Colloidal Quantum Dots (QDs) are an established class of optoelectronic materials thanks to their tunable and highly efficient emission. The encapsulation of QDs in silica shells is a well-known procedure to protect them from the external environment and obtain dispersibility in polar solvents. We have coated different types of QDs (CdSe@CdS [1] and InP@ZnS [2]) with silica shells of different thickness and develop a gold film over them.
First, silica shells were synthesized using reverse microemulsion reaction in combination with an experimental design approach [3,4]. By varying the number of reverse microemulsion reactions performed, shells with diameters ranging from 40 nm to 70 nm were obtained. To further increase the nanoparticle size (80 – 110 nm), a Stöber reaction was employed [5]. In this case, to prevent self-nucleation of empty silica particles, the silica precursor (tetraethyl orthosilicate) was slowly injected using a syringe pump. Finally, we deposited a uniform gold film over the 50 nm diameter silica shells to combine the QDs emission with the gold plasmon resonance, thus imparting a variety of effects on the optical properties of the QDs, as already demonstrated [6]. To grow an Au film, preliminary steps are required as functionalization of silica with aminosilane and subsequent addition of gold seeds. We employed (3-Aminopropyl)trimethoxysilane (APTMS), which has an amino group that can coordinate the gold while the silicon binds strongly to the silica surface through a covalent bond. Thanks to the 1H and two-dimensional NOESY (Nuclear Overhauser Effect Spectroscopy) NMR analyses, we assessed the bonding of the APTMS to the silica surface and determined the best ratio of injected molecules to the surface area of the nanoparticle to obtain complete coverage. Then, we attached gold seeds to the silica surface, and we merged them by adding gold precursor through a slow injection, employing a syringe pump. Through this approach, we avoided self-nucleation of gold nanoparticles, achieving uniform gold coverage. Given the tunable thickness of the SiO₂ shell, one can imagine employing the gold film to build an optical microcavity around the QD. In general, silica shells around QDs can be used as a template to build more sophisticated optical structures and consequently to impart additional optical features to the emitter.