In situ electrical monitoring of cation exchange in nanowires

Tracking Cation Exchange in Individual Nanowires via Transistor Characterization

Cation exchange is a versatile method for modifying the material composition and properties of nanostructures. However, control of the degree of exchange and material properties is difficult at the single-particle level. Successive cation exchange from CdSe to Ag2Se has been utilized here on the same individual nanowires to monitor the change of electronic properties in field-effect transistor devices. The transistors were fabricated by direct synthesis of CdSe nanowires on prepatterned substrates followed by optical lithography. The devices were then subjected to cation exchange by submerging them in an exchange solution containing silver nitrate. By removal of the devices from solution and probing the electrical transport properties at different times, the change in electronic properties of individual nanowires could be monitored throughout the entire exchange reaction from CdSe to Ag2Se. Transistor characterization revealed that the electrical conductivity can be tuned by up to 8 orders of magnitude and the charge-carrier mobility by 7 orders of magnitude. While analysis of the material composition by energy dispersive X-ray spectroscopy confirmed successful cation exchange from CdSe to Ag2Se, X-ray fluorescence spectroscopy proved that cation exchange also took place below the contacts. The method presented here demonstrates an efficient way to tune the material composition and access the resulting properties nondestructively at the single-particle level. This approach can be readily applied to many other material systems and can be used to study the electrical properties of nanostructures as a function of material composition or to optimize nanostructure-based devices after fabrication.

A cation exchange strategy to construct Rod-shell CdS/Cu2S nanostructures for broad spectrum photocatalytic hydrogen production

Herein, Cu₂S as the outer shell is grown on CdS nanorods (NRs) to construct rod-shell nanostructures (CdS/Cu₂S) by a rapid, scalable and facile cation exchange reaction. The CdS NRs are firstly synthesized by a hydrothermal route, in which thiourea as the precursor of sulfur and ethylenediamine (EDA) as the solvent. And then, the outer shells of CdS NRs are successfully exchanged by Cu₂S via a cation exchange reaction. The obtained CdS/Cu₂S rod-shell NRs exhibit much enhanced activity of hydrogen production (640.95 μmol h^-1 g^-1) in comparison with pure CdS NRs (74.1 μmol h^-1 g^-1) and pure Cu₂S NRs (0 μmol h^-1 g^-1). The enhanced photocatalytic activity of CdS/Cu₂S rod-shell NRs owns to the following points: i) the photogenerated electrons generated by CdS quickly migrate to Cu₂S without any barrier due to rod-shell structure by the in-situ cation exchange reaction, a decreased carrier recombination is achieved; ii) Cu₂S as outer shells broaden the light absorption range of CdS/Cu₂S rod-shell NRs into visible or even NIR light, which can produce more electrons and holes. This work inspires people to further study the rod-shell structured photocatalyst through the cation exchange strategy to further solar energy conversion.

Superionic phase transition in individual silver selenide nanowires

Silver selenide (Ag₂Se) is a promising material for applications as a solid-state electrolyte, with a superionic phase transition at 133 °C. Here, we studied the temperature dependent transport properties of single Ag₂Se nanowires in a transistor geometry, which allowed us to determine charge carrier type, concentration, and mobility below and above the superionic phase transition temperature. We found the majority charge carriers to be n-type in the temperature range of 30-150 °C. Across the superionic phase-transition, we observed a sudden increase in conductivity by about 30%, which was accompanied by an increase in charge carrier density by about 200% and a decrease in mobility by about 45%. Interestingly, the size dependent shift of the transition temperatures to below 100 °C in our wires is much more pronounced than for nanocrystals of comparable size. This surprising and potentially useful effect could be caused by changes in crystal structure arising from the synthesis process.

Colloidal CdxM1-xTe Nanowires from the Visible to the Near Infrared Region: N,N-Dimethylformamide-Mediated Precise Cation Exchange

Cation exchange has been a successful methodology for tuning the bandgaps of nanomaterials, while the most popular protocol in the toluene/methanol system lacks precise compositional control due to its inherent poor solvent compatibility. We herein report an alternative cation exchange route in N,N-dimethylformamide (DMF) solvent for converting preformed colloidal CdTe nanowires into Cd_xM_1-xTe (M = Pb²⁺, Zn²⁺, Ag⁺, Hg²⁺) nanowires with good batch-to-batch reproducibility. The resulting Cd_xM_1-xTe nanowires show a tunable bandgap from 2.26 to 0.63 eV, and the energy levels of these nanowires can be finely tuned. Furthermore, a comparative study for the cation exchange of CdTe nanowires with Pb²⁺ ions in toluene/methanol and DMF illustrated that the reduction of Cd²⁺ extraction and the Pb²⁺ introduction barrier accounts for precise compositional control. The cation exchange reaction in the DMF phase provides an efficient way to obtain nanomaterials with precise composition control. Moreover, these available high-quality colloidal semiconductor nanowires also pave the way for near-infrared device exploration.

Fabrication of Ag2S/CdS Heterostructured Nanosheets via Self-Limited Cation Exchange

Highly crystalline vertically aligned Ag2S/CdS heterostructured nanosheets with lateral sizes of several micrometers and thicknesses of a few nanometers are prepared directly on silver surfaces by a two-step process. Firstly, Ag2S sheets were prepared by direct reaction of partially dissolved elementary sulfur in methanol with a solid silver surface in methanol at room temperature. The second step involves a self-limited cation exchange of Ag+ vs. Cd2+ to achieve the formation of large-area Ag2S/CdS heteronanosheets on the solid substrate. The cation exchange was proven and investigated over time via several analytical methods, e.g. X-ray diffraction, Raman spectroscopy and three-dimensional photoluminescence mapping.

Forging Colloidal Nanostructures via Cation Exchange Reactions

Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. In this review, we provide an up to date overview of the complex colloidal nanostructures that could be prepared so far by cation exchange. At the same time, the review gives an account of the fundamental thermodynamic and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.

Cation Exchange Synthesis and Unusual Resistive Switching Behaviors of Ag2Se Nanobelts

Ag2Se nanobelts are prepared through employing ZnSe nanobelts as templates via a facile cation exchange approach. The templates are derived from precursor ZnSe·0.5N2 H4 nanobelts, which are synthesized by a simple hydrothermal method. As-synthesized precursor nanobelts are with 200 nm in width and several hundreds of micrometers in length. Annealed in N2 , they are transformed into ZnSe nanobelts with preserving their initial morphology. Following with a complete replacement of Zn(2+) by Ag(+), Ag2Se nanobelts with single crystalline are obtained via a cation-exchange reaction. Combined with the Langmuir-Blodgett assembly technique, regular films of ZnSe nanobelts can be achieved on transparent glass substrates and Si wafers with interdigital Au electrode arrays. Further, the optical and electrical evolutions are investigated from ZnSe nanobelts to Ag2 Se nanobelts. Finally, the resistive switching characteristic are carefully explored for Ag2Se nanobelts regularly arranged on interdigital Au microelectrodes. The results indicate that it is analogous to complementary resistive switching behaviors, which is different from that of traditional two terminal devices about previously reported Ag2Se. In order to clarify this phenomenon, a possible mechanism has been proposed and indirectly demonstrated through in situ SEM (scanning electron microscropy) observation.

Single-nanocrystal reaction trajectories reveal sharp cooperative transitions

Whereas pathways of chemical reactions involving small molecules are well-understood, the dynamics of reactions in extended solids remain difficult to elucidate. Frequently, kinetic studies on bulk materials provide a picture averaged over multiple domains or grains, smearing out interesting dynamics such as critical nucleation phenomena or sharp phase transitions occurring within individual, often nanoscale, grains, or domains. By optically monitoring a solid-state reaction with single nanocrystal resolution, we directly identified a unique, previously unknown, reaction pathway. Reaction trajectories of single cadmium selenide nanocrystals undergoing ion exchange with silver reveal that each individual nanocrystal waits a unique amount of time before making an abrupt switch to the silver selenide phase on a few hundred millisecond time scale. The gradual reaction progress of ensemble-scale cation exchange is actually comprised of these sharp single-nanocrystal switching events. Statistical distributions of waiting times suggest that the reaction is a cooperative transition rather than a diffusion-limited cation-by-cation exchange, which is confirmed by a stochastic reaction model. Such insight, achievable from single nanocrystal reaction studies, furthers mechanistic understanding of heterogeneous reactions, solid-state catalysis, bottom-up nanostructure growth, and materials' transformations and degradation in reactive environments.

25th anniversary article: Ion exchange in colloidal nanocrystals

We review the progress in ion exchange in a variety of nanocrystal structures from the earliest accounts dating back over two decades ago to the present day. In recent years the number of groups using this method to form otherwise difficult or inaccessible nanoparticle shapes and morphologies has increased considerably and the field has experienced a resurgence of interest. Whilst most of the early work on cation exchange centered on II-VI materials, the methodology has been expanded to cover a far broader range of semiconductor nanocrystals including low toxicity I-III-VI materials and the much less facile III-V materials. The extent of exchange can be controlled leading to lightly doped nanoparticles, alloys, core-shells, segmented rods and dots-in-rods. Progress has been driven by a better understanding of the underlying principles of the exchange process - from thermodynamic factors (differences in cation solubilities); the interactions between ions and transfer agents (solvents, ligands, anions, co-dopants); ionic in-diffusion mechanisms and kinetics. More recent availability of very detailed electron microscopy coupled with image reconstruction techniques has been a valuable tool to investigate the resulting heterostructures and internal interfaces. We start by surveying the range of synthetic approaches most often used to carry out ion exchange, mainly focusing on cation replacement strategies, and then describe the rich variety of nanostructures these techniques can bring forth. We also describe some of the principles that are used to establish the relative ease of exchange and to systematically improve the process where the basic energetics are less favorable. To help further the understanding of the underlying fundamentals we have gathered together useful data from the literature on solubilities, cation and anion hardness, ligand and solvent Lewis acid or base strengths for a wide range of chemical species generally used. We offer a perspective on the outlook for the field in terms of the emerging applications and the ion exchange derived materials that will enable them.

Hybrid A–B–A type nanowires through cation exchange

Hybrid A–B–A type nanowires (NWs) with Ag5Te3–HgTe–Ag5Te3 composition have been created by the reaction of Hg2+ with Ag2Te NWs. The NW morphology of Ag2Te is preserved upon reaction with minor changes and the two separate phases formed are spatially separated within the same NW. The reaction of Hg2+ with Ag2Te NWs was monitored at different concentrations and the reactivity was attributed to cationic exchange depending on solubility products. Hybrid NWs were formed by partial cation exchange only at low concentrations (below 50 ppm) resulting in Ag5Te3 and HgTe within the same NW. However, at high concentrations (above 100 ppm), the HgTe phase alone was formed. These studies have been extended to other metal ions such as Pb2+, Cd2+, and Zn2+ whose reactivity towards Ag2Te NWs is different from that of Hg2+. These ions form a passivating Te oxide layer upon reaction with other metal ions. The mechanism of reactivity of Hg2+ is explained on the basis of free energy of formation of the ionic solid. Phase transition of Hg2+-reacted NWs occurs at a lower temperature than the parent (Ag2Te NWs) and other metal ions-reacted Ag2Te NWs. Details of the process were elucidated using microscopic and spectroscopic investigations. The physical and chemical properties of the individual components within a NW are expected to provide a novel functionality to the metal chalcogenide systems.

A facile spin-cast route for cation exchange of multilayer perpendicularly-aligned nanorod assemblies

A facile spin cast route was developed to convert perpendicularly aligned nanorod assemblies of cadmium chalcogenides into their silver and copper analogues. The assemblies are rapidly cation exchanged without affecting either the individual rod dimensions or collective superlattice order extending over several multilayers.

Cation exchange reactions in colloidal branched nanocrystals

Octapod-shaped colloidal nanocrystals composed of a central "core" region of cubic sphalerite CdSe and pods of hexagonal wurtzite CdS are subject to a cation exchange reaction in which Cd(2+) ions are progressively exchanged by Cu(+) ions. The reaction starts from the tip regions of the CdS pods and proceeds toward the center of the nanocrystals. It preserves both the shape and the anionic lattices of the heterostructures. During the exchange, the hexagonal wurtzite CdS pods are converted gradually into pods of hexagonal Cu(2)S chalcocite. Therefore, the partial cation exchange reactions lead to the formation of a ternary nanostructure, consisting of an octapod in which the central core is still CdSe, while the pods have a segmented CdS/Cu(2)S composition. When the cation exchange reaches the core, the cubic sphalerite CdSe core is converted into a core of cubic Cu(2-x)Se berzelianite phase. Therefore fully exchanged octapods are composed of a core of Cu(2-x)Se and eight pods of Cu(2)S. All these structures are stable, and the epitaxial interfaces between the various domains are characterized by low lattice mismatch. The Cu(2-x)Se(core)/Cu(2)S(pods) octapod represents another example of a nanostructure in which branching is achieved by proper organization of cubic and hexagonal domains in a single nanocrystal.

Assembled monolayer nanorod heterojunctions

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.