Diaphite-structured nanodiamonds with six- and twelve-fold symmetries

Nanodiamonds (ND) with 1-5 nm dimensions found in meteorites or produced by chemical vapour deposition (CVD) and detonation synthesis are typically described in terms of an sp³-bonded carbon network. However, ultra-high-resolution transmission electron microscopy (uHRTEM) combined with density functional theory (DFT) modelling leads to a different structural interpretation. uHRTEM imaging and nanodiffraction studies of many NDs show six-fold symmetry features whose identity has long been controversial. We also observe diffraction patterns with twelve equally-spaced and symmetrically but unequally arranged reflections, indicating structures with crystallographically-forbidden ideal and distorted twelve-fold symmetry. Structural models based on our DFT calculations lead to an interpretation of these unusual features found throughout the meteoritic and CVD samples in terms of sp³ domains arranged around and coherently bonded to graphitic domains embedded within the diamond matrix. The bonding at the sp²-sp³ interface can explain the unusual features observed in electron energy-loss spectra (EELS) below the onset of the main diamond C1s core-loss edge leading to predictions of low-dimensional conductivity behaviour. The presence of sp²- as well as sp³-bonded regions allows us to interpret previously unexplained features of the Raman spectra and EELS data of ND materials.