Optical and chemical sensing investigation of InP surface quantum dots

Photophysical properties of semiconductor quantum dots (QDs), such as broad absorption band and size dependent spectral emission, together with the high effective surface area available for interaction with target chemicals are attractive for applications like chem-/bio-sensors and lab-onchips. In the present work the state of the art of the research field will be discussed, with particular emphasis on the physical and chemical properties of near-infrared (NIR) emitting quantum dots. So far most of the reports of chem-/bio-sensing with QDs are based on colloidal nanocrystals synthesized by wet chemistry methods. Recently, a new approach has been introduced, which made use of Surface QDs (SQDs) grown by epitaxy on solid substrates. These QDs are not suitable to be used in solution, and thus cannot be exploited for application like in vivo imaging and diagnostics. However, the peculiarity that they are directly grown on a semiconductor surface represents a major advantage for chem-/bio-sensor and lab-on-chip design. At least in principle all is needed for a fluorescence based sensor is a light source, the quantum dots and a light detector, and all these components can be easily made with well-developed semiconductor fabrication technology. The presented research has given a contribute to this interesting new perspective. In particular this study is focused on InP SQDs. These island-like nanostructures were synthesized by gas source molecular beam epitaxy (GS-MBE) on In0.48Ga0.52P buffer layer lattice matched to Si doped GaAs substrate. They present a room-temperature NIR photoluminescence (PL). The emission wavelength depends on their dimensions (in the range 750-865 nm). For the present study samples presenting high dot coverage have been synthesized and characterized in term of structural, optical and chemical sensing properties. The most important achievement of this study has been the observation of a reversible PL enhancement when the SQDs were exposed to the vapours of different polar protic solvents. The emission energy and shape were not affected by the solvent vapours, while the photoluminescence intensity depends on solvent vapour concentration (with linear law over a limited concentration range). Besides, a clear correlation between the structural parameters of the SQDs and the response to chemicals has been identified. The presented results showed that InP SQDs are suitable materials for application like optical chemical sensors, bio-sensors and lab-onchip devices.