Atomic Alignment in PbS Nanocrystal Superlattices with Compact Inorganic Ligands via Reversible Oriented Attachment of Nanocrystals

TL;DR

This work shows that PbS nanocrystals capped with compact, highly charged metal chalcogenide complex ligands can assemble into all-inorganic superlattices that exhibit both translational and atomic-lattice orientational order. The observed edge-to-edge alignment arises from strong ionic correlations and layered electrolyte structure, which are captured by a Landau-Ginzburg–type framework yielding an orientational phase diagram with edge-to-edge states as a stable configuration. Remarkably, these superlattices are reversible, able to disassemble back into colloidal NCs by solvent and salt control, illustrating reversible oriented attachment (ROA) at the atomic scale. The integrated experimental and theoretical approach establishes a route to reconfigurable, single-crystal-like nanostructures with tunable electronic and optoelectronic properties for dynamic materials and adaptive devices.

Abstract

Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials, with NC self-assembly offering opportunities to enable novel material properties. Here, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as $Sn_2S_6^{4-}$ and $AsS_4^{3-}$, can self-assemble into all-inorganic superlattices with both long-range superlattice translational and atomic-lattice orientational order. Structural characterizations reveal that the NCs adopt unexpected edge-to-edge alignment, and numerical simulation clarifies that orientational order is thermodynamically stabilized by many-body ion correlations originating from the dense electrolyte. Furthermore, we show that the superlattices of $Sn_2S_6^{4-}$-functionalized PbS NCs can be fully disassembled back into the colloidal state, which is highly unusual for orientationally attached superlattices with atomic-lattice alignment. The reversible oriented attachment of NCs, enabling their dynamic assembly and disassembly into effectively single-crystalline superstructures, offers a pathway toward designing reconfigurable materials with adaptive and controllable electronic and optoelectronic properties.

Figures (5)

• Figure 1: (A) Small-angle X-ray scattering (SAXS) pattern of 5.7 nm PbS-Sn2S64- nanocrystal (NC) superlattice. The peak assignment shows that the superlattice adopts a face-centered cubic (fcc) structure. (B) Bright-field transmission electron microscopy (TEM) image of a 5.7 nm PbS-Sn2S64- NC superlattice domain. (C) Selected area electron diffraction (SAED) image of the entire grain. Distinct spot patterns indicate that the PbS NCs are atomically aligned. (D, E) Scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) image of a 5.7 nm PbS-Sn2S64- NC superlattice.
• Figure 2: (A) Transmission electron microscopy (TEM) and (B) High-resolution TEM (HRTEM) image of the superlattice of 5.7 nm PbS-Sn2S64- NCs, viewed along the (110) zone axis. (C) TEM and (D) HRTEM images of the superlattice of the same NC superlattice viewed along the (100) zone axis. (E, F) Fourier-transformed TEM (FT-TEM) images of the superlattice viewed along the (110) zone axis, with the distinct spots indicating highly ordered fcc structures at both nanometer and atomic scales. (G, H) FT-TEM of the superlattice viewed along the (100) zone axis, also with distinct spots indicating a high degree of fcc ordering.
• Figure 3: (A) Illustration of 5.7 nm PbS-Sn2S64- NC superlattice. Illustration of the superlattice viewed along (B) (110) zone axis and (C) (100) zone axis. (D) Illustration of the PbS NC viewed along (E) (110) zone axis and (F) (100) zone axis.
• Figure 4: (A) Free energy difference between two aligned (red line) and nonaligned (blue line) nanocrystals (NCs) as a function of the distance of separation. Top and bottom inset show the asymmetric charge density for the aligned and non-aligned NCs at the distance of minimum free energy. B) Phase diagram for orientational order in a four-site fcc lattice as a function of characteristic length scales related to ion and solvent size ($l_{c}$) and screening length ($l_{s}$). The three phases correspond to diagonal alignment (blue), face-to-face alignment (gold) and edge-to-edge alignment (red). (C) Representative snapshots of the three phases along with the asymmetric charge density. The snapshot consists of truncated octahedral PbS NCs and the charge distributions around the NCs. Red indicates positive charge, blue indicates negative charge.
• Figure 5: (A) Small-angle X-ray scattering (SAXS) patterns of the original colloid solution, as-prepared superlattices (SL) and redispersed colloid of 5.7 nm PbS-Sn2S64- nanocrystals (NCs). The y-axis is vertically offset for clarity. Absence of Bragg peaks from the SAXS pattern of redispersed PbS NCs indicates full disassembly of superlattices. (B) SAXS patterns of as-prepared and washed 5.7 nm PbS-Sn2S64- NC superlattices. (C, D) Transmission electron microscopy (TEM) images of as-prepared (D) and washed (E) 5.7 nm PbS-Sn2S64- NC superlattices. Insets: magnified images of the regions highlighted by red squares. (E) HAADF-STEM image and STEM-EDS elemental mapping of the superlattice. The plot below presents the ratio of the Sn and Pb signal counts across the superlattice. (F) Schematic of self-assembly, washing and redispersion processes.