The utilization of photoelectrochemical cells (PEC) for converting solar energy into fuels (e.g., hydrogen) is a promising method for sustainable energy generation. We demonstrate a strategy to enhance the performance of PEC devices by integrating surface-functionalized zinc selenide (ZnSe) semiconductor nanocrystals (NCs) into porous polymeric carbon nitride (CN) matrices to form a uniformly distributed blend of NCs within the CN layer via electrophoretic deposition (EPD). The achieved type II heterojunction at the CN/NC interface exhibits intimate contact between the NCs and the CN backbone since it does not contain insulating binders. This configuration promotes efficient charge separation and suppresses carrier recombination. The reported CN/NC composite structure serves as a photoanode, demonstrating a photocurrent density of 160 ± 8 μA cm–2 at 1.23 V vs a reversible hydrogen electrode (RHE), 75% higher compared with a CN-based photoelectrode, for approximately 12 h. Spectral and photoelectrochemical analyses reveal extended photoresponse, reduced charge recombination, and successful charge transfer at the formed heterojunction; these properties result in enhanced PEC oxygen production activity with a Faradaic efficiency of 87%. The methodology allows the integration of high-quality colloidal NCs within porous CN-based photoelectrodes and provides numerous knobs for tuning the functionality of the composite systems, thus showing promise for achieving enhanced solar fuel production using PEC.

Colloidal Quantum Dots (QDs), the building blocks of modern displays and optoelectronic devices, have reached the highest level of size and shape control, and stability during the last 30 years. However, full utilization of their potential requires integration or assembly of more than one nanocrystal as in the case of Coupled Quantum Dots Molecules (CQDM), where two core-shell QDs are fused to form two emission centers in close proximity. These CQDMs were recently shown to switch color under an applied electric field at room temperature. Here we use cryogenic single particle spectroscopy of single CQDMs under an electric field to show that various mechanisms can contribute to the spectrum change under an applied electric field at cryogenic temperatures.  The first mechanism is the control of the delocalized electron wave function when the electric field is applied along the dimer axis. The electric field bends the conduction band and forces the electron wave function to localize in one of the QDs yielding preferential emission of that particular center. In addition, we found that QDs and CQDMs could become sensitive to surface traps under an electric field. In the case of CQDMs, that can result in decreasing the intensity of one of the QDs while increasing the other QD's intensity. Moreover, we show that there are surface charges which screen the applied electric field in some of the QDs. This as well can result in electric field-induced color-tuning of CQDMs. Understanding the underlying mechanisms responsible for spectral shifts under applied electric fields is critical for the development of color-tunable devices utilizing CQDMs, including efficient displays and single photon sources.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

A unique on-chip method for the direct correlation of optical properties, with atomic-scale chemical–structural characteristics for a single quantum dot (QD), is developed and utilized in various examples. This is based on performing single QD optical characterization on a modified glass substrate, followed by the extraction of the relevant region of interest by focused-ion-beam–scanning electron microscope processing into a lamella for high resolution scanning transmission electron microscopy (STEM) characterization with atomic scale resolution. The direct correlation of the optical response under an electric field with STEM analysis of the same particle allows addressing several single particle phenomena: first, the direct correlation of single QD photoluminescence (PL) polarization and its response to the external field with the QD crystal lattice alignment, so far inferred indirectly; second, the identification of unique yet rare few-QD assemblies, correlated directly with their special spectroscopic optical characteristics, serving as a guide for future designed assemblies; and third, the study on the effect of metal island growth on the PL behavior of hybrid semiconductor–metal nanoparticles, with relevance for their possible functionality in photocatalysis. This work, therefore, establishes the use of the direct on-chip optical–structural correlation method for numerous scenarios and timely questions in the field of QD research.