A transmission electron microscopy study reveals grain boundaries and their formation mechanisms in 2D polymers.

Despite the rapid development of two-dimensional covalent organic frameworks (2D COFs) in recent years, it remains a great challenge to synthesize highly crystalline COF materials. Here, a CNC-assisted approach was adopted to synthesize high crystallinity COF materials. A series of 2D COF films were synthesized at the air–water interface by using cellulose nanocrystals (CNCs) as the template. The occurrence of Schiff reactions based on the imine bond was demonstrated by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) exhibited the appearances of 2D COF films were flower-like. When CNCs were added to a certain extent, the size of a single petal in the flowers gradually increased with the amount of CNCs. The film with large petals was characterized by Ultraviolet–Visible diffuse reflectance spectroscopy (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). In UV–Vis DRS curves, the S-band of COF-366 film was red-shifted by 24 nm compared with that of 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP), confirming the existence of extended conjugation in COF-366 film. XPS was used to identify the surface composition of the sample. The N1s signal of the film indicated that each TAPP formed four imine bonds with 2,5-dihydroxyterephthalaldehyde (DHTA) in COF-366 film. TEM images showed that CNCs had an influence on the crystal size. It was observed from SAED that the crystallinity of the film with CNCs was higher than the film without CNCs. This work provided a new template for improving the crystallinity of 2D COF films.

Quasi‐2D (q2D) conjugated polymers (CPs) are polymers that consist of linear CP chains assembled through non‐covalent interactions to form a layered structure. In this work, the synthesis of a novel crystalline q2D polypyrrole (q2DPPy) film at the air/H₂SO₄ (95%) interface is reported. The unique interfacial environment facilitates chain extension, prevents disorder, and results in a crystalline, layered assembly of protonated quinoidal chains with a fully extended conformation in its crystalline domains. This unique structure features highly delocalized π‐electron systems within the extended chains, which is responsible for the low effective mass and narrow electronic bandgap. Thus, the temperature‐dependent charge‐transport properties of q2DPPy are investigated using the van der Pauw (vdP) method and terahertz time‐domain spectroscopy (THz‐TDS). The vdP method reveals that the q2DPPy film exhibits a semiconducting behavior with a thermally activated hopping mechanism in long‐range transport between the electrodes. Conversely, THz‐TDS reveals a band‐like transport, indicating intrinsic charge transport up to a record short‐range high THz mobility of ≈107.1 cm² V⁻¹ s⁻¹.

By structural and analytical TEM and scanning electron microscopy experiments we show that atomically-resolved structural characterization of oxidation-sensitive two-dimensional material is strongly hindered when the final step of the preparation process, the transfer to the TEM grid, is performed with a wet etching method involving bases or acids, interacting with the highly reactive sample surface. Here we present an alternative polymer-assisted and mechanical-exfoliation-based sample preparation method and demonstrate it on selected oxidation-sensitive transition metal phosphorus trisulfides and transition metal dichalcogenides. The analysis, obtained from the samples prepared with both of the methods clearly show that oxidation is the origin of discrepancy, the oxidation during the final preparation step is strongly reduced only when the new method is applied, and atomically-resolved structural characterization of the pristine structures is now possible.