Carrier Transport in Heterojunction Nanocrystals Under Strain

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.

Insights of Diffusion Doping in Formation of Dual-Layered Material and Doped Heterostructure SnS-Sn:Sb2S3 for Sodium Ion Storage

Insights into the formation mechanism of a dual-layered and doped heterostructure material Sn^IIS-Sn^IV:Sb₂S₃ are reported. In the presence of mixed alkyl thiols, first nanotubes of Sb₂S₃ were formed, and upon introduction of Sn(IV), Sn^IIS was deposited onto the surface of these tubular structures. Upon further annealing at a constant temperature, sluggish transformation resulted in a Sn(II)S-Sn(IV) doped Sb₂S₃ heterostructure, which finally turned to flake-like layered doped Sb₂S₃ nanostructures. SnS and Sb₂S₃, both being layered materials, were explored for the study of Na-ion storage, and these heterostructures were observed to be superior in comparison to the individual materials as well as the final doped nanostructures. The mechanism of formation of the heterostructures, the epitaxy at the junction, the diffusion doping, and the dopant-induced axial exfoliations leading to the final doped structures were studied. The electrochemical conversions in the presence of Na ions were also investigated, and insights into the mechanisms of both are reported in this Letter.

Collective aspects of singlet fission in molecular crystals

We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently.

Tetracene Aggregation on Polar and Nonpolar Surfaces: Implications for Singlet Fission

In molecular crystals that exhibit singlet fission, quantum yields depend strongly on intermolecular configurations that control the relevant electronic couplings. Here, we explore how noncovalent interactions between molecules and surfaces stabilize intermolecular structures with strong singlet fission couplings. Using molecular dynamics simulations, we studied the aggregation patterns of tetracene molecules on a solid surface as a function of surface polarity. Even at low surface concentrations, tetracene self-assembled into nanocrystallites where about 10-20% of the clustered molecules were part of at least one herringbone structure. The herringbone structure is the native structure of crystalline tetracene, which exhibits a high singlet fission quantum yield. Increasing the polarity of the surface reduced both the amount of clustering and the relative number of herringbone configurations, but only when the dipoles on the surface were orientationally disordered. These results have implications for the application of singlet fission in dye-sensitized solar cells.