Atomic-Scale Time-Resolved Imaging of Krypton Dimers, Chains and Transition to a One-Dimensional Gas

Abstract

Single-atom dynamics of noble-gas elements have been investigated using time-resolved transmission electron microscopy (TEM), with direct observation providing for a deeper understanding of chemical bonding, reactivity, and states of matter at the nanoscale. We report on a nanoscale system consisting of endohedral fullerenes encapsulated within single-walled carbon nanotubes ((Kr@C₆₀)@SWCNT), capable of the delivery and release of krypton atoms on-demand, via coalescence of host fullerene cages under the action of the electron beam (in situ) or heat (ex situ). The state and dynamics of Kr atoms were investigated by energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Kr atom positions were measured precisely using aberration-corrected high-resolution TEM (AC-HRTEM), aberration-corrected scanning TEM (AC-STEM), and single-atom spectroscopic imaging (STEM-EELS). The electron beam drove the formation of 2Kr@C₁₂₀ capsules, in which van der Waals Kr₂ and transient covalent [Kr₂]⁺ bonding states were identified. Thermal coalescence led to the formation of longer coalesced nested nanotubes containing more loosely bound Kr_n chains (n = 3-6). In some instances, delocalization of Kr atomic positions was confirmed by STEM analysis as the transition to a one-dimensional (1D) gas, as Kr atoms were constrained to only one degree of translational freedom within long, well-annealed, nested nanotubes. Such nested nanotube structures were investigated by Raman spectroscopy. This material represents a highly compressed and dimensionally constrained 1D gas stable under ambient conditions. Direct atomic-scale imaging has revealed elusive bonding states and a previously unseen 1D gaseous state of matter of this noble gas element, demonstrating TEM to be a powerful tool in the discovery of chemistry at the single-atom level.

Keywords: carbon nanotubes; endohedral fullerenes; noble gases; one-dimensional gas; single-atom dynamics; time-resolved imaging; transmission electron microscopy.

Figure 1

Endohedral fullerenes are introduced into the internal cavity of a SWCNT via sublimation, forming a one-dimensional, linear “peapod” chain. Highly energetic electron beam irradiation, or heat, promotes the coalescence of adjacent endohedral fullerene molecules to form molecular capsules containing short chains of atom X, providing for the study of bonding-level interactions in isolation.

Scheme 1. Filling of Kr@C₆₀ into Open-Ended SWCNT to Form (Kr@C₆₀)@SWCNT

Figure 2

(a–e) AC-HRTEM data for (Kr@C₆₀)@SWCNT, recorded at an acceleration voltage of 80 kV, and (f–k) HAADF-STEM data for (Kr@C₆₀)@SWCNT, recorded at 60 kV. (a) A freestanding SWCNT fully filled with Kr@C₆₀, illustrating the onset of coalescence between adjacent fullerene pairs (arrowed). Additional HRTEM images are shown in Figures S1 and S2. (b) Schematic cross-section of Kr@C₆₀ depicting the van der Waals diameters of C and Kr (Avogadro software). (c, d) Representative TEM images of pristine (Kr@C₆₀)@SWCNT and C₆₀@SWCNT and stick models of Kr@C₆₀ and C₆₀, respectively. (e) EDS data for (Kr@C₆₀)@SWCNT (enlarged I, Kr Lα (1.6 keV); II, Kr Kα (12.6 keV) peaks inset). Additional recorded signals were attributed to residual Ni catalyst from SWCNT synthesis, O and Cu from the support film and TEM grid, Na and Si from the glass ampule used during SWCNT filling, and Cr from steel in the TEM column. Additional STEM-EDS mapping is shown in Figure S4. (f, g) HAADF-STEM images of bundles of (Kr@C₆₀)@SWCNT peapods (adjusted γ = 0.55) where (g) was acquired simultaneously with the EEL signal. Molecular motion during a scan results in a double point (arrowed). (h, i) EELS maps of the C K-edge (283–394 eV) (h) and the Kr M-edge (89–200 eV) (j) False-colored composite map showing the EELS signal from C (magenta) and Kr (blue). The map was created by integrating the intensity of the C and Kr edges averaged at each pixel of the image spectrum. (k) EEL spectrum following background subtraction showing the Kr M-edge and C K-edge averaged over the pixels of the green box in (j). The EEL spectrum is shown without background subtraction in Figure S5.

Scheme 2. Thermal Coalescence of C₆₀ within SWCNT to Form C_60n Nested Nanotubes

Scheme 3. Thermal Coalescence of Kr@C₆₀ within SWCNT to Form nKr@C_60n Nested Nanotubes

Figure 3

(a–d) 80 kV AC-HRTEM images of (a–c) (nKr@C_60n)@SWCNT and (d) (C_60n)@SWCNT, formed by coalescence of Kr@C₆₀ inside SWCNT at 1200 °C. (a–c) Representative areas of (nKr@C_60n)@SWCNT illustrating different extents of thermal coalescence and annealing of a nested nanotube, from highly corrugated with high barriers for Kr motion (a), to smoother where Kr atoms are partly delocalized (b), to near perfect where Kr can translate freely along the nanotube (c). Figure S6 shows a representative HRTEM survey image highlighting the range of thermally coalesced nested nanotubes. Analogous thermal coalescence of C₆₀ in SWCNT forms C_60n@SWCNT (d), with a similar annealed nested nanotube structure, with arrows denoting localized defects.

Figure 4

60 kV HAADF-STEM data of (nKr@C_60n)@SWCNT formed by coalescence of Kr@C₆₀ inside SWCNT at 1200 °C. (a) HAADF-STEM image of a bundle of (nKr@C_60n)@SWCNT (adjusted γ = 0.40), with bright lines in the center of the nested nanotubes corresponding to highly mobile Kr atoms. (b–d) EELS maps acquired simultaneously with HAADF image (a), showing the C K-edge (283–394 eV) (b) and the Kr M-edge (89–200 eV) (c). (d) False-colored composite map showing the EELS signal from C (magenta) and Kr (blue). The map was created by integrating the intensity of the C and Kr edges averaged at each pixel of the image spectrum. (e) EEL spectrum following background subtraction showing the Kr M-edge and C K-edge averaged over the pixels of the green box in (d). The EEL spectrum is shown without background subtraction in Figure S7. (f, g) HAADF-STEM images of an area of (nKr@C_60n)@SWCNT with a central defect (arrowed), highlighting how the 1D Kr gas cannot transit through such bottlenecks (j), and a short area of 1D Kr gas bounded on either side by stationary pinned Kr atoms (arrowed) (g). (h, i) Calculation of the relative intensity of gaseous Kr atoms (green box) versus stationary Kr atoms (blue boxes) in HAADF-STEM image (h). (i) Histograms of mean per atom integrated intensity in (h), with fitted Gaussian curves. The relative intensity of gas atom intensity to stationary atom intensity is ∼0.66, close to the expected average occupancy of Kr atoms of 2/3 within nested nanotubes.

Figure 5

(a) 660 nm resonance Raman spectra of empty metallic SWCNT (gray), (Kr@C₆₀)@SWCNT (blue), and (nKr@C_60n)@SWCNT (green), highlighting enlarged RBM, D, and 2D bands (inset). The additional RBM associated with the formed nested nanotubes after thermal treatment is marked with an asterisk. Spectra have been normalized to the intensity of the SWCNT G-band and offset on the y-axis for visual clarity. (b) EDS spectrum for (nKr@C_60n)@SWCNT with enlarged Kr Lα (I, 1.6 keV) and Kr Kα (II, 12.6 keV) peaks inset. Additional fluorescent signals were attributed to O and Cu from the support film and TEM grid, with Si from the glass ampule used during SWCNT filling.(c) XPS spectra of the Kr 3d environment for (Kr@C₆₀)@SWCNT (blue) and thermally processed (nKr@C_60n)@SWCNT (green). Wide scan XPS spectra are shown in Figure S12.

Figure 6

(a-f) Time-series AC-HRTEM images (80 kV; 1 × 10⁷ e^– nm^–2 s^–1) illustrating the latter stages of coalescence of two Kr@C₆₀ molecules encapsulated within a SWCNT. Total electron fluence and frame number for each image is shown at the bottom of each panel, with Kr–Kr separations noted at the top left. (g–j) Enlarged views of (a), (d), (e), and (f), respectively, and accompanying structural models showing the relative position and bonding state of the Kr atoms. (k) Kr–Kr separation (green triangles) and C₁₂₀ bottleneck width (blue circles) for this time series, as a function of increasing time or electron fluence. Unfilled data points correspond to Figure 6a–f. Fitted curves for each data set are shown. The horizontal dashed line indicates the theoretical Kr–Kr van der Waals separation. Statistical data corresponding to dimerization event frequency is shown in Table S2.

Figure 7

Adaptation of the Osawa–Tománek mechanism for e-beam coalescence of 2(Kr@C₆₀) encapsulated within SWCNT. The mechanism proceeds via a reversible [2 + 2] cycloaddition, followed by a retro [2 + 2] and 22 subsequent Stone–Wales rearrangements, leading to the formation of a straight-walled C₁₂₀ nanocapsule. Kr atoms are constrained in the [2 + 2] and peanut intermediates, requiring complete annealing to fully integral nested SWCNT in which coencapsulated Kr atoms can translate. The endohedral species is expected to have no effect on this mechanism and to remain entrapped during the process (Supporting Information).

Figure 8

(a-c) Time-series AC-HRTEM images (80 kV; 4.3 × 10⁷ e^– nm^–2 s^–1) charting the electron-beam-induced coalescence of six thermally precoalesced Kr@C₆₀ molecules, highlighting the one-dimensional translation and bonding of the cluster of six Kr atoms. (d, e) Expanded views of (b, c), respectively, with Kr–Kr separations explicitly shown. (f) AC-HRTEM image of a thermally formed nested nanotube with all seven guest Kr atoms visible, six of which form a chain with spacings ranging 0.46–0.56 nm.