Nano Focus: Molecular ordering phenomenon found at interface between liquids and solids
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up made contact between a piece of highly polished sapphire surface and a puddle of liquid alkanol. The researchers shot a beam of high-intensity x-rays through the interface and by measuring how the x-rays reflected off the sample, the group revealed that the alkanol molecules at the sapphire surface behave very differently from those in the bulk liquid. According to ESRF scientist Diego Pontoni, “Surprisingly, the alkanol molecules form a perfect frozen monolayer at the sapphire interface at temperatures where the bulk is still liquid.” At sufficiently high temperatures, about 30°C above the melting temperature of the bulk alkanol, the monolayer also melts. The temperature range over which this frozen monolayer exists is about 10 times greater than what is observed at the liquid-vapor interfaces of similar materials. According to Alexei Tkachenko, a theoretical physicist who works at Brookhaven’s Center for Functional Nanomaterials, “The temperature range of the surface-frozen layer and its temperature-dependent thickness can be described by a very simple model that we developed. What is remarkable is that the surface layer does not freeze abruptly as in the case of ice, or any other crystal. Rather, a smooth transition occurs over a temperature range of several degrees.” Ocko said, “These films are better ordered and smoother than all other organic monolayer films created to date.” Deutsch said, “The results of this study and the theoretical framework which it provides may lead to new ideas on how to make defect-free, single molecule-thick films.”
application are often unsuitable for the other. Highly photoluminescent polymers such as polyfluorenes have long exciton lifetimes which should aid charge separation in an organic solar cell, but instead tend to give poor efficiencies. Recent work by Y.W. Soon and J.R. Durrant of Imperial College London and their co-workers suggests that one cause of this discrepancy may be a fast
energy transfer mechanism which competes with electron transfer to the acceptor material. The group’s article in the online edition of Chemical Science (DOI: 10.1039/ c0sc00606h) compares the photophysics of photovoltaic devices made from blends of the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and either of two indenofluorene-based poly-
Nano Focus
Molecular ordering phenomenon found at interface between liquids and solids
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Energy Focus Why strongly photoluminescent polymers make poor solar cells
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t first glance, the semiconducting polymers used in organic lightemitting diodes and photovoltaics may seem to require similar properties, and yet materials which work well for one
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MRS BULLETIN
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VOLUME 36 • MAY 2011
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www.mrs.org/bulletin
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