  Engineering the structure of core-shell colloidal semiconductor nanoparticles
(CSNPs) is attractive due to the potential to enhance photo-induced charge
transfer (PICT) and induce favourable optical and electronic properties.
Nonetheless, the sensitivity of telluride CSNPs to high temperatures makes it
challenging to precisely modulate their surface crystallinity. Herein, we have
developed an efficient strategy for synthesising telluride CSNPs with thin
amorphous shells using aqueous cation exchange (ACE). By changing the synthesis
temperature in the range 40 to 110C, the crystallinity of the CdTe
nanoparticles was controllable from perfect crystals with no detectable
amorphous shell (c-CdTe) to a core-shell structure with a crystalline CdTe NP
core covered by an amorphous shell of tunable thickness up to 7-8nm (c@a-CdTe)
. A second ACE step transformed the c@a-CdTe to crystalline CdTe@HgTe
core-shell NPs. The c@a-CdTe nanoparticles synthesized at 60C and having a 4-5
nm thick amorphous shell, exhibited the highest surface-enhanced Raman
scattering activity with a high enhancement factor around 8.82x10^5, attributed
to the coupling between the amorphous shell and the crystalline core.
