Study of the Mechanism and Increasing Crystallinity in the Self-Templated Growth of Ultrathin PbS Nanosheets

Abstract

Colloidal 2D semiconductor nanocrystals, the analogue of solid-state quantum wells, have attracted strong interest in material science and physics. Molar quantities of suspended quantum objects with spectrally pure absorption and emission can be synthesized. For the visible region, CdSe nanoplatelets with atomically precise thickness and tailorable emission have been (almost) perfected. For the near-infrared region, PbS nanosheets (NSs) hold strong promise, but the photoluminescence quantum yield is low and many questions on the crystallinity, atomic structure, intriguing rectangular shape, and formation mechanism remain to be answered. Here, we report on a detailed investigation of the PbS NSs prepared with a lead thiocyanate single source precursor. Atomically resolved HAADF-STEM imaging reveals the presence of defects and small cubic domains in the deformed orthorhombic PbS crystal lattice. Moreover, variations in thickness are observed in the NSs, but only in steps of 2 PbS monolayers. To study the reaction mechanism, a synthesis at a lower temperature allowed for the study of reaction intermediates. Specifically, we studied the evolution of pseudo-crystalline templates toward mature, crystalline PbS NSs. We propose a self-induced templating mechanism based on an oleylamine-lead-thiocyanate (OLAM-Pb-SCN) complex with two Pb-SCN units as a building block; the interactions between the long-chain ligands regulate the crystal structure and possibly the lateral dimensions.

Figure 1

Electron microscopy images of PbS NSs prepared at 165 °C. Low-resolution HAADF-STEM images show the rectangular NSs with sharp 90° corners (a). High-resolution HAADF-STEM images show the serrated edges of the NSs (b) and a deformed orthorhombic PbS structure with dumbbell atom columns in the [100] direction (c). The inset shows the corresponding Fourier transform.

Figure 2

Schematic depictions of the cubic (a = b = c) and deformed orthorhombic PbS crystal lattices (a ≠ b ≠ c). The lead and sulfur atoms are represented respectively by the gray and yellow spheres. In panels (a) and (b), the face-down direction ([100] in zone axis) of both crystal structures is shown. In panels (c) and (d), the edge-up orientation on the long ([001] in panel (c)) and short edge ([010] in panel (d)) of the deformed orthorhombic NSs is shown for 4 MLs, one deformed orthorhombic unit cell. The gray octahedra clearly show that deformation occurs only in the [010] direction (d). See Figure S3 for a comparison with the cubic unit cell that consists of only 2 MLs as well as a table comparing the unit cells of the various crystal structures.

Figure 3

In the edge-up orientation ([001] in zone axis), the thickness of the PbS NSs can be studied with atomic precision, taking into account that some of the sheets are tilted. (a) A high-resolution HAADF-STEM image shows there are both sheets with a thickness of 4 and 6 MLs of PbS. No sheets with an intermediate thickness (3, 5, or 7 MLs) were observed. Some of the NSs merge together (blue arrow), indicating a lack of surface passivation (further discussed in Supporting Information Section 1). (b) The corresponding histogram indicates the 1:1 ratio of NSs with an average thickness of 1.2 (4 MLs) and 1.8 nm (6 MLs).

Figure 4

Atomically resolved HAADF-STEM images of PbS NSs, with the various irregularities in the sheets indicated by the red, blue, and green dashed outlines. (a) PbS sheet with an additional lead-rich region (blue dashed circle) and areas with a lower contrast, potentially due to a lead deficiency (red dashed circles). (b) At higher magnification, point defects (red and blue circles) and some irregularities within the structure (blue arrows) are observed. In both panels (a) and (b), a coherently incorporated small domain with a lower contrast is observed (green dashed square).

Figure 5

Three areas within the deformed orthorhombic PbS NS are indicated by the white dashed boxes (a), which are characterized in panel (b). The corresponding Fourier transform of domain 1 has a lattice spacing of 2.9 Å and is indexed according to the deformed orthorhombic crystal structure. Domains 2 and 3 are coherently incorporated in the principal structure, but their low-diffraction contrast and corresponding Fourier transform are different. Domain 2 is indexed as a conventional orthorhombic crystal structure with a lattice spacing of 4.0 Å along the [010] zone axis. Domain 3 is indexed as a cubic crystal structure with a spacing of 3.0 Å along the [100] axis.

Figure 6

HAADF-STEM images of aliquots taken 2.5 and 6.5 min after the reaction mixture reaches 145 °C, see Figure S9 for the other aliquots. (a) Rectangular NSs with a lateral size of 160 ± 27 by 18 ± 3 nm are already present. The image with atomic resolution in panel (b) shows that the sheets are pseudo-crystalline, roughly 23% of the sheet is crystalline (on average the domain size is 25 ± 17 nm², Figure S11). The Fourier transform of the sheet shows clear diffraction maxima but has some broadening in the long lateral direction. (c) After an additional 4 min, the NSs grow to an average size of 293 ± 79 by 38 ± 8 nm, while the crystallinity was estimated to be approximately 67% of the area with limited residual amorphous strips, as determined through direct visualization of the image (d).

Figure 7

Schematic formation of PbS NS via a self-induced templating mechanism. Starting with an OLAM-Pb-SCN complex with a 90° angle between the Pb/S and C/S bonds (a). By cross-coordination, two OLAM-Pb-SCN complexes can form a square, the central building block of the self-templating mechanism (b). Two-dimensional formation of the pseudo-crystalline structure as observed in the edge-up orientation of the PbS NS (c). Growth of the NSs along the lateral directions via cubes (four cross-coordinated OLAM-Pb-SCN complexes) with diagonal alignment of the favorable ligand–ligand interaction resulting in the [011̅] orientation of the lead atoms (d).

Figure 8

When annealing the NSs at 165 °C for an additional 5 or 10 min, the uniformity of the deformed orthorhombic crystal structure increases (a–c). With intensity threshold analysis applied to the low-contrast defect areas in the NSs (indicated by a green outline), a quantitative estimation on the improvement can be made. The defect areas decrease from 10.8% to 3.8% and then 0.2% (c–e).

Figure 9

With 4D STEM the changes in orientation of crystal structure can be tracked in the PbS NSs. (a) The deformed orthorhombic crystal structure is schematically depicted as it tilts in the short (left) and long (right) lateral direction of the sheet. With the [100] direction in zone axis, the diffraction pattern shows 4 spots with a lattice spacing of 0.29 nm contributing equally (middle image inset, yellow color). As the structure tilts, the contribution of the diffraction maxima changes and each pixel in the 4D STEM image was characterized based on the contribution of the diffraction maxima, green for a tilt in the short lateral direction and red for the long lateral direction. The HAADF-STEM image, and its reconstructed 4D STEM map are shown for 0 min (b), 5 min (c), and 10 min (d) at 165 °C. In panel (b) NSs immediately quenched upon reaching 165 °C have some voids in the HAADF-STEM image, while the maps also show large regions with different parallel orientations in the NSs. A significant improvement in the tilt toward zone axis can be observed in panel (b) and even more in panel (c) where large homogeneous areas of dark yellow can be observed.