CdSe-CdTe Heterojunction Nanorods: Role of CdTe Segment in Modulating the Charge Transfer Processes

Tunable Near-Infrared Emission from CdSe/CdTe/CdSe Core/Shell/Shell Quantum Dots

Core/shell nanostructures of type I and type II band-offset materials have been pivotal in improving photoluminescence and imparting charge separation in well-established colloidal quantum dots. In addition to these features, the multishell motif provides a simple means to expand the emission spectral range and spatially fine-tune electron and hole wave functions. Here, we explore the CdSe/CdTe/CdSe core/shell/shell structure with a total diameter of ≲7 nm, within the practical limits of readily accessible synthesis capabilities. Systematic variations of the dimensions of the core and the shells are carried out to examine how each of these structural parameters affects optical properties. Emission in the red to near-infrared range from 677 to 1057 nm (1.83 to 1.17 eV), photoluminescence quantum yields of up to 88%, and line widths as narrow as 109 meV (41.8 nm for the peak position of 677 nm) are achieved. Even with a total size limit of ∼7 nm with an ∼1 nm thick CdTe shell, the reddest emission energy can approach the expected energy separation between the bulk CdSe conduction band edge and CdTe valence band edge to within tens of millielectronvolts. Comparison to simple effective mass approximation provides insights and guidelines on achieving independent control over the effective band gap and the electron-hole overlap.

Evolution of Metal Tellurides for Energy Storage/Conversion: From Synthesis to Applications

Metal telluride (MTe)-based nanomaterials have emerged as a potential alternative for efficient, highly conductive, robust, and durable electrodes in energy storage/conversion applications. Significant progress in the material development of MTe-based electrodes is well-sought, from the synthesis of its nanostructures, integration of MTes with supporting materials, synthesis of their hybrid morphologies, and their implications in energy storage/conversion systems. Herein, an extensive exploration of the recent advancements and progress in MTes-based nanomaterials is reviewed. This review emphasizes elucidating the fundamental properties of MTes and providing a systematic compilation of its wet and dry synthesis methods. The applications of MTes are extensively summarized and discussed, particularly, in energy storage and conversion systems including batteries (Li-ion, Zn-ion, Li-S, Na-ion, K-ion), supercapacitor, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and CO2 reduction. The review also emphasizes the future prospects and urgent challenges to be addressed in the development of MTes, providing knowledge for researchers in utilizing MTes in energy storage and conversion technologies.

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.

Light-driven directional ion transport for enhanced osmotic energy harvesting

Light-driven ion (proton) transport is a crucial process both for photosynthesis of green plants and solar energy harvesting of some archaea. Here, we describe use of a TiO₂/C₃N₄ semiconductor heterojunction nanotube membrane to realize similar light-driven directional ion transport performance to that of biological systems. This heterojunction system can be fabricated by two simple deposition steps. Under unilateral illumination, the TiO₂/C₃N₄ heterojunction nanotube membrane can generate a photocurrent of about 9 μA/cm², corresponding to a pumping stream of ∼5500 ions per second per nanotube. By changing the position of TiO₂ and C₃N₄, a reverse equivalent ionic current can also be realized. Directional transport of photogenerated electrons and holes results in a transmembrane potential, which is the basis of the light-driven ion transport phenomenon. As a proof of concept, we also show that this system can be used for enhanced osmotic energy generation. The artificial light-driven ion transport system proposed here offers a further step forward on the roadmap for development of ionic photoelectric conversion and integration into other applications, for example water desalination.

Gold nanoparticle on semiconductor quantum dot: Do surface ligands influence Fermi level equilibration

Semiconductor-metal heterojunction nanostructures possess an ability to store electrons upon photoexcitation through Fermi level equilibration. The unique role of capping ligands in modulating the equilibration of Fermi level in CdSe-Au heteronanostructures is explored by taking alkyl thiols and alkyl amines as examples. Alkyl thiol having its highest occupied molecular orbital (HOMO) above the valence band of the heterojunction nanostructure inhibits the exciton recombination by scavenging the photogenerated hole. This leads to the elevation in the Fermi level of Au and equilibration with the conduction band of CdSe. The Fermi level equilibrated electrons are further transferred to an acceptor molecule such as methyl viologen, demonstrating the potential of heterojunction nanostructures capped with hole accepting ligands for charge transport application in photovoltaics. In contrast, alkyl amine being a non-hole acceptor ligand with its HOMO placed below its valence band promotes rapid Au mediated exciton recombination, limiting its usefulness in charge transport application. Thus, the energetics of ligands on heterojunction nanostructures plays a decisive role in Fermi level equilibration.