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

Abstract

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 10 8 to 10 10 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 ∼ 10 12 s −1 . Estimations based on the heat conductivity of ligand-stabilized nanocrystal arrays yield even higher values of the effective thermal flux ∼ 10 13 s −1 .

Quantum dot photoluminescence as charge probe for plasma exposed surfaces

“…This value was cross-checked by externally heating the substrate with the QDs without any plasma exposure, where the trend of the slow shift corresponded exactly to that of the temperature [23] while the fast shift observed in the plasma exposure experiments did not occur. Recently, a detailed evaluation of the thermal balance of surface-deposited QDs was published [25]. The results practically rule out any transient thermal effects that could obscure the quantumconfined Stark effect.…”

“…In that work, the idea of using the photoluminescence (PL) of semiconductor QDs as a diagnostic method for surface charging of nanoparticles due to plasma interaction was proposed. Furthermore, Pustylnik et al [24,25] have theoretically shown that a layer of QDs deposited on the surface of a microparticle could provide the possibility of measuring the surface charge of microparticles in plasmas.…”