Quantum dot on a plasma‐facing microparticle surface: Requirement on thermal contact for optical charge measurement

Semiconductor nanocrystals, quantum dots (QDs), are known to exhibit the quantum-confined Stark effect which reveals itself in the shift of their photoluminescence spectra in response to the external electric field. It was, therefore, proposed to use QDs deposited on the microparticle surface for the optical measurement of the charge acquired by the microparticles in low-temperature plasmas. Another physical process leading to the shift of the photoluminescence spectra of the QDs is heating. Charging of plasma-facing surfaces is always accompanied by their heating. Thermal balance of a quantum dot residing on the surface of a microparticle immersed in a plasma is considered in this work. It is shown that under periodically pulsed plasma conditions, the spectral shift of the photoluminescence of the quantum dot caused by the oscillations of its temperature becomes undetectable if the effective thermal flux characterizing the thermal contact between the quantum dot and the microparticle exceeds the value ranging from 108$$ {10}<^>8 $$ to 1010$$ {10}<^>{10} $$ s-1$$ {\mathrm{s}}<^>{-1} $$ depending on the plasma parameters. If this effective thermal flux exceeds the above-mentioned values, the entire spectral shift observed during the period of plasma pulsing should be attributed to the quantum-confined Stark effect due to the microparticle charge. Lower-boundary estimate for the effective thermal flux for the direct contact between the quantum dot and the microparticle is similar to 1012$$ \sim {10}<^>{12} $$ s-1$$ {\mathrm{s}}<^>{-1} $$. Estimations based on the heat conductivity of ligand-stabilized nanocrystal arrays yield even higher values of the effective thermal flux similar to 1013$$ \sim {10}<^>{13} $$ s-1$$ {\mathrm{s}}<^>{-1} $$.