Nanotubes from the Misfit Layered Compound (SmS)1.19TaS2: Atomic Structure, Charge Transfer, and Electrical Properties

Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5dz2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10–4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.


SI text: Characterization details
Scanning electron microscopy (SEM) SEM imaging of the nanotubes was done with a Zeiss Sigma 500 model. A minute quantity of native powder sample was picked up by a capillary tube and spread over carbon tape for the SEM analysis. Energy-dispersive X-ray spectroscopy analysis (EDS) was performed with the Bruker XFlash/60mm retractable detector installed in the Zeiss Sigma SEM. The quantification of the chemical elements is based on standard-less and self-calibrating spectrum analysis, using the ZAF matrix correction formulas. The relative abundance (yield) of the NT was estimated by analyzing many SEM images of the product. ImageJ software 1 has been used for the analysis of the nanotubes' abundance and their size distribution. The determined abundancies were based on counting the number of nanotubes and flakes in each image and dividing the number of nanotubes by the total number of nanotubes and flakes. Similarly, the diameter of the nanotubes (> 100 tubes in each case) was measured using ImageJ software by calibrating the scale in the image. The abundance of nanotubes (number of nanotubes with a given diameter in the total number of nanotubes analyzed) was plotted as a function of the diameter. While being only semiquantitative in nature, the overall yield did not vary appreciably from one batch to the other.

X-ray powder diffraction
X-ray powder diffraction (XRD) measurements were performed using TTRAX III (Rigaku, Tokyo, Japan) theta-theta diffractometer. The set-up was equipped with a rotating copper anode X-ray tube operating at 50 kV/200 mA. The powders were spread on a zero-background Si holder and pressed with glass to flatten the surface. A bent graphite monochromator and a scintillation detector were aligned to the diffracted X-ray beam. They were scanned in specular diffraction (θ/2θ scans) from 3-80° (2θ) with a step size of 0.02° and a scan rate of 0.5° per min in Bragg-Brentano mode with variable slits. The XRD data was analyzed using JADE Pro software and PDF-4+ 2020 database (ICDD).

Transmission electron microscopy, STEM-EDS and EEL Spectroscopy
Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns analyses were performed using a JEOL JEM2100 microscope operated at 200 kV. The analysis of the TEM images, including intensity profiles along the c-axis, and the SAED has been performed with Digital Micrograph 3.1.0 (Gatan) software.
A double aberration-corrected Titan Themis Z microscope (Thermo Fisher Scientific (TFS) Electron Microscopy Solutions, Hillsboro, USA) equipped with a high-brightness field emission gun and a Wiener-type monochromator was employed for the atomic resolution HR-STEM imaging and monochromated EEL spectroscopy at an accelerating voltage of 200 kV. HAADF-STEM images were recorded with a Fischione Model 3000 detector with a semi-convergence angle of 21.4 mrad, a probe current of 40 pA, and an inner collection angle of 70 mrad. Large angle bright-field images were taken with a TFS BF STEM detector with an outer collection angle of 18 mrad. EDS hyperspectral maps were collected with a SuperX G2 four-segment SDD detector with a probe semi-convergence angle of 21 mrad, a beam current of approximately 200 pA, a pixel dwell time of 10-20 μs and a total recording time of typically 10 minutes. Quantitative maps were analyzed with the TFS Velox software, through background subtraction and spectrum deconvolution. A correction of frame-to-frame beam/specimen drift was employed where required using custom software in order to refine net intensity profiles. Monochromated EEL spectra were recorded at a system energy resolution of 80 meV in Dual-EEL spectrum mode on a Gatan Quantum GIF 966ERS energy loss spectrometer (Gatan Inc., Pleasanton, USA) with an Ultrascan1000 CCD camera. The EEL spectra were recorded with a STEM probe with a semi-convergence angle of 24 mrad and a beam current of 200 pA by summing multiple 2 ms spectrum acquisitions from a spectrum image map taken over a larger field of view to distribute the electron exposure. The outer semi-collection angle of the spectrometer was 50 mrad. DigitalMicrograph (Gatan Inc., Pleasanton, USA) was used for the quantification of the chemical shift of the La M core loss from the monochromated EEL spectra. In all cases of atomic-resolution HRSTEM analyses prior specimen cleaning steps, e.g. plasma cleaning, were avoided in order to preserve the surface structure of the nanostructures.

Preparation of nanotube cross-section lamella with focused ion beam (FIB)
A cross-section lamella of (SmS)1.19TaS2 nanotube was prepared using FIB-SEM Helios 660 Dual Beam microscope (Thermo Fisher Scientific (TFS) Electron Microscopy, Hillsboro, USA) equipped with Ga ion source (SmS)1.19TaS2 was dispersed in ethanol, drop cast on Si(100) substrate, and the sample was dried under normal conditions The lamella was prepared by a standard lift-out process using carbon EBID (50 nm) followed by carbon IBID (2 µm) deposited sequentially with stage tilt ±20° to minimize gaps between the protective layer and the nanotube. Final polishing was done at 2 kV ion beam energy. As prepared lamellae of a nanotube cross-sections were further inspected using Talos FX 200 and Titan-Themis Z double corrected transmission electron microscope.

X-ray photoelectron spectroscopy (XPS)
The powder samples were prepared in a glove box, under N2 atmosphere, and put on a carbon tape such as to get a dense yet very thin layer of grains, intentionally a monolayer of grains. Then, sample transfer to the vacuum chamber involved exposure to ambient for only a fraction of a minute. Base pressure below 1·10 -9 torr was kept in the analysis chamber. The XPS measurements were performed on a Kratos AXIS Ultra DLD spectrometer, using a monochromatic Al kα source at 15-75 W and detection pass energies of 20-80 eV. In-situ work-function measurements on the 'as received' samples were conducted under extremely low power of the X-ray source, 0.2 W.
In order to eliminate charging effects, measurements under both positive and negative charging conditions were compared, yielding no observable line shifts of the major lines (yet, some of the oxidized components underwent small shifts, in the range of 70-200 meV). Consequently, for both types of samples, all line-shape analyses relevant to the discussion in this report were practically free of charging-related artefacts. Complementarily repeated scans on given spots were conducted to identify potential beam-induced sample damage during extended exposures to the X-ray irradiation. No observable damage signatures were found.
Curve fitting of the leading signals in the misfit nanotubes yields two components of 'perfect' constituents, SmS and TaS2, with stoichiometries very close to the theoretical ones, however with an additional component of partially oxidized TaS2-xOx. The latter component is associated with surface imperfections, to which XPS presents enhanced sensitivity. Representative atomic concentration ratios are given in Table S1. Note the slight deviation of S/(2Ta+Sm) from the ideal value of unity. Also, note that Sm/Ta = 1.12 is slightly lower than the ideal 1.19 value. Both latter deviations from perfectly stoichiometric ratios are associated with small amounts of oxidized Ta (Ta 4 ) that could be resolved within the Ta 4f spectral window. Once taken into account, the related concentration ratios become very close to the ideal values, as given in brackets in Table S1.

X-ray absorption spectroscopy (XAS): Sample preparation and measurements
Samples were mixed with cellulose binder (mixing ratio 1:3) and pressed with the hydraulic press (pressure: 2 ton) to produce 10 mm pellets suitable for analysis in transmission geometry. Mixing and handling of the powders were performed inside the Ar-filled glove box (H2O and O2 concentration < 1 ppm) to avoid possible oxidation of the samples. Before analysis, the sample pellets were placed between two Kapton foils (thickness: 50 micron) inside a special sample holder forming a closed volume to save from atmospheric air.
XAS experiments in transmission mode were performed at DESY P23 "In-situ and X-ray imaging beamline". The experimental set-up consisted of entrance slits, the first X-ray detector, a rotating sample stage, and a second X-ray detector. Liquid N2 cooled Double Crystal Monochromator (Si 111) was used for choosing the required energy, X-ray mirrors with B4C or Rh coating were used for harmonic rejection depending on the edge. Silicon avalanche photodiodes (APD) were used for measuring the intensity of incoming and transmitted X-ray beam. The sample pellet was mounted on the OWIS DRTM 40 rotary stage and rotated with a speed of 180°/sec during the analysis.
In accordance with the proposed setup, X-ray absorption of the sample was measured at different X-ray energies in the vicinity of Ta L3 and Sm L3 X-ray absorption edges. The X-ray absorption of the sample at a certain energy point was measured for 10 sec. The energy of the incoming X-ray was also corrected by measuring the absorption spectra of pure Ta (99,99 %, L-III edge) and Mn (99,99 %, K-edge) foils. APD detector data were corrected for a dead time, 2 EXAFS spectra processing and analysis were done by Larch, 3 Athena and Artemis software. 4 Fig. S1. (a) SEM micrographs of (SmS) 1.19TaS2