2D Organic Materials Research Articles

Organic steep-slope nano-FETs: A rational design based on two-dimensional covalent-organic frameworks

We report a strategy to design organic steep-slope nanoscale field-effect transistors (nano-FETs) with subthreshold swing (SS) breaking the thermionic limit of 60 mV/dec at room temperature. The devices are based on a family of two-dimensional (2D) covalent-organic frameworks (COFs) with a square lattice and building blocks of metallophthalocyanine (MPc). Our first-principles calculations reveal that lattice mismatch-free metal/semiconductor monolayer heterojunctions, which serve as both electrodes and channel of the devices, can be obtained by modulating monolayer 2D MPc-COFs with suitable MPc. The cold-source mechanism and the interplay between electrodes and channel are of critical importance for realizing doping-free organic steep-slope nano-FETs, of which the SS can reach as small as 37.2 mV/dec at 300 K. Our findings offer a route to overcome the “Boltzmann tyranny” in low-power nanoelectronic devices by rational design of organic 2D materials, which are much more diverse and flexible in material design than their inorganic counterparts. • Organic nano-FETs with subthreshold swing breaking the thermionic limit of 60 mV/dec at room temperature were proposed. • The devices are based on two-dimensional covalent-organic frameworks of metallophthalocyanine. • The cold-source mechanism and the interplay between electrodes and channel are the underlying mechanism for the devices.

Growing two-dimensional single crystals of organic semiconductors on liquid surfaces

Two-dimensional (2D) organic semiconducting single crystals (OSSCs) combine the advantages of OSSCs and 2D molecular materials, and currently, are an important and hot research topic in the field of organic optoelectronics and 2D materials. Growth of large-sized 2D OSSCs with controlled thickness is highly important for fundamental research and practical application, which is still a challenging task. Liquid surfaces can provide ideal platforms for growth of large-scale and layer-controlled 2D OSSCs because their surfaces are molecularly flat and free-defect, the “coffee-ring” effect can be effectively suppressed on liquid surfaces, and the improved spread area of organic semiconductor solutions on liquid surfaces. This perspective concentrates on recent advances in the growth of large-area and layer-controlled 2D OSSCs on liquid surfaces; the optoelectronic applications based on 2D OSSCs, as well as the challenges and outlooks in the field, are also discussed.

The Current Understanding of how 2D Polymers Grow Photochemically

Abstract2D polymers are a relatively new class of macromolecules. Therefore, it is not astounding that so far research focused on how to provide access to this intriguing class of organic 2D materials, how to prove their existence, and how to assess their structural quality. Studies concerning the formation mechanism are comparatively scarce. We here collect and compare all the mechanistic information available for 2D polymer synthesis by photochemical means and point towards research directions to be followed in order to advance the fundamental understanding and, thus, fast development of this field. Because the two current starting situations for the photochemical synthesis of 2D polymers are layered single crystals and surface‐supported monolayers, the prominent analytical tools are X‐ray diffraction (XRD), local vibration spectroscopy, e. g. tip enhanced Raman spectroscopy (TERS), and scanning probe microscopy (SPM), e. g. low temperature scanning tunnelling microscopy (LT STM) in ultra‐high vacuum (UHV), but also atomic force microscopy (AFM). With their advantages and shortcomings, they will therefore play an important role throughout this mini review.

Manifestation of Strongly Correlated Electrons in a 2D Kagome Metal–Organic Framework

Abstract2D and layered electronic materials characterized by a kagome lattice, whose valence band structure includes two Dirac bands and one flat band, can host a wide range of tunable topological and strongly correlated electronic phases. While strong electron correlations have been observed in inorganic kagome crystals, they remain elusive in organic systems, which benefit from versatile synthesis protocols via molecular self‐assembly and metal‐ligand coordination. Here, direct experimental evidence of local magnetic moments resulting from strong electron–electron Coulomb interactions in a 2D metal–organic framework (MOF) is reported. The latter consists of di‐cyano‐anthracene (DCA) molecules arranged in a kagome structure via coordination with copper (Cu) atoms on a silver surface [Ag(111)]. Temperature‐dependent scanning tunneling spectroscopy reveals magnetic moments spatially confined to DCA and Cu sites of the MOF, and Kondo screened by the Ag(111) conduction electrons. By density functional theory and mean‐field Hubbard modeling, it is shown that these magnetic moments are the direct consequence of strong Coulomb interactions between electrons within the kagome MOF. The findings pave the way for nanoelectronics and spintronics technologies based on controllable correlated electron phases in 2D organic materials.

In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)

Plasmon-induced chemical reactions (PICRs) have recently become promising approaches for highly efficient light-chemical energy conversion. However, an in-depth understanding of their mechanisms at the nanoscale still remains challenging. Here, we present an in-situ investigation by tip-enhanced Raman spectroscopy (TERS) imaging of the plasmon-induced [4+4]-cycloaddition polymerization within anthracene-based monomer monolayers physisorbed on Au(111), and complement the experimental results with density functional theory (DFT) calculations. This two-dimensional (2D) polymerization can be flexibly triggered and manipulated by the hot carriers, and be monitored simultaneously by TERS in real time and space. TERS imaging provides direct evidence for covalent bond formation with ca. 3.7 nm spatial resolution under ambient conditions. Combined with DFT calculations, the TERS results demonstrate that the lateral polymerization on Au(111) occurs by a hot electron tunneling mechanism, and crosslinks form via a self-stimulating growth mechanism. We show that TERS is promising to be plasmon-induced nanolithography for organic 2D materials.

Memristor Based on Inorganic and Organic Two-Dimensional Materials: Mechanisms, Performance, and Synaptic Applications.

A memristor is a two-terminal device with nonvolatile resistive switching (RS) behaviors. Recently, memristors have been highly desirable for both fundamental research and technological applications because of their great potential in the development of high-density memory technology and neuromorphic computing. Benefiting from the unique two-dimensional (2D) layered structure and outstanding properties, 2D materials have proven to be good candidates for use in gate-tunable, highly reliable, heterojunction-compatible, and low-power memristive devices. More intriguing, stable and reliable nonvolatile RS behaviors can be achieved in multi- and even monolayer 2D materials, which seems unlikely to be achieved in traditional oxides with thicknesses less than a few nanometers because of the leakage currents. Moreover, such two-terminal devices show a series of synaptic functionalities, suggesting applications in simulating a biological synapse in the neural network. In this review article, we summarize the recent progress in memristors based on inorganic and organic 2D materials, from the material synthesis, device structure and fabrication, and physical mechanism to some versatile memristors based on diverse 2D materials with good RS properties and memristor-based synaptic applications. The development prospects and challenges at the current stage are then highlighted, which is expected to inspire further advancements and new insights into the fields of information storage and neuromorphic computing.

A Two-Dimensional Polyimide-Graphene Heterostructure with Ultra-fast Interlayer Charge Transfer.

Two‐dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic–inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on‐water surface synthesis of large‐area (cm2), monolayer 2D polyimide (2DPI) with 3.1‐nm lattice. Such 2DPI comprises metal‐free porphyrin and perylene units linked by imide bonds. We further achieve a scalable synthesis of 2DPI‐graphene (2DPI‐G) vdWHs via a face‐to‐face co‐assembly of graphene and 2DPI on the water surface. Remarkably, femtosecond transient absorption spectroscopy reveals an ultra‐fast interlayer charge transfer (ca. 60 fs) in the resultant 2DPI‐G vdWH upon protonation by acid, which is equivalent to that of the fastest reports among inorganic 2D vdWHs. Such large interlayer electronic coupling is ascribed to the interlayer cation–π interaction between 2DP and graphene.

A Two‐Dimensional Polyimide‐Graphene Heterostructure with Ultra‐fast Interlayer Charge Transfer

AbstractTwo‐dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic–inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on‐water surface synthesis of large‐area (cm2), monolayer 2D polyimide (2DPI) with 3.1‐nm lattice. Such 2DPI comprises metal‐free porphyrin and perylene units linked by imide bonds. We further achieve a scalable synthesis of 2DPI‐graphene (2DPI‐G) vdWHs via a face‐to‐face co‐assembly of graphene and 2DPI on the water surface. Remarkably, femtosecond transient absorption spectroscopy reveals an ultra‐fast interlayer charge transfer (ca. 60 fs) in the resultant 2DPI‐G vdWH upon protonation by acid, which is equivalent to that of the fastest reports among inorganic 2D vdWHs. Such large interlayer electronic coupling is ascribed to the interlayer cation–π interaction between 2DP and graphene.

CHARACTERIZATION OF ELECTRONIC STRUCTURES AT ORGANIC–2D MATERIALS INTERFACES WITH ADVANCED SYNCHROTRON-BASED SOFT X-RAY SPECTROSCOPY

The discovery of two-dimensional (2D) materials with single or a few-atom-thick layers has presented unprecedented opportunities to study, control and harness the properties of van der Waals (vdW) materials at their 2D limit. It also offers a platform that allows the arbitrary creation of heterostructures in a materials-by-design approach with properties tailored by the constituting components. Combining organic molecules with 2D materials to form hybrid heterointerfaces has recently emerged as a facile and versatile approach to engineer the electronic, optical, magnetic and chemical properties of 2D materials for new or optimized device applications. Underpinning the development of organic–2D materials heterostructures is the ability to interrogate the interfacial electronic structures and properties at multiple dimensions. This review provides a timely update on the application of synchrotron-based soft X-ray spectroscopies (SR-SXS) in the characterization of organic–2D materials interfaces. By harnessing the unparalleled high energy resolution, tunable energy, high brilliance and tunable polarization inherent to synchrotron radiation, electronic structures, charge transfer dynamics, molecular orientations and spin configurations at the hybrid interfaces can be examined, which helps us to formularize a coherent understanding of the organic–2D materials interfaces that will guide the design of new hybrid vdW structures and devices. We also offer our perspective on the future application of SR-SXS in the exploration of organic–2D materials heterostructures.

SCREENING EFFECTS AT ORGANIC–2D MATERIAL HETEROINTERFACES

Organic molecule–two-dimensional material (OM–2D material) heterostructures are relatively an emergent class of hybrid-based electronic devices and yet have gained enormous interests. These systems hold promise towards application in the realization of electronic, optoelectronic and energy conversion devices, due to their tunable thickness, rich raw materials, flexibility, and also their capability to merge the high light absorption efficiency of organic molecules and the high carrier mobility of 2D materials. Dielectric screening effects have a large influence on electronic structure and optical properties at the OM–2D material interface due to the low intrinsic dielectric polarizability of both organic molecules and 2D materials. Here, we will review current knowledge about the dielectric screening behaviors in 2D materials and the screening effect at the OM–2D material heterointerfaces, including the physical origin of the dielectric screening effect across such interfaces, how to investigate it, and the influence of the screening effect on OM–2D material interfaces. Engineering of the dielectric screening effects in OM–2D material systems is also discussed. We conclude the review by illustrating a brief introduction of other important factors, e.g. defects, charge transfer, and hybridization, that will contribute to the screening effect and these features should be considered for the future scientific and device development of OM–2D material heterostructures.

Exploring the fabrication and transfer mechanism of metallic nanostructures on carbon nanomembranes via focused electron beam induced processing.

Focused electron beam-induced processing is a versatile method for the fabrication of metallic nanostructures with arbitrary shape, in particular, on top of two-dimensional (2D) organic materials, such as self-assembled monolayers (SAMs). Two methods, namely electron beam-induced deposition (EBID) and electron beam-induced surface activation (EBISA) are studied with the precursors Fe(CO)5 and Co(CO)3NO on SAMs of 1,1′,4′,1′′-terphenyl-4-thiol (TPT). For Co(CO)3NO only EBID leads to deposits consisting of cobalt oxide. In the case of Fe(CO)5 EBID and EBISA yield deposits consisting of iron nanocrystals with high purity. Remarkably, the EBISA process exhibits a strong time dependence, which is analyzed in detail for different electron doses. This time dependence is a new phenomenon, which, to the best of our knowledge, was not reported before. The electron-induced cross-linking of the SAM caused by the cleavage of C–H bonds and the subsequent formation of new C–C bonds between neighboring molecules also seems to play a crucial role in the EBISA process. Previous studies showed that iron nanostructures fabricated on top of a cross-linked SAM on Au/mica can be transferred to solid substrates and grids without any changes, aside from oxidation. Here we demonstrate that iron as well as cobalt oxide structures on top of a cross-linked SAM on Ag/mica do change more significantly. The Fe(NO3)3 solution used for etching of the Ag layer also dissolves the cobalt oxide structures and causes dissolution and reduction of the iron structures. These results demonstrate that the fabrication of hybrids of metallic nanostructures onto organic 2D materials is an intrinsically complex procedure. The interactions among the metallic deposits, the substrate for the growth of the SAM, and the associated etching/dissolving agent need to be considered and further studied.

Controlling pairing of \u03c0-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

Features that make macromolecules 2D polymers

The article compares two-dimensional (2D) polymers with their congeners, the abundantly available linear polymers (1D) to show that both kinds of macromolecules are in fact polymer molecules. Repeat units and end (or edge) groups fully describe both kinds of structures. Both structures are obtainable as individual entities. Growth mechanisms well known for 1D polymers have now also been either elucidated or postulated for the first few 2D polymers. 2D copolymers are accessible by reacting different monomers in different feed ratios. Molar masses and molar mass distributions are obtainable when analysing 2D polymers in monolayer sheet form.The author therefore concludes that 2D polymers are in fact polymer molecules, very much as their linear 1D counterparts and that both kinds of structures therefore belong in this very subcategory of macromolecules. Dimensionality differences come with significant property differences and may therefore justify dividing the subcategory ‘polymer molecules’ further into 1D, 2D and – in the near future - 3D polymer molecules. In terms of materials, 2D polymers establish a subclass of organic 2D materials with the unique characteristic of meeting the conditions for polymer molecules.

Electrically conductive covalent organic frameworks: bridging the fields of organic metals and 2D materials

We discuss the recent advances and main approaches towards the design of electrically conductive COFs, highlight some of the challenges in such design and give our views on the future directions of this field.

Microscopically tuning the graphene oxide framework for membrane separations: a review.

Membrane-based separations have been widely applied in gas, water and organic solvent purifications to reduce energy consumption and minimize environmental pollution. In recent years, graphene oxide (GO) membranes have attracted increasing attention due to their self-assembly ability and excellent stability. In this review, publications within the last 3 years on microscopically tuning the GO framework are summarized and reviewed. Various materials, including organic molecules, polymers, inorganic particles, ions and 2D materials, have been deployed to intercalate with GO nanosheets. Due to the varied interlayer spacing and packing structure, the developed GO composites exhibit enhanced stabilities and separation performances. In addition, designing horizontal GO membranes and functionalizing GO nanosheets have also been reported to improve the performance. This review sheds light on the techniques to microscopically tune the GO framework and the resulting macroscopic changes in membrane properties and performances.

Development of metal-free layered semiconductors for 2D organic field-effect transistors.

To this day, the active components of integrated circuits consist mostly of (semi-)metals. Concerns for raw material supply and pricing aside, the overreliance on (semi-)metals in electronics limits our abilities (i) to tune the properties and composition of the active components, (ii) to freely process their physical dimensions, and (iii) to expand their deployment to applications that require optical transparency, mechanical flexibility, and permeability. 2D organic semiconductors match these criteria more closely. In this review, we discuss a number of 2D organic materials that can facilitate charge transport across and in-between their π-conjugated layers as well as the challenges that arise from modulation and processing of organic polymer semiconductors in electronic devices such as organic field-effect transistors.

2D Monomolecular Nanosheets Based on Thiacalixarene Derivatives: Synthesis, Solid State Self-Assembly and Crystal Polymorphism.

Synthetic organic 2D materials are attracting careful attention of researchers due to their excellent functionality in various applications, including storage batteries, catalysis, thermoelectricity, advanced electronics, superconductors, optoelectronics, etc. In this work, thiacalix[4]arene derivatives functionalized by geranyl fragments at the lower rim in cone and 1,3-alternate conformations, that are capable of controlled self-assembly in a 2D nanostructures were synthesized. X-ray diffraction analysis showed the formation of 2D monomolecular-layer nanosheets from synthesized thiacalix[4]arenes, the distance between which depends on the stereoisomer used. It was established by DSC, FSC, and PXRD methods that the obtained macrocycles are capable of forming different crystalline polymorphs, moreover dimethyl sulphoxide (DMSO) is contributing to the formation of a more stable polymorph for cone stereoisomer. The obtained crystalline 2D materials based on synthesized thiacalix[4]arenes can find application in material science and medicine for the development of modern pharmaceuticals and new generation materials.

Synthesis methods of organic two‐dimensional materials

AbstractAmong various two‐dimensional (2D) materials, organic 2D polymers have attracted much attention, owing to their specific properties, such as lightweight, good flexibility, adjustable structure, and high adaptability. In recent years, more and more scientists have devoted to the research on their structural design, synthesis, characterization, and potential properties. However, in contrast to traditional one‐dimensional (1D) and three‐dimensional (3D) network macromolecules, the synthesis of 2D structures presents a challenge to polymer chemists, because polymerization usually takes place in a spatially random manner in solution‐phase synthesis. In this review, we will focus on the synthesis methods of organic 2D materials, which have played a pivotal role since the beginning of the development of this field. We will highlight the representative examples according to the different types of polymers, including supramolecular organic 2D layers and covalent organic 2D polymers, and identify possible future research directions.

Retracted: Emerging 2D MXene/Organic Heterostructures for Future Nanodevices

AbstractMXenes are a rapidly growing family of 2D materials. Their unique combination of metallic conductivity, hydrophilicity, and highly charged surfaces endow them with excellent electrochemical performance. Appropriate surface functional groups determine their semiconducting properties. The wide selection of various MXene structures and surface functional groups enable tunable work functions and bandgaps, making them prime candidates for many optoelectronic applications. In addition, the discovery of 2D organic semiconductors having single molecular thickness has also greatly attracted research attention due to their optical, electronic, optoelectronic, and mechatronic properties. Moreover, the formation of MXenes/organic 2D heterostructures enables many interesting phenomena in the 2D quantum limit to be observed. This review aims to increase motivation toward the investigation of newly emerging MXenes, 2D organic materials, and their combinational heterostructures. It overviews recent progress in the fundamental investigations of the optical, electronic, and optoelectronic properties in isolated 2D MXenes and organic materials; then, highlights the major importance of 2D MXene/organic heterostructures for applications in exciton–polariton lasers, light emitting devices, wearable electronics, and spintronic devices. The review aims outlines a future roadmap for 2D MXenes, organic materials, and their heterostructures for advanced nanotechnology applications.

Liquid-phase exfoliation of black sesame to create a nanoplatform for in vitro photoluminescence and photothermal therapy.

Aim: The present study aims to apply the facile liquid-phase exfoliation (LPE) strategy to fabricate 2D organic materials and thus to broaden the family of biocompatible and multifunctional 2D materials. Materials & methods: 2D material-organic melanin and cellulose nanosheets were synthesized from black sesame hull using LPE. Photoluminescence and photothermal properties of the nanosheets were assessed, as well as stability and cell killing ability. Results: The prepared 2D nanoplatform exhibited broad and multiple photoluminescent emission bands. It also demonstrated efficient photothermal cancer therapy with excellent biocompatibility. Conclusion: The present study could open an avenue in exfoliating organic materials using the LPE strategy. This could make the fabrication of multifunctional 2D organic materials more efficient and broaden the family of biocompatible 2D nanomaterials.