Advances in Preparation and Understanding of Ultrasmall Nanodiamonds

Understanding of diamond properties in ultrasmall region of few nanometers is important, yet challenging task. Since spectroscopic characterization reflects the mass/volume-weighted particle size distribution, only samples containing volumetrically dominant sub-5 nm nanodiamonds (NDs) may be used for reliable characterization. However, conventional detonation nanodiamonds (DNDs) and top-down high-pressure high-temperature (TD_HPHT) NDs do not generally meet this criterium and so additional approaches and synthetic methods are needed.<br/>In this work we show i) isolation of volumetrically-dominant sub-5 nm ND fractions from commercially available DNDs and TD_HPHT NDs by combination of ND size reduction followed by ultracentrifugation [1]; ii) bottom-up HPHT synthesis of sub-5 nm NDs (BU_HPHT NDs) from halogenated adamantanes at 8 GPa with excellent size control in the sub-5 nm region achieved by variation of the synthesis time and temperature [2]. Successful preparation of the sub-5 nm samples of three different ND kinds provides great opportunity not only to compare the typical spectral features of each ND kind but also to investigate size-dependent phenomena such as evolution of the Raman diamond peak parameters (position and full width at half maximum) down to 1.2 nm. Comparison of Raman spectra of DND and TD_HPHT NDs on the sub-5 nm scale reveals clear differences between the two types of NDs. We show that diamond core size, structural quality/defects, and temperature (in)stability caused by laser irradiation during the measurement are key features reflected in the particular NDs’ Raman spectrum[1]. Raman spectra of the BU_HPHT NDs exhibit some additional unique features compared to conventional DNDs and TD_HPHT NDs. The most striking is nearly complete absence of sp²-C related features in most of the investigated samples.<br/>The size-dependent evolution of diamond peak parameters was investigated by multi-wavelength (442, 532, and 633 nm) Raman spectroscopy to identify and/or avoid any heating effect of the excitation laser on the diamond line position. Use of multiple wavelengths also enabled to study dispersion of Raman active bands. We show that down to 2.6 nm the diamond Raman line shift is only minor (2-3 cm^-1) which do not corroborate with the current phonon confinement models often applied to NDs. Only when the size decreases below 2 nm, larger shifts are observed (8-9 cm^-1 for 1.4 nm sample).<br/>Finally, we provide analysis of the first-time observed low-frequency (20−200 cm^-1) NDs related Raman scattering signals assigned to “breathing”-like acoustic modes as these signals exhibit clear size dependence. We show that in contrast to diamond Raman line parameters these features are more suitable for size analysis of sub-4 nm NDs as we evidenced a good match with the size values obtained from XRD data (Scherrer’s formula)[3].<br/> <br/>[1] S. Stehlik, M. Mermoux, B. Schummer, O. Vanek, K. Kolarova, P. Stenclova, A. Vlk, M. Ledinsky, R. Pfeifer, O. Romanyuk, I. Gordeev, F. Roussel-Dherbey, Z. Nemeckova, J. Henych, P. Bezdicka, A. Kromka, B. Rezek, Size Effects on Surface Chemistry and Raman Spectra of Sub-5 nm Oxidized High-Pressure High-Temperature and Detonation Nanodiamonds, J. Phys. Chem. C. 125 (2021) 5647–5669. https://doi.org/10.1021/acs.jpcc.0c09190.<br/>[2] E.A. Ekimov, S.G. Lyapin, Yu.V. Grigoriev, I.P. Zibrov, K.M. Kondrina, Size-controllable synthesis of ultrasmall diamonds from halogenated adamantanes at high static pressure, Carbon. 150 (2019) 436–438. https://doi.org/10.1016/j.carbon.2019.05.047.<br/>[3] A. Vlk, M. Ledinsky, A. Shiryaev, E. Ekimov, S. Stehlik, Nanodiamond Size from Low-Frequency Acoustic Raman Modes, J. Phys. Chem. C. 126 (2022) 6318–6324. https://doi.org/10.1021/acs.jpcc.2c00446.