Plasmon Dynamics in Colloidal Au₂Cd Alloy–CdSe Core/Shell Nanocrystals

Metal–semiconductor nanocrystal heterostructures are model systems for understanding the interplay between the localized surface plasmon resonances in the metal domain and the relaxation of the excited carriers in the semiconductor domain. Here we report the synthesis of colloidal Au₂Cd (core)/CdSe (shell) nanocrystal heterostructures, which were characterized extensively with several structural and optical techniques, including time-resolved fluorescence and broad-band transient absorption spectroscopy (both below and above the CdSe band gap). The dynamics of the transient plasmon peak was dominated by the relaxation of hot carriers in the metal core, its spectral shape was independent of the pump wavelength, and the bleaching lifetime was about half a picosecond, comparable with the value found in the AuCd seeds used for the synthesis

Charge Transport in Nanoscale “All-Inorganic” Networks of Semiconductor Nanorods Linked by Metal Domains

Charge transport across metal-semiconductor interfaces at the nanoscale is a crucial issue in nanoelectronics. Chains of semiconductor nanorods linked by Au particles represent an ideal model system in this respect, because the metal–semiconductor interface is an intrinsic feature of the nanosystem and does not manifest solely as the contact to the macroscopic external electrodes. Here we investigate charge transport mechanisms in all-inorganic hybrid metal–semiconductor networks fabricated <i>via</i> self-assembly in solution, in which CdSe nanorods were linked to each other by Au nanoparticles. Thermal annealing of our devices changed the morphology of the networks and resulted in the removal of small Au domains that were present on the lateral nanorod facets, and in ripening of the Au nanoparticles in the nanorod junctions with more homogeneous metal-semiconductor interfaces. In such thermally annealed devices the voltage dependence of the current at room temperature can be well described by a Schottky barrier lowering at a metal semiconductor contact under reverse bias, if the spherical shape of the gold nanoparticles is considered. In this case the natural logarithm of the current does not follow the square-root dependence of the voltage as in the bulk, but that of V^2/3. From our fitting with this model we extract the effective permittivity that agrees well with theoretical predictions for the permittivity near the surface of CdSe nanorods. Furthermore, the annealing improved the network conductance at cryogenic temperatures, which could be related to the reduction of the number of trap states

Colloidal CdSe/Cu₃P/CdSe Nanocrystal Heterostructures and Their Evolution upon Thermal Annealing

We report the synthesis of colloidal CdSe/Cu₃P/CdSe nanocrystal heterostructures grown from hexagonal Cu₃P platelets as templates. One type of heterostructure was a sort of “coral”, formed by vertical pillars of CdSe grown preferentially on both basal facets of a Cu₃P platelet and at its edges. Another type of heterostructure had a “sandwich” type of architecture, formed by two thick, epitaxial CdSe layers encasing the original Cu₃P platelet. When the sandwiches were annealed under vacuum up to 450 °C, sublimation of P and Cd species with concomitant interdiffusion of Cu and Se species was observed by <i>in situ</i> HR- and EFTEM analyses. These processes transformed the starting sandwiches into Cu₂Se nanoplatelets. Under the same conditions, both the pristine (uncoated) Cu₃P platelets and a control sample made of isolated CdSe nanocrystals were stable. Therefore, the thermal instability of the sandwiches under vacuum might be explained by the diffusion of Cu species from Cu₃P cores into CdSe domains, which triggered sublimation of Cd, as well as out-diffusion of P species and their partial sublimation, together with the overall transformation of the sandwiches into Cu₂Se nanocrystals. A similar fate was followed by the coral-like structures. These CdSe/Cu₃P/CdSe nanocrystals are therefore an example of a nanostructure that is thermally unstable, despite its separate components showing to be stable under the same conditions

Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu_2‑xSe Nanocrystals

We studied cation exchange reactions in colloidal Cu_2‑<i>x</i>Se nanocrystals (NCs) involving the replacement of Cu⁺ cations with either Sn²⁺ or Sn⁴⁺ cations. This is a model system in several aspects: first, the +2 and +4 oxidation states for tin are relatively stable; in addition, the phase of the Cu_2‑<i>x</i>Se NCs remains cubic regardless of the degree of copper deficiency (that is, “<i>x</i>”) in the NC lattice. Also, Sn⁴⁺ ions are comparable in size to the Cu⁺ ions, while Sn²⁺ ones are much larger. We show here that the valency of the entering Sn ions dictates the structure and composition not only of the final products but also of the intermediate steps of the exchange. When Sn⁴⁺ cations are used, alloyed Cu_{2–4<i>y</i>}Sn_<i>y</i>Se NCs (with <i>y</i> ≤ 0.33) are formed as intermediates, with almost no distortion of the anion framework, apart from a small contraction. In this exchange reaction the final stoichiometry of the NCs cannot go beyond Cu_0.66Sn_0.33Se (that is Cu₂SnSe₃), as any further replacement of Cu⁺ cations with Sn⁴⁺ cations would require a drastic reorganization of the anion framework, which is not possible at the reaction conditions of the experiments. When instead Sn²⁺ cations are employed, SnSe NCs are formed, mostly in the orthorhombic phase, with significant, albeit not drastic, distortion of the anion framework. Intermediate steps in this exchange reaction are represented by Janus-type Cu_2‑<i>x</i>Se/SnSe heterostructures, with no Cu–Sn–Se alloys

Alloyed Copper Chalcogenide Nanoplatelets <i>via</i> Partial Cation Exchange Reactions

We report the synthesis of alloyed quaternary and quinary nanocrystals based on copper chalcogenides, namely, copper zinc selenide–sulfide (CZSeS), copper tin selenide–sulfide (CTSeS), and copper zinc tin selenide–sulfide (CZTSeS) nanoplatelets (NPLs) (∼20 nm wide) with tunable chemical composition. Our synthesis scheme consisted of two facile steps: <i>i.e.</i>, the preparation of copper selenide–sulfide (Cu_2–<i>x</i>Se_<i>y</i>S_1–<i>y</i>) platelet shaped nanocrystals <i>via</i> the colloidal route, followed by an <i>in situ</i> cation exchange reaction. During the latter step, the cation exchange proceeded through a partial replacement of copper ions by zinc or/and tin cations, yielding homogeneously alloyed nanocrystals with platelet shape. Overall, the chemical composition of the alloyed nanocrystals can easily be controlled by the amount of precursors that contain cations of interest (<i>e.g.</i>, Zn, Sn) to be incorporated/alloyed. We have also optimized the reaction conditions that allow a complete preservation of the size, morphology, and crystal structure as that of the starting Cu_2–<i>x</i>Se_<i>y</i>S_1–<i>y</i> NPLs. The alloyed NPLs were characterized by optical spectroscopy (UV–vis–NIR) and cyclic voltammetry (CV), which demonstrated tunability of their light absorption characteristics as well as their electrochemical band gaps

HCl Flow-Induced Phase Change of α‑, β‑, and ε‑Ga₂O₃ Films Grown by MOCVD

Precise control of the heteroepitaxy on a low-cost foreign substrate is often the key to drive the success of fabricating semiconductor devices in scale when a large low-cost native substrate is not available. Here, we successfully synthesized three different phases of Ga₂O₃ (α, β, and ε) films on <i>c</i>-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A 3-fold increase in the growth rate of pure β-Ga₂O₃ was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of β- and ε-Ga₂O₃ was observed, until the Ga₂O₃ film transformed completely to a pure ε-Ga₂O₃ with a smooth surface and the highest growth rate (∼1 μm/h) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of α- and ε-Ga₂O₃ with a dominant α-Ga₂O₃, while the growth rate dropped significantly (∼0.4 μm/h). The film became rough as a result of the mixture phases since the growth rate of ε-Ga₂O₃ is much higher than that of α-Ga₂O₃. In this HCl-enhanced MOCVD mode, the Cl impurity concentration was almost identical among the investigated samples. On the basis of our density functional theory calculation, we found that the relative energy between β-, ε-, and α-Ga₂O₃ became smaller, thus inducing the phase change by increasing the HCl flow in the reactor. Thus, it is plausible that the HCl acted as a catalyst during the phase transformation process. Furthermore, we revealed the microstructure and the epitaxial relationship between Ga₂O₃ with different phases and the <i>c</i>-plane sapphire substrates. Our HCl-enhanced MOCVD approach paves the way to achieving highly controllable heteroepitaxy of Ga₂O₃ films with different phases for device applications

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Size-Tunable, Hexagonal Plate-like Cu₃P and Janus-like Cu–Cu₃P Nanocrystals

We describe two synthesis approaches to colloidal Cu₃P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu₃P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu₃P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu₃P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu₃P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu₃P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu₃P particles do not display an electrochemical behavior that would make them suitable for applications in batteries

Strongly Fluorescent Quaternary Cu–In–Zn–S Nanocrystals Prepared from Cu_1‑<i>x</i>InS₂ Nanocrystals by Partial Cation Exchange

We report a high-yield, low cost synthesis route to colloidal Cu_1‑<i>x</i>InS₂ nanocrystals with a tunable amount of Cu vacancies in the crystal lattice. These are then converted into quaternary Cu–In–Zn–S (CIZS) nanocrystals by partial exchange of Cu⁺ and In³⁺ cations with Zn²⁺ cations. The photoluminescence quantum yield of these CIZS nanocrystals could be tuned up to a record 80%, depending on the amount of copper vacancies