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  • richardmitnick 3:16 pm on May 5, 2014 Permalink | Reply
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    From Berkeley Lab: “… Better Doping of Semiconductor Nanocrystals” 

    Berkeley Lab

    The icing on the cake for semiconductor nanocrystals that provide a non-damped optoelectronic effect may exist as a layer of tin that segregates near the surface.

    One method of altering the electrical properties of a semiconductor is by introducing impurities called dopants. A team led by Delia Milliron, a chemist at Berkeley Lab’s Molecular Foundry, a U.S Department of Energy (DOE) national nanoscience center, has demonstrated that equally important as the amount of dopant is how the dopant is distributed on the surface and throughout the material. This opens the door for engineering the distribution of the dopant in order to control what wavelength the material will absorb and more generally how light interacts with the nanocrystals.

    Schematic representation of plasmonic nanocrystals with (a) uniform and (b) surface-segregated dopant distributions. In (a), most of the electron cloud is scattered from ionized impurities (green); in (b), most of the electron cloud is oscillating away from the impurities.

    “Doping in semiconductor nanocrystals is still an evolving art,” says Milliron. “Only in the last few years have people begun to observe interesting optical properties as a result of introducing dopants to these materials, but how the dopants are distributed within the nanocrystals remains largely unknown. What sites they occupy and where they are situated throughout the material greatly influences optical properties.”

    Milliron’s most recent claim to fame, a “smart window” technology that not only blocks natural infrared (IR) radiation while allowing the passage of visible light through transparent coated glass, but also allows for independent control over both kinds of radiation, relies on a doped semiconductor called indium tin oxide (ITO).

    (From left) Amy Bergerud, Evan Runnerstrom, Delia Milliron and Sebastien Lounis were part of a team at Berkeley Lab’s Molecular Foundry that demonstrated the importance of dopant distribution in semiconductors. (Photo by Tracy Mattox)

    ITO, in which tin (the dopant) has replaced some of the indium ions in indium oxide (the semiconductor), has become the prototypical doped semiconductor nanocrystal material. It is used in all kinds of electronic devices, including touchscreens displays, smart windows and solar cells.

    “The exciting thing about this class of materials is that the dopants are able to introduce free electrons that form at high density within the material, which makes them conducting and thus useful as transparent conductors,” says Milliron

    But the same electrons cause the materials to be plasmonic in the IR part of the spectrum. This means that light of IR wavelength can be resonant with free electrons in the material: the oscillating electric fields in the light resonate and can cause absorption.

    “[These materials] can absorb IR light in a way that’s tunable by adjusting the doping, while still being transparent to natural visible light. A tunable amount of absorption of IR light allows you to control heating. For us, that’s the driving application,” explains Milliron.

    Until now, adjustments have been made by changing the amount of dopant in the semiconductor. Puzzled by studies in which optical properties did not behave as expected, Milliron and University of California (UC) Berkeley PhD candidate Sebastien Lounis looked to x-ray photoelectron spectroscopy to probe electrons near the surface of the ITO samples and investigate the distribution of elements within the samples at the Stanford Synchrotron Radiation Lightsource (SSRL).

    The SSRL uses a tuneable beam of photons to excite electrons inside the material. If the electrons are close enough to the surface, they can sometimes be emitted and collected by a detector. These electrons provide information about the properties of the material, including the ratio of the amounts of different elements like indium and tin in ITO. Increasing the energy of the x-ray beam shows how the composition of tin and indium changes as one moves deeper into the sample. Ultimately, the spectroscopy technique allowed Milliron and her team to probe the doping distribution as a function of distance from the nanocrystals’ surface.

    Studies of two sets of samples allowed them to correlated tin distribution with optical properties, and showed that the shape and wavelength of plasmon absorption depended on tin distribution. The tin segregated on the surface showed reduced activation of dopants and symmetric plasmon resonances, with no damping caused by the dopants.

    “When the tin sits near the surface, it interacts only weakly with the majority of the free electrons,” explains Lounis. “This gives us the benefits of doping without some of drawbacks.”

    “Now that we know how to probe, we can go after targeted design features for particular applications,” concludes Milliron. Deliberate placement of dopants by design provides a new tool for “dialing in plasmonic materials to do exactly what we want in terms of interaction with light.”

    A paper on this research has been accepted for publication in the Journal of the American Chemical Society (JACS) in April 2014. The paper is titled The influence of dopant distribution on the plasmonic properties of indium tin oxide nanocrystals with Lounis as the lead author and Milliron as the corresponding author. Other authors are Evan Runnerstorm, Amy Bergerud, and Dennis Nordlund.

    This research was primarily supported by the DOE Office of Science.

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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  • richardmitnick 1:55 pm on February 28, 2013 Permalink | Reply
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    From Berkeley Lab: “Engineering Bacterial Live Wires” 

    Berkeley Lab

    February 28, 2013
    Lita Stephenson

    Just like electronics, living cells use electrons for energy and information transfer. Despite electrons being a common ‘language’ of the living and electronic worlds, living cells cannot speak to our largely technological realm. Cell membranes are largely to blame for this inability to plug cells into our computers: they form a greasy barrier that tightly controls charge balance in a cell. Thus, giving a cell the ability to communicate directly with an electrode would lead to enormous opportunities in the development of new energy conversion techniques, fuel production, biological reporters, or new forms of bioelectronic systems.

    Previous studies performed by scientists and collaborators at Lawrence Berkeley National Laboratory’s (Berkeley Lab) Molecular Foundry have made enormous headway toward cellular-electrode communication by using E. coli as a testbed for expressing an electron transfer pathway naturally occurring in a bacterial species called Shewanella oneidensis MR-1. The engineered E. coli was able to use the protein complex to reduce nanocrystalline iron oxide (Jensen, et al. (2010) PNAS.). Building off of this research, a group led by Caroline Ajo-Franklin, a staff scientist in the Biological Nanostructures Facility at Berkeley Lab’s Molecular Foundry studying synthetic biology, has now demonstrated that these engineered E. coli strains can generate measurable current at an anode.

    The results of this new study, Tuning promoter strengths for improved synthesis and function of electron conduits in Escherichia coli, have recently been published in ACS Synthetic Biology, the American Chemical Society’s new flagship journal for synthetic biology.

    Authors of the recent publication in the Biological Nanostructures Laboratory. From left to right: Caroline Ajo-Franklin, Heather Jensen, Matt Hepler, Cheryl Goldbeck

    See the full article here.

    A U.S. Department of Energy National Laboratory Operated by the University of California


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  • richardmitnick 7:58 pm on March 1, 2012 Permalink | Reply
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    From Berkeley Lab: “Solved: The Mystery of the Nanoscale Crop Circles” 

    Berkeley Lab

    In strange patterns of a gold-silicon alloy, Berkeley Lab scientists uncover unsuspected secrets and promising routes to nanoscale semiconductor processing.

    March 01, 2012
    Paul Preuss

    “Almost three years ago a team of scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) was performing an experiment in which layers of gold mere nanometers (billionths of a meter) thick were being heated on a flat silicon surface and then allowed to cool. They watched in surprise as peculiar features expanded and changed on the screen of their electron microscope, finally settling into circles surrounded by irregular blisters.

    When a thin layer of gold anneals on top of a silicon wafer coated with native silicon oxide, randomly distributed pools of eutectic alloy quickly form – and then go through a rapid series of strange changes, leaving behind bare silicon-dioxide circles surrounded by debris. Each denuded circle reveals a perfect square at its center. The area shown is about 107 by 155 micrometers (millionths of a meter) No image credit.

    Until recently the cause of these strange formations remained a mystery. Now theoretical insights have explained what’s happening, and the results have been published online by Physical Review Letters at http://prl.aps.org/abstract/PRL/v108/i9/e096102.

    See the full article here.

    A US Department of Energy National Laboratory Operated by the University of California


  • richardmitnick 9:07 pm on December 8, 2011 Permalink | Reply
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    From Berkeley Lab: “Nanocrystals Go Bare” 

    Berkeley Lab

    Berkeley Lab Researchers Strip Material’s Tiny Tethers

    DECEMBER 08, 2011
    Aditi Risbud

    “Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a universal technique for stripping nanocrystals of tether-like molecules that until now have posed as obstacles for their integration into devices. These findings could provide scientists with a clean slate for developing new nanocrystal-based technologies for energy storage, photovoltaics, smart windows, solar fuels and light-emitting diodes.

    Nanocrystals are typically prepared in a chemical solution using stringy molecules called ligands chemically tethered to their surface. These hydrocarbon-based or organometallic molecules help stabilize the nanocrystal, but also form an undesirable insulating shell around the structure. Efficient and clean removal of these surface ligands is challenging and has eluded researchers for decades.

    Now, using Meerwein’s salt—an organic compound also known by its tongue twisting moniker triethyloxonium tetrafluoroborate—a Berkeley Lab team has stripped away organic ligands tethered to nanocrystals, exposing a bare surface enabling nanocrystals to be used in a variety of applications.

    ‘Our technique basically allows you to take any nanocrystal—metal oxides, metallic, semiconductors—and turn these into dispersions of ligand-free nanocrystal inks for spin or spray coating and even patterning using an ink jet printer,’ says Brett Helms, a staff scientist in the Organic and Macromolecular Synthesis Facility at Berkeley Lab’s Molecular Foundry, a nanoscience research center. ‘What’s more, they retain their structural integrity and exhibit more efficient transport properties in devices.'”

    Vials of ligand-free nanocrystals dispersed in solution for various applications, including energy storage, smart windows and LEDs.

    From left, Brett Helms, Evelyn Rosen, Raffaella Buonsanti, Delia Milliron and Anna Llordes at Berkeley Lab’s Molecular Foundry with vials of bare surface nanocrystals. (Photo by Roy Kaltschmidt, Berkeley Lab)

    See the full article here.

    A US Department of Energy National Laboratory Operated by the University of California


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