Bandlike Transport in PbS Quantum Dot Superlattices with Quantum Confinement

Electronic transport in quantum-dot-in-perovskite solids

We investigate theoretically the band transport of electrons and holes in a "quantum-dot-in-perovskite" solid, a periodic array of semiconductor nanocrystal quantum dots embedded in a matrix of lead halide perovskite. For concreteness we focus on PbS quantum dots passivated by inorganic halogen ligands and embedded in a matrix of CsPbI₃. We find that the halogen ligands play a decisive role in determining the band offset between the dot and matrix and may therefore provide a straightforward way to control transport experimentally. The model and analysis developed here may readily be generalized to analyze band transport in a broader class of dot-in-solid materials.

Emergent properties in supercrystals of atomically precise nanoclusters and colloidal nanocrystals

We provide a comprehensive account of the optical, electrical and mechanical properties that result from the self-assembly of colloidal nanocrystals or atomically precise nanoclusters into crystalline arrays with long-range order....

Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography

The geometry in self-assembled superlattices of colloidal quantum dots (QDs) strongly affects their optoelectronic properties and is thus of critical importance for applications in optoelectronic devices. Here, we achieve the selective control of the geometry of colloidal quasi-spherical PbS QDs in highly-ordered two and three dimensional superlattices: Disordered, simple cubic (sc), and face-centered cubic (fcc). Gel permeation chromatography (GPC), not based on size-exclusion effects, is developed to quantitatively and continuously control the ligand coverage of PbS QDs. The obtained QDs can retain their high stability and photoluminescence on account of the chemically soft removal of the ligands by GPC. With increasing ligand coverage, the geometry of the self-assembled superlattices by solution-casting of the GPC-processed PbS QDs changed from disordered, sc to fcc because of the finely controlled ligand coverage and anisotropy on QD surfaces. Importantly, the highly-ordered sc supercrystal usually displays unique superfluorescence and is expected to show high charge transporting properties, but it has not yet been achieved for colloidal quasi-spherical QDs. It is firstly accessible by fine-tuning the QD ligand density using the GPC method here. This selective formation of different geometric superlattices based on GPC promises applications of such colloidal quasi-spherical QDs in high-performance optoelectronic devices.

Facet-Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodetectors

Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as-synthesized CQDs results in random orientational relationships among facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal-phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post-synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10× increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D* > 10¹²  Jones, each at 1550 nm. The photodetectors feature a 7 ns response time for a 0.01 mm² area-the fastest reported for solution-processed short-wavelength infrared PDs.

Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids

Strongly coupled, epitaxially fused colloidal nanocrystal (NC) solids are promising solution-processable semiconductors to realize optoelectronic devices with high carrier mobilities. Here, we demonstrate sequential, solid-state cation exchange reactions to transform epitaxially connected PbSe NC thin films into Cu₂Se nanostructured thin-film intermediates and then successfully to achieve zinc-blende, CdSe NC solids with wide epitaxial necking along {100} facets. Transient photoconductivity measurements probe carrier transport at nanometer length scales and show a photoconductance of 0.28(1) cm² V^-1 s^-1, the highest among CdSe NC solids reported. Atomic-layer deposition of a thin Al₂O₃ layer infiltrates and protects the structure from fusing into a polycrystalline thin film during annealing and further improves the photoconductance to 1.71(5) cm² V^-1 s^-1 and the diffusion length to 760 nm. We fabricate field-effect transistors to study carrier transport at micron length scales and realize high electron mobilities of 35(3) cm² V^-1 s^-1 with on-off ratios of 10⁶ after doping.

The role of ligands in electron transport in nanocrystal solids

We investigate theoretically the transport of electrons and holes in crystalline solids consisting of three-dimensional arrays of semiconductor nanocrystals passivated by two types of organic ligands-linear chain carboxylates and functionalized aromatic cinnamates. We focus on a critical quantity in transport: the quantum-mechanical overlap of the strongly confined electron and hole wavefunctions on neighboring nanocrystals. Using results from density-functional-theory (DFT) calculations, we construct a one-dimensional model system whose analytic wavefunctions reproduce the full DFT numerical overlap values. By investigating the analytic behavior of this model, we reveal several important features of electron transport. The most significant is that the wavefunction overlap decays exponentially with ligand length, with a characteristic decay length that depends primarily on properties of the ligand and is almost independent of the size and type of nanocrystal. Functionalization of the ligands can also affect the overlap by changing the height of the tunneling barrier. The physically transparent analytic expressions we obtain for the wavefunction overlap and its decay length should be useful for future efforts to control transport in nanocrystal solids.

Mechanistic Insights into Superlattice Transformation at a Single Nanocrystal Level Using Nanobeam Electron Diffraction

Understanding the mechanism and ultimately directing nanocrystal (NC) superlattice assembly and attachment have important implications on future advances in this emerging field. Here, we use 4D-STEM to investigate a monolayer of PbS NCs at various stages of the transformation from a hexatic assembly to a nonconnected square-like superlattice over large fields of view. Maps of nanobeam electron diffraction patterns acquired with an electron microscope pixel array detector (EMPAD) offer unprecedented detail into the 3D crystallographic alignment of the polyhedral NCs. Our analysis reveals that superlattice transformation is dominated by translation of prealigned NCs strongly coupled along the <11n>_AL direction and occurs stochastically and gradually throughout single grains. We validate the generality of the proposed mechanism by examining the structure of analogous PbSe NC assemblies using conventional transmission electron microscopy and selected area electron diffraction. The experimental results presented here provide new mechanistic insights into NC self-assembly and oriented attachment.