Stepan Stehlik1,2,Katerina Dragounova Aubrechtova2,Ekaterina Shagieva2,Rostislav Medlin1,Petr Belsky1,Tomas Kovarik1,Evgeny Ekimov3,Stepan Potocky4,Bohuslav Rezek4,Alexander Kromka2
New Technologies – Research Centre, University of West Bohemia1,Institute of Physics, AS CR2,Institute of High Pressure Physics, Russian Academy of Sciences3,Faculty of Electrical Engineering, Czech Technical University in Prague4
Stepan Stehlik1,2,Katerina Dragounova Aubrechtova2,Ekaterina Shagieva2,Rostislav Medlin1,Petr Belsky1,Tomas Kovarik1,Evgeny Ekimov3,Stepan Potocky4,Bohuslav Rezek4,Alexander Kromka2
New Technologies – Research Centre, University of West Bohemia1,Institute of Physics, AS CR2,Institute of High Pressure Physics, Russian Academy of Sciences3,Faculty of Electrical Engineering, Czech Technical University in Prague4
Boron-doped diamond is an excellent electrode material due to its large potential window, robustness, and antifouling properties. While boron-doped diamond in the form of CVD-grown mono or polycrystalline diamond films is already a well-established material, reports on boron-doped nanodiamonds (BNDs) are rare, and BNDs are not broadly accessible. Yet, their potential is tremendous e.g. in (photo)catalysis and biomedicine. BNDs were at first obtained by delamination and milling of CVD films (10.1021/nn500573x), showing also some positive effect on conductivity when used for nucleation (10.1016/j.carbon.2014.07.048). A more scalable and promising technique for making of BNDs is a high-pressure high-temperature (HPHT) synthesis from organic molecular precursors. This process offers unprecedented control of the ND size, enables the dopant incorporation in a high concentration, and provides monocrystalline NDs (10.1021/acsbiomaterials.0c00505). Here we investigate whether a nucleation layer from these HPHT BNDs, can potentially lead to better quality (morphology, electrical properties) of the thin (50-200 nm) boron-doped CVD diamond films compared to the standard nucleation with detonation nanodiamonds (DNDs).<br/>We used the BNDs that were synthesized from borabicyclo[3.3.1]nonane dimer C<sub>16</sub>H<sub>30</sub>B<sub>2</sub> under HPHT conditions (8-9 GPa, 1250°C). The obtained black powder was analyzed by Raman and FTIR spectroscopies, which showed relatively high sp<sup>2</sup>-C content and an abundance of C-H<sub>x</sub> bonds. To obtain pure BNDs, the powder was boiled under reflux in a mixture of H<sub>2</sub>SO<sub>4</sub> and HNO<sub>3</sub> (3:1) for 6 hours until the bubbling of the mixture disappeared. Surprisingly, boiling in acids led to complete breakage of the interparticle bonds, and after the centrifugal acid removal, well-dispersed deep blue BND colloid was obtained just by gentle shaking of the sediment. Raman spectroscopy showed a spectrum typical for highly boron-doped diamond without any significant sp<sup>2</sup>-C content. FTIR and zeta potential analysis showed an oxidized surface (presence of C-O and C=O bonds and negative zeta potential of -31 mV). To determine the BND size we used small angle X-ray scattering (SAXS) that showed the mode particle size to be 5 nm. TEM analysis confirmed well-crystalline and well-dispersed BNDs. In order to reverse the zeta potential of the BNDs to become positive, the oxidized BNDs were lyophilized and then hydrogenated at 700°C for 3h. After hydrogenation, the zeta potential of BNDs indeed became positive (44 mV), i.e., similar to the intrinsic NDs after hydrogenation which is important for the achievement of densely seeded substrates with a negative zeta potential. The nucleation layer was created on Si substrates as well as on SiO<sub>2</sub>/Au interdigital electrodes. The morphology and density of the nucleation layers obtained with H-BNDs was investigated by AFM and SEM. Finally, boron-doped CVD diamond film was grown over the nucleation layer on both substrate types, and its morphology, structure, and electrical properties were characterized and compared to the standard nucleation obtained with DNDs.