Au@Hg_xCd_1-xTe core@shell nanorods by sequential aqueous cation exchange for near-infrared photodetectors

We have explored the synthesis of Au@Hg_xCd_1-xTe core@shell nanorods by sequential aqueous cation exchange (ACE) for near-infrared photodetector application. A number of related Au@telluride core/shell nanorod structures were put forwarded, taking advantage of multi-step transformations through a binary and then a ternary phase for the telluride shells. The latter have a high degree of crystallinity thanks to the step-wise ACE method. The use of only trace amounts of Cd²⁺ coordinated with tri-n-butylphosphine, assisted the phase transformation from an amorphous Ag₂Te shell to a highly crystalline Ag₃AuTe₂ shell in the first stage; this was followed by a further cation exchange (CE) step with far higher Cd²⁺ levels to fabricate a highly crystalline CdTe shell, and with an additional CE with Hg²⁺ to convert it to a Hg_xCd_1-xTe shell. The composition of the shell components and the well-controlled thickness of the shells enabled tunable surface plasmon resonance properties of the Au@telluride nanorods in the NIR region. Utilizing the enhanced NIR absorption, a hybrid photodetector structure of Au@Hg_xCd_1-xTe nanorods on graphene was fabricated, showing visible to NIR (vis-NIR) broadband detection with high photoresponsivity (~10⁶ A/W).