Spontaneous emission of precisely positioned quantum dots in 3D silicon photonic band gap crystals
Andreas Schulz is a PhD student in the Department of Complex Photonic Systems. Promotors are prof.dr. W.L. Vos, prof.dr.ir. J. Huskens and prof.dr. G.J. Vancso from the Faculty of Science & Technology.
It is well known from quantum mechanics that the control of photonic band gap crystals over the spontaneous emission of embedded quantum emitters - such as semiconductor nanocrystal quantum dots - depends sensitively on where the emitters are located. Therefore, it is an outstanding challenge to derive methods to precisely position emitters inside photonic crystal nanostructures. This thesis takes on this challenge by the use of modern chemical methodologies: we study how to position quantum dots inside photonic band gap crystals with polymer brushes. Therefore, we investigated a suitable surface chemistry via silane monolayer formation, polymer brushes, and the covalent attachment of lead sulfide quantum dots with a suitable organic ligand. The knowledge we initially obtained from flat silicon substrates was used as input to derive advanced methods for complex 2D and 3D silicon photonic crystal nanostructures.
We have shown that the polymer must be judiciously chosen in order to control the layer thickness of the grafted polymer brushes on silicon. By choosing poly(N-isopropylacrylamide) layers, we achieved a precise control over the layer thickness by varying the activating and de-activating catalyst species. For poly(glycidyl methacrylate) (PGMA) we were able to control the polymer layer thickness by the polymerization time. Since the grafting of poly(glycidyl methacrylate) brushes was more reliable, we decided to proceed with the epoxide-containing polymer.
In order to precisely position the quantum dots within the pores of 3D silicon photonic crystals, it was necessary to develop a sophisticated surface chemistry strategy. Since we made the choice for PGMA layers, a suitable organic ligand on the lead sulfide (PbS) quantum dots was important. We reacted the epoxide side chains of PGMA with amine-PEGylated lead sulfide quantum dots. By varying the coupling conditions, we achieved densely packed quantum dots attached to PGMA brushes.
Near-infrared emitting quantum dots were studied in this thesis because their emitted photons are not absorbed by the silicon backbone of our 3D photonic band gap crystals. We chose lead sulfide quantum dots for our studies because they are commercially available with various organic ligands. Quantum dot suspensions were studied to measure spectra and time-resolved emission rates of the nanocrystals. We successfully interpreted the decay rates of the lead sulfide quantum dot suspensions with a model for an ideal two-level exciton, as well as an adapted model.
In order to apply the acquired surface chemistry, we had to ensure that the silicon photonic crystals have very clean silicon-air surfaces after the fabrication. Therefore, we had to overcome the problem that after the fabrication of our silicon photonic crystals, fluorocarbon residues are present on the pore walls of the crystals. These contaminations had to be removed by several harsh cleaning steps. Our cleaning procedure, consisting of RCA-2 cleaning and an oxidation step at 800 °C in a furnace, resulted in ultra-clean silicon photonic crystal samples that could be used for the functionalization with quantum dots by means of elaborate surface chemistry.
One highlight of this thesis consists of the experimental observations of quantum dots inside 3D photonic band gap crystals made at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The silicon photonic crystals were analyzed by X-ray tomography to determine the electron density. We were able to analyze the chemical composition of our crystals using X-ray fluorescence tomography. To the best of our knowledge, our study is the first to obtain experimental data on silicon nanostructures doped with quantum dots whose positions were controlled by polymer brushes. We present the strengths and advantages of X-ray fluorescence tomography compared to other techniques. After a detailed analysis of the acquired data, insights into the quantum dot coverage inside the pores of the photonic crystals were obtained. In addition, we were able to characterize the distinctive bromine atoms of the ATRP initiator molecules and the chlorine atoms of the polymer brushes.
The “pièce de résistance” of the thesis consists of optical measurements on our best samples, which were studied by X-ray imaging. The silicon photonic crystal samples were studied in terms of spectra and time-resolved emission of the embedded PbS quantum dots. We observed a record of strongly inhibited emission power in the range of the 3D photonic band gap, in agreement with theory. The successful results were reproduced by emission measurements on different 3D photonic crystals. The decay rates of quantum dots positioned in photonic crystals were lower than those of dots positioned on a flat polymer layer outside the crystal on flat silicon. Compared to suspensions, the decay rates were very fast and we estimate very low quantum efficiencies of the dots positioned in the photonic crystals. Our results open up new avenues for functionalizing the sample architecture to create novel types of organic solar cells with high efficiencies.
