Toward ultra-thin nanocrystalline diamond film growth: Electrostatic self-assembly of non-aggregated diamond nanoparticles onto substrate surfaces

Diamond nanoparticles (DNPs) are exciting candidates for a diverse array of applications ranging from lubrication [1], qubits in quantum computing [2,3], semiconductor quantum dots for biomedical imaging [4,5], nanoscale magnetic sensors [6-8], drug delivery [9-13], through globular protein mimics [14] and reflectors for low-energy neutrons [15], to nucleation sites for chemical vapor deposition (CVD) of nanocrystalline diamond (NCD) films on non-diamond substrates [16,17]. For both biological applications using colloidal suspensions of DNPs and nanostructural applications based on solid-state substrate-supported DNP systems, a major weakness of DNPs is their tendency to form large, tightly bound aggregates [18]. Since the discovery of detonation-synthesized diamond nanoparticles [19], numerous approaches have been used to disintegrate the aggregates in colloidal suspensions; for example, surface graphitization and oxidation [20], insertion of surfactants into suspending mediums [21], stirred-media milling with microbeads of ceramic [18] or zirconia [22,23], and high-temperature annealing in hydrogen gas [24]. Owing to these attempts, monodisperse colloidal DNPs, having a narrow distribution of particle sizes centered on the core particle size (3-5 nm), are currently available for the biological applications using colloidal suspensions of DNPs [23,24]. Such ideal colloidal DNPs have directly been used in a technique called electrostatic self-assembly [25] in order to obtain non-aggregated DNPs on the substrate surfaces. To date, the electrostatic self-assembly of DNPs has been tested on various substrates such as Si [16], SiO2 [25], Cu [26], and AlN [27,28]. However, the sizes of the electrostatically self-assembled DNPs, on any of those sub-strate surfaces, are typically much larger than the core particle size (3-5 nm) [16,25,26,28]. This indicates that the monodisperse colloidal DNPs tend to re-aggregate during their electrostatic self-assembly onto substrate surfaces. Based on this background, the main focus of this research is to clarify the re-aggregation mechanism of DNPs during their electrostatic self-assembly onto substrate surfaces plus to succeed in forming non-aggregated DNPs on substrate surfaces. On that basis, the final objective of this research is set to be successful CVD of ultra-thin NCD films on substrates, as it is one of the most remarkable applications of non-aggregated DNPs based on solid-state substrate-supported DNP systems. As a substrate material to be focused in this research, Si and/or SiO2 are chosen since they are the most commonly used substrate materials in materials science and engineering. Firstly, the aggregation kinetics of monodisperse colloidal DNPs was studied based on the underlying theory of nanoparticle aggregation in liquids. It was consequently revealed that the monodisperse colloidal DNPs are highly stable in the suspension and do not tend to aggregate owing to the dominantly strong long-ranging electrical double layer (EDL) repulsive interaction between them. Based on this understanding, secondly, the re-aggregation mechanism of DNPs during their electrostatic self-assembly onto SiO2 surfaces was considered. When the minimum surface distance (MSD) between the DNPs during the electrostatic self-assembly was focused, it was turned out that the re-aggregation of DNPs is due to the increased positive interaction potential between the DNPs caused by the forced approach of the DNPs to each other. This promotes the DNPs to get over the interaction potential barrier of ~8.4 kBT so that the entire interaction potential between the DNPs can be reduced. Thirdly, a strategy to succeed the formation of non-aggregated DNPs on SiO2 surfaces was developed. As the characteristic decay length of the dominantly strong positive EDL interaction potential needed to be decreased in order to reduce the high positive interaction potential between the DNPs, increasing the bulk ion concentration of the monodisperse colloidal DNPs could be a promising strategy. This strategy was carried out using KCl as an electrolyte and, as a result, the characteristic decay length of the dominantly strong positive EDL interaction potential was decreased as intended. Fourthly, the novel electrostatic self-assembly, using the monodisperse colloidal DNPs with the increased bulk ion concentration, was examined onto SiO2 surfaces. It was clarified that the bulk ion concentration of 1.0×10-3 M drastically suppresses the re-aggregation of the DNPs during the electrostatic self-assembly onto SiO2 surfaces and results in a successful formation of non-aggregated DNPs (5.6 nm in average size) on SiO2 surfaces. In this manner, non-aggregated DNPs were successfully formed on SiO2 surfaces via the newly developed idea and technology. Fifthly, this achievement was applied for the CVD of ultra-thin NCD films on substrates to complete the final objective of this research. Aside from the novel electrostatic self-assembly, a suitable condition of CVD was considered to realize rapid coalescence of NCD grains during the growth. According to the NCD growth theory, it was turned out that the control of a-parameter is useful for the rapid grain coalescence as the a-parameter represents differences in growth rates of NCD grains for each crystallographic direction. The most appropriate a-parameter (𝛼~1.73) was theoretically and experimentally determined and then the CVD conditions that fulfil such an a-parameter are experimentally clarified. Finally, based on the formation of non-aggregated DNPs on substrate surfaces plus the CVD with the optimized growth conditions for rapid grain coalescence, CVD of ultra-thin NCD films was examined in an ellipsoidal microwave plasma CVD reactor. The resulted films were characterized via scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction (XRD), and electrochemical voltammetry. From these investigations, the excel-lent potential of non-aggregated DNPs placed on substrate surfaces for the CVD of pinhole-free ultra-thin NCD films was revealed. As a final remark, consequently, ~10 nm thick continuous NCD films, containing an extremely low density of pinholes, were successfully obtained and completely pinhole-free ~30 nm thick NCD films be-came reliably accessible via the novel electrostatic self-assembly plus the CVD optimized conditions for rapid grain coalescence.