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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