Real-time dynamics of Se atoms in CNTs observed by S/TEM

May 21, 2025 – A collaborative team from the University of Nottingham (UK), the SuperSTEM Laboratory in Daresbury (UK) and Ulm University (Germany) reports a controllable route to in‑situ nano‑engineering of elemental selenium. By confining Se inside boron nitride and carbon nanotubes and manipulating the filler with a low‑voltage electron beam, the scientists created one‑dimensional phases with optoelectronic properties that can be tuned on demand. This three‑in‑one use of the electron beam – as imaging probe, spectroscopic tool and nanoscale press – opens a pathway toward designer semiconductors for flexible nanoelectronics.

Experiments performed on Se in the 1870s led to the discovery of photoconductivity¹, the realization of the photovoltaic effect in solid semiconductors², and its use as the photoactive component of the first solar cells³. Extensive research has since focused on producing and optimizing selenium-based semiconductors with defined allotropes, driven by the phase- and dimensionality-dependent nature of their electronic and optical properties^4,5.

The cylindrical inner cavities of carbon nanotubes (CNTs) provide confined reaction environments for the templated growth of high aspect ratio nanomaterials and enable atomically resolved studies of such structures using transmission electron microscopy (TEM)^6,7. This approach has been applied to diverse semiconducting materials including SnSe⁸ and CsPbBr₃⁹, as well as to the elemental chalcogens sulfur¹⁰, selenium¹¹, and tellurium¹².

Selenium was first encapsulated inside multi-walled carbon nanotubes (MWCNTs) in 1996¹³, and has since been incorporated into CNTs with a wide range of diameters¹⁴. Fujimori et al. employed small-diameter double-walled CNTs (DWCNTs) to template single- and double-helix Se chains as narrow as 0.7 nm¹¹. As the inner diameter of the host CNT increases, selenium adopts alternative conformations, including multiple trigonal Se (t-Se) chains¹⁵ or liquid-like t-Se phases exhibiting visible menisci¹⁴.

Recent studies have highlighted the advantages of boron nitride nanotubes (BNNTs) over CNTs as host materials. BNNTs offer improved capabilities for extending the fluorescence emission of encapsulated organic dyes¹⁶ and facilitate direct access to the electronic¹⁷ and optical¹⁸ properties of confined semiconductors.

Aberration-corrected transmission electron microscopy (AC-TEM), supported by image simulations, was employed to investigate how the internal diameter of CNTs affects the structure and bonding of encapsulated selenium nanowires (Se NWs). The narrowest CNT studied (Fig. 1), with an inner diameter of 0.73 nm, hosts a linear atomic chain of selenium (l-Se), whose structure is shown in Fig. 1b. TEM simulations for l-Se within a CNT of matching diameter closely reproduce the experimental observations, revealing interatomic distances of 0.23 nm in the image and 0.24 nm in the model. The longest uninterrupted l-Se chain identified by AC-TEM spans at least 28 nm (Fig. 1a).

While previous Raman spectroscopic studies suggested the formation of l-Se chains within cancrinite nanochannels¹⁹, direct imaging evidence had been lacking. The structure observed here for l-Se under extreme confinement is consistent with findings on other chalcogens: both sulfur¹⁰ and tellurium¹² have been shown to adopt linear chain configurations in narrow nanotube environments.

Additionally, the atomic spacing of selenium along the linear chains was measured as 0.23 nm in both experimental and simulated images (Fig. 2). As illustrated by the yellow square in Fig. 2c, two co-linear Se chains exhibit a staggered arrangement, likely resulting from the optimization of van der Waals interactions between adjacent atoms.

In some instances, the two co-linear l-Se chains were observed to twist along the axis of the CNT (Fig. 3), producing variations in their projected inter-chain distances. Similar co-linear atomic arrangements have previously been reported for tellurium encapsulated in 1.2 nm CNTs by Qin et al.²¹ The co-linear Se chains observed here may exhibit similar dynamic behavior to the double helices described by Fujimori et al.¹¹, where rapid motion impeded atomic resolution imaging. In contrast, the present study focuses on stationary chain configurations, enabling precise determination of selenium atomic positions.

Due to van der Waals constraints, the maximum selenium-based structure that can be encapsulated in narrow CNTs consists of two t-Se chains. The internal diameter of the CNT governs the separation between these chains by mediating favorable interactions with the CNT walls²², resulting in increased inter-chain spacing as the CNT diameter increases. Larger diameters allow for the encapsulation of multiple t-Se chains (Fig. 4), as verified by matching simulated and experimental images.

Even in these wider CNTs, encapsulated Se NWs retain high translational and rotational mobility. Once the diameter is sufficient to host two l-Se or t-Se chains, the encapsulated selenium becomes less crystalline, resembling amorphous Se²³. These mobile nanowires exhibit long-range disorder due to irregular twist axes (Fig. 3), while preserving short-range order, such as consistent interatomic spacing (Fig. 2) and defined inter-chain distances (Fig. 4c), which are in agreement with values for crystalline selenium.

This behavior parallels observations made by Laude and Fitton²⁴, who characterized amorphous selenium not as a true amorphous material, but as a structurally disordered crystal. This interpretation is further supported by modeling studies that distinguish between static and dynamic disorder in similar systems²⁵.

To further investigate the electron-beam-induced transformations, Se-filled BNNTs (Se@BNNT) were imaged under 80 kV AC-HRTEM at a controlled electron flux of 1.2 × 10⁸ e⁻nm⁻² s⁻¹. During irradiation, single BNNTs showed the formation of sidewall defects (green arrow, Fig. 5a), which progressively expanded due to direct knock-on (DKO) damage. As previously demonstrated for empty BNNTs by Celik-Aktas et al.²⁶ and Cheng et al.²⁷, the removal of boron atoms can also destabilize neighboring nitrogen atoms, resulting in mass loss from the irradiated nanotube.

This phenomenon progresses faster in inner walls of multi-walled BNNTs due to their higher curvature and strain^26,27. Prolonged imaging (400 s) revealed lines of contrast interpreted as chains of Se forming near BNNT sidewall defects (Fig. 5a). As this contrast proved reversible, it was attributed to a temporary suppression of translational and rotational motion of the encapsulated t-Se, rather than a permanent crystalline phase transformation.

Importantly, the authors observed that vacancy defects in BNNTs not only propagated but also restructured under continuous electron beam exposure, causing the BNNT diameter to shrink. This behavior mirrors the annealing and structural reorganization processes previously observed^26,27. Here, the diameter contraction of BNNTs directly induces a phase transition from t-Se to l-Se in the encapsulated material.

A comparable beam-induced extrusion process has been reported for carbon nanotubes in Fe@MWCNT systems, although it required simultaneous heating to 600 °C²⁸. As the BNNT diameter narrowed under irradiation, increasingly confined Se NWs formed, showing characteristic t-Se interchain distances consistent with those in narrow CNTs. Eventually, single linear Se chains (Fig. 5c,f) were observed in BNNTs with inner diameters as small as 0.8 nm, consistent with the 1D phase diagram derived from CNT-based systems.

The structural motifs—linear chains (Fig. 5c), seven t-Se chains (Fig. 5b), and smeared contrast in wide nanotubes (Fig. 5h)—mirror those found in CNTs. Although selenium interacts more strongly with BNNT walls²², these interactions are insufficient to prevent the mobility of the encapsulated chains. The authors expect the narrowed BNNTs formed during irradiation to be structurally stable^26,27, suggesting that the Se nanowires templated under these dynamic conditions may resemble those observed in CNTs at ambient conditions.

The influence of Se on BNNT contraction under electron irradiation is likely linked to its effective bonding with boron or nitrogen dangling bonds at vacancy sites. This is supported by the emergence of well-defined t-Se chain contrast adjacent to newly formed defects (Fig. 5a). The formation of Se–B or Se–N bonds stabilizes these defects, thereby lowering the energy barrier for further defect formation in BNNTs according to the Bell–Evans–Polanyi principle²⁹.

By contrast, selenium does not appear to promote defect formation in CNTs. This discrepancy may be attributed to the intrinsic ability of the carbon lattice to reorganize through the formation of five-membered rings and the Stone–Wales transformation, allowing strain redistribution and thus slowing diameter constriction under electron beam exposure³⁰.

Ultra-low-loss electron energy-loss spectroscopy (EELS) enables the determination of optical bandgaps in nanoscale materials by probing their electronic excitations with high precision. When combined with aberration-corrected scanning transmission electron microscopy (AC-STEM) and highly monochromated beams, EELS achieves outstanding spatial and energy resolution, offering insight into local structure–property relationships³¹. This approach has been successfully employed in various systems, including empty carbon nanotubes (CNTs)³².

However, in CNTs, the presence of metallic or small-bandgap characteristics leads to overlapping van Hove singularities and π plasmon excitations, which obscure the electronic response of encapsulated materials³². In contrast, BNNTs offer an electronically transparent host matrix, enabling clearer spectral characterization.

To prevent knock-on damage during data acquisition, high-angle annular dark-field (HAADF) AC-STEM imaging (Fig. 6a,c,e,g) and corresponding AC-STEM–EELS analysis (Fig. 6b,d,f,h) were conducted at an acceleration voltage of 60 kV. The internal diameters of the filled BNNTs studied exceeded 1.4 nm, a range which—according to the 1D Se phase diagram—favors the formation of nanowires (NWs) with similar crystalline disorder but differing in diameter.

EELS analysis revealed a maximum bandgap of 2.45 eV in a 1.8 nm diameter BNNT, where the encapsulated t-Se NW consists of approximately 20 helical chains (Fig. 6c,i, inset). This widening of the bandgap is attributed to quantum confinement effects³³.

Interestingly, as the BNNT diameter was reduced from 1.8 nm to 0.8 nm, the observed bandgap narrowed to 2.24 eV, corresponding to a structure containing only four t-Se helices (Fig. 6g,i, inset). This trend highlights the complex interplay between structural dimensionality and optical properties in confined selenium phases.

In summary, recent studies by Qin et al.²¹ and Milligan et al.¹⁸ have demonstrated the successful integration of semiconductors encapsulated in boron nitride nanotubes (BNNTs) into field-effect transistors and optoelectronic nanodevices. These devices combine favorable form factors, chemical inertness, and high performance.

However, broader applicability hinges on the ability to understand and control the intrinsic properties of the confined semiconductors. The present study shows that the dimensions, atomic structures, and bandgaps of semiconducting nanowires within BNNTs can be modulated with precision using low-voltage aberration-corrected transmission electron microscopy (AC-TEM).

This control over key material characteristics paves the way for the targeted design and functionalization of future nanoscale electronic devices. The work also underscores the multifunctionality of the electron beam, which acts simultaneously as an imaging, spectroscopic, and nanofabrication tool—highlighting its central role in advanced nanomaterials research.

Resource:
Cull, W. J., Ramasse, Q. M., Biskupek, J., Rance, G. A., Cardillo-Zallo, I., Weare, B. L., Fay, M. W., Whitney, R. R., Scammell, L. R., Fernandes, J. A., Kaiser, U., Patanè, A., & Khlobystov, A. N. (2025).
Flexible Selenium Nanowires with Tuneable Electronic Bandgaps.
Advanced Materials, 2025, 2501821.
https://doi.org/10.1002/adma.202501821