Newsletter, December 2014

     
    MIT Materials News that Matters

    December 2014
     
     
    Materials Processing Center at MIT MIT Dome
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          Happy Holidays from everyone at the
    Materials Processing Center!                  
    2015 Summer Scholars Registration Now Open
     
    2014 CMSE-MPC Interns
    2014 Summer Scholars

    The Materials Processing Center and the Center for Materials Science and Engineering jointly run the nine-week Summer Scholars research program for undergraduate students at MIT with funding from the National Science Foundation. This next internship program will run from June 8 to August 8, 2015.

    The application deadline is February 13, 2015.

    Faculty Highlight: William A. Tisdale
    Understanding and controlling how energy moves in short bursts in nanostructured materials such as quantum dots motivates Chemical Engineering Professor William A. Tisdale.
    William A. Tisdale, Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT, has found surprising phenomena exhibited by excitons in quantum dots.
    William A. Tisdale, Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT, has found surprising phenomena exhibited by excitons in quantum dots

    Extremely fast movement of energy over extremely short distances underlies the performance of solar cells, LEDs and thermoelectric devices, but different devices demand different, and sometimes opposite, qualities. In the tiny world of nanostructured materials, William A. Tisdale, Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT, has found some surprising phenomena at work.

    Using ultrasensitive spectroscopy and other techniques, Tisdale's research group studies how excitons, which are paired groups of electrons and holes, behave in quantum dots. They measure minute changes that happen as quickly as 2 billionths of a second and cover a distance as short as several nanometers, or several billionths of a meter. Quantum dots are nanoscale crystals made up about of about 1,000 atoms each, and they have unique properties.

      

    Seemingly infinitesimal differences can make a big difference in device performance because kinetics, the rate at which reactions occur, can determine which of several possible outcomes will happen. Chemical engineers use the same mathematical framework to analyze dynamics of excited states in quantum dots as they use to analyze networks of chemical reactions. "The fastest thing that can happen is the most probable thing to happen," Tisdale explains. "Engineering nanoscale materials to do what you want them to do is all about getting the things you want to happen, to happen quickly, and trying to slow down as much as possible the bad things, so that's really the name of the game in all of this work we're doing."

    Shedding New Light on Quantum Dots
    MIT chemistry graduate student A. Jolene Mork examines rate of excitonic energy transfer.
    MIT chemistry graduate student A. Jolene Mork holds bottles of fluorescent quantum dots.
    MIT chemistry graduate student A. Jolene Mork holds bottles of fluorescent quantum dots.
    Energy transfer in light sensitive materials such as quantum dots is of interest for better solar cells, LEDs and other devices. MIT chemistry graduate student A. Jolene Mork examined how fast energy transfers from one quantum dot to another, a phenomenon known as hopping.

    Mork is lead author of a Journal of Physical Chemistry paper that analyzed energy transfer in colloidal quantum dots. "It's not looking at how far can an exciton go within a film; it is how fast does it transfer from one quantum dot to another," she says. Mork is a fifth-year MIT graduate student in the lab of William A. Tisdale,  Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT.

     

    A standard formula for calculating the rate of energy transfer for molecules, Förster theory, may be inadequate to explain excitons moving from one quantum dot to another, Mork's study reveals. "The standard assumption is that it's center-to-center distance that matters. So we think the edge-to-edge distance may also matter," Mork says.

     
    Read more. 
    Calling Quantum Dots to Order
    MIT chemical engineering graduate student Mark C. Weidman and colleagues demonstrate how to synthesize lead sulfide nanocrystals of uniform size. 
    MIT chemical engineering graduate student Mark C. Weidman and colleagues have demonstrated how to synthesize lead sulfide nanocrystals of uniform size.
    MIT chemical engineering graduate student Mark C. Weidman and colleagues have demonstrated how to synthesize lead sulfide nanocrystals of uniform size.

    Lead sulfide nanocrystals suitable for solar cells have a nearly one-to-one ratio of lead to sulfur atoms, but MIT researchers discovered that to make uniformly sized quantum dots, a higher ratio of lead to sulfur precursors - 24-to-1 - is better.

    MIT chemical engineering graduate student Mark C. Weidman developed the synthetic recipe in the lab of William A. Tisdale, Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT, with colleagues Ferry Prins, Rachel S. Hoffman and 2013 Summer Scholar Megan Beck. Uniformity of size can promote long exciton diffusion lengths in (PbS) quantum dot films, Weidman says. 

     
    Summer Scholars Make Impact
    Beck, Arveson contribute as interns to research in Tisdale Lab.
    2013 MPC-CMSE Summer Scholar Megan Beck worked on synthesizing lead sulfide (PbS) quantum dots Synthesis in the lab of William A. Tisdale, the Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT.
    2013 MPC-CMSE Summer Scholar Megan Beck worked on synthesizing lead sulfide (PbS) quantum dots Synthesis in the lab of William A. Tisdale, the Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT.

    MPC-CMSE Summer Scholars Megan Beck (2013) and Sarah Arveson (2014) worked in the Tisdale Lab as interns, drawing praise for their contributions from William A. Tisdale{link to profile}, the Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT.

    * Beck was co-author of a paper with fourth-year MIT chemical engineering graduate student Mark C. Weidman on controlled synthesis of ordered lead sulfide quantum dots.

    * Arveson worked with former Postdoctoral Associate Pooja Tyagi on excitonic properties of organometal halide perovskites. A paper is in preparation.

    IN OTHER NEWS

    New findings could point

    the way to "valleytronics"

    Researchers clear hurdles toward a new kind of 2-D microchip using different electron properties.


     

    Schematic of the Floquet-driven chiral edge state along the boundary of the laser-exposed region. Credit, Courtesy of the researchers.
    Schematic of the Floquet-driven chiral edge state along the boundary of the laser-exposed region. Credit, Courtesy of the researchers.
     

    David L. Chandler

    MIT News Office

    New findings from a team at MIT and other institutions could provide a pathway toward a kind of two-dimensional microchip that would make use of a characteristic of electrons other than their electrical charge, as in conventional electronics. The new approach is dubbed "valleytronics," because it makes use of properties of an electron that can be depicted as a pair of deep valleys on a graph of their traits.  

    Read more. 

     
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    The goals of the Materials Processing Center are to unite the materials research community at MIT and to enhance Institute-industry interactions. Collaboration on research ventures, technology transfer, continuing education of industry personnel, and communication among industrial and governmental entities are our priorities. The MPC 
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