- Email: [email protected]
Cu promises cheaper solar cells
RESEARCH NEWS
How Au goes noble MODELLING AND SIMULATION Au nanoclusters have found use in many contexts, but their structure and electronic properties have until recently remained elusive. Nanoparticles made of S and Au are known to occur in certain highly symmetric configurations, and there has long been a suspicion that there is a ‘divide and protect’ motif in the structures: a core of Au atoms protected by a bound S and Au ‘shell’. Last year, a group from Stanford University published high-resolution structures of a 102-atom Au cluster [Jadzinsky et. al., Science, doi: 10.1126/science.1148624] showing such a structure. Now, one of the Stanford researchers has joined others from the University of Jyväskylä in Finland, Institut für Festkörperforschung in Germany, Georgia Institute of Technology in the US and Chalmers University in Sweden to put the idea on firmer theoretical grounds. The team have employed supercomputers to solve the electronic structures of nanoclusters of varying sizes [Walter et. al., Proc. Nat. Acad. Sci. (2008) 105, 9157]. They found that the stability of the clusters rely on how the Au atoms donate their electrons; the number of valence electrons for each structure corresponds to closed shells like those found in noble gases. In the 102-atom cluster case, each atom donates one valence electron, 44 of which were tied up in thiolate bonds—leaving a neat 58 to form a full valence shell.
D. Jason Palmer Publisher’s note The author of the Opinion article ‘The Disappearance of Louis le Prince’ [Materials Today, 11 (7/8), 48] wishes to state that this was a work of fiction, and submitted to the writing competition as a creative writing story.
Corrugations cap the Casimir force NANOTECHNOLOGY
As devices get ever smaller, nanotechnologists’ concerns about the forces that come into play at the nanoscale grow ever larger. The Casimir force, one such concern, is a purely quantum mechanical effect. Two closely spaced plates constrain the wavelengths of the virtual particles that pop into and out of the vacuum between them, resulting in a net force outside the plates where the wavelengths are not limited. The plates thus appear to attract one another. A collaboration between researchers from the University of Florida and Bell Laboratories in the US has now shown that the magnitude of the Casimir force can be manipulated by changing the surface structure of the plates [Chan et. al., Phys. Rev. Lett. (2008) 101, 030401]. The team prepared highly doped Si plates either smooth or with one micron deep, 200 nanometer wide trenches every 400 nm – reducing the plate’s effective surface area by half. To measure the minuscule Casimir force, the team used tiny Au spheres suspended from a micromechanical torsional oscillator. The experiment measures the shift in the resonant frequency of the oscillator as an
indirect measure of the influence of the corrugated plate on the spheres. The gradient of the force, as a function of distance between plate and sphere, can be compared between the corrugated and flat plates. What the team found was that the measured Casimir effect was indeed lower for the corrugated plates—but only by 30 to 40%, not by the 50% that was expected from surface area considerations. The authors suggest that this deviation belies a complex geometrydependent theory for the effect. While earlier experiments have shown the effect of surface composition on the lateral Casimir force (that is, along the plates rather than between them), these are the first results showing the effects on the normal Casimir force. Much theoretical work remains to devise an analytical expression for the effect as a function of surface structure. But being able to manipulate the effect for the benefit of micro- and nanoelectromechanical systems may leave nanotechnologists with one less concern. D. Jason Palmer
Cu promises cheaper solar cells ENERGY GENERATION MATERIALS Over the last few years, scientists have been working hard to develop dye-sensitized solar cells (DSSCs) that are both cheaper and easier to manufacture than conventional photovoltaic cells. To date, the best results in this field were based on organic compounds of Ru, unfortunately a rather expensive element. Now, researchers from the University of Basel and the École Polytechnique Fédérale de Lausanne in Switzerland have synthesized complexes using oligopyridine ligands and the much cheaper element, Cu, to produce a material that can be used as a dye-stuff in DSSCs [Bessho et al., Chem. Commun., doi: 10.1039/b808491b]. Whereas in conventional photovoltaic systems both light absorption and charge carrier transport occur in the same material, these two tasks are accomplished by separate substances in DSSCs. The sensitizer – a metal-organic dye-stuff – absorbs photons and releases electrons. The semiconductor only provides the potential barrier to separate the charges and conduct the electrons in order to create a current. Since both systems need to interact closely, layers of sensitizer molecules are coated on nanocrystalline particles of the semiconductor, TiO2. This arrangement makes production much easier, and is suited especially for thin-film solar cells. Presently, state-of-the-art Ru-based DSSCs possess conversion efficiencies of about 11% (the values for common low-cost Si
Schematic of a dye-sensitized solar cell. (Courtesy of Edwin Constable.)
cells range from 12 and 15%, flexible thin-film cells operate at around 8%). Their newly-developed Cu-counterparts do not reach that value, but further development and refinement may lead to Cu-DSSCs with at least comparable efficiencies. “A number of factors are in favour of the copper systems,” says Edwin Constable. “The ideal is a lower price and higher efficiency,” he explains.
Michel Fleck
SEPTEMBER 2008 | VOLUME 11 | NUMBER 9
9
How Au goes noble MODELLING AND SIMULATION Au nanoclusters have found use in many contexts, but their structure and electronic properties have until recently remained elusive. Nanoparticles made of S and Au are known to occur in certain highly symmetric configurations, and there has long been a suspicion that there is a ‘divide and protect’ motif in the structures: a core of Au atoms protected by a bound S and Au ‘shell’. Last year, a group from Stanford University published high-resolution structures of a 102-atom Au cluster [Jadzinsky et. al., Science, doi: 10.1126/science.1148624] showing such a structure. Now, one of the Stanford researchers has joined others from the University of Jyväskylä in Finland, Institut für Festkörperforschung in Germany, Georgia Institute of Technology in the US and Chalmers University in Sweden to put the idea on firmer theoretical grounds. The team have employed supercomputers to solve the electronic structures of nanoclusters of varying sizes [Walter et. al., Proc. Nat. Acad. Sci. (2008) 105, 9157]. They found that the stability of the clusters rely on how the Au atoms donate their electrons; the number of valence electrons for each structure corresponds to closed shells like those found in noble gases. In the 102-atom cluster case, each atom donates one valence electron, 44 of which were tied up in thiolate bonds—leaving a neat 58 to form a full valence shell.
D. Jason Palmer Publisher’s note The author of the Opinion article ‘The Disappearance of Louis le Prince’ [Materials Today, 11 (7/8), 48] wishes to state that this was a work of fiction, and submitted to the writing competition as a creative writing story.
Corrugations cap the Casimir force NANOTECHNOLOGY
As devices get ever smaller, nanotechnologists’ concerns about the forces that come into play at the nanoscale grow ever larger. The Casimir force, one such concern, is a purely quantum mechanical effect. Two closely spaced plates constrain the wavelengths of the virtual particles that pop into and out of the vacuum between them, resulting in a net force outside the plates where the wavelengths are not limited. The plates thus appear to attract one another. A collaboration between researchers from the University of Florida and Bell Laboratories in the US has now shown that the magnitude of the Casimir force can be manipulated by changing the surface structure of the plates [Chan et. al., Phys. Rev. Lett. (2008) 101, 030401]. The team prepared highly doped Si plates either smooth or with one micron deep, 200 nanometer wide trenches every 400 nm – reducing the plate’s effective surface area by half. To measure the minuscule Casimir force, the team used tiny Au spheres suspended from a micromechanical torsional oscillator. The experiment measures the shift in the resonant frequency of the oscillator as an
indirect measure of the influence of the corrugated plate on the spheres. The gradient of the force, as a function of distance between plate and sphere, can be compared between the corrugated and flat plates. What the team found was that the measured Casimir effect was indeed lower for the corrugated plates—but only by 30 to 40%, not by the 50% that was expected from surface area considerations. The authors suggest that this deviation belies a complex geometrydependent theory for the effect. While earlier experiments have shown the effect of surface composition on the lateral Casimir force (that is, along the plates rather than between them), these are the first results showing the effects on the normal Casimir force. Much theoretical work remains to devise an analytical expression for the effect as a function of surface structure. But being able to manipulate the effect for the benefit of micro- and nanoelectromechanical systems may leave nanotechnologists with one less concern. D. Jason Palmer
Cu promises cheaper solar cells ENERGY GENERATION MATERIALS Over the last few years, scientists have been working hard to develop dye-sensitized solar cells (DSSCs) that are both cheaper and easier to manufacture than conventional photovoltaic cells. To date, the best results in this field were based on organic compounds of Ru, unfortunately a rather expensive element. Now, researchers from the University of Basel and the École Polytechnique Fédérale de Lausanne in Switzerland have synthesized complexes using oligopyridine ligands and the much cheaper element, Cu, to produce a material that can be used as a dye-stuff in DSSCs [Bessho et al., Chem. Commun., doi: 10.1039/b808491b]. Whereas in conventional photovoltaic systems both light absorption and charge carrier transport occur in the same material, these two tasks are accomplished by separate substances in DSSCs. The sensitizer – a metal-organic dye-stuff – absorbs photons and releases electrons. The semiconductor only provides the potential barrier to separate the charges and conduct the electrons in order to create a current. Since both systems need to interact closely, layers of sensitizer molecules are coated on nanocrystalline particles of the semiconductor, TiO2. This arrangement makes production much easier, and is suited especially for thin-film solar cells. Presently, state-of-the-art Ru-based DSSCs possess conversion efficiencies of about 11% (the values for common low-cost Si
Schematic of a dye-sensitized solar cell. (Courtesy of Edwin Constable.)
cells range from 12 and 15%, flexible thin-film cells operate at around 8%). Their newly-developed Cu-counterparts do not reach that value, but further development and refinement may lead to Cu-DSSCs with at least comparable efficiencies. “A number of factors are in favour of the copper systems,” says Edwin Constable. “The ideal is a lower price and higher efficiency,” he explains.
Michel Fleck
SEPTEMBER 2008 | VOLUME 11 | NUMBER 9
9