2, Institute for Sustainable Systems Engineering (INATECH), Albert-Ludwigs-University Freiburg, Freiburg, , Germany
Diamond nanoparticles (DNPs) are exciting candidates for various applications ranging from lubrication, semiconductor quantum dots, drug delivery, through globular protein mimics and reflectors for low-energy neutrons, to nucleation sites for chemical vapor deposition (CVD) of nanocrystalline diamond (NCD) films. For both biological applications using colloidal suspensions of DNPs and nanostructural applications based on solid-state substrate-supported DNP systems, a major drawback of DNPs is their tendency to form large, tightly bound aggregates. Since the technology to obtain monodisperse colloidal DNPs, having a narrow distribution of particle sizes centered on the core particle size (3 - 5 nm), was established in the first decade of the 21th century, the biological applications using colloidal DNPs have rapidly progressed. Such ideal colloidal DNPs have directly been used in a technique called electrostatic self-assembly for nanostructural applications based on solid-state substrate-supported DNP systems. However, the sizes of the electrostatically self-assembled DNPs on substrate surfaces were typically much larger than the core particle size.
In this talk, the conventional electrostatic self-assembly, which was realized via the control of pH in the colloidal solutions and zeta potentials of DNPs and substrate surfaces, is firstly overviewed. The electrostatic self-assembly of non-aggregated DNPs onto substrate surfaces is then demonstrated by modifying the salt concentration in colloidal DNP solutions. Several salt concentrations of colloidal DNPs were prepared using KCl and were examined with respect to electrostatic self-assembly. In addition, the interaction energies between DNPs in each of the examined colloidal suspensions were considered on the basis of the DLVO theory. To further investigate the interaction between DNPs and SiO2 surfaces in each of the examined colloidal suspensions, NCD-coated silica micro spheres attached to tipless cantilevers (k = 0.03 N/m) were fabricated, which readily allow probing the force interactions of diamond with different substrate materials by means of force spectroscopy. As a result of these measurements, it was clarified that the salt 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 the end, this salt-assisted electrostatic self-assembly was applied for the CVD of ultra-thin NCD films on Si substrates. Aside from the developed electrostatic self-assembly technique, suitable conditions of CVD were identified to promote rapid coalescence of NCD grains. 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 became reliably accessible via the salt-assisted electrostatic self-assembly.