  Core-shell CdSe/CdS nanocrystals are a very promising material for light
emitting applications. Their solution-phase synthesis is based on
surface-stabilizing ligands that makes them soluble in organic solvents, like
toluene or chloroform. However, solubility of these materials in water provides
many advantages, such as additional process routes and easier handling. So far,
solubilization of CdSe/CdS nanocrystals in water that avoids detrimental
effects on the luminescent properties, poses a major challenge. This work
demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred
into water using a ligand exchange method employing mercaptopropionic acid
(MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod
diameter, which prevents potential surface defects formed during the ligand
exchange from affecting the photophysics of the dot-in-rods. Films made from
water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a
similar threshold (130 microJ/cm2) as the pristine material (115 microJ/cm2).
To demonstrate feasibility for lasing applications, self-assembled micro lasers
are fabricated via the coffee-ring effect that display single-mode operation
and a very low threshold of around 10 microJ/cm2. The performance of these
micro lasers is enhanced by the small size of MPA ligands, enabling a high
packing density of the dot-in-rods.

Applied Physics;Materials Science