Peapods: Nanotube Host–Guest Assemblies

Updated 5 July 2026

Peapods are hybrid nanotube host–guest structures where molecules are confined inside nanotubes, resulting in pronounced one-dimensional organization and tailored nanoscale properties.
Synthesis routes enable fullerene peapods to transform into double-walled carbon nanotubes with high filling ratios, significantly enhancing mechanical stability under pressure.
Electronic and spectroscopic studies reveal phenomena such as Coulomb blockade and electron–vibron coupling, while computational implementations extend peapods to advanced Monte Carlo simulations.

Searching arXiv for relevant peapod literature to ground the article and verify the provided corpus. Peapods are hybrid nanotube host–guest structures in which molecules are encapsulated within the hollow core of a nanotube, producing a one-dimensional assembly that resembles peas in a pod. In the nanomaterials literature represented here, the term includes carbon nanotube peapods with C $_{60}$ inside a single-walled carbon nanotube, fullerene peapods used as precursors to double-walled carbon nanotubes, endohedral metallofullerene peapods such as Er $_3$ N@C $_{80}$ @CNT, polythiophene nano-peapods, and boron nitride nanotube peapods. Across these systems, encapsulation modifies mechanical response, electronic structure, spectroscopic signatures, and device-relevant behavior, while preserving a common cylindrical confinement geometry and a strongly one-dimensional organization of the guest species (Sousa et al., 2020, Anis et al., 2012, Fritz et al., 2017, Milko et al., 2013, Sousa et al., 2022).

1. Structural concept and material realizations

A fullerene peapod is a single-walled carbon nanotube (SWCNT) whose hollow core is filled with fullerene molecules, here typically C $_{60}$ . In the “CNT-C $_{60}$ ” form studied under impact loading, a $(12,12)$ SWCNT contains ten C $_{60}$ molecules, creating a one-dimensional host–guest system in which the nanotube acts as a cylindrical shell and the fullerenes are confined inside its hollow core. Related realizations include peapods formed from endohedral metallofullerenes, such as Er $_3$ N@C $_{80}$ inside CNTs, polythiophene encapsulated in zig-zag SWNTs written as pT@(n,0), and boron nitride nanotube peapods in which a BNNT contains a linear arrangement of C $_{60}$ molecules (Sousa et al., 2020, Fritz et al., 2017, Milko et al., 2013, Sousa et al., 2022).

Variant	Host–guest composition	Emphasis in the literature
Fullerene peapod	SWCNT@C $_3$ 0	Mechanical response, transport, precursor to DWCNTs
Metallofullerene peapod	Er $_3$ 1N@C $_3$ 2@CNT	Nanoscale chemical and magnetic characterization
Polymer nano-peapod	pT@(12,0), pT@(13,0), pT@(14,0), pT@(22,0)	Electronic structure and level alignment
BNNT peapod	$_3$ 3 BNNT with 10 C $_3$ 4	Impact fracture and unzipping

This architecture is important because it combines distinct nanoscale constituents in a single object. For carbon nanotube peapods, the combination of nanotube confinement and fullerene encapsulation yields strong confinement effects, unusual mechanical response, and altered electronic or chemical behavior relative to isolated nanotubes or isolated fullerenes. For metallofullerene peapods, the geometry creates an ordered array of magnetic units that could, in principle, allow controlled spin coupling along the tube axis. For polymer peapods, the relevant regime is van der Waals binding rather than strong covalent hybridization, so level alignment becomes the central problem rather than structural reconstruction (Sousa et al., 2020, Fritz et al., 2017, Milko et al., 2013).

2. Formation routes, conversion pathways, and application context

Peapod-like nanostructures have been successfully synthesized experimentally and have been explored for a range of nanotechnology applications, including optical modulation devices, transistors, solar cells, hydrogen storage, lithium-ion batteries, nano-memory devices, and supercapacitors. In this context, encapsulation is not merely a geometric motif: the fullerene chain inside the nanotube can influence mechanical stiffness, fracture behavior, stress distribution, and possibly transport and device-relevant properties (Sousa et al., 2020).

A particularly important synthetic route uses fullerene peapods as precursors to double-walled carbon nanotubes. Bundled SWCNTs with average diameter $_3$ 5 nm are filled with C $_3$ 6 using a sublimation method to form SWCNT@C $_3$ 7 peapods. These peapods are then annealed at $_3$ 8 for 24 h under dynamic vacuum, during which the encapsulated fullerenes coalesce and transform into an inner nanotube, producing DWCNTs. High-resolution transmission electron microscopy and optical spectroscopy were used to monitor this conversion; HRTEM images acquired on a JEOL JEM-2010F with spherical aberration correctors at 80 kV showed DWCNT bundles derived from C $_3$ 9 peapods and supported a filling ratio $_{80}$ 0, consistent with nearly complete conversion of peapods into DWCNTs (Anis et al., 2012).

This precursor role gives peapods a dual status. They are both a nanostructure of direct interest and an intermediate state from which mechanically reinforced coaxial nanotube architectures can be formed. A plausible implication is that peapod research connects synthesis, transformation chemistry, and functional-property engineering more tightly than many other nanotube filling strategies.

3. Electronic structure, level alignment, and resonant transport

For weakly interacting nano-peapods formed by polythiophene inside SWNTs, the electronic structure can be understood starting from a Schottky–Mott-like alignment of the isolated subsystems relative to the vacuum level. In pT@(n,0), the polymer-derived bands are shifted downward by $_{80}$ 1 for pT@(12,0), pT@(13,0), pT@(14,0), and pT@(22,0), respectively. Density functional theory disentangles three contributions: curvature-induced electrostatic potential difference inside versus outside the tube, local charge redistribution upon encapsulation, and polarization or image-charge effects. The dominant term is the curvature-induced electrostatic potential, with $_{80}$ 2, where $_{80}$ 3 is the nanotube diameter. True charge transfer between polymer and tube is negligible; for the off-centered, optimally spaced configuration, the transferred charge is about $_{80}$ 4. Polarization effects are estimated to be minor for these cylindrical systems, with $_{80}$ 5 for the relevant nanotube diameters and dielectric constants (Milko et al., 2013).

For fullerene peapod quantum dots, the relevant issue is not only static level alignment but nanoelectromechanical coupling in transport. Individual peapods deposited on SiO $_{80}$ 6, contacted by Pd/Au source and drain electrodes with 400 nm spacing, and measured at 300 mK in a $_{80}$ 7He cryostat show Coulomb blockade in the weak-coupling regime. The charging energy is $_{80}$ 8 meV and the level spacing is $_{80}$ 9 meV. Compared to empty SWNTs, some Coulomb blockade peaks in C $_{60}$ 0 peapods display an abnormal, non-monotonic temperature dependence, with low-temperature suppression of conductance, modification at intermediate temperatures, and return to ordinary $_{60}$ 1 behavior at higher $_{60}$ 2. The transport model is a Holstein-type electron-vibron coupling problem with

$_{60}$ 3

and dimensionless coupling

$_{60}$ 4

Fits to the temperature dependence yield $_{60}$ 5, and weak quasi-periodic sidebands with spacing $_{60}$ 6 meV support the vibronic interpretation. In this regime, the fullerenes are not passive cargo: resonant transport is reshaped by electron-vibron coupling, producing a Franck–Condon or polaronic blockade signature (Utko et al., 2010).

Taken together, these results show that peapods do not have a single electronic archetype. In vdW-bound polymer peapods, strong hybridization and substantial charge transfer are absent; in fullerene peapod quantum dots, internal molecular motion couples directly to transport and modifies resonant tunneling.

4. Mechanical stabilization under hydrostatic pressure

The stabilization of carbon nanotubes via filling with inner tubes is demonstrated in peapod-derived DWCNT bundles under hydrostatic pressure. Optical spectroscopy over 3500–22000 cm $_{60}$ 7, using a Bruker IFS 66v/S Fourier-transform spectrometer and a Syassen-Holzapfel diamond anvil cell with liquid nitrogen as pressure medium, shows that the pressure-induced redshifts of the optical transitions in the outer tubes are significantly smaller than in SWCNTs below $_{60}$ 8 GPa. The relevant transitions are $_{60}$ 9 for semiconducting tubes and $_{60}$ 0 for metallic tubes, extracted by fitting absorption bands with Lorentzian oscillators. The smaller redshift indicates less $_{60}$ 1 hybridization, less symmetry breaking, and smaller pressure-induced distortion of the outer tube (Anis et al., 2012).

A critical pressure $_{60}$ 2 marks the onset of pressure-induced deformation of the outer-tube cross-section in DWCNTs. The redshift steepens near this pressure and then saturates into a plateau. This value is in very good agreement with theoretical predictions based on the effective radius concept,

$_{60}$ 3

which yields a predicted critical pressure of about 11 GPa for the estimated inner and outer diameters. By contrast, SWCNTs show earlier anomalies at $_{60}$ 4 GPa, $_{60}$ 5 GPa, and $_{60}$ 6 GPa, associated with circular-to-oval, oval-to-race-track or peanut-like, and collapse regimes. The peapod-derived inner tube therefore supports the outer wall mechanically, delaying pressure-induced structural transitions and making the outer tube more resistant to deformation (Anis et al., 2012).

This stabilization result is important because it gives a direct experimental signature of reinforcement induced by a peapod-derived architecture. It also provides a useful point of comparison for impact studies: in both slow compression and high-strain-rate loading, the encapsulated or derived inner component acts as a mechanical support.

5. High-strain-rate deformation and fracture pathways

Under ultrasonic-velocity impact, carbon nanotube peapods exhibit deformation and fracture behavior governed mainly by impact orientation and speed. Fully atomistic reactive molecular dynamics simulations using ReaxFF in LAMMPS modeled a $_{60}$ 7 SWCNT containing ten C $_{60}$ 8 molecules, equilibrated at 10 K in the NVT ensemble with Nosé–Hoover thermostats, then evolved in the NVE ensemble with a time step of 0.025 fs. Impact velocities ranged from 1 to 6 km/s in 1 km/s increments, and both vertical shooting and lateral shooting were examined. Observed outcomes include fullerene ejection, carbon nanotube fracture, fullerene and nanotube coalescence, and formation of amorphous carbon structures. For lateral impacts, only a few new covalent bonds form up to 4 km/s, whereas above 4 km/s bond formation increases significantly and begins to saturate; at 6 km/s, the bond-formation fraction reaches approximately 16% of the total number of atoms. The fullerenes experience higher local von Mises stress than the enclosing CNT, and the structural evolution proceeds from low-speed retention of the initial configuration to C $_{60}$ 9 rearrangement, fracture, and complete amorphization at the highest velocities (Sousa et al., 2020).

A particularly important result is the contrast with an empty nanotube. For the empty CNT, impact leads to radial collapse and unzipping of the tube. In the peapod, the fullerenes act as an internal mechanical obstacle that blocks this radial collapse, keeping the tube more intact and at lower stress for a longer time. This suggests that encapsulated molecules can suppress one fracture pathway while enabling others, rather than simply increasing or decreasing “strength” in a scalar sense (Sousa et al., 2020).

An analogous but materially distinct behavior appears in boron nitride nanotube peapods. In a fully atomistic ReaxFF study of a 14.9 nm long $(12,12)$ 0 BNNT containing 10 C $(12,12)$ 1 molecules with inter-fullerene spacing of 0.837 nm and total system size of 3480 atoms, the system was equilibrated in the NVT ensemble at 300 K for 1000 time steps using a chain of three Nosé–Hoover thermostats and then evolved in the NVE ensemble with a 0.025 fs time step for $(12,12)$ 2 time steps. Impacts from 1 km/s to 6 km/s were applied against a rigid van der Waals wall in vertical and horizontal geometries. Depending on velocity, the observed responses include tube bending, tube fracture, and C $(12,12)$ 3 ejection; under horizontal impact, fracture becomes extensive near the embedded C $(12,12)$ 4 molecules, and at 6 km/s the fullerenes are almost totally fragmented. A representative case at 3.3 km/s shows a maximum tensile stress of 74.6 GPa in the upper BNNT, followed by partial unzipping and formation of a bilayer nanoribbon incrusted with C $(12,12)$ 5 molecules (Sousa et al., 2022).

Across both CNT and BNNT hosts, impact direction is crucial because different orientations couple the incoming kinetic energy into different deformation modes. The recurring theme is that the encapsulated species guide stress localization, bond rearrangement, and fracture topology.

6. Nanoscale spectroscopy and magnetic behavior of metallofullerene peapods

Metallofullerene peapods provide a route to local chemical and magnetic characterization of endohedral molecular units inside CNT hosts. In Er $(12,12)$ 6N@C $(12,12)$ 7@CNT, the encapsulated fullerene cage already contains an Er $(12,12)$ 8N cluster, and the CNT diameter is comparable to the fullerene size, so the molecules line up one-dimensionally inside the tube. High-resolution transmission electron microscopy shows successful filling of CNTs with Er $(12,12)$ 9N@C $_{60}$ 0, the fullerenes arranged in a one-dimensional chain, the erbium atoms appearing as dark spots because of their high atomic mass, and evidence that the endohedral Er $_{60}$ 1N cluster remains intact inside the CNT (Fritz et al., 2017).

The same study combines HRTEM with scanning transmission X-ray microscopy and X-ray magnetic circular dichroism. By correlating HRTEM and STXM images, structures down to 30 nm are resolved with chemical contrast; CNT bundles as small as about 20–30 nm can be mapped at the Er $_{60}$ 2 edge, and a difference map constructed from on-resonance and off-resonance images identifies Er-rich peapod bundles within CNT aggregates. The authors recorded the first nanoscale X-ray absorption spectrum of an endofullerene peapod structure, with a clear Er $_{60}$ 3 peak and visible multiplet structure despite low signal-to-noise (Fritz et al., 2017).

XMCD measurements on bulk Er $_{60}$ 4N@C $_{60}$ 5 and on a macroscopic assembly of Er $_{60}$ 6N@C $_{60}$ 7@CNT peapods show that the magnetic moments extracted from both samples are essentially the same. Encapsulation inside CNTs therefore does not significantly alter the magnetic properties of the Er $_{60}$ 8 ions; the reduced magnetic moment relative to a fully isotropic paramagnet is attributed to ligand-field splitting and magnetic anisotropy, not to quenching by CNT confinement. This result is a useful corrective to a common overgeneralization: confinement in a nanotube can strongly affect mechanics and transport, but it need not erase intrinsic magnetic behavior of an encapsulated endohedral ion (Fritz et al., 2017).

7. Terminology and a separate computational usage

A separate usage of the name “peapods” appears in computational statistical mechanics, where peapods is an open-source Python package for Monte Carlo simulation of Ising spin systems with arbitrary coupling constants on arbitrary-dimensional hypercubic lattices with periodic boundary conditions. Its computational core is written in Rust and exposed to Python via PyO3; implemented algorithms include Metropolis and Gibbs single-spin-flip updates, Swendsen–Wang and Wolff cluster updates, and parallel tempering, with replica-level parallelism handled by the Rayon work-stealing scheduler (Pei, 22 Feb 2026).

The package adopts the standard Ising Hamiltonian

$_{60}$ 9

supports arbitrary bond arrays or built-in ferromagnetic, bimodal $_3$ 0, and Gaussian disorder couplings, and validates its implementation against the exact two-dimensional Ising critical temperature

$_3$ 1

using finite-size scaling of the Binder cumulant

$_3$ 2

For $_3$ 3, using 50,000 sweeps with Metropolis updates supplemented by Swendsen–Wang cluster updates and parallel tempering, the Binder cumulant curves intersect near $_3$ 4 and $_3$ 5, consistent with the exact critical temperature and the universal Binder cumulant value expected for the 2D Ising universality class (Pei, 22 Feb 2026).

This nomenclatural overlap is purely lexical. In the nanoscience literature, peapods denote confined host–guest nanotube assemblies; in the 2026 software literature, the same word names a Rust-accelerated Monte Carlo package.