Decoding the Superlattice and Interface Structure of Truncate PbS Nanocrystal-Assembled Supercrystal and Associated Interaction Forces

Abstract

Large scale three-dimensional supercrystals were grown by controlling evaporation of truncate PbS nanocrystal (NC) dispersed hexane suspensions. Electron microscopy analysis confirmed the nature of single supercrystal with a face-centered cubic (fcc) lattice. Synchrotron small/wide angle scattering (SAXS/WAXS) images from three typical crystallographic projections allowed ultimate reconstruction of shape orientations of NCs at different crystallographic sites. Position exchange of distinctly oriented NCs between crystallographic sites produces two nondegeneration shape-related pseudo-polymorphs of superlattice that accordingly reduce symmetry from Oh to C4h and C2h with various facet-to-facet arrangements, respectively. In situ SAXS measurements of NC-assembled supercrystal and lead oleate and oleic acid upon pressurization provide additional insights into surface ligand density and the nature of ligand-NC interactions and resulting interface structure. These results allow for feasible evaluation of both NC shape and ligand conformation enabled effects that govern the formation and stability of truncate NC assemblies with various superlattice polymorphs and associated NC-ligand interactions in solvent-mediated assembled processes.

Monitoring Nanocrystal Self‐Assembly in Real Time Using In Situ Small‐Angle X‐Ray Scattering

“…The structure and degree of order in NC superlattices depend on several factors, such as NC size and shape, ligand length, grafting density, ligand–solvent interactions, NC concentration, solvent evaporation rate (in case of evaporative assembly), and cell geometry or substrate where the assembly takes place. Thus, recent studies on lead chalcogenide NC assemblies revealed the formation of face‐centered cubic (fcc) or body‐centered cubic (bcc) superstructures as well as lattice distorted face‐centered tetragonal (fct) and body‐centered tetragonal (bct) superlattices . However, their assembling mechanism remains largely unresolved mainly due to the challenge in measuring intermediate transitions between the initial colloidal state and the final superlattice state, especially in real time.…”

X-ray Mapping of Nanoparticle Superlattice Thin Films

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 size of this sample is apparent from the diffraction spots arising from single superlattice grains. 24,25 Further evidence that these spots arise from single crystals is that the spots appear with the appropriate multiplicity about χ (the angle around the ring). The peaks from the (100) and (110) planes exhibiting six-fold symmetry offset by the expected 30°, as seen in Figure 2a.…”

“…Extending the detection range to include wide angles allows determination of orientation of crystal axes within the NPs forming a NSL. 25 In addition to specifying the crystal orientation within a beam spot and correlating that with its position on a sample, TSAXS also provides information about the film uniformity from total scattering and the spatial variation of polymorphs, projections, or impurity phases in the film. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 Figure 3a is a cartoon of the mapping experiment used to analyze NSL films.…”

The Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum Dots

“…Unambiguous differences are observed for 1 HNMR spectra of QDs of increasing size at the same concentration in [D 8 ]toluene (Figure 3a): upon reduction of the QD diameter, the broad oleyl resonances markedly shift upfield and sharpen. These spectral differences cannot be attributed to improper purification because all QD samples exhibit similar oleyl-to-Pb exc ratio and fully Pb-terminated facets.The size-dependent spectral features may instead result from the presence of alarger number of ligands associated with larger QDs,w hich enhances inter-oleyl interactions thus contributing to peak broadening.The 1 HNMR spectra of QD samples in CDCl 3 show oleyl resonances at chemical shifts that coincide with those of the Pb precursor ( Figure S12), which may result from reduced inter-ligand interactions occurring in polar solvents.T ou nderstand these size-and mediumdependent spectral features,weperformed diffusion ordered NMR spectroscopy (DOSY) measurements on solutions of purified PbS QDs at fixed concentration. In [D 8 ]toluene we observed that the QD diffusion coefficient, D,increases with QD size (Figure 3b)y ielding solvodynamic radii consistent with the related excitonic diameters ( Figure S13).…”

“…As-synthesized colloidal quantum dots (QDs) are composed of nanometer-sized crystallites of inorganic semiconductor materials surrounded by organic molecules-and/or metal complexes-as ligands that coordinate the core surface preventing aggregation and ensuring solubility.T he ligand/ core (organic/inorganic) interface exerts arelevant role in the synthetic control of QD size and shape, [1] markedly affects the electronic structure of colloidal QDs, [2] and mediates QD non-covalent bonding interactions with other QDs or different chemical species. [3] Athorough description of the organic/ inorganic interface is therefore essential for the development of refined synthetic strategies [4] and for the effective application of QDs in (opto)electronic devices and as luminophores, [5] among others,towhich aim metal chalcogenides are the most employed colloidal QD systems.A lthough frequently represented as discrete entities stably dispersed in aliquid phase,the static depiction of metal chalcogenide QDs and of their surface chemistry is fallacious:i ndeed, the QD growth mechanism implies ad ynamic organic/inorganic interface at high temperatures, [1] whereas the room-temperature effect of extra added Lewis bases has led to indirectly infer the lability of neutral ligands at the core surface,either as electron-donor organic species [6] or as electron-acceptor metal complexes. [7] Herein we provide direct evidence that archetypal PbS QDs,s ynthesized according to the most widely employed procedure, [8] exist in the solution phase as equilibrium mixtures with their ligand and core components in response to the QD'ssurroundings.Both organic molecules and metal complexes as ligands in mutual exchange are demonstrated to undergo dynamic equilibrium with the PbS core surface.Such interfacial equilibria depend on the solvent polarity and on QD concentration and size,p revalently involving specific nanocrystal facets.…”