Extreme Nanowires: The Smallest Crystals in the Smallest Nanotubes Authors: Jeremy Sloan (1), Reza Kashtiban (1), Sam Marks (1), Richard Beanland (1), Ana Sanchez (1), Sam Brown (1), Andrij Vasylenko (1), Peter Brommer (1), Krzysztof Morawiec (2), Slawomir Kret (2), Paulo Medeiros (3), James Wynn (3), Joe Spencer (4), David Smith (4), Quentin Ramasse (5), Zheng Liu (6), Kazu Suenaga (6), Andrew Morris (3), David Quigley (1), Eric Faulques (7) 1. Department of Physics and School of Engineering, University of Warwick, Coventry, UK 2. Institute of Physics, Lotników 32/46, PL-02-668, Polish Academy of Sciences, Warsaw , POLAND 3. Cavendish Laboratory, J. J. Thomson Avenue, University of Cambridge, Cambridge, UK 4. School of Physics and Astronomy, University of Southampton, Southampton, UK 5. STFC Daresbury Campus, Daresbury WA4 4AD UK, SuperSTEM, Daresbury, UK 6. Nanotube Research Center, Higashi 1-1-1, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, JAPAN 7. Institut des Matériaux Jean Rouxel, University of Nantes CNRS, Nantes, FRANCE DOI: 10.1002/9783527808465.EMC2016.8632 Corresponding email: j.sloan@warwick.ac.uk Keywords: ac-TEM, ac-STEM, Nanotubes, Crystal Growth, DFT A logical extension to fabrication of monolayer 2D materials such as graphene is creation of 'Extreme Nanowires' (i.e. Fig. 1), down to a single atom column in width.[1,2] In this limit, crystals have fundamentally different physical characteristics and properties.[3-5] We and others have created atomically regulated nanowires by confining them within the smallest diameter carbon nanotubes (i.e. either single walled carbon nanotubes (SWNTs) or double walled carbon nanotubes (DWNTs)), and are investigating their structural and electronic properties. These materials also provide an ultimate benchmark for testing the most sensitive characterisation methodologies which, when corroborated with suitable theory, will provide new data on physics at the most fundamental length scale accessible to nanomaterials fabrication. The most powerful investigative tools for structural investigation are aberraton-corrected Transmission Electron Microscopy (ac-TEM) and Scanning Transmission Electron Microscopy (ac-STEM) and, here, we describe the application of these methods to a variety of Extreme Nanowire systems. One of the most crucial aspects of the role of electron microscopy in our investigation is the 2D and 3D elucidation of the structure of quasi- or true 1D nanowires formed in SWNTs as these form the primary source of information for density functional theory (DFT) and other ab initio theoretical approaches to both structure and properties elucidation. When combined with real physical measurements, this combined approach becomes even more powerful as we can start to piece together how the fundamental physics of a crystalline nanowire changes once we constrain its width down to one or two atoms in cross section. For example we recently record Raman Spectra from 2x2 atom thick HgTe nanowires embedded within 1.2-1.4 nm SWNTs[4] and found that we are able to model the Raman-measured lattice phonons of this system based on a simple structural model previously determined from two pairs of Exit Wave Reconstruction images which we also used to make DFT predictions about the altered electronic structure of this system which is predicted to change from a -0.3 eV semi-metal to a ~1.2 eV band gap semiconductor.[3] Following on from the exciting recent work of Senga et al.[2] who imaged the first true 1D crystals of CsI in DWNTs we are now modeling single atomic chain coils of tellurium formed within narrow SWNTs (Fig. 2).[6] References: [1] J. Sloan et al Chem Commun (2002) 1319 [2] R. Senga et al Nature Mater 13 (2014) 1050 [3] R. Carter et al Phys Rev Lett 96 (2006) 21550 [4] J. H. Spencer et al ACS Nano 8 (2014) 9044 [5] C. Giusca et al Nano Lett 13 (2013) 2040 [6] To be published 397