The structural organization of compounds in a confined space of nanometer-scale 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 <110> or a <100> 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 single-walled 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.
The paper reports the relationship between single-walled carbon nanotube (SWNT) doping and capsulate crystal structure in RbxAg1−xI@SWNT composites.
HRTEM and HAADF STEM of 1DTbBrx@SWCNT meta-nanotubes reveal three structural modifications of 1D nanocrystals within single wall carbon nanotube channels attributed to a different stoichiometry of the guest crystal. For SWCNTs with diameters Dm > 1.4 nm a most complete tetragonal unit cell is observed. When crystallization occurs inside SWCNT with Dm < 1.4 nm 1D TbBrx crystal deforms a nanotube to elliptical shape in cross section. In this case the 1D crystal unit cell becomes monoclinic, with possible loss of a part of bromine atoms. Two modifications of a monoclinic unit cell appear. One of them is characterized by single or pair vacancies in the structure of the 1D crystal. Another structure is explained by peripheral and central bromine atoms loss. An appearance of such modifications can be stimulated by electron irradiation. The loss of bromine atoms is in agreement with chemical analysis data. Electronic properties of obtained meta-nanotubes are investigated using optical absorption and Raman spectroscopy. It is shown that intercalation of terbium bromide into SWCNTs leads to acceptor doping of SWCNTs. According to local EDX analysis and elemental mapping this doping can arise from significant stoichiometry change in 1D nanocrystal indicating an average Tb:Br atomic ratio of 1:2.8 ± 0.1.
The paper reports an experimental study of ZnTe and CuI transport through graphene wall of SWNTs by high resolution transmission electron microscopy. It is shown that encapsulated material evacuates the tube through the defects in the nanotube walls, while in-tube diffusion appears high enough to provide matter intake from the nanotube volume. Diffusion kinetics was studied by "atoms count" resulting in ZnTe and CuI diffusivities of 7.67 × 10 −21 and 1.99 × 10 −20 m 2 /s through single defects in SWNT wall. Semiempirical and DFT modeling of potential energy profiles for different types of defects was utilized to propose minimal structural disturbances in a graphene layer to make possible cross-plane transport of matter. The comparison of experimentally observed diffusivities with calculated activation barrier heights was carried out taking into account an effective temperature of substance under electron beam. Neither of the defects including framework disturbance with 5−7 defects or sp 3 -bound carbon atomic pairs give rise to valuable mass-transport efficiencies through graphene layer. Reasonable conformity of the results is only achieved with carbon vacancy pairs in sp 2 -carbon layer, thus, indicating effective transport of matter occurring through the "holes" in graphene.
Here, we show successful filling of 1.4 nm single-walled carbon nanotubes (SWNT) with PbTe nanocrystals. The structure of one-dimensional PbTe in SWNT was determined using high-resolution transmission electron microscopy (HRTEM). The electronic structure of composites was studied by optical absorbance and Raman spectroscopies indicating no noticeable interaction of encapsulated PbTe with SWNT wall. Experimental data are supported by ab-initio calculations, showing non-zero density of states at the Fermi level of PbTe@SWNT(10,10) provided by both SWNT and PbTe states and thus metallic conductivity of the composite.
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