Quantum dot photoluminescence as a versatile probe to visualize the interaction between plasma and nanoparticles on a surface

We experimentally demonstrate that the interaction between plasma and nanometer-sized semiconductor quantum dots (QDs) is directly connected to a change in their photoluminescence (PL) spectrum. This is done by taking in situ, high resolution, and temporally resolved spectra of the light emitted by laser-excited QDs on an electrically floating sample exposed to a low pressure argon plasma. Our results show a fast redshift of the PL emission peak indicating the quantum-confined Stark effect due to plasma-generated excess charges on the substrate and near the QD surface, while other plasma-induced (thermal and ion) effects on longer timescales could clearly be distinguished from these charging effects. The presented results and method open up pathways to direct visualization and understanding of fundamental plasma– particle interactions on nanometer length scales. VC 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0067532 In the field of complex plasmas—i.e., ionized gases containing nanometerto micrometer-sized particles—the charge of plasmaimmersed particles is the parameter of most interest. This is because obtaining elementary knowledge regarding plasma–particle charging is the main key to understanding many fundamental processes, such as those in the planetary rings of Saturn, in strongly coupled collective particle–particle interactions, e.g., in plasma crystals in earth laboratories and in the International Space Station, and in plasma– particle interactions in the form of dust density waves (DDWs). From an application point of view, plasma charging plays an essential role in in situ plasma-assisted synthesis of nanometerto micrometer-sized structures and plasma–particle interaction in nuclear fusion technology. More recently, with the ever-shrinking length scales in nanomanufacturing, using the surface-charge-driven interaction between contaminating nanoparticles and photon-induced plasmas may become the only route to effective nanoparticle contamination control. Although it has drawn the attention of the entire research community, thus far the fundamental interaction between plasma and particles on the nm size scale has been largely unexplored. This is mostly because of the nonexistence of experimental data needed to verify the few modeling efforts available in the literature. The reason for this lack of experimental data for particles of nm length scales is that, currently, all experimental methods to obtain information about the particle charge—such as the particle resonance method, forcebalancing, particle interactions with electric fields, and Mie ellipsometry and dust density waves analysis—are either directly or indirectly based on light scattering techniques, which face a substantial loss of signal-to-noise in the nm regime due to the sixth-powerdependence of the signal intensity I on the particle size r (I / r6). In this Letter, we demonstrate that the interaction between plasma and nm-sized semiconductor quantum dots (QDs) is directly connected to and can be visualized by a change in their photoluminescence (PL) emission spectrum. This is done by bringing an electrically floated QD-deposited sample into contact with a low pressure argon plasma and monitoring the photoluminescence (PL) spectrum of the QDs in a time-resolved fashion before, during, and after plasma exposure. QDs behave as zero-dimensional quantum wells for charge carriers. When these nanostructures are optically excited, an electron– hole pair is generated and confined in the quantum dot. This electron–hole pair relaxes back to the bound states in the quantum well and, finally, recombines. During the latter process, a photon with a well-defined energy is emitted. When considering an ensemble of Appl. Phys. Lett. 119, 254104 (2021); doi: 10.1063/5.0067532 119, 254104-1 VC Author(s) 2021 Applied Physics Letters ARTICLE scitation.org/journal/apl