New Techniques of Depositing Detonation Nanodiamonds onto Substrates and Their Applications

Detonation nanodiamonds (DNDs) are considered exciting candidates for various applications, such as lubricant additives, semiconductor quantum dots, magnetic mapping sensors, protein mimics, and initial seeds for chemical vapor deposition (CVD) of diamond thin films on heterogeneous substrates. Essentially, these potential applications can be categorized into two groups: solution-based applications that use DND colloidal suspensions, and solid-state applications based on systems of DNDs supported by substrates, tips/probes, etc. The former applications have shown tremendous progress toward practical realization over the past decades. This progress is the result of thorough investigations and technological development in material preparation, including disassembling tightly bound aggregates, isolated/separated dispersion through graphitization or oxidization of surfaces, annealing in a hydrogen atmosphere, insertion of appropriate surfactants, surface functionalization with organic molecules, and stirred-media milling with ceramic or zirconia microbeads. Meanwhile, development of the latter applications has been primarily hindered by the technical challenge of depositing/coating DNDs on the surfaces of solid-state materials with the desired layer thickness, particle density, and surface roughness.<br/>Recently, we have established two new techniques for depositing DNDs onto substrates. One technique involves the nanometer-scale ordered arrangement of DNDs on substrates. The signs of surface charges, i.e., zeta potentials, of substrate surfaces are locally inverted through a combination of electron beam lithography and surface functionalization with 3-aminopropyltriethoxysilane. As a result, selective electrostatic deposition of DNDs onto the substrates is performed with line-and-space and dot array patterns at a length scale of ≥20 nm. Another technique involves the electrostatic layer-by-layer deposition of DNDs onto substrate surfaces using DND colloidal suspensions with different zeta potentials. By treating substrate surfaces in DND colloidal suspensions with positive and negative zeta potentials alternately, DNDs are deposited homogeneously on the substrate surfaces layer-by-layer.<br/>During the presentation, these techniques are introduced in detail, along with specific applications we have recently demonstrated. The nanometer-scale ordered arrangement technique is tested for the patterned growth of nanocrystalline diamond films. This could contribute to the manufacture of smaller diamond-based micro/nano electromechanical system devices through a bottom-up process, rather than the conventional top-down method that requires costly polishing and etching of the diamond films. Meanwhile, the electrostatic layer-by-layer deposition technique is applied to fabricate diamond photocatalytic electrodes, enabling CO₂ reduction with visible light irradiation. The results indicate that the DND layer thickness of the electrode strongly affects the photosensitivity of the CO₂ reduction reaction.