Size-Dependent Thermal Stability and Optical Properties of Ultra-Small Nanodiamonds Synthesized under High Pressure

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 the existing 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 the previously proposed explanation, we attribute this phenomenon to the Fano effect caused by resonance of the 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. The mechanism of nanodiamond formation in HPHT synthesis is proposed, explaining peculiarities of their structure and properties.

Absolute energy levels in nanodiamonds of different origins and surface chemistries

“…Interestingly, recent reports reveal that it can also be applied to investigate electrical conductivity in hydrogenated NDs. 57,58 The FTIR spectra of HPHT ND (Fig. 1c) and DND (Fig.…”

Nanodiamonds as Charge Extraction Layer in Organic Solar Cells: The Impact of the Nanodiamond Surface Chemistry

“…Such a finding is in good agreement with reports about hydrogenated nanodiamonds bearing an sp 2 ‐phase at their surface. [ 46,52,57,58 ] The Raman spectra recently reported by Mikesova et al on the same type of nanodiamonds depicted a significant amount of non‐ sp 3 carbon manifested by the presence of a pronounced G‐band and attributed to graphitic content in the case of HPHT ND‐ar. The purification of HPHT ND‐ar by air annealing considerably reduced the intensity of the non‐ sp 3 carbon band and there is only remaining isolated sp 2 carbon on HPHT ND‐O.…”

Nanodiamond Size from Low-Frequency Acoustic Raman Modes

“…Moreover, the lattice defects and/or surface functional groups may influence the Raman signal. Since the BU_HPHT NDs in this study are monocrystals with excellent crystallinity (see the HRTEM images in the SI), we do not expect significant changes in the acoustic Raman modes caused by defects since X-ray fluorescence and FTIR spectroscopy showed that the surface of the BU_HPHT NDs is hydrogen terminated; i.e., the BU_HPHT NDs can be considered as “ligand-free” …”

“…Since the BU_HPHT NDs in this study are monocrystals with excellent crystallinity (see the HRTEM images in the SI), we do not expect significant changes in the acoustic Raman modes caused by defects since X-ray fluorescence and FTIR spectroscopy showed that the surface of the BU_HPHT NDs is hydrogen terminated; i.e., the BU_HPHT NDs can be considered as "ligand-free". 46 Figure 2 shows the low-frequency features of the 1.2−2.6 nm samples. The data were obtained by 442 and 633 nm excitations.…”