Group V (bismuth and antimony) containing copper chalcogenide-based nanostructures are an environmentally benign class of compounds for employment in potential energy storage and conversion applications. We investigate the mechanistic insights in the colloidal synthesis of this class of materials and harness them in various applications. The colloidal approach for synthesis of most heterostructured/ multicomponent nanocrystals (NCs) typically proceeds via the formation of binary semiconductor NC seeds. Contrary to this, a lesser explored pathway involves liquid droplets for catalysing the growth of the semiconductor NCs, known as the solid-liquid-solid (SLS) mechanism This offers facile control over the reaction kinetics with the variation of the nature of the metal seed catalysing the growth process. Using this approach, we synthesized Bi-Cu_2-xS heterostructures with tuneable Cu_2-xS stem. The stability of the Cu thiolate intermediate formed in the reaction could be varied by modification of the alkyl phosphonic acid used, which in turn controls the number of Cu_2-xS stems in the heterostructures. The advantages of the branched morphology were examined by assessing the electrochemical performance of the single stem and multi stem Bi-Cu_2-xS NCs anodes in K⁺ ion batteries. Multiple stem NCs show enhanced cycling stability and rate capability with higher specific capacity (∼170mAh·g−1 after 200 cycles) than the single pods (∼111mAh·g−1 after 200 cycles).[1] Following this work, we synthesized multinary anisotropic Cu-Bi-Zn-S nanorods (NRs) via the SLS mechanism wherein in situ generated Bi NCs catalyses the formation of Bi-Cu_2-xS heterostructures which eventually transforms into homogeneously alloyed quaternary Cu-Bi-Zn-S NRs. We observe that the reaction proceeds through the dissolution of the metallic bismuth seed into a trisegmented heterostructure with a Bi-rich BixCuySz phase, a Cu-rich BixCuySz stem, and an alloyed transitional BixCuySz segment present at the heterointerface. Finally, the formation of the homogenous NRs is facilitated by the gradual dissolution of the Bi-rich seed and recrystallization of the Cu-rich stem into the transitional segment. The NRs exhibit promising thermoelectric properties with very low thermal conductivity values of 0.45 and 0.65 W/mK at 775 and 605 K, respectively, for Zn-poor and Zn-rich NRs.[2]

Our interest in exploring multinary group V containing colloidal nanostructures also led to the direct synthesis of quaternary compositions of Cu-Sb-S-based NCs. We developed a hot injection synthetic pathway for three substituted tetrahedrites compositions i.e., Cu₁₀Zn₂Sb₄S₁₃, Cu₁₀Ni₂Sb₄S₁₃, Cu₁₀Co₂Sb₄S₁₃. Balancing the precursor reactivities of constituent species was crucial for obtaining phase pure and better size distribution of the NC ensemble. All the synthesized substituted tetrahedrites exhibited lower thermal conductivity while Cu₁₀Ni₂Sb₄S₁₃ exhibited the highest electrical conductivity thus making them promising candidates for thermoelectric applications. [3]

Our quest for developing compositionally complex nanostructures led to pushing our limits from quaternary nanostructures to emerging high entropy materials. High entropy materials are defined as materials containing more than 5 constituent elements with 5-35% of each element and crystallizing in a single phase stabilized by a high configurational entropy. The presence of multiple cations with different reactivities towards the chalcogenide species results in the formation of additional byproducts in the reaction leading to the emergence of a multiphase system in a conventional hot injection or heat-up colloidal synthesis pathway for high entropy materials. Therefore, the design of these nanostructures requires strategic techniques to avoid phase separation. Using cation exchange as a tool we use our preformed multicomponent group V containing copper chalcogenide-based NCs as templates for the subsequent diffusion of additional cations in the chalcogenide phase resulting in the synthesis of the target single-phase high entropy NCs.