37 research outputs found
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Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks.
The goal of this work is to identify favored pathways for preparation of defect-resilient attached wurtzite CdX (X = S, Se, Te) nanocrystals. We seek guidelines for oriented attachment of faceted nanocrystals that are most likely to yield pairs of nanocrystals with either few or no electronic defects or electronic defects that are in and of themselves desirable and stable. Using a combination of in situ high-resolution transmission electron microscopy (HRTEM) and electronic structure calculations, we evaluate the relative merits of atomic attachment of wurtzite CdSe nanocrystals on the {11Ì…00} or {112Ì…0} family of facets. Pairwise attachment on either facet can lead to perfect interfaces, provided the nanocrystal facets are perfectly flat and the angles between the nanocrystals can adjust during the assembly. Considering defective attachment, we observe for {11Ì…00} facet attachment that only one type of edge dislocation forms, creating deep hole traps. For {112Ì…0} facet attachment, we observe that four distinct types of extended defects form, some of which lead to deep hole traps whereas others only to shallow hole traps. HRTEM movies of the dislocation dynamics show that dislocations at {11Ì…00} interfaces can be removed, albeit slowly. Whereas only some extended defects at {112Ì…0} interfaces could be removed, others were trapped at the interface. Based on these insights, we identify the most resilient pathways to atomic attachment of pairs of wurtzite CdX nanocrystals and consider how these insights can translate to the creation of electronically useful materials from quantum dots with other crystal structures
Examining the Role of Chloride Ligands on Defect Removal in Imperfectly Attached Semiconductor Nanocrystals for 1D and 2D Attachment Cases
Semiconducting, core-shell nanocrystals (NCs) are promising building blocks
for the construction of higher dimensional artificial nanostructures using
oriented attachment. However, the assembly and epitaxial attachment steps
critical to this construction introduce disorder and defects which inhibit the
observation of desirable emergent electronic phenomena. Consequently,
understanding defect formation and remediation in these systems as a function
of dimensionality is a crucial step to perfecting their synthesis. In this
work, we use in situ high resolution transmission electron microscopy to
examine the role of chloride ligands as remediator agents for imperfect
attachment interfaces between CdSe/CdS core-shell NCs for both 1D and 2D
attachment cases. In the 1D case, we find that the presence of chloride
additives in imperfectly attached NC dimers can result in defect removal speeds
nearly twice as large as those found in their plain, non-chloride treated
counterparts. However, when we increased the dimensionality of the system and
examined 2D NC arrays, we found no statistically significant difference in
attachment interface quality between the chloride and non-chloride treated
samples. We propose that this discongruity arises from fundamental differences
between 1D and 2D NC attachment and discuss synthetic guidelines to inform
future nanomaterial superlattice design.Comment: 35 pages, 6 figures, work conducted at the University of California,
Berkele
Advanced Techniques in Automated High Resolution Scanning Transmission Electron Microscopy
Scanning transmission electron microscopy is a common tool used to study the
atomic structure of materials. It is an inherently multimodal tool allowing for
the simultaneous acquisition of multiple information channels. Despite its
versatility, however, experimental workflows currently rely heavily on
experienced human operators and can only acquire data from small regions of a
sample at a time. Here, we demonstrate a flexible pipeline-based system for
high-throughput acquisition of atomic-resolution structural data using a custom
built sample stage and automation program. The program is capable of operating
over many hours without human intervention improving the statistics of
high-resolution experiments
Photoexcited Small Polaron Formation in Goethite (α-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy
Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV, whereas the polaron formation probability (85 ± 10%) remains constant. By comparison to hematite (α-Fe_2O_3), an oxide analogue, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides
A composite electrodynamic mechanism to reconcile spatiotemporally resolved exciton transport in quantum dot superlattices
Quantum dot (QD) solids are promising optoelectronic materials; further
advancing their device functionality depends on understanding their energy
transport mechanisms. The commonly invoked near-field F\"orster resonance
energy transfer (FRET) theory often underestimates the exciton hopping rate in
QD solids, yet no consensus exists on the underlying cause. In response, we use
time-resolved ultrafast stimulated emission depletion (TRUSTED) microscopy, an
ultrafast transformation of stimulated emission depletion (STED) microscopy to
spatiotemporally resolve exciton diffusion in tellurium-doped
CdSe-core/CdS-shell QD superlattices. We measure the concomitant time-resolved
exciton energy decay due to excitons sampling a heterogeneous energetic
landscape within the superlattice. The heterogeneity is quantified by
single-particle emission spectroscopy. This powerful multimodal set of
observables provides sufficient constraints on a kinetic Monte Carlo simulation
of exciton transport to elucidate a composite transport mechanism that includes
both near-field FRET and previously-neglected far-field emission/reabsorption
contributions. Uncovering this mechanism offers a much-needed unified framework
in which to characterize transport in QD solids and additional principles for
device design.Comment: 47 pages, including supplemen