Two-dimensional MOF and COF nanosheets for next-generation optoelectronic applications

Surveys interfacial, solvothermal, and ionothermal routes that directly grow nanosheets from molecular precursors by limiting out-of-plane growth.

Relevance: Core · 2 · Bottom-up approaches · Fig. 5; Fig. 6; Fig. 7

Frames 2D COFs as pi-conjugated crystalline organic semiconductors whose donor-acceptor designs and ordered columns enable photodetection, photovoltaics, and emission.

Relevance: Core · 6 · Tow-dimensional COF-based optoelectronic applications · Fig. 15; Fig. 16

Reviews conductive-framework transport concepts, including localisation-limited hopping, through-space and through-bond pathways, and band-like transport signatures.

Relevance: Core · 4 · Conductive MOF and COF · Table 2

Organises fabrication into top-down and bottom-up approaches, contrasting scalable exfoliation with bottom-up control over large, low-defect nanosheets.

Relevance: Core · 1 · Fabrication of two-dimensional MOF and COF · Table 1

Frames 2D MOF and COF nanosheets as flexible, structurally tunable optoelectronic materials and motivates the need for a review focused on 2D framework optoelectronics.

Relevance: Core · 1 · Introduction

Discusses MOF band-gap engineering and MOF uses in photodetection, photovoltaics, LEDs, displays, nonlinear optics, and ultrafast photonics.

Relevance: Core · 4 · Two-dimensional MOF-based optoelectronic applications · Fig. 8-Fig. 14

Summarises synthesis, transport, stability, band-gap, and modelling gaps for translating 2D MOF/COF nanosheets into optoelectronic devices.

Relevance: Core · 6 · Conclusions and outlook

Covers liquid-phase, solvent-assisted, freeze-thaw, mechanical, and chemical exfoliation as ways to separate layered MOF/COF crystals into few-layer nanosheets.

Relevance: Core · 1 · Top-down approaches · Fig. 1; Fig. 2; Fig. 3; Fig. 4

Bottom-up nanosheet growth methods

Bottom-up methods are organised by whether growth is confined at an interface or controlled in a solvent/ionic-liquid medium to suppress vertical growth.

Categories: liquid/liquid interfacial synthesis · liquid/gas interfacial synthesis · solvothermal synthesis · ionothermal synthesis

2-3 · Bottom-up approaches · Fig. 5-Fig. 7

Optoelectronic conversion modes

Applications are framed around photodetectors and photovoltaics for optical-to-electrical conversion, LEDs/displays for electrical-to-optical conversion, and nonlinear or mode-locking photonic devices.

Categories: optical-to-electrical conversion · electrical-to-optical conversion · nonlinear optical modulation

1 · Introduction

2D MOF/COF fabrication routes

The review's central synthesis framework contrasts exfoliating bulk layered frameworks with directly growing nanosheets from precursors under interfacial or solvothermal control.

Categories: top-down exfoliation · bottom-up direct growth

1 · Fabrication of two-dimensional MOF and COF · Table 1

Performance-adjusting levers

The review repeatedly links electronic structure and optoelectronic response to structural modification, ligand choice, reaction control, defect density, doping, and interfaces with graphene or semiconductors.

Categories: coordination environment · organic ligand · reaction condition · morphology and defects · doping or guest modification · heterostructure contact

1 · Introduction

Top-down exfoliation methods

Top-down methods are presented as high-yield routes for layered MOF/COF nanosheets, but with risks of fragmentation, restacking, and topological defects.

Categories: liquid-phase exfoliation · solvent-assisted exfoliation · mechanical delamination · chemical exfoliation · freeze-thaw exfoliation

1-2 · Top-down approaches · Fig. 1-Fig. 4

Conductive framework transport mechanisms

The review adopts a four-mechanism framework for intrinsically conductive MOFs, while COF transport is discussed largely through pi-stacked columns, doping, and donor-acceptor designs.

Categories: hopping transport · through-space transport · through-bond transport · band transport

4 · Conductive MOF and COF

Conductive 2D MOFs using thiolate-rich linkers that support strong metal-ligand electronic coupling.

Conduction: Discussed as high-conductivity or band-like systems suitable for transparent electrodes and photodetection.

Representative materials: Cu-BHT · Fe3(THT)2(NH4)3 · Ni3(HITP)2-like thiolate analogues

Nodes / linkers: Cu · Fe · benzenehexathiol · triphenylenehexathiol

4 · Conductive MOF and COF · Table 2

Heterostructures combining semiconducting framework absorbers with graphene channels or electrodes to assist charge separation and transport.

Conduction: Interfaces and built-in fields are presented as routes to separate photocarriers and compensate for poor framework conductivity.

Representative materials: [Sr(ntca)(H2O)2].H2n MOF/graphene · 2D COF/graphene · COF:PC71BM

Nodes / linkers: Sr · none for COF component · naphthalenetetracarboxylate · carboxaldehyde-triazine COF motifs · donor-acceptor COF units

5-6 · Photodetector; Tow-dimensional COF-based optoelectronic applications · Fig. 8; Fig. 15

Robust 2D conductive MOFs derived from conductive hexaaminobenzene linkers.

Conduction: Included among recent 2D MOFs with high carrier mobility and conductivity.

Representative materials: Co-HAB

Nodes / linkers: Co · hexaaminobenzene

4 · Two-dimensional MOF-based optoelectronic applications · Table 2

Extended pi-conjugated MOFs based on HITP or HHTP-type ligands and transition-metal nodes.

Conduction: Conductivity is associated with in-plane charge delocalisation and pi-d orbital coupling through metal nodes.

Representative materials: M3(HITP)2 · Cu3(HITP)2 · Cr3(HITP)2

Nodes / linkers: Ni · Cu · Cr · hexaiminotriphenylene · hexahydroxytriphenylene

4 · Conductive MOF and COF

Bulk 2D COFs whose pi-pi stacked layers can be separated into few-layer COF nanosheets.

Conduction: The review treats pi-pi interaction and ordered columns as relevant to later charge transport, although many COFs remain low-conductivity without modification.

Representative materials: COF-1 · COF-8 · COF-43

Nodes / linkers: none · covalent organic aromatic linkers · pi-stacked COF sheets

2 · Liquid-phase exfoliation · Fig. 2; Table 1

MOFs composed of stacked two-dimensional units held by weak interlayer interactions that can be separated into nanosheets.

Conduction: Primarily discussed as processable 2D morphology; transport depends on later framework composition and device integration.

Representative materials: MOF-2 · [Cu2Br(IN)2]n · MAMS-1 · X 2,3-DMS (X = Mn, Co, Zn)

Nodes / linkers: Cu · Mn · Co · Zn · carboxylates · dimethylsuccinate-derived ligands · stacked 2D coordination units

1-2 · Liquid-phase exfoliation · Fig. 1; Table 1

COFs incorporating extended aromatic macrocycles and electron-rich or electron-deficient units for charge transport and optoelectronic response.

Conduction: Conductivity and mobility are improved by electron-deficient blocks, donor-acceptor arrangements, and iodine or ionic doping.

Representative materials: nickel(II) phthalocyanine-BTDA COF · POR-COF · COF-DC-8 · triphenylene-porphyrin COF

Nodes / linkers: Ni in phthalocyanine centres · none for purely organic COFs · phthalocyanine · porphyrin · benzothiadiazole · tetrathiafulvalene · pyrene-tetraketone

4 · Conductive MOF and COF · Table 2

Chemical exfoliation

Introduce chemically active groups or reactions that weaken interlayer forces, cut cleavable ligands, or disrupt hydrogen bonding and pi-pi stacking.

Claimed effects: Can give controllable ultrathin nanosheets with high yield and stable dispersion.

Controlling variables: interlayer ligand chemistry · reducing agent · cycloaddition chemistry · heteroatom functionalisation · reaction duration

Representative materials: porphyrinic MOF nanosheets · IISERP-CON7 · IISERP-CON8

Caveat: The approach depends on framework-specific cleavable or reactive motifs and is not a universal exfoliation recipe.

2 · Chemical exfoliation · Fig. 3; Fig. 4

Freeze-thaw solvent exfoliation

Use solvent volume change between frozen and thawed states to exert shear on layered crystals without conventional sonication.

Claimed effects: Reported to produce uniform thin MOF nanosheets while avoiding the cracking associated with liquid-phase exfoliation.

Controlling variables: freeze-thaw temperature swing · solvent phase change · cycle repetition · post-exfoliation size purification

Representative materials: MAMS-1

Caveat: The review warns that the very low or high temperatures required may damage many MOF frameworks.

2 · Liquid-phase exfoliation · Fig. 1

Interfacial COF nanosheet synthesis

Grow COF films or nanosheets at liquid/liquid or liquid/gas interfaces using Schiff-base, Knoevenagel, Suzuki, or cycloaddition-enabled chemistry.

Claimed effects: Enables large-area COF nanosheets and thin films compatible with electronic-device architectures.

Controlling variables: interface type · organic and aqueous phase composition · monomer planarity · pi-pi stacking disruption · evaporation and film transfer

Representative materials: DaTp-Cons · 2DCCOF1 · 2DCCOF2

Caveat: Strong covalent bonding makes COF nanosheet growth and exfoliation chemically distinct from MOFs.

2-3 · Interfacial synthesis method · Fig. 6

Liquid/liquid and liquid/gas interfacial MOF growth

React metal ions and ligands at immiscible liquid/liquid or water/air interfaces to confine growth into large-area nanosheets.

Claimed effects: Produces large lateral MOF nanosheets and can allow monolayer formation at water/air interfaces.

Controlling variables: phase boundary area · precursor distribution between phases · organic solvent evaporation · interface cleanliness · reactant supply

Representative materials: NiAT · CuBDC · Cu-based MOF nanosheets

Caveat: Traditional liquid/liquid routes can be production-limited because nanosheet size is restricted by interfacial area.

2 · Interfacial synthesis method · Fig. 5

Ionothermal synthesis

Use ionic liquids as solvent/template media to support crystallinity, thermal stability, and microwave-assisted synthesis of organic frameworks.

Claimed effects: Presented as promising for crystalline, thermally stable MOF/COF materials and potentially improved conductivity in optoelectronic devices.

Controlling variables: ionic-liquid cation · ionic-liquid anion · water content · viscosity · thermal stability · microwave coupling

Representative materials: NH2-MIL-53(Al) · porous MOFs

Caveat: The section is more conceptual for optoelectronics than directly supported by many 2D device demonstrations.

3 · Ionothermal method

Liquid-phase and solvent-assisted exfoliation

Break interlamellar van der Waals or pi-pi interactions in layered MOF/COF bulk crystals using sonication and solvents whose surface energy matches the target material.

Claimed effects: Can yield monolayer or few-layer nanosheets and large-scale production, but may fragment sheets, create defects, or allow reassembly.

Controlling variables: solvent choice · sonication power · sonication time · surface energy match · layer stabilisation by solvent

Representative materials: MOF-2 · MAMS-1 · COF-8

Caveat: Ultrasound shear can create many defects and degrade lateral morphology even under mild conditions.

1-2 · Liquid-phase exfoliation · Fig. 1; Fig. 2

Solvothermal nanosheet growth

Use solvothermal environments or substrates to grow MOF/COF nanosheets and films by controlling anisotropic crystal growth and morphology.

Claimed effects: Can produce layer-stacked MOF nanosheets or COF nanofibres/films and provide morphology control relevant to device use.

Controlling variables: solvent identity · substrate treatment · growth-rate anisotropy · anti-aggregation solvent mixtures · dynamic bond reversibility

Representative materials: Cu-BHT · Zn2(ZnTCPP) · TATF COFs

Caveat: Nanosheet reassembly after sonication and solvent-dependent aggregation remain practical issues.

3 · Solvothermal method · Fig. 7; Table 1

The review identifies a need for ultrafast molecular charge-transfer systems that remain stable under practical stressors.

Proposed direction: Design charge-transfer systems and interfaces stable under high temperature, high humidity, and high voltage.

7 · Conclusions and outlook

Because conductive COFs are described as being at an initial stage, the review calls for more comprehensive understanding of COF electronic structure.

Proposed direction: Clarify the electronic structures, pi-stacking effects, and dopant/guest roles that control COF mobility and conductivity.

4 · Conductive MOF and COF

The review suggests that machine-learning tools may help understand conductivity and absorption beyond existing experimental information.

Proposed direction: Apply machine learning to synthesis conditions, surface chemistry, conductivity, and light absorption to design specific photo-functional units.

7 · Conclusions and outlook

The review notes that nonlinear optical effects in 2D MOF structures have rarely been reported compared with broader 3D MOF nonlinear-optics work.

Proposed direction: Explore 2D MOF composition, morphology, and symmetry design for nonlinear optical and mode-locking applications.

6 · Nonlinear optics and ultrafast photonics

The review calls for refined synthetic strategies to produce larger 2D structures with fewer defects because top-down routes can restack and damage surfaces.

Proposed direction: Develop bottom-up or otherwise controlled routes that limit vertical stacking while preserving crystallinity and lateral size.

6 · Conclusions and outlook

The review states that finding 2D MOF/COF materials with suitable band gaps for effective visible-region optoelectronic applications remains meaningful but challenging.

Proposed direction: Use ligand, coordination, reaction-condition, and computational design to tune band gaps and absorption.

7 · Conclusions and outlook