Abstract. Carbon dioxide (CO 2 ) is a gas that well represents air quality in indoor environments as well as being an important greenhouse gas. However, the reliable and affordable sensing of environmental CO 2 at room temperature, with techniques other than optical spectroscopy, remains an unsolved problem to this day. One major challenge for solid state sensors is the realisation of adequate selectivity, especially towards changing humidity. The thin film bulk acoustic resonator (FBAR) is a MEMS (Microelectromechanical systems) device that can not only detect gas-induced mass changes but also changes in the acoustic velocity and density of its layers. This multi-sensing provides a suitable platform for selective gas sensing. In this work we present studies done on polyaminosiloxane-and ethyl cellulose-functionalised FBARs regarding CO 2 sensitivity, selectivity towards humidity, and stability. We demonstrate how CO 2 and humidity signals can be separated and that CO 2 can be sensed with a resolution of 50 ppm between 400 and 1000 ppm. Using the Mason model, we show how the acoustic velocity and density of an absorption layer can be determined and how changes in those parameters affect the resonance frequency shift. The understanding of these results ultimately presents a tool to theoretically separate any number of gas analytes.

Silicon based slot waveguides with a slot containing Si nanocrystals (Si-nc) and Erbium ions (Er 3þ ) inside a silica matrix were prepared using sputter deposition and low-energy ion implantation. This sequence enabled independent optimization of nanocrystal formation and Er 3þ incorporation parameters. Using a superlattice approach, the size of the Si-nc inside the slot could be controlled and optimized for maximum Er 3þ luminescence yield at 1.54 lm. Er 3þ is found to be efficiently pumped by Si-nc of sizes around 3 to 4 nm. Increasing Er 3þ photoluminescence at 1.54 lm with increasing post-implantation annealing temperatures up to 1000 C is attributed to annealing of matrix or Si-nc interface defects mainly. Additionally, a dependence of the Er 3þ luminescence intensity on both the excitation and emission linear polarization orientation is shown, which demonstrates efficient field enhancement in sputtered slot waveguide structures. V C 2015 AIP Publishing LLC. [http://dx.

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