Size decrease of detonation nanodiamonds by air annealing investigated by AFM
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Size decrease of detonation nanodiamonds by air annealing investigated by AFM Stepan Stehlik1, Daria Miliaieva1,2, Marian Varga1, Alexander Kromka1, Bohuslav Rezek1, 2 1 Institute of Physics, ASCR, Cukrovarnicka 10, Prague 16200, Czech Republic 2 Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 16627 Prague 6, Czech Republic ABSTRACT Nanodiamonds (NDs) represent a novel nanomaterial applicable from biomedicine to spintronics. Here we study ability of air annealing to further decrease the typical 5 nm NDs produced by detonation synthesis. We use atomic force microscopy (AFM) with sub-nm resolution to directly measure individual detonation nanodiamonds (DNDs) on a flat Si substrate. By means of particle analysis we obtain their accurate and statistically relevant size distributions. Using this approach, we characterize evolution of the size distribution as a function of time and annealing temperature: i) at constant time (25 min) with changing temperature (480, 490, 500°C) and ii) at constant temperature (490°C) with changing time (10, 25, 50 min). We show that the mean size of DNDs can be controllably reduced from 4.5 nm to 1.8 nm without noticeable particle loss and down to 1.3 nm with 36% yield. By air annealing the size distribution changes from Gaussian to lognormal with a steep edge around 1 nm, indicating instability of DNDs below 1 nm. INTRODUCTION Nanodiamonds (NDs), i.e. diamond particles with sizes below 100 nm, represent a novel nanomaterial with interesting features applicable from biomedicine to spintronics [1, 2, 3]. NDs with typical size of 5 nm are nowadays routinely produced by detonation from oxygen-deficient explosives on an industrial scale. Such detonation nanodiamonds (DNDs) have about 20% of the total number of atoms on the surface. This number would further increase with decreasing nanodiamond size due to increase in surface to volume ratio. Controllable size decrease of NDs below 2 nm may, therefore, enable fundamental study of their stability, structure, chemical reactivity, bio-interactions as well as quantum phenomena [4]. If such molecular-sized NDs contain a color center such as silicon-vacancy (SiV) center [5], they may serve as excellent fluorescent labels for fluorescence imaging with no bleaching and minimal perturbation to biological mechanisms such as protein interactions in living cells [6]. Several approaches were suggested to decrease the DND size. Pichot et al. [7, 8] decreased the mean size to 2.9 nm by using nanotextured explosives, i.e. by the tuning of the detonation parameters. Etzold et al. [9] used self-limiting layer-by-layer oxidation process and reported size decrease from 5.2 to 4.8 nm after 15 oxidation cycles. Another perspective approach is air annealing which is often used for purification of NDs from non-diamond carbon [10, 11] and for surface oxidation of DNDs [12, 13]. Since diamond in general consists of sp3 hybridized carbon atoms, its reaction with the air oxygen can be in principle used for an oxidative diamond etching of monocry
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