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

Abstract

Colloidal 2D semiconductor nanocrystals, the analogue of solidstate 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.

Cation Exchange and Spontaneous Crystal Repair Resulting in Ultrathin, Planar CdS Nanosheets

“…The voids in the sheets are surrounded by both amorphous and crystalline areas (Figure b), but a corresponding Fourier transform (inset) shows the uniform orientation of the crystalline domains with only a slight broadening of the diffraction maxima. This retention of orientation is reminiscent of the formation of the parent PbS NSs, where early in the synthesis crystallographic domains form with remarkable alignment of orientation in rectangular but amorphous or pseudocrystalline self-induced templates . In the early stages of the cation exchange the number of damaged sheets is high (>80%, n = 18 sheets after 1 and 2.5 min of reaction, see Figure a, Figure S14).…”

“…Moreover, despite the pronounced structural anisotropy, no visible curling or folding of the thin sheets is observed (different from directly synthesized CdS and CdSe nanoplatelets). , By examining vertical stacks of the NSs in HAADF-STEM mode, we determined the thickness of the respective PbS and CdS sheets (Figure c,d). The parent PbS sheets were previously shown to be either 1.2 or 1.8 nm thick; while the CdS sheets are slightly thicker at 2.2 ± 0.4 nm (Figure S2a). As indicated by the selected area electron diffraction (SAED) of a single nanosheet (inset) in Figure e,f, the sharp diffraction maxima of the deformed orthorhombic crystal structure have broadened and shifted, possibly due to misalignments in the crystal structure.…”

“…To study the Cd 2+ -for-Pb 2+ cation exchange in 2D materials, we selected PbS NSs as synthesized by Akkerman et al. , as the parent NSs (Figure a). We then followed a procedure by Casavola et al, developed for cation exchange in anisotropic PbSe/CdSe core/shell nanomaterials .…”

“…As an additional washing step, 5 mL of acetonitrile was added before centrifuging again at 2750 rpm. The supernatant was removed and the precipitate was redispersed in in 5 mL of toluene . See Figures S1 and S2 for the characterization of PbS NSs.…”

“…Recently, there have been significant advances in the synthesis and study of ultrathin, highly anisotropic PbS nanosheets (NSs) with a deformed orthorhombic crystal structure. Despite having a thickness of only a few nanometers and lateral dimensions in the range of 100–200 nm, the PbS NSs exhibit a rectangular and planar shape, appearing completely flat in TEM without visible distortion, or curling. , This makes them compelling candidates to study the crystallographic aspects of cation exchange. Moreover, strain within CdS NSs was previously shown to strongly perturb the band structure and induce band gap variations up to 20 meV, in addition to influencing the self-assembly of the NSs. , Thus, distortion in NSs can hamper potential applications and device design.…”