Phosphorus-lithium double-helix nanoribbons

Abstract

Phosphorus nanoribbons combine the tunable bandgap and high mobility with the inherent anisotropy of one-dimensional systems, offering promise for functional electronics, but their intrinsic low stability hinders practical applications. Here, we report phosphorus-lithium double-helix nanoribbons with a well-ordered helical architecture and high structural stability under harsh conditions such as in air up to 225°C, water, and even acidic solutions. Comprehensive experimental characterizations and theoretical analyses show that the stability arises from a synergistic combination of Zintl phase formation between phosphorus and lithium atoms, noncovalent interhelical interactions, and geometric protection offered by the distinctive helical architecture. The nanoribbons show tunable optical properties dependent on temperature, thickness, and polarization state. It is demonstrated that these properties enabled nanoribbon-based hydrogels with self-healability and highly efficient photothermal conversion, showing a general approach for stabilizing active low-dimensional materials and paving the way for applying phosphorus-based nanostructures in biomedical engineering and quantum technologies.

Phosphorus-lithium double-helix nanoribbons offer exceptional stability with tunable optics and photothermal biomedical potential.

INTRODUCTION

Phosphorus exhibits remarkable structural diversity through its allotropes—white, red, violet, and black phosphorus—each having distinct physicochemical properties (1–3). Among them, black phosphorus has attracted particular interest because of its layered two-dimensional (2D) structure and exceptional electronic characteristics (4–9). When confined to one-dimensional (1D) nanoribbons, this material combines the advantages of 2D systems with directional quantum confinement effects, manifesting characteristic phenomena such as anisotropic carrier mobility, spin-polarized edge states, topological phase transitions, and spin-density waves (4–16). Despite these extraordinary properties, the practical implementation of black phosphorus nanoribbons remains fundamentally limited by their rapid degradation through oxidation in air (17–21)—a critical challenge requiring innovative stabilization strategies.

Theoretical studies have proposed that helical phosphorus nanostructures could offer enhanced stability by altering the bonding topology and steric configuration (22–25). Helical architectures have also been observed in a limited number of 1D van der Waals crystals, including Te, InSeI, GaSI, GaSeI, SnBrP, and SnIP (26–35), exhibiting characteristic properties such as nonlinear optical behavior and spin-dependent phenomena. Among these, SnIP is particularly noteworthy: As the first carbon-free atomic-scale double-helical semiconductor, it comprises covalently intertwined phosphorus and Sn-I subchains stabilized by van der Waals interactions. This pioneering system demonstrated that helicity can endow inorganic semiconductors with mechanical flexibility, chirality, and remarkable stability in both air and aqueous environments, enabling applications in hybrid photocatalytic water splitting (32–35). These findings suggest that introducing helical motifs into phosphorus nanoribbons could similarly enhance their structural stability and uncover distinct physical behaviors. Unlike biological or organic systems, where cooperative interactions such as hydrogen bonding or π-π stacking facilitate the formation of helical structures (36, 37), inorganic crystalline materials often lack these features, making the experimental realization of stable helical structures in phosphorus nanoribbons particularly rare and challenging. Building on these insights, theoretical predictions suggest that lithium can readily form double-helix nanostructures with phosphorus (24, 25). However, the phosphorus-lithium double-helix has yet to be realized experimentally.

In this work, we synthesized phosphorus-lithium double-helix nanoribbons with high structural stability under ambient conditions, at 225°C air exposure, and in strong acidic environments (Fig. 1A), greatly exceeding the stability of conventional black phosphorus nanoribbons, which degrade rapidly in air. The nanoribbons exhibit intriguing optical properties and are enough stable in water to make nanoribbon-based hydrogels with self-healability and highly efficient photothermal conversion. The mechanisms of the stability and excellent properties of the nanoribbons are experimentally and theoretically explored, indicating strong potential for applications.

Fig. 1. Morphology and structure characterization of ultrastable phosphorus-lithium double-helix nanoribbons.

(A) Schematic diagram of the chemically and thermally stable phosphorus-lithium double-helix nanoribbons. The top image depicts the crystal structure, which features an alternating pattern of two left-handed (L) and two right-handed (R) helices. The crystal structure includes a “Chinese knot” motif and a double-helix unit composed of phosphorus chains and lithium atoms. (B) SEM image of the nanoribbons. (C) Aspect ratio distribution of the nanoribbons captured from SEM images. (D) AFM image of the nanoribbons from a typical region. (E) TEM image of a typical double-helix nanoribbon. (F) Experimental and theoretical HRTEM images along with the [100] zone axis. (G) Experimental and theoretical HRTEM images along with the [013] zone axis. (H) Experimental and theoretical SAED patterns along the [031] zone axis, highlighting the atomic-scale helical motif within the phosphorus-lithium double-helix structure.

RESULTS

Synthesis and characterization of phosphorus-lithium double-helix nanoribbons

The stable phosphorus-lithium double-helix nanoribbons were prepared by exfoliating bulk phosphorus-lithium double-helix crystal, which was synthesized using a three-step coheating process involving lithium borohydride (LiBH₄) and red phosphorus powders. Detailed descriptions of the growth process, the influencing factors, and corresponding thermodynamic and kinetic analyses are available in Materials and Methods and sections S1 to S3 (figs. S1 to S20).

Scanning electron microscopy (SEM) images show that the nanoribbons are highly flexible and ultrathin (Fig. 1B and fig. S21). Statistical analysis of SEM data shows an average width of 83.6 nm and a length of 12.5 μm (fig. S22). A total of 83% of the nanoribbons have aspect ratios exceeding 75 (Fig. 1C), substantially higher than those reported for black phosphorus nanoribbons (19). Atomic force microscopy (AFM) measurements further show that the thickness of the nanoribbons ranges from 3.11 to 50 nm, with an average thickness of ~6.23 nm and the thinnest thickness of 3.11 nm (Fig. 1D), corresponding to a five-layer phosphorus-lithium double-helix nanoribbon (figs. S23 and S24).

The chemical bonding and elemental composition of the nanoribbons were examined using x-ray photoelectron spectroscopy (XPS). A full-scale XPS survey of the nanoribbons is presented in fig. S25A. The high-resolution Li 1 s spectrum shows a peak at 55.2 eV (fig. S25B), indicating a reduced binding energy relative to pristine LiBH₄ (fig. S26A). The high-resolution P 2p spectrum shows four distinct peaks at 128.2, 129.6, 130.4, and 133.5 eV (fig. S25C). The lowest binding energy peak at 128.2 eV, absent in pristine red phosphorus (fig. S26B), is attributed to an interaction between phosphorus and lithium in the nanoribbons (38). The peaks at 129.6 and 130.4 eV correspond to neutral P─P bonding states within the polyanionic framework. The minor high-energy feature at 133.5 eV is assigned to surface-oxidized P species, likely arising from trace oxygen in the precursors (fig. S26B) during synthesis.

To unambiguously resolve the crystal structure, we performed high-resolution powder x-ray diffraction (XRD) measurements, followed by Rietveld refinement based on the proposed phosphorus-lithium double-helix model (see section S4 and figs. S27 and S28). The refinement, carried out using the GSAS-II software package, yielded a weighted residual of 4.902 and a goodness of fit of 2.08, confirming good agreement between the model and the experimental data (fig. S28). The refined crystallographic parameters, atomic coordinates, occupancies, positions, and isotropic atomic displacement parameters are provided in tables S1 and S2. This analysis validates the proposed structural framework and provides a robust structural basis for subsequent transmission electron microscopy (TEM) and spectroscopic interpretations.

TEM observations reveal that a typical exfoliated nanoribbon is ~100 nm wide (Fig. 1E). High-resolution TEM (HRTEM) images from two distinct regions along the nanoribbon, alongside selected-area electron diffraction (SAED) patterns (fig. S29), confirm its single-crystalline nature. Reconstructed images from masked 2D fast Fourier transform patterns show lattice spacings of ~6.85 and 7.46 Å, corresponding to the (020) and (002) planes of a tetragonal bulk phosphorus-lithium double-helix crystal. The exfoliated crystals were oriented along the [100] zone axis, indicating that the long axis of the nanoribbons aligns with the covalent c axis, which is consistent with exfoliation along the a and b axes. Comparison of simulated and experimental HRTEM images and SAED patterns (Fig. 1F) validates the successful synthesis of phosphorus-lithium double-helix nanoribbons.

Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the nanoribbons shows a phosphorus-to-lithium elemental ratio of approximately 8:1 (table S3). This slight deviation from the ideal 7:1 stoichiometry is attributed to residual amorphous or unreacted phosphorus, as indicated by SEM and TEM observations of non-nanoribbon regions, which display high phosphorus content in energy-dispersive x-ray spectroscopy spectra and Raman features characteristic of red phosphorus (fig. S30). These phosphorus-rich residues are morphologically and structurally distinct from the crystalline nanoribbons. Together, the ICP-MS overestimation does not alter the conclusion that the crystalline phase corresponds to the 7:1 phosphorus-lithium double-helix, as supported by HRTEM, SAED, and Rietveld refinement of XRD data.

A systematic investigation of the helical motifs in the nanoribbons was conducted on the basis of HRTEM images and SAED patterns. The nanoribbons unit cell features alternating left-handed and right-handed helices (Fig. 1A), propagating along the c axis and crystallizing in the noncentrosymmetric tetragonal space group I4₁/acd (no. 142). A front view of the structure resembles a pattern of “Chinese knot” (Fig. 1A), while the side view reveals that each double-helical unit consists of P₇ cages and lithium atoms (Fig. 1A and fig. S31), extending infinitely along the c axis. Notably, HRTEM images along the [031] zone axis, acquired from multiple positions along different nanoribbons, consistently reveal the periodic double-helical contrast, confirming the reproducibility of the helical motif along the ribbon length (Fig. 1, G and H, and figs. S32 and S33). The additional diffraction spots, variations in SAED peak intensities, and diffuse scattering observed in Fig. 1H can be attributed to double diffraction and dynamical scattering effects under slight off-zone-axis tilts, as well as local helical modulation, consistent with previously reported observations in helical crystal systems (30, 31).

The helical structures, formed by P₇ cages, were analyzed in detail (Fig. 1A). Unlike previously predicted single-stranded helices based on P₈ cages (22), the P₇ cage helical tubes consist of concentric double and triple helices of phosphorus atoms. These structures exhibit distinct helical parameters, including helical radius (1.15 to 3.93 Å), torsion (0.00526 to 0.01682 Å⁻¹), and rise distance (14.76 Å) (see section S5, figs. S34 and S35, and table S4 for more details).

Structural stability of the phosphorus-lithium double-helix nanoribbon

The phosphorus-lithium double-helix nanoribbons exhibit morphological integrity and structural stability under diverse conditions. TEM analysis reveals that the nanoribbons can retain their crystallinity and intact morphology even after immersion in deionized (DI) water for over 30 days (Fig. 2, A and B, and fig. S36). In addition, Raman spectra of the nanoribbons closely resemble those of freshly exfoliated samples, with no notable change or the appearance of new mode (fig. S37), further confirming their stability. Furthermore, after exposure to air for more than a week, the nanoribbons showed minimal thickness variation and maintained structural integrity (fig. S38), demonstrating no observable degradation even after 7 days under ambient conditions.

Fig. 2. Stability of phosphorus-lithium double-helix nanoribbons.

(A and B) Low-resolution TEM (A) and the corresponding HRTEM images (B) of the nanoribbon after immersion in aqueous solution for 30 days. (C and D) Low-resolution TEM (C) and the corresponding HRTEM images (D) of the nanoribbon after immersion in HCl for 1 hour. (E) High-resolution P 2p XPS spectra of the nanoribbons after exposure to HCl for 1 hour. a.u., arbitrary units. (F) Low-resolution TEM image of the nanoribbon annealed at 225°C in air. (G and H) HRTEM images acquired at selected regions (#1 in blue and #2 in cyan). (I) Structure evolution of monolayer phosphorus-lithium double-helix observed during AIMD simulation. (J) Formation energies of the phosphorus-lithium double-helix and black phosphorus structures with different doping ratios of lithium atoms.

The chemical stability of the phosphorus-lithium double-helix nanoribbons was also tested. After the nanoribbons were immersed in 1 M hydrochloric acid for 1 hour, followed by thorough rinsing with DI water (Fig. 2, C and D, and fig. S39), HRTEM images reveal that the crystalline lattice of the nanoribbons remains intact, with no evidence of amorphization or lattice disorder. Complementary spectroscopic analyses—including Raman, photoluminescence (PL), and XPS—exhibit spectra indistinguishable from those of pristine samples (Fig. 2E and fig. S40), indicating no detectable structural or compositional changes. Together, the results confirm that the chemical environment does not degrade the intrinsic crystal structure.

Thermal stability of the phosphorus-lithium double-helix nanoribbons was systematically investigated under ambient air conditions. Complementary TEM analysis confirmed that the crystalline lattice remains intact up to ~225°C (Fig. 2, E and F, and figs. S41 to S44), while partial amorphization occurs at higher temperatures (fig. S45). Consistently, Raman spectra show no discernible peak shifts or additional vibrational modes up to ~225°C (fig. S46), showing no peak shifts or new vibrational modes, thereby validating their thermal robustness. Collectively, these results demonstrate that the nanoribbons exhibit exceptional thermal stability, ultimately constrained by surface-driven amorphization at elevated temperatures. Notably, their performance surpasses that of black phosphorus nanoribbons, which degrade readily under ambient conditions (table S5).

To further validate the stability of phosphorus-lithium double-helix, we compared the formation energies of black phosphorus and phosphorus-lithium double-helix. A single layer of phosphorus-lithium double-helix exhibited negative formation energy, indicating superior stability relative to black phosphorene, which has positive formation energy even in its bulk form (fig. S47A). Ab initio molecular dynamics (AIMD) simulations at 300 K further affirmed the thermal stability of the nanoribbons, showing minimal buckling vibration (Fig. 2G and fig. S47B).

The stability of phosphorus-lithium double-helix nanoribbons arises from a synergistic interplay of Zintl phase formation, noncovalent interactions, and helical packing geometry (Fig. 2H and figs. S47 to S49). Similar with other Zintl phase materials (39), lithium donates electrons to the phosphorus framework, forming a Zintl phase that enhances P─P covalent bonds and intrastrand stability (figs. S48 to S50, A to D). Each lithium atom transfers 0.86 electrons (fig. S49), generating electrostatic interactions akin to hydrogen bonds in DNA. van der Waals forces between helices further stabilize the double-helix structure, similar to interlayer attractions in bulk black phosphorus. Without noncovalent interactions, simulations show that the single-helix formation energy is positive (0.08 eV per atom) and distorts at 300 K (fig. S50, E and F). Helical packing also contributes one-third of the stability by minimizing steric repulsion and internal strain (Fig. 2H). Detailed descriptions are available in section S6 (figs. S48 to S50).

Optical properties of the phosphorus-lithium double-helix nanoribbon

Raman and PL spectra were systematically used to investigate the electronic and thickness-dependent properties of the phosphorus-lithium double-helix nanoribbons (Fig. 3, A to C). Figure 3A presents spatially resolved Raman and PL mapping images, revealing uniform signal distributions that confirm the high crystallinity and structural homogeneity of the nanoribbons. Notably, the Raman, PL, and ultraviolet (UV)–visible spectra of phosphorus-lithium double-helix nanoribbons (Fig. 3, A to C) differ markedly from those of precursor materials (LiBH₄ and red phosphorus; fig. S51, A to C), confirming successfully structural transformation. Compared with pristine red phosphorus, the P-Li double-helix exhibits stronger optical absorption and a blue-shifted PL peak at 686 nm with over fivefold higher intensity (fig. S51, D and E), reflecting more efficient radiative recombination due to its long-range ordered double-helix structure.

Fig. 3. Thickness-dependent, temperature-dependent, and anisotropic optical properties of the nanoribbons.

(A) Optical micrograph, Raman and PL mapping of the nanoribbons. (B) Thickness-dependent Raman spectra of the nanoribbons. (C) Thickness-dependent PL emission spectra of the nanoribbons. (D) Temperature-dependent Raman spectra of the nanoribbons. (E and F) Polar plots of intensity of 157 cm⁻¹ for 15-nm nanoribbon under (E) parallel-scattering and (F) cross-scattering polarization configurations. (G) Polarized Raman signals under a specific rotating angle of ~60° in the bulk counterpart. (H) Angle-resolved polarized PL spectroscopy of the nanoribbon. (I) SHG spectra of the nanoribbons under the different measurement powers. The inset is schematics of SHG in phosphorus-lithium double-helix structure, which illustrates the frequencies of the incident light (ω) and the generated SHG light (2ω). (J and K) Polar plots of SHG intensity versus the polarization angle for (J) copolarized and (K) cross-polarized configurations.

A thickness-dependent weakening of Raman modes was observed as the nanoribbon thickness decreased (Fig. 3B and see more details in section S7). This phenomenon arises from modulations in interlayer vibrational coupling and intralayer bending/stretching modes, suggesting that mechanical and electronic properties can be precisely tuned via thickness control. Complementary PL studies (Fig. 3C) revealed a broad emission peak at 1.78 eV with a full width at half maximum of 0.31 eV in bulk crystals, consistent with defect-mediated luminescence observed in phosphorus-based materials (40, 41). As the nanoribbon thickness decreases, the PL intensity at 1.78 eV reduces, accompanied by a red shift to 1.95 eV. This red-shift phenomenon was further confirmed through UV-visible absorption experiments (fig. S52). First-principles calculations using the Perdew-Burke-Ernzerhof (PBE) functional (fig. S53A) predicted a bandgap narrowing from 2.13 eV (monolayer) to 1.78 eV (tetralayer), in qualitative agreement with the experimental trend (fig. S53, B and C). Although PBE systematically underestimates the bandgaps of nanoscale phosphorus-lithium double helices (42), the unusually close match between the bulk PBE value and experiment—rare in complex, low-symmetry structures—can be rationalized by low exciton binding energy, due to strong dielectric screening in the bulk, and moderate zero-point renormalization, which partly compensates the PBE underestimation (43).

In the experimentally accessible thickness range of 6 to 100 nm, the optical gap increases by only ~0.13 eV, reflecting the weak confinement regime expected for a flat-band, large-gap system with a small Bohr exciton radius. Fitting the data with the physically motivated relation $E_{\mathrm{g}}=E_0+C/d^n$, where E₀ is the bulk bandgap, C is a material-dependent constant, d is the thickness, and n is an effective exponent (44), reveals that the bandgap continues to increase with decreasing thickness, albeit at a very slow rate (fig. S53C). These results establish thickness as a reliable control parameter for tuning the optoelectronic properties of phosphorus-lithium double-helix nanoribbons.

Temperature-dependent Raman studies spanning −190° to 120°C (Fig. 3D and fig. S54) demonstrated exceptional thermal stability in the nanoribbons, with all vibrational modes remaining intact up to 120°C, while simultaneously revealing critical insights into their interlayer vibrational dynamics and thermal expansion behavior. A distinct red shift and nearly linear change were observed across all visible Raman peaks with increasing temperature (see more details in section S8). The slope value (χ; −0.02778) for the nanoribbons is higher than that of other anisotropic nanomaterials (45–47), such as black phosphorus (A_1g mode, −0.023 cm⁻¹ K⁻¹), SiP₂ (A²_g mode, −0.00183 cm⁻¹ K⁻¹), and PdSe₂ (A²_g mode, −0.01221 cm⁻¹ K⁻¹). This enhanced temperature sensitivity originates from strong interlayer coupling, enabling efficient thermal modulation of vibrational states. At elevated temperatures, peak broadening and intensity reduction are also observed, attributable to intrinsic lattice anharmonicity rather than structural degradation (see more details in section S8 and fig. S55).

Polarized Raman spectroscopy (Fig. 3, E and F, and tables S6 to S8) revealed pronounced anisotropy in 15-nm-thick nanoribbons, with Raman intensities showing π-periodic modulation under parallel/cross-polarized configurations, which aligns with theoretical analysis (see more details in section S9). Thicker nanoribbons (28 and 62 nm) exhibited diminished polarization dependence, while the corresponding bulk crystals displayed a 90° rotation of the principal axis for the mode of 396 cm⁻¹ (table S7). The optical anisotropy remained consistent across 532- and 633-nm excitation wavelengths, exhibiting negligible dependence on the excitation wavelength (table S8).

Polarized Raman spectroscopy with varying microscope rotation angles (β) was used to determine the crystallographic orientation. In the polar plot of the Raman peak intensity of 157 cm⁻¹ for the bulk crystal (Fig. 3G), the crystal rotation angle corresponds to half of the observed rotation angle in the polar plot (see more details in section S9). In addition, angle-resolved polarized PL spectroscopy (Fig. 3H) further corroborated the structural anisotropy. PL emission intensity exhibited a fourfold modulation, reaching maxima at 0° and 180° while minimizing at 90° and 270°.

The helical architecture of the nanoribbons enabled nonlinear optical responses. Second-harmonic generation (SHG) intensity exhibited a near-quadratic power dependence (Fig. 3I and fig. S56), consistent with second-order electric-dipole processes. Polarization-resolved SHG measurements (Fig. 3, J and K) revealed distinct anisotropic behavior, mirroring the structural asymmetry observed in Raman and PL studies.

To situate our phosphorus-lithium double-helix within the broader category of inorganic helical materials, we compare it with SnIP (32–35), the first carbon-free atomic-scale double-helical semiconductor. As summarized in table S9, both materials can be synthesized without the use of metal substrates and feature helical phosphorus substructures along with van der Waals–stacked architectures, contributing to environmental stability and structural flexibility. However, they differ fundamentally in morphology, structural organization, stabilization mechanisms, and electronic properties. Morphologically, SnIP forms nanowire-like crystals, while the phosphorus-lithium double-helix exists as ultrathin nanoribbons with high aspect ratios, allowing enhanced tunability and integration. Structurally, the SnIP double-helix consists of infinite 1D ${}_{\infty }{}^1\mathrm{P}^{-}$ and ${}_{\infty }{}^1\left[\text{SnI}\right]^{+}$ chain, stabilized by noncovalent interactions and helical geometry. In contrast, each of phosphorus-lithium double-helix is composed of P₇ cages and lithium atoms, and its stability arises from a synergistic interplay of Zintl phase formation, noncovalent forces, and helical packing geometry. In spatial arrangement, SnIP exhibits helices of uniform handedness aligned along the b axis, while the phosphorus-lithium framework displays alternating left- and right-handed single helices in a 2D lattice (fig. S57). In terms of electronic properties, SnIP is a direct bandgap semiconductor with a gap of ~1.86 eV, whereas the phosphorus-lithium double-helix nanoribbons exhibit thickness-tunable bandgaps ranging from 2.13 to 1.78 eV. This comparison highlights the structural and functional diversity of inorganic helical phosphorus-based materials and underscores the variety of stabilization strategies and tunable properties achievable through different molecular architectures.

Phosphorus-lithium double-helix nanoribbon hydrogel

Hydrogels are versatile materials with high water content and biocompatibility, making them ideal for biomedical applications (48–51). Phosphorus-lithium double-helix nanoribbons exhibit properties similar to black phosphorus but with remarkable stability in aqueous solutions. To leverage this stability, we used the nanoribbons as a filler to create a hydrogel through a biomineralization-inspired assembly process (see more details in Materials and Methods). As illustrated in Fig. 4A, the double-helix architecture plays a dual role that directly enables gelation. First, Zintl-type electron transfer within the P-Li framework, together with noncovalent interhelical contacts and geometric shielding, suppresses hydrolysis and oxidation in water (Fig. 2), maintaining surface integrity during processing. Second, the chiral, high-curvature helical surface furnishes a dense array of electrostatic and hydrogen bond–like interaction sites for poly(acrylic acid) (PAA) carboxylates, promoting multivalent chain bridging, polymer entanglement, and network percolation These interactions drive the cross-linking of PAA chains into a robust 3D network (fig. S58). Thus, the hydrogelation process is intrinsically linked to the specific double-helix architecture, which not only imparts structural integrity but also dictates the molecular-level interactions responsible for macroscopic gel formation.

Fig. 4. Fabrication and performance characterization of phosphorus-lithium double-helix nanoribbon hydrogel.

(A) Schematic illustration of the formation process of the phosphorus-lithium double-helix nanoribbon hydrogel. (B) Electrical conductivity measurements of the hydrogel with varying nanoribbon contents. (C) Tensile stress-strain curves of the phosphorus-lithium double-helix nanoribbon hydrogel. (D) Photographs of the original hydrogel, a broken hydrogel, and a self-healed hydrogel. (E) Stretched hydrogel post–self-healing. (F) Mechanical strength of the self-healed hydrogel. (G) Photothermal conversion curves of hydrogels with (purple) and without (black) the nanoribbons, demonstrating a photothermal conversion efficiency (PCE) of up to 40.4%. (H) Photothermal conversion curves of the nanoribbon hydrogel subjected to 530-nm light at varying power levels. (I) Photothermal cycling curve of the nanoribbon hydrogel under 530-nm irradiation at 0.4 W.

Systematic optimization of nanoribbon content revealed tunable electrical and mechanical properties. Increasing the nanoribbons loading from 0 to 10 mg ml⁻¹ enhances the hydrogel conductivity by 493% from 1.01 to 5.99 S m⁻¹ (Fig. 4B), attributable to percolative electron transport through the nanoribbon network. Concurrently, mechanical testing demonstrated a sevenfold increase in elastic modulus with nanoribbon incorporation (Fig. 4C), albeit with a controlled reduction in ductility, enabling precise modulation of stiffness and flexibility. Because of the noncovalent interactions, the hydrogel also exhibits reliable self-healing capabilities (Fig. 4, D to F): After being cut, it can self-heal at room temperature without external assistance, regaining its electrical conductivity after healing (fig. S59).

Beyond mechanical reinforcement, the helical morphology also governs the hydrogel’s photothermal functionality. The periodic curvature and chiral topology of the nanoribbons amplify light-matter interactions by enhancing multiple scattering, increasing the optical path length, and enabling broadband absorption (fig. S60). Temperature measurements under different wavelengths of light show that nanoribbon-enhanced hydrogels achieve a greater temperature rise compared to pure PAA–ACC (amorphous calcium carbonate‌‌) hydrogels (Fig. 4G and fig. S61). For example, under a 940-nm irradiation with an intensity of 1 W cm⁻², a remarkable temperature increase of 51.2°C was observed in the nanoribbon-reinforced hydrogel (80.1°C) when compared to pristine PAA-ACC hydrogels (28.9°C) (figs. S62 to S66 and see more details in section S10). At 530 nm, our phosphorus-lithium double-helix nanoribbons showed a high photothermal conversion efficiency of 40.4%, surpassing most reported nanomaterials (table S10). The photothermal response of the hydrogel under 530-, 810-, and 940-nm light, as shown in Fig. 4H and fig. S67, demonstrates thermal control over a wide temperature range. Crucially, the hydrogel still maintained original efficiency over three heating-cooling cycles (Fig. 4I and fig. S68), with XPS analysis confirming no structural degradation postirradiation (fig. S69).

The hydrogel’s photothermal capabilities make it a promising candidate for photothermal imaging. This stability enables precision thermal control, demonstrated by wavelength-dependent temperature modulation (Fig. 4H) and high-resolution near-infrared imaging of hydrogel patterns of “NUAA” and “UTokyo” (fig. S70). Enhanced thermal conductivity (fig. S71 and table S11) further supports applications requiring rapid heat dissipation. These findings suggest that the nanoribbon-based hydrogels offer tremendous potential for high-resolution imaging, bioimaging, and biomedical applications such as photothermal therapy.

DISCUSSION

We report the successful synthesis of phosphorus-lithium double-helix nanoribbons, a previously unexplored class of 1D materials distinguished by high air stability, optical anisotropy, and robust nonlinear optical responses. Experimental and theoretical studies reveal that the electronic bandgap of these nanoribbons can be systematically tuned from 2.13 eV (monolayer) to 1.78 eV (tetralayer) through thickness control—a critical feature for tailoring optoelectronic functionality. Unlike conventional black phosphorus nanoribbons that oxidize within 1 hour, the helix nanoribbons exhibit exceptional resilience, maintaining structural integrity under extreme conditions including 225°C air exposure and strongly acidic environments. This stability originates from a synergistic trifecta of protective mechanisms: Zintl phase chemical bonding, noncovalent interhelical interactions, and geometric shielding inherent to the double-helix architecture. Building on these advances, we engineered double-helix nanoribbon–incorporated hydrogels that combine autonomous self-healing with highly photothermal conversion efficiency (40.4% at 530 nm). The hydrogels demonstrate wavelength-programmable thermal responses, cyclic stability, and biocompatibility—properties that bridge the gap between nanomaterial innovation and practical biomedical applications.

This work establishes phosphorus-lithium double-helix nanoribbons as a transformative platform for durable nano-optoelectronics and precision biomedicine. Their combination of environmental robustness, tunable optoelectronic properties, and integration versatility positions them as candidates for high-resolution bioimaging, targeted photothermal therapy, and flexible nanoelectronics.

MATERIALS AND METHODS

Materials

Chemicals were used as received without further purification unless specified otherwise. LiBH₄ (95%), red phosphorus (98.5%), ethanol (95%), isopropanol (IPA; >99.7%), acetonitrile (>99%), dimethylformamide (DMF; 99.5%), N-methyl-2-pyrrolidone (NMP; 99.5%), and tetrahydrofuran (THF; >99.5%) were purchased from Aladdin Reagent (Shanghai) Co. Ltd. Hydrochloric acid (~37.5 aqueous solution) and acetone (99%) were supplied from Sinopharm Chemical Reagent Co. Ltd. PAA (average molecular weight ~ 450,000) were purchased from Sigma-Aldrich. Calcium chloride (CaCl₂; 95%) and sodium carbonate (99.8%) were purchased from FUJIFILM Wako Pure Chemical Industries. Ultrapure water used in the experiments was obtained from a Milli-Q reference water purification system, achieving a resistivity of 18.2 MΩ·cm at room temperature.

Synthesis of bulk phosphorus-lithium double-helix crystal

High-quality bulk phosphorus-lithium double-helix crystal was synthesized via a high-temperature solution growth method. Stoichiometric amounts of LiBH₄ and red phosphorus powder were placed in a quartz tube, which was subsequently evacuated and flame-sealed under high vacuum (~10⁻⁵ torr). The sealed tube was heated stepwise to 600°C in a custom-built furnace, followed by a controlled cooling to room temperature. First, the powder mixture was initially heated from room temperature to 270°C at a rate of 10°C min⁻¹ and held for 2 hours to form an intermediate product (step I). Second, this intermediate was then heated to 450°C at a rate of 10°C min⁻¹ and maintained for 4 hours to obtain bulk phosphorus-lithium double-helix crystal (step II). Last, the temperature was ramped to 600°C at a rate of 10°C min⁻¹ and held for 30 min to synthesize higher-quality bulk phosphorus-lithium double-helix crystal (step III). The growth details can be found in fig. S1. Upon completion of the reaction, the resulting product was purified by washing with DI water.

Formation of phosphorus-lithium double-helix nanoribbons

Solvent-phase exfoliation is widely recognized as a scalable method for the production of low-dimensional materials, with the selection of an appropriate solvent being critical to the process. Eight solvents with varying surface tensions, including DI water, ethanol, IPA, acetone, acetonitrile, DMF, NMP, and THF, were each used to exfoliate bulk phosphorus-lithium double-helix crystals (3 mg) in glass vials. The vials were subjected to sonication in an ultrasonic bath (Skymen JM-03D; 53 W) for 40 min, with the bath temperature maintained below 40°C. The exfoliated phosphorus-lithium double-helix nanoribbons were collected at 12,000 rpm for 20 min (H2-16T centrifuge). Here, the term nanoribbon follows the broader convention in recent literatures (52, 53), where ribbon-like structures with nanoscale thickness are denoted as nanoribbons even when their lateral dimensions extend to tens or hundreds of nanometers. The exfoliated phosphorus-lithium nanoribbons exhibit an average width of ~83.6 nm and an average thickness of ~6.2 nm, thereby meeting the dimensional criteria commonly reported for nanoribbons.

Characterization

Field-emission SEM (FEI Nova NanoSEM 450) was used to examine the morphological characteristics of phosphorus-lithium double-helix nanoribbons. The thickness profile of these nanoribbons was assessed through AFM (Smart SPM, HORIBA Scientific). HRTEM and SAED were performed on a JEOL JEM-F200 cold field-emission transmission electron microscope operating at 200 kV, using low-dose conditions to minimize beam-induced damage. To verify the chemical composition, dark-field scanning TEM coupled with energy-dispersive x-ray spectroscopy was used. The elemental composition and chemical states were analyzed by XPS (ESCALAB 250Xi, Thermo Fisher Scientific Inc.) with monochromatic Al Kα radiation. Raman and PL spectra, including their polarized variants, were obtained with a laser confocal Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon, France), using lasers with excitation wavelengths of 532 and 633 nm. The phosphorus-lithium double-helix nanoribbons were drop-cast on SiO₂/Si for the measurements of temperature-related Raman and PL spectra. To minimize laser-induced heating and sample degradation, all measurements were performed under strictly controlled excitation powers (≤0.25 mW). SHG and angle-resolved SHG measurements were carried out using a MStarter 100 Ultrafast microscopic testing system (Nanjing Maita Optoelectronic Technology Co. Ltd.). UV-visible absorption spectra were recorded at ambient temperature with a Shimadzu UV-3600 spectrometer, and the bandgap was extracted from plots of (αhv)¹/² versus photon energy (hv), where α denotes absorbance. XRD was performed on a Rigaku SmartLab9 system using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 150 mA) at a scanning speed of 4° min⁻¹. To minimize preferred orientation effects, the phosphorus-lithium double-helix crystals were gently ground into fine powders using an agate mortar and pestle. During the θ-2θ scan, the sample holder was continuously rotated about the surface normal to further randomize the orientation of the anisotropic crystallites. These treatments effectively suppressed texture effects and ensured reliable diffraction statistics for subsequent Rietveld refinement. ICP analysis was conducted to further determine the chemical composition of the nanoribbons. Before the all characterizations, the phosphorus-lithium double-helix nanoribbons were stored in ambient environment.

Preparation of phosphorus-lithium double-helix nanoribbon hydrogel

First, 0.1 g of PAA and 0.1 g of CaCl₂ were dissolved in 10 ml of DI water under constant stirring at 500 rpm at 40°C, yielding a PAA-CaCl₂ solution. Subsequently, 1.5 ml of phosphorus-lithium double-helix nanoribbons at varying concentrations (2, 4, 6, 8, and 10 mg ml⁻¹) were separately introduced into 10 ml of the PAA-CaCl₂ solution, followed by stirring for 30 min at 500 rpm to form the nanoribbon-PAA-CaCl₂ solution. Thereafter, 5.5 ml of 0.1 M sodium carbonate (Na₂CO₃) aqueous solution was gradually added to the nanoribbon-PAA-CaCl₂ solution under continuous stirring, leading to the formation of a nanoribbon-PAA-ACC hydrogel. Last, the phosphorus-lithium double-helix hydrogel was collected and thoroughly washed with DI water. For the preparation of the PAA-ACC hydrogel, the process is identical, except that the 1.5 ml of phosphorus-lithium double-helix nanoribbons dispersion is replaced with DI water in the initial step.

Characterization of phosphorus-lithium double-helix nanoribbon hydrogel

The Ossila four-point probe system and the Ossila sheet resistance software were used to measure and analyze the electrical conductivity of phosphorus-lithium double-helix nanoribbon hydrogel. The mechanical tester (EZ-LX Test) was used to evaluate the mechanical properties of the hydrogels in tensile tests. Infrared thermography (FLIR C5) was used to record the real-time temperature of the hydrogel under the irradiation of light-emitting diodes. Hot-plate method was used to measure and analyze the thermal conductivity of the hydrogel.

Density functional theory calculations

In our first-principles calculations, density functional theory is implemented in VASP package (54–57). In the performed calculations, the plane wave basis set was applied while the electron-ion interactions were simulated with the use of projector augmented-wave (PAW) potentials (58). The exchange-correlation effects were described in the framework of generalized gradient approximation in its PBE formulation (59). The plane-wave kinetic-energy cutoff is set at 450 eV. Brillouin zone integrations are performed using a Kmesh-resolved value of 0.02. All the structures are fully optimized until the forces on each atom are less than 0.001 eV Å⁻¹. The energy convergence of the electronic states was controlled with the help of Davison-Block algorithm. To correctly describe the effect of a van der Waals interaction, we used a dispersion-corrected density functional theory method (60). The time step of AIMD simulations was 1.0 fs. We set the Nosé-Hoover thermostat (61) controlling the temperature to 298 K within the canonical ensembles [constant number, volume, and temperature (NVT)]. The formation energies of phosphorus-lithium double-helix crystal with different layers were calculated using the following expression

$$\mathrm{\Delta }{H}_{\mathrm{f}}=[E\left(\text{Phosphorus-lithium double-helix crystal}\right)-E\left(\text{Li}\right)-E\left(\mathrm{P}\right)\times 7]/8$$

where E(Li) and E(P) are the chemical potentials of Li and P, the energy in their elemental states. E(Phosphorus − lithium double-helix crystal) is the energy of the phosphorus-lithium double-helix crystal per chemical formula.

Supplementary Materials

This PDF file includes:

Figs. S1 to S71

Supplementary Text

Tables S1 to S11

References