolate single molecules or just reacting pairs of molecules and catalysts. The “single-molecule catalysis” concept would allow maximum control of the environment surrounding a molecule, the spatial arrangement adopted by its atoms, the type of bonds made available for reaction, and even how the energy is coupled to and transferred to the molecule. Such level of control would result in the ability to break bonds or insert or remove atoms or change the spatial arrangement of atoms in very specific ways and not others. The resulting products would possess properties—chemical, biological, optical, electronic, or mechanical—superior to those achievable through less controllable chemistry. Researchers are beginning to show that this goal may be achievable. So-called supramolecular or larger-than-molecules cages made with organometallic compounds were used to host other organometallic complexes that have catalytic properties, such as the ability to specifically break carbon-hydrogen bonds. They have shown that certain carbon-hydrogen bonds are selectively broken and that only certain members of a chemical family undergo reaction, and not others. They have even shown that the constrained environment also leads to enhanced rate of production of the most desired product, which is in itself a revolutionary discovery.

Controlling the Crash-landing of Biomolecules on Surfaces. Researchers have, for the first time, demonstrated that peptide ions retain at least one proton after soft landing on chemically modified, “fluffy” surfaces. Controlled deposition on surfaces holds great potential for applications such as selective chemical separations and analysis. Soft landing refers to the intact capture of large size-selected, charged molecules on surfaces of liquids or solids. Previous research suggests that soft landing provides a means for highly specific deposition of molecules of any size and complexity on surfaces using only a tiny fraction of material normally used in standard synthetic approaches. In the present studies, peptide ions are attractive as model systems that can provide important insights on the behavior of soft-landed macromolecules. The researchers used a specially designed mass spectrometer configured for studying interactions of large ions with surfaces. The special characteristics of the instrument enabled quantitative investigation of the effect of the speed and mass of ions on the soft landing process. For example, it was determined that even collisions with high energies can result in deposition of intact ions on surfaces.

Removal of Radium Ions from Water using Special “Grabber” Molecules. Researchers demonstrated a process that is highly selective for binding radium cations. It is a significant challenge to remove radioactive radium cations from wastewater since the large excess of other non-radioactive ions in solution can interfere with the selective extraction of radium. In the new work, a specially designed molecule was used to selectively bind radium. This supramolecular assembly made from isoguanosine is just the right size to extract radium in the presence of other cations such as magnesium and sodium.

How Molecules Move through Small Holes. Measurements of transport through 15-nanometer pores have been compared to theoretical results to yield new understanding of differential transport at small scales. This knowledge is important for an understanding of separation processes at the molecular level, and could lead to a new generation of analytical devices based on microfluidic platforms. By adjusting physical parameters such as the channel diameter, and applying the appropriate external electrical potential, arrays of nanochannels—formed by nanocapillary array membranes—can be made to behave like digital fluidic switches, and the movement of molecules from one side of the array to the other side can be controlled. Combining model calculations with experimental characterization provides important insights into the mechanism of molecular transport and, additionally, provides quantitative measures of the surface characteristics of the interior of the pores.

Using Thorium and Uranium to Activate the Carbon-Hydrogen and Carbon-Nitrogen Bonds in Molecules. The extent of electron-sharing in bonds with metals is an important property in catalysis. The correlation of bond covalency with reactivity can be elucidated by determining the reactivity of actinide (thorium, uranium, and other elements in the same row of the periodic table) ions with multiply bonded functional groups. Pyridine N-oxide (C₅H₅N-O), which has a relatively stable benzene-like ring, can transfer oxygen atoms to certain transition metals. Chemists have discovered that some uranium and thorium compounds can make C-H bonds in pyridine N-oxide more reactive by forming metal-carbon bonds. The structures of the products produced in these new reactions have been confirmed by x-ray crystallography. These reactions provide examples of C-H and C=N bond activation that is mediated by actinide metals. These studies may offer insights into catalytic removal of nitrogen-containing compounds from petroleum feedstocks, which is necessary to reduce nitrogen oxide emission in fuels.

Elusive Carbon Dioxide Binding Mode Discovered in New Uranium Complex. Carbon dioxide (CO₂) is a stable molecule with two strong carbon-oxygen bonds. Inorganic chemists seek to mimic the catalytic chemical processes by which carbon dioxide is modified by plants to form sugars. This process can remove CO₂ from the atmosphere and minimize atmospheric release of CO₂ in industrial processes such as refinement of hydrocarbons. A new exquisitely-designed uranium complex has been found to react with CO₂ such that one electron is transferred from the U³⁺ center to CO₂, producing a species with an unusual linear CO₂ that binds to uranium and has one weaker oxygen-carbon bond. Uranium is an essential component of this species because the U³⁺ ion is large, electron-rich, and has the right structure to participate in bonding. This species is unique in that the CO₂ remains linear, with one C-O bond longer and weaker than the other. The molecular structure, bond lengths and oxidation state were established experimentally. The linear M-O-C-O coordination had previously been seen only in an iron enzyme. The new uranium-CO₂ complex represents a chemical image of a catalytic process and may make it possible to design new catalysts to reduce the concentration of CO₂ in the atmosphere.

Plutonium is Caged and Illuminated by Synchrotron Light. A new complexant, which was synthesized to extract plutonium and other actinide elements selectively, has shown promise to remove plutonium from mammals. Microscopic crystals (about the thickness of a human hair) of a plutonium complex have been produced to provide a structural model in order to design new actinide-selective binders. Using the Advanced Light Source, researchers determined the detailed structure of these crystals and showed that individual plutonium ions are trapped in cavities produced by eight oxygen atoms from the binder molecules. This structural determination will serve as a model of such complexes on which to base the design of novel molecules that are cages for toxic metals.

Sheer Energy: Thinner, Cheaper Fuel Cell Catalysts. Fuel cells are a major source of clean energy in the hydrogen economy. Their economic development critically depends on cheaper electrocatalysts for oxygen reduction. The slow nature of this reaction causes a major limit in fuel cell efficiency. High precious metal content is another drawback of existing technology. Researchers coated five cheaper metals with a layer of platinum one atom thick and tested them. For most of the platinum "monolayers," the reaction occurred more slowly than it does on the thicker platinum layer currently used in fuel cells. But adding a monolayer of platinum to the cheaper metal palladium sped up the reaction. Theoretical computations predicted how the platinum monolayers are affected by atoms from the underlying layer of metal. The theory agreed well with the experiments and showed that a platinum monolayer on palladium balances two competing needs: it is reactive enough to break the bonds between oxygen atoms yet does not cling to the oxygen atoms so tightly that it prevents them from reacting with hydrogen. This method can dramatically decrease the expensive metal loading in fuel cells and improve cost and performance.

Advances in Computational Chemistry Research. Basic research in computational chemistry has resulted in a superior method for the prediction of chemical behavior from computational quantum mechanics and statistical mechanics. The method is based on treating the solvent in which a molecule is placed as a continuum, and determining the cavity-formation energy from statistical mechanics, and the electric contributions from quantum mechanics. This work has now been published and a leading chemical process simulation company has incorporated this method into the most recent release of their industry dominating process simulator. This work will impact modern industrial plant and process design and lead to higher energy efficiencies through effective modeling of manufacturing processes.

Is CO₂ Gone When You Put It In The Ground? There are only two options for dealing with increasing CO₂ concentrations in the atmosphere—get rid of new CO₂ actively or discontinue producing it and wait for natural processes to remove the excess over a very long time. Both approaches will likely be needed in the future. Researchers have been developing capabilities for realistic modeling of CO₂ injection into deep geological formations and for understanding dynamic processes associated with the injection in order to provide a scientific basis for evaluating the injections feasibility. Computational models were developed for coupling fluid properties, chemical and thermodynamic data, and rock-fluid interaction measurements. Reservoir dynamics were investigated on different levels of complexity and scale for natural and engineered systems. These types of calculations also form the basis for understanding possible leaks which may be major regulatory and insurance concerns for large scale geological CO₂ sequestration. The improved computational codes from this project were also used as the basis for design calculations for CO₂ injection at the Frio Test Site as part of the Office of Fossil Energy funded Climate Change Technology Program.

Improving Our Vision of the Subsurface. Large scale subsurface seismic measurements, although adequate for simple oil and gas exploration or waste site characterization, are inadequate for high hydrocarbon recovery rates or more effective environmental remediation or monitoring. Research is providing a better understanding of geophysical measurements of compressional and shear wave velocities, elastic moduli, and seismic anisotropy as they vary as functions of porosity, permeability, fluid contents, and stresses. A fiber-optic “optical” strainmeter has been developed that provides spatially averaged properties over a centimeter or “core” length scale intermediate between point measurements and a meter-scale bulk-measurements. The increased accuracy and sensitivity in measuring elastic deformation during applied sinusoidal stress will enable better discrimination between strain (elastic wave transmission efficiency) and phase lags (attenuation indicative of fluid content and type). In addition, the highly precise optical strain gage measurements will allow higher resolution testing of the significance of different types of heterogeneity at the core scale, in order to enable prediction of these properties at larger scales. The fiber optic sensor has been demonstrated to have a significantly higher sensitivity than other strain gages.

The Auxin Receptor: A Holy Grail in Plant Science. The plant growth hormone called auxin is a small molecule, indole acetic acid (IAA)—too small to have the expected breadth of “informational” content to achieve its myriad effects of controlling the growth of leaves, stems, roots, flowers, fruits, and growth changes in response to light and gravity. Recent research demonstrated that IAA interacts directly with a much larger molecule, a protein, which was earlier shown to affect plant growth by stimulating the expression (activation) of certain growth-related genes. Now the solution to the mystery of auxin action is becoming clear. It turns out to be similar to an electric switch, but a bit more complex. We are beginning to unravel the molecular details of auxin’s biological activity.

The Neutron Reflectometer Commissioned. A new instrument, the neutron reflectometer, was commissioned for use in the general user program at the HFIR Center for Neutron Scattering. This machine is optimized for the studies of surfaces and interfaces. It is the fifth Cold Neutron Source instrument fully commissioned and will be used for the studies of polymers, biomaterials, thin solid films, and surfactants.

Selected FY 2004 Scientific Highlights/Accomplishments

Materials Sciences and Engineering Subprogram

The Ultimate Analysis: Single-Atom Spectroscopy in Bulk Solids.  A longstanding dream in materials sciences and engineering has been to see and study those specific individual atoms that are critical to bulk properties and to determine their location and active configuration. Now, through an enhanced scanning transmission electron microscope with improved optics, researchers are able to observe an individual atom within its bulk environment and characterize its chemical state via spectroscopic means, determining its valence and bonding with nearest neighbors. The advance was made possible by correction of lens aberrations in the electron microscope to give a smaller yet brighter beam with a diameter of approximately 1 Ångstrom. Single-atom sensitivity, the ultimate analysis, opens up all areas of materials science and engineering to fundamental investigations in a revolutionary way

New Thin-film Texture Discovered with Potential for Nanotech Applications.  One of the most fundamental structural properties of a thin film is its “texture,” which is the orientation of individual grains with respect to the deposition substrate. Three types of texture are commonly observed: random, where no single orientation is dominant; fiber-texture, where the film grains are parallel to the growth direction, but random about that direction; and epitaxial, where the film orientation is fixed in three dimensions with respect to the substrate. The new, fourth type of texture, named axiotaxy, was observed in a number of thin film systems in which the film and substrate share a common plane orientation as a consequence of crystal lattice matching. This new texture provides a potential method for assembling large numbers of nanocrystals in regular patterns for nanotech applications

Negative Refraction - New Frontier for Superlenses.  The first demonstration of negative and positive refraction of visible light at the same crystal interface was recognized as one of the “Top 15 Physics News Stories of 2003” by the American Institute of Physics. Nature provides us with optical refraction which is always positive: that is, the incident and transmitted light through an interface of two different media are on opposite sides of the interface normal. For negative refraction, they are on the same side of the interface normal. The beauty of negative refraction is total transmission and zero reflection, regardless of the angle of light incidence. These properties lend themselves to the creation of “super lenses.”  Laser beams can be steered in nano-photonic devices without loss, and optical telescopes can be built with higher resolution. The new interface uses a ferroelastic twin domain boundary such as a yttrium vanadate (YVO4) bi-crystal and is applicable to any frequency of the electromagnetic spectrum. As a vision for the future, electron beams could be focused more efficiently in highly sensitive electron microscopes