2D Polymer & Interfacial Synthesis
In this group, we mainly focus on the development of organic 2DMs, which refer to crystalline 2D nanostructures comprising carbon-rich repeated units linked by covalent/noncovalent bonds, including synthetic graphene, crystalline 2D polymers (2DPs), 2D supramolecular polymers (2DSPs), single/few-layer 2D COFs/MOFs, etc. We expect to establish novel chemical/synthetic methodologies toward the controlled synthesis of organic 2DMs and achieve delineation of reliable structure-property relationships and superior physical and chemical performances of 2D polymers.
Research Area I: Development of Interfacial Synthesis Methodologies
An interface is a flat or curved space (or a phase boundary) between two different matters, or two different phases in one matter. The thickness of the interface space can range from several angstroms to nanometer and even to micrometer size. In our work, we aim to develop interfaces as key roles in “bottom-up” synthesis, which is advantageous for directing the pre-orientation of the molecules or precursors for subsequent 2D polymerization toward organic 2DMs. Typically, the reactions at the interfaces between air−water (or gas−liquid, LB), liquid−liquid, liquid−solid, and gas−solid (or vacuum-solid) are explored for the 2D polymerization by linkage chemistry and the paramount control over the morphology and the structure of organic 2DMs. In our latest work, we have established a surfactant-monolayer-assisted interfacial synthesis (SMAIS) method to prepare 2D polymers, like 2D polyimides, 2D polyimines and 2D boronate ester-linked polymers, thus achieving few-layers and micrometer-sized single-crystalline domains.
Research Area II: Synthetic 2D Polymers
Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) have emerged as a class of structurally defined organic 2D materials with exotic physical and chemical properties. These porous crystalline polymers generally comprise repeated units linked via covalent bonds with long-range ordering in two distinct directions and have displayed diverse physical and chemical properties for broad functions in (opto-)electronics, spintronics, membrane, catalysis, and energy storage and conversion. Yet, controlled synthesis of 2DP and 2D COF single crystals still remains an immense challenge.
We aim to explore thermodynamically reversible dynamic covalent reactions (DCRs) for the synthesis of crystalline 2DPs/2D COFs at interfaces and in solution. DCRs allow the atomically precise integration of organic units to create periodic networks with long-range order linked by covalent bonds. A key feature of DCRs is the thermodynamically controlled product distribution at equilibrium. Typical examples of DCRs include the formation of boronate ester, imine, pyrazine, azine, imide, amide and azodioxy bonds. In our recent work, by combining DCRs and SMAIS, we successfully achieved few-layer 2DP single crystals with the domain size up to ~150 μm2.
We aim to explore the utilization of kinetically irreversible covalent reactions (ICRs) for the synthesis of 2DPs and 2D COFs, which is highly attractive but remains challenging. To achieve the long-range order and layered crystals, 2D polymerization will be carried out under kinetic control relying on monomer pre-organization prior to bond formation. Typical ICRs including Knoevenagel, Aldol-type and Horner-Wadsworth-Emmons condensation reactions have been developed for the solvothermal synthesis of vinylene-linked 2D COFs, yielding fully conjugated polymer materials with high chemical and thermal stabilities. On the other hand, structurally-defined conjugated 2DPs can be realized via the on-surface Ullmann coupling under ultrahigh vacuum conditions. Recently, we also performed the oxidation polymerization on water surface by SMAIS and developed wafer-sized, highly crystalline quasi-2D polyaniline thin film, which exhibited a high conductivity up to 160 S/cm.
Synthetic 2D polymers and their layer-stacked 2D COFs feature with intrinsic porosity and flexibility as well as tailored electroactivity and photoactivity. In our recent work, upon I2 doping, a phthalocyanine-based pyrazine-linked 2D COF exhibited high conductivity (10-3 S/cm) and record mobility reaching to 22 cm2/(Vs). These unique features enable 2D polymers and 2D COFs broad functions in transistors, sensors, flexible devices, memory devices, membrane separation, osmotic power generators, electro-/photo-catalysis, batteries and supercapacitors.
Research Area III. 2D van der Waals heterostructures
Two-dimensional (2D) van der Waals heterostructures (vdWHs) are generated by integration of 2D materials with dangling-bond-free surfaces through the weak interlayer vdW interaction along the vertical direction. In recent years, 2D vdWHs have attracted increasing attentions due to the diverse components and novel functions in photodiodes, phototransistors, tunneling devices and memory devices. Through surface reconstruction and proximity effects between the neighboring layers, the optoelectrical properties of vdWHs can be tailored, thus tuning carrier density, improving electron-hole separation and accelerating charge transfer. In our work, we aim to establish bottom-up chemical synthesis of 2D polymer based vdWHs, control the chemical synthesis for high-quality vdWH interface with atomic precision, and explore the unique physical/chemical properties of the synthetic vdWHs.