An international collaboration bringing together J. Sloan and R. J. Kashtiban (University of Warwick, UK), E. Faulques and N. Kalashnyk (University of Lille, IEMN), S. Cordier and Y. Molard (University of Rennes, ISCR), J.-L. Duvail (University of Nantes, IMN), and V. G. Ivanov (University of Sofia, Bulgaria) recently presented a significant breakthrough for nanoscience.

The results are based on observations by high-resolution electron microscopy (HRTEM) and Raman spectroscopy on new hybrids formed by encapsulating an octahedral molybdenum cluster compound, Cs₂Mo₆Br₁₄, which is phosphorescent in the near-infrared, inside single-walled carbon nanotubes (SWCNTs) of various diameters. The insertion of these clusters into the nanotubes could allow tuning of the optical properties of the hybrids for future applications in telecommunications and biomedicine, notably within the transparency windows of this spectral region. However, another remarkable property has been observed: when the inner  diameter of the SWCNTs becomes comparable to the external diameter of the molecular anion [Mo₆Br₁₄]²⁻, the steric confinement imposed by the nanotube triggers an elimination process leading to the formation of [Mo₂Br₆]_x  entities, which can be regarded as one-dimensional (1D) Ising-type structures. Each unit in the chain interacts only with its two immediate neighbors, like aligned nanomagnets.

Such behavior opens up new possibilities in quantum computing and molecular electronics.

This study is part of a larger project aimed at understanding how, in SWCNTs with very small diameters, extreme confinement can profoundly alter the electronic properties of encapsulated molecules or crystals, while inducing the emergence of original 1D structures. For example, HgTe, which is semi-metallic and cubic in its bulk state, becomes a semiconductor with a 1D tetragonal structure. Similarly, SnSe is an orthorhombic semiconductor that adopts several 1D forms under confinement, including a semi-metallic hexagonal phase.