conductive substrate position-sensitive detector -V pulse 3-d Composition Mapping of Semiconductor Nanowires Semiconductor nanowires (NWs) are one- dimensional materials with unique electronic properties and are potential building blocks for new nanotechnologies. The composition of single NWs has not been determined to better than 1%, but impurity atoms strongly influence NW properties at much lower concentrations. For the first time, we have mapped the 3d composition of a NW with sub-nm resolution and single-atom sensitivity using atom-probe tomography (right). Single atoms were sequentially field-evaporated from the tip of a NW placed in a LEAP microscope. § The original position of the atoms in the NW was reconstructed using software to produce 3d visualizations of nanowire composition. Single impurity atoms and buried heterointerfaces were successfully imaged (next page). Lincoln J. Lauhon, MSE, Northwestern University, DMR *, DMR § *CAREER: Nanoscale Composition Control and Characterization of One-Dimensional Semiconductor Nanostructures § Acquisition of a Local-Electrode Atom-Probe (LEAP) Microscope (An Atom-Probe Tomograph) for Three- Dimensional Nanoscale Characterization of Materials, P.I. David Seidman, co-P.I. Lincoln Lauhon. Local Electrode Atom-Probe (LEAP) Microscope local electrode nanowire array

Catalyst-nanowire interface 3-d Composition Mapping of Semiconductor Nanowires 23x14x14 nm 3 10 nm dia X 1 nm thick slices 23x14x14 nm 3 The positions of single gold atoms within an InAs nanowire are visualized below. Only 2% of In and As atoms are shown to highlight the gold. The gold atoms are trapped in the nanowire during the growth, which is directed by a liquid gold catalyst (at right). Au In As 29 nm tall x 27 nm dia. Single impurity atoms Perea et al, submitted to Nano Letters.

3-d Composition Mapping of Semiconductor Nanowires *CAREER: Nanoscale Composition Control and Characterization of One-Dimensional Semiconductor Nanostructures § Acquisition of a Local-Electrode Atom-Probe (LEAP) Microscope (An Atom-Probe Tomograph) for Three- Dimensional Nanoscale Characterization of Materials, P.I. David Seidman, co-P.I. Lincoln Lauhon. Lincoln J. Lauhon,MSE, Northwestern University, DMR *, DMR § Prof. Lauhon is the faculty director of the Research Experience for Science Teachers (REST) program at Northwestern. He recently gave a presentation on nanoscale composition mapping to REST participants as part of a professional development workshop organized by the Nanoscale Science and Engineering Center for Learning and Teaching (NCLT). A video of the presentation will be available on the NCLT website pending publication of the research results. Outreach Education & Training Daniel Perea, a first year grad student of Mexican descent, performed the atom-probe research shown here and has submitted a manuscript to Nano Letters. In October, he will present his work at the annual conference of the Society for the Advancement of Chicanos and Native Americans in Science (SACNAS). Danny was one of two students selected to represent Northwestern University at this conference. Danny Perea