Cu_3-xP Nanocrystals as a Material Platform for Near-Infrared Plasmonics and Cation Exchange Reactions

Abstract

Synthesis approaches to colloidal Cu3P nanocrystals (NCs) have been recently developed, and their optical absorption features in the near-infrared (NIR) have been interpreted as arising from a localized surface plasmon resonance (LSPR). Our pump–probe measurements on platelet-shaped Cu3-xP NCs corroborate the plasmonic character of this absorption. In accordance with studies on crystal structure analysis of Cu3P dating back to the 1970s, our density functional calculations indicate that this material is substoichiometric in copper, since the energy of formation of Cu vacancies in certain crystallographic sites is negative, that is, they are thermodynamically favored. Also, thermoelectric measurements point to a p-type behavior of the majority carriers from films of Cu3-xP NCs. It is likely that both the LSPR and the p-type character of our Cu3-xP NCs arise from the presence of a large number of Cu vacancies in such NCs. Motivated by the presence of Cu vacancies that facilitate the ion diffusion, we have additionally exploited Cu3-xP NCs as a starting material on which to probe cation exchange reactions. We demonstrate here that Cu3-xP NCs can be easily cation-exchanged to hexagonal wurtzite InP NCs, with preservation of the anion framework (the anion framework in Cu3-xP is very close to that of wurtzite InP). Intermediate steps in this reaction are represented by Cu3-xP/InP heterostructures, as a consequence of the fact that the exchange between Cu+ and In3+ ions starts from the peripheral corners of each NC and gradually evolves toward the center. The feasibility of this transformation makes Cu3-xP NCs an interesting material platform from which to access other metal phosphides by cation exchange.

Ga for Zn Cation Exchange Allows for Highly Luminescent and Photostable InZnP-Based Quantum Dots

“…Such core–shell structures are often observed in systems with low ion diffusivity where the reaction starts from the QD surface. 35−37 Indeed, the 30–60 min time required for the CE reaction to reach completion (Figure 2c) suggests slow ion diffusion, an observation supported by other reports of slow CE reactions in III–V QD materials. 37,38 …”

A sustainable future for photonic colloidal nanocrystals

“…It has recently been used to produce GaAs, InAs, GaP, and InP with relatively narrow size dispersions, while avoiding the challenges associated with the traditional so-called hot-injection synthesis. [21][22][23] This synthetic route also has the advantage of preserving the original NC conformation, opening up the control of shape in these zinc blende NCs, including 1D rods and 2D sheets. A potential drawback is the incomplete removal of Cu that is used in an intermediate stage, and often quenches the PL.…”

“…First, the synthesis of high-PL InP QDs 62 with a spectral coverage that rivals CdSe, and converging performance indicates that these materials are accessible via traditional means. Cation exchange, 21,23,58,83 on the other hand, is an emerging route used to obtain III-V NCs, 21,22 Zn-based II-VI 89 and even ternary chalcogenides. 58 A prominent advantage of this approach is that it preserves the narrow size distributions as well as the shape of initial NCs, providing the possibility to obtain shape-controlled NCs that are difficult to grow via direct routes.…”

Direct Cd-to-Pb Exchange of CdSe Nanorods into PbSe/CdSe Axial Heterojunction Nanorods

“…This solid-state reaction draws much attention particularly because design of complex HNCs is possible when anisotropic seed nanoparticles are available, and a relatively mild exchange process keeps the shape anisotropy intact. 6 The versatility of cation exchange has motivated the intensive study of its molecular-level mechanism for the process. 7 High surface area of NCs reduces subreaction activation barriers; therefore, the synthesis of unique metastable products becomes possible.…”