Optical and structural investigation of ex-situ passivated strain coupled InAs surface quantum dots

Uncapped In(Ga)As quantum dots (QDs) have got very little attention in comparison with its enfolded counterpart. The existence of surface states makes it less attractive to the research community. On the other hand, colloidal QDs have immense recognition in the field of bio-sensing and bio-imaging. Various surface passivated stable colloidal QDs are now commercially available, but only in solution form. Stable solid-state QDs are still a virtue. So, there is a huge demand for stable solid-state surface QDs which can be easily coupled with an electronic device for sensing application. Simply we need to change the ligand, corresponding to a particular target molecule, and we can detect various chemical and biological elements from low molar solution (Nano bio-sensor regime). With this motivation, we have epitaxially grown a simple vertically coupled InAs QD structure, where both seed and top dot layers are of 2.7 ML InAs. In addition, the top QDs are left uncapped to form a surface quantum dot layer. The as-grown sample is acid-etched (to remove the native oxides) and passivated in 0.5 M Thiourea solution for one hour. Significant enhancement of ground state photoluminescence peak has been observed after the passivation for both surface and buried QDs. Atomic force microscopic (AFM) images confirm the modulation of the surface before and after the ex-situ treatment.