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A tale of twists and untwists
RESEARCH NEWS
A tale of twists and untwists CHARACTERIZATION A new study has created crystals that can both twist and untwist, revealing that process of crystal growth is more active than was previously thought. Although crystal growth is usually seen as a grouping of individual atoms, molecules or small clusters adding to a larger block that remains in a fixed translational relationship to the rest, a team of scientists from New York University and St. Petersburg State University in Russia have developed a crystal that continually changes its shape as it grows. The team, whose research is published in the Journal of the American Chemical Society [Shtukenberg, et al., J Am Chem Soc (2010) doi: 10.1021/ja101491n], examined crystals from hippuric acid, discovering that when molecules are added to the end of fine crystalline needles, stresses built up at the tips of the crystals, resulting in a helical twist, similar to the double helix in DNA. However, the process was reversed when crystals thickened from the other end of the growing tip, which undid the twisted formations, due to the elasticity of the crystals decreasing as they become thicker. Bart Kahr, a co-author of the study, said “This competition between twisting and untwisting creates needles with a rainbow of colors, which is a characteristic of tightly wound helices, as well as
Crystal growth. ribbons that have become completely untwisted. This is a very strange and new perspective on crystal growth.” With hippuric acid, the twisting is due to the presence of a decomposition product, which causes both mismatches and strain in lattices. If there is a lack of
symmetry of the systems, twists can occur, an internal stress mechanism that could easily apply to many other systems. The concept of helical crystals that twist like a drill bit originated in a book by Ferdinand Bernauer, “Gedrillte” Krystalle, published in the 1920s. Although forgotten for decades, the book was revived when helical crystals became topical again in the 1950s with the emergence of the synthetic polymer industry and the controversy over how and why high-polymers twist. The team investigated the book and the history of crystal growth in order to understand the crystallization process. It is hoped that this study will bring a greater understanding of high-polymers, which are used in a range of consumer products, such as clothing and liquid crystal displays. It would be useful to be able to prevent crystallization in polymers, and to understand the twisting process, or even stop deleterious crystallization processes. The chemists now hope to find further evidence that these mechanisms are brought about by internal stresses, and intend to examine mixed crystal structures and the form of the pointed needle tips as they grow.
Laurie Donaldson
‘White graphene’ to the rescue CARBON It is being suggested that white graphene may be the perfect solution for silicon as a new era unfolds in nanoscale electronics. The single-atom-thick layers of hexagonal boron nitride (h-BN), the material under intense study at Rice University’s Department of Mechanical Engineering and Materials Science, are likely to find some macro applications as well. Researchers across a number of laboratories have announced a method of producing sheets of h-BN, which could turn out to be the turning point in the large scale adoption of this wonder material. [Song et al., Nano Lett (2010) 10, 3209]. Graphene began the trend with its enhanced electronic properties, with new properties still being discovered today. Hexagonal boron nitride, on the other hand, is an insulator. Earlier this year, Rice postdoctoral researchers in Ajayan’s group found a way to implant islands of h-BN into sheets
White graphene. of graphene, a unique way to exert a level of control over the sheets electronic character. The team lead by primary author li Song has figured out how to deposit sheets of pure h-BN, which is naturally white in bulk form, anywhere from one to five atoms thick on a copper substrate. The material can then be transferred to other substrates. They used a chemical vapour deposition technique to grow the h-BN sheets on a 5 by 5 cm copper backing at temperatures of up to 1000 degrees Celsius. The
sheets could then be stripped from the copper and placed on a variety of substrates. Song sees h-BN sheets finding wide use as a highly effective insulator in graphene based electronics. Song also mentioned it should also be possible to draw microscopic patterns of graphene and h-BN, which could be useful in creating nanoscale field-effect transistors, quantum capacitors or biosensors. Strength tests using the tip of an atomic force microscope to push h-BN into holes in a silicon substrate showed it to be highly elastic and nearly as strong as graphene, the single-atom form of pure carbon. Song said the size of h-BN sheets is limited only by the size of the copper foil and furnace used to grow it. The process should be adaptable to the same kind of rollto-roll technique recently used to form 30 inch sheets of graphene.
Jonathan Agbenyega
SEPTEMBER 2010 | VOLUME 13 | NUMBER 9
9
A tale of twists and untwists CHARACTERIZATION A new study has created crystals that can both twist and untwist, revealing that process of crystal growth is more active than was previously thought. Although crystal growth is usually seen as a grouping of individual atoms, molecules or small clusters adding to a larger block that remains in a fixed translational relationship to the rest, a team of scientists from New York University and St. Petersburg State University in Russia have developed a crystal that continually changes its shape as it grows. The team, whose research is published in the Journal of the American Chemical Society [Shtukenberg, et al., J Am Chem Soc (2010) doi: 10.1021/ja101491n], examined crystals from hippuric acid, discovering that when molecules are added to the end of fine crystalline needles, stresses built up at the tips of the crystals, resulting in a helical twist, similar to the double helix in DNA. However, the process was reversed when crystals thickened from the other end of the growing tip, which undid the twisted formations, due to the elasticity of the crystals decreasing as they become thicker. Bart Kahr, a co-author of the study, said “This competition between twisting and untwisting creates needles with a rainbow of colors, which is a characteristic of tightly wound helices, as well as
Crystal growth. ribbons that have become completely untwisted. This is a very strange and new perspective on crystal growth.” With hippuric acid, the twisting is due to the presence of a decomposition product, which causes both mismatches and strain in lattices. If there is a lack of
symmetry of the systems, twists can occur, an internal stress mechanism that could easily apply to many other systems. The concept of helical crystals that twist like a drill bit originated in a book by Ferdinand Bernauer, “Gedrillte” Krystalle, published in the 1920s. Although forgotten for decades, the book was revived when helical crystals became topical again in the 1950s with the emergence of the synthetic polymer industry and the controversy over how and why high-polymers twist. The team investigated the book and the history of crystal growth in order to understand the crystallization process. It is hoped that this study will bring a greater understanding of high-polymers, which are used in a range of consumer products, such as clothing and liquid crystal displays. It would be useful to be able to prevent crystallization in polymers, and to understand the twisting process, or even stop deleterious crystallization processes. The chemists now hope to find further evidence that these mechanisms are brought about by internal stresses, and intend to examine mixed crystal structures and the form of the pointed needle tips as they grow.
Laurie Donaldson
‘White graphene’ to the rescue CARBON It is being suggested that white graphene may be the perfect solution for silicon as a new era unfolds in nanoscale electronics. The single-atom-thick layers of hexagonal boron nitride (h-BN), the material under intense study at Rice University’s Department of Mechanical Engineering and Materials Science, are likely to find some macro applications as well. Researchers across a number of laboratories have announced a method of producing sheets of h-BN, which could turn out to be the turning point in the large scale adoption of this wonder material. [Song et al., Nano Lett (2010) 10, 3209]. Graphene began the trend with its enhanced electronic properties, with new properties still being discovered today. Hexagonal boron nitride, on the other hand, is an insulator. Earlier this year, Rice postdoctoral researchers in Ajayan’s group found a way to implant islands of h-BN into sheets
White graphene. of graphene, a unique way to exert a level of control over the sheets electronic character. The team lead by primary author li Song has figured out how to deposit sheets of pure h-BN, which is naturally white in bulk form, anywhere from one to five atoms thick on a copper substrate. The material can then be transferred to other substrates. They used a chemical vapour deposition technique to grow the h-BN sheets on a 5 by 5 cm copper backing at temperatures of up to 1000 degrees Celsius. The
sheets could then be stripped from the copper and placed on a variety of substrates. Song sees h-BN sheets finding wide use as a highly effective insulator in graphene based electronics. Song also mentioned it should also be possible to draw microscopic patterns of graphene and h-BN, which could be useful in creating nanoscale field-effect transistors, quantum capacitors or biosensors. Strength tests using the tip of an atomic force microscope to push h-BN into holes in a silicon substrate showed it to be highly elastic and nearly as strong as graphene, the single-atom form of pure carbon. Song said the size of h-BN sheets is limited only by the size of the copper foil and furnace used to grow it. The process should be adaptable to the same kind of rollto-roll technique recently used to form 30 inch sheets of graphene.
Jonathan Agbenyega
SEPTEMBER 2010 | VOLUME 13 | NUMBER 9
9