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Holographic Optical Traps Manipulateand Assemble Multiple Nanowires
Nanophotonics
Doping the wide-bandgap semiconductor with transition metal ions wasconsidered as a potential solution,but doing so during the nucleation orgrowth phases of the synthesisprocess for trillions of nanocrystalsin solution had proved difficult, Pengexplained.
Decoupling the doping step fromone or both of these phases appearsto be the solution. The scientists havedeveloped two approaches: nucleation-doping and growth-doping.
In the former, the dopant and hostprecursors are mixed during nucleation. The growth phase then is performed under slightly altered conditions such that the dopant precursors become inactive and the hostovercoats the doped cores.
In the latter, nucleation takes placeas in the standard synthesis process,and growth is permitted to begin.After the formation of small hostnanocrystals, however, the reactiontemperature is reduced to stop thegrowth phase. The doping step is per-
formed, and host growth is subsequently reinitiated.
At this point, Peng said, the researchers can produce doped ZnSenanocrystals that emit at between450 and 600 nm, although workmust be done to narrow their emission bands. The structures are environmentally and thermally stable,displaying no substantial quenchingof emission upon exposure to air,pyridine, thiols, ultraviolet radiationand temperatures of more than 200°C. Reabsorption and energy trans-
Inthe nanoworld, loads of devicescan be packed into a small area.But to get them there, these tiny
objects must be organized into astructure, which is not easy to dowith lithographic methods or evenwith optical tweezers. To address this
fer do not appear to be an issue inthe doped structures as a result oftheir relatively large Stokes shift.
The scientists will explore the relationship between the synthesisprocess and the performance of thedoped nanocrystals and will work todevelop other hosts and dopants.They also are curious to investigatehow the nanocrystals will performin various devices. Peng said. 0
Daniel S. BurgessJournal of the American Chemical Society.
Dec. 21. 2005. pp. 17586-17587.
challenge, researchers at HarvardUniversity in Cambridge, Mass., andat New York University have developed a holographic approach tonanoassembly that enables the simultaneous manipulation of multiple nanowires.
JANUARY 2006
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Nanophotonics
Nanowires can be cut (a), bent and fused (b) with intense, focused beams of light.Courtesy of David G. Grier.
The scientists used a holographictrapping technique that they originally demonstrated by shining abeam of light through an inexpensive toy that they had ordered froma scientific surplus catalog. Whatwas originally a 4 X 4 array generator turned out to be effective for creating arrays of optical traps, explained David G. Grier. a member of
the team from New York University.Mter that, they turned to computer
holography and started projectingsequences of holograms with spatiallight modulators. producing severalhundred traps from a single beamof light. They have patented this technique. which forms the basis of theproduct offerings of Arryx Inc. inChicago.
In their current work. the researchers had the goal of trapping20- to 100-nm-diameter semiconductor nanowires using 400-nmlight. "[It) is like trying to grabspaghetti with an oven mitt," Griersaid.
To prevent the nanowires fromsticking to any surface. they used achemical stabilization process. Getting the structures to stay in placeafter stabilization became a challenge, however. so they also employedoptical and chemical destabilizationto make the nanowires stick wherethey wanted them.
Using the holographic trappingmethod, they trapped 10 to 20 nanowires at a time and manipulated thestructures in 3-D relative to eachother into interesting configurations.It worked so well that their next stepis to understand why. so they canoptimize it. 0
Anne L. FischerOptics Express. Oct. 31. 2005. pp. 89068912.
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