Ibrahim Abdelwahab

A two-dimensional conjugated aromatic polymer via C–C coupling reaction

The fabrication of crystalline 2D conjugated polymers with well-defined repeating units and in-built porosity presents a significant challenge to synthetic chemists. Yet they present an appealing target because of their desirable physical and electronic properties. Here we report the preparation of a 2D conjugated aromatic polymer synthesized via C-C coupling reactions between tetrabromopolyaromatic monomers. Pre-arranged monomers in the bulk crystal undergo C-C coupling driven by endogenous solid-state polymerization to produce a crystalline polymer, which can be mechanically exfoliated into micrometre-sized lamellar sheets with a thickness of 1 nm. Isothermal gas-sorption measurements of the bulk material reveal a dominant pore size of ~0.6 nm, which indicates uniform open channels from the eclipsed stacking of the sheets. When employed as an organic anode in an ambient-temperature sodium cell, the material allows a fast charge/discharge of sodium ions, with impressive reversible capacity, rate capability and stability metrics.

Quasi‐Monolayer Black Phosphorus with High Mobility and Air Stability

Black phosphorus (BP) exhibits thickness-dependent band gap and high electronic mobility. The chemical intercalation of BP with alkali metal has attracted attention recently due to the generation of universal superconductivity regardless of the type of alkali metals. However, both ultrathin BP, as well as alkali metal-intercalated BP, are highly unstable and corrode rapidly under ambient conditions. This study demonstrates that alkali metal hydride intercalation decouples monolayer to few layers BP from the bulk BP, allowing an optical gap of ≈1.7 eV and an electronic gap of 1.98 eV to be measured by photoluminescence and electron energy loss spectroscopy at the intercalated regions. Raman and transport measurements confirm that chemically intercalated BP exhibits enhanced stability, while maintaining a high hole mobility of up to ≈800 cm2 V-1 s-1 and on/off ratio exceeding 103 . The use of alkali metal hydrides as intercalants should be applicable to a wide range of layered 2D materials and pave the way for generating highly stable, quasi-monolayer 2D materials.

Two-Dimensional Polymer Synthesized via Solid-State Polymerization for High-Performance Supercapacitors

Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield via solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C-C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g-1 at a current density of 1.0 A g-1, and shows outstanding cycle performance.

Large Area Synthesis of 1D-MoSe2 Using Molecular Beam Epitaxy.

Large area synthesis of 1D-MoSe2 nanoribbons on both insulating and conducting substrates via molecular beam epitaxy is presented. Dimensional controlled growth of 2D, 1D-MoSe2 , and 1D-2D-MoSe2 hybrid heterostructure is achieved by tuning the growth temperature or Mo:Se precursor ratio.

Homoepitaxial Growth of Large‐Scale Highly Organized Transition Metal Dichalcogenide Patterns

Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with regular morphology and unique edge structure is highly desired and important for fundamental research and potential applications. Here, single-crystalline MoS2 flakes are reported with regular trigonal symmetric patterns that can be homoepitaxially grown on MoS2 monolayer via chemical vapor deposition. The highly organized MoS2 patterns are rhombohedral (3R)-stacked with the underlying MoS2 monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The epitaxial MoS2 crystals can be tailored from compact triangles to fractal flakes, and the pattern formation can be explained by the anisotropic growth rates of the S and Mo edges under low sulfur chemical potential. The 3R-stacked MoS2 pattern demonstrates strong second and third-harmonic-generation signals, which exceed those reported for monolayer MoS2 by a factor of 6 and 4, correspondingly. This homoepitaxial growth approach for making highly organized TMD patterns is also demonstrated for WS2 .

Highly Enhanced Third-Harmonic Generation in 2D Perovskites at Excitonic Resonances

Two-dimensional hybrid organic-inorganic Ruddlesden-Popper perovskites (RPPs) have attracted considerable attention due to their rich photonic and optoelectronic properties. The natural multi-quantum-well structure of 2D RPPs has been predicted to exhibit a large third-order nonlinearity. However, nonlinear optical studies on 2D RPPs have previously been conducted only on bulk polycrystalline samples, in which only weak third-harmonic generation (THG) has been observed. Here, we perform parametric nonlinear optical characterization of 2D perovskite nanosheets mechanically exfoliated from four different lead halide RPP single crystals, from which we observe ultrastrong THG with a maximum effective third-order susceptibility (χ(3)) of 1.12 × 10-17 m2 V-2. A maximum conversion efficiency of 0.006% is attained, which is more than 5 orders of magnitude higher than previously reported values for 2D materials. The THG emission is resonantly enhanced at the excitonic band gap energy of the 2D RPP crystals and can be tuned from violet to red by selecting the RPP homologue with the requisite resonance. Due to signal depletion effects and phase-matching conditions, the strongest nonlinear response is achieved for thicknesses less than 100 nm.

Molecularly thin two-dimensional hybrid perovskites with tunable optoelectronic properties due to reversible surface relaxation

Due to their layered structure, two-dimensional Ruddlesden–Popper perovskites (RPPs), composed of multiple organic/inorganic quantum wells, can in principle be exfoliated down to few and single layers. These molecularly thin layers are expected to present unique properties with respect to the bulk counterpart, due to increased lattice deformations caused by interface strain. Here, we have synthesized centimetre-sized, pure-phase single-crystal RPP perovskites (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1–4) from which single quantum well layers have been exfoliated. We observed a reversible shift in excitonic energies induced by laser annealing on exfoliated layers encapsulated by hexagonal boron nitride. Moreover, a highly efficient photodetector was fabricated using a molecularly thin n = 4 RPP crystal, showing a photogain of 105 and an internal quantum efficiency of ~34%. Our results suggest that, thanks to their dynamic structure, atomically thin perovskites enable an additional degree of control for the bandgap engineering of these materialsReversible structural surface relaxation under laser exposure is observed for monolayers of 2D metal halide perovskites. These structural changes also induce reversible shifts in the photoluminescence peaks of these materials.