Colloidal Synthesis Path to 2D Crystalline Quantum Dot Superlattices

ACS Nano · 2020 · 56 citations

• Materials science
• Nanotechnology
• Optoelectronics

) depending on the specific applications. Finally, cation exchange reactions can be performed on the final QD-in-matrix, as demonstrated herein with a CdSe/CdS to HgSe/HgS conversion.

Design and synthesis of multigrain nanocrystals via geometric misfit strain

Nature · 2020 · 94 citations

• Materials science
• Chemical engineering
• Nanotechnology

Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors

ACS Nano · 2020 · 34 citations

• Chemistry
• Chemical physics
• Molecular physics

Understanding electronic dynamics in multiexcitonic quantum dots (QDs) is important for designing efficient systems useful in high power scenarios, such as solar concentrators and multielectron charge transfer. The multiple charge carriers within a QD can undergo undesired Auger recombination events, which rapidly annihilate carriers on picosecond time scales and generate heat from absorbed photons instead of useful work. Compared to the transfer of multiple electrons, the transfer of multiple holes has proven to be more difficult due to slower hole transfer rates. To probe the competition between Auger recombination and hole transfer in CdSe, CdS, and CdSe/CdS QDs of varying sizes, we synthesized a phenothiazine derivative with optimized functionalities for binding to QDs as a hole accepting ligand and for spectroscopic observation of hole transfer. Transient absorption spectroscopy was used to monitor the photoinduced absorption features from both trapped holes and oxidized ligands under excitation fluences where the averaged initial number of excitons in a QD ranged from ∼1 to 19. We observed fluence-dependent hole transfer kinetics that last around 100 ps longer than the predicted Auger recombination lifetimes, and the transfer of up to 3 holes per QD. Theoretical modeling of the kinetics suggests that binding of hole acceptors introduces trapping states significantly different from those in native QDs passivated with oleate ligands. Holes in these modified trap states have prolonged lifetimes, which promotes the hole transfer efficiency. These results highlight the beneficial role of hole-trapping states in devising hole transfer pathways in QD-based systems under multiexcitonic conditions.

Sub-Bandgap Photoinduced Transient Absorption Features in CdSe Nanostructures: The Role of Trapped Holes

The Journal of Physical Chemistry C · 2020 · 33 citations

• Materials science
• Molecular physics
• Atomic physics

Transient absorption (TA) is widely used to study the dynamics of various processes, such as trapping, nonradiative decay, or transferring of photoexcited carriers in semiconductor nanocrystals. TA spectra of these systems show photoinduced absorption (PA) features that appear lower in energy than those of the band edge, which have been attributed to sub-bandgap absorptions of photoexcited electrons and holes. Here, we perform atomistic, semiempirical pseudopotential calculations in CdSe nanostructures to compute oscillator strengths for sub-bandgap transitions of conduction band electrons, valence band holes, and surface-trapped holes. We find that sharper peaks in the infrared (IR) range and broader features in the near-IR range (0.5–1.0 eV) are due to near-band-edge transitions of electrons and holes, respectively. Additionally, we focus on the region from 1.45 to 1.9 eV (850–650 nm), in which broad features have been observed and assigned to the PA of holes populating surface traps of nanocrystals. While there has been experimental justification of this assignment, there has been little theoretical investigation. We find that, in this region of interest from 1.45 to 1.9 eV, oscillator strengths for transitions of trapped holes are significantly larger than those of electrons or valence band holes. We conclude that the low symmetry of localized surface trap states and optimal spatial overlap with highly oscillatory states deep in the valence band lead to large electric dipole matrix elements and increased oscillator strengths. Our results are consistent for CdSe and CdS cores, CdSe–CdS core–shell quantum dots, and CdSe nanorods.

Critical differences in 3D atomic structure of individual ligand-protected nanocrystals in solution

Science · 2020 · 155 citations

• Nanotechnology
• Chemical physics
• Materials science

Precise three-dimensional (3D) atomic structure determination of individual nanocrystals is a prerequisite for understanding and predicting their physical properties. Nanocrystals from the same synthesis batch display what are often presumed to be small but possibly important differences in size, lattice distortions, and defects, which can only be understood by structural characterization with high spatial 3D resolution. We solved the structures of individual colloidal platinum nanocrystals by developing atomic-resolution 3D liquid-cell electron microscopy to reveal critical intrinsic heterogeneity of ligand-protected platinum nanocrystals in solution, including structural degeneracies, lattice parameter deviations, internal defects, and strain. These differences in structure lead to substantial contributions to free energies, consequential enough that they must be considered in any discussion of fundamental nanocrystal properties or applications.