SCIENCE & TECHNOLOGY CONSTRUCTION ZONE Combining X-ray spectroscopy with computational methods reveals a fullerenetype reconstruction in diamond nanoparticles (yellow) upon loss of surface hydrogen and a change from ordered (middle) to disordered (right) in the surface structure of germanium nanoparticles upon exposure to heat.
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GENTLER X-RAY SPECTROSCOPY Symposium focuses on analytical applications of absorption and emission methods MITCH JACOBY, C&EN CHICAGO
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trons confined to synchrotron storage rings race endlessly around their metal tracks, all the while emitting a spectrum of intense radiation. The light, much of which falls in the X-ray region, is used by scientists to interrogate a wide variety of materials and chemical systems. Compared with the broad range ofwavelengths used in analytical techniques—from radio waves to y-rays—X-rays may seem like a uniformly high-energy band of radiation. But to those who rely on X-rays, differences in X-ray energy are key. "Hard" X-rays, with energies of about 2 keV or higher, tend to be used in diffraction and crystal structure studies, while less energetic "soft" X-rays are the photons of choice for a collection of spectroscopy techniques. Soft X-ray enthusiasts gathered at the 5th International Chemical Congress of Pacific Basin Societies (Pacifichem) in sunny Honolulu last month to share and catch up on the latest advances in their field. At a symposium focusing on X-ray absorption and X-ray emission spectroscopies, photoelectron spectroscopy, and related techniques, researchers reported on a host of topics ranging from semiconductor and carbon
nanoparticles to DNA and from analysis of buried interfaces to instrumentation development. "The number of applications in the field is exploding," said Alexander Moewes, a professor of physics at the University of Saskatchewan, Saskatoon, and one of the symposium organizers. "Today, you see people applying soft X-ray methods to problems in ways that no one was doing just a few years ago." According to Moewes, one of the trends in X-ray analysis nowadays is to focus on very dilute systems. Researchers are using X-ray spectroscopy methods to study low concentrations of analytes in solution or dopants in solid materials, he noted. Until recently, analyzing liquids and other samples containing small numbers of analyte molecules was quite difficult because, as Moewes noted, "there just wasn't enough signal to do these types of experiments. But with the advent of powerful synchrotrons and advanced spectrometers, we can now collect useful analytical information." Injun Kawai's experience, the basic Xray science and X-ray analysis communities tend to remain distinct, with each group convening its own conferences and workshops. "But at this symposium, they've come
together to hold discussions about their work," Kawai commented. Kawai, a professor of materials science and engineering at Kyoto University in Japan and another of the symposium's organizers, noted that the two-day session brought together synchrotron users from various countries. He pointed out that attendees were a mix of Xray physicists, analytical and physical chemists, and materials scientists from university, government, and industry labs. IN A PAIR of related presentations, Louis J. Terminello and Tony W. van Buuren, researchers in the Chemistry & Materials Science Division at Lawrence Livermore National Laboratory, reported on X-ray studies of semiconductor and carbon-based nanoparticles. The studies examined electronic and other properties of group IV semiconductors and their relationship to particle size, surface effects, and other factors that influence those properties. Byway of introduction, Terminello reviewed the particle-in-a-box description of the changes expected in a material's electronic structure as the specimen size is reduced from bulk dimensions to the nanometer scale. Theory predicts that the band gap, the energy required to promote an electron from the occupied valence band to the unoccupied conduction band, increases as the particle size decreases. This effect, which causes the colors of some materials to change with particle size, is a direct result of quantum confinement—the tiny "roaming" volume available to electrons contained in nanoparticles. After having studied band gaps and other electronic and optical properties of silicon and other group IV semiconductor nanoparticles (also known as quantum dots), the Livermore team turned to nanoscale diamond. This material is available commercially, Terminello pointed out, and a simple, yet energetic synthesis method,
"The number of applications in the field is exploding. Today, you see people applying soft X-ray methods to problems in ways that no one was doing just a few years ago." WWW.CEN-0NLINE.ORG
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SCIENCE & TECHNOLOGY involving high explosives, can be used to prepare uniform samples of 4-nm diamond particles. To study the material's electronic structure, the Livermore group measured the nanoparticles' soft X-ray absorption and emission spectra. Terminello explained that X-ray absorption measurements probe unoccupied electronic states in the conduction band by promoting an electron from a core-level ground state to an excited state in the conduction band. In a related manner, occupied states in the valence band can be probed by studying X-ray emission. That spectroscopy method is based on measuring the energies of characteristic soft: X-ray photons that are emitted during a relaxation process that follows an excitation event. Armed with those tools, Terminello and
coworkers at Livermore compared diamond nanoparticles with reference samples of bulk diamond and graphite. As expected, spectra of nanostructured diamond (nanodiamond) and graphite showed major differences, "proving that nanodiamond isn't just a bag of soot," Terminello said. In contrast, spectra of the two forms of diamond looked similar at first glance. Terminello pointed out that upon closer examination, a series of distinguishing features appears at energies just below the steeplyrisingportion of the curve (the absorption edge). Turning to computational methods, the Livermore group found that if some of the hydrogen that typically caps diamond particles and films is removed, the particles can undergo surface reconstruction. The theoretical portion of the study, which was led by Livermore's Giulia Galli, now a chemistry
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TEAMWORK Kawai (left) and Moewes organized a symposium focusing on soft X-ray spectroscopy.
LIKE-MINDED Lawrence Livermore's van Buuren (left) and Terminello probe semiconductors with X-ray methods. 36
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professor at the University of California, Davis, shows that the reconstruction process leads to stable diamond nanoparticles with fullerene-type structures and sp2 bonding characteristics, Terminello noted. He then showed that calculated spectra for the so-called bucky diamonds closely match the spectral features in nanodiamond that distinguish it from the bulk form of the material. Although some spectral differences between the bulk and nanoscale forms of diamond were identified, Terminello stressed that the typical shift in band energies expected of quantum dots compared with their bulk counterparts was nowhere to be seen in the studies of 4-nm diamond samples. So the Livermore team set out to search for the effect in even smaller nanoparticles, around 1 nm in diameter and smaller, as suggested by theoretical results that had just recently been published. The team quickly came up with "an ideal sample set," Terminello said. The group began collaborating with Molecular Diamond Technologies, a Chevron company whose researchers had just developed methods for isolating and crystallizing diamondoids. The term refers to a family of compounds with diamond structures built up from multiple adamantane cages (C&EN, Jan. 5,2004, page 22). As it turns out, even this ideal set of the tiniest nanodiamond particles exhibits very little shift in electronic band energies relative to bulk diamond and essentially no electronic structure changes as a function of particle size (Phys. Rev. Lett. 2005, 95,113401). Basing their results on X-ray spectroscopy of diamondoids ranging from adamantane (a single diamond cage) to cyclohexamantane (six fused diamond cages), the Livermore and Molecular Diamond research team concluded that nanoscale diamond is quite unlike the other group IV semiconductors. Underscoring those differences, Livermore's van Buuren displayed X-ray absorption and emission spectra of a series of silicon nanoparticles that clearly show shifts in the conduction-band and valence-band edges, indicating an increase in band gap with decreasing particle size. Van Buuren emphasized that X-ray spectroscopy methods are powerful and have enabled the Livermore team to study low concentrations of silicon nanoparticles supported on solid substrates. These methods also have been used to monitor the annealing process used to prepare Si02-encapsulated silicon nanoparticles of various sizes. In the case of germanium nanoparticles, the effects of quantum confinement WWW.CEN-0NLINE.ORG
are even more pronounced, van Buuren reported. Compared with silicon particles of a similar size, germanium nanoparticles exhibit greater band shifting for every nanoparticle size studied by the group. Van Buuren reported that relative to the germanium bulk value, the conduction band edge of germanium shifts from 0.2 eV for 2.7-nm particles to 1.1 eV for 1.2nm particles (App/. Phys. Lett. 2 0 0 4 , 84, 4056). In related work, the group showed that heat treatments cause the surfaces of 2- to 3-nm germanium particles to become disordered. According to van Buuren, using the disordered models in calculations leads to theoretical results that match the experimentally measured spectra. Continuing the semiconductor theme, Clemens Heske, a chemistry professor at the University of Nevada, Las Vegas, brought symposium attendees up-to-date on his investigations of compound semiconductors and techniques for probing interfaces and hidden layers. Heske's focus is oncopper-indium-gallium-sulfo-selenide, Cu(In,Ga)(S,Se)2 (CIGSSe for short). He described this family of compounds as "wonderful materials for solar cells" owing
to their efficiency in absorbing sunlight, compatibility with standard thin-film coating methods, and other properties that may lead to cost-effective photovoltaic devices. The layered solar cells Heske studies typically consist of thin films of ZnO, CdS, CIGSSe, and molybdenum (from top to bot-
Heske
Wurzburg, and elsewhere developed X-ray methods to probe the critical interfaces in a chemically sensitive way. ON THE BASIS of those methods, the group showed that sulfur and selenium diffuse across the interface between CdS and
Taniguchi
Nordgren
torn) deposited on glass. Improving cell performance and durability requires a detailed understanding of interfacial processes. So Heske and colleagues at the Hahn-Meitner Institute in Berlin, the University of
Cu(In,Ga)Se 2 and form compounds such as CuInSxSe2_x, CdJiiyS^Se^ and CdS,_zSez. Knowing that the diffusion products are present at the interface is key, Heske emphasized, because their band gaps, which
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SCIENCE & TECHNOLOGY dictate solar-cell performance, differ from those of the starting materials. The techniques also were used to evaluate the effects of various fabrication steps on the interface composition and to assess the role played by humidity in solar-cell aging. In the humidity study, the team used a specially designed cell that enables incident X-rays to probe the interface between a film of water and a CIGSSe layer, and emitted X-rays to be analyzed by a spectrometer. The study revealed a sulfate film that grows on exposure to X-rays, Heske reported. He pointed out that the film can be used diagnostically to searchforunwanted water at buried interfaces. Fuj i na m i Like other scientists at the session, Joseph Nordgren exploits soft X-ray spectroscopy methods to probe materials and chemical systems. But unlike some researchers who coax their samples to emit X-rays in the conventional way (by first ejecting core-level electrons), the Uppsala University physics professor tunes the energy of the excitation in a way that selectively enhances signals from species in particular chemical environments. In one application of the X-ray method, Nordgren and coworkers at Lawrence Berkeley National Laboratory answered a 40-year-old question regarding the molecular structure of liquid alcohol and its water solutions. They found that, in the pure liquid, methanol tends to form hydrogen-bonded chains and rings of six or eight molecules. In solution, however, the six- and eight-membered alcohol chains assemble into rings that are bridged by a small number of water molecules. Using similar techniques, the Uppsala researcher was able to explain the electronic basis of a famous demonstration in which an yttrium mirror exposed to hydrogen becomes transparent. Referring to a series of spectra, Nordgren pointed out that hydrogen uptake in yttrium creates a band gap, which prevents photon absorption in the energy region of visible light, thus rendering the material transparent. In addition to other studies, Nordgren discussed his investigations of atmospheric corrosion of iron and electrochemical processes in lithium-ion batteries. He also reported on X-ray-based measurements of the rate at which U 6+ is reduced to U 4+ in aqueous solution in the presence of iron. According to Nordgren, the experiment is part of a larger study to evaluate environ38
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the choice of buffer material, water content, the state of DNA (solid or in solution), and other factors. Some of the symposium attendees reported on developments in X-ray ins t r u m e n t a t i o n . For example, Kazuo Taniguchi, a professor at Osaka ElectroCommunication University, in Japan, gave an update on his group's effort to build a field-portable spectrometer. Taniguchi explained that the goal is to conduct onsite analysis of femtogram quantities of nanometer-sized dust particles trapped deep in ice cores presently being bored at the South Pole. By studying the particles, researchers aim to find element-specific clues that suggest long-ago volcanic eruptions (suggested by silicon or aluminum) or meteorite impacts (nickel or iron). Taniguchi said that, by the end of the year, he expects to personally deliver a prototype to the South Pole. X-ray spectroscopy was the common theme in all the presentations except one. Masanori Fujinami, a professor of applied WITH A TOUCH of humor, Moewes conchemistry at Chiba University in Japan, fessed that he "naively thought he could described a positron-based method for get the answer in one afternoon" by using detecting crystal vacancies and impuriX-ray absorption and emission methods to ties in silicon and other semiconductors. measure the gap between DNA's highest The imperfections alter the material's occupied and lowest unoccupied molecular electronic properties even if present at orbitals, the molecular equivalent of a band low concentrations. gap. The idea was that the X-ray measureFujinami explained that a radioisotope such as 22 Na is used to inject positrons (posiExcitation tive electrons) into a sample. X-ray In the absence ofvacancies, a positron will diffuse through the solid for a characteristic period of time before underPolyimide going annihilation with an H20 electron, which liberates two Aluminum 511-keV 7-rays. But at the site of a vacancy, electrostatic Cu(lnlGa)(S,Se)2 forces trap positrons, thereby extending their lifetimes and Molybdenum modifying the 7 radiation in a way that depends on the Adapted from J. Chem. Phys. concentration and chemical state of imperfections. PHOTON IN, PHOTON OUT Using specially The ability to probe liqdesigned cells, X-ray emission spectroscopy uids, solids, and buried incan probe the interface between water and a terfaces makes the collection micrometer-thick film of a semiconductor light of soft X-ray spectroscopy absorber used in solar cells (Culn material). methods versatile tools for providing researchers with new insights into chemical and physical ments would avoid the large number of properties of materials. With continued experimental variables fueling the DNA improvements in X-ray sources and speccontroversy. trometers, the analytical techniques are It didn't turn out to be so simple. Moewes sure to be used by a growing number of and coworkers measured a range of values scientists in an expanding assortment of over the course of the study. He concluded applications. • that the energy gap depends strongly on mental conditions relevant to a plan under consideration for long-term underground storage of spent nuclear fuel in Sweden and Finland. Of the wide range of analytical applications discussed at the symposium, the pre_ sentation by the University I of Saskatchewan's Moewes i on the electronic properties i ofDNAwasoneofthemost o biochemical in nature. He | noted that DNA's potential use in nanoelectronics— for example, as a molecular wire—has motivated numerous studies of the molecule's electrical conductivity. The results, which have been published in leading journals, vary widely, he said—placing DNA all over the conductivity map, from insulator to superconductor.
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