Uniform coating of separated nanodiamonds <i>via</i> thin polymer-assisted electrostatic self-assembly for thermal sensing

Nanodiamonds (NDs) with nitrogen-vacancy (NV) centers have shown significant potential for nano- and micro-scale local temperature sensing and imaging. However, the challenge lies in achieving non-invasive measurement, due to the high thermal conductivity of diamond that can cause strong thermal spreading and heat dissipation. A solution to this problem is to prepare separated NDs for thermal sensing. Here, we introduce a method for uniform coating of separated NDs through a rapid thin polymer-assisted electrostatic self-assembly process. This method can be applied on a large scale at low cost under ambient conditions. By testing with negatively charged carboxylated 100 nm NDs, we demonstrate ND coatings with a controlled area density (up to 1.74 × 10⁹ cm^-2) and a low optical transmission loss of 1.21%. Despite the size variation of the NDs, the separated ND coating exhibits a low measurement error of 1.32 °C in thermal mapping between 27 and 80 °C. Furthermore, by steady-state thermal simulations, we show more than a sevenfold reduction in thermal spreading of the separated ND coating compared to a 100 nm thick ND film. This work offers a simple method for large-scale coating of separated NDs, paving the way for achieving non-invasive and accurate ND-based thermal sensing.