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RESEARCH/RESEARCHERS Advanced Techniques Enable the Growth of One-Dimensional Nanowire Heterostructures Recent advances in nanoscale research have resulted in a number of techniques for growing one-dimensional semiconducting nanowire heterostructures, or superlattices. While previous work on semiconducting nanotubes and nanowires dealt largely with homogeneous systems or with heterojunctions between these systems, there has been a strong drive toward realizing one-dimensional (1D) compositionally modulated heterostructures, that is, nanowires in which segments of different composition can be formed in a controlled fashion. Working

independently toward this goal, three teams of researchers—from Harvard University, Lund University in Sweden, and the Lawrence Berkeley National Laboratory—have recently reported the synthesis of new multi-heterostructures. In the February 7 issue of Nature, Mark S. Gudiksen and co-workers at Harvard demonstrated the growth of nanowire superlattices from Group III–V materials using laser-assisted catalytic growth and from Group IV materials using chemical vapor deposition (CVD); in both cases, Au nanoclusters served as the catalyst and the point at which the vapor-phase reactants were added to the growing nanowires and nucleation occurred.

Reported in the February 13 issue of Nano Letters, Yiying Wu and co-workers from UC—Berkeley describe the formation of single-crystalline Si/SiGe nanowire superlattices through a pulsed laser ablation/chemical vapor deposition (PLACVD) technique. Featured in the same issue of Nano Letters, M.T. Björk and coworkers (Lund University) report a vapor–liquid–solid growth method in which a Au nanoparticle is used to catalytically induce growth of 1D InAs/InP nanowhisker heterostructures. In order to fabricate GaAs/GaP superlattices, Gudiksen and co-workers laserablated GaAs and GaP solid targets to introduce the vapor-phase reactants necessary for nanowire growth. To create multiple (1–20) junctions, or GaAs/GaP interfaces, along the wire, the different reactants were periodically introduced during the synthesis process. Transmission electron microscopy (TEM) data confirmed defect-free crystalline nanowire cores and suggested that the GaP/GaAs junctions were abrupt. However, the researchers showed, through x-ray spectroscopy composition mapping, that the transition between GaAs and GaP phases occurs over a length scale of 15–20 nm. By varying the lengths of alternating segments of GaAs (direct bandgap) and GaP (indirect bandgap) along the nanowires, optical “nanobarcodes” were produced. In addition, they fabricated p-n junctions along Si nanowires by Au-nanoclustercatalyzed CVD and dopant modulation without the need for any lithographic steps. Polarized nanoscale light-emitting diodes (LEDs), with emission wavelengths tuned by the nanowire diameter, were created using similar p-n structures in InP nanowires. Operating in an ultrahigh-vacuum chemical-beam-epitaxy chamber, Björk and co-workers supplied Group V precursor atoms as molecular beams to a eutectic