Manipulating Topological Domain Boundaries in the Single-layer Quantum Spin Hall Insulator 1T’–WSe_2

We report the creation and manipulation of structural phase boundaries in the single-layer quantum spin Hall insulator 1T′–WSe_2 by means of scanning tunneling microscope tip pulses. We observe the formation of one-dimensional interfaces between topologically nontrivial 1T′ domains having different rotational orientations, as well as induced interfaces between topologically nontrivial 1T′ and topologically trivial 1H phases. Scanning tunneling spectroscopy measurements show that 1T′/1T′ interface states are localized at domain boundaries, consistent with theoretically predicted unprotected interface modes that form dispersive bands in and around the energy gap of this quantum spin Hall insulator. We observe a qualitative difference in the experimental spectral line shape between topologically “unprotected” states at 1T′/1T′ domain boundaries and protected states at 1T′/1H and 1T′/vacuum boundaries in single-layer WSe_2

Bottom-up graphene nanoribbon field-effect transistors

Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nano-scale chemically synthesized GNR field-effect transistors, made possible by development of a reliable layer transfer process. We observe strong environmental sensitivity and unique transport behavior characteristic of sub-1 nm width GNRs. © 2013 AIP Publishing LLC.Research was supported by the Office of Naval Research BRC Program, by the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and by National Science Foundation award DMR-1206512. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Peer Reviewe

Direct imaging of covalent bond structure in single-molecule chemical reactions

Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2- ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways.Supported by the Office of Naval Research BRC Program (molecular synthesis, characterization, and STM imaging); the Helios Solar Energy Research Center supported by the Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC02-05CH11231 (STM and nc-AFM instrumentation development, AFM operation); NSF grant DMR-1206512 (image analysis); and European Research Council advanced grant DYNamo ERC-2010-AdG-267374 (ab initio calculations). Computing time was provided by the Barcelona Supercomputing Center “Red Española de Supercomputacion.” D.G.d.O. acknowledges fellowship support by the European Union under FP7-PEOPLE-2010-IOF-271909, A.R. by Austrian Science Fund (FWF) grant J3026-N16, and D.J.M. by the Spanish “Juan de la Cierva” program ( JCI-2010-08156).Peer Reviewe

Closing the Nanographene Gap: Surface-Assisted Synthesis of Peripentacene from 6,6'-Bipentacene Precursors.

The thermally induced cyclodehydrogenation reaction of 6,6'-bipentacene precursors on Au(111) yields peripentacene stabilized by surface interactions with the underlying metallic substrate. STM and atomic-resolution non-contact AFM imaging reveal rectangular flakes of nanographene featuring parallel pairs of zig-zag and armchair edges resulting from the lateral fusion of two pentacene subunits. The synthesis of a novel molecular precursor 6,6'-bipentacene, itself a synthetic target of interest for optical and electronic applications, is also reported. The scalable synthetic strategy promises to afford access to a structurally diverse class of extended periacenes and related polycyclic aromatic hydrocarbons as advanced materials for electronic, spintronic, optical, and magnetic devices