Compositional and interfacial control in heterojunction thin films is critical to the performance of complex devices that separate or combine charges. For high performance, these applications require epitaxially matched interfaces, which are difficult to produce. Here, we present a new architecture for producing low-strain, single-crystalline heterojunctions using self-assembly and in-film cation exchange of colloidal nanorods. A systematic set of experiments demonstrates a cation exchange procedure that lends precise control over compositional depths in a monolayer film of vertically aligned nanorods. Compositional changes are reflected by electrical performance as rectification is induced, quenched, and reversed during cation exchange from CdS to Cu(2)S to PbS. As an additional benefit, we achieve this single-crystal architecture via an inherently simple and low-temperature wet chemical process, which is general to a variety of chemistries. This permits ensemble measurement of transport through a colloidal nanoparticle film with no interparticle charge hopping.

Ion exchange of nanocrystals has the potential to emerge as an alternative to conventional routes for synthesis of ionic nanocrystals. [1][2][3][4][5][6][7][8][9] The facile ability to replace all cations of a nanocrystal with another cation, while preserving size and shape, allows us to employ nanocrystals as templates for the fabrication of other nanocrystals of interest.[6] Such a templated synthesis strategy is especially useful when the chemistry or crystallographic phase of the target nanocrystals is difficult to access via hot-injection methods. For instance, we recently showed [10] that Cu I sulfide quantum dots prepared by hot injection mostly result in the highly Cu-deficient djurleite phase. [11,12] The stoichiometric chalcocite phase is achievable, however, by room-temperature cation exchange of template CdS quantum dots with Cu + ions. Cation exchange holds particular promise for the fabrication of multicomponent heterostructured nanocrystals, [6,9] which allow independent tunability of electron and hole wavefunctions, but present potential synthetic challenges due to their greater structural complexity. Here, it is advantageous that the anionic framework of the heterostructure is maintained during cation exchange, allowing structural preservation of interfaces and junctions that define the electronic band alignment within the heterostructure. This has made possible the design and templated fabrication of novel semiconductor heterostructures that can range [13,14] from type-I, with high quantum yield emission useful for imaging and light-emitting diodes, [15,16] to type-II, which allow charge separation for photovoltaic and photocatalytic applications. [9,17] However, the ion exchange technique has been found to present a severe drawback: it results in nanocrystals with poor optoelectronic properties.[1] This is clear from a quantitative comparison of the optical properties of nanocrystals obtained from cation exchange with those prepared by standard hot injection for the model CdSe/CdS dot/rod heterostructure. [13][14][15][16] In this work, we trace the cause of the poor optical properties of cation-exchange-obtained nanocrystals to chemical impurities on the few atom per nanocrystal level. We have also found a method to purify the nanocrystals of these detrimental impurities post exchange and achieve optical properties comparable to those of hot-injection synthesized nanocrystals.Hot-injection synthesis of CdSe/CdS dot/rods with a 3.9 nm dot yields highly photoluminescent nanorods with a quantum yield (QY) of over 55 %, enabled by the type-I band alignment. On the other hand, CdSe/CdS dot/rods obtained from room-temperature exchange of Cu 2 Se/Cu 2 S dot/rods with Cd 2+ (see the Supporting Information) show relatively negligible emission, that is, a quantum yield (QY) of 0.07 %, almost three orders of magnitude smaller. This is despite the fact that the nanorods prepared by the two methods possess similar heterostructure morphologies, especially seed sizes, and consequently identical excitonic stru...

We review the discovery of localized surface plasmon resonances (LSPRs) in doped semiconductor quantum dots (QDs), an advance that has extended nanoplasmonics to materials beyond the classic gamut of noble metals. The initial demonstrations of near‐infrared LSPRs in QDs of heavily self‐doped copper chalcogenides and conducting metal oxides are setting the broad stage for this new field. We describe the key properties of QD LSPRs. Although the essential physics of plasmon resonances are similar to that in metal nanoparticles, the attributes of QD LSPRs represent a paradigm shift from metal nanoplasmonics. Carrier doping of quantum dots allows access to tunable LSPRs in the wide frequency range from the THz to the near‐infrared. Such composition or carrier density tunability is unique to semiconductor quantum dots and not achievable in metal nanoparticles. Most strikingly, semiconductor quantum dots allow plasmon resonances to be dynamically tuned or switched by active control of carriers. Semiconducting quantum dots thus represent the ideal building blocks for active plasmonics. A number of potential applications are discussed, including the use of plasmonic quantum dots as ultrasmall labels for biomedicine and electrochromic materials, the utility of LSPRs for probing nanoscale charge dynamics in semiconductors, and the exploitation of strong coupling between photons and excitons. Further advances in this field necessitate efforts toward generalizing plasmonic phenomena to a wider range of semiconductors, developing strategies for achieving controlled levels of doping and stabilizing them, investigating the spectroscopy of these systems on a fundamental level, and exploring their integration into optoelectronic devices.