Size-dependent thermal stability and optical properties of ultra-small nanodiamonds synthesized under high pressure

TL;DR

This study investigates how the size of ultra-small nanodiamonds affects their thermal stability and optical properties, revealing that smaller than 2 nm do not have enhanced stability and identifying a Fano-effect related to surface states.

Contribution

It provides new insights into the size-dependent properties of nanodiamonds synthesized via HPHT, including the mechanism of their formation and surface effects.

Findings

Nanodiamonds smaller than 2 nm are not more thermally stable.

Enhanced IR transmission near the Raman mode is observed due to Fano-effect.

Surface reconstruction influences Raman peak asymmetry.

Abstract

Diamond properties down to the quantum-size region are still poorly understood. High-pressure high-temperature (HPHT) synthesis from chloroadamantane molecules allows precise control of nanodiamond size. Thermal stability and optical properties of nanodiamonds with sizes spanning range from $<$1 to 8 nm are investigated. It is shown that hypothesis about enhanced thermal stability of nanodiamonds smaller than 2 nm is incorrect. The most striking feature in IR absorption of these samples is the appearance of an enhanced transmission band near the diamond Raman mode (1332 cm$^{-1}$). Following previously proposed explanation, we attribute this phenomenon to the Fano-effect caused by resonance of diamond Raman mode with continuum of conductive surface states. We assume that these surface states may be formed by reconstruction of broken bonds on the nanodiamond surfaces. This effect is also responsible for the observed asymmetry of Raman scattering peak. he mechanism of nanodiamond formation in HPHT synthesis is proposed, explaining pecularities of their structure and properties.