X-ray photoelectron spectroscopy (XPS) is one of the most sensitive and informative surface analysis techniques available. However, XPS requires a high vacuum to operate, which makes analyzing materials in liquid and gaseous environments difficult. Now, researchers from the National Institute of Standards and Technology (NIST), ELETTRA (Italy) and Technical University of Munich (Germany) have found that graphene—a single-atom-thick sheet of carbon—could make using XPS to study materials in these environments much less expensive and complicated than the conventional approach. Their results were published in the journal ("Photoelectron spectroscopy of wet and gaseous samples through graphene membranes"). 
Drawing shows the set-up for an X-ray photoelectron spectroscopy instrument incorporating suspended, electron-transparent graphene membranes - or windows - that separate the sample from the high-vacuum detection system. (Image: NIST) Researchers have analyzed cells and microorganisms using visible light, which, while informative and gentle, cannot be used to probe objects much smaller than about 500 nanometers. But many of life’s most important processes and interactions take place at much smaller length scales. The same is true with batteries: everything that can go wrong with them takes place at the tiny interfaces between the electrodes and the electrolyte—far beyond the reach of optical microscopes. Many researchers would like to use X rays or electrons to look deeper into these materials, but few labs have the sophisticated equipment necessary to do so, and those labs that are so outfitted are often too pricey for today’s budget-conscious scientists. XPS works by bombarding the surface under study with X rays. The atoms on the surface of the material absorb the X-ray energy and re-emit that energy as photoelectrons. Scientists study the kinetic energy and number of the emitted electrons for clues about the sample’s composition and electronic state. Because X rays and photoelectrons interact with the air, XPS has to be performed under high vacuum, which makes it hard to study materials that have to be in a pressurized environment. What researchers needed was a window material that was nearly transparent to X rays and photoelectrons, but impermeable to gases and liquids and strong enough to withstand the mechanical stress of one atmosphere’s worth of pressure. Knowing that graphene, the wonder material of the 21st century, has these properties, the group explored using it as a window to separate their sample stage’s atmospheric pressure liquid compartment from the high-vacuum conditions inside the electron spectrometer. According to NIST researcher Andrei Kolmakov, their results demonstrate that more than enough X rays—and resultant photoelectrons—are able to pass through the graphene window to produce good quality XPS data from liquids and gases. As an added bonus, the group was also able to measure the intensity of radiation needed to create bubbles in water, a frequently unwanted occurrence that happens when the X rays split water into oxygen and hydrogen. Knowing the point at which bubbles form, they were able to define an upper limit on the intensities of the X rays (or electrons) that can be used in this approach. “We think our work could fill a much-needed gap,” says Kolmakov. “There are many scientists whose work would benefit from using XPS at ambient pressure, but there are not enough instruments that are equipped to analyze the samples under these conditions, and the ones out there are often too costly to use. Our design is far simpler and has the potential to reduce costs to the level that this type of measurement could be afforded by many more labs. With this imaging capability, other researchers could, for example, learn much more about how to create longer- lasting batteries and develop safer and more effective drugs.” Of course, as often happens with new technologies, the approach has a few challenges and limitations. Kolmakov says that the adhesion of the graphene to the surface surrounding the opening needs to be improved. Moreover, the barrage of X rays degrades atomically thin graphene over time, so the team is planning to look for ways to mitigate that, if possible.
Drawing shows the set-up for an X-ray photoelectron spectroscopy instrument incorporating suspended, electron-transparent graphene membranes - or windows - that separate the sample from the high-vacuum detection system. (Image: NIST) Researchers have analyzed cells and microorganisms using visible light, which, while informative and gentle, cannot be used to probe objects much smaller than about 500 nanometers. But many of life’s most important processes and interactions take place at much smaller length scales. The same is true with batteries: everything that can go wrong with them takes place at the tiny interfaces between the electrodes and the electrolyte—far beyond the reach of optical microscopes. Many researchers would like to use X rays or electrons to look deeper into these materials, but few labs have the sophisticated equipment necessary to do so, and those labs that are so outfitted are often too pricey for today’s budget-conscious scientists. XPS works by bombarding the surface under study with X rays. The atoms on the surface of the material absorb the X-ray energy and re-emit that energy as photoelectrons. Scientists study the kinetic energy and number of the emitted electrons for clues about the sample’s composition and electronic state. Because X rays and photoelectrons interact with the air, XPS has to be performed under high vacuum, which makes it hard to study materials that have to be in a pressurized environment. What researchers needed was a window material that was nearly transparent to X rays and photoelectrons, but impermeable to gases and liquids and strong enough to withstand the mechanical stress of one atmosphere’s worth of pressure. Knowing that graphene, the wonder material of the 21st century, has these properties, the group explored using it as a window to separate their sample stage’s atmospheric pressure liquid compartment from the high-vacuum conditions inside the electron spectrometer. According to NIST researcher Andrei Kolmakov, their results demonstrate that more than enough X rays—and resultant photoelectrons—are able to pass through the graphene window to produce good quality XPS data from liquids and gases. As an added bonus, the group was also able to measure the intensity of radiation needed to create bubbles in water, a frequently unwanted occurrence that happens when the X rays split water into oxygen and hydrogen. Knowing the point at which bubbles form, they were able to define an upper limit on the intensities of the X rays (or electrons) that can be used in this approach. “We think our work could fill a much-needed gap,” says Kolmakov. “There are many scientists whose work would benefit from using XPS at ambient pressure, but there are not enough instruments that are equipped to analyze the samples under these conditions, and the ones out there are often too costly to use. Our design is far simpler and has the potential to reduce costs to the level that this type of measurement could be afforded by many more labs. With this imaging capability, other researchers could, for example, learn much more about how to create longer- lasting batteries and develop safer and more effective drugs.” Of course, as often happens with new technologies, the approach has a few challenges and limitations. Kolmakov says that the adhesion of the graphene to the surface surrounding the opening needs to be improved. Moreover, the barrage of X rays degrades atomically thin graphene over time, so the team is planning to look for ways to mitigate that, if possible.
Behind the scenes in the Center for Nanoscale Systems, Mikhail Kats (Ph.D. '14) demonstrates the fabrication process for ultrathin coatings that shine in vivid colors. Kats and Prof. Federico Capasso have shown that these interference effects work on rough materials like paper. "I cut a piece of paper out of my notebook and deposited gold and germanium on it," Kats says, "and it worked just the same." That finding, deceptively simple given the physics involved, now suggests that the ultrathin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics. "This can be viewed as a way of coloring almost any object while using just a tiny amount of material," Capasso says. It was not obvious that the same color effects would be visible on rough substrates, because interference effects are usually highly sensitive to the angle of light. And on a sheet of paper, Kats explains, "There are hills and valleys and fibers and little things sticking out—that's why you can't see your reflection in it. The light scatters." On the other hand, the applied films are so extremely thin that they interact with light almost instantaneously, so looking at the coating straight on or from the side—or, as it turns out, looking at those rough imperfections in the paper—doesn't make much difference to the color. And the paper remains flexible, as usual. Demonstrating the technique in the cleanroom at the Center for Nanoscale Systems, a National Science Foundation–supported research facility at Harvard, Kats uses a machine called an electron beam evaporator to apply the gold and germanium coating. He seals the paper sample inside the machine's chamber, and a pump sucks out the air until the pressure drops to a staggering 10^-6 Torr (a billionth of an atmosphere). A stream of electrons strikes a piece of gold held in a carbon crucible, and the metal vaporizes, traveling upward through the vacuum until it hits the paper. Repeating the process, Kats adds the second layer. A little more or a little less germanium makes the difference between indigo and crimson. This particular lab technique, Kats points out, is unidirectional, so to the naked eye very subtle differences in the color are visible at different angles, where slightly less of the metal has landed on the sides of the paper’s ridges and valleys. "You can imagine decorative applications where you might want something that has a little bit of this pearlescent look, where you look from different angles and see a different shade," he notes. "But if we were to go next door and use a reactive sputterer instead of this e-beam evaporator, we could easily get a coating that conforms to the surface, and you wouldn't see any differences." Many different pairings of metal are possible, too. "Germanium's cheap. Gold is more expensive, of course, but in practice we're not using much of it," Kats explains. Capasso’s team has also demonstrated the technique using aluminum. "This is a way of coloring something with a very thin layer of material, so in principle, if it's a metal to begin with, you can just use 10 nanometers to color it, and if it's not, you can deposit a metal that's 30 nm thick and then another 10 nm. That's a lot thinner than a conventional paint coating that might be between a micron and 10 microns thick.” In those occasional situations where the weight of the paint matters, this could be very significant. Capasso remembers, for example, that the external fuel tank of NASA’s space shuttle used to be painted white. After the first two missions, engineers stopped painting it and saved 600 pounds of weight. Because the metal coatings absorb a lot of light, reflecting only a narrow set of wavelengths, Capasso suggests that they could also be incorporated into optoelectronic devices like photodetectors and solar cells. “The fact that these can be deposited on flexible substrates has implications for flexible and maybe even stretchable optoelectronics that could be part of your clothing or could be rolled up or folded,” Capasso says. Harvard's Office of Technology Development continues to pursue commercial opportunities for the new color coating technology and welcomes contact from interested parties. Kats, who concludes his year-long postdoctoral research position at SEAS this month, will become an assistant professor at the University of Wisconsin, Madison, in January. He credits those many hours spent in Harvard’s state-of-the-art laboratory facilities for much of his success in applied physics. "You learn so much while you're doing it," Kats says. "You can be creative, discover something along the way, apply something new to your research. It’s marvelous that we have students and postdocs down here making things."
Plot of radiative quality factor as a function of wave vector for a photonic crystal slab. At five positions, this factor diverges to infinity, corresponding to special solutions of Maxwell equations called bound states in the continuum. These states have enough energy to escape to infinity but remain spatially localized. (Image: Courtesy of the researchers) Light can usually be confined only with mirrors, or with specialized materials such as photonic crystals. Both of these approaches block light beams; last year’s finding demonstrated a new method in which the waves cancel out their own radiation fields. The new work shows that this light-trapping process, which involves twisting the polarization direction of the light, is based on a kind of vortex — the same phenomenon behind everything from tornadoes to water swirling down a drain. In addition to revealing the mechanism responsible for trapping the light, the new analysis shows that this trapped state is much more stable than had been thought, making it easier to produce and harder to disturb. “People think of this [trapped state] as very delicate,” Zhen says, “and almost impossible to realize. But it turns out it can exist in a robust way.” In most natural light, the direction of polarization — which can be thought of as the direction in which the light waves vibrate — remains fixed. That’s the principle that allows polarizing sunglasses to work: Light reflected from a surface is selectively polarized in one direction; that reflected light can then be blocked by polarizing filters oriented at right angles to it. But in the case of these light-trapping crystals, light that enters the material becomes polarized in a way that forms a vortex, Zhen says, with the direction of polarization changing depending on the beam’s direction. Because the polarization is different at every point in this vortex, it produces a singularity — also called a topological defect, Zhen says — at its center, trapping the light at that point. 
Vortices of bound states in the continuum. The left panel shows five bound states in the continuum in a photonic crystal slab as bright spots. The right panel shows the polarization vector field in the same region as the left panel, revealing five vortices at the locations of the bound states in the continuum. These vortices are characterized with topological charges +1 or -1. (Image: Courtesy of the researchers) Hsu says the phenomenon makes it possible to produce something called a vector beam, a special kind of laser beam that could potentially create small-scale particle accelerators. Such devices could use these vector beams to accelerate particles and smash them into each other — perhaps allowing future tabletop devices to carry out the kinds of high-energy experiments that today require miles-wide circular tunnels. The finding, Soljacic says, could also enable easy implementation of super-resolution imaging (using a method called stimulated emission depletion microscopy) and could allow the sending of far more channels of data through a single optical fiber. “This work is a great example of how supposedly well-studied physical systems can contain rich and undiscovered phenomena, which can be unearthed if you dig in the right spot,” says Yidong Chong, an assistant professor of physics and applied physics at Nanyang Technological University in Singapore who was not involved in this research. Chong says it is remarkable that such surprising findings have come from relatively well-studied materials. “It deals with photonic crystal slabs of the sort that have been extensively analyzed, both theoretically and experimentally, since the 1990s,” he says. “The fact that the system is so unexotic, together with the robustness associated with topological phenomena, should give us confidence that these modes will not simply be theoretical curiosities, but can be exploited in technologies such as microlasers.”