Exciton dynamics in cation-exchanged CdSe/PbSe nanorods: The role of defects

Cu1.94S-ZnS-CdS ternary heteronanoplates with efficient carrier transfer for enhanced photocatalytic hydrogen evolution

Incorporating precise morphology control and efficient carrier separation into single-nanoparticle heterojunctions to achieve high photocatalytic efficiency remains a significant challenge. Here, we synthesized Cu1.94S-ZnS-CdS ternary heteronanoplates (HNPs) with a continuous sublattice structure using cation exchange reactions. Femtosecond transient absorption spectroscopy (TAS) confirms that ternary heterojunction enhances carrier separation efficiency, demonstrating both rapid separation (∼0.2 ps) and an extended lifetime (∼1512 ps). The synergistic combination results in a significantly enhanced hydrogen evolution rate of 2.012 mmol·g-1·h-1, which is 17 times and 183 times higher than that achieved by pure CdS and ZnS, respectively. Furthermore, there is no significant decrease in the activity of Cu1.94S-ZnS-CdS in photocatalytic hydrogen evolution after 288 days of placement. Our work offers an alternative approach for designing noble-metal-free photocatalysts with precisely defined materials and interfaces, aiming to enhance both photocatalytic hydrogen evolution efficiency and stability.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Ultrafast spectroscopy studies of carrier dynamics in semiconductor nanocrystals

Semiconductor nanocrystals have become ubiquitous both in scientific research and in applied technologies related to light. When a nanocrystal absorbs a photon an electron-hole pair is created whose fate dictates whether the nanocrystal will be suitable for a particular application. Ultrafast spectroscopy provides a real-time window to monitor the evolution of the electron-hole pair. In this review, we focus on CdSe nanocrystals, the most-studied nanocrystal system to date, and also highlight ultrasmall nanocrystals, "standard nanocrystals" of different binary composition, alloyed nanocrystals, and core/shell nanocrystals and nanorods. We focus on four time-resolved spectroscopies used to interrogate nanocrystals: pump-probe, fluorescence upconversion, time-correlated single photon counting, and non-linear spectroscopies. The basics of the nanocrystals and the spectroscopies are presented, followed by a detailed synopsis of ultrafast spectroscopy studies performed on the various semiconductor nanocrystal systems.

Optimal Blending of PbSe and CdSe in Polycrystalline PbCdSe Nanocomposite Film: Improved Carrier Multiplication and Enhanced Photoconversion Efficiency

The present work reports galvanostatic electro-co-deposition of n-PbCdSe semiconductor (SC) films on FTO substrate from the respective precursors. Self-designed matrices were formulated at variable concentrations of Pb²⁺ in the deposition medium. The semiconductor films constitute an intermixed structure of close-packed PbSe and CdSe nanoparticles (NPs), and the band gap (E_g) was effectively tuned in the range 0.99-1.47 eV for the variable compositions. Energy dispersive spectroscopy studies revealed that Cd exists in low level in the film matrix compared to Pb, presumably due to competitive deposition kinetics of the two chalcogenide compounds and the crystallite sizes determined from XRD studies, ranges between 15 and 12 nm, which corresponds to the size quenching of SC-NPs with increased Pb²⁺ concentration. The durability studies identify the most stable film developed at 0.025 M Pb²⁺ concentration. PbSe materials are typically characterized with impact ionization which effectively induces carrier multiplication (CM) in the quasi Type-II PbCdSe composite, exhibiting reasonably high photoconversion efficiency (PCE) of 6.14% with current output of 19.2 mA cm^-2 for the optimal PbCdSe film.

Direct cation exchange of CdSe nanocrystals into ZnSe enabled by controlled binding between guest cations and organic ligands

Zn chalcogenides are suitable candidates for blue-emitting fluorophores in light-emitting devices. In particular, the efforts to grow ZnSe nanocrystals (NCs) with fine control over size and shape via bottom-up approaches have faced challenges because of the slow decomposition of Zn precursors. In this study, we report direct cation exchange from CdSe NCs to ZnSe. Absorption spectroscopy and density functional theory (DFT) analysis reveal that the reactivity of cation exchange depends on the degree of complexation between organic ligands and Zn halides. We controlled the binding strength of Zn complexes by changing the organic ligands and halogen species that bind with Zn. Appropriate binding strength allows for the release of Zn ions and their facile incorporation into CdSe seed NCs. Under our experimental conditions, trioctylphosphine oxide (TOPO)-ZnI2 drives the efficient cation exchange reaction whereas TOPO-ZnCl2 induces no cation exchange of CdSe NCs. In addition, functional groups vary the binding strength between Zn and ligands. Oleylamine (OLAm)-ZnI2, which has a weaker ligand-ZnI2 binding than TOPO-ZnI2, breaks down the original morphologies of host CdSe NCs due to the very fast exchange rate. On the other hand, the TOPO-ZnI2 complex induces a mild exchange rate, leading to transformation into various morphologies such as CdSe nanorods (NRs) and nanoplatelets (NPLs) into CdSe/ZnSe heterostructures inaccessible via other synthesis methods. The incorporation of Zn into various morphologies of CdSe results in tunable optical transitions in blue-UV regions. The synthesis of heterostructured NCs in an elongated morphology is possible, opening opportunities in photocatalysis, light emitting diodes, and luminescent solar concentrators.

Deposition of metal particles onto semiconductor nanorods using an ionic liquid

The current study investigates whether metal deposition onto an existing nanorod can be carried out using an ionic liquid, and the effect this has on catalytic performance. Platinum, gold, and silver nanoparticles were deposited onto CdSe@CdS (core@shell) nanorods from metal salts in an ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) without additional surfactants or reducing agents. Photocatalytic dye degradation experiments showed that catalysts with platinum particles deposited using the ionic liquid out-performed similar materials synthesized using organic solvents and ligands. We concluded that metal particles can be deposited onto well-defined semiconductor nanorods using ionic liquids and metal salts without the need for additional reagents, and the deposited particles did not cause significant aggregation even when these materials were taken into organic media. It is possible that a broad range of metal/semiconductor heterostructured particles can be prepared using the methods reported here.