An international team of physicists has succeeded in mapping the condensation of individual atoms, or rather their transition from a gaseous state to another state, using a new method. Led by the Swiss Nanoscience Institute and the Department of Physics at the University of Basel, the team was able to monitor for the first time how xenon atoms condensate in microscopic measuring beakers, or quantum wells, thereby enabling key conclusions to be drawn as to the nature of atomic bonding. The researchers published their results in the journal ("Interplay of weak interactions in the atom-by-atom condensation of xenon within quantum boxes"). 
Monitoring of the condensation of xenon: Depicted in scanning tunneling microscopy are three different quantum wells that contain one, two and three xenon atoms. The team headed by Professor Thomas Jung, which consists of researchers from the Swiss Nanoscience Institute, Department of Physics at the University of Basel and the Paul Scherrer Institute, developed a method enabling the condensation of individual atoms to be mapped on a step by step basis for the first time. The researchers allowed atoms of the noble gas xenon to condensate in quantum wells and monitored the resulting accumulations using a scanning tunneling microscope. Quantum wells as beakers The autonomous organization of specifically 'programmed' molecules facilitates the creation of a porous network on a substrate surface – these are the quantum wells used as measuring beakers with a specifically defined size, shape and atomic wall and floor structure. The atoms' freedom of movement is restricted in the quantum wells, allowing the arrangement of the atoms to be closely monitored and mapped depending on the composition. With this data, the researchers were able to show that the xenon atoms always arrange themselves according to a certain principle. For example, some units consisting of four atoms are only formed when there are at least seven atoms in the quantum well. And if there are twelve atoms in the quantum well, this results in the creation of three highly stable four-atom units. Conclusions about the nature of bonding The images and structures of nano-condensates recorded for the first time allow key conclusions to be drawn as to the nature of the physical bonds formed by the xenon atoms. "But this system is not restricted exclusively to noble gases," says Sylwia Nowakowska, lead author of the publication. "We can also use it to research other atoms and the way that they bond." As the newly developed method accurately maps atomic bonding and determines the stability of the various states, it can also be used to verify theoretical calculations about bonds.
Monitoring of the condensation of xenon: Depicted in scanning tunneling microscopy are three different quantum wells that contain one, two and three xenon atoms. The team headed by Professor Thomas Jung, which consists of researchers from the Swiss Nanoscience Institute, Department of Physics at the University of Basel and the Paul Scherrer Institute, developed a method enabling the condensation of individual atoms to be mapped on a step by step basis for the first time. The researchers allowed atoms of the noble gas xenon to condensate in quantum wells and monitored the resulting accumulations using a scanning tunneling microscope. Quantum wells as beakers The autonomous organization of specifically 'programmed' molecules facilitates the creation of a porous network on a substrate surface – these are the quantum wells used as measuring beakers with a specifically defined size, shape and atomic wall and floor structure. The atoms' freedom of movement is restricted in the quantum wells, allowing the arrangement of the atoms to be closely monitored and mapped depending on the composition. With this data, the researchers were able to show that the xenon atoms always arrange themselves according to a certain principle. For example, some units consisting of four atoms are only formed when there are at least seven atoms in the quantum well. And if there are twelve atoms in the quantum well, this results in the creation of three highly stable four-atom units. Conclusions about the nature of bonding The images and structures of nano-condensates recorded for the first time allow key conclusions to be drawn as to the nature of the physical bonds formed by the xenon atoms. "But this system is not restricted exclusively to noble gases," says Sylwia Nowakowska, lead author of the publication. "We can also use it to research other atoms and the way that they bond." As the newly developed method accurately maps atomic bonding and determines the stability of the various states, it can also be used to verify theoretical calculations about bonds.
A closeup shows how the nanotextured square of silicon completely blocks reflection compared with the surrounding silicon wafer. Preventing reflections requires controlling an abrupt change in "refractive index," a property that affects how waves such as light propagate through a material. This occurs at the interface where two materials with very different refractive indices meet, for example at the interface between air and silicon. Adding a coating with an intermediate refractive index at the interface eases the transition between materials and reduces the reflection, Black explained. "The issue with using such coatings for solar cells," he said, "is that we'd prefer to fully capture every color of the light spectrum within the device, and we'd like to capture the light irrespective of the direction it comes from. But each color of light couples best with a different antireflection coating, and each coating is optimized for light coming from a particular direction. So you deal with these issues by using multiple antireflection layers. We were interested in looking for a better way." For inspiration, the scientists turned to a well-known example of an antireflective surface in nature, the eyes of common moths. The surfaces of their compound eyes have textured patterns made of many tiny "posts," each smaller than the wavelengths of light. This textured surface improves moths' nighttime vision, and also prevents the "deer in the headlights" reflecting glow that might allow predators to detect them. "We set out to recreate moth eye patterns in silicon at even smaller sizes using methods of nanotechnology," said Atikur Rahman, a postdoctoral fellow working with Black at the CFN and first author of the study. The scientists started by coating the top surface of a silicon solar cell with a polymer material called a "block copolymer," which can be made to self-organize into an ordered surface pattern with dimensions measuring only tens of nanometers. The self-assembled pattern served as a template for forming posts in the solar cell like those in the moth eye using a plasma of reactive gases—a technique commonly used in the manufacture of semiconductor electronic circuits. The resulting surface nanotexture served to gradually change the refractive index to drastically cut down on reflection of many wavelengths of light simultaneously, regardless of the direction of light impinging on the solar cell. 
Details of the nanotextured antireflective surface as revealed by a scanning electron microscope at the Center for Functional Nanomaterials. The tiny posts, each smaller than the wavelengths of light, are reminiscent of the structure of moths' eyes, an example of an antireflective surface found in nature. "Adding these nanotextures turned the normally shiny silicon surface absolutely black," Rahman said. Solar cells textured in this way outperform those coated with a single antireflective film by about 20 percent, and bring light into the device as well as the best multi-layer-coatings used in the industry. "We are working to understand whether there are economic advantages to assembling silicon solar cells using our method, compared to other, established processes in the industry," Black said. Hidden layer explains better-than-expected performance One intriguing aspect of the study was that the scientists achieved the antireflective performance by creating nanoposts only half as tall as the required height predicted by a mathematical model describing the effect. So they called upon the expertise of colleagues at the CFN and other Brookhaven scientists to help sort out the mystery. "This is a powerful advantage of doing research at the CFN—both for us and for academic and industrial researchers coming to use our facilities," Black said. "We have all these experts around who can help you solve your problems." Using a combination of computational modeling, electron microscopy, and surface science, the team deduced that a thin layer of silicon oxide similar to what typically forms when silicon is exposed to air seemed to be having an outsized effect. "On a flat surface, this layer is so thin that its effect is minimal," explained Matt Eisaman of Brookhaven's Sustainable Energy Technologies Department and a professor at Stony Brook University. "But on the nanopatterned surface, with the thin oxide layer surrounding all sides of the nanotexture, the oxide can have a larger effect because it makes up a significant portion of the nanotextured material." Said Black, "This 'hidden' layer was the key to the extra boost in performance." The scientists are now interested in developing their self-assembly based method of nanotexture patterning for other materials, including glass and plastic, for antiglare windows and coatings for solar panels.