‘Black silicon’ (bSi) samples with surfaces covered in nanoneedles of length ~5 µm were fabricated using a plasma etching process and then coated with a conformal uniform layer of diamond using hot filament chemical vapour deposition to produce ‘black diamond’ (bD) nanostructures. The diamond needles were then chemically terminated with H, O, NH 2 or F using plasma treatment, and the hydrophilicity of the resulting surfaces were assessed using water droplet contact-angle measurements, and scaled in the order O > H ≈NH 2 >F, with the F-terminated surface being superhydrophobic. The effectiveness of these differently terminated bD needles in killing the Gram-negative bacterium E. coli was semi-quantified by Live/Dead staining and fluorescence microscopy, and visualised by environmental scanning electron microscopy. The total number of adhered bacteria was consistent for all the nanostructured bD surfaces at around 50% of the value for the flat diamond control. This, combined with a chemical bactericidal effect of 20–30%, shows that the nanostructured bD surfaces supported significantly fewer viable E. coli than flat surfaces. Moreover, the bD surfaces were particularly effective at preventing the establishment of bacterial aggregates – a precursor to biofilm formation. The percentage of dead bacteria also decreased as a function of hydrophilicity. These results are consistent with a predominantly mechanical mechanism for bacteria death based on the stretching and disruption of the cell membrane, combined with an additional effect from the chemical nature of the surface.

Carbide forming interlayers, such as amorphous silicon nitride, are typically used for GaN-on-diamond heterogenous integration. This interlayer has a low thermal conductivity, introducing an additional extrinsic interfacial thermal resistance. It may therefore be advantageous to omit this layer, directly bonding GaN-to-diamond (van der Waals bond). However, weakly bonded interfaces are known to increase the intrinsic thermal boundary resistance. An adapted acoustic mismatch model has been implemented to assess which bonding approach is the most optimal for low thermal resistance GaN-on-diamond. A high thermal boundary resistance of 200 m2 K GW−1 is predicted for weakly bonded GaN-to-diamond interfaces, which is close to the measured value of 220 ± 70 m2 K GW−1, and ∼7× higher than values measured when a 10’s nm-thick SiN interlayer is included. Covalently bonded interfaces are therefore critical for achieving low thermal resistance GaN-on-diamond.