Exfoliation of Trigonal Tellurium: Toward Ultra‑Thin Single‑Atom Chains

Background

One‑dimensional (1D) materials—such as carbon nanotubes and semiconductor nanowires—have attracted intense research interest due to their remarkable electronic, optical, and mechanical properties [1,2]. Yet practical deployment has been hindered by challenges like chirality randomness in nanotubes and surface‑state degradation in ultrathin nanowires [3–6]. Consequently, the field has largely pivoted toward two‑dimensional (2D) layered materials, which offer atomic‑scale thickness while preserving high performance through weak interlayer van der Waals forces [9–13].

Extending the layered paradigm, 1D weakly bonded solids can be envisioned as materials with strong covalent bonding along one crystallographic direction but only van der Waals interactions between chains. This structural motif opens the possibility of producing true atomic‑scale wires with diameters of only a few Ångströms. Trigonal Se and Te are prime examples: their crystal lattices consist of helical chains aligned along the c‑axis, each chain comprising three atoms per turn, with adjacent chains arranged hexagonally (Fig. 1) [14–19]. These chains are held together by van der Waals forces, or more accurately, by weak inter‑chain bonding [19].

Se and Te offer unique advantages for 1D nanotechnology. Theoretical studies predict direct band gaps of 1 eV (Te) and 2 eV (Se), strongly dependent on thickness, which could enable tunable optoelectronic devices [19]. Their heavy atomic masses and helical geometry confer pronounced spin‑orbit coupling, negative compressibility, and exceptional flexibility [20–22]. Furthermore, a single Te chain has a triangular cross‑section only 2 Å high, with an inter‑chain spacing of 3.4 Å—making it the thinnest known 1D material [23].

Prior demonstrations of atom‑chain manipulation via scanning tunneling microscopy, substrate step decoration, and self‑assembly have showcased the concept but lack scalability and material diversity [24–27]. In contrast, mechanical exfoliation of weakly bonded 1D solids may overcome these limitations, enabling large‑area production of nanoscale chains or nanowires.

Methods

High‑purity trigonal Te single crystals were obtained commercially [36] and cleaned on 90 nm or 300 nm Si/SiO₂ substrates via acetone, isopropanol, and oxygen plasma to enhance adhesion. Exfoliation was performed by sliding a freshly cleaved facet of Te directly onto the substrate—without adhesive tape—while orienting the crystal’s c‑axis perpendicular to the sliding direction. This “rubbing” technique proved significantly more effective than tape exfoliation, likely reflecting the anisotropic bonding in 1D solids [37]. Optical microscopy was used to locate thin flakes, which exhibit a characteristic green‑blue contrast that intensifies as thickness decreases (Fig. 2a).

a Te flakes immediately after exfoliation. b Same flake after 3 weeks in air. c AFM height image of the highlighted region. d Height profile along the white line in (c).

Results and Discussion

Exfoliated Te appears as anisotropic ribbons up to 50 µm long (Fig. 2a). AFM shows typical heights of 10–15 nm, with pronounced ridges that suggest spontaneous breaking of chains both laterally and vertically—an attribute absent in 2D layered exfoliation. Notably, the sliding method yielded wires as thin as 1–2 nm (Fig. 3b–d), corresponding to two or three atomic chains (inter‑chain spacing 3.4 Å) and lengths of 100–200 nm. Height profiles along the c‑axis reveal surface roughness comparable to the SiO₂ substrate, indicating high structural integrity.

Environmental stability was evaluated by imaging the same flake after 3 weeks of ambient exposure (Fig. 2b). No blistering or color change was observed, contrasting sharply with the rapid degradation of black phosphorus [38]. This suggests that ultrathin Te retains chemical robustness over days to weeks, aligning with reports of Te’s limited oxidation in aqueous media [39].

Raman spectroscopy further confirms material quality. Bulk Te displays an A₁ singlet at 120 cm⁻¹ and E doublets at 92 and 141 cm⁻¹ (Fig. 4a). Exfoliated flakes show the same peaks, but shifted upward by 4 cm⁻¹ for A₁ and 2 cm⁻¹ for E, likely due to reduced inter‑chain coupling or substrate‑induced strain. These shifts are consistent with pressure studies and prior measurements of isolated Te chains in zeolite pores, which exhibit hardening of vibrational modes [43,48].

Polarization‑resolved Raman mapping was employed to determine crystal orientation. Rotating the incident beam’s polarization revealed two intensity maxima at 45° and 225°, aligning with the c‑axis of the flakes (Fig. 4b). This method provides a reliable, non‑destructive orientation assessment, especially valuable when AFM and optical imaging are ambiguous.

Conclusions

Trigonal Te demonstrates a robust weakly bonded architecture that can be mechanically exfoliated into ultrathin 2D flakes and 1D nanowires. AFM confirms layers as thin as 1–2 nm and wires below 100 nm. Raman spectra match bulk Te with a slight hardening of 4 cm⁻¹, and polarized Raman reliably identifies crystal orientation. These findings establish exfoliation as a viable route to nanoscale trigonal Te and open the path toward fabricating single‑atom‑wide Te chains, either by continued mechanical processing or by scalable epitaxial growth.