Letter pubs.acs.org/NanoLett
Size-Dependent Structure Relations between Nanotubes and Encapsulated Nanocrystals Andrei A. Eliseev,†,‡ Nikolay S. Falaleev,† Nikolay I. Verbitskiy,†,§ Andrei A. Volykhov,‡,∥ Lada V. Yashina,‡ Andrei S. Kumskov,⊥,# Victoria G. Zhigalina,⊥ Alexander L. Vasiliev,# Alexey V. Lukashin,† Jeremy Sloan,∇ and Nikolay A. Kiselev⊥ †
Department of Materials Science and ‡Department of Chemistry, Moscow State University, 119992 Moscow, Russia § University of Vienna, 1090 Vienna, Austria ∥ Kurnakov Institute of General and Inorganic Chemistry RAS, 119991 Moscow, Russia ⊥ Shubnikov Institute of Crystallography RAS, 119333 Moscow, Russia # NRC Kurchatov Institute, 123182 Moscow, Russia ∇ Department of Physics, University of Warwick, Coventry, Warwickshire CV47AL, U.K. S Supporting Information *
ABSTRACT: The structural organization of compounds in a confined space of nanometerscale cavities is of fundamental importance for understanding the basic principles for atomic structure design at the nanolevel. Here, we explore size-dependent structure relations between one-dimensional PbTe nanocrystals and carbon nanotube containers in the diameter range of 2.0−1.25 nm using high-resolution transmission electron microscopy and ab initio calculations. Upon decrease of the confining volume, one-dimensional crystals reveal gradual thinning, with the structure being cut from the bulk in either a or a growth direction until a certain limit of ∼1.3 nm. This corresponds to the situation when a stoichiometric (uncharged) crystal does not fit into the cavity dimensions. As a result of the in-tube charge compensation, one-dimensional superstructures with nanometer-scale atomic density modulations are formed by a periodic addition of peripheral extra atoms to the main motif. Structural changes in the crystallographic configuration of the composites entail the redistribution of charge density on singlewalled carbon nanotube walls and the possible appearance of the electron density wave. The variation of the potential attains 0.4 eV, corresponding to charge density fluctuations of 0.14 e/atom. KEYWORDS: Single-walled carbon nanotubes, PbTe, confinement, 1D crystals, atomic structure, HRTEM
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One of the smallest types of nanoreactors with a perfect and adjustable cavity configuration are single-walled carbon nanotubes (SWNTs). A large number of one-dimensional nanocrystals, including elemental substances, metal halides, chalcogenides, and even complex compounds have been used to grow 1D crystals within the SWNT channels.11 A significant recent example is the preparation of stable carbyne samples inside nanotubes.12 The structure of the guest compound was reported to depend strongly on the confining nanotube diameter.13 Moreover, the variability of the system, i.e., the possibility of the same compound to form multiple structural polytypes in the tubes of the same diameter, was also discerned.14 Nevertheless, the general principles of the sizedependent structural relations between one-dimensional crystals and SWNT have so far not yet been comprehensively established.15
he question how atoms of two different elements, forming a binary compound, are arranged in a nanometer-sized confined space belongs to one of the fundamental problems of the nanoworld. This issue has already been treated comprehensively for metal clusters,1 binary semiconductor clusters,2,3 and polymers.4 One-dimensional semiconductors present an engaging case due to their size-dependent electronic structure, optical, and electron transport properties.5,6 Atomiclevel structural organization of substance, in turn, is known to govern all of its physical and chemical properties. For example, the electronic, mechanical, or magnetic characteristics of allotropes of the same elemental composition can differ substantially.7 However, for bulk materials, only few parameters are available to affect the atomic structure; these include pressure, temperature, or composition modifications by structure-stabilizing dopants.8 At the nanoscale, additional possibilities to tune the structure of matter are granted by surface modification or spatial confinement.9 For the enabling of spatial confinement at the nanolevel, solid-state nanoreactors have been employed extensively.10 © 2016 American Chemical Society
Received: September 27, 2016 Revised: November 30, 2016 Published: December 22, 2016 805
DOI: 10.1021/acs.nanolett.6b04031 Nano Lett. 2017, 17, 805−810
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Nano Letters
Figure 1. HRTEM images and their simulations for one-dimensional PbTe nanocrystals formed in SWNTs of different diameters: 1.7 nm (a), 1.5 nm (b), 1.36 nm (c), 1.32 nm (d); and 1.2 nm (e). Δf represents defocus value used in simulations.
nanotube diameter variation; the structure changes from a truncated 6 × 6 atomic layer pattern in cross-section for 1.7 nm nanotubes (Structure 1, Table 1) to a 4 × 3 atomic layer pattern for 1.4 nm SWNTs (Structure 3, Table 1). For these structures, a longitudinal crystal orientation is preserved as down to a SWNT diameter of 1.36 nm. Simulated multislice HRTEM images of the corresponding composites are in good agreement with the experimentally observed HRTEM projections recorded during multiple observations. The rocksalt PbTe (110) plane is not the lowest one in terms of the surface energy (these are generally in the order of (100) < (111) < (110)), so therefore, the spatial confinement and atomic packing density plays here a decisive role. Geometry optimization within density functional theory (DFT) modeling also confirms that all the experimentally observed structures are stable, and their formation energies are provided in Table 1. In summary, the first type of structure (i.e., 1−3, Table 1) are originated from bulk lattice by adding or removing atomic planes parallel to the direction depending on the available room in the tube and taking into account bulk crystal stoichiometry. Drastic changes were observed at stronger confinement by nanotubes with diameters of
